Non-hypoxic activation of the negative regulatory feedback loop of prolyl-hydroxylase oxygen sensors

Oxygen Sensing in Innate Immune Cells: How Inflammation Broadens Classical Hypoxia-Inducible Factor Regulation in Myeloid Cells

Significance: Oxygen deprivation (hypoxia) is a common feature at sites of inflammation. Immune cells and all other cells present at the inflamed site have to adapt to these conditions. They do so by stabilization and activation of hypoxia-inducible factor subunit α (HIF-1α and HIF-2α, respectively), enabling constant generation of adenosine triphosphate (ATP) under these austere conditions by the induction of, for example, glycolytic pathways. Recent Advances: During recent years, it has become evident that HIFs play a far more important role than initially believed because they shape the inflammatory phenotype of immune cells. They are indispensable for migration, phagocytosis, and the induction of inflammatory cytokines by innate immune cells and thereby enable a crosstalk between innate and adaptive immunity. In short, they ensure the survival and function of immune cells under critical conditions. Critical Issues: Up to now, there are still open questions regarding the individual roles of HIF-1 and HIF-2 for the different cell types. In particular, the loss of both HIF-1 and HIF-2 in myeloid cells led to unexpected and contradictory results in the mouse models analyzed so far. Similarly, the role of HIF-1 in dendritic cell maturation is unclear due to inconsistent results from in vitro experiments. Future Directions: The HIFs are indispensable for immune cell survival and action under inflammatory conditions, but they might also trigger over-activation of immune cells. Therefore, they might be excellent setscrews to adjust the inflammatory response by pharmaceuticals. China and Japan and very recently (August 2021) Europe have approved prolyl hydroxylase inhibitors (PHIs) to stabilize HIF such as roxadustat for clinical use to treat anemia by increasing the production of erythropoietin, the classical HIF target gene. Nonetheless, we need further work regarding the use of PHIs under inflammatory conditions, because HIFs show specific activation and distinct expression patterns in innate immune cells. The extent to which HIF-1 or HIF-2 as a transcription factor regulates the adaptation of immune cells to inflammatory hypoxia differs not only by the cell type but also with the inflammatory challenge and the surrounding tissue. Therefore, we urgently need isoform- and cell type-specific modulators of the HIF pathway. Antioxid. Redox Signal. 37, 956-971.

Hypoxia-inducible factors not only regulate but also are myeloid-cell treatment targets

Hypoxia describes limited oxygen availability at the cellular level. Myeloid cells are exposed to hypoxia at various bodily sites and even contribute to hypoxia by consuming large amounts of oxygen during respiratory burst. Hypoxia-inducible factors (HIFs) are ubiquitously expressed heterodimeric transcription factors, composed of an oxygen-dependent α and a constitutive β subunit. The stability of HIF-1α and HIF-2α is regulated by oxygen-sensing prolyl-hydroxylases (PHD). HIF-1α and HIF-2α modify the innate immune response and are context dependent. We provide a historic perspective of HIF discovery, discuss the molecular components of the HIF pathway, and how HIF-dependent mechanisms modify myeloid cell functions. HIFs enable myeloid-cell adaptation to hypoxia by up-regulating anaerobic glycolysis. In addition to effects on metabolism, HIFs control chemotaxis, phagocytosis, degranulation, oxidative burst, and apoptosis. HIF-1α enables efficient infection defense by myeloid cells. HIF-2α delays inflammation resolution and decreases antitumor effects by promoting tumor-associated myeloid-cell hibernation. PHDs not only control HIF degradation, but also regulate the crosstalk between innate and adaptive immune cells thereby suppressing autoimmunity. HIF-modifying pharmacologic compounds are entering clinical practice. Current indications include renal anemia and certain cancers. Beneficial and adverse effects on myeloid cells should be considered and could possibly lead to drug repurposing for inflammatory disorders.

