Promoting export of macrophage cholesterol: the physiological role of a major acute-phase protein, serum amyloid A 2.1

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.

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.

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

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

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.

High-density lipoprotein inhibits serum amyloid A-mediated reactive oxygen species generation and NLRP3 inflammasome activation

Serum amyloid A (SAA) is a high-density apolipoprotein whose plasma levels can increase more than 1000-fold during a severe acute-phase inflammatory response and are more modestly elevated in chronic inflammation. SAA is thought to play important roles in innate immunity, but its biological activities have not been completely delineated. We previously reported that SAA deficiency protects mice from developing abdominal aortic aneurysms (AAAs) induced by chronic angiotensin II (AngII) infusion. Here, we report that SAA is required for AngII-induced increases in interleukin-1β (IL-1β), a potent proinflammatory cytokine that is tightly controlled by the Nod-like receptor protein 3 (NLRP3) inflammasome and caspase-1 and has been implicated in both human and mouse AAAs. We determined that purified SAA stimulates IL-1β secretion in murine J774 and bone marrow-derived macrophages through a mechanism that depends on NLRP3 expression and caspase-1 activity, but is independent of P2X7 nucleotide receptor (P2X7R) activation. Inhibiting reactive oxygen species (ROS) by N-acetyl-l-cysteine or mito-TEMPO and inhibiting activation of cathepsin B by CA-074 blocked SAA-mediated inflammasome activation and IL-1β secretion. Moreover, inhibiting cellular potassium efflux with glyburide or increasing extracellular potassium also significantly reduced SAA-mediated IL-1β secretion. Of note, incorporating SAA into high-density lipoprotein (HDL) prior to its use in cell treatments completely abolished its ability to stimulate ROS generation and inflammasome activation. These results provide detailed insights into SAA-mediated IL-1β production and highlight HDL's role in regulating SAA's proinflammatory effects.

Serum amyloid A3 is a high density lipoprotein-associated acute-phase protein

Serum amyloid A (SAA) is a family of acute-phase reactants. Plasma levels of human SAA1/SAA2 (mouse SAA1.1/2.1) can increase ≥1,000-fold during an acute-phase response. Mice, but not humans, express a third relatively understudied SAA isoform, SAA3. We investigated whether mouse SAA3 is an HDL-associated acute-phase SAA. Quantitative RT-PCR with isoform-specific primers indicated that SAA3 and SAA1.1/2.1 are induced similarly in livers (∼2,500-fold vs. ∼6,000-fold, respectively) and fat (∼400-fold vs. ∼100-fold, respectively) of lipopolysaccharide (LPS)-injected mice. In situ hybridization demonstrated that all three SAAs are produced by hepatocytes. All three SAA isoforms were detected in plasma of LPS-injected mice, although SAA3 levels were ∼20% of SAA1.1/2.1 levels. Fast protein LC analyses indicated that virtually all of SAA1.1/2.1 eluted with HDL, whereas ∼15% of SAA3 was lipid poor/free. After density gradient ultracentrifugation, isoelectric focusing demonstrated that ∼100% of plasma SAA1.1 was recovered in HDL compared with only ∼50% of SAA2.1 and ∼10% of SAA3. Thus, SAA3 appears to be more loosely associated with HDL, resulting in lipid-poor/free SAA3. We conclude that SAA3 is a major hepatic acute-phase SAA in mice that may produce systemic effects during inflammation.

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

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

Inflammation, remodeling, and other factors affecting HDL cholesterol efflux

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Serum amyloid A and atherosclerosis

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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

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

The role of serum amyloid A and sphingosine-1-phosphate on high-density lipoprotein functionality

The high-density lipoprotein (HDL) is one of the most important endogenous cardiovascular protective markers. HDL is an attractive target in the search for new pharmaceutical therapies and in the prevention of cardiovascular events. Some of HDL's anti-atherogenic properties are related to the signaling molecule sphingosine-1-phosphate (S1P), which plays an important role in vascular homeostasis. However, for different patient populations it seems more complicated. Significant changes in HDL's protective potency are reduced under pathologic conditions and HDL might even serve as a proatherogenic particle. Under uremic conditions especially there is a change in the compounds associated with HDL. S1P is reduced and acute phase proteins such as serum amyloid A (SAA) are found to be elevated in HDL. The conversion of HDL in inflammation changes the functional properties of HDL. High amounts of SAA are associated with the occurrence of cardiovascular diseases such as atherosclerosis. SAA has potent pro-atherogenic properties, which may have impact on HDL's biological functions, including cholesterol efflux capacity, antioxidative and anti-inflammatory activities. This review focuses on two molecules that affect the functionality of HDL. The balance between functional and dysfunctional HDL is disturbed after the loss of the protective sphingolipid molecule S1P and the accumulation of the acute-phase protein SAA. This review also summarizes the biological activities of lipid-free and lipid-bound SAA and its impact on HDL function.

