Molecular mechanisms of the conjugated alpha,beta-unsaturated carbonyl derivatives: relevance to neurotoxicity and neurodegenerative diseases

Abstract

Conjugated alpha,beta-unsaturated carbonyl derivatives such acrylamide, acrolein, and 4-hydroxy-2-nonenal (HNE) are members of a large class of chemicals known as the type-2 alkenes. Human exposure through diet, occupation, and pollution is pervasive and has been linked to toxicity in most major organs. Evidence suggests that these soft electrophiles produce toxicity by a common mechanism involving the formation of Michael-type adducts with nucleophilic sulfhydryl groups. In this commentary, the adduct chemistry of the alpha,beta-unsaturated carbonyls and possible protein targets will be reviewed. We also consider how differences in electrophilic reactivity among the type-2 alkenes impact corresponding toxicokinetics and toxicological expression. Whereas these concepts have mechanistic implications for the general toxicity of type-2 alkenes, this commentary will focus on the ability of these chemicals to produce presynaptic damage via protein adduct formation. Given the ubiquitous environmental presence of the conjugated alkenes, discussions of molecular mechanisms and possible neurotoxicological risks could be important. Understanding the neurotoxicodynamic of the type-2 alkenes might also provide mechanistic insight into neurodegenerative conditions where neuronal oxidative stress and presynaptic dysfunction are presumed initiating events. This is particularly germane to a recent proposal that lipid peroxidation and the subsequent liberation of acrolein and HNE in oxidatively stressed neurons mediate synaptotoxicity in brains of Alzheimer's disease patients. This endogenous neuropathogenic process could be accelerated by environmental type-2 alkene exposure because common nerve terminal proteins are targeted by alpha,beta-unsaturated carbonyl derivatives. Thus, the protein adduct chemistry of the conjugated type-2 alkenes offers a mechanistic explanation for the environmental toxicity induced by these chemicals and might provide insight into the pathogenesis of certain human neurodegenerative diseases.

FIG. 1.

(A) This figure illustrates the conjugated α,β-unsaturated carbonyl structure of chemicals in the type-2 alkene class. (B) This figure presents the corresponding line structures for acrolein, HNE, and other structurally related type-2 alkenes. Also shown are several examples of nonconjugated structural analogs.

FIG. 2.

The concentration-dependent effects of α,β-unsaturated alkene derivatives on dopamine transport in striatal synaptosomes (A) and corresponding sulfhydryl content (B). Data are expressed as mean percent control ± SEM and calculated IC₅₀’s are provided in parentheses. Results show that exposure of synaptosomes to a relatively broad concentration range (1μM–10mM) of type-2 alkenes produced parallel, concentration-dependent decreases in synaptosomal transport (A). The decreases in neurotransmitter transport induced by each structural congener were highly correlated (r² ≥ 0.91) to corresponding reductions in sulfhydryl content (B). Although differences in potency were evident, all conjugated analogs exhibited comparable neurotoxic efficacy; that is, each chemical was capable of producing maximal inhibitions of the measured neurochemical parameters and correspondingly depleted sulfhydryl contents. These structure-toxicity data are consistent with previous studies and suggest that nerve terminal toxicity mediated by sulfhydryl adduct formation is a class characteristic of the type-2 alkenes (see also Castegna et al., 2004; Keller et al., 1997a,b; Morel et al., 1999; Pocernich et al., 2001). These data also indicate that the synaptosomal toxicity of the type-2 alkenes is related to their common conjugated α,β-unsaturated structure (see LoPachin et al., 2007a,b; LoPachin et al., unpublished data, for more detailed discussions).

FIG. 3.

(A) Several molecular features in the cell body of an oxidatively stressed neuron. Acrolein and HNE are endogenous by-products of membrane lipid peroxidation that develop as a consequence of oxidative stress (a). These type-2 alkenes can form adducts with key sulfhydryl groups of the Keap1–Nrf2 complex (b). This promotes dissociation of this complex and subsequent nuclear translocation of Nrf2 that specifically targets genes with ARE or EpRE, within their promoter regions. These genes encode a subset of drug metabolizing enzymes such as glutathione S-transferases (GST) and NAD(P)H-quinone oxidoreductase 1 (NQO1) and antioxidant molecules such as heme oxygenase 1 (HO-1) and thioredoxin. The detoxifying and antioxidative stress enzymes/proteins generated by the Keap1–Nrf2 pathway offer nerve cell protection from electrophilic attack. The relatively fast turnover of resident proteins also works in favor of the nerve cell body (c), because adducted and dysfunctional proteins can be quickly replaced and the operation of neuroprocesses maintained. (B) The possible toxicological events initiated by oxidative stress (a) in the nerve terminal. Thus, although protein adduction begins in the cell body (A) and progresses during axonal anterograde transport, adduction continues at the nerve terminal and results in a relatively large pool of adducted, dysfunctional proteins (b). These abnormal proteins participate in broad presynaptic functions; for example, neurotransmitter uptake, release and storage. Because turnover is relatively slow at the nerve terminal (b), protein adducts are replaced slowly and, therefore, accumulate. The build-up of functionally compromised proteins produces a progressive, cumulative neurotoxicity. Nerve terminals are also susceptible to electrophilic attack because, in the absence of transcriptional/translational capacity, an ARE/EpRE-type reaction cannot be initiated (c). Finally, many of the adducted cysteine sites are NO acceptors and, as a consequence, irreversible type-2 alkene adduction disrupts transient and spatially precise NO signaling. Nerve terminals are highly vulnerable to such disruption, because NO signaling modulates numerous critical presynaptic processes involved in neurotransmission; for example, the synaptic vesicle cycle (d). Decreased neurotransmission will disrupt regional neural circuits and higher order networks, which might be responsible for the difficulties in declarative memory that characterize AD (Buckner, 2004; Palop et al., 2006; Walsh and Selkoe, 2004). The peroxidation-based electrophile burdens in both nerve terminal and cell body are additive to preexisting type-2 alkene backgrounds that result from lifetime environmental exposure; for example, through food—symbolized by the French fries; through ambient/occupational exposure—symbolized by the industrial plant.