Knock down of p53 levels in human keratinocytes increases susceptibility to type I and type II interferon-induced apoptosis mediated by a TRAIL dependent pathway

Summary Background Keratinocytes (KCs) in healthy skin only undergo death following differentiation to produce stratum corneum. By contrast, in inflammatory pathological conditions featuring type I (IFN-α) and type II (IFN-γ) interferons KCs undergo premature apoptosis. Objective To define apoptotic susceptibility of KCs, response to interferons was examined. Since molecular cross-talk occurs between interferons and p53, potential mechanistic roles for p53 in KC apoptosis were investigated. Methods Knock down of p53 was performed, and apoptotic response to addition of interferons was assessed using FACS and by staining for activated caspase 3 and TUNEL. Elucidation of death pathway was accomplished by using a dominant negative death receptor construct and a neutralizing TRAIL antibody. Results Reduction in p53 levels in KCs by siRNA treatment enhanced, rather than reduced, apoptotic responses to IFN-α plus IFN-γ. In an immortalized human KC cell line (HaCaT cells with both p53 alleles mutated) enhanced apoptotic susceptibility to interferon exposure was also observed. The mechanism for this enhanced apoptosis involved induction of TRAIL and its interaction with death receptors, as blocking the death receptor pathway using dominant negative FADD, or by addition of neutralizing antibody against TRAIL, reduced the apoptotic response to IFN-α and IFN-γ. Conclusion These results indicate IFN-α plus IFN-γ triggers apoptosis independent of p53 in HaCaT cells, and also demonstrate an unexpected survival role for p53 in human KCs as regards apoptotic responsiveness to cytokines such as IFN-α and IFN-γ involving activation of TRAIL-related death receptors. Strategies enhancing p53 regulated survival proteins in KCs may be of therapeutic benefit in skin disorders characterized by activated immunocytes triggering premature KC apoptosis.