[The role of CXCR1/2 in shear stress-induced endothelial cell migration]

CXCR1 and CXCR2 are important receptors in regulating vascular endothelial cell activities. In order to elucidate the role of CXCR1/2 in shear stress-induced endothelial cell migration, we have investigated the expression levels of CXCR1 and CXCR2 in the endothelial cells exposed to shear stress. In the experiment, anti-IL8RA and anti-IL8RB were used to antagonize CXCR1 and CXCR2. Different shear stresses were generated in a flow chamber; scratch test was carried out to compare endothelial cell migration in the control group and the receptor-antagonized groups. The results indicated that the migration of endothelial cells was restrained effectively after CXCR1 and CXCR2 were antagonized by anti-IL8RA and anti-IL8RB. And anti-IL8RA showed a stronger inhibitive effect than did anti-IL8RB (P<0.05). In the group with both receptor antagonisms, the migration was further inhibited. These results suggest that both CXCR1 and CXCR2 are important factors in mediating the migration of endothelial cells induced by shear stress, and CXCR1 fulfills a more important role.

A hCXCR1 transgenic mouse model containing a conditional color-switching system for imaging of hCXCL8/IL-8 functions in vivo

To address the functions of human CXCL8 (hCXCL8)/IL-8 through hCXCR1 in vivo, we have developed a humanized, transgenic mouse for hCXCR1. This mouse line is versatile and allows for a variety of functional analyses using bioimaging, including Cre/loxP-mediated, tissue-specific hCXCR1 expression in a spatiotemporal manner; a color-switching mechanism, which uses spectrum-complementary, genetically encoded green and red fluorescence markers to label the hCXCR1-expressing cells [enhanced GFP (eGFP)] against the background [monomeric red fluorescent protein (mRFP)]; a bioluminescent marker, which is present in the hCXCR1-expressing cells; and an exogenous cell surface marker (eGFP moiety) in the hCXCR1-expressing cells, which facilitates identification, isolation, and targeting of these cells. The established, transgenic founder line RCLG3A (TG(+)) expresses only mRFP and does so ubiquitously. When the RCLG3A mice are crossed with the tamoxifen-inducible, whole-tissue Cre mice (ROSA26-Cre/Esr(+/-)), administration of tamoxifen induces whole-body hCXCR1 expression and color-switching. When RCLG3A mice are crossed with thymocyte-specific Cre mice (Lck-Cre(+/+)), the hCXCR1 expression and color-switching are restricted in a lineage-specific manner. This mouse line can be used to understand the functions of hCXCL-8 in vivo. In addition, our approach and vectors can be used to establish other tissue-specific, transgenic mice in conjunction with multifunctional cell markers, which facilitate cell imaging, tracing, and manipulation in vivo.

Interleukin 8 and cell migration to inflammatory sites: the regulation of focal adhesion kinase under conditions of migratory desensitization

BACKGROUND

Interleukin 8 is a prototypical inflammatory chemokine that induces leukocyte migration to inflammatory sites. Leukocyte recruitment in response to gradients of this chemokine is attenuated at advanced stages of inflammation to prevent damage to surrounding healthy tissues. Our published studies suggest that over-phosphorylation of focal adhesion kinase in migration-desensitizing conditions is involved in cessation of cell motility. This over-phosphorylation of FAK was induced by IL-8 only when the receptor transmitting the chemokine signals was CXCR2 and not CXCR1, indicating that the two IL-8 receptors diverge in their signaling properties.

OBJECTIVES

To analyze the regulation of FAK in CXCR2-expressing hematopoietic cells under conditions of migratory desensitization, focusing on the roles played by adhesion-related components in this process.

METHODS

Under conditions of migratory desensitization, we determined IL-8-induced cell spreading and FAK localization following disruption of actin filaments, and evaluated the role of integrins in FAK phosphorylation.

RESULTS

The disturbance of intact activity of actin filaments resulted in inhibition of cell spreading and modification of FAK intracellular localization upon IL-8 stimulation. Also, adhesion-dependent pre-stimulation of integrins was required for IL-8-induced FAK phosphorylation.

CONCLUSIONS

Intact actin filaments and integrins are required for optimal IL-8-induced FAK phosphorylation in conditions of migratory desensitization. These observations suggest that lack of adequate activity/regulation of adhesion-related components may give rise to FAK activities that are not appropriately controlled, possibly leading to pathological conditions that are associated with perturbed leukocyte migration phenotypes.

The role of CXCR2 in systemic neovascularization of the mouse lung

We previously showed increased expression of the ELR+, CXC chemokines in the lung after left pulmonary artery obstruction. These chemokines have been shown in other systems to bind their G protein-coupled receptor, CXCR2, and promote systemic endothelial cell proliferation, migration, and capillary tube formation. In the present study, we blocked CXCR2 in vivo using a neutralizing antibody and also studied mice that were homozygous null for CXCR2. To estimate the extent of neovascularization in this model, we measured systemic blood flow to the left lung 14 days after left pulmonary artery ligation (LPAL). We found blood flow significantly reduced (67% decrease) with neutralizing antibody treatment compared with controls. However, blood flow was not altered in the CXCR2-deficient mice compared with wild-type controls after LPAL. To test for ligand availability, we measured macrophage inflammatory protein (MIP)-2 in lung homogenates after LPAL, because this is the predominant CXC chemokine previously shown to be increased after LPAL ( 22 ). MIP-2 protein was two- to fourfold higher in the left lung relative to the right lung in all treatment groups 4 h after LPAL and this increase did not differ among groups. We speculate that the CXCR2-deficient mice have compensatory mechanisms that mitigate their lack of gene expression and conclude that CXCR2 contributes to chemokine-induced systemic angiogenesis after pulmonary artery obstruction.

