REGULATION OF FATTY ACID SYNTHETASE DURING ADIPOCYTE DIFFERENTIATION: STUDIES ON 3T3-L1 CELLS

Abstract

The activity, content, and turnover of the key lipogenic enzyme fatty acid synthetase were studied during the differentiation of 3T3-L1 preadipocytes into adipocytes. Cells were induced to differentiate by treatment with dexamethasone and methylisobutylxanthine for 2 days (Days 1-2). The specific activity of fatty acid synthetase began to rise sharply when triglyceride droplets were first detected in the cells (Day 3) and ultimately increased 13-fold to a new steady state level in adipocytes 7 days after the initiation of differentiation. Direct determination of enzyme mass by competitive displacement radioimmunoassays disclosed that the cellular content of fatty acid synthetase increased in parallel with the augmentation in activity. The content of fatty acid synthetase increased 11-fold during differentiation at which time the enzyme accounted for 2.5 to 4.5% of the cytosolic protein in adipocytes. Pulse-labeling experiments revealed that marked elevations in the relative rate of synthetase synthesis preceded increases in enzyme activity and content by approximately 24 h. The relative rate of synthetase synthesis rose from 0.16% in preadipocytes to 2% in developing adipocytes over a period of 5 days and then decreased to a new steady state level of 1.3% in the "mature" adipocytes. In contrast, the half-life of fatty acid synthetase was very similar in preadipocytes (39 h) and adipocytes (45 h), suggesting that the 11-fold increment in enzyme content resulted from an accelerated rate of enzyme synthesis. Treatment of developing adipocytes with adrenocorticotropic hormone, isoproterenol, or dibutyryl cyclic AMP elicited a 4-fold decrement in the rate of synthetase synthesis, suggesting a role for lipolytic hormones in modulating the expression of fatty acid synthetase during development.;RNA from lipogenic rat and mouse tissues was prepared by guanadinium thiocyanate extraction, enriched for polyadenylated molecules and translated in nuclease digested reticulocyte lysates. Cell-free synthesized fatty acid synthetase and ATP-citrate lyase were identified on the basis of immunoreactivity and SDS-polyacrylamide electrophoretic mobility. Preferential translation of FAS by the reticulocyte lysate system occurred in the presence of 105 mM K('+), 60 mM Cl('-), 40 mM PO(,4), 2% glycerol, and 9 mg/ml unlabeled fatty acid synthetase.