Presence of LDL receptor-related protein/alpha 2-macroglobulin receptors in macrophages of atherosclerotic lesions from cholesterol-fed New Zealand and heterozygous Watanabe heritable hyperlipidemic rabbits

Macrophages of genetically characterized familial hypercholesterolaemia patients show up-regulation of LDL-receptor-related proteins

Familial hypercholesterolaemia (FH) is a major risk for premature coronary heart disease due to severe long‐life exposure to high LDL levels. Accumulation of LDL in the vascular wall triggers atherosclerosis with activation of the innate immunity system. Here, we have investigated (i) gene expression of LDLR and LRPs in peripheral blood cells (PBLs) and in differentiated macrophages of young FH‐patients; and (ii) whether macrophage from FH patients have a differential response when exposed to high levels of atherogenic LDL. PBLs in young heterozygous genetically characterized FH patients have higher expression of LRP5 and LRP6 than age‐matched healthy controls or patients with secondary hypercholesterolaemia. LRP1 levels were similar among groups. In monocyte‐derived macrophages (MACs), LRP5 and LRP1 transcript levels did not differ between FHs and controls in resting conditions, but when exposed to agLDL, FH‐MAC showed a highly significant up‐regulation of LRP5, while LRP1 was unaffected. PBL and MAC cells from FH patients had significantly lower LDLR expression than control cells, independently of the lipid‐lowering therapy. Furthermore, exposure of FH‐MAC to agLDL resulted in a reduced expression of CD163, scavenger receptor with anti‐inflammatory and atheroprotective properties. In summary, our results show for first time that LRPs, active lipid‐internalizing receptors, are up‐regulated in innate immunity cells of young FH patients that have functional LDLR mutations. Additionally, their reduced CD163 expression indicates less atheroprotection. Both mechanisms may play a synergic effect on the onset of premature atherosclerosis in FH patients.

Increasing adipocyte lipoprotein lipase improves glucose metabolism in high fat diet-induced obesity

Lipid accumulation in liver and skeletal muscle contributes to co-morbidities associated with diabetes and obesity. We made a transgenic mouse in which the adiponectin (Adipoq) promoter drives expression of lipoprotein lipase (LPL) in adipocytes to potentially increase adipose tissue lipid storage. These mice (Adipoq-LPL) have improved glucose and insulin tolerance as well as increased energy expenditure when challenged with a high fat diet (HFD). To identify the mechanism(s) involved, we determined whether the Adipoq-LPL mice diverted dietary lipid to adipose tissue to reduce peripheral lipotoxicity, but we found no evidence for this. Instead, characterization of the adipose tissue of the male mice after HFD challenge revealed that the mRNA levels of peroxisome proliferator-activated receptor-γ (PPARγ) and a number of PPARγ-regulated genes were higher in the epididymal fat pads of Adipoq-LPL mice than control mice. This included adiponectin, whose mRNA levels were increased, leading to increased adiponectin serum levels in the Adipoq-LPL mice. In many respects, the adipose phenotype of these animals resembles thiazolidinedione treatment except for one important difference, the Adipoq-LPL mice did not gain more fat mass on HFD than control mice and did not have increased expression of genes in adipose such as glycerol kinase, which are induced by high affinity PPAR agonists. Rather, there was selective induction of PPARγ-regulated genes such as adiponectin in the adipose of the Adipoq-LPL mice, suggesting that increasing adipose tissue LPL improves glucose metabolism in diet-induced obesity by improving the adipose tissue phenotype. Adipoq-LPL mice also have increased energy expenditure.

Amlodipine reduces AngII-induced aortic aneurysms and atherosclerosis in hypercholesterolemic mice

Background

The purpose of this study was to determine effects of amlodipine, a dihydropyridine calcium channel blocker, on development of angiotensin II (AngII)-induced vascular pathologies.

