The actions of Ca2+ ionophores on rat basophilic (2H3) cells are dependent on cellular ATP and hydrolysis of inositol phospholipids. A comparison with antigen stimulation

Off-target effect of the cPLA2α inhibitor pyrrophenone: Inhibition of calcium release from the endoplasmic reticulum

Cytosolic phospholipase A2α (cPLA2α) mediates agonist-induced release of arachidonic acid from membrane phospholipid for production of eicosanoids. The activation of cPLA2α involves increases in intracellular calcium, which binds to the C2 domain and promotes cPLA2α translocation from the cytosol to membrane to access substrate. The cell permeable pyrrolidine-containing cPLA2α inhibitors including pyrrophenone have been useful to understand cPLA2α function. Although this serine hydrolase inhibitor does not inhibit other PLA2s or downstream enzymes that metabolize arachidonic acid, we reported that it blocks increases in mitochondrial calcium and cell death in lung fibroblasts. In this study we used the calcium indicators G-CEPIA1er and CEPIA2mt to compare the effect of pyrrophenone in regulating calcium levels in the endoplasmic reticulum (ER) and mitochondria in response to A23187 and receptor stimulation. Pyrrophenone blocked calcium release from the ER and concomitant increases in mitochondrial calcium in response to stimulation by ATP, serum and A23187. In contrast, ER calcium release induced by the sarco/endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin was not blocked by pyrrophenone suggesting specificity for the calcium release pathway. As a consequence of blocking calcium mobilization, pyrrophenone inhibited serum-stimulated translocation of the cPLA2α C2 domain to Golgi. The ability of pyrrophenone to block ER calcium release is an off-target effect since it occurs in fibroblasts lacking cPLA2α. The results implicate a serine hydrolase in regulating ER calcium release and highlight the importance of careful dose-response studies with pyrrophenone to study cPLA2α function.

Methoxychlor enhances degranulation of murine mast cells by regulating FcεRI-mediated signal transduction

Methoxychlor, an organochlorine insecticide developed to replace DDT (dichlorodiphenyltrichloroethane), has been reported to induce mast cell degranulation and to enhance IgE-mediated allergic responses. However, the mechanisms underlying these effects are not clear. To clarify potential mechanisms, the effects of methoxychlor on degranulation of mast cells were examined. Degranulation responses were evaluated using RBL-2H3 cells and mouse bone marrow-derived mast cells with either the antigen-induced or calcium ionophore-induced stimulation. Phosphorylation of enzymes related to signaling events associated with mast cell degranulation was analyzed by immunoblotting. Effects on vascular permeability in the passive cutaneous anaphylaxis reaction were evaluated following oral administration of methoxychlor to BALB/c mice. The results indicated that methoxychlor caused increased mast cell degranulation in the presence of antigen, whereas it had no effect on calcium ionophore-induced degranulation of RBL-2H3 cells. Immunoblot analyses demonstrated that the phosphorylation level of phosphoinositide 3-kinase (which plays a central role in mast cell signaling) was increased by methoxychlor during antigen-induced degranulation. In addition, methoxychlor activated the signaling pathway via the high-affinity IgE receptor by inducing phosphorylation of Syk and PLCγ1/2, which transfer the signal for degranulation downstream. Lastly, oral administration of methoxychlor exhibited a tendency to promote vascular permeability in passive cutaneous anaphylaxis model mice. Taken together, the results here suggested that methoxychlor enhanced degranulation through FcεRI-mediated signaling and promoted allergenic symptoms involved in mast cell degranulation.

Novel PKCα-mediated phosphorylation site(s) on cofilin and their potential role in terminating histamine release

PKCα phosphorylates cofilin at Ser-23 and/or Ser-24 during degranulation, and the novel phosphorylation contributes to F-actin remodeling by regulating the ability of cofilin to bind to 14-3-3ζ and to depolymerize and/or sever F-actin. This highly regulated mechanism is necessary for the proper termination of degranulation.

Using specific inhibitors, kinase-negative mutants, and small interfering RNA against protein kinase Cα (PKCα) or PKCβI, we find that PKCβI positively regulates degranulation in rat basophilic leukemia–2H3 cells, whereas PKCα negatively regulates degranulation. Mass spectrometric and mutagenic analyses reveal that PKCα phosphorylates cofilin at Ser-23 and/or Ser-24 during degranulation. Overexpression of a nonphosphorylatable form (S23,24A), but not that of a mutant-mimicking phosphorylated form (S23,24E), increases degranulation. Furthermore, the S23,24A mutant binds to F-actin and retains its depolymerizing and/or cleavage activity; conversely, the S23,24E mutant is unable to sever actin filaments, resulting in F-actin polymerization. In addition, the S23,24E mutant preferentially binds to the 14-3-3ζ protein. Fluorescence-activated cell sorting analysis with fluorescein isothiocyanate–phalloidin and simultaneous observation of degranulation, PKC translocation, and actin polymerization reveals that during degranulation, actin polymerization is dependent on PKCα activity. These results indicate that a novel PKCα-mediated phosphorylation event regulates cofilin by inhibiting its ability to depolymerize F-actin and bind to 14-3-3ζ, thereby promoting F-actin polymerization, which is necessary for cessation of degranulation.

