Defective lymphocyte chemotaxis in beta-arrestin2- and GRK6-deficient mice

Role of negative regulation of immune signaling pathways in neutrophil function

Polymorphonuclear neutrophils (PMNs) play a critical role in host defense against infection and in the resolution of inflammation. However, immune responses mediated by PMN must be tightly regulated to facilitate elimination of invading pathogens without inducing detrimental inflammation and host tissue damage. Specific engagement of cell surface immunoreceptors by a diverse range of extracellular signals regulates PMN effector functions through differential activation of intracellular signaling cascades. Although mechanisms of PMN activation mediated via cell signaling pathways have been well described, less is known about negative regulation of PMN function by immune signaling cascades. Here, we provide an overview of immunoreceptor-mediated negative regulation of key PMN effector functions including maturation, migration, phagocytosis, reactive oxygen species release, degranulation, apoptosis, and NET formation. Increased understanding of mechanisms of suppression of PMN effector functions may point to possible future therapeutic targets for the amelioration of PMN-mediated autoimmune and inflammatory diseases.

The Diverse Roles of Arrestin Scaffolds in G Protein-Coupled Receptor Signaling

The visual/β-arrestins, a small family of proteins originally described for their role in the desensitization and intracellular trafficking of G protein-coupled receptors (GPCRs), have emerged as key regulators of multiple signaling pathways. Evolutionarily related to a larger group of regulatory scaffolds that share a common arrestin fold, the visual/β-arrestins acquired the capacity to detect and bind activated GPCRs on the plasma membrane, which enables them to control GPCR desensitization, internalization, and intracellular trafficking. By acting as scaffolds that bind key pathway intermediates, visual/β-arrestins both influence the tonic level of pathway activity in cells and, in some cases, serve as ligand-regulated scaffolds for GPCR-mediated signaling. Growing evidence supports the physiologic and pathophysiologic roles of arrestins and underscores their potential as therapeutic targets. Circumventing arrestin-dependent GPCR desensitization may alleviate the problem of tachyphylaxis to drugs that target GPCRs, and find application in the management of chronic pain, asthma, and psychiatric illness. As signaling scaffolds, arrestins are also central regulators of pathways controlling cell growth, migration, and survival, suggesting that manipulating their scaffolding functions may be beneficial in inflammatory diseases, fibrosis, and cancer. In this review we examine the structure-function relationships that enable arrestins to perform their diverse roles, addressing arrestin structure at the molecular level, the relationship between arrestin conformation and function, and sites of interaction between arrestins, GPCRs, and nonreceptor-binding partners. We conclude with a discussion of arrestins as therapeutic targets and the settings in which manipulating arrestin function might be of clinical benefit.

The role and mechanism of β‑arrestins in cancer invasion and metastasis (Review)

β-arrestins are a family of adaptor proteins that regulate the signaling and trafficking of various G protein-coupled receptors (GPCRs). They consist of β-arrestin1 and β-arrestin2 and are considered to be scaffolding proteins. β-arrestins regulate cell proliferation, promote cell invasion and migration, transmit anti-apoptotic survival signals and affect other characteristics of tumors, including tumor growth rate, angiogenesis, drug resistance, invasion and metastatic potential. It has been demonstrated that β-arrestins serve roles in various physiological and pathological processes and exhibit a similar function to GPCRs. β-arrestins serve primary roles in cancer invasion and metastasis via various signaling pathways. The present review assessed the function and mechanism of β-arrestins in cancer invasion and metastasis via multiple signaling pathways, including mitogen-activated protein kinase/extracellular signal regulated kinase, Wnt/β-catenin, nuclear factor-κB and phosphoinositide-3 kinase/Akt.

Role of G protein-coupled receptor kinase-6 in Escherichia coli lung infection model in mice

G protein-coupled receptor kinase-6 (GRK6) is a serine/threonine kinase that is important in inflammatory processes. In this study, we examined the role of GRK6 in Escherichia coli-induced lung infection and inflammation using GRK6 knockout (KO) and wild-type (WT) mice. Intratracheal instillation of E. coli significantly enhanced bacterial load in the bronchoalveolar lavage (BAL) of KO compared with WT mice. Reduced bacterial clearance in the KO mice was not due to an intrinsic defect in neutrophil phagocytosis or killing but as a result of reduced neutrophil numbers in the KO BAL. Interestingly, neutrophil numbers in the lung were increased in the KO compared with WT mice, suggesting a potential dysfunction in transepithelial migration of neutrophils from the lungs to the bronchoalveolar space. This effect was selective for lung tissue because peritoneal neutrophil numbers were similar between the two genotypes following peritoneal infection. Although neutrophil expression of CXCR2/CXCR3 was similar between WT and KO, IL-17A expression was higher in the KO compared with WT mice. These results suggest that enhanced neutrophil count in the KO lungs but reduced numbers in BAL are likely due to transepithelial migration defect and/or altered chemokines/cytokines. Together, our studies suggest a previously unrecognized and novel role for GRK6 in neutrophil migration specific to pulmonary tissue during bacterial infection.

Pericardial interstitial cell senescence responsible for pericardial structural remodeling in idiopathic and postsurgical constrictive pericarditis

OBJECTIVE

Idiopathic and postsurgical constrictive pericarditis is characterized by pericardial structural remodeling that involves fibrosis, calcification, and inflammation. This study aimed to determine whether cell senescence was responsible for pericardial structural remodeling.

METHODS

Pericardial interstitial cells derived from patients with idiopathic or postsurgical pericarditis (pericarditis cells) were harvested. Timing of senescence and differences in telomere length were compared between age- and sex-matched controls (nonpericarditis cells). Pericardial interstitial cells derived from normal pericardia were serially passaged until senescence (senescent cells). Apoptosis, collagen matrix, calcium deposition, chemoattractant properties, gene expression profiles, and paracrine effects of senescent cells were compared with nonsenescent cells of passage 2 (nonsenescent cells).

RESULTS

Pericarditis cells displayed senescent changes, including short telomere length, large flattened cell sizes, positive staining for senescence-associated β-galactosidase, and limited growth capacity. These senescent cells were resistant to apoptosis, produced more collagen matrix, deposited more calcium, and attracted more monocytes/lymphocytes than the nonsenescent cells. A cluster of genes involved in extracellular matrix deposition (connective tissue growth factor, fibronectin, collagen type I, collagen type III, and tissue inhibitors of metalloproteinase-1), calcium deposition (osteopontin, bone sialoprotein, osteonectin, and matrix Gla protein), and inflammatory cell recruitment (interleukin-6, chemoattractant protein-1, and tumor necrosis factor-α) were upregulated in senescent cells, whereas extracellular matrix-degrading enzyme (metalloproteinase-1 and metalloproteinase-3) was downregulated. Furthermore, senescent cells had the ability to promote the proliferation, differentiation, and senescence of neighboring cells.

CONCLUSIONS

These findings suggest that senescent cells have characteristics promoting pericardial structural remodeling, but further work is needed to establish causation.

