A novel lysophospholipid- and pH-sensitive receptor, GPR4, in brain endothelial cells regulates monocyte transmigration

Strengthening the basics: acids and bases influence vascular structure and function, tissue perfusion, blood pressure, and human cardiovascular disease

Acids and their conjugate bases accumulate in or dissipate from the interstitial space when tissue perfusion does not match the metabolic demand. Extracellular acidosis dilates most arterial beds, but associated acid-base disturbances-e.g., intracellular acidification and decreases in HCO3- concentration-can also elicit pro-contractile influences that diminish vasodilation and even dominate in some vascular beds to cause vasoconstriction. The ensemble activities of the acid-base-sensitive reactions in vascular smooth muscle and endothelial cells optimize vascular resistance for blood pressure control and direct the perfusion towards active tissue. In this review, we describe the mechanisms of intracellular pH regulation in the vascular wall and discuss how vascular smooth muscle and endothelial cells sense acid-base disturbances. We further deliberate on the functional effects of local acid-base disturbances and their integrated cardiovascular consequences under physiological and pathophysiological conditions. Finally, we address how mutations and polymorphisms in the molecular machinery that regulates pH locally and senses acid-base disturbances in the vascular wall can result in cardiovascular disease. Based on the emerging molecular insight, we propose that targeting local pH-dependent effectors-rather than systemic acid-base disturbances-has therapeutic potential to interfere with the progression and reduce the severity of cardiovascular disease.

Expression of endogenous epitope-tagged GPR4 in the mouse brain

GPR4 is a proton-sensing G protein-coupled receptor implicated in many peripheral and central physiological processes. GPR4 expression has previously been assessed only via detection of the cognate transcript or indirectly, by use of fluorescent reporters. In this work, CRISPR/Cas9 knock-in technology was used to encode a hemagglutinin (HA) epitope tag within the endogenous locus of Gpr4 and visualize GPR4-HA in the mouse central nervous system using a specific, well characterized HA antibody; GPR4 expression was further verified by complementary Gpr4 mRNA detection. HA immunoreactivity was found in a limited set of brain regions, including in the retrotrapezoid nucleus (RTN), serotonergic raphe nuclei, medial habenula, lateral septum, and several thalamic nuclei. GPR4 expression was not restricted to cells of a specific neurochemical identity as it was observed in excitatory, inhibitory, and aminergic neuronal cell groups. HA immunoreactivity was not detected in brain vascular endothelium, despite clear expression of Gpr4 mRNA in endothelial cells. In the RTN, GPR4 expression was detected at the soma and in proximal dendrites along blood vessels and the ventral surface of the brainstem; HA immunoreactivity was not detected in RTN projections to two known target regions. This localization of GPR4 protein in mouse brain neurons corroborates putative sites of expression where its function has been previously implicated (e.g., CO2-regulated breathing by RTN), and provides a guide for where GPR4 could contribute to other CO2/H+ modulated brain functions. Finally, GPR4-HA animals provide a useful reagent for further study of GPR4 in other physiological processes outside of the brain.Significance Statement GPR4 is a proton-sensing G-protein coupled receptor whose expression is necessary for a number of diverse physiological processes including acid-base sensing in the kidney, immune function, and cancer progression. In the brain, GPR4 has been implicated in the hypercapnic ventilatory response mediated by brainstem neurons. While knockout studies in animals have clearly demonstrated its necessity for normal physiology, descriptions of GPR4 expression have been limited due to a lack of specific antibodies for use in mouse models. In this paper, we implemented a CRISPR/Cas9 knock-in approach to incorporate the coding sequence for a small epitope tag into the locus of GPR4. Using these mice, we were able to describe GPR4 protein expression directly for the first time.

circ_0014736 induces GPR4 to regulate the biological behaviors of human placental trophoblast cells through miR-942-5p in preeclampsia

Previous studies have indicated that the development of preeclampsia (PE) involves the regulation of circular RNA (circRNA). However, the role of hsa_circ_0014736 (circ_0014736) in PE remains unknown. Thus, the study proposes to reveal the function of circ_0014736 in the pathogenesis of PE and the underlying mechanism. The results showed that circ_0014736 and GPR4 expression were significantly upregulated, while miR-942-5p expression was downregulated in PE placenta tissues when compared with normal placenta tissues. circ_0014736 knockdown promoted the proliferation, migration, and invasion of placenta trophoblast cells (HTR-8/SVneo) and inhibited apoptosis; however, circ_0014736 overexpression had the opposite effects. circ_0014736 functioned as a sponge for miR-942-5p and regulated HTR-8/SVneo cell processes by interacting with miR-942-5p. Additionally, GPR4, a target gene of miR-942-5p, was involved in miR-942-5p-mediated actions in HTR-8/SVneo cells. Moreover, circ_0014736 stimulated GPR4 production through miR-942-5p. Collectively, circ_0014736 inhibited HTR-8/SVneo cell proliferation, migration, and invasion and induced cell apoptosis through the miR-942-5p/GPR4 axis, providing a possible target for the treatment of PE.

pH and proton-sensitive receptors in brain ischemia

Extracellular proton concentration is at 40 nM when pH is 7.4. In disease conditions such as brain ischemia, proton concentration can reach µM range. To respond to this increase in extracellular proton concentration, the mammalian brain expresses at least three classes of proton receptors. Acid-sensing ion channels (ASICs) are the main neuronal cationic proton receptor. The proton-activated chloride channel (PAC), which is also known as (aka) acid-sensitive outwardly rectifying anion channel (ASOR; TMEM206), mediates acid-induced chloride currents. Besides proton-activated channels, GPR4, GPR65 (aka TDAG8, T-cell death-associated gene 8), and GPR68 (aka OGR1, ovarian cancer G protein-coupled receptor 1) function as proton-sensitive G protein-coupled receptors (GPCRs). Though earlier studies on these GPCRs mainly focus on peripheral cells, we and others have recently provided evidence for their functional importance in brain injury. Specifically, GPR4 shows strong expression in brain endothelium, GPR65 is present in a fraction of microglia, while GPR68 exhibits predominant expression in brain neurons. Here, to get a better view of brain acid signaling and its contribution to ischemic injury, we will review the recent findings regarding the differential contribution of proton-sensitive GPCRs to cerebrovascular function, neuroinflammation, and neuronal injury following acidosis and brain ischemia.

Lysophosphatidylcholine promotes intercellular adhesion molecule-1 and vascular cell adhesion molecule-1 expression in human umbilical vein endothelial cells via an orphan G protein receptor 2-mediated signaling pathway

The oxLDL-based bioactive lipid lysophosphatidylcholine (LPC) is a key regulator of physiological processes including endothelial cell adhesion marker expression. This study explored the relationship between LPC and the human umbilical vein endothelial cell expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) with a particular focus on the regulation of the LPC-G2A-ICAM-1/VCAM-1 pathway in this context. We explored the LPC-inducible role of orphan G protein receptor 2 (G2A) in associated regulatory processes by using human kidney epithelial (HEK293) cells that had been transfected with pET-G2A, human umbilical vein endothelial cells (HUVECs) in which an shRNA was used to knock down G2A, and western blotting and qPCR assays that were used to confirm changes in gene expression. For in vivo studies, a rabbit model of atherosclerosis was established, with serum biochemistry and histological staining approaches being used to assess pathological outcomes in these animals. The treatment of both HEK293 cells and HUVECs with LPC promoted ICAM-1 and VCAM-1 upregulation, while incubation at a pH of 6.8 suppressed such LPC-induced adhesion marker expression. Knocking down G2A by shRNA and inhibiting NF-κB activity yielded opposite outcomes. The application of a Gi protein inhibitor had no impact on LPC-induced ICAM-1/VCAM-1 expression. Atherosclerotic model exhibited high circulating LDL and LPC levels as well as high aortic wall ICAM-1/VCAM-1 expression. Overall, these results suggested that the LPC-G2A-ICAM-1/VCAM-1 pathway may contribute to the atherogenic activity of oxLDL, with NF-κB antagonists representing potentially viable therapeutic tools for the treatment of cardiovascular disease.

Lysophosphatidylcholine induces apoptosis and inflammatory damage in brain microvascular endothelial cells via GPR4-mediated NLRP3 inflammasome activation

Lysophosphatidylcholine (LPC), as the main active component of oxidized low-density lipoproteins (ox-LDLs), has significant effects in cerebrovascular disease. However, the complex mechanism by which LPC functions in brain microvascular endothelial cells (BMECs) is not clearly understood. In this study, BMECs were transfected with G protein-coupled receptor 4 (GPR4) siRNA or an NLRP3-overexpression plasmid, and GPR4 expression was identified by RT-qPCR and western blotting; IL-1β, IL-18, and IL-33 levels were evaluated by ELISA. Apoptosis was monitored by flow cytometry and Hoechst staining, while Caspase 3, ASC, NLRP3, and GPR4 protein expression were examined by western blotting. Our results showed that LPC significantly increased the levels of inflammatory cytokines (IL-1β, IL-18, and IL-33) and markedly induced apoptosis and NLRP3 inflammasome activation in BMECs. Moreover, LPC notably upregulated GPR4 in BMECs, and knockdown of GPR4 significantly attenuated the effects of LPC in BMECs. Above all, we also proved that LPC induced apoptosis and inflammatory injury in BMECs by causing GPR4 to activate NLRP3 inflammasomes. Therefore, GPR4-mediated activation of NLRP3 inflammasomes might be the underlying mechanism by which LPC promotes the progression of cerebrovascular disease. In summary we found that LPC is an important pathogenic factor in cerebrovascular disease, and can induce GPR4 to active NLRP3 inflammasomes.

Increased proton-sensing receptor GPR4 signalling promotes colorectal cancer progression by activating the hippo pathway

Background

Colorectal cancer (CRC) is one of the high incidences tumours and is ranked second in cancer-related mortality. Even though great progress has been made, there are no effective therapeutic strategies for late stage and metastatic CRC patients. Acidity is one characteristic of the tumour microenvironment. However, how cancer cells respond to this acidic environment surrounding them remains largely unknown, especially in colorectal cancer.

Methods

Proton sensor receptor expression was analysed in GEO and TCGA datasets. The expression of GPR4 in CRC specimens was confirmed by western blotting and immunohistochemistry (IHC). The role of GPR4 in CRC progression was analysed both in vitro and in vivo. Pharmacological intervention, immunofluorescence and gene set enrichment analyses were performed to reveal the underlying molecular mechanisms of GPR4.

Findings

We found that GPR4 was upregulated in CRC samples. In addition, its high expression correlated with late stage tumours and poor overall survival in patients. Furthermore, loss-of-function assays proved that GPR4 promoted CRC carcinogenesis and metastatic ability. Mechanistically, GPR4 was activated by extracellular protons in the tumour microenvironment and enhanced RhoA activation and F-actin rearrangement, leading to LATS activity inhibition, YAP1 nuclear translocation and oncogene transcription.

Interpretation

The expression of GPR4 is upregulated in colorectal cancer and is associated with shorter overall survival time in CRC patients. These findings reveal the novel roles of GPR4 in CRC progression and suggest GPR4 might be a new therapeutic target for CRC treatment.

CNS distribution, signalling properties and central effects of G-protein coupled receptor 4

Information on the distribution and biology of the G-protein coupled receptor 4 (GPR4) in the brain is limited. It is currently thought that GPR4 couples to Gs proteins and may mediate central respiratory sensitivity to CO2. Using a knock-in mouse model, abundant GPR4 expression was detected in the cerebrovascular endothelium and neurones of dorsal raphe, retro-trapezoidal nucleus locus coeruleus and lateral septum. A similar distribution was confirmed using RNAscope in situ hybridisation. In HEK293 cells, overexpressing GPR4, it was highly constitutively active at neutral pH with little further increase in cAMP towards acidic pH. The GPR4 antagonist NE 52-QQ57 effectively blocked GPR4-mediated cAMP accumulation (IC50 26.8 nM in HEK293 cells). In HUVEC which natively express GPR4, physiological acidification (pH 7.4-7.0) resulted in a cAMP increase by ∼55% which was completely prevented by 1 μM NE 52-QQ57. The main extracellular organic acid, l-lactic acid (LL; 1-10 mM), suppressed pH dependent activation of GPR4 in HEK293 and HUVEC cells, suggesting allosteric negative modulation. In unanaesthetised mice and rats, NE 52-QQ57 (20 mg kg-1) reduced ventilatory response to 5 and 10% CO2. In anaesthetised rats, systemic administration of NE 52-QQ57 (up to 20 mg kg-1) had no effect on hemodynamics, cerebral blood flow and blood oxygen level dependent responses. Central administration of NE 52-QQ57 (1 mM) in vagotomised anaesthetised rats did not affect CO2-induced respiratory responses. Our results indicate that GPR4 is expressed by multiple neuronal populations and endothelium and that its pH sensitivity is affected by level of expression and LL. NE 52-QQ57 blunts hypercapnic response to CO2 but this effect is absent under anaesthesia, possibly due to the inhibitory effect of LL on GPR4.

Bioactive lysophosphatidylcholine 16:0 and 18:0 are elevated in lungs of asthmatic subjects

PURPOSE

Asthma is a chronic inflammatory disease of the airways, and is associated with upregulation of phospholipase A2 (PLA2), the enzyme that hydrolyzes phosphatidylcholine, producing lysophosphatidylcholine (LPC) and free fatty acids. LPC is a lipid mediator with known pro-inflammatory and pro-atherogenic properties, and is believed to be a critical factor in cardiovascular diseases. We postulate that asthmatic subjects have an elevated content of LPC in the lung lining fluids.

METHODS

Eight non-asthmatic controls and seven asthmatic subjects were recruited for broncho-alveolar lavage fluids (BALF) collection for analysis of LPC by high performance liquid chromatography-tandem mass spectrometry.

RESULTS

LPC16:0 and LPC18:0 were significantly elevated in the BALF of asthmatics with impaired lung function characteristic of moderate asthma, but not mild asthma. The increased LPC content in BALF was accompanied by increased PLA2 activity. Furthermore, qRT-PCR analysis of the BALF cell fraction indicated increased secretory PLA2-X (sPLA2-X).

CONCLUSIONS

The increased LPC content in the lung lining fluids is a potential critical lipid mediator in the initiation and/or progression of airway epithelial injury in asthma.

Stimulated bronchial epithelial cells release bioactive lysophosphatidylcholine 16:0, 18:0, and 18:1

PURPOSE

In human subjects and animal models with acute and chronic lung injury, the bioactive lysophosphatidylcholine (LPC) is elevated in lung lining fluids. The increased LPC can promote an inflammatory microenvironment resulting in lung injury. Furthermore, pathological lung conditions are associated with upregulated phospholipase A2 (PLA2), the predominant enzyme producing LPC in tissues by hydrolysis of phosphatidylcholine. However, the lung cell populations responsible for increases of LPC have yet to be systematically characterized. The goal was to investigate the LPC generation by bronchial epithelial cells in response to pathological mediators and determine the major LPC species produced.

METHODS

Primary human bronchial epithelial cells (NHBE) were challenged by vascular endothelial growth factor (VEGF) for 1 or 6 h, and condition medium and cells collected for quantification of predominant LPC species by high performance liquid chromatography-tandem mass spectrometry (LC-MS-MS). The cells were analyzed by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) for PLA2. The direct effects of LPC in inducing inflammatory activities on NHBE were assessed by transepithelial resistance as well as expression of interleukin-8 (IL-8) and matrix metalloproteinase-1 (MMP-1).

RESULTS

VEGF stimulation of NHBE for 1 or 6 h, significantly increased concentrations of LPC16:0, LPC18:0, and LPC18:1 in condition medium compared to control. The sPLA2-selective inhibitor (oleyloxyethyl phosphorylcholine) inhibited the VEGF-induced release of LPC16:0 and LPC18:1 and PLA2 activity. In contrast, NHBE stimulated with TNF did not induce LPC release. VEGF did not increase mRNA of PLA2 subtypes sPLA2-X, sPLA2-XIIa, cPLA2-IVa, and iPLA2-VI. Exogenous LPC treatment increased expression of IL-8 and MMP-1, and reduced the transepithelial resistance in NHBE.

CONCLUSIONS

Our findings indicate that VEGF-stimulated bronchial epithelial cells are a key source of extracellular LPCs, which can function as an autocrine mediator with potential to induce airway epithelial inflammatory injury.

