Chemical and biological mechanisms of phytochemical activation of Nrf2 and importance in disease prevention

Abstract

Plants are an incredibly rich source of compounds that activate the Nrf2 transcription factor, leading to upregulation of a battery of cytoprotective genes. This perspective surveys established and proposed molecular mechanisms of Nrf2 activation by phytochemicals with a special emphasis on a common chemical property of Nrf2 activators: the ability as "soft" electrophiles to modify cellular thiols, either directly or as oxidized biotransformants. In addition, the role of reactive oxygen/nitrogen species as secondary messengers in Nrf2 activation is discussed. While the uniquely reactive C151 of Keap1, an Nrf2 repressor protein, is highlighted as a key target of cytoprotective phytochemicals, also reviewed are other stress-responsive proteins, including kinases, which play non-redundant roles in the activation of Nrf2 by plant-derived agents. Finally, the perspective presents two key factors accounting for the enhanced therapeutic windows of effective phytochemical activators of the Keap1-Nrf2 axis: enhanced selectivity toward sensor cysteines and reversibility of addition to thiolate molecules.

Fig. 7.1

Structures of phytochemicals arranged by the chemistry that leads to Nrf2 activation. All compounds shown activate Nrf2. Thiol-reactive chemical motifs are shown in red, and “additive” distribution of electron-donating groups, which can be oxidized to quinoids, are shown in blue.

Fig. 7.2

The three roles a quinoid-forming polyphenol (represented by the R-catechol) can play: prooxidant, direct antioxidant, and indirect antioxidant. As a direct antioxidant, in the presence of a high concentration of free radical species, a polyphenol can trap the radical, forming a relatively stable radical species. As a prooxidant, in the presence of catalytic amounts of a transition metal, a polyphenol can promote the formation of superoxide and other ROS, enroute to formation of a Michael acceptor. An alternate path to oxidation of the polyphenol is catalyzed by a metalloenzyme and occurs without the production of ROS. Once the quinoid group is formed, the Michael acceptor group can react with a thiolate molecule. There is evidence that a quinone reacts with a key Keap1 sensor cysteine, C151, leading to Nrf2 activation, as described in the text. Upon activation, Nrf2 upregulates a battery of antioxidant enzymes and other cytoprotective enzymes, known as the indirect antioxidant effect. Reaction of the quinoid with GSH and subsequent elimination from the cell will lead to dilution of the effect. Alternative mechanisms of Nrf2 activation by radicals or ROS not depicted are oxidation of sensor cysteines, or formation of disulfides among sensor cysteines.

Fig. 7.3

Biotransformations implicated in conversions of quercetin (A), DAS (B), coffee triterpenoids (kahweol shown here) (C), and I3C and DIM (D) into thiol-reactive conjugated electrophiles: quinoids (quercetin), sulfone (DAS), epoxide or γ-ketoenal (kahweol) and indolenines (I3C & DIM). The established or proposed reactive groups are highlighted in red.

Fig 7.4

Structural features of Michael acceptors affecting the rates of forward and reverse reactions with thiols. Keap1 C151 is shown as an example thiol. (A) Brønsted acid catalysis by a neighboring residue (Keap1 K150). (B) Neighboring group (proximity) catalysis through hydrogen bonding with a hydroxyl adjacent to the β-carbon. (C) Alkene polyactivation, favoring addition. (D) Steric congestion, preventing additions by less reactive nucleophiles via transition state crowding. Cross-conjugation (E) and extended conjugation (F) stabilize an electrophile by diluting the partial positive charges at the electrophilic centers and thereby reducing its reactivity and promoting reversibility.

Fig. 7.5

Keap1 cysteines readily modified by phytochemicals, glutathione, and a quinone-forming polyphenol. Abbreviated name, chemical name and reference: XAN, xanthohumol [123]; ISO, isoliquiritigenin [123]; SHO, 10-shogaol [123]; SUL, sulforaphane [124, 125]; tBQ, tert-butylquinone [127]; and GSSG, oxidized glutathione [82]. Cysteines are ranked in order of most readily modified for all but tBQ, as determined by increasing concentrations of the electrophile, with the darkest circles being the most readily modified, and the empty circle indicating weakly modified or not modified cysteines. For tBQ, the cysteines are not ranked.