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

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

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

The role of hypoxia-inducible factor 1 alpha (HIF-1α) modulation in heavy metal toxicity

Hypoxia-inducible factor 1 (HIF-1) is an oxygen-sensing transcriptional regulator orchestrating a complex of adaptive cellular responses to hypoxia. Several studies have demonstrated that toxic metal exposure may also modulate HIF-1α signal transduction pathway, although the existing data are scarce. Therefore, the present review aims to summarize the existing data on the effects of toxic metals on HIF-1 signaling and the potential underlying mechanisms with a special focus on prooxidant effect of the metals. The particular effect of metals was shown to be dependent on cell type, varying from down- to up-regulation of HIF-1 pathway. Inhibition of HIF-1 signaling may contribute to impaired hypoxic tolerance and adaptation, thus promoting hypoxic damage in the cells. In contrast, its metal-induced activation may result in increased tolerance to hypoxia through increased angiogenesis, thus promoting tumor growth and contributing to carcinogenic effect of heavy metals. Up-regulation of HIF-1 signaling is mainly observed upon Cr, As, and Ni exposure, whereas Cd and Hg may both stimulate and inhibit HIF-1 pathway. The mechanisms underlying the influence of toxic metal exposure on HIF-1 signaling involve modulation of prolyl hydroxylases (PHD2) activity, as well as interference with other tightly related pathways including Nrf2, PI3K/Akt, NF-κB, and MAPK signaling. These effects are at least partially mediated by metal-induced ROS generation. Hypothetically, maintenance of adequate HIF-1 signaling upon toxic metal exposure through direct (PHD2 modulation) or indirect (antioxidant) mechanisms may provide an additional strategy for prevention of adverse effects of metal toxicity.

Alterations in telomere length and mitochondrial DNA copy number in human lymphocytes on short-term exposure to moderate hypoxia

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Exposure to moderate hypoxia for 24 h significantly increased telomere length.

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Telomere elongation is related to the duration of hypoxia exposure.

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Mitochondrial DNA copy number was unaffected by hypoxia exposure.

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Mitochondrial DNA copy number is a more stable marker than telomere length alteration under hypoxia.

Hypoxia is related to a variety of diseases, such as cardiovascular and inflammatory diseases and various cancers. Telomere length (TL) may vary according to the hypoxia level and cell types. To the best of our knowledge, no study has investigated the effect of moderate hypoxia on TL and mitochondrial DNA copy number (mtDNAcn) in human lymphocytes. Therefore, in this study, we analyzed the effect of moderate hypoxia on TL in correlation with mtDNAcn. This study included 32 healthy male nonsmoker’s subjects; in this cohort, we had previously studied sister chromatid exchange and microsatellite instability. Blood samples from each subject were divided into three groups: a control group and two experimental groups exposed to moderate hypoxia for 12 or 24 h. Relative TL and mtDNAcn were measured by a quantitative real-time polymerase chain reaction. The TL in the control group did not significantly differ from that in the experimental group subjected to hypoxia for 12 h; however, the TL in the 24 h hypoxia–treated experimental group was significantly higher than that in the control group. The correlation between TL and mtDNAcn was not statistically significant in the two hypoxic states. The increase in TL was observed on exposure to hypoxia for 24 h and not for 12 h; thus, the findings suggest that telomere elongation is related to hypoxia exposure duration. The mtDNAcn in the two experimental groups did not significantly differ from that in the control group. These observations suggest that mtDNAcn alterations show more genetic stability than TL alterations. To the best of our knowledge, this is the first in vitro study on human lymphocytes reporting an increase in TL and no alteration in mtDNAcn after short-time exposure to moderate hypoxia.

HIF-1α ameliorates tubular injury in diabetic nephropathy via HO-1-mediated control of mitochondrial dynamics

Objectives

In diabetic nephropathy (DN), hypoxia‐inducible factor‐1α (HIF‐1α) activation in tubular cells plays an important protective role against kidney injury. The effects may occur via the target genes of HIF‐1α, such as haem oxygenase‐1 (HO‐1), but the exact mechanisms are incompletely understood.

Materials and methods

Mice with proximal tubule‐specific knockout of HIF‐1α (PT‐HIF‐1α^−/− mice) were generated, and diabetes was induced in these mice by streptozotocin (STZ) injection. In addition, to mimic a hypoxic state, cobaltous chloride (CoCl₂) was applied to HK‐2 cells.

Results

Our study first verified that conditional knockout of HIF‐1α worsened tubular injury in DN; additionally, aggravated kidney dysfunction, renal histopathological alterations, mitochondrial fragmentation, ROS accumulation and apoptosis were observed in diabetic PT‐HIF‐1α^−/− mice. In vitro study showed that compared to control group, HK‐2 cells cultured under hypoxic ambiance displayed increased mitochondrial fragmentation, ROS production, mitochondrial membrane potential loss and apoptosis. These increases were reversed by overexpression of HIF‐1α or treatment with a HO‐1 agonist. Importantly, cotreatment with a HIF‐1α inhibitor and a HO‐1 agonist rescued the HK‐2 cells from the negative impacts of the HIF‐1α inhibitor.

Conclusions

These data revealed that HIF‐1α exerted a protective effect against tubular injury in DN, which could be mediated via modulation of mitochondrial dynamics through HO‐1 upregulation.

Time-series proteomic study of the response of HK-2 cells to hyperglycemic, hypoxic diabetic-like milieu

During diabetes, renal proximal tubular cells (PTC) are exposed to a combination of high glucose and hypoxic conditions, which plays a relevant role in the development of diabetic kidney disease (DKD). In this work, a time-series proteomic study was performed to analyse the effect of a diabetic-like microenvironment induced changes on HK-2 cells, a human cell line derived from normal proximal tubular epithelial cells. Cells simultaneously exposed to high glucose (25 mM) and hypoxia (1% O₂) were compared to cells in control conditions for up to 48 h. Diabetic conditions increased the percentage of death cells after 24 and 48 h, but no differences in the protein/cell ratio were found. The relative protein quantification using dimethyl-labeling and UHPLC-MS/MS analysis allowed the identification of 317, 296 and 259 proteins at 5, 24 and 48 h, respectively. The combination of statistical and time expression profile analyses indicated an increased expression of proteins involved in glycolysis, and a decrease of cytoskeletal-related proteins. The exposure of HK-2 cells to high glucose and hypoxia reproduces some of the effects of diabetes on PTC and, with the limitations inherent to in vitro studies, propose new mechanisms and targets to be considered in the management of DKD.

Mechanism and Consequences of The Impaired Hif-1α Response to Hypoxia in Human Proximal Tubular HK-2 Cells Exposed to High Glucose

Renal hypoxia and loss of proximal tubular cells (PTC) are relevant in diabetic nephropathy. Hypoxia inhibits hypoxia-inducible factor-1α (HIF-1α) degradation, which leads to cellular adaptive responses through HIF-1-dependent activation of gene hypoxia-responsive elements (HRE). However, the diabetic microenvironment represses the HIF-1/HRE response in PTC. Here we studied the mechanism and consequences of impaired HIF-1α regulation in human proximal tubular HK-2 cells incubated in hyperglycemia. Inhibition at different levels of the canonical pathway of HIF-1α degradation did not activate the HIF-1/HRE response under hyperglycemia, except when proteasome was inhibited. Further studies suggested that hyperglycemia disrupts the interaction of HIF-1α with Hsp90, a known cause of proteasomal degradation of HIF-1α. Impaired HIF-1α regulation in cells exposed to hyperglycemic, hypoxic diabetic-like milieu led to diminished production of vascular endothelial growth factor-A and inhibition of cell migration (responses respectively involved in tubular protection and repair). These effects, as well as impaired HIF-1α regulation, were reproduced in normoglycemia in HK-2 cells incubated with microparticles released by HK-2 cells exposed to diabetic-like milieu. In summary, these results highlight the role of proteasome-dependent mechanisms of HIF-1α degradation on diabetes-induced HK-2 cells dysfunction and suggest that cell-derived microparticles may mediate negative effects of the diabetic milieu on PTC.

