Monocyte chemoattractant protein-1-induced CCR2B receptor desensitization mediated by the G protein-coupled receptor kinase 2

Baicalein inhibits IL-1β-induced extracellular matrix degradation with decreased MCP-1 expression in primary rat chondrocytes

Baicalein is a flavonoid extracted from the roots of Scutellaria baicalensis and Scutellaria lateriflora. This compound exerts various biochemical activities, including antioxidant and anti-inflammatory effects. The study aimed to investigate the effect of baicalein on articular cartilage cells and elucidate its underlying mechanism. In primary rat chondrocyte cultures, treatment with baicalein demonstrated a reduction in the loss of proteoglycan and extracellular matrix degradation induced by interleukin (IL)-1β. Baicalein suppressed IL-1β-induced catabolic responses, including the expression and activation of matrix metalloproteinase (MMP)-13, MMP-3, and MMP-1. In addition, baicalein effectively reduced nitric oxide and prostaglandin E2 production, and it downregulated the expression of inducible nitric oxide synthase and cyclooxygenase-2 in primary rat chondrocytes. Furthermore, baicalein downregulated IL-1β-induced inflammatory chemokines and cytokines, such as GM-CSF and MCP-1. These findings suggest that baicalein could potentially mitigate the catabolic responses of IL-1β in chondrocytes, making it a promising candidate for both the prevention and treatment of osteoarthritis.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43188-024-00225-4.

Atypical Chemokine Receptor 3 "Senses" CXC Chemokine Receptor 4 Activation Through GPCR Kinase Phosphorylation

Atypical chemokine receptor 3 (ACKR3) is an arrestin-biased receptor that regulates extracellular chemokine levels through scavenging. The scavenging process restricts the availability of the chemokine agonist CXCL12 for the G protein-coupled receptor (GPCR) CXCR4 and requires phosphorylation of the ACKR3 C-terminus by GPCR kinases (GRKs). ACKR3 is phosphorylated by GRK2 and GRK5, but the mechanisms by which these kinases regulate the receptor are unresolved. Here we determined that GRK5 phosphorylation of ACKR3 results in more efficient chemokine scavenging and β-arrestin recruitment than phosphorylation by GRK2 in HEK293 cells. However, co-activation of CXCR4-enhanced ACKR3 phosphorylation by GRK2 through the liberation of Gβγ, an accessory protein required for efficient GRK2 activity. The results suggest that ACKR3 "senses" CXCR4 activation through a GRK2-dependent crosstalk mechanism, which enables CXCR4 to influence the efficiency of CXCL12 scavenging and β-arrestin recruitment to ACKR3. Surprisingly, we also found that despite the requirement for phosphorylation and the fact that most ligands promote β-arrestin recruitment, β-arrestins are dispensable for ACKR3 internalization and scavenging, suggesting a yet-to-be-determined function for these adapter proteins. Since ACKR3 is also a receptor for CXCL11 and opioid peptides, these data suggest that such crosstalk may also be operative in cells with CXCR3 and opioid receptor co-expression. Additionally, kinase-mediated receptor cross-regulation may be relevant to other atypical and G protein-coupled receptors that share common ligands. SIGNIFICANCE STATEMENT: The atypical receptor ACKR3 indirectly regulates CXCR4-mediated cell migration by scavenging their shared agonist CXCL12. Here, we show that scavenging and β-arrestin recruitment by ACKR3 are primarily dependent on phosphorylation by GRK5. However, we also show that CXCR4 co-activation enhances the contribution of GRK2 by liberating Gβγ. This phosphorylation crosstalk may represent a common feedback mechanism between atypical and G protein-coupled receptors with shared ligands for regulating the efficiency of scavenging or other atypical receptor functions.

Neighborhood socioeconomic deprivation and individual-level socioeconomic status are associated with dopamine-mediated changes to monocyte subset CCR2 expression via a cAMP-dependent pathway

Social determinants of health (SDoH) include socioeconomic, environmental, and psychological factors that impact health. Neighborhood socioeconomic deprivation (NSD) and low individual-level socioeconomic status (SES) are SDoH that associate with incident heart failure, stroke, and cardiovascular mortality, but the underlying biological mechanisms are not well understood. Previous research has demonstrated an association between NSD, in particular, and key components of the neural-hematopoietic-axis including amygdala activity as a marker of chronic stress, bone marrow activity, and arterial inflammation. Our study further characterizes the role of NSD and SES as potential sources of chronic stress related to downstream immunological factors in this stress-associated biologic pathway. We investigated how NSD, SES, and catecholamine levels (as proxy for sympathetic nervous system activation) may influence monocytes which are known to play a significant role in atherogenesis. First, in an ex vivo approach, we treated healthy donor monocytes with biobanked serum from a community cohort of African Americans at risk for CVD. Subsequently, the treated monocytes were subjected to flow cytometry for characterization of monocyte subsets and receptor expression. We determined that NSD and serum catecholamines (namely dopamine [DA] and norepinephrine [NE]) associated with monocyte C-C chemokine receptor type 2 (CCR2) expression (p < 0.05), a receptor known to facilitate recruitment of monocytes towards arterial plaques. Additionally, NSD associated with catecholamine levels, especially DA in individuals of low SES. To further explore the potential role of NSD and the effects of catecholamines on monocytes, monocytes were treated in vitro with epinephrine [EPI], NE, or DA. Only DA increased CCR2 expression in a dose-dependent manner (p < 0.01), especially on non-classical monocytes (NCM). Furthermore, linear regression analysis between D2-like receptor surface expression and surface CCR2 expression suggested D2-like receptor signaling in NCM. Indicative of D2-signaling, cAMP levels were found to be lower in DA-treated monocytes compared to untreated controls (control 29.78 pmol/ml vs DA 22.97 pmol/ml; p = 0.038) and the impact of DA on NCM CCR2 expression was abrogated by co-treatment with 8-CPT, a cAMP analog. Furthermore, Filamin A (FLNA), a prominent actin-crosslinking protein, that is known to regulate CCR2 recycling, significantly decreased in DA-treated NCM (p < 0.05), indicating a reduction of CCR2 recycling. Overall, we provide a novel immunological mechanism, driven by DA signaling and CCR2, for how NSD may contribute to atherogenesis. Future studies should investigate the importance of DA in CVD development and progression in populations disproportionately experiencing chronic stress due to SDoH.

Atypical Chemokine Receptor 3 'Senses' CXC Chemokine Receptor 4 Activation Through GPCR Kinase Phosphorylation

Atypical chemokine receptor 3 (ACKR3) is an arrestin-biased receptor that regulates extracellular chemokine levels through scavenging. The scavenging action mediates the availability of the chemokine CXCL12 for the G protein-coupled receptor (GPCR) CXCR4 and requires phosphorylation of the ACKR3 C-terminus by GPCR kinases (GRKs). ACKR3 is phosphorylated by GRK2 and GRK5, but the mechanisms by which these kinases regulate the receptor are unresolved. Here we mapped the phosphorylation patterns and determined that GRK5 phosphorylation of ACKR3 dominates β-arrestin recruitment and chemokine scavenging over GRK2. Co-activation of CXCR4 significantly enhanced phosphorylation by GRK2 through the liberation of Gβγ. These results suggest that ACKR3 'senses' CXCR4 activation through a GRK2-dependent crosstalk mechanism. Surprisingly, we also found that despite the requirement for phosphorylation, and the fact that most ligands promote β-arrestin recruitment, β-arrestins are dispensable for ACKR3 internalization and scavenging, suggesting a yet to be determined function for these adapter proteins.

