Cloning and functional characterization of the human fractalkine receptor promoter regions

CX3CL1 (Fractalkine)-CX3CR1 Axis in Inflammation-Induced Angiogenesis and Tumorigenesis

The chemotactic cytokine fractalkine (FKN, chemokine CX3CL1) has unique properties resulting from the combination of chemoattractants and adhesion molecules. The soluble form (sFKN) has chemotactic properties and strongly attracts T cells and monocytes. The membrane-bound form (mFKN) facilitates diapedesis and is responsible for cell-to-cell adhesion, especially by promoting the strong adhesion of leukocytes (monocytes) to activated endothelial cells with the subsequent formation of an extracellular matrix and angiogenesis. FKN signaling occurs via CX3CR1, which is the only known member of the CX3C chemokine receptor subfamily. Signaling within the FKN-CX3CR1 axis plays an important role in many processes related to inflammation and the immune response, which often occur simultaneously and overlap. FKN is strongly upregulated by hypoxia and/or inflammation-induced inflammatory cytokine release, and it may act locally as a key angiogenic factor in the highly hypoxic tumor microenvironment. The importance of the FKN/CX3CR1 signaling pathway in tumorigenesis and cancer metastasis results from its influence on cell adhesion, apoptosis, and cell migration. This review presents the role of the FKN signaling pathway in the context of angiogenesis in inflammation and cancer. The mechanisms determining the pro- or anti-tumor effects are presented, which are the cause of the seemingly contradictory results that create confusion regarding the therapeutic goals.

Regulation and biological functions of the CX3CL1-CX3CR1 axis and its relevance in solid cancer: A mini-review

CX3CL1 is a transmembrane protein from which a soluble form can be generated by proteolytic shedding. Membranal and soluble forms of CX3CL1 exhibit different functions, although both bind to the CX3CR1 chemokine receptor. The CX3CL1-CX3CR1 axis mediates the adhesion of leukocytes and is also involved in cell survival and recruitment of immune cell subpopulations. The function of CX3CL1 is finely tuned by cytokines and transcription factors regulating its expression and post-translational modifications. On homeostasis, the CX3CL1-CX3CR1 axis participates in the removal of damaged neurons and neurogenesis, and it is also involved on several pathological contexts. The CX3CL1-CX3CR1 axis induces several cellular responses relevant to cancer such as proliferation, migration, invasion and apoptosis resistance. In this review, we address biological aspects of this molecular axis with important therapeutic potential, emphasizing its role in cancer, one of the most prevalent chronic diseases which significantly affect the quality of life and life expectancy of patients.

Fractalkine Regulates HEC-1A/JEG-3 Interaction by Influencing the Expression of Implantation-Related Genes in an In Vitro Co-Culture Model

Embryo implantation is a complex process regulated by a network of biological molecules. Recently, it has been described that fractalkine (CX3CL1, FKN) might have an important role in the feto-maternal interaction during gestation since the trophoblast cells express fractalkine receptor (CX3CR1) and the endometrium cells secrete fractalkine. CX3CR1 controls three major signalling pathways, PLC-PKC pathway, PI3K/AKT/NFκB pathway and Ras-mitogen-activated protein kinases (MAPK) pathways regulating proliferation, growth, migration and apoptosis. In this study, we focused on the molecular mechanisms of FKN treatment influencing the expression of implantation-related genes in trophoblast cells (JEG-3) both in mono-and in co-culture models. Our results reveal that FKN acted in a concentration and time dependent manner on JEG-3 cells. FKN seemed to operate as a positive regulator of implantation via changing the action of progesterone receptor (PR), activin receptor and bone morphogenetic protein receptor (BMPR). FKN modified also the expression of matrix metalloproteinase 2 and 9 controlling invasion. The presence of HEC-1A endometrial cells in the co-culture contributed to the effect of fractalkine on JEG-3 cells regulating implantation. The results suggest that FKN may contribute to the successful attachment and implantation of embryo.

