Influence of receptor number on the stimulation by salmeterol of gene transcription in CHO-K1 cells transfected with the human beta2-adrenoceptor

LASSBio-897 Reduces Lung Injury Induced by Silica Particles in Mice: Potential Interaction with the A2A Receptor

Silicosis is a lethal fibro-granulomatous pulmonary disease highly prevalent in developing countries, for which no proper therapy is available. Among a small series of N-acylhydrazones, the safrole-derived compound LASSBio-897 (3-thienylidene-3, 4-methylenedioxybenzoylhydrazide) raised interest due to its ability to bind to the adenosine A_2A receptor. Here, we evaluated the anti-inflammatory and anti-fibrotic potential of LASSBio-897, exploring translation to a mouse model of silicosis and the A_2A receptor as a site of action. Pulmonary mechanics, inflammatory, and fibrotic changes were assessed 28 days after intranasal instillation of silica particles in Swiss–Webster mice. Glosensor cAMP HEK293G cells, CHO cells stably expressing human adenosine receptors and ligand binding assay were used to evaluate the pharmacological properties of LASSBio-897 in vitro. Molecular docking studies of LASSBio-897 were performed using the genetic algorithm software GOLD 5.2. We found that the interventional treatment with the A_2A receptor agonist CGS 21680 reversed silica particle-induced airway hyper-reactivity as revealed by increased responses of airway resistance and lung elastance following aerosolized methacholine. LASSBio-897 (2 and 5 mg/kg, oral) similarly reversed pivotal lung pathological features of silicosis in this model, reducing levels of airway resistance and lung elastance, granuloma formation and collagen deposition. In competition assays, LASSBio-897 decreased the binding of the selective A_2A receptor agonist [³H]-CGS21680 (IC₅₀ = 9.3 μM). LASSBio-897 (50 μM) induced modest cAMP production in HEK293G cells, but it clearly synergized the cAMP production by adenosine in a mechanism sensitive to the A_2A antagonist SCH 58261. This synergism was also seen in CHO cells expressing the A_2A, but not those expressing A_2B, A₁ or A₃ receptors. Based on the evidence that LASSBio-897 binds to A_2A receptor, molecular docking studies were performed using the A_2A receptor crystal structure and revealed possible binding modes of LASSBio-897 at the orthosteric and allosteric sites. These findings highlight LASSBio-897 as a lead compound in drug development for silicosis, emphasizing the role of the A_2A receptor as its putative site of action.

Enhancing nuclear translocation: perspectives in inhaled corticosteroid therapy

Corticosteroids are widely used in the treatment of asthma and chronic obstructive pulmonary disease (COPD). In contrast to their use in mild-to-moderate asthma, they are less efficacious in improving lung function and controlling the underlying inflammation in COPD. In most clinical trials, corticosteroids have shown little benefit in COPD, but have shown a greater clinical effect in combination with long-acting bronchodilators. Impaired corticosteroid activation of the glucocorticoid receptor (GR) has been reported in corticosteroid-insensitive individuals. Reversal of corticosteroid-insensitivity by enhancing GR nuclear translocation is a potential therapeutic target. Preclinical studies suggest members of the nuclear receptor superfamily may facilitate glucocorticoid receptor nuclear translocation. Unravelling the mechanisms that govern GR nuclear translocation may identify novel therapeutic targets for reversing corticosteroid-insensitivity.

Fluticasone propionate/formoterol: a fixed-combination therapy with flexible dosage

International guidelines describe asthma control as the main outcome of asthma management. Prevention of symptoms, improved quality of life, and reduction of exacerbations are the main components, consequently decreasing health care costs. However, many of these objectives remain unmet in real life: several surveys show that a large proportion of asthmatic patients are not well controlled despite the efficacy of current available treatment. Several randomized controlled clinical trials indicate that combining inhaled corticosteroids and long-acting β2-agonists, by means of a single inhaler, greatly improves the management of the disease. The results of 9 multicenter phase III clinical studies demonstrate that the fixed combination of fluticasone propionate/formoterol in a single inhaler is effective in terms of lung function and symptom control. These studies highlight the dose flexibility, safety and tolerability of this new inhaled combination. These characteristics meet the recommendations of international guidelines, and the preferences of respiratory physicians who identified these aspects as critical components of a successful asthma therapy. Combination of fluticasone propionate/formoterol in a single inhaler provides potent anti-inflammatory activity of fluticasone propionate and rapid onset of action of the β2-agonist formoterol making this association a viable treatment option both in terms of effectiveness and compliance.

Agonist-induced β2-adrenoceptor desensitization and downregulation enhance pro-inflammatory cytokine release in human bronchial epithelial cells

It is not clear whether increased asthma severity associated with long-term use of β2-adrenoceptor (β2-AR) agonists can be attributed to receptor degradation and increased inflammation. We investigated the cross-talk between β-AR agonist-mediated effects on β2-AR function and expression and cytokine release in human bronchial epithelial cells. In 16HBE14o(-) cells grown in the presence and absence of β-AR agonists and/or antagonists, the β2-AR density was assessed by radioligand binding; the receptor protein and mRNA was determined using laser scanning cytometer and RT-PCR; cAMP generation, the cytokines IL-6 and IL-8 release were determined using AlphaScreen Assay and ELISA, respectively. Isoprenaline (ISO) and salbutamol (Salbu) induced a concentration- and time-dependent significant decrease in β2-AR density. Both Salbu and ISO reduced cAMP generation in a concentration-dependent manner while in same cell culture the IL-6 and IL-8 release was significantly enhanced. These effects were antagonized to a greater extent by ICI 118.551 than by propranolol, but CGP 20712A had no effect. Reduction of the β2-AR protein and mRNA could be seen when cells were treated with ISO for 24 h. Our findings indicate a direct link between cytokine release and altered β2-AR expression and function in airway epithelial cells. β2-AR desensitization and downregulation induced by long-term treatment with β2-AR agonists during asthma may account for adverse reactions also due to enhanced release of pro-inflammatory mediators and should, thus, be considered in asthma therapy.

An Aplysia Egr homolog is rapidly and persistently regulated by long-term sensitization training

The Egr family of transcription factors plays a key role in long-term plasticity and memory in a number of vertebrate species. Here we identify and characterize ApEgr (GenBank: KC608221), an Egr homolog in the marine mollusk Aplysia californica. ApEgr codes for a predicted 593-amino acid protein with the highly conserved trio of zinc-fingered domains in the C-terminus that characterizes the Egr family of transcription factors. Promoter analysis shows that the ApEgr protein selectively recognizes the GSG motif recognized by vertebrate Egrs. Like mammalian Egrs, ApEgr is constitutively expressed in a range of tissues, including the CNS. Moreover, expression of ApEgr is bi-directionally regulated by changes in neural activity. Of most interest, the association between ApEgr function and memory may be conserved in Aplysia, as we observe rapid and long-lasting up-regulation of expression after long-term sensitization training. Taken together, our results suggest that Egrs may have memory functions that are conserved from mammals to mollusks.

