Cellular responses to hypoxia after renal segmental infarction

Mechanisms of enhanced renal and hepatic erythropoietin synthesis by sodium-glucose cotransporter 2 inhibitors

Sodium-glucose cotransporter 2 (SGLT2) inhibitors reduce the risk of major heart failure events, an action that is statistically linked to enhanced erythropoiesis, suggesting that stimulation of erythropoietin and cardioprotection are related to a shared mechanism. Four hypotheses have been proposed to explain how these drugs increase erythropoietin production: (i) renal cortical reoxygenation with rejuvenation of erythropoietin-producing cells; (ii) counterregulatory distal sodium reabsorption leading to increased tubular workload and oxygen consumption, and thus, to localized hypoxia; (iii) increased iron mobilization as a stimulus of hypoxia-inducible factor-2α (HIF-2α)-mediated erythropoietin synthesis; and (iv) direct HIF-2α activation and enhanced erythropoietin gene transcription due to increased sirtuin-1 (SIRT1) signaling. The first two hypotheses assume that the source of increased erythropoietin is the interstitial fibroblast-like cells in the deep renal cortex. However, SGLT2 inhibitors do not alter regional tissue oxygen tension in the non-diabetic kidney, and renal erythropoietin synthesis is markedly impaired in patients with anemia due to chronic kidney disease, and yet, SGLT2 inhibitors produce an unattenuated erythrocytic response in these patients. This observation raises the possibility that the liver contributes to the production of erythropoietin during SGLT2 inhibition. Hypoxia-inducible factor-2α and erythropoietin are coexpressed not only in the kidney but also in hepatocytes; the liver is a major site of production when erythropoietin stimulation is maintained for prolonged periods. The ability of SGLT2 inhibitors to improve iron mobilization by derepressing hepcidin and ferritin would be expected to increase cytosolic ferrous iron, which might stimulate HIF-2α expression in both the kidney and liver through the action of iron regulatory protein 1. Alternatively, the established ability of SGLT2 inhibitors to enhance SIRT1 might be the mechanism of enhanced erythropoietin production with these drugs. In hepatic cell lines, SIRT1 can directly activate HIF-2α by deacetylation, and additionally, through an effect of SIRT in the liver, peroxisome proliferator-activated receptor-γ coactivator-1α binds to hepatic nuclear factor 4 to promote transcription of the erythropoietin gene and synthesis of erythropoietin. Since SIRT1 up-regulation exerts direct cytoprotective effects on the heart and stimulates erythropoietin, it is well-positioned to represent the shared mechanism that links erythropoiesis to cardioprotection during SGLT2 inhibition.

Targeting oxidative stress, a crucial challenge in renal transplantation outcome

Disorders characterized by ischemia/reperfusion (I/R) are the most common causes of debilitating diseases and death in stroke, cardiovascular ischemia, acute kidney injury or organ transplantation. In the latter example the I/R step defines both the amplitude of the damages to the graft and the functional recovery outcome. During transplantation the kidney is subjected to blood flow arrest followed by a sudden increase in oxygen supply at the time of reperfusion. This essential clinical protocol causes massive oxidative stress which is at the basis of cell death and tissue damage. The involvement of both reactive oxygen species (ROS) and nitric oxides (NO) has been shown to be a major cause of these cellular damages. In fact, in non-physiological situations, these species escape endogenous antioxidant control and dangerously accumulate in cells. In recent years, the objective has been to find clinical and pharmacological treatments to reduce or prevent the appearance of oxidative stress in ischemic pathologies. This is very relevant because, due to the increasing success of organ transplantation, clinicians are required to use limit organs, the preservation of which against oxidative stress is crucial for a better outcome. This review highlights the key actors in oxidative stress which could represent new pharmacological targets.

Acute renal infarction: long-term renal outcome and prognostic factors

INTRODUCTION

Acute renal infarction is a rare occurence, whose  prognosis and long-term outcomes remain poorly studied. This study evaluated whether clinical and radiological features at diagnosis can be associated with the long-term outcomes (blood pressure, kidney function and mortality).

METHODS

We retrospectively analyzed the demographic, clinical, biological and radiological data of patients with acute renal infarction hospitalized at Rennes University Hospital between 1997 and 2017 (n = 94).

RESULTS

Patients were followed-up for a median of 60 months. At time of diagnosis of acute renal infarction median age was 53 years, 45% of the patients had acute hypertension, and 31% had Acute Kidney Injury (AKI) requiring dialysis in seven patients. The median Lactate DeHydrogenase (LDH) level was 977 IU/mL. The median extent of kidney damage was 14%, with left renal involvement in 51% of patients. At 60 months of follow-up, 66% of patients had developed Chronic Kidney Disease (CKD) stage 3 or higher, and 55% had hypertension since diagnosis. Age, acute development of hypertension and AKI at diagnosis were associated with long-term CKD (stage 3 or higher) in multivariate analyses, but the extent of kidney damage was not. During the follow-up, 21% of patients died, and only age resulted as a predisposing factor. No tested factor was correlated with long-term hypertension.

DISCUSSION

Age, acute development of hypertension, and AKI were correlated with long term CKD, whereas no factor was correlated with long-term hypertension after acute renal infarction.

Immunopathophysiology of trauma-related acute kidney injury

Physical trauma can affect any individual and is globally accountable for more than one in every ten deaths. Although direct severe kidney trauma is relatively infrequent, extrarenal tissue trauma frequently results in the development of acute kidney injury (AKI). Various causes, including haemorrhagic shock, rhabdomyolysis, use of nephrotoxic drugs and infectious complications, can trigger and exacerbate trauma-related AKI (TRAKI), particularly in the presence of pre-existing or trauma-specific risk factors. Injured, hypoxic and ischaemic tissues expose the organism to damage-associated and pathogen-associated molecular patterns, and oxidative stress, all of which initiate a complex immunopathophysiological response that results in macrocirculatory and microcirculatory disturbances in the kidney, and functional impairment. The simultaneous activation of components of innate immunity, including leukocytes, coagulation factors and complement proteins, drives kidney inflammation, glomerular and tubular damage, and breakdown of the blood-urine barrier. This immune response is also an integral part of the intense post-trauma crosstalk between the kidneys, the nervous system and other organs, which aggravates multi-organ dysfunction. Necessary lifesaving procedures used in trauma management might have ambivalent effects as they stabilize injured tissue and organs while simultaneously exacerbating kidney injury. Consequently, only a small number of pathophysiological and immunomodulatory therapeutic targets for TRAKI prevention have been proposed and evaluated.

Role of promoting inflammation of Krüppel-like factor 6 in acute kidney injury

Background

Krüppel-like factor 6 (KLF6) is a transcription factor that participate in various pathophysiological processes, but its contribution in ischemia acute kidney injury (AKI) is lacking so far. The study aimed to investigate the expression and the role of KLF6 in kidney ischemia–reperfusion (IR) injury.

