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

GNAS mutation inhibits growth and induces phosphodiesterase 4D expression in colorectal cancer cell lines

Approximately 5% of colorectal cancers (CRCs) have a gain-of-function mutation in the GNAS gene, which leads to the activation of cAMP-dependent signaling pathways and associates with poor prognosis. We investigated the effect of an activating GNAS mutation in CRC cell lines on gene expression and cell proliferation in vitro, and tumor growth in vivo. GNAS-mutated (GNASmt) HCT116 cells showed stimulated synthesis of cAMP as compared to parental (Par) cells. The most upregulated gene in the GNASmt cells was cAMP-hydrolyzing phosphodiesterase 4D (PDE4D) as detected by RNA sequencing. To further validate our finding, we analyzed PDE4D expression in a set of human CRC tumors (n = 35) and demonstrated overexpression in GNAS mutant CRC tumors as compared to GNAS wild-type tumors. The GNASmt HCT116 cells proliferated more slowly than the Par cells. PDE4 inhibitor Ro 20-1724 and PDE4D subtype selective inhibitor GEBR-7b further suppressed the proliferation of GNASmt cells without an effect on Par cells. The growth inhibitory effect of these inhibitors was also seen in the intrinsically GNAS-mutated SK-CO-1 CRC cell line having high levels of cAMP synthesis and PDE4D expression. In vivo, GNASmt HCT116 cells formed smaller tumors than the Par cells in nude mice. In conclusion, our findings demonstrate that GNAS mutation results in the growth suppression of CRC cells. Moreover, the GNAS mutation-induced overexpression of PDE4D provides a potential avenue to impede the proliferation of CRC cells through the use of PDE4 inhibitors.

Theophylline-Induced Relaxation Is Enhanced after Testosterone Treatment via Increased KV1.2 and KV1.5 Protein Expression in Guinea Pig Tracheal Smooth Muscle

Theophylline is a drug commonly used to treat asthma due to its anti-inflammatory and bronchodilatory properties. Testosterone (TES) has been suggested to reduce the severity of asthma symptoms. This condition affects boys more than girls in childhood, and this ratio reverses at puberty. We reported that guinea pig tracheal tissue chronic exposure to TES increases the expression of β₂-adrenoreceptors and enhances salbutamol-induced K⁺ currents (IK⁺). Herein, we investigated whether the upregulation of K⁺ channels can enhance the relaxation response to methylxanthines, including theophylline. Chronic incubation of guinea pig tracheas with TES (40 nM, 48 h) enhanced the relaxation induced by caffeine, isobutylmethylxanthine, and theophylline, an effect that was abolished by tetraethylammonium. In tracheal myocytes, chronic incubation with TES increased theophylline-induced IK⁺; flutamide reversed this effect. The increase in IK⁺ was blocked by 4-aminopyridine by ~82%, whereas iberiotoxin reduced IK⁺ by ~17%. Immunofluorescence studies showed that chronic TES exposure increased the expression of K_V1.2 and K_V1.5 in airway smooth muscle (ASM). In conclusion, chronic exposure to TES in guinea pig ASM promotes upregulation of K_V1.2 and K_V1.5 and enhances theophylline relaxation response. Therefore, gender should be considered when prescribing methylxanthines, as teenage boys and males are likely to respond better than females.

PDE4B Is a Homeostatic Regulator of Cyclic AMP in Dendritic Cells

Chronic decreases in the second messenger cyclic AMP (cAMP) occur in numerous settings, but how cells compensate for such decreases is unknown. We have used a unique system-murine dendritic cells (DCs) with a DC-selective depletion of the heterotrimeric GTP binding protein Gαs-to address this issue. These mice spontaneously develop Th2-allergic asthma and their DCs have persistently lower cAMP levels. We found that phosphodiesterase 4B (PDE4B) is the primary phosphodiesterase expressed in DCs and that its expression is preferentially decreased in Gαs-depleted DCs. PDE4B expression is dynamic, falling and rising in a protein kinase A-dependent manner with decreased and increased cAMP concentrations, respectively. Treatment of DCs that drive enhanced Th2 immunity with a PDE4B inhibitor ameliorated DC-induced helper T cell response. We conclude that PDE4B is a homeostatic regulator of cellular cAMP concentrations in DCs and may be a target for treating Th2-allergic asthma and other settings with low cellular cAMP concentrations.

The Molecular Biology of Phosphodiesterase 4 Enzymes as Pharmacological Targets: An Interplay of Isoforms, Conformational States, and Inhibitors

The phosphodiesterase 4 (PDE4) enzyme family plays a pivotal role in regulating levels of the second messenger cAMP. Consequently, PDE4 inhibitors have been investigated as a therapeutic strategy to enhance cAMP signaling in a broad range of diseases, including several types of cancers, as well as in various neurologic, dermatological, and inflammatory diseases. Despite their widespread therapeutic potential, the progression of PDE4 inhibitors into the clinic has been hampered because of their related relatively small therapeutic window, which increases the chance of producing adverse side effects. Interestingly, the PDE4 enzyme family consists of several subtypes and isoforms that can be modified post-translationally or can engage in specific protein-protein interactions to yield a variety of conformational states. Inhibition of specific PDE4 subtypes, isoforms, or conformational states may lead to more precise effects and hence improve the safety profile of PDE4 inhibition. In this review, we provide an overview of the variety of PDE4 isoforms and how their activity and inhibition is influenced by post-translational modifications and interactions with partner proteins. Furthermore, we describe the importance of screening potential PDE4 inhibitors in view of different PDE4 subtypes, isoforms, and conformational states rather than testing compounds directed toward a specific PDE4 catalytic domain. Lastly, potential mechanisms underlying PDE4-mediated adverse effects are outlined. In this review, we illustrate that PDE4 inhibitors retain their therapeutic potential in myriad diseases, but target identification should be more precise to establish selective inhibition of disease-affected PDE4 isoforms while avoiding isoforms involved in adverse effects. SIGNIFICANCE STATEMENT: Although the PDE4 enzyme family is a therapeutic target in an extensive range of disorders, clinical use of PDE4 inhibitors has been hindered because of the adverse side effects. This review elaborately shows that safer and more effective PDE4 targeting is possible by characterizing 1) which PDE4 subtypes and isoforms exist, 2) how PDE4 isoforms can adopt specific conformations upon post-translational modifications and protein-protein interactions, and 3) which PDE4 inhibitors can selectively bind specific PDE4 subtypes, isoforms, and/or conformations.

The cAMP-phosphodiesterase 4 (PDE4) controls β-adrenoceptor- and CFTR-dependent saliva secretion in mice

Saliva, while often taken for granted, is indispensable for oral health and overall well-being, as inferred from the significant impairments suffered by patients with salivary gland dysfunction. Here, we show that treatment with several structurally distinct PAN-PDE4 inhibitors, but not a PDE3 inhibitor, induces saliva secretion in mice, indicating it is a class-effect of PDE4 inhibitors. In anesthetized mice, while neuronal regulations are suppressed, PDE4 inhibition potentiates a β-adrenoceptor-induced salivation, that is ablated by the β-blocker Propranolol and is absent from homozygous ΔF508-CFTR mice lacking functional CFTR. These data suggest that PDE4 acts within salivary glands to gate saliva secretion that is contingent upon the cAMP/PKA-dependent activation of CFTR. Indeed, PDE4 contributes the majority of total cAMP-hydrolytic capacity in submandibular-, sublingual-, and parotid glands, the three major salivary glands of the mouse. In awake mice, PDE4 inhibitor-induced salivation is reduced by CFTR deficiency or β-blockers, but also by the muscarinic blocker Atropine, suggesting an additional, central/neuronal mechanism of PDE4 inhibitor action. The PDE4 family comprises four subtypes, PDE4A-D. Ablation of PDE4D, but not PDE4A-C, produced a minor effect on saliva secretion, implying that while PDE4D may play a predominant role, PDE4 inhibitor-induced salivation results from the concurrent inactivation of multiple (at least two) PDE4 subtypes. Taken together, our data reveal a critical role for PDE4/PDE4D in controlling CFTR function in an in vivo model and in inducing salivation, hinting at a therapeutic potential of PDE4 inhibition for cystic fibrosis and conditions associated with xerostomia.

