Thyrotropin-releasing hormone and GTP activate inositol trisphosphate formation in membranes isolated from rat pituitary cells

Juxtamembrane regions in the third intracellular loop of the thyrotropin-releasing hormone receptor type 1 are important for coupling to Gq

Juxtamembrane residues in the putative third intracellular (I3) loops of a number of G protein-coupled receptors (GPCRs) have been shown to be important for coupling to G proteins. According to standard hydropathy analysis, the I3 loop of the mouse TRH receptor type 1 (mTRH-R1) is composed of 51 amino acids from position-213 to position-263. We constructed deletion and site-specific I3 loop TRH-R mutants and studied their binding and TRH-stimulated signaling activities. As expected, the effects of these mutations on TRH binding were small (less than 5-fold decreases in affinity). No effect on TRH-stimulated signaling activity was found in a mutant receptor in which the I3 loop was shortened to 16 amino acids by deleting residues from Asp-226 to Ser-260. In contrast, mutants with deletions from Asp-222 to Ser-260 or from Asp-226 to Gln-263 exhibited reduced TRH-stimulated signaling. In the region near transmembrane helix 6, single site-specific substitution of either Arg-261 or Lys-262 by neutral glutamine had little effect on signaling, but mutant TRH-Rs that were substituted by glutamine at both basic residues exhibited reduced TRH-stimulated activity. The reduced signaling activity of this doubly substituted mutant was reversed by over expressing the a subunit of Gq. These data demonstrate that the juxtamembrane regions in the TRH-R I3 loop are important for coupling to Gq.

Expression analysis of the thyrotropin-releasing hormone receptor (TRHR) in the immune system using agonist anti-TRHR monoclonal antibodies

Monoclonal anti-rat thyrotropin-releasing hormone (TRH) receptor (TRHR)-specific antibodies (mAb) were generated by immunization with synthetic peptides of rat TRHR partial amino acid sequences; one (TRHR01) was directed against a sequence (84-98) in the extracellular portion of the rat TRHR reported to be constant among different species, including man, and the second (TRHR02) recognizes the C-terminal region sequence 399-412. In lysates from GH4C1 cells, a clonal rat pituitary cell line, both mAb recognize the TRHR in Western blot analysis, and TRHR02 immunoprecipitates the TRHR. Incubation of GH4C1 cells with the mAb causes a fluorescence shift in fluorescence-activated cell sorting analysis. The cells were stained specifically by both mAb using immunocytochemical techniques. Furthermore, TRHR01 is agonistic in its ability to trigger Ca2+ flux, and desensitizes the TRH receptor. We tested for TRHR in several rat organs and found expression in lymphoid tissues. TRHR01 recognizes the human TRHR, and analysis of human peripheral blood lymphocyte and tonsil-derived leukocyte populations showed receptor expression in non-activated and phytohemagglutinin-activated T and B cells.

Is signal transduction modulated by an interaction between heterotrimeric G-proteins and tubulin?

Although it is generally accepted that tubulin plays an important role in G-protein-mediated signal transduction in a variety of systems, the mechanism of this phenomenon is not completely understood. G-protein-tubulin interaction at the cell membrane and the cytosol, and the influence of such an interaction on cellular signaling are discussed in this review article. Because the diameter of a microtubule is 25 nm and the plasma membrane is 9-11 nm thick, it is not possible for membrane-associated tubulin to assemble into a complete microtubule in the membrane environment. However, tubulin heterodimers may be able to function in the membrane environment as individual heterodimers or as polymers arranged into short protofilaments. At the cell membrane, membrane-associated tubulin may influence hormone-receptor interaction, receptor-G-protein coupling, and G-protein-effector coupling. Structural proteins, such as tubulin, can participate in cellular signaling by communicating through physical forces. By virtue of its interaction with the submembranous network of cytoskeletal proteins, tubulin, when perturbed in one locus, can transmit large changes in conformations to other points. Thus, GTP binding to membrane-associated tubulin might lead to a conformational change in either receptors or G proteins. This may, in turn, influence the binding of an agonist to its receptor. On the other hand, in the cell cytosol, subsequent to agonist-induced translocation of G-proteins from the membrane compartment to the cytosol, G-proteins may affect microtubule formation. In GH3 and AtT-20 cells (stably expressing TRH receptor), transiently transfected with Gq alpha cDNA, soluble tubulin levels decreased in Gq alpha-transfected GH3 and AtT-20 cells, by 33% and 52%, respectively. These results suggest that G-proteins may have a direct effect on the microtubule function in vivo. Because tubulin and G-protein families are ubiquitous and highly conserved, an interaction between these two protein families may occur in vivo, and this, in turn, can have an impact on signal transduction. However, the physiological significance of this interaction remains to be demonstrated.

