Prostaglandin E2 stimulates cAMP signaling and resensitizes human leukemia cells to glucocorticoid-induced cell death.

Glucocorticoid (GC) resistance remains a clinical challenge in pediatric acute lymphoblastic leukemia where response to GC is a reliable prognostic indicator. To identify GC resistance pathways, we conducted a genome-wide, survival-based, short hairpin RNA screen in murine T-cell acute lymphoblastic leukemia (T-ALL) cells. Genes identified in the screen interfere with cyclic adenosine monophosphate (cAMP) signaling and are underexpressed in GC-resistant or relapsed ALL patients. Silencing of the cAMP-activating Gnas gene interfered with GC-induced gene expression, resulting in dexamethasone resistance in vitro and in vivo. We demonstrate that cAMP signaling synergizes with dexamethasone to enhance cell death in GC-resistant human T-ALL cells. We find the E prostanoid receptor 4 expressed in T-ALL samples and demonstrate that prostaglandin E2 (PGE2) increases intracellular cAMP, potentiates GC-induced gene expression, and sensitizes human T-ALL samples to dexamethasone in vitro and in vivo. These findings identify PGE2 as a target for GC resensitization in relapsed pediatric T-ALL.

Functional evidence for biased inhibition of G protein signaling by YM-254890 in human coronary artery endothelial cells.

Small molecular chemicals targeting individual subtype of G proteins including Gs, Gi/o and Gq has been lacking, except for pertussis toxin being an established selective peptide inhibitor of the Gi/o protein. Recently, a cyclic depsipeptide compound YM-254890 isolated from culture broth of Chromobacterium sp. was reported as a selective inhibitor for the Gq protein by blocking GDP exchange of GTP on the α subunit of Gq complex. However, functional selectivity of YM-254890 towards various G proteins was not fully characterized, primarily due to its restricted availability before 2017. Here, using human coronary artery endothelial cells as a model, we performed a systemic pharmacological evaluation on the functional selectivity of YM-254890 on multiple G protein-mediated receptor signaling. First, we confirmed that YM-254890, at 30 nM, abolished UTP-activated P2Y2 receptor-mediated Ca2+ signaling and ERK1/2 phosphorylation, indicating its potent inhibition on the Gq protein. However, we unexpectedly found that YM-254890 also significantly suppressed cAMP elevation and ERK1/2 phosphorylation induced by multiple Gs-coupled receptors including ß2-adrenegic, adenosine A2 and PGI2 receptors. Surprisingly, although YM-254890 had no impact on CXCR4/Gi/o protein-mediated suppression of cAMP production, it abolished ERK1/2 activation. Further, no cellular toxicity was observed for YM-254890, and it neither affected A23187- or thapsigargin-induced Ca2+ signaling, nor forskolin-induced cAMP elevation and growth factor-induced MAPK signaling. We conclude that YM-254890 is not a selective inhibitor for Gq protein; instead, it acts as a broad-spectrum inhibitor for Gq and Gs proteins and exhibits a biased inhibition on Gi/o signaling, without affecting non-GPCR-mediated cellular signaling.

A novel GNAS-mutated human induced pluripotent stem cell model for understanding GNAS-mutated tumors.

