PKA and Epac cooperate to augment bradykinin-induced interleukin-8 release from human airway smooth muscle cells

The Impact of EPAC2-Associated Junction Plakoglobin on Respiratory Syncytial Virus Infection

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections in infants, young children, and immunocompromised individuals. Currently, FDA-approved monoclonal antibody therapies are limited to infants and young children with severe RSV disease. As a result, there is an urgent need for comprehensive studies of RSV pathogenesis to support the development of new therapeutic strategies. Exchange proteins directly activated by cAMP (EPAC) have recently emerged as key regulators in various viral infections. Our previous work identified EPAC isoform 2 (EPAC2) as a critical factor in RSV replication and host innate immune responses. However, the molecular mechanisms underlying EPAC2's role in RSV infection remain unclear. In this study, we investigated EPAC2-mediated RSV infection by identifying EPAC2-interacting proteins. Proteomics and immunoprecipitation analyses revealed that junction plakoglobin (JUP) interacts with EPAC2 in both mock- and RSV-infected cells, with this interaction notably enhanced during RSV infection. To determine JUP's role in RSV infection, we compared viral replication in JUP-deficient and control cells. JUP downregulation significantly reduced the production of infectious RSV particles, likely by impairing viral budding and viral gene transcription. Moreover, our findings indicate that JUP is essential for an effective cellular immune response to RSV infection. Together, these results suggest that EPAC2 and JUP may cooperatively regulate RSV replication and dissemination.

Pro-inflammatory action of formoterol in human bronchial epithelia

Despite the wide usage of β2-adrenoceptor agonists in asthma treatment, they do have side effects such as aggravating inflammation. We previously reported that isoprenaline induced Cl- secretion and IL-6 release via cAMP-dependent pathways in human bronchial epithelia, but the mechanisms underlying the inflammation-aggravation effects of β2-adrenoceptor agonists remain pooly understood. In this study, we investigated formoterol, a more specific β2-adrenoceptor agonist, -mediated signaling pathways involved in the production of IL-6 and IL-8 in 16HBE14o- human bronchial epithelia. The effects of formoterol were detected in the presence of PKA, exchange protein directly activated by cAMP (EPAC), cystic fibrosis transmembrane conductance regulator (CFTR), extracellular signal-regulated protein kinase (ERK)1/2 and Src inhibitors. The involvement of β-arrestin2 was determined using siRNA knockdown. Our results indicate that formoterol can induce IL-6 and IL-8 secretion in concentration-dependent manner. The PKA-specific inhibitor, H89, partially inhibited IL-6 release, but not IL-8. Another intracellular cAMP receptor, EPAC, was not involved in either IL-6 or IL-8 release. PD98059 and U0126, two ERK1/2 inhibitors, blocked IL-8 while attenuated IL-6 secretion induced by formoterol. Furthermore, formoterol-induced IL-6 and IL-8 release was attenuated by Src inhibitors, namely dasatinib and PP1, and CFTRinh172, a CFTR inhibitor. In addition, knockdown of β-arrestin2 by siRNA only suppressed IL-8 release when a high concentration of formoterol (1 μM) was used. Taken together, our results suggest that formoterol stimulates IL-6 and IL-8 release which involves PKA/Src/ERK1/2 and/or β-arrestin2 signaling pathways.

cAMP-EPAC-PKCε-RIM1α signaling regulates presynaptic long-term potentiation and motor learning

The cerebellum is involved in learning of fine motor skills, yet whether presynaptic plasticity contributes to such learning remains elusive. Here, we report that the EPAC-PKCε module has a critical role in a presynaptic form of long-term potentiation in the cerebellum and motor behavior in mice. Presynaptic cAMP-EPAC-PKCε signaling cascade induces a previously unidentified threonine phosphorylation of RIM1α, and thereby initiates the assembly of the Rab3A-RIM1α-Munc13-1 tripartite complex that facilitates docking and release of synaptic vesicles. Granule cell-specific blocking of EPAC-PKCε signaling abolishes presynaptic long-term potentiation at the parallel fiber to Purkinje cell synapses and impairs basic performance and learning of cerebellar motor behavior. These results unveil a functional relevance of presynaptic plasticity that is regulated through a novel signaling cascade, thereby enriching the spectrum of cerebellar learning mechanisms.

β2-Adrenoceptor Activation Stimulates IL-6 Production via PKA, ERK1/2, Src, and Beta-Arrestin2 Signaling Pathways in Human Bronchial Epithelia

OBJECTIVE

β2-Adrenoceptor agonists are widely used to treat asthma because of their bronchial-dilation effects. We previously reported that isoprenaline, via the apical and basolateral β2-adrenoceptor, induced Cl- secretion by activating cyclic AMP (cAMP)-dependent pathways in human bronchial epithelia. Despite these results, whether and how the β2-adrenoceptor-mediated cAMP-dependent pathway contributes to pro-inflammatory cytokine release in human bronchial epithelia remains poorly understood.

METHODS

We investigated β2-adrenoceptor-mediated signaling pathways involved in the production of two pro-inflammatory cytokines, interleukin (IL)-6 and IL-8, in 16HBE14o- human bronchial epithelia. The effects of isoprenaline or formoterol were assessed in the presence of protein kinase A (PKA), exchange protein directly activated by cAMP (EPAC), Src, and extracellular signal-regulated protein kinase (ERK)1/2 inhibitors. The involvement of β-arrestin2 was examined using siRNA knockdown.

RESULTS

Isoprenaline and formoterol (both β2 agonists) induced IL-6, but not IL-8, release, which could be inhibited by ICI 118,551 (β2 antagonist). The PKA-specific inhibitor, H89, partially inhibited IL-6 release. Another intracellular cAMP receptor, EPAC, was not involved in IL-6 release. Isoprenaline-mediated IL-6 secretion was attenuated by dasatinib, a Src inhibitor, and PD98059, an ERK1/2 inhibitor. Isoprenaline treatment also led to ERK1/2 phosphorylation. In addition, knockdown of β-arrestin2 by siRNA specifically suppressed cytokine release when a high concentration of isoprenaline (1 mM) was used.

CONCLUSION

Our results suggest that activation of the β2-adrenoceptor in 16HBE14o- cells stimulated the PKA/Src/ERK1/2 and/or β-arrestin2 signaling pathways, leading to IL-6 release. Therefore, our data reveal that β2-adrenoceptor signaling plays a role in the immune regulation of human airway epithelia.

