Decreased Macrophage Paraoxonase 2 Expression in Patients With Hypercholesterolemia Is the Result of Their Increased Cellular Cholesterol Content: Effect of Atorvastatin Therapy

Insights into the role of paraoxonase 2 in human pathophysiology

Paraoxonase 2 (PON2) is a ubiquitously expressed intracellular enzyme that is known to have a protective role from oxidative stress. Clinical studies have also demonstrated the significance of PON2 in the manifestation of cardiovascular and several other diseases, and hence, it is considered an important biomarker. Recent findings of its expression in brain tissue suggest its potential protective effect on oxidative stress and neuroinflammation. Polymorphisms of PON2 in humans are a risk factor in many pathological conditions, suggesting a possible mechanism of its anti-oxidative property probably through lactonase activity. However, exogenous factors may also modulate the expression and activity of PON2. Hence, this review aims to report the mechanism by which PON2 expression is regulated and its role in oxidative stress disorders such as neurodegeneration and tumor formation. The role of PON2 owing to its lactonase activity in bacterial infectious diseases and association of PON2 polymorphism with pathological conditions are also highlighted.

Human Paraoxonase-2 (PON2): Protein Functions and Modulation

PON1, PON2, and PON3 belong to a family of lactone hydrolyzing enzymes endowed with various substrate specificities. Among PONs, PON2 shows the highest hydrolytic activity toward many acyl-homoserine lactones (acyl-HL) involved in bacterial quorum-sensing signaling. Accordingly, defense against pathogens, such as Brevundimonas aeruginosa (B. aeruginosa), was postulated to be the principal function of PON2. However, recent findings have highlighted the importance of PON2 in oxidative stress control, inhibition of apoptosis, and the progression of various types of malignancies. This review focuses on all of these aspects of PON2.

The role of paraoxonase in cancer

The paraoxonase (PON) gene family includes three proteins, PON1, PON2 and PON3. PON1 and PON3 are both associated with high-density lipoprotein (HDL) particles and exert anti-oxidant and anti-inflammatory properties. PON2 and PON3 are intracellular enzymes which modulate mitochondrial superoxide anion production and endoplasmic reticulum (ER) stress-induced apoptosis. The pleiotropic roles exerted by PONs have been mainly investigated in cardiovascular and neurodegenerative diseases. In recent years, overexpression of PON2 and PON3 has been observed in cancer cells and it has been proposed that both enzymes could be involved in tumor survival and stress resistance. Moreover, a lower activity of serum PON1 has been reported in cancer patients. This review summarizes literature data on the role of PONs in human cancers and their potential role as a target for antitumor drugs.

Cholesterol reduction and macrophage function: role of paraoxonases

PURPOSE OF REVIEW

Unregulated uptake of oxidized LDL by macrophages to form foam cells is the hallmark for atherosclerosis. The paraoxonase (PON) family of enzymes plays a critical role in attenuating atherosclerotic lesion formation by hydrolyzing lipid peroxides (LOOHs) and preventing the oxidation of LDL particles and by enhancing HDL-mediated cholesterol efflux. Findings in recent years suggest novel mechanisms by which PON isoforms interact with macrophages to regulate cholesterol metabolism and cellular function.

RECENT FINDINGS

The association of PON with HDL particles facilitates binding of the particle to macrophages and ABCA1-dependent cholesterol efflux. The hydrolysis of membrane phospholipids by PON generates lysophosphatidylcholine which is shown to regulate expression of cholesterol transport proteins. The PON family also regulates multiple aspects of macrophage function. PON attenuates inflammation and prevents induction of apoptosis via activation of a scavenger receptor class B type-1-dependent signaling mechanism. PON limits macrophage-dependent oxidant formation by preventing the activation of the membrane-associated NADPH oxidase and by stabilizing mitochondria. PON also promotes the differentiation of macrophages to an anti-inflammatory phenotype. This function appears to be independent of PON enzymatic activity and, rather, is dependent on the ability of endogenous sulfhydryls to neutralize pro-inflammatory peroxides.

SUMMARY

In recent years, the therapeutic efficacy of HDL-based therapies has been subject to dispute. Pharmacological approaches that target an increase in the expression and/or activity of PON may facilitate macrophage cholesterol metabolism and attenuate inflammatory injury.

Metals and Paraoxonases

The paraoxonases (PONs) are a three-gene family which includes PON1, PON2, and PON3. PON1 and PON3 are synthesized primarily in the liver and a portion is secreted in the plasma, where they are associated with high-density lipoproteins (HDLs), while PON2 is an intracellular enzyme, expressed in most tissues and organs, including the brain. PON1 received its name from its ability to hydrolyze paraoxon, the active metabolite of the organophosphorus (OP) insecticide parathion, and also more efficiently hydrolyzes the active metabolites of several other OPs. PON2 and PON3 do not have OP-esterase activity, but all PONs are lactonases and are capable of hydrolyzing a variety of lactones, including certain drugs, endogenous compounds, and quorum-sensing signals of pathogenic bacteria. In addition, all PONs exert potent antioxidant effects. PONs play important roles in cardiovascular diseases and other oxidative stress-related diseases, modulate susceptibility to infection, and may provide neuroprotection (PON2). Hence, significant attention has been devoted to their modulation by a variety of dietary, pharmacological, lifestyle, or environmental factors. A number of metals have been shown in in vitro, animal, and human studies to mostly negatively modulate expression of PONs, particularly PON1, the most studied in this regard. In addition, different levels of expression of PONs may affect susceptibility to toxicity and neurotoxicity of metals due to their aforementioned antioxidant properties.

Paraoxonases-1, -2 and -3: What are their functions?

Paraoxonase-1 (PON1), an esterase/lactonase primarily associated with plasma high-density lipoprotein (HDL), was the first member of this family of enzymes to be characterized. Its name was derived from its ability to hydrolyze paraoxon, the toxic metabolite of the insecticide parathion. Related enzymes PON2 and PON3 were named from their evolutionary relationship with PON1. Mice with each PON gene knocked out were generated at UCLA and have been key for elucidating their roles in organophosphorus (OP) metabolism, cardiovascular disease, innate immunity, obesity, and cancer. PON1 status, determined with two-substrate analyses, reveals an individual's functional Q192R genotype and activity levels. The three-dimensional structure for a chimeric PON1 has been useful for understanding the structural properties of PON1 and for engineering PON1 as a catalytic scavenger of OP compounds. All three PONs hydrolyze microbial N-acyl homoserine lactone quorum sensing factors, quenching Pseudomonas aeruginosa's pathogenesis. All three PONs modulate oxidative stress and inflammation. PON2 is localized in the mitochondria and endoplasmic reticulum. PON2 has potent antioxidant properties and is found at 3- to 4-fold higher levels in females than males, providing increased protection against oxidative stress, as observed in primary cultures of neurons and astrocytes from female mice compared with male mice. The higher levels of PON2 in females may explain the lower frequency of neurological and cardiovascular diseases in females and the ability to identify males but not females with Parkinson's disease using a special PON1 status assay. Less is known about PON3; however, recent experiments with PON3 knockout mice show them to be susceptible to obesity, gallstone formation and atherosclerosis. Like PONs 1 and 2, PON3 also appears to modulate oxidative stress. It is localized in the endoplasmic reticulum, mitochondria and on HDL. Both PON2 and PON3 are upregulated in cancer, favoring tumor progression through mitochondrial protection against oxidative stress and apoptosis.

