The Impairment of Macrophage-to-Feces Reverse Cholesterol Transport during Inflammation Does Not Depend on Serum Amyloid A

Understanding the heterogeneity and dysfunction of HDL in chronic kidney disease: insights from recent reviews

Chronic kidney disease (CKD) is a complex disease that affects the global population's health, with dyslipidemia being one of its major complications. High density lipoprotein (HDL) is regarded as the "hero" in the bloodstream due to its role in reverse cholesterol transport, which lowers cholesterol levels in the blood and prevents atherosclerosis. However, in the complex internal environment of CKD, even this "hero" may struggle to perform its beneficial functions and could potentially become harmful. This article reviews HDL heterogeneity, HDL subclasses, functional changes in HDL during the progression of CKD, and the application of HDL in CKD treatment. This review aims to deepen understanding of lipid metabolism abnormalities in CKD patients and provide a basis for new therapeutic strategies.

Interactions between macrophage membrane and lipid mediators during cardiovascular diseases with the implications of scavenger receptors

The onset and progression of cardiovascular diseases with the major underlying cause being atherosclerosis, occur during chronic inflammatory persistence in the vascular system, especially within the arterial wall. Such prolonged maladaptive inflammation is driven by macrophages and their key mediators are generally attributed to a disparity in lipid metabolism. Macrophages are the primary cells of innate immunity, endowed with expansive membrane domains involved in immune responses with their signalling systems. During atherosclerosis, the membrane domains and receptors control various active organisations of macrophages. Their scavenger/endocytic receptors regulate the trafficking of intracellular and extracellular cargo. Corresponding influence on lipid metabolism is mediated by their dynamic interaction with scavenger membrane receptors and their integrated mechanisms such as pinocytosis, phagocytosis, cholesterol export/import, etc. This interaction not only results in the functional differentiation of macrophages but also modifies their structural configurations. Here, we reviewed the association of macrophage membrane biomechanics and their scavenger receptor families with lipid metabolites during the event of atherogenesis. In addition, the membrane structure of macrophages and the signalling pathways involved in endocytosis integrated with lipid metabolism are detailed. This article establishes future insights into the scavenger receptors as potential targets for cardiovascular disease prevention and treatment.

Renal AA amyloidosis: presentation, diagnosis, and current therapeutic options: a review

Amyloid A amyloidosis is thought to be the second most common form of systemic amyloidosis behind amyloidosis secondary to monoclonal Ig. It is the result of deposition of insoluble fibrils in the extracellular space of tissues and organs derived from the precursor protein serum amyloid A, an acute phase reactant synthesized excessively in the setting of chronic inflammation. The kidney is the most frequent organ involved. Most patients present with proteinuria and kidney failure. The diagnosis is made through tissue biopsy with involvement of the glomeruli in most cases, but also often of the vessels and the tubulointerstitial compartment. The treatment usually targets the underlying etiology and consists increasingly of blocking the inflammatory cascade of cytokines with interleukin-1 inhibitors, interleukin-6 inhibitors, and tumor necrosis factor-α inhibitors to reduce serum amyloid A protein formation. This strategy has also shown efficacy in cases where an underlying etiology cannot be readily identified and has significantly improved the prognosis of this entity. In addition, there has been increased interest at developing effective therapies able to clear amyloid deposits from tissues, albeit with mitigated results so far.

Role of Serum Amyloid A in Abdominal Aortic Aneurysm and Related Cardiovascular Diseases

Epidemiological data positively correlate plasma serum amyloid A (SAA) levels with cardiovascular disease severity and mortality. Studies by several investigators have indicated a causal role for SAA in the development of atherosclerosis in animal models. Suppression of SAA attenuates the development of angiotensin II (AngII)-induced abdominal aortic aneurysm (AAA) formation in mice. Thus, SAA is not just a marker for cardiovascular disease (CVD) development, but it is a key player. However, to consider SAA as a therapeutic target for these diseases, the pathway leading to its involvement needs to be understood. This review provides a brief description of the pathobiological significance of this enigmatic molecule. The purpose of this review is to summarize the data relevant to its role in the development of CVD, the pitfalls in SAA research, and unanswered questions in the field.

