Role of lysophosphatidic acid receptor LPA2 in the development of allergic airway inflammation in a murine model of asthma

Lysophosphatidic acid, a simple phospholipid with myriad functions

Lysophosphatidic acid (LPA) is a simple phospholipid consisting of a phosphate group, glycerol moiety, and only one hydrocarbon chain. Despite its simple chemical structure, LPA plays an important role as an essential bioactive signaling molecule via its specific six G protein-coupled receptors, LPA_1-6. Recent studies, especially those using genetic tools, have revealed diverse physiological and pathological roles of LPA and LPA receptors in almost every organ system. Furthermore, many studies are illuminating detailed mechanisms to orchestrate multiple LPA receptor signaling pathways and to facilitate their coordinated function. Importantly, these extensive "bench" works are now translated into the "bedside" as exemplified by approaches targeting LPA₁ signaling to combat fibrotic diseases. In this review, we discuss the physiological and pathological roles of LPA signaling and their implications for clinical application by focusing on findings revealed by in vivo studies utilizing genetic tools targeting LPA receptors.

Efficacy Comparison of LPA2 Antagonist H2L5186303 and Agonist GRI977143 on Ovalbumin-Induced Allergic Asthma in BALB/c Mice

Lysophosphatidic acid (LPA), an intercellular lipid mediator, is increased in the bronchoalveolar fluids of patients with asthma after allergen exposure. LPA administration exaggerates allergic responses, and the type 2 LPA receptor (LPA₂) has been reported as a therapeutic target for asthma. However, results with LPA₂ agonist and antagonist along with LPA₂ gene deficient mice have been controversial and contradictory. We compared the effects of LPA₂ antagonist (H2L5186303) and agonist (GRI977143) in a single experimental protocol of ovalbumin (OVA)-induced allergic asthma by treating drugs before antigen sensitization or challenge. H2L5186303 showed strong suppressive efficacy when administered before OVA sensitization and challenge, such as suppression of airway hyper responsiveness, inflammatory cytokine levels, mucin production, and eosinophil numbers. However, GRI977143 showed significant suppression when administered before an OVA challenge. Increases in eosinophil and lymphocyte counts in the bronchoalveolar lavage fluid, Th2 cytokine levels, inflammatory scores, and mucin production were differentially ameliorated by the two drugs. The results demonstrate the multiple roles of LPA₂ in asthmatic responses. We suggest that the development of LPA₂ antagonists would achieve better therapeutic efficacy against asthma than agonists.

LPA and Autotaxin: Potential Drug Targets in Asthma?

Lysophosphatidic acid (LPA) is a versatile lysolipid, and activates a variety of signaling cascades in many cell types. Extracellular LPA is produced from lysophosphatidylcholine (LPC) by the enzyme autotaxin (ATX), and binds to a family of G-protein coupled receptors on its target cells. Research by many groups continues to support the idea that LPA, and the ATX-LPA axis, have important roles in asthma and allergic airway inflammation. In vitro studies have shown that LPA activates many cell types implicated in airway inflammation, including eosinophils, mast cells, dendritic cells, lymphocytes, airway epithelial cells, and airway smooth muscle cells. In animal models ATX and LPA receptor antagonists have been shown to attenuate allergic airway inflammation and hyperreactivity, cardinal features of asthma in humans. ATX and LPA antagonists are currently under active development to treat lung fibrosis, cancer, and other conditions. If compounds with acceptable safety profiles can be identified, then it seems likely that they will be useful in inflammatory lung diseases like asthma.

Serum Lysophosphatidic Acid Measurement by Liquid Chromatography-Mass Spectrometry in COPD Patients

Lysophospholipids are bioactive signaling molecules derived from cell membrane glycerophospholipids or sphingolipids and are highly regulated under normal physiological conditions. Lysophosphatidic acids (LPAs) are a class of lysophospholipids that act on G-protein-coupled receptors to exert a variety of cellular functions. Dysregulation of phospholipase activity and consequently LPA synthesis in serum have been linked to inflammation, such as seen in chronic obstructive pulmonary disease (COPD). The accurate measurement of phospholipids is critical for evaluating their dysregulation in disease. In this study, we optimized experimental parameters for the sensitive measurement of LPAs. We validated the method based on matrix, linearity, accuracy, precision, and stability. An investigation into sample extraction processes emphasized that the common practice of including low concentration of hydrochloric acid in the extraction buffer causes an overestimation of lipid recovery. The liquid chromatography gradient was optimized to separate various lysophospholipid classes. After optimization, detection limits of LPA were sufficiently sensitive for subsequent analysis, ranging from 2 to 8 nM. The validated workflow was applied to a cohort of healthy donor and COPD patient sera. Eight LPA species were identified, and five unique species of LPA were quantified. Most LPA species increased significantly in COPD patients compared to healthy donors. The correlation between LPAs and other demographic parameters was further investigated in a sample set of over 200 baseline patient sera from a COPD clinical trial. For the first time, LPAs other than the two most abundant and readily detectable moieties are quantified in COPD patients using validated methods, opening the door to downstream biomarker evaluation in respiratory disease.

Emerging Role of Phospholipase-Derived Cleavage Products in Regulating Eosinophil Activity: Focus on Lysophospholipids, Polyunsaturated Fatty Acids and Eicosanoids

Eosinophils are important effector cells involved in allergic inflammation. When stimulated, eosinophils release a variety of mediators initiating, propagating, and maintaining local inflammation. Both, the activity and concentration of secreted and cytosolic phospholipases (PLAs) are increased in allergic inflammation, promoting the cleavage of phospholipids and thus the production of reactive lipid mediators. Eosinophils express high levels of secreted phospholipase A2 compared to other leukocytes, indicating their direct involvement in the production of lipid mediators during allergic inflammation. On the other side, eosinophils have also been recognized as crucial mediators with regulatory and homeostatic roles in local immunity and repair. Thus, targeting the complex network of lipid mediators offer a unique opportunity to target the over-activation and 'pro-inflammatory' phenotype of eosinophils without compromising the survival and functions of tissue-resident and homeostatic eosinophils. Here we provide a comprehensive overview of the critical role of phospholipase-derived lipid mediators in modulating eosinophil activity in health and disease. We focus on lysophospholipids, polyunsaturated fatty acids, and eicosanoids with exciting new perspectives for future drug development.

Lysophospholipids in Lung Inflammatory Diseases

The lysophospholipids (LPLs) belong to a group of bioactive lipids that play pivotal roles in several physiological and pathological processes. LPLs are derivatives of phospholipids and consist of a single hydrophobic fatty acid chain, a hydrophilic head, and a phosphate group with or without a large molecule attached. Among the LPLs, lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are the simplest, and have been shown to be involved in lung inflammatory symptoms and diseases such as acute lung injury, asthma, and chronic obstructive pulmonary diseases. G protein-coupled receptors (GPCRs) mediate LPA and S1P signaling. In this chapter, we will discuss on the role of LPA, S1P, their metabolizing enzymes, inhibitors or agonists of their receptors, and their GPCR-mediated signaling in lung inflammatory symptoms and diseases, focusing specially on acute respiratory distress syndrome, asthma, and chronic obstructive pulmonary disease.

