The role of hormone receptors and GTP-regulatory proteins in membrane transduction

Cholera toxin can catalyze ADP-ribosylation of cytoskeletal proteins

Cholera toxin catalyzes transfer of radiolabel from [32P]NAD+ to several peptides in particulate preparations of human foreskin fibroblasts. Resolution of these peptides by two-dimensional gel electrophoresis allowed identification of two peptides of Mr = 42,000 and 52,000 as peptide subunits of a regulatory component of adenylate cyclase. The radiolabeling of another group of peptides (Mr = 50,000 to 65,000) suggested that cholera toxin could catalyze ADP-ribosylation of cytoskeletal proteins. This suggestion was confirmed by showing that incubation with cholera toxin and [32P]NAD+ caused radiolabeling of purified microtubule and intermediate filament proteins.

Dopaminergic modulation of adenylate cyclase stimulation by vasoactive intestinal peptide in anterior pituitary

The activation of adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] by vasoactive intestinal peptide (VIP) was used as a model to investigate the molecular mechanisms triggered by the occupancy of dopamine recognition sites in rat anterior pituitary. Dopamine failed to change the basal enzyme activity, but it inhibited the stimulation of adenylate cyclase elicited by VIP. Apomorphine, 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene, and 2-bromo-alpha-ergocryptine mimicked the effect of dopamine, whereas (-)-sulpiride and and classical neuroleptics antagonized it. Dopamine failed to modulate the activation of pituitary adenylate cyclase by prostaglandin E1, which does not increase prolactin secretion. From these results we infer that stimulation of D-2 (dopamine) receptors may affect pituitary secretion by inhibiting the activation of anterior pituitary adenylate cyclase by VIP or other secretagogues.

Molecular complexes involved in the regulation of adenylate cyclase

Selective extraction of the adenylate cyclase regulatory protein (N-protein) from pigeon erythrocyte plasma membranes provided evidence for its cytoskeletal association. Cholate, but not Triton X-100 or digitonin, was effective in solubilizing the ADP-ribosylated N-protein. The labeled protein complex or components thereof that were associated with the Triton-insoluble cytoskeleton (shells) could be partly released by 0.1 mM EDTA; 1 M KCl in the presence of Triton X-100 achieved complete solubilization. 5'-Guanylyl imidodiphosphate (p[NH]ppG) and NaF, activators of adenylate cyclase, promoted the release of the regulatory protein from the cytoskeleton but MnCl2, an "uncoupler" of the adenylate cyclase system, had the opposite effect. The solubilized, labeled N-protein was able to bind specifically to rat erythrocyte inside-out vesicles in the presence of divalent cations. A proteolytic product of inside-out vesicles inhibited the binding of the N-protein to fresh vesicles. Three molecular species which contained the Mr 45,000 polypeptide component of the N-protein were identified by gel permeation chromatography and by sucrose density gradient velocity sedimentation. p[NH]ppG appeared to convert the two larger molecular complexes to a smaller molecular entity. Such a molecular dissociation might be relevant to the effects of guanyl nucleotides on the activity of adenylate cyclase and on the affinity of hormone receptors.

Up-regulation of hepatic prostaglandin E receptors in vivo induced by prostaglandin synthesis inhibitors

Up-regulation in vivo of liver plasma membrane receptors for prostaglandin E (PGE) was studied in Sprague-Dawley rats using the prostaglandin synthesis inhibitors, acetylsalicylic acid (ASA) and indomethacin (Indo). Following 4 days of treatment with ASA, the concentration of receptors and the affinity for binding were both significantly increased (Ro - +37%, KA = +62%). Following 4 days of treatment with Indo, the number of receptors was increased but the binding-site affinity was decreased (Ro = +40%, KA = -71%). Animals were then examined after treatment with either ASA or Indo for 1 day, a time when there was no significant decrease in PGE. After 1 day of treatment, the opposite changes in binding-site affinity were again observed, but there were no changes in the number of receptors with either drug, suggesting that the changes in affinity resulted from non-prostaglandin-related effects of the drugs. To ascertain the physiologic consequences of up-regulation, adenylate cyclase activity was measured in control and up-regulated membranes. There were no significant changes in basal or in PGE-stimulated adenylate cyclase activity. These data demonstrate that decreased endogenous PGE causes up-regulation of PGE receptors, but that this is not accompanied by increased adenylate cyclase activity. These data may indicate that PGE-stimulated adenylate cyclase operates maximally under normal receptor concentrations and that therefore its activity cannot be increased by regulatory changes in receptor density.

