TRANSCRIPTIONAL ACTIVATION BY P53, BUT NOT INDUCTION OF THE P21 GENE,IS ESSENTIAL FOR ONCOGENE-MEDIATED APOPTOSIS

The p53 tumor suppressor limits cellular proliferation by inducing eit her G(1) arrest or apoptosis, depending on the cellular context, To de termine if these pathways are mechanistically distinct, we have examin ed the effects of different p53 mutants in p53 null primary mouse embr yo fibroblasts. We chose this system as it is highly physiological and ensures that the interpretation of the results will not be confounded by the presence of endogenous p53 or oncoproteins which target p53, U sing single cell microinjection assays for both G(1) arrest and apopto sis, with loss-of-function and chimeric gain-of-function mutants, we h ave demonstrated that transcriptional activation is critical for both processes, Replacement of the p53 activation domain with that of VP16, or replacement of the p53 oligomerization domain with that of GCN4, r econstituted both G(1) arrest and apoptosis activities, However, despi te the importance of transcriptional activation in both processes, the target gene requirements are different, The p21 cyclin-dependent kina se inhibitor, which has been shown to be a direct target of p53 and a component of the radiation-induced G(1) arrest response, is dispensabl e for oncogene-induced apoptosis, suggesting that these two p53-depend ent transcriptional pathways are distinct.