Mast cell maturation is driven via a group III phospholipase A2-prostaglandin D2-DP1 receptor paracrine axis.

Microenvironment-based alterations in phenotypes of mast cells influence the susceptibility to anaphylaxis, yet the mechanisms underlying proper maturation of mast cells toward an anaphylaxis-sensitive phenotype are incompletely understood. Here we report that PLA2G3, a mammalian homolog of anaphylactic bee venom phospholipase A2, regulates this process. PLA2G3 secreted from mast cells is coupled with fibroblastic lipocalin-type PGD2 synthase (L-PGDS) to provide PGD2, which facilitates mast-cell maturation via PGD2 receptor DP1. Mice lacking PLA2G3, L-PGDS or DP1, mast cell-deficient mice reconstituted with PLA2G3-null or DP1-null mast cells, or mast cells cultured with L-PGDS-ablated fibroblasts exhibited impaired maturation and anaphylaxis of mast cells. Thus, we describe a lipid-driven PLA2G3-L-PGDS-DP1 loop that drives mast cell maturation.

Stepwise phosphorylation of BLT1 defines complex assemblies with ß-arrestin serving distinct functions.

G protein-coupled receptors (GPCRs) utilize complex cellular systems to respond to diverse ligand concentrations. By taking BLT1, a GPCR for leukotriene B4 (LTB4 ), as a model, our previous work elucidated that this system functions through the modulation of phosphorylation status on two specific residues: Thr308 and Ser310 . Ser310 phosphorylation occurs at a lower LTB4 concentration than Thr308 , leading to a shift in ligand affinity from a high-to-low state. However, the implications of BLT1 phosphorylation in signal transduction processes or the underlying mechanisms have remained unclear. Here, we identify the sequential BLT1-engaged conformations of ß-arrestin and subsequent alterations in signal transduction. Stimulation of the high-affinity BLT1 with LTB4 induces phosphorylation at Ser310 via the ERK1/2-GRK pathway, resulting in a ß-arrestin-bound low-affinity state. This configuration, referred to as the "low-LTB4 -induced complex," necessitates the finger loop region and the phosphoinositide-binding motif of ß-arrestins to interact with BLT1 and deactivates the ERK1/2 signaling. Under high LTB4 concentrations, the low-affinity BLT1 again binds to the ligand and triggers the generation of the low-LTB4 -induced complex into a different form termed "high-LTB4 -induced complex." This change is propelled by The308 -phosphorylation-dependent basal phosphorylation by PKCs. Within the high-LTB4 -induced complex, ß-arrestin adapts a unique configuration that involves additional N domain interaction to the low-affinity BLT1 and stimulates the PI3K/AKT pathway. We propose that the stepwise phosphorylation of BLT1 defines the formation of complex assemblies, wherein ß-arrestins perform distinct functions.

Constitutively active GPR43 is crucial for proper leukocyte differentiation.

The G protein-coupled receptors, GPR43 (free fatty acid receptor 2, FFA2) and GPR41 (free fatty acid receptor 3, FFA3), are activated by short-chain fatty acids produced under various conditions, including microbial fermentation of carbohydrates. Previous studies have implicated this receptor energy homeostasis and immune responses as well as in cell growth arrest and apoptosis. Here, we observed the expression of both receptors in human blood cells and a remarkable enhancement in leukemia cell lines (HL-60, U937, and THP-1 cells) during differentiation. A reporter assay revealed that GPR43 is coupled with Gαi and Gα12/13 and is constitutively active without any stimuli. Specific blockers of GPR43, GLPG0974 and CATPB function as inverse agonists because treatment with these compounds significantly reduces constitutive activity. In HL-60 cells, enhanced expression of GPR43 led to growth arrest through Gα12/13 . In addition, the blockage of GPR43 activity in these cells significantly impaired their adherent properties due to the reduction of adhesion molecules. We further revealed that enhanced GPR43 activity induces F-actin formation. However, the activity of GPR43 did not contribute to butyrate-induced apoptosis in differentiated HL-60 cells because of the ineffectiveness of the inverse agonist on cell death. Collectively, these results suggest that GPR43, which possesses constitutive activity, is crucial for growth arrest, followed by the proper differentiation of leukocytes.