Suppression of mast cell degranulation by a novel ceramide kinase inhibitor, the F-12509A olefin isomer K1

Antigen-induced degranulation of mast cells plays a pivotal role in allergic and inflammatory responses. Recently, ceramide kinase (CERK) and its phosphorylated product ceramide 1-phosphate (C1P) have emerged as important players in mast cell degranulation. Here, we describe the synthesis of a novel F-12509A olefin isomer, K1, as an effective CERK inhibitor. In vitro kinase assays demonstrated that K1 effectively inhibits CERK without inhibiting sphingosine kinase and diacylglycerol kinase. Treating RBL-2H3 cells with K1 reduced cellular C1P levels to 40% yet had no effect on cell growth. Furthermore, treatment with K1 significantly suppressed both calcium ionophore- and IgE/antigen-induced degranulation, indicating that K1 interferes with signals that happen downstream of Ca(2+) mobilization. Finally, we show that K1 affects neither IgE/antigen-induced global tyrosine phosphorylation nor subsequent Ca(2+) elevation, suggesting a specificity for CERK-mediated signals. Our novel CERK inhibitor provides a useful tool for studying the biological functions of CERK and C1P. Moreover, to our knowledge, this is the first report demonstrating that inhibition of CERK suppresses IgE/antigen-induced mast cell degranulation. This finding suggests that CERK inhibitors might be a potential therapeutic tool in the treatment of allergic diseases.

Regulation of ionomycin-mediated granule release from rat basophil leukemia cells

Cross-linking the high affinity IgE receptor on the rat basophil leukemia clone 2H3 (RBL-2H3) cell line, an vitro model for mast cell signaling, results in granule release. A great deal of research has focused on the earliest steps in this signaling cascade resulting in models which include the participation of lyn, syk, phospholipase C (PLC), protein kinase C (PKC) and intracellular calcium mobilization. In an effort to look at pathways downstream of calcium mobilization, ionomycin-mediated granule release was studied. The kinase inhibitors PP1 (src family), GF109203X (PKC), PD98059 (MEK1/2), and U0126 (MEK1/2) substantially inhibited ionomycin-mediated granule release, while the p38 kinase inhibitor SB203580 did not. Both p38 and erk were phosphorylated upon ionomycin treatment, but only extracellular regulated kinase (erk) activation was completely inhibited by PP1 treatment and partially inhibited by the MEK inhibitors, thus, correlating with the granule release data. Interestingly, while GF109203X alone had no affect on erk activation, combining it with U0126 completely blocked this response. This suggests the existence an alternate pathway for erk activation that is MEK independent and PKC dependent. Experiments in which ionomycin and PP1 were titrated (independently) demonstrated a correlation between erk phosphorylation and granule release, implicating erk in a PP1-inhibitable pathway operating downstream of calcium and controlling mast cell degranulation.

Fyn kinase initiates complementary signals required for IgE-dependent mast cell degranulation

Fc epsilon RI activation of mast cells is thought to involve Lyn and Syk kinases proximal to the receptor and the signaling complex organized by the linker for activation of T cells (LAT). We report here that Fc epsilon RI also uses a Fyn kinase-dependent pathway that does not require Lyn kinase or the adapter LAT for its initiation, but is necessary for mast cell degranulation. Lyn-deficiency enhanced Fyn-dependent signals and degranulation, but inhibited the calcium response. Fyn-deficiency impaired degranulation, whereas Lyn-mediated signaling and calcium was normal. Thus, Fc epsilon RI-dependent mast cell degranulation involves cross-talk between Fyn and Lyn kinases.

The potentiation of the histamine release induced by adenosine in mast cells from guinea pig lung and heart: sharp dependence on the time of preincubation

We studied here the effect of a wide range of adenosine concentration and time of preincubation, on the histamine release induced in the guinea pig mast cells by different stimulus. Adenosine (10(-5)-10(-3)m) potentiated the histamine release induced by antigen in the guinea pig heart (isolated and dispersed tissue) and lung mast cells but not induced by ionophore A23197. The potentiation caused by adenosine (10(-4)m) was maximum after 1-3 min of preincubation and is probably an extracellular effect since it was not avoided by dipyridamol (3x10(-7)-10(-6)m) that inhibit the uptake of adenosine. Similar potentiation was also produced by the adenosine mimetic 2-chloroadenosine (10(-5)m) and both effects were inhibited by 8-phenyltheophylline indicating an effect on the type A receptors. It is suggested that the adenosine potentiation may not be related to changes on the cyclic AMP levels. 2000 Academic Press@p$hr

Promotion of HL-60 cell differentiation by 1,25-dihydroxyvitamin D3 regulation of protein kinase C levels and activity

