Introduction
Aortic valve stenosis due to calcification of the valve leaflets is the most common
valve disease in the developed world. It is the third leading cause of cardiovascular
disease.1 Risk factors include male gender, smoking, diabetes mellitus, hypertension,
high levels of circulating lipids, and metabolic syndrome.2 Calcification was earlier
believed to be a passive degenerative process, but it is now recognized as an active
disease process driven by the cells native to the aortic valve.3, 4 The only option
for treatment is heart surgery with implantation of a valve prosthesis. The heart
valve prostheses are either mechanical, requiring life‐long anticoagulation treatment,
or based on biological material, which will degenerate and calcify after 10 to 15 years.
Implantation of heart valve prostheses has been characterized as “replacing one disease
with another.” Understanding the cellular and molecular processes behind valve calcification
may possibly lead to nonsurgical treatment.
The aortic valve is composed of three fine collagenous leaflets attached to the fibrous
ring at the outlet of the left ventricle. The leaflets in young healthy humans are
a fraction of a millimeter thick, and in systole they are washed onto the walls of
the ascending aorta by the jet of blood.5 The leaflets are composed of a dense extracellular
matrix, usually delineated into 3 layers: lamina fibrosa, lamina spongiosa, and lamina
ventricularis5 (Figure 1). All 3 layers are populated with valve interstitial cells
(VICs), with the entire structure covered by valve endothelial cells (VECs). The 3
layers have different matrix compositions: lamina ventricularis is richer in elastin,
lamina spongiosa in proteoglycans, and lamina fibrosa in collagen.6 During embryogenesis
the endothelial cells covering the primordial valve cushions migrate inside the underlying
matrix and undergo endothelial‐to‐mesenchymal transition to become the interstitial
cells.7 Thus, the VICs have an endothelial origin.
Figure 1
Simplified structure of the human aortic valve leaflet. On the left is a schematic
cross section through the noncoronary leaflet of the aortic valve. The blowup on the
right shows the trilayered organization of the extracellular matrix and the localization
of the aortic valve endothelial cells (shortened throughout to VECs) and interstitial
cells (VICs).
The gross pathology of valve calcification has some similarities with atherosclerosis.
One of the early events is disruption of the endothelial layer on the aortic side
of the leaflets.8 This is followed by slow reorganization of the valve matrix via
metalloproteinases on top of subtle matrix changes occurring inevitably with age.
This leads to valve thickening known as aortic valve sclerosis, which usually is asymptomatic.
The leaflet is then infiltrated by immune cells; at the same time angiogenesis occurs
along with deposition of lipids, proteoglycans, and cell debris, which will deform
the leaflet.8, 9, 10 In the course of this process the valve matrix calcifies, and
the aortic leaflets become stiff, causing aortic stenosis and obstruction of blood
flow. With an orifice of under 1 cm2, versus 2.5 to 4.5 cm2 observed in a normal valve,
the stenotic valve generates a pressure gradient of over 40 mm Hg, representing a
severe degree of constriction.11
A hallmark of valve calcification is the inflammatory response. Valves are invaded
by blood‐borne immune cells, but there is gathering evidence that the immune response
in the resident cells plays a major role.12 Sterile inflammation may be triggered
by circulating lipids in the form of low‐density lipoproteids, which infiltrate the
valve matrix.6 A nonsterile inflammation might also occur, as bacterial infections
of the oral cavity may predispose to valve calcification.13
Most of the calcification occurring in the aortic valve is diffuse and is believed
to be secondary to the action of myofibroblasts. These cells stem from pathologically
differentiated VICs as a result of paracrine action, most probably mediated by the
transforming growth factor β‐1 (TGFβ1).14 The myofibroblasts contract the extracellular
matrix, and the cell aggregates embedded in the reorganized matrix undergo apoptosis,
which leads to the sedimentation of calcium salts around “nucleators” such as cleaved
chains of collagen type I and elastin, bone sialoprotein, apoptotic bodies, and others.15
About 13% of calcified aortic valves at the time of surgery are estimated to contain
true bone with osteoblasts and osteoclasts.8 Osteoblastic differentiation of VICs
is similar to physiological osteogenesis and is largely orchestrated by Runt‐related
transcription factor 2 (Runx2), a key transcriptional regulator of osteoblasts, as
well as bone morphogenetic proteins (BMPs). Osteoblasts secrete a number of proteins
highly expressed in bone tissue: osteocalcin, alkaline phosphatase, and osteopontin.
Finally, calcium crystals are laid in the matrix of the valve, and osteoblasts organize
them in structures similar to lamellar bone, while the osteoclasts (likely descendants
of recruited monocytes) control its turnover by resorption.16
Valve Interstitial Cells: A Model to Study Aortic Valve Calcification
Key Sources of Cells
Animal Cells
Most research on aortic valve calcification is performed on cells from large animals.
The primary source of cells due to the anatomical similarity to humans is the porcine
aortic valve.17, 18 Other large animal models include sheep19, 20, 21 and cow.22,
23, 24 Recently VICs were isolated and cultured from mice.25 An advantage of animal
cells over human cells is ease of access because cells can be isolated from all 4
heart valves for comparison, whereas in humans usually a single valve is obtained.20
The key disadvantage of animal cells is that whatever the result, experiments have
to be replicated in human cells before we can proceed any further with the application
of findings. To date, there is no head‐to‐head comparison of animal versus human VIC
with regard to key pathophysiological processes such as osteoblastic and myofibroblastic
differentiation. Another notion is that the most commonly used animals with native
genotype do not develop aortic valve calcification. Pigs on high‐cholesterol diet
develop valve sclerosis and calcific lesions.26 Some mouse strains are capable of
induced valve calcification, but others are not. Genetic modifications may be required
to induce the process roughly similar to that in humans,25 which confounds the model.
Experiments with larger animals require expensive animal housing facilities and dedicated
personnel or access to abattoirs.
Human Cells From Diseased Valves
Human cells can be accessible to researchers working in proximity to cardiac surgery
units. Humans with stenotic aortic valves are far from a uniform population, as are
the calcified valves themselves. The major variables, beside the patient age, sex,
and concomitant diseases, are the amount of calcium, the degree of stenosis (and,
hence, the pressure gradient), and the valvular anatomy.2 By default the aortic valve
is tricuspid; however, congenital anomalies occur. The most common anomaly (around
1.5%) is a bicuspid valve, with extremely rare variants including monocuspid, quadricuspid,
and pentacuspid. Among the valves operated for aortic stenosis, however, bicuspid
and tricuspid valves are represented equally.27 Such overrepresentation indicates
that a bicuspid aortic valve is a major risk factor for aortic stenosis and calcification.
Whether it is a genetic defect that underlies both the bicuspid morphology and the
increased ability for calcification, or altered morphology with different flow patterns
predispose to calcification, the initiating mechanisms of calcification are probably
quite different in bicuspid and tricuspid valves. Therefore, it is crucial to distinguish
between the 2 variants and to avoid pooling them in the same experimental group.
Gender‐related differences between VICs are an interesting and yet little investigated
area. It is known that being male is a risk factor for developing aortic stenosis.28
Transcriptome analysis performed on porcine VICs demonstrated substantial differences
between males and females, where male VICs had higher expression of factors driving
angiogenesis, lipid deposition, and inflammation, and the cells displayed a higher
rate of proliferation and apoptosis.29 However, these findings require verification
in human isolated VICs.
Human Control Cells
A major bottleneck is access to high‐quality control human material, which means healthy
human valves. Cells obtained from cadavers provide 1 option; however, the quality
of cadaver cells is more variable than that in cells isolated from fresh living tissue.
There are a number of available alternatives:
Donor hearts that were considered unusable for transplantation.30, 31 This is an ideal
source of control material but raises some complicated ethical questions. In Norway
the donor card system contains an option to donate one's body to research, but few
organ donors choose it.
Hearts removed from recipients of heart transplantation without a history of aortic
valve disease.32, 33, 34 Hearts in this group often include those from patients with
idiopathic dilated cardiomyopathy and postinfarction heart failure. It can be argued
whether these patients had a genetic or epigenetic abnormality that also affects the
biology of heart valves.
Noncalcified aortic valves removed because of valve insufficiency. Often this pathology
is combined with aneurysm of the aortic root and the ascending portion of the aorta.35
In some cases the surgeon can reconstruct the aortic root without removing the valve,
but many are still removed. This model has the same limitations as the previous one.
Pediatric valves removed due to congenital abnormality.36 This source has the same
weakness as the previous 2 models.
Valves extracted during operation for aortic dissection.35, 37 These valves are perhaps
closest to normal as long as the underlying predisposing cause of dissection is presumed
not to affect the aortic valve. One problem is to obtain the valves: these are acute
patients admitted at any time of the day or night.
Noncalcified cusps (or portions of the cusp) of the calcified valve.38 This is perhaps
the easiest material to obtain. However, its limitations are obvious. Nevertheless,
these cells can be used as an additional control for determining if the process in
question is intrinsic or extrinsic to the valve.39 These cells can also be used as
a reference for gene expression studies in the calcified portion.40
Optimal Conditions for Isolation and Culture of Valve Interstitial Cells
From the point of view of a laboratory technician, the VICs are similar to fibroblasts
elsewhere in the body.41 This includes relative robustness and long survival time
after the cessation of nutrient delivery, as in explanted valves or valves from cadavers.
Following surgery the valve leaflets should be placed in saline, Ringer solution,
or phosphate‐buffered saline, and for optimal results, the cells should be isolated
ex tempore. The first step in isolating VICs is removal of VECs, usually performed
by a 5‐minute exposure to collagenase II and swabbing.42 Despite the removal of VECs,
they still represent a significant (up to 10%) contamination of a porcine VIC culture.43
The 2 major methods of further VIC isolation/enrichment are based on either outgrowth
of cells from the sections onto the cell culture plastic44 or digestion of extracellular
matrix with subsequent centrifugation and seeding of the cell suspension. In the latter,
one can use collagenase I,36 collagenase II,45 or collagenase III.42
The culture of VICs is relatively routine using medium such as Dulbecco Modified Eagle
Medium45 or M199.13 The optimal type of serum is fetal bovine, which is superior to
bovine calf serum and newborn calf serum.18 Ten percent serum is usually supplemented
to the above base medium. In 10% serum, VICs double every 30 hours,46 reach confluence
in about 5 days,18 and should be passaged at 80% confluence. The concentration of
serum is empiric: in 15% to 20% serum VICs engage in active proliferation,47 whereas
at percentages below 10 they gradually become quiescent or differentiate into myofibroblasts.48
It is also possible to maintain proliferative ability in low serum by supplementing
the medium with insulin—in this case the impact of growth factors naturally present
in serum will be reduced.49 The authors of this study also suggest the use of fibroblast
growth factor (FGF) to block the spontaneous myofibroblastic differentiation of human
VICs.49 This is addressed below. VICs are stable for approximately 8 passages, but
the majority of studies are performed between passages 2 and 5 due to the changing
biology of the VICs.50
For some studies the isolation of VICs is not required. By stripping the endothelial
cells from a healthy valve with collagenase, the investigator is left with a leaflet
filled with VICs in situ, in their natural environment. These can then be exposed
to various stressors or treatments, and the gene regulation can only be attributed
to VICs.51 Instead of using a whole leaflet, it is also possible to use strips.52
In this review, however, we focus on studies performed with isolated VICs in culture
and not explanted valves.
