Is GPAT2 a GPAT?

GARCIA FABIANI, MB; CATTANEO, ER; PELLON MAISON, M; MONTANARO, M. A.; COLEMAN RA; GONZALEZ-BARÓ MR

Sonwmass Village, CO

Congreso; FASEB Summer Research Conferences; 2012

FASEB

Is GPAT2 a GPAT? María Belén García-Fabiani, Elizabeth R. Cattaneo, Magalí Pellon-Maison, Mauro A. Montanaro, Rosalind A. Coleman and María R. Gonzalez-Baró De novo glycerolipid synthesis begins with the acylation of glycerol-3 phosphate catalyzed by glycerol-3-phosphate acyltransferase (GPAT). In mammals, at least four GPAT isoforms have been described, differing in their cell and tissue locations and functions. GPAT1, 3 and 4 are expressed in lipogenic organs like liver and adipose tissue, and preferentially use saturated and monounsaturated fatty acyl-CoA substrates. Unlike the other 3 isoforms, GPAT2, an outer mitochondrial membrane isoform, is highly expressed in rat and mouse testis and certain cancer cells, tissues that do not specialize in lipid synthesis and storage. Our aim was to determine GPAT2 function related to its unusual tissue distribution. We determined that GPAT2 mRNA was expressed in mouse spermatocytes by in situ hybridization, although GPAT2 protein was also detected in spermatides. Mouse GPAT2 overexpression in CHO-K1 cells increased the acylation of glycerol-3-phosphate and 1-acylglycerol-3-phosphate exclusively when arachidonoyl-CoA was the substrate, and increased the incorporation of arachidonate into triacylglycerol. GPAT2 overexpression also increased cell number, compared to control cells and cells overexpressing GPAT1. Because Gpat2 expression was not upregulated by fasting and refeeding in mice, we analyzed Gpat2 mRNA expression in 3T3L1 cells as they differentiated into adipocytes. Unlike the expression of the other three GPAT isoforms, Gpat2 mRNA did not increase during differentiation, indicating a different cellular function of GPAT2. We cloned the murine Gpat2 promoter region (~6.5 kbp, 3.4 kbp and 1.5 kpb upstream from the transcription initiation site) into a luciferase-reporter vector and transfected these regions into CHO-K1 cells. Only the 3.4 kbp construct activated transcription. The fact that a CpG island was predicted 2kbp upstream from the transcription initiation site, suggests possible epigenetic regulation. Although Gpat2 is expressed in germ cells and its promoter contains predicted consensus sequences for several steroid hormone receptors, neither testosterone, dihydrotestosterone nor corticosterone activated Gpat2 transcription. We conclude that the role of GPAT2 differs from that of the other 3 GPATs: it can act either as a GPAT or an AGPAT incorporating arachidonic acid to phosphatidic acid, and its unique tissue distribution and behavior suggest a cellular function unrelated to triacylglycerol synthesis as energy stores. Support: NIH (TW8698 and DK56598), CONICET, Argentina (PIP0527), ANPCyT, Argentina (PICT2246)