Phospholipase C beta3 deficiency leads to macrophage hypersensitivity to apoptotic induction and reduction of atherosclerosis in mice

Sinapine targeting PLCβ3 EF hands disrupts Gαq-PLCβ3 interaction and ameliorates cardiovascular diseases

BACKGROUND

The renin-angiotensin-aldosterone system (RAAS) over-activation is highly involved in cardiovascular diseases (CVDs), with the Gαq-PLCβ3 axis acting as a core node of RAAS. PLCβ3 is a potential target of CVDs, and the lack of inhibitors has limited its drug development.

PURPOSE

Sinapine (SP) is a potential leading compound for treating CVDs. Thus, we aimed to elucidate the regulation of SP towards the Gαq-PLCβ3 axis and its molecular mechanism.

STUDY DESIGN

Aldosteronism and hypertension animal models were employed to investigate SP's inhibitory effect on the abnormal activation of the RAAS through the Gαq-PLCβ3 axis. We used chemical biology methods to identify potential targets and elucidate the underlying molecular mechanisms.

METHODS

The effects of SP on aldosteronism and hypertension were evaluated using an established animal model in our laboratory. Target identification and underlying molecular mechanism research were performed using activity-based protein profiling with a bio-orthogonal click chemistry reaction and other biochemical methods.

RESULTS

SP alleviated aldosteronism and hypertension in animal models by targeting PLCβ3. The underlying mechanism for blocking the Gαq-PLCβ3 interaction involves targeting the EF hands through the Asn-260 amino acid residue. SP regulated the Gαq-PLCβ3 axis more precisely than the Gαq-GEFT or Gαq-PKCζ axis in the cardiovascular system.

CONCLUSION

SP alleviated RAAS over-activation via Gαq-PLCβ3 interaction blockade by targeting the PLCβ3 EF hands domain, which provided a novel PLC inhibitor for treating CVDs. Unlike selective Gαq inhibitors, SP reduced the risk of side effects compared to Gαq inhibitors in treating CVDs.

The role of transient receptor potential (TRP) channels in phagocytosis: A comprehensive review

When host cells are exposed to foreign particles, dead cells, or cell hazards, a sophisticated process called phagocytosis begins. During this process, macrophages, dendritic cells, and neutrophils engulf the target by expanding their membranes. Phagocytosis of apoptotic cells is called efferocytosis. This process is of significant importance as billions of cells are eliminated daily without provoking inflammation. Both phagocytosis and efferocytosis depend on Ca2+ signaling. A big family of Ca2+ permeable channels is transient receptor potentials (TRPs) divided into nine subfamilies. We aimed to review their roles in phagocytosis. The present review article shows that various TRP channels such as TRPV1, 2, 3, 4, TRPM2, 4, 7, 8, TRPML1, TRPA1, TRPC1, 3, 5, 6 have roles at various stages of phagocytosis. They are involved in the phagocytosis of amyloid β, α-synuclein, myelin debris, bacteria, and apoptotic cells. In particular, TRPC3 and TRPM7 contribute to efferocytosis. These effects are mediated by changing Ca2+ signaling or targeting intracellular enzymes such as Akt. In addition, they contribute to the chemotaxis of phagocytic cells towards targets. Although a limited number of studies have assessed the role of TRP channels in phagocytosis and efferocytosis, their findings indicate that they have critical roles in these processes. In some cases, their ablation completely abolished the phagocytic function of the cells. As a result, TRP channels are potential targets for developing new therapeutics that modulate phagocytosis.

Phosphatidylinositol-specific phospholipase C can decrease Müller cell viability and suppress its phagocytic activity by modulating PI3K/AKT signaling pathway

Bacillus cereus endophthalmitis is a devastating eye infection that causes rapid blindness through the release of extracellular tissue-destructive exotoxins. The phagocytic and antibacterial functions of ocular cells are the keys to limiting ocular bacterial infections. In a previous study, we identified a new virulence gene, plcA-2 (different from the original plcA-1 gene), that was strongly associated with the plcA gene of Listeria monocytogenes. This plcA gene had been confirmed to play an important role in phagocytosis. However, how the Bc-phosphatidylinositol-specific phospholipase C (PI-PLC) proteins encoded by the plcA-1/2 genes affect phagocytes remains unclear in B. cereus endophthalmitis. Here, we found that the enzymatic activity of Bc-PI-PLC-A2 was approximately twofold higher than that of Bc-PI-PLC-A1, and both proteins inhibited the viability of Müller cells. In addition, PI-PLC proteins reduced phagocytosis of Müller cells by decreasing the phosphorylation levels of key proteins in the PI3K/AKT signaling pathway. In conclusion, we showed that PI-PLC proteins contribute to inhibit the viability of and suppress the phagocytosis of Müller cells, providing new insights into the pathogenic mechanism of B. cereus endophthalmitis.

Activation Mechanisms and Diverse Functions of Mammalian Phospholipase C

Phospholipase C (PLC) plays pivotal roles in regulating various cellular functions by metabolizing phosphatidylinositol 4,5-bisphosphate in the plasma membrane. This process generates two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol, which respectively regulate the intracellular Ca²⁺ levels and protein kinase C activation. In mammals, six classes of typical PLC have been identified and classified based on their structure and activation mechanisms. They all share X and Y domains, which are responsible for enzymatic activity, as well as subtype-specific domains. Furthermore, in addition to typical PLC, atypical PLC with unique structures solely harboring an X domain has been recently discovered. Collectively, seven classes and 16 isozymes of mammalian PLC are known to date. Dysregulation of PLC activity has been implicated in several pathophysiological conditions, including cancer, cardiovascular diseases, and neurological disorders. Therefore, identification of new drug targets that can selectively modulate PLC activity is important. The present review focuses on the structures, activation mechanisms, and physiological functions of mammalian PLC.

Urinary Proteomics Identifying Novel Biomarkers for the Diagnosis and Phenotyping of Carotid Artery Stenosis

Background: Carotid artery stenosis (CAS) is caused by the formation of atherosclerotic plaques inside the arterial wall and accounts for 20–30% of all strokes. The development of an early, noninvasive diagnostic method and the identification of high-risk patients for ischemic stroke is essential to the management of CAS in clinical practice.

Methods: We used the data-independent acquisition (DIA) technique to conduct a urinary proteomic study in patients with CAS and healthy controls. We identified the potential diagnosis and risk stratification biomarkers of CAS. And Ingenuity pathway analysis was used for functional annotation of differentially expressed proteins (DEPs). Furthermore, receiver operating characteristic (ROC) analysis was performed to evaluate the diagnostic values of DEPs.

Results: A total of 194 DEPs were identified between CAS patients and healthy controls by DIA quantification. The bioinformatics analysis showed that these DEPs were correlated with the pathogenesis of CAS. We further identified 32 DEPs in symptomatic CAS compared to asymptomatic CAS, and biological function analysis revealed that these proteins are mainly related to immune/inflammatory pathways. Finally, a biomarker panel of six proteins (ACP2, PLD3, HLA-C, GGH, CALML3, and IL2RB) exhibited potential diagnostic value in CAS and good discriminative power for differentiating symptomatic and asymptomatic CAS with high sensitivity and specificity.

Conclusions: Our study identified novel potential urinary biomarkers for noninvasive early screening and risk stratification of CAS.

