Age-related changes in carboxyl methylation of proteins in the kidney

Age-related changes in the carboxyl methylation activities of L-isoaspartyl/D-aspartyl methyltransferase (PIMT) and C-terminal isoprenylcysteine methyltransferase (PPMT), as well as in the methylation levels of their major substrates, were studied in the soluble and brush border membrane (BBM) fractions of kidney cortex isolated from rats aged 3 weeks, and 2, 7 and 12 months. PIMT activity measured with ovalbumin, an exogenous substrate, decreased by 30% in the soluble fraction, while it increased by 37% in BBM of rats older than 2 months. In the soluble fraction, the affinity of PIMT for the universal methyl donor, S-adenosyl-L-methionine, was unaffected, while the apparent maximal velocity measured with ovalbumin was 30% lower in 7-month-old rats than in 3-week-old rats. However, the amount of PIMT measured by Western blotting with anti-PIMT antibodies in the soluble fraction was not affected by age. These results suggest that a reduction in the specific activity of PIMT in the soluble fraction occurs as a function of age. Stimulation of the methylation of total proteins by guanosine 5'-3-O-(thio) triphosphate (GTP gamma S) increased in the soluble fraction of rats older than 2 months, (30%) and decreased in BBM of rats older than 7 months (25%). The PIMT methylation of endogenous substrates of 48 and 61 kDa in the soluble fraction decreased by 40% in rats older than 2 months, but no significant difference was found for substrates in the BBM fraction as a function of age. On the other hand, the PPMT activity was stable from 3 weeks postnatal to adulthood. The C-terminal carboxyl methylation of the major PPMT substrates in BBM (22, 26, and 44 kDa) remained stable throughout development and in adults. The levels of carboxyl methylation of the 22 and 26 kDa substrates in BBM were GTP gamma S-dependent, but only the effect on the 22 kDa substrate was regulated by age. These data suggest that the activities of PIMT and PPMT are regulated differently during development and aging in the rat kidney cortex.

Correlation between guanine nucleotide effect and reversible binding property of endothelin analogs

[125I]-IRL-1620 and [125I]-ET-1 (readily reversible and essentially irreversible endothelin (ET) receptor agonists, respectively) were used to demonstrate the relationship between the reversible binding nature of ET receptor agonists and guanine nucleotide effect using ETB receptors as the model system. Addition of increasing concentrations of GTP gamma s to membranes prepared from Chinese hamster ovary (CHO) cells stably transfected with human ETB receptors, dog lung and pig lung decreased [125I]-IRL-1620 binding to these membranes between 50% and 60%, whereas [125I]-ET-1 binding to these receptors was unaffected by GTP gamma s. Saturation binding experiments in the absence and presence of 100 microM GTP gamma s indicated that the apparent dissociation constant [Kd(apparent)] for [125I]-IRL-1620 was increased 2 to 2.4-fold in all 3 membrane preparations in the presence of GTP gamma s compared to its absence. There was no difference in the apparent dissociation constants of [125I]-ET-1 in the presence and absence of GTP gamma s in these membrane preparations. This inhibitory effect was specific for guanosine triphosphate since adenine nucleotides failed to decrease the affinity of [125I]-IRL-1620 for the receptors. The correlation between guanine nucleotide effect and reversible binding property of the agonist was further strengthened by the observation that in rat cerebellum and rat renal papilla, where [125I]-IRL-1620 binding was irreversible, guanine nucleotides had no effect on the binding of this ligand. These data clearly indicate that there is a good correlation between the reversible binding property of the ET receptor agonist and the guanine nucleotide effect on the binding of the agonist.

GTP hydrolysis by human tissue transglutaminase homologue

Human tissue transglutaminase homologue cDNA was expressed in E. coli to analyze the catalytic characteristics. The transglutaminase homologue was purified by immunoaffinity chromatography. Specificity of GTP binding by the homologue was demonstrated by photoaffinity labeling in the absence or presence of GTP-gamma-S. The homologue had GTPase activity with an apparent Km value of 1.8 microM, several-fold lower than the reported Km values for the native tissue transglutaminase. GTPase activity was inhibited by guanine nucleotides in order GTP-gamma-S > GDP > GMP. The higher GTPase activity of the homologue may be related to the signaling events function.

Characterisation of the interaction between guanyl nucleotides and AMPA receptors in rat brain

Guanyl nucleotides inhibit the binding of the AMPA receptor agonists [3H]fluorowillardiine and [3H]AMPA and the competitive antagonist [3H]CNQX to rat brain cerebrocortical membranes. The rank order of inhibition for each of the radioligands tested was GTP > GDP > GMP. The nucleotides CTP and ATP showed no effect. GTP inhibition was unaffected by the presence or absence of NaCl and MgCl2. Pre-treatment of the membranes with GTP, and its removal before addition of radioligand, did not inhibit binding. Quantitative autoradiography demonstrated that GTP inhibition occurred throughout the brain. These results are consistent with guanyl nucleotides acting at an extracellular site present on all AMPA receptor subunits, occupation of which inhibits agonist and antagonist binding.

