Catabolism of 4-hydroxy-2-trans-nonenal by THP1 monocytes/macrophages and inactivation of carboxylesterases by this lipid electrophile

Hormetic effects of EGC and EGCG on CES1 activity and its rescue from oxidative stress in rat liver S9

Carboxylesterase 1 (CES1) is a hydrolytic enzyme that plays an important role in the activation or deactivation of many therapeutic agents, thus affecting their pharmacokinetic and pharmacodynamic outcomes. Using rat liver S9 as an enzyme source and enalapril as a CES1 substrate, the present study examined effects of a number of flavonoids on the formation of enalaprilat (the active form of enalapril) produced by CES1-mediated hydrolysis. While a majority of flavonoids tested showed inhibition on CES1, an unexpected hormetic effect was observed for epigallocatechin (EGC) and epigallocatechin gallate (EGCG), i.e., stimulatory effect at low concentrations and enzyme inhibition at high concentrations. Further experiments revealed that oxidative stress caused by hydrogen peroxide, arachidonic acid plus iron, and oxidized low density lipoproteins (oxLOL) reduced CES1 activity in rat liver S9 and the loss of CES1 enzyme activity could be rescued largely by EGC or EGCG. In contrast, such effects were minimal in human liver S9, probably due to the presence of a higher ratio of reduced vs oxidized forms of glutathione. The above findings suggest that the polyphenolic nature of EGC or EGCG might be responsible for rescuing CES1 activity under oxidative stress. Because of the importance of CES1 in drug activation or deactivation and rat liver S9 as a versatile in vitro system used for drug metabolism studies and drug safety assessment, caution should be exercised to avoid potential biases for data interpretation and decision making when CES1 activity in rat liver S9 is evaluated with dependency on experimental conditions.

Reduced carboxylesterase 1 is associated with endothelial injury in methamphetamine-induced pulmonary arterial hypertension

Pulmonary arterial hypertension is a complication of methamphetamine use (METH-PAH), but the pathogenic mechanisms are unknown. Given that cytochrome P450 2D6 (CYP2D6) and carboxylesterase 1 (CES1) are involved in metabolism of METH and other amphetamine-like compounds, we postulated that loss of function variants could contribute to METH-PAH. Although no difference in CYP2D6 expression was seen by lung immunofluorescence, CES1 expression was significantly reduced in endothelium of METH-PAH microvessels. Mass spectrometry analysis showed that healthy pulmonary microvascular endothelial cells (PMVECs) have the capacity to both internalize and metabolize METH. Furthermore, whole exome sequencing data from 18 METH-PAH patients revealed that 94.4% of METH-PAH patients were heterozygous carriers of a single nucleotide variant (SNV; rs115629050) predicted to reduce CES1 activity. PMVECs transfected with this CES1 variant demonstrated significantly higher rates of METH-induced apoptosis. METH exposure results in increased formation of reactive oxygen species (ROS) and a compensatory autophagy response. Compared with healthy cells, CES1-deficient PMVECs lack a robust autophagy response despite higher ROS, which correlates with increased apoptosis. We propose that reduced CES1 expression/activity could promote development of METH-PAH by increasing PMVEC apoptosis and small vessel loss.

Signaling by 4-hydroxy-2-nonenal: Exposure protocols, target selectivity and degradation

4-hydroxy-2-nonenal (HNE), a major non-saturated aldehyde product of lipid peroxidation, has been extensively studied as a signaling messenger. In these studies a wide range of HNE concentrations have been used, ranging from the unstressed plasma concentration to far beyond what would be found in actual pathophysiological condition. In addition, accumulating evidence suggest that signaling protein modification by HNE is specific with only those proteins with cysteine, histidine, and lysine residues located in certain sequence or environments adducted by HNE. HNE-signaling is further regulated through the turnover of HNE-signaling protein adducts through proteolytic process that involve proteasomes, lysosomes and autophagy. This review discusses the HNE concentrations and exposure modes used in signaling studies, the selectivity of the HNE-adduction site, and the turnover of signaling protein adducts.

Oxyradical Stress, Endocannabinoids, and Atherosclerosis

Atherosclerosis is responsible for most cardiovascular disease (CVD) and is caused by several factors including hypertension, hypercholesterolemia, and chronic inflammation. Oxidants and electrophiles have roles in the pathophysiology of atherosclerosis and the concentrations of these reactive molecules are an important factor in disease initiation and progression. Overactive NADPH oxidase (Nox) produces excess superoxide resulting in oxidized macromolecules, which is an important factor in atherogenesis. Although superoxide and reactive oxygen species (ROS) have obvious toxic properties, they also have fundamental roles in signaling pathways that enable cells to adapt to stress. In addition to inflammation and ROS, the endocannabinoid system (eCB) is also important in atherogenesis. Linkages have been postulated between the eCB system, Nox, oxidative stress, and atherosclerosis. For instance, CB₂ receptor-evoked signaling has been shown to upregulate anti-inflammatory and anti-oxidative pathways, whereas CB₁ signaling appears to induce opposite effects. The second messenger lipid molecule diacylglycerol is implicated in the regulation of Nox activity and diacylglycerol lipase β (DAGLβ) is a key biosynthetic enzyme in the biosynthesis eCB ligand 2-arachidonylglycerol (2-AG). Furthermore, Nrf2 is a vital transcription factor that protects against the cytotoxic effects of both oxidant and electrophile stress. This review will highlight the role of reactive oxygen species (ROS) in intracellular signaling and the impact of deregulated ROS-mediated signaling in atherogenesis. In addition, there is also emerging knowledge that the eCB system has an important role in atherogenesis. We will attempt to integrate oxidative stress and the eCB system into a conceptual framework that provides insights into this pathology.