Lysophosphatidic Acid Regulates the Differentiation of Th2 Cells and Its Antagonist Suppresses Allergic Airway Inflammation

BACKGROUND

Lysophosphatidic acid (LPA), a prototypic member of a large family of lysophospholipids, has been recently shown to play a role in immune responses to respiratory diseases. The involvement of LPA in allergic airway inflammation has been reported, but the mechanism remains unclear.

OBJECT

We analyzed the biological activity of LPA in vitro and in vivo and investigated its role in allergic inflammation in mice using an LPA receptor 2 (LPA2) antagonist.

METHODS

We used a murine model with acute allergic inflammation, in which mice are sensitized and challenged with house dust mite, and analyzed airway hyperresponsiveness (AHR), pathological findings, Th2 cytokines, and IL-33 in bronchoalveolar lavage fluid (BALF) and lung homogenates. The effect of LPA on Th2 differentiation and cytokine production was examined in vitro using naive CD4+ T cells isolated from splenocytes. We also investigated in vivo the effects of LPA on intranasal administration in mice.

RESULTS

The LPA2 antagonist suppressed the increase of AHR, the number of total cells, and eosinophils in BALF and lung tissue. It also decreased the production of IL-13 in BALF and IL-33 and CCL2 in the lung. LPA promoted Th2 cell differentiation and IL-13 production by Th2 cells in vitro. Nasal administration of LPA significantly increased the number of total cells and IL-13 in BALF via regulating the production of IL-33 and CCL-2-derived infiltrating macrophages.

CONCLUSION

These findings suggest that LPA plays an important role in allergic airway inflammation and that the blockade of LPA2 might have therapeutic potential for bronchial asthma.

Bioactive lipids, inflammation and chronic diseases

Endogenous bioactive lipids are part of a complex network that modulates a plethora of cellular and molecular processes involved in health and disease, of which inflammation represents one of the most prominent examples. Inflammation serves as a well-conserved defence mechanism, triggered in the event of chemical, mechanical or microbial damage, that is meant to eradicate the source of damage and restore tissue function. However, excessive inflammatory signals, or impairment of pro-resolving/anti-inflammatory pathways leads to chronic inflammation, which is a hallmark of chronic pathologies. All main classes of endogenous bioactive lipids - namely eicosanoids, specialized pro-resolving lipid mediators, lysoglycerophopsholipids and endocannabinoids - have been consistently involved in the chronic inflammation that characterises pathologies such as cancer, diabetes, atherosclerosis, asthma, as well as autoimmune and neurodegenerative disorders and inflammatory bowel diseases. This review gathers the current knowledge concerning the involvement of endogenous bioactive lipids in the pathogenic processes of chronic inflammatory pathologies.

Regulation of Tumor Immunity by Lysophosphatidic Acid

The tumor microenvironment (TME) may be best conceptualized as an ecosystem comprised of cancer cells interacting with a multitude of stromal components such as the extracellular matrix (ECM), blood and lymphatic networks, fibroblasts, adipocytes, and cells of the immune system. At the center of this crosstalk between cancer cells and their TME is the bioactive lipid lysophosphatidic acid (LPA). High levels of LPA and the enzyme generating it, termed autotaxin (ATX), are present in many cancers. It is also well documented that LPA drives tumor progression by promoting angiogenesis, proliferation, survival, invasion and metastasis. One of the hallmarks of cancer is the ability to modulate and escape immune detection and eradication. Despite the profound role of LPA in regulating immune functions and inflammation, its role in the context of tumor immunity has not received much attention until recently where emerging studies highlight that this signaling axis may be a means that cancer cells adopt to evade immune detection and eradication. The present review aims to look at the immunomodulatory actions of LPA in baseline immunity to provide a broad understanding of the subject with a special emphasis on LPA and cancer immunity, highlighting the latest progress in this area of research.

The roles of autotaxin/lysophosphatidic acid in immune regulation and asthma

Lysophosphatidic acid (LPA) species are present in almost all organ systems and play diverse roles through its receptors. Asthma is an airway disease characterized by chronic allergic inflammation where various innate and adaptive immune cells participate in establishing Th2 immune response. Here, we will review the contribution of LPA and its receptors to the functions of immune cells that play a key role in establishing allergic airway inflammation and aggravation of allergic asthma.

Effects of the LPA1 Receptor Deficiency and Stress on the Hippocampal LPA Species in Mice

Lysophosphatidic acid (LPA) is an important bioactive lipid species that functions in intracellular signaling through six characterized G protein-coupled receptors (LPA_1-6). Among these receptors, LPA₁ is a strong candidate to mediate the central effects of LPA on emotion and may be involved in promoting normal emotional behaviors. Alterations in this receptor may induce vulnerability to stress and predispose an individual to a psychopathological disease. In fact, mice lacking the LPA₁ receptor exhibit emotional dysregulation and cognitive alterations in hippocampus-dependent tasks. Moreover, the loss of this receptor results in a phenotype of low resilience with dysfunctional coping in response to stress and induces anxiety and several behavioral and neurobiological changes that are strongly correlated with mood disorders. In fact, our group proposes that maLPA1-null mice represent an animal model of anxious depression. However, despite the key role of the LPA-LPA₁-pathway in emotion and stress coping behaviors, the available information describing the mechanisms by which the LPA-LPA₁-pathway regulates emotion is currently insufficient. Because activation of LPA₁ requires LPA, here, we used a Matrix-Assisted Laser Desorption/ Ionization mass spectrometry-based approach to evaluate the effects of an LPA₁ receptor deficiency on the hippocampal levels of LPA species. Additionally, the impact of stress on the LPA profile was also examined in both wild-type (WT) and the Malaga variant of LPA1-null mice (maLPA₁-null mice). Mice lacking LPA₁ did not exhibit gross perturbations in the hippocampal LPA species, but the LPA profile was modified, showing an altered relative abundance of 18:0 LPA. Regardless of the genotype, restraint stress produced profound changes in all LPA species examined, revealing that hippocampal LPA species are a key target of stress. Finally, the relationship between the hippocampal levels of LPA species and performance in the elevated plus maze was established. To our knowledge, this study is the first to detect, identify and profile LPA species in the hippocampus of both LPA₁-receptor null mice and WT mice at baseline and after acute stress, as well as to link these LPA species with anxiety-like behaviors. In conclusion, the hippocampal LPA species are a key target of stress and may be involved in psychopathological conditions.

ROCK2 Regulates Monocyte Migration and Cell to Cell Adhesion in Vascular Endothelial Cells

The small GTPase Rho and its downstream effector, Rho-kinase (ROCK), regulate various cellular functions, including organization of the actin cytoskeleton, cell adhesion and migration. A pro-inflammatory lipid mediator, lysophosphatidic acid (LPA), is a potent activator of the Rho/ROCK signalling pathway and has been shown to induce the expression of chemokines and cell adhesion molecules (CAMs). In the present study, we aimed to elucidate the precise mechanism by which ROCK regulates LPA-induced expressions and functions of chemokines and CAMs. We observed that ROCK blockade reduced LPA-induced phosphorylation of IκBα and inhibited NF-κB RelA/p65 phosphorylation, leading to attenuation of RelA/p65 nuclear translocation. Furthermore, small interfering RNA-mediated ROCK isoform knockdown experiments revealed that LPA induces the expression of monocyte chemoattractant protein-1 (MCP-1) and E-selectin via ROCK2 in human aortic endothelial cells (HAECs). Importantly, we found that ROCK2 but not ROCK1 controls LPA-induced monocytic migration and monocyte adhesion toward endothelial cells. These findings demonstrate that ROCK2 is a key regulator of endothelial inflammation. We conclude that targeting endothelial ROCK2 is potentially effective in attenuation of atherosclerosis.

