Effects of hypoxia on growth factor expression in the rat kidney in vivo

Regulation, signalling and functions of hormonal peptides in pulmonary vascular remodelling during hypoxia

Hypoxic state affects organism primarily by decreasing the amount of oxygen reaching the cells and tissues. To adjust with changing environment organism undergoes mechanisms which are necessary for acclimatization to hypoxic stress. Pulmonary vascular remodelling is one such mechanism controlled by hormonal peptides present in blood circulation for acclimatization. Activation of peptides regulates constriction and relaxation of blood vessels of pulmonary and systemic circulation. Thus, understanding of vascular tone maintenance and hypoxic pulmonary vasoconstriction like pathophysiological condition during hypoxia is of prime importance. Endothelin-1 (ET-1), atrial natriuretic peptide (ANP), and renin angiotensin system (RAS) function, their receptor functioning and signalling during hypoxia in different body parts point them as disease markers. In vivo and in vitro studies have helped understanding the mechanism of hormonal peptides for better acclimatization to hypoxic stress and interventions for better management of vascular remodelling in different models like cell, rat, and human is discussed in this review.

Mimicking hypoxia to treat anemia: HIF-stabilizer BAY 85-3934 (Molidustat) stimulates erythropoietin production without hypertensive effects

Oxygen sensing by hypoxia-inducible factor prolyl hydroxylases (HIF-PHs) is the dominant regulatory mechanism of erythropoietin (EPO) expression. In chronic kidney disease (CKD), impaired EPO expression causes anemia, which can be treated by supplementation with recombinant human EPO (rhEPO). However, treatment can result in rhEPO levels greatly exceeding the normal physiological range for endogenous EPO, and there is evidence that this contributes to hypertension in patients with CKD. Mimicking hypoxia by inhibiting HIF-PHs, thereby stabilizing HIF, is a novel treatment concept for restoring endogenous EPO production. HIF stabilization by oral administration of the HIF-PH inhibitor BAY 85-3934 (molidustat) resulted in dose-dependent production of EPO in healthy Wistar rats and cynomolgus monkeys. In repeat oral dosing of BAY 85-3934, hemoglobin levels were increased compared with animals that received vehicle, while endogenous EPO remained within the normal physiological range. BAY 85-3934 therapy was also effective in the treatment of renal anemia in rats with impaired kidney function and, unlike treatment with rhEPO, resulted in normalization of hypertensive blood pressure in a rat model of CKD. Notably, unlike treatment with the antihypertensive enalapril, the blood pressure normalization was achieved without a compensatory activation of the renin–angiotensin system. Thus, BAY 85-3934 may provide an approach to the treatment of anemia in patients with CKD, without the increased risk of adverse cardiovascular effects seen for patients treated with rhEPO. Clinical studies are ongoing to investigate the effects of BAY 85-3934 therapy in patients with renal anemia.

Vasotrophic regulation of age-dependent hypoxic cerebrovascular remodeling

Hypoxia can induce functional and structural vascular remodeling by changing the expression of trophic factors to promote homeostasis. While most experimental approaches have been focused on functional remodeling, structural remodeling can reflect changes in the abundance and organization of vascular proteins that determine functional remodeling. Better understanding of age-dependent hypoxic macrovascular remodeling processes of the cerebral vasculature and its clinical implications require knowledge of the vasotrophic factors that influence arterial structure and function. Hypoxia can affect the expression of transcription factors, classical receptor tyrosine kinase factors, non-classical G-protein coupled factors, catecholamines, and purines. Hypoxia's remodeling effects can be mediated by Hypoxia Inducible Factor (HIF) upregulation in most vascular beds, but alterations in the expression of growth factors can also be independent of HIF. PPARγ is another transcription factor involved in hypoxic remodeling. Expression of classical receptor tyrosine kinase ligands, including vascular endothelial growth factor, platelet derived growth factor, fibroblast growth factor and angiopoietins, can be altered by hypoxia which can act simultaneously to affect remodeling. Tyrosine kinase-independent factors, such as transforming growth factor, nitric oxide, endothelin, angiotensin II, catecholamines, and purines also participate in the remodeling process. This adaptation to hypoxic stress can fundamentally change with age, resulting in different responses between fetuses and adults. Overall, these mechanisms integrate to assure that blood flow and metabolic demand are closely matched in all vascular beds and emphasize the view that the vascular wall is a highly dynamic and heterogeneous tissue with multiple cell types undergoing regular phenotypic transformation.

Protective effect of prostaglandin E1 on renal microvascular injury in rats of acute aristolochic acid nephropathy

BACKGROUND

To investigate the renal microvascular injury in acute aristolochic acid nephropathy (AAN) and the protective effects of prostaglandin E1 (PGE1) in acute AAN.