Phosphodiesterase isoforms and cAMP compartments in the development of new therapies for obstructive pulmonary diseases

The second messenger molecule 3'5'-cyclic adenosine monophosphate (cAMP) imparts several beneficial effects in lung diseases such as asthma, chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). While cAMP is bronchodilatory in asthma and COPD, it also displays anti-fibrotic properties that limit fibrosis. Phosphodiesterases (PDEs) metabolize cAMP and thus regulate cAMP signaling. While some existing therapies inhibit PDEs, there are only broad family specific inhibitors. The understanding of cAMP signaling compartments, some centered around lipid rafts/caveolae, has led to interest in defining how specific PDE isoforms maintain these signaling microdomains. The possible altered expression of PDEs, and thus abnormal cAMP signaling, in obstructive lung diseases has been poorly explored. We propose that inhibition of specific PDE isoforms can improve therapy of obstructive lung diseases by amplifying specific cAMP signals in discreet microdomains.

Specificity of NHERF1 regulation of GPCR signaling and function in human airway smooth muscle

Na+/H+ exchanger regulatory factor 1 (NHERF1; also known as ezrin-radixin-moesin-binding phosphoprotein 50) is a PSD-95, disc large, zona occludens-1 adapter that acts as a scaffold for signaling complexes and cytoskeletal-plasma membrane interactions. NHERF1 is crucial to β-2-adrenoceptor (β2AR)-mediated activation of cystic fibrosis transmembrane conductance regulator (CFTR) in epithelial cells, and NHERF1 has been proposed to mediate the recycling of internalized β2AR back to the cell membrane. In the current study, we assessed the role of NHERF1 in regulating cAMP-mediated signaling and immunomodulatory functions in airway smooth muscle (ASM). NHERF1 knockdown attenuated the induction of (protein kinase A) phospho-vasodilator-stimulated phosphoprotein (p-VASP) by isoproterenol (ISO), prostaglandin E2 (PGE2), or forskolin (FSK) as well as the induction of p-heat shock protein 20 after 4 h of stimulation with ISO and FSK. NHERF1 knockdown fully abrogated the ISO-, PGE2-, and FSK-induced IL-6 gene expression and cytokine production without affecting cAMP-mediated phosphodiesterase 4D (PDE4D) gene expression, phospho-cAMP response element-binding protein (p-CREB), and cAMP response element (CRE)-Luc, or PDGF-induced cyclin D1 expression. Interestingly, NHERF1 knockdown prevented ISO-induced chromatin-binding of the transcription factor CCAAT-enhancer-binding protein-β (c/EBPβ). c/EBPβ knockdown almost completely abrogated the cAMP-mediated IL-6 but not PDE4D gene expression. The differential regulation of cAMP-induced signaling and gene expression in our study indicates a role for NHERF1 in the compartmentalization of cAMP signaling in ASM.-Pera, T., Tompkins, E., Katz, M., Wang, B., Deshpande, D. A., Weinman, E. J., Penn, R. B. Specificity of NHERF1 regulation of GPCR signaling and function in human airway smooth muscle.

Small-molecule allosteric activators of PDE4 long form cyclic AMP phosphodiesterases

Cyclic AMP (cAMP) phosphodiesterase-4 (PDE4) enzymes degrade cAMP and underpin the compartmentalization of cAMP signaling through their targeting to particular protein complexes and intracellular locales. We describe the discovery and characterization of a small-molecule compound that allosterically activates PDE4 long isoforms. This PDE4-specific activator displays reversible, noncompetitive kinetics of activation (increased V _max with unchanged K _m), phenocopies the ability of protein kinase A (PKA) to activate PDE4 long isoforms endogenously, and requires a dimeric enzyme assembly, as adopted by long, but not by short (monomeric), PDE4 isoforms. Abnormally elevated levels of cAMP provide a critical driver of the underpinning molecular pathology of autosomal dominant polycystic kidney disease (ADPKD) by promoting cyst formation that, ultimately, culminates in renal failure. Using both animal and human cell models of ADPKD, including ADPKD patient-derived primary cell cultures, we demonstrate that treatment with the prototypical PDE4 activator compound lowers intracellular cAMP levels, restrains cAMP-mediated signaling events, and profoundly inhibits cyst formation. PDE4 activator compounds thus have potential as therapeutics for treating disease driven by elevated cAMP signaling as well as providing a tool for evaluating the action of long PDE4 isoforms in regulating cAMP-mediated cellular processes.

Inhibition of phosphodiesterase 4D decreases the malignant properties of DLD-1 colorectal cancer cells by repressing the AKT/mTOR/Myc signaling pathway

Colorectal cancer (CRC) is a complex disease involving numerous genetic abnormalities. One of the major characteristics of CRC is enhanced Wnt signaling caused by loss-of-function mutations in the adenomatous polyposis coli (APC) gene. Previously, it has been demonstrated that the majority of malignant phenotypes following APC deletion in adult murine small intestines could be rescued when Myc, a downstream target of the Wnt pathway, was deleted. This indicated that Myc is a critical regulator of CRC development following APC loss. Previous studies reported that cyclic adenosine 3',5'-monophosphate (cAMP) can influence the AKT/mammalian target of rapamycin (mTOR) survival pathway in cancer and Myc is a critical downstream molecule of AKT/mTOR signaling. Phosphodiesterase 4D (PDE4D), a member of the cAMP-specific PDE4 family, has been associated with drug resistance in CRC. However, the association between PDE4D and Myc remains unclear. To investigate the potential role of PDE4D in Myc regulation in CRC, the present study evaluated the expression levels of PDE4 subtypes in DLD-1 CRC cells. Additionally, the effects of PDE4 inhibitors on Myc expression and oncogenic properties were analyzed by western blot analysis, reverse transcription-quantitative polymerase chain reaction, colony formation and soft agar assays. It was demonstrated that cAMP/PDE4D signals serve a critical role in regulating Myc expression in DLD-1 CRC cells. Furthermore, PDE4D was identified to be a main hydrolyzer of cAMP and suppression of PDE4D using selective inhibitors of PDE4 increased intracellular cAMP levels, which resulted in a marked decrease in the oncogenic properties of DLD-1 cells, including colony formation, cell proliferation and anchorage-independent growth. Notably, the current data imply that cAMP represses Myc expression via the downregulation of AKT/mTOR signaling, which was abolished by high PDE4D activities in DLD-1 cells. Additionally, a natural polyphenol resveratrol in combination with forskolin elevated the concentration of cAMP and enhanced the expression of Myc and the malignant phenotype of DLD-1 cells, reproducing the effect of known chemical inhibitors of PDE4. In conclusion, the present study identified that cAMP/PDE4D signaling is a critical regulator of Myc expression in DLD-1 and possibly other CRC cells.

Creating a potential diagnostic for prostate cancer risk stratification (InformMDx™) by translating novel scientific discoveries concerning cAMP degrading phosphodiesterase-4D7 (PDE4D7)

Increased PSA-based screening for prostate cancer has resulted in a growing number of diagnosed cases. However, around half of these are ‘indolent’, neither metastasizing nor leading to disease specific death. Treating non-progressing tumours with invasive therapies is currently regarded as unnecessary over-treatment with patients being considered for conservative regimens, such as active surveillance (AS). However, this raises both compliance and protocol issues. Great clinical benefit could accrue from a biomarker able to predict long-term patient outcome accurately at the time of biopsy and initial diagnosis. Here we delineate the translation of a laboratory discovery through to the precision development of a clinically validated, novel prognostic biomarker assay (InformMDx™). This centres on determining transcript levels for phosphodiesterase-4D7 (PDE4D7), an enzyme that breaks down cyclic AMP, a signalling molecule intimately connected with proliferation and androgen receptor function. Quantifiable detection of PDE4D7 mRNA transcripts informs on the longitudinal outcome of post-surgical disease progression. The risk of post-surgical progression increases steeply for patients with very low ‘PDE4D7 scores’, while risk decreases markedly for those patients with very high ‘PDE4D7 scores’. Combining clinical risk variables, such as the Gleason or CAPRA (Cancer of the Prostate Risk Assessment) score, with the ‘PDE4D7 score’ further enhances the prognostic power of this personalized, precision assessment. Thus the ‘PDE4D7 score’ has the potential to define, more effectively, appropriate medical intervention/AS strategies for individual prostate cancer patients.

