Kinetics of "P"-site-mediated inhibition of adenylyl cyclase and the requirements for substrate

Vidarabine, an anti-herpesvirus agent, prevents catecholamine-induced arrhythmias without adverse effect on heart function in mice

Sympathetic activation causes clinically important arrhythmias including atrial fibrillation (AF) and ventricular tachyarrhythmia. Although the usefulness of β-adrenergic receptor blockade therapy is widely accepted, its multiple critical side effects often prevent its initiation or continuation. The aim of this study is to determine the advantages of vidarabine, an adenylyl cyclase (AC)-targeted anti-sympathetic agent, as an alternative treatment for arrhythmia. We found that vidarabine, which we identified as a cardiac AC inhibitor, consistently shortens AF duration and reduces the incidence of sympathetic activation-induced ventricular arrhythmias. In atrial and ventricular myocytes, vidarabine inhibits adrenergic receptor stimulation-induced RyR2 phosphorylation, sarcoplasmic reticulum (SR) Ca²⁺ leakage, and spontaneous Ca²⁺ release from SR, the last of which has been considered as a potential arrhythmogenic trigger. Moreover, vidarabine also inhibits sympathetic activation-induced reactive oxygen species (ROS) production in cardiac myocytes. The pivotal role of vidarabine's inhibitory effect on ROS production with regard to its anti-arrhythmic property has also been implied in animal studies. In addition, as expected, vidarabine exerts an inhibitory effect on AC function, which is more potent in the heart than elsewhere. Indexes of cardiac function including ejection fraction and heart rate were not affected by a dosage of vidarabine sufficient to exert an anti-arrhythmic effect. These findings suggest that vidarabine inhibits catecholamine-induced AF or ventricular arrhythmia without deteriorating cardiac function in mice.

Inactivation of corticotropin-releasing hormone-induced insulinotropic role by high-altitude hypoxia

We have shown that hypoxia reduces plasma insulin, which correlates with corticotropin-releasing hormone (CRH) receptor 1 (CRHR1) in rats, but the mechanism remains unclear. Here, we report that hypobaric hypoxia at an altitude of 5,000 m for 8 h enhances rat plasma CRH, corticosterone, and glucose levels, whereas the plasma insulin and pancreatic ATP/ADP ratio is reduced. In islets cultured under normoxia, CRH stimulated insulin release in a glucose- and CRH-level-dependent manner by activating CRHR1 and thus the cAMP-dependent protein kinase pathway and calcium influx through L-type channels. In islets cultured under hypoxia, however, the insulinotropic effect of CRH was inactivated due to reduced ATP and cAMP and coincident loss of intracellular calcium oscillations. Serum and glucocorticoid-inducible kinase 1 (SGK1) also played an inhibitory role. In human volunteers rapidly ascended to 3,860 m, plasma CRH and glucose levels increased without a detectable change in plasma insulin. By contrast, volunteers with acute mountain sickness (AMS) exhibited a marked decrease in HOMA insulin sensitivity (HOMA-IS) and enhanced plasma CRH. In conclusion, hypoxia may attenuate the CRH-insulinotropic effect by reducing cellular ATP/ADP ratio, cAMP and calcium influx, and upregulated SGK1. Hypoxia may not affect HOMA-IS in healthy volunteers but reduces it in AMS volunteers.

Nucleoside transporters in the purinome

The purinome is a rich complex of proteins and cofactors that are involved in fundamental aspects of cellular homeostasis and cellular responses. The purinome is evolutionarily ancient and is made up of thousands of members. Our understanding of the mechanisms linking some parts of this complex network and the physiological relevance of the various connections is well advanced. However, our understanding of other parts of the purinome is less well developed. Our research focuses on the adenosine or nucleoside transporters (NTs), which are members of the membrane purinome. Nucleoside transporters are integral membrane proteins that are responsible for the flux of nucleosides, such as adenosine, and nucleoside analog drugs, used in a variety of anti-cancer, anti-viral and anti-parasite therapies, across cell membranes. Nucleoside transporters form the SLC28 and SLC29 families of solute carriers and the protein members of these families are widely distributed in human tissues including the central nervous system (CNS). NTs modulate purinergic signaling in the CNS primarily through their effects on modulating prevailing adenosine levels inside and outside the cell. By clearing the extracellular milieu of adenosine, NTs can terminate adenosine receptor-dependent signaling and this raises the possibility of regulatory feedback loops that tie together receptor signaling with transporter function. Despite the important role of NTs as modulators of purinergic signaling in the human body, very little is known about the nature or underlying mechanisms of regulation of either the SLC28 or SLC29 families, particularly within the context of the CNS purinome. Here we provide a brief overview of our current understanding of the regulation of members of the SLC29 family and highlight some interesting avenues for future research.

Adenylyl cyclases as innovative therapeutic goals

Pharmacological modulation of intracellular cyclic AMP (cAMP) signalling could provide new therapeutic and experimental tools. Although drugs interfering with this pathway have traditionally targeted membrane receptors, the effector enzyme adenylyl cyclase (AC), which functions as a signalling catalyst, also presents an interesting target. Thus, development of isoform-selective stimulator and/or inhibitor compounds for AC could lead to organ-specific pharmacotherapeutics for treating heart failure, cancer and neurodegenerative diseases. In this review, the potential of AC as the object of drug therapy is discussed.

Biochemical and pharmacological characterization of P-site inhibitors on homodimeric guanylyl cyclase domain from natriuretic peptide receptor-A

Guanylyl cyclases catalyze the formation of cGMP from GTP. This family of enzymes includes soluble (sGC) and particulate guanylyl cyclases (pGC). The sGC are heterodimers containing one active catalytic site and one inactive pseudo-site. They are activated by nitric oxide. The pGC are homodimers whose activity is notably regulated by peptide binding to the extracellular domain and by ATP binding to the intracellular kinase homology domain (KHD). The catalytic mechanism of the pGC is still not well understood. Homology modeling of the structure of the homodimeric guanylyl cyclase domain, based on the crystal structure of adenylyl cyclase, suggests the existence of two functional sites for the substrate GTP. We used a purified and fully active recombinant catalytic domain from mammalian pGC, to document its enzyme kinetics properties in the absence of the KHD. The enzyme presents positive cooperativity with the substrate Mg-GTP. However, a heterodimeric catalytic domain mutant (GC-HET) containing only one active catalytic site is non-cooperative and is more similar to sGC. Structure-activity studies of purine nucleoside analogs indicate that 2'd3'GMP is the most potent inhibitor of pGC tested. It displays mixed non-competitive inhibition properties that are potentiated by the second catalytic product inorganic pyrophosphate (PPi). It appears to be equivalent to purinergic site (P-site) inhibitors characterized on particulate adenylyl cyclase. Inhibition of pGC by 2'd3'GMP in the presence of PPi is accompanied by a loss of cooperative enzyme kinetics. These results are best explained by an allosteric dimer model with positive cooperativity for both the substrate and inhibitors.

Molecular details of cAMP generation in mammalian cells: a tale of two systems

The second messenger cAMP has been extensively studied for half a century, but the plethora of regulatory mechanisms controlling cAMP synthesis in mammalian cells is just beginning to be revealed. In mammalian cells, cAMP is produced by two evolutionary related families of adenylyl cyclases, soluble adenylyl cyclases (sAC) and transmembrane adenylyl cyclases (tmAC). These two enzyme families serve distinct physiological functions. They share a conserved overall architecture in their catalytic domains and a common catalytic mechanism, but they differ in their sub-cellular localizations and responses to various regulators. The major regulators of tmACs are heterotrimeric G proteins, which transduce extracellular signals via G protein-coupled receptors. sAC enzymes, in contrast, are regulated by the intracellular signaling molecules bicarbonate and calcium. Here, we discuss and compare the biochemical, structural and regulatory characteristics of the two mammalian AC families. This comparison reveals the mechanisms underlying their different properties but also illustrates many unifying themes for these evolutionary related signaling enzymes.