Cytokines use different intracellular mechanisms to upregulate the membrane expression of CX3CR1 in human monocytes

Membrane expression of fractalkine (CX₃CL1)-receptor (CX₃CR1) is relevant in monocytes (Mo) because CX₃CR1-CX₃CL1 interactions might participate on both, homeostatic and pathologic conditions. We have previously demonstrated that CX₃CR1 levels are decreased during culture and when Mo are differentiated into dendritic cells, but enhanced when differentiated into macrophages. Regarding soluble factors, lipopolysaccharide (LPS) accelerated the loss of CX₃CR1, while interleukin (IL)-10 and Interferon-gamma (IFN-γ) prevented it. However, the comprehensive knowledge about the intracellular pathways that underlay the level of CX₃CR1 expression in Mo is still incomplete. In the current work, we studied the effect of anti-inflammatory cytokines (IL-4, IL-13, IL-10), alone or together with IFN- γ on CX₃CR1 expression. We found that only IL-10 and IFN-γ separately were able to prevent CX₃CR1 down-modulation during culture of human Mo. Besides, Mo incubated with IL-10 plus IFN-γ showed the highest CX₃CR1 expression by cell, suggesting cooperation between two different mechanism used by both cytokines. By studying intracellular mechanisms triggered by IL-10 and IFN-γ, we demonstrated that they specifically induced PI3K-dependent serine-phosphorylation of signal transducer and activator of transcription (STAT)3 or STAT1, respectively. Moreover, chemical inhibitors of STAT1 or STAT3 abrogated IFN-γ or IL-10 effects on CX₃CR1 expression. Strikingly, only IL-10 increased CX₃CR1 mRNA level, as consequence of augmenting mRNA stability. CX₃CR1 mRNA increase was PI3K-dependent, supporting the causal link between the action of IL-10 at the CX₃CR1 transcript and CX₃CR1 protein level on Mo. Thus, both cytokines up-regulate CX₃CR1 expression on human Mo by different intracellular mechanisms.

Evolution of the ability to modulate host chemokine networks via gene duplication in human cytomegalovirus (HCMV)

Human cytomegalovirus (HCMV) is a widespread pathogen that is particularly skillful at evading immune detection and defense mechanisms, largely due to extensive co-evolution with its host. One aspect of this co-evolution involves the acquisition of virally encoded G protein-coupled receptors (GPCRs) with homology to the chemokine receptor family. GPCRs are the largest family of cell surface proteins, found in organisms from yeast to humans, and they regulate a variety of cellular processes including development, sensory perception, and immune cell trafficking. The US27 and US28 genes are encoded by human and primate CMVs, but homologs are not found in the genomes of viruses infecting rodents or other species. Phylogenetic analysis was used to investigate the US27 and US28 genes, which are adjacent in the unique short (US) region of the HCMV genome, and their relationship to one another and to human chemokine receptor genes. The results indicate that both US27 and US28 share the same common ancestor with human chemokine receptor CX3CR1, suggesting that a single host gene was captured and a subsequent viral gene duplication event occurred. The US28 gene product (pUS28) has maintained the function of the ancestral gene and has the ability to bind and signal in response to CX3CL1/fractalkine, the natural ligand for CX3CR1. In contrast, pUS27 does not bind to any known chemokine ligand, and the sequence has diverged significantly, highlighted by the fact that pUS27 currently exhibits greater sequence similarity to human CCR1. While the evolutionary advantage of the gene duplication and neofunctionalization event remains unclear, the US27 and US28 genes are highly conserved among different HCMV strains and retained even in laboratory strains that have lost many virulence genes, suggesting that US27 and US28 have each evolved distinct, important functions during virus infection.

[Role of fractalkine/CX3CL1 and its receptor CX3CR1 in allergic diseases]

Allergic asthma and atopic dermatitis are diseases mainly resulting from the activation of Th2 cells, that produce cytokines favouring IgE production and eosinophilia but also of Th1 cells, that contribute to inflammation chronicity. Lymphocyte recruitment and retention of Th cells in target organs are 2 key events for asthma and atopic dermatitis pathogenesis. While lymphocyte migration is regulated by chemokines and lipid mediators such as leukotrienes and prostaglandins, factors involved in lymphocyte retention and survival within inflammatory tissues remain poorly understood. Recent works show that, in allergic diseases, there is an increased expression of fractalkine/CX3CL1 and its unique receptor CX3CR1 and that this chemokine does not act as chemoattractant. In allergic asthma, CX3CR1 expression regulates Th2 and Th1 cell survival in the inflammatory lung, while, in atopic dermatitis, it regulate Th2 and Th1 cell retention into the inflammatory site. Use of peptides blocking fractalkine binding to its receptor is currently tested in the treatment of asthma and atopic dermatitis.