The potential role of phosphodiesterase inhibitors in the management of asthma

Asthma is a chronic inflammatory condition characterised by reversible airflow obstruction and airway hyperreactivity. The course of the illness may be punctuated by exacerbations resulting in deterioration in quality of life and, in some cases, days lost from school or work. That asthma is common and increasingly prevalent magnifies the importance of any potential economic costs, and promoting asthma control represents an important public health agenda. While lifestyle changes represent a valuable contribution in some patients, the majority of asthmatic patients require therapeutic intervention. The recognition of the role of inflammation in the pathogenesis of asthma has led to an emphasis on regular anti-inflammatory therapy, of which inhaled corticosteroid treatment remains the most superior. In selected patients, further improvements in asthma control may be gained by the addition of regular inhaled long-acting beta(2)-adrenoceptor agonists or oral leukotriene receptor antagonists to inhaled corticosteroid therapy. However, a significant minority of patients with asthma remain poorly controlled despite appropriate treatment, suggesting that additional corticosteroid nonresponsive inflammatory pathways may be operative. Furthermore, some patients with asthma display an accelerated decline in lung function, suggesting that active airway re-modeling is occurring. Such observations have focused attention on the potential to develop new therapies which complement existing treatments by targeting additional inflammatory pathways. The central role of phosphodiesterase (PDE), and in particular the PDE4 enzyme, in the regulation of key inflammatory cells believed to be important in asthma - including eosinophils, lymphocytes, neutrophils and airway smooth muscle - suggests that drugs designed to target this enzyme will have the potential to deliver both bronchodilation and modulate the asthmatic inflammatory response. In vivo studies on individual inflammatory cells suggest that the effects are likely to be favorable in asthma, and animal study models have provided proof of concept; however, first-generation PDE inhibitors have been poorly tolerated due to adverse effects. The development of second-generation agents such as cilomilast and roflumilast heralds a further opportunity to test the potential of these agents, although to date only a limited amount of data from human studies has been published, making it difficult to draw firm conclusions.

Phosphodiesterase Inhibition and Regulation of Dopaminergic Frontal and Striatal Functioning: Clinical Implications

BACKGROUND

The fronto-striatal circuits are the common neurobiological basis for neuropsychiatric disorders, including schizophrenia, Parkinson's disease, Huntington's disease, attention deficit hyperactivity disorder, obsessive-compulsive disorder, and Tourette's syndrome. Fronto-striatal circuits consist of motor circuits, associative circuits, and limbic circuits. All circuits share 2 common features. First, all fronto-striatal circuits consist of hyper direct, direct, and indirect pathways. Second, all fronto-striatal circuits are modulated by dopamine. Intracellularly, the effect of dopamine is largely mediated through the cyclic adenosine monophosphate/protein kinase A signaling cascade with an additional role for the cyclic guanosine monophosphate/protein kinase G pathway, both of which can be regulated by phosphodiesterases. Phosphodiesterases are thus a potential target for pharmacological intervention in neuropsychiatric disorders related to dopaminergic regulation of fronto-striatal circuits.

METHODS

Clinical studies of the effects of different phosphodiesterase inhibitors on cognition, affect, and motor function in relation to the fronto-striatal circuits are reviewed.

RESULTS

Several selective phosphodiesterase inhibitors have positive effects on cognition, affect, and motor function in relation to the fronto-striatal circuits.

CONCLUSION

Increased understanding of the subcellular localization and unraveling of the signalosome concept of phosphodiesterases including its function and dysfunction in the fronto-striatal circuits will contribute to the design of new specific inhibitors and enhance the potential of phosphodiesterase inhibitors as therapeutics in fronto-striatal circuits.

PDK1/Akt/PDE4D axis identified as a target for asthma remedy synergistic with β2 AR agonists by a natural agent arctigenin

BACKGROUND

Asthma is a heterogenetic disorder characterized by chronic inflammation with variable airflow obstruction and airway hyper-responsiveness. As the most potent and popular bronchodilators, β2 adrenergic receptor (β2 AR) agonists bind to the β2 ARs that are coupled via a stimulatory G protein to adenylyl cyclase, thereby improving cAMP accumulation and resulting in airway smooth muscle relaxation. We previously demonstrated arctigenin had a synergistic function with the β2 AR agonist, but the target for this remained elusive.

METHOD

Chemical proteomics capturing was used to enrich and uncover the target of arctigenin in human bronchial smooth muscle cells, and reverse docking and molecular dynamic stimulation were performed to evaluate the binding of arctigenin and its target. In vitro enzyme activities and protein levels were demonstrated with special kits and Western blotting. Finally, guinea pig tracheal muscle segregation and ex vivo function were analysed.

RESULTS

Arctigenin bound to PDK1 with an ideal binding free energy -25.45 kcal/mol and inhibited PDK1 kinase activity without changing its protein level. Additionally, arctigenin reduced PKB/Akt-induced phosphorylation of PDE4D, which was first identified in this study. Attenuation of PDE4D resulted in cAMP accumulation in human bronchial smooth muscle. The inhibition of PDK1 showed a synergistic function with β2 AR agonists and relaxed the constriction of segregated guinea pig tracheal muscle.

CONCLUSIONS

The PDK1/Akt/PDE4D axis serves as a novel asthma target, which may benefit airflow obstruction.

PDE4D phosphorylation: A coincidence detector integrating multiple signaling pathways

In Eukaryotes, more than 100 different phosphodiesterase (PDE) proteins serve to fine-tune cyclic nucleotide (cAMP and cGMP) signals and contribute to specificity of signaling. In mammals, PDEs are divided into 11 families, of which PDE4 represents the largest family. Four genes (pde4a, pde4b, pde4c and pde4d) encode for this class of enzymes in mammals and give rise to more than 20 variants. Within this family of genes, PDE4D was discovered on the basis of its regulatory properties and its induction by hormones and cAMP. PDE4D has often been used as the prototype PDE4 and large body of work has been generated on the biochemical, pharmacological, and physiological properties of this enzyme. This review covers the regulation of PDE4D by phosphorylation, the impact of this regulation in the context of the structure of this protein, and the functional consequences of this complex pattern of posttranslational modifications.

Phosphodiesterase inhibitors for the treatment of asthma and chronic obstructive pulmonary disease

Xanthines like theophylline have long been recognised as being effective drugs for the treatment of asthma and chronic obstructive pulmonary disease (COPD). They are of interest as they possess both anti-inflammatory and bronchodilator activity in the same molecule. Since the discovery of phosphodiesterases (PDEs) in the late 1950s, it has been suggested that xanthines work, in part, by acting as non-selective PDE inhibitors. However, it has also been suggested that the ability of xanthines to non-selectively inhibit PDEs contributes to their many unwanted side effects, thus limiting their use since the arrival of inhaled drugs with more favourable safety profiles. As our understanding of PDEs has improved over the last 30 years, and with the recognition that the distribution of different PDEs varies across different cell types, this family of enzymes has been widely investigated as targets for novel drugs. In particular, PDE3 in airway smooth muscle and PDE4 and PDE7 in inflammatory cells have been targeted to provide new bronchodilators and anti-inflammatory agents, respectively. This review discusses the progress made in this field over the last decade in the development of selective PDE inhibitors to treat COPD and asthma.

