Formation of adenosine 3':5'-monophosphate by preparations of guanylate cyclase from rat liver and other tissues

A short history of cGMP, guanylyl cyclases, and cGMP-dependent protein kinases

Here, we review the early studies on cGMP, guanylyl cyclases, and cGMP-dependent protein kinases to facilitate understanding of development of this exciting but complex field of research encompassing pharmacology, biochemistry, physiology, and molecular biology of these important regulatory molecules.

Guanylate cyclase activity and sperm function

In species with external fertilization, the guanylate cyclase family is responsible for the long-distance interaction between gametes, as its activation allows sperm chemotaxis toward egg-derived substances, gamete encounter, and fertilization. In species with internal fertilization, guanylate cyclase-activating substances, which are secreted by several tissues in the genital tracts of both sexes, deeply affect sperm motility, capacitation, and acrosomal reactivity, stimulating sperm metabolism and promoting the ability of the sperm to approach the oocyte, interact with it, and finally fertilize it. A complex system of intracellular pathways is activated by guanylate cyclase agonists in spermatozoa. Sperm motility appears to be affected mainly through an increase in intracellular cAMP, whereas the acrosome reaction depends more directly on cyclic GMP synthesis. Both cyclic nucleotides activate specific kinases and ion signals. A complex cross-talk between cAMP- and cyclic GMP-generating systems occurs, resulting in an upward shift in sperm function. Excessive amounts of certain guanylate cyclase activators might exert opposite, antireproductive effects, increasing the oxidative stress on sperm membranes. In view of the marked influence exerted by guanylate cyclase-activating substances on sperm function, it seems likely that guanylate cyclase activation or inhibition may represent a new approach for the diagnosis and treatment of male and/or female infertility.

Comparison between the effects of the rapid recombinant insulin analog aspart and those of human regular insulin on platelet cyclic nucleotides and aggregation

INTRODUCTION

Insulin aspart is a rapid insulin analog used in clinical practice: aim of the present study is to evaluate in human platelets its influence on: (i). concentrations of guanosine 3':5'-cyclic monophosphate (cGMP) and adenosine 3':5'-cyclic monophosphate (cAMP), mediators of platelet anti-aggregation; (ii). platelet aggregation to adenosine-5 diphosphate.

MATERIALS AND METHODS

In human platelets, incubated with human regular insulin or with insulin aspart, we measured: (1). guanosine 3':5-cyclic monophosphate and adenosine 3':5'-cyclic monophosphate concentrations by radioimmunoassays, with and without nitric oxide synthase (NOS) inhibition by N(G)-monomethyl-L-arginine, and phosphatidylinositol-3-kinase inhibition by wortmannin; (ii). aggregation to adenosine-5 diphosphate by Born's method.

RESULTS

(i). Human regular insulin and insulin aspart increased both cyclic nucleotides; (ii). these effects were dependent on nitric oxide, being inhibited by N(G)-monomethyl-L-arginine, and mediated by the phosphatidylinositol-3-kinase pathway of insulin signalling, being inhibited by wortmannin; (iii). the effects exerted by insulin aspart on both cyclic nucleotides (ANOVA, p=0.0001) were more prolonged than those exerted by regular insulin; (iv) like human regular insulin, insulin aspart significantly decreased platelet response to ADP (ANOVA, p=0.0001): after 60 min of incubation, the anti-aggregating effect exerted by insulin aspart was significantly greater than that exerted by human regular insulin (p=0.027).

CONCLUSIONS

The effects of insulin aspart on platelet cyclic nucleotides and aggregation show kinetic differences compared to those of human regular insulin, resulting in more prolonged effects.

