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Am J Physiol Endocrinol Metab (December 14, 2004). doi:10.1152/ajpendo.00300.2004
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Submitted on July 8, 2004
Accepted on December 4, 2004

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

Tal M. Lewin1, Shuli Wang1, Cynthia A. Nagle1, Cynthia G. Van Horn1, and Rosalind A. Coleman1*

1 Department of Nutrition, University of North Carolina, Chapel Hill, NC, USA

* To whom correspondence should be addressed. E-mail: rcoleman{at}unc.edu.

Because excess triacylglycerol (TAG) in non-adipose tissues is closely associated with the development of insulin resistance, interest has increased in the metabolism of long-chain acyl-CoAs towards {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-1 for acyl-CoAs and whether it contributes to the formation of TAG, we over-expressed rat mtGPAT1 13-fold in primary hepatocytes obtained from fasted rats. When 100, 250, or 750 µM oleate was present, both TAG mass and the incorporation of [14C]oleate into TAG increased more than two-fold in hepatocytes over-expressing mtGPAT1 compared to vector-controls. Although the incorporation of [14C]oleate into CO2 and acid soluble metabolites increased with increasing amounts of oleate in the media, these metabolites were approximately 40% lower in the Ad-mtGPAT1 infected cells, consistent with competition for acyl-CoAs between CPT1 and mtGPAT1. A 50-60% decrease was also observed in [14C]oleate incorporation into cholesteryl ester. With increasing amounts of exogenous oleate, [14C]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 [14C]TAG as the vector-control cells. Thus, in isolated hepatocytes, mtGPAT1 may synthesize a cytosolic pool of TAG that cannot be secreted




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