The dual-function chemokine receptor CCR2 drives migration and chemokine scavenging through distinct mechanisms

C-C chemokine receptor 2 (CCR2) is a dual-function receptor. Similar to other G protein-coupled chemokine receptors, it promotes monocyte infiltration into tissues in response to the chemokine CCL2, and, like atypical chemokine receptors (ACKRs), it scavenges chemokine from the extracellular environment. CCR2 therefore mediates CCL2-dependent signaling as a G protein-coupled receptor (GPCR) and also limits CCL2 signaling as a scavenger receptor. We investigated the mechanisms underlying CCR2 scavenging, including the involvement of intracellular proteins typically associated with GPCR signaling and internalization. Using CRISPR knockout cell lines, we showed that CCR2 scavenged by constitutively internalizing to remove CCL2 from the extracellular space and recycling back to the cell surface for further rounds of ligand sequestration. This process occurred independently of G proteins, GPCR kinases (GRKs), β-arrestins, and clathrin, which is distinct from other "professional" chemokine scavenger receptors that couple to GRKs, β-arrestins, or both. These findings set the stage for understanding the molecular regulators that determine CCR2 scavenging and may have implications for drug development targeting this therapeutically important receptor.

Analysis of CCR2 splice variant expression patterns and functional properties

Background

C–C motif chemokine receptor 2 (CCR2), the main receptor for monocyte chemoattractant protein-1 (MCP-1), is expressed on immune cells, including monocytes, macrophages, and activated T cells, and mediates cell migration toward MCP-1 in inflammation-related diseases. The CCR2 gene encodes two isoforms: CCR2A and CCR2B. The CCR2B open reading frame is localized in a single exon, similar to other chemokine receptors, and CCR2A and CCR2B feature different amino acid sequences in their C-terminal intracellular loops due to alternative splicing. Most biochemical studies on CCR2-related cellular responses in the immune system have focused on CCR2B, with few reports focused on CCR2A. Understanding the functional properties of CCR2A in cellular responses may elucidate the roles played by MCP-1 and CCR2 in pathophysiological responses.

Results

CCR2 gene expression analysis in several cell types revealed that most adherent cells only expressed CCR2A, whereas CCR2B expression was dominant in monocytic cells. The C-terminal Helix 8 region of CCR2A contains few basic amino acids, which may be unfavorable for cell surface localization, as confirmed with the HiBiT assay. CCR2B contains many C-terminal Ser/Thr residues, similar to other chemokine receptors, which may be phosphorylated by G protein–coupled receptor kinases (GRKs) to promote β-arrestin recruitment and subsequent endocytosis. By contrast, CCR2A contains few C-terminal Ser/Thr residues, which are unlikely to be phosphorylated by GRKs. CCR2A localized on the cell surface is resistant to internalization, despite the interaction between Gβ and GRKs induced by ligand binding with CCR2A. CCR2A induced cellular responses at a relatively higher degree than CCR2B, although both receptors mediated signaling events through Gαq and Gαi. HeLa cells lacking CCR2A showed slowed growth compared with parent cells, regardless of MCP-1 stimulation, and their chemotactic activity toward MCP-1, in addition to basal motility, was significantly impaired.

Conclusion

MCP-1 and CCR2 may play pivotal roles in cancer progression by recruiting macrophages into cancer tissue. This study demonstrates that CCR2A but not CCR2B is expressed in solid cancer–derived cells. CCR2A is resistant to internalization by β-arrestin due to a distinct C-terminal region from CCR2B, which enhances MCP-1-stimulated responses, indicating that CCR2A may play essential roles in solid cancer progression.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00787-6.

Raf kinase inhibitory protein reduces bradykinin receptor desensitization

Inflammatory hyperalgesia represents a nociceptive phenotype that can become persistent in nature through dynamic protein modifications. However, a large gap in knowledge exists concerning how the integration of intracellular signaling molecules coordinates a persistent inflammatory phenotype. Herein, we demonstrate that Raf Kinase Anchoring Protein (RKIP) interrupts a vital canonical desensitization pathway to maintain bradykinin (BK) receptor activation in primary afferent neurons. Biochemical analyses of primary neuronal cultures indicate bradykinin-stimulated PKC phosphorylation of RKIP at Ser153. Furthermore, BK exposure increases G-protein Receptor Kinase 2 (GRK2) binding to RKIP, inhibiting pharmacological desensitization of the BK receptor. Additional studies found that molecular RKIP down-regulation increases BK receptor desensitization in real-time imaging of primary afferent neurons, identifying a key pathway integrator in the desensitization process that controls multiple GRK2-sensitive G-protein coupled receptors. Therefore, RKIP serves as an integral scaffolding protein that inhibits BK receptor desensitization.

Identification of a conserved chemokine receptor motif that enables ligand discrimination

Extensive ligand-receptor promiscuity in the chemokine signaling system balances beneficial redundancy and specificity. However, this feature poses a major challenge to selectively modulate the system pharmacologically. Here, we identified a conserved cluster of three aromatic receptor residues that anchors the second extracellular loop (ECL2) to the top of receptor transmembrane helices (TM) 4 and 5 and enables recognition of both shared and specific characteristics of interacting chemokines. This cluster was essential for the activation of several chemokine receptors. Furthermore, characteristic motifs of the ß₁ strand and 30s loop make the two main CC-chemokine subgroups-the macrophage inflammatory proteins (MIPs) and monocyte chemoattractant proteins (MCPs)-differentially dependent on this cluster in the promiscuous receptors CCR1, CCR2, and CCR5. The cluster additionally enabled CCR1 and CCR5 to discriminate between closely related MIPs based on the N terminus of the chemokine. G protein signaling and β-arrestin2 recruitment assays confirmed the importance of the conserved cluster in receptor discrimination of chemokine ligands. This extracellular site may facilitate the development of chemokine-related therapeutics.

Persistent Oxidative Stress and Inflammasome Activation in CD14highCD16- Monocytes From COVID-19 Patients

The poor outcome of the coronavirus disease-2019 (COVID-19), caused by SARS-CoV-2, is associated with systemic hyperinflammatory response and immunopathology. Although inflammasome and oxidative stress have independently been implicated in COVID-19, it is poorly understood whether these two pathways cooperatively contribute to disease severity. Herein, we found an enrichment of CD14highCD16- monocytes displaying inflammasome activation evidenced by caspase-1/ASC-speck formation in severe COVID-19 patients when compared to mild ones and healthy controls, respectively. Those cells also showed aberrant levels of mitochondrial superoxide and lipid peroxidation, both hallmarks of the oxidative stress response, which strongly correlated with caspase-1 activity. In addition, we found that NLRP3 inflammasome-derived IL-1β secretion by SARS-CoV-2-exposed monocytes in vitro was partially dependent on lipid peroxidation. Importantly, altered inflammasome and stress responses persisted after short-term patient recovery. Collectively, our findings suggest oxidative stress/NLRP3 signaling pathway as a potential target for host-directed therapy to mitigate early COVID-19 hyperinflammation and also its long-term outcomes.

Targeted inhibition of GRK2 kinase domain by CP-25 to reverse fibroblast-like synoviocytes dysfunction and improve collagen-induced arthritis in rats

Rheumatoid arthritis (RA) is an autoimmune disease and is mainly characterized by abnormal proliferation of fibroblast-like synoviocytes (FLS). The up-regulated cellular membrane expression of G protein coupled receptor kinase 2 (GRK2) of FLS plays a critical role in RA progression, the increase of GRK2 translocation activity promotes dysfunctional prostaglandin E4 receptor (EP4) signaling and FLS abnormal proliferation. Recently, although our group found that paeoniflorin-6ʹ-O-benzene sulfonate (CP-25), a novel compound, could reverse FLS dysfunction via GRK2, little is known as to how GRK2 translocation activity is suppressed. Our findings revealed that GRK2 expression up-regulated and EP4 expression down-regulated in synovial tissues of RA patients and collagen-induced arthritis (CIA) rats, and prostaglandin E2 (PGE2) level increased in arthritis. CP-25 could down-regulate GRK2 expression, up-regulate EP4 expression, and improve synovitis of CIA rats. CP-25 and GRK2 inhibitors (paroxetine or GSK180736A) inhibited the abnormal proliferation of FLS in RA patients and CIA rats by down-regulating GRK2 translocation to EP4 receptor. The results of microscale thermophoresis (MST), cellular thermal shift assay, and inhibition of kinase activity assay indicated that CP-25 could directly target GRK2, increase the protein stability of GRK2 in cells, and inhibit GRK2 kinase activity. The docking of CP-25 and GRK2 suggested that the kinase domain of GRK2 might be an important active pocket for CP-25. G201, K220, K230, A321, and D335 in kinase domain of GRK2 might form hydrogen bonds with CP-25. Site-directed mutagenesis and co-immunoprecipitation assay further revealed that CP-25 down-regulated the interaction of GRK2 and EP4 via controlling the key amino acid residue of Ala321 of GRK2. Our data demonstrate that FLS proliferation is regulated by GRK2 translocation to EP4. Targeted inhibition of GRK2 kinase domain by CP-25 improves FLS function and represents an innovative drug for the treatment of RA by targeting GRK2.

