Expression of the third member of the serum amyloid A gene family in mouse adipocytes

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

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

The uterine secretory cycle: recurring physiology of endometrial outputs that setup the uterine luminal microenvironment

Conserved in female reproduction across all mammalian species is the estrous cycle and its regulation by the hypothalamic-pituitary-gonadal (HPG) axis, a collective of intersected hormonal events that are crucial for ensuring uterine fertility. Nonetheless, knowledge of the direct mediators that synchronously shape the uterine microenvironment for successive yet distinct events, such as the transit of sperm and support for progressive stages of preimplantation embryo development, remain principally deficient. Toward understanding the timed endometrial outputs that permit luminal events as directed by the estrous cycle, we used Bovidae as a model system to uniquely surface sample and study temporal shifts to in vivo endometrial transcripts that encode for proteins destined to be secreted. The results revealed the full quantitative profile of endometrial components that shape the uterine luminal microenvironment at distinct phases of the estrous cycle (estrus, metestrus, diestrus, and proestrus). In interpreting this comprehensive log of stage-specific endometrial secretions, we define the "uterine secretory cycle" and extract a predictive understanding of recurring physiological actions regulated within the uterine lumen in anticipation of sperm and preimplantation embryonic stages. This repetitive microenvironmental preparedness to sequentially provide operative support was a stable intrinsic framework, with only limited responses to sperm or embryos if encountered in the lumen within the cyclic time period. In uncovering the secretory cycle and unraveling realistic biological processes, we present novel foundational knowledge of terminal effectors controlled by the HPG axis to direct a recurring sequence of vital functions within the uterine lumen.NEW & NOTEWORTHY This study unravels the recurring sequence of changes within the uterus that supports vital functions (sperm transit and development of preimplantation embryonic stages) during the reproductive cycle in female Ruminantia. These data present new systems knowledge in uterine reproductive physiology crucial for setting up in vitro biomimicry and artificial environments for assisted reproduction technologies for a range of mammalian species.

Serum amyloid A and metabolic disease: evidence for a critical role in chronic inflammatory conditions

Serum amyloid A (SAA) subtypes 1-3 are well-described acute phase reactants that are elevated in acute inflammatory conditions such as infection, tissue injury, and trauma, while SAA4 is constitutively expressed. SAA subtypes also have been implicated as playing roles in chronic metabolic diseases including obesity, diabetes, and cardiovascular disease, and possibly in autoimmune diseases such as systemic lupus erythematosis, rheumatoid arthritis, and inflammatory bowel disease. Distinctions between the expression kinetics of SAA in acute inflammatory responses and chronic disease states suggest the potential for differentiating SAA functions. Although circulating SAA levels can rise up to 1,000-fold during an acute inflammatory event, elevations are more modest (∼5-fold) in chronic metabolic conditions. The majority of acute-phase SAA derives from the liver, while in chronic inflammatory conditions SAA also derives from adipose tissue, the intestine, and elsewhere. In this review, roles for SAA subtypes in chronic metabolic disease states are contrasted to current knowledge about acute phase SAA. Investigations show distinct differences between SAA expression and function in human and animal models of metabolic disease, as well as sexual dimorphism of SAA subtype responses.

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.

Accelerated atherosclerosis caused by serum amyloid A response in lungs of ApoE-/- mice

Airway exposure to eg particulate matter is associated with cardiovascular disease including atherosclerosis. Acute phase genes, especially Serum Amyloid A3 (Saa3), are highly expressed in the lung following pulmonary exposure to particles. We aimed to investigate whether the human acute phase protein SAA (a homolog to mouse SAA3) accelerated atherosclerotic plaque progression in Apolipoprotein E knockout (ApoE^-/- ) mice. Mice were intratracheally (i.t.) instilled with vehicle (phosphate buffered saline) or 2 µg human SAA once a week for 10 weeks. Plaque progression was assessed in the aorta using noninvasive ultrasound imaging of the aorta arch as well as by en face analysis. Additionally, lipid peroxidation, SAA3, and cholesterol were measured in plasma, inflammation was determined in lung, and mRNA levels of the acute phase genes Saa1 and Saa3 were measured in the liver and lung, respectively. Repeated i.t. instillation with SAA caused a significant progression in the atherosclerotic plaques in the aorta (1.5-fold). Concomitantly, SAA caused a statistically significant increase in neutrophils in bronchoalveolar lavage fluid (625-fold), in pulmonary Saa3 (196-fold), in systemic SAA3 (1.8-fold) and malondialdehyde levels (1.14-fold), indicating acute phase response (APR), inflammation and oxidative stress. Finally, pulmonary exposure to SAA significantly decreased the plasma levels of very low-density lipoproteins - low-density lipoproteins and total cholesterol, possibly due to lipids being sequestered in macrophages or foam cells in the arterial wall. Combined these results indicate the importance of the pulmonary APR and SAA3 for plaque progression.

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 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.

Production of serum amyloid A in equine articular chondrocytes and fibroblast-like synoviocytes treated with proinflammatory cytokines and its effects on the two cell types in culture

OBJECTIVE

To investigate the role of the major equine acute phase protein serum amyloid A (SAA) in inflammation of equine intraarticular tissues.

SAMPLE

Articular chondrocytes and fibroblast-like synoviocytes (FLSs) from 8 horses (4 horses/cell type).

PROCEDURES

Chondrocytes and FLSs were stimulated in vitro for various periods up to 48 hours with cytokines (recombinant interleukin [IL]-1β, IL-6, tumor necrosis factor-α, or a combination of all 3 [IIT]) or with recombinant SAA. Gene expression of SAA, IL-6, matrix metalloproteinases (MMP)-1 and -3, and cartilage-derived retinoic acid-sensitive protein were assessed by quantitative real-time PCR assay; SAA protein was evaluated by immunoturbidimetry and denaturing isoelectric focusing and western blotting.

RESULTS

All cytokine stimulation protocols increased expression of SAA mRNA and resulted in detectable SAA protein production in chondrocytes and FLSs. Isoforms of SAA in lysed chondrocytes and their culture medium corresponded to those previously detected in synovial fluid from horses with joint disease. When exposed to SAA, chondrocytes and FLSs had increased expression of IL-6, SAA, and MMP3, and chondrocytes had increased expression of MMP-1. Chondrocytes had decreased expression of cartilage-derived retinoic acid-sensitive protein.

CONCLUSIONS AND CLINICAL RELEVANCE

Upregulation of SAA in chondrocytes and FLSs stimulated with proinflammatory cytokines and the proinflammatory effects of SAA suggested that SAA may be involved in key aspects of pathogenesis of the joint inflammation in horses.

