Relationship of electrophilic stress to aging

Abstract

This review begins with the premise that an organism's life span is determined by the balance between two countervailing forces: (i) the sum of destabilizing effects and (ii) the sum of protective longevity-assurance processes. Against this backdrop, the role of electrophiles is discussed, both as destabilizing factors and as signals that induce protective responses. Because most biological macromolecules contain nucleophilic centers, electrophiles are particularly reactive and toxic in a biological context. The majority of cellular electrophiles are generated from polyunsaturated fatty acids by a peroxidation chain reaction that is readily triggered by oxygen-centered radicals, but propagates without further input of reactive oxygen species (ROS). Thus, the formation of lipid-derived electrophiles such as 4-hydroxynon-2-enal (4-HNE) is proposed to be relatively insensitive to the level of initiating ROS, but to depend mainly on the availability of peroxidation-susceptible fatty acids. This is consistent with numerous observations that life span is inversely correlated to membrane peroxidizability, and with the hypothesis that 4-HNE may constitute the mechanistic link between high susceptibility of membrane lipids to peroxidation and shortened life span. Experimental interventions that directly alter membrane composition (and thus their peroxidizability) or modulate 4-HNE levels have the expected effects on life span, establishing that the connection is not only correlative but causal. Specific molecular mechanisms are considered, by which 4-HNE could (i) destabilize biological systems via nontargeted reactions with cellular macromolecules and (ii) modulate signaling pathways that control longevity-assurance mechanisms.

Fig. 1

Examples of reactions of electrophiles with nucleophiles. Panel A: A general reaction scheme in which an electron-rich nucleophile Nu donates an electron pair to form a bond with electrophile El. Panel B: Aldol condensation of acetaldehyde, a reaction in which a carbanion, derived from acetaldehyde by action of a base, attacks an electrophilic carbon of another acetaldehyde molecule. A new carbon-carbon bond is formed in the process. Panel C: Reaction of a nucleophilic thiol group, such as in a cysteine side chain in proteins or in glutathione, with an electrophilic center on carbon 3 of 4-hydroxynon-2-enal (4-HNE). The reaction, a Michael addition, leads to the formation of a thioether. In all three panels, the electrophilic molecule or electrophilic center is highlighted by grey shading.

Fig. 2

The pathway of tyrosine catabolism. Fumarylacetoacetic acid (boxed structure) accumulates in tyrosinemia type I, in which fumarylacetoacetate hydrolase activity is impaired. Fumarylacetoacetate is an electrophile (Michael acceptor) because it contains a double bond conjugated to a carbonyl group.

Fig. 3

Generation of 4-hydroxynon-2-enal (4-HNE) and 4-hydroxyhex-2-enal from PUFAs. Part A: n-6 PUFAs (the examples of linoleic and arachidonic acids are shown) are attacked by ROS, typically a hydroxyl radical ˙OH, or undergo a lipoxygenase-catalyzed reaction (not shown), to form a lipid hydroperoxide. The latter is non-enzymatically converted to end-products that include 4-HNE. Part B: n-3 PUFAs, exemplified by α-linolenic and docosahexaenoic acids, yield 4-hydroxyhex-2-enal in a reaction sequence analogous to that shown for n-6 PUFAs. These reactions convert an initial oxidative stress to electrophilic stress.

Fig. 4

ROS-triggered lipid peroxidation chain reaction. Initiation: A hydroxyl radical (˙OH) reacts with a PUFA, usually part of a phospholipid in a biological membrane, abstracting a hydrogen atom. In this process, ˙OH is converted to water and a carbon-centered radical is formed on the fatty acid. Propagation: Dioxygen is added to the carbon-centered radical, forming in several steps an oxygen-centered peroxyl radical LOO˙. The latter abstracts a hydrogen from another PUFA and, in the process, is converted to a fatty acid hydroperoxide (LOOH). The PUFA from which a hydrogen was abstracted gives rise to a carbon-centered radical, thus completing the reaction cycle. A single initiation can lead to multiple propagation cycles as long as dioxygen and PUFAs are available. Termination: The peroxyl radical LOO˙ can react with another LOO˙ or with another radical, resulting in non-radical end products. Alternatively, LOO˙ can react with a sacrificial radical scavenger (antioxidant) which gives rise to a stable radical that lacks the ability to abstract hydrogen from PUFA. In either case, the lipid peroxidation chain reaction is terminated.

Fig. 5

Relationship between the concentrations of initial ROS and resulting lipid peroxidation products. Panels A and B: Two theoretical extreme cases are schematically depicted in which the lipid peroxidation chain reaction terminates after a single cycle (A), or does not terminate until all PUFA substrate is used up (B). In the first case (panel A), a hydroxyl radical ˙OH attacks a PUFA (denoted by a bent fatty acyl chain in a membrane phospholipid), resulting in the formation of a lipid peroxidation product, for simplicity denoted as 4-HNE. The chain reaction is then immediately terminated (shown by the red “×” symbol). Thus, the formation of each 4-HNE molecule requires a separate attack on a PUFA by ˙OH. At the other extreme (panel B), a single initiation event starts a chain reaction (red arrows) that continues indefinitely, generating a 4-HNE molecule from each PUFA. Panels C and D depict graphically the two idealized relationships (corresponding to A and B, respectively) between the initiating ˙OH concentration and the amount of resulting 4-HNE. The black and blue lines in panels C and D denote, respectively, a higher and lower content of peroxidizable PUFA in the membrane. For rapidly terminating chain reactions (panel C, depicting results of the mechanism shown in panel A), there is a stoichiometric relationship between [˙OH] and [4-HNE] over a wide range of [˙OH], including a range that is physiologically normal (grey zone in panel C). In this range, the amount of 4-HNE formed is sensitive to the level of oxidative stress but not to the PUFA amount in the membrane, except for a very intensive oxidative burst that would deplete all PUFAs simultaneously (saturation point in panel C). Only under conditions of very high oxidative stress does the PUFA amount play a role; for example, less peroxidizable PUFA would lead to less 4-HNE formed under saturating [˙OH] (blue line). In the case of unlimited propagation of the chain reaction (panel D, illustrating results from the mechanism shown in panel B), a very small amount of ˙OH initiates a reaction that uses up all available PUFA and produces a maximal amount of 4-HNE. Thus, under these conditions, the formation of 4-HNE is a function of the content of peroxidizable PUFA in the membrane (black versus blue line in panel D) but is independent on the ˙OH concentration, including the physiological [˙OH] range (grey zone). Note that the two depicted situations are idealized extremes; an actual membrane may exhibit an intermediate behavior.

Fig. 6

Modes of 4-HNE metabolism. Biological elimination of 4-HNE can proceed via four primary reactions: (1) conjugation with glutathione, (2) oxidation of the aldehyde group, (3) reduction of the aldehyde group, and/or (4) reduction of the double bond. Secondary reactions and/or transport usually follow the primary reactions.

Fig. 7

Correlation of C. elegans life span with the overall capacity of the organism to conjugate 4-HNE (panel A) and with the amount of 4-HNE-protein adducts (panel B). The points represent experimental interventions in which the expression of the endogenous gst-10 gene was silenced by RNA interference (gst-10 RNAi), the expression of the same gene was increased by transgenic overexpression (gst-10 tg), or the murine mGsta4 gene was transgenically expressed using the gst-10 promoter (mGsta4 tg). The Pearson correlation coefficients R and the associated P values are given in each panel. Based on data from refs. [110,174,175].

Fig. 8

Modulation by 4-HNE of selected signaling pathways that affect autophagy. Branches of the pathways, labeled 1 through 4 in the scheme, are referred to in the text.