Na+-mimicking ligands stabilize the inactive state of leukotriene B4 receptor BLT1.

Most G-protein-coupled receptors (GPCRs) are stabilized in common in the inactive state by the formation of the sodium ion-centered water cluster with the conserved Asp2.50 inside the seven-transmembrane domain. We determined the crystal structure of the leukotriene B4 (LTB4) receptor BLT1 bound with BIIL260, a chemical bearing a benzamidine moiety. Surprisingly, the amidine group occupies the sodium ion and water locations, interacts with D662.50, and mimics the entire sodium ion-centered water cluster. Thus, BLT1 is fixed in the inactive state, and the transmembrane helices cannot change their conformations to form the active state. Moreover, the benzamidine molecule alone serves as a negative allosteric modulator for BLT1. As the residues involved in the benzamidine binding are widely conserved among GPCRs, the unprecedented inverse-agonist mechanism by the benzamidine moiety could be adapted to other GPCRs. Consequently, the present structure will enable the rational development of inverse agonists specific for each GPCR.

The atypical N-glycosylation motif, Asn-Cys-Cys, in human GPR109A is required for normal cell surface expression and intracellular signaling.

Asparagine-linked glycosylation (N-glycosylation) is necessary for the proper folding of secreted and membrane proteins, including GPCRs. Thus, many GPCRs possess the N-glycosylation motif Asn-X-Ser/Thr at their N-termini and/or extracellular loops. We found that human GPR109A (hGPR109A) has an N-glycosylation site at Asn(17) in the N-terminal atypical motif, Asn(17)-Cys(18)-Cys(19). Why does hGPR109A require the atypical motif, rather than the typical sequence? Here we show that Asn(17)-Cys(18)-Cys(19) sequence of hGPR109A possesses 2 biologic roles. First, Asn(17)-X-Cys(19) contributed to hGPR109A N-glycosylation by acting as an atypical motif. This modification is required for the normal surface expression of hGPR109A, as evidenced by the reduced surface expression of the nonglycosylated mutants, hGPR109A/N17A, and the finding that hGPR109A/C19S and hGPR109A/C19T, which are N-glycosylated at Asn(17), exhibited expression similar to the wild-type receptor. Second, the X-Cys(18)-Cys(19) dicysteine is indispensable for hGPR109A function. Substitution of Cys(18) or Cys(19) residue to Ala impaired Gi-mediated signaling via hGPR109A. We propose the disulfide bond formations of these residues with other Cys existed in the extracellular loops for the proper folding. Together, these results suggest that the atypical motif Asn(17)-Cys(18)-Cys(19) is crucial for the normal surface trafficking and function of hGPR109A.

Interplay between CXCR2 and BLT1 facilitates neutrophil infiltration and resultant keratinocyte activation in a murine model of imiquimod-induced psoriasis.

Psoriasis is an inflammatory skin disease with accelerated epidermal cell turnover. Neutrophil accumulation in the skin is one of the histological characteristics of psoriasis. However, the precise mechanism and role of neutrophil infiltration remain largely unknown. In this article, we show that orchestrated action of CXCR2 and leukotriene B4 receptor BLT1 plays a key role in neutrophil recruitment during the development of imiquimod (IMQ)-induced psoriatic skin lesions in mice. Depletion of neutrophils with anti-Ly-6G Ab ameliorated the disease severity, along with reduced expression of proinflammatory cytokine IL-1ß in the skin. Furthermore, CXCR2 and BLT1 coordinately promote neutrophil infiltration into the skin during the early phase of IMQ-induced inflammation. In vitro, CXCR2 ligands augment leukotriene B4 production by murine neutrophils, which, in turn, amplifies chemokine-mediated neutrophil chemotaxis via BLT1 in autocrine and/or paracrine manners. In agreement with the increased IL-19 expression in IMQ-treated mouse skin, IL-1ß markedly upregulated expression of acanthosis-inducing cytokine IL-19 in human keratinocytes. We propose that coordination of chemokines, lipids, and cytokines with multiple positive feedback loops might drive the pathogenesis of psoriasis and, possibly, other inflammatory diseases as well. Interference to this positive feedback or its downstream effectors could be targets of novel anti-inflammatory treatment.

