Maitotoxin activates cation channels distinct from the receptor-activated non-selective cation channels of HL-60 cells

L-cysteine reversibly inhibits glucose-induced biphasic insulin secretion and ATP production by inactivating PKM2

Increase in the concentration of plasma L-cysteine is closely associated with defective insulin secretion from pancreatic β-cells, which results in type 2 diabetes (T2D). In this study, we investigated the effects of prolonged L-cysteine treatment on glucose-stimulated insulin secretion (GSIS) from mouse insulinoma 6 (MIN6) cells and from mouse pancreatic islets, and found that the treatment reversibly inhibited glucose-induced ATP production and resulting GSIS without affecting proinsulin and insulin synthesis. Comprehensive metabolic analyses using capillary electrophoresis time-of-flight mass spectrometry showed that prolonged L-cysteine treatment decreased the levels of pyruvate and its downstream metabolites. In addition, methyl pyruvate, a membrane-permeable form of pyruvate, rescued L-cysteine-induced inhibition of GSIS. Based on these results, we found that both in vitro and in MIN6 cells, L-cysteine specifically inhibited the activity of pyruvate kinase muscle isoform 2 (PKM2), an isoform of pyruvate kinases that catalyze the conversion of phosphoenolpyruvate to pyruvate. L-cysteine also induced PKM2 subunit dissociation (tetramers to dimers/monomers) in cells, which resulted in impaired glucose-induced ATP production for GSIS. DASA-10 (NCGC00181061, a substituted N,N'-diarylsulfonamide), a specific activator for PKM2, restored the tetramer formation and the activity of PKM2, glucose-induced ATP production, and biphasic insulin secretion in L-cysteine-treated cells. Collectively, our results demonstrate that impaired insulin secretion due to exposure to L-cysteine resulted from its direct binding and inactivation of PKM2 and suggest that PKM2 is a potential therapeutic target for T2D.

Maitotoxin converts the plasmalemmal Ca(2+) pump into a Ca(2+)-permeable nonselective cation channel

Maitotoxin (MTX) activates Ca(2+)-permeable nonselective cation channels and causes a dramatic increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in every cell examined to date, but the molecular identity of the channels involved remains unknown. A clue came from studies of a structurally related marine toxin called palytoxin (PTX). PTX binds to the plasmalemmal Na(+)-K(+)-ATPase (NKA) and converts the Na(+) pump into a nonselective cation channel. Given the high permeability of the MTX channel for Ca(2+), we considered the possibility that MTX may bind to the plasmalemmal Ca(2+)-ATPase (PMCA) pump, and like PTX, convert the pump into a channel. To test this hypothesis, the PMCA was overexpressed in Spodoptera frugiperda (Sf9) insect cells and in human embryonic kidneys (HEK) 293 cells. In both cell types, enhanced expression of the PMCA was associated with a significant increase in MTX-induced whole cell membrane currents. The effect of MTX on whole cell currents in both wild-type and PMCA overexpressing HEK cells was sensitive to pump ligands including Ca(2+) and ATP. MTX-induced currents were significantly reduced by knockdown of PMCA1 in HEK cells using small interfering RNA or in mouse embryonic fibroblasts from genetically modified mice with the PMCA1(+/-) PMCA4(-/-) genotype. Finally, PMCA catalytic activity (i.e., Ca(2+)-ATPase) in isolated membranes, or in purified PMCA preparations, was inhibited by MTX. Together, these results suggest that MTX binds to and converts the PMCA pump into a Ca(2+)-permeable nonselective cation channel.

Maitotoxin induces two dose-dependent conductances in Xenopus oocytes. Comparison with nystatin effects as a pore inductor