A missense mutation of the guanine nucleotide binding protein alpha stimulating activity polypeptide 1 (GNAS) gene, typically Arg201Cys or Arg201His (R201H/R201C), leads to constitutive activation of the Gsα-cyclic AMP (cAMP) signaling pathway that causes several diseases. However, no germline mutations of GNAS have been identified to date, likely due to their lethality, and no robust human cell models have been generated. Therefore, the aim of this study was to generate GNAS-mutated disease-specific induced pluripotent stem cells as a model for these diseases. We then analyzed the functionality of this induced pluripotent stem cell model and differentiated epithelial cells. We generated disease-specific induced pluripotent stem cells by introducing a mutation in GNAS with the clustered regularly interspaced short palindromic repeats (CRISPR) nickase method, which has lower off-target effects than the conventional CRISPR/Cas9 method. We designed the target vector to contain the R201H mutation in GNAS, which was transfected into human control induced pluripotent stem cells (Nips-B2) by electroporation. We confirmed the establishment of GNASR201H-mutated (GNASR201H/+) induced pluripotent stem cells that exhibited a pluripotent stem cell phenotype. We analyzed the effect of the mutation on cAMP production, and further generated teratomas for immunohistochemical analysis of the luminal epithelial structure. GNAS-mutated induced pluripotent stem cells showed significantly higher levels of intracellular cAMP, which remained elevated state for a long time upon hormonal stimulation with parathyroid hormone or adrenocorticotropic hormone. Immunohistochemical analysis revealed that several mucins, including MUC1, 2, and MUC5AC, are expressed in cytokeratin 18 (CK18)-positive epithelial cells. However, we found few CK18-positive cells in mutated induced pluripotent stem cell-derived teratoma tissues, and reduced MUCINs expression in mutated epithelial cells. There was no difference in CDX2 expression; however, mutated epithelial cells were positive for CEA and CA19-9 expression. GNASR201H-mutated induced pluripotent stem cells and GNASR201H-mutated epithelial cells have distinct phenotypic and differentiation characteristics. We successfully established GNASR201H-mutated human induced pluripotent stem cells with increased cAMP production. Considering the differentiation potential of induced pluripotent stem cells, these cells will be useful as a model for elucidating the pathological mechanisms of GNAS-mutated diseases.

Methylglyoxal Impairs ß2-Adrenoceptor-Mediated Vasodilatory Mechanisms in Rat Retinal Arterioles.

Methylglyoxal, a highly reactive dicarbonyl compound, is formed as a by-product of glycolysis and plays an important role in the pathogenesis of diabetic complications, including diabetic retinopathy. However, it remains to be determined how methylglyoxal affects the regulatory mechanisms of retinal blood flow. In this study, we examined the effects of methylglyoxal on ß2-adrenoceptor-mediated vasodilatory mechanisms in rat retinal arterioles. The retinal vasodilator responses were assessed by measuring the diameter of retinal arterioles in the fundus images. Intravitreal injection of methylglyoxal significantly diminished the vasodilation of retinal arterioles induced by the ß2-adrenoceptor agonist salbutamol. The vasodilator effect of BMS-191011, a large-conductance Ca2+-activated K+ (BKCa) channel opener, on retinal arterioles was also attenuated by methylglyoxal. In contrast, methylglyoxal had no significant effect on retinal vasodilator response to forskolin. Methylglyoxal attenuated retinal vasodilator response to salbutamol under blockade of BKCa channels with iberiotoxin, an inhibitor of the channels. These results suggest that methylglyoxal attenuates ß2-adrenoceptor-mediated retinal vasodilation by impairing the coupling of the ß2-adrenoceptor to the guanine nucleotide-binding protein (Gs protein) and the function of the BKCa channel. Increased methylglyoxal in the eyes may contribute to the impairment of regulatory mechanisms of retinal blood flow in patients with diabetic retinopathy.

Proinflammatory switch from Gαs to Gαi signaling by Glucagon-like peptide-1 receptor in murine splenic monocyte following burn injury.

