Increased protein stability as a mechanism that enhances Nrf2-mediated transcriptional activation of the antioxidant response element: Degradation of Nrf2 by the 26 S proteasome

Nrf2 (NF-E2-related factor 2) is a central transcription factor involved in the transcriptional activation of many genes encoding phase II drug-metabolizing enzymes via the antioxidant response element. Nrf2 has previously been found to undergo nuclear translocation by a phos-phorylation-dependent mechanism mediated by protein kinase C in HepG2 cells treated with tert-butylhydroquinone, β-naphthoflavone, or 12-O-tetradecanoylphorbolβ-acetate. In the present report, we have found that the levels of Nrf2 were increased in cells treated with tertbutylhydroquinone or β-naphthoflavone by a post-tran-. scriptional mechanism. Treatment of HepG2 cells with cycloheximide resulted in the loss of Nrf2 within 30 min. By contrast, treatment with the proteasome inhibitors (lactacystin or MG-132) caused an accumulation of Nrf2 as well as an induction of reporter gene activity in cells transfected with the GSTA2 antioxidant response element-chloramphenicol acetyl transferase construct. Similarly, the protein phosphatase inhibitor okadaic acid also caused an accumulation of Nrf2, whereas the reverse effects were observed with PD 98059 and U 0126, two compounds that block the activation of the MAPK/ERK signaling cascade. These data suggest that Nrf2 is degraded by the ubiquitin-dependent pathway and that phosphorylation of Nrf2 leads to an increase in its stability and subsequent transactivation activity.