Strikingly conserved gene expression changes of polyamine regulating enzymes among various forms of acute and chronic kidney injury.

The polyamines spermidine and spermine and their common precursor molecule putrescine are involved in tissue injury and repair. Here, we test the hypothesis that impaired polyamine homeostasis contributes to various kidney pathologies in mice during experimental models of ischemia-reperfusion, transplantation, rhabdomyolysis, cyclosporine treatment, arterial hypertension, diabetes, unilateral ureteral obstruction, high oxalate feeding, and adenine-induced injuries. We found a remarkably similar pattern in most kidney pathologies with reduced expression of enzymes involved in polyamine synthesis together with increased expression of polyamine degrading enzymes. Transcript levels of amine oxidase copper-containing 1 (Aoc1), an enzyme which catalyzes the breakdown of putrescine, were barely detectable by in situ mRNA hybridization in healthy kidneys. Aoc1 was highly expressed upon various experimental kidney injuries resulting in a significant reduction of kidney putrescine content. Kidney levels of spermine were also significantly reduced, whereas spermidine was increased in response to ischemia-reperfusion injury. Increased Aoc1 expression in injured kidneys was mainly accounted for by an Aoc1 isoform that harbors 22 additional amino acids at its N-terminus and shows increased secretion. Mice with germline deletion of Aoc1 and injured kidneys showed no decrease of kidney putrescine content; although they displayed no overt phenotype, they had fewer tubular casts upon ischemia-reperfusion injury. Hyperosmotic stress stimulated AOC1 expression at the transcriptional and post-transcription levels in metanephric explants and kidney cell lines. AOC1 expression was also significantly enhanced after kidney transplantation in humans. These data demonstrate that the kidneys respond to various forms of injury with down-regulation of polyamine synthesis and activation of the polyamine breakdown pathway. Thus, an imbalance in kidney polyamines may contribute to various etiologies of kidney injury.

Daprodustat prevents cyclosporine-A-mediated anemia and peritubular capillary loss.

Chronic Cyclosporine-A treatment is associated with serious side effects, including kidney toxicity and anemia. Although pathophysiology of Cyclosporine-A-induced kidney injury remains incompletely understood, hypoxia is likely involved. Here, we investigated the effect of the hypoxia inducible factor activator daprodustat on Cyclosporine-A -induced kidney toxicity. As Cyclosporine-A profoundly alters protein phosphorylation by inhibiting the phosphatase calcineurin, special attention was directed towards the kidney phospho-proteome. Mice received Cyclosporine-A with or without daprodustat for up to eight weeks. In kidney homogenates, 1360 selected proteins were analyzed at expression and phosphorylation levels. Of these, Cyclosporine-A changed the expression of 79 and the phosphorylation of 86 proteins. However, when Cyclosporine-A treatment was combined with daprodustat, the expression of 95 proteins and phosphorylation of only six proteins was altered suggesting that daprodustat prevented most protein phosphorylation brought about by Cyclosporine-A. Although daprodustat showed only marginal effect on its own, angiogenesis-related pathways were among the most profoundly impacted by daprodustat when given on top of Cyclosporine-A. Additionally, Cyclosporine-A lowered the blood hemoglobin concentration and caused kidney capillary rarefaction, which daprodustat prevented. Thus, combined daprodustat/Cyclosporine-A treatment prevented deleterious Cyclosporine-A effects on microcirculation and hemoglobin, and the protective action of daprodustat involves suppression of broad protein phosphorylation changes caused by Cyclosporine-A.

Semaphorin class 3C, vascular permeability, and the swollen injured kidney.

Class 3 semaphorins (SEMA3) are secreted glycoproteins with established roles in the developing brain, heart, and kidney. In this issue of Kidney International, Cai et al. show that in acutely injured kidneys, semaphorin isoform SEMA3C is expressed de novo in glomeruli and the nephron, secreted into the circulation, and excreted into the urine. Compelling evidence is provided for SEMA3C promoting microvascular permeability, kidney swelling, and acute injury. SEMA3C antagonism may be a treatment option for acute kidney injury.