Dynamic regulation of epigenetic demethylation by oxygen availability and cellular redox

The chromatin structure of the mammalian genome must facilitate both precisely-controlled DNA replication together with tightly-regulated gene transcription. This necessarily involves complex mechanisms and processes which remain poorly understood. It has long been recognised that the epigenetic landscape becomes established during embryonic development and acts to specify and determine cell fate. In addition, the chromatin structure is highly dynamic and allows for both cellular reprogramming and homeostatic modulation of cell function. In this respect, the functions of epigenetic "erasers", which act to remove covalently-linked epigenetic modifications from DNA and histones are critical. The enzymatic activities of the TET and JmjC protein families have been identified as demethylases which act to remove methyl groups from DNA and histones, respectively. Further, they are characterised as members of the Fe(II)- and 2-oxoglutarate-dependent dioxygenase superfamily. This provides the intriguing possibility that their enzymatic activities may be modulated by cellular metabolism, oxygen availability and redox-based mechanisms, all of which are likely to display dynamic cell- and tissue-specific patterns of flux. Here we discuss the current evidence for such [O₂]- and redox-dependent regulation of the TET and Jmjc demethylases and the potential physiological and pathophysiological functional consequences of such regulation.

Role of HIF-1α in Alcohol-Mediated Multiple Organ Dysfunction

Excess alcohol consumption is a global crisis contributing to over 3 million alcohol-related deaths per year worldwide and economic costs exceeding $200 billion dollars, which include productivity losses, healthcare, and other effects (e.g., property damages). Both clinical and experimental models have shown that excessive alcohol consumption results in multiple organ injury. Although alcohol metabolism occurs primarily in the liver, alcohol exposure can lead to pathophysiological conditions in multiple organs and tissues, including the brain, lungs, adipose, liver, and intestines. Understanding the mechanisms by which alcohol-mediated organ dysfunction occurs could help to identify new therapeutic approaches to mitigate the detrimental effects of alcohol misuse. Hypoxia-inducible factor (HIF)-1 is a transcription factor comprised of HIF-1α and HIF-1β subunits that play a critical role in alcohol-mediated organ dysfunction. This review provides a comprehensive analysis of recent studies examining the relationship between HIF-1α and alcohol consumption as it relates to multiple organ injury and potential therapies to mitigate alcohol’s effects.

High-performance reoxygenation from PLGA-PEG/PFOB emulsions: a feedback relationship between ROS and HIF-1α

Background

Hypoxemia is one of the most common pathological processes in various clinical diseases.

Methods

A novel emulsion of poly(lactide-co-glycolide)-poly(ethylene glycol)/perfluorooctyl bromide has been developed to improve arterial hypoxemia through pulmonary drug delivery. Hypoxia-reoxygenation experiment was used to investigate the ability of the emulsion to supply oxygen and the saline lavage acute lung injury model was established to evaluate oxygen supply of the emulsion.

Results

It has been demonstrated that an apparent increase has been detected in the cytotoxicity test of the emulsion, indicating its lower cell toxicity. A hypoxia-reoxygenation experiment uncovered the fact that notable cell growth was observed after reoxygenation with poly(lactide-co-glycolide)-poly(ethylene glycol)/perfluorooctyl bromide emulsion because of the ability of the emulsion to supply oxygen adequately and reasonably. Moreover, the level of intracellular reactive oxygen species was significantly enhanced during hypoxia, which further influenced the concentration and activity of hypoxia-inducible factor-1α (HIF-1α). Furthermore, the upregulated expression of HIF-1α during hypoxia has verified that certain emulsions can increase HIF-1α content and relieve hypoxia, which further indicates HIF-1α plays an essential role in improving cell viability. Afterwards, the saline lavage acute lung injury model was established to evaluate oxygen supply of the emulsion and the result shows considerable improvement of lung ventilation of rabbits.

Conclusion

We recommend that the feedback relationship between reactive oxygen species and HIF-1 plays an essential role in improving cell viability. It is anticipated that the emulsion will be applied in the field of alleviating hypoxemia.

Knockdown of HIF-1α by siRNA-expressing plasmid delivered by attenuated Salmonella enhances the antitumor effects of cisplatin on prostate cancer

Resistance to cisplatin (DDP) and dose-related toxicity remain two important obstacles in the treatment of prostate cancer (PCa) patients with DDP-based chemotherapy. We have investigated whether the knockdown of hypoxia-inducible factor-1 alpha (HIF-1α) by siRNA could enhance the antitumor activity of DDP, and aimed to determine the underlying mechanisms. Intravenous injection of attenuated Salmonella carrying a HIF-1α siRNA-expressing plasmid was used to knockdown HIF-1α in a PC-3 xenograft model. The in vitro and in vivo effects of HIF-1α siRNA treatment and/or DPP on PCa cell proliferation, apoptosis, glycolysis, and production of reactive oxygen species (ROS) were assessed by examining molecular markers specific to each process. The results demonstrated that the treatment of tumor-bearing mice with attenuated Salmonella carrying the HIF-1α siRNA plasmid greatly enhanced the antitumor effects of low-dose DDP. Further mechanistic studies demonstrated that knockdown of HIF-1α improved the response of PCa cells to DDP by redirecting aerobic glycolysis toward mitochondrial oxidative phosphorylation, leading to cell death through overproduction of ROS. Our findings indicate that DDP-based chemotherapy combined with targeting the HIF-1α-regulated cancer metabolism pathway might be an ideal strategy to treat PCa.

The role of hypoxia-inducible factor-1α in zinc oxide nanoparticle-induced nephrotoxicity in vitro and in vivo

Background

Zinc oxide nanoparticles (ZnO NPs) are used in an increasing number of products, including rubber manufacture, cosmetics, pigments, food additives, medicine, chemical fibers and electronics. However, the molecular mechanisms underlying ZnO NP nephrotoxicity remain unclear. In this study, we evaluated the potential toxicity of ZnO NPs in kidney cells in vitro and in vivo.

Results

We found that ZnO NPs were apparently engulfed by the HEK-293 human embryonic kidney cells and then induced reactive oxygen species (ROS) generation. Furthermore, exposure to ZnO NPs led to a reduction in cell viability and induction of apoptosis and autophagy. Interestingly, the ROS-induced hypoxia-inducible factor-1α (HIF-1α) signaling pathway was significantly increased following ZnO NPs exposure. Additionally, connective tissue growth factor (CTGF) and plasminogen activator inhibitor-1 (PAI-1), which are directly regulated by HIF-1 and are involved in the pathogenesis of kidney diseases, displayed significantly increased levels following ZnO NPs exposure in HEK-293 cells. HIF-1α knockdown resulted in significantly decreased levels of autophagy and increased cytotoxicity. Therefore, our results suggest that HIF-1α may have a protective role in adaptation to the toxicity of ZnO NPs in kidney cells. In an animal study, fluorescent ZnO NPs were clearly observed in the liver, lungs, kidneys, spleen and heart. ZnO NPs caused histopathological lesions in the kidney and increase in serum creatinine and blood urea nitrogen (BUN) which indicate possible renal possible damage. Moreover, ZnO NPs enhanced the HIF-1α signaling pathway, apoptosis and autophagy in mouse kidney tissues.

Conclusions

ZnO NPs may cause nephrotoxicity, and the results demonstrate the importance of considering the toxicological hazards of ZnO NP production and application, especially for medicinal use.

Electronic supplementary material

The online version of this article (doi:10.1186/s12989-016-0163-3) contains supplementary material, which is available to authorized users.