Serum amyloid A1: Structure, function and gene polymorphism

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

Human serum amyloid A3 (SAA3) protein, expressed as a fusion protein with SAA2, binds the oxidized low density lipoprotein receptor

Serum amyloid A3 (SAA3) possesses characteristics distinct from the other serum amyloid A isoforms, SAA1, SAA2, and SAA4. High density lipoprotein contains the latter three isoforms, but not SAA3. The expression of mouse SAA3 (mSAA3) is known to be up-regulated extrahepatically in inflammatory responses, and acts as an endogenous ligand for the toll-like receptor 4/MD-2 complex. We previously reported that mSAA3 plays an important role in facilitating tumor metastasis by attracting circulating tumor cells and enhancing hyperpermeability in the lungs. On the other hand, human SAA3 (hSAA3) has long been regarded as a pseudogene, which is in contrast to the abundant expression levels of the other isoforms. Although the nucleotide sequence of hSAA3 is very similar to that of the other SAAs, a single oligonucleotide insertion in exon 2 causes a frame-shift to generate a unique amino acid sequence. In the present study, we identified that hSAA3 was transcribed in the hSAA2-SAA3 fusion transcripts of several human cell lines. In the fusion transcript, hSAA2 exon 3 was connected to hSAA3 exon 1 or hSAA3 exon 2, located approximately 130kb downstream from hSAA2 exon 3 in the genome, which suggested that it is produced by alternative splicing. Furthermore, we succeeded in detecting and isolating hSAA3 protein for the first time by an immunoprecipitation-enzyme linked immune assay system using monoclonal and polyclonal antibodies that recognize the hSAA3 unique amino acid sequence. We also demonstrated that hSAA3 bound oxidized low density lipoprotein receptor (oxLDL receptor, LOX-1) and elevated the phosphorylation of ERK, the intracellular MAP-kinase signaling protein.

Deletion of serum amyloid A3 improves high fat high sucrose diet-induced adipose tissue inflammation and hyperlipidemia in female mice

Serum amyloid A (SAA) increases in response to acute inflammatory stimuli and is modestly and chronically elevated in obesity. SAA3, an inducible form of SAA, is highly expressed in adipose tissue in obese mice where it promotes monocyte chemotaxis, providing a mechanism for the macrophage accumulation that occurs with adipose tissue expansion in obesity. Humans do not express functional SAA3 protein, but instead express SAA1 and SAA2 in hepatic as well as extrahepatic tissues, making it difficult to distinguish between liver and adipose tissue-specific SAA effects. SAA3 does not circulate in plasma, but may exert local effects that impact systemic inflammation. We tested the hypothesis that SAA3 contributes to chronic systemic inflammation and adipose tissue macrophage accumulation in obesity using mice deficient for Saa3 (Saa3(-/-)). Mice were rendered obese by feeding a pro-inflammatory high fat, high sucrose diet with added cholesterol (HFHSC). Both male and female Saa3(-/-) mice gained less weight on the HFHSC diet compared to Saa3(+/+) littermate controls, with no differences in body composition or resting metabolism. Female Saa3(-/-) mice, but not males, had reduced HFHSC diet-induced adipose tissue inflammation and macrophage content. Both male and female Saa3(-/-) mice had reduced liver Saa1 and Saa2 expression in association with reduced plasma SAA. Additionally, female Saa3(-/-) mice, but not males, showed improved plasma cholesterol, triglycerides, and lipoprotein profiles, with no changes in glucose metabolism. Taken together, these results suggest that the absence of Saa3 attenuates liver-specific SAA (i.e., SAA1/2) secretion into plasma and blunts weight gain induced by an obesogenic diet. Furthermore, adipose tissue-specific inflammation and macrophage accumulation are attenuated in female Saa3(-/-) mice, suggesting a novel sexually dimorphic role for this protein. These results also suggest that Saa3 influences liver-specific SAA1/2 expression, and that SAA3 could play a larger role in the acute phase response than previously thought.

Integrating murine gene expression studies to understand obstructive lung disease due to chronic inhaled endotoxin

RATIONALE

Endotoxin is a near ubiquitous environmental exposure that that has been associated with both asthma and chronic obstructive pulmonary disease (COPD). These obstructive lung diseases have a complex pathophysiology, making them difficult to study comprehensively in the context of endotoxin. Genome-wide gene expression studies have been used to identify a molecular snapshot of the response to environmental exposures. Identification of differentially expressed genes shared across all published murine models of chronic inhaled endotoxin will provide insight into the biology underlying endotoxin-associated lung disease.

METHODS

We identified three published murine models with gene expression profiling after repeated low-dose inhaled endotoxin. All array data from these experiments were re-analyzed, annotated consistently, and tested for shared genes found to be differentially expressed. Additional functional comparison was conducted by testing for significant enrichment of differentially expressed genes in known pathways. The importance of this gene signature in smoking-related lung disease was assessed using hierarchical clustering in an independent experiment where mice were exposed to endotoxin, smoke, and endotoxin plus smoke.

RESULTS

A 101-gene signature was detected in three murine models, more than expected by chance. The three model systems exhibit additional similarity beyond shared genes when compared at the pathway level, with increasing enrichment of inflammatory pathways associated with longer duration of endotoxin exposure. Genes and pathways important in both asthma and COPD were shared across all endotoxin models. Mice exposed to endotoxin, smoke, and smoke plus endotoxin were accurately classified with the endotoxin gene signature.

CONCLUSIONS

Despite the differences in laboratory, duration of exposure, and strain of mouse used in three experimental models of chronic inhaled endotoxin, surprising similarities in gene expression were observed. The endotoxin component of tobacco smoke may play an important role in disease development.

Assay validation and diagnostic applications of major acute-phase protein testing in companion animals

The use of major acute-phase proteins (APPs) for assessment of health and disease in companion animals has increased within the last decade because of increased knowledge in the field and increased access to appropriate assay systems for detection of relevant APPs, which are highly species specific. Despite evidence being restricted almost solely to proven excellent overlap performance of these markers in detecting inflammatory activity, clinically relevant studies at higher evidence levels do exist. The available body of literature shows a clear, but seemingly untapped, potential for more extended routine clinical use of major APP testing in companion animal medicine.

