Hypercholesterolemia in low density lipoprotein receptor knockout mice and its reversal by adenovirus-mediated gene delivery

Overexpression of 15-lipoxygenase in the vascular endothelium is associated with increased thymic apoptosis in LDL receptor-deficient mice

BACKGROUND

15-Lipoxygenase (15-LO) is a nonheme iron-containing enzyme that catalyzes the peroxidation of fatty acids. Herein, we studied the effect of 15-LO overexpression in the vascular endothelium on thymocyte apoptosis by evaluating thymuses from low-density lipoprotein receptor-deficient (LDL-RD) mice and LDL-RD/15-LO mice. Thymuses were evaluated by immunohistochemistry and by TUNEL whereas in vitro studies were carried out by employing freshly isolated thymocytes from the respective mice and evaluation of apoptosis by propidium iodide and annexin V cytometry.

METHODS AND RESULTS

The apoptotic index in LDL-RD/15-LO mice was significantly higher than in the LDL-RD mice. In the thymic medulla the difference was smaller, although still significant. Freshly isolated thymus cells from LDL-RD/15-LO mice exhibited a higher rate of spontaneous cell death than controls. Incubation of thymus cells in the presence of the cell-permeable caspase-3 inhibitor DEVD-CMK resulted in a decrease in the frequency of apoptotic cells in LDL-RD/15-LO thymocytes, whereas no effect was evident in control thymocytes. The antioxidant N-acetylcysteine causes the increase in apoptosis in both groups.

CONCLUSION

LDL-RD/15-LO mice exhibit increased thymocyte apoptosis both in vivo and in vitro. These findings may suggest a role for 15-LO in the natural selection of thymocytes.

Physical activity and atherosclerosis: which animal model?

Atherosclerosis is a progressive disease that is the most important single contributor to human cardiovascular morbidity and mortality. Epidemiologic studies show that physical activity, or routine exercise, reduces the risk of developing cardiovascular disease. The mechanisms through which exercise may function in primary or secondary prevention of atherosclerosis remain largely to be established. Most studies in humans are performed after the onset of clinical signs when disease is well advanced and the prescription of exercise is based on empirical evidence of benefit in secondary prevention. Animal models per-mit the study of the initiation and progression of preclinical stages of atherosclerosis. In order to provide information relevant to treatment and prevention, these models should mimic human disease and interactions of physical activity with disease processes as closely as possible. The purpose of this review is to compare animal models of atherosclerosis and to summarize the available data in those models in regard to the effects of exercise.

Macrophage-specific overexpression of group IIa sPLA2 increases atherosclerosis and enhances collagen deposition

Atherosclerosis is a chronic inflammatory disease of the vessel wall characterized by the accumulation of lipid-laden macrophages and fibrotic material. The initiation of the disease is accompanied by the accumulation of modified lipoproteins in the vessel wall. Group IIa secretory phospholipase A2 (sPLA2 IIa) is a key candidate player in the enzymatic modification of low density lipoproteins. To study the role of sPLA2 IIa in macrophages during atherogenesis, transgenic mice were generated using the human sPLA2 IIa gene and the CD11b promoter. Bone marrow transplantation to LDL receptor-deficient mice was performed to study sPLA2 IIa in atherosclerosis. After 10 weeks of high-fat diet, mice overexpressing sPLA2 IIa in macrophages showed 2.3-fold larger lesions compared with control mice. Pathological examination revealed that sPLA2 IIa-expressing mice had increased collagen in their lesions, independent of lesion size. However, smooth muscle cells or fibroblasts in the lesions were not affected. Other parameters studied, including T-cells and cell turnover, were not significantly affected by overexpression of sPLA2 IIa in macrophages. These data clearly show that macrophage sPLA2 IIa is a proatherogenic factor and suggest that the enzyme regulates collagen production in the plaque and thus fibrotic cap development.

T-bet deficiency reduces atherosclerosis and alters plaque antigen-specific immune responses

The influence of the immune system on atherosclerosis involves both helper T (Th) cell and antibody responses to plaque antigens. These responses may have proatherogenic and protective effects. T-bet is a transcription factor required for Th1 differentiation and regulates the balance between Th1 and Th2 responses in inflammatory diseases. To clarify how helper T cell subset differentiation influences atherosclerosis, we compared lesion development and immune responses to plaque antigens in low-density lipoprotein receptor-deficient (Ldlr-/-) mice with or without functional T-bet genes. Atherosclerosis was significantly reduced in T-bet-deficient Ldlr-/- mice compared with Ldlr-/- controls, and the lesions that did develop in the absence of T-bet had less smooth muscle cell content. Furthermore, T-bet deficiency caused a Th2 switch in the response to the atherosclerosis-associated antigen heat shock protein-60, and a change in T-dependent isotypes of oxidized LDL-specific antibodies. Of particular significance, T-bet deficiency caused a >250% increase in the titer of E06 antibodies, which are known to be atheroprotective and whose production by B-1 B cells is enhanced by IL-5. These findings establish that T cell subset differentiation influences both T cell and antibody responses that modulate atherosclerosis, and validate the therapeutic goal of skewing T responses to atherosclerosis-associated antigens.

Delineation of molecular changes in intrahepatic cholesterol metabolism resulting from diminished cholesterol absorption

The absorption of cholesterol by the small intestine is a major route for the net entry of cholesterol into the body and can therefore affect the plasma low density lipoprotein-cholesterol (LDL-C) concentration. These studies used ezetimibe, a potent inhibitor of cholesterol absorption, to delineate the biochemical and molecular changes in intrahepatic metabolism and biliary lipid secretion when there is a major reduction in chylomicron cholesterol delivery to the liver. In female LDL receptor (LDLR)-deficient (LDLR-/-) mice fed a basal diet containing ezetimibe (0-10 mg/day/kg body weight), cholesterol absorption was reduced up to 91%, fecal neutral sterol excretion was increased up to 4.7-fold, and plasma total cholesterol concentrations decreased by up to 18%. Blocking cholesterol absorption prevented the accumulation of very low density lipoproteins and LDL in the circulation of LDLR-/- mice fed a lipid-rich diet. In female LDLR+/+ mice fed the lipid-rich diet with ezetimibe, the relative mRNA level for the LDLR in the liver was 2-fold greater than in matching mice given the lipid-rich diet alone. We conclude that in the mouse the reduction in plasma LDL-C levels induced by blocking cholesterol absorption reflects both a diminished rate of LDL-C production and a modest increase in hepatic LDLR expression.

Complete deficiency of the low-density lipoprotein receptor is associated with increased apolipoprotein B-100 production

OBJECTIVE

We addressed the role of the low-density lipoprotein (LDL) receptor in determining clearance rates and production rate (PR) of apolipoprotein B (apoB) in humans.

METHODS AND RESULTS

Kinetic studies using endogenous labeling of apoB with deuterated leucine were performed in 7 genetically defined patients with homozygous familial hypercholesterolemia (FH) and compared with 4 controls. The fractional catabolic rates (FCR) and PRs for apoB were determined by multicompartmental modeling. The FCRs of very-low-density lipoprotein 1 (VLDL1), VLDL2, intermediate-density lipoprotein (IDL), and LDL apoB were lower in FH than in controls, with the LDL apoB FCR being significantly lower (0.148+/-0.049 versus 0.499+/-0.099 pools x d(-1); P=0.008). Whereas receptor-defective FH patients had a total apoB PR similar to controls, receptor-null FH patients had a significantly greater total apoB PR than controls (35.97+/-10.51 versus 21.32+/-4.21 mg x kg(-1) x d(-1), respectively; P=0.02).

CONCLUSIONS

This first study of apoB metabolism in homozygous FH using endogenous labeling with stable isotopes demonstrates that the LDL receptor contributes significantly to the clearance of LDL from plasma but plays a lesser role in the clearance of larger apoB-containing lipoproteins. Furthermore, these data also indicate that absence of a LDL receptor in humans substantially influences the apoB PR in vivo.