METHODS

Female Sprague-Dawley rats were randomly divided into three groups. The rats in PGE1 group received Caulis Aristolochia manshuriensis (CAM) decoction by gavage for 5 days, and PGE1 was given by vena caudalis before gavage. The rats in model group were gavaged with CAM for 5 days, and the same dose of 0.9% physiologic saline was given by vena caudalis. The rats in control group only received an equal daily volume of saline solution by gavage. Animals were killed at days 3, 5, and 7. Blood urea nitrogen (BUN), serum creatinine, and urinary protein were monitored before killing. Microvascular density was determined by JG12 immunostaining. The expression of angiogenic factor was assessed by vascular endothelial growth factor (VEGF). Tubulointerstitial hypoxia was assessed by hypoxia-inducible factor-1α (HIF-1α) expression.

RESULTS

CAM induced a significant decrease in VEGF expression and microvascular density in the kidney tissue, accompanied by a significant increase in HIF-1α, which reduced renal function and increased 24-h urinary protein excretion rates. PGE1 lessened the capillary loss, relieved hypoxia, and protected renal function. No significant pathological changes were found in control rats.

CONCLUSION

The renal microvascular injury in acute AAN is severe. PGE1 can significantly ameliorate the renal microvascular injury, relieve hypoxia, and protect renal function.

Metal-on-metal hip implants: do they impair renal function in the long-term? A 10-year follow-up study

INTRODUCTION

The aim of our study was to investigate a potential influence of elevated serumcobalt and serumchromiumlevels on renal function at minimum 10 years after implantation of a metal-on-metal hip.

MATERIALS AND METHODS

Between November 1992 and June 1994 98 patients (44 m, 54 f) with an average age of 56 (22-79) years received a metal-on-metal bearing Metasul. At the time of the 10-year follow-up, 15 patients had died and 8 were lost to follow-up. The remaining 75 patients had laboratory analysis including serumcreatinine and full blood cell count as well as chromium and cobalt serum levels.

RESULTS

Ten years postoperatively the median serumcreatinine level was 0.86 (0.55-1.51) mg/dl, the serumcreatinine clearance Ccr was in the normal range. The hemogram did not differ from that measured at the time of surgery. The median serumcobalt concentration was 0.75 (0.3-50.10) microg/l and the serumchromium concentration was 0.95 (0.3-58.6) mug/l, 10 years postoperatively.

CONCLUSION

Our long-term data do not show any influence of serum cobalt or chromium concentrations on renal function following total hip arthroplasty.

Ischemic injury underlies the pathogenesis of aristolochic acid-induced acute kidney injury

Aristolochic acid nephropathy (AAN) is a progressive tubulointerstitial renal disease caused by aristolochic acid intake. To determine the contribution of renal ischemia to the pathogenesis of AAN, we characterized changes in the expression of angiogenic factors and vasoactive substances, and then we evaluated the expression of a marker of hypoxia in an acute AAN rat model. Rats were orally administrated either a decoction of Aristolochiae manshuriensis that contained 20 mg/kg of aristolochic acid-I or an equal volume of distilled water (control group) once daily for 4 days or 7 days. Renal histology and serum creatinine were assessed. Expression of endothelin-1 (ET-1) and hypoxia inducible factor-1 alpha (HIF-1alpha) mRNA within renal cortex were determined by semiquantitative reverse-transcription polymerase chain reaction. Levels of ET-1, nitric oxide (NO), vascular endothelial growth factor (VEGF), and HIF-1alpha in kidneys were determined by radioimmunoassay, Griess method, Western blot, and immunohistochemistry, respectively. Tubular injury scores and ET-1 mRNA expression were increased in the AA-treated rats at both days 4 and 8, whereas serum creatinine level and ET-1 protein expression was increased only at day 4. In contrast, NO production in AA-treated rats was decreased at day 8 compared with the control group. Similarly, VEGF protein expression was reduced in the AA-treated rats at both days 4 and 8. A dramatic increase in nuclear staining for HIF-1alpha was observed mainly in the tubular cells of tubulointerstitial damage area in the AA-treated rats at day 8. The observed increase in HIF-1alpha protein expression, decrease in VEGF protein expression, and imbalance of vasoactive substances after induction of acute kidney injury by AA suggests that ischemic injury contributes to the pathogenesis of AAN.

