Heteroaromatic 4-arylquinols are novel inducers of nuclear factor-erythroid 2-related factor 2 (Nrf2)

The induction of phase 2 and antioxidant enzymes via the transcription factor Nuclear factor-erythroid 2-related factor 2 (Nrf2) is an important chemopreventive strategy in cancer and neurodegenerative diseases. Nrf2 is mainly regulated at the level of its protein stability by the cytosolic protein Keap1, which functions as a substrate recruiting subunit of a Cullin3 E3 ubiquitin ligase to target Nrf2 for ubiquitination and subsequent degradation. Phase 2 inducing agents usually covalently modify cysteine residues in Keap1, leading to inhibition of Nrf2 ubiquitination. Quinols, which due to their Michael acceptor moiety react readily with cysteine residues in selective cellular proteins, are good candidates for potential Nrf2 inducing chemopreventive agents. Indeed, we found that similar to the known phase II inducer sulforaphane, the heteroaromatic 4-arylquinols PMX290 and PMX464 increase both Nrf2 protein concentrations and transcriptional activity. Interestingly, PMX290 had a much stronger effect on the Nrf2 protein concentration, but a weaker effect on Nrf2 transcriptional activity compared to PMX464. Given the marked effect of PMX290 on the Nrf2 protein concentration, we examined its effect on the interaction of Keap1 with its binding partners. While sulforaphane was found to decrease binding of Cullin3 to Keap1, PMX290 markedly increased the interaction between these two proteins in intact cells. PMX464, which increased Nrf2 protein only weakly, also had a much smaller effect on the binding between Keap1 and Cullin3. In conclusion, PMX290 is a novel phase 2 inducing agent which increases the interaction between Keap1 and Cullin3 and may inhibit Nrf2 ubiquitination via a novel mechanism.