Influence of insulin and of insulin resistance on platelet and vascular smooth muscle cell function

In this short review, we present the main results obtained in our laboratory in the last 15 years concerning the influence exerted by insulin on platelets and human vascular smooth muscle cells (VSMCs). In particular, we discuss: (i) the insulin ability to rapidly activate a constitutive nitric oxide synthase (NOS) in both cell types, with a consequent increase of the two nucleotides guanosine-3',5'-cyclic monophosphate (cGMP) and adenosine-3',5'-cyclic monophosphate (cAMP), well-known mediators of antiaggregation and vasodilation; (ii) the interplay of insulin with substances able to activate adenylate cyclase in both cell types; (iii) the impairment of the antiaggregating insulin effects in insulin-resistant subjects; (iv) the insulin-induced increase on endothelin in the VSMCs; (v) the insulin ability to slightly stimulate VSMC proliferation.

Nitric oxide attenuates H(2)O(2)-induced endothelial barrier dysfunction: mechanisms of protection

Nitric oxide (.NO) attenuates hydrogen peroxide (H(2)O(2))-mediated injury in porcine pulmonary artery endothelial cells (PAECs) and modulates intracellular levels of cGMP and cAMP. We hypothesized that.NO attenuates H(2)O(2)-induced PAEC monolayer barrier dysfunction through cyclic nucleotide-dependent signaling mechanisms. To examine this hypothesis, cultured PAEC monolayers were treated with H(2)O(2), and barrier function was measured as transmonolayer albumin clearance. H(2)O(2) caused significant PAEC barrier dysfunction that was attenuated by intracellular as well as extracellular.NO generation.NO increased PAEC cGMP and cAMP levels, but treatment with inhibitors of soluble guanylate cyclase or protein kinase G did not abrogate.NO-mediated barrier protection. In contrast, H(2)O(2) decreased protein kinase A activity, and inhibiting protein kinase A abrogated the protective effect of.NO. H(2)O(2)-induced barrier dysfunction was not associated with decreased levels of cGMP or cAMP. 3-Isobutyl-1-methylxanthine and the cGMP analog 8-bromo-cGMP had little effect on H(2)O(2)-mediated endothelial barrier dysfunction, whereas 8-bromo-cAMP plus 3-isobutyl-1-methylxanthine was protective. These results indicate that.NO modulates vascular endothelial barrier function through cAMP-dependent signaling mechanisms.

Catalytic mechanism of the adenylyl and guanylyl cyclases: modeling and mutational analysis

The adenylyl and guanylyl cyclases catalyze the formation of 3', 5'-cyclic adenosine or guanosine monophosphate from the corresponding nucleoside 5'-triphosphate. The guanylyl cyclases, the mammalian adenylyl cyclases, and their microbial homologues function as pairs of homologous catalytic domains. The crystal structure of the rat type II adenylyl cyclase C2 catalytic domain was used to model by homology a mammalian adenylyl cyclase C1-C2 domain pair, a homodimeric adenylyl cyclase of Dictyostelium discoideum, a heterodimeric soluble guanylyl cyclase, and a homodimeric membrane guanylyl cyclase. Mg2+ATP or Mg2+GTP were docked into the active sites based on known stereochemical constraints on their conformation. The models are consistent with the activities of seven active-site mutants. Asp-310 and Glu-432 of type I adenylyl cyclase coordinate a Mg2+ ion. The D310S and D310A mutants have 10-fold reduced Vmax and altered [Mg2+] dependence. The NTP purine moieties bind in mostly hydrophobic pockets. Specificity is conferred by a Lys and an Asp in adenylyl cyclase, and a Glu, an Arg, and a Cys in guanylyl cyclase. The models predict that an Asp from one domain is a general base in the reaction, and that the transition state is stabilized by a conserved Asn-Arg pair on the other domain.

