Vascular endothelial growth factor accelerates renal recovery in experimental thrombotic microangiopathy

Retinoic acid increases hypoxia-inducible factor-1α through intracrine prostaglandin E(2) signaling in human renal proximal tubular cells HK-2

We have previously shown in HK-2 cells that ATRA (all-trans-retinoic acid) up-regulates HIF-1α (hypoxia-inducible factor-1α) in normoxia, which results in increased production of renal protector VEGF-A (vascular endothelial growth factor-A). Here we investigated the role of COXs (cyclooxygenases) in these effects and we found that, i) ATRA increased the expression of COX-1 and COX-2 mRNA and protein and the intracellular levels (but not the extracellular ones) of PGE(2). Furthermore, inhibitors of COX isoenzymes blocked ATRA-induced increase in intracellular PGE(2), HIF-1α up-regulation and increased VEGF-A production. Immunofluorescence analysis found intracellular staining for EP1-4 receptors (PGE(2) receptors). These results indicated that COX activity is critical for ATRA-induced HIF-1α up-regulation and suggested that intracellular PGE(2) could mediate the effects of ATRA; ii) Treatment with PGE(2) analog 16,16-dimethyl-PGE(2) resulted in up-regulation of HIF-1α and antagonists of EP1-4 receptors inhibited 16,16-dimethyl-PGE(2)- and ATRA-induced HIF-1α up-regulation. These results confirmed that PGE(2) mediates the effects of ATRA on HIF-1α expression; iii) Prostaglandin uptake transporter inhibitor bromocresol green blocked the increase in HIF-1α expression induced by PGE(2) or by PGE(2)-increasing cytokine interleukin-1β, but not by ATRA. Therefore only intracellular PGE(2) is able to increase HIF-1α expression. In conclusion, intracellular PGE(2) increases HIF-1α expression and mediates ATRA-induced HIF-1α up-regulation.

Thrombotic microangiopathy: a role for magnesium?

Despite advances in more recent years, the pathophysiology and especially treatment modalities of thrombotic microangiopathy (TMA) largely remain enigmatic. Disruption of endothelial homeostasis plays an essential role in TMA. Considering the proven causal association between magnesium and both endothelial function and platelet aggregability, we speculate that a magnesium deficit could influence the course of TMA and the related haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura. A predisposition towards TMA is seen in many conditions with both extracellular and intracellular magnesium deficiency. We propose a rationale for magnesium supplementation in TMA, in analogy with its evidence-based therapeutic application in pre-eclampsia and suggest, based on theoretical grounds, that it might attenuate the development of TMA, minimise its severity and prevent its recurrence. This is based on several lines of evidence from both in vitro and in vivo data showing dose-dependent effects of magnesium supplementation on nitric oxide production, platelet aggregability and inflammation. Our hypothesis, which is further amenable to assessment in animal models before therapeutic applications in humans are implemented, could be explored both in vitro and in vivo to decipher the potential role of magnesium deficit in TMA and of the effects of its supplementation.

VEGF ameliorates tubulointerstitial fibrosis in unilateral ureteral obstruction mice via inhibition of epithelial-mesenchymal transition

AIM

Vascular endothelial growth factor (VEGF) has been shown to be a survival factor for renal tubular epithelial cells. In the present study, we investigated whether administration of VEGF ameliorates tubulointerstitial fibrosis in a mouse model of unilateral ureteral obstruction (UUO).

METHODS

Thirty-six male CD-1 mice were randomly divided into three groups: sham-operation, UUO and UUO+VEGF group. VEGF (50 μg/kg) was subcutaneously injected twice daily from d 1 to d 14. Mice in each group were killed at d 3, 7, or 14 after the operation, and the tubulointerstitial fibrosis was histopathologically evaluated. Human proximal tubular epithelial cells (HK-2) were used for in vitro study. The expression levels of α-SMA, E-cadherin, TGF-β1, CTGF, and BMP-7 in the kidney were determined using Western blot and RT-PCR.

RESULTS

In the UUO mice, the degree of interstitial fibrosis was dramatically increased in a time-dependent manner. At d 3, 7, and 14, both the mRNA and protein expression levels for α-SMA, TGF-β1, and CTGF were significantly upregulated, whereas those for E-cadherin and BMP-7 were significantly downregulated. At d 3 and 7, VEGF treatment significantly reduced interstitial fibrosis and the expression levels for α-SMA, TGF-β1, and CTGF, while significantly increased the expression of E-cadherin and BMP-7, as compared with the UUO mice. At d 14 after operation, no significant differences were observed in the expression of the examined markers between VEGF-treated mice and UUO mice, with the exception of CTGF. In HK-2 cells, VEGF blocked TGF-β1-induced α-SMA and vimentin expression and restored E-cadherin expression in a dose-dependent manner.

CONCLUSION

VEGF may ameliorate renal tubulointerstitial fibrosis at the early stage in UUO mice. This effect may be related to inhibition of VEGF on renal tubular epithelial-mesenchymal transition (EMT).

Fluorescent microangiography is a novel and widely applicable technique for delineating the renal microvasculature

Rarefaction of the renal microvasculature correlates with declining kidney function. However, current technologies commonly used for its evaluation are limited by their reliance on endothelial cell antigen expression and assessment in two dimensions. We set out to establish a widely applicable and unbiased optical sectioning method to enable three dimensional imaging and reconstruction of the renal microvessels based on their luminal filling. The kidneys of subtotally nephrectomized (SNx) rats and their sham-operated counterparts were subjected to either routine two-dimensional immunohistochemistry or the novel technique of fluorescent microangiography (FMA). The latter was achieved by perfusion of the kidney with an agarose suspension of fluorescent polystyrene microspheres followed by optical sectioning of 200 µm thick cross-sections using a confocal microscope. The fluorescent microangiography method enabled the three-dimensional reconstruction of virtual microvascular casts and confirmed a reduction in both glomerular and peritubular capillary density in the kidneys of SNx rats, despite an overall increase in glomerular volume. FMA is an uncomplicated technique for evaluating the renal microvasculature that circumvents many of the limitations imposed by conventional analysis of two-dimensional tissue sections.

Bench-to-bedside review: Ventilation-induced renal injury through systemic mediator release--just theory or a causal relationship?

We review the current literature on the molecular mechanisms involved in the pathogenesis of acute kidney injury induced by plasma mediators released by mechanical ventilation. A comprehensive literature search in the PubMed database was performed and articles were identified that showed increased plasma levels of mediators where the increase was solely attributable to mechanical ventilation. A subsequent search revealed articles delineating the potential effects of each mediator on the kidney or kidney cells. Limited research has focused specifically on the relationship between mechanical ventilation and acute kidney injury. Only a limited number of plasma mediators has been implicated in mechanical ventilation-associated acute kidney injury. The number of mediators released during mechanical ventilation is far greater and includes pro- and anti-inflammatory mediators, but also mediators involved in coagulation, fibrinolysis, cell adhesion, apoptosis and cell growth. The potential effects of these mediators is pleiotropic and include effects on inflammation, cell recruitment, adhesion and infiltration, apoptosis and necrosis, vasoactivity, cell proliferation, coagulation and fibrinolysis, transporter regulation, lipid metabolism and cell signaling. Most research has focused on inflammatory and chemotactic mediators. There is a great disparity of knowledge of potential effects on the kidney between different mediators. From a theoretical point of view, the systemic release of several mediators induced by mechanical ventilation may play an important role in the pathophysiology of acute kidney injury. However, evidence supporting a causal relationship is lacking for the studied mediators.

Sirolimus-based regimen is associated with decreased expression of glomerular vascular endothelial growth factor

BACKGROUND

Sirolimus (SRL) is a potent immunosuppressant used in organ transplantation. It is known to decrease vascular endothelial growth factor (VEGF) synthesis, making it an interesting treatment option for transplant patients who develop Kaposi sarcoma or other malignant diseases. Because VEGF plays a key role in glomerular function and vascular remodelling, we determined the effect of SRL on renal VEGF expression.

METHODS

Using immunohistochemistry and quantitative image analysis, we examined renal VEGF expression in routine kidney biopsies performed at 1 year post-transplant in the CONCEPT study, a prospective randomized study comparing a cyclosporine (CsA)-based regimen to a SRL-based regimen in association with mycophenolate mofetil (MMF).

RESULTS

A total of 74 patients were included in this substudy; 35 were randomized to the CsA group and 39 to the SRL group. Using continuous variables, the mean percentage of glomerular VEGF expression at Week 52 was significantly lower in the SRL group (14.7 ± 13%) compared to CsA group (21.2 ± 14%: P = 0.02). The percentage of glomerular VEGF expression at Week 52 was not influenced by recipient or donor age, gender, renal function, CsA dose, CsA blood level, SRL dose or SRL blood level. It was significantly lower in patients with a proteinuria over versus below 0.5 g/day (11.58 ± 7.9 versus 19.45 ± 15.53; P = 0.036).

