Functional differences between human formyl peptide receptor isoforms 26, 98, and G6

An independent predictor of poor prognosis in locally advanced rectal cancer: rs867228 in formyl peptide receptor 1 (FPR1)

Formyl peptide receptor 1 (FPR1) plays a key regulatory role in innate and adaptive immunity. Recently, we reported that the CC genotype of FPR1-E346A (rs867228, c. 1037 A > C) is an independent biomarker for patients with locally advanced rectal cancer (LARC) who received preoperative concurrent chemoradiotherapy (CCRT). Pharmacologic inhibition of FPR1 decreased the migration and infiltration of T lymphocytes into tumor microenvironment after CCRT.

Polymorphism of formyl peptide receptor 1 (FPR1) reduces the therapeutic efficiency and antitumor immunity after neoadjuvant chemoradiotherapy (CCRT) treatment in locally advanced rectal cancer

Immunosurveillance and immunoscavenging prompted by preoperative chemoradiotherapy (CCRT) may contribute to improve local control and increase survival outcomes for patients with locally advanced rectal cancer (LARC). In this study, we investigated several genotypes of pattern recognition receptors (PRRs) and their impact on therapeutic efficacy in LARC patients treated with CCRT. We found that homozygosis of formyl peptide receptor 1 (FPR1) (E346A/rs867228) was associated with reduced 5-year overall survival (OS) by Kaplan-Meier analysis (62% vs. 81%, p = 0.014) and multivariate analysis [hazard ratio (HR) = 3.383, 95% CI = 1.374-10.239, p = 0.007]. Moreover, in an animal model, we discovered that the FPR1 antagonist, Boc-MLF (Boc-1), reduced CCRT therapeutic efficacy and decreased cytotoxic T cells and T effector memory cells after chemoradiotherapy treatment. Pharmacologic inhibition of FPR1 by Boc-1 decreased T lymphocyte migration to irradiated tumor cells. Therefore, these results revealed that the FPR1 genotype participates in CCRT-elicited anticancer immunity by reducing T lymphocytes migration and infiltration, and that the FPR1-E346A CC genotype can be considered an independent biomarker for chemo- and radiotherapy outcomes.

Formyl-Peptide Receptors in Infection, Inflammation, and Cancer

Formyl-peptide receptors (FPRs) recognize bacterial and mitochondrial formylated peptides as well as endogenous non-formylated peptides and even lipids. FPRs are expressed on various host cell types but most strongly on neutrophils and macrophages. After the discovery of FPRs on leukocytes, it was assumed that these receptors predominantly govern a proinflammatory response resulting in chemotaxis, degranulation, and oxidative burst during infection. However, it is clear that the activation of FPRs has more complex consequences and can also promote the resolution of inflammation. Recent studies have highlighted associations between FPR function and inflammatory conditions, including inflammatory disorders, cancer, and infection. In this review we discuss these recent findings.

The role of formylated peptides and formyl peptide receptor 1 in governing neutrophil function during acute inflammation

Neutrophil migration to sites of inflammation and the subsequent execution of multiple functions are designed to contain and kill invading pathogens. These highly regulated and orchestrated processes are controlled by interactions between numerous receptors and their cognate ligands. Unraveling and identifying those that are central to inflammatory processes may represent novel therapeutic targets for the treatment of neutrophil-dominant inflammatory disorders in which dysregulated neutrophil recruitment, function, and elimination serve to potentiate rather than resolve an initial inflammatory insult. The first G protein-coupled receptor to be described on human neutrophils, formyl peptide receptor 1 (FPR1), is one such receptor that plays a significant role in the execution of these functions through multiple intracellular signaling pathways. Recent work has highlighted important observations with regard to both receptor function and the importance and functional relevance of FPR1 in the pathogenesis of a range of both sterile and infective inflammatory conditions. In this review, we explore the multiple components of neutrophil migration and function in both health and disease, with a focus on the role of FPR1 in these processes. The current understanding of FPR1 structure, function, and signaling is examined, alongside discussion of the potential importance of FPR1 in inflammatory diseases suggesting that FPR1 is a key regulator of the inflammatory environment.

