The regulation of store-dependent Ca2+ influx in HL-60 granulocytes involves GTP-sensitive elements

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

Coupling Ca2+ store release to Icrac channel activation in B lymphocytes requires the activity of Lyn and Syk kinases

Activation of the B cell receptor complex in B lymphocytes causes Ca²⁺ release from intracellular stores, which, in turn, activates ion channels known as Icrac. We investigated the mechanisms that link Ca²⁺ store release to channel gating in DT40 B lymphocyte cell lines genetically manipulated to suppress the expression of several tyrosine kinases: Btk, Lyn, Syk, and the Blnk adaptor molecule. The simultaneous but not the independent suppression of Lyn and Syk expression prevents the activation of Icrac without interfering with thapsigargin-sensitive Ca²⁺ store release. Icrac activation by Ca²⁺ is reversed in mutant cells by the homologous expression of the missing kinases. Pharmacological inhibition of kinase activity by LavendustinA and PP2 cause the same functional deficit as the genetic suppression of enzyme expression. Biochemical assays demonstrate that kinase activity is required as a tonic signal: targets must be phosphorylated to link Ca²⁺ store release to Icrac gating. The action of kinases on Icrac activation does not arise from control of the expression level of the stromal interaction molecule 1 and Orai1 proteins.

Stimulation of neutrophils by prenylcysteine analogs: Ca(2+) release and influx

Farnesylthiosalicylic acid (FTS), a synthetic analog of the terminal prenylcysteine present in signaling proteins induces generation of superoxide ions, phospholipase C-driven hydrolysis of inositol lipids and calcium elevation in human neutrophils and DMSO-differentiated HL60 cells. These effects were ascribed to an interaction of the analog with elements responsible for recognition of specific prenylated proteins. The present study demonstrated that in addition to the release of intracellular calcium stores, FTS enhanced entry of Ca(2+) and Mn(2+) from the medium. The biphasic dependence of the influx on the concentration of FTS, as well as its insensitivity to inhibition by PMA and La(3+) suggest that the influx pathway activated by FTS is distinct from the previously described store-operated calcium channels of neutrophils. Consistent with the participation of a cellular membrane component in the interaction, FTS enhanced (45)Ca uptake in neutrophils and neutrophil cell membranes, but not in multilamellar vesicles. To establish specificity of the farnesyl moiety of FTS (C(15)), effects of three other analogs, geranylthiosalicylate, GTS (C(10)), geranylgeranylthiosalicylate, GGTS (C(20)), as well as the carboxymethyl ester FTS-Me on calcium homeostasis and superoxide production were investigated. GGTS dose-dependently elevated [Ca(2+)](i), induced quenching of the 360 nm Fura-2-calcium fluorescence by Mn(2+) and stimulated superoxide release, while GTS and FTS-Me were inactive. These results defined specific structural requirements for the functional interaction of prenylcysteine analogs with myeloid cells.

Reduced capacitative calcium entry correlates with vesicle accumulation and apoptosis

A preneoplastic variant of Syrian hamster embryo cells, sup(+), exhibits decreased endoplasmic reticulum calcium levels and subsequently undergoes apoptosis in low serum conditions (Preston, G. A., Barrett, J. C., Biermann, J. A., and Murphy, E. (1997) Cancer Res. 57, 537-542). This decrease in endoplasmic reticulum calcium appears to be due, at least in part, to reduced capacitative calcium entry at the plasma membrane. Thus we investigated whether inhibition of capacitative calcium entry per se could reduce endoplasmic reticulum calcium and induce apoptosis of cells. We find that treatment with either SKF96365 (30-100 microM) or cell-impermeant 1,2-bis(o-amino-5-bromophenoxy)ethane-N,N,N', N'-tetraacetic acid (5-10 mM) is able to induce apoptosis of cells in conditions where apoptosis does not normally occur. Because previous work has implicated vesicular trafficking as a mechanism of regulating capacitative calcium entry, we investigated whether disruption of vesicular trafficking could lead to decreased capacitative calcium entry and subsequent apoptosis of cells. Coincident with low serum-induced apoptosis, we observed an accumulation of vesicles within the cell, suggesting deregulated vesicle trafficking. Treatment of cells with bafilomycin (30-100 nM), an inhibitor of the endosomal proton ATPase, produced an accumulation of vesicles, decreased capacitative entry, and induced apoptosis. These data suggest that deregulation of vesicular transport results in reduced capacitative calcium entry which in turn results in apoptosis.