Discovery of rare variants associated with blood pressure regulation through meta-analysis of 1.3 million individuals

Genetic studies of blood pressure (BP) to date have mainly analyzed common variants (minor allele frequency > 0.05). In a meta-analysis of up to ~1.3 million participants, we discovered 106 new BP-associated genomic regions and 87 rare (minor allele frequency ≤ 0.01) variant BP associations (P < 5 × 10-8), of which 32 were in new BP-associated loci and 55 were independent BP-associated single-nucleotide variants within known BP-associated regions. Average effects of rare variants (44% coding) were ~8 times larger than common variant effects and indicate potential candidate causal genes at new and known loci (for example, GATA5 and PLCB3). BP-associated variants (including rare and common) were enriched in regions of active chromatin in fetal tissues, potentially linking fetal development with BP regulation in later life. Multivariable Mendelian randomization suggested possible inverse effects of elevated systolic and diastolic BP on large artery stroke. Our study demonstrates the utility of rare-variant analyses for identifying candidate genes and the results highlight potential therapeutic targets.

Phospholipase C families: Common themes and versatility in physiology and pathology

Phosphoinositide-specific phospholipase Cs (PLCs) are expressed in all mammalian cells and play critical roles in signal transduction. To obtain a comprehensive understanding of these enzymes in physiology and pathology, a detailed structural, biochemical, cell biological and genetic information is required. In this review, we cover all these aspects to summarize current knowledge of the entire superfamily. The families of PLCs have expanded from 13 enzymes to 16 with the identification of the atypical PLCs in the human genome. Recent structural insights highlight the common themes that cover not only the substrate catalysis but also the mechanisms of activation. This involves the release of autoinhibitory interactions that, in the absence of stimulation, maintain classical PLC enzymes in their inactive forms. Studies of individual PLCs provide a rich repertoire of PLC function in different physiologies. Furthermore, the genetic studies discovered numerous mutated and rare variants of PLC enzymes and their link to human disease development, greatly expanding our understanding of their roles in diverse pathologies. Notably, substantial evidence now supports involvement of different PLC isoforms in the development of specific cancer types, immune disorders and neurodegeneration. These advances will stimulate the generation of new drugs that target PLC enzymes, and will therefore open new possibilities for treatment of a number of diseases where current therapies remain ineffective.

Interface of Phospholipase Activity, Immune Cell Function, and Atherosclerosis

Phospholipases are a family of lipid-altering enzymes that can either reduce or increase bioactive lipid levels. Bioactive lipids elicit signaling responses, activate transcription factors, promote G-coupled-protein activity, and modulate membrane fluidity, which mediates cellular function. Phospholipases and the bioactive lipids they produce are important regulators of immune cell activity, dictating both pro-inflammatory and pro-resolving activity. During atherosclerosis, pro-inflammatory and pro-resolving activities govern atherosclerosis progression and regression, respectively. This review will look at the interface of phospholipase activity, immune cell function, and atherosclerosis.

Small GTPase ARF6 Is a Coincidence-Detection Code for RPH3A Polarization in Neutrophil Polarization

Cell polarization is a key step for leukocytes adhesion and transmigration during leukocytes' inflammatory infiltration. Polarized localization of plasma membrane (PM) phosphatidylinositol-4-phosphate (PtdIns4P) directs the polarization of RPH3A, which contains a PtdIns4P binding site. Consequently, RPH3A mediates the RAB21 and PIP5K1C90 polarization, which is important for neutrophil adhesion to endothelia during inflammation. However, the mechanism by which RPH3A is recruited only to PM PtdIns4P rather than Golgi PtdIns4P remains unclear. By using ADP-ribosylation factor 6 (ARF6) small interfering RNA, ARF6 dominant-negative mutant ARF6(T27N), and ARF6 activation inhibitor SecinH3, we demonstrate that ARF6 plays an important role in the polarization of RPH3A, RAB21, and PIP5K1C90 in murine neutrophils. PM ARF6 is polarized and colocalized with RPH3A, RAB21, PIP5K1C90, and PM PtdIns4P in mouse and human neutrophils upon integrin stimulation. Additionally, ARF6 binds to RPH3A and enhances the interaction between the PM PtdIns4P and RPH3A. Consistent with functional roles of polarization of RPH3A, Rab21, and PIP5K1C90, ARF6 is also required for neutrophil adhesion on the inflamed endothelial layer. Our study reveals a previously unknown role of ARF6 in neutrophil polarization as being the coincidence-detection code with PM PtdIns4P. Cooperation of ARF6 and PM PtdIns4P direct RPH3A polarization, which is important for neutrophil firm adhesion to endothelia.

Versatile cell ablation tools and their applications to study loss of cell functions

Targeted cell ablation is a powerful approach for studying the role of specific cell populations in a variety of organotypic functions, including cell differentiation, and organ generation and regeneration. Emerging tools for permanently or conditionally ablating targeted cell populations and transiently inhibiting neuronal activities exhibit a diversity of application and utility. Each tool has distinct features, and none can be universally applied to study different cell types in various tissue compartments. Although these tools have been developed for over 30 years, they require additional improvement. Currently, there is no consensus on how to select the tools to answer the specific scientific questions of interest. Selecting the appropriate cell ablation technique to study the function of a targeted cell population is less straightforward than selecting the method to study a gene's functions. In this review, we discuss the features of the various tools for targeted cell ablation and provide recommendations for optimal application of specific approaches.

Leukocyte Cytoskeleton Polarization Is Initiated by Plasma Membrane Curvature from Cell Attachment

Cell polarization is important for various biological processes. However, its regulation, particularly initiation, is incompletely understood. Here, we investigated mechanisms by which neutrophils break their symmetry and initiate their cytoskeleton polarization from an apolar state in circulation for their extravasation during inflammation. We show here that a local increase in plasma membrane (PM) curvature resulting from cell contact to a surface triggers the initial breakage of the symmetry of an apolar neutrophil and is required for subsequent polarization events induced by chemical stimulation. This local increase in PM curvature recruits SRGAP2 via its F-BAR domain, which in turn activates PI4KA and results in PM PtdIns4P polarization. Polarized PM PtdIns4P is targeted by RPH3A, which directs PIP5K1C90 and subsequent phosphorylated myosin light chain polarization, and this polarization signaling axis regulates neutrophil firm attachment to endothelium. Thus, this study reveals a mechanism for the initiation of cell cytoskeleton polarization.

Regulator of G protein signaling 5 restricts neutrophil chemotaxis and trafficking

Neutrophils are white blood cells that are mobilized to damaged tissues and to sites of pathogen invasion, providing the first line of host defense. Chemokines displayed on the surface of blood vessels promote migration of neutrophils to these sites, and tissue- and pathogen-derived chemoattractant signals, including N-formylmethionylleucylphenylalanine (fMLP), elicit further migration to sites of infection. Although nearly all chemoattractant receptors use heterotrimeric G proteins to transmit signals, many of the mechanisms lying downstream of chemoattractant receptors that either promote or limit neutrophil motility are incompletely defined. Here, we show that regulator of G protein signaling 5 (RGS5), a protein that modulates G protein activity, is expressed in both human and murine neutrophils. We detected significantly more neutrophils in the airways of Rgs5^-/- mice than WT counterparts following acute respiratory virus infection and in the peritoneum in response to injection of thioglycollate, a biochemical proinflammatory stimulus. RGS5-deficient neutrophils responded with increased chemotaxis elicited by the chemokines CXC motif chemokine ligand 1 (CXCL1), CXCL2, and CXCL12 but not fMLP. Moreover, adhesion of these cells was increased in the presence of both CXCL2 and fMLP. In summary, our results indicate that RGS5 deficiency increases chemotaxis and adhesion, leading to more efficient neutrophil mobilization to inflamed tissues in mice. These findings suggest that RGS5 expression and activity in neutrophils determine their migrational patterns in the complex microenvironments characteristic of inflamed tissues.

