NADPH oxidase 2 mediates intermittent hypoxia-induced mitochondrial complex I inhibition: relevance to blood pressure changes in rats

Reactive Oxygen species dependent increase in H3K27 acetylation by intermittent hypoxia is regulated by H3S28 phosphorylation

Histones play a crucial role in regulating gene expression through post -translational modifications (PTMS) which include acetylation, methylation and phosphorylation. We have previously identified histone 3 acetylation (H3Kac) and methylation (H3Kme) as an early epigenetic mechanism associated with intermittent hypoxia (IH), a hallmark feature of sleep apnea. The goal of the present study was to determine the molecular mechanisms underlying IH increased H3 acetylation. IH-induced H3 acetylation was blocked by an antioxidant. Conversely, reactive oxygen species (ROS) mimetics, increased H3 acetylated protein expression similar to IH, suggesting a role for ROS. Trichostatin A (TSA), an HDAC (histone deacetylase) inhibitor mimicked IH-induced H3 acetylation under normoxic conditions, while pharmacological blockade of p300/CBP (HAT, histone acetylase) with CTK7A abolished IH-induced H3 acetylation. These results suggest that interplay between HATs and HDACs regulate ROS-dependent H3 acetylation by IH. Lysine 27 (H3K27) on H3 was one of the lysines specifically acetylated by IH and this acetylation was associated with dephsophorylation of H3 at serine 28 (H3S28). Inhibition of S28 dephosphorylation by protein phosphatase inhibitors (PIC or Calyculin A), prevented H3K27 acetylation by IH. Conversely, inhibiting K27 acetylation with CTK7A, increased S28 phosphorylation in IH-exposed cells. These findings highlight the intricate balance between H3 acetylation and phosphorylation in response to IH, shedding light on epigenetic mechanism regulating gene expression. (Supported by NIH-PO1-HL90554).

Advancements in the neurocirculatory reflex response to hypoxia

Hypoxia is a pivotal factor in the pathophysiology of various clinical conditions, including obstructive sleep apnea, which has a strong association with cardiovascular diseases like hypertension, posing significant health risks. Although the precise mechanisms linking hypoxemia-associated clinical conditions with hypertension remains incompletely understood, compelling evidence suggests that hypoxia induces plasticity of the neurocirculatory control system. Despite variations in experimental designs and the severity, frequency, and duration of hypoxia exposure, evidence from animal and human models consistently demonstrates the robust effects of hypoxemia in triggering reflex-mediated sympathetic activation. Both acute and chronic hypoxia alters neurocirculatory regulation and, in some circumstances, leads to sympathetic outflow and elevated blood pressures that persist beyond the hypoxic stimulus. Dysregulation of autonomic control could lead to adverse cardiovascular outcomes and increase the risk of developing hypertension.

Carotid body hypersensitivity in intermittent hypoxia and obtructive sleep apnoea

Carotid bodies are the principal sensory organs for detecting changes in arterial blood oxygen concentration, and the carotid body chemoreflex is a major regulator of the sympathetic tone, blood pressure and breathing. Intermittent hypoxia is a hallmark manifestation of obstructive sleep apnoea (OSA), which is a widespread respiratory disorder. In the first part of this review, we discuss the role of carotid bodies in heightened sympathetic tone and hypertension in rodents treated with intermittent hypoxia, and the underlying cellular, molecular and epigenetic mechanisms. We also present evidence for hitherto-uncharacterized role of carotid body afferents in triggering cellular and molecular changes induced by intermittent hypoxia. In the second part of the review, we present evidence for a contribution of a hypersensitive carotid body to OSA and potential therapeutic intervention to mitigate OSA in a murine model.

NOX2 deficiency exacerbates diet-induced obesity and impairs molecular training adaptations in skeletal muscle

The production of reactive oxygen species (ROS) by NADPH oxidase (NOX) 2 has been linked to both insulin resistance and exercise training adaptations in skeletal muscle. This study explores the previously unexamined role of NOX2 in the interplay between diet-induced insulin resistance and exercise training (ET). Using a mouse model that harbors a point mutation in the essential NOX2 regulatory subunit, p47phox (Ncf1*), we investigated the impact of this mutation on various metabolic adaptations. Wild-type (WT) and Ncf1* mice were assigned to three groups: chow diet, 60% energy fat diet (HFD), and HFD with access to running wheels (HFD + E). After a 16-week intervention, a comprehensive phenotypic assessment was performed, including body composition, glucose tolerance, energy intake, muscle insulin signaling, redox-related proteins, and mitochondrial adaptations. The results revealed that NOX2 deficiency exacerbated the impact of HFD on body weight, body composition, and glucose intolerance. Moreover, in Ncf1* mice, ET did not improve glucose tolerance or increase muscle cross-sectional area. ET normalized body fat independently of genotype. The lack of NOX2 activity during ET reduced several metabolic adaptations in skeletal muscle, including insulin signaling and expression of Hexokinase II and oxidative phosphorylation complexes. In conclusion, these findings suggest that NOX2 mediates key beneficial effects of exercise training in the context of diet-induced obesity.

Oxidative Stress Response Kinetics after 60 Minutes at Different Levels (10% or 15%) of Normobaric Hypoxia Exposure

In this study, the metabolic responses of hypoxic breathing for 1 h to inspired fractions of 10% and 15% oxygen were investigated. To this end, 14 healthy nonsmoking subjects (6 females and 8 males, age: 32.2 ± 13.3 years old (mean ± SD), height: 169.1 ± 9.9 cm, and weight: 61.6 ± 16.2 kg) volunteered for the study. Blood samples were taken before, and at 30 min, 2 h, 8 h, 24 h, and 48 h after a 1 h hypoxic exposure. The level of oxidative stress was evaluated by considering reactive oxygen species (ROS), nitric oxide metabolites (NOx), lipid peroxidation, and immune-inflammation by interleukin-6 (IL-6) and neopterin, while antioxidant systems were observed in terms of the total antioxidant capacity (TAC) and urates. Hypoxia abruptly and rapidly increased ROS, while TAC showed a U-shape pattern, with a nadir between 30 min and 2 h. The regulation of ROS and NOx could be explained by the antioxidant action of uric acid and creatinine. The kinetics of ROS allowed for the stimulation of the immune system translated by an increase in neopterin, IL-6, and NOx. This study provides insights into the mechanisms through which acute hypoxia affects various bodily functions and how the body sets up the protective mechanisms to maintain redox homeostasis in response to oxidative stress.

Chronic intermittent hypobaric hypoxia induces cardiovascular dysfunction in a high-altitude working shift model

AIMS

Chronic intermittent hypobaric hypoxia (CIHH) exposure due to shift work occurs mainly in 4 × 4 or 7 × 7 days shifts in mining, astronomy, and customs activities, among other institutions. However, the long-lasting effects of CIHH on cardiovascular structure and function are not well characterized. We aimed to investigate the effects of CIHH on the cardiac and vascular response of adult rats simulating high-altitude (4600 m) x low-altitude (760 m) working shifts.

MAIN METHODS

We analyzed in vivo cardiac function through echocardiography, ex vivo vascular reactivity by wire myography, and in vitro cardiac morphology by histology and protein expression and immunolocalization by molecular biology and immunohistochemistry techniques in 12 rats, 6 exposed to CIHH in the hypoxic chamber, and respective normobaric normoxic controls (n = 6).

KEY FINDINGS

CIHH induced cardiac dysfunction with left and right ventricle remodeling, associated with an increased collagen content in the right ventricle. In addition, CIHH increased HIF-1α levels in both ventricles. These changes are associated with decreased antioxidant capacity in cardiac tissue. Conversely, CIHH decreased contractile capacity with a marked decreased in nitric oxide-dependent vasodilation in both, carotid and femoral arteries.

SIGNIFICANCE

These data suggest that CIHH induces cardiac and vascular dysfunction by ventricular remodeling and impaired vascular vasodilator function. Our findings highlight the impact of CIHH in cardiovascular function and the importance of a periodic cardiovascular evaluation in high-altitude workers.

Identification of a novel senescence-associated signature to predict biochemical recurrence and immune microenvironment for prostate cancer

Background

Prostate cancer (PCa) is an age-associated malignancy with high morbidity and mortality rate, posing a severe threat to public health. Cellular senescence, a specialized cell cycle arrest form, results in the secretion of various inflammatory mediators. In recent studies, senescence has shown an essential role in tumorigenesis and tumor development, yet the extensive effects of senescence in PCa have not been systematically investigated. Here, we aimed to develop a feasible senescence-associated prognosis model for early identification and appropriate management in patients with PCa.

Method

The RNA sequence results and clinical information available from The Cancer Genome Atlas (TCGA) and a list of experimentally validated senescence-related genes (SRGs) from the CellAge database were first obtained. Then, a senescence-risk signature related with prognosis was constructed using univariate Cox and LASSO regression analysis. We calculated the risk score of each patient and divided them into high-risk and low-risk groups in terms of the median value. Furthermore, two datasets (GSE70770 and GSE46602) were used to assess the effects of the risk model. A nomogram was built by integrating the risk score and clinical characteristics, which was further verified using ROC curves and calibrations. Finally, we compared the differences in the tumor microenvironment (TME) landscape, drug susceptibility, and the functional enrichment among the different risk groups.

