Expression of gamma-glutamyl transpeptidase provides tumor cells with a selective growth advantage at physiologic concentrations of cyst(e)ine

Cells from the GGT-negative mouse hepatoma cell line, Hepa 1-6, were transfected with a human GGT cDNA and stably transformed clones were isolated. In standard tissue culture medium the GGT-positive cells and GGT-negative controls grew equally well. However, when the cysteine concentration of the medium was reduced to physiologic levels the GGT-positive cells had a growth advantage. Further investigation revealed that the medium of the GGT-negative Hepa 1-6 cells contained glutathione that had been excreted by the cells, but no glutathione was present in the medium of the GGT-positive cells. We have previously shown that expression of GGT enables cells to use extracellular glutathione as a source of cysteine (Hanigan and Ricketts, Biochem., 32:6302, 1993). These new data reveal that physiologic levels of cysteine can be limiting for cell growth and expression of GGT can provide the cells with a selective growth advantage. These data explain the observation that cells transfected with GGT grow at the same rate as the GGT-negative controls in tissue culture medium which contains a high level of cysteine, but the GGT-positive cells grow more rapidly than the GGT-negative cells when transplanted into animals (Warren et al., Proc. Soc. Exp. Biol. Med., 202:9, 1993). GGT-positive tumor cells have a selective growth advantage in vivo in comparison to GGT-negative tumor cells because they are able to use serum glutathione as a secondary source of cysteine thereby overcoming the growth restriction imposed by serum levels of cysteine.