Human neutrophils are activated by a peptide fragment of Clostridium difficile toxin B presumably via formyl peptide receptor

Clostridium difficile may induce antibiotic-associated diarrhoea and, in severe cases, pseudomembranous colitis characterized by tremendous neutrophil infiltration. All symptoms are caused by two exotoxins: TcdA and TcdB. We describe here the activation of isolated human blood neutrophils by TcdB and, moreover, by toxin fragments generated by limited proteolytical digestion. Kinetics and profiles of TcdB-induced rise in intracellular-free Ca(2+) and reactive oxygen species production were similar to that induced by fMLF, which activates the formyl peptide receptor (FPR) recognizing formylated bacterial peptide sequences. Transfection assays with the FPR-1 isoform hFPR26 in HEK293 cells, heterologous desensitization experiments and FPR inhibition via cyclosporine H strongly suggest activation of cells via FPR-1. Domain analyses revealed that the N-terminal glucosyltransferase domain of TcdB is a potent activator of FPR pointing towards an additional mechanism that might contribute to pathogenesis. This pro-inflammatory ligand effect can be triggered even by cleaved and, thus, non-cytotoxic toxin. In summary, we report (i) a ligand effect on neutrophils as completely new molecular mode of action, (ii) pathogenic potential of truncated or proteolytically cleaved 'non-cytotoxic' fragments and (iii) an interaction of the N-terminal glucosyltransferase domain instead of the C-terminal receptor binding domain of TcdB with target cells.

The leukocyte chemotactic receptor FPR1 is functionally expressed on human lens epithelial cells

BACKGROUND

Lens degeneration in Fpr1(-/-) mice prompted us to search for functional FPR1 expression directly on lens epithelial cells.

RESULTS

FPR1 is functionally expressed on human lens epithelial cells but has atypical properties compared with hematopoietic cell FPR1.

CONCLUSION

Lens epithelial cell FPR1 may be involved in development and maintenance of the lens.

SIGNIFICANCE

This is the first link between non-hematopoietic expression of FPR1 and an ophthalmologic phenotype. Formyl peptide receptor 1 (FPR1) is a G protein-coupled chemoattractant receptor expressed mainly on leukocytes. Surprisingly, aging Fpr1(-/-) mice develop spontaneous lens degeneration without inflammation or infection (J.-L. Gao et al., manuscript in preparation). Therefore, we hypothesized that FPR1 is functionally expressed directly on lens epithelial cells, the only cell type in the lens. Consistent with this, the human fetal lens epithelial cell line FHL 124 expressed FPR1 mRNA and was strongly FPR1 protein-positive by Western blot and FACS. Competition binding using FPR1 ligands N-formyl-Nle-Leu-Phe-Nle-Tyr-Lys (Nle = Norleucine), formylmethionylleucylphenylalanine, and peptide W revealed the same profile for FHL 124 cells, neutrophils, and FPR1-transfected HEK 293 cells. Saturation binding with fluorescein-labeled N-formyl-Nle-Leu-Phe-Nle-Tyr-Lys revealed ~2500 specific binding sites on FHL-124 cells (K(D) ~ 0.5 nm) versus ~40,000 sites on neutrophils (K(D) = 3.2 nm). Moreover, formylmethionylleucylphenylalanine induced pertussis toxin-sensitive Ca(2+) flux in FHL 124 cells, consistent with classic G(i)-mediated FPR1 signaling. FHL 124 cell FPR1 was atypical in that it resisted agonist-induced internalization. Expression of FPR1 was additionally supported by detection of the intact full-length open reading frame in sequenced cDNA from FHL 124 cells. Thus, FHL-124 cells express functional FPR1, which is consistent with a direct functional role for FPR1 in the lens, as suggested by the phenotype of Fpr1 knock-out mice.

Full characterization of GPCR monomer-dimer dynamic equilibrium by single molecule imaging

Receptor dimerization is important for many signaling pathways. However, the monomer-dimer equilibrium has never been fully characterized for any receptor with a 2D equilibrium constant as well as association/dissociation rate constants (termed super-quantification). Here, we determined the dynamic equilibrium for the N-formyl peptide receptor (FPR), a chemoattractant G protein-coupled receptor (GPCR), in live cells at 37°C by developing a single fluorescent-molecule imaging method. Both before and after liganding, the dimer-monomer 2D equilibrium is unchanged, giving an equilibrium constant of 3.6 copies/µm(2), with a dissociation and 2D association rate constant of 11.0 s(-1) and 3.1 copies/µm(2)s(-1), respectively. At physiological expression levels of ∼2.1 receptor copies/µm(2) (∼6,000 copies/cell), monomers continually convert into dimers every 150 ms, dimers dissociate into monomers in 91 ms, and at any moment, 2,500 and 3,500 receptor molecules participate in transient dimers and monomers, respectively. Not only do FPR dimers fall apart rapidly, but FPR monomers also convert into dimers very quickly.