Results

We established a unique prognostic signature in PCa patients based on eight SRGs, including CENPA, ADCK5, FOXM1, TFAP4, MAPK, LGALS3, BAG3, and NOX4, and validated well prognosis-predictive power in independent datasets. The risk model was associated with age and TNM staging, and the calibration chart presented a high consistency in nomogram prediction. Additionally, the prognostic signature could serve as an independent prediction factor due to its high accuracy. Notably, we found that the risk score was positively associated with tumor mutation burden (TMB) and immune checkpoint, whereas negatively correlated with tumor immune dysfunction and exclusion (TIDE), suggesting that these patients with risk scores were more sensitive to immunotherapy. Drug susceptibility analysis revealed differences in the responses to general drugs (docetaxel, cyclophosphamide, 5-Fluorouracil, cisplatin, paclitaxel, and vincristine) were yielded between the two risk groups.

Conclusion

Identifying the SRG-score signature may become a promising method for predicting the prognosis of patients with PCa and tailoring appropriate treatment strategies.

Adaptive cardiorespiratory changes to chronic continuous and intermittent hypoxia

This chapter reviews cardiorespiratory adaptations to chronic hypoxia (CH) experienced at high altitude and cardiorespiratory pathologies elicited by chronic intermittent hypoxia (CIH) occurring with obstructive sleep apnea (OSA). Short-term CH increases breathing (ventilatory acclimatization to hypoxia) and blood pressure (BP) through carotid body (CB) chemo reflex. Hyperplasia of glomus cells, alterations in ion channels, and recruitment of additional excitatory molecules are implicated in the heightened CB chemo reflex by CH. Transcriptional activation of hypoxia-inducible factors (HIF-1 and 2) is a major molecular mechanism underlying respiratory adaptations to short-term CH. High-altitude natives experiencing long-term CH exhibit blunted hypoxic ventilatory response (HVR) and reduced BP due to desensitization of CB response to hypoxia and impaired processing of CB sensory information at the central nervous system. Ventilatory changes evoked by long-term CH are not readily reversed after return to sea level. OSA patients and rodents subjected to CIH exhibit heightened CB chemo reflex, increased hypoxic ventilatory response, and hypertension. Increased generation of reactive oxygen species (ROS) is a major cellular mechanism underlying CIH-induced enhanced CB chemo reflex and the ensuing cardiorespiratory pathologies. ROS generation by CIH is mediated by nontranscriptional, disrupted HIF-1 and HIF-2-dependent transcriptions as well as epigenetic mechanisms.

Diabetic mellitus, vascular calcification and hypoxia: A complex and neglected tripartite relationship

DM (diabetic mellitus) and its common vascular complications VC (vascular calcification), are increasingly harmful to human health. In recent years, the research on the relationship between DM and VC is also deepening. Hypoxia, as one of the pathogenic factors of many disease models, is also closely related to the occurrence of DM and VC. There are some studies on the role of hypoxia in the pathogenesis of DM and VC respectively, but no one has made an in-depth summary of the systematic connection between hypoxia, DM and VC. Therefore, what we want to review in this article are the relationship between DM, VC and hypoxia, respectively, as well as the role of hypoxia in the development of DM and VC, which has little concern but is a novel and potentially target that may provide some new ideas for the prevention and treatment of DM, VC, especially diabetic VC.

Additive effects of orchiectomy and intermittent hypoxia on lung mechanics and inflammation in C57BL/6J male mice

NEW FINDINGS

What is the central question of this study? Does endogenous testosterone modulate the consequences of intermittent hypoxia (IH) in the lungs of male mice? What is the main finding and its importance? Orchiectomized mice exposed to IH develop a pattern that is similar to emphysema or obstructive lung disease with elevated lung volumes, low pulmonary elastance during a methacholine challenge test and high counts of lymphocytes in bronchoalveolar lavages. Since low testosterone levels and other respiratory diseases are common in sleep apnoea, there is a clear clinical relevance to these results.

ABSTRACT

We tested the hypothesis that low testosterone levels modulate the pulmonary responses to intermittent hypoxia (IH; used as a model of sleep apnoea (SA)) in male mice. We used intact (SHAM) or orchiectomized (ORX) mice exposed to IH for 14 days (12 h/day, 10 cycles/h, 6% oxygen) or to normoxia (Nx). We first measured ventilation and metabolic rates in freely behaving mice (whole-body plethysmography) and then respiratory mechanics in tracheotomized mice (flexiVent). We assessed the respiratory system resistance and elastance (E_rs ), Newtonian resistance (resistance of the large airways), tissue damping and tissue elastance (H) under baseline conditions and during a methacholine challenge test. We also measured the quasi-static compliance and inspiratory capacity with partial pressure-volume loops. Finally, inflammatory cells were counted in the broncho-alveolar lavage (BAL) and we measured lung volume by water displacement. ORX-IH mice had higher tidal volume, inspiratory capacity and lung volume compared to the other groups, but showed signs of low efficiency of O₂ exchange rate relative to minute ventilation. During the methacholine challenge, orchiectomy decreased the values of most mechanical parameters and IH reduced E_rs and H leading to very low values in ORX-IH mice. Finally, the total number of cells and the number of lymphocytes in BAL were both increased by IH in ORX mice. Since reduced lung elasticity, low O₂ extraction, increased lung volumes and inflammation are signs of emphysematous lung disease, we conclude that testosterone might prevent lung emphysema during IH exposures.

Lysine demethylase KDM6B regulates HIF-1α-mediated systemic and cellular responses to intermittent hypoxia

Intermittent hypoxia (IH) is a hallmark manifestation of obstructive sleep apnea (OSA). Rodents treated with IH exhibit hypertension. Hypoxia-inducible factor (HIF)-1-dependent transcriptional activation of NADPH oxidases (Nox) and the resulting increase in reactive oxygen species (ROS) levels is a major molecular mechanism underlying IH/OSA-induced hypertension. Jumanji C (JmjC)-containing histone lysine demethylases (JmjC-KDMs) are coactivators of HIF-1-dependent transcriptional activation. In the present study, we tested the hypothesis that JmjC-KDMs are required for IH-evoked HIF-1 transcriptional activation of Nox4 and the ensuing hypertension. Studies were performed on pheochromocytoma (PC)12 cells and rats. IH increased KDM6B protein and enzyme activity in PC12 cells in an HIF-1-independent manner as evidenced by unaltered KDM6B activation by IH in HIF-1α shRNA-treated cells. Cells treated with IH showed increased HIF-1-dependent Nox4 transcription as indicated by increased HIF-1α binding to hypoxia-responsive element (HRE) sequence of the Nox4 gene promoter demonstrated by chromatin immunoprecipitation (ChiP) assay. Pharmacological blockade of KDM6B with GSKJ4, a specific KDM6 inhibitor, or genetic silencing of KDM6B with shRNA abolished IH-induced Nox4 transcriptional activation by blocking HIF-1α binding to the promoter of the Nox4 gene. Treating IH-exposed rats with GSKJ4 showed: 1) absence of KDM6B activation and HIF-1-dependent Nox4 transcription in the adrenal medullae, and 2) absence of elevated plasma catecholamines and hypertension. Collectively, these findings indicate that KDM6B functions as a coactivator of HIF-1-mediated Nox4 transactivation and demonstrates a hitherto uncharacterized role for KDMs in IH-induced hypertension by HIF-1.

The role of NADPH oxidase in chronic intermittent hypoxia-induced respiratory plasticity in adult male mice

Reactive oxygen species (ROS) are proposed as mediators of chronic intermittent hypoxia (CIH)-induced respiratory plasticity. We sought to determine if NADPH oxidase 2 (NOX2)-derived ROS underpin CIH-induced maladaptive changes in respiratory control. Adult male mice (C57BL/6 J) were assigned to one of three groups: normoxic controls (sham); chronic intermittent hypoxia-exposed (CIH, 12 cycles/hour, 8 h/day for 14 days); and CIH + apocynin (NOX2 inhibitor, 2 mM) given in the drinking water throughout exposure to CIH. In addition, we studied sham and CIH-exposed NOX2-null mice (B6.129S-Cybb^TM1Din/J). Whole-body plethysmography was used to measure breathing and metabolic parameters. Ventilation (V̇_I/V̇CO₂) during normoxia was unaffected by CIH, but apnoea index was increased, which was prevented by apocynin, but not by NOX2 deletion. The ventilatory response to hypercapnia following exposure to CIH was potentiated in NOX2-null mice. Our results reveal ROS-dependent influences on the control of breathing and point to antioxidant intervention as a potential adjunctive therapeutic strategy in respiratory control disorders.

Histone Deacetylase 5 Is an Early Epigenetic Regulator of Intermittent Hypoxia Induced Sympathetic Nerve Activation and Blood Pressure

Intermittent hypoxia (IH) is a hallmark manifestation of obstructive sleep apnea (OSA). Long term IH (LT-IH) triggers epigenetic reprogramming of the redox state involving DNA hypermethylation in the carotid body chemo reflex pathway resulting in persistent sympathetic activation and hypertension. Present study examined whether IH also activates epigenetic mechanism(s) other than DNA methylation. Histone modification by lysine acetylation is another major epigenetic mechanism associated with gene regulation. Equilibrium between the activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs) determine the level of lysine acetylation. Here we report that exposure of rat pheochromocytoma (PC)-12 cells to IH in vitro exhibited reduced HDAC enzyme activity due to proteasomal degradation of HDAC3 and HDAC5 proteins. Mechanistic investigations showed that IH-evoked decrease in HDAC activity increases lysine acetylation of α subunit of hypoxia inducible factor (HIF)-1α as well as Histone (H3) protein resulting in increased HIF-1 transcriptional activity. Trichostatin A (TSA), an inhibitor of HDACs, mimicked the effects of IH. Studies on rats treated with 10 days of IH or TSA showed reduced HDAC activity, HDAC5 protein, and increased HIF-1 dependent NADPH oxidase (NOX)-4 transcription in adrenal medullae (AM) resulting in elevated plasma catecholamines and blood pressure. Likewise, heme oxygenase (HO)-2 null mice, which exhibit IH because of high incidence of spontaneous apneas (apnea index 72 ± 1.2 apnea/h), also showed decreased HDAC activity and HDAC5 protein in the AM along with elevated circulating norepinephrine levels. These findings demonstrate that lysine acetylation of histone and non-histone proteins is an early epigenetic mechanism associated with sympathetic nerve activation and hypertension in rodent models of IH.

