Receptor-coupled activation of phosphoinositide-specific phospholipase C by an N protein

Tyrosine phosphorylation and oxygen consumption induced by G proteins in neutrophils

In neutrophils, receptor-mediated activation of the respiratory burst requires ATP, possibly for phosphotransferase reactions. The oxidative response is only partially inhibited by blockers of protein kinase C, suggesting the involvement of other kinases. Recent evidence has demonstrated activation of tyrosine phosphorylation in chemoattractant-stimulated cells. This effect is likely mediated by G proteins because it is obliterated by pretreatment with pertussis toxin. In this report we have attempted to correlate the respiratory burst and phosphotyrosine accumulation induced by activation of G proteins, accomplished by treatment of electroporated cells with nonhydrolyzable analogues of GTP. In cells stimulated with guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) both responses displayed similar time course and concentration dependence. The guanine nucleotide selectivity sequence and the divalent cation requirements were also similar for both responses. These similarities suggest a relationship between tyrosine phosphorylation and the activation of the NADPH oxidase. GTP gamma S-induced phosphotyrosine accumulation was found to be inhibited by pretreatment of the cells with phorbol esters, underlining the existence of regulatory interactions between different signal transduction pathways in neutrophils.

A role for guanine-nucleotide-binding proteins in mediating T-cell-receptor coupling to inositol phospholipid hydrolysis in a murine T-helper (type II) lymphocyte clone

Perturbation of the T-cell receptor (TCR) complex is followed by the rapid hydrolysis of inositol phospholipids (InsPL) by phospholipase C (PLC), producing diacylglycerol and inositol phosphates, which act as second messengers in signal transduction. The mechanism coupling the TCR to InsPL hydrolysis is not clearly defined, and no information is available on this mechanism in the CD4+ helper subset of T-lymphocytes (Th). We have tested the hypothesis that guanine-nucleotide-binding proteins (G-proteins) may couple the TCR to PLC in a murine Th type II (Th2) cell clone. Cell permeabilization with streptolysin O (SLO) or tetanolysin (TL) was used to allow membrane-impermeable nucleotides access to intracellular sites of action. Exposure of permeabilized Th2 cells to guanosine 5'-[gamma-thio]triphosphate (GTP gamma S), a non-hydrolysable GTP analogue, resulted in a 2.1-2.5-fold increase in inositol phosphate generation. Similarly, perturbation of the TCR with the monoclonal antibody 145.2C11 (directed against the epsilon-chain of the CD3 component of the TCR) resulted in a 3.1-4.2-fold increase in InsPL hydrolysis by permeabilized cells. Both lysins were similarly effective in allowing GTP gamma S induction of InsPL hydrolysis, but TL-permeabilized cells responded better to TCR perturbation than SLO-treated cells. A role for G-proteins in TCR coupling to PLC was further supported by the inhibition of TCR-induced InsPL hydrolysis by guanosine 5'-[beta-thio]diphosphate (GDP beta S), a guanine nucleotide analogue that inhibits G-protein function. ATP was required for TCR-mediated InsPL hydrolysis, and potentiated GTP gamma S-induced hydrolysis. Other nucleotides (i.e. CTP, GDP, GTP, ITP) did not affect the response. These data indicate that G-proteins may contribute to the regulation of PLC activation in Th2 cells, coupling it to the TCR.

The role of G-protein in matrix-mediated motility of highly and poorly invasive melanoma cells

Membranes from 2 K1735 murine melanoma clones of high invasive capacity show increased amounts of pertussis toxin (PT) substrate when compared to a weakly invasive cellular counterpart. Using a panel of specific G-protein antibodies, we identified Gi alpha 2 as the PT-sensitive G-protein uniquely abundant in highly invasive cells. In addition, RNA hybridization results confirm the immunoblot observations that Gi alpha 2 is present at higher levels in strongly invasive cells. This result suggests that the elevated expression of Gi alpha 2 in highly invasive cells is not entirely due to differences in either translational efficiency or protein degradation but is related to altered RNA transcriptional initiation, processing and/or degradation. ADP-ribosylation of Gi alpha-subunits by PT inhibited the fibronectin, laminin and collagen type-IV-stimulated motility of the 2 highly invasive clones, while PT treatment of cells from a poorly invasive clone resulted in little or no reduction of the fibronectin, laminin or collagen type-IV-stimulated lower motility. Furthermore, PT treatment of highly or poorly invasive K1735 clones does not result in any alteration in cellular cAMP accumulation, suggesting that the PT substrate is not linked with the adenylyl cyclase enzyme complex. The data suggest that a PT-sensitive G-protein, probably Gi alpha 2 regulates second messenger pathways that contribute to elevated motility in highly invasive K1735 cells.

