Chemical and functional analysis of components of adenylyl cyclase from human platelets treated with phorbolesters

Inhibitory effect of α-lipoic acid on platelet aggregation is mediated by PPARs

Peroxisome proliferator-activated receptors (PPARs) isoforms (α, β/δ, and γ are present in human platelets, and activation of PPARs inhibits platelet aggregation. α-Lipoic acid (ALA), occurring naturally in human food, has been reported to exhibit an antiplatelet activity. However, the mechanisms underlying ALA-mediated inhibition of platelet aggregation remain unknown. The aim of this study was to investigate whether the antiplatelet activity of ALA is mediated by PPARs. ALA itself significantly induced PPARα/γ activation in platelets and increased intracellular amounts of PPARα/γ by blocking PPARα/γ secretion from arachidonic acid (AA)-activated platelets. Moreover, ALA significantly inhibited AA-induced platelet aggregation, Ca(2+) mobilization, and cyclooxygenase-1 (COX-1) activity, but increased cyclic AMP production in rabbit washed platelets. Importantly, ALA also enhanced interaction of PPARα/γ with protein kinase Cα (PKCα) and COX-1 accompanied by an inhibition of PKCα activity in resting and AA-activated platelets. However, the above effects of ALA on platelets were markedly reversed by simultaneous addition of selective PPARα antagonist (GW6471) or PPARγ antagonist (GW9662). Taken together, the present study provides a novel mechanism by which ALA inhibition of platelet aggregation is mediated by PPARα/γ-dependent processes, which involve interaction with PKCα and COX-1, increase of cyclic AMP formation, and inhibition of intracellular Ca(2+) mobilization.

Protein kinase C-mediated down-regulation of beta1-adrenergic receptor gene expression in rat C6 glioma cells

In the current study, we investigated the mechanism by which protein kinase C (PKC) regulates the expression of beta1-adrenergic receptor (beta1AR) mRNA in rat C6 glioma cells. Exposure of the cells to 4beta-phorbol-12-myristate-13-acetate (PMA), an activator PKC, resulted in a down-regulation of both beta1AR binding sites and mRNA levels in a time- and concentration-dependent manner. This effect was not observed with phorbol esters that do not activate PKC and was blocked by bisindolylmaleimide, a specific PKC inhibitor. Activation of PKC did not reduce the half-life of beta1AR mRNA but significantly decreased the activity of the beta1AR promoter, as determined by reporter analysis. A putative response element, with partial homology to a consensus cAMP response element, was identified by mutation analysis of the promoter at positions -343 to -336, relative to the translational start site. Mutation of this putative regulatory element, referred to as a beta1AR-PKC response element, completely blocked the PKC-mediated down-regulation of beta1AR promoter activity. Gel mobility shift analysis detected two specific bands when C6 cell extracts were incubated with a labeled DNA probe containing the beta1AR-PKC response element sequence. Formation of one of these bands was inhibited by an oligonucleotide probe containing a consensus CRE and disrupted by an antibody for cAMP response element binding protein. Based on these studies, we propose that the PKC-induced down-regulation of beta1AR gene transcription in C6 cells is mediated in part by a cAMP response element binding protein-dependent mechanism acting on a novel response element.

Potentiation of receptor-mediated cAMP production: role in the cross-talk between vasopressin V1a and V2 receptor transduction pathways

Cross-talk between the phospholipase C and adenylyl cyclase signalling pathways was investigated in Chinese hamster ovary (CHO) cells transfected with the V1a and V2 vasopressin receptors. Cell lines expressing V1a, V2, or both V1a and V2 receptors, were established and characterized. Stimulation of V2 receptors by vasopressin induced a dose-dependent increase in cAMP accumulation, whereas stimulation of V1a receptor resulted in an increase in intracellular calcium without any change in basal cAMP. The simultaneous stimulation of V2 and V1a receptors by vasopressin elicited an intracellular cAMP accumulation which was twice that induced by stimulation of V2 receptor alone with deamino-[d-Arg8]vasopressin. This potentiation between V1a and V2 receptors was mimicked by activation of protein kinase C (PKC) with PMA, and was suppressed when PKC activity was inhibited by bisindolylmaleimide. The potentiation was observed in the presence or absence of 1 mM 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, implying that an alteration in cAMP hydrolysis was not involved. Vasopressin, as well as PMA, had no effect on the forskolin-induced cAMP accumulation, suggesting that PKC did not directly stimulate the cyclase activity. On the other hand, vasopressin, like PMA, potentiated the cAMP accumulation induced by cholera toxin, an activator of Galphas protein. These results suggest that, in CHO cells, vasopressin V1a receptor potentiates the cAMP accumulation induced by the V2 receptor through a PKC-dependent increase in the coupling between Gs protein and adenylyl cyclase.