Insights into GPCR pharmacology from the measurement of changes in intracellular cyclic AMP; advantages and pitfalls of differing methodologies

It is clear that the G protein-coupled receptor family play a key role in the pharmaceutical industry, with a significant proportion of approved drugs targeting this protein class. While our growing understanding of the complexity of G protein-coupled receptor pharmacology is playing a key role in the future success of these endeavours, with allosteric mechanisms now well integrated into the industrial community and G protein-independent signalling mechanisms establishing themselves as novel phenomenon to be exploited, it is still possible to underestimate the complexity of G protein signal transduction mechanisms and the impact that inappropriate study of these mechanisms can have on data interpretation. In this manuscript we review different approaches to measuring the cAMP signal transduction pathway, with particular emphasis on key parameters influencing the data quality and biological relevance.

The effects of antidepressants on mitochondrial function in a model cell system and isolated mitochondria

The in vitro effects of antidepressant drugs on mitochondrial function were investigated in a CHOβ(2)SPAP cell line used previously to determine the effects of antidepressants on gene transcription (Abdel-Razaq et al., Biochem Pharmacol 73:1995-2003, 2007) and in rat heart isolated mitochondria. Apoptotic effects of clomipramine (CLOM), desipramine (DMI) and of norfluoxetine (NORF, the active metabolite of fluoxetine), on cellular viability were indicated by morphological changes and concentration-dependent increases in caspase-3 activity in CHO cells after 18 h exposure to CLOM, DMI and NORF. However, tianeptine (TIAN) was without effect. CLOM and NORF both reduced integrated mitochondrial function as shown by marked reductions in membrane potential (MMP) in mitochondria isolated from rat hearts. DMI also showed a similar but smaller effect, whereas, TIAN did not elicit any significant change in MMP. Moreover, micromolar concentrations of CLOM, DMI and NORF caused significant inhibitions of the activities of mitochondrial complexes (I, II/III and IV). The inhibitory effects on complex IV activity were most marked. TIAN inhibited only complex I activity at concentrations in excess of 20 μM. The observed inhibitory effects of antidepressants on the mitochondrial complexes were accompanied by a significant decrease in the mitochondrial state-3 respiration at concentrations above 10 μM. The results demonstrate that the apoptotic cell death observed in antidepressant-treated cells could be due to disruption of mitochondrial function resulting from multiple inhibition of mitochondrial enzyme complexes. The possibility that antimitochondrial actions of antidepressants could provide a potentially protective pre-conditioning effect is discussed.

Agonist efficacy and receptor desensitization: from partial truths to a fuller picture

It has been demonstrated that the degree of agonist-induced desensitization of the beta(2)-adrenoceptor is related to agonist efficacy (strength of signalling), whereby high-efficacy agonists (e.g. formoterol) cause more phosphorylation and internalization of the receptor than low-efficacy agonists (e.g. salmeterol). These early studies, however, used a protocol where agonists were matched for receptor occupancy rather than functional effect. In this issue of the BJP, Duringer and colleagues have extended these studies to compare the ability of agonists to cause desensitization at equi-effective (cAMP signalling) concentrations rather than equal occupancy. Their data and conclusions are quite different from those previously described. After prolonged exposure, all the agonists caused a similar degree of desensitization, whereas a pulse protocol uncovered a greater loss of responsiveness with the low-efficacy ligands. This is consistent with the notion that high-efficacy agonists have 'spare receptors', and are therefore less sensitive to loss of receptors through desensitization. It also reflects experience in the clinic, where both formoterol and salmeterol show a similar early decline in bronchoprotection, after which their effects remain stable. These findings challenge the notion that high-efficacy ligands always cause more functional desensitization.

Therapeutic management of allergic diseases

Allergic diseases are characterized by the activation of inflammatory cells and by a massive release of mediators. The aim of this chapter was to describe succinctly the modes of action, indications, and side effects of the major antiallergic and antiasthmatic drugs. When considering the ideal pharmacokinetic characteristics of a drug, a poorly metabolized drug may confer a lower variability in plasma concentrations and metabolism-based drug interactions, although poorly metabolized drugs may be prone to transporter-based disposition and interactions. The ideal pharmacological properties of a drug include high binding affinity, high selectivity, and appropriate association and dissociation rates. Finally, from a patient perspective, the frequency and route of administration are important considerations for ease of use.

The effect of assay formats on the estimation of melanocortin agonist affinity and efficacy using the operation model of agonism

Melanocortin MC(3) and MC(4) receptor agonists have pharmaceutical benefit in the regulation of energy homeostasis. These agonists are defined by two parameters, their potency and their efficacy. However, these parameters are dependent upon the system in which they are measured. Herein, we have used the operational model of agonism to define agonist properties. We have used two different assay formats, cAMP accumulation and a cAMP response element (CRE)-beta-lactamase gene reporter to measure melanocortin MC(3) and MC(4) receptor agonist profiles, in the presence and absence of the irreversible receptor inactivator N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) and fitted these data to the operational model of agonism to define agonist affinity and efficacy. Data generated using the cAMP accumulation assay fitted well to assumptions made in the operational model and provided estimations of affinity and efficacy in line with those expected. However, data generated in the gene reporter assays showed over a 100-fold increase in agonist affinity compared with cAMP data and unexpectedly low values for efficacy. These data show that the operational model can be used to determine the efficacies of melanocortin agonists which appear as full agonists in cAMP assays, but that this is not the case for gene reporter assays in which agonist efficacies cannot be distinguished.