A mechanism for testosterone modulation of alpha-1 adrenergic receptor expression in the DDT1 MF-2 smooth muscle myocyte

Previous reports have confirmed that steroid hormones modulate the expression of adrenergic receptors on the surface of smooth muscle myocytes. The present study was undertaken to evaluate the mechanism by which testosterone modulates alpha-1 adrenergic receptor expression in the DDT1 MF-2 transformed smooth muscle cell. Utilizing 3H-prazosin radioligand binding studies, alpha-1 adrenergic receptors were noted to increase more than 2 fold in response to incubation with 10(-8)M testosterone for 96 hours. Dihydrotestosterone similarly stimulated a significant increase in alpha-1 receptors; whereas, estradiol and hydrocortisone appeared to suppress the expression of this receptor in DDT myocytes. The testosterone effect was dose related with a maximal response observed in response to 10(-7)M testosterone at both 48 and 96 hours. Kinetic experiments utilizing 10(-8)M testosterone demonstrated a peak effect on alpha-1 receptor expression at 96 hours, and maintenance of the effect for at least 168 hours (7 days). The testosterone effect was completely prevented at both 48 and 96 hours by inhibition of transcription with actinomycin-D, or inhibition of translation with cycloheximide. Consistent with the receptor binding studies, RNA blotting studies have demonstrated maximal alpha-1 receptor mRNA levels at 48-96 hours of testosterone stimulation. In conclusion, these in vitro experiments have confirmed the physiologic concentrations of testosterone stimulate the increased expression of alpha-1 receptors in the DDT1 MF-2 myocyte after a delay of 48-96 hours; and that this effect appears to be mediated by transcription, translation, and synthesis of new proteins in these genital tract myocytes.

Neutrophil beta-adrenergic receptor responses are potentiated by acute exposure to phorbol ester without changes in receptor distribution or coupling

Exposure to the phorbol ester, phorbol 12-myristate, 13-acetate (PMA, 100nM) for 10 minutes enhanced cyclic AMP accumulation in human neutrophils under basal conditions and in response to the beta-adrenergic receptor agonist isoproterenol (ISO), 1 microM) and the adenylate cyclase activator forskolin (FSK, 10mM). Potentiation of responses to ISO by PMA was dose-dependent between 0.1 and 100nM PMA. The diacylglycerol analogue, 1-oleoyl-2-acetylglycerol (OAG) (50 microM) also elevated beta-receptor responses, but 4 beta-phorbol (100nM), lacking the capacity to activate PMA, was ineffective. Short-term exposure (12 seconds) to the peptide n-formylmethionine leucyl-phenylalanine (FMLP, 1 microM) also elevated neutrophil cyclic AMP accumulation. All potentiating effects of PMA on cyclic AMP production were inhibited by the protein kinase inhibitor 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine (H7). Elevation of cyclic AMP by FMLP was insensitive to H7. PMA had no apparent effect on beta-receptor agonist-affinity, distribution between cell-surface and internalised compartments, or the capacity of ISO to induce beta-receptor internalisation. Responses to FSK or ISO in terms of fold-stimulation of basal cyclic AMP accumulation in the presence of PMA were not elevated by PMA. These findings indicate that PMA exerts a potentiating effect on neutrophil adenylate cyclase responses through protein kinase C activation. FMLP elevation of neutrophil cyclic AMP in the absence of other stimuli, appears however, to be insensitive to protein kinase inhibition.

Urea gradient/SDS-PAGE; a useful tool in the investigation of signal transducing G proteins

We describe an updated and improved protocol to perform urea gradient/SDS-PAGE in which proteins are electrophoresed through 9% polyacrylamide gel slabs in the presence of a linear 4 M to 8 M gradient of urea using Laemmli's separation buffers. We provide examples of this technique to separate PTX labeled G protein alpha subunits, as well as unlabeled alpha and beta subunits of G proteins. Applications of the technique are exemplified in which (1) the chromatographic separations of G proteins in DEAE-Toyopearl and MonoQ columns are compared, (2) the complexity of PTX substrates expressed in human erythrocytes, bovine brain, dog ventricle, FRTL-5 cells, HIT cells, GH4C1 cells and RIN cells are compared, and (3) the polypeptide composition of G protein beta gamma subunits, as expressed in several tissues and found in three distinct G proteins from a single cell population, are analyzed.

Tumor cell autocrine motility factor receptor

The ability to locomote and migrate is fundamental to the acquisition of invasive and metastatic properties by tumor cells. Autocrine motility factor (AMF) is a cytokine produced by various tumor cells which stimulates their in vitro motility and in vivo lung-colonizing ability. AMF stimulates cell motility via a receptor-mediated signalling pathway. Signal transduction following binding of AMF to its receptor, a cell surface glycoprotein of 78 kD (gp78), is mediated by a pertussis toxin sensitive G protein, inositol phosphate production and the phosphorylation of gp78. AMF induces gp78 internalization to intracellular tubulovesicles and transport to the leading edge stimulating pseudopodial protrusion and cell motility.

The biology of beta-adrenergic receptors: analysis in human epidermoid carcinoma A431 cells

1. G-protein-linked transmembrane signaling has emerged as a major pathway for information transduction across the cell membrane. 2. In addition to photopigments that propagate the signal from light, cell-surface receptors for hormones, neurotransmitters, and autacoids propagate signals from ligand binding to membrane-bound effector units via G-proteins. 3. Biochemical and molecular features of one prominent member of these receptors, the beta-adrenergic receptor, will be highlighted in the present article. 4. The role of the human epidermoid carcinoma A431 cells as a model for the study of the structure and biology of beta-adrenergic receptors will be emphasized. 5. A model for receptor regulation, gleaned from recent advances in the biochemistry, cell and molecular biology of beta-adrenergic receptors, is discussed.