The endothelial cell in ischemic acute kidney injury: implications for acute and chronic function

Recent evidence suggests that injury to the renal vasculature may play an important role in the pathogenesis of both early and chronic ischemic acute kidney injury (AKI). Established and new data support the suggestion that vascular injury, in particular, endothelial cell injury, participates in the extent and maintenance of AKI by pathways that are related to vascular tone. Early alterations in peritubular capillary blood flow during reperfusion has been documented and associated with loss of normal endothelial cell function, which can be replaced pharmacologically or with cell replacement interventions. Distorted peritubular capillary morphology is associated with loss of barrier function that may contribute to early alterations in vascular stasis. In addition, ischemia induces alterations in endothelial cells that may promote inflammation and procoagulant activity, thus contributing to vascular congestion. Reductions in microvasculature density may play a critical part in the progression of chronic kidney disease following initial recovery from ischemia/reperfusion-induced AKI. The exact nature of how capillary loss alters renal function and predisposes renal disease is thought to be due at least in part to hypoxia. Finally, the loss of endothelial cell function may represent an important therapeutic target in which nitric oxide, vascular trophic support, and/or endothelial progenitor cells may show potential importance in ameliorating the acute and/or chronic effects of ischemic AKI.

Quantitative RT-PCR assays of hypoxia-inducible factor‐1α, erythropoietin and vascular endothelial growth factor mRNA transcripts in the kidneys with regard to the cause of death in medicolegal autopsy

Accumulating studies demonstrate that the expressions of hypoxia-inducible factor 1 (HIF-1), erythropoietin (EPO) and vascular endothelial growth factor (VEGF) depend on cellular oxygen tension, which is involved in the pathological process of tissue hypoxia and/or ischemia. The present study investigated hypoxia-inducible factor-1alpha (HIF-1alpha), EPO and VEGF mRNA expressions in the kidney with regard to the cause of death in medicolegal autopsy. Relative quantifications of HIF-1alpha, EPO and VEGF mRNAs, based on real-time TaqMan reverse transcription-polymerase chain reaction (RT-PCR), were performed on tissue specimens obtained from consistent sites of the bilateral renal cortices. The cases (total, n=245, 6-48h postmortem) included fatal blunt/sharp instrument injuries (n=53/31), asphyxia (n=28: aspiration, n=8; strangulation/hanging, n=20), drowning (n=27), fire fatality (n=62), acute myocardial infarction/ischemia (AMI, n=39), and gastrointestinal hemorrhage (n=5). Both HIF-1alpha and EPO mRNA levels were significantly lower in drowning cases. More characteristic findings were found for VEGF mRNA: it showed higher expression levels for AMI, acute blunt/sharp instrument injury, and aspiration, whereas it was lower for neck compression (strangulation/hanging), drowning, fire fatality with higher blood carboxyhemoglobin (COHb) levels (>60%), peracute blunt injury, and gastrointestinal hemorrhage. Quantitative assays of renal HIF-1alpha, EPO and VEGF mRNA transcripts are potentially useful for investigating the pathophysiology of death, and VEGF mRNA may be especially useful as an indication of acute circulatory failure.

LPS increases hepatic HIF-1alpha protein and expression of the HIF-1-dependent gene aldolase A in rats

BACKGROUND

Cellular adaptation to hypoxia is mediated in part by the transcription factor hypoxia-inducible factor 1 (HIF-1). Accumulating data suggest that pro-inflammatory mediators can up-regulate HIF-1alpha protein expression and HIF-1 DNA-binding activity in the absence of hypoxia. Accordingly, we investigated HIF-1 mediated signaling in endotoxemic rats.

MATERIALS AND METHODS

We studied three groups of male Sprague Dawley rats. Controls (N = 5) were injected i.p. with saline. Endotoxemic rats (N = 9) received a sublethal dose of lipopolysaccaride (Escherichia coli; 5 mg/kg, i.p.). A third group of rats (N = 5) received the HIF-1 stabilizing agent CoCl(2) (14 mg/kg, i.p.) at T = 0 h and T = 16 h. At T = 18 h, liver microvascular perfusion was measured using laser Doppler flowmetry and hepatic tissue samples were obtained. RNA was isolated and mRNA levels of the HIF-1 dependent genes aldolase A and vascular endothelial growth factor (VEGF) were determined using quantitative real-time RT-PCR. HIF-1alpha content was estimated by immunoprecipitation followed by Western blotting.

RESULTS

HIF-1alpha increased in hepatic tissue after treatment with LPS or CoCl(2). LPS markedly increased hepatic expression of aldolase A, but failed to alter expression of VEGF. CoCl(2) increased aldolase A and VEGF mRNA expression. Although hepatic microvascular perfusion was comparable in saline- and LPS-treated rats, hepatic microvascular blood flow and aldolase A expression were significantly inversely correlated among endotoxemic rats (r = 0.773; P = 0.003).

CONCLUSIONS

Increased expression of aldolase A in endotoxemic rats is mediated by both hypoxia-dependent and hypoxia-independent mechanisms.