Hypoxia: The Force that Drives Chronic Kidney Disease

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

Titanium nanoparticle inhalation induces renal fibrosis in mice via an oxidative stress upregulated transforming growth factor-β pathway

Titanium dioxide nanoparticles (Nano-TiO2) are gradually being used extensively in clinical settings, industry, and daily life. Accumulation studies showed that Nano-TiO2 exposure is able to cause injuries in various animal organs, including the lung, liver, spleen, and kidney. However, it remains unclear whether exposure of Nano-TiO2 by inhalation causes renal fibrosis. Here, we investigated the role of reactive oxygen species (ROS)/reactive nitrogen species (RNS) related signaling molecules in chronic renal damage after Nano-TiO2 inhalation in mice. Mice were treated with Nano-TiO2 (0.1, 0.25, and 0.5 mg/week) or microparticle-TiO2 (0.5 mg/week) by nonsurgical intratracheal instillation for 4 weeks. The results showed that Nano-TiO2 inhalation increased renal pathological changes in a dose-dependent manner. No renal pathological changes were observed in microparticle-TiO2-instilled mice. Nano-TiO2 (0.5 mg/week) possessed the ability to precipitate in the kidneys, determined by transmission electron microscopy and increased serum levels of blood urea nitrogen. The expressions of markers of ROS/RNS and renal fibrosis markers, including nitrotyrosine, inducible nitric oxide synthase, hypoxia inducible factor-1α (HIF-1α), heme oxygenase 1, transforming growth factor-β (TGFβ), and collagen I, determined by immunohistochemical staining were increased in the kidneys. Furthermore, Nano-TiO2-induced renal injury could be mitigated by iNOS inhibitor aminoguanidine and ROS scavenger N-acetylcysteine treatment in transcription level. The in vitro experiments showed that Nano-TiO2 significantly and dose-dependently increased the ROS production and the expressions of HIF-1α and TGFβ in human renal proximal tubular cells, which could be reversed by N-acetylcysteine treatment. Taken together, these results suggest Nano-TiO2 inhalation might induce renal fibrosis through a ROS/RNS-related HIF-1α-upregulated TGF-β signaling pathway.

Activation of hypoxia-inducible factors prevents diabetic nephropathy

Hyperglycemia results in increased oxygen consumption and decreased oxygen tension in the kidney. We tested the hypothesis that activation of hypoxia-inducible factors (HIFs) protects against diabetes-induced alterations in oxygen metabolism and kidney function. Experimental groups consisted of control and streptozotocin-induced diabetic rats treated with or without chronic cobalt chloride to activate HIFs. We elucidated the involvement of oxidative stress by studying the effects of acute administration of the superoxide dismutase mimetic tempol. Compared with controls, diabetic rats displayed tissue hypoxia throughout the kidney, glomerular hyperfiltration, increased oxygen consumption, increased total mitochondrial leak respiration, and decreased tubular sodium transport efficiency. Diabetic kidneys showed proteinuria and tubulointerstitial damage. Cobalt chloride activated HIFs, prevented the diabetes-induced alterations in oxygen metabolism, mitochondrial leak respiration, and kidney function, and reduced proteinuria and tubulointerstitial damage. The beneficial effects of tempol were less pronounced after activation of HIFs, indicating improved oxidative stress status. In conclusion, activation of HIFs prevents diabetes-induced alteration in kidney oxygen metabolism by normalizing glomerular filtration, which reduces tubular electrolyte load, preventing mitochondrial leak respiration and improving tubular transport efficiency. These improvements could be related to reduced oxidative stress and account for the reduced proteinuria and tubulointerstitial damage. Thus, pharmacologic activation of the HIF system may prevent development of diabetic nephropathy.

Bilirubin activates transcription of HIF-1α in human proximal tubular cells cultured in the physiologic oxygen content

The expression of hypoxia-inducible factor (HIF) is influenced by reactive oxygen species (ROS). Effect of bilirubin on HIF-1 expression in proximal tubular cells was investigated under physiological oxygen concentration, which is relative hypoxic condition mimicking oxygen content in the medulla of renal tissue. The human kidney (HK2) cells were cultured in 5% oxygen with or without bilirubin. HIF-1α protein expression was increased by bilirubin treatment at 0.01-0.2 mg/dL concentration. The messenger RNA expression of HIF-1α was increased by 1.69±0.05 folds in the cells cultured with 0.1 mg/dL bilirubin, compared to the control cells. The inhibitors of PI3K/mTOR, PI3K/AKT, and ERK 1/2 pathways did not attenuate increased HIF-1α expression by bilirubin. HIF-1α expression decreased by 10 µM exogenous hydrogen peroxide (H2O2); scavenger of ROS with or without bilirubin in the HK2 cells increased HIF-1α concentration more than that in the cells without bilirubin. Exogenous H2O2 decreased the phosphorylation of P70S6 kinase, which was completely reversed by bilirubin treatment. Knockdown of NOX4 gene by small interfering RNA (siRNA) increased HIF-1α mRNA expression. In coonclusion, bilirubin enhances HIF-1α transcription as well as the up-regulation of HIF-1α protein translation through the attenuation of ROS and subunits of NADPH oxidase.

Telomeres are elongated in rats exposed to moderate altitude

Background

Leukocyte telomere length has been shown to be associated with life span. Hypoxia-associated changes of telomere length have been detected in cell cultures, but no in vivo studies have reported the changes of telomere length under different hypoxic conditions. This study aimed to evaluate the effects of altitude on telomere length in rat leukocytes.

Methods

One hundred and ten male Wistar rats were randomized into 3 groups and maintained at sea-level (altitude of 10 m) (SL group, n = 10), moderate altitude (2,260 m) (MA group, n = 50), or simulated high altitude (5,000 m (SHA group, n = 50). The last two groups were further divided into 5 subgroups and exposed to hypoxia for 1, 3, 7, 15, or 30 days (n = 10). The leukocyte telomere length, hemoglobin concentration, red blood cell count, hematocrit, and plasma levels of telomerase reverse transcriptase (TERT), hypoxia-inducible factor 1α (HIF-1α), malondialdehyde (MDA) and superoxide dismutase (SOD) were measured.

Results

Leukocyte telomere length was significantly longer in the MA group than in the SL or SHA groups, and the TERT expression changed in a similar manner as the leukocyte telomere length. However, HIF-1α level was significantly higher in both MA and SHA groups than the SL group. SOD level was decreased and MDA level was elevated in SHA group.

Conclusions

The telomere length of blood leukocytes is elongated at a moderate altitude, but not at a high altitude. A mild hypoxic state may increase telomere length.

Ischemia induces different levels of hypoxia inducible factor-1α protein expression in interneurons and pyramidal neurons

Introduction

Pyramidal (glutamatergic) neurons and interneurons are morphologically and functionally well defined in the central nervous system. Although it is known that glutamatergic neurons undergo immediate cell death whereas interneurons are insensitive or survive longer during cerebral ischemia, the protection mechanisms responsible for this interneuronal survival are not well understood. Hypoxia inducible factor-1 (HIF-1) plays an important role in protecting neurons from hypoxic/ischemic insults. Here, we studied the expression of HIF-1α, the regulatable subunit of HIF-1, in the different neuronal phenotypes under in vitro and in vivo ischemia.

Results

In a primary cortical culture, HIF-1α expression was observed in neuronal somata after hypoxia (1% oxygen) in the presence of 5 or 25 mM glucose but not under normoxia (21% oxygen). Interestingly, only certain MAP2-positive neurons containing round somata (interneuron-like morphology) co-localized with HIF-1α staining. Other neurons such as pyramidal-like neurons showed no expression of HIF-1α under either normoxia or hypoxia. The HIF-1α positive neurons were GAD65/67 positive, confirming that they were interneuron-type cells. The HIF-1α expressing GAD65/67-positive neurons also possessed high levels of glutathione. We further demonstrated that ischemia induced significant HIF-1α expression in interneurons but not in pyramidal neurons in a rat model of middle cerebral artery occlusion.

Conclusion

These results suggest that HIF-1α protein expression induced by ischemia is neuron-type specific and that this specificity may be related to the intracellular level of glutathione (GSH).

