Activation of ERK1/2 by ΔRaf-1 : ER* represses Bim expression independently of the JNK or PI3K pathways

Abstract

CC139 fibroblasts are one of several model systems in which the RafMEKERK1/2 pathway can inhibit apoptosis independently of the PI3K pathway; however, the precise mechanism for this protective effect is not known. Serum withdrawal from CC139 fibroblasts resulted in the rapid onset of apoptosis, which was prevented by actinomycin D or cycloheximide. Serum withdrawal promoted the rapid, de novo accumulation of Bim_EL, a proapoptotic 'BH3-only' member of the Bcl-2 protein family. Bim_EL expression was an early event, occurring several hours prior to caspase activation. In contrast to studies in neurons, activation of the JNKc-Jun pathway was neither necessary nor sufficient to induce Bim_EL expression. Selective inhibition of either the ERK pathway (with U0126) or the PI3K pathway (with LY294002) caused an increase in the expression of Bim_EL. Furthermore, selective activation of the ERK1/2 pathway by Raf-1:ER* substantially reduced Bim_EL expression, abolished conformational changes in Bax and blocked the appearance of apoptotic cells. The ability of Raf-1:ER* to repress Bim_EL expression required the ERK pathway but was independent of the PI3KPDKPKB pathway. Thus, serum withdrawal-induced expression of Bim_EL occurs independently of the JNKc-Jun pathway and can be repressed by the ERK pathway independently of the PI3K pathway. This may contribute to Raf- and Ras-induced cell survival at low serum concentrations.