Inhibition of inositol trisphosphate-stimulated calcium mobilization by calmodulin antagonists in rat liver epithelial cells

Structural and functional insight into a new emerging target IP3R in cancer

Calcium signaling has been identified as an important phenomenon in a plethora of cellular processes. Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ER-residing intracellular calcium (Ca2+) release channels responsible for cell bioenergetics by transferring calcium from the ER to the mitochondria. The recent availability of full-length IP3R channel structure has enabled the researchers to design the IP3 competitive ligands and reveal the channel gating mechanism by elucidating the conformational changes induced by ligands. However, limited knowledge is available for IP3R antagonists and the exact mechanism of action of these antagonists within a tumorigenic environment of a cell. Here in this review a summarized information about the role of IP3R in cell proliferation and apoptosis has been discussed. Moreover, structure and gating mechanism of IP3R in the presence of antagonists have been provided in this review. Additionally, compelling information about ligand-based studies (both agonists and antagonists) has been discussed. The shortcomings of these studies and the challenges toward the design of potent IP3R modulators have also been provided in this review. However, the conformational changes induced by antagonists for channel gating mechanism still display some major drawbacks that need to be addressed. However, the design, synthesis and availability of isoform-specific antagonists is a rather challenging one due to intra-structural similarity within the binding domain of each isoform. HighlightsThe intricate complexity of IP3R's in cellular processes declares them an important target whereby, the recently solved structure depicts the receptor's potential involvement in a complex network of processes spanning from cell proliferation to cell death.Pharmacological inhibition of IP3R attenuates the proliferation or invasiveness of cancers, thus inducing necrotic cell death.Despite significant advancements, there is a tremendous need to design new potential hits to target IP3R, based upon 3D structural features and pharmacophoric patterns.Communicated by Ramaswamy H. Sarma.

ADP-Mediated Upregulation of Expression of CD62P on Human Platelets Is Critically Dependent on Co-Activation of P2Y1 and P2Y12 Receptors

This study probed the differential utilization of P2Y1 and P2Y12 receptors in mobilizing CD62P (P-selectin) from intracellular granules following activation of human platelets with adenosine 5′-diphosphate (ADP, 100 µmol·L⁻¹) Platelet-rich plasma (PRP) was prepared from the blood of adult humans. CD62P was measured by flow cytometry following activation of PRP with ADP in the absence and presence of the selective antagonists of P2Y1 and P2Y12 receptors, MRS2500 and PSB0739 (both 0.155–10 µmol·L⁻¹), respectively. Effects of the test agents on ADP-activated, CD62P-dependent formation of neutrophil:platelet (NP) aggregates were also measured by flow cytometry, while phosphatidylinositol 3-kinase (PI3K) activity was measured according to Akt1 phosphorylation in platelet lysates. Treatment with MRS2500 or PSB0739 at 10 µmol·L⁻¹ almost completely attenuated (94.6% and 86% inhibition, respectively) ADP-activated expression of CD62P and also inhibited NP aggregate formation. To probe the mechanisms involved in P2Y1/P2Y12 receptor-mediated expression of CD62P, PRP was pre-treated with U73122 (phospholipase C (PLC) inhibitor), 2-aminoethoxy-diphenyl borate (2-APB, inositol triphosphate receptor antagonist), calmidazolium chloride (calmodulin inhibitor), or wortmannin (PI3K inhibitor). U73122, 2-APB, and wortmannin caused almost complete inhibition of ADP-activated expression of CD62P, while calmidazolium chloride caused statistically significant, partial inhibition. PSB0739, but not MRS2500, caused potent inhibition of PI3K-mediated phosphorylation of Akt1. Optimal mobilization of CD62P by ADP-stimulated platelets is critically dependent on the co-activation of platelet P2Y1 and P2Y12 receptors. P2Y12 receptor activation is the key event in activation of PI3K, while activation of the P2Y1 receptor appears to create a high cytosolic Ca²⁺ environment conducive to optimum PI3K activity.

