Mastoparan may activate GTP hydrolysis by Gi-proteins in HL-60 membranes indirectly through interaction with nucleoside diphosphate kinase

Clinical manifestations of wasp stings: a case report and a review of literature

Background

Wasp stinging, a neglected tropical entity can have a myriad of local and systemic effects. We present a case of multi-organ injury following multiple wasp stings and a review of literature on the systemic manifestations of wasp stings.

Case presentation

A 48-year-old Sri Lankan male who suffered multiple wasp stings, developed an anaphylactic shock with respiratory failure, which was treated with adrenaline and mechanical ventilation. Within the next 2 days the patient developed acute fulminant hepatitis, stage III acute kidney injury, rhabdomyolysis, haemolysis and thrombocytopenia. The patient was treated in the intensive care unit with ionopressors and continuous renal replacement therapy (CRRT). Haemoadsorbant therapy was used in adjunct with CRRT. There was a gradual recovery of the organ functions over the 1st week. However, the patient succumbed to fungal sepsis on the 16th day despite treatment. We conducted a literature review to identify the various clinical manifestations of wasp stinging. Wasp venom contains enzymes, amines, peptides and other compounds. These proteins can cause type 1 hypersensitive reactions ranging from local skin irritation to anaphylactic shock. Furthermore, the toxins can cause direct organ injury or delayed hypersensitivity reactions. The commonly affected organs are the kidneys, liver, and muscles. The effect on the haematological system manifests as coagulopathy and/or cytopenia. The heart, nervous system, lungs, intestines and skin can be affected rarely. Treatment is mainly supportive.

Conclusion

In conclusion, wasp envenomation can result in multi-organ injury and attention should be paid in doing further research and establishing evidence-based treatment practices.

Mastoparans: A Group of Multifunctional α-Helical Peptides With Promising Therapeutic Properties

Biologically active peptides have been attracting increasing attention, whether to improve the understanding of their mechanisms of action or in the search for new therapeutic drugs. Wasp venoms have been explored as a remarkable source for these molecules. In this review, the main findings on the group of wasp linear cationic α-helical peptides called mastoparans were discussed. These compounds have a wide variety of biological effects, including mast cell degranulation, activation of protein G, phospholipase A₂, C, and D activation, serotonin and insulin release, and antimicrobial, hemolytic, and anticancer activities, which could lead to the development of new therapeutic agents.

MrgX2 is a promiscuous receptor for basic peptides causing mast cell pseudo-allergic and anaphylactoid reactions

Activation of MrgX2, an orphan G protein-coupled receptor expressed on mast cells, leads to degranulation and histamine release. Human MrgX2 binds promiscuously to structurally diverse peptides and small molecules that tend to have basic properties (basic secretagogues), resulting in acute histamine-like adverse drug reactions of injected therapeutic agents. We set out to identify MrgX2 orthologues from other mammalian species used in nonclinical stages of drug development. Previously, the only known orthologue of human MrgX2 was from mouse, encoded by Mrgprb2. MrgX2 genes of rat, dog (beagle), minipig, pig, and Rhesus and cynomolgus monkey were identified by bioinformatic approaches and verified by their ability to mediate calcium mobilization in transfected cells in response to the classical MrgX2 agonist, compound 48/80. The peptide GSK3212448 is an inhibitor of the PRC2 epigenetic regulator that caused profound anaphylactoid reactions upon intravenous infusion to rat. We showed GSK3212448 to be a potent MrgX2 agonist particularly at rat MrgX2. We screened sets of drug-like molecules and peptides to confirm the highly promiscuous nature of MrgX2. Approximately 20% of drug-like molecules activated MrgX2 (pEC₅₀ ranging from 4.5 to 6), with the principle determinant being basicity. All peptides tested of net charge +3 or greater exhibited agonist activity, including the cell penetrating peptides polyarginine (acetyl-Arg₉-amide) and TAT (49-60), a fragment of HIV-1 TAT protein. Finally, we showed that the glycopeptide antibiotic vancomycin, which is associated with clinical pseudo-allergic reactions known as red man syndrome, is an agonist of MrgX2.

Selectivity of pharmacological tools: implications for use in cell physiology. A review in the theme: Cell signaling: proteins, pathways and mechanisms

Pharmacological inhibitors are frequently used to identify the receptors, receptor subtypes, and associated signaling pathways involved in physiological cell responses. Based on the effects of such inhibitors conclusions are drawn about the involvement of their assumed target or lack thereof. While such inhibitors can be useful tools for a better physiological understanding, their uncritical use can lead to incorrect conclusions. This article reviews the concept of inhibitor selectivity and its implication for cell physiology. Specifically, we discuss the implications of using inhibitor vs. activator approaches, issues of direct vs. indirect pathway modulation, implications of inverse agonism and biased signaling, and those of orthosteric vs. allosteric, competitive vs. noncompetitive, and reversible vs. irreversible inhibition. Additional problems can result from inconsistent estimates of inhibitor potency and differences in potency between cell-free systems and intact cells. These concepts are illustrated by several examples of inhibitors displaying affinity for related but distinct targets or even unrelated targets. Of note, many of the issues being addressed are also applicable to genetic inhibition strategies. The main practical conclusion following from these concepts is that investigators should be critical in the choice of inhibitor, its concentrations, and its mode of application. When this advice is adhered to, small-molecule pharmacological inhibitors can be important experimental tools in the hand of physiologists.