Structural basis for the inhibition of mammalian membrane adenylyl cyclase by 2 '(3')-O-(N-Methylanthraniloyl)-guanosine 5 '-triphosphate

Membrane-bound mammalian adenylyl cyclase (mAC) catalyzes the synthesis of intracellular cyclic AMP from ATP and is activated by stimulatory G protein alpha subunits (Galpha(s)) and by forskolin (FSK). mACs are inhibited with high potency by 2 '(3')-O-(N-methylanthraniloyl) (MANT)-substituted nucleotides. In this study, the crystal structures of the complex between Galpha(s).GTPgammaS and the catalytic C1 and C2 domains from type V and type II mAC (VC1.IIC2), bound to FSK and either MANT-GTP.Mg(2+) or MANT-GTP.Mn(2+) have been determined. MANT-GTP coordinates two metal ions and occupies the same position in the catalytic site as P-site inhibitors and substrate analogs. However, the orientation of the guanine ring is reversed relative to that of the adenine ring. The MANT fluorophore resides in a hydrophobic pocket at the interface between the VC1 and IIC2 domains and prevents mAC from undergoing the "open" to "closed" domain rearrangement. The K(i) of MANT-GTP for inhibition of VC1.IIC2 is lower in the presence of mAC activators and lower in the presence of Mn(2+) compared with Mg(2+), indicating that the inhibitor binds more tightly to the catalytically most active form of the enzyme. Fluorescence resonance energy transfer-stimulated emission from the MANT fluorophore upon excitation of Trp-1020 in the MANT-binding pocket of IIC2 is also stronger in the presence of FSK. Mutational analysis of two non-conserved amino acids in the MANT-binding pocket suggests that residues outside of the binding site influence isoform selectivity toward MANT-GTP.

Direct inhibition of type 5 adenylyl cyclase prevents myocardial apoptosis without functional deterioration

Adenylyl cyclase, a major target enzyme of beta-adrenergic receptor signals, is potently and directly inhibited by P-site inhibitors, classic inhibitors of this enzyme, when the enzyme catalytic activity is high. Unlike beta-adrenergic receptor antagonists, this is a non- or uncompetitive inhibition with respect to ATP. We have examined whether we can utilize this enzymatic property to regulate the effects of beta-adrenergic receptor stimulation differentially. After screening multiple new and classic compounds, we found that some compounds, including 1R,4R-3-(6-aminopurin-9-yl)-cyclopentanecarboxylic acid hydroxyamide, potently inhibited type 5 adenylyl cyclase, the major cardiac isoform, but not other isoforms. In normal mouse cardiac myocytes, contraction induced by low beta-adrenergic receptor stimulation was poorly inhibited with this compound, but the induction of cardiac myocyte apoptosis by high beta-adrenergic receptor stimulation was effectively prevented by type 5 adenylyl cyclase inhibitors. In contrast, when cardiac myocytes from type 5 adenylyl cyclase knock-out mice were examined, beta-adrenergic stimulation poorly induced apoptosis. Our data suggest that the inhibition of beta-adrenergic signaling at the level of the type 5 adenylyl cyclase isoform by P-site inhibitors may serve as an effective method to prevent cardiac myocyte apoptosis induced by excessive beta-adrenergic stimulation without deleterious effect on cardiac myocyte contraction.

Effect of thuringiensin on adenylate cyclase in rat cerebral cortex

The purpose of this work is to evaluate the effect of thuringiensin on the adenylate cyclase activity in rat cerebral cortex. The cyclic adenosine 3'5'-monophosphate (cAMP) levels were shown to be dose-dependently elevated 17-450% or 54-377% by thuringiensin at concentrations of 10 microM-100 mM or 0.5-4 mM, due to the activation of basal adenylate cyclase activity of rat cerebral cortical membrane preparation. Thuringiensin also activated basal activity of a commercial adenylate cyclase from Escherichia coli. However, the forskolin-stimulated adenylate cyclase activity in rat cerebral cortex was inhibited by thuringiensin at concentrations of 1-100 microM, thus cAMP production decreased. Furthermore, thuringiensin or adenylate cyclase inhibitor (MDL-12330A) reduced the forskolin (10 microM)-stimulated adenylate cyclase activity at concentrations of 10 microM, 49% or 43% inhibition, respectively. In conclusion, this study demonstrated that thuringiensin could activate basal adenylate cyclase activity and increase cAMP concentrations in rat cerebral cortex or in a commercial adenylate cyclase. Comparing the dose-dependent effects of thuringiensin on the basal and forskolin-stimulated adenylate cyclase activity, thuringiensin can be regarded as a weak activator of adenylate cyclase or an inhibitor of forskolin-stimulated adenylate cyclase.

Differential inhibition of adenylyl cyclase isoforms and soluble guanylyl cyclase by purine and pyrimidine nucleotides

Mammals express nine membranous adenylyl cyclase isoforms (ACs 1-9), a structurally related soluble guanylyl cyclase (sGC) and a soluble AC (sAC). Moreover, Bacillus anthracis and Bacillus pertussis produce the AC toxins, edema factor (EF), and adenylyl cyclase toxin (ACT), respectively. 2'(3')-O-(N-methylanthraniloyl)-guanosine 5'-[gamma-thio]triphosphate is a potent competitive inhibitor of AC in S49 lymphoma cell membranes. These data prompted us to study systematically the effects of 24 nucleotides on AC in S49 and Sf9 insect cell membranes, ACs 1, 2, 5, and 6, expressed in Sf9 membranes and purified catalytic subunits of membranous ACs (C1 of AC5 and C2 of AC2), sAC, sGC, EF, and ACT in the presence of MnCl(2). N-Methylanthraniloyl (MANT)-GTP inhibited C1.C2 with a K(i) of 4.2 nm. Phe-889 and Ile-940 of C2 mediate hydrophobic interactions with the MANT group. MANT-inosine 5'-[gamma-thio]triphosphate potently inhibited C1.C2 and ACs 1, 5, and 6 but exhibited only low affinity for sGC, EF, ACT, and G-proteins. Inosine 5'-[gamma-thio]triphosphate and uridine 5'-[gamma-thio]triphosphate were mixed G-protein activators and AC inhibitors. AC5 was up to 15-fold more sensitive to inhibitors than AC2. EF and ACT exhibited unique inhibitor profiles. At sAC, 2',5'-dideoxyadenosine 3'-triphosphate was the most potent compound (IC(50), 690 nm). Several MANT-adenine and MANT-guanine nucleotides inhibited sGC with K(i) values in the 200-400 nm range. UTP and ATP exhibited similar affinities for sGC as GTP and were mixed sGC substrates and inhibitors. The exchange of MnCl(2) against MgCl(2) reduced inhibitor potencies at ACs and sGC 1.5-250-fold, depending on the nucleotide and cyclase studied. The omission of the NTP-regenerating system from cyclase reactions strongly reduced the potencies of MANT-ADP, indicative for phosphorylation to MANT-ATP by pyruvate kinase. Collectively, AC isoforms and sGC are differentially inhibited by purine and pyrimidine nucleotides.

Alterations in EDHF-type relaxation and phosphodiesterase activity in mesenteric arteries from diabetic rats

In isolated superior mesenteric artery rings from age-matched control rats and streptozotocin (STZ)-induced diabetic rats, we investigated the role of cAMP in endothelium-derived hyperpolarizing factor (EDHF)-type relaxation. The ACh-induced EDHF-type relaxation was significantly weaker in STZ-induced diabetic rats than in control rats, and in both groups of rats it was attenuated by 18alpha-glycyrrhetinic acid (18alpha-GA), an inhibitor of gap junctions, and enhanced by IBMX, a cAMP-phosphodiesterase (PDE) inhibitor. These enhanced EDHF-type responses were very similar in magnitude between diabetic and age-matched control rats. The EDHF-type relaxation was enhanced by cilostamide, a PDE3-selective inhibitor, but not by Ro 20-1724, a PDE4-selective inhibitor. The expression levels of the mRNAs and proteins for two cAMP PDEs (PDE3A, PDE3B) were significantly increased in STZ-induced diabetic rats, but those for PDE4D were not. We conclude that the impairment of EDHF-type relaxations in STZ-induced diabetic rats may be attributed to a reduction in the action of cAMP via increased PDE activity.