New insights into the emerging effects of inflammatory response on HDL particles structure and function

According to the increasing results, it has been well-demonstrated that the chronic inflammatory response, including systemic lupus erythematosus, rheumatoid arthritis, and inflammatory bowel disease are associated with an increased risk of atherosclerotic cardiovascular disease. The mechanism whereby inflammatory response up-regulates the risk of cardio-metabolic disorder disease is multifactorial; furthermore, the alterations in high density lipoprotein (HDL) structure and function which occur under the inflammatory response could play an important modulatory function. On the other hand, the serum concentrations of HDL cholesterol (HDL-C) have been shown to be reduced significantly under inflammatory status with remarked alterations in HDL particles. Nevertheless, the potential mechanism whereby the inflammatory response reduces serum HDL-C levels is not simply defined but reduces apolipoprotein A1 production. The alterations in HDL structure mediated by the inflammatory response has been also confirmed to decrease the ability of HDL particle to play an important role in reverse cholesterol transport and protect the LDL particles from oxidation. Recently, it has been shown that under the inflammatory condition, diverse alterations in HDL structure could be observed which lead to changes in HDL function. In the current review, the emerging effects of inflammatory response on HDL particles structure and function are well-summarized to elucidate the potential mechanism whereby different inflammatory status modulates the pathogenic development of dyslipidemia.

Sexually Dimorphic Relationships Among Saa3 (Serum Amyloid A3), Inflammation, and Cholesterol Metabolism Modulate Atherosclerosis in Mice

[Figure: see text].

High-Density Lipoproteins and Serum Amyloid A (SAA)

PURPOSE OF REVIEW

Serum amyloid A (SAA) is a highly sensitive acute phase reactant that has been linked to a number of chronic inflammatory diseases. During a systemic inflammatory response, liver-derived SAA is primarily found on high-density lipoprotein (HDL). The purpose of this review is to discuss recent literature addressing the pathophysiological functions of SAA and the significance of its association with HDL.

RECENT FINDINGS

Studies in gene-targeted mice establish that SAA contributes to atherosclerosis and some metastatic cancers. Accumulating evidence indicates that the lipidation state of SAA profoundly affects its bioactivities, with lipid-poor, but not HDL-associated, SAA capable of inducing inflammatory responses in vitro and in vivo. Factors that modulate the equilibrium between lipid-free and HDL-associated SAA have been identified. HDL may serve to limit SAA's bioactivities in vivo. Understanding the factors leading to the release of systemic SAA from HDL may provide insights into chronic disease mechanisms.

Synergy between serum amyloid A and secretory phospholipase A2

Serum amyloid A (SAA) is an evolutionally conserved enigmatic biomarker of inflammation. In acute inflammation, SAA plasma levels increase ~1,000 fold, suggesting that this protein family has a vital beneficial role. SAA increases simultaneously with secretory phospholipase A₂ (sPLA₂), compelling us to determine how SAA influences sPLA₂ hydrolysis of lipoproteins. SAA solubilized phospholipid bilayers to form lipoproteins that provided substrates for sPLA₂. Moreover, SAA sequestered free fatty acids and lysophospholipids to form stable proteolysis-resistant complexes. Unlike albumin, SAA effectively removed free fatty acids under acidic conditions, which characterize inflammation sites. Therefore, SAA solubilized lipid bilayers to generate substrates for sPLA₂ and removed its bioactive products. Consequently, SAA and sPLA₂ can act synergistically to remove cellular membrane debris from injured sites, which is a prerequisite for tissue healing. We postulate that the removal of lipids and their degradation products constitutes a vital primordial role of SAA in innate immunity; this role remains to be tested in vivo.