A membrane-associated, fluorogenic reporter for mammalian phospholipase C isozymes

A diverse group of cell-surface receptors, including many G protein-coupled receptors and receptor tyrosine kinases, activate phospholipase C (PLC) isozymes to hydrolyze phosphatidylinositol 4,5-bisphosphate into the second messengers diacylglycerol and 1,4,5-inositol trisphosphate. Consequently, PLCs control various cellular processes, and their aberrant regulation contributes to many diseases, including cancer, atherosclerosis, and rheumatoid arthritis. Despite the widespread importance of PLCs in human biology and disease, it has been impossible to directly monitor the real-time activation of these enzymes at membranes. To overcome this limitation, here we describe XY-69, a fluorogenic reporter that preferentially partitions into membranes and provides a selective tool for measuring the real-time activity of PLCs as either purified enzymes or in cellular lysates. Indeed, XY-69 faithfully reported the membrane-dependent activation of PLC-β3 by Gα_q Therefore, XY-69 can replace radioactive phosphatidylinositol 4,5-bisphosphate used in conventional PLC assays and will enable high-throughput screens to identify both orthosteric and allosteric PLC inhibitors. In the future, cell-permeable variants of XY-69 represent promising candidates for reporting the activation of PLCs in live cells with high spatiotemporal resolution.

The macrophage C-type lectin receptor CLEC5A (MDL-1) expression is associated with early plaque progression and promotes macrophage survival

Background

Biomarkers of early plaque progression are still elusive. Myeloid DAP12-associating lectin-1 (MDL-1), also called CLEC5A, is a C-type lectin receptor implicated in the progression of multiple acute and chronic inflammatory diseases. However, the relationship between its level and atherosclerosis is unknown. In this study, we aimed to investigate the correlation between macrophage MDL-1 expression and early atherosclerosis progression.

Methods

Immunofluorescence staining, real-time PCR and western blot were performed to analyze MDL-1 expression in aorta or mice macrophages. The role of MDL-1 in macrophage survival was further investigated by adenovirus infection and TUNEL assay.

Results

Significant MDL-1 expression was found in advanced human and apoE−/− mice atherosclerotic plaques, especially in lesional macrophages. In the model of atherosclerosis regression, we found MDL-1 expression was highly downregulated in lesional macrophages from ldlr−/− mouse regressive plaques, coincident with a reduction in lesional macrophage content and marker of M1 proinflammatory macrophages. Furthermore, we found MDL-1 was significantly expressed in inflammatory M1 subtype polarized bone marrow-derived macrophages. In vitro experiments, the level of MDL-1 was remarkably elevated in macrophages treated with pathophysiological drivers of plaque progression, such as oxidized low-density lipoprotein (ox-LDL) and hypoxia. Mechanistically, we demonstrated that MDL-1 overexpression notably promoted macrophage survival and decreased cleaved caspase-3 expression under ox-LDL stimulation, which suggested that it could maintain lesional macrophage survival and cause its accumulation.

Conclusions

This study firstly demonstrated that MDL-1 is mainly expressed in atherosclerotic lesional macrophages and increased macrophage MDL-1 expression is associated with early plaque progression and promotes macrophage survival.

Electronic supplementary material

The online version of this article (10.1186/s12967-017-1336-z) contains supplementary material, which is available to authorized users.

The Complement System Component C5a Produces Thermal Hyperalgesia via Macrophage-to-Nociceptor Signaling That Requires NGF and TRPV1

The complement cascade is a principal component of innate immunity. Recent studies have underscored the importance of C5a and other components of the complement system in inflammatory and neuropathic pain, although the underlying mechanisms are largely unknown. In particular, it is unclear how the complement system communicates with nociceptors and which ion channels and receptors are involved. Here we demonstrate that inflammatory thermal and mechanical hyperalgesia induced by complete Freund's adjuvant was accompanied by C5a upregulation and was markedly reduced by C5a receptor (C5aR1) knock-out or treatment with the C5aR1 antagonist PMX53. Direct administration of C5a into the mouse hindpaw produced strong thermal hyperalgesia, an effect that was absent in TRPV1 knock-out mice, and was blocked by the TRPV1 antagonist AMG9810. Immunohistochemistry of mouse plantar skin showed prominent expression of C5aR1 in macrophages. Additionally, C5a evoked strong Ca(2+) mobilization in macrophages. Macrophage depletion in transgenic macrophage Fas-induced apoptosis mice abolished C5a-dependent thermal hyperalgesia. Examination of inflammatory mediators following C5a injection revealed a rapid upregulation of NGF, a mediator known to sensitize TRPV1. Preinjection of an NGF-neutralizing antibody or Trk inhibitor GNF-5837 prevented C5a-induced thermal hyperalgesia. Notably, NGF-induced thermal hyperalgesia was unaffected by macrophage depletion. Collectively, these results suggest that complement fragment C5a induces thermal hyperalgesia by triggering macrophage-dependent signaling that involves mobilization of NGF and NGF-dependent sensitization of TRPV1. Our findings highlight the importance of macrophage-to-neuron signaling in pain processing and identify C5a, NGF, and TRPV1 as key players in this cross-cellular communication.

SIGNIFICANCE STATEMENT

This study provides mechanistic insight into how the complement system, a key component of innate immunity, regulates the development of pain hypersensitivity. We demonstrate a crucial role of the C5a receptor, C5aR1, in the development of inflammatory thermal and mechanical sensitization. By focusing on the mechanisms of C5a-induced thermal hyperalgesia, we show that this process requires recruitment of macrophages and initiation of macrophage-to-nociceptor signaling. At the molecular level, we demonstrate that this signaling depends on NGF and is mediated by the heat-sensitive nociceptive channel TRPV1. This deeper understanding of how immune cells and neurons interact to regulate pain processing is expected to facilitate mechanism-based approaches in the development of new analgesics.

Macrophages and Phospholipases at the Intersection between Inflammation and the Pathogenesis of HIV-1 Infection

Persistent low grade immune activation and chronic inflammation are nowadays considered main driving forces of the progressive immunologic failure in effective antiretroviral therapy treated HIV-1 infected individuals. Among the factors contributing to this phenomenon, microbial translocation has emerged as a key driver of persistent immune activation. Indeed, the rapid depletion of gastrointestinal CD4⁺ T lymphocytes occurring during the early phases of infection leads to a deterioration of the gut epithelium followed by the translocation of microbial products into the systemic circulation and the subsequent activation of innate immunity. In this context, monocytes/macrophages are increasingly recognized as an important source of inflammation, linked to HIV-1 disease progression and to non-AIDS complications, such as cardiovascular disease and neurocognitive decline, which are currently main challenges in treated patients. Lipid signaling plays a central role in modulating monocyte/macrophage activation, immune functions and inflammatory responses. Phospholipase-mediated phospholipid hydrolysis leads to the production of lipid mediators or second messengers that affect signal transduction, thus regulating a variety of physiologic and pathophysiologic processes. In this review, we discuss the contribution of phospholipases to monocyte/macrophage activation in the context of HIV-1 infection, focusing on their involvement in virus-associated chronic inflammation and co-morbidities.