Method

Microarray data were collected from GSE58438 and GSE52004. The rat IR model was established to evaluate the mRNA and protein expression of KLF6 and inflammatory cytokines in serum and kidney tissues. SiRNA-KLF6 was transfected with HK-2 cells, and then a cell-based hypoxia-reoxygenation (HR) model was established.

Results

Bioinformatics showed KLF6 mRNA in kidney tissue is up-regulated in 3 h after IR in rat kidney, which involved in cell activation, leukocyte activation, and response to hydrogen peroxide after IR. The rat IR model results showed that KLF6 expression was peaking at 6 h, and the expression of pro-inflammatory cytokines MCP-1 and TNF-α was increased both in serum and kidney tissues, while anti-inflammatory cytokine IL-10 was decreased after IR. Furthermore, in vitro results showed that KLF6 knock-down reduced the pro-inflammatory cytokines expression.

Conclusion

These results suggest that (1) KLF6 might be a novel biomarker for early diagnosis of AKI and (2) KLF6 may play a role in promoting inflammation in AKI.

Ischemia and Reperfusion Injury in Kidney Transplantation: Relevant Mechanisms in Injury and Repair

Ischemia and reperfusion injury (IRI) is a complex pathophysiological phenomenon, inevitable in kidney transplantation and one of the most important mechanisms for non- or delayed function immediately after transplantation. Long term, it is associated with acute rejection and chronic graft dysfunction due to interstitial fibrosis and tubular atrophy. Recently, more insight has been gained in the underlying molecular pathways and signalling cascades involved, which opens the door to new therapeutic opportunities aiming to reduce IRI and improve graft survival. This review systemically discusses the specific molecular pathways involved in the pathophysiology of IRI and highlights new therapeutic strategies targeting these pathways.

Evaluation of Renal Pathophysiological Processes Induced by an Iodinated Contrast Agent in a Diabetic Rabbit Model Using Intravoxel Incoherent Motion and Blood Oxygenation Level-Dependent Magnetic Resonance Imaging

OBJECTIVE

To examine the potential of intravoxel incoherent motion (IVIM) and blood oxygen level-dependent (BOLD) magnetic resonance imaging for detecting renal changes after iodinated contrast-induced acute kidney injury (CI-AKI) development in a diabetic rabbit model.

MATERIALS AND METHODS

Sixty-two rabbits were randomized into 2 groups: diabetic rabbits with the contrast agent (DCA) and healthy rabbits with the contrast agent (NCA). In each group, 6 rabbits underwent IVIM and BOLD imaging at 1 hour, 1 day, 2 days, 3 days, and 4 days after an iohexol injection while 5 rabbits were selected to undergo blood and histological examinations at these specific time points. Iohexol was administrated at a dose of 2.5 g I/kg of body weight. Further, the apparent transverse relaxation rate (R2*), average pure molecular diffusion coefficient (D), pseudo-diffusion coefficient (D*), and perfusion fraction (f) were calculated.

RESULTS

The D and f values of the renal cortex (CO) and outer medulla (OM) were significantly decreased compared to baseline values in the 2 groups 1 day after the iohexol injection (p < 0.05). A marked reduction in the D* values for both the CO and OM was also observed after 1 hour in each group (p < 0.05). In the OM, a persistent elevation of the R2* was detected for 4 days in the DCA group (p < 0.05). Histopathological changes were prominent, and the pathological features of CI-AKI aggravated in the DCA group until day 4. The D, f, and R2* values significantly correlated with the histological damage scores, hypoxia-inducible transcription factor-1α expression scores, and serum creatinine levels.

CONCLUSION

A combination of IVIM and BOLD imaging may serve as a noninvasive method for detecting and monitoring CI-AKI in the early stages in the diabetic kidney.

Sodium pentobarbital dosages for exsanguination affect biochemical, molecular and histological measurements in rats

Rodents are widely used for animal research in Egypt. Pentobarbital is the most common anesthetic agent; however overdoses may affect the experimental outcomes and limit the use of tissues. To investigate the effects of sodium pentobarbital overdoses during exsanguination, three groups (6 rats/group) of male and female rats were injected i.p. with 50, 100 and 150 mg/kg of sodium pentobarbital, then carotid exsanguination was performed immediately after loss of consciousness. Hypoxia-inducible factor 1-alpha (Hif1a) and tumor necrosis factor-alpha (Tnfa) mRNA expressions in liver and kidney organs were evaluated. As well as, serum aminotransferase activities (AST&ALT), glucose, urea, creatinine, malondialdehyde (MDA), reduced glutathione (GSH) and catalase (CAT) levels were determined. The histological alterations in liver, kidney and spleen were studied. It was found that Hif1a and Tnfa were significantly overexpressed in the studied organs and serum AST, glucose, creatinine and urea levels were significantly increased after sodium pentobarbital overdoses (100 and 150 mg/kg) compared to 50 mg/kg dose. Similarly, significant increase in MDA and GSH levels of liver, kidney and spleen were noticed. Results showed gender difference where Hif1a and Tnfa levels were significantly overexpressed at high dose of sodium pentobarbital of liver and kidney organs in female more than male rats. Since euthanasia protocol may influence the physiological variables and affect genes' expression, it is recommended to avoid sodium pentobarbital overdose during euthanasia as it may interfere with the biochemical, molecular and histological measurements.

Analysis of the critical determinants of renal medullary oxygenation

We have previously developed a three-dimensional computational model of oxygen transport in the renal medulla. In the present study, we used this model to quantify the sensitivity of renal medullary oxygenation to four of its major known determinants: medullary blood flow (MBF), medullary oxygen consumption rate (V̇o2,M), hemoglobin (Hb) concentration in the blood, and renal perfusion pressure. We also examined medullary oxygenation under special conditions of hydropenia, extracellular fluid volume expansion by infusion of isotonic saline, and hemodilution during cardiopulmonary bypass. Under baseline (normal) conditions, the average medullary tissue Po2 predicted for the whole renal medulla was ~30 mmHg. The periphery of the interbundle region in the outer medulla was identified as the most hypoxic region in the renal medulla, which demonstrates that the model prediction is qualitatively accurate. Medullary oxygenation was most sensitive to changes in renal perfusion pressure followed by Hb, MBF, and V̇o2,M, in that order. The medullary oxygenation also became sensitized by prohypoxic changes in other parameters, leading to a greater fall in medullary tissue Po2 when multiple parameters changed simultaneously. Hydropenia did not induce a significant change in medullary oxygenation compared with the baseline state, while volume expansion resulted in a large increase in inner medulla tissue Po2 (by ~15 mmHg). Under conditions of cardiopulmonary bypass, the renal medulla became severely hypoxic, due to hemodilution, with one-third of the outer stripe of outer medulla tissue having a Po2 of <5 mmHg.

