INVOLVEMENT OF VACUOLAR H+-ADENOSINE TRIPHOSPHATASE-ACTIVITY IN MULTIDRUG RESISTANCE IN HL-60 CELLS

HL60 cells isolated for resistance to vincristine (HL60/Vinc cells) or doxorubicin (HL60/Adr cells) contain enhanced levels of an energy-dependent drug efflux pump. HL60/Vinc cells contain the drug transporter P-glycoprotein, whereas the HL60/Adr isolate does not. In the present study, we examined the possible involvement of vacuolar H+-adenosine triphosphatase (H+-ATPase) activity in drug resistance in HL60 cells. We utilized bafilomycin A1, an agent which selectively inhibits vacuolar H+-ATPase activity at low concentrations. The results showed that bafilomycin A1 induced a major increase in drug accumulation and inhibited drug efflux in both HL60/Adr cells and HL60/Vinc cells. Similar results were obtained with 7-chloro-4-nitrobenz-2-oxa 1,3 diazole, an agent which is also capable of inhibiting vacuolar H+-ATPase. Azide, an inhibitor of F1F(o) mitochondrial ATPase, and vanadate and ouabain, which are inhibitors of E1E2-type ATPase, did not affect drug levels in resistant cells. We also observed that bafilomycin A1 did not compete with [H-3]azidopine binding to P-glycoprotein. Thus, bafilomycin A1 does not appear to function as a substrate for P-glycoprotein. These results suggest an involvement of vacuolar H+-ATPase activity in the pathway of drug efflux from HL60/Adr cells and HL60/Vinc cells. The mechanism of this action remains to be determined.