Rapid regulation of PDE-2 and PDE-4 cyclic AMP phosphodiesterase activity following ligation of the T cell antigen receptor on thymocytes: analysis using the selective inhibitors erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA) and rolipram

Cyclic nucleotide phosphodiesterases as drug targets

Cyclic nucleotides are synthesized by adenylyl and/or guanylyl cyclase, and downstream of this synthesis, the cyclic nucleotide phosphodiesterase families (PDEs) specifically hydrolyze cyclic nucleotides. PDEs control cyclic adenosine-3',5'monophosphate (cAMP) and cyclic guanosine-3',5'-monophosphate (cGMP) intracellular levels by mediating their quick return to the basal steady state levels. This often takes place in subcellular nanodomains. Thus, PDEs govern short-term protein phosphorylation, long-term protein expression, and even epigenetic mechanisms by modulating cyclic nucleotide levels. Consequently, their involvement in both health and disease is extensively investigated. PDE inhibition has emerged as a promising clinical intervention method, with ongoing developments aiming to enhance its efficacy and applicability. In this comprehensive review, we extensively look into the intricate landscape of PDEs biochemistry, exploring their diverse roles in various tissues. Furthermore, we outline the underlying mechanisms of PDEs in different pathophysiological conditions. Additionally, we review the application of PDE inhibition in related diseases, shedding light on current advancements and future prospects for clinical intervention. SIGNIFICANCE STATEMENT: Regulating PDEs is a critical checkpoint for numerous (patho)physiological conditions. However, despite the development of several PDE inhibitors aimed at controlling overactivated PDEs, their applicability in clinical settings poses challenges. In this context, our focus is on pharmacodynamics and the structure activity of PDEs, aiming to illustrate how selectivity and efficacy can be optimized. Additionally, this review points to current preclinical and clinical evidence that depicts various optimization efforts and indications.

A mini-review: phosphodiesterases in charge to balance intracellular cAMP during T-cell activation

T-cell activation is a pivotal process of the adaptive immune response with 3',5'-cyclic adenosine monophosphate (cAMP) as a key regulator of T-cell activation and function. It governs crucial control over T-cell differentiation and production of pro-inflammatory cytokines, such as IFN-γ. Intriguingly, levels of intracellular cAMP differ between regulatory (Treg) and conventional T-cells (Tcon). During cell-cell contact, cAMP is transferred via gap junctions between these T-cell subsets to mediate the immunosuppressive function of Treg. Moreover, the activation of T-cells via CD3 and CD28 co-stimulation leads to a transient upregulation of cAMP. Elevated intracellular cAMP levels are balanced precisely by phosphodiesterases (PDEs), a family of enzymes that hydrolyze cyclic nucleotides. Various PDEs play distinct roles in regulating cAMP and cyclic guanosine monophosphate (cGMP) in T-cells. Research on PDEs has gained growing interest due to their therapeutic potential to manipulate T-cell responses. So far, PDE4 is the best-described PDE in T-cells and the first PDE that is currently targeted in clinical practice to treat autoimmune diseases. But also, other PDE families harbor additional therapeutic potential. PDE2A is a dual-substrate phosphodiesterase which is selectively upregulated in Tcon upon activation. In this Mini-Review, we will highlight the impact of cAMP regulation on T-cell activation and function and summarize recent findings on different PDEs regulating intracellular cAMP levels in T-cells.

Cone photoreceptor phosphodiesterase PDE6H inhibition regulates cancer cell growth and metabolism, replicating the dark retina response

BACKGROUND

PDE6H encodes PDE6γ', the inhibitory subunit of the cGMP-specific phosphodiesterase 6 in cone photoreceptors. Inhibition of PDE6, which has been widely studied for its role in light transduction, increases cGMP levels. The purpose of this study is to characterise the role of PDE6H in cancer cell growth.

METHODS

From an siRNA screen for 487 genes involved in metabolism, PDE6H was identified as a controller of cell cycle progression in HCT116 cells. Role of PDE6H in cancer cell growth and metabolism was studied through the effects of its depletion on levels of cell cycle controllers, mTOR effectors, metabolite levels, and metabolic energy assays. Effect of PDE6H deletion on tumour growth was also studied in a xenograft model.

RESULTS

PDE6H knockout resulted in an increase of intracellular cGMP levels, as well as changes to the levels of nucleotides and key energy metabolism intermediates. PDE6H knockdown induced G1 cell cycle arrest and cell death and reduced mTORC1 signalling in cancer cell lines. Both knockdown and knockout of PDE6H resulted in the suppression of mitochondrial function. HCT116 xenografts revealed that PDE6H deletion, as well as treatment with the PDE5/6 inhibitor sildenafil, slowed down tumour growth and improved survival, while sildenafil treatment did not have an additive effect on slowing the growth of PDE6γ'-deficient tumours.

CONCLUSIONS

Our results indicate that the changes in cGMP and purine pools, as well as mitochondrial function which is observed upon PDE6γ' depletion, are independent of the PKG pathway. We show that in HCT116, PDE6H deletion replicates many effects of the dark retina response and identify PDE6H as a new target in preventing cancer cell proliferation and tumour growth.

Subcellular second messenger networks drive distinct repellent-induced axon behaviors

Second messengers, including cAMP, cGMP and Ca2+ are often placed in an integrating position to combine the extracellular cues that orient growing axons in the developing brain. This view suggests that axon repellents share the same set of cellular messenger signals and that axon attractants evoke opposite cAMP, cGMP and Ca2+ changes. Investigating the confinement of these second messengers in cellular nanodomains, we instead demonstrate that two repellent cues, ephrin-A5 and Slit1, induce spatially segregated signals. These guidance molecules activate subcellular-specific second messenger crosstalk, each signaling network controlling distinct axonal morphology changes in vitro and pathfinding decisions in vivo.

Perspectives of PDE inhibitor on treating idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive interstitial lung disease (ILD) without an identifiable cause. If not treated after diagnosis, the average life expectancy is 3-5 years. Currently approved drugs for the treatment of IPF are Pirfenidone and Nintedanib, as antifibrotic drugs, which can reduce the decline rate of forced vital capacity (FVC) and reduce the risk of acute exacerbation of IPF. However these drugs can not relieve the symptoms associated with IPF, nor improve the overall survival rate of IPF patients. We need to develop new, safe and effective drugs to treat pulmonary fibrosis. Previous studies have shown that cyclic nucleotides participate in the pathway and play an essential role in the process of pulmonary fibrosis. Phosphodiesterase (PDEs) is involved in cyclic nucleotide metabolism, so PDE inhibitors are candidates for pulmonary fibrosis. This paper reviews the research progress of PDE inhibitors related to pulmonary fibrosis, so as to provide ideas for the development of anti-pulmonary fibrosis drugs.