The calponin homology domain of Vav1 associates with calmodulin and is prerequisite to T cell antigen receptor-induced calcium release in Jurkat T lymphocytes

Vav1 is a guanine nucleotide exchange factor that is expressed specifically in hematopoietic cells and plays important roles in T cell development and activation. Vav1 consists of multiple structural domains so as to facilitate both its guanine nucleotide exchange activity and scaffold function following T cell antigen receptor (TCR) engagement. Previous studies demonstrated that the calponin homology (CH) domain of Vav1 is required for TCR-stimulated calcium mobilization and thus downstream activation of nuclear factor of activated T cells. However, it remained obscure how Vav1 functions in regulating calcium flux. In an effort to explore molecules interacting with Vav1, we found that calmodulin bound to Vav1 in a calcium-dependent and TCR activation-independent manner. The binding site was mapped to the CH domain of Vav1. Reconstitution of vav1-null Jurkat T cells (J.Vav1) with CH-deleted Vav1 exhibited a severe deficiency in calcium release to the same extent as that of Jurkat cells treated with the calmodulin inhibitor or J.Vav1 cells. The defect persisted even when phospholipase-Cgamma1 was fully activated, indicating a prerequisite role of Vav1 CH domain in calcium signaling. The results suggest that Vav1 and calmodulin function cooperatively to potentiate TCR-induced calcium release. This study unveiled a mechanism by which the Vav1 CH domain is involved in calcium signaling and provides insight into our understanding of the role of Vav1 in T cell activation.

The role of calmodulin for inositol 1,4,5-trisphosphate receptor function

Intracellular calcium release is a fundamental signaling mechanism in all eukaryotic cells. The ryanodine receptor (RyR) and inositol 1,4,5-trisphosphate receptor (IP(3)R) are intracellular calcium release channels. Both channels can be regulated by calcium and calmodulin (CaM). In this review we will first discuss the role of calcium as an activator and inactivator of the IP(3)R, concluding that calcium is the most important regulator of the IP(3)R. In the second part we will further focus on the role of CaM as modulator of the IP(3)R, using results of the voltage-dependent Ca(2+) channels and the RyR as reference material. Here we conclude that despite the fact that different CaM-binding sites have been characterized, their function for the IP(3)R remains elusive. In the third part we will discuss the possible functional role of CaM in IP(3)-induced Ca(2+) release (IICR) by direct and indirect mechanisms. Special attention will be given to the Ca(2+)-binding proteins (CaBPs) that were shown to activate the IP(3)R in the absence of IP(3).

Modulation of intracellular Ca(2+) concentration by vitamin B12 in rat thymocytes

We have studied several novel effects of vitamin B12 (cyanocobalamin) on cellular Ca(2+) homeostasis in rat thymocytes. We determined the effect of various concentrations of vitamin B12 on intracellular Ca(2+) concentration ([Ca(2+)]i) and parameters of Ca(2+)in signaling using the fluorescent dye Fura-2. The basal [Ca(2+)]i in Ca(2+)-containing media was 115 +/- 5 nM but in vitamin B12 (10 nM)-treated thymocytes [Ca(2+)]i was decreased to 60 +/- 15 nM (mean +/- SEM) during the first 5 min. The decline in [Ca(2+)]i was accompanied by an increase in the endoplasmic reticulum Ca(2+) store, presumably as a result of Ca-ATPase activation. At the same time 100 nM-10 mM B12 induced the accumulation of Ca(2+) in mitochondria. Somewhat higher concentrations of B12 (1-10 microM) had no effect on [Ca(2+)]i. A further increase in B12 concentration with range from 50 microM to 1 mM caused a dose-dependent elevation of [Ca(2+)]i from the basal level (115 +/- 5 nM) up to 200 +/- 50 nM in thymocytes, and this elevation was partially blocked in Ca(2+)-free media. This high concentration of vitamin B12 caused a gradual decrease of endoplasmic reticulum Ca(2+) stores by means of Ca-ATPase inhibition. The B12-induced increase in [Ca(2+)]i was not observed after depletion of intracellular Ca(2+) stores, induced by addition of 2',5'-di(tert-butyl)-1,4-benzohydroquinone (BHQ), an inhibitor of endoplasmic reticulum Ca (2+)-ATPase, concanavalin A, or arachidonic acid. These studies show that vitamin B12 regulates [Ca(2+)]i via several different mechanisms at different B12 concentrations. Participation of G proteins and calmodulin activity in B12-mediated [Ca(2+)]i increase is discussed.