OBJECTIVE: Glucagon-like peptide-1 (GLP-1)-based therapy via G protein-coupled receptor (GPCR) GLP-1R, to attenuate hyperglycemia in critical care has attracted great attention. However, the exaggerated inflammation by GLP-1R agonist, Exendin-4, in a mouse model of burn injury was quite unexpected. Recent studies found that GPCR might elicit proinflammatory effects by switching from Gαs to Gαi signaling in the immune system. Thus, we aimed to investigate the possible Gαs to Gαi switch in GLP-1R signaling in monocyte following burn injury. MATERIALS AND METHODS: Splenic monocytes from sham and burn mice 24 h following burn injury were treated with consecutive doses of Exendin-4 alone or in combination with an inhibitor of Gαi signaling (pertussis toxin, PTX), or a blocker of protein kinase A (H89). Cell viability was assessed by CCK-8, and the supernatant was collected for cytokine measurement by ELISA. Intracellular cAMP level, phosphorylated PKA activity, and nuclear NF-κB p65 were determined by ELISA, ERK1/2 activation was analyzed by Western blot. The expression of GLP-1R downstream molecules, Gαs, Gαi and G-protein coupled receptor kinase 2 (GRK2) were examined by immunofluorescence staining and Western blot. RESULTS: Exendin-4 could inhibit the viability of monocyte from sham rather than burn mice. Unexpectedly, it could also reduce TNF-α secretion from sham monocyte while increase it from burn monocyte. The increased secretion of TNF-α by Exendin-4 from burn monocyte could be reversed by pretreatment of PTX or H89. Accordingly, Exendin-4 could stimulates cAMP production dose dependently from sham instead of burn monocyte. However, the blunt cAMP production from burn monocyte was further suppressed by pretreatment of PTX or H89 after 6-h incubation. Nevertheless, phosphorylated PKA activity was significantly increased by low dose of Exendin-4 in sham monocyte, by contrast, it was enhanced by high dose of Exendin-4 in burn monocyte after 1-h incubation. Following Exendin-4 treatment for 2 h ex vivo, total nuclear NF-κB and phosphorylated NF-κB activity, as well as cytoplasmic pERK1/2 expressions were reduced in sham monocyte, however, only pERK1/2 was increased by Exendin-4 in burn monocytes. Moreover, reduced expressions of GLP-1R, GRK-2 and Gαs in contrast with increased expression of Gαi were identified in burn monocyte relative to sham monocyte. CONCLUSIONS: This study presents an unexpected proinflammatory switch from Gαs to Gαi signaling in burn monocyte, which promotes ERK1/2 and NF-κB activation and the downstream TNF-α secretion. This phenomenon is most probably responsible for proinflammatory response evoked by Gαs agonist Exendin-4 following burn injury.

Inhibition of fibrous dysplasia via blocking Gsα with suramin sodium loaded with an alendronate-conjugated polymeric drug delivery system.

Suramin sodium (SS), which can directly inhibit the committed step of Gsα activation, seems to be a promising drug for treating fibrous dysplasia (FD). Therefore, how to efficiently deliver SS to the lesion site becomes an urgent problem to be solved. Here a bone-targeted and pH-sensitive drug delivery system was constructed to deliver SS for treating FD with high efficiency. The novel type of bone-targeted cationic hyperbranched poly(amine-ester) (HBPAE) was synthesized by the proton-transfer polymerization of triethanolamine and glycidyl methacrylate, followed by surface carboxyl-modification and then conjugation of an alendronate (ALE) bone-targeting moiety. The resultant Suc-HBPAE-ALE formed nanoparticles in aqueous solution, and SS could be encapsulated into the Suc-HBPAE-ALE nanoparticles via electrostatic attraction. The dynamic light scattering (DLS) and transmission electron microscopy (TEM) assays showed that the SS-loaded nanoparticles had a spherical morphology with a mean diameter of 65 nm. The strong affinity of Suc-HBPAE-ALE nanoparticles to bone was verified by the hydroxyapatite (HA) adsorbing experiment. The therapeutic potential of the SS-loaded Suc-HBPAE-ALE nanoparticles was evaluated via the methylthiazoletetrazolium (MTT) assay and flow cytometry (FCM) analysis against FD cells. The experimental results indicated that the SS-loaded Suc-HBPAE-ALE nanoparticles were a highly promising drug delivery system with high efficiency for inhibiting the proliferation of diseased FD cells.

Chronic loss of noradrenergic tone produces ß-arrestin2-mediated cocaine hypersensitivity and alters cellular D2 responses in the nucleus accumbens.

