Cellular cholesterol accumulation modulates high fat high sucrose (HFHS) diet-induced ER stress and hepatic inflammasome activation in the development of non-alcoholic steatohepatitis.

Non-alcoholic steatohepatitis (NASH), is the form of non-alcoholic fatty liver disease posing risk to progress into serious long term complications. Human and pre-clinical models implicate cellular cholesterol dysregulation playing important role in its development. Mouse model studies suggest synergism between dietary cholesterol and fat in contributing to NASH but the mechanisms remain poorly understood. Our laboratory previously reported the primary importance of hepatic endoplasmic reticulum cholesterol (ER-Chol) in regulating hepatic ER stress by comparing the responses of wild type, Ldlr-/-xLcat+/+ and Ldlr-/-xLcat-/- mice, to a 2% high cholesterol diet (HCD). Here we further investigated the roles of ER-Chol and ER stress in HFHS diet-induced NASH using the same strains. With HFHS diet feeding, both WT and Ldlr-/-xLcat+/+ accumulate ER-Chol in association with ER stress and inflammasome activation but the Ldlr-/-xLcat-/- mice are protected. By contrast, all three strains accumulate cholesterol crystal, in correlation with ER-Chol, albeit less so in Ldlr-/-xLcat-/- mice. By comparison, HCD feeding per se (i) is sufficient to promote steatosis and activate inflammasomes, and (ii) results in dramatic accumulation of cholesterol crystal which is linked to inflammasome activation in Ldlr-/-xLcat-/- mice, independent of ER-Chol. Our data suggest that both dietary fat and cholesterol each independently promote steatosis, cholesterol crystal accumulation and inflammasome activation through distinct but complementary pathways. In vitro studies using palmitate-induced hepatic steatosis in HepG2 cells confirm the key roles by cellular cholesterol in the induction of steatosis and inflammasome activations. These novel findings provide opportunities for exploring a cellular cholesterol-focused strategy for treatment of NASH.

Paraoxonase 1 Phenotype and Mass in South Asian versus Caucasian Renal Transplant Recipients.

South Asian renal transplant recipients have a higher incidence of cardiovascular disease compared with Caucasian renal transplant recipients. We carried out a study to determine whether paraoxonase 1, a novel biomarker for cardiovascular risk, was decreased in South Asian compared with Caucasian renal transplant recipients. Subjects were matched two to one on the basis of age and sex for a total of 129 subjects. Paraoxonase 1 was measured by mass, arylesterase activity, and two-substrate phenotype assay. Comparisons were made by using a matched design. The frequency of PON1 QQ, QR and RR phenotype was 56%, 37%, and 7% for Caucasian subjects versus 35%, 44%, and 21% for South Asian subjects (χ(2) = 7.72, P = 0.02). PON1 mass and arylesterase activity were not significantly different between South Asian and Caucasian subjects. PON1 mass was significantly associated with PON1 phenotype (P = 0.0001), HDL cholesterol (P = 0.009), LDL cholesterol (P = 0.02), and diabetes status (P < 0.05). Arylesterase activity was only associated with HDL cholesterol (P = 0.003). Thus the frequency of the PON1 RR phenotype was higher and that of the QQ phenotype was lower in South Asian versus Caucasian renal transplant recipients. However, ethnicity was not a significant factor as a determinant of PON1 mass or arylesterase activity, with or without analysis including PON1 phenotype. The two-substrate method for determining PON1 phenotype may be of value for future studies of cardiovascular complications in renal transplant recipients.

Lecithin:cholesterol acyltransferase deficiency protects against cholesterol-induced hepatic endoplasmic reticulum stress in mice.

