Thrombin- and nucleotide-activated phosphatidylinositol 4,5-bisphosphate phospholipase C in human platelet membranes

Decrease in acetylcholine-induced current by neomycin in PC12 cells

The effects of neomycin, one of the aminoglycoside antibiotics, on the acetylcholine (ACh)-induced current (I(ACh)) were studied in pheochromocytoma cells by using the whole-cell clamp technique. The I(ACh) proved to be generated through neuronal nicotinic receptor. ACh (30 microM) induced an inward current at a holding potential of -80 mV. When cells were treated with neomycin (0.01-1 mM) and ACh (30 microM) simultaneously, an inhibitory effect of neomycin on the peak of I(ACh) was found. This effect was fast, reversible, and concentration dependent. Pretreatment with neomycin for 3-8 min had no effect on the inhibition of I(ACh) induced by neomycin. External application of 0.1 mM neomycin neither shifted the dose-response curve of the peak I(ACh) to the right (dissociation constant (K(d)) = 16.5 microM) nor affected its coefficient (1.8) but inhibited the curve amplitudes by approximately 33%. Stimulated protein kinase C activation by using an exogenous activator produced inhibition of I(ACh), while using protein kinase C inhibitor (PKCI 19-31) had no effect on the inhibition of I(ACh) induced by neomycin. These results suggest that neomycin has an inhibitory effect on I(ACh) without the involvement of phospholipase C. It indicates that neomycin binds to a specific site on the cell membrane, probably on the neuronal nicotinic receptor-coupled channel, and inhibits the I(ACh) in a noncompetitive manner, thus controlling the immediate catecholamine release from the sympathetic cells.

Epidermal growth factor induces rapid and transient association of phospholipase C-gamma 1 with EGF-receptor and filamentous actin at membrane ruffles of A431 cells

Addition of epidermal growth factor to A431 cells results in dramatic changes in cell morphology. Initially the cells form membrane ruffles accompanied by increased actin polymerization, followed by cell rounding. Activation of the tyrosine kinase of the receptor by binding epidermal growth factor leads also to phosphorylation and activation of phospholipase C-gamma 1, a key enzyme in the phosphoinositide pathway. In this study we have investigated the localization of phospholipase C-gamma 1 during cell activation by epidermal growth factor. It is shown that addition of the growth factor to A431 cells leads to a translocation of phospholipase C-gamma 1 from the cytosol to the membrane fraction. Interestingly, this relocation is exclusively directed to the membrane ruffles. Most of the phospholipase C-gamma 1 associates to the membrane and a small fraction to the underlying skeleton. Immunocytochemical studies demonstrated that phospholipase C-gamma 1 co-localizes with the epidermal growth factor receptor and also filamentous actin at the membrane ruffles. Moreover, using anti-phosphotyrosine antibodies we found that the membrane ruffles are significantly enriched in phosphotyrosyl proteins. Between 5 and 10 minutes after stimulation the membrane ruffles disappear and also the co-localization of phospholipase C-gamma 1 with the epidermal growth factor receptor and filamentous actin. These results support the notion that activation of A431 cells by epidermal growth factor leads to the formation of a signalling complex of its receptor, phospholipase C-gamma 1 and filamentous actin which is primarily localized at membrane ruffles.

Phospholipase C activates protein kinase C during induction of slow Na current in Xenopus oocytes

Protein phosphorylation by protein kinase C (PKC) has recently been shown to be a key event in the induction of the slow inward Na current observed during sustained depolarization of the Xenopus oocyte membrane. The present work investigates the possible pathways leading to PKC activation. PKC is activated by a series of phospholipid metabolites, such as diacylglycerol (DAG) and arachidonic acid produced by phospholipases C (PLC) and A2 (PLA2) respectively. To test whether PKC activation was dependent upon the phospholipid metabolites produced either by PLC or by PLA2, enzyme activity was reduced using selective inhibitors. Results indicated that inhibition of PLA2 activity and inhibition of the enzymes involved in the arachidonic acid cascade failed to affect Na current amplitude. On the other hand, PLC inhibition caused a marked decrease of Na current amplitude. In another series of experiments, Na current was fully restored, in spite of PLC inhibition, by directly enhancing PKC activity with a powerful activator phorbol 12-myristate 13-acetate. These data strongly suggest that PLC is involved in PKC activation during Na channel induction.

Inhibitors of phospholipid intracellular signaling as antiproliferative agents

The improved understanding of oncogenesis and the involvement of oncogenes and tumor suppressor genes, has led to a rational approach of specific target-directed anti-cancer drug development. Cancer genes have been found to be important not only in the control of cell proliferation but also in the mediation of processes such as drug resistance, metastasis, neo-vascularization (angiogenesis), and apoptosis. These are all important targets in their own right and the development of drugs against specific "upstream" targets in oncogenic or growth factor signal transduction cascades it may be possible to inhibit multiple "downstream" targets. Ultimately, to test the hypothesis that signaling pathways offer good targets for anticancer drug development will take several years of careful clinical study and we cannot say at this time whether the approach will work. There are a small number of compounds in the early stages of clinical development as anticancer agents that may act by inhibiting growth factor signaling pathways. In all cases the activity of the compounds on intracellular signaling pathways was discovered after their identification as antiproliferative agents. There are also compounds in preclinical development that have been specifically developed as inhibitors of growth factor signaling, although their selectivity for tumor cells compared to normal tissue remains to be investigated fully in appropriate animal tumor models. It is possible that a single antisignaling drug by itself may not have the power to completely inhibit tumor growth and a combination of drugs may be needed. It may also take a combination of drugs to prevent the emergence of resistance. Clearly there are several challenges to developing this new class of anticancer drugs, and there will undoubtedly be others that must be faced.