Local pharmacological preconditioning increases the survival of experimental skin flaps in rats

Skin flap survival is dependent on an adequate blood supply and on the resistance to ischemia. Experimentally we tested the effect of pharmacological preconditioning on the survival of skin flaps. The survival of an ischemic skin flap (McFarlane flap) was tested using the local application of cobalt gel in three groups of Wistar rats. The mean flap necrosis of the control group was 59.15%. The rats treated by the cobalt gel once a week showed mean necrosis in 39.09%; the rats receiving the application of the cobalt gel three times a week in 26.33%. The treated groups presented with significantly lower flap necrosis in comparison to the untreated controls. There was a significant difference in the flap necrosis occurrence according to the application frequency of the cobalt gel. The expression of the genes involved in angiogenetic processes encoding vascular endothelial growth factor (VEGF) and glycolytic enzymes was influenced in a non-mitochondrial way in this study. The results show that non-mitochondrial preconditioning could prolong the survival of an ischemic flap.

Rarefaction of peritubular capillaries following ischemic acute renal failure: a potential factor predisposing to progressive nephropathy

PURPOSE OF REVIEW

Long-term renal complications of acute renal failure have generally not been expected in patients that recover from acute renal failure. However, as the incidence of acute renal failure is rising, the incidence of long-term complications is likely to increase. As a corollary to ischemic acute renal failure, ischemic injury in the setting of transplant is a leading cause of delayed graft function. Unlike acute renal failure in native kidneys, delayed graft function is highly predictive of chronic nephropathy and organ failure. It is generally well accepted that acute reversible injuries mediated by ischemia render grafts susceptible toward future demise. The nature of the susceptibility that is conveyed to grafts following ischemic injury is not well understood.

RECENT FINDINGS

Evidence from animal models suggests that acute injury results in microvascular damage and vessel loss in the kidney, which, as opposed to tubular damage, is largely persistent. In addition, various studies of biopsies of renal transplants suggest that ischemia imposes an early and sustained loss in peritubular capillaries in the transplanted graft. The loss of peritubular capillaries has been associated with nephropathies of diverse etiologies and may represent a single, common pathway towards progressive damage.

SUMMARY

It is hypothesized that rarefaction of peritubular capillaries represents a critical event, following ischemic injury, that permanently alters renal function and predisposes patients to the development of chronic renal insufficiency. Factors that affect vascular reactivity or the structural dynamics of the kidney vascular system following injury may represent future treatment modalities following renal injury.

HIF-1alpha expression follows microvascular loss in advanced murine adriamycin nephrosis

Cellular hypoxia has been proposed as a major factor in the pathogenesis of chronic renal injury, yet to date there has been no direct evidence to support its importance. Therefore, we examined cortical hypoxia in an animal model of chronic renal injury (murine adriamycin nephrosis; AN) by assessing nuclear localization of the oxygen-dependent alpha-subunit of hypoxia-inducible factor-1 (HIF-1alpha) in animals 7, 14, and 28 days after adriamycin. Results were assessed in conjunction with quantitation of the cortical microvasculature (by CD34 immunostaining) and cortical expression of VEGF. Cortical apoptosis was also examined by terminal deoxynucleotidyl transferase dUTP nick-end labeling staining. A dramatic and significant increase in nuclear localization of HIF-1alpha was seen 28 days after adriamycin in the context of severe glomerular and tubulointerstitial damage. Areas of nuclear HIF-1alpha staining did not colocalize with areas of cellular apoptosis. AN was also associated with a significant attenuation of the peritubular capillaries that was significant at 14 and 28 days after adriamycin. Cortical VEGF expression fell in a stepwise manner from day 7 until day 28 after adriamycin. In conclusion, these data are consistent with a significant increase in cellular hypoxia occurring in the advanced stages of murine AN. Increased cortical hypoxia was preceded by significant reductions in both the number of peritubular capillaries (i.e., oxygen supply) and the angiogenic cytokine VEGF. Apart from providing the first direct evidence for cellular hypoxia in a model of chronic renal disease, these results suggest that a primary dysregulation of angiogenesis may be the cause of increased hypoxia in this model.

17Beta-estradiol decreases hypoxic induction of erythropoietin gene expression

Exposure to chronic hypoxia induces erythropoietin (EPO) production to facilitate oxygen delivery to hypoxic tissues. Previous studies from our laboratory found that ovariectomy (OVX) exacerbates the polycythemic response to hypoxia and treatment with 17beta-estradiol (E2-beta) inhibits this effect. We hypothesized that E2-beta decreases EPO gene expression during hypoxia. Because E2-beta can induce nitric oxide (NO) production and NO can attenuate EPO synthesis, we further hypothesized that E2-beta inhibition of EPO gene expression is mediated by NO. These hypotheses were tested in OVX catheterized rats treated with E2-beta (20 microg/day) or vehicle for 14 days and exposed to 8 or 12 h of hypoxia (12% O(2)) or normoxia. We found that E2-beta treatment significantly decreased EPO synthesis and gene expression during hypoxia. E2-beta treatment did not induce endothelial NO synthase (eNOS) expression in the kidney but potentiated hypoxia-induced increases in plasma nitrates. We conclude that E2-beta decreases hypoxic induction of EPO. However, this effect does not appear to be related to changes in renal eNOS expression.