Actin dynamics rapidly reset chemoattractant receptor sensitivity following adaptation in neutrophils

Neutrophils are cells of the innate immune system that hunt and kill pathogens using directed migration. This process, known as chemotaxis, requires the regulation of actin polymerization downstream of chemoattractant receptors. Reciprocal interactions between actin and intracellular signals are thought to underlie many of the sophisticated signal processing capabilities of the chemotactic cascade including adaptation, amplification and long-range inhibition. However, with existing tools, it has been difficult to discern actin's role in these processes. Most studies investigating the role of the actin cytoskeleton have primarily relied on actin-depolymerizing agents, which not only block new actin polymerization but also destroy the existing cytoskeleton. We recently developed a combination of pharmacological inhibitors that stabilizes the existing actin cytoskeleton by inhibiting actin polymerization, depolymerization and myosin-based rearrangements; we refer to these processes collectively as actin dynamics. Here, we investigated how actin dynamics influence multiple signalling responses (PI3K lipid products, calcium and Pak phosphorylation) following acute agonist addition or during desensitization. We find that stabilized actin polymer extends the period of receptor desensitization following agonist binding and that actin dynamics rapidly reset receptors from this desensitized state. Spatial differences in actin dynamics may underlie front/back differences in agonist sensitivity in neutrophils.

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).

Initiation of Plasmodium sporozoite motility by albumin is associated with induction of intracellular signalling

Malaria infection is initiated when a mosquito injects Plasmodium sporozoites into a mammalian host. Sporozoites exhibit gliding motility both in vitro and in vivo. This motility is associated with the secretion of at least two proteins, circumsporozoite protein (CSP) and thrombospondin-related anonymous protein (TRAP). Both derive from micronemes, which are organelles that empty out of the apical end of the sporozoite. Sporozoite motility can be initiated in vitro by albumin added to the medium. To investigate how albumin functions in this process, we studied second messenger signalling within the sporozoite. Using pharmacological activators and inhibitors, we have concluded that gliding motility is initiated when albumin interacts with the surface of the sporozoite and that this leads to a signal transduction cascade within the sporozoite, including the elevation of intracellular cAMP, the modulation of sporozoite motility by Ca(2+) and the release of microneme proteins.

Identification of novel synthetic peptide showing angiogenic activity in human endothelial cells

A novel synthetic hexapeptide (SFKLRY-NH(2)) that displays angiogenic activity has been identified by positional scanning of a synthetic peptide combinatorial library (PS-SPCL). The peptide induced proliferation, migration, and capillary-like tube formation in primary cultured HUVECs, and augmented vessel sprouting ex vivo while attenuated by the treatment with pertussis toxin (PTX) or U73122 (PLC-inhibitor) suggesting the influence of PTX-sensitive G-proteins and PLC. In addition, SFKLRY-NH(2) up-regulated the expression of VEGF-A in HUVECs and the neutralizing antibody against VEGF suppressed SFKLRY-NH(2)-induced tube formation activity. Taken together, these results suggest that SFKLRY-NH(2) may induce blood vessel formation by PTX-sensitive G protein-coupled receptor-PLC-Ca(2+) signaling cascade leading into VEGF-A expression in HUVECs.

Insulin inhibits tissue factor expression in monocytes

OBJECTIVES

Platelets from healthy subjects are inhibited by insulin but type 2 diabetes mellitus (T2DM) platelets have become insulin-resistant, which might explain their hyperactivity. In the present study we investigated whether monocytes are responsive to insulin.

METHODS AND RESULTS

LPS-induced tissue factor (TF) upregulation was measured in human monocytes and monocytic THP-1 cells in a factor Xa generation assay. Insulin (0.1-100 nmol L(-1)) induced a dose-dependent inhibition in both cell types and in monocytes 100 nmol L(-1) insulin inhibited cytosolic, membrane-bound and microparticle TF by 32 +/- 2, 27 +/- 3 and 52 +/- 4% (n = 3). Insulin induced Tyr phosphorylation of the insulin receptor (INS-R) and formation of an INS-R - G(i)alpha(2) complex, suggesting interference with LPS-induced cAMP control. Indeed, insulin interfered with LPS-induced cAMP decrease and TF upregulation in a manner similar to an inhibitor of G(i) (pertussis toxin) and agents that raise cAMP (iloprost, forskolin, IBMX) reduced TF upregulation. Although LPS failed to raise cytosolic Ca(2+), quenching of Ca(2+) increases (BAPTA-AM) reduced and induction of Ca(2+) entry (ionophore, P2X7 activation) enhanced upregulation of TF mRNA and procoagulant activity. Insulin interfered with MCP-1-induced Ca(2+) mobilization but not with ATP-induced Ca(2+) rises.

CONCLUSIONS

Insulin inhibits TF expression in monocytes and monocyte-derived microparticles through interference with G(i)alpha(2)-mediated cAMP suppression, which attenuates Ca(2+)-mediated TF synthesis.

Lipid and peptide control of phosphatidylinositol 4-kinase IIalpha activity on Golgi-endosomal Rafts

The most abundant and widely expressed mammalian phosphoinositide kinase activity is contributed by phosphatidylinositol 4-kinase IIalpha (PI4KIIalpha). In this study we demonstrate that PI4KIIalpha is a novel GTP-independent target of the wasp venom tetradecapeptide mastoparan and that different mechanisms of activation occur in different subcellular membranes. Following cell membrane fractionation mastoparan specifically stimulated a high activity Golgi/endosomal pool of PI4KIIalpha independently of exogenous guanine nucleotides. Conversely, GTPgammaS stimulated a low activity pool of PI4KIIalpha in a separable dense membrane fraction and this response was further enhanced by mastoparan. Overexpression of PI4KIIalpha increased the basal phosphatidylinositol 4-kinase activity of each membrane pool, as well as the mastoparan-dependent activities, thereby demonstrating that mastoparan specifically activates this isozyme. Both mastoparan and M7, at concentrations known to invoke secretion, stimulated PI4KIIalpha with similar efficacies, resulting in an increase in the apparent V(max) and decrease in K(m) for exogenously added PI. Mastoparan also stimulated PI4KIIalpha immunoprecipitated from the raft fraction, indicating that PI4KIIalpha is a direct target of mastoparan. Finally we reveal a striking dependence of both basal and mastoparan-stimulated PI4KIIalpha activity on endogenous cholesterol concentration and therefore conclude that changes in membrane environment can regulate PI4KIIalpha activity.

