S. cerevisiae induces apoptosis in human metastatic breast cancer cells by altering intracellular Ca2+ and the ratio of Bax and Bcl-2

We have recently demonstrated that non-metastatic human breast cancer cell lines undergo apoptosis following phagocytosis of S. cerevisiae. In this study, we investigated the apoptotic effect of heat-killed yeast against human metastatic breast cancer (MBC) cells, MDA-MB-231 in vitro, and the underlying mechanistic bases of this effect. Results show that monolayer MDA-MB-231 cells phagocytized yeast (50% at 16 h) and underwent apoptosis (32% compared with 7.6% of untreated cells, representing a 4.2-fold increase). The increase in apoptosis was associated with an elevation of [Ca2+](I). Addition of 2-aminoethoxydiphenyl borate (2APB), a pharmacological inhibitor of Ca2+ release from the endoplasmic reticulum, effectively diminished yeast-induced apoptosis. Furthermore, yeast caused a substantial decrease in expression of Bcl-2 and an increase in Bax resulting in alteration in the Bax:Bcl-2 ratio. However, yeast had no effect on NO levels. In conclusion, yeast induces apoptosis of human MBC cells in vitro by a mechanism involving intracellular Ca2+ and Bax:Bcl-2.