Hypoxia-inducible factors and obstructive sleep apnea

Intermittent hypoxia (IH) is a hallmark manifestation of obstructive sleep apnea (OSA), a widespread disorder of breathing. This Review focuses on the role of hypoxia-inducible factors (HIFs) in hypertension, type 2 diabetes (T2D), and cognitive decline in experimental models of IH patterned after O2 profiles seen in OSA. IH increases HIF-1α and decreases HIF-2α protein levels. Dysregulated HIFs increase reactive oxygen species (ROS) through HIF-1-dependent activation of pro-oxidant enzyme genes in addition to reduced transcription of antioxidant genes by HIF-2. ROS in turn activate chemoreflex and suppress baroreflex, thereby stimulating the sympathetic nervous system and causing hypertension. We also discuss how increased ROS generation by HIF-1 contributes to IH-induced insulin resistance and T2D as well as disrupted NMDA receptor signaling in the hippocampus, resulting in cognitive decline.

Mechanisms of intermittent hypoxia-mediated macrophage activation - potential therapeutic targets for obstructive sleep apnoea

Intermittent hypoxia (IH) plays a key role in the pathogenesis of insulin resistance (IR) in obstructive sleep apnoea (OSA). IH induces a pro-inflammatory phenotype of the adipose tissue with M1 macrophage polarisation, subsequently impeding adipocyte insulin signalling, and these changes are in striking similarity to those seen in obesity. However, the detailed molecular mechanisms of IH-induced macrophage polarisation are unknown and identification of same should lead to the identification of novel therapeutic targets. In the present study, we tested the hypothesis that IH acts through similar mechanisms as obesity, activating Toll-like-receptor (TLR)4/nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3) signalling pathways leading to the upregulation and secretion of the key cytokines interleukin (IL)-1β and IL-6. Bone-marrow derived macrophages (BMDMs) from lean and obese C57BL/6 male mice were exposed to a state-of-the-art in vitro model of IH. Independent of obesity, IH led to a pro-inflammatory M1 phenotype characterised by increased inducible nitric oxide synthase and IL-6 mRNA expression, robust increase in NF-κB DNA-binding activity and IL-6 secretion. Furthermore, IH significantly increased pro-IL-1β mRNA and protein expression and mature IL-1β secretion compared to control treatment. Providing mechanistic insight, pre-treatment with the TLR4 specific inhibitor, TAK-242, prevented IH-induced M1 polarisation and upregulation of IL-1β mRNA and pro-IL-1β protein expression. Moreover, IH-induced increase in IL-1β secretion was prevented in BMDMs isolated from NLRP3 knockout mice. Thus, targeting TLR4/NF-κB and NLRP3 signalling pathways may provide novel therapeutic options for metabolic complications in OSA.

Hypoxia-inducible factor-1 mediates pancreatic β-cell dysfunction by intermittent hypoxia

The role of hypoxia-inducible factor (HIF)-1 in pancreatic β-cell response to intermittent hypoxia (IH) was examined. Studies were performed on adult wild-type (WT), HIF-1α heterozygous (HET), β-cell-specific HIF-1^-/- mice and mouse insulinoma (MIN6) cells exposed to IH patterned after blood O₂ profiles during obstructive sleep apnea. WT mice treated with IH showed insulin resistance, and pancreatic β-cell dysfunction manifested as augmented basal insulin secretion, and impaired glucose-stimulated insulin secretion and these effects were absent in HIF-1α HET mice. IH increased HIF-1α expression and elevated reactive oxygen species (ROS) levels in β-cells of WT mice. The elevated ROS levels were due to transcriptional upregulation of NADPH oxidase (NOX)-4 mRNA, protein and enzymatic activity, and these responses were absent in HIF-1α HET mice as well as in β-HIF-1^-/- mice. IH-evoked β-cell responses were absent in adult WT mice treated with digoxin, an inhibitor of HIF-1α. MIN6 cells treated with in vitro IH showed enhanced basal insulin release and elevated HIF-1α protein expression, and these effects were abolished with genetic silencing of HIF-1α. IH increased NOX4 mRNA, protein, and enzyme activity in MIN6 cells and disruption of NOX4 function by siRNA or scavenging H₂O₂ with polyethylene glycol catalase blocked IH-evoked enhanced basal insulin secretion. These results demonstrate that HIF-1-mediated transcriptional activation of NOX4 and the ensuing increase in H₂O₂ contribute to IH-induced pancreatic β-cell dysfunction.

The role of reactive oxygen species in cognitive impairment associated with sleep apnea

Obstructive sleep apnea (OSA), a common breathing and sleeping disorder, is associated with a broad range of neurocognitive difficulties. Intermittent hypoxia (IH), one major characteristic of OSA, has been shown to impair learning and memory due to increased levels of reactive oxygen species (ROS). Under normal conditions, ROS are produced in low concentrations and act as signaling molecules in different processes. However, IH treatment leads to elevated ROS production via multiple pathways, including mitochondrial electron transport chain dysfunction and in particular complex I dysfunction, and induces oxidative tissue damage. Moreover, elevated ROS results in the accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) and increased activity of peroxisomes, such as NADPH oxidase, xanthine oxidase and phospholipase A2. Furthermore, oxidative tissue damage has been found in regions of the brains of patients with OSA, including the cortex and hippocampus, which are associated with memory and executive function. Furthermore, increased ROS levels in these regions of the brain induce damage via inflammation, apoptosis, ER stress and neuronal activity disturbance. The present review focuses on the mechanism of excessive ROS production in an OSA model and the relationship between ROS and cognitive impairment.

Progesterone decreases apnoea and reduces oxidative stress induced by chronic intermittent hypoxia in ovariectomized female rats

NEW FINDINGS

What is the central question of this study? Does progesterone reduce the effect of chronic intermittent hypoxia (CIH) on arterial blood pressure, respiratory control and oxidative stress in the central nervous system in ovariectomized rats? What is the main finding and its importance? Progesterone does not prevent the elevation of arterial blood pressure in rats exposed to CIH, but normalizes respiratory control, and reduces cerebral oxidative stress. This study draws focus to a potential role of progesterone and the consequences of sleep apnoea in menopausal women.

ABSTRACT

We tested the hypothesis that progesterone (Prog) reduces the effect of chronic intermittent hypoxia (CIH) on arterial blood pressure, respiratory chemoreflexes and oxidative stress in the central nervous system. Ovariectomized female rats were implanted with osmotic pumps delivering vehicle (Veh) or Prog (4 mg kg^-1  day^-1 ). Two weeks following the surgery, rats were exposed to room air (Air) or CIH (7 days, 10% O₂ , 10 cycles h^-1 , 8 h day^-1 ). We studied three groups: Veh-Air, Veh-CIH and Prog-CIH. After the CIH exposures, we measured the mean arterial pressure (MAP; tail cuff) and assessed the frequency of apnoeas at rest and ventilatory responses to hypoxia and hypercapnia (whole body plethysmography). The activities of the pro-oxidant enzyme NADPH oxidase (NOX) and antioxidant enzymes superoxide dismutase (SOD; in mitochondrial and cytosolic fractions) and glutathione peroxidase (GPx), as well as the concentration of malondialdehyde (MDA), a marker of lipid peroxidation, were measured in brain cortex and brainstem samples. CIH exposure increased the MAP, the frequency of apnoeas, and the respiratory frequency response to hypoxia and hypercapnia. Prog did not prevent the CIH-induced elevation in MAP, but it reduced the CIH-induced frequency of apnoeas and increased hypoxic and hypercapnic ventilatory responses. In the brain cortex, CIH increased NOX activity, and decreased the cytosolic and mitochondrial SOD activities. These effects were prevented by Prog. NOX activity was increased by CIH in the brainstem, and this was also blocked by Prog. The study draws focus to the links between ovarian hormones and the consequences of sleep apnoea in women.