The effects of antidepressants on cyclic AMP-response element-driven gene transcription in a model cell system

The effects of the antidepressant drugs clomipramine (CLOM), desipramine (DMI), tianeptine (TIAN) and of norfluoxetine (NORF, the active metabolite of fluoxetine), were investigated in CHO cells expressing human beta2 adrenoceptors and a secreted placental alkaline phosphatase (SPAP) reporter gene to determine their actions on cyclic AMP-driven gene transcription. After 18 h of exposure, CLOM, DMI and NORF, but not TIAN, had biphasic effects on 1 microM isoprenaline-stimulated SPAP fsproduction with concentrations between 10 nM and 1 microM enhancing the maximal (E(max)) SPAP response, without changing EC50 values, but higher concentrations produced marked inhibitory effects. At nanomolar concentrations, CLOM and DMI increased expression of phospho-CREB (cyclic AMP response element binding protein). NORF was less effective but did significantly increase phospho-CREB at a concentration of 200 nM. TIAN had no effect. None of the antidepressants had any effect on CREB expression, nor on the accumulation of cyclic AMP. After prolonged exposure (7-21 days) to a low concentration (200 nM) of the antidepressants, the enhanced E(max) values for SPAP production evident after 18 h were not maintained but CLOM and DMI induced a significant leftward shift in the isoprenaline EC50 after a 7-day period of treatment and this was sustained at the 21 day time point. TIAN did not produce any significant changes. The results demonstrate that, in vitro, some but not all antidepressants can modify gene transcription via monoamine and cyclic AMP-independent mechanisms. The in vivo adaptive responses to TIAN probably involve alterations in different gene sets to those affected by other antidepressants.

Allosteric activation of the follicle-stimulating hormone (FSH) receptor by selective, nonpeptide agonists

The pituitary glycoprotein hormones, luteinizing hormone and follicle-stimulating hormone (FSH), act through their cognate receptors to initiate a series of coordinated physiological events that results in germ cell maturation. Given the importance of FSH in regulating folliculogenesis and fertility, the development of FSH mimetics has been sought to treat infertility. Currently, purified and recombinant human FSH are the only FSH receptor (FSH-R) agonists available for infertility treatment. By screening unbiased combinatorial chemistry libraries, using a cAMP-responsive luciferase reporter assay, we discovered thiazolidinone agonists (EC50's = 20 microm) of the human FSH-R. Subsequent analog library screening and parallel synthesis optimization resulted in the identification of a potent agonist (EC50 = 2 nm) with full efficacy compared with FSH that was FSH-R-selective and -dependent. The compound mediated progesterone production in Y1 cells transfected with the human FSH-R (EC50 = 980 nm) and estradiol production from primary rat ovarian granulosa cells (EC50 = 10.5 nm). This and related compounds did not compete with FSH for binding to the FSH-R. Use of human FSH/thyroid-stimulating hormone (TSH) receptor chimeras suggested a novel mechanism for receptor activation through a binding site independent of the natural hormone binding site. This study is the first report of a high affinity small molecule agonist that activates a glycoprotein hormone receptor through an allosteric mechanism. The small molecule FSH receptor agonists described here could lead to an oral alternative to the current parenteral FSH treatments used clinically to induce ovarian stimulation for both in vivo and in vitro fertilization therapy.

The effects of desmethylimipramine on cyclic AMP-stimulated gene transcription in a model cell system

The present study utilised an in vitro cell model of the cAMP signalling pathway to investigate the actions of desipramine (DMI) and other psychoactive agents on cAMP-driven gene transcription. The model comprised CHObeta2 SPAP cells; Chinese hamster ovary cells expressing human beta2 adrenoceptors and a secreted placental alkaline phosphatase (SPAP) reporter gene with multiple cAMP response elements (CREs) in its promoter region. SPAP assays showed DMI to inhibit isoprenaline or forskolin-enhanced gene transcription in a time and concentration-dependent manner (IC50=16.6+/-2.0 microM after 18 h). This effect of DMI was not dependent upon activity at the levels of the beta2 receptor, cAMP accumulation or phosphorylation of the transcription factor, cAMP response element binding protein (CREB). The inhibitory effects were maintained in the presence of DMI for at least 3 weeks and were mimicked by exposure to norfluoxetine (the major metabolite of fluoxetine; IC50=7.2+/-1.8 microM) and the neuroleptics, chlorpromazine and clozapine, all at a concentration of 10 microM. Amphetamine (10 microM, 18 h) enhanced SPAP gene transcription. Ca2+ imaging experiments ruled out an inhibitory effect of DMI on Ca2+ influx as concluded by previous studies. The results suggest a molecular target for DMI that lies downstream of CREB phosphorylation. Whether the inhibitory action of DMI is common to naturally expressed CRE-driven genes involved in adaptive responses to antidepressants in vivo remains to be determined.

Site of action of beta-ligands at the human beta1-adrenoceptor

Antagonist affinity measurements have traditionally been considered important in defining the receptor or receptor subtypes present within cells or tissues. Any change in this value has normally been taken as evidence for the presence of a second receptor. However, highly efficacious ligands induce a time and phosphorylation-dependent change in the beta2-adrenoceptor resulting in 10-fold lower affinity for antagonists. Also the beta1-adrenoceptor is now considered to exist in two different active conformations which are distinguished by their pharmacological properties. In this study, the site of action of a range of beta-agonists and beta-antagonists was determined using the human beta1-adrenoceptor stably expressed in Chinese hamster ovary cells with cyclic AMP response element reporter genes. Adrenaline and noradrenaline were confirmed as having agonist actions via the catecholamine site, whereas all antagonists had higher affinity for the catecholamine rather than secondary site. However, the rank order of affinity for the two sites was different suggesting that they are indeed separate entities. The measurements of antagonist affinity at the catecholamine site, however, were found to depend upon the agonist present. For example, xamoterol, cimaterol, terbutaline, and formoterol agonist responses were more readily antagonized by CGP 20712A[2-hydroxy-5-(2-[{hydroxy-3-(4-[1-methyl-4-trifluoromethyl-2-imidazolyl]phenoxy)propyl}amino]ethoxy)benzamide] than the catecholamine responses themselves. This, however, was not related to agonist efficacy as has previously been reported for the human beta2-adrenoceptor. Therefore, it may be that some agonists (e.g., cimaterol) purely activate the catecholamine site and others purely activate the secondary site (e.g., CGP 12177 [(-)-4-(3-tert-butylamino-2-hydroxypropoxy)-benzimidazol-2-one]), whereas the others (e.g., catecholamines) activate both sites to differing degrees.

Influence of receptor number on the cAMP response to forskolin in Chinese hamster ovary cells transfected with human beta2-adrenoceptor

We examined the basal adenosine 3',5'-cyclic monophosphate (cAMP) levels and forskolin-induced cAMP accumulation in cultured Chinese hamster ovary cells (CHO) expressing different levels of human beta(2)-adrenoceptors. Both the basal and forskolin-induced cAMP accumulation in the cells that express higher density of beta(2)-adrenoceptors (CHO-beta(2)/H; 560 fmol/mg protein) were larger than those in the cells that express lower density of beta(2)-adrenoceptors (CHO-beta(2)/L; 270 fmol/mg protein). In addition, isoproterenol-induced cAMP accumulation was also augmented as the number of beta(2)-adrenoceptors was increased. ICI 118,551, a selective beta(2)-adrenoceptor antagonist with inverse agonist properties, decreased all the levels of cAMP observed in both cell lines. These results suggest that the agonist-independent (constitutive) activity of beta(2)-adrenoceptors plays a key role in the control of forskolin-induced cAMP accumulation.

