Molecular Pharmacology of the Four Histamine Receptors

Mast Cell-Derived Histamine Regulates Liver Ketogenesis via Oleoylethanolamide Signaling

The conversion of lipolysis-derived fatty acids into ketone bodies (ketogenesis) is a crucial metabolic adaptation to prolonged periods of food scarcity. The process occurs primarily in liver mitochondria and is initiated by fatty-acid-mediated stimulation of the ligand-operated transcription factor, peroxisome proliferator-activated receptor-α (PPAR-α). Here, we present evidence that mast cells contribute to the control of fasting-induced ketogenesis via a paracrine mechanism that involves secretion of histamine into the hepatic portal circulation, stimulation of liver H₁ receptors, and local biosynthesis of the high-affinity PPAR-α agonist, oleoylethanolamide (OEA). Genetic or pharmacological interventions that disable any one of these events, including mast cell elimination, deletion of histamine- or OEA-synthesizing enzymes, and H₁ blockade, blunt ketogenesis without affecting lipolysis. The results reveal an unexpected role for mast cells in the regulation of systemic fatty-acid homeostasis, and suggest that OEA may act in concert with lipolysis-derived fatty acids to activate liver PPAR-α and promote ketogenesis.

Histamine Excites Rat GABAergic Ventral Pallidum Neurons via Co-activation of H1 and H2 Receptors

The ventral pallidum (VP) is a crucial component of the limbic loop of the basal ganglia and participates in the regulation of reward, motivation, and emotion. Although the VP receives afferent inputs from the central histaminergic system, little is known about the effect of histamine on the VP and the underlying receptor mechanism. Here, we showed that histamine, a hypothalamic-derived neuromodulator, directly depolarized and excited the GABAergic VP neurons which comprise a major cell type in the VP and are responsible for encoding cues of incentive salience and reward hedonics. Both postsynaptic histamine H1 and H2 receptors were found to be expressed in the GABAergic VP neurons and co-mediate the excitatory effect of histamine. These results suggested that the central histaminergic system may actively participate in VP-mediated motivational and emotional behaviors via direct modulation of the GABAergic VP neurons. Our findings also have implications for the role of histamine and the central histaminergic system in psychiatric disorders.

Influence of Adalimumab on the Expression Profile of Genes Associated with the Histaminergic System in the Skin Fibroblasts In Vitro

Objective

The aim of this study was to evaluate the influence of adalimumab on expression profile of genes associated with the histaminergic system in Normal Human Dermal Fibroblast (NHDF) cells stimulated with 8.00 μg/ml of adalimumab and the identification of miRNAs regulating these genes' expression.

Methods

NHDFs were cultured with or without the presence of adalimumab for 2, 8, and 24 hours. The expression profile of genes and miRNA were determined with the use of microarray technology.

Results

Among 22283 ID mRNA, 65 are associated with the histaminergic system. It can be observed that 15 mRNAs differentiate NHDFs cultures with adalimumab form control. The analysis of miRNAs showed that, among 1105 ID miRNA, 20 miRNAs are differentiating in cells treated with adalimumab for 2 hours, 9 miRNA after 8 hours, and only 3 miRNAs after 24 hours.

Conclusion

It was also determined that miRNAs play certain role in the regulation of the expression of genes associated with the histaminergic system. The results of this study confirmed the possibility of using both genes associated with this system as well as miRNAs regulating their expression, as complementary molecular markers of sensitivity to the adalimumab treatment.

The histamine H4-receptor and the central and peripheral nervous system: A critical analysis of the literature

Expression and function of histamine H4R in central and peripheral nervous system have been a matter of controversy for more than a decade. The scientific discussion is often limited to a few publications postulating the presence of functional H4R on neurons of the central and peripheral nervous system, but the even larger number of reports showing negative data is often neglected. In this article, we critically review the existing literature on H4R in central and peripheral nervous system and discuss the weak points often overlooked by the community. We identified as most important problems (i) insufficient validation or quality of antibodies, (ii) missing knockout controls, (iii) uncritical interpretation of RT-PCR results instead of qPCR experiments, (iv) insufficient controls to confirm specificity of pharmacological tools, (v) uncritical reliance on results produced by a single method and (vi) uncritical reliance on results not reproduced by independent research groups. Additionally, there may be a publication as well as a citation bias favoring the awareness of positive results, but neglecting negative data. We conclude that H4R expression on neurons of the brain is not convincingly supported by the current literature, at least as long as the positive data are not reproduced by independent research groups. Expression and function of H4R on peripheral neurons or non-neuronal cells of the nervous system, specifically on microglia is an interesting alternative hypothesis that, however, requires further verification. This article is part of a Special Issue entitled 'Histamine Receptors'.

