Serum amyloid A1 (SAA1) is a protein that in humans is encoded by the SAA1 gene. [5] [6] [7] SAA1 is a major acute-phase protein mainly produced by hepatocytes in response to infection, tissue injury and malignancy. [8] When released into blood circulation, SAA1 is present as an apolipoprotein associated with high-density lipoprotein (HDL). [9] SAA1 is a major precursor of amyloid A (AA), the deposit of which leads to inflammatory amyloidosis. [10] [11]
The gene coding for human SAA1 is one of the 4 SAA genes mapped to a region in the short arm on Chromosome 11. [12] Two of these genes, SAA1 and SAA2, are inducible during acute-phase response, whereas SAA3 is a pseudogene in humans [13] and SAA4 is constitutively expressed in a variety of tissues and cells. Single nucleotide polymorphisms (SNPs) are found in SAA1 in both coding and non-coding sequences, with those located in the coding sequence defining 5 isoforms of SAA1 (SAA1.1 – 1.5). Genetic studies have shown association of some of these SNPs with the disposition to several human diseases including familiar Mediterranean fever, coronary artery diseases, cerebral infarction, and osteoporosis. Mice also have 4 Saa genes. A major difference between human and mouse SAA genes is the expression of the mouse Saa3 gene for a functional protein, generally considered an inducible SAA in inflammatory tissues.
The product of human SAA1 is a pre-protein of 122 amino acids, with a cleavable signal peptide of 18 amino acids. Mature SAA1 consists of 104 amino acids with an apparent molecular weight of 12,500. A crystal structure of SAA1.1 has been solved recently (Figure 1). Native SAA1 is a hexamer with each subunit assuming an antiparallel 4-helix bundle structure. [14] The structure is cone-shaped with its apex forming a binding site for HDL and heparin. The N-terminal helices 1 and 3 have been identified as amyloidogenic peptides of SAA1.1, that are not presence on protein surface in native SAA1 protein. These findings provide the structural basis for the formation of amyloid A fibrils. The human SAA1.1 is comparable at the subunit level with the recently solved structure of mouse Saa3. [15]
SAA1 and SAA2 are highly inducible and hence called acute-phase SAA. Inflammatory cytokines such as IL-1β, IL-6 and TNF-α are major stimulants for hepatocyte expression of the SAA1 gene. [16] Inducible expression of the acute-phase SAA genes is mainly regulated at the transcription level and involves the transcription factors C/EBP, NF-κB, AP2, SAF, Sp1 and STAT3. Elevation of the transcript of SAA1 is often seen in cDNA arrays used for detection of proinflammatory cytokine expression. SAA1 protein level correlates with its transcript level, and has long been considered a clinical indicator for inflammatory conditions.
In addition to its association with HDL, SAA1 interacts with a number of mammalian proteins, mostly cell surface proteins such as receptors. SAA1 binding to the αvβ3 integrin produces an inhibitory effect on the growth of nasopharyngeal carcinoma. [17] Several receptors for SAA1 have been identified using an SAA1 hybrid protein containing two amino acid substitutions from SAA2. [18] These receptors include the G protein-coupled chemoattractant receptor FPR2 (formyl peptide receptor 2), [19] believed to mediate the chemotactic activity of the recombinant SAA1; the murine scavenger receptor SR-BI [20] and the human equivalent CLA-1., [21] for a possible role in SAA1-dependent cholesterol metabolism. Moreover, the Toll-like receptors TLR2 [22] and TLR4 [23] mediate SAA1-induced cytokine gene expression. The P2X7 purinergic receptor is another receptor used by SAA1 for a number of cellular functions including the activation of NLRP3 inflammasomes. [24]
SAA1 has been found to interact with outer membrane protein A (ompA) of several Gram-negative bacteria including E. coli, Salmonella typhimurium , Shigella flexneri , Vibrio cholerae and P. aeruginosa. [25] Exposure of these Gram-negative bacteria to SAA1 promotes uptake of the bacteria by neutrophils, suggesting that SAA1 serves as an opsonin that enhances bacteria clearance. [26] A more recent study identified SAA1 interaction with retinol, resulting in reduced bacterial burden. [27] These findings suggest that SAA1 has a function in host defense against bacterial infection.