Multiple proteases to localize oxidation sites

Proteins present in cellular environments with high levels of reactive oxygen and nitrogen species and/or low levels of antioxidants are highly susceptible to oxidative post-translational modification (PTM). Irreversible oxidative PTMs can generate a complex distribution of modified protein molecules, recently termed as proteoforms. Using ubiquitin as a model system, we mapped oxidative modification sites using trypsin, Lys-C, and Glu-C peptides. Several M+16 Da proteoforms were detected as well as proteoforms that include other previously unidentified oxidative modifications. This work highlights the use of multiple protease digestions to give insights to the complexity of oxidative modifications possible in bottom-up analyses.

Effects of toxicologically relevant xenobiotics and the lipid-derived electrophile 4-hydroxynonenal on macrophage cholesterol efflux: silencing carboxylesterase 1 has paradoxical effects on cholesterol uptake and efflux

Cholesterol cycles between free cholesterol (unesterified) found predominantly in membranes and cholesteryl esters (CEs) stored in cytoplasmic lipid droplets. Only free cholesterol is effluxed from macrophages via ATP-binding cassette (ABC) transporters to extracellular acceptors. Carboxylesterase 1 (CES1), proposed to hydrolyze CEs, is inactivated by oxon metabolites of organophosphorus pesticides and by the lipid electrophile 4-hydroxynonenal (HNE). We assessed the ability of these compounds to reduce cholesterol efflux from foam cells. Human THP-1 macrophages were loaded with [³H]-cholesterol/acetylated LDL and then allowed to equilibrate to enable [³H]-cholesterol to distribute into its various cellular pools. The cholesterol-engorged cells were then treated with toxicants in the absence of cholesterol acceptors for 24 h, followed by a 24 h efflux period in the presence of toxicant. A concentration-dependent reduction in [³H]-cholesterol efflux via ABCA1 (up to 50%) was found for paraoxon (0.1–10 μM), whereas treatment with HNE had no effect. A modest reduction in [³H]-cholesterol efflux via ABCG1 (25%) was found after treatment with either paraoxon or chlorpyrifos oxon (10 μM each) but not HNE. No difference in efflux rates was found after treatments with either paraoxon or HNE when the universal cholesterol acceptor 10% (v/v) fetal bovine serum was used. When the re-esterification arm of the CE cycle was disabled in foam cells, paraoxon treatment increased CE levels, suggesting the neutral CE hydrolysis arm of the cycle had been inhibited by the toxicant. However, paraoxon also partially inhibited lysosomal acid lipase, which generates cholesterol for efflux, and reduced the expression of ABCA1 protein. Paradoxically, silencing CES1 expression in macrophages did not affect the percent of [³H]-cholesterol efflux. However, CES1 mRNA knockdown markedly reduced cholesterol uptake by macrophages, with SR-A and CD36 mRNA reduced 3- and 4-fold, respectively. Immunoblots confirmed SR-A and CD36 protein downregulation. Together, these results suggest that toxicants, e.g., oxons, may interfere with macrophage cholesterol homeostasis/metabolism.

Identification of palmitoyl protein thioesterase 1 in human THP1 monocytes and macrophages and characterization of unique biochemical activities for this enzyme