Capacitative calcium entry channels

In the phospholipase C signaling system, Ca(2+) is mobilized from intracellular stores by an action of inositol 1,4,5-trisphosphate. The depletion of intracellular calcium stores activates a calcium entry mechanism at the plasma membrane called capacitative calcium entry. The signal for activating the entry is unknown but likely involves either the generation or release, or both, from the endoplasmic reticulum of some diffusible signal. Recent research has focused on mammalian homologues of the Drosophila TRP protein as potential candidates for capacitative calcium entry channels. This review summarizes current knowledge about the nature of capacitative calcium entry signals, as well as the potential role of mammalian TRP proteins as capacitative calcium entry channel molecules.

Opposing effects of protein kinase A and C on capacitative calcium entry into HL-60 promyelocytes

Treatment of HL-60 cells with thapsigargin, a microsomal Ca2+/ATPase inhibitor, led to depletion of intracellular calcium stores followed by capacitative calcium entry. Stimulation of adenylyl cyclase with forskolin enhanced thapsigargin-induced Ca2+ influx. The forskolin effect was confirmed by enhanced fluorescence quenching induced by Mn2+ entry into fura-2 loaded cells. 1,9-Dideoxy-forskolin, an inactive analog of forskolin, did not affect capacitative calcium entry. On the other hand, phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, inhibited thapsigargin-induced Ca2+ entry. Histamine and prostaglandin E2 (PGE2) elevated intracellular adenosine 3':5'-cyclic monophosphate (cAMP) levels and enhanced the thapsigargin-induced capacitative calcium entry. Incubation with N-[2-(p-bromocynnamylamino)ethyl]-5-isoquinolinesulfonamide (H89), an inhibitor of protein kinase A (PKA), blocked the forskolin effect, and GF109203X, an inhibitor of protein kinase C (PKC), blocked the phorbol 12-myristate 13-acetate effect. The results suggest that protein kinase A regulates capacitative calcium entry positively, but that protein kinase C regulates Ca2+ influx negatively. Furthermore, after differentiation of HL-60 promyelocytes with dimethylsulfoxide to granulocytes, the inhibitory effect of phorbol 12-myristate 13-acetate became more pronounced, whereas the stimulatory effect of prostaglandin E2 did not change. This result suggests that the regulation of capacitative calcium entry by protein kinase C and protein kinase A develops differently during differentiation.

Leukotriene D4 induces a rapid increase in cAMP in the human epithelial cell line, Int 407: a potential role for this signal in the regulation of calcium influx through the plasma membrane

Although the LTD4-induced Ca2+ influx in human epithelial cells has been shown to be regulated by a pertussis toxin-sensitive heterotrimeric G-protein, most likely a G alpha i3 protein [Adolfsson J.L.P., Ohd J.F., Sjölander A. Leukotriene D4-induced activation and translocation of the G-protein alpha i3-subunit in human epithelial cells. Biochem Biophys Res Commun 1996; 226: 413-419], the signalling pathway further downstream is still unclear. In the present study, we investigated the possible involvement of cAMP and protein kinase A activity in the LTD4-induced Ca2+ influx in the epithelial cell line Int 407. Stimulation with LTD4, but not with the calcium ionophore ionomycin, triggered a rapid increase (peak at 7 s) in the cellular cAMP level, an effect that was totally abolished by pertussis toxin. Furthermore, the LTD4-induced Ca2+ signal was reduced by 60% when cells that had been pre-incubated with the protein kinase A inhibitor Rp-cAMPS (50 microM for 30 min) were stimulated in a calcium containing medium. In contrast, Rp-cAMPS had no apparent effect on the LTD4-induced Ca2+ signal when the cells were stimulated in a calcium-depleted medium. The immediate LTD4-induced protein tyrosine phosphorylation (15 s), previously shown to be necessary for the subsequent Ca2+ influx, was abolished not only by pretreatment with pertussis toxin but also by exposure to Rp-cAMPS. Furthermore, direct activation of the cellular adenylyl cyclase activity by treatment with forskolin alone induced a prompt Ca2+ signal in the presence, but not in the absence, of extracellular Ca2+, identical results were obtained with the cell permeable cAMP analogue 8-bromo-cAMP. In addition, forskolin induced protein tyrosine phosphorylation similar to that seen with LTD4. These results suggest that protein kinase A activity participates in the regulation of the LTD4-induced Ca2+ influx at a site that is downstream of the activation of the pertussis toxin-sensitive G-protein but upstream of a LTD4-stimulated tyrosine kinase(s).

