Characterization of the molecular mechanisms for p53-mediated differentiation

The p53 tumor suppressor protein can induce both apoptosis and cell cycle a rrest. Moreover, we and others have shown previously that p53 is a potent m ediator of differentiation, For example, expression of ptsp53, a temperatur e-inducible form of p53, induces differentiation of leukemic monoblastic U- 937 cells. The functions of p53 have for long been believed to be dependent on the transactivating capacity of p53, However, recent data show that bot h p83-induced cell cycle arrest and apoptosis can be induced independently of p53-mediated transcriptional activation, indicating alternative pathways for p53-induced apoptosis and cell cycle arrest. The bcl-2 proto-oncogene contributes to the development of certain malignancies, probably by inhibit ion of apoptosis, Interestingly, Bcl-2 has been shown to inhibit p53-mediat ed apoptosis as well as p53-mediated transcriptional activation, Asking whe ther Bcl-2 would interfere with the p53-mediated differentiation of U-937 c ells, we stably transfected bcl-2 to U-937 cells inducibly expressing p53. Although the established Bcl-2-expressing clones were resistant to p53-medi ated apoptosis, we did not observe any interference of Bcl-2 with the p53-m ediated differentiation, suggesting separable pathways for p53 in mediating apoptosis and differentiation of U-937 cells, Neither did expression of Bc l-2 interfere with p53-induced expression of endogenous p21, suggesting tha t p53-induced differentiation might be dependent on the transcriptional act ivity of p53, To further investigate whether the p53-mediated differentiati on of U-937 cells depends on the transcriptional activity of p53, we overex pressed transactivation-deficient p53, a transcriptionally inactive p53 mut ant in these cells. However, in contrast to the effects of wild-type p53, e xpression of trans-activation-deficient p53 did neither induce signs of apo ptosis nor of differentiation in U-937 cells. Our results indicate that the transcriptional activity of p53 is essential both for p53-mediated apoptos is and differentiation of U-937 cells.