Analysis of gene expression during differentiation of adipogenic cells in culture and hormonal control of the developmental program.

Treatment of 10T1/2 mouse embryo fibroblasts with 5-azacytidine, an inhibitor of mammalian DNA methylation, leads to the appearance of several new cell types, including adipocytes. We have isolated several such adipogenic cell lines and characterized two of them, TA1 and TA2. When subconfluent these cells resemble fibroblasts. After growth is arrested at high density, both clones express a functional adipose phenotype characterized by accumulation of lipid droplets. This in vitro differentiation is accompanied by a greater than 100-fold increase in glycerol phosphate dehydrogenase activity, an enzyme characteristic of mature adipocytes. Consistent with these morphologic and enzymatic changes, differentiated TA1 cells show a widespread alteration in protein composition as well as a substantial change in the pattern of secreted proteins. We have constructed a cDNA library of TA1 adipocytes and have isolated 12 different cDNA clones corresponding to mRNAs that are induced during adipogenesis. Among these RNAs, some are not expressed prior to initiating differentiation whereas others are expressed in 10T1/2 cells and TA1 preadipocytes. Treatment of TA1 cells with insulin and the synthetic glucocorticoid dexamethasone leads to an acceleration of the phenotypic changes observed during adipogenesis. We have found that hormone treatment leads to a precocious accumulation of specific RNA for all of the clones studied. Analysis of the temporal control of RNA accumulation during differentiation indicates that there are different categories of RNAs, some of which accumulate by day 1 after treatment while others are not apparent until day 3.