Store-operated Ca2+ influx and stimulation of exocytosis in HL-60 granulocytes

This study addresses the role of store-operated Ca2+ influx in the regulation of exocytosis in inflammatory cells. In HL-60 granulocytes, which do not possess voltage-operated Ca2+ channels, the chemotactic peptide fMet-Leu-Phe (fMLP) was able to stimulate store-operated Ca2+ influx and to trigger exocytosis of primary granules. An efficient triggering of exocytosis by fMLP required the presence of extracellular Ca2+ and was inhibited by blockers of store-operated Ca2+ influx. However, receptor-independent activation of store-operated Ca2+ influx through thapsigargin did not trigger exocytosis. fMLP was unable to stimulate exocytosis in the absence of cytosolic free Ca2+ concentration [Ca2+]c elevations. However, a second signal generated by fMLP synergized with store-operated Ca2+ influx to trigger exocytosis and led to a left shift of the exocytosis/[Ca2+]c relationship in ionomycin-stimulated cells. The synergistic fMLP-generated signaling cascade was long-lasting, involved a pertussis toxin-sensitive G protein and a phosphatidylinositol 3-kinase. In summary, store-operated Ca2+ influx is crucial for the efficient triggering of exocytosis in HL-60 granulocytes, but, as opposed to Ca2+ influx through voltage-operated Ca2+ channels in neurons, it is not a sufficient stimulus by itself and requires synergistic receptor-generated signals.

Feedback regulation of ATP-induced Ca2+ signaling in HL-60 cells is mediated by protein kinase A- and C-mediated changes in capacitative Ca2+ entry

Extracellular ATP increases intracellular Ca2+ ([Ca2+]i) in HL-60 cells. When cells are stimulated with supramaximal concentrations of ATP, although the initial [Ca2+]i increase is similar over a range of 30, 100, and 300 microM ATP, the rate of the return to basal [Ca2+]i level is faster in cells treated with higher concentrations of ATP. This probably results from differences in Ca2+ influx rather than Ca2+ release, since the influx of the unidirectional Ca2+ surrogates Ba2+ and Mn2+ also exhibit similar responses. Furthermore, while 300 microM ATP had an inhibitory effect on the thapsigargin-induced capacitative Ca2+ entry, 30 microM ATP potentiated the response. However, the inhibitory action of 300 microM ATP was blocked by protein kinase C (PKC) inhibitors, such as GF 109203X and chelerythrine, and the potentiating action of 30 microM ATP was blocked by protein kinase A (PKA) inhibitors H89 and Rp-cAMPS. The PKC inhibitors also slowed the decay rate of the Ca2+ response induced by 300 microM ATP, and the PKA inhibitors increased it when induced by 30 microM ATP. In the measurements of PKA and PKC activity, 30 microM ATP activates only PKA, while 300 microM ATP activates both kinases. Taken together, these data suggest that the changes in the ATP-induced Ca2+ response result from differential modulation of ATP-induced capacitative Ca2+ entry by PKC and PKA in HL-60 cells.

An evaluation of strategies available for the identification of GTP-binding proteins required in intracellular signalling pathways

Strategies which can be used to elucidate the nature of a GTP-binding regulatory protein (G-protein) involved in an intracellular pathway of interest in the complex environment of the cell are described and evaluated. A desirable strategy is considered to be one in which the first stage indicates a requirement for one or more G-proteins, provides information on whether a monomeric, trimeric or other type of G-protein is involved, and gives some idea of the G-protein sub-class. In the second stage the specific G-protein involved is identified. Approaches available for investigations in the first stage include the use of analogues of GTP and GDP, AlF4-, inhibitors of G-protein isoprenylation, bacterial toxins which covalently modify G-proteins, and the introduction of a purified GDP dissociation inhibitor, GDP exchange and/or GTP-ase activating protein. Identification of the specific G-protein in the second stage can be achieved using anti G-protein antibodies, G-protein-or receptor-derived peptides, antisense G-protein RNA and over-expressed, constitutively-active or dominant-negative G-protein mutants. The correct interpretation of results obtained with GTP and GDP analogues and AlF4- in the first stage is complex and often difficult, and requires a thorough understanding of the functions and mechanisms of activation of G-proteins. Nevertheless, it is important to reach the correct conclusion at this stage since considerable time and expense are usually required for investigations in the second stage.

On the molecular basis and regulation of cellular capacitative calcium entry: roles for Trp proteins

During the last 2 years, our laboratory has worked on the elucidation of the molecular basis of capacitative calcium entry (CCE) into cells. Specifically, we tested the hypothesis that CCE channels are formed of subunits encoded in genes related to the Drosophila trp gene. The first step in this pursuit was to search for mammalian trp genes. We found not one but six mammalian genes and cloned several of their cDNAs, some in their full length. As assayed in mammalian cells, overexpression of some mammalian Trps increases CCE, while expression of partial trp cDNAs in antisense orientation can interfere with endogenous CCE. These findings provided a firm connection between CCE and mammalian Trps. This article reviews the known forms of CCE and highlights unanswered questions in our understanding of intracellular Ca2+ homeostasis and the physiological roles of CCE.

