Gefitinib (Iressa(R), ZD1839) is a selective epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor that blocks growth factor-mediated cell proliferation and extracellular signal-regulated kinases 1/2 (ERK1/2) signaling activation. Rad51 is an essential component of the homologous recombination repair pathway. High level of Rad51 expression has been reported in chemo- or radioresistant carcinomas. We hypothesized that gefitinib may enhance the effects of the alkylating agent cisplatin- or the antitumor antibiotic mitomycin C (MMC)-mediated cytotoxicity by decreasing ERK1/2 activation and Rad51 expression. Exposure of human non-small lung cancer cells to gefitinib decreased cisplatin- or MMC-elicited ERK1/2 activation and Rad51 protein induction. Neither cisplatin nor MMC treatment affected Rad51 messenger RNA (mRNA). However, gefitinib cotreatment with cisplatin or MMC significantly decreased Rad51 mRNA levels. In addition, gefitinib decreased cisplatin- or MMC-elicited Rad51 protein levels by increasing Rad51 protein instability. Enhancement of ERK1/2 signaling by constitutively active mitogen-activated protein kinase kinase 1/2 (MKK1/2-CA) increased Rad51 protein levels and protein stability in gefitinib and cisplatin or MMC cotreated cells. Moreover, the synergistic cytotoxic effects induced by gefitinib cotreatment with cisplatin or MMC were remarkably decreased by MKK1-CA-mediated enhancement of ERK1/2 activation. Depletion of endogenous Rad51 expression by si-Rad51 RNA transfection significantly enhanced lung cancer cell death upon treatment with cisplatin or MMC. We conclude that Rad51 protein protects lung cancer cells from synergistic cytotoxic effects induced by gefitinib and chemotherapeutic agents. Suppression of Rad51 expression may be a novel lung cancer therapeutic modality to overcome drug resistance to EGFR inhibitors and chemotherapeutic agents.

Gefitinib (Iressa, ZD1839) is a selective epidermal growth factor receptor tyrosine kinase inhibitor that can block growth factor-mediated cell proliferation and extracellular signal-regulated kinases 1/2 (ERK1/2) activation. Highlevel Rad51 expression has been reported in chemoresistant or radioresistant carcinomas. In this study, we examined the role of Rad51 in regulating the response to gefitinib among different human lung cancer cell lines. The H520 line (human squamous cell carcinoma) was less sensitive to gefitinib compared with the H1650 (human adenocarcinoma) or A549 (human bronchioloalveolar carcinoma) lines. In H1650 and A549 cells but not in H520 cells, gefitinib decreased cellular levels of phospho-ERK1/2 and Rad51 protein and message levels. Moreover, gefitinib decreased Rad51 protein levels by enhancing Rad51 protein instability through 26S proteasomemediated degradation. Inhibition of endogenous Rad51 levels by si-Rad51 RNA transfection significantly enhanced gefitinib-induced cytotoxicity. In contrast, transfection with constitutively active MKK1 vector could restore both Rad51 protein levels and cell survival inhibited by gefitinib.

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