Phosphorylation of the spliced variant forms of the recombinant stimulatory guanine-nucleotide-binding regulatory protein (Gs alpha) by protein kinase C

Bronchoprotection by olodaterol is synergistically enhanced by tiotropium in a guinea pig model of allergic asthma

The novel once-daily β₂-agonist bronchodilator drug olodaterol has recently been shown to be effective in patients with allergic asthma for >24 hours. An increased cholinergic tone common to these patients may decrease the effectiveness of β₂-agonists. This could provide a rationale for combination therapy with olodaterol and the long-acting anticholinergic tiotropium to aim for a once-daily treatment regimen. In guinea pigs, we evaluated the protective effects of olodaterol, alone and in combination with tiotropium, on airway responsiveness to histamine, which is partially mediated by a cholinergic reflex mechanism. In addition, using a guinea pig model of acute allergic asthma, we examined the cooperative effects of these bronchodilators on allergen-induced early (EAR) and late (LAR) asthmatic reactions, airway hyper-responsiveness (AHR) to histamine, and airway inflammation. It was demonstrated that the protective effect of olodaterol against histamine-induced bronchoconstriction was synergistically enhanced and prolonged in the presence of tiotropium. In addition, tiotropium synergistically augmented both the reversal of and the protection against the allergen-induced AHR after the EAR by olodaterol. Olodaterol and tiotropium were highly effective in inhibiting the magnitude of the allergen-induced EAR and LAR, and both reactions were fully inhibited by the combination of these drugs. It is remarkable that these effects were not associated with an effect on inflammatory cell infiltration in the airways. In conclusion, the results indicate that combination therapy with olodaterol and tiotropium may be highly effective in the treatment of allergen-induced asthmatic reactions and AHR.

Expression of the GTP-binding protein Gαs in human myometrial cells is regulated by ubiquitination and protein degradation: involvement of proteasomal inhibition by trichostatin A

In this study, we show that myometrial transcriptional complexes consisting of Sp1, Sp3, histone deacetylase (HDAC)1/2, RbAp48, and mSin3A are recruited to 4 out of the 6 Sp1-4 sites within the Gαs promoter. Moreover disruption in the binding of these complexes via mithramycin administration results in a substantial decrease in expression of Gαs proteins in myometrial cell cultures. In many instances, these transcriptional regulatory complexes repress expression of genes having a high CG content within their promoter region. This repression can be attenuated by inhibition of HDAC activity by the class I/II HDAC inhibitor trichostatin A (TSA) resulting in increased gene transcription. However, although a substantial increase in Gαs protein levels was observed upon administration of TSA to primary cultures of human myometrial cells, this was not preceded by an increase in messenger RNA (mRNA) and thus an elevation in gene transcription. Importantly the increase in Gαs protein levels occurred via ubiquitination and inhibition of proteasomal activity, indicating that this pathway is also involved in regulating Gαs protein expression during pregnancy and parturition.

Muscarinic receptors on airway mesenchymal cells: novel findings for an ancient target

Since ancient times, anticholinergics have been used as a bronchodilator therapy for obstructive lung diseases. Targets of these drugs are G-protein-coupled muscarinic M(1), M(2) and M(3) receptors in the airways, which have long been recognized to regulate vagally-induced airway smooth muscle contraction and mucus secretion. However, recent studies have revealed that acetylcholine also exerts pro-inflammatory, pro-proliferative and pro-fibrotic actions in the airways, which may involve muscarinic receptor stimulation on mesenchymal, epithelial and inflammatory cells. Moreover, acetylcholine in the airways may not only be derived from vagal nerves, but also from non-neuronal cells, including epithelial and inflammatory cells. Airway smooth muscle cells seem to play a major role in the effects of acetylcholine on airway function. It has become apparent that these cells are multipotent cells that may reversibly adopt (hyper)contractile, proliferative and synthetic phenotypes, which are all under control of muscarinic receptors and differentially involved in bronchoconstriction, airway remodeling and inflammation. Cholinergic contractile tone is increased by airway inflammation associated with asthma and COPD, resulting from exaggerated acetylcholine release as well as increased expression of contraction related proteins in airway smooth muscle. Moreover, muscarinic receptor stimulation promotes proliferation of airway smooth muscle cells as well as fibroblasts, and regulates cytokine, chemokine and extracellular matrix production by these cells, which may contribute to airway smooth muscle growth, airway fibrosis and inflammation. In line, animal models of chronic allergic asthma and COPD have recently demonstrated that tiotropium may potently inhibit airway inflammation and remodeling. These observations indicate that muscarinic receptors have a much larger role in the pathophysiology of obstructive airway diseases than previously thought, which may have important therapeutic implications.