The autocrine and paracrine roles of adipokines

Obesity, defined by an excess of adipose tissue, is often associated with the development of various metabolic diseases. The increased and inappropriate deposition of this tissue contributes to hyperglycemia, hyperlipidemia, insulin resistance, endothelial dysfunction and chronic inflammation. Recent evidence suggests that factors expressed and secreted by the adipose tissue, adipokines, may contribute to the development of these abnormalities by mechanisms including inhibition of adipogenesis, adipocyte hypertrophy and death, immune cell infiltration and disruption of tissue metabolism. The presence of adipokine receptors in adipocytes renders these cells available to autocrine and paracrine effects of adipokines. In this review the reported local effects of adipokines on adipose tissue structure, inflammation and regulation of metabolic functions, in the face of over-nutrition and consequent obesity, are outlined. Elucidating the local regulation of white adipocyte development and function could help in the design of effective, tissue-specific therapies for obesity-associated diseases.

Adipose tissue depots of Holstein cows are immune responsive: inflammatory gene expression in vitro

The transcriptional response of adipose tissue depots with respect to their immune responsiveness in dairy cows remains largely unknown. Thus, we examined mRNA expression and responsiveness of subcutaneous (SUB) and mesenteric (MES) adipose tissue from nonpregnant dairy cows to a short-term (2 h), in vitro lipopolysaccharide (LPS) challenge (20 microg/mL in physiological saline). Abundance of mRNA for tumor necrosis factor-alpha (TNFA), interleukin-6 (IL6), serum amyloid A3 (SAA3), toll-like receptor 4 (TLR4), monocyte chemoattractant protein-1 (CCL2), and RANTES/chemokine C-C motif ligand 5 (CCL5) were analyzed using quantitative polymerase chain reaction (PCR) from tissue samples collected at slaughter from 5 nonpregnant/nonlactating Holstein cows. Prior to LPS challenge, SAA3 mRNA abundance was greater in MES than SUB tissue. Regardless of depot site, LPS led to greater mRNA abundance of TNFA and IL6 and was more pronounced for IL6 in MES. We also observed a marked increased in expression of CCL2, CCL5, TLR4, IL6, and TNFA in both MES and SUB during the 2-h incubation with saline alone (ie, the control). Because mRNA expression of the apoptotic markers B-cell CLL/lymphoma 2 (BCL2) and tumor protein p53 (TP53) did not differ during the 2-h incubation, it is less likely that the response to saline was a result of increased rate of cell death during incubation. Analysis using semiquantitative PCR of the 16s rRNA gene in cDNA from tissue explants revealed the presence of bacteria likely arising from contamination during sample collection. Furthermore, surfactant medium from about 50% of explant cultures had viable aerobic bacteria without differences between treatments or tissue samples. Thus, the presence of bacteria could partly explain the large increase in inflammatory-related genes after 2-h incubation with saline. The higher SAA3 expression in MES suggests that this acute-phase protein has a role in lipid metabolism and/or transport during an immune challenge. Overall, results provided evidence that adipose depots of dairy cows are capable of synthesizing chemokines and are immune responsive when exposed to inflammatory conditions that can arise from a pathogenic insult or during and soon after parturition.

Microbial regulation of SAA3 expression in mouse colon and adipose tissue

Recently, we demonstrated that colonic and adipose expression of SAA3 was modulated by the gut microbiota and Toll-like receptor signaling in mice. We observed that SAA3 was expressed by colonic epithelial cells and that its expression was induced in a murine colonocyte cell line following lipopolysaccharide stimulation and nuclear NFκB translocation. In this addendum, we extend this initial study and suggest that SAA3 (1) resembles human SAA1 both in amino acid homology and tissue distribution, (2) appears to have autocrine or paracrine effects rather than endocrine, and (3) binds to bacteria within the gastrointestinal tract. Although speculative, these observations raise the possibility that SAA3 may promote local inflammation in adipose tissue that affects insulin signaling and also function as an antimicrobial agent in the colon.

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.

Serum amyloid A attenuates cellular insulin sensitivity by increasing JNK activity in 3T3-L1 adipocytes

A permanent increase in acute-phase serum amyloid A (A-SAA) level is observed in obesity and insulin resistance. Recently, A-SAA has been shown to correlate with obesity and insulin resistance in human. However, what triggers A-SAA up-regulation is poorly understood, and the mechanism of elevated A-SAA to insulin resistance has not been elucidated. In this study, we used two cellular models of insulin resistance, one induced by treatment with tumor necrosis factor-alpha (TNF-alpha) and the other with the glucocorticoid dexamethasone. Gene expression analysis showed that SAA3 mRNA levels were increased in both models of insulin resistance, and ELISA showed that A-SAA levels were increased in both models too. To assess the potential impact of A-SAA on insulin resistance, we treated 3T3-L1 adipocytes with recombinant human SAA (Rh-SAA) and found that Rh-SAA attenuated cellular insulin sensitivity, up-regulated the level of phosphor-JNK, and down-regulated the level of phosphotyrosine-IRS-1 and the expression of glucose transporter 4 (GLUT4) in 3T3-L1 adipocytes. Pre-treatment of cells with C-Jun amino-terminal kinases (JNK) inhibitor brought about partial restoration of Rh-SAA-induced insulin resistance. In sum, our findings suggest that serum amyloid A might be a marker of insulin resistance, and it might play a major role in the development of obesity-related insulin resistance. Moreover, in our study it has been proved that JNK is indeed a crucial component of the pathway responsible for SAA-induced insulin resistance in 3T3-L1 adipocytes, which suggests that a selective interference with JNK activity might be a useful strategy in the treatment of Type 2 diabetes and other insulin-resistant states.

Regulation of serum amyloid A3 (SAA3) in mouse colonic epithelium and adipose tissue by the intestinal microbiota

The gut microbiota has been proposed as an environmental factor that affects the development of metabolic and inflammatory diseases in mammals. Recent reports indicate that gut bacteria-derived lipopolysaccharide (LPS) can initiate obesity and insulin resistance in mice; however, the molecular interactions responsible for microbial regulation of host metabolism and mediators of inflammation have not been studied in detail. Hepatic serum amyloid A (SAA) proteins are markers and proposed mediators of inflammation that exhibit increased levels in serum of insulin-resistant mice. Adipose tissue-derived SAA3 displays monocyte chemotactic activity and may play a role in metabolic inflammation associated with obesity and insulin resistance. To investigate a potential mechanistic link between the intestinal microbiota and induction of proinflammatory host factors, we performed molecular analyses of germ-free, conventionally raised and genetically modified Myd88-/- mouse models. SAA3 expression was determined to be significantly augmented in adipose (9.9+/-1.9-fold; P<0.001) and colonic tissue (7.0+/-2.3-fold; P<0.05) by the presence of intestinal microbes. In the colon, we provided evidence that SAA3 is partially regulated through the Toll-like receptor (TLR)/MyD88/NF-kappaB signaling axis. We identified epithelial cells and macrophages as cellular sources of SAA3 in the colon and found that colonic epithelial expression of SAA3 may be part of an NF-kappaB-dependent response to LPS from gut bacteria. In vitro experiments showed that LPS treatments of both epithelial cells and macrophages induced SAA3 expression (27.1+/-2.5-fold vs. 1.6+/-0.1-fold, respectively). Our data suggest that LPS, and potentially other products of the indigenous gut microbiota, might elevate cytokine expression in tissues and thus exacerbate chronic low-grade inflammation observed in obesity.