Inflammatory remodeling of the HDL proteome impairs cholesterol efflux capacity

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

Serum amyloid A receptor blockade and incorporation into high-density lipoprotein modulates its pro-inflammatory and pro-thrombotic activities on vascular endothelial cells

The acute phase protein serum amyloid A (SAA), a marker of inflammation, induces expression of pro-inflammatory and pro-thrombotic mediators including ICAM-1, VCAM-1, IL-6, IL-8, MCP-1 and tissue factor (TF) in both monocytes/macrophages and endothelial cells, and induces endothelial dysfunction—a precursor to atherosclerosis. In this study, we determined the effect of pharmacological inhibition of known SAA receptors on pro-inflammatory and pro-thrombotic activities of SAA in human carotid artery endothelial cells (HCtAEC). HCtAEC were pre-treated with inhibitors of formyl peptide receptor-like-1 (FPRL-1), WRW4; receptor for advanced glycation-endproducts (RAGE), (endogenous secretory RAGE; esRAGE) and toll-like receptors-2/4 (TLR2/4) (OxPapC), before stimulation by added SAA. Inhibitor activity was also compared to high-density lipoprotein (HDL), a known inhibitor of SAA-induced effects on endothelial cells. SAA significantly increased gene expression of TF, NFκB and TNF and protein levels of TF and VEGF in HCtAEC. These effects were inhibited to variable extents by WRW4, esRAGE and OxPapC either alone or in combination, suggesting involvement of endothelial cell SAA receptors in pro-atherogenic gene expression. In contrast, HDL consistently showed the greatest inhibitory action, and often abrogated SAA-mediated responses. Increasing HDL levels relative to circulating free SAA may prevent SAA-mediated endothelial dysfunction and ameliorate atherogenesis.

Intrinsic Stability, Oligomerization, and Amyloidogenicity of HDL-Free Serum Amyloid A

Serum amyloid A (SAA) is an acute-phase reactant protein predominantly bound to high-density lipoprotein in serum and presumed to play various biological and pathological roles. Upon tissue trauma or infection, hepatic expression of SAA increases up to 1,000 times the basal levels. Prolonged increased levels of SAA may lead to amyloid A (AA) amyloidosis, a usually fatal systemic disease in which the amyloid deposits are mostly comprised of the N-terminal 1-76 fragment of SAA. SAA isoforms may differ across species in their ability to cause AA amyloidosis, and the mechanism of pathogenicity remains poorly understood. In vitro studies have shown that SAA is a marginally stable protein that folds into various oligomeric species at 4 °C. However, SAA is largely disordered at 37 °C, reminiscent of intrinsically disordered proteins. Non-pathogenic murine (m)SAA2.2 spontaneously forms amyloid fibrils in vitro at 37 °C whereas pathogenic mSAA1.1 has a long lag (nucleation) phase, and eventually forms fibrils of different morphology than mSAA2.2. Remarkably, human SAA1.1 does not form mature fibrils in vitro. Thus, it appears that the intrinsic amyloidogenicity of SAA is not a key determinant of pathogenicity, and that other factors, including fibrillation kinetics, ligand binding effects, fibril stability, nucleation efficiency, and SAA degradation may play key roles. This chapter will focus on the known structural and biophysical properties of SAA and discuss how these properties may help better understand the molecular mechanism of AA amyloidosis.

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.

N-terminal hydrophobic amino acids of activating transcription factor 5 (ATF5) protein confer interleukin 1β (IL-1β)-induced stabilization

Activating transcription factor 5 (ATF5) is a stress-response transcription factor that responds to amino acid limitation and exposure to cadmium chloride (CdCl2) and sodium arsenite (NaAsO2). The N-terminal amino acids contribute to the destabilization of the ATF5 protein in steady-state conditions and serve as a stabilization domain in the stress response after CdCl2 or NaAsO2 exposure. In this study, we show that interleukin 1β (IL-1β), a proinflammatory cytokine, increases the expression of ATF5 protein in HepG2 hepatoma cells in part by stabilizing the ATF5 protein. The N-terminal domain rich in hydrophobic amino acids that is predicted to form a hydrophobic network was responsible for destabilization in steady-state conditions and served as an IL-1β response domain. Furthermore, IL-1β increased the translational efficiency of ATF5 mRNA via the 5' UTRα and phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α). ATF5 knockdown in HepG2 cells up-regulated the IL-1β-induced expression of the serum amyloid A 1 (SAA1) and SAA2 genes. Our results show that the N-terminal hydrophobic amino acids play an important role in the regulation of ATF5 protein expression in IL-1β-mediated immune response and that ATF5 is a negative regulator for IL-1β-induced expression of SAA1 and SAA2 in HepG2 cells.