Phosphoinositide hydrolysis by phospholipase C modulated by multivalent cations La(3+), Al(3+), neomycin, polyamines, and melittin

Second messenger production from phosphoinositide hydrolysis is regulated by different pathways, such as G-proteins or tyrosine phosphorylation of phosphoinositide phospholipase C (PI-PLC). Another means of altering the activity of PI-PLC is through cation interaction with the phosphoinositide substrate. A variety of organic and inorganic multi-valent cations were examined for their effects on the activity of purified PI-PLC delta. Surprisingly, the cations produced both stimulation and inhibition of PI-PLC catalyzed phosphoinositide hydrolysis, depending on the substrate and the ion to phosphoinositide stoichiometry. These data support the hypothesis that ionic complexes with phosphoinositides may alter their hydrolysis by PI-PLC.

Signalling pathways as targets for anticancer drug development

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach, it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. In this review, the abilities of some antiproliferative drugs to inhibit signalling targets are considered. There are some anticancer drugs already in clinical trial that may act by inhibiting signalling targets, as well as drugs in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

Inhibitors of phosphatidylinositol signalling as antiproliferative agents

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. Phosphatidylinositol (PtdIns) is a key component of two growth factor signalling pathways. It acts as a substrate for PtdIns specific phospholipase C (PtdInsPLC) and for PtdIns-3-kinase. In this review the antiproliferative properties of some inhibitors of PtdInsPLC and PtdIns-3-kinase are considered. There are some compounds already in clinical trial as anticancer drugs that may act by inhibiting PtdIns signalling, as well as several compounds in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

Thrombin and NaF, but not epinephrine, raise cytosolic free Na+ in human platelets

We have investigated changes in [Na+]i in SBFI-loaded platelets stimulated at 37 degrees C with thrombin, epinephrine, and NaF. Basal [Na+]i was 4.9 +/- 1.3 mM (n = 70). Stimulation of platelets with thrombin (0.1 U/ml) in the presence of 1 mM extracellular Ca2+ rapidly raised [Na+]i by 27.3 +/- 6 mM (n = 16). Part of this increase (approx. 20-30%) is caused by Na+/H+ exchange, the rest is predominantly due to Na+ influx. Epinephrine (20 microM) failed to change [Na+]i both in the absence and presence of fibrinogen. This is in agreement with earlier reports showing that epinephrine also fails to activate Na+/H+ exchange in human platelets. NaF which activates platelets via a direct effect on GTP-binding proteins induced a slow rise in [Na+]i to 9.5 +/- 2.5 mM (n = 4) and 33.0 +/- 3.6 mM (n = 12) at 10 and 20 mM NaF, respectively. This effect was completely blocked by SK&F 96365, a blocker of receptor-mediated Ca2+ entry. Hence, the NaF-induced increase in [Na+]i is exclusively due to the opening of non-selective cation channels. This latter finding agrees with earlier observations which showed that NaF does not induce activation of Na+/H+ exchange in platelets.

Multiple pathways of thrombin-induced platelet activation differentiated by desensitization and a thrombin exosite inhibitor

Recently a thrombin receptor with a unique mechanism of activation was cloned from a megakaryocyte-like cell line (Vu et al., Cell 64:1057-1068, 1991). Thrombin cleaves a portion of this receptor creating a new N-terminus that acts as a "tethered-ligand" to activate the receptor. A thrombin receptor activating peptide (SFLLRNPNDKYEPF) homologous to the new N-terminus was shown to activate platelets. We synthesized this peptide and demonstrated that it desensitized platelets to activation by low concentrations of alpha-thrombin but not gamma-thrombin. We also synthesized a thrombin exosite inhibitor (BMS 180742) that inhibited platelet aggregation induced by low, but not high, concentrations of alpha-thrombin. In contrast, a thrombin active site inhibitor, N alpha-(2-naphthylsulfonyl-glycyl)-D,L-amidinophenylalanylpiperi dide, competitively inhibited thrombin-induced platelet aggregation. We conclude that thrombin-induced platelet activation is mediated by at least two pathways: one activated by low concentrations of alpha-thrombin and blocked by a thrombin exosite inhibitor that appears to be coupled to the "tethered-ligand" thrombin receptor, and another that is stimulated by higher concentrations of alpha-thrombin and by gamma-thrombin and does not require the thrombin exosite for activation. Both pathways are blocked by a thrombin active site inhibitor.