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.

Activation of CCK-B receptors elevates cytosolic Ca2+ levels in a pituitary cell line

Cytosolic Ca2+ levels ([Ca2+]i) in GH3 cells, a rat anterior pituitary tumor cell line, were monitored with fura-2 by fluorescence measurements. Cholecystokinin octapeptide (CCK-8) produced a transient elevation of [Ca2+]i. The elevation of [Ca2+]i by CCK-8 was inhibited by L-365,260, but not by devazepide. It was still observed when extracellular Ca2+ was eliminated, indicating that CCK-8 mobilizes Ca2+ from intracellular storage sites after interaction with CCK-B receptors. Cholecystokinin octapeptide increased the turnover of phosphatidylinositol, but it did not affect cyclic AMP levels. A possible involvement of phosphatidylinositol breakdown and calcium mobilization in the transduction system of CCK-B receptors in GH3 cells is suggested.

'Cross-talk' between phospholipase C and adenylyl cyclase involves regulation of G-protein levels in GH3 rat pituitary cells

We have investigated the possibility that adenylyl cyclase (AC) activity and membrane protein levels of the alpha-subunits of the stimulatory and inhibitory G-proteins of AC (Gs alpha and G(i)-2 alpha) in cultured prolactin-producing rat pituitary adenoma cells (GH3 cells) are modulated by phospholipase C (PLC)-generated second messengers. Pretreatment of cells (6-48 h) with ionomycin (1 microM) or 1-oleoyl-2-acetylglycerol (OAG; 1 microM) showed that ionomycin regulated Gs alpha levels in a time-dependent, biphasic manner; a two-fold increase followed a 40% initial reduction, while OAG lowered Gs alpha levels by more than 50% at all time-points. G(i)-2 alpha levels remained unchanged by both pretreatments. OAG, but not ionomycin, increased basal AC activity without increasing enzyme protein levels. Alterations in AC responsiveness to peptide hormones (e.g. thyroliberin and vasoactive intestinal peptide) correlated to membrane Gs protein alpha-subunit content. These results demonstrate the involvement of G-protein translation regulation as one mechanism of 'cross-talk' between the PLC- and AC-dependent signalling pathways.

Hypothalamic hormones modulate G protein levels and second messenger responsiveness in GH3 rat pituitary tumour cells

Thyroliberin (TRH), vasoactive intestinal peptide (VIP) and somatostatin (SRIF) act through receptors that are coupled to guanine nucleotide-binding regulatory proteins (G proteins). Regulation of hormone action may occur at the level of G protein coupling to the receptor or effector systems. In this study we demonstrate that prolonged exposure (for up to 48 hr) of cultured rat pituitary adenoma GH3 cells to these hormones caused homologous and to some extent heterologous attenuation of the adenylyl cyclase (AC) (EC 4.6.1.1) responsiveness. In addition, TRH and SRIF diminished both TRH- and guanosine 5'-[beta gamma-imido]-triphosphate-enhanced phospholipase C (PLC) (EC 3.1.4.3) activity within the same time-course. Measurements of cells membrane levels of Gs protein alpha-subunit (Gs alpha), G(i)-1 alpha/G(i)-2 alpha, G(i)-3 alpha, G(o) alpha and G beta by immunoblotting were performed. TRH and VIP upregulated levels of all G proteins except G(o) alpha and G beta. In contrast, SRIF caused a marked reduction of G beta levels. Thus, TRH and VIP, both acting through Gs, both modulated the alpha-subunit levels of this signal transducer, whereas SRIF, which possibly acts through G(i)-2, did not change the steady state level of G(i)-2 alpha. The actions of TRH, VIP and SRIF are multifaceted at the G protein level, where modulations of subtypes not directly involved in their actions may occur. These findings emphasize the complexity expected to be found in the in vivo situation.