Autophagy in hypoxic ovary

Oxygen deprivation affects human health by modulating system as well as cellular physiology. Hypoxia generates reactive oxygen species (ROS), causes oxidative stress and affects female reproductive health by altering ovarian as well as oocyte physiology in mammals. Hypoxic conditions lead to several degenerative changes by inducing various cell death pathways like autophagy, apoptosis and necrosis in the follicle of mammalian ovary. The encircling somatic cell death interrupts supply of nutrients to the oocyte and nutrient deprivation may result in the generation of ROS. Increased level of ROS could induce granulosa cells as well as oocyte autophagy. Although autophagy removes damaged proteins and subcellular organelles to maintain the cell survival, irreparable damages could induce cell death within intra-follicular microenvironment. Hypoxia-induced autophagy is operated through 5' AMP activated protein kinase-mammalian target of rapamycin, endoplasmic reticulum stress/unfolded protein response and protein kinase C delta-c-junN terminal kinase 1 pathways in a wide variety of somatic cell types. Similar to somatic cells, we propose that hypoxia may induce granulosa cell as well as oocyte autophagy and it could be responsible at least in part for germ cell elimination from mammalian ovary. Hypoxia-mediated germ cell depletion may cause several reproductive impairments including early menopause in mammals.

Genetic disruption of calpain-1 and calpain-2 attenuates tumorigenesis in mouse models of HER2+ breast cancer and sensitizes cancer cells to doxorubicin and lapatinib

Calpains are a family of calcium activated cysteine proteases which participate in a wide range of cellular functions including migration, invasion, autophagy, programmed cell death, and gene expression. Calpain-1 and calpain-2 isoforms are ubiquitously expressed heterodimers composed of isoform specific catalytic subunits coupled with an obligate common regulatory subunit encoded by capns1. Here, we report that conditional deletion of capns1 disrupted calpain-1 and calpain-2 expression and activity, and this was associated with delayed tumorigenesis and altered signaling in a transgenic mouse model of spontaneous HER2⁺ breast cancer and effectively blocked tumorigenesis in an orthotopic engraftment model. Furthermore, capns1 knockout in a tumor derived cell line correlated with enhanced sensitivity to the chemotherapeutic doxorubicin and the HER2/EGFR tyrosine kinase inhibitor lapatinib. Collectively, these results indicate pro-tumorigenic roles for calpains-1/2 in HER2⁺ breast cancer and provide evidence that calpain-1/2 inhibitors could have anti-tumor effects if used either alone or in combination with chemotherapeutics and targeted agents.

Macrophage NOS2 in Tumor Leukocytes

SIGNIFICANCE

Leukocytes and especially macrophages are a major cellular constituent of the tumor mass. The tumor microenvironment not only determines their activity but in turn these cells also contribute to tumor initiation and progression. Recent Advances: Proinflammatory stimulated macrophages upregulate inducible nitric oxide synthase (NOS2) and produce high steady-state NO concentrations. NO provokes tumor cell death by initiating apoptosis and/or necrosis. Mechanisms may comprise p53 accumulation, immunestimulatory activities, and an increased efficacy of chemo- and/or radiotherapy. However, the potential cytotoxic activity of macrophages often is compromised in the tumor microenvironment and instead a protumor activity of macrophages dominates. Contributing factors are signals generated by viable and dying tumor cells, attraction and activation of myeloid-derived suppressor cells, and hypoxia. Limited oxygen availability not only attenuates NOS2 activity but also causes accumulation of hypoxia-inducible factors 1 and 2 (HIF-1/HIF-2). Activation of the HIF system is tightly linked to NO formation and affects the expression of macrophage phenotype markers that in turn add to tumor progression.

CRITICAL ISSUES

To make use of the cytotoxic arsenal of activated macrophages directed against tumor cells, it will be critical to understand how, when, and where these innate immune responses are blocked and whether it will be possible to reinstall their full capacity to kill tumor cells.

FUTURE DIRECTIONS

Low-dose irradiation or proinflammatory activation of macrophages in the tumor microenvironment may open options to boost NOS2 expression and activity and to initiate immunestimulatory features of NO that may help to restrict tumor growth. Antioxid. Redox Signal. 26, 1023-1043.