Association between bronchodilating response to short-acting beta-agonist and non-synonymous single-nucleotide polymorphisms of beta-adrenoceptor gene

BACKGROUND

With beta-agonists being the most widely used agents in the treatment of asthma, in vitro studies reported that beta(2)-adrenergic receptor (ADRB2) polymorphisms are associated with agonist-promoted down-regulation.

OBJECTIVE

The present population-based study aimed to evaluate the association between bronchodilating response to inhaled short-acting beta-agonist and two non-synonymous single-nucleotide polymorphisms (SNPs) of ADRB2 (ADRB2-16 and ADRB2-27).

METHODS

Two hundred and nine children with reduction in forced expiratory volume in 1 s of more than 20% on methacholine bronchial challenge underwent bronchodilating response testing 5 min after the inhalation of 200 mug of albuterol. Of these 209, 195 gave peripheral blood for genotyping of ADRB2 polymorphisms.

RESULTS

The bronchodilating response was significantly higher in subjects with the homozygous Arg16 than in those with the homozygous Gly16. It was further demonstrated that haplotype pairs of the homozygous Arg16Gln27 and of the heterozygous Arg16Gln27/Gly16Glu27 showed the highest bronchodilating responses, and the haplotype pairs of the homozygous Gly16Gln27 the lowest response. As a whole, the bronchodilating response was more positively associated with the combined quantity of Arg16 and Glu27 polymorphisms than with that of Arg16 alone.

CONCLUSION

Non-synonymous SNPs of ADRB2 at codons 16 and 27 is significantly associated with bronchodilating response to inhaled short acting beta-agonists.

Modulation of beta2- and beta3-adrenoceptor-mediated relaxation of rat oesophagus smooth muscle by protein kinase C

Although a prominent role for protein kinase C (PKC) in the cross-talk between the phosphoinositide pathway and beta2-adrenoceptor signalling has been indicated, modulation of beta3-adrenoceptor function by PKC has not been studied thus far. In the present study, we have compared the relative capacity of PKC in modulating beta2- and beta3-adrenoceptor-mediated relaxation of methacholine-contracted rat oesophagus smooth muscle. To this purpose the effects of the PKC-inhibitor GF 109203X (2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide) on relaxation induced by fenoterol, formoterol, (-)-noradrenaline, BRL 35135 (4-[2-[(2-hydroxy-2-(chlorophenyl)ethyl)amino]-propyl]-phenoxyacetic-acidmethylester) and IBMX (3-isobutyl-1-methyl-xanthine) were studied, in the absence and presence of the selective beta2-adrenoceptor antagonist ICI 118,551 (erythro-1(7-methylindan-4-yloxy)-3-(isopropylamin)-butan-2-ol). Our results show that inhibition of PKC resulted in differential augmentation of both beta2- and beta3-adrenoceptor-mediated relaxation. In contrast, relaxation induced by IBMX was not influenced at all by GF 109203X. The beta2-adrenoceptor bears phosphorylation sites for several kinases, including PKC. Since the beta3-adrenoceptor lacks these consensus sites, the results may also indicate that PKC-mediated Galphas phosphorylation is involved in the cross-talk between the muscarinic receptor-mediated phosphoinositide pathway and beta2- and, particularly, beta3-adrenoceptor signalling.

Quantification of G protein Gaalphas subunit splice variants in different human tissues and cells using pyrosequencing

The G protein Galphas is derived from four alternatively spliced transcripts, two long variants (Galphas(L)+CAG and Galphas(L)-CAG), which include an extra 45-bp segment, and two short variants (Galphas(S)+CAG and Galphas(S)-CAG). The long and short forms differ in each case by splicing in or out of a serine residue encoded at the 3' end of the variable exon 3. The relative expression of all four variants in human tissues is poorly investigated due to experimental limitations. We therefore established a method for reliable relative mRNA quantification of these splice variants based on the Pyrosequencing technology, and determined Galphas transcript ratios in various human tissues and cells. Galphas(S)/Galphas ratio was highest in blood mononuclear cells (0.84 +/- 0.02, n = 16) and lowest in the brain (0.51 +/- 0.14, n = 3). The different ranges resulted from differences in Galphas(S)+CAG ratios, which ranged from a total Galphas ratio of 0.32 +/- 0.07 (n = 12) in heart tissue to 0.57 +/- 0.03 (n = 16) in blood mononuclear cells (p < 0.0001), whereas the Galphas(S)-CAG ratio was rather constant and ranged from 0.22 +/- 0.04 (n = 7) in retinoblastoma cells to 0.27 +/- 0.04 in lymphocytes (p = 0.19). The Galphas(L)+CAG ratio ranged from 0.02 +/- 0.02 in heart tissue to 0.05 +/- 0.01 in retinoblastoma cells, with a varying proportion of Galphas(L)-CAG, which ranged from 0.14 +/- 0.02 in blood mononuclear cells to 0.41 +/- 0.08 in heart tissue. Stimulation of immortalized B lymphoblasts with isoproterenol resulted in significant changes of splice variant ratios. Our data indicate that changes of long and short ratios of Galphas in different tissues affected Galphas(L)-CAG and Gas(S)+CAG rather than Galphas(L)+CAG and Galphas(S-)CAG. Furthermore, stimulation of cells seemed to affect splice variant ratios. These results are, therefore, suggestive of different biological functions of these variants.

