Inactivation of CES1 Blocks Prostaglandin D2 Glyceryl Ester Catabolism in Monocytes/Macrophages and Enhances Its Anti-inflammatory Effects, Whereas the Pro-inflammatory Effects of Prostaglandin E2 Glyceryl Ester Are Attenuated

Abstract

Human monocytic cells in blood have important roles in host defense and express the enzyme carboxylesterase 1 (CES1). This metabolic serine hydrolase plays a critical role in the metabolism of many molecules, including lipid mediators called prostaglandin glyceryl esters (PG-Gs), which are formed during cyclooxygenase-mediated oxygenation of the endocannabinoid 2-arachidonoylglycerol. Some PG-Gs have been shown to exhibit anti-inflammatory effects; however, they are unstable compounds, and their hydrolytic breakdown generates pro-inflammatory prostaglandins. We hypothesized that by blocking the ability of CES1 to hydrolyze PG-Gs in monocytes/macrophages, the beneficial effects of anti-inflammatory prostaglandin D₂-glyceryl ester (PGD₂-G) could be augmented. The goals of this study were to determine whether PGD₂-G is catabolized by CES1, evaluate the degree to which this metabolism is blocked by small-molecule inhibitors, and assess the immunomodulatory effects of PGD₂-G in macrophages. A human monocytic cell line (THP-1 cells) was pretreated with increasing concentrations of known small-molecule inhibitors that block CES1 activity [chlorpyrifos oxon (CPO), WWL229, or WWL113], followed by incubation with PGD₂-G (10 μM). Organic solvent extracts of the treated cells were analyzed by liquid chromatography with tandem mass spectrometry to assess levels of the hydrolysis product PGD₂. Further, THP-1 monocytes with normal CES1 expression (control cells) and "knocked-down" CES1 expression (CES1KD cells) were employed to confirm CES1's role in PGD₂-G catabolism. We found that CES1 has a prominent role in PGD₂-G hydrolysis in this cell line, accounting for about 50% of its hydrolytic metabolism, and that PGD₂-G could be stabilized by the inclusion of CES1 inhibitors. The inhibitor potency followed the rank order: CPO > WWL113 > WWL229. THP-1 macrophages co-treated with WWL113 and PGD₂-G prior to stimulation with lipopolysaccharide exhibited a more pronounced attenuation of pro-inflammatory cytokine levels (interleukin-6 and TNFα) than by PGD₂-G treatment alone. In contrast, prostaglandin E₂-glyceryl ester (PGE₂-G) had opposite effects compared to those of PGD₂-G, which appeared to be dependent on the hydrolysis of PGE₂-G to PGE₂. These results suggest that the anti-inflammatory effects induced by PGD₂-G can be further augmented by inactivating CES1 activity with specific small-molecule inhibitors, while pro-inflammatory effects of PGE₂-G are attenuated. Furthermore, PGD₂-G (and/or its downstream metabolites) was shown to activate the lipid-sensing receptor PPARγ, resulting in altered "alternative macrophage activation" response to the Th2 cytokine interleukin-4. These findings suggest that inhibition of CES1 and other enzymes that regulate the levels of pro-resolving mediators such as PGD₂-G in specific cellular niches might be a novel anti-inflammatory approach.