Chemoattractant receptors activate distinct pathways for chemotaxis and secretion. Role of G-protein usage

Stepwise phosphorylation of leukotriene B4 receptor 1 defines cellular responses to leukotriene B4

Leukotriene B₄ (LTB₄) receptor type 1 (BLT1) is abundant in phagocytic and immune cells and plays crucial roles in various inflammatory diseases. BLT1 is phosphorylated at several serine and threonine residues upon stimulation with the inflammatory lipid LTB₄ Using Phos-tag gel electrophoresis to separate differentially phosphorylated forms of BLT1, we identified two distinct types of phosphorylation, basal and ligand-induced, in the carboxyl terminus of human BLT1. In the absence of LTB₄, the basal phosphorylation sites were modified to various degrees, giving rise to many different phosphorylated forms of BLT1. Different concentrations of LTB₄ induced distinct phosphorylation events, and these ligand-induced modifications facilitated additional phosphorylation events at the basal phosphorylation sites. Because neutrophils migrate toward inflammatory sites along a gradient of LTB₄, the degree of BLT1 phosphorylation likely increases in parallel with the increase in LTB₄ concentration as the cells migrate. At high concentrations of LTB₄, deficiencies in these two types of phosphorylation events impaired chemotaxis and β-hexosaminidase release, a proxy for degranulation, in Chinese hamster ovary (CHO-K1) and rat basophilic leukemia (RBL-2H3) cells, respectively. These results suggest that an LTB₄ gradient around inflammatory sites enhances BLT1 phosphorylation in a stepwise manner to facilitate the precise migration of phagocytic and immune cells and the initiation of local responses, including degranulation.

Decoding the chemotactic signal

From an individual bacterium to the cells that compose the human immune system, cellular chemotaxis plays a fundamental role in allowing cells to navigate, interpret, and respond to their environments. While many features of cellular chemotaxis are shared among systems as diverse as bacteria and human immune cells, the machinery that guides the migration of these model organisms varies widely. In this article, we review current literature on the diversity of chemoattractant ligands, the cell surface receptors that detect and process chemotactic gradients, and the link between signal recognition and the regulation of cellular machinery that allow for efficient directed cellular movement. These facets of cellular chemotaxis are compared among E. coli, Dictyostelium discoideum, and mammalian neutrophils to derive organizational principles by which diverse cell systems sense and respond to chemotactic gradients to initiate cellular migration.

The role of the LTB4-BLT1 axis in chemotactic gradient sensing and directed leukocyte migration

Directed leukocyte migration is a hallmark of inflammatory immune responses. Leukotrienes are derived from arachidonic acid and represent a class of potent lipid mediators of leukocyte migration. In this review, we summarize the essential steps leading to the production of LTB₄ in leukocytes. We discuss the recent findings on the exosomal packaging and transport of LTB₄ in the context of chemotactic gradients formation and regulation of leukocyte recruitment. We also discuss the dynamic roles of the LTB₄ receptors, BLT1 and BLT2, in mediating chemotactic signaling in leukocytes and contrast them to other structurally related leukotrienes that bind to distinct GPCRs. Finally, we highlight the specific roles of the LTB₄-BLT1 axis in mediating signal-relay between chemotaxing neutrophils and its potential contribution to a wide variety of inflammatory conditions including tumor progression and metastasis, where LTB₄ is emerging as a key signaling component.

Platelet-activating factor receptor-mediated PI3K/AKT activation contributes to the malignant development of esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC) is one of the most common malignancies worldwide and occurs at a relatively high frequency in China, yet the mechanisms underlying its devastating outcome remain unclear. Here we report that platelet-activating factor receptor (PAFR), a type of G-protein-coupled receptor, was upregulated in ESCC tumors and cell lines, compared with controls; PAFR levels were positively correlated with ESCC clinical stages and survival time. Overexpression of PAFR promoted the malignant development of ESCC in vitro and in vivo, whereas depletion of PAFR suppressed these effects. Interestingly, PAFR was observed to activate PI3K/AKT (phosphatidylinositol 3-kinase/AKT) through the upregulation of FAK kinase activity. AKT-triggered nuclear factor-κB transcriptionally activated PAFR expression. This mutual positive regulation between PAFR and AKT was required for the aggressiveness of ESCC cells both in vitro and in vivo. Furthermore, treating mice bearing ESCC tumors with cholesterol-conjugated PAFR small interfering RNA effectively inhibited tumor progression and the expression of AKT-mediated oncogenic proteins. Taken together, we made the first demonstration that dysregulation of PAFR and the positive regulatory loop between PAFR and pAKT contribute to malignant progression of ESCC.

The role of leukotrienes in allergic diseases

Leukotrienes (LTs), both LTB4 and the cysteinyl LTs (CysLTs) LTC4, LTD4 and LTE4, are implicated in a wide variety of inflammatory disorders. These lipid mediators are generated from arachidonic acid via multistep enzymatic reactions through which arachidonic acid is liberated from membrane phospholipids through the action of phospholipase A2. LTB4 and CysLTs exert their biological effects by binding to cognate receptors, which belong to the G protein-coupled receptor superfamily. LTB4 is widely considered to be a potent chemoattractant for most subsets of leukocytes, whereas CysLTs are potent bronchoconstrictors that have effects on airway remodeling. LTs play a central role in the pathogenesis of asthma and many other inflammatory diseases. This review will provide an update on the synthesis, biological function, and relevance of LTs to the pathobiology of allergic diseases, and examine the current and future therapeutic prospects of LT modifiers.

Ras promotes transforming growth factor-β (TGF-β)-induced epithelial-mesenchymal transition via a leukotriene B4 receptor-2-linked cascade in mammary epithelial cells

Inflammation and inflammatory mediators are inextricably linked with epithelial-mesenchymal transition (EMT) through complex pathways in the tumor microenvironment. However, the mechanism by which inflammatory mediators, such as the lipid inflammatory mediators, eicosanoids, contribute to EMT is largely unknown. In the present study we observed that BLT2, leukotriene B4 receptor-2, is markedly up-regulated by oncogenic Ras and promotes EMT in response to transforming growth factor-β (TGF-β) in mammary epithelial cells. Blockade of BLT2 by the BLT2 inhibitor LY255283 or by siRNA reduced EMT induced by Ras in the presence of TGF-β. In addition, stimulation of BLT2 by the addition of a BLT2 ligand, such as leukotriene B4, restored EMT in the presence of TGF-β in human immortalized mammary epithelial MCF-10A cells. We further searched BLT2 downstream components and identified reactive oxygen species and nuclear factor κB as critical components that contribute to EMT. Taken together, these results demonstrate for the first time that a BLT2-linked inflammatory pathway contributes to EMT. This provides valuable insight into the mechanism of EMT in mammary epithelial cells. In addition, considering the implications of EMT with the stemness of cancer cells, our finding may contribute to a better understanding of tumor progression.

RanBPM protein acts as a negative regulator of BLT2 receptor to attenuate BLT2-mediated cell motility

BLT2, a low affinity receptor for leukotriene B4 (LTB4), is a member of the G protein-coupled receptor family and is involved in many signal transduction pathways associated with various cellular phenotypes, including chemotactic motility. However, the regulatory mechanism for BLT2 has not yet been demonstrated. To understand the regulatory mechanism of BLT2, we screened and identified the proteins that bind to BLT2. Using a yeast two-hybrid assay with the BLT2 C-terminal domain as bait, we found that RanBPM, a previously proposed scaffold protein, interacts with BLT2. We demonstrated the specific interaction between BLT2 and RanBPM by GST pulldown assay and co-immunoprecipitation assay. To elucidate the biological function of the RanBPM-BLT2 interaction, we evaluated the effects of RanBPM overexpression or knockdown. We found that BLT2-mediated motility was severely attenuated by RanBPM overexpression and that knockdown of endogenous RanBPM by shRNA strongly promoted BLT2-mediated motility, suggesting a negative regulatory function of RanBPM toward BLT2. Furthermore, we observed that the addition of BLT2 ligands caused the dissociation of BLT2 and RanBPM, thus releasing the negative regulatory effect of RanBPM. Finally, we propose that Akt-induced BLT2 phosphorylation at residue Thr(355), which occurs after the addition of BLT2 ligands, is a potential mechanism by which BLT2 dissociates from RanBPM, resulting in stimulation of BLT2 signaling. Taken together, our results suggest that RanBPM acts as a negative regulator of BLT2 signaling to attenuate BLT2-mediated cell motility.