Murine CXCR1 is a functional receptor for GCP-2/CXCL6 and interleukin-8/CXCL8

Functional interleuin-8 (IL-8) receptors (IL-8RA and IL-8RB: CXCR1 and CXCR2, respectively) have been described in human, monkey, dog, rabbit, and guinea pig. Although three IL-8R homologues have been found in rat, only one of these, rat CXCR2, appears to be functional based on responsiveness to ligands. Similarly, CXC chemokines induce biological responses through the murine homolog of CXCR2, but the identification of functional rodent CXCR1 homologues has remained elusive. We have identified and characterized the mouse CXCR1 homologue (mCXCR1). Murine CXCR1 shares 68 and 88% amino acid identity with its human and rat counterparts, respectively. Similar to the tissue distribution pattern of rat CXCR1, we found murine CXCR1 mRNA expression predominantly in lung, stomach, bone marrow, and leukocyte-rich tissues. In contrast to previous reports, we determined that mCXCR1 is a functional receptor. We show predominant engagement of this receptor by mouse GCP-2/CXCL6, human GCP-2, and IL-8/CXCL8 by binding, stimulation of GTPgammaS exchange, and chemotaxis of mCXCR1-transfected cells. Furthermore, murine CXCR1 is not responsive to the human CXCR2 ligands ENA-78/CXCL5, NAP-2/CXCL7, GRO-alpha, -beta, -gamma/CXCL1-3, or rat CINC-1-3. In addition, we show concomitant elevation of mCXCR1 and its proposed major ligand, GCP-2, positively correlated with paw swelling in murine collagen-induced arthritis. This report represents the first description of a functional CXCR1-like receptor in rodents.

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.

Phagocyte cell migration is mediated by phospholipases PLD1 and PLD2

We have investigated whether the signaling protein phospholipase D is implicated in leukocyte cell motility. Treating differentiated HL-60 cells with small interfering RNAs (siRNAs), to deplete endogenous expression of the PLD1 isoform, led to an abolishment of basal chemokinesis that could not be rescued with chemoattractants ENA-78, FMLP, and IL-8. Transient overexpression of PLD1 increased both chemokinesis and chemotaxis toward IL-8 and FMLP but not toward ENA-78. Chemokinesis was not mediated by the enzymatic activity of PLD1, but the chemotactic response was, because a lipase-inactive mutant (PLD1-K830R) negated all chemokine-induced potentiating actions and because IL-8 and FMLP increased activity in vitro. Gene expression silencing of the other mammalian isoform, PLD2, also led to cell migration arrest, whereas ENA-78 selectively increased endogenous PLD2 activity and chemotaxis of HL-60 cells overexpressing a myc-pcDNA-PLD2 construct. Thus, PLD1 is differentially activated by CXCR-1, whereas CXCR-2 (and possibly CXCR-1) mediates PLD2 activation. Finally, immunofluorescence microscopy showed that both isoforms were associated with cell polarity and directionality concomitantly with adhesion and F-actin polymerization in response to IL-8. These data represent the first demonstration of the involvement of PLD and its enzymatic activity toward chemokines in the key physiologic process of leukocyte migration.

CXCL8-induced FAK phosphorylation via CXCR1 and CXCR2: Cytoskeleton- and integrin-related mechanisms converge with FAK regulatory pathways in a receptor-specific manner

CXCL8 is a potent chemokine, inducing focal adhesion kinase (FAK) phosphorylation, and migration via a FAK-mediated pathway. Since, unlike growth factors, chemokines directly control integrins and cytoskeleton rearrangements, we determined whether these elements regulate CXCL8-induced FAK phosphorylation. The analysis intentionally dissociated between the CXCL8 receptors CXCR1 and CXCR2. In both CXCR1- and CXCR2-expressing cells, actin and microtubules were required for CXCL8-induced FAK phosphorylation, and CXCL8-induced cell spreading was accompanied by concordant re-localization of FAK with actin and beta-tubulin. The phosphorylation of five FAK sites depended on intact actin filaments and microtubules. While in CXCR2-expressing cells FAK phosphorylation was adhesion-dependent and was stimulated by fibronectin, in CXCR1-expressing cells FAK phosphorylation was adhesion-independent. Of note, even in the absence of integrin stimulation, the CXCL8-induced phosphorylation of FAK in CXCR1-expressing cells required cytoskeletal elements. CXCL8-induced migration in both cell types was highly reliant on actin filaments, but only the migration of CXCR1-expressing cells was fully dependent on microtubules. Overall, several aspects of CXCL8-induced FAK phosphorylation and migration are regulated in a receptor-specific manner. These observations lay the basis for future investigation of the equilibrium between CXCR1 and CXCR2 in cells expressing both receptors together, such as neutrophils, endothelial cells and tumor cells.

Cross-Desensitization among CXCR1, CXCR2, and CCR5: Role of Protein Kinase C-ε

The IL-8 (or CXCL8) chemokine receptors, CXCR1 and CXCR2, activate protein kinase C (PKC) to mediate leukocyte functions. To investigate the roles of different PKC isoforms in CXCL8 receptor activation and regulation, human mononuclear phagocytes were treated with CXCL8 or CXCL1 (melanoma growth-stimulating activity), which is specific for CXCR2. Plasma membrane association was used as a measure of PKC activation. Both receptors induced time-dependent association of PKCalpha, -beta1, and -beta2 to the membrane, but only CXCR1 activated PKCepsilon. CXCL8 also failed to activate PKCepsilon in RBL-2H3 cells stably expressing CXCR2. DeltaCXCR2, a cytoplasmic tail deletion mutant of CXCR2 that is resistant to internalization, activated PKCepsilon as well as CXCR1. Expression of the PKCepsilon inhibitor peptide epsilonV1 in RBL-2H3 cells blocked PKCepsilon translocation and inhibited receptor-mediated exocytosis, but not phosphoinositide hydrolysis or peak intracellular Ca(2+) mobilization. epsilonV1 also inhibited CXCR1-, CCR5-, and DeltaCXCR2-mediated cross-regulatory signals for GTPase activity, Ca(2+) mobilization, and internalization. Peritoneal macrophages from PKCepsilon-deficient mice (PKCepsilon(-/-)) also showed decreased CCR5-mediated cross-desensitization of G protein activation and Ca(2+) mobilization. Taken together, the results indicate that CXCR1 and CCR5 activate PKCepsilon to mediate cross-inhibitory signals. Inhibition or deletion of PKCepsilon decreases receptor-induced exocytosis and cross-regulatory signals, but not phosphoinositide hydrolysis or peak intracellular Ca(2+) mobilization, suggesting that cross-regulation is a Ca(2+)-independent process. Because DeltaCXCR2, but not CXCR2, activates PKCepsilon and cross-desensitizes CCR5, the data further suggest that signal duration leading to activation of novel PKC may modulate receptor-mediated cross-inhibitory signals.