A variant of the alpha 2 subunit of soluble guanylyl cyclase contains an insert homologous to a region within adenylyl cyclases and functions as a dominant negative protein

A variant of the alpha 2 subunit of soluble guanylyl cyclase (alpha 2i) containing 31 additional amino acids was identified in a number of cell lines and tissues. The in-frame sequence of the insert was within the proposed catalytic domain of guanylyl cyclases and was homologous to a region within the putative catalytic domain of adenylyl cyclases. Messenger RNA for the new variant was detected in some but not all cell lines and tissues expressing the alpha 2 subunit. The novel form, as well as the alpha 2 subunit lacking the insert, were coexpressed with the beta 1 subunit in Sf9 and COS-7 cells; alpha 2/beta 1 coexpression yielded a NO-sensitive recombinant protein, whereas the coexpressed alpha 2i/beta 1 subunits exhibited no guanylyl or adenylyl cyclase activities. Because both subunits (alpha 2i/beta 1) copurified, the novel variant retains its ability to heterodimerize. In coexpression experiments, the alpha 2i subunit competed with the alpha 2 subunit for dimerization with the beta 1 subunit, thereby reducing alpha 2/beta 1-catalyzed guanylyl cyclase activity. These data show that the novel variant functions as a dominant negative protein and that post-transcriptional mRNA processing represents a potential mechanism for regulation of NO-sensitive guanylyl cyclase activity.

Oxygen-radical/nitric oxide mediate calcium-dependent hormone action on cyclic GMP system: a novel concept in signal transduction mechanisms

The broad objective of these studies was to understand the nature of cyclic GMP system and the mechanism(s) whereby hormone, autacoids and drugs alter this signal in various physiological systems. Studies were undertaken on the modulation of guanylate cyclase activity by oxygen-radicals/nitric oxide and the mechanism(s) of generation of nitric oxide by receptor-selective hormones. We observed that cytosolic guanylate cyclase undergoes significant stimulation in the presence of oxygen-radicals/nitric oxide. This activation by nitric oxide can be reversed by hemeproteins, thus, enabling guanylate cyclase system to cycle between activated and deactivated state. The evidence is presented that oxygen-radicals are required for the synthesis of nitric oxide by NO synthase as demonstrated by inhibition of NO formation by oxygen-radical scavengers. And finally, the data is presented that acetylcholine-induced elevations of intracellular levels of cyclic GMP can be attenuated by muscarinic antagonist, atropine and superoxide anion scavenger, nitroblue tetrazolium. These observations establish a novel concept that activation of hormone receptors on the cell surface, triggers generation of oxygen radicals and hydrogen peroxide which participates in the catalytic conversion of L-arginine to nitric oxide by nitric oxide synthase in the presence of calcium ion. The oxygen-radicals/NO, thus formed, oxidatively activate guanylate cyclase and transduce the message of calcium-dependent hormones.

Direct inhibition of platelet function by organic nitrates via nitric oxide formation

This study investigates the mechanisms of platelet inhibition by the nitrate esters isosorbide dinitrate, isoidide dinitrate, isomannide dinitrate, isosorbide 2-mononitrate and isosorbide 5-mononitrate as compared to the spontaneous nitric oxide (NO)-donor linsidomine, the active metabolite of molsidomine. Nitrates and linsidomine dose-dependently inhibited aggregation, ATP secretion and thromboxane formation of washed human platelets at a rank order of potency, identical with that for stimulation of cyclic GMP in cultured rat lung fibroblasts. While linsidomine (0.1 mM) caused a 3-fold platelet cGMP elevation, there was a weak (< or = 30%) but significant cGMP stimulation by organic nitroesters, which was tightly correlated with inhibition of platelet aggregation (r = 0.926, P = 0.008). Zaprinast (2 microM) potentiated, while methylene blue (1 microM) and oxyhemoglobin (10 microM) reversed the antiaggregatory effects. Linsidomine (0.5 microM-0.1 mM) dose-dependently released NO in a cell-free system. No spontaneous NO release was detected with organic nitroesters (0.1 mM). These data suggest that, to some extent, bioactivation of organic nitroesters occurs in platelets, resulting in platelet inhibition via the NO/cGMP system.

