Overexpression of 1-Acyl-Glycerol-3-Phosphate Acyltransferase-αEnhances Lipid Storage in Cellular Models of Adipose Tissue and Skeletal Muscle

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Abstract
Plasma nonesterified fatty acids (NEFA) at elevated concentrations antagonize insulin action and thus may play a critical role in the development of insulin resistance in type 2 diabetes. Plasma NEFA and glucose concentrations are regulated, in part, by their uptake into peripheral tissues. Cellular energy uptake can be increased by enhancing either energy transport or metabolism. The effects of overexpression of 1-acylglycerol-3-phosphate acyltransferase (AGAT)-α, which catalyzes the second step in triglyceride formation from glycerol-3-phosphate, was studied in 3T3-L1 adipocytes and C2C12 myotubes. In myotubes, overexpression of AGAT-α did not affect total [14C]glucose uptake in the presence or absence of insulin, whereas insulin-stimulated[14C]glucose conversion to cellular lipids increased significantly(33%, P = 0.004) with a concomitant decrease (-30%, P =0.005) in glycogen formation. [3H]oleic acid (OA) uptake in AGAT-overexpressing myotubes increased 34% (P = 0.027) upon insulin stimulation. AGAT-α overexpression in adipocytes increased basal (130%, P = 0.04) and insulin-stimulated (27%, P = 0.01)[3H]OA uptake, increased insulin-stimulated glucose uptake (56%, P = 0.04) and conversion to cellular lipids (85%, P =0.007), and suppressed basal (-44%, P = 0.01) and isoproterenol-stimulated OA release (-45%, P = 0.03) but not glycerol release. Our data indicate that an increase in metabolic flow to triglyceride synthesis can inhibit NEFA release, increase NEFA uptake, and promote insulin-mediated glucose utilization in 3T3-L1 adipocytes. In myotubes,however, AGAT-α overexpression does not increase basal cellular energy uptake, but can enhance NEFA uptake and divert glucose from glycogen synthesis to lipogenesis upon insulin stimulation.