Temporal characteristics of cAMP response element-mediated gene transcription: requirement for sustained cAMP production

Many clinically used drugs are G-protein-coupled receptor (GPCR) antagonists and are given long-term to prevent receptor activation by endogenous agonists. Most GPCR antagonists are considered to have little agonist efficacy of their own. However, many beta antagonists do stimulate very small beta(2) adrenoceptor-mediated cAMP responses, but these responses become substantial at the level of cAMP response element (CRE)-gene transcription. Here, we compared the temporal characteristics of these beta(2) adrenoceptor-mediated cAMP and CRE-gene transcription responses with ligands of differing agonist efficacy. Within a minute, full agonists (e.g., isoprenaline) stimulated large increases in intracellular and exported cAMP. Very weak partial agonists (e.g., alprenolol) did not increase intracellular cAMP (only stimulating a small export). However, all agonists (regardless of efficacy) stimulated an increase in CRE-gene transcription after a 2-h incubation. An initial 30-min continual stimulation was required to initiate the process of CRE-gene transcription for all ligands. Longer agonist incubations resulted in larger gene transcription responses in a proportional manner for both weak and full agonists alike, and this was despite the lack of intracellular cAMP detection for the weaker ligands. Thus, the major initiator for CRE-gene transcription was not cAMP concentration or total quantity generated but a sustained turnover of intracellular cAMP and hence sustained stimulation of CREB phosphorylation. Thus, long-acting agonists and long-term treatments with very weak partial agonists (including many drugs classified previously as antagonists based on traditional second-messenger assays, e.g., several clinically used "beta-blockers") may cause more substantial gene transcription than previously believed.

AICA riboside both activates AMP-activated protein kinase and competes with adenosine for the nucleoside transporter in the CA1 region of the rat hippocampus

5-Aminoimidazole-4-carboxamide riboside (AICA riboside; Acadesine) activates AMP-activated protein kinase (AMPK) in intact cells, and is reported to exert protective effects in the mammalian CNS. In rat cerebrocortical brain slices, AMPK was activated by metabolic stress (ischaemia > hypoxia > aglycaemia) and AICA riboside (0.1-10 mm). Activation of AMPK by AICA riboside was greatly attenuated by inhibitors of equilibrative nucleoside transport. AICA riboside also depressed excitatory synaptic transmission in area CA1 of the rat hippocampus, which was prevented by an adenosine A1 receptor antagonist and reversed by application of adenosine deaminase. However, AICA riboside was neither a substrate for adenosine deaminase nor an agonist at adenosine receptors. We conclude that metabolic stress and AICA riboside both stimulate AMPK activity in mammalian brain, but that AICA riboside has an additional effect, i.e. competition with adenosine for uptake by the nucleoside transporter. This results in an increase in extracellular adenosine and subsequent activation of adenosine receptors. Neuroprotection by AICA riboside could be mediated by this mechanism as well as, or instead of, by AMPK activation. Caution should therefore be exercised in ascribing an effect of AICA riboside to AMPK activation, especially in systems where inhibition of adenosine re-uptake has physiological consequences.

Influence of agonist efficacy and receptor phosphorylation on antagonist affinity measurements: differences between second messenger and reporter gene responses

The ability of an antagonist to bind to a receptor is an innate property of that ligand-receptor chemical interaction. Provided no change in the antagonist or receptor chemical nature occurs, this affinity should remain constant for a given antagonist-receptor interaction, regardless of the agonists used. This fundamental assumption underpins the classification of receptors. Here, measurements of beta2-adrenoceptor-mediated cAMP accumulation and cAMP response-element (CRE)-mediated reporter-gene transcription revealed differences in antagonist affinity that depended upon agonist incubation time and the efficacy of the competing agonist. In cAMP accumulation studies (10-min agonist incubation), antagonist affinities were the same regardless of the agonist used. The CRE-reporter gene assay (5 h of incubation) antagonist affinities were 10-fold lower in the presence of isoprenaline and adrenaline than when salbutamol or terbutaline were present (e.g., log KD propranolol -8.65 +/- 0.08, n = 22, and -9.68 +/- 0.07, n = 17, for isoprenaline and salbutamol-induced responses, respectively). Isoprenaline and adrenaline were more efficacious in functional studies, and their ability to internalize GFP-tagged human beta2-adrenoceptors. Longer-term cAMP studies also showed significant differences in KD values moving toward that seen with gene transcription. Agonist-dependent differences in antagonist affinity were reduced for reporter-gene responses when a phosphorylation-deficient mutant of the beta2-adrenoceptor was used. This study suggests that high-efficacy agonists induce a chemical modification in beta2-adrenoceptors (via phosphorylation) that reduces antagonist affinities. Because reporter-gene assays are used for high-throughput screening in drug discovery, less efficacious or partial agonists may be more reliable than highly efficacious agonists when reporter-gene techniques are used to estimate antagonist affinity.

Agonist actions of "beta-blockers" provide evidence for two agonist activation sites or conformations of the human beta1-adrenoceptor

Previous work with 4-[3-[(1,1-dimethylethyl)amino]2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one (CGP 12177) has led to the suggestion that there are two different agonist conformations of the human beta1-adrenoceptor: 1) where classic agonists (catecholamines) and beta-antagonists act, and 2) where CGP 12177 is an agonist and relatively resistant to inhibition by beta-adrenoceptor antagonists. In the present study, we have used studies of cAMP response element-regulated gene transcription to confirm the presence of these two beta1-adrenoceptor sites/conformations and to provide strong evidence that a range of clinically used beta-adrenoceptor blockers (beta-blockers) exhibit differential agonists and/or antagonist actions at the two sites. [2-(3-Carbamoyl-4-hydroxyphenoxy)-ethylamino]-3-[4-(1-methyl-4-trifluormethyl-2-imidazolyl)-phenoxy]-2-propanolmethanesulphonate (CGP 20712A) and atenolol act as classic antagonists at the catecholamine binding site but have much lower affinity for the secondary CGP 12177 site. CGP 12177 and carvedilol are potent antagonists at the catecholamine site but mediate substantial agonist actions on gene transcription via the secondary antagonist-resistant site at higher concentrations. Agonist effects of beta-blockers are not, however, confined to this secondary site, and we show that some (particularly acebutolol and labetolol) act primarily via the catecholamine site, whereas others (pindolol and alprenolol) can stimulate both. The different responses to beta-blockers seen in the clinic may therefore be caused in part by these beta-blocker agonist responses and the differential activation of the two sites or conformations.

