Differential modes of regulation of cxc chemokine-induced internalization and recycling of human CXCR1 and CXCR2

Biased agonism at chemokine receptors

In the human chemokine system, interactions between the approximately 50 known endogenous chemokine ligands and 20 known chemokine receptors (CKRs) regulate a wide range of cellular functions and biological processes including immune cell activation and homeostasis, development, angiogenesis, and neuromodulation. CKRs are a family of G protein-coupled receptors (GPCR), which represent the most common and versatile class of receptors in the human genome and the targets of approximately one third of all Food and Drug Administration-approved drugs. Chemokines and CKRs bind with significant promiscuity, as most CKRs can be activated by multiple chemokines and most chemokines can activate multiple CKRs. While these ligand-receptor interactions were previously regarded as redundant, it is now appreciated that many chemokine:CKR interactions display biased agonism, the phenomenon in which different ligands binding to the same receptor signal through different pathways with different efficacies, leading to distinct biological effects. Notably, these biased responses can be modulated through changes in ligand, receptor, and or the specific cellular context (system). In this review, we explore the biochemical mechanisms, functional consequences, and therapeutic potential of biased agonism in the chemokine system. An enhanced understanding of biased agonism in the chemokine system may prove transformative in the understanding of the mechanisms and consequences of biased signaling across all GPCR subtypes and aid in the development of biased pharmaceuticals with increased therapeutic efficacy and safer side effect profiles.

Rat cytomegalovirus-encoded γ-chemokine vXCL1 is a highly adapted, species-specific agonist for rat XCR1-positive dendritic cells

Dendritic cells (DCs) expressing the chemokine receptor XCR1 are specialized in antigen cross-presentation to control infections with intracellular pathogens. XCR1-positive (XCR1⁺) DCs are attracted by XCL1, a γ-chemokine secreted by activated CD8⁺ T cells and natural killer cells. Rat cytomegalovirus (RCMV) is the only virus known to encode a viral XCL1 analog (vXCL1) that competes for XCR1 binding with the endogenous chemokine. Here we show that vXCL1 from two different RCMV strains, as well as endogenous rat XCL1 (rXCL1) bind to and induce chemotaxis exclusively in rat XCR1⁺ DCs. Whereas rXCL1 activates the XCR1 Gi signaling pathway in rats and humans, both of the vXCL1s function as species-specific agonists for rat XCR1. In addition, we demonstrate constitutive internalization of XCR1 in XCR1-transfected HEK293A cells and in splenic XCR1⁺ DCs. This internalization was independent of β-arrestin 1 and 2 and was enhanced after binding of vXCL1 and rXCL1; however, vXCL1 appeared to be a stronger agonist. These findings suggest a decreased surface expression of XCR1 during DC cultivation at 37°C, and subsequent impairment of chemotactic activity and XCR1⁺ DC function.

Corticosteroid use and increased CXCR2 levels on leukocytes are associated with lumacaftor/ivacaftor discontinuation in cystic fibrosis patients homozygous for the F508del CFTR mutation

Cystic fibrosis (CF) is the most common life-shortening genetic disease and is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Several current therapies aim at improving availability and/or function of the mutant CFTR proteins. The combination therapeutic lumacaftor/ivacaftor (Orkambi, luma/iva) partially corrects folding and potentiates CFTR function impaired by the F508del mutation. Despite the potential for clinical benefit, a substantial number of patients discontinue treatment due to intolerable adverse effects. The aim of the present study is to identify differences between individuals who continued treatment and those who discontinued due to adverse respiratory effects to potentially inform treatment decisions. Clinical data from the year prior to treatment initiation were analyzed from 82 patients homozygous for the F508del mutation treated at the Colorado Adult CF Program. Blood samples were collected from 30 of these subjects before initiation of treatment to examine expression of circulating leukocyte surface antigens and cytokines. Clinical and demographic characteristics were analyzed along with inflammatory markers to determine biomarkers of drug discontinuation. The use of oral prednisone and/or nasal budesonide in the year prior to luma/iva initiation was more prevalent in CF subjects who did not tolerate luma/iva (82% vs. 43%). Increased age, but not gender or initial lung function, was associated with higher probability of discontinuing treatment due to side effects overall. Worse lung function (lower ppFEV₁, ppFEF_25-75 ≤ 60%) was associated with higher incidence of discontinuing treatment due to pulmonary adverse effects. In a nested cohort of patients, increased surface levels of CXCR2 on CD14+CD16- monocytes were associated with discontinuation. Overall, the patients who tolerated luma/iva were distinguishable from those who did not tolerate the drug based on clinical and cellular markers obtained prior to treatment initiation.

Differential Effects of Posttranslational Modifications of CXCL8/Interleukin-8 on CXCR1 and CXCR2 Internalization and Signaling Properties

CXCL8 or interleukin (IL)-8 directs neutrophil migration and activation through interaction with CXCR1 and CXCR2 that belong to the family of G protein-coupled receptors (GPCRs). Naturally occurring posttranslational modifications of the NH₂-terminal region of CXCL8 affect its biological activities, but the underlying molecular mechanisms are only partially understood. Here, we studied the implications of site-specific citrullination and truncation for the signaling potency of CXCL8. Native CXCL8(1-77), citrullinated [Cit5]CXCL8(1-77) and the major natural isoform CXCL8(6-77) were chemically synthesized and tested in internalization assays using human neutrophils. Citrullinated and truncated isoforms showed a moderately enhanced capacity to induce internalization of CXCR1 and CXCR2. Moreover, CXCL8-mediated activation of Gαi-dependent signaling through CXCR1 and CXCR2 was increased upon modification to [Cit5]CXCL8(1-77) or CXCL8(6-77). All CXCL8 variants promoted recruitment of β-arrestins 1 and 2 to CXCR1 and CXCR2. Compared to CXCL8(1-77), CXCL8(6-77) showed an enhanced potency to recruit β-arrestin 2 to both receptors, while for [Cit5]CXCL8(1-77) only the capacity to induce β-arrestin 2 recruitment to CXCR2 was increased. Both modifications had no biasing effect, i.e., did not alter the preference of CXCL8 to activate either Gαi-protein or β-arrestin-dependent signaling through its receptors. Our results support the concept that specific chemokine activities are fine-tuned by posttranslational modifications.

Lung Defense through IL-8 Carries a Cost of Chronic Lung Remodeling and Impaired Function

IL-8-dependent inflammation is a hallmark of host lung innate immunity to bacterial pathogens, yet in many human lung diseases, including chronic obstructive pulmonary disease, bronchiectasis, and pulmonary fibrosis, there are progressive, irreversible, pathological changes associated with elevated levels of IL-8 in the lung. To better understand the duality of IL-8-dependent host immunity to bacterial infection and lung pathology, we expressed human IL-8 transgenically in murine bronchial epithelium, and investigated the impact of overexpression on lung bacterial clearance, host immunity, and lung pathology and function. Persistent IL-8 expression in bronchial epithelium resulted in neutrophilia, neutrophil maturation and activation, and chemotaxis. There was enhanced protection against challenge with Pseudomonas aeruginosa, and significant changes in baseline expression of innate and adaptive immunity transcripts for Ccl5, Tlr6, IL-2, and Tlr1. There was increased expression of Tbet and Foxp3 in response to the Pseudomonas antigen OprF, indicating a regulatory T-cell phenotype. However, this enhanced bacterial immunity came at a high price of progressive lung remodeling, with increased inflammation, mucus hypersecretion, and fibrosis. There was increased expression of Ccl3 and reduced expression of Claudin 18 and F11r, with damage to epithelial organization leading to leaky tight junctions, all of which resulted in impaired lung function with reduced compliance, increased resistance, and bronchial hyperreactivity as measured by whole-body plethysmography. These results show that IL-8 overexpression in the bronchial epithelium benefits lung immunity to bacterial infection, but specifically drives lung damage through persistent inflammation, lung remodeling, and damaged tight junctions, leading to impaired lung function.

The role of inflammation in the development of epilepsy

Epilepsy, a neurological disease characterized by recurrent seizures, is often associated with a history of previous lesions in the nervous system. Impaired regulation of the activation and resolution of inflammatory cells and molecules in the injured neuronal tissue is a critical factor to the development of epilepsy. However, it is still unclear as to how that unbalanced regulation of inflammation contributes to epilepsy. Therefore, one of the goals in epilepsy research is to identify and elucidate the interconnected inflammatory pathways in systemic and neurological disorders that may further develop epilepsy progression. In this paper, inflammatory molecules, in neurological and systemic disorders (rheumatoid arthritis, Crohn’s, Type I Diabetes, etc.) that could contribute to epilepsy development, are reviewed.

Understanding the neurobiology of inflammation in epileptogenesis will contribute to the development of new biomarkers for better screening of patients at risk for epilepsy and new therapeutic targets for both prophylaxis and treatment of epilepsy.

