EXPRESSION OF GAMMA-GLUTAMYL-TRANSPEPTIDASE PROVIDES TUMOR-CELLS WITHA SELECTIVE GROWTH ADVANTAGE AT PHYSIOLOGICAL CONCENTRATIONS OF CYST(E)INE

Cells from the GGT-negative mouse hepatoma cell line, Hepa 1-6, were t ransfected with a human GGT cDNA and stably transformed clones were is olated. In standard tissue culture medium the GGT-positive cells and G GT-negative controls grew equally well. However, when the cysteine con centration of the medium was reduced to physiologic levels the GGT-pos itive cells had a growth advantage. Further investigation revealed tha t 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 e xpression of GGT enables cells to use extracellular glutathione as a s ource of cysteine (Hanigan and Ricketts, Biochem., 32:6302, 1993). The se new data reveal that physiologic levels of cysteine can be limiting for cell growth and expression of GGT can provide the cells with a se lective growth advantage. These data explain the observation that cell s transfected with GGT grow at the same rate as the GGT-negative contr ols in tissue culture medium which contains a high level of cysteine, but the GGT-positive cells grow more rapidly than the GGT-negative cel ls when transplanted into animals (Warren et al., Proc. Sec. 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 th ey are able to use serum glutathione as a secondary source of cysteine thereby overcoming the growth restriction imposed by serum levels of cysteine.