Mechanisms of altered sequestration and efflux of chemotherapeutic drugs by multidrug-resistant cells.

This review considers the mechanisms associated with the pleiotropic resistance of cancer cells to chemotherapeutic drugs, and more particularly those related to intracellular pH (pHi). The multidrug resistance (MDR) phenomenon responsible for the decreased accumulation and increased efflux of cytotoxic drugs is generally associated with excess levels of P-glycoproteins (Pgps) encoded by MDR genes and/or the multidrug resistance-associated protein (MRP). MDR cell lines, derived from normal or tumor cells, frequently exhibit abnormally elevated pHi and changes in the production of various proteins. Recent studies have suggested that, in addition to the impact of the ATP-dependent membrane transporters Pgp and MRP on drug transport, other mechanisms linked to pHi changes in MDR cells may play an important role in drug resistance. We have shown that alkalinization of the acidic compartments (endosomes and lysosomes) by lysosomotropic agents could stimulate the efflux of vinblastine from drug-resistant mouse renal proximal tubule cells. The fact that weak base chemotherapeutic drugs can be sequestered within the acidic organelles of MDR cells sheds new light on the cellular mechanisms of drug resistance.

Lysosomotropic agents increase vinblastine efflux from mouse MDR proximal kidney cells exhibiting vectorial drug transport.

Vinblastine (VBL) transport and efflux were studied in mouse proximal tubule PKSV-PR cells and in their multidrug-resistant derivatives PKSV-PRcol50 cells. The PKSV-PRcol50 cells produced more mdr1b transcripts and had higher resistance to various drugs. PKSV-PRcol50 cells had a predominantly basal-to-apical flux of [3H]VBL, 2.7 times larger than that in PKSV-PR cells. This flux was partially inhibited by verapamil (VRP) (10 microM) and cyclosporin A (CsA) (200 nM). [3H]VBL efflux was also greater in PKSV-PRcol50 than in PKSV-PR cells. Treatment with NH4Cl (30 mM), a lysosomotropic weak base, and concanamycin A (CCM A) (20 nM), an inhibitor of the vacuolar H+/ATPase, further increased [3H]VBL efflux from PKSV-PRcol50 cells. The cytoplasmic pH (pHcyt) of these drug-resistant cells transiently increased in the presence of NH4Cl deltapHcyt: +0.4). CCM A caused a moderate, delayed increase in pHcyt (deltapHcyt: +0.1) and made the acidic intralysosomal compartment more alkaline (deltapHlys: +1.3). VRP and CsA prevented the NH4Cl- and CCM A-induced [3H]VBL efflux from PKSV-PRcol50 cells. However, VRP (10 microM) did not significantly affect pHcyt of PKSV-PRcol50 cells, the NH4Cl-and CCM A-induced pHcyt responses, and the effect of CCMA on pHlys. Thus, lysosomotropic agents may affect the kinetics of [3H]VBL efflux. Our results also suggest that the inhibitory action of VRP on VBL efflux was not directly mediated by a pH-dependent process in these drug-resistant renal proximal tubule cells.