Transcriptional and translational control of Mcl-1 during apoptosis

Mcl-1 is an antiapoptotic member of the Bcl-2 family whose protein and mRNA have a short half-life. In this report, we studied the changes in Mcl-1 protein and mRNA expression induced by staurosporine and aspirin. Both drugs induced apoptosis in Jurkat cells and reduced the levels of Mcl-1 protein. The caspase inhibitor Z-VAD.fmk and the proteasome inhibitor MG132 partially protected Mcl-1 from decay, indicating that both caspase-dependent and proteasome pathways are involved during apoptosis. Staurosporine also reduced Mcl-1 mRNA levels and this reduction was mostly caspase-dependent. In addition, staurosporine reduced the transcriptional activity of the Mcl-1 promoter fused to a luciferase gene reporter more than actinomycin D, a general inhibitor of transcription. Thus, we conclude that staurosporine down-regulates Mcl-1 mRNA levels by inhibiting transcription in a caspase-dependent manner and reduces Mcl-1 protein levels by a caspase-independent post-transcriptional mechanism. In contrast aspirin, at doses and times that induced loss of viability and decay of Mcl-1 protein, had no effect on Mcl-1 mRNA levels. Aspirin rapidly inhibited de novo protein synthesis before caspase activation. Moreover, the translational factor eIF2alpha was transiently phosphorylated and therefore inhibited very soon after aspirin treatment. Aspirin also inhibited the luciferase reporter activity of several attached promoter constructs, but it did not affect the luciferase activity of a construct containing an internal ribosome entry site (IRES) in its mRNA 5(')UTR. We conclude that staurosporine inhibits transcription and translation, whereas aspirin only inhibits cap-dependent translation. Treatment with cycloheximide, at doses that inhibit protein synthesis without affecting cell viability, also induced Mcl-1 protein decay. Mcl-1 disappearance might be necessary but not sufficient for the induction of apoptosis by staurosporine and aspirin. A model for the control of Mcl-1 during drug-induced apoptosis is presented.