ACKR3 expression on diffuse large B cell lymphoma is required for tumor spreading and tissue infiltration

Diffuse large B cell lymphoma (DLBCL) is the most frequent lymphoma accounting for more than the 30% of the cases. Involvement of extranodal sites, such as bone marrow and central nervous system, is associated with poor prognosis. A contribution of the chemokine system in these processes is assumed as it is known as a critical regulator of the metastatic process in cancer. The atypical chemokine receptor 3 (ACKR3), which does not couple to G-proteins and does not mediate cell migration, acts as a scavenger for CXCL11 and CXCL12, interfering with the tumor homing CXCL12/CXCR4 axis. Here, functional expression of ACKR3 in DLBCL cells was necessary for colonization of the draining lymph node in an in vivo subcutaneous lymphoma model. Moreover, in a disseminated in vivo lymphoma model, ACKR3 expression was required for bone marrow and brain invasion and local tumor growth. The present data unveil ACKR3 as potential therapeutic target for the control of tumor dissemination in DLBCL.

Canonical and Noncanonical Signaling Roles of β-Arrestins in Inflammation and Immunity

β-Arrestins are a highly conserved family of cytosolic adaptor proteins that contribute to many immune functions by orchestrating the desensitization and internalization of cell-surface G protein-coupled receptors (GPCRs) via well-studied canonical interactions. In cells of the innate and adaptive immune system, β-arrestins also subserve a parallel but less understood role in which they propagate, rather than terminate, intracellular signal transduction cascades. Because β-arrestins are promiscuous in their binding, they are capable of interacting with several different GPCRs and downstream effectors; in doing so, they vastly expand the repertoire of cellular responses evoked by agonist binding and the scope of responses that may contribute to inflammation during infectious and sterile insults. In this chapter, we attempt to provide an overview of the canonical and noncanonical roles of β-arrestins in inflammatory diseases.

G Protein-Coupled Receptor Kinases in the Inflammatory Response and Signaling

G protein-coupled receptor kinases (GRKs) are serine/threonine kinases that regulate a large and diverse class of G protein-coupled receptors (GPCRs). Through GRK phosphorylation and β-arrestin recruitment, GPCRs are desensitized and their signal terminated. Recent work on these kinases has expanded their role from canonical GPCR regulation to include noncanonical regulation of non-GPCR and nonreceptor substrates through phosphorylation as well as via scaffolding functions. Owing to these and other regulatory roles, GRKs have been shown to play a critical role in the outcome of a variety of physiological and pathophysiological processes including chemotaxis, signaling, migration, inflammatory gene expression, etc. This diverse set of functions for these proteins makes them popular targets for therapeutics. Role for these kinases in inflammation and inflammatory disease is an evolving area of research currently pursued in many laboratories. In this review, we describe the current state of knowledge on various GRKs pertaining to their role in inflammation and inflammatory diseases.

C-X-C Motif Chemokine Receptor 3 Splice Variants Differentially Activate Beta-Arrestins to Regulate Downstream Signaling Pathways

Biased agonism, the ability of different ligands for the same receptor to selectively activate some signaling pathways while blocking others, is now an established paradigm for G protein-coupled receptor signaling. One group of receptors in which endogenous bias is critical is the chemokine system, consisting of over 50 ligands and 20 receptors that bind one another with significant promiscuity. We have previously demonstrated that ligands for the same receptor can cause biased signaling responses. The goal of this study was to identify mechanisms that could underlie biased signaling between different receptor splice variants. The C-X-C motif chemokine receptor 3 (CXCR3) has two splice variants, CXCR3A and CXCR3B, which differ by 51 amino acids at its N-terminus. Consistent with an earlier study, we found that C-X-C motif chemokine ligands 4, 9, 10, and 11 all activated G _αi at CXCR3A, while at CXCR3B these ligands demonstrated no measurable G _αi or G _αs activity. β-arrestin (βarr) was recruited at a reduced level to CXCR3B relative to CXCR3A, which was also associated with differences in βarr2 conformation. βarr2 recruitment to CXCR3A was attenuated by both G protein receptor kinase (GRK) 2/3 and GRK5/6 knockdown, while only GRK2/3 knockdown blunted recruitment to CXCR3B. Extracellular regulated kinase 1/2 phosphorylation downstream from CXCR3A and CXCR3B was increased and decreased, respectively, by βarr1/2 knockout. The splice variants also differentially activated transcriptional reporters. These findings demonstrate that differential splicing of CXCR3 results in biased responses associated with distinct patterns of βarr conformation and recruitment. Differential splicing may serve as a common mechanism for generating biased signaling and provides insights into how chemokine receptor signaling can be modulated post-transcriptionally.

G Protein-Coupled Receptor Kinase 3 and Protein Kinase C Phosphorylate the Distal C-Terminal Tail of the Chemokine Receptor CXCR4 and Mediate Recruitment of β-Arrestin

Phosphorylation of G protein-coupled receptors (GPCRs) is a key event for cell signaling and regulation of receptor function. Previously, using tandem mass spectrometry, we identified two phosphorylation sites at the distal C-terminal tail of the chemokine receptor CXCR4, but were unable to determine which specific residues were phosphorylated. Here, we demonstrate that serines (Ser) 346 and/or 347 (Ser-346/7) of CXCR4 are phosphorylated upon stimulation with the agonist CXCL12 as well as a CXCR4 pepducin, ATI-2341. ATI-2341, a Gα_iβγ heterotrimer-biased CXCR4 agonist, induced more robust phosphorylation of Ser-346/7 compared with CXCL12. Knockdown of G protein-coupled receptor kinase (GRK) 2, GRK3, or GRK6 reduced CXCL12-induced phosphorylation of Ser-346/7 with GRK3 knockdown having the strongest effect, while inhibition of the conventional protein kinase C (PKC) isoforms, particularly PKCα, reduced phosphorylation of Ser-346/7 induced by either CXCL12 or ATI-2341. The loss of GRK3- or PKC-mediated phosphorylation of Ser-346/7 impaired the recruitment of β-arrestin to CXCR4. We also found that a pseudo-substrate peptide inhibitor for PKCζ effectively inhibited CXCR4 phosphorylation and signaling, most likely by functioning as a nonspecific CXCR4 antagonist. Together, these studies demonstrate the role Ser-346/7 plays in arrestin recruitment and initiation of receptor desensitization and provide insight into the dysregulation of CXCR4 observed in patients with various forms of WHIM syndrome.

Regulation of inflammation by β-arrestins: Not just receptor tales

The ubiquitously expressed, multifunctional scaffolding proteins β-arrestin1 and β-arrestin2 each affect inflammatory signaling in a variety of cell lines. In addition to binding the carboxyl-terminal tails of innumerable 7-transmembrane receptors, β-arrestins scaffold untold numbers of other plasma membrane and cytoplasmic proteins. Consequently, the effects of β-arrestins on inflammatory signaling are diverse, and context-specific. This review highlights the roles of β-arrestins in regulating canonical activation of the pro-inflammatory transcription factor NFκB.

