| The p53 tumor suppressor is the most commonly inactivated gene in human cancers. The ability of p53, a transcription factor, to regulate genes that mediate cell cycle arrest, apoptosis, DNA repair, and other p53-dependent activities is well known. However, the mechanisms by which p53 induces a specific activity over another are unclear. Here, I have investigated mechanisms of differential target gene regulation by p53 family proteins and have found that stringent regulation of and by p53 family proteins, the characteristics of the target gene promoter, and histone deacetylases (HDACs) all play important roles in this process. Through the use of engineered mutants, I found that the p53 N-terminal activation domain 1 (AD1) and C-terminal basic domain (BD) inhibited the ability of p53 to induce Insulin-like Growth Factor Binding Protein 3 (IGFBP3), a proapoptotic p53 target gene. p53(DeltaNDeltaBD), a natural occurring p53 form in which the inhibitory functional domains are deleted, was able to activate the IGFBP3 promoter. Similar to p53, the C termini of p63 and p73 were also inhibitory. The regulation of IGFBP3 by C-terminally truncated p53 family proteins was dependent upon the unique organization of the p53-responsive element half-sites within the IGFBP3 promoter. Furthermore, I found that HDAC activity, and not p53-DNA binding activity, governed the regulation of IGFBP3 by full-length p53 family proteins, since inhibition of HDACs restored the induction of IGFBP3 by exogenous full-length p53, p63, and p73 proteins. To determine if HDAC1 or HDAC2 was the HDAC involved, I generated stable cell lines which inducibly knockdown HDAC1 or HDAC2. Although knockdown of HDAC1 or HDAC2 did not affect the regulation of IGFBP3 by p53, I found that HDAC2 plays novel roles in the p53 pathway. Importantly, knockdown of HDAC2 inhibited MCF7 cell proliferation in a dose-dependent manner, induced cellular senescence, and inhibited p53 transcriptional activities through modulation of p53-DNA binding activity. In sum, I have found that HDACs facilitate the differential regulation of the pro-apoptotic target gene Insulin-like Growth Factor Binding Protein 3 (IGFBP3) by p53 family isoforms and that HDAC2 impacts the transcriptional activities of p53 by modulation of p53-DNA binding activity. |
