Mitogens increase phosphorylation of phosphoinositides in thymocytes

Distinct plasma-membrane PtdIns(4)P and PtdIns(4,5)P2 dynamics in secretagogue-stimulated beta-cells

Phosphoinositides regulate numerous processes in various subcellular compartments. Whereas many stimuli trigger changes in the plasma-membrane PtdIns(4,5)P(2) concentration, little is known about its precursor, PtdIns(4)P, in particular whether there are stimulus-induced alterations independent of those of PtdIns(4,5)P(2). We investigated plasma-membrane PtdIns(4)P and PtdIns(4,5)P(2) dynamics in insulin-secreting MIN6 cells using fluorescent translocation biosensors and total internal reflection microscopy. Loss of PtdIns(4,5)P(2) induced by phospholipase C (PLC)-activating receptor agonists or stimulatory glucose concentrations was paralleled by increased PtdIns(4)P levels. In addition, glucose-stimulated cells regularly showed anti-synchronous oscillations of the two lipids. Whereas glucose-induced PtdIns(4)P elevation required voltage-gated Ca(2+) entry and was mimicked by membrane-depolarizing stimuli, the receptor-induced response was Ca(2+) independent, but sensitive to protein kinase C (PKC) inhibition and mimicked by phorbol ester stimulation. We conclude that glucose and PLC-activating receptor stimuli trigger Ca(2+)- and PKC-dependent changes in the plasma-membrane PtdIns(4)P concentration that are independent of the effects on PtdIns(4,5)P(2). These findings indicate that enhanced formation of PtdIns(4)P, apart from ensuring efficient replenishment of the PtdIns(4,5)P(2) pool, might serve an independent signalling function by regulating the association of PtdIns(4)P-binding proteins with the plasma membrane.

Effects of PKI55 protein, an endogenous protein kinase C modulator, on specific PKC isoforms activity and on human T cells proliferation

PKI55 protein, coded by the recently identified KI55 gene [R. Selvatici, E. Melloni, M. Ferrati, C. Piubello, F.C. Marincola, E. Gandini, J. Mol. Evol. 57 (2003) 131-139] is synthesized following protein kinase C (PKC) activation and acts as a PKC modulator, establishing a feedback loop of inhibition. In this work, PKI55 was found to inhibit recombinant alpha, beta(1), beta(2), gamma, delta, zeta and eta PKC isoforms; the effect on conventional PKC was lost in the absence of calcium. Confocal immunofluorescence experiments showed that PKI55 can penetrate into peripheral blood mononuclear cells (PBMC), following a coordinated movement of calcium ions. The addition of PKI55 protein down-regulated the PKC enzyme activity in phytohaemagglutinin-activated PBMC, decreasing the activity of alpha, beta(1) and beta(2) PKC isoforms. Moreover, inhibition in PBMC proliferation was observed. Similar effects were detected in Jurkat T cells transfected with a plasmid containing the coding sequence of PKI55. The PKI55 protein functional role could be to control the pathological over-expression of specific PKC isoforms and to regulate proliferation.

Phosphatidylinositol 4,5-bisphosphate anchors cytosolic group IVA phospholipase A2 to perinuclear membranes and decreases its calcium requirement for translocation in live cells

The eicosanoids are centrally involved in the onset and resolution of inflammatory processes. A key enzyme in eicosanoid biosynthesis during inflammation is group IVA phospholipase A2 (also known as cytosolic phospholipase A2alpha, cPLA2alpha). This enzyme is responsible for generating free arachidonic acid from membrane phospholipids. cPLA2alpha translocates to perinuclear membranes shortly after cell activation, in a process that is governed by the increased availability of intracellular Ca2+. However, cPLA2alpha also catalyzes membrane phospholipid hydrolysis in response to agonists that do not mobilize intracellular Ca2+. How cPLA2alpha interacts with membranes under these conditions is a major, still unresolved issue. Here, we report that phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] promotes translocation of cPLA2alpha to perinuclear membranes of intact cells in a manner that is independent of rises in the intracellular Ca2+ concentration. PtdIns(4,5)P2 anchors the enzyme to perinuclear membranes and allows for a proper interaction with its phospholipid substrate to release arachidonic acid.

Localization and functional interrelationships among cytosolic Group IV, secreted Group V, and Ca2+-independent Group VI phospholipase A2s in P388D1 macrophages using GFP/RFP constructs

P388D(1) cells exposed to bacterial lipopolysaccharide (LPS) mobilize arachidonic acid (AA) for prostaglandin synthesis in two temporally distinct pathways. The "immediate pathway" is triggered within minutes by receptor agonists such as platelet-activating factor (PAF) but only if the cells have previously been primed with LPS for 1 h. The "delayed pathway" occurs in response to LPS alone over the course of several hours. We have now investigated the subcellular localization of both the Group IV cytosolic phospholipase A(2) (cPLA(2)) and the Group V secreted PLA(2) (sPLA(2)) during these two temporally distinct routes of AA release. We have prepared cells overexpressing fusion proteins of sPLA(2)-GFP and cPLA(2)-RFP. In the resting cells, cPLA(2)-RFP was uniformly located throughout the cytoplasm, and short-term treatment with LPS did not induce translocation to perinuclear and/or Golgi membranes. However, such a translocation occurred almost immediately after the addition of PAF to the cells. Long-term exposure of the cells to LPS led to the translocation of cPLA(2)-RFP to intracellular membranes after 3 h, and correlates with a significant release of AA in a cPLA(2)-dependent manner. At the same time period that the delayed association of cPLA(2) with perinuclear membranes is detected, an intense fluorescence arising from the sPLA(2)-GFP was found around the nucleus in the sPLA(2)-GFP stably transfected cells. In parallel with these changes, significant AA release was detected from the sPLA(2)-GFP transfectants in a cPLA(2)-dependent manner, which may reflect cross-talk between sPLA(2) and cPLA(2). The subcellular localization of the Group VIA Ca(2+)-independent PLA(2) (iPLA(2)) was also investigated. Cells overexpressing iPLA(2)-GFP showed no fluorescence changes under any activation condition. However, the iPLA(2)-GFP-expressing cells showed relatively high basal AA release, confirming a role for iPLA(2) in basal deacylation reactions. These new data illustrate the subcellular localization changes that accompany the distinct roles that each of the three kinds of PLA(2) present in P388D(1) macrophages play in AA mobilization.

Interaction of the CC-chemokine RANTES with glycosaminoglycans activates a p44/p42 mitogen-activated protein kinase-dependent signaling pathway and enhances human immunodeficiency virus type 1 infectivity

The interaction of the CC-chemokine RANTES with its cell surface receptors transduces multiple intracellular signals: low concentrations of RANTES (1 to 10 nM) stimulate G-protein-coupled receptor (GPCR) activity, and higher concentrations (1 microM) activate a phosphotyrosine kinase (PTK)-dependent pathway. Here, we show that the higher RANTES concentrations induce rapid tyrosine phosphorylation of multiple proteins. Several src-family kinases (Fyn, Hck, Src) are activated, as is the focal adhesion kinase p125 FAK and, eventually, members of the p44/p42 mitogen-activated protein kinase (MAPK) family. This PTK signaling pathway can be activated independently of known seven-transmembrane GPCRs for RANTES because it occurs in cells that lack any such RANTES receptors. Instead, activation of the PTK signaling pathway is dependent on the expression of glycosaminoglycans (GAGs) on the cell surface, in that it could not be activated by RANTES in GAG-deficient cells. We have previously demonstrated that RANTES can both enhance and inhibit infection of cells with human immunodeficiency virus type 1 (HIV-1). Here we show that activation of both PTK and MAPK is involved in the enhancement of HIV-1 infectivity caused by RANTES in cells that lack GPCRs for RANTES but which express GAGs.