Upregulation of Phosphodiesterase 2A Augments T Cell Activation by Changing cGMP/cAMP Cross-Talk

3′,5′-cyclic adenosine monophosphate (cAMP) is well-known for its diverse immunomodulatory properties, primarily inhibitory effects during T cell activation, proliferation, and production of pro-inflammatory cytokines. A decrease in cAMP levels, due to the hydrolyzing activity of phosphodiesterases (PDE), is favoring inflammatory responses. This can be prevented by selective PDE inhibitors, which makes PDEs important therapeutic targets for autoimmune disorders. In this study, we investigated the specific roles of PDE2A and PDE3B in the regulation of intracellular cAMP levels in different mouse T cell subsets. Unexpectedly, T cell receptor (TCR) activation led to a selective upregulation of PDE2A at the protein level in conventional T cells (Tcon), whereas no changes were detected in regulatory T cells (Treg). In contrast, protein expression of PDE3B was significantly higher in both non-activated and activated Tcon subsets as compared to Treg, with no changes upon TCR engagement. Live-cell imaging of T cells expressing a highly sensitive Förster resonance energy transfer (FRET)-based biosensor, Epac1-camps, has enabled cAMP measurements in real time and revealed stronger responses to the PDE2A inhibitors in activated vs non-activated Tcon. Importantly, stimulation of intracellular cGMP levels with natriuretic peptides led to an increase of cAMP in non-activated and a decrease of cAMP in activated Tcon, suggesting that TCR activation changes the PDE3B-dependent positive to PDE2A-dependent negative cGMP/cAMP cross-talk. Functionally, this switch induced higher expression of early activation markers CD25 and CD69. This constitutes a potentially interesting feed-forward mechanism during autoimmune and inflammatory responses that may be exploited therapeutically.

The Role of PDE8 in T Cell Recruitment and Function in Inflammation

Inhibitors targeting cyclic nucleotide phosphodiesterases (PDEs) expressed in leukocytes have entered clinical practice to treat inflammatory disorders, with three PDE4 inhibitors currently in clinical use as therapeutics for psoriasis, psoriatic arthritis, atopic dermatitis and chronic obstructive pulmonary disease. In contrast, the PDE8 family that is upregulated in pro-inflammatory T cells is a largely unexplored therapeutic target. It was shown that PDE8A plays a major role in controlling T cell and breast cancer cell motility, including adhesion to endothelial cells under physiological shear stress and chemotaxis. This is a unique function of PDE8 not shared by PDE4, another cAMP specific PDE, employed, as noted, as an anti-inflammatory therapeutic. Additionally, a regulatory role was shown for the PDE8A-rapidly accelerated fibrosarcoma (Raf)-1 kinase signaling complex in myelin antigen reactive CD4⁺ effector T cell adhesion and locomotion by a mechanism differing from that of PDE4. The PDE8A-Raf-1 kinase signaling complex affects T cell motility, at least in part, via regulating the LFA-1 integrin mediated adhesion to ICAM-1. The findings that PDE8A and its isoforms are expressed at higher levels in naive and myelin oligodendrocyte glycoprotein (MOG)_35–55 activated effector T (Teff) cells compared to regulatory T (Treg) cells and that PDE8 inhibition specifically affects MOG_35–55 activated Teff cell adhesion, indicates that PDE8A could represent a new beneficial target expressed in pathogenic Teff cells in CNS inflammation. The implications of this work for targeting PDE8 in inflammation will be discussed in this review.

Therapeutic Implications for PDE2 and cGMP/cAMP Mediated Crosstalk in Cardiovascular Diseases

Phosphodiesterases (PDEs) are the principal superfamily of enzymes responsible for degrading the secondary messengers 3',5'-cyclic nucleotides cAMP and cGMP. Their refined subcellular localization and substrate specificity contribute to finely regulate cAMP/cGMP gradients in various cellular microdomains. Redistribution of multiple signal compartmentalization components is often perceived under pathological conditions. Thereby PDEs have long been pursued as therapeutic targets in diverse disease conditions including neurological, metabolic, cancer and autoimmune disorders in addition to numerous cardiovascular diseases (CVDs). PDE2 is a unique member of the broad family of PDEs. In addition to its capability to hydrolyze both cAMP and cGMP, PDE2 is the sole isoform that may be allosterically activated by cGMP increasing its cAMP hydrolyzing activity. Within the cardiovascular system, PDE2 serves as an integral regulator for the crosstalk between cAMP/cGMP pathways and thereby may couple chronically adverse augmented cAMP signaling with cardioprotective cGMP signaling. This review provides a comprehensive overview of PDE2 regulatory functions in multiple cellular components within the cardiovascular system and also within various subcellular microdomains. Implications for PDE2- mediated crosstalk mechanisms in diverse cardiovascular pathologies are discussed highlighting the prospective use of PDE2 as a potential therapeutic target in cardiovascular disorders.

Role of cyclic nucleotides and their downstream signaling cascades in memory function: Being at the right time at the right spot

A plethora of studies indicate the important role of cAMP and cGMP cascades in neuronal plasticity and memory function. As a result, altered cyclic nucleotide signaling has been implicated in the pathophysiology of mnemonic dysfunction encountered in several diseases. In the present review we provide a wide overview of studies regarding the involvement of cyclic nucleotides, as well as their upstream and downstream molecules, in physiological and pathological mnemonic processes. Next, we discuss the regulation of the intracellular concentration of cyclic nucleotides via phosphodiesterases, the enzymes that degrade cAMP and/or cGMP, and via A-kinase-anchoring proteins that refine signal compartmentalization of cAMP signaling. We also provide an overview of the available data pointing to the existence of specific time windows in cyclic nucleotide signaling during neuroplasticity and memory formation and the significance to target these specific time phases for improving memory formation. Finally, we highlight the importance of emerging imaging tools like Förster resonance energy transfer imaging and optogenetics in detecting, measuring and manipulating the action of cyclic nucleotide signaling cascades.

Comparative Assessment of the New PDE7 Inhibitor - GRMS-55 and Lisofylline in Animal Models of Immune-Related Disorders: A PK/PD Modeling Approach

Purpose

This study aimed to assess the activity of two phosphodiesterase (PDE) inhibitors, namely GRMS-55 and racemic lisofylline ((±)-LSF)) in vitro and in animal models of immune-mediated disorders.

Methods

Inhibition of human recombinant (hr)PDEs and TNF-alpha release from LPS-stimulated whole rat blood by the studied compounds were assessed in vitro. LPS-induced endotoxemia, concanavalin A (ConA)-induced hepatitis, and collagen-induced arthritis (CIA) animal models were used for in vivo evaluation. The potency of the investigated compounds was evaluated using PK/PD and PK/PD/disease progression modeling.

Results

GRMS-55 is a potent hrPDE7A and hrPDE1B inhibitor, while (±)-LSF most strongly inhibits hrPDE3A and hrPDE4B. GRMS-55 decreased TNF-alpha levels in vivo and CIA progression with IC₅₀ of 1.06 and 0.26 mg/L, while (±)-LSF with IC₅₀ of 5.80 and 1.06 mg/L, respectively. Moreover, GRMS-55 significantly ameliorated symptoms of ConA-induced hepatitis.