Pathophysiology of unilateral ischemia-reperfusion injury: importance of renal counterbalance and implications for the AKI-CKD transition.

Unilateral ischemia-reperfusion (UIR) injury leads to progressive renal atrophy and tubulointerstitial fibrosis (TIF) and is commonly used to investigate the pathogenesis of the acute kidney injury-chronic kidney disease transition. Although it is well known that contralateral nephrectomy (CNX), even 2 wk post-UIR injury, can improve recovery, the physiological mechanisms and tubular signaling pathways mediating such improved recovery remain poorly defined. Here, we examined the renal hemodynamic and tubular signaling pathways associated with UIR injury and its reversal by CNX. Male Sprague-Dawley rats underwent left UIR or sham UIR and 2 wk later CNX or sham CNX. Blood pressure, left renal blood flow (RBF), and total glomerular filtration rate were assessed in conscious rats for 3 days before and over 2 wk after CNX or sham CNX. In the presence of a contralateral uninjured kidney, left RBF was lower (P < 0.05) from 2 to 4 wk following UIR (3.6 ± 0.3 mL/min) versus sham UIR (9.6 ± 0.3 mL/min). Without CNX, extensive renal atrophy, TIF, and tubule dedifferentiation, but minimal pimonidazole and hypoxia-inducible factor-1α positivity in tubules, were present at 4 wk post-UIR injury. Conversely, CNX led (P < 0.05) to sustained increases in left RBF (6.2 ± 0.6 mL/min) that preceded the increases in glomerular filtration rate. The CNX-induced improvement in renal function was associated with renal hypertrophy, more redifferentiated tubules, less TIF, and robust pimonidazole and hypoxia-inducible factor-1α staining in UIR injured kidneys. Thus, contrary to expectations, indexes of hypoxia are not observed with the extensive TIF at 4 wk post-UIR injury in the absence of CNX but are rather associated with the improved recovery of renal function and structure following CNX.

Near-drowning: new perspectives for human hypoxic acute kidney injury.

Concepts regarding hypoxic acute kidney injury (AKI) are derived from widely used warm ischemia-reflow (WIR) models, characterized by extensive proximal tubular injury and associated with profound inflammation. However, there is ample clinical and experimental data indicating that hypoxic AKI may develop without total cessation of renal blood flow, with a different injury pattern that principally affects medullary thick limbs in the outer medulla. This injury pattern likely reflects an imbalance between blood and oxygen supply and oxygen expenditure, principally for tubular transport. Experimental models of hypoxic AKI other than WIR are based on mismatched oxygen delivery and consumption, particularly within the physiologically hypoxic outer medulla. However, evidence for such circumstances in human AKI is lacking. Recent analysis of the clinical course and laboratory findings of patients following near-drowning (ND) provides a rare glimpse into such a scenario. This observation supports the role of renal hypoxia in the evolution of AKI, as renal impairment could be predicted by the degree of whole-body hypoxia (reflected by lactic acidosis). Furthermore, there was a close association of renal functional impairment with indices of reduced oxygen delivery (respiratory failure and features of intense sympathetic activity) and of enhanced oxygen consumption for active tubular transport (extrapolated from the calculated volume of consumed hypertonic seawater). This unique study in humans supports the concept of renal oxygenation imbalance in hypoxic AKI. The drowning scenario, particularly in seawater, may serve as an archetype of this disorder, resulting from reduced oxygen delivery, combined with intensified oxygen consumption for tubular transport.

Oxygenation of the Transplanted Kidney.

Although kidney oxygen tensions are heterogenous, and mostly below renal vein level, the nephron is highly dependent on aerobic metabolism for active tubular transport. This renders the kidney particularly susceptible to hypoxia, which is considered a main characteristic and driver of acute and chronic kidney injury, albeit the evidence supporting this assumption is not entirely conclusive. Kidney transplants are exposed to several conditions that may interfere with the balance between oxygen supply and consumption, and enhance hypoxia and hypoxic injury. These include conditions leading to and resulting from brain death of kidney donors, ischemia and reperfusion during organ donation, storage and transplantation, postoperative vascular complications, vasoconstriction induced by immunosuppression, and impaired perfusion resulting from interstitial edema, inflammation, and fibrosis. Acute graft injury, the immediate consequence of hypoxia and reperfusion, results in delayed graft function and increased risk of chronic graft failure. Although current strategies to alleviate hypoxic/ischemic graft injury focus on limiting injury (eg, by reducing cold and warm ischemia times), experimental evidence suggests that preconditioning through local or remote ischemia, or activation of the hypoxia-inducible factor pathway, can decrease hypoxic injury. In combination with ex vivo machine perfusion such approaches hold significant promise for improving transplantation outcomes.

