Transforming growth factor B-1 decreases uptake of glutathione precursor amino acids in bovine pulmonary artery endothelial cells

We have previously observed that transforming growth factor beta(1) (TGF beta(1)) produces a pro-oxidant effect and decreases cellular glutathione (GSH) levels of cultured bovine pulmonary artery endothelial cells (BPAEC) (White A. C., S. K. Das, and B. L. Fanburg. Am. J. Respir. Cell Mel. Biol. 6:364-368, 1992). In the present studies we demonstrate that 2 ng/ml TGF beta(1) reduces the uptake of two GSH precursor amino acids (cystine and glutamate) by 50% (cystine; control 359.35 +/- 100, TGF beta(1) 187.7 +/- 26 pmol/10 min/10(6) cells, p < 0.05; glutamate; control 215.15 +/- 18, TGF beta(1) 110.2 +/- 16 pmol/10 min/10(6) cells, p < 0.001). The inhibitory effect of TGF beta(1) on the uptake of GSH precursor amino acids persisted in the presence of buthionine sulfoximine (inhibits gamma-glutamyl cysteine synthetase, the rate limiting step in GSH synthesis) or acivicin (inhibits gamma-glutamyl transpeptidase). The uptake of leucine, an amino acid that does not serve as a precursor for GSH, was unaffected by TGF beta(1). In additional experiments TGF beta(1) decreased the levels of cellular and medium GSH-indicating that TGF beta(1) did not increase efflux of GSH from BPAEC. We propose from these observations that TGF beta(1) decreases cellular glutathione, at least in part, through down regulation of precursor amino acid transport and, thereby, its rate of synthesis.