Diabetic retinopathy is the leading cause of blindness in the Western world (1) and
is characterized by abnormal angiogenesis driven by several factors, including tissue
ischemia and hyperglycemia. This abnormal angiogenesis results in new vessels that
are often immature and play a pathological role in retinopathy, contributing to both
vitreous hemorrhage and fibrosis (2). In addition, increased vascular permeability
leading to plasma leakage accounts for the development of macula edema, disrupting
visual function (2). These evidences have led to the development of several therapeutic
strategies targeting angiogenesis in diabetic retinopathy (3).
Abnormal angiogenesis also occurs in diabetic nephropathy; therefore, the overriding
question is whether new vessel formation in the kidney plays a pathological role in
diabetic nephropathy similar to that observed in retinopathy. Intriguingly, the progression
of both diabetic retinopathy and nephropathy is altered by vascular growth factor
signaling through receptor tyrosine kinases, specifically involving the vascular endothelial
growth factor (VEGF)-A and angiopoietin families. This review discusses abnormal angiogenesis
and the role of both VEGF-A and angiopoietins in diabetic nephropathy.
Evidence of abnormal angiogenesis in diabetic nephropathy.
In 1987, Osterby and Nyberg (4) described abnormal blood vessels in glomeruli of patients
with long-term type 1 diabetes, and later these findings were shown to occur in type
2 diabetic patients (5,6) (Fig. 1
A). The abnormal vessels occupied 1–5% of glomerular capillary area, they were occasionally
dilated, and the glomerular basement membrane adjacent to them was found to be focally
extremely thin. Abnormal vessels were also present in Bowman's capsule or in the glomerular
vascular pole, the latter of which could often be detected as an “extra efferent arteriole”
(4,7). Min and Yamanaka (8) then performed detailed analyses of computer-generated
three-dimensional images in 94 patients with diabetic nephropathy and found the presence
of extravessels. Intriguingly, in this study the abnormal vessels anastomosed to the
lobular structure of the intraglomerular capillary network, mainly to afferent branches
through the widened vascular hilus, while the distal end of the vessels connected
to the peritubular capillary. In these vessels, native endothelial cell function was
likely impaired, with the endothelial cells initially swollen and endothelial thickness
gradually decreasing as diabetes progressed (9,10). It was also documented that the
vascular wall was thickened, owing to an accumulation of matrix in these arterioles
(10). Of importance was the finding that these vessels were observed in diabetic patients
during the first 2 years of disease (8), which supports the contention that the development
of these vessels occurs even in the early phases of diabetic nephropathy.
FIG. 1.
Abnormal angiogenesis in diabetic nephropathy. A: Extraglomerular neovascularization
(black arrows) are found in type 2 diabetic patients. Reprinted with permission from
ref. 6. B: Similarly, immunohistochemistry for CD34, a marker for endothelial cells,
indicates the normal glomerular capillary pattern (brown) in nondiabetic C57BL6 mice
(a). Alternatively, abnormal capillary formation is observed around glomerulus in
diabetic mice lacking endothelial nitric oxide synthase (b), resembling abnormal angiogenesis
in human diabetic nephropathy. Reprinted with permission from ref. 33. C: The association
of abnormal angiogenesis with VEGF expression in diabetic eNOSKO mice. In contrast
to glomerulus in normal mice, glomerular endothelial staining is increased along with
VEGF expression. However, hydralazine treatment attenuates the increase in glomerular
capillary number and VEGF expression in diabetic eNOSKO mice. D: Glomerular filtrate
and atubular glomerulus in diabetic eNOSKO mice. Glomerular filtrates are delivered
to the outside of the glomerulus (arrow in a) and spread to the glomerulotubular junction
(arrows in a and b). This filtrate may lead to a disconnection between glomerulus
and proximal tubules (b). Ballooning of Bowman's capsule is observed in c. Glomeurlar
filtrate can be observed outside of Bowman's capsule in the tubular pole, where the
proximal tubulus is completely disconnected from the glomerulus. Bar = 40 μm. (A high-quality
digital representation of this figure is available in the online issue.)