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.

G alpha 11 and G alpha q guanine nucleotide regulatory proteins differentially modulate the response to thyrotropin-releasing hormone in Xenopus oocytes

Xenopus oocytes that express mouse thyrotropin-releasing hormone receptors (TRH-Rs) after injection if RNA transcribed from TRH-R cDNA respond to THR by a depolarizing current. This response is transduced by activation of phosphoinositide-specific phospholipase C and utilizes an as yet unidentified endogenous guanine nucleotide-binding regulatory (G) protein(s). The alpha subunit of G11 and Gq have recently been shown to couple receptors to activation of phospholipase C. To determine whether there are functional differences between these proteins, we have co-expressed the TRH-R with either alpha 11 or alpha q. alpha 11 potentiated the response to TRH (by 61 +/- 16%), while alpha q inhibited the response (by 37 +/- 9%). The changes in amplitudes were accompanied by inverse changes in response latencies. These data show that alpha 11 and alpha q differentially modulate signal transduction in Xenopus oocytes.

The thyroliberin receptor interacts directly with a stimulatory guanine-nucleotide-binding protein in the activation of adenylyl cyclase in GH3 rat pituitary tumour cells. Evidence obtained by the use of antisense RNA inhibition and immunoblocking of the stimulatory guanine-nucleotide-binding protein

The thyroliberin receptor in GH3 pituitary tumour cells is known to couple to phospholipase C via a guanine-nucleotide-binding protein (G protein). Thyroliberin is postulated also to activate adenylyl cyclase, via the stimulatory G protein (Gs). In order to study this coupling, we constructed an antisense RNA expression vector that contained part of the Gs alpha-subunit cDNA clone (Gs alpha) in an inverted orientation relative to the mouse metallothionein promoter. The cDNA fragment included part of the coding region and all of the 3' non-translated region. Transient expression of Gs alpha antisense RNA in GH3 cells resulted in the specific decrease of Gs alpha mRNA levels, followed by decreased Gs alpha protein levels. Thyroliberin-elicited adenylyl cyclase activation in membrane preparations showed a reduction of up to 85%, whereas phospholipase C stimulation remained unaffected. Activation of adenylyl cyclase by vasoactive intestinal peptide was reduced by 30-40%. Investigation of the effects of thyroliberin and vasoactive intestinal peptide on adenylyl cyclase in GH3 cell membranes pretreated with antisera against Gs alpha and Gi-1 alpha/Gi-2 alpha support the results obtained by the use of the antisense technique. We conclude that thyroliberin has a bifunctional effect on GH3 cells, in activating adenylyl cyclase via Gs or a Gs-like protein in addition to the coupling to phospholipase C.

Structural heterogeneity of membrane receptors and GTP-binding proteins and its functional consequences for signal transduction

Recent information obtained, mainly by recombinant cDNA technology, on structural heterogeneity of hormone and transmitter receptors, of GTP-binding proteins (G-proteins) and, especially, of G-protein-linked receptors is reviewed and the implications of structural heterogeneity for diversity of hormone and transmitter actions is discussed. For the future, three-dimensional structural analysis of membrane proteins participating in signal transmission and transduction pathways is needed in order to understand the molecular basis of allosteric regulatory mechanisms governing the interactions between these proteins including hysteretic properties and cell-cybernetic aspects.

Characterization of fMet-Leu-Phe-stimulated phospholipase C in streptolysin-O-permeabilised cells