EPAC2: A new and promising protein for glioma pathogenesis and therapy

Gliomas are prime brain cancers which are initiated by malignant modification of neural stem cells, progenitor cells and differentiated glial cells such as astrocyte, oligodendrocyte as well as ependymal cells. Exchange proteins directly activated by cAMP (EPACs) are crucial cyclic adenosine 3’,5’-monophosphate (cAMP)-determined signaling pathways. Cyclic AMP-intermediated signaling events were utilized to transduce protein kinase A (PKA) leading to the detection of EPACs or cAMP-guanine exchange factors (cAMP-GEFs). EPACs have been detected as crucial proteins associated with the pathogenesis of neurological disorders as well as numerous human diseases. EPAC proteins have two isoforms. These isoforms are EPAC1 and EPAC2. EPAC2 also known as Rap guanine nucleotide exchange factor 4 (RAPGEF4) is generally expression in all neurites. Higher EAPC2 levels was detected in the cortex, hippocampus as well as striatum of adult mouse brain. Activation as well as over-secretion of EPAC2 triggers apoptosis in neurons and EPAC-triggered apoptosis was intermediated via the modulation of Bcl-2 interacting member protein (BIM). EPAC2 secretory levels has proven to be more in low-grade clinical glioma than high-grade clinical glioma. This review therefore explores the effects of EPAC2/RAPGEF4 on the pathogenesis of glioma instead of EPAC1 because EPAC2 and not EPAC1 is predominately expressed in the brain. Therefore, EPAC2 is most likely to modulate glioma pathogenesis rather than EPAC1.

Exchange protein directly activated by cAMP (Epac) protects against airway inflammation and airway remodeling in asthmatic mice

Background

β₂ receptor agonists induce airway smooth muscle relaxation by increasing intracellular cAMP production. PKA is the traditional downstream signaling pathway of cAMP. Exchange protein directly activated by cAMP (Epac) was identified as another important signaling molecule of cAMP recently. The role of Epac in asthmatic airway inflammation and airway remodeling is unclear.

Methods

We established OVA-sensitized and -challenged acute and chronic asthma mice models to explore the expression of Epac at first. Then, airway inflammation and airway hyperresponsiveness in acute asthma mice model and airway remodeling in chronic asthma mice model were observed respectively after treatment with Epac-selective cAMP analogue 8-pCPT-2′-O-Me-cAMP (8pCPT) and Epac inhibitor ESI-09. Next, the effects of 8pCPT and ESI-09 on the proliferation and apoptosis of in vitro cultured mouse airway smooth muscle cells (ASMCs) were detected with CCK-8 assays and Annexin-V staining. Lastly, the effects of 8pCPT and ESI-09 on store-operated Ca²⁺ entry (SOCE) of ASMCs were examined by confocal Ca²⁺ fluorescence measurement.

Results

We found that in lung tissues of acute and chronic asthma mice models, both mRNA and protein expression of Epac1 and Epac2, two isoforms of Epac, were lower than that of control mice. In acute asthma mice model, the airway inflammatory cell infiltration, Th2 cytokines secretion and airway hyperresponsiveness were significantly attenuated by 8pCPT and aggravated by ESI-09. In chronic asthma mice model, 8pCPT decreased airway inflammatory cell infiltration and airway remodeling indexes such as collagen deposition and airway smooth muscle cell proliferation, while ESI-09 increased airway inflammation and airway remodeling. In vitro cultured mice ASMCs, 8pCPT dose-dependently inhibited, whereas ESI-09 promoted ASMCs proliferation. Interestingly, 8pCPT promoted the apoptosis of ASMCs, whereas ESI-09 had no effect on ASMCs apoptosis. Lastly, confocal Ca²⁺ fluorescence examination found that 8pCPT could inhibit SOCE in ASMCs at 100 μM, and ESI-09 promoted SOCE of ASMCs at 10 μM and 100 μM. In addition, the promoting effect of ESI-09 on ASMCs proliferation was inhibited by store-operated Ca²⁺ channel blocker, SKF-96365.

Conclusions

Our results suggest that Epac has a protecting effect on asthmatic airway inflammation and airway remodeling, and Epac reduces ASMCs proliferation by inhibiting SOCE in part.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

CB1 Receptors Mediated Inhibition of ATP-Induced [Ca2+]i Increase in Cultured Rat Spinal Dorsal Horn Neurons

Spinal cannabinoid receptor 1 (CB₁R) and purinergic P2X receptors (P2XR) play a critical role in the process of pathological pain. Both CB₁R and P2XR are expressed in spinal dorsal horn (DH) neurons. It is not clear whether CB₁ receptor activation modulates the function of P2X receptor channels within dorsal horn. For this reason, we observed the effect of CP55940 (cannabinoid receptor agonist) on ATP-induced Ca²⁺ mobilization in cultured rat DH neurons. The changes of intracellular calcium concentration ([Ca²⁺]i) were detected with confocal laser scanning microscopy using fluo-4/AM as a calcium fluorescent indicator. 100 μM ATP caused [Ca²⁺]i increase in cultured DH neurons. ATP-evoked [Ca²⁺]i increase in DH neurons was blocked by chelating extracellular Ca²⁺ and P2 purinoceptor antagonist PPADS. At the same time, ATP-γ-S (a non-hydrolyzable ATP analogue) mimicked the ATP action, while P2Y receptor agonist ADP failed to evoke [Ca²⁺]i increase in cultured DH neurons. These data suggest that ATP-induced [Ca²⁺]i elevation in cultured DH neurons is mediated by P2X receptor. Subsequently, we noticed that, in cultured rat DH neurons, ATP-induced Ca²⁺ mobilization was inhibited after pretreated with CP55940 with a concentration-dependent manner, which implies that the opening of P2X receptor channels are down-regulated by activation of cannabinoid receptor. The inhibitory effect of CP55940 on ATP-induced Ca²⁺ response was mimicked by ACEA (CB₁R agonist), but was not influenced by AM1241 (CB₂R agonist). Moreover, the inhibitory effect of CP55940 on ATP-induced Ca²⁺ mobilization was blocked by AM251 (CB₁ receptor antagonist), but was not influenced by AM630 (CB₂ receptor antagonist). In addition, we also observed that forskolin (an activator of adenylate cyclase) and 8-Br-cAMP (a cell-permeable cAMP analog) reversed the inhibitory effect of CP55940, respectively. In a summary, our observations raise a possibility that CB₁R rather than CB₂R can downregulate the opening of P2X receptor channels in DH neurons. The reduction of cAMP/PKA signaling is a key element in the inhibitory effect of CB₁R on P2X-channel-induced Ca²⁺ mobilization.

