Serum amyloid A, but not C-reactive protein, stimulates vascular proteoglycan synthesis in a pro-atherogenic manner

Serum Amyloid A: A Double-Edged Sword in Health and Disease

Despite more than fifty years since its discovery in the 1970s, Serum Amyloid A (SAA)'s true biological functions remain enigmatic. The research so far has primarily associated SAA with chronic inflammatory conditions such as cardiovascular disease, obesity, and type 2 diabetes; its role in acute inflammation is less understood. Unlike the modest elevations observed in chronic conditions, SAA levels surge dramatically during acute inflammatory responses. Notably, approximately 2.5% of hepatic protein synthesis is devoted to SAA production during acute inflammation-despite the high energy demands required for synthesizing pro-inflammatory cytokines and immune cell activation-leaving its precise necessity unclear. Elucidating SAA's physiological role in acute inflammation is crucial to determine the therapeutic potential of SAA inhibition for chronic inflammatory diseases, such as atherosclerosis and abdominal aortic aneurysms. The evidence suggests that SAA may play a protective role in acute inflammation, positioning it as a "double-edged sword": detrimental in chronic inflammation, yet potentially beneficial in acute settings. This review explores the divergent roles of SAA in chronic versus acute inflammation, proposing that while SAA inhibition could offer therapeutic benefits for chronic conditions, it might pose risks during acute inflammation. As the primary transporter of SAA in circulation, high-density lipoprotein (HDL) has been shown to become dysfunctional in chronic inflammation, at least partly due to SAA's effects. However, we propose that SAA may confer functional properties to HDL during acute inflammatory states, such as sepsis, thereby highlighting the context-dependent nature of its impact.

Podoplanin Positive Cell-derived Extracellular Vesicles Contribute to Cardiac Amyloidosis After Myocardial Infarction

Background

Amyloidosis is a major long-term complication of chronic disease; however, whether it represents one of the complications of post-myocardial infarction (MI) is yet to be fully understood.

Methods

Using wild-type and knocked-out MI mouse models and characterizing in vitro the exosomal communication between bone marrow-derived macrophages and activated mesenchymal stromal cells (MSC) isolated after MI, we investigated the mechanism behind Serum Amyloid A 3 (SAA3) protein overproduction in injured hearts.

Results

Here, we show that amyloidosis occurs after MI and that amyloid fibers are composed of macrophage-derived SAA3 monomers. SAA3 overproduction in macrophages is triggered by exosomal communication from a subset of activated MSC, which, in response to MI, acquire the expression of a platelet aggregation-inducing type I transmembrane glycoprotein named Podoplanin (PDPN). Cardiac MSC PDPN+ communicate with and activate macrophages through their extracellular vesicles or exosomes. Specifically, MSC PDPN+ derived exosomes (MSC PDPN+ Exosomes) are enriched in SAA3 and exosomal SAA3 protein engages with Toll-like receptor 2 (TRL2) on macrophages, triggering an overproduction and impaired clearance of SAA3 proteins, resulting in aggregation of SAA3 monomers as rigid amyloid deposits in the extracellular space. The onset of amyloid fibers deposition alongside extra-cellular-matrix (ECM) proteins in the ischemic heart exacerbates the rigidity and stiffness of the scar, hindering the contractility of viable myocardium and overall impairing organ function. Using SAA3 and TLR2 deficient mouse models, we show that SAA3 delivered by MSC PDPN+ exosomes promotes post-MI amyloidosis. Inhibition of SAA3 aggregation via administration of a retro-inverso D-peptide, specifically designed to bind SAA3 monomers, prevents the deposition of SAA3 amyloid fibrils, positively modulates the scar formation, and improves heart function post-MI.

Conclusion

Overall, our findings provide mechanistic insights into post-MI amyloidosis and suggest that SAA3 may be an attractive target for effective scar reversal after ischemic injury and a potential target in multiple diseases characterized by a similar pattern of inflammation and amyloid deposition.

NOVELTY AND SIGNIFICANCE

What is known? Accumulation of rigid amyloid structures in the left ventricular wall impairs ventricle contractility.After myocardial infarction cardiac Mesenchymal Stromal Cells (MSC) acquire Podoplanin (PDPN) to better interact with immune cells.Amyloid structures can accumulate in the heart after chronic inflammatory conditions. What information does this article contribute? Whether accumulation of cumbersome amyloid structures in the ischemic scar impairs left ventricle contractility, and scar reversal after myocardial infarction (MI) has never been investigated.The pathophysiological relevance of PDPN acquirement by MSC and the functional role of their secreted exosomes in the context of post-MI cardiac remodeling has not been investigated.Amyloid structures are present in the scar after ischemia and are composed of macrophage-derived Serum Amyloid A (SAA) 3 monomers, although mechanisms of SAA3 overproduction is not established.

SUMMARY OF NOVELTY AND SIGNIFICANCE

Here, we report that amyloidosis, a secondary phenomenon of an already preexisting and prolonged chronic inflammatory condition, occurs after MI and that amyloid structures are composed of macrophage-derived SAA3 monomers. Frequently studied cardiac amyloidosis are caused by aggregation of immunoglobulin light chains, transthyretin, fibrinogen, and apolipoprotein in a healthy heart as a consequence of systemic chronic inflammation leading to congestive heart failure with various types of arrhythmias and tissue stiffness. Although chronic MI is considered a systemic inflammatory condition, studies regarding the possible accumulation of amyloidogenic proteins after MI and the mechanisms involved in that process are yet to be reported. Here, we show that SAA3 overproduction in macrophages is triggered in a Toll-like Receptor 2 (TLR2)-p38MAP Kinase-dependent manner by exosomal communication from a subset of activated MSC, which, in response to MI, express a platelet aggregation-inducing type I transmembrane glycoprotein named Podoplanin. We provide the full mechanism of this phenomenon in murine models and confirm SAA3 amyloidosis in failing human heart samples. Moreover, we developed a retro-inverso D-peptide therapeutic approach, "DRI-R5S," specifically designed to bind SAA3 monomers and prevent post-MI aggregation and deposition of SAA3 amyloid fibrils without interfering with the innate immune response.

Multi-proteomic Biomarker Risk Scores for Predicting Risk and Guiding Therapy in Patients with Coronary Artery Disease

PURPOSE OF REVIEW

Patients with established coronary artery disease (CAD) are at high residual risk for adverse events, despite guideline-based treatments. Herein, we aimed to determine whether risk scores based on multiple circulating biomarkers that represent activation of various pathophysiologically important pathways involved in atherosclerosis and myocardial dysfunction help identify those at greatest residual risk.

RECENT FINDINGS

Numerous circulating proteins, representing dysregulation of the pathways involved in the development and stability of coronary and myocardial diseases, have been identified. When aggregated together, biomarker risk scores (BRS) more accurately stratify patients with established CAD that may help target interventions in those individuals who are at elevated risk. Moreover, intensification of guideline-based therapies has been associated with parallel improvements in both BRS and outcomes, indicating that these risk scores may be employed clinically to target therapy. Multi-protein BRS are predictive of risk, independent of, and in addition to traditional risk factor assessments in patients with CAD. Those with elevated risk may benefit from optimization of therapies, and improvements in the BRS will identify those with improved outcomes.