Methods and Results

Male LDL receptor -/- mice were infused with vehicle, amlodipine (5 mg/kg/d), AngII (1,000 ng/kg/min), or AngII + amlodipine for 4 weeks through osmotic pumps (n=10/group). Mice were fed a saturated fat-enriched diet for 1 week prior to pump implantation and during 4 weeks of infusion. Infusion of amlodipine resulted in plasma concentrations of 32 ± 2 ng/ml and 27 ± 2 ng/ml for mice in saline + amlodipine and AngII + amlodipine groups, respectively. This infusion rate of amlodipine did not affect AngII-induced increases in systolic blood pressure. Three of 10 (30%) mice infused with AngII died of aortic rupture, while aortic rupture did not occur in mice co-infused with AngII + amlodipine. Suprarenal aortic width and intimal area of ascending aortas were measured to define aortic aneurysms. In the absence of AngII infusion, amlodipine did not change suprarenal aortic width and ascending aortic area. Infusion of AngII led to profound increases of suprarenal aortic width (saline + vehicle versus AngII + vehicle: 0.86 ± 0.02 versus 1.72 ± 0.26 mm; P=0.0006), whereas co-infusion of AngII and amlodipine diminished abdominal dilation (1.02 ± 0.14 mm; P=0.003). As expected, AngII infusion increased mean intimal area of ascending aortas (saline + vehicle versus AngII + vehicle: 8.5 ± 0.3 versus 12.5 ± 1.1 mm²; P=0.001), while co-infusion of AngII and amlodipine ablated dilation of the ascending aorta (8.6 ± 0.2 mm²; P=0.03). Co-administration of amlodipine also significantly attenuated AngII-induced atherosclerosis in the thoracic region as quantified by percent lesion area (AngII + vehicle versus AngII + amlodipine: 5.8 ± 2.1 % versus 0.3 ± 0.1%; P=0.05).

Conclusions

Amlodipine inhibited AngII-induced aortic aneurysms in both the abdominal and ascending regions, and atherosclerosis in hypercholesterolemic mice.

PD123319 augments angiotensin II-induced abdominal aortic aneurysms through an AT2 receptor-independent mechanism

Background

AT2 receptors have an unclear function on development of abdominal aortic aneurysms (AAAs), although a pharmacological approach using the AT2 receptor antagonist PD123319 has implicated a role. The purpose of the present study was to determine the role of AT2 receptors in AngII-induced AAAs using a combination of genetic and pharmacological approaches. We also defined effects of AT2 receptors in AngII-induced atherosclerosis and thoracic aortic aneurysms.

Methods and Results

Male AT2 receptor wild type (AT2 +/y) and deficient (AT2 -/y) mice in an LDL receptor −/− background were fed a saturated-fat enriched diet, and infused with either saline or AngII (500 ng/kg/min). AT2 receptor deficiency had no significant effect on systolic blood pressure during AngII-infusion. While AngII infusion induced AAAs, AT2 receptor deficiency did not significantly affect either maximal width of the suprarenal aorta or incidence of AAAs. The AT2 receptor antagonist PD123319 (3 mg/kg/day) and AngII were co-infused into male LDL receptor −/− mice that were either AT2 +/y or −/y. PD123319 had no significant effect on systolic blood pressure in either wild type or AT2 receptor deficient mice. Consistent with our previous findings, PD123319 increased AngII-induced AAAs. However, this effect of PD123319 occurred irrespective of AT2 receptor genotype. Neither AT2 receptor deficiency nor PD123319 had any significant effect on AngII-induced thoracic aortic aneurysms or atherosclerosis.

Conclusions

AT2 receptor deficiency does not affect AngII-induced AAAs, thoracic aortic aneurysms and atherosclerosis. PD123319 augments AngII-induced AAAs through an AT2 receptor-independent mechanism.