Protein kinase C activation generates superoxide and contributes to impairment of cerebrovasodilation induced by G protein activation after brain injury

Previous studies have observed that activation of protein kinase C (PKC) contributes to generation of superoxide anion (O(-)(2)) after fluid percussion brain injury (FPI). This study was designed to characterize the effects of FPI on the vascular activity of two activators of a pertussis toxin sensitive G protein, mastoparan and mastoparan-7, and the role of PKC dependent O(-)(2) generation in such effects in newborn pigs equipped with a closed cranial window. Mastoparan (10(-8), 10(-6) M) elicited pial artery dilation that was blunted by FPI and partially restored by the PKC inhibitor chelerythrine (10(-7) M) or the O(-)(2) free radical scavengers polyethylene glycol superoxide dismutase and catalase (SODCAT) (9+/-1 and 16+/-1, sham control; 3+/-1 and 5+/-1, FPI; and 7+/-1 and 11+/-1%, FPI SODCAT pretreated). Similar results were observed for mastoparan-7 but the inactive analogue mastoparan-17 had no effect on pial artery diameter. Exposure of the cerebral cortex to a xanthine oxidase O(-)(2) generating system blunted mastoparan induced pial artery dilation similar to FPI (10+/-1 and 17+/-1 vs. 2+/-1 and 3+/-1%). Pertussis toxin (1 microg/ml) exposure blocked mastoparan and mastoparan-7 vasodilation. These data show that pertussis toxin sensitive G protein activation elicits cerebrovasodilation that is blunted following FPI in a PKC dependent manner. These data also show that O(-)(2) generation similarly blunts G protein mediated cerebrovasodilation. These data suggest that PKC dependent O(-)(2) generation contributes to impaired G protein mediated cerebrovasodilation after FPI.

Regulation of mastoparan-induced increase of paracellular permeability in T84 cells by RhoA and basolateral potassium channels

Mastoparan, a polypeptide known to activate heterotrimeric GTP-binding proteins, enhances the transport of Ca2+ and K+ across membranes. In the present study we investigated the influence of mastoparan on transepithelial resistance (TER) and on short circuit current (SCC) of the intestinal cell line T84. Mastoparan decreased the TER by 80% of baseline and induced a SCC of 8.34+/-1.38 microAcm(-2). The changes in paracellular conductance were estimated using the nystatin technique and showed that mastoparan increased the paracellular conductance 4-fold. Basolateral Cl(-)-free medium, or blockade of the basolateral Cl(-) uptake via the Na+/K+/2Cl(-) co-transporter with bumetanide, reduced SCC of T84 cells, but did not abolish the effect of mastoparan on the TER. Luminal addition of the Cl(-)-channel blocker DIDS or NPPB had no effect on the increase in SCC. In contrast, blocking the basolateral K(+)-channels by 2mM Ba2+ inhibited both the resistance decrease and elevation of the SCC, and further inhibited the mastoparan-induced increase in intracellular free Ca2. This indicates that mastoparan acts primarily via activating K+ channels with a secondary Cl(-) secretion and Ca2+ influx. Reduction of intracellular free Ca2+ did not alter the effect of mastoparan on TER. Stimulation with mastoparan led to a biphasic rearrangement of actin filaments and increased globular actin content in T84 cells. Depolymerization of actin filaments also correlated with inactivation of Rho-proteins, which are known regulators of the cytoskeleton. Mastoparan induced a 2-fold increase in GDI-complexed Rho. We conclude that mastoparan-induced changes in paracellular permeability are mediated via enhanced basolateral K+ conductance and Rho-protein inactivation. A secondary increase in intracellular Ca2+ or direct interaction of small GTPases with the cytoskeleton are likely mediators of the remodeling of the cytoskeleton with subsequent changes in paracellular permeability.

Activation of heterotrimeric G proteins by a high energy phosphate transfer via nucleoside diphosphate kinase (NDPK) B and Gbeta subunits. Complex formation of NDPK B with Gbeta gamma dimers and phosphorylation of His-266 IN Gbeta

G protein betagamma dimers can be phosphorylated in membranes from various tissues by GTP at a histidine residue in the beta subunit. The phosphate is high energetic and can be transferred onto GDP leading to formation of GTP. Purified Gbetagamma dimers do not display autophosphorylation, indicating the involvement of a separate protein kinase. We therefore enriched the Gbeta-phosphorylating activity present in preparations of the retinal G protein transducin and in partially purified G(i/o) proteins from bovine brain. Immunoblots, autophosphorylation, and enzymatic activity measurements demonstrated enriched nucleoside diphosphate kinase (NDPK) B in both preparations, together with residual Gbetagamma dimers. In the retinal NDPK B-enriched fractions, a Gbeta-specific antiserum co-precipitated phosphorylated NDPK B, and an antiserum against the human NDPK co-precipitated phosphorylated Gbetagamma. In addition, the NDPK-containing fractions from bovine brain reconstituted the phosphorylation of purified Gbetagamma. For identification of the phosphorylated histidine residue, bovine brain Gbetagamma and G(t)betagamma were thiophosphorylated with guanosine 5'-O-(3-[(35)S]thio)triphosphate, followed by digestion with endoproteinase Glu-C and trypsin, separation of the resulting peptides by gel electrophoresis and high pressure liquid chromatography, respectively, and sequencing of the radioactive peptides. The sequence information produced by both methods identified specific labeled fragments of bovine Gbeta(1) that overlapped in the heptapeptide, Leu-Met-Thr-Tyr-Ser-His-Asp (amino acids 261-267). We conclude that NDPK B forms complexes with Gbetagamma dimers and contributes to G protein activation by increasing the high energetic phosphate transfer onto GDP via intermediately phosphorylated His-266 in Gbeta(1) subunits.

