Kang Y J, Feng Y, Hatcher E L
Department of Pharmacology and Toxicology, University of North Dakota School of Medicine, Grand Forks, 58202-9037.
J Cell Physiol. 1994 Dec;161(3):589-96. doi: 10.1002/jcp.1041610323.
Extracellular glutathione (GSH) is degraded by an external cell-surface enzyme, gamma-glutamyltranspeptidase (gamma-GT). The products are transported into cells to participate in important cellular processes. In the present study, we tested the hypothesis that extracellular GSH is a source of glutamic acid for cells that express gamma-GT. Under a glutamine-deficient culture condition, the extracellular GSH-supplemented glutamic acid would enhance intracellular glutamine synthesis, thereby stimulating cell proliferation. Human lung carcinoma A549 cells were cultured in glutamine-deficient Dulbecco's modified Eagle medium, and they did not proliferate unless glutamine was supplemented. Extracellular GSH, however, provoked a partial proliferation. The GSH effect correlated with a high level of gamma-GT activity and an increased intracellular level of glutamic acid. A constituent amino acid of GSH, glutamic acid but not cysteine, produced the same growth-stimulatory effect as GSH. Furthermore, neither oxothiazolidine-4-carboxylate (OTC), a cellular cysteine-delivery compound, nor cysteinylglycine, a dipeptide released from the gamma-GT reaction, stimulated cell proliferation. Moreover, buthionine sulfoximine (BSO), a selective inhibitor of gamma-glutamylcysteine synthetase, enhanced the GSH growth stimulatory effect, suggesting that increased cellular GSH synthesis does not correlate with cell growth stimulation. The results obtained demonstrated that glutamine is required for A549 cell proliferation and exogenous GSH partially substitutes for the growth stimulatory action of glutamine. It also suggests that the glutamic acid rather than the cysteine released from the GSH is responsible for the cell proliferation.
细胞外谷胱甘肽（GSH）可被一种细胞表面外酶γ-谷氨酰转肽酶（γ-GT）降解。其产物被转运到细胞内参与重要的细胞过程。在本研究中，我们验证了这样一个假设：细胞外GSH是表达γ-GT的细胞的谷氨酸来源。在谷氨酰胺缺乏的培养条件下，补充细胞外GSH的谷氨酸会增强细胞内谷氨酰胺的合成，从而刺激细胞增殖。人肺癌A549细胞在缺乏谷氨酰胺的杜氏改良 Eagle培养基中培养，除非补充谷氨酰胺，否则它们不会增殖。然而，细胞外GSH引发了部分增殖。GSH的作用与高水平的γ-GT活性和细胞内谷氨酸水平的增加相关。GSH的一种组成氨基酸谷氨酸而非半胱氨酸产生了与GSH相同的生长刺激作用。此外，细胞半胱氨酸递送化合物氧噻唑烷-4-羧酸（OTC）和γ-GT反应释放的二肽半胱氨酰甘氨酸均未刺激细胞增殖。此外，γ-谷氨酰半胱氨酸合成酶的选择性抑制剂丁硫氨酸亚砜胺（BSO）增强了GSH的生长刺激作用，这表明细胞内GSH合成的增加与细胞生长刺激无关。所获得的结果表明，A549细胞增殖需要谷氨酰胺，外源性GSH部分替代了谷氨酰胺的生长刺激作用。这也表明，GSH释放的谷氨酸而非半胱氨酸是细胞增殖的原因。
