Investigational Agent MLN9708 Target Tumor Suppressor MicroRNA-33b in Multiple Myeloma Cells
Authors: Tian Ze , Zhao Jian-Jun , Lin Jianhong , Chauhan Dharminder , Anderson Kenneth C. 
Published: October 2019

Abstract
Abstract 136
Investigational Agent MLN9708 Target Tumor Suppressor MicroRNA-33b in Multiple Myeloma Cells
Ze Tian, Jianjun Zhao, Jianhong Lin, Dharminder Chauhan, Kenneth C. Anderson
Medical Oncology, Dana Farber Cancer Institute and Harvard Medical School, Boston, MA, 02115
MicroRNAs (miRNAs) are 19–25 nucleotide-long noncoding RNA molecules that regulate gene expression both at the level of messenger RNA degradation and translation. Emerging evidence shows that miRNAs play a critical role in tumor pathogenesis by functioning as either oncogene or tumor suppressor genes. The role of miRNA and their regulation in response to proteasome inhibitors treatment in Multiple Myeloma (MM) is unclear. Here, we utilized MLN9708, a selective orally bio-available proteasome inhibitor to examine its effects on miRNA alterations in MM.1S MM cells. Upon exposure to aqueous solutions or plasma, MLN9708 rapidly hydrolyzes to its biologically active form MLN2238. Our previous study using both in vitro and in vivo models showed that MLN2238 inhibits tumor growth and triggers apoptosis via activation of caspases. Moreover, MLN2238 triggered apoptosis in bortezomib-resistant MM cells, and induced synergistic anti-MM activity when combined with HDAC inhibitor SAHA, dexamethasone, and lenalidomide. In the current study, we treated MM.1S cells with MLN2238 (12 nM) for 3 hours and harvested; total RNA was subjected to miRNA profiling using TaqMan® Array Human miRNA A-Card Set v3.0 and the data was analyzed using dChip analysis. Results showed that MLN2238 modulates miRNA expression with a total of 36 miRNA changing their expression profiling (δδCT>1.5 or δδCT <-1.5; 19 were upregulated and 17 showed a downregulation). Among all miRNA, miR-33b was highly (δδCT>7) upregulated in response to MLN2238 treatment. We therefore hypothesized that miR-33b may play a role in MM pathogenesis as well as during MLN2238-induced proteasome inhibition in MM cells. We first utilized quantitative polymerase chain reaction (q-PCR) to validate the changes in miRNA expression profiling. Results confirmed that MLN2238 treatment triggers significant increase in the miR-33b expression in MM.1S cells (2.1 and 2.2 folds at 3h and 6h, respectively; P<0.001). Examination of normal PBMCs and plasma cells showed higher expression of miR-33b than patient MM cells (P<0.001). We further investigated the functional role of miR-33b in MM cells at baseline and during MLN2238 treatment. Drug sensitivity, cell viability, apoptosis, colony formation, and migration assays were performed using cell TilTer-Glo, Annexin V-FITC/PI staining, MTT staining, and Transwell assays, respectively. Signaling pathways modulated post miR-33b overexpression were evaluated by q-PCR, immunoblot, and reporter assays. Our findings show that overexpression of miR-33b significantly decreased cell viability, cell migration, colony formation, as well as increased apoptosis and sensitivity of MM cells to MLN2238 treatment. Targetscan analysis predicted pim-1 as a putative downstream target of miR-33b. Overexpression of miR-33b downregulated pim-1 mRNA and protein expression.
To further corroborate these data, we co-tranfected miR-33b and Pim-1-wt or Pim-1-mt in 293T and MM.1S cell lines. In concert with our earlier findings, miR-33b decreases pim-1-wt, but not pim-1-mt reporter activity in both cell lines. Reflecting the overexpression study results, MLN2238 treatment also decreases pim-1-wt, but not pim1-mt reporter activity. Moreover, a biochemical inhibitor of pim1/2 triggered apoptosis in MM cells. Finally, overexpression of miR-33b inhibits tumor growth (P<0.001) and prolongs survival (P<0.001) in both subcutaneous and disseminated human MM xenograft models. In summary, our study suggests that miR-33b is a tumor suppressor, which plays a role during MLN2238-induced apoptotic signaling in MM cells, and provide the basis for novel therapeutic strategies targeting miR-33b in MM.

Disclosures:
Anderson: Millennium: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees; Acetylon: Equity Ownership.