Bimodal regulation of adenylate cyclase

Are guanine nucleotide binding proteins a distinct class of regulatory proteins?

Proteins which bind guanine nucleotides are found in a diverse set of key regulatory positions. They are involved in hormone action, visual transduction, protein synthesis and microtubule assembly. In addition to their ability to bind guanine nucleotides these proteins possess several other common features. (i) They all have similar subunit composition, (ii) they can be ADP-ribosylated, (iii) their conformation changes depending on the nucleotide bound. These regulatory G-proteins have close functional homologies. Do they form a general class of regulatory proteins, like the protein kinases? Do they have a common evolutionary ancestry?

Enkephalin-mediated inhibition of forskolin-stimulated rabbit luteal adenylyl cyclase activity

Forskolin at 25-100 microM elicited 10- to 15-fold stimulation of rabbit luteal adenylyl cyclase activity in the absence of guanine nucleotides. Addition of saturating concentrations of GTP or guanyl-5'-yl imidodiphosphate [GMP-P(NH)P] inhibited forskolin stimulation by 15-25% and 35-45%, respectively, in Na+-free media. The further addition of 8 microM [D-Ala2, Met5] enkephalin amide (Da-ENK) caused an additional 16-24% inhibition of activity in the presence of GTP plus forskolin, but did not alter enzymatic activity in the presence of forskolin alone or forskolin plus GMP-P(NH)P. Inhibition by guanine nucleotide alone or Da-ENK plus GTP was only observed in the presence of forskolin. Maximal inhibition by Da-ENK was observed at 25 microM forskolin. Da-ENK reduced the IC50 for GTP by 2.3-fold but did not alter the IC50 for GMP-P(NH)P. Addition of Na+ above 3 mM attenuated the inhibitory responses to GTP and GTP plus Da-ENK, but not to GMP-P(NH)P or GMP-P(NH)P plus Da-ENK. Above 100 mM, Na+ inhibited enzymatic activity in the presence of forskolin, forskolin plus GTP and forskolin plus GMP-P(NH)P in the absence and presence of Da-ENK. These findings suggest that the rabbit corpus luteum contains an inhibitory receptor for opiate peptides that couples to adenylyl cyclase.

Effect of pertussis toxin on the hormonal regulation of cyclic AMP levels in hamster fat cells

Pertussis toxin was purified approx. 1800-fold from pertussis vaccine. Administration of as little as 1 microgram of toxin/100 g body weight to hamsters markedly decreased the sensitivity of their adipocytes to agents that inhibit adenylate cyclase through receptor-mediated, GTP-dependent mechanisms such as alpha 2-adrenergic amines, prostaglandins, phenylisopropyladenosine and nicotinic acid. On the contrary, the inhibitory effect of 2',5'-dideoxyadenosine on cyclic AMP accumulation was not affected by the toxin. Activation of adenylate cyclase by isoproterenol, ACTH or forskolin was not diminished by the toxin but the maximum cyclic AMP accumulation was consistently increased. Furthermore, the dose-response curves for ACTH and forskolin were clearly shifted to the left in adipocytes from toxin-treated hamsters as compared to control adipocytes. It is concluded that pertussis toxin blocks the transfer of inhibitory information from the receptors to adenylate cyclase.

Insulin exerts actions through a distinct species of guanine nucleotide regulatory protein: inhibition of adenylate cyclase

Insulin failed to exert an effect on the basal and glucagon- and guanosine 5'-[beta, gamma-imido]-triphosphate-stimulated adenylate cyclase activities of hepatocyte membranes. In the presence of high GTP (0.1 mM) concentrations, however, insulin was shown to inhibit adenylate cyclase activity. This effect was dose-dependent, exhibiting an EC50 (median effective concentration) of 3 microM for GTP. Elevated glucagon concentrations blocked the inhibitory effect of insulin in a dose-dependent fashion, with an EC50 of 1 nM. The insulin inhibition was dose-dependent (EC50 = 90 pM). The inhibitory effects of insulin were abolished using membranes from either glucagon-desensitized hepatocytes or cholera-toxin-treated hepatocytes. If either Mn2+ replaced Mg2+ in adenylate cyclase assays or Na+ was removed from the assay mixtures then insulin failed to exert any inhibitory effect. It is suggested that insulin exerts its action on adenylate cyclase through an inhibitory guanine nucleotide protein. This is integrated with the proposal [Heyworth, Rawal & Houslay (1983) FEBS Lett. 154, 87-91; Heyworth, Wallace & Houslay (1983) Biochem. J. in the press] that insulin mediates a variety of cellular effects through a specific guanine nucleotide regulatory protein and associated protein kinase(s).