Long-term facilitation of catecholamine secretion from adrenal chromaffin cells of neonatal rats by chronic intermittent hypoxia

In neonates, catecholamine (CA) secretion from adrenal medullary chromaffin cells (AMC) is an important mechanism for maintaining homeostasis during hypoxia. Nearly 90% of premature infants experience chronic intermittent hypoxia (IH) because of high incidence of apnea of prematurity, which is characterized by periodic stoppage of breathing. The present study examined the effects of repetitive hypoxia, designed to mimic apnea of prematurity, on CA release from AMC of neonatal rats. Neonatal rats were exposed to either control conditions or chronic intermittent hypoxia (IH) from ages postnatal days 0-5 (P0-P5), and CA release from adrenal medullary slices was measured after challenge with repetitive hypoxia (5 episodes of 30-s hypoxia, Po₂ ~35 mmHg). In response to repetitive hypoxia, chronic IH-treated AMC exhibited sustained CA release, and this phenotype was not seen in control AMC. The sustained CA release was associated with long-lasting elevation of intracellular Ca²⁺ concentration ([Ca²⁺]_i), which was due to store-operated Ca²⁺ entry (SOCE). 2-Aminoethoxydiphenyl borate, an inhibitor of SOCE, prevented the long-lasting [Ca²⁺]_i elevation and CA release. Repetitive hypoxia increased H₂O₂ abundance, and polyethylene glycol (PEG)-catalase, a scavenger of H₂O₂ blocked this effect. PEG-catalase also prevented repetitive hypoxia-induced SOCE activation, sustained [Ca²⁺]_i elevation, and CA release. These results demonstrate that repetitive hypoxia induces long-term facilitation of CA release in chronic IH-treated neonatal rat AMC through sustained Ca²⁺ influx mediated by SOCE.NEW & NOTEWORTHY Apnea of prematurity and the resulting chronic intermittent hypoxia are major clinical problems in neonates born preterm. Catecholamine release from adrenal medullary chromaffin cells maintains homeostasis during hypoxia in neonates. Our results demonstrate that chronic intermittent hypoxia induces a hitherto uncharacterized long-term facilitation of catecholamine secretion from neonatal rat chromaffin cells in response to repetitive hypoxia, simulating hypoxic episodes encountered during apnea of prematurity. The sustained catecholamine secretion might contribute to cardiovascular morbidities in infants with apnea of prematurity.

Tempol Alleviates Chronic Intermittent Hypoxia-Induced Pancreatic Injury Through Repressing Inflammation and Apoptosis

Obstructive sleep apnea (OSA) has been demonstrated to be implicated in disorder of insulin secretion and diabetes mellitus. In this study, we aimed to evaluate the protective role of tempol, a powerful antioxidant, in chronic intermittent hypoxia (IH)-induced pancreatic injury. The rat model of OSA was established by IH exposure. The pathological changes, increased blood-glucose level, and raised proinsulin/insulin ratio in pancreatic tissues of rats received IH were effectively relieved by tempol delivery. In addition, the enhanced levels of pro-inflammatory cytokines, TNF-α, IL-1β, IL-6, and inflammatory mediators, PGE2, cyclooxygenase-2 (COX-2), NO, and inducible nitric oxide synthase (iNOS) in pancreatic tissue were suppressed by tempol. Moreover, tempol inhibited IH-induced apoptosis in pancreatic tissue as evidenced by upregulated Bcl-2 level, and downregulated Bax and cleaved caspase-3 levels. Finally, the abnormal activation of mitogen-activated protein kinase (MAPK) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling pathways induced by IH was restrained by tempol administration. In summary, our study demonstrates that tempol relieves IH-induced pancreatic injury by inhibiting inflammatory response and apoptosis, which provides theoretical basis for tempol as an effective treatment for OSA-induced pancreatic injury.

Neural Activation of Molecular Circuitry in Intermittent Hypoxia

People living at sea level experience intermittent hypoxia (IH) as a consequence of sleep apnea, which is a highly prevalent respiratory disorder. Sleep apnea patients and rodents exposed to IH exhibit autonomic dysfunction manifested as increased sympathetic nerve activity and hypertension. This article highlights physiologic basis of autonomic disturbances by IH, which involves abnormal activation of the carotid body (CB) chemo reflex by reactive oxygen species (ROS).We further evaluate major molecular mechanisms underlying IH-induced ROS generation including transcriptional activation of genes encoding pro-oxidant enzymes by hypoxia-inducible factor (HIF)-1 and transcriptional repression of anti-oxidant enzyme genes by DNA methylation. Lastly, evidence is presented for CB neural activity as a major regulator of HIF-1 activation and DNA methylation by IH in the chemo reflex pathway.

NOX2 oxidase expressed in endosomes promotes cell proliferation and prostate tumour development

Reactive oxygen species (ROS) promote growth factor signalling including for VEGF-A and have potent angiogenic and tumourigenic properties. However, the precise enzymatic source of ROS generation, the subcellular localization of ROS production and cellular targets in vivo that influence tumour-promoting processes, are largely undefined. Here, using mRNA microarrays, we show increased gene expression for NOX2, the catalytic subunit of the ROS-generating NADPH oxidase enzyme, in human primary prostate cancer compared to non-malignant tissue. In addition, NOX4 gene expression was markedly elevated in human metastatic prostate cancers, but not in primary prostate tumours. Using a syngeneic, orthotopic mouse model of prostate cancer the genetic deletion of NOX2 (i.e. NOX2 -/y mouse) resulted in reduced angiogenesis and an almost complete failure in tumour development. Furthermore, pharmacological inhibition of NOX2 oxidase suppressed established prostate tumours in mice. In isolated endothelial cells, and in human normal and prostate cancer cells, NOX2 co-located to varying degrees with early endosome markers including EEA1, Appl1 and Rab5A and the late endosome marker Rab7A, and this correlated with significant VEGF-A-dependent ROS production within acidified endosomal compartments and endothelial cell proliferation that was NOX2 oxidase- and hydrogen peroxide dependent. We concluded that NOX2 oxidase expression and endosomal ROS production were important for prostate cancer growth and that this was required to positively regulate the VEGF pathway. The research provides a paradigm for limiting tumour growth through a better understanding of NOX2 oxidase's effect on VEGF signalling and how controlling the development of tumour vasculature can limit prostate tumour development and metastasis.

DNA methylation in the central and efferent limbs of the chemoreflex requires carotid body neural activity

KEY POINTS

The mechanisms underlying long-term (30 days) intermittent hypoxia (LT-IH)-evoked DNA methylation of anti-oxidant enzyme (AOE) gene repression in the carotid body (CB) reflex pathway were examined. LT-IH-treated rats showed increased reactive oxygen species (ROS) levels in the CB reflex pathway. Administration of a ROS scavenger or CB ablation blocked LT-IH-evoked DNA methylation and AOE gene repression in the central and efferent limbs of the CB reflex. LT-IH increased DNA methyltransferase (Dnmt) activity through upregulation of Dnmt1 and 3b proteins by ROS-dependent inactivation of glycogen synthase kinase 3β (GSK3β) by Akt. A pan-Akt inhibitor prevented LT-IH-induced GSK3β inactivation, elevated Dnmt protein expression and activity, AOE gene methylation, sympathetic activation and hypertension.

ABSTRACT

Long-term exposure to intermittent hypoxia (LT-IH; 30 days), simulating blood O2 profiles during sleep apnoea, has been shown to repress anti-oxidant enzyme (AOE) gene expression by DNA methylation in the carotid body (CB) reflex pathway, resulting in persistent elevation of plasma catecholamine levels and blood pressure. The present study examined the mechanisms by which LT-IH induces DNA methylation. Adult rats exposed to LT-IH showed elevated reactive oxygen species (ROS) in the CB, nucleus tractus solitarius (nTS) and rostroventrolateral medulla (RVLM) and adrenal medulla (AM), which represent the central and efferent limbs of the CB reflex, respectively. ROS scavenger treatment during the first ten days of IH exposure prevented ROS accumulation, blocked DNA methylation, and normalized AOE gene expression, suggesting that ROS generated during the early stages of IH activate DNA methylation. CB ablation prevented the ROS accumulation, normalized AOE gene expression in the nTS, RVLM, and AM and blocked DNA methylation, suggesting that LT-IH-induced DNA methylation in the central and efferent limbs of the CB reflex is indirect and requires CB neural activity. LT-IH increased DNA methyl transferase (Dnmt) activity through upregulation of Dnmt1 and 3b protein expression due to ROS-dependent inactivation of glycogen synthase kinase 3β (GSK3β) by protein kinase B (Akt). Treating rats with the pan-Akt inhibitor GSK690693 blocked the induction of Dnmt activity, Dnmt protein expression, and DNA methylation, leading to normalization of AOE gene expression as well as plasma catecholamine levels and blood pressure.

Carotid body chemoreflex: a driver of autonomic abnormalities in sleep apnoea

What is the topic of this review? This article presents emerging evidence for heightened carotid body chemoreflex activity as a major driver of sympathetic activation and hypertension in sleep apnoea patients. What advances does it heighlight? This article discusses the recent advances on cellular, molecular and epigenetic mechanisms underlying the exaggerated chemoreflex in experimental models of sleep apnoea. The carotid bodies are the principal peripheral chemoreceptors for detecting changes in arterial blood oxygen concentration, and the resulting chemoreflex is a potent regulator of the sympathetic tone, blood pressure and breathing. Sleep apnoea is a disease of the respiratory system that affects several million adult humans. Apnoeas occur during sleep, often as a result of obstruction of the upper airway (obstructive sleep apnoea) or because of defective respiratory rhythm generation by the CNS (central sleep apnoea). Patients with sleep apnoea exhibit several co-morbidities, with the most notable among them being heightened sympathetic nerve activity and hypertension. Emerging evidence suggests that intermittent hypoxia resulting from periodic apnoea stimulates the carotid body, and the ensuing chemoreflex mediates the increased sympathetic tone and hypertension in sleep apnoea patients. Rodent models of intermittent hypoxia that simulate the O2 saturation profiles encountered during sleep apnoea have provided important insights into the cellular and molecular mechanisms underlying the heightened carotid body chemoreflex. This article describes how intermittent hypoxia affects the carotid body function and discusses the cellular, molecular and epigenetic mechanisms underlying the exaggerated chemoreflex.