Alternative splicing of the adenylyl cyclase stimulatory G-protein G alpha(s) is regulated by SF2/ASF and heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1) and involves the use of an unusual TG 3'-splice Site

The factors involved in regulating alternative splicing of the human adenylyl cyclase stimulatory G-protein G alpha(s) in different cell types remain undefined. We have designed a G alpha(s) minigene that retains the signals required for G alpha(s) alternative splicing in vivo. Employing transient transfection of human myometrial smooth muscle cells and HeLa cells, as well as in vitro splicing assays, we have provided evidence that the antagonistic splicing factors SF2/ASF and hnRNPA1 act as potent regulators of G alpha(s) isoform expression in these cells. Both SF2/ASF and hnRNPA1 control the selection of competing 5'-splice sites and respectively promote inclusion or skipping of the small cassette-type exon 3 of G alpha(s) transcripts, resulting in the generation of G alpha(s)-long and G alpha(s)-short mRNA isoforms. We have also provided evidence that SF2/ASF and hnRNPA1 play a role in 3'-splice site selection involving the use of a non-canonical TG 3'-splice site preceding exon 4. Using a score-matrix analysis to identify putative exonic enhancer sequences (ESEs), we found multiple high score ESE motifs for SF2/ASF, SC35, and SRp40 in exon 3 of G alpha(s). These results suggest that tissue-specific expression of SF2/ASF and hnRNPA1 governs the expression of alternative isoforms of G alpha(s) in these different cells types.

G(s) protein dysfunction in allergen-challenged human isolated passively sensitized bronchi

We studied the intracellular mechanisms of allergen-induced beta(2)-adrenoceptor dysfunction in human isolated passively sensitized bronchi. Sensitization was obtained by overnight incubation of bronchial rings with serum containing a high specific IgE level to Dermatophagoides but a low total IgE level. Allergen challenge was done by incubation with a Dermatophagoides mix. The G(s) protein stimulant cholera toxin (2 microg/ml) displaced the carbachol (CCh) concentration-response curves of control and sensitized but not of challenged rings to the right. Cholera toxin (10 microg/ml) displaced the concentration-response curves to CCh of control, sensitized, and challenged rings to the right, but this effect was less in challenged rings. The effects of the G(i) protein inhibitor pertussis toxin (250 ng/ml or 1 microg/ml) on salbutamol concentration-relaxation curves did not differ significantly between challenged and sensitized rings. The adenylyl cyclase activator forskolin and the Ca(2+)-activated K(+)-channel opener NS-1619 relaxed CCh-contracted bronchial rings without significant differences between control, sensitized, and challenged rings. Neither G(i) nor G(s) alpha-subunit expression differed between control, sensitized, and challenged tissues. We conclude that G(s) protein dysfunction may be a mechanism of allergen-induced beta(2)-adrenoceptor dysfunction in human isolated passively sensitized bronchi.

Prostanoids mediate IL-1beta-induced beta-adrenergic hyporesponsiveness in human airway smooth muscle cells

We have previously reported that pretreatment of cultured human airway smooth muscle (HASM) cells with interleukin-1beta (IL-1beta) results in decreased beta-adrenergic responsiveness. The purpose of this study was to determine whether prostanoids released as a result of cyclooxygenase-2 (COX-2) induction by IL-1beta contribute to this effect of the cytokine. Confluent serum-deprived HASM cells were studied in passages 4-7. IL-1beta (20 ng/ml for 22 h) reduced the ability of the beta-agonist isoproterenol (Iso) to decrease stiffness of HASM cells as measured by magnetic twisting cytometry. The effect of IL-1beta on Iso-induced changes in cell stiffness was abolished by nonselective [indomethacin (Indo), 10(-6) M] and selective (NS-398, 10(-5) M) COX-2 inhibitors. Indo and NS-398 also inhibited both the increased basal cAMP and the decreases in Iso-stimulated cAMP production induced by IL-1beta. IL-1beta (20 ng/ml for 22 h) caused an increase in both basal (15-fold) and arachidonic acid (AA)-stimulated (10-fold) PGE2 release. Indo blocked basal and AA-stimulated PGE2 release in both control and IL-1beta-treated cells. NS-398 also markedly reduced basal and AA-stimulated PGE2 release in IL-1beta-treated cells but had no significant effect on AA-stimulated PGE2 release in control cells. Western blot analysis confirmed the induction of COX-2 by IL-1beta. Exogenously administered PGE2 (10(-7) M, 22 h) caused a significant reduction in the ability of Iso to decrease cell stiffness, mimicking the effects of IL-1beta. Cycloheximide (10 microg/ml for 24 h), an inhibitor of protein synthesis, also abolished the effects of IL-1beta on Iso-induced cell stiffness changes and cAMP formation. In summary, our results indicate that IL-1beta significantly increases prostanoid release by HASM cells as a result of increased COX-2 expression. The prostanoids appear to contribute to beta-adrenergic hyporesponsiveness, perhaps by heterologous desensitization of the beta2 receptor.