Inhibition of Hypoxia-Induced Retinal Angiogenesis by Specnuezhenide, an Effective Constituent of Ligustrum lucidum Ait., through Suppression of the HIF-1α/VEGF Signaling Pathway

Specnuezhenide (SPN), one of the main ingredients of Chinese medicine “Nü-zhen-zi”, has anti-angiogenic and vision improvement effects. However, studies of its effect on retinal neovascularization are limited so far. In the present study, we established a vascular endothelial growth factor A (VEGFA) secretion model of human acute retinal pigment epithelial-19 (ARPE-19) cells by exposure of 150 μM CoCl₂ to the cells and determined the VEGFA concentrations, the mRNA expressions of VEGFA, hypoxia inducible factor-1α (HIF-1α) & prolyl hydroxylases 2 (PHD-2), and the protein expressions of HIF-1α and PHD-2 after treatment of 3-(5′-hydroxymethyl-2′-furyl)-1-benzylindazole (YC-1, 1.0 μg/mL) or SPN (0.2, 1.0 and 5.0 μg/mL). Furthermore, rat pups with retinopathy were treated with SPN (5.0 and 10.0 mg/kg) in an 80% oxygen atmosphere and the retinal avascular areas were assessed through visualization using infusion of ADPase and H&E stains. The results showed that SPN inhibited VEGFA secretion by ARPE-19 cells under hypoxia condition, down-regulated the mRNA expressions of VEGFA and PHD-2 slightly, and the protein expressions of VEGFA, HIF-1α and PHD-2 significantly in vitro. SPN also prevented hypoxia-induced retinal neovascularization in a rat model of oxygen-induced retinopathy in vivo. These results indicate that SPN ameliorates retinal neovascularization through inhibition of HIF-1α/VEGF signaling pathway. Therefore, SPN has the potential to be developed as an agent for the prevention and treatment of diabetic retinopathy.

Aloe-emodin suppresses hypoxia-induced retinal angiogenesis via inhibition of HIF-1α/VEGF pathway

Background: Aloe-emodin (AE) has been reported to possess the antiangiogenic effect on laser induced choroidal neovascularization. AE inhibits the vessel formation in the zebrafish embryos. However, it is still unclear whether AE can alleviate neovascularization. Here, we investigated the inhibitory effect of AE on the hypoxia-induced retinal neovascularization and the possible mechanisms. Methods: We established a vascular endothelial growth factor (VEGF) secretion model under chemical induced hypoxia by exposure of 150 µM CoCl₂ to the ARPE-19 cells, then treated the cells with different concentrations of AE (0.2, 1.0 and 5.0 µg/mL) or a special hypoxia-inducible factor 1α (HIF-1α) inhibitor [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole, YC-1, 1.0 µg/mL]. The cellular supernatants were collected 48 h later to measure the VEGFA concentrations by human VEGFA enzyme-linked immunosorbent assay (ELISA) kits, the mRNA expressions of VEGFA, HIF-1α and prolyl hydroxylase-2 (PHD-2) by quantitative reverse transcription-PCR (qRT-PCR) and the protein expressions of HIF-1α and PHD-2 by Western blots. For in vivo study, the rat pups with oxygen-induced retinopathy were treated with Conbercept ophthalmic injection (1.0 mg/kg) or AE (5.0 and 10.0 mg/kg) for five days, then the retinal avascular areas were assessed via visualization of the retinal vasculature with ADPase and hematoxylin & eosin (H&E) stains. Results: AE inhibits the VEGFA secretion of ARPE-19 cells under hypoxia condition, decreases the mRNA expressions of VEGFA and PHD-2 and the protein expressions of VEGFA, HIF-1α and PHD-2 in vitro and prevents hypoxia-induced retinal neovascularization in vivo.Conclusions: AE ameliorates retinal neovascularization throuth inhibition of the HIF-1α/VEGF signaling pathway. AE may be developed as a potential drug for the prevention and treatment of diabetic retinopathy.

Formononetin, an active compound of Astragalus membranaceus (Fisch) Bunge, inhibits hypoxia-induced retinal neovascularization via the HIF-1α/VEGF signaling pathway

BACKGROUND

It has been reported that formononetin (FMN), one of the main ingredients from famous traditional Chinese medicine "Huang-qi" (Astragalus membranaceus [Fisch] Bunge) for Qi-tonifying, exhibits the effects of immunomodulation and tumor growth inhibition via antiangiogenesis. Furthermore, A. membranaceus may alleviate the retinal neovascularization (NV) of diabetic retinopathy. However, the information of FMN on retinal NV is limited so far. In the present study, we investigated the effects of FMN on the hypoxia-induced retinal NV and the possible related mechanisms.