The role of mitochondrial ROS in the aging brain

The brain is the most complex human organ, consuming more energy than any other tissue in proportion to its size. It relies heavily on mitochondria to produce energy and is made up of mitotic and postmitotic cells that need to closely coordinate their metabolism to maintain essential bodily functions. During aging, damaged mitochondria that produce less ATP and more reactive oxygen species (ROS) accumulate. The current consensus is that ROS cause oxidative stress, damaging mitochondria and resulting in an energetic crisis that triggers neurodegenerative diseases and accelerates aging. However, in model organisms, increasing mitochondrial ROS (mtROS) in the brain extends lifespan, suggesting that ROS may participate in signaling that protects the brain. Here, we summarize the mechanisms by which mtROS are produced at the molecular level, how different brain cells and regions produce different amounts of mtROS, and how mtROS levels change during aging. Finally, we critically discuss the possible roles of ROS in aging as signaling molecules and damaging agents, addressing whether age-associated increases in mtROS are a cause or a consequence of aging.

Estradiol Protects Against Cardiorespiratory Dysfunctions and Oxidative Stress in Intermittent Hypoxia

Study Objectives

We tested the hypothesis that estradiol (E2) protects against cardiorespiratory disorders and oxidative stress induced by chronic intermittent hypoxia (CIH) in adult female rats.

Methods

Sprague-Dawley female rats (230-250 g) were ovariectomized and implanted with osmotic pumps delivering vehicle or E2 (0.5 mg/kg/d). After 14 days of recovery, the rats were exposed to CIH (21%-10% O2: 8 h/d, 10 cycles per hour) or room air (RA). After 7 days of CIH or RA exposure, we measured arterial pressures (tail cuff), metabolic rate (indirect calorimetry), minute ventilation, the frequency of sighs and apneas at rest, and ventilatory responses to hypoxia and hypercapnia (whole body plethysmography). We collected the cerebral cortex, brainstem, and adrenal glands to measure the activity of NADPH and xanthine oxidase (pro-oxidant enzymes), glutathione peroxidase, and the mitochondrial and cytosolic superoxide dismutase (antioxidant enzymes) and measured lipid peroxidation and advanced oxidation protein products (markers of oxidative stress).

Results

CIH increased arterial pressure, the frequency of apnea at rest, and the hypoxic and hypercapnic ventilatory responses and reduced metabolic rate. CIH also increased oxidant enzyme activities and decreased antioxidant activity in the cortex. E2 treatment reduced body weight and prevented the effects of CIH.

Conclusions

E2 prevents cardiorespiratory disorders and oxidative stress induced by CIH. These observations may help to better understand the underlying mechanisms linking menopause and occurrence of sleep apnea in women and highlight a potential advantage of hormone therapy.

NADPH oxidase 4 modulates hepatic responses to lipopolysaccharide mediated by Toll-like receptor-4

Chronic inflammation plays a key role in development of many liver diseases. Stimulation of Toll-like receptor 4 (TLR4) by bacterial lipopolysaccharide (LPS) initiates inflammation and promotes development of hepatocellular carcinoma and other liver diseases. NADPH oxidases contribute to LPS-induced reactive oxygen species (ROS) production and modulate TLR responses, but whether these enzymes function in TLR4 responses of hepatocytes is unknown. In the present work, we examined the role of NADPH oxidase 4 (Nox4) in LPS-induced TLR4 responses in human hepatoma cells and wildtype and Nox4-deficient mice. We found that LPS increased expression of Nox4, TNF-α, and proliferating cell nuclear antigen (PCNA). Nox4 silencing suppressed LPS-induced TNF-α and PCNA increases in human cells. The LPS-induced TNF-α increases were MyD88-dependent, and were attenuated in primary hepatocytes isolated from Nox4-deficient mice. We found that Nox4 mediated LPS-TLR4 signaling in hepatocytes via NF-ĸB and AP-1 pathways. Moreover, the effect of Nox4 depletion was time-dependent; following six weeks of repeated LPS stimulation in vivo, hepatic TNF-α and PCNA responses subsided in Nox4-deficient mice compared with wildtype mice. Therefore, our data suggest that Nox4 mediates LPS-TLR4 signaling in human hepatoma cells and murine hepatocytes and may contribute to the ability of LPS to stimulate liver pathology.

PKA activity exacerbates hypoxia-induced ROS formation and hypoxic injury in PC-12 cells

Hypoxia is a primary factor in many pathological conditions. Hypoxic cell death is commonly attributed to metabolic failure and oxidative injury. cAMP-dependent protein kinase A (PKA) is activated in hypoxia and regulates multiple enzymes of the mitochondrial electron transport chain, thus may be implicated in cellular energy depletion and hypoxia-induced cell death. Wild type (WT) PC-12 cells and PKA activity-deficient 123.7 PC-12 cells were exposed to 3, 6, 12 and 24h hypoxia (0.1% or 5% O2). Hypoxia, at 24h 0.1% O2, induced cell death and increased reactive oxygen species (ROS) in WT PC-12 cells. Despite lower ATP levels in normoxic 123.7 cells than in WT cells, hypoxia only decreased ATP levels in WT cells. However, menadione-induced oxidative stress similarly affected both cell types. While mitochondrial COX IV expression remained consistently higher in 123.7 cells, hypoxia decreased COX IV expression in both cell types. N-acetyl cysteine antioxidant treatment blocked hypoxia-induced WT cell death without preventing ATP depletion. Transient PKA catα expression in 123.7 cells partially restored hypoxia-induced ROS but did not alter ATP levels or COX IV expression. We conclude that PKA signaling contributes to hypoxic injury, by regulating oxidative stress rather than by depleting ATP levels. Therapeutic strategies targeting PKA signaling may improve cellular adaptation and recovery in hypoxic pathologies.

Protein Thiol Redox Signaling in Monocytes and Macrophages

SIGNIFICANCE

Monocyte and macrophage dysfunction plays a critical role in a wide range of inflammatory disease processes, including obesity, impaired wound healing diabetic complications, and atherosclerosis. Emerging evidence suggests that the earliest events in monocyte or macrophage dysregulation include elevated reactive oxygen species production, thiol modifications, and disruption of redox-sensitive signaling pathways. This review focuses on the current state of research in thiol redox signaling in monocytes and macrophages, including (i) the molecular mechanisms by which reversible protein-S-glutathionylation occurs, (ii) the identification of bona fide S-glutathionylated proteins that occur under physiological conditions, and (iii) how disruptions of thiol redox signaling affect monocyte and macrophage functions and contribute to atherosclerosis. Recent Advances: Recent advances in redox biochemistry and biology as well as redox proteomic techniques have led to the identification of many new thiol redox-regulated proteins and pathways. In addition, major advances have been made in expanding the list of S-glutathionylated proteins and assessing the role that protein-S-glutathionylation and S-glutathionylation-regulating enzymes play in monocyte and macrophage functions, including monocyte transmigration, macrophage polarization, foam cell formation, and macrophage cell death.

CRITICAL ISSUES

Protein-S-glutathionylation/deglutathionylation in monocytes and macrophages has emerged as a new and important signaling paradigm, which provides a molecular basis for the well-established relationship between metabolic disorders, oxidative stress, and cardiovascular diseases.

FUTURE DIRECTIONS

The identification of specific S-glutathionylated proteins as well as the mechanisms that control this post-translational protein modification in monocytes and macrophages will facilitate the development of new preventive and therapeutic strategies to combat atherosclerosis and other metabolic diseases. Antioxid. Redox Signal. 25, 816-835.

Moderate inhibition of mitochondrial function augments carotid body hypoxic sensitivity

A functional role for the mitochondria in acute O2 sensing in the carotid body (CB) remains undetermined. Whilst total inhibition of mitochondrial activity causes intense CB stimulation, it is unclear whether this response can be moderated such that graded impairment of oxidative phosphorylation might be a mechanism that sets and modifies the O2 sensitivity of the whole organ. We assessed NADH autofluorescence and [Ca2+]i in freshly dissociated CB type I cells and sensory chemoafferent discharge frequency in an intact CB preparation, in the presence of varying concentrations of nitrite (NO2 −), a mitochondrial nitric oxide (NO) donor and a competitive inhibitor of mitochondrial complex IV. NO2 − increased CB type I cell NADH in a manner that was dose-dependent and rapidly reversible. Similar concentrations of NO2 − raised type I cell [Ca2+]i via L-type channels in a PO2-dependent manner and increased chemoafferent discharge frequency. Moderate inhibition of the CB mitochondria by NO2 − augmented chemoafferent discharge frequency during graded hypoxia, consistent with a heightened CB O2 sensitivity. Furthermore, NO2 − also exaggerated chemoafferent excitation during hypercapnia signifying an increase in CB CO2 sensitivity. These data show that NO2 − can moderate the hypoxia sensitivity of the CB and thus suggest that O2 sensitivity could be set and modified in this organ by interactions between NO and mitochondrial complex IV.