Developmental enhancement of secretory response to isoproterenol coupled with increases in beta-adrenoceptor density and Gs protein function in rat parotid tissues

The beta-adrenergic agonist, isoproterenol (IPR), stimulated more significantly and sensitively amylase secretion from both the tissues of 7- and 56-day-old rats than a cholinergic agonist, carbachol, at the same concentration. The EC50 value of amylase secretion with IPR decreased significantly during development but that with carbachol did not change. Estimation by measuring bindings of [3H]dihydroalprenolol and [3H]quinuclidynylbenzylate indicated the marked increases in the numbers of both beta-adrenoceptors and muscarinic receptors in the tissues during development. The affinity of beta-adrenoceptors for the agonist was also enhanced during development, but that of muscarinic receptors for the agonist was not. These developmental changes in the number and affinity of beta-adrenoceptors and muscarinic receptors paralleled those in amylase secretory response of the tissues to their agonists. The response of adenylate cyclase (AC) of the tissues to 1 microM IPR was steadily enhanced after birth. In contrast, the response of AC to 1 microM forskolin was high until 14 days old, but markedly decreased at 28 days old and thereafter maintained this level. The increase in cholera toxin-catalyzed ADP-ribosylation (AR) of stimulatory GTP binding proteins (Gs proteins) in the tissues was apparent at 14 days old, reaching a maximum at 56 days old and thereafter decreasing with age. On the other hand, pertussis toxin-catalyzed AR of inhibitory GTP binding proteins (Gi proteins) did not change after birth. Thus, the ratio of apparent levels of Gs to Gi proteins increased significantly after birth, reaching a maximum at 56 days old, but decreased rapidly till 84 days old and thereafter maintained this level. These changes in the ratio paralleled those in the response of AC to IPR. These results showed that the rapid and marked increases in the number and affinity of beta-adrenoceptors and the ratio of apparent levels of Gs to Gi proteins in rat parotid tissues during development had a key role in the enhancement of the secretory response of the tissues to beta-agonists.

Altered cyclic AMP-dependent human chorionic gonadotropin production in cultured human placental trophoblasts exposed to ethanol

Chronic ethanol abuse during pregnancy can cause fetal injury, including the fetal alcohol syndrome (FAS). A contributing factor in this fetal injury may be the effect of ethanol on placental function. Previous studies have shown that ethanol treatment increases human chorionic gonadotropin (hCG) production by cultured human placental trophoblasts. In this study, we demonstrated that the stimulation of hCG production correlates with the ethanol concentration. Ethanol treatment enhanced intracellular adenosine 3':5'-cyclic monophosphate (cAMP) levels in response to either cholera toxin (CTX) or forskolin (FSK). Moreover, basal (i.e. unstimulated) cAMP levels were increased at 2 hr of ethanol exposure. However, this effect did not persist throughout the 24-hr incubation period. Therefore, ethanol treatment appears to induce increased hCG production, secondary to enhanced basal or stimulated cAMP production. The effect of ethanol was not associated with changes in Gs or Gi2 expression, as determined by northern blot and western blot analyses. In plasma membrane preparations from ethanol-treated cells, cAMP production was higher in response to Mn2+, a direct stimulator of adenylyl cyclase. Inclusion of Rp-cAMP, a protein kinase A inhibitor, eliminated the ethanol effect on hCG production. Treatment of cells with 8-Br-cAMP stimulated hCG production, but there was no difference between the ethanol-naive control and the ethanol-treated cells. These data suggest that ethanol treatment increases in vitro hCG production in human placental trophoblasts by enhancing cAMP production. Ethanol treatment appears to increase trophoblast adenylyl cyclase activity.

