5-Hydroxytryptamine stimulates inositol phosphate production in a cell-free system from blowfly salivary glands. Evidence for a role of GTP in coupling receptor activation to phosphoinositide breakdown

Role of Gi proteins in the regulation of blood pressure and vascular remodeling

Heterotrimeric guanine nucleotide regulatory proteins (G-proteins) through the activation of several signaling mechanisms including adenylyl cyclase/cAMP and phospholipase C (PLC)/phosphatidyl inositol (PI) turnover. regulate a variety of cellular functions, including vascular reactivity, proliferation and hypertrophy of VSMC. Activity of adenylyl cyclase is regulated by two G proteins, stimulatory (Gsα) and inhibitory (Giα). Gsα stimulates adenylyl cyclase activity and increases the levels of cAMP, whereas Giα inhibits the activity of adenylyl cyclase and results in the reduction of cAMP levels. Abnormalities in Giα protein expression and associated adenylyl cyclase\cAMP levels result in the impaired cellular functions and contribute to various pathological states including hypertension. The expression of Giα proteins is enhanced in various tissues including heart, kidney, aorta and vascular smooth muscle cells (VSMC) from genetic (spontaneously hypertensive rats (SHR)) and experimentally - induced hypertensive rats and contribute to the pathogenesis of hypertension. In addition, the enhanced expression of Giα proteins exhibited by VSMC from SHR is also implicated in the hyperproliferation and hypertrophy, the two key players contributing to vascular remodelling in hypertension. The enhanced levels of endogenous vasoactive peptides including angiotensin II (Ang II), endothelin-1 (ET-1) and growth factors contribute to the overexpression of Giα proteins in VSMC from SHR. In addition, enhanced oxidative stress, activation of c-Src, growth factor receptor transactivation and MAP kinase/PI3kinase signaling also contribute to the augmented expression of Giα proteins in VSMC from SHR. This review summarizes the role of Giα proteins, and the underlying molecular mechanisms implicated in the regulation of high blood pressure and vascular remodelling.

Resveratrol prevents the development of high blood pressure in spontaneously hypertensive rats through the inhibition of enhanced expression of Giα proteins

Resveratrol (RV), a polyphenolic component of red wine, has been shown to attenuate high blood pressure (BP) in spontaneously hypertensive rats (SHRs). We previously found that the enhanced expression of Giα proteins plays a role in the pathogenesis of hypertension in SHRs. In the present study, we investigated whether this RV-induced decrease in BP in SHRs can be attributed to the ability of RV to inhibit the enhanced expression of Giα proteins and the upstream signaling molecules implicated in the overexpression of Giα proteins. Administration of RV (50 mg/kg per day) to prehypertensive 2-week-old SHRs for 6 weeks prevented the development of high BP and inhibited the enhanced expression of Giα proteins, the enhanced levels of superoxide anion (O2−) and NADPH oxidase activity, the enhanced activation (phosphorylation) of c-Src and growth factor receptors, as well as the enhanced levels of extracellular signal-regulated kinase 1/2 (ERK1/2) and protein kinase B (Akt) exhibited by vascular smooth muscle cells isolated from SHRs. In conclusion, these results indicate that RV attenuates the development of high BP in SHRs through the inhibition of enhanced levels of Giα proteins, oxidative stress, and the upstream signaling molecules that contribute to the overexpression of Giα proteins. These findings suggest that RV could potentially be used as a therapeutic agent in the treatment of cardiovascular complications including hypertension.

Knockdown of Inhibitory Guanine Nucleotide Binding Protein Giα-2 by Antisense Oligodeoxynucleotides Attenuates the Development of Hypertension and Tachycardia in Spontaneously Hypertensive Rats

BACKGROUND

We previously showed that the levels of both Giα-2 and Giα-3 proteins were augmented in spontaneously hypertensive rats (SHRs) before the onset of hypertension. In addition, intraperitoneal injection of pertussis toxin, which inactivates both Giα proteins, prevented the development of hypertension in SHRs. The aim of the present study was to determine the specific contributions of Giα-2 and Giα-3 proteins to the development of hypertension.

METHODS AND RESULTS

Antisense oligodeoxynucleotide of Giα-2 and Giα-3 encapsulated in PEG/DOTAP/DOPE cationic liposomes were administrated intravenously into 3-week-old prehypertensive SHRs and Wistar Kyoto rats, whereas the control Wistar Kyoto rats and SHRs received PBS, empty liposomes, or sense. The knockdown of Giα-2 but not Giα-3 protein attenuated tachycardia and prevented the development of hypertension up to age 6 weeks; thereafter, blood pressure started increasing and reached the same level as that of untreated SHRs at 9 weeks. Furthermore, Giα-2 and Giα-3 antisense oligodeoxynucleotide treatments significantly decreased the enhanced levels of Giα-2 and Giα-3 proteins, respectively, and enhanced levels of superoxide anion and NADPH oxidase activity in heart, aorta, and kidney and hyperproliferation of vascular smooth muscle cells from SHRs aged 6 weeks. In addition, antisense oligodeoxynucleotide treatment with Giα-2 but not Giα-3 restored enhanced inhibition of adenylyl cyclase by oxotremorine to WKY levels.

CONCLUSIONS

These results suggested that the enhanced expression of Giα-2 but not Giα-3 protein plays an important role in the pathogenesis of hypertension and tachycardia in SHRs.

