Paraoxonase 1 in endothelial cells impairs vasodilation induced by arachidonic acid lactone metabolite

Pleiotropic functions and clinical importance of circulating HDL-PON1 complex

High density lipoprotein (HDL) functions are mostly mediated through a complex proteome, particularly its enzymes. HDL can provide a scaffold for the assembly of several proteins that affect each other's function. HDL particles, particularly small, dense HDL3, are rich in paraoxonase 1 (PON1), which is an important enzyme in the functionality of HDL, so the antioxidant and antiatherogenic properties of HDL are largely attributed to this enzyme. There is an increasing need to represent a valid, reproducible, and reliable method to assay HDL function in routine clinical laboratories. In this context, HDL-associated proteins may be key players; notably PON1 activity (its arylesterase activity) may be a proper candidate because its decreased activity can be considered an important risk factor for HDL dysfunctionality. Of note, automated methods have been developed for the measurement of serum PON1 activity that facilitates its assay in large sample numbers. Arylesterase activity is proposed as a preferred activity among the different activities of PON1 for its assay in epidemiological studies. The binding of PON1 to HDL is critical for the maintenance of its activity and it appears apolipoprotein A-I plays an important role in HDL-PON1 interaction as well as in the biochemical and enzymatic properties of PON1. The interrelationships between HDL, PON1, and HDL's other components are complex and incompletely understood. The purpose of this review is to discuss biochemical and clinical evidence considering the interactions of PON1 with HDL and the role of this enzyme as an appropriate biomarker for HDL function as well as a potential therapeutic target.

PON1 has palmitoyl-protein thioesterase (PPT) activity, and can affect the presence of SR-B1 on the endothelial cell membrane

The high-density lipoprotein (HDL)-associated enzyme paraoxonase 1 (PON1) is expressed almost exclusively in the liver and is then transported by HDL to the peripheral tissues. The lipophilic nature of PON1 enables its easy exchange between the lipoprotein and cell membranes in a process that is dependent on the HDL receptor scavenger receptor class B, type 1 (SR-B1). In endothelial cells, PON1 binding to the cell membrane leads to its internalization by endocytosis and subsequent lysosomal degradation. PON1 is a "promiscuous" enzyme with unusually broad substrate specificity in vitro, but its actual function and substrate are still unknown. The enzyme requires a lipid environment and becomes completely inactive upon delipidation. However, when PON1 binds HDL, its active site faces the lipoprotein's core and is inaccessible to external substrates. Hence, the HDL-bound PON1 is inactive toward substrates outside the particle's lipid core and is rapidly degraded and becomes inactive upon internalization. Consequently, the enzyme is only active in the cell membrane during its transit from HDL to the cytoplasm. To assign a function to PON1, we investigated whether it is a palmitoyl-protein thioesterase (PPT) that can hydrolyze the palmitoyl moieties of membrane proteins involved in HDL and cholesterol transport, such as SR-B1, ABCA1, or their neighboring caveola proteins to facilitate the release of HDL or trigger its endocytosis. This study shows that PON1 can hydrolyze palmitoyl-cysteine thioester bonds in vitro, has direct or indirect PPT activity in vivo, and can significantly decrease the presence of SR-B1 in the endothelial membrane.

Paraoxonase 1 and atherosclerosis

Paraoxonase 1 (PON1), residing almost exclusively on HDL, was discovered because of its hydrolytic activity towards organophosphates. Subsequently, it was also found to hydrolyse a wide range of substrates, including lactones and lipid hydroperoxides. PON1 is critical for the capacity of HDL to protect LDL and outer cell membranes against harmful oxidative modification, but this activity depends on its location within the hydrophobic lipid domains of HDL. It does not prevent conjugated diene formation, but directs lipid peroxidation products derived from these to become harmless carboxylic acids rather than aldehydes which might adduct to apolipoprotein B. Serum PON1 is inversely related to the incidence of new atherosclerotic cardiovascular disease (ASCVD) events, particularly in diabetes and established ASCVD. Its serum activity is frequently discordant with that of HDL cholesterol. PON1 activity is diminished in dyslipidaemia, diabetes, and inflammatory disease. Polymorphisms, most notably Q192R, can affect activity towards some substrates, but not towards phenyl acetate. Gene ablation or over-expression of human PON1 in rodent models is associated with increased and decreased atherosclerosis susceptibility respectively. PON1 antioxidant activity is enhanced by apolipoprotein AI and lecithin:cholesterol acyl transferase and diminished by apolipoprotein AII, serum amyloid A, and myeloperoxidase. PON1 loses this activity when separated from its lipid environment. Information about its structure has been obtained from water soluble mutants created by directed evolution. Such recombinant PON1 may, however, lose the capacity to hydrolyse non-polar substrates. Whilst nutrition and pre-existing lipid modifying drugs can influence PON1 activity there is a cogent need for more specific PON1-raising medication to be developed.