In diabetic animals, Nyengaard and Rasch (11) identified abnormal glomerular capillaries
in an animal rat model induced by streptozotocin. They determined that after both
10 and 50 days following injection, the average total surface area, length, and numbers
of glomerular capillaries were elevated compared with those of controls. Similarly,
db/db mice were found to exhibit increased endothelial cell number and elongation
of capillaries in their glomeruli (12,13). Recently, the occurrence of excessive blood
vessel formation in diabetes has been demonstrated by immunohistochemistry using endothelial
cell markers. As shown in Fig. 1
B, endothelial cell staining was increased in streptozotocin-induced diabetic animals
(13
–16). However, it should be noted that the later stages of diabetic nephropathy are
accompanied by capillary loss and rarefaction in both humans and animal models, a
concept that is discussed below (5,15,17).
The pathological role of abnormal angiogenesis in diabetic nephropathy.
While the pathological role of abnormal vessels remains unclear, it has been demonstrated
that neovascularization is associated with glomerular hypertrophy in diabetic nephropathy.
Morphological changes in capillaries such as elongation and increased number contribute
to glomerular hypertrophy in both humans and animals with diabetes, whereas changes
in mean capillary diameter do not correlate with alterations in glomerular volume
(7,12). Interestingly, the development of abnormal vessels was observed in the extraglomerular
area and associated with glomerular hypertrophy in both diabetic animals and patients
(6,10). Osterby et al. (7) performed a series of studies using electron microscopy
and found that abnormal vessels in the vascular pole were associated with enhanced
glomerular hypertrophy and increased frequency of glomerular capillary occlusion,
fibrinoid lesions, tubulointerstitial injury, and urinary albumin excretion (6,10,18).
Additionally, recent evidence has indicated that blocking angiogenesis attenuated
glomerular basement membrane thickening, mesangial expansion, and transforming growth
factor (TGF)-β1 expression in diabetic animals (13,14,16), suggesting that these vessels
have a causal role in the development of early features of diabetic nephropathy.
The abnormal additional vessels found in diabetes possess a thin wall at the basement
membrane, while endothelial cells are swollen, suggesting that they are structurally
immature and capable of causing an increase in vascular permeability (4,9,19). An
increase in capillary permeability often results in the extravasation of plasma protein
as well as the forming of lesions in diabetic nephropathy. For example, the capsular
drop, fibrin cap, and arteriolar hyalinosis, all of which characterize human diabetic
nephropathy, can be considered a consequence of plasma leakage (20). Furthermore,
recent studies documented that leakage of plasma protein results in the development
of atubular glomeruli in type 1 diabetic patients (21). A potential mechanism for
this observation could be due to the glomerular and peritubular filtrate spreading
into the glomerulo-tubular junction to eventually separate proximal tubules from the
glomerulus (22). Given these facts, the extravasations from immature vessels could
have a causal role in the development of diabetic nephropathy.
Mechanisms for the development of abnormal angiogenesis in the diabetic kidney.
Angiogenesis is often associated with an increase in endothelial number caused by
an imbalance in cell proliferation and apoptosis. Recently, Hohenstein et al. demonstrated
that in type 2 diabetic patients, an increased endothelial number was observed and
early glomerular lesions were caused by a combination of increased proliferation and
decreased apoptosis in glomerular endothelial cells (5). A major driver in this process
appears to be VEGF-A expression, which is induced by high glucose levels in the early
phases of diabetes and can stimulate endothelial cell proliferation and inhibit apoptosis.
In addition, high glucose levels alone can enhance endothelial cell proliferation
(23). Therefore, the beneficial effect of insulin treatment to block the progression
of extravessels in patients with type 1 diabetes (7) could be attributed to reduced
blood glucose levels and inhibition of VEGF-A expression (15).
Glomerular hypertension may be another important driver in the progression of abnormal
angiogenesis in diabetes. Osterby et al. (7) demonstrated that treatment with ACE
inhibitors or β-blockers for 8 years to reduce hypertension in diabetic patients suppressed
progression of glomerular lesions and extravessel formation. In a similar fashion,
we found that lowering blood pressure in a novel animal model of diabetic nephropathy
using endothelial nitric oxide synthase (eNOS) knockout (eNOSKO) mice and steptozotocin
injection led to attenuated progression of abnormal angiogenesis. These mice developed
abnormal vessels accompanied by advanced lesions including nodular lesions and mesangiolysis
(15). As shown in Fig. 1
C, lowering blood pressure in these animals using hydralazine blocked the development
of abnormal angiogenesis and inhibited glomerular VEGF-A expression (24). These data
suggest that the beneficial effect of lowering blood pressure could be mediated by
VEGF-A inhibition. Alternatively, one could postulate that these vessels function
as a by-pass to reduce intraglomerular pressure given that abnormal vessels were found
to connect intraglomerular capillaries to peritubular capillaries (8). Hence, reducing
intraglomerular pressure as a consequence of lowering systemic blood pressure might
reduce the need for the development of by-pass vessels. A depiction of the factors
affecting abnormal angiogenesis and their pathological effects is shown in Fig. 2.
