The Role of Ca²⁺ in Baicalein-induced Apoptosis in Human Breast MDA-MB-231 Cancer Cells through Mitochondria- and Caspase-3-dependent Pathway

Abstract

Baicalein was investigated for tumor cell-specific cytotoxicity, apoptosis-inducing activity and signal pathway against the MDA-MB-231 human breast cancer cell line. After the MDA-MB-231 cells had been treated with baicalein, trypan blue exclusion, propidium iodide (PI) assay and 4′,6-diamidino-2-phenylindole (DAPI) were used to stain the dead cells and detect apoptosis, respectively. The effects of baicalein on the levels of reactive oxygen species (ROS), Ca²⁺ and mitochondrial membrane potential (ΔΨ_m) on MDA-MB-231 cells were examined by flow cytometric assays. The ROS caused endoplasmic reticulum (ER) stress, confirmed by the increase of GADD153 and GRP78 in the examined cells. GADD153 and GRP78 increases were also confirmed by confocal laser microscopy examination and indicated that both proteins translocated to the nucleus. The effects of baicalein on the expression of apoptotic-regulated genes, such as Bcl-2 family and caspase, were detected by Western blotting. To further investigate the apoptotic pathway and the role of Ca²⁺ induced by baicalein, a caspase-3 inhibitor and Ca²⁺ chelator were used to block caspase-3 activity and Ca²⁺ in MDA-MB-231 cells. Baicalein induced apoptosis in a time-dependent effect through the inhibition of Bcl-2 expression, increased the levels of Bax, reduced the level of ΔΨ_m, and promoted the cytochrome c release and caspase-3 activation. MDA-MB- 231 cells were pretreated with BAPTA which reduced the levels of Ca²⁺, ΔΨ_m and apoptosis. In conclusion, baicalein induced apoptosis via Ca²⁺ production, mitochondria-dependent and caspase-3 activation in MDA-MB-231 cells.