CONCLUSIONS

There is emerging evidence that the VEGF system can play either a beneficial or a detrimental role depending on the specific pathologic situations. Therefore, modulating the renal VEGF axis by using an SRL-based regimen may influence the evolution of kidney injury associated with renal transplantation.

Selective stimulation of VEGFR2 accelerates progressive renal disease

Vascular endothelial growth factor A (VEGF-A) can play both beneficial and deleterious roles in renal diseases, where its specific function might be determined by nitric oxide bioavailability. The complexity of VEGF-A in renal disease could in part be accounted for by the distinct roles of its two receptors; VEGFR1 is involved in the inflammatory responses, whereas VEGFR2 predominantly mediates angiogenesis. Because nondiabetic chronic renal disease is associated with capillary loss, we hypothesized that selective stimulation of VEGFR2 could be beneficial in this setting. However, VEGFR2 activation may be deleterious in the presence of nitric oxide deficiency. We systematically overexpressed a mutant form of VEGF-A binding only VEGFR2 (Flk-sel) using an adeno-associated virus-1 vector in wild-type and eNOS knockout mice and then induced renal injury by uninephrectomy. Flk-sel treatment increased angiogenesis and lowered blood pressure in both mouse types. Flk-sel overexpression caused mesangial injury with increased proliferation associated with elevated expression of PDGF, PDGF-β receptor, and VEGFR2; this effect was greater in eNOS knockout than in wild-type mice. Flk-sel also induced tubulointerstitial injury, with some tubular epithelial cells expressing α-smooth muscle actin, indicating a phenotypic evolution toward myofibroblasts. In conclusion, prestimulation of VEGFR2 can potentiate subsequent renal injury in mice, an effect enhanced in the setting of nitric oxide deficiency.

Mutual regulation of hypoxic and retinoic acid related signalling in tubular proximal cells

Hypoxia-inducible factor-1α (HIF-1α) and all-trans retinoic acid (ATRA) afford protection in several experimental models of kidney disease. HIF-1α protein is degraded under normoxia but stabilized by hypoxia, which activates its transcription factor function. ATRA activates another set of transcription factors, the retinoic acid receptors (RAR) α, β and γ, which mediate its effects on target genes. ATRA also up-regulates the expression of RAR α, β and γ at the transcriptional level. Here we demonstrate the presence of mutual regulation of hypoxic and retinoic acid related signalling in tubular proximal cells. In human proximal tubular HK-2 cells we have found that: (i) ATRA treatment induces HIF-1α under normoxic conditions and also synergizes with hypoxia leading to the over-expression of HIF-1α and vascular endothelial growth factor-A, a HIF-1α-regulated renal protector. ATRA-induced HIF-1α expression involved stabilization of HIF-1α mRNA but not of HIF-1α protein. (ii) Expression of HIF-1α is an absolute requirement for the transcriptional up-regulation of RARβ by ATRA. Transfection with HIF-1α siRNA abolished the induction by ATRA of the expression of both RARβ mRNA and protein while treatment with HIF-1α inhibitor YC-1 results in the abolishment of ATRA-induced activity of a retinoic acid-response element (RARE) construct from the RARβ promoter. (iii) Hypoxia up-regulates RARβ through HIF-1α since this effect was inhibited by HIF-1α knockdown. In contrast to ATRA-induced RARβ up-regulation, induction of RARβ expression by ATRA did not involve transcriptional up-regulation as hypoxia did not increase the expression of RARβ mRNA or the activity of the RARE construct. These results suggest the presence of crosstalk between hypoxia/HIF-1α and ATRA/RARβ that may be physiologically and pharmacologically relevant.

Renal involvement in preeclampsia: similarities to VEGF ablation therapy

Glomerular VEGF expression is critical for the maintenance and function of an intact filtration barrier. Alterations in glomerular VEGF bioavailability result in endothelial as well as in podocyte damage. Renal involvement in preeclampsia includes proteinuria, podocyturia, elevated blood pressure, edema, glomerular capillary endotheliosis, and thrombotic microangiopathy. At least the renal signs, symptoms, and other evidence can sufficiently be explained by reduced VEGF levels. The aim of this paper was to summarize our pathophysiological understanding of the renal involvement of preeclampsia and point out similarities to the renal side effects of VEGF-ablation therapy.

Inhibition of capillary repair in proliferative glomerulonephritis results in persistent glomerular inflammation with glomerular sclerosis

The pathological process of glomerulonephritis (GN) includes glomerular capillary damage, and vascular endothelial growth factor (VEGF) has an important role in glomerular capillary repair in GN. We examined the effect of inhibition of glomerular capillary repair after capillary injury in GN. Experimental Thy-1 GN was induced in rats that were divided into two groups: rats that received anti-VEGF neutralizing antibody (50 μg per 100 g body weight per day) and those treated with the vehicle from day 2 to day 9. We assessed the renal function and histopathology serially until week 6. Rats of the Thy-1 GN group showed diffuse glomerular mesangiolysis with ballooning destruction of the capillary network by day 3. VEGF(164) protein levels increased in the damaged glomeruli during days 5 to 10, and endothelial-cell proliferation increased with capillary repair in the vehicle-injected group. Proliferative GN resolved subsequently with decreased mesangial hypercellularity, and recovery of most of the glomeruli to the normal structure was evident by week 6. In contrast, administration of anti-VEGF antibody significantly decreased endothelial-cell proliferation and capillary repair in glomeruli by week 2. Thereafter, glomerular mesangial-cell proliferation and activation continued with persistent infiltration of macrophages. At week 6, segmental glomerular sclerosis developed with mesangial matrix accumulation and proteinuria. Deposition of type I collagen was also noted in sclerotic lesions. We conclude that impaired capillary repair was the underlying mechanism in the prolongation of glomerular inflammation in proliferative GN and in the development of glomerular sclerosis. Capillary repair has an important role in the recovery of glomerular damage and in the resolution of proliferative GN.

The suffocating kidney: tubulointerstitial hypoxia in end-stage renal disease

Chronic kidney disease (CKD) is characterized by irreversible pathological processes that result in the development of end-stage renal disease (ESRD). Accumulating evidence has emphasized the important role of chronic hypoxia in the tubulointerstitium in the final common pathway that leads to development of ESRD. The causes of chronic hypoxia in the tubulointerstitium are multifactorial and include mechanisms such as hemodynamic changes and disturbed oxygen metabolism of resident kidney cells. Epidemiological studies have revealed an association between CKD and systemically hypoxic conditions, such as chronic obstructive pulmonary disease and sleep apnea syndrome. In addition to tubulointerstitial hypoxia, glomerular hypoxia can occur and is a crucial factor in the development of glomerular disorders. Chemical compounds, polarographic sensors, and radiographical methods can be used to detect hypoxia. Therapeutic approaches that target chronic hypoxia in the kidney should be effective against a broad range of kidney diseases. Amelioration of hypoxia is one mechanism of inhibiting the renin-angiotensin system, the current gold standard of CKD therapy. Future therapeutic approaches include protection of the vascular endothelium and appropriate activation of hypoxia-inducible factor, a key transcription factor involved in adaptive responses against hypoxia.

The role of endothelial cell injury in thrombotic microangiopathy

Thrombotic microangiopathy (TMA) refers to a clinical and pathologic syndrome in which endothelial injury results in the manifestations of thrombocytopenia, microangiopathic hemolytic anemia, and kidney injury. A host of causes may induce endothelial injury and TMA, including enteric bacterial toxins, deficiency or dysfunction of complement regulatory proteins, deficiency or inhibition of von Willebrand factor-cleaving proteases, and factors that inhibit endothelial cell proliferation and turnover. This has led specialists to concentrate on these specific inciting factors in terms of designing treatment and management. However, a key and less recognized factor is the underlying level of endothelial health. Many persons with hereditary causes may remain disease free for years or may never develop disease. Others with acute inciting events, such as Escherichia coli O157 enteritis, never manifest TMA. Experimental studies document the importance of specific factors, such as endothelial nitric oxide levels, in helping protect animals from TMA. This suggests that one might approach the management of TMA not simply with specific treatments aimed at the underlying hereditary cause or inciting event, but rather at general measures that may improve overall endothelial health. We propose studies to determine whether interventions that improve endothelial health, such as the administration of angiotensin-converting enzyme inhibitors, statins, vitamin C, allopurinol, or nitric oxide-producing drugs, may be able to prevent TMA, even in persons with underlying hereditary conditions that otherwise would predispose them to these diseases.