Cholesterol cell content modulates GTPase activity of G proteins in GH4C1 cell membranes

Previous results from our laboratory showed that GH(4)C(1) cells with low-cholesterol cell content had increased adenylyl cyclase (AC) activity with a parallel increase in G protein alpha subunits associated to the plasma membrane. This effect was directly related to mevalonate availability. In the present report, we characterized the high-affinity GTPase activity present in GH(4)C(1) cell membranes and studied its regulation by cholesterol cell content. The high-affinity GTPase activity, measured as the [gamma32P]GTP hydrolysis rate, was both time-dependent and protein concentration-dependent. Cultured cells with lipoprotein-deficient serum (LPDS) showed decreased cholesterol cell content and decreased GTPase activity. The kinetic analysis, as interpreted by Lineweaver-Burk plots, indicated that low-cholesterol cell content had no effect on the apparent affinity for GTP, but resulted in a 47% decrease in the maximal velocity of the reaction. Addition of 25-hydroxycholesterol (25-HC), an inhibitor of the expression of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and synthetase to cells in LPDS, further decreased GTPase activity in a dose-dependent manner. This effect was reverted by exogenous cholesterol, but not by mevalonate. Studies with bacterial toxins revealed that neither cholera toxin (CTX) nor pertussis toxins (PTX) were able to revert the inhibition produced by low-cholesterol cell content. These results allowed us to postulate that cholesterol modulates GTPase activity in both Gs and Gi protein families. To analyse further the mechanism of modulation of GTPase activity by cholesterol cell content, [35S]GTPgammaS binding in membranes of GH(4)C(1) cells was studied. Changes in cholesterol cell content did not have any effect on GTP binding. Data demonstrated that high-affinity GTPase activity in plasma membrane of GH(4)C(1) cells is direct stimulated by cholesterol cell content and not by mevalonate availability. This example provides a mechanism by which cholesterol cell content can modulate signal transduction mediating by G proteins.

ET-1 contributes to age-dependent G protein impairment after brain injury

Previous studies have observed that endothelin-1 (ET-1) concentration is elevated in CSF and contributes to impaired cerebral hemodynamics following fluid percussion brain injury (FPI) in an age-dependent manner. This study was designed to characterize the effects of FPI on the vascular activity of two activators of a pertussin toxin-sensitive G protein, mastoparan and mastoparan-7, as a function of age and the role of ET-1 in such effects in newborn (1-5 days old) and juvenile (3-4 weeks old) pigs equipped with a closed cranial window. Mastoparan (10(-8), 10(-6) M) elicited pial artery dilation that was blunted more by FPI in newborn versus juvenile pigs (9 +/- 1 and 16 +/- 1 vs. 3 +/- 1 and 5 +/- 1%, newborn; 9 +/- 1 and 15 +/- 1 vs. 6 +/- 1 and 9 +/- 1%, juvenile). Similar results were observed for mastoparan-7, but the inactive analogue mastoparan-17 had no effect on pial diameter. BQ123 (10(-6) M), an ET-1 antagonist, partially restored impaired mastoparan dilation after FPI in the newborn but not in the juvenile (3 +/- 1 and 5 +/- 1 vs. 7 +/- 1 and 11 +/- 1%, newborn; 6 +/- 1 and 9 +/- 1 vs. 6 +/- 1 and 10 +/- 1%, juvenile). These data show that G protein activation elicits cerebrovasodilation that is blunted following FPI in an age-dependent manner. These data suggest that ET-1 contributes to the impairment of G protein-mediated vasodilation in an age-dependent manner after FPI.

G protein activation elicits cerebrovasodilation through interaction with K(ATP) and K(Ca) channels

This study was designed to characterize the vascular activity of two activators of a pertussis toxin-sensitive G protein, mastoparan and mastoparan-7, on pial artery diameter and their interaction with the ATP sensitive K (K(ATP)) and calcium sensitive K (K(Ca)) channel in newborn pigs equipped with a closed cranial window. Mastoparan (10(-8), 10(-6) M) elicited pial artery dilation that was blunted by the K(ATP) channel antagonist glibenclamide (10(-6) M) and attenuated by the K(Ca) channel antagonist iberiotoxin (10(-7) M) (10+/-1 and 17+/-1 vs. 3+/-1 and 6+/-1 vs. 6+/-1 and 10+/-2% for mastoparan in the absence and presence of glibenclamide or iberiotoxin, respectively). Similar results were observed for mastoparan-7 but the inactive analogue mastoparan-17 had no effect on pial artery diameter. These data show that G protein activation elicits cerebrovasodilation through interaction with K(ATP) and K(Ca) channels.