Calcineurin activity augments cAMP/PKA-dependent activation of V-ATPase in blowfly salivary glands

We have examined the role of the Ca(2+)-dependent protein phosphatase 2B (calcineurin) in the regulation of the vacuolar H(+)-ATPase (V-ATPase) in blowfly salivary glands. In response to the neurohormone serotonin [5-hydroxytryptamine (5-HT)] and under the mediation of the cAMP/PKA signaling pathway, the secretory cells assemble and activate V-ATPase molecules at the apical membrane. We demonstrate that the inhibition of calcineurin activity by cyclosporin A, by FK-506, or by prevention of the elevation of Ca(2+) diminishes the 5-HT-induced assembly and activation of V-ATPase. The effect of calcineurin on V-ATPase is mediated by the cAMP/PKA signaling pathway, with calcineurin acting upstream of PKA, because 1) cyclosporin A does not influence the 8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphate (8-CPT-cAMP)-induced activation of V-ATPase, and 2) the 5-HT-induced rise in cAMP is highly reduced in the presence of cyclosporin A. Moreover, a Ca(2+) rise evoked by the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) inhibitor cyclopiazonic acid leads to an increase in intracellular cAMP concentration and a calcineurin-mediated PKA-dependent activation of V-ATPase. We propose that calcineurin activity mediates cross talk between the inositol 1,4,5-trisphosphate/Ca(2+) and the cAMP/PKA signaling pathways, thereby augmenting the 5-HT-induced rise in cAMP and thus the cAMP/PKA-mediated activation of V-ATPase.

Biochemistry and molecular biology of receptors for biogenic amines in locusts

The biochemistry and molecular biology of biogenic amines and their metabotropic receptors in insects, with a focus on locusts, is reviewed. These compounds are known to be responsible for the control of a huge variety of different behaviours. Receptors for these amines usually belong to the class of G-protein coupled receptors (GPCR) and transmit all known functions of these compounds. The physiological significance of biogenic amine neurotransmission in insects, especially in locusts is briefly summarised. Regarding the corresponding receptors, their pharmacological features and the molecular properties are described in detail.

Affinity purification of pancreastatin receptor-Gq/11 protein complex from rat liver membranes

Pancreastatin, a chromogranin A derived peptide, exerts a glycogenolytic effect on the hepatocyte. This effect is initiated by binding to membrane receptors which are coupled to pertussis toxin insensitive G proteins belonging to the Gq/11 family. We have recently solubilized active pancreastatin receptors from rat liver membranes still functionally coupled to G proteins. Here, we have purified pancreastatin receptors by a two-step procedure. First, pancreastatin receptors with their associated Gq/11 regulatory proteins were purified from liver membranes by lectin absorption chromatography on wheat germ agglutinin immobilized on agarose. A biotinylated rat pancreastatin analog was tested for binding to liver membranes before using it for affinity purification. Unlabeled biotinylated rat pancreastatin competed for 125I-labeled [Tyr0]PST binding to solubilized receptors with a Kd = 0.27 nM, comparable to that of native pancreastatin. The biotinylated analog was immobilized on streptavidin-coated Sepharose beads and used to further affinity purify wheat germ agglutinin eluted receptor material. Specific elution at low pH showed that the receptor protein was purified as an 80-kDa protein in association with a G protein of the q/11 family, as demonstrated by specific immunoblot analysis. The specificity of the receptor band was assessed by chemical cross-linking of the purified material followed by SDS-PAGE and autoradiography. In conclusion, we have purified pancreastatin receptor as a glycoprotein of 80 kDa physically associated with a Gq/11 protein.

The role of inositol 1,4,5-trisphosphate 5-phosphatase in inositol signaling in the CNS of larval Manduca sexta

Production of inositol 1,4,5-trisphosphate (IP3) in cells results in the mobilization of intracellular calcium. Therefore, the dynamics of IP3 metabolism is important for calcium dependent processes in cells. This report investigates the coupling of mAChRs to the inositol lipid pathway in the CNS of the larval Manduca sexta. Stimulation of intact abdominal ganglia prelabeled with [3H]-inositol using a muscarinic agonist, oxotremorine-M (oxo-M), increased total inositol phosphate levels in a dose dependent manner (EC50 = 4.23 microM). These inositol phosphates consisted primarily of inositol 1,4-bisphosphate (IP2) and inositol monophosphate (IP1). Similarly, when nerve cord homogenates were provided with [3H]-phosphatidylinositol 4,5-bisphosphate ([3H]-PIP2) (10-13 microM) the predominant products were IP2 and IP1. In contrast, incubation of purified membranes with 1 mM oxo-M in the presence of 100 microM GTP gamma S and [3H]-PIP2 increased IP3 levels, suggesting that the direct activation of phospholipase C (PLC) by mAChRs occurs in a membrane delimited process. Together, these results suggest that in the intact nerve cord and in crude homogenates, a cytosolic 5-phosphatase quickly metabolizes IP3 to produce to IP2 and IP1. This enzyme was kinetically characterized using IP3 (Km = 43.7 microM, Vmax = 864 pmoles/min/mg) and IP4 (Km = 0.93 microM; Vmax = 300pmoles/min/mg) as substrates. The enzyme activity can be potently inhibited by two IP thiol compounds; IP3S3 (1,4,6) and IP3S3 (2,3,5), that show complex binding kinetics (Hill numbers < 1) and can distinguish different forms of the 5-phosphatase in purified membranes. These two inhibitors could be very useful tools to determine the role of the inositol lipid pathway in neuroexcitability.