Muscarinic actions in hippocampus are probably not mediated by cyclic GMP

It has been proposed that, in a variety of tissues, guanosine 3':5'-monophosphate (cyclic GMP) is the intracellular mediator of muscarinic effects. This hypothesis was tested in the CA1 region of the hippocampus, in urethane-anaesthetized rats, by studying extracellularly muscarinic disinhibition of disfacilitation and the effect of dibutyryl cyclic GMP, muscarinic agents and an inhibitor of cyclic nucleotide-dependent kinase (H-8), all applied by microiontophoresis. The main findings were: (a) cyclic GMP analogues do not mimic disfacilitation or disinhibition produced by muscarinic agents; (b) N-(2-(methylamino)ethyl)-5-isoquinoline sulfonamide (H-8) does not prevent the excitatory actions of muscarinic agents; and (c) H-8 alone does not change the field responses. In conclusion, cyclic nucleotide-dependent kinases do not seem to play a major role in the on-going modulation of excitability in the hippocampus and cyclic GMP is unlikely to be a major intracellular messenger mediating directly or indirectly the excitatory actions of acetylcholine.

Guanylate cyclase in olfactory cilia from rat and pig

A guanylate cyclase was identified in cilia from rat and pig olfactory epithelia. Enzyme activities were 200-250 and 90-100 pmol/min.mg-1, respectively. Activity required the presence of non-ionic detergents, e.g., 0.1% Lubrol PX. MnGTP, not MgGTP was used as a substrate. Furthermore, 0.9 mM free Mn2+ was necessary for optimal activity indicating a regulatory site for a divalent cation. The guanylate cyclase displayed sigmoidal Michaelis-Menten kinetics suggesting cooperativity between MnGTP and enzyme. S0.5 was 160 microM MnGTP. The Hill coefficient of 1.7 indicates that more than one class of substrate-binding sites interact in a positive cooperative manner. ATP inhibited the enzyme and linearized plots of substrate kinetics with MnGTP. SH-Blocking agents reversibly inhibited enzyme activity. Sodium azide and nitroprusside were without effect as were several odorants. A guanylate cyclase activity in cilia from tracheal tissue had properties similar to the olfactory enzyme.

Molecular basis of signalling in the spermatozoon

The spermatozoon contains cell surface receptors for various egg-associated molecules, one of which has now been identified as the enzyme guanylate cyclase. A single membrane-spanning region divides the enzyme such that about one-half is extracellular and one-half is intracellular. A new paradigm for signal/transduction is established by these observations, in that a hormone or effector molecule binding to an extracellular site activates the catalytic domain of the same protein, resulting in the increased formation of a low molecular weight second messenger. Research on signalling mechanisms in the spermatozoon clearly apply to a diversity of cellular processes as well as to fertilization.

Negative regulation of cyclic-AMP levels by carbamylcholine in dog thyroid is not mediated by cyclic-GMP

Carbamylcholine, through calcium, enhances cyclic-GMP accumulation and depresses cyclic-AMP accumulation in TSH stimulated dog thyroid. The results presented show that compounds which can be transformed to nitric oxide increase cyclic-GMP accumulation in the dog thyroid. These compounds do not require extracellular calcium for their action. In thyroid stimulated by TSH, these compounds do not depress AMP accumulation. Cyclic-GMP is not the main intracellular signal involved in the negative regulation of cyclic-AMP levels in dog thyroid.

Alterations in the subcellular distribution of Guanylate cyclase and its responsiveness to nitric oxide in diethylstilbestrol-induced renal tumors