FIG. 2.
Mechanism and pathogenic role of abnormal angiogenesis in diabetic nephropathy.
VEGF as a mediator of abnormal angiogenesis in diabetic nephropathy.
The VEGF-A family has a role in the development, maintenance, and remodeling of the
vasculature, acting through the receptor tyrosine kinases VEGFR-1 and VEGFR-2 (25).
The VEGF-A family is very complex with several isoforms generated by alternative splicing
of exons 6 and 7. In diabetes, the VEGF-A164 and VEGF-A188 isoforms are increased
and can be reduced by insulin treatment (26). Additional isoforms with anti-angiogenic
properties termed VEGF-Axxxb occur due to exon 8 distal splice site selection (25),
leading to an unique carboxy-terminal sequence.
Several studies have examined the expression pattern of the VEGF-A family in diabetic
animals and patients. Cooper et al. (27) examined VEGF-A and VEGFR-2 in short- and
long-term diabetic rats (3 and 32 weeks following streptozotocin injection, respectively).
Short-term diabetes led to elevated VEGF-A and VEGFR-2 mRNA, whereas in long-term
diabetic animals VEGF-A remained elevated and VEGFR-2 was unaltered. VEGF-A was localized
to podocytes and, to a lesser extent, tubular epithelial cells, whereas VEGFR-2 was
expressed in glomerular and peritubular capillaries. Elevated VEGF-A has been confirmed
in our animal model of diabetic nephropathy using eNOSKO mice (15). The mice developed
excessive vessels in glomeruli and tubulointerstitium that were associated with upregulation
of glomerular VEGF-A expression. Elevation of VEGF-A has also been observed in human
biopsy samples where the number of extravessels around the glomerular vascular pole
was associated with upregulation of VEGF-A expression in the kidney (6). Finally,
examination of urinary VEGF-A showed significant elevations in type 2 diabetic patients
(28), compared with healthy control subjects, that positively correlated with urinary
albumin-to-creatinine ratio and negatively correlated with creatinine clearance.
A potential consequence of high levels of VEGF-A will be enhanced vascular permeability
in the glomerulus (29). In addition, low NO (nitric oxide) bioavailability observed
in diabetes (30,31) could be an additional contributor to the increased vascular permeability.
Predescu et al. (32) documented that low levels of endothelial-derived NO altered
the integrity of interendothelial junctions in capillaries, resulting in an increase
in vascular permeability. As such, a low NO bioavailability along with high VEGF-A
expression (we term this condition “uncoupling of VEGF-A with NO”) observed in the
diabetic milieu of eNOSKO mice could potentiate the vascular permeability in the glomerulus
and cause glomerular injury in diabetic nephropathy (15,33). Intriguingly, this uncoupling
condition could also cause the development of abnormal angiogenesis. This notion can
be supported by recent evidences from our laboratory and other groups that NO can
negatively regulate VEGF-A–induced endothelial proliferation (34), whereas NO deficiency
enhances VEGF-A activity, leading to endothelial proliferation (35). We have extensively
reviewed a causal role of this uncoupling condition in other types of vascular diseases,
including coronary artery disease, remnant kidney, and angiotensin II–induced renal
injury in previous work (33). It should be noted that while the aforementioned studies
indicate low NO contributing to capillary hyperpermeability, Tilton et al. (36) demonstrated
that supra-physiological NO positively mediates hyperpermeability in response to exogenous
VEGF-A in several different nondiabetic tissues. Therefore, it is likely that physiological
levels of NO are required to maintain low vascular permeability and that NO levels
that are either too high or too low (depending on the biological situation) may lead
to hyperpermeability.