Down-regulation of protein kinase C by parathyroid hormone and mezerein differentially modulates cAMP production and phosphate transport in opossum kidney cells

We examined the effects of prolonged exposure to parathyroid hormone (PTH) and the protein kinase C (PKC) activator mezerein (MEZ) on cyclic adenosine monophosphate (cAMP) production, PKC activity, and Na(+)-dependent phosphate (Na/Pi) transport in an opossum kidney cell line (OK/E). A 5 minute exposure to PTH stimulated, while a 6 h incubation reduced, cAMP production, Na/Pi transport was maximally inhibited under desensitizing conditions and was not affected by reintroduction of the hormone. MEZ pretreatment (6 h) enhanced PTH-, cholera toxin (CTX)-, and forskolin (FSK)-stimulated cAMP production, suggesting enhanced Gs alpha coupling and increased adenylyl cyclase activity. However, PKA- and PKC-dependent regulation of Na/Pi were blocked in MEZ-treated cells. The PTH-induced decrease in cAMP production was associated with a reduction in membrane-associated PKC activity while MEZ-induced increases in cAMP production were accompanied by decreases in membrane and cytosolic PKC activity. Enhanced cAMP production was not accompanied by significant changes in PTH/PTH related peptide (PTHrP) receptor affinity or number, nor was the loss of Na/Pi transport regulation associated with changes in PKA activity. The results indicate that down-regulation of PKC by PTH or MEZ differentially modulates cAMP production and regulation of Na/Pi transport. The distinct effects of PTH and MEZ on PKC activity suggest that agonist-specific activation and/or down-regulation of PKC isozyme(s) may be involved in the observed changes in cAMP production and Na/Pi transport.

Signalling functions and biochemical properties of pertussis toxin-resistant G-proteins

Pertussis toxin (PTX) has been widely used as a reagent to characterize the involvement of heterotrimeric G-proteins in signalling. This toxin catalyses the ADP-ribosylation of specific G-protein alpha subunits of the Gi family, and this modification prevents the occurrence of the receptor-G-protein interaction. This review focuses on the biochemical properties and signalling of those G-proteins historically classified as 'PTX-resistant' due to the inability of the toxin to influence signalling through them. These G-proteins include members of the Gq and G12 families and one Gi family member, i.e. Gz. Signalling pathways controlled by these G-proteins are well characterized only for Gq family members, which activate specific isoforms of phospholipase C, resulting in increases in intracellular calcium and activation of protein kinase C (PKC), among other responses. While members of the G12 family have been implicated in processes that regulate cell growth, and Gz has been shown to inhibit adenylate cyclase, the specific downstream targets to these G-proteins in vivo have not been clearly established. Since two of these proteins, G12 alpha and Gz alpha, are excellent substrates for PKC, there is the potential for cross-talk between their signalling and Gq-dependent processes leading to activation of PKC. In tissues that express these G-proteins, a number of guanine-nucleotide-dependent, PTX-resistant, signalling pathways have been defined for which the G-protein involved has not been identified. This review summarizes these pathways and discusses the evidence both for the participation of specific PTX-resistant G-proteins in them and for the regulation of these processes by PKC.

Activation of protein kinase C by lysophosphatidic acid: dependence on composition of phospholipid vesicles

Lysophosphatidic acid (LPA) has attracted recent attention as a major serum-derived regulator implicated in responses to vascular injury and inflammation, in tumour invasiveness and in neuronal signalling and remodelling. Although the possibility of a specific G-protein-coupled LPA receptor protein has been suggested, characterization of such a receptor is lacking. Since LPA can activate protein kinase C (PKC) pathways in many cells and PKC activators mimic many LPA effects, the possibility of more direct LPA effects on PKC was investigated. Phosphatidylcholine (PC)/phosphatidylserine (PS)/diacylglycerol (DAG) lipid vesicles of defined acyl chain composition were used to activate the enzyme. At total concentrations of saturated PC/PS + DAG vesicles (2-3 mM) that provided maximal PKC activation, 1-10 mol % [18:1]-LPA led to a further approx. 2-fold activation of PKC alpha. At lower lipid concentrations, a greater increase was observed with LPA concentrations up to 16-20 mol %. Higher concentrations of LPA were inhibitory. The LPA activation of PKC was dependent on the presence of DAG, PS and Ca2+. [18:1]-Lysophosphatidylcholine produced similar PKC activation in PC/PS/DAG vesicles. [14:0]-LPA was less effective, and longer-chain saturated lysolipids were ineffective. In unsaturated PC/PS vesicles, very little to no effect of LPA was discernable. These results suggest that physiologically or pathologically relevant concentrations of LPA can contribute to PKC activation depending on the composition of the lipid membrane. We hypothesize that LPA may affect the formation of lipid domains that are recognized by the enzyme.

