Uptake and metabolism of exogenous gangliosides by cultured cells: effect of choleragen on the turnover of GM1.

When added to the culture medium, 3H-labeled GM1 (tritiated predominantly in the terminal galactose residue) was taken up by murine NCTC 2071 and rat glioma C6 cells, both of which are GM1-deficient. Upon incubating the labeled cells in fresh medium, the cell-associated GM1 was metabolized by the cells with a half-life of 1 to 2 days. Some of the GM1 was converted to GD1a but the bulk of the label appeared in the medium as degradation products. When GM1 labeled in the sialic acid or lipid portion of the molecule was utilized, GM2 also was detected with time in the cells and only a small fraction of the radioactivity was detected in the medium. The rat glioma C6 cells appeared unable to degrade the GM2 that they accumulated; this was demonstrated directly by incubating the cells with labeled GM2. The uptake and subsequent metabolism of GM1 was observed over a wide range of GM1 concentrations (10(-8) to 10(-4) M). The GM1-treated cells initially bound more iodinated choleragen than did untreated cells; but with time, binding capacity decreased. When GM1-treated cells were transferred to fresh medium in the presence of excess choleragen, the amount of cell-associated GM1 remained relatively constant for several days; the conversion of GM1 to GD1a also was blocked. Although labeled GM3 and GD1b also were taken up by the cells, choleragen had no effect on their subsequent metabolism. Choleragenoid, the binding subunit of choleragen, also inhibited GM1 metabolism without activating adenylate cyclase. These results indicate that exogenous gangliosides taken up by cultured cells are metabolized and that choleragen, which binds with high affinity to GM1, specifically prevents the metabolism of this ganglioside.