Episodic Hypoxia Promotes Defence Against Cellular Stress.

BACKGROUND/AIMS: Recently, we have demonstrated that episodic hypoxia occurs in kidneys of mice challenged repetitively with the immunosuppressant cyclosporine A (CsA), in analogy to humans on CsA treatment. However, the molecular consequences of episodic hypoxia remain poorly defined, as is its impact on cell survival. Here, we systematically study cell response to episodic, as compared to single course hypoxia. METHODS: In vivo, kidneys of mice challenged daily with CsA for one week were analyzed by microarray analysis, gene ontology analysis, and qPCR. In vitro, renal cells were subjected to hypoxia (1 % O2) which was either episodic (4 h for 6 consecutive days), short-term (4 h), or sustained (24 h). Western blot analysis quantified hypoxia-inducible factor-1α (HIF-1α). 2',7'-dichlorofluorescein diacetate detected intracellular ROS. After re-oxygenation, staurosporine served to induce apoptosis, quantified by active caspase-3. RESULTS: In vivo, HIF target gene expression was suppressed by daily CsA treatment. Yet, we found up-regulation of genes involved in defence against cellular stress, notably against ROS. Renal cells in vitro behaved largely different under episodic and sustained hypoxia, while their response to short-term hypoxia oscillated between the previous two. Episodic hypoxia exhibited the highest total HIF-1α protein level, lowest nucleus-to-cytoplasm ratio, and lowest HIF target gene expression. When compared with normoxia, re-oxygenation after sustained hypoxia increased ROS by 3.04 ± 1.04 fold (p<0.001), and re-oxygenation after episodic hypoxia by 1.26 ± 0.16 fold (p<0.01). Staurosporine-induced active caspase-3 was highest after sustained, and lowest after episodic hypoxia. CONCLUSION: In vitro episodic hypoxia mimics the largely HIF-independent transcriptome observed after repetitive CsA treatment in vivo. In vitro preconditioning with episodic hypoxia protects against stress-induced apoptosis. Despite of its long-term adverse effects, CsA derived episodic hypoxia induces a unique renal hypoxia response that provides adaptation to re-oxygenation mediated ROS damage.

First-in-human Phase I studies of PRS-080#22, a hepcidin antagonist, in healthy volunteers and patients with chronic kidney disease undergoing hemodialysis.

In chronic kidney disease both renal insufficiency and chronic inflammation trigger elevated hepcidin levels, which impairs iron uptake, availability. and erythropoiesis. Here we report the two first-in-human phase 1 trials of PRS-080#22, a novel, rationally engineered Anticalin protein that targets and antagonizes hepcidin. A single intravenous infusion of placebo or PRS-080#22 was administered to 48 healthy volunteers (phase 1a) and 24 patients with end stage chronic kidney disease (CKD) on hemodialysis (phase 1b) at different doses (0.08-16mg/kg for the phase 1a study and 2-8mg/kg for the phase 1b study) in successive dosing cohorts. The primary endpoint for both randomized, double-blind, phase 1 trials was safety and tolerability. Following treatment, all subjects were evaluable, with none experiencing dose limiting toxicities. Most adverse events were mild. One serious adverse event occurred in the phase 1b (CKD patient) study. There were no clinically significant changes in safety laboratory values or vital signs. PRS-080#22 showed dose-proportional pharmacokinetics (PK), with a terminal half-life of approximately three days in healthy volunteers and 10 to 12 days in CKD patients. Serum hepcidin levels were suppressed in a dose dependent manner and remained low for up to 48 hours after dosing. PRS-080#22 dose-dependently mobilized serum iron with increases in both serum iron concentration and transferrin saturation. No consistent changes were observed with regard to ferritin, reticulocytes, hemoglobin, and reticulocyte hemoglobin. Low titer anti-drug-antibodies were detected in five healthy volunteers but in none of the CKD patients. PRS-080#22, a novel Anticalin protein with picomolar affinity for hepcidin, was safe and well-tolerated when administered to healthy volunteers and CKD patients at all doses tested. The drug exhibited linear pharmacokinetics, longer half-life in CKD patients in comparison to healthy volunteers as well as expected pharmacodynamic effects which hold promise for further clinical studies.