Assay of a phosphatidylinositol bisphosphate phospholipase C activity in postmortem human brain

The activity of a phospholipase C which hydrolyses exogenous phosphatidylinositol-4,5-bisphosphate [( 3H]PtdIns(4,5)P2) in membranes prepared from frozen postmortem human brain and rat brain was investigated. Enzyme characteristics were essentially similar in membranes prepared from frozen postmortem brain and fresh or frozen rat brain. The [3H]PtdIns(4,5)P2 solubilization and assay procedure employed resulted in an efficient availability of the substrate for the enzyme. The non-hydrolysable guanosine triphosphate analogue guanosine 5'-[beta gamma-imido]diphosphate (Gpp[NH]p) stimulated hydrolysis rapidly with a half maximum activity of approximately 25 microM. This stimulation was not specific for guanine nucleotides as ATP, imidodiphosphate and pyrophosphate also caused enzyme activation. However these activation effects could be distinguished by the polyanion spermine. The non-hydrolysable guanine dinucleotide analogue guanosine 5'-[beta-thio]diphosphate acted as a partial agonist thereby inhibiting the stimulatory effect of Gpp[NH]p. Gpp[NH]p-stimulated enzyme activity showed a maximum response in the presence of 1 mM deoxycholate and displayed a pH optima in the range 7.0-7.5. PtdIns(4,5)P2 hydrolysis was observed in the absence of added calcium, but hydrolytic cleavage was inhibited in the presence of divalent ion chelators. Magnesium inhibited PtdIns(4,5)P2 hydrolysis in a concentration-dependent manner. Elucidation of these aspects of the phosphatidylinositol cycle in normal human postmortem brain will permit comparative studies in CNS disease states.

Characterization of multiple forms of phosphoinositide-specific phospholipase C from bovine aorta

Three forms (I, II and III) of phospholipase C were separated from the cytosol of bovine aorta by chromatography on Blue Sepharose. All three forms showed an increase of enzyme activity when free Ca2+ in the assay was raised between 40 microM and 9 mM. The pH optimum was in the range of 6.0 to 6.5 for each subtype. Marked differences in thermostability were found when the three enzyme forms were pre-incubated at 50 degrees C prior to the assay. All three forms were able to hydrolyse phosphatidylinositol as well as phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. In contrast, when phosphatidylcholine was used as substrate, no enzyme activity was observed. Spermine and spermidine, but not putrescine, were able to stimulate form I and III; neomycin sulphate inhibited all three subtypes.

Receptor regulation of phosphoinositidase C

Numerous hormones, neurotransmitters and growth factors regulate intracellular events by acting at cell surface receptors which are coupled to the generation of inositol phospholipid-derived intracellular messengers. Receptors trigger the hydrolysis of inositol phospholipids by activating phosphoinositidase C (PIC) enzymes. At least four families of genes encode structurally distinct PIC enzymes and it is likely that distinct PIC isoenzymes participate in different pathways of signal transduction. Two different modes of receptor regulation have been identified and these involve distinct PIC isoenzymes. In the first of these, PIC-gamma is a substrate for growth factor receptor protein-tyrosine kinases. The second of these pathways involves PIC-beta plus other isoenzymes whose activities are regulated by G proteins in response to agonist binding to G protein-linked receptors. At least two types of G proteins regulate PIC activity and each may control the activity of different PIC isoenzymes.

Activation of phospholipase C in rabbit brain membranes by carbachol in the presence of GTP gamma S; effects of biological detergents

Rabbit brain cortical membranes incubated with carbachol in the presence of GTP gamma S show a marked increase in the degradation of exogenous phosphatidylinositol 4,5-bisphosphate. This activation of phospholipase C is dependent on the presence of deoxycholate and maximal at 0.8-1 mM deoxycholate. There is negligible activation by carbachol alone but in the presence of GTP gamma S a carbachol effect can be readily demonstrated. Optimal activation of phospholipase C by carbachol was seen at 10 to 100 nM free Ca2+. Washing cortical membranes with hypertonic buffer extracted 60% of the membrane protein yet the carbachol and GTP gamma S coupling remained intact. Incubation of the membranes with lysophosphatidylcholine, Nonidet P-40, sodium deoxycholate or digitonin at concentrations considerably less than those frequently used to solubilize membrane proteins abolished the carbachol response. Octyl glucoside and sodium cholate also uncoupled receptor regulation of phospholipase C but only at concentrations where solubilization of membrane proteins occurred. Prior exposure of membranes to carbachol did not prevent the uncoupling observed as a result of detergent treatment. Incubation of the membranes with carbachol and GTP gamma S did not appear to be accompanied by specific release of either active phospholipase C or inhibitors of phospholipase C activity.