The absence of the leukotriene B4 receptor BLT1 attenuates peripheral inflammation and spinal nociceptive processing following intraplantar formalin injury.

BACKGROUND: Leukotriene B4 (LTB4) is a potent lipid mediator of inflammation, and its biological effects are mediated primarily through the high affinity LTB4 receptor BLT1. Although numerous studies have reported that LTB4-BLT1 signaling is involved in inflammatory diseases, the role of BLT1 signaling in pain remains undefined. To clarify the role of LTB4-BLT1 signaling in acute inflammatory pain induced by tissue injury, we performed pain behavioral analysis and assessment of local inflammation induced by peripheral formalin injections in BLT1 knockout mice. We examined the phosphorylation of cAMP response element-binding protein (CREB) in the spinal cord both in wild-type and BLT1 knockout mice because phosphorylation of CREB in spinal cord neurons is important for nociceptive sensitization following peripheral injury. We also examined the effect of a BLT1 antagonist on formalin-induced pain responses in mice. RESULTS: BLT1 knockout mice exhibited markedly attenuated nociceptive responses induced by intraplantar formalin injections. Edema formation and neutrophil infiltration in the paw were significantly decreased in BLT1 knockout mice compared with wild-type mice. Phosphorylation of CREB in the spinal cord after the intraplantar formalin injection was decreased in BLT1 knockout mice. In addition, mice pretreated with a BLT1 antagonist showed reduced nociception and attenuated CREB phosphorylation in the spinal cord after the formalin injection. CONCLUSIONS: Our data suggest that LTB4-BLT1 axis contributes not only to the peripheral inflammation but also to the neuronal activation in the spinal cord induced by intraplantar formalin injections. Thus, LTB4-BLT1 signaling is a potential target for therapeutic intervention of acute and persistent pain induced by tissue injury.

Visualization of the pH-dependent dynamic distribution of G2A in living cells.

G2A (from G2 accumulation) receptor is a member of the proton-sensing G-protein coupled receptor (GPCR) family and induces signal transduction events that regulate the cell cycle, proliferation, oncogenesis, and immunity. The mechanism by which G2A-mediated signal transduction is regulated by the extracellular pH remains unresolved. Here, we first visualize the pH-dependent G2A distribution change in living cells by a sortase A-mediated pulse labeling technology: the short-peptide tag-fused human G2A on human embryo kidney HEK293T cell surfaces was labeled with a small fluorescent dye in the presence of lysophosphatidylcholine, and the labeled G2A was chased at acidic and neutral pHs in real time by microscope time course observations. G2A internalization from cell surfaces into intracellular compartments was observed to be inhibited under acidic pH conditions, and this inhibition was relieved at neutral pH. Additionally, the internalized G2A was redistributed onto cell surfaces by jumping from a neutral to an acidic pH. From quantitative image analysis data, we conclude the amount of G2A on the cell surface was controlled by suppressing the G2A internalization rate by one-tenth in response to the extracellular acidic pH, and this acidic pH-induced G2A accumulation on cell surfaces may be explained by proton-induced dissociation of G2A from endocytic machinery.

Discovery of INT131: a selective PPARγ modulator that enhances insulin sensitivity.

PPARγ is a member of the nuclear hormone receptor family and plays a key role in the regulation of glucose homeostasis. This Letter describes the discovery of a novel chemical class of diarylsulfonamide partial agonists that act as selective PPARγ modulators (SPPARγMs) and display a unique pharmacological profile compared to the thiazolidinedione (TZD) class of PPARγ full agonists. Herein we report the initial discovery of partial agonist 4 and the structure-activity relationship studies that led to the selection of clinical compound INT131 (3), a potent PPARγ partial agonist that displays robust glucose-lowering activity in rodent models of diabetes while exhibiting a reduced side-effects profile compared to marketed TZDs.