2'(3')-O-(N-methylanthraniloyl)-substituted GTP analogs: a novel class of potent competitive adenylyl cyclase inhibitors

2'(3')-O-(N-Methylanthraniloyl)-(MANT)-substituted nucleotides are fluorescent and widely used for the kinetic analysis of enzymes and signaling proteins. We studied the effects of MANT-guanosine 5'-[gamma-thio]triphosphate (MANT-GTP gamma S) and MANT-guanosine 5'-[beta,gamma-imido]triphosphate (MANT-GppNHp) on G alpha(s)- and G alpha(i)-protein-mediated signaling. MANT-GTP gamma S/MANT-GppNHp had lower affinities for G alpha(s) and G alpha(i) than GTP gamma S/GppNHp as assessed by inhibition of GTP hydrolysis of receptor-G alpha fusion proteins. MANT-GTP gamma S was much less effective than GTP gamma S at disrupting the ternary complex between the formyl peptide receptor and G alpha(i2). MANT-GTP gamma S/MANT-GppNHp non-competitively inhibited GTP gamma S/GppNHp-, AlF(4)(-)-, beta(2)-adrenoceptor plus GTP-, cholera toxin plus GTP-, and forskolin-stimulated adenylyl cyclase (AC) in G alpha(s)-expressing Sf9 insect cell membranes and S49 wild-type lymphoma cell membranes. AC inhibition by MANT-GTP gamma S/MANT-GppNHp was not due to G alpha(s) inhibition because it was also observed in G alpha(s)-deficient S49 cyc(-) lymphoma cell membranes. Mn(2+) blocked AC inhibition by GTP gamma S/GppNHp in S49 cyc(-) membranes but enhanced the potency of MANT-GTP gamma S/MANT-GppNHp at inhibiting AC by approximately 4-8-fold. MANT-GTP gamma S and MANT-GppNHp competitively inhibited forskolin/Mn(2+)-stimulated AC in S49 cyc(-) membranes with K(i) values of 53 and 160 nm, respectively. The K(i) value for MANT-GppNHp at insect cell AC was 155 nm. Collectively, MANT-GTP gamma S/MANT-GppNHp bind to G alpha(s)- and G alpha(i)-proteins with low affinity and are ineffective at activating G alpha. Instead, MANT-GTP gamma S/MANT-GppNHp constitute a novel class of potent competitive AC inhibitors.

Small ligands modulating the activity of mammalian adenylyl cyclases: a novel mode of inhibition by calmidazolium

Molecular cloning of membrane-spanning mammalian adenylyl cyclases (ACs) has led to the discovery of nine different isotypes, making ACs potentially useful therapeutic targets. This study investigated the mechanism by which fungicidal nitroimidazole compounds modulate AC activity. Current evidence indicates that biological control of AC activity occurs through the cytosolic domains. Hence, full-length ACII, ACIX, and recombinant fusion proteins composed of the cytoplasmic loops of human ACIX or the first and second cytoplasmic loops of rat ACV and ACII, respectively, were expressed in human embryonic kidney 293 cells. The AC activities of the respective proteins were characterized, and their modulation by nitroimidazoles was investigated. Calmidazolium inhibited the activities of both full-length ACs and soluble fusion proteins (IC(50), approximately 10 microM). Inhibition of ACIX by calmidazolium was mediated by direct interaction with the catalytic core in a noncompetitive fashion. ACIX was essentially insensitive to 2'-deoxyadenosine 3'-monophosphate, a known blocker of AC activity. The ACV-ACII fusion protein was inhibited by calmidazolium (IC(50), approximately 20 microM) as well as by 2'-deoxyadenosine 3'-AMP (IC(50), approximately 2 microM), in a manner indicating independent mechanisms of action. Taken together, the data demonstrate that ACIX is insensitive to adenosine analogs and that calmidazolium inhibits AC activity by a novel, noncompetitive mechanism.

Coordinate regulation of membrane cAMP by Ca2+-inhibited adenylyl cyclase and phosphodiesterase activities

Activation of store-operated Ca(2+) entry inhibits type 6 adenylyl cyclase (EC; AC(6); Yoshimura M and Cooper DM. Proc Natl Acad Sci USA 89: 6712-6720, 1992) activity in pulmonary artery endothelial cells. However, in lung microvascular endothelial cells (PMVEC), which express AC(6) and turn over cAMP at a rapid rate, inhibition of global (whole cell) cAMP is not resolved after direct activation of store-operated Ca(2+) entry using thapsigargin. Present studies sought to determine whether the high constitutive phosphodiesterase activity in PMVECs rapidly hydrolyzes cAMP so that Ca(2+) inhibition of AC(6) is difficult to resolve. Direct stimulation of adenylyl cyclase using forskolin and inhibition of type 4 phosphodiesterases using rolipram increased cAMP and revealed Ca(2+) inhibition of AC(6). Enzyme activity was assessed using PMVEC membranes, where Ca(2+) and cAMP concentrations were independently controlled. Endogenous AC(6) activity exhibited high- and low-affinity Ca(2+) inhibition, similar to that observed in C6-2B cells, which predominantly express AC(6). Ca(2+) inhibition of AC(6) in PMVEC membranes was observed after enzyme activation and inhibition of phosphodiesterase activity and was independent of the free cAMP concentration. Thus, under basal conditions, the constitutive type 4 phosphodiesterase activity rapidly hydrolyzes cAMP so that Ca(2+) inhibition of AC(6) is difficult to resolve, indicating that high phosphodiesterase activity works coordinately with AC(6) to regulate membrane-delimited cAMP concentrations, which is important for control of cell-cell apposition.

Adenosine as a signaling molecule in the retina: biochemical and developmental aspects

The nucleoside adenosine plays an important role as a neurotransmitter or neuromodulator in the central nervous system, including the retina. In the present paper we review compelling evidence showing that adenosine is a signaling molecule in the developing retina. In the chick retina, adenosine transporters are present since early stages of development before the appearance of adenosine A1 receptors modulating dopamine-dependent adenylate cyclase activity or A2 receptors that directly activate the enzyme. Experiments using retinal cell cultures revealed that adenosine is taken up by specific cell populations that when stimulated by depolarization or neurotransmitters such as dopamine or glutamate, release the nucleoside through calcium-dependent transporter-mediated mechanisms. The presence of adenosine in the extracellular medium and the long-term activation of adenosine receptors is able to regulate the survival of retinal neurons and blocks glutamate excitoxicity. Thus, adenosine besides working as a neurotransmitter or neuromodulator in the mature retina, is considered as an important signaling molecule during retinal development having important functions such as regulation of neuronal survival and differentiation.

Kinetic and thermodynamic characterization of adenylyl cyclase from Euglena gracilis

Some kinetic and thermodynamic properties of the plasma membrane adenylyl cyclase (AC) from the protist Euglena gracilis were examined. The AC kinetics for Mg-ATP was hyperbolic with a K(m) value of 0.33-0.43 mM, whereas the inhibition exerted by 2('),5(')-dideoxyadenosine was of the mixed type with a K(i) of 80-147 microM. The V(m) value (0.9 or 1.8 nmol(mg protein)(-1)min(-1)) changed, depending upon the carbon source in the growth medium (lactic acid or glutamate plus malate). Lactic acid membrane AC was slightly more thermolabile (from 28 to 40 degrees C) and showed higher activation energy (range 15-25 degrees C). With lactate, the total and saturated fatty acid percentage content in the plasma membrane was significantly greater than with glutamate plus malate, whereas the percentage content of polyunsaturated (n-3) fatty acids was lower. The data suggest that the fatty acid composition, as changed by the carbon source in the growth medium, may modulate the AC activity in Euglena.

