Oral administration of a PPAR-δ agonist to rodents worsens, not improves, maximal insulin-stimulated glucose transport in skeletal muscle of different fibers

Abstract

Agonists targeting the nuclear receptor peroxisome proliferator-activated receptors (PPAR)-delta may be potential therapeutic agents for insulin-resistant related conditions, as they may be able to stimulate fatty acid (FA) oxidation and attenuate the accumulation of harmful lipid species in skeletal muscle. Several reports have demonstrated that PPAR-delta agonists improve whole body insulin sensitivity. However, whether these agonists exert their direct effects on glucose and FA metabolism in skeletal muscle, and specifically with different fiber types, is unknown. This study was undertaken to determine the effects of oral treatment with the PPAR-delta agonist, GW 501516, in conjunction with the administration of a high-saturated-fat diet on insulin-stimulated glucose transport in isolated oxidative (soleus) and glycolytic (epitrochlearis) rodent skeletal muscle in vitro. High-fat feeding significantly decreased maximal insulin-stimulated glucose transport in soleus, but not epitrochlearis muscle, and was associated with increased skeletal muscle diacylglycerol and ceramide content. Unexpectedly, treatment with the PPAR-delta agonist significantly reduced insulin-stimulated glucose transport in both soleus and epitrochlearis muscles, regardless of dietary fat content. The reduction in insulin-stimulated glucose transport induced by the agonist was associated with large increases in total muscle fatty acid translocase (FAT)/CD36protein content, but not diacylglycerol or ceramide contents. Agonist treatment did not alter the protein content of PPAR-delta, GLUT4, or insulin-signaling proteins (IRS-1, p85 PI3-K, Akt). Agonist treatment led to a small, but significant increase, in the oxidative capacity of glycolytic but not oxidative muscle. We propose that chronic treatment with the PPAR-delta agonist GW 501516 may induce or worsen insulin resistance in rodent skeletal muscle by increasing the capacity for FA transport across the sarcolemma without a sufficient compensatory increase in FA oxidation. However, an accumulation of diacylglycerol and ceramide, while associated with diet-induced insulin resistance, does not appear to be responsible for the agonist-induced reduction in insulin-stimulated glucose transport.