Renal ischemia-reperfusion increases endothelial VEGFR-2 without increasing VEGF or VEGFR-1 expression

BACKGROUND

Hypoxia is a potent stimulus to angiogenesis. Expression of the angiogenic growth factor vascular endothelial growth factor (VEGF) and its receptors (VEGFR-1 and VEGFR-2) is up-regulated by hypoxia in a variety of organs and cell lines. We have previously reported that VEGF expression is not increased in renal ischemia-reperfusion injury, although tubular cells concentrate VEGF at their basolateral surface. In this study we assess whether altered VEGF receptor expression compensates for the lack of VEGF regulation during renal ischemia-reperfusion injury.

METHODS

VEGFR-1 mRNA expression was assessed by Northern blotting and semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). VEGFR-2 mRNA expression was analyzed by Northern blotting and in situ hybridization (ISH), while VEGFR-2 protein expression was studied using immunohistochemistry. VEGF mRNA expression was assessed by ISH.

RESULTS

VEGFR-2 mRNA and protein expression were up-regulated without an increase in VEGF or VEGFR-1 expression. Normal kidneys showed low-level VEGFR-2 mRNA and protein expression in glomerular and peritubular endothelium. Following ischemia and ischemia-reperfusion, a marked increase in VEGFR-2 mRNA and protein expression was seen (2- to 4-fold). Most prominent was VEGFR-2 mRNA up-regulation in the glomerulus although, surprisingly, increased protein was not demonstrated here. ISH showed that VEGF mRNA was not up-regulated in this model, confirming our previous findings for VEGF.

CONCLUSION

VEGF and VEGFR-1 expression are not increased by renal ischemia and ischemia-reperfusion injury. Instead, endothelial expression of VEGFR-2 is increased. VEGFR-2 up-regulation in renal ischemia-reperfusion may be important in mediating the mitogenic and anti-apoptotic actions of VEGF on endothelial cells, thereby preserving the integrity of the endothelium and the potential for blood supply to ischemic tissues.

Heme oxygenase activity modulates vascular endothelial growth factor synthesis in vascular smooth muscle cells

Hypoxia, cytokines, and nitric oxide (NO) stimulate the generation of vascular endothelial growth factor (VEGF) and induce heme oxygenase-1 (HO-1) expression in vascular tissue. HO-1 degrades heme to carbon monoxide (CO), iron, and biliverdin, the latter being reduced to bilirubin by biliverdin reductase. In the present study, we investigated the role of HO-1 in the modulation of VEGF synthesis in rat vascular smooth muscle cells (VSMC). In VSMC stimulated with cytokines, inhibition of NO production significantly, but not completely, reduced VEGF release. In contrast, inhibition of HO activity by tin protoporphyrin IX (SnPPIX) totally prevented cytokine-induced increase in VEGF, despite an augmented synthesis of intracellular NO. Stimulation of HO-1 activity by hemin enhanced VEGF production; this effect was abrogated by blockade of the HO pathway. Similarly, VEGF synthesis induced by hypoxia was down-regulated by SnPPIX, but not by inhibitors of NO synthase. To elucidate further a direct involvement of HO-1 in the observed effects, we generated transfected cells that overexpressed the HO-1 gene. Notably, these cells synthesized significantly more VEGF protein than cells transfected with a control gene. Among the products of HO-1, biliverdin and bilirubin showed no effect, whereas iron ions inhibited VEGF synthesis. Exposure of cells to 1% CO resulted in a marked accumulation of VEGF (20-fold increase) over the basal level. Our data indicate that HO-1 activity influences the generation of VEGF in VSMC in both normoxic and hypoxic conditions. As CO and iron, respectively the inducer and the inhibitor of VEGF synthesis, are concomitantly produced during the degradation of heme, these data indicate that HO by-products may differentially modulate VEGF production.

Osteopontin mediates hypoxia-induced proliferation of cultured mesangial cells: role of PKC and p38 MAPK

BACKGROUND

We previously reported that hypoxia induces the proliferation of cultured mesangial cells mediated by the stimulation of intracellular calcium and the activation of protein kinase C (PKC). In the present study, we examined the roles of mesangial cell specific growth factors (platelet-derived growth factor and endothelin-1) and osteopontin (OPN) in hypoxia-induced proliferation of mesangial cells. In addition, we determined the effect of hypoxia on p38 mitogen-activated protein (MAP) kinase activity and the roles of both PKC and p38 MAP kinase in hypoxia-induced alterations in OPN and mesangial cell growth.