Forskolin-stimulated platelet adenylyl cyclase activity is lower in persons with major depression

We investigated platelet adenylyl cyclase activity in 17 subjects with a history of major depression ("depressed subjects") and 20 controls. Forskolin was used to directly activate adenylyl cyclase, while guanine nucleotides (Gpp(NH)p) and fluoride ions were used to measure adenylyl cyclase activity modulated through the G proteins. Forskolin-stimulated adenylyl cyclase was significantly lower in the depressed subjects (p < 0.0005). There was a statistically significant difference in basal adenylyl cyclase activity between male depressed subjects and male controls. The basal adenylyl cyclase activity was also lower in female depressed subjects, but this difference did not reach statistical significance (p < 0.2). The adenylyl cyclase activity measured after stimulation with a guanine nucleotide or cesium fluoride did not differ between control and depressed male or female subjects. Severity of current depression and the current use of antidepressant medication were not related to the lower forskolin-stimulated enzyme activity in the depressed subjects. The difference in forskolin-stimulated adenylyl cyclase activity appears to reflect a qualitative difference in the adenylyl cyclase enzyme activity in persons with a history of major depression.

Identification and characterization of a widely expressed form of adenylyl cyclase

A novel mammalian adenylyl cyclase was identified by reverse transcription-polymerase chain reaction amplification using degenerate primers based on a conserved region of previously described adenylyl cyclases (Premont, R. T. (1994) Methods Enzymol. 238, 116-127). The full-length cDNA sequence obtained from mouse brain predicts a 1353-amino acid protein possessing a 12-membrane span topology, and containing two regions of high similarity with the catalytic domains of adenylyl cyclases. Comparison of this novel adenylyl cyclase with the eight previously described mammalian enzymes indicates that this type 9 adenylyl cyclase sequence is the most divergent, defining a sixth distinct subclass of mammalian adenylyl cyclases. The AC9 gene has been localized to human chromosome band 16p13.3-13.2. The 8.5-kb mRNA encoding the type 9 adenylyl cyclase is widely distributed, being readily detected in all tissues tested, and is found at very high levels in skeletal muscle and brain. AC9 mRNA is found throughout rat brain but is particularly abundant in hippocampus, cerebellum, and neocortex. An antiserum directed against the carboxyl terminus of the type 9 adenylyl cyclase detects native and expressed recombinant AC9 protein in tissue and cell membranes. Levels of the AC9 protein are highest in mouse brain membranes. Characterization of expressed recombinant AC9 reveals that the protein is a functional adenylyl cyclase that is stimulated by Mg2+, forskolin, and mutationally activated Gsalpha. AC9 activity is not affected by Ca2+/calmodulin or by G protein betagamma-subunits. Thus AC9 represents a functional G protein-regulated adenylyl cyclase found in brain and in most somatic tissues.

Acylation of adenylyl cyclase catalyst is important for enzymic activity

Incubation of human thrombocytes in the presence of [3H]palmitic acid leads to incorporation of this fatty acid into the alpha subunit of Gs as described [Linder et al., Proc. Natl. Acad. Sci. USA 90 (1993) 3675-3679; Degtyarev et al., Biochemistry 32 (1993) 8057-8061] but also into the catalyst of adenylyl cyclase which has not been recognized before. Treatment of labeled membranes with hydroxylamine released the label from both components. Label incorporated into the catalyst could be identified as [3H]palmitate. At the same time chemical deacylation caused partial loss of adenylyl cyclase activity.

