Expression of histamine H1 receptors on cultured histiocytic lymphoma cells

Effects of histamine H1 receptor signaling on glucocorticoid receptor activity. Role of canonical and non-canonical pathways

Histamine H1 receptor (H1R) antagonists and glucocorticoid receptor (GR) agonists are used to treat inflammatory conditions such as allergic rhinitis, atopic dermatitis and asthma. Consistent with the high morbidity levels of such inflammatory conditions, these receptors are the targets of a vast number of approved drugs, and in many situations their ligands are co-administered. However, this drug association has no clear rationale and has arisen from clinical practice. We hypothesized that H1R signaling could affect GR-mediated activity, impacting on its transcriptional outcome. Indeed, our results show a dual regulation of GR activity by the H1R: a potentiation mediated by G-protein βγ subunits and a parallel inhibitory effect mediated by Gαq-PLC pathway. Activation of the H1R by its full agonists resulted in a composite potentiating effect. Intriguingly, inactivation of the Gαq-PLC pathway by H1R inverse agonists resulted also in a potentiation of GR activity. Moreover, histamine and clinically relevant antihistamines synergized with the GR agonist dexamethasone to induce gene transactivation and transrepression in a gene-specific manner. Our work provides a delineation of molecular mechanisms underlying the widespread clinical association of antihistamines and GR agonists, which may contribute to future dosage optimization and reduction of well-described side effects associated with glucocorticoid administration.

Antihistamine response: a dynamically refined function at the host-tick interface

Background

Ticks counteract host inflammatory responses by secreting proteins from their saliva that compete for histamine binding. Among these tick salivary proteins are lipocalins, antiparallel beta-barrel proteins that sequester small molecules. A tick salivary lipocalin has been structurally resolved and experimentally shown to efficiently compete for histamine with its native receptor (e.g., H1 histamine receptor). To date, molecular dynamics simulations focus on protein-protein and protein-ligand interactions, but there are currently no studies for simultaneous ligand exploration between two competing proteins.

Methods

Aided by state-of-the-art, high-throughput computational methods, the current study simulated and analyzed the dynamics of competitive histamine binding at the tick-host interface using the available crystal structures of both the tick salivary lipocalin histamine-binding protein from Rhipicephalus appendiculatus and the human histamine receptor 1.

Results

The attraction towards the tick salivary lipocalin seems to depend on the protonated (adding a hydrogen ion) state of histamine since the current study shows that as histamine becomes more protonated it increases its exploration for the tick salivary lipocalin. This implies that during tick feeding, histamine may need to be protonated for the tick salivary lipocalin to efficiently sequester it in order to counteract inflammation. Additionally, the beta-hairpin loops (at both ends of the tick salivary lipocalin barrel) were reported to have a functional role in sequestering histamine and the results in the current study concur and provide evidence for this hypothesis. These beta-hairpin loops of the tick salivary lipocalin possess more acidic residues than a structurally similar but functionally unrelated lipocalin from the butterfly, Pieris brassicae; comparative results indicate these acidic residues may be responsible for the ability of the tick lipocalin to out-compete the native (H1) receptor for histamine.

Conclusions

Three explanatory types of data can be obtained from the current study: (i) the dynamics of multiple binding sites, (ii) competition between two proteins for a ligand, and (iii) the intrinsic molecular components involved in the competition. These data can provide further insight at the atomic level of the host-tick interface that cannot be experimentally determined. Additionally, the methods used in this study can be applied in rationally designing drugs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0491-9) contains supplementary material, which is available to authorized users.