New paradigms in chemokine receptor signal transduction: Moving beyond the two-site model

Chemokine receptor (CKR) signaling forms the basis of essential immune cellular functions, and dysregulated CKR signaling underpins numerous disease processes of the immune system and beyond. CKRs, which belong to the seven transmembrane domain receptor (7TMR) superfamily, initiate signaling upon binding of endogenous, secreted chemokine ligands. Chemokine-CKR interactions are traditionally described by a two-step/two-site mechanism, in which the CKR N-terminus recognizes the chemokine globular core (i.e. site 1 interaction), followed by activation when the unstructured chemokine N-terminus is inserted into the receptor TM bundle (i.e. site 2 interaction). Several recent studies challenge the structural independence of sites 1 and 2 by demonstrating physical and allosteric links between these supposedly separate sites. Others contest the functional independence of these sites, identifying nuanced roles for site 1 and other interactions in CKR activation. These developments emerge within a rapidly changing landscape in which CKR signaling is influenced by receptor PTMs, chemokine and CKR dimerization, and endogenous non-chemokine ligands. Simultaneous advances in the structural and functional characterization of 7TMR biased signaling have altered how we understand promiscuous chemokine-CKR interactions. In this review, we explore new paradigms in CKR signal transduction by considering studies that depict a more intricate architecture governing the consequences of chemokine-CKR interactions.

Biased signaling pathways via CXCR3 control the development and function of CD4+ T cell subsets

Structurally related chemotactic cytokines (chemokines) regulate cell trafficking through interactions with 7-transmembrane domain, G protein-coupled receptors. Biased signaling or functional selectivity is a concept that describes a situation where a 7-transmembrane domain receptor preferentially activates one of several available cellular signaling pathways. It can be divided into 3 distinct cases: ligand bias, receptor bias, and tissue or cell bias. Many studies, including those coming from our lab, have shown that only a limited number of chemokines are key drivers of inflammation. We have referred to them as "driver chemokines." They include the CXCR3 ligands CXCL9 and CXCL10, the CCR2 ligand CCL2, all 3 CCR5 ligands, and the CCR9 ligand CCL25. As for CXCR3, despite the proinflammatory nature of CXCL10 and CXCL9, transgenic mice lacking CXCR3 display an aggravated manifestation of different autoimmune disease, including Type I diabetes and experimental autoimmune encephalomyelitis. Recently, we showed that whereas CXCL9 and CXCL10 induce effector Th1/Th17 cells to promote inflammation, CXCL11, with a relatively higher binding affinity to CXCR3, drives the development of the forkhead box P3-negative IL-10(high) T regulatory 1 cell subset and hence, dampens inflammation. We also showed that CXCL9/CXCL10 activates a different signaling cascade than CXCL11, despite binding to the same receptor, CXCR3, which results in these diverse biologic activities. This provides new evidence for the role of biased signaling in regulating biologic activities, in which CXCL11 induces ligand bias at CXCR3 and receptor-biased signaling via atypical chemokine receptor 3.

A Neutrophil Phenotype Model for Extracorporeal Treatment of Sepsis

Neutrophils play a central role in eliminating bacterial pathogens, but may also contribute to end-organ damage in sepsis. Interleukin-8 (IL-8), a key modulator of neutrophil function, signals through neutrophil specific surface receptors CXCR-1 and CXCR-2. In this study a mechanistic computational model was used to evaluate and deploy an extracorporeal sepsis treatment which modulates CXCR-1/2 levels. First, a simplified mechanistic computational model of IL-8 mediated activation of CXCR-1/2 receptors was developed, containing 16 ODEs and 43 parameters. Receptor level dynamics and systemic parameters were coupled with multiple neutrophil phenotypes to generate dynamic populations of activated neutrophils which reduce pathogen load, and/or primed neutrophils which cause adverse tissue damage when misdirected. The mathematical model was calibrated using experimental data from baboons administered a two-hour infusion of E coli and followed for a maximum of 28 days. Ensembles of parameters were generated using a Bayesian parallel tempering approach to produce model fits that could recreate experimental outcomes. Stepwise logistic regression identified seven model parameters as key determinants of mortality. Sensitivity analysis showed that parameters controlling the level of killer cell neutrophils affected the overall systemic damage of individuals. To evaluate rescue strategies and provide probabilistic predictions of their impact on mortality, time of onset, duration, and capture efficacy of an extracorporeal device that modulated neutrophil phenotype were explored. Our findings suggest that interventions aiming to modulate phenotypic composition are time sensitive. When introduced between 3–6 hours of infection for a 72 hour duration, the survivor population increased from 31% to 40–80%. Treatment efficacy quickly diminishes if not introduced within 15 hours of infection. Significant harm is possible with treatment durations ranging from 5–24 hours, which may reduce survival to 13%. In severe sepsis, an extracorporeal treatment which modulates CXCR-1/2 levels has therapeutic potential, but also potential for harm. Further development of the computational model will help guide optimal device development and determine which patient populations should be targeted by treatment.

Sepsis occurs when a patient develops a whole body immune response due to infection. In this condition, white blood cells called neutrophils circulate in an active state, seeking and eliminating invading bacteria. However, when neutrophils are activated, healthy tissue is inadvertently targeted, leading to organ damage and potentially death. Even though sepsis kills millions worldwide, there are still no specific treatments approved in the United States. This may be due to the complexity and diversity of the body’s immune response, which can be managed well using computational modeling. We have developed a computational model to predict how different levels of neutrophil activation impact survival in an overactive inflammatory conditions. The model was utilized to assess the effectiveness of a simulated experimental sepsis treatment which modulates neutrophil populations and activity. This evaluation determined that treatment timing plays a critical role in therapeutic effectiveness. When utilized properly the treatment drastically improves survival, but there is also risk of causing patient harm when introduced at the wrong time. We intend for this computational model to support and guide further development of sepsis treatments and help translate these preliminary results from bench to bedside.

Immunomodulatory oligonucleotides inhibit neutrophil migration by decreasing the surface expression of interleukin-8 and leukotriene B4 receptors

Neutrophils play important roles in many inflammatory diseases. The migration of neutrophils to the inflammatory site is tightly regulated by specific chemokines, of which interleukin-8 (IL-8) and leukotriene B4 (LTB4 ) constitute key mediators by binding to the surface receptors CXCR1/2 and BLT1, respectively. Oligonucleotides (ODN) containing CpG motifs mediate potent immunomodulatory effects through binding to Toll-like receptor 9. So far, knowledge on how ODN can affect neutrophil migration during inflammation is lacking. This study demonstrates that several novel CpG ODN significantly down-regulate the surface expression of CXCR1/2 and BLT1. In addition, the ODN significantly blocked IL-8-induced and LTB4 -induced neutrophil migration in vitro, as well as leucocyte migration in vivo demonstrated in mice by intravital microscopy and in a model of airway inflammation. The down-regulation of CXCR1 is rapid, occurring 15 min after ODN stimulation, and can be mediated through an endosomally independent mechanism. Inhibition of the IL-8 and LTB4 pathways may provide new opportunities of therapeutic intervention using ODN to reduce neutrophil infiltration during inflammation.

CXCR1/CXCR2 antagonist CXCL8(3-74)K11R/G31P blocks lung inflammation in swine barn dust-instilled mice

Inhalation of agricultural occupational dusts from swine confinement facilities can result in lung inflammation. The innate immune response to organic barn dusts results in production of a number of pro-inflammatory factors in the lungs of barn workers such as cytokines, chemokines, and an influx of neutrophils. Many of these inflammatory factors are influenced by the chemokine CXCL8/IL-8 (KC or MIP-2 in mice). Previously, we have demonstrated that an endotoxin-independent component of swine barn dust extract (SBE) elevates lung chemokines in a protein kinase C (PKC)-dependent manner resulting in the significant formation of lung inflammatory cell infiltrates in a mouse model of SBE injury. In this study we test the ability of a CXCR1/CXCR2 antagonist, CXCL8(3-74)K11R/G31P (G31P) to block many of the features of lung-inflammation in response to challenge with SBE in an established mouse exposure system. Injection of G31P concurrent with SBE nasal instillation over a course of 3 weeks significantly reduced neutrophil accumulation in the lungs of barn dust exposed animals compared to those given SBE alone. There was a similar reduction in pro-inflammatory cytokines and chemokines IL-6, KC, and MIP-2 in SBE plus G31P-treated mice. In addition to excreted products, the receptors ICAM-1, CXCR1, and CXCR2, which all were elevated with SBE exposure, were also decreased with G31P treatment. SBE activation of PKCα and PKCε was reduced as well with G31P treatment. Thus, G31P was found to be highly effective at reducing several features of lung inflammation in mice exposed to barn dust extracts.

Regulation of CXCR2 expression and function by a disintegrin and metalloprotease-17 (ADAM17)

The chemokine receptor CXCR2 is expressed at high levels on circulating neutrophils and is critical for directing their migration to sites of inflammation. CXCR2 surface levels are rapidly modulated by 2 mechanisms-cell internalization and recycling upon ligand binding-and by a metalloprotease activity following overt neutrophil activation by nonligand stimuli. The latter process has only been described in human neutrophils, and essentially, nothing is known about its functional relevance and the specific protease involved. We show that targeting ADAM17 in mouse and human neutrophils blocks CXCR2 down-regulation induced by nonligand stimuli but not by chemokine ligands. This was determined by use of a selective ADAM17 inhibitor, an ADAM17 function-blocking antibody, and ADAM17 gene-targeted mice. CXCR2 is known to undergo a marked down-regulation during various inflammatory disorders, and this is associated with impaired neutrophil recruitment. We show that blocking ADAM17 activity reduced CXCR2 down-regulation on circulating neutrophils and enhanced their recruitment during acute inflammation, which was reversed by a CXCR2 inhibitor. Taken together, our findings demonstrate that unlike CXCR2 internalization, ADAM17 induction down-regulates the receptor in an irreversible manner and may serve as a master switch in controlling CXCR2 function, but may also contribute to neutrophil dysfunction during excessive inflammation.