Hypoxia-inducible factor 1 contributes to N-acetylcysteine's protection in stroke

Stroke is a leading cause of adult morbidity and mortality with very limited treatment options. Evidence from preclinical models of ischemic stroke has demonstrated that the antioxidant N-acetylcysteine (NAC) effectively protects the brain from ischemic injury. Here, we evaluated a new pathway through which NAC exerted its neuroprotection in a transient cerebral ischemia animal model. Our results demonstrated that pretreatment with NAC increased protein levels of hypoxia-inducible factor-1α (HIF-1α), the regulatable subunit of HIF-1, and its target proteins erythropoietin (EPO) and glucose transporter (GLUT)-3, in the ipsilateral hemispheres of rodents subjected to 90min middle cerebral artery occlusion (MCAO) and 24h reperfusion. Interestingly, after NAC pretreatment and stroke, the contralateral hemisphere also demonstrated increased levels of HIF-1α, EPO, and GLUT-3, but to a lesser extent. Suppressing HIF-1 activity with two widely used pharmacological inhibitors, YC-1 and 2ME2, and specific knockout of neuronal HIF-1α abolished NAC's neuroprotective effects. The results also showed that YC-1 and 2ME2 massively enlarged infarcts, indicating that their toxic effect was larger than just abolishing NAC's neuroprotective effects. Furthermore, we determined the mechanism of NAC-mediated HIF-1α induction. We observed that NAC pretreatment upregulated heat-shock protein 90 (Hsp90) expression and increased the interaction of Hsp90 with HIF-1α in ischemic brains. The enhanced association of Hsp90 with HIF-1α increased HIF-1α stability. Moreover, Hsp90 inhibition attenuated NAC-induced HIF-1α protein accumulation and diminished NAC-induced neuroprotection in the MCAO model. These results strongly indicate that HIF-1 plays an important role in NAC-mediated neuroprotection and provide a new molecular mechanism involved in the antioxidant's neuroprotection in ischemic stroke.

Renal overexpression of atrial natriuretic peptide and hypoxia inducible factor-1α as adaptive response to a high salt diet

In the kidney, a high salt intake favors oxidative stress and hypoxia and causes the development of fibrosis. Both atrial natriuretic peptide (ANP) and hypoxia inducible factor (HIF-1α) exert cytoprotective effects. We tested the hypothesis that renal expression of ANP and HIF-1α is involved in a mechanism responding to the oxidative stress produced in the kidneys of rats chronically fed a high sodium diet. Sprague-Dawley rats were fed with a normal salt (0.4% NaCl) (NS) or a high salt (8% NaCl) (HS) diet for 3 weeks, with or without the administration of tempol (T), an inhibitor of oxidative stress, in the drinking water. We measured the mean arterial pressure (MAP), glomerular filtration rate (GFR), and urinary sodium excretion (UV_Na). We evaluated the expression of ANP, HIF-1α, and transforming growth factor (TGF-β1) in renal tissues by western blot and immunohistochemistry. The animals fed a high salt diet showed increased MAP and UV_Na levels and enhanced renal immunostaining of ANP, HIF-1α, and TGF-β1. The administration of tempol together with the sodium overload increased the natriuresis further and prevented the elevation of blood pressure and the increased expression of ANP, TGF-β1, and HIF-1α compared to their control. These findings suggest that HIF-1α and ANP, synthesized by the kidney, are involved in an adaptive mechanism in response to a sodium overload to prevent or attenuate the deleterious effects of the oxidative stress and the hypoxia on the development of fibrosis.

Contribution of endogenously produced reactive oxygen species to the activation of podocyte NLRP3 inflammasomes in hyperhomocysteinemia

Hyperhomocysteinemia (hHcys) is an important pathogenic factor contributing to the progression of end-stage renal disease. Recent studies have demonstrated the implication of nicotinamide adenine dinucleotide phosphate oxidase-mediated NLRP3 inflammasome activation in the development of podocyte injury and glomerular sclerosis during hHcys. However, it remains unknown which reactive oxygen species (ROS) are responsible for this activation of NLRP3 inflammasomes and how such action of ROS is controlled. This study tested the contribution of common endogenous ROS including superoxide (O2(-)), hydrogen peroxide (H2O2), peroxynitrite (ONOO(-)), and hydroxyl radical (OH) to the activation of NLRP3 inflammasomes in mouse podocytes and glomeruli. In vitro, confocal microscopy and size-exclusion chromatography demonstrated that dismutation of O2(-) by 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (Tempol) and decomposition of H2O2 by catalase prevented Hcys-induced aggregation of NLRP3 inflammasome proteins and inhibited Hcys-induced caspase-1 activation and IL-1β production in mouse podocytes. However, scavenging of ONOO(-) or OH had no significant effect on either Hcys-induced NLRP3 inflammasome formation or activation. In vivo, scavenging of O2(-) by Tempol and removal of H2O2 by catalase substantially inhibited NLRP3 inflammasome formation and activation in glomeruli of hHcys mice as shown by reduced colocalization of NLRP3 with ASC or caspase-1 and inhibition of caspase-1 activation and IL-1β production. Furthermore, Tempol and catalase significantly attenuated hHcys-induced glomerular injury. In conclusion, endogenously produced O2(-) and H2O2 primarily contribute to NLRP3 inflammasome formation and activation in mouse glomeruli resulting in glomerular injury or consequent sclerosis during hHcys.

Antioxidant therapeutics: Pandora's box

Evolution has favored the utilization of dioxygen (O2) in the development of complex multicellular organisms. O2 is actually a toxic mutagenic gas that is highly oxidizing and combustible. It is thought that plants are largely to blame for polluting the earth's atmosphere with O2 owing to the development of photosynthesis by blue-green algae over 2 billion years ago. The rise of the plants and atmospheric O2 levels placed evolutionary stress on organisms to adapt or become extinct. This implies that all the surviving creatures on our planet are mutants that have adapted to the "abnormal biology" of O2. Much of the adaptation to the presence of O2 in biological systems comes from well-coordinated antioxidant and repair systems that focus on converting O2 to its most reduced form, water (H2O), and the repair and replacement of damaged cellular macromolecules. Biological systems have also harnessed O2's reactive properties for energy production, xenobiotic metabolism, and host defense and as a signaling messenger and redox modulator of a number of cell signaling pathways. Many of these systems involve electron transport systems and offer many different mechanisms by which antioxidant therapeutics can alternatively produce an antioxidant effect without directly scavenging oxygen-derived reactive species. It is likely that each agent will have a different set of mechanisms that may change depending on the model of oxidative stress, organ system, or disease state. An important point is that all biological processes of aerobes have coevolved with O2 and this creates a Pandora's box for trying to understand the mechanism(s) of action of antioxidants being developed as therapeutic agents.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

Role of haem oxygenase in the renoprotective effects of soluble epoxide hydrolase inhibition in diabetic spontaneously hypertensive rats

We have shown previously that inhibition of sEH (soluble epoxide hydrolase) increased EETs (epoxyeicosatrienoic acids) levels and reduced renal injury in diabetic mice and these changes were associated with induction of HO (haem oxygenase)-1. The present study determines whether the inhibition of HO negates the renoprotective effect of sEH inhibition in diabetic SHR (spontaneously hypertensive rats). After 6 weeks of induction of diabetes with streptozotocin, SHR were divided into the following groups: untreated, treated with the sEH inhibitor t-AUCB {trans-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid}, treated with the HO inhibitor SnMP (stannous mesoporphyrin), and treated with both inhibitors for 4 more weeks; non-diabetic SHR served as a control group. Induction of diabetes significantly increased renal sEH expression and decreased the renal EETs/DHETEs (dihydroxyeicosatrienoic acid) ratio without affecting HO-1 activity or expression in SHR. Inhibition of sEH with t-AUCB increased the renal EETs/DHETEs ratio and HO-1 activity in diabetic SHR; however, it did not significantly alter systolic blood pressure. Treatment of diabetic SHR with t-AUCB significantly reduced the elevation in urinary albumin and nephrin excretion, whereas co-administration of the HO inhibitor SnMP with t-AUCB prevented these changes. Immunohistochemical analysis revealed elevations in renal fibrosis as indicated by increased renal TGF-β (transforming growth factor β) levels and fibronectin expression in diabetic SHR and these changes were reduced with sEH inhibition. Co-administration of SnMP with t-AUCB prevented its ability to reduce renal fibrosis in diabetic SHR. In addition, SnMP treatment also prevented t-AUCB-induced decreases in renal macrophage infiltration, IL-17 expression and MCP-1 levels in diabetic SHR. These findings suggest that HO-1 induction is involved in the protective effect of sEH inhibition against diabetic renal injury.