Profile of the Paraoxonase 1 (PON1) Gene 192Q/R Polymorphism and Clinical Associations among Older Singaporean Chinese with Alzheimer's and Mixed Dementia

BACKGROUND

To examine the prevalence of the Paraoxonase1 (PON1) gene 192Q/R polymorphism amongst Singaporean Chinese with Alzheimer's disease (AD) and mixed dementia and possible clinical associations.

METHODS

We examined the presence of the PON1 192Q/R polymorphism together with cognitive status, functional status and neuropsychiatric symptoms among 186 older Singaporean Chinese with AD (n = 109) and mixed dementia (n = 77).

RESULTS

The R allele predominated in 67% of the AD patients and 63.1% of the patients with mixed dementia. Within the mixed dementia subgroup, the R allele was significantly associated with a higher BADLS score, NPI-Q scores and CDR scores.

CONCLUSION

Among older Singaporean Chinese with AD and mixed dementia, the R allele was predominant. In particular, within the mixed dementia subgroup, the R allele carrier status was associated with poorer functional status, greater presence of neuropsychiatric symptoms and a more severe stage of dementia. Further studies should be conducted.

High intrinsic aerobic capacity and pomegranate juice are protective against macrophage atherogenecity: studies in high- vs. low-capacity runner (HCR vs. LCR) rats

We studied the rat model system of high- vs. low-capacity runner (HCR vs. LCR) rats to question the atherogenic properties (oxidative stress, triglycerides and cholesterol metabolism) in the rat macrophages, serum, liver and heart. Half of the LCR or HCR rats consumed pomegranate juice (PJ; 15 μmol of gallic acid equivalents/rat/day) for 3 weeks and were compared to placebo-treated rats. At the end of the study blood samples, peritoneal macrophages (RPM), livers, and hearts were harvested from the rats. RPM harvested from HCR vs. LCR demonstrated reduced cellular oxidation (21%), increased paraoxonase 2 activity (28%) and decreased triglycerides mass (44%). Macrophage uptake rates of fluorescein-isothiocyanate-labeled low-density lipoprotein (LDL) or oxidized LDL were significantly lower, by 37% or by 18%, respectively, in HCR vs. LCR RPM. PJ consumption significantly decreased all the above atherogenic parameters with more substantial beneficial effects observed in the LCR vs. the HCR rats (~80% vs. ~40% improvement, respectively). Similar hypo-triglyceridemic pattern was noted in serum from HCR vs. LCR. In contrast to the above results, liver oxidation and triglycerides mass were both minimally increased in HCR vs. LCR rats by 31% and 28%, respectively. In the heart, lipid content was very low, and interestingly, an absence of any significant oxidative stress, along with modest triglyceride accumulation, was observed. We conclude that HCR vs. LCR rats demonstrate reduced atherogenicity, mostly in their macrophages. PJ exerts a further improvement, mostly in macrophages from LCR rats.

Paraoxonase and arylesterase activities in dipper and non-dipper prehypertensive subjects

Paraoxonase-1, a high-density lipoprotein linked enzyme complex, was shown to be decreased in several cardiovascular diseases. We aimed to explore whether serum paraoxonase and arylesterase activities differ in dipper and non-dipper prehypertensive subjects compared to healthy controls.Sixty prehypertensive subjects and 30 controls were enrolled. All subjects underwent echocardiographic assessment and 24-hour ambulatory blood pressure monitoring (ABPM). According to the blood pressure (BP) course on ABPM, prehypertensive subjects were categorized into two: non-dipper prehypertensive (NDPH) and dipper prehypertensive (DPH) groups. Serum paraoxonase and arylesterase activities were detected spectrophotometrically.Paraoxonase and arylesterase activities were significantly lower in patients with NDPH compared to both DPH and control groups. Both paraoxonase and arylesterase activities showed significant negative correlations with BP and left ventricular mass index.We have demonstrated that NDPH subjects have lower paraoxonase and arylesterase activities compared to DPH subjects and normotensives. Further prospective studies are needed to clarify the role of paraoxonase and arylesterase activities in the development of overt hypertension in prehypertensive subjects.

HDL3 stimulates paraoxonase 1 antiatherogenic catalytic and biological activities in a macrophage model system: in vivo and in vitro studies

We analyzed in-vivo and in-vitro high density lipoprotein (HDL) effects on paraoxonase 1 (PON1) antiatherogenic properties in serum and in macrophages. Intraperitoneal injection to C57BL/6 mice of recombinant PON1 (rePON1) + HDL, in comparison to HDL or to rePON1 alone, significantly increased serum PON1 arylesterase activity (by 20%), and serum-mediated cholesterol efflux from J774A.1 macrophages (by 18%). Similarly, in peritoneal macrophages (MPM) harvested from mice injected with HDL + rePON1 versus rePON1 alone, we observed reduction in oxidative stress (by 11%), increase in cellular PON1 activity (by 14%) and in HDL-mediated cholesterol efflux (by 38%). Incubation of serum or HDL with rePON1, substantially increased PON1 arylesterase activity, two-fold more than the expected additive values. HDL2 and HDL3 increased PON1 activity by 199% or 274%, respectively. Macrophage (J774A.1) cholesterol efflux rate significantly increased by HDL3 + rePON1 versus HDL3 alone (by 19%), but not by HDL2 + rePON1 versus HDL2 alone. Oxidation of HDL3 reduced its ability to induce macrophage cholesterol efflux, and abolished HDL3 stimulatory effects on rePON1. Addition of exogenous polyphenol quercetin (60 µM), but not phosphatidylcholine or apolipoprotein A1, to HDL + rePON1 increased PON1 activity (by 404%), increased the ability to reduce oxidative stress in J774A.1 macrophages (by 53%) and to stimulate macrophage cholesterol efflux (by 14%). Upon adding the hypocholesterolemic drug simvastatin (15 µg/mL) to HDL + rePON1, PON1 activity and the ability to induce macrophage cholesterol efflux increased, in comparison to HDL + rePON1. We thus concluded that HDL (mostly HDL3), stimulates PON1 antiatherogenic activities in macrophages, and these PON1 activities were further stimulated by quercetin, or by simvastatin.

Paraoxonase-2 (PON2) in brain and its potential role in neuroprotection

Paraoxonase 2 (PON2) is a member of a gene family which also includes the more studied PON1, as well as PON3. PON2 is unique among the three PONs, as it is expressed in brain tissue. PON2 is a lactonase and displays anti-oxidant and anti-inflammatory properties. PON2 levels are highest in dopaminergic regions (e.g. striatum), are higher in astrocytes than in neurons, and are higher in brain and peripheral tissues of female mice than male mice. At the sub-cellular level, PON2 localizes primarily in mitochondria, where it scavenges superoxides. Lack of PON2 (as in PON2(-/-) mice), or lower levels of PON2 (as in male mice compared to females) increases susceptibility to oxidative stress-induced toxicity. Estradiol increases PON2 expression in vitro and in vivo, and provides neuroprotection against oxidative stress. Such neuroprotection is not present in CNS cells from PON2(-/-) mice. Similar results are also found with the polyphenol quercetin. PON2, given its cellular localization and antioxidant and anti-inflammatory actions, may represent a relevant enzyme involved in neuroprotection, and may represent a novel target for neuroprotective strategies. Its differential expression in males and females may explain gender differences in the incidence of various diseases, including neurodevelopmental, neurological, and neurodegenerative diseases.