Cell boundaries are made up of fatty substances known as lipids. When cells get severely damaged, their lipid membranes break apart. These broken fragments of membrane become highly toxic, and must be removed as soon as possible to allow the tissue to heal. A small protein called serum amyloid A, SAA for short, was recently proposed to play a pivotal role in this process. In humans, SAA levels in the blood rapidly spike to over a thousand times their normal level following inflammation, injury or infection. Combined with the fact SAA has been conserved for over 500 million years, this suggests that SAA must be important for survival. But, it is not entirely clear how this protein works.

One clue for how SAA works is its relationship to another ancient protein called secretory phospholipase A₂. This protein, also known as sPLA₂, is part of a big family of enzymes that break down lipids in the cell membrane. Notably, sPLA₂ levels rise at the same time and place as SAA during inflammation. This led Jayaraman et al. to ask whether SAA and sPLA₂ might be working together to clean up the cell membrane debris.

To find out, Jayaraman et al. mixed mouse SAA with vesicles of membrane lipids, and then added sPLA₂. This revealed that SAA reshapes the lipid membrane into smaller ‘nanoparticles’ with tightly curved surfaces that are easier for sPLA₂ to break down. As the sPLA₂ breaks up these particles, SAA then gathers up and gets rid of the leftover toxic fragments. This suggests that SAA has two roles: helping sPLA₂ break down the membrane, and removing any toxic debris.

Clearing debris after injury is essential for proper healing. So, understanding how it works is crucial to find new ways to treat inflammation. Further work to understand SAA and sPLA₂ could improve our understanding of how to treat acute and chronic inflammation and its life-threatening complications.

Apoproteins E, A-I, and SAA in Macrophage Pathobiology Related to Atherogenesis

Macrophages are core cellular elements of both early and advanced atherosclerosis. They take up modified lipoproteins and become lipid-loaded foam cells and secrete factors that influence other cell types in the artery wall involved in atherogenesis. Apoproteins E, AI, and SAA are all found on HDL which can enter the artery wall. In addition, apoE is synthesized by macrophages. These three apoproteins can promote cholesterol efflux from lipid-loaded macrophages and have other functions that modulate macrophage biology. Mimetic peptides based on the sequence or structure of these apoproteins replicate some of these properties and are potential therapeutic agents for the treatment of atherosclerosis to reduce cardiovascular diseases.

Structure and Expression of Different Serum Amyloid A (SAA) Variants and their Concentration-Dependent Functions During Host Insults

Serum amyloid A (SAA) is, like C-reactive protein (CRP), an acute phase protein and can be used as a diagnostic, prognostic or therapy follow-up marker for many diseases. Increases in serum levels of SAA are triggered by physical insults to the host, including infection, trauma, inflammatory reactions and cancer. The order of magnitude of increase in SAA levels varies considerably, from a 10- to 100-fold during limited inflammatory events to a 1000-fold increase during severe bacterial infections and acute exacerbations of chronic inflammatory diseases. This broad response range is reflected by SAA gene duplications resulting in a cluster encoding several SAA variants and by multiple biological functions of SAA. SAA variants are single-domain proteins with simple structures and few post-translational modifications. SAA1 and SAA2 are inducible by inflammatory cytokines, whereas SAA4 is constitutively produced. We review here the regulated expression of SAA in normal and transformed cells and compare its serum levels in various disease states. At low concentrations (10-100 ng/ml), early in an inflammatory response, SAA induces chemokines or matrix degrading enzymes via Toll-like receptors and functions as an activator and chemoattractant through a G protein-coupled receptor. When an infectious or inflammatory stimulus persists, the liver continues to produce more SAA (> 1000 ng/ml) to become an antimicrobial agent by functioning as a direct opsonin of bacteria or by interference with virus infection of host cells. Thus, SAA regulates innate and adaptive immunity and this information may help to design better drugs to treat specific diseases.

Inflammation, remodeling, and other factors affecting HDL cholesterol efflux

PURPOSE OF REVIEW

The ability of HDL to promote cholesterol efflux from macrophages is a predictor of cardiovascular risk independent of HDL cholesterol levels. However, the molecular determinants of HDL cholesterol efflux capacity (CEC) are largely unknown.