Significance of chemokine receptor expression in aggressive NK cell leukemia

Natural killer (NK) cell-type lymphoproliferative diseases of granular lymphocytes can be subdivided into aggressive NK cell leukemia (ANKL) and chronic NK cell lymphocytosis (CNKL). One reason for the poor outcome in ANKL is leukemic infiltration into multiple organs. The mechanisms of cell trafficking associated with the chemokine system have been investigated in NK cells. To clarify the mechanism of systemic migration of leukemic NK cells, we enrolled nine ANKL and six CNKL cases, and analyzed the expression profiles and functions of chemokine receptors by flowcytometry and chemotaxis assay. CXCR1 was detected on NK cells in all groups, and CCR5 was positive in all ANKL cells. Proliferating NK cells were simultaneously positive for CXCR1 and CCR5 in all ANKL patients examined, and NK cells with this phenotype did not expand in CNKL patients or healthy donors. ANKL cells showed enhanced chemotaxis toward the ligands of these receptors. These results indicated that the chemokine system might play an important role in the pathophysiology of ANKL and that chemokine receptor profiling might be a novel tool for discriminating ANKL cells from benign NK cells.

Reversing systemic inflammatory response syndrome with chemokine receptor pepducins

We describe a new therapeutic approach for the treatment of lethal sepsis using cell-penetrating lipopeptides-termed pepducins-that target either individual or multiple chemokine receptors. Interleukin-8 (IL-8), a ligand for the CXCR1 and CXCR2 receptors, is the most potent endogenous proinflammatory chemokine in sepsis. IL-8 levels rise in blood and lung fluids to activate neutrophils and other cells, and correlate with shock, lung injury and high mortality. We show that pepducins derived from either the i1 or i3 intracellular loops of CXCR1 and CXCR2 prevent the IL-8 response of both receptors and reverse the lethal sequelae of sepsis, including disseminated intravascular coagulation and multi-organ failure in mice. Conversely, pepducins selective for CXCR4 cause a massive leukocytosis that does not affect survival. CXCR1 and CXCR2 pepducins conferred nearly 100% survival even when treatment was postponed, suggesting that our approach might be beneficial in the setting of advanced disease.

IL-8 increases transmission of HIV type 1 in cervical explant tissue

Interleukin-8 (IL-8) is the predominant cytokine expressed in the female genital tract of women with certain infectious/inflammatory conditions. IL-8 increased HIV-1 replication in T cells and to a greater extent in monocytes/macrophages in vitro. Physiological levels of IL-8 increased susceptibility to HIV-1 infection 5- to 8-fold in cervical explant tissues. Competitive inhibition of the IL-8 receptor CXCR2 with the small molecule inhibitor SB225002 resulted in a 45-70% decrease in cervical explant susceptibility to HIV-1 infection.

IL-8 responsiveness defines a subset of CD8 T cells poised to kill

CD8 T cells play a key role in host defense against intracellular pathogens. Efficient migration of these cells into sites of infection is therefore intimately linked to their effector function. The molecular mechanisms that control CD8 T-cell trafficking into sites of infection and inflammation are not well understood, but the chemokine/chemokine receptor system is thought to orchestrate this process. Here we systematically examined the chemokine receptor profile expressed on human CD8 T cells. Surprisingly, we found that CXC chemokine receptor 1 (CXCR1), the predominant neutrophil chemokine receptor, defined a novel interleukin-8/CXC ligand 8 (IL-8/CXCL8)–responsive CD8 T-cell subset that was enriched in perforin, granzyme B, and interferon-γ (IFNγ), and had high cytotoxic potential. CXCR1 expression was down-regulated by antigen stimulation both in vitro and in vivo, suggesting antigen-dependent shaping of the migratory characteristics of CD8 T cells. On virus-specific CD8 T cells from persons with a history of Epstein-Barr virus (EBV) and influenza infection, CXCR1 expression was restricted to terminally differentiated effector memory cells. In HIV-1 infection, CXCR1-expressing HIV-1–specific CD8 T cells were present only in persons who were able to control HIV-1 replication during structured treatment interruptions. Thus, CXCR1 identifies a subset of CD8 T cells poised for immediate cytotoxicity and early recruitment into sites of innate immune system activation.

Rabbit neutrophil chemotactic protein (NCP) activates both CXCR1 and CXCR2 and is the functional homologue for human CXCL6

Neutrophil chemotactic protein (NCP) is a rabbit CXC chemokine with activating and chemotactic properties on neutrophilic granulocytes. Although its selective activity on neutrophils is demonstrated, its interactions with specific chemokine receptors are not defined. For further functional characterization, NCP was chemically synthesized and was found to be equipotent as natural NCP in neutrophil chemotaxis. To identify its human homologue, we separately expressed two potential rabbit NCP receptors (CXCR1 and CXCR2) in Jurkat cells. Pure synthetic NCP was equally efficient to promote chemotaxis through either rabbit CXCR1 or CXCR2. Moreover, chemotaxis assays on rabbit CXCR1 and CXCR2 transfectants showed that NCP uses the same receptors as interleukin-8 (IL-8), a major rabbit CXC chemokine, but not rabbit GROalpha, which only recognized CXCR2. In addition, specific inhibitors for CXCR1 or CXCR2 reduced rabbit neutrophil chemotaxis induced by NCP and rabbit IL-8. Furthermore, NCP and the structurally related human CXCR1/CXCR2 agonist CXCL6/GCP-2 (granulocyte chemotactic protein-2) cross-desensitized each other in intracellular calcium release assays on human neutrophils, further indicating that both chemokines share the same receptors. The inflammatory role of NCP was also evidenced by its potent granulocytosis inducing capacity in rabbits upon systemic administration. This study provides in vitro and in vivo evidences that NCP is the functional rabbit homologue for human CXCL6/GCP-2 rather than the most related CXCR2 agonist CXCL5/ENA-78 (epithelial cell-derived neutrophil activating peptide-78). It is concluded that the rabbit is a better model to study human neutrophil activation compared to mice, which lack CXCL8/IL-8.