Heterozygous mutations in cyclic AMP phosphodiesterase-4D (PDE4D) and protein kinase A (PKA) provide new insights into the molecular pathology of acrodysostosis

Acrodysostosis without hormone resistance is a rare skeletal disorder characterized by brachydactyly, nasal hypoplasia, mental retardation and occasionally developmental delay. Recently, loss-of-function mutations in the gene encoding cAMP-hydrolyzing phosphodiesterase-4D (PDE4D) have been reported to cause this rare condition but the pathomechanism has not been fully elucidated. To understand the pathogenetic mechanism of PDE4D mutations, we conducted 3D modeling studies to predict changes in the binding efficacy of cAMP to the catalytic pocket in PDE4D mutants. Our results indicated diminished enzyme activity in the two mutants we analyzed (Gly673Asp and Ile678Thr; based on PDE4D4 residue numbering). Ectopic expression of PDE4D mutants in HEK293 cells demonstrated this reduction in activity, which was identified by increased cAMP levels. However, the cells from an acrodysostosis patient showed low cAMP accumulation, which resulted in a decrease in the phosphorylated cAMP Response Element-Binding Protein (pCREB)/CREB ratio. The reason for this discrepancy was due to a compensatory increase in expression levels of PDE4A and PDE4B isoforms, which accounted for the paradoxical decrease in cAMP levels in the patient cells expressing mutant isoforms with a lowered PDE4D activity. Skeletal radiographs of 10-week-old knockout (KO) rats showed that the distal part of the forelimb was shorter than in wild-type (WT) rats and that all the metacarpals and phalanges were also shorter in KO, as the name acrodysostosis implies. Like the G-protein α-stimulatory subunit and PRKAR1A, PDE4D critically regulates the cAMP signal transduction pathway and influences bone formation in a way that activity-compromising PDE4D mutations can result in skeletal dysplasia. We propose that specific inhibitory PDE4D mutations can lead to the molecular pathology of acrodysostosis without hormone resistance but that the pathological phenotype may well be dependent on an over-compensatory induction of other PDE4 isoforms that can be expected to be targeted to different signaling complexes and exert distinct effects on compartmentalized cAMP signaling.

Arrestin-dependent localization of phosphodiesterases

Many G-protein-coupled receptors trigger the synthesis of cAMP in order to transduce signals from the membrane into the cell cytoplasm. As stimulation of each receptor type results in a specific physiological outcome, compartmentalization of proteins that make, break, and are activated by cAMP underpin receptor-specific responses. Until 2002, it was thought that static compartmentalization of phosphodiesterase 4 (PDE4), conferred by N-terminal targeting sequences, was one way to shape intricate cAMP gradients that formed after receptor activation. Discovery of the PDE4-β-arrestin complex represented a major breakthrough in cAMP signaling, as it spurred the initial realization that PDE4s could be transported to sites of high cAMP to orchestrate destruction of the second messenger at the same time as the receptor's signal to the G-protein is silenced. This chapter charts the scientific process that led to the discovery and characterization of the PDE4-β-arrestin interaction and discusses the known functions of this signaling complex.

Dual PDE3/4 and PDE4 inhibitors: novel treatments for COPD and other inflammatory airway diseases

Selective phosphodiesterase (PDE) 4 and dual PDE3/4 inhibitors have attracted considerable interest as potential therapeutic agents for the treatment of respiratory diseases, largely by virtue of their anti-inflammatory (PDE4) and bifunctional bronchodilator/anti-inflammatory (PDE3/4) effects. Many of these agents have, however, failed in early development for various reasons, including dose-limiting side effects when administered orally and lack of sufficient activity when inhaled. Indeed, only one selective PDE4 inhibitor, the orally active roflumilast-n-oxide, has to date received marketing authorization. The majority of the compounds that have failed were, however, orally administered and non-selective for either PDE3 (A,B) or PDE4 (A,B,C,D) subtypes. Developing an inhaled dual PDE3/4 inhibitor that is rapidly cleared from the systemic circulation, potentially with subtype specificity, may represent one strategy to improve the therapeutic index and also exhibit enhanced efficacy versus inhibition of either PDE3 or PDE4 alone, given the potential positive interactions with regard to anti-inflammatory and bronchodilator effects that have been observed pre-clinically with dual inhibition of PDE3 and PDE4 compared with inhibition of either isozyme alone. This MiniReview will summarize recent clinical data obtained with PDE inhibitors and the potential for these drugs to treat COPD and other inflammatory airways diseases such as asthma and cystic fibrosis.

PGE 2 desensitizes β -agonist effect on human lung fibroblast-mediated collagen gel contraction through upregulating PDE4

In the current study, we investigated the effect of a long-acting β-agonist (salmeterol) and a phosphodiesterase 4 (PDE4) inhibitor (cilomilast) on human lung fibroblast-mediated collagen gel contraction. Higher concentrations of salmeterol (10⁻⁷ and 10⁻⁶ M) inhibited fibroblast-mediated collagen gel contraction. No effect was observed with cilomilast alone (up to 10⁻⁵ M). In the presence of 10⁻⁸ M salmeterol, however, cilomilast could significantly inhibit fibroblast-mediated collagen gel contraction in a concentration-dependent manner (10⁻⁷~10⁻⁵ M). Blockade of endogenous PGE₂ by indomethacin further potentiated the inhibitory effect of salmeterol on fibroblast-mediated collagen gel contraction, but it did not affect cilomilast's effect. Pretreatment with PGE₂ abolished the inhibitory effect of salmeterol, but it potentiated the inhibitory effect of cilomilast on fibroblast-mediated collagen gel contraction. Finally, indomethacin slightly inhibited PDE4C expression, while PGE₂ stimulated the expression of PDE4A and -4C in human lung fibroblasts. These findings suggest that long-acting β-agonist and PDE4 inhibitor have a synergistic effect in regulating fibroblast tissue repair functions and that PGE₂ can modulate the effect of β-agonist and PDE4 inhibitor at least in part through the mechanism of regulating PDE4 expression.

Targeting protein-protein interactions within the cyclic AMP signaling system as a therapeutic strategy for cardiovascular disease

The cAMP signaling system can trigger precise physiological cellular responses that depend on the fidelity of many protein-protein interactions, which act to bring together signaling intermediates at defined locations within cells. In the heart, cAMP participates in the fine control of excitation-contraction coupling, hence, any disregulation of this signaling cascade can lead to cardiac disease. Due to the ubiquitous nature of the cAMP pathway, general inhibitors of cAMP signaling proteins such as PKA, EPAC and PDEs would act non-specifically and universally, increasing the likelihood of serious 'off target' effects. Recent advances in the discovery of peptides and small molecules that disrupt the protein-protein interactions that underpin cellular targeting of cAMP signaling proteins are described and discussed.

DISC1-ATF4 transcriptional repression complex: dual regulation of the cAMP-PDE4 cascade by DISC1

Disrupted-In-Schizophrenia 1 (DISC1), a risk factor for major mental illnesses, has been studied extensively in the context of neurodevelopment. However, the role of DISC1 in neuronal signaling, particularly in conjunction with intracellular cascades that occur in response to dopamine, a neurotransmitter implicated in numerous psychiatric disorders, remains elusive. Previous data suggest that DISC1 interacts with numerous proteins that impact neuronal function, including activating transcription factor 4 (ATF4). In this study, we identify a novel DISC1 and ATF4 binding region in the genomic locus of phosphodiesterase 4D (PDE4D), a gene implicated in psychiatric disorders. We found that the loss of function of either DISC1 or ATF4 increases PDE4D9 transcription, and that the association of DISC1 with the PDE4D9 locus requires ATF4. We also show that PDE4D9 is increased by D1-type dopamine receptor dopaminergic stimulation. We demonstrate that the mechanism for this increase is due to DISC1 dissociation from the PDE4D locus in mouse brain. We further characterize the interaction of DISC1 with ATF4 to show that it is regulated via protein kinase A-mediated phosphorylation of DISC1 serine-58. Our results suggest that the release of DISC1-mediated transcriptional repression of PDE4D9 acts as feedback inhibition to regulate dopaminergic signaling. Furthermore, as DISC1 loss-of-function leads to a specific increase in PDE4D9, PDE4D9 itself may represent an attractive target for therapeutic approaches in psychiatric disorders.

Bronchial epithelium as a target for innovative treatments in asthma

Increasing evidence of a critical role played by the bronchial epithelium in airway homeostasis is opening new therapeutic avenues. Its unique situation at the interface with the environment suggests that the subtle regulation orchestrated by the epithelium between tolerance and specific immune response might be impaired in asthma. Airway mucus is acting as a physical and a biological fluid between the environment and the epithelium, synergistically moved by the cilia. In asthma, excessive mucus production is a hallmark of airway remodeling. Since many years we tried to therapeutically target mucus hypersecretion, but actually this option is still not achieved. The present review discusses the dynamic processes regulating airway mucus production. Airway inflammation is central in current asthma management. Understanding of how the airway epithelium influences the TH2 paradigm in response to deleterious agents is improving. The multiple receptors expressed by the airway epithelium are the transducers of the biological signals induced by various invasive agents to develop the most adapted response. Airway remodeling is observed in severe chronic airway diseases and may result from ongoing disturbance of signal transduction and epithelial renewal. Chronic airway diseases such as asthma will require assessment of these epithelial abnormalities to identify phenotypic characteristics associated with predicting a clinical benefit for epithelial-directed therapies.

