Quercetin inhibits histamine-induced calcium influx in human keratinocyte via histamine H4 receptors.

Histamine is released from mast cells when tissues are inflamed or stimulated by allergens. Activation of histamine receptors and calcium influx via TRPV1 could be related to histamine-induced itch and skin inflammation. Quercetin is known to have anti-inflammatory and anti-itching effects. This study aims to understand whether quercetin can directly affect histamine-induced calcium influx in human keratinocyte. In it, we investigated quercetin, which acts on histamine-induced intracellular free calcium ([Ca2+]i) elevation in human keratinocyte. Changes in [Ca2+]i were measured using spectrofluorometry and confocal Imaging. We detected the expression of IL-8 after treatment of quercetin using qRT-PCR and evaluated its anti-itching effect in BALB/c mice. We also performed a docking study to estimate the binding affinity of quercetin to H4 receptors. We found that quercetin pretreatment decreased histamine-induced [Ca2+]i elevation in a concentration-dependent manner. The inhibitory effect of quercetin on histamine-induced [Ca2+]i elevation was blocked by JNJ7777120, a selective H4 antagonist, as well as by U73122, a PLC inhibitor, and by GF109203X, a PKC inhibitor. We also found that H4 agonist (4-methylhistamine)-induced [Ca2+]i elevation could be inhibited by quercetin. Moreover, the selective TRPV1 blocker capsazepine significantly suppressed the quercetin-mediated inhibition of histamine-induced [Ca2+]i elevation, whereas the TRPV4 blocker GSK2193874 had no effect. Last, quercetin decreased histamine and H4 agonist-induced IL-8 expression in keratinocyte and inhibited the scratching behavior-induced compound 48/80 in BALB/c mice. The molecular docking study also showed that quercetin exhibited high binding affinities with H4 receptors (autodock scores for H4 = -8.7 kcal/mol). These data suggest that quercetin could decrease histamine 4 receptor-induced calcium influx through the TRPV1 channel and could provide a molecular mechanism of quercetin in anti-itching, anti-inflammatory, and unpleasant sensations.

Evolutionary history of histamine receptors: Early vertebrate origin and expansion of the H3-H4 subtypes.

Histamine receptors belonging to the superfamily of G protein-coupled receptors (GPCRs) mediate the diverse biological effects of biogenic histamine. They are classified into four phylogenetically distinct subtypes H1-H4, each with a different binding affinity for histamine and divergent downstream signaling pathways. Here we present the evolutionary history of the histamine receptors using a phylogenetic approach complemented with comparative genomics analyses of the sequences, gene structures, and synteny of gene neighborhoods. The data indicate the earliest emergence of histamine-mediated GPCR signaling by a H2 in a prebilaterian ancestor. The analyses support a revised classification of the vertebrate H3-H4 receptor subtypes. We demonstrate the presence of the H4 across vertebrates, contradicting the currently held notion that H4 is restricted to mammals. These non-mammalian vertebrate H4 orthologs have been mistaken for H3. We also identify the presence of a new H3 subtype (H3B), distinct from the canonical H3 (H3A), and propose that the H3A, H3B, and H4 likely emerged from a H3 progenitor through the 1R/2R whole genome duplications in an ancestor of the vertebrates. It is apparent that the ability of the H1, H2, and H3-4 to bind histamine was acquired convergently. We identified genomic signatures suggesting that the H1 and H3-H4 shared a last common ancestor with the muscarinic receptor in a bilaterian predecessor whereas, the H2 and the α-adrenoreceptor shared a progenitor in a prebilaterian ancestor. Furthermore, site-specific analysis of the vertebrate subtypes revealed potential residues that may account for the functional divergence between them.

Design and synthesis of histamine H3/H4 receptor ligands with a cyclopropane scaffold.

We previously designed and synthesized a series of histamine analogues with an imidazolylcyclopropane scaffold and identified potent non-selective antagonists for histamine H3 and H4 receptor subtypes. In this study, to develop H4 selective ligands, we newly designed and synthesized cyclopropane-based derivatives having an indole, benzimidazole, or piperazine structure, which are components of representative H4 selective antagonists such as JNJ7777120 and JNJ10191584. Among the synthesized derivatives, imidazolylcyclopropanes 12 and 13 conjugated with a benzimidazole showed binding affinity to the H3 and H4 receptors comparable to that of a well-known non-selective H3/H4 antagonist, thioperamide. These results suggest that the binding modes of the cyclopropane-based H3/H4 ligands in the H4 receptor can be different from those of the indole/benzimidazole-piperazine derivatives.