Therapeutic target of leukotriene B4 receptors, BLT1 and BLT2: Insights from basic research.

Leukotriene B4 (LTB4) is a lipid mediator rapidly generated from arachidonic acid in response to various stimuli. This lipid mediator exerts its biological activities by binding to cognate receptors. Two LTB4 receptors have been cloned; BLT1 and BLT2 as a high- and a low-affinity receptors, respectively. In numerous analyses, physiological and pathophysiological importance of LTB4 and cognate receptors in various diseases has been clarified. For example, disruption of the BLT1 gene or treatment with blockers for this receptor reduced various diseases such as rheumatoid arthritis and bronchial asthma in mice, in contrast BLT2 deficiency facilitated several diseases in the small intestine and the skin. These data support the idea that BLT1 blockers and BLT2 agonists could be useful for the cure of these diseases. Thus, various drugs targeting each receptor are being developed by many pharmaceutical companies. In this review, we focus on our current knowledge of the biosynthesis and physiological roles of LTB4 through cognate receptors. We further describe the effects of these receptor deficiencies on several pathophysiological conditions, including the potential of LTB4 receptors as therapeutic targets for the cure of the diseases. Moreover, current information on the structure and post-translational modification of BLT1 and BLT2 is discussed.

Trehalose treatment suppresses inflammation, oxidative stress, and vasospasm induced by experimental subarachnoid hemorrhage.

BACKGROUND: Subarachnoid hemorrhage (SAH) frequently results in several complications, including cerebral vasospasm, associated with high mortality. Although cerebral vasospasm is a major cause of brain damages after SAH, other factors such as inflammatory responses and oxidative stress also contribute to high mortality after SAH. Trehalose is a non-reducing disaccharide in which two glucose units are linked by α,α-1,1-glycosidic bond, and has been shown to induce tolerance to a variety of stressors in numerous organisms. In the present study, we investigated the effect of trehalose on cerebral vasospasm, inflammatory responses, and oxidative stress induced by blood in vitro and in vivo. METHODS: Enzyme immunoassay for eicosanoids, pro-inflammatory cytokines, and endothelin-1, and western blotting analysis for cyclooxygenase-2, inducible nitric oxide synthase, and inhibitor of NF-κB were examined in macrophage-like cells treated with hemolysate. After treatment with hemolysate and hydrogen peroxide, the levels of lipid peroxide and amounts of arachidonic acid release were also analyzed. Three hours after the onset of experimental SAH, 18 Japanese White rabbits received an injection of saline, trehalose, or maltose into the cisterna magna. Angiographic and histological analyses of the basilar arteries were performed. In a separate study, the femoral arteries from 60 rats were exposed to fresh autologous blood. At 1, 3, 5, 7, 10, and 20 days after treatment, cryosections prepared from the femoral arteries were histologically analyzed. RESULTS: When cells were treated with hemolysate, trehalose inhibited the production of several inflammatory mediators and degradation of the inhibitor of NF-κB and also suppressed the lipid peroxidation, the reactive oxygen species-induced arachidonic acid release in vitro. In the rabbit model, trehalose produced an inhibitory effect on vasospasm after the onset of experimental SAH, while maltose had only a moderate effect. When the rat femoral arteries exposed to blood were investigated for 20 days, histological analysis revealed that trehalose suppressed vasospasm, inflammatory response, and lipid peroxidation. CONCLUSIONS: These data suggest that trehalose has suppressive effects on several pathological events after SAH, including vasospasm, inflammatory responses, and lipid peroxidation. Trehalose may be a new therapeutic approach for treatment of complications after SAH.

Recent advances in function and structure of two leukotriene B4 receptors: BLT1 and BLT2.