The hormone 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] promotes differentiation of a number of cell types including HL-60 promyelocytic leukemia cells. It is now established that protein kinase Cbeta (PKCbeta) plays a critical role in HL-60 cell maturation to a monocyte/macrophage phenotype. In the present study, we investigated the importance of PKCbeta levels and activation in 1,25-(OH)2D3-mediated differentiation of HL-60 cells. Cell differentiation promoted by 1,25-(OH)2D3 at 48 hr was 39 +/- 3% (mean +/- SEM) nitroblue tetrazolium (NBT) positive and at 72 hr it was 35 +/- 2% NBT positive and 70% CD14 positive. Thus, promotion of cell differentiation by 20 nM 1,25-(OH)2D3 treatment was maximal at 48-72 hr. When PKCbeta levels and cell differentiation were assayed at 72 hr, treatment with 20 nM 1,25-(OH)2D3 for the initial 6 hr increased PKCbeta levels by 175% but had little effect on cell differentiation (7 +/- 2% NBT positive; 11% CD14 positive). The effect of ionomycin, a calcium ionophore, on PKCbeta levels and cell differentiation also was examined. Alone, 5 microM ionomycin promoted few cells (3% CD14 positive) to differentiate. In contrast, cells treated with 5 microM ionomycin for 66 hr after a 6-hr pretreatment with 20 nM 1,25-(OH)2D3 resulted in 34 +/- 5% NBT positive cells and 73% CD14 positive cells. Quantitatively, this induction of differentiation was identical to that observed in cultures continuously treated with 1,25-(OH)2D3 (35 +/- 2% NBT positive; 70% CD14 positive). Therefore, ionomycin seemed to replace the requirement for the continuous presence of 1,25-(OH)2D3. Chelerythrine chloride (3 microM), a specific PKC inhibitor, blocked differentiation promoted by 1,25-(OH)2D3 alone (82 +/- 2% inhibition) or in sequence with ionomycin (86 +/- 3% inhibition). Taken together, our data show that the capacity of 1,25-(OH)2D3 to both increase PKCbeta levels and activate PKC is utilized to promote HL-60 cell differentiation. These data further suggest that 1,25-(OH)2D3 has a genomic action to increase PKCbeta levels and also a nongenomic action requiring its continuous presence to promote HL-60 cell differentiation.

Thrombin primes responsiveness of selective chemoattractant receptors at a site distal to G protein activation

To define the molecular basis of human chemoattractant receptor regulation, rat basophilic leukemia RBL-2H3 cells, which are thrombin-responsive, were transfected to stably express epitope-tagged receptors for C5a, interleukin-8 (IL-8), formylpeptides (e.g. N-formyl-methionyl-leucyl-phenylalanine (fMLP)), and platelet-activating factor (PAF). Here we demonstrate that both thrombin and a synthetic peptide ligand for the thrombin receptor (sequence SFLLRN) caused phosphorylation and heterologous desensitization of the receptors for C5a, IL-8, and PAF but not that for formylpeptides as measured by agonist-stimulated [35S]guanosine 5'-3-O-(thio)triphosphate binding to membranes. Consistent with the PAF receptor phosphorylation, both thrombin and thrombin receptor peptide inhibited phosphoinositide hydrolysis, Ca2+ mobilization, and degranulation stimulated by PAF. Unexpectedly, despite heterologous desensitization at the level of receptor/G protein activation, there was enhancement ("priming") by thrombin of subsequent activities stimulated by C5a and IL-8 as well as fMLP. The priming effect of thrombin was blocked by its inhibitor, hirudin. However, two other activators of the thrombin receptor, the peptide SFLLRN and trypsin, stimulated Ca2+ mobilization in RBL-2H3 cells but did not cause priming. In addition, SFLLRN and the thrombin receptor antagonist peptide FLLRN both inhibited thrombin-induced Ca2+ mobilization but not priming. Furthermore, the proteolytically active gamma-thrombin, which does not stimulate the tethered ligand thrombin receptor and caused little or no Ca2+ mobilization in RBL-2H3 cells, effectively primed the response to fMLP. These data demonstrate that heterologous receptor phosphorylation and attenuation of G protein activation are not, by themselves, sufficient for the inhibition of biological responses mediated by C5a and IL-8. Moreover, thrombin appears to utilize mechanism(s) independent of its tethered ligand receptor to selectively prime phospholipase C-mediated biological responses of the C5a, IL-8, and formylpeptide receptors but not PAF. Because C5a, IL-8, and formylpeptide activate phospholipase Cbeta2, whereas PAF stimulates a different phospholipase C, the striking selectivity of thrombin's priming may be mediated via its ability to enhance receptor-mediated activation of phospholipase Cbeta2.

ATP depletion inhibits capacitative Ca2+ entry in rat thymic lymphocytes

The present study investigates the requirement for cellular ATP in the increase in plasma membrane Ca2+ permeability activated by the release of Ca2+ from intracellular stores in rat thymic lymphocytes (capacitative Ca2+ entry). The permeability state of this pathway following activation with thapsigargin was probed in control and ATP-depleted cells using fluorometric measurements of intracellular Ca2+, Mn2+ entry, and membrane potential, and unidirectional measurements of Ca2+ uptake using 45Ca2+. The capacitative Ca(2+)-entry pathway was markedly inhibited in cells depleted of ATP by incubation in glucose-free solution containing oligomycin, antimycin A, and 2-deoxy-D-glucose. These data cannot be explained on the basis of a loss of the transmembrane electrochemical gradient for Ca2+, alterations in intracellular pH or cellular Na+ content, a direct effect of the inhibitors of ATP production on the capacitative Ca(2+)-entry pathway, or the ability of thapsigargin to release Ca2+ from intracellular stores. Rather, these data are consistent with a requirement for ATP or a high-energy phosphate donor in the activation and/or maintained activation of capacitative Ca2+ entry.