Staphylococcus aureus lipoteichoic acid inhibits platelet activation and thrombus formation via the Paf receptor

Impaired healing is common in wounds infected with the major human pathogen Staphylococcus aureus, although the underlying mechanisms are poorly understood. Here, we show that S. aureus lipoteichoic acid (LTA) inhibits platelet aggregation caused by physiological agonists and S. aureus and reduced platelet thrombus formation in vitro. The presence of D-alanine on LTA is necessary for the full inhibitory effect. Inhibition of aggregation was blocked using a monoclonal anti-platelet activating factor receptor (PafR) antibody and Ginkgolide B, a well-defined PafR antagonist, demonstrating that the LTA inhibitory signal occurs via PafR. Using a cyclic AMP (cAMP) assay and a Western blot for phosphorylated VASP, we determined that cAMP levels increase upon platelet incubation with LTA, an effect which inhibits platelet activation. This was blocked when platelets were preincubated with Ginkgolide B. Furthermore, LTA reduced hemostasis in a mouse tail-bleed assay.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.

Defective macrophage migration in Gαi2- but not Gαi3-deficient mice

Various heterotrimeric G(i) proteins are considered to be involved in cell migration and effector function of immune cells. The underlying mechanisms, how they control the activation of myeloid effector cells, are not well understood. To elucidate isoform-redundant and -specific roles for Gα(i) proteins in these processes, we analyzed mice genetically deficient in Gα(i2) or Gα(i3). First, we show an altered distribution of tissue macrophages and blood monocytes in the absence of Gα(i2) but not Gα(i3). Gα(i2)-deficient but not wild-type or Gα(i3)-deficient mice exhibited reduced recruitment of macrophages in experimental models of thioglycollate-induced peritonitis and LPS-triggered lung injury. In contrast, genetic ablation of Gα(i2) had no effect on Gα(i)-dependent peritoneal cytokine production in vitro and the phagocytosis-promoting function of the Gα(i)-coupled C5a anaphylatoxin receptor by liver macrophages in vivo. Interestingly, actin rearrangement and CCL2- and C5a anaphylatoxin receptor-induced chemotaxis but not macrophage CCR2 and C5a anaphylatoxin receptor expression were reduced in the specific absence of Gα(i2). Furthermore, knockdown of Gα(i2) caused decreased cell migration and motility of RAW 264.7 cells, which was rescued by transfection of Gα(i2) but not Gα(i3). These results indicate that Gα(i2), albeit redundant to Gα(i3) in some macrophage activation processes, clearly exhibits a Gα(i) isoform-specific role in the regulation of macrophage migration.

CCR1 as a target for multiple myeloma

INTRODUCTION

By directing cell trafficking, differentiation and growth, chemokines modulate the immune response and are involved in the pathogenesis of autoimmune diseases and cancers, including multiple myeloma (MM). MM, the second most common hematological malignancy in the US, is characterized by disordered plasma cell growth within the bone marrow microenvironment. CCL3 and its receptors, CCR1 in particular, play a central role in the pathogenesis of MM and MM-induced osteolytic bone disease.

AREAS COVERED

This review describes the functional role of CCR1 in MM and the preclinical results observed with CCR1 antagonists. CCL3 and CCR1 stimulate tumor growth, both directly and indirectly, via upregulation of cell adhesion and cytokine secretion. In addition, they modulate the osteoclast/osteoblast balance, by inducing osteoclast differentiation and inhibiting osteoblast function. Targeting either ligand or receptor reverses these effects, leading to in vivo tumor burden control and prevention of osteolysis, as confirmed in both murine and humanized mouse models.

EXPERT OPINION

These promising data set the stage for clinical trials to assess the effects of CCR1 inhibitors in MM. The success of these studies depends on the development of novel antagonists with improved chemical/physical properties and careful selection of the patient population who may benefit the most from these agents.

BLT2 Is upregulated in allergen-stimulated mast cells and mediates the synthesis of Th2 cytokines

Mast cells are effector cells that mediate the allergic response through Ag stimulation of IgE bound to FcεRI. In allergic reactions, cross-linking of the surface receptors for IgE on mast cells results in the synthesis of Th2 cytokines such as IL-4 and IL-13, which are critical for the initiation and progression of the allergic response. Despite the important roles of these cytokines, the signaling mechanism by which Ag stimulation mediates the production of IL-4 and IL-13 in mast cells is not clearly understood. In the present study, we found that Ag-stimulated bone marrow-derived mast cells (BMMCs) highly upregulated the expression of BLT2, a leukotriene B(4) receptor, and that blockade of BLT2 with the specific antagonist LY255283 or small interfering RNA knockdown completely abolished the production of Th2 cytokines. Furthermore, BMMCs overexpressing BLT2 showed significantly enhanced production of Th2 cytokines compared with wild-type BMMCs. Additionally, we found that the generation of Nox1-derived reactive oxygen species occurs downstream of BLT2, thus mediating the synthesis of Th2 cytokines. Taken together, our results suggest that the BLT2-Nox1-reactive oxygen species cascade is a previously unsuspected mediatory signaling mechanism to Th2 cytokine production in Ag-stimulated BMMCs, thus contributing to allergic response.

Blockade of LTB4-induced chemotaxis by bioactive molecules interfering with the BLT2-Galphai interaction

BLT2, a low-affinity leukotriene B4 (LTB4) receptor, is a member of the G-protein coupled receptor (GPCR) family and is involved in the pathogenesis of inflammatory diseases such as asthma. Despite its clinical implications, however, no pharmacological inhibitors are available. In the present study, we screened for small molecules that interfere with the interaction between the third intracellular loop region of BLT2 (BLT2iL3) and the Galphai3 protein subunit (Galphai3), using a high-throughput screening (HTS) assay with a library of 1040 FDA-approved drugs and bioactive compounds. We identified two small molecules-purpurin [1,2,4-trihydroxy-9,10-anthraquinone; IC50 = 1.6 microM for BLT2] and chloranil [tetrachloro-1,4-benzoquinone; IC50 = 0.42 microM for BLT2]-as specific BLT2-blocking agents. We found that blockade of the BLT2iL3-Galphai3 interaction by these small molecules inhibited the BLT2-downstream signaling cascade. For example, BLT2-signaling to phosphoinositide-3 kinase (PI3K)/Akt phosphorylation was completely abolished by these molecules. Furthermore, we observed that these small molecules blocked LTB4-induced chemotaxis by inhibiting the BLT2-PI3K/Akt-downstream, Rac1-reactive oxygen species-dependent pathway. Taken together, our results show that purpurin and chloranil interfere with the interaction between BLT2iL3 and Galphai3 and thus block the biological functions of BLT2 (e.g., chemotaxis). The present findings suggest a potential application of purpurin and chloranil as pharmacological therapeutic agents against BLT2-associated inflammatory human diseases.

Relationship between calcium release and NADPH oxidase inhibition in human neutrophils

The aim of this study was to investigate the possible relationship between NADPH oxidase activity and changes in cytosolic Ca(2+) in response to different agonists. Treatment of neutrophils with leukotriene B4 (LTB(4)) demonstrated characteristic changes to cytoslic Ca(2+) yielding an EC(50) of 4nM. The pA(2) values for the specific LTB(4) receptor (BLT) antagonists, U-75302 and LY-255283 were 6.32 and 6.38, respectively. Similarly, neutrophils treated with N-formyl-l-methionyl-l-leucyl-l-phenylalanine (FMLP) and platelet activating factor (PAF) exhibited changes in cytoslic Ca(2+) in a dose dependant manner with pD(2) values of 9.0 and 9.9, respectively. The phorbol ester PMA prevented elevations in cytosolic Ca(2+) in response to LTB(4), FMLP and PAF with IC(50) values of 5.88, 1.44 and 5.71nM, respectively. In addition, potent NADPH oxidase inhibitors apocynin and diphenyleneiodonium (DPI) inhibited FMLP mediated cytosolic Ca(2+) release. These results demonstrate that inhibition of the NADPH oxidase suppresses cytosolic Ca(2+) release in FMLP activated human neutrophils.

