Abstract 1092: miR-603 acts as a tumor suppressor in triple-negative breast cancer and inhibits cell proliferation, invasiveness and tumorigenesis by targeting elongation factor 2-kinase (eEF2K)

Triple Negative Breast Cancer (TNBC) is characterized by heterogeneous disease due to its unique molecular profile, aggressiveness, distinct metastatic patterns and lack of molecular targets such as ER, PR and HER2/neu for effective targeted therapies including tamoxifen and herceptin reduced sensitivity to chemotherapies, representing unmet therapeutic challenge. Eukaryotic elongation factor-2 kinase (eEF2K) is an atypical calcium/calmodulin dependent serine/tyrosine kinase that regulates protein synthesis through phosphorylation of EF2. Recently, we reported that eEF2K promotes TNBC cell proliferation, colony formation, migration/invasion, angiogeneis and tumorigenesis and drug resistance by inducing PI3K/Akt, mTOR, IGF1R, Src/FAK, C-myc,, Bcl-2, CyclinD1, and VEGF (Tekedereli 2012). We also demonstrated that in vivo therapeutic targeting of eEF2K by systemically administered liposomal eEF2KsiRNA nanoparticles significantly inhibited growth of highly aggressive MDA-MB-231 tumor xenografts in and enhanced the efficacy of standard chemoagents (ie, doxorubicin, paclitaxel), representing a potential molecular target in TNBC. However, currently the transcriptional regulation of eEF2K is unknown. Here, we report that eEF2K expression is highly upregulated in TNBC cells and associated with poor patient survival and clinical outcome in these patients.. Thorough extensive analysis of databases and predictive algorithms, and in vitro and in vivo correlation studies we identified that miR-603 inversely correlated with eEF2K and demonstrated miR603 as the first microRNA that directly binds to the 3’-UTR of eEF2K gene and suppresses its expression, though gene reporter assays. Introduction of mutations to the miR-603 binding site completely reversed eEF2K expression in gene reporter assays. We also showed that ectopic expression of miR-603 in TNBC cell lines resulted in the inhibition of cell growth, colony formation, migration and invasion of TNBC cells, recapitulating the effects of eEF2K inhibition by siRNA. Systemic administration of miR-603 mimics incorporated in liposomal nanoparticles (0.3 mg/kg once a week for 4 weeks) in an orthotopic xenograft TNBC mouse model led to significant inhibition of eEF2K protein expression and tumor growth. Analysis of the tumors after the end of study reveled that miR-603-induced downregulation of eEF2K expression was correlated with Src and Akt. In conclusion, our findings suggest for the first time that miR-603 function as a tumor suppressor and loss of its expression contributes to upregulation of eEF2K, tumor growth and progression of TNBC and miR-603 based gene targeting strategy may be used to control eEF2K in TNBC, representing a novel molecularly targeted therapy. Citation Format: Recep Bayraktar, Martin Pichler Pichler, Cristina Ivan, George Calin, Bulent Ozpolat. miR-603 acts as a tumor suppressor in triple-negative breast cancer and inhibits cell proliferation, invasiveness and tumorigenesis by targeting elongation factor 2-kinase (eEF2K). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1092.