The common pathway for alpha- and gamma-thrombin-induced platelet activation is independent of GPIb: a study of Bernard-Soulier platelets

The responses to alpha- and gamma-thrombin were studied in normal and Bernard-Soulier platelets labelled with [32P]phosphate, to investigate the relationship between thrombin binding to the platelet membrane glycoprotein Ib (GPIb) and thrombin-induced platelet activation. For this purpose we conducted parallel studies of the kinetics of platelet aggregation, granule secretion, hydrolysis of polyphosphoinositides, formation of phosphatidic acid, phosphorylation of the myosin light chain (p20) and of the 43 kDa protein (p43), and thromboxane B2 formation. Like alpha-thrombin, gamma-thrombin activated control platelets via all the above metabolic responses, but only after a prolonged lag. In Bernard-Soulier platelets, alpha-thrombin induced polyphosphoinositide hydrolysis and phosphatidic acid formation, p20 and p43 phosphorylation, thromboxane B2 formation, secretion and to a lesser extent aggregation, but only after a prolonged lag. The metabolic responses of Bernard-Soulier platelets to gamma-thrombin were very similar to those of control platelets. We have previously showed that GPIb which is not present in Bernard-Soulier platelets binds alpha- but not gamma-thrombin. The present results indicate that thrombin binding to GPIb is not directly coupled either with the activation of phospholipase C specific to polyphosphoinositides, or with the activation of protein kinase C and phospholipase A2. However, thrombin binding to GPIb appears to promote an early mechanism which accelerates all the platelet responses.

Second messengers in thrombin-stimulated bone resorption

Characterized human thrombins and two commercial bovine thrombin preparations were examined for their effects on bone resorption and on the cyclic AMP and phosphoinositide second messenger systems in bone. Human alpha- and gamma-thrombins, as well as both bovine thrombin preparations, stimulated bone resorption in vitro, whereas catalytically inactivated human diisopropylfluorophosphate (DIP)-alpha-thrombin did not significantly stimulate resorption. Human alpha-thrombin and a commercial bovine thrombin preparation increased cyclic AMP production in fetal rat limb bones, but another bovine commercial thrombin preparation and gamma-thrombin did not. Except for DIP-alpha-thrombin, all thrombins increased production of inositol phosphates in fetal rat limb bones at concentrations that stimulated resorption. In time course studies, bovine thrombin increased label in inositol trisphosphate at 30 s, with decreasing effects at later times. Inositol monophosphate increased progressively over 30 min. Our results are consistent with thrombin-stimulated bone resorption being mediated at least partially through the inositol phosphate pathway.

Persistent activation of platelet membrane phospholipase C by proteolytic action of trypsin and thrombin

Trypsin causes rapid activation of intact platelets that mimics many actions of thrombin, including the stimulation of phospholipase C (PLC). We have examined the effects of thrombin and trypsin on PLC in a platelet membrane preparation using exogenous [3H]-phosphatidylinositol 4,5-bisphosphate (PIP2) as substrate. Trypsin induced PIP2 breakdown, which was maximal at 20 micrograms/ml, but was reduced at higher concentrations. alpha- and gamma-Thrombins also stimulated PLC-induced hydrolysis of PIP2 in membranes. This effect was inhibited by leupeptin. Exogenous [3H]phosphatidylinositol 4-monophosphate (PIP) was hydrolyzed in response to both thrombin and trypsin in the same ratio as PIP2. Activation of membrane-bound PLC persisted after removal of thrombin and trypsin. The hydrolysis of [3H]phosphatidylinositol was not activated by alpha-thrombin and trypsin. We examined the question of whether calpain was involved in the observed PLC activation by thrombin and trypsin. Although dibucaine activated a Ca2(+)-dependent protease as judged by the hydrolysis of actin-binding protein and by the activation of phosphoprotein phosphatases, it failed to stimulate the generation of phosphatidic acid in 32P-prelabeled platelets. Moreover, when PLC was assayed in the membranes, the addition of Ca2(+)-activated neutral proteinases did not increase the rate of hydrolysis of either PIP or PIP2. Our results show that proteases such as trypsin and thrombin are able to stimulate membrane-bound PLC, but this activation does not seem to be related to calpain.

Pertussis toxin-sensitive GTP-binding proteins may regulate phospholipase A2 in response to thrombin in rabbit platelets

Incubation of rabbit platelets with thrombin resulted in rapid accumulations of inositol trisphosphate (IP3) in [3H]inositol-labeled platelets, increases of [3H]arachidonic acid [( 3H]AA) release, and [3H]serotonin secretion from the platelets prelabeled with these labeled compounds. The experiments using phospholipase A2 or C inhibitor suggested that not only phospholipase C but also phospholipase A2 activity plays an important role in serotonin secretion. We then studied the regulatory mechanisms of phospholipase A2 activity. Guanosine 5'-(3-O-thio)triphosphate (GTP gamma S), guanyl-5'-(beta,gamma-iminio)triphosphate), or AlF4- caused a significant liberation of AA in digitonin-permeabilized platelets but not in intact platelets. Thrombin-stimulated AA release was not observed in permeabilized platelets, whereas thrombin acted synergistically with GTP or GTP analogs to stimulate AA release. GTP analog-stimulated AA release was inhibited by guanosine 5'-(2-O-thio)diphosphate) and was also inhibited by decreased Mg2+ concentrations. Thrombin-induced, GTP-dependent AA release, but not IP3 formation, was diminished by 100 ng/ml of pertussis toxin, associated with ADP-ribosylation of membrane 41-kDa protein(s). Thrombin-stimulated AA release from intact platelets and GTP gamma S-stimulated release from permeabilized platelets were both markedly dependent on Ca2+. However, Ca2+ addition could not enhance AA release without GTP gamma S even when Ca2+ was increased up to 10(-4) M in permeabilized platelets. The results show that thrombin-stimulated AA release from rabbit platelets is mainly mediated by phospholipase A2 activity, not by phospholipase C activity, and that Ca2+ is an important factor to the activation of phospholipase A2 but is not the sole factor to the regulation. GTP-binding protein(s) is involved in receptor-mediated activation of phospholipase A2.

