Agonist-induced activation of histamine H3 receptor signals to extracellular signal-regulated kinases 1 and 2 through PKC-, PLD-, and EGFR-dependent mechanisms

Histamine H3 Receptor Isoforms: Insights from Alternative Splicing to Functional Complexity

Alternative splicing significantly enhances the diversity of the G protein-coupled receptor (GPCR) family, including the histamine H3 receptor (H3R). This post-transcriptional modification generates multiple H3R isoforms with potentially distinct pharmacological and physiological profiles. H3R is primarily involved in the presynaptic inhibition of neurotransmitter release in the central nervous system. Despite the approval of pitolisant for narcolepsy (Wakix®) and daytime sleepiness in adults with obstructive sleep apnea (Ozawade®) and ongoing clinical trials for other H3R antagonists/inverse agonists, the functional significance of the numerous H3R isoforms remains largely enigmatic. Recent publicly available RNA sequencing data have confirmed the expression of multiple H3R isoforms in the brain, with some isoforms exhibiting unique tissue-specific distribution patterns hinting at isoform-specific functions and interactions within neural circuits. In this review, we discuss the complexity of H3R isoforms with a focus on their potential roles in central nervous system (CNS) function. Comparative analysis across species highlights evolutionary conservation and divergence in H3R splicing, suggesting species-specific regulatory mechanisms. Understanding the functionality of H3R isoforms is crucial for the development of targeted therapeutics. This knowledge will inform the design of more precise pharmacological interventions, potentially enhancing therapeutic efficacy and reducing adverse effects in the treatment of neurological and psychiatric disorders.

Histamine and receptors in neuroinflammation: Their roles on neurodegenerative diseases

Histamine, an auto-reactive substance and mediator of inflammation, is synthesized from histidine through the action of histidine decarboxylase (HDC). It primarily acts on histamine receptors in the central nervous system (CNS). Increasing evidence suggests that histamine and its receptors play a crucial role in neuroinflammation, thereby modulating the pathology of neurodegenerative diseases. Recent studies have demonstrated that histamine regulates the phenotypic switching of microglia and astrocytes, inhibits the production of pro-inflammatory cytokines, and alleviates inflammatory responses. In the CNS, our research group has also found that histamine and its receptors are involved in regulating inflammatory responses and play a central role in ameliorating chronic neuroinflammation in neurodegenerative diseases. In this review, we will discuss the role of histamine and its receptors in neuroinflammation associated with neurodegenerative diseases, potentially providing a novel therapeutic target for the treatment of chronic neuroinflammation-related neurodegenerative diseases in clinical settings.

Postsynaptic histamine H3 receptors in ventral basal forebrain cholinergic neurons modulate contextual fear memory

Overly strong fear memories can cause pathological conditions. Histamine H3 receptor (H3R) has been viewed as an optimal drug target for CNS disorders, but its role in fear memory remains elusive. We find that a selective deficit of H3R in cholinergic neurons, but not in glutamatergic neurons, enhances freezing level during contextual fear memory retrieval without affecting cued memory. Consistently, genetically knocking down H3R or chemogenetically activating cholinergic neurons in the ventral basal forebrain (vBF) mimics this enhanced fear memory, whereas the freezing augmentation is rescued by re-expressing H3R or chemogenetic inhibition of vBF cholinergic neurons. Spatiotemporal regulation of H3R by a light-sensitive rhodopsin-H3R fusion protein suggests that postsynaptic H3Rs in vBF cholinergic neurons, but not presynaptic H3Rs of cholinergic projections in the dorsal hippocampus, are responsible for modulating contextual fear memory. Therefore, precise modulation of H3R in a cell-type- and subcellular-location-specific manner should be explored for pathological fear memory.

