Somatotropin (ST) markedly decreases lipogenesis, fatty acid synthase (FAS) enzyme activity and mRNA abundance in pig adipocytes. The present 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 abundance 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 transcription; ST significantly shortened the half-life of FAS mRNA from 35 to 11 h. To corroborate the run-on analyses, cells were stably transfected with a pFAS–CAT5 (in which CAT stands for chloramphenicol acetyltransferase) reporter construct that contained 2195 bp of the 5´ flanking 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 is the result of a marked decrease in both transcription of the FAS gene and stability of the FAS mRNA.

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