Biased and g protein-independent signaling of chemokine receptors

Biased signaling or functional selectivity occurs when a 7TM-receptor preferentially activates one of several available pathways. It can be divided into three distinct forms: ligand bias, receptor bias, and tissue or cell bias, where it is mediated by different ligands (on the same receptor), different receptors (with the same ligand), or different tissues or cells (for the same ligand–receptor pair). Most often biased signaling is differentiated into G protein-dependent and β-arrestin-dependent signaling. Yet, it may also cover signaling differences within these groups. Moreover, it may not be absolute, i.e., full versus no activation. Here we discuss biased signaling in the chemokine system, including the structural basis for biased signaling in chemokine receptors, as well as in class A 7TM receptors in general. This includes overall helical movements and the contributions of micro-switches based on recently published 7TM crystals and molecular dynamics studies. All three forms of biased signaling are abundant in the chemokine system. This challenges our understanding of “classic” redundancy inevitably ascribed to this system, where multiple chemokines bind to the same receptor and where a single chemokine may bind to several receptors – in both cases with the same functional outcome. The ubiquitous biased signaling confers a hitherto unknown specificity to the chemokine system with a complex interaction pattern that is better described as promiscuous with context-defined roles and different functional outcomes in a ligand-, receptor-, or cell/tissue-defined manner. As the low number of successful drug development plans implies, there are great difficulties in targeting chemokine receptors; in particular with regard to receptor antagonists as anti-inflammatory drugs. Un-defined and putative non-selective targeting of the complete cellular signaling system could be the underlying cause of lack of success. Therefore, biased ligands could be the solution.

Constitutively active chemokine CXC receptors

Chemokines are low-molecular-weight, secreted proteins that act as leukocyte-specific chemoattractants. The chemokine family has more than 40 members. Based on the position of two conserved cysteines in the N-terminal domain, chemokines can be divided into the CXC, C, CC, and CX3C subfamilies. The interaction of chemokines with their receptors mediates signaling pathways that play critical roles in cell migration, differentiation, and proliferation. The receptors for chemokines are G protein-coupled receptors (GPCRs), and thus far, seven CXC receptors have been cloned and are designated CXCR1-7. Constitutively active GPCRs are present in several human immune-mediated diseases and in tumors, and they have provided valuable information in understanding the molecular mechanism of GPCR activation. Several constitutively active CXC chemokine receptors include the V6.40A and V6.40N mutants of CXCR1; the D3.49V variant of CXCR2; the N3.35A, N3.35S, and T2.56P mutants of CXCR3; the N3.35 mutation of CXCR4; and the naturally occurring KSHV-GPCR. Here, we review the regulation of CXC chemokine receptor signaling, with a particular focus on the constitutive activation of these receptors and the implications in physiological conditions and in pathogenesis. Understanding the mechanisms behind the constitutive activation of CXC chemokine receptors may aid in pharmaceutical design and the screening of inverse agonists and allosteric modulators for the treatment of autoimmune diseases and cancers.

Depletion of blood neutrophils from patients with sepsis: treatment for the future?

Organ failure arising from severe sepsis accounts for nearly 6 million deaths worldwide per annum. At present there are no specific pharmacological agents available for its treatment and identifying a suitable therapeutic target is urgently needed. Neutrophils appear to be contributing directly to pulmonary damage in severe forms of lung injury and indirectly to the failure of other organs. Blood neutrophils from patients with sepsis possess a phenotype that is indicative of activation and our results show that neutrophils isolated from patients with sepsis exhibit a supranormal adherence to endothelial monolayers treated with pro-inflammatory cytokines. Additional studies reveal that the patients' cells are highly efficient at releasing IL-8. We also demonstrate that organ function is improved upon removing neutrophils from the circulation. In this article we propose that in severe sepsis there is a subpopulation of neutrophils which is actively engaged in pathological insult. The phenotypic characterisation of this subset may provide a novel therapeutic strategy for sepsis that could lead to patient benefit.

Neuroendocrine-immune interaction: regulation of inflammation via G-protein coupled receptors

Neuroendocrine- and immune systems interact in a bi-directional fashion to communicate the status of pathogen recognition to the brain and the immune response is influenced by physiological changes. The network of ligands and their receptors involved includes cytokines and chemokines, corticosteroids, classical pituitary hormones, catecholamines and neuropeptides (e.g. opioids), as well as neural pathways. We studied the role of opioid, adrenergic and melatonin G-protein coupled receptors (GPCR) on carp (Cyprinus carpio) leucocytes. Ligand interaction by morphine and adrenaline both in vitro and in vivo resulted in considerable decrease of chemotaxis and expression of CXC chemokines and chemokine CXC receptors. These effects may have substantial influence on the process of inflammation, the efficacy of which is crucial for an effective immune response. Both opioid receptors and chemokine receptors are G-protein coupled receptors (GPCRs), and were classically assumed to function as monomers. This paradigm is now challenged by the emerging concept of homo- and hetero dimerization which may represent the native form of many receptors. G-protein coupling, downstream signaling and regulatory processes such as receptor internalization are largely influenced by the dimeric nature. The true functional importance of GPCR interactions remains enigmatic, but it certainly has implications with respect to the specificity of currently used medications. This review focuses on the important function of chemokine GPCRs during inflammation and the potential neuroendocrine modulation of this process through "neuroendocrine" GPCRs.

The Hyper-IgE Syndromes: Lessons in Nature, From Bench to Bedside

: Hyper-IgE syndrome is a primary immunodeficiency marked by abnormalities in the coordination of cell-cell signaling with the potential to affect TH17 cell, B cell, and neutrophil responses. Clinical manifestations include recurrent skin and lung infections, serum IgE elevation, connective tissue repair and development alterations, and the propensity for vascular abnormalities and tumor development. Signal transducer and activator of transcription 3 (STAT3) signaling, dedicator of cytokinesis 8 (DOCK8) signaling, and tyrosine kinase 2 (TYK2) signaling alterations have been implicated in 3 forms of hyper-IgE syndrome.

Overexpression of chemokine ligand 7 is associated with the progression of canine transmissible venereal tumor

BACKGROUND

Chemokines play multiple roles in the development and progression in a variety of tumors. Chemokine (C-X-C motif) ligand 7 (CXCL7) has been found associated with pro-inflammatory responses, but its role in cancer growth remains unclear. Our previous study showed that R phase tumor infiltrating lymphocytes (TILs) produced large amounts of interleukin (IL)-6 which antagonized transforming growth factor (TGF)-β derived from CTVT to diminish the immune-suppressive microenvironment. Now we intend to determine the expression pattern of CXCL7 and the role of IL-6/TGF-β in CXCL7 induction during spontaneous progressive (P) and regressive (R) phases in canine transmissible venereal tumor (CTVT).

RESULTS

We have demonstrated that CXCL7 expressed at high level in P phase and down-regulated in R phase by western blot and real-time PCR. This suggested that CXCL7 expression was negatively correlated with the tumor growth. Co-culturing TILs with CTVT cells was found to reduce CXCL7 expression, while adding IL-6 blocking antibody reversed it. Moreover, in P phase CTVT, while IL-1β and TGF-β had no obvious effect on CXCL7 expression, IL-6 was found significantly to reduce CXCL7 expression in a dose-dependent manner. The mRNA expression results of CXCL7 receptor, CXCR2, further confirmed the effects of IL-6 concentration on the CXCL7 expression.

CONCLUSION

CXCL7 overexpression might be associated with the progressive growth of CTVT. The results shown here also suggest the role of CXCL7 in cancer development and the potential as the anti-cancer therapeutic target.

Leu128(3.43) (l128) and Val247(6.40) (V247) of CXCR1 are critical amino acid residues for g protein coupling and receptor activation

CXCR1, a classic GPCR that binds IL-8, plays a key role in neutrophil activation and migration by activating phospholipase C (PLC)β through Gα₁₅ and Gα_i which generates diacylglycerol and inositol phosphates (IPs). In this study, two conserved amino acid residues of CXCR1 on the transmembrane domain (TM) 3 and TM6, Leu128^3.43 (L128) and Val247^6.40 (V247), respectively, were selectively substituted with other amino acids to investigate the role of these conserved residues in CXCR1 activation. Although two selective mutants on Leu128, Leu128Ala (L128A) and Leu128Arg (L128R), demonstrated high binding affinity to IL-8, they were not capable of coupling to G proteins and consequently lost the functional response of the receptors. By contrast, among the four mutants at residue Val247 (TM6.40), replacing Val247 with Ala (V247A) and Asn (V247N) led to constitutive activation of mutant receptors when cotransfected with Gα₁₅. The V247N mutant also constitutively activated the Gα_i protein. These results indicate that L128 on TM3.43 is involved in G protein coupling and receptor activation but is unimportant for ligand binding. On the other hand, V247 on TM6.40 plays a critical role in maintaining the receptor in the inactive state, and the substitution of V247 impaired the receptor constraint and stabilized an active conformation. Functionally, there was an increase in chemotaxis in response to IL-8 in cells expressing V247A and V247N. Our findings indicate that Leu128^3.43 and Val247^6.40 are critical for G protein coupling and activation of signaling effectors, providing a valuable insight into the mechanism of CXCR1 activation.