MATERIALS AND METHODS

The VEGF secretion model of acute retinal pigment epithelial-19 (ARPE-19) cells under chemical hypoxia was established by the exposure of cells to 150 μM CoCl₂ and then cells were treated with 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1, a potent HIF-1α inhibitor, 1.0 μg/mL) or different concentrations of FMN (0.2 μg/mL, 1.0 μg/mL, and 5.0 μg/mL). The supernatants of cells were collected 48 hours later to measure the VEGF concentrations, following the manufacturer's instruction. The mRNA expressions of VEGF, HIF-1α, PHD-2, and β-actin were analyzed by quantitative reverse transcription polymerase chain reaction, and the protein expressions of HIF-1α and PHD-2 were determined by Western blot analysis. Furthermore, the rats with retinopathy were treated by intraperitoneal administration of conbercept injection (1.0 mg/kg) or FMN (5.0 mg/kg and 10.0 mg/kg) in an 80% oxygen atmosphere. The retinal avascular areas were assessed through visualization of the retinal vasculature by adenosine diphosphatase staining and hematoxylin and eosin staining.

RESULTS

FMN can indeed inhibit the VEGF secretion of ARPE-19 cells under hypoxia, downregulate the mRNA expression of VEGFA and PHD-2, and decrease the protein expression of VEGF, HIF-1α, and PHD-2 in vitro. Furthermore, FMN can prevent hypoxia-induced retinal NV in vivo.

CONCLUSION

FMN can ameliorate retinal NV via the HIF-1α/VEGF signaling pathway, and it may become a potential drug for the prevention and treatment of diabetic retinopathy.

Overexpression of HIF prolyl-hydoxylase-2 transgene in the renal medulla induced a salt sensitive hypertension

Renal medullary hypoxia-inducible factor (HIF)-1α and its target genes, such as haem oxygenase and nitric oxide synthase, have been indicated to play an important role in the regulation of sodium excretion and blood pressure. HIF prolyl hydroxylase domain-containing proteins (PHDs) are major enzymes to promote the degradation of HIF-1α. We recently reported that high salt intake suppressed the renal medullary PHD2 expression and thereby activated HIF-1α-mediated gene regulation in the renal medulla in response to high salt. To further define the functional role of renal medullary PHD2 in the regulation of renal adaptation to high salt intake and the longer term control of blood pressure, we transfected PHD2 expression plasmids into the renal medulla in uninephrectomized rats and determined its effects on pressure natriuresis, sodium excretion after salt overloading and the long-term control of arterial pressure after high salt challenge. It was shown that overexpression of PHD2 transgene increased PHD2 levels and decreased HIF-1α levels in the renal medulla, which blunted pressure natriuresis, attenuated sodium excretion, promoted sodium retention and produced salt sensitive hypertension after high salt challenge compared with rats treated with control plasmids. There was no blood pressure change in PHD2-treated rats that were maintained in low salt diet. These results suggested that renal medullary PHD2 is an important regulator in renal adaptation to high salt intake and a deficiency in PHD2-mediated molecular adaptation in response to high salt intake in the renal medulla may represent a pathogenic mechanism producing salt sensitive hypertension.

Diabetic nephropathy: are there new and potentially promising therapies targeting oxygen biology?

The multipronged drug approach targeting blood pressure and serum levels of glucose, insulin, and lipids fails to fully prevent diabetic nephropathy (DN). Recently, a broad range of anomalies associated with oxygen biology, such as hypoxia, oxidative stress (OS), and dyserythropoiesis, have been implicated in DN. This review delineates the cellular mechanisms of these anomalies to pinpoint novel therapeutic approaches. The PHD-HIF system mitigates hypoxia: HIF activates a broad range of reactions against hypoxia whereas PHD is an intracellular oxygen sensor negatively regulating HIF. The Keap1-Nrf2 system mitigates OS: Nrf2 activates cellular reactions against OS whereas Keap1 negatively regulates Nrf2. Clinical trials of PHD inhibitors to correct anemia in patients with CKD as well as of a Nrf2 activator, bardoxolone methyl, for DN are under way, even if the latter has been recently interrupted. A specific PHD1 inhibitor, a Keap1 inhibitor, and an allosteric effector of hemoglobin may offer alternative, novel therapies. Erythropoietin (EPO) is critical for the development of erythroid progenitors and thus for tissue oxygen supply. Renal EPO-producing (REP) cells, originating from neural crests, but not fibroblasts from injured tubular epithelial cells, transdifferentiate into myofibroblasts and contribute to renal fibrosis. Agents restoring the initial function of REP cells might retard renal fibrosis. These newer approaches targeting oxygen biology may offer new treatments not only for DN but also for several diseases in which hypoxia and/or OS is a final, common pathway.