Soluble guanylate cyclase is allosterically inhibited by direct interaction with 2-substituted adenine nucleotides

Nitric oxide (NO), the principal endogenous ligand for soluble guanylate cyclase (sGC), stimulates that enzyme and accumulation of intracellular cGMP, which mediates many of the (patho) physiological effects of NO. Previous studies demonstrated that 2-substituted adenine nucleotides, including 2-methylthioATP (2MeSATP) and 2-chloroATP (2ClATP), allosterically inhibit guanylate cyclase C, the membrane-bound receptor for the Escherichia coli heat-stable enterotoxin in the intestine. The present study examined the effects of 2-substituted adenine nucleotides on crude and purified sGC. 2-Substituted nucleotides inhibited basal and NO-activated crude and purified sGC, when Mg2+ served as the substrate cation cofactor. Similarly, 2-substituted adenine nucleotides inhibited those enzymes when Mn2+, which activates sGC in a ligand-independent fashion, served as the substrate cation cofactor. Inhibition of sGC by 2-substituted nucleotides was associated with a decrease in Vmax, consistent with a noncompetitive mechanism. In contrast to guanylate cyclase C, 2-substituted nucleotides inhibited sGC by a guanine nucleotide-independent mechanism. These studies demonstrate that 2-substituted adenine nucleotides allosterically inhibit basal and ligand-stimulated sGC. They support the suggestion that allosteric inhibition by adenine nucleotides is a general characteristic of the family of guanylate cyclases. This allosteric inhibition is mediated by direct interaction of adenine nucleotides with sGC, likely at the catalytic domain in a region outside the substrate-binding site.

Type-specific regulation of adenylyl cyclase. Selective pharmacological stimulation and inhibition of adenylyl cyclase isoforms

Crystallographic studies have elucidated the binding mechanism of forskolin and P-site inhibitors to adenylyl cyclase. Accordingly, computer-assisted drug design has enabled us to identify isoform-selective regulators of adenylyl cyclase. After examining more than 200 newly synthesized derivatives of forskolin, we found that the modification at the positions of C6 and C7, in general, enhances isoform selectivity. The 6-(3-dimethylaminopropionyl) modification led to an enhanced selectivity for type V, whereas 6-[N-(2-isothiocyanatoethyl) aminocarbonyl] and 6-(4-acrylbutyryl) modification led to an enhanced selectivity for type II. In contrast, 2'-deoxyadenosine 3'-monophosphate, a classical and 3'-phosphate-substituted P-site inhibitor, demonstrated a 27-fold selectivity for inhibiting type V relative to type II, whereas 9-(tetrahydro-2-furyl) adenine, a ribose-substituted P-site ligand, showed a markedly increased, 130-fold selectivity for inhibiting type V. Consequently, on the basis of the pharmacophore analysis of 9-(tetrahydro-2-furyl) adenine and adenylyl cyclase, a novel non-nucleoside inhibitor, 2-amino-7-(2-furanyl)-7,8-dihydro-5(6H)-quinazolinone (NKY80), was identified after virtual screening of more than 850,000 compounds. NKY80 demonstrated a 210-fold selectivity for inhibiting type V relative to type II. More importantly, the combination of a type III-selective forskolin derivative and 9-(tetrahydro-2-furyl) adenine or NKY80 demonstrated a further enhanced selectivity for type III stimulation over other isoforms. Our data suggest the feasibility of adenylyl cyclase isoform-targeted regulation of cyclic AMP signaling by pharmacological reagents, either alone or in combination.

Molecular biological approaches to unravel adenylyl cyclase signaling and function

Signal transduction through the cell membrane requires the participation of one or more plasma membrane proteins. For many transmembrane signaling events adenylyl cyclases (ACs) are the final effector enzymes which integrate and interpret divergent signals from different pathways. The enzymatic activity of adenylyl cyclases is stimulated or inhibited in response to the activation of a large number of receptors in virtually all cells of the human body. To date, ten different mammalian isoforms of adenylyl cyclase (AC) have been cloned and characterized. Each isoform has its own distinct tissue distribution and regulatory properties, providing possibilities for different cells to respond diversely to similar stimuli. The product of the enzymatic reaction catalyzed by ACs, cyclic AMP (cAMP) has been shown to play a crucial role for a variety of fundamental physiological cell functions ranging from cell growth and differentiation, to transcriptional regulation and apoptosis. In the past, investigations into the regulatory mechanisms of ACs were limited by difficulties associated with their purification and the availability of the proteins in any significant amount. Moreover, nearly every cell expresses several AC isoforms. Therefore, it was difficult to perform biochemical characterization of the different AC isoforms and nearly impossible to assess the physiological roles of the individual isoforms in intact cells, tissue or organisms. Recently, however, different molecular biological approaches have permitted several breakthroughs in the study of ACs. Recombinant technologies have allowed biochemical analysis of adenylyl cyclases in-vitro and the development of transgenic animals as well as knock-out mice have yielded new insights in the physiological role of some AC isoforms. In this review, we will focus mainly on the most novel approaches and concepts, which have delineated the mechanisms regulating AC and unravelled novel functions for this enzyme.

Synthesis and use of 3'-(azidoiodosalicyl) derivatives of 2', 5'-dideoxyadenosine as photoaffinity ligands for adenylyl cyclase

3'-[(4-Azidosalicyl)glycyl]-2',5'-dideoxyadenosine (1), 3'- [(4-azidosalicyl)-gamma-aminobutyryl]-2',5'-dideoxyadenosine (2), and the (125)I-labeled mono- and diiodinated analogs of 1 were synthesized and tested as photoaffinity probes for adenylyl cyclases. Kinetics for inhibition of purified type I enzyme by 1 was noncompetitive with respect to Mn(*)5'-ATP in the absence of light, implying a P-site mechanism of inhibition. In a UV-dependent manner both 1 and 2 and the iodinated derivative of 1 irreversibly inactivated membrane-bound and purified forms of recombinant type I bovine adenylyl cyclase expressed in ovarian cells of either the fall armyworm (Sf9) or Trichoplasia ni (High Five). Irreversible inactivation was independent of 5'-ATP and was prevented by 2', 5'-dideoxyadenosine. Adenylyl cyclase, whether purified from bovine brain or in membranes from High Five cells expressing type I enzyme, when subjected to UV irradiation in the presence of (125)I-labeled 1 resulted in radioactive incorporation into protein migrating at approximately 116 kDa. The cross-linking of 1 and its iodinated derivative with adenylyl cyclase suggests potential for such compounds to be useful in structural studies of adenylyl cyclases or of other proteins for which adenine nucleosides are substrates or allosteric regulators.

Lys-Ala mutations of type I adenylyl cyclase result in altered susceptibility to inhibition by adenine nucleoside 3'-polyphosphates

Native and recombinant wild type and mutant forms of type I adenylyl cyclase, expressed in fall army worm ovarian cells (Sf9) cells, with mutations Lys-923-Ala, Lys-921-Ala, and Lys-350-Ala, retained the characteristic noncompetitive inhibition by adenine nucleoside 3'-polyphosphates, but exhibited substantially different sensitivities to inhibition by them. The type I K923A enzyme resulted in increased IC(50) values, e.g., >100-fold for 2'-deoxyadenosine-3'-monophosphate, but the shift diminished as the number of 3'-phosphates increased. The K921A mutation increased IC(50) values approximately 5-fold for all adenine nucleosides tested, whereas the K350A mutation increased IC(50) values approximately 6- to 8-fold for all adenine nucleosides tested except 2'-deoxyadenosine-3'-diphosphate, which was increased >/=2-fold. The data suggest that 3'-phosphates sufficiently increase binding affinity of these ligands to compensate for the reduced coordination of the adenine moiety induced by the K923A mutation. Moreover, the altered structures induced by both K350A and K921A mutations impair ligand binding in general, but paradoxically those resulting from the K350A change minimally affected nucleoside 3'-diphosphate binding, implying that selective changes in ligand binding can be induced by this site-specific mutation.