RECENT FINDINGS

The term HDL defines a heterogeneous population of particles with distinct size, shape, protein, and lipid composition. Cholesterol efflux is mediated by multiple pathways that may be differentially modulated by HDL composition. Furthermore, different subpopulations of HDL particles mediate CEC via specific pathways, but the molecular determinants of CEC, either proteins or lipids, are unclear. Inflammation promotes a profound remodeling of HDL and impairs overall HDL CEC while improving ATP-binding cassette transporter G1-mediated efflux. This review discusses recent findings that connect HDL composition and CEC.

SUMMARY

Data from recent animal and human studies clearly show that multiple factors associate with CEC including individual proteins, lipid composition, as well as specific particle subpopulations. Although acute inflammation remodels HDL and impairs CEC, chronic inflammation has more subtle effects. Standardization of assays measuring HDL composition and CEC is a necessary prerequisite for understanding the factors controlling HDL CEC. Unraveling these factors may help the development of new therapeutic interventions improving HDL function.

Quercetin attenuates high fructose feeding-induced atherosclerosis by suppressing inflammation and apoptosis via ROS-regulated PI3K/AKT signaling pathway

Quercetin is a dietary flavonoid compound extracted from various plants, such as apple and onions. Previous studies have revealed its anti-inflammatory, anti-cancer, antioxidant and anti-apoptotic activities. This study investigated the ability of quercetin to inhibit high fructose feeding- or LPS-induced atherosclerosis through regulating oxidative stress, apoptosis and inflammation response in vivo and in vitro experiments. 50 and 100mg/kg quercetin were used in our study, showing significant inhibitory role in high fructose-induced atherosclerosis via reducing reactive oxygen species (ROS) levels, Caspase-3 activation, inflammatory cytokines releasing, the number of terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL)-positive cells and collagen contents as well as modulating apoptosis- and inflammation-related proteins expression. We also explored the protective effects of quercetin on atherosclerosis by phosphatidylinositide 3-kinases (PI3K)/Protein kinase B (AKT)-associated Bcl-2/Caspase-3 and nuclear factor kappa B (NF-κB) signal pathways activation, promoting AKT and Bcl-2 expression and reducing Caspase-3 and NF-κB activation. Quercetin reduced the atherosclerotic plaque size in vivo in high fructose feeding-induced mice assessed by oil red O. Also, in vitro experiments, quercetin displayed inhibitory role in LPS-induced ROS production, inflammatory response and apoptosis, which were linked with PI3K/AKT-regulated Caspase-3 and NF-κB activation. In conclusion, our results showed that quercetin inhibited atherosclerotic plaque development in high fructose feeding mice via PI3K/AKT activation regulated by ROS.

Effect of inflammation on HDL structure and function

PURPOSE OF REVIEW

Studies have shown that chronic inflammatory disorders, such as rheumatoid arthritis, systemic lupus erythematosus, and psoriasis are associated with an increased risk of atherosclerotic cardiovascular disease. The mechanism by which inflammation increases cardiovascular disease is likely multifactorial but changes in HDL structure and function that occur during inflammation could play a role.

RECENT FINDINGS

HDL levels decrease with inflammation and there are marked changes in HDL-associated proteins. Serum amyloid A markedly increases whereas apolipoprotein A-I, lecithin:cholesterol acyltransferase, cholesterol ester transfer protein, paraoxonase 1, and apolipoprotein M decrease. The exact mechanism by which inflammation decreases HDL levels is not defined but decreases in apolipoprotein A-I production, increases in serum amyloid A, increases in endothelial lipase and secretory phospholipase A2 activity, and decreases in lecithin:cholesterol acyltransferase activity could all contribute. The changes in HDL induced by inflammation reduce the ability of HDL to participate in reverse cholesterol transport and protect LDL from oxidation.

