Abstract P5-03-02: Targeting mRNA Translation to Enhance the Radiosensitivity of Inflammatory Breast Cancer Stem Cells

Abstract

Purpose/Objective(s): Inflammatory breast cancer (IBC) is a highly aggressive and radiation resistant malignancy with a dismal prognosis despite multimodality therapy, including ionizing radiation. We have previously shown that the unique pathogenic properties of IBC result in part from over-expression of translation initiation factor eIF4G1, which is part of the eIF4F translation initiation complex, along with eIF4E and eIF4A. eIF4F is regulated by mTOR, providing a promising target for anti-cancer therapeutics. We demonstrated that protein synthesis is highly regulated during IR by the DNA-damage response (DDR) pathway through mTOR signaling. Many key proteins required for the DDR pathway are encoded by mRNAs that require high levels of the eIF4F complex and mTOR activity for their efficient translation. We hypothesized that upregulation of eIF4F in IBC plays a crucial role in the radio-resistance of disease. Materials/Methods: Experiments were conducted in IBC SUM149 cells. eIF4G1, eIF4E and eIF4A were silenced through the generation of stable cell lines that express tetracycline-inducible shRNAs. eIF4A was also inhibited using the pharmacologic investigational inhibitor DAMD-PatA. Radiation sensitivity in vitro was determined by cell survival assay. Tumor xenografts were generated by the injection of stable shRNA inducible cell lines into nude mice. IBC SUM149 cancer stem cells (CSC) from both in vitro and in vivo experiments were analyzed by a combination of cell surface marker analysis, mammosphere formation and Aldefluor assays. Results: We show that moderate inhibition by silencing of individual components (or by pharmacologic inhibition of eIF4A) of the eIF4F complex prevents IBC xenograft tumor growth and strongly enhances radiosensitivity. In contrast to results obtained for non-transformed breast epithelial cells, reducing the high levels of eIF4G1 in epithelial IBC cells in 2D cultures provides no enhancement in radiation sensitivity. Rather, SUM149 IBC cells harbor a substantial population of CSCs, which are the cells that are strongly dependent on high levels of eIF4G1, and which are selectively radio-sensitized as a result of eIF4G1-silencing. CSCs also require eIF4E and eIF4A activity in order to survive radiation treatment. We also demonstrate that silencing of eIF4G1 radio-sensitizes the stem cell population within IBC tumor xenografts. Radio-resistance of IBC cells is likely mediated by differential responses to the DDR in the stem-cell populations and by selective mRNA translation of proteins involved in the DDR pathway. Conclusions: Our results demonstrate that regulation of mRNA translation plays an important role in conferring radio-resistance to advanced breast cancers, particularly by allowing the survival of the CSC compartment. While inhibition of eIF4F enhances radiation sensitivity in non-transformed cells, this process is abrogated in IBC due to enrichment of a radiation resistant CSC population, demonstrating translational control of the breast cancer stem cell population, and providing a novel understanding of the role of the regulation of mRNA translation in radiation resistance of breast cancer. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P5-03-02.