On the origins and present state of the art of G protein research

G proteins are central to the transduction of many receptor signals. I review the "firsts" that have led to our current knowledge and pose some of the questions presently investigated in several laboratories around the world.

Cyclic AMP agonists induce the phosphorylation of phospholipase C-tau and of a 76 kDa protein co-precipitated by anti-(phospholipase C-tau) monoclonal antibodies in BALB/c-3T3 cells. Relationship to inositol phosphate formation

Previous studies have demonstrated enhanced phosphorylation of phospholipase C-tau (PLC-tau), a key regulatory enzyme in phosphoinositide metabolism, in cells treated with platelet-derived growth factor (PDGF) and epidermal growth factor, both of which act via specific receptor tyrosine kinases. Our studies on BALB/c-3T3 cells show that agents that promote cellular cyclic AMP accumulation also increase the phosphorylation, specifically the serine phosphorylation, of this enzyme. Increased phosphorylation of PLC-t (2-3-fold) was evident within 5-10 min of addition of isobutylmethylxanthine (IBMX) and either cholera toxin or forskolin to cells, and persisted for at least 3 h. Treatment of cells with cyclic AMP agonists also enhanced, with similar kinetics, the phosphorylation of a 76 kDa protein co-precipitated by anti-PLC-tau monoclonal antibodies. Brief exposure of cells to cholera toxin/IBMX or forskolin/IBMX decreased inositol phosphate formation induced by the GTP-binding protein (G-protein) activator aluminium fluoride by approx. 50%, but was without effect on PDGF-stimulated inositol phosphate formation. These findings suggest that PLC-tau, and perhaps the 76 kDa co-precipitated protein, are substrates of cyclic AMP-dependent protein kinase in BALB/c-3T3 cells: however, the lack of effect of cyclic AMP elevation on PDGF-stimulated inositol phosphate formation indicates that the intrinsic activity of PLC-tau is unaltered by cyclic AMP-mediated phosphorylation.

PT pretreatment inhibits 48/80-induced activation of Ca(+)-permeable channels in rat peritoneal mast cells

We recently reported that the secretagogue, compound 48/80, activated Ca2(+)-permeable channels of mast cells possibly via a second messenger [Kuno, Okada, and Shibata. Am. J. Physiol. 256 (Cell Physiol. 25): C560-C568, 1989]. The effects of pretreatment with pertussis toxin (PT) on compound 48/80 (48/80)-induced activation of the Ca2(+)-permeable channel have now been investigated by measuring Ca2+ signals of cell suspensions using the Ca2+ indicator fura-2 and by recording Ba2+ currents through the channel using the patch-clamp technique. In the presence of extracellular Ca2+, the fluorescence change was biphasic, with an immediate rise and a delayed peak at room temperature. The delayed peak, mainly due to Ca2+ entry through plasma membranes, was greatly reduced by pretreatment with PT. The quenching of the fluorescence by 48/80-induced Mn2+ influx was also decreased by PT, whereas the Ca2+ transients due to Ca2+ release from the intracellular stores apparently did not change. In patch-clamp recordings from cell-attached patches with pipettes containing isotonic Ba2+, the 48/80-induced Ba2+ currents were either suppressed or delayed in the PT-treated cells, under conditions where degranulation was absent. These results suggest that PT-sensitive GTP-binding protein is involved in activating the Ca2(+)-permeable channel in mast cells during stimulus-secretion coupling.

Phospholipases in biology and medicine

Phospholipases, a group of enzymes that catalyze the hydrolysis of membrane phospholipids, are classified according to the bond cleaved in a phospholipid into PLA1 (EC 3.1.1.3), PLA2 (EC 3.1.1.4), PLB (EC 3.1.1.5), PLC (EC 3.1.4.3), and PLD (EC 3.1.4.4). This paper reviews source and structure of PLA2 and the involvement of PLA2 and PLC in several biological phenomena, such as, signal transduction, photoreception, biosynthesis of lung surfactant, sperm motility, and fertilization. New assays for PLA2 activity and concentration in biological fluids are discussed. Phospholipases are involved in many inflammatory reactions by making arachidonate available for eicosanoid biosynthesis. The determination of PLA2 activity and mass concentration in plasma is useful in the diagnosis and prognosis of pancreatitis and of septic shock. Naturally occurring phospholipase inhibitors, such as lipocortins act as second messengers in the anti-inflammatory response to steroids. Lipocortins may be valuable therapeutic agents, because they are more specific in their anti-inflammatory action than glucocorticoids; therefore, they are less likely to produce harmful side effects.

