Evidence for functional discrimination between leukemic cells overexpressing multidrug-resistance associated protein and P-glycoprotein.

Multidrug-resistance (MDR), caused by overexpression of either P-glycoprotein (Pgp) or the multidrug-resistance associated protein (MRP), is characterised by a decreased cellular drug accumulation. One form of MDR is the sequestration of the drug inside cytoplasmic vesicles followed by an a exocytotic and/or efflux process. In some studies, increased intracellular glutathione (GSH) has been associated with MDR. In this study, we examined the effects of 7-chloro-4-nitrobenz-2-oxa-1,3-diazole or NBD (a H(+)-ATPase pump inhibitor) and buthionine sulphoximine or BSO (an inhibitor of GSH biosynthesis) on the subcellular distribution of daunorubicin or DNR in two leukemic homoharringtonine-resistant K562 cell lines, overexpressing MRP (K-H30) and Pgp (K-H300). DNR nuclear accumulation was carried out using microspectrofluorometry. Our results show that DNR nuclear accumulation and sensitivity of K-H30 cells were increased by NBD and BSO whereas in K-H300 cells, NBD and BSO were unable to increase the DNR nuclear accumulation and sensitivity of these cells. This study demonstrates clearly that even if vesicular sequestration can happen in cells overexpressing MRP and Pgp proteins, only the MRP protein is able to extrude the drug through intracellular vesicles and efflux. In addition, GSH plays an important part in the pathway of drug transport in cells overexpressing MRP. Data entrain also the notion of functional discrimination between the MDR and MRP phenotype.