Inhibition of NS-protein-stimulated human-platelet adenylate cyclase by epinephrine and stable GTP analogs

The influence of purified, preactivated NS protein, mediating adenylate cyclase stimulation by hormones and guanine nucleotides, was studied on adenylate cyclase activity in membranes of human platelets. The preactivated coupling protein caused a large increase in platelet enzyme activity. The activation occurred after a short lag phase and exhibited saturation. NS protein increased the V of the platelet adenylate cyclase without change in the enzyme's affinity for the substrate, MgATP, whereas the apparent affinity for Mg2+ was increased by more than one order of magnitude. The NS-protein-activated human platelet adenylate cyclase was inhibited by the hormone, epinephrine, and by the stable GTP analogs, guanosine 5'-[beta, gamma-imido]triphosphate and guanosine 5'-[gamma-thio]triphosphate. The stable GTP analog or hormone-induced adenylate cyclase inhibition was not competitive with regard to the concentration of NS protein. Inhibition of NS-protein-stimulated platelet adenylate cyclase caused by stable GTP analogs appeared to be persistent, was amplified by epinephrine and was accompanied by a large decrease in the enzyme's apparent affinity for Mg2+. The data suggest that the hormone-sensitive adenylate cyclase is regulated by two guanine-nucleotide-binding sites, NS and Ni, mediating enzyme stimulation and inhibition, respectively, by hormones and guanine nucleotides, and that the two coupling components interact in a non-competitive manner with the adenylate cyclase, exhibiting high and low affinity for Mg2+ when affected by NS and Ni, respectively.

Adenylate cyclase inhibition and GTPase stimulation by somatostatin in S49 lymphoma cyc- variants are prevented by islet-activating protein

cyc--Variants of S49 lymphoma cells are defective in the stimulatory guanine nucleotide site of the adenylate cyclase but contain an inhibitory site. Treatment of cyc- cells with islet-activating protein (IAP), which causes ADP-ribosylation of an Mr 40 000 polypeptide in cyc- membranes, abolishes adenylate cyclase inhibition by GTP and the peptide hormone, somatostatin, but not that induced by GTP gamma S. Furthermore, somatostatin-induced stimulation of GTP hydrolysis is lost. Thus, the data indicate that IAP interferes with the adenylate cyclase system by an action at the inhibitory guanine nucleotide site.

Occurrence of a hormone-sensitive inhibitory coupling component of the adenylate cyclase in S49 lymphoma cyc- variants

The influence of somatostatin was studied on cyclic AMP levels and adenylate cyclase activity in cyc- variants of S49 lymphoma cells. These cells are deficient in the guanine nucleotide site that mediates hormone-induced adenylate cyclase stimulation, but their cyclase can be stimulated by forskolin. Somatostatin maximally decreased the 30 microM forskolin-stimulated cyclic AMP levels by 35%. Half-maximal suppression occurred at about 0.1 nM somatostatin. Somatostatin (up to 1 microM) had no effect on the 100 microM forskolin-stimulated adenylate cyclase activity in cyc- membrane preparations when guanine nucleotides were not present. In the presence of GTP, however, which by itself caused a small decrease in activity, somatostatin maximally inhibited the enzyme by 20-25%. GTP was half-maximally effective at 0.1 microM, and half-maximal inhibition by somatostatin was observed at 0.1- 1 nM. In the presence of the stable GTP analog guanosine 5'-O-(3-thiotriphosphate) (1 microM), which decreased the stimulated activity by about 40% after a short lag period, somatostatin (1 microM) did not cause a further decrease in final activity but reduced the lag period by about 50%. The data indicate that membranes of cyc- variants contain a regulatory site that mediates both guanine nucleotide and hormone-induced inhibition of the adenylate cyclase and suggest that the mechanisms of activation and inactivation of this inhibitory site are similar to those of the stimulatory component missing in cyc-membranes.

