Buthionine sulfoximine enhances glutathione-but attenuates glutamate-stimulated cell proliferation

Buthionine sulfoximine (BSO) inhibits proliferation of human lung carcinoma A549 cells, and exogenous glutathione (GSH) overcomes the antiproliferative effect. The BSO antiproliferation may result from inhibition of cellular uptake of amino acids, and the antagonistic effect of GSH would result from supplementation of amino acids via the gamma-glutamyl cycle. To explore these possibilities, the present study was undertaken to determine effects of BSO on glutamate- and GSH-stimulated cell proliferation. A549 cells were cultured in a glutamine-deficient Dulbecco's modified Eagle's medium (Gln-(-)DMEM), in which they did not proliferate. Addition of glutamate or GSH in the medium to a concentration of 4 mM stimulated cell proliferation. BSO of 0.1 mM enhanced the GSH-stimulated cell proliferation and attenuated the glutamate-stimulated cell proliferation. This BSO effect correlated with changes in cellular glutamate levels; that is, BSO increased and decreased glutamate concentrations, respectively, in GSH- and glutamate-stimulated cells. GSH or glutamate alone significantly increased cellular GSH levels. BSO depleted cellular GSH in both GSH- and glutamate-stimulated cells to the same level. These changes in GSH levels did not correlate with the respective growth modulatory effect. Because BSO inhibits cellular uptake of some amino acids and the A549 cells contain high levels of gamma-glutamyl transpeptidase activity, the results suggest that the BSO inhibition of glutamate-stimulated cell proliferation may result from decreased glutamate uptake. GSH would supplement the cells with glutamate via the gamma-glutamyl pathway to bypass the inhibition of amino acid uptake and overcome the BSO-antiproliferative effect.