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ABC drug transporter, MDR1, is a drug flippase that moves a variety of hydrophobic molecules from the inner to the outer leaflet of the plasma membrane. We have previously reported that MDR1 can function as a glycolipid flippase, being one of the mechanisms responsible for the translocation of glucosylceramide into the Golgi for neutral, but not acidic, glycosphingolipids (GSLs) synthesis. The interplay between GSLs and MDR1 could provide a whole new spectrum of innovative therapeutic options. We found that cell surface MDR1 partially co-localized with globotriaosyl ceramide (Gb3) in MDR1 transfected cells. Inhibition of GSL biosynthesis results in the loss of drug resistance and of cell surface MDR1. We speculated that an association of MDR1 and cell surface GSLs, in particular Gb3, may be functional at the cell surface, as MDR1 partitions into plasma membrane lipid rafts regulating MDR1 function. We therefore tested adamantyl Gb3 (adaGb3), a water soluble analog of Gb3, on MDR1 functions. AdaGb3 was able to inhibit MDR1-mediated rhodamine 123 drug efflux from MDR1 expressing cells, like cyclosporin A (CsA), a classical MDR1 inhibitor. AdaGb3 was also able to reverse vinblastine drug resistance in cell culture, whereas adamantyl galactosylceramide had no effect on drug resistance. The strong MDR1 reversal effects of adaGb3, as well as its favourable in vivo features make it a possible choice for inhibition of MDR1 to increase bioavailability of drugs across the intestinal epithelium (De Rosa et al., 2008). Thus, specific GSL analogs provide a new approach to MDR reversal. We have previously shown that MDR1 inhibitor CsA depletes Fabry cell lines of Gb3, the characteristic GSL accumulated in this disease, by preventing its de novo synthesis, and can also deplete Gaucher lymphoid cell lines of accumulated GlcCer (Mattocks et al., 2006). Liver and heart sections of Fabry mice treated with third generation MDR1 inhibitors showed significantly less Gb3 than liver and heart sections of untreated Fabry mice. Thus, MDR1 inhibition offers a potential alternative therapeutic approach not only for Fabry disease given the extraordinary cost of conventional enzyme replacement therapy, but also for other neutral GSL storage diseases, such as Gaucher disease. |