IL-8-mediated cell migration in endothelial cells depends on cathepsin B activity and transactivation of the epidermal growth factor receptor

Microvascular endothelial cells (HMECs) express both the CXCR1 and the CXCR2, but cell migration is almost entirely mediated by the CXCR2. Similarly, NIH 3T3 cells transfected with the CXCR2 migrated toward IL-8, whereas CXCR1-transfected cells failed to do so. This situation differs from that seen in leukocytes, where chemotaxis is primarily a function of the CXCR1. To define signal transduction pathways that explain this difference in behavior, various inhibitors were used to block cell migration. Apart from inhibitors of phosphatidylinositol 3-kinase, which blocked migration in all cases, inhibition of the epidermal growth factor (EGF) receptor blocked IL-8-mediated cell migration in HMECs and in CXCR2-transfected NIH 3T3 cells, but not in RBL2H3 cells, which do not express an EGFR. Blocking Abs against the EGFR or against heparin-binding EGF-like growth factor similarly blocked IL-8-mediated cell migration and in vitro tubulogenesis in HMECs. Furthermore, inhibition of the EGFR also attenuated focus formation in NIH 3T3 expressing the CXCR2. Immunoprecipitations of the EGFR in HMECs and in NIH 3T3 cells expressing the CXCR2 confirmed that the EGFR was phosphorylated following stimulation with IL-8. However, in contrast to previous reports, e.g., for the thrombin receptor, inhibition of matrix metalloproteases blocked IL-8-mediated cell migration only partially, whereas it was ablated by inhibition of cathepsin B. These results indicate that IL-8-induced transactivation of the EGFR is mediated by the CXCR2 and involves cathepsin B, and that this pathway is important for the migratory and tumorigenic effects of IL-8.

Molecular characterization of the chemokine receptor CXCR3: evidence for the involvement of distinct extracellular domains in a multi-step model of ligand binding and receptor activation

CXCR3 is a chemokine receptor predominantly expressed on T lymphocytes, and binds the chemokines CXCL9 (Mig), CXCL10 (IP-10) and CXCL11 (I-TAC). Here, we have investigated the role of the extracellular domains of CXCR3 in ligand selectivity and receptor activation by assessing the ligand binding and chemotactic responses of chimeric CXCR3/CXCR1 constructs. Our data reveal that the secondextracellular loop of CXCR3 is essential for receptor activation in response to all CXCR3 ligands. In contrast, the N terminus and first extracellular loop of CXCR3 play some role in CXCL10- and CXCL11-mediated activation but are dispensable for CXCL9-induced signaling. The third extracellular loop of CXCR3 is important only for CXCL9- and CXCL10-induced chemotaxis. Binding studies suggest that the CXCR3 ligands bind to distinct sites composed of multiple domains of CXCR3 and that high-affinity binding and receptor activation are disparate functions. Collectively, our data support a multi-site model for CXCR3 interactions with its agonists, in which several extracellular domains of CXCR3 contribute to ligand binding and the induction of receptor activation. The development of antagonists targeting the second extracellular loop of CXCR3 should impede receptor activation and aid the treatment of several human inflammatory disorders.

Inflammatory chemoreceptor cross-talk suppresses leukotriene B4 receptor 1-mediated neutrophil calcium mobilization and chemotaxis after trauma

G protein-coupled chemoattractants recruit neutrophils (PMN) to sites of injury and infection. The leukotrienes (LT) and CXC chemokines (CXC) and their receptors (BLT1/BLT2 and CXCR1/CXCR2) are all known to play roles in these responses. Each system has been studied separately in vitro, but in vivo they act concurrently, and the clinical interactions between the two systems are unstudied. We prospectively studied calcium mobilization and chemotactic responses to LTB(4) in PMN from major trauma patients. The responses of the high affinity BLT1 receptor were suppressed at the 3-day postinjury time point, but recovered by 1 wk. Trauma patients had transient elevations of plasma LT and CXC levels. Functional deficits identical with those in trauma PMN were reproduced in vitro by exposing healthy PMN to CXCs at the elevated plasma concentrations found. Functional responses to LTB(4) were suppressed by cross-talk with CXC and BLT2 receptors that desensitize BLT1. Since the suppression of intracellular calcium mobilization was prominent, we also studied the role of suppressed cell calcium mobilization in the defective chemotactic responses to LTB(4). We noted that PMN chemotaxis to LTB(4) showed far more dependence on store-operated calcium entry than on the release of cellular calcium stores, and that store-operated calcium responses to BLT1 activation were markedly inhibited during the same time period as was chemotaxis. The intermittent release of inflammatory mediators after injury can blunt PMN responses to LTs by suppressing BLT1 as well as downstream calcium entry. Diminished LT receptor activity due to cross-talk with CXC receptors can inhibit PMN recruitment to infective sites. This may predispose injured patients to septic complications.

Blood monocytes consist of two principal subsets with distinct migratory properties

Peripheral blood monocytes are a heterogeneous population of circulating leukocytes. Using a murine adoptive transfer system to probe monocyte homing and differentiation in vivo, we identified two functional subsets among murine blood monocytes: a short-lived CX(3)CR1(lo)CCR2(+)Gr1(+) subset that is actively recruited to inflamed tissues and a CX(3)CR1(hi)CCR2(-)Gr1(-) subset characterized by CX(3)CR1-dependent recruitment to noninflamed tissues. Both subsets have the potential to differentiate into dendritic cells in vivo. The level of CX(3)CR1 expression also defines the two major human monocyte subsets, the CD14(+)CD16(-) and CD14(lo)CD16(+) monocytes, which share phenotype and homing potential with the mouse subsets. These findings raise the potential for novel therapeutic strategies in inflammatory diseases.

