Oxygen-dependent expression of hypoxia-inducible factor-1alpha in renal medullary cells of rats

Anemia and Hypoxia Impact on Chronic Kidney Disease Onset and Progression: Review and Updates

Chronic kidney disease (CKD) is caused by hypoxia in the renal tissue, leading to inflammation and increased migration of pathogenic cells. Studies showed that leukocytes directly sense hypoxia and respond by initiating gene transcription, encoding the 2-integrin adhesion molecules. Moreover, other mechanisms participate in hypoxia, including anemia. CKD-associated anemia is common, which induces and worsens hypoxia, contributing to CKD progression. Anemia correction can slow CKD progression, but it should be cautiously approached. In this comprehensive review, the underlying pathophysiology mechanisms and the impact of renal tissue hypoxia and anemia in CKD onset and progression will be reviewed and discussed in detail. Searching for the latest updates in PubMed Central, Medline, PubMed database, Google Scholar, and Google search engines were conducted for original studies, including cross-sectional studies, cohort studies, clinical trials, and review articles using different keywords, phrases, and texts such as "CKD progression, anemia in CKD, CKD, anemia effect on CKD progression, anemia effect on CKD progression, and hypoxia and CKD progression". Kidney tissue hypoxia and anemia have an impact on CKD onset and progression. Hypoxia causes nephron cell death, enhancing fibrosis by increasing interstitium protein deposition, inflammatory cell activation, and apoptosis. Severe anemia correction improves life quality and may delay CKD progression. Detection and avoidance of the risk factors of hypoxia prevent recurrent acute kidney injury (AKI) and reduce the CKD rate. A better understanding of kidney hypoxia would prevent AKI and CKD and lead to new therapeutic strategies.

Hypoxia-inducible factors and essential hypertension: narrative review of experimental and clinical data

Hypoxia-inducible factor (HIFs) is a new class of drug developed for the management of anemia in chronic kidney disease (CKD) patients. HIFs increase the production of erythropoietin in the kidney and liver, enhance the absorption and utilization of iron, and stimulate the maturation and proliferation of erythroid progenitor cells. Besides, HIFs regulate many physiologic processes by orchestrating the transcription of hundreds of genes. Essential hypertension (HT) is an epidemic worldwide. HIFs play a role in many biological processes involved in the regulation of blood pressure (BP). In the current review, we summarize pre-clinical and clinical studies investigating the relationship between HIFs and BP regulation in patients with CKD, conflicting issues, and discuss future potential strategies.

The hypoxia-inducible factor prolyl hydroxylase inhibitor FG4592 promotes natriuresis through upregulation of COX2 in the renal medulla

The renal medulla is a key site for the regulation of renal sodium excretion. However, the molecular mechanism remains unclear. Cyclooxygenase 2 (COX2) is specifically expressed in the renal medulla and contributes to the maintenance of the electrolyte/water balance in the body. Hypoxia-inducible factors (HIFs) have also been found to be expressed in the renal medulla, probably owing to the hypoxic conditions in the renal medulla. This study was designed to test the effects of HIF activation on renal sodium handling and renal medullary COX2 expression. Our data showed that HIF activation by the prolyl hydroxylase inhibitor (PHI) FG4592 enhanced natriuresis in mice challenged with a high-salt diet. In addition, FG4592 upregulated the expression of COX2 in the renal medulla. An in vitro study further supported the finding that HIF can induce the expression of COX2 and that this induction is mediated through direct binding to the promoter region of the Cox2 gene, facilitating its transcription. In addition, the COX2 inhibitor celecoxib diminished the natriuretic effect of FG4592. Together, these results suggest that HIF activation promotes sodium excretion through upregulation of COX2 in the renal medulla and therefore maintains sodium homeostasis in the body.

Effects of empagliflozin on nondiabetic salt-sensitive hypertension in uninephrectomized rats

Impaired pressure natriuresis (PN) underlies salt-sensitive hypertension, and renal inflammation and hypoxia-inducible factor-1 (HIF-1) have been implicated in the modulation of systemic hypertension. Although sodium-glucose cotransporter-2 (SGLT2) inhibitors were reported to lower blood pressure (BP) in type 2 diabetes mellitus, whether they have a role in nondiabetic hypertensive kidney diseases is unclear. The present study was undertaken to investigate whether nondiabetic salt-sensitive hypertension and accompanying renal inflammation are ameliorated by SGLT2 inhibition. Male Sprague-Dawley rats were randomly divided into three groups: sham controls (SCs), uninephrectomized controls (UCs), and empagliflozin-treated rats (ETs). All rats were fed a rodent diet with 8% NaCl throughout the study period. Empagliflozin was orally administered for 3 weeks after uninephrectomy. Systolic blood pressure was recorded weekly, and kidneys were harvested for immunoblotting, immunohistochemistry, and quantitative PCR analysis at the end of the animal experiment. Systolic BP was significantly decreased in ETs that were orally given empagliflozin for 3 weeks after uninephrectomy. Although ETs did not show any increase in weekly measured urine sodium, the right-shifted PN relationship in UCs was improved by empagliflozin treatment. The expression of HIF-1α was increased in the renal outer medulla of ETs. Consistent with this, HIF prolyl-hydroxylase-2 protein and mRNA were decreased in ETs. The abundance of CD3 and ED-1 immunostaining in UCs was reduced by empagliflozin treatment. The increased IL-1ß, gp91phox, and NOX4 mRNA levels in UCs were also reversed. Empagliflozin restored impaired PN in nondiabetic hypertensive kidney disease in association with increased renal medullary expression of HIF-1α and amelioration of renal inflammation.

Diuretic, Natriuretic, and Vasodepressor Activity of a Lipid Fraction Enhanced in Medium of Cultured Mouse Medullary Interstitial Cells by a Selective Fatty Acid Amide Hydrolase Inhibitor

The relationship between the endocannabinoid system in the renal medulla and the long-term regulation of blood pressure is not yet understood. To investigate the possible role of the endocannabinoid system in renomedullary interstitial cells, mouse medullary interstitial cells (MMICs) were obtained, cultured, and characterized for their responses to treatment with a selective inhibitor of fatty acid amide hydrolase, PF-3845 (N-3-pyridinyl-4-[[3-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenyl]methyl]-1-piperidinecarboxamide). Treatment of MMICs with PF-3845 increased cytoplasmic lipid granules detected by Sudan Black B staining and multilamellar bodies identified by transmission electron microscopy. High-performance liquid chromatography (HPLC) analyses of lipid extracts of MMIC culture medium revealed a 205-nm absorbing peak that showed responsiveness to PF-3845 treatment. The biologic activities of the PF-3845-induced product (PIP) isolated by HPLC were investigated in anesthetized, normotensive surgically instrumented mice. Intramedullary and intravenous infusion of PIP at low dose rates (0.5-1 area units under the peak/10 min) stimulated diuresis and natriuresis, whereas these parameters returned toward baseline at higher doses but mean arterial pressure (MAP) was lowered. Whereas intravenous bolus doses of PIP stimulated diuresis, the glomerular filtration rate, and medullary blood flow (MBF) and reduced or had no effect on MAP, an intraperitoneal bolus injection of PIP reduced MAP, increased MBF, and had no effect on urine parameters. These data support a model whereby PF-3845 treatment of MMICs results in increased secretion of a neutral lipid that acts directly to promote diuresis and natriuresis and indirectly through metabolites to produce vasodepression. Efforts to identify the structure of the PF-3845-induced lipid and its relationship to the previously proposed renomedullary antihypertensive lipids are ongoing.

Regulation of hypoxia inducible factor/prolyl hydroxylase binding domain proteins 1 by PPARα and high salt diet

BACKGROUND

Hypoxia inducible factor (HIF)/prolyl hydroxylase domain (PHD)-containing proteins are involved in renal adaptive response to high salt (HS). Peroxisome proliferator activated receptor alpha (PPARα), a transcription factor involved in fatty acid oxidation is implicated in the regulation of renal function. As both HIF-1α/PHD and PPARα contribute to the adaptive changes to altered oxygen tension, this study tested the hypothesis that PHD-induced renal adaptive response to HS is PPARα-dependent.

