Structure of serum amyloid A suggests a mechanism for selective lipoprotein binding and functions: SAA as a hub in macromolecular interaction networks

Longitudinal Fluctuations in Protein Concentrations and Higher-Order Structures in the Plasma Proteome of Kidney Failure Patients Subjected to a Kidney Transplant

Using proteomics and complexome profiling, we evaluated in a year-long study longitudinal variations in the plasma proteome of kidney failure patients, prior to and after a kidney transplantation. The post-transplant period was complicated by bacterial infections, resulting in dramatic changes in the proteome, attributed to an acute phase response (APR). As positive acute phase proteins (APPs), being elevated upon inflammation, we observed the well-described C-reactive protein and Serum Amyloid A (SAA), but also Fibrinogen, Haptoglobin, Leucine-rich alpha-2-glycoprotein, Lipopolysaccharide-binding protein, Alpha-1-antitrypsin, Alpha-1-antichymotrypsin, S100, and CD14. As negative APPs, being downregulated upon inflammation, we identified the well-documented Serotransferrin and Transthyretin, but added Kallistatin, Heparin cofactor 2, and interalpha-trypsin inhibitor heavy chain H1 and H2 (ITIH1, ITIH2). For the patient with the most severe APR, we performed plasma complexome profiling by SEC-LC-MS on all longitudinal samples. We observed that several plasma proteins displaying alike concentration patterns coelute and form macromolecular complexes. By complexome profiling, we expose how SAA1 and SAA2 become incorporated into high-density lipid particles, replacing largely Apolipoprotein (APO)A1 and APOA4. Overall, our data highlight that the combination of in-depth longitudinal plasma proteome and complexome profiling can shed further light on correlated variations in the abundance of several plasma proteins upon inflammatory events.

Pleiotropic functions and clinical importance of circulating HDL-PON1 complex

High density lipoprotein (HDL) functions are mostly mediated through a complex proteome, particularly its enzymes. HDL can provide a scaffold for the assembly of several proteins that affect each other's function. HDL particles, particularly small, dense HDL3, are rich in paraoxonase 1 (PON1), which is an important enzyme in the functionality of HDL, so the antioxidant and antiatherogenic properties of HDL are largely attributed to this enzyme. There is an increasing need to represent a valid, reproducible, and reliable method to assay HDL function in routine clinical laboratories. In this context, HDL-associated proteins may be key players; notably PON1 activity (its arylesterase activity) may be a proper candidate because its decreased activity can be considered an important risk factor for HDL dysfunctionality. Of note, automated methods have been developed for the measurement of serum PON1 activity that facilitates its assay in large sample numbers. Arylesterase activity is proposed as a preferred activity among the different activities of PON1 for its assay in epidemiological studies. The binding of PON1 to HDL is critical for the maintenance of its activity and it appears apolipoprotein A-I plays an important role in HDL-PON1 interaction as well as in the biochemical and enzymatic properties of PON1. The interrelationships between HDL, PON1, and HDL's other components are complex and incompletely understood. The purpose of this review is to discuss biochemical and clinical evidence considering the interactions of PON1 with HDL and the role of this enzyme as an appropriate biomarker for HDL function as well as a potential therapeutic target.

Structural studies of a serum amyloid A octamer that is primed to scaffold lipid nanodiscs

Serum amyloid A (SAA) is a highly conserved acute-phase protein that plays roles in activating multiple pro-inflammatory pathways during the acute inflammatory response and is commonly used as a biomarker of inflammation. It has been linked to beneficial roles in tissue repair through improved clearance of lipids and cholesterol from sites of damage. In patients with chronic inflammatory diseases, elevated levels of SAA may contribute to increased severity of the underlying condition. The majority of circulating SAA is bound to lipoproteins, primarily high-density lipoprotein (HDL). Interaction with HDL not only stabilizes SAA but also alters its functional properties, likely through altered accessibility of protein-protein interaction sites on SAA. While high-resolution structures for lipid-free, or apo-, forms of SAA have been reported, their relationship with the HDL-bound form of the protein, and with other possible mechanisms of SAA binding to lipids, has not been established. Here, we have used multiple biophysical techniques, including SAXS, TEM, SEC-MALS, native gel electrophoresis, glutaraldehyde crosslinking, and trypsin digestion to characterize the lipid-free and lipid-bound forms of SAA. The SAXS and TEM data show the presence of soluble octamers of SAA with structural similarity to the ring-like structures reported for lipid-free ApoA-I. These SAA octamers represent a previously uncharacterized structure for lipid-free SAA and are capable of scaffolding lipid nanodiscs with similar morphology to those formed by ApoA-I. The SAA-lipid nanodiscs contain four SAA molecules and have similar exterior dimensions as the lipid-free SAA octamer, suggesting that relatively few conformational rearrangements may be required to allow SAA interactions with lipid-containing particles such as HDL. This study suggests a new model for SAA-lipid interactions and provides new insight into how SAA might stabilize protein-lipid nanodiscs or even replace ApoA-I as a scaffold for HDL particles during inflammation.