Interleukin-8-mediated heterologous receptor internalization provides resistance to HIV-1 infectivity. Role of signal strength and receptor desensitization

Human immunodeficiency virus type 1 (HIV-1) entry into CD4(+) cells requires the chemokine receptors CCR5 or CXCR4 as co-fusion receptors. We have previously demonstrated that chemokine receptors are capable of cross-regulating the functions of each other and, thus, affecting cellular responsiveness at the site of infection. To investigate the effects of chemokine receptor cross-regulation in HIV-1 infection, monocytes and MAGIC5 and rat basophilic leukemia (RBL-2H3) cell lines co-expressing the interleukin-8 (IL-8 or CXCL8) receptor CXCR1 and either CCR5 (ACCR5) or CXCR4 (ACXCR4) were generated. IL-8 activation of CXCR1, but not the IL-8 receptor CXCR2, cross-phosphorylated CCR5 and CXCR4 and cross-desensitized their responsiveness to RANTES (regulated on activation normal T cell expressed and secreted) (CCL5) and stromal derived factor (SDF-1 or CXCL12), respectively. CXCR1 activation internalized CCR5 but not CXCR4 despite cross-phosphorylation of both. IL-8 pretreatment also inhibited CCR5- but not CXCR4-mediated virus entry into MAGIC5 cells. A tail-deleted mutant of CXCR1, DeltaCXCR1, produced greater signals upon activation (Ca(2+) mobilization and phosphoinositide hydrolysis) and cross-internalized CXCR4, inhibiting HIV-1 entry. The protein kinase C inhibitor staurosporine prevented phosphorylation and internalization of the receptors by CXCR1 activation. Taken together, these results indicate that chemokine receptor-mediated HIV-1 cell infection is blocked by receptor internalization but not desensitization alone. Thus, activation of chemokine receptors unrelated to CCR5 and CXCR4 may play a cross-regulatory role in the infection and propagation of HIV-1. Since DeltaCXCR1, but not CXCR1, cross-internalized and cross-inhibited HIV-1 infection to CXCR4, the data indicate the importance of the signal strength of a receptor and, as a consequence, protein kinase C activation in the suppression of HIV-1 infection by cross-receptor-mediated internalization.

Potential involvement of IL-8 and its receptors in the invasiveness of pancreatic cancer cells

The purpose of the present study was to examine the expression of interleukin (IL)-8 and IL-8 receptors and to evaluate the effects of IL-8 on human pancreatic cancer. We examined the expression of IL-8 and its two receptors (CXCR1 and CXCR2) in 40 surgically resected human pancreatic cancer tissues and in three different human pancreatic cancer cell lines (PANC-1, MIAPaCa-2 and Capan-2). The immunohistochemical analysis using specific antibodies demonstrated that positive staining for IL-8, CXCR1 and CXCR2 in surgically resected human pancreatic cancer was 50, 55 and 65%, respectively. Moreover, 40% of these cases were positive for both IL-8 and IL-8 receptors. In contrast, immunoreactive signals for those proteins were extremely suppressed in normal pancreatic tissues. All of the pancreatic cancer cell lines expressed IL-8 and IL-8 receptors at the RNA and protein levels. Receptor binding experiments using 125I-labeled IL-8 showed that PANC-1 cells had specific binding sites for IL-8. The cell proliferation assay demonstrated that IL-8 did not affect the growth of the three cell lines. However, treatment with IL-8 enhanced the invasiveness into Matrigel and increased the activity of matrix metalloproteinase (MMP)-2 in supernatants of the PANC-1 cells. These results demonstrate that IL-8 and IL-8 receptors are over-expressed in pancreatic cancer, and suggest that IL-8 regulates MMP-2 activity and plays an important role in the invasiveness of human pancreatic cancer.

IL-8-induced migratory responses through CXCR1 and CXCR2: association with phosphorylation and cellular redistribution of focal adhesion kinase

CXCR1 and CXCR2 mediate migratory activities in response to IL-8 and other ELR+-CXC chemokines (e.g., GCP-2 and NAP-2). In vitro, activation of migration is induced by low IL-8 concentrations (10-50 ng/mL), whereas migratory shut-off is induced by high IL-8 concentrations (1000 ng/mL). The stimulation of CXCR1 and CXCR2 by IL-8 concentrations that result in migratory activation induced focal adhesion kinase (FAK) phosphorylation in a G(alpha)i-dependent manner. The expression of FRNK, a dominant negative mutant of FAK, perturbed migratory responses to the activating dose of 50 ng/mL IL-8. The migration-activating concentrations of 50 ng/mL GCP-2 and NAP-2 induced less potent migratory responses and FAK phosphorylation in CXCR2-expressing cells as compared with IL-8. These results indicate that FAK is phosphorylated, and required, for the chemotactic response under conditions of migratory activation by ELR+-CXC chemokines. In addition, FAK phosphorylation was determined following exposure to migration-attenuating concentrations of IL-8. In CXCR1-RBL cells this treatment resulted in FAK phosphorylation, in similar levels to those induced by activating concentrations of IL-8. In contrast, in CXCR2-RBL cells the migration-attenuating concentrations of IL-8 induced promoted levels of FAK phosphorylation and different patterns of FAK phosphorylation on its six potential tyrosine phosphorylation sites, as compared to activating concentrations of the chemokine. Exposure to IL-8 resulted not only in FAK phosphorylation but also in its cellular redistribution, indicated by the formation of defined contact regions with the substratum, enriched in phosphorylated FAK and vinculin. Overall, FAK phosphorylation was associated with, and found to be differently regulated upon, ELR+-CXC chemokine-induced migration.

Role of the cytoplasmic tails of CXCR1 and CXCR2 in mediating leukocyte migration, activation, and regulation

IL-8 (or CXCL8) activates the receptors CXCR1 (IL-8RA) and CXCR2 (IL-8RB) to induce chemotaxis in leukocytes, but only CXCR1 mediates cytotoxic and cross-regulatory signals. This may be due to the rapid internalization of CXCR2. To investigate the roles of the intracellular domains in receptor regulation, wild-type, chimeric, phosphorylation-deficient, and cytoplasmic tail (C-tail) deletion mutants of both receptors were expressed in RBL-2H3 cells and studied for cellular activation, receptor phosphorylation, desensitization, and internalization. All but one chimeric receptor bound IL-8 and mediated signal transduction, chemotaxis, and exocytosis. Upon IL-8 activation, the chimeric receptors underwent receptor phosphorylation and desensitization. One was resistant to internalization, yet it mediated normal levels of beta-arrestin 2 (beta arr-2) translocation. The lack of internalization by this receptor may be due to its reduced association with beta arr-2 and the adaptor protein-2 beta. The C-tail-deleted and phosphorylation-deficient receptors were resistant to receptor phosphorylation, desensitization, arrestin translocation, and internalization. They also mediated greater phosphoinositide hydrolysis and exocytosis and sustained Ca(2+) mobilization, but diminished chemotaxis. These data indicate that phosphorylation of the C-tails of CXCR1 and CXCR2 are required for arrestin translocation and internalization, but are not sufficient to explain the rapid internalization of CXCR2 relative to CXCR1. The data also show that receptor internalization is not required for chemotaxis. The lack of receptor phosphorylation was correlated with greater signal transduction but diminished chemotaxis, indicating that second messenger production, not receptor internalization, negatively regulates chemotaxis.

