Inhibition of mitochondrial β-oxidation by miR-107 promotes hepatic lipid accumulation and impairs glucose tolerance in vivo

BACKGROUND: Hepatic expression of microRNA-107 is ubiquitously upregulated in various metabolic diseases. In our previous study, we had demonstrated that fatty acid synthase (FASN) is a target of miR-107. miR-107-FASN interaction, by inducing endoplasmic reticulum (ER) stress, promotes lipid accumulation in hepatocytes. Here, we explore the possible mechanism(s) of the miR-107-FASN-ER stress on hepatic lipid metabolism. METHODS: HepG2 cells were transfected with the scramble or miR-107 and/or its inhibitor. Transcript levels of lipid droplet membrane proteins, apolipoproteins and beta-oxidation genes were quantified by quantitative reverse transcription-PCR. Cells were treated with tunicamycin (Tm, 1 h) and 4-PBA (4-phenyl butyric acid, 8 h) or transfected with hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase/enoyl-CoA hydratase, apha subunit (HADHA) short interfering RNA or its overexpression vector. Mice were injected with the scramble or mmu-miR-107 (2.5 mg kg(-1) body weight) and random glucose levels were measured and oral glucose tolerance test was performed. Serum levels of cholesterol, triglyceride and serum glutamic oxaloacetic transaminase/serum glutamicpyruvic transaminase (SGOT/SGPT) were evaluated. Hepatic tissues were collected to estimate levels of miR-107 and mitochondrial beta-oxidation genes. Six-micrometer-thick cryosections of hepatic tissues were prepared and stained with Oil Red O for lipid accumulation. RESULTS: miR-107 does not alter the expression of lipid metabolism-related transcription factors, lipid droplet components and apolipiprotein B. miR-107 significantly decreased the levels of the mitochondrial beta-oxidation enzyme, HADHA in HepG2 cells (P < 0.01), which was prevented by the miR-107 inhibitor. Similar decrease was observed with Tm (P < 0.001), suggesting that HADHA inhibition is promoted by ER stress induction. Interestingly, miR-107-mediated HADHA suppression was rescued by the ER stress inhibitor, 4-PBA (Po0.01). HADHA overexpression rescued miR-107-induced lipid accumulation (Po0.01). miR-107 injection in mice increased random blood glucose levels (P < 0.05) and impaired glucose tolerance (P < 0.05). Hepatic levels of Hadha were significantly decreased (P < 0.001 and P < 0.05) accompanied by increased lipid accumulation (P < 0.001). CONCLUSIONS: miR-107 promotes hepatic lipid accumulation by suppressing transcript levels of HADHA, induces hyperglycemia and impairs glucose tolerance. We conclude that miR-107 regulation of fatty acid oxidation is an important contributor toward hepatic lipid accumulation.