JNK Cascade-Induced Apoptosis-A Unique Role in GqPCR Signaling

The response of cells to extracellular signals is mediated by a variety of intracellular signaling pathways that determine stimulus-dependent cell fates. One such pathway is the cJun-N-terminal Kinase (JNK) cascade, which is mainly involved in stress-related processes. The cascade transmits its signals via a sequential activation of protein kinases, organized into three to five tiers. Proper regulation is essential for securing a proper cell fate after stimulation, and the mechanisms that regulate this cascade may involve the following: (1) Activatory or inhibitory phosphorylations, which induce or abolish signal transmission. (2) Regulatory dephosphorylation by various phosphatases. (3) Scaffold proteins that bring distinct components of the cascade in close proximity to each other. (4) Dynamic change of subcellular localization of the cascade's components. (5) Degradation of some of the components. In this review, we cover these regulatory mechanisms and emphasize the mechanism by which the JNK cascade transmits apoptotic signals. We also describe the newly discovered PP2A switch, which is an important mechanism for JNK activation that induces apoptosis downstream of the Gq protein coupled receptors. Since the JNK cascade is involved in many cellular processes that determine cell fate, addressing its regulatory mechanisms might reveal new ways to treat JNK-dependent pathologies.

The histamine H3 receptor modulates dopamine D2 receptor-dependent signaling pathways and mouse behaviors

The histamine H3 receptor (H3R) is highly enriched in the spiny projection neurons (SPNs) of the striatum, in both the D1 receptor (D1R)-expressing and D2 receptor (D2R)-expressing populations. A crossantagonistic interaction between H3R and D1R has been demonstrated in mice, both at the behavioral level and at the biochemical level. Although interactive behavioral effects have been described upon coactivation of H3R and D2R, the molecular mechanisms underlying this interaction are poorly understood. Here, we show that activation of H3R with the selective agonist R-(-)-α-methylhistamine dihydrobromide mitigates D2R agonist-induced locomotor activity and stereotypic behavior. Using biochemical approaches and the proximity ligation assay, we demonstrated the existence of an H3R-D2R complex in the mouse striatum. In addition, we examined consequences of simultaneous H3R-D2R agonism on the phosphorylation levels of several signaling molecules using immunohistochemistry. H3R agonist treatment modulated Akt (serine/threonine PKB)-glycogen synthase kinase 3 beta signaling in response to D2R activation via a β-arrestin 2-dependent mechanism in D2R-SPNs but not in D1R-SPNs. Phosphorylation of mitogen- and stress-activated protein kinase 1 and rpS6 (ribosomal protein S6) was largely unchanged under these conditions. As Akt-glycogen synthase kinase 3 beta signaling has been implicated in several neuropsychiatric disorders, this work may help clarify the role of H3R in modulating D2R function, leading to a better understanding of pathophysiology involving the interaction between histamine and dopamine systems.

Inhibition of histamine receptor H3 suppresses the growth and metastasis of human non-small cell lung cancer cells via inhibiting PI3K/Akt/mTOR and MEK/ERK signaling pathways and blocking EMT

Recent evidence shows that the expression levels of histamine receptor H3 (Hrh3) are upregulated in several types of cancer. However, the role of Hrh3 in non-small cell lung cancer (NSCLC) has not been elucidated. In the present study, we showed that the expression levels of Hrh3 were significantly increased in NSCLC samples, and high levels of Hrh3 were associated with poor overall survival (OS) in NSCLC patients. In five human NSCLC cell lines tested, Hrh3 was significantly upregulated. In NSCLC cell lines H1975, H460, and A549, Hrh3 antagonist ciproxifan (CPX, 10-80 μM) exerted moderate and concentration-dependent inhibition on the cell growth and induced apoptosis, whereas its agonist RAMH (80 μM) reversed these effects. Furthermore, inhibition of Hrh3 by CPX or siRNA retarded the migration and invasion of NSCLC cells through inhibiting epithelial-mesenchymal transition (EMT) progression via reducing the phosphorylation of PI3K/Akt/mTOR and MEK/ERK signaling pathways. In nude mice bearing H1975 cell xenograft or A549 cell xenograft, administration of CPX (3 mg/kg every other day, intraperitoneal) significantly inhibited the tumor growth with increased E-cadherin and ZO-1 expression and decreased Fibronectin expression in tumor tissue. In conclusion, this study reveals that Hrh3 plays an important role in the growth and metastasis of NSCLC; it might be a potential therapeutic target against the lung cancer.

