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Failure of chemotherapy in cancer patients because of development of drug resistance is a major problem. Alterations of DNA repair mechanisms and drug targets are among the important resistance mechanisms which are developed against topoisomerase II inhibitors etoposide and doxorubicin. Modifications in the expression levels of mismatch repair (MMR) genes due to resistance to topoisomerase II inhibitors are involved in breast cancer. In this study, etoposide resistant sublines were developed from MCF7 breast cancer cell line (MCF7/S) and the expression levels of TOP2A and two important MMR genes MSH2 and MLH1 were examined by real time qPCR. Previously developed doxorubicin resistant cells were also studied for comparison. Etoposide resistant sublines MCF7/1000E, MCF7/1250E and MCF7/2000E were approximately 2, 3 and 4 fold resistant relative to parental MCF7/S cells, respectively. MLH1, MSH2 and TOP2A expressions decreased in both etoposide and doxorubicin resistant sublines relative to MCF7/S cells. Expression levels of TOP2A in resistant sublines differ between 10-95 percent of the expression levels in the parental cells. In the sublines MCF7/200E, MCF7/500E, MCF7/750E and MCF7/1000E a decrease in TOP2A gene expression was determined. In sublines MCF7/1250E and MCF7/2000E fluctuations in the expression levels were observed. Among the doxorubicin resistant sublines (MCF7/600D and MCF7/1000D), in MCF7/1000D which is more resistant to doxorubicin, TOP2A expression level was higher. Expression levels of MSH2 decreased regularly as the resistance increased. However, in MCF7/1250E significant increase relative to MCF7/1000E was observed. In MCF7/2000E, expression levels of MSH2 again significantly decreased to 41 percent of the levels in parental cell line. Expression levels of MLH1 decreased significantly (18-58 percent) in etoposide resistant sublines relative to MCF7/S cells. In doxorubicin resistant sublines, a decrease in MLH1 gene expression was observed in MCF7/1000D. It can be concluded from the results that decrease in the expression levels of TOP2A, MSH2 and MLH1 genes may contribute to resistance together. Above a certain resistance level, sublines may develop new strategies for acquiring higher resistance. Whenever a strategy becomes limited, new strategies emerge. New approaches developed to overcome resistance in cancer chemotherapy should consider the molecular basis of resistance in different stages of the disease. |