Modulation of paraoxonase 2 (PON2) in mouse brain by the polyphenol quercetin: a mechanism of neuroprotection?

Quercetin is a common flavonoid polyphenol which has been shown to exert neuroprotective actions in vitro and in vivo. Though quercetin has antioxidant properties, it has been suggested that neuroprotection may be ascribed to its ability of inducing the cell's own defense mechanisms. The present study investigated whether quercetin could increase the levels of paraoxonase 2 (PON2), a mitochondrial enzyme expressed in brain cells, which has been shown to have potent antioxidant properties. PON2 protein, mRNA, and lactonase activity were highest in mouse striatal astrocytes. Quercetin increased PON2 levels, possibly by activating the JNK/AP-1 pathway. The increased PON2 levels induced by quercetin resulted in decreased oxidative stress and ensuing toxicity induced by two oxidants. The neuroprotective effect of quercetin was significantly diminished in cells from PON2 knockout mice. These findings suggest that induction of PON2 by quercetin represents an important mechanism by which this polyphenol may exert its neuroprotective action.

Triglyceride accumulation in macrophages upregulates paraoxonase 2 (PON2) expression via ROS-mediated JNK/c-Jun signaling pathway activation

The aim of this study was to analyze the effect and mechanism of action of macrophage triglyceride accumulation on cellular PON2 expression. Incubation of J774A.1 (murine macrophages) with VLDL (0-75 μg protein/mL) significantly and dose-dependently increased cellular triglyceride mass, and reactive oxygen species (ROS) formation, by up to 3.3- or 1.8-fold, respectively. PON2 expression (mRNA, protein, activity) in cells treated with VLDL (50 μg protein/mL) was higher by 2- to 3-fold, as compared with control cells. Similar effects were noted upon using THP-1 (human macrophages). Incubation of macrophages with synthetic triglyceride or triglyceride fraction from carotid lesion resulted in similar effects, as shown for VLDL. Upon using specific inhibitors of MEK1/2 (UO126, 10 μM), p38 (SB203580, 10 μM), or JNK (SP600125, 20 μM), we demonstrated that MEK, as well as JNK, but not p38, are involved in VLDL-induced macrophage PON2 upregulation. VLDL activated JNK (but not ERK), which resulted in c-Jun phosphorylation. This signaling pathway is probably activated by ROS, since the antioxidant reduced glutathione (GSH), significantly decreased VLDL-induced macrophage ROS formation, c-Jun phosphorylation and PON2 overexpression. We conclude that macrophage triglyceride accumulation upregulates PON2 expression via MEK/ JNK/c-Jun pathway, and these effects could be related, at least in part, to cellular triglycerides-induced ROS formation. ©

Regulation of FMO and PON detoxication systems in ALS human tissues

Amyotrophic lateral sclerosis (ALS) is an adult-onset, progressive, and fatal neurodegenerative disease with unknown etiology. Recent evidence suggests an association between the exposure to toxic environmental factors and sporadic ALS. The flavin-containing monooxygenases (FMOs) and paraoxonase (PONs) genes encode enzymes involved in xenobiotic detoxication and are associated with ALS. FMO and PON gene expression has been examined in the human central nervous system including human brain subregions defined as the spinal cord, medulla, and cerebral cortex and in the peripheral tissues (lymphocytes, fibroblasts) in ALS patients and normal control subjects. FMO expression was generally higher in tissues from ALS subjects than in control tissues, with the largest increases in FMO expression detected in the spinal cord. In peripheral tissues, the FMO mRNA level was found to be lower compared with FMO expression in brain tissue, and no differences were detected between ALS patients and the control tissue. FMO and PON gene expression was low in peripheral tissues. In contrast to FMO5 expression, the PON2 gene was down-regulated in ALS patients compared to the controls. Because FMO and PON are involved in the detoxication processes and their functional activity to bioactivate chemicals to toxins has been documented, the data herein suggest that environmental toxin exposure may play a role in a subset of individuals who contract ALS by altering FMO and PON gene expression. Although the precise pathogenic link is presently unknown, these findings suggest a role at FMO and PON genes in the development of ALS.

Improving Total-Cholesterol/HDL-Cholesterol Ratio Results in an Endothelial Dysfunction Recovery in Peripheral Artery Disease Patients

Aims. To evaluate the effects of variations of total-cholesterol/HDL-cholesterol ratio and the effects of the atorvastatin on endothelial function in peripheral artery disease (PAD). Material and Methods. A prospective, randomised controlled study was carried out in 150 PAD patients. Patients randomized to the control group (n = 75) were treated with antiplatelet drugs, angiotensin-converting-enzyme inhibitors and cardiovascular-risk-factor control. Experimental group (n = 75) also received treatment with atorvastatin for a month. It was determined baseline nitrite plasma levels and total-cholesterol/HDL-cholesterol ratio and after one month of treatment in both groups. It was also analysed the correlation between the gradient of nitrite levels and the differential of total-cholesterol/HDL ratio in treatment group. Results. After a month, a reduction in nitrite levels was detected in treatment group (11.88 ± 7.8 μM versus 5.7 ± 1.8 μM, P < 0.0001). It was shown a higher decrease in nitrite plasma levels in the atorvastatin group finding lower levels assessments (5.7 ± 1.8 μM versus 13.1 ± 9.1 μM, resp., P < 0.001). A significant reduction in total-cholesterol/HDL-cholesterol ratio was observed in statin group after treatment (P < 0.0001). A strong correlation was found between the gradient of nitrite levels and the differential of total-cholesterol/HDL-cholesterol ratio in atorvastatin group (r = 0.7; P < 0.001). Conclusions. Improvement of nitrite levels are associated with decreased total cholesterol/HDL ratio values in PAD patients treated with atorvastatin.

Regulation of gene expression in atherosclerosis: insights from microarray studies in monocytes/macrophages

Atherosclerosis is a pathological phenomenon in which the walls of large arteries thicken and lose elasticity as a result of the growth of atheromatous lesions. It is a complex, multifactorial disease that involves several cell types and various pathobiological processes. Its genetic basis has not yet been deciphered, but it is related to complex multigene patterns influenced by environmental interactions. In this review, we focus specifically on the application of microarrays to atherosclerosis research using monocytes and monocyte-derived macrophages, as these are key cells in all phases of atherosclerosis, from the formation of foam cells to the destabilization and rupture of the atherosclerotic plaque. These studies have provided relevant information on genes involved in atherosclerosis development, contributing to our understanding of the molecular mechanisms that underlie this complex disease.