Two naturally occurring mutations of human GPR103 define distinct G protein selection bias

The neuropeptide 26RFa plays important roles in the regulation of many physiological functions. 26RFa has been recognized as an endogenous ligand for receptor GPR103. In the present study, we demonstrate that GPR103 dually couples to Gαq and Gαi/o proteins. However, two naturally occurring missense mutations were identified from a young male patient. In the first, Y68H, induction of Ca²⁺ mobilization was noted without detection of ERK1/2 activation. In the second, R371W, the potential to activate ERK1/2 signaling was retained but with failure to evoke Ca²⁺ mobilization. Further analysis provides evidence that Gαq, L-type Ca²⁺ channel and PKCβI and βII are involved in the Y68H-mediated signaling pathway, whereas Gαi/o, Gβγ, and PKCζ are implicated in the R371W-induced signaling. Our results demonstrate that two point mutations, Y68H and R371W, affect the equilibrium between the different receptor conformations, leading to alteration of G protein-coupling preferences. Importantly, these findings provide a foundation for future elucidation of GPCR-mediated biased signaling and the physiological implications of their bias.

Activation of the δ opioid receptor relieves cerebral ischemic injury in rats via EGFR transactivation

Delta opioids are thought to relieve ischemic injury and have tissue-protective properties. However, the detailed mechanisms of delta opioids have not been well identified. Receptor tyrosine kinases (RTKs), such as epidermal growth factor receptor (EGFR), have been shown to mediate downstream signals of δ opioid receptor (δOR) activation through the metalloproteinase (MMP)-dependent EGF-like growth factor (HB-EGF) excretion pathway, which is called transactivation. In this study, to investigate the role of EGFR in δOR-induced anti-ischemic effects in the brain, we applied the middle cerebral artery occlusion (MCAO) model followed by reperfusion to mimic ischemic stroke injury in rats. Pre-treatment with the δOR agonist [D-ala², D-leu⁵] enkephalin (DADLE) improved the neurologic deficits and the decreased infarct volume caused by cerebral ischemia/reperfusion injury, which were blocked by the EGFR inhibitor AG1478 and the MMP inhibitor GM6001, respectively. Further results indicated that DADLE activated EGFR, Akt and ERK1/2 and upregulated EGFR expression in the hippocampus in a time-dependent manner, which were inhibited by AG1478 and GM6001. The enzyme-linked immunosorbent assay (ELISA) results showed that δOR activation led to an increase in HB-EGF release, but HB-EGF in tissue was downregulated at the mRNA and protein levels. Moreover, this protective action caused by δOR agonists may involve attenuated hippocampal cellular apoptosis. Overall, these results demonstrate that MMP-mediated transactivation of EGFR is essential for δOR agonist-induced MCAO/reperfusion injury relief. These findings provide a potential molecular mechanism for the neuroprotective property of δOR and may add new insight into mitigating or preventing injury.

Ligand-dependent internalization of Bombyx mori tachykinin-related peptide receptor is regulated by PKC, GRK5 and β-arrestin2/BmKurtz

Tachykinin signaling system is present in both vertebrates and invertebrates, and functions as neuromodulator responsible for the regulation of various physiological processes. In human, the internalization of G protein-coupled receptors has been extensively characterized; however, the insect GPCR internalization has been rarely investigated. Here, we constructed two expression vectors of Bombyx tachykinin-related peptide receptor (BmTKRPR) fused with Enhanced Green Fluorescent Protein (EGFP) at the C-terminal end for direct visualization of receptor expression, localization, and trafficking in cultured mammalian HEK293 and insect Sf21 cells. Our results demonstrated that agonist-activated BmTKRPR underwent rapid internalization in a dose-and time-dependent manner via a clathrin-dependent pathway in both HEK293 and Sf21 cells. Further investigation via RNAi or specific inhibitors, or co-immunoprecipitation demonstrated that agonist-induced BmTKRPR internalization was mediated by PKC, GRK5 and β-arrestin2/BmKurtz. In addition, we also observed that most of the internalized BmTKRP receptors were recycled to the cell surface via early endosomes upon peptide ligand removal. Our study provides the first in-depth information on mechanisms underlying insect TKRP receptor internalization and perhaps aids in the interpretation of the signaling in the regulation of physiological processes.