Pancreastatin activates beta3 isoform of phospholipase C via G(alpha)11 protein stimulation in rat liver membranes

Pancreastatin (PST) receptors have been recently shown to mediate activation of phospholipase C (PLC) in rat liver membranes. There is evidence that the G protein that links pancreastatin receptor with PLC-beta is pertussis toxin-insensitive and belongs to the G(alpha)q family. Here, we have employed blocking antisera to sort out the specific PLC-beta isoform as well as the specific G(alpha) subunit activated by PST receptor in rat liver membranes. The presence of different PLC-beta isoforms was checked by immunoblot analysis. Only PLC-beta4 was not detected, whereas PLC-beta1, beta2 and beta3 were abundant in rat liver membranes. However, only anti-PLC-beta3 serum was able to block the PST receptor response. We also checked the expression of G(alpha)q and Galpha11 in rat liver membranes by immunoblot. Even though both isoforms were present. only anti-Galpha11 serum was able to block the PST receptor response. In order to check the specificity of the blocking antisera, we employed them to block the effect of ADP and thrombin stimulating PLC activity in platelet membranes, a system lacking Galpha11. Anti-G(alpha)q but not anti-Galpha11 sera were able to block the agonist stimulated PLC activity. These data suggest that PST receptor response is mediated by the activation of the beta3 isoform of PLC via Galpha11 protein stimulation in rat liver membranes.

Pancreastatin receptor is coupled to a guanosine triphosphate-binding protein of the G(q/11)alpha family in rat liver membranes

Pancreastatin (PST), a recently discovered regulatory peptide derived from chromogranin A, has been shown to have a glycogenolytic effect in the hepatocyte that is mediated by increasing intracellular calcium. Our previous studies on pancreastatin signaling suggested that PST receptor is coupled to some G proteins in the plasma membrane of the hepatocyte. The nature of this interaction was investigated using antisera against G(q/11)alpha by different approaches. Indirect evidence of a pertussis toxin (PT)-insensitive G protein of the family of G(q/11)alpha was obtained by measuring high-affinity guanosine triphosphatase (GTPase) activity in soluble rat liver membranes. PST increased GTPase activity in a dose-dependent manner. This effect was only slightly inhibited by PT pretreatment of the membranes, whereas anti-G(q/11)alpha antisera blocked most of the PST-stimulated GTPase activity. The selective association of the PST receptor with this G protein was further studied by the coelution in wheat germ agglutinin semipurification of the receptor and by immunoprecipitation of the G protein-PST receptor complexes using G-protein-specific antisera. A G protein of the family of G(q/11)alpha was found to be associated with the semipurified PST receptor. Moreover, anti-G(q/11)alpha antisera immunoprecipitated most PST-binding activity (95%), bringing down most of the specific G protein, whereas anti-G(il,2)alpha and -G(o,i3)alpha failed to immunoprecipitate the PST-binding activity. Finally, the coupling of the PST receptor with the effector phospholipase C was disrupted by blocking with G(q/11)alpha antisera, suggesting that a G protein of the family of G(q/11)alpha is a signal mediator from PST receptors to phospholipase C activation in rat liver membranes.

Solubilization and molecular characterization of active pancreastatin receptors from rat liver membranes

Pancreastatin receptors were solubilized from rat liver membranes with the nonionic detergent Triton X-100. Binding of a iodinated analog of rat pancreastatin ([125I-Tyr0]pancreastatin) to the soluble fraction was time dependent, saturable, and reversible. Scatchard analysis of binding under equilibrium conditions indicated that the soluble extracts contained a single class of pancreastatin-binding sites, with a binding capacity of 14 fmol/mg protein and a Kd of 0.3 nM. As observed with membrane-bound receptors, binding of [125I]pancreastatin to soluble extracts was inhibited by guanine nucleotides with the following rank order of potency: guanyl-5'-yl-imidodiphosphate > GTP > GDP > GMP, indicating that the soluble receptors are functionally linked to G proteins. Molecular analysis of the soluble pancreastatin receptor by covalent cross-linking to [125I]pancreastatin using disuccinimidyl suberate and further identification on SDS-PAGE indicated a single band of 85,000 Mr. Gel filtration of soluble extracts on Sephacryl S-300 revealed two molecular components with binding abilities (Mr 80,000 and 170,000). The higher molecular mass component was more sensitive to guanine nucleotides, and covalent cross-linking of both components to [125I]pancreastatin and further SDS-PAGE analysis revealed again a single band of 85,000 Mr, suggesting an association of the receptor with a G protein. Moreover, direct evidence that a Gq was present in the same chromatographic fraction was obtained by specific immunodetection. The soluble receptor is a glycoprotein that can be specifically bound to the wheat-germ agglutinin lectin. We conclude that we solubilized active pancreastatin receptors from rat liver membranes, and these results support the conclusion that the liver pancreastatin receptor consists of a 80,000 Mr glycoprotein associated with G proteins.