Depletion of endothelial or smooth muscle cell-specific angiotensin II type 1a receptors does not influence aortic aneurysms or atherosclerosis in LDL receptor deficient mice

BACKGROUND

Whole body genetic deletion of AT1a receptors in mice uniformly reduces hypercholesterolemia and angiotensin II-(AngII) induced atherosclerosis and abdominal aortic aneurysms (AAAs). However, the role of AT1a receptor stimulation of principal cell types resident in the arterial wall remains undefined. Therefore, the aim of this study was to determine whether deletion of AT1a receptors in either endothelial cells or smooth muscle cells influences the development of atherosclerosis and AAAs.

METHODOLOGY/PRINCIPAL FINDINGS

AT1a receptor floxed mice were developed in an LDL receptor -/- background. To generate endothelial or smooth muscle cell specific deficiency, AT1a receptor floxed mice were bred with mice expressing Cre under the control of either Tie2 or SM22, respectively. Groups of males and females were fed a saturated fat-enriched diet for 3 months to determine effects on atherosclerosis. Deletion of AT1a receptors in either endothelial or smooth muscle cells had no discernible effect on the size of atherosclerotic lesions. We also determined the effect of cell-specific AT1a receptor deficiency on atherosclerosis and AAAs using male mice fed a saturated fat-enriched diet and infused with AngII (1,000 ng/kg/min). Again, deletion of AT1a receptors in either endothelial or smooth muscle cells had no discernible effects on either AngII-induced atherosclerotic lesions or AAAs.

CONCLUSIONS

Although previous studies have demonstrated whole body AT1a receptor deficiency diminishes atherosclerosis and AAAs, depletion of AT1a receptors in either endothelial or smooth muscle cells did not affect either of these vascular pathologies.

Regional variation in aortic AT1b receptor mRNA abundance is associated with contractility but unrelated to atherosclerosis and aortic aneurysms

Background

Angiotensin II (AngII), the main bioactive peptide of the renin angiotensin system, exerts most of its biological actions through stimulation of AngII type 1 (AT1) receptors. This receptor is expressed as 2 structurally similar subtypes in rodents, termed AT1a and AT1b. Although AT1a receptors have been studied comprehensively, roles of AT1b receptors in the aorta have not been defined.

Methodology/Results

We initially compared the regional distribution of AT1b receptor mRNA with AT1a receptor mRNA in the aorta. mRNA abundance of both subtypes increased from the proximal to the distal aorta, with the greatest abundance in the infra-renal region. Corresponding to the high mRNA abundance for both receptors, only aortic rings from the infra-renal aorta contracted in response to AngII stimulation. Despite the presence of both receptor transcripts, deletion of AT1b receptors, but not AT1a receptors, diminished AngII-induced contractility. To determine whether absence of AT1b receptors influenced aortic pathologies, we bred AT1b receptor deficient mice into an LDL receptor deficient background. Mice were fed a diet enriched in saturated fat and infused with AngII (1,000 ng/kg/min). Parameters that could influence development of aortic pathologies, including systolic blood pressure and plasma cholesterol concentrations, were not impacted by AT1b receptor deficiency. Absence of AT1b receptors also had no effect on size of aortic atherosclerotic lesions and aortic aneurysms in both the ascending and abdominal regions.

Conclusions/Significance

Regional abundance of AT1b receptor mRNA coincided with AngII-induced regional contractility, but it was not associated with AngII-induced aortic pathologies.

Doxycycline does not influence established abdominal aortic aneurysms in angiotensin II-infused mice

Background

There is no proven medical approach to attenuating expansion and rupture of abdominal aortic aneurysms (AAAs). One approach that is currently being investigated is the use of doxycycline. Despite being primarily used as an antimicrobial drug, doxycycline has been proposed to function in reducing AAA expansion. Doxycycline is effective in reducing the formation in the most commonly used mouse models of AAAs when administered prior to the initiation of the disease. The purpose of the current study was to determine the effects of doxycycline on established AAAs when it was administered at a dose that produces therapeutic serum concentrations.