Pertussis toxin signals through the TCR to initiate cross-desensitization of the chemokine receptor CXCR4

Pertussis toxin (PTx) has been shown to exert a variety of effects on immune cells independent of its ability to ADP-ribosylate G proteins. Of these effects, the binding subunit of PTx (PTxB) has been shown to block signaling via the chemokine receptor CCR5, but the mechanism involved in this process is unknown. Here, we show that PTxB causes desensitization of a related chemokine receptor, CXCR4, and explore the mechanism by which this occurs. CXCR4 is the receptor for the chemokine stromal cell-derived factor 1alpha (SDF-1alpha) and elicits a number of biological effects, including stimulation of T cell migration. PTxB treatment causes a decrease in CXCR4 surface expression, inhibits G protein-associated signaling, and blocks SDF-1alpha-mediated chemotaxis. We show that PTxB mediates these effects by activating the TCR signaling network, as the effects are dependent on TCR and ZAP70 expression. Additionally, the activation of the TCR with anti-CD3 mAb elicits a similar set of effects on CXCR4 activity, supporting the idea that TCR signaling leads to cross-desensitization of CXCR4. The inhibition of CXCR4 by PTxB is rapid and transient; however, the catalytic activity of PTx prevents CXCR4 signaling in the long term. Thus, the effects of PTx holotoxin on CXCR4 signaling can be divided into two phases: short term by the B subunit, and long term by the catalytic subunit. These data suggest that TCR crosstalk with CXCR4 is likely a normal cellular process that leads to cross-desensitization, which is exploited by the B subunit of PTx.

Receptor "hijacking" by malignant glioma cells: a tactic for tumor progression

Gliomas are the most common and deadly tumors in the central nervous system (CNS). In the course of studying the role of chemoattractant receptors in tumor growth and metastasis, we discovered that highly malignant human glioblastoma and anaplastic astrocytoma specimens were stained positively for the formylpeptide receptor (FPR), which is normally expressed in myeloid cells and accounts for their chemotaxis and activation induced by bacterial peptides. Screening of human glioma cell lines revealed that FPR was expressed selectively in glioma cell lines with a more highly malignant phenotype. FPR expressed in glioblastoma cell lines mediates cell chemotaxis, proliferation and production of an angiogenic factor, vascular endothelial growth factor (VEGF), in response to agonists released by necrotic tumor cells. Furthermore, FPR in glioblastoma cells activates the receptor for epidermal growth factor (EGFR) by increasing the phosphorylation of a selected tyrosine residue in the intracellular tail of EGFR. Thus, FPR hijacked by human glioblastoma cells exploits the function of EGFR to promote rapid tumor progression.

Specific leukotriene receptors couple to distinct G proteins to effect stimulation of alveolar macrophage host defense functions

Leukotrienes (LTs) are lipid mediators implicated in asthma and other inflammatory diseases. LTB(4) and LTD(4) also participate in antimicrobial defense by stimulating phagocyte functions via ligation of B leukotriene type 1 (BLT1) receptor and cysteinyl LT type 1 (cysLT1) receptor, respectively. Although both Galpha(i) and Galpha(q) proteins have been shown to be coupled to both BLT1 and cysLT1 receptors in transfected cell systems, there is little known about specific G protein subunit coupling to LT receptors, or to other G protein-coupled receptors, in primary cells. In this study we sought to define the role of specific G proteins in pulmonary alveolar macrophage (AM) innate immune responses to LTB(4) and LTD(4). LTB(4) but not LTD(4) reduced cAMP levels in rat AM by a pertussis toxin (PTX)-sensitive mechanism. Enhancement of FcgammaR-mediated phagocytosis and bacterial killing by LTB(4) was also PTX-sensitive, whereas that induced by LTD(4) was not. LTD(4) and LTB(4) induced Ca(2+) and intracellular inositol monophosphate accumulation, respectively, highlighting the role of Galpha(q) protein in mediating PTX-insensitive LTD(4) enhancement of phagocytosis and microbicidal activity. Studies with liposome-delivered G protein blocking Abs indicated a dependency on specific Galpha(q/11) and Galpha(i3) subunits, but not Galpha(i2) or G(beta)gamma, in LTB(4)-enhanced phagocytosis. The selective importance of Galpha(q/11) protein was also demonstrated in LTD(4)-enhanced phagocytosis. The present investigation identifies differences in specific G protein subunit coupling to LT receptors in antimicrobial responses and highlights the importance of defining the specific G proteins coupled to heptahelical receptors in primary cells, rather than simply using heterologous expression systems.

Lysophosphatidylglycerol stimulates chemotactic migration and tube formation in human umbilical vein endothelial cells

In this study, we observed that lysophosphatidylglycerol (LPG) stimulates extracellular signal-regulated kinase (ERK) in human umbilical vein endothelial cells (HUVECs). LPG-stimulated ERK activity was not inhibited by pertussis toxin (PTX), indicating PTX-sensitive G-proteins-independent manner. In terms of functional aspect, LPG induced chemotactic migration of HUVECs in a PTX-insensitive manner. Preincubation of HUVECs with an ERK inhibitor (PD98059) completely inhibited LPG-induced chemotactic migration, suggesting the crucial role of ERK in the process. LPG-induced ERK activation and chemotactic migration in HUVECs were not affected by an lysophosphatidic acid receptor-selective antagonist (Ki16425), indicating lysophosphatidic acid receptors-independency. We also found that LPG stimulated tube formation in HUVECs. Taken together we suggest that LPG stimulates HUVECs and result in chemotactic migration and tube formation, suggesting a new aspect of LPG as a modulator of endothelial cell functioning.

Identification of an alternative G{alpha}q-dependent chemokine receptor signal transduction pathway in dendritic cells and granulocytes

CD38 controls the chemotaxis of leukocytes to some, but not all, chemokines, suggesting that chemokine receptor signaling in leukocytes is more diverse than previously appreciated. To determine the basis for this signaling heterogeneity, we examined the chemokine receptors that signal in a CD38-dependent manner and identified a novel "alternative" chemokine receptor signaling pathway. Similar to the "classical" signaling pathway, the alternative chemokine receptor pathway is activated by Galpha(i2)-containing Gi proteins. However, unlike the classical pathway, the alternative pathway is also dependent on the Gq class of G proteins. We show that Galpha(q)-deficient neutrophils and dendritic cells (DCs) make defective calcium and chemotactic responses upon stimulation with N-formyl methionyl leucyl phenylalanine and CC chemokine ligand (CCL) 3 (neutrophils), or upon stimulation with CCL2, CCL19, CCL21, and CXC chemokine ligand (CXCL) 12 (DCs). In contrast, Galpha(q)-deficient T cell responses to CXCL12 and CCL19 remain intact. Thus, the alternative chemokine receptor pathway controls the migration of only a subset of cells. Regardless, the novel alternative chemokine receptor signaling pathway appears to be critically important for the initiation of inflammatory responses, as Galpha(q) is required for the migration of DCs from the skin to draining lymph nodes after fluorescein isothiocyanate sensitization and the emigration of monocytes from the bone marrow into inflamed skin after contact sensitization.

Cryptotanshinone inhibits chemotactic migration in macrophages through negative regulation of the PI3K signaling pathway

BACKGROUND AND PURPOSE

Cryptotanshinone, the major tanshinone isolated from Salvia miltiorrhiza Bunge, exhibits anti-inflammatory activity. However, there is no report on the effect of cryptotanshinone on recruitment of leukocytes to inflammatory sites. We therefore assessed the effects of cryptotanshinone on macrophage chemotaxis.

EXPERIMENTAL APPROACH

Macrophage migration induced by complement 5a (C5a) or macrophage inflammatory protein-1alpha (MIP-1alpha) was measured in vitro. Intracellular kinase translocation and phosphorylation was assessed by Western blotting.

