EGFR-dependent downregulation of Bim in epithelial cells requires MAPK and PKC-delta activities

Activation of the epidermal growth factor receptor (EGFR) provides a measure of protection to immortalized epidermal keratinocytes (HaCaT cells) against apoptosis induced by diverse cellular stressors. This effect is due, in part, to sustained MAPK-dependent Bcl-xL expression. Here, we report a second EGFR/MAPK-dependent signaling event that protects HaCaT cells against apoptosis incurred during forced suspension culture (anoikis). This pathway targets Bim, a pro-apoptotic BH3-only Bcl-2 family member. Bim expression was functionally relevant to HaCaT cell survival as demonstrated by partial protection against anoikis provided by siRNA-induced Bim downregulation. Growth factor starvation of attached and suspended cells was associated with enhanced Bim expression whereas EGFR activation reduced Bim expression by inducing Bim phosphorylation and proteasomal degradation. EGFR-dependent Bim phosphorylation required MAPK activation. Furthermore, PKC-delta activity contributed to both MEK/MAPK phosphorylation and Bim phosphorylation as demonstrated using both pharmacological inhibitors of PKC-delta and siRNA-mediated PKC-delta knockdown. In addition to HaCaT cells, EGFR activation supported survival and induced Bim phosphorylation in several squamous carcinoma cell lines in a strictly MAPK-dependent fashion. These results establish that EGFR activation attenuates susceptibility of immortalized and malignant keratinocytes to apoptosis by post-translational control of Bim-EL expression through a pathway requiring PKC-delta and MEK/MAPK activation.