VEGF-A is lowered in the advanced stage of diabetic nephropathy.
The study by Cooper et al. (27) suggested that although VEGF-A may be elevated in
the initial phases of diabetic nephropathy, it may not be maintained as more chronic
fibrotic changes occur in the kidney. Indeed, in many animal models of chronic kidney
disease, VEGF-A levels are reduced, correlating with the progression of renal damage
(37,38). To examine this in diabetic nephropathy, Baelde et al. (17) used laser-capture
microdissection to determine gene expression in glomeruli from 28 diabetic patients.
They observed a reduction of 2.5-fold in VEGF-A expression in severely injured glomeruli
as evidenced by a loss of endothelial cells and a reduction in podocyte markers (WT-1,
nephrin, and podocin mRNAs) (17). Given that podocytes and tubular epithelial cells
are the primary source of VEGF-A in the kidney, the mechanism for a reduction in VEGF-A
expression in severe renal injury could be attributed to the inability of these cells
to produce VEGF-A due to advanced stages of cellular injury. Other studies have found
that VEGF-A expression was decreased in sclerotic areas and in nodular lesions of
diabetic nephropathy (39,40). In addition, Zucker diabetic fatty rats exhibited a
decline in renal VEGF-A expression in advanced stages of diabetic nephropathy (41,42).
This interesting concept was highlighted in an elegant study by Hohenstein et al.
(5) where they used specific antibodies to examine not only VEGF-A expression but
also receptor-bound VEGF-A as a marker of bioactivity in diabetic patients. In their
study, although VEGF-A expression was increased in all diabetic glomeruli by many
cell types, VEGF-A activity was only increased in the endothelium of mildly injured
glomeruli and significantly decreased in more severe glomeruli (5). This data suggests
that the upregulation of VEGF-A in early stages of diabetic nephropathy may provide
a mechanism for the initial progression of the disease, leading to excessive blood
vessel formation. The decline of VEGF-A in the later phase of diabetic nephropathy
may reflect a loss of endogenous VEGF-A due to the disruption of podocytes and tubular
cells in chronic kidney damage (Table 1).
TABLE 1
Reduction of VEGF expression in diabetic nephropathy
Renal VEGF expression
Endothelium
Diabetes history
Stage of nephropathy
Ref.
Human
Decreased (mRNA, protein)
NE
Not mentioned
Sclerotic glomerulus
40
Human
Decreased (mRNA)
NE
Sclerotic glomerulus with heavy proteinuria
70
Human type 2 diabetes
Decreased (mRNA)
NE
>5 years
Glomerulosclerosis (24–33%) and tubulointerstitial injury (25–50%)
71
Human type 2 diabetes
Decreased mRNA in VEGF165, increased mRNA in VEGF121
NE
>2 years
Microalbuminuria, 10 of 17 patients; macroalbuminuria, 7 of 17 patients
72
Human type 2 diabetes
Decreased (protein)
Decreased EC proliferation, increased EC apoptosis
12.8 ± 7.8 years
Advanced lesions
5
Human type 2 diabetes
Decreased (mRNA, protein)
Decreased CD31 (+) EC
>5 years
Interstitial fibrosis, podocyte loss
17
Human type 1 and 2 diabetes
Decreased (mRNA, protein)
Decreased CD31 (+) EC
Established diabetes or early diabetes
EC loss in peritubular capillary or tubulointerstitial injury
73
Zucker rat
Decreased (mRNA, protein)
NE
22 weeks old
Advanced renal injury
41
EC, endothelial cell; NE, not examined.
Alterations in angiopoietin balance as a molecular mechanism of diabetic nephropathy.