Coexpression systems as models for the analysis of constitutive GPCR activity

The investigation of constitutive activity of GPCRs in transfected mammalian cells is often hampered by the presence of other constitutively active receptors that generate a high background signal. This impairs the measurement of constitutive activity and of inverse agonistic effects, both of which often occur in a relatively small signal range. Moreover, constitutive activity of a GPCR depends on the interacting G-protein. Since the commonly used mammalian cells contain a set of several different G-protein types, it is very difficult to investigate the influence of specific Gα and Gβγ subunits on constitutive activity in more detail in these expression systems. Here, we show that the Sf9 cell/baculovirus expression system provides excellent conditions for the characterization of constitutively active GPCRs. Sf9 cells express a restricted set of G-protein subtypes that show only a limited capability of interacting with mammalian GPCRs. Moreover, the Sf9 cell/baculovirus expression system allows the combined expression of up to four different proteins encoded by the respective genetically modified baculoviruses. Using the highly constitutively active human histamine H₄R (hH₄R) as a paradigm, we demonstrate how the coexpression of hH₄R with different signaling proteins (Gα, Gβγ, and RGS-proteins) in combination with sensitive functional assays (high-affinity agonist binding and steady-state GTPase- and GTPγS-binding assays) allows in-depth studies of constitutive activity. The preparation of Sf9 cell membranes, coexpressing hH₄R and various additional proteins, is described in detail as well as the procedures of the different functional assays. Moreover, we show that coexpression of GPCRs with signal transduction components in Sf9 cells can also be applied to the characterization of other constitutively active receptors, for example, the formyl peptide receptor and β₂-adrenoceptor.

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

Formylpeptide receptor single nucleotide polymorphism 348T>C and its relationship to polymorphonuclear leukocyte chemotaxis in aggressive periodontitis

BACKGROUND

Aggressive periodontitis (AgP) is associated with impaired polymorphonuclear leukocyte (PMN) chemotaxis toward bacterial N-formylpeptides. Formylpeptide receptors (FPRs) play a major role in guiding PMNs to infection sites. Previous work revealed a significant association between FPR1 single nucleotide polymorphism (SNP) 348T>C and AgP in African Americans. We tested the hypothesis that 348T impairs PMN chemotaxis by decreasing FPR mRNA expression, thereby increasing susceptibility to AgP.

METHODS

Blood samples were obtained from African American subjects (37 AgP cases and 38 controls). Chemotaxis to N-formyl-methionine-leucine-phenylalanine by freshly isolated PMNs was assayed in a modified Boyden chamber. RNA was isolated from PMNs, and FPR1 gene expression was quantified by real-time polymerase chain reaction (PCR). To detect FPR1 5' SNPs, genomic DNA was isolated, and four fragments spanning the FPR1 5' region were PCR-amplified and sequenced. Haplotype associations between SNP 348T>C and 5' SNPs were analyzed.

RESULTS

The homozygous 348T genotype was only found in AgP cases (P = 0.017; odds ratio, 18.9). Subjects with this genotype exhibited a significantly lower PMN chemotactic response relative to controls and to subjects with the 348C/C or 348T/C genotype (P <0.05). There were no significant differences in PMN FPR1 expression among subjects with the 348C/C, 348T/C, and 348T/T genotypes. Eleven FPR1 5' SNPs were detected, but none of the predicted haplotypes reflected associations with AgP or with 348T.

CONCLUSIONS

Although the 348T/T genotype is relatively rare, it is associated with significantly impaired PMN chemotaxis and an increased risk for developing AgP in African Americans. These associations do not seem to be related to significant reductions in FPR1 transcripts in subjects expressing 348T.