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.

Cryo-EM structure of ex vivo fibrils associated with extreme AA amyloidosis prevalence in a cat shelter

AA amyloidosis is a systemic disease characterized by deposition of misfolded serum amyloid A protein (SAA) into cross-β amyloid in multiple organs in humans and animals. AA amyloidosis occurs at high SAA serum levels during chronic inflammation. Prion-like transmission was reported as possible cause of extreme AA amyloidosis prevalence in captive animals, e.g. 70% in cheetah and 57-73% in domestic short hair (DSH) cats kept in zoos and shelters, respectively. Herein, we present the 3.3 Å cryo-EM structure of AA amyloid extracted post-mortem from the kidney of a DSH cat with renal failure, deceased in a shelter with extreme disease prevalence. The structure reveals a cross-β architecture assembled from two 76-residue long proto-filaments. Despite >70% sequence homology to mouse and human SAA, the cat SAA variant adopts a distinct amyloid fold. Inclusion of an eight-residue insert unique to feline SAA contributes to increased amyloid stability. The presented feline AA amyloid structure is fully compatible with the 99% identical amino acid sequence of amyloid fragments of captive cheetah.

Human Serum Amyloid a Impaired Structural Stability of High-Density Lipoproteins (HDL) and Apolipoprotein (Apo) A-I and Exacerbated Glycation Susceptibility of ApoA-I and HDL

Human serum amyloid A (SAA) is an exchangeable apolipoprotein (apo) in high-density lipoprotein (HDL) that influences HDL quality and functionality, particularly in the acute phase of inflammation. On the other hand, the structural and functional correlations of HDL containing SAA and apoA-I have not been reported. The current study was designed to compare the change in HDL quality with increasing SAA content in the lipid-free and lipid-bound states in reconstituted HDL (rHDL). The expressed recombinant human SAA1 (13 kDa) was purified to at least 98% and characterized in the lipid-free and lipid-bound states with apoA-I. The dimyristoyl phosphatidylcholine (DMPC) binding ability of apoA-I was impaired severely by the addition of SAA, while SAA alone could not bind with DMPC. The recombinant human SAA1 was incorporated into the rHDL (molar ratio 95:5:1, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC): cholesterol: apoA-I) with various apoA-I:SAA molar ratios from 1:0 to 1:0.5, 1:1 and 1:2. With increasing SAA1 content, the rHDL particle size was reduced from 98 Å to 93 Å, and the α-helicity of apoA-I:SAA was decreased from 73% to 40% for (1:0) and (1:2), respectively. The wavelength maximum fluorescence (WMF) of tryptophan in rHDL was red-shifted from 339 nm to 345 nm for (1:0) and (1:2) of apoA-I:SAA, respectively, indicating that the addition of SAA to rHDL destabilized the secondary structure of apoA-I. Upon denaturation by urea treatment from 0 M to 8 M, SAA showed only a 3 nm red-shift in WMF, while apoA-I showed a 16 nm red-shift in WMF, indicating that SAA is resistant to denaturation and apoA-I had higher conformational flexibility than SAA. The glycation reaction of apoA-I in the presence of fructose was accelerated up to 1.8-fold by adding SAA in a dose-dependent manner than that of apoA-I alone. In conclusion, the incorporation of SAA in rHDL impaired the structural stability of apoA-I and exacerbated glycation of HDL and apoA-I.

Serum amyloid protein (SAA) as a healthy marker for immune function in Tridacna crocea

Serum amyloid protein (SAA) is known as an acute reactive protein of innate immunity in mammals. However, in invertebrates, the role of SAA in innate immunity is still unclear. In this study, a full-length cDNA of the SAA gene (named TcSAA) was cloned from Tridacna crocea, mollusca. The gene includes a 193 bp 5' untranslated region (UTR) and a 129 bp 3' UTR sequence, and the open reading frame (ORF) with 393 bp nucleotides encodes a polypeptide of 130 amino acids. TcSAA contains a typical signal peptide and an SAA functional domain. The mRNA expression of TcSAA was detected in all 12 selected tissues and 7 different developmental stages. Furthermore, the expression of TcSAA was increased quickly in hemocytes after challenge with V. coralliilyticus or LPS. Furthermore, rTcSAA could bind V. coralliilyticus and V. alginolyticus, and the protein could reduce the lethality rate of the clams from 80% to 55% which caused by V. coralliilyticus about 48 h after injection. In summary, these results indicate that TcSAA may act as a marker for monitoring health and protecting T. crocea.