Urinary Calprotectin Differentiates Between Prerenal and Intrinsic Acute Renal Allograft Failure.

BACKGROUND: Urinary calprotectin has recently been identified as a promising biomarker for the differentiation between prerenal and intrinsic acute kidney injury (AKI) in the nontransplant population. The present study investigates whether calprotectin is able to differentiate between these 2 entities in transplant recipients as well. METHODS: Urinary calprotectin was assessed by enzyme-linked immunosorbent assay in 328 subjects including 125 cases of intrinsic acute allograft failure, 27 prerenal graft failures, 118 patients with stable graft function, and 58 healthy controls. Acute graft failure was defined as AKI stages 1 to 3 (Acute Kidney Injury Network criteria), exclusion criteria were obstructive uropathy, urothelial carcinoma, and metastatic cancer. The clinical differentiation of prerenal and intrinsic graft failure was performed either by biopsy or by a clinical algorithm including response to fluid repletion, history, physical examination, and urine dipstick examination. RESULTS: Reasons for intrinsic graft failure comprised rejection, acute tubular necrosis, urinary tract infection/pyelonephritis, viral nephritis, and interstitial nephritis. Calprotectin concentrations of patients with stable graft function (50.4 ng/mL) were comparable to healthy controls (54.8 ng/mL, P = 0.70) and prerenal graft failure (53.8 ng/mL, P = 0.62). Median urinary calprotectin was 36 times higher in intrinsic AKI (1955 ng/mL) than in prerenal AKI (P < 0.001). Receiver-operating characteristic curve analysis revealed a high accuracy of calprotectin (area under the curve, 0.94) in the differentiation of intrinsic versus prerenal AKI. A cutoff level of 134.5 ng/mL provided a sensitivity of 90.4% and a specificity of 74.1%. Immunohistochemical stainings for calprotectin in renal allograft biopsy specimens confirmed the serological results. CONCLUSIONS: Urinary calprotectin is a promising biomarker for the differentiation of prerenal and intrinsic acute renal allograft failure.

Endothelin-converting enzyme is a plausible target gene for hypoxia-inducible factor.

Renal endothelin-converting enzyme (ECE)-1 is induced in experimental diabetes and following radiocontrast administration, conditions characterized by renal hypoxia, hypoxia-inducible factor (HIF) stabilization, and enhanced endothelin synthesis. Here we tested whether ECE-1 might be a HIF-target gene in vitro and in vivo. ECE-1 transcription and expression increased in cultured vascular endothelial and proximal tubular cell lines, subject to hypoxia, to mimosine or cobalt chloride. These interventions are known to stabilize HIF signaling by inhibition of HIF-prolyl hydroxylases. In rats, HIF-prolyl-hydroxylase inhibition by mimosine or FG-4497 increased HIF-1α immunostaining in renal tubules, principally in distal nephron segments. This was associated with markedly enhanced ECE-1 protein expression, predominantly in the renal medulla. A progressive and dramatic increase in ECE-1 immunostaining over time, in parallel with enhanced HIF expression, was also noted in conditional von Hippel-Lindau knockout mice. Since HIF and STAT3 are cross-stimulated, we triggered HIF expression by STAT3 activation in mice, transfected by or injected with a chimeric IL-6/IL-6-receptor protein, and found a similar pattern of enhanced ECE-1 expression. Chromatin immunoprecipitation sequence (ChIP-seq) and PCR analysis in hypoxic endothelial cells identified HIF binding at the ECE-1 promoter and intron regions. Thus, our findings suggest that ECE-1 may be a novel HIF-target gene.