High constitutive activity and a G-protein-independent high-affinity state of the human histamine H(4)-receptor

The human histamine H(4)-receptor (hH(4)R) is expressed in mast cells and eosinophils and mediates histamine (HA)-induced chemotaxis via G(i)-proteins. For a detailed investigation of hH(4)R/G(i)-protein interaction, we coexpressed the hH(4)R with Galpha(i2) and Gbeta(1)gamma(2) as well as an hH(4)R-Galpha(i2) fusion protein with Gbeta(1)gamma(2) in Sf9 insect cells. The agonist radioligand [(3)H]HA showed a K(D) value of approximately 10 nM at hH(4)R and hH(4)R-Galpha(i2). The high-affinity states of hH(4)R and hH(4)R-Galpha(i2) were insensitive to guanosine 5'-[gamma-thio]triphosphate (GTPgammaS). The affinity of [(3)H]HA for hH(4)R was retained in the absence of mammalian G(i)-proteins. In steady-state GTPase- and [(35)S]GTPgammaS-binding assays, hH(4)R exhibited high constitutive activity and uncommon insensitivity to Na(+). Thioperamide (THIO) was only a partial inverse agonist. Addition of HA or THIO to baculovirus-infected (hH(4)R + Galpha(i2) + Gbeta(1)gamma(2)) Sf9 cells increased the B(max) in [(3)H]HA binding, but not in immunoblots, suggesting conformational instability and ligand-induced stabilization of membrane-integrated hH(4)R. No effect was observed on hH(4)R-Galpha(i2) expression, neither in [(3)H]HA binding nor in immunoblot. However, the expression level of hH(4)R-Galpha(i2) was consistently higher compared to hH(4)R, suggesting chaperone-like or stabilizing effects of Galpha(i2) on hH(4)R. In 37 degrees C stability assays, HA stabilized hH(4)R, and THIO even restored misfolded [(3)H]HA binding sites. Inhibition of hH(4)R glycosylation by tunicamycin reduced the [(3)H]HA binding B(max) value. In conclusion, (i) hH(4)R shows high constitutive activity and structural instability; (ii) hH(4)R shows a G-protein-independent high-affinity state; (iii) hH(4)R conformation is stabilized by agonists, inverse agonists and G-proteins; (iv) hH(4)R glycosylation is essential for cell-surface expression of intact hH(4)R.

The N-formyl peptide receptors and the anaphylatoxin C5a receptors: an overview

Leukocyte recruitment to sites of inflammation and infection is dependent on the presence of a gradient of locally produced chemotactic factors. This review is focused on current knowledge about the activation and regulation of chemoattractant receptors. Emphasis is placed on the members of the N-formyl peptide receptor family, namely FPR (N-formyl peptide receptor), FPRL1 (FPR like-1) and FPRL2 (FPR like-2), and the complement fragment C5a receptors (C5aR and C5L2). Upon chemoattractant binding, the receptors transduce an activation signal through a G protein-dependent pathway, leading to biochemical responses that contribute to physiological defense against bacterial infection and tissue damage. C5aR, and the members of the FPR family that were previously thought to be restricted to phagocytes proved to have a much broader spectrum of cell expression. In addition to N-formylated peptides, numerous unrelated ligands were recently found to interact with FPR and FPRL1. Novel agonists include both pathogen- and host-derived components, and synthetic peptides. Antagonistic molecules have been identified that exhibit limited receptor specificity. How distinct ligands can both induce different biological responses and produce different modes of receptor activation and unique sets of cellular responses are discussed. Cell responses to chemoattractants are tightly regulated at the level of the receptors. This review describes in detail the regulation of receptor signalling and the multi-step process of receptor inactivation. New concepts, such as receptor oligomerization and receptor clustering, are considered. Although FPR, FPRL1 and C5aR trigger similar biological functions and undergo a rapid chemoattractant-mediated phosphorylation, they appear to be differentially regulated and experience different intracellular fates.

