Regulation of fatty acid synthetase concentration and activity during adipocyte differentiation. Studies on 3T3-L1 cells.

The activity, content, and turnover of the key lipogenic enzyme fatty acid synthetase were studied during the differentiation of 3T3-Ll preadipwytes into adipocytes. The specific activity of the enzyme 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 determinations of enzyme mass by competitive ~splacement radioimmunoassays disclosed that the cellular content of fatty acid synthetase increased in parallel with augmentations in activity. Fatty acid synthetase content increased 11-fold during differentiation and 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 results from a comparable acceleration in the 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, thereby suggesting a possible role for lipolytic hormones in modulating the expression of fatty acid synthetase during development.