KEY RESULTS

RAW264.7 cell migration towards C5a (1 microg ml(-1)) was significantly inhibited by cryptotanshinone (1, 3, 10 and 30 microM) in a concentration-dependent manner. Primary human macrophages stimulated by C5a were similarly inhibited. C5a-evoked migration in RAW264.7 cells was significantly suppressed by wortmannin (phosphatidylinositol 3-kinase (PI3K) inhibitor), PD98059 (MEK1/2 inhibitor) and SB203580 (p38 mitogen-activated protein kinase (MAPK) inhibitor), but not by SP600125 (c-Jun N-terminal kinase (JNK) inhibitor), suggesting that activation of PI3K, ERK1/2 and p38 MAPK signal pathways was involved in responses to C5a. Western blotting revealed that cryptotanshinone significantly inhibited PI3K-p110gamma membrane translocation and phosphorylation of Akt (PI3K downstream effector protein) and ERK1/2 induced by C5a. However, neither p38 MAPK nor JNK phosphorylation was affected by cryptotanshinone. Wortmannin significantly attenuated C5a-induced PI3K-p110gamma translocation, Akt and ERK1/2 phosphorylation. PD98059 suppressed ERK1/2 phosphorylation but failed to modify PI3K-p110gamma translocation by C5a stimulation. Furthermore, MIP-1alpha-induced cell migration and PI3K-p110gamma translocation were also inhibited by cryptotanshinone in a concentration-dependent manner.

CONCLUSIONS AND IMPLICATIONS

Inhibition of macrophage migration by cryptotanshinone involved inhibition of PI3K activation with consequent reduction of phosphorylation of Akt and ERK1/2.

Expression and functional role of formyl peptide receptor in human bone marrow-derived mesenchymal stem cells

We investigated the expression of formyl peptide receptor (FPR) and its functional role in human bone marrow-derived mesenchymal stem cells (MSCs). We analyzed the expression of FPR by using ligand-binding assay with radio-labeled N-formyl-met-leu-phe (fMLF), and found that MSCs express FPR. FMLF stimulated intracellular calcium increase, mitogen-activated protein kinases activation, and Akt activation, which were mediated by G(i) proteins. MSCs were chemotactically migrated to fMLF. FMLF-induced MSC chemotaxis was also completely inhibited by pertussis toxin, LY294002, and PD98059, indicating the role of G(i) proteins, phosphoinositide 3-kinase, and extracellular signal regulated protein kinase. N-terminal fragment of annexin-1, Anx-1(2-26), an endogenous agonist for FPR, also induced chemotactic migration of MSCs. Thus MSCs express functional FPR, suggesting a new (patho)physiological role of FPR and its ligands in regulating MSC trafficking during induction of injured tissue repair.

An apparent paradox: Chemokine receptor agonists can be used for anti-inflammatory therapy

Inflammation plays an important role in a wide range of human diseases. Chemokines are a group of proteins which control the migration and activation of the immune cells involved in all aspects of the inflammatory response. Chemokines bind to specific receptors of the seven-transmembrane spanning type on target leukocytes and also bind to cell-surface glycosaminoglycans (GAG). Leukocytes express a range of chemokine receptors which can cross-desensitise each other, potentially allowing a single chemokine receptor agonist to desensitise all the chemokine receptors on a cell. If an appropriate single receptor agonist is engineered to be non-chemotactic itself, then a treated cell will lose the potential to migrate in response to chemokines towards any developing site of inflammation. A non-GAG-binding but receptor agonistic form of the chemokine CCL7 can inhibit leukocyte recruitment in response to a diverse range of chemokines in vitro and in vivo. We hypothesise that this modified chemokine mediates its effect by inducing homologous and heterologous receptor desensitisation and further propose that other suitable candidates could include agonistic chemokine receptor-specific antibodies or small molecule chemokine receptor agonists. Hence, an appropriate chemokine receptor agonist could be used to inhibit multiple chemokine receptors, thereby producing a powerful and robust anti-inflammatory effect. This review considers the mechanisms leading to chemokine receptor desensitisation and discusses the potential to develop a new class of anti-inflammatory agents based on targeted stimulation of chemokine receptors.

Critical role for polar residues in coupling leukotriene B4 binding to signal transduction in BLT1

Leukotriene B(4) (LTB(4)) mediates a variety of inflammatory diseases such as asthma, arthritis, atherosclerosis, and cancer through activation of the G-protein-coupled receptor, BLT1. Using in silico molecular dynamics simulations combined with site-directed mutagenesis we characterized the ligand binding site and activation mechanism for BLT1. Mutation of residues predicted as potential ligand contact points in transmembrane domains (TMs) III (H94A and Y102A), V (E185A), and VI (N241A) resulted in reduced binding affinity. Analysis of arginines in extracellular loop 2 revealed that mutating arginine 156 but not arginine 171 or 178 to alanine resulted in complete loss of LTB(4) binding to BLT1. Structural models for the ligand-free and ligand-bound states of BLT1 revealed an activation core formed around Asp-64, displaying multiple dynamic interactions with Asn-36, Ser-100, and Asn-281 and a triad of serines, Ser-276, Ser-277, and Ser-278. Mutagenesis of many of these residues in BLT1 resulted in loss of signaling capacity while retaining normal LTB(4) binding function. Thus, polar residues within TMs III, V, and VI and extracellular loop 2 are critical for ligand binding, whereas polar residues in TMs II, III, and VII play a central role in transducing the ligand-induced conformational change to activation. The delineation of a validated binding site and activation mechanism should facilitate structure-based design of inhibitors targeting BLT1.

The Immunosuppressant Cyclosporin A Antagonizes Human Formyl Peptide Receptor through Inhibition of Cognate Ligand Binding

Cyclosporin A (CsA) is a fungus-derived cyclic undecapeptide with potent immunosuppressive activity. Its analog, cyclosporin H (CsH), lacks immunosuppressive function but can act as an antagonist for the human formyl peptide receptor (FPR). More recent studies have shown that CsA also inhibits fMLF-induced degranulation in differentiated HL-60 promyelocytic leukemia cells. However, it is unclear whether CsA interferes with ligand-receptor interaction, G protein activation, or other downstream signaling events. In this study we used human neutrophils, differentiated HL-60 cells, and rat basophilic leukemia (RBL)-2H3 cells expressing human FPR (RBL-FPR) to identify the action site of CsA. In functional assays, CsA inhibited fMLF-stimulated degranulation, chemotaxis, calcium mobilization, and phosphorylation of the MAPKs ERK 1/2 and the serine/threonine protein kinase Akt. CsA also blocked Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm)-induced functions in RBL-FPR cells. Concentrations for half-maximal inhibition with CsA are generally 6- to 50-fold higher than that of CsH. CsA was compared with another immunosuppressant, ascomycin, relative to the inhibitory effects on FPR-mediated chemotaxis, calcium mobilization, and degranulation. In these experiments, ascomycin produced no inhibitory effects at low micromolar concentrations (1-4 microM), whereas the inhibitory effects of CsA were prominent at comparable concentrations. Finally, CsA dose-dependently inhibited the uptake of fNle-Leu-Phe-Nle-Tyr-Lys-fluoresceine and [3H]fMLF or [125I]WKYMVm binding to FPR. However, CsA and CsH did not show any obvious inhibitory effect on FPR-like 1-mediated cellular functions. These results demonstrate that CsA is a selective antagonist of FPR and that its inhibition of fMLF-stimulated leukocyte activation is at the level of cognate ligand binding.

Activation and regulation of platelet-activating factor receptor: role of G(i) and G(q) in receptor-mediated chemotactic, cytotoxic, and cross-regulatory signals

Platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycerolphosphocholine; PAF) induces leukocyte accumulation and activation at sites of inflammation via the activation of a specific cell surface receptor (PAFR). PAFR couples to both pertussis toxin-sensitive and pertussis toxin-insensitive G proteins to activate leukocytes. To define the role(s) of G(i) and G(q) in PAF-induced leukocyte responses, two G-protein-linked receptors were generated by fusing G alpha(i3) (PAFR-G alpha(i3)) or G alpha(q) (PAFR-G alpha(q)) at the C terminus of PAFR. Rat basophilic leukemia cell line (RBL-2H3) stably expressing wild-type PAFR, PAFR-G alpha(i3), or PAFR-G alpha(q) was generated and characterized. All receptor variants bound PAF with similar affinities to mediate G-protein activation, intracellular Ca2+ mobilization, phosphoinositide (PI) hydrolysis, and secretion of beta-hexosaminidase. PAFR-G alpha(i3) and PAFR-G alpha(q) mediated greater GTPase activity in isolated membranes than PAFR but lower PI hydrolysis and secretion in whole cells. PAFR and PAFR-G alpha(i3), but not PAFR-G alpha(q), mediated chemotaxis to PAF. All three receptors underwent phosphorylation and desensitization upon exposure to PAF but only PAFR translocated beta arrestin to the cell membrane and internalized. In RBL-2H3 cells coexpressing the PAFRs along with CXCR1, IL-8 (CXCL8) cross-desensitized Ca2+ mobilization to PAF by all the receptors but only PAFR-G alpha(i3) activation cross-inhibited the response of CXCR1 to CXCL8. Altogether, the data indicate that G(i) exclusively mediates chemotactic and cross-regulatory signals of the PAFR, but both G(i) and G(q) activate PI hydrolysis and exocytosis by this receptor. Because chemotaxis and cross-desensitization are exclusively mediated by G(i), the data suggest that differential activation of both G(i) and G(q) by PAFR likely mediate specific as well as redundant signaling pathways.