PKN1 Directs Polarized RAB21 Vesicle Trafficking via RPH3A and Is Important for Neutrophil Adhesion and Ischemia-Reperfusion Injury

Polarized vesicle transport plays an important role in cell polarization, but the mechanisms underlying this process and its role in innate immune responses are not well understood. Here, we describe a phosphorylation-regulated polarization mechanism that is important for neutrophil adhesion to endothelial cells during inflammatory responses. We show that the protein kinase PKN1 phosphorylates RPH3A, which enhances binding of RPH3A to guanosine triphosphate (GTP)-bound RAB21. These interactions are important for polarized localization of RAB21 and RPH3A in neutrophils, which leads to PIP5K1C90 polarization. Consistent with the roles of PIP5K1C90 polarization, the lack of PKN1 or RPH3A impairs neutrophil integrin activation, adhesion to endothelial cells, and infiltration in inflammatory models. Furthermore, myeloid-specific loss of PKN1 decreases tissue injury in a renal ischemia-reperfusion model. Thus, this study characterizes a mechanism for protein polarization in neutrophils and identifies a potential protein kinase target for therapeutic intervention in reperfusion-related tissue injury.

Nuclear factor-κB regulates expression of platelet phospholipase C-β2 (PLCB2)

Phospholipase C (PLC)-β2 (gene PLCB2) is a critical regulator of platelet responses upon activation. Mechanisms regulating of PLC-β2 expression in platelets/MKs are unknown. Our studies in a patient with platelet PLC-β2 deficiency revealed the PLCB2 coding sequence to be normal and decreased platelet PLC-β2 mRNA, suggesting a defect in transcriptional regulation. PLCB2 5'- upstream region of the patient revealed a heterozygous 13 bp deletion (-1645/-1633 bp) encompassing a consensus sequence for nuclear factor-κB (NF-κB). This was subsequently detected in three of 50 healthy subjects. To understand the mechanisms regulating PLC-β2, we studied the effect of this variation in the PLCB2. Gel-shift studies using nuclear extracts from human erythroleukaemia (HEL) cells or recombinant p65 showed NF-κB binding to oligonucleotide with NF-κB site; in luciferase reporter studies its deletion reduced PLCB2 promoter activity. PLCB2 expression was decreased by siRNA knockdown of NF-κB p65 subunit and increased by p65 overexpression. By immunoblotting platelet PLC-β2 in 17 healthy subjects correlated with p65 (r=0.76, p=0.0005). These studies provide the first evidence that NF-κB regulates MK/platelet PLC-β2 expression. This interaction is important because of the major role of PLC-β2 in platelet activation and of NF-κB in processes, including inflammation and atherosclerosis, where both are intimately involved.

ORP4L Facilitates Macrophage Survival via G-Protein-Coupled Signaling: ORP4L-/- Mice Display a Reduction of Atherosclerosis

RATIONALE

Macrophage survival within the arterial wall is a central factor contributing to atherogenesis. Oxysterols, major components of oxidized low-density lipoprotein, exert cytotoxic effects on macrophages.

OBJECTIVE

To determine whether oxysterol-binding protein-related protein 4 L (ORP4L), an oxysterol-binding protein, affects macrophage survival and the pathogenesis of atherosclerosis.

METHODS AND RESULTS

By hiring cell biological approaches and ORP4L^-/- mice, we show that ORP4L coexpresses with and forms a complex with Gα_q/11 and phospholipase C (PLC)-β3 in macrophages. ORP4L facilitates G-protein-coupled ligand-induced PLCβ3 activation, IP₃ production, and Ca²⁺ release from the endoplasmic reticulum. Through this mechanism, ORP4L sustains antiapoptotic Bcl-XL expression through Ca²⁺-mediated c-AMP responsive element binding protein transcriptional regulation and thus protects macrophages from apoptosis. Excessive stimulation with the oxysterol 25-hydroxycholesterol disassembles the ORP4L/Gα_q/11/PLCβ3 complexes, resulting in reduced PLCβ3 activity, IP₃ production, and Ca²⁺ release, as well as decreased Bcl-XL expression and increased apoptosis. Overexpression of ORP4L counteracts these oxysterol-induced defects. Mice lacking ORP4L exhibit increased apoptosis of macrophages in atherosclerotic lesions and a reduced lesion size.

CONCLUSIONS

ORP4L is crucial for macrophage survival. It counteracts the cytotoxicity of oxysterols/oxidized low-density lipoprotein to protect macrophage from apoptosis, thus playing an important role in the development of atherosclerosis.

Phosphoinositide-specific phospholipase C in health and disease

Phospholipases are widely occurring and can be found in several different organisms, including bacteria, yeast, plants, animals, and viruses. Phospholipase C (PLC) is a class of phospholipases that cleaves phospholipids on the diacylglycerol (DAG) side of the phosphodiester bond producing DAGs and phosphomonoesters. Among PLCs, phosphoinositide-specific PLC (PI-PLC) constitutes an important step in the inositide signaling pathways. The structures of PI-PLC isozymes show conserved domains as well as regulatory specific domains. This is important, as most PI-PLCs share a common mechanism, but each of them has a peculiar role and can have a specific cell distribution that is linked to a specific function. More importantly, the regulation of PLC isozymes is fundamental in health and disease, as there are several PLC-dependent molecular mechanisms that are associated with the activation or inhibition of important physiopathological processes. Moreover, PI-PLC alternative splicing variants can play important roles in complex signaling networks, not only in cancer but also in other diseases. That is why PI-PLC isozymes are now considered as important molecules that are essential for better understanding the molecular mechanisms underlying both physiology and pathogenesis, and are also potential molecular targets useful for the development of innovative therapeutic strategies.

Pivotal Role of Serum- and Glucocorticoid-Inducible Kinase 1 in Vascular Inflammation and Atherogenesis

Objective—

Atherosclerosis, an inflammatory disease of arterial vessel walls, requires migration and matrix metalloproteinase (MMP)-9–dependent invasion of monocytes/macrophages into the vascular wall. MMP-9 expression is stimulated by transcription factor nuclear factor-κB, which is regulated by inhibitor κB (IκB) and thus IκB kinase. Regulators of nuclear factor-κB include serum- and glucocorticoid-inducible kinase 1 (SGK1). The present study explored involvement of SGK1 in vascular inflammation and atherogenesis.

Approach and Results—

Gene-targeted apolipoprotein E (ApoE)–deficient mice without ( apoe −/− sgk1 +/+ ) or with ( apoe −/− sgk1 −/− ) additional SGK1 knockout received 16-week cholesterol-rich diet. According to immunohistochemistry atherosclerotic lesions in aorta and carotid artery, vascular CD45 + leukocyte infiltration, Mac-3 + macrophage infiltration, vascular smooth muscle cell content, MMP-2, and MMP-9 positive areas in atherosclerotic tissue were significantly less in apoe −/− sgk1 −/− mice than in apoe −/− sgk1 +/+ mice. As determined by Boyden chamber, thioglycollate-induced peritonitis and air pouch model, migration of SGK1-deficient CD11b + F4/80 + macrophages was significantly diminished in vitro and in vivo. Zymographic MMP-2 and MMP-9 production, MMP-9 activity and invasion through matrigel in vitro were significantly less in sgk1 −/− than in sgk1 +/+ macrophages and in control plasmid–transfected or inactive K127N SGK1-transfected than in constitutively active S422D SGK1-transfected THP-1 cells. Confocal microscopy revealed reduced macrophage number and macrophage MMP-9 content in plaques of apoe −/− sgk1 −/− mice. In THP-1 cells, MMP-inhibitor GM6001 (25 μmol/L) abrogated S422D SGK1-induced MMP-9 production and invasion. According to reverse transcription polymerase chain reaction, MMP-9 transcript levels were significantly reduced in sgk1 −/− macrophages and strongly upregulated in S422D SGK1-transfected THP-1 cells compared with control plasmid–transfected or K127N SGK1-transfected THP-1 cells. According to immunoblotting and confocal microscopy, phosphorylation of IκB kinase and inhibitor κB and nuclear translocation of p50 were significantly lower in sgk1 −/− macrophages than in sgk1 +/+ macrophages and significantly higher in S422D SGK1-transfected THP-1 cells than in control plasmid–transfected or K127N SGK1-transfected THP-1 cells. Treatment of S422D SGK1-transfected THP-1 cells with IκB kinase-inhibitor BMS-345541 (10 μmol/L) abolished S422D SGK1-induced increase of MMP-9 transcription and gelatinase activity.