AA-amyloidosis can be transferred by peripheral blood monocytes

Spongiform encephalopathies have been reported to be transmitted by blood transfusion even prior to the clinical onset. Experimental AA-amyloidosis shows similarities with prion disease and amyloid-containing organ-extracts can prime a recipient for the disease. In this systemic form of amyloidosis N-terminal fragments of the acute-phase reactant apolipoprotein serum amyloid A are the main amyloid protein. Initial amyloid deposits appear in the perifollicular region of the spleen, followed by deposits in the liver. We used the established murine model and induced AA-amyloidosis in NMRI mice by intravenous injections of purified amyloid fibrils ('amyloid enhancing factor') combined with inflammatory challenge (silver nitrate subcutaneously). Blood plasma and peripheral blood monocytes were isolated, sonicated and re-injected into new recipients followed by an inflammatory challenge during a three week period. When the animals were sacrificed presence of amyloid was analyzed in spleen sections after Congo red staining. Our result shows that some of the peripheral blood monocytes, isolated from animals with detectable amyloid, contained amyloid-seed that primed for AA-amyloid. The seeding material seems to have been phagocytosed by the cells since the AA-precursor (SAA1) was found not be expressed by the monocytes. Plasma recovered from mice with AA amyloidosis lacked seeding capacity. Amyloid enhancing activity can reside in monocytes recovered from mice with AA-amyloidosis and in a prion-like way trigger amyloid formation in conjunction with an inflammatory disorder. Human AA-amyloidosis resembles the murine form and every individual is expected to be exposed to conditions that initiate production of the acute-phase reactant. The monocyte-transfer mechanism should be eligible for the human disease and we point out blood transfusion as a putative route for transfer of amyloidosis.

Induction of serum amyloid A genes is associated with growth and apoptosis of HC11 mammary epithelial cells

In this study, we examined the expression and functions of serum amyloid A (SAA) isoforms during apoptosis of HC11 mammary gland epithelial cells. Expression of SAA mRNAs and apoptosis were increased in HC11 cells by serum withdrawal and gradually decreased upon the addition of serum, or epidermal growth factor (EGF). TNFalpha treatment of HC11 cells also induced expression of SAA genes, and the effect on SAA1 and SAA2 expression was suppressed by treatment with MG132, and in cells transfected with a dominant negative mutant form of IkappaBalpha. Similar results were observed in response to interleukin-1 (IL-1), IL-6 and interferon gamma (IFNgamma). Furthermore, overexpression of the SAA1 and SAA2 isoforms suppressed growth and accelerated apoptosis of HC11 cells by increasing caspase 3/7 and caspase 8 activities, but the apoptotic effect of tumor necrosis factor alpha (TNFalpha) on HC11 cells was not enhanced. We found that expression of SAA1 and SAA2, but not SAA3, was regulated by an NFkappaB-dependent pathway, and that overexpression of SAA isoforms accelerated the apoptosis of HC11 cells.

Pulmonary and systemic induction of SAA3 after ventilation and endotoxin in preterm lambs

Serum amyloid A (SAA), an acute phase reactant (APR) protein, is induced in liver during systemic inflammation. Serum amyloid A3 (SAA3), an isoform of SAA, is induced in both liver and extra hepatic sites in response to proinflammatory stimuli such as cytokines. Previously, we showed a modest increase in plasma cytokine levels in a preterm lamb model of lung injury. The study objective was to determine the relative contributions of lung and liver to the acute phase response during postnatal lung injury. Preterm (130d) and near term (141d) newborn lambs (term=150d) were randomized to either no ventilation (controls), ventilation+intratracheal (IT) endotoxin (endo) or ventilation+IT saline. A group of near term lambs were exposed to ventilation+IV endotoxin. In the lungs, ventilation alone increased SAA3 mRNA 3- and 13-fold while ventilation+IT endotoxin increased SAA3 mRNA 64 and 366-fold above controls in preterm and near term lambs, respectively. In the liver, SAA3 mRNA was induced by ventilation alone (three-fold) and ventilation+IT endotoxin (45-fold) above controls in both preterm and near term animals. Ventilation + IV endotoxin caused the highest increase in SAA3 mRNA (212-fold) in the liver of near term animals. A different isoform, identified as SAA-Liver inducible was maximally induced in liver by ventilation alone with minimal further response to endotoxin. Lung SAA3 mRNA expression was detected primarily in airway epithelium, bronchial glands, perichondrium of bronchial cartilage and vascular smooth muscle cells. Our experiments show rapid induction of an APR gene in lung in response to proinflammatory stimuli.

Serum amyloid A production by chicken fibroblast-like synoviocytes

In brown chicken with chronic inflammatory processes of the joints amyloid arthropathy easily develops. The amyloid has been shown to be of the AA type which is derived from serum amyloid A (SAA). The aim of the present study was to investigate whether fibroblast-like synoviocytes (FLS) originating from brown chicken and other chicken breeds express SAA mRNA and produce SAA protein. FLS were isolated from the knee joint synovium of healthy brown chickens, white chickens, and broilers. The absence of macrophages in FLS cultures was confirmed by assessment of the phagocytic capability and by immunohistochemistry. Additionally, cultured cells were identified by electron microscopy and immunohistochemical staining. Expression of SAA mRNA in normal and lipopolysaccharide (LPS)-stimulated cells was assessed by in situ hybridization, Northern blot analysis, reverse transcriptase-polymerase chain reaction (RT-PCR), Southern blot analysis and real-time quantitative PCR. SAA protein production was analyzed by Western blotting and ELISA. SAA mRNA was detected in unstimulated FLS isolated from the three different chicken breeds and more abundantly in those stimulated with LPS. However, SAA protein production was only detected in culture medium and cell lysate of LPS-stimulated FLS. Furthermore, FLS produced SAA in a concentration-dependent manner after stimulation with different amounts of LPS. The data suggest that during infection and inflammation chicken FLS may act as a source of articular SAA. This process may enhance development of amyloid from SAA in the joint.

