Characterization of 3T3-L1 storage lipid metabolism: effect of somatotropin and insulin on specific pathways.

TLC of total cellular lipids showed that 3T3-L1 cells predominantly accumulate triglycerides. GH and insulin principally regulated glucose utilization for synthesis of the fatty acid portion of 3T3-L1 triglycerides, with little effect on glucose utilization for synthesis of other lipids or triglyceride glycerol. Gas chromatographic mass spectrometry (MS) showed that 50% of 3T3-L1 triglyceride fatty acids are 16-carbon chains. However, 3T3-L1 adipocytes were unusual, in that 20% of their triglyceride fatty acids were C15 or C17 in length, observed by both MS of saponified triglycerides and electrospray MS of intact triglycerides. Gas chromatographic analysis of 14C-labeled fatty acids showed 3T3-L1 utilization of [14C]glucose for synthesis of C15 and C16 fatty acids. Insulin and GH regulated the amount of [14C]glucose incorporation into C15 and C16 fatty acids without altering their relative ratio. GH antagonized all studied insulin-regulated events. GH antagonism of insulin-stimulated 3T3-L1 glucose transport, glucose oxidation, and glucose utilization for lipid synthesis reached a plateau of 50-60% inhibition at 0.1-0.2 nM. Insulin, at 0.1 nM, suppressed 3T3-L1 generation of glycerol from lipolysis by approximately 50%. GH, between 0.04-1.0 nM, fully reversed the insulin-inhibited lipolysis, although GH did not stimulate lipolysis beyond that in untreated control cultures. The results suggest that GH regulates a very early event in the insulin signal transduction pathway, such that is affects all insulin-responsive processes to essentially the same extent.