Inhibition of mediator release in RBL-2H3 cells by some H1-antagonist derived anti-allergic drugs: relation to lipophilicity and membrane effects

In a model for mucosal mast cells (RBL-2H3 cells) a set H1-antagonist derived anti-allergic drugs containing a diphenylmethyl piperazinyl moiety was examined for their ability to inhibit release of the mediator beta-hexosaminidase. Cells were activated with antigen or the calcium ionophore A23187, whether or not in combination with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Oxatomide, hydroxyzine and cetirizine inhibit the antigen induced beta-hexosaminidase release. The release triggered by A23187, whether or not in combination with TPA is hardly influenced by the compounds. A biphasic dependence of the inhibition of exocytosis in RBL cells on lipophilicity is observed with the optimum at log P is 5-6. The extremely lipophilic compounds meclozine and buclizine are not active in this model. pH dependence of the effect of the drugs shows that especially the uncharged species are active in inhibiting exocytosis. The investigated compounds show an effect on phase transitions in L-alpha-phosphatidylcholine dipalmitoyl liposomes as assayed with differential scanning calorimetry (DSC). For the less extremely lipophilic compounds the induced changes in the phospholipid membranes increased with lipophilicity. The relation between structural features of the drug and the interaction with phospholipids is discussed in view of the DSC results. We conclude that location of the active drugs at the membrane or the membrane/protein interface is important for the inhibiting activity on exocytosis. This could affect several membrane related processes, which are abundant in the early phases of the IgE-mediated signal transduction process.

Mutation of aspartate 82 of the human C5a receptor abolishes the secretory response to human C5a in transfected rat basophilic leukemia cells

C5a is a potent chemoattractant for monocytes, neutrophils and other leukocytes. The receptor for human C5a is a member of the rhodopsin superfamily of G protein-coupled receptors and contains an aspartate residue (Asp82) within the putative second transmembrane domain conserved in all other G protein-linked receptors. We investigated the role of this residue and also the carboxy-terminal 23 residues of the C5a receptor in ligand binding and signal transduction by expressing wild-type and mutant receptors in the rat basophilic leukemia cell line RBL-2H3. Wild-type and truncated receptors coupled efficiently to effector systems, resulting in the C5a-dependent discharge of granule contents. In contrast RBL cells transfected with receptors in which Asp82 had been mutated to asparagine did not respond to human C5a by secretion despite binding human C5a with high affinity. We conclude therefore that Asp82 is not involved in the interaction with ligand but is essential for the proper transduction of the ligand binding signal.

Histamine release and calcium concentrations in rat mast cells are dependent on intracellular ATP: effects of prostaglandin D2

When PGD2 (10 microM), was added to rat mast cells, it caused a rapid increase in adenosine 3',5'-cyclic monophosphate (cyclic AMP) and decrease in adenosine 5'-triphosphate (ATP), both of which recovered to their original levels within 2 min. The accumulation of cyclic AMP was maximal at 30 s after challenge with PGD2. The minimum level of ATP was observed at 30 s after addition of PGD2. The initial rise in [Ca2+]i and the histamine release induced by anti-IgE (200 micrograms/ml) were strongly inhibited at 30 s after incubation of the mast cells with PGD2. Removal of glucose from Tyrode-Hepes solution caused a rapid decrease on ATP level in mast cells, and showed strong inhibition on the rise in [Ca2+]i and histamine release induced by anti-IgE. Addition of glucose to the mast cells induced a time-dependent increase in ATP, and the rises in [Ca2+]i and histamine release were closely correlated with the recovery of ATP. These results suggested that the inhibitory mechanism of PGD2 on the initial rise in [Ca2+]i and histamine release induced by anti-IgE was due to the inhibition of ATP-dependent CA(2+)-release from the intracellular Ca(2+)-stores.

86Rb+ ion fluxes in resting and immunologically stimulated mucosal mast cells

We studied fluxes of Rb+ ions, using its 86Rb isotope as a radioactive tracer in living rat mucosal mast cell cultures (RBL-2H3 line) grown to high density on beads. Continuously perfused samples of these cells could be immunologically stimulated by antigen clustering of IgE bound to the cells type I Fc epsilon receptors (Fc epsilon RI) and both the cellular response, as measured by the secreted mediators, as well as the uptake of 86Rb+ of the perfused sample could be monitored. The following results were obtained. (i) In resting cells, 86Rb+ influx is observed upon exposure to extracellular 86Rb+. It proceeds with a monoexponential time course (tau = 30.6 +/- 8 min) reaching a steady-state distribution of [86Rb+]int/[86Rb+]ext = 31.6 +/- 6.4 and can be inhibited by ouabain. (ii) Fc epsilon RI clustering-mediated stimulation of these cells causes an immediate and marked increase in both amplitude and rate of 86Rb+ uptake, which also fits a monoexponential function (tau = 26.8 +/- 8.6 min). (iii) This stimulated 86Rb+ uptake can also be inhibited by ouabain. It is not caused by Ca2+ influx or by the exocytotic process as evidenced by the fact that it is also observed in buffer to which no Ca2+ ions were added. Analysis of these results by a simple model taking into account unidirectional 86Rb+ influx by the Na+/K(+)-dependent ATPase and its efflux by K+ channels yields a resting cells unidirectional K+ uptake of 3.0 +/- 1.1 10(7) ions/cell/s, which is increased by ca. 10% upon clustering of the Fc epsilon RI by IgE and antigen. The stimulated influx is suggested to be due to enhanced activity of the Na+/K(+)-dependent ATPase, reflecting increased permeability for Na+ ions.