Histamine H1 and H3 receptor activation increases the expression of Glucose Transporter 1 (GLUT-1) in rat cerebro-cortical astrocytes in primary culture

Astrocytes take up glucose via the 45 kDa isoform of the Glucose Transporter 1 (GLUT-1), and in this work we have investigated whether histamine regulates GLUT-1 expression in rat cerebro-cortical astrocytes in primary culture. Cultured astrocytes expressed histamine H₁ and H₃ receptors (H₁Rs and H₃Rs) as evaluated by radioligand binding. Receptor functionality was confirmed by the increase in the intracellular concentration of Ca²⁺ (H₁R) and the inhibition of forskolin-induced cAMP accumulation (H₃R). Quantitative RT-PCR showed that histamine and selective H₁R and H₃R agonists (1 h incubation) significantly increased GLUT-1 mRNA to 153 ± 7, 163 ± 2 and 168 ± 13% of control values, respectively. In immunoblot assays, incubation (3 h) with histamine or H₁R and H₃R agonists increased GLUT-1 protein levels to 224 ± 12, 305 ± 11 and 193 ± 13% of control values, respectively, an action confirmed by inmunocytochemistry. The effects of H₁R and H₃R agonists were blocked by the selective antagonists mepyramine (H₁R) and clobenpropit (H₃R). The pharmacological inhibition of protein kinase C (PKC) prevented the increase in GLUT-1 protein induced by either H₁R or H₃R activation. Furthermore, histamine increased ERK-1/2 phosphorylation, and the effect of H₁R and H₃R activation on GLUT-1 protein levels was reduced or prevented, respectively, by MEK-1/2 inhibition. These results indicate that by activating H₁Rs and H₃Rs histamine regulates the expression of GLUT-1 by astrocytes. The effect appears to involve the phospholipase C (PLC) → diacylglycerol (DAG)/Ca²⁺→ PKC and PLC → DAG/Ca²⁺ → PKC → MAPK pathways.

Histamine, histamine receptors, and neuropathic pain relief

Histamine, acting via distinct histamine H₁, H₂, H₃, and H₄ receptors, regulates various physiological and pathological processes, including pain. In the last two decades, there has been a particular increase in evidence to support the involvement of H₃ receptor and H₄ receptor in the modulation of neuropathic pain, which remains challenging in terms of management. However, recent data show contrasting effects on neuropathic pain due to multiple factors that determine the pharmacological responses of histamine receptors and their underlying signal transduction properties (e.g., localization on either the presynaptic or postsynaptic neuronal membranes). This review summarizes the most recent findings on the role of histamine and the effects mediated by the four histamine receptors in response to the various stimuli associated with and promoting neuropathic pain. We particularly focus on mechanisms underlying histamine‐mediated analgesia, as we aim to clarify the analgesic potential of histamine receptor ligands in neuropathic pain.

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This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc

Role of PLD-PKCζ signaling axis in p47phox phosphorylation for activation of NADPH oxidase by angiotensin II in pulmonary artery smooth muscle cells