Analysis of the Role of CX3CL1 (Fractalkine) and Its Receptor CX3CR1 in Traumatic Brain and Spinal Cord Injury: Insight into Recent Advances in Actions of Neurochemokine Agents

CX3CL1 (fractalkine) is the only member of the CX3C (delta) subfamily of chemokines which is unique and combines the properties of both chemoattractant and adhesion molecules. The two-form ligand can exist either in a soluble form, like all other chemokines, and as a membrane-anchored molecule. CX3CL1 discloses its biological properties through interaction with one dedicated CX3CR1 receptor which belongs to a family of G protein-coupled receptors (GPCR). The CX3CL1/CX3CR1 axis acts in many physiological phenomena including those occurring in the central nervous system (CNS), by regulating the interactions between neurons, microglia, and immune cells. Apart from the role under physiological conditions, the CX3CL1/CX3CR1 axis was implied to have a role in different neuropathologies such as traumatic brain injury (TBI) and spinal cord injury (SCI). CNS injuries represent a serious public health problem, despite improvements in therapeutic management. To date, no effective treatment has been determined, so they constitute a leading cause of death and severe disability. The course of TBI and SCI has two consecutive poorly demarcated phases: the initial, primary injury and secondary injury. Recent evidence has implicated the role of the CX3CL1/CX3CR1 axis in neuroinflammatory processes occurring after CNS injuries. The importance of the CX3CL1/CX3CR1 axis in the pathophysiology of TBI and SCI in the context of systemic and direct local immune response is still under investigation. This paper, based on a review of the literature, updates and summarizes the current knowledge about CX3CL1/CX3CR1 axis involvement in TBI and SCI pathogenesis, indicating possible molecular and cellular mechanisms with a potential target for therapeutic intervention.

DNA Methylation Regulates the Differential Expression of CX3CR1 on Human IL-7Rαlow and IL-7Rαhigh Effector Memory CD8+ T Cells with Distinct Migratory Capacities to the Fractalkine

DNA methylation is an epigenetic mechanism that modulates gene expression in mammalian cells including T cells. Memory T cells are heterogeneous populations. Human effector memory (EM) CD8(+) T cells in peripheral blood contain two cell subsets with distinct traits that express low and high levels of the IL-7Rα. However, epigenetic mechanisms involved in defining such cellular traits are largely unknown. In this study, we use genome-wide DNA methylation and individual gene expression to show the possible role of DNA methylation in conferring distinct traits of chemotaxis and inflammatory responses in human IL-7Rα(low) and IL-7Rα(high) EM CD8(+) T cells. In particular, IL-7Rα(low) EM CD8(+) T cells had increased expression of CX3CR1 along with decreased DNA methylation in the CX3CR1 gene promoter compared with IL-7Rα(high) EM CD8(+) T cells. Altering the DNA methylation status of the CX3CR1 gene promoter changed its activity and gene expression. IL-7Rα(low) EM CD8(+) T cells had an increased migratory capacity to the CX3CR1 ligand fractalkine compared with IL-7Rα(high) EM CD8(+) T cells, suggesting an important biological outcome of the differential expression of CX3CR1. Moreover, IL-7Rα(low) EM CD8(+) T cells induced fractalkine expression on endothelial cells by producing IFN-γ and TNF-α, forming an autocrine amplification loop. Overall, our study shows the role of DNA methylation in generating unique cellular traits in human IL-7Rα(low) and IL-7Rα(high) EM CD8(+) T cells, including differential expression of CX3CR1, as well as potential biological implications of this differential expression.