The proximal tubule in the pathophysiology of the diabetic kidney

Diabetic nephropathy is a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved in the early changes of the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. This review focuses on the proximal tubule in the early diabetic kidney, particularly on its exposure and response to high glucose levels, albuminuria, and other factors in the diabetic glomerular filtrate, the hyperreabsorption of glucose, the unique molecular signature of the tubular growth phenotype, including aspects of senescence, and the resulting cellular and functional consequences. The latter includes the local release of proinflammatory chemokines and changes in proximal tubular salt and fluid reabsorption, which form the basis for the strong tubular control of glomerular filtration in the early diabetic kidney, including glomerular hyperfiltration and odd responses like the salt paradox. Importantly, these early proximal tubular changes can set the stage for oxidative stress, inflammation, hypoxia, and tubulointerstitial fibrosis, and thereby for the progression of diabetic renal disease.

Hypoxia. 1. Intracellular sensors for oxygen and oxidative stress: novel therapeutic targets

A variety of human disorders, e.g., ischemic heart disease, stroke, kidney disease, eventually share the deleterious consequences of a common, hypoxic and oxidative stress pathway. In this review, we utilize recent information on the cellular defense mechanisms against hypoxia and oxidative stress with the hope to propose new therapeutic tools. The hypoxia-inducible factor (HIF) is a key player as it activates a broad range of genes protecting cells against hypoxia. Its level is determined by its degradation rate by intracellular oxygen sensors prolyl hydroxylases (PHDs). There are three different PHD isoforms (PHD1-3). Small molecule PHD inhibitors improve hypoxic injury in experimental animals but, unfortunately, may induce adverse effects associated with PHD2 inhibition, e.g., angiogenesis. As yet, no inhibitor specific for a distinct PHD isoform is currently available. Still, the specific disruption of the PHD1 gene is known to induce hypoxic tolerance, without angiogenesis and erythrocytosis, by reprogramming basal oxygen metabolism with an attendant decreased oxidative stress in hypoxic mitochondria. A specific PHD1 inhibitor might therefore offer a novel therapy against hypoxia. The nuclear factor-erythroid 2 p45-related factor 2 (Nrf2) regulates the basal and inducible expression of numerous antioxidant stress genes. Disruption of its gene exacerbates oxidative tissue injury. Nrf2 activity is modulated by Kelch-like ECH-associated protein 1 (Keap1), an intracellular sensor for oxidative stress. Inhibitors of Keap 1 may prove therapeutic against oxidative tissue injury.

Eicosapentaenoic acid restores diabetic tubular injury through regulating oxidative stress and mitochondrial apoptosis

The present study was designed to elucidate a possible mechanism of hyperglycemia-induced tubular injury and to examine a therapeutic potential of dietary eicosapentaenoic acid (EPA) for the prevention of diabetic kidney disease. Utilizing streptozotocin-induced diabetic mice, the extents of albuminuria and histological injuries were monitored at 2 wk after diabetic induction. Reactive oxygen species (ROS) production, apoptosis, and hypoxia in the kidney were evaluated by immunohistochemistry and Western blotting. An in vitro study was performed using rat proximal tubular cells (NRK-52E) to confirm the protective effect of EPA for methylglyoxal (MG)-induced ROS generation and staurosporine (STS)-induced mitochondrial apoptosis. The extents of albuminuria and histological tubular injuries were significantly lower in EPA-treated diabetic mice compared with untreated diabetic mice. The levels of lipid peroxidation product (4-hydroxy-2-nonenal), oxidative DNA damage (8-hydoxy-deoxyguanosine), and mitochondrial apoptosis (TUNEL, caspase-9, cleaved caspase-3, and cytochrome c release) in the tubular cells were also significantly lower in EPA-treated diabetic mice. Furthermore, hypoxia-inducible factor (HIF)-1α expression was significantly upregulated in the kidney tissues from EPA-treated mice compared with untreated diabetic mice. MG-induced ROS overproduction and STS-induced mitochondrial apoptosis in NRK-52E cells were significantly reduced by EPA treatment in vitro. These results indicated that the ROS generation and mitochondrial apoptosis were involved in hyperglycemia-induced tubular injury and EPA had a beneficial effect by suppressing ROS generation and mitochondrial apoptosis partly through augmentation of an HIF-1α response in diabetic kidney disease.

Altered hypoxia-inducible factor-1 alpha expression levels correlate with coronary vessel anomalies

The outflow tract myocardium and other regions corresponding to the location of the major coronary vessels of the developing chicken heart, display a high level of hypoxia as assessed by the hypoxia indicator EF5. The EF5-positive tissues were also specifically positive for nuclear-localized hypoxia inducible factor-1 alpha (HIF-1alpha), the oxygen-sensitive component of the hypoxia inducible factor-1 (HIF-1) heterodimer. This led to our hypothesis that there is a "template" of hypoxic tissue that determines the stereotyped pattern of the major coronary vessels. In this study, we disturbed this template by altering ambient oxygen levels (hypoxia 15%; hyperoxia 75-40%) during the early phases of avian coronary vessel development, in order to alter tissue hypoxia, HIF-1alpha protein expression, and its downstream target genes without high mortality. We also altered HIF-1alpha gene expression in the embryonic outflow tract cardiomyocytes by injecting an adenovirus containing a constitutively active form of HIF-1alpha (AdCA5). We assayed for coronary anomalies using anti-alpha-smooth muscle actin immunohistology. When incubated under abnormal oxygen levels or injected with a low titer of the AdCA5, coronary arteries displayed deviations from their normal proximal connections to the aorta. These deviations were similar to known clinical anomalies of coronary arteries. These findings indicated that developing coronary vessels may be subject to a level of regulation that is dependent on differential oxygen levels within cardiac tissues and subsequent HIF-1 regulation of gene expression.

Diabetic nephropathy: a disorder of oxygen metabolism?

Chronic hypoxia induces sequential abnormalities in oxygen metabolism (for example, oxidative stress, nitrosative stress, advanced glycation, carbonyl stress, endoplasmic reticulum stress) in the kidneys of individuals with diabetes. Identification of these abnormalities improves our understanding of therapeutic benefits that can be achieved with antihypertensive agents, the control of hyperglycemia and/or hyperinsulinemia and the dietary correction of obesity. Key to the body's defense against hypoxia is hypoxia-inducible factor, the activity of which is modulated by prolyl hydroxylases (PHDs)-oxygen sensors whose inhibition may prove therapeutic. Renal benefits of small-molecule PHD inhibitors have been documented in several animal models, including those of diabetic nephropathy. Three different PHD isoforms have been identified (PHD1, PHD2 and PHD3) and their respective roles have been delineated in knockout mouse studies. Unfortunately, none of the current inhibitors is specific for a distinct PHD isoform. Nonspecific inhibition of PHDs might induce adverse effects, such as those associated with PHD2 inhibition. Specific disruption of PHD1 induces hypoxic tolerance, without angiogenesis and erythrocytosis, through the reprogramming of basal oxygen metabolism and decreased generation of oxidative stress in hypoxic mitochondria. A specific PHD1 inhibitor might, therefore, offer a novel therapy for abnormal oxygen metabolism not only in the diabetic kidney, but also in other diseases for which hypoxia is a final, common pathway.