Varying the ratio of dietary n−6/n−3 polyunsaturated fatty acid alters the tendency to thrombosis and progress of atherosclerosis in apoE−/− LDLR−/− double knockout mouse

We have investigated the influence of dietary n-6/n-3 (ù-6/ù-3) polyunsaturated fatty acid-balance on the tendency to arterial thrombosis and the progress of atherosclerosis in apoE-/- LDLR-/- double knockout mouse. Homozygous apoE-/- LDLR-/- double knockout mouse (DKO mice, 129XC57BL/6J background) and male C57BL/6 mice aged 6 weeks were divided into four groups. Each group was fed a diet containing a different n-6/n-3 ratio (Group l: 0.29; Group 2: 1.43; Group 3: 5.00; Group 4: 8), prepared with high linolenic (LNA) flaxseed oil (n-3 rich) and high linoleic (LA) safflower oil (n-6 rich). There were no statistical differences in the gain in body weight between the four groups. After 16 weeks, plasma triglyceride and LDL levels in Group 1 were significantly lower than in the other groups. Conversely, HDL was the highest. After 8 and 16 weeks, the tendency to arterial thrombosis was assessed using a He-Ne laser-induced thrombosis model. The degree of atherosclerosis was measured using the entire aorta method employing image analysis software. The n-6/n-3 ratio had a dose-dependent antithrombotic effect (thrombus volume decreased 23%, Group 1 vs. Group 4), In addition, the extent of atherosclerosis was less in the animals fed a low n-6/n-3 ratio compared with the high n-6/n-3 ratio group (atherosclerotic area decreased 40%, Group 1 vs. Group 4). The lowest n-6/n-3 ratio tested (0.29) was the most effective in suppressing the thrombotic and atherosclerotic parameters in these DKO mice.

Inhibition and regression of atherosclerotic lesions

Atherosclerosis, once believed to be a result of a slow, irreversible process resulting from lipid accumulation in arterial walls, is now recognized as a dynamic process with reversibility. Liver-directed gene therapy for dyslipidemia aims to treat patients who are not responsive to currently available primary and secondary prevention. Moreover, gene therapy strategies have also proved valuable in studying the dynamics of atherosclerotic lesion formation, progression, and remodeling in experimental animals. Recent results on the long-term effect of gene therapy suggest that hepatic expression of therapeutic genes suppresses inflammation and has profound effects on the nature of the atherogenic process.

Understanding hyperlipidemia and atherosclerosis: lessons from genetically modified apoe and ldlr mice

Hyperlipidemia is the most important risk factor for atherosclerosis, which is the major cause of cardiovascular disease. The etiology of hyperlipidemia and atherosclerosis is complex and governed by multiple interacting genes. However, mutations in two genes have been shown to be directly involved, i.e., the low-density lipoprotein receptor (LDLR) and apolipoprotein E (ApoE). Genetically modified mouse models have been instrumental in elucidating the underlying molecular mechanisms in lipid metabolism. In this review, we focus on the use of two of the most widely used mouse models, ApoE- and LDLR-deficient mice. After almost a decade of applications, it is clear that each model has unique strengths and drawbacks when carrying out studies of the role of additional genes and environmental factors such as nutrition and lipid-lowering drugs. Importantly, we elaborate on mice expressing mutant forms of APOE, including the

Low-Density Lipoprotein From Apolipoprotein E-Deficient Mice Induces Macrophage Lipid Accumulation in a CD36 and Scavenger Receptor Class A-Dependent Manner

Objective— To investigate the potential of circulating low-density lipoprotein (LDL), isolated from apolipoprotein E (apoE)-deficient mice (E−/−LDL) and from LDL receptor-deficient mice (Lr−/−LDL), to induce foam cell formation.

Methods and Results— Binding studies using COS-7 cells overexpressing CD36, J774 cells, and mouse peritoneal macrophages (MPMs) unexpectedly showed for the first time that E−/−LDL, which is enriched in cholesterol, is a high-affinity ligand for CD36 and exhibited greater macrophage uptake than Lr−/−LDL or normal LDL. Minimal copper-mediated oxidization of Lr−/−LDL or C57LDL in vitro resulted in increased ligand internalization, although cell uptake of these oxidized LDLs was lower than that of E−/−LDL, even at oxidation levels similar to that found in E−/−LDL. Treatment of MPMs with E−/−LDL and Lr−/−LDL (to a 2- to 3-fold lesser extent), but not normal LDL, resulted in significant cellular cholesteryl ester accumulation and foam cell formation. Experiments using MPMs lacking CD36, scavenger receptor class A (SR-A), or both, indicated a major contribution of CD36 (≈50%), and to a lesser extent, SR-A (24% to 30%), to E−/−LDL uptake.

Conclusions— Because of its increased state of oxidation and high cholesterol content, LDL in apoE-deficient mice acts in a proatherogenic manner, without requiring further modification in the vascular wall, to induce foam cell formation through its uptake by scavenger receptors.

Roles of thromboxane A(2) and prostacyclin in the development of atherosclerosis in apoE-deficient mice

Production of thromboxane (TX) A2 and PG I2/prostacyclin (PGI2) is increased in patients with atherosclerosis. However, their roles in atherogenesis have not been critically defined. To examine this issue, we cross-bred atherosclerosis-prone apoE-deficient mice with mice deficient in either the TXA receptor (TP) or the PGI receptor (IP). Although they showed levels of serum cholesterol and triglyceride similar to those of apoE-deficient mice, apoE-/-TP-/- mice exhibited a significant delay in atherogenesis, and apoE-/-IP-/- mice exhibited a significant acceleration in atherogenesis compared with mice deficient in apoE alone. The plaques in apoE-/-IP-/- mice showed partial endothelial disruption and exhibited enhanced expression of ICAM-1 and decreased expression of platelet endothelial cell adhesion molecule 1 (PECAM-1) in the overlying endothelial cells compared with those of apoE-/-TP-/- mice. Platelet activation with thrombin ex vivo revealed higher and lower sensitivity for surface P-selectin expression in platelets of apoE-/-IP-/- and apoE-/-TP-/- mice, respectively, than in those of apoE-/- mice. Intravital microscopy of the common carotid artery revealed a significantly greater number of leukocytes rolling on the vessel walls in apoE-/-IP-/- mice than in either apoE-/-TP-/- or apoE-/- mice. We conclude that TXA2 promotes and PGI2 prevents the initiation and progression of atherogenesis through control of platelet activation and leukocyte-endothelial cell interaction.

Technical Assessment of the First 20 Years of Research Using Mouse Embryonic Stem Cell Lines

This review assesses the effect that mouse embryonic stem (ES) cells have had on biomedical research during the 20 years that followed their isolation in 1981. Notable scientific discoveries enabled by these cell lines--including insights into cell cycle regulation, spatial and temporal relationships during development, and the roles of transcription factors and homeobox genes in developmental pathways--are discussed. The acceleration of basic discovery of gene function and the genetic basis of disease using a breakthrough technology (homologous recombination between modified gene constructs and the ES cell genome) became the principal enabling method to establish transgenic laboratory animals with single targeted genetic change. This review also examines the widespread influence of mouse ES cells as an enabling technology by highlighting their effect on drug development paradigms, directed differentiation to treat specific diseases, nuclear transfer protocols used in cloning, and establishment of methodologies for isolating non-rodent ES cells. This review concludes with a brief analysis of the most influential mouse ES cell lines of the first 20 years as viewed within the twin contexts of human disease application and contributions to the primary literature.

Oxidative stress in atherosclerosis‐prone mouse is due to low antioxidant capacity of mitochondria

Atherosclerotic disease remains a leading cause of death in westernized societies, and reactive oxygen species (ROS) play a pivotal role in atherogenesis. Mitochondria are the main intracellular sites of ROS generation and are also targets for oxidative damage. Here, we show that mitochondria from atherosclerosis-prone, hypercholesterolemic low-density lipoprotein (LDL) receptor knockout mice have oxidative phosphorylation efficiency similar to that from control mice but have a higher net production of ROS and susceptibility to develop membrane permeability transition. Increased ROS production was observed in mitochondria isolated from several tissues, including liver, heart, and brain, and in intact mononuclear cells from spleen. In contrast to control mitochondria, knockout mouse mitochondria did not sustain a reduced state of matrix NADPH, the main source of antioxidant defense against ROS. Experiments in vivo showed faster liver secretion rates and de novo synthesis of triglycerides and cholesterol in knockout than in control mice, suggesting that increased lipogenesis depleted the reducing equivalents from NADPH and generated a state of oxidative stress in hypercholesterolemic knockout mice. These data provide the first evidence of how oxidative stress is generated in LDL receptor defective cells and could explain the increased LDL oxidation, cell death, and atherogenesis seen in familiar hypercholesterolemia.