Intracellular cross-talk between thyrotropin receptor and A1 adenosine receptor in regulation of phospholipase C and adenylate cyclase in COS-7 cells transfected with their receptor genes

COS-7 cells were transiently transfected with human thyrotropin receptor (TSHR) and dog A1 adenosine receptor (A1R) cDNA. TSH stimulated both inositol phosphate production and cyclic AMP (cAMP) accumulation in the cells. An A1 agonist, N6-(L-2-phenylisopropyl)adenosine (PIA), which is ineffective alone, significantly enhanced TSH-induced inositol phosphate production, but insignificantly inhibited TSH-induced cAMP accumulation was revealed by short-term treatment with the protein kinase C inhibitors, staurosporine and K252a, or long-term treatment with 12-myristate 13-acetate, suggesting that endogenous protein kinase C inhibits the A1R-mediated inhibition of the TSHR-adenylate cyclase system. In staurosporine-treated cells, the stimulatory and inhibitory permissive actions of PIA on TSH-induced phospholipase C and adenylate cyclase activation respectively were completely reversed by pretreatment with pertussis toxin whereas intrinsic TSH-induced effects were hardly affected by the toxin. The cross-talk between the signalling pathway for TSHR and that for A1R was not detected in a mixture of cells expressing either TSHR or A1R. We conclude that a single species of A1R, via pertussis-toxin-sensitive GTP-binding proteins, not only inhibits adenylate cyclase but also stimulates phospholipase C in collaboration with an activated TSHR within a single cell expressing both types of receptor.

Phorbol ester-induced stimulation and phosphorylation of adenylyl cyclase 2

Adenylyl cyclase 2 was expressed in Sf9 cells by recombinant baculovirus infection. Phorbol 12-myristate 13-acetate (PMA) treatment of cells expressing adenylyl cyclase 2 (AC2) increased basal activity. This increase was blocked by staurosporine, a protein kinase C inhibitor. PMA treatment increased Vmax without affecting Km. Greatest increase in basal activity was seen at physiologically relevant Mg2+ concentrations. PMA treatment did not alter sensitivity to guanine nucleotide stimulatory factor (Gs) but enhanced stimulation at all concentrations of activated Gs alpha subunit tested. AC2 was tagged at the N terminus with an 8-amino acid epitope. Epitope-tagged AC2 was purified to apparent homogeneity in a single step by using an antiepitope antibody-affinity column. The eluate was resolved by SDS/PAGE. Silver staining of the gel showed a 106-kDa band. The purified protein was recognized by antipeptide antibody against a region common to all mammalian adenylyl cyclases. The epitope-tagged enzyme expressed in Sf9 cells was also stimulated by PMA. When cells were labeled with 32P and treated with PMA, a 3-fold increase in 32P incorporation of purified epitope-tagged AC2 was observed. We conclude that activation of protein kinase C results in phosphorylation and stimulation of AC2, a cell-surface G protein effector enzyme. Thus, covalent modification of cell-surface effectors may provide an independent mode for signal transmission through G protein pathways.

Receptor crosstalk: effects of prolonged carbachol exposure on beta 1-adrenoceptors and adenylyl cyclase activity in neonatal rat ventricular myocytes

