Activation of the NRF2 Signaling Pathway by Copper-Mediated Redox Cycling of Para- and Ortho-Hydroquinones

Transcription factor NF-E2 p45-related factor 2 (Nrf2) mediates adaptation to oxidants and electrophiles through up-regulating genes that contain antioxidant response elements (AREs) in their promoters. Using the stably transfected human AREc32 reporter cell line, we found that copper and other transition metals enhanced induction of ARE-driven luciferase by 2-tert-butyl-1,4-hydroquinone (tBHQ) as a result of increased oxidation to 2-tert-butyl-1,4-benzoquinone (tBQ). Following exposure to tBHQ for 30 min, ARE-luciferase activity measured after 24 hr was dependent on the presence of Cu²⁺. In contrast, tBQ-induced activity was Cu²⁺-independent. The metal-catalyzed oxidation of tBHQ to tBQ occured rapidly and stoichiometrically. Compounds that share para- or ortho-hydroquinone structures, such as catechol estrogens, dopamine, and l-DOPA, also induced ARE-driven luciferase in a Cu²⁺-dependent manner. Thus, the oxidation of para- and ortho-hydroquinones to quinones represents the rate-limiting step in the activation of Nrf2.