Abstract B16: The pro-apoptotic effect of dexamethasone mediated by GILZ and Bim up-regulation is related to genetic heterogeneity of multiple myeloma.
Authors: Kervoëlen Charlotte , Ménoret Emmanuelle , Gomez-Bougie Patricia , Bataille Régis , Moreau Philippe , Pellat-Deceunynck Catherine , Amiot Martine 
Published: August 2015

Multiple myeloma (MM) is heterogeneous with respect to its causative molecular abnormalities and the treatment response of patients. Briefly, MM that are hyperdiploid or have a t(11;14) translocation have a better prognosis than those with t(4;14) or t(14;16) translocations. Dexamethasone (Dex) is widely used in all phases of MM treatment as induction, consolidation or maintenance. However, the mechanism of Dex sensitivity still remains 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 carrying t(11;14), t(4;14) or t(14;16) translocations, which deregulate CCND1, MMSET and c-MAF, respectively. The mechanisms controlling Dex-induced apoptosis were evaluated by analyzing glucocorticoid receptor (GR), glucocorticoid-induced leucin zipper (GILZ) and Bcl-2 protein family expression in the different MM molecular subtypes. Transient knock-down of glucocorticoid-responsive proteins were performed to define their role in the pro-apoptotic effect of Dex.

We first demonstrated that direct pro-apoptotic effect of Dex was related to the genetic heterogeneity of MM, since sensitive cell lines were restricted to t(14;16) and t(4;14) subgroups. We next demonstrated by transcriptomic Affymetrix analysis that GR expression was heterogeneous among the different molecular subtypes of HMCLs. MAF subgroup significantly expressed higher levels of GR than all other subgroups (p=0.02). This result was also confirmed on 300 newly diagnosed MM patients (p&lt;.0001). We also demonstrated that Dex sensitivity was associated with its ability to down-regulate MAF protein level and its well-known target CCND2. This result was in agreement with the fact that elevated levels of MAF were associated not only with t(14;16) translocation but also with MMSET over-expression, which is a characteristic of t(4;14) subgroup. We next demonstrated that Dex induced an up-regulation of GILZ, a glucocorticoid-responsive molecule, in dex-sensitive and insensitive HMCLs. Nevertheless, resistant HMCLs, which expressed low levels of GR, failed to up-regulate GILZ to the same extent than sensitive HMCLs, suggesting that GR levels could be an important limiting factor for GILZ up-regulation. Of note, GILZ silencing induced a strong reduction of Dex-induced cell death. These results suggest that transactivation has a pivotal role in the induction of cell death by Dex in myeloma cells. Dex also induced an up-regulation of Bim isoforms and a down-regulation of Bcl-xL in all sensitive cell lines. We further demonstrated that Bim up-regulation is required for the apoptotic program in response to Dex. Finally, the silencing of GILZ impaired the up-regulation of Bim and the down-regulation of Bcl-xL under Dex treatment.

In conclusion, Dex exerted a direct anti-tumor effect on HMCLs of t(14;16) and t(4;14) molecular subgroups while t(11;14) subgroup was insensitive to it. This result suggested that conventional therapeutic approaches should be re-evaluated within molecular subgroups of MM patients to favor potential molecular subtype therapy based on rational preclinical data. Finally, while Dex interferes with multiple pathways, we begin to unravel its complex mechanism of action demonstrating that transactivation, inducing GILZ up-regulation, plays a pivotal role in Dex induced cell death through the regulation of the Bcl-2 protein network.

Citation Format: Charlotte Kervoëlen, Emmanuelle Ménoret, Patricia Gomez-Bougie, Régis Bataille, Philippe Moreau, Catherine Pellat-Deceunynck, Martine Amiot. The pro-apoptotic effect of dexamethasone mediated by GILZ and Bim up-regulation is related to genetic heterogeneity of multiple myeloma. [abstract]. In: Proceedings of the AACR Special Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(17 Suppl):Abstract nr B16.