IL-8 directly enhanced endothelial cell survival, proliferation, and matrix metalloproteinases production and regulated angiogenesis

IL-8, a member of the chemokine family, has been shown to play an important role in tumor growth, angiogenesis, and metastasis. The objective of this study was to determine the mechanism of IL-8-mediated angiogenesis. We examined the direct role of IL-8 in angiogenesis by examining IL-8 receptor expression on endothelial cells and their proliferation, survival, and matrix metalloproteinases (MMPs) production. We demonstrate that HUVEC and human dermal microvascular endothelial cells constitutively express CXCR1 and CXCR2 mRNA and protein. Recombinant human IL-8 induced endothelial cell proliferation and capillary tube organization while neutralization of IL-8 by anti-IL-8 Ab blocks IL-8-mediated capillary tube organization. Incubation of endothelial cells with IL-8 inhibited endothelial cell apoptosis and enhanced antiapoptotic gene expression. Endothelial cells incubated with IL-8 had higher levels of Bcl-x(L):Bcl-x(S) and Bcl-2:Bax ratios. Furthermore, incubation of endothelial cells with IL-8 up-regulated MMP-2 and MMP-9 production and mRNA expression. Our data suggest that IL-8 directly enhanced endothelial cell proliferation, survival, and MMP expression in CXCR1- and CXCR2-expressing endothelial cells and regulated angiogenesis.

Molecular cloning and genomic structure of an interleukin-8 receptor-like gene from homozygous clones of rainbow trout (Oncorhynchus mykiss)

Chemokines are small proteins (70-100 amino acids) which play an important role in recruitment and activation of leucocytes to migrate to the site of inflammation. Based on the position of the first two conserved cysteines, chemokines are classified into four subfamilies: C, CC, CXC and CX3C. To date, many members of CC and CXC have been found and studied extensively [1]. Chemokines exert effects on their target cell via chemokine receptors, which are G-protein coupled receptors containing seven transmembrane domains with an extracellular N-terminus and an intracellular C-terminus [2]. Interleukin 8 (IL-8) belongs to the CXC chemokine subfamily. It can activate and attract migratory neutrophils to an inflammation site. Two IL-8 receptors, CXCR1 and CXCR2, have been identified in mammals [3-6]; both of these receptors have high affinity for IL-8 and are expressed on the neutrophil. CXCR1 just binds IL-8; however, CXCR2 binds IL-8 and other structurally related chemokines such as growth-related oncogene (GRO) a, GRObeta, GROgamma, neutrophil-activating peptide-2 (NAP-2) and epithelial cell-derived neutrophil activating peptide-78 (ENA-78) [7, 8]. Several studies on fish chemokine receptors have been reported [9-11]. Thus far, however, IL-8 and CXCR1 and CXCR2 proteins from rainbow trout have not been reported: however, the sequence of a rainbow trout IL-8 has been noted (GenBank Accession No. AJ279069 [12]). Cloning of the IL-8 receptor is important to study the function of IL-8/CXCR1 and (CXCR2) in inflammation and signal transduction in fish. This paper reports the molecular cloning and genomic structure of an IL-8 receptor-like gene from four homozygous clones of rainbow trout: Oregon State University (OSU), Hot Creek (HC), Arlee (AR) and Swanson (SW).

Identification of a signal transduction switch in the chemokine receptor CXCR1

Chemokine receptors belong to the superfamily of G protein-coupled receptors, which regulate the trafficking and activation of leukocytes, and operate as coreceptors in the entry of HIV-1. To investigate the early steps in the signal transmission from the chemokine-binding site to the G protein-coupling region we engineered metal ion-binding sites at putative extracellular sites in the chemokine receptor CXCR1. We introduced histidines into sites located in the second and third putative extracellular loops of CXCR1, creating single, double, and triple mutant receptors: R199H, R203H, D265H, R199H/R203H, R199H/D265H, R203H/D265H, R203H/H207Q, and R199H/R203H/D265H. Cells expressing the double mutants R199H/D265H and R203H/D265H and the triple mutant R199H/R203H/D265H failed to trigger interleukin 8-dependent calcium responses. Interestingly, calcium responses mediated by the single mutant R203H and the double mutants R199H/R203H and R203H/H207Q were blocked by Zn(II), indicating the creation of a functional metal ion-binding site. On the other hand, cells expressing all single, double, or triple histidine-substituted CXCR1 demonstrated high affinity binding to interleukin 8 in the presence and absence of metal ions. These findings indicate that occupation of the engineered metal-binding site uncouples the chemokine-binding site from the activation mechanism in CXCR1. Most importantly, we identify for the first time elements of an early signal transduction switch of chemokine receptors before the activation of cytoplasmic G proteins.

