The beta-TrCP-mediated Pathway Cooperates with the Keap1-mediated Pathway in Nrf2 Degradation in Vivo. Mus musculus

Nrf2 activates cytoprotective gene expression, and Nrf2 activity is regulated through two protein degradation pathways: the Keap1-mediated and beta-TrCP-mediated pathways. To address the relative contributions of these pathways, we generated knock-in mouse lines expressing an Nrf2SA mutant that harbored two substitution mutations of serine residues interacting with beta-TrCP. The homozygous (Nrf2SA/SA) mice grew normally, with Nrf2 levels comparable to those of WT (wild-type) mice under unstressed conditions. However, when Keap1 activity was suppressed, high levels of Nrf2 accumulated in Nrf2SA/SA macrophages compared with that in WT macrophages. We crossed Nrf2SA/SA mice with mice in which Keap1 was knocked down to two levels and found that the Nrf2SA/SA mutation induced higher Nrf2 activity when the Keap1 level was strongly reduced, and these mice showed severe growth retardation. However, activation and growth retardation were not evident when Keap1 was moderately suppressed. These increases in Nrf2 activity induced by the Nrf2SA mutation caused severe hyperplasia and hyperkeratosis in the esophageal epithelium but did not cause abnormalities in the other tissues/organs examined. These results indicate that the beta-TrCP-mediated pathway cooperates with the Keap1-mediated pathway to regulate Nrf2 activity, which is apparent when the Keap1-mediated pathway is profoundly suppressed.