Inhibition of adenylyl cyclase by acyclic nucleoside phosphonate antiviral agents

Acyclic derivatives of adenine, known as highly effective nucleotide analogs with broad spectrum antiviral activity, were evaluated for potential cross-reactivity with adenylyl cyclases, a family of membrane-bound enzymes that share putative topologies at their catalytic sites with oligonucleotide polymerases and reverse transcriptases. A series of derivatives of 9-(2-phosphonylmethoxyethyl)adenine (PMEA) inhibited a preparation of adenylyl cyclase derived from rat brain with IC(50) values that ranged from 66 microM (PMEA) to 175 nM for its diphosphate derivative (PMEApp) and mimics of it. PMEApp mimics included PMEAp(NH)p, PMEAp(CH(2))p, PMEAp(CX(2))p (X = fluorine, chlorine, or bromine), PMEAp(CHX)pp, and PMEAp(C(OH)CH(3)pp. The data suggest that inhibition of adenylyl cyclases may contribute to the therapeutic action of some of these or similar compounds or constitute part of their side effects in therapeutic settings.

Enzymatic synthesis of unlabeled and beta-(32)P-labeled beta-L-2', 3'-dideoxyadenosine-5'-triphosphate as a potent inhibitor of adenylyl cyclases and its use as reversible binding ligand

beta-L-2',3'-Dideoxyadenosine-5'-triphosphate (beta-L-2', 3'-dd-5'-ATP) was prepared enzymatically from the corresponding monophosphate by the use of adenylate kinase, creatine phosphate, and creatine kinase in a single step. The beta-(32)P-labeled analog was prepared similarly, but in a two step reaction. beta-L-2', 3'-dd-5'-ATP inhibited adenylyl cyclase from rat brain competitively with respect to substrate (5'-ATP.Mn(2+)) and exhibited an IC(50) approximately 24 nM. The labeled ligand was used in the development of a reversible binding assay for adenylyl cyclases. Binding of beta-L-2',3'-dd-[beta-(32)P]5'-ATP was saturable with increasing concentrations of ligand and increased in proportion to membrane protein, and was enhanced by Mn(2+) to a greater extent than by Mg(2+). Binding was displaced with adenine nucleotides known to be either competitive or noncompetitive inhibitors but not by agents known not to act on the cyclase, or by 3-isobutyl-1-methylxanthine, creatine phosphate, or creatine kinase. Binding was rapid, with a half-time for the on-rate <1.8 min and for the off-rate <0.8 min. The potency and mechanism of the inhibition of this ligand and the pattern of agents that displace binding suggest an interaction with adenylyl cyclase per se and to a configuration of the enzyme consistent with an interaction at the catalytic active site. The data suggest that this is a pretransition state inhibitor and contrasts with the equipotent 2',5'-dd-3'ATP, a post-transition state noncompetitive inhibitor.

Covalent labeling of adenylyl cyclase cytosolic domains with gamma-methylimidazole-2',5'-dideoxy-[gamma-(32)P]3'-ATP and the mechanism for P-site-mediated inhibition

A truncated first cytosolic domain of type V adenylyl cyclase (VC(1)) and a truncated second cytosolic domain of type II adenylyl cyclase (IIC(2)) were used alone and in the readily reversible complex (VC(1).IIC(2)) to evaluate interactions with each other and with reversible and irreversible P-site ligands. Enzyme activity was used to assess formation and dissolution of VC(1).IIC(2). The data suggest that binding of 2',5'-dideoxy-3'-ATP to VC(1) and IIC(2) prevented formation of VC(1).IIC(2) and that 2',5'-dideoxy-3'-ATP dissociation occurred slowly. To enable configuration specific cross-linking to the catalytic site, 2',5'-dideoxyadenosine 3'-[gamma-(1-methylimidazole)-triphosphate] (gamma-MetIm-2', 5'-dd-3'-ATP) and 2',5'-dd-adenosine 3'-(gamma-azidoanilido)-triphosphate (gamma-azidoanilido-2', 5'-dd-3'-ATP) were synthesized, the former also as its gamma-(32)P-labeled analog. gamma-Azidoanilido-2',5'-dd-3'-ATP exhibited an inhibitory potency comparable with that of 2', 5'-dd-3'-ATP. gamma-MetIm-2',5'-dd-[gamma-(32)P]3'-ATP labeled the individual VC(1) and IIC(2) domains comparably and covalently to approximately 20% within 1 h. Formation of VC(1).IIC(2) resulted in reduced labeling of VC(1) but enhanced labeling of IIC(2). The data imply that formation of the catalytically active VC(1).IIC(2) complex affects the interaction of each domain with the 2', 5'-dd-3'-ATP, the binding of which also affects the interaction between the two cytosolic domains, leading to a pseudo-irreversible inhibition.

Cyclic AMP mediates EDHF-type relaxations of rabbit jugular vein

Isolated rings of rabbit jugular vein have been used to test the hypothesis that formation of cAMP within the endothelial cell contributes to relaxations that are attributable to the endothelium-derived hyperpolarizing factor, EDHF. Relaxations induced by acetylcholine under conditions of combined NO synthase and cyclooxygenase blockade were almost abolished by inhibition of adenylate cyclase with the selective P-site agonist 2', 3'-dideoxyadenosine (2',3'-DDA). They were similarly attenuated by the gap junction inhibitors 18alpha-glycyrrhetinic acid (18alpha-GA) and Gap 27 peptide which interrupt direct endothelium-smooth muscle communication without themselves affecting smooth muscle tone. By contrast, stimulation of adenylate cyclase with forskolin promoted gap junction-dependent relaxations, with concentration-relaxation curves to this agent exhibiting an equivalent rightward shift in the presence of 18alpha-GA and following endothelial denudation. The findings suggest that cAMP may cross from the endothelium to smooth muscle via gap junction channels and/or enhance the endothelial hyperpolarization normally associated with agonist stimulation. Both mechanisms may contribute to EDHF/gap junction-dependent relaxations.

The interactions of adenylate cyclases with P-site inhibitors

Recent kinetic, binding and crystallographic studies using P-site inhibitors of mammalian adenylate bases provide new insights into the catalytic mechanism of these highly regulated enzymes. Here, Carmen Dessauer and colleagues discuss the conformational states of adenylate cyclase, the structural determinants of inhibitor binding and the potential uses of these inhibitors as pharmacological agents.

Enzymatic preparation of 32P-labeled beta-L-2',3',-dd-5'ATP and its use as a high-affinity, conformation-specific ligand for labeling adenylyl cyclases

An enzymatic method was developed for the preparation of unlabeled and [beta-32P]-labeled beta-L-2',3'-dd-5'ATP from the monophosphate with near quantitative yields. beta-L-2',3'-dd-5'ATP was a competitive and potent inhibitor of adenylyl cyclases (IC5 approximately 30 nM). Upon uv-irradiation beta-L-2',3'-dd-[beta-32P]-5'ATP directly crosslinked to a chimeric construct of this enzyme. Data suggest that this is a pre-transition state inhibitor and contrasts with the equipotent 2',5'-dd-3'ATP, a post-transition state, noncompetitive inhibitor.