The profiles of serine hydrolases in human and mouse macrophages are similar yet different. For instance, human macrophages express high levels of carboxylesterase 1 (CES1), whereas mouse macrophages have minimal amounts of the orthologous murine CES1. On the other hand, macrophages from both species exhibit limited expression of the canonical 2-arachidonoylglycerol (2-AG) hydrolytic enzyme, MAGL. Our previous study showed CES1 was partly responsible for the hydrolysis of 2-AG (50%) and prostaglandin glyceryl esters (PG-Gs) (80-95%) in human THP1 monocytes and macrophages. However, MAGL and other endocannabinoid hydrolases, FAAH, ABHD6, and ABHD12, did not have a role because of limited expression or no expression. Thus, another enzyme was hypothesized to be responsible for the remaining 2-AG hydrolysis activity following chemical inhibition and immunodepletion of CES1 (previous study) or CES1 gene knockdown (this study). Here we identified two candidate serine hydrolases in THP1 cell lysates by activity-based protein profiling (ABPP)-MUDPIT and Western blotting: cathepsin G and palmitoyl protein thioesterase 1 (PPT1). Both proteins exhibited electrophoretic properties similar to those of a serine hydrolase in THP1 cells detected by gel-based ABPP at 31-32 kDa; however, only PPT1 exhibited lipolytic activity and hydrolyzed 2-AG in vitro. Interestingly, PPT1 was strongly expressed in THP1 cells but was significantly less reactive than cathepsin G toward the activity-based probe, fluorophosphonate-biotin. KIAA1363, another serine hydrolase, was also identified in THP1 cells but did not have significant lipolytic activity. On the basis of chemoproteomic profiling, immunodepletion studies, and chemical inhibitor profiles, we estimated that PPT1 contributed 32-40% of 2-AG hydrolysis activity in the THP1 cell line. In addition, pure recombinant PPT1 catalyzed the hydrolysis of 2-AG, PGE2-G, and PGF2α-G, although the catalytic efficiency of hydrolysis of 2-AG by PPT1 was ~10-fold lower than that of CES1. PPT1 was also insensitive to several chemical inhibitors that potently inhibit CES1, such as organophosphate poisons and JZL184. This is the first report to document the expression of PPT1 in a human monocyte and macrophage cell line and to show PPT1 can hydrolyze the natural substrates 2-AG and PG-Gs. These findings suggest that PPT1 may participate in endocannabinoid metabolism within specific cellular contexts and highlights the functional redundancy often exhibited by enzymes involved in lipid metabolism.

Metabolic impact of 4-hydroxynonenal on macrophage-like RAW 264.7 function and activation

Metabolic profiling of macrophage metabolic response upon exposure to 4-hydroxynonenal (HNE) demonstrates that HNE does not simply inactivate superoxide-generating enzymes but also could be responsible for the impairment of downfield signaling pathways. Multianalyte microphysiometry (MAMP) was employed to simultaneously measure perturbations in extracellular acidification, lactate production, and oxygen consumption for the examination of aerobic and anaerobic pathways. Combining the activation of oxidative burst with phorbol myristate acetate (PMA) and the immunosuppression with HNE, the complex nature of HNE toxicity was determined to be concentration- and time-dependent. Further analysis was utilized to assess the temporal effect of HNE on reactive oxygen species (ROS) production and on protein kinase C (PKC). Increased levels of HNE with decreasing PKC activity suggest that PKC is a target for HNE adductation prior to oxidative burst. Additionally, localization of PKC to the cell membrane was prevented with the introduction of HNE, demonstrating a consequence of HNE adductation on NADPH activation. The impairment of ROS by HNE suggests that HNE has a greater role in foam cell formation and tissue damage than is already known. Although work has been performed to understand the effect of HNE's regulation of specific signaling pathways, details regarding its involvement in cellular metabolism as a whole are generally unknown. This study examines the impact of HNE on macrophage oxidative burst and identifies PKC as a key protein for HNE suppression and eventual metabolic response.

Inhibition of recombinant human carboxylesterase 1 and 2 and monoacylglycerol lipase by chlorpyrifos oxon, paraoxon and methyl paraoxon

Oxons are the bioactivated metabolites of organophosphorus insecticides formed via cytochrome P450 monooxygenase-catalyzed desulfuration of the parent compound. Oxons react covalently with the active site serine residue of serine hydrolases, thereby inactivating the enzyme. A number of serine hydrolases other than acetylcholinesterase, the canonical target of oxons, have been reported to react with and be inhibited by oxons. These off-target serine hydrolases include carboxylesterase 1 (CES1), CES2, and monoacylglycerol lipase. Carboxylesterases (CES, EC 3.1.1.1) metabolize a number of xenobiotic and endobiotic compounds containing ester, amide, and thioester bonds and are important in the metabolism of many pharmaceuticals. Monoglyceride lipase (MGL, EC 3.1.1.23) hydrolyzes monoglycerides including the endocannabinoid, 2-arachidonoylglycerol (2-AG). The physiological consequences and toxicity related to the inhibition of off-target serine hydrolases by oxons due to chronic, low level environmental exposures are poorly understood. Here, we determined the potency of inhibition (IC(50) values; 15 min preincubation, enzyme and inhibitor) of recombinant CES1, CES2, and MGL by chlorpyrifos oxon, paraoxon and methyl paraoxon. The order of potency for these three oxons with CES1, CES2, and MGL was chlorpyrifos oxon>paraoxon>methyl paraoxon, although the difference in potency for chlorpyrifos oxon with CES1 and CES2 did not reach statistical significance. We also determined the bimolecular rate constants (k(inact)/K(I)) for the covalent reaction of chlorpyrifos oxon, paraoxon and methyl paraoxon with CES1 and CES2. Consistent with the results for the IC(50) values, the order of reactivity for each of the three oxons with CES1 and CES2 was chlorpyrifos oxon>paraoxon>methyl paraoxon. The bimolecular rate constant for the reaction of chlorpyrifos oxon with MGL was also determined and was less than the values determined for chlorpyrifos oxon with CES1 and CES2 respectively. Together, the results define the kinetics of inhibition of three important hydrolytic enzymes by activated metabolites of widely used agrochemicals.