Conclusions—

SGK1 plays a pivotal role in vascular inflammation during atherogenesis. SGK1 participates in the regulation of monocyte/macrophage migration and MMP-9 transcription via regulation of nuclear factor-κB.

Front-signal-dependent accumulation of the RHOA inhibitor FAM65B at leading edges polarizes neutrophils

A hallmark of neutrophil polarization is the back localization of active RHOA and phosphorylated myosin light chain (pMLC, also known as MYL2). However, the mechanism for the polarization is not entirely clear. Here, we show that FAM65B, a newly identified RHOA inhibitor, is important for the polarization. When FAM65B is phosphorylated, it binds to 14-3-3 family proteins and becomes more stable. In neutrophils, chemoattractants stimulate FAM65B phosphorylation largely depending on the signals from the front of the cells that include those mediated by phospholipase Cβ (PLCβ) and phosphoinositide 3-kinase γ (PI3Kγ), leading to FAM65B accumulation at the leading edge. Concordantly, FAM65B deficiency in neutrophils resulted in an increase in RHOA activity and localization of pMLC to the front of cells, as well as defects in chemotaxis directionality and adhesion to endothelial cells under flow. These data together elucidate a mechanism for RHOA and pMLC polarization in stimulated neutrophils through direct inhibition of RHOA by FAM65B at the leading edge.

Immune regulation by phospholipase C-β isoforms

Rapid progress has recently been made regarding how phospholipase C (PLC)-β functions downstream of G protein-coupled receptors and how PLC-β functions in the nucleus. PLC-β has also been shown to interplay with tyrosine kinase-based signaling pathways, specifically to inhibit Stat5 activation by recruiting the protein-tyrosine phosphatase SHP-1. In this regard, a new multimolecular signaling platform, named SPS complex, has been identified. The SPS complex has important regulatory roles in tumorigenesis and immune cell activation. Furthermore, a growing body of work suggests that PLC-β also participates in the differentiation and activation of immune cells that control both the innate and adaptive immune systems.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Phospholipase C-β in immune cells

Great progress has recently been made in structural and functional research of phospholipase C (PLC)-β. We now understand how PLC-β isoforms (β1-β4) are activated by GTP-bound Gαq downstream of G protein-coupled receptors. Numerous studies indicate that PLC-βs participate in the differentiation and activation of immune cells that control both the innate and adaptive immune systems. The PLC-β3 isoform also interplays with tyrosine kinase-based signaling pathways, to inhibit Stat5 activation by recruiting the protein-tyrosine phosphatase SHP-1, with which PLC-β3 and Stat5 form a multi-molecular signaling platform, named SPS complex. The SPS complex has important regulatory roles in tumorigenesis and immune cell activation.

Phosphatidic acid is required for the constitutive ruffling and macropinocytosis of phagocytes

Phagocytes spontaneously ruffle as they probe their environment and take up antigens. These cells are uniquely enriched in phosphatidic acid, which is necessary for ruffling and macropinocytosis.

Macrophages and dendritic cells continuously survey their environment in search of foreign particles and soluble antigens. Such surveillance involves the ongoing extension of actin-rich protrusions and the consequent formation of phagosomes and macropinosomes. The signals inducing this constitutive cytoskeletal remodeling have not been defined. We report that, unlike nonphagocytic cells, macrophages and immature dendritic cells have elevated levels of phosphatidic acid (PA) in their plasma membrane. The plasmalemmal PA is synthesized by phosphorylation of diacylglycerol, which is in turn generated by a G protein–stimulated phospholipase C. Inhibition of diacylglycerol kinase activity results in the detachment of T-cell lymphoma invasion and metastasis–inducing protein 1 (TIAM1)—a Rac guanine exchange factor—from the plasma membrane, thereby depressing Rac activity and abolishing the constitutive ruffling and macropinocytosis that characterize macrophages and immature dendritic cells. Accumulation of PA and binding of TIAM1 to the membrane require the activity of phosphatidylinositol-4,5-bisphosphate 3-kinase. Thus a distinctive, constitutive pathway of PA biosynthesis promotes the actin remodeling required for immune surveillance.

Small molecule inhibitors of phospholipase C from a novel high-throughput screen

Phospholipase C (PLC) isozymes are important signaling molecules, but few small molecule modulators are available to pharmacologically regulate their function. With the goal of developing a general approach for identification of novel PLC inhibitors, we developed a high-throughput assay based on the fluorogenic substrate reporter WH-15. The assay is highly sensitive and reproducible: screening a chemical library of 6280 compounds identified three novel PLC inhibitors that exhibited potent activities in two separate assay formats with purified PLC isozymes in vitro. Two of the three inhibitors also inhibited G protein-coupled receptor-stimulated PLC activity in intact cell systems. These results demonstrate the power of the high-throughput assay for screening large collections of small molecules to identify novel PLC modulators. Potent and selective modulators of PLCs will ultimately be useful for dissecting the roles of PLCs in cellular processes, as well as provide lead compounds for the development of drugs to treat diseases arising from aberrant phospholipase activity.

A novel sarcoidosis risk locus for Europeans on chromosome 11q13.1

RATIONALE

Sarcoidosis is a complex inflammatory disease with a heterogeneous clinical picture. Among others, an acute and chronic clinical course can be distinguished, for which specific genetic risk factors are known.

OBJECTIVES

To identify additional risk loci for sarcoidosis and its acute and chronic subforms, we analyzed imputed data from a genome-wide association scan for these phenotypes.

METHODS

After quality control, the genome-wide association scan comprised nearly 1.3 million imputed single-nucleotide polymorphisms based on an Affymetrix 6.0 Gene Chip dataset of 564 German sarcoidosis cases, including 176 acute and 354 chronic cases and 1,575 control subjects.

MEASUREMENTS AND MAIN RESULTS

We identified chromosome 11q13.1 (rs479777) as a novel locus influencing susceptibility to sarcoidosis with genome-wide significance. The marker was significantly associated in three distinct German case-control populations and in an additional German family sample with odds ratios ranging from 0.67 to 0.77. This finding was further replicated in two independent European case-control populations from the Czech Republic (odds ratio, 0.75) and from Sweden (odds ratio, 0.79). In a meta-analysis of the included European case-control samples the marker yielded a P value of 2.68 × 10(-18). The locus was previously reported to be associated with Crohn disease, psoriasis, alopecia areata, and leprosy. For sarcoidosis, fine-mapping and expression analysis suggest KCNK4, PRDX5, PCLB3, and most promising CCDC88B as candidates for the underlying risk gene in the associated region.

CONCLUSIONS

This study provides striking evidence for association of chromosome 11q13.1 with sarcoidosis in Europeans, and thus identified a further genetic risk locus shared by sarcoidosis, Crohn disease and psoriasis.