Cellular regulation of cytosolic group IV phospholipase A2 by phosphatidylinositol bisphosphate levels

Cytosolic group IV phospholipase A2 (cPLA2) is a ubiquitously expressed enzyme with key roles in intracellular signaling. The current paradigm for activation of cPLA2 by stimuli proposes that both an increase in intracellular calcium and mitogen-activated protein kinase-mediated phosphorylation occur together to fully activate the enzyme. Calcium is currently thought to be needed for translocation of the cPLA2 to the membrane via a C2 domain, whereas the role of cPLA2 phosphorylation is less clearly defined. Herein, we report that brief exposure of P388D1 macrophages to UV radiation results in a rapid, cPLA2-mediated arachidonic acid mobilization, without increases in intracellular calcium. Thus, increased Ca2+ availability is a dispensable signal for cPLA2 activation, which suggests the existence of alternative mechanisms for the enzyme to efficiently interact with membranes. Our previous in vitro data suggested the importance of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) in the association of cPLA2 to model membranes and hence in the regulation of cPLA2 activity. Experiments described herein show that PtdInsP2 also serves a similar role in vivo. Moreover, inhibition of PtdInsP2 formation during activation conditions leads to inhibition of the cPLA2-mediated arachidonic acid mobilization. These results suggest that cellular PtdInsP2 levels are involved in the regulation of group IV cPLA2 activation.

Effects of polyamines and calcium and sodium ions on smooth muscle cytoskeleton-associated phosphatidylinositol (4)-phosphate 5-kinase

In many different cell types, including smooth muscle cells (Baron et al., 1989, Am. J. Physiol., 256: C375-383; Baron et al., J. Pharmacol. Exp. Ther. 266: 8-15), phosphatidylinositol (4)-phosphate 5-kinase plays a critical role in the regulation of membrane concentrations of phosphatidylinositol (4,5)-bisphosphate and formation of inositol (1,4,5)-trisphosphate. In unstimulated porcine trachealis smooth muscle, 70% of total cellular phosphatidylinositol (4)-phosphate 5-kinase activity was associated with cytoskeletal proteins and only trace activity was detectable in isolated sarcolemma. Using two different preparations, we studied cytoskeleton-associated phosphatidyl inositol (4)-phosphate 5-kinase under conditions that attempted to mimic the ionic and thermal cytoplasmic environment of living cells. The cytoskeleton-associated enzyme, studied using phosphatidylinositol (4)-phosphate substrate concentrations that produced phosphatidylinositol 4,5-bisphosphate at about 10% of the maximal rate, was sensitive to free [Mg2+], had an absolute requirement for phosphatidylserine, phosphatidic acid, or phosphatidylinositol, and included type I isoforms. At 0.5 mM free [Mg2+], physiological spermine concentrations, 0.2-0.4 mM, increased phosphatidylinositol (4)-phosphate 5-kinase activity two to four times compared to controls run without spermine. The EC50 for spermine-evoked increases in activity was 0.17 +/- 0.02 mM. Spermine-evoked enzyme activity was a function of both free [Mg2+] and substrate concentration. Cytoskeleton-associated phosphatidylinositol (4)-phosphate 5-kinase was inhibited by free [Ca2+] over a physiological range for cytoplasm--10(-8) to 10(-5) M, an effect independent of the presence of calmodulin. Na+ over the range 20 to 50 mM also inhibited this enzyme activated by 5 mM Mg2+ but had no effect on spermine-activated enzyme. Na+, Ca2+, and spermine appear to be physiological modulators of smooth muscle cytoskeleton-bound phosphatidylinositol (4)-phosphate 5-kinase.

Concanavalin A modulates tyrosine phosphorylation and activation of a type II phosphatidylinositol 4-kinase in rat splenic lymphocytes

Activation of rat splenic lymphocytes by concanavalin A resulted in two-fold increase in Ptdlns 4-kinase activity and rapid tyrosine phosphorylation of the enzyme. The activation kinetics showed a strong correlation with tyrosine phosphorylation state of the enzyme. Characterization of the enzyme activity suggests that it is a type II PtdIns 4-kinase. Kinetic analysis of the enzyme reaction showed three-fold decrease in Km for PtdIns and two-fold increase in Vmax in Con A stimulated cells. These results suggest that a type II PtdIns 4-kinase is an integral component of the early signal transduction machinery during T-cell activation by concanavalin A and is actively regulated by protein tyrosine phosphorylation-dephosphorylation.

Rapid regulation of PDE-2 and PDE-4 cyclic AMP phosphodiesterase activity following ligation of the T cell antigen receptor on thymocytes: analysis using the selective inhibitors erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA) and rolipram

The PDE2, cyclic GMP-stimulated, and the PDE4, cyclic AMP-specific enzymes provide the major, detectable cyclic AMP phosphodiesterase activities in murine thymocytes. In the absence of the cyclic GMP, PDE4 activity predominated (approximately 80% total) but in the presence of low (10 microM) cyclic GMP concentrations, PDE2 activity constituted the major PDE activity in thymocytes (approximately 80% total). The PDE4 selective inhibitor rolipram dose-dependently inhibited thymocyte PDE4 activity (IC50 approximately 65 nM). PDE2 was dose-dependently activated (EC50 approximately 1 microM) by cyclic GMP and inhibited by erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA) (IC50 approximately 4 microM). EHNA was shown to serve as a selective inhibitor of PDE-2 activity as assessed from studies using separated PDE1, PDE2, PDE3 and PDE4 species from hepatocytes as well as human PDE2 and PDE4 enzymes. EHNA completely ablated the ability of cyclic GMP to activate PDE2 activity, whilst having a much smaller inhibitory effect on the unstimulated PDE2 activity. EHNA exhibited normal Michaelian kinetics of inhibition for the cyclic GMP-stimulated PDE2 activity with Hill plots near unity. Apparent negative co-operative effect were seen in the absence of cyclic GMP with Hill coefficients of approximately 0.3 for inhibition of PDE2 activity. Within 5 min of challenge of thymocytes with the lectin phytohaemagglutinin (PHA) there was a transient decrease (approximately 83%) in PDE-4 activity and in PDE2 activity (approximately 40%). Both anti-TCR antibodies also caused an initial reduction in the PDE4 activity which was followed by a sustained and profound increase in activity. In contrast to that observed with PHA, anti-TCR/CD3 antisera had little effect on PDE2 activity. It is suggested that, dependent upon the intracellular concentrations of cyclic GMP, thymocyte cyclic AMP metabolism can be expected to switch from being under the predominant control of PDE4 activity to that determined predominantly by PDE2 activity. These activities may be rapidly and differentially regulated following ligation of different cell surface receptors.