Monocyte Chemoattractant Protein-1 promotes cancer cell migration via c-Raf/MAPK/AP-1 pathway and MMP-9 production in osteosarcoma

Background

Osteosarcoma is generally reported among younger individuals and has a very poor prognosis, particularly for the development of metastasis. However, more effective metastatic biomarkers and therapeutic methods are absent. Monocyte chemoattractant protein-1 (MCP-1) is involved in cancer progression and inflammatory recruitment. Although previous studies have reported higher serum MCP-1 levels in patients with osteosarcoma, the role of MCP-1 in osteosarcoma progression remains to be addressed.

Methods

The osteosarcoma cell migratory ability was assessed by transwell migration assay. The MCP-1 and MMP-9 expression levels were analyzed by Western blot and qPCR. The signal activation was conducted by Western blot. The in vivo mouse experiment and tumor tissue array were performed to confirm our findings in vitro.

Results

The present study demonstrates that MCP-1 regulates cell mobility through matrix metalloproteinase (MMP)-9 expression in osteosarcoma cells. Moreover, MCP-1 promotes MMP-9 expression, cell migration, and cell invasion by mediating CCR2, c-Raf, MAPK, and AP-1 signal transduction. Using MCP-1 knockdown stable cell lines, we found that MCP-1 knockdown reduces MMP-9 expression and cell mobility. Finally, we found high MCP-1 expression levels in osteosarcoma specimens.

Conclusions

Our results provide prognostic value of MCP-1 in osteosarcoma by promoting MMP-9 expression.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-020-01756-y.

Forsythoside A inhibits adhesion and migration of monocytes to type II alveolar epithelial cells in lipopolysaccharide-induced acute lung injury through upregulating miR-124

Acute lung injury (ALI) is a severe disease for which effective drugs are still lacking at present. Forsythia suspensa is a traditional Chinese medicine commonly used to relieve respiratory symptoms in China, but its functional mechanisms remain unclear. Therefore, forsythoside A (FA), the active constituent of F. suspensa, was studied in the present study. Inflammation models of type II alveolar epithelial MLE-12 cells and BALB/c mice stimulated by lipopolysaccharide (LPS) were established to explore the effects of FA on ALI and the underlying mechanisms. We found that FA inhibited the production of monocyte chemoattractant protein-1 (MCP-1/CCL2) in LPS-stimulated MLE-12 cells in a dose-dependent manner. Moreover, FA decreased the adhesion and migration of monocytes to MLE-12 cells. Furthermore, miR-124 expression was upregulated after FA treatment. The luciferase report assay showed that miR-124 mimic reduced the activity of CCL2 in MLE-12 cells. However, the inhibitory effects of FA on CCL2 expression and monocyte adhesion and migration to MLE-12 cells were counteracted by treatment with a miR-124 inhibitor. Critically, FA ameliorated LPS-induced pathological damage, decreased the serum levels of tumor necrosis factor-α and interleukin-6, and inhibited CCL2 secretion and macrophage infiltration in lungs in ALI mice. Meanwhile, administration of miR-124 inhibitor attenuated the protective effects of FA. The present study suggests that FA attenuates LPS-induced adhesion and migration of monocytes to type II alveolar epithelial cells though upregulating miR-124, thereby inhibiting the expression of CCL2. These findings indicate that the potential application of FA is promising and that miR-124 mimics could also be used in the treatment of ALI.

β-arrestin 1 transfection induced cell death in high grade glioma in vitro

The best known functions of β-arrestins (β-arr) are to regulate G protein-coupled receptors (GPCR) signaling through receptor desensitization and internalization. Many reports also suggest that β-arrs play important role in immune regulation and inflammatory responses, under physiological and pathological conditions. Recent studies have shown that β-arr 1 silencing halts proliferation and increases temozolomide (TMZ) response in glioblastoma (GBM) cells. The focus of this paper is to analyze the role of β-arr 1 overexpression in the 18 high grade glioma (HGG) cell line in terms of viability and their response to TMZ treatment. For this reason, the cell line was transfected with β-arr 1 and the effect was analyzed after 24 h, 48 h and 72 h in terms of proliferation and treatment response. We observed that β-arr 1 overexpression induced a time and dose dependant inhibition in the HGG cells. Unexpectedly, β-arr transfection resulted in a very mild increase in TMZ toxicity after 24 h, becoming non-statistically significant at 72 h. In conclusion, we showed that β-arr 1 overexpression inhibits cell proliferation in the 18 cell line but only has a very modest effect on treatment response with the alkylating agent TMZ.

An Overview on G Protein-coupled Receptor-induced Signal Transduction in Acute Myeloid Leukemia

BACKGROUND

Acute Myeloid Leukemia (AML) is a genetically heterogeneous disease characterized by uncontrolled proliferation of precursor myeloid-lineage cells in the bone marrow. AML is also characterized by patients with poor long-term survival outcomes due to relapse. Many efforts have been made to understand the biological heterogeneity of AML and the challenges to develop new therapies are therefore enormous. G Protein-coupled Receptors (GPCRs) are a large attractive drug-targeted family of transmembrane proteins, and aberrant GPCR expression and GPCR-mediated signaling have been implicated in leukemogenesis of AML. This review aims to identify the molecular players of GPCR signaling, focusing on the hematopoietic system, which are involved in AML to help developing novel drug targets and therapeutic strategies.

METHODS

We undertook an exhaustive and structured search of bibliographic databases for research focusing on GPCR, GPCR signaling and expression in AML.

RESULTS AND CONCLUSION

Many scientific reports were found with compelling evidence for the involvement of aberrant GPCR expression and perturbed GPCR-mediated signaling in the development of AML. The comprehensive analysis of GPCR in AML provides potential clinical biomarkers for prognostication, disease monitoring and therapeutic guidance. It will also help to provide marker panels for monitoring in AML. We conclude that GPCR-mediated signaling is contributing to leukemogenesis of AML, and postulate that mass spectrometrybased protein profiling of primary AML cells will accelerate the discovery of potential GPCR related biomarkers for AML.

G protein-coupled receptor kinase 2 (GRK2) as a multifunctional signaling hub

Accumulating evidence indicates that G protein-coupled receptor kinase 2 (GRK2) is a versatile protein that acts as a signaling hub by modulating G protein-coupled receptor (GPCR) signaling and also via phosphorylation or scaffolding interactions with an extensive number of non-GPCR cellular partners. GRK2 multifunctionality arises from its multidomain structure and from complex mechanisms of regulation of its expression levels, activity, and localization within the cell, what allows the precise spatio-temporal shaping of GRK2 targets. A better understanding of the GRK2 interactome and its modulation mechanisms is helping to identify the GRK2-interacting proteins and its substrates involved in the participation of this kinase in different cellular processes and pathophysiological contexts.