Methods and Results

LDL receptor −/− male mice fed a saturated-fat supplemented diet were infused with AngII (1,000 ng/kg/min) via mini-osmotic pumps for 28 days. Upon verification of AAA formation by noninvasive high frequency ultrasonography, mice were stratified based on aortic lumen diameters, and continuously infused with AngII while also administered either vehicle or doxycycline (100 mg/kg/day) in drinking water for 56 days. Administration of doxycycline led to serum drug concentrations of 2.3±0.6 µg/ml. Doxycycline administration had no effect on serum cholesterol concentrations and systolic blood pressures. Doxycycline administration did not prevent progressive aortic dilation as determined by temporal measurements of lumen dimensions using high frequency ultrasound. This lack of effect on AAA regression and progression was confirmed at the termination of the study by ex vivo measurements of maximal width of suprarenal aortas and AAA volumes. Also, doxycycline did not reduce AAA rupture. Medial and adventitial remodeling was not overtly changed by doxycycline as determined by immunostaining and histological staining.

Conclusions

Doxycycline administration did not influence AngII-induced AAA progression and aortic rupture when administered to mice with established AAAs.

Deficiency of receptor-associated protein attenuates angiotensin II-induced atherosclerosis in hypercholesterolemic mice without influencing abdominal aortic aneurysms

OBJECTIVE

Receptor-associated protein (RAP) was initially described as a regulator of low density lipoprotein receptor-related protein 1 (LRP1), but is now known to regulate many proteins. Since the direct effects of RAP on vascular pathologies have not been studied, this study determined whether RAP deficiency influenced angiotensin II (AngII)-induced atherosclerosis and abdominal aortic aneurysms (AAAs) in hypercholesterolemic mice.

METHODS AND RESULTS

Male LDL receptor -/- mice that were either RAP +/+ or -/- were infused with AngII (500 ng/kg/min) for 4 weeks while consuming a saturated fat-enriched diet. RAP deficiency had no effects on body weight or AngII-induced increases of systolic blood pressure. Despite increased plasma cholesterol concentrations, RAP deficiency reduced atherosclerotic lesion size in aortic arches, while having no effect on AngII-induced AAAs. RAP deficiency profoundly reduced LRP1 protein abundance in macrophages, but did not change its abundance in aortic smooth muscle cells. Also, RAP deficiency had no effects on mRNA abundance of LRP1 or lipoprotein lipase in macrophages. To determine whether RAP deficiency in leukocytes influenced AngII-induced atherosclerosis, irradiated male LDL receptor -/- mice were repopulated with bone marrow-derived cells from either RAP +/+ or -/- male mice. The chimeric mice were infused with AngII (500 ng/kg/min) for 4 weeks while fed the saturated fat-enriched diet. RAP deficiency in bone marrow-derived cells did not influence either plasma cholesterol concentrations or atherosclerotic lesion size.

CONCLUSIONS

Whole body RAP deficiency attenuated atherosclerosis without influencing AAAs in hypercholesterolemic mice infused with AngII. The anti-atherogenic effect was not attributable to RAP deficiency in bone marrow-derived cells.

Thiol oxidative stress induced by metabolic disorders amplifies macrophage chemotactic responses and accelerates atherogenesis and kidney injury in LDL receptor-deficient mice

BACKGROUND

Strengthening the macrophage glutathione redox buffer reduces macrophage content and decreases the severity of atherosclerotic lesions in LDL receptor-deficient (LDLR(-/-)) mice, but the underlying mechanisms were not clear. This study examined the effect of metabolic stress on the thiol redox state, chemotactic activity in vivo, and the recruitment of macrophages into atherosclerotic lesions and kidneys of LDL-R(-/-) mice in response to mild, moderate, and severe metabolic stress.