We recently reported that lecithin:cholesterol acyltransferase (LCAT) knock-out mice, particularly in the LDL receptor knock-out background, are hypersensitive to insulin and resistant to high fat diet-induced insulin resistance (IR) and obesity. We demonstrated that chow-fed Ldlr-/-xLcat+/+ mice have elevated hepatic endoplasmic reticulum (ER) stress, which promotes IR, compared with wild-type controls, and this effect is normalized in Ldlr-/-xLcat-/- mice. In the present study, we tested the hypothesis that hepatic ER cholesterol metabolism differentially regulates ER stress using these models. We observed that the Ldlr-/-xLcat+/+ mice accumulate excess hepatic total and ER cholesterol primarily attributed to increased reuptake of biliary cholesterol as we observed reduced biliary cholesterol in conjunction with decreased hepatic Abcg5/g8 mRNA, increased Npc1l1 mRNA, and decreased Hmgr mRNA and nuclear SREBP2 protein. Intestinal NPC1L1 protein was induced. Expression of these genes was reversed in the Ldlr-/-xLcat-/- mice, accounting for the normalization of total and ER cholesterol and ER stress. Upon feeding a 2% high cholesterol diet (HCD), Ldlr-/-xLcat-/- mice accumulated a similar amount of total hepatic cholesterol compared with the Ldlr-/-xLcat+/+ mice, but the hepatic ER cholesterol levels remained low in conjunction with being protected from HCD-induced ER stress and IR. Hepatic ER stress correlates strongly with hepatic ER free cholesterol but poorly with hepatic tissue free cholesterol. The unexpectedly low ER cholesterol seen in HCD-fed Ldlr-/-xLcat-/- mice was attributable to a coordinated marked up-regulation of ACAT2 and suppressed SREBP2 processing. Thus, factors influencing the accumulation of ER cholesterol may be important for the development of hepatic insulin resistance.

Mouse serum paraoxonase-1 lactonase activity is specific for medium-chain length fatty acid lactones.

Recent studies suggest that paraoxonase-1 (PON1), complexed with high-density lipoproteins, is the major lactonase in the circulation. Using 5-hydroxy eicosatetraenoate δ-lactone (5-HETEL) as the substrate, we observed lactonase activity in serum from Pon1-/- mice. However, 6-12 carbon fatty acid γ- and δ-lactones were not hydrolyzed in serum from Pon1-/- mice. Serum from both wild-type and Pon1-/- mice contained a lactonase activity towards 5-HETEL and 3-oxo-dodecanoyl-homoserine lactone that was resistant to inactivation by EDTA. This lactonase activity was sensitive to the serine esterase inhibitor phenyl methyl sulfonyl fluoride and co-eluted with carboxylesterase activity by size-exclusion chromatography. Analysis of serum from the Es1e mouse strain, which has a deficiency in the carboxylesterase, ES-1, proved that this activity was due to ES-1. PON1 activity predominated at early time points (30 s), whereas both PON1 and ES-1 contributed equally at later time points (15 min). When both PON1 and ES-1 were inhibited, 5-HETEL was stable in mouse serum. Thus, while long-chain fatty acid lactones are substrates for PON1, they can be hydrolyzed by ES-1 at neutral pH. In contrast, medium-chain length fatty acid lactones are stable in mouse serum in the absence of PON1, suggesting that PON1 plays a specific role in the metabolism of these compounds.

Association of the novel cardiovascular risk factors paraoxonase 1 and cystatin C in type 2 diabetes.

Paraoxonase 1 (PON1) has been reported to be associated with proteinuria in subjects with type 2 diabetes mellitus (T2DM). Plasma cystatin C is more accurate than creatinine for identifying stage 3 kidney disease in T2DM. We tested the hypothesis that PON1 and cystatin C would be associated in T2DM subjects from an Aboriginal Canadian community, who are at high risk for the development of nephropathy. PON1 A(-162)G and PON2 Ala148Gly genotypes, cystatin C, HbA1c, high density lipoprotein cholesterol (HDLC), waist circumference (waist), and duration of diabetes were included in the regression analysis with log(e) (ln) of PON1 mass as the dependent variable. A regression model including PON2 Ala148Gly genotype, HDLC, and ln cystatin C explained 25.8% of the variance in PON1 mass. Conversely, waist, age, ln HbA1c, ln duration of diabetes, and ln PON1 mass, but not PON2 genotype, explained 38% of the variance in cystatin C. Subjects with cystatin C estimated glomerular filtration rate (eGFR) <60 ml/min per 1.73 m(2) (stage 3 kidney disease) had significantly lower PON1 mass compared with subjects with cystatin C-eGFR >60 ml/min per 1.73 m(2). The lower mass of PON1, an anti-inflammatory HDL-associated enzyme, in T2DM with cystatin C-eGFR <60 ml/min per 1.73 m(2) may contribute to their increased risk for cardiovascular disease.