Detection of cellular hypoxia by pimonidazole adduct immunohistochemistry in kidney disease: methodological pitfalls and their solution

Pimonidazole adduct immunohistochemistry is one of the few available methods for assessing renal tissue hypoxia at the cellular level. It appears to be prone to artifactual false positive staining under some circumstances. Here, we assessed the nature of this false positive staining and, having determined how to avoid it, reexamined the nature of cellular hypoxia in rat models of kidney disease. When a mouse-derived anti-pimonidazole primary antibody was used, two types of staining were observed. First, there was diffuse staining of the cytoplasm of tubular epithelial cells, which was largely absent when the primary antibody was omitted from the incubation protocol or in tissues known not to contain pimonidazole adducts. Second, there was staining of the apical membranes of tubular epithelial cells, debris within the lumen of renal tubules, including tubular casts, and the interstitium; this latter staining was present even when the primary antibody was omitted from the incubation protocol. Such false positive staining was particularly prominent in acutely injured kidneys. It could not be avoided by preincubation of sections with a mouse IgG blocking reagent. Furthermore, preadsorption of the secondary antibody against rat Ig abolished all staining; however, when a rabbit-derived polyclonal anti-pimonidazole primary antibody was used, the false positive staining was largely avoided. Using this method, we confirmed the presence of hypoxia, localized mainly to the tubular epithelium, in the acute phase of severe renal ischemia-reperfusion injury, adenine-induced chronic kidney disease, and polycystic kidney disease. We conclude that this new method provides improved detection of renal cellular hypoxia.

Immunopathogenesis of Acute Kidney Injury

Pathophysiologically, the classification of acute kidney injury (AKI) can be divided into three categories: (1) prerenal, (2) intrinsic, and (3) postrenal. Emerging evidence supports the involvement of renal tubular epithelial cells and the innate and adaptive arms of the immune system in the pathogenesis of intrinsic AKI. Pro-inflammatory damage-associated molecular patterns, pathogen-associated molecular patterns, hypoxia inducible factors, toll-like receptors, complement system, oxidative stress, adhesion molecules, cell death, resident renal dendritic cells, neutrophils, T and B lymphocytes, macrophages, natural killer T cells, cytokines, and secreted chemokines contribute to the immunopathogenesis of AKI. However, other immune cells and pathways such as M2 macrophages, regulatory T cells, progranulin, and autophagy exhibit anti-inflammatory properties and facilitate kidney tissue repair after AKI. Thus, therapies for AKI include agents such as anti-inflammatory (e.g., recombinant alkaline phosphatase), antioxidants (iron chelators), and apoptosis inhibitors. In preclinical toxicity studies, drug-induced kidney injury can be seen after exposure to a nephrotoxicant test article due to immune mechanisms and dysregulation of innate, and/or adaptive cellular immunity. The focus of this review will be on intrinsic AKI, as it relates to the immune and renal systems cross talks focusing on the cellular and pathophysiologic mechanisms of AKI.

Enhanced lipoxygenase/LTD4 signaling accounts for the exaggerated hypertensive and nephrotoxic effects of cyclosporine plus indomethacin in rats

The combined use of cyclosporine (CSA) and nonsteroidal antiinflammatory drugs (NSAIDs) causes exaggerated rises in systolic blood pressure (SBP) and nephrotoxicity. We examined whether these influences relate to the arachidonate/5-lipoxygenase (LOX) pathway. Rats were treated with CSA (20 mg kg^-1 day^-1), indomethacin (5 mg kg^-1 day^-1), or their combination for 10 days. Changes in SBP and renal biochemical/histopathological characteristics along with leukotriene levels were determined in rats treated with or without LT receptor antagonists. CSA or indomethacin caused: (i) renal tubular atrophy and interstitial fibrosis, (ii) increases in serum creatinine, blood urea nitrogen (BUN), and renal LTD4, LTB4, TNF-α, TGF-β1, and caspase-3, and (iii) decreases in renal PGE2 and total antioxidant capacity (TAC). SBP measured by tail-cuff plethysmography was increased by CSA but not indomethacin. These effects were mostly intensified in rats treated with CSA plus indomethacin. The co-treatment with montelukast (cysteinyl LT receptor blocker), but not ONO-4057 (LTB4 receptor blocker), ameliorated CSA/indomethacin-evoked hypertension, renal structural/biochemical deterioration, and LTD4 levels. Moreover, montelukast exhibited a greater capacity in reversing inflammatory, oxidative, apoptotic, and fibrotic abnormalities induced by CSA/indomethacin. Overall, lipoxygenase/LTD4 upregulation contributes to the exaggerated hypertension and nephrotoxicity caused by CSA/indomethacin. The therapeutic potential of cysteinyl LT receptor antagonism in rectifying CSA/NSAIDs-evoked anomalies is warranted.

GYY4137, a Hydrogen Sulfide Donor Modulates miR194-Dependent Collagen Realignment in Diabetic Kidney

The relationship between hydrogen sulfide (H₂S), microRNAs (miRs), matrix metalloproteinases (MMPs) and poly-ADP-ribose-polymerase-1 (PARP-1) in diabetic kidney remodeling remains mostly obscured. We aimed at investigating whether alteration of miR-194-dependent MMPs and PARP-1 causes renal fibrosis in diabetes kidney, and whether H₂S ameliorates fibrosis. Wild type, diabetic Akita mice as well as mouse glomerular endothelial cells (MGECs) were used as experimental models, and GYY4137 as H₂S donor. In diabetic mice, plasma H₂S levels were decreased while ROS and expression of its modulator (ROMO1) were increased. In addition, alteration of MMPs-9, −13 and −14 expression, PARP-1, HIF1α, and increased collagen biosynthesis as well as collagen cross-linking protein, P4HA1 and PLOD2 were observed along with diminished vascular density in diabetic kidney. These changes were ameliorated by GYY4137. Further, downregulated miRNA-194 was normalized by GYY4137 in diabetic kidney. Similar results were obtained in in vitro condition. Interestingly, miR-194 mimic also diminished ROS production, and normalized ROMO1, MMPs-9, −13 and −14, and PARP-1 along with collagen biosynthesis and cross-linking protein in HG condition. We conclude that decrease H₂S diminishes miR-194, induces collagen deposition and realignment leading to fibrosis and renovascular constriction in diabetes. GYY4137 mitigates renal fibrosis in diabetes through miR-194-dependent pathway.

