Drug transport mechanisms in HL60 cells isolated for resistance to adriamycin : evidence for nuclear drug accumulation and redistribution in resistant cells

HL60 cells isolated for resistance to Adriamycin are multidrug resistant and defective in the cellular accumulation of drug. These cells do not contain detectable levels of P-glycoprotein. At the present time the mechanism by which HL60/Adr cells reduce drug levels is not known. To gain insight into the molecular basis of this system we have analyzed transport pathways and the distribution of daunomycin in drug-resistant HL60 cells. Using a cell fractionation technique we find that the major portion of daunomycin accumulates in the nucleus of both sensitive and resistant cells. Further studies reveal, however, that under efflux conditions drug is retained in the nuclei of sensitive cells but rapidly removed from the nuclei of the resistant isolate. Essentially identical results are obtained when daunomycin distribution and transport are analyzed by fluorescence microscopy. A number of agents which alter transport processes have been tested for their effect on drug accumulation in resistant cells. Thus we find that brefeldin A, which disassembles Golgi, and various lysosomotropic agents such as chloroquine and methylamine do not affect drug levels. In contrast the protonophores nigericin and monensin induce an increase in drug accumulation and inhibit efflux. The results of this study thus suggest that resistance in HL60/Adr cells is related to a mechanism whereby drug is transported to the nucleus and thereafter rapidly redistributed to the extracellular space. The molecular basis of this transport pathway is not known.