Signal processing in migrating T24 human bladder carcinoma cells: role of the autocrine interleukin-8 loop

T24 human bladder carcinoma cells reveal a high locomotor activity (70% locomoting cells) within a 3-dimensional collagen matrix. This high migratory activity is induced by an autocrine engagement of the interleukin-8 receptor A, as was shown by antibodies neutralizing the secreted interleukin-8. Treatment of the cells with these specific antibodies reduced the locomotor activity by half. The intracellular signal transduction underlying the interleukin-8-induced T24 locomotion involves the activity of protein tyrosine kinases (PTKs), the phospholipase Cgamma (PLCgamma) and the protein kinase C (PKC), as proven by the use of specific enzyme inhibitors. These results suggest the following model for the regulatory signal transduction of interleukin-8-induced human T24 bladder carcinoma cell migration: The engagement of the interleukin-8-receptor, a receptor of the serpentine family, leads to the beta-arrestin-mediated activation of PTKs. These kinases phosphorylate the PLCgamma, which generates the second messengers diacylglycerol (DAG) and inositol-1,4,5-trisphosphate (IP(3)). DAG activates the PKC, whereas IP(3) mediates the release of calcium from the endoplasmatic reticulum. By means of confocal laser microscopy, we observed an oscillation of the cytosolic calcium concentration in migrating T24 cells, which were loaded with the calcium-dye fluo-3/AM. Here, we report on a new autocrine function of secreted interleukin-8 and the intracellular signal transduction leading to the regulation of cytosolic calcium and to a migratory tumor cell phenotype.

[The role of rat cytokine-induced neutrophil chemoattractants (CINCs) in inflammation]

Rat cytokine-induced neutrophil chemoattractants (CINCs) are the members of the CXC chemokine family. Four neutrophil chemokines, CINC-1, CINC-2 alpha, CINC-2 beta and CINC-3, were purified from the conditioned medium of granulation-tissue culture. CINC-2 alpha and CINC-2 beta differ only in the sequence of three carboxy-terminal residues and are produced by alternative splicing. CINC-3 had neutrophil chemotactic activity similar to that of CINC-1 and CINC-2, but induced greater calcium mobilization than CINC-1 and CINC-2. CINC-1, -2 and -3 induced calcium flux in CXCR2-transfected HEK293 cells. In addition, anti-CXCR2 serum inhibited neutrophil chemotactic activities of the three types of CINCs almost completely. These results indicate that rat CXCR2 is a unique receptor for CINC-1, -2 and -3. CINCs induced calcium mobilization through pertussis toxin-insensitive G-protein but induced chemotaxis through pertussis toxin-sensitive G-protein. CINC-1/-2 and CINC-3 may stimulate both G-proteins with distinct efficiency. The concentration of CINC-1 increased transiently in rat air pouch/lipopolysaccharide inflammation, whereas the CINC-2 level increased linearly. The number of infiltrated cells increased up to 8 h. The increase in cell number was correlated with the total concentration of CINC-1 and CINC-2. Northern blot analyses and enzyme-linked immunosorbent assay showed that CINC expression was very low in rat macrophages without stimulation and increased after lipopolysaccharide stimulation. These data suggest that CINCs are expressed by inflammatory cells such as macrophages at a site of inflammation and play important roles in neutrophil infiltration.

Signal transduction by the chemokine receptor CXCR5: structural requirements for G protein activation analyzed by chimeric CXCR1/CXCR5 molecules

The human chemokine receptors CXCR5 and CXCR1 activate signaling pathways via pertussis toxin-sensitive as well as insensitive G proteins. CXCR5 induces Ca2+ signaling and chemotaxis independently of inhibitory G proteins, whereas the same signaling pathways are entirely dependent on inhibitory G proteins for CXCR1. In contrast, activation of the MAP kinase cascade via ERK1/2 is a pertussis toxin-sensitive signaling event for both receptors. Using chimeric CXCR1/CXCR5 receptors we investigated structural requirements for the activation of signal transduction pathways by CXCR5. Individual or multiple intracellular domains of CXCR1 were exchanged for the corresponding sequences of CXCR5, leading to receptors resembling CXCR5 at the cytoplasmic surface to a varying extent. Replacing the second intracellular domain of CXCR1 had a major influence on signaling mediated by inhibitory G proteins, whereas the exchange of the third or carboxy-terminal intracellular domain had only minor effects on signal transduction. Activation of the MAP kinase cascade via ERK1/2 and chemotaxis are largely reduced in chimeras comprising the second intracellular domain of CXCR5, although coupling to inhibitory G proteins is retained in all chimeric receptors. In summary, these data characterize the contribution of the intracellular domains of CXCR5 to receptor signaling, thereby disclosing unique structural requirements that modulate G protein coupling by the receptor.

Interleukin-8 receptor antagonists in pulmonary diseases

Inflammatory cells are thought to be instrumental in the pathophysiology of pulmonary diseases, and control of their recruitment and activation in the lung would appear to be an attractive strategy for therapeutic intervention. Interleukin-8 and related CXC chemokines are involved in the function of neutrophils and T cells, and have been implicated in several lung diseases. Small-molecule antagonists of the interleukin-8 receptors have been identified, which may help elucidate the role of interleukin-8 and related chemokines in the pathophysiology of lung diseases.

Neutrophil recruitment, chemokine receptors, and resistance to mucosal infection

Neutrophil migration to infected mucosal sites involves a series of complex interactions with molecules in the lamina propria and at the epithelial barrier. Much attention has focussed on the vascular compartment and endothelial cells, but less is known about the molecular determinants of neutrophil behavior in the periphery. We have studied urinary tract infections (UTIs) to determine the events that initiate neutrophil recruitment and interactions of the recruited neutrophils with the mucosal barrier. Bacteria activate a chemokine response in uroepithelial cells, and the chemokine repertoire depends on the bacterial virulence factors and on the specific signaling pathways that they activate. In addition, epithelial chemokine receptor expression is enhanced. Interleukin (IL)-8 and CXCR1 direct neutrophil migration across the epithelial barrier into the lumen. Indeed, mIL-8Rh knockout mice showed impaired transepithelial neutrophil migration, with tissue accumulation of neutrophils, and these mice developed renal scarring. They had a defective antibacterial defense and developed acute pyelonephritis with bacteremia. Low CXCR1 expression was also detected in children with acute pyelonephritis. These results demonstrate that chemokines and chemokine receptors are essential to orchestrate a functional antimicrobial defense of the urinary tract mucosa. Mutational inactivation of the IL-8R caused both acute disease and chronic tissue damage.

