Cardiac response to chronic intermittent hypoxia with a transition from adaptation to maladaptation: the role of hydrogen peroxide

Chronic intermittent hypoxia-induced cardiovascular and renal dysfunction: from adaptation to maladaptation

Chronic intermittent hypoxia (CIH) is the dominant pathological feature of human obstructive sleep apnoea (OSA), which is highly prevalent and associated with cardiovascular and renal diseases. CIH causes hypertension, centred on sympathetic nervous overactivity, which persists following removal of the CIH stimulus. Molecular mechanisms contributing to CIH-induced hypertension have been carefully delineated. However, there is a dearth of knowledge on the efficacy of interventions to ameliorate high blood pressure in established disease. CIH causes endothelial dysfunction, aberrant structural remodelling of vessels and accelerates atherosclerotic processes. Pro-inflammatory and pro-oxidant pathways converge on disrupted nitric oxide signalling driving vascular dysfunction. In addition, CIH has adverse effects on the myocardium, manifesting atrial fibrillation, and cardiac remodelling progressing to contractile dysfunction. Sympatho-vagal imbalance, oxidative stress, inflammation, dysregulated HIF-1α transcriptional responses and resultant pro-apoptotic ER stress, calcium dysregulation, and mitochondrial dysfunction conspire to drive myocardial injury and failure. CIH elaborates direct and indirect effects in the kidney that initially contribute to the development of hypertension and later to chronic kidney disease. CIH-induced morphological damage of the kidney is dependent on TLR4/NF-κB/NLRP3/caspase-1 inflammasome activation and associated pyroptosis. Emerging potential therapies related to the gut-kidney axis and blockade of aryl hydrocarbon receptors (AhR) are promising. Cardiorenal outcomes in response to intermittent hypoxia present along a continuum from adaptation to maladaptation and are dependent on the intensity and duration of exposure to intermittent hypoxia. This heterogeneity of OSA is relevant to therapeutic treatment options and we argue the need for better stratification of OSA phenotypes.

Effect of renal denervation on cardiac remodelling and function in rats with chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) mimicking obstructive sleep apnea elicits divergent outcomes in the cardiovascular systems. The effect of renal denervation (RDN) on the heart during CIH remains unclear. We aimed to explore the effect of RDN on cardiac remodelling in rats exposed to CIH and to discuss the underlying mechanisms. Adult Sprague Dawley rats were divided into four groups: control, control+RDN, CIH (CIH exposure for 6 weeks, nadir of 5%-7% to peak of 21% O2, 20 cycles/h, 8 h/day) and CIH+ RDN group. Echocardiography, cardiac fibrosis, left ventricle (LV) expressions of nuclear factor-E2-related factor 2 (Nrf2)/heme oxygenase-1 (HO-1) pathway and inflammatory factors were tested at the end of the study. Cardiac structural remodelling and dysfunction were induced by CIH and attenuated by RDN. Myocardial fibrosis was more severe in the CIH group than in the control group and improved in the CIH + RDN group. Sympathetic activity reflected by tyrosine hydroxylase (TH) expression and noradrenaline were significantly elevated after CIH but blunted by RDN. CIH downregulated LV protein expressions of Nrf2 and HO-1, which was activated by RDN. The downstream of Nrf2/HO-1, such as NQO1 and SOD expression, elevated following RDN. RDN also decreased the mRNA expression of IL-1β and IL-6. Notably, control+RDN did not affect cardiac remodelling and Nrf2/HO-1 compared with the control. Taken together, we found that RDN exerted cardio-protective effects in a rat model of CIH involving Nrf2/HO-1 pathway and inflammation.

Intermittent exposure to chlorpyrifos results in cardiac hypertrophy and oxidative stress in rats

Forbidden in some countries due to its proven toxicity to humans, chlorpyrifos (CPF) still stands as an organophosphate pesticide (OP) highly used worldwide. Cardiotoxicity assessment is an unmet need in pesticide regulation and should be deeply studied through different approaches to better inform and generate an appropriate regulatory response to OP use. In the present study, we used our 4-week intermittent OP exposure model in rats to address the CPF effects on cardiac morphology allied with cardiovascular functional and biomolecular evaluation. Rats were intermittently treated with CPF at doses of 7 mg/kg and 10 mg/kg or saline (i.p.) and assessed for cardiac morphology (cardiomyocyte diameter and collagen content), cardiopulmonary Bezold-Jarisch reflex (BJR) function, cardiac autonomic tone, left ventricle (LV) contractility, cardiac expression of NADPH oxidase (Nox2), catalase (CAT), superoxide dismutase 1 (SOD1), superoxide dismutase 2 (SOD2) and cardiac levels of advanced oxidation protein products (AOPP) and thiobarbituric acid reactive substances (TBARS). Plasma butyrylcholinesterase (BuChE) and brainstem acetylcholinesterase (AChE) were also measured. Intermittent exposure to CPF induced cardiac hypertrophy, increasing cardiomyocyte diameter and collagen content. An impairment of cardioinhibitory BJR responses and an increase in cardiac vagal tone were also observed in CPF-treated animals without changes in LV contractility. CPF exposure increased cardiac Nox-2, CAT, SOD1, and TBARS levels and inhibited plasma BuChE and brainstem AChE activities. Our data showed that intermittent exposure to CPF induces cardiac hypertrophy together with cardiovascular reflex impairment, imbalance of autonomic tone and oxidative stress, which may bring significant cardiovascular risk to individuals exposed to OP compounds seasonally.