Arginine 313 of the putative 8th helix mediates Gαq/14 coupling of human CC chemokine receptors CCR2a and CCR2b

In man, two CC chemokine receptor isoforms, CCR2a and CCR2b, are present that belong to the rhodopsin-like G protein-coupled receptor family, and couple to G_i and G_q family members. The CCR2 receptors are known to regulate canonical functions of chemokines such as directed migration of leukocytes, and to potentially control non-canonical functions such as differentiation, proliferation, and gene transcription of immune and non-immune cells. We recently reported on the activation of phospholipase C isoenzymes and RhoA GTPases by coupling of the two CCR2 receptors to members of the G_q family, in particular Gα_q and Gα₁₄. So far little is known about the structural requirements for the CCR2/G_q/14 interaction. Interestingly, the CCR2 receptor isoforms are identical up to arginine 313 (R³¹³) that is part of the putative 8th helix in CCR2 receptors, and the 8th helix has been implicated in the interaction of rhodopsin-like G protein-coupled receptors with Gα_q. In the present work we describe that the 8th helix of both CCR2a and CCR2b is critically involved in selectively activating Gα_q/14-regulated signaling. Refined analysis using various CCR2a and CCR2b mutants and analyzing their cellular signaling, e.g. ligand-dependent (i) activation of phospholipase C isoenzymes, (ii) stimulation of serum response factor-mediated gene transcription, (iii) activation of mitogen-activated protein kinases, (iv) internalization, and (v) changes in intracellular calcium concentrations, identified arginine 313 within the amino terminal portion of helix 8 to play a role for the agonist-mediated conformational changes and the formation of a Gα_q/14 binding surface. We show that R³¹³ determines Gα_q/14 protein-dependent but not G_i protein-dependent cellular signaling, and plays no role in G_q/G_i-independent receptor internalization, indicating a role of R³¹³ in biased signaling of CCR2 receptors.

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.

Protein Mimetic and Anticancer Properties of Monocyte-Targeting Peptide Amphiphile Micelles

Monocyte chemoattractant protein-1 (MCP-1) stimulates the migration of monocytes to inflammatory sites, leading to the progression of many diseases. Recently, we described a monocyte-targeting peptide amphiphile micelle (MCP-1 PAM) incorporated with the chemokine receptor CCR2 binding motif of MCP-1, which has a high affinity for monocytes in atherosclerotic plaques. We further report here the biomimetic components of MCP-1 PAMs and the influence of the nanoparticle upon binding to monocytes. We report that MCP-1 PAMs have enhanced secondary structure compared to the MCP-1 peptide. As a result, MCP-1 PAMs displayed improved binding and chemoattractant properties to monocytes, which upregulated the inflammatory signaling pathways responsible for monocyte migration. Interestingly, when MCP-1 PAMs were incubated in the presence of prostate cancer cells in vitro, the particle displayed anticancer efficacy by reducing CCR2 expression. Given that monocytes play an important role in tumor cell migration and invasion, our results demonstrate that PAMs can improve the native biofunctional properties of the peptide and may be used as an effective inhibitor to prevent chemokine-receptor interactions that promote disease progression.

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.

Mincle activation enhances neutrophil migration and resistance to polymicrobial septic peritonitis

Sepsis is a systemic inflammatory response to bacterial infection. The therapeutic options for treating sepsis are limited. Impaired neutrophil recruitment into the infection site is directly associated with severe sepsis, but the precise mechanism is unclear. Here, we show that Mincle plays a key role in neutrophil migration and resistance during polymicrobial sepsis. Mincle-deficient mice exhibited lower survival rates in experimental sepsis from cecal ligation and puncture and Escherichia coli–induced peritonitis. Mincle deficiency led to higher serum inflammatory cytokine levels and reduced bacterial clearance and neutrophil recruitment. Transcriptome analyses revealed that trehalose dimycolate, a Mincle ligand, reduced the expression of G protein–coupled receptor kinase 2 (GRK2) in neutrophils. Indeed, GRK2 expression was upregulated, but surface expression of the chemokine receptor CXCR2 was downregulated in blood neutrophils from Mincle-deficient mice with septic injury. Moreover, CXCL2-mediated adhesion, chemotactic responses, and F-actin polymerization were reduced in Mincle-deficient neutrophils. Finally, we found that fewer Mincle-deficient neutrophils infiltrated from the blood circulation into the peritoneal fluid in bacterial septic peritonitis compared with wild-type cells. Thus, our results indicate that Mincle plays an important role in neutrophil infiltration and suggest that Mincle signaling may provide a therapeutic target for treating sepsis.

Cytokine Regulation of Metastasis and Tumorigenicity

The human body combats infection and promotes wound healing through the remarkable process of inflammation. Inflammation is characterized by the recruitment of stromal cell activity including recruitment of immune cells and induction of angiogenesis. These cellular processes are regulated by a class of soluble molecules called cytokines. Based on function, cell target, and structure, cytokines are subdivided into several classes including: interleukins, chemokines, and lymphokines. While cytokines regulate normal physiological processes, chronic deregulation of cytokine expression and activity contributes to cancer in many ways. Gene polymorphisms of all types of cytokines are associated with risk of disease development. Deregulation RNA and protein expression of interleukins, chemokines, and lymphokines have been detected in many solid tumors and hematopoetic malignancies, correlating with poor patient prognosis. The current body of literature suggests that in some tumor types, interleukins and chemokines work against the human body by signaling to cancer cells and remodeling the local microenvironment to support the growth, survival, and invasion of primary tumors and enhance metastatic colonization. Some lymphokines are downregulated to suppress tumor progression by enhancing cytotoxic T cell activity and inhibiting tumor cell survival. In this review, we will describe the structure/function of several cytokine families and review our current understanding on the roles and mechanisms of cytokines in tumor progression. In addition, we will also discuss strategies for exploiting the expression and activity of cytokines in therapeutic intervention.

Chapter Three - Ubiquitination and Protein Turnover of G-Protein-Coupled Receptor Kinases in GPCR Signaling and Cellular Regulation

G-protein-coupled receptors (GPCRs) are responsible for regulating a wide variety of physiological processes, and distinct mechanisms for GPCR inactivation exist to guarantee correct receptor functionality. One of the widely used mechanisms is receptor phosphorylation by specific G-protein-coupled receptor kinases (GRKs), leading to uncoupling from G proteins (desensitization) and receptor internalization. GRKs and β-arrestins also participate in the assembly of receptor-associated multimolecular complexes, thus initiating alternative G-protein-independent signaling events. In addition, the abundant GRK2 kinase has diverse "effector" functions in cellular migration, proliferation, and metabolism homeostasis by means of the phosphorylation or interaction with non-GPCR partners. Altered expression of GRKs (particularly of GRK2 and GRK5) occurs during pathological conditions characterized by impaired GPCR signaling including inflammatory syndromes, cardiovascular disease, and tumor contexts. It is increasingly appreciated that different pathways governing GRK protein stability play a role in the modulation of kinase levels in normal and pathological conditions. Thus, enhanced GRK2 degradation by the proteasome pathway occurs upon GPCR stimulation, what allows cellular adaptation to chronic stimulation in a physiological setting. β-arrestins participate in this process by facilitating GRK2 phosphorylation by different kinases and by recruiting diverse E3 ubiquitin ligase to the receptor complex. Different proteolytic systems (ubiquitin-proteasome, calpains), chaperone activities and signaling pathways influence the stability of GRKs in different ways, thus endowing specificity to GPCR regulation as protein turnover of GRKs can be differentially affected. Therefore, modulation of protein stability of GRKs emerges as a versatile mechanism for feedback regulation of GPCR signaling and basic cellular processes.