Ca2+-induced contraction of cat esophageal circular smooth muscle cells

ACh-induced contraction of esophageal circular muscle (ESO) depends on Ca2+ influx and activation of protein kinase Cepsilon (PKCepsilon). PKCepsilon, however, is known to be Ca2+ independent. To determine where Ca2+ is needed in this PKCepsilon-mediated contractile pathway, we examined successive steps in Ca2+-induced contraction of ESO muscle cells permeabilized by saponin. Ca2+ (0.2-1.0 microM) produced a concentration-dependent contraction that was antagonized by antibodies against PKCepsilon (but not by PKCbetaII or PKCgamma antibodies), by a calmodulin inhibitor, by MLCK inhibitors, or by GDPbetas. Addition of 1 microM Ca2+ to permeable cells caused myosin light chain (MLC) phosphorylation, which was inhibited by the PKC inhibitor chelerythrine, by D609 [phosphatidylcholine-specific phospholipase C inhibitor], and by propranolol (phosphatidic acid phosphohydrolase inhibitor). Ca2+-induced contraction and diacylglycerol (DAG) production were reduced by D609 and by propranolol, alone or in combination. In addition, contraction was reduced by AACOCF(3) (cytosolic phospholipase A(2) inhibitor). These data suggest that Ca2+ may directly activate phospholipases, producing DAG and arachidonic acid (AA), and PKCepsilon, which may indirectly cause phosphorylation of MLC. In addition, direct G protein activation by GTPgammaS augmented Ca2+-induced contraction and caused dose-dependent production of DAG, which was antagonized by D609 and propranolol. We conclude that agonist (ACh)-induced contraction may be mediated by activation of phospholipase through two distinct mechanisms (increased intracellular Ca2+ and G protein activation), producing DAG and AA, and activating PKCepsilon-dependent mechanisms to cause contraction.

Ca2+/calmodulin inhibition and phospholipase C-linked Ca2+ Signaling in clonal beta-cells

Neurotransmitters and hormones, such as arginine vasopressin (AVP) and bombesin, evoke frequency-modulated repetitive Ca2+ transients in insulin-secreting HIT-T15 cells by binding to receptors linked to phospholipase C (PLC). The role of calmodulin (CaM)-dependent mechanisms in the generation of PLC-linked Ca2+ transients was investigated by use of the naphthalenesulfonamide CaM antagonists W-7 and W-13 and their dechlorinated control analogs W-5 and W-12. W-7 (10-30 microM) and W-13 (30-100 microM), but not W-5 (100 microM) and W-12 (300 microM), reversibly inhibited the AVP- and bombesin-induced Ca2+ transients. As the generation of PLC-linked Ca2+ transients requires mobilization of internal Ca2+ and Ca2+ influx through voltage-sensitive (VSCC) and -insensitive (VICC) Ca2+ channels, the effects of the W compounds on these processes were further investigated. First, W-7 dose dependently diminished K+ (45 mM)-induced Ca2+ signals (IC50, approximately 25 microM), and W-13 (100 microM) reduced the K+ (45 mM)-induced [Ca2+]i rise by about 40-60%, whereas W-5 (100 microM) and W-12 (300 microM) had no effect. In addition, W-7 (100 microM) inhibited whole cell Ca2+ currents in mouse beta-cells by about 60%. Second, pretreatment of cells (5 min) with W-7 (30 microM), but not W-5 (30 microM), inhibited agonist-induced internal Ca2+ mobilization by about 75% in Ca2+-free medium. Neither W-7 (30 microM) nor W-5 (30 microM) affected AVP (100 nM)-stimulated formation of IP3. Third, capacitative Ca2+ influx through VICC activated by thapsigargin (2 microM) in the presence of verapamil (50 microM) was inhibited by W-7 (30 microM) but not by W-5 (30 microM). As all of the W compound effects corresponded well to their reported anticalmodulin activity, a specific anticalmodulin action can be assumed. Thus, Ca2+ via activation of CaM-dependent processes could provide positive feedback on the generation of PLC-linked Ca2+ transients in HIT-T15 cells. This appears to involve CaM-dependent regulation of both mobilization of internal Ca2+ and Ca2+ influx through VSCC and VICC.

Increased cytosolic Ca(2+) concentration in endothelial cells by calmodulin antagonists

Many functions of endothelial cells are Ca(2+)/calmodulin dependent, whereas the role of calmodulin in the regulation of cytosolic Ca(2+) ([Ca(2+)](i)) remains largely unexplained. In the present study, effects of various calmodulin antagonists on [Ca(2+)](i) were investigated in cultured aortic endothelial cells loaded with the Ca(2+)-sensitive dye fura-2/AM, and were compared with those of calmodulin-dependent protein kinase II (CaM kinase II) inhibitors. The calmodulin antagonists W-7, calmidazolium and fendiline provoked dose-dependent increases in [Ca(2+)](i). However, the CaM kinase II inhibitors KN-93 and lavendustin C had no effect on [Ca(2+)](i). In the absence of extracellular Ca(2+), pretreatment of cells with bradykinin (BK) and thapsigargin completely prevented W-7-stimulated increase in [Ca(2+)](i). Alternatively, pretreatment with W-7 also completely blocked BK- and thapsigargin-stimulated increases in [Ca(2+)](i). The time course of the Ca(2+)-response in W-7 treated cells was identical to that in thapsigargin-treated cells, but not that in BK-stimulated cells, suggesting that calmodulin antagonists could share a common signaling pathway with thapsigargin to increase [Ca(2+)](i) in endothelial cells. These findings indicate that calmodulin is involved in the regulation of [Ca(2+)](i), and may play an important role in the uptake of Ca(2+) to intracellular stores.