The cloning and sequence of the C isoform of PtdIns4P 5-kinase

In this study we describe the purification and sequencing of the C isoform of platelet PtdIns4P 5-kinase. Subsequently a cDNA was isolated from a human circulating-leucocyte library, which when sequenced was shown to contain all of the peptides identified in the purified protein. In addition, expression of this cDNA in bacteria led to the production of a protein which was recognized by specific monoclonal antibodies raised to the bovine brain enzyme [Brooksbank, Hutchings, Butcher, Irvine and Divecha (1993) Biochem. J. 291, 77-82] and also led to the appearance of PtdIns4P 5-kinase activity in the bacterial lysates. Interestingly, the cDNA showed no similarity to any of the previously cloned inositide kinases. A search of the DNA databases showed that two proteins from Saccharomyces cerevisiae shared close similarity to this enzyme, one of which, the mss4 gene product, has been implicated in the yeast inositol lipid pathway. These data suggest that the PtdIns4P 5-kinases are a new family of inositide kinases unrelated to the previously cloned phosphoinositide 3/4-kinases.

Inhibition of angiotensin II-stimulated inositol phosphate production by D2-dopamine receptor is calcium-dependent in human trophoblastic cells

We previously reported that dopamine (DA) acted via D2-dopamine receptors in human trophoblastic cells to inhibit All-stimulated inositol phosphate (InsP) accumulation. However, the mechanism by which DA inhibited All-stimulated InsP accumulation is still unknown except that this inhibitory effect was sensitive to Pertussis toxin (PTX). In this study, we characterize this DA-mediated inhibition of All-stimulated InsP production in human placenta. Freshly isolated human term placental cells were prelabeled with myo-[2-3H]inositol and incubated with various stimuli in the presence of 10 mM LiCl. All (10(-6) M) stimulated 1.5 times the human trophoblastic cell InsP production whereas DA (10(-4) M) inhibited this All-stimulated InsP production by 54 +/- 7%. This inhibitory effect was mimicked by bromocriptine (53 +/- 3% of inhibition), a D2-dopamine agonist. We recently reported that bromocriptine inhibited human placental adenosine 3',5'-cyclic monophosphate (cAMP) production. Increasing the intracellular concentration of cAMP levels by adding forskolin did not modify the effect of DA and bromocriptine on InsP accumulation. On the other hand, the effect of DA and bromocriptine on All-stimulated InsP production were greatly affected by treatments that modify the cytosolic free Ca2+ concentration. Specifically, the D2-dopaminergic mediated inhibition was prevented by treatment of cells with the Ca2+ ionophore ionomycin (10(-5) M) and was mimicked either by removal of Ca2+ from incubation medium (53 +/- 6%) or by blockage of voltage-gated Ca2+ channels with nifedipine (51 +/- 7%). Our data indicate that the inhibitory effect of D2-dopamine agonists on All-stimulated InsP production is an indirect event probably due to the DA-inhibition of calcium influx.

Relationship of growth stimulated by lithium, estradiol, and EGF to phospholipase C activity in MCF-7 human breast cancer cells

Lithium-stimulated MCF-7 cell proliferation was compared to proliferation stimulated by other mitogens for this cell line-estradiol (E2) and epidermal growth factor (EGF)-and lithium was found to be effective within a narrow concentration range. Mitogenic effects of lithium on proliferation stimulated by E2 and EGF were additive below maximum, but were not synergistic. The phosphoinositide pathway is a cell signaling system involved in cell proliferation, within which phospholipase C (PLC)-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] leads to the production of the second messengers inositol-1,4,5-trisphosphate [Ins(1,4,5)P3] and diacylglycerol (DAG), as well as to calcium mobilization. At mitogen concentrations which maximally stimulated cell growth, estradiol stimulated both growth and PLC activity, while EGF and lithium stimulated cell growth but had little effect on the activity of the enzyme. Dose-responses with EGF revealed that a low concentration (0.1 ng/ml, 0.017 nM) of EGF appeared to stimulate both PLC activity and cell growth, but that higher concentrations of EGF which stimulated greater proliferation inhibited PLC activity. Steady-state levels of inositol phosphates including inositol trisphosphate were increased by all three mitogens. In growth assays, the phorbol ester phorbol 12-myristate-13-acetate (PMA), which mimics the actions of DAG, stimulated some cell growth, but dioctanoylglycerol, an additional DAG analog, and the calcium ionophore A23187, alone or with the DAG analogs, had no effect. These results suggest that PLC-mediated PtdIns(4,5)P2 hydrolysis is not primarily associated with signaling proliferation by lithium or EGF in MCF-7 breast cancer cells.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

Influence of hexachlorocyclohexane on phosphoinositides in rat erythrocyte membranes and brain

Single exposure of rats to hexachlorocyclohexane (100 mg/kg) did not cause any significant change in phosphoinositide levels in rat erythrocyte membrane and cerebrum (fore brain) 2 or 24 h after exposure. However, the phosphoinositide turnover and generation of second messengers from phosphoinositides were increased in the treated erythrocyte membranes as judged from a marked increase in the incorporation of [2-3H]inositol into phosphoinositides 24 h after the treatment. A significant decrease in phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) was observed in the erythrocyte membrane and cerebrum of rats repeatedly exposed to the pesticide for 3 or 6 months. This drastic reduction in phosphoinositide levels suggests adverse effects on vital membrane and cell functions modulated by phosphoinositides.

Stimulation of human lymphocytes by cathepsin G

We investigated the effect of cathepsin G, a serine protease in polymorphonuclear granulocytes, on the function of human lymphocytes. Cathepsin G increased the [3H]thymidine incorporation into human lymphocytes. This mitogenic activity was due to the proteolytic activity of cathepsin G. Both B and T cells showed increased [3H]thymidine incorporation, and this effect was more remarkable for T cells than for B cells. Among the T cell subsets, CD4+ T cells showed the increase in DNA synthesis, but CD8+ T cells did not. When human lymphocytes were stimulated with cathepsin G, intracellular free Ca2+ concentration ([Ca2+]i) increased in B and T cells, including CD4+ T cells and CD8+ T cells. The change in intracellular Ca2+ was due to Ca2+ influx and release of intracellular stores. Cathepsin G also induced the production of inositol 1,4,5-trisphosphate (IP3) in B cells, CD4+ T cells, and CD8+ T cells, leading to the release of Ca2+ from intracellular stores. Moreover, the stimulation with cathepsin G resulted in alkalization of the cytosol of B cells, CD4+ T cells, and CD8+ T cells as the result of Na+/H+ antiport activation. The change in intracellular Ca2+, production of IP3, and cytoplasmic alkalization in lymphocytes were due to its proteolytic activity. Cathepsin G released from granulocytes is considered to act on human lymphocytes in vivo and lead to the increase in DNA synthesis of B cells and CD4+ T cells through IP3 production, an increase in [Ca2+]i, and alkalization. However, these second messengers do not lead to the increase in DNA synthesis of CD8+ T cells.