The transcription factor PU.1 is critical for viability and function of human brain microglia

Microglia are the predominant resident immune cells of the brain and can assume a range of phenotypes. They are critical for normal brain development and function but can also contribute to many disease processes. Although they are widely studied, the transcriptional control of microglial phenotype and activation requires further research. PU.1 is a key myeloid transcription factor expressed by peripheral macrophages and rodent microglia. In this article, we report the presence of PU.1 specifically in microglia of the adult human brain and we examine its functional role in primary human microglia. Using siRNA, we achieved substantial PU.1 protein knock-down in vitro. By assessing a range of characteristic microglial proteins we found decreased viability of adult human microglia with reduced PU.1 protein expression. This observation was confirmed with PU.1 antisense DNA oligonucleotides. An important function of microglia is to clear debris by phagocytosis. We assessed the impact of loss of PU.1 on microglial phagocytosis and show that PU.1 siRNA reduces the ability of adult human microglia to phagocytose amyloid-beta1-42 peptide. These results show that PU.1 controls human microglial viability and function and suggest PU.1 as a molecular target for manipulation of human microglial phenotype.

Inflammatory effects of resistin on human smooth muscle cells: up-regulation of fractalkine and its receptor, CX3CR1 expression by TLR4 and Gi-protein pathways

In the atherosclerotic plaque, smooth muscle cells (SMC) acquire an inflammatory phenotype. Resistin and fractalkine (CX3CL1) are found in human atheroma and not in normal arteries. CX3CL1 and CX3CR1 are predominately associated with SMC. We have questioned whether resistin has a role in the expression of CX3CL1 and CX3CR1 in SMC thus contributing to the pro-inflammatory status of these cells. Cultured human aortic SMC were stimulated with 100 ng/ml resistin for 4, 6, 12, and 24 h, and then CX3CL1 and CX3CR1 expression was assessed by quantitative reverse transcription with the polymerase chain reaction and Western blot. We found that resistin up-regulated CX3CL1 and CX3CR1 in SMC and induced the phosphorylation of p38MAPK and STAT3. Inhibitors of p38MAPK, JAK-STAT, NF-kB, and AP-1 significantly reduced CX3CL1 and CX3CR1 expression. Knockdown of STAT1 and STAT3 with decoy oligodeoxinucleotides and the silencing of p65 and cjun with short interfering RNA decreased CX3CL1 and CX3CR1 expression. Anti-TLR4 antibody and pertussis toxin also reduced CX3CL1 and CX3CR1 protein expression. xCELLigence experiments revealed that resistin probably uses Gi-proteins for its effect on SMC. The CX3CL1 induced by resistin exhibited a chemotactic effect on monocyte transmigration. Thus, (1) resistin contributes to the pro-inflammatory state of SMC by the up-regulation of CX3CL1 and CX3CR1 expression via a mechanism involving NF-kB, AP-1, and STAT1/3 transcription factors, (2) resistin employs TLR4 and Gi-protein signaling for its effect on SMC, (3) CX3CL1 induced by resistin is functional in monocyte chemotaxis. The data reveal new mechanisms by which resistin promotes the inflammatory phenotype of SMC.

CX3CL1 in allergic diseases: not just a chemotactic molecule

Allergic asthma and atopic dermatitis (AD) are two allergic diseases that are primarily driven by the activation of T helper (Th)2 cells. Th2 cells produce cytokines that directly contribute to the symptoms of these diseases. The recruitment and maintenance of Th2 cells into the target tissues are two key events in the pathogenesis of allergic asthma and AD. While migration is mediated by both chemokines and lipid mediators such as leukotrienes and prostaglandins, very little is known about the molecules involved in lymphocyte survival and maintenance in inflamed tissues. However, chemokines could also play a role in this phenomenon. An example of this could be illustrated by CX3CL1, also known as fractalkine. CX3CL1 is a chemokine that is upregulated in some inflammatory diseases including allergic pathologies and that was recently demonstrated to provide a survival signal upon binding to its unique receptor CX3CR1.