Downregulation of miR-34c-5p alleviates chronic intermittent hypoxia-induced myocardial damage by targeting sirtuin 1

Numerous microRNAs (miRs) are abnormally expressed in response to hypoxia-induced myocardial damage. Herein, miR-34c-5p as a potential pharmaco-target was investigated in a mouse model of chronic intermittent hypoxia (CIH)-induced myocardial damage. A mouse model of myocardial damage was established using CIH with 7% or 21% O₂ alternating 60 s for 12 h/day, 21% O₂ for 12 h/day. AntagomiR-34c-5p (20 nM/0.1 ml; once a week for 12 weeks) was used as a miR-34c-5p inhibitor in a mouse model with tail-vein injection. In another experiment, mice were administrated with Sirt1 activator SRT1720 (50 mg/kg/day) by intraperitoneal injection. Gene Expression Omnibus database showed a significant upregulation of miR-34c-5p expression in the ischemic myocardium of male mice. In CIH-stimulated mice, miR-34c-5p expression was also significantly increased compared with normal mice. Treatment of antagomiR-34c-5p significantly restrained CIH-triggered myocardial apoptosis. After administration of antagomiR-34c-5p or Sirt1 activator SRT1720, cardiac hypertrophy and oxidative stress were attenuated in CIH-stimulated mice. We also found sirtuin 1 (Sirt1) as a direct target of miR-34c-5p, which was able to mediate Sirt1 protein expression in cardiomyocytes. AntagomiR-34c-5p injection markedly elevated Sirt1 protein expression in CIH-stimulated mice. AntagomiR-34c-5p or Sirt1 activator SRT1720 administration exhibited the antioxidative activity and cardioprotective roles in CIH-stimulated mice.

Short-term intermittent hypoxia induces simultaneous systemic insulin resistance and higher cardiac contractility in lean mice

BACKGROUND

Intermittent hypoxia (IH) is the major feature of obstructive sleep apnea syndrome, well-known to induce cardiometabolic complications. We previously demonstrated that IH induces hyperinsulinemia and associated altered insulin signaling in adipose tissue, liver, and skeletal muscle, but impact of IH on cardiac insulin signaling and functional/structural consequences remains unknown. Therefore, the aims of this study were to investigate in both lean and obese mice the effects of chronic IH on the following: (1) cardiac insulin signaling and (2) cardiac remodeling and function.

METHODS

C57BL/6 J male mice were fed low-fat (LFD) or high-fat (HFD) diet for 20 weeks, and exposed to IH (21-5% FiO2, 60 s cycle, 8 h/day) or normoxia (N) for the last 6 weeks. Systemic insulin sensitivity was evaluated by an insulin tolerance test. Cardiac remodeling and contractile function were assessed by cardiac ultrasonography. Ultimately, hearts were withdrawn for biochemical and histological analysis.

RESULTS

In LFD mice, IH-induced hyperinsulinemia and systemic insulin resistance that were associated with increased phosphorylations of cardiac insulin receptor and Akt on Tyr1150 and Ser473 residues, respectively. In addition, IH significantly increased cardiac interstitial fibrosis and cardiac contractility. In the HFD group, IH did not exert any additional effect, nor on insulin/Akt signaling, nor on cardiac remodeling and function.

CONCLUSION

Our study suggests that, despite systemic insulin resistance, IH exposure mediates an adaptive cardiac response in lean but not in obese mice. Further studies are needed to investigate which specific mechanisms are involved and to determine the long-term evolution of cardiac responses to IH.

Hypoxia and its preconditioning on cardiac and vascular remodelling in experimental animals

Since oxygen (O₂) is indispensable for mammalian life, every cell in the body is endowed with mechanisms to detect and to respond to changes in the O₂ levels in the microenvironment. The heart and the brain are the two most vital, life-supporting organs requiring a continuous supply of O₂ to sustain their high metabolic rate. On being challenged with hypoxia, maintenance of O₂ supply to these organs even at the cost of others becomes a priority. This review describes the cardiovascular, skeletal muscle vascular, pulmonary vascular and cerebrovascular remodelling in face of chronic mild hypoxia exposure and the underlying mechanisms, with special reference to the role of oxidative stress, hypoxia signalling, autonomic nervous mechanisms. The significance of the normalized wall index (NWI) in assessing the remodelling of the vessels particularly of the intramyocardial coronary artery has been underscored. The review also highlights the basic concepts of hypoxic preconditioning and the subsequent protection of the brain against an acute ischemic insult in preclinical studies hinting towards its possible therapeutic potential in the management of ischemic stroke.

Ling-Gui-Zhu-Gan Decoction Protects H9c2 Cells against H2O2-Induced Oxidative Injury via Regulation of the Nrf2/Keap1/HO-1 Signaling Pathway

OBJECTIVES

Ling-Gui-Zhu-Gan decoction (LGZGD) is a potentially effective treatment for heart failure, and it showed significant anti-inflammatory potential in our previous studies. However, its ability to ameliorate heart failure through regulation of oxidative stress response is still unknown. This study was aimed to investigate the protective effect of LGZGD-containing serum on H2O2-induced oxidative injury in H9c2 cells and explore the underlying mechanism.

METHODS

Eighteen rats were randomly divided into two groups: the blank control group and LGZGD group. The LGZGD group rats were administrated with 8.4 g/kg/d LGZGD for seven consecutive days through gavage, while the blank control group rats were given an equal volume of saline. The serum was extracted from all the rats. To investigate the efficacy and the underlying mechanism of LGZGD, we categorized the H9c2 cells into groups: the control group, model group, normal serum control (NSC) group, LGZGD group, LGZGD + all-trans-retinoic acid (ATRA) group, and ATRA group. Malonedialdehyde (MDA) and superoxide dismutase (SOD) were used as markers for oxidative stress. Dichlorodihydrofluorescin diacetate (DCFH-DA) staining was used to measure the levels of reactive oxygen species (ROS). The apoptosis rate was detected using flow cytometry. The expression levels of pro-caspase-3, cleaved-caspase-3, Bcl-2, Bax, Keap1, Nrf2, and HO-1 were measured using western blotting. The mRNA levels of Keap1, Nrf2, and HO-1 were measured using RT-qPCR.

RESULTS

The LGZGD attenuated injury to H9c2 cells and reduced the apoptosis rate. It was also found to upregulate the SOD activity and suppress the formation of MDA and ROS. The expression levels of pro-caspase-3 and Bcl-2 were significantly increased, while those of cleaved-caspase-3 and Bax were decreased in the LGZGD group compared with the model group. As compared with the model group, the LGZGD group demonstrated decreased Keap1 protein expression and significantly increased Nrf2 nuclear expression and Nrf2-mediated transcriptional activity. ATRA was found to reverse the LGZGD-mediated antioxidative and antiapoptotic effect on injured H9c2 cells induced by H2O2.