The human paraoxonase gene cluster as a target in the treatment of atherosclerosis

The paraoxonase (PON) gene cluster contains three adjacent gene members, PON1, PON2, and PON3. Originating from the same fungus lactonase precursor, all of the three PON genes share high sequence identity and a similar β propeller protein structure. PON1 and PON3 are primarily expressed in the liver and secreted into the serum upon expression, whereas PON2 is ubiquitously expressed and remains inside the cell. Each PON member has high catalytic activity toward corresponding artificial organophosphate, and all exhibit activities to lactones. Therefore, all three members of the family are regarded as lactonases. Under physiological conditions, they act to degrade metabolites of polyunsaturated fatty acids and homocysteine (Hcy) thiolactone, among other compounds. By detoxifying both oxidized low-density lipoprotein and Hcy thiolactone, PONs protect against atherosclerosis and coronary artery diseases, as has been illustrated by many types of in vitro and in vivo experimental evidence. Clinical observations focusing on gene polymorphisms also indicate that PON1, PON2, and PON3 are protective against coronary artery disease. Many other conditions, such as diabetes, metabolic syndrome, and aging, have been shown to relate to PONs. The abundance and/or activity of PONs can be regulated by lipoproteins and their metabolites, biological macromolecules, pharmacological treatments, dietary factors, and lifestyle. In conclusion, both previous results and ongoing studies provide evidence, making the PON cluster a prospective target for the treatment of atherosclerosis.

The effect of a platelet cholesterol modulation on the acetylsalicylic acid-mediated blood platelet inhibition in hypercholesterolemic patients

Aspirin (acetylsalicylic acid, ASA) is widely used in the prevention of cardiovascular disease, but its beneficial effects may be restrained in some individuals, where the reduced ability of ASA to protect against arterial thrombotic events is observed. We analyzed the influence of the treatment with atorvastatin (10mg/day) on the platelet sensitivity to ASA monitored under in vitro conditions in hypercholesterolemic patients. The associations between plasma or platelet cholesterol parameters and the ASA-mediated inhibition of platelet reactivity or the extent of platelet protein acetylation by ASA were estimated in the patients treated with atorvastatin for 1, 3, or 6 months. Out of 27 patients, in 17 individuals platelets appeared significantly more sensitive to 50 μM ASA in arachidonic acid- or collagen-induced whole blood aggregation following 1 month atorvastatin therapy (inhibition by 60.9 ± 5.6% vs. 48.8 ± 5.4%, P<0.05 for 0.5mM arachidonic acid, 40.8 ± 2.9% vs. 27.0 ± 4.1%, P<0.05 for 1 μg/ml collagen), and this effect lasted for 3 and 6 months, remaining in a weak, although significant, relation to the reduction of platelet cholesterol content (R(S)=-0.277, P<0.002 for arachidonic acid, R(S)=-0.197, P<0.02 for collagen). It was, however, not dependent upon either antiplatelet action or plasma lipid-lowering activity of atorvastatin. In addition, in about 50% of patients, we noticed that ASA (50 μM) significantly and time-dependently diminished thromboxane B(2) concentration in atorvastatin-treated patients. The ASA-induced acetylation of platelet proteins significantly increased in the course of atorvastatin therapy and was associated with reduced platelet cholesterol (R(S)=-0.598, P<0.0001). In conclusion, statin therapy may improve platelet sensitivity to ASA in some hypercholesterolemic patients. This effect may extend beyond the action of atorvastatin as merely a lipid-lowering agent. The mechanisms of resistance of some patients to such a combined ASA-statin treatment remain to be elucidated.

Comparative modeling of PON2 and analysis of its substrate binding interactions using computational methods

Paraoxonase (PON) constitutes a family of calcium-dependent mammalian enzymes comprising of PON1, PON2, and PON3. PON family shares ~60% sequence homology. These enzymes exhibit multiple activities like paraoxonase, arylesterase, and lactonase in a substrate dependent manner. Decreased PON activity has been reported in diseases like cardiovascular disease, atherosclerosis, and diabetes. Even though, PON2 is the oldest member of the family, PON1 is the only member studied in silico. In this study, the structure of PON2 was modeled using MODELLER 9v7 and its interactions with relevant ligands and it's physiological substrate homocysteine thiolactone was performed using AutoDock 4.0. The results reveal that PON1 and PON2 share common ligand binding patterns for arylesterase and lactonase activity, whereas in case of paraoxon binding, the residues involved in the interactions were different. Interestingly, the substrate HCTL was found to have the lowest free energy of binding (ΔG) and highest affinity for PON2 than PON1.

Macrophage endoplasmic reticulum (ER) proteins and reducing elements stabilize paraoxonase 2 (PON2)

OBJECTIVE

To analyze the ability of macrophage sub-cellular fractions to stabilize paraoxonase 2 (PON2).

METHODS

Nuclei, mitochondria, lysosomes, endoplasmic reticulum (ER) and cytosol were isolated from J774A.1 macrophage cell line and incubated with recombinant PON2.

RESULTS

Among the fractions analyzed the ER contains the highest PON2 lactonase activity, and was the most potent one in stabilizing recombinant PON2 (rePON2). Whereas control rePON2 activity was decreased by 40% after 20 h of incubation at 37°C, in the presence of ER it decreased by only 15%. This effect could be attributed to the ER aqueous phase, and not to the ER lipids. The ER proteins fraction was responsible for PON2 stabilization, since heated ER or proteinase K-treated ER was not able to protect rePON2 from inactivation, while the protein fraction (after ammonium sulfate precipitation) completely prevented rePON2 inactivation. Since in the macrophage ER, there are increased levels of NADPH, secondary to glutathione reductase deficiency, we next studied the effect of the redox environment on PON2 inactivation. Incubation of rePON2 with DTT protected PON2 from inactivation. Similarly, NADPH, but not NADP, significantly increased rePON2 lactonase activity by up to 19%, after 20h of incubation as compared to control rePON2. Unlike ER from non-treated macrophages, ER harvested from oxidized-, or from cholesterol loaded-macrophages showed a significant lower basal PON2 lactonase activity, and did not protect PON2 from inactivation but rather increased it.

CONCLUSION

Under normal conditions macrophage ER stabilizes PON2 activity, and this effect could be attributed to ER proteins and redox status.

Macrophages from alpha 7 nicotinic acetylcholine receptor knockout mice demonstrate increased cholesterol accumulation and decreased cellular paraoxonase expression: a possible link between the nervous system and atherosclerosis development

OBJECTIVE

The parasympathetic nervous system regulates inflammation in peripheral tissues through a pathway termed the "cholinergic anti-inflammatory reflex" (CAIR). Mice deficient in the alpha 7 nicotinic acetylcholine receptor (alpha7(-/-)) have an impaired CAIR due to decreased signaling through this pathway. The purpose of this study was to determine if the increased inflammation in alpha7(-/-) mice is associated with enhanced serum and macrophage atherogenicity.

METHODS

We measured serum markers of inflammation and oxidative stress, and macrophage atherogenicity in mouse peritoneal macrophages harvested from alpha7(-/-) mice on the background of C57BL/6 mice, as well as on the background of the atherosclerotic Apolipoprotein E-deficient (ApoE(-/-)) mice.

