Nrf2 regulates the expression of genes encoding antioxidant proteins involved in cellular redox homeostasis. Previous studies have suggested that activation of Nrf2 is mediated by mechanisms promoting its dissociation from Keap1, a cytosolic repressor that acts to sequester the transcription factor in the cytoplasm. As a short-lived protein, Nrf2 is also activated by mechanisms leading to its stabilization in cells under stress, and recent evidence indicates that Keap1 has an active role in the control of its stability. In this report, using immunocytochemistry, cell fractionation, and chromatin immunoprecipitation analyses, we found that Nrf2 is primarily a nuclear protein and that it is expressed and recruited to the chromatin constitutively to drive basal gene expression. Furthermore, we found evidence indicating that Keap1 may repress Nrf2 activity by transiently shuttling into the nucleus to promote its ubiquitylation. The data suggested that the steady-state level of Nrf2 is maintained by a dynamic pathway that balances its constitutive expression with a Keap1-regulated degradation process downstream of its role as a transcriptional activator. We suggest that the stabilization of Nrf2 in cells under stress represents the central regulatory response mediated by mechanisms that interfere with its interaction with Keap1, leading to the induction of antioxidant enzymes important to maintain cellular redox homeostasis.
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