Macrophage cholesterol efflux and the active domains of serum amyloid A 2.1

Serum amyloid A 2.1 (SAA2.1) suppresses ACAT and stimulates cholesteryl ester hydrolase (CEH) activities in cholesterol-laden macrophages, and in the presence of a cholesterol transporter and an extracellular acceptor, there is a marked increase in the rate of cholesterol export in culture and in vivo. The stimulation of CEH activity by SAA2.1 is not affected by chloroquine, suggesting that it operates on neutral CEH rather than the lysosomal form. With liposomes containing individual peptides of SAA2.1, residues 1-20 inhibit ACAT activity, residues 74-103 stimulate CEH activity, and each of residues 1-20 and 74-103 promotes macrophage cholesterol efflux to HDL in culture media. In combination, these peptides exhibit a profound effect, so that 55-70% of cholesterol is exported to media HDL in 24 h. The effect is also demonstrable in vivo. [3H]cholesterol-laden macrophages injected intravenously into mice were allowed to establish themselves for 24 h. Thereafter, the mice received a single intravenous injection of liposomes containing intact SAA1.1, SAA2.1, peptides composed of SAA2.1 residues 1-20, 21-50, 51-80, 74-103, or SAA1.1 residues 1-20. Only liposomes containing intact SAA2.1 or its residues 1-20 or 74-103 promoted the efflux of cholesterol in vivo. A single injection of each of the active peptides is effective in promoting cholesterol efflux in vivo for at least 4 days.

Human serum amyloid A3 peptide enhances intestinal MUC3 expression and inhibits EPEC adherence

We previously determined that the N-terminal region of bovine mammary-associated serum amyloid A3 (M-SAA3) increased intestinal mucin MUC3 levels in HT29 human intestinal cells by approximately 2.5-fold, relative to untreated cells. This study shows that the human M-SAA3 N-terminal peptide further enhances MUC3 transcript levels by approximately 4.3-fold in these cells (p<0.02), implicating a species-specific interaction. Furthermore, immunofluorescence and immunoblot analysis using a MUC3-specific monoclonal antibody confirms that the human M-SAA3 peptide stimulates MUC3 protein expression and secretion by the HT29 cells. More importantly, pretreatment of the cells with the peptide causes a subsequent 73% decrease in the adherence of enteropathogenic Escherichia coli (EPEC) to these cells, relative to untreated cells (p<0.01). The intestinal mucin MUC3 has been shown to provide a protective barrier in the gut and inhibit adherence of pathogens to the gut wall. Therefore, a means to increase MUC3 protein expression by a colostrum-associated peptide or protein may be a highly effective prophylactic treatment for the prevention of gastrointestinal diseases such as necrotizing enterocolitis and infectious diarrhea.

The conserved TFLK motif of mammary-associated serum amyloid A3 is responsible for up-regulation of intestinal MUC3 mucin expression in vitro

In various mammalian species, an isoform of serum amyloid A is secreted at high concentrations into colostrum. A conserved four-amino-acid motif (TFLK) is contained within the first eight N-terminal amino acid residues of this mammary-associated serum amyloid A isoform 3 (M-SAA3). Peptides derived from the bovine N-terminal amino acid sequence of M-SAA3 were produced and added to cell culture medium of HT29 cells to study the effects on intestinal mucin gene expression. HT29 cells were grown to enhance expression of either MUC2 or MUC3 intestinal mucins. After incubation, total RNA was isolated for Northern blot analyses using MUC2 or MUC3 mucin cDNA probes. Signals were detected by autoradiography with mRNA levels expressed relative to 28S rRNA. The 10-mer peptides containing the intact TFLK-motif or a TFLK 4-mer peptide increased MUC3 mRNA expression compared with control cells (p < 0.05). There was no effect of these peptides on MUC2 mRNA expression. Cells that were incubated with 10-mer N-terminal derived peptides containing a scrambled TFLK motif, with all 10 amino acid residues scrambled or derived from a C-terminal region of M-SAA3, did not show increased MUC3 expression. Inhibition of enteropathogenic Escherichia coli strain E2348/69 adhesion to HT29 cells grown to enhance MUC3 expression was reduced by a similar amount when either peptides containing the intact TFLK motif or probiotic microbes were added to cell culture medium compared with control cells. M-SAA3 is a bioactive peptide secreted into colostrums that can up-regulate mucin expression and thereby may enhance innate protective mechanisms that limit access of deleterious microbes to intestinal mucosal epithelial cells in the postparturition period.

Functional analysis of a minimal mouse serum amyloid A3 promoter in transgenic mice

We report the generation of transgenic mice harboring the SAA3/LacZ transgene and analysis of its expression patterns in vivo following LPS-induced inflammation. Our results show that a 210-bp fragment of the mouse SAA3 promoter when placed in front of the LacZ gene was sufficient to confer basal and inflammation-induced reporter gene expression. Consistent with endogenous SAA3 expression, the basal level of LacZ expression was high in the lung and liver of newborn and 1-week-old transgenic mice. Its expression however decreased with increasing age and at 3-weeks ofage, LacZ expression was very low in the lung and was essentially undetectable in the liver. When SAA3/LacZ transgenic mice were injected with lipopolisaccharide to induce inflammation, beta-gal activities were increased approximately 6- and 16-fold in the lung and liver, respectively. Histological examination of lung and liver tissues stained with X-gal revealed that the LacZ transgene was expressed primarily in the macrophages. Thus, this minimal SAA3 promoter fragment contains the necessary regulatory sequences for its expression and cytokine responsiveness in macrophages albeit is insufficient to confer expression in hepatocytes.

Elevated extrahepatic expression and secretion of mammary-associated serum amyloid A 3 (M-SAA3) into colostrum

Mammary-associated serum amyloid A 3 (M-SAA3) was secreted at highly elevated levels in bovine, equine and ovine colostrum and found at lower levels in milk 4 days postparturition. N-terminal sequencing of the mature M-SAA3 protein from all the three species revealed a conserved four amino acid motif (TFLK) within the first eight residues. This motif has not been reported to be present in any of the hepatically-produced acute phase SAA (A-SAA) isoforms. Cloning of the bovine M-Saa3 cDNA from mammary gland epithelial cells revealed an open reading frame that encoded a precursor protein of 131 amino acids which included an 18 amino acid signal peptide. The predicted 113 residue mature M-SAA3 protein had a theoretical molecular mass of 12,826Da that corresponded with the observed 12.8kDa molecular mass obtained for M-SAA3 in immunoblot analysis. The high abundance of this extrahepatically produced SAA3 isoform in the colostrum of healthy animals suggests that M-SAA3 may play an important functional role associated with newborn adaptation to extrauterine life and possibly mammary tissue remodeling.