Diverse roles of macrophages in atherosclerosis: from inflammatory biology to biomarker discovery

Cardiovascular disease, a leading cause of mortality in developed countries, is mainly caused by atherosclerosis, a chronic inflammatory disease. Macrophages, which differentiate from monocytes that are recruited from the blood, account for the majority of leukocytes in atherosclerotic plaques. Apoptosis and the suppressed clearance of apoptotic macrophages (efferocytosis) are associated with vulnerable plaques that are prone to rupture, leading to thrombosis. Based on the central functions of macrophages in atherogenesis, cytokines, chemokines, enzymes, or microRNAs related to or produced by macrophages have become important clinical prognostic or diagnostic biomarkers. This paper discusses the impact of monocyte-derived macrophages in early atherogenesis and advanced disease. The role and possible future development of macrophage inflammatory biomarkers are also described.

Signalling from dead cells drives inflammation and vessel remodelling

Death of vascular smooth muscle cells (VSMCs) has been demonstrated in vessel development and in disease, most notably in atherosclerosis, but also after injury and remodelling. VSMC death promotes multiple features of vulnerable plaques, but also induces features of normal vessel ageing and cystic medial necrosis, including loss of VSMCs, elastin fragmentation and loss, increased glycosaminoglycans and speckled calcification. VSMC apoptosis in the absence of efficient phagocytosis also produces inflammation due to secondary necrosis; in contrast, VSMC apoptosis in normal vessels can be silent. We have investigated the consequences of VSMC apoptosis in both disease and during vessel remodelling. We find that VSMCs release specific cytokines dependent upon the mode of cell death; IL-1β predominates during apoptosis, whilst IL-1α predominates during necrosis. Both IL-1α and β promote release of further cytokines from adjacent live cells, in particular IL-6 and MCP-1. The balance of cytokines results in pathology with differing compositions, including inflammation or neointima formation/vascular repair, via direct promotion of VSMC proliferation and migration. Thus, VSMC death can promote either pathology or repair, depending upon the context and cytokine signalling.

Cell death, damage-associated molecular patterns, and sterile inflammation in cardiovascular disease

Cell death and inflammation are ancient processes of fundamental biological importance in both normal physiology and pathology. This is evidenced by the profound conservation of mediators, with ancestral homologues identified from plants to humans, and the number of diseases driven by aberrant control of either process. Apoptosis is the most well-studied cell death, but many forms exist, including autophagy, necrosis, pyroptosis, paraptosis, and the obscure dark cell death. Cell death occurs throughout the cardiovascular system, from initial shaping of the heart and vasculature during development to involvement in pathologies, including atherosclerosis, aneurysm, cardiomyopathy, restenosis, and vascular graft rejection. However, determining whether cell death primarily drives pathology or is a secondary bystander effect is difficult. Inflammation, the primary response of innate immunity, is considered essential in initiating and driving vascular diseases. Cell death and inflammation are inextricably linked with their effectors modulating the other process. Indeed, an evolutionary link between cell death and inflammation occurs at caspase-1 (which activates interleukin-1β), which can induce death by pyroptosis, and is a member of the caspase family vital for apoptosis. This review examines cell death in vascular disease, how it can induce inflammation, and finally the emergence of inflammasomes in vascular pathology.

Phospholipase Cβ3 mediates LH-induced granulosa cell differentiation

Previous studies showed that under certain conditions LH can stimulate not only adenylate cyclase (AC) but also phospholipase Cβ (PLCβ) signaling in target cells; however, the physiological involvement of PLCβ in LH-induced ovarian follicular cell differentiation has not been determined. To address this, ex vivo expression analyses and specific PLCβ targeting were performed in primary bovine granulosa cells. Expression analyses in cells from small (2.0-5.9 mm), medium (6.0-9.9 mm), and ovulatory-size (10.0-13.9 mm) follicles revealed an increase in mRNA and protein levels of heterotrimeric G protein subunits-αs, -αq, -α11, and -αi2 in ovulatory-size follicles, simultaneous with a substantial increase in LH receptor expression. Among the four known PLCβ isoforms, PLCβ3 (PLCB3) was specifically up-regulated in cells from ovulatory-size follicles, in association with a predominantly cytoplasmic location of PLCB3 in these cells and a significant inositol phosphate response to LH stimulation. Furthermore, RNA interference-mediated PLCB3 down-regulation reduced the ability of LH to induce hallmark differentiation responses of granulosa cells, namely transcriptional up-regulation of prostaglandin-endoperoxide synthase 2 and down-regulation of both aromatase expression and estradiol production. Responses to the AC agonist, forskolin, however, were not affected. In addition, PLCB3 down-regulation did not alter cAMP responses to LH in granulosa cells, ruling out a primary involvement of AC in mediating the effects of PLCB3. In summary, we provide evidence of a physiological involvement of PLCβ signaling in ovulatory-size follicles and specifically identify PLCB3 as a mediator of LH-induced differentiation responses of granulosa cells.

A requirement for the p85 PI3K adapter protein BCAP in the protection of macrophages from apoptosis induced by endoplasmic reticulum stress

Macrophages are innate immune cells that play key roles in regulation of the immune response and in tissue injury and repair. In response to specific innate immune stimuli, macrophages may exhibit signs of endoplasmic reticulum (ER) stress and progress to apoptosis. Factors that regulate macrophage survival under these conditions are poorly understood. In this study, we identified B cell adapter protein (BCAP), a p85 PI3K-binding adapter protein, in promoting survival in response to the combined challenge of LPS and ER stress. BCAP was unique among nine PI3K adapter proteins in being induced >10-fold in response to LPS. LPS-stimulated macrophages incubated with thapsigargin, a sarcoplasmic/endoplasmic reticulum calcium ATPase inhibitor that induces ER stress, underwent caspase-3 activation and apoptosis. Macrophages from BCAP(-/-) mice exhibited increased apoptosis in response to these stimuli. BCAP-deficient macrophages demonstrated decreased activation of Akt, but not ERK, and, unlike BCAP-deficient B cells, expressed normal amounts of the NF-κB subunits, c-Rel and RelA. Retroviral transduction of BCAP-deficient macrophages with wild-type BCAP, but not a Y4F BCAP mutant defective in binding the SH2 domain of p85 PI3K, reversed the proapoptotic phenotype observed in BCAP-deficient macrophages. We conclude that BCAP is a nonredundant PI3K adapter protein in macrophages that is required for maximal cell survival in response to ER stress. We suggest that as macrophages engage their pathogenic targets, innate immune receptors trigger increased expression of BCAP, which endows them with the capacity to withstand further challenges from ongoing cellular insults, such as ER stress.

Impairment of survival signaling and efferocytosis in TRPC3-deficient macrophages

We have recently shown that in macrophages proper operation of the survival pathways phosphatidylinositol-3-kinase (PI3K)/AKT and nuclear factor kappa B (NFkB) has an obligatory requirement for constitutive, non-regulated Ca(2+) influx. In the present work we examined if Transient Receptor Potential Canonical 3 (TRPC3), a member of the TRPC family of Ca(2+)-permeable cation channels, contributes to the constitutive Ca(2+) influx that supports macrophage survival. We used bone marrow-derived macrophages obtained from TRPC3(-/-) mice to determine the activation status of survival signaling pathways, apoptosis and their efferocytic properties. Treatment of TRPC3(+/+) macrophages with the pro-apoptotic cytokine TNFα induced time-dependent phosphorylation of IκBα, AKT and BAD, and this was drastically reduced in TRPC3(-/-) macrophages. Compared to TRPC3(+/+) cells TRPC3(-/-) macrophages exhibited reduced constitutive cation influx, increased apoptosis and impaired efferocytosis. The present findings suggest that macrophage TRPC3, presumably through its constitutive function, contributes to survival signaling and efferocytic properties.