Dynamic Changes in the Microbiome and Mucosal Immune Microenvironment of the Lower Respiratory Tract by Influenza Virus Infection

Influenza is a major public health concern, and the high mortality rate is largely attributed to secondary bacterial infections. There are several mechanisms through which the virus increases host susceptibility to bacterial colonization, but the micro-environment in lower respiratory tract (LRT) of host, infected with influenza virus, is unclear. To this end, we analyzed the LRT microbiome, transcriptome of lung and metabolome of bronchoalveolar lavage fluid (BALF) in mice inoculated intra-nasally with H1N1 to simulate human influenza, and we observed significant changes in the composition of microbial community and species diversity in the acute (7 days post inoculation or dpi), convalescent (14 dpi) and the recovery (28 dpi) periods. The dominant bacterial class shifted from Alphaproteobacteria to Gammaproteobacteria and Actinobacteria in the infected mice, with a significant increase in the relative abundance of anaerobes and facultative anaerobes like Streptococcus and Staphylococcus. The dysbiosis in the LRT of infected mice was not normalized even in the recovery phase of the infection. In addition, the infected lung transcriptome showed significant differences in the expression levels of genes associated with bacterial infection and immune responses. Finally, the influenza virus infection also resulted in significant changes in the metabolome of the BALF. These alterations in the microbiome, transcriptome, and metabolome of infected lungs were not only appeared at the acute period, but also observed at the recovery period. Furthermore, the infection of influenza virus induced a long-term effect in LRT micro-environmental homeostasis, which may give a chance for the invasion of potential pathogens.

Function of secreted phospholipase A2 group-X in asthma and allergic disease

Elevated secreted phospholipase A₂ (sPLA₂) activity in the airways has been implicated in the pathogenesis of asthma and allergic disease for some time. The identity and function of these enzymes in asthma is becoming clear from work in our lab and others. We focused on sPLA₂ group X (sPLA₂-X) after identifying increased levels of this enzyme in asthma, and that it is responsible for a large portion of sPLA₂ activity in the airways and that the levels are strongly associated with features of airway hyperresponsiveness (AHR). In this review, we discuss studies that implicated sPLA₂-X in human asthma, and murine models that demonstrate a critical role of this enzyme as a regulator of type-2 inflammation, AHR and production of eicosanoids. We discuss the mechanism by which sPLA₂-X acts to regulate eicosanoids in leukocytes, as well as effects that are mediated through the generation of lysophospholipids and through receptor-mediated functions. This article is part of a Special Issue entitled Novel functions of phospholipase A2 Guest Editors: Makoto Murakami and Gerard Lambeau.

Novel mechanisms regulating endothelial barrier function in the pulmonary microcirculation

The pulmonary epithelial and vascular endothelial cell layers provide two sequential physical and immunological barriers that together form a semi-permeable interface and prevent alveolar and interstitial oedema formation. In this review, we focus specifically on the continuous endothelium of the pulmonary microvascular bed that warrants strict control of the exchange of gases, fluid, solutes and circulating cells between the plasma and the interstitial space. The present review provides an overview of emerging molecular mechanisms that permit constant transcellular exchange between the vascular and interstitial compartment, and cause, prevent or reverse lung endothelial barrier failure under experimental conditions, yet with a clinical perspective. Based on recent findings and at times seemingly conflicting results we discuss emerging paradigms of permeability regulation by altered ion transport as well as shifts in the homeostasis of sphingolipids, angiopoietins and prostaglandins.

Autotaxin in Pathophysiology and Pulmonary Fibrosis

Lysophospholipid signaling is emerging as a druggable regulator of pathophysiological responses, and especially fibrosis, exemplified by the relative ongoing clinical trials in idiopathic pulmonary fibrosis (IPF) patients. In this review, we focus on ectonucleotide pyrophosphatase-phosphodiesterase 2 (ENPP2), or as more widely known Autotaxin (ATX), a secreted lysophospholipase D (lysoPLD) largely responsible for extracellular lysophosphatidic acid (LPA) production. In turn, LPA is a bioactive phospholipid autacoid, forming locally upon increased ATX levels and acting also locally through its receptors, likely guided by ATX's structural conformation and cell surface associations. Increased ATX activity levels have been detected in many inflammatory and fibroproliferative conditions, while genetic and pharmacologic studies have confirmed a pleiotropic participation of ATX/LPA in different processes and disorders. In pulmonary fibrosis, ATX levels rise in the broncheoalveolar fluid (BALF) and stimulate LPA production. LPA engagement of its receptors activate multiple G-protein mediated signal transduction pathways leading to different responses from pulmonary cells including the production of pro-inflammatory signals from stressed epithelial cells, the modulation of endothelial physiology, the activation of TGF signaling and the stimulation of fibroblast accumulation. Genetic or pharmacologic targeting of the ATX/LPA axis attenuated disease development in animal models, thus providing the proof of principle for therapeutic interventions.

Three lysophosphatidic acids with a distinct long chain moiety differently affect cell differentiation of human colon epithelial cells to goblet cells

AIM

The intestinal mucus layer helps maintain intestinal homeostasis. In this study, we investigated the effects of lysophosphatidic acids (LPA) on differentiation of human colon carcinoma cell line, HT-29, to goblet cells with and without sodium butyrate, a known differentiation factor for intestinal cells.

MAIN METHODS

Number and average size of cells with goblet-like morphology in five photographs per dish were measured for assessment of differentiation of HT-29 cells to goblet cells as well as their relative portion of surface of to whole surface area of the photograph.

KEY FINDINGS

Our results revealed that 18:1 LPA enhanced butyrate-induced differentiation of HT-29 cells. Because increased mRNA expression of LPA₅ and decreased mRNA expression of LPA₆ were observed in HT-29 cells after treatment with butyrate, we explored the effects of alkyl LPA and 20:4 LPA, which show preferentially higher affinities to LPA₅ and LPA₆, respectively. As a result, the cell differentiation to goblet cell was increased by alkyl LPA but decreased by 20:4 LPA. Further, alkyl LPA and 18:1 LPA, but not 20:4 LPA, were found to reduce the numbers of cells surviving after incubation in a standard culture medium containing 10% fetal calf serum.

SIGNIFICANCE

We suggest that the three LPAs positively and negatively affect the differentiation of HT-29 cells to goblet cells, which may be associated with their reduced survival through the activation of distinct LPA receptor(s).