The biological function of SAA1 has not been fully understood despite intensive research in the last three decades. Research tools such as the SAA1 knockout mice and transgenic mice have become available only recently. It has been well established, however, that elevated plasma concentration of SAA1 is associated with a multitude of inflammatory conditions. As a result, SAA1 has been a clinical indicator and reliable biomarker for inflammatory diseases, chronic metabolic disorders and late-stage malignancy. [28] Inflammatory amyloidosis results from chronic inflammation with increased production of SAA1, which is a major precursor of amyloid A fibril deposit in various tissues. [29]
SAA1 has been extensively studied for its binding to HDL, with results suggesting a role in lipid metabolism. During the acute-phase response, elevated levels of SAA1 in the plasma displaces ApoA-I and becomes a major apolipoprotein of HDL. [30] The exact biological consequence of HDL remodeling by SAA1 is still under investigation, using recently developed tools such as the Saa1 and Saa2 knockout mice. SAA1 is also believed to contribute to the development of atherosclerosis. [31] [32] However, in an ApoE-deficient mouse model, deletion of the Saa1/Saa2 genes does not appear to affect atherosclerotic lesions. [33]
Ex vivo and in vitro studies have shown that the recombinant human SAA1 hybrid protein has strong chemotactic activity for neutrophils and macrophages. [34] This effect is believed to be mediated through FPR2, a G protein-coupled chemoattractant receptor. [35] The same receptor also mediates the cytokine-like activity of the recombinant SAA1, resulting in an elevated expression of IL-8 in neutrophils. [36] The recombinant SAA1 has been reported to induce the expression of a variety of inflammatory cytokines including IL-1β, TNF-α, IL-6, IL-12p40, [37] [38] as well as immunoregulatory cytokines such as IL-23, [39] IL-33 [40] and growth-stimulatory cytokines such as G-CSF. [41]
SAA1 may also be produced by macrophages and epithelial cells in various tissues. It has been shown to promote local Th17 response in the gut. [42] This finding, which is based on both Saa1/Saa2 knockout mice and ex vivo studies of T cells, strongly suggest a local immunomodulatory function of SAA1 as opposed to its established role as an acute-phase protein produced in the liver and present in the plasma as an apolipoprotein of HDL. Transgenic expression of human SAA1.1 in mouse liver aggravates T cell-mediated hepatitis through elevated production of chemokines, [43] which involves the SAA1 receptor TLR2. Secretion of SAA1 by melanoma cells may induce anti-inflammatory IL-10-secreting neutrophils that interact with invariant natural killer T cells (iNKT cells). [44] In addition, SAA1 can skew macrophages to a M2 phenotype. [45]
Published reports link SAA1 to a number of malignancies, but a causal relationship has not been established. SAA1 has been associated with tumor pathogenesis, [46] and its gene polymorphism is a contributing factor to certain types of malignant tumors. [47] SAA1 has also been shown to affect the tumor microenvironment and contribute to tumor cell metastasis. [48]
Some of the results obtained with the recombinant human SAA1 hybrid remain controversial, as the protein does not have exactly the same sequence of human SAA1 and its properties may be different from the native SAA1. [49] Other studies have shown that native human SAA1 retains some of the cytokine-like activities such as the G-CSF-induction capability [50]
Recent studies using the Saa1/Saa2 knockout mice showed weakened Th17 response in gut epithelial cells, [51] suggesting that SAA1 plays a role in vivo in the regulation of immunity.
 | The 2016 version of this article was updated by an external expert under a dual publication model. The corresponding academic peer reviewed article was published in Gene and can be cited as: Lei Sun; Richard D Ye (2 March 2016). "Serum amyloid A1: Structure, function and gene polymorphism". Gene . Gene Wiki Review Series. 583 (1): 48–57. doi:10.1016/J.GENE.2016.02.044. ISSN 0378-1119. PMC 5683722 . PMID 26945629. Wikidata Q38761950. |
Acute-phase proteins (APPs) are a class of proteins whose concentrations in blood plasma either increase or decrease in response to inflammation. This response is called the acute-phase reaction. The acute-phase reaction characteristically involves fever, acceleration of peripheral leukocytes, circulating neutrophils and their precursors. The terms acute-phase protein and acute-phase reactant (APR) are often used synonymously, although some APRs are polypeptides rather than proteins.
Interleukins (ILs) are a group of cytokines that are expressed and secreted by white blood cells (leukocytes) as well as some other body cells. The human genome encodes more than 50 interleukins and related proteins.
A lipoxin (LX or Lx), an acronym for lipoxygenase interaction product, is a bioactive autacoid metabolite of arachidonic acid made by various cell types. They are categorized as nonclassic eicosanoids and members of the specialized pro-resolving mediators (SPMs) family of polyunsaturated fatty acid (PUFA) metabolites. Like other SPMs, LXs form during, and then act to resolve, inflammatory responses. Initially, two lipoxins were identified, lipoxin A4 (LXA4) and LXB4, but more recent studies have identified epimers of these two LXs: the epi-lipoxins, 15-epi-LXA4 and 15-epi-LXB4 respectively.
Interleukin 8 is a chemokine produced by macrophages and other cell types such as epithelial cells, airway smooth muscle cells and endothelial cells. Endothelial cells store IL-8 in their storage vesicles, the Weibel-Palade bodies. In humans, the interleukin-8 protein is encoded by the CXCL8 gene. IL-8 is initially produced as a precursor peptide of 99 amino acids which then undergoes cleavage to create several active IL-8 isoforms. In culture, a 72 amino acid peptide is the major form secreted by macrophages.