Cocaine blocks plasma membrane monoamine transporters and increases extracellular levels of dopamine (DA), norepinephrine (NE) and serotonin (5-HT). The addictive properties of cocaine are mediated primarily by DA, while NE and 5-HT play modulatory roles. Chronic inhibition of dopamine ß-hydroxylase (DBH), which converts DA to NE, increases the aversive effects of cocaine and reduces cocaine use in humans, and produces behavioral hypersensitivity to cocaine and D2 agonism in rodents, but the underlying mechanism is unknown. We found a decrease in ß-arrestin2 (ßArr2) in the nucleus accumbens (NAc) following chronic genetic or pharmacological DBH inhibition, and overexpression of ßArr2 in the NAc normalized cocaine-induced locomotion in DBH knockout (Dbh -/-) mice. The D2/3 agonist quinpirole decreased excitability in NAc medium spiny neurons (MSNs) from control, but not Dbh -/- animals, where instead there was a trend for an excitatory effect. The Gαi inhibitor NF023 abolished the quinpirole-induced decrease in excitability in control MSNs, but had no effect in Dbh -/- MSNs, whereas the Gαs inhibitor NF449 restored the ability of quinpirole to decrease excitability in Dbh -/- MSNs, but had no effect in control MSNs. These results suggest that chronic loss of noradrenergic tone alters behavioral responses to cocaine via decreases in ßArr2 and cellular responses to D2/D3 activation, potentially via changes in D2-like receptor G-protein coupling in NAc MSNs.

EFFECT OF PEPTIDOGLYCANE OF STAPHYLOCOCCUS AUREUS CELL WALL ON THE MECHANISM OF REGULATION OF CONTRACTILE ACTIVITY OF RAT MYOMETRIUM BY ADENYLATE CYCLASE SYSTEM.

The revue deals with the role of each component of adenylate cyclase regulatory system in the rat myometrial contractile activity modulation by the peptidoglycane of Staphylococcus aureus. Noradrenalin and salbutamol were used to investigate peptidoglycane impact on the myometrial ß-adrenergic receptors. It was shown that inhibited by these substances myometrial contractility increased to the initial level after peptidoglycane application. The same effect we observed under the cAMP level elevation by forscolin. Peptidoglycan' s ability to strengthen contractions was inhibited by the 8-brom-cAMP and papaverine application. Stimulation of Gs-protein by the cholera toxin didn't influence on the peptidoglycane effect while the blocking of Gi/o-protein by the pertussis toxin caused stopping it's manifestation. We concluded that the modulating effect of peptidoglycane implemented via Gi/o-protein activation, which causes adenilatcyclase desensitization.

A Girl With Beckwith-Wiedemann Syndrome and Pseudohypoparathyroidism Type 1B Due to Multiple Imprinting Defects.

CONTEXT: Several patients with Beckwith-Wiedemann Syndrome (BWS) with multiple imprinting defects found by genetic analysis have been described. However, only two cases have been described with both genetic and clinical signs and symptoms of multiple diseases caused by imprinting defects. CASE DESCRIPTION: The girl in this case presented at the age of 6 months with morbid obesity (body mass index, +7.5 SDS) and a large umbilical hernia. Genetic analysis showed BWS (hypomethylation of the KCNQ1OT1 gene). Calcium homeostasis was normal, and she had no signs of Albright hereditary osteodystrophy. At the age of 10 years, she presented with fatigue, and laboratory analyses showed marked hypocalcemia with signs of PTH resistance, but without evidence for Albright hereditary osteodystrophy, thus suggesting pseudohypoparathyroidism type 1B. Consistent with this diagnosis, methylation analysis of the GNAS complex revealed hypomethylation (about 20%) of the GNAS exon 1A, NESPAS, and GNASXL loci and hypermethylation (100% methylation) of the NESP locus. CONCLUSIONS: Imprinting defects at several different loci can occur in some patients, thus causing multiple different diseases. Symptoms of pseudohypoparathyroidism type 1B may be absent at diagnosis of BWS, yet prolonged subclinical hypocalcemia and/or hyperphosphatemia can have negative consequences (eg, intracerebral calcifications, myocardial dysfunction). We therefore suggest that patients with an imprinting disorder should be monitored for elevations in PTH, and epigenetic analysis of the GNAS complex locus should be considered.

ß2 adrenergic receptor activation governs cardiac repolarization and arrhythmogenesis in a guinea pig model of heart failure.