SUMMARY

During inflammation multiple changes in HDL structure occur leading to alterations in HDL function. In the short term, these changes may be beneficial resulting in an increase in cholesterol in peripheral cells to improve host defense and repair but over the long term these changes may increase the risk of atherosclerosis.

Serum amyloid A and atherosclerosis

PURPOSE OF REVIEW

Atherosclerosis is a chronic inflammation associated with increased expression of the acute phase isoforms of serum amyloid A (SAA) and in humans is a plasma biomarker for future cardiovascular events. However, whether SAA is only a biomarker or participates in the development of cardiovascular disease is not well characterized. The purpose of this review is to summarize putative functions of SAA relevant to atherogenesis and in-vivo murine studies that directly examine the effect of SAA on atherosclerosis.

RECENT FINDINGS

Modulation of the expression of SAA1 and/or SAA2 in murine models of atherosclerosis suggests that SAA promotes early atherogenesis. SAA secreted from bone-marrow-derived cells contributes to this antiatherogenic phenotype. SAA also promotes angiotensin-induced abdominal aneurysm in atherogenic mouse models. The reduction in atherosclerosis may be due, at least in part, to remodeling of the acute phase HDL to reduce its capacity to promote cholesterol efflux and reduce its anti-inflammatory ability.

SUMMARY

SAA is more than a marker of cardiovascular disease and is a participant in the early atherogenic process.

Impact of individual acute phase serum amyloid A isoforms on HDL metabolism in mice

The acute phase (AP) reactant serum amyloid A (SAA), an HDL apolipoprotein, exhibits pro-inflammatory activities, but its physiological function(s) are poorly understood. Functional differences between SAA1.1 and SAA2.1, the two major SAA isoforms, are unclear. Mice deficient in either isoform were used to investigate plasma isoform effects on HDL structure, composition, and apolipoprotein catabolism. Lack of either isoform did not affect the size of HDL, normally enlarged in the AP, and did not significantly change HDL composition. Plasma clearance rates of HDL apolipoproteins were determined using native HDL particles. The fractional clearance rates (FCRs) of apoA-I, apoA-II, and SAA were distinct, indicating that HDL is not cleared as intact particles. The FCRs of SAA1.1 and SAA2.1 in AP mice were similar, suggesting that the selective deposition of SAA1.1 in amyloid plaques is not associated with a difference in the rates of plasma clearance of the isoforms. Although the clearance rate of SAA was reduced in the absence of the HDL receptor, scavenger receptor class B type I (SR-BI), it remained significantly faster compared with that of apoA-I and apoA-II, indicating a relatively minor role of SR-BI in SAA's rapid clearance. These studies enhance our understanding of SAA metabolism and SAA's effects on AP-HDL composition and catabolism.

Caveolae and Caveolin-1 Integrate Reverse Cholesterol Transport and Inflammation in Atherosclerosis

Lipid disorder and inflammation play critical roles in the development of atherosclerosis. Reverse cholesterol transport is a key event in lipid metabolism. Caveolae and caveolin-1 are in the center stage of cholesterol transportation and inflammation in macrophages. Here, we propose that reverse cholesterol transport and inflammation in atherosclerosis can be integrated by caveolae and caveolin-1.

Serum amyloid A1: Structure, function and gene polymorphism

Inducible expression of serum amyloid A (SAA) is a hallmark of the acute-phase response, which is a conserved reaction of vertebrates to environmental challenges such as tissue injury, infection and surgery. Human SAA1 is encoded by one of the four SAA genes and is the best-characterized SAA protein. Initially known as a major precursor of amyloid A (AA), SAA1 has been found to play an important role in lipid metabolism and contributes to bacterial clearance, the regulation of inflammation and tumor pathogenesis. SAA1 has five polymorphic coding alleles (SAA1.1-SAA1.5) that encode distinct proteins with minor amino acid substitutions. Single nucleotide polymorphism (SNP) has been identified in both the coding and non-coding regions of human SAA1. Despite high levels of sequence homology among these variants, SAA1 polymorphisms have been reported as risk factors of cardiovascular diseases and several types of cancer. A recently solved crystal structure of SAA1.1 reveals a hexameric bundle with each of the SAA1 subunits assuming a 4-helix structure stabilized by the C-terminal tail. Analysis of the native SAA1.1 structure has led to the identification of a competing site for high-density lipoprotein (HDL) and heparin, thus providing the structural basis for a role of heparin and heparan sulfate in the conversion of SAA1 to AA. In this brief review, we compares human SAA1 with other forms of human and mouse SAAs, and discuss how structural and genetic studies of SAA1 have advanced our understanding of the physiological functions of the SAA proteins.