Phorbol esters modulate the high Ca2(+)-stimulated accumulation of inositol phosphates in bovine parathyroid cells

We examined the effects of TPA on the high Ca2(+)-stimulated accumulation of inositol phosphates in bovine parathyroid cells to determine whether protein kinase C modulates phosphoinositide turnover in a fashion similar to that observed in other cell types stimulated by more classic Ca2+ mobilizing hormones. Following exposure of parathyroid cells to TPA (10(-6) M) for 10 or 30 minutes, there was a time- and dose-dependent inhibition of the accumulation of inositol monophosphate (IP), inositol bisphosphate (IP2) and inositol trisphosphate (IP3) stimulated by 3 mM Ca2+. Half the maximal observed inhibition took place at 1-10 nM TPA, with 50-60% inhibition of high Ca2(+)-stimulated accumulation of inositol phosphates at 10(-6) M TPA. The active phorbol ester, 4 beta-phorbol didecanoate, produced similar effects; the inactive derivative, 4 alpha-phorbol didecanoate, was without effect. When parathyroid cells were exposed to TPA (10(-6) M) for varying times and were then incubated with high (3 mM) Ca2+, inhibition of inositol phosphate accumulation was observed with 10 or 30 minutes preincubation. In contrast, preincubation of cells with TPA for 3 or 18 h markedly enhanced the high (3 mM) Ca2(+)-induced increase in inositol phosphates. In cells preincubated with TPA for 18 h, binding sites for [3H]phorbol dibutyrate and total protein kinase C (PKC) activity were reduced by greater than 95% and by 71%, respectively, consistent with downregulation of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulatory effect of staphylococcal leukocidin on phosphoinositide metabolism in rabbit polymorphonuclear leukocytes

When rabbit polymorphonuclear leukocytes (PMNs) were incubated with staphylococcal leukocidin (F and S components) in the presence of 32Pi at 37 degrees C, incorporation of 32Pi into phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) occurred after a lag phase of 10 s and reached a maximal level at 60 s of 50- and 30-fold increase, respectively, compared with that of the control in the absence of the toxin. Whereas the amount of 32P radioactivity incorporated in PIP and PIP2 decreased to control levels in a few minutes, 32P incorporation into phosphatidic acid (PA) continuously increased over 3 min. These findings suggested an early activation of phosphoinositide-specific phospholipase C in rabbit PMNs by leukocidin as shown by the rapid breakdown of PIP and PIP2 accompanied by the appearance of PA. The stimulatory effect of leukocidin on some enzymatic activities of the phosphatidylinositol pathway was further investigated by using PMN cell membrane preparations. In the presence of both the F and S components, enhanced 32P incorporation was observed not only in PIP2 and PA but also in PIP. While the F component mainly enhanced 32P incorporation into PIP2 and PA, the S component alone had no effect on 32P incorporation into PIP, PIP2, and PA. The F component alone enhanced conversion of PIP to [32P]PIP2 in the presence of unlabeled PIP and [gamma-32P]ATP, through the activation of PIP kinase. PIP kinase activity was potentiated by the addition of NAD and GTP. Subsequent formation of [32P]PA was also enhanced by the F component, resulting from activation of the phosphoinositide-specific phospholipase C. These results suggested that the F component of staphylococcal leukocidin is responsible for the enhancement of phosphoinositide metabolism in rabbit PMN cell membranes.

Desensitization of muscarinic M1 receptors of murine neuroblastoma cells (clone N1E-115) without receptor down-regulation and protein kinase C activity

Acute desensitization of M1 muscarinic receptor-mediated responses (cyclic GMP formation and inositol phosphate release) was studied in murine neuroblastoma cells (N1E-115 clone). After a 45-min incubation at 37 degrees of N1E-115 cells either in monolayer or in suspension, with the muscarinic agonist carbachol (1 mM), the receptor-mediated cyclic GMP response to carbachol was nearly completely lost. This loss was associated with greater than 80% loss of carbachol-mediated inositol phosphate release. The protein kinase C activator phorbol 12-myristate 13-acetate (PMA) inhibited both responses with similar potencies. Carbachol or PMA reduced by 30-40% the number of muscarinic receptor sites for antagonist and agonist on intact cells (determined in binding assays using [3H]N-methylscopolamine) only for cells in monolayer and not for those in suspension. PMA but not carbachol pretreatment of cells in monolayer or in suspension caused a translocation of [3H]phorbol 12,13-dibutyrate binding and protein kinase C activity. In addition, desensitization to carbachol occurred in cells largely depleted of protein kinase C by chronic exposure to PMA. Thus, agonist-mediated down-regulation is not needed for muscarinic M1 receptor desensitization, which may be a result of the activation of a receptor-activated kinase different from protein kinase C.

