Abstract 1268: Anoikis evasion in inflammatory breast cancer cells is mediated by Bim-EL sequestration

Abstract

Abstract Inflammatory breast cancer (IBC) is a rare and highly invasive type of breast cancer, and patients diagnosed with IBC often face a very poor prognosis. IBC is characterized by the lack of primary tumor formation and the rapid accumulation of cancerous epithelial cells in the dermal lymphatic vessels. Given that normal epithelial cells require attachment to the extracellular matrix (ECM) for survival, a comprehensive examination of the molecular mechanisms underlying IBC cell survival in the lymphatic vessels is of paramount importance to our understanding of IBC pathogenesis. Due to the inherent invasive nature of IBC cells, we hypothesized that IBC cells could evade detachment-induced apoptosis (anoikis), and rely on tightly regulated intracellular signaling pathways to do so. Here we demonstrate that in contrast to normal mammary epithelial cells, IBC cells robustly survive in ECM-detached conditions. ErbB2 and EGFR knockdown in KPL-4 and SUM149 cells, respectively, causes decreased colony growth in soft agar and increased caspase activation following ECM detachment, suggesting overexpression/hyperactivation of these oncogenes is vital for IBC evasion of anoikis. Downstream of ErbB2 and EGFR, ERK/MAPK signaling was found to protect cells from anoikis by phosphorylating Bim-EL at serine 59. This phosphorylation of Bim-EL results in a physical interaction between Bim-EL, LC8, and Beclin-1 and prevents Bim-EL from localizing to the mitochondria and promoting cell death. Mutation of the serine 59 site results in increased cell death and a diminished capacity to form a protein complex with LC8 and Beclin-1. Bim-EL knockdown also rescued ECM-detachment-induced death in IBC cells, suggesting Bim-EL is necessary for the induction of anoikis in IBC cells. Interestingly, Bim-EL levels were found to be elevated in ECM-detached IBC cells and present in a significant percentage of IBC patient samples, despite their evasion of anoikis. Therefore, the sequestration of Bim-EL away the mitochondria provides a compelling rationale for how IBC cells survive in the presence of high Bim-EL levels. Furthermore, this mechanism provides a unique and novel opportunity for chemotherapeutic approaches aimed at freeing the already elevated Bim-EL levels from sequestration to specifically induce cell death in IBC cells. Citation Format: Cassandra Buchheit, Brittany Angarola, Allison Steiner, Kelsey Weigel, Zachary Schafer. Anoikis evasion in inflammatory breast cancer cells is mediated by Bim-EL sequestration. [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 1268. doi:10.1158/1538-7445.AM2015-1268