Development of an immunoblot assay with infrared fluorescence to quantify paraoxonase 1 in serum and plasma.

Paraoxonase 1 (PON1) requires calcium for activity and is inactivated in the presence of EDTA. Because of this, studies to date have used serum or heparinized plasma for both activity and mass assays of PON1. Whole serum and EDTA plasma were analyzed by SDS-electrophoresis and Western blot using anti-PON1 monoclonal antibody 4C10. Because PON1 has one disulfide and one free cysteine residue, the samples were reduced with dithiothreitol before electrophoresis. Western blot identified a major PON1 band with a molecular mass of approximately 45 kDa and two minor bands of approximately 40 and 35 kDa in both serum and EDTA plasma. This established that PON1 is inactive, but structurally intact, in EDTA plasma and suggested that a mass assay could be developed based on SDS-electrophoresis and Western blot. Linearity was established for plasma and for a PON1 standard. Quantification was based on the major PON1 band at 45 kDa. The correlation between serum and plasma PON1 mass was 0.9553. The between-run variation was determined with a serum pool to be 7.8%. The mass of PON1 in serum was significantly correlated with arylesterase activity (r = 0.85). Thus, we have demonstrated the feasibility of measuring PON1 mass in either serum or EDTA plasma.

Paraoxonase-1 does not reduce or modify oxidation of phospholipids by peroxynitrite.

Serum paraoxonase (PON1) is a high-density lipoprotein (HDL)-associated esterase/lactonase implicated to play a role in protection against atherosclerosis. However, the exact mechanism(s) and substrates for PON1 are still uncertain. In this article, we review some of the evidence for PON1's antioxidant activity, as well as our efforts to identify the actual substrates and products for this activity. We originally reported that PON1 had phospholipase activity toward oxidized phosphatidylcholine (J. Biol. Chem. 276:24473-24481; 2001). Subsequently, Marathe et al. (J. Biol. Chem. 278:3937-3947; 2003) reported that this activity was due to a contaminating lipase. However, that article did not replicate the conditions used in our previous study. To address this controversy, we purified serum PON1 by a modified method that separates the paraoxonase activity from an activity detectable as platelet-activating factor acetyl hydrolase (PAF-AH) (Teiber et al., J. Lipid. Res. 2004; Epub ahead of print, PMID 15342686) and reexamined the oxidation of phosphatidylcholine by peroxynitrite using 3-morpholinosydnonimine as a peroxynitrite generator and apolipoprotein AI-phosphatidylcholine- PON1 complexes. The phosphatidylcholines were studied by electrospray ionization tandem mass spectrometry. PON1 preparations free of PAF-AH activity showed no phospholipase activity when reconstituted into apolipoprotein AI-phosphatidylcholine complexes. We conclude that PON1 does not affect the accumulation of phosphatidylcholine oxidation products. Further, we have no evidence that PON1 has an intrinsic phospholipase A2 activity toward oxidized phospholipids.

A novel in vivo lecithin-cholesterol acyltransferase (LCAT)-deficient mouse expressing predominantly LpX is associated with spontaneous glomerulopathy.

