Activation of ATF4 mediates the unwanted Mcl-1accumulation by proteasome inhibition.

Myeloid cell leukemia-1 (Mcl-1) protein is an anti-apoptotic Bcl-2 family protein that plays essential roles in multiple myeloma (MM) survival and drug resistance. In MM, it has been demonstrated that proteasome inhibition can trigger the accumulation of Mcl-1, which has been shown to confer MM cells resistance to bortezomib induced lethality. However, the mechanisms involved in this unwanted Mcl-1 accumulation are still unclear. The aim of the current study was to test whether the unwanted Mcl-1 accumulation could be induced by the unfolded protein response (UPR), and to elucidate the role of ER stress response in regulating Mcl-1 expression. By using qRT-PCR and Western blot, we found that the translation of activating transcription factor-4 (ATF4), an important effector of UPR, was also greatly enhanced by proteasome inhibition. Chromatin immunoprecipitation (ChIP) further revealed that bortezomib stimulated binding of ATF4 to a regulatory site (at position -332 to -324) at the promoter of Mcl-1 gene. Knocking down ATF4 was paralleled by down-regulation of Mcl-1 induction by bortezomib. Furthermore, knocking down ATF4 significantly increased the bortezomib induced apoptosis. Taken together, these data identify UPR and more specifically its ATF4 branch as an important mechanism mediating up-regulation of Mcl-1 by proteasome inhibition.