Adenylyl cyclase P-site ligands accelerate differentiation in Ob1771 preadipocytes

Differentiation of Ob1771 preadipocytes to adipocytes was characterized by morphological changes and elevated expression of the specific marker enzyme, glycerol-3-phosphate dehydrogenase. A differentiation response substantially more complete and rapid than that obtained with insulin and 3,5,3'-triiodothyronine was observed with established inhibitors of adenylyl cyclases: 2', 5'-dideoxyadenosine (2',5'-dd-Ado), 9-(cyclopentyl)adenine (9-CP-Ade), and 9-(arabinofuranosyl)adenine (9-Ara-Ade), coincident with decreased cellular cAMP levels. These ligands inhibit adenylyl cyclases noncompetitively, via a domain referred to as the P-site because of its requirement for an intact purine moiety. Differentiation was not induced by inosine, a nucleoside known not to act at the P-site, or by N6-(2-phenylisopropyl)adenosine or 1, 3-diethyl-8-phenylxanthine, agonist and antagonist, respectively, for adenosine A1 receptors. Also ineffective were IBMX or forskolin, agents that can raise intracellular cAMP levels. Potency of the differentiation response followed the order 2',5'-dd-Ado (1-20 microM) > 9-CP-Ade (10-100 microM) = 9-Ara-Ade (10-100 microM) >> inosine, consistent with their potencies to inhibit adenylyl cyclases. The data suggest that inhibition of adenylyl cyclase via the P-site and the consequent reduction in cell cAMP levels facilitate the induction of differentiation in Ob1771 cells. The findings raise the question whether the known endogenous P-site ligands participate in the differentiation response induced by hormones.

Direct photoaffinity labeling of individual cytosolic domains of adenylyl cyclase by [32P]2'-deoxy-3'-AMP and [alpha-32P]5'-ATP

The susceptibility of purines to form a covalent attachment with proteins upon exposure to UV irradiation was applied to adenylyl cyclase by use of [32P]2'-d-3'-AMP, a dead-end inhibitor that binds to the post-transition configuration of the enzyme. [32P]2'-d-3'-AMP was synthesized enzymatically. It and [alpha-32P]5'-ATP were used for direct photocross-linking to individually expressed cytosolic domains of adenylyl cyclase. Both the C1 domain of the type V isozyme (VC1) and the C2 domain of the type II isozyme (IIC2) were labeled, whether alone or combined, upon photolysis of [32P]2'-d-3'-AMP in the presence of acetone. Labeling of VC1 and IIC2 was greatly enhanced in the presence of PPi, was almost completely suppressed by 50 microM 2',5'-dideoxy-3'-ATP, the most potent reported P-site inhibitor of adenylyl cyclases, but was partially suppressed by 1 mM 3'-IMP, a ligand that does not inhibit the enzyme via the P-site. Neither 3':5'-cAMP nor 5'-ATP had a major effect on labeling by [32P]2'-d-3'-AMP. Direct cross-linking of VC1 with [alpha-32P]5'-ATP was substantially suppressed by 2', 5'-dideoxy-3'-ATP and partially suppressed by 2'-d-3'-AMP, whereas cross-linking of IIC2 was less affected by the 3'-triphosphate. The data imply that either cytosolic domain can interact directly with either substrate or P-site ligand and that subunit interaction modifies the susceptibility of each domain to UV-induced covalent modification by either [alpha-32P]5'-ATP or [32P]2'-d-3'-AMP.

Adenine nucleoside 3'-tetraphosphates are novel and potent inhibitors of adenylyl cyclases

2'-Deoxyadenosine 3'-tetraphosphate (2'-deoxy-3'-A4P) and 2', 5'-dideoxyadenosine 3'-tetraphosphate (2',5'-dideoxy-3'-A4P) were synthesized, and their effects were tested on crude and purified forms of native adenylyl cyclases isolated from brain. Syntheses combined the method of alkoxide activation with the use of tribromoethyl phosphoromorpholino-chloridate as an initial phosphorylating agent. Inhibition of adenylyl cyclase was rapid in onset. With 2'-d-3'-A4P or 2',5'-dd-3'-A4P inhibition of a purified native enzyme conformed to a linear noncompetitive behavior with respect to substrate, metal-5'ATP. Order of potency was 2', 5'-dideoxy- > 2'-deoxyadenosine and 3'-tetraphosphate > 3'-triphosphate. Both mechanism of inhibition and rank order of potency were consistent with inhibition via the 3'-nucleotide-(P)-site on adenylyl cyclase. Neither 2',5'-dd-3'-ATP nor 2',5'-dd-3'-A4P had any effect on the activities of other adenosine nucleotide binding proteins such as Ca2+/calmodulin-sensitive cyclic nucleotide phosphodiesterase, Na+/K+-ATPase, or cAMP-dependent protein kinase. With purified adenylyl cyclase from bovine brain 2',5'-dd-3'-A4P and 2'-d-3'-A4P gave, respectively, IC50 values of 9.3 and 15 nM and Ki values of 23 and 53 nM. These 3'-nucleotides are the most potent regulators described for adenylyl cyclases.

Exchange of substrate and inhibitor specificities between adenylyl and guanylyl cyclases

The active sites of guanylyl and adenylyl cyclases are closely related. The crystal structure of adenylyl cyclase and modeling studies suggest that specificity for ATP or GTP is dictated in part by a few amino acid residues, invariant in each family, that interact with the purine ring of the substrate. By exchanging these residues between guanylyl cyclase and adenylyl cyclase, we can completely change the nucleotide specificity of guanylyl cyclase and convert adenylyl cyclase into a nonselective purine nucleotide cyclase. The activities of these mutant enzymes remain fully responsive to their respective stimulators, sodium nitroprusside and Gsalpha. The specificity of nucleotide inhibitors of guanylyl and adenylyl cyclases that do not act competitively with respect to substrate are similarly altered, indicative of their action at the active sites of these enzymes.

Inhibition of adenylyl cyclase by caveolin peptides

Caveolae and their principal component caveolin have been implicated in playing a major role in G protein-mediated transmembrane signaling. We examined whether caveolin interacts with adenylyl cyclase, an effector of G protein signaling, using a 20-mer peptide derived from the N-terminus scaffolding domain of caveolin-1. When tissue adenylyl cyclases were examined, cardiac adenylyl cyclase was inhibited more potently than other tissue adenylyl cyclases. The caveolin-1 peptide inhibited type V, as well as type III adenylyl cyclase, overexpressed in insect cells, whereas the same peptide had no effect on type II. The caveolin-3 scaffolding domain peptide similarly inhibited type V adenylyl cyclase. In contrast, peptides derived from the caveolin-2 scaffolding domain and a caveolin-1 nonscaffolding domain had no effect. Kinetic studies showed that the caveolin-1 peptide decreased the maximal rate (Vmax) value of type V without changing the Michaelis constant (Km) value for the substrate ATP. Studies with various truncations and point mutations of this peptide revealed that a minimum of 16 amino acid residues and intact aromatic residues are important for the inhibitory effect. The potency of inhibition was greater when adenylyl cyclase was in stimulated condition vs. basal condition. Thus, caveolin may be another cellular component that regulates adenylyl cyclase catalytic activity. Our results also suggest that the caveolin peptide may be used as an isoform-selective inhibitor of adenylyl cyclase.

The catalytic mechanism of mammalian adenylyl cyclase. Equilibrium binding and kinetic analysis of P-site inhibition

The mechanism of P-site inhibition of adenylyl cyclase has been probed by equilibrium binding measurements using 2'-[3H]deoxyadenosine, a P-site inhibitor, and by kinetic analysis of both the forward and reverse reactions (i.e. cyclic AMP and ATP synthesis, respectively). There is one binding site for 2'-deoxyadenosine per C1/C2 heterodimer; the Kd is 40 +/- 3 microM. Binding is observed only in the presence of one of the products of the adenylyl cyclase reaction, pyrophosphate (PPi). A substrate analog, Ap(CH2)pp (alpha,beta-methylene adenosine 5'-triphosphate), and cyclic AMP compete for the P-site in the presence of PPi, but P-site analogs do not compete for substrate binding (in the absence of PPi). Kinetic analysis indicates that release of products from the enzyme is random. These facts permit formulation of a model for the adenylyl cyclase reaction, for which we provide substantial kinetic support. We propose that P-site analogs act as dead-end inhibitors of product release, stabilizing an enzyme-product (E-PPi) complex by binding at the active site. Although product release is random, cyclic AMP dissociates from the enzyme preferentially. Release of PPi is slow and partially rate-limiting.