G-protein-coupled receptors in HL-60 human leukemia cells

1. HL-60 human leukemia cells are a widely employed model system for the analysis of signal transduction processes mediated via regulatory heterotrimeric guanine nucleotide-binding proteins (G-proteins). HL-60 promyelocytes are pluripotent and can be differentiated into neutrophilic or monocytic cells. 2. HL-60 cells express formyl peptide-, complement C5a-, leukotriene B4 (LTB4)- and platelet-activating factor receptors, receptors for purine and pyrimidine nucleotides, histamine H1- and H2-receptors, beta 2-adrenoceptors and prostaglandin receptors. 3. The major G-proteins in HL-60 cells are pertussis toxin (PTX)-sensitive Gi-proteins (Gi2 > Gi3). Gs-proteins and G-proteins of the Gq-family (e.g., G16) are expressed, too. 4. G-protein-regulated effector systems in HL-60 cells are adenylyl cyclase and phospholipase C-beta 2 (PLC-beta 2) and, possibly, phospholipase D (PLD), nonselective cation (NSC) channels and NADPH oxidase. 5. The expression of signal transduction pathways in HL-60 cells strongly depends on the differentiation state of cells. 6. Formyl peptides, via Gi-proteins, mediate activation of PLC, PLD, NSC channels, NADPH oxidase and azurophilic granule release and are referred to as full secretagogues. In dibutyryl cAMP (Bt2cAMP)-differentiated HL-60 cells, C5a and LTB4 are partial and incomplete secretagogues, respectively. There are substantial differences in the Gi-protein activations induced by formyl peptides, C5a and LTB4. 7. In HL-60 promyelocytes, purine and pyrimidine nucleotides mediate activation of PLC and NSC channels largely via PTX-insensitive G-proteins and induce functional differentiation. In Bt2cAMP-differentiated HL-60 cells, they additionally activate PLD, NADPH oxidase and granule release via PTX-sensitive and -insensitive pathways. ATP and UTP are partial secretagogues. Multiple types of receptors (i.e., P2Y- and P2U-receptors and pyrimidinocyeptors) may mediate the effects of nucleotides in HL-60 cells. 8. Bt2cAMP- and 1 alpha,25-dihydroxycholecalciferol-differentiated HL-60 cells express H1-receptors coupled to Gi-proteins and PTX-insensitive G-proteins. In the former cells, histamine mediates activation of PLC and NSC channels, and in the latter, activation of NSC channels. Histamine is an incomplete secretagogue in these cells. 9. HL-60 promyelocytes express H2-receptors coupled to adenylyl cyclase, PLC, and NSC channels. There are substantial differences in the agonist/antagonist profiles of H2-receptor-mediated cAMP formation and rises in cytosolic Ca2+ concentration, indicative of the involvement of different H2-receptor subtypes. H2-receptors mediate functional differentiation of HL-60 cells. 10. Certain cationic-amphiphilic histamine receptor ligands (i.e., 2-substituted histamines, lipophilic guanidines, and a histamine trifluoromethyl-toluidide derivative) show stimulatory effects in HL-60 cells that are attributable to receptor-independent activation of Gi-proteins.

Histamine H1-receptors in HL-60 monocytes are coupled to Gi-proteins and pertussis toxin-insensitive G-proteins and mediate activation of Ca2+ influx without concomitant Ca2+ mobilization from intracellular stores

The results of binding studies suggest the presence of histamine H1-receptors in human monocytes, but it is not known whether these receptors are functionally active. This prompted us to study the effects of histamine (HA) on cytosolic Ca2+ concentration ([Ca2+]i) and superoxide anion (O2-) formation in HL-60 cells differentiated towards monocytes with 1 alpha,25-dihydroxycholecalciferol. In HL-60 monocytes, HA increased [Ca2+]i with a half-maximal effect at 8 microM and a maximum at 30-100 microM. Pertussis toxin (PTX) partially inhibited the stimulatory effects of HA on [Ca2+]i. Betahistine, a weak partial H1-receptor agonist, also increased [Ca2+]i, whereas H2- and H3-receptor agonists were ineffective. H1- but not H2- and H3-receptor antagonists inhibited HA-induced rises in [Ca2+]i. HA-induced rises in [Ca2+]i were desensitized in a homologous manner and were also inhibited by the activator of protein kinase C, 4 beta-phorbol 12-myristate 13-acetate. Various protein kinase C inhibitors did not interfere with homologous desensitization. The stimulatory effects of HA on [Ca2+]i were completely dependent on the presence of extracellular Ca2+ and were inhibited by the blocker of non-selective cation (NSC) channels, 1-(beta-[3-(4-methoxyphenyl)propoxyl]-4-methoxyphenethyl)-1 H-imidazole hydrochloride (SK & F 96365). HA was much less effective than the chemotactic peptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), to induce rises in [Ca2+]i. Unlike fMLP, HA did not activate O2- formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Desensitization of histamine H1 receptor-mediated inositol phosphate production in HeLa cells

