Abstract 4990: PP2A signaling is overridden by MEK/ERK activity leading to suppression of Bim and protection of melanoma cells from ER stress-induced apoptosis

Endoplasmic reticulum (ER) stress triggers apoptosis by activating Bim in diverse types of cells. This involves dephosphorylation of Bim by protein phosphatase 2A (PP2A). However, melanoma cells are largely resistant to ER stress-induced apoptosis, indicating that Bim activation is suppressed in melanoma cells undergoing ER stress. We show here that this is due to increased activation of MEK/ERK signaling that overrides the PP2A activity leading to enhanced Bim phosphorylation and subsequent proteosomal degradation. Upon pharmacological ER stress, the Bim protein expression was rapidly downregulated after an initial increase, despite its sustained upregulation at the transcriptional level. Although PP2A exerted the dephosphorylating effect on BimEL, its activity appeared to be superseded by increased activation of MEK/ERK signaling that led to enchanced phosphorylation and accelerated turnover of the protein. Moreover, in contrast to its upregulation by ER stress in MCF-7 and HEK293 cells, PP2A was downregulated in melanoma cells undergoing ER stress. Pharmacological activation or enforced expression of PP2A decreased ERK and Bim phosphorylation, up-regulated Bim expression, and rendered melanoma cells sensitive to ER stress-induced apoptosis. Notably, instead of direct interaction with Bim, PP2A reduced Bim phosphorylation indirectly by dephosphorylating ERK in melanoma cells. Together, these results identify a key resistance mechanism of melanoma cells against apoptosis induced by ER stress. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4990. doi:1538-7445.AM2012-4990