Hepatic fibrosis is a hallmark feature of chronic liver diseases of various etiologies
including genetic deficiency, metabolic derangement, infection, autoimmunity, and
exposure to physical and chemical agents. This condition yearly affects millions of
patients in the USA and across the world. Progressive hepatic fibrosis, leading to
cirrhosis, is the most common cause of liver failure.
Essentially, hepatic fibrosis represents a pathophysiological process triggered in
response to liver injury, to mediate tissue repair and wound healing. It is primarily
driven by specialized effector cells called liver myofibroblasts and characterized
by sustained extracellular matrix deposition, leading to excessive scar formation,
organ dysfunction, and eventually end-stage liver disease cirrhosis. As fibrosis takes
place, persistent cell damage and distinctive changes in liver tissue architecture
trigger in parenchymal and nonparenchymal hepatic cells several signal transduction
pathways that regulate pathophysiological mechanisms promoting tissue repair, such
as inflammation, proliferation, remodeling, and angiogenesis. Concomitantly, the newly
formed extracellular matrix scaffold evolves into a dynamic and complex microenvironment
that orchestrates, with damage-activated hepatic cells such as leukocytes and liver
myofibroblasts, key functional aspects of the response to liver injury such as regulated
release of fibrogenic/proliferative factors and cell activation/migration, to restore
tissue homeostasis and integrity. Yet, in spite of significant advances in the identification
of key molecular targets and signaling pathways, there is presently no effective and
well-tolerated curative strategy for hepatic fibrosis in patients, beyond liver transplantation.
This special issue focuses on current concepts relevant to the pathogenesis of hepatic
fibrosis. We hope that these articles will generate continued interest in understanding
of the molecular mechanisms driving hepatic fibrosis progression.
Three articles will focus on how molecular changes of the homeostatic status of extracellular
matrix (ECM) network, including ECM transcriptome alterations (L.G. Poole and G. E.
Arteel), ECM-modifying protein activities (T. Saneyasu et al.), or ECM remodeling
regulation (C. L. Lamb et al.), significantly impact the progression of hepatic fibrosis.
The extracellular matrix (ECM) is more than an inert scaffold on which epithelial
cells are anchored and through/over which cells travel. It is increasingly appreciated
that the ECM is a dynamic entity in all tissues and serves as a critical depot for
growth factor sequestration required for rapid response to tissue injury or other
cues. In addition, degradation of ECM components themselves leads to the liberation
of bioactive fragments that modulate response to tissue injury. Critically, direct
interactions between cells and the ECM, mediated predominantly by integrins, induce
potent signals, which dictate cell activity such as migration. These, and other examples,
demonstrate the critical importance of the ECM in regulation of tissue homeostasis
in healthy tissue and in return to homeostasis after tissue injury. When dysregulated,
as in the case of chronic liver injury, ECM remodeling favors the accumulation of
ECM components. In liver disease, much effort is placed on understanding the pathomechanisms
that drive fibrosis, focusing on development of myofibroblasts, synthesis of fibrillar
collagens, and scar “maturation.” This strategy is employed to help in the development
of bona fide antifibrosis therapies. While it is indisputable that a fibrotic ECM
is responsible for organ dysfunction in advanced liver disease, early, “transitional”
changes to the ECM likely drive fibrogenesis. Understanding these transitional changes
may reveal novel points of intervention to prevent the development of liver fibrosis
in the first place. In their contribution to this special issue, L. G. Poole and G.
E. Arteel provide strong evidence to support this idea. Specifically, several changes
in the ECM and associated molecules, referred to as the “matrisome,” are known in
alcoholic liver disease and these changes occur prior to histologic evidence of fibrosis.
Further discussed are how the current approaches to studying the ECM are flawed as
they often consider single ECM components in isolation and at the expense of understanding
the myriad of other ECM changes which happen concurrently. This reductionist approach
fails to fully appreciate the incredible complexity associated with a full understanding
of matrisome-modifying events in early liver disease. To fill this important gap,
an interesting proposition is to utilize a robust “omics” approach to more thoroughly
understand transitional ECM changes associated with early stages of liver disease
progression, a novel and intriguing idea.
