THE GANGLIOSIDES AS A POSSIBLE MOLECULAR COUPLING FACTOR BETWEEN THE PROPORTION OF RADIOSENSITIVE CELLS IN-VITRO AND THE METASTATIC POTENTIAL IN-VIVO WITHIN A HUMAN-MELANOMA CELL-LINE

With an experimental model of spontaneous lung metastases in immunosup pressed newborn rats, seven clones and variants with different metasta tic potential and gangliosides expression were derived from a single p arental human melanoma cell line M4Be. The cellular radiosensitivity o f M4Be and its seven sublines was estimated using an in vitro colony a ssay. The total amount of gangliosides in M4Be and its seven sublines was determined by cell extraction and thin-layer chromatography, while the expression of GD3 gangliosides was estimated by flow cytometry wi th a monoclonal antibody. The radiation-cell survival curves of most c lones and variants derived from M4Be showed a zero dose extrapolation clearly lower than 100%, suggesting that two populations of cells of v ery different radiosensitivity coexist within each of these clones and variants. Although the proportion of radiosensitive cells could be es timated from the shape of the survival curve; its radiosensitivity is too high to be properly evaluated by the colony assay. The eight survi val curves differ essentially in the proportion of radiosensitive cell s - which varied from 0% to 40% among M4Be and its seven sublines - wh ereas the cellular radiosensitivity of the radioresistant population w as similar among them. The metastatic potential in vivo of M4Be and it s seven sublines was not significantly related to the cellular radiose nsitivity of their corresponding radioresistant population, but signif icantly increased with the fraction of radiosensitive cells. This rela tionship is valid only when the highly metastatic cells are cultured f or no more than five passages in vitro as the fraction of radiosensiti ve cells is rapidly lost during subcultures. The relationship remains valid in vivo as metastatic melanoma-bearing newborn rats whole body i rradiated with 20 cGy show no lung metastasis compared with controls. The radiosensitive cell fraction is inversely correlated with both the total ganglioside content (r=0.84, P<0.02) and the number of cells po sitively labelled with the monoclonal antibody directed to GD3 (r=0.92 , P<0.001). The incubation of a radiosensitive clone with the exogenou s bovine brain ganglioside GM1 significantly increases the proportion of radioresistant cells and suppresses its metastatic potential, while the inhibition of the endogenous gangliosides synthesis in the radior esistant cell line M4Be increases the proportion of radiosensitive cel ls. This study provides a possible explanation for the correlation bet ween the metastatic potential and the proportion of radiosensitive cel ls within the seven sublines derived from a single parental human mela noma cell line.