Abstract 1734: Drug- induced generation of Mcl-1128-350 fragment triggers apoptosis in multiple myeloma cells through upregulation of c-Jun.
Authors: Fan Fengjuan , Tonon Giovanni , Sattler Martin , Goldschmidt Hartmut , Opferman Joseph , Anderson Kenneth C. , Jäger Dirk , Podar Klaus 
Published: August 2014

Abstract
Myeloid cell leukemia-1 (Mcl-1) gene, a pro-survival member of the Bcl-2 family, has been suggested to play a key role in Multiple Myeloma (MM) pathogenesis and drug resistance. Our present data demonstrate significant overexpression of Mcl-1 gene in a data set comparing samples from more than 500 MM patients with samples from healthy donors. These findings indicate Mcl-1 as a potential therapeutic target in MM. One strategy to inhibit Mcl-1 activity in MM and other malignancies is the induction of proteasome- independent Mcl-1 degradation. Indeed, our own and other previous studies demonstrate that a caspase- induced 28 kD Mcl-1 fragment, Mcl-1128-350 inhibits MM cell proliferation and survival. Importantly, our present results show that bortezomib, staurosporine and adaphostin, but not doxorubicin, thalidomide or enzastaurin induce generation of Mcl-1128-350 in MM cells. We next sought to identify molecular sequelae downstream of Mcl-1128-350, which mediate its pro-apoptotic effect. Surprisingly, we observed nuclear accumulation of drug- induced or transfected Mcl-1128-350, followed by elevated mRNA and protein levels of c-Jun as well as enhanced AP-1 reporter activity. Supporting a key role of Mcl-1128-350 in c-Jun transcription, drug- induced c-Jun upregulation was blocked after introduction of point mutations into the two highly conserved Mcl-1 caspase- cleavage sites Asp127 and Asp157. Consistent with these data, treatment with bortezomib triggered c-Jun upregulation followed by apoptosis in Mcl-1wt/wt, but not Mcl-1d/null murine embryonic fibroblasts (MEFs). Conversely, transfection of a plasmid carrying Mcl-1wt into Mcl-1d/null MEFs restored bortezomib- induced Mcl-1 fragementation, c-Jun upregulation and cell apoptosis. Importantly, Mcl-1 sequencing in several MM cell lines and primary cells didn't show the existence of point mutations in Mcl-1 caspase- cleavage sites, excluding a potential mechanism of drug resistance. In order to identify c-Jun downstream effectors of MM cell apoptosis, gene array profiling was finally performed on MM cells transfected with Mcl-1wt or Mcl-1128-350. Taken together, the present study demonstrates a promising therapeutic role of Mcl-1128-350 in MM. Moreover, our preliminary data indicate that a similar therapeutic role of Mcl-1128-350 may also exist in a variety of solid and other hematologic malignancies.
Citation Format: Fengjuan Fan, Giovanni Tonon, Martin Sattler, Hartmut Goldschmidt, Joseph Opferman, Kenneth C. Anderson, Dirk Jäger, Klaus Podar. Drug- induced generation of Mcl-1128-350 fragment triggers apoptosis in multiple myeloma cells through upregulation of c-Jun. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1734. doi:10.1158/1538-7445.AM2013-1734