Dyslipidemia and atherosclerosis induced by chronic intermittent hypoxia are attenuated by deficiency of stearoyl coenzyme A desaturase.

Obstructive sleep apnea leads to chronic intermittent hypoxia (CIH) and is associated with atherosclerosis. We have previously shown that C57BL/6J mice exposed to CIH and a high-cholesterol diet develop dyslipidemia, atherosclerosis of the aorta, and upregulation of a hepatic enzyme of lipoprotein secretion, stearoyl coenzyme A desaturase 1 (SCD-1). We hypothesized that (1) SCD-1 deficiency will prevent dyslipidemia and atherosclerosis during CIH; and (2) human OSA is associated with dyslipidemia and upregulation of hepatic SCD. C57BL/6J mice were exposed to CIH or normoxia for 10 weeks while being treated with either SCD-1 or control antisense oligonucleotides. Obese human subjects underwent sleep study and bariatric surgery with intraoperative liver biopsy. In mice, hypoxia increased hepatic SCD-1 and plasma very-low-density lipoprotein cholesterol levels and induced atherosclerosis lesions in the ascending aorta (the cross-section area of 156514+/-57408 microm(2)), and descending aorta (7.0+/-1.2% of the total aortic surface). In mice exposed to CIH and treated with SCD-1 antisense oligonucleotides, dyslipidemia and atherosclerosis in the ascending aorta were abolished, whereas lesions in the descending aorta showed 56% reduction. None of the mice exposed to normoxia developed atherosclerosis. In human subjects, hepatic SCD mRNA levels correlated with the degree of nocturnal hypoxemia (r=0.68, P=0.001). Patients exhibiting oxyhemoglobin desaturations at night showed higher plasma triglyceride and low-density lipoprotein cholesterol levels, compared to subjects without hypoxemia. In conclusion, CIH is associated with dyslipidemia and overexpression of hepatic SCD in both humans and mice alike; SCD-1 deficiency attenuates CIH-induced dyslipidemia and atherosclerosis in mice.

Chronic intermittent hypoxia and acetaminophen induce synergistic liver injury in mice.

Obstructive sleep apnoea (OSA) leads to chronic intermittent hypoxia (CIH) during sleep. Obstructive sleep apnoea has been associated with liver injury. Acetaminophen (APAP; known as paracetamol outside the USA) is one of the most commonly used drugs which has known hepatotoxicity. The goal of the present study was to examine whether CIH increases liver injury, hepatic oxidative stress and inflammation induced by chronic APAP treatment. Adult C57BL/6J mice were exposed to CIH or intermittent air (IA) for 4 weeks. Mice in both groups were treated with intraperitoneal injections of either APAP (200 mg kg(-1)) or normal saline daily. A combination of CIH and APAP caused liver injury, with marked increases in serum alanine aminotransferase, aspartate aminotransferase (AST), gamma-glutamyl transferase and total bilirubin levels, whereas CIH alone induced only elevation in serum AST levels. Acetaminophen alone did not affect serum levels of liver enzymes. Histopathology revealed hepatic necrosis and increased apoptosis in mice exposed to CIH and APAP, whereas the liver remained intact in all other groups. Mice exposed to CIH and APAP exhibited decreased hepatic glutathione in conjunction with a fivefold increase in nitrotyrosine levels, suggesting formation of toxic peroxynitrite in hepatocytes. Acetaminophen or CIH alone had no effect on either glutathione or nitrotyrosine. A combination of CIH and APAP caused marked increases in pro-inflammatory chemokines, monocyte chemoattractant protein-1 and macrophage inflammatory protein-2, which were not observed in mice exposed to CIH or APAP alone. We conclude that CIH and chronic APAP treatment lead to synergistic liver injury, which may have clinical implications for patients with OSA.

Effect of deficiency in SREBP cleavage-activating protein on lipid metabolism during intermittent hypoxia.

