Intermittent hypoxia exacerbates metabolic effects of diet-induced obesity

Abstract

Obesity causes insulin resistance (IR) and nonalcoholic fatty liver disease (NAFLD), but the relative contribution of sleep apnea is debatable. The main aim of this study is to evaluate the effects of chronic intermittent hypoxia (CIH), a hallmark of sleep apnea, on IR and NAFLD in lean mice and mice with diet-induced obesity (DIO). Mice (C57BL/6J), 6-8 weeks of age were fed a high fat (n = 18) or regular (n = 16) diet for 12 weeks and then exposed to CIH or control conditions (room air) for 4 weeks. At the end of the exposure, fasting (5 h) blood glucose, insulin, homeostasis model assessment (HOMA) index, liver enzymes, and intraperitoneal glucose tolerance test (1 g/kg) were measured. In DIO mice, body weight remained stable during CIH and did not differ from control conditions. Lean mice under CIH were significantly lighter than control mice by day 28 (P = 0.002). Compared to lean mice, DIO mice had higher fasting levels of blood glucose, plasma insulin, the HOMA index, and had glucose intolerance and hepatic steatosis at baseline. In lean mice, CIH slightly increased HOMA index (from 1.79 ± 0.13 in control to 2.41 ± 0.26 in CIH; P = 0.05), whereas glucose tolerance was not affected. In contrast, in DIO mice, CIH doubled HOMA index (from 10.1 ± 2.1 in control to 22.5 ± 3.6 in CIH; P < 0.01), and induced severe glucose intolerance. In DIO mice, CIH induced NAFLD, inflammation, and oxidative stress, which was not observed in lean mice. In conclusion, CIH exacerbates IR and induces steatohepatitis in DIO mice, suggesting that CIH may account for metabolic dysfunction in obesity.

Figure 1

Indices of insulin resistance during chronic intermittent hypoxia. (a) Glucose, (b) insulin, and (c) homeostasis model assessment (HOMA) index in lean and diet-induced obesity (DIO) mice during chronic intermittent hypoxia (CIH) or control conditions. P < 0.05 obese vs. lean mice.

Figure 2

Intraperitoneal glucose tolerance test in lean and diet-induced obesity (DIO) mice submitted to chronic intermittent hypoxia (CIH) or control conditions. Overall, P < 0.05 obese vs. lean mice. *P < 0.05 vs. CIH and control lean mice; ^†P < 0.05 vs. control DIO mice; ^‡P < 0.01 vs. CIH and control lean mice; ^§P < 0.01 vs. control DIO mice; P < 0.05 vs. control lean mice.

Figure 3

Area under the curve (AUC) for the intraperitoneal glucose tolerance test in lean and diet-induced obesity (DIO) mice submitted to chronic intermittent hypoxia (CIH) or control conditions. P < 0.05 obese vs. lean mice.

Figure 4

Levels of (a) serum triglycerides, (b) free-fatty acids, (c) corticosterone, and (d–f) liver enzymes in lean and diet-induced obesity (DIO) mice submitted to chronic intermittent hypoxia (CIH) or control conditions. P < 0.05 obese vs. lean mice (except for serum triglycerides—P > 0.05). ALT, alanine aminotransaminase; AST, aspartate aminotransaminase; AP, alkaline phosphatase.

Figure 5

Levels of (a) liver triglycerides and (b) malondialdehyde (MDA) in lean and diet-induced obesity (DIO) mice submitted to chronic intermittent hypoxia (CIH) or control conditions. P < 0.05 obese vs. lean mice.

Figure 6

Representative images of the liver in lean (a and c) and diet-induced obesity (DIO) mice (b and d) submitted to chronic intermittent hypoxia (CIH) or control conditions. No significant changes were observed in (a) control-lean. (b) Control-DIO mice presented microvesicular hepatic steatosis. (c) No significant changes were observed in CIH-lean. (d) CIH-DIO mice presented a robust increase in macrovesicular hepatic steatosis and lobular inflammation. H&E, hematoxylin and eosin.