Pertussis toxin substrate, the putative Ni component of adenylyl cyclases, is an alpha beta heterodimer regulated by guanine nucleotide and magnesium

The final step in a scheme for the purification of the guanine nucleotide- and Mg2+-binding stimulatory regulatory component (Ns) of adenylyl cyclase [adenylate cyclase; ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] from human erythrocyte membranes involves chromatography over hydroxylapatite (HAP) which yields two fractions. The first fraction (HAP I) contains predominantly two peptides that, upon sodium dodecyl sulfate/polyacrylamide gel electrophoresis, migrate with Mr values of 39,000 and 35,000. The second fraction (HAP II) contains predominantly Ns formed of two peptides of Mr 42,000 and 35,000. The HAP I, Mr 39,000 peptide is shown to be a substrate for the ADP-ribosylating toxin of Bordetella pertussis (pertussis toxin). Upon sucrose density gradient centrifugation, both the Mr 39,000 and the Mr 35,000 peptides of HAP I migrate at about 4 S. Treatment of HAP I with guanine nucleotide and Mg2+ prior to centrifugation results in a coordinated change in the migration of both peptides to 2 S. It is postulated that HAP I contains an alpha beta heterodimeric protein composed of an alpha subunit of Mr 39,000 and a beta subunit of Mr 35,000. Further, this protein dissociates under the influence of guanine nucleotides and Mg2+ into its individual alpha and beta subunits. Because previous studies have shown that treatment of cells and cell membranes with pertussis toxin results in attenuation of the effects of hormones that inhibit adenylyl cyclase activity, and because this effect correlates with the ADP-ribosylation of a Mr approximately equal to 40,000 peptide, we believe that we have purified a guanine nucleotide- and Mg2+-binding inhibitory regulatory component of adenylyl cyclases--i.e., the Ni.

Effect of islet-activating protein (IAP) on contractile responses of rat vas deferens: evidence for participation of Ni (inhibitory GTP binding regulating protein) in the alpha 2-adrenoceptor-mediated response

The effects of islet-activating protein (IAP) on the contractile responses of vas deferens to exogenous norepinephrine (NE) and electric stimulation (ES) were investigated. IAP treatment had no effect on the contractile response to NE. Pretreatment of the vas deferens with IAP (2.0 micrograms/ml) for 2 h however significantly attenuated the inhibitory effect of clonidine on ES-induced contractile responses, suggesting that feedback inhibition of presynaptic alpha 2-adrenoceptors is mediated by the Ni subunit of presynaptic adenylate cyclase.

A nucleotide regulatory site for somatostatin inhibition of adenylate cyclase in S49 lymphoma cells

The cyc- variants of S49 lymphoma cells have served as powerful tools for studying the components and mechanisms of hormone-induced adenylate cyclase stimulation, as these cells are deficient in the guanine nucleotide regulatory site (Ns) mediating hormone, guanine nucleotide, cholera toxin and fluoride-induced stimulations of the enzyme. Because of this deficiency, membranes of these cells have been used for reconstitution of the system by inserting the coupling component derived from other cell types. The hormone-sensitive adenylate cyclase is not only stimulated by hormones but can also be inhibited by a wide variety of hormones and neurotransmitters, and there is some evidence that hormonal inhibition may be mediated by a distinct guanine nucleotide regulatory site. Studies in cyc- cells lacking a functional Ns may therefore answer this unresolved, important question. We have recently observed that stable GTP analogues can inhibit cyc- adenylate cyclase stimulated by purified, preactivated Ns or forskolin, which can activate adenylate cyclase even in the absence of a functional Ns (ref. 10). The data indicated that these Ns-deficient cells contain an inhibitory guanine nucleotide site, Ni. To strengthen this concept, we investigated whether the cyc- adenylate cyclase can be inhibited by a hormone. We report here that somatostatin decreases cyclic AMP levels in cyc- cells, inhibits the forskolin-stimulated adenylate cyclase and causes a concomitant increase in a high affinity GTPase activity in cyc- membranes. The data strongly suggest that both the hormone- and guanine nucleotide-induced adenylate cyclase inhibitions in cyc- cells are mediated by Ni and that the mechanisms of activation and inactivation of Ni are similar to those established for Ns.

