Regulation of Ubiquitin-conjugating Enzymes by Glutathione Following Oxidative Stress

Abstract

Upon oxidative stress cells show an increase in the oxidized glutathione (GSSG) to reduced glutathione (GSH) ratio with a concomitant decrease in activity of the ubiquitinylation pathway. Because most of the enzymes involved in the attachment of ubiquitin to substrate proteins contain active site sulfhydryls that might be covalently modified (thiolated) upon enhancement of GSSG levels (glutathiolation), it appeared plausible that glutathiolation might alter ubiquitinylation rates upon cellular oxidative stress. This hypothesis was explored using intact retina and retinal pigment epithelial (RPE) cell models. Exposure of intact bovine retina and RPE cells to H 2 O 2 (0.1-1.7 mol/mg) resulted in a dose-dependent increase in the GSSG:GSH ratio and coincident dose-dependent reductions in the levels of endogenous ubiquitin-activating enzyme (E1)-ubiquitin thiol esters and endogenous protein-ubiquitin conjugates and in the ability to form de novo retinal protein-125 I-labeled ubiquitin conjugates. Oxidant-induced decrements in ubiquitin conjugates were associated with 60 -80% reductions in E1 and ubiquitin-conjugating enzyme (E2) activities as measured by formation of ubiquitin thiol esters. When GSH levels in RPE cells recovered to preoxidation levels following H 2 O 2 removal, endogenous E1 activity and protein-ubiquitin conjugates were restored. Evidence that S thiolation of E1 and E2 enzymes is the biochemical link between cellular redox state and E1/E2 activities includes: (i) 5-fold increases in levels of immunoprecipitable, dithiothreitollabile 35 S-E1 adducts in metabolically labeled, H 2 O 2 -treated, RPE cells; (ii) diminished formation of E1-and E2-125 I-labeled ubiquitin thiol esters, oligomerization of E2 25K , and coincident reductions in protein-125 I-labeled ubiquitin conjugates in supernatants from nonstressed retinas upon addition of levels of GSSG equivalent to levels measured in oxidatively stressed retinas; and (iii) partial restoration of E1 and E2 activities and levels of protein-125 I-labeled ubiquitin conjugates in supernatants from H 2 O 2 -treated retinas when GSSG:GSH ratios were restored to preoxidation levels by the addition of physiological levels of GSH. These data suggest that the cellular redox status modulates protein ubiquitinylation via reversible S thiolation of E1 and E2 enzymes, presumably by glutathione.Oxidative stress damages cells, and this damage is causally implicated in "normal" aging (1, 2) and in the pathogenesis of human diseases, including eye lens cataract and retinopathy (1), neurodegenerative diseases (reviewed in Ref. 3), diabetes (4), and cancer (5). Thus, elucidation of biochemical mechanisms that protect cells from or promote cellular recovery following oxidative stress are of vital importance. Studies suggest that ubiquitin, a highly conserved 76-residue protein, is essential for viability following stress (6 -8) and that ubiquitin-dependent processes play an important role in cellular resistance to oxidative insult (6, 9, 10).The principle mechanism of ubiqui...

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Proteasome-dependent regulation of signal transduction in retinal pigment epithelial cells

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