Epigenetic regulation of redox state mediates persistent cardiorespiratory abnormalities after long-term intermittent hypoxia

KEY POINTS

The effects of short-term (ST; 10 days) and long-term (LT; 30 days) intermittent hypoxia (IH) on blood pressure (BP), breathing and carotid body (CB) chemosensory reflex were examined in adult rats. ST- and LT-IH treated rats exhibited hypertension, irregular breathing with apnoea and augmented the CB chemosensory reflex, with all these responses becoming normalized during recovery from ST- but not from LT-IH. The persistent cardiorespiratory responses to LT-IH were associated with elevated reactive oxygen species (ROS) levels in the CB and adrenal medulla, which were a result of DNA methylation-dependent suppression of genes encoding anti-oxidant enzymes (AOEs). Treating rats with decitabine either during LT-IH or during recovery from LT-IH prevented DNA methylation of AOE genes, normalized the expression of AOE genes and ROS levels, reversed the heightened CB chemosensory reflex and hypertension, and also stabilized breathing.

ABSTRACT

Rodents exposed to chronic intermittent hypoxia (IH), simulating blood O₂ saturation profiles during obstructive sleep apnoea (OSA), have been shown to exhibit a heightened carotid body (CB) chemosensory reflex and hypertension. CB chemosensory reflex activation also results in unstable breathing with apnoeas. However, the effect of chronic IH on breathing is not known. In the present study, we examined the effects of chronic IH on breathing along with blood pressure (BP) and assessed whether the autonomic responses are normalized after recovery from chronic IH. Studies were performed on adult, male, Sprague-Dawley rats exposed to either short-term (ST; 10 days) or long-term (LT, 30 days) IH. Rats exposed to either ST- or LT-IH exhibited hypertension, irregular breathing with apnoeas, an augmented CB chemosensory reflex as indicated by elevated CB neural activity and plasma catecholamine levels, and elevated reactive oxygen species (ROS) levels in the CB and adrenal medulla (AM). All these effects were normalized after recovery from ST-IH but not from LT-IH. Analysis of the molecular mechanisms underlying the persistent effects of LT-IH revealed increased DNA methylation of genes encoding anti-oxidant enzymes (AOEs). Treatment with decitabine, a DNA methylation inhibitor, either during LT-IH or during recovery from LT-IH, prevented DNA methylation, normalized the expression of AOE genes, ROS levels, CB chemosensory reflex and BP, and also stabilized breathing. These results suggest that persistent cardiorespiratory abnormalities caused by LT-IH are mediated by epigenetic re-programming of the redox state in the CB chemosensory reflex pathway.

Complement-induced activation of the cardiac NLRP3 inflammasome in sepsis

Cardiac dysfunction develops during sepsis in humans and rodents. In the model of polymicrobial sepsis induced by cecal ligation and puncture (CLP), we investigated the role of the NLRP3 inflammasome in the heart. Mouse heart homogenates from sham-procedure mice contained high mRNA levels of NLRP3 and IL-1β. Using the inflammasome protocol, exposure of cardiomyocytes (CMs) to LPS followed by ATP or nigericin caused release of mature IL-1β. Immunostaining of left ventricular frozen sections before and 8 h after CLP revealed the presence of NLRP3 and IL-1β proteins in CMs. CLP caused substantial increases in mRNAs for IL-1β and NLRP3 in CMs which are reduced in the absence of either C5aR1 or C5aR2. After CLP, NLRP3^-/- mice showed reduced plasma levels of IL-1β and IL-6. In vitro exposure of wild-type CMs to recombinant C5a (rC5a) caused elevations in both cytosolic and nuclear/mitochondrial reactive oxygen species (ROS), which were C5a-receptor dependent. Use of a selective NOX2 inhibitor prevented increased cytosolic and nuclear/mitochondrial ROS levels and release of IL-1β. Finally, NLRP3^-/- mice had reduced defects in echo/Doppler parameters in heart after CLP. These studies establish that the NLRP3 inflammasome contributes to the cardiomyopathy of polymicrobial sepsis.-Kalbitz, M., Fattahi, F., Grailer, J. J., Jajou, L., Malan, E. A., Zetoune, F. S., Huber-Lang, M., Russell, M. W., Ward, P. A. Complement-induced activation of the cardiac NLRP3 inflammasome in sepsis.

Calpain activation by ROS mediates human ether-a-go-go-related gene protein degradation by intermittent hypoxia

Human ether-a-go-go-related gene (hERG) channels conduct delayed rectifier K(+) current. However, little information is available on physiological situations affecting hERG channel protein and function. In the present study we examined the effects of intermittent hypoxia (IH), which is a hallmark manifestation of sleep apnea, on hERG channel protein and function. Experiments were performed on SH-SY5Y neuroblastoma cells, which express hERG protein. Cells were exposed to IH consisting of alternating cycles of 30 s of hypoxia (1.5% O2) and 5 min of 20% O2. IH decreased hERG protein expression in a stimulus-dependent manner. A similar reduction in hERG protein was also seen in adrenal medullary chromaffin cells from IH-exposed neonatal rats. The decreased hERG protein was associated with attenuated hERG K(+) current. IH-evoked hERG protein degradation was not due to reduced transcription or increased proteosome/lysomal degradation. Rather it was mediated by calcium-activated calpain proteases. Both COOH- and NH2-terminal sequences of the hERG protein were the targets of calpain-dependent degradation. IH increased reactive oxygen species (ROS) levels, intracellular Ca(2+) concentration ([Ca(2+)]i), calpain enzyme activity, and hERG protein degradation, and all these effects were prevented by manganese-(111)-tetrakis-(1-methyl-4-pyridyl)-porphyrin pentachloride, a membrane-permeable ROS scavenger. These results demonstrate that activation of calpains by ROS-dependent elevation of [Ca(2+)]i mediates hERG protein degradation by IH.

Peripheral chemoreception and arterial pressure responses to intermittent hypoxia

Carotid bodies are the principal peripheral chemoreceptors for detecting changes in arterial blood oxygen levels, and the resulting chemoreflex is a potent regulator of blood pressure. Recurrent apnea with intermittent hypoxia (IH) is a major clinical problem in adult humans and infants born preterm. Adult patients with recurrent apnea exhibit heightened sympathetic nerve activity and hypertension. Adults born preterm are predisposed to early onset of hypertension. Available evidence suggests that carotid body chemoreflex contributes to hypertension caused by IH in both adults and neonates. Experimental models of IH provided important insights into cellular and molecular mechanisms underlying carotid body chemoreflex-mediated hypertension. This article provides a comprehensive appraisal of how IH affects carotid body function, underlying cellular, molecular, and epigenetic mechanisms, and the contribution of chemoreflex to the hypertension.

CaV3.2 T-type Ca2+ channels mediate the augmented calcium influx in carotid body glomus cells by chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) is a hallmark manifestation of sleep apnea. A heightened carotid body activity and the resulting chemosensory reflex mediate increased sympathetic nerve activity by CIH. However, the mechanisms underlying heightened carotid body activity by CIH are not known. An elevation of intracellular calcium ion concentration ([Ca(2+)]i) in glomus cells, the primary oxygen-sensing cells, is an essential step for carotid body activation by hypoxia. In the present study, we examined the effects of CIH on the glomus cell [Ca(2+)]i response to hypoxia and assessed the underlying mechanisms. Glomus cells were harvested from adult rats or wild-type mice treated with 10 days of either room air (control) or CIH (alternating cycles of 15 s of hypoxia and 5 min of room air; 9 episodes/h; 8 h/day). CIH-treated glomus cells exhibited an enhanced [Ca(2+)]i response to hypoxia, and this effect was absent in the presence of 2-(4-cyclopropylphenyl)-N-((1R)-1-[5-[(2,2,2-trifluoroethyl)oxo]-pyridin-2-yl]ethyl)acetamide (TTA-A2), a specific inhibitor of T-type Ca(2+) channels, and in voltage-gated calcium channel, type 3.2 (CaV3.2), null glomus cells. CaV3.2 knockout mice exhibited an absence of CIH-induced hypersensitivity of the carotid body. CIH increased reactive oxygen species (ROS) levels in glomus cells. A ROS scavenger prevented the exaggerated TTA-A2-sensitive [Ca(2+)]i response to hypoxia. CIH had no effect on CaV3.2 mRNA levels. CIH augmented Ca(2+) currents and increased CaV3.2 protein in plasma membrane fractions of human embryonic kidney-293 cells stably expressing CaV3.2, and either a ROS scavenger or brefeldin-A, an inhibitor of protein trafficking, prevented these effects. These findings suggest that CIH leads to an augmented Ca(2+) influx via ROS-dependent facilitation of CaV3.2 protein trafficking to the plasma membrane.

SENP1 inhibits the IH-induced apoptosis and nitric oxide production in BV2 microglial cells

To reveal SUMOylation and the roles of Sentrin-specific proteases (SENP)s in microglial cells under Intermittent hypoxia (IH) condition would provide more intensive view of understanding the mechanisms of IH-induced central nervous system (CNS) damage. Hence, in the present study, we detected the expression levels of SENPs in microglial cells under IH and normoxia conditions via RT-PCR assay. We found that SENP1 was significantly down-regulated in cells exposure to IH. Subsequently, the effect of IH for the activation of microglia and the potential roles of SENP1 in the SENP1-overexpressing cell lines were investigated via Western blotting, RT-PCR and Griess assay. The present study demonstrated the apoptosis-inducing and activating role of IH on microglia. In addition, we revealed that the effect of IH on BV-2 including apoptosis, nitric oxide synthase (iNOS) expression and nitric oxide (NO) induction can be attenuated by SENP1 overexpression. The results of the present study are of both theoretical and therapeutic significance to explore the potential roles of SENP1 under IH condition and elucidated the mechanisms underlying microglial survival and activation.

