Breast cancer resistance protein-mediated topotecan resistance in ovarian cancer cells

Overexpression of breast cancer resistance protein ( BCRP) and mitoxantrone ( MX) resistance protein can confer resistance to a variety of cytostatic drugs, such as MX, topotecan ( TPT), doxorubicin, and daunorubicin. This study investigates the role of BCRP in resistance of ovarian cancer to TPT treatment. We have developed TPT- resistant human ovarian cancer cell line. Intracellular concentration of fluorescent dye rhodamine 123 ( Rh123) was measured by flow cytometry. The expression of several membrane transporter proteins including P- glycoprotein ( P- gp), multidrug resistance protein 1 ( MRP1), and BCRP were determined by reverse transcription - polymerase chain reaction and Western blotting. The Rh123 concentration in parental cells was approximately three times of those in TPT- resistant cells. In contrast to undetectable level of P- gp messenger RNA ( mRNA) and minimal level of MRP1 expression in TPT- resistant cells, overexpression of both the BCRP mRNA and the protein was detected in these cells. Introduction of antisense- phosphorothioate oligonucleotide derived from BCRP mRNA into TPT- resistant cells resulted in a significant increase in the concentration of intracellular Rh123. These results suggested a novel mechanism in which a reduced intracellular drug concentration may be mediated by BCRP gene products in human ovarian cancer cells.