Ceramide toxicity and metabolism differ in wild-type and multidrug-resistant cancer cells

Previously we demonstrated that multidrug-resistant (MDR) cancer cells have elevated levels of a glycosylated form of ceramide, glucosylceramide. Here we compared ceramide metabolism and ceramide toxicity in MCF-7 and in adri amycin-resistant (MCF-7-AdrR) human breast cancer cells. MCF-7-AdrR cells w ere resistant to C-6-ceramide (1-10 mu M); however, in MCF-7 cells treated with C-6-ceramide, viability dropped sharply. Ceramide, when supplemented, was not metabolized by MCF-7 cells. In contrast, ceramide was efficiently c onverted to glucosyl-ceramide by MCF-7-AdrR cells. Analysis of extracellula r [H-3]ceramide in radiolabeled cells showed that MCF-7-AdrR cells do not h ave an enhanced capacity to efflux ceramide compared with MCF-7 cells. Trip henylethylene anti-estrogens, known modulators of drug resistance, were eff ective inhibitors of ceramide conversion to glucosyl-ceramide, suggesting t hat blocking ceramide metabolism plays a role in chemosensitization. The an ti-progestine, RU486, also blocked glucosylceramide synthesis in cells; how ever, LY117018, a raloxifene analog, was without influence. We propose that an enhanced capacity to glycosylate ceramide as evidenced in MCF-7-AdrR ce lls, is a molecular determinant of drug resistance, particularly as regards resistance to ceramide-enhancing agents such as anthracyclines, ionizing r adiation, and tumor necrosis factor-alpha.