Upregulation of Epac-1 in Hepatic Stellate Cells by Prostaglandin E2 in Liver Fibrosis Is Associated with Reduced Fibrogenesis

Exchange protein activated by cAMP (Epac-1) is an important signaling mechanism for cAMP-mediated effects, yet factors that change Epac-1 levels are unknown. Such factors are relevant because it has been postulated that Epac-1 directly affects fibrogenesis. Prostaglandin E2 (PGE2) is a well-known cAMP activator, and we therefore studied the effects of this cyclo-oxygenase product on Epac-1 expression and on fibrogenesis within the liver. Liver fibrosis was induced by 8 weeks carbon tetrachloride (CCL4) administration to mice. In the last 2 weeks, mice received vehicle, PGE2, the cyclo-oxygenase-2 inhibitor niflumic acid (NFA), or PGE2 coupled to cell-specific carriers to hepatocytes, Kupffer cells, or hepatic stellate cells (HSC). Results showed antifibrotic effects of PGE2 and profibrotic effects of NFA in CCL4 mice. Western blot analysis revealed reduced Epac-1 protein expression in fibrotic livers of mice and humans compared with healthy livers. PGE2 administration to fibrotic mice completely restored intrahepatic Epac-1 levels and also led to reduced Rho kinase activity, a downstream target of Epac-1. Cell-specific delivery of PGE2 to either hepatocytes, Kupffer cells, or HSC identified the latter cell as the key player in the observed effects on Epac-1 and Rho kinase. No significant alterations in protein kinase A expressions were found. In primary isolated HSC, PGE2 elicited Rap1 translocation reflecting Epac-1 activation, and Epac-1 agonists attenuated platelet-derived growth factor-induced proliferation and migration of these cells. These studies demonstrate that PGE2 enhances Epac-1 activity in HSC, which is associated with significant changes in (myo)fibroblast activities in vitro and in vivo. Therefore, Epac-1 is a potential target for antifibrotic drugs.

EPAC1 activation by cAMP stabilizes CFTR at the membrane by promoting its interaction with NHERF1

Cyclic AMP (cAMP) activates protein kinase A (PKA) but also the guanine nucleotide exchange factor 'exchange protein directly activated by cAMP' (EPAC1; also known as RAPGEF3). Although phosphorylation by PKA is known to regulate CFTR channel gating - the protein defective in cystic fibrosis - the contribution of EPAC1 to CFTR regulation remains largely undefined. Here, we demonstrate that in human airway epithelial cells, cAMP signaling through EPAC1 promotes CFTR stabilization at the plasma membrane by attenuating its endocytosis, independently of PKA activation. EPAC1 and CFTR colocalize and interact through protein adaptor NHERF1 (also known as SLC9A3R1). This interaction is promoted by EPAC1 activation, triggering its translocation to the plasma membrane and binding to NHERF1. Our findings identify a new CFTR-interacting protein and demonstrate that cAMP activates CFTR through two different but complementary pathways - the well-known PKA-dependent channel gating pathway and a new mechanism regulating endocytosis that involves EPAC1. The latter might constitute a novel therapeutic target for treatment of cystic fibrosis.

cAMP effects in neuroendocrine tumors: The role of Epac and PKA in cell proliferation and adhesion

cAMP effects have been initially attributed to protein kinase A (PKA) activation. Subsequently, two exchange proteins directly activated by cAMP (Epac1/2) have been identified as cAMP targets. Aim of this study was to investigate cAMP effects in pancreatic-NET (P-NET) and bronchial carcinoids and in corresponding cell lines (QGP-1 and H727) on cell proliferation and adhesion and to determine PKA and Epac role in mediating these effects. We found that cAMP increased cyclin D1 expression in P-NET and QGP-1 cells, whereas it had opposite effects on bronchial carcinoids and H727 cells and it promoted cell adhesion in QGP-1 and H727 cells. These effects are mimicked by Epac and PKA specific analogs, activating the small GTPase Rap1. In conclusion, we demonstrated that cAMP exerted divergent effects on proliferation and promoted cell adhesion of different neuroendocrine cell types, these effects being mediated by both Epac and PKA and involving the same effector GTPase Rap1.

cAMP-mediated secretion of brain-derived neurotrophic factor in developing airway smooth muscle

Moderate hyperoxic exposure in preterm infants contributes to subsequent airway dysfunction and to risk of developing recurrent wheeze and asthma. The regulatory mechanisms that can contribute to hyperoxia-induced airway dysfunction are still under investigation. Recent studies in mice show that hyperoxia increases brain-derived neurotrophic factor (BDNF), a growth factor that increases airway smooth muscle (ASM) proliferation and contractility. We assessed the mechanisms underlying effects of moderate hyperoxia (50% O2) on BDNF expression and secretion in developing human ASM. Hyperoxia increased BDNF secretion, but did not alter endogenous BDNF mRNA or intracellular protein levels. Exposure to hyperoxia significantly increased [Ca2+]i responses to histamine, an effect blunted by the BDNF chelator TrkB-Fc. Hyperoxia also increased ASM cAMP levels, associated with reduced PDE4 activity, but did not alter protein kinase A (PKA) activity or adenylyl cyclase mRNA levels. However, 50% O2 increased expression of Epac2, which is activated by cAMP and can regulate protein secretion. Silencing RNA studies indicated that Epac2, but not Epac1, is important for hyperoxia-induced BDNF secretion, while PKA inhibition did not influence BDNF secretion. In turn, BDNF had autocrine effects of enhancing ASM cAMP levels, an effect inhibited by TrkB and BDNF siRNAs. Together, these novel studies suggest that hyperoxia can modulate BDNF secretion, via cAMP-mediated Epac2 activation in ASM, resulting in a positive feedback effect of BDNF-mediated elevation in cAMP levels. The potential functional role of this pathway is to sustain BDNF secretion following hyperoxic stimulus, leading to enhanced ASM contractility and proliferation.