METHODS

PPARα wild type (WT) and knock out (KO) mice were fed a low salt (LS) (0.03% NaCl) or a HS (8% NaCl) diet for 8 days and treated with hydralazine. PPARα and heme oxygenase (HO)-1 expression were evaluated in the kidney cortex and medulla. A 24-h urinary volume (UV), sodium excretion (UNaV), and nitrite excretion (UNOx V) were also determined.

RESULTS

PHD1 expression was greater in the medulla as compared to the cortex of PPARα WT mice (p<0.05) fed with a LS (0.03% NaCl) diet. The HS diet (8% NaCl) downregulated PHD1 expression in the medulla (p<0.05) but not the cortex of WT mice whereas expression was downregulated in the cortex (p<0.05) and medulla (p<0.05) of KO mice. These changes were accompanied by HS-induced diuresis (p<0.05) and natriuresis (p<0.05) that were greater in WT mice (p<0.05). Similarly, UNOx V, index of renal nitric oxide synthase (NOS) activity or availability and heme oxygenase (HO)-1 expression was greater in WT (p<0.05) but unchanged in KO mice on HS diet. Hydralazine, a PHD inhibitor, did not affect diuresis or natriuresis in LS diet-fed WT or KO mice but both were increased (p<0.05) in HS diet-fed WT mice. Hydralazine also increased UNOx V (p<0.05) with no change in diuresis, natriuresis, or HO-1 expression in KO mice on HS diet.

CONCLUSIONS

These data suggest that HS-induced PPARα-mediated downregulation of PHD1 is a novel pathway for PHD/HIF-1α transcriptional regulation for adaptive responses to promote renal function via downstream signaling involving NOS and HO.

Characterization and Activation of NLRP3 Inflammasomes in the Renal Medulla in Mice

BACKGROUND/AIMS

Recent studies have indicated that local inflammatory mediators are importantly involved in the regulation of renal function. However, it remains unknown how such local inflammation is triggered intracellularly in the kidney. The present study was designed to characterize the inflammasome centered by Nlrp3 in the kidney and also test the effect of its activation in the renal medulla.

METHODS AND RESULTS

By immunohistochemistry analysis, we found that inflammasome components, Nlrp3, Asc and caspase-1, were ubiquitously distributed in different kidney areas. The caspase-1 activity and IL-1β production were particularly high in the renal outer medulla compared to other kidney regions. Further confocal microscopy and RT-PCR analysis showed that Nlrp3, Asc and caspase-1 were particularly enriched in the thick ascending limb of Henle's loop. In anesthetized mice, medullary infusion of Nlrp3 inflammasome activator, monosodium urate (MSU), induced significant decreases in sodium excretion and medullary blood flow without changes in mean arterial blood pressure and renal cortical blood flow. Caspase-1 inhibitor, Ac-YVAD-CMK and deletion of Nlrp3 or Asc gene abolished MSU-induced decreases in renal sodium excretion and MBF.

CONCLUSION

Our results indicate that renal medullary Nlrp3 inflammasomes represent a new regulatory mechanism of renal MBF and sodium excretion which may not depend on classical inflammatory response.

Hypoxia: The Force that Drives Chronic Kidney Disease

In the United States the prevalence of end-stage renal disease (ESRD) reached epidemic proportions in 2012 with over 600,000 patients being treated. The rates of ESRD among the elderly are disproportionally high. Consequently, as life expectancy increases and the baby-boom generation reaches retirement age, the already heavy burden imposed by ESRD on the US health care system is set to increase dramatically. ESRD represents the terminal stage of chronic kidney disease (CKD). A large body of evidence indicating that CKD is driven by renal tissue hypoxia has led to the development of therapeutic strategies that increase kidney oxygenation and the contention that chronic hypoxia is the final common pathway to end-stage renal failure. Numerous studies have demonstrated that one of the most potent means by which hypoxic conditions within the kidney produce CKD is by inducing a sustained inflammatory attack by infiltrating leukocytes. Indispensable to this attack is the acquisition by leukocytes of an adhesive phenotype. It was thought that this process resulted exclusively from leukocytes responding to cytokines released from ischemic renal endothelium. However, recently it has been demonstrated that leukocytes also become activated independent of the hypoxic response of endothelial cells. It was found that this endothelium-independent mechanism involves leukocytes directly sensing hypoxia and responding by transcriptional induction of the genes that encode the β2-integrin family of adhesion molecules. This induction likely maintains the long-term inflammation by which hypoxia drives the pathogenesis of CKD. Consequently, targeting these transcriptional mechanisms would appear to represent a promising new therapeutic strategy.

Low glucose microenvironment of normal kidney cells stabilizes a subset of messengers involved in angiogenesis

As glucose is a mandatory nutrient for cell proliferation and renewal, it is suspected that glucose microenvironment is sensed by all cell types to regulate angiogenesis. Several glucose-sensing components have been partially described to respond to high glucose levels. However, little is known about the response to low glucose. Here, we used well-differentiated isolated normal rat renal tubules under normal oxygenation conditions to assess the angiogenic response to low glucose. In apparent paradox, but confirming observations made separately in other models, high glucose but also low glucose increased mRNA level of vascular endothelial growth factor A (VEGFA). A subset of mRNAs including hypoxia-inducible factor 1A (HIF1A), angiopoietin receptor (TIE-2), and VEGF receptor 2 (FLK1) were similarly glucose-sensitive and responded to low glucose by increased stability independently of HIF1A and HIF2A proteins. These results contribute to gain some insights as to how normal cells response to low glucose may play a role in the tumor microenvironment.

The requirement of c-Jun N-terminal kinase 2 in regulation of hypoxia-inducing factor-1α mRNA stability

The mRNA of hif-1α is considered as being constitutively and ubiquitously expressed, regardless of the level of oxygen tension. However many recent reports have showed that hif-1α mRNA could be regulated by natural antisense transcripts, potential microRNAs, and low O(2). In this study, it was found that a deficiency of JNK2 expression reduced HIF-1α protein induction in response to nickel treatment resulting from the impaired expression of hif-1α mRNA. Both the promoter luciferase assay and mRNA degradation assay clearly showed that depletion of JNK2 affected stability of hif-1α mRNA, rather than regulated its transcription. In addition, nucleolin, a classic histone chaperone, was demonstrated to physically bind to hif-1α mRNA and maintain its stability. Further investigation indicated that JNK2 regulated nucleolin expression and might in turn stabilize hif-1α mRNA. Collectively, we provided one more piece of evidence for the oncogenic role of JNK2 and nucleolin in regulating the cancer microenvironments by controlling HIF-1α expression.

Production and actions of the anandamide metabolite prostamide E2 in the renal medulla

Medullipin has been proposed to be an antihypertensive lipid hormone released from the renal medulla in response to increased arterial pressure and renal medullary blood flow. Because anandamide (AEA) possesses characteristics of this purported hormone, the present study tested the hypothesis that AEA or one of its metabolites represents medullipin. AEA was demonstrated to be enriched in the kidney medulla compared with cortex. Western blotting and enzymatic analyses of renal cortical and medullary microsomes revealed opposite patterns of enrichment of two AEA-metabolizing enzymes, with fatty acid amide hydrolase higher in the renal cortex and cyclooxygenase-2 (COX-2) higher in the renal medulla. In COX-2 reactions with renal medullary microsomes, prostamide E2, the ethanolamide of prostaglandin E₂, was the major product detected. Intramedullarily infused AEA dose-dependently increased urine volume and sodium and potassium excretion (15-60 nmol/kg/min) but had little effect on mean arterial pressure (MAP). The renal excretory effects of AEA were blocked by intravenous infusion of celecoxib (0.1 μg/kg/min), a selective COX-2 inhibitor, suggesting the involvement of a prostamide intermediate. Plasma kinetic analysis revealed longer elimination half-lives for AEA and prostamide E2 compared with prostaglandin E₂. Intravenous prostamide E2 reduced MAP and increased renal blood flow (RBF), actions opposite to those of angiotensin II. Coinfusion of prostamide E2 inhibited angiotensin II effects on MAP and RBF. These results suggest that AEA and/or its prostamide metabolites in the renal medulla may represent medullipin and function as a regulator of body fluid and MAP.

