The role of ceramide in the antagonism of insulin signal transduction by free fatty acids in skeletal muscle

Abstract

Obesity is highly associated with insulin resistance and is the major risk factor for Type 2 diabetes mellitus and Metabolic Syndrome X. However, the connection between expanded adipose mass and the development of insulin resistance in skeletal muscle, the tissue responsible for the majority of postprandial glucose uptake, is not clearly established. Recent studies suggest that circulating factors released by adipose tissue such as free fatty acids (FFAs) induce insulin resistance in peripheral organs by promoting the accumulation of ectopic fat-derived metabolites capable of attenuating insulin action. Specifically, numerous studies implicate the sphingolipid ceramide as a potential mediator linking lipid oversupply to the antagonism of insulin action. Ceramide is a direct product of fatty acyl-CoA that has been shown to accumulate in insulin resistance states and to inhibit early steps in insulin signaling. The studies described herein investigate the hypothesis that overaccumulation of ceramide is an important event in the development of muscle insulin resistance. We first demonstrated that induced ceramide buildup by exposing cultured C2C12 myotubes to long-chain saturated FFAs inhibits insulin stimulation of Akt/PKB, a key regulator of glucose uptake and anabolic metabolism. To further evaluate the contribution of ceramide to the saturated FFA-dependant inhibition of insulin signaling, we employed different strategies to modulate intracellular ceramide levels in C2C12 myotubes. First, inhibitors of the de novo ceramide synthesis prevented the antagonistic effects of saturated FFAs on insulin signaling. Second, inhibitors of ceramide catabolism recapitulated and augmented the effects of saturated FFAs on both intracellular ceramide accumulation and the inhibition of insulin signaling. Third, overexpression of human acid ceramidase, an enzyme that catalyzes the lysosomal degradation of ceramide, protected cells from the inhibitory effects of long-chain saturated FFAs. Collectively, these studies demonstrated that ceramide is the principal intermediate linking saturated FFAs to the inhibition of insulin signaling, and implicate aberrant ceramide accumulation in the development of muscle insulin resistance.