Rapid Turnover of Mcl-1 Couples Translation to Cell Survival and Apoptosis

Kenneth W Adams,Geoffrey M Cooper

doi:10.1074/jbc.m610643200

Abstract

Inhibition of translation plays a role in apoptosis induced by a variety of stimuli, but the mechanism by which it promotes apoptosis has not been established. We have investigated the hypothesis that selective degradation of anti-apoptotic regulatory protein(s) is responsible for apoptosis resulting from translation inhibition. Induction of apoptosis by cycloheximide was detected within 2-4 h and blocked by proteasome inhibitors, indicating that degradation of short-lived protein(s) was required. Caspase inhibition and overexpression of Bcl-x(L) blocked cycloheximide-induced apoptosis. In addition, cycloheximide induced rapid activation of Bak and Bax, which required proteasome activity. Mcl-1 was degraded by the proteasome with a half-life of approximately 30 min following inhibition of protein synthesis, preceding Bak/Bax activation and the onset of apoptosis. Overexpression of Mcl-1 blocked apoptosis induced by cycloheximide, whereas RNA interference knockdown of Mcl-1 induced apoptosis. Knockdown of Bim and Bak, downstream targets of Mcl-1, inhibited cycloheximide-induced apoptosis, as did knockdown of Bax. Apoptosis resulting from inhibition of translation thus involves the rapid degradation of Mcl-1, leading to activation of Bim, Bak, and Bax. Because of its rapid turnover, Mcl-1 may serve as a convergence point for signals that affect global translation, coupling translation to cell survival and the apoptotic machinery.

Highlights

Signaling pathways that regulate apoptosis can directly modify Bcl-2 family proteins, as well as alter the expression of Bcl-2 family members at both the transcriptional and translational levels
Inhibition of eIF2 is mediated by four eIF2␣ kinases that are activated in response to different stress stimuli: the double-stranded RNA-activated protein kinase PKR, which is activated during viral infection; GCN2, which is activated under conditions of amino acid starvation; PERK, which is activated by accumulation of unfolded proteins in the endoplasmic reticulum; and HRI, which couples globin synthesis to heme availability in reticulocytes
We have focused on induction of apoptosis by cycloheximide to analyze the events responsible for apoptosis resulting from inhibition of protein synthesis

Summary

Introduction

Signaling pathways that regulate apoptosis can directly modify Bcl-2 family proteins, as well as alter the expression of Bcl-2 family members at both the transcriptional and translational levels. We show that translation inhibition activates the mitochondrial pathway of apoptosis because of the loss of a regulatory protein(s) via proteasome-mediated degradation and identify the anti-apoptotic Bcl-2 family member Mcl-1 as a key regulatory protein that couples cell survival to translational regulation.

Results

Conclusion