Evidence for receptor and G-protein regulation of a phosphatidylethanolamine-hydrolysing phospholipase A1 in guinea-pig heart microsomes: stimulation of phospholipase A1 activity by DL-isoprenaline and guanine nucleotides

While evidence has been presented for the receptor-mediated activation of phospholipases A2, C and D, the activation of phospholipase A1 subsequent to receptor activation has not been established. Phospholipase A1-catalysed hydrolysis of 1-palmitoyl-2-linoleoyl-glycerophosphoethanolamine (GPE) by guinea-pig heart microsomes was stimulated 40-60% by isoprenaline. This isoprenaline-mediated increase in activity was blocked by propranolol and butoxamine, a specific beta 2-adrenergic antagonist, but not by atenolol, a specific beta 1-adrenergic antagonist. Neither clonidine nor phenylephrine, alpha 1- and alpha 2-adrenergic agonists respectively, had a stimulatory effect on the hydrolysis of the PE substrate. Guanosine 5'(-)[gamma-thio]triphosphate (GTP[S]) and guanosine 5'(-)[beta,gamma-imido]triphosphate, but not guanosine 5'(-)[beta-thio]diphosphate (GDP[S]) or adenosine 5'(-)[gamma-thio]triphosphate, stimulated the hydrolysis of 1-palmitoyl-2-linoleoyl-GPE by phospholipase A1. GDP[S] inhibited the isoprenaline-mediated stimulation of phospholipase A1 activity. Phospholipase A1 hydrolysis of 1-palmitoyl-2-linoleoyl-GPE was not dependent on cations; however, the stimulatory effects of isoprenaline and GTP[S] on the hydrolytic activity were abolished by cation chelators. The above data suggest that phospholipase A1 activity in guinea-pig heart microsomes is activated by the binding of isoprenaline to beta 2-adrenergic receptors. Furthermore the stimulation of phospholipase A1 activity by the agonist may be mediated via activation of G-proteins.

Dynamin forms polymeric complexes in the presence of lipid vesicles. Characterization of chemically cross-linked dynamin molecules

Dynamin is a GTP-binding protein that is involved in the release of coated endocytic vesicles from the plasma membrane. We have been characterizing the enzymatic properties of purified rat brain dynamin to better understand how GTP binding and hydrolysis relate to its proposed function. Previously, we have demonstrated that activation of dynamin GTPase results from positive cooperative associations between dynamin molecules as they are bound to a polymeric surface. Our present report has extended these studies and has examined the structural features of dynamin self-association. After treatment with the zero-length protein cross-linking reagent, 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide, dynamin in solution was found cross-linked into dimers. This homodimer likely reflects the native soluble state of the molecule. After binding to brain vesicles, dynamin was cross-linked into higher order oligomers of greater than 800 kDa. Dynamin, copurified on brain membranous organelles, also formed multimeric complexes when cross-linked suggesting dynamin exists in polymeric form in vivo. No cross-linked species other than homo-oligomers were observed, providing no evidence for close interactions between dynamin and membrane proteins. From experiments examining the effects of GTP, GDP, guanosine 5'-3-O-(thio)triphosphate, and 5'-guanylyl-beta,gamma-imidodiphosphate on cross-linking, we have determined that both dynamin membrane binding and self-association occur independently from the nucleotide-bound state of the enzyme. An 80-kDa dynamin fragment that is lacking its carboxyl-terminal domain is not cross-linked into higher order oligomers, suggesting that this domain is required for binding of dynamin to membranes and the subsequent enhancement of oligomerization. However, the dynamin fragment was found to form dimers indicating that this domain is not required for dynamin dimerization. Cross-linked dynamin was able to cooperatively bind microtubules, but did not exhibit GTPase activation. We propose that intramolecular cross-links in the dynamin monomer impart structural constraints that prevent the enhancement of GTP hydrolysis. We describe a model of the dynamin activation process to be considered in further investigations of the role for dynamin in endocytic vesicle formation.

Characterisation of recombinant serotonin 5-HT1A receptors expressed in Chinese hamster ovary cells: the agonist [3H]lisuride labels free receptor and receptor coupled to G protein

