Thioesterase superfamily member 2 promotes hepatic insulin resistance in the setting of glycerol-3-phosphate acyltransferase 1–induced steatosis

Veronika Tillander,Akihiro Miniami,Michele Alves-Bezerra,Rosalind A Coleman,David E Cohen

doi:10.1074/jbc.ra118.005184

Abstract

Hepatic insulin resistance in the setting of steatosis is attributable at least in part to the accumulation of bioactive lipids that suppress insulin signaling. The mitochondria-associated glycerol-3-phosphate acyltransferase 1 (GPAT1) catalyzes the first committed step in glycerolipid synthesis, and its activity diverts fatty acids from mitochondrial β-oxidation. GPAT1 overexpression in mouse liver leads to hepatic steatosis even in the absence of overnutrition. The mice develop insulin resistance owing to the generation of saturated diacylglycerol and phosphatidic acid molecular species that reduce insulin signaling by activating PKCϵ and by suppressing mTORC2, respectively. Them2, a mitochondria-associated acyl-CoA thioesterase, also participates in the trafficking of fatty acids into oxidative versus glycerolipid biosynthetic pathways. Them2-/- mice are protected against diet-induced hepatic steatosis and insulin resistance. To determine whether Them2 contributes to hepatic insulin resistance due to hepatic overexpression of GPAT1, recombinant adenovirus was used to overexpress GPAT1 in livers of chow-fed Them2+/+ and Them2-/- mice. Hepatic GPAT1 overexpression led to steatosis in both genotypes. In the setting of GPAT1 overexpression, glucose tolerance was reduced in Them2+/+ but not Them2-/- mice, without influencing whole-body insulin sensitivity or basal hepatic glucose production. Improved glucose tolerance in Them2-/- mice was associated with reduced PKCϵ translocation. Preserved insulin receptor activity was supported by Thr-308 phosphorylation of Akt following GPAT1 overexpression in Them2-/- hepatocytes. These findings suggest a pathogenic role of Them2 in the biosynthesis of glycerolipid metabolites that promote hepatic insulin resistance.

Highlights

Hepatic insulin resistance in the setting of steatosis is attributable at least in part to the accumulation of bioactive lipids that suppress insulin signaling
These findings suggest a pathogenic role of Thioesterase superfamily member 2 (Them2) in the biosynthesis of glycerolipid metabolites that promote hepatic insulin resistance
Current concepts suggest that the activity of Glycerol-3-phosphate acyltransferase (GPAT) isoform 1 (GPAT1), which is located at the mitochondrial outer membrane, is a key determinant of fatty acid trafficking between glycerolipid synthesis and mitochondrial ␤-oxidation [3]

Summary

To whom correspondence should be addressed

Nonalcoholic fatty liver disease (NAFLD) has emerged as a global epidemic, with a world-wide prevalence of ϳ25% [1]. The mechanisms are not fully understood, bioactive lipid species play key roles in suppressing insulin signaling These include free fatty acids (FFA) and the glycerolipids diacylglycerols (DAG) and phosphatidic acids (PA) [2]. GPAT1-induced lipid accumulation leads to decreased insulin signaling and reduced hepatic glucose tolerance, which are attributable to the hepatocellular accumulation of DAG and PA (6, 8 –10). In keeping with this observation, mice lacking GPAT1 (Gpat1Ϫ/Ϫ) are protected against hepatic steatosis and insulin resistance [11]. To test whether Them may function upstream of GPAT1 to provide fatty acids for incorporation into bioactive glycerolipid molecular species that suppress insulin signaling, we utilized recombinant adenovirus to overexpress GPAT1 in livers and hepatocytes of Them2Ϫ/Ϫ mice and their respective controls. Hepatic GPAT1 overexpression promoted hepatic steatosis irrespective of Them expression, the absence of Them expression improved glucose tolerance in a manner consistent with decreased DAG-mediated suppression of hepatic insulin signaling

Results

Discussion

Experimental procedures