Phospholipase C (specific for inositol lipids) is known to be present both in membranes and cytosol. Receptor-mediated activation of this enzyme occurs via a guanine nucleotide regulatory protein (G-protein), designated Gp. We have compared the stimulation of this enzyme by fMet-Leu-Phe via the G-protein in HL60 membranes and in permeabilised cells. fMet-Leu-Phe stimulated phospholipase C in membranes at 2 min and the response was dependent on exogenously added GTP. GTP alone also stimulated phospholipase C activity such that at 10 min the response to fMet-Leu-Phe was minimal. In comparison, the response to fMet-Leu-Phe in permeabilised cells was greater in extent but did not require added GTP. However, it was antagonized by GDP analogues (GDP[beta S] greater than GDP greater than dGDP) and by pertussis toxin pretreatment, indicating that fMet-Leu-Phe-stimulated phospholipase C activity was also mediated via Gp. GTP and its analogue GTP[gamma S] also stimulated phospholipase C and their effects were strictly additive to the stimulation obtained with fMet-Leu-Phe. Such additivity was also observed when two receptor-directed agonists, fMet-Leu-Phe and ATP, were used to stimulate intact cells. It is concluded that (a) the size of the response with fMet-Leu-Phe in membranes is limited by the loss of a component, possibly phospholipase C, and (b) stoichiometry and physical organisation of multiple species of G-proteins and/or phospholipases C may explain the independent nature of phospholipase C activation by fMet-Leu-Phe, ATP and guanine nucleotides.

Gonadotropin-releasing hormone and thyrotropin-releasing hormone regulation of g protein function in the rat anterior pituitary lobe

Abstract In order to evaluate the role of guanine nucleotide-dependent transducer proteins (G proteins) in hormone-mediated signal transduction in the anterior pituitary lobe, we examined the effect of gonadotropin-releasing hormone (GnRH) and thyrotropin-releasing hormone (TRH) on two parameters of G protein function, namely [(35) S]GTP(gamma)S binding and low K(m)GTPase activity. Plasma membranes were prepared from anterior pituitary lobes of adult male rats using conventional procedures. GTP binding was determined by incubating 2 to 5 mug membrane protein with approximately 100,000 cpm [(35) S]GTP(gamma)S in a buffer containing 20 mM Tris- HCl, 1 mM EDTA, 1 mM dithiothreitol, and 100 mM NaCI at a pH of 7.4 for 10 or 15 min at 37 degrees C GnRH agonist and TRH stimulated high affinity [(35) S]GTP(gamma)S binding in a concentration-dependent manner. GTP binding was maximally stimulated by GnRH agonist (1 muM) and TRH (0.1 muM) by up to 27% and 34%, respectively. A time-course study revealed that 1 muM GnRH agonist stimulated GTP binding by 30% at 15 min; 0.1 muM TRH stimulated GTP binding by 23% at 1 min, 18% at 5 min and 25% at 10 min. A stable GTP analog, 5'-guanylylimidodiphosphate, inhibited GnRH- as well as TRH-stimulated GTP binding. GnRH antagonist did not affect GTP binding. However, in the presence of the antagonist, stimulation of GTP binding by the GnRH agonist was completely blocked. The low K(m)GTPase activity (EC 3.6.1.-), another parameter of G protein function, was assayed in 2 to 5 mug membrane protein using [gamma-(32) P]GTP at 37 degrees C in an ATP-regenerating buffer containing 1 muM unlabeled GTP. GnRH agonist (0.1 muM) and TRH (1 muM) maximally stimulated this GTPase activity by up to 50% and 40%, respectively. GnRH agonist (1 muM) stimulated the GTPase activity by 30% at 10 min and 48% at 30 min. TRH (1 muM) stimulated the GTPase activity at all time points monitored; stimulation was 46% at 5 min, 49% at 20 min, and 41% at 30 min. Interestingly, the GnRH antagonist stimulated GTPase activity by about 20%, but inhibited GnRH agonist-stimulated GTPase activity in a concentration-dependent manner. These results indicate that the binding of GnRH and TRH to their receptors results in interaction of the receptor with a G protein and activation of the G protein cycle.

Receptor-effector coupling by G proteins

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

Substance P and related tachykinins induce receptor-mediated hydrolysis of polyphosphoinositides in the rat anterior pituitary