Associations of high altitude polycythemia with polymorphisms in EPHA2 and AGT in Chinese Han and Tibetan populations

High altitude polycythemia (HAPC) refers to the long-term living in the plateau of the hypoxia environment is not accustomed to cause red blood cell hyperplasia. The pathological changes are mainly the various organs and tissue congestion, blood stasis and hypoxia damage. Although chronic hypoxia is the main cause of HAPC, the related molecular mechanisms remain largely unclear. This study aims to explore the genetic basis of HAPC in the Chinese Han and Tibetan populations. We enrolled 100 patients (70 Han, 30 Tibetan) with HAPC and 100 healthy control subjects (30 Han, 70 Tibetan). To explore the hereditary basis of HAPC and investigate the association between EPHA2 with AGT and HAPC in Chinese Han and Tibetan populations. Using the Chi-squared test and analyses of genetic models, rs2291804, rs2291805, rs3768294, rs3754334, rs6603856, rs6669624, rs11260742, rs13375644 and rs10907223 in EPHA2, and rs699, rs4762 and rs5051 in AGT showed associations with reduced HAPC susceptibility in Han populations. Additionally, in Tibetan populations, rs2478523 in AGT showed an increased the risk of HAPC. Our study suggest that polymorphisms in the EPHA2 and AGT correlate with susceptibility to HAPC in Chinese Han and Tibetan populations.

The multifaceted role of the renal microvasculature during acute kidney injury

Pediatric acute kidney injury (AKI) represents a complex disease process for clinicians as it is multifactorial in cause and only limited treatment or preventatives are available. The renal microvasculature has recently been implicated in AKI as a strong therapeutic candidate involved in both injury and recovery. Significant progress has been made in the ability to study the renal microvasculature following ischemic AKI and its role in repair. Advances have also been made in elucidating cell-cell interactions and the molecular mechanisms involved in these interactions. The ability of the kidney to repair post AKI is closely linked to alterations in hypoxia, and these studies are elucidated in this review. Injury to the microvasculature following AKI plays an integral role in mediating the inflammatory response, thereby complicating potential therapeutics. However, recent work with experimental animal models suggests that the endothelium and its cellular and molecular interactions are attractive targets to prevent injury or hasten repair following AKI. Here, we review the cellular and molecular mechanisms of the renal endothelium in AKI, as well as repair and recovery, and potential therapeutics to prevent or ameliorate injury and hasten repair.

Bidirectional signalling between EphA2 and ephrinA1 increases tubular cell attachment, laminin secretion and modulates erythropoietin expression after renal hypoxic injury

Acute kidney injury (AKI) is common in hospitalized patients and has a poor prognosis, the severity of AKI being linked to progression to chronic kidney disease. This stresses the need to search for protective mechanisms during the acute phase. We investigated kidney repair after hypoxic injury using a rat model of renal artery branch ligation, which led to an oxygen gradient vertical to the corticomedullary axis. Three distinct zones were observed: tubular necrosis, infarction border zone and preserved normal tissue. EphA2 is a receptor tyrosine kinase with pivotal roles in cell architecture, migration and survival, upon juxtacrine contact with its membrane-bound ligand EphrinA1. Following hypoxia, EphA2 was up-regulated in cortical and medullary tubular cells, while EphrinA1 was up-regulated in interstitial cells adjacent to peritubular capillaries. Moreover, erythropoietin (EPO) messenger RNA (mRNA) was strongly expressed in the border zone of infarcted kidney within the first 6 h. To gain more insight into the biological impact of EphA2 and EphrinA1 up-regulation, we activated the signalling pathways in vitro using recombinant EphrinA1/Fc or EphA2/Fc proteins. Stimulation of EphA2 forward signalling in the proximal tubular cell line HK2 increased cell attachment and laminin secretion at the baso-lateral side. Conversely, activation of reverse signalling through EphrinA1 expressed by Hep3B cells promoted EPO production at both the transcriptional and protein level. Strikingly, in co-culture experiments, juxtacrine contact between EphA2 expressing MDCK and EphrinA1 expressing Hep3B was sufficient to induce a significant up-regulation of EPO mRNA production in the latter cells, even in the absence of hypoxic conditions. The synergistic effects of EphA2 and hypoxia led to a 15-20-fold increase of EPO expression. Collectively, our results suggest an important role of EphA2/EphrinA1 signalling in kidney repair after hypoxic injury through stimulation of (i) tubular cell attachment, (ii) secretion of basal membrane proteins and (iii) EPO production. These findings could thus pave the way to new therapeutic approaches.

Endothelin ETA receptor/lipid peroxides/COX-2/TGF-β1 signalling underlies aggravated nephrotoxicity caused by cyclosporine plus indomethacin in rats

BACKGROUND AND PURPOSE

Cyclosporine (CSA) and non-steroidal anti-inflammatory drugs (NSAIDs) are co-prescribed for some arthritic conditions. We tested the hypothesis that this combined regimen elicits exaggerated nephrotoxicity in rats via the up-regulation of endothelin (ET) receptor signalling.

EXPERIMENTAL APPROACH

The effects of a 10 day treatment with CSA (20 mg · kg(-1) · day(-1)), indomethacin (5 mg · kg(-1) · day(-1)) or their combination on renal biochemical, inflammatory, oxidative and structural profiles were assessed. The roles of ETA receptor and COX-2 pathways in the interaction were evaluated.

KEY RESULTS

Oral treatment with CSA or indomethacin elevated serum urea and creatinine, caused renal tubular atrophy and interstitial fibrosis, increased renal TGF-β1, and reduced immunohistochemical expressions of ETA receptors and COX-2. CSA, but not indomethacin, increased renal ET-1, the lipid peroxidation product malondialdehyde (MDA) and GSH activity. Compared with individual treatments, simultaneous CSA/indomethacin exposure caused: (i) greater elevations in serum creatinine and renal MDA; (ii) loss of the compensatory increase in GSH; (iii) renal infiltration of inflammatory cells and worsening of fibrotic and necrotic profiles; and (iv) increased renal ET-1 and decreased ETA receptor and COX-2 expressions. Blockade of ETA receptors by atrasentan ameliorated the biochemical, structural, inflammatory and oxidative abnormalities caused by the CSA/indomethacin regimen. Furthermore, atrasentan partly reversed the CSA/indomethacin-evoked reductions in the expression of ETA receptor and COX-2 protein.

CONCLUSIONS AND IMPLICATIONS

The exaggerated oxidative insult and associated dysregulation of the ETA receptor/COX-2/TGF-β1 signalling might account for the aggravated nephrotoxicity caused by the CSA/indomethacin regimen. The potential renoprotective effect of ETA receptor antagonism might be exploited therapeutically.

