Gemcitabine (2',2'-difluorodeoxycytidine), a deoxycytidine analog, and erlotinib, an epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitor (TKI), are clinically used to treat patients with non-small-cell lung cancer (NSCLC). However, the molecular mechanisms for the drug-resistant of gemcitabine in NSCLC cells are poorly understood. In this study, we used constructs containing human Rad51 cDNA or specific Rad51 small interfering RNA to examine the role of Rad51 in chemoresistant of gemcitabine in three different human NSCLC cell lines. Exposure of human NSCLC cells lines to gemcitabine increased the phosphorylation levels of MKK1/2-ERK1/2 and AKT in a time- and dose-dependent manner, which was accompanied by an induction of Rad51 mRNA and protein expression. Gemcitabine increased the expression of Rad51 by increasing its mRNA and protein stability. Blockage of ERK1/2 or AKT activation by U0126 (MKK1/2 inhibitor) or LY294002 (PI3K inhibitor), respectively, decreased the gemcitabine-induced Rad51 expression. Gemcitabine-induced cytotoxicity was significantly increased using siRNA depletion of Rad51 or blockage of ERK1/2 and AKT activation. Erlotinib enhanced the gemcitabine-induced cytotoxicity via the inactivation of ERK1/2 and AKT, and the downregulation of Rad51. Enforced expression of constitutively active MKK1/2 or AKT rescued cell viability and Rad51 protein levels that were decreased by the combination of erlotinib and gemcitabine. Suppression of Rad51 expression or the inactivation of ERK1/2 or AKT signaling may be considered as potential therapeutic modalities for gemcitabine-resistant lung cancer.
- Received July 20, 2010.
- Revision received September 16, 2010.
- Accepted September 17, 2010.
- The American Society for Pharmacology and Experimental Therapeutics