Heterotrimeric G-proteins in plant cell signaling

Heterotrimeric G-proteins, which couple cell surface receptors with internal effectors, are evident in all eukaryotes. Their operation involves receptor activation, GTP/GDP exchange and modulation of effector activity; deactivation occurs by an intrinsic GTPase activity. Structurally, G-proteins comprise three dissimilar subunits; Gα, Gβ and Gγ. The Gα subunit consists of an α-helical and a GTPase domain, the latter is responsible for interaction with Gβγ, receptor and effector. Gβ and Gγ form a tightly associated heterodimer which can also modulate effector activity when released by the activated Gα. Genome sequence and other data suggest that, in plants, there are several (~8-10?) Gα, one or two Gβ and one Gγ. These proteins are expressed throughout the plant, mainly in the plasma membrane and endoplasmic reticulum. In vivo, there is strong evidence for G-protein control of ion channels, particularly K⁺ , in the response pathways to fungal and bacterial pathogens as well as in some aspects of gibberellin, abscisic acid and auxin signaling pathways. Finally, future prospects for understanding plant G-protein linked signaling will rely on new and emerging technologies; these include antisense suppression, gene knockouts, yeast two-hybrid and phage display molecular approaches, intracellular immunization using recombinant single chain antibodies and expression of peptide encoding minigenes.

Molecular analysis of beta(2)-adrenoceptor coupling to G(s)-, G(i)-, and G(q)-proteins

The beta(2)-adrenoceptor (beta(2)AR) couples to the G-protein G(s) to activate adenylyl cyclase. Intriguingly, several studies have demonstrated that the beta(2)AR can also interact with G-proteins of the G(i)- and G(q)-family. To assess the efficiency of beta(2)AR interaction with various G-protein alpha-subunits (G(xalpha)), we expressed fusion proteins of the beta(2)AR with the long (G(salphaL)) and short (G(salphaS)) splice variants of G(salpha), the G(i)-proteins G(ialpha2) and G(ialpha3), and the G(q)-proteins G(qalpha) and G(16alpha) in Sf9 cells. Fusion proteins provide a rigorous approach for comparing the coupling of a given receptor to G(xalpha) because of the defined 1:1 stoichiometry of receptor and G-protein and the efficient coupling. Here, we show that the beta(2)AR couples to G(s)-, G(i)-, and G(q)-proteins as assessed by ternary complex formation and ligand-regulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding. The combined analysis of ternary complex formation, GTPgammaS binding, agonist efficacies, and agonist potencies revealed substantial differences in the interaction of the beta(2)AR with the various classes of G-proteins. Comparison of the coupling of the beta(2)AR and formyl peptide receptor to G(ialpha2) revealed receptor-specific differences in the kinetics of GTPgammaS binding. We also detected highly efficient stimulation of GTPgammaS dissociation from G(salphaL), but not from G(qalpha) and G(16alpha), by a beta(2)AR agonist. Moreover, we show that the 1:1 stoichiometry of receptor to G-protein in fusion proteins reflects the in vivo stoichiometry of receptor/G-protein coupling more closely than was previously assumed. Collectively, our data show 1) that the beta(2)AR couples differentially to G(s)-, G(i)-, and G(q)-proteins, 2) that there is ligand-specific coupling of the beta(2)AR to G-proteins, 3) that receptor-specific G-protein conformational states may exist, and 4) that nucleotide dissociation is an important mechanism for G-protein deactivation.

Histamine release from mast cells of EAE rats by Gi protein-dependent and IgE-dependent pathways

We investigated both Gi protein-dependent and IgE-dependent pathways that control release of histamine by PMCs derived from EAE or Complete Freund's Adjuvant (CFA) immunized rats. The number and histamine content of MCs per rat were the same between normal and EAE rats. Activation of Gi pathway by substance P (SP), DSA, 48/80, and mastoparan resulted in a dose-dependent increase in release of histamine by PMCs in normal, EAE-, and CFA-immunized rats. In EAE and CFA rats, however, the induction was decreased by 10-20% compared to normal rats. The histamine release induced by MP was decreased at a concentration of 3 microM, but not at 10 microM in severe active EAE rats. Activation of the IgE pathway by MAM and concanavalin A (Con A) in the presence of phosphatidylserine led to dose-dependent histamine release in normal rats, and a 10-25% lower level of induction was observed in EAE rats. In CFA rats, the induction of histamine release was equivalent to normal rats. There was an increase in intracellular calcium stores following activation of both pathways in normal rats, whereas depletion of calcium stores by ryanodine reduced the level of induction by 48/80 and MP by 9-11% in normal rats. In EAE rats, 48/80, Con A, and MAM induced a smaller increase, but SP and MP induced larger or similar increases in calcium stores compared to normal rats. It was unlikely that the calcium stores of the PMCs from EAE rats were depleted, because MP stimulated calcium movement subsequent to the release of histamine. These results suggested that the Gi pathway may not be correlated to clinical manifestation of EAE, but cold be involved in the inflammatory process, and that the IgE pathway is better associated with clinical symptoms of EAE and may be more directly related to disease outcome.

Nucleoside diphosphate kinase activity in soluble transducin preparations biochemical properties and possible role of transducin-beta as phosphorylated enzyme intermediate