The Impact of Aerobic Exercise on HDL Quantity and Quality: A Narrative Review

High-density lipoproteins comprise roughly 25-30% of the circulating proteins involved in the transport of lipids in circulation. These particles differ in size and lipid composition. Recent evidence suggests that the quality of HDL particles (which depends on shape, size and the composition of proteins and lipids determining HDL functionality) may be more important than their quantity. The functionality of HDL is mirrored by its cholesterol efflux activity, as well as its antioxidant (including the protection of LDL against oxidation), anti-inflammatory and antithrombotic properties. The results of many studies and meta-analyses imply the beneficial impact of aerobic exercise on HDL-C levels. Physical activity was found to be usually associated with an increase in HDL cholesterol and a decrease in LDL cholesterol and triglycerides. Exercise, apart from inducing quantitative alterations in serum lipids, exerts a beneficial impact on HDL particle maturation, composition and functionality. The Physical Activity Guidelines Advisory Committee Report underlined the importance of establishing a program recommending exercises that enable attainment of maximal advantage at the lowest level of risk. The aim of this manuscript is to review the impact of different types of aerobic exercise (various intensities and durations) on the level and quality of HDL.

Acute-serum amyloid A and A-SAA-derived peptides as formyl peptide receptor (FPR) 2 ligands

Originally, it was thought that a single serum amyloid A (SAA) protein was involved in amyloid A amyloidosis, but in fact, SAA represents a four-membered family wherein SAA1 and SAA2 are acute phase proteins (A-SAA). SAA is highly conserved throughout evolution within a wide range of animal species suggestive of an important biological function. In fact, A-SAA has been linked to a number of divergent biological activities wherein a number of these functions are mediated via the G protein-coupled receptor (GPCR), formyl peptide receptor (FPR) 2. For instance, through the activation of FPR2, A-SAA has been described to regulate leukocyte activation, atherosclerosis, pathogen recognition, bone formation and cell survival. Moreover, A-SAA is subject to post-translational modification, primarily through proteolytic processing, generating a range of A-SAA-derived peptides. Although very little is known regarding the biological effect of A-SAA-derived peptides, they have been shown to promote neutrophil and monocyte migration through FPR2 activation via synergy with other GPCR ligands namely, the chemokines CXCL8 and CCL3, respectively. Within this review, we provide a detailed analysis of the FPR2-mediated functions of A-SAA. Moreover, we discuss the potential role of A-SAA-derived peptides as allosteric modulators of FPR2.

Serum amyloid A, ferritin and carcinoembryonic antigen as biomarkers of severity in patients with COVID-19

In view of the rapid spread of COVID-19 and the high mortality rate of severe cases, reliable risk stratifying indicators of prognosis are necessary to decrease morbidity and mortality. The aim of the present study was to evaluate the value of serum amyloid A (SAA) and carcinoembryonic antigen (CEA) as prognostic biomarkers in comparison to other predictors, including C-reactive protein (CRP) and ferritin levels. This study included 124 patients diagnosed with COVID-19, and they were assigned to one of two groups: Mild and severe, based on the severity of the infection. Radiological and laboratory investigations were performed, including evaluation of CRP, ferritin, D-Dimer, SAA and CEA levels. Significantly higher levels of CRP, ferritin, D-Dimer, SAA and CEA were observed in severe cases. SAA was significantly correlated with CRP (r=0.422, P<0.001), ferritin (r=0.574, P<0.001), CEA (r=0.514, P<0.001) and computed tomography severity score (CT-SS; r=0.691, P<0.001). CEA was correlated with CRP (r=0.441, P<0.001), ferritin (r=0.349, P<0.001) and CT-SS (r=0.374, P<0.001). Receiver operator characteristic (ROC) curves for performance of SAA, CEA, ferritin, CRP and SAA showed the highest AUC value of 0.928, with a specificity of 93.1%, and a sensitivity of 98.5% at a cut-off of 16 mg/l. The multi-ROC curve for SAA and ferritin showed 100% specificity, 100% sensitivity and 100% efficiency, with an AUC of 1.000. Thus, combining SAA and ferritin may have guiding significance for predicting COVID-19 severity. SAA alone showed the highest prognostic significance. Both SAA and CEA were positively correlated with the CT-SS. Early monitoring of these laboratory markers may thus provide significant input for halting disease progression and reducing mortality rates.

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.

Serum Amyloid A Is Present in Human Saccular Intracranial Aneurysm Walls and Associates With Aneurysm Rupture

Saccular intracranial aneurysm (sIA) rupture leads to a disabling subarachnoid hemorrhage. Chronic inflammation and lipid accumulation in the sIA wall contribute to wall degenerative remodeling that precedes its rupture. A better understanding of the pathobiological process is essential for improved future treatment of patients carrying sIAs. Serum amyloid A (SAA) is an acute-phase protein produced in response to acute and chronic inflammation and tissue damage. Here, we studied the presence and the potential role of SAA in 36 intraoperatively resected sIAs (16 unruptured and 20 ruptured), that had previously been studied by histology and immunohistochemistry. SAA was present in all sIAs, but the extent of immunopositivity varied greatly. SAA immunopositivity correlated with wall degeneration (p = 0.028) and rupture (p = 0.004), with numbers of CD163-positive and CD68-positive macrophages and CD3-positive T lymphocytes (all p < 0.001), and with the expression of myeloperoxidase, matrix metalloproteinase-9, prostaglandin E-2 receptor, and cyclo-oxygenase 2 in the sIA wall. Moreover, SAA positivity correlated with the accumulation of apolipoproteins A-1 and B-100. In conclusion, SAA occurs in the sIA wall and, as an inflammation-related factor, may contribute to the development of a rupture-prone sIA.

Adipokines and Arterial Stiffness in Obesity

Adipokines are active molecules with pleiotropic effects produced by adipose tissue and involved in obesity-related metabolic and cardiovascular diseases. Arterial stiffness, which is a consequence of arteriosclerosis, has been shown to be an independent predictor of cardiovascular morbidity and mortality. The pathogenesis of arterial stiffness is complex but incompletely understood. Adipokines dysregulation may induce, by various mechanisms, vascular inflammation, endothelial dysfunction, and vascular remodeling, leading to increased arterial stiffness. This article summarizes literature data regarding adipokine-related pathogenetic mechanisms involved in the development of arterial stiffness, particularly in obesity, as well as the results of clinical and epidemiological studies which investigated the relationship between adipokines and arterial stiffness.