Cyclic Adenosine Monophosphate-Mediated Enhancement of Vascular Endothelial Growth Factor Released by Differentiated Human Monocytic Cells: The Role of Protein Kinase A

OBJECTIVE

Our investigation was designed to examine the signaling pathway involved in the enhancement of vascular endothelial growth factor (VEGF) release by β-adrenoceptor agonists.

MATERIALS AND METHODS

Human U937 cells differentiated into macrophages were primed with lipopolysaccharide (LPS) in the absence or presence of β-adrenoceptor agonists and antagonists. The VEGF released and the intracellular cyclic adenosine monophosphate (cAMP) generated were assayed by ELISA. Where necessary, differences between mean values were tested for significance using Student's t test.

RESULTS

Isoprenaline, procaterol and salbutamol concentration-dependently enhanced the release of VEGF induced by LPS in U937 cells. R*,R*-(±)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid (BRL 37344), a selective β3-adrenoceptor agonist, did not enhance VEGF release. Using isoprenaline as an agonist, propranolol, ICI 118551 and atenolol produced a parallel rightward shift of the concentration-response curve with no reduction in the maximum response. The -logKB values were 8.12 ± 0.17, 8.03 ± 0.05 and 7.23 ± 0.05 for propranolol, ICI 118551 and atenolol, respectively, indicating the possible involvement of both β1- and β2-adrenoceptor subtypes. Isoprenaline and prostaglandin E2 concentration-dependently increased cAMP generation in U937 cells. Isoprenaline, db-cAMP and 6-Bnz-cAMP, a protein kinase A (PKA) activator, all enhanced VEGF release induced by LPS, and this effect was abolished by KT 5720 and Rp-cAMPS, which are both selective PKA inhibitors, suggesting that PKA is the downstream effector of cAMP activity. 8-CPT-cAMP, a selective activator of the Epac system, had no effect on VEGF release induced by LPS, indicating that the Epac pathway played no role in the release process.

CONCLUSION

In this study, we established that β1- and β2- but not β3-adrenoceptors mediated cAMP-dependent enhancement of VEGF release induced by LPS in differentiated U937 cells, and that PKA was the downstream effector of cAMP activity.

A-kinase anchoring proteins: cAMP compartmentalization in neurodegenerative and obstructive pulmonary diseases

The universal second messenger cAMP is generated upon stimulation of Gs protein-coupled receptors, such as the β2 -adreneoceptor, and leads to the activation of PKA, the major cAMP effector protein. PKA oscillates between an on and off state and thereby regulates a plethora of distinct biological responses. The broad activation pattern of PKA and its contribution to several distinct cellular functions lead to the introduction of the concept of compartmentalization of cAMP. A-kinase anchoring proteins (AKAPs) are of central importance due to their unique ability to directly and/or indirectly interact with proteins that either determine the cellular content of cAMP, such as β2 -adrenoceptors, ACs and PDEs, or are regulated by cAMP such as the exchange protein directly activated by cAMP. We report on lessons learned from neurons indicating that maintenance of cAMP compartmentalization by AKAP5 is linked to neurotransmission, learning and memory. Disturbance of cAMP compartments seem to be linked to neurodegenerative disease including Alzheimer's disease. We translate this knowledge to compartmentalized cAMP signalling in the lung. Next to AKAP5, we focus here on AKAP12 and Ezrin (AKAP78). These topics will be highlighted in the context of the development of novel pharmacological interventions to tackle AKAP-dependent compartmentalization.

A-kinase anchoring proteins contribute to loss of E-cadherin and bronchial epithelial barrier by cigarette smoke

Airway epithelium, which forms the first barrier towards environmental insults, is disturbed by cigarette smoking, a major risk factor for developing chronic obstructive pulmonary disease (COPD). A-kinase anchoring proteins (AKAP) maintain endothelial barrier function and coordinate subcellular localization of protein kinase A (PKA). However, the role of AKAPs in epithelial barrier function is unknown. We studied the role of AKAPs in regulating human bronchial epithelial (Hogg JC, Timens W. Annu Rev Pathol 4: 435-459, 2009; HBE) barrier. Cigarette smoke extract (CSE) reduced barrier function in 16HBE cells and the expression of the adhesion molecule E-cadherin specifically at the cell membrane. In addition, CSE reduced the protein expression of the AKAP family member AKAP9 at the cell membrane. The expression of AKAP5 and AKAP12 was unaffected by CSE. AKAP9 interacted and colocalized with E-cadherin at the cell membrane, suggesting that the reduction of both proteins may be related. Interestingly, disruption of AKAP-PKA interactions by st-Ht31 prevented the CSE-induced reduction of E-cadherin and AKAP9 protein expression and subsequent loss of barrier function. Silencing of AKAP9 reduced the functional epithelial barrier and prevented the ability of st-Ht31 to restore membrane localization of E-cadherin. Our data suggest the possibility of a specific role for AKAP9 in the maintenance of the epithelial barrier. E-cadherin, but not AKAP9, protein expression was reduced in lung tissue from COPD patients compared with controls. However, AKAP9 mRNA expression was decreased in primary bronchial epithelial cells from current smokers compared with non/ex-smokers. In conclusion, our results indicate that AKAP proteins, most likely AKAP9, maintain the bronchial epithelial barrier by regulating the E-cadherin expression at the cell membrane.