Known nucleoside diphosphate kinases (NDPKs) are oligomers of 17-23-kDa subunits and catalyze the reaction N1TP + N2DP --> N1DP + N2TP via formation of a histidine-phosphorylated enzyme intermediate. NDPKs are involved in the activation of heterotrimeric GTP-binding proteins (G-proteins) by catalyzing the formation of GTP from GDP, but the properties of G-protein-associated NDPKs are still incompletely known. The aim of our present study was to characterize NDPK in soluble preparations of the retinal G-protein transducin. The NDPK is operationally referred to as transducin-NDPK. Like known NDPKs, transducin-NDPK utilizes NTPs and phosphorothioate analogs of NTPs as substrates. GDP was a more effective phosphoryl group acceptor at transducin-NDPK than ADP and CDP, and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) was a more effective thiophosphoryl group donor than adenosine 5'-[gamma-thio]triphosphate (ATP[S]). In contrast with their action on known NDPKs, mastoparan and mastoparan 7 had no stimulatory effect on transducin-NDPK. Guanosine 5'-[beta, gamma-imido]triphosphate (p[NH]ppG) potentiated [3H]GTP[S] formation from [3H]GDP and ATP[S] but not [3H]GTP[S] formation from [3H]GDP and GTP[S]. Depending on the thiophosphoryl group acceptor and donor, [3H]NTP[S] formation was differentially regulated by Mg2+, Mn2+, Co2+, Ca2+ and Zn2+. [gamma-32P]ATP and [gamma-32P]GTP [32P]phosphorylated, and [35S]ATP[S] [35S]thiophosphorylated, a 36-kDa protein comigrating with transducin-beta. p[NH]ppG potentiated [35S]thiophosphorylation of the 36-kDa protein. 32P-labeling of the 36-kDa protein showed characteristics of histidine phosphorylation. There was no evidence for (thio)phosphorylation of 17-23-kDa proteins. Our data show the following: (a) soluble transducin preparations contain a GDP-prefering and guanine nucleotide-regulated NDPK; (b) transducin-beta may serve as a (thio)phosphorylated NDPK intermediate; (c) transducin-NDPK is distinct from known NDPKs and may consist of multiple kinases or a single kinase with multiple regulatory domains.

Mechanism of peptide-induced mast cell degranulation. Translocation and patch-clamp studies

Substance P and other polycationic peptides are thought to stimulate mast cell degranulation via direct activation of G proteins. We investigated the ability of extracellularly applied substance P to translocate into mast cells and the ability of intracellularly applied substance P to stimulate degranulation. In addition, we studied by reverse transcription--PCR whether substance P-specific receptors are present in the mast cell membrane. To study translocation, a biologically active and enzymatically stable fluorescent analogue of substance P was synthesized. A rapid, substance P receptor- and energy-independent uptake of this peptide into pertussis toxin-treated and -untreated mast cells was demonstrated using confocal laser scanning microscopy. The peptide was shown to localize preferentially on or inside the mast cell granules using electron microscopic autoradiography with 125I-labeled all-D substance P and 3H-labeled substance P. Cell membrane capacitance measurements using the patch-clamp technique demonstrated that intracellularly applied substance P induced calcium transients and activated mast cell exocytosis with a time delay that depended on peptide concentration (delay of 100-500 s at concentrations of substance P from 50 to 5 microM). Degranulation in response to intracellularly applied substance P was inhibited by GDPbetaS and pertussis toxin, suggesting that substance P acts via G protein activation. These results support the recently proposed model of a receptor-independent mechanism of peptide-induced mast cell degranulation, which assumes a direct interaction of peptides with G protein alpha subunits subsequent to their translocation across the plasma membrane.

High constitutive activity of the human formyl peptide receptor

The formyl peptide receptor (FPR) couples to pertussis toxin (PTX)-sensitive Gi-proteins to activate chemotaxis and exocytosis in neutrophils. PTX reduces not only formyl peptide-stimulated but also agonist-independent ("basal") Gi-protein activity, suggesting that the FPR is constitutively active. We aimed at identifying an inverse FPR agonist, i.e. a compound that suppresses constitutive FPR activity. In Sf9 insect cell membranes, the G-protein heterotrimer Gialpha2beta1gamma2 reconstituted N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP)-stimulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding and GTPgammaS-sensitive high affinity [3H]FMLP binding. The FPR "antagonist" cyclosporin H (CsH) potently and efficiently reduced basal GTPgammaS binding in Sf9 membranes. Another FPR antagonist, N-t-butoxycarbonyl-L-phenylalanyl-L-leucyl-L-phenylalanyl-L-leucyl-L- phenylalanine did not inhibit basal GTPgammaS binding but blocked the inhibitory effect of CsH on GTPgammaS binding. Na+ reduced basal GTPgammaS binding and eliminated the inhibitory effect of CsH. Similar effects of FMLP, CsH, and Na+ as in Sf9 membranes were observed with FPR expressed in the mammalian cell line HEK293. Our data show that the human FPR possesses high constitutive activity. CsH is an inverse FPR agonist and stabilizes the FPR in an inactive state. Na+ also stabilizes the FPR in an inactive state and, thereby, diminishes inverse agonist efficacy.

Mastoparan-induced apoptosis of cultured cerebellar granule neurons is initiated by calcium release from intracellular stores

We have recently reported that mastoparan, a peptide toxin isolated from wasp venom, induces apoptosis in cultured cerebellar granule neurons that can be blocked by cholera toxin, an activator of Gs. Measurements of intracellular free calcium concentration ([Ca2+]i) reveal that mastoparan induces a dramatic elevation of [Ca2+]i that is frequently followed by enhanced leakage of fura-2 out of the neurons, suggesting that this rise in [Ca2+]i may be due to a more generalized change in membrane permeability. However, the mastoparan-induced initial elevation of [Ca2+]i is maintained in the absence of extracellular Ca2+, suggesting that the rise of [Ca2+]i is from intracellular stores. This conclusion is supported by the observation that depletion of [Ca2+]i stores by pretreatment with either caffeine or thapsigargin attenuates both the rise in [Ca2+]i and cell death induced by mastoparan. Phospholipase C (PLC) inhibitors, neomycin and U73122 block mastoparan-induced increases of [Ca2+]i and protect against neuronal death. Pretreatment with cholera toxin, but not pertussis toxin, reduced the mastoparan-induced rise in [Ca2+]i. Taken together, our data suggest that mastoparan initiates cell death in cerebellar granule neurons by inducing Ca2+ release from intracellular stores, probably via activation of PLC and IP3. A secondary or parallel process results in disruption of plasma membrane integrity and may be ultimately responsible for the death of these neurons by mastoparan.

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.