Association of filtered sodium load with medullary volumes and medullary hypoxia in hypertensive African Americans as compared with whites

BACKGROUND

African Americans develop hypertension earlier with more target manifestations than whites despite having a higher glomerular filtration rate (GFR) for any level of serum creatinine. STUDY DESIGN & PARTICIPANTS: This study tested the hypothesis that increased GFR and sodium reabsorption in African Americans is associated with increased metabolic work and medullary hypoxia in 49 nondiabetic patients with essential hypertension (29 whites and 20 African Americans) following a constant-sodium diet (150 mEq/d) and renin-angiotensin system blockade.

PREDICTORS

Ethnicity, age, measured GFR, sodium excretion, and body mass index.

OUTCOMES

We examined cortical and medullary volumes and blood flows using multidetector computed tomography and intrarenal deoxyhemoglobin (R2*) using blood oxygen level-dependent magnetic resonance.

RESULTS

Blood pressure and sodium excretion were similar, whereas African Americans were more obese and had higher iothalamate GFRs. Renal cortical volumes did not differ, but medullary volumes adjusted for body size and age were higher in African Americans (32.3 ± 11.2 vs 25.1 ± 7.4 cm(3)/m(2) body surface area; P < 0.001). Sodium reabsorption and blood flows were higher in African Americans. Basal cortical deoxyhemoglobin values were similar between ethnic groups, whereas medullary R2* was higher in African Americans (39.7 ± 5.1 vs 36.3 ± 6.5/s; P = 0.02), but decreased to levels similar to whites after furosemide treatment. Levels of the circulating isoprostane prostaglandin F(2α) were higher in African Americans and daily urinary prostaglandin F(2α) excretion in African Americans correlated directly with renal blood flow (R = 0.71; P < 0.01).

LIMITATIONS

Studies were limited to treated volunteers with normal kidney function without knowledge of prior nutrient intake.

CONCLUSIONS

These data show for the first time that increased sodium reabsorption in obese African American patients with hypertension was associated with enlarged medullary volumes, functional hypoxia related to solute reabsorption, and a direct relationship between blood flows and urinary isoprostane levels. Our results support a model of increased oxygen consumption and oxidative stress in African Americans that may accelerate hypertension and target-organ injury compared with white patients with essential hypertension.

A novel lipid natriuretic factor in the renal medulla: sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite formed by phosphorylation of sphingosine. S1P has been indicated to play a significant role in the cardiovascular system. It has been shown that the enzymes for S1P metabolism are expressed in the kidneys. The present study characterized the expression of S1P receptors in the kidneys and determined the role of S1P in the control of renal hemodynamics and sodium excretion. Real-time RT-PCR analyses showed that S1P receptors S1P1, S1P2, and S1P3 were most abundantly expressed in the renal medulla. Immunohistochemistry revealed that all three types of S1P receptors were mainly located in collecting ducts. Intramedullary infusion of FTY720, an S1P agonist, produced a dramatic increase in sodium excretion by twofold and a small but significant increase in medullary blood flow (16%). Administration of W146, an S1P1 antagonist, into the renal medulla blocked the effect of FTY720 and decreased the sodium excretion by 37% when infused alone. The antagonists of S1P2 and S1P3 had no effect. FTY720 produced additive natriuretic effects in combination with different sodium transporter inhibitors except amiloride, an epithelial sodium channel blocker. In the presence of nitric oxide synthase inhibitor l-NAME, FTY720 still increased sodium excretion. These data suggest that S1P produces natriuretic effects via activation of S1P1 in the renal medulla and this natriuretic effect may be through inhibition of epithelial sodium channel, which is nitric oxide independent. It is concluded that S1P is a novel diuretic factor in the renal medulla and may be an important regulator of sodium homeostasis.

Repetitive hypoxic preconditioning attenuates renal ischemia/reperfusion induced oxidative injury via upregulating HIF-1 alpha-dependent bcl-2 signaling

BACKGROUND

In response to ischemic/hypoxic preconditioning, tissues/organs exhibit protective responses to subsequent and severe ischemic stress. We hypothesized that repetitive hypoxic preconditioning (RHP) may provide long-lasting protection than single preconditioning against ischemia/reperfusion injury in rat kidneys through hypoxia-induced factor (HIF)-1-dependent pathway.

METHODS

For RHP induction, female Wistar rats were subjected to intermittent hypoxic exposure (380 Torr) 15 hr/day for 28 days.

RESULTS

RHP increased renal HIF-1 alpha mRNA and protein expression and triggered HIF-1 alpha-dependent renal Bcl-2 protein expression in a time-dependent manner. When returning to normoxia, increased RHP exposure prolonged renal Bcl-2 expression. Forty-five minutes of renal ischemia with 4 hr of reperfusion enhanced O2- levels and proapoptotic mechanisms, including enhanced cytosolic Bax translocation to mitochondria, release of cytochrome c to cytosol, activation of caspase 3, poly-(ADP-ribose)-polymerase fragments, tubular apoptosis, blood urea nitrogen, and creatinine level. RHP treatment depressed renal O2- production, mitochondrial Bax translocation and cytochrome c release, and tubular apoptosis. In the primary tubular cultures from RHP-treated kidneys, antisense oligodeoxyribonucleotides of bcl-2 abrogated this protection.

CONCLUSIONS

RHP activates an HIF-1 alpha-dependent signaling cascade leading to an increase in Bcl-2 protein expression, an inhibition in cytosolic Bax and mitochondrial cytochrome c translocation, and a hypoxic/ischemia tolerance against renal ischemia/reperfusion injury.

Hypoxia-Inducible Factor in Ringed Seal (Phoca hispida) Tissues

Tissue hypoxia and ischemia-reperfusion pose a dangerous situation for oxidative stress. However, diving mammals and birds show pronounced resistance to oxidative injury under such conditions, which are a consequence of selective vasoconstriction during a dive. As the function of Hypoxia-Inducible Factor-1alpha (HIF-1alpha) in protection against and adaptation to hypoxia has been recognized in terrestrial animals, we have investigated the genomics and expression of this protein in ringed seal (Phoca hispida) in order to determine if it may play a protective role in this diving mammal. PCR studies using primers based on sequences from mouse HIF-1alpha exons 3, 4, 5, 6, 9, 10, 11, 12 and 15 showed that DNA from seal lung generated PCR products similar to those from mouse DNA. These studies have established that a putative HIF-1alpha gene exists in the seal genome that appears to have a similar but not identical sequence to the mouse gene. Seal lung and skeletal muscle tissues showed the highest relative levels of HIF-1alpha protein expression, with heart muscle showing significantly lower levels, and levels of HIF-1beta protein expression paralleled this situation. Analysis of oxidized cellular protein levels indicated that seal lung and heart muscle had the lowest levels of oxidized proteins. Thus, as seal lung tissue had the highest level of HIF-1alpha protein expression and the second lowest level of protein oxidation, this suggests that HIF-1alpha expression may have an important protective effect in this tissue in diving mammals. Our results support the hypothesis that HIF-1alpha expression is dependent on both tissue-specific energy requirements and adequate metabolic supply-to-demand ratio. Combined, the evidence available suggests that diving mammals have an overall anticipatory response to avoid the ill effects of dive-associated ischemia-reperfusion which may involve the HIF-1 system.

p53 regulates renal expression of HIF-1{alpha} and pVHL under physiological conditions and after ischemia-reperfusion injury

Ischemia-reperfusion injury (IRI) is a common cause of acute kidney injury (AKI) and is characterized by widespread tubular and microvascular damage. The tumor suppressor p53 is upregulated after IRI and contributes to renal injury in part by promoting apoptosis. Acute, short-term inhibition of p53 with pifithrin-alpha conveys significant protection after IRI. The hypoxia-inducible factor-1 (HIF-1) pathway is also activated after IRI and has opposing effects to those promoted by p53. The balance between the HIF-1 and p53 responses can determine the outcome of IRI. In this manuscript, we investigate whether p53 regulates the HIF-1 pathway in a rodent model of IRI. HIF-1alpha is principally expressed in the collecting tubules (CT) and thick ascending limbs (TAL) under physiological conditions. However, inhibition of p53 with pifithrin-alpha increases the faint expression of HIF-1alpha in proximal tubules (PT) under physiological conditions. Twenty-four hours after IRI, HIF-1alpha expression is decreased in both CT and TAL. HIF-1alpha expression in the PT is not significantly altered after IRI. Acute inhibition of p53 significantly increases HIF-1alpha expression in the PT after IRI. Additionally, pifithrin-alpha prevents the IRI-induced decrease in HIF-1alpha in the CT and TAL. Parallel changes are observed in the HIF-1alpha transcriptive target, carbonic anhydrase-9. Finally, inhibition of p53 prevents the dramatic changes in Von Hippel-Lindau protein morphology and expression after IRI. We conclude that activation of p53 after IRI mitigates the concomitant activation of the protective HIF-1 pathway. Modulating the interactions between the p53 and HIF-1 pathway can provide novel options in the treatment of AKI.