Serotonin 5-HT1A receptors expressed stably in recombinant Chinese hamster ovary cells have been studied using radioligand binding with the radiolabelled agonist [3H]lisuride. Competition studies with a range of antagonists versus [3H]lisuride confirmed that all of the specific [3H]lisuride binding was to 5-HT1A receptors on the cells. Competition studies with the antagonist spiperone and several agonists gave data that fitted best to two-binding-site models. The affinities of these competing ligands at the two classes of sites were generally in agreement with their corresponding affinities determined in previous work with either 8-[3H]hydroxydipropylaminotetralin ([3H]8-OH-DPAT; labels receptor coupled to G protein) or [3H]spiperone (labels free receptor). Saturation analyses with [3H]lisuride showed that this radioligand labels a single class of binding sites, but the level of radioligand binding was approximately twice that seen when either [3H]8-OH-DPAT or [3H]spiperone was used. [3H]Lisuride binding was partially inhibited by addition of guanine nucleotides, and the extent of inhibition decreased as the [3H]lisuride concentration was increased. This inhibition was due to the effect of guanine nucleotide to decrease slightly the affinity of [3H]lisuride for binding to the 5-HT1A receptors on the cells. It is concluded that [3H]lisuride can label both the free receptor and the receptor coupled to G proteins but with slightly different affinities and that these two states of the receptor exist in roughly equal amounts in the cells. Agonists generally have a higher affinity for the receptor coupled to G protein, whereas antagonists, with the exception of spiperone (which has a higher affinity for the free receptor), have roughly equal affinities for the free receptor and the receptor coupled to G proteins.

Characterization of the human oxytocin receptor stably expressed in 293 human embryonic kidney cells

The human oxytocin (OT) receptor was stably expressed in 293 embryonic kidney cells (293/OTR), characterized pharmacologically and compared to human uterine myometrial receptors. The cloned receptor is expressed at a reasonably high density (0.82 fmole/microgram protein) and exhibits high affinity for [3H]OT (Kd = 0.32nM), similar to the value found in human myometrial tissue. The rank-order of potency for various antagonist and agonist ligands from several structural classes is also similar between the cloned and native receptor, as seen in a comparison of their inhibitory constants for [3H]OT binding. Agonist affinity at the cloned OT receptor is decreased by guanine nucleotide analogs, demonstrating functional G-protein-coupling. The OT receptor in 293 cells, like in human myometrium, is also coupled to the inositol phosphate pathway. In 293/OTR cells, OT stimulates inositol phosphate accumulation with an EC50 = 4.1 nM, an effect blocked by a potent and selective OT antagonist, L-366,948. Additionally, the cloned receptor in 293 cells desensitizes to high concentrations of OT, similar to the desensitization in myometrial tissue and also described for several other G-protein-coupled receptors. These results illustrate the utility of the 293 cell line for expressing human OT receptors in an environment quite comparable to the native myometrial tissue.

Modulation of agonist binding by guanine nucleotides in CHO cells expressing muscarinic m1-m5 receptors

In membranes prepared from CHO-m2 cells, inhibition of [3H]-N-methylscopolamine ([3H]NMS) binding by several muscarinic agonists resulted in competition curves with Hill slopes significantly different from unity. Addition of 5'-guanylylimidodiphosphate (Gpp(NH)p) led to an increase in the IC50 value of the agonists with significant steepening of the inhibition curves. The shift in potency induced by Gpp(NH)p differed among the agonists with a rank order of oxotremorine-M = carbachol > oxotremorine > McN-A-343 = pilocarpine. In CHO-m4 membranes, Gpp(NH)p was less efficacious than in CHO-m2 membranes whereas no effect of the guanine nucleotide was found in membranes prepared from CHO-m1, -m3, and -m5 cells. No major differences in the effect of Gpp(NH)p among agonists were found in CHO-m4 cells. Atropine binding was not affected by the guanine nucleotide. Together, these results indicate that coupling of G-proteins to muscarinic receptors linked to inhibition of cyclic adenosine monophosphate (cAMP) (m2 and m4) but not of those linked to phosphoinositol turnover (m1, m3 and m5) can be perturbed by Gpp(NH)p. The differential effects observed with Gpp(NH)p between agonist binding to m2 and m4 receptors appear to be receptor-specific and may reflect differences in the G proteins activated by these receptors in CHO cells.

Characterisation of inositol 1,4,5-trisphosphate binding sites in rabbit aortic smooth muscle

The present study investigated the characteristics of D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) binding sites in crude membrane preparations of rabbit aortic smooth muscle. A particular aim was to demonstrate if increases in cytoplasmic cyclic guanosine 3':5' monophosphate (cGMP), which mediates the effect of nitrovasodilators, may cause smooth muscle relaxation in part by the displacement of Ins(1,4,5)P3 binding. Negligible Ins(1,4,5)P3 binding was observed at pH < 7, while maximum binding occurred over the pH range 8-9. Saturation analysis of isotopic dilution binding data revealed an apparently homogenous population of Ins(1,4,5)P3 binding sites with a KD of 4.02 +/- 0.53 nM and a Bmax of 27.7 +/- 4.6 fmol/mg protein. Heparin, an Ins(1,4,5)P3 receptor antagonist, inhibited binding with an IC50 of 11.43 +/- 2.81 micrograms/ml. The ability of other polyphosphate compounds to inhibit Ins(1,4,5)P3 binding in this preparation was also examined. D-myo-Inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), adenosine 5'-triphosphate (ATP) and guanosine 5'-triphosphate (GTP) inhibited Ins(1,4,5)P3 binding, although each was significantly less potent that Ins(1,4,5)P3. In contrast, cyclic guanosine 3':5' monophosphate (cGMP) did not significantly alter Ins(1,4,5)P3 binding in rabbit aortic smooth muscle. This observation suggests that competitive inhibition of Ins(1,4,5)P3 receptor binding is not an important consideration in cGMP-mediated vascular smooth muscle cell relaxation.