Interleukin-1 alpha also known as hematopoietin 1 is a cytokine of the interleukin 1 family that in humans is encoded by the IL1A gene. In general, Interleukin 1 is responsible for the production of inflammation, as well as the promotion of fever and sepsis. IL-1α inhibitors are being developed to interrupt those processes and treat diseases.
Interleukin-15 (IL-15) is a protein that in humans is encoded by the IL15 gene. IL-15 is an inflammatory cytokine with structural similarity to Interleukin-2 (IL-2). Like IL-2, IL-15 binds to and signals through a complex composed of IL-2/IL-15 receptor beta chain (CD122) and the common gamma chain. IL-15 is secreted by mononuclear phagocytes following infection by virus(es). This cytokine induces the proliferation of natural killer cells, i.e. cells of the innate immune system whose principal role is to kill virally infected cells.
Interleukin-18 (IL-18), also known as interferon-gamma inducing factor is a protein which in humans is encoded by the IL18 gene. The protein encoded by this gene is a proinflammatory cytokine. Many cell types, both hematopoietic cells and non-hematopoietic cells, have the potential to produce IL-18. It was first described in 1989 as a factor that induced interferon-γ (IFN-γ) production in mouse spleen cells. Originally, IL-18 production was recognized in Kupffer cells, liver-resident macrophages. However, IL-18 is constitutively expressed in non-hematopoietic cells, such as intestinal epithelial cells, keratinocytes, and endothelial cells. IL-18 can modulate both innate and adaptive immunity and its dysregulation can cause autoimmune or inflammatory diseases.
Interleukin 33 (IL-33) is a protein that in humans is encoded by the IL33 gene.
Interleukin-26 (IL-26) is a protein that in humans is encoded by the IL26 gene.
Interleukin-22 (IL-22) is protein that in humans is encoded by the IL22 gene.
Interleukin 17 family is a family of pro-inflammatory cystine knot cytokines. They are produced by a group of T helper cell known as T helper 17 cell in response to their stimulation with IL-23. Originally, Th17 was identified in 1993 by Rouvier et al. who isolated IL17A transcript from a rodent T-cell hybridoma. The protein encoded by IL17A is a founding member of IL-17 family. IL17A protein exhibits a high homology with a viral IL-17-like protein encoded in the genome of T-lymphotropic rhadinovirus Herpesvirus saimiri. In rodents, IL-17A is often referred to as CTLA8.
Serum amyloid A (SAA) proteins are a family of apolipoproteins associated with high-density lipoprotein (HDL) in plasma. Different isoforms of SAA are expressed constitutively at different levels or in response to inflammatory stimuli. These proteins are produced predominantly by the liver.
Chemokine ligands 4 previously known as macrophage inflammatory protein (MIP-1β), is a protein which in humans is encoded by the CCL4 gene. CCL4 belongs to a cluster of genes located on 17q11-q21 of the chromosomal region. Identification and localization of the gene on the chromosome 17 was in 1990 although the discovery of MIP-1 was initiated in 1988 with the purification of a protein doublet corresponding to inflammatory activity from supernatant of endotoxin-stimulated murine macrophages. At that time, it was also named as "macrophage inflammatory protein-1" (MIP-1) due to its inflammatory properties.
CCAAT/enhancer-binding protein beta is a protein that in humans is encoded by the CEBPB gene.
Pentraxin-related protein PTX3 also known as TNF-inducible gene 14 protein (TSG-14) is a protein that in humans is encoded by the PTX3 gene.
N-formyl peptide receptor 2 (FPR2) is a G-protein coupled receptor (GPCR) located on the surface of many cell types of various animal species. The human receptor protein is encoded by the FPR2 gene and is activated to regulate cell function by binding any one of a wide variety of ligands including not only certain N-Formylmethionine-containing oligopeptides such as N-Formylmethionine-leucyl-phenylalanine (FMLP) but also the polyunsaturated fatty acid metabolite of arachidonic acid, lipoxin A4 (LXA4). Because of its interaction with lipoxin A4, FPR2 is also commonly named the ALX/FPR2 or just ALX receptor.
Serum amyloid A protein is a protein that in humans is encoded by the SAA2 gene.
Interleukin 37 (IL-37), also known as Interleukin-1 family member 7 (IL-1F7), is an anti-inflammatory cytokine important for the downregulation of pro-inflammatory cytokine production as well as the suppression of tumor cell growth.
The Interleukin-1 family is a group of 11 cytokines that play a central role in the regulation of immune and inflammatory responses to infections or sterile insults.
Interleukin 36, or IL-36, is a group of cytokines in the IL-1 family with pro-inflammatory effects. The role of IL-36 in inflammatory diseases is under investigation.