Chemistry and antihypertensive effects of tempol and other nitroxides

Nitroxides can undergo one- or two-electron reduction reactions to hydroxylamines or oxammonium cations, respectively, which themselves are interconvertible, thereby providing redox metabolic actions. 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol) is the most extensively studied nitroxide. It is a cell membrane-permeable amphilite that dismutates superoxide catalytically, facilitates hydrogen peroxide metabolism by catalase-like actions, and limits formation of toxic hydroxyl radicals produced by Fenton reactions. It is broadly effective in detoxifying these reactive oxygen species in cell and animal studies. When administered intravenously to hypertensive rodent models, tempol caused rapid and reversible dose-dependent reductions in blood pressure in 22 of 26 studies. This was accompanied by vasodilation, increased nitric oxide activity, reduced sympathetic nervous system activity at central and peripheral sites, and enhanced potassium channel conductance in blood vessels and neurons. When administered orally or by infusion over days or weeks to hypertensive rodent models, it reduced blood pressure in 59 of 68 studies. This was accompanied by correction of salt sensitivity and endothelial dysfunction and reduced agonist-evoked oxidative stress and contractility of blood vessels, reduced renal vascular resistance, and increased renal tissue oxygen tension. Thus, tempol is broadly effective in reducing blood pressure, whether given by acute intravenous injection or by prolonged administration, in a wide range of rodent models of hypertension.

Metallothionein is upregulated by hypoxia and stabilizes hypoxia-inducible factor in the kidney

Recent studies underscore that chronic hypoxia in the tubulointerstitium is a final common pathway to progression to end-stage renal failure regardless of etiology. We used microarray analysis of rat kidneys made hypoxic by unilateral renal artery stenosis to measure transcriptomic events and clarify pathophysiological mechanisms of renal injury induced by chronic hypoxia. Many genes were upregulated in the kidney by chronic hypoxia, but we focused on metallothionein due to its antioxidative properties. Using tubular epithelial cells transfected with a reporter construct of luciferase, driven by the hypoxia-responsive elements (HRE), we found that addition of metallothionein to the culture media increased luciferase activity. This was associated with upregulation of the target genes of hypoxia-inducible factor (HIF), such as vascular endothelial growth factor and glucose transporter-1. Stimulation of the HIF-HRE pathway by metallothionein was confirmed by metallothionein overexpression. Hypoxia and exogenous metallothionein increased HIF-1alpha protein without changes in its mRNA levels, suggesting protein stabilization. Upregulation of the HIF-HRE system by metallothionein was associated with phosphorylation of ERK but not Akt. MEK inhibition and rapamycin decreased metallothionein-induced HIF activity. Our study shows that upregulation of metallothionein expression by hypoxia activates the HIF-HRE system through the ERK/mTOR pathway and may be a novel defense against hypoxia.

2-Methoxyestradiol (2-ME) reduces the airway inflammation and remodeling in an experimental mouse model

Patients with asthma experience airway structural changes, termed airway remodeling, in response to persistent inflammation. 2-Methoxyestradiol (2-ME) is an anti-angiogenic agent and downregulates hypoxia-inducible factor 1 (HIF-1) and inhibits HIF-1alpha-induced transcriptional activation of vascular endothelial growth factor (VEGF) expression. We hypothesized that 2-ME may interfere with the development of the clinical manifestations of asthma. We used a chronic murine model of allergic airway inflammation with subepithelial fibrosis in BALB/c mice. Mice were sensitized with ovalbumin (OVA) that was administered intraperitoneally at days 0-5 and challenged intratracheally (IT) with OVA on days 12-22. The mice received 2-ME IT at days 24, 26 and 28 and sacrificed at day 32. The sensitized/challenged mice developed an extensive cell inflammatory response of the airways. 2-ME administration significantly reduced the cellular infiltrate in the perivascular and peribronchial lung tissues, reduced goblet mucous production, reduced airway fibrosis and thickness of smooth muscle and blood vessels, and reduced eosinophil infiltration. Mice treated with 2-ME had a significant decrease of HIF-1 and VEGF expression in the perivascular, peribronchial, and interstitium of lung tissues. Collagen IV expression was also significantly reduced in 2-ME treated mice compared to untreated mice. The 2-ME treatment was associated with a significant decrease of OVA-specific IgE antibodies. These findings provide the first indication that IT administration of 2-ME is effective in preventing and reversing antigen-induced airway remodeling in the OVA allergen inflammatory murine model. The potential role of 2-ME in patients is discussed.

Reactive oxygen species regulate hypoxia-inducible factor 1alpha differentially in cancer and ischemia

In exercise, as well as cancer and ischemia, hypoxia-inducible factor 1 (HIF1) transcriptionally activates hundreds of genes vital for cell homeostasis and angiogenesis. While potentially beneficial in ischemia, upregulation of the HIF1 transcription factor has been linked to inflammation, poor prognosis in many cancers, and decreased susceptibility of tumors to radiotherapy and chemotherapy. Considering HIF1's function, HIF1alpha protein and its hydroxylation cofactors look increasingly attractive as therapeutic targets. Independently, antioxidants have shown promise in lowering the risk of some cancers and improving neurological and cardiac function following ischemia. The mechanism of how different antioxidants and reactive oxygen species influence HIF1alpha expression has drawn interest and intense debate. Here we present an experimentally based computational model of HIF1alpha protein degradation that represents how reactive oxygen species and antioxidants likely affect the HIF1 pathway differentially in cancer and ischemia. We use the model to demonstrate effects on HIF1alpha expression from combined doses of five potential therapeutically targeted compounds (iron, ascorbate, hydrogen peroxide, 2-oxoglutarate, and succinate) influenced by cellular oxidation-reduction and involved in HIF1alpha hydroxylation. Results justify the hypothesis that reactive oxygen species work by two opposite ways on the HIF1 system. We also show how tumor cells and cells under ischemic conditions would differentially respond to reactive oxygen species via changes to HIF1alpha expression over the course of hours to days, dependent on extracellular hydrogen peroxide levels and largely independent of initial intracellular levels, during hypoxia.

Up-regulation of hepatic heme oxygenase-1 expression by locally induced interleukin-6 in rats administered carbon tetrachloride intraperitoneally

We previously reported that interleukin-6 (IL-6) was locally produced in the early period after intraperitoneal (i.p.) or subcutaneous carbon tetrachloride (CCl4) administration, but not after oral (p.o.) administration. In the present study, we focused on the up-regulation of stress-inducible proteins induced by IL-6 after i.p. CCl4 administration. The expression of heme oxygenase-1 (HO-1) (EC 1.14.99.3) mRNA and protein were induced more in rats administered CCl4 via the i.p. route, compared with the p.o. route; however, expression of heat shock protein (HSP) 72 and HSP90 mRNA were increased to similar extents in both experimental groups. The induction of HO-1 mRNA and protein after i.p. CCl4 administration were significantly reduced after pretreatment with anti-rat IL-6 antibody. Activation of the signal transducer and activator of transcription factor 3 (STAT3), which promotes HO-1 expression, peaked together with plasma levels of IL-6 after i.p. CCl4 administration, suggesting that hepatic HO-1 expression was increased by IL-6 via the Janus kinase/STAT3 pathway. The present data indicate that hepatic HO-1 is up-regulated by endogenously produced IL-6, in addition to its up-regulation by heme derived from cytochrome P450 which has already been reported in rats administered i.p. CCl4. The up-regulation of hepatic HO-1 expression may reduce the tissue injury to livers caused by CCl4.