Maitotoxin (MTX)-induced conductances in Xenopus oocytes were thoroughly characterized using the two-electrode voltage clamp technique with a hyperpolarizing voltage protocol. MTX 5-100pM induced an inward current with maximal amplitude between 0.1 and 10microA. The kinetics of this current had rising and decaying phases, which were non-voltage dependent. Its reversal potential (Erev) was close to 0mV in high K+ or Na+ external solution, indicating the participation of non-selective cation channels (NSCC). A second conductance was developed at MTX doses higher than 200pM whose amplitude increased continuously. This current showed a large instantaneous component and a voltage-independent decay, as well as similar selectivity for Na+ and K+ ions (Erev approximately 0 mV). Moreover, the maximal current amplitude was about 34% bigger in high K+ than in high Na+. The MTX effect was reversible at all doses in pM range. All the properties found are similar to those of NSCC. The differences in the current kinetics suggest that the MTX-elicited currents reflect the activation of two sets of voltage-independent NSCC. As MTX has been proposed to act by forming pores directly into the plasma membrane, we compared its effects with those of nystatin, a well-known membrane pore inductor. We found strong differences between the effects of both substances suggesting different mechanisms for these drugs.

Alteration of the Fc gamma RIIa dimer interface affects receptor signaling but not ligand binding

The aggregation of cell surface FcRs by immune complexes induces a number of important Ab-dependent effector functions. However, despite numerous studies that examine receptor function, very little is known about the molecular organization of these receptors within the cell. In this study, protein complementation, mutagenesis, and ligand binding analyses demonstrate that human FcgammaRIIa is present as a noncovalent dimer form. Protein complementation studies found that FcgammaRIIa molecules are closely associated. Mutagenesis of the dimer interface, as identified by crystallographic analyses, did not affect ligand binding yet caused significant alteration to the magnitude and kinetics of receptor phosphorylation. The data suggest that the ligand binding and the dimer interface are distinct regions within the receptor, and noncovalent dimerization of FcgammaRIIa may be an essential feature of the FcgammaRIIa signaling cascade.

Blockade of maitotoxin-induced endothelial cell lysis by glycine and L-alanine

The maitotoxin (MTX)-induced cell death cascade in bovine aortic endothelial cells (BAECs) is a model for oncotic/necrotic cell death. The cascade is initiated by an increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)), which is followed by the biphasic uptake of vital dyes. The initial phase of dye entry reflects activation of large pores and correlates with surface membrane bleb formation; the second phase reflects cell lysis. In the present study, the effect of the cytoprotective amino acid glycine was examined. Glycine had no effect on MTX-induced change in [Ca(2+)](i) or on the first phase of vital dye uptake but produced a concentration-dependent (EC(50) approximately 1 mM) inhibition of the second phase of dye uptake. No cytoprotective effect was observed with l-valine, l-proline, or d-alanine, whereas l-alanine was equieffective to glycine. Furthermore, glycine had no effect on MTX-induced bleb formation. To test the hypothesis that glycine specifically blocks formation of a lytic "pore," the loss of fluorescence from BAECs transiently expressing GFP and concatemers of GFP ranging in size from 27 to 162 kDa was examined using time-lapse videomicroscopy. MTX-induced loss of GFP was rapid, correlated with the second phase of dye uptake, and was relatively independent of molecular size. The MTX-induced loss of GFP from BAECs was completely blocked by glycine. The data suggest that the second "lytic" phase of MTX-induced endothelial cell death reflects formation of a novel permeability pathway that allows macromolecules such as GFP or LDH to escape, yet can be prevented by the cytoprotective agents glycine and l-alanine.

Modulation of the maitotoxin response by intracellular and extracellular cations

The aim of the present study was to characterize the role played by intracellular and extracellular calcium and sodium on the maitotoxin (MTX) response in Chinese hamster ovary (CHO) cells. The results presented here indicated that: (1) MTX activates calcium and sodium influx in a concentration-dependent manner; (2) extracellular calcium is required for the sodium influx; (3) removal of the extracellular sodium did not prevent the MTX-induced calcium influx; (4) elevation in the intracellular calcium concentration potentiates the MTX response; and (5) MTX, at the concentrations tested, did not compromise cell viability.

Blockade of maitotoxin-induced oncotic cell death reveals zeiosis

BACKGROUND

Maitotoxin (MTX) initiates cell death by sequentially activating 1) Ca2+ influx via non-selective cation channels, 2) uptake of vital dyes via formation of large pores, and 3) release of lactate dehydrogenase, an indication of cell lysis. MTX also causes formation of membrane blebs, which dramatically dilate during the cytolysis phase. To determine the role of phospholipase C (PLC) in the cell death cascade, U73122, a specific inhibitor of PLC, and U73343, an inactive analog, were examined on MTX-induced responses in bovine aortic endothelial cells.