ECL1i, d(LGTFLKC), a novel, small peptide that specifically inhibits CCL2-dependent migration

CC chemokine receptor type 2 (CCR2) is a key molecule in inflammatory diseases and is an obvious drug target for the treatment of inflammation. A number of nonpeptidic, competitive CCR2 antagonists have been developed, but none has yet been approved for clinical use. Our aim was to identify a short peptide that showed allosteric antagonism against human and mouse CCR2. On the basis of sequence analysis and 3-dimensional modeling, we identified an original 7-d-amino acid peptidic CCR2 inhibitor that we have called extracellular loop 1 inverso (ECL1i), d(LGTFLKC). In vitro, ECL1i selectively and potently inhibits CC chemokine ligand type 2 (CCL2)-triggered chemotaxis (IC50, 2 µM) but no other conventional CCL2-associated events. We used the classic competitive CCR2 antagonist, BMS22 {2-[(isopropylaminocarbonyl)amino]-N-[2-[[cis-2-[[4-(methylthio)benzoyl]amino]cyclohexyl]amino]-2-oxoethyl]-5-(trifluoromethyl)benzamide}, as positive control and inhibited CCL2-dependent chemotaxis with an IC50 of 18 nM. As negative control, we used a peptide with the same composition as ECL1i, but in a different sequence, d(FKLTLCG). In vivo, ECL1i (4 mg/kg) interfered with CCR2-positive cell recruitment and attenuated disease progression in experimental autoimmune encephalomyelitis, a mouse model of multiple sclerosis. This study establishes ECL1i as the first allosteric inhibitor of CCR2 with functional selectivity. ECL1i is a promising new agent in therapeutic development, and it may, by its selective effect, increase our understanding of CCR2 signaling pathways and functions.-Auvynet, C., Baudesson de Chanville, C., Hermand, P., Dorgham, K., Piesse, C., Pouchy, C., Carlier, L., Poupel, L., Barthélémy, S., Felouzis, V., Lacombe, C., Sagan, S., Salomon, B., Deterre, P., Sennlaub, F., Combadière, C. ECL1i, d(LGTFLKC), a novel, small peptide that specifically inhibits CCL2-dependent migration.

Prenatal fat-rich diet exposure alters responses of embryonic neurons to the chemokine, CCL2, in the hypothalamus

Maternal consumption of a high-fat diet (HFD) during pregnancy is found to stimulate the genesis of hypothalamic orexigenic peptide neurons in the offspring, while HFD intake in adult animals produces a systemic low-grade inflammation which increases neuroimmune factors that may affect neurogenesis and neuronal migration. Building on this evidence and our recent study showing that the inflammatory chemokine, CCL2, stimulates the migration of hypothalamic neurons and expression of orexigenic neuropeptides, we tested here the possibility that prenatal exposure to a HFD in rats affects this chemokine system, both CCL2 and its receptors, CCR2 and CCR4, and alters its actions on hypothalamic neurons, specifically those expressing the neuropeptides, enkephalin (ENK) and galanin (GAL). Using primary dissociated hypothalamic neurons extracted from embryos on embryonic day 19, we found that prenatal HFD exposure compared to chow control actually reduces the expression of CCL2 in these hypothalamic neurons, while increasing CCR2 and CCR4 expression, and also reduces the sensitivity of hypothalamic neurons to CCL2. The HFD abolished the dose-dependent, stimulatory effect of CCL2 on the number of migrated neurons and even shifted its normal stimulatory effect on migrational velocity and distance traveled by control neurons to an inhibition of migration. Further, it abolished the dose-dependent, stimulatory effect of CCL2 on neuronal expression of ENK and GAL. These results demonstrate that prenatal HFD exposure greatly disturbs the functioning of the CCL2 chemokine system in embryonic hypothalamic neurons, reducing its endogenous levels and ability to promote the migration of neurons and their expression of orexigenic peptides.

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.

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.

Macrophages are essential for CTGF-mediated adult β-cell proliferation after injury

Objective

Promotion of endogenous β-cell mass expansion could facilitate regeneration in patients with diabetes. We discovered that the secreted protein CTGF (aka CCN2) promotes adult β-cell replication and mass regeneration after injury via increasing β-cell immaturity and shortening the replicative refractory period. However, the mechanism of CTGF-mediated β-cell proliferation is unknown. Here we focused on whether CTGF alters cells of the immune system to enhance β-cell replication.

Methods

Using mouse models for 50% β-cell ablation and conditional, β-cell-specific CTGF induction, we assessed changes in immune cell populations by performing immunolabeling and gene expression analyses. We tested the requirement for macrophages in CTGF-mediated β-cell proliferation via clodronate-based macrophage depletion.

Results

CTGF induction after 50% β-cell ablation increased both macrophages and T-cells in islets. An upregulation in the expression of several macrophage and T-cell chemoattractant genes was also observed in islets. Gene expression analyses suggest an increase in M1 and a decrease in M2 macrophage markers. Depletion of macrophages (without changes in T cell number) blocked CTGF-mediated β-cell proliferation and prevented the increase in β-cell immaturity.

Conclusions

Our data show that macrophages are critical for CTGF-mediated adult β-cell proliferation in the setting of partial β-cell ablation. This is the first study to link a specific β-cell proliferative factor with immune-mediated β-cell proliferation in a β-cell injury model.

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Following partial beta cell ablation, induction of CTGF in beta cells increases the number of pancreatic macrophages and T cells.

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Increased macrophages are found preferentially within islets.

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Expression of genes involved in macrophage and T cell chemoattraction is increased in islets following CTGF induction.

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Depletion of macrophages completely abrogates CTGF-mediated increases in beta cell proliferation after injury.

Role of G protein-coupled receptor kinase 2 in tumoral angiogenesis

Downregulation of G protein-coupled receptor kinase 2 (GRK2) in endothelial cells has recently been identified as a relevant event in the tumoral angiogenic switch. Based on the effects of altering GRK2 dosage in cell and animal models, this kinase appears to act as a hub in key signaling pathways involved in vascular stabilization and remodeling. Accordingly, decreased GRK2 expression in endothelial cells accelerates tumor growth in mice by impairing the pericytes ensheathing the vessels, thereby promoting hypoxia and macrophage infiltration. These results raise new questions regarding the mechanisms by which transformed cells trigger the decrease in GRK2 observed in human breast cancer vessels and how GRK2 modulates the interactions between different cell types that occur in the tumor microenvironment.

Stimulatory role of the chemokine CCL2 in the migration and peptide expression of embryonic hypothalamic neurons

Neuroinflammation is a feedback mechanism against infection, with recent studies suggesting a neuromodulatory role. The chemokine, (C-C motif) ligand 2 (CCL2), and its receptor, (C-C motif) receptor type 2 (CCR2), affect neuromodulation and migration in response to damage. Although CCL2 co-localizes with neuropeptides in the hypothalamus that control voluntary behavior, the function of CCL2/CCR2 is unknown. This led us to consider the possibility that CCL2 acting through CCR2, under natural conditions, may affect the migration and peptide levels of hypothalamic neurons that control voluntary behavior. This study used primary embryonic hypothalamic neurons to examine the effect of CCL2 on migratory behavior and on levels of the peptides, enkephalin (ENK) and galanin. Treatment with CCL2 led to a significant, dose-dependent increase in the number of migrated neurons and an increase in the velocity and distance traveled. CCL2 also significantly increased the number of ENK-expressing and CCR2/ENK co-expressing neurons and the percentage of neurons that contain higher levels of ENK. Lastly, CCL2 produced a dose-dependent increase in expression of ENK and galanin. These results provide evidence for a stimulatory effect of CCL2 on embryonic hypothalamic neurons involving changes in migratory behavior, expression, and synthesis of neuropeptides that function in controlling behavior. Our results demonstrate that the chemokine, CCL2, functions through its receptor, CCR2, to stimulate the migration and expression of the orexigenic peptides, enkephalin (ENK) and galanin (GAL), in developing embryonic hypothalamic neurons that are important for controlling ingestive behavior. This evidence reveals broad effects of CCL2 in the developing hypothalamus, showing this chemokine system to be tightly linked to the hypothalamic peptide neurons.

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.