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.

Impact of serum amyloid A on cellular cholesterol efflux to serum in type 2 diabetes mellitus

OBJECTIVE

Serum amyloid A (SAA) is an acute phase response protein and has apolipoprotein properties. Since type 2 diabetes is associated with chronic subclinical inflammation, the objective of this study is to investigate the changes in SAA level in type 2 diabetic patients and to evaluate the relationship between SAA and the capacity of serum to induce cellular cholesterol efflux via the two known cholesterol transporters, scavenger receptor class B type I (SR-BI) and ATP-binding cassette transporter G1 (ABCG1).

METHODS

264 patients with type 2 diabetes mellitus (42% with normoalbuminuria, 30% microalbuminuria, and 28% proteinuria) and 275 non-diabetic controls were recruited. SAA was measured by ELISA. SR-BI and ABCG1-mediated cholesterol efflux to serum were determined by measuring the transfer of [(3)H]cholesterol from Fu5AH rat hepatoma cells expressing SR-BI and from human ABCG1-transfected CHO-K1 cells to the medium containing the tested serum respectively.

RESULTS

SAA was significantly increased in diabetic patients with incipient or overt nephropathy. Both SR-BI and ABCG1-mediated cholesterol efflux to serum were significantly impaired in all three groups of diabetic patients (p < 0.01). SAA inversely correlated with SR-BI-mediated cholesterol efflux (r = -0.36, p < 0.01) but did not correlate with ABCG1-mediated cholesterol efflux. Stepwise linear regression analysis showed that HDL, the presence or absence of diabetes, and log(SAA) were significant independent determinants of SR-BI-mediated cholesterol efflux to serum.

CONCLUSION

SAA was increased in type 2 diabetic patients with incipient or overt nephropathy, and SAA was associated with impairment of SR-BI-mediated cholesterol efflux to serum.

No evidence for a role of adipose tissue-derived serum amyloid a in the development of insulin resistance or obesity-related inflammation in hSAA1(+/-) transgenic mice

Obesity is associated with a low-grade inflammation including moderately increased serum levels of the acute phase protein serum amyloid A (SAA). In obesity, SAA is mainly produced from adipose tissue and serum levels of SAA are associated with insulin resistance. SAA has been described as a chemoattractant for inflammatory cells and adipose tissue from obese individuals contains increased numbers of macrophages. However, whether adipose tissue-derived SAA can have a direct impact on macrophage infiltration in adipose tissue or the development of insulin resistance is unknown. The aim of this study was to investigate the effects of adipose tissue-derived SAA1 on the development of insulin resistance and obesity-related inflammation. We have previously established a transgenic mouse model expressing human SAA1 in the adipose tissue. For this report, hSAA1^+/− transgenic mice and wild type mice were fed with a high fat diet or normal chow. Effects of hSAA1 on glucose metabolism were assessed using an oral glucose tolerance test. Real-time PCR was used to measure the mRNA levels of macrophage markers and genes related to insulin sensitivity in adipose tissue. Cytokines during inflammation were analyzed using a Proinflammatory 7-plex Assay. We found similar insulin and glucose levels in hSAA1 mice and wt controls during an oral glucose tolerance test and no decrease in mRNA levels of genes related to insulin sensitivity in adipose tissue in neither male nor female hSAA1 animals. Furthermore, serum levels of proinflammatory cytokines and mRNA levels of macrophage markers in adipose tissue were not increased in hSAA1 mice. Hence, in this model we find no evidence that adipose tissue-derived hSAA1 influences the development of insulin resistance or obesity-related inflammation.

Characterization of the oligomerization and aggregation of human Serum Amyloid A

The fibrillation of Serum Amyloid A (SAA) – a major acute phase protein – is believed to play a role in the disease Amyloid A (AA) Amyloidosis. To better understand the amyloid formation pathway of SAA, we characterized the oligomerization, misfolding, and aggregation of a disease-associated isoform of human SAA – human SAA1.1 (hSAA1.1) – using techniques ranging from circular dichroism spectroscopy to atomic force microscopy, fluorescence spectroscopy, immunoblot studies, solubility measurements, and seeding experiments. We found that hSAA1.1 formed alpha helix-rich, marginally stable oligomers in vitro on refolding and cross-beta-rich aggregates following incubation at 37°C. Strikingly, while hSAA1.1 was not highly amyloidogenic in vitro, the addition of a single N-terminal methionine residue significantly enhanced the fibrillation propensity of hSAA1.1 and modulated its fibrillation pathway. A deeper understanding of the oligomerization and fibrillation pathway of hSAA1.1 may help elucidate its pathological role.