The chemokine receptors CXCR1 and CXCR2 couple to distinct G protein-coupled receptor kinases to mediate and regulate leukocyte functions

The chemokine receptors, CXCR1 and CXCR2, couple to Gαi to induce leukocyte recruitment and activation at sites of inflammation. Upon activation by CXCL8, these receptors become phosphorylated, desensitized, and internalized. In this study, we investigated the role of different G protein-coupled receptor kinases (GRKs) in CXCR1- and CXCR2-mediated cellular functions. To that end, short hairpin RNA was used to inhibit GRK2, 3, 5, and 6 in RBL-2H3 cells stably expressing CXCR1 or CXCR2, and CXCL8-mediated receptor activation and regulation were assessed. Inhibition of GRK2 and GRK6 increased CXCR1 and CXCR2 resistance to phosphorylation, desensitization, and internalization, respectively, and enhanced CXCL8-induced phosphoinositide hydrolysis and exocytosis in vitro. GRK2 depletion diminished CXCR1-induced ERK1/2 phosphorylation but had no effect on CXCR2-induced ERK1/2 phosphorylation. GRK6 depletion had no significant effect on CXCR1 function. However, peritoneal neutrophils from mice deficient in GRK6 (GRK6(-/-)) displayed an increase in CXCR2-mediated G protein activation but in vitro exhibited a decrease in chemotaxis, receptor desensitization, and internalization relative to wild-type (GRK6(+/+)) cells. In contrast, neutrophil recruitment in vivo in GRK6(-/-) mice was increased in response to delivery of CXCL1 through the air pouch model. In a wound-closure assay, GRK6(-/-) mice showed enhanced myeloperoxidase activity, suggesting enhanced neutrophil recruitment, and faster wound closure compared with GRK6(+/+) animals. Taken together, the results indicate that CXCR1 and CXCR2 couple to distinct GRK isoforms to mediate and regulate inflammatory responses. CXCR1 predominantly couples to GRK2, whereas CXCR2 interacts with GRK6 to negatively regulate receptor sensitization and trafficking, thus affecting cell signaling and angiogenesis.

Dissociating quaternary structure regulates cell-signaling functions of a secreted human tRNA synthetase

Many tRNA synthetases are homodimers that are catalytically inactive as monomers. An example is the 528-amino acid human tyrosyl-tRNA synthetase, which is made up of an N-terminal catalytic unit (TyrRS(Mini)) and a 164-amino acid C-domain. Although native TyrRS has no known cytokine functions, natural proteolysis of secreted TyrRS releases TyrRS(Mini), which not only has the same aminoacylation activity as native TyrRS but also has strong activity for stimulating migration of polymorphonuclear leukocytes. The migration-stimulating activity is dependent on an ELR tripeptide motif, similar to that in CXC cytokines like IL-8, and also has the familiar bell-shaped concentration dependence seen for CXC cytokines. Here we show that in contrast to IL-8, where the bell-shaped dependence arises from the effects of CXCR1/2 receptor internalization, TyrRS(Mini) does not induce internalization of CXCR1/2. A rationally designed non-associating monomer and a non-dissociating dimer were constructed. With these constructs, the bell-shaped concentration dependence of leukocyte migration was shown to arise from the agonist (for migration) activity of the catalytically inactive monomer and the antagonist activity of the catalytically active dimer. Thus, the dissociating quaternary structure of TyrRS(Mini) regulates two opposing cytokine activities and suggests the possibility of dissociating quaternary structures regulating novel functions of other tRNA synthetases.

Expression of G protein-coupled receptors and related proteins in HEK293, AtT20, BV2, and N18 cell lines as revealed by microarray analysis

BACKGROUND

G protein coupled receptors (GPCRs) are one of the most widely studied gene superfamilies. Thousands of GPCR research studies have utilized heterologous expression systems such as human embryonic kidney cells (HEK293). Though often treated as 'blank slates', these cell lines nevertheless endogenously express GPCRs and related signaling proteins. The outcome of a given GPCR study can be profoundly influenced by this largely unknown complement of receptors and/or signaling proteins. Little easily accessible information exists that describes the expression profiles of the GPCRs in cell lines. What is accessible is often limited in scope - of the hundreds of GPCRs and related proteins, one is unlikely to find information on expression of more than a dozen proteins in a given cell line. Microarray technology has allowed rapid analysis of mRNA levels of thousands of candidate genes, but though often publicly available, the results can be difficult to efficiently access or even to interpret.

RESULTS

To bridge this gap, we have used microarrays to measure the mRNA levels of a comprehensive profile of non-chemosensory GPCRs and over a hundred GPCR signaling related gene products in four cell lines frequently used for GPCR research: HEK293, AtT20, BV2, and N18.

CONCLUSIONS

This study provides researchers an easily accessible mRNA profile of the endogenous signaling repertoire that these four cell lines possess. This will assist in choosing the most appropriate cell line for studying GPCRs and related signaling proteins. It also provides a better understanding of the potential interactions between GPCRs and those signaling proteins.

Hyperoxia sensing: from molecular mechanisms to significance in disease

Oxygen therapy using mechanical ventilation with hyperoxia is necessary to treat patients with respiratory failure and distress. However, prolonged exposure to hyperoxia leads to the generation of excessive reactive oxygen species (ROS), causing cellular damage and multiple organ dysfunctions. As the lungs are directly exposed, hyperoxia can cause both acute and chronic inflammatory lung injury and compromise innate immunity. ROS may contribute to pulmonary oxygen toxicity by overwhelming redox homeostasis, altering signaling cascades that affect cell fate, ultimately leading to hyperoxia-induced acute lung injury (HALI). HALI is characterized by pronounced inflammatory responses with leukocyte infiltration, injury, and death of pulmonary cells, including epithelia, endothelia, and macrophages. Under hyperoxic conditions, ROS mediate both direct and indirect modulation of signaling molecules such as protein kinases, transcription factors, receptors, and pro- and anti-apoptotic factors. The focus of this review is to elaborate on hyperoxia-activated key sensing molecules and current understanding of their signaling mechanisms in HALI. A better understanding of the signaling pathways leading to HALI may provide valuable insights on its pathogenesis and may help in designing more effective therapeutic approaches.

Targeting CXCR1/CXCR2 receptor antagonism in malignant melanoma

IMPORTANCE OF THE FIELD

The incidence of malignant melanoma is increasing throughout the world and is currently rising faster than any other cancer in men and second only to lung cancer in women. Current strategies focused on systemic therapy for treatment of melanoma have shown no effect on survival. Therefore there is a pressing need for developing novel targeted therapeutics.

AREAS COVERED IN THIS REVIEW

Our goal is to provide an overview regarding targeting CXCR1/2 in malignant melanoma, the rationale behind these approaches and the future perspective.

WHAT THE READER WILL GAIN

This review illustrates our current understanding of CXCR1/2 receptor in melanoma progression and metastasis. We describe approaches that are being developed to block CXCR1/2 activation, including low-molecular-weight antagonists, modified chemokines and antibodies directed against ligands and receptors.

TAKE HOME MESSAGE

The chemokine receptors CXCR1 and CXCR2 and their ligands play an important role in the pathogenesis of malignant melanoma. Recent reports demonstrated that CXCR1 is constitutively expressed in all melanoma cases irrespective of stage and grade, however, CXCR2 expression was restricted to aggressive melanoma tumors,. Furthermore, modulation of CXCR1/2 expression and/or activity has been shown to regulate malignant melanoma growth, angiogenesis and metastasis, suggesting CXCR1/2 targeting as a novel therapeutic approach for malignant melanoma.

Short-term IL-1beta blockade reduces monocyte CD11b integrin expression in an IL-8 dependent fashion in patients with type 1 diabetes

OBJECTIVE

Interleukin 1-beta (IL-1beta) is a major inflammatory cytokine. Blockade of the IL-1beta pathway is therapeutically efficacious in type 2 diabetes, but the mechanistic effects on the immune system are incompletely understood.

RESEARCH DESIGN

We administered an IL-1 receptor antagonist, anakinra, to 7 type 1 diabetes patients in order to investigate the immunologic and metabolic effects of this drug. Mechanistic assays were performed before and after drug administration.

RESULTS

A novel signature was observed, with reduced serum interleukin 8 (IL-8) levels and reduced CD11b integrin expression on monocytes associated with increased CXCR1 expression.