Pharmacology and direct visualisation of BODIPY-TMR-CGP: a long-acting fluorescent beta2-adrenoceptor agonist

1 Fluorescence techniques offer a way to circumvent several problems associated with many radioligand binding and functional assays and the need for large numbers of cells. Fluorescent ligands also offer the advantage of allowing real time direct visualisation of ligand - receptors interactions. A fluorescent analogue of CGP 12177 (BODIPY-TMR-CGP) has thus been evaluated as a beta(2)-adrenoceptor ligand in CHO-K1 cells expressing the human beta(2)-adrenoceptor. 2 Studies of (3)H-cAMP accumulation showed that BODIPY-TMR-CGP stimulated an increase in cAMP accumulation and cyclic AMP response element (CRE)-mediated gene transcription with an EC(50) of 21-28 nM. Both of these responses were antagonised by the selective beta(2)-adrenoceptor antagonist ICI 118551. 3 Binding studies with (3)H-CGP 12177, and functional studies of CRE-regulated gene transcription showed that the BODIPY-TMR-CGP interaction with the human beta(2)-adrenoceptor is of very long duration. 4 Visualisation of the binding of BODIPY-TMR-CGP to single living mammalian cells was clearly demonstrated by confocal microscopy and showed that this ligand was able to selectively label cell surface beta(2)-adrenoceptors in living CHO-K1 cells transfected with the human beta(2)-adrenoceptor with an apparent K(D) of 27 nM. Studies with cells expressing a beta(2)-adrenoceptor-green fluorescent protein (GFP) fusion protein provided further strong evidence that BODIPY-TMR-CGP was binding to the beta(2)-adrenoceptor. 5 BODIPY-TMR-CGP is therefore a long-acting fluorescent beta(2)-adrenoceptor agonist that can be used to label beta(2)-adrenoceptors in the plasma membrane of living cells.

Pharmacological characterization of CGP 12177 at the human beta(2)-adrenoceptor

1 It has recently been reported that CGP 12177 can act as an agonist at a novel secondary site within the human beta(1)-adrenoceptor. The aim of this study was to undertake a detailed pharmacological study of the effects of CGP 12177 on the human beta(2)-adrenoceptor. 2 CGP 12177 acted as a potent partial agonist of (3)H-cyclic AMP accumulation (log EC(50)-8.90+/-0.06) and CRE-mediated reporter gene transcription (log EC(50)-9.66+/-0.04) in CHO-K1 cells expressing the human beta(2)-adrenoceptor. These CGP-induced responses were antagonized by the beta(2)-selective antagonist ICI 118551 (apparent log K(D) values of -8.84+/-0.15 and -9.51+/-0.02 for the cyclic AMP and reporter gene responses respectively). 3 CGP 12177 was also able to antagonize both cyclic AMP and reporter gene responses to more efficacious beta(2)-agonists with similar log K(D) values (e.g. -9.57+/-0.15 and -10.04+/-0.096 respectively with salbutamol as agonist). 4 (3)H-CGP 12177 binding to beta(2)-adrenoceptors in intact CHO-beta(2) cells yielded a log K(D) value of -9.84+/-0.06, but indicated that the ligand dissociates very slowly from the receptor (t(1/2) for dissociation=65 min). However, studies with a Green Fluorescent Protein (GFP)-tagged beta(2)-adrenoceptor indicated that CGP 12177 does not stimulate beta(2)-adrenoceptor internalization. 5 This study demonstrates that CGP 12177 is a high affinity partial agonist of both cAMP accumulation and CRE-mediated gene transcription at the human beta(2)-adrenoceptor. It provides no evidence that CGP 12177 can discriminate a secondary site on the beta(2)-adrenoceptor analogous to that observed for the human beta(1)-adrenoceptor. However, despite its very weak actions on cAMP accumulation, the potent agonist effects of CGP 12177 on CRE-mediated gene transcription at the human beta(2)-adrenoceptor, coupled with its long duration of action, offers a potential lead for drug development for the treatment of chronic inflammatory airway diseases.

Cyclic AMP-dependent transcriptional up-regulation of phosphodiesterase 4D5 in human airway smooth muscle cells. Identification and characterization of a novel PDE4D5 promoter

Phosphodiesterase 4D (PDE4D), part of the complex cAMP-specific PDE4 family, plays a pivotal role in the regulation of airway smooth muscle relaxation by catalyzing the hydolysis of cAMP. Its gene on chromosome 5q12 encodes 5 splice variants, which show tissue-dependent expression and regulation. The genomic arrangement of PDE4D was determined using in silico methods, and a putative promoter of one of the protein kinase A-activated, long isoforms, PDE4D5 was identified. Promoter-luciferase constructs, transiently transfected into a beta(2) adrenoreceptor-expressing CHO-K1 cell line, were used to demonstrate that the PDE4D5 promoter up-regulated reporter gene expression in response to increased cell cAMP. Site-directed mutagenesis of the cAMP-response element (CRE) at position -201 identified this as the principal component of the mechanism underlying this cAMP responsiveness. In the second part of this study, cAMP-dependent induction of PDE4D5 transcript in primary cultured human airway smooth muscle cells (hASMs) was demonstrated using both qualitative reverse-transcriptase PCR and quantitative real-time PCR. Isolated PDE4D5 isoenzyme activity, measured after selective immunoprecipitation from hASMs, confirmed that this increase in expression led to an up-regulation of functional activity. We present evidence for cAMP-driven PDE4D5 up-regulation in hASMs and suggest a CRE-containing, isoform-specific promoter as the primary mechanism.