Activation of adenylate cyclase in bovine corpus-luteum membranes by human choriogonadotropin, guanine nucleotides and NaF

1. Preincubation of luteal membranes with human choriogonadotropin results in the formation of an activated state of adenylate cyclase which is not reversed by washing and which is limited only by the absence of guanine nucleotides, whereas preincubation with GTP yields only a partially activated adenylate cyclase which requires the presence of both GTP and human choriogonadotropin during assay to demonstrate maximal activity. 2. Preincubation of luteal membranes with GTP and human choriogonadotropin does not lead to a synergistic increase in wash-resistant activity. 3. Luteal membranes that had been preincubated with GTP and hormone exhibited a decreasing rate of cyclic AMP synthesis during the adenylate cyclase assay incubation; addition of GTP during the assay incubation reversed the decrease. 4. Membranes that had been preincubated in the absence of guanine nucleotide and hormone showed a ;burst' phase of cyclic AMP synthesis when GTP was present in the assay incubation and a ;lag' phase with p[NH]ppG (guanosine 5'-[beta,gamma-imido]triphosphate) present in the assay. The presence of human choriogonadotropin with either nucleotide in the assay incubation eliminated the curvatures in plots observed with guanine nucleotides alone. 5. Luteal adenylate cyclase was persistently activated by preincubation with p[NH]ppG alone or in combination with human choriogonadotropin; the activation caused by p[NH]ppG alone was still increasing after 70min of preincubation, whereas that caused by p[NH]ppG in the presence of hormone was essentially complete within 10min of preincubation. 6. Luteal adenylate cyclase that had been partially preactivated by preincubation with p[NH]ppG was slightly increased in activity by the inclusion of further p[NH]ppG in the adenylate cyclase assay incubation, but more so with p[NH]ppG and hormone. Human choriogonadotropin alone caused no further increase in the activity of the partially stimulated preparation unless p[NH]ppG was also added to the assay incubation. 7. GTP decreased the activity of adenylate cyclase in membranes that had been partially preactivated in the presence of p[NH]ppG; the decrease in activity was greater when GTP and hormone were present simultaneously in the assay. 8. The results indicate that stable activation states of adenylate cyclase can be induced by preincubation of luteal membranes in vitro with human choriogonadotropin or p[NH]ppG, and that in the presence of p[NH]ppG the hormone may accelerate events subsequent to guanine nucleotide binding. Stable activation of luteal adenylate cyclase by prior exposure to GTP is not achieved. The involvement of GTPase activity and of hormone-promoted guanine nucleotide exchange in the modulation of luteal adenylate cyclase activity is discussed.

Effect of guanine nucleotides on dopaminergic agonist and antagonist affinity for [3H]sulpiride binding sites in rat striatal membrane preparations

[3H]Sulpiride bound to rat striatal membrane preparations with a saturable, high affinity component. This binding was displaced potently by dopamine antagonists (both classic neuroleptics and the benzamide, sulpiride) and less potently by dopamine agonists. GTP and its stable analogue Gpp(NH)p did not affect [3H]sulpiride binding to the membranes but altered the affinity for dopaminergic agonists. This effect was specific in that antagonist binding was not affected and only GTP, GDP, and Gpp(NH)p produced the effect. Similar alterations in ligand binding affinity caused by guanine nucleotides have been observed for binding sites linked to an adenylate cyclase. Such an interpretation for the case of [3H]sulpiride is contrary to suggestions that sulpiride labels only those dopamine receptors that are not cyclase linked.

Interconverting mu and delta forms of the opiate receptor in rat striatal patches

The binding of a radiolabeled "mu receptor" prototype opiate, dihydromorphine (H2morphine), and the binding of a "delta receptor" prototype, [D-Ala2,D-Leu5]enkephalin (D-Enk), to slide-mounted rat caudate slices were simultaneously compared quantitatively and visualized by autoradiography. Generally, D-Enk bound to opiate receptors distributed evenly throughout the entire striatum (type 2 pattern), whereas H2morphine labeled discrete islands or patches of receptors (type 1 pattern). In the presence of Mn2+ (3 mM) or other divalent cations, however, Na+ and GTP at 25 degrees C caused an increase in D-Enk binding at the expense of H2morphine binding at striatal opiate receptor patches. Thus, these conditions shifted D-Enk binding from an even pattern to one that included both an even and patchy distribution. These incubation conditions not only promoted D-Enk binding to striatal patches but also enabled the opiate receptor to regulate adenylate cyclase with the same (P less than 0.01) ligand selectivity pattern as that obtained by the displacement of D-Enk binding. The relative affinity of opiate receptors in striatal patches for opiate peptides, naloxone, and morphine appears to be a function of incubation conditions and coupling to adenylate cyclase and is not indicative of distinctly different opiate receptors. We postulate a three-state allosteric model consisting of mu agonist-, mu antagonists-, and adenylate cyclase-coupled delta-agonist-preferring states, whose equilibrium may be regulated by a sulfhydryl group mechanism.