Murine retrovirus-induced depletion of T cells is mediated through activation-induced death by apoptosis

ts1, a mutant of Moloney murine leukemia virus, causes neurologic disorders and acute immunodeficiency associated with the destruction of thymocytes and helper T cells. In this study, we examined whether apoptosis was involved in ts1-induced killings of T cells. Neonatal mice were inoculated with ts1, and 20 to 23 days postinoculation, when cytopathic effects on T cells normally appear, thymocytes and splenic lymphocytes were isolated and examined. Our results showed that several features of apoptosis were present in ts1-infected thymocytes and splenic lymphocytes. Apoptotic fragmented DNA, condensation of the chromatin, and enhanced cell death after stimulation with mitogens which was preventable with protein synthesis inhibitors, all of which are common features of apoptotic cell death, were observed in ts1-infected cells. Several other viruses, including human immunodeficiency virus, have been shown to cause apoptotic death of T cells. Here we show for the first time that a murine retrovirus which also induces immunodeficiency can cause apoptotic T-cell death. Future studies with this murine retrovirus may provide important results to help us better understand the mechanisms of retrovirus-induced apoptosis of T cells.

Metabotropic ATP receptor in hippocampal and thalamic neurones: pharmacology and modulation of Ca2+ mobilizing mechanisms

Changes in cytoplasmic free Ca2+ concentration, [Ca]i, elicited by ATP, were studied in neurones cultured from rat hippocampus and thalamus. ATP evoked [Ca]i increases in about 30% of all cells tested and suppressed [Ca]i transients in responsive cells. The number of responses to ATP markedly increased after pretreatment of cells with inhibitors of protein kinase C, H-7 or staurosporine. The potentiation was blocked by a phorbol ester and by dioleylglycerol. In pretreated cells both once peak [Ca]i and the number of successive trials were augmented by an [ATP] increase. The former effect can be described by the Michaelis-Menten equation whereas the latter one has a steeper, leftward-shifted dependence. Both concentration dependences are explained with a model, describing Ca2+ release as a threshold phenomena. ATP analogues had the rank of potency: ATP approximately ADP >> AMP > alpha, beta-MeATP. A single ATP application depleted internal Ca2+ stores which could be replenished by brief membrane depolarization with high-K+. ATP- and caffeine-induced [Ca]i transients were independent, indicating two non-overlapping Ca2+ storage sites. Only caffeine effects were potentiated at an elevated [Ca]i level, showing a Ca(2+)-induced Ca2+ release. Inhibitors of the Ca2+ pump in internal stores, ryanodine and sulphydryl reagents suppressed the ATP-induced [Ca]i transients, acting via different mechanisms.

Isolation and characterization of membrane potential changes associated with release of calcium from intracellular stores in rat thymic lymphocytes

Membrane potential changes accompanying Ca2+ influx stimulated by release of Ca2+ from intracellular stores (store-regulated Ca2+ uptake) were monitored in BAPTA-loaded rat thymic lymphocytes using the fluorescent indicator bis(1,3-diethylthiobarbituric acid)trimethine oxonol. Depletion of [Ca2+]i stores by the application of thapsigargin, ionomycin or cyclopiazonic acid induced a depolarization which was (i) dependent upon BAPTA-loading, (ii) dependent upon extracellular Ca2+, (iii) independent of extracellular Na+ and (iv) abolished by 5 mM extracellular Ni2+. This depolarization was followed by a charybdotoxin-sensitive repolarization and subsequent hyperpolarization to values approximating the K+ equilibrium potential, consistent with secondary activation of a K+ conductance. These membrane potential changes temporally correlated with Ca2+ influx from the extracellular medium as measured fluorimetrically with indo-1. The divalent cation permeability sequence was investigated by monitoring the magnitude of the depolarization observed following the addition of 4 mM Ca2+, Mn2+, Ba2+ or Sr2+ to cells pretreated with doses of thapsigargin or ionomycin known to activate the store-regulated calcium uptake pathway. On the basis of these experiments, we conclude that the store-regulated Ca2+ uptake pathway has the following permeability sequence: Ca2+ > Mn2+ >> Ba2+, Sr2+ with Mn2+ displaying significant permeability relative to Ca2+. This pathway is distinguishable from other divalent cation uptake pathways reported in other cells types on the basis of its activation by thapsigargin and its high Mn2+ permeability.

Activation of phosphatidylinositol 4,5-bisphosphate supply by agonists and non-hydrolysable GTP analogues

PtdIns(4,5)P2 serves as a precursor of a diverse family of signalling molecules, including diacylglycerol (and hence phosphatidic acid), Ins(1,4,5)P3 [and hence Ins(1,3,4,5)P4] and PtdIns(3,4,5)P3. The production of these messengers can be activated by agonists, and therefore the rate of utilization of PtdIns(4,5)P2 can vary dramatically. Although cells can only meet these large changes in demand for PtdIns(4,5)P2 by increasing its synthesis and/or by continuously cycling it at a rate that exceeds its potential consumption (avoiding the need for a co-ordinated activation mechanism), no satisfactory explanation for how this is achieved in agonist-stimulated cells has yet been provided. We show here that, in streptolysin-O-permeabilized neutrophils, N-formylmethionyl-leucyl-phenylalanine (FMLP), platelet-activating factor (PAF) and non-hydrolysable GTP analogues can cause large activations of PtdIns4P 5-kinase, suggesting that cells can accommodate agonist-activated rates of consumption of PtdIns(4,5)P2 without having to sustain continuous, comparably rapid and energetically expensive 'futile cycling' reactions.

Effects of fluorinated inositols on the proliferation of Swiss 3T3 fibroblasts

The six monodeoxyfluoro-myo-inositols (nFIns) have previously been synthesized as potential inhibitors of signalling pathways mediated by phosphoinositides and their derivatives. Each of the six nFIns isomers was introduced into Swiss 3T3 fibroblasts by the techniques of microinjection or scrape loading at intracellular concentrations of approx. 2-4 mM. Of the six nFIns analogues, only 3FIns and 5FIns inhibited the serum-stimulated proliferation of 3T3 fibroblasts assayed by cell counting. Proliferation was inhibited to a similar extent by 3FIns or 5FIns, irrespective of which technique was used to introduce the nFIns analogues into the cells. Proliferation of cells 35 h after serum stimulation (i.e. when the first cell cycle was completed in control cells) was inhibited by approx. 50% by both 3FIns and 5FIns, and entry into S phase in the first cell cycle was inhibited to the same extent. This indicated that the nFIns analogues were inhibiting proliferation in the G1 phase of the cell cycle. Proliferation during the second cell cycle (35-60 h after stimulation) was inhibited by 75-85%. The inhibitory nFIns analogues were not toxic to the cells, nor did they affect the cellular ATP/ADP ratio. The effectiveness of the nFIns analogues in inhibiting proliferation was directly correlated with their ability to be incorporated into phosphatidylinositol analogues, suggesting that they may act by modulating phosphoinositide signalling pathways or other functions essential for DNA synthesis.

