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). [5] [6] Because of its interaction with lipoxin A4, FPR2 is also commonly named the ALX/FPR2 or just ALX receptor.
The FPR2 receptor is expressed on human neutrophils, eosinophils, monocytes, macrophages, T cells, synovial fibroblasts, and intestinal and airway epithelium. [7]
Many oligopeptides that possess an N-Formylmethionine N-terminal residue such as the prototypical tripeptide N-Formylmethionine-leucyl-phenylalanine (i.e. FMLP), are products of the protein synthesis conducted by bacteria. They stimulate granulocytes to migrate directionally (see chemotaxis) and become active in engulfing (see phagocytosis) and killing bacteria and thereby contribute to host defense by directing the innate immune response of acute inflammation to sites of bacterial invasion. Early studies suggested that these formyl oligopeptides operated by a Receptor (biochemistry) mechanism. Accordingly, the human leukocyte cell line, HL-60 promyelocytes (which do not respond to FMLP), was purposely differentiated to granulocytes (which do respond to FMLP) and used to partially purify [8] and clone a gene that when transfected into FMLP-unresponsive cells bestowed responsiveness to this and other N-formyl oligopeptides. [9] [10] [11] [12] [13] This receptor was initially named the formyl peptide receptor (i.e. FPR). However, a series of subsequent studies cloned two genes that encoded receptor-like proteins with amino acid sequences very similar to that of FPR. [14] [15] [16] The three receptors had been given various names but are now termed formyl peptide receptor 1 (i.e. FPR1) for the first defined receptor, FPR2, and Formyl peptide receptor 3 (i.e. FPR3). FPR2 and FPR3 are termed formyl peptide receptors base on the similarities of their amino acid sequences to that of FPR1 rather than any preferences for binding formyl peptides. Indeed, FPR2 prefers a very different set of ligands and has some very different functions than FPR1 while FPR3 does not bind FMLP or many other N-formyl peptides which bind to FPR1 or FPR2. [17] A major function for FPR2 is binding certain specialized pro-resolving mediators (SPMs), i.e. lipoxin (Lx)A4, and AT-LxA4 (metabolites of arachidonic acid) as well as resolvin D1 (RvD)1, RvD2, and AT-RvD1 (metabolites of docosahexaenoic acid) and thereby to mediate these metabolites activities in inhibiting and resolving inflammation (see Specialized pro-resolving mediators). However, FPR2 also mediates responses to a wide range of polypeptides and proteins which may serve to promote inflammation or regulate activities not directly involving inflammation. The function of FPR3 is not clear.
Confusingly, there are two "standard" nomenclatures for FPR receptors and their genes, the first used, FPR, FPR1, and FPR2 and its replacement, FPR1, FPR2, and FPR3. The latter nomenclature is recommended by the International Union of Basic and Clinical Pharmacology [17] and is used here. Other previously used names for FPR1 are NFPR, and FMLPR; for FPR2 are FPRH1, FPRL1, RFP, LXA4R, ALXR, FPR2/ALX, HM63, FMLPX, and FPR2A; and for FPR3 are FPRH2, FPRL2, and FMLPY. [17]
The human FPR2 gene encodes the 351 amino acid receptor, FPR2, within an intronless open reading frame. It forms a cluster with FPR1 and FPR3 genes on chromosome 19q.13.3 in the order of FPR1, FPR2, and FPR3; this cluster also includes the genes for two other chemotactic factor receptors, the G protein-coupled C5a receptor (also termed CD88) and a second C5a receptor, GPR77 (i.e. C5a2 or C5L2), which has the structure of G protein receptors but apparently does not couple to G proteins and is of uncertain function. [18] The FPR1, FPR2, and FPR3 paralogs, based on phylogenetic analysis, originated from a common ancestor with early duplication of FPR1 and FPR2/FPR3 splitting with FPR3 originating from the latest duplication event near the origin of primates. [19]
Mice have no less than 7 FPR receptors encoded by 7 genes that localize to chromosome 17A3.2 in the following order: Fpr1, Fpr-rs2 (or fpr2), Fpr-rs1 (or LXA4R), Fpr-rs4, Fpr-rs7, Fpr-rs7, Fpr-rs6, and Fpr-rs3; this locus also contains Pseudogenes ψFpr-rs2 and ψFpr-rs3 (or ψFpr-rs5) which lie just after Fpr-rs2 and Fpr-rs1, respectively. The 7 mouse FPR receptors have ≥50% amino acid sequence identity with each other as well as with the three human FPR receptors. [20] Fpr2 and mFpr-rs1 bind with high affinity and respond to lipoxins but have little or no affinity for, and responsiveness to, formyl peptides; they thereby share key properties with human FPR2; [21] [22] [23]
The large number of mouse compared to human FPR receptors makes it difficult to extrapolate human FPR functions based on genetic (e.g. gene knockout or forced overexpression) or other experimental manipulations of the FPR receptors in mice. In any event, combined disruption of the Fpr2 and Fpr3 genes causes mice to mount enhanced acute inflammatory responses as evidenced in three models, intestine inflammation caused by mesenteric artery ischemia-reperfusion, paw swelling caused by carrageenan injection, and arthritis caused by the intraperatoneal injection of arthritis-inducing serum. [24] Since Fpr2 gene knockout mice exhibit a faulty innate immune response to intravenous listeria monocytogenes injection, [25] these results suggest that the human FPR2 receptor and mouse Fpr3 receptor have equivalent functions in dampening at least certain inflammatory response.