METHODS AND RESULTS

Reduced glutathione (GSH) and glutathione disulfide (GSSG) levels in peritoneal macrophages isolated from mildly, moderately, and severe metabolically-stressed LDL-R(-/-) mice were measured by HPLC, and the glutathione reduction potential (E(h)) was calculated. Macrophage E(h) correlated with the macrophage content in both atherosclerotic (r(2)=0.346, P=0.004) and renal lesions (r(2)=0.480, P=0.001) in these mice as well as the extent of both atherosclerosis (r(2)=0.414, P=0.001) and kidney injury (r(2)=0.480, P=0.001). Compared to LDL-R(-/-) mice exposed to mild metabolic stress, macrophage recruitment into MCP-1-loaded Matrigel plugs injected into LDL-R(-/-) mice increased 2.6-fold in moderately metabolically-stressed mice and 9.8-fold in severely metabolically-stressed mice. The macrophage E(h) was a strong predictor of macrophage chemotaxis (r(2)=0.554, P<0.001).

CONCLUSIONS

Thiol oxidative stress enhances macrophage recruitment into vascular and renal lesions by increasing the responsiveness of macrophages to chemoattractants. This novel mechanism contributes at least in part to accelerated atherosclerosis and kidney injury associated with dyslipidemia and diabetes in mice.

Influence of low-density lipoprotein (LDL) receptor on lipid composition, inflammation and parasitism during Toxoplasma gondii infection

Intracellular replication of Toxoplasma gondii requires cholesterol uptake by host cell low-density lipoprotein receptor (LDLr), a critical element in atherosclerosis. We evaluated host parasitism, inflammatory responses and development of atherosclerosis in LDLr knockout (LDLr(-/-)) and their controls C57BL/6 mice infected with T. gondii. Our results show that T. gondii cysts were reduced in LDLr(-/-) mice when compared to C57BL/6 mice. However, in presence of hypercholesterolemic diet, parasite growth in LDLr(-/-) mice was similar to that seen in infected C57BL/6 mice. In presence of a hypercholesterolemic diet, T. gondii infection leads to a 60% reduction of serum triacylglycerol, total and atherogenic lipoprotein cholesterol. When aortic valve lesion was analyzed, infected mice showed a reduction of atherosclerotic lesion area as well as CD36 expression. MCP-1, SRA-I, SRA-II, ICAM-1 and VCAM-1 mRNA expression was kept similar between infected and control groups. Thus, despite the intense inflammatory process, the drastic reduction in serum lipids seems to limit the development of atherosclerosis in LDLr(-/-) mice infected with T. gondii. In conclusion, our results indicate that T. gondii employs host LDLr to acquire cholesterol and favor its growth. However, in the presence of hypercholesterolemia, T. gondii parasites are able to acquire cholesterol-rich lipoproteins through an alternative host receptor, and overcome LDLr deficiency, favoring host parasitism and impairing lipid loading of foam cells.

Infection with Toxoplasma gondii increases atherosclerotic lesion in ApoE-deficient mice

Toxoplasma gondii is an intracellular protozoan that elicits a potent inflammatory response during the acute phase of infection. Herein, we evaluate whether T. gondii infection alters the natural course of aortic lesions. ApoE knockout mice were infected with T. gondii, and at 5 weeks of infection, serum, feces, and liver cholesterol; aortic lesion size, cellularity, and inflammatory cytokines; and levels of serum nitrite and gamma interferon (IFN-gamma) were analyzed. Our results showed that serum cholesterol and atherogenic lipoproteins were reduced after T. gondii infection. The reduction of serum levels of total cholesterol and atherogenic lipoproteins was associated with increases in the aortic lesion area, numbers of inflammatory cells, and expression of monocyte chemoattractant protein 1 and inducible nitric oxide synthase mRNA in the site of lesions as well as elevated concentrations of IFN-gamma and nitrite in sera of T. gondii-infected animals. These results suggest that infection with T. gondii accelerates atherosclerotic development by stimulating the proinflammatory response and oxidative stress, thereby increasing the area of aortic lesion.