Adhesion to fibronectin stimulates inositol lipid synthesis and enhances PDGF-induced inositol lipid breakdown

The aim of these experiments was to investigate whether inositol lipids might mediate some of the effects of extracellular matrix (ECM) on cellular form and functions. The lipid phosphatidylinositol bisphosphate (PIP2) plays a role in cytoskeletal regulation while its hydrolysis products, diacylglycerol and inositol triphosphate, serve as second messengers. We therefore measured the effect of adhesion to fibronectin (FN) on PIP2 and its hydrolysis products, in the presence and absence of the soluble mitogen PDGF. PDGF induced a threefold increase in release of water-soluble inositol phosphates in C3H 10T1/2 fibroblasts when cells were attached to FN, but had little effect in suspended cells. Suppression of inositol phosphate release in unattached cells was not due to dysfunction of the PDGF receptor or failure to activate phospholipase C-gamma; PDGF induced similar tyrosine phosphorylation of PLC-gamma under both conditions. By contrast, the total mass of phosphatidylinositol bisphosphate (PIP2), the substrate for PLC-gamma, was found to decrease by approximately 80% when cells were detached from their ECM attachments and placed in suspension in the absence of PDGF. PIP2 levels were restored when suspended cells were replated on FN, demonstrating that the effect was reversible. Furthermore, a dramatic increase in synthesis of PIP2 could be measured in cells within 2 min after reattachment to FN in the absence of PDGF. These results show that FN acts directly to stimulate PIP2 synthesis, and that it also enhances PIP2 hydrolysis in response to PDGF. The increase in PIP2 induced by adhesion may mediate some of the known effects of FN on cell shape and cytoskeletal organization, while regulation of inositol lipid hydrolysis may provide a means for integrating hormone- and ECM-dependent signaling pathways.

The uptake of 3H-labelled monodeoxyfluoro-myo-inositols into thymocytes and their incorporation into phospholipid in permeabilized cells

Monodeoxyfluoro-myo-inositols were applied to electropermeabilized and intact thymocyte preparations to study their metabolism and uptake in order to investigate their suitability as potential inhibitors of phosphoinositide-mediated cellular responses. Only three of the monodeoxyfluoro-myo-inositols were incorporated into the phospholipids of thymocytes: 1D-3-deoxy-3-fluoro-myo-inositol, 5-deoxy-5-fluoro-myo-inositol and 1D-6-deoxy-6-fluoro-myo-inositol, all of which were weaker substrates for phosphatidylinositol synthase than was myo-inositol. The 3-, 5- and 6-fluoro analogues also behaved as competitive inhibitors, with K1 values of 350 +/- 5 microM, 350 +/- 5 microM and 2.9 +/- 2 mM respectively, compared with a Km for myo-inositol of 31 +/- 4 microM. When incubated with electropermeabilized thymocyte preparations, these three analogues of myo-inositol all formed phospholipids with chromatographic properties which corresponded to those of substituted phosphatidylinositol and phosphatidylinositol monophosphate. The uptake of myo-inositol and of the monodeoxyfluoro-myo-inositols into intact thymocytes was studied by a dual-label technique. All the monodeoxyfluoro-myo-inositols were taken up to some extent, but only 2-deoxy-2-fluoro-myo-inositol and 1D-3-deoxy-3-fluoro-myo-inositol were actively concentrated. The monodeoxyfluoro-myo-inositols were also assayed for their ability to inhibit the uptake of myo-inositol into cells. Both 2-deoxy-2-fluoro-myo-inositol and 1D-3-deoxy-3-fluoro-myo-inositol were effective inhibitors of myo-inositol uptake. Furthermore, 1D-1-deoxy-1-fluoro-myo-inositol, which was not taken up actively, was an effective inhibitor of myo-inositol uptake. The three effective inhibitors all showed Ki values of approximately 150 microM, close to the apparent Km for inositol uptake of 180 microM, and the 4-, 5- and 6-fluoro analogues had Ki values in excess of 10 mM.

Inhibition of Ca2+ transport pathways in thymic lymphocytes by econazole, miconazole, and SKF 96365

Cytochrome P-450 has been proposed to underlie the mechanism of regulation of the plasma membrane Ca2+ permeability by the Ca2+ content of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool. We have investigated the effects on divalent cation uptake in rat thymic lymphocytes of three structurally related imidazole reagents reported to inhibit redox mechanisms. Changes in intracellular Ca2+ concentration and intracellular Mn2+ concentration were measured fluorimetrically with indo-1 and/or quin-2. Econazole, miconazole, and SKF 96365 were found to be potent blockers of Ca2+ and Mn2+ uptake activated by release of Ca2+ from intracellular stores induced by thapsigargin. Additionally, we found that concentrations of these agents required to abolish divalent cation uptake also released Ca2+ from the thapsigargin-sensitive intracellular stores, consistent with inhibition of the endosomal Ca(2+)-ATPase. In agreement with this suggestion, we have found that all three of these agents are potent inhibitors of isolated sarcoplasmic reticulum Ca(2+)-ATPase. We conclude that econazole, miconazole, and SKF 96365 inhibit cytochrome P-450-independent filling of intracellular Ca2+ pools, as well as store-regulated Ca2+ entry, and caution against the use of these compounds as selective inhibitors of cytochrome P-450.

Cyclic AMP is not a direct regulator of calcium flux and hydrolysis of phosphoinositides in human lymphocytes

The regulatory effects of adenosine 3',5'-cyclic monophosphate (cAMP) on Ca2+ flux and phosphatidylinositol (PI) turnover in human lymphocytes were studied. cAMP did not affect the intracellular accumulation of Ca2+ induced by phytohemagglutinin (PHA) and histamine-trifluoromethyl toluidide derivative (HTMT) in peripheral blood lymphocytes (PBL). In addition, cAMP also did not alter Ca2+ flux induced by PHA, anti-CD3, or PAF in T cells, or by anti-IgM and HTMT in non-rosetted cells. Similarly, cAMP did not inhibit IP accumulation induced by HTMT in PBL, anti-CD3 in T cells, and by anti-IgM or HTMT in non-rosetted cells. The only exception was the synthesis of IP induced by PHA in T cells that was inhibited by cAMP. Furthermore, prolonged treatment of T cells with cholera toxin inhibited Ca2+ accumulation in response to CD3. The degree of inhibition of Ca2+ and IP responses was not proportional to the levels of intracellular cAMP generated.

Activation of calcium (Ca)-dependent protein kinase C in aged mesenteric lymph node T and B cells

We studied the effects of aging on the activities and translocation of Ca-dependent protein kinase C (PKC) in resting mesenteric lymph node (MLN) T and B cells during the activation process induced by T and B cell mitogens and B cell-stimulatory interleukins, including IL-4, IL-5 and IL-6. The activation process in senescent, resting (high density (HD)) MLN T cells is impaired, when these cells are stimulated with T cell mitogen, Con A. The defect in activation is associated with a reduction in both the new production of inositol-1,4,5-triphosphate (IP3) (an indicator for the production of intracellular free Ca) and the induction of Ca-dependent PKC. In contrast, the activation of the aged B cells with LPS plus/minus interleukins (IL-4, IL-5 and IL-6) is not impaired, being at least associated with a Ca-independent pathway of PKC activation. The elevated IP3 content and total (cytosol plus membrane) PKC activity in both resting T and B cells from aged MLN along with the greater difference in T cells than in B cells suggest that the in vivo Go cell cycle status of these cells may differ from that of the young, involving more in T cells. Finally, the MLN and splenic T cell Ca-dependent and B cell Ca-independent PKC activation do not differ between both age groups.