We sought to determine the mechanism by which angiotensin II (ANGII) stimulates NADPH oxidase-mediated superoxide (O₂ ^.- ) production in bovine pulmonary artery smooth muscle cells (BPASMCs). ANGII-induced increase in phospholipase D (PLD) and NADPH oxidase activities were inhibited upon pretreatment of the cells with chemical and genetic inhibitors of PLD2, but not PLD1. Immunoblot study revealed that ANGII treatment of the cells markedly increases protein kinase C-α (PKC-α), -δ, -ε, and -ζ levels in the cell membrane. Pretreatment of the cells with chemical and genetic inhibitors of PKC-ζ, but not PKC-α, -δ, and -ε, attenuated ANGII-induced increase in NADPH oxidase activity without a discernible change in PLD activity. Transfection of the cells with p47phox small interfering RNA inhibited ANGII-induced increase in NADPH oxidase activity without a significant change in PLD activity. Pretreatment of the cells with the chemical and genetic inhibitors of PLD2 and PKC-ζ inhibited ANGII-induced p47phox phosphorylation and subsequently translocation from cytosol to the cell membrane, and also inhibited its association with p22phox (a component of membrane-associated NADPH oxidase). Overall, PLD-PKCζ-p47phox signaling axis plays a crucial role in ANGII-induced increase in NADPH oxidase-mediated O₂ ^.- production in the cells.

Orexin facilitates GABAergic IPSCs via postsynaptic OX1 receptors coupling to the intracellular PKC signalling cascade in the rat cerebral cortex

Orexin has multiple physiological functions including wakefulness, appetite, nicotine intake, and nociception. The cerebral cortex receives abundant orexinergic projections and expresses both orexinergic receptor 1 (OX₁R) and 2 (OX₂R). However, little is known about orexinergic regulation of GABA-mediated inhibitory synaptic transmission. In the cerebral cortex, there are multiple GABAergic neural subtypes, each of which has its own morphological and physiological characteristics. Therefore, identification of presynaptic GABAergic neural subtypes is critical to understand orexinergic effects on GABAergic connections. We focused on inhibitory synapses at pyramidal neurons (PNs) from fast-spiking GABAergic neurons (FSNs) in the insular cortex by a paired whole-cell patch-clamp technique, and elucidated the mechanisms of orexin-induced IPSC regulation. We found that both orexin A and orexin B enhanced unitary IPSC (uIPSC) amplitude in FSN→PN connections without changing the paired-pulse ratio or failure rate. These effects were blocked by SB-334867, an OX₁ receptor (OX₁R) antagonist, but not by TCS-OX2-29, an OX₂R antagonist. [Ala¹¹, D-Leu¹⁵]-orexin B, a selective OX₂R agonist, had little effect on uIPSCs. Variance-mean analysis demonstrated an increase in quantal content without a change in release probability or the number of readily releasable pools. Laser photolysis of caged GABA revealed that orexin A enhanced GABA-mediated currents in PNs. Downstream blockade of G_q/11 protein-coupled OX₁Rs by IP₃ receptor or protein kinase C (PKC) blockers and BAPTA injection into postsynaptic PNs diminished the orexin A-induced uIPSC enhancement. These results suggest that the orexinergic uIPSC enhancement is mediated via postsynaptic OX₁Rs, which potentiate GABA_A receptors through PKC activation.

Crosstalk between receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCR) in the brain: Focus on heteroreceptor complexes and related functional neurotrophic effects

Neuronal events are regulated by the integration of several complex signaling networks in which G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) are considered key players of an intense bidirectional cross-communication in the cell, generating signaling mechanisms that, at the same time, connect and diversify the traditional signal transduction pathways activated by the single receptor. For this receptor-receptor crosstalk, the two classes of receptors form heteroreceptor complexes resulting in RTKs transactivation and in growth-promoting signals. In this review, we describe heteroreceptor complexes between GPCR and RTKs in the central nervous system (CNS) and their functional effects in controlling a variety of neuronal effects, ranging from development, proliferation, differentiation and migration, to survival, repair, synaptic transmission and plasticity. In this interaction, RTKs can also recruit components of the G protein signaling cascade, creating a bidirectional intricate interplay that provides complex control over multiple cellular events. These heteroreceptor complexes, by the integration of different signals, have recently attracted a growing interest as novel molecular target for depressive disorders. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Role of catechins on ET-1-induced stimulation of PLD and NADPH oxidase activities in pulmonary smooth muscle cells: determination of the probable mechanism by molecular docking studies