Conclusions

PDE4B but not PDE4D inhibition appears to be mainly engaged in anti-inflammatory activity of the studied compounds. GRMS-55 and (±)-LSF seem to be promising candidates for future studies on the treatment of immune-related diseases. The developed PK/PD models may be used to assess the anti-inflammatory and anti-arthritic potency of new compounds for the treatment of rheumatoid arthritis and other inflammatory disorders.

Electronic supplementary material

The online version of this article (10.1007/s11095-019-2727-z) contains supplementary material, which is available to authorized users.

cAMP Catalyzing Phosphodiesterases Control Cholinergic Muscular Activity But Their Inhibition Does Not Enhance 5-HT4 Receptor-Mediated Facilitation of Cholinergic Contractions in the Murine Gastrointestinal Tract

Background: As the signal transduction of 5-HT₄ receptors on cholinergic neurons innervating smooth muscle is controlled by phosphodiesterase (PDE) 4 in porcine stomach and colon, and human large intestine, the in vivo gastroprokinetic effects of a 5-HT₄ receptor agonist might be enhanced by combination with a selective PDE4 inhibitor. The presence of 5-HT₄ receptors on cholinergic neurons towards murine gastrointestinal circular muscle was recently shown. If the control of this receptor pathway by PDE4 is also present in mice, this might be a good model for in vivo testing of the combination therapy. Therefore this study investigates the role of cAMP catalyzing PDEs in smooth muscle cell activity and in the intraneuronal signal transduction of the 5-HT₄ receptors in the gastrointestinal tract of C57Bl/6J mice.

Methods: In circular smooth muscle strips from murine fundus, jejunum, and colon, submaximal cholinergic contractions were induced by either electrical field stimulation (EFS) or by carbachol (muscarinic receptor agonist). The influence of the PDE inhibitors IBMX (non-selective), vinpocetine (PDE1), EHNA (PDE2), cilostamide (PDE3), and rolipram (PDE4) was tested on these contractions and on the facilitating effect of a submaximal concentration of prucalopride (5-HT₄ receptor agonist) on EFS-induced contractions.

Results: In the three gastrointestinal regions, IBMX and cilostamide concentration-dependently decreased carbachol- as well as EFS-induced contractions. Some inhibitory effect was also observed with rolipram. In the fundus a non-significant trend for an enhancement of the facilitating effect of prucalopride on EFS-induced contractions was observed with IBMX, but none of the selective PDE inhibitors enhanced the facilitating effect of prucalopride in fundus, jejunum or colon.

Conclusion: In analogy with the porcine gastrointestinal tract, in murine fundus, jejunum, and colon circular smooth muscle PDE3 is the main regulator of the cAMP turnover, with some contribution of PDE4. In contrast to the porcine gastrointestinal tract, the in vitro facilitation of electrically induced cholinergic contractions by 5-HT₄ receptor stimulation could not be enhanced by specific PDE inhibition. The C57Bl/6J murine model is thus not suitable for in vivo testing of a 5-HT₄ receptor agonist combined with a selective PDE4 inhibitor.

Synergy between 5-HT4 receptor stimulation and phosphodiesterase 4 inhibition in facilitating acetylcholine release in human large intestinal circular muscle

BACKGROUND

Gastroprokinetic properties of 5-HT₄ receptor agonists, such as prucalopride, are attributed to activation of 5-HT₄ receptors on cholinergic nerves innervating smooth muscle in the gastrointestinal smooth muscle layer, increasing acetylcholine release and muscle contraction. In porcine stomach and colon, phosphodiesterase (PDE) 4 has been shown to control the signaling pathway of these 5-HT₄ receptors. The aim of this study was to investigate the PDE-mediated control of these 5-HT₄ receptors in human large intestine.

METHODS

Circular smooth muscle strips were prepared from human large intestine; after incubation with [³H]-choline, electrically induced tritium outflow was determined as a measure for acetylcholine release. The influence of PDE inhibition on the facilitating effect of prucalopride on electrically induced acetylcholine release was studied.

KEY RESULTS

The non-selective PDE inhibitor IBMX enhanced the facilitating effect of prucalopride on electrically induced acetylcholine release. The selective inhibitors vinpocetine (PDE1), EHNA (PDE2) and cilostamide (PDE3) did not influence, while rolipram and roflumilast (PDE4) enhanced the prucalopride-induced facilitation to the same extent as IBMX.

CONCLUSIONS & INFERENCES

In human large intestinal circular muscle, the intracellular pathway of 5-HT₄ receptors facilitating cholinergic neurotransmission to large intestinal circular smooth muscle is controlled by PDE4. If the synergy between 5-HT₄ receptor agonism and PDE4 inhibition is confirmed in a functional assay with electrically induced cholinergic contractions of human large intestinal circular smooth muscle strips, combination of a selective 5-HT₄ receptor agonist with a selective PDE4 inhibitor might enhance the in vivo prokinetic effect of the 5-HT₄ receptor agonist in the large intestine.

PDE2 at the crossway between cAMP and cGMP signalling in the heart

The cyclic nucleotides cAMP and cGMP are central second messengers in cardiac cells and critical regulators of cardiac physiology as well as pathophysiology. Consequently, subcellular compartmentalization allows for spatiotemporal control of cAMP/cGMP metabolism and subsequent regulation of their respective effector kinases PKA or PKG is most important for cardiac function in health and disease. While acute cAMP-mediated signalling is a mandatory prerequisite for the physiological fight-or-flight response, sustained activation of this pathway may lead to the progression of heart failure. In contrast, acute as well as sustained cGMP-mediated signalling can foster beneficial features, e.g. anti-hypertrophic and vasodilatory effects. These two signalling pathways seem to be intuitively counteracting and there is increasing evidence for a functionally relevant crosstalk between cAMP and cGMP signalling pathways on the level of cyclic nucleotide hydrolysing phosphodiesterases (PDEs). Among this diverse group of enzymes, PDE2 may fulfill a unique integrator role. Equipped with dual substrate specificity for cAMP as well as for cGMP, it is the only cAMP hydrolysing PDE, which is allosterically activated by cGMP. Recent studies have revealed strongly remodelled cAMP/cGMP microdomains and subcellular concentration profiles in different cardiac pathologies, leading to a putatively enhanced involvement of PDE2 in cAMP/cGMP breakdown and crosstalk compared to the other cardiac PDEs. This review sums up the current knowledge about molecular properties and regulation of PDE2 and explains the complex signalling network encompassing PDE2 in order to better understand the functional role of PDE2 in distinct cell types in cardiac health and disease. Moreover, this review gives an outlook in which way PDE2 may serve as a therapeutic target to treat cardiac disease.