Mechanism of action of MY-1250, an active metabolite of Repirinast, in inhibiting histamine release from rat mast cells

1. When MY-1250 (3.6 x 10(-5) M) was added to mast cells, it caused a rapid increase in adenosine 3':5'-cyclic monophosphate (cyclic AMP) and decrease in adenosine 5'-triphosphate (ATP), both of which recovered to their original levels within 2 min. The accumulation of cyclic AMP was maximal at 20 s after challenge with MY-1250. The minimum level of ATP was observed at 20 s after addition of MY-1250. 2. The initial rise in [Ca2+]i and the histamine release induced by DNP-AS antigen (40 micrograms ml-1) was most strongly inhibited at 20 s after incubation of the mast cells with MY-1250. 3. MY-1250 strongly and dose-dependently inhibited the histamine release from rat mast cells induced by antigen. Moreover, MY-125 strongly inhibited calcium ion mobilization from the intracellular Ca(2+)-store. 4. These results suggested that the inhibitory mechanism of MY-1250 on the initial rise in [Ca2+]i and histamine release induced by antigen was due to the inhibition of ATP-dependent Ca(2+)-release from the intracellular Ca(2+)-stores.

Inhibition of IgE-mediated histamine release by myosin light chain kinase inhibitors

Wortmannin, a specific inhibitor of myosin light chain kinase (MLCK) blocked IgE mediated histamine release from rat basophilic leukemia cell (RBL-2H3) and human basophils dose-dependently. Its IC50 was 20 nM for RBL-2H3 cells and 30 nM for human basophils. There was complete inhibition at the concentration of 1 microM. Wortmannin inhibited partially the A23187 induced histamine release from RBL-2H3 cells (40% inhibition at 1 microM). This inhibition was not accompanied by any significant effect on cytosolic free calcium concentration [( Ca2+]i). KT5926, another MLCK inhibitor, inhibited histamine release comparably with wortmannin and blocked to some degree the increase of [Ca2+]i in RBL-2H3 cells. Thus, the phosphorylation of myosin seems to be involved in signal transduction through Fc epsilon RI.

Differential effects of propranolol on the IgE-dependent, or calcium ionophore-stimulated, phosphoinositide hydrolysis and calcium mobilization in a mast (RBL 2H3) cell line

Our previous studies demonstrated that propranolol, an inhibitor of phosphatidic acid phosphohydrolase (PAPase) (EC 3.1.3.4) blocks the IgE-dependent mediator release from a rat mast (RBL 2H3) cell line. To continue these studies, we examined the ability of propranolol to inhibit the IgE-dependent or ionomycin-mediated phosphoinositide hydrolysis and calcium mobilization in RBL 2H3 cells. RBL 2H3 cells, sensitized with mouse monoclonal anti-trinitrophenol IgE (anti-TNP IgE), were stimulated to release both histamine and peptidoleukotrienes (LT) in response to a suboptimal concentration of trinitrophenol-ovalbumin conjugate (TNP-OVA) or ionomycin. Preincubation of the cells with d,l-propranolol (300 microM) significantly (P less than 0.05) inhibited the effects of both TNP-OVA and ionomycin on histamine and LT release. There was no difference in potency for the different isomers of propranolol, indicating that these effects were not a consequence of an effect on beta 2-adrenergic receptors. TNP-OVA produced a rapid hydrolysis of phosphoinositides resulting in a time-dependent increase in mono- (IP1), di- (IP2), tri- (IP3), and total inositol phosphate production. Ionomycin also produced a rapid increase in total inositol phosphate production; however, this largely reflected an accumulation of IP1. Both secretagogues produced a rapid elevation in cytosolic free calcium ([Ca2+]i); however, the effect of ionomycin maximized within a much shorter time frame than the effect of TNP-OVA. The effects of TNP-OVA on phosphoinositide hydrolysis and increase in [Ca2+]i were inhibited by propranolol over exactly the same concentration range as the effects of this compound on TNP-OVA-stimulated mediator release. In contrast, propranolol had no effect on the increase in [Ca2+]i and phosphoinositide hydrolysis in response to ionomycin. Taken together, these results suggest that PAPase/phospholipase D (PLD) (EC 3.1.4.4) activation may be a prerequisite for both IgE-dependent and ionomycin-stimulated mediator release from RBL 2H3 cells. Although other explanations are possible, the data further suggest that receptor-mediated, but not ionophore-stimulated, phosphoinositide hydrolysis and [Ca2+]i in RBL 2H3 cells may be regulated by a propranolol-sensitive pathway involving possible activation of PAPase.