An important issue is associated infection in VICs. Mycoplasma is especially common
in freshly isolated valves. Therefore, effective testing and quarantine measures are
to be taken when harvesting a fresh batch of valvular cells. If infected, the cells
should be discarded or, if especially valuable, may be treated with, for example,
Plasmocin (InvivoGen, San Diego, CA) for 2 weeks to eradicate Mycoplasma. However,
results obtained with these cells should be interpreted with caution. Whether the
source of contamination is the patient or it occurs somewhere on the way to the bench
in unclear.
Key Properties of Valvular Interstitial Cells
A number of experimental studies have addressed the biological properties of VIC and
developed original methodology now forming the backbone of the VIC experimental repertoire.
We chose the studies we deem most significant in this respect, and present a short
list in chronological order in Table 1.
Table 1
Key Studies With Original Methodology Revealing Important Properties of Isolated,
Cultured VICs
Reference No.
Author, Year
VIC Source
Technique
Key Results
20
Merryman, 2006
Ovine
VICs isolated from all 4 heart valves are tested for stiffness using a pipette aspirator
The left‐sided cells are stiffer, express more αSMA and heat shock protein 47, a chaperone
of collagen
17
Butcher, 2006
Porcine
Cocultures of VIC with VEC in proprietary bioreactor with tubular molds exposed to
shear stress
VECs downregulate expression of αSMA in VICs and oppose proliferation of VICs in response
to shear stress
38
Clark‐Greuel, 2007
Ovine
Timeline of gene expression and calcium accumulation in response to TGFβ1 stimulation
Calcium deposits appear and alkaline phosphatase activity increases after 72 h; matrix
metalloproteinase 9 expression is increased after 7 d
18
Bond, 2007
Porcine
Comparison of different serum variants for VIC culture
Cells prefer fetal bovine serum of all sera, reach confluence within 5 d
53
Merryman, 2007
Porcine
Study of inherent stiffness of aortic and pulmonary VIC and ability to contract collagen
gels.
Aortic VICs are stiffer than pulmonary and contract gel matrix more quickly
54
Benton, 2008
Porcine
Test of substrate coatings for VIC: plastic vs polyethylene glycol hydrogel, bare
or coated with fibronectin or fibrin
VIC form nodules better on bare plastic and fibrin, but not on fibronectin; polyethylene
glycol hydrogel, coated or uncoated, suppresses calcification
55
Yip, 2009
Porcine
Comparison of substrate (matrix) of variable stiffness for culturing and osteogenic
differentiation.
VIC proliferated better on compliant matrix and calcify more readily in osteogenic
medium
56
Benton, 2009
Porcine
Timeline and mechanism of nodule formation in response to TGFβ1
The VICs are torn from culture plastic by contraction and clump together into a nodule
48
Gu, 2010
Porcine
A comparison of 3 peptide coatings for their ability to induce nodule formation
RGD‐containing peptide is the most procalcific; RGD is common for fibrin, collagen,
fibronectin, laminin
57
Hakuno, 2010
Murine (rat)
A large study on periostin in valve disease; includes isolation of VIC from rats
58
Bertacco, 2010
Bovine
Proteomic analysis of the calcifying clones of VIC, 3D culture (collagen I sponges)
Calcification is triggered by exposure to lipopolysaccharide and inorganic phosphate.
21
Gwanmesia, 2010
Ovine
Comparison of different coatings for VICs
VICs grown on fibronectin formed very few nodules and showed no apoptosis
59
Rodriguez, 2011
Porcine
VIC cultured on hyaluronic acid of variable molecular weight
Culturing VICs on hyaluronic acid reduces nodule formation; optimal hyaluronan MW
64 kDa
35
Yu, 2011
Human
Stimulation of control and calcified VICs with tumor necrosis factor α
TNFα induces calcification only in calcified VIC, the effect is mediated by bone morphogenetic
protein 2 and nuclear factor κB
31
Ferdous, 2011
Human
Healthy VIC and aortic smooth muscle cells are stretched in tubular molds with collagen.
Stretch triggers calcification in both cell types
26
Yip, 2011
Porcine
Cultured VIC from normal and high‐cholesterol‐fed pigs
The aortic valves are stenotic in high‐cholesterol‐fed pigs, cells are thus a feasible
model of human disease
60
Hutcheson, 2012
Porcine
Steady stretch in 2D cultures using FlexCell®, and in 3D cultures in proprietary bioreactor
Steady stretch induces intracellular calcium accumulation, which triggers apoptosis
in VIC
61
Wyss, 2012
Porcine
Elastic modulus of VICs
Elastic modulus in cultured VICs increases over time and passaging, proportional to
αSMA expression
62
Gould, 2012
Porcine
A proprietary bioreactor for application of isotropic (circular molds) and anisotropic
(oval molds) stretch to a 3D culture
VICs orient along the longer axis of anisotropy and remodel the matrix the same way;
anisotropy also increases apoptosis and proliferation in VICs
63
Fisher, 2013
Porcine
FlexCell® treatment of VIC alongside stimulation with TGFβ1
Both TGFβ1 and the degree of stretch are proportional to the quantity of calcific
nodules
64
Quinlan, 2012
Porcine
Effect of stiffness of collagen‐coated polyacrylamide gel substrate on VIC proliferation
Loose gels: cuboidal cells, no stress fibers. Stiff gels: sprouting cells with stress
fibers made of αSMA
29
McCoy, 2012
Porcine
Microarray study of untreated VIC and ingenuity pathway analysis with emphasis on
gender differences
Male VICs have higher expression of genes responsible for proliferation and apoptosis;
functional implications are confirmed by cell behavior in culture
65
Monzack, 2012
Porcine
Time‐dependent response of VICs to growth, myofibroblast and osteogenic medium
Cells in osteogenic medium have increased alkaline phosphatase activity, high proliferation,
downregulation of αSMA.
66
Moraes, 2013
Porcine
A proprietary bioreactor to stretch VICs over the polyurethane membranes; isolation
of VIC from lamina ventricularis and lamina fibrosa
The VICs adhere better to polyurethane than plastic; stretch triggers myofibroblastic
differentiated VIC from ventricularus express more αSMA than those from fibrosa, also
in response to TGFβ1.
67
Richards, 2013
Porcine
Cocultures of VIC and VEC, both attached, detached, and treated with osteogenic medium
Osteogenic medium triggers calcification only in attached VIC monocultures; VEC inhibit
calcification driven by osteogenic medium
68
Ferdous, 2013
Human
Long‐term (3 w) cyclic stretch using a proprietary bioreactor for tubular molds
Different stretch amplitudes are tested; 10% stretch was most procalcific. Arsenazo
dye used for calcium.
24
Duan, 2013
Bovine
VIC cultured in hyaluronic acid‐based hydrogels; VIC culture using hanging drop method
43
Wang, 2013
Porcine
Flow cytometry analysis of isolated VIC using markers specific for various cell types
Under 10% of isolated cells are positive for other markers than fibroblasts. Some
of the cells are VEC; some myofibroblasts express endothelial markers too.
69
Gould, 2014
Porcine
Isolation and coculture of VIC and VEC using peptide‐functionalized polyethylene glycol
gels.
Harder substrates promote myofibroblast differentiation, softer ones purely fibroblastic.
VECs in coculture inhibit myofibroblast differentiation of VIC via nitric oxide signaling.
25
El Husseini, 2014
Murine (mouse)
Isolation and culture of murine VICs
49
Latif, 2016
Human
Optimal conditions for culture of human VIC
Dulbecco modified Eagle medium induces myofibroblast differentiation in VIC. Medium
that does not contains 2% fetal calf serum, 50 ng/mL insulin, 10 nm/mL fibroblast
growth factor.
70
Porras, 2017
Porcine
Optimal conditions for culture of porcine VIC
As in previous study, to prevent myofibroblast differentiation one should use 2% fetal
calf serum, 5.25 μg/mL insulin, 10 nm/mL fibroblast growth factor
3D indicates 3‐dimensional; 2D, 2‐dimensional; αSMA, α smooth muscle actin; MW, molecular
weight; TGFβ1, transforming growth factor β1; TNFα, tumor necrosis factor α; VEC,
valve endothelial cells; VIC, valve interstitial cells.
Cell‐Specific Markers of VICs
A practical concern during isolation of VICs is cross‐contamination with VECs. Healthy
human VICs can be distinguished from VECs by positive α‐smooth muscle actin (αSMA)
and prolyl‐4‐hydrolase staining and/or absence of CD31 and von Willebrand factor.71
VECs are also positive for VE‐cadherin, whereas VICs are not.72 These markers can
be used for separating live cells using, for example, magnetic assisted cell sorting.
After the VICs have been separated from VECs, it is important to be able to define
the cell populations within the VIC culture. VICs have a differentiation potential
reminiscent of mesenchymal stem cells: in the Petri dish they can differentiate into
adipocytes, osteoblasts, and chondrocytes,73 depending on the molecular triggers and
physical environment. In diseased valves the physical and biochemical milieu changes
so radically over time that the resident cells can undergo differentiation into other
mesenchymal cell types. Therefore, it is crucial to have a definition for what an
actual VIC is. The most commonly used markers that define VICs, differentiating them
from VECs, are αSMA, smooth muscle myosin, vimentin, and desmin.33, 47 However, expression
levels need to be taken into account. At higher levels, αSMA is a marker of myofibroblastic
differentiation; this is why it is imperative to use additional markers such as desmin,
which is low in VICs isolated from healthy human valves and high in stenotic valve
cusps.41 Another consideration is interspecies differences. Ovine valves express desmin
and calponin but not vimentin or αSMA.19 Sixty‐four percent of healthy pig VICs express
αSMA, 98% express vimentin, and 90% express smooth muscle myosin.52 The expression
of αSMA in porcine VICs is generally low initially after isolation, but a small population
of cells can express αSMA to the level of myofibroblasts.46
Interestingly, expression of αSMA increases in the course of primary isolation of
cells, up to 4‐fold by 7 days. However, if the cells are appropriately passaged, the
expression level remains low.46 Recently developed medium formulations allow keeping
levels of αSMA consistently low, maintaining the native phenotype over a long time.49,
70 Finally, VICs are also a heterogeneous population when it comes to valve‐side–specific
differences. Porcine VICs from the ventricular side, populating the “lamina ventricularis,”
express more αSMA and respond with higher levels of αSMA to TGFβ1 stimulation .66
The observed heterogeneity of VICs has led to attempts at segregating these subpopulations.