In the present study characterization of phosphatidylinositol 4,5-bisphosphate-specific phospholipase C (PIP2-PLC) activity and receptor-mediated hydrolysis of PIP2 in rat anterior pituitary membranes were investigated. Incubation of the membrane fraction of anterior pituitary homogenate with [3H]inositol-labeled PIP2 in the presence of calcium increased the concentration of the water-soluble degradation product inositol trisphosphate (IP3) in a time-dependent manner. PIP2-PLC in the rat anterior pituitary had a pH optimum at 5.5 and a requirement for cations. Ca2+ and Mg2+ could activate the enzyme. Activity was maximal at a total magnesium concentration of 1 mM and at a free Ca2+ concentration of 100 microM. The addition of the detergent Triton X-100 (0.05% w/v) to the membrane fraction resulted in a 50% decrease of PIP2-PLC activity, whereas the presence of sodium deoxycholate (1 mg/ml) in the membrane fraction increased the PIP2-PLC activity by 100%. The tachykinins substance P, 8-Tyr-substance P, physalaemin, neurokinin A, eledoisin, kassinin and neurokinin B induced receptor-mediated breakdown of [3H]inositol-labeled PIP2 in the membrane fraction in a concentration-dependent manner, but with different potencies. The tachykinins displayed the following rank order of potencies: substance P greater than 8-Tyr-substance P greater than physalaemin greater than neurokinin A greater than eledoisin greater than kassinin greater than neurokinin B, which is consistent with the involvement of a NK-1 receptor. Combined treatment of anterior pituitary membranes by substance P and thyrotropin-releasing hormone (TRH) resulted in an additional increase in PIP2-PLC activity compared to stimulation with TRH alone.

Guanine nucleotides stimulate hydrolysis of phosphatidyl inositol bis phosphate in human myelin membranes

Phosphodiesterase activity was stimulated in myelin membranes in the presence of guanine nucleotide analogues. This activity was reduced in myelin membranes which had been adenosine diphosphate ribosylated in the presence of cholera toxin which ADP-ribosylated three proteins of Mr 46,000, 43,000 and 18,500. Aluminum fluoride treatment of myelin had the same stimulatory effects on phosphodiesterase activity as did the guanine nucleotides.

Evidence of an auxin-mediated phosphoinositide turnover and an inositol (1,4,5)trisphosphate effect on isolated membranes of Daucus carota L

Microsomal membranes from carrot suspension cells were phosphorylated in vitro with [gamma-32P]ATP. In the presence of submicromolar concentrations of the natural auxin indoleacetic acid (IAA), a rapid, but transient decrease of the [32P] label could be detected in the phospholipid extracts of the membranes. The phytohormone effect was not the result of an inhibition of the lipid phosphorylation reactions, but was caused by a simultaneous release of water-soluble compounds, which, according to their chromatographic properties, were assumed to contain inositol polyphosphates. Although the [32P]-labeled lipids, as well as the inositol polyphosphates, were not identified unequivocally by chemical analysis, these findings point to an auxin-mediated control of a phosphoinositidase C-like reaction similar to the hormone-stimulated phosphoinositide response in animals. Exogenously applied inositol (1,4,5)trisphosphate [(1,4,5)IP3] was found to release 45Ca2+ from preloaded membrane vesicles of carrot cells. Both the detection of the auxin-stimulated phosphoinositide response and the (1,4,5)IP3-mediated Ca2+ release on isolated cell membranes offer new experimental approaches for the identification of the putative auxin receptor and its signal transduction pathway.

Stimulation of inositol phosphate formation in ROS 17/2.8 cell membranes by guanine nucleotide, calcium, and parathyroid hormone

In addition to stimulation of cyclic AMP, parathyroid hormone (PTH) may influence cellular events by utilizing other pathways of hormone action, such as the generation of inositol phosphates (IPs). We sought to examine this potential action of PTH by assessing the formation of inositol phosphates in PTH-sensitive ROS 17/2.8 cells. The polyphosphoinositides were labeled by growing the cells with [3H]inositol following which cell homogenates were prepared. The nonhydrolyzable guanine nucleotide, GTP gamma S, and calcium ion, alone and together, stimulated all three IPs, IP1, IP2, and IP3. IP1 formation was linear over 30 minutes but IP2 and IP3 accumulated more rapidly peaking by 5 minutes for all agonist conditions. The proportion of total P as IP3 was enhanced when the cells were grown with retinoic acid (1 microM) or when the assay was conducted at pH 4.5. In addition, the lower pH was associated with much more enzyme activity. PTH agonists, bPTH-(1-84) and bPTH-(1-34), both caused a small but significant stimulation of IP3 formation. When bPTH-(1-84), and the analog bPTH-(3-34)amide, that inhibits PTH-mediated adenylate cyclase activity were present together, there was additive stimulation of IP3 formation compared with that with either agent alone. The results demonstrate that inositol phosphate formation can be stimulated directly in a membrane preparation of ROS cells by GTP gamma S, calcium ion, and PTH and that the enzyme mediating this activity, phospholipase C, is regulated by a guanine nucleotide binding protein.

