The role of RAD51 in etoposide (VP16) resistance in small cell lung cancer

Etoposide (VP16) is a potent inducer of DNA double-strand breaks (DSBs) and is efficiently used in small cell lung cancer (SCLC) therapy. However, acquired VP16 resistance remains an important barrier to effective treatment. To understand the underlying mechanisms for VP16 resistance in SCLC, we investigated DSB repair and cellular VP16 sensitivity of SCLC cells. VP16 sensitivity and RAD5I, DNA-PKcs, topoisomerase IIa and P-glycoprotein protein levels were determined in 17 SCLC cell lines. In order to unravel the role of RAD5I in VP16 resistance, we cloned the human RAD5I gene, transfected SCLC cells with RAD5I sense or antisense constructs and measured the VP16 resistance. Finally, we measured VP16-induced DSBs in the 17 SCLC cell lines. Two cell lines exhibited a multidrug-resistant phenotype. In the other SCLC cell lines, the cellular VP16 resistance was positively correlated with the RAD5I protein level. In addition, downregulation or overexpression of the RAD5I gene altered the VP16 sensitivity. Furthermore, the levels of the RAD5I and DNA-PKcs proteins were related to VP16-induced DSBs. The results suggest that repair of VP16-induced DSBs is mediated through both RAD5I-dependent homologous recombination and DNA-PKcs-dependent non-homologous end-joining and may be a determinant of the variation in clinical treatment effect observed in human SCLC tumors of identical histologic subtype. Finally, we propose RAD5I as a potential target to improve VP16 efficacy and predict tumor resistance in the treatment of SCLC patients. (C) 2003 Wiley-Liss, Inc.