Obstructive sleep apnea (OSA), a condition leading to intermittent hypoxia (IH) during sleep, has been associated with dyslipidemia, atherosclerosis, and increased cardiovascular mortality. We previously showed in C57BL/6J mice that IH causes hypercholesterolemia and upregulation of sterol regulatory element binding protein (SREBP)-1, a transcription factor of lipid biosynthesis in the liver. The goal of the present study was to provide mechanistic evidence that IH causes hypercholesterolemia via the SREBP-1 pathway. We utilized mice with a conditional knockout of SREBP cleavage-activating protein (SCAP) in the liver (L-Scap- mice), which exhibit low levels of an active nuclear isoform of SREBP-1 (nSREBP-1). We exposed L-Scap- mice and wild-type (WT) littermates to IH or intermittent air control for 5 days. IH was induced during the 12-h light phase by decreasing Fi(O(2)) from 20.9% to 5% for a period of 30 s with rapid reoxygenation to 20.9% through the subsequent 30 s. In WT mice, IH increased fasting levels of serum total and HDL cholesterol, serum triglycerides, serum and liver phospholipids, mRNA levels of SREBP-1 and mitochondrial glycerol-3-phosphate acyltransferase (mtGPAT), and protein levels of SCAP, nSREBP-1, and mtGPAT in the liver. In L-Scap- mice, IH did not have any effect on serum and liver lipids, and expression of lipid metabolic genes was not altered. We conclude that hyperlipidemia in response to IH is mediated via the SREBP-1 pathway. Our data suggest that the SREBP-1 pathway could be used as a therapeutic target in patients with both OSA and hyperlipidemia.

Hyperlipidemia and lipid peroxidation are dependent on the severity of chronic intermittent hypoxia.

Obstructive sleep apnea (OSA) is characterized by chronic intermittent hypoxia (CIH) and associated with dysregulation of lipid metabolisms and atherosclerosis. Causal relationships between OSA and metabolic abnormalities have not been established because of confounding effects of underlying obesity. The goal of the study was to determine if CIH causes lipid peroxidation and dyslipidemia in the absence of obesity and whether the degrees of dyslipidemia and lipid peroxidation depend on the severity of hypoxia. Lean C57BL/6J mice were exposed to CIH for 4 wk with a fractional inspired O2 (FI(O2)) nadir of either 10% (moderate CIH) or 5% (severe CIH). Mice exposed to severe CIH exhibited significant increases in fasting serum levels of total cholesterol (129 +/- 2.9 vs. 113 +/- 2.8 mg/dl in control mice, P < 0.05) and low-density lipoprotein cholesterol (85.7 +/- 8.9 vs. 56.4 +/- 9.7 mg/dl, P < 0.05) in conjunction with a 1.5- to 2-fold increase in lipoprotein secretion, and upregulation of hepatic stearoyl coenzyme A desaturase 1 (SCD-1). Severe CIH also markedly increased lipid peroxidation in the liver (malondialdehyde levels of 94.4 +/- 5.4 vs. 57.4 +/- 5.2 nmol/mg in control mice, P < 0.001). In contrast, moderate CIH did not induce hyperlipidemia or change in hepatic SCD-1 levels but did cause lipid peroxidation in the liver at a reduced level relative to severe CIH. In conclusion, CIH leads to hypercholesterolemia and lipid peroxidation in the absence of obesity, and the degree of metabolic dysregulation is dependent on the severity of the hypoxic stimulus.

Altered metabolic responses to intermittent hypoxia in mice with partial deficiency of hypoxia-inducible factor-1alpha.