Thromboxane A2 activates phospholipase C in astrocytoma cells via pertussis toxin-insensitive G-protein

The properties of thromboxane A2 (TXA2) receptors were examined in 1321N1 human astrocytoma cells. 9,11-Epithio-11,12-methanothromboxane A2 (STA2), a stable analogue of TXA2, stimulated the accumulation of inositol phosphates (IPs) with an EC50 of about 50 nM. The STA2-induced accumulation of IPs was inhibited concentration dependently by ONO3708, a TXA2 receptor antagonist, with an inhibition constant (Ki) of about 10 nM. Inositol trisphosphate (IP3) was accumulated more rapidly than inositol bisphosphate (IP2) in response to STA2. HPLC analysis indicated that inositol 1,4,5-trisphosphate accumulated in the presence of STA2. STA2 alone had no effect on the accumulation of IPs in membrane preparations but it potentiated the accumulation induced by GTP gamma S. [3H]SQ29548, a TXA2 receptor antagonist, bound specifically to TXA2 receptors, expressing a single binding site with a dissociation constant (Kd) of 10.9 nM. The competition curve for STA2 inhibition of [3H]SQ29548 binding was shifted to the right and was steeper in the presence of GTP gamma S. Pertussis toxin (IAP) elicited ADP-ribosylation of 41KD protein but had no effect on the sensitivity to GTP of the STA2 inhibition of SQ29548 binding or of STA2-induced accumulation of IPs. It is concluded from these results that the stimulation of TXA2 receptors results in activation of phospholipase C via a GTP binding protein and that the protein is not a substrate for IAP.

Effect of guanine nucleotides on phospholipase C activity in permeabilized pituitary cells: possible involvement of an inhibitory GTP-binding protein

Cultured pituitary cells prelabeled with myo-[2-3H] inositol were permeabilized by ATP4-, exposed to guanine nucleotides and resealed by Mg2+. Addition of guanosine 5'-0-(3-thio triphosphate) (GTP gamma S) to permeabilized cells, or gonadotropin releasing hormone (GnRH) to resealed cells, resulted in enhanced phospholipase C activity as determined by [3H] inositol phosphate (Ins-P) production. The effect was not additive, but the combined effect was partially inhibited by guanosine 5'-0-(2-thiodiphosphate) (GDP beta S) or by neomycin. Surprisingly, addition of GDP beta S (100-600 microM) on its own resulted in a dose-related increase in [3H]Ins-P accumulation. Several nucleoside triphosphates stimulated phospholipase C activity in permeabilized pituitary cells with the following order: UTP greater than GTP gamma S greater than ATP greater than CTP. The stimulatory effect of UTP, ATP and CTP, but not GTP gamma S or GDP beta S, could also be demonstrated in normal pituitary cells suggesting a receptor-activated mechanism. GTP and GTP gamma S decreased the affinity of GnRH binding to pituitary membranes and stimulated LH secretion in permeabilized cells. These results suggest the existence of at least two G-proteins (stimulatory and inhibitory) which are involved in phospholipase C activation and GnRH action in pituitary cells.

Changes in polyphosphoinositide metabolism during mediator release from stimulated rat mast cells

The metabolism of the polyphosphoinositides, diphosphoinositide (DPI) and triphosphoinositide (TPI), was studied during mediator release from rat mast cells. Serosal mast cells were purified by density gradient centrifugation and prelabeled with 32PO4. Incorporation of 32PO4 into DPI and TPI was determined by thin-layer chromatography on oxalic acid impregnated silica gel plates. 32PO4 incorporation into DPI and TPI was increased by Concanavalin A (ConA) or compound 48/80. The concentration of ConA causing a half-maximal increase in DPI labeling was less than that required for a comparable change in histamine release. The increases in DPI labeling and histamine release in response to ConA were enhanced by phosphatidylserine. The addition of alpha-methylmannoside to mast cells after challenge with ConA rapidly halted DPI and TPI labeling. The results of these studies indicate that changes in the metabolism of polyphosphoinositides may be an intrinsic part of the biochemical mechanisms that control mediator release from mast cells.