Angiogenesis and chronic kidney disease

The number of patients requiring renal replacement therapy due to end-stage renal disease (ESRD) is increasing worldwide. The prevalence of chronic kidney disease (CKD), and the importance of CKD as a risk factor in development of ESRD and in complicating cardiovascular disease (CVD) have been confirmed. In recent years, the involvement of angiogenesis-related factors in the progression of CKD has been studied, and the potential therapeutic effects on CKD of modulating these factors have been identified. Vascular endothelial growth factor (VEGF)-A, a potent pro-angiogenic factor, is involved in the development of the kidney, in maintenance of the glomerular capillary structure and filtration barrier, and in the renal repair process after injury. VEGF-A is also involved in the development of early diabetic nephropathy, demonstrated by the therapeutic effects of anti-VEGF-A antibody. Angiopoietin (Ang)-1 induces the maturation of newly formed blood vessels, and the therapeutic effects of Ang-1 in diabetic nephropathy have been described. In experimental models of diabetic nephropathy, the therapeutic effects of angiogenesis inhibitors, including angiostatin, endostatin and tumstatin peptides, the isocoumarin NM-3, and vasohibin-1, have been reported.

Further analysis of the involvement of angiogenesis-related factors in the development of CKD is required. Determining the disease stage at which therapy is most effective and developing an effective drug delivery system targeting the kidney will be essential for pro-or anti-angiogenic strategies for patients with CKD.

Soluble Flt-1 gene therapy ameliorates albuminuria but accelerates tubulointerstitial injury in diabetic mice

VEGF is recognized as a major mediator in the development of diabetic nephropathy. Soluble Flt-1 (sFlt-1) is the endogenous inhibitor of VEGF, and recently genetic overexpression of sFlt-1 in the podocyte was shown to be protective in murine diabetic nephropathy. In this study, we performed a translational study to determine whether an intramuscular gene transfer of sFlt-1 can prevent the progression of renal disease in diabetic db/db mice. Adeno-associated virus-1 (AAV1) encoding human sFlt-1 in two different doses was intramuscularly administrated in db/db and wild-type mice. The sFlt-1-AAV1 treatment significantly increased serum sFlt-1 level at 4 and 8 wk. A dose that was developed in this study caused minimal abnormalities in normal mice but reduced albuminuria in diabetic db/db mice. In renal histology, sFlt-1 treatment at this dose had minimal effects on mesangial expansion in diabetic mice, whereas podocyte injury was significantly improved, at 8 wk. Unfortunately, tubulointerstitial injury was markedly exacerbated by sFlt-1 treatment in association with a reduction in endogenous VEGF expression and peritubular capillary loss. In conclusion, gene therapy with sFlt-1-AAV1 protects podocytes but accelerates tubulointerstitial injury in diabetic db/db mice. These data suggest systemic overexpression of sFlt-1 will not likely be useful for treating diabetic nephropathy.

Intermedin ameliorates vascular and renal injury by inhibition of oxidative stress

Intermedin (IMD) is a newly discovered peptide related to calcitonin gene-related peptide and adrenomedullin, and has been shown to reduce blood pressure and reactive oxygen species formation in vivo. In this study, we determined whether IMD exerts vascular and renal protection in DOCA-salt hypertensive rats by intravenous injection of adenovirus harboring the human IMD gene. Expression of human IMD was detected in the rat kidney via immunohistochemistry. IMD administration significantly lowered blood pressure, increased urine volume, and restored creatinine clearance. IMD also dramatically decreased superoxide formation and media thickness in the aorta. Vascular injury in the kidney was reduced by IMD gene delivery as evidenced by the prevention of glomerular and peritubular capillary loss. Moreover, IMD lessened morphological damage of the renal tubulointerstitium and reduced glomerular injury and hypertrophy. Attenuation of inflammatory cell accumulation in the kidney by IMD was accompanied by inhibition of p38MAPK activation and intercellular adhesion molecule 1 expression. In addition, IMD gene transfer resulted in a marked decline in myofibroblast and collagen accumulation in association with decreased transforming growth factor-beta1 levels. Furthermore, IMD increased nitric oxide excretion in the urine and lowered the amount of lipid peroxidation. These results demonstrate that IMD is a powerful renal protective agent with pleiotropic effects by preventing endothelial cell loss, kidney damage, inflammation, and fibrosis in hypertensive DOCA-salt rats via inhibition of oxidative stress and proinflammatory mediator pathways.

Renal hypoxia and dysoxia after reperfusion of the ischemic kidney

Ischemia is the most common cause of acute renal failure. Ischemic-induced renal tissue hypoxia is thought to be a major component in the development of acute renal failure in promoting the initial tubular damage. Renal oxygenation originates from a balance between oxygen supply and consumption. Recent investigations have provided new insights into alterations in oxygenation pathways in the ischemic kidney. These findings have identified a central role of microvascular dysfunction related to an imbalance between vasoconstrictors and vasodilators, endothelial damage and endothelium-leukocyte interactions, leading to decreased renal oxygen supply. Reduced microcirculatory oxygen supply may be associated with altered cellular oxygen consumption (dysoxia), because of mitochondrial dysfunction and activity of alternative oxygen-consuming pathways. Alterations in oxygen utilization and/or supply might therefore contribute to the occurrence of organ dysfunction. This view places oxygen pathways' alterations as a potential central player in the pathogenesis of acute kidney injury. Both in regulation of oxygen supply and consumption, nitric oxide seems to play a pivotal role. Furthermore, recent studies suggest that, following acute ischemic renal injury, persistent tissue hypoxia contributes to the development of chronic renal dysfunction. Adaptative mechanisms to renal hypoxia may be ineffective in more severe cases and lead to the development of chronic renal failure following ischemia-reperfusion. This paper is aimed at reviewing the current insights into oxygen transport pathways, from oxygen supply to oxygen consumption in the kidney and from the adaptation mechanisms to renal hypoxia. Their role in the development of ischemia-induced renal damage and ischemic acute renal failure are discussed.

Renal hypoxia and dysoxia after reperfusion of the ischemic kidney

Ischemia is the most common cause of acute renal failure. Ischemic-induced renal tissue hypoxia is thought to be a major component in the development of acute renal failure in promoting the initial tubular damage. Renal oxygenation originates from a balance between oxygen supply and consumption. Recent investigations have provided new insights into alterations in oxygenation pathways in the ischemic kidney. These findings have identified a central role of microvascular dysfunction related to an imbalance between vasoconstrictors and vasodilators, endothelial damage and endothelium-leukocyte interactions, leading to decreased renal oxygen supply. Reduced microcirculatory oxygen supply may be associated with altered cellular oxygen consumption (dysoxia), because of mitochondrial dysfunction and activity of alternative oxygen-consuming pathways. Alterations in oxygen utilization and/or supply might therefore contribute to the occurrence of organ dysfunction. This view places oxygen pathways' alterations as a potential central player in the pathogenesis of acute kidney injury. Both in regulation of oxygen supply and consumption, nitric oxide seems to play a pivotal role. Furthermore, recent studies suggest that, following acute ischemic renal injury, persistent tissue hypoxia contributes to the development of chronic renal dysfunction. Adaptative mechanisms to renal hypoxia may be ineffective in more severe cases and lead to the development of chronic renal failure following ischemia-reperfusion. This paper is aimed at reviewing the current insights into oxygen transport pathways, from oxygen supply to oxygen consumption in the kidney and from the adaptation mechanisms to renal hypoxia. Their role in the development of ischemia-induced renal damage and ischemic acute renal failure are discussed.

Alteration of microvascular permeability in acute kidney injury

Functional and structural abnormalities in the renal microvasculature are important processes contributing to the pathophysiology of ischemic acute kidney injury (AKI). Renewed interest in the complex interplay between tubular injury, inflammation and microvascular alterations has emerged in order to gain a better understanding of acute kidney injury syndromes. This review examines alterations of the renal microvasculature as they relate to ischemic and septic AKI with an emphasis on the mechanisms involved in altered microvascular permeability.