Leukotriene B4 (LTB4) is generated by the enzymatic oxidation of arachidonic acid, which is then released from the cell membrane and acts as a potent activator of leukocytes and other inflammatory cells. Numerous studies have demonstrated the physiological and pathophysiological significance of this lipid in various diseases. LTB4 exerts its activities by binding to its specific G protein-coupled receptors (GPCRs): BLT1 and BLT2. In mouse disease models, treatment with BLT1 antagonists or BLT1 gene ablation attenuated various diseases, including bronchial asthma, arthritis, and psoriasis, whereas BLT2 deficiency exacerbated several diseases in the skin, cornea, and small intestine. Therefore, BLT1 inhibitors and BLT2 activators could be beneficial for the treatment of several inflammatory and immune disorders. As a result, attractive compounds targeting LTB4 receptors have been developed by several pharmaceutical companies. This review aims to understand the potential of BLT1 and BLT2 as therapeutic targets for the treatment of various inflammatory diseases. In addition, recent topics are discussed with major focuses on the structure and post-translational modifications of BLT1 and BLT2. Collectively, current evidence on modulating LTB4 receptor functions provides new strategies for the treatment of various diseases.

Bitter taste receptor T2R38 is expressed on skin-infiltrating lymphocytes and regulates lymphocyte migration.

Bitter taste receptors (T2Rs) are G protein-coupled receptors involved in the perception of bitter taste on the tongue. In humans, T2Rs have been found in several sites outside the oral cavity. Although T2R38 has been reported to be expressed on peripheral lymphocytes, it is poorly understood whether T2R38 plays immunological roles in inflammatory skin diseases such as atopic dermatitis (AD). Then, we first confirmed that T2R38 gene expression was higher in lesional skin of AD subjects than healthy controls. Furthermore, skin T2R38 expression levels were correlated with serum thymus and activation-regulated chemokine and IgE levels in AD patients. In lesional skin of AD, section staining revealed that CD3+ T cells in the dermis were T2R38 positive. In addition, flow cytometry analysis showed T2R38 expression in skin T cells. Migration assays using T2R38-transduced Jurkat T cell leukemia cells revealed that T2R38 agonists exerted a dose-dependent migration inhibitory effect. Moreover, skin tissue extracts, as well as supernatants of cultured HaCaT keratinocytes, caused T2R38-dependent migration inhibition, indicating that there should be an endogenous ligand for T2R38 in the skin epidermis. These findings implicate T2R38 as a migratory inhibitory receptor on the skin-infiltrating lymphocytes and as a therapeutic target for allergic/inflammatory skin diseases.

Role of leukotriene B4 (LTB4)-LTB4 receptor 1 signaling in post-incisional nociceptive sensitization and local inflammation in mice.

Leukotriene B4 (LTB4) is a potent lipid mediator involved in the recruitment and activation of neutrophils, which is an important feature of tissue injury and inflammation. The biological effects of LTB4 are primarily mediated through the high-affinity LTB4 receptor, BLT1. Postoperative incisional pain is characterized by persistent acute pain at the site of tissue injury and is associated with local inflammation. Here, we compared the role of LTB4-BLT1 signaling in postoperative incisional pain between BLT1-knockout (BLT1KO) and wild-type (BLT1WT) mice. A planter incision model was developed, and mechanical pain hypersensitivity was determined using the von Frey test before and after incision. Local infiltration of neutrophils and inflammatory monocytes was quantified by flow cytometry. Inflammatory cytokine levels in the incised tissue were also determined. Mechanical pain hypersensitivity was significantly reduced in BLT1KO mice compared to BLT1WT mice at 2, 3, and 4 days after incision. LTB4 levels in the tissue at the incision site peaked 3 hours after the incision. Infiltrated neutrophils peaked 1 day after the incision in both BLT1KO and BLT1WT mice. The accumulation of inflammatory monocytes increased 1-3 days after the incision and was significantly more reduced in BLT1KO mice than in BLT1WT mice. In BLT1KO mice, Interleukin-1ß and Tumor Necrosis Factor-α levels 1 day after the incision were significantly lower than those of BLT1WT mice. Our data suggest that LTB4 is produced and activates its receptor BLT1 in the very early phase of tissue injury, and that LTB4-BLT1 signaling exacerbates pain responses by promoting local infiltration of inflammatory monocytes and cytokine production. Thus, LTB4-BLT1 signaling is a potential target for therapeutic intervention of acute and persistent pain induced by tissue injury.