Hypoxia-inducible factor prolyl-hydroxylase-2 mediates transforming growth factor beta 1-induced epithelial-mesenchymal transition in renal tubular cells

Transforming growth factor beta 1 (TGF-β1)-induced epithelial-mesenchymal transition (EMT) in kidney epithelial cells plays a key role in renal tubulointerstitial fibrosis in chronic kidney diseases. As hypoxia-inducible factor (HIF)-1α is found to mediate TGF-β1-induced signaling pathway, we tested the hypothesis that HIF-1α and its upstream regulator prolyl hydroxylase domain-containing proteins (PHDs) are involved in TGF-β1-induced EMT using cultured renal tubular cells. Our results showed that TGF-β1 stimulated EMT in renal tubular cells as indicated by the significant decrease in epithelial marker P-cadherin, and the increase in mesenchymal markers α-smooth muscle actin (α-SMA) and fibroblast-specific protein 1 (FSP-1). Meanwhile, we found that TGF-β1 time-dependently increased HIF-1α and that HIF-1α siRNA significantly inhibited TGF-β1-induced EMT, suggesting that HIF-1α mediated TGF-β1 induced-EMT. Real-time PCR showed that PHD1 and PHD2, rather than PHD3, could be detected, with PHD2 as the predominant form of PHDs (PHD1:PHD2=0.21:1.0). Importantly, PHD2 mRNA and protein, but not PHD1, were decreased by TGF-β1. Furthermore, over-expression of PHD2 transgene almost fully prevented TGF-β1-induced HIF-1α accumulation and EMT marker changes, indicating that PHD2 is involved in TGF-β1-induced EMT. Finally, Smad2/3 inhibitor SB431542 prevented TGF-β1-induced PHD2 decrease, suggesting that Smad2/3 may mediate TGF-β1-induced EMT through PHD2/HIF-1α pathway. It is concluded that TGF-β1 decreased PHD2 expression via an Smad-dependent signaling pathway, thereby leading to HIF-1α accumulation and then EMT in renal tubular cells. The present study suggests that PHD2/HIF-1α is a novel signaling pathway mediating the fibrogenic effect of TGF-β1, and may be a new therapeutic target in chronic kidney diseases.

Nitric oxide modulates hypoxia-inducible factor-1 and poly(ADP-ribose) polymerase-1 cross talk in response to hypobaric hypoxia

The physiological response to hypobaric hypoxia represents a complex network of biochemical pathways in which the nitrergic system plays an important role. Previous studies have provided evidence for an interplay between the hypoxia-inducible factor-1 (HIF-1) and poly(ADP-ribose) polymerase-1 (PARP-1) under hypoxia. Here, we evaluate the potential involvement of nitric oxide (NO) in the cross talk between these two proteins. With this aim, we studied comparatively the effect of pharmacological inhibitors of NO production or PARP activity in the response of the mouse cerebral cortex to 4 h of exposure to a simulated altitude of 31,000 ft. Particularly, we analyzed the NO and reactive oxygen species production, the expression of NO synthase (NOS) isoforms, PARP-1 activity, HIF-1α expression and HIF-1 transcriptional activity, the protein level of the factor inhibiting HIF, and, finally, beclin-1 and fractin expression, as markers of cellular damage. Our results demonstrate that the reduction of NO level did not affect reactive oxygen species production but significantly 1) dampened the posthypoxic increase in neuronal NOS and inducible NOS expression without altering endothelial NOS protein level; 2) prevented PARP activation; 3) decreased HIF-1α response to hypoxia; 4) achieved a higher long-term HIF-1 transcriptional activity by reducing factor inhibiting HIF expression; and 5) reduced hypoxic damage. The pharmacological inhibition of PARP reproduced the NOS expression pattern and the HIF-1α response observed in NOS-inhibited mice, supporting its involvement in the NO-dependent regulation of hypoxia. As a whole, these results provide new data about the molecular mechanism underlying the beneficial effects of controlling NO production under hypobaric hypoxic conditions.