Complete lecithin cholesterol acyltransferase (LCAT) deficiency is a rare genetic cause of extreme reduction in high density lipoproteins and there is a high prevalence of chronic renal dysfunction that may progress to renal failure. Previous in vitro studies suggest the vesicular lipoprotein X (LpX) particles commonly seen in LCAT-deficient plasmas may be causative. To test this hypothesis, we have generated a novel murine model that selectively accumulate LpX in the circulation by cross breeding the sterol regulatory element binding protein (SREBP) 1a transgenic mice (S+) with the LCAT knockout (lcat-/-) mice. Fast protein liquid chromatography fractionation of pooled plasma lipids revealed that virtually all cholesterol is concentrated in the very low density lipoprotein (VLDL)-sized fractions. These fractions are enriched in free cholesterol and phospholipid but extremely poor in triglyceride. Electron microscopy of the d <1.063 g/ml fraction of the S+lcat-/- mice revealed abnormal large vesicular particles, suggestive of LpX. The S+lcat-/- mice developed glomerular lesions spontaneously evident at 6 months with glomerular and tubulointerstitial lipid-deposits. Immunohistochemical staining with RhoA showed marked positive focal staining in glomeruli in the S+lcat-/- mice and undetectable in the S+/lcat+/+ control. By 10 months of age, the kidneys showed progressive glomerular injury including segmental foam cell infiltrates, mesangial expansion, and hyalinosis. Renal abnormalities are very similar to those seen in human LCAT deficiency. We conclude that the selective high-level accumulation of plasma LpX in the S+lcat-/- mice is strongly associated with a spontaneous glomerulopathy, providing in vivo evidence that LpX contributes to the LCAT deficiency-related nephropathy.

Separation and quantitative recovery of mouse serum arylesterase and carboxylesterase activity.

Paraoxonase-1 (PON1) is known to be associated with high density lipoproteins. We optimized buffer conditions to obtain quantitative recovery of PON1 (arylesterase) activity and analyzed the distribution of PON1 in mice using a combination of size-exclusion chromatography and ultracentrifugation. Size-exclusion chromatography of mouse serum separated the esterase activity into two peaks, one overlapping the high density lipoproteins and a second peak of lower molecular weight, consistent with serum carboxylesterase, which accounted for approximately 20% of the total esterase activity of normal mouse serum. Using conditions for the quantitative recovery of arylesterase activity, we fractionated serum by ultracentrifugation into d < 1.21 g/ml, d < 1.25 g/ml, d > 1.21 g/ml, and d > 1.25 g/ml fractions. We observed that PON1 arylesterase activity and mass were isolated in the d < 1.21 g/ml fraction and that serum carboxylesterase was recovered in the d > 1.25 g/ml fraction. The significance of the confounding of PON1 arylesterase activity by serum carboxylesterase was demonstrated by studying mice challenged with a high-fat, high-cholate diet for 14 days. It was shown that all of the decrease in arylesterase activity in response to this diet is attributable to the HDL-associated arylesterase activity (PON1). We conclude that mouse PON1 is quantitatively associated with high density lipoproteins. The contribution of serum carboxylesterase to the total esterase activity significantly confounds the interpretation of total arylesterase activity in mouse serum.

Hypertriglyceridemia in lecithin-cholesterol acyltransferase-deficient mice is associated with hepatic overproduction of triglycerides, increased lipogenesis, and improved glucose tolerance.