Phospholipase C-β3 is a key modulator of IL-8 expression in cystic fibrosis bronchial epithelial cells

Respiratory insufficiency is the major cause of morbidity and mortality in patients affected by cystic fibrosis (CF). An excessive neutrophilic inflammation, mainly orchestrated by the release of IL-8 from bronchial epithelial cells and amplified by chronic bacterial infection with Pseudomonas aeruginosa, leads to progressive tissue destruction. The anti-inflammatory drugs presently used in CF patients have several limitations, indicating the need for identifying novel molecular targets. To address this issue, we preliminarily studied the association of 721 single nucleotide polymorphisms from 135 genes potentially involved in signal transduction implicated in neutrophil recruitment in a cohort of F508del homozygous CF patients with either severe or mild progression of lung disease. The top ranking association was found for a nonsynonymous polymorphism of the phospholipase C-β3 (PLCB3) gene. Studies in bronchial epithelial cells exposed to P. aeruginosa revealed that PLCB3 is implicated in extracellular nucleotide-dependent intracellular calcium signaling, leading to activation of the protein kinase Cα and Cβ and of the nuclear transcription factor NF-κB p65. The proinflammatory pathway regulated by PLCB3 acts by potentiating the Toll-like Receptors' signaling cascade and represents an interesting molecular target to attenuate the excessive recruitment of neutrophils without completely abolishing the inflammatory response.

Selective macrophage ascorbate deficiency suppresses early atherosclerosis

To test whether severe ascorbic acid deficiency in macrophages affects progression of early atherosclerosis, we used fetal liver cell transplantation to generate atherosclerosis-prone apolipoprotein E-deficient (apoE(-/-)) mice that selectively lacked the ascorbate transporter (SVCT2) in hematopoietic cells, including macrophages. After 13 weeks of chow diet, apoE(-/-) mice lacking the SVCT2 in macrophages had surprisingly less aortic atherosclerosis, decreased lesion macrophage numbers, and increased macrophage apoptosis compared to control-transplanted mice. Serum lipid levels were similar in both groups. Peritoneal macrophages lacking the SVCT2 had undetectable ascorbate; increased susceptibility to H(2)O(2)-induced mitochondrial dysfunction and apoptosis; decreased expression of genes for COX-2, IL1β, and IL6; and decreased lipopolysaccharide-stimulated NF-κB and antiapoptotic gene expression. These changes were associated with decreased expression of both the receptor for advanced glycation end products and HIF-1α, either or both of which could have been the proximal cause of decreased macrophage activation and apoptosis in ascorbate-deficient macrophages.

Macrophage adipose triglyceride lipase deficiency attenuates atherosclerotic lesion development in low-density lipoprotein receptor knockout mice

OBJECTIVE

The consequences of macrophage triglyceride (TG) accumulation on atherosclerosis have not been studied in detail so far. Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme for the initial step in TG hydrolysis. Because ATGL knockout (KO) mice exhibit massive TG accumulation in macrophages, we used ATGL KO mice to study the effects of macrophage TG accumulation on atherogenesis.

METHODS AND RESULTS

Low-density lipoprotein receptor (LDLr) KO mice were transplanted with bone marrow from ATGL KO (ATGL KO→LDLr KO) or wild-type (WT→LDLr KO) mice and challenged with a Western-type diet for 9 weeks. Despite TG accumulation in ATGL KO macrophages, atherosclerosis in ATGL KO→LDLr KO mice was 43% reduced associated with decreased plasma monocyte chemoattractant protein-1 (MCP-1) and macrophage interleukin-6 concentrations. This coincided with a reduced amount of macrophages, possibly because of a 39% increase in intraplaque apoptosis and a decreased migratory capacity of ATGL KO macrophages. The reduced number of white blood cells might be due to a 36% decreased Lin(-)Sca-1(+)cKit(+) hematopoietic stem cell population.

CONCLUSIONS

We conclude that the attenuation of atherogenesis in ATGL KO→LDLr KO mice is due to decreased infiltration of less inflammatory macrophages into the arterial wall and increased macrophage apoptosis.

Integrin-induced PIP5K1C kinase polarization regulates neutrophil polarization, directionality, and in vivo infiltration

Neutrophils are important in innate immunity and acute inflammatory responses. However, the regulation of their recruitment to sites of inflammation has not been well characterized. Here, we investigated the kinase PIP5K1C and showed that PIP5K1C deficiency impaired neutrophil recruitment because of an adhesion defect. PIP5K1C regulated the adhesion through facilitating RhoA GTPase and integrin activation by chemoattractants. Integrins could induce polarization of an isoform of PIP5K1C, PIP5K1C-90, in neutrophils through intracellular vesicle transport independently of exogenous chemoattractant. PIP5K1C-90 polarization was required for polarized RhoA activation at uropods and provided an initial directional cue for neutrophil polarization on the endothelium. Importantly, the polarization was also required for circumventing the inhibition of lamellipodium formation by RhoA so that neutrophils could form leading edges required for transendothelial migration. Because integrins are not known to regulate neutrophil polarization, our study revealed a previously underappreciated role of integrin signaling in neutrophil regulation.

Phospholipase C is a key enzyme regulating intracellular calcium and modulating the phosphoinositide balance

Spatial and temporal activation of phosphoinositide turnover enables eukaryotic cells to perform various functions such as cell proliferation/differentiation, fertilization, neuronal functions, and cell motility. In this system, phospholipase C (PLC) is a key enzyme, which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) into two second messengers, inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) and diacylglycerol (DAG). Ins(1,4,5)P(3) triggers the release of calcium from intracellular stores, and DAG mediates the activation of protein kinase C (PKC). In parallel, PI(4,5)P(2) also directly regulates a variety of cellular functions, including cytoskeletal remodeling, cytokinesis, phagocytosis, membrane dynamics, and channel activity, in addition to its role as a substrate for PLC and phosphatidylinositol 3-kinase (PI3K), which generates PI(3,4,5)P(3). An imbalance of these phosphoinositides contributes to the pathogeneses of various human diseases. Therefore, strict regulation of the levels of PI(4,5)P(2) and PI(3,4,5)P(3) by PLC or other interconverting enzymes is necessary for cellular functions. In this review, we focus on the roles of PLC as a calcium-regulating enzyme and as a modulator of the phosphoinositide balance.

Clostridium difficile toxin B differentially affects GPCR-stimulated Ca2+ responses in macrophages: independent roles for Rho and PLA2

Clostridium difficile toxins cause acute colitis by disrupting the enterocyte barrier and promoting inflammation. ToxB from C. difficile inactivates Rho family GTPases and causes release of cytokines and eicosanoids by macrophages. We studied the effects of ToxB on GPCR signaling in murine RAW264.7 macrophages and found that ToxB elevated Ca(2+) responses to Galphai-linked receptors, including the C5aR, but reduced responses to Galphaq-linked receptors, including the UDP receptors. Other Rho inhibitors also reduced UDP Ca(2+) responses, but they did not affect C5a responses, suggesting that ToxB inhibited UDP responses by inhibiting Rho but enhanced C5a responses by other mechanisms. By using PLCbeta isoform-deficient BMDM, we found that ToxB inhibited Ca(2+) signaling through PLCbeta4 but enhanced signaling through PLCbeta3. Effects of ToxB on GPCR Ca(2+) responses correlated with GPCR use of PLCbeta3 versus PLCbeta4. ToxB inhibited UDP Ca(2+) signaling without reducing InsP3 production or the sensitivity of cellular Ca(2+) stores to exogenous InsP3, suggesting that ToxB impairs UDP signaling at the level of InsP3/Ca(2+)coupling. In contrast, ToxB elevated InsP3 production by C5a, and the enhancement of Ca(2+) signaling by C5a was prevented by inhibition of PLA(2) or 5-LOX but not COX, implicating LTs but not prostanoids in the mechanism. In sum, ToxB has opposing, independently regulated effects on Ca(2+) signaling by different GPCR-linked PLCbeta isoforms in macrophages.