Phosphorylation of elongation factor 1 (EF-1) by protein kinase C stimulates GDP/GTP-exchange activity

Phosphorylation of the alpha, beta and delta subunits of elongation factor (EF) 1 by protein kinase C results in stimulation of elongation activity up to threefold both in vivo and in vitro [Venema, R. C., Peters, H. I. & Traugh, J. A. (1991) J. Biol. Chem. 266, 11,993-11,998, Venema, R. C., Peters, H. I. & Traugh, J. A. (1991) J. Biol. Chem. 266, 12,574-12,580]. The alpha subunit catalyzes the GTP-dependent binding of amino-acyl-tRNA to the ribosome, while the beta gamma and delta subunits of EF-1 catalyze exchange of the residual GDP on EF-1 alpha for GTP. To determine whether the change in elongation rate following phosphorylation by protein kinase C is due to stimulation of GDP/GTP exchange activity, EF-1 and EF-1.valyl-tRNA-synthetase have been purified from rabbit reticulocytes, phosphorylated in vitro by protein kinase C and the effect of phosphorylation on nucleotide-exchange activity analyzed. The alpha, beta and delta subunits are phosphorylated only on serine, and phosphopeptide maps show distinct phosphopeptides for each subunit. Following quantitative phosphorylation of EF-1 by protein kinase C on the alpha, beta, and delta subunits, a twofold enhancement of the rate of nucleotide exchange over the non-phosphorylated controls is observed with EF-1 and EF-1.valyl-tRNA synthetase. Stimulation of nucleotide exchange results in a two-fold increase in the formation of EF-1 alpha.GTP.Phe-tRNA, leading to an increased rate of binding of Phe-tRNA to ribosomes. The magnitude of stimulation of the exchange rate is similar to that reported previously for the rate of elongation following phosphorylation of EF-1 by protein kinase C. Thus, the enhancement of EF-1 activity in response to 4 beta-phorbol 12-myristate 13-acetate appears to be due to stimulation of the rate of GDP/GTP exchange following phosphorylation of EF-1 by protein kinase C.

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.

Modulation of airway smooth muscle beta-adrenoceptor function by a muscarinic agonist

We have investigated the effect of a muscarinic agonist and protein kinase C (PKC) activation on beta-adrenoceptors and their coupling to adenylyl cyclase in bovine tracheal smooth muscle. There was a significant reduction in maximum binding capacity of [125I]iodocyanopindolol ([125I]ICYP) after exposure to carbachol and 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA). Similarly both carbachol and PMA inhibited the 5'-guanylylimidodiphosphate-induced shift in [125I]ICYP binding by isoproterenol and significantly decreased isoproterenol-induced cyclic AMP accumulation. A phorbol ester, 4 alpha-phorbol 12,13-didecanoate which does not activate PKC had no effect on beta-receptor binding or coupling. These results suggest that PKC activation directly via a phorbol ester and indirectly via muscarinic receptor stimulation may lead to reduction and uncoupling of beta-receptors in airway smooth muscle. We suggest that this mechanism may be relevant to the reduction in beta-receptor coupling in asthmatic airways as an effect of PKC activation by inflammatory mediators and neurotransmitters.

Early events of human T lymphocyte activation are associated with type I protein kinase A activity

Human T lymphocytes possess both the type I and II isozymes of protein kinase A (PKA). The type I (PKA-I) isozyme is predominantly associated with the plasma membrane, whereas the type II (PKA-II) isozyme is primarily localized to the cytosol. Because the functions of both PKA-I and PKA-II isozymes in the biochemical events of T lymphocyte activation have not been clearly elucidated, we tested the hypothesis that very early events of normal human T lymphocyte activation are mediated by the PKA-I and/or PKA-II isozyme(s). Fresh normal human T cells and a normal human CD4+ T cell line (GK606) activated with anti-CD3-epsilon and recombinant interleukin 1 alpha (rIL-1 alpha) exhibited a peak six- to sevenfold increase of PKA phosphotransferase activity at 5 min that returned to baseline by 60 min. Similarly, both fresh T cells and the T cell line activated by phorbol myristate acetate and ionomycin demonstrated a peak eightfold increase of PKA activity by 15 min that returned toward baseline by 60 min. Chromatographic separation of the PKA isozymes and quantification of phosphotransferase activities after T cell activation by either agonist pair showed preferential activation of the PKA-I isozyme, resulting in a significant reduction in the ratio of PKA-I to PKA-II isozyme activity from 3.1:1-6.2:1 to 1.1:1-3.2:1. PKA-I isozyme activation resulted in the release of free catalytic (C) subunit, an increase in C subunit phosphotransferase activity, and the phosphorylation of T cell plasma membrane-associated proteins, p14, p17, p20, p21, p38, and p48. However, activation of the PKA-I isozyme did not appear to be required for the transcription of IL-2 mRNA, an event necessary for mitosis. These data indicate that ligand-induced T cell activation is associated with rapid activation of the PKA-I, but not PKA-II, isozyme that results in phosphorylation of plasma membrane-associated proteins. The involvement of the PKA-I isozyme during the very early events of T cell activation suggests that this isozyme may be an antigen- or mitogen-stimulated protein kinase.