Differential effects of exercise on aortic mitochondria

Routine exercise is widely recognized as cardioprotective. Exercise induces a variety of effects within the cardiovasculature, including decreased mitochondrial damage and improved aerobic capacity. It has been generally thought that the transient increase in oxidative stress associated with exercise initiates cardioprotective processes. Somewhat paradoxically, increased oxidative stress associated with cardiovascular disease (CVD) risk factors is thought to play an important role in the promotion and development of CVD. Hence, it is possible that CVD risk factors that increase oxidative stress (e.g., hypercholesterolemia) may modulate the cardioprotective effects of exercise. In this regard, the interaction between CVD risk factors and exercise on atherosclerotic lesion development and basal oxidant load is less defined. To determine the influence of preexistent hypercholesterolemia on cardioprotective effects of exercise, atherosclerotic lesion formation, oxidant load, mitochondrial damage, protein nitration (3-nitrotyrosine levels), and mitochondrial enzyme activities were determined in aortic tissues from normocholesterolemic (C57 control) and hypercholesterolemic [apoliprotein E-deficient (apoE(-/-))] mice after 16 wk of regular exercise. In normocholesterolemic mice, regular exercise was associated with decreased mitochondrial damage and oxidant load and increased SOD2 and adenine nucleotide translocator activities. Exercise did not decrease endogenous oxidant load and mitochondrial damage in hypercholesterolemic mice and did not reduce atherosclerotic lesion development. These data are consistent with the notion that CVD risk factors associated with increased oxidative stress can alter the benefits of exercise and that mitochondrial damage appears to be correlated with the cardiovascular effects of exercise.

Distribution of the lipolysis stimulated receptor in adult and embryonic murine tissues and lethality of LSR-/- embryos at 12.5 to 14.5 days of gestation

The lipolysis stimulated receptor (LSR) recognizes apolipoprotein B/E-containing lipoproteins in the presence of free fatty acids, and is thought to be involved in the clearance of triglyceride-rich lipoproteins (TRL). The distribution of LSR in mice was studied by Northern blots, quantitative PCR and immunofluorescence. In the adult, LSR mRNA was detectable in all tissues tested except muscle and heart, and was abundant in liver, lung, intestine, kidney, ovaries and testes. During embryogenesis, LSR mRNA was detectable at 7.5 days post-coitum (E7) and increased up to E17 in parallel to prothrombin, a liver marker. In adult liver, immunofluorescence experiments showed a staining at the periphery of hepatocytes as well as in fetal liver at E12 and E15. These results are in agreement with the assumption that LSR is a plasma membrane receptor involved in the clearance of lipoproteins by liver, and suggest a possible role in steroidogenic organs, lung, intestine and kidney). To explore the role of LSR in vivo, the LSR gene was inactivated in 129/Ola ES cells by removing a gene segment containing exons 2-5, and 129/Ola-C57BL/6 mice bearing the deletion were produced. Although heterozygotes appeared normal, LSR homozygotes were not viable, with the exception of three males, while the total progeny of genotyped wild-type and heterozygote pups was 345. Mortality of the homozygote embryos was observed between days 12.5 and 15.5 of gestation, a time at which their liver was much smaller than that of their littermates, indicating that the expression of LSR is critical for liver and embryonic development.

MR and fluorescent imaging of low-density lipoprotein receptors

RATIONALE AND OBJECTIVES

Over-expression of low-density lipoprotein receptors (LDLRs) occurs in many types of malignancies and is related to the requirement for lipids for rapid proliferation of the tumors. On the other hand, LDLRs that are unable to bind LDL are found on hepatocytes of patients with familial hypercholesterolemia (FH), a genetic disease that leads to premature atherosclerosis and death. The highly selective binding of LDL to LDLR makes these particles ideal carriers of therapeutic and diagnostic contrast agents into the targeted cells. The objectives of this paper are to examine whether a prototype contrast agent (PTIR267) with dual detection properties is suitable for labeling of LDL particles for in vivo detection of LDLR by magnetic resonance imaging (MRI) and for in vitro monitoring of cellular localization by confocal fluorescence microscopy.

MATERIALS AND METHODS

PTIR267 is a lipophilic GdDTPA derivative conjugated to a fluorescent dye. The conjugated dye molecule makes the probe sufficiently water soluble to allow labeling of LDL by a brief incubation of LDL with PTIR267 dissolved in PBS at 37 degrees C (mole ratio LDL: PTIR267 = 0.09:1). The molar relaxivity of PTIR267 in saline is 26 mM(-1)s(-1). Specific LDLR-mediated uptake of PTIR267-labeled LDL was demonstrated in vitro by confocal fluorescence imaging of B16 melanoma cells using confocal fluorescence imaging. In vivo uptake of PTIR267-labeled LDL by a subcutaneously implanted B16 melanoma in mice leads to 30% decrease in longitudinal relaxation time (T(1)) in the tumor. In vivo uptake of PTIR267-labeled LDL leads to 70% decrease in T(1) in a normal C57BL/6 mouse liver; however, in the liver of LDL receptor gene knockout (LDLr-/-) mice with C57BL/6 background, only 12% decrease in T(1) is observed.

CONCLUSIONS

The dual fluorescence and MR imaging properties of PTIR267, combined with the ease of LDL labeling, suggest that it will be a useful tool for optimization of LDLR-targeted cancer diagnosis or therapy and for monitoring the efficacy of gene therapy of FH.

Post-transcriptional Regulation of Low Density Lipoprotein Receptor Protein by Proprotein Convertase Subtilisin/Kexin Type 9a in Mouse Liver

Lipid homeostasis is transcriptionally regulated by three DNA-binding proteins, designated sterol regulatory element-binding protein (SREBP)-1a, -1c, and -2. Oligonucleotide arrays hybridized with RNA made from livers of transgenic SREBP-1a, transgenic SREBP-2, and SREBP cleavage-activating protein knockout mice recently identified 33 genes regulated by SREBPs in liver, four of which had no known connection to lipid metabolism. One of the four genes was PCSK9, which encodes proprotein convertase subtilisin/kexin type 9a, a protein that belongs to the proteinase K subfamily of subtilases. Mutations in PCSK9 are associated with an autosomal dominant form of hypercholesterolemia. Here, we demonstrate that hepatic overexpression of either wild-type or mutant PCSK9 in mice results in hypercholesterolemia. The hypercholesterolemia is due to a post-transcriptional event causing a reduction in low density lipoprotein (LDL) receptor protein prior to the internalization and recycling of the receptor. Overexpression of PCSK9 in primary hepatocytes and in mice lacking the LDL receptor does not alter apolipoprotein B secretion. These data are consistent with PCSK9 affecting plasma LDL cholesterol levels by altering LDL receptor protein levels via a post-transcriptional mechanism.

Disruption of Autosomal Recessive Hypercholesterolemia Gene Shows Different Phenotype In Vitro and In Vivo

We previously characterized the patients with autosomal recessive hypercholesterolemia (ARH) as having severe hypercholesterolemia and retarded plasma low-density lipoprotein (LDL) clearance despite normal LDL receptor (LDLR) function in their cultured fibroblasts, and we identified a mutation in the ARH locus in these patients. ARH protein is an adaptor protein of the LDL and reportedly modulates its internalization. We developed ARH knockout mice ( ARH −/− ) to study the function of this protein. Plasma total cholesterol level was higher in ARH −/− mice than that in wild-type mice ( ARH +/+ ), being attributed to a 6-fold increase of LDL, whereas plasma lipoprotein was normal in the heterozygotes ( ARH +/− ). Clearance of 125 I-LDL from plasma was retarded in ARH −/− mice, as much as that found in LDLR −/− mice. Fluorescence activity of the intravenously injected 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI)-LDL was recovered in the cytosol of the hepatocytes of ARH +/+ mice, but not in those of ARH −/− or LDLR −/− mice. Also, less radioactivity was recovered in the liver of ARH −/− or LDLR −/− mice when [ 3 H]cholesteryl oleyl ether (CE)-labeled LDL was injected. In contrast, uptakes of [ 3 H]CE-labeled LDL, 125 I-LDL, and DiI-LDL were all normal or slightly subnormal when the ARH −/− hepatocytes were cultured. We thus concluded that the function of the hepatic LDLR is impaired in the ARH −/− mice in vivo, despite its normal function in vitro. These findings were consistent with the observations with the ARH homozygous patients and suggested that certain cellular environmental factors modulate the requirement of ARH for the LDLR function.

Quantitative trait locus mapping of genetic modifiers of metabolic syndrome and atherosclerosis in low-density lipoprotein receptor-deficient mice: identification of a locus for metabolic syndrome and increased atherosclerosis on chromosome 4

OBJECTIVE

The purpose of this study was to examine genetic factors responsible for metabolic syndrome and atherosclerosis in a setting of low-density lipoprotein (LDL) receptor deficiency in a cross between C57BL/6J (B6) and PERA/Ei (PERA) inbred mouse strains.