Urinary calprotectin: a new diagnostic marker in urothelial carcinoma of the bladder.

PURPOSE: Recently, a proteomic study of sera from patients with bladder cancer identified S100A8 and S100A9 as tumor-associated proteins. The present cross-sectional study investigates whether calprotectin, the heterodimer of S100A8/S100A9 may serve as a urinary biomarker for the detection of urothelial bladder cancer. METHODS: Urinary calprotectin concentrations were assessed in a population of 181 subjects including 46 cases of bladder cancer. 41 cases of renal cell cancer, 54 cases of prostate cancer, and 40 healthy subjects served as control. Acute kidney injury, urinary tract infection, previous BCG-treatment and secondary transurethral resection of the bladder tumor were defined as exclusion criteria. Assessment was performed by enzyme-linked immunosorbent assay and immunohistochemistry detecting calprotectin. RESULTS: Median calprotectin concentrations (ng/ml) were significantly higher in patients with bladder cancer than in healthy controls (522.3 vs. 51.0, p < 0.001), renal cell cancer (90.4, p < 0.001), and prostate cancer (71.8, p < 0.001). In urothelial carcinoma prominent immunostaining occurred in a subset of tumor cells and in infiltrating myeloid cells. Receiver operating characteristic analysis provided an area under the curve of 0.88 for the differentiation of bladder cancer and healthy control. A cut-off value of 140 ng/ml (determined by Youden's index) resulted in sensitivity and specificity values of 80.4 and 92.5 %. Low grade tumors were associated with significantly lower calprotectin concentrations than high grade tumors (351.9 vs. 1635.2 ng/ml, p = 0.004). CONCLUSIONS: Urothelial malignancies are associated with highly increased concentrations of calprotecin in the urine. In absence of renal failure and pyuria, calprotectin constitutes a promising biomarker for the detection of bladder cancer.

Stabilization of hypoxia inducible factor-1α ameliorates acute renal neurogenic hypertension.

OBJECTIVES: Renal neurogenic hypertension (RNH) contributes to cardiovascular morbidity. Renal hypoxia may cause RNH and vice versa, leading to a vicious circle. Hypoxia adaptation is conferred through hypoxia-inducible factors (HIFs). We hypothesized that acute RNH is accompanied by increased renal vascular resistance (RVR) and that hypertension and increased RVR are countered by increasing HIF-1α by cobalt chloride (CoCl2) preconditioning. METHODS: First, we studied mean arterial pressure (MAP) and RVR in innervated or denervated contralateral kidneys in anesthetized rats before and after unilateral intrarenal injection of phenol, a manoeuvre known to elicit acute RNH. Then HIFα was induced by CoCl2 in drinking water (2 mM, 10 days) after which we compared intrarenal isotonic saline or phenol injection on MAP and RVR in CoCl2 preconditioned and control rats. HIF-1α was determined by immunohistochemistry. RESULTS: Unilateral intrarenal phenol induced immediate rise in MAP and contralateral RVR, and comparable HIF-1α upregulation in both kidneys, consistent with bi-renal hypoxia. Removing the phenol-injected kidney immediately normalized MAP. Contralateral renal denervation had no effect on the rise in MAP, but abrogated the contralateral increase in RVR, suggesting mediation by increased efferent nerve activity. Strong renal staining for HIF-1α confirmed efficacy of CoCl2 preconditioning, and time-dependent increase in heme oxygenase-1 gene expression stabilization of HIFα. CoCl2 preconditioning prior to phenol reduced both ΔMAP (+10 ± 2 vs. +20 ± 3%, P=0.015) and ΔRVR (+21 ± 11 vs. +90 ± 26%, P=0.003). CONCLUSION: Acute RNH leads to renal vasoconstriction and increased renal HIF-1α. Increasing HIF-1α by CoCl2 preconditioning ameliorates intrarenal phenol-induced RNH and renal vasoconstriction.

Why is diabetes mellitus a risk factor for contrast-induced nephropathy?