VEGF-121 preserves renal microvessel structure and ameliorates secondary renal disease following acute kidney injury

Acute kidney injury induced by renal ischemia-reperfusion (I/R) compromises microvascular density and predisposes to chronic kidney disease (CKD) and sodium-dependent hypertension. VEGF-121 was administered to rats fed a standard (0.4%) sodium diet at various times following recovery from I/R injury for up to 35 days. VEGF-121 had no effect on the initial loss of renal function, as indicated by serum creatinine levels measured 24 h after injury. Serum creatinine levels declined thereafter, indicative of renal repair. Rats were then switched to an elevated (4.0%) sodium diet for an additional 28 days to induce CKD. The 4.0% sodium diet enhanced renal hypertrophy, interstitial volume, albuminuria, and cardiac hypertrophy relative to postischemic animals maintained on the 0.4% sodium diet. Administration of VEGF-121 from day 0 to 14, day 0 to 35, or day 3 to 35 after I/R suppressed the effects of sodium diet on CKD development, while delayed administration of VEGF-121 from day 21 to 35 had no effect. Endothelial nitric oxide synthase protein levels were upregulated in postischemic animals, and this effect was significantly increased by the 4.0% sodium diet but was not influenced by prior treatment with VEGF. Conversely, microvascular density was preserved in postischemic animals treated with VEGF-121 relative to vehicle-treated postischemic animals. These data suggest that early, but not delayed, treatment with VEGF-121 can preserve vascular structure after ischemia and influence chronic renal function in response to elevated sodium intake.

PDE-5 inhibition impedes TSP-1 expression, TGF-beta activation and matrix accumulation in experimental glomerulonephritis

BACKGROUND

Matrix expansion and mesangial proliferation are hallmarks of mesangial proliferative glomerulonephritis. Specific inhibition of PDE-5, an enzyme catalyzing the intracellular degradation of cyclic GMP, can be achieved by the inhibitor vardenafil. In this study, we investigated the effects of PDE-5 inhibition in the anti-Thy1 model in the rat in vivo.

METHODS

After disease induction, rats received 10 mg/kg bw vardenafil twice a day via gavage. On Days 2 and 6, renal biopsies, as well as glomerular isolates, urine and blood samples were taken to compare vardenafil- and placebo-treated groups during the course of disease.

RESULTS

Small amounts of PDE-5 were detected in healthy kidneys, but induced in a typical mesangial pattern during disease (by IHC and WB). Specific PDE-5 inhibition resulted in increased glomerular levels of cGMP. Treated animals demonstrated inhibition of MC proliferation and matrix accumulation while renal function and influx of inflammatory cells were not affected. Due to PDE-5 inhibition, the endogenous TGF-beta-activating protein TSP-1 and the TGF-beta-signalling protein p-smad-2/3 were decreased suggesting this as an antifibrotic mechanism of action of vardenafil in this model.

CONCLUSION

Considering the availability and safety profile of vardenafil, the beneficial antiproliferative and antifibrotic effect in experimental glomerulonephritis may potentially be applicable to the treatment of mesangial proliferative glomerulonephritis in man.

Hypoxia-inducible factor signaling in the development of tissue fibrosis

Capillary rarefaction is a hallmark of fibrotic diseases and results in reduced blood perfusion and oxygen delivery. In the kidney, tubulointerstitial fibrosis, which leads to the destruction of renal tissue and the irreversible loss of kidney function, is associated with hypoxia and the activation of Hypoxia-Inducible-Factor (HIF) signaling. HIF-1 and HIF-2 are basic-helix-loop-helix transcription factors that allow cells to survive in a low oxygen environment by regulating energy metabolism, vascular remodeling, erythropoiesis, cellular proliferation and apoptosis. Recent studies suggest that HIF activation promotes epithelial to mesenchymal transition (EMT) and renal fibrogenesis. These findings raise the possibility that the spectrum of HIF activated biological responses to hypoxic stress may differ under conditions of acute and chronic hypoxia. Here we discuss the role of HIF signaling in the pathogenesis and progression of chronic kidney disease.

Vascular endothelial growth factor protein expression in a renal ablation rabbit model under prolonged warm and cold ischemia

BACKGROUND/AIMS

To establish a potential correlation between renal and systemic production of vascular endothelial growth factor (VEGF) protein after prolonged ischemia in a renal ablation model under normothermic and hypothermic conditions.

METHODS

38 uninephrectomized New Zealand rabbits were divided into 5 groups. The rabbits of each group underwent partial nephrectomy under 90 and 60 min of warm and 90 and 120 min of cold ischemia, except for the sham group (S), which served as control. Serum creatinine (SCr) and blood-urea-nitrogen (BUN) levels were assessed. On the 15th postoperative day (POD), the animals were euthanized and the remaining kidneys were evaluated. VEGF immunohistochemistry and serum Western blot analysis were performed.

RESULTS

In comparison to the control group, groups 60W, 90C and 120C showed 1.6-, 1.14- and 1.75-fold decreases, respectively, while the production of VEGF was significantly declined by 7.4-fold in group 90W (p < 0.05). Immunohistochemistry revealed prominent VEGF staining in the above-mentioned three groups, while in group 90W staining was negative. Serum biochemistry and microscopic evaluation verified the same differentiation.

CONCLUSION

Renal and serum VEGF seem to have an analogous expression under conditions of prolonged ischemia. VEGF is overexpressed in hypothermic conditions compared to warm ischemia exceeding 60 min. Hypothermia can be more advantageous in a procedure applying prolonged ischemia.

Heme oxygenase-1 modulates the expression of the anti-angiogenic chemokine CXCL-10 in renal tubular epithelial cells

The turnover and repair of peritubular capillaries is essential for the maintenance of normal renal tubular structure and function. Following injury, ineffective capillary repair/angiogenesis may result in chronic disease, whereas effective repair attenuates the injury process. Thus the process of healing in the kidney is likely dependent on an intricate balance between angiogenic and anti-angiogenic factors to maintain the renal microvasculature. We investigated the role of cytoprotective heme oxygenase-1 (HO-1) in the regulation of chemokines in human renal proximal tubular epithelial cells (RPTEC). Transfection of RPTEC with a HO-1 overexpression plasmid promoted a marked induction in the mRNA expression of the anti-angiogenic chemokine CXCL-10, along with angiogenic chemokines CXCL-8 and CCL-2. Utilizing a CXCL-10 promoter luciferase construct, we observed that HO-1-induced CXCL-10 expression is regulated at the transcriptional level. However, with increases in concentrations and time intervals of HO-1 induction, there was a marked decrease in CXCL-10 expression. Using pharmacological inhibitors, we found that HO-1-induced early robust CXCL-10 transcription is mediated through the PKC signaling pathway. To evaluate the functional significance of HO-1-induced CXCL-10 release, we cultured human vascular endothelial cells in the absence and presence of culture supernatants of the HO-1 plasmid-transfected RPTEC. We found that early (24 h) supernatants of the HO-1 plasmid-transfected cells (RPTEC) inhibited endothelial cell proliferation, and this effect was blocked by addition of a CXCL-10 neutralizing antibody. Thus HO-1 can regulate the expression of the anti-angiogenic CXCL-10 and may alter a critical balance between angiogenic vs. anti-angiogenic factors that are important to maintain renal microvasculature during injury.

Role of VEGF in maintaining renal structure and function under normotensive and hypertensive conditions

Inhibiting the actions of VEGF is a new therapeutic paradigm in cancer management with antiangiogenic therapy also under intensive investigation in a range of nonmalignant diseases characterized by pathological angiogenesis. However, the effects of VEGF inhibition on organs that constitutively express it in adulthood, such as the kidney, are mostly unknown. Accordingly, we examined the effect of VEGF inhibition on renal structure and function under physiological conditions and in the setting of the common renal stressors: hypertension and activation of the renin-angiotensin system. When compared with normotensive Sprague-Dawley (SD) rats, glomerular VEGF mRNA was increased 2-fold in transgenic (mRen-2)27 rats that overexpress renin with spontaneously hypertensive rat (SHR) kidneys showing VEGF expression levels that were intermediate between them. Administration of either an orally active inhibitor of the type 2 VEGF receptor (VEGFR-2) tyrosine kinase or a VEGF neutralizing antibody to TGR(mRen-2)27 rats resulted in loss of glomerular endothelial cells and transformation to a malignant hypertensive phenotype with severe glomerulosclerosis. VEGFR-2 kinase inhibition treatment was well tolerated in SDs and SHRs; although even in these animals there was detectable endothelial cell loss and rise in albuminuria. Mild mesangial expansion was also noted in hypertensive SHR, but not in SD rats. These studies illustrate: (i) VEGF has a role in the maintenance of glomerular endothelial integrity under physiological circumstances, (ii) glomerular VEGF is increased in response to hypertension and activation of the renin-angiotensin system, and (iii) VEGF signaling plays a protective role in the setting of these renal stressors.