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

Studies suggest that inflammation impairs reverse cholesterol transport (RCT). We investigated whether serum amyloid A (SAA) contributes to this impairment using an established macrophage-to-feces RCT model. Wild-type (WT) mice and mice deficient in SAA1.1 and SAA2.1 (SAAKO) were injected intraperitoneally with ³H-cholesterol-labeled J774 macrophages 4 hr after administration of LPS or buffered saline. ³H-cholesterol in plasma 4 hr after macrophage injection was significantly reduced in both WT and SAAKO mice injected with LPS, but this was not associated with a reduced capacity of serum from LPS-injected mice to promote macrophage cholesterol efflux in vitro. Hepatic accumulation of ³H-cholesterol was unaltered in either WT or SAAKO mice by LPS treatment. Radioactivity present in bile and feces of LPS-injected WT mice 24 hr after macrophage injection was reduced by 36% (P < 0.05) and 80% (P < 0.001), respectively. In contrast, in SAAKO mice, LPS did not significantly reduce macrophage-derived ³H-cholesterol in bile, and fecal excretion was reduced by only 45% (P < 0.05). Injection of cholesterol-loaded allogeneic J774 cells, but not syngeneic bone-marrow-derived macrophages, transiently induced SAA in C57BL/6 mice. Our study confirms reports that acute inflammation impairs steps in the RCT pathway and establishes that SAA plays only a minor role in this impairment.

Heparan sulfate dissociates serum amyloid A (SAA) from acute-phase high-density lipoprotein, promoting SAA aggregation

Inflammation-related (AA) amyloidosis is a severe clinical disorder characterized by the systemic deposition of the acute-phase reactant serum amyloid A (SAA). SAA is normally associated with the high-density lipoprotein (HDL) fraction in plasma, but under yet unclear circumstances, the apolipoprotein is converted into amyloid fibrils. AA amyloid and heparan sulfate (HS) display an intimate relationship in situ, suggesting a role for HS in the pathogenic process. This study reports that HS dissociates SAA from HDLs isolated from inflamed mouse plasma. Application of surface plasmon resonance spectroscopy and molecular modeling suggests that HS simultaneously binds to two apolipoproteins of HDL, SAA and ApoA-I, and thereby induce SAA dissociation. The activity requires a minimum chain length of 12-14 sugar units, proposing an explanation to previous findings that short HS fragments preclude AA amyloidosis. The results address the initial events in the pathogenesis of AA amyloidosis.

Acute-phase serum amyloid A: perspectives on its physiological and pathological roles

Serum amyloid A (SAA), a protein originally of interest primarily to investigators focusing on AA amyloidogenesis, has become a subject of interest to a very broad research community. SAA is still a major amyloid research topic because AA amyloid, for which SAA is the precursor, is the prototypic model of in vivo amyloidogenesis and much that has been learned with this model has been applicable to much more common clinical types of amyloid. However, SAA has also become a subject of considerable interest to those studying (i) the synthesis and regulation of acute phase proteins, of which SAA is a prime example, (ii) the role that SAA plays in tissue injury and inflammation, a situation in which the plasma concentration of SAA may increase a 1000-fold, (iii) the influence that SAA has on HDL structure and function, because during inflammation the majority of SAA is an apolipoprotein of HDL, (iv) the influence that SAA may have on HDL's role in reverse cholesterol transport, and therefore, (v) SAA's potential role in atherogenesis. However, no physiological role for SAA, among many proposed, has been widely accepted. None the less from an evolutionary perspective SAA must have a critical physiological function conferring survival-value because SAA genes have existed for at least 500 million years and SAA's amino acid sequence has been substantially conserved. An examination of the published literature over the last 40 years reveals a great deal of conflicting data and interpretation. Using SAA's conserved amino acid sequence and the physiological effects it has while in its native structure, namely an HDL apolipoprotein, we argue that much of the confounding data and interpretation relates to experimental pitfalls not appreciated when working with SAA, a failure to appreciate the value of physiologic studies done in the 1970-1990 and a current major focus on putative roles of SAA in atherogenesis and chronic disease. When viewed from an evolutionary perspective, published data suggest that acute-phase SAA is part of a systemic response to injury to recycle and reuse cholesterol from destroyed and damaged cells. This is accomplished through SAA's targeted delivery of HDL to macrophages, and its suppression of ACAT, the enhancement of neutral cholesterol esterase and ABC transporters in macrophages. The recycling of cholesterol during serious injury, when dietary intake is restricted and there is an immediate and critical requirement of cholesterol in the generation of myriads of cells involved in inflammation and repair responses, is likely SAA's important survival role. Data implicating SAA in atherogenesis are not relevant to its evolutionary role. Furthermore, in apoE(-/-) mice, domains near the N- and C- termini of SAA inhibit the initiation and progression of aortic lipid lesions illustrating the conflicting nature of these two sets of data.