Potential for pharmacological manipulation of human embryonic stem cells

The therapeutic potential of human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) is vast, allowing disease modelling, drug discovery and testing and perhaps most importantly regenerative therapies. However, problems abound; techniques for cultivating self-renewing hESCs tend to give a heterogeneous population of self-renewing and partially differentiated cells and general include animal-derived products that can be cost-prohibitive for large-scale production, and effective lineage-specific differentiation protocols also still remain relatively undefined and are inefficient at producing large amounts of cells for therapeutic use. Furthermore, the mechanisms and signalling pathways that mediate pluripotency and differentiation are still to be fully appreciated. However, over the recent years, the development/discovery of a range of effective small molecule inhibitors/activators has had a huge impact in hESC biology. Large-scale screening techniques, coupled with greater knowledge of the pathways involved, have generated pharmacological agents that can boost hESC pluripotency/self-renewal and survival and has greatly increased the efficiency of various differentiation protocols, while also aiding the delineation of several important signalling pathways. Within this review, we hope to describe the current uses of small molecule inhibitors/activators in hESC biology and their potential uses in the future.

Clinical and molecular genetics of the phosphodiesterases (PDEs)

Cyclic nucleotide phosphodiesterases (PDEs) are enzymes that have the unique function of terminating cyclic nucleotide signaling by catalyzing the hydrolysis of cAMP and GMP. They are critical regulators of the intracellular concentrations of cAMP and cGMP as well as of their signaling pathways and downstream biological effects. PDEs have been exploited pharmacologically for more than half a century, and some of the most successful drugs worldwide today affect PDE function. Recently, mutations in PDE genes have been identified as causative of certain human genetic diseases; even more recently, functional variants of PDE genes have been suggested to play a potential role in predisposition to tumors and/or cancer, especially in cAMP-sensitive tissues. Mouse models have been developed that point to wide developmental effects of PDEs from heart function to reproduction, to tumors, and beyond. This review brings together knowledge from a variety of disciplines (biochemistry and pharmacology, oncology, endocrinology, and reproductive sciences) with emphasis on recent research on PDEs, how PDEs affect cAMP and cGMP signaling in health and disease, and what pharmacological exploitations of PDEs may be useful in modulating cyclic nucleotide signaling in a way that prevents or treats certain human diseases.

The Concise Guide to PHARMACOLOGY 2013/14: enzymes

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Enzymes are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, nuclear hormone receptors, catalytic receptors and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Profiling of cAMP and cGMP phosphodiesterases in isolated ventricular cardiomyocytes from human hearts: comparison with rat and guinea pig

AIMS

Phosphodiesterases (PDEs) are key enzymes controlling cAMP and cGMP levels and spatial distribution within cardiomyocytes. Despite the clinical importance of several classes of PDE inhibitor there has not been a complete characterization of the PDE profile within the human cardiomyocyte, and no attempt to assess which species might best be used to model this for drug evaluation in heart disease.

MAIN METHODS

Ventricular cardiomyocytes were isolated from failing human hearts of patients with various etiologies of disease, and from rat and guinea pig hearts. Expression of PDE isoforms was determined using RT-PCR. cAMP- and cGMP-PDE hydrolytic activity was determined by scintillation proximity assay, before and after treatment with PDE inhibitors for PDEs 1, 2, 3, 4, 5 and 7. Functional effects of cAMP PDEi were determined on the contraction of single human, rat and guinea pig cardiomyocytes.

KEY FINDINGS

The presence and activity of PDE5 were confirmed in ventricular cardiomyocytes from failing and hypertrophied human heart, as well as PDE3, with ventricle-specific results for PDE4 and a surprisingly large contribution from PDE1 for hydrolysis of both cAMP and cGMP. The total PDE activity of human cardiomyocytes, and the profile of inhibition by PDE1, 3, 4, and 5 inhibitors, was modelled well in guinea pig but not rat cardiomyocytes.

SIGNIFICANCE

Our results provide the first full characterisation of human cardiomyocyte PDE isoforms, and suggest that guinea pig myocytes provide a better model than rat for PDE levels and activity.

cGMP signals modulate cAMP levels in a compartment-specific manner to regulate catecholamine-dependent signaling in cardiac myocytes

RATIONALE

cAMP and cGMP are intracellular second messengers involved in heart pathophysiology. cGMP can potentially affect cAMP signals via cGMP-regulated phosphodiesterases (PDEs).

OBJECTIVE

To study the effect of cGMP signals on the local cAMP response to catecholamines in specific subcellular compartments.

METHODS AND RESULTS

We used real-time FRET imaging of living rat ventriculocytes expressing targeted cAMP and cGMP biosensors to detect cyclic nucleotides levels in specific locales. We found that the compartmentalized, but not the global, cAMP response to isoproterenol is profoundly affected by cGMP signals. The effect of cGMP is to increase cAMP levels in the compartment where the protein kinase (PK)A-RI isoforms reside but to decrease cAMP in the compartment where the PKA-RII isoforms reside. These opposing effects are determined by the cGMP-regulated PDEs, namely PDE2 and PDE3, with the local activity of these PDEs being critically important. The cGMP-mediated modulation of cAMP also affects the phosphorylation of PKA targets and myocyte contractility.

CONCLUSIONS

cGMP signals exert opposing effects on local cAMP levels via different PDEs the activity of which is exerted in spatially distinct subcellular domains. Inhibition of PDE2 selectively abolishes the negative effects of cGMP on cAMP and may have therapeutic potential.

Erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) blocks differentiation and maintains the expression of pluripotency markers in human embryonic stem cells

hESCs (human embryonic stem cells) have enormous potential for use in pharmaceutical development and therapeutics; however, to realize this potential, there is a requirement for simple and reproducible cell culture methods that provide adequate numbers of cells of suitable quality. We have discovered a novel way of blocking the spontaneous differentiation of hESCs in the absence of exogenous cytokines by supplementing feeder-free conditions with EHNA [erythro-9-(2-hydroxy-3-nonyl)adenine], an established inhibitor of ADA (adenosine deaminase) and cyclic nucleotide PDE2 (phosphodiesterase 2). hESCs maintained in feeder-free conditions with EHNA for more than ten passages showed no reduction in hESC-associated markers including NANOG, POU5F1 (POU domain class 5 transcription factor 1, also known as Oct-4) and SSEA4 (stage-specific embryonic antigen 4) compared with cells maintained in feeder-free conditions containing bFGF (basic fibroblast growth factor). Spontaneous differentiation was reversibly suppressed by the addition of EHNA, but, upon removing EHNA, hESC populations underwent efficient spontaneous, multi-lineage and directed differentiation. EHNA also acts as a strong blocker of directed neuronal differentiation. Chemically distinct inhibitors of ADA and PDE2 lacked the capacity of EHNA to suppress hESC differentiation, suggesting that the effect is not driven by inhibition of either ADA or PDE2. Preliminary structure–activity relationship analysis found the differentiation-blocking properties of EHNA to reside in a pharmacophore comprising a close adenine mimetic with an extended hydrophobic substituent in the 8- or 9-position. We conclude that EHNA and simple 9-alkyladenines can block directed neuronal and spontaneous differentiation in the absence of exogenous cytokine addition, and may provide a useful replacement for bFGF in large-scale or cGMP-compliant processes.