2-[125I]iodomelatonin binding sites in the bovine hippocampus are not sensitive to guanine nucleotides

The discrete distribution and pharmacological characteristics of melatonin binding sites in the bovine hippocampus were determined. Autoradiography revealed the presence of melatonin binding sites in the stratum lacunosum-molecularis of the hippocampus (CA1), stratum molecularis of the subiculum and in the enthorhinal cortex. Analysis of the kinetic parameters demonstrated that the binding was stable and reversible, represented by a single class high affinity binding sites (Kd 40 pM, Bmax = 3.9 fmol/mg protein). However, 2-iodomelatonin and 2-bromomelatonin inhibited 2-[125I]iodomelatonin binding in a biphasic manner. The presence of 4 mM CaCl2 did not cause changes in the affinity constant values. Finally, experiments performed with GTP gamma S revealed that binding affinity was not decreased even with high concentrations of the nucleotide. These findings show that 2-[125I]iodomelatonin binding sites in the bovine parahippocampal-hippocampal region possess some binding features not common to melatonin receptors described so far; moreover they seem not to be linked to a regulatory G-protein.

A soluble protein negatively regulates phospholipase D activity. Partial purification and characterization

Phosphatidylcholine-specific phospholipase D (PLD) is an important signalling phospholipase in mammalian cells. Recently, PLD activity has been shown to be positively regulated by the GTP-binding protein ARF (ADP-ribosylating factor). In the present work, we document the presence of a factor negatively regulating PLD activity in bovine brain cytosol. The inhibitory factor is characterized as a large protein or a complex of proteins with a molecular mass higher than 300 kDa. Using permeabilized and pre-permeabilized HL-60 cells depleted of their cytosol, we demonstrate that the inhibitor acts on GTP[S]-stimulated PLD activity. This effect is immediate, persistent and dose dependent for GTP[S]-stimulated PLD. Different possibilities for a mechanism of action of the inhibitory factor on the regulation of GTP binding of ARF were investigated. This inhibitory factor is not the guanine-dissociating inhibitor (GDI) for the small G-binding proteins Rho (Rho-GDI), reported to be a PLD inhibitor, since specific antibodies against this protein did not recognize a protein in the peak containing the inhibitory factor for PLD activity. Furthermore, the inhibitory factor does not prevent the binding of GTP[S] to ARF in the presence of HL-60 membranes. This excludes its possible role as an inhibitor of an ARF/guanine exchange factor. The inhibitory factor not only inhibits a pathway of PLD through GTP[S] activation in particular of the small GTP-binding protein, ARF, but it also inhibits PLD activated via either protein kinase C (PKC) or tyrosine kinase activation. The inhibitory factor also decreases PLC activity and this effect seems to be secondary to the inhibition of PLD activity. We discuss a mechanism of action of the inhibitor on PLD and the importance of this enzyme activity for membrane traffic.

Characterization of a rat type 2 angiotensin II receptor stably expressed in 293 cells

A cDNA clone for the rat type 2 (AT2) angiotensin II receptor was stably transfected into human embryonic kidney 293 cells. Binding characteristics of CGP42112A (Kd = 0.18 nM, Bmax = 10.8 pmol/mg protein) and ligand specificity were indistinguishable from those obtained with the whole rat fetus and with transiently transfected COS-7 cells. Non-hydrolyzable guanine nucleotide analogs did not affect the ligand binding curve; interestingly, the guanine nucleotide analog's effect was observed in the presence of sulfhydryl reducing agent, suggesting that a certain redox condition may affect G protein coupling to this receptor. Using the established cell line, several second messenger systems were assessed. None of cAMP levels, cGMP levels, arachidonic acid release, or phosphotyrosine phosphatase activity was affected by angiotensin II stimulation of this receptor. Furthermore, the AT2 receptor did not undergo agonist-stimulated internalization. These results using the cloned receptor suggest that the transfected AT2 receptor fails to effectively couple to the major G protein-mediated signaling mechanisms and ligand-activated internalization in transfected 293 cells.

Dopamine-induced reduction in the density of guanine nucleotide-sensitive D1 receptors in human postmortem brain in the absence of apparent D1: D2 interactions

The effects of dopamine and guanine nucleotides on the binding of the D1 dopamine receptor antagonist ligand [3H]SCH 23390 were examined in membranes prepared from putamen, caudate and nucleus accumbens of human postmortem brain. Dopamine induced a concentration-dependent decrease in the apparent maximum number of binding sites (Bmax) in each brain region studied, and displaced binding in a biphasic manner consistent with the presence of both high and low affinity states of the D1 receptor; the GTP analogue Gpp(NH)p transformed this biphasic displacement to a monophasic pattern consistent with a shift of high affinity sites to a low affinity state. However, the selective D2 antagonist eticlopride did not reverse the action of dopamine to decrease Bmax. These data suggest that dopamine decreases Bmax for D1 receptors through a high affinity, guanine nucleotide-sensitive agonist binding site, but fail to reveal D1:D2 interactions at this synaptic level.