Vinconate, a cognitive enhancer, and PI turnover-phospholipase C systems in the brain

The molecular mechanisms for the stimulation of inositol 1-phosphate (IP1) formation by vinconate were investigated using preparations of rat brain. Vinconate (10(-8)-10(-3) M) dose-dependently inhibited the binding of [3H]quinuclidinyl benzilate ([3H]QNB) to muscarinic acetylcholine receptors and its IC50 value for [3H]QNB binding was 1.7 x 10(-5) M. The rightward shift of carbachol displacement curve of [3H]QNB binding by GTP (10(-4) M) was completely abolished by vinconate (10(-5) M). Carbachol (10(-8)-10(-2) M) increased [3H]IP1 formation in a dose-dependent manner and the carbachol-induced [3H]IP1 formation was significantly accentuated by vinconate (10(-5) M). The enhancement of [3H]IP1 accumulation by vinconate was inhibited by approximately 50% in the presence of atropine (10(-5) M), although phentolamine and ketanserin had no effects on the stimulatory effect of vinconate on [3H]IP1 formation. Vinconate showed no alteration in the binding of [3H]guanosine 5'-(beta, gamma-imino) triphosphate ([3H]Gpp(NH)p) to the crude synaptic membranes. The enhancement of phosphatidylinositol 4,5-biphosphate (PIP2)-specific phospholipase C (PLC) activity by GTP was unaffected in the presence of 10(-3) M vinconate, whereas vinconate alone dose-dependently enhanced the activities of both PIP2-specific and cytosolic PLC. These results suggest that vinconate may induce the facilitation of phosphatidylinositide (PI) turnover via the stimulation of muscarinic acetylcholine receptors, the enhancement of coupling between muscarinic acetylcholine receptors and GTP-binding protein, and the direct activations of PIP2-specific and cytosolic PLC.

Investigation of presumptive mobilization pathways for calcium in the steroidogenic action of big prothoracicotropic hormone

Ecdysteroidogenesis in the prothoracic glands of the tobacco hornworm Manduca sexta is stimulated by the cerebral neuropeptide prothoracicotropic hormone (PTTH). PTTH-stimulated cAMP synthesis and ecdysone secretion are dependent on the presence of extracellular calcium, suggesting that PTTH enhances calcium entry into the cytosol. Such entry into the cytosol might involve the opening of a plasma membrane calcium channel, or a mechanism dependent upon prior inositol triphosphate (IP3)-mediated release of intracellularly stored calcium. In pupal prothoracic glands, PTTH does not increase IP3 or other inositol phosphates over-times ranging from seconds up to 30 min, even in the presence of lithium. However, the L-type calcium channel antagonist nitrendipine completely prevents PTTH-stimulated ecdysone synthesis. A 41 kDa G-protein in prothoracic glands is ADP-ribosylated by pertussis toxin. However, PTTH-stimulated ecdysone synthesis is unaffected by prior exposure to pertussis toxin, indicating that the 41 kDa protein is not involved in the acute stimulation of steroidogenesis. By contrast, cholera toxin has a stimulatory effect on ecdysone secretion suggesting the involvement of a Gs-like protein. Based on the absence of PTTH-stimulated inositol phosphate formation in pupal prothoracic glands, it is suggested that calcium mobilization may occur through the opening of a calcium channel, possibly regulated by Gs.

The stimulatory effect of L-glutamate and related agents on inositol 1,4,5-trisphosphate production in the cestode Hymenolepis diminuta

The effects of L-glutamate, acetylcholine, and serotonin (5HT) were examined on generation of inositol 1,4,5-triphosphate [Ins(1,4,5)P3], in membrane preparations of the cestode Hymenolepis diminuta. Only L-glutamate and acetylcholine stimulated a significant elevation in Ins(1,4,5)P3. The response to L-glutamate was stereospecific; D-glutamate or L-aspartate were not as potent. A role for G-protein(s) was supported by the observations that sodium fluoride stimulated Ins(1,4,5)P3 generation, and the L-glutamate response was potentiated by GTP and GTP-S and was suppressed by GDPS. However, studies with pertussis and cholera toxins indicated that the putative G-protein(s) was not pertussis or cholera toxin sensitive. The pharmacological profile of the L-glutamate response was examined partially. Trans-ACPD was a very effective agonist at 10(-5)M. While 10(-3)M L-glutamate, NMDA, and AMPA significantly elevated Ins(1,4,5)P3 levels, quisqualate and kainate did not. The elevation of Ins(1,4,5)P3 levels by L-glutamate and NMDA was antagonized by the specific glutamatergic antagonists AP-5, AP-7, CNQX, and CPP. While the response to ACPD was antagonized by AP5, CPP and CPG, CNQX was without effect. Collectively, the data support the hypothesis that in the cestode H. diminuta, L-glutamate activation of a metabotropic (ACPD) and/or ionotropic-like AMPA/NMDA receptor subtypes proceeds via a G protein(s) to enhance phospholipase C activity, ultimately resulting in the elevation of Ins(1,4,5)P3 levels in the tissues.

Purification of chlorpromazine-sensitive GTPase from rat cerebral cortex

The chlorpromazine-sensitive GTPase from the cell membrane of rat cerebral cortex was purified to homogenity by using DEAE Bio-Gel A agarose, hydroxyapatite and heparin agarose chromatography. The purified chlorpromazine-sensitive GTPase was purified 370-fold to obtain a final specific activity of 40 mumol GTP hydrolyzed2min/mg protein. The purified enzyme was inhibited by chlorpromazine but not by compound 48/80. Magnesium was required for its activity instead of calcium. The purified enzyme had an apparent pH optimum of 8.0, and molecular weight was estimated to be 58,000.