β-Arrestin1 and Signal-transducing Adaptor Molecule 1 (STAM1) Cooperate to Promote Focal Adhesion Kinase Autophosphorylation and Chemotaxis via the Chemokine Receptor CXCR4

The chemokine receptor CXCR4 and its chemokine ligand CXCL12 mediate directed cell migration during organogenesis, immune responses, and metastatic disease. However, the mechanisms governing CXCL12/CXCR4-dependent chemotaxis remain poorly understood. Here, we show that the β-arrestin1·signal-transducing adaptor molecule 1 (STAM1) complex, initially identified to govern lysosomal trafficking of CXCR4, also mediates CXCR4-dependent chemotaxis. Expression of minigene fragments from β-arrestin1 or STAM1, known to disrupt the β-arrestin1·STAM1 complex, and RNAi against β-arrestin1 or STAM1, attenuates CXCL12-induced chemotaxis. The β-arrestin1·STAM1 complex is necessary for promoting autophosphorylation of focal adhesion kinase (FAK). FAK is necessary for CXCL12-induced chemotaxis and associates with and localizes with β-arrestin1 and STAM1 in a CXCL12-dependent manner. Our data reveal previously unknown roles in CXCR4-dependent chemotaxis for β-arrestin1 and STAM1, which we propose act in concert to regulate FAK signaling. The β-arrestin1·STAM1 complex is a promising target for blocking CXCR4-promoted FAK autophosphorylation and chemotaxis.

Exploiting agonist biased signaling of chemokines to target cancer

As knowledge of growth-independent functions of cancer cells is expanding, exploration into the role of chemokines in modulating cancer pathogenesis, particularly metastasis, continues to develop. However, more study into the mechanisms whereby chemokines direct the migration of cancer cells is needed before specific therapies can be generated to target metastasis. Herein, we draw attention to the longstanding conundrum in the field of chemokine biology that chemokines stimulate migration in a biphasic manner; and explore this phenomenon's impact on chemokine function in the context of cancer. Typically, low concentrations of chemokines lead to chemotactic migration and higher concentrations halt migration. The signaling mechanisms that govern this phenomenon remain unclear. Over the last decade, we have defined a novel signaling mechanism for regulation of chemokine migration through ligand oligomerization and biased agonist signaling. We provide insight into this new paradigm for chemokine signaling and discuss how it will impact future exploration into chemokine function and biology. In the pursuit of producing more novel cancer therapies, we suggest a framework for pharmaceutical application of the principles of chemokine oligomerization and biased agonist signaling in cancer. © 2016 Wiley Periodicals, Inc.

β-Arrestin-mediated Angiotensin II Signaling Controls the Activation of ARF6 Protein and Endocytosis in Migration of Vascular Smooth Muscle Cells

Angiotensin II (Ang II) is a vasopressive hormone but is also a potent activator of cellular migration. We have previously shown that it can promote the activation of the GTPase ARF6 in a heterologous overexpressing system. The molecular mechanisms by which receptors control the activation of this small G protein remain, however, largely unknown. Furthermore, how ARF6 coordinates the activation of complex cellular responses needs to be further elucidated. In this study, we demonstrate that Ang II receptors engage β-arrestin, but not Gq, to mediate ARF6 activation in HEK 293 cells. To further confirm the key role of β-arrestin proteins, we overexpressed β-arrestin2-(1-320), a dominant negative mutant known to block receptor endocytosis. We show that expression of this truncated construct does not support the activation of the GTPase nor cell migration. Interestingly, β-arrestin2 can interact with the ARF guanine nucleotide exchange factor ARNO, although the C-terminally lacking mutant does not. We finally examined whether receptor endocytosis controlled ARF6 activation and cell migration. Although the clathrin inhibitor PitStop2 did not impact the ability of Ang II to activate ARF6, cell migration was markedly impaired. To further show that ARF activation regulates key signaling events leading to migration, we also examined MAPK activation. We demonstrate that this signaling axis is relevant in smooth muscle cells of the vasculature. Altogether, our findings show for the first time that Ang II receptor signaling to β-arrestin regulates ARF6 activation. These proteins together control receptor endocytosis and ultimately cell migration.

Multifaceted role of β-arrestins in inflammation and disease

Arrestins are intracellular scaffolding proteins known to regulate a range of biochemical processes including G protein-coupled receptor (GPCR) desensitization, signal attenuation, receptor turnover and downstream signaling cascades. Their roles in regulation of signaling network have lately been extended to receptors outside of the GPCR family, demonstrating their roles as important scaffolding proteins in various physiological processes including proliferation, differentiation and apoptosis. Recent studies have demonstrated a critical role for arrestins in immunological processes including key functions in inflammatory signaling pathways. In this review, we provide a comprehensive analysis of the different functions of the arrestin family of proteins especially related to immunity and inflammatory diseases.

Probing Biased Signaling in Chemokine Receptors

The chemokine system mediates leukocyte migration during homeostatic and inflammatory processes. Traditionally, it is described as redundant and promiscuous, with a single chemokine ligand binding to different receptors and a single receptor having several ligands. Signaling of chemokine receptors occurs via two major routes, G protein- and β-arrestin-dependent, which can be preferentially modulated depending on the ligands or receptors involved, as well as the cell types or tissues in which the signaling event occurs. The preferential activation of a certain signaling pathway to the detriment of others has been termed signaling bias and can accordingly be grouped into ligand bias, receptor bias, and tissue bias. Bias has so far been broadly overlooked in the process of drug development. The low number of currently approved drugs targeting the chemokine system, as well as the broad range of failed clinical trials, reflects the need for a better understanding of the chemokine system. Thus, understanding the character, direction, and consequence of biased signaling in the chemokine system may aid the development of new therapeutics. This review describes experiments to assess G protein-dependent and -independent signaling in order to quantify chemokine system bias.

Targeting chemokine receptors in disease--a case study of CCR4

Since their early 1990s, the chemokine receptor family of G protein-coupled receptors (GPCRs) has been the source of much pharmacological endeavour. Best known for their key roles in recruiting leukocytes to sites of infection and inflammation, the receptors present themselves as plausible drug targets for therapeutic intervention. In this article, we will focus our attention upon CC Chemokine Receptor Four (CCR4) which has been implicated in diseases as diverse as allergic asthma and lymphoma. We will review the discovery of the receptors and their ligands, their perceived roles in disease and the successful targeting of CCR4 by both small molecule antagonists and monoclonal antibodies. We will also discuss future directions and strategies for drug discovery in this field.

Process of hepatic metastasis from pancreatic cancer: biology with clinical significance

PURPOSE

Pancreatic cancer shows a remarkable preference for the liver to establish secondary tumors. Selective metastasis to the liver is attributed to the development of potential microenvironment for the survival of pancreatic cancer cells. This review aims to provide a full understanding of the hepatic metastatic process from circulating pancreatic cancer cells to their settlement in the liver, serving as a basic theory for efficient prediction and treatment of metastatic diseases.

METHODS

A systematic search of relevant original articles and reviews was performed on PubMed, EMBASE and Cochrane Library for the purpose of this review.

RESULTS

Three interrelated phases are delineated as the contributions of the interaction between pancreatic cancer cells and the liver to hepatic metastasis process. Chemotaxis of disseminated pancreatic cancer cells and simultaneous defensive formation of platelets or neutrophils facilitate specific metastasis toward the liver. Remodeling of extracellular matrix and stromal cells in hepatic lobules and angiogenesis induced by proangiogenic factors support the survival and growth of clinical micrometastasis colonizing the liver. The bimodal role of the immune system or prevalence of cancer cells over the immune system makes metastatic progression successfully proceed from micrometastasis to macrometastasis.