RESULTS

Addition of either U73122 or U73343, prior to MTX, produced a concentration-dependent inhibition of the cell death cascade (IC50 asymptotically equal to 1.9 and 0.66 microM, respectively) suggesting that the effect of these agents was independent of PLC. Addition of U73343 shortly after MTX, prevented or attenuated the effects of the toxin, but addition at later times had little or no effect. Time-lapse videomicroscopy showed that U73343 dramatically altered the blebbing profile of MTX-treated cells. Specifically, U73343 blocked bleb dilation and converted the initial blebbing event into "zeiosis", a type of membrane blebbing commonly associated with apoptosis. Cells challenged with MTX and rescued by subsequent addition of U73343, showed enhanced caspase-3 activity 48 hr after the initial insult, consistent with activation of the apoptotic program.

CONCLUSIONS

Within minutes of MTX addition, endothelial cells die by oncosis. Rescue by addition of U73343 shortly after MTX showed that a small percentage of cells are destined to die by oncosis, but that a larger percentage survive; cells that survive the initial insult exhibit zeiosis and may ultimately die by apoptotic mechanisms.

Maitotoxin activates an endogenous non-selective cation channel and is an effective initiator of the activation of the heterologously expressed hTRPC-1 (transient receptor potential) non-selective cation channel in H4-IIE liver cells

The structures and mechanisms of activation of non-selective cation channels (NSCCs) are not well understood although NSCCs play important roles in the regulation of metabolism, ion transport, cell volume and cell shape. It has been proposed that TRP (transient receptor potential) proteins are the molecular correlates of some NSCCs. Using fura-2 and patch-clamp recording, it was shown that the maitotoxin-activated cation channels in the H4-IIE rat liver cell line admit Ca(2+), Mn(2+) and Na(+), have a high selectivity for Na(+) compared with Ca(2+), and are inhibited by Gd(3+) (half-maximal inhibition at 1 microM). Activation of the channels by maitotoxin was inhibited by increasing the extracellular Ca(2+) concentration or by inclusion of 10 mM EGTA in the patch pipette. mRNA encoding TRP proteins 1, 2 and 3 at levels comparable with those in brain was detected using reverse transcriptase-polymerase chain reaction in poly(A)(+) RNA prepared from H4-IIE cells and freshly-isolated rat hepatocytes. In H4-IIE cells transiently transfected with cDNA encoding hTRPC-1, the expressed hTRPC-1 protein was chiefly located at intracellular sites and at the plasma membrane. Cells expressing hTRPC-1 exhibited a substantial enhancement of maitotoxin-initiated Ca(2+) inflow and a modest enhancement of thapsigargin-initiated Ca(2+) inflow (measured using fura-2) and no enhancement of the highly Ca(2+)-selective store-operated Ca(2+) current (measured using patch-clamp recording). In cells expressing hTRPC-1, maitotoxin activated channels which were not found in untransfected cells, have an approximately equal selectivity for Na(+) and Ca(2+), and are inhibited by Gd(3+) (half-maximal inhibition at 3 microM). It is concluded that in liver cells (i) maitotoxin initiates the activation of endogenous NSCCs with a high selectivity for Na(+) compared with Ca(2+); (ii) TRP proteins 1, 2 and 3 are expressed; (iii) maitotoxin is an effective initiator of activation of heterologously expressed hTRPC-1 channels; and (iv) the endogenous TRP-1 protein is unlikely to be the molecular counterpart of the maitotoxin-activated NSCCs nor the highly Ca(2+)-selective store-operated Ca(2+) channels.