An early study described 2 types of cells isolated from porcine valves: secretory
(containing vesicles) and contractile, where contraction was enhanced in response
to angiotensin II, bradykinin, epinephrine, and isoproterenol.47 One may speculate
that the latter population represented myofibroblasts. A later report outlined 3 cell
types derived from both human and porcine valve cusps: fibroblasts, myofibroblasts,
and cells resembling smooth muscle cells, which expressed both αSMA and smoothelin
and displayed contractile properties.44 A recent study of bovine VICs revealed 4 unique
cell populations with differential expression of αSMA, SM22, smooth muscle myosin,
osteocalcin, and osteopontin.22 Fluorescence‐assisted cell sorting analysis of porcine
VICs, obtained from different layers of the valve leaflet, also revealed 4 populations
distinguished by cell type–specific markers: contaminating endothelial cells (about
10% positive cells), myofibroblasts, cells bearing known markers of embryonic stem
cells, and progenitor cells. Some VECs expressed markers of myofibroblasts; thus,
part of the latter might have endothelial origin—cells that underwent endothelial‐to‐mesenchymal
transition. The myofibroblasts were mainly localized on the ventricular side, and
the cells positive for markers of embryonic stem cells were found in the spongiosa.43
In all, there is probably a continuous spectrum of cells covering a smooth transition
between more defined cell types such as the VICs, smooth muscle cells, osteoblasts,
and potentially others as well. The reason for this might be the anisotropy of forces
that the valve cusps are exposed to. Thus, cells with different properties are needed
to maintain the distension, compression, shear stress, etc. There are no parts of
our body that are exposed to so much beating continuously during our whole lives.
Mammalian evolution had to develop an extraordinary structure in its light weight,
robustness, and precision, and we are yet to understand how the aortic valve is built
to these specifications using the cells we find in it.
Mechanical Properties of VICs
All VICs are robust cells, but they are different across the different valves, even
though by gross appearance these cells appear similar.74 Porcine VICs isolated from
the left side of the heart (mitral and aortic valves) have higher stiffness than the
pulmonary and tricuspid VICs, as tested by an aspirator pipette, an effect attributed
to higher expression of αSMA and heat shock protein 47.20 The observed differences
in mechanical properties were further confirmed with an atomic force microscope.53
Porcine aortic VICs also display a greater ability to contract the extracellular matrix
than their pulmonary counterparts. However, dermal fibroblasts contract even more
efficiently.53 Interestingly, the mechanical stiffness of porcine VICs in culture
increases with passaging and with culturing on harder substrates (stiffer collagen
gels).61 The tougher character of the aortic VICs is understandable because the pressure
gradient across the aortic valve is greater.
Choice of Substrate for VICs
Biology of VICs is highly dependent on the substrate they are maintained on as well
as the configuration of the culture. Two‐dimensional cell cultures are the least representative
of the in vivo situation; however, because of their simplicity they are widely employed
to study VICs. Following isolation the VICs are spindle‐shaped, whereupon they assume
a flattened sprouting shape and create a monolayer in the dish. VICs have been grown
on glass, plastic alone, and plastic coated with collagen, elastin, fibronectin, and
so on.66 In addition, the cells can be grown on elastic silicone membranes to facilitate
studies of biomechanics. Porcine VICs grown on silicone rubber demonstrate an ability
to compress and wrinkle the substrate.47 The latter creates an opportunity to study
the interaction of the cells with the substrate and measures it by the extent of deformation.
A more advanced and “physiological” option is to study the cells in the 3‐dimensional
(3D) environment. The most basic of these is a 3D collagen hydrogel culture, where
the cells are essentially cast in a collagen solution, which is then allowed to stiffen/gel.
This model allows for measurement of the contractile ability of the fibroblasts by
studying the dynamics of gel compaction.18 Different densities of collagen gel have
different effects on cell viability and proliferation. A comparison of collagen gels
(at 1%, 2%, and 5%) demonstrated that the maximum effect on proliferation of healthy
human VICs was observed in a 1% gel; however, these VICs also had an increased expression
of αSMA, collagen I and III, and a number of matrix metalloproteinases (MMPs), possibly
reflecting myofibroblastic differentiation.75 Such phenotypes may result in unwanted
matrix remodeling, and cell behavior will be further from the physiological situation.
Human VICs grown in 2% collagen displayed a more quiescent phenotype and gene expression
pattern,75 and this density is optimal for studying gel compression.18, 76 A more
sophisticated model to study the response of VICs to stretch involves casting a 3D
collagen gel culture with VICs in elastic molds and subjecting them to stretch.62
For studying the interplay between VICs and VECs, collagen gels populated with VICs
can be further seeded with VECs, thereby creating a coculture.17 As an alternative
to casting cells in a collagen gel, they can be seeded on microfibrillar sponges made
from collagen.58 The newest development in VIC collagen culture constructs is stacked
layers of filter paper embedded in collagen and seeded with the cells. The porcine
VICs not only proliferate in this environment but also show a phenotypic consistency
characteristic of healthy cells.77
Collagen is not the only option for creating a 3D culture of VICs. The porcine VICs
thrive in a polyethylene glycol hydrogel uncoated or coated with fibronectin or fibrin
just as well as on the tissue culture plastic.48, 54 Polyethylene glycol hydrogels
can be dynamically stiffened: after the cells have been seeded, the cross‐linker is
added, and the elastic modulus can be brought to the desired modality.78
In another study a polyacrylamide gel of variable stiffness was coated with collagen
and seeded with porcine VICs. The cells grown on looser gels displayed a cuboidal
shape, with the absence of stress fibers, but VICs maintained on stiffer matrices
expressed αSMA and sprouting morphology.64
Culturing bovine VICs on hyaluronic acid–based hydrogels yielded viability greater
than 75% over 14 days but affected cell spreading, keeping them in a spherical shape
for several days. The advantage of matrices made of hyaluronic acid is their improved
stability over time compared with collagen.24
The choice of substrate for VICs is important because the aortic valve is a very attractive
target for bioengineering and 1 of the structures that possibly will be bioprinted.
However, it needs to be self‐renewing and self‐maintaining. When we know more about
the substrate preference of VICs, we may preseed the bioprinted valves with the mesenchymal
stem cells from the same patient and hopefully expect them to differentiate into the
fully functional VICs.
Pathological Differentiation of VICs
Today's understanding of aortic valve calcification builds on the concept of diffuse
calcification secondary to myofibroblast differentiation of VICs (sometimes called
the dystrophic calcification) and ossification carried out in a regulated fashion
by osteoblasts, also stemming from resident interstitial cells. The relative contribution
of each of these 2 processes is at present unknown. For now, this dichotomy leads
research into 2 main courses; pursuing 1 or another is for now a matter of preference
or belief.
Myofibroblastic Differentiation
Activation of interstitial cells (fibroblasts) and myofibroblastic transformation
are normal regenerative processes throughout the body.79 As shown above, myofibroblasts
are claimed to be present among the interstitial cells populating the aortic valves
normally.44 Myofibroblasts can be defined as fibroblasts that assume some of the properties
of smooth muscle cells and are able to contract the extracellular matrix. Myofibroblast‐specific
factors are often listed as follows: αSMA, smooth muscle myosin, and vimentin.52 Strikingly,
these are the same factors that are often used to distinguish the VICs themselves.
However, myofibroblasts usually present with much higher levels of αSMA, and they
are much more active at contraction than the healthy VICs. Recent studies point out
that native VICs should not be positive for αSMA, and it is a sign of unwanted phenotype
change. By maintaining the culture in low serum and additionally stimulating cells
with FGF‐2, human VICs in culture can be prevented from activation and spontaneous
myofibroblast differentiation.49
The most commonly used approach to differentiate VICs into myofibroblasts is to stimulate
them with TGFβ1. The first measurable effect of TGFβ1 on VICs is increased expression
of αSMA as assessed by quantitative PCR, immunostaining, or Western blot. αSMA organizes
into bundles, which are often called “stress fibers.” The next major event in myofibroblast
differentiation in vitro is formation of nodules, a process more pronounced when the
cells are cultured on laminin and fibrin80 but that can also be present on plastic,
where 5 ng/mL of TGFβ1 is sufficient for maximal effect.81 The nodules form over the
course of several days. The process is more or less rapid depending on the substrate
and is driven by αSMA, which literally tears the cells from the surface.56 Finally,
after nodule formation and compaction, the cells within the nodule undergo dystrophic
changes and apoptosis, leading to leak of factors that stimulate nucleation of calcium
crystals.79 The process of calcification in TGFβ1‐treated cultures takes days. The
first calcium deposits and nodules are observed after 72 hours and increase with time.
Ovine VICs in the nodule become apoptotic from around day 14.38 The calcification
in culture is usually quantified after 21 days.
TGFβ1 belongs to the transforming growth factor superfamily, which signals through
the cell surface TGFβ receptor. The signal is further conveyed via the canonical pathway
involving small mothers against decapentaplegic (SMADs), or a noncanonical pathway
involving mitogen‐activated protein kinase p38. SMAD2 and 3 are activated in both
porcine and calcified human VICs in response to TGFβ1 stimulation.42 The procalcific
effects include enhanced binding of low‐density lipoproteins to glycosaminoglycan
chains; this effect is higher the longer the chains are.42 Nodule formation and alkaline
phosphatase (ALP) activity in porcine VICs are mediated by the noncanonical pathway
of TGFβ1.82 Mitogen‐activated kinase p42/44 (ERK) activated downstream of the TGFβ1
receptor triggers expression of cadherin‐11, which enables nodule formation by binding
to αSMA. Blocking ERK phosphorylation or knocking down cadherin‐11 expression with
small interfering RNA (siRNA) abolishes nodule formation.79 Other members of the TGFβ
superfamily can also cause calcification as shown by the observation that healthy
human VICs treated with TGFβ1, TGFβ3, BMP‐2, ‐4, or ‐7 show increased ALP activity
and osteocalcin expression comparable to that triggered by osteogenic medium.32
TGFβ1 treatment is not absolutely necessary to achieve nodule formation. Instead,
a combination of low serum concentration (1% or even 0.5%) and an appropriate extracellular
matrix is enough to stimulate this response.21, 48 Porcine VICs display dramatic nodule
formation after just 5 days of culture on fibrin‐coated or untreated plastic in 1%
serum. Ovine VICs form nodules after just 24 hours in 0.5% serum.21 The characteristic
of human VIC culture in low‐serum conditions is the associated increased apoptosis,
which is believed to be integral to calcification.39
One possible reason substrate is important for nodule formation is the tensile stress
experienced by VICs. Growing VICs on a hard/stiff substrate in 3D (thicker collagen
gel matrix) is essential for αSMA expression.46 Tissue culture on plastic (stiff compliance)
favors calcification, whereas cultures of porcine VICs on softer polyethylene glycol
hydrogel calcify considerably less on treatment with TGFβ1.54 Myofibroblast differentiation
of porcine VICs is activated on stiff collagen matrices in the presence of osteogenic
medium and the absence of TGFβ1. These cells express αSMA in stress fibers and display
high elastic modulus. The same cells on lower‐compliance gels aggregate and differentiate
into osteoblasts.61 Porcine VICs cultured on stiff polyacrylamide matrices were avidly
stimulated by TGFβ1 to express stress fibers.64 Of note, tensile stress is not unique
in this ability to stimulate myofibroblast differentiation. Shear stress applied to
VICs in collagen also triggers expression of αSMA.17
Furthermore, the effect varies depending on the extracellular matrix coating of the
surface. For example, fibronectin efficiently inhibits nodule formation and expression
of both Runx2 and αSMA in porcine VICs, whereas fibrin, on the other hand, promotes
calcification.54 Collagen, fibronectin, and laminin oppose the nodule formation in
porcine cells, with fibronectin also opposing it in ovine VICs.21 Integrin receptors
that interact with specific peptides of the extracellular matrix are believed to explain
this difference; the RGDS peptide was most procalcific of the ones tested using porcine
VICs.48 The results obtained on 2D‐cultures were reproduced in a pilot 3D culture
experiment with polyethylene glycol hydrogel coated with different peptides; again,
the RGDS peptide was procalcific.48 Further development of this model, with dynamically
stiffened hydrogels, showed that progressive decline in αSMA expression correlated
with the increase in gel stiffness, which is contradictory to the studies involving
collagen hydrogels.78
We regard calcium accumulation and its possible inhibition as the primary end points
when studying mechanisms of calcification. An important point about models of myofibroblastic
differentiation is that calcium accumulation is usually not measured in these studies.