Rats express an ortholog of FPR2 (74% amino acid sequence identity) with high affinity for lipoxin A4. [20]
FPL2 is often co-expressed with FPR1. It is widely expressed by circulating blood neutrophils, eosinophils, basophils, and monocytes; lymphocyte T cells and B cells; tissue Mast cells, macrophages, fibroblasts, and immature dendritic cells; vascular endothelial cells; neural tissue glial cells, astrocytes, and neuroblastoma cells; liver hepatocytes; various types of epithelial cells; and various types of multicellular tissues. [20] [26] [27] [28] [29]
FPR2 is also known as the LXA4 or ALX/FPR2 receptor based on studies finding that is a high affinity receptor for the arachidonic acid metabolite, lipoxin A4 (LXA4), and thereafter for a related arachidonic acid metabolite, the Epi-lipoxin, aspirin-triggered lipoxin A4 (i.e. ATL, 15-epi-LXA4) and a docosahexaenoic acid metabolite, resolvin D1 (i.e. RvD1); these three cell-derived fatty acid metabolites act to inhibit and resolve inflammatory responses. [30] [31] [32] [33] [34] This receptor was previously known as an orphan receptor, termed RFP, obtained by screening myeloid cell-derived libraries with a FMLP-like probe. [35] [36] [37] In addition to LXA4, LTA, RvD1, and FMLP, FPR2 binds a wide range of polypeptides, proteins, and products derived from these polypeptides and proteins. One or more of these various ligands may be involved not only in regulating inflammation but also be involved in the development of obesity, cognitive decline, reproduction, neuroprotection, and cancer. [38] However, the most studied and accepted role for FPR2 receptors is in mediating the actions of the cited lipoxins and resolvins in dampening and resolving a wide range of inflammatory reactions (see lipoxin, Epi-lipoxin, and resolvin). [39] [40]
The following is a list of FPR2/ALX ligands and in parentheses their suggested pro-inflammatory or anti-inflammatory actions base on in vitro and animal model studies: a) bacterial and mitochondrial N-formyl peptides such as FMLP (pro-inflammatory but perhaps less significant or insignificant compared to the actions of LXA4, ATL, and RvD1 on FPR2);
b) Hp(2-20), a non-formyl peptide derived from Helicobacter pylori (pro-inflammatory by promoting inflammatory responses against this stomach ulcer-causing pathogen);
c) T21/DP107 and N36, which are N-acetylated polypeptides derived from the gp41 envelope protein of the HIV-1 virus, F peptide, which is derived from gp120 protein of the HIV-1 Bru strain virus, and V3 peptide, which is derived from a linear sequence of the V3 region of the HIV-1 MN strain virus (unknown effect on inflammation and HIV infection);
d) the N-terminally truncated form of the chemotactic chemokine, CCL23, termed CCL23 splice variant CCL23β(amino acids 22–137) and SHAAGtide, which is a product of CCL23β cleavage by pro-inflammatory proteases (pro-inflammatory); e) two N-acetyl peptides, Ac2–26 and Ac9–25 of Annexin A1 (ANXA1 or lipocortin 1), which at high concentrations fully stimulate neutrophil functions but at lower concentrations leave neutrophils desensitized (i.e. unresponsive) to the chemokine IL-8 (CXCL8) (pro-inflammatory and anti-inflammatory, respectively, highlighting the duality of FPR2/ALX functions in inflammation);
f) Amyloid beta(1–42) fragment and prion protein fragment PrP(106–126) (pro-inflammatory, suggesting a role for FPR2/ALX in the inflammatory components of diverse amyloid-based diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, prion-based diseases such as Transmissible spongiform encephalopathy, Creutzfeldt–Jakob disease, and Kuru), and numerous other neurological and non-neurological diseases [see amyloid]);
g) the neuroprotective peptide, Humanin (anti-inflammatory by inhibiting the pro-inflammatory effects of Amalyoid beta(1-42) in promoting Alzheimer's disease-related inflammation);
h) two cleaved soluble fragments of UPARAP which is the Urokinase-type plasminogen activator receptor (uPAR), D2D3(88–274) and uPAR(84–95) (pro-inflammatory);
i) LL-37 and CRAMP, which are enzymatic cleavage products of human and rat, respectively, Cathelicidin-related antimicrobial peptides, numerous Pleurocidins which are a family of cationic antimicrobial peptides found in fish and other vertebrates structurally and functionally similar to cathelicidins, [29] and Temporin A, which is a frog-derived antimicrobial peptide ((pro-inflammatory products derived from host anti-microbial proteins); and
j) Pituitary adenylate cyclase-activating polypeptide 27 (pro-inflammatory). [17] [41]
