Critical to the function of mast cells in immune responses including allergy is their production of lipid mediators, among which only omega-6 (omega-6) arachidonate-derived eicosanoids have been well characterized. Here, by employing comprehensive lipidomics, we identify omega-3 (omega-3) fatty acid epoxides as new mast cell-derived lipid mediators and show that they are produced by PAF-AH2, an oxidized-phospholipid-selective phospholipase A2. Genetic or pharmacological deletion of PAF-AH2 reduced the steady-state production of omega-3 epoxides, leading to attenuated mast cell activation and anaphylaxis following FcvarepsilonRI cross-linking. Mechanistically, the omega-3 epoxides promote IgE-mediated activation of mast cells by downregulating Srcin1, a Src-inhibitory protein that counteracts FcvarepsilonRI signaling, through a pathway involving PPARg. Thus, the PAF-AH2-omega-3 epoxide-Srcin1 axis presents new potential drug targets for allergic diseases.
Serine hydrolases are a diverse enzyme class representing approximately 1% of all human proteins. The biological functions of most serine hydrolases remain poorly characterized owing to a lack of selective inhibitors to probe their activity in living systems. Here we show that a substantial number of serine hydrolases can be irreversibly inactivated by 1,2,3-triazole ureas, which show negligible cross-reactivity with other protein classes. Rapid lead optimization by click chemistry-enabled synthesis and competitive activity-based profiling identified 1,2,3-triazole ureas that selectively inhibit enzymes from diverse branches of the serine hydrolase class, including peptidases (acyl-peptide hydrolase, or APEH), lipases (platelet-activating factor acetylhydrolase-2, or PAFAH2) and uncharacterized hydrolases (alpha,beta-hydrolase-11, or ABHD11), with exceptional potency in cells (sub-nanomolar) and mice (<1 mg kg(-1)). We show that APEH inhibition leads to accumulation of N-acetylated proteins and promotes proliferation in T cells. These data indicate 1,2,3-triazole ureas are a pharmacologically privileged chemotype for serine hydrolase inhibition, combining broad activity across the serine hydrolase class with tunable selectivity for individual enzymes.