ß2-AR activation increases the risk of sudden cardiac death (SCD) in heart failure (HF) patients. Non-selective ß-AR blockers have greater benefits on survival than selective ß1-AR blockers in chronic HF patients, indicating that ß2-AR activation contributes to SCD in HF. This study investigated the role of ß2-AR activation on repolarization and ventricular arrhythmia (VA) in the experimental HF model. The guinea pig HF was induced by descending aortic banding. The effective refractoriness period (ERP), corrected QT (QTc) and the incidence of VA were examined using Langendorff and programmed electrical stimulation. Ikr and APD were recorded by the whole cell patch clamp. Selective ß2-AR agonist salbutamol significantly increased the incidence of VA, prolonged QTc and shortened ERP. These effects could be prevented by the selective ß2-AR antagonist, ICI118551. Salbutamol prolonged APD90 and reduced Ikr in guinea pig HF myocytes. The antagonists of cAMP (Rp-cAMP) and PKA (KT5720) attenuated Ikr inhibition and APD prolongation induced by salbutamol. However, the antagonists of Gi protein (PTX) and PDE III (amrinone) showed opposite effects. This study indicates that ß2-AR activation increases the incidence of VA in the experimental HF model via activation of Gs/cAMP/PKA and/or inhibition of Gi/PDE pathways.

Molecular cloning and expression analysis of the GNAS gene in pig and porcine fibroblast cells.

The Alpha subunit of the stimulatory guanine nucleotide-binding protein (GNAS) is a complex imprinted gene. The major product of the GNAS gene is the α-subunit of the guanine nucleotide-binding protein (Gas), which plays a key role in multiple signal transduction pathways. Gas is required for the production of the receptor-stimulated intracellular cyclic adenosine monophosphate (cAMP). It has been demonstrated that an increase in the concentration of the intracellular second messenger cAMP promotes apoptosis in different tumor entities. Mutations of GNAS have also been identified in many tumors. This study aimed to investigate the expression pattern and the apoptosis effect in fibroblast cells for porcine GNAS. The results show that GNAS mRNA was detected in a wide range of tissues, especially in the longissimus dorsi muscle and thyroid gland. The developmental pattern of GNAS mRNA in the thyroid gland of Jinhua pigs was then examined; however, there was no significant difference (P > 0.05) among any of the stages. GNAS gene expression was relatively stable in the thyroid gland during the entire growth and development process. The developmental pattern of GNAS mRNA in the longissimus dorsi muscle was significantly different among the various developmental stages (P < 0.01). GNAS mRNA was strongly expressed at 60 days, 90 days, and 150 days after birth, whereas the expression level was very low during the embryo stages. Target RNA interference of GNAS in porcine fibroblast cells leads to lower mRNA expression of Bcl-2, Fas, and Caspase-3, which are recognized as apoptosis related markers.

A high throughput screening assay system for the identification of small molecule inhibitors of gsp.

Mis-sense mutations in the α-subunit of the G-protein, Gsα, cause fibrous dysplasia of bone/McCune-Albright syndrome. The biochemical outcome of these mutations is constitutively active Gsα and increased levels of cAMP. The aim of this study was to develop an assay system that would allow the identification of small molecule inhibitors specific for the mutant Gsα protein, the so-called gsp oncogene. Commercially available Chinese hamster ovary cells were stably transfected with either wild-type (WT) or mutant Gsα proteins (R201C and R201H). Stable cell lines with equivalent transfected Gsα protein expression that had relatively lower (WT) or higher (R201C and R201H) cAMP levels were generated. These cell lines were used to develop a fluorescence resonance energy transfer (FRET)-based cAMP assay in 1536-well microplate format for high throughput screening of small molecule libraries. A small molecule library of 343,768 compounds was screened to identify modulators of gsp activity. A total of 1,356 compounds with inhibitory activity were initially identified and reconfirmed when tested in concentration dose responses. Six hundred eighty-six molecules were selected for further analysis after removing cytotoxic compounds and those that were active in forskolin-induced WT cells. These molecules were grouped by potency, efficacy, and structural similarities to yield 22 clusters with more than 5 of structurally similar members and 144 singleton molecules. Seven chemotypes of the major clusters were identified for further testing and analyses.