Efficacy of the Piperidine Nitroxide 4-MethoxyTEMPO in Ameliorating Serum Amyloid A-Mediated Vascular Inflammation

Intracellular redox imbalance in endothelial cells (EC) can lead to endothelial dysfunction, which underpins cardiovascular diseases (CVD). The acute phase serum amyloid A (SAA) elicits inflammation through stimulating production of reactive oxygen species (ROS). The cyclic nitroxide 4-MethoxyTEMPO (4-MetT) is a superoxide dismutase mimetic that suppresses oxidant formation and inflammation. The aim of this study was to investigate whether 4-MetT inhibits SAA-mediated activation of cultured primary human aortic EC (HAEC). Co-incubating cells with 4-MetT inhibited SAA-mediated increases in adhesion molecules (VCAM-1, ICAM-1, E-selectin, and JAM-C). Pre-treatment of cells with 4-MetT mitigated SAA-mediated increases in transcriptionally activated NF-κB-p65 and P120 Catenin (a stabilizer of Cadherin expression). Mitochondrial respiration and ROS generation (mtROS) were adversely affected by SAA with decreased respiratory reserve capacity, elevated maximal respiration and proton leakage all characteristic of SAA-treated HAEC. This altered respiration manifested as a loss of mitochondrial membrane potential (confirmed by a decrease in TMRM fluorescence), and increased mtROS production as assessed with MitoSox Red. These SAA-linked impacts on mitochondria were mitigated by 4-MetT resulting in restoration of HAEC nitric oxide bioavailability as confirmed by assessing cyclic guanosine monophosphate (cGMP) levels. Thus, 4-MetT ameliorates SAA-mediated endothelial dysfunction through normalising EC redox homeostasis. Subject to further validation in in vivo settings; these outcomes suggest its potential as a therapeutic in the setting of cardiovascular pathologies where elevated SAA and endothelial dysfunction is linked to enhanced CVD.

Molecular regulation of autophagy in a pro-inflammatory tumour microenvironment: New insight into the role of serum amyloid A

Chronic inflammation, systemic or local, plays a vital role in tumour progression and metastasis. Dysregulation of key physiological processes such as autophagy elicit unfavourable immune responses to induce chronic inflammation. Cytokines, growth factors and acute phase proteins present in the tumour microenvironment regulate inflammatory responses and alter crosstalk between various signalling pathways involved in the progression of cancer. Serum amyloid A (SAA) is a key acute phase protein secreted by the liver during the acute phase response (APR) following infection or injury. However, cancer and cancer-associated cells produce SAA, which when present in high levels in the tumour microenvironment contributes to cancer initiation, progression and metastasis. SAA can activate several signalling pathways such as the PI3K and MAPK pathways, which are also known modulators of the intracellular degradation process, autophagy. Autophagy can be regarded as having a double edged sword effect in cancer. Its dysregulation can induce malignant transformation through metabolic stress which manifests as oxidative stress, endoplasmic reticulum (ER) stress and DNA damage. On the other hand, autophagy can promote cancer survival during metabolic stress, hypoxia and senescence. Autophagy has been utilised to promote the efficiency of chemotherapeutic agents and can either be inhibited or induced to improve treatment outcomes. This review aims to address the known mechanisms that regulate autophagy as well as illustrating the role of SAA in modulating these pathways and its clinical implications for cancer therapy.

Pseudogenes in Cardiovascular Disease

Cardiovascular disease is the main disease that affects human life span. In recent years, the disease has been increasingly addressed at the molecular levels, for example, pseudogenes are now known to be involved in the pathogenesis and development of cardiovascular diseases. Pseudogenes are non-coding homologs of protein-coding genes and were once called “junk gene.” Since they are highly homologous to their functional parental genes, it is somewhat difficult to distinguish them. With the development of sequencing technology and bioinformatics, pseudogenes have become readily identifiable. Recent studies indicate that pseudogenes are closely related to cardiovascular diseases. This review provides an overview of pseudogenes and their roles in the pathogenesis of cardiovascular diseases. This new knowledge adds to our understanding of cardiovascular disease at the molecular level and will help develop new biomarkers and therapeutic approaches designed to prevent and treat the disease.

Network analysis of transcriptomics data for the prediction and prioritization of membrane-associated biomarkers for idiopathic pulmonary fibrosis (IPF) by bioinformatics approach

Idiopathic pulmonary fibrosis (IPF) is a rare yet crucial persistent lung disorder that actuates scarring of lung tissues, which makes breathing difficult. Smoking, environmental pollution, and certain viral infections could initiate lung scarring. However, the molecular mechanism involved in IPF remains elusive. To develop an efficient therapeutic arsenal against IPF, it is vital to understand the pathology and deviations in biochemical pathways that lead to disorder. In this study, we availed network analysis and other computational pipelines to delineate the prominent membrane proteins as diagnostic biomarkers and therapeutic targets for IPF. This study yielded a significant role of glycosaminoglycan binding, endothelin, and GABA-B receptor signaling pathway in IPF pathogenesis. Furthermore, ADCY8, CRH, FGB, GPR17, MCHR1, NMUR1, and SAA1 genes were found to be immensely involved with IPF, and the enrichment pathway analysis suggests that most of the pathways were corresponding to membrane transport and signal transduction functionalities. This analysis could help in better understanding the molecular mechanism behind IPF to develop an efficient therapeutic target or biomarkers for IPF.

Serum amyloid A is not incorporated into HDL during HDL biogenesis

Liver-derived serum amyloid A (SAA) is present in plasma where it is mainly associated with HDL and from which it is cleared more rapidly than are the other major HDL-associated apolipoproteins. Although evidence suggests that lipid-free and HDL-associated forms of SAA have different activities, the pathways by which SAA associates and disassociates with HDL are poorly understood. In this study, we investigated SAA lipidation by hepatocytes and how this lipidation relates to the formation of nascent HDL particles. We also examined hepatocyte-mediated clearance of lipid-free and HDL-associated SAA. We prepared hepatocytes from mice injected with lipopolysaccharide or an SAA-expressing adenoviral vector. Alternatively, we incubated primary hepatocytes from SAA-deficient mice with purified SAA. We analyzed conditioned media to determine the lipidation status of endogenously produced and exogenously added SAA. Examining the migration of lipidated species, we found that SAA is lipidated and forms nascent particles that are distinct from apoA-I-containing particles and that apoA-I lipidation is unaltered when SAA is overexpressed or added to the cells, indicating that SAA is not incorporated into apoA-I-containing HDL during HDL biogenesis. Like apoA-I formation, generation of SAA-containing particles was dependent on ABCA1, but not on scavenger receptor class B type I. Hepatocytes degraded significantly more SAA than apoA-I. Taken together, our results indicate that SAA's lipidation and metabolism by the liver is independent of apoA-I and that SAA is not incorporated into HDL during HDL biogenesis.

Role of serum amyloid A in atherosclerosis

PURPOSE OF REVIEW

Acute phase serum amyloid A (SAA) is persistently elevated in chronic inflammatory conditions, and elevated levels predict cardiovascular risk in humans. More recently, murine studies have demonstrated that over-expression of SAA increases and deficiency/suppression of SAA attenuates atherosclerosis. Thus, beyond being a biomarker, SAA appears to play a causal role in atherogenesis. The purpose of this review is to summarize the data supporting SAA as a key player in atherosclerosis development.

RECENT FINDINGS

A number of pro-inflammatory and pro-atherogenic activities have been ascribed to SAA. However, the literature is conflicted, as recombinant SAA, and/or lipid-free SAA, used in many of the earlier studies, do not reflect the activity of native human or murine SAA, which exists largely lipid-associated. Recent literatures demonstrate that SAA activates the NLRP3 inflammasome, alters vascular function, affects HDL function, and increases thrombosis. Importantly, SAA activity appears to be regulated by its lipid association, and HDL may serve to sequester and limit SAA activity.

SUMMARY

SAA has many pro-inflammatory and pro-atherogenic activities, is clearly demonstrated to affect atherosclerosis development, and may be a candidate target for clinical trials in cardiovascular diseases.