Fluoride-mediated activation of the respiratory burst in electropermeabilized neutrophils

Electropermeabilization creates small pores in the plasma membrane allowing the introduction of low-molecular-weight modulatory components, such as ions and nucleotides, into the cytosol. The present study investigates fluoride-mediated stimulation of the signal transduction pathway that activates the respiratory burst in electropermeabilized neutrophils. In marked contrast to intact (i.e., non-electropermeabilized) neutrophils, cells permeabilized by this technique demonstrated an immediate and potent stimulation of the superoxide (O2-)-generating NADPH oxidase in response to the addition of fluoride. Furthermore, permeabilization of neutrophils in the presence of exogenously added ATP enhanced the rate of F(-)-mediated O2- production. Fluoride-stimulated O2- production in electropermeabilized neutrophils was antagonized by GDP beta S and dependent upon the presence of Mg2+ in the medium, but was insensitive to pertussis toxin treatment, consistent with the hypothesis that fluoride activates a G protein, probably Gp, by interacting with the nucleotide-binding site on the G alpha subunit. In addition, electropermeabilized neutrophil O2- release triggered by F- was blocked by staurosporine and H-7, indicating that this pathway proceeds largely through protein kinase C activation. However, nucleotide-enhanced O2- production was only partially blocked by these inhibitors, suggesting that under such conditions ATP either competes with the inhibitor-protein kinase interaction or affects the signaling pathway(s) in such a way that protein kinase C may no longer be necessary for the activation of NADPH oxidase.

Exogenous gangliosides stimulate breakdown of neuro-2A phosphoinositides in a manner unrelated to neurite outgrowth

Gangliosides administered exogenously are well-known effectors of differentiation in many neuroblastoma lines and primary neuronal cultures. Previous studies suggested the phosphoinositide signaling mechanism could be a contributing factor. We have found that treatment of Neuro-2A cells with bovine brain ganglioside mixture (BBG) causes breakdown of phosphoinositides, as measured by increased levels of inositol phosphates. The effect was optimal at 60 min and required a minimal BBG concentration of 25 microM. However, addition of neomycin, which blocked phosphoinositide breakdown, had no observable effect on ganglioside-stimulated neurite outgrowth. A similar result was obtained with psi-tectorigenin, which also inhibited phosphoinositide hydrolysis. When cells were treated with maitotoxin, an agent that promotes phosphoinositide breakdown, there was no enhancement of neurite outgrowth. These findings indicate that although exogenous gangliosides elevate inositol phosphate formation over a prolonged period in neuro-2A cells, this reaction is not integral to the differentiation of these cells. The possibility of secondary effects influencing neurite type and structure cannot be excluded.

Intracellular regulation of ion channels in cell membranes

Cells communicate with their environment through receptor proteins on the cell membrane. Some ion channels are receptors, whereas others are linked to receptors through guanine nucleotide-binding proteins (G proteins). Ion channels control intracellular concentrations of ions such as calcium, and these concentrations control cell functions such as secretion and cell division. This review summarizes the current state of knowledge about the control of ion channels.

Mg.ATP primes superoxide-generating responses in electropermeabilized neutrophils

The present study utilizes an electropermeabilized cell system to determine the effect of Mg.ATP on neutrophil superoxide (O2-)-generating responses stimulated by suboptimal concentrations of fMLP, GTP gamma S and PMA. Permeabilization in the presence of exogenously added Mg.ATP was neither sufficient to initiate O2- release nor necessary for stimulated O2- production. However, the inclusion of Mg.ATP in the permeabilization medium primed the O2(-)-generating responses mediated by suboptimal concentrations of these stimuli. The site of action of Mg.ATP is intracellular. Moreover, the fact that Mg.ATP primes responses stimulated by fMLP, GTP gamma S and PMA suggests that the modulatory effect is at the level of protein kinase C.

P2-purinergic receptors activate a guanine nucleotide-dependent phospholipase C in membranes from HL-60 cells

We have previously determined that human neutrophils and monocytes, as well as neutrophil/monocyte progenitor cells, express a subtype of P2-purinergic receptors (for ATP) which activate the inositol phospholipid signalling system. In the present study, membranes prepared from HL-60 promyelocytic leukemia cells were used to examine the mechanism by which these ATP receptors activate phosphatidylinositol-specific phospholipase C (PI-PLC) under defined in vitro conditions. Micromolar concentrations of the receptor agonists ATP, UTP, and ATP gamma S stimulated the GTP-dependent formation of inositol bisphosphate (IP2) and inositol trisphosphate (IP3) in washed membranes prepared from undifferentiated HL-60 cells prelabeled with [3H]inositol. The stimulatory effects of these nucleotides on PI-PLC appeared to be mediated through a GTP binding protein since minimal inositol polyphosphate accumulation was observed in the absence of guanine nucleotides. The increased inositol polyphosphate formation triggered by these nucleotide receptor agonists did not result from inhibition of GTP breakdown. Neither was it a consequence of increased [3H]polyphosphatidylinositol levels resulting from enhanced activity of membrane-associated PI- or PIP-kinases. Instead, the stimulated phospholipase activity was apparently receptor-mediated. The rank order of potency observed in these in vitro membrane assays (ATP = UTP greater than ATP gamma S much greater than TTP greater than CTP much greater than beta, gamma-CH-ATP) was similar to that observed with intact HL-60 cells. This order of potency appears to distinguish the P2-purinergic receptors expressed by human phagocytic leukocytes from the P2 gamma-purinergic receptors which activate PI-PLC in turkey erythrocyte membranes.