Sensitivity to theta-burst timing permits LTP in dorsal striatal adult brain slice.

Long-term potentiation (LTP) of excitatory afferents to the dorsal striatum likely occurs with learning to encode new skills and habits, yet corticostriatal LTP is challenging to evoke reliably in brain slice under physiological conditions. Here we test the hypothesis that stimulating striatal afferents with theta-burst timing, similar to recently reported in vivo temporal patterns corresponding to learning, evokes LTP. Recording from adult mouse brain slice extracellularly in 1 mM Mg(2+), we find LTP in dorsomedial and dorsolateral striatum is preferentially evoked by certain theta-burst patterns. In particular, we demonstrate that greater LTP is produced using moderate intraburst and high theta-range frequencies, and that pauses separating bursts of stimuli are critical for LTP induction. By altering temporal pattern alone, we illustrate the importance of burst-patterning for LTP induction and demonstrate that corticostriatal long-term depression is evoked in the same preparation. In accord with prior studies, LTP is greatest in dorsomedial striatum and relies on N-methyl-d-aspartate receptors. We also demonstrate a requirement for both Gq- and Gs/olf-coupled pathways, as well as several kinases associated with memory storage: PKC, PKA, and ERK. Our data build on previous reports of activity-directed plasticity by identifying effective values for distinct temporal parameters in variants of theta-burst LTP induction paradigms. We conclude that those variants which best match reports of striatal activity during learning behavior are most successful in evoking dorsal striatal LTP in adult brain slice without altering artificial cerebrospinal fluid. Future application of this approach will enable diverse investigations of plasticity serving striatal-based learning.

Biphasic Erk1/2 activation sequentially involving Gs and Gi signaling is required in beta3-adrenergic receptor-induced primary smooth muscle cell proliferation.

The beta3 adrenergic receptor (B3-AR) reportedly induces cell proliferation, but the signaling pathways that were proposed, involving either Gs or Gi coupling, remain controversial. To further investigate the role of G protein coupling in B3-AR induced proliferation, we stimulated primary human myometrial smooth muscle cells with SAR150640 (B3-AR agonist) in the absence or presence of variable G-protein inhibitors. Specific B3-AR stimulation led to an Erk1/2 induced proliferation. We observed that the proliferative effects of B3-AR require two Erk1/2 activation peaks (the first after 3min, the second at 8h). Erk1/2 activation at 3min was mimicked by forskolin (adenylyl-cyclase activator), and was resistant to pertussis toxin (Gi inhibitor), suggesting a Gs protein signaling. This first signaling also required the downstream Gs signaling effectors PKA and Src. However, Erk1/2 activation at 8h turned out to be pertussis toxin-dependent, and PKA-independent, indicating a Gi signaling pathway in which Src and PI3K were required. The pharmacological inhibition of both the Gs and Gi pathway abolished B3-AR-induced proliferation. Altogether, these data indicate that B3-AR-induced proliferation depends on the biphasic activation of Erk1/2 sequentially induced by the Gs/PKA/Src and Gi/Src/PI3K signaling pathways.

SASH1 regulates melanocyte transepithelial migration through a novel Gαs-SASH1-IQGAP1-E-Cadherin dependent pathway.

One important function of melanocytes (MCs) is to produce and transfer melanin to neighbouring keratinocytes (KCs) to protect epithelial cells from UV radiation. The mechanisms regulating the specific migration and localisation of the MC lineage remain unknown. We have found three heterozygous mutations that cause amino acid substitutions in the SASH1 gene in individuals with a kind of dyschromatosis. In epidermal tissues from an affected individual, we observed the increased transepithelial migration of melanocytes. Functional analyses indicate that these SASH1 mutations not only cause the increased migration of A375 cells and but also induce intensive bindings with two novel cell adhesion partners IQGAP1 and Gαs. Further, SASH1 mutations induce uniform loss of E-Cadherin in human A375 cells. Our findings suggest a new scaffold protein SASH1 to regulate IQGAP1-E-Cadherin signalling and demonstrate a novel crosstalking between GPCR signalling, calmodulin signalling for the modulation of MCs invasion.