The treatment of human pulmonary artery smooth muscle cells with ET-1 stimulates the activity of PLD and NADPH oxidase, but this stimulation is inhibited by pretreatment with bosentan (ET-1 receptor antagonist), FIPI (PLD inhibitor), apocynin (NADPH oxidase inhibitor), and EGCG and ECG (catechins having a galloyl group), but not EGC and EC (catechins devoid of a galloyl group). Herein, using molecular docking analyses based on our biochemical studies, we determined the probable mechanism by which the catechins containing a galloyl group inhibit the stimulation of PLD activity induced by ET-1. The ET-1-induced stimulation of PLD activity was inhibited by SecinH3 (inhibitor of cytohesin). Arf6 and cytohesin-1 are associated in the cell membrane, which is not inhibited by the catechins during ET-1 treatment of the cells. However, EGCG and ECG inhibited the binding of GTPγS with Arf6, even in the presence of cytohesin-1. The molecular docking analyses revealed that the catechins containing a galloyl group (EGCG and ECG) with cytohesin-1–Arf6GDP, but not the catechins without a galloyl group (EGC and EC), prevent GDP–GTP exchange in Arf6, which seems to be an important mechanism for inhibiting the activation of PLD induced by ET-1, and subsequently increases the activity of NADPH oxidase.

ERK and p38 Upregulation versus Bcl-6 Downregulation in Rat Kidney Epithelial Cells Exposed to Prolonged Hypoxia

Hypoxia is a common cause of kidney injury and a major issue in kidney transplantation. Mitogen-activated protein kinases (MAPKs) are involved in the cellular response to hypoxia, but the precise roles of MAPKs in renal cell reactions to hypoxic stress are not well known yet. This work was conducted to investigate the regulation of extracellular signal-regulated kinase-1 and -2 (ERK1/2) and p38 and their signaling-relevant molecules in kidney epithelial cells exposed to prolonged hypoxia. Rat kidney epithelial cells Normal Rat Kidney (NRK)-52E were exposed to hypoxic conditions (1% O₂) for 24 to 72 h. Cell morphology was examined by light microscopy, and cell viability was checked by 3-[4,5-dimethylthiazol-2-yl]-5-[3-carboxymethoxypheny]-2-[4-sulfophenyl]-2H-tetrazolium (MTS). The expression of ERK1/2 and p38 MAPK, as well as their signaling-related molecules, was measured by Western blot and real-time polymerase chain (RT-PCR) reaction. At the 1% oxygen level, cell morphology had no appreciable changes compared to the control up to 72 h of exposure under light microscopy, whereas the results of MTS showed a slight but significant reduction in cell viability after 72 h of hypoxia. On the other hand, ERK1/2 and p38 phosphorylation remarkably increased in these cells after 24 to 72 h of hypoxia. In sharp contrast, the expression of transcription factor B-cell lymphoma 6 (Bcl-6) was significantly downregulated in response to hypoxic stress. Other intracellular molecules relevant to the ERK1/2 and p38 signaling pathway, such as protein kinase A, protein kinase C, Bcl-2, nuclear factor erythroid 2-related factor 2, tristetraprolin, and interleukin-10(IL-10), had no significant alterations after 24 to 72 h of hypoxic exposure. We conclude that hypoxic stress increases the phosphorylation of both ERK1/2 and p38 but decreases the level of Bcl-6 in rat kidney epithelial cells.

A synthetic intrabody-based selective and generic inhibitor of GPCR endocytosis

Beta-arrestins (βarrs) critically mediate desensitization, endocytosis and signalling of G protein-coupled receptors (GPCRs), and they scaffold a large number of interaction partners. However, allosteric modulation of their scaffolding abilities and direct targeting of their interaction interfaces to modulate GPCR functions selectively have not been fully explored yet. Here we identified a series of synthetic antibody fragments (Fabs) against different conformations of βarrs from phage display libraries. Several of these Fabs allosterically and selectively modulated the interaction of βarrs with clathrin and ERK MAP kinase. Interestingly, one of these Fabs selectively disrupted βarr-clathrin interaction, and when expressed as an intrabody, it robustly inhibited agonist-induced endocytosis of a broad set of GPCRs without affecting ERK MAP kinase activation. Our data therefore demonstrate the feasibility of selectively targeting βarr interactions using intrabodies and provide a novel framework for fine-tuning GPCR functions with potential therapeutic implications.