METHODS

Quiescent cultures of mesangial cells were exposed to hypoxia (3% O2) or normoxia (18% O2) in a serum-free medium, and [3H]-thymidine incorporation, OPN protein and mRNA expression, and p38 MAP kinase activity were assessed.

RESULTS

Hypoxic-conditioned medium mimicked the effect of hypoxia on thymidine incorporation, suggesting the release of diffusable growth promoting factor(s) by hypoxia. Neither anti-endothelin-1 nor anti-platelet-derived growth factor-neutralizing antibodies had an effect on increased thymidine incorporation induced by hypoxia. However, blocking the effects of OPN either with anti-OPN antibody or its beta3 integrin receptor antibody completely prevented the hypoxia-induced increase in thymidine incorporation. Hypoxia also stimulated OPN protein and mRNA levels. Hypoxia caused an acute activation of p38 MAP kinase, which was inhibited by both verapamil and an inhibitor of PKC (calph C). PKC inhibitor and an inhibitor of p38 MAP kinase (SB203580) reduced the hypoxia-induced stimulation of both OPN and cell growth.

CONCLUSIONS

These studies provide, to our knowledge, the first evidence demonstrating the role of OPN in hypoxia-induced proliferation of mesangial cells. In addition, hypoxia causes an activation of p38 MAP kinase in a calcium- and PKC-dependent manner, and the activation of PKC and p38 MAP kinase appears to be involved in the stimulation of both OPN and mesangial cell proliferation induced by hypoxia.

Redistribution of cytoplasmic VEGF to the basolateral aspect of renal tubular cells in ischemia-reperfusion injury

BACKGROUND

Vascular endothelial growth factor (VEGF) mRNA and protein expression are increased by hypoxia in a variety of cell types and organs. In the kidney, however, chronic hypoxia does not up-regulate VEGF mRNA. This suggests that VEGF may be regulated by unique mechanisms in the kidney.

METHODS

Unilateral ischemia was induced in rats by vascular cross-clamping (40 min) followed by reperfusion (0, 20, 40, and 80 min). The distribution of VEGF protein was determined by immunohistochemical staining and Western blotting. mRNA was detected by Northern blotting and semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). Immunohistochemical staining for VEGF was verified using two VEGF antibodies. To further substantiate the immunohistochemical findings, laser scanning confocal fluorescence microscopy was used to demonstrate the distribution of VEGF protein in rat renal tubular epithelial cells (NRK52-E) subjected to hypoxia (40 min) and re-oxygenation (0, 5, 20, 40 and 80 min).

RESULTS

Normal kidneys showed diffuse immunohistochemical staining for VEGF in all tubules of the renal cortex and medulla. Following ischemia, staining demonstrated a prominent shift of cytoplasmic VEGF to the basolateral aspect of tubular cells with both VEGF antibodies. The distribution of cytoplasmic VEGF returned to normal following 40 and 80 minutes of reperfusion. Western blots of cytoplasmic samples from ischemic kidneys reperfused for 0 and 20 minutes showed decreased levels of VEGF164 compared with normal (P < 0.01). VEGF164 and VEGF188 levels in the membrane fraction showed no change. Northern blots and semiquantitative RT-PCR showed no significant up-regulation of VEGF mRNA or change in the splice pattern. NRK52-E cells subjected to hypoxia and re-oxygenation for 0 and 5 minutes showed increased staining for VEGF compared with normal, with prominent VEGF staining at the periphery of the cell, similar to the appearance in ischemic kidneys. VEGF staining became more diffuse with further re-oxygenation.

CONCLUSION

Although synthesis of VEGF mRNA and protein is not increased during ischemia reperfusion injury, pre-existing VEGF in the tubular cell cytoplasm redistributes to the basolateral aspect of the cells. These data suggest that the kidney may have evolved unique patterns of VEGF regulation to cope with acute hypoxia.

Effects of chronic hypoxia on renal renin gene expression in rats

BACKGROUND

The effects of hypoxia on renin secretion and renin gene expression have been controversial. In recent studies, we have demonstrated that acute hypoxia of 6 h duration caused a marked stimulation of renin secretion and renal renin gene expression. This hypoxia-induced stimulation of the renin-angiotensin system might contribute, for example, to the progression of chronic renal failure and to the development of hypertension in the sleep-apnoea syndrome. For this reason, we were interested in the more chronic effects of hypoxia on renal renin gene expression and its possible regulation.

METHODS

Male rats were exposed to chronic normobaric hypoxia (10% O(2)) for 2 and 4 weeks. Additional groups of rats were treated with an endothelin ET(A) receptor antagonist, LU135252, or a NO donor, molsidomine, respectively. Systolic blood pressure and right ventricular pressures were measured. Renal renin, endothelin-1 and endothelin-3 gene expression were quantitated using RNAase protection assays.