In vivo evidence suggesting reciprocal renal hypoxia-inducible factor-1 upregulation and signal transducer and activator of transcription 3 activation in response to hypoxic and non-hypoxic stimuli

In vitro studies suggest that combined activation of hypoxia-inducible factor (HIF) and signal transducer and activator of transcription 3 (STAT3) promotes the hypoxia response. However, their interrelationship in vivo remains poorly defined. The present study investigated the possible relationship between HIF-1 upregulation and STAT3 activation in the rodent kidney in vivo. Activation of HIF-1 and STAT3 was analysed by immunohistochemical staining and western blot analysis in: (i) models of hypoxia-associated kidney injury induced by radiocontrast media or rhabdomyolysis; (ii) following activation of STAT3 by the interleukin (IL)-6-soluble IL-6 receptor complex; or (iii) following HIF-1α stabilization using hypoxic and non-hypoxic stimuli (mimosine, FG-4497, CO, CoCl(2)) and in targeted von Hippel-Lindau-knockout mice. Western blot analysis and immunostaining revealed marked induction of both transcription factors under all conditions tested, suggesting that in vivo STAT3 can trigger HIF and vice versa. Colocalization of HIF-1α and phosphorylated STAT3 was detected in some, but not all, renal cell types, suggesting that in some cells a paracrine mechanism may be responsible for the reciprocal activation of the two transcription factors. Nevertheless, in several cell types spatial concordance was observed under the majority of conditions tested, suggesting that HIF-1 and STAT3 may act as cotranscription factors. These in vivo studies suggest that, in response to renal hypoxic-stress, upregulation of HIF-1 and activation of STAT3 may be both reciprocal and cell type dependent.

Immune cells in experimental acute kidney injury

Acute kidney injury (AKI) prolongs hospital stay and increases mortality in various clinical settings. Ischaemia-reperfusion injury (IRI), nephrotoxic agents and infection leading to sepsis are among the major causes of AKI. Inflammatory responses substantially contribute to the overall renal damage in AKI. Both innate and adaptive immune systems are involved in the inflammatory process occurring in post-ischaemic AKI. Proinflammatory damage-associated molecular patterns, hypoxia-inducible factors, adhesion molecules, dysfunction of the renal vascular endothelium, chemokines, cytokines and Toll-like receptors are involved in the activation and recruitment of immune cells into injured kidneys. Immune cells of both the innate and adaptive immune systems, such as neutrophils, dendritic cells, macrophages and lymphocytes contribute to the pathogenesis of renal injury after IRI, and some of their subpopulations also participate in the repair process. These immune cells are also involved in the pathogenesis of nephrotoxic AKI. Experimental studies of immune cells in AKI have resulted in improved understanding of the immune mechanisms underlying AKI and will be the foundation for development of novel diagnostic and therapeutic targets. This Review describes what is currently known about the function of the immune system in the pathogenesis and repair of ischaemic and nephrotoxic AKI.

HIF-1α, VEGF, and EGFR: contributing factors in the pathogenesis of necrotizing sialometaplasia

OBJECTIVES

Necrotizing sialometaplasia (NS) is an uncommon reactive lesion involving the minor salivary glands. This study aimed to investigate the expression of hypoxia-inducible factor alpha (HIF-1α), vascular endothelial growth factor (VEGF), and epithelial growth factor receptor (EGFR) in the pathogenesis of NS.

METHODS

Paraffin-embedded tissue sections from 10 cases of NS were immunohistochemically stained for HIF-1α, VEGF, and EGFR. A semiquantitative morphometric analysis was performed and compared with normal palatal salivary glands and traumatic ulcerations.

RESULTS

Hypoxia-inducible factor alpha staining was observed in most elements of the affected area, the acini and ducts of the involved salivary glands as well as in the inflammatory infiltrate, the endothelial cells, and stromal cells. HIF-1α was almost absent in the control glands (P < 0.0001). VEGF staining was positive in the stromal capillaries and in the inflammatory infiltrate. The expression was higher in cases of NS compared with the normal salivary glands (P < 0.001). EGFR was expressed in the surface epithelium, the pseudo-epitheliomatous hyperplasia, and the islands of squamous metaplasia. VEGF expression in traumatic ulcerations was lower than that in cases of NS.

CONCLUSION

This study provides molecular evidence to the role of hypoxia in NS; HIF-1α, the main regulator of hypoxia, was expressed in the infarcted salivary glands, EGFR in the metaplastic epithelium and VEGF in the stromal capillaries, all three components are the key factors induced by hypoxia.

Chronic activation of vasopressin V2 receptor signalling lowers renal medullary oxygen levels in rats

AIM

In the present study, we aimed to elucidate the effects of chronic vasopressin administration on renal medullary oxygen levels.

METHODS

Adult Sprague Dawley or vasopressin-deficient Brattleboro rats were treated with the vasopressin V2 receptor agonist, desmopressin (5 ng/h; 3d), or its vehicle via osmotic minipumps. Immunostaining for pimonidazole and the transcription factor HIF-1α (hypoxia-inducible factor-1α) were used to identify hypoxic areas. Activation of HIF-target gene expression following desmopressin treatment was studied by microarray analysis.

RESULTS

Pimonidazole staining was detected in the outer and inner medulla of desmopressin-treated rats, whereas staining in control animals was weak or absent. HIF-1α immunostaining demonstrated nuclear accumulation in the papilla of desmopressin-treated animals, whereas no staining was observed in the controls. Gene expression analysis revealed significant enrichment of HIF-target genes in the group of desmopressin-regulated gene products (P = 2.6*10(-21) ). Regulated products included insulin-like growth factor binding proteins 1 and 3, angiopoietin 2, fibronectin, cathepsin D, hexokinase 2 and cyclooxygenase 2.

CONCLUSION

Our results demonstrate that an activation of the renal urine concentrating mechanism by desmopressin causes renal medullary hypoxia and an upregulation of hypoxia-inducible gene expression.

Type 2 diabetes mellitus in mice aggravates the renal impact of hemorrhagic shock

The objectives of this study were to determine whether type 2 diabetic mice would exhibit a more severe renal impact of hemorrhagic shock (HS) based on a recently described model of acute kidney injury and to determine the impact of HS on renal responses to hypoxia. We induced HS or sham procedure in type 2 diabetic and obese db/db mice. Creatininemia, glomerular filtration rate, urine output, histologic injury score, and kidney inductible molecule 1 mRNA were used to investigate the renal impact of HS. Tissular hypoxia and its impact were quantified using pimonidazole immunostaining and mRNA of hypoxic inducible factor, vascular endothelial growth factor receptors 1 and 2, Tie-2, endothelial nitric oxide synthase, and inducible nitric oxide synthase. Diabetic mice exhibiting mild diabetic nephropathy express hypoxic signals at baseline. The renal impact of HS was more severe in diabetic mice, with a worsening of tissular hypoxia and an altered response to hypoxia. Furthermore, endothelial nitric oxide synthase was highly overexpressed in diabetic shocked mice when compared with nondiabetic shocked mice. Renal impact of HS in type 2 diabetic mice is more intense than in nondiabetic ones. Preexisting hypoxia during diabetes could result in a renal preconditioning that modifies endothelial and tissular responses to acute kidney injury.