CONCLUSION

Our results demonstrated that LGZGD attenuated the H2O2-induced injury to H9c2 cells by inhibiting oxidative stress and apoptosis via the Nrf2/Keap1/HO-1 pathway. These observations suggest that LGZGD might prevent and treat heart failure through regulation of the oxidative stress response.

Combination of Broccoli Sprout Extract and Zinc Provides Better Protection against Intermittent Hypoxia-Induced Cardiomyopathy Than Monotherapy in Mice

Nuclear factor-E2-related factor 2 (Nrf2) and metallothionein have each been reported to protect against chronic intermittent hypoxia- (IH-) induced cardiomyopathy. Sulforaphane-rich broccoli sprout extract (BSE) and zinc can effectively induce Nrf2 and metallothionein, respectively, to protect against IH-induced cardiomyopathy via antioxidative stress. However, whether the cardiac protective effects of the combination of BSE and zinc can be synergistic or the same has not been evaluated. In this study, we treated 8-week-old C57BL/6J mice with BSE and/or zinc during exposure to IH for 8 weeks. Cardiac dysfunction, as determined by echocardiography, and pathological remodeling and abnormalities, including cardiac fibrosis, inflammation, and oxidative damage, examined by histopathology and western blotting, were clearly observed in IH mice but were not significant in IH mice treated with either BSE, zinc, or zinc/BSE. Furthermore, the effects of the combined treatment with BSE and zinc were always greater than those of single treatments. Nrf2 function and metallothionein expression in the heart increased to a greater extent using the combination of BSE and zinc than using BSE or zinc alone. These findings for the first time indicate that the dual activation of Nrf2 and metallothionein by combined treatment with BSE and zinc may be more effective than monotherapy at preventing the development of IH-induced cardiomyopathy.

Effect of age on the cardiovascular remodelling induced by chronic intermittent hypoxia as a murine model of sleep apnoea

BACKGROUND AND OBJECTIVE

Chronic intermittent hypoxia (CIH) is a major determinant of the cardiovascular morbidity associated with obstructive sleep apnoea (OSA), and the magnitude of CIH impact may be influenced by ageing. Here, we assessed the role of ageing in the early cardiovascular structural remodelling induced by severe CIH in a murine model of OSA.

METHODS

Cardiovascular remodelling was assessed in young (2 months old, n = 20) and aged (18 months old, n = 20) C57BL/6 female mice exposed to CIH (20% O₂ for 40 s, 5% O₂ for 20 s) or normoxia (room air) for 8 weeks (6 h/day).

RESULTS

Early vascular remodelling was observed in young mice exposed to CIH as illustrated by intima-media thickening (mean change: 4.6 ± 2.6 μm; P = 0.02), elastin fibre disorganization (mean change: 9.2 ± 4.5%; P = 0.02) and fragmentation (mean change: 2.5 ± 0.8%; P = 0.03), and collagen (mean change: 3.2 ± 0.6%; P = 0.001) and mucopolysaccharide accumulation (mean change: 2.4 ± 0.8%; P = 0.01). In contrast, vascular remodelling was not apparent in aged mice exposed to CIH. Furthermore, left ventricular perivascular fibrosis (mean change: 0.71 ± 0.1; P < 0.001) and hypertrophy (mean change: 0.17 ± 0.1; P = 0.038) were increased by CIH exposure in young mice, but not in aged mice. Principal component analysis identified similar cardiovascular alterations among the young mice exposed to CIH and both older mouse groups, suggesting that CIH induces premature cardiovascular senescence.

CONCLUSION

Cardiovascular remodelling induced by severe CIH is affected by the age at which CIH onset occurs, suggesting that the deleterious cardiovascular effects associated with CIH may be more pronounced in younger populations, and such changes resemble chronological age-related declines in cardiovascular structural integrity.

Astragaloside IV Exerts a Myocardial Protective Effect against Cardiac Hypertrophy in Rats, Partially via Activating the Nrf2/HO-1 Signaling Pathway

Previous evidence suggested that astragaloside IV (ASIV) had a cardioprotective effect, but the potential mechanisms were undetermined. This study is aimed at validating the prevention of cardiac hypertrophy in chronic heart failure (CHF) rats and hypertrophy in H9c2 cardiomyocytes by ASIV and at exploring the potential mechanism involved. CHF rat models of abdominal aortic constriction (AAC) were used with the aim of determining the protective effect of ASIV in cardiac hypertrophy in the rats. We proved that ASIV could attenuate cardiac hypertrophy by improving left ventricular function and structure and showed that the expression of nuclear factor-erythroid 2-related factor 2 (Nrf2) and its downstream gene heme oxygenase-1 (HO-1) increased in the high-dose ASIV intervention group. To further investigate the specific mechanism of ASIV, we hypothesized that ASIV might prevent cardiac hypertrophy via activating the Nrf2/HO-1 signaling pathway. We established a cardiomyocyte hypertrophy model induced by angiotensin II (Ang II), which was then transfected with Nrf2 shRNA, to knock down the expression of the Nrf2 gene. We found that the protective effect of ASIV against Ang II-induced cardiomyocyte hypertrophy was abolished in the Nrf2 shRNA transfection group, ultimately aggravating cardiomyocyte hypertrophy induced by Ang II, and it is possible that oxidative stress may be involved in this process. Our results demonstrated that ASIV improved cardiac function and inhibited cardiac hypertrophy by upregulating Nrf2, and this effect was partially achieved by stimulating the Nrf2/HO-1 signaling pathway, suggesting that ASIV could have therapeutic potential for the treatment of cardiac hypertrophy and CHF.

Intermittent hypoxia induces beneficial cardiovascular remodeling in left ventricular function of type 1 diabetic rat

OBJECTIVE

Depressed mechanical activity is a marked complication in diabetics. Hypoxia has properties for novel diagnostic and therapeutic strategies, while intermittent hypoxia (IH) provides early functional and histologic remodeling, including some cardio benefits in early hemodynamic alterations with histologic remodeling and delayed changes in peripheral vasoreactivity. Therefore, we aimed to examine whether IH application presents a cardioprotective effect, via stabilization of hypoxia-inducible factor (HIF) in streptozotocin (STZ)-induced diabetic rat heart.

