Vascular endothelial growth factor accelerates renal recovery in experimental thrombotic microangiopathy

Human podocytes express angiopoietin 1, a potential regulator of glomerular vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is abundantly expressed by podocytes, but its role in glomeruli is unknown. Angiopoietins are endothelial cell growth factors that function in concert with VEGF but have not previously been observed in human glomeruli. Angiopoietin 1 (Ang1) acts via the endothelial receptor Tie2 to promote maturation and stabilization of blood vessels, resisting angiogenesis and opposing some actions of VEGF. Ang1, Ang2, Tie2, and VEGF expression in normal human renal cortex was examined with immunofluorescence and immunohistochemical analyses. High-power, multiple-color, immunofluorescence analyses and additional antibodies (specific for particular components of the glomerular filtration barrier) were used to determine the exact locations of Ang1 and Tie2 in the glomerular capillary wall. Immuno-electron-microscopic analysis of rat glomeruli was used to further localize endothelial Tie2 expression. RNA and protein extracted from human glomeruli, cultured human podocytes, and cultured human endothelial cells were analyzed for Ang1, Ang2, and Tie2 by using reverse transcription-PCR and Western blotting. Ang1 was detected in podocytes in normal glomeruli and, with VEGF, was concentrated in podocyte foot processes. Tie2 was demonstrated on glomerular capillary endothelial cells, particularly on the abluminal surface. Reverse transcription-PCR and Western blotting analyses confirmed the expression of Ang1 and Tie2 in glomeruli and of Ang1 in cultured podocytes. These findings suggest a role for Ang1 in the maintenance of the glomerular endothelium and make it a good candidate to be a regulator of the actions of VEGF on glomerular permeability, resisting angiogenesis while allowing the induction of high permeability to water and small solutes.

Targeting growth factors to the kidney: myth or reality?

Growth factors and cytokines play a crucial role in the progression of renal diseases. A growing body of evidence has been obtained from experimental studies, suggesting that manipulation of the activity of growth factors and cytokines is a potential form of therapy for renal diseases. To preserve the renal function structure in progressive renal diseases, this approach is achieved by inhibition of apoptosis of renal intrinsic cells and by decrease in the fibrotic signal. Inhibition of transforming growth factor beta, platelet-derived growth factor, interleukin-1 and tumor necrosis factor alpha, and supplementation of hepatocyte growth factor, vascular endothelial growth factor and bone morphogenic protein-7 may be beneficial. Recent progress in therapeutic implements including humanized antibodies, chimeric soluble receptors, aptamers, antisense oligonucleotides, and gene therapy allow us to target the causal molecules. Administration of a combination of growth factors and cytokines is a potential therapeutic approach. Targeting signal transduction molecules and their co-factors and regulators is another possibility because the signals from various growth factors use a common pathway. Thus, targeting growth factors and cytokines in renal diseases could be a promising therapeutic approach.

Increased renal angiopoietin-1 expression in folic acid-induced nephrotoxicity in mice

Growth factors affect epithelial regeneration after acute renal injury, but less is known regarding the expression of vascular growth factors in this setting. A mouse model of folic acid (FA)-induced nephrotoxicity was used to study the expression of angiopoietins (Ang), factors that bind the Tie-2 receptor and modulate endothelial growth. Tubular damage was detected 1 d after FA administration; in the next 14 d, most tubules regenerated but patchy atrophy, with interstitial fibrosis, was also observed. Ang-1 immunostaining was detected between cortical tubules and in the vasa rectae of vehicle-treated animals. FA-induced nephropathy was associated with the acquisition of Ang-1 immunostaining in renal arterial walls and in a subset of injured cortical tubules that failed to stain with periodic acid-Schiff stain, which indicated that they were distal tubules. Renal Ang-1 protein levels were significantly increased after FA administration, compared with time-matched control values, as assessed by Western blotting. Capillaries between regenerating tubules expressed both Tie-2 and platelet-endothelial cell adhesion molecule. A subset of these endothelia expressed proliferating cell nuclear antigen, whereas capillary proliferation was absent in control samples. Therefore, FA-induced nephropathy is associated with increased Ang-1 protein expression in renal epithelia and arteries. In addition, Tie-2-expressing capillaries near damaged cortical tubules undergo proliferation. Further experiments are required to establish whether these events are functionally related.

