Deletion of the G2A receptor fails to attenuate experimental autoimmune encephalomyelitis

Immunomodulation of Proton-activated G Protein-coupled Receptors in Inflammation

Proton-activated G protein-coupled receptors (GPCRs), initially discovered by Ludwig in 2003, are widely distributed in various tissues. These receptors have been found to modulate the immune system in several inflammatory diseases, including inflammatory bowel disease, atopic dermatitis, and asthma. Proton-activated GPCRs belong to the G protein-coupled receptor family and can detect alternations in extracellular pH. This detection triggers downstream signaling pathways within the cells, ultimately influencing the function of immune cells. In this review, we specifically focused on investigating the immune response of proton-activated GPCRs under inflammatory conditions.

Lipid Receptor G2A-Mediated Signal Pathway Plays a Critical Role in Inflammatory Response by Promoting Classical Macrophage Activation

Macrophage polarization is a dynamic and integral process in tissue inflammation and remodeling. In this study, we describe that lipoprotein-associated phospholipase A₂ (Lp-PLA₂) plays an important role in controlling inflammatory macrophage (M1) polarization in rodent experimental autoimmune encephalomyelitis (EAE) and in monocytes from multiple sclerosis (MS) patients. Specific inhibition of Lp-PLA₂ led to an ameliorated EAE via markedly decreased inflammatory and demyelinating property of M1. The effects of Lp-PLA₂ on M1 function were mediated by lysophosphatidylcholine, a bioactive product of oxidized lipids hydrolyzed by Lp-PLA₂ through JAK2-independent activation of STAT5 and upregulation of IRF5. This process was directed by the G2A receptor, which was only found in differentiated M1 or monocytes from MS patients. M1 polarization could be inhibited by a G2A neutralizing Ab, which led to an inhibited disease in rat EAE. In addition, G2A-deficient rats showed an ameliorated EAE and an inhibited autoimmune response. This study has revealed a mechanism by which lipid metabolites control macrophage activation and function, modification of which could lead to a new therapeutic approach for MS and other inflammatory disorders.

CD36, but not G2A, modulates efferocytosis, inflammation, and fibrosis following bleomycin-induced lung injury

Macrophage G2A and CD36 lipid receptors are thought to mediate efferocytosis following tissue injury and thereby prevent excessive inflammation that could compromise tissue repair. To test this, we subjected mice lacking G2A or CD36 receptor to bleomycin-induced lung injury and measured efferocytosis, inflammation, and fibrosis. Loss of CD36 (but not G2A) delayed clearance of apoptotic alveolar cells (mean 78% increase in apoptotic cells 7 days postinjury), potentiated inflammation (mean 56% increase in lung neutrophils and 75% increase in lung KC levels 7 days postinjury, 51% increase in lung macrophages 14 days postinjury), and reduced lung fibrosis (mean 41% and 29% reduction 14 and 21 days postinjury, respectively). Reduced fibrosis in CD36(-/-) mice was associated with lower levels of profibrotic TH2 cytokines (IL-9, IL-13, IL-4), decreased expression of the M2 macrophage marker Arginase-1, and reduced interstitial myofibroblasts. G2A, on the other hand, was required for optimal clearance of apoptotic neutrophils during zymosan-induced peritoneal inflammation (50.3% increase in apoptotic neutrophils and 30.6% increase in total neutrophils 24 h following zymosan administration in G2A(-/-) mice). Thus, CD36 is required for timely removal of apoptotic cells in the context of lung injury and modulates subsequent inflammatory and fibrotic processes relevant to fibrotic lung disease.

Oxidized lipids and lysophosphatidylcholine induce the chemotaxis and intracellular calcium influx in natural killer cells

We previously reported that human NK cells express G2A and they respond to LPC. Here, we report that oxidized lipids such as 9-R-HODE, 9-S-HODE and 13-R-HODE, as well as LPC induced the in vitro chemotaxis of human NK cells, although with variable efficacies. The chemotactic effects of these lipids were inhibited by prior treatment of NK cells with pertussis toxin (PTX). 9-S-HODE, 9-R-HODE and LPC optimally induced the influx of intracellular Ca(2+) in NK cells. Addition of 9-S-HODE prior to the addition of LPC inhibited more than 50% of the effect of LPC, whereas addition of LPC prior to the addition of 9-S-HODE completely inhibited the effect of the latter lipid. Also, there was a complete reciprocal desensitization among 9-R-HODE and LPC on the influx of intracellular Ca(2+). Further analysis showed that the four lipids did not affect NK cell lysis of tumor target cells. 9-R-HODE but not any other lipid increased the percentages of NK cells producing IFN-γ and is the only lipid that enhanced the release of this cytokine by these cells. In conclusion, we provide novel evidence showing that oxidized lipids and LPC exert important functions for cells of innate immune system.