Age-related impairment of early and late events of signal transduction in mouse immune cells

The effects of aging were studied on both early and late [subsequent to activation of protein kinase C (PKC)] events of signal transduction in mouse T cell subsets. Aged C57Bl/6 mice showed significant suppression of Con A-stimulated Ca2+ mobilization in both CD4+ and CD8+T cells compared to young mice. When early signaling was bypassed by stimulating cells with the combination of a calcium ionophore and a phorbol ester. [3H]thymidine incorporation was comparable between young and aged mice while Con A-stimulated [3H]thymidine uptake remained depressed in aged mice. These results suggest that impairment of early (but not late) events may be responsible for the suppression of T cell proliferation in aged animals. Further, aged mice exhibited a significant increase in spontaneous proliferation resulting in a robust reduction of the stimulation index of both Con A and PMA/A23187-induced proliferation. Similarly, aged mice exhibited a significant increase in spontaneous IL-2R expression in both CD4+ and CD8+ T cells. No differences in IL-2 secretion were found between young and aged mice. The change in IL-2R expression was positively correlated with increased spontaneous proliferation. Overall these results suggest that: 1) impaired early (pre-PKC) rather than later (post-PKC) events of signaling may be responsible for poor proliferation of T cells, and 2) endogenous activation of cells related to increased level of IL-2R expression may play an important role in aging-associated immunosuppression.

Augmentation of phorbol ester-induced T cell proliferation by agents which raise intracellular cyclic adenosine monophosphate

Although raising intracellular cyclic adenosine monophosphate (cAMP) levels is generally considered to be inhibitory on the mitogen-induced T cell proliferation, in this study we have shown that the addition of either dbcAMP (50 microM) or cholera toxin (1 ng/ml) resulted in an increase in [3H]thymidine uptake in PBMC cultures stimulated with phorbol ester, 12-tetradecanoylphorbol 13-acetate (TPA), or with a combination of TPA plus anti-CD3 mAb (mAb 235). In contrast, under similar culture conditions, the phytohemagglutinin-P (PHA-P) response was inhibited by these agents as has been reported. The augmentative effect of dbcAMP in PBMC cultures was due to an increase in IL-2 production and not to increased in IL-2R-alpha chain expression. The enhancing effect of dbcAMP and CT observed with PBMC was monocyte dependent and not seen with purified T cell preparations. The addition of monocytes reconstituted the ability of intracellular cAMP elevating agents to augment the T cell response to TPA with and without mAb to CD3. The monocytes mediate their action via soluble factor(s) with molecular weight (m.w.) of more than 10 kDa. Neither rIL-1, rIL-6, nor rTNF-alpha have any augmentative effect as contrast with the supernatant from treated monocytes. Taken together, our results indicate that cAMP can play a positive regulatory role in T cell proliferation due to factor(s) secreted by dbcAMP-treated monocytes resulting in increased IL-2 synthesis in T cells.

Purification and characterization of phosphatidylinositol 4-phosphate 5-kinases

We detail the purification and characterization of three distinct isoforms of PtdIns4P 5-kinase present in bovine brain. One of these, PtdIns4P 5-kinase C, was purified to apparent homogeneity, and SDS/PAGE analysis demonstrated a single polypeptide and molecular mass 53 KDa. These three isoforms were shown to differ in their kinetic properties, and immunological characterization with an antibody raised to PtdIns4P 5-kinase C demonstrated that this isoform was unrelated to the other two. Furthermore, PtdIns4P 5-kinase C was shown to be the bovine brain homologue of the Type II PtdIns4P 5-kinase previously purified from human erythrocytes [Bazenet, Ruano, Brockman & Anderson (1990) J. Biol. Chem. 265, 18012-18022].

Multiple effects of staurosporine, a kinase inhibitor, on thymocyte functions. Comparison with the effect of tyrosine kinase inhibitors

The effects of staurosporine, a protein kinase inhibitor, on the signal transduction and proliferation of thymocytes were studied. Signal transduction in response to Concanavalin A (Con A) as well as Concanavalin A (Con A)-induced augmentation of [3H]inositol incorporation into phospholipids were inhibited by staurosporine (> or = 10(-8) M). Staurosporine inhibited thymocyte proliferation in response to Con A in the presence or absence of the phorbol ester, phorbol myristate acetate (TPA) (10 nM). This inhibition was observed regardless of whether staurosporine was added together with Con A or 3 hr later. High concentrations of staurosporine (> 10(-8) M) inhibited thymocyte proliferation induced by the calcium ionophore A23187 and the phorbol ester TPA, whereas lower concentrations of the inhibitor (< or = 10(-8) M) enhanced thymidine incorporation in response to these activators. This dual effect of staurosporine was also observed in the presence of the staurosporine-related kinase inhibitor, K252a. In contrast, the tyrosine kinase inhibitor, tyrphostin AG490, inhibited the response to A23187 and TPA at all concentrations of the inhibitor and no augmentation was seen. Interleukin 2 (IL-2)-driven mitogenesis in IL-2-dependent cells was also inhibited by staurosporine. We suggest that the inhibition of thymocyte proliferation by staurosporine results from inhibition of both protein kinase C and tyrosine kinase: the augmentation of the response to A23187 and TPA results from inhibition of protein kinase C. Inhibition of signal transduction as well as inhibition of IL-2-driven mitogenesis result from inhibition of tyrosine kinase.

Effects of phorbol myristate and ionomycin on in vitro growth of aged Peyer's patch T and B cells

The proliferative responses of Peyer's patch (PP) T cells from aged BALB/c mice to concanavalin A (Con A) are considerably reduced, as compared to those of the young (P < 0.001). This reduced reactivity of aged T cells could be partly, but not entirely, corrected by interleukin 2 (IL-2) (P < 0.001). PP T cells from aged mice responded synergistically to a protein kinase C (PKC) activator, phorbol myristate acetate (PHA), plus a calcium ionophore, ionomycin, at much lower concentrations than to Con A (P < 0.001); however, the maximal proliferative response still remained nearly at 8/10th of the young (P < 0.01) and higher levels of PMA (but not of ionomycin) were required (P < 0.001). Addition of IL-2 restored the diminished response to the levels of the young T cells (P < 0.05), but that of Con A did not (P > 0.05). The proliferative responses of PP B cells to lipopolysaccharide (LPS) do not differ from those of the young (P > 0.05), but the spontaneous proliferation of aged (unstimulated) B cells is enhanced nearly twofold versus that of the young (P < 0.001). Like the PP T cells, PP B cells from aged mice also responded synergistically to PMA plus ionomycin but to a lesser degree than those of the young under the same stimulation (P < 0.01). Their maximal proliferation required higher levels of PMA, but not of ionomycin and was also diminished (P < 0.01), compared to that of the young. B cell stimulatory co-factors, IL-4 and IL-6, failed to affect the response of aged and young B cells to PMA plus ionomycin (P > 0.05), whereas LPS remediates the reduced response of aged B cells to PMA plus ionomycin. Thus, T and B cells from senescent PP demonstrate an impaired proliferative responsiveness via the Ca-dependent PKC pathway. A T cell mitogen and B cell stimulatory cytokines did not alter this activation pathway, once optimally stimulated. Whereas, T cell stimulatory cytokine IL-2 and B cell mitogen LPS could restore the age-associated decline of the corresponding lymphocyte subsets, T and B cells, in activation of the Ca-dependent pathway. The altered transmembrane signal transduction appears to be intrinsically defective in these aged PP T and B cells.