The cytoplasmic domain of the low density lipoprotein (LDL) receptor-related protein, but not that of the LDL receptor, triggers phagocytosis

The macrophage LDL receptor and LDL receptor-related protein (LRP, CD91) mediate the phagocytic-like uptake of atherogenic lipoproteins and apoptotic cells, yet the structural basis of their phagocytic functions is not known. To address this issue, we transfected macrophages with chimeric proteins containing the cytoplasmic tails and transmembrane regions of the LDL receptor or LRP and the ectodomain of CD2, which can bind non-opsonized sheep red blood cells (SRBCs). Macrophages expressing receptors containing the LDL receptor domains were able to bind but not internalize SRBCs. In contrast, macrophages expressing receptors containing the cytoplasmic tail of LRP were able to bind and internalize SRBCs. Chimeras in which the LRP cytoplasmic tail was mutated in two di-leucine motifs and a tyrosine in an NPXYXXL motif were able to endocytose anti-CD2 antibody and bind SRBCs, but SRBC phagocytosis was decreased by 70%. Thus, the phagocytic-like functions of LRP, but not those of the LDL receptor, can be explained by the ability of the LRP cytoplasmic tail to trigger phagocytosis. These findings have important implications for atherogenesis and apoptotic cell clearance and for a fundamental cell biological understanding of how the LDL receptor and LRP function in internalization processes.

Exogenous interferon-gamma enhances atherosclerosis in apolipoprotein E-/- mice

A role for interferon-gamma (IFN-gamma) has been implied in the atherogenic process. To determine whether exogenously administered IFN-gamma exerts an effect on the development of atherosclerosis, we intraperitoneally administered either recombinant IFN-gamma (100 U/g body weight) or phosphate buffered saline daily for 30 days to atherosclerosis-susceptible apolipoprotein E-/- mice (16-week-old male mice, n = 11 per group) fed a normal diet. Atherosclerotic lesion size was quantified in the ascending aorta. The number of T lymphocytes and major histocompatibility complex (MHC) class II-positive cells within lesions were also quantified in this region. IFN-gamma administration reduced serum cholesterol concentrations by 15% (P = 0.02). For both groups, the majority of cholesterol was present in very low density lipoproteins, which were modestly reduced in mice receiving IFN-gamma. Despite the decrease in serum cholesterol concentrations, IFN-gamma injections significantly increased lesion size twofold compared to controls (119,980 +/- 18, 536 vs. 59,396 +/- 20,017 micrometer(2); P = 0.038). IFN-gamma also significantly increased the mean number of T lymphocytes (19 +/- 4 vs. 7 +/- 1 cells; P = 0.03) and MHC class II-positive cells (10 +/- 3 vs. 3 +/- 1 cells; P = 0.04) within lesions. These data lend further support to a pro-atherogenic role of IFN-gamma.

Effect of dietary cholesterol on low density lipoprotein-receptor, 3-hydroxy-3-methylglutaryl-CoA reductase, and low density lipoprotein receptor-related protein mRNA expression in healthy humans

We investigated the possibility that dietary cholesterol downregulates the expression of low density lipoprotein (LDL) receptor and 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase genes of circulating mononuclear cells in vivo in healthy humans. We also studied the variations of the LDL receptor-related protein (LRP) gene in the same conditions. Dieters (n = 5) were submitted to a 4-d fat restriction (mean cholesterol intake: 6+/-4 mg/d), followed by a 7-d cholesterol (a mean of 791+/-150 mg/d) supplementation. Controls (n = 3) did not change their diet. During fat restriction, serum total and LDL cholesterol decreased significantly (P < 0.05), and LDL receptor and HMG-CoA reductase mRNA copy numbers in mononuclear cells increased by 57 and 147%, respectively (P < 0.05). After reintroducing cholesterol, serum cholesterol was stable whereas LDL receptor and HMG-CoA reductase mRNA decreased by 46 and 72% (P < 0.05) and LRP mRNA increased by 59% (P < 0.005). The changes in LDL receptor and HMG-CoA reductase mRNA abundance were correlated (r = +0.79, P = 0.02) during cholesterol reintroduction as were LDL receptor and LRP mRNA levels, but negatively (r = -0.70, P = 0.05). Also, 70% of the variability in LRP mRNA (P < 0.005) was explained by dietary cholesterol. Thus, the basic mechanisms regulating cellular cholesterol content, the coordinate feedback repression of genes governing the synthesis and uptake of cholesterol, are operating in vivo in humans. However, serum cholesterol did not increase in response to dietary cholesterol, suggesting that these mechanisms may not play as predominant a role as previously believed in the short-term control of serum cholesterol in vivo in humans. A new finding is that LRP gene is also sensitive to dietary cholesterol, suggesting that it may participate in the control of serum cholesterol. Further in vivo studies in humans are warranted to explore the molecular mechanisms of the physiological response to dietary cholesterol in humans.

