p38 protects human melanoma cells from UV-induced apoptosis through down-regulation of NF-kappaB activity and Fas expression.

Identifying mechanisms that underlie the resistance of human melanoma to radiation and chemotherapy is expected to assist in developing new strategies for the treatment of this tumor type. We recently demonstrated that through up-regulation of TNFalpha, ATF2 increases the resistance of late stage melanoma cells to apoptosis induced by UV-irradiation. In elucidating the role of ATF2 kinases, we now demonstrate that ASK1/MKK6/p38 elicits suppression of Fas expression. ASK1/p38 downregulates the expression of a Fas via NF-kappaB/SP1 site on the Fas promoter. Deletion or mutation of NF-kappaB/SP1 within the Fas promoter abrogates p38 effect. ASK1/p38 silences the Fas promoter by inhibition of IkappaBalpha phosphorylation - thereby limiting NF-kappaB activity. Forced expression of a dominant negative form of p38 (p38-ASP) or treatment with p38 pharmacological inhibitor, SB203580, increases NF-kappaB activity, Fas expression and the levels of UVC-induced apoptosis in late stage melanoma cells. Inhibition of p38 activity also restored NF-kappaB activity and Fas expression in early-phase melanoma cells, suggesting that p38 elicited suppression of Fas expression is not restricted to late phase melanoma. Identifying p38-mediated down-regulation of Fas expression illustrates a novel regulatory pathway by which ASK1/MKK6/p38 alters the degree and nature of the UV-induced apoptosis of melanoma cells. Oncogene (2000).