CONCLUSIONS

This set of linked phenotypes suggests that blockade of the IL-1beta pathway results in the reduced ability of mononuclear cells to traffic to sites of inflammation. Mechanistic studies from large scale trials using IL-1 blockade in type 1 diabetes should focus on changes in monocyte trafficking and the IL-8 pathway.

Reduced expression of chemoattractant receptors by polymorphonuclear leukocytes in Hyper IgE Syndrome patients

Hyper IgE Syndrome (HIES) is a rare genetic disorder, characterized by elevated serum IgE levels and reduced inflammatory responses to bacterial infections. This leads to dermatitis, recurrent lung infections and "cold abscesses". Recently, progress was made in HIES research, when mutations in STAT3 were found in the autosomal dominant form of HIES, and impaired responses of T helper 17 cells were reported. However, the causes for reduced inflammatory responses in these patients were not fully elucidated. In view of studies that indicated that polymorphonuclear leukocytes (PMN) of HIES patients are defective in their chemotactic properties, we asked if the PMN of these patients have reduced expression of receptors for chemoattractants. To analyze this possibility, we focused on fMLP and ELR(+)-CXC chemokines - which are essential for mounting acute inflammatory responses - and determined the coding sequences and expression levels of their corresponding receptors: FPR (for fMLP) as well as CXCR1 and CXCR2 (the receptors for ELR(+)-CXC chemokines). The analyses of these receptors in HIES patients indicated that their coding sequences were intact and normal. However, the percentages of PMN that expressed FPR, CXCR1 and CXCR2 were significantly lower in HIES patients. In addition, lower expression levels per cell were denoted for CXCR1 in PMN of the patients. A cumulative score that was calculated for the three chemoattractant receptors together indicated that in some of the patients there were prominent reductions, of up to approximately 50% in the overall expression of the receptors (indicated by % positive cells and mean expression levels per cell). In addition, we asked whether deregulation of PMN activities in HIES may result from binding of IgE to corresponding receptors on HIES PMN. Our findings indicate that this is probably not the case, because similarly to normal PMN, the cells of HIES patients did not express notable levels of the IgE receptors FcvarepsilonRI and FcvarepsilonRII. Together, these results provide novel information on the expression of key determinants in PMN migration in HIES, suggesting that a defect in the expression of chemoattractant receptors may lead to impaired chemotaxis found in HIES patients, and to decreased inflammatory responses.

Canine CXCL7 and its functional expression in dendritic cells undergoing maturation

Many cells, including leucocytes and stromal cells, express CXCL7, a member of the CXC chemokine family, also known as platelet basic protein. CXCL7 is a potent chemoattractant and activator of neutrophil function. Dendritic cells (DCs) play a pivotal role in antigen processing and presentation. Very little information is available on the ability of DCs to recruit neutrophils by producing chemokines. In this work, we have cloned canine CXCL7. Based on the predicted gene sequence and using the 3'RACE technique, the full-length gene was amplified from LPS-treated canine peripheral blood mononuclear cells. The cloned cDNA sequence consisted of 357 nucleotides and encoded a 118 amino acid protein, including a 38 amino acid signal peptide. The use of CXCL7-containing supernatants from CXCL7-transfected BALB/3T3 in the neutrophil migration assay confirmed that canine CXCL7 had chemoattractive activity for neutrophils. We then used canine monocyte-derived DCs to generate CXCL7 for the rest of the experiment. Expression of CXCL7 by DCs treated with LPS, IL-1beta, IL-6, TGF-beta, TNF-alpha, or IFN-gamma was compared using real-time RT-PCR and Western blotting. When treated with IL-1beta, IL-6, TNF-alpha, or TGF-beta, canine DCs expressed significantly higher levels of CXCL7 mRNA and protein than when treated with IFN-gamma or LPS. It is concluded that dog DCs express high levels of the neutrophil chemotactic factor CXCL7 when stimulated by proinflammatory cytokines, including IL-1beta, IL-6, TNF-alpha, or TGF-beta, and may play an important role in modulating inflammatory responses.

Differential activation and regulation of CXCR1 and CXCR2 by CXCL8 monomer and dimer

CXCL8 (also known as IL-8) activates CXCR1 and CXCR2 to mediate neutrophil recruitment and trigger cytotoxic effect at sites of infection. Under physiological conditions, CXCL8 could exist as monomers, dimers, or a mixture of monomers and dimers. Therefore, both forms of CXCL8 could interact with CXCR1 and CXCR2 with different affinities and potencies to mediate different cellular responses. In the present study, we have used a "trapped" nonassociating monomer (L25NMe) and a nondissociating dimer (R26C) to investigate their activities for human neutrophils that express both receptors and for RBL-2H3 cells stably expressing either CXCR1(RBL-CXCR1) or CXCR2 (RBL-CXCR2). The monomer was more active than the dimer for activities such as intracellular Ca(2+) mobilization, phosphoinositide hydrolysis, chemotaxis. and exocytosis. Receptor regulation, however, is distinct for each receptor. The rate of monomer-mediated regulation of CXCR1 is greater for activities such as phosphorylation, desensitization, beta-arrestin translocation, and internalization. In contrast, for CXCR2, both monomeric and dimeric CXCL8 mediate these activities to a similar extent. Interestingly, receptor-mediated signal-regulated kinase (ERK) phosphorylation in response to all three CXCL8 variants was more sustained for CXCR2 relative to CXCR1. Taken together, the results indicate that the CXCL8 monomer and dimer differentially activate and regulate CXCR1 and CXCR2 receptors. These distinct properties of the ligand and the receptors play a critical role in orchestrating neutrophil recruitment and eliciting cytotoxic activity during an inflammatory response.

The functional significance behind expressing two IL-8 receptor types on PMN

PMN are critical to innate immunity and are fundamental to antibacterial defense. To localize to sites of infection, PMN possess receptors that detect chemoattractant stimuli elicited at the site, such as chemokines, complement split products, or bioactive lipids. Signaling through these receptors stimulates chemotaxis toward the site of infection but also activates a number of biochemical processes, with the result that PMN kill invading bacteria. PMN possess two receptors, CXCR1 and CXCR2, for the N-terminal ELR motif-containing CXC chemokines, although only two chemokine members bind both receptors and the remainder binding only CXCR2. This peculiar pattern in receptor specificity has drawn considerable interest and investigation into whether signaling through each receptor might impart unique properties on the PMN. Indeed, at first glance, CXCR1 and CXCR2 appear to be functionally redundant; however, there are differences. Considering these proinflammatory activities of activating PMN through chemokine receptors, there has been great interest in the possibility that blocking CXCR1 and CXCR2 on PMN will provide a therapeutic benefit. The literature examining CXCR1 and CXCR2 in PMN function during human and modeled diseases will be reviewed, asking whether the functional differences can be perceived based on alterations in the role PMN play in these processes.

CXCR2 antagonists for the treatment of pulmonary disease

Chemokines have long been implicated in the initiation and amplification of inflammatory responses by virtue of their role in leukocyte chemotaxis. The expression of one of the receptors for these chemokines, CXCR2, on a variety of cell types and tissues suggests that these receptors may have a broad functional role under both constitutive conditions and in the pathophysiology of a number of acute and chronic diseases. With the development of several pharmacological, immunological and genetic tools to study CXCR2 function, an important role for this CXC chemokine receptor subtype has been identified in chronic obstructive pulmonary disease (COPD), asthma and fibrotic pulmonary disorders. Interference with CXCR2 receptor function has demonstrated different effects in the lungs including inhibition of pulmonary damage induced by neutrophils (PMNs), antigen or irritant-induced goblet cell hyperplasia and angiogenesis/collagen deposition caused by lung injury. Many of these features are common to inflammatory and fibrotic disorders of the lung. Clinical trials evaluating small molecule CXCR2 antagonists in COPD, asthma and cystic fibrosis are currently underway. These studies hold considerable promise for identifying novel and efficacious treatments of pulmonary disorders.

Defects in actin dynamics lead to an autoinflammatory condition through the upregulation of CXCL5

BACKGROUND

Destrin (DSTN) is a member of the ADF/cofilin family of proteins and is an important regulator of actin dynamics. The primary function of destrin is to depolymerize filamentous actin into its monomeric form and promote filament severing. While progress has been made in understanding the biochemical functions of the ADF/cofilin proteins, the study of an animal model for cells deficient for DSTN provides an opportunity to investigate the physiological processes regulated by proper actin dynamics in vivo. A spontaneous mouse mutant, corneal disease 1(corn1), is deficient for DSTN, which causes epithelial hyperproliferation and neovascularization in the cornea. Dstn(corn1) mice exhibit an actin dynamics defect in the cornea as evidenced by the formation of actin stress fibers in the epithelial cells. Previously, we observed a significant infiltration of leukocytes into the cornea of Dstn(corn1) mice as well as the upregulation of proinflammatory molecules. In this study, we sought to characterize this inflammatory condition and explore the physiological mechanism through which a loss of Dstn function leads to inflammation.

METHODOLOGY/PRINCIPAL FINDINGS

Through immunofluorescent analyses, we observed a significant recruitment of neutrophils and macrophages to the Dstn(corn1) cornea, demonstrating that the innate immune system is spontaneously activated in this mutant. The inflammatory chemokine, CXCL5, was ectopically expressed in the corneal epithelial cells of Dstn(corn1) mice, and targeting of the receptor for this chemokine inhibited neutrophil recruitment. An inflammatory reaction was not observed in the cornea of allelic mutant strain, Dstn(corn1-2J), which has a milder defect in actin dynamics in the corneal epithelial cells.