RESULTS

alpha7-Deficiency had no significant effects on serum cholesterol, or on markers of serum oxidative stress (TBARS and paraoxonase1 activities). However, alpha7-deficiency significantly increased serum CRP and IL-6 (p<0.05) levels in atherosclerotic mice, confirming an anti-inflammatory role for the alpha7 receptor. Macrophage cholesterol mass was increased by 25% in both normal and atherosclerotic mice in the absence of the alpha7 receptor (p<0.05). This was accompanied by conditional increases in oxidized LDL uptake and in macrophage total peroxide levels. Furthermore, alpha7-deficiency reduced macrophage paraoxonase2 mRNA and activity by 50-100% in normal and atherosclerotic mice (p<0.05 for each), indicating a reduction in macrophage anti-oxidant capacity in the alpha7(-/-) mice.

CONCLUSION

The above results suggest an anti-atherogenic role for the macrophage alpha7nAchr, through a mechanism that involves attenuated macrophage oxidative stress and decreased uptake of oxidized LDL.

Effect of quercetin on paraoxonase 2 levels in RAW264.7 macrophages and in human monocytes--role of quercetin metabolism

There is increasing evidence that the intracellular antioxidant enzyme paraoxonase 2 (PON2) may have a protective function in the prevention of atherogenesis. An enhancement of PON2 activity by dietary factors including flavonoids is therefore of interest. In the present study we determined the effect of quercetin on paraoxonase 2 levels in cultured murine macrophages in vitro and in overweight subjects with a high cardiovascular risk phenotype supplemented with 150 mg quercetin/day for 42 days in vivo. Supplementation of murine RAW264.7 macrophages in culture with increasing concentrations of quercetin (1, 10, 20 μmol/L) resulted in a significant increase in PON2 mRNA and protein levels, as compared to untreated controls. Unlike quercetin, its glucuronidated metabolite quercetin-3-glucuronide did not affect PON2 gene expression in cultured macrophages. However the methylated quercetin derivative isorhamnetin enhanced PON2 gene expression in RAW264.7 cells to similar extent like quercetin. Although supplementing human volunteers with quercetin was accompanied by a significant increase in plasma quercetin concentration, dietary quercetin supplementation did not change PON2 mRNA levels in human monocytes in vivo. Current data indicate that quercetin supplementation increases PON2 levels in cultured monocytes in vitro but not in human volunteers in vivo.

Urokinase activates macrophage PON2 gene transcription via the PI3K/ROS/MEK/SREBP-2 signalling cascade mediated by the PDGFR-beta

AIMS

We have recently shown that urokinase plasminogen activator (uPA) increases oxidative stress (OS), cholesterol biosynthesis, and paraoxonase 2 (PON2) expression in macrophages via binding to its receptor, the uPAR. Since PON2 is regulated by both OS and cholesterol content, we hypothesized that uPA elicits a cascade of signal transduction events shared by NADPH oxidase and cholesterol biosynthesis that culminates in PON2 gene expression. Here, we investigated the signalling pathway that leads to the expression of PON2 in macrophages in response to uPA.

METHODS AND RESULTS

The increase in macrophage PON2 mRNA levels in response to uPA was shown to depend on PON2 gene promoter activation and mRNA transcription. LDL abolished these effects, suggesting a possible role for a transcription factor involved in cellular cholesterogenesis. Indeed, uPA upregulated PON2 expression in a sterol regulatory binding protein-2 (SREBP-2)-dependent manner, since blocking SREBP-2 maturation by 4-(2-aminoethyl)-benzenesulfonyl fluoride abolished uPA-stimulation of PON2, whereas inhibition of SREBP-2 catabolism by N-acetyl-leucyl-norleucinal had an opposite effect. The upstream signalling mechanisms include uPA activation of extracellular signal-regulated kinases (ERK1/2), which was dependent on NADPH oxidase and phosphatidylinositol 3-kinase activation, and these latter effects were mediated by the tyrosine kinase activity of the platelet-derived growth factor receptor-beta.

CONCLUSION

These findings provide a framework linking interactions among cellular signalling pathways associated with reactive oxygen species production, macrophage cholesterol biosynthesis, and cellular PON2 expression in vascular pathophysiology.

Paraoxonases role in the prevention of cardiovascular diseases

Increased oxidative stress is a characteristic of patients with high risk for atherosclerosis development (hypercholesterolemic, hypertensive, diabetic), and the above phenomenon was shown to be associated with attenuated antioxidative status. The increased oxidative stress in atherosclerotic patients is present in their blood, as well as in their arterial wall cells, including macrophages, the hallmark of foam cells formation during early atherogenesis. Serum high density lipoprotein (HDL)-associated paraoxonase 1 (PON1) reduces oxidative stress in lipoproteins, in macrophages, and in the atherosclerotic lesion, whereas paraoxonase 2 (PON2, which is present in tissues, but not in serum) acts as an antioxidant at the cellular and not humoral level. Both PON1 and PON2 protect against atherosclerosis development, and this phenomenon could be related to their antioxidative properties. The use of nutritional antioxidants such as vitamin E, carotenoids (lycopene and beta-carotene), and mainly polyphenols (such as those present in red wine, licorice root ethanolic extract, or in pomegranate) by atherosclerotic animals and also by cardiovascular patients, leads to a reduction in oxidative stress and to the attenuation of atherosclerosis development. These latter phenomena could be related to the nutritional antioxidants-induced increase in HDL PON1 activity (effects on gene expression, on preventing enzyme inactivation, and on increasing PON1 stability through its binding to HDL), as well as an increase in macrophage PON2 activation (at the gene expression level).

Paraoxonase 2 attenuates macrophage triglyceride accumulation via inhibition of diacylglycerol acyltransferase 1

This study questioned the role of paraoxonase 2 (PON2) in attenuation of macrophage lipids accumulation. Mouse peritoneal macrophages (MPMs) harvested from PON2-deficient mice versus control C57BL/6 mice, look like foam cells and were larger in size and filled with lipid droplets. Macrophage triglyceride (but not cholesterol) content, biosynthesis rate, and microsomal acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) activity (not mRNA and protein) in PON2-deficient versus control MPM were all significantly increased by 4.6-, 3.6-, and 4.4-fold, respectively. Similarly, microsomal DGAT1 activity and cellular triglyceride content were significantly decreased in human PON2-transfected cells as well as upon incubation of PON2-deficient MPM with recombinant PON2. In all the above experimental systems, PON2 also decreased macrophage oxidative state. Incubation of PON2-deficient MPM with the free radicals generator 2,2'-amidinopropane hydrochloride increased cellular oxidative stress and DGAT1 activity by 2.2- and 3.4-fold, respectively, whereas incubation of microsomes from PON2-deficient MPM with superoxide dismutase decreased DGAT1 activity by 40%. We thus conclude that PON2 attenuates macrophage triglyceride accumulation and foam cell formation via inhibition of microsomal DGAT1 activity, which appears to be sensitive to oxidative state.