Response of layer and broiler strain chickens to parenteral administration of a live Salmonella Typhimurium vaccine

Responses to the parenteral administration of a live aroA deletion Salmonella serovar Typhimurium vaccine given to three brown egg layer strains and two broiler strains were studied. Twenty-five birds of each strain were reared together in floor pens to 6 weeks of age and then moved as individual strains to new floor pens and injected with 10(8) colony forming units (CFU) per bird of the vaccine bacteria intramuscularly or subcutaneously, 10(6) CFU per bird subcutaneously, or phosphate buffered saline (PBS) subcutaneously as a vaccination control. Three birds of one layer strain were injected intramuscularly with 0.5mg/ bird S. Typhimurium lipopolysaccharide (LPS) to evaluate whether response was similar for vaccine and endotoxin. Birds were weighed, and rectal temperatures recorded at the time of injection, then observed over 24 hours. Rectal temperatures were measured and blood samples collected for serum IL-6 assay at 3 hours post injection (PI). At 12 hours PI blood samples were drawn for analyses for plasma phosphorus (P), glucose (Glu), cholesterol (Cho), aspartate transaminase (AST), total protein (Ptn) and creatinine kinase (CK). Blood was sampled 14 days PI and tested for serum antibody to S. Typhimurium. Vaccination resulted in significant seroconversion by 14 days PI in all strains compared to the controls. The three layer strains exhibited a clinical malaise, evident within 90 minutes of injection, lasting for 12 hours, with complete recovery by 24 hours PI. Only the 10(8) CFU dose given subcutaneously produced an increase in rectal temperature 3 hours PI. Vaccination had no effect on IL-6 or Ptn. All vaccine doses increased P and the higher dose by either route decreased Cho in all bird strains. The 10(8) vaccine dose increased Glu and intramuscular injection markedly elevated CK only in the layer strains. The response was not completely congruous with that to LPS alone. The results highlight the need for consideration of differences in response of bird strain when consideration is given to the parenteral administration of live Salmonella vaccines.

Inflammatory remodeling of the HDL proteome impairs cholesterol efflux capacity

Recent studies demonstrate that HDL's ability to promote cholesterol efflux from macrophages associates strongly with cardioprotection in humans independently of HDL-cholesterol (HDL-C) and apoA-I, HDL's major protein. However, the mechanisms that impair cholesterol efflux capacity during vascular disease are unclear. Inflammation, a well-established risk factor for cardiovascular disease, has been shown to impair HDL's cholesterol efflux capacity. We therefore tested the hypothesis that HDL's impaired efflux capacity is mediated by specific changes of its protein cargo. Humans with acute inflammation induced by low-level endotoxin had unchanged HDL-C levels, but their HDL-C efflux capacity was significantly impaired. Proteomic analyses demonstrated that HDL's cholesterol efflux capacity correlated inversely with HDL content of serum amyloid A (SAA)1 and SAA2. In mice, acute inflammation caused a marked impairment of HDL-C efflux capacity that correlated with a large increase in HDL SAA. In striking contrast, the efflux capacity of mouse inflammatory HDL was preserved with genetic ablation of SAA1 and SAA2. Our observations indicate that the inflammatory impairment of HDL-C efflux capacity is due in part to SAA-mediated remodeling of HDL's protein cargo.