Exchange protein activated by cAMP (Epac) induces vascular relaxation by activating Ca2+-sensitive K+ channels in rat mesenteric artery

Vasodilator-induced elevation of intracellular cyclic AMP (cAMP) is a central mechanism governing arterial relaxation but is incompletely understood due to the diversity of cAMP effectors. Here we investigate the role of the novel cAMP effector exchange protein directly activated by cAMP (Epac) in mediating vasorelaxation in rat mesenteric arteries. In myography experiments, the Epac-selective cAMP analogue 8-pCPT-2-O-Me-cAMP-AM (5 μM, subsequently referred to as 8-pCPT-AM) elicited a 77.6 ± 7.1% relaxation of phenylephrine-contracted arteries over a 5 min period (mean ± SEM; n = 6). 8-pCPT-AM induced only a 16.7 ± 2.4% relaxation in arteries pre-contracted with high extracellular K(+) over the same time period (n = 10), suggesting that some of Epac's relaxant effect relies upon vascular cell hyperpolarization. This involves Ca(2+)-sensitive, large-conductance K(+) (BK(Ca)) channel opening as iberiotoxin (100 nM) significantly reduced the ability of 8-pCPT-AM to reverse phenylephrine-induced contraction (arteries relaxed by only 35.0 ± 8.5% over a 5 min exposure to 8-pCPT-AM, n = 5; P < 0.05). 8-pCPT-AM increased Ca(2+) spark frequency in Fluo-4-AM-loaded mesenteric myocytes from 0.045 ± 0.008 to 0.103 ± 0.022 sparks s(-1) μm(-1) (P < 0.05) and reversibly increased both the frequency (0.94 ± 0.25 to 2.30 ± 0.72 s(-1)) and amplitude (23.9 ± 3.3 to 35.8 ± 7.7 pA) of spontaneous transient outward currents (STOCs) recorded in isolated mesenteric myocytes (n = 7; P < 0.05). 8-pCPT-AM-activated STOCs were sensitive to iberiotoxin (100 nM) and to ryanodine (30 μM). Current clamp recordings of isolated myocytes showed a 7.9 ± 1.0 mV (n = 10) hyperpolarization in response to 8-pCPT-AM that was sensitive to iberiotoxin (n = 5). Endothelial disruption suppressed 8-pCPT-AM-mediated relaxation in phenylephrine-contracted arteries (24.8 ± 4.9% relaxation after 5 min of exposure, n = 5; P < 0.05), as did apamin and TRAM-34, blockers of Ca(2+)-sensitive, small- and intermediate-conductance K(+) (SK(Ca) and IK(Ca)) channels, respectively, and N(G)-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase (NOS). In Fluo-4-AM-loaded mesenteric endothelial cells, 8-pCPT-AM induced a sustained increase in global Ca(2+). Our data suggest that Epac hyperpolarizes smooth muscle by (1) increasing localized Ca(2+) release from ryanodine receptors (Ca(2+) sparks) to activate BK(Ca) channels, and (2) endothelial-dependent mechanisms involving the activation of SK(Ca)/IK(Ca) channels and NOS. Epac-mediated smooth muscle hyperpolarization will limit Ca(2+) entry via voltage-sensitive Ca(2+) channels and represents a novel mechanism of arterial relaxation.

Correlation between pathogenesis of dampness syndrome and interleukin-2, interleukin-8 in rats

OBJECTIVE

To explore the pathogenesis of dampness syndrome by detecting the changes of interleukin-2 (IL-2) and interleukin-8 (IL-8) levels.

METHODS

Female Sprague Dawley rats were divided randomly into five groups according to the random number table: a normal group (Group I), an external dampness group (Group II), an internal dampness group (Group III), and an external and internal dampness group (Group IV). Twenty days after the model made, IL-2 and IL-8 levels were detected by radioimmunoassay method.

RESULTS

The IL-2 and IL-8 levels among groups were significant (F = 3.102, P < 0.05; F = 2.657, P < 0.05, respectively). The level of IL-2 in Group II and Group III were higher than that in Group I (P < 0.05, P < 0.01, respectively), especially higher in the Group III compared with Group II (P < 0.05). The level of IL-8 in Group III were higher than those in Group I, Group II and Group IV (P < 0.05, P < 0.01, P < 0.05, respectively). In the Group III, the 24-hour water and body weight were higher than that in the Group IV (all P < 0.05), and spontaneous movement frequency was higher than those in Group II and Group IV (P < 0.05).

CONCLUSION

Immune activation and inflammatory reaction might be easily caused by external dampness other than internal dampness.

Exchange protein directly activated by cAMP (epac): a multidomain cAMP mediator in the regulation of diverse biological functions

Since the discovery nearly 60 years ago, cAMP is envisioned as one of the most universal and versatile second messengers. The tremendous feature of cAMP to tightly control highly diverse physiologic processes, including calcium homeostasis, metabolism, secretion, muscle contraction, cell fate, and gene transcription, is reflected by the award of five Nobel prizes. The discovery of Epac (exchange protein directly activated by cAMP) has ignited a new surge of cAMP-related research and has depicted novel cAMP properties independent of protein kinase A and cyclic nucleotide-gated channels. The multidomain architecture of Epac determines its activity state and allows cell-type specific protein-protein and protein-lipid interactions that control fine-tuning of pivotal biologic responses through the "old" second messenger cAMP. Compartmentalization of cAMP in space and time, maintained by A-kinase anchoring proteins, phosphodiesterases, and β-arrestins, contributes to the Epac signalosome of small GTPases, phospholipases, mitogen- and lipid-activated kinases, and transcription factors. These novel cAMP sensors seem to implement certain unexpected signaling properties of cAMP and thereby to permit delicate adaptations of biologic responses. Agonists and antagonists selective for Epac are developed and will support further studies on the biologic net outcome of the activation of Epac. This will increase our current knowledge on the pathophysiology of devastating diseases, such as diabetes, cognitive impairment, renal and heart failure, (pulmonary) hypertension, asthma, and chronic obstructive pulmonary disease. Further insights into the cAMP dynamics executed by the Epac signalosome will help to optimize the pharmacological treatment of these diseases.

cAMP regulation of airway smooth muscle function

Agonists activating β(2)-adrenoceptors (β(2)ARs) on airway smooth muscle (ASM) are the drug of choice for rescue from acute bronchoconstriction in patients with both asthma and chronic obstructive pulmonary disease (COPD). Moreover, the use of long-acting β-agonists combined with inhaled corticosteroids constitutes an important maintenance therapy for these diseases. β-Agonists are effective bronchodilators due primarily to their ability to antagonize ASM contraction. The presumed cellular mechanism of action involves the generation of intracellular cAMP, which in turn can activate the effector molecules cAMP-dependent protein kinase (PKA) and Epac. Other agents such as prostaglandin E(2) and phosphodiesterase inhibitors that also increase intracellular cAMP levels in ASM, can also antagonize ASM contraction, and inhibit other ASM functions including proliferation and migration. Therefore, β(2)ARs and cAMP are key players in combating the pathophysiology of airway narrowing and remodeling. However, limitations of β-agonist therapy due to drug tachyphylaxis related to β(2)AR desensitization, and recent findings regarding the manner in which β(2)ARs and cAMP signal, have raised new and interesting questions about these well-studied molecules. In this review we discuss current concepts regarding β(2)ARs and cAMP in the regulation of ASM cell functions and their therapeutic roles in asthma and COPD.

