Quadros Marlene R, Connelly Sharon, Kari Csaba, Abrams Marc T, Wickstrom Eric, Rodeck Ulrich
Department of Dermatology and Cutaneous Biology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA.
Cancer Biol Ther. 2006 May;5(5):498-504. doi: 10.4161/cbt.5.5.2567. Epub 2006 May 6.
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.
表皮生长因子受体(EGFR)的激活为永生化表皮角质形成细胞(HaCaT细胞)提供了一定程度的保护,使其免受多种细胞应激源诱导的细胞凋亡。这种效应部分归因于持续的丝裂原活化蛋白激酶(MAPK)依赖性Bcl-xL表达。在此,我们报告了第二种EGFR/MAPK依赖性信号转导事件,该事件可保护HaCaT细胞免受强制悬浮培养(失巢凋亡)过程中发生的细胞凋亡。该途径作用于Bim,一种仅含BH3结构域的促凋亡Bcl-2家族成员。Bim表达与HaCaT细胞存活功能相关,siRNA诱导的Bim下调对失巢凋亡提供的部分保护证明了这一点。贴壁和悬浮细胞的生长因子饥饿与Bim表达增强相关,而EGFR激活通过诱导Bim磷酸化和蛋白酶体降解降低Bim表达。EGFR依赖性Bim磷酸化需要MAPK激活。此外,使用PKC-δ的药理学抑制剂和siRNA介导的PKC-δ敲低均证明,PKC-δ活性对MEK/MAPK磷酸化和Bim磷酸化均有贡献。除了HaCaT细胞外,EGFR激活还以严格的MAPK依赖性方式支持几种鳞状癌细胞系的存活并诱导Bim磷酸化。这些结果表明,EGFR激活通过一种需要PKC-δ和MEK/MAPK激活的途径,通过对Bim-EL表达的翻译后控制,减弱了永生化和恶性角质形成细胞对细胞凋亡的敏感性。