METHODS AND RESULTS

Comparison of metabolic phenotypes in B6 and PERA strains revealed the PERA genetic background to be dramatically more susceptible to hyperleptinemia, hyperglycemia, hypertriglyceridemia, elevated insulin levels, and body fat increase than the B6 background. To facilitate genetic analysis, metabolic syndrome-related traits and atherosclerotic lesion area were measured in 167 [(PERAxB6.129S7-Ldlr(tm1Her))xB6.129S7-Ldlr(tm1Her)]N2 male and female backcross mice that were homozygous for the Ldlr null allele. Quantitative trait locus analysis was performed using 153 polymorphic microsatellite markers spanning the genome. On chromosome 4, we identified a locus influencing plasma triglyceride, insulin, and leptin concentrations, body weight, and atherosclerosis. Several other genetic loci were identified with separate effects on plasma insulin, body weight, high-density lipoprotein cholesterol, and atherosclerosis.

CONCLUSIONS

The PERA strain is highly susceptible to the development of metabolic syndrome after feeding a Western-type diet. This susceptibility is due, in part, to a locus on murine chromosome 4 in which PERA alleles predispose to adiposity, increased insulin, and accelerated atherogenesis in the absence of marked hyperlipidemia.

Cellular and molecular mechanisms of atherosclerosis with mouse models

Recently, there has been an explosion in the number of in vivo studies using genetically engineered mouse models. Atherosclerosis research using mice began with the invention of traditional atherosclerotic mice including low-density lipoprotein receptor knockout (LDLR(-/-)) and apolipoprotein E knockout (apoE(-/-)) mice, which provided tremendous progress in atherosclerosis research. Since then, a number of modified atherosclerotic mouse models have been reported to generate lesions that more closely characterize human atherosclerotic lesions. Those modifications include inflammation, hypertension, proteinases and extracellular matrix, glucose metabolism, and immune systems. This article focuses on various kinds of mouse models with atherosclerosis and their contributions to the current advances of research.

Antigen-tailored therapy based on the inflammatory theory of atherosclerosis

In recent years, considerable data has been provided to support the role of the immune system in atherosclerosis. These reports came from studies involving knockout murine models of atherosclerosis and human subjects. The notion that inflammatory mechanisms are operable in atherogenesis has fueled a series of studies demonstrating that autoimmune responses are also evident in the atherosclerotic lesions and appear to influence the initiation and progression of the plaque. The principal autoantigens that have been suggested as potential triggers of autoimmune responses in atherosclerosis are modified forms of low-density lipoproteins, heat shock proteins and beta2 glycoprotein I. Immunization against these antigens influenced the generation of lesions and adoptive transfer studies of lymphocytes reactive to some of these antigens have also been demonstrated to enhance the growth of the plaques. The realization that autoimmune-like responses may play a role in the progression of atherosclerotic lesions has paved the way for exciting modes of manipulating lesions, irrespective of the effect on the lipid profile. Thus, recent studies indicate that oral tolerance with antigens or derivatives could suppress the respective immune responses and reduce the extent of the lesions in transgenic atherosclerosis-prone mice. These observations should be further explored and if validated, could represent novel modalities for influencing atherogenessis in humans.

Reflecting on 80 years of excellence

A small group of members of the American Society for Clinical Investigation began chatting in 1916 about the possibility of launching a new biomedical research journal. By October 1924, they managed to make the idea a reality with the publication of the first issue of the Journal of Clinical Investigation. Our 80th birthday seems an appropriate time to reflect on the history of biomedical science as it has been played out on our pages.

Blasts from the past

With this issue of the JCI, we celebrate the 80th anniversary of the Journal. While 80 years is not a century, we still feel it is important to honor what the JCI has meant to the biomedical research community for 8 decades. To illustrate why the JCI is the leading general-interest translational research journal edited by and for biomedical researchers, we have asked former JCI editors-in-chief to reflect on some of the major scientific advances reported in the pages of the Journal during their tenures.

Dual role for scavenger receptor class B, type I on bone marrow-derived cells in atherosclerotic lesion development

The function of scavenger receptor class B, type I (SR-BI) in the liver as a high-density lipoprotein receptor that promotes the selective uptake of cholesteryl esters is well defined. Its role in macrophages, however, is primarily unknown, because it functions in the uptake of (modified) lipoproteins as well as the secretion of cholesterol to high-density lipoproteins. In this study, the biological role of SR-BI on bone marrow-derived cells, including macrophages, in lipid metabolism and atherosclerosis was assessed by selective disruption of SR-BI in bone marrow in two established models of atherosclerosis: low-density lipoprotein (LDL) receptor-deficient mice that develop extensive atherosclerosis on a Western-type diet and wild-type mice that develop fatty streak lesions when fed a high-cholesterol diet containing 0.5% cholate. The presence of SR-BI in bone marrow-derived cells in LDLr-/- mice decreased lesion development after 9 and 12 weeks of Western-type diet feeding, indicating that macrophage SR-BI protects against lesion development. At 6 weeks, no significant effect of SR-BI in bone marrow-derived cells on lesion development was observed. Interestingly, after only 4 weeks of Western-type diet feeding of transplanted LDLr-/- mice and in wild-type mice on a high-cholesterol/cholate diet, the presence of SR-BI in bone marrow-derived cells increased the development of small fatty streak lesions. It thus appears that, depending on the stage of atherosclerotic lesion development, SR-BI in bone marrow-derived cells is either pro-atherogenic or anti-atherogenic, indicating a unique dual role in the pathogenesis of atherosclerosis.

Binding of low density lipoprotein to platelet apolipoprotein E receptor 2' results in phosphorylation of p38MAPK

Binding of low density lipoprotein (LDL) to platelets enhances platelet responsiveness to various aggregation-inducing agents. However, the identity of the platelet surface receptor for LDL is unknown. We have previously reported that binding of the LDL component apolipoprotein B100 to platelets induces rapid phosphorylation of p38 mitogen-activated protein kinase (p38MAPK). Here, we show that LDL-dependent activation of this kinase is inhibited by receptor-associated protein (RAP), an inhibitor of members of the LDL receptor family. Confocal microscopy revealed a high degree of co-localization of LDL and a splice variant of the LDL receptor family member apolipoprotein E receptor-2 (apoER2') at the platelet surface, suggesting that apoER2' may contribute to LDL-induced platelet signaling. Indeed, LDL was unable to induce p38MAPK activation in platelets of apoER2-deficient mice. Furthermore, LDL bound efficiently to soluble apoER2', and the transient LDL-induced activation of p38MAPK was mimicked by an anti-apoER2 antibody. Association of LDL to platelets resulted in tyrosine phosphorylation of apoER2', a process that was inhibited in the presence of PP1, an inhibitor of Src-like tyrosine kinases. Moreover, phosphorylated but not native apoER2' co-precipitated with the Src family member Fgr. This suggests that exposure of platelets to LDL induces association of apoER2' to Fgr, a kinase that is able to activate p38MAPK. In conclusion, our data indicate that apoER2' contributes to LDL-dependent sensitization of platelets.

Adenovirus-mediated overexpression of tissue inhibitor of metalloproteinases-1 in the liver: efficient protection against T-cell lymphoma and colon carcinoma metastasis

BACKGROUND

Matrix metalloproteinases (MMPs) are critical for metastasis of tumor cells. Tissue inhibitor of metalloproteinases-1 (TIMP-1), a natural MMP inhibitor, was shown to reduce metastasis in different models. Here, we investigated whether increased TIMP-1 levels in the liver achieved by adenoviral gene transfer will effectively inhibit liver metastasis of two independent tumor cell lines.

METHOD

TIMP-1 was transferred with adenoviral vectors into the livers of DBA/2 and Balb/c mice, which were subsequently challenged by hematogenous experimental metastases of the T-cell lymphoma cell line L-CI.5s or the colorectal carcinoma cell line CT-26, respectively.

RESULTS

MMP-9 expression in the liver was induced upon metastasis in both tumor types. Adenoviral gene transfer led to high transduction efficacy as indicated by lacZ expression in 60% of hepatocytes. TIMP-1, a key inhibitor of MMP-9, was expressed at 10(5)-fold higher levels by adenoviral gene transfer as compared with levels achieved in TIMP-1 transgenic mice, previously shown to be inefficient to reduce T-cell lymphoma metastasis. High local and systemic (serum) levels of TIMP-1 led to substantial (94%) reduction of T-cell lymphoma and colorectal carcinoma (73%) experimental liver metastasis.