Dopamine D4 receptors and effects of guanine nucleotides on [3H]raclopride binding in postmortem caudate nucleus of subjects with schizophrenia or major depression

The densities of dopamine-D4 receptors were determined in postmortem samples of caudate nucleus from patients with schizophrenia (n = 9) and age-matched controls (n = 10). D4 receptor binding was defined as the difference between binding sites labeled by [3H]YM-09151-2 (D2 + D3 + D4 receptors) and those by [3H]raclopride, in the presence of 5'-guanylylimidodiphosphate (Gpp(NH)p) (D2 + D3 receptors). D4 receptor binding was measurable in all the subjects with schizophrenia (mean = 3.8 pmol/g tissue) but only in 3/10 controls. To determine the specificity of these findings for schizophrenia, D4 receptor binding was also measured in the caudate nucleus of suicide victims with major depression (n = 6) and age-matched controls (n = 6). A small amount of D4 binding was noted in some of the controls + depressed subjects and there was no significant difference between controls and patients with major depression. The addition of 200 microM Gpp(NH)p to the assay significantly increased the amount of specific binding of [3H]raclopride in control tissues, but not in tissues from subjects with schizophrenia, suggesting an abnormality in the G-protein component coupled to the D2 receptor. [3H]Raclopride binding was also significantly increased by Gpp(NH)p in subjects with major depression. These results confirm a previous report of Seeman et al. (1993) and suggest that measurable D4 receptor binding in the caudate nucleus is more frequent in patients with schizophrenia as compared with normal controls and subjects with major depression and that guanine nucleotides do not enhance [3H]raclopride binding in schizophrenia.(ABSTRACT TRUNCATED AT 250 WORDS)

Translocation of p21rac2 from cytosol to plasma membrane is neither necessary nor sufficient for neutrophil NADPH oxidase activity

Activation of the membrane-associated NADPH oxidase of neutrophils requires several cytosolic factors including p47phox, p67phox and p21rac2. We compared NADPH oxidase activity with the membrane translocation of p47phox, p67phox, and p21rac2. In a cell-free system, GTP gamma S stimulated translocation of p47phox and p67phox to the plasma membrane only in the presence of arachidonate, and this translocation correlated with NADPH oxidase activity of the reisolated plasma membranes (R = 0.94 and 0.97, respectively). In contrast, GTP gamma S-stimulated p21rac2 translocation with or without arachidonate, and the extent of translocation did not correlate with oxidase activity (R = 0.17). Neutrophil cytoplasts were used to relate membrane translocation of p47phox, p67phox and p21rac2 to membrane oxidase activity in response to the inflammatory agonists. Whereas N-formyl-methionyl-leucyl-phenylalanine stimulated equimolar, transient membrane translocation of p47phox and p67phox which kinetically paralleled NADPH oxidase activity, relatively little p21rac2 translocated (moles of p47phox/p21rac2 = 16.6). Moreover, although phorbol 12-myristate 13-acetate stimulated both the stable translocation of p47phox and p67phox and sustained NADPH oxidase activity, it did not stimulate p21rac2 translocation. From these data we conclude that membrane translocation of p21rac2 does not regulate NADPH oxidase activity stoichiometrically.

Characterization of the signal transduction of prostaglandin E receptor EP1 subtype in cDNA-transfected Chinese hamster ovary cells

We examined the signal transduction of mouse prostaglandin E receptor EP1 subtype using Chinese hamster ovary cells stably expressing the cloned EP1. Sulprostone, an EP1 agonist, induced a rapid increase in intracellular Ca2+ concentration in the EP1-expressing cells. Most of the increase was abolished by removal of extracellular Ca2+, and was insensitive to U-73122, a phospholipase C inhibitor. Sulprostone stimulated phosphatidylinositol hydrolysis, but this stimulation was abolished by removal of extracellular Ca2+, indicating that EP1-stimulated phosphatidylinositol hydrolysis is the result of extracellular Ca2+ influx. Thus, the signal transduction of EP1 is extracellular Ca2+ entry through a pathway independent of phospholipase C activation. We further examined the regulation of the signal transduction of EP1 having potential phosphorylation sites for either protein kinase C or protein kinase A. Short-term exposure of the cells to 12-O-tetradecanoylphorbol 13-acetate (TPA) completely suppressed the sulprostone-induced increase in intracellular Ca2+ concentration, while forskolin or dibutyryl cAMP did not affect it, suggesting that protein kinase C but not protein kinase A is involved in the regulation of the EP1 signal transduction. Furthermore, long-term exposure to TPA decreased PGE2 protein kinase A is involved in the regulation of the EP1 signal transduction. Furthermore, long-term exposure to TPA decreased PGE2 binding activity of EP1 due to the reduction of the EP1 mRNA level. Protein kinase C induces short- and long-term desensitization of EP1.