Characteristics of NaF-induced differentiation of HL-60 cells

Sodium fluoride (NaF) is known to stimulate osteoblastic bone formation, but little attention has been given to the possibility that NaF also affects bone resorption and the differentiation of osteoclastic progenitor cells. When human promyelocytic HL-60 cells were treated with NaF (0.5 mM, 0-4 days), cell proliferation was inhibited, and the addition of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) (10nM, 0-4 days) augmented this antiproliferative effect. NaF increased cellular reduction of nitroblue tetrazolium (NBT), and this effect was strongly augmented by 1,25(OH)2D3. In addition, NaF produced marked changes in cellular morphology, increased cellular adhesion to plastic, reduced the nuclear/cytoplasmic ratio, and increased cellular expression of chloroacetate esterase, but failed to alter cellular nonspecific esterase activity. Furthermore, NaF increased expression of CD11b and CD66b, and this stimulation was enhanced by adding 1,25(OH)2D3. The sum of these changes in classical promyelocytic cellular indices suggest: (1) that NaF stimulates the early stages of HL-60 differentiation toward a granulocyte-like cell and (2) that 1,25(OH)2D3 promotes these actions of NaF. Additional experiments aimed at further understanding the NaF-induced conversion of HL-60 cells identified further changes. NaF also increased cellular production of prostaglandin E2 (PGE2) and nitric oxide (NO) and induced expression of inducible nitric oxide synthase (iNOS); 1,25(OH)2D3 once again augmented these NaF-induced effects. Similarly, NaF stimulated the production of interleukin 1 alpha (IL-1 alpha), IL-6, and tumor necrosis factor-alpha, and 1,25(OH)2D3 again strongly enhanced these effects. Indomethacin completely blocked stimulation of NBT reduction, NO production, and iNOS expression induced by NaF plus 1,25(OH)2D3; adding exogenous PGE2 (0.1-10 ng/ml) to these indomethacin-blocked cultures dose-dependently restored NO production. These additional findings together with the observed slow onset (24-48 h) of NaF and 1,25(OH)2D3 interaction strongly suggest that 1,25(OH)2D3 acts as a cofactor with NaF primarily through interaction with an endogenous NaF-induced cyclo-oxygenase product(s), quite possibly PGE2 itself. Such a mechanism for NaF and 1,25(OH)2D3 interaction would be strongly analogous to the interaction we have recently demonstrated between 1,25(OH)2D3 and PGE1 on the differentiation of HL-60 cells.

Role of nonspecific cross-reacting antigen, a CD66 cluster antigen, in activation of human granulocytes

Nonspecific cross-reacting antigen (NCA) is the name of a family of highly glycosylated bacterial-binding receptors found on human granulocytes and other tissues. These glycoproteins are members of the immunoglobulin supergene family and are related structurally to carcinoembryonic antigen. In this study, we demonstrate that ligation of granulocyte NCA results in the activation of the cells, as measured by degranulation and the flux of intracellular calcium. These studies further the proposition that NCA has a function in the immune response of granulocytes against bacterial infections.

A common low-affinity binding site for primary prostanoids on bovine aortic endothelial cells

[3H]PGE2 and [3H]PGF2 alpha were shown to bind with similar binding capacity and dissociation constants to bovine aorta endothelial cells. The similarity in the binding parameters suggests that both agonists may bind to the same binding site. Displacement of [3H]PGE2 performed with PGE2, PGF2 alpha or U-46619, a thromboxane agonist, shows that all three prostanoids displaced the bound [3H]PGE2 with comparable potency (IC50 = 10(-7) M). These results indicated that the three different prostanoids, which serve as specific agonists to different prostanoid receptors, also compete for the same binding site in bovine endothelial cells with similar affinity. Comparison of the displacement of [3H]PGE2 or [3H]PGF2 alpha by a number of prostaglandin agonists and antagonists further supports the notion that the natural prostanoids bind with similar affinities to the same binding site. Thus, sulprostone, an EP1/EP3 agonist, displaced bound [3H]PGE2 and [3H]PGF2 alpha with IC50 of about 10(-7) M. On the other hand, thromboxane antagonists (BAY u-3405 and GR-32191B), EP1 specific antagonist (SC-19220) EP1/DP antagonist (AH-6809) and iloprost, a stable prostacyclin agonist, failed to displace bound [3H]PGE2 or [3H]PGF2 alpha at a concentration range of 10(-9)-10(-6) M. Gradual increase of sodium fluoride (NaF), a general activator of G binding proteins, or incubation of permeabilized cells with GTP gamma S resulted in a decrease in [3H]PGE2 binding, suggesting that the binding site represents a low-affinity common prostanoid receptor which, similar to other prostanoid receptors, is probably coupled with G binding proteins.