Caveolin-1 and lipid microdomains regulate Gs trafficking and attenuate Gs/adenylyl cyclase signaling.

Lipid rafts and caveolae are specialized membrane microdomains implicated in regulating G protein-coupled receptor signaling cascades. Previous studies have suggested that rafts/caveolae may regulate beta-adrenergic receptor/Galpha(s) signaling, but underlying molecular mechanisms are largely undefined. Using a simplified model system in C6 glioma cells, this study disrupts rafts/caveolae using both pharmacological and genetic approaches to test whether caveolin-1 and lipid microdomains regulate G(s) trafficking and signaling. Lipid rafts/caveolae were disrupted in C6 cells by either short-term cholesterol chelation using methyl-beta-cyclodextrin or by stable knockdown of caveolin-1 and -2 by RNA interference. In imaging studies examining Galpha(s)-GFP during signaling, stimulation with the betaAR agonist isoproterenol resulted in internalization of Galpha(s)-GFP; however, this trafficking was blocked by methyl-beta-cyclodextrin or by caveolin knockdown. Caveolin knockdown significantly decreased Galpha(s) localization in detergent insoluble lipid raft/caveolae membrane fractions, suggesting that caveolin localizes a portion of Galpha(s) to these membrane microdomains. Methyl-beta-cyclodextrin or caveolin knockdown significantly increased isoproterenol or thyrotropin-stimulated cAMP accumulation. Furthermore, forskolin- and aluminum tetrafluoride-stimulated adenylyl cyclase activity was significantly increased by caveolin knockdown in cells or in brain membranes obtained from caveolin-1 knockout mice, indicating that caveolin attenuates signaling at the level of Galpha(s)/adenylyl cyclase and distal to GPCRs. Taken together, these results demonstrate that caveolin-1 and lipid microdomains exert a major effect on Galpha(s) trafficking and signaling. It is suggested that lipid rafts/caveolae are sites that remove Galpha(s) from membrane signaling cascades and caveolins might dampen globally Galpha(s)/adenylyl cyclase/cAMP signaling.

Bradykinin inhibits the transient outward K+ current in mouse Schwann cells via the cAMP/PKA pathway.

Bradykinin (BK) is an endogenous peptide with diverse biological actions and is considered to be an important mediator of the inflammatory response in both the peripheral and the central nervous systems. BK has attracted recent interest as a potential mediator of K(+) conductance, Cl(-) channels, and Ca(2+)-activated K(+) channels. However, few reports have associated BK with the voltage-gated K(+) current. In this study, we demonstrated that BK suppressed the transient outward potassium current (I(A)) in mouse Schwann cells using whole cell recording techniques. At a concentration of 0.1 muM to 5 muM, BK reversibly inhibited I(A) in a dose-dependent manner with the modulation of steady-state activation and inactivation properties. The effect of BK on I(A) current was abolished after preincubation with a B(2) receptor antagonist but could not be eliminated by B(1) receptor antagonist. Intracellular application of GTP-gammaS induced an irreversible decrease in I(A), and the inhibition of G(s) using NF449 provoked a gradual augmentation in I(A) and eliminated the BK-induced effect on I(A,) while the G(i)/(o) antagonist NF023 did not. The application of forskolin or dibutyryl-cAMP mimicked the inhibitory effect of BK on I(A) and abolished the BK-induced effect on I(A). H-89, an inhibitor of PKA, augmented I(A) amplitude and completely eliminated the BK-induced inhibitory effect on I(A). In contrast, activation of PKC by PMA augmented I(A) amplitude. A cAMP assay revealed that BK significantly increased intracellular cAMP level. It is therefore concluded that BK inhibits the I(A) current in Schwann cells by cAMP/PKA-dependent pathways via activation of the B(2) receptor.

Intracellular signal transduction by the extracellular calcium-sensing receptor of Xenopus melanotrope cells.

