Mcl-1 Fragment Mcl-1(128–350) Induces Inhibition of Multiple Myeloma Cell Proliferation and Apoptosis Via Both Upregulation of C-Jun as Well as Modulation of Its Transcriptional Activity

Myeloid cell leukaemia-1 (Mcl-1) is an anti- apoptotic member of the Bcl-2 family commonly expressed in multiple myeloma (MM). Drugs (e.g. bortezomib) can induce a 28kD Mcl-1 fragment (Mcl-1Δ128–350) in a caspase- dependent manner, which induces inhibition of MM cell proliferation and apoptosis. Here we sought to delineate molecular sequelae downstream of Mcl-1Δ128–350 which mediate its anti- proliferative and pro-apoptotic effects in MM and other malignant cells. Our results demonstrate that exogenous Mcl-1Δ128–350 induces upregulation and nuclear accumulation of c-Jun, as well as generation of a pro-apoptotic 60kD c-Abl fragment (c-Abl Δ). Bortezomib treatment triggered c-Jun upregulation in Mcl-1wt/wt, but not Mcl-1 null, murine embryonic fibroblasts (MEFs), and neither transfection with exogenous c-Jun nor with exogenous 60kD c-Abl Δ triggered the generation of Mcl-1Δ128–350. Moreover, drug-induced generation of Mcl-1Δ128–350 was not abrogated by specific transient knockdown of c-Jun or c-Abl by small interfering RNA, further supporting the requirement of Mcl-1Δ128–350 for c-Jun upregulation. Our studies also identified mechanisms downstream of upregulated c-Jun which trigger inhibition of MM cell proliferation and apoptosis. Interestingly and similar to c-Jun and c-Abl Δ, Mcl-1Δ128–350 accumulates within the nuclear fraction. Indeed, interaction of Mcl-1Δ128–350 with c-Jun, as well as subsequent enhanced AP-1 reporter activity, demonstrate a direct regulatory role of Mcl-1Δ128–350 in c-Jun- dependent gene transcription. Finally, gene profiles in MM cells transfected with either Mcl-1wt or Mcl-1Δ128–350 identify differentially expressed genes associated with MM cell proliferation, survival and drug resistance. Taken together, these data both delineate the role of Mcl-1 in MM pathogenesis and further support targeting Mcl-1 in novel MM treatment strategies.