Amino acid residues critical for endoplasmic reticulum export and trafficking of platelet-activating factor receptor.

Several residues are conserved in the transmembrane domains (TMs) of G-protein coupled receptors. Here we demonstrate that a conserved proline, Pro(247), in TM6 of platelet-activating factor receptor (PAFR) is required for endoplasmic reticulum (ER) export and trafficking after agonist-induced internalization. Alanine-substituted mutants of the conserved residues of PAFRs, including P247A, were retained in the ER. Because a PAFR antagonist, Y-24180, acted as a pharmacological chaperone to rescue ER retention, this retention is due to misfolding of PAFR. Methylcarbamyl (mc)-PAF, a PAFR agonist, did not increase the cell surface expression of P247A, even though another ER-retained mutant, D63A, was effectively trafficked. Signaling and accumulation of the receptors in the early endosomes were observed in the mc-PAF-treated P247A-expressing cells, suggesting that P247A was trafficked to the cell surface by mc-PAF, and thereafter disappeared from the surface due to aberrant trafficking, e.g. enhanced internalization, deficiency in recycling, and/or accelerated degradation. The aberrant trafficking was confirmed with a sortase-A-mediated method for labeling cell surface proteins. These results demonstrate that the conserved proline in TM6 is crucial for intracellular trafficking of PAFR.

AML1 enhances the expression of leukotriene B4 type-1 receptor in leukocytes.

Leukotriene B(4) type-1 receptor (BLT1), which plays a role in various inflammatory diseases, is exclusively expressed in peripheral leukocytes, which suggests that its expression is stringently regulated. However, the precise mechanism of BLT1 expression is not fully understood. Here we report that acute myeloid leukemia 1 (AML1/Runx1) is involved in the enhancement of BLT1 expression in leukocytes. In retinoic acid (RA)-stimulated human promyelocytic leukemia (HL-60) cells, the transcription of the BLT1 gene was found to be significantly activated. RA did not directly modulate the BLT1 promoter, suggesting enhancers in other loci. DNase I-hypersensitivity analyses revealed an activated region, termed AE-BLex, at the intron-I:exon-II boundary. AE-BLex acts as an enhancer for the BLT1 promoter and possesses 2 AML1 recognition sites. The importance of AML1 was determined using electrophoretic mobility shift assays, reporter assays, and knockdown experiments. We demonstrated that the enhancement of BLT1 expression during the RA-induced differentiation of HL-60 cells is due to a loosening of the chromatin structure around AE-BLex, which leads to the incremental binding of AML1. The AML1/AE-BLex complex was confirmed in other BLT1- expressing leukemia cell lines and human peripheral leukocytes. Thus, AML1 enhances BLT1 expression by binding to AE-BLex, which is accessible in leukocytes.

Protective role of the leukotriene B4 receptor BLT2 in murine inflammatory colitis.

BLT2 is a low-affinity leukotriene B(4) (LTB(4)) receptor that is activated by 12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT) and LTB(4). Despite the well-defined proinflammatory roles of BLT1, the in vivo functions of BLT2 remain elusive. To clarify the role of BLT receptors in intestinal inflammation, we assessed susceptibility to dextran sodium sulfate (DSS)-induced colitis in mice lacking either BLT1 or BLT2. BLT2(-/-) mice exhibited increased sensitivity to DSS as compared to wild-type and BLT1(-/-) mice, with more severe body weight loss and inflammation. Expression of inflammatory cytokines such as interferon (IFN)-γ, interleukin (IL)-1ß, and IL-6, chemokines such as CXC chemokine ligand 9 (CXCL9) and C-C motif chemokine 19 (CCL19), and metalloproteinases was highly up-regulated in the colons of DSS-treated BLT2(-/-) mice, and there was an enhanced accumulation of activated macrophages. Phosphorylation of the signal transducer and activator of transcription 3 (STAT3) was also markedly accelerated in the crypts of DSS-treated BLT2(-/-) mice. Madin-Darby canine kidney II (MDCKII) cells transfected with BLT2 exhibited enhanced barrier function as measured by transepithelial electrical resistance (TER) and FITC-dextran leakage through MDCK monolayers. Thus, BLT2 is expressed in colon cryptic cells and appears to protect against DSS-induced colitis, possibly by enhancing barrier function in epithelial cells of the colon. These novel results suggest a direct anti-inflammatory role of BLT2 that is distinct from the proinflammatory roles of BLT1.