Sodium saccharin inhibits adenylyl cyclase activity in non-taste cells

We have studied the in vitro effect of sodium saccharin (NaSacch) on the rat adipocyte adenylyl cyclase complex. NaSacch (2.5-50 mM) inhibited significantly in a dose-dependent manner basal and isoproterenol-stimulated cAMP accumulation on isolated rat adipocytes. Similarly, NaSacch (2.5-50 mM) inhibited forskolin-stimulated adenylyl cyclase activity measured in the presence of Mg(2+)-ATP on adipocyte, astrocyte and thyrocyte membrane fractions. In contrast, NaSacch did not inhibit but slightly increased the forskolin-stimulated adenylyl cyclase activity measured in the presence of Mn(2+)-ATP and GDP beta S, a stable GDP analogue. The effect of NaSacch was not mediated through either the A1-adenosine receptor (A1R) or the alpha 2-adrenergic receptor (alpha 2AR). The inhibitory effect of NaSacch was additive to that of A1R agonist and was not blocked by the addition of the alpha 2AR antagonist RX 821002. Pretreatment of adipocytes with pertussis toxin slightly attenuated but did not abolish the inhibitory effect of NaSacch on forskolin-stimulated adenylyl cyclase activity on membrane fractions. These data suggest that the inhibitory effect of NaSacch on forskolin stimulated-adenylyl cyclase in adipocytes does not imply only Gi protein but also other direct or indirect inhibitory pathway(s) which remain to be determined.

Effects of the wasp venom peptide, mastoparan, on GTP hydrolysis in rat brain membranes

1. The effects of mastoparan, a wasp venom toxin, on GTP hydrolyzing activity were examined in rat brain membranes. 2. Mastoparan inhibited the low-affinity GTPase activity, defined as the amount of 32Pi released from 0.3 microm [gamma-32P]-GTP in the presence of 100 microM unlabelled GTP, in a concentration-dependent manner. This inhibitory effect of mastoparan on low-affinity GTPase activity was diminished by increasing concentrations of UDP and was completely attenuated at 20 mM, indicating that activation of nucleoside diphosphokinase (NDPK) is inolved in the phenomenon. 3. In the presence of 20 mM UDP, mastoparan stimulated the high-affinity GTPase activity by increasing the Vmax value without affecting the apparent K(M) for GTP. Mastoparan-stimulated high-affinity GTPase activity was apparent at concentrations higher than 1 microM, in a concentration-dependent manner, but without saturation even at 100 microM. 4. Mastoparan-induced high-affinity GTPase activity showed a characteristic sensitivity to MgCl2, quite different from that seen in L-glutamate-stimulated activity, a representative of receptor-mediated G-protein activation. 5. There appeared to be a simple additive interaction between mastoparan- and L-glutamate-stimulated high-affinity GTPase activities, indicting that distinct pools of G-proteins are involved in receptor-independent and receptor-mediated G-protein activation. 6. These results suggest that G-proteins in brain membranes are functionally altered by mastoparan through multiple mechanisms of action and that the mastoparan-induced, direct G-protein activating process lacks a synergistic or antagonistic interaction with an agonist-induced, receptor-mediated activation of G-proteins.

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.

A 16-kDa protein functions as a new regulatory protein for Hsc70 molecular chaperone and is identified as a member of the Nm23/nucleoside diphosphate kinase family

Cytoplasmic Hsc70 is a multifunctional molecular chaperone. It is hypothesized that accessory proteins are used to specify the diverse chaperone activities of Hsc70. A 16-kDa cytosolic protein (p16) co-purified with Hsc70 obtained from a fish hepatocyte cell line, PLHC-1. Hsc70 also co-immunoprecipitated with p16 from PLHC-1 cells and fish liver. p16 was identified as a member of the Nm23/nucleoside diphosphate (NDP) kinase family based on its amino acid sequence similarity, NDP kinase activity, and recognition by anti-human NDP kinase-A antibody. This antibody also co-immunoprecipitated Hsc70 and NDP kinase from human HepG2 cells. p16 monomerized Hsc70 and released Hsc70 from pigeon cytochrome c peptide (Pc) but not from FYQLALT, a peptide specifically designed for high affinity binding. Therefore, p16 may modulate Hsc70 function by maintaining Hsc70 in a monomeric state and by dissociating unfolded proteins from Hsc70 either through protein-protein interactions or by supplying ATP indirectly through phosphate transfer. p16 did not affect basal or unfolded protein-stimulated ATPase activity of bovine brain Hsc70 using in vitro assays. Interestingly, bovine liver NDP kinase did not dissociate the Hsc70.Pc complex. In addition, two nonconservative amino acid subsitutions were found near the amino terminus of p16. Therefore, p16 may be a unique Nm23/NDP kinase that functions as an accessory protein for cytosolic Hsc70 in eukaryotes.

Induction of cytosolic Ca2+ elevation mediated by Mas-7 occurs through membrane pore formation

Mas-7, a mastoparan derivative, induces elevation of intracellular free Ca2+ concentration ([Ca2+]i) along two independent pathways. The minor contribution occurs via phospholipase C activation and is negatively regulated by treatment with phorbol 12-myristate 13-acetate, a protein kinase C activator. The major contribution involves plasma membrane pores allowing not only Ca2+, Mn2+, and Na+ to enter but also the uptake of ethidium bromide (314 Da) and lucifer yellow (457 Da), but not fura-2 (831 Da), Evans blue (961 Da), and fluorescein-conjugate phalloidin (1,175 Da). Mas-7-induced current, as measured in planar lipid bilayers, reveals that Mas-7-induced pores have two slope conductances, 290 and 94 pS, and that the pores are nonselective for cations. The results also indicate that Mas-7 can produce pores by direct interaction with the plasma membrane without the involvement of membrane proteins and cytosolic factors. Besides in human neuroblastoma cells, similar Mas-7 effects were also observed in other cell lines such as HL-60, 1321N1 human astrocytoma, and bovine chromaffin cells. The data suggest that the Mas-7-induced [Ca2+]i elevation is the combined result of Ca2+ release from stores via phosphoinositide turnover and prolonged Ca2+ influx through membrane pores.

