SOMATOTROPIN-DEPENDENT DECREASE IN FATTY-ACID SYNTHASE MESSENGER-RNA ABUNDANCE IN 3T3-F442A ADIPOCYTES IS THE RESULT OF A DECREASE IN BOTH GENE-TRANSCRIPTION AND MESSENGER-RNA STABILITY

Somatotropin (ST) markedly decreases lipogenesis, fatty acid synthase (FAS) enzyme activity and mRNA abundance in pig adipocytes. The presen t study was conducted to determine whether the decrease in FAS mRNA in 3T3-F442A adipocytes was the result of a decrease in transcription of the FAS gene and/or a change in FAS mRNA stability. Insulin increased the abundance of FAS mRNA 2-13-fold and fatty acid synthesis 3-7-fold . Somatotropin decreased the stimulatory effect of insulin on the abun dance of FAS mRNA and lipogenesis by 40-70 % and 20-60 % respectively. Subsequent run-on analyses demonstrated that the decrease observed in FAS mRNA in response to ST was associated with an 82 % decrease in tr anscription; ST significantly shortened the half-life of FAS mRNA from 35 to 11 h. To corroborate the run-on analyses, cells were stably tra nsfected with a pFAS-CAT5 tin which CAT stands for chloramphenicol ace tyltransferase) reporter construct that contained 2195 bp of the 5' fl anking region of the rat FAS gene. Insulin treatment increased FAS-CAT activity 4.7-fold. When ST was added to the insulin-containing medium there was an approx. 60 % reduction in FAS-CAT activity. In summary, our results indicate that ST decreases FAS mRNA levels and that this i s the result of a marked decrease in both transcription of the FAS gen e and stability of the FAS mRNA.