Macrophage paraoxonase 2 (PON2) expression is upregulated by unesterified cholesterol through activation of the phosphatidylinositol 3-kinase (PI3K) pathway

Advanced atherosclerotic lesions are characterized by a progressive increase in the unesterified cholesterol (UC) content and a decrease in its cholesteryl ester (CE) content. In the present study, we examined mechanisms involved in the effect of UC and CE on the expression of paraoxonase 2 (PON2) in macrophages. J774A.1 macrophages were enriched with CE or UC by incubation for 14-48 h with 50 microg acetylated low-density lipoprotein in the absence or presence of the acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor 58035 (50 microg/ml), respectively. Macrophage PON2 mRNA expression, protein abundance and activity were increased only in the UC-enriched cells. In UC-enriched cells, inhibition of phosphatidylinositol 3-kinase (PI(3)K; using wortmannin or LY294002) attenuated the increase in PON2 mRNA expression by 50%, compared to untreated cells. In addition, we evidenced an increased phosphorylation of Akt in UC-enriched cells. Thus, we conclude from our data that macrophage PON2 expression is upregulated in UC-enriched macrophages through activation of the PI(3)K signal pathway.

Effects of atorvastatin therapy on hypercholesterolemic rabbits with respect to oxidative stress, nitric oxide pathway and homocysteine

Hypercholesterolemia is characterized with changes in lipid profile, nitric oxide pathway and oxidative stress markers. This study is designed to evaluate the effects of hypercholesterolemic diet and atorvastatin therapy on oxidative stress, lipid peroxide and thiobarbituric acid reactive substances (TBARS), NO pathway markers, nitric oxide(NO) and asymmetric dimethylarginine (ADMA), homocysteine, and paraoxonase activity (PON1) in rabbits. Twenty rabbits fed with high-cholesterol diet for 8 weeks were randomly divided into 2 groups on the fourth week of the hypercholesterolemic diet. First group was fed with high-cholesterol diet alone, whereas the second group with the same cholesterol diet plus atorvastatin (0.3 mg/kg/day) for 4 weeks. High-cholesterol diet increased total cholesterol, low density lipoprotein (LDL-C), high density lipoprotein (HDL-C), ADMA, TBARS and lipid peroxide levels and reduced PON1 activity and NO levels in rabbits. Four weeks of atorvastatin therapy significantly increased HDL-C, PON1 activity and reduced LDL-C, TBARS and lipid peroxide concentrations. Atorvastatin therapy is beneficial in decreasing oxidative stress related with hypercholesterolemia, mainly affecting lipid profile and PON1 activity.

Paraoxonase 2 gene C311S polymorphism is associated with a risk of large vessel disease stroke in a Polish population

BACKGROUND

Oxidative stress plays a role in atherosclerosis. Human paraoxonase (PON) gene products exhibit antioxidant properties. We studied the significance of the Q192R and M55L polymorphisms of the PON1 gene and the C311S polymorphism of the PON2 gene in different etiologies of ischemic stroke.

METHODS

One hundred and thirty-six patients with large vessel disease (LVD) stroke, 140 with small vessel disease stroke, 272 with cardioembolic stroke, and their age- and sex-matched controls were included. PON genotypes were evaluated by PCR-RFLP analyses.

RESULTS

The distribution of PON1 polymorphisms was similar in each stroke group and in the respective controls. Genotypes with the C allele of the PON2 gene C311S polymorphism were overrepresented in LVD stroke patients as compared with their controls, both in univariate and multivariate (dominant model: OR = 1.58, 95% CI: 1.006-2.48) analyses.

CONCLUSION

The genotype with the C allele of the PON2 gene is a risk factor for LVD stroke in a Polish population.

TagSNP analyses of the PON gene cluster: effects on PON1 activity, LDL oxidative susceptibility, and vascular disease

Paraoxonase 1 (PON1) activity is consistently predictive of vascular disease, although the genotype at four functional PON1 polymorphisms is not. To address this inconsistency, we investigated the role of all common PON1 genetic variability, as measured by tagging single-nucleotide polymorphisms (tagSNPs), in predicting PON1 activity for phenylacetate hydrolysis, LDL susceptibility to oxidation ex vivo, plasma homocysteine (Hcy) levels, and carotid artery disease (CAAD) status. The biological goal was to establish whether additional common genetic variation beyond consideration of the four known functional SNPs improves prediction of these phenotypes. PON2 and PON3 tagSNPs were secondarily evaluated. Expanded analysis of an additional 26 tagSNPs found evidence of previously undescribed common PON1 polymorphisms that affect PON1 activity independently of the four known functional SNPs. PON1 activity was not significantly correlated with LDL oxidative susceptibility, but genotypes at the PON1(-108) promoter polymorphism and several other PON1 SNPs were. Neither PON1 activity nor PON1 genotype was significantly correlated with plasma Hcy levels. This study revealed previously undetected common functional PON1 polymorphisms that explain 4% of PON1 activity and a high rate of recombination in PON1, but the sum of the common PON1 locus variation does not explain the relationship between PON1 activity and CAAD.

Paraoxonase 1 (PON1) attenuates macrophage oxidative status: studies in PON1 transfected cells and in PON1 transgenic mice

OBJECTIVE

High density lipoprotein (HDL)-associated paraoxonase 1 (PON1), hydrolyzes oxidized lipids in oxidized low density lipoprotein (LDL) and thus protects against atherosclerosis development. Increased susceptibility to atherosclerosis observed in PON1 knockout (PON1(0)) mice was associated with increased LDL lipid peroxidation as well as increased macrophage oxidative stress. Thus, the aim of the present study is to characterize the direct effect of PON1 on oxidative status processes in macrophages.

METHODS AND RESULTS

We used in vitro and in vivo models of PON1 expression in macrophages, as PON1 is not synthesized by these cells. Peritoneal macrophages (MPM) harvested from PON1(0) mice were transfected with human (hPON1). These cells exhibited reduced total peroxide levels by 47% and decreased capacity to release superoxide anions by 69%, associated with a small but significant increment of the reduced form of glutathione (GSH), a major cellular anti-oxidant, compared to control cells. MPM were also harvested from PON1 transgenic (PON1Tg) mice. Unexpectedly, these cells expressed hPON1 (mRNA and activity). Compared to MPM derived from control C57BL/6J mice, PON1Tg mouse MPM exhibited 35% decreased cellular total peroxide levels, decreased capacity to produce superoxide anions and 47% decreased capacity to oxidize LDL. PON1Tg mouse MPM were also characterized by 51% increased levels of GSH, compared to control MPM. Similarly, MPM harvested from PON1Tg on the genetic background of the atherosclerotic apolipoprotein E knockout (PON1Tg/E(0)) mice also exhibited decreased oxidative stress, compared to E(0) mouse MPM. Aortas obtained from these mice were characterized by decreased lipid peroxide levels, decreased capacity to oxidize LDL, and also increased GSH levels, compared to aortas obtained from E(0) mice. The decreased macrophage and aortic oxidative stress in PON1Tg/E(0) mice was associated with 2.7-fold decreased atherosclerotic lesion size in comparison to E(0) mice.