Corticosteroid insensitivity of chemokine expression in airway smooth muscle of patients with severe asthma

BACKGROUND

Patients with severe asthma are less responsive to the beneficial effects of corticosteroid therapy.

OBJECTIVE

We investigated whether corticosteroid insensitivity was present in airway smooth muscle cells (ASMCs) of patients with severe asthma.

METHODS

ASMCs cultured from bronchial biopsy specimens of nonasthmatic control subjects (n = 12) and patients with nonsevere (n = 10) or severe (n = 10) asthma were compared for the effect of dexamethasone on suppression of TNF-α- and IFN-γ-induced CCL11 (eotaxin), CXCL8 (IL-8), and CX3CL1 (fractalkine) expression. The mechanisms of corticosteroid insensitivity are also determined.

RESULTS

CCL11 release was higher in ASMCs of patients with nonsevere but not severe asthma and nonasthmatic control subjects; CXCL8 and CX3CL1 release were similar in all groups. In patients with severe asthma, dexamethasone caused less suppression of CCL11 and CXCL8 release induced by TNF-α. Dexamethasone potentiated TNF-α- and IFN-γ-induced CX3CL1 release equally in all 3 groups. TNF-α-induced phosphorylated p38 mitogen-activated protein kinase levels were increased in ASMCs from patients with severe asthma compared with those from patients with nonsevere asthma and nonasthmatic subjects, whereas TNF-α-induced phosphorylated c-Jun N-terminal kinase and phosphorylated extracellular signal-related kinase levels were increased in all asthmatic groups. A p38 inhibitor increased the inhibitory effect of dexamethasone.

CONCLUSIONS

ASMCs of patients with severe asthma are corticosteroid insensitive; this might be secondary to heightened p38 mitogen-activated protein kinase levels.

Hypoxia-inducible factor-1α regulates chemotactic migration of pancreatic ductal adenocarcinoma cells through directly transactivating the CX3CR1 gene

CX3CR1 is an important chemokine receptor and regulates the chemotactic migration of pancreatic ductal adenocarcinoma (PDAC) cells. Up to now, its regulatory mechanism remains largely undefined. Here, we report that hypoxia upregulates the expression of CX3CR1 in pancreatic cancer cells. When hypoxia-inducible factor (HIF)-1α expression was knocked down in vitro and in vivo, the expression of CX3CR1 was significantly decreased. Chromatin immunoprecipitation assay demonstrated that HIF-1α bound to the hypoxia-response element (HRE; 5'-A/GCGTG-3') of CX3CR1 promoter under normoxia, and this binding was significantly enhanced under hypoxia. Overexpression of HIF-1α significantly upregulated the expression of luciferase reporter gene under the control of the CX3CR1 promoter in pancreatic cancer cells. Importantly, we demonstrated that HIF-1α may regulate cancer cell migration through CX3CR1. The HIF-1α/CX3CR1 pathway might represent a valuable therapeutic target to prevent invasion and distant metastasis in PDAC.

CX3CR1 is required for airway inflammation by promoting T helper cell survival and maintenance in inflamed lung

Allergic asthma is a T helper type 2 (T(H)2)-dominated disease of the lung. In people with asthma, a fraction of CD4(+) T cells express the CX3CL1 receptor, CX3CR1, and CX3CL1 expression is increased in airway smooth muscle, lung endothelium and epithelium upon allergen challenge. Here we found that untreated CX3CR1-deficient mice or wild-type (WT) mice treated with CX3CR1-blocking reagents show reduced lung disease upon allergen sensitization and challenge. Transfer of WT CD4(+) T cells into CX3CR1-deficient mice restored the cardinal features of asthma, and CX3CR1-blocking reagents prevented airway inflammation in CX3CR1-deficient recipients injected with WT T(H)2 cells. We found that CX3CR1 signaling promoted T(H)2 survival in the inflamed lungs, and injection of B cell leukemia/lymphoma-2 protein (BCl-2)-transduced CX3CR1-deficient T(H)2 cells into CX3CR1-deficient mice restored asthma. CX3CR1-induced survival was also observed for T(H)1 cells upon airway inflammation but not under homeostatic conditions or upon peripheral inflammation. Therefore, CX3CR1 and CX3CL1 may represent attractive therapeutic targets in asthma.