trp, a novel mammalian gene family essential for agonist-activated capacitative Ca2+ entry

SUMMARY

Capacitative calcium entry (CCE) describes CA2+ influx into cells that replenishes CA2+ stores emptied through the action of IP3 and other agents. It is an essential component of cellular responses to many hormones and growth factors. The molecular basis of this form of Ca2+ entry is complex and may involve more than one type of channel. Studies on visual signal transduction in Drosophila led to the hypothesis that a protein encoded in trp may be a component of CCE channels. We reported the existence of six trp-related genes in the mouse genome. Expression in L cells of small portions of these genes in antisense orientation suppressed CCE. Expression in COS cells of two full-length cDNAs encoding human trp homologs, Htrp1 and Htrp3, increased CCE. This identifies mammalian gene products that participate in CCE. We propose that trp homologs are subunits of CCE channels, not unlike those of classical voltage-gated ion channels.

Calcium store depletion potentiates a phosphodiesterase inhibitor- and dibutyryl cGMP-evoked calcium influx in rat pituitary GH3 cells

A role for cGMP in the control of capacitative Ca2+ influx was identified in rat pituitary GH3 cells. Application of 50 microM - 1 mM of the non-specific phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), or the specific cGMP-phosphodiesterase inhibitor, zaprinast, induced a dose-dependent increase in the intracellular free Ca2+ concentration [Ca2+]i of the pituitary cell line, as assessed by video ratio imaging using fura-2. Response onset times were identical and response profiles were similar in all cells analysed. Application of 50 microM dibutyryl cGMP to GH3 cells resulted in heterogeneous Ca2+ responses, consisting of single or multiple transients with varying onset times. In all cases, increases in [Ca2+]i were predominantly due to Ca2+ influx, since no responses were detected in low Ca2+ medium, or following pre-incubation of cells with 1 microM verapamil, or nicardipine. Depleting intracellular Ca2+ stores by prior treatment of cells with 1 microM thapsigargin resulted in a dramatic potentiation in the Ca2+ influx mediated by both phosphodiesterase inhibitors and dibutyryl cGMP, suggesting that cGMP modulates a dihydropyridine-sensitive Ca2+ entry mechanism in GH3 cells which is possibly regulated by the state of filling of Ca2+ stores.

Intracellular ADP modulates the Ca2+ release-activated Ca2+ current in a temperature- and Ca2+-dependent Way

The rat basophilic cell line RBL-1 is known to express high levels of the Ca2+ current activated by store depletion, known as Ca2+ release-activated Ca2+ current (ICRAC), the main Ca2+ influx pathway so far identified in nonexcitable cells. We show here that, as reported in other cell types, metabolic drugs strongly inhibit the Ca2+ influx operated by store depletion in RBL-1 cells also. We have tested the hypothesis that intracellular adenine and/or guanine nucleotide levels act as coupling factors between ICRAC and cell metabolism. Using the whole cell configuration of the patch-clamp technique, we demonstrate that addition of ADP to the intracellular solution significantly reduces ICRAC induced by inositol 1,4,5-trisphosphate. This phenomenon differs from other regulatory pathways of ICRAC, since it is highly temperature-dependent, is observable only in the presence of low intracellular Ca2+ buffering capacity, and requires a cytosolic factor(s) which is rapidly lost during cell dialysis. Moreover, the inhibition is specific for ADP and is partially mimicked by ADPbetaS and AMP, but not by GDP or GTP.

Pharmacological and functional properties of voltage-independent Ca2+ channels

During the last few years, considerable progress has taken place in our knowledge of the molecular and functional properties of the various voltage-independent Ca2+ channels. In addition to the ionotropic receptor-channels (ROCs), that are not discussed in the present review, these channels include the SMOCs, activated via second messengers or other transducing processes directly triggered by receptor activation; and the SOCCs, activated as a consequence of depletion of the rapidly exchanging Ca2+ stores in the cytoplasm. In parallel, a pharmacological approach to the study of these channels has been developed, based primarily on heterogeneous drugs already known for different biological effects, and subsequently recognized as voltage-independent Ca(2+)-channel blockers. From the systematic analysis of the effects of these drugs new information has emerged about SMOCs and SOCCs function. In addition, pharmacological blockade of these channels appears to have beneficial therapeutic effects in pathological conditions such as tumoral cell growth, inflammation and immunity. At the moment the field is rapidly evolving, with major developments expected in the years ahead.

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.

