Keeping Wavering Bonds: Deactivation-Induced Signaling by Reactive Electrophiles

Discovered ∼60years ago, the lipid metabolite 4-hydroxynonenal (HNE) is linked to a plethora of macromolecular targets and biological functions. For a molecule that weighs 156 Da and possesses a single H-bond donor, this is quite a feat. Despite its chemical simplicity, HNE contains an α,β-unsaturated aldehyde system, endowing it with the capability to react covalently with numerous biological functional groups and bestowing on it pleiotropic properties. Regardless of the specific entity engaging with HNE, it is covalent bond formation that has dominated thought on HNE behavior. Indeed, cells possess a flurry of detoxifying enzymes that convert HNE to less reactive chemicals lacking the α,β-unsaturated aldehyde. For instance, the cell can either reduce or oxidize the aldehyde within HNE, deactivating HNE's chemical reactivity. Here, we discuss one of our recent papers that discovered that HNE can modify the detoxification enzyme, Cyp-33e1, in Caenorhabditis elegans, using a customized tissue-specific screen for HNE-sensor proteins. Consistent with the concepts of active site partitioning, HNE also emerged as a substrate of Cyp-33e1. We next discovered that HNE changed lipid storage in worms in a Cyp-33e1-dependent manner. We proposed that the product of Cyp-33e1 detoxifying HNE was responsible for this change in lipid storage and were able to show that 4-hydroxynonenoic acid (HNA), the product of Cyp-33e1 oxidation of HNE, causes this phenotype. We have dubbed this new signaling mode, "deactivation signaling". It sets an important precedent for how the bioactivity of HNE is considered, and we discuss the ramifications of this result in this perspective.