Novel Inhibitory Effect of a Lysophosphatidic Acid 2 Agonist on Allergen-Driven Airway Inflammation

Lysophosphatidic acid (LPA) is a pleiotropic lipid signaling molecule associated with asthma pathobiology. LPA elicits its effects by binding to at least six known cell surface G protein-coupled receptors (LPA1-6) that are expressed in the lung in a cell type-specific manner. LPA2 in particular has emerged as an attractive therapeutic target in asthma because it appears to transduce inhibitory or cell-protective signals. We studied a novel and specific small molecule LPA2 agonist (2-[4-(1,3-dioxo-1H,3H-benzoisoquinolin-2-yl)butylsulfamoyl] benzoic acid [DBIBB]) in a mouse model of house dust mite-induced allergic airway inflammation. Mice injected with DBIBB developed significantly less airway and lung inflammation compared with vehicle-treated controls. Levels of lung Th2 cytokines were also significantly attenuated by DBIBB. We conclude that pharmacologic activation of LPA2 attenuates Th2-driven allergic airway inflammation in a mouse model of asthma. Targeting LPA receptor signaling holds therapeutic promise in allergic asthma.

Polyunsaturated lysophosphatidic acid as a potential asthma biomarker

Lysophosphatidic acid (LPA), a lipid mediator in biological fluids and tissues, is generated mainly by autotaxin that hydrolyzes lysophosphatidylcholine to LPA and choline. Total LPA levels are increased in bronchoalveolar lavage fluid from asthmatic lung, and are strongly induced following subsegmental bronchoprovocation with allergen in subjects with allergic asthma. Polyunsaturated molecular species of LPA (C22:5 and C22:6) are selectively synthesized in the airways of asthma subjects following allergen challenge and in mouse models of allergic airway inflammation, having been identified and quantified by LC/MS/MS lipidomics. This review discusses current knowledge of LPA production in asthmatic lung and the potential utility of polyunsaturated LPA molecular species as novel biomarkers in bronchoalveolar lavage fluid and exhaled breath condensate of asthma subjects.

The Role of Lysophosphatidic Acid on Airway Epithelial Cell Denudation in a Murine Heterotopic Tracheal Transplant Model

Supplemental digital content is available in the text.

Background

Chronic rejection is the major leading cause of morbidity and mortality after lung transplantation. Obliterative bronchiolitis (OB), a fibroproliferative disorder of the small airways, is the main manifestation of chronic lung allograft rejection. However, there is currently no treatment for the disease. We hypothesized that lysophosphatidic acid (LPA) participates in the progression of OB. The aim of this study was to reveal the involvement of LPA on the lesion of OB.

Methods

Ki16198, an antagonist specifically for LPA₁ and LPA₃, was daily administered into the heterotopic tracheal transplant model mice at the day of transplantation. At days 10 and 28, the allografts were isolated and evaluated histologically. The messenger RNA levels of LPAR in microdissected mouse airway regions were assessed to reveal localization of lysophosphatidic acid receptors. The human airway epithelial cell was used to evaluate the mechanism of LPA-induced suppression of cell adhesion to the extracellular matrix (ECM).

Results

The administration of Ki16198 attenuated airway epithelial cell loss in the allograft at day 10. Messenger RNAs of LPA₁ and LPA₃ were detected in the airway epithelial cells of the mice. Lysophosphatidic acid inhibited the attachment of human airway epithelial cells to the ECM and induced cell detachment from the ECM, which was mediated by LPA₁ and Rho-kinase pathway. However, Ki16198 did not prevent obliteration of allograft at day 28.

Conclusions

The LPA signaling is involved in the status of epithelial cells by distinct contribution in 2 different phases of the OB lesion. This finding suggests a role of LPA in the pathogenesis of OB.

Autotaxin-LPA receptor axis in the pathogenesis of lung diseases

Lysophosphatidic acid (LPA) is a small lipid which mediates a variety of cellular functions via the activation of LPA receptors. LPA is generated from lysophosphatidylcholine by the extracellular enzyme, autotaxin (ATX). Elevated ATX expression, LPA production and their signaling pathways have been reported in multiple pathological conditions of lung tissue, including inflammation, fibrosis and cancer. Emerging evidence has highlighted the importance of ATX and LPA receptors in the pathogenesis of lung diseases. Here, we briefly review the current knowledge of different roles of the ATX-LPA receptor axis in lung diseases focusing on inflammation, fibrosis and cancer.

Expression of autotaxin and lysophosphatidic acid receptors 1 and 3 in the developing rat lung and in response to hyperoxia

We sought to evaluate lysophosphatidic acid (LPA) signaling improvement in lung development by assessing the expression of autotaxin and LPA receptor 1 and 3 (LPAR1 and LPAR3) in the neonatal rat lung during normal perinatal development and in response to hyperoxia. In the developmental study, rats were sacrificed on days 17, 19, and 21 of gestation; on postnatal days 1, 4, and 7; and at adulthood (postnatal 9 weeks). In the hyperoxia study, 42 postnatal 4-day-old rat pups were divided into seven groups and exposed to either 85% O2 for 24, 72, or 120 h or room air for 0, 24, 72, or 120 h. The rats were then euthanized after 0, 24, 72, and 120 h of exposure. Immunofluorescence demonstrated that autotaxin, LPAR1, and LPAR3 proteins were broadly colocalized in airway epithelial cells, but mainly distributed in vascular endothelial and mesenchymal cells during the first postnatal week. The expression of autotaxin, LPAR1, and LPAR3 were increased during late gestation and then decreased after birth. Autotaxin expression and enzymatic activity were significantly increased at 72 and 120 h after exposure to hyperoxia. LPAR1 and LPAR3 expression was also increased after 120 h of hyperoxic exposure. These findings suggest that LPA-associated molecules were upregulated at birth and induced by hyperoxia in the developing rat lung. Therefore, the LPA pathway may be involved in normal lung development, including vascular development, as well as wound-healing processes of injured neonatal lung tissue, which is at risk of neonatal hyperoxic acute lung injury.

Chronic rhinosinusitis with nasal polyps and without nasal polyps is associated with increased expression of lysophosphatidic acid-related molecules

BACKGROUND

Chronic sinusitis with nasal polyps (CRSwNPs) or CRS without NPs (CRSsNPs) is associated with expression of various cytokines. Lysophosphatidic acid (LPA) generated by autotaxin (ATX), LPA-producing enzyme, initiates signaling cascade involved in the inflammatory responses and participates in diverse biological processes through LPA receptors, including cytokine production. We analyzed the expression and distribution patterns of LPA-related molecules in nasal secretion and sinus mucosa of normal controls and patients with CRSwNPs and CRSsNPs, to evaluate the possible effects of the ATX-LPA receptor axis on the pathogenesis of CRS.

METHODS

LPA levels in nasal secretion and the expression and distribution patterns of ATX and LPA receptors 1-3 (LPA1-3) in sinus mucosa were investigated using ELISA, real-time polymerase chain reaction, Western blot, and immunohistochemistry. We elucidated the effect of CRS-relevant cytokines on the expression of ATX and LPA receptors, using cultured sinus epithelial cells, and investigated the effect of LPA on the expression of CRS-relevant cytokines, using sinus mucosa explant culture.

