Search

Free fatty acids (FFA) are considered as a causative link between obesity and diabetes. In various animal models and in humans FFA can stimulate hepatic gluconeogenesis. Although the in vivo role of FFA in hepatic gluconeogenesis has been clearly established, the intracellular role of FFA and related signaling pathway remain unclear in the regulation of hepatic gluconeogenic gene transcription. In this study, we have identified p38 mitogen-activated protein kinase (p38) as a critical signaling component in FFA-induced transcription of key gluconeogenic genes. We show in primary hepatocytes that both mid- and long-chain fatty acids (saturated or unsaturated) could activate p38 and increase levels of phosphoenolpyruvate carboxykinase (PEPCK), glucose-6-phosphatase, and peroxisome proliferator-activated receptor γ coactivator α (PGC-1α) gene transcripts. The FFA-induced expression of PEPCK and PGC-1α genes and gluconeogenesis in isolated hepatocytes could be blocked by the inhibition of p38. Furthermore, PGC-1α phosphorylation by p38 was necessary for FFA-induced activation of the PEPCK promoter. Additionally, FFA stimulated phosphorylation of cAMP-response element-binding protein (CREB) through p38. The overexpression of the dominant-negative CREB prevented FFA-induced activation of the PEPCK promoter. Finally, we show that FFA activation of p38 requires protein kinase C5. Together, our results indicate that p38 plays a critical role in FFA-induced transcription of gluconeogenic genes, and the known gluconeogenic regulators, PGC-1α and CREB, are also integral parts of FFA-stimulated transcription of gluconeogenic genes. [ABSTRACT FROM AUTHOR]

Free fatty acid (FFA) is believed to be a major environmental factor linking obesity to Type II diabetes. We have recently reported that FFA can induce gluconeogenesis in hepatocytes through p38 mitogen-activated protein kinase (p38). In this study, we have investigated the role of p38 in oleate-induced hepatic insulin resistance. Our results show that a prolonged treatment of primary hepatocytes with oleate blunted insulin suppression of hepatic gluconeogenesis, and decreased insulin-induced phosphorylation of Akt in a p38-dependent manner. Reduction of the insulin-induced Akt phosphorylation by oleate correlated with activation of p38. In the presence of p38 inhibition, prolonged exposure of hepatocytes to oleate failed to reduce insulin-stimulated phosphorylation of Akt. An siRNA against p38α prevented oleate suppression of the insulin-induced phosphorylation of Akt. Furthermore, a prolonged exposure of hepatocytes to oleate decreased insulin-induced tyrosine phosphorylation of IRS1/2, while slightly increasing serine phosphorylation of IRS. The decrease of insulin-stimulated tyrosine phosphorylation of IRS1/2 in hepatocytes by oleate was reversed by the inhibition of p38. We further show that a prolonged exposure of primary hepatocytes to oleate elevated the protein level of the phosphatase and tensin homolog deleted on chromosome 10 (PTEN) gene in a p38-dependent manner, but had no effect on the mRNA level of PTEN. Knocking down the PTEN gene prevented oleate to inhibit insulin activation of Akt and insulin suppression of gluconeogenesis. Together, results from this study demonstrate a critical role for p38 in oleate-induced hepatic insulin resistance. [ABSTRACT FROM AUTHOR]