Ca2+-independent inhibition of inositol trisphosphate receptors by calmodulin: redistribution of calmodulin as a possible means of regulating Ca2+ mobilization

The interactions between calmodulin, inositol 1,4,5-trisphosphate (InsP3), and pure cerebellar InsP3 receptors were characterized by using a scintillation proximity assay. In the absence of Ca2+, 125I-labeled calmodulin reversibly bound to multiple sites on InsP3 receptors and Ca2+ increased the binding by 190% +/- 10%; the half-maximal effect occurred when the Ca2+ concentration was 184 +/- 14 nM. In the absence of Ca2+, calmodulin caused a reversible, concentration-dependent (IC50 = 3.1 +/- 0.2 microM) inhibition of [3H]InsP3 binding by decreasing the affinity of the receptor for InsP3. This effect was similar at all Ca2+ concentrations, indicating that the site through which calmodulin inhibits InsP3 binding has similar affinities for calmodulin and Ca2+-calmodulin. Calmodulin (10 microM) inhibited the Ca2+ release from cerebellar microsomes evoked by submaximal, but not by maximal, concentrations of InsP3. Tonic inhibition of InsP3 receptors by the high concentrations of calmodulin within cerebellar Purkinje cells may account for their relative insensitivity to InsP3 and limit spontaneous activation of InsP3 receptors in the dendritic spines. Inhibition of InsP3 receptors by calmodulin at all cytosolic Ca2+ concentrations, together with the known redistribution of neuronal calmodulin evoked by protein kinases and Ca2+, suggests that calmodulin may also allow both feedback control of InsP3 receptors and integration of inputs from other signaling pathways.

Regulation of protein kinases in steady-state contraction of cat gastric smooth muscle

Cat gastric smooth muscle strips were used to investigate the involvement of protein kinases in the steady-state contraction induced by 1 microM acetylcholine or 20 mM KCI. The steady-state contraction induced by acetylcholine or KCl was inhibited by EGTA dose dependently. Voltage-dependent Ca2+ channel antagonists dose dependently inhibited the contractions induced by KCI as well as by acetylcholine. Inhibitory effects of voltage-dependent Ca2+ channel antagonists were significantly more prominent on KCI-induced contractions than on acetylcholine-induced contractions. The acetylcholine-induced contraction was dose dependently inhibited by 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8, a blocker of intracellular Ca2+ release), but the KCl-induced contraction was not inhibited at all. Therefore both intracellular Ca2+ release and extracellular Ca2+ influx seem to be necessary for the acetylcholine-induced contraction, but intracellular Ca2+ release is not necessary for the KCl-induced contraction. Protein kinase C inhibitors, 10 microM 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine 2HCl (H-7) and 1 microM staurosporine, significantly inhibited the contraction induced by acetylcholine or KCl. Calmodulin antagonists, 30 microM trifluoperazine and 50 microM N-(6-aminohexyl)-5-chloro-2-naphthalenesulfonamide HCI (W-7), however, significantly inhibited the contraction induced by acetylcholine but not by KCl. A tyrosine kinase inhibitor, 50 microM genistein, did not affect the acetylcholine-induced contraction but significantly inhibited the KCl-induced contraction. These results strongly suggest that the involvement of protein kinases in regulation of the steady-state contraction may be agonist-dependent.

Serotonin uptake inhibitors modulate intracellular Ca2+ mobilization in platelets

The serotonin uptake inhibitors sertraline, paroxetine and fluoxetine were compared with imipramine and the calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W-7) and calmidazolium, for their effects on intracellular Ca2+ mobilization in human platelets. All serotonin uptake inhibitors and calmodulin antagonists augmented thrombin-mediated increases in intracellular Ca2+. Sertraline, calmidazolium and W-7 also caused large dose-dependent increases in baseline levels of intracellular Ca2+. There was a rough correlation between the ability to elevate intracellular Ca2+ and potencies for inhibition of calmodulin. Neomycin, an inhibitor of inositol trisphosphate (IP3) generation, significantly inhibited the effects of sertaline. This is consistent with a role of IP3 and calmodulin in the effects of these drugs.