RESULTS

LPA, ATX, and LPA1-3 levels are increased in CRSwNPs and CRSsNPs. ATX and LPA1-3 were localized to superficial epithelium, submucosal glands in normal and inflammatory mucosa, but in inflammatory mucosa, they were found in inflammatory cells. LPA1-3 were noted in endothelium. Sinus mucosa explant stimulated with LPA increasingly produced IL-4, IL-5, interferon gamma, and TNF-alpha, and in cultured epithelial cells stimulated with CRS-relevant cytokines, ATX, and LPA1-3 were differentially induced.

CONCLUSION

LPA in human sinus mucosa may play important roles in the pathogenesis of CRS, contributing to produce CRS-related cytokines. LPA-related molecules were increased in CRS, which may attribute to CRS-related cytokines.

Regulation of T cell motility in vitro and in vivo by LPA and LPA2

Lysophosphatidic acid (LPA) and the LPA-generating enzyme autotaxin (ATX) have been implicated in lymphocyte trafficking and the regulation of lymphocyte entry into lymph nodes. High local concentrations of LPA are thought to be present in lymph node high endothelial venules, suggesting a direct influence of LPA on cell migration. However, little is known about the mechanism of action of LPA, and more work is needed to define the expression and function of the six known G protein-coupled receptors (LPA 1-6) in T cells. We studied the effects of 18∶1 and 16∶0 LPA on naïve CD4+ T cell migration and show that LPA induces CD4+ T cell chemorepulsion in a Transwell system, and also improves the quality of non-directed migration on ICAM-1 and CCL21 coated plates. Using intravital two-photon microscopy, lpa2-/- CD4+ T cells display a striking defect in early migratory behavior at HEVs and in lymph nodes. However, later homeostatic recirculation and LPA-directed migration in vitro were unaffected by loss of lpa2. Taken together, these data highlight a previously unsuspected and non-redundant role for LPA2 in intranodal T cell motility, and suggest that specific functions of LPA may be manipulated by targeting T cell LPA receptors.

Challenges in accurate quantitation of lysophosphatidic acids in human biofluids

Lysophosphatidic acids (LPAs) are biologically active signaling molecules involved in the regulation of many cellular processes and have been implicated as potential mediators of fibroblast recruitment to the pulmonary airspace, pointing to possible involvement of LPA in the pathology of pulmonary fibrosis. LPAs have been measured in various biological matrices and many challenges involved with their analyses have been documented. However, little published information is available describing LPA levels in human bronchoalveolar lavage fluid (BALF). We therefore conducted detailed investigations into the effects of extensive sample handling and sample preparation conditions on LPA levels in human BALF. Further, targeted lipid profiling of human BALF and plasma identified the most abundant lysophospholipids likely to interfere with LPA measurements. We present the findings from these investigations, highlighting the importance of well-controlled sample handling for the accurate quantitation of LPA. Further, we show that chromatographic separation of individual LPA species from their corresponding lysophospholipid species is critical to avoid reporting artificially elevated levels. The optimized sample preparation and LC/MS/MS method was qualified using a stable isotope-labeled LPA as a surrogate calibrant and used to determine LPA levels in human BALF and plasma from a Phase 0 clinical study comparing idiopathic pulmonary fibrosis patients to healthy controls.

Lysophosphatidic acid receptor-2 deficiency confers protection against bleomycin-induced lung injury and fibrosis in mice

Idiopathic pulmonary fibrosis is a devastating disease characterized by alveolar epithelial cell injury, the accumulation of fibroblasts/myofibroblasts, and the deposition of extracellular matrix proteins. Lysophosphatidic acid (LPA) signaling through its G protein-coupled receptors is critical for its various biological functions. Recently, LPA and LPA receptor 1 were implicated in lung fibrogenesis. However, the role of other LPA receptors in fibrosis remains unclear. Here, we use a bleomycin-induced pulmonary fibrosis model to investigate the roles of LPA2 in pulmonary fibrogenesis. In the present study, we found that LPA2 knockout (Lpar2(-/-)) mice were protected against bleomycin-induced lung injury, fibrosis, and mortality, compared with wild-type control mice. Furthermore, LPA2 deficiency attenuated the bleomycin-induced expression of fibronectin (FN), α-smooth muscle actin (α-SMA), and collagen in lung tissue, as well as levels of IL-6, transforming growth factor-β (TGF-β), and total protein in bronchoalveolar lavage fluid. In human lung fibroblasts, the knockdown of LPA2 attenuated the LPA-induced expression of TGF-β1 and the differentiation of lung fibroblasts to myofibroblasts, resulting in the decreased expression of FN, α-SMA, and collagen, as well as decreased activation of extracellular regulated kinase 1/2, Akt, Smad3, and p38 mitogen-activated protein kinase. Moreover, the knockdown of LPA2 with small interfering RNA also mitigated the TGF-β1-induced differentiation of lung fibroblasts. In addition, LPA2 deficiency significantly attenuated the bleomycin-induced apoptosis of alveolar and bronchial epithelial cells in the mouse lung. Together, our data indicate that the knockdown of LPA2 attenuated bleomycin-induced lung injury and pulmonary fibrosis, and this may be related to an inhibition of the LPA-induced expression of TGF-β and the activation and differentiation of fibroblasts.

Autotaxin production of lysophosphatidic acid mediates allergic asthmatic inflammation

RATIONALE

Bioactive lipid mediators, derived from membrane lipid precursors, are released into the airway and airspace where they bind high-affinity cognate receptors and may mediate asthma pathogenesis. Lysophosphatidic acid (LPA), a bioactive lipid mediator generated by the enzymatic activity of extracellular autotaxin (ATX), binds LPA receptors, resulting in an array of biological actions on cell proliferation, migration, survival, differentiation, and motility, and therefore could mediate asthma pathogenesis.

OBJECTIVES

To define a role for the ATX-LPA pathway in human asthma pathogenesis and a murine model of allergic lung inflammation.

METHODS

We investigated the profiles of LPA molecular species and the level of ATX exoenzyme in bronchoalveolar lavage fluids of human patients with asthma subjected to subsegmental bronchoprovocation with allergen. We interrogated the role of the ATX-LPA pathway in allergic lung inflammation using a murine allergic asthma model in ATX-LPA pathway-specific genetically modified mice.

MEASUREMENTS AND MAIN RESULTS

Subsegmental bronchoprovocation with allergen in patients with mild asthma resulted in a remarkable increase in bronchoalveolar lavage fluid levels of LPA enriched in polyunsaturated 22:5 and 22:6 fatty acids in association with increased concentrations of ATX protein. Using a triple-allergen mouse asthma model, we showed that ATX-overexpressing transgenic mice had a more severe asthmatic phenotype, whereas blocking ATX activity and knockdown of the LPA2 receptor in mice produced a marked attenuation of Th2 cytokines and allergic lung inflammation.

CONCLUSIONS

The ATX-LPA pathway plays a critical role in the pathogenesis of asthma. These preclinical data indicate that targeting the ATX-LPA pathway could be an effective antiasthma treatment strategy.

