抄録
The Keap1-Nrf2 system plays a central role in cytoprotection against oxidative and electrophilic insults. Under normal conditions Keap1 serves as a substrate adaptor for Cullin3-based ubiquitin E3 ligase and promotes proteasomal degradation of Nrf2. Keap1 is inactivated upon the exposure to environmental stimuli such as electrophiles, and in this situation Nrf2 is stabilized and activates transcription of cytoprotective genes. Nrf2 knockout mouse (Nrf2–/–) is very sensitive to toxicants like acetaminophen. On the other hand, Keap1 knockdown mouse (Keap1flox/flox) in which Nrf2 is activated is resistant to toxicants. Whereas the turnover of Nrf2 has been well analyzed, little is known about the Keap1 turnover or recovery of the Keap1 activity after the electrophilic insults. We found that Keap1 is accumulated when autophagy is impaired in the liver of mice. Treatment of cells with an autophagy inhibitor or inducer markedly increased or decreased the Keap1 level, respectively. The Keap1 degradation was accelerated upon the exposure to certain type of electrophiles, implying that inactivated Keap1 after modification with electrophiles becomes a preferred substrate of autophagy. We also found that the electrophilic challenge enhances the transcription of Keap1 gene, suggesting that the Keap1 activity is recovered by de novo synthesis of Keap1. Consequently, Keap1 protein was kept in constant level even in the presence of electrophiles. These results thus demonstrate that Keap1 is degraded through autophagy and that this degradation machinery in concert with the de novo synthesis of Keap1 maintains the active Keap1 level and the redox homeostasis regulated by Nrf2.