Acellular haemoglobin attenuates hypoxia-inducible factor-1α (HIF-1α) and its target genes in haemodiluted rats

Hb (haemoglobin)-based blood substitutes represent a class of therapeutics designed to correct oxygen deficit under conditions of anaemia and traumatic blood loss. The influences of these agents on HIF-1α (hypoxia-inducible factor-1α) target genes involved in adaptation to hypoxia have so far not been studied. In the study presented here, rats underwent 80% ET (exchange transfusion) with either HS (hetastarch) or a polymerized Hb OG (Oxyglobin®). HS induced dramatic EPO (erythropoietin) gene transcription, reaching a maximum at 4 h post-ET. In contrast, OG suppressed EPO transcription until approx. 24 h post-ET. Large plasma EPO levels that were observed post-ET with HS were significantly blunted in animals transfused with OG. OG, unlike HS, induced a sharp increase in HO-1 (haem oxygenase-1) transcription at 4 h, which declined rapidly within 24 h, whereas modest increases in iNOS [inducible (nitric oxide synthase)] and constitutive NOS [eNOS (endothelial NOS)] were detected over the control. Our results demonstrate for the first time that severe haemodilution-induced erythropoietic responses in kidneys were attenuated by a low-oxygen-affinity cell-free Hb and suggest that tissue-specific oxygen-sensing pathways can be influenced by allosterically modified Hbs.

Salt-sensitive hypertension induced by decoy of transcription factor hypoxia-inducible factor-1alpha in the renal medulla

Hypoxia inducible factor (HIF)-1alpha, a transcription factor, is abundantly expressed in the renal medulla and regulates many oxygen-sensitive genes such as nitric oxide synthase, cyclooxygenase-2, and heme oxygenase-1. Given the important roles of these genes in the control of arterial pressure, the present study was to test the hypothesis that HIF-1alpha-mediated gene activation serves as an antihypertensive pathway by regulating renal medullary function and sodium excretion. HIF-1alpha decoy oligodeoxynucleotides (ODNs) or scrambled ODNs were transfected into the renal medulla in uninephrectomized Sprague-Dawley rats. Two weeks after ODN transfection, the HIF-1alpha binding activities were significantly inhibited by 45%, and high salt-induced increases of nitric oxide synthase-2 and heme oxygenase-1 transcriptions were also inhibited by 70% and 61% in the renal medulla from decoy rats. The natriuretic responses and increases of renal medullary blood flow responding to the elevations of renal perfusion pressure were significantly blunted by 50% and 37% in decoy rats. Intravenously acute sodium loading increased medullary blood flow and urinary sodium excretion, which was remarkably attenuated in decoy rats. In decoy rats, high salt intake caused a greater positive sodium balance. Consequently, arterial pressure was remarkably increased (from 118+/-1.9 to 154+/-6.3 mm Hg) in decoy rats but not in control rats when the rats were challenged with a high salt diet. There was no blood pressure change in decoy rats that were maintained in normal salt diet. In conclusion, HIF-1alpha-mediated gene activation importantly participates in the regulation of renal medullary function and long-term arterial blood pressure.

Experimental strategies to improve in vitro models of renal ischemia

Ischemia has elicited a great deal of interest among the scientific community due to its role in life-threatening pathologies such as cancer, stroke, acute renal failure, and myocardial infarction. Oxygen deprivation (hypoxia) associated with ischemia has recently become a subject of intense scrutiny. New investigators may find it challenging to induce hypoxic injury in vitro. Researchers may not always be aware of the experimental barriers that contribute to this phenomenon. Furthermore, ischemia is associated with other major insults, such as excess carbon dioxide (hypercapnia), nutrient deprivation, and accumulation of cellular wastes. Ideally, these conditions should also be incorporated into in vitro models. Therefore, the motivation behind this review is to: i. delineate major in vivo ischemic insults; ii. identify and explain critical in vitro parameters that need to be considered when simulating ischemic pathologies; iii. provide recommendations to improve experiments; and as a result, iv. enhance the validity of in vitro results for understanding clinical ischemic pathologies. Undoubtedly, it is not possible to completely replicate the in vivo environment in an ex vivo model system. In fact, the primary goal of many in vitro studies is to elucidate the role of specific stimuli during in vivo pathological events. This review will present methodologies that may be implemented to improve the applicability of in vitro models for understanding the complex pathological mechanisms of ischemia. Finally, although these topics will be discussed within the context of renal ischemia, many are pertinent for cellular models of other organ systems and pathologies.

Modulation of mRNA stability as a novel therapeutic approach

During the last decade evidence has accumulated that modulation of mRNA stability plays a central role in cellular homeostasis, including cell differentiation, proliferation and adaptation to external stimuli. The functional relevance of posttranscriptional gene regulation is highlighted by many pathologies, wherein occurrence tightly correlates with a dysregulation in mRNA stability, including chronic inflammation, cardiovascular diseases and cancer. Most commonly, the cis-regulatory elements of mRNA decay are represented by the adenylate- and uridylate (AU)-rich elements (ARE) which are specifically bound by trans-acting RNA binding proteins, which finally determine whether mRNA decay is delayed or facilitated. Regulation of mRNA decay by RNA stabilizing and RNA destabilizing factors is furthermore controlled by different intrinsic and environmental stimuli. The modulation of mRNA binding proteins, therefore, illuminates a promising approach for the pharmacotherapy of those key pathologies mentioned above and characterized by a posttranscriptional dysregulation. Most promisingly, intracellular trafficking of many of the mRNA stability regulating factors is, in turn, regulated by some major signaling pathways, including the mitogen-activated protein kinase (MAPK) cascade, the AMP-activated kinase (AMPK) and the protein kinase (PK) C (PKC) family. In this review, we present timely examples of genes regulated by mRNA stability with a special focus on signaling pathways involved in the ARE-dependent mRNA decay. A better understanding of these processes may form the basis for the development of novel therapeutics to treat major human diseases.

Role of Renal Medullary Heme Oxygenase in the Regulation of Pressure Natriuresis and Arterial Blood Pressure

Recent studies have demonstrated that inhibition of renal medullary heme oxygenase (HO) activity and carbon monoxide (CO) significantly decreases renal medullary blood flow and sodium excretion. Given the crucial role of renal medullary blood flow in the control of pressure natriuresis, the present study was designed to determine whether renal medullary HO activity and resulting CO production participate in the regulation of pressure natriuresis and thereby the long-term control of arterial blood pressure. In anesthetized Sprague-Dawley rats, increases in renal perfusion pressure induced significant elevations of CO concentrations in the renal medulla. Renal medullary infusion of chromium mesoporphyrin (CrMP), an inhibitor of HO activity, remarkably inhibited HO activity and the renal perfusion pressure-dependent increases in CO levels in the renal medulla and significantly blunted pressure natriuresis. In conscious Sprague-Dawley rats, continuous infusion of CrMP into the renal medulla significantly increased mean arterial pressure (129±2.5 mm Hg in CrMP group versus 118±1.6 mm Hg in vehicle group) when animals were fed a normal salt diet (1% NaCl). After rats were switched to a high-salt diet (8% NaCl) for 10 days, CrMP-treated animals exhibited further increases in mean arterial pressure compared with CrMP-treated animals that were kept on normal salt diet (152±4.1 versus 130±4.2 mm Hg). These results suggest that renal medullary HO activity plays a crucial role in the control of pressure natriuresis and arterial blood pressure and that impairment of this HO/CO-mediated antihypertensive mechanism in the renal medulla may result in the development of hypertension.