Negative regulation of ligand-initiated Ca(2+) uptake by PKC-beta II in differentiated HL60 cells

In phagocytic cells, fMet-Leu-Phe triggers phosphoinositide remodeling, activation of protein kinase C (PKC), release of intracellular Ca(2+) and uptake of extracellular Ca(2+). Uptake of extracellular Ca(2+) can be triggered by store-operated Ca(2+) channels (SOCC) and via a receptor-operated nonselective cation channel(s). In neutrophilic HL60 cells, the PKC activator phorbol myristate acetate (PMA) activates multiple PKC isotypes, PKC-alpha, PKC-beta, and PKC-delta, and inhibits ligand-initiated mobilization of intracellular Ca(2+) and uptake of extracellular Ca(2+). Therefore PKC is a negative regulator at several points in Ca(2+) mobilization. In contrast, selective depletion of PKC-beta in HL60 cells by an antisense strategy enhanced fMet-Leu-Phe-initiated Ca(2+) uptake but not mobilization of intracellular Ca(2+). Thapsigargin-induced Ca(2+) uptake through SOCC was not affected by PKC-beta II depletion. Thus PKC-beta II is a selective negative regulator of Ca(2+) uptake but not release of intracellular Ca(2+) stores. PKC-beta II inhibits a receptor-operated cation or Ca(2+) channel, thus inhibiting ligand-initiated Ca(2+) uptake.

Maitotoxin-induced membrane blebbing and cell death in bovine aortic endothelial cells

BACKGROUND

Maitotoxin, a potent cytolytic agent, causes an increase in cytosolic free Ca2+ concentration ([Ca2+]i) via activation of Ca2+-permeable, non-selective cation channels (CaNSC). Channel activation is followed by formation of large endogenous pores that allow ethidium and propidium-based vital dyes to enter the cell. Although activation of these cytolytic/oncotic pores, or COP, precedes release of lactate dehydrogenase, an indication of oncotic cell death, the relationship between CaNSC, COP, membrane lysis, and the associated changes in cell morphology has not been clearly defined. In the present study, the effect maitotoxin on [Ca2+]i, vital dye uptake, lactate dehydrogenase release, and membrane blebbing was examined in bovine aortic endothelial cells.

RESULTS

Maitotoxin produced a concentration-dependent increase in [Ca2+]i followed by a biphasic uptake of ethidium. Comparison of ethidium (Mw 314 Da), YO-PRO-1 (Mw 375 Da), and POPO-3 (Mw 715 Da) showed that the rate of dye uptake during the first phase was inversely proportional to molecular weight, whereas the second phase appeared to be all-or-nothing. The second phase of dye uptake correlated in time with the release of lactate dehydrogenase. Uptake of vital dyes at the single cell level, determined by time-lapse videomicroscopy, was also biphasic. The first phase was associated with formation of small membrane blebs, whereas the second phase was associated with dramatic bleb dilation.

CONCLUSIONS

These results suggest that maitotoxin-induced Ca2+ influx in bovine aortic endothelial cells is followed by activation of COP. COP formation is associated with controlled membrane blebbing which ultimately gives rise to uncontrolled bleb dilation, lactate dehydrogenase release, and oncotic cell death.

Maitotoxin stimulates Cd influx in Madin-Darby kidney cells by activating Ca-permeable cation channels

This study examined the role of calcium channels for the uptake of cadmium (Cd) into Madin-Darby canine kidney (MDCK) cells. Maitotoxin, an activator of different types of calcium channels, increased accumulation of 109Cd and 45Ca in MDCK cells. We found that maitotoxin increased accumulation by stimulating 109Cd influx because it did not affect efflux. An inhibitor of store-operated Ca channels, SKF96365, partially blocked 45Ca influx but did not affect 109Cd influx. Ni and Mn, and loperamide and proadifen (SKF 525a), inhibited 45Ca and 109Cd influx in cells stimulated with maitotoxin, but La and nifedipine did not. Overnight treatment with phorbol 12, 13-ibutyrate (PDBu) to activate protein kinase C resulted in a decrease in the concentration of maitotoxin needed to stimulate 45Ca and 109Cd influx. The effect of PDBu was blocked by treating cells with the protein kinase C inhibitor GF109203X. Additionally, the effect of PDBu was lost in cells treated with an inhibitor of RNA synthesis actinomycin D. These results suggest that a Ca permeable cation channel different from voltage-dependent and store-operated Ca channels mediates the uptake of Cd in MDCK cells. The expression of this channel is regulated by protein kinase C.

