Etoposide resistance in MCF-7 breast cancer cell line is marked by multiple mechanisms

Acquired or intrinsic drug resistance is one of the major handicaps in the success of chemotherapy. Etoposide is a topoisomerase II poison widely used in chemotherapy. Similar to other topoisomerase inhibitors and DNA damaging agents, resistance to etoposide may arise as a result of alterations in target expression and activity, increased drug efflux and alterations in DNA damage response mechanisms. Here, we tested the involvement of such mechanisms in etoposide-resistant MCF-7 breast cancer cells.

Relative etoposide resistance was determined by XTT cell proliferation assay. For gene expression analysis, total RNA was extracted from each cell line and gene expression was quantified by real-time PCR following reverse transcription. Topoisomerase II activities of each cell line were compared by using in vitro topoisomerase II activity assay.

Etoposide-resistant sublines MCF-7/1E and MCF-7/4E are 2.6- and 4.6-fold more resistant to etoposide compared to parental cell line MCF-7/S. TOP2A, the gene encoding the topoisomerase II alpha, is significantly downregulated in drug resistant sublines while topoisomerase II activity seemed similar among cell lines. MRP1, which encodes an etoposide efflux pump, is significantly upregulated in etoposide-resistant sublines. Two DNA damage response proteins TOPBP1 and EDD were found to be downregulated in etoposide-resistant sublines.

This study sheds light into the etoposide resistance in breast cancer by investigating previously proposed and novel factors that may have a role in development or progression of etoposide resistance which can be considered as diagnostic markers and therapy targets.