Role of NF-κB in the apoptotic-resistant phenotype of keratinocytes

Abstract

Several studies point to a role for NF-κB in modulating epidermal thickness and apoptotic susceptibility of keratinocytes. When phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate (TPA) are topically applied, prominent epidermal thickening occurs, and exposure to interferon (IFN)-γ promotes increased epidermal thickness producing psoriatic lesions. While keratinocytes derived from psoriatic plaque resist apoptosis, and combination of TPA and IFN-γ activates NF-κB, the molecular mechanism linking NF-κB activation and keratinocyte apoptosis resistance was unknown. Therefore, we examined the ability of IFN-γ plus TPA to influence NF-κB activity, gene expression, and response to UV light-induced apoptosis. These responses in normal keratinocytes were compared with immortalized keratinocytes (HaCaT cells). Exposure of normal keratinocytes to IFN-γ plus TPA produced a synergistic activation of NF-κB, compared with when each reagent was used individually. Normal keratinocytes when exposed to IFN-γ plus TPA acquired a resistance to UV light-induced apoptosis, which was dependent on NF-κB because expression of a dominant negative form of IκBα overcame the resistance. Compared with normal keratinocytes, HaCaT cells have a dysfunctional constitutive NF-κB signaling pathway not induced by IFN-γ and TPA, rendering HaCaT cells highly susceptible to UV-induced apoptosis. Thus, immortalized HaCaT cells have an abnormal constitutive and dysfunctional NF- κB signaling system. These results provide evidence that activation and proper regulation of NF-κB is essential for acquisition of an apoptotic- resistant phenotype for epidermal-derived keratinocytes.