Regulation of BAX and BCL-2 expression in breast cancer cells by chemotherapy

Optimizing chemotherapeutic drug delivery strategies relies, in part, on id entification of the most clinically effective sequence, dose, and duration of drug exposure. The combination of dose intensive etoposide (VP-16) follo wed by cyclophosphamide has clinical efficacy in the treatment of advanced breast cancer. However, molecular mechanisms that underlie the effectivenes s of this combination of chemotherapeutic agents have not been investigated . In this study we investigated regulation of BAX and BCL-2 expression by V P-16 and cyclophosphamide as a potential mechanism for the induction of bre ast cancer cell death induced by this regimen. There was a dose and time dependent increase in BAX expression in the breas t cancer cell lines MCF-7, MDA-MB-435S, and MDA-MB-A231 following in vitro treatment with 50-100 mu M VP-16. Elevation of BAX protein expression in th e presence of VP-16 alone did not correlate with reduced viability or induc tion of apoptosis in MCF-7, MDA-MB-435S, or MDA-MB-A231. VP-16 did effectiv ely block the breast cancer cell lines evaluated (MCF-7 and MDA-MB-435S) at G(2)/M phase of the cell cycle, confirming activity of the drug in vitro. MCF-7 and MDA-MB-435S cells that were pre-treated with VP-16 and subsequent ly exposed to 1.0-12.0 mu g/m1 4-hydroperoxycyclophosphamide (4HC), an acti ve metabolite of cyclophosphamide, had markedly reduced viability when comp ared to matched controls treated with either VP-16 or 4HC individually. Con sistent with this loss of viability, exposure of all three cell lines to th e combination of VP-16 and 4HC resulted in higher BAX protein levels than t hose observed following treatment with either single agent. This combinatio n of chemotherapeutic agents also resulted in reduced BCL-2 expression. These observations suggest that combination chemotherapy may derive its eff icacy, in part, through coordinated regulation of specific gene products as sociated with apoptosis. Characterization of molecular events that underlie susceptibility of specific tumor cells to combination chemotherapeutic reg imens may lead to additional improvements in treatment strategies for this disease.