Rosiglitazone activation of PPARgamma suppresses fractalkine signaling

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is a key transcriptional regulator of both lipid metabolism and inflammation. The importance of PPARgamma is accentuated by the widespread use of synthetic PPARgamma agonists, thiazolidinediones (such as rosiglitazone), as drugs for insulin resistance and type II diabetes. Fractalkine (FKN) and FKN receptor (FR) play an important role in the immune responses by regulating leukocyte migration and adhesion to inflamed peripheral tissues. In this study, we have identified a novel link between PPARgamma activation and FKN signaling. On one hand, the activation of PPARgamma by rosiglitazone in macrophages not only represses the transcription of the FR gene, but also prevents the plasma membrane translocation of the FR protein. On the other hand, the activation of PPARgamma by rosiglitazone in endothelial cells also impedes the nuclear export of FKN. Together, these data suggest that PPARgamma activation represses FKN signaling. These findings indicate a previously unrecognized mechanism that may contribute to the anti-inflammatory effect of PPARgamma.

Fractalkine upregulates intercellular adhesion molecule-1 in endothelial cells through CX3CR1 and the Jak Stat5 pathway

Fractalkine (FKN) is a membrane-bound chemokine that can be released by proteolysis to produce soluble FKN (s-FKN). FKN and its receptor, CX3CR1, are believed to be important factors in atherosclerosis and may play a role in acute inflammatory responses. Although FKN is expressed on endothelial cells (ECs), CX3CR1 is reported to reside mainly on certain leukocyte populations. RT-PCR and Western blotting demonstrated that both human coronary artery and umbilical vein ECs expressed CX3CR1 mRNA and protein. Confocal microscopy showed that CX3CR1 was located at the cell membrane and to a lesser extent in the cytoplasm. Following exposure of both types of ECs to hypoxia and reoxygenation, FKN expression increased rapidly and s-FKN was shed into the culture medium. The addition of s-FKN protein to cultured ECs resulted in a dose-dependent increase in intercellular adhesion molecule (ICAM)-1 mRNA. Perfusion of mouse hearts with s-FKN protein increased expression of ICAM-1 protein in vascular endothelium. Transfection of ECs with CX3CR1-interfering RNA to knockdown the receptor resulted in decreased ICAM-1 expression after s-FKN stimulation. In addition, when ECs were stimulated with s-FKN, greater adhesion of human neutrophils to the ECs was observed. This increase was ICAM-1 dependent and was blocked by CX3CR1 knockdown. Following exposure to s-FKN, ECs exhibited increased phosphorylation of Jak2 and Stat5 and the ICAM-1 expression induced by s-FKN was blocked by silencing of Stat5 with small interfering RNA. Vascular ECs express both FKN and its receptor CX3CR1. s-FKN is shed from ECs following hypoxia/reoxygenation and acts through CX3CR1 on ECs to increase ICAM-1 expression and promote neutrophil adhesion through activation of the Jak-Stat5 pathway.

Oxidized Lipid-Driven Chemokine Receptor Switch, CCR2 to CX3CR1, Mediates Adhesion of Human Macrophages to Coronary Artery Smooth Muscle Cells Through a Peroxisome Proliferator-Activated Receptor γ–Dependent Pathway

Background— Recent genetic data in mouse and humans suggest that the chemokine receptors CCR2 and CX3CR1 are involved in atherogenesis; however, detailed molecular and cellular mechanisms have not been fully delineated.

Methods and Results— Here, we show that oxidized linoleic acid metabolites, which are components of oxidized LDL found in large amounts in atherosclerotic plaque, were able to specifically induce differentiation of human monocytes to macrophages with decreased expression of CCR2, confirming a previous report, and increased expression of CX3CR1. These macrophages acquired the ability to adhere to coronary artery smooth muscle cells. The adhesion was mediated directly and predominantly by CX3CR1. Reciprocal effects of these lipids on CCR2 and CX3CR1 expression were mediated by the nuclear receptor peroxisome proliferator-activated receptor (PPAR) γ, and targeting the PPARγ gene with sRNAi dramatically reduced macrophage adhesion to coronary artery smooth muscle cells.

