Hypoxia-inducible factors (HIFs) are oxygen-sensitive transcription factors that mediate cellular adaptation to hypoxia. Depending on the type of injury, activation of HIF signaling in renal cells may be renoprotective or promote fibrosis. Isoe and colleagues demonstrate that hyperglycemia activates HIF-1 in mesangial cells via carbohydrate response element binding protein (ChREBP), thus providing a novel link between alterations in systemic glucose homeostasis and HIF-regulated gene expression.
Capillary rarefaction is a hallmark of fibrotic diseases and results in reduced blood perfusion and oxygen delivery. In the kidney, tubulointerstitial fibrosis, which leads to the destruction of renal tissue and the irreversible loss of kidney function, is associated with hypoxia and the activation of Hypoxia-Inducible-Factor (HIF) signaling. HIF-1 and HIF-2 are basic-helix-loop-helix transcription factors that allow cells to survive in a low oxygen environment by regulating energy metabolism, vascular remodeling, erythropoiesis, cellular proliferation and apoptosis. Recent studies suggest that HIF activation promotes epithelial to mesenchymal transition (EMT) and renal fibrogenesis. These findings raise the possibility that the spectrum of HIF activated biological responses to hypoxic stress may differ under conditions of acute and chronic hypoxia. Here we discuss the role of HIF signaling in the pathogenesis and progression of chronic kidney disease.
hypoxia-inducible factor (HIF); hypoxia; chronic kidney disease; fibrosis; epithelial to mesenchymal transition (EMT); epithelial cell plasticity; lysyl oxidases
Over the last two decades molecular studies of inherited tumor syndromes, which are associated with the development of kidney cancer have led to the identification of genes and biochemical pathways that play key roles in the malignant transformation of renal epithelial cells. Some of these findings have broad biological impact and extend beyond renal cancer. This review’s focus is on the von Hippel-Lindau (VHL)/hypoxia-inducible factor (HIF) oxygen-sensing pathway and its role in physiology, energy metabolism and tumorigenesis.
renal cell cancer; VHL tumor suppressor; hypoxia-inducible factor; oxygen; glucose metabolism; mitochondria
Epithelial-to-mesenchymal transition (EMT) is a developmentally vital, molecularly complex cellular process by which epithelial cells lose apico–basal polarity and cell–cell contact, become motile, and acquire mesenchymal characteristics. Under pathophysiological conditions EMT has a central role in cancer progression and metastasis, and has been associated with fibrotic disorders. Microenvironmental changes such as alterations in oxygen levels and activation of hypoxic signaling through hypoxia-inducible factor (HIF) are emerging as important triggers and modulators of EMT. Recent insights into potential molecular mechanisms underlying oxygen-dependent regulation of this process and their relevance to disease are discussed.
chronic kidney disease; EMT; extracellular matrix; fibrosis; hypoxia; hypoxia-indacible factors
Angiotensin II (Ang II) is a major contributor to the progression of renal fibrosis. Wang and colleagues provide evidence that signaling through the prolyl-4-hydroxylase domain (PHD)–hypoxia-inducible factor-1 (HIF-1) pathway mediates profibrotic effects of Ang II in rat renal medullary interstitial cells under normoxic conditions, thus placing the HIF oxygen-sensing pathway into the center of an Ang II-induced profibrotic signaling cascade.
Inactivation of the von Hippel-Lindau tumor suppressor, pVHL, is associated with both hereditary and sporadic renal cysts and renal cell carcinoma, which are commonly thought to arise from the renal proximal tubule. pVHL regulates the protein stability of hypoxia-inducible factor (HIF)-α subunits and loss of pVHL function leads to HIF stabilization. The role of HIF in the development of VHL-associated renal lesions remains to be determined. To investigate the functional consequences of pVHL inactivation and the role of HIF signaling in renal epithelial cells, we used the phosphoenolpyruvate carboxykinase (PEPCK) promoter to generate transgenic mice in which Cre-recombinase is expressed in the renal proximal tubule and in hepatocytes. We found that conditional inactivation of VHL in PEPCK-Cre mutants resulted in renal cyst development that was associated with increased erythropoietin levels and polycythemia. Increased expression of the HIF target gene erythropoietin was limited to the liver, whereas expression of carbonic anhydrase 9 and multidrug resistance gene 1 was up-regulated in the renal cortex of mutant mice. Inactivation of the HIF-α binding partner, arylhydrocarbon receptor nuclear translocator (Arnt), but not Hif-1α, suppressed the development of renal cysts. Here, we present the first mouse model of VHL-associated renal disease that will provide a basis for further genetic studies to define the molecular events that are required for the progression of VHL-associated renal cysts to clear cell renal cell carcinoma.
