Molecular basis of electrophilic and oxidative defense: promises and perils of Nrf2

Abstract

Induction of drug-metabolizing enzymes through the antioxidant response element (ARE)-dependent transcription was initially implicated in chemoprevention against cancer by antioxidants. Recent progress in understanding the biology and mechanism of induction revealed a critical role of induction in cellular defense against electrophilic and oxidative stress. Induction is mediated through a novel signaling pathway via two regulatory proteins, the nuclear factor erythroid 2-related factor 2 (Nrf2) and the Kelch-like erythroid cell-derived protein with CNC homology-associated protein 1 (Keap1). Nrf2 binds to Keap1 at a two site-binding interface and is ubiquitinated by the Keap1/cullin 3/ring box protein-1-ubiquitin ligase, resulting in a rapid turnover of Nrf2 protein. Electrophiles and oxidants modify critical cysteine thiols of Keap1 and Nrf2 to inhibit Nrf2 ubiquitination, leading to Nrf2 activation and induction. Induction increases stress resistance critical for cell survival, because knockout of Nrf2 in mice increased susceptibility to a variety of toxicity and disease processes. Collateral to diverse functions of Nrf2, genome-wide search has led to the identification of a plethora of ARE-dependent genes regulated by Nrf2 in an inducer-, tissue-, and disease-dependent manner to control drug metabolism, antioxidant defense, stress response, proteasomal degradation, and cell proliferation. The protective nature of Nrf2 could also be hijacked in a number of pathological conditions by means of somatic mutation, epigenetic alteration, and accumulation of disruptor proteins, promoting drug resistance in cancer and pathologic liver features in autophagy deficiency. The repertoire of ARE inducers has expanded enormously; the therapeutic potential of the inducers has been examined beyond cancer prevention. Developing potent and specific ARE inducers and Nrf2 inhibitors holds certain new promise for the prevention and therapy against cancer, chronic disease, and toxicity.

Fig. 1

Representative chemical types of ARE inducers. HQ, hydroquinone; BQ, benzoquinone; PDA, phenylenediamine; NCHN, 1-nitrocyclohex-1-ene; PCDT, pyrrolidine-1-carbodithioate; BAL, 2,3-dimercaptopropanol; HPPB, (2-hydroperoxypropan-2-yl)benzene; PAO, phenylarsine oxide; PMC, phenylmercury(II) chloride; TMONTMC, 2,2′-((1E,3E,5E,7E,9E,11E,13E,15E,17E)-3,7,12,16-tetramethyloctadeca-1,3,5,7,9,11,13,15,17-nonaene-1,18-diyl)bis(1,3,3-trimethylcyclohex-1-ene).

Fig. 2

ARE inducers from different sources. PEITC, phenethyl isothiocyanate; 15d-PGJ₂, 15-deoxy-Δ^12,14-prostaglandin J₂.

Fig. 3

Domain structures of mouse Nrf2, small Mafs, and Keap1. NTR, N-terminal region.

Fig. 4

Models of Nrf2 regulation. Under basal conditions, Nrf2 is rapidly polyubiquitinated through the Keap1/Cul3/Rbx1–E3 and degraded by the 26S-proteasomes resulting in a short half-life. A small portion of Nrf2 accumulates in the nucleus to mediate basal expression of ARE-dependent genes to contribute to maintenance of homeostasis. Under stress conditions or in the presence of chemoprotective agents, ARE inducers modify the cysteine codes of Keap1 and Nrf2 to inhibit polyubiquitination of Nrf2 resulting in activation of Nrf2 and induction of ARE-dependent genes to resist stress. Nrf2 plays a more important role in the induction of genes, such as NQO1 and GST, than in their basal expression, because, presumably, other signaling pathways support basal expression of ARE-dependent genes in addition to Nrf2, which may explain why Nrf2 KO mice have a largely normal phenotype in unstressed conditions. In certain pathological scenarios, such as cancer and autophagy deficiency, interaction between Nrf2 and Keap1 or between Keap1 and Cul3 is altered through mutations, epigenetic changes, accumulation of disruptor proteins, leading to inhibition of Nrf2 ubiquitination and persistent hyperactivation of Nrf2, a condition that promotes tumor growth, drug resistance, and toxicity.

Fig. 5

Cysteine residues of Nrf2 and Keap1.

Fig. 6

Triterpenoid inducers and Michael reaction with cysteine thiols. A, structures of selected triterpenoids and tricyclic and monocyclic cyano enones. Two Michael accepter motifs of triterpenoids are shown in boxs with broken lines. B, illustration of Michael adduct formation between Keap1 cysteine thiols and TBE 31 at two cyano enone sites.