A second family of growth factors implicated in the progression of diabetic nephropathy
are the angiopoietins, which are critical for the normal vascular differentiation,
maintenance, and turnover of blood vessels in mature animals (43). Angiopoietin-1
and -2 are ligands for the Tie-2 receptor tyrosine kinase, expressed mainly by endothelia;
angiopoietin-1 stimulates receptor activation, leading to promotion of endothelial
survival and stabilization. Angiopoietin-2 is considered a natural antagonist of angiopoietin-1
(44), although other data suggest that high concentrations of angiopoietin-2 may activate
Tie-2 (45). Alterations in the expression of the angiopoietins have been implicated
in the progression of diabetic nephropathy (rev. in (43). In addition, transgenic
mice with inducible overexpression of angiopoietin-2 in podocytes in otherwise normal
healthy adult animals develop significant increases in albuminuria (46), a parameter
that correlates with, and can predict, the progression of renal damage in diabetes
(47). Collectively, these observations suggest that a decreased ratio of angiopoietin-1
to angiopoietin-2 might play a role alongside VEGF-A in the pathobiology of diabetic
nephrology. Importantly, the biological effects of angiopoietin-2 are context dependent
and, in vivo, depend on ambient levels of VEGF-A, such that vessel regression occurs
if VEGF-A is lacking, whereas vessel destabilization followed by angiogenesis occurs
if the local milieu is rich in VEGF-A (44). It could be postulated that the increased
levels of angiopoietin-2 alongside a VEGF-A–rich milieu in glomeruli during the initial
phases of diabetes will lead to the destabilization of blood vessels and hence excessive
angiogenesis. Therefore, it is possible that modulation of the balance between angiopoietin-1
and -2 may have therapeutic potential in diabetic nephropathy.
Targeting angiogenesis to treat diabetic nephropathy.
Given the evidence above, there is a rationale for targeting angiogenic pathways to
prevent diabetic nephropathy, and several studies have now blocked VEGF-A activity
as a therapy to prevent abnormal angiogenesis. An elegant genetic approach was recently
taken by Gnudi and colleagues (48) by blocking VEGF-A signaling in mice administered
streptozotocin through overexpressing soluble VEGFR-1, specifically in podocytes.
Diabetic mice that overexpressed soluble VEGFR-1 had attenuated albumin excretion,
mesangial expansion, glomerular basement membrane thickening, podocyte foot process
fusion, and TGF-β1 expression (48). de Vriese's group (49) examined the effect of
treatment with a monoclonal anti–VEGF-A antibody in the early phase of diabetes induced
by streptozotocin. Administration of the antibody decreased hyperfiltration, albuminuria,
and glomerular hypertrophy in diabetic rats. Although the effect on angiogenesis was
not specifically examined in this study, VEGF-A blockade prevented the upregulation
of eNOS associated with this model (49). Other studies were performed in db/db mice
and the Zucker diabetic fatty rat (50,51). In db/db mice, VEGF-A antibody treatment
resulted in a reduction in kidney weight, glomerular volume, basement membrane thickness,
and urinary albumin excretion (50); in the Zucker diabetic fatty rat, VEGF-A antibody
treatment prevented glomerular hypertrophy. However, neither of these studies examined
the effect of reducing VEGF-A on abnormal angiogenesis. Similarly, Sung et al. (52)
blocked the phosphorylation of the VEGF-A receptors using the pharmacological kinase
inhibitor SU5416 in db/db mice and found that this approach prevented the development
of albuminuria and glomerular basement membrane thickening. Interestingly, blocking
VEGF-A activation prevented the loss of nephrin and improved structural changes in
podocyte foot processes in db/db mice. These results suggest that VEGF-A could impair
podocyte function, which may be an additional mechanism by which VEGF-A causes urinary
protein excretion. However, since these studies did not examine the process of abnormal
angiogenesis per se, further experiments are required to determine whether this beneficial
effect of anti–VEGF-A therapy could be due to the blocking of VEGF-associated angiogenesis.
Currently VEGF-A inhibitors are classified into four groups (Table 2) and have been
used in clinical practice. Importantly, the efficacy of these individual compounds
is not identical. For instance, the tyrosine kinase inhibitors have greater anti-tumor
efficacy only at early stages of cancer progression (53), whereas monoclonal antibodies
are capable of regressing tumor growth (54). In the kidney, VEGF-A function is also
complicated given that it has been found to exhibit both deleterious and beneficial
effects (rev. in (33). In fact, VEGF-A is found to be deleterious in diabetic nephropathy
but largely beneficial in nondiabetic animal models of renal disease. Hence, we need
to be cautious before using VEGF-A inhibitors in the diseased kidney. Previously,
the beneficial effect of anti–VEGF-antibodies was shown in two diabetic animal models:
streptozotocin-induced diabetic rats and db/db mice (49,55). On the contrary, it has
been postulated that a potential adverse effect with VEGF-A inhibitors could be endothelial
injury because endothelial cells require VEGF-A in physiological conditions. Eremina
et al. (56) demonstrated that bevacizumab, the anti–VEGF-antibody, causes renal thrombotic
microangiopathy partly due to endothelial injury in patients. Similarly, Advani et
al. (57) demonstrated that VEGFR-2 tyrosine kinase inhibitors exacerbated hypertension
and renal disease in hypertensive rats. Likewise, systemic overexpression of soluble
VEGFR-1 in normal animals was found to cause endotheliosis and podocyte injury, leading
to proteinuria and hypertension (58,59). In addition, the deleterious effect of anti-VEGF
antibodies could be attributed to the deposition of VEGF–anti-VEGF complex, C3 deposition,
and endothelial swelling (54). However, in some experiments, it was also shown that
normal kidneys did not have any side effects from VEGF inhibitors treatment (60,61).