CONCLUSIONS/SIGNIFICANCE

This study shows that severe defects in actin dynamics lead to an autoinflammatory condition that is mediated by the expression of CXC chemokines.

Curcumin (1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) blocks the chemotaxis of neutrophils by inhibiting signal transduction through IL-8 receptors

We investigated the impact of curcumin on neutrophils. Chemotactic activity via human recombinant IL-8 (hrIL-8) was significantly inhibited by curcumin. Curcumin reduced calcium ion flow induced by internalization of the IL-8 receptor. We analyzed flow cytometry to evaluate the status of the IL-8 receptor after curcumin treatment. The change in the distribution of receptors intracellularly and on the cell surface suggested that curcumin may affect the receptor trafficking pathway intracellulary. Rab11 is a low molecular weight G protein associated with the CXCR recycling pathway. Following curcumin treatment, immunoprecipitation studies showed that the IL-8 receptor was associated with larger amounts of active Rab11 than that in control cells. These data suggest that curcumin induces the stacking of the Rab11 vesicle complex with CXCR1 and CXCR2 in the endocytic pathway. The mechanism for antiinflammatory response by curcumin may involve unique regulation of the Rab11 trafficking molecule in recycling of IL-8 receptors.

Scavenging roles of chemokine receptors: chemokine receptor deficiency is associated with increased levels of ligand in circulation and tissues

In vitro studies have implicated chemokine receptors in consumption and clearance of specific ligands. We studied the role that various signaling chemokine receptors play during ligand homeostasis in vivo. We examined the levels of ligands in serum and CNS tissue in mice lacking chemokine receptors. Compared with receptor-sufficient controls, Cx3cr1(-/-) mice exhibited augmented levels of CX3CL1 both in serum and brain, and circulating levels of CXCL1 and CXCL2 were increased in Cxcr2(-/-) mice. CCR2-deficient mice showed significantly increased amounts of circulating CCL2 compared with wild-type mice. Cxcr3(-/-) mice revealed increased levels of circulating and brain CXCL10 after experimental autoimmune encephalomyelitis (EAE) induction. CCR2-deficient peripheral blood and resident peritoneal cells exhibited reduced binding capacity and biologic responses to the CCR1 ligand CCL3, suggesting that elevated levels of CCR2 ligands had down-regulated CCR1. The results indicate that signaling chemokine receptors clear chemokines from circulation and tissues. These homeostatic functions of signaling chemokine receptors need to be integrated into safety and efficacy calculations when considering therapeutic receptor blockade.

Intracellular cross-talk between the GPCR CXCR1 and CXCR2: role of carboxyl terminus phosphorylation sites

In the present study, we used the human chemokine receptors CXCR1 and CXCR2 as a model system for the study of intracellular cross-talk between two closely related G protein-coupled receptors (GPCR). In cells expressing either CXCR1 or CXCR2, exposure to the CXCL8 ligand resulted in prominent reduction in cell surface expression of the receptors. We have shown previously that the reduction in cell surface expression of CXCR1 and CXCR2, to be termed herein "down-regulation", is significantly lower in cells expressing both receptors together. Now we show that reduced receptor down-regulation was specific to the CXCR1+CXCR2 pair. Also, CXCR2 carboxyl terminus phosphorylation sites were required for inducing inhibition of CXCR1 down-regulation, and vice versa. Accordingly, phosphorylation of CXCR2 carboxyl terminus domain was intact when expressed together with CXCR1. Moreover, specific carboxyl terminus phosphorylation sites on each of the wild type receptors protected them from more severe inhibition of down-regulation, induced by joint expression with the other receptor. When concomitantly expressed, CXCR1 and CXCR2 were impaired in recycling to the plasma membrane, despite their undergoing intact dephosphorylation. Overall, we show that cross-talk between two GPCR is manifested by impairment of their intracellular trafficking, primarily of ligand-induced down-regulation, via carboxyl terminus phosphorylation sites.

CXCR1 and CXCR2 Activation and Regulation

CXCL8 (interleukin-8) interacts with two receptors, CXCR1 and CXCR2, to activate leukocytes. Upon activation, CXCR2 internalizes very rapidly relative to CXCR1 ( approximately 90% versus approximately 10% after 5 min). The C termini of the receptors have been shown to be necessary for internalization but are not sufficient to explain the distinct kinetics of down-regulation. To determine the structural determinant(s) that modulate receptor internalization, various chimeric and point mutant receptors were generated by progressively exchanging specific domains or amino acids between CXCR1 and CXCR2. The receptors were stably expressed in rat basophilic leukemia 2H3 cells and characterized for receptor binding, intracellular Ca(2+) mobilization, phosphoinositide hydrolysis, phosphorylation, internalization, and MAPK activation. The data herein indicate that the second extracellular loop (2ECL) of the receptors is critical for the distinct rate of internalization. Replacing the 2ECL of CXCR2 with that of CXCR1 (B(2ECL)A) or Asp(199) with its CXCR1 valine counterpart (B(D199V)A) delayed CXCR2 internalization similarly to CXCR1. Replacing Asp(199) with Asn (B(D199N)) restored CXCR2 rapid internalization. Structure modeling of the 2ECL of the receptors also suggested that Asp(199) plays a critical role in stabilizing and modulating CXCR2 rapid internalization relative to CXCR1. B(D199N) internalized rapidly but migrated as a single phosphorylated form like CXCR1 ( approximately 75 kDa), whereas B(2ECL)A and B(D199V)A showed slow and fast migrating forms like CXCR2 ( approximately 45 and approximately 65 kDa, respectively) but internalized like CXCR1. These data further undermine the role of receptor oligomerization in CXCL8 receptor internalization. Like CXCR1, B(D199V)A also induced sustained ERK activation and cross-desensitized Ca(2+) mobilization to CCR5 relative to B(D199N) and CXCR2. Altogether, the data suggest that the 2ECL of the CXCL8 receptors is important in modulating their distinct rate of down-regulation and thereby signal length and post-internalization activities.

Neutrophil recruitment in immunized mice depends on MIP-2 inducing the sequential release of MIP-1alpha, TNF-alpha and LTB(4)

Neutrophils are thought to play an important role in the tissue damage observed in various autoimmune diseases. Chemokines, cytokines and leukotrienes have recognized roles in the orchestration of neutrophil migration. We have recently shown that antigen-induced neutrophil migration into the peritoneum of immunized mice is mediated by macrophage-inflammatory protein (MIP)-1alpha which interacts with CCR1 and induces the sequential release of TNF-alpha and leukotriene B(4) (LTB(4)). The present study investigates the role of MIP-2 and CXCR2 in the cascade of events leading to mediator generation and neutrophil influx. Antigen challenge of immunized mice induced the expression of CXCR2 and the production of KC and MIP-2 proteins. Antigen-induced neutrophil migration was inhibited by a CXCR2 receptor antagonist (repertaxin) or an anti-MIP-2 antibody, but not by an anti-KC antibody. Administration of MIP-2 promoted a dose-dependent neutrophil migration in naive mice which was inhibited by repertaxin, anti-TNF-alpha, anti-MIP-1alpha antibodies or by MK886 (leukotriene synthesis inhibitor). MIP-2 administration induced the release of MIP-1alpha, TNF-alpha and LTB(4), and the release of the latter two was inhibited by anti-MIP-1alpha antibody treatment. Our studies highlight the intricate balance between mediator production and action during an immune-mediated inflammatory response and suggest a mediator cascade leading to neutrophil influx following antigen challenge of immunized mice: MIP-2 --> MIP-1alpha --> TNF-alpha --> LTB(4).

Neutrophil activation induced by the lectin KM+ involves binding to CXCR2

The lectin KM+ from Artocarpus integrifolia, also known as artocarpin, induces neutrophil migration by haptotaxis. The interactions of KM+ with both neutrophils and the extracellular matrix depend on the lectin's ability to recognize mannose-containing glycans. In the present study, we characterized the binding of KM+ to human neutrophils and the responses stimulated by this binding. Exposure to KM+ results in cell polarization, formation of a lamellipodium, and induction of deep ruffles on the cell surface. By fluorescence microscopy, we observed that KM+ is distributed homogeneously over the cell surface. KM+/ligand complexes are rapidly internalized, reaching maximum intracellular concentrations at 120 min, and decreasing thereafter. Furthermore, KM+ binding to the surface of human neutrophils is inhibited by the specific sugars, d-mannose or mannotriose. KM+-induced neutrophil migration is inhibited by pertussis toxin as well as by inhibition of CXCR2 activity. These results suggest that the KM+ ligand on the neutrophil surface is a G protein-coupled receptor (GPCR). The results also suggest that neutrophil migration induced by KM+ involves binding to CXCR2.