Indeed, data on calcium deposition measured by alizarin red are scarce. The main end
points are the nodule formation or expression of procalcific genes. This may be a
major weakness of these studies, and they should be interpreted with caution.
Osteoblastic Differentiation
Practically any interstitial cells can differentiate into osteoblasts when stimulated
with appropriate stimuli. This explains a large versatility of ectopic calcification
that may occur after tissue injury in most parts of the body. The VICs are no exception.
In cell cultures the earliest signs of calcification are usually flagged by increased
activity of ALP and activation of the osteoblastic factors Runx2 and BMP2.36 Runx2
is the master transcriptional regulator of the osteoblastic lineage, and its expression
manifests commitment of the cells. The late and the ultimate indicator of calcification
is calcium as detected by alizarin red,54 Von Kossa staining,58, 73 or arsenazo dye.68
The osteoblastic pathway does not normally include activation of αSMA or the contraction
of extracellular matrix, although there is a certain degree of overlap.61
Standard osteogenic medium contains β‐glycerophosphate, dexamethasone, and ascorbic
acid. β‐Glycerophosphate is an organic donor of phosphate groups, necessary for the
formation of calcium phosphate crystals.13 Osteogenic medium applied to human VICs
for 21 days induces increased ALP activity and osteocalcin32 as well as Runx2 expression.40
Effects similar to osteogenic medium can be achieved in human VICs by adenosine triphosphate.71
Osteogenic medium can also be supplemented with 50 ng/mL of BMP2.39
The variability described cell specific markers of VIC with respect to VICs after
isolation is also observed in their response to osteogenic medium, and heterogeneity
is found even among VICs isolated from the same valve. Porcine VICs were shown to
form 4 types of colonies, with only 1 of them being responsive to calcification.73
The proportion of calcifying colonies decreased with passage number; therefore, experiments
with osteogenic medium should be carried out on low‐passage VICs. In response to osteogenic
medium the cells form calcifying colonies as well as nodules grossly similar to those
triggered by TGFβ1. However, the cells within the nodule present with a cuboidal morphology
characteristic of osteoblasts.73 Flow cytometry established that the porcine VICs
most active in calcium accumulation were positive for the transporter ABCG2 (ATP‐binding
cassette, sub‐family G, member 2), a side population progenitor cell marker.43
The response of VICs to osteogenic medium is dependent on the underlying surface as
observed with the response to TGFβ1. Only soft collagen matrices promoted osteoblastic
differentiation, in contrast to the stiffer matrices, whereas the opposite was observed
with TGFβ1‐induced aggregation of VICs into nodules. VICs cultured in stiff matrices
also expressed higher levels of TGFβ receptor.73 Again, calcium accumulation occurs
in both models but by different mechanisms: apoptosis in TGFβ1‐treated porcine cultures
or via osteoblast differentiation in osteogenic medium‐treated cells.55
Interesting data have been obtained when porcine VICs were compared side by side to
murine calvarian preosteoblasts MC3T3 and embryonic derived fibroblasts C3H10T for
their capacity to respond to osteogenic medium.83 The end points were ALP, osteocalcin,
and αSMA expression, respectively. The porcine VICs downregulated αSMA and osteocalcin
but upregulated ALP, whereas MC3T3 cells downregulated αSMA but upregulated ALP and
osteocalcin. In murine embryonic fibroblasts C3H10T, osteogenic medium upregulated
all 3 markers.83
Overlap Between Myofibroblast and Osteoblast Pathways
The 2 pathways of valve calcification are not necessarily mutually exclusive in the
same cell population. It is unknown whether the given cell assumes characteristics
of both myofibroblast and osteoblast, or 2 cells from the same source, treated with
the same cocktail, react differently to it. In any event, on treatment with osteogenic
medium, murine embryonic fibroblasts, C3H10T,83 and porcine VICs in 3D collagen matrices
expressed increased levels of αSMA indicative of myofibroblast differentiation.61
Cloyd and co‐authors treated porcine VICs and murine primary neonatal calvarian osteoblasts
with a combination of osteogenic medium and TGFβ1. The derived nodules were analyzed
after 21 days. The nodules formed by the VICs were rich in collagen type 1 and stained
positive for calcium, similarly to the nodules produced in mouse osteoblast culture.
However, the expression of osteocalcin was lower in all cells treated with the combination
of osteogenic medium and TGFβ1.84 The authors stated that this combination of treatments
resulted in a myofibroblastic and not osteoblastic phenotype.
Role of Mechanical Stress in Differentiation of VICs
As with most fibroblasts, VIC phenotype is dependent on mechanical stimulation. Valve
leaflets are exposed to the highest pressure gradients and jet speeds in the entire
circulatory system, and the mechanobiology of VICs is crucial for their phenotype.
Models of mechanical stress of isolated VICs can be employed alone or in combination
with pathological differentiation (osteogenic medium and TGFβ1 stimulation).
A popular in vitro model for studying stretch in VICs is FlexCell® bioreactor by FlexCell
International (Burlington, NC). The magnitude of stretch required to induce calcification
is an open question. Some believe that 10% is actually a physiological distention
for the aortic valve, and for an “overstretch” one should go up to 15%.68 Porcine
VICs grow eagerly on both collagen‐ and laminin‐coated Bioflex plates and respond
to stretch by expressing collagen III. The effect is both time and intensity dependent.45
Furthermore, the FlexCell treatment of human VICs from stenotic valves at 1 Hz, 10%
distention for 72 hours upregulates osteocalcin, osteopontin, ALP, tenascin‐C, and
biglycan.85 Shorter stretching times in porcine VICs showed intracellular accumulation
of calcium that leads to apoptosis; calcium influx is also proportional to the degree
of strain.60 The osteoblastic differentiation triggered by stretch can be further
augmented by combining it with TGFβ1. The result is a marked enhancement of nodule
formation in porcine VICs proportional to the degree of stretch and TGFβ1 concentration.
For the granules to calcify, TGFβ1 has to be added first, followed by the applied
stress.63 It should be noted that for these experiments the authors chose BioFlex
plates that had been coated with Pronectin, a “positively charged, protein polymer
which incorporates multiple copies of the RGD cell attachment epitope derived from
human fibronectin between repeated structural peptide units” (Sanyo Chemical Industries,
Kyoto, Japan). It should be reiterated that fibronectin inhibits calcification in
the experiments where stretch is not applied.21, 54
Mechanical stress in the valve leaflet is anisotropic: it is different for the cells
on 2 opposite sides as well as for cells near the free edge versus cells near the
aortic wall. Porcine VICs subjected to anisotropic stress (oval molds) in 3D collagen
gels align along the longer axis of anisotropy and remodel the surrounding matrix
in the same way. Anisotropic stress, compared with a uniform one, also promotes VIC
apoptosis and proliferation.62 This finding demonstrates how the leaflet morphology
can affect cell function; this may contribute to the tendency of abnormally shaped
bicuspid valves to calcify faster than their tricuspid counterparts.
Stretch is not the only mechanical stress modality that influences VICs. Pressure
is indispensable for the calcification of bone, a principle known as the Wolff law.
For example, in vivo bending stress applied to the rat tibia increases the proliferation
of preosteoblasts86 and ALP activity in the bone.87 To date there is limited information
on the effect of pressure on calcification of VICs. Porcine valve leaflets stripped
of endothelium were subjected to pressure in a custom‐built bioreactor, and mRNA was
analyzed by microarray. A number of inflammatory factors, mainly centering around
tumor necrosis factor alpha (TNFα), were upregulated in the compressed cells.51
Key Findings Regarding Aortic Valve Calcification Obtained Using Valvular Interstitial
Cells
VICs provide an attractive model for studying aortic valve calcification, and it has
been around for at least 20 years. In this section we highlight the most interesting
findings obtained using these cells. The most prominent findings are summarized in
chronological order in Table 2 and are also represented in graphical form in Figure 2.
Table 2
Key Findings Obtained Using VIC Regarding Calcific Aortic Valve Disease
Reference No.
Author, Date
VIC Source
Factor
Key Result, End Point
19
Jian, 2002
Ovine
Serotonin
Serotonin induces TGFβ1, and they both trigger matrix remodeling.
32
Osman, 2006
Human, noncalcified
TGFβ family cytokines, statins
TGFβ family cytokines increase osteoblast differentiation; atorvastatin inhibits it.
71
Osman, 2006
Human, noncalcified
Adenosine triphosphate, statins
Adenosine triphosphate activates osteoblast differentiation; atorvastatin inhibits
this effect.
40
Osman, 2007
Human, calcified
β1‐, β2‐, β3‐Adrenoreceptors
β1‐Adrenoreceptor mRNA is upregulated during osteoblast differentiation; salmeterol
(selective β2‐agonist) reduces osteoblastic differentiation.
76
Cushing, 2008
Porcine
Fibroblast growth factor 2
Fibroblast growth factor 2, via mitogen‐associated protein kinases, inhibits myofibroblast
differentiation induced by TGFβ1.
81
Kennedy, 2009
Porcine
Nitric oxide signaling
Nodule formation induced by TGFβ1 is inhibited by nitric oxide donor via cyclic guanosine
monophosphate signalling.