Protective effect of high-dose montelukast on salbutamol-induced homologous desensitisation in airway smooth muscle

Montelukast (MK) is a potent cysteinyl-leukotriene receptor antagonist that causes dose-related improvements in chronic asthma. We sought to determine whether MK was able to prevent salbutamol-induced tolerance in airway smooth muscle. Homologous β2-adrenoceptor desensitisation models were established in guinea-pigs and in human bronchial smooth muscle cells (BSMC) by chronic salbutamol administration. Characterisation tools included measurement of the response of tracheal smooth muscle tissues to salbutamol, analysis of gene expression and receptor trafficking, evaluation of intracellular cAMP levels and phosphodiesterase (PDE) activity in human bronchial smooth muscle cells. Salbutamol-induced β2-adrenoceptor desensitisation was characterised by β2-agonist hyporesponsiveness (-30%, p < 0.001) in desensitised tracheal smooth muscle, as compared to controls. MK, given intraperitoneally at 5 mg/kg/day for 6 consecutive days, completely restored tissue responsiveness to salbutamol. Prolonged salbutamol treatment significantly decreased cAMP synthesis, induced a complete removal of the β2-adrenoceptor from plasma membrane with a parallel increase in the cytosol and increased PDE4D5 gene transcription and PDE activity in human bronchial smooth muscle cells. In homologously desensitised BSMC, MK 30 μM for 24 h was able to prevent salbutamol subsensitivity and such an effect was associated with inhibition of salbutamol-induced PDE4 activity and restoration of membrane β2-adrenoceptor expression and function. These findings suggest the presence of a favourable interaction between MK and β2-adrenoceptor agonists that might improve the therapeutic index of bronchodilators in patients with chronic respiratory diseases.

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.

Novel cAMP signalling paradigms: therapeutic implications for airway disease

Since its discovery over 50 years ago, cAMP has been the archetypal second messenger introducing students to the concept of cell signalling at the simplest level. As explored in this review, however, there are many more facets to cAMP signalling than the path from Gs-coupled receptor to adenylyl cyclase (AC) to cAMP to PKA to biological effect. After a brief description of this canonical cAMP signalling pathway, a snapshot is provided of the novel paradigms of cAMP signalling. As in the airway the cAMP pathway relays the major bronchorelaxant signal and as such is the target for frontline therapy for asthma and COPD, particular emphasis is given to airway disease and therapy. Areas discussed include biased agonism, continued signalling following internalization, modulation of cAMP by AC, control of cAMP degradation, cAMP and calcium crosstalk, Epac-mediated signalling and finally the implications of altered genotypes will be considered. LINKED ARTICLES This article is part of a themed section on Novel cAMP Signalling Paradigms. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-2.

Proinflammatory cytokine regulation of cyclic AMP-phosphodiesterase 4 signaling in microglia in vitro and following CNS injury

Cyclic AMP suppresses immune cell activation and inflammation. The positive feedback loop of proinflammatory cytokine production and immune activation implies that cytokines may not only be regulated by cyclic AMP but also conversely regulate cyclic AMP. This study examined the effects of tumor necrosis factor (TNF)-α and interleukin (IL)-1β on cyclic AMP-phosphodiesterase (PDE) signaling in microglia in vitro and after spinal cord injury (SCI) or traumatic brain injury (TBI). TNF-α or IL-1β stimulation produced a profound reduction (>90%) of cyclic AMP within EOC2 microglia from 30 min that then recovered after IL-1β but remained suppressed with TNF-α through 24 h. Cyclic AMP was also reduced in TNF-α-stimulated primary microglia, albeit to a lesser extent. Accompanying TNF-α-induced cyclic AMP reductions, but not IL-1β, was increased cyclic AMP-PDE activity. The role of PDE4 activity in cyclic AMP reductions was confirmed by using Rolipram. Examination of pde4 mRNA revealed an immediate, persistent increase in pde4b with TNF-α; IL-1β increased all pde4 mRNAs. Immunoblotting for PDE4 showed that both cytokines increased PDE4A1, but only TNF-α increased PDE4B2. Immunocytochemistry revealed PDE4B nuclear translocation with TNF-α but not IL-1β. Acutely after SCI/TBI, where cyclic AMP levels are reduced, PDE4B was localized to activated OX-42(+) microglia; PDE4B was absent in OX-42(+) cells in uninjured spinal cord/cortex or inactive microglia. Immunoblotting showed PDE4B2 up-regulation from 24 h to 1 wk post-SCI, the peak of microglia activation. These studies show that TNF-α and IL-1β differentially affect cyclic AMP-PDE signaling in microglia. Targeting PDE4B2 may be a putative therapeutic direction for reducing microglia activation in CNS injury and neurodegenerative diseases.

G Protein βγ-subunit signaling mediates airway hyperresponsiveness and inflammation in allergic asthma

Since the Gβγ subunit of Gi protein has been importantly implicated in regulating immune and inflammatory responses, this study investigated the potential role and mechanism of action of Gβγ signaling in regulating the induction of airway hyperresponsiveness (AHR) in a rabbit model of allergic asthma. Relative to non-sensitized animals, OVA-sensitized rabbits challenged with inhaled OVA exhibited AHR, lung inflammation, elevated BAL levels of IL-13, and increased airway phosphodiesterase-4 (PDE4) activity. These proasthmatic responses were suppressed by pretreatment with an inhaled membrane-permeable anti-Gβγ blocking peptide, similar to the suppressive effect of glucocorticoid pretreatment. Extended mechanistic studies demonstrated that: 1) corresponding proasthmatic changes in contractility exhibited in isolated airway smooth muscle (ASM) sensitized with serum from OVA-sensitized+challenged rabbits or IL-13 were also Gβγ-dependent and mediated by MAPK-upregulated PDE4 activity; and 2) the latter was attributed to Gβγ-induced direct stimulation of the non-receptor tyrosine kinase, c-Src, resulting in downstream activation of ERK1/2 and its consequent transcriptional upregulation of PDE4. Collectively, these data are the first to identify that a mechanism involving Gβγ-induced direct activation of c-Src, leading to ERK1/2-mediated upregulation of PDE4 activity, plays a decisive role in regulating the induction of AHR and inflammation in a rabbit model of allergic airway disease.

β2-Agonists upregulate PDE4 mRNA but not protein or activity in human airway smooth muscle cells from asthmatic and nonasthmatic volunteers

β(2)-Adrenergic receptor (β2AR) agonists induce airway relaxation via cAMP. Phosphodiesterase (PDE)s degrade and regulate cAMP, and in airway smooth muscle (ASM) cells PDE4D degrades cAMP. Long-acting β(2)-agonists are now contraindicated as monotherapy for asthma, and increased PDE4D has been speculated to contribute to this phenomenon. In this study we investigated the expression of PDE4D in asthmatic and nonasthmatic ASM cells and its regulation by formoterol and budesonide. Primary ASM cells from people with or without asthma were stimulated with transforming growth factor (TGF)-β(1), formoterol, and/or budesonide. PDE4D mRNA was assessed by real-time PCR, or PCR to assess splice variant production. PDE4D protein was assessed by Western blotting, and we investigated the effect of formoterol on cAMP production and PDE activity. Interleukin (IL)-6 was assessed using ELISA. PDE4D mRNA was dose dependently upregulated by formoterol, with a single splice variant, PDE4D5, present. Formoterol did not induce PDE4D protein at time points between 3 to 72 h, whereas it did induce and increase IL-6 secretion. We pretreated cells with actinomycin D and a proteasome inhibitor, MG132, and found no evidence of alterations in mRNA, protein expression, or degradation of PDE4D. Finally PDE activity was not altered by formoterol. This study shows, for the first time, that PDE4D5 is predominantly expressed in human ASM cells from people with and without asthma and that formoterol does not upregulate PDE4D protein production. This leads us to speculate that continual therapy with β2AR agonists is unlikely to cause PDE4-mediated tachyphylaxis.