Physiological oxygenation status is required for fully differentiated phenotype in kidney cortex proximal tubules

Hypoxia has been suspected to trigger transdifferentiation of renal tubular cells into myofibroblasts in an epithelial-to-mesenchymal transition (EMT) process. To determine the functional networks potentially altered by hypoxia, rat renal tubule suspensions were incubated under three conditions of oxygenation ranging from normoxia (lactate uptake) to severe hypoxia (lactate production). Transcriptome changes after 4 h were analyzed on a high scale by restriction fragment differential display. Among 1,533 transcripts found, 42% were maximally expressed under severe hypoxia and 8% under mild hypoxia (Po2 = 48 mmHg), suggesting two different levels of oxygen sensing. Normoxia was required for full expression of the proximal tubule-specific transcripts 25-hydroxyvitamin D 1-hydroxylase ( Cyp27b1) and l-pyruvate kinase ( Pklr), transcripts involved in tissue cohesion such as fibronectin ( Fn1) and N-cadherin ( Cdh2), and non-muscle-type myosin transcripts. Mild hypoxia increased myogenin transcript level. Conversely, severe hypoxia increased transcripts involved in extracellular matrix remodeling, those of muscle-type myosins, and others involved in creatine phosphate synthesis and lactate transport ( Slc16a7). Accordingly, microscopy showed loss of tubule aggregation under hypoxia, without tubular disruption. Hypoxia also increased the levels of kidney-specific transcripts normally restricted to the less oxygenated medullary zone and others specific for the distal part of the nephron. We conclude that extensive oxygen supply to the kidney tubule favors expression of its differentiated functions specifically in the proximal tubule, whose embryonic origin is mesenchymal. The phenotype changes could potentially permit transient adaptation to hypoxia but also favor pathological processes such as tissue invasion.

Expression and actions of HIF prolyl-4-hydroxylase in the rat kidneys

Hypoxia inducible factor (HIF) prolyl-4-hydroxylase domain-containing proteins (PHDs) promote the degradation of HIF-1alpha. Because HIF-1alpha is highly expressed in the renal medulla and HIF-1alpha-targeted genes such as nitric oxide synthase, cyclooxygenase, and heme oxygenase are important in the regulation of renal medullary function, we hypothesized that PHD regulates HIF-1alpha levels in the renal medulla and, thereby, participates in the control of renal Na(+) excretion. Using real-time RT-PCR, Western blot, and immunohistochemical analyses, we have demonstrated that all three isoforms of PHD, PHD1, PHD2, and PHD3, are expressed in the kidneys and that PHD2 is the most abundant isoform. Regionally, all PHDs exhibited much higher levels in renal medulla than cortex. A furosemide-induced increase in renal medullary tissue Po(2) significantly decreased PHD levels in renal medulla, whereas hypoxia significantly increased mRNA levels of PHDs in cultured renal medullary interstitial cells, indicating that O(2) regulates PHDs. Functionally, the PHD inhibitor l-mimosine (l-Mim, 50 mg x kg(-1) x day(-1) i.p. for 2 wk) substantially upregulated HIF-1alpha expression in the kidneys, especially in the renal medulla, and remarkably enhanced (by >80%) the natriuretic response to renal perfusion pressure in Sprague-Dawley rats. Inhibition of HIF transcriptional activity by renal medullary transfection of HIF-1alpha decoy oligodeoxynucleotides attenuated l-Mim-induced enhancement of pressure natriuresis, which confirmed that HIF-1alpha mediated the effect of l-Mim. These results indicate that highly expressed PHDs in the renal medulla make an important contribution to the control of renal Na(+) excretion through regulation of HIF-1alpha and its targeted genes.

Survival in Hostile Environments: Strategies of Renal Medullary Cells

Cells in the renal medulla exist in a hostile milieu characterized by wide variations in extracellular solute concentrations, low oxygen tensions, and abundant reactive oxygen species. This article reviews the strategies adopted by these cells to allow them to survive and fulfill their functions under these extreme conditions.

Current understanding of HIF in renal disease

Hypoxia-inducible factors (HIF) are ubiquitous transcription factors regulated by oxygen-dependent proteolysis, and hence rapidly mount an adaptational response to hypoxia. The HIF system is apparently more complex than initially considered in the perspective of the increasing number of HIF target genes, and the inter-relationship with various additional regulatory pathways. Regional hypoxia is believed to play a major role in renal disease. Experimental data confirm a role for HIF in renal pathophysiology. The discovery of HIF prolyl-hydroxylases as key enzymes of oxygen sensing and HIF proteolysis offer new possibilities to therapeutically target HIF. Herein, we review basic concepts of HIF regulation, and existing data on HIF activation in renal disease.

A biologic role of HIF-1 in the renal medulla

BACKGROUND

Activation of hypoxia-inducible factor-1 (HIF-1) is the primary defensive mechanism against hypoxia. HIF-1 activation generally occurs in pathologic disruption of tissue oxygenation. However, a biologic role of HIF-1 in the medulla of the kidney, which is considered perpetually hypoxic under physiologic conditions due to its unique circulation, remains to be elucidated.

METHODS

The expression of HIF-1alpha was detected by immunohistochemical analysis. Functional studies of HIF in medulla were carried out by gene transfer of various plasmids by retrograde injection via ureter.

RESULTS

Our immunohistochemical analysis detected HIF-1alpha in the inner stripe and the inner medulla of normal rats. Water deprivation increased the number of HIF-1alpha-positive cells, which may be mediated by an increase in medullar workload and a decrease in local blood flow. To perform functional studies, we performed gene transfer. Efficient expression of the transgene was confirmed using an enhanced green fluorescent protein (E-GFP) expressing vector. Our histologic and immunoblotting analysis detected the transgene product at the inner medulla and the inner stripe 48 hours after injection. Administration of negative-dominant HIF induced severe damage in the medulla of normal rats. In contrast, gene transfer of constitutively active HIF (HIF/VP16) induced expression of various HIF-regulated genes and protected the medulla against ischemic insults.

CONCLUSION

Our studies demonstrated a crucial role of HIF in the renal medulla under normal and hypoxic circumstances.

Cloning and characterization of the rat HIF-1α prolyl-4-hydroxylase-1 gene

Prolyl-4-hydroxylase domain-containing enzymes (PHDs) mediate the oxygen-dependent regulation of the heterodimeric transcription factor hypoxia-inducible factor-1 (HIF-1). Under normoxic conditions, one of the subunits of HIF-1, HIF-1alpha, is hydroxylated on specific proline residues to target HIF-1alpha for degradation by the ubiquitin-proteasome pathway. Under hypoxic conditions, the hydroxylation by the PHDs is attenuated by lack of the oxygen substrate, allowing HIF-1 to accumulate, translocate to the nucleus, and mediate HIF-mediated gene transcription. In several mammalian species including humans, three PHDs have been identified. We report here the cloning of a full-length rat cDNA that is highly homologous to the human and murine PHD-1 enzymes and encodes a protein that is 416 amino acids long. Both cDNA and protein are widely expressed in rat tissues and cell types. We demonstrate that purified and crude baculovirus-expressed rat PHD-1 exhibits HIF-1alpha specific prolyl hydroxylase activity with similar substrate affinities and is comparable to human PHD-1 protein.