30
Yang, 2009
Human, calcified and noncalcified
LPS and peptidoglycan
LPS and peptidoglycan stimulate osteoblast differentiation via toll‐like receptors
2 and 4.
88
Yang, 2009
Human, calcified and noncalcified
BMP2
BMP2 induces initial stages of osteoblast differentiation via canonical and noncanonical
pathways.
56
Benton, 2009
Porcine
Statins
Pravastatin inhibits myofibroblast differentiation via Rho kinase, HMG‐CoA, and myosin
light chain kinase.
89
Nigam, 2009
Ovine
Notch1
Notch cleavage inhibitor and siRNA both cause osteoblast differentiation in sheep
VIC through BMP2.
58
Bertacco, 2010
Bovine
l‐Arginine LPS
LPS increases osteoblast differentiation in calcifying VIC; l‐arginine diminishes
this effect, possibly via nitric oxide signaling.
21
Gwanmesia, 2010
Ovine
Vascular endothelial growth factor
Vascular endothelial growth factor treatment combined with fibronectin coating prevents
calcified nodule formation, calcification, and apoptosis.
59
Rodriguez, 2011
Porcine
Hyaluronan
Culturing VICs on hyaluronic acid reduces nodule formation; adding it to the medium
reduces calcification.
90
Chen, 2011
Porcine
Wnt3, β‐catenin
TGFβ1 and Wnt3A synergistically induce myofibroblast differentiation via β‐catenin.
35
Yu, 2011
Human, calcified and noncalcified
Tumor necrosis factor α, BMP2
Tumor necrosis factor α induces osteoblast differentiation only in calcified VICs
through BMP2 and NFkB signalling.
91
Carthy, 2012
Human, noncalcified
Versican
Versican is secreted by VICs in wound assay; blocking its receptor CD44 decreases
stress fiber (αSMA formation in migrating VIC and inhibits collagen gel contraction.
26
Yip, 2011
Porcine, normal and high‐cholesterol‐fed pigs
C‐natriuretic peptide
C‐natriuretic peptide inhibits both osteoblastic and myofibroblastic differentiation
of VIC; simvastatin upregulates C‐natriuretic peptide mRNA.
52
Witt, 2012
Porcine
Sphingosine
Sphingosine increases nodule formation in a concentration‐dependent manner, acting
via S1P2 receptor, RhoA, and ROCK kinases, executed by calcium release from internal
cellular stores.
92
Yanagawa, 2012
Porcine
MicroRNA 141,
MicroRNA 141 inhibits TGFβ1‐induced nodule formation and alkaline phosphatase activity
by inhibition of BMP2 and Runx2 expression.
93
Xu, 2013
Porcine
β‐Catenin, Wnt3
Wnt3a increases VIC proliferation, the mechanism involves β‐catenin
82
Hutcheson, 2012
Porcine
Serotonin, 5‐HT2b (serotonin receptor)
Antagonists of 5‐HT2b counteract myofibroblast differentiation induced by TGFβ1, likely
by blocking noncanonical and enhancing canonical TGFβ1 signaling.
94
Song, 2012
Human, calcified and noncalcified
Biglycan
VICs from calcified valves have increased biglycan expression; biglycan induces osteoblast
differentiation via toll‐like receptor 2 and ERK. Biglycan expression and calcification
are stimulated by oxidized low‐density lipopolysaccharides.
95
Zeng, 2012
Human, calcified and noncalcified
LPS, toll‐like receptor 4, Notch
LPS via toll‐like receptor 4 activates inflammatory phenotype in VIC. In calcified
VIC Notch1 sensitizes toll‐like receptor 4 to LPS by means of NFκB.
96
Nadlonek, 2012
Human, noncalcified
γ‐Radiation
Irradiation of cultured VICs increases osteoblast differentiation.
79
Hutcheson, 2013
Porcine
Cadherin‐11
Cadherin‐11 is activated by TGFβ1 via phosphorylation of ERK. Cadherin‐11 is essential
for calcified nodule formation as it increases intercellular tension.
97
Branchetti, 2013
Human, calcified
DNA damage and repair mechanisms, antioxidants
DNA repair mechanisms are compromised in calcified VIC; cells are vulnerable to H2O2
‐induced damage. Catalase adenovirus transfection reverses this.
50
Poggio, 2013
Human, calcified and noncalcified
Bone morphogenetic protein 4
Bone morphogenetic protein 4 triggers osteoblast differentiation only in noncalcified
VIC, to levels higher than osteogenic medium alone.
67
Richards, 2013
Porcine, VIC and VEC
Nitric oxide signaling from VEC to VIC
Osteogenic medium causes osteoblast differentiation in attached VIC 3D monocultures.
This is inhibited by VEC by means of nitric oxide signaling.
98
Zeng, 2013
Human, calcified and noncalcified
LPS, Notch1
LPS stimulates cleavage and nuclear translocation of Notch1 intracellular domain which
then leads to osteoblast differentiation through ERK and NFκB pathways.
34
Nadlonek, 2013
Human, noncalcified
Interleukin‐1β
Interleukin‐1β induces an inflamatory phenotype in VIC via NFκB.
39
Zhang, 2014
Human, noncalcified
MicroRNA 30b
BMP2 triggers osteoblastic differentiation in VIC and inhibits expression of microRNA
30b. MicroRNA 30b suppresses osteoblastic differentiation and apoptosis.
72
Farrar, 2014
Porcine, VIC and VEC
TNFα
TNFα stimulates endothelial‐to‐mesenchymal transition in VEC, TNFα‐treated VECs have
similar gene expression profile to TNFα‐treated VICs.
99
Galeone, 2013
Human, calcified and noncalcified
TNF‐related apoptosis‐inducing ligand (TRAIL)
Calcified VICs express TRAIL receptors. Adding TRAIL to osteogenic medium increases
calcified nodule formation and apoptosis.
69
Gould, 2014
Porcine, VIC and VEC
Role of VEC
VECs in coculture inhibit myofibroblast differentiation in VIC through nitric oxide
signaling.
25
El Husseini, 2014
Human, noncalcified; murine from wild type and P2Y2−/−
AKT kinase, P2Y2 receptor
Both AKT kinase and P2Y2 receptor via NFκB pathway inhibit expression of interleukin
6, which is necessary for mineralization.
Cells from P2Y2
−/− mice are prone to osteoblast differentiation.
100
Zhang, 2014
Human, from noncalcified areas of calcified valves
Transcription factor Twist
Osteogenic medium upregulates Twist. Overexpression of Twist decreased other calcification
genes, and Twist siRNA triggers osteoblast differentiation.
101
Carrion, 2014
Human, noncalcified
Long noncoding RNA HOTAIR
HOTAIR is downregulated by stretch via Wnt signaling; siRNA to HOTAIR upregulates
BMP2 and alkaline phosphatase expression.
102
Zeng, 2014
Human, noncalcified
Oxidized low‐density lipoproteins, LPS, Notch1
Oxidized low‐density lipoproteins augment osteoblastic differentiation triggered by
LPS through NFκB and Notch1 cleavage.
103
Witt, 2014
Porcine, Human, noncalcified
Polyunsaturated fatty acids
Several polyunsaturated fatty acids reversibly inhibit myofibroblast activation via
Rho kinase and ROCK kinase.
104
Song, 2014
Human, noncalcified
Biglycan
Biglycan is a ligand for toll‐like receptors 2 and 4 in activation of inflammation
in VIC; effect is mediated by NFκB and ERK.
αSMA indicates α smooth muscle actin; 3D, 3‐dimensional; BMP2, bone morphogenetic
protein 2; ERK, extracellular signal‐regulated kinase; LPS, lipopolysaccharide; NFκB,
nuclear factor κB; TGFβ1, transforming growth factor β1; TNFα, tumor necrosis factor
α; VEC, valve endothelial cells; VIC, valve interstitial cells.
Figure 2
A current understanding of the pathological differentiation of valvular interstitial
cells in aortic valve calcification. The cell types are given in blue. Quiescent valvular
interstitial cels (VICs) as an effect of exogenous stimuli (given in yellow fields)
differentiate into myofibroblasts (left) or preosteoblasts (right). Differentiation
process is shown as gray arrows. The myofibroblasts can further assemble themselves
into nodules, which undergo apoptosis and provide substrate for diffuse calcification
(bottom left). The process is negatively regulated by valve endothelial cells (far
left). The preosteoblasts can further differentiate into osteoblasts, which in turn
synthesize ordinary bone (bottom right). The processes are orchestrated by a complex
network of factors. The ligands stimulate surface receptors (white circles), which
further relay to the signaling networks (black arrows). The signals can be inhibitory
(stump arrows) or stimulatory (arrows with a “+”). The signal can constitute stimulation
of expression of certain factors, a process shown as blue arrows. Generally the procalcific
stimuli are shown with red arrows, and anticalcific are given in green. Due to the
scheme complexity, several factors appear in multiple places on the scheme. ALP indicates
alkaline phosphatase; aSMA, α‐smooth muscle actin; BMP, bone morphogenetic protein;
BMPR, bone morphogenetic protein receptor; DAPT, inhibitor of γ‐secretase; ENOS, endothelial
nitric oxide synthase; ENPP, ectonucleotide pyrophosphatase/phosphodiesterase 1; ERK,
extracellular signal‐regulated kinase; FGF, fibroblast growth factor; IL, interleukin;
LPS, lipopolysaccharide; MAPK, mitogen‐associated protein kinase; MMP, matrix metalloproteinase;
NFkB Nuclear factor κB; NICD, Notch intracellular domain; NO, nitric oxide; NotchR,
Notch receptor; oxLDL, oxidized low‐density lipoproteids; PGN, peptidoglycan; Runx2,
runt‐related transcriptional factor 2; SMAD, small mothers against decapentaplegic;
TGF, transforming growth factor; TLR, toll‐like receptor; TNF, tumor necrosis factor.
Role of Nonsterile Inflammation
Even though the direct link between bacterial infection and aortic valve calcification
is not established, bacteria have been found in aortic valve cusps on autopsy. This
puts bacterial toxins, such as lipopolysacharide (LPS) and peptidoglycan, on the tentative
list of potential culprits.98 LPS is a Gram‐negative bacterial toxin, and peptidoglycan
is the prevailing component of Gram‐positive bacterial cell walls. The innate immune
system recognizes these 2 compounds via the toll‐like receptors TLR2 and TLR4. Healthy
human VICs isolated from explanted hearts were found to express both of these receptors.
Stimulation of VICs with LPS triggered a nuclear factor κB (NFκB) signaling response,
involving nuclear translocation of p65 subunit, secretion of interleukins IL‐6 and
IL‐8, resulting in increased expression of both BMP2 and Runx2.33 This effect was
abolished by the administration of siRNA against TLR2 and 4.30 VICs from calcified
human valves had a stronger response to LPS stimulation in terms of IL‐8, monocyte
chemoattractant protein‐1, and intercellular adhesion molecule 1 (ICAM‐1) expression
than healthy ones.95 Microarray analysis revealed that LPS affected human VICs in
several ways similarly to β‐glycerophosphate (a critical component of osteogenic medium).