Once the ECM proteins are secreted, enzymatic cross-linking of the major ECM proteins
(collagens, elastin, and others) is an essential process for fibrotic matrix stabilization,
which in turn contributes to fibrosis progression and limits reversibility of liver
fibrosis once the causative agent is removed. Moreover, collagen cross-linking confers
increased stiffness to fibrotic matrix, promoting activation of fibrogenic effector
cells through mechanosensing. As T. Saneyasu et al. review in this issue, several
enzymes, including tissue transglutaminase (TG2) and the lysyl oxidase (LOX) family,
are overexpressed in hepatic fibrosis and able to catalyze the formation of collagen
cross-links. However, TG2-deficient mice display unaltered collagen cross-linking,
develop liver fibrosis normally, and do not show improved fibrosis reversal, casting
doubt on the functional significance of TG2 in fibrotic matrix stabilization. Instead,
lysyl oxidases directly participate in fibrotic matrix cross-linking and stabilization,
since LOX inhibition favorably affects the architecture of liver scar tissue, rendering
fibrosis more readily reversible after cessation of the fibrogenic stimulus. In the
liver, hepatic stellate cells and portal fibroblasts are major producers of LOX family
proteins, including LOXL2 that has recently been implicated to promote lung and liver
fibrosis and tumorigenesis. Anti-LOXL2 therapeutic antibody (simtuzumab) is currently
undergoing testing in phase II clinical studies in patients with primary sclerosing
cholangitis, nonalcoholic steatohepatitis, and hepatitis C/HIV coinfection (NCT identifier:
01672866, 01672879, 01672853, and 01707472), while several small molecule LOX/LOXL2
inhibitors are currently in early stages (preclinical/Phase 1) of development.
There is a notable paucity of studies examining the effect of environmental/nutritional
toxins or xenobiotics (e.g., smoke toxicants, drugs) on hepatic fibrosis, a dysregulated
tissue repair process associated with the development of chronic liver conditions.
Because xenobiotics often tend to be considered as disease-enhancing rather than disease-causing
agents, their regulatory role in hepatic fibrosis has been underappreciated to an
extent. However, that view has slowly evolved with the advent of functional studies
directly linking the biological activities of xenobiotic-sensing nuclear receptors
to alterations of liver homeostasis including organ toxicity, tissue damage, and tissue
repair. Recently, environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD),
a xenobiotic acting as ligand for aryl hydrocarbon receptor, has been shown to exacerbate
development of hepatic fibrosis; however the mechanisms by which this occurs have
not been fully explored. The study by C. L. Lamb et al. in this issue examined the
impact of TCDD-induced activation of aryl hydrocarbon receptor on the regulation of
ECM synthesis, deposition, and breakdown during chronic liver injury. The authors
described three components of ECM homeostasis, namely, collagen synthesis, ECM metabolism,
and plasminogen activator/plasmin system, that were regulated by TCDD. Further insights
into the mechanisms responsible for this dysregulation will be important contributions
to our understanding of ECM homeostasis in hepatic fibrosis following exposure to
environmental pollutants.
The next three articles will highlight the complexity of functional cooperation between
various specialized liver cell populations and its critical importance in hepatic
fibrosis progression, in the context of inherited liver disease (L. Jiang et al.),
metabolic dysfunction (N. Magee et al.) specifically, or chronic liver diseases (V.
Natarajan et al.) in general.
In addition to chronic overnutrition and hepatotoxins (e.g., alcohol exposure), genetic
diseases exist in which hepatic fibrosis is a major contributor to disease morbidity
and mortality. One such disease is congenital hepatic fibrosis found in patients with
autosomal recessive polycystic kidney disease (CHF/ARPKD). Disease pathogenesis is
linked to mutations in a protein called fibrocystin, which normally localizes to primary
cilia found on cholangiocytes and renal tubule epithelial cells. Those mutations lead
to a lack of fibrocystin localization to primary cilia and malformed primary cilia,
which suffer functional defects leading to the development of large, fluid-filled
cysts and a robust pericystic fibrosis. However, a clear understanding of the mechanisms
driving CHF/ARPKD progression remains elusive in this understudied and rare disease.