Effects of GTP gamma S on muscarinic receptor-stimulated inositol phospholipid hydrolysis in permeabilized smooth muscle from the small intestine

1. Smooth muscle fragments from the longitudinal layer of the small intestine of the guinea-pig were permeabilized with Staphylococcus aureus alpha toxin (alpha-toxin) and used to investigate the role of G-protein activation in the regulation of muscarinic acetylcholine receptor (AChR)-stimulated inositol phospholipid hydrolysis. 2. The efficiency of alpha-toxin permeabilization was estimated by the release of [3H]-2-deoxyglucose ([3H]-2DG) after prior loading or lactate dehydrogenase (LDH) enzyme release from the smooth muscle fragments. 3. In alpha-toxin-permeabilized smooth muscle, but not in non-permeabilized muscle, GTP gamma S induced time- and concentration-dependent increases in labelled inositol phosphates. Carbachol (CCh) increased labelled inositol phosphates in both permeabilized and non-permeabilized muscle, although the increases were greater in non-permeabilized smooth muscle. The response to 100 microM CCh was severely reduced by 0.5 microM atropine. 4. In permeabilized muscle the effects of GTP gamma S or CCh on inositol phosphate levels were reduced by treatment with pertussis toxin (PTX) and completely inhibited by GDP beta S. 5. GTP gamma S caused a concentration-dependent inhibition of the CCh-induced increases in the levels of labelled inositol phosphates. Dibutyryl cyclic AMP or Sp-cAMPs (adenosine-3',5'-cyclic phosphorothiolate-Sp) reduced the effects of CCh on inositol phosphate levels. 6. The results suggest that muscarinic AChR activation induces inositol phospholipid hydrolysis via more than one G-protein in this smooth muscle and that several mechanisms may contribute to the modulation of both stimulatory and inhibitory responses observed.

Nobel Lecture. G proteins and regulation of adenylyl cyclase

The function and structures of G proteins and their role in the regulation of adenylyl cyclase is reviewed.

Bacterial stimulators of macrophages

Our current understanding of the interaction between bacteria and macrophages, cells of the immune system that play a major role in the defense against infection, is summarized. Cell-surface structures of Gram-negative and Gram-positive bacteria that account for these interactions are described in detail. Besides surface structures, soluble bacterial molecules, toxins that are derived from pathogenic bacteria, are also shown to modulate macrophage functions. In order to affect macrophage functions, bacterial surface structures have to be recognized by the macrophage and toxins have to be taken up. Subsequently, signal transduction mechanisms are initiated that enable the macrophage to respond to the invading bacteria. To destroy bacteria, macrophages employ many strategies, among which antigen processing and presentation to T cells, phagocytosis, chemotaxis, and different bactericidal mechanisms are considered to be the main weapons.

G-proteins coupled to phosphoinositide hydrolysis in the cochlear and vestibular sensory epithelia of the rat are insensitive to cholera and pertussis toxins

In the cochlear (CSE) and vestibular sensory epithelia (VSE), phosphoinositides are hydrolyzed in response to stimulation of phospholipase C (PLC) by cholinergic muscarinic and purinergic P2y agonists. Such receptor-mediated activation of PLC is expected to be coupled through guanine nucleotide-binding proteins (G-proteins). Although several classes of G-proteins have been identified in the inner ear, nothing is known about the type of G-proteins associated with the phosphoinositide second messenger system in CSE and VSE. Phosphoinositide hydrolysis was determined by the release of radiolabeled inositol phosphates (InsPs). Ten mM NaF plus 10 microM AlCl3 increased basal InsPs accumulation 2-fold in both CSE and VSE of the rat. Release of InsPs was also enhanced by guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) in saponin-permeabilized tissues. Furthermore, release of InsPs stimulated by both carbamylcholine (CCh) and adenosine 5'-O-[3-thiotriphosphate] (ATP-gamma-S) was significantly inhibited by 100 microM guanosine 5'-O-[2-thiodiphosphate] (GDP-beta-S). These results strongly suggest the involvement of G-proteins in the receptor-PLC coupling in CSE and VSE. ADP-ribosylation in membrane fractions of CSE and VSE in the presence of cholera toxin (CTX) or pertussis toxin (PTX) indicated the existence of Gs- and G(i)-type G-proteins. However, neither CTX nor PTX affected basal or agonist-stimulated release of InsPs. These observations suggest that muscarinic and P2y purinergic receptors are coupled to PLC via CTX- and PTX-insensitive G-proteins in CSE and VSE.

Guanosine-5'-(3-O-thio)triphosphate-mediated stimulation of phosphoinositidase C in solubilized rat peripheral nerve myelin and its alteration in streptozotocin-induced diabetes

The regulation of phosphoinositidase C (PIC) activity by guanosine-5'-(3-O-thio)triphosphate (GTP gamma S) was characterized in a cholate-solubilized peripheral myelin-enriched fraction from rat sciatic nerve. The GTP analog maximally enhanced PIC-catalyzed hydrolysis of exogenous phosphatidylinositol-4,5-bisphosphate (PIP2) in a dose-dependent manner only within a narrow range of cholate concentrations. Maximal stimulation was attained at 0.6 microM GTP gamma S and could be completely prevented by 1 microM guanosine-5'-(2-O-thio)diphosphate. Neither adenylyl-imidodiphosphate nor adenosine triphosphate (ATP) enhanced PIC activity. Carbamoylcholine (1 mM) added together with GTP gamma S increased the extent of PIP2 hydrolysis over that elicited by GTP gamma S alone and this stimulation was blocked by the muscarinic receptor antagonist, atropine (50 microM). In detergent-solubilized myelin preparations from streptozotocin-induced diabetic rats, a higher concentration of the guanine nucleotide analog was required to achieve stimulation comparable to that obtained with corresponding preparations from normal animals. These results suggest that sciatic nerve myelin possesses muscarinic receptors coupled via a GTP-binding protein to PIC and that this system can be reconstituted in detergent-solubilized extracts. It is possible that the function of G proteins in cell signaling is impaired in experimental diabetic neuropathy.