CONCLUSIONS

Pancreatic cancer is an appropriate research object of cancer metastasis representing more than a straight cascade. If any of the successive or simultaneous phases, especially tumor-induced immunosuppression, is totally disrupted, hepatic metastasis will be temporarily under control or even cancelled forever. To shrink cancers on multiple fronts and prolong survival for patients, novel oral or intravenous anti-cancer agents covering one or different phases of metastatic pancreatic cancer are expected to be integrated into innovative strategies on the premise of safety and efficacious biostability.

G-protein-coupled receptor kinases in inflammation and disease

G-protein-coupled receptor kinases (GRKs) are serine/threonine protein kinases originally discovered for their role in G-protein-coupled receptor (GPCR) phosphorylation. Recent studies have demonstrated a much broader function for this kinase family including phosphorylation of cytosolic substrates involved in cell signaling pathways stimulated by GPCRs, as well as by non-GPCRs. In addition, GRKs modulate signaling via phosphorylation-independent functions. Because of these various biochemical functions, GRKs have been shown to affect critical physiological and pathophysiological processes, and thus are considered as drug targets in diseases such as heart failure. Role of GRKs in inflammation and inflammatory diseases is an evolving area of research and several studies including work from our lab in the recent years have demonstrated critical role of GRKs in the immune system. In this review, we discuss the classical and the newly emerging functions of GRKs in the immune system and their role in inflammation and disease processes.

Novel Human Cytomegalovirus Viral Chemokines, vCXCL-1s, Display Functional Selectivity for Neutrophil Signaling and Function

Human CMV (HCMV) uses members of the hematopoietic system including neutrophils for dissemination throughout the body. HCMV encodes a viral chemokine, vCXCL-1, that is postulated to attract neutrophils for dissemination within the host. The gene encoding vCXCL-1, UL146, is one of the most variable genes in the HCMV genome. Why HCMV has evolved this hypervariability and how this affects the virus' dissemination and pathogenesis is unknown. Because the vCXCL-1 hypervariability maps to important binding and activation domains, we hypothesized that vCXCL-1s differentially activate neutrophils, which could contribute to HCMV dissemination, pathogenesis, or both. To test whether these viral chemokines affect neutrophil function, we generated vCXCL-1 proteins from 11 different clades from clinical isolates from infants infected congenitally with HCMV. All vCXCL-1s were able to induce calcium flux at a concentration of 100 nM and integrin expression on human peripheral blood neutrophils, despite differences in affinity for the CXCR1 and CXCR2 receptors. In fact, their affinity for CXCR1 or CXCR2 did not correlate directly with chemotaxis, G protein-dependent and independent (β-arrestin-2) activation, or secondary chemokine (CCL22) expression. Our data suggest that vCXCL-1 polymorphisms affect the binding affinity, receptor usage, and differential peripheral blood neutrophil activation that could contribute to HCMV dissemination and pathogenesis.

Cell, isoform, and environment factors shape gradients and modulate chemotaxis

Chemokine gradient formation requires multiple processes that include ligand secretion and diffusion, receptor binding and internalization, and immobilization of ligand to surfaces. To understand how these events dynamically shape gradients and influence ensuing cell chemotaxis, we built a multi-scale hybrid agent-based model linking gradient formation, cell responses, and receptor-level information. The CXCL12/CXCR4/CXCR7 signaling axis is highly implicated in metastasis of many cancers. We model CXCL12 gradient formation as it is impacted by CXCR4 and CXCR7, with particular focus on the three most highly expressed isoforms of CXCL12. We trained and validated our model using data from an in vitro microfluidic source-sink device. Our simulations demonstrate how isoform differences on the molecular level affect gradient formation and cell responses. We determine that ligand properties specific to CXCL12 isoforms (binding to the migration surface and to CXCR4) significantly impact migration and explain differences in in vitro chemotaxis data. We extend our model to analyze CXCL12 gradient formation in a tumor environment and find that short distance, steep gradients characteristic of the CXCL12-γ isoform are effective at driving chemotaxis. We highlight the importance of CXCL12-γ in cancer cell migration: its high effective affinity for both extracellular surface sites and CXCR4 strongly promote CXCR4+ cell migration. CXCL12-γ is also more difficult to inhibit, and we predict that co-inhibition of CXCR4 and CXCR7 is necessary to effectively hinder CXCL12-γ-induced migration. These findings support the growing importance of understanding differences in protein isoforms, and in particular their implications for cancer treatment.

The evolving impact of g protein-coupled receptor kinases in cardiac health and disease

G protein-coupled receptors (GPCRs) are important regulators of various cellular functions via activation of intracellular signaling events. Active GPCR signaling is shut down by GPCR kinases (GRKs) and subsequent β-arrestin-mediated mechanisms including phosphorylation, internalization, and either receptor degradation or resensitization. The seven-member GRK family varies in their structural composition, cellular localization, function, and mechanism of action (see sect. II). Here, we focus our attention on GRKs in particular canonical and novel roles of the GRKs found in the cardiovascular system (see sects. III and IV). Paramount to overall cardiac function is GPCR-mediated signaling provided by the adrenergic system. Overstimulation of the adrenergic system has been highly implicated in various etiologies of cardiovascular disease including hypertension and heart failure. GRKs acting downstream of heightened adrenergic signaling appear to be key players in cardiac homeostasis and disease progression, and herein we review the current data on GRKs related to cardiac disease and discuss their potential in the development of novel therapeutic strategies in cardiac diseases including heart failure.

CXCL13 responsiveness but not CXCR5 expression by late transitional B cells initiates splenic white pulp formation

Secondary lymphoid organs (SLO) provide the structural framework for coconcentration of Ag and Ag-specific lymphocytes required for an efficient adaptive immune system. The spleen is the primordial SLO, and evolved concurrently with Ig/TCR:pMHC-based adaptive immunity. The earliest cellular/histological event in the ontogeny of the spleen's lymphoid architecture, the white pulp (WP), is the accumulation of B cells around splenic vasculature, an evolutionarily conserved feature since the spleen's emergence in early jawed vertebrates such as sharks. In mammals, B cells are indispensable for both formation and maintenance of SLO microarchitecture; their expression of lymphotoxin α1β2 (LTα1β2) is required for the LTα1β2:CXCL13 positive feedback loop without which SLO cannot properly form. Despite the spleen's central role in the evolution of adaptive immunity, neither the initiating event nor the B cell subset necessary for WP formation has been identified. We therefore sought to identify both in mouse. We detected CXCL13 protein in late embryonic splenic vasculature, and its expression was TNF-α and RAG-2 independent. A substantial influx of CXCR5(+) transitional B cells into the spleen occurred 18 h before birth. However, these late embryonic B cells were unresponsive to CXCL13 (although responsive to CXCL12) and phenotypically indistinguishable from blood-derived B cells. Only after birth did B cells acquire CXCL13 responsiveness, accumulate around splenic vasculature, and establish the uniquely splenic B cell compartment, enriched for CXCL13-responsive late transitional cells. Thus, CXCL13 is the initiating component of the CXCL13:LTα1β2 positive feedback loop required for WP ontogeny, and CXCL13-responsive late transitional B cells are the initiating subset.

