T-type voltage-activated calcium channel Cav3.1, but not Cav3.2, is involved in the inhibition of proliferation and apoptosis in MCF-7 human breast cancer cells

T-type voltage-gated Ca2+ channels have unique electrophysiological properties, suitable for generating Ca2+ oscillations and waves and thus controlling the proliferation of various tumor cells. In the present study, we investigated the role of Cav3.1, a candidate tumor suppressor gene, in neoplastic processes, and compared the differences between Cav3.1 with Cav3.2 channels. While the overexpression of a full-length Cav3.1 clone suppressed cell proliferation, the knockdown of the Cav3.1 gene by siRNA, or treatment with ProTx-I, a relatively selective inhibitor for Cav3.1, promoted the cell proliferation of MCF-7 cells (a human breast adenocarcinoma cell line). Although Cav3.1 and Cav3.2 channels possess comparable biophysical properties and are often co-expressed in various tissues, gene knockdown or the overexpression of Cav3.2 channels exhibited no effect on cell proliferation. Using immunocytochemical co-staining, the Cav3.1 channels were specifically visualized in the plasma membranes of apoptotic cells, identified by Annexin V and terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) assays and nuclear condensation. On the contrary, Cav3.2 channels were expressed at the membrane of large portions of cells, with no likely relation to Cav3.1 expression or apoptosis. An apoptosis assay revealed that the overexpression of the Cav3.1 clone caused an increase in the number of apoptotic cells. Furthermore, Cav3.1 knockdown blocked cyclophosphamide-induced apoptosis. These results suggest that Cav3.1 channels may contribute to the repression of tumor proliferation and the promotion of apoptosis mediated via Cav3.1-specific Ca2+ influx.