Year: 2008
Abstract Number: 290-OR
Authors: JIANRONG YAO, SHANMING HU, ALEX HOWE, ARTHUR A. SPECTOR, ANDREW W. NORRIS, Iowa City, IA
Institutions: Iowa City, IA
Results: We and others have shown that PPARγ in skeletal muscle protects against whole-body adiposity and insulin resistance, in part by maintaining the ability of oxidative muscle to utilize lipid fuel. To better define the direct actions of PPARγ on muscle lipid metabolism, we have transfected C2C12 myotubes with adenovirus expressing PPARγ (adPPARγ). Control myotubes, transfected with adenovirus expressing green fluorescent protein (adGFP), had no detectable endogenous PPARγ mRNA, protein, or activity. Consistent with in vivo findings, cellular uptake of the monounsaturated fatty acid oleate was increased 22±2% in response to adPPARγ (p<0.0005). Rosiglitazone further enhanced cellular uptake of oleate in adPPARγ transfected myotubes, to 51±2% above baseline, with an EC50 of 11 nM. The enhanced fatty acid uptake stimulated by PPARγ plus rosiglitazone led to increased integration of oleate into multiple cellular lipids, including 2.5- and 3.0-fold higher rates of triglyceride (p<0.005) and phosphatidylinositol incorporation (p<0.001) respectively. To determine the mechanism of the increased fatty acid uptake induced by PPARγ, we surveyed the expression of candidate genes. Notably, there was a 153±34 fold increase in the expression of CD36 induced by adPPARγ plus 0.5 µM rosiglitazone. CD36 protein levels were likewise increased. To determine the role of CD36 in the PPARγ enhancement of fatty acid uptake, we used short-hairpin RNA interference directed against CD36 (shRNA-CD36). Two independent C2C12 lines stably transfected with shRNA-CD36 demonstrated a nearly-complete lack of CD36 protein, even in response to adPPARγ and rosiglitazone. Myotubes from these two cell lines demonstrated a 90% reduction in the normal enhancement of oleate uptake induced by PPARγ and rosiglitazone (p<0.0001). Uptake of the saturated fatty acid palmitate was analogously stimulated by PPARγ and rosiglitazone in a CD36 dependent fashion. In conclusion, fatty acid uptake in muscle is stimulated by PPARγ action via an upregulation of CD36. This action of PPARγ is essential to allow muscle enhanced access to lipid energy, thus helping prevent lipid overload, adiposity, and insulin resistance in other tissues.
Category: Integrated Physiology - Nutrient Metabolism (Amino Acid and Fatty Acid combined)