Targeted Disruption of Leukotriene B4 Receptors BLT1 and BLT2: A Critical Role for BLT1 in Collagen-Induced Arthritis in Mice

Leukotriene B(4) mediates diverse inflammatory diseases through the G protein-coupled receptors BLT1 and BLT2. In this study, we developed mice deficient in BLT1 and BLT2 by simultaneous targeted disruption of these genes. The BLT1/BLT2 double-deficient mice developed normally and peritoneal exudate cells showed no detectable responses to leukotriene B(4) confirming the deletion of the BLT1/BLT2 locus. In a model of collagen-induced arthritis on the C57BL/6 background, the BLT1/BLT2(-/-) as well as the previously described BLT1(-/-) animals showed complete protection from disease development. The disease severity correlated well with histopathology, including loss of joint architecture, inflammatory cell infiltration, fibrosis, pannus formation, and bone erosion in joints of BLT1/BLT2(+/+) animals and a total absence of disease pathology in leukotriene receptor-deficient mice. Despite these differences, all immunized BLT1(-/-) and BLT1/BLT2(-/-) animals had similar serum levels of anti-collagen Abs relative to BLT1/BLT2(+/+) animals. Thus, BLT1 may be a useful target for therapies directed at treating inflammation associated with arthritis.

Lysophosphatidylserine Stimulates L2071 Mouse Fibroblast Chemotactic Migration via a Process Involving Pertussis Toxin-Sensitive Trimeric G-Proteins

Lysophosphatidylserine (LPS) may be generated after phosphatidylserine-specific phospholipase A2 activation. However, the effects of LPS on cellular activities and the identities of its target molecules have not been fully elucidated. In this study, we observed that LPS stimulates an intracellular calcium increase in L2071 mouse fibroblast cells, and that this increase was inhibited by 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione (U-73122) but not by pertussis toxin, suggesting that LPS stimulates calcium signaling via G-protein coupled receptor-mediated phospholipase C activation. Moreover, LPS-induced calcium mobilization was not inhibited by the lysophosphatidic acid receptor antagonist, (S)-phosphoric acid mono-{2-octadec-9-enoylamino-3-[4-(pyridine-2-ylmethoxy)-phenyl]-propyl} ester (VPC 32183), thus indicating that LPS binds to a receptor other than lysophosphatidic acid receptors. It was also found that LPS stimulates two types of mitogen-activated protein kinase [i.e., extracellular signal-regulated protein kinase (ERK) and p38 kinase] in L2071 cells. Furthermore, these LPS-induced ERK and p38 kinase activations were inhibited by pertussis toxin, which suggests the role of pertussis toxin-sensitive G-proteins in the process. In terms of functional issues, LPS stimulated L2071 cell chemotactic migration, which was completely inhibited by pertussis toxin, indicating the involvement of pertussis toxin-sensitive G(i) protein(s). This chemotaxis of L2071 cells induced by LPS was also dramatically inhibited by 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) and by 2'-amino-3'-methoxyflavone (PD98059). This study demonstrates that LPS stimulates at least two different signaling cascades, one of which involves a pertussis toxin-insensitive but phospholipase C-dependent intracellular calcium increase, and the other involves a pertussis toxin-sensitive chemotactic migration mediated by phosphoinositide 3-kinase and ERK.

TG1019/OXE, a Gαi/o-protein-coupled receptor, mediates 5-oxo-eicosatetraenoic acid-induced chemotaxis

We have previously identified a Galpha(i/o)-protein-coupled receptor (TG1019/OXE) using 5-oxo-6E,8Z,11Z,14Z-eicosatetraenoic acid (5-oxo-ETE) as its ligand. We investigated signal transduction from TG1019 following stimulation with 5-oxo-ETE and role of TG1019 in 5-oxo-ETE-induced chemotaxis, using Chinese hamster ovary cells expressing TG1019 (CHO/TG1019 cells). 5-Oxo-ETE induced intracellular calcium mobilization and rapid activation of MEK/ERK and PI3K/Akt pathways in CHO/TG1019 cells. CHO/TG1019 cells stimulated with 5-oxo-ETE and other eicosanoids exhibited chemotaxis with efficacies related to agonistic activity of each eicosanoid for TG1019. Pretreatment of the cells with pertussis toxin, a phospholipase C (PLC) inhibitor (U73122) or a PI3K inhibitor (LY294002), markedly suppressed 5-oxo-ETE-induced chemotaxis, whereas pretreatment with a MEK inhibitor (PD98059) had no significant effect on the chemotaxis. Our results show that TG1019 mediates 5-oxo-ETE-induced chemotaxis and that signals from TG1019 are transduced via Galpha(i/o) protein to PLC/calcium mobilization, MEK/ERK, and PI3K/Akt, among which PLC and PI3K would play important roles in the chemotaxis.

Leukotriene B4 receptor and the function of its helix 8

More than 30 lipid ligands, which express their biological activities through cognate G-protein-coupled receptors (GPCRs), have been reported. Among them, leukotriene B(4) (LTB(4)) is a potent lipid mediator involved in host defense, inflammation, and the immune responses. Two GPCRs for LTB(4) (BLT1 and BLT2) have been cloned and analyzed. Recent studies using genetically engineered mice suggest that BLT1 plays an important role in several inflammatory diseases including ischemic reperfusion tissue injury, atherosclerosis, and bronchial asthma. BLT1 is also a good tool to study the molecular mechanism of GPCR activation and inactivation in vitro. In this brief review, we focus on the biological and biochemical properties of BLT1 with special attention to the putative helix 8 of the receptor.

GRO family chemokines are specialized for monocyte arrest from flow

Chemokines participate in various processes of monocyte recruitment including monocyte arrest and migration. Our group and others have demonstrated that growth-related oncogene (GRO)-alpha (CXCL1) can support monocyte arrest in models of inflammation. Here we employed a parallel plate-flow chamber and Transwell reconstitution assay to test whether GRO family chemokines were sufficient for Mono Mac 6 (a human monocytic cell line) and isolated human monocyte recruitment. Our study shows that 1) GRO-alpha, -beta (CXCL2), and -gamma (CXCL3) all act as arrest chemokines for monocyte adhesion on vascular cell adhesion molecule (VCAM)-1 under flow in the presence of P-selectin; 2) CXCR2 is the functional receptor for GRO-family chemokines in monocyte arrest; however, CXCR2 is not an arrest chemokine receptor in general, since epithelial neutrophil-activating peptide ENA-78 failed to arrest monocytes; 3) GRO-alpha, -beta, and -gamma all fail to increase intracellular free Ca2+ or mediate monocyte chemotaxis; and 4) signaling through G alpha(i) protein, phosphoinositide 3-kinase, and actin polymerization but not Ca2+ mobilization or the mitogen-activated kinases p38 and MAPK/extracellular signal-related kinase are necessary for GRO-alpha-mediated Mono Mac 6 cell arrest under flow. We conclude that the GRO-family chemokines are specialized monocyte-arrest chemokines. Their role in monocyte recruitment in inflammation can be inhibited by blocking CXCR2 function or downstream signaling events.