Evidence that the pertussis toxin-sensitive trimeric GTP-binding protein Gi2 is required for agonist- and store-activated Ca2+ inflow in hepatocytes

The role of a trimeric GTP-binding protein (G-protein) in the mechanism of vasopressin-dependent Ca2+ inflow in hepatocytes was investigated using both antibodies against the carboxyl termini of trimeric G-protein alpha subunits, and carboxyl-terminal alpha-subunit synthetic peptides. An anti-Gi1-2 alpha antibody and a Gi2 alpha peptide (Gi2 alpha) Ile345-Phe355), but not a Gi3 alpha peptide (Gi3 alpha Ile344-Phe354), inhibited vasopressin- and thapsigargin-stimulated Ca2+ inflow, had no effect on vasopressin-stimulated release of Ca2+ from intracellular stores, and caused partial inhibition of thapsigargin-stimulated release of Ca2+. An anti-Gq alpha antibody also inhibited vasopressin-stimulated Ca2+ inflow and partially inhibited vasopressin-induced release of Ca2+ from intracellular stores. Immunofluorescence measurements showed that Gi2 alpha is distributed throughout much of the interior of the hepatocyte as well as at the periphery of the cell. By contrast, Gq/11 alpha was found principally at the cell periphery. It is concluded that the trimeric G-protein, Gi2, is required for store-activated Ca2+ inflow in hepatocytes and acts between the release of Ca2+ from the endoplasmic reticulum (presumably adjacent to the plasma membrane) and the receptor-activated Ca2+ channel protein(s) in the plasma membrane.

Ca2+ influx induced by store release and cytosolic Ca2+ chelation in Ht29 colonic carcinoma cells

Cl- secretion in HT29 cells is regulated by agonists such as carbachol, neurotensin and adenosine 5'-triphosphate (ATP). These agonists induce Ca2+ store release as well as Ca2+ influx from the extracellular space. The increase in cytosolic Ca2+ enhances the Cl- and K+ conductances of these cells. Removal of extracellular Ca2+ strongly attenuates the secretory response to the above-mentioned agonists. The present study utilises patch-clamp methods to characterise the Ca2+ influx pathway. Inhibitors which have been shown previously to inhibit non-selective cation channels, such as flufenamate (0.1 mmol.l-1, n = 6) and Gd3+ (10 micromol.l-1, n=6) inhibited ATP (0.1 mmol.l-1) induced increases in whole-cell conductance (Gm). When Cl- and K+ currents were inhibited by the presence of Cs2SO4 in the patch pipette and gluconate in the bath, ATP (0.1 mmol.l-1) still induced a significant increase in Gm from 1.2 +/- 0.3 nS to 4.7 +/- 1 nS (n = 24). This suggests that ATP induces a cation influx with a conductance of approximately 3-4 nS. This cation influx was inhibited by flufenamate (0.1 mmol.l-1, n = 6) and Gd3+ (10 micromol.l-1, n = 9). When Ba2+ (5 mmol.l-1) and 4,4'-diisothiocyanato-stilbene-2-2'-disulphonic acid (DIDS, 0.1 mmol.l-1) were added to the KCl/K-gluconate pipette solution to inhibit K+ and Cl- currents and the cells were clamped to depolarised voltages, ATP (0.1 mmol.l-1) reduced the membrane current (Im) significantly from 86 +/- 14 pA to 54 +/- 11 pA (n = 13), unmasking a cation inward current. In another series, the cation inward current was activated by dialysing the cell with a KCl/K-gluconate solution containing 5-10 mmol.l-1 1,2-bis-(2-aminoethoxy)ethane-N,N,N',N'-tetraacetic acid (EGTA) or 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA). The zero-current membrane voltage (Vm) and Im (at a clamp voltage of +10 mV) were monitored as a function of time. A new steady-state was reached 30-120 s after membrane rupture. Vm depolarised significantly from -33 +/- 2 mV to -12 +/- 1 mV, and Im fell significantly from 17 +/- 2 pA to 8.9 +/- 1.0 pA (n = 71). This negative current, representing a cation inward current, was activated when Ca2+ stores were emptied and was reduced significantly ( Im) when Ca2+ and/or Na+ were removed from the bathing solution: removal of Ca2+ in the absence of Na+ caused a Im of 5.0 +/- 1.2 pA (n = 12); removal of Na+ in the absence of Ca2+ caused a Im of 12.8 +/- 3.5 pA (n = 4). The cation inward current was also reduced significantly by La3+, Gd3+, and flufenamate. We conclude that store depletion induces a Ca2+/Na+ influx current in these cells. With 145 mmol.l-1 Na+ and 1 mmol.l-1 Ca2+, both ions contribute to this cation inward current. This current is an important component in the agonist-regulated secretory response.