Uncoupling of the VEGF-endothelial nitric oxide axis in diabetic nephropathy: an explanation for the paradoxical effects of VEGF in renal disease

In many forms of experimental kidney diseases, renal VEGF is low, and administering VEGF can be shown to be protective. A paradox occurs in diabetes, in which renal VEGF levels are high and a deleterious effect of VEGF on kidney disease has been shown. We have hypothesized that endothelial dysfunction induced by hyperglycemia or other factors may underlie the pathogenic mechanisms of a high VEGF state. VEGF normally stimulates endothelial nitric oxide (NO) release and acts in concert with elevated NO levels as a trophic factor for vascular endothelium. The increased NO derived from the endothelial cell acts as an inhibitory factor that prevents excess endothelial cell proliferation, vascular smooth muscle cell proliferation, and macrophage infiltration. In the setting where NO bioavailability is reduced in diabetes, high levels of VEGF lead to excessive endothelial cell proliferation, stimulation of macrophage chemotaxis, and vascular smooth muscle cell activation. Consistent with this hypothesis is our recent observation that diabetes induced in endothelial NO-deficient mice results in clinical and histological features identical to human diabetic nephropathy. The discovery of the key role for impaired endothelial NO bioavailability in the stimulation of VEGF and VEGF-dependent disease may provide key insights into not only the pathogenesis of diabetic nephropathy but also the utility and hazard of administering VEGF as a treatment for kidney disease.

Thrombospondin 2 Functions as an Endogenous Regulator of Angiogenesis and Inflammation in Experimental Glomerulonephritis in Mice

The role of thrombospondin 2 (TSP2) was investigated in an anti-glomerular basement membrane (GBM) nephritis model that compared TSP2-null mice with wild-type (WT) controls. TSP2-null mice were analyzed for kidney function, renal cortical matrix expansion, influx of inflammatory cells, proliferation, and apoptosis, as well as for capillary rarefaction after induction of anti-GBM disease. Whereas the renal cortex of normal control WT mice did not show any detectable TSP2 staining above background, TSP2 protein expression was clearly upregulated in anti-GBM disease. TSP2 deficiency led to an accelerated and enhanced inflammatory response, as indicated by the influx of CD4(+) and CD8a(+) cells and monocytes/macrophages. Glomerular fibrin deposition and a matrix-remodeling response were also observed, as indicated by collagens I and IV staining and a proliferative response within the renal interstitium. These changes were accompanied by increased matrix metalloproteinase 2 activity and enhanced alpha-smooth muscle actin staining in the TSP2-null mice. Neither a compensatory increase in TSP1 nor increased phosphorylation of Smad 2/3, an indicator for TGF-beta activity, was observed. The proliferative response of the peritubular endothelium was accelerated and enhanced, leading to a reversal of capillary rarefaction in TSP2-null mice, whereas interstitial cell death was equivalent to that in WT mice. In conclusion, the lack of the matricellular protein TSP2 in mice accelerates and enhances several responses to renal injury and reveals an important role for TSP2 as a major endogenous antiangiogenic and matrix metalloproteinase 2-regulating factor in renal disease.

P2Y1 Gene Deficiency Protects from Renal Disease Progression and Capillary Rarefaction during Passive Crescentic Glomerulonephritis

The metabotropic receptor P2Y1 is necessary for full ADP-induced platelet activation and is localized on various intrinsic renal cells, including mesangial cells, podocytes, and endothelial cells. To date, nothing is known about the role of the P2Y1 receptor during inflammatory renal disease. The role of the P2Y1 receptor was investigated using 22 P2Y1 gene-deficient (-/-) and 27 wild-type (wt) mice during the time course of passive crescentic nephrotoxic glomerulonephritis. Six P2Y1 -/- and six wt mice served as undiseased controls. Renal tissues were harvested on days 1, 10, and 28 after disease induction. No renal phenotype was found in P2Y1 -/- versus wt mice. In contrast, during crescentic glomerulonephritis, approximately 50% of all wt mice died, whereas all P2Y1 -/- mice survived. Renal function as assessed by creatinine clearance measurements, glomerulosclerosis, and tubulointerstitial injury indices as well as glomerular and interstitial matrix expansion were improved significantly in P2Y1 -/- compared with wt mice. These changes were preceded by reduced glomerular and peritubular capillary rarefaction indices in P2Y1 -/- compared with wt mice. The alteration of the rates of both peritubular apoptosis and endothelial cell proliferation suggests improved capillary preservation in P2Y1 -/- mice early in disease (day 10) and an additional enhanced repair reaction in P2Y1 -/- mice at the late time point (day 28), whereas injury on day 1 seemed to be equivalent in both groups. It is concluded that loss of P2Y1 receptor function safeguards against capillary loss, fibrosis, and death by renal failure during experimental crescentic glomerulonephritis.

Vascular endothelial growth factor administration does not improve microvascular disease in the salt-dependent phase of post-angiotensin II hypertension

Renal microvascular injury and tubulointerstitial inflammation may provide a potential mechanism for the development of salt-sensitive hypertension. Therefore, we hypothesized that vascular endothelial growth factor (VEGF) administration would prevent the development of salt-sensitive hypertension induced by ANG II. Infusion of ANG II in rats for 2 wk led to an elevation in blood pressure and an increase in blood urea nitrogen. Prominent tubular injury, focal areas of peritubular capillary loss accompanied by a decrease in urinary nitrites, thickening of the afferent arteriole, and an elevation in systemic and renal VEGF protein levels also occurred. In separate studies, animals were infused with ANG II and then placed on a low-salt diet for 1 wk. At this point, the animals were paired on the basis of weight and blood pressure and treated with either VEGF121or vehicle subcutaneously for 8 wk while being fed a high-salt diet. During the treatment period, a spontaneous improvement in many parameters, including both renal function and healing of the peritubular capillaries, occurred to the same degree in both vehicle- and VEGF121-treated rats. VEGF121significantly reduced blood pressure and accelerated the recovery of tubular injury. In contrast, vehicle-treated rats demonstrated a persistent increase in afferent arteriolar media-to-lumen ratio, which was further enhanced in rats treated with VEGF121. Therefore, VEGF therapy has only limited benefits on the healing of renal lesions in the salt-dependent phase of post-ANG II-mediated hypertension.

Analysis of glomerular VEGF mRNA and protein expression in murine mesangioproliferative glomerulonephritis

Capillary repair is crucial in the healing of glomerulonephritis (GN). The vascular endothelial growth factor (VEGF) has pro-angiogenic properties and plays an important role in glomerular capillary regeneration. Habu Snake Venom (HSV) GN, a murine model for mesangioproliferative GN, was induced in uninephrectomized C57/BL6 mice. Glomerular damage and capillary repair were assessed using morphometry, stereology, and confocal laser scanning microscopy. Mesangiolytic glomeruli were microdissected (days 1,3,7,14) using laser capture microdissection technique. VEGF mRNA expression was analyzed by real-time polymerase chain reaction and compared to intact glomeruli of healthy controls. Spatiotemporal VEGF gene and protein expression was determined using nonradioactive in situ hybridization and immunohistochemistry. On day 1, diseased animals developed focal mesangiolysis paralleled by a significant decrease in length density of glomerular capillaries that gradually returned to baseline levels thereafter, indicating capillary growth in response to initial injury. Glomerular VEGF mRNA expression increased on day 3 and returned back to baseline and beyond at day 14 when the glomerular recovery process was completed. Similarly, glomerular VEGF protein expression tended to be higher on day 3. The present study documents temporarily increased glomerular VEGF gene and protein expression during the healing of HSV GN, suggesting a potential role of VEGF in the repair of mesangiolytic glomerular damage.

Role of peritubular capillary loss and hypoxia in progressive tubulointerstitial fibrosis in a rat model of aristolochic acid nephropathy

BACKGROUND/AIMS

To investigate the effects of peritubular capillary (PTC) loss and hypoxia on the progression of tubulointerstitial fibrosis in a rat model of aristolochic acid nephropathy (AAN).

METHODS

Female Wistar rats received Caulis aristolochiae manshuriensis (CAM) decoction by gavage for 8 weeks, and were sacrificed at 8, 12 and 16 weeks, respectively, after administration. Blood urea nitrogen (BUN), serum creatinine (Scr) and urinary protein were monitored prior to sacrifice. PTC loss and tubulointerstitial hypoxia were assessed by CD34 immunostaining and hypoxia-inducible factor-alpha subunit 1 (HIF-1alpha) expression, respectively. Myofibroblasts were assessed by alpha-smooth muscle actin (alpha-SMA) expression. The expression of angiogenic factor was assessed by vascular endothelial growth factor (VEGF).