The imidazoline derivative calmidazolium inhibits voltage-gated Ca(2+)-channels and insulin release but has no effect on the phospholipase C system in insulin producing RINm5F-cells

The present study shows that the calmodulin antagonist calmidazolium inhibited influx of Ca2+ through voltage-gated Ca(2+)-channels in clonal insulin producing RINm5F-cells. The mechanism of inhibition may involve both Ca(2+)-calmodulin-dependent protein kinases and direct binding of calmidazolium to the Ca(2+)-channel. Calmidazolium did not affect uptake of Ca2+ into intracellular Ca(2+)-pools, inositol 1,4,5-trisphosphate (InsP3) formation or action on intracellular Ca(2+)-pools. The calmodulin inhibitor also did not affect glucose utilization or oxidation in RINm5F-cells, speaking against an unspecific toxic effect of the compound. KCl-and ATP-stimulated insulin release from RINm5F-cells was attenuated by calmidazolium, whereas basal hormone secretion was unaffected.

Homologous desensitization of serotonin 5-HT2 receptor-stimulated intracellular calcium mobilization in C6BU-1 glioma cells via a mechanism involving a calmodulin pathway

Serotonin 5-HT2 receptor-mediated intracellular Ca2+ mobilization was investigated in rat glioma C6BU-1 cells. The receptors became desensitized after previous exposure to 5-HT in a time- and concentration-dependent manner. The desensitization of 5-HT2 receptor-mediated intracellular signaling appeared to be homologous because previous exposure to 5-HT did not alter the response to other transmitters such as thrombin or isoproterenol and because previous exposure to thrombin or isoproterenol did not diminish the response to 5-HT. The desensitization induced by pretreatment with 5-HT was potently prevented by the naphthalenesulfonamide derivative W-7, a calmodulin antagonist, when it was cosupplied with 5-HT. Furthermore, the preventive effect of W-7 was greater than that of W-5, a weak analogue of W-7, and than that of H-7, a nonselective inhibitor of protein kinases. These results suggest that 5-HT2 receptor-mediated Ca2+ mobilization can be desensitized homologously after prolonged exposure to 5-HT in a calmodulin-dependent manner in rat glioma C6BU-1 cells.

Role of Ca(2+)-calmodulin dependent phospholamban phosphorylation on the relaxant effect of beta-adrenergic agonists

The role of the Ca(2+)-calmodulin dependent pathway of phospholamban phosphorylation on the relaxant effect of beta-adrenergic agonists was studied in isolated perfused rat heart. Administration of the calmodulin antagonist W7 or lowering [Ca]o from 1.35 mM (control) to 0.25 mM, were used as experimental tools to inhibit the Ca(2+)-calmodulin dependent protein kinase activity. 3 x 10(-8) M isoproterenol increased cAMP levels from 0.613 +/- 0.109 pmol/mg wet weight to 1.581 +/- 0.123, phospholamban phosphorylation from 36 +/- 6 pmol 32P/mg protein to 277 +/- 26 and decreased time to half relaxation (t1/2) from 61 +/- 2 msec to 39 +/- 2. Simultaneous perfusion of isoproterenol with 10(-6) M W7, decreased phospholamban phosphorylation to 170 +/- 23 and prolongated t1/2 to 47 +/- 3 but did not affect the increase either in cAMP levels or myocardial contractility produced by isoproterenol. Similar effects on phospholamban phosphorylation and myocardial relaxation were obtained when isoproterenol was perfused in low [Ca]o. Low [Ca]o did not affect the increase in cAMP elicited by isoproterenol but offset the positive inotropic effect of the beta-agonist. The results suggest a physiological role of the Ca(2+)-calmodulin dependent phospholamban phosphorylation pathway as a mechanism that supports, in part, the beta-adrenergic cardiac relaxant effect.