Comparison of maitotoxin with thromboxane A2 in rabbit platelet activation

Maitotoxin (MTX), a Ca2+ channel-activating marine toxin, caused shape change followed by aggregation in rabbit platelets, like U46619, a thromboxane A2 analogue. Although both drugs failed to cause aggregation in the absence of external Ca2+, U46619, but not maitotoxin, elicited shape change in the absence of external Ca2+. The observations of platelets with a scanning electron microscope showed that both drugs caused contraction of platelets and extension of pseudopodia (shape change) followed by aggregation with a clot in the presence of Ca2+. It is noteworthy that long term exposure to MTX caused the lysis of platelets in the presence of Ca2+. While U46619 transiently increased the internal Ca2+ concentration ([Ca2+]i), maitotoxin slowly but irreversibly increased [Ca2+]i in an external Ca2(+)-dependent manner. MTX-induced phosphoinositide hydrolysis was totally dependent on the presence of external Ca2+, but U46619-induced phosphoinositide hydrolysis was still observed in the absence of external Ca2+. MTX-induced phosphoinositide hydrolysis was partly inhibited by SK&F96365, a voltage-independent Ca2+ channel antagonist, or by genistein, a tyrosine kinase inhibitor. MTX caused phosphorylation of tyrosine residues of several proteins, like U46619. Thus, MTX is similar to U46619 in functions of Ca2+ mobilization, phosphoinositide hydrolysis and tyrosine phosphorylation, but MTX-induced actions are strictly dependent on the presence of external Ca2+.

Maitotoxin activates a nonselective cation channel and a P2Z/P2X(7)-like cytolytic pore in human skin fibroblasts

Maitotoxin (MTX), a potent cytolytic agent, activates Ca(2+) entry via nonselective cation channels in virtually all types of cells. The identity of the channels involved and the biochemical events leading to cell lysis remain unknown. In the present study, the effect of MTX on plasmalemmal permeability of human skin fibroblasts was examined. MTX produced a time- and concentration-dependent increase in cytosolic free Ca(2+) concentration that depended on extracellular Ca(2+) and was relatively insensitive to blockade by extracellular lanthanides. MTX also produced a time- and concentration-dependent increase in plasmalemma permeability to larger molecules as indicated by 1) uptake of ethidium (314 Da), 2) uptake of YO-PRO-1 (375 Da), 3) release of intracellular fura 2 (636 Da), 4) uptake of POPO-3 (715 Da), and, ultimately, 5) release of lactate dehydrogenase (relative molecular weight of 140,000). At the single cell level, uptake of YO-PRO-1 correlated in time with the appearance of large MTX-induced membrane currents carried by the organic cation, N-methyl-D-glucamine (167 Da). Thus MTX initially activates Ca(2+)-permeable cation channels and later induces the formation of large pores. These effects of MTX on plasmalemmal permeability are similar to those seen on activation of P2Z/P2X(7) receptors in a variety of cell types, raising the intriguing possibility that MTX and P2Z/P2X(7) receptor stimulation activate a common cytolytic pore.

Transient Ca2+-dependent activation of ERK1 and ERK2 in cytotoxic responses induced by maitotoxin in breast cancer cells

Treatment of MCF-7 breast cancer cells with the marine toxin maitotoxin (MTX) induces cell death. The cytotoxic effects are clearly detectable within 2-4 h after cell treatment with 10(-10)-10(-9) M concentrations of MTX. The response was found to depend on extracellular Ca2+, inasmuch as cell death was prevented when culture dishes received MTX, following addition of EGTA. MTX caused transient phosphorylation of extracellular signal-regulated kinase isoforms 1 and 2 (ERK1 and ERK2) mitogen-activated protein kinase isoforms in MCF-7 cells, which was maximal 15 min after toxin addition to culture vessels. The effect was dependent on influx of extracellular Ca2+, as it was abolished by EGTA, and was induced by ionophores, such as A23187 and ionomycin. Our findings show that signaling pathways involving Ca2+ ions may cause activation of ERK1 and ERK2 in cell death responses.