Eicosanoids are signaling molecules made by the enzymatic or non-enzymatic oxidation of arachidonic acid or other polyunsaturated fatty acids (PUFAs) that are, similar to arachidonic acid, around 20 carbon units in length. Eicosanoids are a sub-category of oxylipins, i.e. oxidized fatty acids of diverse carbon units in length, and are distinguished from other oxylipins by their overwhelming importance as cell signaling molecules. Eicosanoids function in diverse physiological systems and pathological processes such as: mounting or inhibiting inflammation, allergy, fever and other immune responses; regulating the abortion of pregnancy and normal childbirth; contributing to the perception of pain; regulating cell growth; controlling blood pressure; and modulating the regional flow of blood to tissues. In performing these roles, eicosanoids most often act as autocrine signaling agents to impact their cells of origin or as paracrine signaling agents to impact cells in the proximity of their cells of origin. Some eicosanoids, such as prostaglandins, may also have endocrine roles as hormones to influence the function of distant cells.
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.
Resolvins are specialized pro-resolving mediators (SPMs) derived from omega-3 fatty acids, primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), as well as from two isomers of docosapentaenoic acid (DPA), one omega-3 and one omega-6 fatty acid. As autacoids similar to hormones acting on local tissues, resolvins are under preliminary research for their involvement in promoting restoration of normal cellular function following the inflammation that occurs after tissue injury. Resolvins belong to a class of polyunsaturated fatty acid (PUFA) metabolites termed specialized proresolving mediators (SPMs).
N-Formylmethionine is a derivative of the amino acid methionine in which a formyl group has been added to the amino group. It is specifically used for initiation of protein synthesis from bacterial and organellar genes, and may be removed post-translationally.
The formyl peptide receptors (FPR) belong to a class of G protein-coupled receptors involved in chemotaxis. In humans, there are three formyl peptide receptor isoforms, each encoded by a separate gene that are named FPR1, FPR2, and FPR3. These receptors were originally identified by their ability to bind N-formyl peptides such as N-formylmethionine produced by the degradation of either bacterial or host cells. Hence formyl peptide receptors are involved in mediating immune cell response to infection. These receptors may also act to suppress the immune system under certain conditions. The close phylogenetic relation of signaling in chemotaxis and olfaction was recently proved by detection formyl peptide receptor like proteins as a distinct family of vomeronasal organ chemosensors in mice.
Most of the eicosanoid receptors are integral membrane protein G protein-coupled receptors (GPCRs) that bind and respond to eicosanoid signaling molecules. Eicosanoids are rapidly metabolized to inactive products and therefore are short-lived. Accordingly, the eicosanoid-receptor interaction is typically limited to a local interaction: cells, upon stimulation, metabolize arachidonic acid to an eicosanoid which then binds cognate receptors on either its parent cell or on nearby cells to trigger functional responses within a restricted tissue area, e.g. an inflammatory response to an invading pathogen. In some cases, however, the synthesized eicosanoid travels through the blood to trigger systemic or coordinated tissue responses, e.g. prostaglandin (PG) E2 released locally travels to the hypothalamus to trigger a febrile reaction. An example of a non-GPCR receptor that binds many eicosanoids is the PPAR-γ nuclear receptor.
Arachidonate 5-lipoxygenase, also known as ALOX5, 5-lipoxygenase, 5-LOX, or 5-LO, is a non-heme iron-containing enzyme that in humans is encoded by the ALOX5 gene. Arachidonate 5-lipoxygenase is a member of the lipoxygenase family of enzymes. It transforms essential fatty acids (EFA) substrates into leukotrienes as well as a wide range of other biologically active products. ALOX5 is a current target for pharmaceutical intervention in a number of diseases.
Transient receptor potential cation channel subfamily V member 4 is an ion channel protein that in humans is encoded by the TRPV4 gene.
Transient receptor potential cation channel, subfamily V, member 3, also known as TRPV3, is a human gene encoding the protein of the same name.
ALOX15 is, like other lipoxygenases, a seminal enzyme in the metabolism of polyunsaturated fatty acids to a wide range of physiologically and pathologically important products. ▼ Gene Function
N-Formylmethionyl-leucyl-phenylalanine is an N-formylated tripeptide and sometimes simply referred to as chemotactic peptide is a potent polymorphonuclear leukocyte (PMN) chemotactic factor and is also a macrophage activator.
