The electrophilic metabolite of kynurenine, kynurenine-CKA, requires C151 in Keap1 to derepress Nrf2

The Kelch-like ECH-associated protein 1/Nuclear factor-erythroid 2 p45-related factor 2 (Keap1/Nrf2) system responds to a wide array of structurally diverse small molecules, of both exogenous and endogenous origin, by inducing a robust cytoprotective program that allows adaptation during oxidative, metabolic and inflammatory stress. Here, we report that exposure to the tryptophan metabolite kynurenine and its electrophilic derivative kynurenine-carboxyketoalkene (Kyn-CKA) leads to an increase in the abundance of transcription factor Nrf2 and induction of Nrf2-target genes, including NAD(P)H:quinone oxidoreductase 1 (NQO1), in murine and human cells. Additionally, both kynurenine and Kyn-CKA activate the aryl hydrocarbon receptor (AhR). Using cellular thermal shift assays, we found that Kyn-CKA increases the thermal stability of Keap1-mCherry fusion protein, but not free mCherry, indicating target engagement of Keap1, the principal repressor of Nrf2. Critically, the ability of Kyn-CKA to increase the abundance of Nrf2 and expression of NQO1 in mouse embryonic fibroblasts (MEFs) expressing wild-type Keap1 was greatly diminished in C151S-Keap1 mutant MEFs. Furthermore, Kyn-CKA reacts with Keap1 C151 much faster in vitro than with the small molecule thiol N-acetyl cysteine, suggesting that Kyn-CKA is targeted to C151 by the surrounding active site. Experiments in wild-type, AhR-knockout, and Nrf2-knockout primary murine bone marrow-derived macrophages showed that Nrf2 is required for the acute anti-inflammatory activity of Kyn-CKA, whereas AhR is dispensable. Together, these findings demonstrate that Kyn-CKA requires C151 in Keap1 to derepress Nrf2 and reveal that Nrf2, but not AhR, is a main contributor to the anti-inflammatory activity of Kyn-CKA in macrophages.