Role of CXCR3 carboxyl terminus and third intracellular loop in receptor-mediated migration, adhesion and internalization in response to CXCL11

The chemokine receptor CXCR3 is predominantly expressed on activated T and natural killer (NK) cells. CXCR3 and its ligands, CXCL11, CXCL10, and CXCL9, play a major role in T-helper 1 (Th1)-dependent inflammatory responses. CXCL11 is the most dominant physiological inducer of adhesion, migration, and internalization of CXCR3. To study the role of CXCR3 carboxyl-terminus and the third intracellular (3i) loop in chemokine-mediated migration, adhesion, and CXCR3 internalization, we generated CXCR3 receptors mutated in their distal (Ser-Thr domain) or proximal (trileucine domain) membrane carboxyl terminus, and/or the third intracellular loop. We found that migration of CXCR3-expressing HEK 293 cells toward CXCL11 was pertussis toxin-dependent and required the membrane proximal carboxyl terminus of CXCR3. Internalization induced by CXCL11 and protein kinase C (PKC) activation was also regulated by the membrane proximal carboxyl terminus; however, only CXCL11-induced internalization required the LLL motif of this region. Internalization and Ca(2+) flux induced by CXCL11 were independent of the 3i loop S245, whereas migration at high CXCL11 concentrations, integrin-dependent adhesion, and actin polymerization were S245 dependent. Our findings indicate that CXCL11-dependent CXCR3 internalization and cell migration are regulated by the CXCR3 membrane proximal carboxyl terminus, whereas adhesion is regulated by the 3i loop S245. Thus, distinct conformational changes induced by a given CXCR3 ligand trigger different downstream effectors of adhesion, motility, and CXCR3 desensitization.

Modulation of neutrophil apoptosis in plasma of patients after orthognathic surgery

BACKGROUND

Human neutrophils undergo rapid apoptosis during in vitro culture. The aim of this study was to investigate the role of interleukin-8 (IL-8) on neutrophil apoptosis in surgery-induced inflammation.

MATERIALS AND METHODS

Blood samples were drawn from 21 patients with mandibular prognathism 2 days before, and 1 and 5 days after orthognathic surgery. The IL-8 levels in the separated plasma were measured using an ELISA kit. The expression of two receptors for IL-8, CXCR1, and CXCR2, and their role in neutrophil apoptosis was evaluated using a flow cytometer.

RESULTS

The IL-8 levels in the plasma were correlated with acute inflammatory markers, such as peripheral blood neutrophil counts and C-reactive protein levels. Both IL-8 receptors were markedly raised in patient-derived neutrophils 1 day post-operatively. Recombinant IL-8 (0-100 ng/ml) suppressed apoptosis in fresh-isolated neutrophils from healthy donors dose-dependently. Neutrophil apoptosis 1 day post-operatively was slightly accelerated in the presence of fetal bovine serum compared to the value 2 days pre-operatively and 5 days post-operatively. In contrast, in the presence of autogenous plasma, neutrophil apoptosis was significantly suppressed 1 day post-operatively compared to the value 2 days pre-operatively and 5 days post-operatively. Moreover, the anti-apoptotic effect of plasma on neutrophil apoptosis was partially decreased by the addition of anti-IL-8 neutralizing antibody.

CONCLUSIONS

These results suggest that circulating neutrophils are susceptible to augmentation by IL-8 through the reinforcement of IL-8 receptors in acute inflammatory conditions. Furthermore, IL-8 may, in part, contribute to the regulation of neutrophil survival during the inflammatory response.

The macrophage-derived lectin, MNCF, activates neutrophil migration through a pertussis toxin-sensitive pathway

The macrophage-derived neutrophil chemotactic factor (MNCF) is a D-galactose-binding lectin that induces neutrophil migration in vitro and in vivo. Neutrophil recruitment induced by MNCF is resistant to glucocorticoid treatment and is inhibited by the lectin-specific sugar, D-galactose. In the present study, we characterized the binding of MNCF to neutrophils and the responses triggered by this binding. Exposure to MNCF resulted in cell polarization, formation of a lamellipodium, and deep ruffles on the cell surface. By confocal microscopy, we observed that MNCF was evenly distributed on the cell surface after 30 min of incubation. The labeling intensity progressively diminished with longer incubations. Internalization kinetics showed that MNCF/ligand complexes were rapidly internalized, reaching maximum intracellular concentrations at 120 min and then decreased thereafter. The binding and internalization of MNCF were selectively inhibited by D-galactose. MNCF-induced neutrophil chemotaxis was inhibited by pertussis toxin. This fact strongly suggests that the MNCF-ligand on the neutrophil surface is a G-protein-coupled receptor (GPCR), similar to receptors for well-established neutrophil attractants. Our observations on the ability of MNCF to activate neutrophils are consistent with the increasing evidence for the participation of animal lectins in the innate immune response.

Chemokine receptor internalization and intracellular trafficking

The internalization and intracellular trafficking of chemokine receptors have important implications for the cellular responses elicited by chemokine receptors. The major pathway by which chemokine receptors internalize is the clathrin-mediated pathway, but some receptors may utilize lipid rafts/caveolae-dependent internalization routes. This review discusses the current knowledge and controversies regarding these two different routes of endocytosis. The functional consequences of internalization and the regulation of chemokine receptor recycling will also be addressed. Modifications of chemokine receptors, such as palmitoylation, ubiquitination, glycosylation, and sulfation, may also impact trafficking, chemotaxis and signaling. Finally, this review will cover the internalization and trafficking of viral and decoy chemokine receptors.

Regulation of human neutrophil chemokine receptor expression and function by activation of Toll-like receptors 2 and 4

Neutrophil chemokine receptor expression can be altered by exposure to Toll-like receptor (TLR) agonists, a process that is thought to have the potential to localize neutrophils to sites of infection. In order to investigate this process in more detail, we examined the regulation of highly pure neutrophil CXCR1 and CXCR2 expression and function by selective agonists of TLR2 (Pam(3)CSK(4)) and TLR4 (lipopolysaccharide, LPS). CXCR1 and CXCR2 were down-regulated by TLR engagement. CXCR2 loss was more rapid and showed a dependence upon soluble helper molecules (LPS binding protein and CD14) that was not evident for CXCR1, suggesting differential coupling of LPS signalling to CXCR1 and CXCR2 loss. However, TLR engagement in highly pure neutrophils did not result in complete loss of chemokine receptors, and LPS-treated neutrophils remained able to mount a respiratory burst to CXCL8 and CXCL1, and were able to migrate towards CXCL8 in assays of under-agarose chemotaxis. Thus, although treatment of purified human neutrophils with TLR2 and TLR4 agonists modifies chemokine receptor expression, remaining receptors remain functionally competent.

Cysteine residues are critical for chemokine receptor CXCR2 functional properties

We examined the role of cysteine (Cys) residues present in chemokine receptor CXCR2 for proper surface expression, dimerization, signaling, and chemotaxis. To address this issue, serine or leucine residues were substituted for Cys, generating nine CXCR2 mutants transiently expressed in HEK cells. Single substitution of Cys residues present in the three extracellular loops (C119L, C196L, C286S) or in the seventh-transmembrane (TM) domain (C308L) abolished CXCL8 agonist binding, while no Cys substitution abolished surface receptor expression. We have previously demonstrated that CXCR2 dimerizes under reducing conditions, due to hydrophobic interactions that involve TM3 regions, and here we show that the dimer/monomer CXCR2 ratio drastically increases when analyzed under non-reducing conditions. We report that none of the Cys-deficient CXCR2 mutants abolishes receptor dimerization, demonstrating that Cys-Cys bonds are not the exclusive determinant of CXCR2 dimerization. Furthermore, both wt- and Cys-mutated CXCR2 dimers are expressed at the cell surface, indicating that receptor dimers are efficiently transferred at the plasma membrane. We also show that every Cys substitution in CXCR2, including those that still bind CXCL8, results in an impairment of receptor activity, analyzed as cell chemotaxis and intracellular signaling, suggesting that some structural requirement is likely fulfilled by Cys presence.

The effects and comparative differences of neutrophil specific chemokines on neutrophil chemotaxis of the neonate

Neutrophil specific chemokines are potent chemoattractants for neutrophils. IL-8/CXCL8 is the most extensively studied member of this group, and its concentrations increase during inflammatory conditions of the newborn infant including sepsis and chronic lung disease. A significant amount of information exists on the effects of IL-8/CXCL8 on neutrophil chemotaxis of neonates, but little is known about the other neutrophil specific chemokines. The aim of this study was to determine the relative potency of the neutrophil specific chemokines on chemotaxis of neonatal neutrophils and to compare this effect with the effect on adult neutrophils. Neutrophils were isolated from cord blood or healthy adult donors and incubated in a Neuroprobe chemotaxis chamber. Chemokine concentrations ranging from 1-1000 ng/mL were used. Differences in chemotactic potency existed among the seven neutrophil specific chemokines. Specifically, at 100 ng/mL, the order was IL-8/CXCL8>GRO-alpha/CXCL1>GCP-2/CXCL6>NAP-2/CXCL7>ENA-78/CXCL5>GRO-gamma/CXCL2>GRO-beta/CXCL3. This pattern was observed for adult and neonatal neutrophils. We conclude that (1) neutrophils from cord blood exhibit the same pattern of potency for each ELR chemokine as neutrophils from adults, and (2) migration of neonatal neutrophils is significantly less than that of adults at every concentration examined except the lowest (1 ng/mL).