The cyclic GMP content of diethylstilbestrol-induced renal tumors in the male golden hamster was increased nearly 130-fold over that in kidney from control animals. Cyclic GMP in tumors was 91.80 +/- 19.18 pmoles cyclic GMP/mg protein compared to 0.72 +/- 0.07 in control kidneys. Cyclic AMP in tumors was also increased over control, however, to a much lesser degree (2.7-fold). In control kidneys, 84.6% of homogenate guanylate cyclase activity was recovered in the 100,000 X g supernatant fraction. Total homogenate guanylate cyclase activity from diethylstilbestrol-induced renal tumors was increased 5.5-fold over that in control kidneys and only 8.1% was associated with the 100,000 X g supernatant fraction. Neither the soluble or particulate guanylate cyclase from renal tumors could be activated by nitric oxide. The unresponsiveness of tumor guanylate cyclase to nitric oxide was independent of the cation cofactor, and not due to a shift in the dose response curve for nitric oxide. Responsiveness to nitric oxide was not restored by thiols, sugars, other proteins, or hemoglobin. Basal cyclic AMP formation by soluble guanylate cyclase from renal tumors was dramatically increased over that observed in control kidneys, and could not be increased further by nitric oxide. This is the first study of cyclic GMP and guanylate cyclase in a primary estrogen-induced tumor. The possibility that the changes observed in guanylate cyclase from diethylstilbestrol-induced renal tumors are related to in vivo activation of the enzyme by epoxide metabolites of diethylstilbestrol is discussed.

Correlation between vascular smooth muscle relaxation and increase in cyclic GMP induced by some organic nitro esters

Three different nitro compounds glyceryl mononitrate (GMN), glyceryl dinitrate (GDN) and ethylene glycol dinitrate (EGDN) were tested on histamine contracted bovine mesenteric artery. GDN and EGDN caused a dose-dependent relaxation which was accompanied by an increase in the endogenous cGMP level. The ED50-value for GDN and EGDN with regard to the relaxant action was 1.06 X 10(-6) M and 4.20 X 10(-8), respectively. GMN was almost completely ineffective as a relaxing agent and did not cause any significant change in cGMP. Regression analysis revealed a significant correlation between relaxation and increase in cGMP for both GDN and EGDN. The regression model for EGDN could be significantly improved by including the squared cGMP change, indicating a non-linear relationship. With regard to GDN a hyperbolic relationship between relaxation and cGMP increase was found. A slight improvement of the regression model was found for EGDN when the change in cAMP was included. For GMN and GDN no improvement of the regression model could be revealed by including the change of cAMP. It is suggested that the present data give further evidence for cGMP as a mediator of vascular smooth muscle relaxation induced by nitro compounds.

Purification of a soluble, sodium-nitroprusside-stimulated guanylate cyclase from bovine lung

A soluble, sodium-nitroprusside-stimulated guanylate cyclase as been purified from bovine lung by DEAE-cellulose chromatography, ammonium sulfate precipitation, chromatography on Blue Sepharose CL-6B and preparative gel electrophoresis. Apparent homogeneity was obtained after at least 7000-fold purification with a yield of 3%. A single stained band (Mr 72000) was observed after gel electrophoresis in the presence of sodium dodecyl sulfate. The purified enzyme migrated as one band also under non-denaturing conditions in acrylamide gels (5-12%). The mobility of this band corresponded to an Mr of 145000. The enzyme sedimented on sucrose gradients with an S20, w of 7.0 S. Gel filtration yielded a Stokes' radius of 4.6 nm. These data suggest that the enzyme has an Mr of approximately 150000 and consists of two, presumably identical, subunits of Mr 72000. Sodium nitroprusside stimulated the purified enzyme 15-fold and 140-fold to specific activities of 8.5 and 15.7 mumol of cGMP formed min-1 mg-1 in the presence of Mn2+ and Mg2+, respectively. Formation of cGMP was proportional to the incubation time and to the amount of enzyme added. The stimulatory effect of sodium nitroprusside was half-maximal at about 2 microM, was observed immediately after addition and could be reversed either by dilution or by removal of sodium nitroprusside on a Sephadex G-25 column. The purified enzyme in the absence of catalase was stimulated by sodium nitroprusside, N-methyl-N'-nitro-N-nitrosoguanidine and 3-morpholino-sydnonimine and in the presence of catalase by sodium nitrite and sodium azide. In the presence of Mn2+ and sodium nitroprusside, the purified enzyme catalyzed the formation of cAMP from ATP at a rate of 0.6 mumol min-1 mg-1.