Stimulation and inhibition of adenylyl cyclases mediated by distinct regulatory proteins

Adenylyl cyclases are under positive and negative control by guanine nucleotides and hormones. Stimulatory responses are mediated by a guanine nucleotide- and Mg-binding regulatory component (Ns), a protein that has been purified to homogeneity. Inhibitory responses have been hypothesized to be mediated by an analogous regulatory component (Ni) distinct from Ns, but definitive proof for this is lacking and these effects may result from modulation of Ns activity. Recently, Bordetella pertussis toxin has been shown to ADP-ribosylate a peptide that is not part of Ns, and this coincides with attenuation of hormonal inhibition of adenylyl cyclase. We show here that cyc- S49 cells contain a substrate for ADP-ribosylation by pertussis toxin and that the toxin alters GTP dependent inhibition of cyc- adenyl cyclase activity. As cyc- S49 cells do not contain Ns by several criteria, we conclude that Ni is a distinct and separate regulatory component of adenylyl cyclase.

Potentiation of monovalent cation effects on ligand binding to cardiac muscarinic receptors in N-ethylmaleimide treated membranes

Guanine nucleotides and monovalent cations decrease the affinity of cardiac muscarinic receptors for agonists and are required for muscarinic receptor mediated inhibition of adenylate cyclase. N-ethylmaleimide abolished the effects of Gpp(NH)p on the ability of the agonist oxotremorine to inhibit the binding of the antagonist [3H]quinuclidinyl benzilate to purified chick heart membranes. However, the effects of NH4+ to decrease the IC50 for oxotremorine were retained in N-ethylmaleimide treated membranes. The N-ethylmaleimide treatment mimicked the effects of Gpp(NH)p and the oxotremorine inhibition curves obtained with treated membranes in the presence of NH4+ were identical to those obtained in control membranes in the presence of NH4+ and Gpp(NH)p. The results suggest that monovalent cation effects on muscarinic receptors are mediated at a site distinct from effects produced by guanine nucleotides and are greater on free receptors than on receptors coupled to guanine nucleotide binding proteins.

Inactivation of the guanine nucleotide regulatory site mediating inhibition of the adenylate cyclase in hamster adipocytes

The influence of N-ethylmaleimide and trypsin was studied on stimulatory and inhibitory regulations of the hamster adipocyte adenylate cyclase. Treatment of intact adipocytes or adipocyte ghosts with N-ethylmaleimide decreased basal and forskolin-stimulated adenylate cyclase activities. In the pretreated membrane preparations, inhibition of the enzyme by GTP and by stable GTP analogues was abolished. Concomitantly, activation of the adenylate cyclase by NaCl and its inhibition by the antilipolytic agents, prostaglandin E1 and nicotinic acid, were obliterated. In contrast, adenylate cyclase stimulation by ACTH and stable GTP analogues was not impaired but rather increased. Similarly, the NaCl-induced attenuation of the ACTH-stimulated enzyme activity was increased by the N-ethylmaleimide treatment. Limited proteolysis of hamster adipocyte ghosts with trypsin also obliterated GTP and prostaglandin E1-induced inhibitions and NaCl-induced activation of the adenylate cyclase. In contrast, adenylate cyclase activity stimulated by isoproterenol was increased after trypsin treatment. The data suggest that the activity of the adenylate cyclase is regulated via two distinct guanine nucleotide sites and that treatment with N-ethylmaleimide and limited proteolysis with trypsin functionally eliminates the regulatory site mediating adenylate cyclase inhibition, leading to a state where the enzyme activity is regulated only via the stimulatory site. The differential effects of these treatments on NaCl-induced activation and attenuation of the adenylate cyclase suggest that sodium acts on both regulatory sites in an inhibitory manner, and that by the functional elimination of the inhibitory site, only the sodium-induced attenuation of the adenylate cyclase via the stimulatory site is observed.