Maitotoxin induces calpain but not caspase-3 activation and necrotic cell death in primary septo-hippocampal cultures

Maitotoxin is a potent toxin that activates voltage and receptor-mediated Ca2+ channels, resulting in Ca2+ overload and rapid cell death. We report that maitotoxin-induced cell death is associated with activation of calpain but not caspase-3 proteases in septo-hippocampal cell cultures. Calpain and caspase-3 activation were examined by accumulation of protease-specific breakdown products to alpha-spectrin. Cell death manifested exclusively necrotic-like characteristics including round, shrunken nuclei, even distribution of chromatin, absence of DNA fragmentation and failure of protein synthesis inhibition to reduce cell death. Necrotic cell death was observed in neurons and astroglia. Calpain inhibitor II inhibited calpain-specific processing of alpha-spectrin and significantly reduced cell death. The pan-caspase inhibitor, Z-D-DCB, nominally attenuated cell death. Results suggest that: (1) calpain, but not caspase-3, is activated as a result of maitotoxin-induced Ca2+ influx; (2) necrotic cell death caused by maitotoxin exposure is partially mediated by calpain activation; (3) maitotoxin is a useful tool to investigate pathological mechanisms of necrosis.

Characterization of a Ca2+ release-activated nonselective cation current regulating membrane potential and [Ca2+]i oscillations in transgenically derived beta-cells

Although stimulation of insulin secretion by glucose is regulated by coupled oscillations of membrane potential and intracellular Ca2+ ([Ca2+]i), the membrane events regulating these oscillations are incompletely understood. In the presence of glucose and tetraethylammonium, transgenically derived beta-cells (betaTC3-neo) exhibit coupled voltage and [Ca2+]i oscillations strikingly similar to those observed in normal islets in response to glucose. Using these cells as a model system, we investigated the membrane conductance underlying these oscillations. Alterations in delayed rectifier or Ca2+-activated K+ channels were excluded as a source of the conductance oscillations, as they are completely blocked by tetraethylammonium. ATP-sensitive K+ channels were also excluded, since the ATP-sensitive K+ channel blocker tolbutamide substituted for glucose in inducing [Ca2+]i oscillations. Thapsigargin, which depletes intracellular Ca2+ stores, and maitotoxin, an activator of nonselective cation channels, both converted the glucose-dependent [Ca2+]i oscillations into a sustained elevation. On the other hand, both SKF 96365, a blocker of Ca2+ store-operated channels, and external Na+ removal suppressed the glucose-stimulated [Ca2+]i oscillations. Maitotoxin activated a nonselective cation current in betaTC3 cells that was attenuated by removal of extracellular Na+ and by SKF 96365, in the same manner to a current activated in mouse beta-cells following depletion of intracellular Ca2+ stores. Currents similar to these are produced by the mammalian trp-related channels, a gene family that includes Ca2+ store-operated channels and inositol 1,4,5-trisphosphate-activated channels. We found several of the trp family genes were expressed in betaTC3 cells by reverse transcriptase polymerase chain reaction using specific primers, but by Northern blot analysis, mtrp-4 was the predominant message expressed. We conclude that a conductance underlying glucose-stimulated oscillations in beta-cells is provided by a Ca2+ store depletion-activated nonselective cation current, which is plausibly encoded by homologs of trp genes.

Characterization of the maitotoxin-induced calcium influx pathway from human skin fibroblasts

Maitotoxin (MTX), a water-soluble polyether obtained from the marine dinoflagellate Gambierdiscus toxicus increased intracellular calcium in a concentration-dependent manner in fibroblasts obtained from human skin. The effect of this toxin was both saturable and of high affinity, showing an apparent half activation constant of 450 fM. The toxin did not release intracellular calcium storage compartments nor did the release of these compartments with thapsigargin or ionomycin affect the toxin response. The toxin effect was reduced significantly by pre-incubating the cells with 0.1% trypsin for 30 min, strongly suggesting that the toxin receptor is a plasmalemmal protein. The effect of MTX was partially inhibited by diphenoxylate.

Calcium is permeable through a maitotoxin-activated nonselective cation channel in mouse L cells

The shellfish poison maitotoxin causes the irreversible opening of nonselective cation channels in mouse L cell fibroblasts, consistent with the action of this toxin in other cell types and the previously demonstrated existence of 28-pS voltage-insensitive nonselected cation channels that are activated by platelet-derived growth factor in these cells. Toxin-induced opening of these nonselective cation channels led to increases of intracellular calcium and secondary activation of calcium-activated potassium channel. These effects were completely dependent on influx of extracellular calcium, supporting the conclusion that the maitotoxin-activated nonselective cation channels are permeable to calcium as well as to sodium and potassium. The implication of this finding is that calcium signaling through this channel underlies its links into the growth factor response.