Vascular endothelial growth factor (VEGF121) protects rats from renal infarction in thrombotic microangiopathy

BACKGROUND

Renal thrombotic microangiopathy, typified by the hemolytic uremic syndrome, is associated with endothelial cell injury in which the presence of cortical necrosis, extensive glomerular involvement, and arterial occlusive lesions correlates with a poor clinical outcome. We hypothesized that the endothelial survival factor vascular endothelial growth factor (VEGF) may provide protection.

METHOD

Severe, necrotizing, thrombotic microangiopathy was induced in rats by the renal artery perfusion of antiglomerular endothelial antibody, followed by the administration of VEGF or vehicle, and renal injury was evaluated.

RESULTS

Control rats developed severe glomerular and tubulointerstitial injury with extensive renal necrosis. The administration of VEGF significantly reduced the necrosis, preserved the glomerular endothelium and arterioles, and reduced the number of apoptotic cells in glomeruli (at 4 hours) and in the tubulointerstitium (at 4 days). The prosurvival effect of VEGF for endothelium may relate in part to the ability of VEGF to protect endothelial cells from factor-induced apoptosis, as demonstrated for tumor necrosis factor-alpha (TNF-alpha), which was shown to be up-regulated through the course of this model of renal microangiopathy. Endothelial nitric oxide synthase expression was preserved in VEGF-treated rats compared with its marked decrease in the surviving glomeruli and interstitium of the antibody-treated rats that did not receive VEGF.

CONCLUSIONS

VEGF protects against renal necrosis in this model of thrombotic microangiopathy. This protection may be mediated by maintaining endothelial nitric oxide production and/or preventing endothelial cell death.

Protective role of nitric oxide in a model of thrombotic microangiopathy in rats

Anew model of thrombotic microangiopathy (TMA) was previously developed, and it was demonstrated that endothelial nitric oxide (NO) synthase (NOS) is upregulated in glomeruli in this model. It was hypothesized that the synthesis of NO, a potent vasodilator and platelet inhibitory factor, is induced as a defense mechanism. The goal of this study was to clarify the role of NO in this model. Ex vivo experiments using Western blotting and functional assays demonstrated upregulation of endothelial NOS in isolated glomeruli from TMA rats. In in vivo experiments, five groups of rats were studied, including rats with TMA treated with vehicle, N(G)-nitro-L-arginine methyl ester (L-NAME) (a NOS inhibitor), or L-N(6)-(1-iminoethyl)lysine (L-NIL) (a specific inducible NOS inhibitor) and normal control rats treated with vehicle or L-NAME. Blood urea nitrogen levels, BP, urinary nitrate/nitrite excretion, and proteinuria were measured. Histologic assessments using periodic acid-Schiff staining and immunohistologic studies with markers for endothelium, platelets, fibrin, cell proliferation, and apoptosis were also performed. L-NAME inhibition of NO synthesis in rats with TMA resulted in more severe glomerular and tubulointerstitial injury, which was accompanied by thrombus formation and a marked loss of endothelial cells, with more apoptotic cells. These changes were associated with severe renal function deterioration. In contrast, these features were less pronounced in the vehicle- or L-NIL-treated rats with TMA and were absent in the control animals. In conclusion, inhibition of NO production in this model of TMA markedly exacerbated renal injury. The absence of effects with L-NIL treatment suggests a minor role for inducible NOS in this model. These results suggest that production of NO, most likely by endothelial cells, is an important protective mechanism in TMA.

Glomerular endothelial cells are maintained by vascular endothelial growth factor in the adult kidney

Vascular endothelial growth factor (VEGF) is known to maintain endothelial cells of immature vessels and is constitutively expressed in the kidney from the embryo to adult. We tested the hypothesis that VEGF activity is needed to maintain glomerular endothelial cells in the adult. Neutralizing antibody to VEGF165 was intraperitoneally administered to mice for 3 days to strongly suppress its intrinsic activity. On the fourth day, mice were sacrificed and tissues were examined by light and electron microscopies. Vascular casts of renal vessels were observed by a scanning electron microscopy. Distribution of the administered antibody and expressions of VEGF and Flk-1 were examined immunohistochemically. The suppression of endogenous VEGF activity caused swelling and vacuolation of endothelial cells and obstruction of capillaries in the glomerulus. Other tissues were not impaired significantly. The administered antibody was specifically localized to the glomerulus, and was found more predominantly in the juxta-medullary than in the cortical glomerulus. This pattern of antibody deposition was similar to that of Flk-1. VEGF expression in the glomerulus was compensatively elevated by the antibody treatment. These results show that demand for VEGF signaling in the glomerulus is much higher than in other tissues, probably to protect its endothelial cells against high tension for blood filtration. This demand may be fulfilled by enriched signaling through the Flk-1 in the glomerulus.