Andrographolide acts through inhibition of ERK1/2 and Akt phosphorylation to suppress chemotactic migration

We now evaluated the anti-inflammatory mechanisms of andrographolide on complement 5a (C5a)-induced macrophage recruitment in vitro. Andrographolide concentration dependently inhibited cell migration toward C5a with an IC50 of 5.6+/-0.7 microM. With relatively specific kinase inhibitors (PD98059, SB203580, SP600125, wortmannin and LY294002, respectively) the results showed that extracellular signal-regulated kinase1/2 (ERK1/2), p38 mitogen-activated protein kinase (p38 MAPK) and phosphatidylinositol-3-kinase (PI3K) were necessary for C5a-induced migration, whereas c-Jun N-terminal kinase (JNK) was nonessential. Andrographolide significantly attenuated C5a-stimulated phosphorylation of ERK1/2, and of its upstream activator, MAP kinase-ERK kinase (MEK1/2). C5a-activated ERK1/2 phosphorylation was 86+/-9% inhibited by 30 microM andrographolide. Under the same conditions, however, andrographolide failed to affect C5a-stimulated p38 MAPK and JNK phosphorylation. Andrographolide also strongly abolished C5a-stimulated Akt phosphorylation, a downstream target protein for PI3K. These results indicate that inhibition of cell migration by interfering with ERK1/2 and PI3K/Akt signal pathways may contribute to the anti-inflammatory activity of andrographolide.

C5a differentially stimulates the ERK1/2 and p38 MAPK phosphorylation through independent signaling pathways to induced chemotactic migration in RAW264.7 macrophages

We elucidate the roles of various protein kinases involved in complement 5a (C5a)-induced cell migration. Results showed that extracellular signal-regulated kinase1/2 (ERK1/2), p38 mitogen-activated protein kinase (p38 MAPK) and phosphatidylinositol 3-kinase (P13K) were necessary for C5a-induced migration, whereas protein kinase C and c-Jun N-terminal kinase (JNK) were nonessential. C5a-induced migration was also suppresses by phospholipase C (PLC) inhibitor U73122 and pertussis toxin (PTX). We found that C5a-induced, time-dependent (1) ERK1/2 phosphorylation was markedly diminished by PTX, U73122, P13K inhibitors wortmannin and LY294002 and ERK1/2 inhibitor PD98059; (2) Akt phosphorylation was also attenuated by the above inhibitors except PD98059; (3) p38 MAPK phosphorylation was only affected by PTX. Furthermore, C5a also stimulated PLCbeta(2) membrane translocation in a time-dependent manner that occurred early prior to Akt phosphorylation and could be abolished only by PTX and U73122. These results suggest that C5a, through the activation of PTX-sensitive G protein, to differentially stimulate ERK1/2 and p38 MAPK phosphorylation and evoke cell migration. That is, ERK1/2 but not p38 MAPK phosphorylation is down stream of P13K/Akt and modulated by PLC. Additionally, beta(2) isoform may be one of the participates in C5a signal and acts more upstream of P13K/Akt.

Controlled pseudopod extension of human neutrophils stimulated with different chemoattractants

The formation of pseudopods and lamellae after ligation of chemoattractant sensitive G-protein coupled receptors (GPCRs) is essential for chemotaxis. Here, pseudopod extension was stimulated with chemoattractant delivered from a micropipet. The chemoattractant diffusion and convection mass transport were considered, and it is shown that when the delivery of chemoattractant was limited by diffusion there was a strong chemoattractant gradient along the cell surface. The diffusion-limited delivery of chemoattractant from a micropipet allowed for maintaining an almost constant chemoattractant concentration at the leading edge of single pseudopods during their growth. In these conditions, the rate of pseudopod extension was dependent on the concentration of chemoattractant in the pipet delivering chemoattractant. The pseudopod extension induced using micropipets was oscillatory even in the presence of a constant delivery of chemoattractant. This oscillatory pseudopod extension was controlled by activated RhoA and its downstream effector kinase ROCK and was abolished after the inhibition of RhoA activation with Clostridium botulinium C3 exoenzyme (C3) or the blocking of ROCK activation with Y-27632. The ability of the micropipet assay to establish a well-defined chemoattractant distribution around the activated cell over a wide range of molecular weights of the used chemoattractants allowed for comparison of the effect of chemoattractant stimulation on the dynamics of pseudopod growth. Pseudopod growth was stimulated using N-formylated peptide (N-formyl-methionyl-leucyl-phenylalanine (fMLP)), platelet activating factor (PAF), leukotriene B4 (LTB(4)), C5a anaphylotoxin (C5a), and interleukin-8 (IL-8), which represent the typical ligands for G-protein coupled chemotactic receptors. The dependence of the rate of pseudopod extension on the concentration of these chemoattractants and their equimolar mixture was measured and shown to be similar for all chemoattractants. The inhibition of the activity of phosphoinositide-3 kinase (PI3K) with wortmannin showed that 72%-80% of the rate of pseudopod extension induced with N-formyl-methionyl-leucyl-phenylalanine, platelet activating factor, and leukotriene B4 was phosphoinositide-3 kinase-dependent, in contrast to 55% of the rate of pseudopod extension induced with interleukin-8. The dependence of the rate of pseudopod extension on the concentration of individual chemoattractants and their equimolar mixture suggests that there is a common rate-limiting mechanism for the polymerization of cytoskeletal F-actin in the pseudopod region induced by G-protein coupled chemoattractant receptors.

Phosphorylation-independent beta-arrestin translocation and internalization of leukotriene B4 receptors

Leukotriene B4 (LTB4) activates the G-protein-coupled receptor leukotriene B4 receptor 1 (BLT1) to mediate a diverse array of cellular responses in leukocytes including chemotaxis, calcium mobilization, degranulation, and gene expression. To determine the role of phosphorylation in BLT1 regulation, we generated mutants of BLT1 in which all of the serine/threonine residues in the C-tail are converted to alanine or to aspartate/glutamate. These mutants expressed in rat basophilic leukemia RBL-2H3 cells bound LTB4 with similar affinity and activated all of the known functional activities of BLT1, albeit at different levels. The conversion of phosphorylation sites to alanine resulted in enhanced G-protein-mediated activities, whereas conversion to aspartate/glutamate resulted in reduced responses and a right shift in dose response, indicating that receptor phosphorylation is a critical regulator of G-protein-mediated pathways. Surprisingly, translocation of beta-arrestin and receptor internalization was completely independent of BLT1 phosphorylation. Real-time analysis of beta-arrestin translocation and receptor internalization using digital fluorescence video microscopy in cells expressing a red fluorescent protein labeled BLT1 and a green fluorescent protein-tagged beta-arrestin confirmed phosphorylation-independent beta-arrestin translocation and internalization of BLT1. In beta-arrestin-deficient mouse embryo fibroblasts, the BLT1 receptors failed to display endosomal localization upon stimulation. In these cells, co-expression of beta-arrestin-green fluorescent protein with BLT1-red fluorescent protein resulted in co-localization of BLT1 and beta-arrestin upon activation. Thus, receptor phosphorylation-dependent mechanisms regulate G-protein-mediated pathways; however, phosphorylation-independent mechanisms regulate beta-arrestin association and internalization of BLT1.

Leukotrienes and atherosclerosis: new roles for old mediators

Lipid mediators generated from arachidonic acid through the action of 5-lipoxygenase have been known for over two decades and are implicated in a wide variety of inflammatory disorders. G-protein-coupled receptors mediate the effects of different leukotrienes in distinct cell types. Novel cellular and molecular targets were recently discovered for these mediators, with important consequences for the function of both adaptive and innate immune systems. These studies have outlined crucial new roles for leukotrienes in the recruitment of T lymphocytes and in the development of atherosclerotic lesions, suggesting novel mechanisms for their actions. Through the development of appropriate animal models, leukotrienes are becoming renewed targets for treatment of many inflammatory diseases including atherosclerosis.

