Effects of Six Weeks of Hypoxia Exposure on Hepatic Fatty Acid Metabolism in ApoE Knockout Mice Fed a High-Fat Diet

Abstract

Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease with a characteristic of abnormal lipid metabolism. In the present study, we employed apolipoprotein E knockout (ApoE KO) mice to investigate the effects of hypoxia exposure on hepatic fatty acid metabolism and to test whether a high-fat diet (HFD) would suppress the beneficial effect caused by hypoxia treatment. ApoE KO mice were fed a HFD for 12 weeks, and then were forwarded into a six-week experiment with four groups: HFD + normoxia, normal diet (ND) + normoxia, HFD + hypoxia exposure (HE), and ND + HE. The C57BL/6J wild type (WT) mice were fed a ND for 18 weeks as the baseline control. The hypoxia exposure was performed in daytime with normobaric hypoxia (11.2% oxygen, 1 h per time, three times per week). Body weight, food and energy intake, plasma lipid profiles, hepatic lipid contents, plasma alanine aminotransferase (ALT) and aspartate aminotransferase (AST), and molecular/biochemical makers and regulators of the fatty acid synthesis and oxidation in the liver were measured at the end of interventions. Six weeks of hypoxia exposure decreased plasma triglycerides (TG), total cholesterol (TC), and low-density lipoprotein cholesterol (LDL-C) contents but did not change hepatic TG and non-esterified fatty acid (NEFA) levels in ApoE KO mice fed a HFD or ND. Furthermore, hypoxia exposure decreased the mRNA expression of Fasn, Scd1, and Srebp-1c significantly in the HFD + HE group compared with those in the HFD + normoxia group; after replacing a HFD with a ND, hypoxia treatment achieved more significant changes in the measured variables. In addition, the protein expression of HIF-1α was increased only in the ND + HE group but not in the HFD + HE group. Even though hypoxia exposure did not affect hepatic TG and NEFA levels, at the genetic level, the intervention had significant effects on hepatic metabolic indices of fatty acid synthesis, especially in the ND + HE group, while HFD suppressed the beneficial effect of hypoxia on hepatic lipid metabolism in male ApoE KO mice. The dietary intervention of shifting HFD to ND could be more effective in reducing hepatic lipid accumulation than hypoxia intervention.

Keywords: ApoE KO mice; diet; hepatic lipid metabolism; hypoxia.

Figure 1

A schematic figure of the study protocol.

Figure 2

Changes in plasma lipid profiles (A), ALT (B), AST (C), and hepatic lipid profiles (D–F) between WT fed a ND and ApoE KO mice fed HFD. * p < 0.05, ** p < 0.01 vs. WT mice.

Figure 3

Changes in body weight (A), food (B), and energy (C) intake. * p < 0.05, ** p < 0.01 vs. HFD groups; # p < 0.05, ## p < 0.01 vs. normoxia groups.

Figure 4

Changes in plasma lipid profiles (A–E), ALT (F), AST (G), and hepatic lipid profiles (H–J). * p < 0.05, ** p < 0.01 vs. HFD groups; # p < 0.05, ## p < 0.01 vs. normoxia groups.

Figure 5

Changes in hepatic PPARα protein expression (A), mRNA expression levels of genes involved in mitochondrial fatty acid oxidation (B–D) and synthesis (E–H), and activities of FASN (I) and ACC (J). * p < 0.05, ** p < 0.01 vs. HFD groups; # p < 0.05, ## p < 0.01 vs. normoxia groups.

Figure 6

Changes in hepatic p-AMPKα(Thr172)/AMPKα (A) and p-ACC(Ser79)/ACCα (B) ratios.

Figure 7

Changes in protein expression of HIF-1α (A), DEC1 (B), and SREBP-1 (D), and mRNA expression of Srebp-1c (C). ** p < 0.01 vs. HFD groups; # p < 0.05, ## p < 0.01 vs. normoxia groups.