Lecithin-cholesterol acyltransferase deficiency is frequently associated with hypertriglyceridemia (HTG) in animal models and humans. We investigated the mechanism of HTG in the ldlr-/- x lcat-/- (double knockout (dko)) mice using the ldlr-/- x lcat+/+ (knock-out (ko)) littermates as control. Mean fasting triglyceride (TG) levels in the dko mice were elevated 1.75-fold compared with their controls (p < 0.002). Both the very low density lipoprotein and the low density lipoprotein/intermediate density lipoprotein fractions separated by fast protein liquid chromatography were TG-enriched in the dko mice. In vitro lipolysis assay revealed that the dko mouse very low density lipoprotein (d < 1.019 g/ml) fraction separated by ultracentrifugation was a more efficient substrate for lipolysis by exogenous bovine lipoprotein lipase. Post-heparin lipoprotein lipase activity was reduced by 61% in the dko mice. Hepatic TG production rate, determined after intravenous Triton WR1339 injection, was increased 8-fold in the dko mice. Hepatic mRNA levels of sterol regulatory element binding protein-1 (srebp-1) and its target genes acetyl-CoA carboxylase-1 (acc-1), fatty acid synthase (fas), and stearoyl-CoA desaturase-1 (scd-1) were significantly elevated in the dko mice compared with the ko control. The hepatic mRNA levels of LXRalpha (lxralpha) and its target genes including angiopoietin-like protein 3 (angptl-3) in the dko mice were unchanged. Fasting glucose and insulin levels were reduced by 31 and 42%, respectively in the dko mice, in conjunction with a 49% reduction in hepatic pepck-1 mRNA (p = 0.014). Both the HTG and the improved fasting glucose phenotype seen in the dko mice are at least in part attributable to an up-regulation of the hepatic srebp-1c gene.

Formation of apolipoprotein AI-phosphatidylcholine core aldehyde Schiff base adducts promotes uptake by THP-1 macrophages.

OBJECTIVE: High-density lipoprotein (HDL) is believed to protect against development of atherosclerosis by inhibiting the accumulation of oxidized lipids in low-density lipoprotein (LDL). Paradoxically, HDL lipid is more susceptible to oxidation than LDL lipid. In the present study, we examined the effect of oxidized phospholipids on the uptake of HDL by macrophages. METHODS AND RESULTS: Oxidation of HDL increased formation of phosphatidylcholine core aldehydes that was paralleled by increased covalent binding of phospholipids to HDL protein from 0.96+/-0.44 to 8.5+/-1.76 phosphorus/HDL protein (mol/mol). Incubation of apolipoprotein AI with synthetically prepared phosphatidylcholine core aldehydes, 1-palmitoyl-2-[5-oxo]valeroyl-sn-glycero-3-phosphocholine or 1-palmitoyl-2-[9-oxo] nonanoyl-sn-glycero-3-phosphocholine, significantly increased the phosphorus:apolipoprotein AI ratio from 1.1+/-0.5 to 7.2+/-2.0 and from 0.9+/-0.6 to 8.5+/-0.8, respectively. The binding and uptake of phosphatidylcholine core aldehyde-apolipoprotein AI proteoliposomes, by THP-1 macrophages, was similar to that observed for oxidized HDL and oxidized LDL. CONCLUSION: We conclude that oxidation of HDL increased formation of phosphatidylcholine core aldehyde-apolipoprotein AI Schiff base adducts and enhanced uptake of oxidized HDL by THP-1 macrophages.

Influence of interferon-gamma on the extent and phenotype of diet-induced atherosclerosis in the LDLR-deficient mouse.

OBJECTIVE: The aim of this study was to investigate the influence of interferon-gamma (IFN-gamma) on atherosclerosis in low density lipoprotein receptor (LDLR)-null mice. METHODS AND RESULTS: We cross-bred IFN-gamma-deficient mice with LDLR-null mice and analyzed lipoprotein profiles and atherosclerosis in the compound mutant progeny after 8 and 20 weeks on a cholesterol-enriched diet. IFN-gamma deficiency did not affect serum cholesterol levels or lipoprotein profiles, but it did affect the extent and phenotype of atherosclerosis. Atherosclerotic lesions in IFN-gamma-deficient mice were reduced by 75% in the aortic arch and by 46% in the descending aorta compared with control mice after 8 weeks on the diet. After 20 weeks, arch lesions were reduced by 43%, and descending aorta lesions were reduced by 65% in IFN-gamma-deficient mice compared with controls. At 8 weeks, percent lesional macrophage and smooth muscle content was significantly less in the IFN-gamma-deficient mice, but not at 20 weeks. Although there were fewer class II major histocompatibility complex-positive cells in the lesions of IFN-gamma-deficient animals compared with controls, class II major histocompatibility complex expression on endothelial cells overlying lesions persisted in the absence of IFN-gamma. CONCLUSIONS: These data provide direct evidence that IFN-gamma influences atherosclerosis development and phenotype in the LDLR-deficient mouse, independent of changes in blood lipoprotein profiles.

