Selenium compounds are substrates for glutaredoxins: a novel pathway for selenium metabolism and a potential mechanism for selenium-mediated cytotoxicity

The Grx (glutaredoxin) proteins are oxidoreductases with a central function in maintaining the redox balance within the cell. In the present study, we have explored the reactions between selenium compounds and the glutaredoxin system. Selenite, GS-Se-SG (selenodiglutathione) and selenocystine were all shown to be substrates of human Grx1, implying a novel role for the glutaredoxins in selenium metabolism. During the past few years, selenium has further evolved as a potential therapeutic agent in cancer treatment, and a leading mechanism of cytotoxicity is the generation of ROS (reactive oxygen species). Both selenite and GS-Se-SG were reduced by Grx 1 and Grx2 in a non-stoichiometric manner due to redox cycling with oxygen, which in turn generated ROS. The role of Grx in selenium toxicity was therefore explored. Cells were treated with the selenium compounds in combination with transient overexpression of, or small interfering RNA against, Grx1. The results demonstrated an increased viability of the cells during silencing of Grx1, indicating that Grx1 is contributing to selenium toxicity. This is in contrast with TrxR (thioredoxin reductase), which previously was shown to protect cells from selenium cytotoxicity, verifying a diverse role between Grx and TrxR in selenium-mediated cytotoxicity. Furthermore, selenium treatment led to a marked increase in protein glutathionylation and cysteinylation that potentially can influence the activity and function of several proteins within the cell.