Dynamic protein structures in normal function and pathologic misfolding in systemic amyloidosis

Dynamic and disordered regions in native proteins are often critical for their function, particularly in ligand binding and signaling. In certain proteins, however, such regions can contribute to misfolding and pathologic deposition as amyloid fibrils in vivo. For example, dynamic and disordered regions can promote amyloid formation by destabilizing the native structure, by directly triggering the aggregation, by promoting protein condensation, or by acting as sites of early proteolytic cleavage that favor a release of aggregation-prone fragments or facilitate fibril maturation. At the same time, enhanced dynamics in the native protein state accelerates proteolytic degradation that counteracts amyloid accumulation in vivo. Therefore, the functional need for dynamic protein regions must be balanced against their inherently labile nature. How exactly this balance is achieved and how is it shifted upon amyloidogenic mutations or post-translational modifications? To illustrate possible scenarios, here we review the beneficial and pathologic roles of dynamic and disordered regions in the native states of three families of human plasma proteins that form amyloid precursors in systemic amyloidoses: immunoglobulin light chain, apolipoproteins, and serum amyloid A. Analysis of structure, stability and local dynamics of these diverse proteins and their amyloidogenic variants exemplifies how disordered/dynamic regions can provide a functional advantage as well as an Achilles heel in pathologic amyloid formation.

Single-cell transcriptomic sequencing analyses of cell heterogeneity during osteogenesis of human adipose-derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) are known for their multilineage differentiation potential with immune-modulatory properties. The molecular underpinnings of differentiation remain largely undefined. In this study, we investigated the cellular and molecular features of chemically induced osteogenesis from MSC isolated from human adipose tissue (human adipose MSCs, hAMSCs) using single-cell RNA-sequencing (scRNA-seq). We found that a near complete differentiation of osteogenic clusters from hAMSCs under a directional induction. Both groups of cells are heterogeneous, and some of the hAMSCs cells are intrinsically prepared for osteogenesis, while variant OS clusters seems in cooperation with a due division of the general function. We identified a set of genes related to cell stress response highly expressed during the differentiation. We also characterized a series of transitional transcriptional waves throughout the process from hAMSCs to osteoblast and specified the unique gene networks and epigenetic status as key markers of osteogenesis.

The turning away of serum amyloid A biological activities and receptor usage

Serum amyloid A (SAA) is an acute-phase protein (APP) to which multiple immunological functions have been attributed. Regardless, the true biological role of SAA remains poorly understood. SAA is remarkably conserved in mammalian evolution, thereby suggesting an important biological function. Since its discovery in the 1970s, the majority of researchers have investigated SAA using recombinant forms made available through bacterial expression. Nevertheless, recent studies indicate that these recombinant forms of SAA are unreliable. Indeed, commercial SAA variants have been shown to be contaminated with bacterial products including lipopolysaccharides and lipoproteins. As such, biological activities and receptor usage (TLR2, TLR4) revealed through the use of commercial SAA variants may not reflect the inherent nature of this APP. Within this review, we discuss the biological effects of SAA that have been demonstrated through more solid experimental approaches. SAA takes part in the innate immune response via the recruitment of leucocytes and executes, through pathogen recognition, antimicrobial activity. Knockout animal models implicate SAA in a range of functions, such as regulation of T-cell-mediated responses and monopoiesis. Moreover, through its structural motifs, not only does SAA function as an extracellular matrix protein, but it also binds extracellular matrix proteins. Finally, we here also provide an overview of definite SAA receptor-mediated functions and highlight those that are yet to be validated. The role of FPR2 in SAA-mediated leucocyte recruitment has been confirmed; nevertheless, SAA has been linked to a range of other receptors including CD36, SR-BI/II, RAGE and P2RX7.

Acquired and inherited amyloidosis: Knowledge driving patients' care

Until recently, systemic amyloidoses were regarded as ineluctably disabling and life-threatening diseases. However, this field has witnessed major advances in the last decade, with significant improvements in therapeutic options and in the availability of accurate and non-invasive diagnostic tools. Outstanding progress includes unprecedented hematological response rates provided by risk-adapted regimens in light chain (AL) amyloidosis and the approval of innovative pharmacological agents for both hereditary and wild-type transthyretin amyloidosis (ATTR). Moreover, the incidence of secondary (AA) amyloidosis has continuously reduced, reflecting advances in therapeutics and overall management of several chronic inflammatory diseases. The identification and validation of novel therapeutic targets has grounded on a better knowledge of key molecular events underlying protein misfolding and aggregation and on the increasing availability of diagnostic, prognostic and predictive markers of organ damage and response to treatment. In this review, we focus on these recent advancements and discuss how they are translating into improved outcomes. Neurological involvement dominates the clinical picture in transthyretin and gelsolin inherited amyloidosis and has a significant impact on disease course and management in all patients. Neurologists, therefore, play a major role in improving patients' journey to diagnosis and in providing early access to treatment in order to prevent significant disability and extend survival.

