Prolonged exposure to palmitate impairs fatty acid oxidation despite activation of AMP-activated protein kinase in skeletal muscle cells.

The aim of this study was to investigate the chronic effects of palmitate on fatty acid (FA) oxidation, AMPK/ACC phosphorylation/activation, intracellular lipid accumulation, and the molecular mechanisms involved in these processes in skeletal muscle cells. Exposure of L6 myotubes for 8 h to 200, 400, 600, and 800 microM of palmitate did not affect cell viability but significantly reduced FA oxidation by approximately 26.5%, approximately 43.5%, approximately 50%, and approximately 47%, respectively. Interestingly, this occurred despite significant increases in AMPK ( approximately 2.5-fold) and ACC ( approximately 3-fold) phosphorylation and in malonyl-CoA decarboxylase activity ( approximately 38-60%). Low concentrations of palmitate (50-100 microM) caused an increase ( approximately 30%) in CPT-1 activity. However, as the concentration of palmitate increased, CPT-1 activity decreased by approximately 32% after exposure for 8 h to 800 microM of palmitate. Although FA uptake was reduced ( approximately 35%) in cells exposed to increasing palmitate concentrations, intracellular lipid accumulation increased in a dose-dependent manner, reaching values approximately 2.3-, approximately 3-, and 4-fold higher than control in muscle cells exposed to 400, 600, and 800 microM palmitate, respectively. Interestingly, myotubes exposed to 400 microM of palmitate for 1 h increased basal glucose uptake and glycogen synthesis by approximately 40%. However, as time of incubation in the presence of palmitate progressed from 1 to 8 h, these increases were abolished and a time-dependent inhibition of insulin-stimulated glucose uptake ( approximately 65%) and glycogen synthesis ( approximately 30%) was observed in myotubes. These findings may help explain the dysfunctional adaptations that occur in glucose and FA metabolism in skeletal muscle under conditions of chronically elevated circulating levels of non-esterified FAs, such as in obesity and Type 2 Diabetes. (c) 2008 Wiley-Liss, Inc