Mouse beta 3a- and beta 3b-adrenoceptors expressed in Chinese hamster ovary cells display identical pharmacology but utilize distinct signalling pathways

1. This study characterizes the mouse beta(3a)-adrenoceptor (AR) and the splice variant of the beta(3)-AR (beta(3b)-AR) expressed in Chinese hamster ovary cells (CHO-K1). 2. Stable clones with high (approximately 1200), medium (approximately 500) or low receptor expression (approximately 100 fmol mg protein(-1)) were determined by saturation binding with [(125)I]-(-)-cyanopindolol. Competition binding studies showed no significant differences in affinity of beta-AR ligands for either receptor. 3. Several functional responses of each receptor were measured, namely extracellular acidification rate (EAR; cytosensor microphysiometer), cyclic AMP accumulation, and Erk1/2 phosphorylation. The beta(3)-AR agonists BRL37344, CL316243, GR265162X, L755507, SB251023, the non-conventional partial beta-AR agonist CGP12177 and the beta-AR agonist (-)-isoprenaline caused concentration-dependent increases in EAR in cells expressing either splice variant. CL316243 caused concentration-dependent increases in cyclic AMP accumulation and Erk1/2 phosphorylation in cells expressing either receptor. 4. PTX treatment increased maximum EAR and cyclic AMP responses to CL316243 in cells expressing the beta(3b)-AR but not in cells expressing the beta(3a)-AR at all levels of receptor expression. 5. CL316243 increased Erk1/2 phosphorylation with pEC(50) values and maximum responses that were not significantly different in cells expressing either splice variant. Erk1/2 phosphorylation was insensitive to PTX or H89 (PKA inhibitor) but was inhibited by LY294002 (PI3K gamma inhibitor), PP2 (c-Src inhibitor), genistein (tyrosine kinase inhibitor) and PD98059 (MEK inhibitor). 6. The adenylate cyclase activators forskolin or cholera toxin failed to increase Erk1/2 levels although both treatments markedly increased cyclic AMP accumulation in both beta(3a)- or beta(3b)-AR transfected cells. 7. These results suggest that in CHO-K1 cells, the beta(3b)-AR, can couple to both G(s) and G(i) to stimulate and inhibit cyclic AMP production respectively, while the beta(3a)-AR, couples solely to G(s) to increase cyclic AMP levels. However, the increase in Erk1/2 phosphorylation following receptor activation is not dependent upon coupling of the receptors to G(i) or the generation of cyclic AMP.

Pharmacological characterisation of the beta-adrenoceptor expressed by human lung mast cells

The nonselective beta-adrenoceptor agonist, isoprenaline (pD2; 8.8 +/- 0.2), and selective beta2-adrenoceptor agonists, clenbuterol (9.2 +/- 0.4) and salbutamol (7.1 +/- 0.1), inhibited the immunoglobulin E-mediated release of histamine from human lung mast cells in a concentration-dependent manner. The beta2-adrenoceptor-selective antagonist, ICI118551 (erythro-(+/- )-1-(7-methylindan-4-yloyl)-3-isopropylaminobutan-2-ol HCl), antagonised the isoprenaline inhibition of histamine release from human lung mast cells with high affinity (apparent pK(B); 9.5 +/- 0.2), whereas high concentrations of the beta1-adrenoceptor-selective antagonist, CGP20712A (2-hydroxy-5-(2-(hydroxy-3-(4((1-methyl-4-trifluoromethyl)-1-H-imidazol-2-yl)-phenoxy)-propyl)-aminoethoxyl)-benzamide), were required to reverse the isoprenaline inhibition (apparent pK(B); 6.5 +/- 0.3). Radioligand binding studies using [125I]-iodocyanopindolol ([125I]CYP) were performed on membranes derived from purified mast cells (>90% purity). Binding of [125I]CYP to mast cell membranes was displaced from a single binding site with a high affinity for ICI118551 (pK(i); 8.9 +/- 0.1) and low affinity for CGP20712A (pK(i); 6.0 +/- 0.03), indicative of a homogeneous population of beta2-adrenoceptors. In contrast, in human lung membranes, these antagonists displaced [125I]CYP from two sites indicative of a heterogeneous population of beta1-adrenoceptors (20%) and beta2-adrenoceptors (80%). These data indicate that the beta-adrenoceptor expressed by human lung mast cells and mediating inhibition of mediator release from these cells is the beta2-adrenoceptor.

Modulation by cellular cholesterol of gene transcription via the cyclic AMP response element

The effect of rapid changes in cellular cholesterol content on adenosine 3',5'-cyclic monophosphate (cAMP) response element-mediated gene transcription was investigated. The study was carried out in Chinese hamster ovary (CHO-K1) cells permanently expressing the human beta(2)-adrenoceptor. Gene transcription was quantified using a reporter gene (secreted placental alkaline phosphatase) under the transcriptional control of cAMP response element (CRE) sequences. Cellular cholesterol was reduced by 42% or elevated by 47% by incubating cells for 1 hr with methyl-beta-cyclodextrin alone or methyl-beta-cyclodextrin complexed with cholesterol, respectively. There was a significant negative correlation between the free cholesterol content of the cells and CRE-mediated gene expression in response to 10(-6) M isoprenaline (slope = -4.57 +/- 0.73, P < 0.001), indicating that beta(2)-adrenoceptor-mediated activation of the CRE is inhibited by cholesterol. Cyclic AMP accumulation in response to isoprenaline (10(-12) to 10(-5) M) was also inhibited in cholesterol-enriched cells and enhanced in cholesterol-depleted cells compared to controls (P < 0.05, two-way ANOVA). Cholesterol also inhibited serum-mediated enhancement of CRE-driven gene expression, and we present data suggesting that the pathway activated by serum and inhibited by cholesterol could be independent of adrenoceptor activation and protein kinase A. We conclude that in CHO-K1 cells cholesterol inhibits at least two processes that can stimulate CRE-mediated gene expression. One is isoprenaline activation of cAMP synthesis, the other is activated by serum. These findings demonstrate that activation of gene transcription by extracellular stimuli could be influenced by cellular cholesterol content.

Influence of receptor number on functional responses elicited by agonists acting at the human adenosine A(1) receptor: evidence for signaling pathway-dependent changes in agonist potency and relative intrinsic activity

Activation of A(1) adenosine receptors leads to the inhibition of cAMP accumulation and the stimulation of inositol phosphate accumulation via pertussis toxin-sensitive G-proteins. In this study we have investigated the signaling of the A(1) adenosine receptor in Chinese hamster ovary (CHO) cells, when expressed at approximately 203 fmol/mg (CHOA1L) and at approximately 3350 fmol/mg (CHOA1H). In CHOA1L cells, the agonists N(6)-cyclopentyladenosine (CPA), (R)-N(6)-(2-phenylisopropyl)adenosine, and 5'-(N-ethylcarboxamido)adenosine (NECA) inhibited cAMP production in a concentration-dependent manner. After pertussis toxin treatment, the agonist NECA produced a stimulation of cAMP production, whereas CPA and (R)-N(6)-(2-phenylisopropyl)adenosine were ineffective. In CHOAIH cells, however, all three agonists produced both an inhibition of adenylyl cyclase and a pertussis toxin-insensitive stimulation of adenylyl cyclase. All three agonists were more potent at inhibiting adenylyl cyclase in CHOA1H cells than in CHOA1L cells. In contrast, A(1) agonists (and particularly NECA) were less potent at stimulating inositol phosphate accumulation in CHOA1H cells than in CHOA1L cells. After pertussis toxin treatment, agonist-stimulated inositol phosphate accumulation was reduced in CHOA1H cells and abolished in CHOA1L cells. The relative intrinsic activity of NECA in stimulating inositol phosphate accumulation, compared to CPA (100%), was much greater in the presence of pertussis toxin (289.6%) than in the absence of pertussis toxin (155.2%). These data suggest that A(1) adenosine receptors can couple to both pertussis toxin-sensitive and -insensitive G-proteins in an expression level-dependent manner. These data also suggest that the ability of this receptor to activate different G-proteins is dependent on the agonist present.