CONCLUSIONS

PON1 directly reduced macrophage and aortic oxidative status, which was associated with decreased superoxide anion production and increased glutathione content. These phenomena could be responsible for the observed attenuated atherosclerosis development in PON1Tg mice in comparison to control mice.

Paraoxonase 1 (PON1) is a more potent antioxidant and stimulant of macrophage cholesterol efflux, when present in HDL than in lipoprotein-deficient serum: relevance to diabetes

The present study analyzed serum paraoxonase 1 (PON1) distribution among HDL and lipoprotein-deficient serum (LPDS) in atherosclerotic patients, and compared PON1 biological functions in these fractions. Serum HDL and LPDS fractions were isolated from control healthy subjects, diabetic and hypercholesterolemic patients. PON1 activities and protein in HDL/LPDS, as well as its ability to protect against lipid peroxidation and to stimulate HDL/LPDS-mediated macrophage cholesterol efflux were measured. In LPDS from controls, PON1 protein and a significant paraoxonase activity were found, whereas arylesterase and lactonase activities were substantially reduced compared to HDL, by 78% and 88%, respectively. In diabetic patients, PON1 protein and paraoxonase activity in HDL were significantly decreased by 2.8- and 1.7-fold, respectively, compared with controls' HDL. In parallel, in these patient's LPDS, PON1 protein and paraoxonase activity were markedly increased by 3.7- and 1.7-fold, respectively, compared with controls' LPDS. PON1 in HDL (but not PON1 in LPDS) significantly decreased AAPH-induced lipid peroxides formation by 33%, and increased macrophage cholesterol efflux by 31%. We conclude that PON1 is less antiatherogenic when present in LPDS than in HDL. The abnormal serum PON1 distribution in diabetic patients, could be responsible for the accelerated atherosclerosis development in these patients.

Paraoxonases and cardiovascular diseases: pharmacological and nutritional influences

PURPOSE OF REVIEW

To summarize the new articles published in the last year on paraoxonases, including their expression in cardiovascular diseases, and regulation by pharmacological and nutritional means.

RECENT FINDINGS

The elucidation of the crystal structure of the paraoxonase 1 (PON1) gene, obtained by directed evolution, shows that it consists of a six-bladed beta-propeller with a unique active site. PON1 is present in HDL but also in lipoprotein-deficient serum, in VLDL and in chylomicrons. PON1 protects lipids in lipoproteins, in macrophages and in erythrocytes from oxidation. Cellular PON2 and PON3 were also shown to reduce oxidative stress. Beyond its antioxidative properties, PON1 possesses additional antiatherogenic properties against macrophage foam cell formation: attenuation of cholesterol and oxidized lipids influx, inhibition of macrophage cholesterol biosynthesis and stimulation of macrophage cholesterol efflux. The PON1 gene is regulated by Sp1 and protein kinase C, whereas the PON2 gene in macrophages is regulated by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. PON1 activity and mass are both reduced in cardiovascular diseases and the hypocholesterolemic drugs, statins, increase serum PON1 activity (by reducing oxidative stress, or by upregulating hepatic PON1 expression). Expression of cellular PON2, like PON1, was upregulated by statins. Nutritional antioxidants, such as polyphenols, increase PON1 mRNA expression and activity, by an aryl hydrocarbon receptor-dependent mechanism.

SUMMARY

The elucidation of PON1 structure and its active center has enabled a better understanding of its mechanism of action, including its physio-pathological substrate(s). Some drugs and nutrients including dietary antioxidants and polyphenols considerably increase the activities of paraoxonases which, in turn, can reduce oxidative stress and atherosclerosis development.

Atorvastatin

The introduction of statins has drastically changed the treatment and prevention of atherosclerotic vascular disease. By lowering lipid levels and reducing the risk of coronary heart disease, these drugs are among the most effective at reducing morbidity and mortality available to clinical practice. In fact, these compounds have demonstrated the reversible nature of the process of atherosclerosis and can be considered the most useful drugs we currently have in our armamentarium in the prevention of atherosclerosis and its clinical sequelae. Atorvastatin provides pronounced lipid lowering in a broad range of individuals with hypercholesterolaemia and, as such, is an appropriate first-line therapy for patients at low to high risk of coronary heart disease. Reductions in total and low-density lipoprotein cholesterol achieved with atorvastatin have been shown to translate into reductions in risk of cardiovascular morbidity and mortality in both primary and secondary prevention settings. Significant clinical benefits have specifically been observed among patients with Type 2 diabetes and in those with acute coronary syndromes. In common with other members of the statin class, atorvastatin is well tolerated, and adverse events are generally mild and transient in nature. Despite the significant clinical benefits provided by atorvastatin, its full potential in the management of atherosclerotic disease has yet to be wholly explored; however, studies currently ongoing will answer many of the outstanding questions.

Antioxidant effect of atorvastatin is independent of PON1 gene T(-107)C, Q192R and L55M polymorphisms in hypercholesterolaemic patients

BACKGROUND

Serum paraoxonase (PON1), a high density lipoprotein (HDL)-bound antioxidant enzyme, plays a role in atherosclerosis. An increase in PON1 activity has been reported following statin treatment.

OBJECTIVE

In the present study the following factors were evaluated: the influence of PON1 gene Q192R, L55M and T(-107)C polymorphisms on the response of LDL oxidisability and PON1 activity to atorvastatin treatment.

RESEARCH DESIGN AND METHODS

205 Sicilian subjects with primary hypercholesterolaemia (HCh) and 69 healthy subjects as controls were concurrently enrolled. Hypercholesterolaemic patients were randomly divided into two groups: an atorvastatin group (10 mg/day atorvastatin) and a placebo group. Lipid profile, markers of LDL resistance to in vitro oxidation (lag-phase, oxidation rate and thiobarbituric acid-reactive substances), vitamin E content in LDL, PON1 activity and genotypes in both HCh and control subjects were determined at baseline. The same parameters were measured again after 3 weeks of treatment in both the atorvastatin and placebo groups.

RESULTS

HCh subjects showed significantly lower LDL resistance to oxidation, vitamin E content and PON1 activity levels than controls. A strong association was found among PON1 T(-107)C genotypes, LDL susceptibility to oxidation, vitamin E content and PON1 activity. After treatment, the atorvastatin group displayed a significant decrease in total cholesterol, LDL-cholesterol levels, and LDL susceptibility to oxidation, and an increase in vitamin E content and PON1 activity, compared with baseline values. Unlike PON1 activity levels, no difference among PON1 gene polymorphisms and reduction in markers of LDL oxidisability was observed.

CONCLUSIONS

These results show, for the first time, that atorvastatin is able to improve the resistance to LDL oxidation independently of PON1 gene polymorphism.