Functional regulation of HIF-1α under normoxia--is there more than post-translational regulation?

The hypoxia-inducible factor-1 (HIF-1) is an oxygen-regulated transcriptional activator playing a pivotal role in mammalian physiology and disease pathogenesis, e.g., HIF-1 is indispensable in a broad range of developmental stages in different tumors. Its post-translational regulation via PHDs under the influence of hypoxia is widely investigated and accepted. Different non-hypoxic stimuli such as lipopolysaccharides (LPS), thrombin, and angiotensin II (Ang II), have been proven to enhance HIF-1 levels through activation of regulative mechanisms distinct from protein stabilization. Some of these stimuli specifically regulate HIF-1α at the transcriptional, post-transcriptional, or translational level, whereas others additionally influence post-translational modifications. Thus, it is difficult for the investigators to discern the impact of the different mechanisms leading to functional HIF-1 protein. Nevertheless, profound knowledge of additional regulatory networks appears to depict new therapeutic opportunities and thus is an interesting and important field for further investigations.

Silencing of hypoxia-inducible factor-1α gene attenuated angiotensin II-induced renal injury in Sprague-Dawley rats

Although it has been shown that upregulation of hypoxia-inducible factor (HIF)-1α is protective in acute ischemic renal injury, long-term overactivation of HIF-1α is implicated to be injurious in chronic kidney diseases. Angiotensin II (Ang II) is a well-known pathogenic factor producing chronic renal injury and has also been shown to increase HIF-1α. However, the contribution of HIF-1α to Ang II-induced renal injury has not been evidenced. The present study tested the hypothesis that HIF-1α mediates Ang II-induced renal injury in Sprague-Dawley rats. Chronic renal injury was induced by Ang II infusion (200 ng/kg per minute) for 2 weeks in uninephrectomized rats. Transfection of vectors expressing HIF-1α small hairpin RNA into the kidneys knocked down HIF-1α gene expression by 70%, blocked Ang II-induced HIF-1α activation, and significantly attenuated Ang II-induced albuminuria, which was accompanied by inhibition of Ang II-induced vascular endothelial growth factor, a known glomerular permeability factor, in glomeruli. HIF-1α small hairpin RNA also significantly improved the glomerular morphological damage induced by Ang II. Furthermore, HIF-1α small hairpin RNA blocked Ang II-induced upregulation of collagen and α-smooth muscle actin in tubulointerstitial region. There was no difference in creatinine clearance and Ang II-induced increase in blood pressure. HIF-1α small hairpin RNA had no effect on Ang II-induced reduction in renal blood flow and hypoxia in the kidneys. These data suggested that overactivation of HIF-1α-mediated gene regulation in the kidney is a pathogenic pathway mediating Ang II-induced chronic renal injuries, and normalization of overactivated HIF-1α may be used as a treatment strategy for chronic kidney damages associated with excessive Ang II.

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.

Hypoxia-inducible factor prolyl-hydroxylase 2 senses high-salt intake to increase hypoxia inducible factor 1alpha levels in the renal medulla

High salt induces the expression of transcription factor hypoxia-inducible factor (HIF) 1alpha and its target genes in the renal medulla, which is an important renal adaptive mechanism to high-salt intake. HIF prolyl-hydroxylase domain-containing proteins (PHDs) have been identified as major enzymes to promote the degradation of HIF-1alpha. PHD2 is the predominant isoform of PHDs in the kidney and is primarily expressed in the renal medulla. The present study tested the hypothesis that PHD2 responds to high salt and mediates high-salt-induced increase in HIF-1alpha levels in the renal medulla. In normotensive rats, high-salt intake (4% NaCl, 10 days) significantly inhibited PHD2 expressions and enzyme activities in the renal medulla. Renal medullary overexpression of the PHD2 transgene significantly decreased HIF-1alpha levels. PHD2 transgene also blocked high-salt-induced activation of HIF-1alpha target genes heme oxygenase 1 and NO synthase 2 in the renal medulla. In Dahl salt-sensitive hypertensive rats, however, high-salt intake did not inhibit the expression and activities of PHD2 in the renal medulla. Correspondingly, renal medullary HIF-1alpha levels were not upregulated by high-salt intake in these rats. After transfection of PHD2 small hairpin RNA, HIF-1alpha and its target genes were significantly upregulated by high-salt intake in Dahl salt-sensitive rats. Overexpression of PHD2 transgene in the renal medulla impaired renal sodium excretion after salt loading. These data suggest that high-salt intake inhibits PHD2 in the renal medulla, thereby upregulating the HIF-1alpha expression. The lack of PHD-mediated response to high salt may represent a pathogenic mechanism producing salt-sensitive hypertension.