RESULTS

AAN rats differed from controls by increased BUN, Scr and 24-hour urinary protein excretion rates. There was a progressive loss of PTCs in the AAN model, which was associated with the decreased expression of VEGF. A significant increase in nuclear localization of HIF-1alpha was seen 16 weeks after treatment with CAM decoction in the context of severe tubulointerstitial damage. Multifocal tubulointerstitial fibrosis was seen in AAN rats at weeks 12 and 16, predominantly in the area of the outer stripe and outer medulla. No significant pathologic changes were found in control rats.

CONCLUSION

Following the reduction of PTCs density and up-regulation of HIF-1alpha, the tubulointerstitial fibrosis area increased. Ischemia and hypoxia are the important causes of severe tubulointerstitial fibrosis in AAN rats.

Hypoxia-inducible factors in the kidney

Tissue hypoxia not only occurs under pathological conditions but is also an important microenvironmental factor that is critical for normal embryonic development. Hypoxia-inducible factors HIF-1 and HIF-2 are oxygen-sensitive basic helix-loop-helix transcription factors, which regulate biological processes that facilitate both oxygen delivery and cellular adaptation to oxygen deprivation. HIFs consist of an oxygen-sensitive alpha-subunit, HIF-alpha, and a constitutively expressed beta-subunit, HIF-beta, and regulate the expression of genes that are involved in energy metabolism, angiogenesis, erythropoiesis and iron metabolism, cell proliferation, apoptosis, and other biological processes. Under conditions of normal Po(2), HIF-alpha is hydroxylated and targeted for rapid proteasomal degradation by the von Hippel-Lindau (VHL) E3-ubiquitin ligase. When cells experience hypoxia, HIF-alpha is stabilized and either dimerizes with HIF-beta in the nucleus to form transcriptionally active HIF, executing the canonical hypoxia response, or it physically interacts with unrelated proteins, thereby enabling convergence of HIF oxygen sensing with other signaling pathways. In the normal, fully developed kidney, HIF-1alpha is expressed in most cell types, whereas HIF-2alpha is mainly found in renal interstitial fibroblast-like cells and endothelial cells. This review summarizes some of the most recent advances in the HIF field and discusses their relevance to renal development, normal kidney function and disease.

Everolimus inhibits glomerular endothelial cell proliferation and VEGF, but not long-term recovery in experimental thrombotic microangiopathy

BACKGROUND

Everolimus is a potent immunosuppressant used in renal transplant therapy, but its effects on renal endothelial cell regeneration after injury are unknown. The effects of an everolimus therapy were investigated in a model of renal thrombotic microangiopathy (TMA) with specific endothelial cell (EC) injury in the rat in vivo as well as in glomerular ECs in vitro.

METHODS

During the early regenerative phase (day 3) of the renal microvascular injury model in vivo, everolimus inhibited glomerular EC proliferation by up to 60% compared with vehicle-treated rats, whereas apoptosis was not different in these groups. This decreased EC proliferation was associated with an enhanced deposition of fibrin in everolimus treated animals on day 3. In cultured glomerular endothelial cells, everolimus effectively and dose dependently inhibited cellular proliferation. This anti-proliferative effect was associated with a reduced phosphorylation of the p70S6 kinase and reduction of the pro-angiogenic factor VEGF in glomeruli in vivo and in cultured podocytes in vitro.

RESULTS

Despite the prolonged EC repair and in contrast to the anti-Thy1 nephritis model, everolimus therapy did not disturb the long-term repair reaction in this thrombotic microangiopathy model.

CONCLUSION

Everolimus is anti-proliferative for glomerular EC in vitro and in vivo and does not seem to have detrimental long-term effects in experimental renal TMA, when only the glomerular endothelium, but not the mesangium is severely injured.

Local VEGF activity but not VEGF expression is tightly regulated during diabetic nephropathy in man

Several studies have implicated the angiogenic cytokine vascular endothelial growth factor (VEGF) in the development of diabetic nephropathy, but no data are available about its local activity during human disease. Glomeruli from 52 archival biopsies from type II diabetics were evaluated and compared to 10 renal biopsies without kidney disease (controls). Glomerulosclerosis, capillary rarefaction, glomerular and endothelial cell proliferation, apoptosis, VEGF expression, as well as receptor-bound VEGF indicating local VEGF activity, and phosphorylation of the signal transduction molecule Akt were investigated. Owing to substantial heterogeneity of glomerular lesions in individual biopsies, these parameters were correlated with the degree of injury in individual glomeruli rather than biopsies. Severe glomerular capillary rarefaction was linked to the degree of glomerulosclerosis. While cellular apoptosis was detected independent of the stage of injury, endothelial cell proliferation indicating capillary repair was markedly increased only in mildly/moderately injured glomeruli. In controls, VEGF was predominantly expressed in podocytes, whereas receptor-bound VEGF was confined to the glomerular endothelium. VEGF expression was increased in all diabetic glomeruli by many different cell types. In contrast, VEGF receptor activation was increased predominantly in the endothelium of only mildly injured glomeruli, but significantly decreased in more severely injured glomeruli. Diabetic nephropathy is associated with glomerular capillary rarefaction. Despite overall increased glomerular VEGF, the decreased receptor-bound VEGF on the endothelium may be an indicator of an insufficient capillary repair reaction.

Transplanted mesenchymal stem cells accelerate glomerular healing in experimental glomerulonephritis

Bone marrow-derived cells contribute to glomerular cell turnover and repair, but the cell types involved are unknown. Whether rat mesenchymal stem cells (MSC) can accelerate recovery from damage in rat mesangioproliferative anti-Thy1.1 glomerulonephritis was studied. After injection into the left renal artery on day 2 after disease induction, fluorescently labeled MSC were detected in 20 to 50% of glomeruli and rare intrarenal vessels but not in the tubulointerstitium, in contralateral kidneys, or in medium controls. In control experiments, injected mesangial cells were detected less frequently in glomeruli in comparison with injected MSC. In nephritic outbred Wistar rats, MSC injection led to an approximately 50% reduction of mesangiolysis on days 4 and 6 after disease induction, accompanied by three- to four-fold higher intraglomerular cell proliferation on day 4 and more rapid mesangial reconstitution as detected by alpha-smooth muscle actin expression. Injection of MSC into tail veins or intra-arterial injection of mesangial cells instead of MSC failed to reproduce any of these findings. In inbred Lewis rats, anti-Thy1.1 nephritis followed an aggravated course with transient acute renal failure. Acute renal failure was ameliorated by MSC injection into the left renal artery on day 2 after disease induction. Again, MSC led to more rapid recovery from mesangiolysis, increased glomerular cell proliferation, and reduction of proteinuria by 28%. Double immunostaining of 5-bromo-2'-deoxyuridine-labeled MSC for endothelial, mesangial, or monocyte/macrophage antigens showed that 85 to 95% of MSC that localized in glomeruli on day 6 failed to express these markers. In vitro, MSC secreted high amounts of vascular endothelial growth factor and TGF-beta1 but not PDGF-BB. In conclusion, even low numbers of MSC can markedly accelerate glomerular recovery from mesangiolytic damage possibly related to paracrine growth factor release and not to differentiation into resident glomerular cell types or monocytes/macrophages.

Angiotensin Type 1 Receptor Blocker Restores Podocyte Potential to Promote Glomerular Endothelial Cell Growth

Both podocytes and glomerular endothelial cells (GEN) are postulated to play important roles in the progression and potential regression of glomerulosclerosis. Inhibition of angiotensin is crucial in treatment of chronic kidney disease, presumably via effects on BP and extracellular matrix. This study aimed to investigate how angiotensin inhibition altered the interactions between podocytes and GEN. The effects of supernatants from primary cultured mouse podocytes, before or after sublethal injury by puromycin aminonucleoside, in the presence or absence of angiotensin type 1 receptor blocker (ARB), on GEN sprouting and growth were assessed. Supernatant from normal podocytes significantly increased GEN sprouting, whereas puromycin aminonucleoside-injured podocyte supernatant decreased these GEN responses. These effects were linked to decreased vascular endothelial growth factor A (VEGF-A) and angiopoietin-1 (Ang-1) protein from injured podocytes. This downregulation of VEGF-A and Ang-1 protein was reversed when injured podocytes were treated with ARB. Inhibition of VEGF-A or Ang-1 prevented this restored response by ARB. Activation of intracellular kinases (p38, extracellular signal-regulated kinase, and AKT) was suppressed in GEN that were treated with medium from injured podocytes but restored by medium from ARB-treated injured podocytes. Therefore, injured podocytes are ineffective in promoting GEN sprouting, and this effect is reversed by ARB treatment of the injured podocyte. These data support the idea that ARB effects on podocytes may mediate capillary remodeling in vivo.