RESULTS

During chronic hypoxia, haematocrit increased to 72+/-2%, and right ventricular pressure increased by a mean of 26 mmHg. Renal renin gene expression was halved during 4 weeks of chronic hypoxia. This decrease was reversed by endothelin receptor blockade (105 or 140% of baseline values after treatment for weeks 3-4 or 1-4). Furthermore, there was a trend of increasing renal endothelin-1 gene expression (to 173% of baseline values) after 4 weeks of hypoxia. Systolic blood pressure increased moderately during 4 weeks of chronic hypoxia from 129+/-2 to 150+/-4 mmHg. This blood pressure increase was higher in rats treated for 4 weeks with an endothelin receptor antagonist (196+/-11 mmHg).

CONCLUSIONS

Chronic hypoxia (in contrast to acute hypoxia) suppresses renal renin gene expression. This inhibition presumably is mediated by endothelins.

Brainstem activation of platelet-derived growth factor-beta receptor modulates the late phase of the hypoxic ventilatory response

The early phase of the biphasic ventilatory response to hypoxia in mammals is critically dependent on NMDA glutamate receptor activation within the nucleus of the solitary tract. However, the mechanisms underlying the subsequent development of the typical ventilatory roll-off are unclear and could underlie important roles in the functional and molecular adaptation to oxygen deprivation. Because the growth factor platelet-derived growth factor (PDGF)-BB can modulate the open channel probability of NMDA receptors by activating PDGF-beta receptors, its contribution to hypoxic ventilatory roll-off was examined. Administration of PDGF-BB, but not PDGF-AA, in the nucleus of the solitary tract was associated with significant attenuations of the early hypoxic ventilatory response in conscious rats. Furthermore, marked reductions in the magnitude of hypoxic ventilatory roll-off occurred in mice heterozygous for a mutation in the PDGF-beta receptor. Administration of a PDGF-beta receptor antagonist to wild-type littermates elicited similar declines in hypoxic ventilatory roll-off. The relative abundance of PDGF-beta receptors was confirmed in the nucleus of the solitary tract and other nuclei implicated in the hypoxic ventilatory response. In nucleus of the solitary tract lysates, PDGF-beta receptor tyrosine phosphorylation was temporally correlated with hypoxic ventilatory roll-off formation. Increased PDGF-B chain mRNA expression was induced by hypoxia in the nucleus of the solitary tract, and PDGF-B chain immunoreactivity colocalized with approximately 40% of nucleus of the solitary tract neurons, demonstrating hypoxia-induced c-Fos enhancements. Thus, PDGF-BB release and PDGF-beta receptor activation in the nucleus of the solitary tract are critical components of hypoxic ventilatory roll-off and may have important functional implications in processes underlying survival and acclimatization to hypoxic environments.

Tissue oxygen levels control astrocyte movement and differentiation in developing retina

Astrocytes play a key role in the development of retinal vessels by detecting hypoxia in developing retina and secreting the hypoxia-induced angiogenic factor VEGF to induce vessel formation. The astrocytes which play this role are themselves spreading over the retina, just ahead of the growing vessels. To understand the mechanisms which keep astrocytes in this strategic 'just ahead' position we have studied the effects of hyperoxia and hypoxia on astrocyte differentiation and movement in situ in neonatal rat retina and in primary culture. Hyperoxia in situ inhibited the stellation of astrocytes, so that they persisted in a relatively unbranched form, which accumulated at the edge of their spreading population; hyperoxia permitted but did not accelerate migration. Conversely, hypoxia induced unstellated astrocytes to stellate within 6 h. If the hypoxia was abnormally severe, it caused the astrocytes to hyperstellate and slowed their spread. Astrocytes in primary culture did not change morphology or motility when challenged by hypoxia. When treated with medium conditioned by retina however, astrocytes became mobile and, if the medium was conditioned by hypoxic retina, became stellate. These results suggest that the oxygen released by retinal vessels maintains the mobility of astrocytes, via a diffusible factor released by other retinal cells. Conversely, naturally generated hypoxia of developing retina plays a triple role, inducing astrocytes to stellate, to end their migration and to produce VEGF, thereby inducing vessel formation. The induction of stellation is mediated by a diffusible factor released by other retinal cells. Thus hypoxia of the retina generated by neural maturation induces key events in both the differentiation of astrocytes and the formation of blood vessels.