Multiple facets of cAMP signalling and physiological impact: cAMP compartmentalization in the lung

Therapies involving elevation of the endogenous suppressor cyclic AMP (cAMP) are currently used in the treatment of several chronic inflammatory disorders, including chronic obstructive pulmonary disease (COPD). Characteristics of COPD are airway obstruction, airway inflammation and airway remodelling, processes encompassed by increased airway smooth muscle mass, epithelial changes, goblet cell and submucosal gland hyperplasia. In addition to inflammatory cells, airway smooth muscle cells and (myo)fibroblasts, epithelial cells underpin a variety of key responses in the airways such as inflammatory cytokine release, airway remodelling, mucus hypersecretion and airway barrier function. Cigarette smoke, being next to environmental pollution the main cause of COPD, is believed to cause epithelial hyperpermeability by disrupting the barrier function. Here we will focus on the most recent progress on compartmentalized signalling by cAMP. In addition to G protein-coupled receptors, adenylyl cyclases, cAMP-specific phospho-diesterases (PDEs) maintain compartmentalized cAMP signalling. Intriguingly, spatially discrete cAMP-sensing signalling complexes seem also to involve distinct members of the A-kinase anchoring (AKAP) superfamily and IQ motif containing GTPase activating protein (IQGAPs). In this review, we will highlight the interaction between cAMP and the epithelial barrier to retain proper lung function and to alleviate COPD symptoms and focus on the possible molecular mechanisms involved in this process. Future studies should include the development of cAMP-sensing multiprotein complex specific disruptors and/or stabilizers to orchestrate cellular functions. Compartmentalized cAMP signalling regulates important cellular processes in the lung and may serve as a therapeutic target.

Distinct PKA and Epac compartmentalization in airway function and plasticity

Asthma and chronic obstructive pulmonary disease (COPD) are obstructive lung diseases characterized by airway obstruction, airway inflammation and airway remodelling. Next to inflammatory cells and airway epithelial cells, airway mesenchymal cells, including airway smooth muscle cells and (myo)fibroblasts, substantially contribute to disease features by the release of inflammatory mediators, smooth muscle contraction, extracellular matrix deposition and structural changes in the airways. Current pharmacological treatment of both diseases intends to target the dynamic features of the endogenous intracellular suppressor cyclic AMP (cAMP). This review will summarize our current knowledge on cAMP and will emphasize on key discoveries and paradigm shifts reflecting the complex spatio-temporal nature of compartmentalized cAMP signalling networks in health and disease. As airway fibroblasts and airway smooth muscle cells are recognized as central players in the development and progression of asthma and COPD, we will focus on the role of cAMP signalling in their function in relation to airway function and plasticity. We will recapture on the recent identification of cAMP-sensing multi-protein complexes maintained by cAMP effectors, including A-kinase anchoring proteins (AKAPs), proteins kinase A (PKA), exchange protein directly activated by cAMP (Epac), cAMP-elevating seven-transmembrane (7TM) receptors and phosphodiesterases (PDEs) and we will report on findings indicating that the pertubation of compartmentalized cAMP signalling correlates with the pathopysiology of obstructive lung diseases. Future challenges include studies on cAMP dynamics and compartmentalization in the lung and the development of novel drugs targeting these systems for therapeutic interventions in chronic obstructive inflammatory diseases.

Thiazolidinediones inhibit airway smooth muscle release of the chemokine CXCL10: in vitro comparison with current asthma therapies

BACKGROUND

Activated mast cells are present within airway smooth muscle (ASM) bundles in eosinophilic asthma. ASM production of the chemokine CXCL10 plays a role in their recruitment. Thus the effects of glucocorticoids (fluticasone, budesonide), long-acting β2-agonists (salmeterol, formoterol) and thiazolidinediones (ciglitazone, rosiglitazone) on CXCL10 production by ASM cells (ASMC) from people with and without asthma were investigated in vitro.

METHODS

Confluent serum-deprived cells were treated with the agents before and during cytokine stimulation for 0-24 h. CXCL10 protein/mRNA, IκB-α levels and p65 activity were measured using ELISA, RT PCR, immunoblotting and p65 activity assays respectively. Data were analysed using ANOVA followed by Fisher's post-hoc test.

RESULTS

Fluticasone and/or salmeterol at 1 and 100 nM inhibited CXCL10 release induced by IL-1β and TNF-α, but not IFNγ or all three cytokines (cytomix). The latter was also not affected by budesonide and formoterol. In asthmatic ASMC low salmeterol, but not formoterol, concentrations increased cytomix-induced CXCL10 release and at 0.01 nM enhanced NF-κB activity. Salmeterol 0.1 nM together with fluticasone 0.1 and 10 nM still increased CXCL10 release. The thiazolidinediones ciglitazone and rosiglitazone (at 25 and 100 μM) inhibited cytomix-induced CXCL10 release but these inhibitory effects were not prevented by the PPAR-g antagonist GW9662. Ciglitazone did not affect early NF-κB activity and CXCL10 mRNA production.

CONCLUSIONS

Thus the thiazolidinediones inhibited asthmatic ASMC CXCL10 release under conditions when common asthma therapies were ineffective or enhanced it. They may provide an alternative strategy to reduce mast cell-ASM interactions and restore normal airway physiology in asthma.

Models to study airway smooth muscle contraction in vivo, ex vivo and in vitro: implications in understanding asthma

Asthma is a chronic obstructive airway disease characterised by airway hyperresponsiveness (AHR) and airway wall remodelling. The effector of airway narrowing is the contraction of airway smooth muscle (ASM), yet the question of whether an inherent or acquired dysfunction in ASM contractile function plays a significant role in the disease pathophysiology remains contentious. The difficulty in determining the role of ASM lies in limitations with the models used to assess contraction. In vivo models provide a fully integrated physiological response but ASM contraction cannot be directly measured. Ex vivo and in vitro models can provide more direct assessment of ASM contraction but the loss of factors that may modulate ASM responsiveness and AHR, including interaction between multiple cell types and disruption of the mechanical environment, precludes a complete understanding of the disease process. In this review we detail key advantages of common in vivo, ex vivo and in vitro models of ASM contraction, as well as emerging tissue engineered models of ASM and whole airways. We also highlight important findings from each model with respect to the pathophysiology of asthma.