Iron demand in bone marrow increases when erythropoiesis is stimulated by hypoxia via increased erythropoietin (EPO) synthesis in kidney and liver. Hepcidin, a small polypeptide produced by hepatocytes, plays a central role in regulating iron uptake by promoting internalization and degradation of ferroportin, the only known cellular iron exporter. Hypoxia suppresses hepcidin, thereby enhancing intestinal iron uptake and release from internal stores. While HIF, a central mediator of cellular adaptation to hypoxia, directly regulates renal and hepatic EPO synthesis under hypoxia, the molecular basis of hypoxia/HIF-mediated hepcidin suppression in the liver remains unclear. Here, we used a genetic approach to disengage HIF activation from EPO synthesis and found that HIF-mediated suppression of the hepcidin gene (Hamp1) required EPO induction. EPO induction was associated with increased erythropoietic activity and elevated serum levels of growth differentiation factor 15. When erythropoiesis was inhibited pharmacologically, Hamp1 was no longer suppressed despite profound elevations in serum EPO, indicating that EPO by itself is not directly involved in Hamp1 regulation. Taken together, we provide in vivo evidence that Hamp1 suppression by the HIF pathway occurs indirectly through stimulation of EPO-induced erythropoiesis.
The von Hippel–Lindau tumor suppressor pVHL plays a critical role in the pathogenesis of familial and sporadic clear cell carcinomas of the kidney and hemangioblastomas of the retina and central nervous system. pVHL targets the oxygen sensitive alpha subunit of hypoxia-inducible factor (HIF) for proteasomal degradation, thus providing a direct link between tumorigenesis and molecular pathways critical for cellular adaptation to hypoxia. Cell type specific gene targeting of VHL in mice has demonstrated that proper pVHL mediated HIF proteolysis is fundamentally important for survival, proliferation and differentiation of many cell types and furthermore, that inactivation of pVHL may, unexpectedly, inhibit tumor growth under certain conditions. Mouse knock out studies have provided novel mechanistic insights into VHL associated tumorigenesis and established a central role for HIF in the development of the VHL phenotype.
von Hippel–Lindau (VHL) tumor suppressor; Hypoxia-inducible factor (HIF); Conditional knock out mice; Renal cell cancer; Hemangioma
Renal fibrosis and inflammation are associated with hypoxia, and tissue pO2 plays a central role in modulating the progression of chronic kidney disease. Key mediators of cellular adaptation to hypoxia are hypoxia-inducible factor (HIF)-1 and -2. In the kidney they are expressed in a cell type-specific manner; to what degree activation of each homolog modulates renal fibrogenesis and inflammation has not been established. To address this issue, we used Cre-loxP recombination to activate or to delete both Hif-1 and Hif-2 either globally or cell type-specifically in myeloid cells. Global activation of Hif suppressed inflammation and fibrogenesis in mice subjected to unilateral ureteral obstruction, while activation of Hif in myeloid cells suppressed inflammation only. Suppression of inflammatory cell infiltration was associated with down-regulation of CC chemokine receptors in renal macrophages. Conversely, global deletion or myeloid-specific inactivation of Hif promoted inflammation. Furthermore, prolonged hypoxia suppressed the expression of multiple inflammatory molecules in non-injured kidneys. Collectively, we provide experimental evidence that hypoxia and/or myeloid cell-specific HIF activation attenuates renal inflammation associated with chronic kidney injury.