Endotoxemia and acute-phase proteins in major abdominal surgery

BACKGROUND

Translocation of endotoxin is a controversial issue. The ability of plasma to inactivate endotoxin is an indirect measure of endotoxemia. Endotoxin is a potent stimulator of the inflammatory response and affects the innate immune system.

OBJECTIVE

To elucidate the kinetics of endotoxemia and the ability of plasma to inactivate endotoxin in patients with major abdominal operations. To demonstrate the early time course of the acute-phase proteins C-reactive protein (CRP), serum amyloid A (SAA), alpha(1)-antitrypsin, alpha(2)-macroglobulin, transferrin, and interleukin 6 (IL-6), and to correlate them with the amount of endotoxemia.

METHODS

Twenty patients with elective major abdominal operation and 10 healthy controls were investigated. Blood was collected preoperatively, during the operation and regularly up to 12 days after surgery. Endotoxin was measured by Limulus amebocyte lysate test (LAL), the ability of plasma to inactivate endotoxin by modified LAL, the acute-phase proteins nephelometrically, and IL-6 by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Preoperative endotoxin plasma level (0.026 +/- 0.004 EU/mL) did not differ from healthy volunteers but increased during operation (0.09 +/- 0.02 EU/mL, P = 0.02). Endotoxemia peaked 1 hour after the surgical procedure (0.16 +/- 0.03 EU/mL; P <0.0001 versus preoperative) and decreased to almost normal values after 48 hours. The capability of plasma to inactivate endotoxin was significantly reduced during (recovery, 0.16 +/- 0.03 EU/mL), 1 hour (0.25 +/- 0.04 EU/mL) and 24 hours (0.16 +/- 0.02 EU/mL) after the operation compared with preoperative (0.068 +/- 0.01 EU/mL) values. Plasma IL-6 was significantly increased for 48 hours with a peak 1 hour after surgery (470 +/- 108 pg/mL). CRP peaked at 210 +/- 19 mg/L (P <0.0001 versus preoperative) 48 hours after operation and was significantly elevated for the rest of the observation period. SAA was significantly increased 24 hours after surgery (249 +/- 45 mg/L) and peaked additional 48 hours later (456 +/- 86 mg/L). alpha(1)-Antitrypsin, although a positive acute-phase protein, decreased initially to 1.38 +/- 0.1 g/L (preoperative, 2.33 +/- 0.18 g/L; P <0.0001) and increased thereafter until day 12 (3.05 +/- 0.35 g/L, P = 0.11 versus preoperative). The same was true for alpha(2)-macroglobulin (preoperative, 2.2 +/- 0.16 g/L; intraoperative, 1.36 +/- 0.13 g/L; day 5, 2.8 +/- 0.4 g/L). Transferrin decreased already during surgery (1.6 +/- 0.1 g/L versus preoperative 2.8 +/- 0.17 g/L, P <0.0001) and remained on this level for 5 days. Correlation analysis revealed a relationship between endotoxemia and the ability of plasma to inactivate endotoxin (r = 0.67, P <0.0001) and also a relation between intraoperative endotoxemia on one hand and alpha(2)-macroglobulin (-0.53 > r > -0.6, P <0.05) as well as alpha(1)-antitrypsin (0.64 > r >0.55, P <0.05) on the other.

CONCLUSION

Major abdominal surgery is associated with transient endotoxemia and a transient reduced endotoxin inactivation capacity of the plasma. Endotoxemia correlates with the endotoxin inactivation capacity. The surgical procedure causes substantial changes in plasma concentrations of acute-phase proteins. alpha(2)-Macroglobulin and alpha(1)-antitrypsin correlate moderately with endotoxemia.

Serum amyloid A, the major vertebrate acute-phase reactant

The serum amyloid A (SAA) family comprises a number of differentially expressed apolipoproteins, acute-phase SAAs (A-SAAs) and constitutive SAAs (C-SAAs). A-SAAs are major acute-phase reactants, the in vivo concentrations of which increase by as much as 1000-fold during inflammation. A-SAA mRNAs or proteins have been identified in all vertebrates investigated to date and are highly conserved. In contrast, C-SAAs are induced minimally, if at all, during the acute-phase response and have only been found in human and mouse. Although the liver is the primary site of synthesis of both A-SAA and C-SAA, extrahepatic production has been reported for most family members in most of the mammalian species studied. In vitro, the dramatic induction of A-SAA mRNA in response to pro-inflammatory stimuli is due largely to the synergistic effects of cytokine signaling pathways, principally those of the interleukin-1 and interleukin-6 type cytokines. This induction can be enhanced by glucocorticoids. Studies of the A-SAA promoters in several mammalian species have identified a range of transcription factors that are variously involved in defining both cytokine responsiveness and cell specificity. These include NF-kappaB, C/EBP, YY1, AP-2, SAF and Sp1. A-SAA is also post-transcriptionally regulated. Although the precise role of A-SAA in host defense during inflammation has not been defined, many potential clinically important functions have been proposed for individual SAA family members. These include involvement in lipid metabolism/transport, induction of extracellular-matrix-degrading enzymes, and chemotactic recruitment of inflammatory cells to sites of inflammation. A-SAA is potentially involved in the pathogenesis of several chronic inflammatory diseases: it is the precursor of the amyloid A protein deposited in amyloid A amyloidosis, and it has also been implicated in the pathogenesis of atheroscelerosis and rheumatoid arthritis.

