Thiol redox status can affect important functions both intracellularly and extracellularly. The plasma membrane enzyme gamma-glutamyl transpeptidase (GIST), which plays a crucial role in cellular handling of thiols, is often expressed in malignant tumors, including melanoma, although its expression levels may vary widely among different tumors or cells of the same tumor. In an attempt to better understand the functional significance of GIST overexpression, we have examined the relationships between intra- and extra-cellular thiol metabolism and GIST expression. intra- and extra-cellular distribution of glutathione and other low mol. wt. thiols and disulfides was investigated in two different Me665/2 human melanoma clones that originated from the same metastasis but exhibiting high (2/60 clone) and low (2/21 clone) GIST activity. Intracellular content of glutathione was lower in GIST-rich 2160 cells, in spite of high GIST expression. A lower utilization of extracellular cystine was also observed in these cells. In both clones, a direct secretion of cysteine in the extracellular medium was detected, which was independent of GIST-mediated catabolism of extracellular glutathione. Substantial amounts of glutathione, GSSG and glutathione-cysteine disulfide were accumulated extracellularly only in the case of GIST-poor 2/21 cells, while the same event was apparent in 2/60 cells only after the following inhibition of GIST activity. When exposed to the trinuclear platinum compound 13131131 13464 3113464 or hydrogen peroxide, which are very reactive for sulfur-containing nucleophiles, the 2/60 clone showed higher sensitivity than the 2/21 clone to both agents. These results suggest that the clone-specific balance between transport of sulfur aminoacids and GIST activity results in profound differences in the capability of each clone to modify the thiol redox status of the extracellular milieu. The finding may have important implications in tumor cell behavior with particular reference to chemosensitivity, since thiols are recognized factors in modulation of cell sensitivity to platinum-based anticancer drugs. (C) 2002 Wiley-Liss, Inc.