CONCLUSIONS

Adenoviral gene transfer led to systemic and local TIMP-1 levels sufficient to inhibit metastasis of a highly aggressive T-cell lymphoma, pointing at the requirement of threshold levels for effective anti-metastatic efficacy. This approach was also efficient in a colon carcinoma solid tumor model. We propose that viral gene transfer of TIMP-1 can provide a suitable defense strategy to prevent metastatic spread to the liver.

High-level expression of ABCG5 and ABCG8 attenuates diet-induced hypercholesterolemia and atherosclerosis in Ldlr−− mice

Transgenic mice expressing human ABCG5 (G5) and ABCG8 (G8) have decreased fractional absorption and increased biliary secretion of cholesterol, but their plasma cholesterol levels are unchanged (males) or modestly reduced (females). To determine whether increased expression of G5 and G8 can ameliorate hypercholesterolemia in mice lacking LDL receptors (LDLRs), we examined the effects of G5G8 transgene expression on cholesterol metabolism and atherosclerosis in Ldlr-/- mice. In chow-fed Ldlr-/- mice, the G5G8 transgene reduced fractional absorption of dietary cholesterol by 50% and increased biliary cholesterol levels by 60% but did not affect plasma cholesterol levels. On a Western diet (21% fat, 0.2% cholesterol), G5G8Tg; Ldlr-/- mice had a 30% reduction in the level of hepatic cholesterol and 45% lower plasma cholesterol levels than the Ldlr-/- mice. After 6 months on the Western diet, the atherosclerotic lesion area in the aortic root and arch was approximately 70% lower in the G5G8Tg;Ldlr-/- than in the Ldlr-/- mice and was correlated with the plasma cholesterol levels. These results demonstrate that increased expression of G5 and G8 attenuates diet-induced hypercholesterolemia in Ldlr-/- mice, resulting in a significant reduction in plasma levels of cholesterol and aortic atherosclerotic lesion area.

Defective VLDL metabolism and severe atherosclerosis in mice expressing human apolipoprotein E isoforms but lacking the LDL receptor

Differences in affinity of human apolipoprotein E (apoE) isoforms for the low density lipoprotein receptor (LDLR) are thought to result in the differences in lipid metabolism observed in humans with different APOE genotypes. Mice expressing three common human apoE isoforms, E2, E3, and E4, in place of endogenous mouse apoE were used to investigate the relative roles of apoE isoforms in LDLR- and non-LDLR-mediated very low density lipoprotein (VLDL) clearance. While both VLDL particles isolated from mice expressing apoE3 and apoE4 bound to mouse LDLR with affinity and Bmax similar to VLDL containing mouse apoE, VLDL with apoE2 bound with only half the Bmax. In the absence of the LDLR, all lines of mice expressing human apoE showed dramatic increases in VLDL cholesterol and triglycerides (TG) compared to LDLR knockout mice expressing mouse apoE. The mechanism of the hyperlipidemia in mice expressing human apoE isoforms is due to impairment of non-LDL-receptor-mediated VLDL clearance. This results in the severe atherosclerosis observed in mice expressing human apoE but lacking the LDLR, even when fed normal chow diet. Our data show that defects in LDLR independent pathway(s) are a potential factor that trigger hyperlipoproteinemia when the LDLR pathway is perturbed, as in E2/2 mice.

Low-density lipoprotein receptor gene therapy using helper-dependent adenovirus produces long-term protection against atherosclerosis in a mouse model of familial hypercholesterolemia

We tested the efficacy of low-density lipoprotein receptor (LDLR) therapy using helper-dependent adenovirus (HD-Ad), comparing it with that of very low-density lipoprotein receptor (VLDLR), an LDLR homolog. We treated high cholesterol diet fed LDLR-/- mice with a single intravenous injection of HD-Ad expressing monkey LDLR (1.5 x 10(13) or 5 x 10(12) VP/kg) or VLDLR. Throughout the 24-week experiment, plasma cholesterol of LDLR-treated mice was lower than that of VLDLR-treated mice, which was in turn lower than that of PBS-treated mice. Anti-LDLR antibodies developed in 2/10 mice treated with high-dose HD-Ad-LDLR but in none (0/14) of the other treatment groups. HD-Ad-treated mice displayed significant retardation of atherosclerotic lesion progression. We next tested the long-term efficacy of low-dose HD-Ad-LDLR injected into 12-week-old LDLR-/- mice. After 60 weeks, atherosclerosis lesions covered approximately 50% of the surface of aortas of control mice, whereas aortas of treated mice were essentially lesion-free. The lipid lowering effect of HD-Ad-LDLR lasted at least 108 weeks (>2 years) when all control mice had died. In addition to retarding lesion progression, treatment caused lesion remodeling from a vulnerable-looking to a more stable-appearing phenotype. In conclusion, HD-Ad-mediated LDLR gene therapy is effective in conferring long-term protection against atherosclerosis in a mouse model of familial hypercholesterolemia.

Identification of the human rhinovirus serotype 1A binding site on the murine low-density lipoprotein receptor by using human-mouse receptor chimeras

Human rhinovirus serotype 1A (HRV1A) binds more strongly to the mouse low-density lipoprotein receptor (LDLR) than to the human homologue (M. Reithmayer, A. Reischl, L. Snyers, and D. Blaas, J. Virol. 76:6957-6965, 2002). Here, we used this fact to determine the binding site of HRV1A by replacing selected ligand binding modules of the human receptor with the corresponding ligand binding modules of the mouse receptor. The chimeric proteins were expressed in mouse fibroblasts deficient in endogenous LDLR and LDLR-related protein, both used by minor group HRVs for cell entry. Binding was assessed by virus overlay blots, by immunofluorescence microscopy, and by measuring cell attachment of radiolabeled virus. Replacement of ligand binding repeat 5 of the human LDLR with the corresponding mouse sequence resulted in a substantial increase in HRV1A binding, whereas substitution of repeats 3 and 4 was without effect. Replacement of human receptor repeats 1 and 2 with the murine homologues also increased virus binding. Finally, murine receptor modules 1, 2, and 5 simultaneously introduced into the human receptor resulted in HRV1A binding indistinguishable from mouse wild-type receptor. Thus, repeats 1 and/or 2 and repeat 5 are involved in HRV1A attachment. Changing CDGGPD in the acidic cluster of module 5 in the human receptor to CDGEAD present in the mouse receptor led to substantially increased binding of HRV1A, indicating an important role of the glutamate residue in HRV1A recognition.

Low-density lipoprotein receptor-knockout mice display impaired spatial memory associated with a decreased synaptic density in the hippocampus

The low-density lipoprotein receptor (LDLR) is the first described receptor for apolipoprotein E (apoE). We hypothesize that the absence of the LDLR, similar to the absence of apoE, results in impaired learning and memory processes. Six-month-old homozygous Ldlr-/- and wild-type littermates (Ldlr+/+), maintained on a standard lab chow diet, were used. Unlike humans, Ldlr-/- mice, under these conditions, do not develop atherosclerosis. The results of the Morris water escape task revealed an impaired spatial memory in the Ldlr-/- mice in comparison with Ldlr+/+ mice. Also in a T-maze task, the working memory performance of the Ldlr-/- mice was impaired. Furthermore, Ldlr-/- mice, in comparison with Ldlr+/+ mice, display a decreased number of synaptophysin-immunoreactive presynaptic boutons in the hippocampus CA1. In conclusion, the results show in mice deficiency for the LDLR results in impaired hippocampal-dependent memory functions. A decrease in the number of presynaptic boutons may underlay these behavioral alterations. Therefore, the LDLR may be an important receptor for apoE in the central nervous system.