Serum amyloid A1 (SAA1) is a protein that in humans is encoded by the SAA1 gene. SAA1 is a major acute-phase protein mainly produced by hepatocytes in response to infection, tissue injury and malignancy. When released into blood circulation, SAA1 is present as an apolipoprotein associated with high-density lipoprotein (HDL). SAA1 is a major precursor of amyloid A (AA), the deposit of which leads to inflammatory amyloidosis.
Chemokine like receptor 1 also known as ChemR23 is a protein that in humans is encoded by the CMKLR1 gene. Chemokine receptor-like 1 is a G protein-coupled receptor for the chemoattractant adipokine chemerin and the omega-3 fatty acid eicosapentaenoic acid-derived specialized pro-resolving molecule, resolvin E1. The murine receptor that shares almost 80% homology with the human receptor, is called Dez.
N-formyl peptide receptor 3 (FPR3) is a receptor protein that in humans is encoded by the FPR3 gene.
N-Arachidonyl glycine receptor, also known as G protein-coupled receptor 18 (GPR18), is a protein that in humans is encoded by the GPR18 gene. Along with the other previously "orphan" receptors GPR55 and GPR119, GPR18 has been found to be a receptor for endogenous lipid neurotransmitters, several of which also bind to cannabinoid receptors. It has been found to be involved in the regulation of intraocular pressure.
G protein-coupled receptor 32, also known as GPR32 or the RvD1 receptor, is a human receptor (biochemistry) belonging to the rhodopsin-like subfamily of G protein-coupled receptors.
Maresin 1 (MaR1 or 7R,14S-dihydroxy-4Z,8E,10E,12Z,16Z,19Z-docosahexaenoic acid) is a macrophage-derived mediator of inflammation resolution coined from macrophage mediator in resolving inflammation. Maresin 1, and more recently defined maresins, are 12-lipoxygenase-derived metabolites of the omega-3 fatty acid, docosahexaenoic acid (DHA), that possess potent anti-inflammatory, pro-resolving, protective, and pro-healing properties similar to a variety of other members of the specialized proresolving mediators (SPM) class of polyunsaturated fatty acid (PUFA) metabolites. SPM are dihydroxy, trihydroxy, and epoxy-hydroxy metabolites of long chain PUFA made by certain dioxygenase enzymes viz., cyclooxygenases and lipoxygenases. In addition to the maresins, this class of mediators includes: the 15-lipoxygenase (i.e. ALOX15 and/or possibly ALOX15B)-derived lipoxin A4 and B4 metabolites of the omega 6 fatty acid, arachidonic acid; the cyclooxygenase 2-derived resolvin E series metabolites of the omega 3 fatty acid, eicosapentaenoic acid; certain 15-lipoxygenase-derived resolvin D series metabolites of DHA; certain other 15-lipoxygenase-derived protectin D1 and related metabolites of DHA; and the more recently defined and therefore less fully studied 15-lipoxygenase-derived resolvin Dn-3DPA metabolites of the omega-3 fatty acid n-3 docosapentaenoic acid (n-3 DPA or clupanodonic acid), the cyclooxygenase 2-derived resolvin T metabolites of this clupanodonic acid, and the 15-lipoxygenase-derived products of the N-acetylated fatty acid amide of the DHA metabolite, docosahexaenoyl ethanolamide.
Formyl peptide receptor 1 is a cell surface receptor protein that in humans is encoded by the formyl peptide receptor 1 (FPR1) gene. This gene encodes a G protein-coupled receptor cell surface protein that binds and is activated by N-Formylmethionine-containing oligopeptides, particularly N-Formylmethionine-leucyl-phenylalanine (FMLP). FPR1 is prominently expressed by mammalian phagocytic and blood leukocyte cells where it functions to mediate these cells' responses to the N-formylmethionine-containing oligopeptides which are released by invading microorganisms and injured tissues. FPR1 directs these cells to sites of invading pathogens or disrupted tissues and then stimulates these cells to kill the pathogens or to remove tissue debris; as such, it is an important component of the innate immune system that operates in host defense and damage control.
Heme binding protein 1 is a protein that in humans is encoded by the HEBP1 gene.
Specialized pro-resolving mediators are a large and growing class of cell signaling molecules formed in cells by the metabolism of polyunsaturated fatty acids (PUFA) by one or a combination of lipoxygenase, cyclooxygenase, and cytochrome P450 monooxygenase enzymes. Pre-clinical studies, primarily in animal models and human tissues, implicate SPM in orchestrating the resolution of inflammation. Prominent members include the resolvins and protectins.
This article incorporates text from the United States National Library of Medicine, which is in the public domain.