Modulation of erythropoiesis in rat bone marrow erythroblastic islands by cyclooxygenase inhibition

We designed our study to explore how the inhibition of prostaglandins (PGs) could affect erythropoiesis in bone marrow erythroblastic islands (EIs). To this end, we used hypoxic-stimulated rats-hypobaric hypoxia (42.55 kPa/6 h)-pretreated or not with indomethacin (4 mg/kg/3 days). Blood sampling was done at 0 h, 24 h, and 72 h after hypoxia. The study included estimations of the plasma erythropoietin (EPO) level (by radioimmunoassay), peripheral blood, number of EI from classes I to V per femur, rate of immature cell's differentiation into erythroblasts, and rate of repeated participation of macrophages in new EI reconstruction. Plasma EPO rose significantly (p < 0.01) in all hypoxic rats: 40.5+/-10.15 mU/ml and 46.75+/-16.28 mU/ml and at 0 h versus 13.83+/-6.82 mU/ml in controls. An increased rate of cell differentiation into erythroblasts in EIs (p < 0.01), an enhanced reconstruction in involuted EIs, and a reduced number of maturing EIs (p < 0.01) were observed in all hypoxic animals. However, in indomethacin-pretreated rats, the stimulation of bone marrow erythropoiesis was better expressed. Our results favor the concept that PG inhibition does not attenuate the erythropoietic response to hypoxia and support the hypothesis about the important role of EI macrophages as a local regulator of bone marrow erythropoiesis.

Effects of hypoxia on renin secretion and renal renin gene expression

Plasma renin activity (PRA) and renal renin mRNA levels were measured in male rats exposed to hypoxia (8% O2) or to carbon monoxide (CO; 0.1%) for six hours. PRA increased fourfold and 3.3-fold, and renin mRNA levels increased to 220% and 200% of control, respectively. In primary cultures of renal juxtaglomerular (JG) cells, hypoxia (lowering medium O2 from 20% to 3% or 1%) for 6 or 20 hours did not affect renin secretion or gene expression. Renal denervation did not prevent stimulation of the renin system by hypoxia. Because norepinephrine increased 1.7-fold and 3.2-fold and plasma epinephrine increased 3.9-fold and 7.8-fold during hypoxia and CO inhalation, respectively, circulating catecholamines might mediate the stimulatory effects of hypoxia on renin secretion and renin gene expression. Stimulation of beta-adrenergic receptors by continuous infusion of 160 microg/kg/hr isoproterenol increased PRA 17-fold and 20-fold after three and six hours, respectively, and renin mRNA by 130% after six hours. In rats with a stimulated renin system (low-sodium diet), isoproterenol did not stimulate PRA or renal renin mRNA further. In summary, both arterial and venous hypoxia can stimulate renin secretion and renin gene expression powerfully in vivo but not in vitro. These effects seem not to be mediated by renal nerves or by a direct effect on JG cells but might be mediated by circulating catecholamines.

Mesangiolytic glomerulopathy in severe congestive heart failure

To study the glomerular morphological abnormalities in congestive heart failure (CHF), we analyzed 27 autopsy cases without other causes of renal disease. Their mean age was 59 years, and they showed mild prerenal azotemia. They had generally been treated with digitalis and diuretics, and a few of them with captopril or nifedipine. The abnormal glomerular findings of enlargement, hyperemia, and mesangial thickening were observed at high frequencies (61%, 64%, and 57%, respectively). They characteristically showed mesangiolysis (ML) by the findings of microaneurysms (81%) and mesangial degeneration (70%) such as loose reticular matrix and poor matrix area. In addition, glomerular infiltration of mononuclear leukocytes including macrophages was noted in 70% of the cases. Glomerular enlargement was not correlated with the grade of hyperemia, but it was correlated with the grade of ML index of % glomeruli with microaneurysms (F = 7.22, p < 0.004). There was an inverse relationship between the grades of mesangial thickening and of the ML index (P < 0.005). The number of glomerular leukocytes was positively correlated with mean glomerular size (P < 0.002) and with the ML index (P < 0.03). Notably, the glomerular macrophage-positive cases showed a prominently higher mean ML index than the negative cases (P < 0.005). There was an inverse correlation between the mean glomerular size and the partial oxygen pressure in arterial blood (PaO2; P < 0.01), and a positive correlation between the mean glomerular size and hematocrit (Hct) levels (P < 0.02). The cases positive for mesangiolytic mesangial degeneration showed significantly lower PaO2 values than the cases negative for this lesion (P < 0.04). In the analysis of the various causes of CHF, the patients with congenital cardiac anomalies showed mean levels of the lowest PaO2 (P < 0.02) and the highest Hct (P < 0.03) and histologically the largest mean glomerular size (P < 0.04). There was no difference in the ML index and the glomerular leukocyte number among the subgroups classified by the causes. These results indicate that ML associated with glomerular enlargement is the major glomerular abnormality characteristic in patients with severe CHF and suggest that glomerular infiltration of leukocytes, especially of macrophages, should play an important role in the progression of both ML and glomerulomegaly. The contributions of persistent hypoxia and up-regulated angiotensin II as the causative factors of these glomerular abnormalities in congestive heart failure are discussed.