Abstract P3-07-03: Targeting low voltage-activated calcium channels in HER2-positive breast cancer

Under physiological conditions low voltage-activated calcium channels (LVA, also called T-type channels) occur mostly in the brain, peripheral nervous system, smooth muscles, and pacemaker of the heart. However, LVA are often aberrantly expressed in various cancers and their overexpression is inversely correlated with outcomes. Active LVA contribute to proliferation, progression and resistance to therapy in several human cancers, including breast cancer (BC). The molecular mechanism of this activity is still not fully understood, and there is a lack of pre-clinical studies that target LVA in animal models of BC.Data from the Cancer Genome Atlas (TCGA) show a positive correlation between HER2 gene amplification and LVA channel overexpression, particularly of Cav3.1 (CACNA1G) subunit. The goal of this study is to identify functional links between HER2 receptor signaling and LVA channels, and validate LVA Ca2+ channels as a potential target for BC therapy. Inhibition of LVA, by small molecule antagonist mibefradil (MIB) or shRNA, significantly decreased the activity of PI3K/mTOR/AKT pathway in HER2-positive BC cell lines, blocked cell cycle progression, and at higher concentrations induced apoptotic cell death. Furthermore, LVA antagonist, mibefradil, sensitized cells to trastuzumab, lapatinib and paclitaxel, commonly used standard treatments for HER2-positive BC. To test the effects of LVA inhibition in vivo, the MMTV-PyMT mouse model of BC was used. In this model several features of HER2-positive BC are recapitulated, such as activation of PI3K/AKT/mTOR pathway, overexpression of HER2, BIRC5 and cyclin D1, and low expression of ERα, PGR and FOXA1. In PyMT animals palpable tumors can be detected at the age of 10-12 weeks, and 100% of animals will develop high tumor burden, including lung metastases, at the age of 15-20 weeks. Expression of CACNA1G subunit was detected in breast epithelial cells of 7 week old mice, and throughout the adult life, in both wild-type and PyMT animals, but CACNA1G was not expressed in lung or skeletal muscles. In in vitro experiments, cell lines derived from PyMT tumors were sensitive to MIB. In in vivo experiment, 5 to 7 weeks-old PyMT mice were fed either control or MIB-containing diet ad lib. Analysis of mouse serum revealed stable MIB concentration of ˜1500 ng/mL achieved within one week of treatment. In MIB-fed animals tumor appearance was delayed by 2 weeks, and the tumors were smaller as compared to control, thus suggesting a supporting role for LVA channels in breast tumor development and progression. Together, our observations provide new insights into the role of Ca2+ channels in HER2 driven BC and posit a future use of LVA channel antagonists for the treatment HER2-positive breast tumors. Citation Format: Dziegielewska B, Dunlap-Brown ME, Warnock WT, Dillon PM, Bouton AH, Dziegielewski J. Targeting low voltage-activated calcium channels in HER2-positive breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-07-03.