Altering bioelectricity on inhibition of human breast cancer cells

Abstract

Background: Membrane depolarization is associated with breast cancer. Depolarization-activated voltage-gated ion channels are directly implicated in the initiation, proliferation, and metastasis of breast cancer. Methods: In this study, the role of voltage-gated potassium and calcium ion channel modulation was explored in two different invasive ductal human carcinoma cell lines, MDA-MB-231 (triple-negative) and MCF7 (estrogen-receptor-positive). Results: Resting membrane potential is more depolarized in MCF7 and MDA-MB-231 cells compared to normal human mammary epithelial cells. Increasing extracellular potassium concentration up to 50mM depolarized membrane potential and greatly increased cell growth. Tetraethylammonium (TEA), a non-specific blocker of voltage-gated potassium channels, stimulated growth of MCF7 cells (control group grew by 201%, 1mM TEA group grew 376%). Depolarization-induced calcium influx was hypothesized as a requirement for growth of human breast cancer. Removing calcium from culture medium stopped growth of MDA and MCF7 cells, leading to cell death after 1week. Verapamil, a blocker of voltage-gated calcium channels clinically used in treating hypertension and coronary disease, inhibited growth of MDA cells at low concentration (10-20μM) by 73 and 92% after 1 and 2days, respectively. At high concentration (100μM), verapamil killed >90% of MDA and MCF7 cells after 1day. Immunoblotting experiments demonstrated that an increased expression of caspase-3, critical in apoptosis signaling, positively correlated with verapamil concentration in MDA cells. In MCF7, caspase-9 expression is increased in response to verapamil. Conclusions: Our results support our hypotheses that membrane depolarization and depolarization-induced calcium influx stimulate proliferation of human breast cancer cells, independently of cancer subtypes. The underlying mechanism of verapamil-induced cell death involves different caspases in MCF7 and MDA-MB-231. These data suggest that voltage-gated potassium and calcium channels may be putative targets for pharmaceutical remediation in human invasive ductal carcinomas.