Functional consequences of perturbed CXCL12 signal processing: analyses of immature hematopoiesis in GRK6-deficient mice

Hematopoietic stem and progenitor cells (HSPCs) reside in bone marrow (BM) in an environment rich in CXCL12, the ligand for CXCR4, which is constitutively expressed on all immature hematopoietic cells in BM. This ligand-receptor pair critically controls HSPC retention and (relative) quiescence in BM. Interestingly, in a chemokine-abundant environment, CXCR4 surface expression and CXCL12 sensitivity of BM-residing HSPCs are continuously maintained. The mechanisms underlying this peculiar pattern of G-protein signal integration by BM-HSPCs are unknown. G-protein receptor kinases (GRKs) control receptor function by phosphorylating the intracellular domains upon ligand-induced activation, which results in receptor internalization and transient refractoriness. Using, therefore, a GRK6-deficient (GRK6(-/-)) mouse, we sought to address how perturbed ligand-induced CXCR4 (in)activation affects HSPC behavior in vitro and in vivo. In vitro, GRK6(-/-) HSPCs were characterized by hyper-responsiveness to CXCL12, as expected. In vivo, GRK6(-/-) immature hematopoiesis was characterized by a marked expansion of immature hematopoiesis in spleens and a modest repopulation defect in serial competitive transplantation. Enforced mobilization with granulocyte colony-stimulating factor (G-CSF) and AMD3100 was normal, as was hematopoietic regeneration after noncompetitive transplantation or pharmacological myelosuppression. These observations illustrate that GRK-mediated restriction of CXCR4 signal input after ligand engagement is largely dispensable for BM-resident HSPCs, which may explain how continuous CXCL12 responsiveness of BM-HSPCs can be maintained.

β-arrestin 2 inhibits proinflammatory chemokine production and attenuates contact allergic inflammation in the skin

β-Arrestins participate in G-protein receptor signaling and act as adapter proteins that direct the recruitment, activation, and scaffolding of various cytoplasmic signaling complexes. β-Arrestin 2-deficient (Arrb2(-/-)) mice show decreased T-cell recruitment into allergic lung tissue but increased neutrophil infiltration into wounded skin. Given these opposing effects in different immune cell subsets, we investigated the role of β-arrestin 2 in the regulation of contact hypersensitivity responses. We observed significantly increased allergic ear swelling to the obligate contact sensitizers DNFB and FITC in Arrb2(-/-) compared with wild-type mice. Immunohistological analyses revealed strikingly increased neutrophil infiltration with abundant subcorneal pustules in inflamed ear tissue of DNFB-allergic Arrb2(-/-) mice. Experiments involving adoptive transfers of sensitized lymphocytes and bone marrow chimeric mice indicated that β-arrestin 2 exerts its anti-inflammatory effects predominantly through radioresistant, skin-resident cells in the challenge phase of contact hypersensitivity. As a potential mechanism, we found that primary cultures of β-arrestin 2-deficient keratinocytes secreted higher levels of neutrophil-attracting chemokines including CXCL1/KC in response to T cell-derived cytokines in vitro. These experimental results support a model in which β-arrestin 2 inhibits the production of proinflammatory chemokines, which limits the recruitment of myeloid immune cells and thereby attenuates allergic skin inflammation.

Arrestin-dependent activation of ERK and Src family kinases

The four members of the mammalian arrestin family, two visual and two nonvisual, share the property of stimulus-dependent docking to G protein-coupled receptors. This conformational selectivity permits them to function in receptor desensitization, as arrestin binding sterically inhibits G protein coupling. The two nonvisual arrestins further act as adapter proteins, linking receptors to the clathrin-dependent endocytic machinery and regulating receptor sequestration, intracellular trafficking, recycling, and degradation. Arrestins also function as ligand-regulated scaffolds, recruiting catalytically active proteins into receptor-based multiprotein "signalsome" complexes. Arrestin binding thus marks the transition from a transient G protein-coupled state on the plasma membrane to a persistent arrestin-coupled state that continues to signal as the receptor internalizes. Two of the earliest discovered and most studied arrestin-dependent signaling pathways involve regulation of Src family nonreceptor tyrosine kinases and the ERK1/2 mitogen-activated kinase cascade. In each case, arrestin scaffolding imposes constraints on kinase activity that dictate signal duration and substrate specificity. Evidence suggests that arrestin-bound ERK1/2 and Src not only play regulatory roles in receptor desensitization and trafficking but also mediate longer term effects on cell growth, migration, proliferation, and survival.

Molecular mechanisms underlying beta-arrestin-dependent chemotaxis and actin-cytoskeletal reorganization

β-Arrestins play a crucial role in cell migration downstream of multiple G-protein-coupled receptors (GPCRs) through multiple mechanisms. There is considerable evidence that β-arrestin-dependent scaffolding of actin assembly proteins facilitates the formation of a leading edge in response to a chemotactic signal. Conversely, there is substantial support for the hypothesis that β-arrestins facilitate receptor turnover through their ability to desensitize and internalize GPCRs. This chapter discusses both theories for β-arrestin-dependent chemotaxis in the context of recent studies, specifically addressing known actin assembly proteins regulated by β-arrestins, chemokine receptors, and signaling by chemotactic receptors.

G protein-coupled receptor kinase-3-deficient mice exhibit WHIM syndrome features and attenuated inflammatory responses

Chemokine receptor interactions coordinate leukocyte migration in inflammation. Chemokine receptors are GPCRs that when activated, are phosphorylated by GRKs to turn off G protein-mediated signaling yet recruit additional signaling machinery. Recently, GRK3 was identified as a negative regulator of CXCL12/CXCR4 signaling that is defective in human WHIM syndrome. Here, we report that GRK3-/- mice exhibit numerous features of human WHIM, such as impaired CXCL12-mediated desensitization, enhanced CXCR4 signaling to ERK activation, altered granulocyte migration, and a mild myelokathexis. Moreover, GRK3-/- protects mice from two acute models of inflammatory arthritis (K/BxN serum transfer and CAIA). In these granulocyte-dependent disease models, protection of GRK3-/- mice is mediated by retention of cells in the marrow, fewer circulating granulocytes in the peripheral blood, and reduced granulocytes in the joints during active inflammation. In contrast to WHIM, GRK3-/- mice have minimal hypogammaglobulinemia and a peripheral leukocytosis with increased lymphocytes and absent neutropenia. Thus, we conclude that the loss of GRK3-mediated regulation of CXCL12/CXCR4 signaling contributes to some, but not all, of the complete WHIM phenotype and that GRK3 inhibition may be beneficial in the treatment of inflammatory arthritis.

Role of G protein-coupled receptor kinases in cell migration

G protein-coupled receptor kinases (GRKs) are emerging as important integrative nodes in cell migration processes. Recent evidence links GRKs (particularly the GRK2 isoform) to the complex modulation of diverse aspects of cell motility. In addition to its well-established role in the desensitization of G protein-coupled receptors involved in chemotaxis, GRK2 can play an effector role in the organization of actin and microtubule networks and in adhesion dynamics, by means of novel substrates and transient interacting partners, such as the GIT1 scaffold or the cytoplasmic α-tubulin deacetylase histone deacetylase 6 (HDAC6). The overall effect of altering GRK levels or activity on chemotaxis would depend on how such different roles are integrated in a given cell type and physiological context, and may have relevant implications in inflammatory diseases or cancer progression.

