Differential regulation of cellular target genes by p53 devoid of the PXXPmotifs with impaired apoptotic activity

Activation of the p53 tumor suppressor protein can lead to either cell cycl e arrest or apoptosis. Several functional domains necessary for mediating c ell cycle arrest and apoptosis in p53 have been mapped, e.g., the proline-r ich domain. The proline-rich domain is located within residues 60-90, which comprise five PXXP motifs (where P represents proline and X any amino acid ). To further delineate the function of the proline-rich domain and its pot ential role in transactivation, we generated several groups of cell lines t hat inducibly express various p53 mutants using a tetracycline-regulated ex pression system. We found that p53(Delta 62-91), which lacks all five PXXP motifs in human p53, is capable of inducing cell cycle arrest but not apopt osis, while p53(gln22-ser23/Delta 62-91), which contains a double point mut ation in the activation domain as well as deletion of the proline-rich doma in, completely loses its activity. However, p53(Delta 74-91), which contain s only one PXXP motif at its N-terminus, is not only capable of inducing ce ll cycle arrest but also retains a partial apoptotic activity. Furthermore, we found that deletion of the proline-rich region has no or very mild effe cts on activation of several transiently transfected p53 target gene promot ers, i.e., the p21, MDM2, BAX, and GADD45 promoters. However, such deletion differentially affects p53 induction of endogenous target genes, i.e., ind uction of p21, MDM2, BTG2, p85, PIG3, PIG6 and PIG11 was reduced or abrogat ed but induction of BAX, KILLER/DR5, PIG2, PIG7 and PIGS was not substantia lly affected. Interestingly, induction of GADD45 was enhanced. These result s suggest that the proline-rich region may play a role in chromatin remodel ing, which counteracts chromatin-mediated repression for some of the endoge nous p53 target genes.