Supersensitivity of adenylyl cyclase after exposure to inhibitory agonists is a general means of cellular adaptation. We hypothesized that such "crosstalk" between muscarinic cholinergic agonists, beta 1-adrenoceptors, and adenylyl cyclase may be an important mechanism of cardiac adaptation to interventions that enhance vagal activity. We used primary cultures of neonatal rat ventricular myocytes and measured beta-adrenoceptors by radioligand binding and adenylyl cyclase activity by a single column method. Carbachol induced a time- and dose-dependent reversible decrease in cell surface beta 1-adrenoceptors. The peak effect occurred after 20 h of exposure to 100 microM carbachol which caused a decrease in the maximum number of binding sites for the beta-adrenoceptor antagonist 3H-CGP-12177 from 42.3 +/- 3.4 to 33.0 +/- 2.6 fmol/mg protein (n = 12, P < 0.03) without a change in antagonist affinity. Loss of cell surface receptors was prevented by atropine and by the protein kinase C inhibitor H7. The decrease in cell surface receptors was not accompanied by receptor internalization as assessed by equilibrium binding experiments in a cytosolic fraction using 125I-iodocyanopindolol. In contrast to the well-known acute inhibitory effects of carbachol on adenylyl cyclase activation, prolonged carbachol exposure preserved (-)-isoprenaline-stimulated adenylyl cyclase activity and enhanced postreceptor stimulated adenylyl cyclase activity. Carbachol did not further enhance adenylyl cyclase activity after pretreatment with pertussis toxin. The protein kinase C inhibitor chelerythrine prevented the carbachol induced enhancement of forskolin-stimulated adenylyl cyclase activity. We conclude that prolonged incubation with carbachol in rat neonatal ventricular myocytes causes a reduction in cell surface beta 1-Adrenoceptor density. beta 1-Adrenoceptor-mediated adenylyl cyclase activity is preserved and postreceptor-mediated adenylyl cyclase activity is augmented. Our data suggest that carbachol-stimulated protein kinase C activity may play a key role in the prolonged muscarinic regulation of adenylyl cyclase activity.

Bradykinin-dependent activation of adenylate cyclase activity and cyclic AMP accumulation in tracheal smooth muscle occurs via protein kinase C-dependent and -independent pathways

Treatment of cultured tracheal smooth-muscle cells (TSM) with phorbol 12-myristate 13-acetate (PMA) (100 nM) or bradykinin (100 nM) elicited enhanced basal and guanosine 5'-[beta gamma-imido]-triphosphate-stimulated adenylate cyclase activities in subsequently isolated membranes. Combined stimulation of cells was non-additive, indicating that both agents activate adenylate cyclase via similar routes. Both PMA (100 nM) and bradykinin (100 nM) allowed the alpha subunit of Gs to act as a more favourable substrate for its cholera-toxin-catalysed ADP-ribosylation in vitro. PMA was without effect on intracellular cyclic AMP in control cells. However, constitutive activation of Gs by treatment in vivo with cholera toxin (0.5 ng/ml, 18 h) sensitized the cells to PMA stimulation, resulting in a concentration-dependent increase in intracellular cyclic AMP accumulation (EC50 = 7.3 +/- 2.5 nM, n = 5). Bradykinin also elicited a concentration-dependent increase in intracellular cyclic AMP (EC50 = 63.3 +/- 14.5 nM, n = 3). Constitutive activation of Gs resulted in an increased maximal response (10-fold) and potency (EC50 = 6.17 +/- 1.6 nM, n = 3) to bradykinin. This response was not affected by the B2-receptor antagonist, NPC567 [which selectively blocks bradykinin-stimulated phospholipase C (PLC), with minor activity against phospholipase D (PLD) activity]. Des-Arg9-bradykinin (a B1-receptor agonist) was without activity. These results suggest that the receptor sub-type capable of activating PLD may also be stimulatory for cyclic AMP accumulation. Furthermore, pre-treatment of the cells with butan-l-ol (0.3%, v/v), which traps phosphatidate derived from PLD reactions, blocked the bradykinin-stimulated increase in intracellular cyclic AMP. These studies suggest that there may be a causal link between PLD-derived phosphatidate and the positive modulation of adenylate cyclase activity. In support of this, the concentration-dependence for bradykinin-stimulated adenylate cyclase activity was identical with that of bradykinin-stimulated phospholipase D activity (EC50 = 5 nM). Bradykinin, but not PMA, was also capable of eliciting the inhibition of cyclic AMP phosphodiesterase activity in TSM cells (EC50 > 100 nM) via an unidentified mechanism. These studies indicate that cross-regulation between the cyclic AMP pathway and phospholipid-derived second messengers in TSM cells does not occur as a consequence of PLC-catalysed PtdIns(4,5)P2 hydrolysis, but may involve, in part, PLD-catalysed phosphatidylcholine hydrolysis.