Muscarinic-dopaminergic synergism on retinal cyclic AMP formation

The cholinergic agonist, carbachol, produces a small increase in cyclic AMP concentration in the isolated rat retina, and markedly potentiates dopamine-stimulated cyclic AMP formation. This effect of carbachol is mediated through a muscarinic receptor, is calcium-independent, and is not due to inhibition of phosphodiesterase activity. Activation of muscarinic receptors may potentiate dopaminergic responses in the retina by enhancing coupling of the dopamine receptor to adenylate cyclase.

Forskolin: unique diterpene activator of adenylate cyclase in membranes and in intact cells

The diterpene, forskolin [half-maximal effective concentration (EC50), 5-10 microM] activates adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] in rat cerebral cortical membranes in a rapid and reversible manner. Activation is not dependent on exogenous guanyl nucleotides and is not inhibited by guanosine 5'-O-(2-thiodiphosphate) when assayed with adenosine 5'-[beta, gamma-imido]triphosphate as substrate. GTP and GDP potentiate responses to forskolin. The activations of adenylate cyclase by forskolin and guanosine 5'-[beta, gamma-imido]triphosphate p[NH]ppG are not additive, whereas activations by forskolin and fluoride are additive or partially additive. The responses of adenylate cyclase to forskolin or fluoride are not inhibited by manganese ions, whereas the response to p[NH]ppG is completely blocked. Activation of adenylate cyclase by forskolin is considerably greater than the activation by fluoride in membranes from rat cerebellum, striatum, heart, and liver, while being about equal or less than the activation by fluoride in other tissues. Forskolin (EC50, 25 microM) causes a rapid and readily reversible 35-fold elevation of cyclic AMP in rat cerebral cortical slices that is not blocked by a variety of neurotransmitter antagonists. Low concentrations of forskolin (1 microM) augment the response of cyclic AMP-generating systems in brain slices to norepinephrine, isoproterenol, histamine, adenosine, prostaglandin E2, and vasoactive intestinal peptide. Forskolin would appear to activate adenylate cyclase through a unique mechanism involving both direct activation of the enzyme and facilitation or potentiation of the modulation of enzyme activity by receptors or the guanyl nucleotide-binding subunit, or both.

Characterization of adenylate cyclase purified from rat brain by hydrophobic chromatography

Hydrophobic chromatography of detergent-solubilized rat brain adenylate cyclase on dodecyl-Sepharose produced a species that was soluble in the absence of detergent and could be manipulated like a conventional hydrophilic protein. Sevenfold purification was achieved by this technique. Further purification could then be effected by affinity chromatography on ATP-Sepharose. The purified enzyme was no longer sensitive to fluoride or guanyl nucleotides. No interaction of brain adenylate cyclase was observed with immobilized triazinyl dyes such as Cibacron Blue 3GA nor with con-canavalin A-Sepharose. The molecular weight of the fluoride-activated catalytic complex in a freeze-dried membrane preparation was estimated to be 133,000 by irradiation inactivation.

Age-associated decrease of adenylate cyclase activity in rat myocardium

Myocardial inotropic and chronotropic responses to beta-adrenergic agonists are diminished with aging. Since myocardial beta-adrenergic receptors are unaltered with age, we tested the hypothesis that this decreased responsiveness is related to a defect in the adenylate cyclase system. Isoproterenol-stimulated adenylate cyclase activity was assessed in myocardial membranes from Fischer 344 rats of 3, 12, and 24 months of age. Basal, as well as F-, GTP-, and hormone-stimulated adenylate cyclase activity was decreased by 20-30% with age. The Km of the enzyme for ATP was not found to be statistically different for any age group studied. These data support the hypothesis that the diminished responsiveness seen in senescence is a result of an alteration in either the catalytic subunit or the coupling protein (N) of the adenylate cyclase complex.

