Metabolism of prostaglandin glyceryl esters (PG‐Gs) by human carboxylesterases, CES1 and CES2, and its inhibition by bioactive metabolites of organophosphate (OP) insecticides

Abstract

PG‐Gs are derived from the oxidation of 2‐arachidonoylglycerol by COX‐2 and have roles in inflammation and pain. Currently, the enzyme(s) responsible for degrading PG‐Gs are not well defined. Carboxylesterases (CES) are nonspecific hydrolases and candidate enzymes. Two CES isoforms are found in human tissues, CES1 and CES2. Here, we determined if PG‐Gs are hydrolyzed by human CES and then investigated the hydrolytic metabolism of these compounds in human THP1 monocytes/macrophages. Further, we determined if metabolites of OP insecticides could inhibit PG‐G metabolism in these cells. We show that recombinant human CES1 and CES2 effectively hydrolyze PGF2α‐G and PGE2‐G [CES1: (kcat/Km)PGF2α‐G = 0.60 min−1 μM−1and (kcat/Km)PGE2‐G = 0.37 min−1μM−1; CES2: (kcat/Km)PGF2α‐G = 1.7 min−1μM−1and (kcat/Km)PGE2‐G = 0.23 min−1μM−1]. In THP1 monocytes/macrophages, we demonstrated using selective CES1 inhibitors, immunoprecipitation, and immunoblotting that CES1 is likely the main enzyme responsible for hydrolyzing PGE2‐Gs, and that MAGL and FAAH do not have significant roles. In addition, pre‐treatment of intact cells with OP metabolite (0.1–10 μM paraoxon) significantly reduced the hydrolysis rate of exogenously added PGE2‐G (~20–70%, p<0.05). These findings suggest that exposure of CES1‐ and/or CES2‐expressing cells to bioactive OP metabolites may perturb the PG‐G tone in tissues composed of these cells, thus potentially inducing a pro‐inflammatory state.Supported by NIH R15 ES015348.