Adenylyl cyclase in lung from hypersensitive guinea pig displays increased responsiveness to guanine nucleotides and isoprenaline: the role of the G proteins Gs and Gi

Basal adenylyl cyclase activity in lung membranes isolated from hypersensitive guinea pigs was increased and more sensitive to stimulation by isoprenaline, GTP and GppNHp when compared to adenylyl cyclase in lung membranes isolated from normal healthy guinea pigs. Maximal forskolin-stimulated adenylyl cyclase activity was unaltered. There was no change in the immunological quantitative amounts of either alpha subunits of the G proteins GiII and Gs (G(o), GiI and GiIII were not present). Maximal pertussis-toxin- and cholera-toxin-catalyzed ADP-ribosylation of Gi alpha and Gs alpha respectively were not significantly altered. The addition of purified protein kinase C to isolated lung membranes resulted in the phosphorylation of the alpha subunit of Gs (stoichiometry was 0.53 mol of 32P incorporated/mol of Gs alpha). Addition of protein kinase C to lung membranes isolated from hypersensitive guinea pigs was equally effective at catalysing the phosphorylation of the alpha subunit of Gs. GppNHp-stimulated and basal adenylyl cyclase activity was also enhanced in isolated tracheal smooth-muscle membranes from hypersensitive guinea pigs. These results suggest that hypersensitive reactions are associated with the improved coupling of the stimulatory G protein (Gs) with adenylyl cyclase.

Protein kinase C sensitizes olfactory adenylate cyclase

Effects of neurotransmitters on cAMP-mediated signal transduction in frog olfactory receptor cells (ORCs) were studied using in situ spike recordings and radioimmunoassays. Carbachol, applied to the mucosal side of olfactory epithelium, amplified the electrical response of ORCs to cAMP-generating odorants, but did not affect unstimulated cells. A similar augmentation of odorant response was observed in the presence of phorbol dibutyrate (PDBu), an activator of protein kinase C (PKC). The electrical response to forskolin, an activator of adenylate cyclase (AC), was also enhanced by PDBu, and it was attenuated by the PKC inhibitor Goe 6983. Forskolin-induced accumulation of cAMP in olfactory tissue was potentiated by carbachol, serotonin, and PDBu to a similar extent. Potentiation was completely suppressed by the PKC inhibitors Goe 6983, staurosporine, and polymyxin B, suggesting that the sensitivity of olfactory AC to stimulation by odorants and forskolin was increased by PKC. Experiments with deciliated olfactory tissue indicated that sensitization of AC was restricted to sensory cilia of ORCs. To study the effects of cell Ca2+ on these mechanisms, the intracellular Ca2+ concentration of olfactory tissue was either increased by ionomycin or decreased by BAPTA/AM. Increasing cell Ca2+ had two effects on cAMP production: (a) the basal cAMP production was enhanced by a mechanism sensitive to inhibitors of calmodulin; and (b) similar to phorbol ester, cell Ca2+ caused sensitization of AC to stimulation by forskolin, an effect sensitive to Goe 6983. Decreasing cell Ca2+ below basal levels rendered AC unresponsive to stimulation by forskolin. These data suggest that a crosstalk mechanism is functional in frog ORCs, linking the sensitivity of AC to the activity of PKC. At increased activity of PKC, olfactory AC becomes more responsive to stimulation by odorants, forskolin, and cell Ca2+. Neurotransmitters appear to use this crosstalk mechanism to regulate olfactory sensitivity.