Hypoxia-inducible factors (HIFs) are heterodimeric oxygen-sensitive basic helix-loop-helix transcription factors that play central roles in cellular adaptation to low oxygen environments. The von-Hippel Lindau tumor suppressor (pVHL) is the substrate recognition component of an E3 ubiquitin ligase and functions as a master regulator of HIF activity by targeting the hydroxylated HIF-alpha subunit for ubiquitylation and rapid proteasomal degradation under normoxic conditions. Mutations in pVHL can be found in familial and sporadic hemangioblastomas, clear cell carcinomas of the kidney, pheochromocytomas and inherited forms of erythrocytosis, illustrating the importance of disrupted molecular oxygen sensing in the pathogenesis of these diseases. Tissue-specific gene targeting of pVHL in mice has demonstrated that efficient execution of HIF proteolysis is critically important for normal tissue physiology, and has provided novel insights into the functional consequences of HIF activation on the cellular and tissue level. Here we focus on the contribution of individual HIF transcription factors to the development of VHL phenotypes and discuss how the pVHL/HIF axis could be exploited pharmacologically.
von Hippel-Lindau (VHL) tumor suppressor; hypoxia-inducible factor (HIF); renal cell cancer; hemangioblastoma; erythropoietin; anemia; metabolism; kidney cysts; mouse model
Renal ischemia reperfusion injury is a major cause of acute kidney injury. We previously found that renal A1 adenosine receptor (A1AR) activation attenuated multiple cell death pathways including necrosis, apoptosis and inflammation. Here, we tested whether induction of cytoprotective sphingosine kinase (SK)-1 and sphingosine-1 phosphate (S1P) synthesis might be the mechanism of protection. A selective A1AR agonist (CCPA) increased the synthesis of S1P and selectively induced SK-1 in mouse kidney and HK-2 cells. This agonist failed to protect SK1-knockout but protected SK2-knockout mice against renal ischemia reperfusion injury indicating a critical role of SK1 in A1AR-mediated renal protection. Inhibition of SK prevented A1AR-mediated defense against necrosis and apoptosis in HK-2 cells. A selective S1P1R antagonist (W146) and global in vivo gene knockdown of S1P1Rs with small interfering RNA completely abolished the renal protection provided by CCPA. Mice selectively deficient in renal proximal tubule S1P1Rs (S1P1Rflox/flox PEPCKCre/−) were not protected against renal ischemia reperfusion injury by CCPA. Mechanistically, CCPA increased nuclear translocation of hypoxia inducible factor-1α in HK-2 cells and selective hypoxia inducible factor-1α inhibition blocked A1AR-mediated induction of SK1. Thus, proximal tubule SK-1 has a critical role in A1AR-mediated protection against renal ischemia reperfusion injury.
apoptosis; hypoxia inducible factor-1α; inflammation; necrosis; sphingosine 1-phosphate
A discrepancy between oxygen availability and demand has been found in most chronic kidney diseases (CKD) irrespective of etiology. This results from a combination of structural and functional changes that are commonly associated with the development of fibrosis, which include a reduction in peritubular blood flow, luminal narrowing of atherosclerotic vessels, capillary rarefaction and vascular constriction due to altered expression of vasoactive factors and signaling molecules (e.g. angiotensin II, endothelin, nitric oxide). Consistent with decreased renal oxygenation in CKD is the increased expression of the oxygen-sensitive α-subunit of hypoxia-inducible factor (HIF)-1. HIF transcription factors are members of the Per-ARNT-Sim (PAS) family of heterodimeric basic helix-loop-helix transcription factors and consist of an oxygen-sensitive α-subunit and a constitutively expressed β-unit, also known as the aryl-hydrocarbon-receptor nuclear translocator (ARNT) or HIF-β. Recent experimental evidence suggests that prolonged activation of HIF signaling in renal epithelial cells enhances maladaptive responses, which lead to fibrosis and further tissue destruction. Cell type-specific functions of individual HIF transcription factors and their relevant transcriptional targets are discussed in the context of renal fibrogenesis.