Phosphodiesterase type 4D gene polymorphism: association with the response to short-acting bronchodilators in paediatric asthma patients

Short-acting b2-adrenergic receptor agonists are commonly used bronchodilators for symptom relief in asthmatics. The aim of this study was to test whether genetic variants in PDE4D gene, a key regulator of b2-adrenoceptor-induced cAMP turnover in airway smooth muscle cells, affect the response to short-acting b2-agonists. Bronchodilator responsiveness was assessed in 133 asthmatic children by % change in baseline forced expiratory volume in one second (FEV₁) after administration of albuterol. The analyses were performed in patients with airway obstruction (FEV₁/FVC ratio below 90%, n = 93). FEV₁  % change adjusted for baseline FEV₁ values was significantly different between genotypes of rs1544791 G/A polymorphism (P = 0.006) and −1345 C/T (rs1504982) promoter variation (P = 0.03). The association remained significant with inclusion of age, sex, atopy, and controller medication into multivariate model (P = 0.004 and P = 0.02, resp.). Our work identifies new genetic variants implicated in modulation of asthma treatment, one of them (rs1544791) previously associated with asthma phenotype.

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.

An evolutionary analysis of cAMP-specific Phosphodiesterase 4 alternative splicing

BACKGROUND

Cyclic nucleotide phosphodiesterases (PDEs) hydrolyze the intracellular second messengers: cyclic adenosine monophosphate (cAMP) and cyclic guanine monophosphate (cGMP). The cAMP-specific PDE family 4 (PDE4) is widely expressed in vertebrates. Each of the four PDE4 gene isoforms (PDE4 A-D) undergo extensive alternative splicing via alternative transcription initiation sites, producing unique amino termini and yielding multiple splice variant forms from each gene isoform termed long, short, super-short and truncated super-short. Many species across the vertebrate lineage contain multiple splice variants of each gene type, which are characterized by length and amino termini.

RESULTS

A phylogenetic approach was used to visualize splice variant form genesis and identify conserved splice variants (genome conservation with EST support) across the vertebrate taxa. Bayesian and maximum likelihood phylogenetic inference indicated PDE4 gene duplication occurred at the base of the vertebrate lineage and reveals additional gene duplications specific to the teleost lineage. Phylogenetic inference and PDE4 splice variant presence, or absence as determined by EST screens, were further supported by the genomic analysis of select vertebrate taxa. Two conserved PDE4 long form splice variants were found in each of the PDE4A, PDE4B, and PDE4C genes, and eight conserved long forms from the PDE4 D gene. Conserved short and super-short splice variants were found from each of the PDE4A, PDE4B, and PDE4 D genes, while truncated super-short variants were found from the PDE4C and PDE4 D genes. PDE4 long form splice variants were found in all taxa sampled (invertebrate through mammals); short, super-short, and truncated super-short are detected primarily in tetrapods and mammals, indicating an increasing complexity in both alternative splicing and cAMP metabolism through vertebrate evolution.

CONCLUSIONS

There was a progressive independent incorporation of multiple PDE4 splice variant forms and amino termini, increasing PDE4 proteome complexity from primitive vertebrates to humans. While PDE4 gene isoform duplicates with limited alternative splicing were found in teleosts, an expansion of both PDE4 splice variant forms, and alternatively spliced amino termini predominantly occurs in mammals. Since amino termini have been linked to intracellular targeting of the PDE4 enzymes, the conservation of amino termini in PDE4 splice variants in evolution highlights the importance of compartmentalization of PDE4-mediated cAMP hydrolysis.

Cyclic nucleotide signaling in polycystic kidney disease

Increased levels of 3'-5'-cyclic adenosine monophosphate (cAMP) stimulate cell proliferation and fluid secretion in polycystic kidney disease. Levels of this molecule are more sensitive to inhibition of phosphodiesterases (PDEs), whose activity far exceeds the rate of cAMP synthesis by adenylyl cyclase. Several PDEs exist, and here we measured the activity and expression of PDE families, their isoforms, and the expression of downstream effectors of cAMP signaling in the kidneys of rodents with polycystic kidney disease. We found a higher overall PDE activity in kidneys from mice as compared with rats, as well as a higher contribution of PDE1, relative to PDE4 and PDE3, to total PDE activity of kidney lysates and lower PDE1, PDE3, and PDE4 activities in the kidneys of cystic as compared with wild-type mice. There were reduced amounts of several PDE1, PDE3, and PDE4 proteins, possibly due to increased protein degradation despite an upregulation of their mRNA. Increased levels of cGMP were found in the kidneys of cystic animals, suggesting in vivo downregulation of PDE1 activity. We found an additive stimulatory effect of cAMP and cGMP on cystogenesis in vitro. Cyclic AMP-dependent protein kinase subunits Ialpha and IIbeta, PKare, the transcription factor CREB-1 mRNA, and CREM, ATF-1, and ICER proteins were upregulated in the kidneys of cystic as compared with wild-type animals. Our study suggests that alterations in cyclic nucleotide catabolism may render cystic epithelium particularly susceptible to factors acting on Gs-coupled receptors. This may account, in part, for increased cyclic nucleotide signaling in polycystic kidney disease and contribute substantially to disease progression.

Mechanism regulating proasthmatic effects of prolonged homologous beta2-adrenergic receptor desensitization in airway smooth muscle

Use of long-acting beta(2)-adrenergic receptor (beta2AR) agonists to treat asthma incurs an increased risk of asthma morbidity with impaired bronchodilation and heightened bronchoconstriction, reflecting the adverse effects of prolonged homologous beta2AR desensitization on airway smooth muscle (ASM) function. Since phosphodiesterase 4 (PDE4) regulates ASM relaxation and contractility, we examined whether the changes in ASM function induced by prolonged homologous beta2AR desensitization are attributed to altered expression and action of PDE4. Cultured human ASM cells and isolated rabbit ASM tissues exposed for 24 h to the long-acting beta2AR agonist salmeterol exhibited impaired acute beta2AR-mediated cAMP accumulation and relaxation, respectively, together with ASM constrictor hyperresponsiveness. These proasthmatic-like changes in ASM function were associated with upregulated PDE4 activity due to enhanced expression of the PDE4D5 isoform and were prevented by pretreating the ASM preparations with the PDE4 inhibitor rolipram or with inhibitors of either PKA or ERK1/2 signaling. Extended studies using gene silencing and pharmacological approaches demonstrated that: 1) the mechanism underlying upregulated PDE4D5 expression following prolonged beta2AR agonist exposure involves PKA-dependent activation of G(i) protein signaling via its betagamma-subunits, which elicits downstream activation of ERK1/2 and its induction of PDE4D5 transcription; and 2) the induction of PDE4 activity and consequent changes in ASM responsiveness are prevented by pretreating the beta2AR agonist-exposed ASM preparations with inhibitors of G(i)-betagamma signaling. Collectively, these findings identify that the proasthmatic changes in ASM function resulting from prolonged homologous beta2AR desensitization are attributed to upregulated PDE4 expression induced by G(i)-betagamma-mediated cross-talk between the PKA and ERK1/2 signaling pathways.

Dual PDE3/4 inhibitors as therapeutic agents for chronic obstructive pulmonary disease

Phosphodiesterase (PDE)4, and to a lesser extent, PDE3/4 inhibitors have attracted considerable interest as potential therapeutic agents for diseases including chronic obstructive pulmonary disease. Indeed, ibudilast and theophylline are utilized clinically, and roflumilast is in late-stage clinical development. Unfortunately, however many PDE4 and dual PDE3/4 inhibitors have failed in early development due to low therapeutic ratios. The majority of these compounds are however orally administered and non-selective for either PDE3(A, B) or PDE4(A, B, C, D) subtypes. Developing an inhaled dual PDE3/4 inhibitor with subtype specificity may represent one strategy to improve the therapeutic index. Indeed combined inhibition of PDE3 and PDE4 inhibitor has additive and synergistic anti-inflammatory and bronchodilatory effects versus inhibition of either PDE3 or PDE4 alone. Given that synergy has been seen in terms of efficacy end points, an obvious concern is that synergy may also be observed in side effects. Interestingly, however, no synergy or additive effects with a combination of a PDE3 and PDE4 inhibitor in a cardiomyocyte assay were observed. This review will summarize the rationale for developing an inhaled dual PDE3/4 inhibitor, as a treatment for chronic obstructive pulmonary disease together with recent advances in trying to understand the pathogenesis of PDE inhibitor-induced mesenteric vasculitis (a key potential dose-limiting side effect of these agents), highlighting potential early and sensitive predictive biomarkers.