Biased agonism as a mechanism for differential signaling by chemokine receptors

Chemokines display considerable promiscuity with multiple ligands and receptors shared in common, a phenomenon that is thought to underlie their biochemical "redundancy." Their receptors are part of a larger seven-transmembrane receptor superfamily, commonly referred to as G protein-coupled receptors, which have been demonstrated to be able to signal with different efficacies to their multiple downstream signaling pathways, a phenomenon referred to as biased agonism. Biased agonism has been primarily reported as a phenomenon of synthetic ligands, and the biologic prevalence and importance of such signaling are unclear. Here, to assess the presence of biased agonism that may underlie differential signaling by chemokines targeting the same receptor, we performed a detailed pharmacologic analysis of a set of chemokine receptors with multiple endogenous ligands using assays for G protein signaling, β-arrestin recruitment, and receptor internalization. We found that chemokines targeting the same receptor can display marked differences in their efficacies for G protein- or β-arrestin-mediated signaling or receptor internalization. This ligand bias correlates with changes in leukocyte migration, consistent with different mechanisms underlying the signaling downstream of these receptors induced by their ligands. These findings demonstrate that biased agonism is a common and likely evolutionarily conserved biological mechanism for generating qualitatively distinct patterns of signaling via the same receptor in response to different endogenous ligands.

β-Arrestins 1 and 2 are critical regulators of inflammation

β-Arrestins 1 and 2 couple to seven trans-membrane receptors and regulate G protein-dependent signaling, receptor endocytosis and ubiquitylation. Recent studies have uncovered several unanticipated functions of β-arrestins, suggesting that the role of β-arrestins in cell signaling is much broader than originally thought. It is now recognized that β-arrestins can transduce receptor signaling independent of G proteins. The expression of β-arrestins is differentially regulated in immune cells and tissues in response to specific inflammatory stimuli, and β-arrestins are critical regulators of the inflammatory response. This review will focus on β-arrestins in immune cells and the impact of altered expression on the pathogenesis of specific inflammatory diseases. Understanding the role of β-arrestins in inflammation may lead to new strategies to treat inflammatory diseases, such as sepsis, rheumatoid arthritis, asthma, multiple sclerosis, inflammatory bowel disease and atherosclerosis.

GRK6 expression in patients with hepatocellular carcinoma

OBJECTIVE

To investigate the expression and potential roles of G protein-coupled receptor kinase 6 (GRK6) in hepatocellular carcinoma (HCC) patients.

METHODS

Immunohistochemistry and Western blot was performed to determine GRK6 expression in 73 HCC samples. And the correlation with clinicopathological features was also analyzed.

RESULTS

GRK6 expression was significantly higher in HCC than that in normal hepatic tissue. GRK6 was positively correlated with proliferation marker Ki-67, clinical stage, metastasis and survival time.

CONCLUSIONS

Our results suggested that GRK6 overexpression plays an important role in HCC. Monitoring the expression of GRK6 maybe helpful in early diagnosis and prognosis of HCC.

Phosphorylation of chemoattractant receptors regulates chemotaxis, actin reorganization and signal relay

Migratory cells, including mammalian leukocytes and Dictyostelium, use G-protein-coupled receptor (GPCR) signaling to regulate MAPK/ERK, PI3K, TORC2/AKT, adenylyl cyclase and actin polymerization, which collectively direct chemotaxis. Upon ligand binding, mammalian GPCRs are phosphorylated at cytoplasmic residues, uncoupling G-protein pathways, but activating other pathways. However, connections between GPCR phosphorylation and chemotaxis are unclear. In developing Dictyostelium, secreted cAMP serves as a chemoattractant, with extracellular cAMP propagated as oscillating waves to ensure directional migratory signals. cAMP oscillations derive from transient excitatory responses of adenylyl cyclase, which then rapidly adapts. We have studied chemotactic signaling in Dictyostelium that express non-phosphorylatable cAMP receptors and show through chemotaxis modeling, single-cell FRET imaging, pure and chimeric population wavelet quantification, biochemical analyses and TIRF microscopy, that receptor phosphorylation is required to regulate adenylyl cyclase adaptation, long-range oscillatory cAMP wave production and cytoskeletal actin response. Phosphorylation defects thus promote hyperactive actin polymerization at the cell periphery, misdirected pseudopodia and the loss of directional chemotaxis. Our data indicate that chemoattractant receptor phosphorylation is required to co-regulate essential pathways for migratory cell polarization and chemotaxis. Our results significantly extend the understanding of the function of GPCR phosphorylation, providing strong evidence that this evolutionarily conserved mechanism is required in a signal attenuation pathway that is necessary to maintain persistent directional movement of Dictyostelium, neutrophils and other migratory cells.

β-arrestin2 in infiltrated macrophages inhibits excessive inflammation after myocardial infarction

Beta-arrestins (β-arrestin1 and β-arrestin2) are known as cytosolic proteins that mediate desensitization and internalization of activated G protein-coupled receptors. In addition to these functions, β-arrestins have been found to work as adaptor proteins for intracellular signaling pathways. β-arrestin1 and β-arrestin2 are expressed in the heart and are reported to participate in normal cardiac function. However, the physiological and pathological roles of β-arrestin1/2 in myocardial infarction (MI) have not been examined. Here, we demonstrate that β-arrestin2 negatively regulates inflammatory responses of macrophages recruited to the infarct area. β-arrestin2 knockout (KO) mice have higher mortality than wild-type (WT) mice after MI. In infarcted hearts, β-arrestin2 was strongly expressed in infiltrated macrophages. The production of inflammatory cytokines was enhanced in β-arrestin2 KO mice. In addition, p65 phosphorylation in the macrophages from the infarcted hearts of β-arrestin2 KO mice was increased in comparison to that of WT mice. These results suggest that the infiltrated macrophages of β-arrestin2 KO mice induce excessive inflammation at the infarct area. Furthermore, the inflammation in WT mice transplanted with bone marrow cells of β-arrestin2 KO mice is enhanced by MI, which is similar to that in β-arrestin2 KO mice. In contrast, the inflammation after MI in β-arrestin2 KO mice transplanted with bone marrow cells of WT mice is comparable to that in WT mice transplanted with bone marrow cells of WT mice. In summary, our present study demonstrates that β-arrestin2 of infiltrated macrophages negatively regulates inflammation in infarcted hearts, thereby enhancing inflammation when the β-arrestin2 gene is knocked out. β-arrestin2 plays a protective role in MI-induced inflammation.

Gene dosage-dependent negative regulatory role of β-arrestin-2 in polymicrobial infection-induced inflammation

β-arrestin-2 (β-arr2) is a scaffolding protein of the arrestin family with a wide variety of cellular functions. Recent studies have demonstrated differential roles for β-arr2 in inflammation following endotoxemia and cecal ligation and puncture (CLP) models of sepsis. Because CLP-induced inflammation involves response to fecal contents and necrotic cecum in addition to microbial challenge, in this study, we examined the role of β-arr2 in an exclusively polymicrobial infection (PMI) model. In addition, we examined the role of gene dosage of β-arr2 in polymicrobial sepsis. Our studies demonstrate that β-arr2 is a negative regulator of systemic inflammation in response to polymicrobial infection and that one allele is sufficient for this process. Our results further reveal that loss of β-arr2 leads to increased neutrophil sequestration and overt inflammation specifically in the lungs following polymicrobial infection. Consistent with this, specific NF-κB and mitogen-activated protein kinase (MAPK) signaling pathways were differentially activated in the β-arr2 knockout (KO) mice lungs compared to the wild type (WT) following PMI. Associated with enhanced inflammation in the KO mice, PMI-induced mortality was also significantly higher in KO mice than in WT mice. To understand the differential role of β-arr2 in different sepsis models, we used cell culture systems to evaluate inflammatory cytokine production following endotoxin and polymicrobial stimulation. Our results demonstrate cell-type- as well as stimulus-specific roles for β-arr2 in inflammation. Taken together, our results reveal a negative regulatory role for β-arr2 in polymicrobial infection-induced inflammation and further demonstrate that one allele of β-arr2 is sufficient to mediate most of these effects.