Acceleration of amyloid protein A amyloidosis by amyloid-like synthetic fibrils

Amyloid protein A (AA) amyloidosis is a consequence of some long-standing inflammatory conditions, and subsequently, an N-terminal fragment of the acute phase protein serum AA forms beta-sheet fibrils that are deposited in different tissues. It is unknown why only some individuals develop AA amyloidosis. In the mouse model, AA amyloidosis develops after approximately 25 days of inflammatory challenge. This lag phase can be shortened dramatically by administration of a small amount of amyloid extract containing an as yet undefined amyloid-enhancing factor. In the present study, we show that preformed amyloid-like fibrils made from short synthetic peptides corresponding to parts of several different amyloid fibril proteins exert amyloidogenic enhancing activity when given i.v. to mice at the induction of inflammation. We followed i.v. administered, radiolabeled, heterologous, synthetic fibrils to the lung and to the perifollicular area in the spleen and found that new AA-amyloid fibrils developed on these preformed fibrils. Our findings thus show that preformed, synthetic, amyloid-like fibrils have an in vivo nidus activity and that amyloid-enhancing activity may occur, at least in part, through this mechanism. Our findings also show that fibrils of a heterologous chemical nature exert amyloid-enhancing activity.

The molecular basis of reactive amyloidosis

Reactive amyloidosis is a disease occurring in patients suffering from chronic infections, inflammation, and certain malignant conditions that are characterized by a considerable elevation of the acute phase reactant serum amyloid A (SAA). It is defined by the presence of extracellular deposits of fibrillar material containing amyloid A (AA) as its main component. AA is an 8.5-kd protein structurally identical to the NH2-terminal of the acute phase reactant SAA. SAA consists of a group of evolutionally conserved amphipathic proteins, encoded by a large number of genes and produced abundantly during inflammation, all suggesting an important role, probably of a neutralizing (anti-inflammatory) nature. An analysis of various aspects of SAA provides no clues to the mechanism of amyloid production, its occurrence in only selected individuals, and its preferential relationship to one isotype of SAA. Until more data is available, the present view on AA amyloidogenesis remains hypothetical.

NF-kappa B and C/EBP transcription factor families synergistically function in mouse serum amyloid A gene expression induced by inflammatory cytokines

Mouse serum amyloid A proteins (SAA) are encoded by multiple genes and the expression of these SAA genes is highly induced during inflammation. We demonstrate that the expression of one of SAA genes (SAA3) is induced by interleukin-1 (IL-1), and that other inflammatory cytokines such as IL-6 and leukemia inhibitory factor, while they themselves are without any effects, enhanced IL-1 induced SAA3 gene expression. The results of mutational analysis on the SAA3 promoter indicate that both the NF-kappa B and C/EBP transcription factor-binding motifs are essential for cytokine-induced SAA3 gene expression in Hep3B cells. To study further roles of NF-kappa B and C/EBP transcription factor family members in SAA3 gene activation, expression vectors for NF-kappa B subunits (p50 and p65) and C/EBP family members (C/EBP-alpha and NFIL-6, also called C/EBP-beta) were co-transfected into Hep3B hepatoma and F9 embryonic carcinoma cells. The results show that, while the expression of p65 alone strongly transactivated a SAA3 gene, p50 did not induce a significant transactivation, and NFIL-6 and C/EBP-alpha induced only a marginal transactivation when expressed alone. However, the co-expression of p50 or p65 with C/EBP family members did result in the efficient induction of SAA3 gene expression, indicating that the synergy between NF-kappa B and C/EBP transcription factor families is essential for SAA3 gene expression during inflammation.

Serum amyloid A: an acute phase apolipoprotein and precursor of AA amyloid

Serum amyloid A is an acute phase protein complexed to HDL as an apoprotein. The molecular weight is 11.4-12.5 kDa in different species and the protein has from 104 to 112 amino acids, without or with an insertion of eight amino acids at position 72. The protein is very well conserved throughout evolution, indicating an important biological function. The N-terminal part of the molecule is hydrophobic and probably responsible for the lipid binding properties. The most conserved part is from position 38 to 52 and this part is therefore believed to be responsible for the until now unknown biological function. The protein is coded on chromosome 11p in man, and chromosome 7 in mice, and found in all mammals until now investigated, and also in the Peking duck. In the rat a truncated SAA mRNA has been demonstrated, but no equivalent serum protein has been reported. Acute phase SAA is first of all produced in hepatocytes after induction by cytokines, but extrahepatic expression of both acute phase and constitutive SAA proteins have been demonstrated. Several cytokines, first of all IL-1, IL-6 and TNF are involved in the induction of SAA synthesis, but the mutual importance of these cytokines seems to be cell-type specific and to vary in various experimental settings. The role of corticosteroids in SAA induction is somewhat confusing. In most in vitro studies corticosteroids show an enhancing or synergistic effect with cytokines on SAA production in cultured cell. However, in clinical studies and in vivo studies in animals an inhibitory effect of corticosteroids is evident, probably due to the all over anti-inflammatory effect of the drug. Until now no drug has been found that selectively inhibits SAA production by hepatocytes. Effective anti-inflammatory or antibacterial treatment is the only tool for reducing SAA concentration in serum and reducing the risk of developing secondary amyloidosis. The function of SAA is still unclear. Interesting theories, based on current knowledge of the lipid binding properties of the protein and the relation to macrophages, in the transportation of cholesterol from damaged tissues has been advanced. A putative role in cholesterol metabolism is supported by the findings of SAA as an inhibitor of LCAT. The potential that SAA is a modifying protein in inflammation influencing the function of neutrophils and platelets is interesting and more directly related to the inflammatory process itself.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of apolipoprotein serum amyloid A mRNA in human atherosclerotic lesions and cultured vascular cells: implications for serum amyloid A function

Altered lipoprotein metabolism and vascular injury are considered to be major parts of the pathogenesis of atherosclerotic lesions. Serum amyloid A (SAA) is a family of acute-phase reactants found residing mainly on high density lipoproteins (HDL) in the circulation. Several functions for the SAAs have been proposed that could be important in atherosclerosis. These include involvement in cholesterol metabolism, participation in detoxification, depression of immune responses, and interference with platelet functions. Like other acute-phase reactants, the liver is a major site of SAA synthesis. However, studies in the mouse have revealed that several cell types including macrophages express SAA. Furthermore, we recently found that SAA mRNA expression can be induced in the human monocyte/macrophage cell line, THP-1. In the present study, human atherosclerotic lesions of coronary and carotid arteries were examined for expression of SAA mRNA by in situ hybridization. Surprisingly, SAA mRNA was found in most endothelial cells and some smooth muscle cells as well as macrophage-derived "foam cells," adventitial macrophages, and adipocytes. In addition, cultured smooth muscle cells expressed SAA1, SAA2, and SAA4 mRNAs when treated with interleukin 1 or 6 (IL-1 or IL-6) in the presence of dexamethasone. These findings give further credence to the notion that the SAAs are involved in lipid metabolism or transport at sites of injury and in atherosclerosis or may play a role in defending against viruses or other injurious agents such as oxidized lipids. Furthermore, expression of SAAs by endothelial cells is compatible with the evidence that SAA modulates platelet aggregation and function and possibly adhesion at the endothelial cell surface.