Vascular endothelial growth factor enhances glomerular capillary repair and accelerates resolution of experimentally induced glomerulonephritis

Vascular endothelial growth factor (VEGF) regulates angiogenesis through endothelial cell proliferation and plays an important role in capillary repair in damaged glomeruli. We tested the hypothesis that VEGF might be beneficial in rats with severe glomerular injury in glomerulonephritis (GN) based on its angiogenic and vascular remodeling properties. Acute GN with severe glomerular destruction was induced in rats by injection of anti-Thy-1.1 antibody (day 0) and Habu-snake venom (day 1). Rats were intraperitoneally injected with recombinant human VEGF(165) (10 microg/100 g body wt/day) or vehicle from day 2 to day 9, and monitored changes in glomerular capillaries, development of glomerular inflammation, and progression to glomerular sclerosis after acute glomerular destruction in both groups. Rats that received anti-Thy-1.1 antibody and Habu-snake venom showed severe mesangiolysis and marked destruction of capillary network on day 2. VEGF was expressed on glomerular epithelial cells, proliferating mesangial cells, and some infiltrating leukocytes, and VEGF(165) protein levels increased in damaged glomeruli during day 5 to day 7. Normal, damaged, and regenerating glomerular endothelial cells expressed VEGF receptor flk-1. However, endothelial cell proliferation and capillary repair was rare in vehicle-treated rats with severe glomerular damage, which progressed to global sclerosis and chronic renal failure by week 8. In contrast, in the VEGF-treated group, VEGF(165) significantly enhanced endothelial cell proliferation and capillary repair in glomeruli by day 9 (proliferating endothelial cells: VEGF(165), 4.3 +/- 1.1; control, 2.2 +/- 0.9 cells on day 7, P < 0.001; and glomerular capillaries: VEGF(165), 24.6 +/- 4.8; control, 16.9 +/- 3.4 capillaries on day 7, P < 0.01). Thereafter, damaged glomeruli gradually recovered after development of capillary network by week 8, and significant improvement of renal function was evident in the VEGF-treated group during week 8 (creatinine: VEGF(165), 0.3 +/- 0.1; control, 2.6 +/- 0.9 mg/dl, P < 0.001; proteinuria: VEGF(165), 54 +/- 15; control, 318 +/- 60 mg/day, P < 0.001). We conclude that the beneficial effect of VEGF(165) in severe glomerular injury in GN emphasizes the importance of capillary repair in the resolution of GN, and may allow the design of new therapeutic strategies against severe GN.

Impaired angiogenesis in the remnant kidney model: II. Vascular endothelial growth factor administration reduces renal fibrosis and stabilizes renal function

Impaired angiogenesis and decreased vascular endothelial growth factor (VEGF) expression were recently documented in the remnant kidney (RK) model of progressive renal failure. VEGF (50 microg/kg, twice daily) was administered to RK rats between weeks 4 and 8 after surgery, and rats were euthanized at week 8 for histologic study. During the administration of VEGF (n = 7) or vehicle (n = 6), systemic BP was comparable in the two groups. VEGF treatment resulted in improved renal function and lower mortality rates, compared with the vehicle-treated group. Renal histologic analyses confirmed a 3.5-fold increase in glomerular endothelial cell proliferation (0.14 +/- 0.03 versus 0.04 +/- 0.02 proliferating endothelial cells/glomerulus, VEGF versus vehicle, P < 0.05), a twofold increase in peritubular capillary endothelial cell proliferation (1.60 +/- 0.30 versus 0.78 +/- 0.17 cells/mm(2), VEGF versus vehicle, P < 0.01), a threefold decrease in peritubular capillary rarefaction (P < 0.01), and a twofold increase in endothelial nitrix oxide synthase expression (P < 0.05) in the VEGF-treated group; an eightfold increase in urinary nitrate/nitrite levels (P < 0.05) was also noted. Although the difference in glomerulosclerosis scores did not reach statistical significance (0.67 +/- 0.42 versus 1.22 +/- 0.63, VEGF versus vehicle; range, 0 to 4; P = NS), VEGF-treated rats exhibited less interstitial collagen type III deposition (9.32 +/- 3.26 versus 17.45 +/- 7.50%, VEGF versus vehicle, P < 0.01) and reduced tubular epithelial cell injury, as manifested by osteopontin expression (5.57 +/- 1.60 versus 9.58 +/- 3.45%, VEGF versus vehicle, P < 0.01). In conclusion, VEGF treatment reduces fibrosis and stabilizes renal function in the RK model. The use of angiogenic factors may represent a new approach to the treatment of kidney disease.