G protein-coupled receptors as therapeutic targets for multiple sclerosis

G protein-coupled receptors (GPCRs) mediate most of our physiological responses to hormones, neurotransmitters and environmental stimulants. They are considered as the most successful therapeutic targets for a broad spectrum of diseases. Multiple sclerosis (MS) is an inflammatory disease that is characterized by immune-mediated demyelination and degeneration of the central nervous system (CNS). It is the leading cause of non-traumatic disability in young adults. Great progress has been made over the past few decades in understanding the pathogenesis of MS. Numerous data from animal and clinical studies indicate that many GPCRs are critically involved in various aspects of MS pathogenesis, including antigen presentation, cytokine production, T-cell differentiation, T-cell proliferation, T-cell invasion, etc. In this review, we summarize the recent findings regarding the expression or functional changes of GPCRs in MS patients or animal models, and the influences of GPCRs on disease severity upon genetic or pharmacological manipulations. Hopefully some of these findings will lead to the development of novel therapies for MS in the near future.

Autoimmune-mediated reduction of high-density lipoprotein-cholesterol and paraoxonase 1 activity in systemic lupus erythematosus-prone gld mice

OBJECTIVE

To characterize modifications of high-density lipoprotein (HDL) in autoimmune gld mice that may be relevant to premature atherosclerosis in systemic lupus erythematosus, and to assess their relationship to specific aspects of autoimmune disease.

METHODS

HDL cholesterol (HDL-C), apolipoprotein A-I (Apo A-I), paraoxonase 1 (PON1) activity, hepatic gene expression, and HDL biogenesis were measured in aging female gld and wild-type congenic mice. Autoantibodies, lymphoid organs, and cytokines were analyzed by enzyme-linked immunosorbent assay, flow cytometry, and multiplex assay, respectively.

RESULTS

Plasma HDL-C, HDL Apo A-I, and HDL-associated PON1 activity were reduced in aging gld mice in association with the development of autoimmunity, independent of changes in hepatic Apo A-I and PON1 expression or HDL biogenesis. Hepatic induction of the acute-phase reactant serum amyloid A1 resulted in its incorporation into HDL in gld mice. Deletion of the lipid-sensitive receptor G2A in gld mice (G2A-/- gld) attenuated reductions in HDL-C and PON1 activity without altering hepatic Apo A-I and PON1 expression, HDL biogenesis, or levels of acute-phase proinflammatory cytokines. Plasma anti-Apo A-I autoantibodies were elevated in aging gld mice commensurate with detectable increases in Apo A-I immune complexes. Autoantibody levels were lower in aging G2A-/- gld mice compared with gld mice, and anti-Apo A-I autoantibody levels were significantly related to HDL-C concentrations (r=-0.645, P<0.00004) and PON1 activity (r=-0.555, P<0.0007) among autoimmune gld and G2A-/- gld mice.

CONCLUSION

Autoantibodies against Apo A-I contribute to reducing HDL-C and PON1 activity in autoimmune gld mice independently of hepatic HDL biogenesis, suggesting that functional impairment and premature clearance of HDL immune complexes may be principal mechanisms involved.

G2A and LPC: regulatory functions in immunity

The G2A receptor was originally identified by virtue of its transcriptional induction in murine B lymphoid cells in response to oncogenic transformation and treatment with various DNA-damaging agents. While preliminary characterization of cellular responses to G2A overexpression in fibroblastic cell lines suggested that this receptor may negatively regulate cell growth under conditions of proliferative and genotoxic stress, subsequent studies driven by the discovery of lysophosphatidylcholine (LPC) as a regulator of G2A signaling in immunoregulatory cells point to an important role for this receptor in innate and adaptive immunity.