The role of fat storage-inducing transmembrane protein 2 in regulating energy metabolism

Abstract

The ability to store triglyceride in cytosolic lipid droplets is essential in mammalian organisms to maintain energy homeostasis during periods of fasting. Adipose tissue serves as the main storage depot of triglyceride rich lipid droplets, where upon increased energy demand triglyceride is hydrolyzed by lipases to release fatty acids that serve as energy substrates for peripheral tissue. However, in obesity excessive accumulation of triglyceride in adipose tissue results in deposition of fatty acids in peripheral tissues such as skeletal muscle and liver. A large body of literature supports the hypothesis that adipose tissue expansion and storage of fatty acyl-CoAs in the form of triglyceride in cytosolic lipid droplets attenuates the accumulation of intermediate lipid metabolites such as fatty acyl-CoAs, ceramides and diacyglycerol, which have been shown to attenuate insulin signal transduction in skeletal muscle and liver. Fat storage-Inducing Transmembrane Protein 2 (FITM2/FIT2) is the anciently conserved member of the FIT family of proteins implicated to be important in the formation of lipid droplets. Overexpression of FIT2 in vitro results in the accumulation of lipid droplets with a modest 2-fold increase in cellular triglyceride. The opposite phenotype occurs when FIT2 is knock down in 3T3-L1 adipocytes, causing decreased lipid droplet number and cellular triglyceride levels. Based on these results we hypothesized that FIT2 plays an important role in the maintenance of triglyceride in cytosolic lipid droplets in vivo. Therefore, to test this hypothesis in vivo we generated muscle specific FIT2 transgenic mice (CKF2) as well as adipose-specific FIT2 knockout mice (AF2KO). CKF2 mice exhibited a significant increase in intramyocellular triglyceride accumulation and complete protection from diet induced weight gain due to increased energy expenditure. Metabolic profiling of CKF2 muscle indicated increased oxidation of branched chain amino acids (BCAAs) and a decrease in fatty acid oxidation. Further analysis indicated that CFK2 mice were diverting glycolytic intermediates and fatty acids for the purpose of triglyceride synthesis while heavily relying on BCAAs as an oxidative substrate. This correlated with decreased energy charge in CKF2 skeletal muscle as indicated by decreased ATP levels and increased phosphorylated AMP-activated protein kinase. Since most of the phenotypes observed in CKF2 mice may have been adaptations as a result of chronic FIT2 expression in skeletal muscle, we generated an inducible muscle specific FIT2 transgenic using a Tet-on system which allowed us to determine the proximal effects of FIT2 expression in skeletal muscle. Transient expression of FIT2 in skeletal muscle resulted in increased intramyocellular triglyceride accumulation, increased BCAA oxidation and decreased ATP. Taken together our results indicate that FIT2 expression in skeletal muscle reprograms energy metabolism and supports an important role for the lipid droplet in regulating energy metabolism. We then studied the role of FIT2 in the maintenance of triglyceride in mature adipocytes in FIT2 adipose-specific deficient mice using a Cre-lox system (AF2KO). AF2KO mice exhibited decreased adipose accumulation in response to high fat feeding, which correlated with decreased adipocyte area and decreased accumulation of triglyceride in adipose tissue. This correlated with increased basal and isoproterenol-stimulated lipolysis in white adipose tissue fat pads isolated from AF2KO mice. AF2KO mice exhibited a significant increase in adipose tissue macrophage accumulation, which correlated with impaired glucose tolerance. Taken together our studies in the AF2KO mouse indicate that FIT2 plays an essential role in triglyceride storage in mature adipocytes in vivo under conditions of excess caloric intake.