Inhibition of polymorphonuclear leukocyte oxidative metabolism by exogenous phospholipase C

We studied the effects of exogenous, purified phospholipase C (PLC) on neutrophil oxidative metabolism, lysosomal enzyme release and aggregation. We found that PLC inhibited O2- and H2O2 generation and oxygen consumption, but did not alter glucose oxidation via the hexose monophosphate shunt. In contrast, we found a striking stimulation of aggregation and release of the lysosomal enzymes lysozyme and beta-glucuronidase. In experiments designed to further characterize the mechanism of the PLC effect on membrane activation we studied the effect of PLC on intracellular calcium concentration [Ca2+]i and found that PLC did not interfere with the fMLP-mediated rise in [Ca2+]i, suggesting that its inhibitory effect on the respiratory burst does not involve inhibition of early signal transduction events. In addition, we found that PLC alone results in mobilization of intracellular Ca2+ stores, consistent with its stimulatory effect on aggregation and lysosomal enzyme release.

Phosphatidylinositol-4,5-bisphosphate phospholipase C in bovine rod outer segments

Preparations of rod outer segments from cattle retinas contained soluble and particulate phospholipase C activities which hydrolyzed phosphatidylinositol 4,5-bisphosphate (PIP2) and the other phosphoinositides. Ca2+ was required for PIP2 hydrolysis, but high (greater than 300 microM) concentrations were inhibitory. Mg2+ and spermine at low concentrations stimulated the particulate activity but inhibited the soluble. Mn2+ inhibited both. High (greater than 100 microM) concentrations of the nonhydrolyzable GTP analogue guanylyl beta,gamma-methylenediphosphonate inhibited PIP2 hydrolysis by both the soluble and particulate activities, but guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), fluoride, and cholera and pertussis toxins were without effect. Overall phospholipase C activity in ROS was unaffected by light. Evidence was found for multiple forms of the enzyme, requiring isolation and separate characterization before ruling out regulation by light or G-protein.

Receptor-effector coupling by G proteins

The primary structure of G proteins as deduced from purified proteins and cloned subunits is presented. When known, their functions are discussed, as are recent data on direct regulation of ionic channels by G proteins. Experiments on expression of alpha subunits, either in bacteria or by in vitro translation of mRNA synthesized from cDNA are presented as tools for definitive assignment of function to a given G protein. The dynamics of G protein-mediated signal transduction are discussed. Key points include the existence of two superimposed regulatory cycles in which upon activation by GTP, G proteins dissociate into alpha and beta gamma and their dissociated alpha subunits hydrolyze GTP. The action of receptors to catalyze rather than regulate by allostery the activation of G proteins by GTP is emphasized, as is the role of subunit dissociation, without which receptors could not act as catalysts. To facilitate the reading of this review, we have presented the various subtopics of this rapidly expanding field in sections 1-1X, each of which is organized as a self-contained sub-chapter that can be read independently of the others.

The glycosyl phosphatidylinositol-linked Fc gamma RIIIPMN mediates transmembrane signaling events distinct from Fc gamma RII

To investigate the ability of FcgammaRIII(PMN), the GPI-anchored isoform of FcgammaRIII (CD16) in polymorphonuclear leukocytes (PMN), to mediate transmembrane signaling events, we measured changes in membrane potential with DiOC(5) and in intracellular calcium with indo-1. FcgammaR were ligated by anti-FcgammaRIII mAb 3G8 (IgG and Fab), anti-FcgammaRII mAb IV.3 (IgG and Fab), and human IgG aggregates. Cell bound mAbs were also crosslinked by goat F(ab')(2) anti-mouse IgG. 3G8 IgG elicited a rapid change in [Ca(2+)](i), which was unaffected by EGTA, Vibrio cholerae toxin (CT), or Bordetella pertussis toxin (PT), and was abolished by BAPTA . Univalent receptor binding with 3G8 Fab gave no response but crosslinking with F(aV)2 GAM gave a rapid [Ca2,](i) response. Neither IV.3 Fab, IV.3 IgG, nor crosslinking of IV.3 Fab elicited a calcium signal. PI-PLC-treated PMN with the density of FcgammaRIII(PMN) reduced to that of FcgammaRII showed an unattenuated change in [Ca(2+)](i), with a 3G8 stimulus. The effects of IgG aggregates paralleled those of 3G8 mAb. These data indicate that multivalent ligation of FcgammaRIII(PMN) initiates an increase in [Ca(2+)];, derived from intracellular stores, that is distinct from both the FMLP- and FcgammaRII-induced responses. Ligand-dependent interaction with FcgammaRII is not required. Since FcgammaRIII(PMN) can internalize the FcgammaRIII-specific probe Con A-opsonized E and lyse anti-FcgammaRIII heteroantibody-opsonized chick E, this GPI-anchored molecule mediates both signal transduction and integrated cell responses.