Potential roles of heterotrimeric G proteins of the endomembrane system

Heterotrimeric G proteins are recognized as versatile switches linking cell surface receptors to cellular effectors. Beside their location at the plasma membrane G proteins are found on intracellular membranes. Studies with modulators of G protein activity suggest that G proteins associated with organelle membranes are involved in various steps of secretion and vesicular function. In contrast to hormonal responses involving G proteins little is currently known about possible receptors or activators and effectors interacting with intracellular G proteins. This short review focuses on recent developments elucidating the role of organelle-associated G proteins.

Phosphotransfer reactions as a means of G protein activation

Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) serve to transduce information from agonist-bound receptors to effector enzymes or ion channels. Current models of G protein activation-deactivation indicate that the oligomeric GDP-bound form must undergo release of GDP, bind GTP and undergo subunit dissociation, in order to be in active form (GTP bound alpha subunits and free beta gamma dimers) and to regulate effectors. The effect of receptor occupation by an agonist is generally accepted to be promotion of guanine nucleotide exchange thus allowing activation of the G protein. Recent studies indicate that transphosphorylation leading to the formation of GTP from GDP and ATP in the close vicinity, or even at the G protein, catalysed by membrane-associated nucleoside diphosphate kinase, may further activate G proteins. This activation is demonstrated by a decreased affinity of G protein-coupled receptors for agonists and an increased response of G protein coupled effectors. In addition, a phosphorylation of G protein beta subunits and consequent phosphate transfer reaction resulting in G protein activation has also been demonstrated. Finally, endogenously formed GTP was preferentially effective in activating some G proteins compared to exogenous GTP. The aim of this report is to present an overview of the evidence to date for a transphosphorylation as a means of G protein activation (see also refs [1 and 2] for reviews).

Neutralizing antibodies to nucleoside diphosphate kinase inhibit the enzyme in vitro and in vivo: evidence for two distinct mechanisms of activation of atrial currents by ATPgammaS

Nucleoside diphosphate kinase (NDPK) participates in multiple cellular functions, yet the molecular mechanisms of its involvement are often unknown, given that there are no specific inhibitors for the enzyme from vertebrates. We developed antibodies against NDPK by immunization of rabbits with the enzyme from bullfrog skeletal muscle. The antibodies specifically recognized the enzyme from frog tissues, and cross-reacted with NDPK from Xenopus. In contrast to mammalian NDPK, the amphibian enzyme elicited antibodies that inhibit potently its catalytic function. We utilized the inhibitory properties of these immunoglobulins to examine the role of NDPK on the ATPgammaS-induced stimulation of Ca2+ and K+ currents of cardiac myocytes. Injection of NDPK-neutralizing Fab fragments into atrial cells reduced considerably the effect of ATPgammaS on muscarinic K+ currents, but not on Ca2+ currents. Therefore, ATPgammaS increases calcium and potassium currents of atrial cells by two distinct mechanisms. NDPK is essential for the conversion of ATPgammaS into GTPgammaS which leads to muscarinic K+ channel activation but not for the stimulation of Ca2+ currents by ATPgammaS. The results demonstrate that antibodies to frog NDPK block the activity of the enzyme in vivo and in vitro, and can be used to determine the relevance of NDPK and its catalytic activity to the function of vertebrate cells.

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)

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

In dibutyryl cAMP-differentiated human leukemia (HL-60) cells, the potent histamine H1-receptor agonist, 2-(3-chlorophenyl)histamine, activates pertussis toxin (PTX)-sensitive guanine nucleotide-binding proteins (G-proteins) of the Gi-subfamily by a mechanism which is independent of known histamine receptor subtypes (Seifert et al. Mol Pharmacol 45: 578-586, 1994). In order to learn more about this G-protein activation, we studied the effects of histamine and various 2-substituted histamine derivatives in various cell types and on purified G-proteins. In HL-60 cells, histamine and 2-methylhistamine increased cytosolic Ca2+ concentration ([Ca2+]i) in a clemastine-sensitive manner. Phenyl- and thienyl-substituted histamines increased [Ca2+]i as well, but their effects were not inhibited by histamine receptor antagonists. 2-Substituted histamines activated high-affinity GTPase in HL-60 cell membranes in a PTX-sensitive manner, with the lipophilicity of substances increasing their effectiveness. Although HEL cells do not possess histamine receptors mediating rises in [Ca2+]i, 2-(3-bromophenyl)histamine increased [Ca2+]i in a PTX-sensitive manner. It also increased GTP hydrolysis by Gi-proteins in HEL cell membranes. All these stimulatory effects of 2-substituted histamine derivatives were seen at concentrations higher than those required for activation of H1-receptors. In various other cell types and membrane systems, 2-substituted histamine derivatives showed no or only weak stimulatory effects on G-proteins. 2-Substituted histamine derivatives activated GTP hydrolysis by purified bovine brain Gi/Go-proteins and by pure Gi2 (the major PTX-sensitive G-protein in HL-60 and HEL cells). Our data suggest the following: (1) histamine and 2-methylhistamine act as H1-receptor agonists in HL-60 cells; (2) incorporation of bulky and lipophilic groups results in loss of H1-agonistic activity of 2-substituted histamine derivatives in HL-60 cells but causes a receptor-independent G-protein-stimulatory activity; (3) the effects of 2-substituted histamine derivatives on G-proteins are cell-type specific.