Quantitative Single-Cell Analysis of Signaling Pathways Activated Immediately Downstream of Histamine Receptor Subtypes

Genetically encoded biosensors based on Förster resonance energy transfer (FRET) can visualize responses of individual cells in real time. Here, we evaluated whether FRET-based biosensors provide sufficient contrast and specificity to measure activity of G-protein-coupled receptors. The four histamine receptor subtypes (H1R, H2R, H3R, and H4R) respond to the ligand histamine by activating three canonical heterotrimeric G-protein-mediated signaling pathways with a reported high degree of specificity. Using FRET-based biosensors, we demonstrate that H1R activates Gαq. We also observed that H1R activates Gαi, albeit at a 10-fold lower potency. In addition to increasing cAMP levels, most likely via Gαs, we found that the H2R induces Gαq-mediated calcium release. The H3R and H4R activated Gαi with high specificity and a high potency. We demonstrate that a number of FRET sensors provide sufficient contrast to: 1) analyze the specificity of the histamine receptor subtypes for different heterotrimeric G-protein families with single-cell resolution, 2) probe for antagonist specificity, and 3) allow the measurement of single-cell concentration-response curves.

H4 histamine receptors inhibit steroidogenesis and proliferation in Leydig cells

The histamine H4 receptor (HRH4), discovered only 13 years ago, is considered a promising drug target for allergy, inflammation, autoimmune disorders and cancer, as reflected by a steadily growing number of scientific publications and patent applications. Although the presence of HRH4 has been evidenced in the testis, its specific localization or its role has not been established. Herein, we sought to identify the possible involvement of HRH4 in the regulation of Leydig cell function. We first evaluated its expression in MA-10 Leydig tumor cells and then assessed the effects of two HRH4 agonists on steroidogenesis and proliferation. We found that HRH4 is functionally expressed in MA-10 cells, and that its activation leads to the inhibition of LH/human chorionic gonadotropin-induced cAMP production and StAR protein expression. Furthermore, we observed decreased cell proliferation after a 24-h HRH4 agonist treatment. We then detected for the sites of HRH4 expression in the normal rat testis, and detected HRH4 immunostaining in the Leydig cells of rats aged 7-240 days, while 21-day-old rats also presented HRH4 expression in male gametes. Finally, we evaluated the effect of HRH4 activation on the proliferation of normal progenitor and immature rat Leydig cell culture, and both proved to be susceptible to the anti-proliferative effect of HRH4 agonists. Given the importance of histamine (2-(1H-imidazol-4-yl)ethanamine) in human (patho)physiology, continued efforts are directed at elucidating the emerging properties of HRH4 and its ligands. This study reveals new sites of HRH4 expression, and should be considered in the design of selective HRH4 agonists for therapeutic purposes.

Comparison of airway responses in sheep of different age in precision-cut lung slices (PCLS)

Background

Animal models should display important characteristics of the human disease. Sheep have been considered particularly useful to study allergic airway responses to common natural antigens causing human asthma. A rationale of this study was to establish a model of ovine precision-cut lung slices (PCLS) for the in vitro measurement of airway responses in newborn and adult animals. We hypothesized that differences in airway reactivity in sheep are present at different ages.

Methods

Lambs were delivered spontaneously at term (147d) and adult sheep lived till 18 months. Viability of PCLS was confirmed by the MTT-test. To study airway provocations cumulative concentration-response curves were performed with different allergic response mediators and biogenic amines. In addition, electric field stimulation, passive sensitization with house dust mite (HDM) and mast cells staining were evaluated.

