Genetic regulation of fatty acid synthetase expression in adipose tissue: overtranscription of the gene in genetically obese rats.

We have investigated the molecular mechanism of the overactivity of fatty acid synthetase (FAS) in adipose tissue from the genetically obese Zucker rat. Purified FAS from lean and obese rat adipose tissues displayed kinetics constants, molecular weight, and immunological properties that were identical. Western blot analysis revealed that FAS overactivity in obese versus lean rat adipose tissue was paralleled by a proportionate increase in FAS mass, i.e., 4-fold increase in suckling normoinsulinemic 16-day-old pups and 25-fold in weaned hyperinsulinemic 30-day-old rats. The determination of absolute FAS mass disclosed that FAS was quantitatively a major protein in obese rat adipose tissue accounting for 13% of cytosolic proteins versus 2% in lean rat at 30 days of age. FAS hyperabundance could be ascribed to an increased relative rate of FAS synthesis that was 6-fold higher in obese than in lean rat adipose tissue. Northern blot analysis demonstrated that FAS mRNA levels in obese rats were increased 4- and 14-fold over those of lean rats at 16 and 30 days of age, respectively, in very close proportion to the 3- and 15-fold increases in FAS gene transcription rates revealed by nuclear run-on assays. Southern analysis of genomic DNA did not allow for detecting amplification or any major structural changes in the FAS gene. It is concluded that FAS overactivity, shown here to be a life-long and general feature of all adipose tissue sites in the obese rat, arises primarily from FAS gene overtranscription.