Partial characterization of protein kinase C from an insect cell line

The characteristics of protein kinase C (EC 2.7.1.37) from an insect cell line (Choristoneura fumiferana) have been described. DEAE-cellulose chromatography produced a major peak of activity which eluted at 0.04-0.055 M NaCl. The enzyme was sensitive to phosphatidylserine in the presence of calcium. Phorbol 12-myristate 13-acetate (PMA) in nanomolar concentrations stimulated protein kinase C activity 8-fold over basal levels and reduced the enzymes requirement for Ca2+. The enzyme had a Ka of 10 nM for PMA. Diacylglycerols tested included diolein, dilinolein, diarachidonin, oleoyl-acetyl-glycerol, dioctonoyl-sn-glycerol, dipalmitin and distearin. A 2.5- to 3-fold activation was obtained in the presence of 26 microM diolein, 40 microM oleoyl-acetyl-glycerol and 46 microM dioctonoyl-sn-glycerol. The enzyme activity was sensitive to the inhibitor H-7 and 50% inhibition was achieved at a concentration of 52 microM H-7. Phosphatidylinositol enhanced enzyme activity in the absence of phosphatidylserine but phosphatidylethanolamine had no effect.

Phosphatidylinositol kinase and phosphatidylinositol 4-phosphate kinase activities in Md 66 insect cell line

Phosphatidylinositol kinase (PI kinase) and phosphatidylinositol 4-phosphate kinase (PIP kinase) activities in an insect cell line (Md 66) were studied by measuring the incorporation of 32P from [gamma-32P] ATP into phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) respectively. The apparent Km values of phosphatidylinositol kinase and phosphatidylinositol 4-phosphate kinase for ATP were 78.5 microM and 71.7 microM respectively. PIP kinase activity was enhanced by GTP analogue guanosine 5-O(thio)triphosphate (GTP gamma S).

Altered levels of phosphatidylinositol phospholipase C activity in rat liver cells in response to insulin, epinephrine, acetylcholine and bacterial phospholipase C

Rat liver cells were homogenized and subsequently fractionated by a simplified method based on microfiltration, which proved to give a high recovery of membrane-bound phosphatidylinositol phospholipase C (PI-PLC) activity. The effects of insulin, acetylcholine (AC), epinephrine (EN) and bacterial phospholipase C (bPLC) on the PI-PLC activity were studied after in vitro treatment of isolated membranes or after in situ application in rat liver. A dose-dependent increase of membranous PI-PLC (up to 3-fold) and corresponding 36 to 72% decline of the cytosolic activity were established upon treatment with supraphysiological doses of insulin or with bPLC, respectively. AC induced a biphasic response with a maximal stimulation in the micromolar range. On the other hand EN promoted a slight but significant dose-dependent inhibition of PI-PLC in both cytosol and membranes. Sodium fluoride was also a potent inhibitor of the membrane-associated PI-PLC with an EC50 value of about 5 mM. The combined assay with NaF and EN revealed no additivity between their inhibitory effects, suggesting a common step in the mechanism(s) of inhibition caused by the two agents. The stimulatory effects of insulin and AC were partially reduced by soluble cytosolic factors, which still remain to be identified. When insulin and AC were applied in combination in the presence of cytosol, this resulted in a 56% inhibition of PI-PLC below the control level.

Increased phospholipase C activity in neoplastic thyroid membrane

The phospholipase C (PLC)-protein kinase C (PKC) signal transduction pathway appears to be important for cellular growth of many normal and neoplastic tissues. Because alterations in the thyroid-stimulating hormone (TSH) receptor-adenylate cyclase-protein kinase A system in some thyroid tumors do not correlate with tumor size, invasiveness, or metastatic potential, we studied the PLC activity in both normal and neoplastic thyroid tissues from 11 patients. Five of these patients had follicular adenomas and 6 had papillary carcinomas. An 8,000 x g membrane fraction and a 105,000 x g cytosol fraction were prepared from the normal and neoplastic human thyroid tissues. PLC hydrolyzes phosphatidylinositol, 4,5-diphosphate (PIP2) to diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). Phospholipase C activity was determined measuring the hydrolysis of [3H]-PIP2. The activity of PLC in the neoplastic thyroid tissue membrane fraction (20.91 +/- 2.28 nmol PIP2 hydrolyzed/mg protein/120 min) was higher than that in normal thyroid membrane (14.27 +/- 0.82) (p < 0.05). In contrast, PLC activity was similar in the neoplastic (16.12 +/- 0.86 nmol PIP2 hydrolyzed/mg protein/120 min) and normal (16.66 +/- 0.60) cytosol. There was no difference between PLC activity in the membrane fraction from adenomas (21.21 +/- 3.71 nmol PIP2 hydrolyzed/mg protein/120 min) when compared with thyroid carcinomas (20.67 +/- 3.14). Neoplastic thyroid membranes have greater PLC activity than that found in normal thyroid membranes from the same patients. Although PLC activity in benign and malignant thyroid membranes was similar, the increased PLC activity in thyroid neoplasms may be responsible for or contribute to the enhanced growth of some thyroid tumors.