Agonist receptors and G proteins as mediators of platelet activation

Recent studies have helped to define the earliest events of signal transduction in platelets, particularly those involved in the generation of second messengers. The best-understood of these events are those which involve guanine nucleotide binding regulatory proteins. G proteins are heterotrimers comprised of alpha, beta and gamma subunits, each of which can exist in multiple forms. Some, but not all, of the known variants of G alpha are substrates for ADP-ribosylation by pertussis toxin, a modification which disrupts the flow of information from receptor to effector. The G proteins that have been identified in platelets to date are Gs, Gi1, Gi2, Gi3, Gz and Gq. Gs and one or more of the Gi family members regulate cAMP formation by adenylylcyclase. Gi may also be responsible for the pertussis toxin-sensitive activation of phospholipase C which occurs when platelets are activated by thrombin. Gq is thought to be responsible for the pertussis toxin-resistant activation of phospholipase C by TxA2. Gz does not have an established role, but has the unique property of being phosphorylated by protein kinase C during platelet activation. Recent efforts to clone the receptors that interact with G proteins in platelets have been successful for epinephrine, thrombin, TxA2 and platelet activating factor. Each of these resembles other G protein-coupled receptors, being comprised of a single polypeptide with 7 transmembrane domains. In the case of thrombin, receptor activation is thought to involve a unique mechanism in which thrombin cleaves its receptor, creating a new N-terminus that can serve as a tethered ligand. Peptides corresponding to the tethered ligand can mimic the effects of thrombin, while antibodies to the same domain inhibit platelet activation. Shortly after activation, thrombin receptors become resistant to re-activation by thrombin. This desensitization, which appears to be due to a combination of proteolysis, phosphorylation and internalization, provides a potential mechanism for limiting the duration of thrombin-initiated signals in platelets.

The alpha subunit of the stimulatory guanine-nucleotide-binding regulatory protein forms high-molecular-mass aggregates, concomitant with iloprost-induced desensitization of human platelet adenylyl cyclase

Prolonged treatment of human platelets with the prostacyclin analog iloprost led to desensitization of the response to various prostaglandin derivatives. However, basal adenylyl cyclase activity and stimulation by agents acting directly via Gs, the stimulatory guanine-nucleotide-binding regulatory protein of adenylyl cyclase, were likewise decreased. Reconstitution of desensitized membranes with purified Gs from turkey erythrocytes indicated no alteration in the catalyst itself. However, the function of Gs (in cholate extracts) appeared to be severely impaired when reconstituted with adenylyl cyclase catalyst. Modification of Gs was also indicated by its altered sedimentation in sucrose density gradients. From Western blots, the alpha subunit of Gs, alpha s, from control platelets sedimented as a 5.6S species, while that from desensitized cells appeared at higher S values (in a polydisperse distribution). Activation by guanosine 5'-[gamma-thio]triphosphate of Gs from control platelets shifted alpha s to 3.5-3.7S, while activation of Gs from desensitized platelets induced such shift only for a minor portion of alpha s. This small fraction alone appeared to be susceptible to ADP-ribosylation by cholera toxin/[32P]NAD. Furthermore, an antibody directed against the C-terminal hexadecapeptide of alpha s precipitated much less alpha s from cholate extracts derived from desensitized platelets. Modification of alpha s during desensitization was also suggested from cross-linking experiments using the homobifunctional agent bismaleimidohexane: alpha s from desensitized platelets formed a single product of 80 kDa, while that from untreated platelets yielded a doublet (100 kDa and 110 kDa).

Prostaglandin-concentration-dependent desensitization of adenylate cyclase in human erythroleukaemia (HEL) cells is abolished by pertussis toxin and enhanced by induction by dimethyl sulphoxide

Prostaglandin-regulated cyclic AMP metabolism in human erythroleukaemia (HEL) cells was similar to that previously described in platelets [Ashby (1989) Mol. Pharmacol. 36, 866-873], displaying prostaglandin-concentration-dependent desensitization that could be explained by the presence of separate stimulatory and inhibitory prostaglandin receptors. Pertussis toxin abolished prostaglandin-concentration-dependent desensitization, indicating that the process is mediated through a pertussis toxin-sensitive GTP-binding protein. Treatment of HEL cells for 4 days with the inducer dimethyl sulphoxide enhanced prostaglandin-concentration-dependent desensitization, but did not alter the initial rate of cyclic AMP synthesis or the amount of Gi2 alpha measured by immunoblotting, suggesting that the inhibitory receptor was selectively induced by changing the cells to a more platelet-like form.