Identification of novel and direct target genes of p73

The p53 paralogues p73, p63 and their respective truncated isoforms have been shown to be critical regulators of developmental and differentiation processes. Indeed, both p73 and p63 deficient mice exhibit severe developmental defects. Here, we show that the S100A2 gene, whose transcript and protein are induced during keratinocyte differentiation of HaCaT cells, is a direct transcriptional target of p73β and ΔNp63α and is required for proper keratinocyte differentiation. Transactivation assays reveal that p73β and ΔNp63α exert opposite transcriptional effects on the S100A2 gene. While ΔNp63α is found in vivo onto S100A2 regulatory regions predominantly in proliferating cells, p73β is recruited in differentiating cells. Silencing of p73 impairs the induction of S100A2 during the differentiation of HaCaT cells. Moreover, silencing of p73 or S100A2 impairs the proper expression of keratinocyte differentiation markers. Of note, p53 family members do not trigger S100A2 gene expression in response to apoptotic doses of cisplatin and doxorubicin. The p53 family is also known to be involved in the transcriptional control of growth arrest and apoptosis. Despite the recent identification of specific p73- target genes by genome-wide expression profile techniques, p73-mediated apoptosis occurs mostly through the activation of a set of genes that were originally found to be activated by p53. This suggests that promoter selectivity by both p53 and p73 might be the result of biochemical events such as post-translational modifications and specific protein-protein interactions. The transcriptional coactivator Yes-associated protein (YAP) has been demonstrated to interact with and to enhance p73-dependent apoptosis in response to DNA damage. Here we show the existence of specific target genes whose transcriptional activation during the apoptotic response requires both p73 and YAP. In particular p73 and YAP are concomitantly recruited onto the regulatory regions of the promyelocytic leukemia gene (PML); an essential event for PML induction after cisplatin treatment. Moreover, sequestring YAP into the cytoplasm by a constitutively active mutant of AKT leads to a reduction of p300 recruitment onto the PML regulatory regions which correlates with a reduction in histone acetylation and a reduction in PML expression. Finally, we show that PML binds to YAP and plays a role in the regulation of YAP half-life, preventing its ubiquitinylation and subsequent degradation.