Pertussis toxin and H-7 distinguish mechanisms involved in eicosanoid release from lipopolysaccharide-primed macrophages. Eicosanoid release from lipopolysaccharide-primed macrophages

Release of eicosanoids is an important response of macrophages to inflammation and bacterial infection. At low concentrations, bacterial lipopolysaccharide (1-2 micrograms/ml) fails to stimulate eicosanoid release in resident peritoneal macrophages but primes the macrophages for a greatly enhanced release of eicosanoids on stimulation with the calcium ionophore A23187 (0.1 microM) or with phorbol 12-myristate 13-acetate (50 nM), an activator of protein kinase C. Incubation of macrophages with Bordetella pertussis toxin, prior to priming with lipopolysaccharide, inhibited the release of both cyclooxygenase and lipoxygenase products upon A23187 stimulation. Pertussis toxin treatment of macrophages had no effect on eicosanoid release when the stimulus was phorbol 12-myristate 13-acetate. The presence of 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), an effective inhibitor of protein kinase C, during lipopolysaccharide priming and subsequent stimulation significantly inhibited eicosanoid release when phorbol 12-myristate 13-acetate was the stimulus, but did not affect eicosanoid release stimulated by A23187. Based on these results, at least two mechanisms, distinguished by apparent differences in sensitivity to pertussis-toxin-sensitive, guanine-nucleotide-binding proteins and protein kinase C, are involved in eicosanoid secretion by lipopolysaccharide-activated macrophages in response to A23187 and phorbol 12-myristate 13-acetate.

Turning on the respiratory burst

The respiratory burst is a distinguishing property of phagocytes. It is induced by chemotactic stimulation or phagocytosis and reflects the activation of a membrane-bound enzyme system that transfers electrons from cytosolic NADPH to extracellular oxygen, producing superoxide. The products of the burst are essential for the killing of microorganisms, but are also a cause of tissue damage and inflammation. Studies aimed at a better understanding of the regulation of the respiratory burst should help in the search for new ways to treat infections and inflammation.

Nonsteroidal antiinflammatory drugs exert differential effects on neutrophil function and plasma membrane viscosity. Studies in human neutrophils and liposomes

Nonsteroidal antiinflammatory drugs (NSAIDs) inhibit neutrophil functions via mechanisms separate from their capacity to inhibit prostaglandin synthesis. We have studied discrete events in the process of signal transduction: NSAIDs but not a related analgesic drug (acetaminophen), inhibited aggregation in response to the chemoattractants f-Met-Leu-Phe (FMLP), leukotriene B4, and C5a. NSAIDs, but not acetaminophen, inhibited binding of radiolabeled FMLP to purified neutrophil membranes. Gpp(NH)p, a GTPase insensitive analog of GTP, also inhibited the binding of FMLP but, paradoxically, enhanced superoxide anion generation and lysozyme release. The inhibition of ligand binding by NSAIDs did not correlate with their capacity to inhibit FMLP-induced increments in diacylglycerol (DG): piroxicam, but not salicylate effectively inhibited appearance of label ([3H]arachidonate, [14C]glycerol) in DG. Finally, NSAIDs exerted differential effects on the viscosity of neutrophil plasma membranes and multilamellar vesicles (liposomes): membrane viscosity was increased by piroxicam and indomethacin, decreased by salicylate, and unaffected by acetaminophen. Thus, the different effects of NSAIDs on discrete pathways are not due to their shared capacity to reduce ligand binding but rather to a capacity to uncouple postreceptor signaling events that depend upon the state of membrane fluidity.

Autocrine motility factor stimulates a three-fold increase in inositol trisphosphate in human melanoma cells

The biochemical pathways through which tumor cell locomotion is mediated are poorly understood. Autocrine motility factor (AMF), which is produced by and stimulates motility in A2058 human melanoma cells, was used to characterize phosphoinositide (PtdIns) metabolism activated in association with tumor cell motility. AMF stimulated up to a 400% increase in de novo incorporation of 3H-myo-inositol into cellular lipids beginning 40 minutes after exposure. In cells prelabeled with 3H-myo-inositol, AMF stimulated a 200% increase in total inositol phosphates (inositol monophosphate, InsP1; inositol bisphosphate, InsP2; inositol trisphosphate, InsP3) after 90 minutes of exposure, with a 300% maximal increase in InsP3 at 120 minutes. InsP1 and InsP2 were maximally increased 130% of control values. Treatment with AMF stimulated a parallel dose-dependent increase in both motility and PtdIns levels. We have shown previously that the A2058 motile response to AMF is inhibited markedly by cell pretreatment with pertussis toxin (PT). Inositol phosphate production was inhibited by a 2-hour pretreatment of cells with PT (0.5 microgram/ml). PT treatment of A2058 membranes was associated with ADP-ribosylation of a 40-kDa protein consistent with the presence of an alpha subunit of a guanine nucleotide-binding protein (G protein). These data indicate that AMF elicits increases in cell motility and phosphoinositide metabolism via a PT-sensitive G protein signal transduction pathway.