Pseudohypoparathyroidism: inheritance of deficient receptor-cyclase coupling activity

Pseudohypoparathyroidism, type I (PHP-I) is an inherited disorder of primary resistance to multiple hormones that work by stimulating adenylate cyclase. In an attempt to clarify the mode of inheritance of PHP-I, we measured the activity of the N protein, a receptor-cyclase coupling component, in erythrocyte membranes. Erythrocyte N-protein activity was reduced by approximately 50% in erythrocytes of 15 PHP-I patients and was normal in 19 of their clinically normal first degree relatives. Reduced N-protein activity and the PHP-I phenotype in these families exhibited both dominant and recessive patterns of inheritance. This suggests that at least two distinct genetic loci are involved in inheritance of N-protein deficiency. In two additional families, dominant inheritance of the PHP-I phenotype was associated with normal activities of erythrocyte N protein. Thus, it appears that mutation of at least one additional genetic locus, not involving the N protein, can produce PHP-I.

Endogenously formed PGE2 mediates the increase in cAMP accumulation in rabbit splenic fibroblasts following alpha-receptor stimulation

Prostaglandin E2 synthesis in rabbit splenic fibroblasts was stimulated by noradrenaline but not by isoprenaline. Noradrenaline, isoprenaline and prostaglandin E2 increased intracellular cAMP accumulation. Noradrenaline-, but not isoprenaline-evoked cAMP accumulation was reduced by indomethacin; only combinations of phentolamine or indomethacin with propranolol abolished the noradrenaline effect. We conclude that the noradrenaline-induced increase in cAMP levels of rabbit splenic fibroblasts was due to stimulation of both alpha- and beta-adrenoceptors. The response to alpha-receptor stimulation was possibly mediated by prostaglandin E2.

Covalent crosslinking of human chorionic gonadotropin to its receptor in rat testes

The bifunctional crosslinking reagents disuccinimidyl suberate and dithiobis(succinimidyl propionate) were used to attach 125I-labeled human chorionic gonadotropin (125I-hCG) covalently to rat testicular membranes. The extent of crosslinking was dependent on time and concentration; routinely, 30% of the specifically bound hormone was covalently linked to the membranes in the presence of 0.5 mM crosslinking reagent when incubated at 25 degrees C for 15 min. Excess unlabeled hCG blocked the crosslinking of 125I-hCG to the membranes. When solubilized with Triton X-100 and analyzed by sucrose density gradient centrifugation, both the native and the crosslinked hormone-receptor complex sedimented with an apparent Mr of 220,000. Thus, the receptor itself would have Mr 180,000. When the crosslinked complex was analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, the predominant species had a Mr of 123,000 and appeared to represent the labeled alpha subunit of hCG covalently linked to a membrane component. The Mr of this receptor component would be 100,000, a value approximately half that of the Triton X-100-solubilized receptor. Thus, the membrane receptor for hCG may consist of a dimer of two binding subunits or a binding subunit associated with one or more additional subunits that might play a coupling or regulatory function.

Specific binding of solubilized adenylate cyclase to the erythrocyte cytoskeleton

Concepts and criteria that have been developed for the study of the molecular organization of membrane-associated proteins are employed here to investigate the interaction of adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] with other membrane components. Detergent-solubilized adenylate cyclase can be shown to bind to erythrocyte-derived Triton X-100 shells containing cytoskeletal elements. This binding appears to be saturable with respect to adenylate cyclase concentration, and it is enhanced by the presence of divalent cations. Preactivation of the enzyme with 5'-guanylyl imidodiphosphate and isoproterenol, or with NaF, is a prerequisite for effective binding. Two exceptions to this general observation are noted: rat brain adenylate cyclase, which binds without prestimulation, and rat testicular cytosolic adenylate cyclase, which fails to bind under any of the conditions tried. The binding sites of the Triton X-100 shells are inactivated or released by treatment with various concentrations of trypsin or KCl. Moreover, exposure of the Triton X-100 shells to increasing temperatures results in a progressive loss of the adenylate cyclase binding capacity. On the basis of these and other findings, it is suggested that the adenylate cyclase complex possesses two principal domains that allow it to interact with both cytoskeletal elements and the lipid bilayer. The specific modulation of these interactions may be involved in the hormonal regulation of adenylate cyclase activity.

Stimulation of glucose incorporation and amino acid transport by insulin and an insulin-like growth factor in fibroblasts with defective insulin receptors cultured from a patient with leprechaunism

Fibroblasts cultured from an infant with leprechaunism and insulin resistance have been reported to exhibit profound, selective defect in insulin binding. We now examine the effect of this defect on two acute metabolic actions of insulin thought to be mediated by the insulin receptor, glucose incorporation and N-methyl-alpha-aminoisobutyric acid (Me-AiBu) uptake. In the patient's fibroblasts, maximal insulin-stimulated glucose incorporation was less than 25% of that in control fibroblasts, whereas stimulation by hydrogen peroxide, an insulinomimetic agent that acts distal to the insulin receptor, was normal. By contrast, insulin stimulated Me-AiBu uptake to the same extent in patient's and control fibroblasts. Impaired glucose incorporation and relatively normal Me-AiBu uptake also were observed in the patient's cells with multiplication-stimulating activity, an insulin-like growth factor, despite the fact that multiplication-stimulating activity appeared to stimulate both responses in normal fibroblasts via an insulin-like growth factor receptor. The divergent effects on two hormone-stimulated functions in the patient's cells suggests differences in the coupling of a receptor to different effectors. The same coupling mechanisms appear to be used by insulin receptors and receptors for insulin-like growth factors.