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

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

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.

Stimulation of histamine H2- (and H1)-receptors activates Ca2+ influx in all-trans-retinoic acid-differentiated HL-60 cells independently of phospholipase C or adenylyl cyclase

In human neutrophils, histamine H2-receptors mediate activation of adenylyl cyclase (AC) and inhibition of N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP)-induced superoxide anion (O2-) formation, and in HL-60 promyelocytes, H2-receptors mediate parallel activation of AC, phospholipase C (PLC) and non-selective cation (NSC) channels. As all-trans-retinoic acid (RA) is successfully used in the differentiation therapy of acute promyelocytic leukaemia, we studied signal transduction in RA-differentiated HL-60 cells. Histamine and the H2-receptor agonist, impromidine, induced both rises in cAMP levels and cytosolic Ca2+ ([Ca2+]i). Substances acting at post-receptor sites to increase cAMP did not increase [Ca2+]i. H2- but not H1-receptor antagonists inhibited histamine-induced cAMP accumulation and rises in [Ca2+]i were more effectively inhibited by H2- than by H1-receptor antagonists. Histamine-induced rises in [Ca2+]i were completely dependent on the presence of extracellular Ca2+ and were abolished by the blocker of NSC channels, Gd3+, but were resistant to inhibition by pertussis toxin. Unlike FMLP, histamine did not activate PLC. The effects of FMLP on [Ca2+]i were less sensitive to blockade by Gd3+ than those of histamine, and there was no cross-desensitization between the two stimuli. FMLP, but not histamine, inhibited transiently thapsigargin-induced rises in [Ca2+]. Taken together, our results show that histamine activates AC-mediated cAMP accumulation in RA-differentiated HL-60 cells via H2-receptors and NSC channel-mediated Ca2+ influx via H2- (and H1)-receptors. Histamine-induced NSC channel activation is not the consequence of AC- or PLC stimulation and occurs, directly or indirectly, via pertussis toxin-insensitive guanine nucleotide-binding proteins. FMLP and histamine activate Ca2+ influx by different mechanisms. There are similarities in H2-receptor-mediated signal transduction between RA-differentiated HL-60 cells and HL-60 promyelocytes and differences between the former cells and neutrophils, indicating that RA-differentiated HL-60 cells must be considered as partially immature.

Maitotoxin-elicited calcium influx in cultured cells. Effect of calcium-channel blockers

Maitotoxin elicited a marked influx of 45Ca2+ into NIH 3T3 fibroblast cells. The influx was blocked by imidazoles (econazole, miconazole, SKF 96365, clotrimazole, calmidazolium) with IC50 values from 0.56 to 3 microM. Phenylalkylamines (verapamil, methoxyverapamil) and nitrendipine were less potent, and diltiazem was very weak. Among other calcium blockers, the diphenylbutylpiperidines fluspirilene and penfluridol, the diphenylpropylpiperidine loperamide, and the local anesthetic proadifen were quite active with IC50 values of 2-4 microM. The pattern of inhibition of maitotoxin-elicited calcium influx did not correspond to the ability of the agents to block elevation of calcium that ensues through calcium-release activated calcium (CRAC) channels after activation of phosphoinositide breakdown by ATP in HL-60 cells. The imidazoles did block CRAC channels, but fluspirilene, penfluridol, loperamide and proadifen were ineffective. Loperamide actually appeared to enhance influx of calcium via the activated CRAC channels. The imidazoles, in particular calmidazolium, caused an apparent influx of calcium and caused a stimulation of phosphoinositide breakdown in HL-60 cells.