Conclusions— These data suggest that in atherogenesis oxidized lipid-driven activation of macrophage PPARγ in the intima may result in a proadhesive chemokine receptor switch–CCR2 off, CX3CR1 on–causing cessation of CCR2-dependent migration and activation of CX3CR1-dependent retention mechanisms, which together promote macrophage accumulation in vessel wall. Our results may explain at the molecular and cell biology levels the genetic link between CX3CR1 and atherosclerosis. Moreover, they identify macrophage binding to coronary artery smooth muscle cells as the first primary cell setting in which CX3CR1 functions as the major adhesion system.

The mouse CCR2 gene is regulated by two promoters that are responsive to plasma cholesterol and peroxisome proliferator-activated receptor γ ligands

We have previously shown that the expression of monocyte CCR2, the receptor for monocyte chemoattractant protein-1, is induced by plasma cholesterol. The present study examines the mechanisms that regulate monocyte CCR2 expression in hypercholesterolemia using a mouse model. Our data demonstrate that in the mouse, CCR2 expression in circulating monocytes is controlled by two promoters P1 and P2. The two distinct transcripts, which encode the same protein, are produced by alternative splicing in the 5'-untranslated region. Both promoters are constitutively active, but only P2 is stimulated by cholesterol. However, both promoters are repressed by peroxisome proliferator-activated receptor gamma.

Interleukin (IL)-15 and IL-2 Reciprocally Regulate Expression of the Chemokine Receptor CX3CR1 through Selective NFAT1- and NFAT2-dependent Mechanisms

We have recently reported that interleukin (IL)-15 and IL-2, which signal through IL-2Rbetagamma, oppositely regulate expression of the proinflammatory chemokine receptor CX3CR1. Here we delineate molecular mechanisms responsible for this paradox. By using a luciferase reporter plasmid, we identified a 433-bp region spanning the major transcriptional start point of human CX3CR1 that, when expressed in human peripheral blood mononuclear cells (PBMCs), possessed strong constitutive promoter activity. IL-2 and IL-15 treatment increased and abolished this activity, respectively, mimicking their effects on endogenous CX3CR1. IL-2 and IL-15 have been reported to also have opposite effects on the immunoregulatory transcription factor NFAT (nuclear factor of activated T cells), and the 433-bp region contains a kappaB-like NFAT site. The effects of IL-15 and IL-2 on both CX3CR1 reporter activity and endogenous CX3CR1 transcription in PBMCs were abolished by the NFAT inhibitors cyclosporin A and VIVIT. Moreover, mutation of the kappaB-like NFAT sequence markedly attenuated IL-2 and IL-15 modulation of CX3CR1 promoter-reporter activity in PBMCs. Furthermore, chromatin immunoprecipitation revealed that IL-15 promoted specific recruitment of NFAT1 but not NFAT2 to the CX3CR1 promoter, whereas IL-2 had the converse effect. This appears to be relevant in vivo because mouse CX3CR1 mRNA was expressed in both PBMCs and splenocytes from NFAT1-/- mice injected with recombinant IL-15 but was undetectable in cells from IL-15-injected NFAT1+/+ BALB/c mice; as predicted, IL-2 up-regulated cx3cr1 in both mouse strains to a similar extent. Thus, by pharmacologic, genetic, and biochemical criteria in vitro and in vivo, our results suggest that IL-15 and IL-2 oppositely regulate CX3CR1 gene expression by differentially recruiting NFAT1 and NFAT2 to a kappaB-like NFAT site within the CX3CR1 promoter. We propose that expression of CX3CR1 and possibly other immunoregulatory genes may be determined in part by the balance of NFAT1 and NFAT2 activity in leukocytes.