Methods to study the structure of misfolded protein states in systemic amyloidosis

Systemic amyloidosis is defined as a protein misfolding disease in which the amyloid is not necessarily deposited within the same organ that produces the fibril precursor protein. There are different types of systemic amyloidosis, depending on the protein constructing the fibrils. This review will focus on recent advances made in the understanding of the structural basis of three major forms of systemic amyloidosis: systemic AA, AL and ATTR amyloidosis. The three diseases arise from the misfolding of serum amyloid A protein, immunoglobulin light chains or transthyretin. The presented advances in understanding were enabled by recent progress in the methodology available to study amyloid structures and protein misfolding, in particular concerning cryo-electron microscopy (cryo-EM) and nuclear magnetic resonance (NMR) spectroscopy. An important observation made with these techniques is that the structures of previously described in vitro formed amyloid fibrils did not correlate with the structures of amyloid fibrils extracted from diseased tissue, and that in vitro fibrils were typically more protease sensitive. It is thus possible that ex vivo fibrils were selected in vivo by their proteolytic stability.

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.

High-Density Lipoprotein (HDL) in Allergy and Skin Diseases: Focus on Immunomodulating Functions

From an evolutionary perspective, lipoproteins are not only lipid transporters, but they also have important functions in many aspects of immunity. High-density lipoprotein (HDL) particles are the most abundant lipoproteins and the most heterogeneous in terms of their composition, structure, and biological functions. Despite strong evidence that HDL potently influences the activity of several immune cells, the role of HDL in allergies and skin diseases is poorly understood. Alterations in HDL-cholesterol levels have been observed in allergic asthma, allergic rhinitis, atopic dermatitis (eczema), psoriasis, urticaria, and angioedema. HDL-associated apolipoprotein (apo) A-I, apoA-IV, and apoC-III, and lyso-phosphatidylcholines potently suppress immune cell effector responses. Interestingly, recent studies provided evidence that allergies and skin diseases significantly affect HDL composition, metabolism, and function, which, in turn, could have a significant impact on disease progression, but may also affect the risk of cardiovascular disease and infections. Interestingly, not only a loss in function, but also, sometimes, a gain in function of certain HDL properties is observed. The objective of this review article is to summarize the newly identified changes in the metabolism, composition, and function of HDL in allergies and skin diseases. We aim to highlight the possible pathophysiological consequences with a focus on HDL-mediated immunomodulatory activities.

Structural Basis for Vital Function and Malfunction of Serum Amyloid A: an Acute-Phase Protein that Wears Hydrophobicity on Its Sleeve

PURPOSE OF REVIEW

This review addresses normal and pathologic functions of serum amyloid A (SAA), an enigmatic biomarker of inflammation and protein precursor of AA amyloidosis, a life-threatening complication of chronic inflammation. SAA is a small, highly evolutionarily conserved acute-phase protein whose plasma levels increase up to one thousand-fold in inflammation, infection, or after trauma. The advantage of this dramatic but transient increase is unclear, and the complex role of SAA in immune response is intensely investigated. This review summarizes recent advances in our understanding of the structure-function relationship of this intrinsically disordered protein, outlines its newly emerging beneficial roles in lipid transport and inflammation control, and discusses factors that critically influence its misfolding in AA amyloidosis.

RECENT FINDINGS

High-resolution structures of lipid-free SAA in crystals and fibrils have been determined by x-ray crystallography and electron cryo-microscopy. Low-resolution structural studies of SAA-lipid complexes, together with biochemical, cell-based, animal model, genetic, and clinical studies, have provided surprising new insights into a wide range of SAA functions. An emerging vital role of SAA is lipid encapsulation to remove cell membrane debris from sites of injury. The structural basis for this role has been proposed. The lysosomal origin of AA amyloidosis has solidified, and its molecular and cellular mechanisms have emerged. Recent studies have revealed molecular underpinnings for understanding complex functions of this Cambrian protein in lipid transport, immune response, and amyloid formation. These findings help guide the search for much-needed targeted therapies to block the protein deposition in AA amyloidosis.

Clinical Value of Serum Amyloid-A Protein, High-density Lipoprotein Cholesterol and Apolipoprotein-A1 in the Diagnosis and Follow-up of Neonatal Sepsis

BACKGROUND

To evaluate the performance of serum amyloid-A (SAA), high-density lipoprotein cholesterol (HDL-C) and apolipoprotein-A1 (Apo-A1) levels in the identification and monitoring of neonatal sepsis.

METHODS

This prospective study included 113 full-term septic neonates (postnatal age 4-28 days) admitted to the Special Care Neonatal Unit of a University Hospital from January 1, 2016, to April 30, 2019, and 68 healthy neonates (controls). Blood samples were drawn serially in septic neonates at enrollment and on days 1, 3 and 7, and once in controls, for SAA, HDL-C and Apo-A1 determination.

RESULTS

At enrollment, SAA levels were significantly higher in septic neonates in comparison with controls (median 50.7 vs. 3.5 mg/L; P < 0.0001); HDL-C and Apo-A1 levels were significantly lower in patients than in controls (P < 0.001 and P < 0.006, respectively). SAA levels were higher in culture-positive compared with culture-negative sepsis (median 202.0 vs. 14.2 mg/L; P < 0.0001). HDL-C and Apo-A1 levels did not differ significantly between culture-positive and culture-negative sepsis. Receiver operating characteristic curve analysis of SAA levels at enrollment resulted in significant areas under the curve (AUC) for detecting sepsis {AUC = 0.929 [95% confidence interval: 0.885-0.973]; P < 0.0001} and also for discriminating between culture-positive and culture-negative sepsis [AUC = 0.933 (95% confidence interval: 0.882-0.984); P < 0.0001]. The combination of HDL-C and Apo-A1 with SAA increased its diagnostic performance. Furthermore, serial SAA levels following enrollment could indicate clinical response in septic neonates.