Transcriptional regulation of human G2A in monocytes/ macrophages: involvement of c/EBPs, Runx and Pu.1.

G2 accumulation (G2A) is a G-protein coupled receptor, activated by several ligands and stimuli, such as lysophosphatidylcholine (LPC), extracellular low pH and oxidized phospholipids including 9- and 13-hydroxyoctadecadienoic acid, and has been implicated in mediating inflammatory process under oxidative conditions. Recently, it was demonstrated that G2A in monocytes/macrophages plays critical roles in atherosclerosis deterioration, and therefore its transcriptional regulation in monocytes/macrophages is of great interest. Here, we first confirmed the expression of human G2A (hG2A) in lymph nodes, spleen and peripheral blood leukocytes, including monocytes. Thereafter, transcription start site (TSS) of hG2A was determined by 5'-rapid amplification of cDNA ends analysis. In the course of the analysis, we found that two transcriptional variants, hG2A-a and -b, are produced by alternative splicing, resulting in the production of N-terminal different hG2A proteins with similar sensitivity to low pH and LPC. Using a monocytic cell line THP-1 as a model, transcription of hG2A was precisely examined, and we demonstrated that it is dependent both on the chromatin structure around TSS, and on the binding of the transcription factors (c/EBPalpha and beta, Runx1 and Pu.1) to their cis-elements, located at the core promoter just upstream of TSS.

Specific ligands as pharmacological chaperones: The transport of misfolded G-protein coupled receptors to the cell surface.

In the endoplasmic reticulum (ER), quality control mechanisms distinguish between correctly and incorrectly folded structures to ensure that aberrant proteins are not processed along the secretory pathway. Numerous studies have demonstrated the functional rescue of ER-retained, aberrant proteins by small membrane permeable molecules called pharmacological chaperones. Pharmacological chaperones can bind to misfolded proteins, including G-protein coupled receptors (GPCRs), and promote their correct folding and export from the ER. Recently, common structural features of GPCRs have been uncovered, including the eighth helical domain in the C-terminal tail and conserved residues in the transmembrane domains. However, little is known about the importance of these features in signaling and intracellular trafficking, because receptors deficient in these domains are likely retained in the ER due to misfolding. In this review, we summarize the current knowledge about the requirement of these consensus domains and amino acid residues for the passing through the quality control of the ER. Furthermore, we propose the utilization of membrane permeable ligands for the transport of their cognate, ER-retained GPCRs to the cell surface. The chaperone activity of these ligands allows us to perform functional analyses of the structure-deficient receptors after their trafficking to the cell surface.

Helix 8 of leukotriene B4 type-2 receptor is required for the folding to pass the quality control in the endoplasmic reticulum.

Many G protein-coupled receptors (GPCRs) possess a putative cytoplasmic helical domain, termed helix 8 (H8), at the proximal region of the C-terminal tail. However, the significance of this domain is not fully understood. Here, we demonstrate the requirement of H8 for the proper folding of GPCRs for passage through the quality control in the endoplasmic reticulum (ER). In the human leukotriene B(4) type-2 receptor (hBLT2), lack of H8 led to an accumulation of the receptor (hBLT2/DeltaH8) in the ER. Similar results were obtained in two representative human GPCRs, dopamine type-1 and lysophosphatidic acid type-2 receptors, which were engineered to lack H8. Treatment with the several ligands, which act as pharmacological chaperones, facilitated the surface expression of hBLT2/DeltaH8. The surface-trafficked hBLT2/DeltaH8 exhibited an agonist-evoked increase in Ca(2+), demonstrating that H8 is not critical for ligand binding and activation of coupled G proteins. Thus, these results suggest that the H8 region of hBLT2 plays an important role in transport-competent receptor folding.