Connecting p63 to Cellular Proliferation

An unresolved issue regards the role of p73 and p63, the two homologs of the p53 oncosuppressor gene, in normal cells and in tumor development. Specific target genes for each protein need to be identified and characterized in order to understand the specific role of each protein in tumor initiation and progression as well as in oncosuppression and development. We tested whether p63 is implicated in transcriptional events related to sustaining cell proliferation by transactivation of antiapoptotic and cell survival target genes such as Adenosine Deaminase (ADA), an important gene involved in cell proliferation. We demonstrate that ADA is a direct target gene of p63 isoforms. In human keratinocytes, the rate of proliferation and the high level of ADA transcript diminished upon elimination of p63 by small interfering RNA. Reporter assays and chromatin immunoprecipitation experiments indicate a physical interaction of p63 with the two putative p53 binding sites we identified in the ADA gene. Moreover, in response to p53 stabilization and ∆Np63 downregulation in normal keratinocytes after U.V. treatment, we found a change in the transcriptional pattern of the p53 family target genes, consistent with the different roles played by p53 and p63 in tumor suppression and cellular proliferation. In fact p53 upregulation determined an increase in p21, which in turn mediated the cell cycle arrest, while the downregulation of ∆Np63 determined a marked decrease in ADA transcript. The experiments reported here support the hypothesis that TAp63 and ∆Np63 might contribute to tumor genesis not exclusively by antagonizing p53, but by conferring a proliferative potential on cancer cells through the transactivation of target genes indispensable for cell division, such as the Adenosine Deaminase gene.