Desensitization by protein kinase C activation differentially uncouples formyl peptide receptors from effector enzymes in HL-60 granulocytes

The hypothesis that protein kinase C (PKC) participates in agonist-mediated desensitization of formyl peptide receptors in HL-60 granulocytes was tested. fMet-Leu-Phe and leukotriene B4(LTB4) produced homologous desensitization of agonist-stimulated intracellular calcium transients. Pre-treatment with the PKC activator, phorbol myristate acetate (PMA; 10 nM), abolished both fMet-Leu-Phe and LTB4-stimulated calcium transients. Membranes prepared from control HL-60 granulocytes (NM) or cells treated with 10 nM PMA (PMA-M) demonstrated increased formyl peptide receptor and G protein density, as determined by radioligand binding and pertussis toxin- and cholera toxin-catalysed ADP ribosylation. fMet-Leu-Phe stimulation of guanosine 5'-[gamma-thio]-triphosphate (GTP gamma S) binding and GTP hydrolysis and GDP inhibition of fMet-Leu-Phe binding were not different between NM and PMA-M. Pre-treatment with 10 nM PMA did not inhibit subsequent fMet-Leu-Phe-stimulated superoxide generation or phospholipase D activation. We conclude that PKC desensitizes fMet-Leu-Phe-stimulated phospholipase C, but not phospholipase D, responses and that PKC activation does not mediate agonist-induced desensitization of formyl peptide receptors.

Modulation of Ca2+ channels by protein kinase C in rat central and peripheral neurons: disruption of G protein-mediated inhibition

Activation of protein kinase C (PKC) reduced G protein-dependent inhibition of Ca2+ channels by glutamate, GA-BAB, adenosine, muscarinic, alpha-adrenergic, and LHRH receptors in a variety of central and peripheral neurons. PKC stimulation also relieved the inhibitory effect of internal GTP gamma S and reduced tonic G protein-mediated inhibition observed with internal GTP in the absence of transmitter receptor agonist. Basal Ca2+ channel currents were enhanced by PKC stimulation in most neurons studied. The PKC-induced enhancement of basal current was voltage dependent, and enhanced currents displayed altered kinetics. Inhibition of G proteins with GDP beta S attenuated the PKC-induced enhancement of basal Ca2+ channel current. These results show that PKC regulates the inhibitory effects of G proteins, possibly by disrupting the coupling of G proteins to Ca2+ channels. The PKC-induced enhancement of Ca2+ channel current results, at least in part, from the removal of tonic G protein-mediated inhibition.

The sites of phosphorylation by protein kinase C and an intact SH2 domain are required for the enhanced response to beta-adrenergic agonists in cells overexpressing c-src

Previously we demonstrated that C3H10T1/2 murine fibroblasts overexpressing avian c-src exhibit elevated levels of cyclic AMP (cAMP) in response to beta-adrenergic agonists compared with that in control cells and that this enhanced response requires c-src kinase activity (W. A. Bushman, L. K. Wilson, D. K. Luttrell, J. S. Moyers, and S. J. Parsons, Proc. Natl. Acad. Sci. USA 87:7462-7466, 1990). However, it is not yet known which components of the beta-adrenergic receptor pathway, if any, interact with pp60c-src. It has recently been shown that immune complexes of pp60c-src phosphorylate recombinant G alpha proteins in vitro to stoichiometric levels, resulting in alterations of GTP binding and GTPase activity (W. P. Hausdorff, J. A. Pitcher, D. K. Luttrell, M. E. Linder, H. Kurose, S. J. Parsons, M. G. Caron, and R. J. Lefkowitz, Proc. Natl. Acad. Sci. USA 89:5720-5724, 1992), raising the possibility that the Gs alpha protein may be an in vivo target for the interaction with pp60c-src. To further characterize the involvement of pp60c-src in the beta-adrenergic signalling pathway, we have overexpressed, in 10T1/2 cells, pp60c-src containing mutations in several domains which are believed to be important for signalling processes. In this study we show that the sites of phosphorylation by protein kinase C (PKC) (Ser-12 and Ser-48) as well as the SH2 region of pp60c-src are required for the enhanced response of c-src overexpressors to beta-agonist stimulation. Mutation at the site of myristylation (Gly-2) results in a decrease in the enhanced response, while mutation at the site of phosphorylation by cAMP-dependent protein kinase (Ser-17) has no effect. Two-dimensional phosphotryptic analyses indicate that phosphorylation on Ser-12 and Ser-48 in unstimulated cells is associated with the ability of overexpressed pp60c-src to potentiate beta-adrenergic signalling. Cells overexpressing wild-type c-src also exhibit enhanced cAMP accumulation upon treatment with cholera toxin, an effect that is abated in cells overexpressing pp60c-src defective in the kinase or SH2 domains or altered at the sites of phosphorylation by PKC. These studies provide the first evidence for the physiological significance of the pp60c-src sites of PKC phosphorylation. In addition, they show that the SH2, Ser-12/48, and myristylation regions may be important for efficient interaction of pp60c-src with components of the beta-adrenergic pathway. Our data also support the possibility that the Gs alpha protein may be an in vivo target for alteration by pp60c-src.