We have previously shown that exposure of C57BL/6J mice to intermittent hypoxia (IH) leads to 1) hypertriglyceridemia due to upregulation of pathways of lipid biosynthesis, including sterol regulatory element binding protein (SREBP)-1 and stearoyl CoA desaturase (SCD)-1; and 2) hypercholesterolemia due to impaired cholesterol uptake. The goal of the present study was to examine whether hypoxia-inducible factor (HIF)-1 is implicated in changes in lipid metabolism induced by IH. Lean HIF-1alpha (Hif1a)(+/-) mice, which are heterozygous for a null allele at the locus encoding the HIF-1alpha subunit, and their wild-type (WT) Hif1a(+/+) littermates were exposed to IH or control conditions for 5 days. IH increased fasting blood glucose, serum total cholesterol, and high-density lipoprotein-cholesterol, phospholipids, triglycerides (TG), and leptin in mice of both genotypes, whereas serum insulin and interleukin-6 were elevated only in WT mice. The impact of IH on serum TG levels in WT mice was significantly greater than that in Hif1a(+/-) mice (95 +/- 9 vs. 66 +/- 6 mg/dl, P < 0.05), whereas cholesterol and glucose levels were affected independently of genotype. Under hypoxic conditions, mRNA and protein levels of SREBP cleavage-activating protein (SCAP) and SCD-1 and protein levels of nuclear isoform of SREBP-1 in the liver were induced to significantly higher levels in WT mice than in Hif1a(+/-) mice. We conclude that 1) the effect of IH on serum TG levels is mediated through HIF-1, 2) HIF-1 may impact on posttranscriptional regulation of SREBP-1, and 3) the effect of IH on serum cholesterol levels was not altered by partial HIF-1alpha deficiency.

Intermittent hypoxia has organ-specific effects on oxidative stress.

Obstructive sleep apnea is characterized by upper airway collapse, leading to intermittent hypoxia (IH). It has been postulated that IH-induced oxidative stress may contribute to several chronic diseases associated with obstructive sleep apnea. We hypothesize that IH induces systemic oxidative stress by upregulating NADPH oxidase, a superoxide-generating enzyme. NADPH oxidase is regulated by a cytosolic p47(phox) subunit, which becomes phosphorylated during enzyme activation. Male C57BL/6J mice were exposed to IH with an inspired O(2) fraction nadir of 5% 60 times/h during the 12-h light phase (9 AM-9 PM) for 1 or 4 wk. In the aorta and heart, IH did not affect lipid peroxidation [malondialdehyde (MDA) level], nitrotyrosine level, or p47(phox) expression and phosphorylation. In contrast, in the liver, exposure to IH for 1 wk resulted in a trend to an increase in MDA levels, whereas IH for 4 wk resulted in a 38% increase in MDA levels accompanied by upregulation of p47(phox) expression and phosphorylation. Administration of an NADPH oxidase inhibitor, apocynin, during IH exposure attenuated IH-induced increases in hepatic MDA. In p47(phox)-deficient mice, MDA levels were higher at baseline and, unexpectedly, decreased during IH. In conclusion, oxidative stress levels and pathways under IH conditions are organ and duration specific.

Chronic intermittent hypoxia causes hepatitis in a mouse model of diet-induced fatty liver.

Obstructive sleep apnea (OSA) causes chronic intermittent hypoxia (CIH) during sleep. OSA is associated with nonalcoholic steatohepatitis (NASH) in obese individuals and may contribute to progression of nonalcoholic fatty liver disease from steatosis to NASH. The purpose of this study was to examine whether CIH induces inflammatory changes in the liver in mice with diet-induced hepatic steatosis. C57BL/6J mice (n = 8) on a high-fat, high-cholesterol diet were exposed to CIH for 6 mo and were compared with mice on the same diet exposed to intermittent air (control; n = 8). CIH caused liver injury with an increase in serum ALT (461 +/- 58 U/l vs. 103 +/- 16 U/l in the control group; P < 0.01) and AST (637 +/- 37 U/l vs. 175 +/- 13 U/l in the control group; P < 0.001), whereas alkaline phosphatase and total bilirubin levels were unchanged. Histology revealed hepatic steatosis in both groups, with mild accentuation of fat staining in the zone 3 hepatocytes in mice exposed to CIH. Animals exposed to CIH exhibited lobular inflammation and fibrosis in the liver, which were not evident in control mice. CIH caused significant increases in lipid peroxidation in serum and liver tissue; significant increases in hepatic levels of myeloperoxidase and proinflammatory cytokines IL-1beta, IL-6, and CXC chemokine MIP-2; a trend toward an increase in TNF-alpha; and an increase in alpha1(I)-collagen mRNA. We conclude that CIH induces lipid peroxidation and inflammation in the livers of mice on a high-fat, high-cholesterol diet.

