FATTY-ACIDS RAPIDLY INDUCE THE CARNITINE PALMITOYLTRANSFERASE-I GENE IN THE PANCREATIC BETA-CELL LINE INS-1

Fatty acids are important metabolic substrates for the pancreatic beta -cell, and long term exposure of pancreatic islets to elevated concent rations of fatty acids results in an alteration of glucose-induced ins ulin secretion. Previous work suggested that exaggerated fatty acid ox idation may be implicated in this process by a mechanism requiring cha nges in metabolic enzyme expression. We have therefore studied the reg ulation of carnitine palmitoyltransferase I (CPT I) gene expression by fatty acids in the pancreatic beta-cell line INS-I since this enzyme catalyzes the limiting step of fatty acid oxidation in various tissues . Palmitate, oleate, and linoleate (0.35 mar) elicited a 4-6-fold incr ease 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 aci ds did not require their oxidation, and 2-bromopalmitate, a nonoxidiza ble fatty acid, increased CPT I mRNA to the same extent as palmitate. The induction was not prevented by cycloheximide treatment of cells in dicating that it was mediated by pre-existing transcription factors. N either glucose nor pyruvate and various secretagogues had a significan t effect except glutamine (7 mM) which slightly induced CPT I mRNA. Th e 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 pronounc ed transcriptional activation of the CPT I gene by fatty acids. The in crease in CPT I mRNA was followed by a 2-3-fold increase in CPT I enzy matic activity measured in isolated mitochondria. The increase in acti vity was time-dependent, detectable after 4 h, and close to maximal af ter 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 compar ison with control cells. Long term exposure of INS-1 cells to fatty ac id was associated with elevated secretion of insulin at a low (5 mM) c oncentration of glucose and a decreased effect of higher glucose conce ntrations. It also resulted in a decreased oxidation of glucose. The r esults 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 in duction is implicated in the process whereby fatty acids alter glucose -induced insulin secretion.