Serum Amyloid A Stimulates Vascular and Renal Dysfunction in Apolipoprotein E-Deficient Mice Fed a Normal Chow Diet

Elevated serum amyloid A (SAA) levels may promote endothelial dysfunction, which is linked to cardiovascular and renal pathologies. We investigated the effect of SAA on vascular and renal function in apolipoprotein E-deficient (ApoE^−/−) mice. Male ApoE^−/− mice received vehicle (control), low-level lipopolysaccharide (LPS), or recombinant human SAA by i.p. injection every third day for 2 weeks. Heart, aorta and kidney were harvested between 3 days and 18 weeks after treatment. SAA administration increased vascular cell adhesion molecule (VCAM)-1 expression and circulating monocyte chemotactic protein (MCP)-1 and decreased aortic cyclic guanosine monophosphate (cGMP), consistent with SAA inhibiting nitric oxide bioactivity. In addition, binding of labeled leukocytes to excised aorta increased as monitored using an ex vivo leukocyte adhesion assay. Renal injury was evident 4 weeks after commencement of SAA treatment, manifesting as increased plasma urea, urinary protein, oxidized lipids, urinary kidney injury molecule (KIM)-1 and multiple cytokines and chemokines in kidney tissue, relative to controls. Phosphorylation of nuclear-factor-kappa-beta (NFκB-p-P65), tissue factor (TF), and macrophage recruitment increased in kidneys from ApoE^−/− mice 4 weeks after SAA treatment, confirming that SAA elicited a pro-inflammatory and pro-thrombotic phenotype. These data indicate that SAA impairs endothelial and renal function in ApoE^−/− mice in the absence of a high-fat diet.

Acute phase reactant serum amyloid A in inflammation and other diseases

Acute-phase reactant serum amyloid A (A-SAA) plays an important role in acute and chronic inflammation and is used in clinical laboratories as an indicator of inflammation. Although both A-SAA and C-reactive protein (CRP) are acute-phase proteins, the detection of A-SAA is more conclusive than the detection of CRP in patients with viral infections, severe acute pancreatitis, and rejection reactions to kidney transplants. A-SAA has greater clinical diagnostic value in patients who are immunosuppressed, patients with cystic fibrosis who are treated with corticoids, and preterm infants with late-onset sepsis. Nevertheless, for the assessment of the inflammation status and identification of viral infection in other pathologies, such as bacterial infections, the combinatorial use of A-SAA and other acute-phase proteins (APPs), such as CRP and procalcitonin (PCT), can provide more information and sensitivity than the use of any of these proteins alone, and the information generated is important in guiding antibiotic therapy. In addition, A-SAA-associated diseases and the diagnostic value of A-SAA are discussed. However, the relationship between different A-SAA isotypes and their human diseases are mostly derived from research laboratories with limited clinical samples. Thus, further clinical evaluations are necessary to confirm the clinical significance of each A-SAA isotype. Furthermore, the currently available A-SAA assays are based on polyclonal antibodies, which lack isotype specificity and are associated with many inflammatory diseases. Therefore, these assays are usually used in combination with other biomarkers in the clinic.

Serum amyloid A - a review

Serum amyloid A (SAA) proteins were isolated and named over 50 years ago. They are small (104 amino acids) and have a striking relationship to the acute phase response with serum levels rising as much as 1000-fold in 24 hours. SAA proteins are encoded in a family of closely-related genes and have been remarkably conserved throughout vertebrate evolution. Amino-terminal fragments of SAA can form highly organized, insoluble fibrils that accumulate in “secondary” amyloid disease. Despite their evolutionary preservation and dynamic synthesis pattern SAA proteins have lacked well-defined physiologic roles. However, considering an array of many, often unrelated, reports now permits a more coordinated perspective. Protein studies have elucidated basic SAA structure and fibril formation. Appreciating SAA’s lipophilicity helps relate it to lipid transport and metabolism as well as atherosclerosis. SAA’s function as a cytokine-like protein has become recognized in cell-cell communication as well as feedback in inflammatory, immunologic, neoplastic and protective pathways. SAA likely has a critical role in control and possibly propagation of the primordial acute phase response. Appreciating the many cellular and molecular interactions for SAA suggests possibilities for improved understanding of pathophysiology as well as treatment and disease prevention.

Serum Amyloid A Is an Exchangeable Apolipoprotein

Objective- SAA (serum amyloid A) is a family of acute-phase reactants that have proinflammatory and proatherogenic activities. SAA is more lipophilic than apoA-I (apolipoprotein A-I), and during an acute-phase response, <10% of plasma SAA is found lipid-free. In most reports, SAA is found exclusively associated with high-density lipoprotein; however, we and others have reported SAA on apoB (apolipoprotein B)-containing lipoproteins in both mice and humans. The goal of this study was to determine whether SAA is an exchangeable apolipoprotein. Approach and Results- Delipidated human SAA was incubated with SAA-free human lipoproteins; then, samples were reisolated by fast protein liquid chromatography, and SAA analyzed by ELISA and immunoblot. Both in vitro and in vivo, we show that SAA associates with any lipoprotein and does not remain in a lipid-free form. Although SAA is preferentially found on high-density lipoprotein, it can exchange between lipoproteins. In the presence of CETP (cholesterol ester transfer protein), there is greater exchange of SAA between lipoproteins. Subjects with diabetes mellitus, but not those with metabolic syndrome, showed altered SAA lipoprotein distribution postprandially. Proteoglycan-mediated lipoprotein retention is thought to be an underlying mechanism for atherosclerosis development. SAA has a proteoglycan-binding domain. Lipoproteins containing SAA had increased proteoglycan binding compared with SAA-free lipoproteins. Conclusions- Thus, SAA is an exchangeable apolipoprotein and increases apoB-containing lipoproteins' proteoglycan binding. We and others have previously reported the presence of SAA on low-density lipoprotein in individuals with obesity, diabetes mellitus, and metabolic syndrome. We propose that the presence of SAA on apoB-containing lipoproteins may contribute to cardiovascular disease development in these populations.

Systemic serum amyloid A as a biomarker for exposure to zinc and/or copper-containing metal fumes

Zinc- and copper-containing welding fumes increase systemic C-reactive protein (CRP). The aim of this study was to investigate the performance of the biomarkers serum amyloid A (SAA) and soluble vascular cell adhesion molecule-1 (VCAM-1) in this regard. Fifteen male subjects were exposed under controlled conditions to welding fumes containing either zinc, or copper, or copper and zinc for 6 h. Plasma samples were collected before, 6 and 24 h after start of exposure and biomarkers therein were measured by electrochemiluminescent assay. For each exposure, systemic concentrations of systemic SAA, but not VCAM-1, increased significantly at 24 h after exposure start compared with baseline ("copper only": P=0.0005, "zinc only": P=0.027, "copper and zinc": P=0.001). SAA showed a wider range of concentrations than did CRP and its levels increased up to 19-fold after welding fume exposure. The recognition of copper as a potential harmful component in welding fumes, also independent from zinc, deserves further consideration. SAA might represent a new sensitive biomarker for potential subclinical sterile inflammation after inhalation of copper- and/or zinc-containing welding fumes. As elevations of CRP and SAA protein have both been linked to a higher risk for cardiovascular disease, these findings might particularly be important for long-term welders.