Oxygen-sensing under the influence of nitric oxide

The transcription factor complex Hypoxia inducible factor 1 (HIF-1) controls the expression of most genes involved in adaptation to hypoxic conditions. Oxygen-dependency is maintained by prolyl- and asparagyl-4-hydroxylases (PHDs/FIH-1) belonging to the superfamily of iron(II) and 2-oxoglutarate dependent dioxygenases. Hydroxylation of the HIF-1alpha subunit by PHDs and FIH-1 leads to its degradation and inactivation. By hydroxylating HIF-1alpha in an oxygen-dependent manner PHDs and FIH-1 function as oxygen-sensing enzymes of HIF signalling. Besides molecular oxygen nitric oxide (NO), a mediator of the inflammatory response, can regulate HIF-1alpha accumulation, HIF-1 activity and HIF-1 dependent target gene expression. Recent studies addressing regulation of HIF-1 by NO revealed a complex and paradoxical picture. Acute exposure of cells to high doses of NO increased HIF-1alpha levels irrespective of the residing oxygen concentration whereas prolonged exposure to NO or low doses of this radical reduced HIF-1alpha accumulation even under hypoxic conditions. Several mechanisms were found to contribute to this paradoxical role of NO in regulating HIF-1. More recent studies support the view that NO regulates HIF-1 by modulating the activity of the oxygen-sensor enzymes PHDs and FIH-1. NO dependent HIF-1alpha accumulation under normoxia was due to direct inhibition of PHDs and FIH-1 most likely by competitive binding of NO to the ferrous iron in the catalytically active center of the enzymes. In contrast, reduced HIF-1alpha accumulation by NO under hypoxia was mainly due to enhanced HIF-1alpha degradation by induction of PHD activity. Three major mechanisms are discussed to be involved in enhancing the PHD activity despite the lack of oxygen: (1) NO mediated induction of a HIF-1 dependent feedback loop leading to newly expressed PHD2 and enhanced nuclear localization, (2) O2-redistribution towards PHDs after inhibition of mitochondrial respiration by NO, (3) reactivation of PHD activity by a NO mediated increase of iron and 2-oxoglutarate and/or involvement of reactive oxygen and/or nitrogen species.

Nanoscopy of the cellular response to hypoxia by means of fluorescence resonance energy transfer (FRET) and new FRET software

Background

Cellular oxygen sensing is fundamental to all mammalian cells to adequately respond to a shortage of oxygen by increasing the expression of genes that will ensure energy homeostasis. The transcription factor Hypoxia-Inducible-Factor-1 (HIF-1) is the key regulator of the response because it coordinates the expression of hypoxia inducible genes. The abundance and activity of HIF-1 are controlled through posttranslational modification by hydroxylases, the cellular oxygen sensors, of which the activity is oxygen dependent.

Methods

Fluorescence resonance energy transfer (FRET) was established to determine the assembly of the HIF-1 complex and to study the interaction of the α-subunit of HIF-1 with the O₂-sensing hydroxylase. New software was developed to improve the quality and reliability of FRET measurements.

Results

FRET revealed close proximity between the HIF-1 subunits in multiple cells. Data obtained by sensitized FRET in this study were fully compatible with previous work using acceptor bleaching FRET. Interaction between the O₂-sensing hydroxylase PHD1 and HIF-1α was demonstrated and revealed exclusive localization of O₂-sensing in the nucleus. The new software FRET significantly improved the quality and speed of FRET measurements.

Conclusion

FRET measurements do not only allow following the assembly of the HIF-1 complex under hypoxic conditions but can also provide important information about the process of O₂-sensing and its localisation within a cell.

MCS codes: 92C30, 92C05, 92C40

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.