Serum amyloid A 2.2 refolds into a octameric oligomer that slowly converts to a more stable hexamer

Serum amyloid A (SAA) is an inflammatory protein predominantly bound to high-density lipoprotein in plasma and presumed to play various biological and pathological roles. We previously found that the murine isoform SAA2.2 exists in aqueous solution as a marginally stable hexamer at 4-20°C, but becomes an intrinsically disordered protein at 37°C. Here we show that when urea-denatured SAA2.2 is dialyzed into buffer (pH 8.0, 4°C), it refolds mostly into an octameric species. The octamer transitions to the hexameric structure upon incubation from days to weeks at 4°C, depending on the SAA2.2 concentration. Thermal denaturation of the octamer and hexamer monitored by circular dichroism showed that the octamer is ∼10°C less stable, with a denaturation mid point of ∼22°C. Thus, SAA2.2 becomes kinetically trapped by refolding into a less stable, but more kinetically accessible octameric species. The ability of SAA2.2 to form different oligomeric species in vitro along with its marginal stability, suggest that the structure of SAA might be modulated in vivo to form different biologically relevant species.

ATP binding cassette G1-dependent cholesterol efflux during inflammation

ATP binding cassette transporter G1 (ABCG1) mediates the transport of cellular cholesterol to HDL, and it plays a key role in maintaining macrophage cholesterol homeostasis. During inflammation, HDL undergoes substantial remodeling, acquiring lipid changes and serum amyloid A (SAA) as a major apolipoprotein. In the current study, we investigated whether remodeling of HDL that occurs during acute inflammation impacts ABCG1-dependent efflux. Our data indicate that lipid free SAA acts similarly to apolipoprotein A-I (apoA-I) in mediating sequential efflux from ABCA1 and ABCG1. Compared with normal mouse HDL, acute phase (AP) mouse HDL containing SAA exhibited a modest but significant 17% increase in ABCG1-dependent efflux. Interestingly, AP HDL isolated from mice lacking SAA (SAAKO mice) was even more effective in promoting ABCG1 efflux. Hydrolysis with Group IIA secretory phospholipase A(2) (sPLA(2)-IIA) significantly reduced the ability of AP HDL from SAAKO mice to serve as a substrate for ABCG1-mediated cholesterol transfer, indicating that phospholipid (PL) enrichment, and not the presence of SAA, is responsible for alterations in efflux. AP human HDL, which is not PL-enriched, was somewhat less effective in mediating ABCG1-dependent efflux compared with normal human HDL. Our data indicate that inflammatory remodeling of HDL impacts ABCG1-dependent efflux independent of SAA.

Serum amyloid A as a marker and mediator of acute coronary syndromes

Inflammation promotes acute coronary syndromes and ensuing clinical complications. An emerging downstream marker of inflammation is serum amyloid A (SAA). Elevated plasma SAA levels predict increased cardiovascular risk and portend worse prognosis in patients with acute coronary artery disease (CAD). The pathophysiological role of SAA remains enigmatic. SAA plays a role in host defense, but it might also be atherogenic. SAA affects cholesterol transport, contributes to endothelial dysfunction, promotes thrombosis, evokes recruitment of inflammatory cells, activates neutrophils and suppresses neutrophil apoptosis, key events underlying acute coronary syndromes. These results provide a potential link between SAA and CAD and suggest that reducing SAA levels and/or opposing the actions of SAA may have beneficial effects in patients with acute CAD.

High-density lipoprotein and the acute phase response

PURPOSE OF REVIEW

Inflammation and the concomitant acute phase response induce marked changes in the lipoprotein profile, particularly the high-density lipoprotein (HDL) fraction. The present review describes the transfer proteins and lipases that remodel HDL and regulate its plasma levels, discusses the changes occurring in their activities during inflammation, and the influence of this altered remodeling on HDL function. The review will also discuss the contribution of the ATP-binding-membrane-cassette transporters to the protective actions of HDL.

RECENT FINDINGS

Studies using different models showed that remodeling of acute phase HDL in vitro generates pre-beta migrating particles capable of cholesterol efflux. Induction of the acute phase response in humans resulted in a reduction of HDL phospholipids without a change in HDL-cholesterol. However, the capacity of HDL to promote cholesterol efflux ex vivo was impaired. Studies with ATP-binding-membrane-cassette transporter A1 and ATP-binding-membrane-cassette transporter G1 knockout mice demonstrated anti-inflammatory roles for these transporters by virtue of reducing cell-membrane-free cholesterol and lipid raft content, thus attenuating proinflammatory signaling pathways.

SUMMARY

It is well known that HDL has anti-inflammatory properties that are diminished during inflammation. Acute phase HDL contains serum amyloid A that can be liberated during remodeling by cholesteryl ester transfer protein and secretory phospholipase A2, or other inflammatory factors. The ability of serum amyloid A and apolipoprotein A-I to promote cholesterol efflux may confer protective effects during the acute phase response.

