Deregulated signaling through the epidermal growth factor receptor (EGFR) is involved in chemoresistance. To identify the molecular determinants of sensitivity to the EGFR inhibitor gefitinib (Iressa, ZD1839) in chemoresistance, we compared the response of matched chemosensitive and chemoresistant glioma and ovarian cancer cell lines. We found that chemoresistant cell lines were 2- to 3-fold more sensitive to gefitinib growth-inhibitory effects, because of decreased proliferation rather than survival. Sensitivity to gefitinib correlated with overexpression and constitutive phosphorylation of HER2 and HER3, but not EGFR, altered HER ligand expression, and enhanced activation of EGF-triggered EGFR pathway. No activating mutations were found in EGFR. Gefitinib fully inhibited EGF-induced and constitutive Akt activation only in chemoresistant cells. In parallel, gefitinib downregulated constitutively phosphorylated HER2 and HER3, and activated GSK3beta with a concomitant degradation of cyclin D1. Ectopically overexpressed HER2 on its own was insufficient to sensitize chemonaive cells to gefitinib. pHER3 coimmunoprecipitated with p85-PI3K in chemoresistant cells and gefitinib dissociated these complexes. siRNA-mediated inhibition of HER3 decreased constitutive activation of Akt and sensitivity to gefitinib in chemoresistant cells. Our study indicates that in chemoresistant cells gefitinib inhibits both an enhanced EGF-triggered pathway and a constitutive HER3-mediated Akt activation, indicating that inhibition of HER3 together with that of EGFR could be relevant in chemorefractory tumors. Furthermore, in combination experiments gefitinib enhanced the effects of coadministered drugs more in chemoresistant than chemosensitive ovarian cancer cells. Combined treatment might be therapeutically beneficial in chemoresistant tumors from ovary and likely from other tissues.
(c) 2008 Wiley-Liss, Inc.