Development of a facile method for high throughput screening with reporter gene assays

This report describes a facile methodology for high throughput screening with stable mammalian cell reporter gene assays. We have adapted a 96-well adherent cell method to an assay in which cells propagated in suspension are dispensed into 96- or 384-well plates containing test compounds in 100% DMSO. The validation of a stable CHO cell line that expresses 6xCRE-luciferase for use as a reporter gene host cell line is described. The reporter gene, when expressed in this particular CHO cell line, appears to respond specifically to modulation of cAMP levels, thus the cell line is appropriate for screening and pharmacological analysis of Galpha(s)- and Galpha(i)-coupled seven-transmembrane receptors. The development of the new suspension cell assay in both 96- and 384-well formats was performed using a derivative of the CHO host reporter cell line that was stably transfected with human melanocortin-1 receptor. The response of this cell line to NDP-alpha-melanocyte-stimulating hormone and forskolin was nearly identical between the adherent and suspension methods. The new method offers improvements in cost, throughput, cell culture effort, compound stability, accuracy of compound delivery, and hands-on time. The 384-well assay can be performed at high capacity in any laboratory without the use of expensive automation systems such that a single person can screen 100 plates per day with 3.5-4 h hands-on time. Although the system has been validated using Galpha(s)-coupled receptor-mediated activation of a cAMP response element, the method can be applied to other types of targets and/or transcriptional response elements.

The generation and characterization of antagonist RNA aptamers to human oncostatin M

Oncostatin M (OSM) is a multifunctional member of the interleukin-6 cytokine family. OSM has been implicated as a powerful proinflammatory mediator and may represent a potentially important, novel therapeutic opportunity for treatment of established rheumatoid arthritis. To further investigate the role of OSM in inflammatory disorders, we have isolated a series of RNA aptamers that bind specifically to human OSM. The highest affinity aptamer, designated ADR58, has been characterized in a series of in vitro and cell based assays. ADR58 has an affinity of 7 nm for human OSM, and it can antagonize OSM binding to the gp130 receptor and specifically antagonize OSM mediated signaling. The aptamer has been truncated in length to 33 bases, all pyrimidine positions are substituted with 2' fluorine, and 14 of 18 purine positions have been substituted with 2' O-methyl to increase stability toward nucleases. This truncated, modified form of ADR58 retains complete affinity and functional activity for OSM. This aptamer may be used as a tool to further investigate the role of OSM in inflammatory disorders and may also have role as a therapeutic agent.

A reporter gene assay for high-throughput screening of G-protein-coupled receptors stably or transiently expressed in HEK293 EBNA cells grown in suspension culture

We describe in detail a robust, sensitive, and versatile functional assay for G-protein-coupled receptors (GPCRs) expressed in human embryonic kidney (HEK) 293-EBNA (Epstein-Barr virus nuclear antigen) (designated 293E) cells. The ability to grow these cells in suspension, in conjunction with the use of the secreted form of the human placental alkaline phosphatase (SEAP) as the reporter enzyme transcriptionally regulated by 5-cyclic AMP (cAMP) response elements (CREs) (Chen et al., Anal. Biochem. 226, 349-354 (1995)), makes this CRE-SEAP assay potentially attractive for high-throughput screening (HTS). A 293E clonal cell line, stably transfected with the CRE-SEAP plasmid, was initially characterized with compounds known to activate intracellular signal transduction pathways similar to those activated by GPCRs. Forskolin and cAMP analogues were potent at inducing SEAP expression but calcium ionophores (A23187 and ionomycin) were without effect. The forskolin response was also potentiated by the protein kinase C activator phorbol myristate acetate as well as the phosphodiesterase inhibitor isobutylmethylxanthine. Previously established cell lines expressing the G(alphas)-coupled DP or the G(alphaq)-coupled-EP(1) prostanoid receptors were stably transfected with the reporter gene construct and clones were selected based on their ability to secrete SEAP upon agonist challenge. Pharmacological characterization of the DP and EP(1) receptors displayed a similar rank order of potency for several known prostanoids and related compounds to that previously reported using classical binding assays or other functional assays. The CRE-SEAP assay was also used to characterize the EP(1) receptor antagonists SC-51322, SC-51089, and AH6809. In summary, we have established a reporter gene assay for GPCRs that couple to both G(alphas) and G(alphaq) and is amenable to HTS of both agonists and antagonists.

Non-competitive antagonism of beta(2)-agonist-mediated cyclic AMP accumulation by ICI 118551 in BC3H1 cells endogenously expressing constitutively active beta(2)-adrenoceptors

Constitutive activity of the beta(2)-adrenoceptor, which is sensitive to inhibition by an inverse agonist such as ICI 118551, has been readily demonstrated in recombinant systems expressing constitutively-active mutant receptors or over-expressing the wild-type beta(2)-adrenoceptor. Here we demonstrate the presence of constitutive beta(2)-adrenoceptor activity in BC3H1 cells which endogenously express this receptor. In BC3H1 cells, only ICI 118551 behaved as an inverse agonist at beta(2)-adrenoceptors, while propranolol, ICI 118551, atenolol and, to a lesser extent, alprenolol exhibited inverse agonism in CHO-beta(2)4 cells transfected with cDNA for the human beta(2)-adrenoceptor (310 fmol. mg protein(-1)). The level of expression of beta2-adrenoceptors in BC3H1 cells was not high (78 fmol.mg protein-1) and the efficiency of receptor - effector coupling in this cell line was much lower than in the recombinant CHO-beta(2)4 cells (as judged by the partial agonist nature of both salbutamol and clenbuterol). ICI 118551 (log K(D)-9.73+/-0.07) and propranolol (log K(D)-9.25+/-0.12) both behaved as conventional competitive antagonists of isoprenaline-stimulated cyclic AMP accumulation in high expressing CHO-beta(2)4 cells. In contrast, ICI 118551 appeared to act as a non-competitive antagonist in BC3H1 cells and in low expressing CHO-beta(2)6 cells (50 fmol.mg protein(-1)). This non-competitive effect of ICI 118551 in BC3H1 cells was also observed when either salbutamol was used as agonist, or the incubation period with isoprenaline was extended to 30 min. The possibility that these effects of ICI 118551 are due to an interaction with different affinity states (R, R* and R') of the receptor is discussed.

