Fatty acids rapidly induce the carnitine palmitoyltransferase I gene in the pancreatic beta-cell line INS-1

Abstract

Fatty acids are important metabolic substrates for the pancreatic beta-cell, and long term exposure of pancreatic islets to elevated concentrations of fatty acids results in an alteration of glucose-induced insulin secretion. Previous work suggested that exaggerated fatty acid oxidation may be implicated in this process by a mechanism requiring changes in metabolic enzyme expression. We have therefore studied the regulation of carnitine palmitoyltransferase I (CPT I) gene expression by fatty acids in the pancreatic beta-cell line INS-1 since this enzyme catalyzes the limiting step of fatty acid oxidation in various tissues. Palmitate, oleate, and linoleate (0.35 mM) elicited a 4-6-fold increase in CPT I mRNA. The effect was dose-dependent and was similar for saturated and unsaturated fatty acids. It was detectable after 1 h and reached a maximum after 3 h. The induction of CPT I mRNA by fatty acids did not require their oxidation, and 2-bromopalmitate, a nonoxidizable fatty acid, increased CPT I mRNA to the same extent as palmitate. The induction was not prevented by cycloheximide treatment of cells indicating that it was mediated by pre-existing transcription factors. Neither glucose nor pyruvate and various secretagogues had a significant effect except glutamine (7 mM) which slightly induced CPT I mRNA. The half-life of the CPT I transcript was unchanged by fatty acids, and nuclear run-on analysis showed a rapid (less than 45 min) and pronounced transcriptional activation of the CPT I gene by fatty acids. The increase in CPT I mRNA was followed by a 2-3-fold increase in CPT I enzymatic activity measured in isolated mitochondria. The increase in activity was time-dependent, detectable after 4 h, and close to maximal after 24 h. Fatty acid oxidation by INS-1 cells, measured at low glucose, was also 2-3-fold higher in cells cultured with fatty acid in comparison with control cells. Long term exposure of INS-1 cells to fatty acid was associated with elevated secretion of insulin at a low (5 mM) concentration of glucose and a decreased effect of higher glucose concentrations. It also resulted in a decreased oxidation of glucose. The results indicate that the CPT I gene is an early response gene induced by fatty acids at the transcriptional level in beta- (INS-1) cells. It is suggested that exaggerated fatty acid oxidation caused by CPT-1 induction is implicated in the process whereby fatty acids alter glucose-induced insulin secretion.