Autotaxin and lysophosphatidic acid signalling in lung pathophysiology

Autotaxin (ATX or ENPP2) is a secreted glycoprotein widely present in biological fluids. ATX primarily functions as a plasma lysophospholipase D and is largely responsible for the bulk of lysophosphatidic acid (LPA) production in the plasma and at inflamed and/or malignant sites. LPA is a phospholipid mediator produced in various conditions both in cells and in biological fluids, and it evokes growth-factor-like responses, including cell growth, survival, differentiation and motility, in almost all cell types. The large variety of LPA effector functions is attributed to at least six G-protein coupled LPA receptors (LPARs) with overlapping specificities and widespread distribution. Increased ATX/LPA/LPAR levels have been detected in a large variety of cancers and transformed cell lines, as well as in non-malignant inflamed tissues, suggesting a possible involvement of ATX in chronic inflammatory disorders and cancer. In this review, we focus exclusively on the role of the ATX/LPA axis in pulmonary pathophysiology, analysing the effects of ATX/LPA on pulmonary cells and leukocytes in vitro and in the context of pulmonary pathophysiological situations in vivo and in human diseases.

Cutting-edge issues in primary biliary cirrhosis

Several crucial issues remain open in our understanding of primary biliary cirrhosis (PBC), an autoimmune liver disease targeting the small- and medium-sized intrahepatic bile ducts. These issues include the high tissue specificity of the autoimmune injury despite the nontraditional autoantigens found in all mitochondria recognized by PBC-associated autoantibodies, the causes of the commonly observed pruritus, and the disease etiology per se. In all these fields, there has been recent interest secondary to the use of large-scale efforts (such as genome-wide association studies) that were previously considered poorly feasible in a rare disease such as PBC as well as other intuitions. Accordingly, there are now fascinating theories to explain the onset and severity of pruritus due to elevated autotaxin levels, the peculiar apoptotic features of bile duct cells to explain the tissue specificity, and genomic and epigenetic associations contributing to disease susceptibility. We have arbitrarily chosen these four aspects as the most promising in the PBC recent literature and will provide herein a discussion of the recent data and their potential implications.

Lysophospholipids and their receptors in the central nervous system

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), two of the best-studied lysophospholipids, are known to influence diverse biological events, including organismal development as well as function and pathogenesis within multiple organ systems. These functional roles are due to a family of at least 11 G protein-coupled receptors (GPCRs), named LPA(1-6) and S1P(1-5), which are widely distributed throughout the body and that activate multiple effector pathways initiated by a range of heterotrimeric G proteins including G(i/o), G(12/13), G(q) and G(s), with actual activation dependent on receptor subtypes. In the central nervous system (CNS), a major locus for these signaling pathways, LPA and S1P have been shown to influence myriad responses in neurons and glial cell types through their cognate receptors. These receptor-mediated activities can contribute to disease pathogenesis and have therapeutic relevance to human CNS disorders as demonstrated for multiple sclerosis (MS) and possibly others that include congenital hydrocephalus, ischemic stroke, neurotrauma, neuropsychiatric disorders, developmental disorders, seizures, hearing loss, and Sandhoff disease, based upon the experimental literature. In particular, FTY720 (fingolimod, Gilenya, Novartis Pharma, AG) that becomes an analog of S1P upon phosphorylation, was approved by the FDA in 2010 as a first oral treatment for MS, validating this class of receptors as medicinal targets. This review will provide an overview and update on the biological functions of LPA and S1P signaling in the CNS, with a focus on results from studies using genetic null mutants for LPA and S1P receptors. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Lysophosphatidic acid (LPA) and its receptors: role in airway inflammation and remodeling

Lysophosphatidic acid (LPA), a simple bioactive phospholipid, is present in biological fluids such as plasma and bronchoalveolar lavage (BAL). It appears to have both pro- and anti-inflammatory roles in inflammatory lung diseases. Exogenous LPA promotes inflammatory responses by regulating the expression of chemokines, cytokines, and cytokine receptors in lung epithelial cells. In addition to the modulation of inflammatory responses, LPA regulates cytoskeleton rearrangement and confers protection against lung injury by enhancing lung epithelial cell barrier integrity and remodeling. The biological effects of LPA are mediated through its cell surface G-protein coupled LPA(1-7) receptors. The roles of LPA receptors in lung fibrosis, asthma, and acute lung injury have been investigated using genetically engineered LPA receptor deficient mice and there appears to be a definitive role for endogenous LPA and its receptors in the pathogenesis of pulmonary inflammatory diseases. This review summarizes recent reports on the role of LPA and its receptors in the regulation of lung epithelial inflammatory responses and remodeling. This article is part of a Special Issue entitled: Advances in Lysophospholipid Research.

Lysophosphatidic acid activates lipogenic pathways and de novo lipid synthesis in ovarian cancer cells

One of the most common molecular changes in cancer is the increased endogenous lipid synthesis, mediated primarily by overexpression and/or hyperactivity of fatty acid synthase (FAS) and acetyl-CoA carboxylase (ACC). The changes in these key lipogenic enzymes are critical for the development and maintenance of the malignant phenotype. Previous efforts to control oncogenic lipogenesis have been focused on pharmacological inhibitors of FAS and ACC. Although they show anti-tumor effects in culture and in mouse models, these inhibitors are nonselective blockers of lipid synthesis in both normal and cancer cells. To target lipid anabolism in tumor cells specifically, it is important to identify the mechanism governing hyperactive lipogenesis in malignant cells. In this study, we demonstrate that lysophosphatidic acid (LPA), a growth factor-like mediator present at high levels in ascites of ovarian cancer patients, regulates the sterol regulatory element binding protein-FAS and AMP-activated protein kinase-ACC pathways in ovarian cancer cells but not in normal or immortalized ovarian epithelial cells. Activation of these lipogenic pathways is linked to increased de novo lipid synthesis. The pro-lipogenic action of LPA is mediated through LPA(2), an LPA receptor subtype overexpressed in ovarian cancer and other malignancies. Downstream of LPA(2), the G(12/13) and G(q) signaling cascades mediate LPA-dependent sterol regulatory element-binding protein activation and AMP-activated protein kinase inhibition, respectively. Moreover, inhibition of de novo lipid synthesis dramatically attenuated LPA-induced cell proliferation. These results demonstrate that LPA signaling is causally linked to the hyperactive lipogenesis in ovarian cancer cells, which can be exploited for development of new anti-cancer therapies.

Lpa2 is a negative regulator of both dendritic cell activation and murine models of allergic lung inflammation

Negative regulation of innate immune responses is essential to prevent excess inflammation and tissue injury and promote homeostasis. Lysophosphatidic acid (LPA) is a pleiotropic lipid that regulates cell growth, migration, and activation and is constitutively produced at low levels in tissues and in serum. Extracellular LPA binds to specific G protein-coupled receptors, whose function in regulating innate or adaptive immune responses remains poorly understood. Of the classical LPA receptors belonging to the Edg family, lpa2 (edg4) is expressed by dendritic cells (DC) and other innate immune cells. In this article, we show that DC from lpa2(-/-) mice are hyperactive compared with their wild-type counterparts and are less susceptible to inhibition by different LPA species. In transient-transfection assays, we found that lpa2 overexpression inhibits NF-κB-driven gene transcription. Using an adoptive-transfer approach, we found that allergen-pulsed lpa2(-/-) DC induced substantially more lung inflammation than did wild-type DC after inhaled allergen challenge. Finally, lpa2(-/-) mice develop greater allergen-driven lung inflammation than do their wild-type counterparts in models of allergic asthma involving both systemic and mucosal sensitization. Taken together, these findings identify LPA acting via lpa2 as a novel negative regulatory pathway that inhibits DC activation and allergic airway inflammation.