Human basophil histamine release is differently affected by inhibitors of calmodulin, diacylglycerol kinase and peptidyl prolyl cis-trans isomerase in a secretagogue specific manner

To assess the role of calmodulin in human basophil histamine release, we triggered leukocytes with different secretagogues in the presence of putative inhibitors of calmodulin. Calcium ionophore-induced histamine release was reduced or blocked by calmidazolium, CGS 9343B, felodipine, metofenazate, and Ro 22-4839. H 186/86, a felodipine-related dihydropyridine derivative, blocked A23187-but not ionomycin-triggered histamine release, suggesting a difference in the mode of action of these ionophores. In contrast, leukocyte histamine release triggered by the purported protein kinase C (PKC) activator, 1,2-isopropylidene-3-decanoyl-sn-glycerol (IpOCOC9), was enhanced by calmidazolium, CGS 9349B and metofenazate but not affected by felodipine or Ro 22-4839, whereas the response triggered by 4 beta-phorbol 12-myristate 13-acetate (PMA) was reduced by metofenazate and Ro 22-4839 but not consistently affected by calmidazolium, CGS 9343B or felodipine. The PMA-induced histamine release was enhanced by H 186/86. Anti-IgE- and FMLP-induced responses were either unaffected or slightly enhanced by the examined calmodulin antagonists. In comparison with the calmodulin antagonists, R 59022, an inhibitor of diacylglycerol kinase, failed to reduce calcium ionophore-triggered histamine release, whereas FK506, an inhibitor of peptidyl prolyl cis-trans isomerase (PPI), reduced both anti-IgE- and ionophore-triggered responses. These results indicate that calmodulin constitutes an obligate link in signal transduction pathways leading to human leukocyte histamine release if the trigger is a calcium ionophore but not when responses are induced by anti-IgE, FMLP or PMA; a calmodulin-dependent component may rather balance responses induced by IpOCOC9.(ABSTRACT TRUNCATED AT 250 WORDS)

Oscillatory muscarinic acetylcholine responses of Xenopus oocytes are desensitized by protein kinase C and sensitized by protein phosphatase 2B

The oscillatory current response to acetylcholine (ACh) in Xenopus laevis oocytes, mediated by endogenous muscarinic ACh receptors, is known to be mildly desensitized by repetitive ACh applications. Pretreatment of oocytes with staurosporine (an inhibitor of protein kinases) was found not only to abolish this desensitization but also to positively and progressively potentiate oscillatory ACh responses. This sensitization by staurosporine was suppressed by 12-O-tetradecanoylphorbol 13-acetate (an activator of protein kinase C). In staurosporine-untreated (control) oocytes, intracellularly injected calcineurin (an isozyme of Ca2+/calmodulin-dependent protein phosphatase 2B) or Ca2+ enhanced oscillatory ACh responses, while trifluoperazine (a calmodulin inhibitor) suppressed the ACh responses but did not affect oscillatory responses to intracellularly injected inositol 1,4,5-trisphosphate. These results suggest that, as far as short-term changes in receptor responsiveness are concerned, endogenous muscarinic ACh receptors in Xenopus oocytes are desensitized by phosphorylation by protein kinase C and sensitized by dephosphorylation by Ca2+/calmodulin-dependent protein phosphatase 2B.

Role of calcium in regulation of phosphoinositide signaling pathway

Using primary neuronal cultures we have examined the role of extracellular Ca2+ in a receptor-regulated phosphoinositide turnover. We report that receptor (glutamic acid and acetylcholine)-activated phosphoinositide turnover requires the presence of extracellular Ca2+ (EC50 = 21.1 microM). The requirement for Ca2+ appears to be at an intracellular level and is highly selective for Ca2+. We also found that several inorganic and organic Ca2+ channel blockers, including La3+ and verapamil, inhibit phosphoinositide turnover. However, the pharmacological profile of these agents in this regard was distinct from their actions at the voltage-sensitive Ca2+ channels. To explain the above requirement for extracellular Ca2+ in agonist-stimulated phosphoinositide turnover and its sensitivity to Ca(2+)-channel blockers, we propose a hypothetical model suggesting that Ca2+, following IP-3-mediated mobilization, exerts a facilitatory action on the activity of receptor-phospholipase C complex. We further propose that in the absence of extracellular Ca2+ or in the presence of certain Ca(2+)-channel blockers, refilling of calciosomes is ineffectual or inhibited, causing its depletion and subsequent inactivation of agonist-stimulated phosphoinositide turnover.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.

Calmodulin- and Ca2(+)-insensitive fatty acid methyltransferase from RINm5F cells. Inhibition by trifluoperazine and W7

1. Methylation of endogenous lipids by homogenates of rat insulinoma cells was studied. 2. 3H-methyl groups (38 pmol/mg protein per 10 min) from [3H-methyl]S-adenosyl-L-methionine were incorporated into endogenous lipids, mainly (greater than 80%) into the neutral lipid fraction. 3. The reaction was sensitive to heat, was almost abolished by S-adenosyl-L-homocysteine, but insensitive to the addition of EGTA (5 mM), Ca2+ (5-100 microM) and/or calmodulin (15 microns). 4. At concentrations relevant for calmodulin antagonistic activity strong inhibition by W7 and trifluoperazine (25-100 microM each), but not by CGS 9343B (10 microM), was observed. 5. Calmodulin antagonists of phenothiazine- and sulfonamide-type appear to block the fatty acid methyltransferase in a way unrelated to calmodulin.