Involvement of multiple protein kinase C isozymes in the ACTH secretory pathway of AtT-20 cells

1. The mouse AtT-20/D16-16 anterior pituitary tumour cell line was used as a model system for the study of protein kinase C (PKC)-mediated enhancement of calcium- and guanine nucleotide-evoked adrenocorticotrophin (ACTH) secretion. 2. A profile of the PKC isozymes present in AtT-20 cells was obtained by Western blotting analysis and it was found that AtT-20 cells express the alpha, beta, epsilon and zeta isoforms of PKC. 3. PKC isozymes were activated by the use of substances reported to activate particular isoforms of the enzyme. The effects of these substances were investigated in both intact and electrically-permeabilized cells. Phorbol 12-myristate 13-acetate (PMA, EC50 = 1 +/- 0.05 nM, which activates all isozymes of PKC, except the zeta isozyme), thymeleatoxin (TMX, EC50 = 10 +/- 0.5 nM, which activates the alpha, beta and gamma isozymes) and 12-deoxyphorbol 13-phenylacetate 20-acetate (dPPA, EC50 = 3 +/- 0.5 nM, a beta 1-selective isozyme activator) all stimulated ACTH secretion from intact cells in a concentration-dependent manner. Maximal TMX stimulated ACTH secretion was of a similar degree to that obtained in response to PMA but maximal dPPA-stimulated ACTH secretion was only 60-70% of that obtained in response to PMA or TMX. 4. Calcium stimulated ACTH secretion from electrically-permeabilized cells over the concentration-range of 100 nM to 10 microM. PMA (100 nM), TMX (100 nM) but not dPPA (100 nM) enhanced the amount of ACTH secreted at every concentration of calcium investigated. PMA (100 nM) and TMX (100 nM)significantly enhanced ACTH secretion in the effective absence of calcium (i.e. where the free calcium concentration is nM).5. GTP-gamma-S stimulated ACTH secretion from permeabilized cells in a concentration-dependent manner with a threshold of 1 micro M. PMA (100 nM), TMX (100 nM) but not dPPA (100 nM) increased the amount of ACTH secretion evoked by every concentration of GTP-gamma-S investigated.6. The PKC inhibitor, chelerythrine chloride (10 micro M), blocked the PMA (100 nM)-evoked enhancement of calcium- and GTP-micro-S-stimulated ACTH secretion but did not significantly alter calcium- or GTP-micro-S-evoked secretion itself.7. The present paper indicates that AtT-20 cells express multiple isoforms of PKC and that these act at different sites in the secretory pathway for ACTH secretion. The alpha and epsilon isozymes of PKC can act distal to calcium entry to modulate the ability of increased cytosolic calcium concentrations to stimulate ACTH secretion. This site of action is either at the level of, or at some stage distal to, a GTP-binding protein which mediates the effects of calcium upon ACTH secretion. The beta isozyme of PKC may act ata stage early in the secretory pathway to regulate the cytosolic calcium concentration.

Evidence for regulated coupling of A1 adenosine receptors by phosphorylation in Zucker rats

Studies were designed to find the molecular basis for previous observations that lipolysis is less active and A1 adenosine receptor signaling is more active in adipocytes from obese than from lean Zucker rats. With quantitative immunoblot procedures for detection, Gi alpha 1 and Gs alpha 45 levels were found anomalously low in obese compared with lean membranes (50 and 30%, respectively), but other G alpha subunit levels were normal. However, the sensitivity of the receptor-Gi protein to GTP was about 5- to 10-fold higher in obese compared with lean membranes when assessed from 1) the ability of GTP to inhibit forskolin-stimulated adenylyl cyclase in the presence of an adenosine receptor agonist and 2) the ability of a nonhydrolyzable guanine nucleotide analogue to alter A1 adenosine receptor agonist binding. Alkaline phosphatase treatment of isolated adipocyte membranes from obese but not lean animals decreased guanine nucleotide sensitivity of agonist binding. Surprisingly, solubilized adipocyte A1 adenosine receptors from all animals exhibited the same high sensitivity to guanine nucleotides as that of intact obese membranes, and this high sensitivity could be decreased 20-fold by treatment with alkaline phosphatase. These data suggest that protein phosphorylation may regulate coupling of the A1 adenosine receptor in rat adipocyte membranes.