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.

The sites of phosphorylation by protein kinase C and an intact SH2 domain are required for the enhanced response to beta-adrenergic agonists in cells overexpressing c-src

Previously we demonstrated that C3H10T1/2 murine fibroblasts overexpressing avian c-src exhibit elevated levels of cyclic AMP (cAMP) in response to beta-adrenergic agonists compared with that in control cells and that this enhanced response requires c-src kinase activity (W. A. Bushman, L. K. Wilson, D. K. Luttrell, J. S. Moyers, and S. J. Parsons, Proc. Natl. Acad. Sci. USA 87:7462-7466, 1990). However, it is not yet known which components of the beta-adrenergic receptor pathway, if any, interact with pp60c-src. It has recently been shown that immune complexes of pp60c-src phosphorylate recombinant G alpha proteins in vitro to stoichiometric levels, resulting in alterations of GTP binding and GTPase activity (W. P. Hausdorff, J. A. Pitcher, D. K. Luttrell, M. E. Linder, H. Kurose, S. J. Parsons, M. G. Caron, and R. J. Lefkowitz, Proc. Natl. Acad. Sci. USA 89:5720-5724, 1992), raising the possibility that the Gs alpha protein may be an in vivo target for the interaction with pp60c-src. To further characterize the involvement of pp60c-src in the beta-adrenergic signalling pathway, we have overexpressed, in 10T1/2 cells, pp60c-src containing mutations in several domains which are believed to be important for signalling processes. In this study we show that the sites of phosphorylation by protein kinase C (PKC) (Ser-12 and Ser-48) as well as the SH2 region of pp60c-src are required for the enhanced response of c-src overexpressors to beta-agonist stimulation. Mutation at the site of myristylation (Gly-2) results in a decrease in the enhanced response, while mutation at the site of phosphorylation by cAMP-dependent protein kinase (Ser-17) has no effect. Two-dimensional phosphotryptic analyses indicate that phosphorylation on Ser-12 and Ser-48 in unstimulated cells is associated with the ability of overexpressed pp60c-src to potentiate beta-adrenergic signalling. Cells overexpressing wild-type c-src also exhibit enhanced cAMP accumulation upon treatment with cholera toxin, an effect that is abated in cells overexpressing pp60c-src defective in the kinase or SH2 domains or altered at the sites of phosphorylation by PKC. These studies provide the first evidence for the physiological significance of the pp60c-src sites of PKC phosphorylation. In addition, they show that the SH2, Ser-12/48, and myristylation regions may be important for efficient interaction of pp60c-src with components of the beta-adrenergic pathway. Our data also support the possibility that the Gs alpha protein may be an in vivo target for alteration by pp60c-src.

Muscarinic blockade of beta-adrenoceptor-stimulated adenylyl cyclase: the role of stimulatory and inhibitory guanine-nucleotide binding regulatory proteins (Gs and Gi)

1. The functional antagonism that exists between muscarinic and beta-adrenoceptor function in guinea-pig tracheal smooth muscle was investigated by assessing Gs and Gi regulated adenylyl cyclase activity in isolated membranes. 2. Membranes from guinea-pig tracheal smooth muscle contain both Gi alpha and Gs alpha as assessed by Western blots with anti-G-protein antibodies. 3. GppNHp, a non-hydrolysable analogue of guanosine 5'-triphosphate (GTP), was shown to stimulate adenylyl cyclase activity at high concentrations (10(-6)-10(-4) M). GppNHp also produced a concentration-dependent reduction in pertussis toxin-catalysed adenosine diphosphate (ADP)-ribosylation of Gi alpha. 4. Pretreatment of tracheal smooth muscle slices with methacholine (10(-6) M) provoked a blockade of isoprenaline plus GTP, GppNHp- and GTP-stimulated adenylyl cyclase. 5. Addition of methacholine to membranes did not trigger inhibition of GTP-stimulated adenylyl cyclase activity but did block the isoprenaline-mediated augmentation of GTP-stimulated adenylyl cyclase activity. 6. Pretreatment of tracheal smooth muscle with methacholine (10(-6) M) provoked a blockade of cholera toxin-catalysed NAD(+)-dependent ADP-ribosylation of Gs alpha. 7. Phorbol-12-myristate 13-acetate (PMA)-treatment of tracheal smooth muscle slices actually enhanced GppNHp-stimulated adenylyl cyclase activity in subsequently prepared membranes. 8. We suggest that methacholine in addition to inhibiting adenylyl cyclase via a Gi-dependent mechanism induces a functional inactivation of Gs activity. These results together may explain the functional antagonism that exists between increased muscarinic tone and the ability of beta-adrenoceptor agonists to provoke excitation-contraction uncoupling.