CONCLUSIONS

SAA seems to be a useful biomarker for identification and monitoring of neonatal sepsis, and also for discriminating between culture-positive and culture-negative sepsis. HDL-C and Apo-A1 could be used as complementary markers.

Serum Amyloid A1 (SAA1) Revisited: Restricted Leukocyte-Activating Properties of Homogeneous SAA1

Infection, sterile injury, and chronic inflammation trigger the acute phase response in order to re-establish homeostasis. This response includes production of positive acute phase proteins in the liver, such as members of the serum amyloid A (SAA) family. In humans the major acute phase SAAs comprise a group of closely related variants of SAA1 and SAA2. SAA1 was proven to be chemotactic for several leukocyte subtypes through activation of the G protein-coupled receptor FPRL1/FPR2. Several other biological activities of SAA1, such as cytokine induction, reported to be mediated via TLRs, have been debated recently. Especially commercial SAA1, recombinantly produced in Escherichia coli, was found to be contaminated with bacterial products confounding biological assays performed with this rSAA1. We purified rSAA1 by RP-HPLC to homogeneity, removing contaminants such as lipopolysaccharides, lipoproteins and formylated peptides, and re-assessed several biological activities attributed to SAA1 (chemotaxis, cytokine induction, MMP-9 release, ROS generation, and macrophage differentiation). The homogeneous rSAA1 (hrSAA1) lacked most cell-activating properties, but its leukocyte-recruiting capacity in vivo and it’s in vitro synergy with other leukocyte attractants remained preserved. Furthermore, hrSAA1 maintained the ability to promote monocyte survival. This indicates that pure hrSAA1 retains its potential to activate FPR2, whereas TLR-mediated effects seem to be related to traces of bacterial TLR ligands in the E. coli-produced human rSAA1.

Structural Basis for Lipid Binding and Function by an Evolutionarily Conserved Protein, Serum Amyloid A

Serum amyloid A (SAA) is a plasma protein that transports lipids during inflammation. To explore SAA solution conformations and lipid-binding mechanism, we used hydrogen-deuterium exchange mass spectrometry, lipoprotein reconstitution, amino acid sequence analysis, and molecular dynamics simulations. Solution conformations of lipid-bound and lipid-free mSAA1 at pH~7.4 agreed in details with the crystal structures but also showed important differences. The results revealed that amphipathic α-helices h1 and h3 comprise a lipid-binding site that is partially pre-formed in solution, is stabilized upon binding lipids, and shows lipid-induced folding of h3. This site sequesters apolar ligands via a concave hydrophobic surface in SAA oligomers. The largely disordered/dynamic C-terminal region is conjectured to mediate the promiscuous binding of other ligands. The h1-h2 linker region is predicted to form an unexpected β-hairpin that may represent an early amyloidogenic intermediate. The results help establish structural underpinnings for understanding SAA interactions with its key functional ligands, its evolutional conservation, and its transition to amyloid.

Expression and clinical significance of serum amyloid A and interleukin-6 in patients with acute exacerbation of chronic obstructive pulmonary disease

Expression and clinical significance of serum amyloid A (SAA) and interleukin-6 (IL-6) in patients with acute exacerbation of chronic obstructive pulmonary disease (AECOPD) were investigated. Fifty-one patients with AECOPD (acute group, AG) and 51 patients with stable chronic obstructive pulmonary disease (stable group, SG) admitted to Qingdao Eighth People's Hospital were selected. The expression levels of SAA and IL-6 in peripheral blood of patients in the two groups were detected and compared before and after treatment. Pearson analysis was used in the correlation between SAA and IL-6 and Receiver operating characteristic (ROC) curve was employd to analyze the predictive value of SAA and IL-6 for AECOPD progression. Logistic regression analysis was used to analyze the risk factors of AECOPD patients. The expression levels of SAA and IL-6 of patients in AG were significantly higher than those in SG (P<0.05). Pearson analysis showed that SAA was positively correlated with IL-6 expression (P<0.05). ROC curve analysis showed that AUC predicted by SAA and IL-6 for AECOPD progress was 0.789 and 0.762 (P<0.05). Logistic regression analysis showed that SAA and IL-6 were prediction indexes of AECOPD progression. The levels of SAA and IL-6 were significantly increased during AECOPD and effectively predicted the progress of AECOPD and is a risk factor affecting AECOPD patients.