High density lipoprotein structure-function and role in reverse cholesterol transport

High density lipoprotein (HDL) possesses important anti-atherogenic properties and this review addresses the molecular mechanisms underlying these functions. The structures and cholesterol transport abilities of HDL particles are determined by the properties of their exchangeable apolipoprotein (apo) components. ApoA-I and apoE, which are the best characterized in structural terms, contain a series of amphipathic alpha-helical repeats. The helices located in the amino-terminal two-thirds of the molecule adopt a helix bundle structure while the carboxy-terminal segment forms a separately folded, relatively disorganized, domain. The latter domain initiates lipid binding and this interaction induces changes in conformation; the alpha-helix content increases and the amino-terminal helix bundle can open subsequently. These conformational changes alter the abilities of apoA-I and apoE to function as ligands for their receptors. The apoA-I and apoE molecules possess detergent-like properties and they can solubilize vesicular phospholipid to create discoidal HDL particles with hydrodynamic diameters of ~10 nm. In the case of apoA-I, such a particle is stabilized by two protein molecules arranged in an anti-parallel, double-belt, conformation around the edge of the disc. The abilities of apoA-I and apoE to solubilize phospholipid and stabilize HDL particles enable these proteins to be partners with ABCA1 in mediating efflux of cellular phospholipid and cholesterol, and the biogenesis of HDL particles. ApoA-I-containing nascent HDL particles play a critical role in cholesterol transport in the circulation whereas apoE-containing HDL particles mediate cholesterol transport in the brain. The mechanisms by which HDL particles are remodeled by lipases and lipid transfer proteins, and interact with SR-BI to deliver cholesterol to cells, are reviewed.

Secretory phospholipase A2, group IIA is a novel serum amyloid A target gene: activation of smooth muscle cell expression by an interleukin-1 receptor-independent mechanism

Atherosclerosis is a multifactorial vascular disease characterized by formation of inflammatory lesions. Elevated circulating acute phase proteins indicate disease risk. Serum amyloid A (SAA) is one such marker but its function remains unclear. To determine the role of SAA on aortic smooth muscle cell gene expression, a preliminary screen of a number of genes was performed and a strong up-regulation of expression of secretory phospholipase A(2), group IIA (sPLA(2)) was identified. The SAA-induced increase in sPLA(2) was validated by real time PCR, Western blot analysis, and enzyme activity assays. Demonstrating that SAA increased expression of sPLA(2) heteronuclear RNA and that inhibiting transcription eliminated the effect of SAA on sPLA(2) mRNA suggested that the increase was transcriptional. Transient transfections and electrophoretic mobility shift assays identified CAAT enhancer-binding protein (C/EBP) and nuclear factor kappaB (NFkappaB) as key regulatory sites mediating the induction of sPLA(2). Moreover, SAA activated the inhibitor of NF-kappaB kinase (IKK) in cultured smooth muscle cells. Previous reports showed that interleukin (IL)-1beta up-regulates Pla2g2a gene transcription via C/EBPbeta and NFkappaB. Interestingly, SAA activated smooth muscle cell IL-1beta mRNA expression, however, blocking IL-1 receptors had no effect on SAA-mediated activation of sPLA(2) expression. Thus, the observed changes in sPLA(2) expression were not secondary to SAA-induced IL-1 receptor activation. The association of SAA with high density lipoprotein abrogated the SAA-induced increase in sPLA(2) expression. These data suggest that during atherogenesis, SAA can amplify the involvement of smooth muscle cells in vascular inflammation and that this can lead to deposition of sPLA(2) and subsequent local changes in lipid homeostasis.

Acute-phase response protein serum amyloid A stimulates renal tubule formation: studies in vitro and in vivo

Serum amyloid A protein (SAA) surges 1,000-fold in the blood of acute-phase animals, and yet its function during these acute events remains unknown. We report herein that SAA stimulates a developmental program in cultured NRK-52E cells that culminates in differentiated and functional tubules that feature a proximal tubule phenotype. We also found strong SAA expression in states of tubule formation (in utero stage) and regeneration (recovery from ischemia-reperfusion injury). These data lend support to a novel view of a more localized renal acute-phase reaction, where renal SAA may act as a paracrine or autocrine molecule that promotes tubule formation during development and repair.