Clonidine and cirazoline inhibit activation of nicotinic channels in PC-12 cells

Clonidine and cirazoline bind with high affinity to a nonadrenergic site in the brain stem, the so-called imidazoline I1 receptor. Our aim was to determine the mechanism by which these receptors act and their possible linkage to signal-transducing heterotrimeric G-proteins. We examined the effects of clonidine and cirazoline on PC-12 cells, a neuronal cell line that is reported to possess the I1 site and have no alpha 2-adrenoceptors. In undifferentiated PC-12 cells loaded with the Ca2+ indicator dye fura-2, clonidine and cirazoline (10-100 microM) inhibited the increase in [Ca2+]i produced by nicotine (10 microM). This inhibition was not reversed by yohimbine (100 microM), and adrenaline and BHT 920 were ineffective at 100 microM. This effect was not inhibited by pretreatment with pertussis toxin (24 hours, 100 ng/ml) and not modulated by pretreatment with IBMX (100 microM). The nicotine-induced increase in [Ca2+]i is apparently due to Ca2+ entering via the intrinsic ion channel of the nicotinic acetylcholine receptor. Clonidine and cirazoline inhibited the inward current produced by nicotine (10 microM) as measured by the whole cell patch-clamp technique in differentiated PC-12 cells, recorded at a holding potential of -60 mV. In agreement with the results found with fura-2, inhibition of inward current was concentration dependent and not blocked by yohimbine (100 microM) or mimicked by adrenaline (100 microM). Pretreatment of PC-12 cells with pertussis toxin or infusion of GDP-beta-S (2 mM) via the patch pipette did not alter the inhibition of the nicotine-induced inward current by clonidine or cirazoline. Clonidine and cirazoline, but not adrenaline, displayed [3H]phencyclidine from Torpedo electroplaque membranes enriched in nicotinic acetylcholine receptors in a concentration-dependent manner (10-100 microM). Taken together, these results suggest that clonidine and cirazoline inhibit Na+ and Ca2+ entry through the nicotinic acetylcholine receptor via a nonadrenergic mechanism that is independent of G-proteins and cyclic nucleotides, presumably by direct blockade of the intrinsic ion channel of the nicotinic acetylcholine receptor.

Alpha 2A-adrenoceptors mediate activation of non-selective cation channels via Gi-proteins in human erythroleukaemia (HEL) cells. No evidence for a functional role of imidazoline receptors in modulating calcium

Human erythroleukaemia (HEL) cells were investigated to characterize their alpha 2-adrenoceptor and imidazoline receptor sites. Membranes from HEL cells bound [3H]2-(2-methoxy-1, 4-benzodioxan-2yl)-2-imidazoline ([3H]RX821002) in a saturable and specific manner with a KD of 0.64 +/- 0.07 nM and a Bmax of 126 +/- 4 fmol/mg protein. [3H]RX821002 was displaced from HEL membranes by adrenergic drugs with the order of potency being yohimbine approximately oxymetazoline >> prazosin = 2-[2-[4-(o-methoxyphenyl)piperazin-1-yl]ethyl]-4,4-dimethyl- 1,3(2H,4H)-isochinolindione HCl (ARC 239), consistent with this site being an alpha 2A-adrenoceptor. HEL membranes also bound [3H]idazoxan in the presence of adrenaline to block alpha 2-adrenoceptors. This binding was saturable and specific with a KD of 3.5 +/- 1.0 nM and a Bmax of 31 +/- 6 fmol/mg protein. Adrenergic drugs from both the phenylethylamine and imidazoline classes increased high-affinity GTPase activity, an index of activation of regulatory heterotrimeric guanine-nucleotide binding proteins (G-proteins), and produced increases in cytosolic free calcium concentration ([Ca2+]i). The effects of these agonists in both systems were abolished by pertussis toxin pretreatment, and oxymetazoline and clonidine were antagonists. The potency of adrenergic drugs to inhibit 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304)-induced increases in [Ca2+]i was yohimbine approximately oxymetazoline >> ARC 239, consistent with the binding data and an action at alpha 2A-adrenoceptors. No evidence was found for a role of imidazoline receptors in stimulating G-proteins or modulating [Ca2+]i. The adrenergic agonist-induced increases in [Ca2+]i were due to both release of Ca2+ from intracellular stores and entry of extracellular Ca2+. Ca2+ entry was blocked by 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenylethyl)-1H- imidazole hydrochloride (SKF 96365), but not by nitrendipine. Adrenaline also stimulated Mn2+ entry in HEL cells. Taken together, these results suggest that HEL cells have alpha 2A-adrenoceptors that activate non-selective cation channels via pertussis toxin-sensitive G-proteins, i.e. Gi-proteins.