PDGF-induced activation of phospholipase C is not required for induction of DNA synthesis

Platelet-derived growth factor (PDGF) induction of DNA synthesis is believed to involve activation of phospholipase C (PLC) and subsequent accumulation of inositol 1,4,5-triphosphate [I(1,4,5)P3], increase in intracellular Ca2+, activation of protein kinase C (PKC), and receptor down regulation. Generation of these events is triggered by the tyrosine protein kinase (TPK) activity of the PDGF receptor. The TPK inhibitor genistein blocked PDGF induction of these events, including DNA synthesis, with the exception of receptor down regulation. PDGF-induced phosphotyrosine phosphorylations, including receptor autophosphorylation, were inhibited by genistein. Removal of genistein and PDGF resulted in DNA synthesis without the occurrence of PLC activation. These findings indicate that these early events, with the exception of receptor down regulation, are not necessary for PDGF-induced DNA synthesis.

The effect of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) on muscarinic receptor-induced Ca2+ mobilization in a human salivary epithelial cell line

We have investigated the effect of W-7, a calmodulin (CaM) antagonist, on Ca2+ mobilization in a human salivary epithelial cell line, HSG-PA, after muscarinic receptor stimulation. In a medium containing 1.5 mmol/l Ca2+, W-7 reduced both the maximum peak increase in cytosolic Ca2+ [( Ca2+]i) which follows stimulation by carbachol (Cch, 100 mumol/l) and the sustained nature of the response. Using an experimental approach which allows separate visualization of the intracellular Ca2+ release and extracellular Ca2+ entry phases, W-7 was shown preferentially to inhibit Ca2+ release. At 100 mumol/l W-7, Cch-induced Ca2+ release was completely inhibited, but Cch-induced Ca2+ entry was partially (approximately 40%) maintained. This W-7 residual Ca2+ entry response was abolished when cells were depolarized with high K+ or gramicidin D. W-7 also substantially inhibited Cch-induced inositol trisphosphate (IP3) production (approximately 5%). W-5, a less potent CaM antagonist than W-7, had markedly smaller effects on Cch-induced Ca2+ mobilization and IP3 formation. W-7 (100 mumol/l) completely blocked (comparable to 10 mumol/l atropine) the binding of the muscarinic antagonist [3H] quinuclidinyl benzilate (QNB) to muscarinic receptors on cell membranes, whereas Cch (at 100 mumol/l) had minimal effects on ligand binding. W-7 and W-5 were equipotent in their ability to inhibit [3H] QNB binding. These results suggest that W-7 reduces Ca2+ mobilization in HSG-PA cells by a mechanism which likely involves the antagonism of a CaM regulatory step(s) but may also involve at least a partial blockade of the muscarinic receptor.

Inositol tetrakisphosphate-induced sequestration of Ca2+ replenishes an intracellular pool sensitive to inositol trisphosphate

In a permeable neoplastic rat liver epithelial (261B) cell system, inositol 1,3,4,5-tetrakisphosphate--Ins(1,3,4,5)P4--induces sequestration of Ca2+ released by inositol 2,4,5-trisphosphate--Ins(2,4,5)P3; a non-metabolized inositol trisphosphate (InsP3) isomer--and Ca2+ added exogenously in the form of CaCl2. Studies were performed to identify the Ca2+ pool filled after Ins(1,3,4,5)P4 treatment. Both Ins(2,4,5)P3 and inositol 1,4,5-trisphosphate--Ins(1,4,5)P3--dose-dependently release Ca2+ from permeable 261B cells--Ins(1,4,5)P3 having a threefold greater potency--but differ in that Ca2+ released by Ins(1,4,5)P3 is readily sequestered, while the Ca2+ released by Ins(2,4,5)P3 is not. Maximal release of Ca2+ by 6 microM Ins(2,4,5)P3 blocked the action of Ins(1,4,5)P3, demonstrating that these two isomers influence the same intracellular Ca2+ pool through a shared membrane receptor. Addition of 2 microM Ins(2,4,5)P3 to discharge partially the Ca2+ pool reduced the amount of Ca2+ released by a maximal dose of Ins(1,4,5)P3 (2 microM). Ins(1,3,4,5)P4 combined with Ins(2,4,5)P3 produced a Ca2+ release and sequestration response, which replenished the InsP3-sensitive pool as indicated by a recovery of full Ca2+ release by 2 microM Ins(1,4,5)P3. Induction of Ca2+ sequestration by Ins(1,3,4,5)P4 occurred dose-dependently, with a half-maximal response elicited at a dose of 0.9 microM. Further studies of the effect of Ins(1,3,4,5)P4 apart from the influence of Ins(2,4,5)P3 using a model in which the Ca2+ levels are raised by an exogenous addition of CaCl2 showed that Ins(1,4,5)P3 released twice the amount of Ca2+ from the storage pool following Ins(1,3,4,5)P4-induced Ca2+ sequestration. These results demonstrate that the Ca2+ uptake induced by Ins(1,3,4,5)P4 preferentially replenishes the intracellular Ca2+ storage sites regulated by Ins(1,4,5)P3 and Ins(2,4,5)P3.