Properties of membranous phospholipase C from WRK1 cell: sensitivity to guanylnucleotides and bacterial toxins

As previously described, WRK1 plasma membrane possesses a vasopressin-sensitive phospholipase C [G. Guillon et al., 1986, FEBS Lett. 196, 155-159]. In the present study, we examined the sensitivity of this enzyme to guanylnucleotides. GTP gamma S induces a time- and dose-dependent stimulation of Ins(1,4,5)P3 and Ins(1,4)P2 accumulation. No accumulation of InsP1, Ins(1,3,4)P3 or Ins(1,3,4,5)P4 occurred under similar conditions. Gpp(NH)p produced the same effect but was less potent. GTP and a nonhydrolyzable analogue of ATP, App(NH)p, were without effect. Calcium also stimulated the phospholipase C activity in a time- and dose-dependent manner. In the absence of calcium, the activity of GTP gamma S was considerably reduced. Physiological calcium concentrations (between 10(-8) and 10(-7) M), allowed maximal GTP gamma S stimulation of phospholipase C activity. In this system, the presence of vasopressin alone did not generate inositol phosphate accumulation. However, this hormone: (i) reduced the lag-time observed during GTP gamma S stimulation, (ii) increased the sensitivity of phospholipase C to GTP and to GTP gamma S, and (iii) did not modify the stimulation of phospholipase C induced by maximal doses of GTP gamma S. Unlike sodium fluoride, GTP gamma S elicited an irreversible activation of phospholipase C. Calcium, GTP gamma S and sodium fluoride stimulated the phospholipase C activity via mechanisms sharing a common step, since their maximal effects were not additive. Cholera toxin treatment, known to produce complete ADP-ribosylation of 'alpha s' subunits, partially reduced the basal and the maximal GTP gamma S-mediated stimulation of phospholipase C activity as well as that caused by vasopressin. This inhibition was not mimicked by treatment with either forskolin or pertussis toxin.

Cholinergic-induced [3H] noradrenaline release in rat brain cortical slices is mediated via a pertussis toxin sensitive GTP binding protein and involves activation of protein kinase C

The involvement of a GTP-binding protein (G-protein) in the process of neurotransmitter release was examined using pertussis toxin and cholera toxin. Cholinergic agonists are shown to mediate [3H]noradrenaline release in rat brain slices via a pertussis toxin (1.2 micrograms/ml) sensitive, and cholera toxin (0.5 microgram/ml) insensitive G-protein. An indication for the involvement of a G-protein and phospholipase C activation in the release process was implied from the inhibitory effect of neomycin on K+-, veratridine- and carbachol-induced-norepinephrine release. Depolarizing agents mediate a neomycin-sensitive release, which is not which is not affected either by pertussis toxin or cholera toxin, suggesting a different mode of phospholipase C activation, unlike carbachol-induced release, which is both neomycin and pertussis toxin sensitive. Similarly, a hormone-sensitive carrier activated by phenylephrine not via alpha 1-adrenergic receptors, mediates a non-exocytosis efflux which is not affected by neomycin and is shown to be pertussis toxin-insensitive. The inhibitory action of protein kinase C inhibitors polymyxin B, K252a and H-7 [(1-(5-isoquinolinesulphonyl)-2-methyl-piperazine] on release, strongly suggests its participation in the process. Polymyxin B, a relatively selective protein kinase C inhibitor, inhibited carbachol-induced release (IC50 = 0.53 microM) as well as the K+ and the veratridine induced [3H] noradrenaline release, K252a, an inhibitor of various protein kinases at the ATP site, and H-7, another protein kinase C inhibitor, inhibited carbachol-induced noradrenaline released with IC50 = 35 nM and 3 microM respectively. Consistent with its inability to activate phospholipase C, phenylephrine-induced noradrenaline efflux was unaffected by polymyxin B (greater than 70 microM). These results offer more supportive evidence for a major role played by the dual messengers inositol trisphosphate and diacylglycerol (IP3/DG) in the mechanisms of neuronal release.