Insights into the pharmacological relevance of lysophospholipid receptors

The discovery of lysophospholipid (LP) 7-transmembrane, G protein-coupled receptors (GPCRs) that began in the 1990s, together with research into the functional roles of the major LPs known as lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), have opened new research avenues into their biological processes and mechanisms. Major examples of LP signalling effects include embryogenesis, nervous system development, vascular development, uterine implantation, immune cell trafficking, and inflammatory reactions. LP signalling also influences the pathophysiology of many diseases including cancer, autoimmune and inflammatory diseases, which indicate that LP receptors may be attractive targets for pharmacological therapies. A key example of such a therapeutic agent is the S1P receptor modulator FTY720, which upon phosphorylation and continued drug exposure, acts as an S1P receptor functional antagonist. This compound (also known as fingolimod or Gilenya) has recently been approved by the FDA for the treatment of relapsing forms of multiple sclerosis. Continued basic and translational research on LP signalling should provide novel insights into both basic biological mechanisms, as well as novel therapeutic approaches to combat a range of human diseases.

Lysophosphatidic acid increases soluble ST2 expression in mouse lung and human bronchial epithelial cells

Lysophosphatidic acid (LPA), a naturally occurring bioactive lysophospholipid increases the expression of both pro-inflammatory and anti-inflammatory mediators in airway epithelial cells. Soluble ST2 (sST2), an anti-inflammatory mediator, has been known to function as a decoy receptor of interleukin (IL)-33 and attenuates endotoxin-induced inflammatory responses. Here, we show that LPA increased sST2 mRNA expression and protein release in a dose and time dependent manner in human bronchial epithelial cells (HBEpCs). LPA receptors antagonist and Gαi inhibitor, pertussis toxin, attenuated LPA-induced sST2 release. Inhibition of NF-κB or JNK pathway reduced LPA-induced sST2 release. LPA treatment decreased histone deacetylase 3 (HDAC3) expression and enhanced acetylation of histone H3 at lysine 9 that binds to the sST2 promoter region. Furthermore, limitation of intracellular LPA generation by the down-regulation of acetyl glycerol kinase attenuated exogenous LPA-induced histone H3 acetylation on sST2 promoter region, as well as sST2 gene expression. Treatment of HBEpCs with recombinant sST2 protein or sST2-rich cell culture media attenuated endotoxin-induced phosphorylation of PKC and airway epithelial barrier disruption. These results unravel a novel sST2 mediated signaling pathway that has physiological relevance to airway inflammation and remodeling.

Lysophosphatidic acid receptor 1 modulates lipopolysaccharide-induced inflammation in alveolar epithelial cells and murine lungs

Lysophosphatidic acid (LPA), a bioactive phospholipid, plays an important role in lung inflammation by inducing the release of chemokines and lipid mediators. Our previous studies have shown that LPA induces the secretion of interleukin-8 and prostaglandin E(2) in lung epithelial cells. Here, we demonstrate that LPA receptors contribute to lipopolysaccharide (LPS)-induced inflammation. Pretreatment with LPA receptor antagonist Ki16425 or downregulation of LPA receptor 1 (LPA(1)) by small-interfering RNA (siRNA) attenuated LPS-induced phosphorylation of p38 MAPK, I-κB kinase, and I-κB in MLE12 epithelial cells. In addition, the blocking of LPA(1) also suppressed LPS-induced IL-6 production. Furthermore, LPS treatment promoted interaction between LPA(1) and CD14, a LPS coreceptor, in a time- and dose-dependent manner. Disruption of lipid rafts attenuated the interaction between LPA(1) and CD14. Mice challenged with LPS increased plasma LPA levels and enhanced expression of LPA receptors in lung tissues. To further investigate the role of LPA receptors in LPS-induced inflammation, wild-type, or LPA(1)-deficient mice, or wild-type mice pretreated with Ki16425 were intratracheally challenged with LPS for 24 h. Knock down or inhibition of LPA(1) decreased LPS-induced IL-6 release in bronchoalveolar lavage (BAL) fluids and infiltration of cells into alveolar space compared with wild-type mice. However, no significant differences in total protein concentration in BAL fluids were observed. These results showed that knock down or inhibition of LPA(1) offered significant protection against LPS-induced lung inflammation but not against pulmonary leak as observed in the murine model for lung injury.

Non-Edg family LPA receptors: the cutting edge of LPA research

Lysophosphatidic acid (LPA) is a bioactive lipid mediator with diverse physiological and pathological actions on many types of cells. Originally, LPA was thought to elicit its biological functions through three subtypes of endothelial differentiation gene (Edg) family G protein-coupled receptors (LPA1, LPA2 and LPA3) until our group identified a fourth subtype, LPA4. The discovery of this receptor, which is structurally distinct from the Edg family LPA receptors, led to the identification of two additional LPA receptors, LPA5 and LPA6, homologous to LPA4. These 'non-Edg family' LPA receptors now provide a new framework for understanding the diverse functions of LPA, including vascular development, platelet activation and hair growth. In this review, we summarize the identification, intracellular signalling and biological functions of this novel cluster of LPA receptors.

LPA5 is abundantly expressed by human mast cells and important for lysophosphatidic acid induced MIP-1β release

Background

Lysophosphatidic acid (LPA) is a bioactive lipid inducing proliferation, differentiation as well as cytokine release by mast cells through G-protein coupled receptors. Recently GPR92/LPA5 was identified as an LPA receptor highly expressed by cells of the immune system, which prompted us to investigate its presence and influence on mast cells.

Principal Findings

Transcript analysis using quantitative real-time PCR revealed that LPA5 is the most prevalent LPA-receptor in human mast cells. Reduction of LPA5 levels using shRNA reduced calcium flux and abolished MIP-1β release in response to LPA.

Conclusions

LPA5 is a bona fide LPA receptor on human mast cells responsible for the majority of LPA induced MIP-1β release.

NF-κB signaling pathways regulated by CARMA family of scaffold proteins

The NF-κB family of transcription factors plays a crucial role in cell activation, survival and proliferation. Its aberrant activity results in cancer, immunodeficiency or autoimmune disorders. Over the past two decades, tremendous progress has been made in our understanding of the signals that regulate NF-κB activation, especially how scaffold proteins link different receptors to the NF-κB-activating complex, the IκB kinase complex. The growing number of these scaffolds underscores the complexity of the signaling networks in different cell types. In this review, we discuss the role of scaffold molecules in signaling cascades induced by stimulation of antigen receptors, G-protein-coupled receptors and C-type Lectin receptors, resulting in NF-κB activation. Especially, we focus on the family of Caspase recruitment domain (CARD)-containing proteins known as CARMA and their function in activation of NF-κB, as well as the link of these scaffolds to the development of various neoplastic diseases through regulation of NF-κB.