Regulation of IgE receptor-mediated secretion from RBL-2H3 mast cells by GTP binding-proteins and calcium

In RBL-2H3 rat basophilic leukemia cells, cholera toxin does not per se stimulate secretion but it enhances secretion stimulated by antigens that crosslink IgE receptors, by the Ca2+ ionophore, ionomycin, and by thapsigargin, a tumor promoter that releases cytoplasmic Ca2+ stores. Calmodulin inhibitors reduce both the basal and cholera toxin-enhanced secretory responses to antigen and Ca2(+)-mobilizing agents. These synergistic effects suggest that the activation of a Gs-like GTP-binding protein, together with a (probably calmodulin-dependent) event activated by an increase in cytoplasmic Ca2+ levels, may jointly provide a sufficient signal for secretion. Antigen-stimulated secretion is inhibited by depleting cells of GTP with mycophenolic acid but is maximal in cells treated with mycophenolic acid plus cholera toxin. The simplest explanation is that cholera toxin selectively reactivates the Gs-coupled pathway leading to secretion in GTP-depleted cells without restoring the activity of a separate GTP-binding protein(s) that constrains antigen-stimulated secretion.

31P and 23Na nuclear magnetic resonance studies of resting and stimulated mast cells

Exocytosis induced by crosslinking the type I receptor for Fc epsilon domains present on rat mucosal mast cells (RBL-2H3-line) requires the influx of Ca2+ ions and is markedly influenced by the concentration of monovalent cations (K+, Na+ and protons) in their medium. We investigated the role of these ions in coupling the immunological stimulus to secretion using NMR spectroscopy to monitor simultaneously intracellular pH, ATP and Na+ concentrations and the secretory response of living adherent mast cells. Using this methodology we observed that: (i) ATP concentration and intracellular pH are highly regulated and no changes could be resolved in them upon stimulation and during exocytosis. (ii) In the absence of potassium ions in the cells' medium, a decrease is observed in the intracellular pH and ATP concentration and an increase in the Na+ concentration. (iii) From the influx of extracellular Na+ following inhibition of the Na+, K(+)-ATPase by ouabain, we estimated the inward Na+ current of resting cells to 5 x 10(7) ions/(cell.s). This value does not vary by more than 10% during exocytosis.

The effects of the protein kinase C inhibitors staurosporine and H7 on the IgE dependent mediator release from RBL 2H3 cells

RBL 2H3 cells, a model for mast cell function, sensitized with rat IgE, released histamine and peptidoleukotrienes (LT) in response to rabbit anti-rat IgE in a concentration-dependent manner. The calcium ionophore, A23187 also stimulated the release of both mediators but to a greater extent. The protein kinase C activator, 12-O-tetradecanoyl phorbol-13-acetate (TPA) failed to influence mediator release when added alone, but when added with either A23187 or anti-IgE, TPA significantly enhanced the release of both histamine and LT. The effects of anti-IgE, TPA and A23187 were completely inhibited by prior addition of the protein kinase C inhibitors staurosporine and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H7) but not by N-(2-guanidinoethyl)-5-isoquinoline-sulfonamide dihydrochloride (HA1004), a compound which has similar potency to H7 as an inhibitor of some protein kinases but is less potent as a protein kinase C inhibitor. Although other explanations are possible, these results support the hypothesis that the release of histamine and leukotrienes from RBL 2H3 cells resulting from the cross bridging of the IgE receptors, is dependent on activation of protein kinase C.

Histamine release from rat mast cells induced by metabolic activation of polyunsaturated fatty acids into free radicals

Polyunsaturated fatty acids (PUFA: arachidonic and linoleic acid) release histamine from isolated purified rat serosal mast cells only in the presence of oxidizing systems such as phenobarbital-induced rat liver microsomes, prostaglandin-H-synthetase (PHS) or soybean lipoxygenase. The release of mast cell histamine by activated PUFA has a long time-course and the electron microscopical features are consistent with an exocytotic secretion in the case of arachidonic acid and cell lysis in the case of linoleic acid. The phenomenon is associated with a significant increase in malonyldialdehyde (MDA) and conjugated diene generation, suggesting a relationship between histamine release and membrane lipid peroxidation. The secretion of histamine was inhibited by anti-free radical interventions such as D-mannitol, reduced glutathione and alpha-tocopherol. Some cyclooxygenase and lipoxygenase inhibitors, cimetidine and carnitine derivatives, are differentially active in the inhibition of mast cell histamine release by activated arachidonic acid. These results suggest that free radical derivatives of PUFA, generated by metabolic activation, trigger mast cell histamine release.

The effect of mitochondrial inhibitors on calcium homeostasis in tumor mast cells

The depletion of intracellular ATP by mitochondrial inhibitors in a glucose-free saline solution inhibited antigen-stimulated 45Ca uptake, the rise in cytoplasmic calcium, measured by fura-2, and secretion in rat basophilic leukemia cells. Lowering the intracellular ATP concentration also released calcium from an intracellular store and made further 45Ca efflux from the cells unresponsive to subsequent antigen stimulation. Antigen-stimulated 45Ca efflux could be restored by the addition of glucose. The ATP-sensitive calcium store appeared to be the same store that releases calcium in response to antigen. In contrast, intracellular ATP was not lowered, and antigen-stimulated secretion was unaffected by mitochondrial inhibitors, provided that glucose was present in the bathing solution. Similarly, antigen-stimulated 45Ca uptake, 45Ca efflux, and the rise in free ionized calcium were unaffected by individual mitochondrial inhibitors in the presence of glucose. However, when the respiratory chain inhibitor antimycin A was used in combination with the ATP synthetase inhibitor oligomycin in the presence of glucose, antigen-stimulated 45Ca uptake was inhibited, whereas the rise in free ionized calcium and secretion were unaffected. Also, antigen-induced depolarization (an indirect measurement of Ca2+ influx across the plasma membrane) was not affected. The inhibition of antigen-stimulated 45Ca uptake could, however, be overcome if a high concentration of the Ca2+ buffer quin2 was present in the cells to buffer the incoming 45Ca. These results suggest that in fully functional rat basophilic leukemia cells the majority of the calcium entering in response to antigen stimulation is initially buffered by a calcium store sensitive to antimycin A and oligomycin, presumably the mitochondria.