Deficiency of Endogenous Acute-Phase Serum Amyloid A Protects apoE-/- Mice From Angiotensin II-Induced Abdominal Aortic Aneurysm Formation

OBJECTIVE

Rupture of abdominal aortic aneurysm (AAA), a major cause of death in the aged population, is characterized by vascular inflammation and matrix degradation. Serum amyloid A (SAA), an acute-phase reactant linked to inflammation and matrix metalloproteinase induction, correlates with aortic dimensions before aneurysm formation in humans. We investigated whether SAA deficiency in mice affects AAA formation during angiotensin II (Ang II) infusion.

APPROACH AND RESULTS

Plasma SAA increased ≈60-fold in apoE(-/-) mice 24 hours after intraperitoneal Ang II injection (100 μg/kg; n=4) and ≈15-fold after chronic 28-day Ang II infusion (1000 ng/kg per minute; n=9). AAA incidence and severity after 28-day Ang II infusion was significantly reduced in apoE(-/-) mice lacking both acute-phase SAA isoforms (SAAKO; n=20) compared with apoE(-/-) mice (SAAWT; n=20) as assessed by in vivo ultrasound and ex vivo morphometric analyses, despite a significant increase in systolic blood pressure in SAAKO mice compared with SAAWT mice after Ang II infusion. Atherosclerotic lesion area of the aortic arch was similar in SAAKO and SAAWT mice after 28-day Ang II infusion. Immunostaining detected SAA in AAA tissues of Ang II-infused SAAWT mice that colocalized with macrophages, elastin breaks, and enhanced matrix metalloproteinase activity. Matrix metalloproteinase-2 activity was significantly lower in aortas of SAAKO mice compared with SAAWT mice after 10-day Ang II infusion.

CONCLUSIONS

Lack of endogenous acute-phase SAA protects against experimental AAA through a mechanism that may involve reduced matrix metalloproteinase-2 activity.

Alginic acid cell entrapment: a novel method for measuring in vivo macrophage cholesterol homeostasis

Macrophage conversion to atherosclerotic foam cells is partly due to the balance of uptake and efflux of cholesterol. Cholesterol efflux from cells by HDL and its apoproteins for subsequent hepatic elimination is known as reverse cholesterol transport. Numerous methods have been developed to measure in vivo macrophage cholesterol efflux. Most methods do not allow for macrophage recovery for analysis of changes in cellular cholesterol status. We describe a novel method for measuring cellular cholesterol balance using the in vivo entrapment of macrophages in alginate, which retains incorporated cells while being permeable to lipoproteins. Recipient mice were injected subcutaneously with CaCl2 forming a bubble into which a macrophage/alginate suspension was injected, entrapping the macrophages. Cells were recovered after 24 h. Cellular free and esterified cholesterol mass were determined enzymatically and normalized to cellular protein. Both normal and cholesterol loaded macrophages undergo measureable changes in cell cholesterol when injected into WT and apoA-I-, LDL-receptor-, or apoE-deficient mice. Cellular cholesterol balance is dependent on initial cellular cholesterol status, macrophage cholesterol transporter expression, and apolipoprotein deficiency. Alginate entrapment allows for the in vivo measurement of macrophage cholesterol homeostasis and is a novel platform for investigating the role of genetics and therapeutic interventions in atherogenesis.

A brief elevation of serum amyloid A is sufficient to increase atherosclerosis

Serum amyloid A (SAA) has a number of proatherogenic effects including induction of vascular proteoglycans. Chronically elevated SAA was recently shown to increase atherosclerosis in mice. The purpose of this study was to determine whether a brief increase in SAA similarly increased atherosclerosis in a murine model. The recombination activating gene 1-deficient (rag1(-/-)) × apolipoprotein E-deficient (apoe(-/-)) and apoe(-/-) male mice were injected, multiple times or just once respectively, with an adenoviral vector encoding human SAA1 (ad-SAA); the injected mice and controls were maintained on chow for 12-16 weeks. Mice receiving multiple injections of ad-SAA, in which SAA elevation was sustained, had increased atherosclerosis compared with controls. Strikingly, mice receiving only a single injection of ad-SAA, in which SAA was only briefly elevated, also had increased atherosclerosis compared with controls. Using in vitro studies, we demonstrate that SAA treatment leads to increased LDL retention, and that prevention of transforming growth factor beta (TGF-β) signaling prevents SAA-induced increases in LDL retention and SAA-induced increases in vascular biglycan content. We propose that SAA increases atherosclerosis development via induction of TGF-β, increased vascular biglycan content, and increased LDL retention. These data suggest that even short-term inflammation with concomitant increase in SAA may increase the risk of developing CVD.