Use of a dual firefly and Renilla luciferase reporter gene assay to simultaneously determine drug selectivity at human corticotrophin releasing hormone 1 and 2 receptors

The aim of this study was to investigate and validate the use of a dual glow-signal luciferase reporter gene assay to simultaneously evaluate drug activity at two different seven-transmembrane receptor subtypes. Stable cell lines (CHO) transfected with either human corticotrophin releasing hormone 1 (hCRH1) receptors and a firefly luciferase reporter gene or hCRH2 and a Renilla luciferase reporter gene were created to provide different luciferase readouts for CRH1 and CRH2 receptors, respectively. Cells were combined for stimulation and measurement of luciferase luminescence in a 96-well plate format assay. The nonselective CRH agonists rat/human CRH and sauvagine caused concentration-dependent increases in luminescence via activation of CRH1 (firefly luciferase; pEC50 = 8.40 +/- 0.06 and 8.39 +/- 0.08, respectively, n = 8) and CRH2 (Renilla luciferase; pEC50 = 8.89 +/- 0.14 and 8.92 +/- 0.13, respectively, n = 8) receptors. The nonselective CRH antagonist astressin blocked these agonist-induced increases in luciferase at both CRH1 and CRH2 receptors. The selective CRH1 antagonist CP154,526 blocked r/hCRH- and sauvagine-induced increases in luciferase at CRH1 receptors only. These data report the expected pharmacology for CRH1 and CRH2 receptors. This assay enabled two receptor subtypes to be studied simultaneously in the same 96-well plate and generated robust data with low variability. It has the potential advantage of significant time and cost savings, with application to both basic research and compound screening.

A combination assay for simultaneous assessment of multiple signaling pathways

We have developed an assay in which modulation of two or more signaling pathways can be assessed concurrently by combining reporter gene systems with fluorescent probe technology. The validation of this method was achieved by indirect analysis of adenylyl cyclase activation with the use of a cyclic AMP response element (CRE)-luciferase reporter system in combination with the measurement of calcium mobilization by Calcium Green-1 AM fluorescence on a fluorescent imaging plate reader. To demonstrate the utility of the method in studying the pharmacology of receptors that couple to more than one G protein, Chinese hamster ovary (CHO) cells, which stably expressed both the CRE-luciferase reporter gene and the human pituitary adenylyl cyclase-activating peptide (PACAP) receptor, were treated with PACAP 1-27 and 1-38. Calcium mobilization and the induction of adenylyl cyclase activity in response to each concentration of peptide were assessed in individuals wells. This assay may also be used to screen for ligands of two or more unrelated receptors simultaneously without compromising the assessment of either signaling pathway. To illustrate this point, Rat-1 fibroblasts, which expressed human alpha1A receptors, were cocultured with CRE-luciferase CHO cells, which expressed human GLP-1 receptors. Calcium mobilization elicited by phenylephrine agonism of the alpha1A receptor was assessed in the same assay as GLP-1-induced activation of adenylyl cyclase. The pEC(50) for each agonist was similar to that observed when the cell lines were not cocultured. The number of different receptors that can be screened per well is limited only by the ability to distinguish different reporter gene signals and fluorescent indicators.

Constitutive receptor systems for drug discovery

This paper discusses the use of constitutively active G-protein-coupled receptor systems for drug discovery. Specifically, the ternary complex model is used to define the two major theoretical advantages of constitutive receptor screening-namely, the ability to detect antagonists as well as agonists directly and the fact that constitutive systems are more sensitive to agonists. In experimental studies, transient transfection of Chinese hamster ovary cyclic AMP response element (CRE) luciferase reporter cells with cDNA for human parathyroid hormone receptor, glucagon receptor, and glucagon-like peptide (GLP-1) receptor showed cDNA concentration-dependent constitutive activity with parathyroid hormone (PTH-1) and glucagon. In contrast, no constitutive activity was observed for GLP-1 receptor, yet responses to GLP-1 indicated that receptor expression had taken place. In another functional system, Xenopus laevi melanophores transfected with cDNA for human calcitonin receptor showed constitutive activity. Nine ligands for the calcitonin receptor either increased or decreased constitutive activity in this assay. The sensitivity of the system to human calcitonin increased with increasing constitutive activity. These data indicate that, for those receptors which naturally produce constitutive activity, screening in this mode could be advantageous over other methods.

Effects of a range of beta2 adrenoceptor agonists on changes in intracellular cyclic AMP and on cyclic AMP driven gene expression in cultured human airway smooth muscle cells

1. The effects of the selective beta2 adrenoceptor agonists salbutamol, terbutaline and salmeterol and the non-selective beta adrenoceptor agonist isoprenaline on [3H]-cyclic AMP formation and cyclic AMP response element (CRE) driven luciferase expression, assessed using the construct p6CRE/luc, were studied in primary cultures of human airway smooth muscle (HASM) cells. 2. Optimal transfection conditions for transient expression of pGL3 Control were 4 microg DNA/well71 in a 6 well plate and 1.8 microl Transfectam/microg DNA. Expression was maximal at 48 - 72 h. 3. Salbutamol (maximum response 19%, EC50 0.6 microM), terbutaline (maximum response 38%, EC50 2.3 microM) and salmeterol (maximum response 18%, EC50 0.0012 microM) were all partial agonists for cyclic AMP formation compared with isoprenaline (EC50 0.08 microM). However, all of the beta2 adrenoceptor agonists produced increases in CRE-driven luciferase activity, in cultured HASM transfected with the vector p6CRE/luc, which were equivalent or greater (salmeterol) than those seen with isoprenaline. 4. Both salbutamol and salmeterol were more potent at increasing luciferase expression than in elevating cyclic AMP levels in these cells. The potency ratios (EC50 (cyclic AMP)/EC50 (LUC)) for the agents studied were isoprenaline: 0. 2 fold, terbutaline: 3 fold, salbutamol: 24 fold, salmeterol: 38 fold. 5. These data suggest that important quantitative differences exist in the ability of beta2 adrenoceptor agonists to increase whole cell cyclic AMP levels in airway smooth muscle and to drive gene expression via a CRE-driven mechanism.