Phenolic Michael Reaction Acceptors: Combined Direct and Indirect Antioxidant Defenses Against Electrophiles and Oxidants

Abstract

The implications of oxidative stress in the pathogenesis of many chronic human diseases has led to the widely accepted view that low molecular weight antioxidants could be beneficial and postpone or even prevent these diseases. Small molecules of either plant or synthetic origins, which contain Michael acceptor functionalities (olefins or acetylenes conjugated to electron-withdrawing groups) protect against the toxicity of oxidants and electrophiles indirectly, i.e., by inducing phase 2 cytoprotective enzymes. Some of these molecules, e.g., flavonoid and curcuminoid analogues that have phenolic hydroxyl groups in addition to Michael acceptor centers, are also potent direct antioxidants, and may therefore be appropriately designated: bifunctional antioxidants. By use of spectroscopic methods we identified phenolic chalcone and bis(benzylidene)acetone analogues containing one or two Michael acceptor groups, respectively, as very efficient scavengers of two different types of radicals: (a) the nitrogen-centered 2,2'-azinobis-(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS.+) radical cation, and (b) the oxygen-centered galvinoxyl (phenoxyl) radical. The most potent scavengers are those also bearing hydroxyl substituents on the aromatic ring(s) at the ortho-position(s). The initial reaction velocities are very rapid and concentration-dependent. In the human keratinocyte cell line HaCaT, the same compounds coordinately increase the intracellular levels of glutathione, glutathione reductase, and thioredoxin reductase. Thus, such bifunctional antioxidants could exert synergistic protective effects against oxidants and electrophiles which represent the principal biological hazards by: (i) scavenging hazardous oxidants directly and immediately; and (ii) inducing the phase 2 response to prevent and resolve the consequences of hazardous processes that are already in progress, i.e., acting indirectly, but with much more diverse and long-lasting effects.

Direct and indirect antioxidant properties of inducers of cytoprotective proteins

“…In certain cases (i. e., within the class of cyclic and acyclic bis(benzylidene)alkanones), the ratio of the potencies of the hydroxylated and the nonhydroxylated derivatives is greater than 200 times. In addition to NQO1, these compounds coordinately increase the intracellular levels of glutathione and the activities of glutathione reductase and thioredoxin reductase [33,79]. The plant phenylpropenoids are abundant natural products that occur in various species and their inducer properties have been reviewed [85].…”

“…They inhibit the chemiluminescence resulting from the reaction of lucigenin with the superoxide radical, generated in the xanthine/xanthine oxidase system [83], quench the oxygen-centered galvinoxyl (phenoxyl) radical, and the nitrogen-centered 2,29-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS9 + ) radical cation [33]. In all three assays, the most potent scavengers are those also bearing hydroxyl substituents on the aromatic ring(s) at the ortho-position(s).…”

“…These inducers in turn can be considered indirect antioxidants. Similarly, glutathione reductase, which plays a major role in regenerating reduced glutathione, and glutathione peroxidase are also elevated by inducers of the phase 2 response through the Keap1/Nrf2/ARE system [31][32][33]. Further complexity is added to this relationship by the fact that GSH is an obligatory participant in the reactions catalyzed by several cytoprotective enzymes (e.g., glutathione transferases, glutathione peroxidase), as well as in both the nonenzymatic and the enzymatic reductions of dehydroascorbate that maintain ascorbate in its reduced state [34,35].…”

“…Importantly, the levels of both thioredoxin and thioredoxin reductase are transcriptionally upregulated by inducers of cytoprotective enzymes through their respective AREs [33,[41][42][43][44].…”

Cinnamamides, Novel Liver X Receptor Antagonists that Inhibit Ligand-Induced Lipogenesis and Fatty Liver

“…This aromatic fatty acid, which has an a,b-unsaturated carbonyl moiety, generally exhibits what are termed Michael acceptor functionalities. Many phase 2 enzyme inducers and anticancer drugs, including hydroquinones and phenylpropenoids (chalcones and flavonoids), are cinnamic acid derivatives (Dinkova-Kostova et al, 2007). Recently, cinnamic acid amides and related compounds have attracted increasing attention because of their antimicrobial, antioxidant, and anticancer properties (Patel and Telvekar, 2014;Natella et al, 1999;De et al, 2011).…”