Serum amyloid A3 is pro-atherogenic

BACKGROUND AND AIMS

Serum amyloid A (SAA) predicts cardiovascular events. Overexpression of SAA increases atherosclerosis development; however, deficiency of two of the murine acute phase isoforms, SAA1.1 and SAA2.1, has no effect on atherosclerosis. SAA3 is a pseudogene in humans, but is an expressed acute phase isoform in mice. The goal of this study was to determine if SAA3 affects atherosclerosis in mice.

METHODS

ApoE-/- mice were used as the model for all studies. SAA3 was overexpressed by an adeno-associated virus or suppressed using an anti-sense oligonucleotide approach.

RESULTS

Over-expression of SAA3 led to a 4-fold increase in atherosclerosis lesion area compared to control mice (p = 0.01). Suppression of SAA3 decreased atherosclerosis in mice genetically deficient in SAA1.1 and SAA2.1 (p < 0.0001).

CONCLUSIONS

SAA3 augments atherosclerosis in mice. Our results resolve a previous paradox in the literature and support extensive epidemiological data that SAA is pro-atherogenic.

Prevention of renal apoB retention is protective against diabetic nephropathy: role of TGF-β inhibition

Animal studies demonstrate that hyperlipidemia and renal lipid accumulation contribute to the pathogenesis of diabetic nephropathy (DN). We previously demonstrated that renal lipoproteins colocalize with biglycan, a renal proteoglycan. The purpose of this study was to determine whether prevention of renal lipid (apoB) accumulation attenuates DN. Biglycan-deficient and biglycan wild-type Ldlr^-/- mice were made diabetic via streptozotocin and fed a high cholesterol diet. As biglycan deficiency is associated with elevated transforming growth factor-β (TGF-β), in some experiments mice were injected with either the TGF-β-neutralizing antibody, 1D11, or with 13C4, an irrelevant control antibody. Biglycan deficiency had no significant effect on renal apoB accumulation, but led to modest attenuation of DN with ∼30% reduction in albuminuria; however, biglycan deficiency caused a striking elevation in TGF-β. Use of 1D11 led to sustained suppression of TGF-β for approximately 8 weeks at a time. The 1D11 treatment caused decreased renal apoB accumulation, decreased albuminuria, decreased renal hypertrophy, and improved survival, compared with the 13C4 treatment. Thus, prevention of renal apoB accumulation is protective against development of DN. Furthermore, this study demonstrates that prevention of renal apoB accumulation is a mechanism by which TGF-β inhibition is nephroprotective.

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.

The cytoprotective effect of biglycan core protein involves Toll-like receptor 4 signaling in cardiomyocytes

AIMS

Exogenously administered biglycan (core protein with high-molecular weight glycosaminoglycan chains) has been shown to protect neonatal cardiomyocytes against simulated ischemia/reperfusion injury (SI/R), however, the mechanism of action is not clear. In this study we aimed to investigate, which structural component of biglycan is responsible for its cardiocytoprotective effect and to further explore the molecular mechanisms involved in the cytoprotection.

METHODS AND RESULTS

A pilot study was conducted to demonstrate that both native (glycanated) and deglycanated biglycan can attenuate cell death induced by SI/R in a dose-dependent manner in primary neonatal cardiomyocytes isolated from Wistar rats. In separate experiments, we have shown that similarly to glycanated biglycan, recombinant human biglycan core protein (rhBGNc) protects cardiomyocytes against SI/R injury. In contrast, the glycosaminoglycan component dermatan sulfate had no significant effect on cell viability, while chondroitin sulfate further enhanced cell death induced by SI/R. Treatment of cardiomyocytes with rhBGNc reverses the effect of SI/R upon markers of necrosis, apoptosis, mitochondrial membrane potential, and autophagy. We have also shown that pharmacological blockade of Toll-like receptor 4 (TLR4) signaling or its downstream mediators (IRAK1/4, ERK, JNK and p38 MAP kinases) abolished the cytoprotective effect of rhBGNc against SI/R injury. Pretreatment of cardiomyocytes with rhBGNc for 20h resulted in increased Akt phosphorylation and NO production without having significant effect on phosphorylation of ERK1/2, STAT3, and on the production of superoxide. Treatment over 10min and 1h with rhBGNc increased ERK1 phosphorylation, while the SI/R-induced increase in superoxide production was attenuated by rhBGNc. Blockade of NO synthesis also prevented the cardiocytoprotective effect of rhBGNc.

CONCLUSIONS

The core protein of exogenous biglycan protects myocardial cells from SI/R injury via TLR4-mediated mechanisms involving activation of ERK, JNK and p38 MAP kinases and increased NO production. The cytoprotective effect of rhBGNc is due to modulation of SI/R-induced changes in necrosis, apoptosis and autophagy.

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.

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.

Serum Amyloid A Facilitates Early Lesion Development in Ldlr-/- Mice

BACKGROUND

Atherosclerosis is a chronic inflammatory disorder, and several studies have demonstrated a positive association between plasma serum amyloid A (SAA) levels and cardiovascular disease risk. The aim of the study was to examine whether SAA has a role in atherogenesis, the underlying basis of most cardiovascular disease.

METHODS AND RESULTS

Mice globally deficient in acute-phase isoforms Saa1 and Saa2 (Saa(-/-)) were crossed to Ldlr(-/-) mice (Saa(-/-)Ldlr(-/-)). Saa(-/-)Ldlr(-/-) mice demonstrated a 31% reduction in lesional area in the ascending aorta but not in the aortic root or innominate artery after consuming a high-fat, high-cholesterol Western-type diet for 6 weeks. The lesions were predominantly macrophage foam cells. The phenotype was lost in more mature lesions in mice fed a Western-type diet for 12 weeks, suggesting that SAA is involved in early lesion development. The decreased atherosclerosis in the Saa(-/-)Ldlr(-/-) mice occurred despite increased levels of blood monocytes and was independent of plasma lipid levels. SAA is produced predominantly by hepatocytes and macrophages. To determine which source of SAA may have a dominant role in lesion development, bone marrow transplantation was performed. Ldlr(-/-) mice that received bone marrow from Saa(-/-)Ldlr(-/-) mice had slightly reduced ascending aorta atherosclerosis compared with Saa(-/-)Ldlr(-/-) mice receiving bone marrow from Ldlr(-/-) mice, indicating that the expression of SAA by macrophages may have an important influence on atherogenesis.

CONCLUSIONS

The results indicate that SAA produced by macrophages promotes early lesion formation in the ascending aorta.

Emerging functions of serum amyloid A in inflammation

SAA is a major acute-phase protein produced in large quantity during APR. The rise of SAA concentration in blood circulation during APR has been a clinical marker for active inflammation. In the past decade, research has been conducted to determine whether SAA plays an active role during inflammation and if so, how it influences the course of inflammation. These efforts have led to the discovery of cytokine-like activities of rhSAA, which is commercially available and widely used in most of the published studies. SAA activates multiple receptors, including the FPR2, the TLRs TLR2 and TLR4, the scavenger receptor SR-BI, and the ATP receptor P2X7. More recent studies have shown that SAA not only activates transcription factors, such as NF-κB, but also plays a role in epigenetic regulation through a MyD88-IRF4-Jmjd3 pathway. It is postulated that the activation of these pathways leads to induced expression of proinflammatory factors and a subset of proteins expressed by the M2 macrophages. These functional properties set SAA apart from well-characterized inflammatory factors, such as LPS and TNF-α, suggesting that it may play a homeostatic role during the course of inflammation. Ongoing and future studies are directed to addressing unresolved issues, including the difference between rSAA and native SAA isoforms and the exact functions of SAA in physiologic and pathologic settings.