Chronic intermittent hypoxia induces atherosclerosis.

RATIONALE: Obstructive sleep apnea, a condition leading to chronic intermittent hypoxia (CIH), is associated with hyperlipidemia, atherosclerosis, and a high cardiovascular risk. A causal link between obstructive sleep apnea and atherosclerosis has not been established. OBJECTIVES: The objective of the present study was to examine whether CIH may induce atherosclerosis in C57BL/6J mice. METHODS: Forty male C57BL/6J mice, 8 weeks of age, were fed either a high-cholesterol diet or a regular chow diet and subjected either to CIH or intermittent air (control conditions) for 12 weeks. MEASUREMENTS AND MAIN RESULTS: Nine of 10 mice simultaneously exposed to CIH and high-cholesterol diet developed atherosclerotic lesions in the aortic origin and descending aorta. In contrast, atherosclerosis was not observed in mice exposed to intermittent air and a high-cholesterol diet or in mice exposed to CIH and a regular diet. A high-cholesterol diet resulted in significant increases in serum total and low-density lipoprotein cholesterol levels and a decrease in high-density lipoprotein cholesterol. Compared with mice exposed to intermittent air and a high-cholesterol diet, combined exposure to CIH and a high-cholesterol diet resulted in marked progression of dyslipidemia with further increases in serum total cholesterol and low-density lipoprotein cholesterol (124 +/- 4 vs. 106 +/- 6 mg/dl; p < 0.05), a twofold increase in serum lipid peroxidation, and up-regulation of an important hepatic enzyme of lipoprotein secretion, stearoyl-coenzyme A desaturase-1. CONCLUSIONS: CIH causes atherosclerosis in the presence of diet-induced dyslipidemia.

Chronic intermittent hypoxia predisposes to liver injury.

UNLABELLED: Obstructive sleep apnea (OSA) is characterized by chronic intermittent hypoxia (CIH). OSA is associated with nonalcoholic steatohepatitis (NASH) in obese subjects. The aim of this study was to investigate the effects of CIH on the liver in the absence of obesity. Lean C57BL/6J mice (n = 15) on a regular chow diet were exposed to CIH for 12 weeks and compared with pair-fed mice exposed to intermittent air (IA, n = 15). CIH caused liver injury with an increase in serum ALT (224 +/- 39 U/l versus 118 +/- 22 U/l in the IA group, P < 0.05), whereas AST and alkaline phosphatase were unchanged. CIH also induced hyperglycemia, a decrease in fasting serum insulin levels, and mild elevation of fasting serum total cholesterol and triglycerides (TG). Liver TG content was unchanged, whereas cholesterol content was decreased. Histology showed swelling of hepatocytes, no evidence of hepatic steatosis, and marked accumulation of glycogen in hepatocytes. CIH led to lipid peroxidation of liver tissue with a malondialdehyde (MDA)/free fatty acids (FFA) ratio of 0.54 +/- 0.07 mmol/mol versus 0.30 +/- 0.01 mmol/mol in control animals (P < 0.01), and increased levels of active nuclear factor kappaB (NF-kappaB) in the nuclear fraction of hepatocytes, suggesting that CIH induced oxidative stress in the liver. Finally, CIH greatly exacerbated acetaminophen-induced liver toxicity, causing fulminant hepatocellular injury. CONCLUSION: In the absence of obesity, CIH leads to mild liver injury via oxidative stress and excessive glycogen accumulation in hepatocytes and sensitizes the liver to a second insult, whereas NASH does not develop.