Acute-phase-HDL remodeling by heparan sulfate generates a novel lipoprotein with exceptional cholesterol efflux activity from macrophages

During episodes of acute-inflammation high-density lipoproteins (HDL), the carrier of so-called good cholesterol, experiences a major change in apolipoprotein composition and becomes acute-phase HDL (AP-HDL). This altered, but physiologically important, HDL has an increased binding affinity for macrophages that is dependent on cell surface heparan sulfate (HS). While exploring the properties of AP-HDLratioHS interactions we discovered that HS caused significant remodeling of AP-HDL. The physical nature of this change in structure and its potential importance for cholesterol efflux from cholesterol-loaded macrophages was therefore investigated. In the presence of heparin, or HS, AP-HDL solutions at pH 5.2 became turbid within minutes. Analysis by centrifugation and gel electrophoresis indicated that AP-HDL was remodeled generating novel lipid poor particles composed only of apolipoprotein AI, which we designate beta2. This remodeling is dependent on pH, glycosaminoglycan type, is promoted by Ca(2+) and is independent of protease or lipase activity. Compared to HDL and AP-HDL, remodeled AP-HDL (S-HDL-SAA), containing beta2 particles, demonstrated a 3-fold greater cholesterol efflux activity from cholesterol-loaded macrophage. Because the identified conditions causing this change in AP-HDL structure and function can exist physiologically at the surface of the macrophage, or in its endosomes, we postulate that AP-HDL contains latent functionalities that become apparent and active when it associates with macrophage cell surface/endosomal HS. In this way initial steps in the reverse cholesterol transport pathway are focused at sites of injury to mobilize cholesterol from macrophages that are actively participating in the phagocytosis of damaged membranes rich in cholesterol. The mechanism may also be of relevance to aspects of atherogenesis.

Serum amyloid A, in vivo splenic cholesterol export and its potential implications in hemolytic disorders

A model to examine the in vivo relationship of acute phase serum amyloid A (SAA) to spleen cholesterol mobilisation was devised. Reticuloendothelial cells in vivo were loaded with a known quantity of cholesterol (1.5 mg) by infusing fragmented red blood cell membranes, which consist of approximately 50% cholesterol by dry weight. Following infusion, 7% of the infused cholesterol was in the spleen and significantly increased (by 35%) spleen cholesterol concentration above the baseline. An acute inflammatory reaction was induced by the subcutaneous injection of AgNO(3) which also raised spleen cholesterol values, but not significantly. Both treatments were also administered together and the increase in spleen cholesterol concentration after 1 h was equivalent to the sum of the individual treatments. In all the treatment groups, the spleen cholesterol concentration and the plasma SAA values were then followed over a period of 24 h. In all treatment groups the spleen cholesterol values fell to baseline values primarily between 18 and 24 h which coincided with significantly raised levels of plasma SAA. In the case of the dual treatment, between 4 and 18 h, SAA increased from 92.1 +/- 12.3 to 478 +/- 58.3 microg/ml, respectively and depletion of spleen cholesterol occurred gradually reaching baseline values after 24 h. The significant flux of cholesterol though the spleen raises the distinct possibility that the spleen is much more involved in cholesterol metabolism than previously appreciated. Furthermore, the speed with which plasma SAA increases following the infusion of fragmented red blood cell membranes and the role that SAA plays in cholesterol mobilisation raise issues that may be relevant to alterations in plasma acute phase protein and lipid parameters in patients undergoing transfusions or suffering from hemolytic disorders.

Primer: the practical use of biological markers of rheumatic and systemic inflammatory diseases

The assessment of systemic inflammation by means of laboratory tests often complements the results of medical examination. Traditionally, the erythrocyte sedimentation rate and leukocytosis with left shift are diagnostic markers for inflammatory and infectious diseases. The levels of acute-phase proteins--especially C-reactive protein--are used to assess both the presence of inflammation and any response to treatment. The determination of C-reactive protein levels may be advised in three types of pathological situation: infection, acute or chronic inflammation, and evaluation of metabolic risk. Procalcitonin is useful as a marker of sepsis and severe infection. The concentration of serum amyloid A predicts the chances of survival of patients with secondary (AA) amyloidosis. Ferritin and its glycosylated form are of interest in the study of specific diseases such as adult-onset Still's disease. Markers of cartilage and bone turnover are complementary to these markers of inflammation. Although cytokine serum levels are transiently crucial to the generation of inflammation, their usefulness in the clinic is still under investigation. Serum concentrations of cytokine inhibitors or soluble cytokine receptors, as well as the clinical response of patients to treatment with cytokine antagonists, might generate important information for monitoring autoinflammatory diseases.

Molecular interactions of acute phase serum amyloid A: possible involvement in carcinogenesis

Acute phase serum amyloid A (A-SAA) is a well-known marker of inflammation. The present review summarizes data on the regulation of A-SAA expression, signaling pathways which it is involved in, its effects, and possible influences on progression of malignant tumors.

HDL cholesterol transport during inflammation

PURPOSE OF REVIEW

The aim of this article is to review recent advances made towards understanding how inflammation and acute phase proteins, particularly serum amyloid A and group IIa secretory phospholipase A2, may alter reverse cholesterol transport by HDL during inflammation and the acute phase response.