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.

Serum amyloid A is a marker for pulmonary involvement in systemic sclerosis

Inflammation in systemic sclerosis (SSc) is a prominent, but incompletely characterized feature in early stages of the disease. The goal of these studies was to determine the circulating levels, clinical correlates and biological effects of the acute phase protein serum amyloid A (SAA), a marker of inflammation, in patients with SSc. Circulating levels of SAA were determined by multiplex assays in serum from 129 SSc patients and 98 healthy controls. Correlations between SAA levels and clinical and laboratory features of disease were analyzed. The effects of SAA on human pulmonary fibroblasts were studied ex vivo. Elevated levels of SAA were found in 25% of SSc patients, with the highest levels in those with early-stage disease and diffuse cutaneous involvement. Significant negative correlations of SAA were found with forced vital capacity and diffusion capacity for carbon monoxide. Patients with elevated SAA had greater dyspnea and more frequent interstitial lung disease, and had worse scores on patient-reported outcome measures. Incubation with recombinant SAA induced dose-dependent stimulation of IL-6 and IL-8 in normal lung fibroblasts in culture. Serum levels of the inflammatory marker SAA are elevated in patients with early diffuse cutaneous SSc, and correlate with pulmonary involvement. In lung fibroblasts, SAA acts as a direct stimulus for increased cytokine production. These findings suggest that systemic inflammation in SSc may be linked to lung involvement and SAA could serve as a potential biomarker for this complication.

FPR2/ALX activation reverses LPS-induced vascular hyporeactivity in aorta and increases survival in a pneumosepsis model

The formylpeptide receptor 2 (FPR2/ALX) is a very promiscuous receptor, utilized by lipid and protein ligands that trigger pro- or anti-inflammatory responses. FPR2/ALX expression is increased in lung tissues of septic animals and its activation has a beneficial therapeutic effect by controlling exacerbated inflammation. Although FPR2/ALX expression was observed in vascular smooth muscle cells, its role in vascular reactivity in inflammatory conditions has not been studied. In this study, we report that LPS increases FPR2/ALX expression in vascular smooth muscle cells (A7r5 cells) and aorta tissue, and that the selective agonist WKYMVm reverses LPS-induced vascular hyporeactivity in mouse aorta rings. Mice bearing pneumosepsis by Klebsiella pneumoniae and treated with WKYMVm recovered the reactivity to vasoconstrictors and the survival improved by 40%. As for the mechanisms involved, FPR2/ALX activation decreases NO production in LPS-stimulated cells and aorta, but it does not seem involve the regulation of NOS-2 expression. The molecular mechanism by which the peptide inhibits NO production still needs to be elucidated, but our data suggests an important role for NO in the WKYMVm beneficial effect observed in LPS injury and sepsis. In conclusion, our data suggest, for the first time, that a receptor, primarily described as a mediator of immune responses, may have an important role in the vascular dysfunctions observed in sepsis and may be a possible target for new therapeutic interventions.

Serum amyloid A as a marker of persistent inflammation and an indicator of cardiovascular and renal involvement in patients with rheumatoid arthritis

OBJECTIVES

Rheumatoid arthritis (RA) is a systemic, inflammatory disease. Serum amyloid A (SAA) is an acute-phase protein, involved in pathogenesis of atherosclerosis. The aim of the study was to assess serum concentration of SAA in RA patients, with reference to other inflammatory parameters and markers of extra-articular involvement.

METHODS

The study population consisted of 140 RA patients, low/moderate disease activity (L/MDA) in 98 (70%) patients and high disease activity (HDA) in 42 (30%). Comprehensive clinical and laboratory assessment was performed with evaluation of electrocardiogram and carotid intima-media thickness.

RESULTS

The mean SAA concentration [327.0 (263.4) mg/L] was increased highly above the normal value, even in patients with L/MDA. Simultaneously, SAA was significantly higher in patients with HDA versus L/MDA. The mean SAA concentration was significantly higher in patients treated with glucocorticoids, was inversely associated with QTc duration, and was markedly higher in patients with atherosclerotic plaques, emphasizing increased CV risk. SAA was significantly higher in patients with increased cystatin-C level.

CONCLUSIONS

In RA patients, high serum SAA concentration was strongly associated with activity of the disease and risk of CV and renal involvement. Recurrent assessment of SAA may facilitate searching patients with persistent inflammation and risk of extra-articular complications.

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.

Serum amyloid A and pairing formyl peptide receptor 2 are expressed in corneas and involved in inflammation-mediated neovascularization

AIM

To solidify the involvement of Saa-related pathway in corneal neovascularization (CorNV). The pathogenesis of inflammatory CorNV is not fully understood yet, and our previous study implicated that serum amyloid A (Saa) 1 (Saa1) and Saa3 were among the genes up-regulated upon CorNV induction in mice.

METHODS

Microarray data obtained during our profiling project on CorNV were analyzed for the genes encoding the four SAA family members (Saa1-4), six reported SAA receptors (formyl peptide receptor 2, Tlr2, Tlr4, Cd36, Scarb1, P2rx7) and seven matrix metallopeptidases (Mmp) 1a, 1b, 2, 3, 9, 10, 13 reportedly to be expressed upon SAA pathway activation. The baseline expression or changes of interested genes were further confirmed in animals with CorNV using molecular or histological methods. CorNV was induced in Balb/c and C57BL/6 mice by placing either three interrupted 10-0 sutures or a 2 mm filter paper soaked with sodium hydroxide in the central area of the cornea. At desired time points, the corneas were harvested for histology examination or for extraction of mRNA and protein. The mRNA levels of Saa1, Saa3, Fpr2, Mmp2 and Mmp3 in corneas were detected using quantitative reverse transcription-PCR, and SAA3 protein in tissues detected using immunohistochemistry or western blotting.

RESULTS

Microarray data analysis revealed that Saa1, Saa3, Fpr2, Mmp2, Mmp3 messengers were readily detected in normal corneas and significantly up-regulated upon CorNV induction. The changes of these five genes were confirmed with real-time PCR assay. On the contrary, other SAA members (Saa2, Saa4), other SAA receptors (Tlr2, Tlr4, Cd36, P2rx7, etc), or other Mmps (Mmp1a, Mmp1b, Mmp9, Mmp10, Mmp13) did not show consistent changes. Immunohistochemistry study and western blotting further confirmed the expression of SAA3 products in normal corneas as well as their up-regulation in corneas with CorNV.

CONCLUSION

SAA-FPR2 pathway composing genes were expressed in normal murine corneas and, upon inflammatory stimuli challenge to the corneas, their expressions were up-regulated, suggesting their roles in pathogenesis of CorNV. The potential usefulness of SAA-FPR2 targets in future management of CorNV-related diseases deserves investigation.