Gi-independent macrophage chemotaxis to lysophosphatidylcholine via the immunoregulatory GPCR G2A

G2A is a G-protein-coupled receptor (GPCR) involved in immune regulation. Previous studies have shown that lysophosphatidylcholine (LPC), a bioactive lipid associated with atherosclerosis and autoimmunity, acts through G2A to induce diverse biologic effects. Production of LPC during cell apoptosis serves as a chemotactic signal for macrophage recruitment. Here we demonstrate that macrophage chemotaxis to LPC is dependent on G2A function. Wild-type but not G2A-deficient mouse peritoneal macrophages migrated toward LPC. RNAi-mediated knockdown of G2A in J774A.1 macrophages abolished LPC-induced chemotaxis, whereas overexpression of G2A significantly enhanced this process. Mutation of the conserved DRY motif of G2A resulted in loss of chemotaxis to LPC, suggesting a requirement for G-protein signaling. Unlike most GPCRs, including the chemokine receptors, coupling to G(i) is not required for LPC/G2A-mediated chemotaxis, but coupling to G(q/11) and G(12/13) is necessary as judged by inhibition with dominant negative forms of these alpha subunits or with regulators of G-protein signaling (RGS) constructs. Collectively, these data establish that pertussis toxin-insensitive G2A signaling regulates macrophage chemotaxis to LPC. Defects in this signaling pathway may be related to the pathogenesis of systemic autoimmune disease.

Heterodimerization and cross-desensitization between the mu-opioid receptor and the chemokine CCR5 receptor

Cross-desensitization between micro-opioid receptor agonists and CC chemokines was shown to occur in immune cells and in the central nervous system. However, these cells do not permit examination of potential mechanisms at cellular levels due to low levels and mixed populations of receptors. In this study, we investigated possible interactions and biochemical mechanisms of cross-desensitization between the mu-opioid and chemokine CCR5 receptors coexpressed in Chinese hamster ovary (CHO) cells. Hemagglutinin (HA)-tagged micro-opioid receptor coimmunoprecipitated with FLAG (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys)-tagged chemokine receptor CCR5 in cells expressing the two receptors, but not in a mixture of cells transfected with one of the two receptors, indicating that the two receptors form heterodimers. Treatment with the mu-opioid receptor agonist DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin), the chemokine RANTES (Regulated on Activation, Normal T cell-Expressed and -Secreted) (CCL5), or both, did not affect the level of coimmunoprecipitation. DAMGO and RANTES (CCL5) induced chemotaxis in CHO cells coexpressing both receptors, and preincubation with either DAMGO or RANTES (CCL5) profoundly inhibited chemotaxis caused by the other. DAMGO pretreatment enhanced phosphorylation of the chemokine CCR5 receptor and reduced RANTES (CCL5)-promoted [35S]GTP gamma S binding. Conversely, RANTES (CCL5) preincubation slightly increased phosphorylation of the mu-opioid receptor and significantly reduced DAMGO-induced [35S]GTP gamma S binding. These results indicate that activation of either receptor affected G protein coupling of the other, likely due to enhanced phosphorylation of the receptor. Heterodimerization between the two receptors may contribute to the observed cross-desensitization.

Exploring the pharmacology of the leukotriene B4 receptor BLT1, without the confounding effects of BLT2

Most previous studies of leukotriene B4 (LTB4) pharmacology using primary leukocyte cultures and myeloid cell lines do not differentiate between leukotriene BLT1 and BLT2 receptor activation because both receptors are often expressed by these cells. Here we show that in HeLa cells expressing BLT1 but not BLT2 receptors, BLT1 receptor activation resulted in IP3 mediated calcium release from intracellular stores initially, followed by calcium influx through cell membrane channels. BLT1 calcium signalling was sensitive to the activity of protein kinase C (PKC), protein kinase A (PKA) and protein-tyrosine kinases (PTKs), as well as changes in membrane cholesterol levels and treatments that are known to disrupt normal membrane physiology and/or lipid rafts. Inhibition of MAP kinases, Rho-associated kinases, or phosphoinositol-3-kinases (PI3K) had no effect on BLT1 receptor induced calcium signalling, and the receptor was insensitive to the redox state of the extracellular compartment.

Platelet-activating factor-induced chemokine gene expression requires NF-kappaB activation and Ca2+/calcineurin signaling pathways. Inhibition by receptor phosphorylation and beta-arrestin recruitment

Previously, we reported that platelet-activating factor (PAF) stimulates higher G protein activation and a more robust Ca2+ mobilization in RBL-2H3 cells expressing carboxyl terminus deletion, phosphorylation-deficient mutant of PAF receptor (mPAFR) when compared with the wild-type receptor (PAFR). However, PAF did not provide sufficient signal for CC chemokine receptor ligand 2 (CCL2) production in cells expressing mPAFR. Based on these findings, we hypothesized that receptor phosphorylation provides a G protein-independent signal that synergizes with Ca2+ mobilization to induce CCL2 production. Here, we show that a mutant of PAFR (D289A), which does not couple to G proteins, was resistant to agonist-induced receptor phosphorylation. Unexpectedly, we found that when this mutant was coexpressed with mPAFR, it restored NF-kappaB activation and CCL2 production. PAF caused translocation of beta-arrestin from the cytoplasm to the membrane in cells expressing PAFR but not a phosphorylation-deficient mutant in which all Ser/Thr residues were replaced with Ala (DeltaST-PAFR). Interestingly, PAF induced significantly higher NF-kappaB and nuclear factor of activated T cells (NFAT)-luciferase activity as well as CCL2 production in cells expressing DeltaST-PAFR than those expressing PAFR. Furthermore, a Ca2+/calcineurin inhibitor completely inhibited PAF-induced NFAT activation and CCL2 production but not NF-kappaB activation. These findings suggest that the carboxyl terminus of PAFR provides a G protein-independent signal for NF-kappaB activation, which synergizes with G protein-mediated Ca2+/calcineurin activation to induce CCL2 production. However, receptor phosphorylation and beta-arrestin recruitment inhibit CCL2 production by blocking both NF-kappaB activation and Ca2+/calcineurin-dependent signaling pathways.

Dual signaling and effector pathways mediate human eosinophil activation by platelet-activating factor

Platelet-activating factor (PAF) induces various cellular functions in eosinophils including chemotaxis, adhesion, superoxide anion (O2-) production, and degranulation. While PAF shares many biological effects with other chemotactic factors such as N-formyl-methionyl-leucyl-phenylalanine, complement fragments, and lipid mediators, PAF is unique in that its action is relatively resistant to pertussis toxin (PTX), and in activating eosinophils more strongly than neutrophils. In this review we consider how PAF might activate human eosinophils in preference to neutrophils, and discuss possible mechanisms of PAF-induced activation of human eosinophils via two distinct signaling and effector pathways. Recently we analyzed O2- production by eosinophils using a sensitive, real-time chemiluminescence method. Our results showed that in human eosinophils PAF activates two distinct signaling and effector pathways coupled to the PAF receptor: one linked to PTX-sensitive G protein(s) and another to PTX-resistant G protein(s), phosphatidylinositol 3-kinase, and cellular adhesion. This activation of two different G proteins by the eosinophil PAF receptor may explain the strong and diverse biological responses of human eosinophils to PAF.

beta-Arrestin-dependent constitutive internalization of the human chemokine decoy receptor D6

Seven transmembrane receptors mediate diverse physiological responses including hormone action, olfaction, neurotransmission, and chemotaxis. Human D6 is a non-signaling seven-transmembrane receptor expressed on lymphatic endothelium interacting with most inflammatory CC-chemokines resulting in their rapid internalization. Here, we demonstrate that this scavenging activity is mediated by continuous internalization and constant surface expression of the receptor, a process involving the clathrin-coated pit-dependent pathway. D6 constitutively associates with the cytoplasmic adaptor beta-arrestin, and this interaction is essential for D6 internalization. An acidic region, but not the putative phosphorylation sites in the cytoplasmic tail of D6, is critical for receptor interaction with beta-arrestin and subsequent internalization. Neither the native D6 nor mutants uncoupled from beta-arrestin activate any G-protein-mediated signaling pathways. Therefore, D6 may be considered a decoy receptor structurally adapted to perform chemokine scavenging.