METHODS

Male 10-week-old Wistar rats were randomly assigned as control group (C), IH group, (STZ)-induced diabetic group (DM) and IH applied DM group (DM+IH). Diabetes duration was kept 6 weeks and IH groups were exposed to hypobaric hypoxia at about 70 kPa (including ~14% PO2; 6 h/day for 6-weeks).

RESULTS

Depressed left ventricular developed pressure (LVDP) and prolonged contraction and relaxation of Langendorff-perfused hearts, as well as increased total oxidative status from streptozotocin (STZ)-induced diabetic rats were markedly prevented with IH application. IH application induced significant increase in protein expression levels of both HIF-1α and vascular endothelial growth factor (VEGF), in both control and diabetic rat hearts, whereas there were significant decreases in the protein levels of prolyl-4 hydroxylase domain enzymes, PHD2, and PHD3 in diabetic hearts. Furthermore, IH application induced marked increases in protein levels of matrix metalloproteinases, MMP-2 and MMP-9 and capillary density in left ventricle of diabetic rats.

CONCLUSION

Overall, we presented how IH application has a beneficial cardiovascular remodeling effect in left ventricular function of diabetic rats, at most, via affecting increased oxidative stress and HIF-VEGF related angiogenesis, providing information on hyperglycemia associated new targets and therapeutic strategies.

Protective effects of quercetin from oxidative/nitrosative stress under intermittent hypobaric hypoxia exposure in the rat's heart

BACKGROUND

Exposure to high altitude in hypobaric hypoxia (HH) is considered to be a physiological oxidative/nitrosative stress. Quercetin (Que) is an effective antioxidant and free radical scavenger against oxidative/nitrosative stress.

AIMS

The aim of this study was to investigate the cardioprotective effects of Que in animals exposed to intermittent HH (IHH) and therefore exposed to oxidative/nitrosative stress.

MATERIALS AND METHODS

Wistar albino male rats were exposed to short-term (2 days) or long-term (4 weeks; 5 days/week) IHH in a hypobaric chamber (5,500 m, 8 h/day, 380 mmHg, 12% O₂, and 88% N₂). Half of the animals received natural antioxidant Que (body weight: 30 mg/kg) daily before each IHH exposure and the remaining rats received vehicle (carboxymethylcellulose solution). Control rats were kept under normobaric normoxia (Nx) and treated in a corresponding manner. One day after the last exposure to IHH, we measured the cardiac hypoxia-induced oxidative/nitrosative stress biomarkers: the malondialdehyde (MDA) level and protein carbonyl (PC) content, the activity of some antioxidant enzymes [superoxide dismutase (SOD) and catalase (CAT)], the nitrite plus nitrate (NOx) production, and the inducible nitric oxide synthase (iNOS) protein expression.

RESULTS

Heart tissue MDA and PC levels, NOx level, and iNOS expression of IHH-exposed rats had increased, and SOD and CAT activities had decreased compared with those of the Nx-exposed rats (control groups). MDA, CP, NOx, and iNOS levels had decreased in Que-treated IHH-exposed rats compared with IHH-exposed rats (control groups). However, Que administration increased SOD and CAT activities of the heart tissue in the IHH-exposed rats.

CONCLUSION

HH exposure increases oxidative/nitrosative stress in heart tissue and Que is an effective cardioprotective agent, which further supports the oxidative cardiac dysfunction induced by hypoxia.

Association of sleep problems with neuroendocrine hormones and coagulation factors in patients with acute myocardial infarction

Background

Obstructive sleep apnea (OSA) and insomnia are frequent sleep problems that are associated with poor prognosis in patients with coronary heart disease. The mechanisms linking poor sleep with an increased cardiovascular risk are incompletely understood. We examined whether a high risk of OSA as well as insomnia symptoms are associated with neuroendocrine hormones and coagulation factors in patients admitted with acute myocardial infarction.

Methods

We assessed 190 patients (mean age 60 years, 83% men) in terms of OSA risk (STOP screening tool for the assessment of high vs. low OSA risk) and severity of insomnia symptoms (Jenkins Sleep Scale for the assessment of subjective sleep difficulties) within 48 h of an acute coronary intervention. Circulating concentrations of epinephrine, norepinephrine, cortisol, fibrinogen, D-dimer, and von Willebrand factor were measured the next morning. The association of OSA risk and insomnia symptoms with neuroendocrine hormones and coagulation factors was computed using multivariate models adjusting for demographic factors, health behaviors, somatic and psychiatric comorbidities, cardiac disease-related variables, and OSA risk in the model for insomnia symptoms, respectively, for insomnia symptoms in the model for OSA risk.

Results

High OSA risk was identified in 41% of patients and clinically relevant insomnia symptoms were reported by 27% of patients. Compared to those with low OSA risk, patients with high OSA risk had lower levels of epinephrine (p = 0.015), norepinephrine (p = 0.049) and cortisol (p = 0.001). More severe insomnia symptoms were associated with higher levels of fibrinogen (p = 0.037), driven by difficulties initiating sleep, and with lower levels of norepinephrine (p = 0.024), driven by difficulties maintaining sleep.

Conclusions

In patients with acute myocardial infarction, sleep problems are associated with neuroendocrine hormones and coagulation activity. The pattern of these relationships is not uniform for patients with a high risk of OSA and those with insomnia symptoms, and whether they contribute to adverse cardiovascular outcomes needs to be established.

Trial registration

ClinicalTrials.gov NCT01781247.