Adaptation to hypoxia in the diabetic rat kidney

Hypoxia of the kidney in diabetes could predispose it to develop acute and chronic renal failure. To examine the relationship between renal hypoxia and renal failure, we measured hypoxia (as a pimonidazole adducts), hypoxia-inducible factors (HIFs), and a hypoxia target gene heme oxygenase-1. The studies were performed in rats with streptozotocin (STZ)-induced diabetes, Cohen diabetes sensitive rats, and during short-term artificial hyperglycemia in rats induced by intravenous glucose and octreotide. STZ-treated rats received insulin, the superoxide dismutase mimetic tempol, or contrast medium. Radiocontrast media causes hypoxia and HIF induction. Hypoxia, HIFs, and heme oxygenase were undetectable in controls, but transiently activated in STZ-treated and the Cohen diabetes sensitive rats. Different patterns of HIFs and pimonidazole were observed between the three models. Insulin abolished pimonidazole and HIF induction, whereas tempol lead to increased HIFs and heme oxygenase induction at similar levels of pimonidazole. When compared with control rats, STZ-treated rats exhibited more intense and protracted renal pimonidazole, with augmented hypoxia inducible factor production and reduced GFR following contrast media. Our data suggest that both regional hypoxia and hypoxia adaptation transiently occur in early stages of experimental diabetes, largely dependent on hyperglycemia or after contrast media. Tempol may augment the HIF response in diabetes.

Antioxidant vitamins induce angiogenesis in the normal pig kidney

The effects of chronic supplementation with antioxidant vitamins on angiogenesis are controversial. The aim of the present study was to evaluate in kidneys of normal pigs the effect of chronic supplementation with vitamins E and C, at doses that are effective in reducing oxidative stress and attenuating angiogenesis under pathological conditions. Domestic pigs were randomized to receive a 12-wk normal diet without ( n = 6) or with antioxidant vitamins supplementation (1g/day vitamin C, 100 IU·kg−1·day−1 vitamin E; n = 6). Electron beam computed tomography (CT) was used to evaluate renal cortical vascular function in vivo, and micro-CT was to assess the spatial density and average diameter of cortical microvessels (diameter <500 μm) ex vivo. Oxidative stress and expressions of vascular endothelial growth factor (VEGF) and hypoxia-inducible factor (HIF)-1α were evaluated in renal tissue. The effects of increasing concentrations of the same vitamins on redox status and angiogenesis were also evaluated in human umbilical vascular endothelial cells (HUVEC). Compared with normal pigs, the density of cortical transmural microvessels was significantly greater in vitamin-supplemented pigs (149.0 ± 11.7 vs. 333.8 ± 48.1 vessel/cm2, P < 0.05), whereas the cortical perfusion response to ACh was impaired. This was accompanied by a significant increase in tissue oxidative stress and levels of VEGF and HIF-1α. A low dose of antioxidant decreased, whereas a high dose increased, HUVEC oxidative stress and angiogenesis, which was partly mediated by hydrogen peroxide. Antioxidant vitamin supplementation can increase tissue oxidative redox and microvascular proliferation in the normal kidney, probably due to a biphasic effect that depends on basal redox balance.

Autocrine/paracrine pattern of superoxide production through NAD(P)H oxidase in coronary arterial myocytes

The present study tested the hypothesis that membrane-bound NAD(P)H oxidase in coronary arterial myocytes (CAMs) is capable of producing superoxide (O(2)(*-)) toward extracellular space to exert an autocrine- or paracrine-like action in these cells. Using a high-speed wavelength-switching fluorescent microscopic imaging technique, we simultaneously monitored the binding of dihydroethidium-oxidizing product to exogenous salmon testes DNA trapped outside CAMs and to nuclear DNA as indicators of extra- and intracellular O(2)(*-) production. It was found that a muscarinic agonist oxotremorine (OXO; 80 microM) increased O(2)(*-) levels more rapidly outside than inside CAMs. In the presence of superoxide dismutase (500 U/ml) plus catalase (400 U/ml) and NAD(P)H oxidase inhibitor diphenylene iodonium (50 microM) or apocynin (100 microM), these increases in extra- and intracellular O(2)(*-) levels were substantially abolished or attenuated. The O(2)(*-) increase outside CAMs was also confirmed by detecting oxidation of nitro blue tetrazolium and confocal microscopic localization of Matrigel-trapped OxyBURST H(2)HFF Green BSA staining around these cells. By electron spin resonance spectrometry, the extracellular accumulation of O(2)(*-) was demonstrated as a superoxide dismutase-sensitive component outside CAMs. Furthermore, RNA interference of NAD(P)H oxidase subunits Nox1 or p47 markedly blocked OXO-induced increases in both extra- and intracellular O(2)(*-) levels, whereas small inhibitory RNA of Nox4 only attenuated intracellular O(2)(*-) accumulation. These results suggest that Nox1 represents a major NAD(P)H oxidase isoform responsible for extracellular O(2)(*-) production. This rapid extracellular production of O(2)(*-) seems to be unique to OXO-induced M(1)-receptor activation, since ANG II-induced intra- and extracellular O(2)(*-) increases in parallel. It is concluded that the outward production of O(2)(*-) via NAD(P)H oxidase in CAMs may represent an important producing pattern for its autocrine or paracrine actions.

Reactive oxygen species in the control of hypoxia-inducible factor-mediated gene expression

Reactive oxygen species (ROS) have long been considered as cytotoxic. However, recent evidence indicates a prominent role of ROS as signaling molecules in the response to hormones, growth and coagulation factors, cytokines and other factors as well as to changes in oxygen tension. The hypoxia-inducible transcription factors (HIFs) are key players in the cellular response to changes in oxygen tension. Recently, HIFs have also been shown to respond to the above-mentioned non-hypoxic stimuli. In this article, the role of ROS in the regulation of HIF-1 under hypoxic and non-hypoxic conditions is summarized.

Hypoxia-inducible factors and tubular cell survival in isolated perfused kidneys

Adaptation to hypoxic environment is conferred through hypoxia-inducible transcription factors (HIFs). We have previously shown that the HIF system is transiently activated in vivo in radiocontrast-induced acute renal failure, associated with profound hypoxia in the renal medulla. Medullary thick ascending limbs (mTALs), the most affected nephron segments in this model, were virtually unable to mount an adaptive HIF response. Here, we study correlations between oxygenation, HIF activation, and cell viability in a related ex vivo model, the isolated perfused rat kidney (IPK). In IPKs perfused with cell-free oxygenated medium, severe medullary hypoxic damage developed, affecting 42+/-9% of mTALs in the mid-inner stripe. HIF-1alpha tubular immunostaining was noted with a zonal and tubular pattern largely similar to our findings in vivo: in 34+/-3% of collecting ducts (CDs) within the mid-inner stripe and extensively in the papillary tip, whereas mTALs were all HIF-negative. In IPKs supplemented with RBCs (improved oxygen supply), mTAL damage was totally prevented and CDs' HIF expression was attenuated (22+/-4%). By contrast, although measures designed to reduce medullary hypoxia by decreasing tubular reabsorptive activity (furosemide, ouabain, or high-albumin-non-filtering system) reduced mTAL damage, all paradoxically resulted in increased HIF expression in CDs (51+/-4%), and 17+/-3% of mTALs became immunostained as well. Our data confirm that CDs and mTALs have markedly different HIF responses, which correlate with their viability under hypoxic stress. mTALs transcriptional adaptation occurs within a narrow hypoxic range, and it appears that workload reduction can shift mTALs into this window of opportunity for HIF activation and survival.