Vimentin is a target of PKCβ phosphorylation in MCP-1-activated primary human monocytes

OBJECTIVE AND DESIGN

We designed a study to detect downstream phosphorylation targets of PKCβ in MCP-1-induced human monocytes.

METHODS

Two-dimensional gel electrophoresis was performed for monocytes treated with MCP-1 in the presence or absence of PKCβ antisense oligodeoxyribonucleotides (AS-ODN) or a PKCβ inhibitor peptide, followed by phospho- and total protein staining. Proteins that stained less intensely with the phospho-stain, when normalized to the total protein stain, in the presence of PKCβ AS-ODN or the PKCβ inhibitor peptide, were sequenced.

RESULTS

Of the proteins identified, vimentin was consistently identified using both experimental approaches. Upon (32)P-labeling and vimentin immunoprecipitation, increased phosphorylation of vimentin was observed in MCP-1 treated monocytes as compared to the untreated monocytes. Both PKCβ AS-ODN and the PKCβ inhibitor reduced MCP-1-induced vimentin phosphorylation. The IP of monocytes with anti-vimentin antibody and immunoblotting with a PKCβ antibody revealed that increased PKCβ becomes associated with vimentin upon MCP-1 activation. Upon MCP-1 treatment, monocytes were shown to secrete vimentin and secretion depended on PKCβ expression and activity.

CONCLUSIONS

We conclude that vimentin, a major intermediate filament protein, is a phosphorylation target of PKCβ in MCP-1-treated monocytes and that PKCβ phosphorylation is essential for vimentin secretion. Our recently published studies have implicated vimentin as a potent stimulator of the innate immune receptor Dectin-1 as reported by Thiagarajan et al. (Cardiovasc Res 99:494-504, 2013). Taken together our findings suggest that inhibition of PKCβ regulates vimentin secretion and, thereby, its interaction with Dectin-1 and downstream stimulation of superoxide anion production. Thus, PKCβ phosphorylation of vimentin likely plays an important role in propagating inflammatory responses.

CCL2 increases αvβ3 integrin expression and subsequently promotes prostate cancer migration

BACKGROUND

Chemokine ligand 2 (CCL2), also known as monocyte chemoattractant protein-1 (MCP-1), belongs to the CC chemokine family which is associated with the disease status and outcomes of cancers. Prostate cancer is the most commonly diagnosed malignancy in men and shows a predilection for metastasis to the bone. However, the effect of CCL2 on human prostate cancer cells is largely unknown. The aim of this study was to examine the role of CCL2 in integrin expression and migratory activity in prostate cancers.

METHODS

Prostate cancer migration was examined using Transwell, wound healing, and invasion assay. The PKCδ and c-Src phosphorylations were examined by using western blotting. The qPCR was used to examine the mRNA expression of integrins. A transient transfection protocol was used to examine AP-1 activity.

RESULTS

Stimulation of prostate cancer cell lines (PC3, DU145, and LNCaP) induced migration and expression of integrin αvβ3. Treatment of cells with αvβ3 antibody or siRNA abolished CCL2-increased cell migration. CCL2-increased migration and integrin expression were diminished by CCR2 but not by CCR4 inhibitors, suggesting that the CCR2 receptor is involved in CCL2-promoted prostate cancer migration. CCL2 activated a signal transduction pathway that includes PKCδ, c-Src, and AP-1. Reagents that inhibit specific components of this pathway each diminished the ability of CCL2 to effect cell migration and integrin expression.

CONCLUSIONS

Interaction between CCL2 and CCR2 enhances migration of prostate cancer cells through an increase in αvβ3 integrin production.

GENERAL SIGNIFICANCE

CCL2 is a critical factor of prostate cancer metastasis.

TLR4 Signaling augments monocyte chemotaxis by regulating G protein-coupled receptor kinase 2 translocation

Monocytes are critical effector cells of the innate immune system that protect the host by migrating to inflammatory sites, differentiating to macrophages and dendritic cells, eliciting immune responses, and killing pathogenic microbes. MCP-1, also known as CCL2, plays an important role in monocyte activation and migration. The chemotactic function of MCP-1 is mediated by binding to the CCR2 receptor, a member of the G protein-coupled receptor (GPCR) family. Desensitization of GPCR chemokine receptors is an important regulator of the intensity and duration of chemokine stimulation. GPCR kinases (GRKs) induce GPCR phosphorylation, and this leads to GPCR desensitization. Regulation of subcellular localization of GRKs is considered an important early regulatory mechanism of GRK function and subsequent GPCR desensitization. Chemokines and LPS are both present during Gram-negative bacterial infection, and LPS often synergistically exaggerates leukocyte migration in response to chemokines. In this study, we investigated the role and mechanism of LPS-TLR4 signaling on the regulation of monocyte chemotaxis. We demonstrate that LPS augments MCP-1-induced monocyte migration. We also show that LPS, through p38 MAPK signaling, induces phosphorylation of GRK2 at serine 670, which, in turn, suppresses GRK2 translocation to the membrane, thereby preventing GRK2-initiated internalization and desensitization of CCR2 in response to MCP-1. This results in enhanced monocyte migration. These findings reveal a novel function for TLR4 signaling in promoting innate immune cell migration.

Inefficient antimicrobial functions of innate phagocytes render infant mice more susceptible to bacterial infection

Neonates and infants, due to the immaturity in their adaptive immunity, are thought to depend largely on the innate immune system for protection against bacterial infection. However, the innate immunity-mediated antimicrobial response in neonates and infants is incompletely characterized. Here, we report that infant mice were more susceptible to microbial sepsis than adult mice, with significantly reduced bacterial clearance from the circulation and visceral organs. Infant PMNs exhibited less constitutive expression of the chemokine receptor CXCR2, and bacterial infection caused further reduction of PMN CXCR2 in infant mice compared with adult mice. This correlates with diminished in vitro chemotaxis of infant PMNs toward the chemoattractant CXCL2 and impaired in vivo recruitment of infant PMNs into the infectious site. Furthermore, consistent with the reduced antimicrobial response in vivo, infant macrophages displayed an impaired bactericidal activity with a defect in phagosome maturation after ingestion of either gram-positive or gram-negative bacteria. Thus, infant mice exhibit an increased vulnerability to microbial infection with delayed bacterial clearance, which is associated with the inefficiency in their innate phagocyte-associated antimicrobial functions characterized by defects in PMN recruitment and macrophage phagosome maturation during microbial sepsis.

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.

The CCL2/CCR2 axis enhances vascular cell adhesion molecule-1 expression in human synovial fibroblasts

Background

Chemokine ligand 2 (CCL2), also known as monocyte chemoattractant protein-1 (MCP-1), belongs to the CC chemokine family that is associated with the disease status and outcomes of osteoarthritis (OA). Here, we investigated the intracellular signaling pathways involved in CCL2-induced vascular cell adhesion molecule-1 (VCAM-1) expression in human OA synovial fibroblasts (OASFs).

Methodology/Principal Findings

Stimulation of OASFs with CCL2 induced VCAM-1 expression. CCL2-mediated VCAM-1 expression was attenuated by CCR2 inhibitor (RS102895), PKCδ inhibitor (rottlerin), p38MAPK inhibitor (SB203580), and AP-1 inhibitors (curcumin and tanshinone IIA). Stimulation of cells with CCL2 increased PKCδ and p38MAPK activation. Treatment of OASFs with CCL2 also increased the c-Jun phosphorylation and c-Jun binding to the AP-1 element on the VCAM-1 promoter. Moreover, CCL2-mediated CCR2, PKCδ, p38MAPK, and AP-1 pathway promoted the adhesion of monocytes to the OASFs monolayer.

Conclusions/Significance

Our results suggest that CCL2 increases VCAM-1 expression in human OASFs via the CCR2, PKCδ, p38MAPK, c-Jun, and AP-1 signaling pathway. The CCL2-induced VCAM-1 expression promoted monocytes adhesion to human OASFs.