Experimental Evidence for the Limiting Role of Enzymatic Reactions in Chemoattractant-induced Pseudopod Extension in Human Neutrophils

Chemoattractant-stimulated pseudopod growth in human neutrophils was used as a model system to study the rate-limiting mechanism of cytoskeleton rearrangement induced by activated G-protein-coupled receptors. Cells were activated with N-formyl-Met-Leu-Phe, and the temperature dependence of the rate of pseudopod extension was measured in the presence of pharmacological inhibitors with known mechanisms of action. Three groups of inhibitors were used: (i) inhibitors sequestering substrates involved in F-actin polymerization (latrunculin A for G-actin and cytochalasin D for actin filament-free barbed ends) or sequestering secondary messengers (PIP-binding peptide for phosphoinositide lipids); (ii) competitively binding inhibitors (Akt-inhibitor for Akt/protein kinase B); and (iii) inhibitors that reduce enzyme activity (wortmannin for phosphoinositide 3-kinase and chelerythrine for protein kinase C). The experimental data are consistent with a model in which the relative involvement of a given pathway of F-actin polymerization to the measured rate of pseudopod extension is limited by a slowest (bottleneck) reaction in the cascade of reactions involved in the overall signaling pathway. The approach we developed was used to demonstrate that chemoattractant-induced pseudopod growth and mechanically stimulated cytoskeleton rearrangement are controlled by distinct pathways of F-actin polymerization.

Role of Leukotriene B 4 Receptors in the Development of Atherosclerosis: Potential Mechanisms

Objective— Leukotriene B 4 (LTB 4 ), a potent leukocyte chemoattractant, is known to promote several inflammatory diseases, including atherosclerosis. We sought to determine mechanisms through which LTB 4 modulates atherosclerosis in cell lines expressing LTB 4 receptors, BLT-1, and in mice deficient in BLT-1 as well as macrophage cell lines derived from BLT-1 +/+ and BLT-1 −/− mice.

Methods and Results— Analysis of global changes in gene expression induced by LTB 4 in rat basophilic leukemia cells (RBL-2H3) expressing the human BLT-1 showed highest-fold increase in expression of fatty acid translocase/CD36 and the chemokine MCP1/JE/CCL2 , which are critical in atherogenesis. To determine the importance of BLT-1 in atherogenesis, we crossed BLT-1-null mice with apolipoprotein (apo)-E-deficient mice, which develop severe atherosclerosis. Deletion of BLT-1 significantly reduced the lesion formation in apo-E −/− mice only during initiating stages (4 and 8 weeks) but had no effect on the lesion size in mice fed atherogenic diet for 19 weeks. Macrophage cell lines from BLT-1-deficient mice expressed the low-affinity LTB 4 receptor, BLT-2, and exhibited chemotaxis to LTB 4 .

Conclusions— The effects of LTB 4 in atherosclerosis are likely mediated through the high-affinity BLT-1 and the low-affinity BLT-2 receptors. LTB 4 promotes atherosclerosis by chemo-attracting monocytes, by providing an amplification loop of monocyte chemotaxis via CCL2 production, and by converting monocytes to foam cells by enhanced expression of CD36 and fatty acid accumulation.

Differential Activation of Formyl Peptide Receptor-Like 1 by Peptide Ligands

Formyl peptide receptor-like 1 (FPRL1) plays a key role in the regulation of immune responses. The activation of FPRL1 induces a complicated pattern of cellular signaling, which results in the regulation of several immune responses, such as chemotactic migration and the production of reactive oxygen species (ROS). Because some of these cellular responses are not beneficial to the host, ligands that selectively modulate these cellular responses are useful. His-Phe-Tyr-Leu-Pro-Met (HFYLPM) is a synthetic peptide that binds to FPRL1. In this study, we generated various HFYLPM analogues and examined their effects on cellular responses via FPRL1 in FPRL1-expressing rat basophilic leukemia-2H3 cells or in primary human neutrophils. Among the HXYLPM analogues, His-Arg-Tyr-Leu-Pro-Met (HRYLPM) activated a broad spectrum of cellular signaling events, including an intracellular Ca(2+) concentration increase, phosphoinositide 3-kinase, extracellular signal-regulated kinase, and Akt activation, however, His-Glu-Tyr-Leu-Pro-Met (HEYLPM) activated only intracellular Ca(2+) concentration and Akt but did not increase Ca(2+). In addition, HRYLPM was found to stimulate chemotaxis and ROS generation via phosphoinositide 3-kinase and an intracellular Ca(2+) concentration increase, respectively, whereas HEYLPM stimulated chemotaxis but not ROS generation. With respect to the molecular mechanisms involved in the differential action of HRYLPM and HEYLPM, we found that HRYLPM but not HEYLPM competitively inhibited the binding of (125)I-labeled Trp-Lys-Tyr-Met-Val-D-Met-NH(2) (WKYMVm, a FPRL1 ligand) to FPRL1. This study demonstrates that the important chemoattractant receptor, FPRL1, may be differentially modulated by distinct peptide ligands. We also suggest that HRYLPM and HEYLPM may be used to selectively modulate FPRL1.

Activation of platelet-activating factor receptor-coupled G alpha q leads to stimulation of Src and focal adhesion kinase via two separate pathways in human umbilical vein endothelial cells

Platelet-activating factor (PAF), a phospholipid second messenger, has diverse physiological functions, including responses in differentiated endothelial cells to external stimuli. We used human umbilical vein endothelial cells (HUVECs) as a model system. We show that PAF activated pertussis toxin-insensitive G alpha(q) protein upon binding to its seven transmembrane receptor. Elevated cAMP levels were observed via activation of adenylate cyclase, which activated protein kinase A (PKA) and was attenuated by a PAF receptor antagonist, blocking downstream activity. Phosphorylation of Src by PAF required G alpha(q) protein and adenylate cyclase activation; there was an absolute requirement of PKA for PAF-induced Src phosphorylation. Immediate (1 min) PAF-induced STAT-3 phosphorylation required the activation of G alpha(q) protein, adenylate cyclase, and PKA, and was independent of these intermediates at delayed (30 min) and prolonged (60 min) PAF exposure. PAF activated PLC beta 3 through its G alpha(q) protein-coupled receptor, whereas activation of phospholipase C gamma 1 (PLC gamma 1) by PAF was independent of G proteins but required the involvement of Src at prolonged PAF exposure (60 min). We demonstrate for the first time in vascular endothelial cells: (i) the involvement of signaling intermediates in the PAF-PAF receptor system in the induction of TIMP2 and MT1-MMP expression, resulting in the coordinated proteolytic activation of MMP2, and (ii) a receptor-mediated signal transduction cascade for the tyrosine phosphorylation of FAK by PAF. PAF exposure induced binding of p130(Cas), Src, SHC, and paxillin to FAK. Clearly, PAF-mediated signaling in differentiated endothelial cells is critical to endothelial cell functions, including cell migration and proteolytic activation of MMP2.

Differential activation of formyl peptide receptor signaling by peptide ligands

Formyl peptide receptor (FPR) and formyl peptide receptor like 1 (FPRL1) play important roles in inflammation and immunity. Stimulation of FPR and FPRL1 initiates a cascade of signaling events, leading to activation of various phagocyte responses, including chemotaxis, superoxide generation, and exocytosis. Trp-Lys-Tyr-Met-Val-d-Met-NH2 (WKYMVm) is a synthetic peptide that binds to and activates FPR and FPRL1. To develop agonists that selectively activate phagocyte functions and therefore protect host from unwanted tissue damage, we generated various WKYMVm analogs and examined their effects on cellular responses in FPR- or FPRL1-expressing RBL-2H3 cells. Analogs with substitution at the third position such as WKGMVm, WKRMVm, as well as analogs with substitution at the sixth d-Met, selectively altered calcium mobilization in cells expressing FPRL1 but not in cells expressing FPR. Whereas binding of WKYMVm to FPR activates a broad spectrum of cellular signaling events, including phospholipase C-mediated intracellular calcium concentration ([Ca2+]i) mobilization and activation of extracellular signal-regulated kinase (ERK) and Akt, WKGMVm and WKRMVm could only activate ERK and Akt but did not induce [Ca2+]i mobilization. With respect to phagocyte functions, WKYMVm could induce both chemotaxis and exocytosis, but the two analogs WKGMVm and WKRMVm could only induce chemotaxis but not exocytosis. This study demonstrates that a major phagocyte chemoattractant receptor FPR may be activated differentially by distinct peptide ligands. Our results suggest that WKGMVm and WKRMVm may be useful model for further development of pharmacological agents that selectively activate FPR-mediated functions.