IS REDUCED ACCUMULATION OF HOECHST-33342 IN MULTIDRUG-RESISTANT CELLSRELATED TO P-GLYCOPROTEIN ACTIVITY

Although bisbenzimidazole-DNA interactions have been studied in soluti on, little information has been available in living cells. The reduced accumulation of the nuclear dye Hoechst 33342 (H342) in cells with mu ltidrug resistant (MDR) phenotype suggested its possible use in a func tional test for detection of these cells. We performed experiments to elucidate the mechanisms involved in the H342-exclusion from resistant cells. As contradictory results have been reported in Literature, we compared the entire fluorescence spectra of H342 in solution and in in tact living cells under different experimental conditions. The study w as performed by fluorescence image cytometry. This technique allow acc urate quantification of the amount of H342 bound to DNA in living cell s. The dye uptake was followed in sensitive and resistant cells, a lym phoblastoid cell line, CCRF-CEM, and its resistant variant selected wi th vinblastine CEM/VLB100 under conditions that could modulate H342-ce ll binding. Competition experiments with sodium azide, verapamil, and vinblastine indicated that resistant cells did not differ in the numbe r of possible binding sites for H342. The obtained results ruled out t he possibility of discriminating cells on the basis of a spectral shif t. Two modes of binding, differing in their affinity for the dye, seem to co-exist in intact cells. Although it clearly appeared that the P- glycoprotein expressed in MDR cells was mainly responsible for the H34 2-exclusion, other mechanisms might also be involved. (C) 1995 Wiley-L iss, Inc.