Characterization of GTP-gamma-S binding to isolated human platelet plasma membranes and its relationship with the stimulation of a phospholipase C activity

Binding parameters for the interaction of GTP-gamma-[35S] with isolated platelet plasma membranes have been studied. Analysis of the data by a non-linear curve fitting program indicates that the interaction can be satisfactory described by a model with a single, high affinity binding site (Kd = 0.3 +/- 0.07 microM and Bm = 0.4 +/- 0.2 nmoles of GTP-gamma-S/mg of membrane protein). Binding is selectively inhibited by GDP-beta-S and GMP-PNP (1 microM), but not affected by ATP, CTP, ITP, or UTP, even at mM concentration. Optimal conditions for the interaction were 30 degrees C and pH 8.0. Incubation of the isolated membranes with GTP-gamma-S results in a measurable phospholipase C activity (as detected both by a breakdown of phosphoinositides and an increase of inositide phosphates) which under our experimental conditions is only slightly enhanced by addition of cytosolic proteins. Our results indicate that platelet plasma membranes contain all the necessary elements for signal transduction through the diacylglycerol/inositolphosphates pathway.

Mammalian phosphoinositide-specific phospholipase C isoenzymes

Procaryotic and eucaryotic cells have evolved multiple pathways for communication with their external environment. The inositol 1,4,5-trisphosphate/diacylglycerol second messenger system is an example of such a signal transduction pathway which is present in multicellular eucaryotic organisms. Binding of an agonist to a specific cell surface receptor promotes rapid hydrolysis of phosphatidylinositol 4,5-bisphosphate. The pivotal enzyme for this second messenger system is phosphoinositide-specific phospholipase C which hydrolyzes phosphatidylinositol 4,5-bisphosphate to generate the two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. Recently, much progress has been made in the purification, characterization and cDNA cloning of multiple PI-PLC isoenzymes. The results of the recent studies on phosphoinositide-specific phospholipase C are reviewed.

GTP-dependent hydrolysis of phosphatidylinositol-4,5-bisphosphate by soluble phospholipase C from adult human epidermis

The regulation of soluble phosphoinositide-specific phospholipase C from adult human epidermis by guanine nucleotide was investigated. In the presence of physiologic concentrations of Ca++ (1 microM) and Mg++ (1.5 mM), neither phosphatidylinositol (PI) nor phosphatidylinositol-4,5-bisphosphate (PIP2) were appreciably hydrolyzed. Addition of guanosine-5'-triphosphate (GTP) or guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S) significantly stimulated hydrolysis of PIP2, but not PI. Stimulation of PIP2 hydrolysis by GTP was dose-dependent between 1-100 microM GTP. Other nucleoside triphosphates and nucleotide analogues were unable to substitute for GTP or GTP-gamma-S. A GTP-gamma-S-stimulated PIP2 hydrolysis was inhibited by guanosine-5'-O-(2-thiodiphosphate (GDP-beta-S). The phospholipase C preparation specifically bound [35S]GTP-gamma-S and this binding was also inhibited by GDP-beta-S. In addition to a 41,000-dalton pertussis toxin substrate, the phospholipase C preparation contained 3-4 GTP binding proteins with molecular weights between 20,000-30,000. These data demonstrate that human epidermis contains a soluble GTP-dependent phospholipase C activity that specifically hydrolyzes PIP2 and suggest that this reaction is regulated by a GTP-binding protein(s).

Characterization of phospholipase C activity of the plasma membrane and cytosol of an osteoblast-like cell line

