p16^INK4A enhances the transcriptional and the apoptotic functions of p53 through DNA-dependent interaction

p16^INK4A and p53 are two important tumor suppressor proteins that play essential roles during cell proliferation and aging through regulating the expression of several genes. Here, we report that p16^INK4A and p53 co-regulate a plethora of transcripts. Furthermore, both proteins colocalize in the nucleus of human primary skin fibroblasts and breast luminal cells, and form a heteromer whose level increases in response to genotoxic stress as well as aging of human fibroblasts and various mouse organs. CDK4 is also present in this heteromeric complex, which is formed only in the presence of DNA both in vitro using pure recombinant proteins and in vivo. We have also shown that p16^INK4A enhances the binding efficiency of p53 to its cognate sequence presents in the CDKN1A promoter in vitro, and both proteins are present at the promoters of CDKN1A and BAX in vivo. Importantly, the fourth ankyrin repeat of p16^INK4A and the C-terminal domain of p53 were necessary for the physical association between these two proteins. The physiologic importance of this association was revealed by the inability of cancer-associated p16^INK4A mutants to interact with p53 and to transactivate the expression of its major targets CDKN1A and BAX in the p16-defective U2OS cells expressing either wild-type or mutated p16^INK4A . Furthermore, the association between p16^INK4A and p53 was capital for their nuclear colocalization, the X-ray-dependent induction of p21 and Bax proteins as well as the induction of apoptosis in various types of cells. Together, these results show DNA-dependent physical interaction between p16^INK4A and p53.