Hierarchical organization of multi-site phosphorylation at the CXCR4 C terminus

The chemokine receptor CXCR4 regulates cell migration during ontogenesis and disease states including cancer and inflammation. Upon stimulation by the endogenous ligand CXCL12, CXCR4 becomes phosphorylated at multiple sites in its C-terminal domain. Mutations in the CXCR4 gene affecting C-terminal phosphorylation sites are a hallmark of WHIM syndrome, a genetic disorder characterized by a gain-of-CXCR4-function. To better understand how multi-site phosphorylation of CXCR4 is organized and how perturbed phosphorylation might affect CXCR4 function, we developed novel phosphosite-specific CXCR4 antibodies and studied the differential regulation and interaction of three C-terminal phosphorylation sites in human embryonic kidney cells (HEK293). CXCL12 promoted a robust phosphorylation at S346/347 which preceded phosphorylation at S324/325 and S338/339. After CXCL12 washout, the phosphosites S338/339 and S324/325 were rapidly dephosphorylated whereas phosphorylation at S346/347 was long-lasting. CXCL12-induced phosphorylation at S346/347 was staurosporine-insensitive and mediated by GRK2/3. WHIM syndrome-associated CXCR4 truncation mutants lacking the S346/347 phosphosite and the recently identified E343K WHIM mutant displayed strongly impaired phosphorylation at S324/325 and S338/339 as well as reduced CXCL12-induced receptor internalization. Relevance of the S346-S348 site was confirmed by a S346-348A mutant showing strongly impaired CXCL12-promoted phosphorylation at S324/325 and S338/339, defective internalization, gain of calcium mobilization, and reduced desensitization. Thus, the triple serine motif S346-S348 contains a major initial CXCR4 phosphorylation site and is required for efficient subsequent multi-site phosphorylation and receptor regulation. Hierarchical organization of CXCR4 phosphorylation explains why small deletions at the extreme CXCR4 C terminus typically associated with WHIM syndrome severely alter CXCR4 function.

G protein-coupled receptor kinase 6 deficiency promotes angiogenesis, tumor progression, and metastasis

G protein-coupled receptor kinases (GRKs) phosphorylate the activated form of G protein-coupled receptors leading to receptor desensitization and downregulation. We have recently shown that the chemokine receptor, CXCR2, couples to GRK6 to regulate cellular responses including chemotaxis, angiogenesis, and wound healing. In this study, we investigate the role of GRK6 in tumorigenesis using murine models of human lung cancer. Mice deficient in GRK6 (GRK6(-/-)) exhibited a significant increase in Lewis lung cancer growth and metastasis relative to control littermates (GRK6(+/+)). GRK6 deletion had no effect on the expression of proangiogenic chemokine or vascular endothelial growth factor, but upregulated matrix metalloproteinase (MMP)-2 and MMP-9 release, tumor-infiltrating PMNs, and microvessel density. Because β-arrestin-2-deficient (βarr2(-/-)) mice exhibited increased Lewis lung cancer growth and metastasis similar to that of GRK6(-/-), we developed a double GRK6(-/-)/βarr2(-/-) mouse model. Surprisingly, GRK6(-/-)/βarr2(-/-) mice exhibited faster tumor growth relative to GRK6(-/-) or βarr2(-/-) mice. Treatment of the mice with anti-CXCR2 Ab inhibited tumor growth in both GRK6(-/-) and GRK6(-/-)/βarr2(-/-) animals. Altogether, the results indicate that CXCR2 couples to GRK6 to regulate angiogenesis, tumor progression, and metastasis. Deletion of GRK6 increases the activity of the host CXCR2, resulting in greater PMN infiltration and MMP release in the tumor microenvironment, thereby promoting angiogenesis and metastasis. Because GRK6(-/-)/βarr2(-/-) showed greater tumor growth relative to GRK6(-/-) or βarr2(-/-) mice, the data further suggest that CXCR2 couples to different mechanisms to mediate tumor progression and metastasis.

Growth factor receptor-Src-mediated suppression of GRK6 dysregulates CXCR4 signaling and promotes medulloblastoma migration

BACKGROUND

Metastasis in medulloblastoma (MB) is associated with poor survival. Recent genetic studies revealed MB to comprise distinct molecular subgroups, including the sonic hedgehog (SHH) subgroup that exhibits a relatively high rate of progression. To identify targeted therapeutics against metastasis, a better understanding of the regulation of MB cell migration is needed. G protein-coupled receptor kinases (GRKs) have been implicated in cancer metastasis through their regulation of G-protein coupled receptors (GPCRs) involved in growth factor (GF)-mediated cell migration. However, the specific roles and regulation of GRKs in MB have not been investigated.

METHODS

Microarray mRNA analysis was performed for GRKs, GPCRs, and GFs in 29 human MB, and real time RT-PCR was used to detect GRK6 expression in MB cells. Lenti- or retro-virus infection, and siRNA or shRNA transfection, of MB cells was used to overexpress and knockdown target genes, respectively. Western blot was used to confirm altered expression of proteins. The effect of altered target protein on cell migration was determined by Boyden chamber assay and xCELLigence migration assays.

RESULTS

We observed co-overexpression of PDGFRA, CXCR4, and CXCL12 in the SHH MB subtype compared to non-SHH MB (5, 7, and 5-fold higher, respectively). GRK6, which typically acts as a negative regulator of CXCR4 signaling, is downregulated in MB, relative to other GRKs, while the percentage of GRK6 expression is lower in MB tumors with metastasis (22%), compared to those without metastasis (43%). In SHH-responsive MB cells, functional blockade of PDGFR abolished CXCR4-mediated signaling. shPDGFR transfected MB cells demonstrated increased GRK6 expression, while PDGF or 10% FBS treatment of native MB cells reduced the stability of GRK6 by inducing its proteosomal degradation. Overexpression or downregulation of Src, a key mediator of GF receptor/PDGFR signaling, similarly inhibited or induced GRK6 expression, respectively. siRNA downregulation of GRK6 enhanced CXCR4 signaling and promoted MB migration, while lentiviral-GRK6 overexpression suppressed CXCR4 signaling, potentiated the effect of AMD3100, a CXCR4 antagonist, and impaired migration.

CONCLUSIONS

Our findings demonstrate a novel mechanism of GF receptor/PDGFR-Src-mediated dysregulation of CXCR4 signaling that promotes MB cell migration, which could potentially be exploited for therapeutic targeting in SHH MB.