Nrf2 expression and function, but not MT expression, is indispensable for sulforaphane-mediated protection against intermittent hypoxia-induced cardiomyopathy in mice

We reported previously that nuclear factor erythroid 2-related factor 2 (Nrf2) and metallothionein (MT) play critical roles in preventing intermittent hypoxia (IH)-induced cardiomyopathy. In addition, positive feedback regulation between Nrf2 and MT is required for the efficient compensative responses of the heart to IH. As an activator of Nrf2, sulforaphane (SFN) has attracted attention as a potential protective agent against cardiovascular disease. Here, we investigated whether SFN can up-regulate cardiac Nrf2 expression and function, as well as MT expression, to prevent IH-induced cardiomyopathy, and if so, whether Nrf2 and MT are indispensable for this preventive effect. Nrf2-knock-out (Nrf2-KO) or MT-KO mice and their wild-type (WT) equivalents were exposed to IH for 4 weeks with or without SFN treatment. SFN almost completely prevented IH-induced cardiomyopathy in WT mice, and this preventive effect was abolished in Nrf2-KO mice but retained in MT-KO mice. In IH-exposed WT mice, SFN induced significant increases in the expression levels of Nrf2 and its downstream antioxidant target genes, as well as those of MT, but these effects were not seen in IH-exposed Nrf2-KO mice. By contrast, KO of MT did not affect the ability of SFN to up-regulate the expression of Nrf2 and its downstream antioxidant targets. These results suggest that SFN-induced MT expression is Nrf2-dependent, and SFN prevents IH-induced cardiomyopathy in a Nrf2-dependent manner, for which MT is dispensable. This study provides important information that is relevant to the potential use of SFN to prevent IH-induced cardiomyopathy.

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Sulforaphane (SFN) protects from intermittent-hypoxia (IH)-induced cardiomyopathy;

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SFN can increase both Nrf2 and metallothionein (MT) but the latter is Nrf2 dependent.

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SFN protects the heart from IH in wild-type and MT-KO mice, but not in Nrf2 mice.

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Nrf2 is indispensable, but not MT, for SFN to protect from IH-induced cardiomyopathy.

Intermittent hypoxia-induced cardiomyopathy and its prevention by Nrf2 and metallothionein

The mechanism for intermittent hypoxia (IH)-induced cardiomyopathy remains obscure. We reported the prevention of acute and chronic IH-induced cardiac damage by selective cardiac overexpression of metallothionein (MT). Herein we defined that MT-mediated protection from IH-cardiomyopathy is via activation of nuclear factor erythroid 2-related factor 2 (Nrf2), a critical redox-balance controller in the body. For this, mice were exposed to IH for 3 days (acute) or 4 or 8 weeks (chronic). Cardiac Nrf2 and MT expression in response to IH were significantly increased acutely yet decreased chronically. Interestingly, cardiac overexpression (Nrf2-TG) or global deletion of the Nrf2 gene (Nrf2-KO) made mice highly resistant or highly susceptible, respectively, to IH-induced cardiomyopathy and MT expression. Mechanistically, 4-week IH exposure significantly decreased cardiac Nrf2 binding to the MT gene promoter, and thus, depressed both MT transcription and translation in WT mice but not Nrf2-TG mice. Likewise, cardiac MT overexpression prevented chronic IH-induced cardiomyopathy and down-regulation of Nrf2 likely via activation of a PI3K/Akt/GSK-3β/Fyn-dependent signaling pathway. These results reveal an integrated relationship between cardiac Nrf2 and MT expression in response to IH -- acute compensatory up-regulation followed by chronic down-regulation and cardiomyopathy. Cardiac overexpression of either Nrf2 or MT offered cardioprotection from IH via complicated PI3K/Akt/GSK3B/Fyn signaling. Potential therapeutics may target either Nrf2 or MT to prevent chronic IH-induced cardiomyopathy.

Tauroursodeoxycholic acid attenuates endoplasmic reticulum stress and protects the liver from chronic intermittent hypoxia induced injury

Obstructive sleep apnea that characterized by chronic intermittent hypoxia (CIH) has been reported to associate with chronic liver injury. Tauroursodeoxycholic acid (TUDCA) exerts liver-protective effects in various liver diseases. The purpose of this study was to test the hypothesis that TUDCA could protect liver against CIH injury. C57BL/6 mice were subjected to intermittent hypoxia for eight weeks and applied with TUDCA by intraperitoneal injection. The effect of TUDCA on liver histological changes, liver function, oxidative stress, inflammatory response, hepatocyte apoptosis and endoplasmic reticulum (ER) stress were investigated. The results showed that administration of TUDCA attenuated liver pathological changes, reduced serum alanine aminotransferase and aspartate aminotransferase level, suppressed reactive oxygen species activity, decreased tumor necrosis factor-α and interleukin-1β level and inhibited hepatocyte apoptosis induced by CIH. TUDCA also inhibited CIH-induced ER stress in liver as evidenced by decreased expression of ER chaperone 78 kDa glucose-related protein, unfolded protein response transducers and ER proapoptotic proteins. Altogether, the present study described a liver-protective effect of TUDCA in CIH mice model, and this effect seems at least partly through the inhibition of ER stress.

Re-evaluation of the WHO (2010) formaldehyde indoor air quality guideline for cancer risk assessment

In 2010, the World Health Organization (WHO) established an indoor air quality guideline for short- and long-term exposures to formaldehyde (FA) of 0.1 mg/m3 (0.08 ppm) for all 30-min periods at lifelong exposure. This guideline was supported by studies from 2010 to 2013. Since 2013, new key studies have been published and key cancer cohorts have been updated, which we have evaluated and compared with the WHO guideline. FA is genotoxic, causing DNA adduct formation, and has a clastogenic effect; exposure-response relationships were nonlinear. Relevant genetic polymorphisms were not identified. Normal indoor air FA concentrations do not pass beyond the respiratory epithelium, and therefore FA's direct effects are limited to portal-of-entry effects. However, systemic effects have been observed in rats and mice, which may be due to secondary effects as airway inflammation and (sensory) irritation of eyes and the upper airways, which inter alia decreases respiratory ventilation. Both secondary effects are prevented at the guideline level. Nasopharyngeal cancer and leukaemia were observed inconsistently among studies; new updates of the US National Cancer Institute (NCI) cohort confirmed that the relative risk was not increased with mean FA exposures below 1 ppm and peak exposures below 4 ppm. Hodgkin's lymphoma, not observed in the other studies reviewed and not considered FA dependent, was increased in the NCI cohort at a mean concentration ≥0.6 mg/m3 and at peak exposures ≥2.5 mg/m3; both levels are above the WHO guideline. Overall, the credibility of the WHO guideline has not been challenged by new studies.