Release of calcium from intracellular stores in rat basophilic leukemia cells monitored with the fluorescent probe chlortetracycline

Release of calcium from intracellular stores of rat basophilic leukemia cells was monitored using the fluorescent probe chlortetracycline. The ability of chlortetracycline to indicate release from intracellular calcium stores was initially validated. The decrease of chlortetracycline fluorescence upon antigen-stimulation was not the result of secretion of granule-associated dye or of changes in the properties of the membranes. The chlortetracycline fluorescence signal was not influenced by Ca2+ influx across the plasma membrane. Results obtained from these chlortetracycline fluorescence measurements corresponded well with 45Ca efflux data, an indirect measurement of release of calcium from stores. Chlortetracycline was used to examine the rate of antigen-induced release of calcium from stores, the depletion of intracellular calcium stores by EGTA, and the relationship between the antigen-stimulated release of stored calcium and exocytosis. Chlortetracycline was shown to be a useful qualitative indicator for the release of intracellular calcium with a relatively rapid response time.

Modulation by phorbol 12-myristate 13-acetate of arachidonic acid release from rat basophilic leukemia cells stimulated with A23187

Rat basophilic leukemia (RBL-2H3) cells were cultured in medium containing [3H]arachidonic acid and labelling of the different lipid fractions was followed with time. After up to 4 h of culture, the label was found mostly in phosphatidylcholine. After 8 h, labelling of phosphatidylethanolamine gradually exceeded that of phosphatidylcholine, until at 24 h, approximate equilibrium labelling of the lipid fractions was attained and 45% of the label was found in phosphatidylethanolamine, 35% in phosphatidylcholine, 18% in the phosphatidylserine/inositide fraction and the remainder in the neutral lipid fraction. Stimulation of cells with A23187 after 30 min of labelling caused release of [3H]arachidonic acid which was accountable by a decrease in radioactivity of phosphatidylcholine, whereas stimulation of cells after 24 h of labelling caused the release of radioactive arachidonic acid, which was accompanied by a decrease of label in both phosphatidylcholine and phosphatidylethanolamine. Incubation of the labelled cells with phorbol 12-myristate 13-acetate prior to ionophore addition enhanced both the release of [3H]arachidonic acid and its metabolites and the decrease in label of the same phospholipids as those affected by ionophore alone. Under our conditions, the enhancement effects of phorbol ester were greatest after 2-5 min of preincubation, prior to ionophore addition. The results suggest that in basophilic leukemia cells, arachidonic acid release proceeds from several pools of phospholipids and that the activity of the phospholipase(s) involved is modulated by protein kinase C.

Receptor-mediated release of inositol 1,4,5-trisphosphate and inositol 1,4-bisphosphate in rat basophilic leukemia RBL-2H3 cells permeabilized with streptolysin O

Antigen-mediated exocytosis in intact rat basophilic leukemia (RBL-2H3) cells is associated with substantial hydrolysis of membrane inositol phospholipids and an elevation in concentration of cytosol Ca2+ ([ Ca2+i]). Paradoxically, these two responses are largely dependent on external Ca2+. We report here that cells labeled with myo-[3H]inositol and permeabilized with streptolysin O do release [3H]inositol 1,4,5-trisphosphate upon stimulation with antigen or guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) at low (less than 100 nM) concentrations of free Ca2+. The response, however, is amplified by increasing free Ca2+ to 1 microM. The subsequent conversion of the trisphosphate to inositol 1,3,4,5-tetrakisphosphate is enhanced also by the increase in free Ca2+. Although [3H]inositol 1,4,5-trisphosphate accumulates in greater amounts than is the case in intact cells, [3H]inositol 1,4-bisphosphate is still the major product in permeabilized cells even when the further metabolism of [3H]inositol 1,4,5-trisphosphate is suppressed (by 77%) by the addition of excess (1000 microM) unlabeled inositol 1,4,5-trisphosphate and the phosphatase inhibitor 2,3-bisphosphoglycerate. It would appear that either the activity of the membrane 5-phosphomonoesterase allows virtually instantaneous dephosphorylation of the inositol 1,4,5-trisphosphate under all conditions tested or both phosphatidylinositol 4-monophosphate and the 4,5-bisphosphate are substrates for the activated phospholipase C. The latter alternative is supported by the finding that permeabilized cells, which respond much more vigorously to high (supraoptimal) concentrations of antigen than do intact RBL-2H3 cells, produce substantial amounts of [3H]inositol 1,4-bisphosphate before any detectable increase in levels of [3H]inositol 1,4,5-trisphosphate.

