Histamine receptor-dependent and/or -independent activation of guanine nucleotide-binding proteins by histamine and 2-substituted histamine derivatives in human leukemia (HL-60) and human erythroleukemia (HEL) cells

Molecular and cellular analysis of human histamine receptor subtypes

The human histamine receptors hH(1)R and hH(2)R constitute important drug targets, and hH(3)R and hH(4)R have substantial potential in this area. Considering the species-specificity of pharmacology of H(x)R orthologs, it is important to analyze hH(x)Rs. Here, we summarize current knowledge of hH(x)Rs endogenously expressed in human cells and hH(x)Rs recombinantly expressed in mammalian and insect cells. We present the advantages and disadvantages of the various systems. We also discuss problems associated with the use of hH(x)R antibodies, an issue of general relevance for G-protein-coupled receptors (GPCRs). There is much greater overlap in activity of 'selective' ligands for other hH(x)Rs than the cognate receptor subtype than generally appreciated. Studies with native and recombinant systems support the concept of ligand-specific receptor conformations, encompassing agonists and antagonists. It is emerging that for characterization of hH(x)R ligands, one cannot rely on a single test system and a single parameter. Rather, multiple systems and parameters have to be studied. Although such studies are time-consuming and expensive, ultimately, they will increase drug safety and efficacy.

Synthesis and structure-activity relationships of cyanoguanidine-type and structurally related histamine H4 receptor agonists

Recently, we identified high-affinity human histamine H3 (hH3R) and H4 receptor (hH4R) ligands among a series of NG-acylated imidazolylpropylguanidines, which were originally designed as histamine H2 receptor (H2R) agonists. Aiming at selectivity for hH4R, the acylguanidine group was replaced with related moieties. Within a series of cyanoguanidines, 2-cyano-1-[4-(1H-imidazol-4-yl)butyl]-3-[(2-phenylthio)ethyl]guanidine (UR-PI376, 67) was identified as the most potent hH4R agonist (pEC50 = 7.47, alpha = 0.93) showing negligible hH1R and hH2R activities and significant selectivity over the hH3R (pKB = 6.00, alpha = -0.28), as determined in steady-state GTPase assays using membrane preparations of hH(x)R-expressing Sf9 cells. In contrast to previously described selective H4R agonists, this compound and other 3-substituted derivatives are devoid of agonistic activity at the other HR subtypes. Modeling of the binding mode of 67 suggests that the cyanoguanidine moiety forms charge-assisted hydrogen bonds not only with the conserved Asp-94 but also with the hH4R-specific Arg-341 residue. 2-Carbamoyl-1-[2-(1H-imidazol-4-yl)ethyl]-3-(3-phenylpropyl)guanidine (UR-PI97, 88) was unexpectedly identified as a highly potent and selective hH3R inverse agonist (pKB = 8.42, >300-fold selectivity over the other HR subtypes).

N1-(3-Cyclohexylbutanoyl)-N2-[3-(1H-imidazol-4-yl)propyl]guanidine (UR-AK57), a Potent Partial Agonist for the Human Histamine H1- and H2-Receptors

Both the histamine H1-receptor (H1R) and H2-receptor (H2R) exhibit pronounced species selectivity in their pharmacological properties; i.e., bulky agonists possess higher potencies and efficacies at guinea pig (gp) than at the corresponding human (h) receptor isoforms. In this study, we examined the effects of NG-acylated imidazolylpropylguanidines substituted with a single phenyl or cyclohexyl substituent on H1R and H2R species isoforms expressed in Sf9 insect cells. N1-(3-Cyclohexylbutanoyl)-N2-[3-(1H-imidazol-4-yl)propyl]guanidine (UR-AK57) turned out to be the most potent hH2R agonist identified so far (EC50 of 23 nM in the GTPase assay at the hH2R-Gsalpha fusion protein expressed in Sf9 insect cells). UR-AK57 was almost a full-hH2R agonist and only slightly less potent and efficacious than at gpH2R-Gsalpha. Several NG-acylated imidazolylpropylguanidines showed similar potency at hH2R and gpH2R. Most unexpectedly, UR-AK57 exhibited moderately strong partial hH1R agonism with a potency similar to that of histamine, whereas at gpH1R, UR-AK57 was only a very weak partial agonist. Structure/activity relationship studies revealed that both the alkanoyl chain connecting the aromatic or alicyclic substituent with the guanidine moiety and the nature of the carbocycle (cyclohexyl versus phenyl ring) critically determine the pharmacological properties of this class of compounds. Collectively, our data show that gpH1R and gpH R do not necessarily exhibit preference for bulky agonists (2) compared with hH1R and hH2R, respectively, and that UR-AK57 is a promising starting point for the development of both potent and efficacious hH1R and hH2R agonists.