Adenosine receptor A2A deficiency in leukocytes increases arterial neointima formation in apolipoprotein E-deficient mice

OBJECTIVE

To use the mice deficient in both adenosine receptor A(2A)(A(2A)R(-/-)) and apolipoprotein E (apoE(-/-)) to investigate the role of A(2A)R in mediating the interactions of leukocytes with injured arterial walls and the formation of arterial neointima induced by a guide wire.

METHODS AND RESULTS

In apoE(-/-) mice, A(2A)R deficiency increased the size of the arterial neointima in injured carotid arteries by 83%. Arterial neointima formation was also enhanced in chimeric mice that underwent bone marrow transplantation (these mice lacked A(2A)R in their bone marrow-derived cells). Epifluorescence intravital microscopy showed that neutrophil rolling and adherence to the injured arterial area were enhanced by 80% and 110% in A(2A)R(-/-)/apoE(-/-) mice, respectively. This phenomenon occurred even though the protein levels of homing molecules on A(2A)R-deficient neutrophils were unchanged from those of wild-type neutrophils. A(2A)R-deficient neutrophils exhibited an increase in the phosphorylation of p38 mitogen-activated protein kinase, P-selectin glycoprotein ligand-1 (PSGL-1) clustering, and the affinity of b(2) integrins. The inhibition of p38 phosphorylation abrogated the increased PSGL-1 clustering and beta(2) integrin affinity, thus reversing the increased homing ability of A(2A)R-deficient leukocytes.

CONCLUSION

A(2A)R plays a complex role in inflammation and tissue injury. The deficiency of A(2A)R enhances the homing ability of leukocytes and increases the formation of the arterial neointima after injury. A(2A)R antagonists are being tested for the treatment of neurodegenerative and other chronic diseases. An evaluation of the effect of A(2A)R antagonists on arterial restenosis after arterial angioplasty should be conducted.

Vascular smooth muscle cell apoptosis induces interleukin-1-directed inflammation: effects of hyperlipidemia-mediated inhibition of phagocytosis

RATIONALE

Atherosclerosis is characterized by lipid accumulation in the vessel wall, inflammation, and both macrophage and vascular smooth muscle cell (VSMC) apoptosis. However, whereas VSMC apoptosis in mice with established atherosclerotic plaques or hyperlipidemia increases serum levels of the proatherogenic cytokines monocyte chemotactic protein (MCP)-1, tumor necrosis factor alpha, and interleukin (IL)-6, the link between hyperlipidemia, apoptosis and inflammation, and the mechanisms by which apoptotic cells promote inflammation in atherosclerosis are unknown.

OBJECTIVE

To determine whether hyperlipidemia affects apoptotic cell clearance, and identify the molecular pathways downstream of VSMC apoptosis that may promote inflammation.

METHODS AND RESULTS

We find that human VSMCs are potent and efficient phagocytes of apoptotic human VSMCs, but phagocytosis is significantly reduced by oxidized low-density lipoprotein in vitro or hyperlipidemia in vivo. Necrotic human aortic VSMCs release IL-1alpha, which induces IL-6 and MCP-1 production from viable human VSMCs in vitro. In contrast, secondary necrotic VSMCs release both IL-1alpha and caspase-activated IL-1beta, augmenting IL-6 and MCP-1 production. Conditionally inducing VSMC apoptosis in situ in hyperlipidemic SM22alpha-hDTR/ApoE(-/-) mice to levels seen in human plaques increases serum MCP-1, tumor necrosis factor alpha, and IL-6, which is prevented by blocking IL-1.

CONCLUSIONS

We conclude that VSMC necrosis releases IL-1alpha, whereas secondary necrosis of apoptotic VSMCs releases both IL-1alpha and beta. IL-1 from necrotic VSMCs induces the surrounding viable VSMCs to produce proinflammatory cytokines. Thus, failed clearance of apoptotic VSMCs caused by hyperlipidemia in vivo may promote the increased serum cytokines and chronic inflammation associated with atherosclerosis.

Suppression of PLCbeta2 by endotoxin plays a role in the adenosine A(2A) receptor-mediated switch of macrophages from an inflammatory to an angiogenic phenotype

Toll-like receptor (TLR) 2, 4, 7, and 9 agonists, together with adenosine A(2A) receptor (A(2A)R) agonists, switch macrophages from an inflammatory (M1) to an angiogenic (M2-like) phenotype. This switch involves induction of A(2A)Rs by TLR agonists, down-regulation of tumor necrosis factor alpha (TNFalpha) and interleukin-12, and up-regulation of vascular endothelial growth factor (VEGF) and interleukin-10 expression. We show here that the TLR4 agonist lipopolysaccharide (LPS) induces rapid and specific post-transcriptional down-regulation of phospholipase C(PLC)beta1 and beta2 expression in macrophages by de-stabilizing their mRNAs. The PLCbeta inhibitor U73122 down-regulates TNFalpha expression by macrophages, and in the presence of A(2A)R agonists, up-regulates VEGF, mimicking the synergistic action of LPS with A(2A)R agonists. Selective down-regulation of PLCbeta2, but not PLCbeta1, using small-interfering RNA resulted in increased VEGF expression in response to A(2A)R agonists, but did not suppress TNFalpha expression. Macrophages from PLCbeta2(-/-) mice also expressed increased VEGF in response to A(2A)R agonists. LPS-mediated suppression of PLCbeta1 and beta2 is MyD88-dependent. In a model of endotoxic shock, LPS (35 microg/mouse, i.p.) suppressed PLCbeta1 and beta2 expression in spleen, liver, and lung of wild-type but not MyD88(-/-) mice. These studies indicate that LPS suppresses PLCbeta1 and beta2 expression in macrophages in vitro and in several tissues in vivo. These results suggest that suppression of PLCbeta2 plays an important role in switching M1 macrophages into an M2-like state.

Immune and inflammatory mechanisms of atherosclerosis (*)

Atherosclerosis is an inflammatory disease of the wall of large- and medium-sized arteries that is precipitated by elevated levels of low-density lipoprotein (LDL) cholesterol in the blood. Although dendritic cells (DCs) and lymphocytes are found in the adventitia of normal arteries, their number is greatly expanded and their distribution changed in human and mouse atherosclerotic arteries. Macrophages, DCs, foam cells, lymphocytes, and other inflammatory cells are found in the intimal atherosclerotic lesions. Beneath these lesions, adventitial leukocytes organize in clusters that resemble tertiary lymphoid tissues. Experimental interventions can reduce the number of available blood monocytes, from which macrophages and most DCs and foam cells are derived, and reduce atherosclerotic lesion burden without altering blood lipids. Under proatherogenic conditions, nitric oxide production from endothelial cells is reduced and the burden of reactive oxygen species (ROS) and advanced glycation end products (AGE) is increased. Incapacitating ROS-generating NADPH oxidase or the receptor for AGE (RAGE) has beneficial effects. Targeting inflammatory adhesion molecules also reduces atherosclerosis. Conversely, removing or blocking IL-10 or TGF-beta accelerates atherosclerosis. Regulatory T cells and B1 cells secreting natural antibodies are atheroprotective. This review summarizes our current understanding of inflammatory and immune mechanisms in atherosclerosis.