Increased atherosclerosis in mice with increased vascular biglycan content

OBJECTIVE

The response to retention hypothesis of atherogenesis proposes that atherosclerosis is initiated via the retention of atherogenic lipoproteins by vascular proteoglycans. Co-localization studies suggest that of all the vascular proteoglycans, biglycan is the one most closely co-localized with LDL. The goal of this study was to determine if over-expression of biglycan in hyperlipidemic mice would increase atherosclerosis development.

METHODS

Transgenic mice were developed by expressing biglycan under control of the smooth muscle actin promoter, and were crossed to the LDL receptor deficient (C57BL/6 background) atherosclerotic mouse model. Biglycan transgenic and non-transgenic control mice were fed an atherogenic Western diet for 4-12 weeks.

RESULTS

LDL receptor deficient mice overexpressing biglycan under control of the smooth muscle alpha actin promoter had increased atherosclerosis development that correlated with vascular biglycan content.

CONCLUSION

Increased vascular biglycan content predisposes to increased lipid retention and increased atherosclerosis development.

Localized thymic amyloidosis presenting with myasthenia gravis: case report

A mediastinal mass was incidentally found on chest radiography in a 46-yr-old woman who had had myasthenia gravis (MG) for 2 months. Computed tomography revealed a 4-cm in size, well-defined, and lobulating mass with nodular calcification that was located in the thymus. Microscopically, the mass consisted of diffuse amorphous eosinophilic materials. These deposits exhibited apple-green birefringence under polarized light microscopy after Congo red staining. Immunohistochemical analysis revealed that they were positive for both kappa and lambda light chains and negative for amyloid A. A diagnosis of localized primary thymic amyloidosis was finally made. After thymectomy, the symptoms of MG were controlled with reduced corticosteroid requirements. Localized thymic amyloidosis associated with MG has not been reported to date.

Proteomic analysis of plasma-purified VLDL, LDL, and HDL fractions from atherosclerotic patients undergoing carotid endarterectomy: identification of serum amyloid A as a potential marker

Apolipoproteins are very heterogeneous protein family, implicated in plasma lipoprotein structural stabilization, lipid metabolism, inflammation, or immunity. Obtaining detailed information on apolipoprotein composition and structure may contribute to elucidating lipoprotein roles in atherogenesis and to developing new therapeutic strategies for the treatment of lipoprotein-associated disorders. This study aimed at developing a comprehensive method for characterizing the apolipoprotein component of plasma VLDL, LDL, and HDL fractions from patients undergoing carotid endarterectomy, by means of two-dimensional electrophoresis (2-DE) coupled with Mass Spectrometry analysis, useful for identifying potential markers of plaque presence and vulnerability. The adopted method allowed obtaining reproducible 2-DE maps of exchangeable apolipoproteins from VLDL, LDL, and HDL. Twenty-three protein isoforms were identified by peptide mass fingerprinting analysis. Differential proteomic analysis allowed for identifying increased levels of acute-phase serum amyloid A protein (AP SAA) in all lipoprotein fractions, especially in LDL from atherosclerotic patients. Results have been confirmed by western blotting analysis on each lipoprotein fraction using apo AI levels for data normalization. The higher levels of AP SAA found in patients suggest a role of LDL as AP SAA carrier into the subendothelial space of artery wall, where AP SAA accumulates and may exert noxious effects.

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.

Serum amyloid A activation of human coronary artery endothelial cells exhibits a neutrophil promoting molecular profile

BACKGROUND

Serum amyloid A (SAA) has been shown to be an active participant in atherosclerosis and cardiovascular diseases. SAA-stimulated human coronary artery endothelial cells (HCAEC) were reported to release pro-inflammatory cytokines, chemokines and adhesion molecules; however it remains unclear which putative SAA receptors are present in these cells and how they act. We investigated the effects of inflammatory stimuli on the expression of SAA receptors, signaling pathways and molecular profiles in HCAEC.

METHODOLOGY/PRINCIPLE FINDINGS

HCAEC were cultured in vitro and stimulated with SAA (1000nM) or IL-1β (1000pg/ml). Expression of mRNA was determined by qPCR, and expression and quantification of proteins were assessed by dot array blots and ELISA, respectively. Protein phosphorylation was determined by dot blot arrays and Western blots. We report that all potential SAA receptors tested (FPR2/ALX, RAGE, TANIS, TLR2, TLR4 and CLA-1/hSR-B1) are expressed in HCAEC. Importantly, IL-1β or SAA significantly increased solely the expression of the innate immune receptor TLR2. SAA upregulated the phosphorylation of ERK1/2, NF-κB (p65, p105) and JNK, as well as expression/release of IL-6, IL-8, G-CSF, GM-CSF, ICAM-1 and VCAM-1, all potent molecules involved in neutrophil-related activities. A TLR2-dependent positive feedback mechanism of SAA expression was found.

CONCLUSION/SIGNIFICANCE

SAA stimulated responses in HCAEC target neutrophil rather than monocyte/macrophage activation.

Serum amyloid A is found on ApoB-containing lipoproteins in obese humans with diabetes

OBJECTIVE

In murine models of obesity/diabetes, there is an increase in plasma serum amyloid A (SAA) levels along with redistribution of SAA from high-density lipoprotein (HDL) to apolipoprotein B (apoB)-containing lipoprotein particles, namely, low-density lipoprotein and very low-density lipoprotein. The goal of this study was to determine if obesity is associated with similar SAA lipoprotein redistribution in humans.

DESIGN AND METHODS

Three groups of obese individuals were recruited from a weight loss clinic: healthy obese (n = 14), metabolic syndrome (MetS) obese (n = 8), and obese with type 2 diabetes (n = 6). Plasma was separated into lipoprotein fractions by fast protein liquid chromatography, and SAA was measured in lipid fractions using enzyme-linked immunosorbent assay and Western blotting.

RESULTS

Only the obese diabetic group had SAA detectable in apoB-containing lipoproteins, and SAA reverted back to HDL with active weight loss.

CONCLUSIONS

In human subjects, SAA is found in apoB-containing lipoprotein particles only in obese subjects with type 2 diabetes, but not in healthy obese or obese subjects with MetS.

Plasma proteomics of differential outcome to long-term therapy in children with idiopathic pulmonary arterial hypertension

PURPOSE

The prognosis for children with IPAH unresponsive to therapy is poor. We investigated the plasma proteome for a molecular basis of good versus poor outcome to long-term vasodilator therapy.

EXPERIMENTAL DESIGN

Plasma was collected at baseline or shortly after therapy initiation and following chronic vasodilator therapy, then divided into those with good outcome (n = 8), and those with a poor outcome (n = 7). To identify proteins unique to either outcome, we used differential gel electrophoresis and mass spectrometry. Results were confirmed by commercial enzyme-linked immunosorbent assay.

RESULTS

Before and after therapy, SAA-4 was 4-fold lower in those with good outcome compared to those with poor outcome, while serum paraoxonase/arylesterase-1 was increased 2-fold in those with good outcome versus poor outcome. After therapy, haptoglobin and hemopexin were 1.45- and 1.8-fold lower, respectively, in those with a good versus poor outcome. Among those with a good outcome, SAP was 1.3-fold lower prior to therapy.

CONCLUSIONS AND CLINICAL RELEVANCE

SAP and SAA-4 regulate circulating mononuclear phagocytes. As such, they may contribute to the differential response to chronic vasodilator therapy in the context of inflammation in IPAH.