Paraoxonase 1 hydrolysis of EPA-derived lactone impairs endothelial-mediated vasodilation

Human serum paraoxonase-1 (PON1) is a lactonase that plays a significant role in anti-atherosclerotic high-density lipoprotein (HDL) activity. PON1 is also localized in endothelial cell membranes, where it is enzymatically active and regulates endothelial signals. PON1 has a high specificity for lipophilic lactones and has been shown to hydrolyze and regulate lactone lipid mediators derived from arachidonic polyunsaturated fatty acids (PUFA). Previously, we showed that an arachidonic acid lactone metabolite (AA-L) dose-dependently dilates PON1 gene deletion (PON1KO) mouse mesenteric arteries significantly more than wild-type arteries. In contrast, preincubation with HDL or rePON1 reduced AA-L-dependent vasodilation. Recently we showed that an additional δ-lactone metabolite derived from the eicosapentaenoic acid lactone, 5,6-δ-DiHETE lactone (EPA-L) reduced blood pressure by dilating microvessels of hypertensive rats. However, whether PON1 regulates the activity of the EPA-L lipid mediator is unknown.

AIM

To demonstrate that PON1 hydrolyzes EPA-L and to reveal the effect of this hydrolysis on endothelial-dependent vascular dilation.

METHODS AND RESULTS

In vascular reactivity experiments, EPA-L dose-dependently dilated PON1KO mouse mesenteric arteries significantly more than wild-type mesenteric arteries. This dilation was not affected by nitric oxide inhibition. PON1 impaired the cellular calcium increase mediated by EPA-L in endothelial cells, though this impairment decreased with PON1 internalization to the cell.

CONCLUSION

These findings support that PUFA-lactones are physiological substrates of PON1, and that PON1 activity in the endothelial membrane affects the dilation of microvessels that is induced by these endothelial-derived hyperpolarizing PUFA-lactones.

Alternative Method for HDL and Exosome Isolation with Small Serum Volumes and Their Characterizations

High-density lipoprotein (HDL) and exosomes are promising sources of biomarkers. However, the limited sample volume and access to the ultracentrifuge equipment are still an issue during HDL and exosome isolation. This study aimed to isolate HDL and exosomes using an ultracentrifugation-free method with various small serum volumes. HDL was isolated from 200 µL (HDL200) and 500 µL (HDL500) of sera. Three different volumes: 50 µL (Exo50), 100 µL (Exo100), and 250 µL (Exo250) were used for exosome isolation. HDL and exosomes were isolated using commercial kits with the modified method and characterized by multiple approaches. The HDL levels of HDL200 and HDL500 were not significantly different (p > 0.05), with percent recoveries of >90%. HDL200 and HDL500 had the same protein pattern with a biochemical similarity of 99.60 ± 0.10%. The particle sizes of Exo50, Exo100, and Exo250 were in the expected range. All isolated exosomes exhibited a similar protein pattern with a biochemical similarity of >99%. In conclusion, two different serum volumes (200 and 500 µL) and three different serum volumes (50, 100, and 250 µL) can be employed for HDL and exosome isolation, respectively. The possibility of HDL and exosome isolation with small volumes will accelerate biomarker discoveries with various molecular diagnostic approaches.

Vasodilation and blood pressure-lowering effect mediated by 5,6-EEQ lactone in 5/6 nephrectomy hypertensive rats

Microvascular dysfunction is a key contributor to vascular hypertension, one of the most common chronic diseases in the world. Microvascular dysfunction leads to the loss of nitric oxide-mediated endothelial dilation and the subsequent compensatory function of endothelium-derived hyperpolarizing (EDH) factors in the regulation of vascular tone. Previously, we showed that lactone metabolite derived from arachidonic acid induces endothelial-dependent vasodilation in isolated human microvessels. Based on structural similarities, we hypothesize that additional lactone metabolites formed from eicosapentaenoic fatty acid (EPA) may bear EDH properties.