Antioxidant and renoprotective effects of paricalcitol on experimental contrast-induced nephropathy model

OBJECTIVES

The aim of the study was to assess the effect of paricalcitol on the experimental contrast-induced nephropathy (CIN) model. We hypothesised that paricalcitol may prevent CIN.

METHODS

32 Wistar albino rats were divided into four groups (n=8 each): control group, paricalcitol group, CIN group and paricalcitol plus CIN group. Paricalcitol (0.4 µg kg(-1) day(-1)) was given intraperitoneally for 5 consecutive days prior to induction of CIN. CIN was induced at day 4 by intravenous injection of indometacin (10 mg kg(-1)), Nω-nitro-L-arginine methyl ester (L-NAME, 10 mg kg(-1)) and meglumine amidotrizoate (6 ml kg(-1)). Renal function parameters, oxidative stress biomarkers, histopathological findings and vascular endothelial growth factor (VEGF) immunoexpression were evaluated.

RESULTS

The paricalcitol plus CIN group had lower mean serum creatinine levels (p=0.034) as well as higher creatinine clearance (p=0.042) than the CIN group. Serum malondialdehyde and kidney thiobarbituric acid-reacting substances levels were significantly lower in the paricalcitol plus CIN group than in the CIN group (p=0.024 and p=0.042, respectively). The mean scores of tubular necrosis (p=0.024), proteinaceous casts (p=0.038), medullary congestion (p=0.035) and VEGF immunoexpression (p=0.018) in the paricalcitol plus CIN group were also significantly lower.

CONCLUSION

This study demonstrates the protective effect of paricalcitol in the prevention of CIN in an experimental model.

Phosphodiesterase inhibitor improves renal tubulointerstitial hypoxia of the diabetic rat kidney

BACKGROUND/AIMS

Renal hypoxia is involved in the pathogenesis of diabetic nephropathy. Pentoxifyllin (PTX), a nonselective phosphodiesterase inhibitor, is used to attenuate peripheral vascular diseases. To determine whether PTX can improve renal hypoxia, we investigated its effect in the streptozocin (STZ)-induced diabetic kidney.

METHODS

PTX (40 mg/kg, p.o.) was administered to STZ-induced diabetic rats for 8 weeks. To determine tissue hypoxia, we examined hypoxic inducible factor-1α (HIF-1α), heme oxygenase-1 (HO-1), vascular endothelial growth factor (VEGF), and glucose transporter-1 (GLUT-1) levels. We also tested the effect of PTX on HIF-1α in renal tubule cells.

RESULTS

PTX reduced the increased protein creatinine ratio in diabetic rats at 8 weeks. HIF-1α, VEGF, and GLUT-1 mRNA expression increased significantly, and the expression of HO-1 also tended to increase in diabetic rats. PTX significantly decreased mRNA expression of HIF-1α and VEGF at 4 and 8 weeks, and decreased HO-1 and GLUT-1 at 4 weeks. The expression of HIF-1α protein was significantly increased at 4 and 8 weeks in tubules in the diabetic rat kidney. PTX tended to decrease HIF-1α protein expression at 8 weeks. To examine whether PTX had a direct effect on renal tubules, normal rat kidney cells were stimulated with CoCl(2) (100 µM), which enhanced HIF-1α mRNA and protein levels under low glucose conditions (5.5 mM). Their expressions were similar even after high glucose (30 mM) treatment. PTX had no effect on HIF-1α expression.

CONCLUSIONS

PTX attenuates tubular hypoxia in the diabetic kidney.

Survival and function of mesenchymal stem cells (MSCs) depend on glucose to overcome exposure to long-term, severe and continuous hypoxia

Use of mesenchymal stem cells (MSCs) has emerged as a potential new treatment for various diseases but has generated marginally successful results. A consistent finding of most studies is massive death of transplanted cells. The present study examined the respective roles of glucose and continuous severe hypoxia on MSC viability and function with respect to bone tissue engineering. We hereby demonstrate for the first time that MSCs survive exposure to long-term (12 days), severe (pO₂ < 1.5 mmHg) hypoxia, provided glucose is available. To this end, an in vitro model that mimics the hypoxic environment and cell-driven metabolic changes encountered by grafted sheep cells was established. In this model, the hallmarks of hypoxia (low pO₂, hypoxia inducible factor-1α expression and anaerobic metabolism) were present. When conditions switched from hypoxic (low pO₂) to ischemic (low pO₂ and glucose depletion), MSCs exhibited shrinking, decreased cell viability and ATP content due to complete exhaustion of glucose at day 6; these results provided evidence that ischemia led to the observed massive cell death. Moreover, MSCs exposed to severe, continuous hypoxia, but without any glucose shortage, remained viable and maintained both their in vitro proliferative ability after simulation with blood reperfusion at day 12 and their in vivo osteogenic ability. These findings challenge the traditional view according to which severe hypoxia per se is responsible for the massive MSC death observed upon transplantation of these cells and provide evidence that MSCs are able to withstand exposure to severe, continuous hypoxia provided that a glucose supply is available.

Cytoglobin, a novel globin, plays an antifibrotic role in the kidney

Cytoglobin (Cygb), a novel member of the globin superfamily, is expressed by fibroblasts in various organs. However, its function remains unknown. Because of its localization, we speculated that a biological role of Cygb may be related to fibrogenesis. To clarify the role of Cygb in kidney fibrosis, we employed the remnant kidney model in rats. Immunohistochemical analysis showed an increase in Cygb expression in parallel with disease progression. To investigate the functional consequence of Cygb upregulation, we established transgenic rats overexpressing rat Cygb. Overexpression of Cygb improved histological injury, preserved renal function, and ameliorated fibrosis, as estimated by the accumulation of collagen I and IV as well as Masson trichrome staining. These protective effects of Cygb were associated with a decrease in nitrotyrosine deposition in the kidney and urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) excretion as a marker of oxidative stress. We also performed in vitro studies utilizing a rat kidney fibroblast cell line transiently overexpressing Cygb, an inducible kidney cell transfected with Cygb, and primary cultured fibroblasts isolated from the kidneys of the transgenic rats. These different experimental systems consistently showed that Cygb inhibited collagen synthesis. Furthermore, mutant disruption of heme in Cygb that impaired its antioxidant properties led to the loss of antifibrotic effects, suggesting that Cygb reduces fibrosis via a radical scavenging function. In conclusion, we showed that Cygb plays an important role in protection of the kidney against fibrosis via the amelioration of oxidative stress both in vitro and in vivo. Cygb might represent a good therapeutic target in chronic kidney disease.