RECENT FINDINGS

Findings suggest that the decreased apoA-I content and markedly increased serum amyloid A content in HDL during the acute phase response result from reciprocal and coordinate transcriptional regulation of these proteins as well as HDL remodeling by group IIa secretory phospholipase A2. Serum amyloid A functions efficiently in a lipid-free or lipid-poor form to promote cholesterol efflux by ATP binding cassette protein ABCA1, evidently by functioning directly as an acceptor for cholesterol efflux as well as by increasing the availability of cellular free cholesterol. Serum amyloid A increases the ability of acute phase HDL to serve as an acceptor for SR-BI-dependent cellular cholesterol efflux. Altered remodeling of HDL by group IIa secretory phospholipase A2 in concert with cholesterol ester transfer protein may contribute to the generation of lipid-poor apoA-I and serum amyloid A acceptors for cholesterol efflux.

SUMMARY

Current data support a model for the acute phase response in which serum amyloid A and sPLA2-IIa, present at sites of inflammation and tissue damage, play a protective role by enhancing cellular cholesterol efflux, thereby promoting the removal of excess cholesterol from macrophages.

The lipidation status of acute-phase protein serum amyloid A determines cholesterol mobilization via scavenger receptor class B, type I

During the acute-phase reaction, SAA (serum amyloid A) replaces apoA-I (apolipoprotein A-I) as the major HDL (high-density lipoprotein)-associated apolipoprotein. A remarkable portion of SAA exists in a lipid-free/lipid-poor form and promotes ABCA1 (ATP-binding cassette transporter A1)-dependent cellular cholesterol efflux. In contrast with lipid-free apoA-I and apoE, lipid-free SAA was recently reported to mobilize SR-BI (scavenger receptor class B, type I)-dependent cellular cholesterol efflux [Van der Westhuyzen, Cai, de Beer and de Beer (2005) J. Biol. Chem. 280, 35890-35895]. This unique property could strongly affect cellular cholesterol mobilization during inflammation. However, in the present study, we show that overexpression of SR-BI in HEK-293 cells (human embryonic kidney cells) (devoid of ABCA1) failed to mobilize cholesterol to lipid-free or lipid-poor SAA. Only reconstituted vesicles containing phospholipids and SAA promoted SR-BI-mediated cholesterol efflux. Cholesterol efflux from HEK-293 and HEK-293[SR-BI] cells to lipid-free and lipid-poor SAA was minimal, while efficient efflux was observed from fibroblasts and CHO cells (Chinese-hamster ovary cells) both expressing functional ABCA1. Overexpression of SR-BI in CHO cells strongly attenuated cholesterol efflux to lipid-free SAA even in the presence of an SR-BI-blocking IgG. This implies that SR-BI attenuates ABCA1-mediated cholesterol efflux in a way that is not dependent on SR-BI-mediated re-uptake of cholesterol. The present in vitro experiments demonstrate that the lipidation status of SAA is a critical factor governing cholesterol acceptor properties of this amphipathic apolipoprotein. In addition, we demonstrate that SAA mediates cellular cholesterol efflux via the ABCA1 and/or SR-BI pathway in a similar way to apoA-I.

Cellular events associated with the initial phase of AA amyloidogenesis: insights from a human monocyte model

Reactive amyloidosis is a systemic protein deposition disease that develops in association with chronic inflammation. The deposits are composed of extracellular, fibrillar masses of amyloid A (AA) protein, an N-terminal fragment of the acute-phase serum protein serum amyloid A (SAA). The pathogenic conversion of SAA into amyloid has been studied in two human cell culture models, peritoneal cells and peripheral blood monocytes. Human monocyte cultures proved more robust than either mouse or human peritoneal cells at initiating amyloid formation in the absence of a preformed nidus such as amyloid-enhancing factor and particularly well suited for examination of individual cells undergoing amyloid formation. Amyloid-producing monocyte cultures were stained with Congo red and Alcian blue for detection of amyloid and glycosaminglycans, respectively; immunocytochemistry was performed to identify SAA/AA, CD68, CD14, lysosomal protein Lamp-1, and early endosomal protein EEA1. SAA interaction with monocytes was also visualized directly via fluorescence confocal microscopy. Amyloid was initially detected only in intracellular vesicles, but with time was seen extracellularly. Morphologic changes in lysosomes were noted during the early phase of amyloid formation, suggesting that exocytosis of fibrils may occur via lysosome-derived vesicles. Cultures engaged in amyloid formation remained metabolically active; no cytotoxic effects were observed. Mimicking in vivo phenomena, amyloid formation was accompanied by increased glycosaminoglycan content and C-terminal processing of SAA. The ability of human monocytes to endocytose and intracellularly transform SAA into amyloid via a mechanism that requires and maintains, rather than compromises, metabolic activity distinguishes them as a useful model for probing earliest events in the disease process.