Inositol polyphosphates and intracellular calcium release

The hydrolysis of inositol lipids triggered by the occupation of cell surface receptors generates several intracellular messengers. Many different inositol phosphate isomers accumulate in stimulated cells. Of these D-myo-inositol 1,4,5-trisphosphate (Ins 1,4,5-P3) is responsible for discharging Ca2+ from intracellular stores. Specific membrane binding sites for Ins 1,4,5-P3 have been detected. The properties of these sites and their possible relationship to the calcium release process is reviewed. Ins 1,4,5-P3 binding sites may be present in discrete subcellular structures ("calciosomes"). Kinetic and some electrophysiological evidence indicates that Ins 1,4,5-P3 acts to open a Ca2+ channel. Recent progress on the purification of the receptor from neuronal tissues is summarized. Phosphorylation of Ins 1,4,5-P3 by a specific kinase results in the production of D-myo-inositol 1,3,4,5-tetraphosphate (Ins 1,3,4,5-P4). This inositol phosphate has been reported to increase the entry of Ca2+ across the plasma membrane, activate nonspecific ion channels in the plasma membrane, alter the Ca2+ content of the Ins 1,4,5-P3-releasable store, and bind to and alter the activity of certain enzymes. These data and the possible biological significance of Ins 1,3,4,5-P4 are discussed.

Inositol 1,3,4,5-tetrakisphosphate stimulates the initiation of DNA synthesis in Ca2+-deprived rat liver cells

The G1-S boundary of non-neoplastic cells requires extracellular Ca2+ for successful transition. Inositol 1,3,4,5-tetrakisphosphate but not inositol 1,4,5-trisphosphate can partially replace Ca2+ and stimulate the initiation of DNA synthesis of Ca2+-deprived T51B rat liver cells but only if sufficient extracellular Ca2+ (i.e., 0.075 mM) is present. The potent tumor promoter and protein kinase C activator 12-O-tetradecanoylphorbol acetate is also capable of replacing extracellular Ca2+ and partially stimulating the initiation of DNA synthesis. In addition, both inositol-1,3,4,5-tetrakisphosphate and 12-O-tetradecanoylphorbol acetate added together elicit a full DNA synthetic response.

Inositol 1,3,4,5-tetrakisphosphate induces Ca2+ sequestration in rat liver cells

Inositol 1,4,5-trisphosphate [I(1,4,5)P3] is a second messenger generated along with diacylglycerol upon the binding of various physiological agents with their cell surface receptors. I(1,4,5)P3 mobilizes Ca2+ from intracellular storage sites through a receptor-coupled mechanism, and the subsequent increased intracellular free calcium ion concentration [( Ca2+]i) activates a multitude of cellular responses. Electropermeabilized neoplastic rat liver epithelial (261B) cells were used to study Ca2+ sequestration, a process that reverses the elevated [Ca2+]i to resting levels and replenishes intracellular Ca2+ pools. Although I (1,4,5)P3-mobilized Ca2+ is readily sequestered into storage pools by the action of Ca2+-adenosine triphosphatases, Ca2+ mobilized by addition of the nonmetabolized inositol trisphosphate isomer I(2,4,5)P3 is not sequestered, suggesting that metabolism is necessary to eliminate the stimulus for Ca2+ release. Several inositol phosphate compounds were examined for their ability to lower the buffer [Ca2+] to determine if a specific I(1,4,5)P3 metabolite might be involved in stimulating Ca2+ sequestration; of these, I(1,3,4,5)P4 alone was found to induce Ca2+ sequestration, demonstrating a physiological role for this inositol trisphosphate metabolite.