Results

PCLS from sheep were viable for at least three days. PCLS of newborn and adult sheep responded equally strong to methacholine and endothelin-1. The responses to serotonin, leukotriene D₄ and U46619 differed with age. No airway contraction was evoked by histamine, except after cimetidine pretreatment. In response to EFS, airways in PCLS from adult and newborn sheep strongly contracted and these contractions were atropine sensitive. Passive sensitization with HDM evoked a weak early allergic response in PCLS from adult and newborn sheep, which notably was prolonged in airways from adult sheep. Only few mast cells were found in the lungs of non-sensitized sheep at both ages.

Conclusion

PCLS from sheep lungs represent a useful tool to study pharmacological airway responses for at least three days. Sheep seem well suited to study mechanisms of cholinergic airway contraction. The notable differences between newborn and adult sheep demonstrate the importance of age in such studies.

G proteins Gαi1/3 are critical targets for Bordetella pertussis toxin-induced vasoactive amine sensitization

Pertussis toxin (PTX) is an AB5-type exotoxin produced by the bacterium Bordetella pertussis, the causative agent of whooping cough. In vivo intoxication with PTX elicits a variety of immunologic and inflammatory responses, including vasoactive amine sensitization (VAAS) to histamine (HA), serotonin (5-HT), and bradykinin (BDK). Previously, by using a forward genetic approach, we identified the HA H1 receptor (Hrh1/H1R) as the gene in mice that controls differential susceptibility to B. pertussis PTX-induced HA sensitization (Bphs). Here we show, by using inbred strains of mice, F1 hybrids, and segregating populations, that, unlike Bphs, PTX-induced 5-HT sensitivity (Bpss) and BDK sensitivity (Bpbs) are recessive traits and are separately controlled by multiple loci unlinked to 5-HT and BDK receptors, respectively. Furthermore, we found that PTX sensitizes mice to HA independently of Toll-like receptor 4, a purported receptor for PTX, and that the VAAS properties of PTX are not dependent upon endothelial caveolae or endothelial nitric oxide synthase. Finally, by using mice deficient in individual Gαi/o G-protein subunits, we demonstrate that Gαi1 and Gαi3 are the critical in vivo targets of ADP-ribosylation underlying VAAS elicited by PTX exposure.

Antinociceptive activity of CC44, a biotinylated improgan congener

Improgan, a non-opioid, antinociceptive drug, activates descending analgesic circuits following brain administration, but the improgan receptor remains unidentified. Since biotinylation of drugs can enhance drug potency or facilitate discovery of new drug targets, a biotinylated congener of improgan (CC44) and several related compounds were synthesized and tested for antinociceptive activity. In rats and mice, intracerebroventricular (i.c.v.) administration of CC44 produced dose-dependent reductions in thermal nociceptive (tail flick and hot plate) responses, with 5-fold greater potency than improgan. CC44 also robustly attenuated mechanical (tail pinch) nociception in normal rats and mechanical allodynia in a spinal nerve ligation model of neuropathic pain. Similar to the effects of improgan, CC44 antinociception was reversed by the GABAA agonist muscimol (consistent with activation of analgesic circuits), and was resistant to the opioid antagonist naltrexone (implying a non-opioid mechanism). Also like improgan, CC44 produced thermal antinociception when microinjected into the rostral ventromedial medulla (RVM). Unlike improgan, CC44 (i.c.v.) produced antinociception which was resistant to antagonism by the cannabinoid CB1 antagonist/inverse agonist rimonabant. CC44 was inactive in mice following systemic administration, indicating that CC44 does not penetrate the brain. Preliminary findings with other CC44 congeners suggest that the heteroaromatic nucleus (imidazole), but not the biotin moiety, is required for CC44's antinociceptive activity. These findings demonstrate that CC44 is a potent analgesic compound with many improgan-like characteristics. Since powerful techniques are available to characterize and identify the binding partners for biotin-containing ligands, CC44 may be useful in searching for new receptors for analgesic drugs.