Identification and characterization of small-molecule ligands that maintain pluripotency of human embryonic stem cells

hESCs (human embryonic stem cells) offer great potential for pharmaceutical research and development and, potentially, for therapeutic use. However, improvements in cell culture are urgently required to allow the scalable production of large numbers of cells that maintain pluripotency. Supplementing feeder-free conditions with either EHNA [erythro-9-(2-hydroxy-3-nonyl)adenine] or readily synthesized analogues of this compound maintains hESC pluripotency in the absence of exogenous cytokines. When the hESC lines SA121 or SA461 were maintained in feeder-free conditions with EHNA they displayed no reduction in stem-cell-associated markers such as Nanog, Oct4 (octamer-binding protein 4) and SSEA4 (stage-specific embryonic antigen 4) when compared with cells maintained in full feeder-free conditions that included exogenously added bFGF (basic fibroblast growth factor). Spontaneous differentiation was reversibly suppressed by the addition of EHNA, but EHNA did not limit efficient spontaneous or directed differentiation following its removal. We conclude that EHNA or related compounds offers a viable alternative to exogenous cytokine addition in maintaining hESC cultures in a pluripotent state and might be a particularly useful replacement for bFGF for large-scale or GMP (good manufacturing practice)-compliant processes.

Chronic lymphocytic leukemia and B and T cells differ in their response to cyclic nucleotide phosphodiesterase inhibitors

Phosphodiesterase (PDE)4 inhibitors, which activate cAMP signaling by reducing cAMP catabolism, are known to induce apoptosis in B lineage chronic lymphocytic leukemia (CLL) cells but not normal human T cells. The explanation for such differential sensitivity remains unknown. In this study, we report studies contrasting the response to PDE4 inhibitor treatment in CLL cells and normal human T and B cells. Affymetrix gene chip analysis in the three cell populations following treatment with the PDE4 inhibitor rolipram identified a set of up-regulated transcripts with unusually high fold changes in the CLL samples, several of which are likely part of compensatory negative feedback loops. The high fold changes were due to low basal transcript levels in CLL cells, suggesting that cAMP-mediated signaling may be unusually tightly regulated in this cell type. Rolipram treatment augmented cAMP levels and induced ATF-1/CREB serine 63/133 phosphorylation in both B lineage cell types but not T cells. As treatment with the broad-spectrum PDE inhibitor 3-isobutyl-1-methylxanthine induced T cell CREB phosphorylation, we tested a series of family-specific PDE inhibitors for their ability to mimic 3-isobutyl-1-methylxanthine-induced ATF-1/CREB phosphorylation. Whereas PDE3 inhibitors alone had no effect, the combination of PDE3 and PDE4 inhibitors induced ATF-1/CREB serine 63/133 phosphorylation in T cells. Consistent with this observation, PDE3B transcript and protein levels were low in CLL cells but easily detectable in T cells. Combined PDE3/4 inhibition did not induce T cell apoptosis, suggesting that cAMP-mediated signal transduction that leads to robust ATF-1/CREB serine 63/133 phosphorylation is not sufficient to induce apoptosis in this lymphoid lineage.

Increased particulate phosphodiesterase 4 in the prefrontal cortex supports 5-HT4 receptor-induced improvement of object recognition memory in the rat

RATIONALE

Serotonin receptors (5-HT4Rs) are critical to both short-term and long-term memory processes. These receptors mainly trigger the cyclic adenosine monophosphate (cAMP)/protein kinase A signaling pathway, which is regulated by cAMP phosphodiesterases (PDEs).

OBJECTIVES

We investigated the mechanisms underlying the effect of the selective activation of 5-HT4R on information acquisition in an object recognition memory task and the putative regulation of PDE.

MATERIALS AND METHODS

The effect of RS 67333 (1 mg/kg, intraperitoneally [i.p.], injected 30 min before the sample phase) was examined at different delay intervals in an object recognition task in Sprague-Dawley rats. After the testing trial, PDE activity of brain regions implicated in this task was assayed.

RESULTS

RS 67333-treated rats spent more time exploring the novel object after a 15-min (P < 0.001) or 4-h delay (P < 0.01) but not after a 24-h delay, whereas control animals showed no preference for the novel object for delays greater than 15 min. We characterized the specific patterns and kinetic properties of PDE in the prefrontal and perirhinal cortices as well as in the hippocampus. We demonstrated that particulate PDE activities increase in both the prefrontal cortex and hippocampus following 5-HT4R stimulation. In the prefrontal cortex, PDE4 activities support the RS 67333-induced modification of PDE activities, whereas in the hippocampus, all cAMP-PDE activities varied. In contrast, particulate PDE variation in the hippocampus was not found to support improvement of recognition memory after a 4-h delay.

CONCLUSIONS

We provide evidence that the increase in particulate PDE4 activity in the prefrontal cortex supports the 5-HT4R-induced increase in information acquisition.

cAMP and cGMP signaling cross-talk: role of phosphodiesterases and implications for cardiac pathophysiology

Cyclic nucleotide phosphodiesterases regulate cAMP-mediated signaling by controlling intracellular cAMP content. The cAMP-hydrolyzing activity of several families of cyclic nucleotide phosphodiesterases found in human heart is regulated by cGMP. In the case of PDE2, this regulation primarily involves the allosteric stimulation of cAMP hydrolysis by cGMP. For PDE3, cGMP acts as a competitive inhibitor of cAMP hydrolysis. Several cGMP-mediated responses in cardiac cells, including a potentiation of Ca(2+) currents and a diminution of the responsiveness to beta-adrenergic receptor agonists, have been shown to result from the effects of cGMP on cAMP hydrolysis. These effects appear to be dependent on the specific spatial distribution of the cGMP-generating and cAMP-hydrolyzing proteins, as well as on the intracellular concentrations of the two cyclic nucleotides. Gaining a more precise understanding of how these cross-talk mechanisms are individually regulated and coordinated is an important direction for future research.

Phosphodiesterase 8 (PDE8) regulates chemotaxis of activated lymphocytes

The immune system depends on chemokines to recruit lymphocytes to tissues in inflammatory diseases. This study identifies PDE8 as a new target for inhibition of chemotaxis of activated lymphocytes. Chemotactic responses of unstimulated and concanavalin A-stimulated mouse splenocytes and their modulation by agents that stimulate the cAMP signaling pathway were compared. Dibutyryl cAMP inhibited migration of both cell types. In contrast, forskolin and 3-isobutyl-1-methylxanthine each inhibited migration of unstimulated splenocytes, with little effect on migration of stimulated splenocytes. Only dipyridamole alone, a PDE inhibitor capable of inhibiting PDE8, strongly inhibited migration of stimulated and unstimulated splenocytes and this inhibition was enhanced by forskolin and reversed by a PKA antagonist. Following concanavalin A stimulation, mRNA for PDE8A1 was induced. These results suggest that in employing PDE inhibitor therapy for inflammatory illnesses, inhibition of PDE8 may be required to inhibit migration of activated lymphocytes to achieve a full therapeutic effect.

