Differential regulation of stimulated glucose transport by free fatty acids and PPARα or -δ agonists in cardiac myocytes

Stimulation of glucose transport in response to insulin or metabolic stress is an important determinant of cardiac myocyte function and survival, particularly during ischemia-reperfusion episodes. The impact of dyslipidemia and its consequence PPAR activation on stimulated glucose transport in cardiac myocytes remains unknown. Isolated adult rat cardiac myocytes were chronically exposed to free fatty acids (FFA) or PPAR agonists. Insulin- (ISGT) and oligomycin-stimulated glucose transport (OSGT) and related cell signaling were analyzed. Exposure of cardiac myocytes to FFA reduced both ISGT and OSGT. Exposure to either PPARα or PPARδ agonists, but not to a PPARγ agonist, reduced ISGT but not OSGT and increased fatty acid oxidation (FAO). The reduction in ISGT was associated with impaired insulin signaling and, in the case of PPAR stimulation, overexpression of SOCS-3, a protein known to hinder proximal insulin signaling. In contrast, the reduction of OSGT could not be explained by a reduced activity of the cellular energy-sensing system, as assessed from the maintained phosphorylation state of AMPK. Inhibition of FAO at the level of mitochondrial acylcarnitine uptake restored OSGT but not ISGT. Seemingly paradoxically, further stimulation of FAO with PPARα or PPARδ agonists also restored OSGT but not ISGT. Together, these results suggest that inhibition of OSGT occurs downstream of energy gauging and is caused by some intermediate(s) of fatty acid oxidation, which does not appear to be acylcarnitines. The results indicate that the mechanisms underlying FFA-mediated inhibition of ISGT and OSGT differ remarkably.