Evidence for an increase in transcription of specific mRNAs during differentiation of 3T3-L1 preadipocytes.

Differentiation of 3T3-L1 preadipocytes in culture is accompanied by alterations in the abundance of several mRNAs and by the appearance of many new adipocyte-specific mRNAs. To investigate the processes responsible for these alterations, the kinetics of accumulation of several specific mRNAs were compared with their respective rates of nuclear runoff transcription. The mRNAs for fructose-1,6-bisphosphate aldolase and an unidentified 4800-base mRNA increase in abundance only moderately (2-4-fold) during differentiation. Runoff transcription by nuclei isolated from 3T3-L1 cells during the course of differentiation revealed very little or no change in the rates of transcription of these mRNAs. Similar results were obtained for the beta, alpha-actin and beta-tubulin mRNAs where no difference in nuclear runoff transcription rates were observed even though a 2-fold decrease in the steady-state levels of these mRNAs accompanies differentiation. In contrast, the steady-state levels of mRNAs for 3T3-L1 P2 protein, an adipocyte homologue of myelin P2 protein, and an unidentified 5000-base mRNA increased dramatically (greater than 20-fold) during adipose conversion. These large increases in abundance were correlated with marked rises (greater than 10-fold) in nuclear runoff transcription rates for these mRNAs during differentiation of 3T3-L1 preadipocytes. No change in runoff transcription activity for these mRNAs was detected by nuclei from control nondifferentiating 3T3-C2 cells. These results strongly suggest that an increased rate of specific transcription is primarily responsible for the accumulation of these mRNAs during preadipocyte differentiation.