Cyclic nucleotide phosphodiesterases as targets for treatment of haematological malignancies

The cAMP signalling pathway has emerged as a key regulator of haematopoietic cell proliferation, differentiation and apoptosis. In parallel, general understanding of the biology of cyclic nucleotide PDEs (phosphodiesterases) has advanced considerably, revealing the remarkable complexity of this enzyme system that regulates the amplitude, kinetics and location of intracellular cAMP-mediated signalling. The development of therapeutic inhibitors of specific PDE gene families has resulted in a growing appreciation of the potential therapeutic application of PDE inhibitors to the treatment of immune-mediated illnesses and haematopoietic malignancies. This review summarizes the expression and function of PDEs in normal haematopoietic cells and the evidence that family-specific inhibitors will be therapeutically useful in myeloid and lymphoid malignancies.

Compartmentalized phosphodiesterase-2 activity blunts beta-adrenergic cardiac inotropy via an NO/cGMP-dependent pathway

beta-Adrenergic signaling via cAMP generation and PKA activation mediates the positive inotropic effect of catecholamines on heart cells. Given the large diversity of protein kinase A targets within cardiac cells, a precisely regulated and confined activity of such signaling pathway is essential for specificity of response. Phosphodiesterases (PDEs) are the only route for degrading cAMP and are thus poised to regulate intracellular cAMP gradients. Their spatial confinement to discrete compartments and functional coupling to individual receptors provides an efficient way to control local [cAMP]i in a stimulus-specific manner. By performing real-time imaging of cyclic nucleotides in living ventriculocytes we identify a prominent role of PDE2 in selectively shaping the cAMP response to catecholamines via a pathway involving beta3-adrenergic receptors, NO generation and cGMP production. In cardiac myocytes, PDE2, being tightly coupled to the pool of adenylyl cyclases activated by beta-adrenergic receptor stimulation, coordinates cGMP and cAMP signaling in a novel feedback control loop of the beta-adrenergic pathway. In this, activation of beta3-adrenergic receptors counteracts cAMP generation obtained via stimulation of beta1/beta2-adrenoceptors. Our study illustrates the key role of compartmentalized PDE2 in the control of catecholamine-generated cAMP and furthers our understanding of localized cAMP signaling.

Irsogladine, an anti-ulcer drug, suppresses superoxide production by inhibiting phosphodiesterase type 4 in human neutrophils

Neutrophil superoxide production is implicated in the pathogenesis of gastric mucosal damage induced by various ulcerative agents and Helicobacter pylori infection. We investigated here the effects of an anti-ulcer drug irsogladine [2, 4-diamino-6-(2, 5-dichlorophenyl)-s-triazine maleate] on cAMP formation in isolated human neutrophils. The cAMP level in human neutrophils was elevated by a phosphodiesterase (PDE) type 4 selective inhibitor rolipram, but not by any inhibitors of PDE1, PDE2 and PDE3. Irsogladine also increased cAMP formation in a concentration-dependent manner in neutrophils. A non-selective PDE inhibitor 3-isobutyl-1-methylxanthine (IBMX) alone significantly increased cAMP level, whereas irsogladine was unable to further increase cAMP level in the presence of IBMX. Irsogladine inhibited concentration-dependently the superoxide (O(2)(-)) production induced by various stimuli including formyl-methionyl-leucyl-phenylalanine, opsonized zymosan, guanosine 5'-[gamma-thio] triphosphate, A23187 and phorbol 12-myristate 13-acetate. These effects of irsogladine were mimicked by rolipram, IBMX and dibutyryl cAMP. The inhibitory effects of irsogladine and rolipram on the O(2)(-) production were reversed by a protein kinase A inhibitor H-89. These results indicate that irsogladine inhibits the superoxide production in human neutrophils by the increase of cAMP content by PDE 4 inhibition, which in turn contributing to the anti-ulcer effects of irsogladine on gastric mucosal lesions associated with oxidative stress.

Phosphodiesterase inhibition by a gastroprotective agent irsogladine: preferential blockade of cAMP hydrolysis

The effect of irsogladine [2,4-diamino-6-(2,5-dichlorophenyl)-s-triazine maleate], an antiulcer drug, on contents of cyclic nucleotides including cAMP and cGMP was investigated in rat stomachs. Irsogladine concentration-dependently increased cAMP content in rat glandula stomach. However, irsogladine at higher concentration (10(-5) M) was unable to further increase cAMP level in the presence of non-selective phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine, although 3-isobutyl-1-methylxanthine by itself increased cAMP level. On the other hand, irsogladine had no effect on the glandula cGMP content. Subsequently, the effect of irsogladine on the cyclic nucleotide degradation by purified bovine brain and heart PDEs was investigated. The cAMP degradation by purified bovine brain PDE was partially suppressed by PDE1 inhibitor vinpocetin, PDE2 inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride and PDE4 inhibitor rolipram but not by PDE3 inhibitor cilostamide, and completely inhibited by 3-isobutyl-1-methylxanthine, suggesting that is attributed almost exclusively to PDE1, PDE2 and PDE4. Meanwhile, cGMP degradation by purified bovine brain PDE was partially suppressed by erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride. Irsogladine preferentially inhibited the response to cAMP degradation compared with cGMP degradation by this brain PDE. The cAMP degradation by bovine heart PDE was almost completely inhibited by the combination with vinpocetine and cilostamide, indicating that is mediated almost exclusively by PDE1 and PDE3. Irsogladine suppressed this cAMP degradation measured in the presence of vinpocetine to almost the same extent as that determined in the presence of cilostamide. These results indicate that irsogladine produces the increase of intracellular cAMP content via non-selective inhibition of PDE isozymes, which may be a key mechanism involved in its gastroprotective actions.

Phosphoinositide 3-kinase is involved in Xenopus and Labrus melanophore aggregation

Melanophores are pigmented cells capable of quick colour changes through coordinated transport of their intracellular pigment granules. We demonstrate the involvement of phosphoinositide 3-kinase (PI3-K) in Xenopus and Labrus aggregation by the use of the PI3-K inhibitor, LY-294002. In Xenopus, wortmannin-insensitive PI3-K was found to be essential for the aggregation, mitogen-activated protein kinase (MAPK) activation and tyrosine phosphorylation of a 280-kDa protein, and for the maintenance of low cyclic adenosine 3':5'-monophosphate (cAMP) during the aggregated state. Pre-aggregated cells disperse completely to LY-294002 at 50-100 muM, involving a transient elevation in cAMP due to adenylate cyclase (AC) stimulation or to inhibition of cyclic nucleotide phosphodiesterase (PDE). The inactive analogue LY-303511 did not induce dispersion at the same concentrations. PDE4 and/or PDE2 was found to be involved in melanosome aggregation. The similar kinetics of LY-294002 and various PDE inhibitors indicates that the elevation of cAMP might be due to inhibition of PDE. In Labrus melanophores, LY-294002 had a less dramatic effect, probably due to less dependence on PDE in regulation of cAMP levels. In Xenopus aggregation, we suggest that melatonin stimulation of the Mel1c receptor via G(beta gamma) activates PI3-K that, directly or indirectly via MAPK, activates PDE.