Synthesis of adenosine 3',5'-monophosphate by guanylate cyclase, a new pathway for its formation

The 105 000 X g gupernatant fractions from homogenates of various rat tissues catalyzed the formation of both cyclic GMP and cyclic AMP from GTP and ATP, respectively. Generally cyclic AMP formation with crude or purified preparations of soluble guanylate cyclase was only observed when enzyme activity was increased with sodium azide, sodium nitroprusside, N-methyl-N'-nitro-N-nitrosoguanidine, sodium nitrite, nitric oxide gas, hydroxyl radical and sodium arachidonate. Sodium fluoride did not alter the formation of either cyclic nucleotide. After chromatography of supernatant preparations on Sephadex G-200 columns or polyacrylamide gel electrophoresis, the formation of cyclic AMP and cyclic GMP was catalyzed by similar fractions. These studies indicate that the properties of guanylate cyclase are altered with activation. Since the synthesis of cyclic AMP and cyclic GMP reported in this study appears to be catalyzed by the same protein, one of the properties of activated guanylate cyclase is its ability to catalyze the formation of cyclic AMP from ATP. The properties of this newly described pathway for cyclic AMP formation are quite different from those previously described for adenylate cyclase preparations. The physiological significance of this pathway for cyclic AMP formation is not known. However, these studies suggest that the effects of some agents and processes to increase cyclic AMP accumulation in tissue could result from the activation of either adenylate cyclase or guanylate cyclase.

Effects of thiol inhibitors on hepatic guanylate cylase activity

Several thiol blocking agents inhibit basal guanylate cyclase activity of 100 000 X g hepatic supernatant fractions and the stimulation of enzyme activity by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), NaN3, NaNO2 and nitroprusside. The relative potency of the thiol blockers as inhibitors was CdCl2 greater than p-hydroxymercuribenzoate greater than N-ethylmaleimide greater than arsenite greater than iodoacetamide. Inhibition of basal and MNNG-responsive soluble guanylate cyclase activities by arsenite was markedly potentiated by an equimolar concentration of 2,3-dimercaprol, but not by mercaptoethanol. Inhibition of soluble guanylate cyclase by either arsenite or CdCl2 was completely reversed by excess 2,3-dimercaprol. Qualitatively similar effects were observed with DE-52 cellulose purified soluble hepatic guanylate cyclase, and suggested an involvement of closely juxtaposed thiol groups in the regulation of enzyme activity. For several reasons inhibition by thiol blockers appeared to be mediated through multiple mechanisms and/or sites of interaction: (1) Concentrations of the thiol inhibitors which had no effect on basal activity strikingly inhibited the responsiveness of the enzyme to a submaximal concentration of MNNG. (2) CdCl2 abolished the action of excess MnCl2 to stimulate purified guanylate cyclase, but was a relatively ineffective inhibitor when MnCl2 and GTP were present in equimolar concentrations. By contrast, arsenite-2,3-dimercaprol was uniformly effective in inhibiting guanylate cyclase activity in the presence or absence of excess MnCl2. (3) Arsenite-2,3-dimercaprol increased the Km for MnGTP (control, 0.13 +/- 0.02 mM; 0.2 mM arsenite-2,3-dimercaprol, 0.31 +/- 0.03 mM), whereas CdCl2 had no effect on this parameter. (4) Hepatic particulate guanylate cyclase activity was significantly inhibited by arsenite 2,3-dimercaprol but not by CdCl2. Thus, the data not only indicate that vicinal dithiol groups are required for expression of basal guanylate cyclase activity and enzyme responses to agonists, but strongly suggest the involvement of more than one interacting site containing free thiol residues.

Cigarette smoke activates guanylate cyclase and increases guanosine 3',5'-monophosphate in tissues

The gaseous phase of cigarette smoke induced a 2- to 36-fold increase in the activity of guanylate cyclase in supernatant and particulate fractions from various rat and bovine tissues over basal activity. The characteristics of this phenomenon paralleled those of the activation of guanylate cyclase by nitric oxide, which is a component of tobacco smoke.