Grk2 is an essential regulator of CXCR7 signalling in astrocytes

We previously demonstrated that in astrocytes, SDF-1/CXCL12 exclusively signals through CXCR7 despite the additional presence of the alternate SDF-1/CXCL12 receptor, CXCR4. In addition, we provided evidence that astrocytic CXCR7-signalling involves a G protein-dependent mechanism. This is insofar remarkable as in all other cell types studied to date, CXCR7 either acts as a scavenger chemokine receptor, a modulator of CXCR4, or a non-classical chemokine receptor, signalling through ß-arrestin. To begin to unravel the molecular framework impinging the selective function of CXCR7 on a given cell type, we have now analysed the role of G protein-coupled receptor kinases (Grks) in astrocytic CXCR7 signalling. We demonstrate that Grk2 mediates signalling of SDF-1/CXCL12-bound CXCR7 as suggested by the finding that SDF-1/CXCL12-induced activation of Erk1/2 and Akt is abrogated following RNAi-mediated inhibition of Grk2, but not of Grk3, Grk5, or Grk6. We further unravel that Grk2 additionally controls signalling of SDF-1/CXCL12-bound CXCR7 in astrocytes by mediating internalization and subsequent silencing of CXCR7. Finally, we demonstrate that Grk2 is likewise expressed by microglial cells and Schwann cells, cell types in which CXCR7 does not act as a classical chemokine receptor. In conclusion, our findings establish that Grk2 tightly controls CXCR7 signalling in astrocytes, but does not imprint the cell type-specific function of this chemokine receptor.

β-Arrestins in the immune system

β-Arrestins regulate G protein-coupled receptors through receptor desensitization while also acting as signaling scaffolds to facilitate numerous effector pathways. Recent studies have provided evidence that β-arrestins play a key role in inflammatory responses. Here, we summarize these advances on the roles of β-arrestins in immune regulation and inflammatory responses under physiological and pathological conditions, with an emphasis on translational implications of β-arrestins on human diseases.

Arrestins as regulators of kinases and phosphatases

The discovery that, in addition to mediating G protein-coupled receptor (GPCR) desensitization and endocytosis, arrestins bind to diverse catalytically active nonreceptor proteins and act as ligand-regulated signaling scaffolds led to a paradigm shift in the study of GPCR signal transduction. Research over the past decade has solidified the concept that arrestins confer novel GPCR-signaling capacity by recruiting protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into receptor-based multiprotein "signalsome" complexes. Signalsomes regulate downstream pathways controlled by Src family nonreceptor tyrosine kinases, mitogen-activated protein kinases, protein kinase B (AKT), glycogen synthase kinase 3, protein phosphatase 2A, nuclear factor-κB, and several others, imposing spatial and temporal control on their function. While many arrestin-bound kinases and phosphatases are involved in the control of cytoskeletal rearrangement, vesicle endocytosis, exocytosis, and cell migration, other signals reach into the nucleus, affecting cell proliferation, apoptosis, and survival. Indeed, the kinase/phosphatase network regulated by arrestins may be fully as diverse as that regulated by heterotrimeric G proteins.

Arrestins in actin reorganization and cell migration

Arrestins have emerged as important regulators of actin reorganization and cell migration. Both in their classical roles as mediators of receptor desensitization and internalization, and in their newer role as signaling scaffolds, β-arrestins help orchestrate the cellular response to chemotactic signals. However, there is still a considerable amount to be learned about the precise molecular mechanisms underlying these processes. This review discusses how, by regulating receptor internalization and by scaffolding of signaling molecules in discrete cellular locations, arrestins facilitate gradient sensing and cytoskeletal reorganization, ultimately resulting in cell migration. In addition, putative new targets of β-arrestin regulation that may play important roles in cell migration are discussed, as continued research on these targets may provide important details to fill in the current gaps in our understanding of these processes.

Pharmacological characterization of a small-molecule agonist for the chemokine receptor CXCR3

BACKGROUND AND PURPOSE

The chemokine receptor CXCR3 is a GPCR found predominantly on activated T cells. CXCR3 is activated by three endogenous peptides; CXCL9, CXCL10 and CXCL11. Recently, a small-molecule agonist, VUF10661, has been reported in the literature and synthesized in our laboratory. The aim of the present study was to provide a detailed pharmacological characterization of VUF10661 by comparing its effects with those of CXCL11.

EXPERIMENTAL APPROACH

Agonistic properties of VUF10661 were assessed in a chemotaxis assay with murine L1.2 cells transiently transfected with cDNA encoding the human CXCR3 receptor and in binding studies, with [(125)I]-CXCL10 and [(125)I]-CXCL11, on membrane preparations from HEK293 cells stably expressing CXCR3. [(35)S]-GTPγS binding was used to determine its potency to induce CXCR3-mediated G protein activation and BRET-based assays to investigate its effects on intracellular cAMP levels and β-arrestin recruitment.

KEY RESULTS

VUF10661 acted as a partial agonist in CXCR3-mediated chemotaxis, bound to CXCR3 in an allosteric fashion in ligand binding assays and activated G(i) proteins with the same efficacy as CXCL11 in the [(35)S]-GTPγS binding and cAMP assay, while it recruited more β-arrestin1 and β-arrestin2 to CXCR3 receptors than the chemokine.

CONCLUSIONS AND IMPLICATIONS

VUF10661, like CXCL11, activates both G protein-dependent and -independent signalling via the CXCR3 receptor, but probably exerts its effects from an allosteric binding site that is different from that for CXCL11. It could stabilize different receptor and/or β-arrestin conformations leading to differences in functional output. Such ligand-biased signalling might offer interesting options for the therapeutic use of CXCR3 agonists.

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.

Functional characterization of an arrestin gene on insecticide resistance of Culex pipiens pallens

Background

Continuous and excessive application of insecticides has resulted in the rapid development of insecticide resistance in several mosquito species, including Culex pipiens pallens. Previous studies in our laboratory found that arrestin gene expression was higher in the deltamethrin-resistant (DR) strain than in the deltamethrin-susceptible (DS) strain of Cx. pipiens pallens. Similarly, other studies reported that arrestin was highly expressed in permethrin-resistant Cx. quinquefasciatus and in dichlorodiphenyltrichloroethane (DDT)-resistant Drosophila melanogaster.

Methods

Full-length cDNAs of an arrestin gene were cloned from Cx. pipiens pallens via polymerase chain reaction (PCR) and rapid amplification of cDNA end (RACE). The mRNA levels of the arrestin gene in the whole life cycle of DR and DS strains of Cx. pipiens pallens were investigated via quantitative real-time PCR. In addition, the relationship between arrestin and deltamethrin (DM) resistance were identified using genetic overexpression strategies and arrestin RNAi in mosquito cells. Cell viability was analyzed with cholecystokinin octapeptide after DM treatment. Moreover, the mRNA levels of cytochrome P450 6A1 (CYP6A1) and opsin in the transfected cells and controls were analyzed.

Results

Complete arrestin gene sequence was cloned and expressed throughout the life cycle of Cx. pipiens pallens. Moreover, arrestin was significantly upregulated in the DR strain, compared with that in the DS strain at the egg, pupae, and adult stages. Arrestin overexpression comparably increased the mosquito cell viability, whereas arrestin knockdown by siRNA decreased mosquito cell viability with deltamethrin (DM) treatment. Meanwhile, the mRNA levels of CYP6A1 and opsin were upregulated in mosquito cells transfected with arrestin and downregulated in mosquito cells with arrestin knockdown.

Conclusion

This study presented the first evidence that arrestin might be associated with insecticide resistance in Cx. pipiens pallens.