AIM

To elucidate the vasodilatory and blood pressure (BP)-reducing characteristics of the 5,6-EEQ (5,6-epoxyeicosatetraenoic acids) lactone (EPA-L) in hypertensive 5/6 nephrectomy (5/6Nx) rats.

METHODS

5/6Nx hypertensive rats intravenously administrated with EPA-L for five days. BP, blood and urine chemistry, and kidney function were detected and analyzed. Vascular dilation was detected using a pressure myograph with or without Ca²⁺ - activated K⁺ (K_Ca) endothelial channel inhibitors. KCNN3 and KCNN4 gene expression (mRNA) detected in mesenteric arteries from 5/6Nx and NT rats.

RESULTS

EPA-L administration to 5/6Nx rats significantly (p < 0.05) reduced BP and heart rate without affecting kidney function. 5/6Nx rat mesenteric arterioles exhibited a lower dilation response to acetylcholine (10^-7 mol/l) than normotensive (NT) vessels, while EPA-L administration restored the vessel relaxation response. The EPA-L-driven relaxation of mesenteric arteries was significantly reduced by pretreatment with TRAM-34 and apamin. However, K_Ca channel expression did not significantly differ between 5/6Nx and NT mesenteric arteries.

CONCLUSION

EPA-L reduces BP by improving microvessel dilation involving calcium-dependent potassium endothelial channels.

The uptake mechanism and intracellular fate of Paraoxonase-1 in endothelial cells

Paraoxonase-1 (PON1) is a high-density lipoprotein (HDL)-associated lactonase that plays a significant role in the anti-atherosclerotic activity of HDL. However, several studies have shown that PON1 localizes in cells, where it operates independently of HDL. Previously, we showed that PON1 localizes in endothelial cells (ECs), and impairs vasodilation mediated by the endothelium-derived hyperpolarizing factor (EDHF) 5,6-δ-DHTL. However, the internalization pathway of PON1 into ECs, and the intracellular fate of PON1 are unknown. Therefore, the present study aimed to elucidate the uptake mechanism, intracellular trafficking and the function of PON1 in ECs. ‏We conducted a series of inhibition experiments of fluorescently labeled recombinant PON1 (rePON1) in ECs, followed by FACS analyses. We found that rePON1 binds the EC membrane via specific binding sites located in lipid-rafts/caveolae microdomains that are shared with HDL, and internalized through dynamin-dependent endocytosis. Qualitative assessments of the intracellular trafficking of rePON1, using confocal z-stack images, showed colocalization of the labeled rePON1 with early and late endosome/lysosome markers. Accordingly, a "pulse-chase" incubation of rePON1, followed by lactonase activity measurement in EC lysate, revealed that rePON1 retains its lactonase activity after binding to the cells. However, this activity decreases over time. Finally, induction of endothelial dysfunction with high glucose, angiotensin II, or palmitic acid increased rePON1 uptake by ECs. In conclusion, these results indicate that free PON1 interacts with ECs via binding sites located in lipid-rafts/caveolae, where it is enzymatically active and regulates endothelial functions. However, once internalized, PON1 is degraded. Additionally, alteration in endothelial function affects PON1 uptake by ECs.

Mutual Influences between Nitric Oxide and Paraoxonase 1

One of the best consolidated paradigms in vascular pharmacology is that an uncontrolled excess of oxidizing chemical species causes tissue damage and loss of function in the endothelial and subendothelial layers. The fact that high-density lipoproteins play an important role in preventing such an imbalance is integrated into that concept, for which the expression and activity of paraoxonases is certainly crucial. The term paraoxonase (aryldialkyl phosphatase, EC 3.1.8.1) encompasses at least three distinct isoforms, with a wide variation in substrate affinity, cell and fluid localization, and biased expression of polymorphism. The purpose of this review is to determine the interactions that paraoxonase 1 has with nitric oxide synthase, its reaction product, nitric oxide (nitrogen monoxide, NO), and its derived reactive species generated in an oxidative medium, with a special focus on its pathological implications.