Aluminum fluoride reveals a phosphoinositide system within the suprachiasmatic region of rat hypothalamus

The phosphoinositide (PI) transduction system has proven to be of major importance in several regions of mammalian brain. In this report, we examined in rats whether a PI system is present in the hypothalamic suprachiasmatic nuclei (SCN), the site of a biological clock that generate circadian rhythms. Autoradiographic localization of phorbol ester binding revealed moderate levels of protein kinase C, a component of the PI system, in the SCN. Hypothalamic explants containing SCN showed substantial incorporation of [3H]myoinositol into lipids. AlF4-, a non-specific activator of G proteins, produced a dose-dependent increase in inositol monophosphate (IP1) levels in the explants in calcium-free medium, with a maximum increase of 216% of control at 50 mM NaF. Medium containing 1.8 mM calcium stimulated a similar increase in IP1 levels, but the stimulatory effects of AlF4- and calcium were not additive, so that the effect of Al4- was obscured in medium containing calcium. AlF4- stimulated accumulation of IP1, as well as inositol bis-, and trisphosphate, over a 40-min time course in the presence and absence of lithium (10 mM LiCl). Lithium, a known inhibitor of phosphatases in the inositol phosphate recycling pathway, raised levels of all 3 inositol phosphates in SCN explants both at baseline (without A1F4-) and after 30 min AlF4- stimulation. The results show the existence of a lithium-sensitive PI system within the suprachiasmatic region of the rat hypothalamus.

Thrombin-induced prostacyclin biosynthesis in human endothelium: role of guanine nucleotide regulatory proteins in stimulus/coupling responses

The regulation of prostacyclin (PGI2) synthesis by cultured human umbilical vein endothelium (HUVEC) was investigated. HUVEC monolayer generation of PGI2 was monitored by RIA of 6-keto PGF1 alpha and dose-dependent increases observed with human alpha- and gamma-thrombins, histamine, or arachidonate. Alpha thrombin (10 nM) produced levels of 6-keto PGF1 alpha approximating responses with 1 microM gamma-thrombin, 5 microM arachidonate, or 10 microM histamine. Diisopropyl phosphorofluoridate-inactivated alpha-thrombin did not stimulate PGI2 release, demonstrating that catalytic activity was required for thrombin-stimulated PGI2 release. Sodium fluoride (NaF), at concentrations known to activate guanine nucleotide regulatory proteins (G proteins), directly stimulated HUVEC PGI2 synthesis in a dose-dependent and time-dependent manner (20 mM NaF, 4.4 +/- 0.5-fold increase at 10 min, 11.9 +/- 1.5-fold increase at 30 min). Neither alpha-thrombin nor NaF-stimulated PGI2 release was dependent upon the availability of extracellular Ca++). The hypothesis that G proteins are involved in agonist-stimulated PGI2 synthesis was further supported by studies using digitonin-permeabilized HUVEC monolayers challenged with another G protein activator, guanosine 5'-0-3-thiotrisphosphate (GTP gamma S), which effected significant dose-dependent increases in PGI2 synthesis compared with control levels of 6-keto PGF1 alpha. In contrast, the G-protein inhibitor GDP beta S, (guanosine 5'-0-2-thiodiphosphate), attenuated alpha-thrombin-mediated prostaglandin generation. Treatment of HUVEC monolayers with pertussis toxin (1 microgram/ml) did not inhibit the PGI2 synthesis stimulated by either alpha-thrombin, NaF, or histamine but catalyzed the ADP ribosylation of a 40 kDa membrane protein which cross-reacted with antisera against a synthetic peptide corresponding to an amino acid sequence common to the alpha-subunit of other G-proteins. Preincubation of HUVEC microsomal membranes with alpha-thrombin diminished pertussis toxin-catalyzed ADP ribosylation in a time-dependent manner. These data suggest that thrombin stimulation of PGI2 synthesis by HUVEC monolayers requires the catalytically functional enzyme and further suggests that the thrombin-occupied receptor is coupled to phospholipase activities by a pertussis toxin-insensitive guanine nucleotide regulatory protein in human endothelial cell membranes.

Inosine monophosphate dehydrogenase inhibitors. Probes for investigations of the functions of guanine nucleotide binding proteins in intact cells

Taken together, the above reports indicate that the IMP dehydrogenase inhibitors are valuable probes for investigation of the biological functions of guanine nucleotides in intact cells. While these agents have minor effects on levels of other nucleotides and enzymes, non-specific effects can be monitored by addition of guanine or guanosine to provide substrates for the salvage pathway of guanine nucleotide synthesis. The most important question yet to be resolved in employing these agents is why incomplete depletion of intracellular guanine nucleotides results in such dramatic effects on G-protein function. Since the level of GTP in resting cells is approximately 0.5 mM, even a 90% reduction in GTP levels should leave enough nucleotide to adequately activate most known G-proteins, as the latter display high binding affinities for guanine nucleotides in cell free systems. Several explanations have been proposed to account for this disparity. Much of the intracellular guanine nucleotide may be bound or compartmentalized and therefore unable to interact with certain G-proteins. Possibly, G-proteins in the intracellular environment possess a much lower affinity for GTP that they do in cell free system. It may be to the cells' advantage that relatively minor fluctuations in levels of GTP result in pronounced alterations in the biological function of G-proteins as this effect may provide a physiologically important mechanism for the regulation of G-proteins in vivo. Further studies are necessary to clarify the mechanisms involved in the regulation of the biological function of G-proteins and oncogene products by guanine nucleotides in intact cells.