Contrast-induced nephropathy (CIN) remains a leading cause of iatrogenic acute kidney injury, as the usage of contrast media for imaging and intravascular intervention keeps expanding. Diabetes is an important predisposing factor for CIN, particularly in patients with renal functional impairment. Renal hypoxia, combined with the generation of reactive oxygen species, plays a central role in the pathogenesis of CIN, and the diabetic kidney is particularly susceptible to intensified hypoxic and oxidative stress following the administration of contrast media. The pathophysiology of this vulnerability is complex and involves various mechanisms, including a priori enhanced tubular transport activity, oxygen consumption, and the generation of reactive oxygen species. The regulation of vascular tone and peritubular blood flow may also be altered, particularly due to defective nitrovasodilation, enhanced endothelin production, and a particular hyperresponsiveness to adenosine-related vasoconstriction. In addition, micro- and macrovascular diseases and chronic tubulointerstitial changes further compromise regional oxygen delivery, and renal antioxidant capacity might be hampered. A better understanding of these mechanisms and their control in the diabetic patient may initiate novel strategies in the prevention of contrast nephropathy in these susceptible patients.

Tubular von Hippel-Lindau knockout protects against rhabdomyolysis-induced AKI.

Renal hypoxia occurs in AKI of various etiologies, but adaptation to hypoxia, mediated by hypoxia-inducible factor (HIF), is incomplete in these conditions. Preconditional HIF activation protects against renal ischemia-reperfusion injury, yet the mechanisms involved are largely unknown, and HIF-mediated renoprotection has not been examined in other causes of AKI. Here, we show that selective activation of HIF in renal tubules, through Pax8-rtTA-based inducible knockout of von Hippel-Lindau protein (VHL-KO), protects from rhabdomyolysis-induced AKI. In this model, HIF activation correlated inversely with tubular injury. Specifically, VHL deletion attenuated the increased levels of serum creatinine/urea, caspase-3 protein, and tubular necrosis induced by rhabdomyolysis in wild-type mice. Moreover, HIF activation in nephron segments at risk for injury occurred only in VHL-KO animals. At day 1 after rhabdomyolysis, when tubular injury may be reversible, the HIF-mediated renoprotection in VHL-KO mice was associated with activated glycolysis, cellular glucose uptake and utilization, autophagy, vasodilation, and proton removal, as demonstrated by quantitative PCR, pathway enrichment analysis, and immunohistochemistry. In conclusion, a HIF-mediated shift toward improved energy supply may protect against acute tubular injury in various forms of AKI.

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.

Regulation of hypoxia-inducible factor in kidney disease.

Hypoxia plays a crucial role in the pathophysiology of acute kidney injury (AKI) and presumably also chronic kidney disease (CKD). Hypoxia-inducible factor (HIF) is the master transcription factor that regulates adaptive responses against hypoxia. Under hypoxic conditions, HIF activates target genes with hypoxia-responsive elements in their regulatory regions. The HIF isoforms and regulators of HIF (i.e. prolyl hydroxylases) show cell type-specific distributions. Hypoxia is observed in both ischaemic and so-called non-ischaemic forms of AKI. In addition to the acute phase, hypoxia may ensue during the recovery phase of AKI, possibly due to the oxygen-consuming processes of cell growth and proliferation for repair. Although HIF protects the kidney against AKI, intrinsic HIF activation is submaximal in AKI and further augmentation of HIF ameliorates disease manifestations. The kidney in CKD also suffers from hypoxia caused by multiple mechanisms, including sustained oxygen demands in the remaining nephrons due to maladaptive tubuloglomerular feedback. Whether HIF is chronically upregulated in CKD is contentious. Hypoxia-inducible factor activation is a promising therapeutic approach to CKD, but excessive activation of HIF may be deleterious. It is likely that there is a therapeutic window of HIF activation in chronic conditions. Under certain circumstances, animals with CKD are protected against AKI and this may be explained by non-physiological hypoxia of the kidney and subsequent HIF expression. In addition, an acute hypoxic insult may induce long-lasting changes, possibly including epigenetic modifications induced by HIF. These observations suggest a complex interaction between AKI and CKD via hypoxia and HIF activation.