Epidermal growth factor receptor (EGFR) signaling plays important roles in the survival and proliferation of cancer cells. Overexpression of EGFR is found in many cancer types and has been associated with poor patient prognosis. Although it is well known that activation of EGFR signaling decreases apoptosis in tumor cells, the mechanism of resistance to apoptosis has yet to be elucidated. Recent studies have shown that survivin, a member of the inhibitor of apoptosis (IAP) family, is highly expressed in over 70% of breast cancers, although not in normal breast tissue. The role of survivin in blocking apoptosis and decreasing chemo- or radiotherapeutic response has been demonstrated in cancer cell lines and animal tumor models. In our study, we have found that EGF stimulation increases the level of survivin gene expression in breast cancer cell lines such as MDA-MB-231, MCF-7, and SKBr-3; but not in immortalized normal breast epithelial cell line MCF-10A. After EGF treatment, a 20% reduction of apoptotic rate is presented in breast cancer cells treated by chemotherapy drug docetaxel. As expected, we found that EGF stimulation increases AKT and MAPK activity while EGF-induced upregulation of survivin is blocked by the PI3K inhibitor LY294002, the MAPK inhibitor PD98059, and the EGFR inhibitor AG1478. We have previously shown that the level of survivin gene expression in breast cancer cells is increased under hypoxic conditions. In this study, we further found that under normoxic conditions, EGF treatment increases HIF-1 α, which then induces survivin gene expression by direct binding to a hypoxia-responsive element (HRE) located at -81 to -85 nt of the 5’-flanking region of the survivin gene as evidenced by a modified McKay assay. Furthermore, we showed that the effects of EGF on survivin gene expression and apoptosis resistance are blocked by HIF-1 α SiRNA administration. Therefore, HIF-1 α and survivin appear to be promising molecular targets for the development of novel therapeutic approaches to enhance chemosensitivity in human tumor cells receiving EGF/EGFR signaling.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]