Constitutive activity and ligand selectivity of human, guinea pig, rat, and canine histamine H2 receptors

Previous studies revealed pharmacological differences between human and guinea pig histamine H(2) receptors (H(2)Rs) with respect to the interaction with guanidine-type agonists. Because H(2)R species variants are structurally very similar, comparative studies are suited to relate different properties of H(2)R species isoforms to few molecular determinants. Therefore, we systematically compared H(2)Rs of human (h), guinea pig (gp), rat (r), and canine (c). Fusion proteins of hH(2)R, gpH(2)R, rH(2)R, and cH(2)R, respectively, and the short splice variant of G(salpha), G(salphaS), were expressed in Sf9 insect cells. In the membrane steady-state GTPase activity assay, cH(2)R-G(salphaS) but neither gpH(2)R-G(salphaS) nor rH(2)R-G(salphaS) showed the hallmarks of increased constitutive activity compared with hH(2)R-G(salphaS), i.e., increased efficacies of partial agonists, increased potencies of agonists with the extent of potency increase being correlated with the corresponding efficacies at hH(2)R-G(salphaS), increased inverse agonist efficacies, and decreased potencies of antagonists. Furthermore, in membranes expressing nonfused H(2)Rs without or together with mammalian G(salphaS) or H(2)R-G(salpha) fusion proteins, the highest basal and GTP-dependent increases in adenylyl cyclase activity were observed for cH(2)R. An example of ligand selectivity is given by metiamide, acting as an inverse agonist at hH(2)R-G(salphaS), gpH(2)R-G(salphaS), and rH(2)R-G(salphaS) in the GTPase assay in contrast to being a weak partial agonist with decreased potency at cH(2)R-G(salphaS). In conclusion, the cH(2)R exhibits increased constitutive activity compared with hH(2)R, gpH(2)R, and rH(2)R, and there is evidence for ligand-specific conformations in H(2)R species isoforms.

Chemotaxis Receptors and Signaling

Chemotaxis is an important cellular response common in biology. In many chemotaxing cells the signal that regulates movement is initiated by G protein-coupled receptors on the cell surface that bind specific chemoattractants. These receptors share important structural similarities with other G protein-coupled receptors, including rhodopsin, which currently serves as the best starting point for modeling their structures. However, the chemotaxis receptors also share a number of relatively unique structural features that are less common in other GPCRs. The chemoattractant ligands of chemotaxis receptors exhibit a broad variety of sizes and chemical properties, ranging from small molecules and peptides to protein ligands. As a result, different chemotaxis receptors have evolved specialized mechanisms for the early steps of ligand binding and receptor activation. The mechanism of transmembrane signaling is currently under intensive study and several alternate mechanisms proposing different conformational rearrangements of the transmembrane helices have been proposed. Some chemotaxis receptors are proposed to form dimers, and in certain cases dimer formation is proposed to play a role in transmembrane signaling. In principle the structural and dynamical changes that occur during transmembrane signaling could be specialized for different receptors, or could be broadly conserved. Extensive mutagenesis studies have been carried out, and have begun to identify critical residues involved in ligand binding, receptor activation, and transmembrane signaling.

The human formyl peptide receptor as model system for constitutively active G-protein-coupled receptors

According to the two-state model of G-protein-coupled receptor (GPCR) activation, GPCRs isomerize from an inactive (R) state to an active (R*) state. In the R* state, GPCRs activate G-proteins. Agonist-independent R/R* isomerization is referred to as constitutive activity and results in an increase in basal G-protein activity, i.e. GDP/GTP exchange. Agonists stabilize the R* state and further increase, whereas inverse agonists stabilize the R state and decrease, basal G-protein activity. Constitutive activity is observed in numerous wild-type GPCRs and disease-causing GPCR mutants with increased constitutive activity. The human formyl peptide receptor (FPR) exists in several isoforms (FPR-26, FPR-98 and FPR-G6) and activates chemotaxis and cytotoxic cell functions of phagocytes through G(i)-proteins. Studies in HL-60 leukemia cell membranes demonstrated inhibitory effects of Na(+) and pertussis toxin on basal G(i)-protein activity, suggesting that the FPR is constitutively active. However, since HL-60 cells express several constitutively active chemoattractant receptors, analysis of constitutive FPR activity was difficult. Sf9 insect cells do not express chemoattractant receptors and G(i)-proteins and provide a sensitive reconstitution system for FPR/G(i)-protein coupling. Such expression studies showed that FPR-26 is much more constitutively active than FPR-98 and FPR-G6 as assessed by the relative inhibitory effects of Na(+) and of the inverse agonist cyclosporin H on basal G(i)-protein activity. Site-directed mutagenesis studies suggest that the E346A exchange in the C-terminus critically determines dimerization and constitutive activity of FPR. Moreover, N-glycosylation of the N-terminus seems to be important for constitutive FPR activity. Finally, we discuss some future directions of research.