Ras modulates commitment and maturation of 10T1/2 fibroblasts to adipocytes

The positive association of the ras oncogene with human cancer and the recognition that malignancy may, in part, represent the imbalance between cell proliferation and differentiation have generated intense interest in the potential role of ras in cell differentiation. We investigated this possibility utilizing as a model system the differentiation of the mesenchymal cell line C3H 10T1/2 (10T1/2) to adipocytes, and a series of transfectants of 10T1/2 cells in which the level of the ras gene product (p21ras; Ras) can be effectively up- or down-modulated. In agreement with previous reports, we found that 10T1/2 cultures, propagated in the resting state for several weeks, spontaneously convert to fat cells at a very low frequency. Downmodulation of endogenous p21ras levels, as a consequence of expression of antisense ras, markedly increased the rapidity and frequency of adipose conversion (6- to 10-fold), which was equivalent in magnitude to that effected by the potent differentiating agent 5-azacytidine. Conversely, overexpression of ras completely inhibited cell differentiation. In addition, adipocytes derived from antisense-ras expressing lines were characterized by a decrease in hormone responsiveness, as well as an apparent deficiency in attaining the terminally differentiated state. These findings suggest that Ras may be a negative regulator of the decision-making step of fibroblast differentiation to adipocytes. In addition, Ras may play an essential positive role in the transduction of hormonal signals necessary for full adipocyte maturation during later progression along the differentiation pathway.