The glycosphingolipid patterns were analyzed on two clones derived from a human melanoma cell line and selected for their respectively high and low metastatic ability in immunosuppressed newborn rats. Conversely to the weakly metastatic cells which exhibited a pattern similar to that of the parental cell line, highly metastatic human melanoma cells appeared to be deficient in ganglioside biosynthesis. An accumulation of lactosylceramide was found in the latter cells, with low amounts of GM3 as the only ganglioside detected and a fourfold decreased activity of GM3 synthase (EC 2.4.99.9). After subcutaneous injection of metastatic cells in newborn rats, the cells proliferating in the tumor induced at the injection site re-expressed the four common gangliosides of melanoma: GM3, GM2, GD3 and GD2, whereas the cells growing in the lungs as metastatic nodules were deficient in ganglioside synthesis and showed an accumulation of lactosylceramide. Taken together, our results suggest that the human melanoma cells which are able to escape from the primary tumor and invade the lungs have an impaired ganglioside biosynthesis with a deficient GM3 synthase.

With an experimental model of spontaneous lung metastases of melanoma developed in this laboratory, a range of sublines (variants and clones) with different metastatic potential and ganglioside expression was established from a single human melanoma cell line M4Be. Using an in vitro clonogenic assay and provided that cells were cultured for no more than five passages, variations in cellular radioresistance of M4Be and seven sublines derived from M4Be were detected. This study shows a positive correlation between the cell intrinsic radioresistance of M4Be and its seven sublines and their total ganglioside content. More precisely, the proportion of radioresistant cells in M4Be and the seven sublines correlated with the number of cells determined by flow cytometry that were positively labelled with a monoclonal antibody directed to GD3 disialoganglioside. Blocking the cellular biosynthesis of gangliosides with the inhibitor Fumonisin B1 or cleaving with Vibrio cholerae neuraminidase the cell surface ganglioside-bound sialic acid in a radioresistant poorly metastatic subline increased its radiosensitivity in vitro. In contrast, enrichment of a radiosensitive metastatic subline with exogenous bovine brain GM1 increased its radioresistance in vitro. These results suggest that, in the radiation dose range important for radioprotection (0-1 Gy), membrane gangliosides radioprotect human melanoma cells in vitro.

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