Regulation of expression of the fatty acid synthase gene in 3T3-L1 cells by differentiation and triiodothyronine.

We have previously reported that fatty acid synthase mRNA levels increase 10-15-fold during the differentiation of 3T3-L1 cells to adipocytes, correlating well with the increase in the relative rate of synthesis for this enzyme. Here we show by transcription run-on assays that fatty acid synthase is highly transcribed in both preadipocytes and adipocytes. Furthermore, the transcription rate of the fatty acid synthase gene increased only 1.5-fold during the adipose conversion, whereas the apparent mRNA half-life increased from 2.5 h in preadipocytes to approximately 20 h in adipocytes. These results indicate that the increase in mRNA level during adipose conversion is not only due to the transcriptional activation of this gene but reflects the post-transcriptional stabilization of the message in adipocytes compared to preadipocytes. As thyroid hormone has been reported to increase lipogenic enzyme activities including fatty acid synthase in adipose tissue and in differentiating adipocytes in vitro, we used fully differentiated 3T3-L1 adipocytes to study T3 regulation of mammalian fatty acid synthase expression. We measured the effect of T3 on the relative rate of protein synthesis, mRNA content, and transcription rate of this gene. When mature adipocytes were treated with 10 nM T3, the relative rate of synthesis of fatty acid synthase increased 1.9-fold at 6 h, and it reached a maximum of 2.9-fold at 12 h. In addition, Northern blot analysis showed that T3 increased the steady-state mRNA level for fatty acid synthase by 2.4-fold at 12 h and 4.5-fold at 24 h. Furthermore, run-on transcription analysis with isolated nuclei from cells treated with T3 showed that the transcription rate of the fatty acid synthase gene increased 4.1-fold after 6 h of T3 treatment and remained at the stimulated level for 24 h. These results demonstrate that the increase in transcription of the fatty acid synthase gene preceded that of the steady-state mRNA level, indicating that T3 regulates expression of fatty acid synthase primarily by modulating the transcription rate of the gene. In conclusion, while the differentiation-dependent increase in fatty acid synthase is mediated by both transcriptional and posttranscriptional processes, T3 regulation is primarily at the transcriptional level.