Epidermal growth factor receptor signaling in rat pancreatic acinar cells

Epidermal growth factor (EGF) regulates pancreatic acinar enzyme secretion. The mechanism of action of EGF in pancreatic acinar cells is not clear. In the present study we investigated the role of heterotrimeric GTP-binding proteins (G proteins) in EGF receptor signal transduction. Pancreatic acini were isolated from rat pancreas by collagenase digestion and permeabilized by digitonin. Activation of phosphatidylinositol 4,5-bisphosphate-specific phospholipase C (PLC) was assessed using a radioreceptor assay specific for inositol 1,4,5-trisphosphate [IP3(1,4,5)]. For measurement of amylase secretion isolated pancreatic acini were incubated with secretagogues for 30 min at 37 degrees C. Amylase released into the medium was assessed by monitoring the hydrolysis rate of p-nitrophenyl-alpha,D-maltohepatoside. The weakly hydrolyzable GTP analogue guanosine 5'-[3-O-thio]triphosphate (GTP gamma S) and guanosine 5'-diphosphate (GDP) were used to activate and inhibit G protein-mediated signal transduction, respectively. EGF (90 nM) stimulated amylase release in isolated pancreatic acini. This effect was enhanced by guanosine 5'-[3-O-thio]triphosphate (0.1 mM), which stimulates G proteins. Guanosine 5'-diphosphate (1 mM), which inhibits the activity of heterotrimeric G proteins, had no effect on basal and EGF-induced amylase release. Lower EGF concentrations (20 nM) inhibited COOH-terminal cholecystokinin octapeptide (CCK8)-induced IP3(1,4,5) production and amylase release in pancreatic acini). However, in the presence of GDP, EGF had no significant effect on CCK8-stimulated amylase release. Furthermore, coincubation of the acini with CCK8, EGF, and GDP revealed that GDP reduces the inhibitory effect of EGF on CCK8-induced IP3(1,4,5) production.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of calyculin A upon calcium-, guanine nucleotides- and phorbol 12-myristate 13-acetate-stimulated ACTH secretion from AtT-20 cells

1. The mouse AtT-20/D16-16 anterior pituitary tumour cell line was used as a model system for the study of protein phosphatase involvement in the late stages of the secretory pathway for adrenocorticotrophin (ACTH) secretion. The effects of the type 1 and 2 phosphatase inhibitor calyculin A upon calcium-, guanine nucleotide- and phorbol 12-myristate 13-acetate (PMA)-stimulated ACTH secretion from electrically-permeabilized AtT-20 cells were studied. 2. Calyculin A (1 nM-1 microM) inhibited both calcium (10 microM)- and guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) (100 microM)-evoked ACTH secretion from permeabilized cells in a concentration-dependent manner. These effects were maximal with 100 nM calyculin A. 3. ACTH secretion was stimulated from electrically-permeabilized cells when the cytosolic free calcium ion concentration, controlled by calcium-EGTA buffers, was raised over the concentration range of 100 nM to 10 microM. This calcium-stimulated ACTH secretion was inhibited by co-incubation with calyculin A (100 nM). 4. GTP-gamma-S (10 nM-100 microM) stimulated ACTH secretion from permeabilized cells at concentrations greater than 1 microM GTP-gamma-S. Co-incubation with calyculin A (100 nM) inhibited this stimulation of ACTH secretion observed at these concentrations of GTP-gamma-S. 5. PMA (100 nM) significantly stimulated ACTH secretion from permeabilized cells in the absence of either calcium and guanine nucleotides and this action was enhanced by calyculin A (100 nM). Furthermore, an inhibition of GTP-gamma-S (100 microM)-stimulated ACTH secretion observed in the presence of calyculin A (100 nM) was not observed in the presence of PMA (100 nM). 6. The results of the present study indicate that dephosphorylation by phosphatases plays an important role in stimulus-secretion coupling in AtT-20 cells and is involved in mediating the effects of GE upon the secretory apparatus in these cells. Furthermore, the point of regulation of the secretory response by PKC which underlies the ability of PKC to amplify the calcium/GE system may lie distal to both GE and these phosphatases.

Activation-dependent carboxyl methylation of neutrophil G-protein gamma subunit

The gamma subunits of heterotrimeric guanine nucleotide-binding regulatory (G) proteins (G gamma) are post-translationally processed at their C termini by prenylation, proteolysis, and carboxyl methylation. Whereas prenylation of G gamma is required for membrane association of G proteins, the role of carboxyl methylation is unknown. Here we show that human neutrophils express G gamma 2 but not G gamma 3 or G gamma 7 and that carboxyl methylation of G gamma 2 is associated with signal transduction. In a reconstituted cell-free system, neutrophil G gamma 2 was labeled by the methyl donor S-[methyl-3H]adenosyl-L-methionine. Carboxyl methylation was confirmed by alkaline hydrolysis and quantitation of volatile [3H]methanol. Neutrophil G gamma 2 methylation was stimulated by activation of G protein with guanosine 5'-[beta, gamma-thio]triphosphate. We estimate that after 1 hr of G-protein activation at least 6% of the total pool of G gamma 2 was carboxyl-methylated. The inflammatory agonist fMet-Leu-Phe stimulated guanosine 5'-[beta,gamma-thio]triphosphate-dependent carboxyl methylation. Methylation of G gamma 2 was inhibited by the carboxyl methyltransferase inhibitor N-acetyl-S-trans,trans-farnesylcysteine at concentrations that affected signal transduction in neutrophils. These results demonstrate that activation of neutrophil Gi is associated with alpha-carboxyl methyl esterification of G gamma 2 and suggest that carboxyl methylation of G gamma may play a role in signal transduction.