Dietary antioxidants and paraoxonases against LDL oxidation and atherosclerosis development

Oxidative modification of low-density lipoprotein (LDL) in the arterial wall plays a key role in the pathogenesis of atherosclerosis. Under oxidative stress LDL is exposed to oxidative modifications by arterial wall cells including macrophages. Oxidative stress also induces cellular-lipid peroxidation, resulting in the formation of 'oxidized macrophages', which demonstrate increased capacity to oxidize LDL and increased uptake of oxidized LDL. Macrophage-mediated oxidation of LDL depends on the balance between pro-oxidants and antioxidants in the lipoprotein and in the cells. LDL is protected from oxidation by antioxidants, as well as by a second line of defense--paraoxonase 1 (PON1), which is a high-density lipoprotein-associated esterase that can hydrolyze and reduce lipid peroxides in lipoproteins and in arterial cells. Cellular paraoxonases (PON2 and PON3) may also play an important protective role against oxidative stress at the cellular level. Many epidemiological studies have indicated a protective role for a diet rich in fruits and vegetables against the development and progression of cardiovascular disease. A large number of studies provide data suggesting that consumption of dietary antioxidants is associated with reduced risk for cardiovascular diseases. Basic research provides plausible mechanisms by which dietary antioxidants might reduce the development of atherosclerosis. These mechanisms include inhibition of LDL oxidation, inhibition of cellular lipid peroxidation and consequently attenuation of cell-mediated oxidation of LDL. An additional possible mechanism is preservation/increment of paraoxonases activity by dietary antioxidants. This review chapter presents recent data on the anti-atherosclerotic effects and mechanism of action of three major groups of dietary antioxidants-vitamin E, carotenoids and polyphenolic flavonoids.

Paraoxonase 2 (PON2) expression is upregulated via a reduced-nicotinamide-adenine-dinucleotide-phosphate (NADPH)-oxidase-dependent mechanism during monocytes differentiation into macrophages

Paraoxonase 2 (PON2) is a member of the paraoxonases gene family. PON2 is ubiquitously present in cells, including macrophages, and it was shown to protect against cellular oxidative stress. The aim of the present study was to analyze mechanisms involved in PON2 expression during monocyte/macrophage differentiation. PON2 expression was analyzed in vitro in THP-1 cells differentiated with 1alpha,25-dihydroxyvitamin D3 and in vivo in mouse peritoneal macrophages (MPM) isolated at increasing time intervals after intraperitoneal thioglycollate injection. PON2 expression (mRNA and protein) and activity gradually increased during monocyte/macrophage differentiation, up to five fold and eight fold in vitro and in vivo, respectively. This effect was associated with a gradual increase in cellular superoxide anion production. Supplementation of vitamin E to Balb/C mice inhibited the reduced nicotinamide adenine dinuleotide phosphate (NADPH)-oxidase-dependent increase in cellular superoxide anion production by 50% and down-regulated PON2 mRNA expression and activity by 30 and 60%, respectively. Furthermore, PON2 expression was lower by nine fold in MPM isolated from P47(phox-/-) (inactive NADPH oxidase) mice, in comparison to MPM from control mice. PON2 expression was found to be regulated, at least in part, by the transcription factor AP-1, as suggested by decreased JDP2 (AP-1 repressor) protein expression in the nucleus and by decreased PON2 expression in the presence of a Jun N-terminal kinase inhibitor (SP600125). The present study demonstrates, for the first time, that PON2 expression increases in monocytes during their maturation into macrophage as a result of NADPH-oxidase activation, and this process is partly regulated by the transcription factor AP-1. PON2 stimulation may represent a compensatory mechanism against the increase in cellular superoxide anion production and atherogenesis.

Paraoxonases 1, 2, and 3, oxidative stress, and macrophage foam cell formation during atherosclerosis development

Paraoxonases PON1 and PON3, which are both associated in serum with HDL, protect the serum lipids from oxidation, probably as a result of their ability to hydrolyze specific oxidized lipids. The activity of HDL-associated PON1 seems to involve an activity (phospholipase A2-like activity, peroxidase-like activity, lactonase activity) which produces LPC. To study the possible role of PON1 in macrophage foam cell formation and atherogenesis we used macrophages from control mice, from PON1 knockout mice, and from PON1 transgenic mice. Furthermore, we analyzed PON1-treated macrophages and PON1-transfected cells to demonstrate the contribution of PON1 to the attenuation of macrophage cholesterol and oxidized lipid accumulation and foam cell formation. PON1 was shown to inhibit cholesterol influx [by reducing the formation of oxidized LDL (Ox-LDL), increasing the breakdown of specific oxidized lipids in Ox-LDL, and decreasing macrophage uptake of Ox-LDL]. PON1 also inhibits cholesterol biosynthesis and stimulates HDL-mediated cholesterol efflux from macrophages. PON2 and PON3 protect against oxidative stress, with PON2 acting mainly at the cellular level. Whereas serum PON1 and PON3 were inactivated under oxidative stress, macrophage PON2 expression and activity were increased under oxidative stress, probably as a compensatory mechanism against oxidative stress. Intervention to increase the paraoxonases (cellular and humoral) by dietary or pharmacological means can reduce macrophage foam cell formation and attenuate atherosclerosis development.

Paraoxonase, a cardioprotective enzyme: continuing issues

PURPOSE OF REVIEW

The paraoxonase family consists of three members (PON1, PON2 and PON3) that share structural properties and enzymatic activities, among which is the ability to hydrolyze oxidized lipids in LDL. The exact function of the different family members is not clear although the conservation among the individual family members across species suggests a strong evolutionary pressure to preserve these functional differences. The purpose of this review is to highlight several problems with respect to the mechanism of action of paraoxonase and differences between the family members that merit further study.

RECENT FINDINGS

PON1 transgenic mice are at lower risk for atherosclerosis, which is consistent with PON1 gene knockout studies in mice and human genetic polymorphism studies. The exact mechanism by which paraoxonase is cardioprotective is not clear, although it is likely to be related to its antioxidant properties especially on LDL. PON1 levels are influenced by a variety of environmental factors, including statins and cytokines. The preferential association of PON1 with HDL is mediated in part by its signal peptide and by desorption from the plasma membrane of expressing cells by HDL or phospholipid. Apolipoprotein A-I is not necessary for PON1 association with HDL, but its activity is stabilized in the presence of the apolipoprotein. Only in the absence of both lecithin cholesterol acyltransferase and apolipoprotein E is paraoxonase associated with non-HDL lipoproteins. The displacement of paraoxonase by serum amyloid A may explain in part the proinflammatory nature of HDL in the acute phase. The mechanism by which PON3 associates with HDL has not been studied. In addition to the ability to hydrolyze oxidized lipids in LDL, paraoxonase also alters the oxidative state of macrophages. Exogenous PON1 is able to reverse the oxidative stress in macrophages in aged apolipoprotein E deficient and PON1 deficient mice. The increase in oxidative stress in macrophages from PON1 deficient mice occurs despite the expression of PON2 and PON3 in macrophages. PON1 has recently been shown to contain phospholipase A2 activity, with the subsequent release of lysophosphatidylcholine that influences macrophage cholesterol biosynthesis.

SUMMARY

PON1 mass and activity in the plasma significantly influence the risk of developing cardiovascular disease. This is likely mediated by its antioxidation properties on LDL and/or macrophages. The precise mechanism by which this HDL associated protein prevents or attenuates oxidation of LDL and the oxidative stress of macrophages remains to be clarified. The role of PON2 and PON3 in atherosclerosis and their antioxidant properties with respect to LDL and macrophages also merit further investigation.