Hematopoietic G-protein-coupled receptor kinase 2 deficiency decreases atherosclerotic lesion formation in LDL receptor-knockout mice

Leukocyte chemotaxis is deemed instrumental in initiation and progression of atherosclerosis. It is mediated by G-protein-coupled receptors (e.g., CCR2 and CCR5), the activity of which is controlled by G-protein-coupled receptor kinases (GRKs). In this study, we analyzed the effect of hematopoietic deficiency of a potent regulator kinase of chemotaxis (GRK2) on atherogenesis. LDL receptor-deficient (LDLr(-/-)) mice with heterozygous hematopoietic GRK2 deficiency, generated by bone marrow transplantation (n=15), displayed a dramatic attenuation of plaque development, with 79% reduction in necrotic core and increased macrophage content. Circulating monocytes decreased and granulocytes increased in GRK2(+/-) chimeras, which could be attributed to diminished granulocyte colony-forming units in bone marrow. Collectively, these data pointed to myeloid cells as major mediators of the impaired atherogenic response in GRK2(+/-) chimeras. LDLr(-/-) mice with macrophage/granulocyte-specific GRK2 deficiency (LysM-Cre GRK2(flox/flox); n=8) failed to mimic the aforementioned phenotype, acquitting these cells as major responsible subsets for GRK2 deficiency-associated atheroprotection. To conclude, even partial hematopoietic GRK2 deficiency prevents atherosclerotic lesion progression beyond the fatty streak stage, identifying hematopoietic GRK2 as a potential target for intervention in atherosclerosis.

Milk fat globule-epidermal growth factor-factor 8 attenuates neutrophil infiltration in acute lung injury via modulation of CXCR2

Excessive neutrophil infiltration to the lungs is a hallmark of acute lung injury (ALI). Milk fat globule epidermal growth factor-factor 8 (MFG-E8) was originally identified for phagocytosis of apoptotic cells. Subsequent studies revealed its diverse cellular functions. However, whether MFG-E8 can regulate neutrophil function to alleviate inflammation is unknown. We therefore aimed to reveal MFG-E8 roles in regulating lung neutrophil infiltration during ALI. To induce ALI, C57BL/6J wild-type (WT) and Mfge8(-/-) mice were intratracheally injected with LPS (5 mg/kg). Lung tissue damage was assessed by histology, and the neutrophils were counted by a hemacytometer. Apoptotic cells in lungs were determined by TUNEL, whereas caspase-3 and myeloperoxidase activities were assessed spectrophotometrically. CXCR2 and G protein-coupled receptor kinase 2 expressions in neutrophils were measured by flow cytometry. Following LPS challenge, Mfge8(-/-) mice exhibited extensive lung damage due to exaggerated infiltration of neutrophils and production of TNF-α, MIP-2, and myeloperoxidase. An increased number of apoptotic cells was trapped into the lungs of Mfge8(-/-) mice compared with WT mice, which may be due to insufficient phagocytosis of apoptotic cells or increased occurrence of apoptosis through the activation of caspase-3. In vitro studies using MIP-2-mediated chemotaxis revealed higher migration of neutrophils of Mfge8(-/-) mice than those of WT mice via increased surface exposures to CXCR2. Administration of recombinant murine MFG-E8 reduces neutrophil migration through upregulation of GRK2 and downregulation of surface CXCR2 expression. Conversely, these effects could be blocked by anti-α(v) integrin Abs. These studies clearly indicate the importance of MFG-E8 in ameliorating neutrophil infiltration and suggest MFG-E8 as a novel therapeutic potential for ALI.

Role of myeloid-specific G-protein coupled receptor kinase-2 in sepsis

Previous studies have implicated a critical role for G-protein coupled receptor kinase-2 (GRK2) in sepsis owing to its ability to regulate inflammatory response and chemotaxis of immune cells. We therefore, hypothesized that deletion of GRK2 in myeloid cells would significantly modulate the pathogenesis of polymicrobial sepsis. To test this hypothesis, we induced cecal ligation and puncture (CLP), in mice with myeloid-specific deletion of GRK2 and the corresponding GRK2 wild type littermates and determined the inflammatory response (IL-6 and IL-10), immune cell infiltration, bacterial load and survival. Six hours after surgery, plasma IL-6 and IL-6:IL-10 ratios were significantly enhanced in the GRK2 knockouts compared to the GRK2 wild type mice. Compared to these effects, IL-6was significantly elevated in the bronchoalveolar lavage but not in the peritoneal fluid of the GRK2 knockout mice. On the other hand, peritoneal IL-10 was significantly elevated in the GRK2 knockout mice compared to the GRK2 wild type. Even though GRK2 knockout mice exhibited an exaggerated cytokine response, there was no difference in immune cell infiltration into the primary site of infection or in bacterial clearance when compared between the GRK2 wild type and GRK2 knockout mice after surgery. Furthermore, in spite of the enhanced pro-inflammatory profile early after surgery, there was only a modest increase in mortality in the GRK2 knockout compared to the GRK2 wild type mice after CLP. Together, our studies demonstrate that myeloid-specific knockout of GRK2 renders the mice more susceptible to an early pro-inflammatory state. However, myeloid-specific GRK2 is not involved in immune cell infiltration to the primary site of infection or in bacterial clearance and does not significantly modulate mortality in the cecal ligation puncture model of polymicrobial sepsis.

G protein-coupled receptor kinases: more than just kinases and not only for GPCRs

G protein-coupled receptor (GPCR) kinases (GRKs) are best known for their role in homologous desensitization of GPCRs. GRKs phosphorylate activated receptors and promote high affinity binding of arrestins, which precludes G protein coupling. GRKs have a multidomain structure, with the kinase domain inserted into a loop of a regulator of G protein signaling homology domain. Unlike many other kinases, GRKs do not need to be phosphorylated in their activation loop to achieve an activated state. Instead, they are directly activated by docking with active GPCRs. In this manner they are able to selectively phosphorylate Ser/Thr residues on only the activated form of the receptor, unlike related kinases such as protein kinase A. GRKs also phosphorylate a variety of non-GPCR substrates and regulate several signaling pathways via direct interactions with other proteins in a phosphorylation-independent manner. Multiple GRK subtypes are present in virtually every animal cell, with the highest expression levels found in neurons, with their extensive and complex signal regulation. Insufficient or excessive GRK activity was implicated in a variety of human disorders, ranging from heart failure to depression to Parkinson's disease. As key regulators of GPCR-dependent and -independent signaling pathways, GRKs are emerging drug targets and promising molecular tools for therapy. Targeted modulation of expression and/or of activity of several GRK isoforms for therapeutic purposes was recently validated in cardiac disorders and Parkinson's disease.

High-Content Analysis of CCR2 Antagonists on Human Primary Monocytes

The monocyte chemoattractant protein 1 (MCP-1)–driven activation of CC-type chemokine receptor 2 (CCR2) is one of the early key events to induce monocyte migration toward centers of inflammation. In this work, the authors analyzed MCP-1 internalization into primary human monocytes using partially automated liquid handling, automated fluorescence microscopic imaging, and a specific image analysis algorithm. A fluorophore-conjugated form of MCP-1 was rapidly endocytosed and retained by the monocytes. The CCR2 dependency of the MCP-1 internalization was demonstrated by the use of BMS CCR2 22, a CCR2-specific antagonist. The apparent inhibitory potencies of a series of small-molecule CCR2 antagonists were determined and compared in five assay formats, including the high-content analysis assay described in this work. Interestingly, some but not all antagonists showed markedly different inhibitory behaviors in the five readout systems, with an up to more than 100-fold difference between the highest and the lowest apparent inhibitory potencies. These findings raise the distinct possibility that some CCR2 antagonists are capable of discriminating between different functional states of the CCR2 receptor(s) and suggest strategies for the identification of functionally selective CCR2 antagonists with increased therapeutic advantage over nonselective antagonists.

