Abstract 2248: Mechanism of action of RPL39 in breast cancer initiation and metastasis

Abstract

Abstract Introduction: Triple negative breast cancer (TNBC) is the most aggressive and lethal form of cancer with no targeted therapy due to absence of the expression of ER, PR and HER. As such, treatment of patients with TNBC has been challenging due to heterogeneity of the disease and lack of well-defined molecular targets.. New targeted therapies are therefore an urgent unmet medical need for this patient population. In order to identify novel targets and better understand the mechanism of survival of these cancers, we identified a gene signature for breast cancer stem cells (BCSC) derived from patient biopsies. In our recently published study, we describe the ribosomal protein like 39 (RPL39) as one potentially useful target which was found to be responsible for therapy resistance and BCSC self-renewal. RPL39 is a specific protein over-expressed in breast cancer and is regulated by nitric oxide signaling (NOS). Knockdown of RPL39 in human-in-mouse patient derived tumor xenografts that mimic human TNBC showed reduced tumor volume and lung metastases with concomitant decrease in BCSC markers. In the present study we examine the detailed mechanism of action of RPL39 in promoting tumorigenesis and metastasis. Our study was designed to identify the mechanism by which RPL39 affects NOS signaling leading to its previously reported effects on self renewal, migration and metastasis. Methods and Results: Mass spectrometric analysis following immunoprecipiation of RPL39, in two breast cancer cell lines SUM159 and MDAMB231 indicated that proteins involved in translation, specifically the mTOR dependent translational pathway, were part of the complex. Overexpression of RPL39 in breast cancer cell lines (SUM159, MDAMB231, and MDAMB436) demonstrated an increase in EIF4G, p4EBP1 which are components of the selective translation initiation complex. In order to verify that the results we observed were due to targeting of the translational arm of mTOR signaling pathway, we knocked down both the 4EBP-1 (translation) and S6K (cell size) arms of the mTOR pathway in three breast cancer cell lines (SUM159, MDAMB231 and MDAMB436). Analysis of these pathways by proliferation, apoptosis, sphere formation and western blotting in all three breast cancer cell lines demonstrated that the translation arm was significantly altered: self-renewal (p<0.05, student's t test) and proliferation (p<0.05, student's t test) in a NOS signaling dependent manner. Additional analyses of breast cancer cell lines overexpressing RPL39 (SUM159, MDAMB231 and MDAMB436) demonstrated that RPL39 affected these components of the selective translation initiation complex through a mismatch repair pathway. Conclusion: RPL39 gene is modulated by the selective translational initiation complex which affects self renewal and proliferation through NOS signaling. The significance of targeting of RPL39 and/or NOS pathway allows a potentially novel therapeutic strategy for the treatment of breast cancer. Citation Format: Bhuvanesh Dave, Jenny Chang. Mechanism of action of RPL39 in breast cancer initiation and metastasis. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2248. doi:10.1158/1538-7445.AM2015-2248