Interactions of forskolin and ATP with the cytosolic domains of mammalian adenylyl cyclase

Fragments of the two cytoplasmic domains of mammalian adenylyl cyclases can be synthesized independently (and abundantly) as soluble proteins; Gsalpha- and forskolin-stimulated enzymatic activity is restored upon their mixture. We have utilized this system to characterize the interactions of adenylyl cyclase with forskolin and its substrate, ATP. In the presence of Gsalpha, adenylyl cyclase is activated in response to occupation of only one forskolin-binding site. A single binding site for forskolin was identified by equilibrium dialysis; its Kd (0.1 microM) corresponds to the EC50 for enzyme activation. The affinity of forskolin for adenylyl cyclase is greatly reduced in the absence of Gsalpha ( approximately 40 microM). Binding of forskolin to the individual cytoplasmic domains of the enzyme was not detected. A single binding site for the ATP analog, alpha,beta-methylene ATP (Ap(CH2)pp), was also detected by equilibrium dialysis. Such binding was not observed with the individual domains. Binding of Ap(CH2)pp was unaffected by P-site inhibitors of adenylyl cyclase. A modified P-loop sequence located near the carboxyl terminus of adenylyl cyclase has been implicated in ATP binding. Mutation of the conserved, non-glycine residues within this region caused no significant changes in the Km for ATP or the Ki for Ap(CH2)pp. It thus seems unlikely that this region is part of the active site. However, a mutation in the C1 domain (E518A) causes a 10-fold decrease in the binding affinity for Ap(CH2)pp. This residue and the active site of the enzyme may lie at the interface between the two cytosolic domains.

Isozyme-dependent sensitivity of adenylyl cyclases to P-site-mediated inhibition by adenine nucleosides and nucleoside 3'-polyphosphates

Recombinant adenylyl cyclase isozyme Types I, II, VI, VII, and three splice variants of Type VIII were compared for their sensitivity to P-site-mediated inhibition by several adenine nucleoside derivatives and by the family of recently synthesized adenine nucleoside 3'-polyphosphates (Désaubry, L., Shoshani, I., and Johnson, R. A. (1996) J. Biol. Chem. 271, 14028-14034). Inhibitory potencies were dependent on isozyme type, the mode of activation of the respective isozymes, and on P-site ligand. For the nucleoside derivatives potency typically followed the order 2',5'-dideoxyadenosine (2',5'-ddAdo) > beta-adenosine > 9-(cyclopentyl)-adenine (9-CP-Ade) >/= 9-(tetrahydrofuryl)-adenine (9-THF-Ade; SQ 22,536), with the exception of Type II adenylyl cyclase, which was essentially insensitive to inhibition by 9-CP-Ade. For the adenine nucleoside 3'-polyphosphates inhibitory potency followed the order Ado < 2'-dAdo < 2',5'-ddAdo and 3'-mono- < 3'-di- < 3'-triphosphate. Differences in potency of these ligands were noted between isozymes. The most potent ligand was 2',5'-dd-3'-ATP with IC50 values of 40-300 nM. The data demonstrate isozyme selectivity for some ligands, suggesting the possibility of isozyme-selective inhibitors to take advantage of differences in P-site domains among adenylyl cyclase isozymes. Differential expression of adenylyl cyclase isozymes may dictate the physiological sensitivity and hence importance of this regulatory mechanism in different cells or tissues.

Protein kinase C alters the responsiveness of adenylyl cyclases to G protein alpha and betagamma subunits

The ability of protein kinase C (PKC) to regulate the responsiveness of adenylyl cyclase to different activators was assessed. Membranes prepared from Sf9 cells infected with recombinant baculoviruses encoding either type II or IV adenylyl cyclase were incubated with recombinant PKCalpha (purified from Sf9 cells), and the effects on adenylyl cyclase activity were measured after reconstitution with Gsalpha, Gbetagamma, or forskolin. PKCalpha treatment of type II adenylyl cyclase leads to increases in basal, forskolin-stimulated, and betagamma-stimulated activities and greater sensitivity to stimulation by Gsalpha. Paradoxically, most of the betagamma potentiation of Gsalpha-stimulated activity is eliminated by pretreatment with PKCalpha. By contrast, treatment of type IV adenylyl cyclase with PKCalpha has little effect on the basal, forskolin-stimulated, or betagamma-stimulated activities but markedly reduces the Gsalpha-stimulated and betagamma-potentiated activity of this isoform. These studies demonstrate that protein kinases can alter both the activity of adenylyl cyclase isoforms and their responsiveness to G protein regulation, thereby altering the ability of adenylyl cyclases to integrate signals derived from multiple hormonal inputs.

Purification and characterization of a soluble form of mammalian adenylyl cyclase

An engineered, soluble form of mammalian adenylyl cyclase has been expressed in Escherichia coli and purified by three chromatographic steps. The enzyme utilizes one molecule of ATP to synthesize one molecule of cyclic AMP and pyrophosphate at a maximal specific activity of 12.8 micromol/min/mg, corresponding to a turnover number of 720 min-1. Although devoid of membrane spans, the enzyme displays all of the regulatory properties that are common to mammalian adenylyl cyclases. It is activated synergistically by Gsalpha and forskolin and is inhibited by adenosine (P-site) analogs with kinetic patterns that are identical to those displayed by the native enzymes. The purified enzyme is also inhibited directly by the G protein betagamma subunit complex. After adenovirus-mediated expression in adenylyl cyclase-deficient HC-1 cells, the enzyme can be stimulated synergistically by Gs-coupled receptors and forskolin.

Inhibition of adenylyl cyclase by a family of newly synthesized adenine nucleoside 3'-polyphosphates

The synthesis of a number of adenine nucleoside 3'-polyphosphates has been devised via a phosphotriester approach that combines the method of alkoxide activation with the use of 2,2,2-tribromoethyl phosphoromorpholinochloridate as a phosphorylating agent. The family of compounds included 3'ADP, 3'ATP, 2'-deoxy-3'ADP, 2'-deoxy-3'ATP, 2',5'-dideoxy-3'ADP, and 2',5'-dideoxy-3'ATP. Potency as inhibitors of adenylyl cyclases followed the order: 3'-mono- < 3'-di- < 3'-triphosphate and adenosine (Ado) < 2'-d-Ado < 2',5'-dd-Ado derivatives, with 2',5'-dideoxy-3'ATP exhibiting an IC50 of approximately 40 nM. This order was maintained with purified and recombinant forms of the type I enzyme. The nucleoside 3'-phosphates caused noncompetitive inhibition of the type I adenylyl cyclase from bovine brain, consistent with inhibition via the P-site. Inhibition was not due to hydrolytic products because this was minimal and inhibition kinetics by inorganic polyphosphates were inconsistent with those caused by the nucleoside 3'-polyphosphates. Only 3'ATP underwent cation-catalyzed, nonenzymatic hydrolysis, with the primary product being 2':3'-cAMP. Because 3'-ADP and 3'-ATP are naturally occurring, this class of compounds may physiologically regulate adenylyl cyclases and possibly other enzymes, mediating responses that include a reduction in 3':5'-cAMP levels and consequent reductions in protein kinase A-activated pathways.