Targeting mitochondrial reactive oxygen species to modulate hypoxia-induced pulmonary hypertension

Pulmonary hypertension (PH) is characterized by increased pulmonary vascular remodeling, resistance, and pressures. Reactive oxygen species (ROS) contribute to PH-associated vascular dysfunction. NADPH oxidases (Nox) and mitochondria are major sources of superoxide (O(2)(•-)) and hydrogen peroxide (H(2)O(2)) in pulmonary vascular cells. Hypoxia, a common stimulus of PH, increases Nox expression and mitochondrial ROS (mtROS) production. The interactions between these two sources of ROS generation continue to be defined. We hypothesized that mitochondria-derived O(2)(•-) (mtO(2)(•-)) and H(2)O(2) (mtH(2)O(2)) increase Nox expression to promote PH pathogenesis and that mitochondria-targeted antioxidants can reduce mtROS, Nox expression, and hypoxia-induced PH. Exposure of human pulmonary artery endothelial cells to hypoxia for 72 h increased mtO(2)(•-) and mtH(2)O(2). To assess the contribution of mtO(2)(•-) and mtH(2)O(2) to hypoxia-induced PH, mice that overexpress superoxide dismutase 2 (Tg(hSOD2)) or mitochondria-targeted catalase (MCAT) were exposed to normoxia (21% O(2)) or hypoxia (10% O(2)) for three weeks. Compared with hypoxic control mice, MCAT mice developed smaller hypoxia-induced increases in RVSP, α-SMA staining, extracellular H(2)O(2) (Amplex Red), Nox2 and Nox4 (qRT-PCR and Western blot), or cyclinD1 and PCNA (Western blot). In contrast, Tg(hSOD2) mice experienced exacerbated responses to hypoxia. These studies demonstrate that hypoxia increases mtO(2)(•-) and mtH(2)O(2). Targeting mtH(2)O(2) attenuates PH pathogenesis, whereas targeting mtO(2)(•-) exacerbates PH. These differences in PH pathogenesis were mirrored by RVSP, vessel muscularization, levels of Nox2 and Nox4, proliferation, and H(2)O(2) release. These studies suggest that targeted reductions in mtH(2)O(2) generation may be particularly effective in preventing hypoxia-induced PH.

Reactive oxygen species are involved in BMP-induced dendritic growth in cultured rat sympathetic neurons

Previous studies have shown that bone morphogenetic proteins (BMPs) promote dendritic growth in sympathetic neurons; however, the downstream signaling molecules that mediate the dendrite promoting activity of BMPs are not well characterized. Here we test the hypothesis that reactive oxygen species (ROS)-mediated signaling links BMP receptor activation to dendritic growth. In cultured rat sympathetic neurons, exposure to any of the three mechanistically distinct antioxidants, diphenylene iodinium (DPI), nordihydroguaiaretic acid (NGA) or desferroxamine (DFO), blocked de novo BMP-induced dendritic growth. Addition of DPI to cultures previously induced with BMP to extend dendrites caused dendritic retraction while DFO and NGA prevented further growth of dendrites. The inhibition of the dendrite promoting activity of BMPs by antioxidants was concentration-dependent and occurred without altering axonal growth or neuronal cell survival. Antioxidant treatment did not block BMP activation of SMAD 1,5 as determined by nuclear localization of these SMADs. While BMP treatment did not cause a detectable increase in intracellular ROS in cultured sympathetic neurons as assessed using fluorescent indicator dyes, BMP treatment increased the oxygen consumption rate in cultured sympathetic neurons as determined using the Seahorse XF24 Analyzer, suggesting increased mitochondrial activity. In addition, BMPs upregulated expression of NADPH oxidase 2 (NOX2) and either pharmacological inhibition or siRNA knockdown of NOX2 significantly decreased BMP-7 induced dendritic growth. Collectively, these data support the hypothesis that ROS are involved in the downstream signaling events that mediate BMP7-induced dendritic growth in sympathetic neurons, and suggest that ROS-mediated signaling positively modulates dendritic complexity in peripheral neurons.

Relationship between expression of NADPH oxidase 2 and invasion and prognosis of human gastric cancer

AIM: To assess the expression and prognostic value of nicotinamide adenine dinucleotide phosphate oxidase 2 (NOX2) in gastric cancer, and its correlation with vascular endothelial growth factor (VEGF) and epidermal growth factor receptor (EGFR).

METHODS: Tumor and adjacent tissues were obtained from 123 patients who underwent radical surgery for gastric cancer at Renmin Hospital of Wuhan University from 2008-2009. The expression of NOX2, VEGF, EGFR and CD68 in tumor tissues was detected by immunohistochemistry. The expression of NOX2 in gastric cancer and adjacent tissues was detected by Western blot analysis. Spearman^’s correlation was performed to elucidate the relationship of NOX2 with VEGF and EGFR. The Kaplan-Meier method was used to calculate survival time, and the log-rank test was used to evaluate differences in survival. Cox‘s proportional hazards regression model was applied in a stepwise manner to analyze the independent prognostic factors.

RESULTS: NOX2 exhibited positive expression in 47.2% (58/123) of the gastric cancer tissues. Western blot analysis revealed that NOX2 was up-regulated in tumor tissues compared to the adjacent tissue [39.0% (48/123)]. Immunohistochemistry staining revealed that CD68, which is a specific marker of macrophages, and NOX expression presented a similar localization and staining intensity. The expression of NOX2 was positively correlated with that of VEGF and EGFR. Comparison of the 5-year survival rates of the NOX2 positive and NOX2 negative groups showed that the NOX2 positive group presented a poor prognosis.

CONCLUSION: NOX2 positively correlates with the levels of VEGF and EGFR. NOX2 may be used as a new biomarker and a potential therapeutic target for gastric cancer.

Understanding the biology of reactive oxygen species and their link to cancer: NADPH oxidases as novel pharmacological targets

Reactive oxygen species (ROS), the cellular products of myriad physiological processes, have long been understood to lead to cellular damage if produced in excess and to be a causative factor in cancer through the oxidation and nitration of various macromolecules. Reactive oxygen species influence various hallmarks of cancer, such as cellular proliferation and angiogenesis, through the promotion of cell signalling pathways intrinsic to these processes and can also regulate the function of key immune cells, such as macrophages and regulatory T cells, which promote angiogenesis in the tumour environment. Herein we emphasize the family of NADPH oxidase enzymes as the most likely source of ROS, which promote angiogenesis and tumourigenesis through signalling pathways within endothelial, immune and tumour cells. In this review we focus on the pharmacological inhibitors of NADPH oxidases and suggest that, compared with traditional anti-oxidants, they are likely to offer better alternatives for suppression of tumour angiogenesis. Despite the emerging enthusiasm towards the use of NADPH oxidase inhibitors for cancer therapy, this field is still in its infancy; in particular, there is a glaring lack of knowledge of the roles of NADPH oxidases in in vivo animal models and in human cancers. Certainly a clearer understanding of the relevant signalling pathways influenced by NADPH oxidases during angiogenesis in cancer is likely to yield novel therapeutic approaches.

Neural regulation of hypoxia-inducible factors and redox state drives the pathogenesis of hypertension in a rodent model of sleep apnea

Obstructive sleep apnea (OSA) is one of the most common causes of hypertension in western societies. OSA causes chronic intermittent hypoxia (CIH) in specialized O2-sensing glomus cells of the carotid body. CIH generates increased reactive oxygen species (ROS) that trigger a feedforward mechanism in which increased intracellular calcium levels ([Ca(2+)]i) trigger increased HIF-1α synthesis and increased HIF-2α degradation. As a result, the normal homeostatic balance between HIF-1α-dependent prooxidant and HIF-2α-dependent antioxidant enzymes is disrupted, leading to further increases in ROS. Carotid body sensory nerves project to the nucleus tractus solitarii, from which the information is relayed via interneurons to the rostral ventrolateral medulla in the brain stem, which sends sympathetic neurons to the adrenal medulla to stimulate the release of epinephrine and norepinephrine, catecholamines that increase blood pressure. At each synapse, neurotransmitters trigger increased [Ca(2+)]i, HIF-1α:HIF-2α, and Nox2:Sod2 activity that generates increased ROS levels. These responses are not observed in other regions of the brain stem that do not receive input from the carotid body or signal to the sympathetic nervous system. Thus sympathetic nervous system homeostasis is dependent on a balance between HIF-1α and HIF-2α, disruption of which results in hypertension in OSA patients.

Hypoxia-inducible factors and hypertension: lessons from sleep apnea syndrome

Systemic hypertension is one of the most prevalent cardiovascular diseases. Sleep-disordered breathing (SDB) with recurrent apnea is a major risk factor for developing essential hypertension. Chronic intermittent hypoxia (CIH) is a hallmark manifestation of recurrent apnea. Rodent models patterned after the O2 profiles seen with SDB patients showed that CIH is the major stimulus for causing systemic hypertension. This article reviews the physiological and molecular basis of CIH-induced hypertension. Physiological studies have identified that augmented carotid body chemosensory reflex and the resulting increase in sympathetic nerve activity are major contributors to CIH-induced hypertension. Analysis of molecular mechanisms revealed that CIH activates hypoxia-inducible factor (HIF)-1 and suppresses HIF-2-mediated transcription. Dysregulation of HIF-1- and HIF-2-mediated transcription leads to imbalance of pro-oxidant and anti-oxidant enzyme gene expression resulting in increased reactive oxygen species (ROS) generation in the chemosensory reflex which is central for developing hypertension.