PDE4 associates with different scaffolding proteins: modulating interactions as treatment for certain diseases

cAMP is an ubiquitous second messenger that is crucial to many cellular processes. The sole means of terminating the cAMP signal is degradation by cAMP phosphodiesterases (PDEs). The PDE4 family is of particular interest because PDE4 inhibitors have therapeutic potential for the treatment of various inflammatory and auto-immune diseases and also have anti-depressant and memory-enhancing effects. The subcellular targeting of PDE4 isoforms is fundamental to the compartmentalization of cAMP signaling pathways and is largely achieved via proteinprotein interactions. Increased knowledge of these protein-protein interactions and their regulatory properties could aid in the design of novel isoform-specific inhibitors with improved efficacy and fewer prohibitive side effects.

In cardiac myocytes, cAMP elevation triggers the down-regulation of transcripts and promoter activity for cyclic AMP phosphodiesterase-4A10 (PDE4A10)

Transcripts for the PDE4A10 cyclic AMP phosphodiesterase isoform are present in a wide variety of rat tissues including the heart. Sequence comparisons between the putative human and mouse promoters revealed a number of conserved regions including both an Sp1 and a CREB-binding site. The putative mouse PDE4A10 promoter was amplified from genomic DNA and sub-cloned into a luciferase reporter vector for investigation of activity in neonatal cardiac myocytes. Transfection with this construct identified a high level of luciferase expression in neonatal cardiac myocytes. Surprisingly, this activity was down-regulated by elevation of intracellular cAMP through a process involving PKA, but not EPAC, signalling. Such inhibition of the rodent PDE4A10 promoter activity in response to elevated cAMP levels is in contrast to the PDE4 promoters so far described. Site-directed mutagenesis revealed that the Sp1 binding site at promoter position -348 to -336 is responsible for the basal constitutive expression of murine PDE4A10. The conserved CREB-binding motif at position -370 to -363 also contributes to basal promoter activity but does not in itself confer cAMP inhibition upon the PDE4A10 promoter. EMSA analysis confirmed the authenticity of CREB and Sp1 binding sites. The transcriptional start site was identified to be an adenine residue at position -55 in the mouse PDE4A10 promoter. We present evidence that this novel down-regulation of PDE4A10 is mediated by the transcription factor ICER in a PKA dependent manner. The pool of cAMP in cardiac myocytes that down-regulates PDE4A10 is regulated by beta-adrenoceptor coupled adenylyl cyclase activity and via hydrolysis determined predominantly by the action of PDE4 (cAMP phosphodiesterase-4) and not PDE3 (cAMP phosphodiesterase-3). We suggest that increased cAMP may remodel cAMP-mediated signalling events by not only increasing the expression of specific PDE4 cAMP phosphodiesterases but also by down-regulating specific isoforms, such as is shown here for PDE4A10 in cardiac myocytes.

Prolonged heterologous beta2-adrenoceptor desensitization promotes proasthmatic airway smooth muscle function via PKA/ERK1/2-mediated phosphodiesterase-4 induction

Beta2-adrenergic receptor (beta2AR) agonists acutely relieve bronchoconstriction via cAMP-mediated relaxation of airway smooth muscle (ASM). Airway constrictor responsiveness may be significantly heightened, however, following protracted exposure to these agents, presumably reflecting the effects of beta2AR desensitization in ASM accompanying prolonged cAMP signaling. Because cAMP phosphodiesterase (PDE) activity can significantly modulate ASM contractility, we investigated the mechanism regulating PDE expression and its potential role in mediating changes in agonist-induced constrictor and relaxation responsiveness in ASM following its heterologous beta2AR desensitization by prolonged exposure to cAMP-elevating agents. Isolated rabbit ASM tissues and cultured human ASM cells treated for 24 h with the receptor- or nonreceptor-coupled cAMP-stimulating agent, prostaglandin E(2) (PGE(2)) or forskolin, respectively, exhibited constrictor hyperresponsiveness to acetylcholine and impaired beta2AR-mediated relaxation and cAMP accumulation. These proasthmatic-like changes in ASM function were associated with upregulated PDE4 activity, reflective of increased transcription of the PDE4D5 isoform, and were prevented by pretreatment of the ASM with a PDE4 inhibitor. Extended studies using gene silencing and pharmacological approaches to inhibit specific intracellular signaling molecules demonstrated that the mechanism underlying PGE(2)-induced transcriptional upregulation of PDE4D5 involves PKA-dependent activation of G(i) protein signaling via the betagamma-subunits, the latter eliciting downstream activation of ERK1/2 and its consequent induction of PDE4D5 transcription. Collectively, these findings identify that beta2AR desensitization in ASM following prolonged exposure to cAMP-elevating agents is associated with proasthmatic-like changes in ASM responsiveness that are mediated by upregulated PDE4 expression induced by activated cross talk between the PKA and ERK1/2 signaling pathways.

A whole genome association study of neuroticism using DNA pooling

We describe a multistage approach to identify single nucleotide polymorphisms (SNPs) associated with neuroticism, a personality trait that shares genetic determinants with major depression and anxiety disorders. Whole genome association with 452 574 SNPs was performed on DNA pools from approximately 2000 individuals selected on extremes of neuroticism scores from a cohort of 88 142 people from southwest England. The most significant SNPs were then genotyped on independent samples to replicate findings. We were able to replicate association of one SNP within the PDE4D gene in a second sample collected by our laboratory and in a family-based test in an independent sample; however, the SNP was not significantly associated with neuroticism in two other independent samples. We also observed an enrichment of low P-values in known regions of copy number variations. Simulation indicates that our study had approximately 80% power to identify neuroticism loci in the genome with odds ratio (OR)>2, and approximately 50% power to identify small effects (OR=1.5). Since we failed to find any loci accounting for more than 1% of the variance, the heritability of neuroticism probably arises from many loci each explaining much less than 1%. Our findings argue the need for much larger samples than anticipated in genetic association studies and that the biological basis of emotional disorders is extremely complex.

cAMP-specific phosphodiesterase-4D5 (PDE4D5) provides a paradigm for understanding the unique non-redundant roles that PDE4 isoforms play in shaping compartmentalized cAMP cell signalling

The PDE4 (phosphodiesterase-4) enzyme family consists of a distinct array of N-terminal splice variant isoforms arising from four subfamily genes (4A, 4B, 4C and 4D). These all hydrolyse specifically the intracellular second messenger cAMP. Although identical in catalytic function, each isoform appears to serve a non-superfluous regulatory role. For example, a beta-arrestin-sequestered subpopulation of the PDE4D5 isoform specifically regulates the phosphorylation of the beta(2)-AR (beta(2)-adrenergic receptor) by PKA (protein kinase A; also called cAMP-dependent protein kinase). This was elucidated by the use of novel technologies, including dominant-negative approaches, siRNA (small interfering RNA) knockdown and spot-immobilized peptide array analyses. Functional phenotypes uncovered using these methodologies have shown that beta-arrestin-sequestered PDE4D5 shapes the spatial cAMP gradient around the membrane-bound beta(2)-AR, regulating its phosphorylation by PKA and its ability to activate ERK (extracellular-signal-regulated kinase) through G(i) in cardiomyocytes and HEK-293 (human embryonic kidney)-B2 cells. This approach has provided the very first identification of a non-redundant and specific role for a PDE isoform. The fact that phenotypes can be uncovered by displacing PDE4 isoforms from specific anchor sites using dominant-negative constructs and cell-permeable peptides points to novel means for developing therapeutics aimed at disrupting specifically sequestered PDE isoforms and even specifically sequestered subpopulations of individual isoforms.