Subacute toxicity and toxicokinetics of CJ-10882, a type IV phosphodiesterase inhibitor, after 4-week repeated oral administration in dogs

The subacute toxicity and toxicokinetics of a type IV phosphodiesterase inhibitor, CJ-10882, were evaluated after single (on the 1st day) and 4-week (on the 27th day) oral administration of the drug, in doses of 0 (to serve as a control), 2, 10 and 50 mg/kg/day, to male and female dogs (n=3 for male and female dogs for each dose). During the test period, clinical signs, mortality, body weight, food consumption, ophthalmoscopy, urinalysis, hematology, serum biochemistry, gross findings, organ weight and histopathology were examined. The 4-week repeated oral doses of CJ-10882 resulted in salivation, vomiting, and atrophy of the thymus. The absolute toxic dose was 50 mg/kg/day and the level at which no adverse effects were observed was 2 mg/kg/day for male and female dogs. There were no significant gender differences in the pharmacokinetic parameters of CJ-10882 for each dose after both single and 4-week oral administration. The pharmacokinetic parameters of CJ-10882 were dose independent after a single oral administration; the time to reach a peak plasma concentration (T(max)) and the dose-normalized area under the plasma concentration-time curve from time zero to 8 h in plasma (AUC(0-8 h)) were not significantly different among three doses. The accumulation of CJ-10882 after 4-week oral administration was not notable at the toxic dose of 50 mg/kg/day. For example, after 4-week administration, the dose-normalized AUC(0-8 h) value at 50 mg/kg/day (0.132 microg h/ml) was not significantly greater than that at 10 mg/kg/day (0.131 microg h/ml). After 4-week oral administration, the dose-normalized C(max) and AUC(0-8 h) at 50 mg/kg/day were not significantly higher and greater, respectively, than those after the single oral administration.

Localization of the nitric oxide/cGMP signaling pathway-related genes and influences of morpholino knock-down of soluble guanylyl cyclase on medaka fish embryogenesis

To better understand the nitric oxide (NO) / cyclic GMP (cGMP) signaling pathway during embryogenesis, we examined the spatial and temporal expression pattern of the genes for neuronal nitric oxide synthase (nNOS), soluble guanylyl cyclase (soluble GC) subunit (OlGCS-alpha(1), OlGCS-alpha(2), and OlGCS-beta(1)), and cGMP-dependent protein kinase (cGK) I and II (cGK I and cGK II) in the medaka fish embryos. OlGCS-beta(1) and nNOS were expressed maternally and OlGCS-alpha(1), OlGCS-alpha(2),and cGK II were expressed zygotically. The zygotic expression of OlGCS-alpha(1) and cGK I was detected at stage 19, while that of OlGCS-alpha(2) was detected at stage 16. Whole-mount in situ hybridization showed that the expression of nNOS or cGK I was localized in tail bud, otic vesicles, thyroid, and brain ventricle, or in thymus, gill arch, and olfactory pits, respectively, and that of OlGCS-alpha(1), OlGCS-alpha(2), or OlGCS-beta(1) was dim and dispersed throughout the embryos. To clarify the "role of the NO/cGMP signaling pathway in embryogenesis, we examined the influences of morpholino antisense oligonucleotide of the soluble GC subunit gene (alpha(1)-MO, alpha(2)-MO or beta(1)-MO) on development of medaka fish embryos. Embryos injected with alpha(1)-MO or alpha(2)-MO mainly exhibited abnormalities in the central nervous system, including defects in the formation of forebrain, eye, and otic vesicles. alpha(2)-MO injection caused cell death at the tail bud of the embryos at stage 22, and beta(1)-MO injection inhibited the development of the embryos at late blastula. These results suggest that the NO/cGMP signaling pathway plays critical roles in early embryogenesis.

Kinetics of nitric oxide-cyclic GMP signalling in CNS cells and its possible regulation by cyclic GMP

Physiologically, nitric oxide (NO) signal transduction occurs through soluble guanylyl cyclase (sGC), which catalyses cyclic GMP (cGMP) formation. Knowledge of the kinetics of NO-evoked cGMP signals is therefore critical for understanding how NO signals are decoded. Studies on cerebellar astrocytes showed that sGC undergoes a desensitizing profile of activity, which, in league with phosphodiesterases (PDEs), was hypothesized to diversify cGMP responses in different cells. The hypothesis was tested by examining the kinetics of cGMP in rat striatal cells, in which cGMP accumulated in neurones in response to NO. Based on the effects of selective PDE inhibitors, cGMP hydrolysis following exposure to NO was attributed to a cGMP-stimulated PDE (PDE 2). Analysis of NO-induced cGMP accumulation in the presence of a PDE inhibitor indicated that sGC underwent marked desensitization. However, the desensitization kinetics determined under these conditions described poorly the cGMP profile observed in the absence of the PDE inhibitor. An explanation shown plausible theoretically was that cGMP determines the level of sGC desensitization. In support, tests in cerebellar astrocytes indicated an inverse relationship between cGMP level and recovery of sGC from its desensitized state. We suggest that the degree of sGC desensitization is related to the cGMP concentration and that this effect is not mediated by (de)phosphorylation.

Type IV phosphodiesterase activity specifically regulates cAMP-stimulated casein secretion in the rat mammary gland

This study investigates the regulation of cAMP-stimulated casein secretion in rat mammary explants by cAMP phosphodiesterase (cAMP-PDE) activity. cAMP-PDE activity of the lactating rat mammary gland is shown to be provided by three families, types II, III and IV. In mammary explants, general inhibition of the cAMP-PDE activity significantly increased the rate of cAMP-stimulated casein secretion. This effect could be mimicked using the type-IV specific inhibitor rolipram but not by the specific, or combined, inhibition of the type II and type III activity. Only type IV activity significantly affected intracellular accumulation of cAMP whereas all three cAMP-PDE activities were shown to influence the PKA activation ratio in cells. RtPCR analysis showed that the mammary gland apparently expresses just three type IV isozymes, RNPDE4A5, RNPDE4A8 and RNPDE4D3. A specific role for type IV cAMP-PDE activity in the regulation of casein secretion is suggested and possible mechanisms for the effects of PDEIV activity discussed.

Cyclic nucleotide phosphodiesterases

Cyclic nucleotide second messengers (cAMP and cGMP) play a central role in signal transduction and regulation of physiologic responses. Their intracellular levels are controlled by the complex superfamily of cyclic nucleotide phosphodiesterase (PDE) enzymes. Continuing advances in our understanding of the molecular pharmacology of these enzymes has led to the development of selective inhibitors as therapeutic agents for disease states ranging from cancer and heart failure to depression and sexual dysfunction. Several PDE types have been identified as therapeutic targets for immune/inflammatory diseases. This article briefly reviews the available in vitro, preclinical, and clinical data supporting the potential for selective PDE inhibitors as immunomodulatory agents.