Proliferative effect of histamine on MA-10 Leydig tumor cells mediated through HRH2 activation, transient elevation in cAMP production, and increased extracellular signal-regulated kinase phosphorylation levels

Mast cells (MC) occur normally in the testis with a species-specific distribution, yet their precise role remains unclear. Testicular MC express histidine decarboxylase (HDC), the unique enzyme responsible for histamine (HA) generation. Evidence to date supports a role for HA as a local regulator of steroidogenesis via functional H₁ and H₂ receptor subtypes (HRH1 and HRH2, respectively) present in Leydig cells. Given that HA is a well-known modulator of physiological and pathological proliferation in many different cell types, we aimed in the present study to evaluate whether HA might contribute to the regulation of Leydig cell number as well as to the control of androgen production. Herein, we demonstrate, to our knowledge for the first time, that MA-10 Leydig tumor cells, but not normal immature Leydig cells (ILC), exhibit a proliferative response upon stimulation with HA that involves HRH2 activation, transient elevation of cAMP levels, and increased extracellular signal-regulated kinase (ERK) phosphorylation. Our results also reveal that MA-10 cells show significantly heightened HDC expression compared to normal ILC or whole-testicular lysate and that inhibition of HDC activity decreases MA-10 cell proliferation, suggesting a possible correlation between autocrine overproduction of HA and abnormally increased proliferation in Leydig cells. The facts that germ cells are also both source and target of HA and that multiple testicular cells are susceptible to HA action underline the importance of the present study, which we hope will serve as a first step for further research into regulation of non-MC-related HDC expression within the testis and its significance for testicular function.

Species-dependent activities of G-protein-coupled receptor ligands: lessons from histamine receptor orthologs

Histamine is a biogenic amine that exerts its biological effects as a neurotransmitter and local mediator via four histamine receptor (HR) subtypes (H(x)Rs) - H(1)R, H(2)R, H(3)R, and H(4)R - belonging to the superfamily of G-protein-coupled receptors (GPCRs). All four H(x)Rs exhibit pronounced differences in agonist and/or antagonist pharmacology among various species orthologs. The species differences constitute a problem for animal experiments and drug development. This problem applies to GPCRs with diverse ligands. Here, we summarize our current knowledge on H(x)R orthologs as a case study for species-dependent activity of GPCR ligands. We show that species-specific pharmacology also provides unique opportunities to study important aspects of GPCR pharmacology in general, including ligand-binding sites, the roles of extracellular domains in ligand binding and receptor activation, agonist-independent (constitutive) receptor activity, thermodynamics of ligand/receptor interaction, receptor-activation mechanisms, and ligand-specific receptor conformations.

Regulation of mast cell responses in health and disease

Mast cells are multifunctional cells that initiate not only IgE-dependent allergic diseases but also play a fundamental role in innate and adaptive immune responses to microbial infection. They are also thought to play a role in angiogenesis, tissue remodeling, wound healing, and tumor repression or growth. The broad scope of these physiologic and pathologic roles illustrates the flexible nature of mast cells, which is enabled in part by their phenotypic adaptability to different tissue microenvironments and their ability to generate and release a diverse array of bioactive mediators in response to multiple types of cell-surface and cytosolic receptors. There is increasing evidence from studies in cell cultures that release of these mediators can be selectively modulated depending on the types or groups of receptors activated. The intent of this review is to foster interest in the interplay among mast cell receptors to help understand the underlying mechanisms for each of the immunological and non-immunological functions attributed to mast cells. The second intent of this review is to assess the pathophysiologic roles of mast cells and their products in health and disease. Although mast cells have a sufficient repertoire of bioactive mediators to mount effective innate and adaptive defense mechanisms against invading microorganisms, these same mediators can adversely affect surrounding tissues in the host, resulting in autoimmune disease as well as allergic disorders.

Inflammation dampened by gelatinase A cleavage of monocyte chemoattractant protein-3

Tissue degradation by the matrix metalloproteinase gelatinase A is pivotal to inflammation and metastases. Recognizing the catalytic importance of substrate-binding exosites outside the catalytic domain, we screened for extracellular substrates using the gelatinase A hemopexin domain as bait in the yeast two-hybrid system. Monocyte chemoattractant protein-3 (MCP-3) was identified as a physiological substrate of gelatinase A. Cleaved MCP-3 binds to CC-chemokine receptors-1, -2, and -3, but no longer induces calcium fluxes or promotes chemotaxis, and instead acts as a general chemokine antagonist that dampens inflammation. This suggests that matrix metalloproteinases are both effectors and regulators of the inflammatory response.