Chapter 1 The Multidrug Transporter: Mechanistic Considerations

Publisher Summary The investigation of how human cancers elude chemotherapy has uncovered the existence of an energy-dependent multidrug transporter. This transporter belongs to a superfamily of proteins, many of which are other transporters that use the energy of adenosine triphosphate (ATP) to move drugs, peptides, pigments, nutrients, and other metabolites across the cell membranes. The cloning and sequencing of a complementary DNA (cDNA) for the multidrug transporter, from the mouse and human, made it possible to generate a model for its primary structure. The human multidrug transporter, also known as P-glycoprotein, is glycosylated and consists of 1280 amino acids encoded by the MDR1 gene. Analysis of the mechanism of action of the multidrug transporter reveals some unusual mechanistic features, including accumulating evidence that it recognizes and extrudes drugs directly from the plasma membrane. Use of confocal epifluorescence microscopy has confirmed that, in drug-sensitive cells, several P-glycoprotein substrates, including the anticancer anthracyclines and the mitochondrial laser dye rhodamine 123, are predominantly localized in the plasma membrane and intracellular membranous structures in addition to their normal cytoplasmic targets. The wide variety of drugs handled by the transporter, including amphipathic hydrophobic natural products and hydrophobic peptides (gramicidin D, valinomycin, and cyclosporine A), suggests that a simple model of substrate recognition and transport is not likely to be correct. Detailed kinetic analyses of drug accumulation and efflux by cells, expressing the multidrug transporter, consistently suggest that there is a decrease in drug influx as well as an increase in efflux. One interpretation of these results is that decreased influx results from genetic changes unrelated to the expression of the mdr gene in some of these cell lines.