Multidrug-resistant (MDR)
tumors and
cancer cell lines demonstrate a wide variety of biochemical changes. In this study we used
drug-sensitive wild-type (wt)
cancer cell lines and respective MDR subclones, and we demonstrate the accumulation of distinct
lipids in MDR cells. These
lipids were either absent or present at very low levels in
drug-sensitive cells. The compounds, termed lipid-1 and lipid-2, migrated on thin-layer chromatography as a doublet. They could be radiolabeled by incubating MCF-7-AdrR (
Adriamycin-resistant)
breast cancer cells with [3H]
serine, [3H]
palmitic acid, or [3H]
galactose. Utilization of these precursors by MCF-7-wt cells for synthesis of lipid-1 and -2 was minimal. Two inhibitors of
sphingolipid biosynthesis, L-
cycloserine and
fumonisin B1, prevented the observed accumulation of the
lipid compounds. An inhibitor of
glucosylceramide synthesis,
1-phenyl-2-palmitoylamino-3-morpholino-1-propanol, completely abolished the formation of lipid-1 and -2 in MCF-7-AdrR cells and, to a lesser extent, inhibited the formation of
lactosylceramides and
gangliosides. Utilizing mass spectrometry, the multidrug resistance-associated
lipids were further characterized as monoglycosylceramides of two major species that contained either 16-carbon (palmitic) or 24-carbon (lignoceric and nervonic)
fatty acids. The
carbohydrate head group of glycosylceramides was identified as
glucose, not
galactose, thus designating the accumulated
lipids as
glucosylceramides. Incorporation of [3H]
palmitic acid into
glucosylceramide was strikingly higher (8-10 times) in MCF-7-AdrR cells compared with MCF-7-wt cells. Since the rate of
glucosylceramide degradation in MCF-7-AdrR cells was not attenuated, accelerated
glycosphingolipid synthesis in MDR cells is suggested.
Glucosylceramide also accumulated in KB-V-1, a
vinblastine-resistant
epidermoid carcinoma but not in KB-3-1,
drug-sensitive wt cells. MDR ovarian
adenocarcinoma cells (NIH:OVCAR-3) also contained elevated levels of
glucosylceramide. Our results demonstrate a correlation between cellular drug resistance and alterations in
glucosylceramide metabolism.