Pharmacological characterization of a 5-HT receptor in locust nervous tissue

A 5-HT receptor in the nervous tissue of the desert locust (Schistocerca gregaria Forsk.) was investigated, using [3H]LSD (lysergic acid diethylamide) as the radioligand. [3H]LSD labels in addition a putative dopamine receptor whose specific [3H]LSD binding nevertheless could easily be diminished by co-incubation with 1 microM dopamine. The binding site was characterized by a KD of 1.64 nM, and a maximal concentration of binding sites of 79.8 fmol/mg protein. Pharmacological investigation revealed a relatively low affinity for the putative natural agonist, serotonin (KI = 0.209 microM). In contrast to the high affinity of classical serotonergic antagonists (e.g. dihydroergotamine or (+)-butaclamol) substances with subtype specificity such as 8-OH-DPAT (8-hydroxyl-1-(N,N-dipropyl)-aminotetralin) or ketanserin have only moderate affinities. Quantitative comparison of the pharmacological data demonstrated that there is obviously no pharmacological homology with vertebrate 5-HT receptors characterized so far. The only receptors with a close pharmacological relationship to the 5-HT receptor of locusts are the 5-HTdro1 receptor expressed in Drosophila nervous tissue and a 5-HT receptor in snail nervous tissue which might be homologous to that of locusts. The 5-HT receptor investigated, was shown to be G-protein-coupled, as addition of stable GTP analogues or depletion of Mg2+ ions from the incubation medium led to agonist-specific lowering of the affinity.

The generation of inositolglycan mediators from rat liver plasma membranes: the role of guanine nucleotide binding proteins

The guanine nucleotide dependence for the generation of inositolglycan second messengers from rat liver plasma membranes has been investigated. Plasma membranes, when treated with insulin release a soluble mediator substance which activates pyruvate dehydrogenase (PDH). Guanosine 5'-[3-thio]triphosphate (GTP gamma S) was found to be as potent as insulin in stimulating mediator release. The stimulatory effects of GTP gamma S required the presence of magnesium and following preincubation of membranes with guanosine 5'-[2-thio]diphosphate (GDP beta S) the stimulation of mediator release by either insulin or GTP gamma S was blocked. The activation of PDH by mediator fractions produced in response to either insulin or GTP gamma S was abolished following treatment of the fractions with anti-inositolglycan antibodies. The significance of these observations with respect to the possible involvement of a regulatory guanine-nucleotide binding protein (G-protein) in the generation of insulin mediators is discussed.

Regulation of phosphoinositide hydrolysis in cultured astrocytes by sphingosine and psychosine

The effects of sphingosine and psychosine on phosphoinositide hydrolysis in primary cultured astrocytes were determined. Exposure to sphingosine produced a dose-dependent stimulation of phosphoinositide hydrolysis requiring the presence of external Ca++ for optimal activity. The addition of 10 microM norepinephrine resulted in a stimulation additional to that with sphingosine. The alpha 1-antagonist prazosin completely inhibited norepinephrine-induced phosphoinositide hydrolysis but had no effect on that produced by sphingosine. Psychosine (108 microM), when co-incubated with sphingosine, produced complete inhibition of sphingosine-induced phosphoinositide hydrolysis at all doses of sphingosine tested (33-668 microM). Likewise, psychosine totally inhibited norepinephrine-induced phosphoinositide hydrolysis. The protein kinase C inhibitor staurosporine (1 microM) had no effect on sphingosine-induced phosphoinositide hydrolysis. These findings suggest that lysosphingolipids such as sphingosine and psychosine may play an important role in the regulation of phosphoinositide turnover in astrocytes by a mechanism dependent on extracellular Ca++ and independent of the alpha 1-adrenergic receptor and protein kinase C.

Regulation of inositol 1,4,5-trisphosphate metabolism by guanine nucleotides in membranes of cultured newborn rat cardiomyocytes

Membranes of cultured newborn rat cardiomyocytes contain enzymatic activities that regulate the formation and the breakdown of inositol 1,4,5-trisphosphate (1,4,5-IP3). GTP gamma S increased the rate of exogenous [3H]phosphatidyl 4,5-bisphosphate ([3H]PIP2) hydrolysis (EC50: 40 microM). This effect was dependent on the presence of deoxycholate and maximal at 2 mM deoxycholate. GTP gamma S increased the efficacy of phospholipase C (PLC) (by 2.3-fold), but did not alter the apparent affinity of the enzyme for PIP2. Other nucleotides, GDP beta S and ATP gamma S, and pyrophosphate also stimulated PIP2 hydrolysis, while AlF4- was ineffective. The effect of GTP gamma S was not inhibited by GDP beta S. The agonists norepinephrine and thrombin, which by themselves had no effect, did not potentiate the response to GTP gamma S. In contrast, 1,4,5-IP3 hydrolysis was decreased by GTP gamma S (EC50: 100 microM) as well as by other nucleotides and by pyrophosphate, but not by AlF4-. GDP beta S did not antagonize the GTP gamma S-induced inhibition of IP3 hydrolysis. These results suggest that GTP can stimulate the hydrolysis of exogenous PIP2 by an action on membrane-bound PLC at a site beyond the G protein activating PLC and inhibit the hydrolysis of 1,4,5-IP3 by a mechanism common to all nucleotides. Thus, GTP can regulate 1,4,5-IP3 metabolism by stimulating its formation and inhibiting its breakdown.

