Multidrug Resistance‐associated Protein‐mediated Multidrug Resistance Modulated by Cyclosporin A in a Human Bladder Cancer Cell Line

A doxorubicin‐resistant subline (5637/DR5.5) from human bladder cancer cells (5637) was induced by stepwise increase in the doxorubicin concentration. 5637/DR5.5 cells were cross‐resistant to vinblastine and etoposide but not to mitomycin C and cisplatin. We analyzed the mdr1, MRP (multidrug resistance‐associated protein), and DNA topoisomerase II gene expression using the reverse transcription polymerase chain reaction assay (RT‐PCR) and investigated possible differences in the accumulation and efflux of radiolabeled daunorubicin. 5637/DR5.5 cells do not express the mdr1 gene, but the expression levels of MRP are markedly higher than in drug‐sensitive 5637 cells. The intracellular accumulation of radiolabeled daunorubicin was markedly decreased in the 56377 DR5.5 cells in comparison with the parent cells. This reduced drug accumulation was associated with an enhanced drug efflux, but was reversed when cells were incubated with cyclosporin A. Cyclosporin A at the concentration of 5 (M caused 3.4‐fold enhancement of daunorubicin‐sensitivity in the 56377 DR5.5 cells. On the other hand, there was no difference in DNA‐topoisomerase II activity between the parent and resistant cells. The resistance of the 5637/DR5.5 cells is therefore associated with an enhanced drug efflux mediated by the MRP gene overexpression, as distinct from P‐glycoprotein, and is modulated by cyclosporin A.