Slow calcium-dependent inactivation of depletion-activated calcium current. Store-dependent and -independent mechanisms

Feedback regulation of Ca2+ release-activated Ca2+ (CRAC) channels was studied in Jurkat leukemic T lymphocytes using whole cell recording and [Ca2+]i measurement techniques. CRAC channels were activated by passively depleting intracellular Ca2+ stores in the absence of extracellular Ca2+. Under conditions of moderate intracellular Ca2+ buffering, elevating [Ca2+]o to 22 mM initiated an inward current through CRAC channels that declined slowly with a half-time of approximately 30 s. This slow inactivation was evoked by a rise in [Ca2+]i, as it was effectively suppressed by an elevated level of EFTA in the recording pipette that prevented increases in [Ca2+]i. Blockade of Ca2+ uptake into stores by thapsigargin with or without intracellular inositol 1,4,5-trisphosphate reduced the extent of slow inactivation by approximately 50%, indicating that store refilling normally contributes significantly to this process. The store-independent (thapsigargin-insensitive) portion of slow inactivation was largely prevented by the protein phosphatase inhibitor, okadaic acid, and by a structurally related compound, 1-norokadaone, but not by calyculin A nor by cyclosporin A and FK506 at concentrations that fully inhibit calcineurin (protein phosphatase 2B) in T cells. These results argue against the involvement of protein phosphatases 1, 2A, 2B, or 3 in store-independent inactivation. We conclude that calcium acts through at least two slow negative feedback pathways to inhibit CRAC channels. Slow feedback inhibition of CRAC current is likely to play important roles in controlling the duration and dynamic behavior of receptor-generated Ca2+ signals.

Inositol phosphate structural requisites for Ca2+ influx

To understand how inositol phosphates (InsP) cause Ca2+ influx, we injected 37 highly purified compounds containing a total of 49 InsP positional isomers into Xenopus oocytes. The eight InsP that stimulated Ca2+ influx were those that had the highest potency at releasing intracellular Ca2+, indicating that their common target was the inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptor. To cause Ca2+ influx, these InsP had to be injected in a much higher concentration than the minimal concentration required to release intracellular Ca2+. Such high InsP concentrations could inhibit ongoing oscillatory intracellular Ca2+ release. In addition, we found that InsPs could not elicit further intracellular Ca2+ release during the course of Ca2+ influx. Our data are consistent with the "capacitative Ca2+ entry" hypothesis, which states that InsP stimulate Ca2+ influx by depleting the InsP-sensitive intracellular Ca2+ store. In this context, we would suggest that to deplete the InsP-sensitive intracellular Ca2+ store, InsP may have to be present in a sufficiently high concentration to override the oscillatory Ca(2+)-refilling mechanisms of the stores.

Inositol trisphosphate receptor mediated spatiotemporal calcium signalling

Spatiotemporal Ca2+ signalling in the cytoplasm is currently understood as an excitation phenomenon by analogy with electrical excitation in the plasma membrane. In many cell types, Ca2+ waves and Ca2+ oscillations are mediated by inositol 1,4,5-trisphosphate (IP3) receptor/Ca2+ channels in the endoplasmic reticulum membrane, with positive feedback between cytosolic Ca2+ and IP3-induced Ca2+ release creating a regenerative process. Remarkable advances have been made in the past year in the analysis of subcellular Ca2+ microdomains using confocal microscopy and of Ca2+ influx pathways that are functionally coupled to IP3-induced Ca2+ release. Ca2+ signals can be conveyed into the nucleus and mitochondria. Ca2+ entry from outside the cell allows repetitive Ca2+ release by providing Ca2+ to refill the endoplasmic reticulum stores, thus giving rise to frequency-encoded Ca2+ signals.

The regulation of leukotriene D4-induced calcium influx in human epithelial cells involves protein tyrosine phosphorylation