The binding of [3H]-prostacyclin to membranes of a neuronal somatic hybrid

1 [3H]-prostacyclin bound to a washed membrane preparation of the NCB-20 neuronal hybrid cell line. 2 Kinetic analysis of [3H]-prostacyclin binding suggested a simple, non-cooperative bimolecular interaction between the ligand and a single receptor population. The equilibrium dissociation constant was 16.6 nM, and binding at a saturating [3H]-prostacyclin concentration enabled the receptor density of 2.57 x 10(5) receptor molecules per cell to be calculated. 3 At 20 degrees C the rate constant for the forward reaction (K+1) was 2.26 x 10(5) M-1 S-1, and the rate constant for the dissociation of the ligand-receptor complex (k-1) was 3.85 x 10(-3) S-1. Thus the dissociation constant (k-1/k+1) was 17.0 nM. 4 Prostaglandin E1 and prostacyclin compete for a single receptor in these cells, and comparison of other prostaglandins as inhibitors of [3H]-prostacyclin binding revealed some of the structural requirements for high-affinity occupation of prostacyclin receptors.

Characterization by [3H]dihydroergocryptine binding of alpha-adrenergic receptors in neuroblastoma X glioma hybrid cells

[3H]Dihydroergocryptine ([3H]DHE) was shown to bind to sites in membranes from neuroblastoma X glioma hybrid cells (NG 108-15) that had the characteristics expected of alpha-adrenergic receptors. The binding was saturable with 0.3 pmol [3H]DHE bound per mg of protein and of high affinity, with an apparent dissociation constant (KD) of 1.8 nM. The specificity of the binding site for various ligands was more similar to that of alpha 2 receptors than to that of alpha 1. No specific binding of [3H]WB-4101 was found in the membranes derived from NG 108 cells. This finding also indicated that the [3H]DHE binding site in the cell is the alpha 2 receptor. GTP lowered the affinity of agonists for the [3H]DHE binding site, although the nucleotide hardly affected the affinity of antagonists including [3H]DHE.

Structure of the turkey erythrocyte adenylate cyclase system

Target analysis of the turkey erythrocyte adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] system showed that the molecular weight of the ground state enzyme increases from 92,000 with MnATP as substrate and no stimulatory ligands to 226,000 when activated by fluoride ion or by 5'-guanyl imidodiphosphate (p[NH]ppG) subsequent to clearance of previously bound GDP. The identical increment in size (130,000) suggests that the same regulatory unit is involved in the activation by both effectors. When assayed with isoproterenol and p[NH]ppG, the enzyme system displayed a further increment in size of 90,000 daltons. Based on binding of the antagonist 125I-labeled hydroxybenzylpindolol, the beta-adrenergic receptor is about 90,000 daltons or the same as that seen for activation of the enzyme by isoproterenol through the beta-adrenergic-receptor. Because single targets were seen for the ground state enzyme system under all conditions, it would appear that the various regulatory and catalytic components are structurally linked prior to activation by hormone, guanine nucleotides, and fluoride ion. Furthermore, based on reported subunit sizes of the nucleotide regulatory and receptor components are composed of multiple subunits, either homologous or heterologous in structure.

Effects of guanine nucleotides on CNS neuropeptide receptors

The effect of nucleotides on central nervous system neuropeptide receptor binding was investigated. The guanine nucleotides, guanosine-5'-triphosphate and guanylyl-5'-imidodiphosphate, significantly inhibited the binding of radiolabeled vasoactive intestinal polypeptide but not that of [Tyr4]bombesin to rat brain membranes. Vasoactive intestinal polypeptide binding was inhibited by guanine nucleotides in a dose-dependent manner. Using a 20 microM dose, 60% of the specific vasoactive intestinal polypeptide binding was inhibited by guanylyl-5'-imido-diphosphate, which was more potent than guanosine-5'-triphosphate, whereas other nucleotides were not effective. This reduction in binding was a consequence of lower affinity of the receptor for vasoactive intestinal polypeptide, which in turn resulted from an increased rate of dissociation.