Submitted on December 20, 2007
Revised on January 10, 2008
Accepted on January 11, 2008
Identification of a novel sn-glycerol-3-phosphate acyltransferase isoform, GPAT4 as the enzyme deficient in Agpat6-/- mice
Cynthia A. Nagle, Laurent Vergnes, Hendrik DeJong, Shuli Wang, Tal M. Lewin, Karen Reue, and Rosalind A. Coleman
Nutrition, University of North Carolina, Chapel Hill, NC 27599
Corresponding Author: rcoleman{at}unc.edu
Elucidation of the metabolic pathways of triacylglycerol synthesis is critical to the understanding of chronic metabolic disorders such as obesity, cardiovascular disease, and diabetes. Glycerol-sn-3-phosphate acyltransferase (GPAT) and sn-1-acylglycerol-3-phosphate acyltransferase (AGPAT) catalyze the first and second steps in de novo triacylglycerol synthesis. AGPAT6 is one of eight AGPAT isoforms identified through sequence homology, but the enzyme activity for AGPAT6 has not been confirmed. We found that in liver and brown adipose tissue from Agpat6-/- mice NEM-sensitive GPAT specific activity was 65% lower than in tissues from wildtype mice, but that AGPAT specific activity was similar. Over-expression of Agpat6 in Cos-7 cells increased an NEM-sensitive GPAT specific activity, but AGPAT specific activity was not increased. Agpat6 and Gpat1 overexpression in Cos-7 cells increased the incorporation of [14C]oleate into diacylglycerol (DAG) or into DAG and triacylglycerol, respectively, suggesting that the LPA, PA, and DAG intermediates initiated by each of these isoforms lie in different cellular pools. Taken as a whole, the data show that “Agpat6-/- mice” are actually deficient in a novel NEM-sensitive GPAT, GPAT4, and indicate that the alterations in lipid metabolism in adipose tissue, liver, and mammary epithelium of these mice are attributable to the absence of GPAT4.
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