Receptor-mediated and receptor-independent activation of G-proteins in rat brain membranes

High-affinity GTPase activity intrinsic to G-proteins, which serves as an index of G-protein activation elicited through agonist-stimulated receptors as well as by receptor-independent direct G-protein activators like mastoparan, was measured in rat brain membranes. Receptor-mediated high-affinity GTPase activity was detectable preferentially for the Gi subfamily associated with adenylyl cyclase inhibition mediated by group II metabotropic glutamate, pirenzepine-insensitive muscarinic acetylcholine, GABA(B), adenosine A1, dopamine D2-like (striatum), and serotonin 5-HT1A (hippocampus) receptors. The pharmacological characteristics of such receptor-mediated high-affinity GTPase activities were presented. Mastoparan, a tetradecapeptide from wasp venom which has been shown to directly activate Gi and Go, inhibited low-affinity GTP hydrolyzing activity, probably due to its activating effect on nucleoside diphosphokinase (NDPK). When NDPK activity was inhibited completely by UDP, mastoparan stimulated high-affinity GTPase activity in a concentration-dependent manner. There are many compounds other than mastoparan with apparently diverse structural properties which have been shown to directly activate G-proteins. The relevance and possible participation of receptor-independent mode of G-protein activation for some neuropeptides were discussed.

Alkyl-substituted amino acid amides and analogous di- and triamines: new non-peptide G protein activators

Synthesis and pharmacological properties of new potent direct activators of heterotrimeric G proteins are described. Compounds were synthesized from protected amino acids with alkylamines using coupling reagents (CDI, DCC, and EDC). Alkyl-substituted amino acid amides and their corresponding di- and triamines were subjected to structure-activity analysis. All compounds activated membrane-bound HL-60 GTPases in a pertussis toxin-sensitive fashion. This suggests a specific effect of compounds on the carboxy terminus of a defined subclass of heterotrimeric G proteins, i.e., members of the G alpha i subfamily. Elongation of the alkyl chain and increasing the number of amino groups enhanced the potency of compounds on HL-60 membrane-bound GTPase. N-(2,5-Diaminopentyl)dodecylamine (21) was selected to study its mode of action employing purified pertussis toxin-sensitive G proteins. It stimulated G alpha subunits by inducing the release of bound GDP. In contrast to receptors G beta gamma complexes were not required for 21-mediated activation of G alpha. Moderate isoform selectivity of its action was observed within a group of highly homologous members of the Gi subfamily with G alpha o1 being activated at lowest concentrations, whereas higher concentrations were necessary for the stimulation of G alpha i1 or transducin. We conclude that these compounds represent important tools for studying G protein-dependent cellular functions.

Activation by beta-carbolines of G-proteins in HL-60 membranes and the bovine retinal G-protein transducin in a receptor-independent manner