Chemokines in cutaneous wound healing

Healing of wounds is one of the most complex biological events after birth as a result of the interplay of different tissue structures and a large number of resident and infiltrating cell types. The latter are mainly constituted by leukocyte subsets (neutrophils, macrophages, mast cells, and lymphocytes), which sequentially infiltrate the wound site and serve as immunological effector cells but also as sources of inflammatory and growth-promoting cytokines. Recent data demonstrate that recruitment of leukocyte subtypes is tightly regulated by chemokines. Moreover, the presence of chemokine receptors on resident cells (e.g., keratinocytes, endothelial cells) indicates that chemokines also contribute to the regulation of epithelialization, tissue remodeling, and angiogenesis. Thus, chemokines are in an exclusive position to integrate inflammatory events and reparative processes and are important modulators of human-skin wound healing. This review will focus preferentially on the role of chemokines during skin wound healing and intends to provide an update on the multiple functions of individual chemokines during the phases of wound repair.

Transepithelial neutrophil migration is CXCR1 dependent in vitro and is defective in IL-8 receptor knockout mice

Neutrophil migration across infected mucosal surfaces is chemokine dependent, but the role of chemokine receptors has not been investigated. In this study, chemokine receptors were shown to be expressed by epithelial cells lining the urinary tract, and to play an essential role for neutrophil migration across the mucosal barrier. Uroepithelial CXCR1 and CXCR2 expression was detected in human urinary tract biopsies, and in vitro infection of human uroepithelial cell lines caused a dramatic increase in both receptors. As a consequence, there was higher binding of IL-8 to the cells and the IL-8-dependent neutrophil migration across the infected epithelial cell layers was enhanced. Abs to IL-8 or to the CXCR1 receptor inhibited this increase by 60% (p<0.004), but anti-CXCR2 Abs had no effect, suggesting that CXCR1 was the more essential receptor in this process. Similar observations were made in the mouse urinary tract, where experimental infection stimulated epithelial expression of the murine IL-8 receptor, followed by a rapid flux of neutrophils into the lumen. IL-8 receptor knockout mice, in contrast, failed to express the receptor, their neutrophils were unable to cross the epithelial barrier, and accumulated in massive numbers in the tissues. These results demonstrate that epithelial cells express CXC receptors and that infection increases receptor expression. Furthermore, we show that CXCR1 is required for neutrophil migration across infected epithelial cell layers in vitro, and that the murine IL-8 receptor is needed for neutrophils to cross the infected mucosa of the urinary tract in vivo.

Neutrophil gelatinase B potentiates interleukin-8 tenfold by aminoterminal processing, whereas it degrades CTAP-III, PF-4, and GRO-alpha and leaves RANTES and MCP-2 intact

Chemokines are mediators in inflammatory and autoimmune disorders. Aminoterminal truncation of chemokines results in altered specific activities and receptor recognition patterns. Truncated forms of the CXC chemokine interleukin (IL)-8 are more active than full-length IL-8 (1-77), provided the Glu-Leu-Arg (ELR) motif remains intact. Here, a positive feedback loop is demonstrated between gelatinase B, a major secreted matrix metalloproteinase (MMP-9) from neutrophils, and IL-8, the prototype chemokine active on neutrophils. Natural human neutrophil progelatinase B was purified to homogeneity and activated by stromelysin-1. Gelatinase B truncated IL-8(1-77) into IL-8(7-77), resulting in a 10- to 27-fold higher potency in neutrophil activation, as measured by the increase in intracellular Ca(++) concentration, secretion of gelatinase B, and neutrophil chemotaxis. This potentiation correlated with enhanced binding to neutrophils and increased signaling through CXC chemokine receptor-1 (CXCR1), but it was significantly less pronounced on a CXCR2-expressing cell line. Three other CXC chemokines-connective tissue-activating peptide-III (CTAP-III), platelet factor-4 (PF-4), and GRO-alpha-were degraded by gelatinase B. In contrast, the CC chemokines RANTES and monocyte chemotactic protein-2 (MCP-2) were not digested by this enzyme. The observation of differing effects of neutrophil gelatinase B on the proteolysis of IL-8 versus other CXC chemokines and on CXC receptor usage by processed IL-8 yielded insights into the relative activities of chemokines. This led to a better understanding of regulator (IL-8) and effector molecules (gelatinase B) of neutrophils and of mechanisms underlying leukocytosis, shock syndromes, and stem cell mobilization by IL-8. (Blood. 2000;96:2673-2681)

Interleukin 8 receptor deficiency confers susceptibility to acute experimental pyelonephritis and may have a human counterpart

Neutrophils migrate to infected mucosal sites that they protect against invading pathogens. Their interaction with the epithelial barrier is controlled by CXC chemokines and by their receptors. This study examined the change in susceptibility to urinary tract infection (UTI) after deletion of the murine interleukin 8 receptor homologue (mIL-8Rh). Experimental UTIs in control mice stimulated an epithelial chemokine response and increased chemokine receptor expression. Neutrophils migrated through the tissues to the epithelial barrier that they crossed into the lumen, and the mice developed pyuria. In mIL-8Rh knockout (KO) mice, the chemokine response was intact, but the epithelial cells failed to express IL-8R, and neutrophils accumulated in the tissues. The KO mice were unable to clear bacteria from kidneys and bladders and developed bacteremia and symptoms of systemic disease, but control mice were fully resistant to infection. The experimental UTI model demonstrated that IL-8R-dependent mechanisms control the urinary tract defense, and that neutrophils are essential host effector cells. Patients prone to acute pyelonephritis also showed low CXC chemokine receptor 1 expression compared with age-matched controls, suggesting that chemokine receptor expression may also influence the susceptibility to UTIs in humans. The results provide a first molecular clue to disease susceptibility of patients prone to acute pyelonephritis.

Regulation of tyrosine kinase activation and granule release through beta-arrestin by CXCRI

Chemoattractant-stimulated granule release from neutrophils, basophils and eosinophils is critical for the innate immune response against infectious bacteria. Interleukin 8 (IL-8) activation of the chemokine receptor CXCRI was found to stimulate rapid formation of beta-arrestin complexes with Hck or c-Fgr. Formation of beta-arrestin-Hck complexes led to Hck activation and trafficking of the complexes to granule-rich regions. Granulocytes expressing a dominant-negative beta-arrestin-mutant did not release granules or activate tyrosine kinases after IL-8 stimulation. Thus, beta-arrestins regulate chemokine-induced granule exocytosis, indicating a broader role for beta-arrestins in the regulation of cellular functions than was previously suspected.