Adipose tissue-derived human serum amyloid a does not affect atherosclerotic lesion area in hSAA1+/-/ApoE-/- mice

Chronically elevated serum levels of serum amyloid A (SAA) are linked to increased risk of cardiovascular disease. However, whether SAA is directly involved in atherosclerosis development is still not known. The aim of this study was to investigate the effects of adipose tissue-derived human SAA on atherosclerosis in mice. hSAA1+/- transgenic mice (hSAA1 mice) with a specific expression of human SAA1 in adipose tissue were bred with ApoE-deficient mice. The hSAA1 mice and their wild type (wt) littermates were fed normal chow for 35 weeks. At the end of the experiment, the mice were euthanized and blood, gonadal adipose tissue and aortas were collected. Plasma levels of SAA, cholesterol and triglycerides were measured. Atherosclerotic lesion areas were analyzed in the aortic arch, the thoracic aorta and the abdominal aorta in en face preparations of aorta stained with Sudan IV. The human SAA protein was present in plasma from hSAA1 mice but undetectable in wt mice. Similar plasma levels of cholesterol and triglycerides were observed in hSAA1 mice and their wt controls. There were no differences in atherosclerotic lesion areas in any sections of the aorta in hSAA1 mice compared to wt mice. In conclusion, our data suggest that adipose tissue-derived human SAA does not influence atherosclerosis development in mice.

Deficiency of endogenous acute phase serum amyloid A does not affect atherosclerotic lesions in apolipoprotein E-deficient mice

OBJECTIVE

Although elevated plasma concentrations of serum amyloid A (SAA) are associated strongly with increased risk for atherosclerotic cardiovascular disease in humans, the role of SAA in the pathogenesis of lesion formation remains obscure. Our goal was to determine the impact of SAA deficiency on atherosclerosis in hypercholesterolemic mice.

APPROACH AND RESULTS

Apolipoprotein E-deficient (apoE(-/-)) mice, either wild type or deficient in both major acute phase SAA isoforms, SAA1.1 and SAA2.1, were fed a normal rodent diet for 50 weeks. Female mice, but not male apoE-/- mice deficient in SAA1.1 and SAA2.1, had a modest increase (22%; P≤0.05) in plasma cholesterol concentrations and a 53% increase in adipose mass compared with apoE-/- mice expressing SAA1.1 and SAA2.1 that did not affect the plasma cytokine levels or the expression of adipose tissue inflammatory markers. SAA deficiency did not affect lipoprotein cholesterol distributions or plasma triglyceride concentrations in either male or female mice. Atherosclerotic lesion areas measured on the intimal surfaces of the arch, thoracic, and abdominal regions were not significantly different between apoE-/- mice deficient in SAA1.1 and SAA2.1 and apoE-/- mice expressing SAA1.1 and SAA2.1 in either sex. To accelerate lesion formation, mice were fed a Western diet for 12 weeks. SAA deficiency had effect neither on diet-induced alterations in plasma cholesterol, triglyceride, or cytokine concentrations nor on aortic atherosclerotic lesion areas in either male or female mice. In addition, SAA deficiency in male mice had no effect on lesion areas or macrophage accumulation in the aortic roots.

CONCLUSIONS

The absence of endogenous SAA1.1 and 2.1 does not affect atherosclerotic lipid deposition in apolipoprotein E-deficient mice fed either normal or Western diets.