Expression of serum amyloid A genes in mink during induction of inflammation and amyloidosis

Serum amyloid A (SAA) is an acute phase protein and the precursor of amyloid protein A (AA) in deposits of secondary amyloidosis. Several isotypes exist in mink, but previous studies suggest that mink AA is derived from only one. To assess the effect of repeated episodes of inflammation and induction of amyloidosis, qualitative and quantitative changes in hepatic and extrahepatic SAA mRNA were studied. Young female mink received subcutaneous lipopolysaccharide injections for amyloid induction. Studies were performed using RNA probes and oligonucleotide probes specific for each of two SAA mRNA species. Northern blot hybridization showed that hepatic SAA1 and SAA2 mRNA levels increased dramatically after inflammatory stimulation, and were subsequently maintained at elevated levels, showing considerable interindividual variation, but only a slight decrease during repeated inflammatory stimuli and the early stages of amyloid deposition. No preferential accumulation of mRNA specifying a particular isotype was found during the experiment. Differential expression of mink SAA mRNA during repeated inflammatory stimulation does not seem to explain why only SAA2-derived AA is found in amyloid deposits. Extrahepatic SAA mRNA seemed to be independently regulated and may thus represent another, yet not characterized, SAA isotype.

Differential expression of rabbit serum amyloid A genes in response to various inflammatory agents

Serum amyloid A (SAA) is an acute-phase plasma protein which increases up to 1000-fold after an acute-phase stimulus. Several SAA genes and corresponding protein isotypes exist in individual species. Liver is the main source of production, but extra-hepatic SAA expression has been described. In this study inflammation was induced in rabbits with lipopolysaccharide, turpentine, or casein. Transcription of SAA mRNA was studied using Northern blot analysis with probes specific for three different rabbit SAA isotypes and analysed by scanning densitometry. In the stimulated liver slight variation in SAA mRNA transcription level was seen after stimulation with different inflammatory agents. After lipopolysaccharide-stimulation SAA gene expression was also seen in most of the extra-hepatic organs. After turpentine stimulation SAA mRNA was seen in the liver, the ovary, and the small intestines, and after casein stimulation only in the liver and the ovary. SAA1 and SAA2 were induced exclusively in the liver, while SAA3 was induced mainly in the extra-hepatic organs. This indicates that the SAA genes probably are independently regulated both in relation to stimulus, gene- and tissue-specificity.

Serum amyloid A (SAA): an acute phase protein and apolipoprotein

Serum amyloid A (SAA) proteins comprise a family of apolipoproteins coded for by at least three genes with allelic variation and a high degree of homology between species. The synthesis of certain members of the family is greatly increased in inflammation. However, SAA is not often used as an acute-phase marker despite being at least as sensitive as C-reactive protein. SAA proteins can be considered as apolipoproteins since they associate with plasma lipoproteins mainly within the high density range, perhaps through amphipathic alpha-helical structure. It is not known why certain subjects expressing SAA develop secondary systemic amyloidosis. There is still no specific function attributed to SAA; however, a popular hypothesis suggests that SAA may modulate metabolism of high density lipoproteins (HDL). This may impede the protective function of HDL against the development of atherosclerosis. The potential significance of the association between SAA and lipoproteins needs further evaluation.

Murine serum amyloid A3 is a high density apolipoprotein and is secreted by macrophages

The serum amyloid A (SAA) proteins make up a multigene family of apolipoproteins associated with high density lipoproteins. They are of ancient origin; the finding of a highly homologous protein in mammals and ducks indicates that SAAs have been in existence for at least 300 million years. The interspecies similarity among the SAAs makes the mouse, in which they have been most thoroughly studied, a reasonable model to use for defining the function(s) of this family of proteins in humans. Originally it was observed that the SAA proteins were made in the liver and represented a set of proteins belonging to acute-phase reactants. SAA3 is a unique member of the SAA multigene family in mice in that its mRNA is also expressed in extrahepatic tissues by a variety of cell types, mainly macrophages and adipocytes. To date, nothing has been reported regarding the fate or function of the SAA3 translation product. To identify the SAA3 protein, we developed SAA3-specific antibodies by immunizing rabbits against a portion of SAA3 protein synthesized in a bacterial fusion protein expression system. Electroimmunoblot analysis of serum and lipoprotein fractions of it showed SAA3 to be associated with high density lipoproteins of mice treated with lipopolysaccharide. Furthermore, a continuous mouse macrophage cell line (J-774.1), when exposed to lipopolysaccharide, expressed SAA3 mRNA in a dose-dependent manner and secreted SAA3 protein. The expression and secretion of SAA3 by macrophages stimulated with lipopolysaccharide suggest a role for this SAA in local responses to injury and inflammation.

Serum amyloid A (SAA), a protein without a function: some suggestions with reference to cholesterol metabolism

Serum amyloid A, as an apolipoprotein, is present on high density lipoprotein only during inflammatory states. When viewed from HDL's established function as a mechanism for reverse cholesterol transportation, it is postulated that serum amyloid A represents a signal to redirect HDL to sites of tissue destruction where cholesterol is being collected by macrophages. The object is to direct the reverse cholesterol transporter to sites of cholesterol accumulation for the subsequent removal of these cholesterol stores. The hypothesis has relevance to the process of atheroma formation.

Serum amyloid A in the mouse. Sites of uptake and mRNA expression

Murine serum amyloid A1 (SAA1) and serum amyloid A2 (SAA2) are circulating, acute phase, high density apolipoproteins of unknown function. To pursue issues relating to their possible function their uptake and formation were studied. Kinetics of SAA protein distribution and gene expression after acute phase stimulation by casein or lipopolysaccharide were examined using immunocytochemistry for protein and RNA blot and in situ hybridization with probes for SAA1 and SAA2 mRNA. After casein injection, interstitial cells of testes, cells of adrenal cortex, kidney proximal convoluted tubule epithelia, and some parafollicular cells of spleen took up SAA in a time pattern related to plasma SAA levels. Extrahepatic SAA1 and SAA2 mRNA were induced by lipopolysaccharide in kidney proximal and distal convoluted tubule epithelia, and SAA1 mRNA was induced in epithelial lining the mucosa of the ileum and large intestine, indicating that there may be more than one function for the apoSAA gene family related to site of and stimulus for expression.