1H NMR structural and functional characterisation of a cAMP-specific phosphodiesterase-4D5 (PDE4D5) N-terminal region peptide that disrupts PDE4D5 interaction with the signalling scaffold proteins, beta-arrestin and RACK1

The unique 88 amino acid N-terminal region of cAMP-specific phosphodiesterase-4D5 (PDE4D5) contains overlapping binding sites conferring interaction with the signaling scaffold proteins, betaarrestin and RACK1. A 38-mer peptide, whose sequence reflected residues 12 through 49 of PDE4D5, encompasses the entire N-terminal RACK1 Interaction Domain (RAID1) together with a portion of the beta-arrestin binding site. (1)H NMR and CD analyses indicate that this region has propensity to form a helical structure. The leucine-rich hydrophobic grouping essential for RACK1 interaction forms a discrete hydrophobic ridge located along a single face of an amphipathic alpha-helix with Arg34 and Asn36, which also play important roles in RACK1 binding. The Asn22/Pro23/Trp24/Asn26 grouping, essential for RACK1 interaction, was located at the N-terminal head of the amphipathic helix that contained the hydrophobic ridge. RAID1 is thus provided by a distinct amphipathic helical structure. We suggest that the binding of PDE4D5 to the WD-repeat protein, RACK1, may occur in a manner akin to the helix-helix interaction shown for G(gamma) binding to the WD-repeat protein, G(beta). A more extensive section of the PDE4D5 N-terminal sequence (Thr11-Ala85) is involved in beta-arrestin binding. Several residues within the RAID1 helix contribute to this interaction however. We show here that these residues form a focused band around the centre of the RAID1 helix, generating a hydrophobic patch (from Leu29, Val30 and Leu33) flanked by polar/charged residues (Asn26, Glu27, Asp28, Arg34). The interaction with beta-arrestin exploits a greater circumference on the RAID1 helix, and involves two residues (Glu27, Asp28) that do not contribute to RACK1 binding. In contrast, the interaction of RACK1 with RAID1 is extended over a greater length of the helix and includes Leu37/Leu38, which do not contribute to beta-arrestin binding. A membrane-permeable, stearoylated Val12-Ser49 38-mer peptide disrupted the interaction of both beta-arrestin and RACK1 with endogenous PDE4D5 in HEKB2 cells, whilst a cognate peptide with a Glu27Ala substitution selectively failed to disrupt PDE4D5/RACK1 interaction. The stearoylated Val12-Ser49 38-mer peptide enhanced the isoprenaline-stimulated PKA phosphorylation of the beta(2)-adrenergic receptors (beta(2)AR) and its activation of ERK, whilst the Glu27Ala peptide was ineffective in both these regards.

A major functional role for phosphodiesterase 4D5 in human airway smooth muscle cells

Relaxation of airway smooth muscle is dependent predominantly upon elevation of cell cAMP content. Although the processes involved in elevation of cell cAMP content are reasonably well established, the mechanisms governing subsequent control of cAMP turnover are less clear. Breakdown of cAMP is solely regulated by phosphodiesterase (PDE) isoenzymes. We have previously reported that PDE4 family members are likely to be important in this process, and that expression of PDE4D variants is actively regulated at the transcriptional level. Here, we demonstrate a key role for PDE4D5 in the control of beta(2)-adrenoceptor (beta(2)AR)-stimulated cAMP activity in human airway smooth muscle cells using splice variant-specific small interfering RNA knockdown. Furthermore, we show, using an Epac (exchange protein directly activated by cAMP)-based, cAMP-sensitive fluorescent probe, that these intracellular cAMP gradients are controlled both temporally and dynamically by PDE4D5. Elevation of cAMP within the cytoplasm after beta(2)AR stimulation is rapid and shows no distinct spatial compartmentalization in these cells. These data suggest that PDE4D5, despite being a minor component of the tissue PDE pool, is the key physiological regulator of beta(2)AR-induced cAMP turnover within human airway smooth muscle.

Changes in beta 2-adrenoceptor and other signaling proteins produced by chronic administration of 'beta-blockers' in a murine asthma model

BACKGROUND

We have previously reported that chronic treatment with certain 'beta-blockers' reduces airway hyperresponsiveness (AHR) to methacholine in a murine model of asthma.

METHODS

Airway resistance was measured using the forced oscillation technique in ovalbulmin-sensitized and ovalbulmin-challenged mice treated with several beta-adrenoceptor (beta-AR) ligands. We used the selective beta 2-AR ligand ICI 118,551 and the preferential beta 1-AR ligand metoprolol to investigate the receptor subtype mediating the beneficial effect. Expression of beta-ARs was evaluated using immunofluorescence. We evaluated several signaling proteins by western blot using lung homogenates, and measured the relaxation of the isolated trachea produced by EP2 and IP receptor agonists.

RESULTS

Four findings were associated with the decreased AHR after chronic beta-blocker treatment: (1) the highly selective beta 2-AR antagonist/inverse agonist, ICI 118,551 produced the bronchoprotective effect; (2) beta 2-AR up-regulation resulted from chronic 'beta-blocker' treatment; (3) reduced expression of certain proteins involved in regulating bronchial tone, namely, Gi, phosphodiesterase 4D and phospholipase C-beta 1; and (4) an enhanced bronchodilatory response to prostanoid agonists for the IP and EP2 receptors.

CONCLUSIONS

These data suggest that in the murine model of asthma, several compensatory changes associated with either increased bronchodilator signaling or decreased bronchoconstrictive signaling, result from the chronic administration of certain 'beta-blockers'.

cAMP-Specific Phosphodiesterase-4 Enzymes in the Cardiovascular System

Cyclic AMP regulates a vast number of distinct events in all cells. Early studies established that its hydrolysis by cyclic nucleotide phosphodiesterases (PDEs) controlled both the magnitude and the duration of its influence. Recent evidence shows that PDEs also act as coincident detectors linking cyclic-nucleotide- and non-cyclic-nucleotide-based cellular signaling processes and are tethered with great selectively to defined intracellular structures, thereby integrating and spatially restricting their cellular effects in time and space. Although 11 distinct families of PDEs have been defined, and cells invariably express numerous individual PDE enzymes, a large measure of our increased appreciation of the roles of these enzymes in regulating cyclic nucleotide signaling has come from studies on the PDE4 family. Four PDE4 genes encode more than 20 isoforms. Alternative mRNA splicing and the use of different promoters allows cells the possibility of expressing numerous PDE4 enzymes, each with unique amino-terminal-targeting and/or regulatory sequences. Dominant negative and small interfering RNA-mediated knockdown strategies have proven that particular isoforms can uniquely control specific cellular functions. Thus the protein kinase A phosphorylation status of the beta(2) adrenoceptor and, thereby, its ability to switch its signaling to extracellular signal-regulated kinase activation, is uniquely regulated by PDE4D5 in cardiomyocytes. We describe how cardiomyocytes and vascular smooth muscle cells selectively vary both the expression and the catalytic activities of PDE4 isoforms to regulate their various functions and how altered regulation of these processes can influence the development, or resolution, of cardiovascular pathologies, such as heart failure, as well as various vasculopathies.

Overview of PDEs and their regulation

Contraction and relaxation of vascular smooth muscle and cardiac myocytes are key physiological events in the cardiovascular system. These events are regulated by second messengers, cAMP and cGMP, in response to extracellular stimulants. The strength of signal transduction is controlled by intracellular cyclic nucleotide concentrations, which are determined by a balance in production and degradation of cAMP and cGMP. Degradation of cyclic nucleotides is catalyzed by 3',5'-cyclic nucleotide phosphodiesterases (PDEs), and therefore regulation of PDEs hydrolytic activity is important for modulation of cellular functions. Mammalian PDEs are composed of 21 genes and are categorized into 11 families based on sequence homology, enzymatic properties, and sensitivity to inhibitors. PDE families contain many splice variants that mostly are unique in tissue-expression patterns, gene regulation, enzymatic regulation by phosphorylation and regulatory proteins, subcellular localization, and interaction with association proteins. Each unique variant is closely related to the regulation of a specific cellular signaling. Thus, multiple PDEs function as a particular modulator of each cardiovascular function and regulate physiological homeostasis.