The regulatory effect of nucleoside diphosphate kinase on G-protein and G-protein mediated phospholipase C

The effect of nucleoside diphosphate kinase (NDPK) on the activity of guanine nucleotide regulatory protein (G-protein) mediated phospholipase C (PLC) and on the [35S] GTPT tau S binding of G-protein was investigated in this work in order to demonstrate the mechanism behind the regulation of G-protein and its effector PLC by NDPK. The stimulation of PLC in turkey erythrocyte membrane by both GTP and GTP tau S indicated that the PLC stimulation was mediated by G-protein. NDPK alone stimulated PLC activity, as well as the stimulation in the presence of GTP and GDP, in a dose-dependent manner. However, NDPK inhibited GTP tau S-stimulated PLC. Furthermore, NDPK inhibited [35S]GTP tau S binding of purified Gi-protein in a non-competitive manner. A hypothesis implying an important role of direct interaction of G-protein and NDPK in the regulation of their functions is suggested and discussed.

Activity of cAMP-dependent protein kinase is reduced in protein-energy malnourished rats

Glucagon decreases glutathione synthesis in hepatocytes from well-nourished rats. However, in hepatocytes from malnourished rats, glucagon does not inhibit glutathione synthesis, suggesting a desensitization of cAMP-mediated signal transduction. We investigated the mechanism for this desensitization of cAMP-mediated responsiveness in malnourished rats by comparing the signal transduction pathways in rats fed very low protein diets (0.5 g protein/100 g diet) with those of rats fed diets adequate in protein (15 g protein/100 g diet) for 2 wk. Glucagon receptor and forskolin-stimulated cAMP production were greater in hepatocytes from malnourished rats. Stimulation of adenylyl cyclase with forskolin, guanine nucleotides or manganese in hepatic membranes was also enhanced after malnutrition. Moreover, quantity of the stimulatory guanine nucleotide regulatory protein was 70-80% greater in hepatocytes from malnourished rats but the inhibitory guanine nucleotide regulatory protein was not different. These results suggested that desensitization of cAMP-mediated signal transduction after malnutrition occurred at a site distal to cAMP production. Maximal activity of cAMP-dependent protein kinase was 60% lower in liver homogenates from malnourished rats compared with controls. This difference in activity was confined to the cytosolic compartment, with no difference in activity observed in the particulate fraction. Lower activity of cAMP-dependent protein kinase in the cytosol of malnourished rats was associated with a 43% reduction in the quantity of regulatory subunit type I, with no difference in the regulatory subunit type II. These data indicate that desensitization of cAMP signal transduction in rat liver after malnutrition is due to a decrease in the quantity and activity of cAMP-dependent protein kinase.

The inhibition of adenylyl cyclase activity in isolated lung membranes by muscarinic and alpha-adrenoceptor agonists: role of G-protein alpha and beta gamma sub-units

Forskolin (10 microM) failed to abolish the GppNHp- (0.1 nM) dependent inhibition of adenylyl cyclase activity in isolated membranes. Whilst clonidine (1 microM), an alpha 2-adrenoceptor agonist, inhibited adenylyl cyclase activity it did not protentiate the GppNHp-dependent inhibition. This indicates that low concentrations of the guanine-nucleotide activate sufficient Gi to inhibit adenylyl cyclase activity maximally and that clonidine inhibits this enzyme via a similar route. These data support a role of alpha i sub-unit inhibition of adenylyl cyclase activity. In contrast, forskolin (10 microM) abolished the GppNHp (0.001-0.1 nM) dependent inhibition of adenylyl cyclase activity in membranes where adenylyl cyclase activity is limited, i.e. where activity has been depleted by approximately 80%. In this case, inhibition of adenylyl cyclase by beta gamma sub-units is implicated and only becomes evident under these conditions. Adenylyl cyclase is also inhibited by muscarinic receptor agonist, methacholine and by the alpha 2-adrenoceptor agonists, clonidine and nor-adrenaline. Both classes of agonist also elicit an increase in the cholera toxin-catalysed ADP-ribosylation of the splice variant forms of Gs alpha and of a polypeptide of 41,000 M(r). The ADP-ribosylation of the 41,000 M(r) polypeptide is inhibited by GTP (100 microM) and therefore displays characteristics similar to Gi alpha. Muscarinic receptor and alpha 2-adrenoceptor agonists appear to inhibit adenylyl cyclase activity in lung membranes predominantly via Gi alpha. Lung expresses both type II and IV adenylyl cyclase which are stimulated by direct interaction with beta gamma sub-units and this is conditional upon the co-incident activation of Gs alpha.(ABSTRACT TRUNCATED AT 250 WORDS)