Activation of BKCa Channels in Zoledronic Acid-Induced Apoptosis of MDA-MB-231 Breast Cancer Cells

Background: Zoledronic acid, one of the most potent nitrogen-containing biphosphonates, has been demonstrated to have direct anti-tumor and anti-metastatic properties in breast cancer in vitro and in vivo. In particular, tumor-cell apoptosis has been recognized to play an important role in the treatment of metastatic breast cancer with zoledronic acid. However, the precise mechanisms remain less clear. In the present study, we investigated the specific role of large conductance Ca2+-activated potassium (BKCa) channel in zoledronic acid-induced apoptosis of estrogen receptor (ER)-negative MDA-MB-231 breast cancer cells. Methodology/Principal Findings: The action of zoledronic acid on BKCa channel was investigated by whole-cell and cell-attached patch clamp techniques. Cell apoptosis was assessed with immunocytochemistry, analysis of fragmented DNA by agarose gel electrophoresis, and flow cytometry assays. Cell proliferation was investigated by MTT test and immunocytochemistry. In addition, such findings were further confirmed with human embryonic kidney 293 (HEK293) cells which were transfected with functional BKCa alpha-subunit (hSlo alpha). Our results clearly indicated that zoledronic acid directly increased the activities of BKCa channels, and then activation of BKCa channel by zoledronic acid contributed to induce apoptosis in MDA-MB-231 cells. The possible mechanisms were associated with the elevated level of intracellular Ca2+ and a concomitant depolarization of mitochondrial membrane potential (Delta psi m) in MDA-MB-231 cells. Conclusions: Activation of BKCa channel was here shown to be a novel molecular pathway involved in zoledronic acid-induced apoptosis of MDA-MB-231 cells in vitro.