Impaired angiogenesis in the remnant kidney model: I. Potential role of vascular endothelial growth factor and thrombospondin-1

Few studies have examined the role of the microvasculature in progressive renal disease. It was hypothesized that impaired angiogenesis might occur in the diseased kidney and could contribute to renal scarring. Progressive renal disease was induced in rats by 5/6 renal ablation and those rats were compared with sham-operated control animals at multiple time points, for examination of changes in the microvasculature and the expression of angiogenic factors. An early angiogenic response was documented in remnant kidneys, with increases in the proliferation of peritubular (1 wk) and glomerular (2 wk) endothelial cells. Subsequently, however, there was a decrease in endothelial cell proliferation, which was reduced to levels below those of sham-treated animals, in conjunction with interstitial expression of the antiangiogenic factor thrombospondin-1 (TSP-1) and decreased tubular expression of the proangiogenic factor vascular endothelial growth factor (VEGF). Both the increase in TSP-1 expression and the loss of VEGF expression were correlated with capillary loss and the development of glomerulosclerosis and interstitial fibrosis. Progressive macrophage infiltration was correlated both spatially and quantitatively with the sites of absent or diminished VEGF expression. In addition, macrophage-associated cytokines (interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha) inhibited VEGF mRNA expression and protein secretion by cultured tubular epithelial cells of the medullary thick ascending limb, under both normoxic and hypoxic conditions. Impaired angiogenesis characterizes the remnant kidney model and is correlated with progression. The impaired angiogenesis may be mediated by alterations in the renal expression of TSP-1 and VEGF, with the latter being regulated by macrophage-associated cytokines.

Impaired angiogenesis in the aging kidney: vascular endothelial growth factor and thrombospondin-1 in renal disease

We investigated the relationship of changes in the microvasculature to age-related structural and functional changes in the kidney to determine whether there was evidence of impaired angiogenesis and whether the loss of microvasculature could be accounted for by changes in the local production of angiogenic or antiangiogenic factors. Glomerular and peritubular capillary number, density, and endothelial cell proliferation were determined in aging (24 months; n = 9) and young (3 months; n = 8) rat kidneys and correlated with renal functional and structural changes and alterations in renal expression of vascular endothelial growth factor (VEGF) and thrombospondin-1 (TSP-1). Aging rats showed a focal decrease in both peritubular capillary (peritubular capillary staining, 5.4% +/- 1.8% versus 11.3% +/- 2.0% per 100 tubules; rarefaction index, 10.6% +/- 4.6% versus 0.6% +/- 0.1%, aging versus young rats; P < 0.05 and P: < 0.001, respectively) and glomerular capillary loops (27.3 +/- 6.9 versus 50.7 +/- 7.4/glomerulus, aging versus young rats; P < 0.001). The number of proliferating endothelial cells was decreased in aging rats compared with young rats (glomerular, 0.04 +/- 0.01 versus 0.15 +/- 0.03 positive cells/glomerular cross-section; peritubular, 0.7 +/- 0.2 versus 4.3 +/- 2.6 positive cells/mm(2); P < 0.05). In the aging kidney, VEGF expression was focally increased in the cortex compared with young rats, whereas a profound decrease was observed in the outer and inner medulla (total area of VEGF expression, 19.2% +/- 11.4% versus 39.3% +/- 7.6%; P < 0.05). Tubular VEGF expression correlated with peritubular capillary density (r(2) = 0.57; P < 0.01) and inversely correlated with tubular osteopontin (r(2) = -0.55; P < 0.05) and macrophage infiltration (r(2) = -0.64; P < 0.01). TSP-1 staining was increased in the glomeruli and tubulointerstitium of the aging rats. Glomerular TSP-1 score correlated inversely with glomerular capillary number (r(2) = -0.89; P < 0.001). Tubulointerstitial TSP-1 also correlated with percentage of positive staining of peritubular capillary (r(2) = -0.59; P < 0.001). Glomerular capillary number showed significant correlation with glomerulosclerosis score, as well as with 24-hour urinary protein excretion. Peritubular capillary density also inversely correlated with interstitial fibrosis score and urinary protein excretion. In conclusion, glomerular and peritubular capillary loss in the aging kidney correlate with alterations in VEGF and TSP-1 expression and also with the development of glomerulosclerosis and tubulointerstitial fibrosis. These findings suggest that impaired angiogenesis associated with progressive loss in renal microvasculature may have a pivotal role in age-related nephropathy.