TABLE 2
VEGF inhibitors
Function
Drug name
Inhibition of VEGF secretion
Iressa*
Tarceva*
Sequestration of VEGF
Bevacizumab*
Ranibizumab*
Pegaptanib*
Blocking binding of VEGF to VEGFR
DC101
Inhibitior of receptor tyrosine kinase
Sunitinib*
Sorafenib*
*Approved by the U.S. Food and Drug Administration.
Since endothelial cells require VEGF-A in physiological conditions, substantial inhibition
can cause endothelial injury. In this regard, it may not be adequate to use VEGF-A
inhibitors in patients with normal kidney function or in nondiabetic renal injury
in which VEGF-A expression is downregulated. In contrast, Gnudi and colleagues (48)
succeeded in treating diabetic nephropathy using podocyte-specific overexpression
of soluble VEGFR-1. In this study, neither VEGF-A expression nor VEGFR-2 phosphorylation
was significantly blocked by overexpression of soluble VEGFR-1 in the diabetic kidney,
suggesting that VEGF-A function was partially inhibited. Thus, the “partial” inhibition
might be a means to treat diabetic nephropathy without any adverse effects. Further
clarification on the adverse effects of VEGF-A inhibitors is required before they
may be used to treat diabetic patients.
Angiopoietins have been used therapeutically in several diabetes situations. Administration
of angiopoietin-1 has been shown to suppress diabetic retinopathy by preventing leukocyte
adhesion, endothelial cell injury, and blood-retinal barrier breakdown (62). With
regard to diabetic nephropathy, Lee et al. (63) demonstrated that systemic adenoviral
delivery of COMP-Ang-1 (a modified form of angiopoietin-1) reduced renal fibrosis
in db/db mice. However, this strategy also caused a significant improvement in hyperglycemia,
an event possibly related to the systemic administration of angiopoietin-1, which
could itself, at least partly, account for the amelioration of diabetic nephropathy.
Therefore, further experiments are required to examine whether modulation of this
pathway could be a future treatment for patients with diabetic nephropathy.
Several studies have attempted to block angiogenesis using other anti-angiogenic molecules
in animal models, as shown in Table 3 (13,14,16,64). Angiostatin is a potent angiogenic
inhibitor that blocks proliferation, induces apoptosis, and prevents migration of
endothelial cells in vitro. In addition, angiostatin has anti-inflammatory actions
by inhibiting leukocyte recruitment and both neutrophil and macrophage migration.
In streptozotocin-induced diabetic nephropathy, adenoviral-mediated delivery of angiostatin
was found to alleviate albuminuria and glomerular hypertrophy (64). We also found
a similar advantage of angiostatin treatment in the remnant kidney model (65). Similarly,
endostatin, a potent inhibitor of angiogenesis derived from type XVIII collagen (14),
and tumstatin, an angiogenic inhibitor derived from type IV collagen (16), were both
able to prevent glomerular hypertrophy, hyperfiltration, and albuminuria in type 1
diabetic mice. Interestingly, these treatments were shown to prevent mesangial expansion
and inflammation and also to attenuate the increase in levels of VEGF-A and angiopoietin-2
normally observed in this model (14,16) independent of blood pressure and blood glucose
levels (Fig. 2). Similar observations were made with 2-(8-hydroxy-6-methoxy-1-oxo1H-2-benzopyran-3-yl)
propionic acid (a small molecule with anti-angiogenic activity) in db/db mice (13).