Deciduogenic effects of mediators of the polyphosphatidylinositol pathway in pseudopregnant mice

Intraluminal injections (15 microliters) of either concanavalin A (125 micrograms) or ionophore A 23,187 (0.01 mumol) induced a decidual cell reaction (DCR) in the uterus of day 4.5 pseudopregnant mice. However, when these agents were administered in different combinations with each other or with CaCl2 (15 mumol) and phorbol-12-myristate-13-acetate (1.6 nmol), interacting effects occurred to either enhance or inhibit each of the others' independent deciduogenic capacities. The results suggest that the polyphosphatidylinositol pathway and Ca2+ are involved in the induction of the DCR in mice with complex interactions occurring between the active components of the pathway to modulate the outcome of the transformation process.

Ziskind-Somerfeld research Award. The involvement of guanine nucleotide binding proteins in the pathogenesis and treatment of affective disorders

Guanine nucleotide binding (G) proteins play a pivotal role in postreceptor information transduction. An important characteristic of G proteins is their increased guanine nucleotide binding following agonist stimulation, which in turn leads to their activation. We have developed a method that enables the measurement of early events in signal transduction beyond receptors, through activated receptor-coupled guanine nucleotide exchange on G proteins. Using this method, lithium was recently demonstrated to inhibit the coupling of both muscarinic cholinergic and beta-adrenergic receptors to pertussis toxin-sensitive and cholera toxin-sensitive G proteins, respectively, thus suggesting alteration of the function of G protein by lithium, as the single site for both the antimanic and antidepressant effects of this drug. One of the most puzzling aspects of the ability of lithium to ameliorate the manic-depressive condition is its relatively selective action upon the central nervous system (CNS). It was previously shown that lithium selectively attenuated the function of Gs proteins in the CNS. In the present study, we show that inhibition by lithium of muscarinic receptor-coupled G protein function is also selective to the CNS. The clinical profile of lithium, carbamazepine, and electroconvulsive treatment (ECT), agents that are effective in the prevention and treatment of bipolar affective disorder, differs from that of purely antidepressant drugs. Antidepressant drugs are effective in the acute treatment and prevention of depression only, and can even precipitate hypomanic or manic "switches," or "rapid cycling" between mania and depression. We have investigated and compared the effects of chronic antibipolar and antidepressant treatments on receptor-coupled G protein function. Antibipolar treatments (lithium, carbamazepine, ECT) attenuate both receptor-coupled Gs and non-Gs (i.e., Gi, Go) proteins function; in contrast, only Gs protein function is inhibited by antidepressant drugs [either tricyclics or monoamine oxidase (MAO) inhibitors]. Moreover, an integral adrenergic neuronal system is required for antidepressant inhibition of Gs protein function, as pretreatment with the noradrenergic neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) specifically abolishes the effects of antidepressant drugs on Gs protein, whereas antibipolar drug effects on G protein function are unaffected by DSP-4. Our results suggest that attenuation of beta-adrenergic receptor-coupled Gs protein function, which is common to both antidepressant and antibipolar treatments, may be the mechanism underlying their antidepressant therapeutic efficacy.(ABSTRACT TRUNCATED AT 400 WORDS)

Alteration in Ca2+ homeostasis by a trauma peptide

Postinjury tissue inflammation with PMN elastase proteolysis generates immunosuppressive fibronectin peptides (FNDP) impairing chemotaxis, T-cell activation, and proliferation. Excess intracellular Ca2+ ([Ca2+]i) impairs T-cell activation. This study quantifies the changes in [Ca2+]i following exposure to a degradation peptide of fibronectin to determine the mechanism of action of these peptides on calcium homeostasis. Isolated human PBLs were exposed to immunosuppressive concentrations of FNDP after loading with the [Ca2+]i probe FURA-2AM. Resting and sustained [Ca2+]i concentrations were calculated and compared to buffer control. The mechanism of action was determined by pretreatment with: (1) EDTA binding extra cellular Ca2+: [Ca2+]e, (2) the Ca2+ channel blockers verapamil and nifedipine, and (3) inhibition of [Ca2+]i released by dantrolene. Inositol triphosphate (IP3) essential for [Ca2+]i release was measured following T-cell stimulation as well. FNDP caused 200-400% increases in [Ca2+]i concentration relative to buffer control at known suppressive doses. Verapamil and nifedipine partially block [Ca2+]i influx by as much as 50% suggesting the slow Ca2+ (voltage independent) channels are partially responsible for the increased [Ca2+]i seen following FNDP. EDTA completely suppressed [Ca2+]e influx but did not completely inhibit the release of [Ca2+]i although IP3 was 80% suppressed. The increase in [Ca2+]i following FNDP stimulation is due to release of intracellular stores.

A defect in the protein kinase C system in T cells from patients with systemic lupus erythematosus

To determine whether there is an intrinsic defect in T cells from patients with systemic lupus erythematosus (SLE), we studied signal transduction systems, assaying the total protein kinase C (PKC) levels and the phorbol myristate acetate (PMA)-induced activation of PKC in PHA-treated T cells. T cells from SLE patients showed a decrease in proliferation in response to PMA, but not to PHA, thereby suggesting the existence of an intrinsic abnormality in the PKC-mediated activation pathway. Total PKC activity in the T cells from SLE patients was significantly decreased. Although stimulation with PMA induced a translocation of PKC from the cytosol to the particulate fraction, translocated PKC activity after 2 nM PMA treatment was decreased in the SLE T cells. Furthermore, PMA-induced phosphorylation of 80-kDa substrates was also decreased in SLE T cells. These results suggest that there is a reduced PKC activity and an impaired PKC activation in response to PMA in the SLE T cells, a finding which may explain, if partially, the defect in T cell activation in patients with SLE.

Hepatic phosphatidylinositol kinase activity in continuously endotoxemic rats

The activity of phosphatidylinositol (PI) kinase and the content and fatty acid composition of inositol phospholipids (IPLs) were analyzed in the livers of rats that had been continuously infused with Escherichia coli endotoxin (ET) or saline for 30 h. Maximal enzymatic activity in total liver membrane fractions was observed in the presence of 1 mM ATP, 20 mM MgCl2, exogenously added 0.3 mM PI and Triton X-100 (0.25%). The activity of PI kinase for endogenous and exogenous PI was 43 and 79% higher respectively, in ET- as compared with saline-infused rats. The Km of the enzyme for ATP was not altered (0.175 mM), while the apparent Vmax was higher for ET- as compared with saline-infused rats (0.48 and 0.38 nmol of phosphatidylinositol 4-phosphate formed/mg protein per min, respectively). The ET-induced higher activity of PI kinase was paralleled by a 68-78% increase in the content of polyphosphoinositides (PPI), while PI content was unchanged. All IPLs from livers of endotoxemic rats had a lower content of arachidonic acid. We demonstrate for the first time that ET can directly and/or indirectly stimulate the net synthesis of PPI in liver cells. This effect could serve to modulate the PPI derived signals by increasing the availability of the substrate phosphatidylinositol 4,5-bisphosphate.

Lithium sensitive G protein hyperfunction: a dynamic model for the pathogenesis of bipolar affective disorder

Guanine nucleotide binding proteins (G proteins) play a pivotal role in information transduction from various membrane receptors to a variety of intracellular effector systems. By influencing the metabolism of adenylate cyclase and phosphatidylinositol, G proteins affect the activities of both cAMP-dependent protein kinase (kinase A) and protein kinase C. The hypothesis of this present study addresses the oscillatory behavior of symptoms observed in manic-depressive patients by suggesting that the cellular phosphorylation state in the central nervous system, which results from the relative activity of protein kinase A and protein kinase C, determines the affective state. From this hypothesis, we developed a kinetic model based on self- and inter-regulatory steps between these two protein kinase systems. The solutions of the differential equations governing this kinetic model can describe oscillatory pathological affective states. More specifically, we show that hyperfunction of G proteins leads to an unstable 'catastrophic' dynamic system characteristic of a manic or depressive state, and that lithium treatment attenuates G protein function and damps the oscillatory system to yield a stable state.