For instance, LPS activated genes of BMP2, MMP2, platelet‐derived growth factor, and
FGF, and the latter 2 were activated by β‐glycerophosphate as well.13 BMP2 was upregulated
in human VICs following treatment with LPS, and peptidoglycan stimulated ALP activity
via toll‐like receptors TLR2 and TLR4. This effect can be blocked by Noggin (a BMP2
signaling inhibitor).30 The signaling immediately downstream of BMP2 includes the
canonical pathway involving the SMADs, in which SMAD1 activates expression of osteopontin,
and the noncanonical pathway, in which ERK activates expression of Runx2. The phosphorylation
of ERK following LPS stimulation was higher and more sustained in human VICs obtained
from already calcified aortic valves.88 TLR4 stimulation in healthy human VICs also
activated Il‐1β, a known proinflammatory cytokine. It further activated ICAM‐1, MCP‐1,
IL‐6 and ‐8, and ultimately NFκB. ERK inhibition did not affect the calcification
caused by IL‐1β, suggesting that the 2 pathways are parallel to each other.34
As indicated above, all VICs are not equal, and bacterial toxins do not trigger calcification
uniformly in all of them. The response of VICs to both LPS and peptidoglycan was found
to be higher in human aortic VICs compared with those isolated from the pulmonary
valve, which was reflected by increased transcription of BMP2 and Runx2,30 although
the expression of TLR4 was similar in human VICs from all 4 cardiac valve sites.74
VICs from all 4 sites reacted with LPS by upregulation of ICAM‐1 and MCP‐1, but BMP2
was induced only in aortic VICs.74 Furthermore, VICs from diseased human aortic valves
exhibited a higher baseline expression of TLR2, TLR4, and BMP2.30, 88 A study of different
cell populations among the bovine VICs revealed 4 distinct colony types, and the calcific
response to LPS (ALP activity) exclusively happened in cells expressing osteocalcin
but not αSMA, SM22, SMM, or osteopontin.22 In the calcifying clones, a downregulation
of the l‐arginine‐metabolizing enzymes was found. Supplementation with l‐arginine
and dimethylarginine countered ALP activity in the clones. The effect was replicated
in 3D cultures using collagen I sponges.58
An important mediator of inflammation is TNFα. Human VICs isolated from calcified
and noncalcified (aortic regurgitation, aortic dissection patients) valves revealed
that TNFα triggered calcification in the calcified VICs via NFκB signaling to BMP2.
Blocking NFκB signaling abolished this effect.35 It is plausible that TNFα signaling
is the link between LPS and BMP2 in other studies as well. TNFα‐related apoptosis‐inducing
ligand is a member of the TNFα superfamily of ligands and is expressed by T‐lymphocytes,
macrophages, and VICs. Expression of TNFα‐related apoptosis‐inducing ligand death
receptors DR4, DR5, and the decoy receptors DcR1 and DcR2 is increased in calcified
human VICs. Osteogenic medium increased DR4 receptor expression, and adding TNFα‐related
apoptosis‐inducing ligand to the osteogenic medium augmented human VIC calcification.99
Regarding the bulk of studies on LPS mentioned in this article, the main end point
in many of them was protein expression of BMP2 in a short (days) time frame. Whether
it is an adequate reflection of calcific response in VICs can be questioned. After
all, as stated above, the primary end point in studying valve calcification must be
qualitative and quantitative assessment of calcium.
Role of Wnt in Osteoblastic Differentiation of VIC
Wnt (wingless‐related integration site) is a secreted glycoprotein that signals through
several pathways, one of them called “canonical,”105 which involves the second messenger
β‐catenin. In the absence of signal, β‐catenin forms a complex with glycogen synthase
kinase β, Axin, and APC (destruction complex), which leads to β‐catenin being degraded.
However, on stimulation of the Wnt pathway through Wnt ligands, β‐catenin is stabilized
by disruption of the distruction complex. The Wnt signaling pathway has multifaceted
roles in differentiation, proliferation, and death as well as development and homeostasis.
Wnt/β‐catenin signaling is known to play a critical role in cardiac development, in
particular regulating cardiac valve formation, and also in regeneration and repair
in frogs and fish.106, 107, 108 On the flip side Wnt/β‐catenin signaling is implicated
in cancer and a number of diseases that are outside the scope of this review.109 The
role of Wnt signaling has been studied for over a decade in the regulation of osteoblastic
differentiation in bone and regulation of bone mass, as well as its involvement in
disorders of bone.110 There is emerging evidence for the role of Wnt/β‐catenin signaling
in calcification of aortic valves.111
In healthy VICs, β‐catenin is found in the cytoplasm, but Wnt3a ligand stimulates
its nuclear translocation. A similar effect was observed in VICs treated with TGFβ1,
and the blocking of β‐catenin translocation was shown to cancel the ensuing myofibroblast
differentiation. In stenotic human valves αSMA, β‐catenin, TGFβ1, Wnt3a, and SMADs,
2/3 colocalize in the same areas.90 Porcine VICs stimulated with Wnt3 increased levels
of β‐catenin both in the cytoplasm and in the nucleus and increased proliferative
activity.93 In addition, clues can be found to the role of Wnt by looking at the associated
risk factors of aortic valve disease, which include elevated levels of low‐density
lipoprotein (LDL) and familial hypercholesterolemia, where patients develop vascular
disease, coronary artery disease, and aortic valve lesions that calcify over time.112
The work of Rajamannan and colleagues further implicates the participation of the
Wnt pathway via the low‐density receptor‐related protein Lrp5, which is a component
of the Wnt receptor (comprised of LPR5/6 and Frizzled). Using a rabbit model they
investigated the role of cholesterol and statins. They showed that a high‐cholesterol
diet induced bone formation in aortic valves but that the administration of atorvastatin
along with cholesterol suppressed the observed formation of bone.111
The Statin Paradox
There are many histopathological similarities between aortic valve calcification and
atherosclerosis, which have led to a number of attempts at comparing the two diseases.
It was shown that a high‐cholesterol diet in pigs leads to increased αSMA expression
in aortic valve leaflets.46 Human VICs were used by the Yacoub group to investigate
the effect of HMG‐CoA reductase inhibitors/statins on the osteogenic response. They
showed that atorvastatin inhibited both ALP activity and cytokine expression triggered
by TGFβ1, TGFβ3, TNFα, and BMP4. This effect was also alleviated by atorvastatin in
healthy human VICs.71 In addition to inhibiting the osteoblast pathway, statins exerted
an effect on myofibroblastic differentiation. In cultured porcine VICs simvastatin
completely inhibited collagen gel contraction, even when the cells were stimulated
with TGFβ1.80 Nodule formation by cultured porcine VICs was inhibited by pravastatin,
acting via Rho kinase, HMG‐CoA, and myosin light chain kinase, inhibiting expression
of αSMA. However, although the formation of nodules was suppressed, the already formed
nodules were not resolved.56 The effect of statins (simvastatin at least) on nodule
formation could possibly be mediated by the C‐natriuretic peptide, which is expressed
by VECs populating the ventricular side of the valve.26
The data on statins remain highly controversial as 3 randomized clinical trials using
statins for treatment of aortic valve calcification—SALTIRE (Scottish Aortic Stenosis
and Lipid Lowering Trial, Impact on Regression), SEAS (Simvastatin and Ezetimibe in
Aortic Stenosis), and ASTRONOMER (Aortic Stenosis Progression Observation: Measuring
Effects of Rosuvastatin)—failed to demonstrate any beneficial effects.113 Instead,
we now have a statin paradox—although vascular calcification is reduced by statin
use, bone mineralization and aortic valve calcification are actually increased.6 A
recent study by Monzack and colleagues showed that in porcine VICs treated with osteogenic
medium or under reduced serum conditions (where the cells expressed high levels of
αSMA), statins actually increased ALP activity and the expression of osteocalcin.65
Statins are now the front line in prevention of atherosclerosis, but the present conclusion
is that they have no effect in preventing aortic valve calcification. Possibly the
statins might inhibit development in healthy valves, but once the calcification process
has started, the statins cannot stop it.
Role of Notch
Notch is a key signaling pathway in embryonic development, ensuring cross talk between
different types of cells and their physiological differentiation.114 Notch is particularly
important during cardiac valvulogenesis as well as in the pathogenesis of aortic valve
calcification. In a seminal study Garg and co‐authors showed that Notch1 haploinsufficiency
results in aortic valve calcification.115 Mutations in Notch1 are associated with
bicuspid aortic valves and consequent valve calcification. Later Notch1 has been shown
to repress osteogenic pathways in aortic valve cells.26 However, the exact mechanisms
of Notch1 action in aortic valve calcification remain unknown, and the existing evidence
is rather controversial. Some reports show that Notch activation prevents osteogenic
differentiation but that the Notch ligand Jag1 may promote osteogenic differentiation.116,
117
The lab of Srivastava cultured both sheep VICs and endocardial cells from mice. Using
a transgenic model with a heterozygous knockout of Notch1 they showed that these mice
developed valve stenosis if fed with the high‐fat (“Western”) diet. Inhibition of
Notch1 with a siRNA or using its inhibitor DAPT increased Runx2 expression; however,
this effect was abolished when siRNA against BMP2 was used simultaneously.89
It would seem that Notch is a clear anticalcification factor. However, Zeng and colleagues
showed that Notch1 increased the sensitivity of TLR4 to LPS stimulation in human VICs
through the activation of NFκB signaling, effectively linking TLR4 and NFκB. Notch1
intracellular domain cleavage (required for Notch1 signal transduction) was proportional
to the dose of LPS. The effect was inhibited by DAPT, an inhibitor of γ‐secretase,
an enzyme that cleaves the Notch1 intracellular domain from the membrane domain.95
A follow‐up study showed that Notch1 maintained the phosphorylation of NFκB and ERK
(mediator of the noncanonical BMP2 signaling) via MEK1/2 kinase. Surprisingly, ERK
and NFκB activation were found to be upstream of BMP2 activation, and they could activate
them without Notch1, but to a lesser degree.98 Notch cleavage, subsequent ALP activation,
and BMP2 expression were also triggered by a combination of LPS and oxidized LDL,
higher than the LPS alone. Also NFκB activation gave an equivalent response.102
New data on the role of Notch in aortic valve calcification have been obtained recently
with the help of Notch1
+/− mice. As outlined above, an important aspect of myofibroblast activation and contraction
is the interaction between αSMA and cadherin‐11.79
Notch1
+/− mice display aortic valve mineralization. Paradoxically, the VICs from these mice
have a decrease of both Runx2 and αSMA, and a stark increase of cadherin‐11. Notch1
+/− VICs are hypersensitive to mechanical stress, as stretch stimulates an increased
response in αSMA levels compared with wild‐type cells. This response is likely to
explain the valve calcification observed in these mice in vivo. Thus, Notch1 harnesses
the myofibroblastic differentiation of VICs and prevents dystrophic mineralization.118
In line with the reports mentioned above it has been shown that inhibition of Notch
activity by γ‐secretase inhibitor in rat aortic VICs causes significant downregulation
of transcription factor Sox9 along with several cartilage‐specific genes. In porcine
VICs Notch inhibition resulted in accelerated calcification, whereas stimulation of
Notch signaling attenuated the calcific process. The addition of Sox9 to the medium
was able to prevent calcification that occurs at Notch inhibition.119 Altogether,
it seems that Notch has a complex role in aortic valve calcification, which should
be interpreted with respect to the process timeline, the species, and the end point
employed in the study.
