Control and virus-transformed baby hamster kidney cells resistant to ethidium bromide. I. Characterization and the respiratory enzymes

Ethidium bromide resistance in a rat liver epithelial cell line: association with enhanced drug efflux

Ethidium bromide-resistant cell strains were obtained by continuous selection of an adult rat liver-derived cell line (ARL6T) grown in the continuous presence of 200 ng/ml ethidium bromide. Comparison of resistant strains and parental (sensitive) cells was made for uptake and binding of ethidium bromide, visualized as fluorescent ethidium bromide-nucleic acid complexes. Although uptake of ethidium bromide was similar in parental and resistant cells, efflux kinetics were markedly different. Over a three-hour period, parental (sensitive) cells maintained fluorescence following a short ethidium bromide pulse (100 micrograms/ml ethidium bromide). In contrast, ethidium bromide-resistant cell lines eliminated photographically detectable fluorescent complexes within three hours following pulse exposure to ethidium bromide. The rapid elimination of ethidium bromide-fluorescent complexes in all (5) resistant cell strains examined supports an efflux mechanism as contributing to the resistance of ethidium bromide cytotoxicity in these cells.

Control and virus-transformed baby hamster kidney cells resistant to ethidium bromide. II. Membrane properties

Aspects of membrane structure and functions were studied in ethidium bromide resistant cells. Submitochondrial particles were solubilized and electrophoresed. The gel patterns, representing mitochondrial membrane proteins, demonstrated qualitative and quantitative alterations in mitochondrial preparations derived from virus-transformed cells and ethidium bromide resistant cells as compared to the control cells. The plasma membrane glycoproteins were labelled by the sodium borohydride method. The glycoproteins were pleased with Triton X-100 and electrophoresed. Fluorograms of the gels demonstrated some marked differences between the ethidium bromide resistant cells and their parental strain. The observed alterations in the membrane glycoproteins did not result in altered glucose transport properties or in the elution patterns of plasma membrane glycopeptides as analyzed by Sephadex G-50 chromatography. Dye uptake and binding studies with intact parental and drug resistant cells and their isolated mitochondria demonstrated no alteration of the membrane permeability or the number of binding sites for ethidium bromide. Similar results were also obtained with a cyanine dye. This latter finding was significant in that it permitted one to exclude dye exclusion as a mechanism for ethidium bromide resistance.