Cyclic AMP counteracts mitogen-induced inositol phosphate generation and increases in intracellular Ca2+ concentrations in human lymphocytes

1. The effects of increases in intracellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) on mitogen-induced generation of inositol phosphates and increases in intracellular Ca2+ concentration were investigated in human peripheral blood mononuclear leukocytes (MNL). 2. The mitogens concanavalin A (Con A), pokeweed mitogen (PWM) and phytohaemagglutinin (PHA) concentration-dependently stimulated generation of inositol phosphates. Catecholamines inhibited this process with an order of potency: isoprenaline greater than adrenaline greater than noradrenaline indicating involvement of beta 2-adrenoceptors. This order of potency was also consistent with the catecholamine potencies for stimulating the generation of cyclic AMP. 3. In addition to catecholamines, the cyclic AMP formation-stimulating agents prostaglandin E1 (PGE1) and forskolin concentration-dependently inhibited mitogen-induced inositol phosphate generation, too. Moreover, the inhibitory effect of isoprenaline was potentiated by co-incubation with the phosphodiesterase inhibitor isobutylmethylxanthine demonstrating that these inhibitory effects were mediated by cyclic AMP. 4. Con A and PHA concentration-dependently increased the intracellular Ca2+ concentration in human MNL (assessed by the fluorescent indicator dye Fura-2). This increase was almost completely blocked by chelation of extracellular Ca2+, demonstrating influx rather than mobilization from intracellular stores. 5. The elevation of intracellular Ca2+ was not blocked by pretreatment with pertussis toxin, 100 ng ml-1, for 16 h. 6. Isoprenaline, PGE1, and forskolin, however, inhibited the mitogen-stimulated elevation of intracellular Ca2+. This inhibition was enhanced by the phosphodiesterase inhibitors isobutylmethylxanthine and Ro 20-1724, demonstrating mediation by cyclic AMP. 7. We conclude that catecholamines and other cyclic AMP increasing agents can inhibit mitogen-stimulated generation of inositol phosphates and elevation of intracellular Ca2+ in resting human MNL.

Proteolytic fragment of protein kinase C (kinase M) phosphorylates in vitro phosphatidylinositol-4-phosphate

Limited tryptic proteolysis of homogeneous protein kinase C induces the formation of a catalytically active fragment of 50 kDa (kinase M) which, unlike native PK C acquires the ability to phosphorylate PIP. Both ATP and GTP were found to be capable of serving as phosphate donors in this process. Incubation of purified kinase M with a preparation of rat brain membrane fraction enhanced the level of phosphorylation of PIP in the presence and in the absence of exogenous PIP. A scheme of the interrelationship of phosphoinositide metabolism and the proteolytic processing of protein kinase C is proposed.

Altered levels of phosphoinositide metabolites and activation of guanine-nucleotide dependent phospholipase C in rat hepatic tumors

The metabolism of phosphatidylinositol was studied in normal quiescent hepatocytes, hepatocellular carcinomas induced by single dose of diethylnitrosamine, followed by 2-acetylaminofluorene and partial hepatectomy (Solt-Farber model), and in an established hepatoma cell line, JB1. The JB1 hepatoma cell line and hepatocellular carcinomas demonstrated a 4- to 5-fold higher rate of turnover of [3H]-inositol and [3H]-glycerol than the control hepatocytes. Significantly, elevated levels of second messengers inositol 1,4,5-trisphosphate and sn-1,2-diacylglycerol were noted in hepatic tumor cells within 4 hr of labeling with precursor molecules, whereas no detectable level of 3H-labeled inositol trisphosphate was noted in quiescent hepatocytes, even after incubation with 10 mM LiCl for 30 min. Approximately 2.5-fold higher specific activities of a guanine nucleotide and Ca+2 dependent phosphatidylinositol 4,5-bisphosphate specific phospholipase C were detected in the hepatocellular carcinoma cells. The cellular location of the phospholipase C activity was also different, being membrane bound in hepatocytes and equally distributed between cytosolic and membrane factions in the hepatomas. These data are consistent with the hypothesis that the enhanced production of diacylglycerol and inositol 1,4,5-trisphosphate in hepatocellular carcinomas may be due to the activation of a guanine nucleotide dependent phosphatidylinositol 4,5-bisphosphate specific phospholipase C. These data are the first to compare phosphoinositide turnover in normal liver and hepatic tumor cells and suggest that the sustained levels of second messengers is closely associated with the transformation and enhanced growth rate in hepatic tumor cells.

Regulation of the antigen-induced F-actin response in rat basophilic leukemia cells by protein kinase C

Multivalent antigen that is capable of binding to and crosslinking the IgE receptors on rat basophilic leukemia (RBL) cells, induces a rapid and sustained rise in the content of filamentous actin. This reorganization of the actin may be responsible for changes in cellular morphology during the degranulation process. The antigen-stimulated polymerization of actin can be blocked in a dose-dependent manner by protein kinase inhibitors which also block degranulation. Conversely, reagents such as PMA, 1,2-dioctanoyl-sn-glycerol (diC8), and 1-oleoyl-2-acetyl-glycerol (OAG) which stimulate protein kinase C (PKC) also activate the rise in F-actin, although they have no effect on degranulation by themselves. The actin response which can be stimulated by the PKC activators can also be blocked by protein kinase inhibitors indicating that the PMA- and OAG-induced response is probably through activation of a protein kinase. Depletion of PKC activity through long term (20 h) exposure of RBL cells to PMA, also inhibited the F-actin response when the cells were stimulated with either multivalent antigen or OAG. External Ca++, which is an absolute requirement for degranulation, is not necessary for the rise in F-actin, but may modulate the response. Furthermore, ionomycin, which induces a large Ca++ influx, does not stimulate the F-actin increase even at doses that cause degranulation. These results suggest that activation of a protein kinase, such as PKC, may be responsible for signaling the polymerization of actin in RBL cells and that a rise in intracellular Ca++ is neither necessary nor sufficient for this response.

Implications of prostaglandin E2 synthesis and phospholipase C activation in potentiation of T-cell proliferation by LF 1695

Murine spleen cells, T-enriched by nylon wool filtration, proliferate in the presence of a protein kinase C stimulator and a calcium ionophore. Using this cell proliferation system, we show that LF 1695 can potentiate phorbol myristate acetate (PMA) action in the presence of A 23187. This potentiation can be due to PGE2 inhibition since it is found that lipopolysaccharide (LPS) or A 23187 induced PGE2 release from spleen cells is inhibited by LF 1695. Indomethacin and LF 1695 gave similar stimulation of spleen cell proliferation, and exogeneously added PGE2 inhibits this phenomenon. Considering two of the main early components of intracellular signal transduction, LF 1695 induces IP3 release and calcium mobilization. However, the compound is not mitogenic per se. These results show that LF 1695 behaves only as a costimulant for T-cell proliferation.