1. Histamine stimulated the accumulation of total [3H]-inositol phosphates (IPn) in control HeLa cells with an EC50 of 3.7 +/- 0.7 microM in the presence of 10 mM LiCl. The maximum response to histamine after 15 min incubation was 43 +/- 5% over basal accumulation and occurred at a concentration of 1 mM histamine. 2. The histamine-induced IPn production in HeLa cells was confirmed as H1 receptor-mediated, since the H1 antagonist mepyramine (10(-6) M) inhibited the histamine response (10(-4) M) by 83 +/- 7%, whereas the H2 antagonist, ranitidine (10(-4) M), and H3 antagonist, thioperamide (10(-6) M), were ineffective. 3. Histamine (10(-4) M) pretreatment of HeLa cells for 30 min desensitized the subsequent histamine-induced IPn accumulation. The desensitized cells accumulated IPn in response to histamine with an EC50 of 1.7 +/- 0.7 microM after 15 min incubation. The maximum histamine-induced IPn accumulation at 10(-4) M was 19 +/- 5% over basal and was significantly lower (P < 0.03) than the maximum response in control cells. 4. The desensitization of histamine-induced IPn accumulation was time-dependent and, at a desensitizing histamine concentration of 10(-4) M, the half-maximal attenuation occurred after approximately 9 min and maximum desensitization was achieved by 15-20 min. The desensitization of the IPn accumulation was a reversible phenomenon and full recovery of the response occurred 150 min after the removal of the desensitizing histamine-containing medium. The half-time for the recovery of the histamine-induced response was estimated at 120 min. 5. Bradykinin stimulated IPn, accumulation in HeLa cells, and the ECm in control cells of 1.9 +/- 0.2 nM was not significantly different from the EC50 value from histamine-pretreated cells of 1.6 +/- 0.9 nM. The bradykinin response at 1 microM was 194 +/- 48% over basal IPn accumulation in control cells and this value was significantly different (P <0.04) from the 1 microM bradykinin-induced IPn accumulation in histamine pretreated HeLa cells of 143 +/- 38% over basal.6. NaF stimulated IP,, accumulation in control HeLa cells in a dose-related manner, with the maximum effect occurring at 15-20 mM. The EC50 value for NaF-stimulated IPn accumulation in control cells was 10.5 +/- 1.1 mm and the maximum response was 136 +/- 41% over basal after 20 min incubation. In histamine desensitized HeLa cells the EC50 value for NaF was 12.3 +/- 0.4 mM after 20 min stimulation,which was not significantly different from the value obtained in control cells. The maximum NaF stimulated IPn formation in desensitized cells of 68 +/- 23% over basal occurred at 15 -20 mM and was significantly lower (P<0.01) than that obtained in control cells.7. We show here that the acute histamine pretreatment of HeLa cells results in the desensitization of histamine H1 receptor-mediated IPn production. The desensitization was not restricted to the H1 receptor-mediated signal transduction pathway, but also includes both the bradykinin- and NaF mediated responses, supporting a heterologous desensitization mechanism. Our results are consistent with the site of attenuation being at or distal to the G-protein and the underlying mechanism being a slowed time-course for the production of inositol phosphates.