Leukotriene D4 (LTD4) has been found to induce calcium signalling in the intestinal epithelial cell line Int 407, and this action involves the activation of both different GTP-binding proteins (G-proteins) and phospholipase C of the gamma-subtype (PLC-gamma). With this knowledge as the incentive, we investigated the possible regulatory role of protein tyrosine kinase activities in the calcium signalling system of the LTD4 receptor. The tyrosine kinase inhibitors genistein and herbimycin. A both reduced the LTD4-induced calcium signal by 70% when Int 407 cells were stimulated in the presence of extracellular calcium, but had no effect on the signal when the cells were stimulated in a calcium-free medium. In accordance with these findings, pretreatment with a tyrosine kinase inhibitor also blocked thapsigargin-induced cellular influx of calcium. These inhibitors had no effect on the intracellular mobilisation of calcium, which was supported by the findings that LTD4 was able to induce an increase in the tyrosine phosphorylation of PLC-gamma even when one of the tyrosine kinase inhibitors was present. Of possible interest regarding the effect of genistein on LTD4-induced calcium influx is that two major tyrosine phosphorylated protein bands were detected in immunoprecipitates obtained with PLC-gamma antibodies from LTD4-stimulated cells. These proteins, which associate with PLC-gamma, have estimated molecular weights of 84 and 97 kD. Preincubation with genistein completely abolished the LTD4-induced increase in tyrosine phosphorylation of the major 97 kD band, whereas the 84 kD protein band, like the PLC-gamma band, still exhibited an increased phosphorylation of tyrosine residues in response to LTD4. Neither this effect nor any of the other effects of genistein were induced when cells were preincubated with daidzein, an inactive analogue of genistein. The present results suggest that LTD4-induced calcium signalling in epithelial cells involves not only tyrosine phosphorylation of PLC-gamma, but also a tyrosine kinase-dependent step which occurs downstream of PLC-gamma activation and is directly implicated in the regulation of agonist-mediated calcium influx.

G-protein- and capacitatively regulated Ca2+ entry pathways are activated by muscarinic receptor stimulation in a human submandibular ductal cell line

In the human submandibular ductal cell line (HSG) thapsigargin and carbachol stimulated Ca2+ release from the internal Ca2+ pool, resulting in the activation of capacitatively regulated Ca2+ entry (CRCE). This entry pathway was permeant to both Ca2+ and Mn2+, blocked by Ni2+ and insensitive to the muscarinic antagonist, atropine. Carbachol also stimulated an increase in cytosolic [Ca2+] in internal Ca(2+)-pool-depleted (i.e. thapsigargin-treated) cells which was dependent on the presence of external Ca2+ and blocked by Ni2+, demonstrating that it was due to Ca2+ entry. However, under the same experimental conditions, carbachol was unable to stimulate Mn2+ entry. Additionally, this latter carbachol-stimulated Ca2+ entry pathway was blocked by atropine. Pretreatment of HSG cells with AlF4-increased basal rates of Mn2+ entry due to CRCE activation, but attenuated carbachol-stimulated Ca2+ entry into thapsigargin-treated cells. The data suggest that two distinct divalent cation entry pathways are activated in muscarinic-receptor-stimulated HSG cells; a CRCE mechanism, permeable to both Mn2+ and Ca2+, and a second entry mechanism, permeable only to Ca2+. The latter does not depend on internal pool depletion, but appears to be regulated via G-protein activation.

A role for a pertussis toxin-sensitive trimeric G-protein in store-operated Ca2+ inflow in hepatocytes

The mechanism of store-operated Ca2+ inflow in hepatocytes was investigated using fluo-3 and fura-2 to monitor changes in the concentration of intracellular free Ca2+ in single cells, and 1-(alpha-glycerophosphoryl)-myo-inositol 4,5-diphosphate, P4(5)-1-(2-nitrophenyl)ethyl ester ('caged' GPIP2) and 'caged' guanosine 5'-[gamma thio]triphosphate (GTP gamma S) (introduced into the cytoplasmic space by microinjection), thapsigargin and 2,5-di-tert- butylhydroquinone (DBHQ) to stimulate Ca2+ inflow. Photolysis of 'caged' GPIP2 or 'caged' GTP gamma S stimulated Ca2+ inflow. The abilities of GPIP2, thapsigargin and DBHQ to stimulate Ca2+ inflow were inhibited by the pre-treatment of hepatocytes with pertussis toxin in vivo for 36 h. Thapsigargin-stimulated Ca2+ inflow was also inhibited by guanosine 5'-[beta-thio]diphosphate (GDP beta S) (introduced by microinjection). It is concluded that, in hepatocytes, store-operated Ca2+ inflow induced by the actions of either inositol 1,4,5-trisphosphate, thapsigargin or DBHQ requires a pertussis toxin-sensitive trimeric G-protein.

Receptor-activated Ca2+ influx: how many mechanisms for how many channels?

Receptors that are coupled to the production of inositol (1,4,5)-trisphosphate cause an increase in cytosolic free Ca2+ concentration as a consequence of both Ca2+ mobilization from intracellular stores and Ca2+ influx through the plasma membrane. Although this latter phenomenon appears attributable to the activation of a number of Ca(2+)-permeable channels, the channels that are controlled by the Ca2+ content of the intracellular stores have recently received much attention. In this review, Cristina Fasolato, Barbara Innocenti and Tullio Pozzan summarize the characteristics of this Ca(2+)-influx pathway and discuss the hypotheses about its mechanism of activation and its relationship with other receptor-activated Ca2+ channels.