Hormonal regulation of stearoyl coenzyme A desaturase activity and lipogenesis during adipose conversion of 3T3-L1 cells.

Abstract

During differentiation of 3T3-Ll cells to adipocytes the desaturation of exogenous ['4C]palmitate and of de novo synthesized fatty acids from exogenous [14C]acetate to monounsaturated fatty acids increased by 5.9and 255-fold, respectively.Under similar conditions, fully differentiated 3T3-Ll adipocytes showed a maximal induction of loo-, 63-, and 50-fold, respectively, in the activities of stearoyl-CoA desaturase, fatty acid synthetase, and malic enzyme.The extent of differentiation into adipocytes and the increase in enzyme activities occurred more slowly when the differentiation was induced by insulin alone than when differentiation was induced by dexamethasone, 1-methyl-3-isobutylxanthine, and insulin.In one experiment a 30-fold induction of overall stearoyl-CoA desaturation was correlated with a 32-fold increase in the activity of terminal component of the desaturase system on differentiation of 3T3-Ll cells.The activities of NADH-ferricyanide reductase, NADH-cytochrome c reductase, and the content of cytochrome b5 increased by 3.4-, 2.3-, and 6.6-fold, respectively, on differentiation of 3T3-Ll cells.However, reconstitution of the stearoyl-CoA desaturation in various preparations of microsomes with excess NADH-cytochrome b5 reductase and cytochrome b6 indicated that the changes in the reductase activity and cytochrome b5 content are not influencing the expression of overall stearoyl-CoA desaturation in microsomes.The induction of stearoyl-CoA desaturation during adipose conversion of 3T3-Ll cells is primarily due to an increase in the activity of A' terminal desaturase.Biotin deficiency in 3T3-Ll adipocytes inhibited de novo fatty acid synthesis from ['%]acetate but did not inhibit phenotypic expression of lipid accumulation as well as activities of stearoyl-CoA desaturase and fatty acid synthetase, suggesting that continued synthesis of saturated fatty acid substrate is not required for the induction of stearoyl-CoA desaturase activity.The induction of stearoyl-CoA desaturase and fatty acid synthetase activities as well as enhanced synthesis of fatty acids and sterols during 3T3-Ll cell differentiation is inhibited by anti-insulin serum, and this inhibition is relieved by the addition of exogenous insulin.These results indicate that insulin supports phenotypic expression as well as the induction of lipogenesis and the activities of stearoyl-CoA desaturase and fatty acid synthetase during adipose conversion of 3T3-Ll cells.