The properties of phospholipase C (PL-C) in the plasma membranes (PM) and the cytosol of osteoblast-like osteosarcoma cells, UMR-106, were analyzed to see if separate enzymes or similar enzymes were involved in signalling, transduction, and arachidonate release. The cytosolic PL-C displayed substrate affinities in the order of phosphatidylinositol (PI) greater than phosphatidylinositol-4-phosphate (PIP) or phosphatidylinoisitol-4, 5-bisphosphate (PIP2). Hydrolysis of PI, PIP, and PIP2 by cytosolic PL-C was not affected by GTP or GTP gamma S and other nucleotides. PI hydrolysis by PM and cytosolic PL-C was undetectable in the presence of 500 microM EGTA and displayed two activity plateaus at various concentrations of Ca2+. The Km for Ca2+ in the PL-C activity of the first plateau was 0.08 microM. Significant hydrolysis of PIP2 by cytosolic PL-C was observed in the absence of Ca2+. In contrast to the enzyme(s) predominant in the cytosol, the order of substrate affinities for PM PL-C was PIP2 greater than PIP greater than PI. Only PIP2 hydrolysis by PM PL-C was stimulated by both GTP and GTP gamma S in a dose-dependent manner. PIP2 hydrolysis by PL-C of the PM was not observed in the absence of Ca2+, serving to further discriminate this enzyme activity from that of the cytosol. PIP2 hydrolysis by PL-C of the PM also was biphasic in the dependence on Ca2+. At resting cytosolic Ca2+ levels, the Vmax of the high affinity activity already had been achieved. Guanine nucleotide stimulation of PIP2 hydrolysis by PM PL-C was characterized by increased maximum activity with an unchanged Km for Ca2+ or for PIP2. The pH optimum of PIP2 hydrolysis was similar between cytosolic and PM forms of PL-C. PIP2 hydrolysis with production of IP3 (PL-C activity) in UMR-106 cells treated with [2-3H]-myoinositol was stimulated by PTH, and this stimulation was not inhibited by pertussis toxin. These data suggest that UMR-106 cells possess at least two distinct PL-C activities, one predominant in the cytosol and activated by increasing cytosolic Ca2+ with PI as the substrate. The second enzyme, a GTP-activated PIP2-specific PL-C in the plasma membranes may play an important role in hormone-induced PIP2 hydrolysis mediated through guanine nucleotide regulatory proteins and may participate in the hormonal regulation of osteoblast cytosolic Ca2+ and bone remodeling functions.

Binding of neomycin to phosphatidylinositol 4,5-bisphosphate (PIP2)

Schacht (Schacht, J. (1976) J. Neurochem. 27, 1119-1124) demonstrated that neomycin, an aminoglycoside antibiotic, binds with high affinity to phosphatidylinositol 4,5-bisphosphate (PIP2). We investigated the binding of neomycin to PIP2 by making electrophoretic mobility measurements with multilamellar bilayer vesicles and surface potential measurements with monolayers. The bilayers and monolayers were formed from mixtures of PIP2 and egg phosphatidylcholine (PC) in 0.1 M KCl at pH 7. Neomycin does not bind to PC; 10(-3) M neomycin affects neither the zeta potential of PC vesicles nor the surface potential of PC monolayers. In contrast, 10(-6) M neomycin reduces the magnitude of the zeta potential of PC/PIP2 vesicles (5, 9, and 17 mol% PIP2) and the surface potential of monolayers (17 mol% PIP2) to less than 50% of their initial values. The electrophoretic mobility results indicate that neomycin forms an electroneutral complex with PIP2; high concentrations (greater than 10(-4) M) of neomycin reduce the zeta potential of the PC/PIP2 vesicles to zero. We could describe our data with the Gouy-Chapman-Stern theory assuming the intrinsic association constant of the 1:1 neomycin-PIP2 complex is 10(5) M-1. Neomycin is widely used in cell biology to interfere with the generation of second messengers; we discuss the relevance of our results to these studies. Specifically, 10(-6) M neomycin binds greater than 50% of the PIP2 in a bilayer or monolayer but 10(-5)-10(-3) M neomycin is required to affect the turnover of PIP2 in permeabilized platelets, mast cells, and sea urchin eggs. This result is consistent with a hypothesis that most of the PIP2 in the inner leaflet of these plasma membranes is not accessible to neomycin because it is associated with proteins.

Stimulation of phosphatidylinositol 4,5-bisphosphate phospholipase C activity by phosphatidic acid

Phosphatidic acid was a potent activator of the phosphatidylinositol 4,5-bisphosphate (PtdIns-P2) phospholipase C activity associated with human platelet membranes. Lysophosphatidic acid was half as active as phosphatidic acid, and shortening the fatty acid chain reduced the effectiveness of the corresponding phosphatidic acid. Compounds lacking either the phosphate group (diacylglycerol or phorbol ester) or the fatty acid (glycerol phosphate) were not activators. When the negative charge was contributed by a carboxyl group (fatty acid or phosphatidylserine), stimulation of phospholipase C was weak but detectable. Structural analogs of phosphatidic acid (lipopolysaccharide, lipid A, and 2,3-diacylglucosamine 1-phosphate) were less effective but also enhanced PtdIns-P2 hydrolysis. Phosphatidic acid potentiated the activation of phospholipase C by alpha-thrombin, chelators, and guanine nucleotides. Phosphatidylinositol 4-phosphate and PtdIns-P2 were also effective activators of PtdIns-P2 degradation. Other phospholipids were without effect. The production of inositol 1,4,5-trisphosphate and diacylglycerol via the activation of phospholipase C provides a rationale for the cellular responses evoked by phosphatidic acid and the ability of this phospholipid to potentiate and initiate hormonal responses.