The HIF-2α-driven pseudo-hypoxic phenotype in tumor aggressiveness, differentiation, and vascularization

Cellular adaptation to diminished tissue oxygen tensions, hypoxia, is largely governed by the hypoxia inducible transcription factors, HIF-1 and HIF-2. Tumor hypoxia and high HIF protein levels are frequently associated with aggressive disease. In recent years, high tumor cell levels of HIF-2 and the oxygen sensitive subunit HIF-2α have been associated with unfavorable disease and shown to be highly expressed in tumor stem/initiating cells originating from neuroblastoma and glioma, respectively. In these cells, HIF-2 is active under nonhypoxic conditions as well, creating a pseudo-hypoxic phenotype with clear influence on tumor behavior. Neuroblastoma tumor initiating cells are immature with a neural crest-like phenotype and downregulation of HIF-2α in these cells results in neuronal sympathetic differentiation and the cells become phenotypically similar to the bulk of neuroblastoma cells found in clinical specimens. Knockdown of HIF-2α in neuroblastoma and glioma tumor stem/initiating cells leads to reduced levels of VEGF and poorly vascularized, highly necrotic tumors. As high HIF-2α expression further correlates with disseminated disease as demonstrated in neuroblastoma, glioma, and breast carcinoma, we propose that targeting HIF-2α and/or the pseudo-hypoxic phenotype induced by HIF-2 under normoxic conditions has great clinical potential.

Hypoxia-inducible factor-2alpha-expressing interstitial fibroblasts are the only renal cells that express erythropoietin under hypoxia-inducible factor stabilization

The adaptation of erythropoietin production to oxygen supply is determined by the abundance of hypoxia-inducible factor (HIF), a regulation that is induced by a prolyl hydroxylase. To identify cells that express HIF subunits (HIF-1alpha and HIF-2alpha) and erythropoietin, we treated Sprague-Dawley rats with the prolyl hydroxylase inhibitor FG-4497 for 6 h to induce HIF-dependent erythropoietin transcription. The kidneys were analyzed for colocalization of erythropoietin mRNA with HIF-1alpha and/or HIF-2alpha protein along with cell-specific identification markers. FG-4497 treatment strongly induced erythropoietin mRNA exclusively in cortical interstitial fibroblasts. Accumulation of HIF-2alpha was observed in these fibroblasts and in endothelial and glomerular cells, whereas HIF-1alpha was induced only in tubular epithelia. A large proportion (over 90% in the juxtamedullary cortex) of erythropoietin-expressing cells coexpressed HIF-2alpha. No colocalization of erythropoietin and HIF-1alpha was found. Hence, we conclude that in the adult kidney, HIF-2alpha and erythropoietin mRNA colocalize only in cortical interstitial fibroblasts, which makes them the key cell type for renal erythropoietin synthesis as regulated by HIF-2alpha.

Morg1 heterozygous mice are protected from acute renal ischemia-reperfusion injury

Renal ischemia and reperfusion injury leads to acute renal failure when proinflammatory and apoptotic processes in the kidney are activated. The increase in hypoxia-inducible transcription factor-alpha (HIF-alpha), an important transcription factor for several genes, can attenuate ischemic renal injury. We recently identified a novel WD-repeat protein designated Morg1 (MAPK organizer 1) that interacts with prolyl hydroxylase 3 (PHD3), an important enzyme involved in the regulation of HIF-1alpha and HIF-2alpha expression. While homozygous Morg1 -/- mice are embryonic lethal, heterozygous Morg1 +/- mice have a normal phenotype. We show here that Morg1 +/- were partially protected from renal ischemia-reperfusion injury compared with wild-type Morg1 +/+ animals. Morg1 +/- mice compared with wild-type animals revealed a stronger increase in HIF-1alpha and HIF-2alpha expression in the ischemic-reperfused kidney associated with enhanced serum erythropoietin levels. However, no significant expression of HIF-1alpha and HIF-2alpha was found in nonischemic kidneys without any difference between Morg1 +/- and Morg1 +/+ mice. Ischemic kidneys of Morg1 +/- mice expressed more erythropoietin mRNA than ischemic kidneys from wild-type animals. Renal ischemia in Morg1 +/- mice resulted in a decrease in renal inflammation and reduction of proinflammatory cytokines (MCP-1, IP-10, MIP-2) compared with wild-type mice. Furthermore, there was significantly less apoptosis and tubular damage in Morg1 +/- kidneys after ischemia-reperfusion, and this was also reflected in significantly improved renal function compared with wild-type. Thus Morg1 may be a novel therapeutic target to limit renal injury after ischemia-reperfusion.

The interaction between ischemia-reperfusion and immune responses in the kidney

Kidney ischemia-reperfusion injury (IRI) engages both the innate and adaptive immune responses. Cellular mediators of immunity, such as dendritic cells, neutrophils, macrophages, natural killer T, T, and B cells, contribute to the pathogenesis of renal injury after IRI. Postischemic kidneys express increased levels of adhesion molecules on endothelial cells and toll-like receptors on tubular epithelial cells. Soluble components of the immune system, such as complement activation proteins and cytokines, also participate in injury/repair of postischemic kidneys. Experimental studies on the immune response in kidney IRI have resulted in better understanding of the mechanisms underlying IRI and led to the discovery of novel therapeutic and diagnostic targets.

Drug discovery for overcoming chronic kidney disease (CKD): prolyl-hydroxylase inhibitors to activate hypoxia-inducible factor (HIF) as a novel therapeutic approach in CKD

Hypoxia-inducible factor (HIF) is a heterodimeric transcription factor composed of an oxygen-dependent alpha-subunit and constitutively expressed beta subunit, which plays a central role in cellular adaptation to hypoxia by transcriptionally upregulating its target genes involved in angiogenesis, erythropoiesis, glycolysis, and so on. Recent studies demonstrated that hypoxia in the tubulointerstitium is involved in the pathology of progressive renal diseases and that HIF, which is activated in experimental kidney diseases, may serve to protect tubulointerstitium from the ischemic insult. The expression of HIF alpha-chains is post-translationally regulated and hydroxylation at one or two of the conserved proline residues by prolyl-hydroxylase domains (PHDs) is a critical step for the oxygen-dependent recruitment of the von Hippel-Lindau gene product (pVHL), a recognition component of the E3 ubiquitin ligase complex, and degradation of HIF-alpha. Conversely, modalities to inhibit the enzymatic activities of PHDs have been shown to activate HIF irrespective of oxygenation status and are regarded as candidate targets of pharmacological approaches against chronic kidney diseases characterized by hypoxia.