Signaling through the neuropeptide GPCR PAC₁ induces neuritogenesis via a single linear cAMP- and ERK-dependent pathway using a novel cAMP sensor

Both cAMP and ERK are necessary for neuroendocrine cell neuritogenesis, and pituitary adenylate cyclase-activating polypeptide (PACAP) activates each. It is important to know whether cAMP and ERK are arranged in a novel, linear pathway or in two parallel pathways using known signaling mechanisms. Native cellular responses [cAMP elevation, ERK phosphorylation, cAMP responsive element binding (CREB) phosphorylation, and neuritogenesis] and promoter-reporter gene activation after treatment with forskolin, cAMP analogs, and PACAP were measured in Neuroscreen-1 (NS-1) cells, a PC12 variant enabling simultaneous morphological, molecular biological, and biochemical analysis. Forskolin (25 μM) and cAMP analogs (8-bromo-cAMP, dibutyryl-cAMP, and 8-chlorophenylthio-cAMP) stimulated ERK phosphorylation and neuritogenesis in NS-1 cells. Both ERK phosphorylation and neuritogenesis were MEK dependent (blocked by 10 μM U0126) and PKA independent (insensitive to 30 μM H-89 or 100 nM myristoylated protein kinase A inhibitor). CREB phosphorylation induced by PACAP was blocked by H-89. The exchange protein activated by cAMP (Epac)-selective 8-(4-chlorophenylthio)-2'-O-Me-cAMP (100-500 μM) activated Rap1 without affecting the other cAMP-dependent processes. Thus, PACAP-38 potently stimulated two distinct and independent cAMP pathways leading to CREB or ERK activation in NS-1 cells. Drug concentrations for appropriate effect were derived from control data for all compounds. In summary, a novel PKA- and Epac-independent signaling pathway: PACAP → adenylate cyclase → cAMP → ERK → neuritogenesis has been identified.

Anti-inflammatory role of the cAMP effectors Epac and PKA: implications in chronic obstructive pulmonary disease

Cigarette smoke-induced release of pro-inflammatory cytokines including interleukin-8 (IL-8) from inflammatory as well as structural cells in the airways, including airway smooth muscle (ASM) cells, may contribute to the development of chronic obstructive pulmonary disease (COPD). Despite the wide use of pharmacological treatment aimed at increasing intracellular levels of the endogenous suppressor cyclic AMP (cAMP), little is known about its exact mechanism of action. We report here that next to the β(2)-agonist fenoterol, direct and specific activation of either exchange protein directly activated by cAMP (Epac) or protein kinase A (PKA) reduced cigarette smoke extract (CSE)-induced IL-8 mRNA expression and protein release by human ASM cells. CSE-induced IκBα-degradation and p65 nuclear translocation, processes that were primarily reversed by Epac activation. Further, CSE increased extracellular signal-regulated kinase (ERK) phosphorylation, which was selectively reduced by PKA activation. CSE decreased Epac1 expression, but did not affect Epac2 and PKA expression. Importantly, Epac1 expression was also reduced in lung tissue from COPD patients. In conclusion, Epac and PKA decrease CSE-induced IL-8 release by human ASM cells via inhibition of NF-κB and ERK, respectively, pointing at these cAMP effectors as potential targets for anti-inflammatory therapy in COPD. However, cigarette smoke exposure may reduce anti-inflammatory effects of cAMP elevating agents via down-regulation of Epac1.

Epac as a novel effector of airway smooth muscle relaxation

Dysfunctional regulation of airway smooth muscle tone is a feature of obstructive airway diseases such as asthma and chronic obstructive pulmonary disease. Airway smooth muscle contraction is directly associated with changes in the phosphorylation of myosin light chain (MLC), which is increased by Rho and decreased by Rac. Although cyclic adenosine monophosphate (cAMP)-elevating agents are believed to relieve bronchoconstriction mainly via activation of protein kinase A (PKA), here we addressed the role of the novel cAMP-mediated exchange protein Epac in the regulation of airway smooth muscle tone. Isometric tension measurements showed that specific activation of Epac led to relaxation of guinea pig tracheal preparations pre-contracted with methacholine, independently of PKA. In airway smooth muscle cells, Epac activation reduced methacholine-induced MLC phosphorylation. Moreover, when Epac was stimulated, we observed a decreased methacholine-induced RhoA activation, measured by both stress fibre formation and pull-down assay whereas the same Epac activation prevented methacholine-induced Rac1 inhibition measured by pull-down assay. Epac-driven inhibition of both methacholine-induced muscle contraction by Toxin B-1470, and MLC phosphorylation by the Rac1-inhibitor NSC23766, were significantly attenuated, confirming the importance of Rac1 in Epac-mediated relaxation. Importantly, human airway smooth muscle tissue also expresses Epac, and Epac activation both relaxed pre-contracted human tracheal preparations and decreased MLC phosphorylation. Collectively, we show that activation of Epac relaxes airway smooth muscle by decreasing MLC phosphorylation by skewing the balance of RhoA/Rac1 activation towards Rac1. Therefore, activation of Epac may have therapeutical potential in the treatment of obstructive airway diseases.

Protein kinase A and the exchange protein directly activated by cAMP (Epac) modulate phenotype plasticity in human airway smooth muscle

BACKGROUND AND PURPOSE

Platelet-derived growth factor (PDGF) modulates the airway smooth muscle (ASM) 'contractile' phenotype to a more 'proliferative' phenotype, resulting in increased proliferation and reduced contractility. Such phenotypic modulation may contribute to airway remodelling in asthma. We have previously shown that the cAMP effector molecules, protein kinase A (PKA) and the exchange protein directly activated by cAMP (Epac) inhibited PDGF-induced phenotypic modulation in bovine ASM. Here, we investigated these mechanisms in human ASM strips and cells.