Inhibition by cyclic AMP of guanine nucleotide-induced activation of phosphoinositide-specific phospholipase C in human platelets

Phosphoinositide-specific phospholipase C (PLC) activity of human platelet membranes was activated by the nonhydrolyzable guanine nucleotide GTP gamma S. This activation did not occur in either membranes prepared from dibutyryl cyclic AMP-pretreated platelets (A-membranes) or those prepared from untreated cells and subsequently incubated with cyclic AMP (cAMP) (B-membranes). This cAMP-mediated inhibition was abolished in the presence of inhibitors of cAMP-dependent protein kinase (A-kinase), suggesting that the inhibition was due to phosphorylation of (a) protein component(s). No significant differences were observed in the basal PLC activity and the extent of pertussis toxin-catalyzed ADP-ribosylation among control membranes and the two types of phosphorylated membranes (A- and B-membranes). GTP-binding activities of Gs, Gi and GTP-binding proteins of lower molecular masses were not altered by the phosphorylation of the membranes. These findings suggest that a GTP-binding protein is involved in the GTP gamma S-mediated activation of PLC and that cAMP (plus A-kinase) inhibits this activation by phosphorylating a membrane protein (probably a 240-kDa protein), rather than the GTP-binding protein or PLC itself. It is likely that this phosphorylation uncouples the GTP-binding protein from PLC.

Purification and characterization of phosphatidylinositol-specific phospholipase C from bovine spermatozoa

1. The distribution of phosphatidylinositol3, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate hydrolysis or phosphatidylinositol-specific phospholipase C (PI-PLC), activity in the bull reproductive system showed the highest specific activity in the isolated spermatozoa (SZ) followed by testis and different epididymal segments. Both the head and tail fractions of SZ were active. 2. The optimal solubilization of the enzyme from SZ was obtained with 0.2% Triton X-100 or at 0.05% detergent concentration when combined with a 60 sec sonication. The sucrose gradient centrifugation showed that PI-PLC was enriched in membrane fraction distinct from mitochondria and acrosomes. 3. The enzyme was purified by ammonium sulphate precipitation and fractionations by hydrophobic interaction chromatography, gel filtration, Con A-Sepharose affinity and chromatofocusing columns. The purified enzyme was able to hydrolyse all phosphatidylinositol substrates with optimum at pH 7.0 and activation by Ca2+, Cd2+ and Mn2+ but not phospholipids lacking the inositol residue. 4. In PAGE (8-25% gradient) the purified (aggregated) enzyme did not enter the gel. In SDS-PAGE two closely located bands were found with Mr-values of 15,000 and 18,000. Isoelectric focusing showed a wide band at pl 4.5-5.1. 5. Gel filtration resulted in a broad elution peak indicating multiple molecular forms (aggregates); the basic form had an apparent molecular weight of 100,000. The binding of the enzyme to Con A-Sepharose indicated that the enzyme is a glycoprotein.

Protein kinase C is not involved in secretion by permeabilized human neutrophils

The generally accepted sequence of intracellular signal transduction involves: (1) cell surface receptor-ligand interactions; (2) activation of G-proteins; (3) activation of phospholipase C, leading to inositol phosphate (IP3), and diacylglycerol production; (4) parallel mobilization of intracellular Ca2+ by IP3, and; (5) activation of protein kinase C (PKC) by diacylglycerol and Ca2+, leading to; (6) cellular responses. Human neutrophils appear to utilize this cascade, at least in general, and some, but not all, elements of the intracellular signal cascade known to be operating in intact cells also function in permeabilized cell systems. We have previously shown that permeabilized neutrophils can be induced to secrete lysosomal enzymes in response to elevated levels of Ca2+ alone and this secretion can be synergistically enhanced by the presence of guanine nucleotides. We now show that Ca2+, in the presence and absence of guanine nucleotides, can stimulate the production of soluble inositol phosphates. Furthermore, neomycin, a putative inhibitor of phospholipase C, can block Ca2(+)-induced secretion. These data thus suggest a role for phospholipase C activity or its products in the transduction process. The next enzymatic activity 'downstream' is PKC. Consequently, we looked at the role Mg-ATP, one of the substrates of PKC, plays in degranulation by permeabilized neutrophils, We found no obligatory role for this nucleotide in the secretory process. We then looked at the activity of oleoyl-acetyl-glycerol (OAG), a synthetic diacylglycerol and PKC agonist, on degranulation. We found that OAG was largely additive with Ca2+. Another PKC agonist, phorbol myristate acetate (PMA), also did not display notable synergy. Finally, inhibitors of PKC activity were not capable of blocking secretion, either in the presence or absence of guanine nucleotides. Thus, while circumstantial evidence seems to point towards a requirement for phospholipase C activation and diacylglycerol production in secretion, we were unable to demonstrate the next putative step in signal transduction, namely activation of PKC.

