Redox modulation of cell surface protein thiols in U937 lymphoma cells: The role of gamma-glutamyl transpeptidase-dependent H2O2 production and S-thiolation

The expression of gamma-glutamyl transpeptidase (GGT), a plasma membrane ec toenzyme involved in the metabolism of extracellular reduced glutathione (G SH), is a marker of neoplastic progression in several experimental models, and occurs in a number of human malignant neoplasms and their metastases. B ecause it favors the supply of precursors for the synthesis of GSH, GGT exp ression has been interpreted as a member in cellular antioxidant defense sy stems. However, thiol metabolites generated at the cell surface during GGT activity can induce prooxidant reactions, leading to production of free rad ical oxidant species. The present study was designed to characterize the pr ooxidant reactions occurring during GGT ectoactivity, and their possible ef fects on the thiol redox status of proteins of the cell surface. Results in dicate that: (i) in U937 cells, expressing significant amounts of membrane- bound GGT, GGT-mediated metabolism of GSH is coupled with the extracellular production of hydrogen peroxide; (ii) GGT activity also results in decreas ed levels of protein thiols at the cell surface; (iii) GGT-dependent decrea se in protein thiols is due to sulfhydryl oxidation and protein S-thiolatio n reactions; and (iv) GGT irreversible inhibition by acivicin is sufficient to produce an increase of protein thiols at the cell surface. Membrane rec eptors and transcription factors have been shown to possess critical thiols involved in the transduction of proliferative signals. Furthermore, it was suggested that S-thiolation of cellular proteins may represent a mechanism for protection of vulnerable thiols against irreversible damage by prooxid ant agents. Thus, the findings reported here provide additional explanation s for the envisaged role played by membrane-bound GGT activity in the proli ferative attitude of malignant cells and their resistance to prooxidant dru gs and radiation therapy. (C) 1999 Elsevier Science Inc.