EXPERIMENTAL APPROACH

ASM strips were incubated with PDGF in the absence or presence of the activators of Epac (8-pCPT-2'-O-Me-cAMP) or of PKA (6-Bnz-cAMP) for 4 days. Strips were mounted for isometric contraction experiments or analysed for the expression of contractile markers. Cell proliferation was measured and proliferative markers were analysed under similar conditions.

KEY RESULTS

Activation of Epac and PKA prevented PDGF-induced ASM strip hypocontractility, and restored the expression of smooth muscle actin, myosin and calponin, which had been markedly diminished by PDGF. Epac and PKA activation inhibited the PDGF-induced ASM cell proliferation and G(1)/S phase transition and the expression and phosphorylation of cell cycle regulators.

CONCLUSIONS AND IMPLICATIONS

Epac and PKA maintain a normally contractile ASM phenotype in a mitogenic environment, suggesting that specific activators of Epac and PKA may be beneficial in the treatment of airway remodelling in asthma.

Real time analysis of β(2)-adrenoceptor-mediated signaling kinetics in human primary airway smooth muscle cells reveals both ligand and dose dependent differences

BACKGROUND

β2-adrenoceptor agonists elicit bronchodilator responses by binding to β2-adrenoceptors on airway smooth muscle (ASM). In vivo, the time between drug administration and clinically relevant bronchodilation varies significantly depending on the agonist used. Our aim was to utilise a fluorescent cyclic AMP reporter probe to study the temporal profile of β2-adrenoceptor-mediated signaling induced by isoproterenol and a range of clinically relevant agonists in human primary ASM (hASM) cells by using a modified Epac protein fused to CFP and a variant of YFP.

METHODS

Cells were imaged in real time using a spinning disk confocal system which allowed rapid and direct quantification of emission ratio imaging following direct addition of β2-adrenoceptor agonists (isoproterenol, salbutamol, salmeterol, indacaterol and formoterol) into the extracellular buffer. For pharmacological comparison a radiolabeling assay for whole cell cyclic AMP formation was used.

RESULTS

Temporal analysis revealed that in hASM cells the β2-adrenoceptor agonists studied did not vary significantly in the onset of initiation. However, once a response was initiated, significant differences were observed in the rate of this response with indacaterol and isoproterenol inducing a significantly faster response than salmeterol. Contrary to expectation, reducing the concentration of isoproterenol resulted in a significantly faster initiation of response.

CONCLUSIONS

We conclude that confocal imaging of the Epac-based probe is a powerful tool to explore β2-adrenoceptor signaling in primary cells. The ability to analyse the kinetics of clinically used β2-adrenoceptor agonists in real time and at a single cell level gives an insight into their possible kinetics once they have reached ASM cells in vivo.

Does airway smooth muscle express an inflammatory phenotype in asthma?

In addition to hyperresponsiveness in asthma, airway smooth muscle (ASM) also manifests an inflammatory phenotype characterized by augmented expression of mediators that enhance inflammation, contribute to tissue remodelling and augment leucocyte trafficking and activity. Our present review summarizes contemporary understanding of ASM-derived mediators and their paracrine and autocrine actions in airway diseases.

cAMP inhibits modulation of airway smooth muscle phenotype via the exchange protein activated by cAMP (Epac) and protein kinase A

BACKGROUND AND PURPOSE

Changes in airway smooth muscle (ASM) phenotype may contribute to the pathogenesis of airway disease. Platelet-derived growth factor (PDGF) switches ASM from a contractile to a proliferative, hypo-contractile phenotype, a process requiring activation of extracellular signal-regulated kinase (ERK) and p70(S6) Kinase (p70(S6K) ). The effects of cAMP-elevating agents on these processes is unknown. Here, we investigated the effects of cAMP elevation by prostaglandin E(2) (PGE(2) ) and the activation of the cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP (Epac) on PDGF-induced phenotype switching in bovine tracheal smooth muscle (BTSM).

EXPERIMENTAL APPROACH

Effects of long-term treatment with the PGE(2) analogue 16,16-dimethyl-PGE(2) , the selective Epac activator, 8-pCPT-2'-O-Me-cAMP and the selective PKA activator, 6-Bnz-cAMP were assessed on the induction of a hypo-contractile, proliferative BTSM phenotype and on activation of ERK and p70(S6K) , both induced by PDGF.

KEY RESULTS

Treatment with 16,16-dimethyl-PGE(2) inhibited PDGF-induced proliferation of BTSM cells and maintained BTSM strip contractility and contractile protein expression in the presence of PDGF. Activation of both Epac and PKA similarly prevented PDGF-induced phenotype switching and PDGF-induced activation of ERK. Interestingly, only PKA activation resulted in inhibition of PDGF-induced phosphorylation of p70(S6K) .

CONCLUSIONS AND IMPLICATIONS

Our data indicate for the first time that both Epac and PKA regulated switching of ASM phenotype via differential inhibition of ERK and p70(S6K) pathways. These findings suggest that cAMP elevation may be beneficial in the treatment of long-term changes in airway disease.

Anti-mitogenic effects of β-agonists and PGE2 on airway smooth muscle are PKA dependent

Inhaled β-agonists are effective airway smooth muscle (ASM)-relaxing agents that help reverse bronchoconstriction in asthma, but their ability to affect the aberrant ASM growth that also occurs with asthma is poorly understood. β-Agonists exhibit PKA-dependent antimitogenic effects in several cell types. However, recent studies suggest that Epac, and not PKA, mediates the antimitogenic effect of cAMP in both ASM and fibroblasts. This study aims to clarify the role of PKA in mediating the effect of G(s)-coupled receptors on human ASM growth. Pretreatment of ASM cultures with β-agonists albuterol, isoproterenol, or salmeterol (100 nM to 10 μM) caused a significant (∼ 25-30%) inhibition of EGF-stimulated ASM thymidine incorporation and cell proliferation, whereas a much greater inhibition was observed from pretreatment with PGE(2) (75-80%). However, all agents were ineffective in cells expressing GFP chimeras of either PKI (a PKA inhibitor) or a mutant PKA regulatory subunit relative to the control cells expressing GFP. The antimitogenic efficacy of PGE(2) in inhibiting control cultures was associated with greater ability to stimulate sustained PKA activation and greater inhibition of late-phase promitogenic p42/p44 and PI3K activities. These findings suggest that therapeutic approaches enabling superior PKA activation in ASM will be most efficacious in deterring ASM growth.