Two cases of renovascular hypertension and ischemic renal dysfunction: reliable choice of examinations and treatments

We experienced two aged patients with atherosclerotic renovascular stenosis associated with hypertension and ischemic nephropathy. Both patients exhibited sudden rise in blood pressure (BP) and progressive aggravation of renal dysfunction. In these patients, the use of contrast medium to screen for renal artery stenosis (RAS) ran the risk of further deterioration of renal function. We therefore used magnetic resonance angiography (MRA), which is less conducive to renal damage, to screen for RAS. One-sided RAS was treated by percutaneous transluminal angioplasty of the renal artery (PTRA) and stenting. As a result, BP decreased in both patients. Serum creatinine (Cr) decreased slightly in one patient, whereas, in the other, serum Cr increased transiently and then decreased and stabilized to pre-treatment levels. Thus, although it is unclear whether the combination of PTRA and stenting is among the best treatments for patients with RAS and moderate-to-severe renal dysfunction, PTRA and stenting are clearly of benefit in selected patients. In addition, recent progress in characterizing the pathophysiology of ischemic nephropathy associated with renovascular hypertension has created interest in the therapeutic potential of angiotensin II receptor antagonists, sympatholytic agents, and antioxidants. Therefore, we discuss the therapeutic utility of PTRA and stenting and the above-mentioned medications in patients with RAS and renal dysfunction.

Mechanism of hypertensive nephropathy in the Dahl/Rapp rat: a primary disorder of vascular smooth muscle

The Dahl/Rapp salt-sensitive (S) rat is a model of salt-sensitive hypertension and hypertensive renal disease. This study explored the role of vascular remodeling in the development of renal failure in S rats. Groups of S and Sprague-Dawley rats were given 0.3 and 8.0% NaCl diets for up to 21 days and evidence of smooth muscle proliferation identified using immunohistochemistry that showed nuclear accumulation of proliferating cell nuclear antigen and 5-bromo-2′-deoxy-uridine. Compared with the other three groups, S rats on 8.0% NaCl diet showed increased nuclear labeling of cells of the aorta and arteries and arterioles of the kidney by the end of the first week of study. Progressive luminal narrowing of the interlobular arteries and preglomerular arterioles occurred in S rats over the 3 wk on the 8.0% NaCl diet. Accumulation of pimonidazole adducts and nuclear accumulation of hypoxia-inducible factor-1α (HIF-1α) were used as markers of tissue hypoxia. By the end of the second week of study, pimonidazole levels increased in S rats on 8.0% NaCl diet and deposition was apparent in tubular cells in the cortex and medulla. At the completion of the experiment, HIF-1α levels were increased in nuclear extracts from the cortex and medulla of S rats on this diet, compared with the other three groups of rats. The data demonstrated a disorder of the vascular remodeling process with proliferation of vascular smooth muscle cells temporally followed by development of tissue hypoxia in the hypertensive nephropathy of S rats on 8.0% NaCl diet.

Natural antisense transcripts of HIF-1alpha are conserved in rodents

A natural antisense transcript (aHIF), which sequence is strictly complementary to the 3' untranslated region (3'UTR) of HIF-1alpha mRNA, has been identified in human and shown to be overexpressed in renal carcinomas. We searched for aHIF in different rodent tissues. Two candidate expressed sequence tag (EST) were identified in silico and their PCR products (1.1 and 1.0 kb) were cloned and sequenced in mouse and rat, respectively. These transcripts were rigorously complementary to the 3'UTR of rodent HIF-1alpha mRNA and were broadly expressed in all mouse and rat tissues we tested. The conservation of aHIF in rodents underlined its potential importance in cell regulations. Therefore the responses of aHIF and HIF-1alpha transcripts were investigated in various types of hypoxic conditions. In freshly isolated rat renal tubules, aHIF RNA level was increased by acute hypoxia and low in normal supply of oxygen. In a rat strain raised in chronic hypobaric altitude hypoxia, aHIF transcript was greatly induced in the oxidative-type soleus and heart muscles of 3 month-old animals. By contrast, in the glycolytic-type extensor digitorum longus muscle aHIF transcript amount was lowered by hypoxia whereas HIF-1alpha transcript was highly expressed. In brain, where oxidative glycolysis takes place, HIF-1alpha mRNA and its antisense transcript levels were high and not significantly changed by altitude. Tumour cell lines cultured for 6 h in conditions mimicking hypoxia expressed lower amounts of HIF-1alpha mRNA. In two rat cell lines, aHIF transcript levels were greatly augmented after a 6-h incubation in these conditions, whereas in a mouse cell line, aHIF level was significantly reduced.

Effect of sepsis on skeletal muscle oxygen consumption and tissue oxygenation: interpreting capillary oxygen transport data using a mathematical model

Inherent in the inflammatory response to sepsis is abnormal microvascular perfusion. Maldistribution of capillary red blood cell (RBC) flow in rat skeletal muscle has been characterized by increased 1) stopped-flow capillaries, 2) capillary oxygen extraction, and 3) ratio of fast-flow to normal-flow capillaries. On the basis of experimental data for functional capillary density (FCD), RBC velocity, and hemoglobin O2 saturation during sepsis, a mathematical model was used to calculate tissue O2 consumption (V̇o2), tissue Po2 (Pt) profiles, and O2 delivery by fast-flow capillaries, which could not be measured experimentally. The model describes coupled capillary and tissue O2 transport using realistic blood and tissue biophysics and three-dimensional arrays of heterogeneously spaced capillaries and was solved numerically using a previously validated scheme. While total blood flow was maintained, capillary flow distribution was varied from 60/30/10% (normal/fast/stopped) in control to 33/33/33% (normal/fast/stopped) in average sepsis (AS) and 25/25/50% (normal/fast/stopped) in extreme sepsis (ES). Simulations found approximately two- and fourfold increases in tissue V̇o2 in AS and ES, respectively. Average (minimum) Pt decreased from 43 ( 40 ) mmHg in control to 34 ( 27 ) and 26 ( 15 ) mmHg in AS and ES, respectively, and clustering fast-flow capillaries (increased flow heterogeneity) reduced minimum Pt to 14.5 mmHg. Thus, although fast capillaries prevented tissue dysoxia, they did not prevent increased hypoxia as the degree of microvascular injury increased. The model predicts that decreased FCD, increased fast flow, and increased V̇o2 in sepsis expose skeletal muscle to significant regions of hypoxia, which could affect local cellular and organ function.

Androgens regulate the binding of endogenous HuR to the AU-rich 3'UTRs of HIF-1alpha and EGF mRNA

The 3'UTRs of mammalian HIF-1alpha and EGF mRNA contain several highly conserved AU-rich elements (ARE) known to control the turnover of labile mRNAs by binding ARE-binding proteins that regulate nucleocytoplasmic shuttling, translation, and degradation. Androgens regulate the level and subcellular shuttling of HuR, a major ARE-binding protein that stabilizes many ARE-mRNAs. Pull down of biotinylated 3'UTRs of HIF-1alpha or EGF enriches HuR on blots from Jurkat cell lysates 5-fold, and enriches the amount of RNase-protected biotinylated RNA that comigrates with HuR approximately 10-fold. Dihydrotestosterone treatment decreases the HuR-protected riboprobe pulled down from total Jurkat cell lysates by 30-40%, apparently reflecting shifts in HuR from the nucleus to the cytoplasm. Androgen treatment also changes the amount of HuR-protected riboprobe pulled down from a PC-3 clone expressing a functional androgen receptor. The shift in the amount of riboprobe bound by HuR suggests that androgen is up-regulating endogenous ARE-mRNAs that can compete for binding endogenous HuR. These changes in the shuttling and ARE-binding of endogenous HuR indicate that androgen can act posttranscriptionally to regulate ARE-mRNAs, including HIF-1alpha and EGF.

Cortical microvascular remodeling in the stenotic kidney: role of increased oxidative stress

OBJECTIVE

Mechanisms of renal injury distal to renal artery stenosis (RAS) remain unclear. We tested the hypothesis that it involves microvascular remodeling consequent to increased oxidative stress.

METHODS AND RESULTS

Three groups of pigs (n=6 each) were studied after 12 weeks of RAS, RAS+antioxidant supplementation (100 IU/kg vitamin E and 1 g vitamin C daily), or controls. The spatial density and tortuousity of renal microvessels (<500 microm) were tomographically determined by 3D microcomputed tomography. The in situ production of superoxide anion and the expression of vascular endothelial growth factor (VEGF), its receptor VEGFR-2, hypoxia-inducible-factor (HIF)-1alpha, von Hippel-Lindau (VHL) protein, and NAD(P)H oxidase (p47phox and p67phox subunits) were determined in cortical tissue. RAS and RAS+antioxidant groups had similar degrees of stenosis and hypertension. The RAS group showed a decrease in spatial density of cortical microvessels, which was normalized in the RAS+antioxidant group, as was arteriolar tortuousity. RAS kidneys also showed tissue fibrosis (by trichrome and Sirius red staining), increased superoxide anion abundance, NAD(P)H oxidase, VHL protein, and HIF-1alpha mRNA expression. In contrast, expression of HIF-1alpha, VEGF, and VEGFR-2 protein was downregulated. These were all significantly improved by antioxidant intervention.

