A comparison of rhodamine 123 accumulation and efflux in cells with P-glycoprotein-mediated and MRP-associated multidrug resistance phenotypes.

Rhodamine 123 (Rh123) is a fluorescent dye which locates in the mitochondria of cells. It is a substrate for P-glycoprotein (Pgp) and can, therefore, be used as a molecular probe in studies of the multidrug resistance (MDR) phenotype. However, not all MDR cells overexpress Pgp. In some, the MDR phenotype is associated with expression of an alternative transporter molecule, the multidrug resistance-associated protein (MRP). We have studied the accumulation and efflux of Rh123 in MDR cells having both Pgp-mediated and MRP-associated phenotypes. In the mouse tumour parental cell line, EMT6/P, Rh123 accumulates rapidly to reach plateau levels by 90 min. Confocal microscopy confirms a localisation to the mitochondria. In the MDR subline, EMT6/AR1.0, which overexpresses Pgp and which is 10-fold resistant to Rh123 cytotoxicity, accumulation is dramatically reduced. Efflux of Rh123 from both resistant and parental lines is rapid but can be inhibited by reduced temperature or by the presence of cyclosporin A (5 micrograms/ml). Efflux from the parental line is probably due to the presence of very low, but detectable, levels of Pgp but the existence of other mechanisms cannot be ruled out. In contrast, the human lung cancer parental cell line COR-L23/P, and its MRP-associated (but Pgp-negative) MDR subline, COR-L23/R (which is 23-fold resistant to Rh123 cytotoxicity), accumulate Rh123 at similar rates for the first 30 min. The curves then diverge so that, at 180 min, the resistant cells contain only 70% of the Rh123 of parental cells. Confocal microscopy demonstrates a similar distribution of fluorescence in resistant and parental cells. Essentially no efflux of Rh123 occurs from parental cells, whereas 70% of the content is lost from resistant cells over a period of 150 min. Such efflux may again be inhibited by reduced temperature but cyclosporin A (5 micrograms/ml) has little effect. These observations should be borne in mind when interpreting Rh123 efflux data in terms of MDR mechanisms.