Serum amyloid A exhibits pH dependent antibacterial action and contributes to host defense against Staphylococcus aureus cutaneous infection

Serum amyloid A (SAA), one of the major highly conserved acute-phase proteins in most mammals, is predominantly produced by hepatocytes and also by a variety of cells in extrahepatic tissues. It is well-known that the expression of SAA is sharply increased in bacterial infections. However, the exact physiological function of SAA during bacterial infection remains unclear. Herein, we showed that SAA expression significantly increased in abscesses of Staphylococcus aureus cutaneous infected mice, which exert direct antibacterial effects by binding to the bacterial cell surface and disrupting the cell membrane in acidic conditions. Mechanically, SAA disrupts anionic liposomes by spontaneously forming small vesicles or micelles under acidic conditions. Especially, the N-terminal region of SAA is necessary for membrane disruption and bactericidal activity. Furthermore, we found that mice deficient in SAA1/2 were more susceptible to infection by S. aureus In addition, the expression of SAA in infected skin was regulated by interleukin-6. Taken together, these findings support a key role of the SAA in host defense and may provide a novel therapeutic strategy for cutaneous bacterial infection.

Cytokines and serum amyloid A in the pathogenesis of hepatitis C virus infection

Expression of the acute phase protein serum amyloid A (SAA) is dependent on the release of the pro-inflammatory cytokines IL-1, IL-6 and TNF-α during infection and inflammation. Hepatitis C virus (HCV) upregulates SAA-inducing cytokines. In line with this, a segment of chronically infected individuals display increased circulating levels of SAA. SAA has even been proposed to be a potential biomarker to evaluate treatment efficiency and the course of disease. SAA possesses antiviral activity against HCV via direct interaction with the viral particle, but might also divert infectivity through its function as an apolipoprotein. On the other hand, SAA shares inflammatory and angiogenic activity with chemotactic cytokines by activating the G protein-coupled receptor, formyl peptide receptor 2. These latter properties might promote chronic inflammation and hepatic injury. Indeed, up to 80 % of infected individuals develop chronic disease because they cannot completely clear the infection, due to diversion of the immune response. In this review, we summarize the interconnection between SAA and cytokines in the context of HCV infection and highlight the dual role SAA could play in this disease. Nevertheless, more research is needed to establish whether the balance between those opposing activities can be tilted in favor of the host defense.

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.

Expression of SAA1, SAA2 and SAA4 genes in human primary monocytes and monocyte-derived macrophages

Circulating serum amyloid A (SAA) is increased in various inflammatory conditions. The human SAA protein family comprises the acute phase SAA1/SAA2, known to activate a large set of innate and adaptive immune cells, and the constitutive SAA4. The liver synthesis of SAA1/SAA2 is well-established but there is still an open debate on extrahepatic SAA expression especially in macrophages. We aimed to investigate the ability of human primary monocytes and monocyte-derived macrophages to express SAA1, SAA2 and SAA4 at both the transcriptional and protein levels, as previous studies almost exclusively dealt with monocytic cell lines. Monocytes and derived macrophages from healthy donors were stimulated under various conditions. In parallel with SAA, pro-inflammatory IL1A, IL1B and IL6 cytokine expression was assessed. While LPS alone was non-effective, a combined LPS/dexamethasone treatment induced SAA1 and to a lesser extent SAA2 transcription in human monocytes and macrophages. In contrast, as expected, pro-inflammatory cytokine expression was strongly induced following stimulation with LPS, an effect which was dampened in the presence of dexamethasone. Furthermore, in monocytes polarized towards a pro-inflammatory M1 phenotype, SAA expression in response to LPS/dexamethasone was potentiated; a result mainly seen for SAA1. However, a major discrepancy was observed between SAA mRNA and intracellular protein levels under the experimental conditions used. Our results demonstrate that human monocytes and macrophages can express SAA genes, mainly SAA1 in response to an inflammatory environment. While SAA is considered as a member of a large cytokine network, its expression in the monocytes-macrophages in response to LPS-dexamethasone is strikingly different from that observed for classic pro-inflammatory cytokines. As monocytes-macrophages are major players in chronic inflammatory diseases, it may be hypothesized that SAA production from macrophages may contribute to the local inflammatory microenvironment, especially when macrophages are compactly organized in granulomas as in sarcoidosis.

Intestinal Serum amyloid A suppresses systemic neutrophil activation and bactericidal activity in response to microbiota colonization

The intestinal microbiota influences the development and function of myeloid lineages such as neutrophils, but the underlying molecular mechanisms are unresolved. Using gnotobiotic zebrafish, we identified the immune effector Serum amyloid A (Saa) as one of the most highly induced transcripts in digestive tissues following microbiota colonization. Saa is a conserved secreted protein produced in the intestine and liver with described effects on neutrophils in vitro, however its in vivo functions remain poorly defined. We engineered saa mutant zebrafish to test requirements for Saa on innate immunity in vivo. Zebrafish mutant for saa displayed impaired neutrophil responses to wounding but augmented clearance of pathogenic bacteria. At baseline, saa mutants exhibited moderate neutrophilia and altered neutrophil tissue distribution. Molecular and functional analyses of isolated neutrophils revealed that Saa suppresses expression of pro-inflammatory markers and bactericidal activity. Saa's effects on neutrophils depended on microbiota colonization, suggesting this protein mediates the microbiota's effects on host innate immunity. To test tissue-specific roles of Saa on neutrophil function, we over-expressed saa in the intestine or liver and found that sufficient to partially complement neutrophil phenotypes observed in saa mutants. These results indicate Saa produced by the intestine in response to microbiota serves as a systemic signal to neutrophils to restrict aberrant activation, decreasing inflammatory tone and bacterial killing potential while simultaneously enhancing their ability to migrate to wounds.

