Abstract C28: Dexamethasone-induced apoptosis in multiple myeloma cells is restricted to t(14;16) and t(4;14) molecular subgroups due to a combined down-regulation of MAF and an up-regulation of Bim.

Multiple myeloma (MM) is heterogeneous with respect to its causative chromosomal abnormalities and the treatment response of patients. Briefly, MM patients with chromosomal hyperdiploidy or those carrying a t(11.14) translocation have a better prognosis than those with a t(4;14) or t(14;16) translocation. Dexamethasone (Dex) is widely used in all phases of the treatment of MM as induction, consolidation or maintenance. However, the mechanisms of sensitivity and resistance to Dex still remain elusive. In the present study, we analyzed the ability of Dex to induce cell death by Apo-2.7 staining in 33 human myeloma cell lines (HMCLs) representative of the molecular subsets i.e., carrying a t(11;14), t(4;14) or t(14;16) translocation, which deregulates the expression of CCND1, MMSET, and c-MAF respectively. The mechanisms controlling Dex-induced apoptosis were evaluated by analyzing the nuclear translocation of glucocorticoid receptor (GR), signalling pathways and modulation of Bcl-2 protein family in the different MM molecular subtypes. We first demonstrated by transcriptomic analysis (Affymetrix) that GR expression was heterogeneous among the different molecular subtypes of HMCLs and patients. Indeed, t(14;16) c-MAF subgroup significantly expressed higher levels of GR than all other subgroups in both HMCLs (Kruskall-Wallis p<.02) and primary MM cells from patients at diagnosis (n=309, Kruskall-Wallis p<.0001). We further demonstrated that the direct pro-apoptotic effect of Dex was related to the genetic heterogeneity of MM, since sensitive HMCLs were restricted to t(14;16) and t(4;14) subgroups. Dex-induced apoptosis was associated with caspase 6 and 3 activation. We next demonstrated that Dex sensitivity depended on its ability to induce the down-regulation of c-MAF protein, which led to a decrease in CCND2 expression, a well-known target gene of c-MAF. The down-regulation of c-MAF mainly occurred by the increase of its proteasomal degradation. Altogether, these results were in agreement with the elevated levels of c-MAF found not only in t(14;16) translocated primary cells and HMCLs but also in t(4;14) translocated cells and HMCLs. While Bim expression (all isoforms) was up-regulated in most of cell lines by Dex irrespectively of its capacity to induce apoptosis, we demonstrated by Bim silencing that its up-regulation was required for initiating Dex-induced apoptotic program. Of note, by a long-term culture in the presence of low doses of Dex, we generated a Dex-resistant t(14;16) which was characterized by a decreased expression of GR. Dex treatment of this Dex-resistant HMCL neither induced any down-regulation of c-MAF nor any up-regulation of Bim, showing that a minimal expression level of GR was the first limiting step to induce a death response. In the present study, we demonstrated that Dex exerted a direct anti-tumor effect on myeloma cells from t(14;16) and t(4;14) molecular subgroups by controlling c-MAF level while those belonging to the t(11;14) subgroup were insensitive to Dex. This result highlights the fact that conventional therapeutic approaches could be re-evaluated in concert with the definition of molecular subgroups of MM patients to favor molecular subtype-specific therapy based on rational preclinical data. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C28. Citation Format: Charlotte Kervoelen, Emmanuelle Menoret, Patricia Gomez-Bougie, Regis Bataille, Catherine Pellat-Deceunynck, Martine Amiot. Dexamethasone-induced apoptosis in multiple myeloma cells is restricted to t(14;16) and t(4;14) molecular subgroups due to a combined down-regulation of MAF and an up-regulation of Bim. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C28.