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)

Regulation of carbachol- and histamine-induced inositol phospholipid hydrolysis in a human oligodendroglioma

A stable cell line derived from a human oligodendroglioma (HOG) was used to study the regulation of muscarinic- and histamine receptor-mediated phosphoinositide hydrolysis. Both carbachol and histamine increased inositol monophosphate (InsP) accumulation in a dose- and time-dependent manner in the presence of lithium and the effect of simultaneous addition of carbachol and histamine was additive, implying independent signal transduction pathways. Homologous desensitization of muscarinic, but not histamine receptors, could be demonstrated although neither receptor type appeared to be heterologously desensitized. [3H]InsP accumulation in HOG cells was also stimulated by fluoride, suggesting guanosine triphosphate (GTP)-binding protein involvement, but phosphoinositide (PtdIns) hydrolysis was not sensitive to pertussis toxin. Phorbol ester-activation of protein kinase C (PKC) inhibited both muscarinic and histamine receptor-stimulated InsP release but did not attenuate either the fluoride-induced release of InsP nor beta-adrenergic receptor-mediated stimulation of adenylate cyclase activity. Taken together, we conclude that muscarinic and histamine receptors are differentially regulated through both PKC-dependent and -independent mechanisms, and that feedback inhibition of PtdIns turnover occurs proximal to the GTP binding proteins.

Presence of phospholipase C in coated vesicles from bovine brain. Dual regulatory effects of GTP-analogs

Bovine brain coated vesicles display free calcium-dependent phospholipase C activity. Gpp(NH)p and GTP gamma S inhibited phospholipase C at nanomolar concentrations. Increasing concentrations of Gpp(NH)p and GTP gamma S reversed the inhibitory effects and stimulated phospholipase C activity. Preincubation of coated vesicles with pertussis toxin blocked the poorly-hydrolyzable GTP-analogs' inhibitory effects on phospholipase C. These data indicate that guanine nucleotides exert a dual regulatory control of phospholipase C in coated vesicles and that the inhibitory pathway is mediated by a pertussis toxin-sensitive G-protein.

Muscarinic cholinergic stimulation of exogenous phosphatidylinositol hydrolysis is regulated by guanine nucleotides in rabbit brain cortical membranes

Rabbit brain cortical membranes, which have been extracted with 2 M KCl, hydrolyze exogenously added [3H]phosphatidylinositol [( 3H]PI) in a guanine nucleotide- and carbachol-dependent manner. Both oxotremorine-M and carbachol are full agonists with EC50 values of 8 and 73 microM, respectively. Pirenzepine and atropine inhibit carbachol-stimulated [3H]PI hydrolysis. The hydrolysis-resistant guanine nucleotide analog guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) is the most potent in supporting carbachol-stimulated hydrolysis of PI. There is no effect of carbachol in the absence of guanine nucleotides or in the presence of 100 microM adenosine 5'-O-(3-thiotriphosphate), adenosine-5'-(beta, gamma-imido)triphosphate, or sodium pyrophosphate. Guanylyl-5'-(beta,gamma-imido)triphosphate [Gpp(NH)p] in the presence of carbachol also stimulates PI hydrolysis although much less than that seen with GTP gamma S. GDP and Gpp(NH)p are potent antagonists of the GTP gamma S-dependent carbachol response. Optimal stimulation by carbachol and GTP gamma S was observed at 0.3-1 microM free Ca2+ and 6 mM MgCl2. Limited trypsinization resulted in loss of receptor-regulated PI breakdown and a slight decrease in basal activity. These results demonstrate that phospholipase C hydrolysis of exogenous PI by rabbit cortical membranes may be stimulated by carbachol in a guanine nucleotide-dependent manner.

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.

NaF-induced amylase release from rat parotid cells is mediated by PI breakdown leading to Ca2+ mobilization

We have examined the effects of sodium fluoride (NaF) on amylase release, cellular adenosine 3',5'-cyclic monophosphate (cAMP) level, inositol phosphate formation, and cytosolic free Ca2+ concentration ([Ca2+]i) in dispersed rat parotid acini or cells. At concentrations greater than 1 mM, NaF significantly increased amylase release. The maximum response was observed at 10 mM NaF and was comparable to that of the muscarinic-cholinergic agonist carbachol. Removal of extracellular Ca2+ with EGTA markedly suppressed the NaF-induced secretory response. At concentrations up to 10 mM, NaF did not increase the cellular level of cAMP, indicating that the NaF-induced amylase release is not mediated by cAMP. NaF (1-20 mM) caused a slow increase in [Ca2+]i in a concentration-dependent manner, as monitored with the fluorescent Ca2+ indicator fura-2, and the increased [Ca2+]i did not decline for at least 10 min after addition of NaF. In the absence of extracellular Ca2+, NaF evoked only a small and transient increase in [Ca2+]i. The addition of 10 mM NaF produced a significant accumulation of inositol monophosphate, inositol bisphosphate, and inositol trisphosphate. These results suggest that the NaF-induced amylase release is mediated by a breakdown of phosphoinositides leading to Ca2+ mobilization. The effects of fluoride may be through the action of F- on the GTP-binding protein(s) coupled to phospholipase C.