Exercise Attenuates Intermittent Hypoxia-Induced Cardiac Fibrosis Associated with Sodium-Hydrogen Exchanger-1 in Rats

Purpose: To investigate the role of sodium–hydrogen exchanger-1 (NHE-1) and exercise training on intermittent hypoxia-induced cardiac fibrosis in obstructive sleep apnea (OSA), using an animal model mimicking the intermittent hypoxia of OSA.

Methods: Eight-week-old male Sprague–Dawley rats were randomly assigned to control (CON), intermittent hypoxia (IH), exercise (EXE), or IH combined with exercise (IHEXE) groups. These groups were randomly assigned to subgroups receiving either a vehicle or the NHE-1 inhibitor cariporide. The EXE and IHEXE rats underwent exercise training on an animal treadmill for 10 weeks (5 days/week, 60 min/day, 24–30 m/min, 2–10% grade). The IH and IHEXE rats were exposed to 14 days of IH (30 s of hypoxia—nadir of 2–6% O₂—followed by 45 s of normoxia) for 8 h/day. At the end of 10 weeks, rats were sacrificed and then hearts were removed to determine the myocardial levels of fibrosis index, oxidative stress, antioxidant capacity, and NHE-1 activation.

Results: Compared to the CON rats, IH induced higher cardiac fibrosis, lower myocardial catalase, and superoxidative dismutase activities, higher myocardial lipid and protein peroxidation and higher NHE-1 activation (p < 0.05 for each), which were all abolished by cariporide. Compared to the IH rats, lower cardiac fibrosis, higher myocardial antioxidant capacity, lower myocardial lipid, and protein peroxidation and lower NHE-1 activation were found in the IHEXE rats (p < 0.05 for each).

Conclusion: IH-induced cardiac fibrosis was associated with NHE-1 hyperactivity. However, exercise training and cariporide exerted an inhibitory effect to prevent myocardial NHE-1 hyperactivity, which contributed to reduced IH-induced cardiac fibrosis. Therefore, NHE-1 plays a critical role in the effect of exercise on IH-induced increased cardiac fibrosis.

Sheng-Mai-San attenuates contractile dysfunction and structural damage induced by chronic intermittent hypoxia in mice

Sheng-Mai-San (SMS), a well-known Chinese medicinal plant formula, is widely used for the treatment of cardiac diseases characterized by deficiency of Qi and Yin syndrome. A mouse chronic intermittent hypoxia (CIH) model was established to mimic the primary clinical features of deficiency of Qi and Yin syndrome. Mice experienced CIH for 28 days (nadir 7% to peak 8% oxygen, 20 min per day), resulting in left ventricle (LV) dysfunction and structure abnormalities. After administration of SMS (0.55, 1.1, and 5.5 g·kg(-1)·d(-1)) for four weeks, improved cardiac function was observed, as indicated by the increase in the ejection fraction from the LV on echocardiography. SMS also preserved the structural integrity of the LV against eccentric hypotrophy, tissue vacuolization, and mitochondrial injury as measured by histology, electron microscopy, and ultrasound assessments. Mechanistically, the antioxidant effects of SMS were demonstrated; SMS was able to suppress mitochondrial apoptosis as indicated by the reduction of several pro-apoptotic factors (Bax, cytochrome c, and cleaved caspase-3) and up-regulation of the anti-apoptosis factor Bcl-2. In conclusion, these results demonstrate that SMS treatment can protect the structure and function of the LV and that the protective effects of this formula are associated with the regulation of the mitochondrial apoptosis pathway.

Protective Effects of Sheng-Mai-San on Right Ventricular Dysfunction during Chronic Intermittent Hypoxia in Mice

Right ventricular (RV) dysfunction and failure contribute to the increasing morbidity and mortality of cardiovascular diseases; however, current treatment strategies are grossly inadequate. Sheng-Mai-San (SMS) has been used to treat heart diseases for hundreds of years in China, and its protective effects on RV have not been observed. The present study was to investigate the protective effects of SMS aqueous extract on RV dysfunction in chronic intermittent hypoxia (CIH) mice model. The results showed that CIH mice model presented RV dysfunction and maladaptive compensation after 28-day-CIH and SMS treatment significantly reversed these changes. Diastolic function of RV was restored and systolic dysfunction was attenuated, including elevation of RV stroke volume and fractional shortening, as well as pulmonary circulation. Structurally, SMS treatment inhibited RV dilation, cardiomyocytes vacuolization, ultrastructure abnormalities, and the expression of cleaved caspase-3. Of importance, SMS showed remarkable antioxidant activity by decreasing the levels of malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE), increasing the levels of superoxide dismutase (SOD) and heme oxygenase-1 (HO-1), as well as inhibiting the overexpression of 3-NT in RV. Our results indicate that SMS preserve RV structure and function in CIH-exposed mice by involving regulation in both ROS and Reactive Nitrogen Species (RNS) production.

HypoxamiRs: regulators of cardiac hypoxia and energy metabolism

Hypoxia and its intricate regulation are at the epicenter of cardiovascular research. Mediated by hypoxia-inducible factors as well as by several microRNAs, recently termed 'hypoxamiRs', hypoxia affects several cardiac pathophysiological processes. Hypoxia is the driving force behind the regulation of the characteristic metabolic switch from predominant fatty acid oxidation in the healthy heart to glucose utilization in the failing myocardium, but also instigates reactivation of the fetal gene program, induces the cardiac hypertrophy response, alters extracellular matrix composition, influences mitochondrial biogenesis, and impacts upon myocardial contractility. HypoxamiR regulation adds a new level of complexity to this multitude of hypoxia-mediated effects, rendering the understanding of the hypoxic response a fundamental piece in solving the cardiovascular disease puzzle.