Characterization of an in vivo hormonally regulated phosphodiesterase 3 (PDE3) associated with a liver Golgi-endosomal fraction

The biochemical properties of an in vivo hormonally regulated low Km cAMP phosphodiesterase (PDE) activity associated with a liver Golgi-endosomal (GE) fraction have been characterized. DEAE-Sephacel chromatography of a GE fraction solubilized by a lysosomal extract resulted in the sequential elution of three peaks of activity (numbered I, II, and III), while ion-exchange HPLC resolved five peaks of activity (numbered 1, 2, 3, 4, and 5). Based on the sensitivity of the eluted activity to cGMP and selected phosphodiesterase inhibitors, two phosphodiesterase isoforms were resolved: a cGMP-stimulated and EHNA-inhibited PDE2, eluted in DEAE-Sephacel peak I and HPLC peak 2 and a cGMP-, a cilostamide-, and ICI 118233-inhibited PDE3, eluted in DEAE-Sephacel peak III and HPLC peaks 3, 4, and 5. GE fractions isolated after acute treatments with insulin, tetraiodoglucagon, and growth hormone displayed an increase in phosphodiesterase activity relative to saline-injected controls, as did GE fractions from genetically obese and hyperinsulinemic rats relative to lean littermates. In all experimental rats, an increase in PDE3 activity associated with DEAE-Sephacel peak III and HPLC peaks 4 and 5 was observed relative to control animals. Furthermore, in genetically obese Zucker rats, an increase in the sensitivity of PDE activity to cilostamide and in the amount of PDE activity immunoprecipitated by an antibody to adipose tissue PDE3 was observed relative to lean littermates. These results extend earlier studies on isolated hepatocytes and show that liver PDE3 is the main if not sole PDE isoform activated by insulin, glucagon, and growth hormone in vivo.

The effects of phosphodiesterase inhibition on cyclic GMP and cyclic AMP accumulation in the hippocampus of the rat

The effects of selective and non-selective 3',5'-cyclic nucleotide phosphodiesterase (PDE) inhibitors on cGMP and cAMP accumulation were studied in rat hippocampal slices incubated in vitro. The following PDE inhibitors were used: vinpocetine and calmidazolium (PDE1 selective), erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, PDE2 selective), SK&F 95654 (PDE3 selective), rolipram (PDE4 selective), SK&F 96231 (PDE5 selective), the mixed type inhibitors zaprinast and dipyridamole, and the non-selective inhibitors 3-isobutyl-1-metylxanthine (IBMX) and caffeine. cGMP levels were increased in the presence of different concentrations of IBMX, EHNA, dipyridamole, vinpocetine and rolipram. cGMP immunocytochemistry showed that incubation with different inhibitors in the presence and/or absence of sodium nitroprusside resulted in pronounced differences in the extent and regional localization of the cGMP response and indicate that PDE activity in the hippocampus is high and diverse in nature. The results suggest an interaction between cGMP and cAMP signalling pathways in astrocytes of the rat hippocampus.

Evidence for increased dorsal hippocampal adenosine release and metabolism during pharmacologically induced seizures in rats

There is growing pharmacological evidence from several animal models of seizure disorder that adenosine possesses endogenous anticonvulsant activity. In order to further evaluate the role of adenosine in seizure activity, we monitored adenosine and its major biochemical metabolites inosine, xanthine, and hypoxanthine in the dorsal hippocampus by in vivo microdialysis before and during the induction of generalized seizures. Seizures were induced pharmacologically in groups of urethane-anesthetized rats by the administration of bicuculline (0.5 mg/kg, i.v.), kainic acid (12.0 mg/kg, i.v.) or pentylenetetrazol (100-250 mg/kg, i.p). Seizure activity was monitored electrophysiologically from the dorsal hippocampus. Dialysate hippocampal purine levels increased during all three seizure types. The largest increases were for the adenosine metabolites hypoxanthine and inosine, with smaller increases observed for adenosine and xanthine. Intra-hippocampal perfusion with the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl-adenine, (EHNA, 300 microM), only slightly increased basal hippocampal adenosine. Guanosine levels in the hippocampus, a purine not directly related to adenosine metabolism, were unaffected by all treatments. These findings demonstrate that an increase in hippocampal adenosine release and metabolism is associated with seizure activity and support the hypothesis that the increased adenosine levels may attenuate hippocampal seizure activity, possibly by terminating ongoing seizures and altering the pattern of subsequent seizures.

The cAMP signaling pathway as a therapeutic target in lymphoid malignancies

Certain subsets of lymphoid cells, such as thymocytes or peripheral B cells, undergo apoptosis after treatment with agents which elevate intracellular 3',5' cyclic adenosine monophosphate (cAMP). Investigators have also noted induction of apoptosis of chronic lymphocytic leukemia (CLL) cells following treatment with methylxanthines, a phenomenon that may, at least in part, be due to the activity of these drugs as non-specific phosphodiesterase (PDE) inhibitors. We discuss three general strategies for altering cAMP-mediated signal transduction in lymphoid cells. After a review of what is known about the expression and regulation of PDE families in human lymphoid cells, we focus on the use of isoform-specific PDE inhibitors as potential therapeutic agents in CLL. Our work has suggested that despite the presence of PDE1, PDE3B, PDE4 and PDE7 enzymes in CLL, inhibition of PDE4 results in uniquely potent induction of apoptosis in CLL cells. This effect is relatively specific as comparable treatment of human peripheral blood T cells does not induce apoptosis. Clinical trials utilizing PDE4 inhibitors are indicated in the therapy of CLL patients resistant to standard therapy.

Microvascular permeability

This review addresses classical questions concerning microvascular permeabiltiy in the light of recent experimental work on intact microvascular beds, single perfused microvessels, and endothelial cell cultures. Analyses, based on ultrastructural data from serial sections of the clefts between the endothelial cells of microvessels with continuous walls, conform to the hypothesis that different permeabilities to water and small hydrophilic solutes in microvessels of different tissues can be accounted for by tortuous three-dimensional pathways that pass through breaks in the junctional strands. A fiber matrix ultrafilter at the luminal entrance to the clefts is essential if microvascular walls are to retain their low permeability to macromolecules. Quantitative estimates of exchange through the channels in the endothelial cell membranes suggest that these contribute little to the permeability of most but not all microvessels. The arguments against the convective transport of macromolecules through porous pathways and for the passage of macromolecules by transcytosis via mechanisms linked to the integrity of endothelial vesicles are evaluated. Finally, intracellular signaling mechanisms implicated in transient increases in venular microvessel permeability such as occur in acute inflammation are reviewed in relation to studies of the molecular mechanisms involved in signal transduction in cultured endothelial cells.

Cyclic nucleotide phosphodiesterase (PDE) inhibitors and immunomodulation

Intracellular levels of cyclic nucleotide second messengers are regulated predominantly by the complex superfamily of cyclic nucleotide phosphodiesterase (PDE) enzymes. Recent advances in our understanding of the molecular pharmacology of these enzymes has led to their identification as biologic regulators of certain disease states and the development of isozyme-selective inhibitors as potential therapeutic agents. A large body of in vitro and preclinical data suggests the therapeutic utility of PDE4 inhibitors as potent anti-inflammatory agents. