Metal-Stimulated Interleukin-6 Production Through a Proton-Sensing Receptor, Ovarian Cancer G Protein-Coupled Receptor 1, in Human Bronchial Smooth Muscle Cells: A Response Inhibited by Dexamethasone

Purpose

Human bronchial smooth muscle cells (BSMCs) contribute to airway obstruction and hyperresponsiveness in patients with bronchial asthma. BSMCs also generate cytokines and matricellular proteins in response to extracellular acidification through the ovarian cancer G protein-coupled receptor 1 (OGR1). Cobalt (Co) and nickel (Ni) are occupational agents, which cause occupational asthma. We examined the effects of Co and Ni on interleukin-6 (IL-6) secretion by human BSMCs because these metals may act as ligands of OGR1.

Methods

Human BSMCs were incubated in Dulbecco's Modified Eagle Medium (DMEM) containing 0.1% bovine serum albumin (BSA) (0.1% BSA-DMEM) for 16 hours and stimulated for the indicated time by exchanging the medium with 0.1% BSA-DMEM containing any of the metals or pH-adjusted 0.1% BSA-DMEM. IL-6 mRNA expression was quantified via reverse transcription polymerase chain reaction (RT-PCR) using the real-time TaqMan technology. IL-6 was measured using an enzyme-linked immunosorbent assay. Dexamethasone (DEX) was added 30 minutes before each stimulation. To knock down the expression of OGR1 in BSMCs, small interfering RNA (siRNA) targeting OGR1 (OGR1-siRNA) was transfected to the cells and non-targeting siRNA (NT-siRNA) was used as a control.

Results

Co and Ni both significantly increased IL-6 secretion in human BSMCs at 300 μM. This significant increase in IL-6 mRNA expression was observed 5 hours after stimulation. BSMCs transfected with OGR1-siRNA produced less IL-6 than BSMCs transfected with NT-siRNA in response to either Co or Ni stimulation. DEX inhibited Co- and Ni-stimulated IL-6 secretion by human BSMCs as well as pH 6.3-stimulated IL-6 secretion in a dose-dependent manner. DEX did not decrease phosphorylation of ERK1/2, p38 MAP kinase, and NF-κB p65 induced by either Co or Ni stimulation.

Conclusion

Co and Ni induce secretion of IL-6 in human BSMCs through activation of OGR1. Co- and Ni-stimulated IL-6 secretion is inhibited by DEX.

The protective role of proton-sensing TDAG8 in the brain injury in a mouse ischemia reperfusion model

Extracellular acidification in the brain has been observed in ischemia; however, the physiological and pathophysiological implications of the pH reduction remain largely unknown. Here, we analyzed the roles of proton-sensing G protein-coupled receptors, including T-cell death-associated gene 8 (TDAG8), ovarian cancer G protein-coupled receptor 1 (OGR1), and G protein-coupled receptor 4 (GPR4) in a mouse ischemia reperfusion model. Cerebral infarction and dysfunctional behavior with transient middle cerebral artery occlusion (tMCAO) and subsequent reperfusion were exacerbated by the deficiency of TDAG8, whereas no significant effect was observed with the deficiency of OGR1 or GPR4. We confirmed that the pH of the predicted infarction region was 6.5. TDAG8 mRNA was observed in Iba1-positive microglia in the mouse brain. The tMCAO increased the mRNA expression of tumor necrosis factor-α in the ipsilateral cerebral hemisphere and evoked morphological changes in microglia in an evolving cerebral injury. These tMCAO-induced actions were significantly enhanced by the TDAG8 deficiency. Administration of minocycline, which is known to inhibit microglial activation, improved the cerebral infarction and dysfunctional behavior induced by tMCAO in the TDAG8-deficient mouse. Thus, acidic pH/TDAG8 protects against cerebral infarction caused by tMCAO, at least due to the mechanism involving the inhibition of microglial functions.

Species-Dependent Enhancement of Ovarian Cancer G Protein-Coupled Receptor 1 Activation by Ogerin

Ogerin is a positive allosteric modulator of human and mouse ovarian cancer G protein-coupled receptors (OGR1s). In the present study, we found that ogerin differentially enhances the activation of OGR1 in various animal species. Amino acid residues of OGR1 that are associated with ogerin are conserved among the species. This suggests that other amino acid residues may be involved in the action of ogerin. Chimeric receptors between human and zebrafish OGR1s showed that the amino acid residues that determine the species specificity of ogerin-induced enhancement reside in the transmembrane and/or intracellular regions of OGR1. This result highlights the importance of first verifying the effectiveness of ogerin to the OGR1 of the species of interest at the cellular level prior to analyzing the physiological and pathophysiological roles of OGR1 in the species.

Involvement of GPR4 in increased growth hormone and prolactin expressions by extracellular acidification in MtT/S cells

Hormone-secreting pituitary adenomas show unregulated hormonal hypersecretion and cause hyperpituitarism. However, the mechanism of the unregulated hormone production and secretion has not yet been fully elucidated. Solid tumors show reduced extracellular pH, partly due to lactate secretion from anaerobic glycolysis. It is known that extracellular acidification affects hormone secretion. However, whether and how the extracellular acidification influences the unregulated hormone production and secretion remain unknown. In the present study, we found that GPR4, a proton-sensing G protein-coupled receptor, was highly expressed in MtT/S cells, a growth hormone-producing and prolactin-producing pituitary tumor cell line. When we reduced the extracellular pH, growth hormone and prolactin mRNA expressions increased in the cells. Both increased expressions were partially suppressed by a GPR4 antagonist. We also found that extracellular acidification enhanced growth hormone-releasing factor-induced growth hormone secretion from MtT/S cells. These results suggest that GPR4 may play a role in hypersecretion of the hormone from hormone-producing pituitary tumors. A GPR4 antagonist will be a useful tool for preventing the hypersecretion.

Characterization of molecular mechanisms of extracellular acidification-induced intracellular Ca2+ increase in LβT2 cells

Extracellular acidification regulates endocrine cell functions. Ovarian cancer G protein-coupled receptor 1 (OGR1), also known as GPR68, is a proton-sensing G protein-coupled receptor and is activated by extracellular acidification, resulting in the activation of multiple intracellular signaling pathways. In the present study, we found that OGR1 was expressed in some gonadotropic cells in rat anterior pituitary and in LβΤ2 cells, which are used as a model of gonadotropic cells. When we reduced extracellular pH, a transient intracellular Ca²⁺ increase was detected in LβT2 cells. The Ca²⁺ increase was inhibited by a G_q/11 inhibitor and Cu²⁺, which is known as an OGR1 antagonist. We also found that extracellular acidification enhanced GnRH-induced Gaussia luciferase secretion from LβT2 cells. These results suggest that OGR1 may play a role in the regulation of gonadotropic cell function such as its hormone secretion.

Protons Differentially Activate TDAG8 Homologs from Various Species

Mammalian T cell death-associated gene 8 (TDAG8)s are activated by extracellular protons. In the present study, we examined whether the TDAG8 homologs of other species are activated by protons as they are in mammals. We found that Xenopus TDAG8 also stimulated cAMP response element (CRE)-driven promoter activities reflecting the activation of Gs/cAMP signaling pathways when they are stimulated by protons. On the other hand, the activities of chicken and zebrafish TDAG8s are hardly affected by protons. Results using chimeric receptors of human and zebrafish TDAG8s indicate that the specificity of the proton-induced activation lies in the extracellular region. These results suggest that protons are not an evolutionarily conserved agonist of TDAG8.

Metals Differentially Activate Ovarian Cancer G Protein-Coupled Receptor 1 in Various Species

Human, mouse, and zebrafish ovarian cancer G protein-coupled receptors (OGR1s) are activated by both metals and extracellular protons. In the present study, we examined whether pig, rat, chicken, and Xenopus OGR1 homologs could sense and be activated by protons and metals. We found that all homologs stimulated serum response element (SRE)-driven promoter activities when they are stimulated by protons. On the other hand, metals differentially activated the homologs. The results using chimeric receptors of human and zebrafish OGR1s indicate that the specificity of the metal-induced activation lies in the extracellular region. These results suggest that protons are an evolutionally conserved agonist of OGR1. However, the types of metals that activated the receptor differed among the homologs.

Extracellular acidification-induced CXCL8 production through a proton-sensing receptor OGR1 in human airway smooth muscle cells: a response inhibited by dexamethasone

Background

Human airway smooth muscle cells (ASMCs) contribute to bronchial contraction and airway hyperresponsiveness in patients with bronchial asthma. They also generate cytokines, chemokines, and matricellular proteins. Ovarian cancer G protein-coupled receptor 1 (OGR1) senses extracellular protons and mediates the production of interleukin-6 (IL-6) and connective tissue growth factor (CTGF) in ASMCs.

Methods

ASMCs were stimulated for the indicated time by pH 6.3 or pH 7.4-adjusted Dulbecco’s Modified Eagle Medium (DMEM) containing 0.1% bovine serum albumin (BSA) (0.1% BSA-DMEM). As a control stimulant, pH 7.4-adjusted 0.1% BSA-DMEM containing 10 ng/mL tumor necrosis factor-α (TNF-α) was used. Interleukin-8/C-X-C motif chemokine ligand 8 (CXCL8) mRNA expression in ASMCs was quantified by RT-PCR using real-time TaqMan technology. CXCL8 secreted from ASMCs was measured by enzyme-linked immunosorbent assay (ELISA). Phosphorylation at serine 536 of NF-κB p65 and binding of p65 to oligonucleotide containing an NF-κB consensus binding site were analyzed by Western blotting and an ELISA-based kit.

Results

Acidic pH induced a significant increase of CXCL8 mRNA expression and CXCL8 protein secretion in ASMCs. ASMCs transfected with small interfering RNA (siRNA) targeted for OGR1 produced less CXCL8 compared with those transfected with non-targeting siRNA. Protein kinase C (PKC) inhibitor, MEK1/2 inhibitor, and the inhibitor of IκB phosphorylation reduced acidic pH-stimulated CXCL8 production in ASMCs. Dexamethasone also inhibited acidic pH-stimulated CXCL8 production of ASMCs in a dose-dependent manner. Dexamethasone did not affect either phosphorylation or binding to the consensus DNA site of NF-κB p65.

Conclusions

CXCL8 released from ASMCs by extracellular acidification may play a pivotal role in airway accumulation of neutrophils. Glucocorticoids inhibit acidic pH-stimulated CXCL8 production independent of serine 536 phosphorylation and the binding to DNA of NF-κB p65, although NF-κB activity is essential for CXCL8 production in ASMCs.

The proton-sensing G protein-coupled receptor T-cell death-associated gene 8 (TDAG8) shows cardioprotective effects against myocardial infarction

Myocardial infarction (MI) is an ischaemic heart condition caused by the occlusion of coronary arteries. Following MI, lactic acid from anaerobic glycolysis increases and infiltrating immune cells produce severe inflammation, which leads to acidosis in the ischaemic heart. However, the physiological implication of this pH reduction remains largely unknown. T-cell death-associated gene 8 (TDAG8) is a proton-sensing G protein-coupled receptor found on cardiac macrophages that recognise increases in extracellular protons. We demonstrated that TDAG8 negatively regulates the transcription of the chemokine Ccl20. The infarcted hearts of TDAG8 KO mice showed an increase in CCL20 expression and the number of infiltrating IL-17A-producing γδT cells that express CCR6, a receptor for CCL20. Accordingly, excessive IL-17A production, which is linked to the functional deterioration after MI, was observed in MI-operated TDAG8 KO mice. The survival rate and cardiac function significantly decreased in TDAG8 KO mice compared with those in wild-type mice after MI. Thus, our results suggest that TDAG8 is a key regulator of MI and a potential therapeutic target.

Reactive astrocytes function as phagocytes after brain ischemia via ABCA1-mediated pathway

Astrocytes become reactive following various brain insults; however, the functions of reactive astrocytes are poorly understood. Here, we show that reactive astrocytes function as phagocytes after transient ischemic injury and appear in a limited spatiotemporal pattern. Following transient brain ischemia, phagocytic astrocytes are observed within the ischemic penumbra region during the later stage of ischemia. However, phagocytic microglia are mainly observed within the ischemic core region during the earlier stage of ischemia. Phagocytic astrocytes upregulate ABCA1 and its pathway molecules, MEGF10 and GULP1, which are required for phagocytosis, and upregulation of ABCA1 alone is sufficient for enhancement of phagocytosis in vitro. Disrupting ABCA1 in reactive astrocytes result in fewer phagocytic inclusions after ischemia. Together, these findings suggest that astrocytes are transformed into a phagocytic phenotype as a result of increase in ABCA1 and its pathway molecules and contribute to remodeling of damaged tissues and penumbra networks.

Astrocytic phagocytosis has been shown to play a role in synaptic pruning during development, but whether adult astrocytes possess phagocytic ability is unclear. Here the authors show that following brain ischemia, reactive astrocytes become phagocytic and engulf debris via the ABCA1 pathway.

ω-3 Polyunsaturated fatty acids accelerate airway repair by activating FFA4 in club cells

A G protein-coupled receptor (GPCR) named free fatty acid receptor 4 (FFA4, also known as GPR120) was found to act as a GPCR for ω-3 polyunsaturated fatty acids. Its expression has been reported in lung epithelial club cells. We investigated whether supplementation of the ω-3 fatty acids benefits lung health. Omacor (7.75 mg/kg), clinically prescribed preparation of ω-3 fatty acids, and FFA4-knockout mice were utilized in a naphthalene-induced mouse model of acute airway injury (1 injection of 30 mg/kg ip). Naphthalene injection induced complete destruction of bronchiolar epithelial cells within a day. Appearance of bronchiolar epithelial cells was observed after 21 days in control mice. It was found, however, that supplementation of Omacor accelerated the recovery. The appearance of bronchiolar epithelial cells was observed between 7 and 14 days after naphthalene injury in Omacor-treated mice. In isolated club cells, ω-3 fatty acids were found to stimulate cell proliferation and migration but to inhibit cell differentiation. With the use of pharmacological tools and FFA4-knockout mice, FFA4 was found to be responsible for ω-3 fatty acids-induced proliferation in vitro in club cells. Furthermore, accelerated recovery from naphthalene-induced airway injury in Omacor-treated mice was not observed in FFA4-knockout mice in vivo. Present findings indicate that ω-3 fatty acids-induced proliferation of bronchiole epithelial cells through FFA4 is responsible for Omacor-induced accelerated recovery from airway injury. Therefore, intermittent administration of Omacor needs to be tested for acute airway injury because ω-3 fatty acids stimulate proliferation but inhibit differentiation of club cells.

Manganese and cobalt activate zebrafish ovarian cancer G-protein-coupled receptor 1 but not GPR4

Mammalian ovarian G-protein-coupled receptor 1 (OGR1) is activated by some metals in addition to extracellular protons and coupling to multiple intracellular signaling pathways. In the present study, we examined whether zebrafish OGR1, zebrafish GPR4, and human GPR4 (zOGR1, zGPR4, and hGPR4, respectively) could sense the metals and activate the intracellular signaling pathways. On one hand, we found that only manganese and cobalt of the tested metals stimulated SRE-promoter activities in zOGR1-overexpressed HEK293T cells. On the other hand, none of the metals tested stimulated the promoter activities in zGPR4- and hGPR4-overexpressed cells. The OGR1 mutant (H4F), which is lost to activation by extracellular protons, did not stimulate metal-induced SRE-promoter activities. These results suggest that zOGR1, but not GPR4, is also a metal-sensing G-protein-coupled receptor in addition to a proton-sensing G-protein-coupled receptor, although not all metals that activate hOGR1 activated zOGR1.

Mutations in the pH-Sensing G-protein-Coupled Receptor GPR68 Cause Amelogenesis Imperfecta

Amelogenesis is the process of dental enamel formation, leading to the deposition of the hardest tissue in the human body. This process requires the intricate regulation of ion transport and controlled changes to the pH of the developing enamel matrix. The means by which the enamel organ regulates pH during amelogenesis is largely unknown. We identified rare homozygous variants in GPR68 in three families with amelogenesis imperfecta, a genetically and phenotypically heterogeneous group of inherited conditions associated with abnormal enamel formation. Each of these homozygous variants (a large in-frame deletion, a frameshift deletion, and a missense variant) were predicted to result in loss of function. GPR68 encodes a proton-sensing G-protein-coupled receptor with sensitivity in the pH range that occurs in the developing enamel matrix during amelogenesis. Immunohistochemistry of rat mandibles confirmed localization of GPR68 in the enamel organ at all stages of amelogenesis. Our data identify a role for GPR68 as a proton sensor that is required for proper enamel formation.

Identification of a Potent and Selective GPR4 Antagonist as a Drug Lead for the Treatment of Myocardial Infarction

GPR4, a pH-sensing G protein-coupled receptor, is highly expressed in endothelial cells and may be activated in myocardial infarction due the decreased tissue pH. We are interested in GPR4 antagonists as potential effective pharmacologic tools and/or drug leads for the treatment of myocardial infarction. We investigated the structure-activity relationship of a known GPR4 antagonist 1 as a lead compound to identify 3b as the first potent and selective GPR4 antagonist, whose effectiveness was demonstrated in a mouse myocardial infarction model.

17(R)-resolvin D1 ameliorates bleomycin-induced pulmonary fibrosis in mice

Idiopathic pulmonary fibrosis (IPF) is a destructive inflammatory disease with limited therapeutic options. Inflammation plays an integral role in the development of pulmonary fibrosis. Unresolved inflammatory responses can lead to substantial tissue injury, chronic inflammation, and fibrosis. The resolvins are a family of endogenous ω‐3 fatty acid derived‐lipid mediators of inflammation resolution. Resolvin D1 (RvD1) displays potent anti‐inflammatory, pro‐resolving activity, without causing immunosuppression. Its epimer, 17(R)‐resolvin D1 (17(R)‐RvD1), exhibits equivalent functionality to RvD1. In addition, 17(R)‐RvD1 is resistant to rapid inactivation by eicosanoid oxidoreductases. In the present study, we tested the hypothesis that 17(R)‐RvD1 can provide a therapeutic benefit in IPF by reducing inflammation and pulmonary fibrosis, while leaving the normal immune response intact. Mice were exposed to bleomycin (BLM) via micro‐osmotic pump to induce pulmonary fibrosis, and were then treated with 17(R)‐RvD1 or vehicle by intraperitoneal injection. Administration of 17(R)‐RvD1 from the start of BLM treatment attenuated neutrophil alveolar infiltration, lung collagen content, and Interleukin‐1β (IL‐1β), transforming growth factor‐β1 (TGF‐β1), connective tissue growth factor (CTGF), and type I collagen mRNA expression, along with subsequent reduction in histologically detectable fibrosis. The 17(R)‐RvD1‐induced infiltration of inflammatory cells was inhibited by an antagonist of lipoxin A4 receptor/formyl peptide receptor 2 (ALX/FPR2). The administration of 17(R)‐RvD1 at the later fibrotic stage also improved the lung failure. These results suggest that 17(R)‐RvD1 attenuates pulmonary fibrosis by promoting the resolution of neutrophilic inflammation and also provides pulmonary restoration. These data highlight the therapeutic potential of 17(R)‐RvD1 in the management of this intractable disease.

Two zebrafish G2A homologs activate multiple intracellular signaling pathways in acidic environment

Human G2A is activated by various stimuli such as lysophosphatidylcholine (LPC), 9-hydroxyoctadecadienoic acid (9-HODE), and protons. The receptor is coupled to multiple intracellular signaling pathways, including the Gs-protein/cAMP/CRE, G12/13-protein/Rho/SRE, and Gq-protein/phospholipase C/NFAT pathways. In the present study, we examined whether zebrafish G2A homologs (zG2A-a and zG2A-b) could respond to these stimuli and activate multiple intracellular signaling pathways. We also examined whether histidine residue and basic amino acid residue in the N-terminus of the homologs also play roles similar to those played by human G2A residues if the homologs sense protons. We found that the zG2A-a showed the high CRE, SRE, and NFAT activities, however, zG2A-b showed only the high SRE activity under a pH of 8.0. Extracellular acidification from pH 7.4 to 6.3 ameliorated these activities in zG2A-a-expressing cells. On the other hand, acidification ameliorated the SRE activity but not the CRE and NFAT activities in zG2A-b-expressing cells. LPC or 9-HODE did not modify any activity of either homolog. The substitution of histidine residue at the 174(th) position from the N-terminus of zG2A-a to asparagine residue attenuated proton-induced CRE and NFAT activities but not SRE activity. The substitution of arginine residue at the 32nd position from the N-terminus of zG2A-a to the alanine residue also attenuated its high and the proton-induced CRE and NFAT activities. On the contrary, the substitution did not attenuate SRE activity. The substitution of the arginine residue at the 10th position from the N-terminus of zG2A-b to the alanine residue also did not attenuate its high or the proton-induced SRE activity. These results indicate that zebrafish G2A homologs were activated by protons but not by LPC and 9-HODE, and the activation mechanisms of the homologs were similar to those of human G2A.

Angiotensin II Reduces Lipoprotein Lipase Expression in Visceral Adipose Tissue via Phospholipase C β4 Depending on Feeding but Increases Lipoprotein Lipase Expression in Subcutaneous Adipose Tissue via c-Src

Metabolic syndrome is characterized by visceral adiposity, insulin resistance, high triglyceride (TG)- and low high-density lipoprotein cholesterol-levels, hypertension, and diabetes-all of which often cause cardiovascular and cerebrovascular diseases. It remains unclear, however, why visceral adiposity but not subcutaneous adiposity causes insulin resistance and other pathological situations. Lipoprotein lipase (LPL) catalyzes hydrolysis of TG in plasma lipoproteins. In the present study, we investigated whether the effects of angiotensin II (AngII) on TG metabolism are mediated through an effect on LPL expression. Adipose tissues were divided into visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT) for comparison. AngII accelerated LPL expression in SAT but, on the contrary, suppressed its expression in VAT. In both SAT and VAT, AngII signaled through the same type 1 receptor. In SAT, AngII increased LPL expression via c-Src and p38 MAPK signaling. In VAT, however, AngII reduced LPL expression via the Gq class of G proteins and the subsequent phospholipase C β4 (PLCβ4), protein kinase C β1, nuclear factor κB, and inducible nitric oxide synthase signaling pathways. PLCβ4 small interfering RNA experiments showed that PLCβ4 expression is important for the AngII-induced LPL reduction in VAT, in which PLCβ4 expression increases in the evening and falls at night. Interestingly, PLCβ4 expression in VAT decreased with fasting, while AngII did not decrease LPL expression in VAT in a fasting state. In conclusion, AngII reduces LPL expression through PLCβ4, the expression of which is regulated by feeding in VAT, whereas AngII increases LPL expression in SAT. The different effects of AngII on LPL expression and, hence, TG metabolism in VAT and SAT may partly explain their different contributions to the development of metabolic syndrome.

Acidic environment augments FcεRI-mediated production of IL-6 and IL-13 in mast cells

Although blood pH is maintained in a narrow range of around pH 7.4 in living organisms, inflammatory loci are characterized by acidic conditions. Mast cells tend to reside close to the surface of the body in areas such as the mucosa and skin where they may be exposed to exogenous acids, and they play an important role in immune responses. However, little is known about the effects of extracellular acidification on the functions of mast cell. Here, we found that extracellular acidification increased the dinitrophenyl-conjugated human serum albumin (DNP-HSA)-induced production of interleukin (IL)-6 and IL-13 in MC/9 cells or bone marrow-derived mouse mast cells sensitized with anti-DNP IgE. Extracellular acidification also inhibited migration of MC/9 cells toward DNP-HSA. In addition, acidic pH stimulated antigen-induced activation of p38 mitogen-activated protein kinase (MAPK) and protein kinase B (Akt). These findings suggest that extracellular acidification augmented antigen/IgE-induced and FcεRI-mediated production of IL-6 and IL-13 in mast cells, and that this was associated with the enhancement of p38 MAPK and Akt activation.

Characterization of Imidazopyridine Compounds as Negative Allosteric Modulators of Proton-Sensing GPR4 in Extracellular Acidification-Induced Responses

G protein-coupled receptor 4 (GPR4), previously proposed as the receptor for sphingosylphosphorylcholine, has recently been identified as the proton-sensing G protein-coupled receptor (GPCR) coupling to multiple intracellular signaling pathways, including the G_s protein/cAMP and G₁₃ protein/Rho. In the present study, we characterized some imidazopyridine compounds as GPR4 modulators that modify GPR4 receptor function. In the cells that express proton-sensing GPCRs, including GPR4, OGR1, TDAG8, and G2A, extracellular acidification stimulates serum responsive element (SRE)-driven transcriptional activity, which has been shown to reflect Rho activity, with different proton sensitivities. Imidazopyridine compounds inhibited the moderately acidic pH-induced SRE activity only in GPR4-expressing cells. Acidic pH-stimulated cAMP accumulation, mRNA expression of inflammatory genes, and GPR4 internalization within GPR4-expressing cells were all inhibited by the GPR4 modulator. We further compared the inhibition property of the imidazopyridine compound with psychosine, which has been shown to selectively inhibit actions induced by proton-sensing GPCRs, including GPR4. In the GPR4 mutant, in which certain histidine residues were mutated to phenylalanine, proton sensitivity was significantly shifted to the right, and psychosine failed to further inhibit acidic pH-induced SRE activation. On the other hand, the imidazopyridine compound almost completely inhibited acidic pH-induced action in mutant GPR4. We conclude that some imidazopyridine compounds show specificity to GPR4 as negative allosteric modulators with a different action mode from psychosine, an antagonist susceptible to histidine residues, and are useful for characterizing GPR4-mediated acidic pH-induced biological actions.

Inhibition of interleukin-1β production by extracellular acidification through the TDAG8/cAMP pathway in mouse microglia

Interleukin-1β (IL-1β) is released from activated microglia and involved in the neurodegeneration of acute and chronic brain disorders, such as stroke and Alzheimer's disease, in which extracellular acidification has been shown to occur. Here, we examined the extracellular acidic pH regulation of IL-1β production, especially focusing on TDAG8, a major proton-sensing G-protein-coupled receptor, in mouse microglia. Extracellular acidification inhibited lipopolysaccharide -induced IL-1β production, which was associated with the inhibition of IL-1β cytoplasmic precursor and mRNA expression. The IL-1β mRNA and protein responses were significantly, though not completely, attenuated in microglia derived from TDAG8-deficient mice compared with those from wild-type mice. The acidic pH also stimulated cellular cAMP accumulation, which was completely inhibited by TDAG8 deficiency. Forskolin and a cAMP derivative, which specifically stimulates protein kinase A (PKA), mimicked the proton actions, and PKA inhibitors reversed the acidic pH-induced IL-1β mRNA expression. The acidic pH-induced inhibitory IL-1β responses were accompanied by the inhibition of extracellular signal-related kinase and c-Jun N-terminal kinase activities. The inhibitory enzyme activities in response to acidic pH were reversed by the PKA inhibitor and TDAG8 deficiency. We conclude that extracellular acidic pH inhibits lipopolysaccharide-induced IL-1β production, at least partly, through the TDAG8/cAMP/PKA pathway, by inhibiting extracellular signal-related kinase and c-Jun N-terminal kinase activities, in mouse microglia.

Role of extracellular proton-sensing OGR1 in regulation of insulin secretion and pancreatic β-cell functions

Insulin secretion with respect to pH environments has been investigated for a long time but its mechanism remains largely unknown. Extracellular pH is usually maintained at around 7.4 and, its change has been thought to occur in non-physiological situations. Acidification takes place under ischemic and inflammatory microenvironments, where stimulation of anaerobic glycolysis results in the production of lactic acid. In addition to ionotropic ion channels, such as transient receptor potential V1 (TRPV1) and acid-sensing ion channels (ASICs), metabotropic proton-sensing G protein-coupled receptors (GPCRs) have also been identified recently as proton-sensing machineries. While ionotropic ion channels usually sense strong acidic pH, proton-sensing GPCRs sense pH of 7.6 to 6.0 and have been shown to mediate a variety of biological actions in neutral and mildly acidic pH environments. Studies with receptor knockout mice have revealed that proton-sensing receptors, including ovarian cancer G protein-coupled receptor 1 (OGR1), a proton-sensing GPCRs, play a role in the regulation of insulin secretion and glucose metabolism under physiological conditions. Small molecule 3,5-disubstituted isoxazoles have recently been identified as OGR1 agonists working at neutral pH and have been shown to stimulate pancreatic β-cell differentiation and insulin synthesis. Thus, proton-sensing OGR1 may be an important player for insulin secretion and a potential target for improving β-cell function.

Proton-sensing ovarian cancer G protein-coupled receptor 1 on dendritic cells is required for airway responses in a murine asthma model

Ovarian cancer G protein-coupled receptor 1 (OGR1) stimulation by extracellular protons causes the activation of G proteins and subsequent cellular functions. However, the physiological and pathophysiological roles of OGR1 in airway responses remain largely unknown. In the present study, we show that OGR1-deficient mice are resistant to the cardinal features of asthma, including airway eosinophilia, airway hyperresponsiveness (AHR), and goblet cell metaplasia, in association with a remarkable inhibition of Th2 cytokine and IgE production, in an ovalbumin (OVA)-induced asthma model. Intratracheal transfer to wild-type mice of OVA-primed bone marrow-derived dendritic cells (DCs) from OGR1-deficient mice developed lower AHR and eosinophilia after OVA inhalation compared with the transfer of those from wild-type mice. Migration of OVA-pulsed DCs to peribronchial lymph nodes was also inhibited by OGR1 deficiency in the adoption experiments. The presence of functional OGR1 in DCs was confirmed by the expression of OGR1 mRNA and the OGR1-sensitive Ca²⁺ response. OVA-induced expression of CCR7, a mature DC chemokine receptor, and migration response to CCR7 ligands in an in vitro Transwell assay were attenuated by OGR1 deficiency. We conclude that OGR1 on DCs is critical for migration to draining lymph nodes, which, in turn, stimulates Th2 phenotype change and subsequent induction of airway inflammation and AHR.

Oxidative stress potentially enhances FcεRI-mediated leukotriene C4 release dependent on the late-phase increase of intracellular glutathione in mast cells

Cysteinyl leukotrienes (cysLTs), which include leukotriene C4 (LTC4), are the predominant class of LTs synthesized by mast cells. CysLTs can induce many of the abnormalities seen in asthma. LTC4 is generated by the conjugation of LTA4 with reduced glutathione (GSH) by LTC4 synthase. During screening of the effects of prostanoids on high-affinity IgE receptor (FcεRI)-mediated LTC4 release from mast cells, we realized that some prostanoids, including ONO-AE1-259-01 and ONO-AE-248, inhibited LTC4 release, which was associated with a decrease in the amount of intracellular total GSH. We ascertained that l-buthionine-S,R-sulfoximine (BSO), a selective inhibitor of glutamate-cysteine ligase, inhibited LTC4 release. In addition, cell-permeable GSH, the glutathione reduced form ethyl ester (GSH-OEt), enhanced LTC4 release in accordance with the change in intracellular total GSH. Depletion of intracellular total GSH induced by ONO-AE-248 or BSO enhanced FcεRI-mediated LTB4 release in contrast to LTC4. Oxidative stress contributes to many pathological conditions including asthma. GSH is a major soluble antioxidant and a cofactor for several detoxifying enzymes including GSH peroxidase. Exposure of mast cells to hydrogen peroxide (H2O2) or diamide to mimic oxidative stress unexpectedly increased rather than decreased the intracellular reduced GSH content as well as total GSH in the late phase (i.e., 24 or 48 h after exposure), which was accompanied by an increase in LTC4 release. In conclusion, FcεRI-mediated LTC4 release from mast cells is mainly regulated by the amount of intracellular GSH. In some cases, oxidative stress may induce a late-phase increase in intracellular GSH, resulting in enhanced LTC4 release from mast cells.

Lysophosphatidylethanolamine utilizes LPA(1) and CD97 in MDA-MB-231 breast cancer cells

Lysophosphatidylethanolamine (LPE) is a lyso-type metabolite of phosphatidylethanolamine (a plasma membrane component), and its intracellular Ca(2+) ([Ca(2+)]i) increasing actions may be mediated through G-protein-coupled receptor (GPCR). However, GPCRs for lysophosphatidic acid (LPA), a structurally similar representative lipid mediator, have not been implicated in LPE-mediated activities in SK-OV3 or OVCAR-3 ovarian cancer cells or in receptor over-expression systems. In the present study, LPE-induced [Ca(2+)]i increase was observed in MDA-MB-231 cells but not in other breast cancer cell lines. In addition, LPE- and LPA-induced responses showed homologous and heterologous desensitization. Furthermore, VPC32183 and Ki16425 (antagonists of LPA1 and LPA3) inhibited LPE-induced [Ca(2+)]i increases, and knockdown of LPA1 by transfection with LPA1 siRNA completely inhibited LPE-induced [Ca(2+)]i increases. Furthermore, the involvement of CD97 (an adhesion GPCR) in the action of LPA1 in MDA-MB-231 cells was demonstrated by siRNA transfection. Pertussis toxin (a specific inhibitor of Gi/o proteins), edelfosine (an inhibitor of phospholipase C), or 2-APB (an inhibitor of IP3 receptor) completely inhibited LPE-induced [Ca(2+)]i increases, whereas HA130, an inhibitor of autotaxin/lysophospholipase D, did not. Therefore, LPE is supposed to act on LPA1-CD97/Gi/o proteins/phospholipase C/IP3/Ca(2+) rise in MDA-MB-231 breast cancer cells.

Deficiency of proton-sensing ovarian cancer G protein-coupled receptor 1 attenuates glucose-stimulated insulin secretion

Ovarian cancer G protein-coupled receptor 1 (OGR1) has been shown as a receptor for protons. In the present study, we aimed to know whether OGR1 plays a role in insulin secretion and, if so, the manner in which it does. To this end, we created OGR1-deficient mice and examined insulin secretion activity in vivo and in vitro. OGR1 deficiency reduced insulin secretion induced by glucose administered ip, although it was not associated with glucose intolerance in vivo. Increased insulin sensitivity and reduced plasma glucagon level may explain, in part, the unusual normal glucose tolerance. In vitro islet experiments revealed that glucose-stimulated insulin secretion was dependent on extracellular pH and sensitive to OGR1; insulin secretion at pH 7.4 to 7.0, but not 8.0, was significantly suppressed by OGR1 deficiency and inhibition of G(q/11) proteins. Insulin secretion induced by KCl and tolbutamide was also significantly inhibited, whereas that induced by several insulin secretagogues, including vasopressin, a glucagon-like peptide 1 receptor agonist, and forskolin, was not suppressed by OGR1 deficiency. The inhibition of insulin secretion was associated with the reduction of glucose-induced increase in intracellular Ca(2+) concentration. In conclusion, the OGR1/G(q/11) protein pathway is activated by extracellular protons existing under the physiological extracellular pH of 7.4 and further stimulated by acidification, resulting in the enhancement of insulin secretion in response to high glucose concentrations and KCl.

Ovarian cancer G-protein-coupled receptor 1 induces the expression of the pain mediator prostaglandin E2 in response to an acidic extracellular environment in human osteoblast-like cells

Although bone pain in osteoporosis and skeletal metastasis is an expected consequence of fracture, there are other underlying causes responsible. Our study demonstrated that ovarian cancer G-protein-coupled receptor 1 detected extracellular protons in MG63 cells, and regulated osteoblast functions, such as prostaglandin E2 production, in response to acidic circumstances. In this work, we measured inositol phosphate production, intracellular Ca(2+) concentration, prostaglandin E2 production, and cyclic adenosine monophosphate accumulation in MG63 cells exposed to extracellular acidification. Extracellular acidity induced a transient increase in Ca(2+) concentration and inositol phosphate production. Acidification also induced prostaglandin E2 production, resulting in cyclic adenosine monophosphate accumulation. A small interfering RNA specific for the ovarian cancer G-protein-coupled receptor 1 markedly inhibited these proton-induced actions in MG63 cells. These results indicated that the involvement of ovarian cancer G-protein-coupled receptor 1 in acidic extracellular environment may be an underlying mechanism responsible for bone pain in osteoporosis or bone metastasis without clinically proved fractures.

Orally active lysophosphatidic acid receptor antagonist attenuates pancreatic cancer invasion and metastasis in vivo

Pancreatic cancer is highly metastatic and has a poor prognosis. However, there is no established treatment for pancreatic cancer. Lysophosphatidic acid (LPA) has been shown to be present in effluents of cancers and involved in migration and proliferation in a variety of cancer cells, including pancreatic cancer cells, in vitro. In the current study, we examined whether an orally active LPA antagonist is effective for pancreatic cancer tumorigenesis and metastasis in vivo. Oral administration of Ki16198, which is effective for LPA(1) and LPA(3), into YAPC-PD pancreatic cancer cell-inoculated nude mice significantly inhibited tumor weight and remarkably attenuated invasion and metastasis to lung, liver, and brain, in association with inhibition of matrix metalloproteinase (MMP) accumulation in ascites in vivo. Ki16198 inhibited LPA-induced migration and invasion in several pancreatic cancer cells in vitro, which was associated with the inhibition of LPA-induced MMP production. In conclusion, Ki16198 is a promising orally active LPA antagonist for inhibiting the invasion and metastasis of pancreatic cancer cells. The inhibitory effects of the antagonist on invasion and metastasis in vivo may be partially explained by the inhibition of motility activity and MMP production in cancer cells.

Critical role of ABCA1 transporter in sphingosine 1-phosphate release from astrocytes

Sphingosine 1-phosphate (S1P) is accumulated in lipoproteins, especially high-density lipoprotein (HDL), in plasma. However, it remains uncharacterized how extracellular S1P is produced in the CNS. The treatment of rat astrocytes with retinoic acid and dibutyryl cAMP, which induce apolipoprotein E (apoE) synthesis and HDL-like lipoprotein formation, stimulated extracellular S1P accumulation in the presence of its precursor sphingosine. The released S1P was present together with apoE particles in the HDL fraction. S1P release from astrocytes was inhibited by the treatment of the cells with glybenclamide or small interfering RNAs specific to ATP-binding cassette transporter A1 (ABCA1). Astrocytes from Abca1-/- mice also showed impairment of retinoic acid/dibutyryl cAMP-induced S1P release in association with the blockage of HDL-like lipoprotein formation. However, the formation of either apoE or lipoprotein itself was not sufficient, and additional up-regulation of ABCA1 was requisite to stimulate S1P release. We conclude that the S1P release from astrocytes is coupled with lipoprotein formation through ABCA1.

Constitutively active heat shock factor 1 enhances glucose-driven insulin secretion

Weak pancreatic β-cell function is a cause of type 2 diabetes mellitus. Glucokinase regulates insulin secretion via phosphorylation of glucose. The present study focused on a system for the self-protection of pancreatic cell by expressing heat shock factor (HSF) and heat shock protein (HSP) to improve insulin secretion without inducing hypoglycemia. We previously generated a constitutively active form of human HSF1 (CA-hHSF1). An adenovirus expressing CA-hHSF1 using the cytomegalovirus promoter was generated to infect mouse insulinoma cells (MIN6 cells). An adenovirus expressing CA-hHSF1 using a human insulin promoter (Ins-CA-hHSF1) was also generated to infect rats. We investigated whether CA-hHSF1 induces insulin secretion in MIN6 cells and whether Ins-CA-hHSF1 can improve blood glucose and serum insulin levels in healthy Wister rats and type 2 diabetes mellitus model rats. CA-hHSF1 expression increased insulin secretion 1.27-fold compared with the overexpression of wild-type hHSF1 in MIN6 cells via induction of HSP90 expression and subsequent activation of glucokinase. This mechanism is associated with activation of both glucokinase and neuronal nitric oxide synthase. Ins-CA-hHSF1 improved blood glucose levels in neonatal streptozotocin-induced diabetic rats. Furthermore, Ins-CA-hHSF1 reduced oral glucose tolerance testing results in healthy Wister rats because of an insulin spike at 15 minutes; however, it did not induce hypoglycemia. CA-hHSF1 induced insulin secretion both in vitro and in vivo. These findings suggest that gene therapy with Ins-CA-hHSF1 will be able to be used to treat patients with type 2 diabetes mellitus and impaired glucose tolerance without causing hypoglycemia at fasting.

p2y5/LPA(6) attenuates LPA(1)-mediated VE-cadherin translocation and cell-cell dissociation through G(12/13) protein-Src-Rap1

AIMS

We investigated the mechanisms of action of lysophosphatidic acid (LPA) to regulate vascular endothelial (VE)-cadherin dynamics and cell-cell contact.

METHODS AND RESULTS

While a low concentration of LPA stimulated VE-cadherin internalization and subsequent cell-cell dissociation, a high concentration of LPA masked the disruptive actions on VE-cadherin and protected the barrier function in human vascular endothelial cells. Knockdown experiments of major LPA receptor subtypes, i.e. LPA(1) and p2y5 (also termed LPA(6)), with their specific small interfering RNAs, showed that LPA(1) and LPA(6) mediate the LPA-induced disruptive and protective actions on barrier integrity, respectively. LPA(6)-mediated tube formation, reflecting stabilization of barrier integrity, was confirmed by in vitro angiogenesis assay. The LPA(1)-mediated disruptive actions were inhibited by pertussis toxin, dominant-negative Rac1, and inhibitors for c-Jun NH(2)-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38MAPK), but not by dominant-negative RhoA. In contrast, the LPA(6)-mediated protective actions were associated with activation of Src and Rap1 and attenuated by abrogation of their activities. Further characterization showed that Rap1 is located downstream of Src and dependent on C3G, a Rap1 guanine nucleotide exchange factor. Finally, an LPA antagonist significantly inhibited lactic acid-induced limb lesions in vivo, which may be attributed to dysfunction of endothelial cells.

CONCLUSION

LPA induced disruption and protection of VE-cadherin integrity through LPA(1)-G(i) protein-Rac1-JNK/p38MAPK and LPA(6)-G(12/13) protein-Src-C3G-Rap1 pathways, respectively.

Unmasking of LPA1 receptor-mediated migration response to lysophosphatidic acid by interleukin-1β-induced attenuation of Rho signaling pathways in rat astrocytes

Action mechanism of lipopolysaccharide (LPS), interleukin-1β (IL-1β), and lysophosphatidic acid (LPA) to regulate motility, an important process of astrogliosis, was investigated in rat astrocytes. While LPA exerted no significant effect on the cell migration, the prior treatment of the cells with LPS or IL-1β resulted in the appearance of migration activity in response to LPA. The LPS induction of the migration response to LPA was associated with the production of IL-1β precursor protein and inhibited by the IL-1 receptor antagonist. The IL-1β treatment also allowed LPA to activate Rac1. The LPA-induced Rac1 activation and migration were inhibited by pertussis toxin, a small interfering RNA specific to LPA(1) receptors, and LPA(1) receptor antagonists, including Ki16425. However, the IL-1β treatment had no appreciable effect on LPA(1) receptor mRNA expression and LPA-induced activation of ERK, Akt, and proliferation. The induction of the migration response to LPA by IL-1β was inhibited by a constitutively active RhoA. Moreover, LPA significantly activated RhoA through the LPA(1) receptor in the control cells but not in the IL-1β-treated cells. These results suggest that IL-1β inhibits the LPA(1) receptor-mediated Rho signaling through the IL-1 receptor, thereby disclosing the LPA(1) receptor-mediated G(i) protein/Rac/migration pathway.

Role of sphingosine 1-phosphate in anti-atherogenic actions of high-density lipoprotein

The reverse cholesterol transport mediated by high-density lipoprotein (HDL) is an important mechanism for maintaining body cholesterol, and hence, the crucial anti-atherogenic action of the lipoprotein. Recent studies, however, have shown that HDL exerts a variety of anti-inflammatory and anti-atherogenic actions independently of cholesterol metabolism. The present review provides an overview of the roles of sphingosine 1-phosphate (S1P)/S1P receptor and apolipoprotein A-I/scavenger receptor class B type I systems in the anti-atherogenic HDL actions. In addition, the physiological significance of the existence of S1P in the HDL particles is discussed.

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.

Cross-talk between exogenous statins and endogenous high-density lipoprotein in anti-inflammatory and anti-atherogenic actions

The reverse cholesterol transport mediated by high-density lipoprotein (HDL) is an important mechanism for maintaining body cholesterol at normal levels and, hence, the critical anti-atherogenic action of the lipoprotein. Recent studies, however, showed that HDL exerts a variety of anti-inflammatory and anti-atherogenic actions independently of the cholesterol metabolism. On the other hand, statins, inhibitors of HMG-CoA reductase, were initially developed to lower low-density lipoprotein cholesterol in plasma, and they are now recognized to exert a variety of pleiotropic or beneficial actions. Thus, although the mechanisms are different, both endogenous HDL and exogenous statins regulate cholesterol balance in a negative manner and exert a variety of beneficial actions independently of their cholesterol-lowering activity. These results raise the possibility that statins act in part through modulating the plasma levels of HDL and/or its actions. Here, we reviewed the cross-talk mechanism between statins and HDL in anti-inflammatory and anti-atherogenic actions, with a focus on scavenger receptor class B type I, one of main players involved in the cholesterol metabolism-independent HDL actions, and its downstream signaling pathway, leading to the activation of endothelial nitric oxide synthase and the inhibition of adhesion molecule expression in endothelial cells.

Protective effect of resolvin E1 on the development of asthmatic airway inflammation

Resolvin E1 (RvE1) is an anti-inflammatory lipid mediator derived from the omega-3 fatty acid eicosapentaenoic acid (EPA), and strongly acts in the resolution of inflammation. We previously reported that RvE1 dampens airway inflammation and hyperresponsiveness in a murine model of asthma. In the present study, to elucidate the effects of RvE1 on the development of asthmatic airway inflammation, we investigated whether RvE1 acts on different phases of an OVA-sensitized and -challenged mouse model of asthma. RvE1 treatments at the time of either OVA sensitization or at the time of OVA challenge were investigated and compared with RvE1 treatments at the time of both OVA sensitization and challenge. After RvE1 was administered to mice intraperitoneally at the time of both OVA sensitization and challenge, there were decreases in airway eosinophil and lymphocyte recruitment, as well as a reduction in Th2 cytokine and airway hyperresponsiveness. RvE1 treatment at the time of either OVA sensitization or challenge also improved AHR and airway inflammation. Our results suggest that RvE1 acts on several phases of asthmatic inflammation and may have anti-inflammatory effects on various cell types.

Extracellular acidification stimulates IL-6 production and Ca(2+) mobilization through proton-sensing OGR1 receptors in human airway smooth muscle cells

The asthmatic airway has been shown to be an acidic environment that may be involved in the pathophysiological features of asthma. However, the mechanism by which an acidic pH modulates the cellular activities involved in the asthmatic airway remains elusive. Here, we characterized acidic pH-induced actions in human airway smooth muscle cells (ASMCs). Extracellular acidification stimulates the mRNA expression and protein production of IL-6, a proinflammatory cytokine, in association with the phosphorylation of extracellular signal-regulated kinase (ERK) and p38MAPK, reflecting the activation of the enzymes. Acidification-induced cytokine production was inhibited by inhibitors of ERK and p38MAPK. Acidification also increased intracellular Ca(2+) concentration, which was accompanied by cell rounding, most likely reflecting contraction. In ASMCs, OGR1 is expressed at by far the highest levels among proton-sensing G protein-coupled receptors. The knockdown of OGR1 and G(q/11) protein with their specific small interfering RNAs and an inhibition of G(q/11) protein with YM-254890 attenuated the acidification-induced actions. We conclude that extracellular acidification stimulates IL-6 production and Ca(2+) mobilization through proton-sensing OGR1 receptors/G(q/11) proteins in human ASMCs.

Ovarian cancer G protein-coupled receptor 1-dependent and -independent vascular actions to acidic pH in human aortic smooth muscle cells

Atherosclerosis is a chronic inflammation disease characterized by acidic micromilieu and the accumulation of numerous bioactive lipid mediators, such as lysophosphatidic acid (LPA) and prostaglandins, in the atherosclerotic lesion. Chronic acidification induced various effects on vascular smooth muscle cells, but the molecular mechanisms underlying these effects remain unknown. In this study, we examine the role of proton-sensing ovarian cancer G protein-coupled receptor 1 (OGR1) in extracellular acidification-induced regulation of cyclooxygenase (COX)-2 induction, PGI(2) production, MAPK phosphatase (MKP)-1 expression, and plasminogen activator inhibitor (PAI)-1 expression and proliferation in human aortic smooth muscle cells (AoSMCs). Experiments with knockdown with small interfering RNA specific to OGR1 and specific inhibitors for G proteins showed that acidification-induced COX-2 expression, PGI(2) production, and MKP-1 expression, but not PAI-1 expression and inhibition of proliferation, were dependent on OGR1 and mainly mediated by G(q/11) protein. LPA remarkably enhanced, through the LPA(1) receptor/G(i) protein, the OGR1-mediated vascular actions to acidic pH. In conclusion, acidic pH-induced vascular actions of AoSMCs can be dissected to OGR1-dependent and -independent pathways: COX-2 expression, PGI(2) production, and MKP-1 expression are mediated by OGR1, but PAI-1 expression and inhibition of proliferation are not. LPA, which is usually thought to be a proatherogenic lipid mediator, may exert antiatherogenic actions under acidic micromilieu through cross-talk between LPA(1)/G(i) protein and OGR1/G(q/11) protein.

Role of Rap1B and tumor suppressor PTEN in the negative regulation of lysophosphatidic acid--induced migration by isoproterenol in glioma cells

The clarification of mechanisms that negatively regulate the invasive behavior of human glioma cells is of great importance in order to find new methods of treatment. In this study, we have focused on the negative regulation of lysophosphatidic acid (LPA)-induced migration in glioma cells. Using small interference RNA and dominant-negative gene strategies in addition to pharmacological tools, we found that isoproterenol (ISO) and sphingosine-1-phosphate (S1P) negatively but differently regulate the LPA-induced migration. ISO-induced suppression of the migration of glioma cells occurs via beta(2)-adrenergic receptor/cAMP/Epac/Rap1B/inhibition of Rac, whereas S1P has been shown to suppress the migration of the cells through S1P(2) receptor/Rho-mediated down-regulation of Rac1. The expression of tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is required for the inhibitory ISO-induced and Rap1B-mediated actions on the migration, Rac1 activation, and Akt activation in response to LPA. Thus, the PTEN-mediated down-regulation of phosphatidylinositol 3-kinase activity may be involved in the regulation of Rap1B-dependent inhibition of Rac1 activity. These findings suggest that there are at least two distinct inhibitory pathways, which are mediated by the S1P(2) receptor and beta(2)-adrenergic receptor, to control the migratory, hence invasive, behavior of glioma cells.

Each one of certain histidine residues in G-protein-coupled receptor GPR4 is critical for extracellular proton-induced stimulation of multiple G-protein-signaling pathways

GPR4, previously proposed as the receptor for sphingosylphosphorylcholine, has recently been identified as the proton-sensing G-protein-coupled receptor coupling to multiple intracellular signaling pathways, including the G(s)-protein/cAMP, G(12/13)-protein/Rho, and G(q)-protein/phospholipase C pathways. In the present study, we examined whether extracellularly located histidine residues of GPR4 sense extracellular protons and, if so, whether a certain histidine residue is critical for coupling to the single or multiple signaling pathway(s). We found that the mutation of histidine residue at 79, 165, or 269 from the N-terminal of GPR4 to phenylalanine shifted the half-maximal effective concentration (EC(50)) of proton-induced signaling activities to the right, including cAMP accumulation, SRE promoter activity reflecting Rho activity, and NFAT promoter activity reflecting phospholipase C signaling activity, without an appreciable change in the maximal activities. These results suggest that the protonation of each one of histidine residues at 79, 165, and 269 in GPR4 may be critical for conformational change of the receptor for coupling to multiple intracellular signaling pathways through G-proteins.

Mechanism and role of high density lipoprotein-induced activation of AMP-activated protein kinase in endothelial cells

The upstream signaling pathway leading to the activation of AMP-activated protein kinase (AMPK) by high density lipoprotein (HDL) and the role of AMPK in HDL-induced antiatherogenic actions were investigated. Experiments using genetic and pharmacological tools showed that HDL-induced activation of AMPK is dependent on both sphingosine 1-phosphate receptors and scavenger receptor class B type I through calcium/calmodulin-dependent protein kinase kinase and, for scavenger receptor class B type I system, additionally serine-threonine kinase LKB1 in human umbilical vein endothelial cells. HDL-induced activation of Akt and endothelial NO synthase, stimulation of migration, and inhibition of monocyte adhesion and adhesion molecule expression were dependent on AMPK activation. The inhibitory role of AMPK in the adhesion molecule expression and monocyte adhesion on endothelium of mouse aorta was confirmed in vivo and ex vivo. On the other hand, stimulation of ERK and proliferation were hardly affected by AMPK knockdown but completely inhibited by an N17Ras, whereas the dominant-negative Ras was ineffective for AMPK activation. In conclusion, dual HDL receptor systems differentially regulate AMPK activity through calcium/calmodulin-dependent protein kinase kinase and/or LKB1. Several HDL-induced antiatherogenic actions are regulated by AMPK, but proliferation-related actions are regulated by Ras rather than AMPK.

Inhibition of superoxide anion production by extracellular acidification in neutrophils

Extracellular acidification inhibited formyl-Met-Leu-Phe- or C5a-induced superoxide anion (O(2)(-)) production in differentiated HL-60 neutrophil-like cells and human neutrophils. A cAMP-increasing agonist, prostaglandin E(1), also inhibited the formyl peptide-induced O(2)(-) production. The inhibitory action on the O(2)(-) production by extracellular acidic pH was associated with cAMP accumulation and partly attenuated by H89, a protein kinase A inhibitor. A significant amount of mRNAs for T-cell death-associated gene 8 (TDAG8) and other proton-sensing ovarian cancer G-protein-coupled receptor 1 (OGR1)-family receptors is expressed in these cells. These results suggest that cAMP/protein kinase A, possibly through proton-sensing G-protein-coupled receptors, may be involved in extracellular acidic pH-induced inhibition of O(2)(-) production.