Phenytoin treatment reduces atherosclerosis in mice through mechanisms independent of plasma HDL-cholesterol concentration

Phenytoin (PHT) increases high density lipoprotein cholesterol (HDL-C) and reduces coronary artery disease mortality in humans. We report the results of PHT treatment on atherosclerosis susceptibility and lipid profile in four different types of mouse: control C57BL/6 mice and cholesteryl ester transfer protein transgenic mice as models of fatty streak, and LDL receptor-deficient mice and apolipoprotein E-deficient mice as models of mature atherosclerosis. Each mouse type was fed an appropriate diet to induce atherosclerosis and prevent liver toxicity. PHT treatment demonstrated a protective effect in all models. Reduction in aortic atherosclerotic area by PHT treatment was more evident in early atherosclerosis (2.3-fold) than in mature atherosclerosis (decreases of 40 and 23%, respectively, but only in mice in the upper 50% percentile of plasma PHT concentration). Atherosclerosis prevention was not concomitant with a consistent increase in HDL-C or any other protective change in the lipid profile. Different analyses of potential antiatherogenic HDL functions did not provide additional information. Microarray liver gene expression analyses identified a potential atheroprotective mechanism characterized by decreased expression of syndecan-4, RhoA2, double LIM protein-1, zeta-chain-associated protein kinase-70 and interleukin 6 receptor-alpha. However, to demonstrate that these changes are part of a PHT-antiatherogenic effect, they will need to be found also in arteries, maintained at protein level and proved to be causal rather than reactive.

Effects of streptozotocin-induced diabetes in apolipoprotein AI deficient mice

During the past decade a number of investigators have attempted to develop mouse models of diabetic macrovascular disease. Hyperglycemia might increase vascular damage because it increases oxidant stress. For this reason we studied animals that were deficient in HDL; HDL is widely believed to protect against oxidant stress. An inbred line of mice doubly deficient in LDL receptor and apoAI was made diabetic with streptozotocin (STZ); control mice had an average glucose of 7.2+/-2mmol/l and STZ-treated mice had an average glucose of 19.4+/-6.5mmol/l. The animals were fed a high cholesterol but low fat diet leading to plasma cholesterol levels of 9.4+/-1.6mmol/l in control animals and 10.1+/-1.8mmol/l in STZ-treated mice. The control and STZ-treated animals had similar plasma lipoprotein profiles. Atherosclerosis assessed at 23 weeks averaged 38154microm(2) in control and 32962microm(2) in STZ-treated mice. Therefore STZ-induced diabetes does not alter plasma lipoproteins or atherosclerosis in HDL deficient mice.

Scavenger receptor expressed by endothelial cells I (SREC-I) mediates the uptake of acetylated low density lipoproteins by macrophages stimulated with lipopolysaccharide

Scavenger receptor expressed by endothelial cells I (SREC-I) is a novel endocytic receptor for acetylated low density lipoprotein (LDL). Here we show that SREC-I is expressed in a wide variety of tissues, including macrophages and aortas. Lipopolysaccharide (LPS) robustly stimulated the expression of SREC-I in macrophages. In an initial attempt to clarify the role of SREC-I in the uptake of modified lipoproteins as well as in the development of atherosclerosis, we generated mice with a targeted disruption of the SREC-I gene by homologous recombination in embryonic stem cells. To exclude the overwhelming effect of the type A scavenger receptor (SR-A) on the uptake of Ac-LDL, we further generated mice lacking both SR-A and SREC-I (SR-A(-/-);SREC-I(-/-)) by cross-breeding and compared the uptake and degradation of Ac-LDL in the isolated macrophages. The contribution of SR-A and SREC-I to the overall degradation of Ac-LDL was 85 and 5%, respectively, in a non-stimulated condition. LPS increased the uptake and degradation of Ac-LDL by 1.8-fold. In this condition, the contribution of SR-A and SREC-I to the overall degradation of Ac-LDL was 90 and 6%, respectively. LPS increased the absolute contribution of SR-A and SREC-I by 1.9- and 2.3-fold, respectively. On the other hand, LPS decreased the absolute contribution of other pathways by 31%. Consistently, LPS did not increase the expression of other members of the scavenger receptor family such as CD36. In conclusion, SREC-I serves as a major endocytic receptor for Ac-LDL in LPS-stimulated macrophages lacking SR-A, suggesting that it has a key role in the development of atherosclerosis in concert with SR-A.

Chronic treatment with bark infusion fromCroton cajucaralowers plasma triglyceride levels in genetic hyperlipidemic mice

Aqueous infusion and preparations containing dehydrocrotonin (DHC) and essential oil from Croton cajucara bark were tested for plasma lipid-lowering effects in genetically modified hyperlipidemic mice. Two mouse models were tested: 1) primary hypercholesterolemia resulting from the LDL-receptor gene knockout, and 2) combined hyperlipidemia resulting from crosses of LDL-receptor knockout mice with transgenic mice overexpressing apolipo protein (apo) CIII and cholesteryl ester-transfer protein. Mice treated with bark infusion, DHC, essential oil, or placebos for 25 days showed no signals of toxicity as judged by biochemical tests for liver and kidney functions. The bark infusion reduced triglyceride plasma levels by 40%, while essential oil and DHC had no significant effects on plasma lipid levels. The bark infusion treatment promoted a redistribution of cholesterol among the lipoprotein fractions in combined hyperlipidemic mice. There was a marked reduction in the VLDL fraction and an increase in the HDL fraction, in such a way that the (VLDL + LDL)/HDL ratio was reduced by half. The bark infusion treatment did not modify cholesterol distribution in hypercholesterolemic mice. In conclusion, C. cajucara bark infusion reduced plasma triglycerides levels and promoted a redistribution of cholesterol among lipoproteins in genetically combined hyperlipidemic mice. These changes modify risk factors for the development of atherosclerotic diseases.Key words: hyperlipidemia, transgenic mice, Croton cajucara, dehydrocrotonin, cholesterol.

Lipid peroxidation and oxidant stress regulate hepatic apolipoprotein B degradation and VLDL production

How omega-3 and omega-6 polyunsaturated fatty acids (PUFAs) lower plasma lipid levels is incompletely understood. We previously showed that marine omega-3 PUFAs (docosahexaenoic acid [DHA] and eicosapentaenoic acid) stimulate a novel pathway, post-ER presecretory proteolysis (PERPP), that degrades apolipoprotein B100 (ApoB100), thereby reducing lipoprotein secretion from liver cells. To identify signals stimulating PERPP, we examined known actions of omega-3 PUFA. In rat hepatoma or primary rodent hepatocytes incubated with omega-3 PUFA, cotreatment with the iron chelator desferrioxamine, an inhibitor of iron-dependent lipid peroxidation, or vitamin E, a lipid antioxidant, suppressed increases in thiobarbituric acid-reactive substances (TBARSs; a measure of lipid peroxidation products) and restored ApoB100 recovery and VLDL secretion. Moreover, omega-6 and nonmarine omega-3 PUFA, also prone to peroxidation, increased ApoB100 degradation via intracellular induction of TBARSs. Even without added fatty acids, degradation of ApoB100 in primary hepatocytes was blocked by desferrioxamine or antioxidant cotreatment. To extend these results in vivo, mice were infused with DHA, which increased hepatic TBARSs and reduced VLDL-ApoB100 secretion. These results establish a novel link between lipid peroxidation and oxidant stress with ApoB100 degradation via PERPP, and may be relevant to the hypolipidemic actions of dietary PUFAs, the basal regulation of ApoB100 secretion, and hyperlipidemias arising from ApoB100 overproduction.

Adenoviral-mediated expression of Pcsk9 in mice results in a low-density lipoprotein receptor knockout phenotype

Proprotein convertase subtilisin kexin 9 (Pcsk9) is a subtilisin serine protease with a putative role in cholesterol metabolism. Pcsk9 expression is down-regulated by dietary cholesterol, and mutations in Pcsk9 have been associated with a form of autosomal dominant hypercholesterolemia. To study the function of Pcsk9 in mice, an adenovirus constitutively expressing murine Pcsk9 (Pcsk9-Ad) was used. Pcsk9 overexpression in wild-type mice caused a 2-fold increase in plasma total cholesterol and a 5-fold increase in non-high-density lipoprotein (HDL) cholesterol, with no increase in HDL cholesterol, as compared with mice infected with a control adenovirus. Fast protein liquid chromatography analysis showed that the increase in non-HDL cholesterol was due to an increase in low-density lipoprotein (LDL) cholesterol. This effect appeared to depend on the LDL receptor (LDLR) because LDLR knockout mice infected with Pcsk9-Ad had no change in plasma cholesterol levels as compared with knockout mice infected with a control adenovirus. Furthermore, whereas overexpression of Pcsk9 had no effect on LDLR mRNA levels, there was a near absence of LDLR protein in animals overexpressing Pcsk9. These results were confirmed in vitro by the demonstration that transfection of Pcsk9 in McA-RH7777 cells caused a reduction in LDLR protein and LDL binding. In summary, these results indicate that overexpression of Pcsk9 interferes with LDLR-mediated LDL cholesterol uptake. Because Pcsk9 and LDLR are coordinately regulated by cholesterol, Pcsk9 may be involved in a novel mechanism to modulate LDLR function by an alternative pathway than classic cholesterol inhibition of sterol regulatory element binding protein-mediated transcription.