The epithelial-mesenchymal transition of colon carcinoma involves expression of IL-8 and CXCR-1-mediated chemotaxis

The epithelial-mesenchymal transition (EMT) is an essential component of embryonic development, tissue remodeling, and wound repair. In addition, many epithelial tumors, including colorectal carcinomas, appear to undergo this transition that may facilitate their invasion. Using a novel model of EMT in colon carcinoma in which the inflammatory cytokine TNF-alpha accelerates this TGF-beta directed process, we report that TNF-alpha stimulation upregulates expression of the chemokine IL-8, and that this upregulation is dependent on the transcription factor NF-kappaB. Significantly, this effect is not merely an inflammatory response by these colon carcinoma cells because IL-8 expression is induced in cells undergoing a TGF-beta-driven EMT in the absence of exogenous TNF-alpha. During the EMT, a concomitant increase in the chemokine receptor CXCR-1, but not CXCR-2, also occurs. Moreover, both IL-8 and CXCR-1 function in the chemokinetic and chemotactic migration of colon carcinoma cells as assessed by antibody inhibition studies. These studies establish that the regulated expression of a specific chemokine and its receptor are linked to the EMT and they provide a biochemical framework for understanding the mechanisms by which the EMT promotes migration.

A library of 7TM receptor C-terminal tails. Interactions with the proposed post-endocytic sorting proteins ERM-binding phosphoprotein 50 (EBP50), N-ethylmaleimide-sensitive factor (NSF), sorting nexin 1 (SNX1), and G protein-coupled receptor-associated sorting protein (GASP)

Adaptor and scaffolding proteins determine the cellular targeting, the spatial, and thereby the functional association of G protein-coupled seven-transmembrane receptors with co-receptors, transducers, and downstream effectors and the adaptors determine post-signaling events such as receptor sequestration through interactions, mainly with the C-terminal intracellular tails of the receptors. A library of tails from 59 representative members of the super family of seven-transmembrane receptors was probed as glutathione S-transferase fusion proteins for interactions with four different adaptor proteins previously proposed to be involved in post-endocytotic sorting of receptors. Of the two proteins suggested to target receptors for recycling to the cell membrane, which is the route believed to be taken by a majority of receptors, ERM (ezrin-radixin-moesin)-binding phosphoprotein 50 (EBP50) bound only a single receptor tail, i.e. the beta(2)-adrenergic receptor, whereas N-ethylmaleimide-sensitive factor bound 11 of the tail-fusion proteins. Of the two proteins proposed to target receptors for lysosomal degradation, sorting nexin 1 (SNX1) bound 10 and the C-terminal domain of G protein-coupled receptor-associated sorting protein bound 23 of the 59 tail proteins. Surface plasmon resonance analysis of the binding kinetics of selected hits from the glutathione S-transferase pull-down experiments, i.e. the tails of the virally encoded receptor US28 and the delta-opioid receptor, confirmed the expected nanomolar affinities for interaction with SNX1. Truncations of the NK(1) receptor revealed that an extended binding epitope is responsible for the interaction with both SNX1 and G protein-coupled receptor-associated sorting protein as well as with N-ethylmaleimide-sensitive factor. It is concluded that the tail library provides useful information on the general importance of certain adaptor proteins, for example, in this case, ruling out EBP50 as being a broad spectrum-recycling adaptor.

Human T lymphocytes and mast cells differentially express and regulate extra- and intracellular CXCR1 and CXCR2

CXCL8 plays a major role in cell recruitment to sites of inflammation. Apart from neutrophils, little is known, however, about the cellular distribution and regulation of CXCL8 receptors in cells involved in acquired and adaptive immune responses. In previous studies, we have demonstrated the extracellular expression and function of CXCR1/2 on mast cells and also detected an intracellular pool of CXCR1/2. Here, we have addressed the question of receptor regulation during stimulation of human mast cells (HMC-1 cell line) and have studied T cells in comparison. Cell permeabilization was performed to detect both surface and possible intracellular receptor pools. HMC-1 cells stained positive for both receptors on the cell surface (CXCR1, 50%; CXCR2, 51%) and also after cell permeabilization (CXCR1, 86%; CXCR2, 74%). Similarly, T cells exhibited both cell-surface receptor expression (CXCR1, 30%; CXCR2, 23%) and higher total receptor expression (CXCR1, 50%; CXCR2, 36%), although overall values were lower than that in HMC-1 cells. On immunoblot, molecular weights of extra- and intracellular receptors on mast cells were the same, excluding altered receptor glycosylation. On stimulation with phorbol 12-myristate 13-acetate plus calcium ionophore, a time-dependent decrease of surface-membrane receptors was observed in both cell types, while total receptor remained the same, suggesting that receptor shedding is not involved. The kinetics of membrane receptor internalization and replenishment differed for the two cell types. Furthermore, receptor internalization was associated with decreased F-actin polymerization, a basic prerequisite for cell migration. These findings demonstrate for the first time the expression of extra- and intracellular CXCR1/2 receptors on T cells and delineate the dynamics of CXCR1/2 receptors on mast cells and T cells. Furthermore, they suggest a cell-type-specific and finely tuned regulation of chemokine responses at the receptor level in the context of inflammation.

The CXC chemokine receptor encoded by herpesvirus saimiri, ECRF3, shows ligand-regulated signaling through Gi, Gq, and G12/13 proteins but constitutive signaling only through Gi and G12/13 proteins

Open reading frame 74 (ORF74) of many gamma(2)-herpesviruses encodes a CXC chemokine receptor. The molecular pharmacological profile of ORF74 from herpesvirus saimiri, ECRF3, is characterized here and compared with that of the well known ORF74 from human herpesvirus 8 (HHV8). The ECRF3 receptor bound the so-called ELR (Glu-Leu-Arg) CXC chemokines (125)I-CXCL1/GRO alpha, (125)I-CXCL6/GCP-2, and (125)I-CXCL8/interleukin-8 with high affinity; but in contrast to ORF74 from HHV8, it did not bind the non-ELR CXC chemokine (125)I-CXCL10/IP10. Interestingly, the B(max) value for CXCL6/GCP-2 was 3-fold higher than the capacity for maximal binding of CXCL1/GRO alpha to ECRF3 and 85-fold higher than that of CXCL8/interleukin-8, despite similar affinities. Like ORF74 from HHV8, ECRF3 activated a broad range of pathways (G(q), G(i), and G(12/13) as well as the cAMP response element-binding protein, NF-kappa B, NFAT, and serum response element transcription factors) in a ligand-regulated manner, with CXCL6/GCP-2 being the most potent and efficacious agonist. ECRF3 signaled constitutively through G(i) and G(12/13), but surprisingly not through G(q). At the level of transcription factor activation, the serum response element was activated constitutively by ECRF3, whereas cAMP response element-binding protein, NFAT, and NF-kappa B were only ligand-regulated. The maximal signaling capacities were similar for the two receptors; however, the ligand-regulated signaling was responsible for the major part of the total ECRF3 signaling and only for a minor part of the total HHV8 ORF74 signaling. The activation pattern of ECRF3 with constitutive activation of some (but not all) of the employed pathways has not been seen before in endogenous or virus-encoded chemokine receptors. The results suggest that the unique ligand selectivity of ECRF3 among ORF74 receptors could reflect differences in the cellular tropism of the gamma(2)-herpesviruses.

Specific CXC but not CC chemokines cause elevated monocyte migration in COPD: a role for CXCR2

Leukocyte migration is critical to maintaining host defense, but uncontrolled cellular infiltration into tissues can lead to chronic inflammation. In the lung, such diseases include chronic obstructive pulmonary disease (COPD), a debilitating, respiratory condition characterized by progressive and largely irreversible airflow limitation for which cigarette smoking is the major risk factor. COPD is associated with an increased inflammatory cell influx including increased macrophage numbers in the airways and tissue. Alveolar macrophages develop from immigrating blood monocytes and have the capacity to cause the pathological changes associated with COPD. This study addressed the hypothesis that increased macrophage numbers in COPD are a result of increased recruitment of monocytes from the circulation. Chemotaxis assays of peripheral blood mononuclear cells (PBMC)/monocytes from nonsmokers, smokers, and COPD patients demonstrated increased chemotactic responses for cells from COPD patients when compared with controls toward growth-related oncogene (GRO)alpha and neutrophil-activating peptide (NAP)-2 but not toward monocyte chemoattractant protein, interleukin-8, or epithelial-derived NAP(ENA)-78. The enhanced chemotactic response toward GROalpha and NAP-2 was not mediated by differences in expression of their cellular receptors, CXCR1 or CXCR2. Receptor expression studies using flow cytometry indicated that in COPD, monocyte expression of CXCR2 is regulated differently from nonsmokers and smokers, which may account for the enhanced migration toward GROalpha and NAP-2. The results highlight the potential of CXCR2 antagonists as therapy for COPD and demonstrate that an enhanced PBMC/monocyte response to specific CXC chemokines in these patients may contribute to increased recruitment and activation of macrophages in the lungs.