Activity of Ubiquitin-dependent Pathway in Response to Oxidative Stress

Abstract

Relations between the ubiquitin pathway and cellular stress have been noted, but data regarding responses of the ubiquitin pathway to oxidative stress are scanty. This paper documents the response of this pathway to oxidative stress in lens cells. A brief exposure of lens epithelial cells to physiologically relevant levels of H2O2 induces a transient increase in activity of the ubiquitin- dependent pathway. Ubiquitin conjugation activity was maximal and increased 3.5–9.2-fold over the activity noted in untreated cells by 4 h after removal of H2O2.By 24 h after removal of H2O2, ubiquitin conjugation activ- ity returned to the level noted in untreated cells. In parallel to the changes in ubiquitin conjugation activity, the activity of ubiquitin-activating enzyme (E1), as de- termined by thiol ester formation, increased 2–6.7-fold during recovery from oxidation. Addition of exogenous E1 resulted in an increase in ubiquitin conjugation ac- tivity and in the levels of ubiquitin carrier protein (E2)- ubiquitin thiol esters in both the untreated cells and the H2O2-treated cells. These data suggest that E1 is the rate-limiting enzyme in the ubiquitin conjugation proc- ess and that the increases in ubiquitin conjugation ac- tivity which are induced upon recovery from oxidation are primarily due to increased E1 activity. The oxida- tion- and recovery-induced up-regulation of E1 activity is primarily due to post-synthetic events. Substrate availability and up-regulation of E2 activities also ap- pear to be related to the enhancement in ubiquitinyla- tion upon recovery from oxidative stress. The oxidation- induced increases in ubiquitin conjugation activity were associated with an increase in intracellular prote- olysis, suggesting that the transient increase in ubiquiti- nylation noted upon recovery from oxidative stress may play a role in removal of damaged proteins from the cells.