Substrate Report for: Prostaglandin-D2-Glyceryl-Ester

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 D(2)-glyceryl ester (PGD(2)-G) could be augmented. The goals of this study were to determine whether PGD(2)-G is catabolized by CES1, evaluate the degree to which this metabolism is blocked by small-molecule inhibitors, and assess the immunomodulatory effects of PGD(2)-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 PGD(2)-G (10 M). Organic solvent extracts of the treated cells were analyzed by liquid chromatography with tandem mass spectrometry to assess levels of the hydrolysis product PGD(2). 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 PGD(2)-G catabolism. We found that CES1 has a prominent role in PGD(2)-G hydrolysis in this cell line, accounting for about 50% of its hydrolytic metabolism, and that PGD(2)-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 PGD(2)-G prior to stimulation with lipopolysaccharide exhibited a more pronounced attenuation of pro-inflammatory cytokine levels (interleukin-6 and TNFalpha) than by PGD(2)-G treatment alone. In contrast, prostaglandin E(2)-glyceryl ester (PGE(2)-G) had opposite effects compared to those of PGD(2)-G, which appeared to be dependent on the hydrolysis of PGE(2)-G to PGE(2). These results suggest that the anti-inflammatory effects induced by PGD(2)-G can be further augmented by inactivating CES1 activity with specific small-molecule inhibitors, while pro-inflammatory effects of PGE(2)-G are attenuated. Furthermore, PGD(2)-G (and/or its downstream metabolites) was shown to activate the lipid-sensing receptor PPARgamma, 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 PGD(2)-G in specific cellular niches might be a novel anti-inflammatory approach.

Cyclooxygenase-2 (COX-2) action on the endocannabinoids, 2-arachidonylglycerol (2-AG) and anandamide (AEA), generates prostaglandin glycerol esters (PG-G) and ethanolamides (PG-EA), respectively. The diversity of PG-Gs and PG-EAs that can be formed enzymatically following COX-2 oxygenation of endocannabinoids was examined in cellular and subcellular systems. In cellular systems, glycerol esters and ethanolamides of PGE(2), PGD(2), and PGF(2alpha) were major products of the endocannabinoid-derived COX-2 products, PGH(2)-G and PGH(2)-EA. The sequential action of purified COX-2 and thromboxane synthase on AEA and 2-AG provided thromboxane A(2) ethanolamide and glycerol ester, respectively. Similarly, bovine prostacyclin synthase catalyzed the isomerization of the intermediate endoperoxides, PGH(2)-G and PGH(2)-EA, to the corresponding prostacyclin derivatives. Quantification of the efficiency of prostaglandin and thromboxane synthase-directed endoperoxide isomerization demonstrated that PGE, PGD, and PGI synthases catalyze the isomerization of PGH(2)-G at rates approaching those observed with PGH(2). In contrast, thromboxane synthase was far more efficient at catalyzing PGH(2) isomerization than at catalyzing the isomerization of PGH(2)-G. These results define the in vitro diversity of endocannabinoid-derived prostanoids and will permit focused investigations into their production and potential biological actions in vivo.