Reactive oxygen species attenuate nitric-oxide-mediated hypoxia-inducible factor-1alpha stabilization

Tissue hypoxia/ischemia are major pathophysiological determinants. Conditions of decreased oxygen availability provoke accumulation and activation of hypoxia-inducible factor-1 (HIF-1). Recent reports demonstrate a crucial role of HIF-1 for inflammatory events. Regulation of hypoxic responses by the inflammatory mediators nitric oxide (NO) and reactive oxygen species (ROS) is believed to be of pathophysiolgical relevance. It is reported that hypoxic stabilization of HIF-1alpha can be antagonized by NO due to its ability to attenuate mitochondrial electron transport. Likely, the formation of ROS could contribute to this effect. As conflicting results emerged from several studies showing either decreased or increased ROS production during hypoxia, we used experiments mimicking hypoxic intracellular ROS changes by using the redox cycling agent 2,3-dimethoxy-1,4-naphthoquinone (DMNQ), which generates superoxide inside cells. Treatment of A549, HEK293, HepG2, and COS cells with DMNQ resulted in a concentration-dependent raise in ROS which correlated with HIF-1alpha accumulation. By using a HIF-1alpha-von Hippel-Lindau tumor suppressor protein binding assay, we show that ROS produced by DMNQ impaired prolyl hydroxylase activity. When HIF-1alpha is stabilized by NO, low concentrations of DMNQ (<1 microM) revealed no effect, intermediate concentrations of 1 to 40 microM DMNQ attenuated HIF-1alpha accumulation and higher concentrations of DMNQ promoted HIF-1alpha stability. Attenuation of NO-induced HIF-1alpha stability regulation by ROS was mediated by an active proteasomal degradation pathway. In conclusion, we propose that scavenging of NO by ROS and vice versa attenuate HIF-1alpha accumulation in a concentration-dependent manner. This is important to fully elucidate HIF-1alpha regulation under inflammatory conditions.

Oxidative stress and nitric oxide deficiency in the kidney: a critical link to hypertension?

There is growing evidence that oxidative stress contributes to hypertension. Oxidative stress can precede the development of hypertension. In almost all models of hypertension, there is oxidative stress that, if corrected, lowers BP, whereas creation of oxidative stress in normal animals can cause hypertension. There is overexpression of the p22(phox) and Nox-1 components of NADPH oxidase and reduced expression of extracellular superoxide dismutase (EC-SOD) in the kidneys of ANG II-infused rodents, whereas there is overexpression of p47(phox) and gp91(phox) and reduced expression of intracellular SOD with salt loading. Several mechanisms have been identified that can make oxidative stress self-sustaining. Reactive oxygen species (ROS) can enhance afferent arteriolar tone and reactivity both indirectly via potentiation of tubuloglomerular feedback and directly by microvascular mechanisms that diminish endothelium-derived relaxation factor/nitric oxide responses, generate a cyclooxygenase-2-dependent endothelial-derived contracting factor that activates thromboxane-prostanoid receptors, and enhance vascular smooth muscle cells reactivity. ROS can diminish the efficiency with which the kidney uses O(2) for Na(+) transport and thereby diminish the P(O(2)) within the kidney cortex. This may place a break on further ROS generation yet could further enhance vasculopathy and hypertension. There is a tight relationship between oxidative stress in the kidney and the development and maintenance of hypertension.

NO restores HIF-1alpha hydroxylation during hypoxia: role of reactive oxygen species

The activity of hypoxia-inducible factor 1 (HIF-1) is primarily determined by stability regulation of its alpha subunit, which is stabilized under hypoxia but degraded during normoxia. Hydroxylation of HIF-1alpha by prolyl hydroxylases (PHDs) recruits the von Hippel-Lindau (pVHL) E3 ubiquitin ligase complex to initiate proteolytic destruction of the alpha subunit. Hypoxic stabilization of HIF-1alpha has been reported to be antagonized by nitric oxide (NO). By using a HIF-1alpha-pVHL binding assay, we show that NO released from DETA-NO restored prolyl hydroxylase activity under hypoxia. Destabilization of HIF-1alpha by DETA-NO was reversed by free radical scavengers such as NAC and Tiron, thus pointing to the involvement of reactive oxygen species (ROS). Therefore, we examined the effects of ROS on HIF-1alpha stabilization. Treatment of cells under hypoxia with low concentrations of the superoxide generator 2,3-dimethoxy-1,4-naphthoquinone lowered HIF-1alpha protein stabilization. In vitro HIF-1alpha-pVHL interaction assays demonstrated that low-level ROS formation increased prolyl hydroxylase activity, an effect antagonized by ROS scavengers. While determining intracellular ROS formation we noticed that reduced ROS production under hypoxia was restored by the addition of DETA-NO. We propose that an increase in ROS formation contributes to HIF-1alpha destabilization by NO donors under hypoxia via modulation of PHD activity.

2-methoxyestradiol inhibits hypoxia-inducible factor 1alpha, tumor growth, and angiogenesis and augments paclitaxel efficacy in head and neck squamous cell carcinoma

PURPOSE

Head and neck squamous cell carcinomas have been reported to overexpress hypoxia-inducible factor (HIF)-1alpha, a transcription factor that promotes expression of angiogenesis factors and resistance to programmed and therapy-induced cell death. 2-Methoxyestradiol (2ME2) is a natural compound with HIF-1alpha inhibitory activity that is currently being evaluated in phase 1 and 2 clinical trials for advanced solid tumors and multiple myeloma. To our knowledge, this is the first study to evaluate the effects of 2ME2 in head and neck squamous cell carcinoma.

EXPERIMENTAL DESIGN

In the present study, we investigated the effects of 2ME2 alone and in combination with paclitaxel, an active agent in recurrent or advanced head and neck squamous cell carcinoma.

RESULTS

2ME2 exhibited antiproliferative and cytotoxic effects in a panel of five head and neck squamous cell carcinoma cell lines in the 0.5 to 10 micromol/L range, including induction of G2-M blockade, caspase-3/7 activation, and apoptosis at 48 hours. 2ME2 resulted in decreased nuclear HIF-1alpha-binding activity and affected the expression of downstream genes, such as bid, a proapoptotic bcl-2 family member, and vascular endothelial growth factor, a proangiogenic cytokine. The up-regulation of Bid (57.5% at 12 hours, P < 0.0006) and inhibition of vascular endothelial growth factor secretion (57.7% at 24 hours, P < 0.015; and 50.3% at 48 hours, P < 0.0006) could be partially attributed to the effects on HIF-1alpha, because HIF-1alpha small interfering RNAs produced similar effects. Finally, in vivo, in a xenograft model of head and neck squamous cell carcinoma using UM-SCC-11A cells, 2ME2 exhibited antitumor and antiangiogenic activity, as measured by CD31 immunostaining.

CONCLUSIONS

These results provide support for the use of 2ME2 in combination with paclitaxel for the treatment of recurrent or advanced head and neck squamous cell carcinoma.

Tempol reduces oxidative stress, improves insulin sensitivity, decreases renal dopamine D1 receptor hyperphosphorylation, and restores D1 receptor-G-protein coupling and function in obese Zucker rats

Oxidative stress plays a pathogenic role in hypertension, particularly the one associated with diabetes and obesity. Here, we test the hypothesis that renal dopamine D1 receptor dysfunction in obese Zucker rats is caused by oxidative stress. One group each from lean and obese Zucker rats received tempol, a superoxide dismutase mimetic in drinking water for 2 weeks. Obese animals were hypertensive, hyperglycemic, and hyperinsulinemic, exhibited renal oxidative stress, and increased protein kinase C activity. Also, there was hyperphosphorylation of D1 receptor, defective receptor-G-protein coupling, blunted dopamine-induced Na+-K+-ATPase inhibition, and diminished natriuretic response to D1 receptor agonist, SKF-38393. However, obese animals had elevated levels of plasma nitric oxide and urinary cGMP. In addition, L-N-nitroarginine and sodium nitroprusside showed similar effect on blood pressure in lean and obese rats. In obese animals, tempol reduced blood pressure, blood glucose, insulin, renal oxidative stress, and protein kinase C activity. Tempol also decreased D1 receptor phosphorylation and restored receptor G-protein coupling. Dopamine inhibited Na+-K+-ATPase activity, and SKF-38393 elicited a natriuretic response in tempol-treated obese rats. Thus in obese Zucker rats, tempol ameliorates oxidative stress and improves insulin sensitivity. Consequently, hyperphosphorylation of D1 receptor is reduced, leading to restoration of receptor-G-protein coupling and the natriuretic response to SKF-38393.