Mitochondrial glycerol-3-phosphate acyltransferase-1 directs the metabolic fate of exogenous fatty acids in hepatocytes

Because excess triacylglycerol ( TAG) in nonadipose tissues is closely associated with the development of insulin resistance, interest has increased in the metabolism of long-chain acyl-CoAs toward beta-oxidation or the synthesis and storage of TAG. To learn whether a mitochondrial isoform of glycerol-3-phosphate acyltransferase (mtGPAT1) competes with carnitine palmitoyltransferase I (CPT I) for acyl-CoAs and whether it contributes to the formation of TAG, we overexpressed rat mtGPAT1 13-fold in primary hepatocytes obtained from fasted rats. When 100, 250, or 750 mu M oleate was present, both TAG mass and the incorporation of [C-14] oleate into TAG increased more than twofold in hepatocytes overexpressing mtGPAT1 compared with vector controls. Although the incorporation of [C-14]oleate into CO2 and acid-soluble metabolites increased with increasing amounts of oleate in the media, these metabolites were similar to 40% lower in the Ad-mtGPAT1 infected cells, consistent with competition for acyl-CoAs between CPT I and mtGPAT1. A 50-60% decrease was also observed in [C-14]oleate incorporation into cholesteryl ester. With increasing amounts of exogenous oleate, [C-14]TAG secretion increased appropriately in vector control-infected hepatocytes, suggesting that the machinery for VLDL-TAG biogenesis and secretion was unaffected. Despite the marked increases in TAG synthesis and storage in the Ad-mtGPAT1 cells, however, the Ad-mtGPAT1 cells secreted the same amount of [C-14]TAG as the vector control cells. Thus, in isolated hepatocytes, mtGPAT1 may synthesize a cytosolic pool of TAG that cannot be secreted.