Naturally occurring beta-carbolines are lipophilic compounds which show psychotropic and physiological effects in mammals. They bind to distinct high-affinity binding sites in various mammalian tissues. However, the mechanism by which the beta-carbolines affect transmembrane signal transduction processes is still unknown. Since beta-carbolines are cationic-amphiphilic substances and since such substances are known to activate heterotrimeric regulatory guanine nucleotide binding proteins (G-proteins) in a receptor-independent manner, we put forward the hypothesis that beta-carbolines act directly on G-proteins. Therefore, we investigated the ability of beta-carbolines to stimulate high-affinity GTP hydrolysis in membranes of dibutyryl-cAMP differentiated HL-60 cells and of the purified bovine G-protein, transducin (TD). The beta-carbolines norharman and harman, stimulated the GTPase in HL-60 membranes with an EC50 of 410 microM and 450 microM, respectively, and a maximum effect at 1 mM each. Norharman and harman stimulated the GTPase of TD with an EC50 of 60 microM and 300 microM, and a maximum at 1 mM for both compounds. The stimulatory effect of norharman in HL-60 membranes was pertussis toxin-sensitive. Structure/activity characteristics of the beta-carbolines showed a specificity of norharman to stimulate the GTPase of TD, because norharman activated GTP hydrolysis in HL-60 membranes approximately 7 times less potently than that of TD. Norharman was a five-fold more potent activator of TD than tetrahydronorharman. Hydroxylation of the beta-carboline molecule in position 6 led to a loss of GTPase-activating properties. Our data suggest that naturally occurring beta-carbolines are a novel class of receptor-independent G-protein activating substances. This mechanism could contribute to their diverse biological effects.

Thapsigargin activates univalent- and bivalent-cation entry in human neutrophils by a SK&F I3 96365- and Gd3+-sensitive pathway and is a partial secretagogue: involvement of pertussis-toxin-sensitive G-proteins and protein phosphatases 1/2A and 2B in the signal-transduction pathway

The Ca2+-ATPase inhibitor thapsigargin (TG) activates bivalent-cation early in human neutrophils via depletion of intracellular Ca2+ stores bu little is known about the underlying mechanism and the functional role of TG-induced cation entry. We studied the effects of TG on univalent- and bivalent cation entry, lysozyme release and superoxide-anion (O2-) formation in human neutrophils. TG, like the chemotactic peptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), stimulated entry of Ca2+, Mn2+, Ba2+, Sr2+ and Na+ in a 1-{beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl}-1H-imidazole hydrochloride (SK&F 96365)- and Gd3+-sensitive manner. The inhibitors of protein phosphates 1/2A, calyculin A and okadaic acid, diminished TG-induced cation influxes, whereas the inhibitors of protein phosphatase 2B, cyclosporin A and FK-506, were potentiators. Pertussis toxin (PTX) partially inhibited the effects of TG on Ca2+ and Mn2+ entry. TG and fMLP activated inward currents with a linear current-voltage relationship and a reversal potential at about 0 mV. TG activated lysozyme release and potentiated fMLP-induced O2- formation. TG-induced lysozyme release was inhibited by SK&F 96365, PTX and the removal of extracellular Ca2+ or Na+. Our data show that TG activates a non-selective and SK&F 96365- and Gd3+-sensitive cation entry pathway and is a partial secretagogue. TG-stimulated cation entry involves PTX-sensitive G-proteins and protein phosphatases, with protein phosphatases 1/2A and 2B playing opposite roles.

Activation of GTP formation and high-affinity GTP hydrolysis by mastoparan in various cell membranes. G-protein activation via nucleoside diphosphate kinase, a possible general mechanism of mastoparan action

The wasp venom, mastoparan (MP), is a direct activator of reconstituted pertussis toxin-sensitive G-proteins and of purified nucleoside diphosphate kinase (NDPK) [E.C. 2.6.4.6.]. In HL-60 membranes, MP activates high-affinity GTPase [E.C. 3.6.1.-] and NDPK-catalyzed GTP formation, but not photolabeling of G-protein alpha-subunits with GTP azidoanilide; this suggests that the venom activates G-proteins in this system indirectly via stimulation of NDPK. Moreover, the MP analogue, mastoparan 7 (MP 7), is a much more effective activator of reconstituted G-proteins than MP, whereas with regard to NDPK and GTPase in HL-60 membranes, the two peptides are similarly effective. In our present study, we investigated NDPK- and G-protein activation by MP in membranes of the human neuroblastoma cell line, SH-SY5Y, the human erythroleukemia cell line, HEL, the rat basophilic leukemia cell line, RBL 2H3, and the hamster ductus deferens smooth muscle cell line, DDT1MF-2. All these membranes exhibited high NDPK activities that were increased by MP. Compared to basal GTP formation rates, basal rates of high-affinity GTP hydrolysis in cell membranes were low. MP activated high-affinity GTP hydrolysis in cell membranes but did not enhance incorporation of GTP azidoanilide into G-protein alpha-subunits. As with HL-60 membranes, MP and MP 7 were similarly effective activators of NDPK and GTPase in SH-SY5Y membranes. Pertussis toxin inhibited MP-stimulated GTP hydrolyses in SH-SY5Y- and HEL membranes, whereas NDPK activations by MP were pertussis toxin-insensitive. Our data suggest that indirect G-protein activation via NDPK is not restricted to HL-60 membranes but is a more general mechanism of MP action in cell membranes. Pertussis toxin-catalyzed ADP-ribosylation of alpha-subunits may inhibit the transfer of GTP from NDPK to G-proteins. NDPK may play a much more important role in transmembrane signal transduction than was previously appreciated and, moreover, the GTPase of G-protein alpha-subunits may serve as GDP-synthase for NDPK.