The atherogenic significance of an elevated plasma triglyceride level

The importance of hypertriglyceridemia as an independent predictor of coronary artery disease (CAD) remains unsettled. Hypertriglyceridemia, with or without associated hypercholesterolemia, occurs more frequently in premature CAD subjects than does hypercholesterolemia alone. With univariate analysis, most studies show a positive correlation between plasma triglyceride (TG) level and risk for CAD, but with multivariate analysis plasma TG level is no longer an independent risk factor except in women and diabetics. Prospective studies have shown that subjects with a high LDL/HDL cholesterol ratio and a high plasma TG level have the highest risk for CAD. Hypertriglyceridemia signifies the presence of excess triglyceride-rich lipoproteins (TRL), including chylomicrons, VLDL, and their remnants. The question then becomes one of whether TRL are directly or indirectly involved in atherogenesis. TRL were thought to be too big to infiltrate the arterial wall, and histopathological studies have shown cholesterol but not triglyceride accumulation in the atherosclerotic plaque. However, there was a recent demonstration of undegraded VLDL and IDL in atherosclerotic plaques. Larger TRL may undergo hydrolysis on the arterial surface to become smaller particles before entry into the intima. Possible cellular pathways for the uptake of TRL by macrophages have been described. The smaller TRL (Sf 20-60), including postprandial chylomicron remnants, are believed to be the most atherogenic of all TRL particles. Because large amounts of TRL are produced in the postprandial period, atherogenesis involving TRL may be primarily a postprandial phenomenon. Once in the intima, TG may undergo hydrolysis, releasing free fatty acids and mono- and diacyl glycerol, accounting for the dearth of TG in atherosclerotic lesions. Particle for particle, VLDL delivers five times as much cholesterol as LDL does to the macrophage.

Absence of T Lymphocyte-Derived Cytokines Fails to Diminish Macrophage 12/15-Lipoxygenase Expression In Vivo

IL-4 and IL-13 are the only known activators of 15-lipoxygenase (LO) expression in cultured macrophages. To determine whether these lymphocyte-derived cytokines regulate 15-LO expression in vivo, the abundance of the murine homologue (12/15-LO) was assessed in peritoneal macrophages from immune-deficient strains of mice. Macrophages were harvested from recombinase activator gene (RAG)-2−/− mice that do not develop mature lymphocytes and cannot secrete activation-dependent cytokines. Unexpectedly, 12/15-LO protein and activity were significantly increased in peritoneal macrophages from RAG-2−/− mice compared with strain-matched controls. This increase was related to phenotypic differences between cells from RAG-2+/+ and RAG-2−/− mice. After 3 h in culture, RAG-2+/+ macrophages were of two distinct sizes, with only the larger cells immunostaining for 12/15-LO. However, all RAG-2−/− cells were distributed in the large size range, and all were immunoreactive for the enzyme. The activation of 12/15-LO expression appears to be related to prolonged residence within the peritoneum, since there were fewer resident peritoneal macrophages in RAG-2−/− than in RAG-2+/+ mice, and newly recruited macrophages elicited by the administration of Sephacryl to RAG-2−/− mice did not immunostain for 12/15-LO. To determine whether 12/15-LO expression was due to IL-4 or IL-13 from nonlymphoid cells, the abundance of the enzyme was quantified in peritoneal macrophages from STAT6−/− mice that have attenuated responses to both cytokines. STAT6 deficiency did not influence the abundance of the protein in macrophages. Therefore, neither IL-4 nor IL-13 secretion is a requirement for macrophage 15-LO expression in vivo.

