Mcl-1 Sequesters Pro-Apoptotic BH3-Only Bim To Inhibit Its Collaboration with Bax in Myeloma Cells.

Multiple Myeloma is a fatal malignancy of B-cell origin characterized by the accumulation of plasma cells within the bone marrow. IL-6 is a major myeloma cell survival factor that induces the activation of both, Ras/MAP kinase and JAK/STAT pathways, the latter being involved in cell survival. Thus, IL-6 starvation or IL-6 signaling blockade by AG490 (a JAK 2 inhibitor) trigger apoptosis. In myeloma cells, others and us have shown that Mcl-1 is the major anti-apoptotic protein, since Mcl-1 antisenses induce apoptosis while Bcl-2 or Bcl-xL antisenses have no effect. In the present study, we examined the pro-apoptotic BH3-only molecule Bim which is implicated in apoptosis induced by growth factor deprivation. The three major Bim isoforms (EL, L and S) are expressed in viable human myeloma cell lines and are negatively regulated by IL-6. Blockade of IL-6 signaling induces the up-regulation of Bim isoforms concomitant to the down-regulation of Mcl-1. Of major interest, in viable myeloma cells, Bim is strongly associated to Mcl-1, to Bcl-2 and to a lesser extent to the dynein light chain, suggesting that Bim is always complexed in viable cells. Bim/Mcl-1 endogenous interaction is disrupted upon apoptosis induction, either by IL-6 starvation or AG 490 treatment. Of note, Bim/Bcl-2 complex is not affected under the same conditions. In parallel to disruption of Bim/Mcl-1 interaction and Bim release, Bax activation was observed. Microinjection of a peptide comprising the minimal BH3 domain from Bim triggered apoptosis in myeloma cells, confirming that free Bim might exert a direct pro-apoptotic effect in these cells. Therefore, in myeloma cells Mcl-1 neutralises Bim through endogenous complex formation. Under apoptosis induction, Mcl-1 drastic down-regulation leads to Bim release, which in turn exerts its pro-apoptotic function through Bax activation.