Renal vascular endothelial growth factor in neonatal obstructive nephropathy. II. Exogenous VEGF

Chronic unilateral ureteral obstruction (UUO) in the neonatal rat causes delayed renal maturation, tubular apoptosis, and interstitial inflammation. Vascular endothelial growth factor (VEGF) acts as a survival factor for tubular cells and reduces renal injury in several models of renal disease. To determine whether exogenous VEGF attenuates renal injury from UUO, rats were subjected within the first 48 h of life to sham operation, partial UUO, or complete UUO. Saline vehicle or VEGF(121) (50 mg/kg) was injected twice daily for 7 days, after which kidneys were harvested for histological study. The density of peritubular capillaries was measured with platelet-endothelial cell adhesion molecule-1 immunostaining, proliferating nuclei were detected by proliferating-cell nuclear antigen staining, apoptosis by the transferase-mediated dUTP nick end-labeling technique, macrophages by ED-1 immunostaining, and collagen by Sirius red staining. Glomerular number and maturation index were also determined in each group. Following chronic complete UUO in the neonatal rat, peritubular capillary density was significantly decreased. Cortical capillary density was further reduced by exogenous VEGF in the partially obstructed kidney. While UUO also decreased glomerular number and delayed glomerular maturation, exogenous VEGF exerted no additional effects. Cellular proliferation and tubular apoptosis increased in proportion to the severity of obstruction, but exogenous VEGF had no additional effects on proliferation, tubular apoptosis, or macrophage infiltration. However, VEGF reduced interstitial apoptosis in the kidney with partial UUO. We conclude that VEGF does not have salutary effects on the renal lesions caused by chronic UUO in the neonatal rat and may actually worsen obstructive nephropathy by aggravating the interstitial lesions.

Plasmid-mediated VEGF gene transfer induces cardiomyogenesis and reduces myocardial infarct size in sheep

We have recently reported that in pigs with chronic myocardial ischemia heart transfection with a plasmid encoding the 165 isoform of human vascular endothelial growth factor (pVEGF165) induces an increase in the mitotic index of adult cardiomyocytes and cardiomyocyte hyperplasia. On these bases we hypothesized that VEGF gene transfer could also modify the evolution of experimental myocardial infarct. In adult sheep pVEGF165 (3.8 mg, n=7) or empty plasmid (n=7) was injected intramyocardially 1 h after coronary artery ligation. After 15 days infarct area was 11.3+/-1.3% of the left ventricle in the VEGF group and 18.2+/-2.1% in the empty plasmid group (P<0.02). The mechanisms involved in infarct size reduction (assessed in additional sheep at 7 and 10 days after infarction) included an increase in early angiogenesis and arteriogenesis, a decrease in peri-infarct fibrosis, a decrease in myofibroblast proliferation, enhanced cardiomyoblast proliferation and mitosis of adult cardiomyocytes with occasional cytokinesis. Resting myocardial perfusion (99mTc-sestamibi SPECT) was higher in VEGF-treated group than in empty plasmid group 15 days after myocardial infarction. We conclude that plasmid-mediated VEGF gene transfer reduces myocardial infarct size by a combination of effects including neovascular proliferation, modification of fibrosis and cardiomyocyte regeneration.

The VHL/HIF oxygen-sensing pathway and its relevance to kidney disease

Over the past decade major advances have been made in our understanding of the molecular machinery that mammalian cells use to sense and to adapt to a low-oxygen environment. A critical mediator of cellular adaptation to hypoxia is hypoxia-inducible factor (HIF), a basic helix-loop-helix transcription factor that consists of an oxygen-sensitive alpha-subunit, HIF-alpha and a constitutively expressed beta-subunit, HIF-beta. Under conditions of normal oxygen tension, the HIF-alpha subunit is hydroxylated by specific prolyl-hydroxylases and targeted for rapid proteasomal degradation by the von Hippel-Lindau (VHL) tumor suppressor, which is the substrate recognition component of an E3-ubiquitin ligase. In a hypoxic environment or in the absence of functional VHL tumor suppressor protein irrespective of oxygen concentration, HIF-alpha is not degraded and translocates to the nucleus, where it dimerizes with HIF-beta to form transcriptionally active HIF. As a transcription factor, HIF is involved in the regulation of many biological processes that facilitate both oxygen delivery and adaptation to oxygen deprivation by regulating genes that are involved in glucose uptake and energy metabolism, angiogenesis, erythropoiesis, cell proliferation and apoptosis, cell-cell and cell-matrix interactions, and barrier function. This review summarizes some of the most recent advances in the VHL/HIF field and discusses their relevance for pathogenesis and treatment of acute ischemic renal failure, renal fibrosis, and renal cancer.

Hypoxic conditions stimulate the production of angiogenin and vascular endothelial growth factor by human renal proximal tubular epithelial cells in culture

BACKGROUND

Chronic low oxygen in the tubulointerstitial area is a crucial cause of renal degradation and tubulointerstitial damage. Previous reports have suggested that the maintenance of renal blood flow plays a role in the suppression of progressive renal damage. Neovascularization is important for the maintenance of blood flow. We studied the production of angiogenic factors by culturing renal proximal tubular epithelial cells (PTEC) under hypoxic conditions.

METHODS

Cultured PTEC were exposed to normal and low-oxygen conditions. The levels of angiogenin (ANG) and vascular endothelial growth factor (VEGF) in the cell supernatants were measured by enzyme-linked immunosorbent assay. The messenger RNAs (mRNAs) of ANG and VEGF in the PTEC were examined by real-time reverse transcriptase polymerase chain reaction (real-time RT-PCR). The presence of ANG, VEGF and hypoxia-inducible factor-1 (HIF-1) was studied by immunofluorescence techniques. The effect of cobalt chloride (CoCl(2)), which is an HIF-1 inducer, on the production of ANG and VEGF was also examined in order to elucidate the contribution of the HIF-1 pathway to the production of these cytokines.

RESULTS

ANG and VEGF were demonstrated to exist in the cell supernatants, and ANG and VEGF mRNAs were detected in the PTEC. Hypoxic conditions stimulated the secretion of ANG (2.5-fold vs normoxia, P<0.001) and VEGF (3.2-fold vs normoxia, P<0.001) by PTEC. Hypoxic conditions increased the mRNA expression of ANG for 6 h (1.38-fold vs normoxia, P<0.05) and VEGF for 24 h (2.04-fold vs normoxia, P<0.01). Hypoxic conditions also enhanced ANG, VEGF and HIF-1 protein expression in PTEC. The CoCl(2) increased the secretion of ANG (5.2-fold vs control, P<0.0001) and VEGF (2.3-fold vs control, P<0.0001) by PTEC.

CONCLUSION

Under hypoxic conditions, the ANG and VEGF secreted by PTEC may modulate angiogenesis and vascular remodeling in the renal interstitium via an increase in the production of HIF-1.

Induction of protective genes by cobalt ameliorates tubulointerstitial injury in the progressive Thy1 nephritis

BACKGROUND

We previously demonstrated that chronic hypoxia has pivotal roles in the progression of tubulointerstitial injury from the early stage of the uninephrectomized Thy1 nephritis model. We have also shown that pretreatment of cobalt confers renoprotection in the ischemia/reperfusion (I/R) injury, in association with the up-regulation of hypoxia-inducible factor (HIF)-regulated genes. Here, we tested the hypothesis that cobalt administration not only attenuates acute ischemic insult, but also ameliorates tubulointerstitial injury secondary to chronic hypoxia.

METHODS

We applied sustained cobalt treatment to the uninephrectomized Thy1 nephritis model at 3 to 5 weeks, when tubular hypoxia appeared. Histologic evaluation, including glomerular and peritubular capillary networks, was made at 8 weeks. HIF activation was confirmed by real-time polymerase chain reaction (PCR) analyses for HIF-regulated genes, such as erythropoietin (EPO), vascular endothelial growth factor (VEGF), and heme oxygenase 1 (HO-1). Up-regulation of HIF-1alpha and HIF-regulated genes was also verified by Western blotting analysis. To elucidate responsible mechanisms of cobalt in the amelioration of tubuloniterstitial injury, terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate (dUTP) nick end labeling (TUNEL) staining was conducted at 5 weeks. A combination therapy with angiotensin receptor blocker (ARB), olmesartan, was also challenged.

RESULTS

Although the intervention did not change glomerular structural damage or urinary protein excretion rate, tubulointerstitial injury was improved in cobalt-treated animals when compared with the vehicle-treated group. The amelioration was associated with the parallel up-regulation of renoprotective, HIF-regulated gene expression. TUNEL staining revealed that the number of apoptotic cells was reduced in the cortex by cobalt administration, suggesting that renoprotection was achieved partly through its antiapoptotic properties. Furthermore, it was demonstrated that cobalt treatment exerts additional renoprotective effects with the ARB treatment in this model.