Attenuation of diet-induced atherosclerosis in rabbits with a highly selective 15-lipoxygenase inhibitor lacking significant antioxidant properties

1. 15-Lipoxygenase (15-LO) has been implicated in the pathogenesis of atherosclerosis because of its localization in lesions and the many biological activities exhibited by its products. To provide further evidence for a role of 15-LO, the effects of PD 146176 on the development of atherosclerosis in cholesterol-fed rabbits were assessed. This novel drug is a specific inhibitor of the enzyme in vitro and lacks significant non specific antioxidant properties. 2. PD 146176 inhibited rabbit reticulocyte 15-LO through a mixed noncompetitive mode with a Ki of 197 nM. The drug had minimal effects on either copper or 2,2'-azobis(2-amidinopropane)hydrochloride (ABAP) induced oxidation of LDL except at concentrations 2 orders higher than the Ki. 3. Control New Zealand rabbits were fed a high-fat diet containing 0.25% wt./wt. cholesterol; treated animals received inhibitor in this diet (175 mg kg-1, b.i.d.). Plasma concentrations of inhibitor were similar to the estimated Ki (197 nM). During the 12 week study, there were no significant differences in weight gain haematocrit, plasma total cholesterol concentrations, or distribution of lipoprotein cholesterol. 4. The drug plasma concentrations achieved in vivo did not inhibit low-density lipoprotein (LDL) oxidation in vitro. Furthermore, LDL isolated from PD 146176-treated animals was as susceptible as that from controls to oxidation ex vivo by either copper or ABAP. 5. PD 146176 was very effective in suppressing atherogenesis, especially in the aortic arch where lesion coverage diminished from 15 +/- 4 to 0% (P < 0.02); esterified cholesterol content was reduced from 2.1 +/- 0.7 to 0 micrograms mg-1 (P < 0.02) in this region. Immunostainable lipid-laden macrophages present in aortic intima of control animals were totally absent in the drug-treated group. 6. Results of these studies are consistent with a role for 15-LO in atherogenesis.

Beta-carotene inhibits atherosclerosis in hypercholesterolemic rabbits

Oxidatively damaged LDL may be of central importance in atherogenesis. Epidemiological evidence suggests that high dietary intakes of beta-carotene and vitamin E decreases the risk for atherosclerotic vascular disease, raising the possibility that lipid-soluble antioxidants slow vascular disease by protecting LDL from oxidation. To test this hypothesis, we fed male New Zealand White rabbits a high-cholesterol diet or the same diet supplemented with either 1% probucol, 0.01% vitamin E, 0.01% all-trans beta-carotene, or 0.01% 9-cis beta-carotene; then we assessed both the susceptibility of LDL to oxidation ex vivo and the extent of aortic atherosclerosis. As in earlier studies, probucol protected LDL from oxidation and inhibited lesion formation. In contrast, vitamin E modestly inhibited LDL oxidation but did not prevent atherosclerosis. While beta-carotene had no effect on LDL oxidation ex vivo, the all-trans isomer inhibited lesion formation to the same degree as probucol. Moreover, all-trans beta-carotene was undetectable in LDL isolated from rabbits fed the compound, although tissue levels of retinyl palmitate were increased. The effect of all-trans beta-carotene on atherogenesis can thus be separated from the resistance of LDL to oxidation, indicating that other mechanisms may account for the ability of this compound to prevent vascular disease. Our results suggest that metabolites derived from all-trans beta-carotene inhibit atherosclerosis in hypercholesterolemic rabbits, possibly via stereospecific interactions with retinoic acid receptors in the artery wall.