In this issue, L. Jiang et al. propose a “pathogenic triumvirate” to explain CHF/ARPKD
progression and link cell proliferation and cyst growth and fibrosis as interrelated
pathomechanisms. Indeed, several identifiable commonalities between molecular drivers
of proliferation, inflammation, and fibrosis in CHF/ARPKD disease exist in support
of this notion and may altogether indicate that the mechanistic relationships between
the major cell types found in each of the triumvirate's vertices including myofibroblasts,
macrophages, mast cells, and proliferating cyst wall epithelial cells that produce
and transduce those shared molecular signals are the integral mediators of disease
progression. In this context, these proven commonalities shared between pathomechanisms
could be either targeted independently, to break the feed-forward cycle of progressive
disease, or targeted by way of combinatorial therapies aimed at each of the vertices,
to further enhance disease regression. Finally, and perhaps even more intriguing,
it is conceivable that each member of the triumvirate is regulated by the same core
mechanism, thus raising the possibility that a single target for therapeutic development
may exist. Overall the review stresses the importance of cell-cell and cell-matrix
cross talk in mechanisms which drive fibrotic disease, emphasizing the importance
of nonreductionist approaches to studying disease pathogenesis and progression.
Nonalcoholic fatty liver disease (NAFLD) is a common liver disorder that describes
a wide range of chronic liver conditions caused by steatosis, a process histologically
characterized by abnormal lipid accumulation within hepatocytes. In a small group
of patients, it can progress to a potentially serious condition called nonalcoholic
steatohepatitis (NASH), which occurs when lipid deposition is combined with sustained
inflammation, severe cell damage, and advanced fibrosis ultimately leading to cirrhosis.
Currently, effective therapeutic options are not available, partly because understanding
of NASH pathogenesis remains limited. From the initial “two-hit hypothesis” proposed
by Day and Jamesto to the current “multiple parallel hits hypothesis” articulated
by Tilg and Moschen, the prevailing line of thought has been that NASH represents
a multifaceted disorder that does not mechanistically revolve around steatosis development
solely. Conceptually, an important question that has not yet been conclusively answered
is whether steatosis development precedes, coincides with, or even follows NASH development.
As N. Magee et al. emphasize in this special issue, NASH is increasingly recognized
as a complex interplay between parenchymal cells and nonparenchymal cells. Early research
efforts have mainly focused on elucidating the genetic factors and molecular signals
associated with lipid toxicity, oxidative stress, and organelle dysfunction (e.g.,
endoplasmic reticulum, mitochondria) in parenchymal hepatocytes that were initially
seen as the primary source of pathogenic factors driving NAFLD/NASH development. More
recently, novel signaling pathways relevant to disease pathogenesis have been uncovered
in nonparenchymal cell populations including immune cells and resident liver fibroblasts
that play critical roles in inflammation initiation/perpetuation and scar tissue formation
process associated with NAFLD progression to NASH, respectively. Innate/acquired immunity
mechanisms such as cytokine production/release, inflammasome activation, and gut dysbiosis
are being recognized as important factors that contribute to prolonging hepatic steatosis
and exacerbating inflammation in NASH. In addition, major signaling molecules such
as Notch and sonic Hedgehog that promote fibrosis progression in NASH have been now
identified as key regulators of activation of hepatic stellate cells, a fibroblast
population from which most of injury-associated myofibroblasts originate in several
chronic liver disease settings. These recent developments shed a new light on key
molecular pathways underlying NAFLD/NASH pathogenesis and will lead to the identification
of novel therapeutic targets and strategies for NASH condition.