High-density lipoprotein loses its anti-inflammatory capacity by accumulation of pro-inflammatory-serum amyloid A

AIMS

High-density lipoprotein (HDL) is known to have potent anti-inflammatory properties. Monocyte chemoattractant protein-1 is an important pro-inflammatory cytokine in early atherogenesis. There is evidence that HDL can lose its protective function during inflammatory disease. In patients with end-stage renal disease (ESRD), epidemiological studies have documented that the inverse correlation between HDL-cholesterol and cardiovascular risk is lost. Many structural modifications leading to reduced HDL function have been characterized, but the functional consequences are not fully understood.

METHODS AND RESULTS

We showed that HDL from patients with ESRD has a lower anti-inflammatory potential by reduced inhibition of monocyte chemoattractant protein-1 formation in vascular smooth muscle cells. Via a proteomic approach, we identified proteins in HDL from ESRD patients exerting pro-inflammatory actions. By chromatographic separation of proteins and mass-spectrometric analysis, we found serum amyloid A (SAA) to be one molecule acting as a potent pro-inflammatory protein. SAA is enriched in HDL from ESRD patients, correlating with reduced anti-inflammatory capacity. In SAA signal transduction, activation of formyl-peptide receptor 2 is involved. SAA enrichment in HDL of healthy subjects reduced the anti-inflammatory capacity of HDL and correlated with its decreased function.

CONCLUSION

These results suggest that SAA enrichment of HDL during disease conditions contributes to the decreased protective function. It is a novel finding that SAA acts as a pro-inflammatory molecule to reduce the anti-inflammatory properties of HDL.

The acute-phase protein serum amyloid A induces endothelial dysfunction that is inhibited by high-density lipoprotein

The acute-phase protein serum amyloid A (SAA) is elevated during inflammation and may be deposited in atheroma where it promotes atherosclerosis. We investigated the proatherogenic effects of SAA on the vascular endothelium and their regulation by high-density lipoprotein (HDL). Exposure of human aortic endothelial cells (HAEC) to SAA (0.25-25μg/ml) decreased nitric oxide ((•)NO) synthesis/bioavailability, although the endothelial NO synthase monomer-to-dimer ratio was unaffected. SAA (10μg/ml) stimulated a Ca(2+) influx linked to apocynin-sensitive superoxide radical anion (O(2)(•-)) production. Gene expression for arginase-1, nuclear factor κB (NF-κB), interleukin-8, and tissue factor (TF) increased within 4h of SAA stimulation. Enzymatically active Arg-1/2 was detected in HAEC cultured with SAA for 24h. Therefore, in addition to modulating (•)NO bioavailability by stimulating O(2)(•-) production in the endothelium, SAA modulated vascular l-Arg bioavailability. SAA also diminished relaxation of preconstricted aortic rings induced by acetylcholine, and added superoxide dismutase restored the vascular response. Preincubation of HAEC with HDL (100 or 200, but not 50, μg/ml) before (not after) SAA treatment ameliorated the Ca(2+) influx and O(2)(•-) production; decreased TF, NF-κB, and Arg-1 gene expression; and preserved overall vascular function. Thus, SAA may promote endothelial dysfunction by modulating (•)NO and l-Arg bioavailability, and HDL pretreatment may be protective. The relative HDL to SAA concentrations may regulate the proatherogenic properties of SAA on the vascular endothelium.

Serum amyloid A directly accelerates the progression of atherosclerosis in apolipoprotein E-deficient mice

Although serum amyloid A (SAA) is an excellent marker for coronary artery disease, its direct effect on atherogenesis in vivo is obscure. In this study we investigated the direct effect of SAA on promoting the formation of atherosclerosis in apolipoprotein E-deficient (ApoE⁻/⁻) mice. Murine SAA lentivirus was constructed and injected into ApoE⁻/⁻ mice intravenously. Then, experimental mice were fed a chow diet (5% fat and no added cholesterol) for 14 wks. The aortic atherosclerotic lesion area was larger with than without SAA treatment. With increased SAA levels, the plasma levels of interleukin-6 and tumor necrosis factor-α were significantly increased. Macrophage infiltration in atherosclerotic regions was enhanced with SAA treatment. A migration assay revealed prominent dose-dependent chemotaxis of SAA to macrophages. Furthermore, the expression of monocyte chemotactic protein-1 and vascular cell adhesion molecule-1 (VCAM-1) was upregulated significantly with SAA treatment. SAA-induced VCAM-1 production was detected in human aortic endothelial cells in vitro. Thus, an increase in plasma SAA directly accelerates the progression of atherosclerosis in ApoE⁻/⁻ mice. SAA is not only a risk marker for atherosclerosis but also an active participant in atherogenesis.

The correlation between the cardio-ankle vascular index (CAVI) and serum amyloid A in asymptomatic Japanese subjects

Chronic inflammation has received a great deal of attention due to the role it plays in cardiovascular disease (CVD). The cardio-ankle vascular index (CAVI) has recently been developed to evaluate arterial stiffness. This index is independent of blood pressure at the time that it is measured, making it a better measure for clinical studies on the prevention of CVD. Information on the association of serum amyloid A (SAA) with arterial stiffness in relatively healthy subjects is still scarce. The aim of the present study was to investigate the potential correlation between SAA and CAVI in asymptomatic Japanese subjects. In addition to SAA and CAVI, data on smoking status, body mass index, blood pressure, and serum/plasma biochemical indices such as glucose and total cholesterol were collected in 387 nonmedicated and CVD-free adult subjects during a health check examination (male/female 191/196, mean age 61.8 years). Among them, a randomly selected subgroup of 256 subjects (male/female 133/123, mean age 62.4 years) had a full dataset, including low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, triglyceride, and hemoglobin A1c. Among the whole population, CAVI levels were significantly higher in males than in females [mean 8.5 ± (SD 1.1) vs. 8.2 ± 1.1, p < 0.05], while SAA levels were slightly but nonsignificantly higher in females than in males [median 6.4 (interquartile range 4.0-9.3) μg/mL vs. 5.1 (3.5-8.4)]. In a multiple linear regression analysis, CAVI was weakly but significantly, independently, and positively correlated with SAA (β-coefficient 0.200, p < 0.01). The results of the same analyses for the randomly selected subgroup were relatively similar to the findings for the whole population. SAA may be a positive inflammatory factor associated with arterial stiffness, and the clinical relevance and the biological mechanism for this relationship should be established in future studies.

Serum amyloid A in atherosclerosis

PURPOSE OF REVIEW

Serum amyloid A (SAA) is a family of acute-phase proteins which are shown to correlate with cardiovascular disease, but whether this SAA contributes causally to atherosclerosis development or reflects underlying disease or risk factors remains unclear.

RECENT FINDINGS

SAA has been detected within atherosclerotic lesions and within adipose tissue where it is hypothesized that it may play a contributory role in disease development. In the acute-phase response SAA is synthesized by the liver and transported primarily in association with HDL. However, there is a growing literature suggesting that localized synthesis of SAA within the vasculature, or adipose tissue, may play a distinct role in disease development. Furthermore, SAA can be found in association with apoB-containing lipoproteins, in which its biological activity may be different.

SUMMARY

This review will discuss recent experimental evidence supporting a causal role of SAA with atherosclerosis.