CONCLUSION

Maneuvers to activate HIF in the ischemic tubulointerstitium will be a new direction to future therapeutic strategies.

Implication of Peritubular Capillary Loss and Altered Expression of Vascular Endothelial Growth Factor in IgA Nephropathy

BACKGROUND/AIMS

To determine the roles of peritubular capillary (PTC) loss and expression of vascular endothelial growth factor (VEGF) and its transcription factor, hypoxia-inducible factor-1 (HIF-1), in the progression of IgA nephropathy (IgAN), we analyzed the expression of VEGF and HIF-1, and the number of PTCs in patients with variable severity of IgAN.

METHODS

Renal biopsy specimens from patients with IgAN (n = 23) were classified according to interstitial injury score: grade 0 (0%), grade 1 (1-25%), grade 2 (25-50%) and grade 3 (50-100%). We examined the immunohistochemical expression of CD34, VEGF and HIF-1alpha.

RESULTS

VEGF was expressed in the cytoplasm of tubular epithelia, and VEGF-positive area significantly expanded in grades 1 (35.5 +/- 5.9%, mean +/- SD) and 2 (32.5 +/- 5.9%) compared with grade 0 (23.4 +/- 4.5%). The numbers of PTCs were significantly lower in grades 2 (559 +/- 49/mm2) and 3 (510 +/- 56/mm2) than grade 0 (708 +/- 49/mm2). HIF-1alpha was weakly expressed in tubular epithelia in grade 0, increased with progression to grade 2, and markedly decreased in grade 3. It was also increased in pericapsular interstitial area in grade 1. The expression pattern of HIF-1alpha did not parallel that of VEGF. In renal biopsies of 5 control patients with minor glomerular abnormality, glomerular expression levels of VEGF and HIF-1alpha were similar to those of IgAN grade 0 kidneys.

CONCLUSION

VEGF production was accelerated in the early stage of IgAN but it did not protect against PTC injury/loss. The lack of correlation between VEGF and HIF-1alpha expression suggests HIF-independent VEGF production in IgAN.

VEGF regulation of endothelial nitric oxide synthase in glomerular endothelial cells

BACKGROUND

Vascular endothelial growth factor (VEGF) regulation of endothelial nitric oxide synthase (eNOS) and signaling pathways involved have not been well studied in glomerular endothelial cells (GENCs).

METHODS

GENCs grown from tsA58 Immortomice were used. Immunoblotting and in-cell Western blot analysis were employed to assess changes in VEGF receptor signaling pathway and eNOS phosphorylation of ser1177. Immunokinase assay and immunoblotting with phosphospecific antibodies were performed to assess activity of kinases.

RESULTS

VEGF rapidly induced tyrosine phosphorylation of type 1 and type 2 VEGF receptors. Physical association between VEGF-receptor 2 (VEGF-R2) and insulin receptor substrate (IRS-1) and phosphatidylinositol 3'-kinase (PI3K) was induced by VEGF, which augmented PI3K activity in VEGF-R2 immunoprecipitates. VEGF stimulated Akt phosphorylation in a PI3K-dependent manner. VEGF increased eNOS phosphorylation on Ser1177. Activation of eNOS was associated with nitric oxide generation as measured by medium nitrite content. Signaling mechanisms involved in VEGF stimulation of eNOS were explored. VEGF-induced eNOS phosphorylation was abolished by SU1498, a VEGF-R2 inhibitor, LY294002, a PI3K inhibitor, and infection of cells with an adenovirus carrying a dominant negative-mutant of Akt, demonstrating the requirement of the VEGF-R2/IRS-1/PI3K/Akt axis for activation of eNOS. VEGF also activated extracellular signal-regulated protein kinase (ERK) in a time-dependent manner; and VEGF-stimulated eNOS phosphorylation on Ser1177 was prevented by PD098059, an upstream inhibitor of ERK, demonstrating that ERK was involved in VEGF regulation of eNOS. ERK phosphorylation was abolished by LY294002, suggesting ERK was downstream of PI3K in VEGF-treated GENC.

CONCLUSIONS

Our data demonstrate that in GENC, VEGF stimulates VEGF-R2/IRS-1/PI3K/Akt axis to regulate eNOS phosphorylation on Ser1177 in conjunction with the ERK signaling pathway.

Protection of HIF-1-deficient primary renal tubular epithelial cells from hypoxia-induced cell death is glucose dependent

Ischemic acute renal failure is a frequent clinical problem in hospitalized patients and is associated with significant mortality. Hypoxia-inducible factor 1 (HIF-1) mediates cellular adaptation to hypoxia by regulating biological processes important for cell survival, which include glycolysis, angiogenesis, erythropoiesis, apoptosis, and proliferation. To investigate the role of HIF-1 in hypoxia-induced renal epithelial cell death, we generated mice that allow inactivation of HIF-1α by tetracycline-inducible Cre-loxP-mediated recombination in primary renal proximal tubule cells (PRPTC), resulting in a suppression of HIF-1-mediated gene transcription during oxygen deprivation. In the absence of glucose, the onset and the degree of hypoxia-induced cell death in HIF-1-deficient PRPTC were comparable to wild-type cells. However, when glucose availability was limited, the onset of cell death was delayed in either PRPTC that were HIF-1 deficient or in wild-type PRPTC when glycolysis or glucose uptake was partially inhibited. Our findings suggest in an in vitro genetic model that 1) the generation of adequate energy levels for the maintenance of PRPTC viability under hypoxia does not require HIF-1 and 2) that HIF-1 regulates the timing of hypoxia-induced cell death and apoptosis onset through its effects on glucose consumption.

Sirolimus-induced thrombotic microangiopathy is associated with decreased expression of vascular endothelial growth factor in kidneys

The aim of this study was to examine the clinical characteristics, the histological features and the renal expression of vascular endothelial growth factor (VEGF) of five patients with sirolimus-associated thrombotic microangiopathy (TMA). Sirolimus-induced TMA occurs preferentially in kidneys with concomitant endothelial injury: it was observed in three patients with acute cellular rejection on calcineurin inhibitor-free regimen, in one patient with chronic graft rejection on a calcineurin inhibitor-free protocol and in one patient with chronic calcineurin inhibitor nephrotoxicity. We found that renal VEGF expression during sirolimus-induced TMA was significantly lower than VEGF expression in normal transplanted kidneys (p < 0.01). Decreased expression of VEGF seems to be a consequence of sirolimus treatment since (i) analysis of two biopsies performed after the switch of sirolimus to calcineurin inhibitor showed reappearance of VEGF expression, (ii) no decreased expression of VEGF was found in five kidneys with classical TMA and, (iii) an increased expression of VEGF was observed in seven kidneys with acute cellular rejection on a sirolimus-free immunosuppressive regimen (p < 0.01). The potential role of sirolimus-induced downregulation of VEGF as a predisposing factor to the development of TMA is discussed.

Cobalt promotes angiogenesis via hypoxia-inducible factor and protects tubulointerstitium in the remnant kidney model

Tubulointerstitial hypoxia has been implicated in a number of progressive renal diseases, and several lines of evidence indicate that the administration of angiogenic growth factors ameliorates tubulointerstitial injury. We hypothesized that induction of hypoxia-inducible factors (HIF) mediates renoprotection by their angiogenic properties. At 5-9 weeks after subtotal nephrectomy, cobalt was administered to rats to activate HIF. Histological evaluation demonstrated that the tubulointerstitial injury was significantly ameliorated in animals that received cobalt (score: 2.51+/-0.12 (cobalt) vs 3.21+/-0.24 (vehicle), P<0.05). Furthermore, animals receiving cobalt had fewer vimentin- and TdT-mediated dUTP nick-end labeling (TUNEL)-positive tubular cells. The renoprotective effect of cobalt was associated with the preservation of peritubular capillary networks (rarefaction index: 13.7+/-0.4 (cobalt) vs 18.6+/-0.9 (vehicle), P<0.01). This improvement in capillary networks was accompanied by an increased number of proliferating (PCNA-positive) glomerular and peritubular endothelial cells. The angiogenesis produced by this method was not accompanied by an increase in vascular permeability. Furthermore, in vitro experiments clarified that HIF-1 in tubular epithelial cells promotes proliferation of endothelial cells and that HIF-2 overexpressed in renal endothelial cells mediates migration and network formation. Collectively, these findings demonstrate a renoprotective role of HIF through angiogenesis and provide a rationale for therapeutic approaches to target HIF for activation.