Cross‐Talk Between Valve Interstitial and Valve Endothelial Cells
VECs probably play a major role in aortic valve calcification, and they deserve a
separate review, although at present there is a scarcity of studies. Only recently
has it been possible to isolate and reliably characterize VECs. Studies now utilize
VECs in coculture systems with VICs and are unveiling multiple effects of endothelium
on the underlying interstitial cells. VECs harnessed the αSMA expression in porcine
VICs, both in control cultures and in those subjected to shear stress.17 It was later
demonstrated that nodule formation, an important step of the myofibroblast pathway,
was inhibited with the nitric oxide donor DETA‐NONOate,81 implying that the effect
may be due to the nitric oxide produced by the VECs. In a key study on VIC‐VEC interaction
Richards and colleagues demonstrated that porcine VECs prevented VICs from calcification
with osteogenic medium, reflected in normal expression of Runx2, αSMA, and osteocalcin.67
The effect was proven by treating a VIC monoculture with DETA‐NONOate and blocked
by the nitric oxide signaling blocker l‐NAME.67 In human valves, VECs were found to
express more endothelial nitric oxide synthase on the ventricular side, where calcification
is not known to occur.67 Similar to the osteogenic differentiation, porcine VECs in
coculture with VICs also inhibited myofibroblast differentiation on hard substrates
(peptide‐functionalized polyethylene glycol) in the absence of TGFβ1. The role of
VECs was again traced to nitric oxide production, as the above effect was negated
with the use of l‐NAME.69 Mice with homozygous deletion of endothelial nitric oxide
synthase are born with bicuspid aortic valves in 30% of the cases, and the majority
of these calcify by 18 months of age. The bicuspid valves had increased expression
of caspase 3, a marker and mediator of apoptosis, and osterix, a cofactor of osteoblast
differentiation.120
The myofibroblasts in aortic valves have commonly been thought to be derived from
VICs. However, a study by Wang and colleagues used flow cytometry to demonstrate that
a proportion of porcine valve myofibroblasts bear endothelial cell markers.43 Farrar
and colleagues investigated whether porcine VECs undergo endothelial‐to‐mesenchymal
transition and contribute to the myofibroblast population.72 VECs were seeded on transwell
membranes on top of a collagen gel and stimulated with TNFα. After 6 days it was shown
that a portion of the VECs had undergone mesenchymal transformation and migrated into
the collagen gel, rearranging the collagen fibrils. These cells expressed low levels
of CD31 and high levels of αSMA. These invading VECs were in many ways similar to
VICs: their response to TNFα showed an upregulation of MMP9, TGFβ1, BMP4, Notch1,
and ICAM‐1 and a downregulation of nitric oxide synthase.72
Endothelial‐to‐mesenchymal transition in VECs has recently been reproduced in cells
isolated from sheep after treatment with TGFβ and osteogenic medium. The transition
was confirmed by a decrease in endothelial markers and an upregulation of αSMA and
MMP‐2 after 7 days of treatment. By day 14 the cells expressed osteopontin, osteocalcin,
and Runx2 as well as increased ALP activity. After 21 days of exposure to osteogenic
medium, the VEC cultures were positive for calcium deposition. When VICs were directly
cocultured with VECs, TGFβ‐induced endothelial‐to‐mesenchymal transition and osteogenic
medium‐induced calcification were attenuated. However, VECs did not oppose the myofibroblastic
or osteogenic differentiation of VICs. The authors showed staining of human aortic
valves in which the same cells were positive for CD31 and αSMA, which they interpreted
as cells undergoing endothelial‐to‐mesenchymal transition.121
Recently it has been shown in a mouse model that VEC‐specific Notch1 signaling regulates
endothelial‐to‐mesenchymal transition in aortic valves through promoting expression
of TNFα in VECs. TNFα interacts with its receptors Tnfr1 and Tnfr2 on VICs and induces
apoptosis in VICs.122
Thus, there is accumulating evidence that VICs do not act alone, but VECs and VICs
interact to ensure the proper development and maintenance of the aortic valve. Probably
a disruption of this interaction could contribute to valve pathology and subsequent
calcification.
Role of Sterile Inflammation
Sterile inflammation, chiefly triggered by uncontained cellular debris or irritant
particles, is currently a hot topic in heart research. Sterile inflammation is by
definition an immune response to signals of damage instead of signals of foreignness.
The concept challenges a lot of preexisting knowledge but is continuously gaining
ground in the area of valve research. A number of intrinsic inflammatory ligands have
been shown to contribute to the pathological differentiation of VICs by initiating
an inflammatory reaction.
Oxidized Low‐Density Lipoproteins
Oxidized low‐density lipoproteins (oxLDL) are known to accumulate in atherosclerotic
plaques as well as in the calcified valves and to take part in tissue degeneration
and calcification.6 OxLDL stimulation of healthy human VICs increased expression of
inorganic phosphate transporters, which play a role in accumulation of calcium phosphate
during mineralization.123 OxLDL treatment of porcine VICs triggered expression of
Runx2 and osteocalcin and calcification via the receptor for advanced glycation end
products and MAP kinases p38 and c‐Jun N‐kinase. Blocking the receptor for advanced
glycation end products with targeted siRNA abolished the calcific response exerted
by oxLDL.124
Biglycan
Biglycan is an extracellular proteoglycan whose expression is correlated with the
degree of stenosis.85 It is elevated in stenotic human VICs compared with the healthy
ones. Biglycan expression in human VICs is stimulated by oxLDLs. Biglycan binds to
TLR2 receptor, where the signal is relayed via ERK kinase to BMP2 and ALP.94 Thus,
it is a link between triggers of nonsterile inflammation and the ensuing osteoblast
differentiation of VICs. The molecular events triggered by exogenous biglycan in human
VICs mimic those exerted by LPS: activation of ICAM‐1, MCP1, and IL‐6 with a likely
involvement of NFκB. The effect was attenuated by siRNA against TLR2 and TLR4 and
blocked by the ERK inhibitor PD98059.104
High‐Mobility Group Box 1
The high‐mobility group box 1 (HMGB1) is a nuclear protein that orchestrates the inflammatory
response in atherosclerosis and is thought to be secreted by macrophages, other immune
cells, or by leakage from dying resident cells. The receptor for HMGB1 is TLR4, and
the signal is further conveyed by NFκB and MAP kinases. Inside the human VICs HMGB1
undergoes translocation from the nucleus to the cytoplasm and out into the extracellular
space to exert its inflammatory effect. This process is dependent on osteopontin,
a known marker of calcification.125 Osteopontin expression is increased in calcified
human valves, concurrent with large amounts of HMGB1 in the extracellular space and
in some VECs. Osteopontin promotes proliferation of healthy VECs and inhibits proliferation
of VECs from calcified valves. It also stimulates VECs to secrete HMGB1.125 Treatment
of human VICs with HMGB1 induces a dose‐dependent calcific response: it induces expression
of MCP1, BMP2, TNFα, and IL‐6 as well as calcium deposition.37 Thus, the HMGB might
engage a similar response as the LPS, but in absence of microorganisms, which elevates
the relevance of studies of TLR2 and TLR4 in calcific aortic valve disease.
Intracellular Adhesion Molecule‐1
ICAM‐1 is an indicator of cellular stress and inflammation. It interacts with integrins
on leukocytes and enhances migration of leukocytes into the tissues. However, ICAM‐1
may have an independent role in calcification of VICs. Exposure of human VICs to lymphocyte
function‐associated molecule‐1, a ligand of ICAM, elicited an osteogenic response
through mechanisms involving BMP2, NFκB, and Notch1. Inhibition of γ‐secretase, an
enzyme necessary for Notch1 cleavage (and subsequent signaling), blocks the osteogenic
response to ICAM‐1.126
Cyclooxygenase 2
Cyclooxygenase 2 (Cox2) is an enzyme that plays a crucial role in synthesis of leukotrienes
and prostaglandins and is a target for most nonsteroidal anti‐inflamatory drugs. Cox2
has a role in bone formation, namely in mechanotransduction stimulating osteoblastogenesis.127
In healthy murine aortic valves Cox2 expression is confined to the endothelial cells.
Mice with a genetic ablation of Klotho manifest early onset of aging and nodular calcification
of the aortic valve and are used as model animals. VICs in calcific nodules in Klotho‐deficient
mice had high levels of Cox2, similar to human histology specimens. Porcine VICs treated
with osteogenic medium and selecoxib, a selective Cox2 inhibitor, showed decreased
osteocalcin and bone sialoprotein expression compared with osteogenic medium alone.
Selecoxib also alleviated aortic valve calcification in Klotho‐deficient mice.128
Interleukin‐6
IL‐6 is a cytokine secreted by many different cells in the course of an immune response.
It is increasingly expressed in VICs from calcified human valves and in vitro promotes
calcification in cultured human VICs. P2Y2 nucleotide receptor was shown to inhibit
the expression of IL‐6 by means of phosphorylation of AKT, which then represses the
NFκB pathway. Calcified valves were found to express less AKT; furthermore, VICs from
P2Y2‐knockout mice showed increased mineralization.25
MicroRNAs and Epigenetic Mechanisms
MicroRNAs (miRNAs) are emerging as the new regulators of genes at the translational
level. Each miRNA regulates many transcripts, and a single transcript often has binding
sites for several miRNAs.129 MiRNA profiling was used to compare human VICs isolated
from calcified and noncalcified valves, both of bicuspid and tricuspid morphology.