Role of ADP ribosylation factor6- Cytohesin1-PhospholipaseD signaling axis in U46619 induced activation of NADPH oxidase in pulmonary artery smooth muscle cell membrane

Treatment of human pulmonary artery smooth muscle cells (HPASMCs) with the thromboxane A2 receptor antagonist, SQ29548 inhibited U46619 stimulation of phospholipase D (PLD) and NADPH oxidase activities in the cell membrane. Pretreatment with apocynin inhibited U46619 induced increase in NADPH oxidase activity. The cell membrane contains predominantly PLD2 along with PLD1 isoforms of PLD. Pretreatment with pharmacological and genetic inhibitors of PLD2, but not PLD1, attenuated U46619 stimulation of NADPH oxidase activity. U46619 stimulation of PLD and NADPH oxidase activities were insensitive to BFA and Clostridium botulinum C3 toxin; however, pretreatment with secinH3 inhibited U46619 induced increase in PLD and NADPH oxidase activities suggesting a major role of cytohesin in U46619-induced increase in PLD and NADPH oxidase activities. Arf-1, Arf-6, cytohesin-1 and cytohesin-2 were observed in the cytosolic fraction, but only Arf-6 and cytohesin-1 were translocated to the cell membrane upon treatment with U46619. Coimmunoprecipitation study showed association of Arf-6 with cytohesin-1 in the cell membrane fraction. In vitro binding of GTPγS with Arf-6 required the presence of cytohesin-1 and that occurs in BFA insensitive manner. Overall, BFA insensitive Arf6-cytohesin1 signaling axis plays a pivotal role in U46619-mediated activation of PLD leading to stimulation of NADPH oxidase activity in HPASMCs.

Role of curcumin in PLD activation by Arf6-cytohesin1 signaling axis in U46619-stimulated pulmonary artery smooth muscle cells

Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine to produce phosphatidic acid (PA) which in some cell types play a pivotal role in agonist-induced increase in NADPH oxidase-derived [Formula: see text]production. Involvement of ADP ribosylation factor (Arf) in agonist-induced activation of PLD is known for smooth muscle cells of systemic arteries, but not in pulmonary artery smooth muscle cells (PASMCs). Additionally, role of cytohesin in this scenario is unknown in PASMCs. We, therefore, determined the involvement of Arf and cytohesin in U46619-induced stimulation of PLD in PASMCs, and the probable mechanism by which curcumin, a natural phenolic compound, inhibits the U46619 response. Treatment of PASMCs with U46619 stimulated PLD activity in the cell membrane, which was inhibited upon pretreatment with SQ29548 (Tp receptor antagonist), FIPI (PLD inhibitor), SecinH3 (inhibitor of cytohesins), and curcumin. Transfection of the cells with Tp, Arf-6, and cytohesin-1 siRNA inhibited U46619-induced activation of PLD. Upon treatment of the cells with U46619, Arf-6 and cytohesin-1 were translocated and associated in the cell membrane, which were not inhibited upon pretreatment of the cells with curcumin. Cytohesin-1 appeared to be necessary for in vitro binding of GTPγS with Arf-6; however, addition of curcumin inhibited binding of GTPγS with Arf-6 even in the presence of cytohesin-1. Our computational study suggests that although curcumin to some extent binds with Tp receptor, yet the inhibition of Arf6_GDP to Arf6_GTP conversion appeared to be an important mechanism by which curcumin inhibits U46619-induced increase in PLD activity in PASMCs.

