We have analyzed the hormonal basis for the acceleration of differentiation by dexamethasone and insulin in the stable adipogenic cell line TA1. These cells, which were derived from 5-azacytidine-treated 10T1/2 mouse embryo fibroblasts, undergo differentiation in culture after reaching confluence. The ensuing morphological changes are accompanied by widespread alterations in the pattern of protein synthesis and the increased accumulation of specific mRNAs. Using cDNA clones corresponding to mRNAs that are induced during adipogenesis, we find that dexamethasone elicits the precocious accumulation of differentiation-specific gene products. This effect appears to be mediated by the glucocorticoid receptor, yet unlike standard steroid inductions, most of the RNAs reach the same maximal levels in the absence of dexamethasone. Glucocorticoids thus may increase the expression of a regulatory factor required for activating the entire set of differentiation-dependent genes. We also describe a gene whose transcription is not only activated during adipogenesis but is also specifically inducible by dexamethasone in the mature adipocyte. Moreover, the glucocorticoid responsiveness of this gene in differentiated cells appears to be dependent on its prior developmental activation.During development, cells that were pluripotent become increasingly restricted to specific differentiation pathways. This process, which culminates with differentiation into adult tissues, undoubtedly involves an ordered sequence of changes in gene expression reflected by synthesis of increasingly specialized proteins. Although little is known about how these changes are regulated, many of them involve interaction with specific inducers or hormones. Examples include induction of amphibian metamorphosis by thyroxine (l 5), stimulation of oviduct development by estrogen and progesterone (37, 41), and triggering of insect molting by ecdysone (49). Such hormonal effects may provide useful insights into the mechanisms underlying the developmental control of gene expression. Analysis of such mechanisms, however, has been hampered by the complexity of the systems being studied and the difficulty in obtaining pure populations of precursor cells that undergo differentiation under controlled conditions. To overcome this problem, we sought a cell line that can be induced to undergo differentiation in culture. We recently described the isolation and characterization of a stable adipogenic cell line, TA1, derived from 5-azacytidine-treated 10Tl/2 mouse embryo fibroblast (7). The cells are preadipocytes which during growth resemble the 10T1/2 fibroblasts. Once growth is arrested by allowing cells to reach confluence, they exhibit (over a period of 1-2 wk) the morphology characteristic of adipocytes and accumulate lipid droplets. This morphological change is accompanied by widespread alterations in the pattern of protein and RNA synthesis, and, as has also been found in other adipogenic cell lines (for review see reference 1), the appearance of enzymatic ac...

The stable adipogenic cell line TA1, spontaneously differentiates into mature adipocytes after several days at confluence. Glucocorticoids (e.g. dexamethasone) accelerate the onset of differentiation by precociously activating the transcription of genes that are expressed in the mature adipocyte but not in the preadipocyte. Thus the hormone may induce a critical regulatory factor required for activating the entire set of differentiation-dependent genes. We have found that the nonsteroidal antiinflammatory drug indomethacin also stimulates differentiation of TA1 cells but even more rapidly and completely than does dexamethasone. Contrary to previous suggestions we find that this activity of indomethacin's cannot be ascribed to inhibition of cyclo-oxygenase, the critical enzyme in prostaglandin biosynthesis. Finally, indomethacin's ability to stimulate TA1 cell differentiation synchronously and rapidly has allowed us to document that cell confluence is required for efficient differentiation and that the drug needs only to trigger rather than maintain the differentiation process.

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