HIF-1α activation by intermittent hypoxia requires NADPH oxidase stimulation by xanthine oxidase

Hypoxia-inducible factor 1 (HIF-1) mediates many of the systemic and cellular responses to intermittent hypoxia (IH), which is an experimental model that simulates O2 saturation profiles occurring with recurrent apnea. IH-evoked HIF-1α synthesis and stability are due to increased reactive oxygen species (ROS) generated by NADPH oxidases, especially Nox2. However, the mechanisms by which IH activates Nox2 are not known. We recently reported that IH activates xanthine oxidase (XO) and the resulting increase in ROS elevates intracellular calcium levels. Since Nox2 activation requires increased intracellular calcium levels, we hypothesized XO-mediated calcium signaling contributes to Nox activation by IH. We tested this possibility in rat pheochromocytoma PC12 cells subjected to IH consisting alternating cycles of hypoxia (1.5% O2 for 30 sec) and normoxia (21% O2 for 5 min). Kinetic analysis revealed that IH-induced XO preceded Nox activation. Inhibition of XO activity either by allopurinol or by siRNA prevented IH-induced Nox activation, translocation of the cytosolic subunits p47phox and p67phox to the plasma membrane and their interaction with gp91phox. ROS generated by XO also contribute to IH-evoked Nox activation via calcium-dependent protein kinase C stimulation. More importantly, silencing XO blocked IH-induced upregulation of HIF-1α demonstrating that HIF-1α activation by IH requires Nox2 activation by XO.

Chronic intermittent hypoxia increases rat sternohyoid muscle NADPH oxidase expression with attendant modest oxidative stress

Chronic intermittent hypoxia (CIH) causes upper airway muscle dysfunction. We hypothesized that the superoxide generating NADPH oxidase (NOX) is upregulated in CIH-exposed muscle causing oxidative stress. Adult male Wistar rats were exposed to intermittent hypoxia (5% O₂ at the nadir for 90 s followed by 210 s of normoxia), for 8 h per day for 14 days. The effect of CIH exposure on the expression of NOX subunits, total myosin and 4-hydroxynonenal (4-HNE) protein adducts in sternohyoid muscle was determined by western blotting and densitometry. Sternohyoid protein free thiol and carbonyl group contents were determined by 1D electrophoresis using specific fluorophore probes. Aconitase and glutathione reductase activities were measured as indices of oxidative stress. HIF-1α content and key oxidative and glycolytic enzyme activities were determined. Contractile properties of sternohyoid muscle were determined ex vivo in the absence and presence of apocynin (putative NOX inhibitor). We observed an increase in NOX 2 and p47 phox expression in CIH-exposed sternohyoid muscle with decreased aconitase and glutathione reductase activities. There was no evidence, however, of increased lipid peroxidation or protein oxidation in CIH-exposed muscle. CIH exposure did not affect sternohyoid HIF-1α content or aldolase, lactate dehydrogenase, or glyceraldehyde-3-phosphate dehydrogenase activities. Citrate synthase activity was also unaffected by CIH exposure. Apocynin significantly increased sternohyoid force and power. We conclude that CIH exposure upregulates NOX expression in rat sternohyoid muscle with concomitant modest oxidative stress but it does not result in a HIF-1α-dependent increase in glycolytic enzyme activity. Constitutive NOX activity decreases sternohyoid force and power. Our results implicate NOX-dependent reactive oxygen species in CIH-induced upper airway muscle dysfunction which likely relates to redox modulation of key regulatory proteins in excitation-contraction coupling.

The decrease of NAD(P)H:quinone oxidoreductase 1 activity and increase of ROS production by NADPH oxidases are early biomarkers in doxorubicin cardiotoxicity

CONTEXT

Doxorubicin cardiotoxicity displays a complex and multifactorial progression.

OBJECTIVE

Identify early biochemical mechanisms leading to a sustained imbalance of cellular bioenergetics.

METHODS

Measurements of the temporal evolution of selected biochemical markers after treatment of rats with doxorubicin (20 mg/kg body weight).

RESULTS

Doxorubicin treatment increased lipid oxidation, catalase activity and production of H₂O₂ by Nox-NADPH oxidases, and down-regulated

NAD(P)H

quinone oxidoreductase-1 prior eliciting changes in reduced glutathione, protein carbonyls and protein nitrotyrosines. Alterations of mitochondrial and myofibrillar bioenergetics biomarkers were detected only after this oxidative imbalance was established.

CONCLUSIONS

NAD(P)H

quinone oxidoreductase-1 activity and increase of hydrogen peroxide production by NADPH oxidases are early biomarkers in doxorubicin cardiotoxicity.

Reactive oxygen species mediated diaphragm fatigue in a rat model of chronic intermittent hypoxia

Respiratory muscle dysfunction documented in sleep apnoea patients is perhaps due to oxidative stress secondary to chronic intermittent hypoxia (CIH). We sought to explore the effects of different CIH protocols on respiratory muscle form and function in a rodent model. Adult male Wistar rats were exposed to CIH (n = 32) consisting of 90 s normoxia-90 s hypoxia (either 10 or 5% oxygen at the nadir; arterial O2 saturation ∼ 90 or 80%, respectively] for 8 h per day or to sham treatment (air-air, n = 32) for 1 or 2 weeks. Three additional groups of CIH-treated rats (5% O2 for 2 weeks) had free access to water containing N-acetyl cysteine (1% NAC, n = 8), tempol (1 mM, n = 8) or apocynin (2 mM, n = 8). Functional properties of the diaphragm muscle were examined ex vivo at 35 °C. The myosin heavy chain and sarco(endo)plasmic reticulum Ca(2+)-ATPase isoform distribution, succinate dehydrogenase and glyercol phosphate dehydrogenase enzyme activities, Na(+)-K(+)-ATPase pump content, concentration of thiobarbituric acid reactive substances, DNA oxidation and antioxidant capacity were determined. Chronic intermittent hypoxia (5% oxygen at the nadir; 2 weeks) decreased diaphragm muscle force and endurance. All three drugs reversed the deleterious effects of CIH on diaphragm endurance, but only NAC prevented CIH-induced diaphragm weakness. Chronic intermittent hypoxia increased diaphragm muscle myosin heavy chain 2B areal density and oxidized glutathione/reduced glutathione (GSSG/GSH) ratio. We conclude that CIH-induced diaphragm dysfunction is reactive oxygen species dependent. N-Acetyl cysteine was most effective in reversing CIH-induced effects on diaphragm. Our results suggest that respiratory muscle dysfunction in sleep apnoea may be the result of oxidative stress and, as such, antioxidant treatment could prove a useful adjunctive therapy for the disorder.

NADPH oxidase-mitochondria axis-derived ROS mediate arsenite-induced HIF-1α stabilization by inhibiting prolyl hydroxylases activity

Arsenic exposure has been shown to induce hypoxia inducible factor 1α (HIF-1α) accumulation, however the underlying mechanism remains unknown. In the present study, we tested the hypothesis that arsenic exposure triggered the interaction between NADPH oxidase and mitochondria to promote reactive oxygen species (ROS) production, which inactivate prolyl hydroxylases (PHDs) activity, leading to the stabilization of HIF-1α protein. Exposure of human immortalized liver cell line HL-7702 cells to arsenite induced HIF-1α accumulation in a dose-dependent manner, which was abolished by SOD mimetic MnTMPyP. Inhibition of NADPH oxidase with diphenyleneiodonium chloride (DPI) or inhibition of mitochondrial respiratory chain with rotenone significantly blocked arsenite-induced ROS production, and the mitochondria appeared to be the major source of ROS production. Arsenite treatment inhibited HIF-1α hydroxylation by prolyl hydroxylases (PHDs) and increased HIF-1α stabilization, but did not affect HIF-1α mRNA expression and Akt activation. Supplementation of ascorbate or Fe(II) completely abolished arsenite-induced PHDs inhibition and HIF-1α stabilization. In conclusion, these results define a unique mechanism of HIF-1α accumulation following arsenic exposure, that is, arsenic activates NADPH oxidase-mitochondria axis to produce ROS, which deplete intracellular ascorbate and Fe(II) to inactivate PHDs, leading to HIF-1α stabilization.

Xanthine oxidase mediates hypoxia-inducible factor-2α degradation by intermittent hypoxia

Sleep-disordered breathing with recurrent apnea produces chronic intermittent hypoxia (IH). We previously reported that IH leads to down-regulation of HIF-2α protein via a calpain-dependent signaling pathway resulting in oxidative stress. In the present study, we delineated the signaling pathways associated with calpain-dependent HIF-2α degradation in cell cultures and rats subjected to chronic IH. Reactive oxygen species (ROS) scavengers prevented HIF-2α degradation by IH and ROS mimetic decreased HIF-2α protein levels in rat pheochromocytoma PC12 cell cultures, suggesting that ROS mediate IH-induced HIF-2α degradation. IH activated xanthine oxidase (XO) by increased proteolytic conversion of xanthine dehydrogenase to XO. ROS generated by XO activated calpains, which contributed to HIF-2α degradation by IH. Calpain-induced HIF-2α degradation involves C-terminus but not the N-terminus of the HIF-2α protein. Pharmacological blockade as well as genetic knock down of XO prevented IH induced calpain activation and HIF-2α degradation in PC12 cells. Systemic administration of allopurinol to rats prevented IH-induced hypertension, oxidative stress and XO activation in adrenal medulla. These results demonstrate that ROS generated by XO activation mediates IH-induced HIF-2α degradation via activation of calpains.