Paraoxonase-1 reduces monocyte chemotaxis and adhesion to endothelial cells due to oxidation of palmitoyl, linoleoyl glycerophosphorylcholine.

OBJECTIVE: High-density lipoprotein (HDL) is postulated to protect against the development of atherosclerosis, in part, by inhibiting the oxidation of low density lipoprotein (LDL) in the sub-endothelial space and thus inhibiting activation of the endothelium. The HDL-associated enzyme, paraoxonase-1, is proposed to be a major protective factor. However, HDL is also prone to oxidation when exposed to peroxynitrite and may therefore, once oxidized, have properties similar to oxidized LDL. METHODS AND RESULTS: We exposed human HDL to the peroxynitrite donor 3-morpholinosydnonimine and incubated oxidized HDL with human umbilical vein endothelial cells (HUVECs). Oxidized HDL increased monocyte binding (P<0.001) and enhanced chemotaxis (P<0.001). The major oxidized phospholipids were 1-palmitoyl (stearoyl)-2-[9-oxo]nanoyl(azelaoyl)-sn-glycero-phosphocholine, derived from linoleate-containing phosphatidylcholines, and 1-palmitoyl(stearoyl)-2-[5-oxo]valeroyl(glutaroyl)-sn-glycero-phosphocholine, derived from arachidonate-containing phosphatidylcholines. Incubation of HUVECs with synthetically prepared 1-palmitoyl-2-[9-oxo]nanoyl(azelaoyl)-sn-glycero-phosphocholine, or 1-palmitoyl-2-[5-oxo]valeroyl(glutaroyl)-sn-glycero-phosphocholine increased binding of monocytes (P<0.001) and chemotaxis (P<0.001). Purified paraoxonase-1 reduced monocyte adhesion and chemotaxis (P<0.001). CONCLUSIONS: (i) HDL can be a source of oxidatively-derived bioactive phospholipids; (ii) the fragmented phospholipids with a 9-carbon aldehyde or acid are as effective as a 5-carbon aldehyde or acid at inducing monocyte adhesion and chemotaxis; and (iii) paraoxonase-1 is effective at reducing the activity of these phospholipid oxidation products.

Influence of C3 deficiency on atherosclerosis.

BACKGROUND: The influence of complement activation on atherosclerosis is not well understood. The purpose of this study was to examine the effects of C3 deficiency on the extent and phenotype of atherosclerosis. METHODS AND RESULTS: Aortic atherosclerosis was analyzed in low-density lipoprotein receptor (ldlr)/C3-deficient mice (ldlr(-/-)C3(-/-)) and ldlr(-/-)C3(+/-) littermate control mice after 15 weeks on a 1.25% (wt/wt) cholesterol diet. Serum lipoprotein profiles and immunoglobulin levels were not significantly different between the 2 experimental groups. The lipid-positive en face lesional area in thoracic and abdominal aorta was greater in C3-deficient mice than in control mice (3.9% versus 2.1%, median, P=0.0076). Similarly, the lipid-positive area in aortic arch sections was greater in C3-deficient mice than in controls (0.04 mm2 versus 0.02 mm2, median, P=0.0089). Analysis of aortic arch sections showed greater lesional macrophage content in C3-deficient versus control mice (8.24+/-1.36% versus 5.9+/-1.63% intimal area, mean+/-SEM, P=0.003), less smooth muscle cell content in C3-deficient versus control mice (0.06+/-0.05% versus 0.92+/-0.32% intimal area, mean+/-SEM, P<0.0001), and less collagen content in C3-deficient versus control mice (0.52+/-1.26% versus 11+/-10.43% intimal area, mean+/-SEM, P=0.008). CONCLUSIONS: The maturation of atherosclerotic lesions beyond the foam cell stage is strongly dependent on an intact complement system.