Receptor-independent G protein activation may account for the stimulatory effects of first-generation H1-receptor antagonists in HL-60 cells, basophils, and mast cells

The first-generation histamine H1-receptor antagonists, chlorpheniramine (CPHE) and diphenhydramine (DPH), may activate histamine release from basophils and mast cells. Because CPHE and DPH are cationic-amphiphilic and because several substances with such physicochemical properties activate heterotrimeric regulatory guanine nucleotide-binding proteins (G-proteins) in a receptor-independent manner, we asked the question of whether or not H1-receptor antagonists could be G-protein activators as well. In dibutyryl cAMP-differentiated HL-60 cells, CPHE and DPH increased cytosolic Ca2+ concentration and azurophilic granule release in pertussis toxin (PTX)-sensitive manners. In HL-60 membranes, PTX-sensitive stimulations of GTPase [E.C. 3.6.1.] and binding of guanosine 5'-[gamma-thio]triphosphate by H1 receptor antagonists were observed. CPHE and DPH also increased GTP hydrolysis by the purified PTX-sensitive G-protein, transducin. In all-trans-retinoic acid-differentiated HL-60 cells and rat basophilic leukemia cells (RBL 2H3 cells), H1-receptor antagonists induced, unlike in dibutyryl cAMP-differentiated HL-60 cells, Ca2+ influx without Ca2+ mobilization from intracellular stores. CPHE and DPH also induced serotonin release from RBL 2H3 cells. Our data indicate that first-generation H1-receptor antagonists are receptor-independent G-protein activators and that such a mechanism of action accounts for their stimulatory effects in HL-60 cells, basophils, and mast cells.

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.

Direct and indirect receptor-independent G-protein activation by cationic-amphiphilic substances. Studies with mast cells, HL-60 human leukemic cells and purified G-proteins

Studies from several laboratories have revealed that structurally diverse substances including the wasp venom, mastoparan (MP), activate purified regulatory heterotrimeric guanine nucleotide-binding proteins (G-proteins) in a receptor-independent manner, presumably by mimicking the effects of heptahelical receptors. Mast cells and differentiated HL-60 human leukemic cells are useful model systems for the analysis of receptor-independent G-protein activation. We compared the effects of 2-phenylhistamines which are cationic-amphiphilic, too, and of MP on G-protein activation in dibutyryl cAMP-differentiated HL-60 cells and in the rat basophilic leukemia cell line, RBL 2H3. In HL-60 cells, 2-phenylhistamines show stimulatory effects which resemble those of formyl peptide receptor agonists but which cannot be attributed to agonism at classical receptors. 2-phenylhistamines do not, however, activate RBL 2H3 cells and various other myeloid cell types, pointing to cell type-specificity of receptor-independent G-protein activation. In HL-60 cells, MP shows effects on G-protein activation which differ substantially from those of formyl peptides. In RBL 2H3 membranes, MP shows similar effects on G-protein activation as in HL-60 membranes. We develop a model according to which receptor-independent G-protein activation can be subdivided into direct and indirect receptor-independent G-protein activation. In case of the former mechanism, substances like 2-phenylhistamines interact with G-protein alpha-subunits and in case of the latter mechanism, substances like MP interact with nucleoside diphosphate kinase which catalyzes the formation of GTP. This newly formed GTP is then transferred to, and cleaved by, G-protein alpha-subunits.(ABSTRACT TRUNCATED AT 250 WORDS)