Hypertension affects more than 1.5 billion people worldwide but the precise cause of elevated blood pressure (BP) cannot be determined in most affected individuals. Nonetheless, blockade of the renin-angiotensin system (RAS) lowers BP in the majority of patients with hypertension. Despite its apparent role in hypertension pathogenesis, the key cellular targets of the RAS that control BP have not been clearly identified. Here we demonstrate that RAS actions in the epithelium of the proximal tubule have a critical and non-redundant role in determining the level of BP. Abrogation of AT1 angiotensin receptor signaling in the proximal tubule alone is sufficient to lower BP, despite intact vascular responses. Elimination of this pathway reduces proximal fluid reabsorption and alters expression of key sodium transporters, modifying pressure-natriuresis and providing substantial protection against hypertension. Thus, effectively targeting epithelial functions of the proximal tubule of the kidney should be a useful therapeutic strategy in hypertension.
Vascular/parenchymal crosstalk is increasingly recognized as important in the development and maintenance of healthy vascularized tissues. The retina is an excellent model in which to study the role of cell type-specific contributions to the process of blood vessel and neuronal growth. During retinal vascular development, glial cells such as astrocytes provide the template over which endothelial cells migrate to form the retinal vascular network, and hypoxia-regulated vascular endothelial growth factor (VEGF) has been demonstrated to play a critical role in this process as well as pathological neovascularization. To investigate the nature of cell-specific contributions to this process, we deleted VEGF and its upstream regulators, the hypoxia-inducible transcription factors HIF-1α and HIF-2α, and the negative regulator of HIFα, von Hippel-Lindau protein (VHL), in astrocytes. We found that loss of hypoxic response and VEGF production in astrocytes does not impair normal development of retinal vasculature, indicating that astrocyte-derived VEGF is not essential for this process. In contrast, using a model of oxygen-induced ischemic retinopathy, we show that astrocyte-derived VEGF is essential for hypoxia-induced neovascularization. Thus, we demonstrate that astrocytes in the retina have highly divergent roles during developmental, physiological angiogenesis and ischemia-driven, pathological neovascularization.
Loss of the VHL tumor suppressor is regarded as an initiating event in the development of clear-cell renal carcinoma. Surprisingly, loss of VHL induces senescence in mouse fibroblasts in vitro, a response that would restrict development of renal carcinoma in vivo. Typical in vitro cell culture levels of oxygen, however, are significantly higher than physiological levels of oxygen, which have been shown to abrogate senescence induced by many stimuli. Therefore, we investigated the oxygen dependence of VHL loss-induced senescence. Using mouse fibroblasts and primary renal epithelial cells in vitro, we found that VHL loss leads to senescence under atmospheric conditions (21% O2), partly through increasing p27 levels, but not under physiological oxygenation (2% to 5% O2), despite maintaining increased p27 expression. This suggests that VHL inactivation sensitizes cells to oxidative stress. In support of this concept, senescence following VHL loss depends on p53 activity, which decreases under the less stressful conditions of mild hypoxia. We confirmed these observations in vivo by treating kidney-specific VHL knockout animals with the potent oxidizer paraquat and observed a robust induction of cellular senescence. Together, these data demonstrate that in vivo oxygenation promotes tolerance of VHL loss in renal epithelia, which may promote the development of renal carcinoma.