Effects of phosphoinositides on calcium movements in human platelet membrane vesicles

In a mixed endoplasmic and surface-type membrane vesicle preparation from human platelets the polyphosphoinositides PIP and PIP2, similarly to IP3, were found to induce a rapid calcium release reaction. At physiological (resting) cytoplasmic calcium concentrations (0.1-0.3 microM) the PIP2 and IP3 concentrations producing half-maximum calcium release were similar (0.7 microM) and both agents could mobilize about 30-40% of the intravesicular calcium. However, the phosphodiesteric degradation of PIP2 in the membrane vesicles was found to be negligible and the ion- and drug-sensitivities of the calcium release reactions were different. The IP3-induced calcium release was selectively inhibited by micromolar calcium concentrations and by cinnarizine, while the PIP2-induced release was blocked by magnesium ions and neomycin. The calcium release evoked by either agent was inhibited by low concentrations of lanthanum but, in contrast to the ATP-dependent calcium pump, it was insensitive to vanadate, quercetin and to the lowering of the incubation temperature. When added simultaneously or in a rapid succession, maximum effective IP3 and PIP2 concentrations produced an additive calcium release reaction. Based on these data we suggest that IP3 and PIP2, respectively, induce rapid transmembrane calcium movements involving different transport pathways and/or membrane calcium pools, which are not related to the active calcium transport systems.

Lithium stimulation of membrane-bound phospholipase C from PC12 cells exposed to nerve growth factor

LiCl stimulated the formation of inositol monophosphate in PC12 cells that had been exposed to nerve growth factor (NGF) for 4-5 days. Half-maximal accumulation was observed at approximately 8 mM LiCl. Stimulation of formation of inositol bisphosphate plus inositol trisphosphate was half-maximal at approximately 1 mM LiCl. With membranes isolated from PC12 cells differentiated with NGF, the hydrolysis of added phosphatidylinositol 4,5-bisphosphate (PIP2) was stimulated by LiCl in a biphasic manner, with the first stimulation half-maximal at approximately 0.7 mM and the second half-maximal at approximately 15 mM LiCl. The apparent Km for PIP2 was lowered in the presence of 1.1 mM LiCl from approximately 200 to approximately 70 microM. Membranes from cells grown in the absence of NGF did not respond to LiCl. Although observations with intact cells are difficult to interpret without ambiguity, the results obtained with isolated membranes support our interpretation of the stimulatory action of lithium in the intact PC12 cells.

Effect of harvesting methods, growth conditions and growth phase on diacylglycerol levels in cultured human adherent cells

The cellular mass of sn-1,2-diacylglycerols, which are intracellular second messengers which activate protein kinase C, were quantitatively determined with an enzymatic assay. The method employed to harvest cultured human skin fibroblasts or human epidermal A431 cells prior to extraction of lipid into chloroform/methanol affected diacylglycerol (DAG) levels. Scraping or trypsinization significantly increased DAG levels. A method was devised to allow reliable and reproducible DAG measurements from adherent cells. The addition of methanol prior to scraping was shown to stop cellular metabolism and to permit accurate quantitation. Importantly, this solvent was compatible with cultures grown on plastic. Using this method, growth conditions which could affect DAG levels were investigated. Changes in the osmolality of the culture medium did not affect the DAG levels of A431 cells; exposure of A431 cells to acidic pH or elevated temperature lowered DAG levels. In contrast to fibroblasts, the total DAG levels of A431 cells continued to increase during serum deprivation. The highest DAG levels, normalized to phospholipids, were observed during the exponential growth phase. This ratio dropped when the cultures reached confluency. These experiments also demonstrated that A431 cells possess higher DAG levels than do normal fibroblasts. The function of DAG in cellular regulation is discussed.

Effect of phorbol 12-myristate 13-acetate on collagen-induced signal transduction in rabbit platelet

Investigations were made on the inhibitory effect of phorbol 12-myristate 13-acetate (PMA), a powerful activator on protein kinase C, on collagen-induced signal transduction in washed rabbit platelets. Upon activation of the platelets with a low-dose of collagen (5 micrograms/ml), which was suppressed by 10 microM indomethacin, pretreatment of the platelets with 2 nM PMA caused prolongation of lag phase (2 min) before the onsets of the aggregation and ATP secretion as compared with PMA-untreated platelets (30 sec). Under this condition, appearance of the cell responses including the phosphatidic acid formation, thromboxane (Tx) generation and Ca2+-influx was similarly retarded for 2-3 min, whereas arachidonic acid liberation from the membrane phospholipids was not significantly affected by the PMA pretreatment. After such lag phase, every response appeared rapidly and reached almost the control value (without PMA). Upon activation of the same platelets with a high-dose of collagen (50 micrograms/ml), which was only half suppressible by indomethacin, PMA in the presence of indomethacin almost completely suppressed the phosphatidic acid formation as well as the aggregation and ATP secretion. Thus, our results suggest that collagen-platelet interaction may elicit direct activation of phospholipase A2 and C, and that the latter enzyme activation may be regulated by a negative effect of protein kinase C. However, the phospholipase A2 activation may be regulated by a mechanism independent of such effect. In PMA-pretreated platelets in response to a low-dose of collagen, the prolonged lag phase for aggregation appears to be due to impaired conversion of liberated arachidonic acid to TxA2.