GABABR-Induced EGFR Transactivation Promotes Migration of Human Prostate Cancer Cells

G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) act in concert to regulate cell growth, proliferation, survival, and migration. Metabotropic GABA_B receptor (GABA_BR) is the GPCR for the main inhibitory neurotransmitter GABA in the central nervous system. Increased expression of GABA_BR has been detected in human cancer tissues and cancer cell lines, but the role of GABA_BR in these cells is controversial and the underlying mechanism remains poorly understood. Here, we investigated whether GABA_BR hijacks RTK signaling to modulate the fates of human prostate cancer cells. RTK array analysis revealed that the GABA_BR-specific agonist baclofen selectively induced the transactivation of EGFR in PC-3 cells. EGFR transactivation resulted in the activation of ERK1/2 by a mechanism that is dependent on G_i/o protein and that requires matrix metalloproteinase-mediated proligand shedding. Positive allosteric modulators (PAMs) of GABA_BR, such as CGP7930, rac-BHFF, and GS39783, can function as PAM agonists to induce EGFR transactivation and subsequent ERK1/2 activation. Moreover, both baclofen and CGP7930 promoted cell migration and invasion through EGFR signaling. In summary, our observations demonstrated that GABA_BR transactivated EGFR in a ligand-dependent mechanism to promote prostate cancer cell migration and invasion, thus providing new insights into developing a novel strategy for prostate cancer treatment by targeting neurotransmitter signaling.

Cyclooxygenase-2 induced β1-integrin expression in NSCLC and promoted cell invasion via the EP1/MAPK/E2F-1/FoxC2 signal pathway

Cyclooxygenase-2 (COX-2) has been implicated in cell invasion in non-small-cell lung cancer (NSCLC). However, the mechanism is unclear. The present study investigated the effect of COX-2 on β1-integrin expression and cell invasion in NSCLC. COX-2 and β1-integrin were co-expressed in NSCLC tissues. COX-2 overexpression or Prostaglandin E2 (PGE2) treatment increased β1-integrin expression in NSCLC cell lines. β1-integrin silencing suppressed COX-2-mediated tumour growth and cancer cell invasion in vivo and in vitro. Prostaglandin E Receptor EP1 transfection or treatment with EP1 agonist mimicked the effect of PGE₂ treatment. EP1 siRNA blocked PGE₂-mediated β1-integrin expression. EP1 agonist treatment promoted Erk1/2, p38 phosphorylation and E2F-1 expression. MEK1/2 and p38 inhibitors suppressed EP1-mediated β1-integrin expression. E2F-1 silencing suppressed EP1-mediated FoxC2 and β1-integrin upregulation. ChIP and Luciferase Reporter assays identified that EP1 agonist treatment induced E2F-1 binding to FoxC2 promotor directly and improved FoxC2 transcription. FoxC2 siRNA suppressed β1-integrin expression and EP1-mediated cell invasion. Immunohistochemistry showed E2F-1, FoxC2, and EP1R were all highly expressed in the NSCLC cases. This study suggested that COX-2 upregulates β1-integrin expression and cell invasion in NSCLC by activating the MAPK/E2F-1 signalling pathway. Targeting the COX-2/EP1/PKC/MAPK/E2F-1/FoxC2/β1-integrin pathway might represent a new therapeutic strategy for the prevention and treatment of this cancer.

The Histamine H3 Receptor: Structure, Pharmacology, and Function

Among the four G protein-coupled receptors (H₁-H₄) identified as mediators of the biologic effects of histamine, the H₃ receptor (H₃R) is distinguished for its almost exclusive expression in the nervous system and the large variety of isoforms generated by alternative splicing of the corresponding mRNA. Additionally, it exhibits dual functionality as autoreceptor and heteroreceptor, and this enables H₃Rs to modulate the histaminergic and other neurotransmitter systems. The cloning of the H₃R cDNA in 1999 by Lovenberg et al. allowed for detailed studies of its molecular aspects. In this work, we review the characteristics of the H₃R, namely, its structure, constitutive activity, isoforms, signal transduction pathways, regional differences in expression and localization, selective agonists, antagonists and inverse agonists, dimerization with other neurotransmitter receptors, and the main presynaptic and postsynaptic effects resulting from its activation. The H₃R has attracted interest as a potential drug target for the treatment of several important neurologic and psychiatric disorders, such as Alzheimer and Parkinson diseases, Gilles de la Tourette syndrome, and addiction.