Hypoxia-inducible factors (HIFs), in particular HIF-1α, have been implicated in tumor biology. However, HIF target genes in the esophageal tumor microenvironment remain elusive. Gene expression profiling was performed upon hypoxia-exposed non-transformed immortalized human esophageal epithelial cells, EPC2-hTERT, and comparing with a gene signature of esophageal squamous cell carcinoma (ESCC). In addition to known HIF-1α target genes such as carbonic anhydrase 9, insulin-like growth factor binding protein-3 (IGFBP3) and cyclooxygenase (COX)-2, prostaglandin E synthase (PTGES) was identified as a novel target gene among the commonly upregulated genes in ESCC as well as the cells exposed to hypoxia. The PTGES induction was augmented upon stabilization of HIF-1α by hypoxia or cobalt chloride under normoxic conditions and suppressed by dominant-negative HIF-1α. Whereas PTGES messenger RNA (mRNA) was negatively regulated by normoxia, PTGES protein remained stable upon reoxygenation. Prostaglandin E2 (PGE2) biosynthesis was documented in transformed human esophageal cells by ectopic expression of PTGES as well as RNA interference directed against PTGES. Moreover, hypoxia stimulated PGE2 production in a HIF-1α-dependent manner. In ESCC, PTGES was overexpressed frequently at the mRNA and protein levels. Finally, COX-2 and PTGES were colocalized in primary tumors along with HIF-1α and IGFBP3. Activation of the COX-2–PTGES axis in primary tumors was further corroborated by concomitant upregulation of interleukin-1β and downregulation of hydroxylprostaglandin dehydrogenase. Thus, PTGES is a novel HIF-1α target gene, involved in prostaglandin E biosynthesis in the esophageal tumor hypoxic microenvironment, and this has implications in diverse tumors types, especially of squamous origin.
Chronic kidney disease is a leading cause of death in the United States. Tubulointerstitial fibrosis (TIF) is considered the final common pathway leading to end-stage renal disease (ESRD). Here, we used pharmacologic, genetic, in vivo, and in vitro experiments to show that activation of the Notch pathway in tubular epithelial cells (TECs) in patients and in mouse models of TIF plays a role in TIF development. Expression of Notch in renal TECs was found to be both necessary and sufficient for TIF development. Genetic deletion of the Notch pathway in TECs reduced renal fibrosis. Consistent with this, TEC-specific expression of active Notch1 caused rapid development of TIF. Pharmacologic inhibition of Notch activation using a γ-secretase inhibitor ameliorated TIF. In summary, our experiments establish that epithelial injury and Notch signaling play key roles in fibrosis development and indicate that Notch blockade may be a therapeutic strategy to reduce fibrosis and ESRD development.
Skin plays an essential role in adaptation to environmental stimuli that is mediated in part by its remarkable vascular plasticity. Certain vertebrates, such as amphibians, respond to hypoxia in part through the skin; but it is unknown whether this tissue can influence mammalian systemic adaptation to low oxygen levels. We have found that epidermal deletion of the hypoxia responsive transcription factor HIF-1α inhibits renal erythropoietin (EPO) synthesis in response to hypoxia. Conversely, mice with an epidermal deletion of the von Hippel Lindau (VHL) factor, a negative regulator of HIF, have increased EPO synthesis and polycythemia. We show that nitric oxide release induced by the HIF pathway acts on cutaneous vascular flow to increase systemic erythropoietin expression. These results demonstrate that in mice the skin is a critical mediator of systemic responses to environmental oxygen.
In mammals, the liver integrates nutrient uptake and delivery of carbohydrates and lipids to peripheral tissues to control overall energy balance. Hepatocytes maintain metabolic homeostasis by coordinating gene expression programs in response to dietary and systemic signals. Hepatic tissue oxygenation is an important systemic signal that contributes to normal hepatocyte function as well as disease. Hypoxia-inducible factors 1 and 2 (HIF-1 and HIF-2, respectively) are oxygen-sensitive heterodimeric transcription factors, which act as key mediators of cellular adaptation to low oxygen. Previously, we have shown that HIF-2 plays an important role in both physiologic and pathophysiologic processes in the liver. HIF-2 is essential for normal fetal EPO production and erythropoiesis, while constitutive HIF-2 activity in the adult results in polycythemia and vascular tumorigenesis. Here we report a novel role for HIF-2 in regulating hepatic lipid metabolism. We found that constitutive activation of HIF-2 in the adult results in the development of severe hepatic steatosis associated with impaired fatty acid β-oxidation, decreased lipogenic gene expression, and increased lipid storage capacity. These findings demonstrate that HIF-2 functions as an important regulator of hepatic lipid metabolism and identify HIF-2 as a potential target for the treatment of fatty liver disease.