International Union of Basic and Clinical Pharmacology. LXXVIII. Lysophospholipid receptor nomenclature

Lysophospholipids are cell membrane-derived lipids that include both glycerophospholipids such as lysophosphatidic acid (LPA) and sphingoid lipids such as sphingosine 1-phosphate (S1P). These and related molecules can function in vertebrates as extracellular signals by binding and activating G protein-coupled receptors. There are currently five LPA receptors, along with a proposed sixth (LPA₁-LPA₆), and five S1P receptors (S1P₁-S1P₅). A remarkably diverse biology and pathophysiology has emerged since the last review, driven by cloned receptors and targeted gene deletion ("knockout") studies in mice, which implicate receptor-mediated lysophospholipid signaling in most organ systems and multiple disease processes. The entry of various lysophospholipid receptor modulatory compounds into humans through clinical trials is ongoing and may lead to new medicines that are based on this signaling system. This review incorporates IUPHAR Nomenclature Committee guidelines in updating the nomenclature for lysophospholipid receptors ( http://www.iuphar-db.org/DATABASE/FamilyMenuForward?familyId=36).

Lysophosphatidic acid inhibits CC chemokine ligand 5/RANTES production by blocking IRF-1-mediated gene transcription in human bronchial epithelial cells

Lysophosphatidic acid (LPA) is a phospholipid mediator that exerts a variety of biological responses through specific G-protein-coupled receptors (LPA(1)-LPA(5) and P2Y5). LPA is thought to be involved in airway inflammation by regulating the expression of anti-inflammatory and proinflammatory genes. Chemokines such as CCL5/RANTES are secreted from airway epithelium and play a key role in allergic airway inflammation. CCL5/RANTES is a chemoattractant for eosinophils, T lymphocytes, and monocytes and seems to exacerbate asthma. We stimulated CCL5/RANTES production in a human bronchial epithelial cell line, BEAS-2B, with IFN-γ and TNF-α. When LPA was added, CCL5/RANTES mRNA expression and protein secretion were inhibited, despite the presence of IFN-γ and TNF-α. The LPA effect was attenuated by Ki16425, a LPA(1)/LPA(3) antagonist, but not by dioctylglycerol pyrophosphate 8:0, an LPA(3) antagonist. Pertussis toxin, the inhibitors for PI3K and Akt also attenuated the inhibitory effect of LPA on CCL5/RANTES secretion. We also identify the transcription factor IFN regulatory factor-1 (IRF-1) as being essential for CCL5/RANTES production. Interestingly, LPA inhibited IFN-γ and TNF-α-induced IRF-1 activation by blocking the binding of IRF-1 to its DNA consensus sequence without changing IRF-1 induction and its nuclear translocation. Ki16425, pertussis toxin, and PI3K inhibitors attenuated the inhibitory effect of LPA on IRF-1 activation. Our results suggest that LPA inhibits IFN-γ- and TNF-α-induced CCL5/RANTES production in BEAS-2B cells by blocking the binding of IRF-1 to the CCL5/RANTES promoter. LPA(1) coupled to G(i) and activation of PI3K is required for this unique effect.

Lysophosphatidic acid is a potential mediator of cholestatic pruritus

BACKGROUND & AIMS

Pruritus is a common and disabling symptom in cholestatic disorders. However, its causes remain unknown. We hypothesized that potential pruritogens accumulate in the circulation of cholestatic patients and activate sensory neurons.

METHODS

Cytosolic free calcium ([Ca(2+)](i)) was measured in neuronal cell lines by ratiometric fluorometry upon exposure to serum samples from pruritic patients with intrahepatic cholestasis of pregnancy (ICP), primary biliary cirrhosis (PBC), other cholestatic disorders, and pregnant, healthy, and nonpruritic disease controls. Putative [Ca(2+)](i)-inducing factors in pruritic serum were explored by analytical techniques, including quantification by high-performance liquid chromatography/mass spectroscopy. In mice, scratch activity after intradermal pruritogen injection was quantified using a magnetic device.

RESULTS

Transient increases in neuronal [Ca(2+)](i) induced by pruritic PBC and ICP sera were higher than corresponding controls. Lysophosphatidic acid (LPA) could be identified as a major [Ca(2+)](i) agonist in pruritic sera, and LPA concentrations were increased in cholestatic patients with pruritus. LPA injected intradermally into mice induced scratch responses. Autotaxin, the serum enzyme converting lysophosphatidylcholine into LPA, was markedly increased in patients with ICP versus pregnant controls (P < .0001) and cholestatic patients with versus without pruritus (P < .0001). Autotaxin activity correlated with intensity of pruritus (P < .0001), which was not the case for serum bile salts, histamine, tryptase, substance P, or mu-opioids. In patients with PBC who underwent temporary nasobiliary drainage, both itch intensity and autotaxin activity markedly decreased during drainage and returned to preexistent levels after drain removal.

CONCLUSIONS

We suggest that LPA and autotaxin play a critical role in cholestatic pruritus and may serve as potential targets for future therapeutic interventions.

Rho-kinase mediates lysophosphatidic acid-induced IL-8 and MCP-1 production via p38 and JNK pathways in human endothelial cells

Lysophosphatidic acid (LPA), an inflammatory mediator that is elevated in multiple inflammatory diseases, is a potent activator of Rho kinase (ROCK) signaling and of chemokine production in endothelial cells. In this study, LPA activated ROCK, p38, JNK and NF-kappaB pathways and induced interleukin-8 (IL-8) and monocyte chemotactic protein-1 (MCP-1) mRNA and protein expression in human endothelial cells. We mapped signaling events downstream of ROCK, driving chemokine production. In summary, MCP-1 production was partly regulated by ROCK acting upstream of p38 and JNK and mediated downstream by NF-kappaB. IL-8 production was largely driven by ROCK through p38 and JNK activation, but with no involvement of NF-kappaB.

Rho kinase-2 activation in human endothelial cells drives lysophosphatidic acid-mediated expression of cell adhesion molecules via NF-kappaB p65

Endothelial cells play an important role in the recruitment of immune cells to a disease locus through the induced expression of chemokines and cell adhesion molecules (CAMs). The proinflammatory lysophospholipid, lysophosphatidic acid (LPA), which is elevated in multiple inflammatory diseases, is a potent activator of the RhoA/Rho kinase signaling pathway and has been shown to induce the expression of CAMs in endothelial cells. The present study was undertaken to map signal transduction downstream of LPA and to investigate the contributions of the Rho kinase isoforms ROCK1 and ROCK2 to adhesion molecule expression in human umbilical vein endothelial cells. LPA activated Rho kinase within minutes and subsequently the NF-kappaB pathway through phosphorylation of the p65 subunit. The lipid also induced the late expression of intracellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1). Pharmacologic inhibition of Rho kinase signaling blocked LPA-induced p65 phosphorylation and suppressed ICAM-1 and VCAM-1 expression. Inhibition of the NF-kappaB pathway had no impact on LPA-induced Rho kinase activation, but inhibited adhesion molecule expression. Small interfering RNA-facilitated knockdown of each isoform identified ROCK2 as the mediator of LPA-driven phosphorylation of NF-kappaB p65 and of ICAM-1 and VCAM-1 mRNA and protein induction. Taken collectively, our data are consistent with Rho kinase being upstream of NF-kappaB in driving LPA-mediated adhesion molecule expression. This study also provides the first evidence of the critical involvement of ROCK2 in LPA-induced CAM expression through activation of the NF-kappaB pathway in human endothelial cells.