Modulation of human leukocyte histamine release by sn-1,2-isopropylidene-3-decanoyl-glycerol and decanoic acid cyclopentyl methylester in comparison with effects of synthetic diacylglycerols and a phorbol ester

Previous studies have shown that the glyceride derivative, sn-1,2-isopropylidene-3-decanoyl-glycerol (IpOCOC9), can trigger human leukocyte histamine release. Approximately 25% of the total cellular histamine content is extruded in the presence of 206 microM of IpOCOC9; at 69 microM, however, the secretagogue action of the compound is marginal. The characteristics of the release induced by IpOCOC9 are closely similar to those reportedly recorded at hyperosmolar triggering of basophils with mannitol, and in many respects they also mimic those observed at phorbol ester-induced histamine release. The compound decanoic acid cyclopentyl methylester (DACPME), a structural analogue of IpOCOC9, fails to induce histamine release. IpOCOC9, but not DACPME, stimulates human polymorphonuclear leukocyte cytosolic Ca2+- and phospholipid-dependent histone III-S kinase activity (unpublished observations). The secretagogue action of IpOCOC9 has therefore tentatively, at least partly, been attributed to a direct protein kinase C activation. In the present studies, we examined the influence of IpOCOC9 and DACPME on histamine release triggered by an ensuing exposure to anti-IgE, the calcium ionophore A23187, formyl-methionyl-leucyl-phenylalanine (FMLP), or 4 beta-phorbol 12-myristate 13-acetate (PMA). It is shown that IpOCOC9-treatment of cells results in either enhancement or reduction of the release induced by anti-IgE or by A23187, whereas FMLP-induced release is consistently reduced and PMA-induced release consistently enhanced by such a treatment. Treatment of cells with DACPME enhances but does not reduce anti-IgE-triggered release, whereas FMLP-induced release is not affected. Pretreatment of the cells with other putative protein kinase C activators like PMA, sn-1-oleoyl-2-acetyl-glycerol (OAG), 1,2-dioctanoyl-glycerol (DiC8) or the glycerol derivative sn-1,2-diacetyl-3-decanoyl-glycerol (DiC2OCOC9) affects secretagogue-induced basophil histamine release according to specific patterns similar to but not identical with those recorded for IpOCOC9 and DACPME. Thus, e.g., DiC2OCOC9 consistently reduces but does not enhance anti-IgE-triggered release. These data show that limited structural changes of IpOCOC9 may qualitatively affect its modulating properties in the human basophil histamine release system.

Induction of human basophil histamine release by a novel protein kinase C activator, sn-1,2-isopropylidene-3-decanoyl-glycerol (IpOCOC9): partial characterization of secretagogue characteristics

Human leukocytes were found to release histamine at exposure for the synthetic glyceride derivative sn-1,2-isopropylidene-3-decanoyl-glycerol (IpOCOC9). The following characteristics for the IpOCOC9-induced basophil histamine release were recorded. A. In the order of 25% of the cellular histamine content was extruded at 206 mumol/l and 45% at 690 mumol/l of the compound, respectively. B. Removal of extracellular Ca2+ variably affected IpOCOC9-triggered release. C. The presence of N-ethylmaleimide (10 mumol/l) or p-bromophenacylbromide (10 mumol/l) markedly reduced IpOCOC9-induced histamine release. D. The time course of the release triggered by IpOCOC9 was intermediate to those characterizing the release triggered by 4 beta-phorbol 12-myristate 13-acetate (PMA) and by formyl-methionyl-leucyl-phenylalanine (FMLP). E. Cells desensitized to IgE-receptor-mediated stimulation were hyperresponsive to stimulation with IpOCOC9. F. Cells treated with a low concentration of 2-deoxyglucose were not hyperresponsive to IpOCOC9. These data show that IpOCOC9, a PMN/leukocyte protein kinase C stimulator, acts as a non-cytotoxic secretagogue for human basophils with a mode of action which in some, but not all respects, mimics that of PMA. In particular, IpOCOC9-triggered release resembles that reported by other authors for hyperosmolar triggering of release by mannitol.

Cardiotoxin from cobra venom increases the level of phosphatidylinositol 4-monophosphate and phosphatidylinositol kinase activity in two cell lines

In rat basophilic leukemia-2H3 (RBL-2H3) and Madin-Darby canine kidney (MDCK) cells, cardiotoxin from cobra venom induced a marked decrease in the level of [3H] phosphatidylinositol and a corresponding increase in the level of [3H]phosphatidylinositol 4-monophosphate over the course of 20 min as demonstrated in cells that had been labeled to equilibrium with [3H]inositol. The effect was dependent on the concentration (5-30 micrograms/ml) of the toxin. In plasma membrane-enriched fractions isolated from the two cell lines, the cardiotoxin enhanced the endogenous activity of phosphatidylinositol kinase especially at temperatures above 14 degrees C. In RBL-2H3 cells, cardiotoxin also induced release of substantial amounts of histamine and lactate dehydrogenase. The release of histamine, but not of lactate dehydrogenase, was totally dependent on external calcium and this release probably represented an exocytotic response of the cells to cardiotoxin. Although, initially, treatment with the toxin did not impair antigen-induced hydrolysis of inositol phospholipids or prevent the antigen-induced rise in the concentration of cytosol Ca2+, prolonged exposure to the toxin did result in a progressive loss of responsiveness of RBL-2H3 cells to antigen.