2',5'-Dideoxyadenosine 3'-polyphosphates are potent inhibitors of adenylyl cyclases

2',5'-Dideoxyadenosine 3'-di- and triphosphates were tested as inhibitors of brain adenylyl cyclases. With an IC50 approximately 40 nM, 2',5'-dideoxy-3'-ATP is the most potent nonprotein synthetic regulator of adenylyl cyclases thus far described. Neither 2',5'-dideoxy-3'-ADP nor 2',5'-dideoxy-3'-ATP inhibited activity by competition with substrate, and the linear noncompetitive inhibition observed was consistent with interaction via a distinct domain. The availability of this ligand will permit the development of a variety of probes that will be extremely useful in investigating adenylyl cyclase structure and the role(s) that this class of compound may play in physiologically regulating cell function.

Insulin sensitizes beta-agonist and forskolin-stimulated lipolysis to inhibition by 2',5'-dideoxyadenosine

In isolated rat adipocytes incubated in the absence of insulin, 2',5'-dideoxyadenosine blocked the increase in total adenosine 3',5'-cyclic monophosphate (cAMP) accumulation due to beta 1- or beta 3-catecholamine agonists and forskolin without affecting their stimulation of lipolysis. The inhibition of cAMP accumulation by 2',5'-dideoxyadenosine was not reflected in the total cytosolic cAMP-dependent protein kinase A activity, suggesting that the inhibition of cAMP occurred in cellular compartments distinct from those involved in the regulation of bulk protein kinase A activity. However, there was a good correlation between effects of lipolytic agents on cytosolic protein kinase A activity in fat cell extracts and lipolysis. Furthermore, it was possible to see an inhibition of the increase due to beta-agonists in cAMP accumulation, protein kinase A activity, and lipolysis by 2',5'-dideoxyadenosine in the presence of insulin. These data suggest that the readily measurable accumulation of cAMP seen with catecholamines in the absence of insulin is in a compartment separate from that involved in protein kinase A activation.

Azido-iodo-phenyl-analogs of 2',5'-dideoxy-adenosine as photoaffinity ligands for adenylyl cyclase

Azidoiodophenyl-analogs of 2',5'-dideoxyadenosine were synthesized and tested as potential 'P'-site selective affinity probes for adenylyl cyclases. The 3'-substituted analogs included: 1: 3'-[(4-nitrophenyl)-acetyl]-2',5'-dideoxy-adenosine 2: 3'-[(4-nitrophenyl)-butyryl]-2',5'-dideoxyadenosine 3: 3'-[(4-azido-3-iodophenyl)-acetyl]-2',5'-dideoxyadenosine and 4: 3'-[(4-azido-3-iodophenyl)-butyryl]-2',5'-dideoxyadenosine. The azidoiodo-phenyl-analogs inactivated adenylyl cyclase irreversibly and in a light-dependent manner. This was observed with detergent-dispersed enzyme from rat brain, purified native enzyme from bovine brain, and recombinant Type I bovine adenylyl cyclase expressed in membranes from fall army worm ovarian (Sf9) cells. Inactivation of the recombinant enzyme was inversely dependent on ATP concentration and was not completely prevented by 2',5'-dideoxyadenosine. Inhibition kinetics with the recombinant enzyme in the absence of light suggested two sites of inhibition, whereas with the native Type I enzyme inhibition kinetics exhibited a straightforward noncompetitive mechanism. Occupation of either or both sites by ligand protected the enzyme against denaturation by UV-irradiation per se. The data are consistent with inactivation of the recombinant enzyme occurring both through the 'P'-site and the catalytic active site, but suggest that this is a characteristic of the recombinant enzyme and is not dependent on the probes per se. The data suggest the potential for independent interactions of such ligands with different sites on a given enzyme and also with other enzymes containing adenosine or adenine nucleotide binding domains.

Construction of a soluble adenylyl cyclase activated by Gs alpha and forskolin

A soluble adenylyl cyclase was constructed by linkage of portions of the cytosolic domains of the mammalian type I and type II enzymes. The soluble enzyme was stimulated by both forskolin and the alpha subunit of the heterotrimeric guanine nucleotide-binding protein (G protein) Gs (Gs alpha). Expression of the construct complemented the catabolic defect in a strain of Escherichia coli that is deficient in adenylyl cyclase activity. The active, approximately 60-kilodalton enzyme accumulated in the cytoplasmic fraction of E. coli to yield activities in excess of 1 nanomole per minute per milligram of protein. The two sets of transmembrane helices of mammalian adenylyl cyclases are thus not necessary for the catalytic or the most characteristic regulatory activities of the enzyme. This system may be useful for both genetic and biochemical analysis of G protein-regulated adenylyl cyclases.

A brain synaptosomal adenylyl cyclase of high specific activity is photolabeled with azido-ATP

Partially purified adenylyl cyclase preparations of high specific activity (60 +/- 10 mumol cAMP/(mg.min)) were obtained from rat brain synaptosomes (Orlando, C., d'Alayer, J., Baillat, G., Castets, F., Jeannequin, O., Mazié, J. C., & Monneron, A. (1992) Biochemistry 31, 3215-3222). Adenylyl cyclase activity was stimulated 4-fold by Ca2+/calmodulin and 2-fold by forskolin or by Mn2+. These preparations contained two major proteins of 140 and 110 kDa. The 140-kDa protein was identified as the neural cell adhesion molecule. The 110-kDa protein was specifically recognized by affinity-purified antibodies directed against a peptide corresponding to sequence 976-1013 of adenylyl cyclase type I. It was photolabeled by [alpha-32P]8- and 2-N3ATP in a light-dependent manner and was by far the most heavily labeled component of FC fractions. Saturation was obtained with 30 microM [32P]8-N3ATP. Photoinsertion of N3ATP into the protein was largely prevented by ATP or adenylyl imidodiphosphate but not by ADP, AMP, or adenosine. A modest incorporation of N3cAMP and photoinsertion of [alpha-32P]N3GTP into the 110-kDa protein were observed. Although some of the properties of the synaptosomal 110-kDa protein described here would match those expected from adenylyl cyclase type I, it appears that its specific activity is on the order of 1 mmol cAMP/(mg.min), about 200-fold that measured for brain adenylyl cyclases type I.

Studies of the thermal inactivation of cardiac adenylyl cyclase: evidence for a conformational change in the reaction mechanism

Membrane bound cardiac adenylyl cyclase was shown to undergo a spontaneous and irreversible thermal inactivation with a t1/2 of approximately 10 min. The loss of activity could not be explained by the action of endogenous proteases. Repeated freeze-thaw of membrane preparations resulted in a much increased rate of thermal inactivation (t1/2 = approx. 2 min). ATP, adenylimidodiphosphate, ADP, and PPi protected the enzyme from thermal inactivation with dissociation constants (Kd) of 193, 5.04, 84.4, and 6.3 microM, respectively. 5'-AMP and cyclic AMP were ineffective as protectors at concentrations as high as 3 mM. Activators of adenylyl cyclase such as Mn2+, forskolin, 5-guanylylimidodiphosphate, and NaF and 9 mM Mg2+ protected against thermal inactivation with Kd of 16.8 microM, 8.81 microM, 0.23 microM and 1.04 mM, respectively. Mg2+ alone was without effect. Thermal inactivation was first order under all conditions tested. Arrhenius plots of the rate constants for inactivation vs temperature were linear. The increased stability of ligand bound adenylyl cyclase was shown to be associated with an increased free energy of activation (delta G 0). These data provide evidence for the existence of two distinct conformations of cardiac adenylyl cyclase based on different susceptibilities to thermal inactivation. These enzyme conformations, termed E1 and E2, may be important reaction intermediates. The thermal stability of E1 was highly influenced by the enzyme's membrane lipid environment. The formation of E2 from E1 was enhanced by interaction with substrate, PPi, activators of adenylyl cyclase, and by interaction with dissociated stimulatory guanine nucleotide binding protein-alpha beta gamma heterotrimers.