A key adaptation to environmental hypoxia is an increase in erythropoiesis, driven by the hormone erythropoietin (EPO) through what is traditionally thought to be primarily a renal response. However, both neurons and astrocytes (the largest subpopulation of glial cells in the CNS) also express EPO following ischemic injury, and this response is known to ameliorate damage to the brain. To investigate the role of glial cells as a component of the systemic response to hypoxia, we created astrocyte-specific deletions of the murine genes encoding the hypoxia-inducible transcription factors HIF-1α and HIF-2α and their negative regulator von Hippel–Lindau (VHL) as well as astrocyte-specific deletion of the HIF target gene Vegf. We found that loss of the hypoxic response in astrocytes does not cause anemia in mice but is necessary for approximately 50% of the acute erythropoietic response to hypoxic stress. In accord with this, erythroid progenitor cells and reticulocytes were substantially reduced in number in mice lacking HIF function in astrocytes following hypoxic stress. Thus, we have demonstrated that the glial component of the CNS is an essential component of hypoxia-induced erythropoiesis.
Background & Aims
Colon epithelial cells are critical for barrier function and contain a highly developed immune response. A previous study has shown hypoxia-inducible factor (HIF) as a critical regulator of barrier protection during colon epithelial injury. However, the role of HIF signaling in colon mucosal immunity is not known.
With the use of cre/loxP technology, intestinal specific disruption of Vhl, Hif-1α, and Arnt were generated. Colon inflammation was induced using a dextran sulfate sodium (DSS)-induced colitis model and the mice analyzed by histology, western blot analysis, and quantitative polymerase chain reactions.
In mice, colonic epithelium disruption of Vhl resulted in constitutively expression of HIF which initiated an increase in inflammatory infiltrates and edema in the colon. These effects were ameliorated in mice by disruption of both Vhl and Arnt/Hif1β (which inactivates HIF). In a DSS-induced colitis model, increased HIF expression correlated with more severe clinical symptoms and an increase in histological damage, while disruption of both Vhl and Arnt in the colon epithelium inhibited these effects. Furthermore, colons with constitutive activation of HIF displayed increased expression of pro-inflammatory mediators which were synergistically potentiated following DSS administration and reduced by inhibition of the pro-inflammatory and direct HIF-target gene macrophage migration inhibitory factor (MIF).
The present study demonstrates that a chronic increase in HIF signaling in the colon epithelial cells initiates a hyper-inflammatory reaction that may have important implications in developing therapeutic strategies for inflammatory bowel disease.
The von Hippel-Lindau tumor suppressor pVHL regulates the stability of Hypoxia-Inducible Factors (HIF) -1 and –2, oxygen-sensitive basic helix-loop-helix transcription factors, which mediate the hypoxic induction of angiogenic growth factors such as vascular endothelial growth factor (VEGF). Loss of VHL function results in constitutive activation of HIF-1 and HIF-2 and is associated with the development of highly vascularized tumors in multiple organs. We have used a conditional gene targeting approach to investigate the relative contributions of HIF-1 and HIF-2 to VHL-associated vascular tumorigenesis in a mouse model of liver hemangiomas. Here we demonstrate genetically that conditional inactivation of HIF-2α suppressed the development of VHL-associated liver hemangiomas and that angiogenic gene expression in hepatocytes is predominantly regulated by HIF-2 and not by HIF-1. These findings suggest that HIF-2 is the dominant HIF in the pathogenesis of VHL-associated vascular tumors and that pharmacologic targeting of HIF-2 may be an effective strategy for their treatment.
