Resistance to UV-induced apoptosis in human keratinocytes during accelerated senescence is associated with functional inactivation of p53

Abstract

Compared to proliferating keratinocytes (KCs), growth-arrested KCs are relatively resistant to UV-light induced apoptosis. When KCs undergo confluency, or following exposure to anti-proliferative agents such as IFN-gamma plus a phorbol ester-12-O-tetradecanoylyphorbol-13-acetate (TPA), they convert from a proliferative to a nonproliferative state resembling senescence. Since p53 regulates UV-induced apoptosis of KCs, this report further characterizes p53 half-life, post-translational modifications, and transcriptional activity using cultured human KCs and living epidermal equivalents. The half-life of p53 in KCs was longer than fibroblasts (greater than approximately 3 h vs. 30 min). Exposure of proliferating KCs to UV-light induces post-translational modifications of p53 including acetylation of lysine-382 residues. By contrast, KCs undergoing irreversible growth arrest following confluency, or exposure to IFN-gamma plus TPA, were resistant to UV-induced apoptosis, and failed to undergo the acetylation modification of p53. Exposure of KCs to IFN-gamma plus TPA reduced total cellular p53 levels and reduced the transcriptional activity of p53. Addition of Trichostatin A (TSA), an inhibitor of de-acetylation, increased acetylation of lysine-382 in confluent KCs, thereby enhancing susceptibility of confluent cultures to UV-induced apoptosis. Pre-treatment of epidermal equivalents with IFN-gamma plus TPA also blocked UV-light induced increase in p53 levels, and reduced apoptosis. In conclusion, these studies demonstrate that growth arrested KCs may resist UV-light induced apoptosis by inactivating the pro-apoptotic function of p53.