Induction of apoptosis in breast cancer cells by Saccharomyces cerevisiae, the baker's yeast, in vitro.

The present study was undertaken to evaluate the effect of phagocytosis of killed yeast on the induction of apoptosis in human metastatic breast cancer cells (MCF-7 and ZR-75-1) and non-metastatic breast cancer cells (HCC70). Heat-killed Saccharomyces cerevisiae, baker's and brewer's yeast, was cultured with cancer cells at a ratio of yeast to cancer cells = 10:1, and the percent apoptotic cancer cells was determined by flow cytometry and cytospin preparation. Upon phagocytosis of yeast, breast cancer cells underwent apoptosis. Induction of apoptosis was time- and dose-dependent. Apoptosis was detected as early as 0.5 h (13%), increased to 19% at 2 h and peaked (38%) at 4 h. Metastatic cancer cells were found to be more susceptible to yeast-induced apoptosis than non-metastatic cells; 629% increase for MCF-7 as compared to cells alone, 258% for ZR-75 cells, while HCC70 cells showed a 178% increase. Phagocytosis is associated with the disruption of mitochondrial membrane potential and activation of initiator and effector caspases 8, 9 and 3. However, inhibitors of these caspases did not inhibit yeast-induced apoptosis in cancer cells, suggesting that yeast induces apoptosis in breast cancer cells by a mechanism that is independent of caspase activation. This data may have clinical implications.