CONCLUSIONS

Increased oxidative stress in the stenotic kidney alters growth factor activity and plays an important role in renal microvascular remodeling, which can be prevented by chronic antioxidant intervention.

Expression of adrenomedullin in hypoxic and ischemic rat kidneys and human kidneys with arterial stenosis

To investigate regional aspects of hypoxic regulation of adrenomedullin (AM) in kidneys, we mapped the distribution of AM in the rat kidney after hypoxia (normobaric hypoxic hypoxia, carbon monoxide, and CoCl2 for 6 h), anemia (hematocrit lowered by bleeding) and after global transient ischemia for 1 h (unilateral renal artery occlusion and reperfusion for 6 and 24 h) and segmental infarct (6 and 24 h). AM expression and localization was determined in normal human kidneys and in kidneys with arterial stenosis. Hypoxia stimulated AM mRNA expression significantly in rat inner medulla (CO 13 times, 8% O2 6 times, and CoCl2 8 times), followed by the outer medulla and cortex. AM mRNA level was significantly elevated in response to anemia and occlusion-reperfusion. Immunoreactive AM was associated with the thin limbs of Henle's loop, distal convoluted tubule, collecting ducts, papilla surface epithelium, and urothelium. AM labeling was prominent in the inner medulla after CO and in the outer medulla after occlusion-reperfusion. The infarct border zone was strongly labeled for AM. In cultured inner medullary collecting duct cells, AM mRNA was significantly increased by hypoxia. AM mRNA was equally distributed in human kidney and AM was localized as in the rat kidney. In human kidneys with artery stenosis, AM mRNA was not significantly enhanced compared with controls, but AM immunoreactivity was observed in tubules, vessels, and glomerular cells. In summary, AM expression was increased in the rat kidney in response to hypoxic and ischemic hypoxia in keeping with oxygen gradients. AM was widely distributed in the human kidney with arterial stenosis. AM may play a significant role to counteract hypoxia in the kidney.

Expression and Localization of Hypoxia-Inducible Factor-1 Subunits in the Adult Rat Epididymis1

The epididymal epithelium contributes to formation of a luminal fluid that is essential for the protection of spermatozoa from a variety of insults including changes in oxygen tension. A key regulator of the response to oxygen debt in many cells is hypoxia-inducible factor-1 (HIF-1). A transcription factor composed of alpha and beta subunits, HIF-1 activates genes that mediate oxygen homeostasis and cell survival pathways or trigger cell death responses. Previously we have shown that HIF-1alpha mRNA is expressed in the adult rat epididymis. Goals of this study were to determine whether HIF-1alpha protein is activated by ischemia in the rat epididymis, to determine whether epididymal HIF-1alpha mRNA expression is androgen dependent, and to identify epididymal cell types expressing HIF-1alpha and beta. Immunoblot analysis revealed that HIF-1alpha protein is primarily present in corpus and cauda of the normoxic epididymis and unaffected by ischemia, whereas HIF-1beta was detected equally in all regions and also unaffected by ischemia. HIF-1alpha mRNA expression in all regions was not affected by 15 days bilateral orchiectomy. Principal cells stained positive for HIF-1alpha by immunocytochemistry, with the epithelium of initial segment and caput epididymidis staining less intensely than corpus and cauda. HIF-1beta immunoreactivity was equally present in principal cells in all regions. Clear, narrow, and basal cells were unreactive for HIF-1alpha and beta. The presence of HIF-1 in normoxic epididymis and the regional distribution of HIF-1alpha suggests fundamental differences in how proximal and distal regions of the epididymis maintain oxygen homeostasis to protect the epithelium and spermatozoa from hypoxia.

Redox regulation of the afferent arteriole and tubuloglomerular feedback

Oxidative stress implies an increased production of reactive oxygen species (ROS) or a decreased capacity to metabolize them. Superoxide anion (O) can bioinactivate nitric oxide (NO). Therefore, many effects of ROS are manifest as NO deficiency. The afferent arteriole and macula densa cell both contain a full complement of components of nicotine adenine dinucleotide phosphate (NADPH) oxidase that generates O. Nitric oxide synthase (NOS) type 1 or neuronal NOS (nNOS) is expressed in the macula densa and NOS type II or endothelial NOS (eNOS) in the afferent arteriole. Whole animal studies in models of hypertension and oxidative stress demonstrate that metabolism of O by a superoxide dismutase (SOD) mimetic can reduce renal vascular resistance. In vivo studies of single nephron function and in vitro studies with the double-perfused juxtaglomerular apparatus preparation have shown extensive interaction between O and NO in macula densa to regulate afferent arteriolar tone mediated by the tubuloglomerular feedback response. In vitro studies of rabbits isolated, perfused afferent arterioles have shown a similar interaction in this vessel. These data indicate important roles for O in the macula densa and afferent arterioles to enhance preglomerular resistance in animal models of oxidative stress. As an increase in afferent arteriolar resistance can precede hypertension, oxidative stress could be important in determining the long-term blood pressure and thereby contribute to hypertension.

Peritubular capillary loss after mouse acute nephrotoxicity correlates with down-regulation of vascular endothelial growth factor-A and hypoxia-inducible factor-1 alpha

Although the response of kidneys acutely damaged by ischemia or toxins is dominated by epithelial destruction and regeneration, other studies have begun to define abnormalities in the cell biology of the renal microcirculation, especially with regard to peritubular capillaries. We explored the integrity of peritubular capillaries in relation to expression of vascular endothelial growth factor (VEGF)-A, hypoxia-inducible factor (HIF)-alpha proteins, and von Hippel-Lindau protein (pVHL) in mouse folic acid nephropathy, a model in which acute tubular damage is followed by partial regeneration and progression to patchy chronic histological damage. Throughout a period of 14 days, in areas of cortical tubular atrophy and interstitial fibrosis, loss of VEGFR-2 and platelet endothelial cell adhesion molecule-expressing peritubular capillaries was preceded by marked decreases in VEGF-A transcript and protein levels. Nephrotoxicity was associated with tissue hypoxia, especially in regenerating tubules, as assessed by an established in situ method. Despite the hypoxia, levels of HIF-1 alpha, a protein known to up-regulate VEGF-A, were reduced. During the course of nephrotoxicity, levels of pVHL, a factor that destabilizes HIF-1 alpha, increased significantly. We speculate that that down-regulation of VEGF-A may be functionally-implicated in the progressive attrition of peritubular capillaries in areas of tubular atrophy and interstitial fibrosis; VEGF-A down-regulation correlates with a loss of HIF-1 alpha expression which itself occurs in the face of increased tissue hypoxia.

Reactive oxygen species and molecular regulation of renal oxygenation

It has been known since the 1940s that a gradient of renal oxygenation exists in the kidney with the lowest PO2 in the renal inner medulla under physiological conditions. Due to a low PO2 milieu in the renal medulla, the cells in this region are at constant risk of hypoxic injury. Although numerous studies have shown that renal medullary cells adapt well to low PO2, the precise mechanism mediating this adaptive response remains poorly understood. Recently, hypoxia-induced molecular adaptation in mammalian tissues or cells has been studied extensively and many studies have indicated that the molecular regulation of gene expression is importantly involved. This paper focuses on the role of a transcription factor, hypoxia-inducible factor-1 (HIF-1)-mediated molecular adaptation and explores the physiological relevance of molecular activation of HIF-1 and its target genes in the renal medulla. Given that this HIF-1-mediated action is associated with local redox status, evidence is presented to indicate that reactive oxygen species (ROS), especially superoxide (O) is importantly involved in HIF-1-mediated molecular adaptation in renal medullary cells. O degrades HIF-1alpha, an HIF-1 subunit, by activating ubiquitin-proteasome and thereby decreases the transcriptional activation of many oxygen-sensitive genes. This action of O disturbs renal medullary adaptation to low PO2 and produces renal medullary dysfunction, resulting in sodium retention and hypertension. This report also provides evidence indicating the primary source of O, enzymatic pathways for O production and activating mechanism of O production in the kidney. It is concluded that HIF-1-mediated molecular adaptation to low PO2 is of importance in the regulation of renal medullary function and that ROS may target this HIF-1-mediated medullary adaptation to damage renal function.