Effects of rhodiola crenulata on mice hearts under severe sleep apnea

BACKGROUND

The goal of this study is to determine if Rhodiola Crenulata (RC) has protective effects on mice hearts with severe sleep apnea model.

METHODS

Sixty-four C57BL/6 J mice 5-6 months old were distributed into 4 groups i.e. Control group (21% O2, 24 h per day, 8 weeks, n=16); Hypoxia group (Hypoxia: 7% O2 60 s, 20% O2 alternating 60 s, 8 h per day, 8 weeks, n=16); Hypoxia+90RC and Hypoxia+270RC group (Hypoxia for 1st 4 weeks and hypoxia pretreated 90 mg/Kg and 270 mg/Kg Rhodiola Crenulata by oral gavage per day for 2nd 4 weeks, each n=16). Excised hearts from 4 groups of mice were analyzed for heart weight index changes using H&E staining, TUNEL-positive assays and Western Blotting protein.

RESULTS

Cardiac widely dispersed TUNEL-positive apoptotic cells in mice hearts were less in Hypoxia+RC90 and Hypoxia+RC270 than those in Hypoxia. Compared with Hypoxia, the protein levels of Fas ligand, Fas death receptors, Fas-Associated Death Domain (FADD), activated caspase 8, and activated caspase 3 (Fas dependent apoptotic pathways) were decreased in Hypoxia+RC90, Hypoxia+RC270. The protein levels of Bad, Bax, t-Bid, activated caspase 9, activated caspase 3 (mitochondria dependent apoptotic pathway) were less in Hypoxia+RC90, Hypoxia+RC270 than those in hypoxia. The protein levels of Bcl2, Bcl-xL, p-Bad (Bcl2-realted anti-apoptotic pathway) and VEGF, p-PI3k, p-AKT (VEGF-related pro-survival pathway) were higher in Hypoxia+RC90, Hypoxia+RC270 than those in hypoxia.

CONCLUSIONS

Our findings suggest that Rhodiola Crenulata have protective effects on chronic intermittent hypoxia-induced cardiac widely dispersed apoptosis via Fas-dependent and mitochondria-dependent apoptotic and VEGF-related pro-survival pathway.

Alterations in left ventricular function during intermittent hypoxia: Possible involvement of O-GlcNAc protein and MAPK signaling

Obstructive sleep apnea, characterized by recurrent episodes of hypoxia [intermittent hypoxia (IH)], has been identified as a risk factor for cardiovascular diseases. The O-linked β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) of proteins has important regulatory implications on the pathophysiology of cardiovascular disorders. In this study, we examined the role of O-GlcNAcylation in cardiac architecture and left ventricular function following IH. Rats were randomly assigned to a normoxia and IH group (2 min 21% O₂; 2 min 6–8% O₂). Left ventricular function, myocardial morphology and the levels of signaling molecules were then measured. IH induced a significant increase in blood pressure, associated with a gradually abnormal myocardial architecture. The rats exposed to 2 or 3 weeks of IH presented with augmented left ventricular systolic and diastolic function, which declined at week 4. Consistently, the O-GlcNAc protein and O-GlcNAcase (OGA) levels in the left ventricular tissues steadily increased following IH, reaching peak levels at week 3. The O-GlcNAc transferase (OGT), extracellular signal-regulated kinase 1/2 (ERK1/2) and the p38 mitogen-activated protein kinase (p38 MAPK) phosphorylation levels were affected in an opposite manner. The phosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) remained unaltered. In parallel, compared with exposure to normoxia, 4 weeks of IH augmented the O-GlcNAc protein, OGT, phosphorylated ERK1/2 and p38 MAPK levels, accompanied by a decrease in OGA levels and an increase in the levels of myocardial nuclear factor-κB (NF-κB), inflammatory cytokines, caspase-3 and cardiomyocyte apoptosis. Taken together, our suggest a possible involvement of O-GlcNAc protein and MAPK signaling in the alterations of left ventricular function and cardiac injury following IH.

Deletion of metallothionein exacerbates intermittent hypoxia-induced oxidative and inflammatory injury in aorta

The present study was to explore the effect of metallothionein (MT) on intermittent hypoxia (IH) induced aortic pathogenic changes. Markers of oxidative damages, inflammation, and vascular remodeling were observed by immunohistochemical staining after 3 days and 1, 3, and 8 weeks after IH exposures. Endogenous MT was induced after 3 days of IH but was significantly decreased after 8 weeks of IH. Compared with the wild-type mice, MT knock-out mice exhibited earlier and more severe pathogenic changes of oxidative damages, inflammatory responses, and cellular apoptosis, as indicated by the significant accumulation of collagen, increased levels of connective tissue growth factor, transforming growth factor β1, tumor necrosis factor-alpha, vascular cell adhesion molecule 1,3-nitrotyrosine, and 4-hydroxy-2-nonenal in the aorta. These findings suggested that chronic IH may lead to aortic damages characterized by oxidative stress and inflammation, and MT may play a pivotal role in the above pathogenesis process.

Protective effect of salidroside on cardiac apoptosis in mice with chronic intermittent hypoxia

BACKGROUND

The goal of this study is to determine if salidroside has protective effects on hypoxia-induced cardiac widely dispersed apoptosis in mice with severe sleep apnea model.

METHODS

Sixty-four C57BL/6J mice 5-6 months of age were divided into four groups, i.e. Control group (21% O2, 24h per day, 8 weeks, n=16); Hypoxia group (Hypoxia: 7% O2 60s, 20% O2 alternating 60s, 8h per day, 8 weeks, n=16); and Hypoxia+S10 and Hypoxia+S 30 groups (Hypoxia for 1st 4 weeks, hypoxia pretreated 10mg/kg and 30 mg/kg salidroside by oral gavage per day for 2nd 4 weeks, n=16 and 16). The excised hearts from four groups were measured by the heart weight index, H&E staining, TUNEL-positive assays and Western blotting.

