Cytotoxicity induced by manipulation of signal transduction pathways is associated with down-regulation of Bcl-2 but not Mcl-1 in MCF-7 human breast cancer

We examined the role of Mcl-1 and Bcl-2 expression in the induction of apop tosis, through blocking protein tyrosine kinase (PTK), protein kinase C (PK C), phosphatidylinositol 3-kinase (PI3-K) and mitogen-activated protein kin ase (MAPK)/Erk kinase (MEK) signaling pathways by various kinase inhibitors in MCF-7 breast cancer cells. The PTK inhibitor genistein (GEN) and PKC in hibitor staurosporine (STP) down-regulated Mcl-1 and Bcl-2 expression, and induced growth inhibition by blocking at the G(2)/M phase of cell cycle, fo llowed by apoptosis, leading to chromatin condensation and DNA fragmentatio n. LY294002 (LY)-mediated inhibition of PI3-K activity down-regulated Bcl-2 but not Mcl-1 expression, triggered growth arrest at the G(1)/G(0) phase o f cell cycle and also led to apoptosis marked with chromatin condensation a nd DNA fragmentation. The MEK inhibitor U0126 (U0) decreased Bcl-2 expressi on but not Mcl-1 expression, inhibited cells growth and induced G(1)/G(0) a rrest, but in this case cell death occurred without significant apoptotic f eatures. The kinase inhibitor concentration dependence of cytotoxicity corr elated well with down-regulation of Bcl-2 but not with changes in Mcl-1 lev els. This suggests that Bcl-2 plays a predominant role in the regulation of cell death induced by cell signaling alterations whereas Mcl-1 does not ap pear to control cell survival under these conditions in MCF-7 cells. Furthe r studies showed that the combination of GEN, STP and LY with U0 can produc e synergetic cytotoxic effects on MCF-7 cells. Our results suggest that PTK , PKC, PI3-K and MEK signaling pathways can regulate Bcl-2 expression and f orm an integrated network that plays a critical role in cell survival.