B7-1/B7-2 costimulation regulates plaque antigen-specific T-cell responses and atherogenesis in low-density lipoprotein receptor-deficient mice

BACKGROUND

Several lines of evidence indicate that T-cell responses influence the progression of atherosclerotic disease. Interferon-gamma (IFN-gamma)-producing T cells specific for lesional antigens, including oxidized LDLs and heat shock protein 60 (HSP60), may promote lesion development as well as plaque instability. B7-1 and B7-2 are closely related molecules expressed on antigen-presenting cells that provide costimulatory signals for T-cell activation. This study tested the hypothesis that the ability of T cells to influence atherosclerosis depends on B7-1/B7-2 costimulation.

METHODS AND RESULTS

B7-1/B7-2/LDL receptor (LDLR)-deficient mice and LDLR-deficient control mice were fed a 1.25% cholesterol or control diet for 8 and 20 weeks. Total serum cholesterol levels and extent and phenotype of atherosclerosis were analyzed. Splenic and lymph node CD4+ T cells from the animals were cultured with mouse recombinant HSP60 or media and antigen-presenting cells and analyzed for IFN-gamma and interleukin-4 production. The absence of B7-1 and B7-2 significantly reduced early cholesterol diet-induced atherosclerotic lesion development in LDLR-deficient mice compared with B7-1/B7-2-expressing control mice. Furthermore, CD4+ T cells from the cholesterol-fed B7-deficient mice secreted a significantly lower amount of IFN-gamma in response to mouse HSP60 in vitro than did T cells from B7-expressing control mice.

CONCLUSIONS

The data show that B7-1 and B7-2 regulated the development of atherosclerotic lesions and the priming of lesional antigen-specific T cells. This study highlights the B7-CD28 pathway as a potentially important target for immunomodulation of atherosclerosis.

Dietary soy beta-conglycinin (7S globulin) inhibits atherosclerosis in mice

Although beta-conglycinin (7S globulin), a major soy storage protein, stimulates the expression of LDL receptors and the degradation of LDL by hepatocytes in vitro, the in vivo effects of dietary beta-conglycinin on the cardiovascular system are unknown. We assessed the effects of dietary beta-conglycinin and other soy peptide fractions on the development of atherosclerosis in atherosclerosis-susceptible mice. At 6 wk of age, male and ovariectomized female apolipoprotein (apo) E-null mice and LDL receptor-null, apoB transgenic mice were assigned randomly to treatment groups that differed only in the source of dietary protein: 1) casein/lactalbumin, 2) isoflavone-containing soy protein isolate, 3) beta-conglycinin, 4) glycinin (11S globulin, another major soy storage protein), 5) beta-conglycinin-devoid soy protein, and 6) W008 (a peptide fraction produced by hydrolysis and precipitation of soy protein isolate). After 4 mo, aortic atherosclerosis (cholesteryl ester content) and plasma lipoprotein cholesterol concentrations were quantified using GLC. Relative to mice fed casein/lactalbumin-based diets, the extent of atherosclerosis was reduced in ovariectomized female mice fed all soy protein-containing diets. Relative to mice fed isoflavone-containing soy protein isolate, atherosclerosis was reduced only in mice fed the beta-conglycinin-containing diet. Mean reductions were 39 and 67% (all P <0.05) in male and ovariectomized female apoE null mice and 66% (P < 0.05) in male LDL receptor null mice. These effects were unrelated to variation in isoflavone content of the protein source and only minimally related to plasma lipoprotein cholesterol concentrations. We conclude that a diet rich in beta-conglycinin has atheroprotective effects that greatly exceed those of isoflavone-containing soy protein isolate and do not depend on LDL receptors or influences on plasma lipoproteins.

Annexin 2-caveolin 1 complex is a target of ezetimibe and regulates intestinal cholesterol transport

Modulation of cholesterol absorption in the intestine, the primary site of dietary cholesterol uptake in humans, can have profound clinical implications. We have undertaken a reverse genetic approach by disrupting putative cholesterol processing genes in zebrafish larvae by using morpholino (MO) antisense oligonucleotides. By using targeted MO injections and immunoprecipitation (IP) experiments coupled with mass spectrometry, we determined that annexin (ANX)2 complexes with caveolin (CAV)1 in the zebrafish and mouse intestine. The complex is heat stable and unaffected by SDS or reducing conditions. MO targeting of anx2b or cav1, which are both strongly expressed in the larval and adult zebrafish intestinal epithelium, prevents formation of the protein heterocomplex. Furthermore, anx2b MO injection prevents processing of a fluorescent cholesterol reporter and results in reduced sterol mass. Pharmacological treatment of mice with ezetimibe disrupts the heterocomplex in only hypercholesterolemic animals. These data suggest that ANX2 and CAV1 are components of an intestinal sterol transport complex.

Scavenger receptor BI plays a role in facilitating chylomicron metabolism

The function of scavenger receptor class B type I (SR-BI) in mediating the selective uptake of high density lipoprotein (HDL) cholesterol esters is well established. However, the potential role of SR-BI in chylomicron and chylomicron remnant metabolism is largely unknown. In the present investigation, we report that the cell association of 160 nm-sized triglyceride-rich chylomicron-like emulsion particles to freshly isolated hepatocytes from SR-BI-deficient mice is greatly reduced (>70%), as compared with wild-type littermate mice. Competition experiments show that the association of emulsion particles with isolated hepatocytes is efficiently competed for (>70%) by the well established SR-BI ligands, HDL and oxidized low density lipoprotein (LDL), whereas LDL is ineffective. Upon injection into SR-BI-deficient mice the hepatic association of emulsion particles is markedly decreased ( approximately 80%) as compared with wild-type mice. The relevance of these findings for in vivo chylomicron (remnant) metabolism was further evaluated by studying the effect of SR-BI deficiency on the intragastric fat load-induced postprandial triglyceride response. The postprandial triglyceride response is 2-fold higher in SR-BI-deficient mice as compared with wild-type littermates (area-under-the-curve 39.6 +/- 1.2 versus 21.1 +/- 3.6; p < 0.005), with a 4-fold increased accumulation of chylomicron (remnant)-associated triglycerides in plasma at 6 h after intragastric fat load. We conclude that SR-BI is important in facilitating chylomicron (remnant) metabolism and might function as an initial recognition site for chylomicron remnants whereby the subsequent internalization can be exerted by additional receptor systems like the LDL receptor and LDL receptor-related protein.

Sterol regulation of scavenger receptor class B type I in macrophages

Scavenger receptor class B type I (SR-BI) is expressed in macrophages, but its role in sterol trafficking in these cells remains controversial. We examined the effect of sterol loading on SR-BI expression in human monocytes/macrophages, mouse peritoneal macrophages, and a cultured mouse macrophage cell line (J774 cells). Sterol loading using either acetylated LDL or 25-hydroxycholesterol resulted in a time- and concentration-dependent decrease in SR-BI protein and mRNA levels. Treatment of lipid-loaded J774 cells with cyclodextrin or HDL to promote cellular sterol efflux was associated with an increase in SR-BI expression. Studies were performed to determine if the sterol-associated downregulation of SR-BI in macrophages was mediated by either sterol regulatory element binding proteins (SREBPs) or the liver X receptor (LXR). Expression of constitutively active SREBPs failed to alter the expression of a luciferase reporter placed downstream of a 2556 bp 5' flanking sequence from the mouse SR-BI gene. Reduction in SR-BI expression was also seen in sterol-loaded peritoneal macrophages from mice expressing no LXRalpha and LXRbeta. We conclude that SR-BI levels in macrophages are responsive to changes in intracellular sterol content and that these sterol-associated changes are not mediated by LXR and are unlikely to be mediated by an SREBP pathway.