IL-15 and IL-2 oppositely regulate expression of the chemokine receptor CX3CR1

The chemokine receptor CX3CR1 (CX3C chemokine receptor 1) is expressed in mouse blood on natural killer (NK) cells and on monocytes. Because interleukin-15 (IL-15) is an essential cytokine for NK cell development and maintenance, we hypothesized that it may induce CX3CR1 expression on this cell type. In contrast, we found that in primary mouse bone marrow-derived NK cells IL-15 specifically inhibited CX3CR1 protein and mRNA accumulation, whereas the related cytokine IL-2 did not inhibit but instead increased CX3CR1 expression. Consistent with this finding, intravenous injection of a single dose of recombinant IL-15 into C57BL/6 mice decreased steady-state CX3CR1 levels 24 hours after injection in freshly isolated peripheral blood mononuclear cells (PBMCs), splenocytes, and bone marrow cells, and treatment of mouse PBMCs with IL-15 in vitro inhibited CX3CL1 (ligand for CX3CR1)-induced chemotaxis. These data suggest that IL-15 may be a negative regulator of innate immunity by inhibiting CX3CR1 expression. These data also suggest that IL-15 inhibition of CX3CR1 may subvert potential cell immunotherapy strategies in which IL-15 is used to expand NK cell populations in vivo or ex vivo. Finally, our results provide additional evidence for differential signaling by IL-2 and IL-15, despite usage of common beta gamma c receptor chains.

Two Novel Fully Functional Isoforms of CX3CR1 Are Potent HIV Coreceptors

We identified two novel isoforms of the human chemokine receptor CX3CR1, produced by alternative splicing and with N-terminal regions extended by 7 and 32 aa. Expression of the messengers coding these isoforms, compared with that of previously described V28 messengers, is lower in monocytes and NK cells, but higher in CD4(+) T lymphocytes. CX3CR1 and its extended isoforms were expressed in HEK-293 cells and compared for expression, ligand binding, and cellular responses. In steady state experiments, all three CX3CR1 isoforms bound CX3CL1 with similar affinity. In kinetic binding studies, however, k(on) and k(off) were significantly greater for the extended CX3CR1 isoforms, thereby suggesting that the N-terminal extensions may alter the functions induced by CX3CL1. In signaling studies, all three CX3CR1 isoforms mediated agonist-dependent calcium mobilization, but the EC(50) was lower for the extended than for the standard isoforms. In addition, chemotactic responses for these extended isoforms shifted left, also indicating a more sensitive response. Finally, the longer variants appeared to be more potent HIV coreceptors when tested in fusion and infection assays. In conclusion, we identified and characterized functionally two novel isoforms of CX3CR1 that respond more sensitively to CX3CL1 and HIV viral envelopes. These data reveal new complexity in CX3CR1 cell activation and confirm the critical role of the N-terminal domain of the chemokine receptors in ligand recognition and cellular response.

Genomic organization and evolution of the CX3CR1/CCR8 chemokine receptor locus

The chemokine receptors CCR8 and CX3CR1 are key players in adaptive immunity and are co-receptors for human immunodeficiency virus. We describe here the genomic organization and evolutionary history of both of these genes. CX3CR1 has three promoters that transcribe three separate exons that are spliced with a fourth exon containing the coding region. CCR8 has two promoters. One promoter produces a transcript of two spliced exons, and the other promoter transcribes an exon containing the coding region and lacks introns. We analyzed these promoters in the context of a luciferase reporter and identified several positive and negative regulatory elements. Identification of the genomic organization of these genes in mouse demonstrates a similar organization for CCR8, but mouse CX3CR1 lacks two of the human promoters and has an additional mouse-specific promoter that transcribes only the exon containing the coding region and therefore resembles the organization of the human and mouse CCR8 genes. We also identify two nontranscribed regions that are highly conserved between human and mouse CX3CR1 containing possible regulatory elements. Examination of the CX3CR1 and CCR8 genes and surrounding genomic regions indicates that these genes are the result of the duplication of an ancestral gene prior to the divergence of teleost fish. We characterize single nucleotide polymorphisms in the promoters of human CCR8 and CX3CR1 and establish linkage relationships between CX3CR1 promoter polymorphisms and two previously described CX3CR1 coding polymorphisms associated with human immunodeficiency virus disease progression and arteriosclerosis susceptibility.