Coenzyme Q (CoQ) is an essential electron carrier in the respiratory chain whose deficiency has been implicated in a wide variety of human mitochondrial disease manifestations. Its multi-step biosynthesis involves production of polyisoprenoid diphosphate in a reaction that requires the enzymes be encoded by PDSS1 and PDSS2. Homozygous mutations in either of these genes, in humans, lead to severe neuromuscular disease, with nephrotic syndrome seen in PDSS2 deficiency. We now show that a presumed autoimmune kidney disease in mice with the missense Pdss2kd/kd genotype can be attributed to a mitochondrial CoQ biosynthetic defect. Levels of CoQ9 and CoQ10 in kidney homogenates from B6.Pdss2kd/kd mutants were significantly lower than those in B6 control mice. Disease manifestations originate specifically in glomerular podocytes, as renal disease is seen in Podocin/cre,Pdss2loxP/loxP knockout mice but not in conditional knockouts targeted to renal tubular epithelium, monocytes, or hepatocytes. Liver-conditional B6.Alb/cre,Pdss2loxP/loxP knockout mice have no overt disease despite demonstration that their livers have undetectable CoQ9 levels, impaired respiratory capacity, and significantly altered intermediary metabolism as evidenced by transcriptional profiling and amino acid quantitation. These data suggest that disease manifestations of CoQ deficiency relate to tissue-specific respiratory capacity thresholds, with glomerular podocytes displaying the greatest sensitivity to Pdss2 impairment.
Coenzyme Q is a critical component of the mitochondrial respiratory chain, the process by which cells make energy. Coenzyme Q deficiency in humans causes a wide range of disease manifestations affecting the nervous system, muscles, and kidneys. Here, we show that the failure to make Coenzyme Q due to a Pdss2 mutation is the cause of a lethal kidney disease in mice that was previously thought to result from an autoimmune process. Studying both a spontaneously occurring missense mutant and a series of mutants generated to have the Coenzyme Q deficiency targeted solely to liver, kidney, or macrophages, we show that the specific cell type in which the kidney disease arises is the glomerular podocyte. No other manifestations of disease are evident in these animals. However, our analysis of livers from these mice reveals that they have significant depletion of Coenzyme Q, impairment of mitochondrial respiratory chain function, and disturbance of many other basic metabolic processes. Similar microarray patterns of cellular alterations to primary mitochondrial dysfunction were seen both in these mice and in a previously reported nematode model, suggesting that a common cellular profile of primary respiratory chain function may exist across evolution.
Hypoxia has been proposed as an important microenvironmental factor in the development of tissue fibrosis; however, the underlying mechanisms are not well defined. To examine the role of hypoxia-inducible factor–1 (HIF-1), a key mediator of cellular adaptation to hypoxia, in the development of fibrosis in mice, we inactivated Hif-1α in primary renal epithelial cells and in proximal tubules of kidneys subjected to unilateral ureteral obstruction (UUO) using Cre-loxP–mediated gene targeting. We found that Hif-1α enhanced epithelial-to-mesenchymal transition (EMT) in vitro and induced epithelial cell migration through upregulation of lysyl oxidase genes. Genetic ablation of epithelial Hif-1α inhibited the development of tubulointerstitial fibrosis in UUO kidneys, which was associated with decreased interstitial collagen deposition, decreased inflammatory cell infiltration, and a reduction in the number of fibroblast-specific protein–1–expressing (FSP-1–expressing) interstitial cells. Furthermore, we demonstrate that increased renal HIF-1α expression is associated with tubulointerstitial injury in patients with chronic kidney disease. Thus, we provide clinical and genetic evidence that activation of HIF-1 signaling in renal epithelial cells is associated with the development of chronic renal disease and may promote fibrogenesis by increasing expression of extracellular matrix–modifying factors and lysyl oxidase genes and by facilitating EMT.
Iron is essential for many biological processes, including oxygen delivery, and its supply is tightly regulated. Hepcidin, a small peptide synthesized in the liver, is a key regulator of iron absorption and homeostasis in mammals. Hepcidin production is increased by iron overload and decreased by anemia and hypoxia; but the molecular mechanisms that govern the hepcidin response to these stimuli are not known. Here we establish that the von Hippel–Lindau/hypoxia-inducible transcription factor (VHL/HIF) pathway is an essential link between iron homeostasis and hepcidin regulation in vivo. Through coordinate downregulation of hepcidin and upregulation of erythropoietin and ferroportin, the VHL-HIF pathway mobilizes iron to support erythrocyte production.