Growth differentiation factor 15 deficiency protects against atherosclerosis by attenuating CCR2-mediated macrophage chemotaxis

The TGF-β family member GDF-15 promotes lesion formation and plaque instability in atherosclerosis-prone LDLr-deficient mice.

Growth differentiation factor (GDF) 15 is a member of the transforming growth factor β (TGF-β) superfamily, which operates in acute phase responses through a currently unknown receptor. Elevated GDF-15 serum levels were recently identified as a risk factor for acute coronary syndromes. We show that GDF-15 expression is up-regulated as disease progresses in murine atherosclerosis and primarily colocalizes with plaque macrophages. Hematopoietic GDF-15 deficiency in low density lipoprotein receptor^−/− mice led to impaired initial lesion formation and increased collagen in later lesions. Although lesion burden in GDF-15^−/− chimeras was unaltered, plaques had reduced macrophage infiltrates and decreased necrotic core formation, all features of improved plaque stability. In vitro studies pointed to a TGFβRII-dependent regulatory role of GDF-15 in cell death regulation. Importantly, GDF-15^−/− macrophages displayed reduced CCR2 expression, whereas GDF-15 promoted macrophage chemotaxis in a strictly CCR2- and TGFβRII-dependent manner, a phenomenon which was not observed in G protein–coupled receptor kinase 2^+/− macrophages. In conclusion, GDF-15 deletion has a beneficial effect both in early and later atherosclerosis by inhibition of CCR2-mediated chemotaxis and by modulating cell death. Our study is the first to identify GDF-15 as an acute phase modifier of CCR2/TGFβRII-dependent inflammatory responses to vascular injury.

Different effects of the different natural CC chemokine receptor 2b ligands on beta-arrestin recruitment, Gαi signaling, and receptor internalization

The chemokine receptor CCR2, which has been implicated in a variety of inflammatory, autoimmune, and cardiovascular conditions, binds several natural chemokine ligands. Here, we assessed the recruitment of β-arrestin to CCR2 in response to these ligands using bioluminescence resonance energy transfer technology. Compared with CCL2, which was considered as a full agonist, other CCR2 ligands were partial agonists with reduced efficacy and potency. Agonist potencies were not a function of their affinity for CCR2. Efficacy of arrestin recruitment matched that of agonist-induced CCR2 internalization. Although the potency and efficacy rank orders of the ligands in arrestin recruitment were similar to those observed for Gα(i1) activation, arrestin recruitment was at least in part resistant to Gα(i/o)-inactivating pertussis toxin, suggesting partial independence from Gα(i/o). The degree of pertussis toxin resistance of arrestin recruitment was different between the chemokines. Moreover, qualitative differences between the arrestin responses to the different ligands were identified in the stability of the response: although CCL7-induced arrestin recruitment had a half-life of less than 15 min, CCL8 and CCL13 induced stable CCR2-arrestin interactions. Finally, the ligands stabilized different conformations of the CCR2 homodimer. Our results support the validity of models for receptor-ligand interactions in which different ligands stabilize different receptor conformations also for endogenous receptor ligands, with corresponding implications for drug development targeting CCR2.

CXC chemokine ligand 4 (CXCL4) down-regulates CC chemokine receptor expression on human monocytes

During acute inflammation, monocytes are essential in abolishing invading micro-organisms and encouraging wound healing. Recruitment by CC chemokines is an important step in targeting monocytes to the inflamed tissue. However, cell surface expression of the corresponding chemokine receptors is subject to regulation by various endogenous stimuli which so far have not been comprehensively identified. We report that the platelet-derived CXC chemokine ligand 4 (CXCL4), a known activator of human monocytes, induces down-regulation of CC chemokine receptors (CCR) 1, -2, and -5, resulting in drastic impairment of monocyte chemotactic migration towards cognate CC chemokine ligands (CCL) for these receptors. Interestingly, CXCL4-mediated down-regulation of CCR1, CCR2 and CCR5 was strongly dependent on the chemokine's ability to stimulate autocrine/paracrine release of TNF-α. In turn, TNF-α induced the secretion CCL3 and CCL4, two chemokines selective for CCR1 and CCR5, while the secretion of CCR2-ligand CCL2 was TNF-α-independent. Culture supernatants of CXCL4-stimulated monocytes as well as chemokine-enriched preparations thereof reproduced CXCL4-induced CCR down-regulation. In conclusion, CXCL4 may act as a selective regulator of monocyte migration by stimulating the release of autocrine, receptor-desensitizing chemokine ligands. Our results stress a co-ordinating role for CXCL4 in the cross-talk between platelets and monocytes during early inflammation.

Filamin a binds to CCR2B and regulates its internalization

The chemokine (C-C motif) receptor 2B (CCR2B) is one of the two isoforms of the receptor for monocyte chemoattractant protein-1 (CCL2), the major chemoattractant for monocytes, involved in an array of chronic inflammatory diseases. Employing the yeast two-hybrid system, we identified the actin-binding protein filamin A (FLNa) as a protein that associates with the carboxyl-terminal tail of CCR2B. Co-immunoprecipitation experiments and in vitro pull down assays demonstrated that FLNa binds constitutively to CCR2B. The colocalization of endogenous CCR2B and filamin A was detected at the surface and in internalized vesicles of THP-1 cells. In addition, CCR2B and FLNa were colocalized in lamellipodia structures of CCR2B-expressing A7 cells. Expression of the receptor in filamin-deficient M2 cells together with siRNA experiments knocking down FLNa in HEK293 cells, demonstrated that lack of FLNa delays the internalization of the receptor. Furthermore, depletion of FLNa in THP-1 monocytes by RNA interference reduced the migration of cells in response to MCP-1. Therefore, FLNa emerges as an important protein for controlling the internalization and spatial localization of the CCR2B receptor in different dynamic membrane structures.

An antimicrobial peptide regulates tumor-associated macrophage trafficking via the chemokine receptor CCR2, a model for tumorigenesis

BACKGROUND

Tumor-associated macrophages (TAMs) constitute a significant part of infiltrating inflammatory cells that are frequently correlated with progression and poor prognosis of a variety of cancers. Tumor cell-produced human beta-defensin-3 (hBD-3) has been associated with TAM trafficking in oral cancer; however, its involvement in tumor-related inflammatory processes remains largely unknown.

METHODOLOGY

The relationship between hBD-3, monocyte chemoattractant protein-1 (MCP-1), TAMs, and CCR2 was examined using immunofluorescence microscopy in normal and oral carcinoma in situ biopsy specimens. The ability of hBD-3 to chemoattract host macrophages in vivo using a nude mouse model and analysis of hBD-3 on monocytic cell migration in vitro, applying a cross-desensitization strategy of CCR2 and its pharmacological inhibitor (RS102895), respectively, was also carried out.

CONCLUSIONS/FINDINGS

MCP-1, the most frequently expressed tumor cell-associated chemokine, was not produced by tumor cells nor correlated with the recruitment of macrophages in oral carcinoma in situ lesions. However, hBD-3 was associated with macrophage recruitment in these lesions and hBD-3-expressing tumorigenic cells induced massive tumor infiltration of host macrophages in nude mice. HBD-3 stimulated the expression of tumor-promoting cytokines, including interleukin-1alpha (IL-1alpha), IL-6, IL-8, CCL18, and tumor necrosis factor-alpha (TNF-alpha) in macrophages derived from human peripheral blood monocytes. Monocytic cell migration in response to hBD-3 was inhibited by cross-desensitization with MCP-1 and the specific CCR2 inhibitor, RS102895, suggesting that CCR2 mediates monocyte/macrophage migration in response to hBD-3. Collectively, these results indicate that hBD-3 utilizes CCR2 to regulate monocyte/macrophage trafficking and may act as a tumor cell-produced chemoattractant to recruit TAMs. This novel mechanism is the first evidence of an hBD molecule orchestrating an in vivo outcome and demonstrates the importance of the innate immune system in the development of tumors.