Liver sinusoidal endothelial cells (LSEC) have recently emerged as an important regulator
of both progression and regression of liver fibrosis. Liver endothelium is positioned
at the functional interface, receiving molecular cues from ECM, nonparenchymal cells,
hepatocytes, and blood, as reviewed in depth by V. Natarajan et al. in this issue.
In the normal liver, quiescent fenestrated LSEC suppress activation of hepatic stellate
cells, losing such ability when liver vasculature undergoes “capillarization” in chronically
injured liver. A recent study demonstrated that manipulation of specific pathways
selectively in LSEC might force the liver to either regenerative (via chemokine receptor
CXCR7) or fibrotic responses (via chemokine receptor CXCR4 and growth factor receptor
FGFR1) to chronic liver injury, thereby postulating the fundamental importance of
liver vasculature in chronic liver disease outcomes. Also, neovascularization (angiogenesis)
has been noted in chronic liver disease for decades, although its important (and complex)
role in liver fibrosis has not been appreciated until recently. During progressive
phase of chronic liver injury, suppression of angiogenesis may impact liver fibrogenesis
differently depending on particular pathway. Thus, vascular endothelial growth factor
(VEGF) neutralization suppresses angiogenesis and fibrosis in mice, but inhibition
of neovascularization via integrin αvβ3 antagonism may worsen fibrotic outcomes. Contrarily,
during recovery phase, VEGF is required for liver repair and fibrosis resolution,
reminiscent of dual, opposing role of Cd11b+ macrophages during injury and repair.
Another article in this special issue will describe the regulation of liver myofibroblast
functions by fibroblast growth factor-associated signaling pathways and its functional
relevance in hepatic fibrosis progression.
In chronic liver diseases, liver myofibroblasts drive the wound healing and tissue
repair response, by releasing large amounts of extracellular matrix (e.g., collagens,
glycosaminoglycans). Several studies have shown that the perennial cellular source
of liver myofibroblasts during fibrosis is the nonparenchymal hepatic stellate cell
population. Following injury, hepatic stellate cells (HSC) undergo several phenotypic
changes including heightened contractility, increased fibrogenic capacity, and enhanced
sensitivity to inflammatory, fibrogenic, and proliferative mediators. Among the latter,
the fibroblast growth factor (FGF) signaling pathway has emerged as an important regulator
of HSC-derived liver myofibroblast functions. As J. D. Schumacher and G. L. Guo review
in this issue, various FGFs (FGF1, FGF2, FGF7, FGF9, FGF15/FGF19, and FGF21 among
others) and their related receptors (FGFR1–4) are expressed by HSC-derived liver myofibroblasts,
in regulated fashion during the development of liver fibrosis. Also, FGF molecules
induce multiple signal transduction cascades within liver myofibroblasts, to control
key cellular functions/processes such as activation state, proliferative ability,
and migratory capacity. Despite potential functional redundancy resulting from pharmacological
promiscuity (e.g., receptor-ligand binding affinity), the central role played by FGFs
and FGFRs activities in the regulation of liver myofibroblasts functions indicates
that these molecules may represent attractive therapeutic candidates for the development
of antifibrotic treatments.
The liver is a functionally complex organ, owing to both its cellular diversity and
its specialization. Hepatic fibrosis, commonly defined as excessive deposition of
ECM proteins in the local microenvironment, is the scar forming process triggered
in response to chronic liver injury. Altogether, the articles included in this special
issue underlines the importance of liver functional complexity during the development
of hepatic fibrosis but also stresses the significance of dynamic interactions existing
between both cellular (parenchymal and nonparenchymal cells) and noncellular (ECM
proteins, ECM-associated proteins, and ECM-modifying proteins) components of the hepatic
microenvironment during hepatic fibrosis. A better understanding of the molecular
mechanisms underlying these cell-to-cell and cell-to-matrix interactions will give
new insights into the pathogenesis of liver fibrosis.
Michel Fausther
Michele T. Pritchard
Yury V. Popov
Kim Bridle