Action of monensin, a monovalent cationophore, on cultured human fibroblasts: evidence that it induces high cellular accumulation of glucosyl- and lactosylceramide (gluco- and lactocerebroside)

Abstract

We have exposed cultured human fibroblasts to micromolar concentrations of the ionophore monensin. A salient result was a rapid accumulation in these cells of glucosylceramide (glucocerebroside, GlcCer) and lactosylceramide (lactocerebroside, LacCer). When we incubated these cells with radioactively labeled galactose, GlcCer and LacCer became highly labeled. These results indicate that monensin greatly increases these simplest glycosphingolipids that are the precursor to the major plasma membrane glycosphingolipids. We observed, simultaneously, a decreased incorporation of labeled galactose into some more highly glycosylated neutral glycosphingolipids and sialoglycosphingolipids (gangliosides), and unlike GlcCer and LacCer, the cellular content of these more highly glycosylated compounds remained the same in the presence or absence of monensin. We have found that cultured Gaucher disease fibroblasts, with genetically impaired lysosomal glucocerebrosidase activity, accumulated even more GlcCer and LacCer than normal cells upon exposure to monensin. This finding shows that monensin affects biosynthesis rather than merely disrupting lysosomal degradation that is already deleted with respect to GlcCer in Gaucher disease cells. These results represent the first indication of an apparently remarkable effect of the monovalent ionophore, monensin, on plasma membrane glycosphingolipid biosynthesis. The evidence suggests a regulatory distinction between initial and higher intracellular glycosylation steps. Monensin does not diminish and may augment initial anabolic mono- and diglycosylations and also appears to inhibit higher glycosylations of glycosphingolipids.