Forskolin and prostacyclin inhibit fluoride induced platelet activation and protein kinase C dependent responses

Platelet activation (cytosolic [Ca2+] increase, aggregation and ATP secretion) was induced with A1F-4. This agent presumably interacts with a G protein which appears to mediate the coupling of the receptors for Ca mobilizing hormones and phospholipase C. All the A1F-4 evoked responses were inhibited by treatment with forskolin or prostacyclin, agents known to increase cellular cAMP. Thus the G protein-phospholipase C system appears to be the site of cAMP inhibition. Unexpectedly forskolin and prostacyclin also inhibited secretion and aggregation induced by the activators of protein kinase C, diglyceride and phorbol ester, suggesting that cAMP can also inhibit directly the protein kinase C dependent responses.

Guanine nucleotides stimulate hydrolysis of phosphatidylinositol and polyphosphoinositides in permeabilized Swiss 3T3 cells

Hydrolysis-resistant analogues of GTP specifically stimulate the formation of [3H]inositol mono-, bis- and trisphosphates by saponin-permeabilized Swiss 3T3 cells prelabelled with [3H]inositol. Each inositol phosphate is formed largely by hydrolysis of its parent lipid and not by dephosphorylation of inositol 1,4,5-trisphosphate [(1,4,5)IP3]. Although hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) is most sensitive to guanine nucleotides, hydrolysis of phosphatidyl-inositol (PI) and phosphatidylinositol 4-phosphate (PIP) is quantitatively more important. These results suggest that a guanine nucleotide-dependent regulatory protein(s) (G-protein) is involved in regulating the hydrolysis of PI and PIP, as well as PIP2, and so may allow formation of diacylglycerol (DG) without simultaneous production of (1,4,5)IP3 and mobilization of intracellular Ca2+.

An analogue of creatine increases TRH-stimulated prolactin secretion and phosphoinositide hydrolysis in rat pituitary tumor cells

Prolactin (PRL)-secreting GH3 cells were grown, in vitro, with the creatine analogue beta-guanidinopropionic acid (GPA) added to the culture medium. After 5 days there was a small increase in basal and greatly increased thyrotropin-releasing hormone (TRH)-stimulated PRL secretion. The site of action of GPA is at the TRH-induced hydrolysis of phosphoinositides, since increased amounts of mono, bis and tris/tetrakis inositol phosphates were found in treated cells, while the PRL secretion induced by a phorbol ester or a calcium ionophore, treatments which mimic the second messages generated by inositol phospholipid hydrolysis, were not enhanced by GPA. The mechanism by which GPA increases phospholipase C activity has not been fully elucidated but may involve the activity of a controlling G protein.

Guanosine 5'-O-thiotriphosphate and NaF stimulation of phosphatidylinositol 4,5-bisphosphate hydrolysis in bovine corneal epithelium

The role of a GTP-binding protein in activation of phospholipase C in bovine corneal epithelium was determined by investigating the effects of non-hydrolyzable GTP analog, GTP-gamma-S, and NaF on breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) in this tissue. GTP-gamma-S (2-50 microM), when introduced into the permeabilized corneal epithelial cells labeled with myo-[3H]inositol, dose-dependently increased the formation of myoinositol trisphosphate (IP3). Other guanine nucleotides and ATP were ineffective. Incubation of 32P-prelabeled corneal epithelium with NaF (2-50 mM) resulted in increased breakdown of PIP2 and increased synthesis of phosphatidic acid. In myo-[3H]inositol-labeled tissue, NaF dose-dependently increased the accumulation of IP3. Microsomal membrane fraction from corneal epithelium was found to contain phospholipase C activity towards endogenous phosphatidylinositol 4-phosphate and PIP2. The enzyme activity was stimulated by Ca2+ (100 microM). Addition of GTP-gamma-S to microsomal fraction containing phosphoinositides which were radiolabeled with 32Pi in situ or with [gamma-32P]ATP in vitro caused a dose dependent hydrolysis of PIP2. These data, taken collectively, suggest that a GTP-binding protein is involved in activation of phospholipase C towards PIP2 in bovine corneal epithelium, and that this guanine nucleotide regulatory protein may serve to couple norepinephrine and 5-hydroxytryptamine receptors to phospholipase C during transmembrane signalling.