Naringin alters the cholesterol biosynthesis and antioxidant enzyme activities in LDL receptor-knockout mice under cholesterol fed condition

The purpose of the current study was to evaluate the lipid lowering and antioxidant capacity of naringin in LDL receptor knockout (LDLR-KO) mice fed a cholesterol (0.1 g/100 g) diet. As such, naringin or lovastatin (0.02 g/100 g) was supplemented in a cholesterol diet for 6 weeks. The naringin and lovastatin supplementation significantly lowered the plasma total cholesterol level compared to the control group. The plasma and hepatic triglyceride level was only lowered by the lovastatin supplement, while the hepatic cholesterol content was lowered by both the naringin and lovastatin supplements compared to the control group. The hepatic HMG-CoA reductase activity was significantly lower in the naringin and lovastatin supplemented groups than in the control group, whereas the ACAT activity was unaffected. The excretion of total sterol was significantly higher in the naringin and lovastatin groups compared to the control group due to significant changes in the acidic and neutral sterol, respectively. When comparing the hepatic antioxidant enzyme activities, the superoxide dismutase, catalase, and glutathione reductase activities were all significantly higher in the naringin-supplemented group than in the control group, while only the lovastatin supplement increased the glutathione reductase activity. Accordingly, the current results confirmed that naringin lowers the plasma cholesterol level via the inhibition of hepatic HMG-CoA reductase activity and increases the excretion of fecal sterol. Naringin was also found to improve the activities of hepatic antioxidant enzymes against oxidative stress in a hypercholesterolemic animal model, i.e. cholesterol-fed LDLR-KO mice.

The role of alpha1-fetoprotein transcription factor/LRH-1 in bile acid biosynthesis: a known nuclear receptor activator that can act as a suppressor of bile acid biosynthesis

Two key regulatory enzymes in the bile acid biosynthesis pathway are cholesterol 7alpha-hydroxylase/CYP7A1 (7alpha-hydroxylase) and sterol 12alpha-hydroxylase/CYP8B1 (12alpha-hydroxylase). It has been shown previously that hepatocyte nuclear factor-4alpha (HNF-4) and the alpha(1)-fetoprotein transcription factor (FTF) are activators of 7alpha-and 12alpha-hydroxylase transcription and that the small heterodimer partner (SHP) suppresses bile acid biosynthesis by heterodimerizing with FTF. However, the role of FTF in bile acid biosynthesis has been studied only in tissue culture systems. In heterozygous FTF knockout mice, 7alpha- and 12alpha-hydroxylase genes were expressed at 5-7-fold higher levels than in wild-type mice, an apparent direct contradiction to previous in vitro observations. This higher expression of the 7alpha- and 12alpha-hydroxylase genes resulted in a 33% higher bile acid pool in their gallbladders, bile more enriched in cholic acid, and a 13% decrease in plasma cholesterol levels. Adenovirus-mediated FTF overexpression in wild-type mice resulted in 10-fold lower expression of the 7alpha- and 12alpha-hydroxylase genes and up to 8-fold higher SHP expression, highlighting the dual role that FTF plays in different promoters. Shorter overexpression times still resulted in lower 7alpha- and 12alpha-hydroxylase expression, but unchanged SHP expression, suggesting that two different mechanisms are involved in the FTF-mediated suppression of 7alpha- and 12alpha-hydroxylase expression. This FTF-mediated suppression of the expression of two bile acid biosynthesis genes resulted in a 3-fold lower rate of bile acid synthesis in a rat bile fistula animal model. Based on these observations and on protein binding studies performed in vitro and by chromatin immunoprecipitation, we hypothesize that FTF has two synergetic effects that contribute to its role in bile acid biosynthesis: 1) it has the ability to activate the expression of SHP, which in turn heterodimerizes and suppresses FTF transactivation activity; and 2) it occupies the FTF/HNF-4 recognition site within the 7alpha- and 12alpha-hydroxylase promoters, which can otherwise be occupied by a factor (HNF-4) that cannot be suppressed by SHP.

Elevation of ceramide in serum lipoproteins during acute phase response in humans and mice: role of serine-palmitoyl transferase

Recent studies have indicated that ceramide generated in the liver is secreted into the bloodstream as component of very-low-density lipoproteins (VLDL) and low-density lipoproteins (LDL). This manuscript investigates the effect of host acute phase response to inflammation on lipoprotein ceramide levels. In humans, two different patterns of responses were found. One group of volunteers experienced transient increases in serum ceramide at 1.5h after LPS administration. Second group showed prolonged increases that reached up to 10-fold above the basal level and continued for up to 24h. Increases in ceramide were found only in VLDL and LDL particles. LPS administration induced similar increases in mice. These increases were accompanied by activation of secreted sphingomyelinase in serum and serine-palmitoyl transferase in liver. ASMase knockout mice retained LPS-induced increases in serum ceramide, thus suggesting that the elevation of VLDL and LDL ceramide content is attributed at least in part to activation of de novo synthesis of ceramide in the liver.

A sensitive noninvasive method for monitoring successful liver-directed gene transfer of the low-density lipoprotein receptor in Watanabe hyperlipidemic rabbits in vivo

Noninvasive tools to quantitate transgene expression directly are a prerequisite for clinical gene therapy. We established a method to determine location, magnitude, and duration of low-density lipoprotein (LDL) receptor (LDLR) transgene expression after adenoviral gene transfer into LDLR-deficient Watanabe hypercholesterolemic rabbits by following tissue uptake of intravenously injected (111)In-labeled LDL using a scintillation camera. Liver-specific tracer uptake was calculated by normalizing the counts measured over the liver to counts measured over the heart that represent the circulating blood pool of the tracer (liver/heart (L/H) ratio). Our results indicate that the optimal time point for transgene imaging is 4 h after the tracer injection. Compared with control virus-injected rabbits, animals treated with the LDLR-expressing adenovirus showed seven-fold higher L/H ratios on day 6 after gene transfer, and had still 4.5-fold higher L/H ratios on day 30. This imaging method might be a useful strategy to obtain reliable data on functional transgene expression in clinical gene therapy trials of familial hypercholesterolemia.

Adenoviral transfer of endothelial nitric oxide synthase attenuates lesion formation in a novel murine model of postangioplasty restenosis

OBJECTIVE

Restenosis remains a major late complication of percutaneous transluminal coronary angioplasty (PTCA), for which the development of prevention strategies has thus far been hampered by the lack of a representative and practical animal model. We have, therefore, developed a murine model of PTCA-induced restenosis.

METHODS AND RESULTS

Rigid probe angioplasty of pre-existing atherosclerotic lesions in the carotid arteries of ApoE-deficient mice was found to result in an increase in lesion size (0.14+/-0.04x10(5) microm2 to 0.42+/-0.09x10(5) microm2, P=0.007) with a smooth muscle cell-rich, fibrotic lesion morphology. In an additional experiment, lesions were incubated immediately after angioplasty with adenovirus bearing an endothelial nitric oxide synthase (eNOS) transgene (Ad.APT.eNOS), or an "empty" control virus (Ad.APT.empty) at a titer of 1.5x10(9) pfu/mL. Ad.APT.eNOS treatment was seen to lead to a 73.1% reduction in plaque size (0.27+/-0.04x10(5) microm2 versus 1.02+/-0.39x10(5) microm2, P=0.07), which translated to a significantly lowered average degree of stenosis (33.6+/-4.1% versus 74.6+/-14.0%, P=0.02). Ad.APT.eNOS also decreased lesional collagen content from 29.1% to 4.8% (P<0.001).

CONCLUSIONS

We believe that we have established a representative murine model of postangioplasty restenosis, which may serve to elucidate the mechanisms underlying restenosis and to evaluate potential antirestenotic therapies.

Inhibition of NF-kappaB activation in macrophages increases atherosclerosis in LDL receptor-deficient mice

Atherosclerosis is now generally accepted as a chronic inflammatory condition. The transcription factor NF-kappaB is a key regulator of inflammation, immune responses, cell survival, and cell proliferation. To investigate the role of NF-kappaB activation in macrophages during atherogenesis, we used LDL receptor-deficient mice with a macrophage-restricted deletion of IkappaB kinase 2 (IKK2), which is essential for NF-kappaB activation by proinflammatory signals. These mice showed increased atherosclerosis as quantified by lesion area measurements. In addition, the lesions were more advanced and showed more necrosis and increased cell number in early lesions. Southern blotting revealed that deletion of IKK2 was approximately 65% in macrophages, coinciding with a reduction of 50% in NF-kappaB activation, as compared with controls. In both groups, the expression of differentiation markers, uptake of bacteria, and endocytosis of modified LDL was similar. Upon stimulation with LPS, production of TNF was reduced by approximately 50% in IKK2-deleted macrophages. Interestingly, we also found a major reduction in the anti-inflammatory cytokine IL-10. Our data show that inhibition of the NF-kappaB pathway in macrophages leads to more severe atherosclerosis in mice, possibly by affecting the pro- and anti-inflammatory balance that controls the development of atherosclerosis.