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 sequesters diverse phospholipids and their hydrolytic products, hampering fibril formation and proteolysis in a lipid-dependent manner

Serum amyloid A action in immune response and deposition in inflammation-linked amyloidosis involve SAA-lipid interactions. We show that SAA sequesters neutral and anionic phospholipids and their hydrolytic products to form nanoparticles, suggesting a synergy with phospholipase A2. The lipid charge and shape affect SAA protection from proteolysis, aggregation and fibrillogenesis.

Apolipoprotein A-II induces acute-phase response associated AA amyloidosis in mice through conformational changes of plasma lipoprotein structure

During acute-phase response (APR), there is a dramatic increase in serum amyloid A (SAA) in plasma high density lipoproteins (HDL). Elevated SAA leads to reactive AA amyloidosis in animals and humans. Herein, we employed apolipoprotein A-II (ApoA-II) deficient (Apoa2 ^-/- ) and transgenic (Apoa2Tg) mice to investigate the potential roles of ApoA-II in lipoprotein particle formation and progression of AA amyloidosis during APR. AA amyloid deposition was suppressed in Apoa2 ^-/- mice compared with wild type (WT) mice. During APR, Apoa2 ^-/- mice exhibited significant suppression of serum SAA levels and hepatic Saa1 and Saa2 mRNA levels. Pathological investigation showed Apoa2 ^-/- mice had less tissue damage and less inflammatory cell infiltration during APR. Total lipoproteins were markedly decreased in Apoa2 ^-/- mice, while the ratio of HDL to low density lipoprotein (LDL) was also decreased. Both WT and Apoa2 ^-/- mice showed increases in LDL and very large HDL during APR. SAA was distributed more widely in lipoprotein particles ranging from chylomicrons to very small HDL in Apoa2 ^-/- mice. Our observations uncovered the critical roles of ApoA-II in inflammation, serum lipoprotein stability and AA amyloidosis morbidity, and prompt consideration of therapies for AA and other amyloidoses, whose precursor proteins are associated with circulating HDL particles.

Serum amyloid A forms stable oligomers that disrupt vesicles at lysosomal pH and contribute to the pathogenesis of reactive amyloidosis

Serum amyloid A (SAA) is an acute-phase plasma protein that functions in innate immunity and lipid homeostasis. SAA is a protein precursor of reactive AA amyloidosis, the major complication of chronic inflammation and one of the most common human systemic amyloid diseases worldwide. Most circulating SAA is protected from proteolysis and misfolding by binding to plasma high-density lipoproteins. However, unbound soluble SAA is intrinsically disordered and is either rapidly degraded or forms amyloid in a lysosome-initiated process. Although acidic pH promotes amyloid fibril formation by this and many other proteins, the molecular underpinnings are unclear. We used an array of spectroscopic, biochemical, and structural methods to uncover that at pH 3.5-4.5, murine SAA1 forms stable soluble oligomers that are maximally folded at pH 4.3 with ∼35% α-helix and are unusually resistant to proteolysis. In solution, these oligomers neither readily convert into mature fibrils nor bind lipid surfaces via their amphipathic α-helices in a manner typical of apolipoproteins. Rather, these oligomers undergo an α-helix to β-sheet conversion catalyzed by lipid vesicles and disrupt these vesicles, suggesting a membranolytic potential. Our results provide an explanation for the lysosomal origin of AA amyloidosis. They suggest that high structural stability and resistance to proteolysis of SAA oligomers at pH 3.5-4.5 help them escape lysosomal degradation, promote SAA accumulation in lysosomes, and ultimately damage cellular membranes and liberate intracellular amyloid. We posit that these soluble prefibrillar oligomers provide a missing link in our understanding of the development of AA amyloidosis.