Novel bioactive glycerol-based lysophospholipids: new data -- new insight into their function

Based on the results of research conducted over last two decades, lysophospholipids (LPLs) were observed to be not only structural components of cellular membranes but also biologically active molecules influencing a broad variety of processes such as carcinogenesis, neurogenesis, immunity, vascular development or regulation of metabolic diseases. With a growing interest in the involvement of extracellular lysophospholipids in both normal physiology and pathology, it has become evident that those small molecules may have therapeutic potential. While lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have been studied in detail, other LPLs such as lysophosphatidylglycerol (LPG), lysophosphatidylserine (LPS), lysophosphatidylinositol (LPI), lysophosphatidylethanolamine (LPE) or even lysophosphatidylcholine (LPC) have not been elucidated to such a high degree. Although information concerning the latter LPLs is sparse as compared to LPA and S1P, within the last couple of years much progress has been made. Recently published data suggest that these compounds may regulate fundamental cellular activities by modulating multiple molecular targets, e.g. by binding to specific receptors and/or altering the structure and fluidity of lipid rafts. Therefore, the present review is devoted to novel bioactive glycerol-based lysophospholipids and recent findings concerning their functions and possible signaling pathways regulating physiological and pathological processes.

Reduced pathological angiogenesis and tumor growth in mice lacking GPR4, a proton sensing receptor

The G protein-coupled receptor GPR4 is activated by acidic pH and recent evidence indicates that it is expressed in endothelial cells. In agreement with these reports, we observe a high correlation of GPR4 mRNA expression with endothelial marker genes, and we confirm expression and acidic pH dependent function of GPR4 in primary human vascular endothelial cells. GPR4-deficient mice were generated; these are viable and fertile and show no gross abnormalities. However, these animals show a significantly reduced angiogenic response to VEGF (vascular endothelial growth factor), but not to bFGF (basic fibroblast growth factor), in a growth factor implant model. Accordingly, in two different orthotopic models, tumor growth is strongly reduced in mice lacking GPR4. Histological analysis of tumors indicates reduced tumor cell proliferation as well as altered vessel morphology, length and density. Moreover, GPR4 deficiency results in reduced VEGFR2 (VEGF Receptor 2) levels in endothelial cells, accounting, at least in part, for the observed phenotype. Our data suggest that endothelial cells sense local tissue acidosis via GPR4 and that this signal is required to generate a full angiogenic response to VEGF.

Loss of p120 catenin upregulates transcription of pro-inflammatory adhesion molecules in human endothelial cells

P120 catenin (p120ctn) is an adherens junction protein recognized to regulate barrier function, but emerging evidence indicates that p120ctn may also exert control on other cellular functions such as transcriptional suppression of genes. We investigated the hypothesis that loss of p120ctn in human endothelial cells activates transcription of pro-inflammatory adhesion molecules. For study, siRNA targeted to p120ctn was transfected into brain microvascular (HBMECs) or pulmonary artery endothelial cells (HPAECs) for 24-120h, which depleted 50-80% of endogenous p120ctn. This loss of p120ctn resulted in increased promoter reporter activity of transcription factors, NFκB, AP-1, and Kaiso, as well as of target genes, MMP-1 and ICAM-1. Real-time RT-PCR analysis indicated that the mRNA for ICAM-1, VCAM-1, and E- and P-selectins were all upregulated during the period of 24-120h of p120ctn depletion, although the time-course and extent of the expression profiles differed. The upregulated mRNA of adhesion molecules corresponded with increased PMN adhesion to the EC surface and elevated ICAM-1 protein expression. We further explored the role of ERK1/2 as a potential signaling mechanism responsible for regulation of transcriptional activities by p120ctn. Results indicated that loss of p120ctn increased phosphorylated ERK1/2, and a MEK1 inhibitor (PD98059) prevented NFκB nuclear translocation. This implicates ERK1/2 in signaling the NFκB activation induced by p120ctn loss. The findings provide strong evidence that deficiency in p120ctn expression in endothelial cells is a potent stimulus for transcriptional upregulation of multiple adhesion molecules. We conclude that p120ctn functions to suppress transcription, which is an important and novel regulation in vascular endothelium.