Oxidative stress is markedly elevated in lecithin:cholesterol acyltransferase-deficient mice and is paradoxically reversed in the apolipoprotein E knockout background in association with a reduction in atherosclerosis.

Complete lecithin:cholesterol acyltransferase (LCAT) deficiency is a rare cause of severe hypoalphalipoproteinemia, but the affected subjects are surprisingly not particularly prone to premature coronary heart disease. We studied oxidative stress in lcat-/- mice and their cross-breed with apolipoprotein-E knockout mice (apoE-/-xlcat-/-) by measuring vascular ring superoxide production and plasma phospholipid (PL)-bound F2-isoprostane levels and their relationship with aortic atherosclerosis. Compared with wild type control (lcat+/+), lcat-/- and lcat+/- mice showed a 4.9- (p = 0.003) and a 2.1-fold (p = 0.04) increase in plasma PL-F2-isoprostane levels, respectively. There was also a 3.6- (p < 0.0001) and 2.9-fold (p = 0.003) increase in the area under the curve for the aortic ring superoxide excursion by lucigenin-derived chemiluminescence. A comparison of apoE-/-xlcat+/+ mice with wild type control mice showed a more modest 2.1- (p = 0.04) and 2.2-fold (p < 0.00001) increase in these respective markers. Surprisingly, the apoE-/-xlcat-/- mice showed a paradoxical normalization in both oxidation markers. Furthermore, by fast protein liquid chromatography separation, we observed an associated retention and redistribution of serum paraoxonase activities to the non-high density lipoprotein fractions in both the apoE-/-xlcat-/- and apoE-/-xlcat+/- mice. Aortic atherosclerotic lesions in male apoE-/-xlcat-/- and apoE-/-xlcat+/- mice were reduced by 52 (p = 0.02) and 24% (p = 0.46), respectively. Our data suggest that LCAT-deficient mice are associated with an increased oxidative stress that is paradoxically reversed in a hyperlipidemic background, possibly due to the redistribution of paraoxonase. This modulation of oxidative stress may in part contribute to the reduced atherosclerosis seen in the apoE-/- xlcat-/- mice.

Multiple substrates for paraoxonase-1 during oxidation of phosphatidylcholine by peroxynitrite.

Paraoxonase (PON-1) is a high-density lipoprotein (HDL)-bound enzyme with activity toward multiple substrates. It hydrolyzes organic phosphate and aromatic carboxylic acid esters. It also inhibits accumulation of oxidized phospholipids in plasma lipoproteins by a mechanism yet to be determined. Therefore, we subjected apolipoprotein A-I proteoliposomes containing either 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine or 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine to oxidation by a peroxynitrite generator, SIN-1, in the presence and absence of purified PON-1. PON-1 modified the proportion of oxidation products without affecting the overall extent of PC oxidation. However, in the presence of PON-1, phosphatidylcholine isoprostanes were hydrolyzed to lysophosphatidylcholine. In addition, PON-1 hydrolyzed the phosphatidylcholine core aldehydes 1-palmitoyl-2-(9-oxo)nonanoyl-sn-glycero-3-phosphocholine and 1-palmitoyl-2-(5-oxo)valeroyl-sn-glycero-3-phosphocholine to lysophosphatidylcholine. This hydrolysis was not affected by pefabloc, a serine esterase inhibitor. There was no detectable release of linoleate, arachidonate, or their hydroperoxy or hydroxy derivatives in the presence of PON-1. We conclude that PON-1 minimizes the accumulation of phosphatidylcholine oxidation products by the hydrolysis of phosphatidylcholine isoprostanes and core aldehydes to lysophosphatidylcholine with a serine esterase-independent mechanism.