Serum amyloid A self-assembles with phospholipids to form stable protein-rich nanoparticles with a distinct structure: A hypothetical function of SAA as a "molecular mop" in immune response

Serum amyloid A (SAA) is an acute-phase protein whose action in innate immunity and lipid homeostasis is unclear. Most circulating SAA binds plasma high-density lipoproteins (HDL) and reroutes lipid transport. In vivo SAA binds existing lipoproteins or generates them de novo upon lipid uptake from cells. We explored the products of SAA-lipid interactions and lipoprotein remodeling in vitro. SAA complexes with palmitoyl-oleoyl phosphocholine (POPC) were analyzed for structure and stability using circular dichroism and fluorescence spectroscopy, electron microscopy, gel electrophoresis and gel filtration. The results revealed the formation of 8-11nm lipoproteins that were∼50% α-helical and stable at near-physiological conditions but were irreversibly remodeled at T_m∼52°C. Similar HDL-size nanoparticles formed spontaneously at ambient conditions or upon thermal remodeling of parent lipoproteins containing various amounts of proteins and lipids, including POPC and cholesterol. Therefore, such HDL-size particles formed stable kinetically accessible structures in a wide range of conditions. Based on their size and stoichiometry, each particle contained about 12 SAA and 72 POPC molecules, with a protein:lipid weight ratio circa 2.5:1, suggesting a structure distinct from HDL. High stability of these nanoparticles and their HDL-like size suggest that similar lipoproteins may form in vivo during inflammation or injury when SAA concentration is high and membranes from dead cells require rapid removal. We speculate that solubilization of membranes by SAA to generate lipoproteins in a spontaneous energy-independent process constitutes the primordial function of this ancient protein, providing the first line of defense in clearing cell debris from the injured sites.

Carbamylation of the amino-terminal residue (Gly1) of mouse serum amyloid A promotes amyloid formation in a cell culture model

Amyloid A (AA) amyloidosis is a fatal protein deposition disease afflicting a small percentage of patients with chronic inflammation. Factors other than inflammation that determine development of AA amyloidosis remain largely unknown. The subunit protein comprising AA amyloid fibrils is derived from serum amyloid A (SAA), specifically its amino-terminal portion. In this in vitro study, carbamylation of residues in this region (primarily Gly1 but also Lys24) was shown to markedly increase amyloid-forming propensity as judged by extensive accumulation of amyloid in cell cultures. Contrastingly, no amyloid deposition occurred in cultures given SAA having a noncarbamylated amino terminus. Carbamylation, known to occur during uremia or inflammation, merits investigation as a potential determinant of AA amyloid fibril formation.

Triglyceride increase in the core of high-density lipoproteins augments apolipoprotein dissociation from the surface: Potential implications for treatment of apolipoprotein deposition diseases

Lipids in the body are transported via lipoproteins that are nanoparticles comprised of lipids and amphipathic proteins termed apolipoproteins. This family of lipid surface-binding proteins is over-represented in human amyloid diseases. In particular, all major proteins of high-density lipoproteins (HDL), including apoA-I, apoA-II and serum amyloid A, can cause systemic amyloidoses in humans upon protein mutations, post-translational modifications or overproduction. Here, we begin to explore how the HDL lipid composition influences amyloid deposition by apoA-I and related proteins. First, we summarize the evidence that, in contrast to lipoproteins that are stabilized by kinetic barriers, free apolipoproteins are labile to misfolding and proteolysis. Next, we report original biochemical and biophysical studies showing that increase in triglyceride content in the core of plasma or reconstituted HDL destabilizes the lipoprotein assembly, making it more labile to various perturbations (oxidation, thermal and chemical denaturation and enzymatic hydrolysis), and promotes apoA-I release in a lipid-poor/free aggregation-prone form. Together, the results suggest that decreasing plasma levels of triglycerides will shift the dynamic equilibrium from the lipid-poor/free (labile) to the HDL-bound (protected) apolipoprotein state, thereby decreasing the generation of the protein precursor of amyloid. This prompts us to propose that triglyceride-lowering therapies may provide a promising strategy to alleviate amyloid diseases caused by the deposition of HDL proteins.

Paradoxical effects of SAA on lipoprotein oxidation suggest a new antioxidant function for SAA

Oxidative stress and inflammation, which involve a dramatic increase in serum amyloid A (SAA) levels, are critical in the development of atherosclerosis. Most SAA circulates on plasma HDL particles, altering their cardioprotective properties. SAA-enriched HDL has diminished anti-oxidant effects on LDL, which may contribute to atherogenesis. We determined combined effects of SAA enrichment and oxidation on biochemical changes in HDL. Normal human HDLs were incubated with SAA, oxidized by various factors (Cu²⁺, myeloperoxidase, H₂O₂, OCl^-), and analyzed for lipid and protein modifications and biophysical remodeling. Three novel findings are reported: addition of SAA reduces oxidation of HDL and LDL lipids; oxidation of SAA-containing HDL in the presence of OCl^- generates a covalent heterodimer of SAA and apoA-I that resists the release from HDL; and mild oxidation promotes spontaneous release of proteins (SAA and apoA-I) from SAA-enriched HDL. We show that the anti-oxidant effects of SAA extend to various oxidants and are mediated mainly by the unbound protein. We propose that free SAA sequesters lipid hydroperoxides and delays lipoprotein oxidation, though much less efficiently than other anti-oxidant proteins, such as apoA-I, that SAA displaces from HDL. These findings prompt us to reconsider the role of SAA in lipid oxidation in vivo.

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.