Hypoxia rapidly activates HIF-3alpha mRNA expression

The role of the hypoxia-inducible factor (HIF) subunits 1alpha and 1beta in cellular response to hypoxia is well established, whereas little is known about HIF-2alpha and HIF-3alpha with respect to organ distribution and transcriptional regulation by hypoxia. We investigated mRNA levels of all HIF subunits and of their target genes erythropoietin (EPO) and glucose-transporter 1 (GLUT1) in rats undergoing systemic hypoxia for 30 or 120 min by quantitative real-time RT-PCR. In normoxia, persistently high mRNA levels of all HIF subunits were detected in cerebral cortex, hippocampus, and lung; the heart contained the lowest amounts. Hypoxia did not affect mRNA levels of HIF-1alpha, -1beta, and -2alpha. HIF-3alpha mRNA levels increased in all organs examined after 2 h of hypoxia. A significant rise of EPO and GLUT1 mRNA levels occurred in cortex, heart, liver, and kidney after 2 h of hypoxia, indicating activation of the HIF system. Protein levels of all HIF subunits, determined in brain and lung by immunoblotting, showed a marked increase corresponding to the duration of hypoxia. Our results suggest that induction at the transcriptional level is a unique feature of HIF-3alpha, which therefore may represent a rapidly reacting component of the HIF system in protection against hypoxic damage.

Redox regulation of HIF-1alpha levels and HO-1 expression in renal medullary interstitial cells

The present study hypothesized that superoxide (O2(-)*) importantly contributes to the regulation of hypoxia-inducible factor (HIF)-1alpha expression at posttranscriptional levels in renal medullary interstitial cells (RMICs) of rats. By Western blot analysis, it was found that incubation of RMICs with O2(-)* generators xanthine/xanthine oxidase and menadione significantly inhibited the hypoxia- or CoCl(2)-induced increase in HIF-1alpha levels and completely blocked the increase in HIF-1alpha levels induced by ubiquitin-proteasome inhibition with CBZ-LLL in the nuclear extracts from these cells. Under normoxic conditions, a cell-permeable O2(-)* dismutase (SOD) mimetic, 4-hydroxyl-tetramethylpiperidin-oxyl (TEMPOL) and PEG-SOD, significantly increased HIF-1alpha levels in RMICs. Two mechanistically different inhibitors of NAD(P)H oxidase, diphenyleneiodonium and apocynin, were also found to increase HIF-1alpha levels in these renal cells. Moreover, introduction of an anti-sense oligodeoxynucleotide specific to NAD(P)H oxidase subunit, p22(phox), into RMICs markedly increased HIF-1alpha levels. In contrast, the OH* scavenger tetramethylthiourea had no effect on the accumulation of HIF-1alpha in these renal cells. By Northern blot analysis, scavenging or dismutation of O2(-)* by TEMPOL and PEG-SOD was found to increase the mRNA levels of an HIF-1alpha-targeted gene, heme oxygenase-1. These results indicate that increased intracellular O2(-)* levels induce HIF-1alpha degradation independently of H(2)O(2) and OH* radicals in RMICs. NAD(P)H oxidase activity may importantly contribute to this posttranscriptional regulation of HIF-1alpha in these cells under physiological conditions.

Homocysteine enhances TIMP-1 expression and cell proliferation associated with NADH oxidase in rat mesangial cells

BACKGROUND

Recent studies in our laboratory demonstrated that chronic hyperhomocysteinemia (hHcys) induced glomerular sclerosis. The mechanism mediating hHcys-induced glomerular damage remains unknown. The present study was designed to test a hypothesis that homocysteine (Hcys) increases the production by nicotinamide adenine dinucleotide (NADH) oxidase and thereby stimulates the formation of tissue inhibitor of metalloproteinase (TIMP-1) in rat mesangial cells, consequently leads to glomerulosclerosis.

METHODS

Rat mesangial cells were incubated with L-homocysteine (L-Hcys) to determine the effects of Hcys on cell proliferation and metabolism of extracellular matrix (ECM). Northern blot, Western blot, oligonucleotide transfection, measurements of NADH oxidase activity and levels, and cell proliferation assay were performed.

RESULTS

In cultured rat mesangial cells, treatment with L-Hcys (40 to 160 micromol/L) markedly increased the mRNA levels of TIMP-1 and Gp91 and led to accumulation of collagen I, which were accompanied by enhanced cell proliferation and NADH oxidase activity in mesangial cells. These Hcys-induced biochemical and functional changes were substantially blocked by a NADH oxidase inhibitor, diphenylene iodonium chloride (DPI) or a superoxide dismutase (SOD) mimetic, hydroxyl-tetramethylpiperidin-oxyl (TEMPOL). Moreover, blockade of NADH oxidase subunit, phox22, by its antisense oligodeoxynucleotide also eliminated the increase in NADH oxidase activity induced by L-Hcys.

CONCLUSION

These results indicate that Hcys-induced alterations of ECM metabolism in mesangial cells are associated with enhanced NADH oxidase activity and that oxidative stress-stimulated up-regulation of TIMP-1 may play an important role in the deposition of collagen or ECM elements in the glomeruli during hHcys.

Importance of the renal medullary circulation in the control of sodium excretion and blood pressure

The control of renal medullary perfusion and the impact of alterations in medullary blood flow on renal function have been topics of research interest for almost four decades. Many studies have examined the vascular architecture of the renal medulla, the factors that regulate renal medullary blood flow, and the influence of medullary perfusion on sodium and water excretion and arterial pressure. Despite these studies, there are still a number of important unanswered questions in regard to the control of medullary perfusion and the influence of medullary blood flow on renal excretory function and blood pressure. This review will first address the vascular architecture of the renal medulla and the potential mechanisms whereby medullary perfusion may be regulated. The known extrarenal and local systems that influence the medullary vasculature will then be summarized. Finally, this review will present an overview of the evidence supporting the concept that selective changes in medullary perfusion can have a potent influence on sodium and water excretion with a long-term influence on arterial blood pressure regulation.

Production of superoxide through NADH oxidase in thick ascending limb of Henle's loop in rat kidney

We recently reported that NADH oxidase is one of the major enzymes responsible for superoxide (O(2)(-)*) production in the rat kidney. However, the functional significance of NADH oxidase-mediated O. production and the mechanisms regulating this enzyme activity are poorly understood. Using fluorescence microscopic imaging analysis, the present study demonstrated that thick ascending limbs of Henle's loop (TALHs) exhibited red fluorescence when incubated with dihydroethidium (DHE), suggesting that O(2)(-)* is produced in this tubular segment. Compared with other nephron segments, TALHs from both renal cortex and medulla showed the highest fluorescence intensity. By incubating cortical TALHs (cTALHs) with the substrates of NADH oxidase, xanthine oxidase, nitric oxide synthase, arachidonic acid-metabolizing enzymes, and intramitochondrial oxidases, NADH oxidase was found to be one of the most important enzymes for O(2)(-)* production in this tubular segment. The NADH oxidase inhibitor diphenyleneiodonium (DPI; 100 microM) completely blocked NADH-induced O(2)(-)* production in cTALHs. Exposure of cTALHs to low PO(2) (5-10 Torr) significantly increased O(2)(-)* production regardless of the absence or presence of NADH. Furthermore, angiotensin II (100 nM) increased NADH oxidase activity by 32%, which was completely blocked by DPI. These results suggest that NADH oxidase is a major enzyme responsible for O(2)(-)* production in the TALHs and that the production of O(2)(-)* via NADH oxidase may be regulated by renal tissue oxygenation and circulating hormones.