Buthionine sulfoximine spares intracellular glutamate: a possible mechanism for cell growth stimulation

Buthionine sulfoximine (BSO), a selective inhibitor of glutathione (GSH) synthesis, has a dual effect on proliferation of human lung carcinoma A549 cells, i.e., at low concentrations it stimulates and at higher concentrations it inhibits A549 cell proliferation. This study was undertaken to test the hypothesis that BSO, by inhibiting the synthesis of GSH, spares its constituent amino acids, particularly glutamate, and thereby stimulates cell proliferation. Treatment of A549 cells with BSO significantly increased intracellular glutamate levels, while it decreased cellular GSH levels. To determine whether the increased glutamate level is responsible for the BSO-stimulated cell proliferation, A549 cells were cultured in glutamine-deficient Dulbecco's modified Eagle's medium. These cells did not proliferate in this medium unless glutamine (4 mM) was supplemented. When glutamine was replaced by glutamate in the medium the cells were also stimulated to proliferate, although this stimulation was not as effective as that of glutamine. Cysteine and its cellular delivery system L-2-oxothiazolidine-4-carboxylate did not stimulate cell proliferation even though BSO would also increase cellular cysteine levels. The results obtained suggest that the BSO-increased cellular glutamate level is likely responsible for the BSO growth-stimulating effect.