RESULTS

TUNEL-positive apoptotic cells in mice heart were less in Hypoxia+S10 and Hypoxia+S30 than those in the Hypoxia group. Compared with Hypoxia, the protein levels of Fas ligand, Fas death receptors, Fas-Associated Death Domain (FADD), activated caspase 8, and activated caspase 3 (Fas pathways) were decreased in Hypoxia+S10 and Hypoxia+S30. In the mitochondria pathway, the protein levels of BcLx, Bcl2, and Bid (anti-apoptotic Bcl2 family) in Hypoxia+S10 and Hypoxia+S30 were more than those in Hypoxia. The protein levels of Bax, t-Bid, activated caspase 9, and activated caspase 3 were less in Hypoxia+S10 and Hypoxia+S30 than those in hypoxia.

CONCLUSIONS

Our findings suggest that salidroside has protective effects on chronic intermittent hypoxia-induced Fas-dependent and mitochondria-dependent apoptotic pathways in mice hearts.

Metallothionein as a compensatory component prevents intermittent hypoxia-induced cardiomyopathy in mice

Obstructive sleep apnea (OSA) causes chronic intermittent hypoxia (IH) to induce cardiovascular disease, which may be related to oxidative damage. Metallothionein (MT) has been extensively proved to be an endogenous and highly inducible antioxidant protein expressed in the heart. Therefore, we tested the hypotheses that oxidative stress plays a critical role in OSA induced cardiac damage and MT protects the heart from OSA-induced cardiomyopathy. To mimic hypoxia/reoxygenation events that occur in adult OSA patients, mice were exposed to IH for 3 days to 8 weeks. The IH paradigm consisted of alternating cycles of 20.9% O₂/8% O₂ F(I)O₂ (30 episodes per hour) with 20s at the nadir F(I)O₂ for 12 h a day during daylight. IH significantly increased the ratio of heart weight to tibia length at 4 weeks with a decrease in cardiac function from 4 to 8 weeks. Cardiac oxidative damage and fibrosis were observed after 4 and 8 weeks of IH exposures. Endogenous MT expression was up-regulated in response to 3-day IH, but significantly decreased at 4 and 8 weeks of IH. In support of MT as a major compensatory component, mice with cardiac overexpression of MT gene and mice with global MT gene deletion were completely resistant, and highly sensitive, respectively, to chronic IH induced cardiac effects. These findings suggest that chronic IH induces cardiomyopathy characterized by oxidative stress-mediated cardiac damage and the antioxidant MT protects the heart from such pathological and functional changes.

Comparison of natural estrogens and synthetic derivative on genioglossus function and estrogen receptors expression in rats with chronic intermittent hypoxia

The pathogenesis of obstructive sleep apnea--hypopnea syndrome (OSAHS) is summarized as the narrow anatomic structure of upper airway (UA) and the defective function of UA dilator muscles. Up to now, there have been no specific treatments for the UA dilator muscle deficiency. We previously found that some estrogen-like compounds exert protective effects on genioglossus, but this protection tends to be less satisfactory. A novel phytoestrogen derivative was synthesized in recent years and was verified to have some cytoprotective activity. This study was designed to compare the effects of natural estrogens and the synthetic resveratrol dimer on genioglossus contraction and expression of estrogen receptors (ERs) under chronic intermittent hypoxia (CIH) condition. Genioglossus myoblasts of rat were isolated and cultured in a culture medium with different agents (estradiol, genistein, resveratrol, and resveratrol dimer, respectively) under hypoxia condition, and ERs expressions were detected. In vivo study, 48 ovariectomized female rats were randomized into six groups. After CIH exposure and agents injection, rats were tested for genioglossus contractile properties and further analysis of ERs expression. Estradiol up-regulated ERα level and exerted the best protective effect of fatigue resistance. Genistein, resveratrol and resveratrol dimer primarily up-regulated the expression of ERβ. Resveratrol dimer exhibited better protection of fatigue resistance than genistein and resveratrol, and expressed higher binding affinity for ERβ than for ERα. Besides estrogenic effects, there may be some other mechanisms for the fatigue resistance improvement contributed by phytoestrogens and their derivatives.

Tissue oxygenation and mitochondrial respiration under different modes of intermittent hypoxia

We compared the results of five modes of intermittent hypoxia training (IHT) on gastrocnemius muscle Po2 and heart and liver mitochondrial respiration in rats. Minutes of hypoxia, %O2, and recovery minutes on air in each mode were: 1) 5, 12%, 5; 2) 15, 12%, 15; 3) 5, 12%, 15; 4) 5, 7%, 5; and 5) 5, 7%, 15. Mode 1 proved best in that Pmo2 dropped minimally at the end of every hypoxic bout and recovered quickly after each bout. One, 2, and 3 week IHT in mode 1 each increased tissue PO2 in both normoxic and 30 min severe hypoxic (7% O2) tests. Adaptation to IHT in Mode 1 caused the substrate-dependent reorganization of liver and heart mitochondrial energy metabolism favoring NADH-dependent oxidation and improving the efficiency of oxidative phosphorylation. Mitochondrial adaptation occurred after 14 days of IHT in liver tissue, but after 21 days in myocardium, and was preserved during the 3 months following IHT termination. When using Mode 2, positive changes were also registered, but were less pronounced. Other IHT modes provoked negative effects on Pmo2 levels, both during hypoxic periods and reoxygenation. In conclusion, the most effective IHT regimen is 5 min 12% O2 with 5 min breaks, five cycles per day during 2 or 3 weeks depending on the task of IHT.

Cardiorespiratory coupling in health and disease

Cardiac and respiratory activities are intricately linked both functionally as well as anatomically through highly overlapping brainstem networks controlling these autonomic physiologies that are essential for survival. Cardiorespiratory coupling (CRC) has many potential benefits creating synergies that promote healthy physiology. However, when such coupling deteriorates autonomic dysautonomia may ensue. Unfortunately there is still an incomplete mechanistic understanding of both normal and pathophysiological interactions that respectively give rise to CRC and cardiorespiratory dysautonomia. Moreover, there is also a need for better quantitative methods to assess CRC. This review addresses the current understanding of CRC by discussing: (1) the neurobiological basis of respiratory sinus arrhythmia (RSA); (2) various disease states involving cardiorespiratory dysautonomia; and (3) methodologies measuring heart rate variability and RSA.