These novel treatments to prevent angiogenesis could be considered for patients in
early stages of diabetic nephropathy.
TABLE 3
Angit-angiogenic therapy in diabetic nephropathy
Anti-VEGF antibody
SU5416
Angiostatin
Endostatin
Tumstatin
NM-3
PEDF
Blocking VEGF
Pan-VEGFR kinase inhibitor
Proteolytic fragment of plasminogen
NC1 domain of type 18 collagen
NC domain of the α3 chain of type 4 collagen
2-(8-hydroxy-6-methoxy-1-oxo-1H-2-benxopyran-3-yl) propionic acid
Pigment epithelium-derived factor
STZ Wister rat*
db/db mouse*
GK rat*
db/db mouse
STZ Brown Norway rat
STZ-C57BL6 mouse*
STZ-C57BL6 mouse*
db/db mouse*
STZ Brown Norway rat
Age or weight
250–280 g
8 weeks
8 weeks
8 weeks
8 weeks
7–8 weeks
7–8 weeks
7–8 weeks
8 weeks
Treatment duration
≤6 weeks
60 days
6 weeks
8 weeks
2–3 weeks
4 weeks
2–3 weeks
8 weeks
3 weeks
Increase in CD31(+) endothelial cell in glomeruli
NE
NE
NE
NE
NE
Blocked
Blocked
Blocked
NE
Renal hypertrophy
NE
Blocked
NE
NE
NE
Blocked
Blocked
Not blocked
NE
Glomerular hypertrophy
Blocked
Blocked
No effect
NE
Blocked
Blocked
Blocked
Blocked
NE
Mesangial expansion
NE
Tended to be lowered
NE
Blocked
NE
Blocked
Blocked
Blocked
NE
Glomerular basement membrane thickening
NE
Blocked
NE
Blocked
NE
NE
NE
NE
NE
Hyperfiltration
Blocked
Blocked
No effect
NE
NE
Blocked
Blocked
Blocked
NE
Urinary albumin
Decreased
Decreased
No effect
Decreased
Decreased
Decreased
Decreased
Decreased
Decreased
Podocyte injury or Nephrin expression
NE
NE
NE
Improved
NE
Recovered (Nephrin expression)
Recovered (Nephrin expression)
Recovered (Nephrin expression)
NE
Macrophage infiltration
NE
NE
NE
NE
NE
Blocked
Blocked
Blocked
NE
VEGF expression
NE
NE
NE
NE
Decreased
Decreased
Decreased
Decreased
NE
TGF-β1 expression
NE
NE
NE
NE
Decreased
Decreased
NE
Decreased
Decreased
Reference
49
50
51
52
64
14
16
13
74, 75
NE, not examined; STZ, streptocotocin.
*Female.
What prospects are there for other novel therapies for diabetic nephropathy? One area
of interest may be in examining the anti-angiogenic isoforms of VEGF-Axxxb in models
of diabetic nephropathy, which may open new avenues of treatment strategies. Another
therapy could be the use of RNA aptamers, which are oligonucleotidue ligands that
bind with high-affinity to molecular targets. One such aptamer that targets VEGF-A165
has been used successfully in clinical trials to block ocular neovascularization (66).
Promising results have also been obtained using small-molecule tyrosine kinase inhibitors
to treat type 1 diabetic mice (67); however, the kidneys were not examined in these
studies. Finally, other molecules involved in angiogenic pathways such as the Notch
family (68) may provide interesting information in the pathobiology and treatment
of diabetic nephropathy. In this regard, studies by Niranjan et al. (69) have already
demonstrated that lack of the Notch1 transcriptional partner Rbpj in podocytes is
able to modulate the progression of albuminuria in diabetic mice.
In conclusion, while the presence of abnormal angiogenesis was demonstrated more than
a decade ago, we are only beginning to unravel the pathophysiological importance of
this event. Anti-angiogenic treatments can prevent the progression of animal models
of diabetic nephropathy, but further studies are required before these treatments
can be used in a clinical setting. The fact that diabetic nephropathy is currently
still the leading cause of end-stage renal disease points to the need for additional
treatment strategies. Thus, novel therapies that target other angiogenic pathways
such as the angiopoietin and Notch families could be an attractive option to block
diabetic nephropathy in the future.