Pimonidazole adduct immunohistochemistry in the rat kidney: detection of tissue hypoxia

Immunohistochemistry for pimonidazole adducts serves to define hypoxia within tissues. For this purpose, pimonidazole is delivered in vivo, binds to thiol groups at oxygen tensions below 10 mmHg, and is visualized with help of commercially available anti-pimonidazole antibodies. Renal parenchymal oxygen distribution is highly variable under normal conditions and during acute renal failure and chronic renal disorders. Pimonidazole immunostaining clearly helps in delineating hypoxic regions within the kidneys, but technical pitfalls should be taken into account. In particular, tissue fixation by in vivo perfusion is strongly recommended in order to eliminate artificial staining, because immersion fixation per se can promote a hypoxic environment within kidney tissue.

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.

The renal cortical interstitium: morphological and functional aspects

The renal interstitial compartment, situated between basement membranes of epithelia and vessels, contains two contiguous cellular networks. One network is formed by interstitial fibroblasts, the second one by dendritic cells. Both are in intimate contact with each other. Fibroblasts are interconnected by junctions and connected to basement membranes of vessels and tubules by focal adhesions. Fibroblasts constitute the “skeleton” of the kidney. In the renal cortex, fibroblasts produce erythropoietin and are distinguished from other interstitial cells by their prominent F-actin cytoskeleton, abundance of rough endoplasmic reticulum, and by ecto-5′-nucleotidase expression in their plasma membrane. The resident dendritic cells belong to the mononuclear phagocyte system and fulfil a sentinel function. They are characterized by their expression of MHC class II and CD11c. The central situation of fibroblasts suggests that signals from tubules, vessels, and inflammatory cells converge in fibroblasts and elicit an integrated response. Following tubular damage and inflammatory signals fibroblasts proliferate, change to the myofibroblast phenotype and increase their collagen production, potentially resulting in renal fibrosis. The acquisition of a profibrotic phenotype by fibroblasts in renal diseases is generally considered a main causal event in the progression of chronic renal failure. However, it might also be seen as a repair process.

Intrarenal oxygenation: unique challenges and the biophysical basis of homeostasis

The kidney is faced with unique challenges for oxygen regulation, both because its function requires that perfusion greatly exceeds that required to meet metabolic demand and because vascular control in the kidney is dominated by mechanisms that regulate glomerular filtration and tubular reabsorption. Because tubular sodium reabsorption accounts for most oxygen consumption (Vo2) in the kidney, renal Vo2 varies with glomerular filtration rate. This provides an intrinsic mechanism to match changes in oxygen delivery due to changes in renal blood flow (RBF) with changes in oxygen demand. Renal Vo2 is low relative to supply of oxygen, but diffusional arterial-to-venous (AV) oxygen shunting provides a mechanism by which oxygen superfluous to metabolic demand can bypass the renal microcirculation. This mechanism prevents development of tissue hyperoxia and subsequent tissue oxidation that would otherwise result from the mismatch between renal Vo2 and RBF. Recent evidence suggests that RBF-dependent changes in AV oxygen shunting may also help maintain stable tissue oxygen tension when RBF changes within the physiological range. However, AV oxygen shunting also renders the kidney susceptible to hypoxia. Given that tissue hypoxia is a hallmark of both acute renal injury and chronic renal disease, understanding the causes of tissue hypoxia is of great clinical importance. The simplistic paradigm of oxygenation depending only on the balance between local perfusion and Vo2 is inadequate to achieve this goal. To fully understand the control of renal oxygenation, we must consider a triad of factors that regulate intrarenal oxygenation: local perfusion, local Vo2, and AV oxygen shunting.

Nitric oxide regulates expression of sonic hedgehog and hypoxia-inducible factor-1alpha in an experimental model of kidney ischemia-reperfusion

This study was designed to determine the effect of L-arginine on hypoxia inducible factor alpha (HIF-1 alpha) and Sonic hedgehog (Shh) levels considered to be involved in the development of ischemia/reperfusion (I/R) injury. Unilaterally nephrectomized Sprague-Dawley rats were subjected to 60 minutes of left renal ischemia followed by 45 minutes of reperfusion. Group 1 were sham-operated animals; group 2, I-R/Untreated animals; and group 3, I-R/L-Arg-treated animals. Serum creatinine, blood urea nitrogen (BUN), and kidney malondialdehyde (MDA) levels were determined as well as examining the kidneys histologically. The treatment of rats with L-Arg produced a significant reduction in the levels of BUN, creatinine, MDA, and histopathological score compared to renal I/R groups. The Shh expression in the tubulus epithelia were intensely increased in the I-R/L-Arg group when compared to that of the Sham-control and the I-R/untreated groups. Additionally, the HIF-1alpha expression in the tubulus epithelia and the interstitial spaces were intensely increased in the I-R/L-Arg group. These findings suggest that NO reduces the renal dysfunction associated with I/R of the kidney and may act as a trigger to induce Shh and HIF-1 activity.

Inhibition of hypoxia inducible factor hydroxylases protects against renal ischemia-reperfusion injury

Acute renal failure resulting from hypoperfusion and hypoxia is a significant clinical problem. Hypoxia activates the heterodimeric transcription factor hypoxia inducible factor (HIF), leading to changes in gene expression that promote tissue adaptation and survival. To determine whether HIF may protect the kidney from ischemia-reperfusion injury, we subjected hif1a(+/-) and hif2a(+/-) mice to renal ischemia-reperfusion injury. Injury was substantially more severe in hif(+/-) than in littermate controls, consistent with a protective role for HIF. Because wild-type mice exhibited submaximal HIF accumulation in response to no-flow ischemia, we tested compounds that might augment the protective HIF response following ischemia-reperfusion in these animals. We found that l-mimosine and dimethyloxalylglycine, two small molecules that activate HIF by inhibiting HIF hydroxylases, protected mouse kidneys from ischemia-reperfusion injury. Therefore, pharmacological activation of HIF may offer an effective strategy to protect the kidney from ischemic injury.

HIF activation protects from acute kidney injury

The contribution of hypoxia to cisplatin-induced renal tubular injury is controversial. Because the hypoxia-inducible factor (HIF) pathway is a master regulator of adaptation to hypoxia, we measured the effects of cisplatin on HIF accumulation in vitro and in vivo, and tested whether hypoxic preconditioning is protective against cisplatin-induced injury. We found that cisplatin did not stabilize HIF-1alpha protein in vitro or in vivo under normoxic conditions. However, hypoxic preconditioning of cisplatin-treated proximal tubular cells in culture reduced apoptosis in an HIF-1alpha-dependent fashion and increased cell proliferation as measured by BrdU incorporation. In vivo, rats preconditioned with carbon monoxide before cisplatin administration had significantly better renal function than rats kept in normoxic conditions throughout. Moreover, the histomorphological extent of renal damage and tubular apoptosis was reduced by the preconditional treatment. Therefore, development of pharmacologic agents to induce renal HIF might provide a new approach to ameliorate cisplatin-induced nephrotoxicity.