The profile of cells from bicuspid aortic valves was quite different from the profile
of tricuspid counterparts. Furthermore, a single miRNA, MiR‐141, was found to inhibit
nodule formation, ALP activity, Runx2, and BMP2 expression induced by TGFβ1 in porcine
VICs.64 Another miRNA, MiR‐30b, was downregulated in calcified valve regions and in
human calcified VICs treated with BMP2. The potential target genes for this miRNA
include transcripts of Runx2, SMAD1, and Caspase 3. Treatment of VICs stimulated with
osteogenic medium (supplemented with BMP2) with MiR‐30b precursor reduced ALP activity,
Runx2 and osteocalcin expression. MiR‐30b also reduced apoptosis in human VICs cultured
for 96 hours in low‐serum medium.39
Mature miRNAs are short, 22 to 23 nucleotides in length, whereas long noncoding RNAs
are longer than 200 nucleotides. A long noncoding RNA, HOTAIR, has been shown to play
a role in aortic calcification. The expression of this long noncoding RNA is lower
in diseased bicuspid valves than tricuspid, and it is downregulated by stretch and
Wnt agonists. HOTAIR targets are, among others, Periostin, MMP‐2, ‐10, and ‐12, collagen
1α1, BMP‐1, ‐4, ‐6, and BMP receptor 2. Thus, HOTAIR may be the endogenous inhibitor
of ectopic mineralization and a potential therapeutic drug target.101
Another regulatory mechanism is epigenetic modification, for example, by DNA methylation,
which is responsible for repressing transcription of a huge variety of factors. One
example is 5‐lipoxygenase, which is involved in leukotriene synthesis, which promotes
inflammation. Calcified human VICs showed decreased methylation at the 5‐lipoxygenase
promoter site and, consequently, increased expression of the 5‐lipoxygenase mRNA,
which may contribute to inflammation.130
Miscellaneous Factors
What follows is a listing of factors that are active at different points of the proposed
mechanisms of calcification. At present it is impossible to outline the most important
ones.
Periostin is found in VICs from healthy bovine aortic valves, but expression increases
following exposure to LPS. Periostin is chiefly expressed in VICs of the lamina ventricularis,
less in the fibrosa, and is coexpressed with elastin.57 Periostin is secreted by macrophages
and myofibroblasts, and it stimulates expression of MMP‐2 and ‐9 in human VICs. Wild‐type
mice fed with the Western diet develop aortic stenosis, but periostin‐knockout mice
do not. In addition, they express lower levels of αSMA, collagen 1, and MMP‐2 and
‐13.57 Periostin may thus play a stimulatory role in the development of valve calcification.
Jian and colleagues showed that whereas healthy aortic valves did not express MMP‐2
or ALP, diseased valves expressed both, as well as tenascin‐C, an extracellular matrix
glycoprotein found in atherosclerotic plaques. Ovine VICs grown on tenascin‐C showed
an increased expression of MMP2.36 Its possible role in calcification is at present
unknown.
Immunohistochemistry has revealed increased expression of β1‐, β2‐, and β3‐adrenoreceptors
in the calcified area of the human valve along with RANK, one of the markers of osteoclasts.
Osteogenic medium applied to VICs harvested from the calcified area stimulated increased
expression of β1‐adrenoreceptor. Treatment of these cells with salmeterol (selective
β2 agonist) reduced ALP activity and osteocalcin expression.40 A possible pathophysiological
role is not known.
Versican is a proteoglycan that is upregulated after tissue injury and binds CD44
receptor. In a wound assay using healthy human VICs, versican was secreted and orchestrated
gel contraction to repair the wound. Blocking the CD44 receptor with an antibody disabled
the gel contraction property of the VICs.91 Gel contraction is one of the key functional
end points for myofibroblasts, thus implying the role of versican in myofibroblast
differentiation of VICs.
C‐Natriuretic peptide is expressed in healthy VICs and is downregulated in VICs from
calcified valves. Experiments using porcine VICs showed that C‐natriuretic peptide
inhibited nodule formation and gel contraction and in addition the osteoblastic differentiation
potential.26 C‐Natriuretic peptide also increased cell elasticity properties as measured
by micropipette aspiration.61
Another factor that promotes nodule formation in porcine VICs is sphingosine‐1. The
small molecule JTE‐013, which blocks the S1P2 sphingosine receptor, abolishes this
effect. The addition of sphingosine‐1 to a culture of VICs activates both RhoA and
ROCK kinases and triggers influx of calcium leading to cell contraction. These results
obtained in VICs were verified using strips of porcine valves stimulated with sphingosine‐1.52
Serotonin is an active vasodilator activating endothelial nitric oxide synthase. Agonists
of the serotonin 2b receptor (norfenfluramine and pergolide) were found to promote
myofibroblastic differentiation of porcine VICs and ensuing calcification.82 Production
of TGFβ1 in ovine VICs is stimulated by serotonin in a dose‐dependent manner through
G‐coupled receptor signaling.19 Serotonin receptor antagonists block the noncanonical
signaling pathway of TGFβ1 in porcine VICs, simultaneously enhancing the canonical
one, inhibiting nodule formation and expression of αSMA and smooth muscle myosin.82
This is in agreement with heart valve damage due to TGFβ1 signaling caused by high
levels of serotonin in the circulation secondary to carcinoids, serotonin‐producing
tumors.19
TGFβ1 stimulation in a 3D collagen culture of porcine VICs also stimulates faster
and more powerful contraction of the extracellular matrix.53 This effect is counteracted
by FGF‐2. The antifibrotic effect of FGF‐2 on VICs is mediated by SMADs and ERK. The
concept of FGF‐2 treatment against leaflet fibrosis was suggested by experiments in
excised pig valve leaflets in the form of reduced αSMA expression.76 FGF is also suggested
as a medium supplement to reduce myofibroblast activation in freshly isolated VICs.49
The role of BMP2 has been amply referred to above in terms of nonsterile inflammation.
However, there are several BMP isoforms, including BMP4. Addition of recombinant BMP4
to strips cut from healthy human aortic valve subjected to cyclic stretch induced
expression of αSMA and calcification. BMP4 antagonist Noggin abolished the effect
of BMP4.50 Stretching healthy human VICs in tubular molds of collagen gel for 3 weeks
at 15% resulted in a modest increase of BMP2 and BMP4 mRNA and BMP2 protein.68 Microarray
studies of human sclerotic, stenotic, and control aortic valves showed an increased
expression of BMP4 in both diseased groups. The additive effect of mechanical stress
and BMP4 is reminiscent of a combination of stretch and TGFβ1.50
Even though BMP2 has a net procalcific effect in valve mineralization, some of its
targets may actually have an opposite, beneficial effect. As stated above, the BMPs
belong to the transforming growth factor superfamily and rely on SMADs for their canonical
signaling pathway. The SMADs fall into an activating and inhibitory group.131 SMAD6
is an inhibitory SMAD activated by BMP2, and SMAD6‐knockout mice have aortic valve
calcification. These mice also display reduced levels of SMAD6 in their valve leaflets.
Treatment of murine VICs with TNFα elicited an osteogenic response and reduced expression
of SMAD6. Knocking down SMAD6 in murine VICs led to mineralization in absence of other
stimuli.132
Twist‐related protein (TWIST) inhibits Runx2 function in preosteoblasts by adherence
to its DNA‐binding domain and recruitment of histone deacetylases.133 Calcified human
VICs express less TWIST than the healthy ones. Immunohistochemistry shows that Runx2
and Twist expression areas are nonoverlapping. Overexpression of TWIST in VICs decreased
expression of Runx2, osteocalcin, osteopontin, and ALP, whereas the knockdown of Twist
with siRNA had the opposite effect.100
Hyaluronan is one of the abundant components of the extracellular matrix in connective
tissues, including the aortic valve leaflets. Porcine VICs grown on collagen were
found to secrete hyaluronan, and adding exogenous hyaluronan at the mean molecular
weight of 64 kDa to the medium reduced nodule formation, although the higher and lower
molecular hyaluronan did not have this effect. Digestion of hyaluronan in situ in
the porcine valve specimens led to increased apoptosis, proliferation, and αSMA expression
in resident VICs.59
Inhibitors of aortic valve calcification can come in many forms, but none are more
attractive than the food supplements. A study of polyunsaturated fatty acids with
fish oil showed that docosahexaenoic acid and arachidonic acid dose‐dependently inhibited
nodule formation in both human and porcine VIC cultures. This inhibition was reversible,
as the nodule formation increased again after the polyunsaturated fatty acid supplementation
was discontinued.103
Radiotherapy is known to produce valve disease: over 60% of patients undergoing radiation
therapy in the mediastinal region developed calcific aortic stenosis over the next
20 years. Aortic valves from irradiated patients express more BMP2 than cells that
received no radiation. γ‐Irradiation of healthy human VICs with 10 Gy induced expression
of BMP2, Runx2, osteopontin, and ALP.96
DNA damage and repair are a routine activities in all cells, but if the balance is
tipped toward damage, the cells may undergo apoptosis. Human VICs from sclerotic and
stenotic aortic valves have increased oxidative DNA damage compared with the healthy
ones, impaired DNA repair enzymes, and decreased expression of superoxide dismutase,
catalase, and other antioxidants. Adenovirus delivery of catalase alleviates the oxidative
damage and the calcific response.97
Conclusion
VICs represent a relevant model for studies of aortic valve calcification, especially
when complemented with VECs. The most relevant models are 3D. The ideal source of
cells is human valves, both calcified and healthy ones, obtained during surgery. The
cells require no specific culturing techniques compared with most fibroblasts; however,
several things are to be kept in mind. The population of VICs is quite heterogeneous
with respect both to capacity to differentiate and to morphology already present at
isolation. The phenotype relevant for the physiological situation changes with passaging,
and the cells should be used at as early a passage as possible. Also, we should be
conscious at the choice of substrate, as its physical properties and chemical composition
heavily influence the biology of VICs.
The key concept of the cellular mechanism leading to aortic valve calcification is
the differentiation of resident interstitial cells into cell types foreign to the
valve itself: osteoblasts and myofibroblasts (although the studies indicate that myofibroblasts
may be present in some quantities even in the healthy valves). It is not known which
mechanism prevails or which comes first and which follows. The conclusions are widely
drawn based on autopsy findings, and the time‐course of the disease is basically unknown.
The concepts of ossification driven by osteoblasts and dystrophic calcification secondary
to formation of nodules by contraction of myofibroblasts (the current view) may be
changed or entirely replaced by more accurate theories.
The future of aortic valve research is likely to elucidate the mechanisms underlying
myofibroblast transformation and osteogenesis but also to go into previously unknown
areas: circulating nucleic acids, epigenetics, unorthodox pathogens, radiation, and
others. This will require that the models used are representative of the clinical
and physiological situation. Unfortunately, the plethora of factors that may influence
the phenotype of VICs represent important limitations of using VICs to clarify the
molecular and cellular mechanisms of heart valve calcification. At the end of the
day, one must create the maximally representative model for human disease, and many
conflicting results can be explained by different protocols, culture conditions, and
choice of cell source. After all, the VICs in culture are not identical to VICs in
the living valve. Consequently, although VICs are the backbone of experimental models,
findings in cultured VICs must be verified in cultured whole leaflets, in vivo animal
models, and ultimately in humans.
Sources of Funding
This work was supported by South‐Eastern Norway Regional Health Authority (grant 2013109),
the National Association (Norway), the University of Oslo, The Norwegian Research
Council, the Government of Russian Federation (grant 074‐U01), and the Russian Foundation
of Basic Research (grant 17‐04‐01318).
Disclosures
None.