The extracellular calcium-sensing receptor (CaR) is expressed in various types of endocrine pituitary cell, but the intracellular mechanism this G protein-coupled receptor uses in these cells is not known. In the present study we investigated possible intracellular signal transduction pathway(s) utilized by the CaR of the endocrine melanotrope cells in the intermediate pituitary lobe of the South African-clawed toad Xenopus laevis. For this purpose, the effects of various pharmacological agents on CaR-evoked secretion of radiolabeled secretory peptides from cultured melanotrope cells were assessed. CaR-evoked secretion, induced by the potent CaR agonist L-phenylalanine (L-Phe), could not be inhibited by cholera toxin, nor by NPC-15437 and PMA, indicating that neither G(s)/PKA nor G(q)/PKC pathways are involved. However, pertussis toxin (G(i/o) protein inhibitor), genistein (inhibitor of PTKs), wortmannin/LY-294002 (PI3-K inhibitor) and U-0126 (inhibitor of extracellular signal-regulated kinase, ERK) all substantially inhibited CaR-evoked secretion, indicating that the Xenopus melanotrope cell possesses a PI3-K/MAPK system that plays some role in CaR-signaling. Since no direct effect of L-Phe on ERK phosphorylation could be shown it is concluded that CaR must act primarily through another, still unknown, signaling pathway in Xenopus melanotropes. Our results indicate that the PI3-K/MAPK system has a facilitating effect on CaR-induced secretion, possibly by sensitizing the CaR.

An inhibitory compound against the interaction between G alpha(s) and the third intracellular loop region of serotonin receptor subtype 6 (5-HT6) disrupts the signaling pathway of 5-HT6.

Serotonin receptor subtype 6 (5-HT(6)) is a neurotransmitter receptor, which is involved in various brain functions such as memory and mood. It mediates signaling via the interaction with a stimulatory G-protein. Especially, the third intracellular loop (iL3) of 5-HT(6) and the alpha subunit of stimulatory G protein (G alpha(s)) are responsible for the signaling process of 5-HT(6). Chemical compounds that could inhibit the interaction between the iL3 region of 5-HT(6) and G alpha(s) were screened from a chemical library consisted of 5,600 synthetic compounds. One of the identified compounds bound to G alpha(s) and effectively blocked the interaction between G alpha(s) and the iL3 region of 5-HT(6). The identified compound was further shown to reduce the serotonin-induced accumulation of cAMP in 293T cells transformed with 5-HT(6) cDNA. It also lowered the Ca(2+) efflux induced by serotonin in cells expressing 5-HT(6) and chimeric G alpha(s5/q). These results indicate that the interaction between the iL3 of 5-HT(6) and G alpha(s) can be exploited for screening of regulatory compounds against the signaling pathway of 5-HT(6).

An inhibitory compound against the interaction between Galpha(s) and the third intracellular loop region of serotonin receptor subtype 6 (5-HT(6)) disrupts the signaling pathway of 5-HT(6)

Serotonin receptor subtype 6 (5-HT(6)) is a neurotransmitter receptor, which is involved in various brain functions such as memory and mood. It mediates signaling via the interaction with a stimulatory G-protein. Especially, the third intracellular loop (iL3) of 5-HT(6) and the alpha subunit of stimulatory G protein (Galpha(s)) are responsible for the signaling process of 5-HT(6). Chemical compounds that could inhibit the interaction between the iL3 region of 5-HT(6) and Galpha(s) were screened from a chemical library consisted of 5,600 synthetic compounds. One of the identified compounds bound to Galpha(s) and effectively blocked the interaction between Galpha(s) and the iL3 region of 5-HT(6). The identified compound was further shown to reduce the serotonin-induced accumulation of cAMP in 293T cells transformed with 5-HT(6) cDNA. It also lowered the Ca2+ efflux induced by serotonin in cells expressing 5-HT(6) and chimeric Galpha(s5/q). These results indicate that the interaction between the iL3 of 5-HT(6) and Galpha(s) can be exploited for screening of regulatory compounds against the signaling pathway of 5-HT(6).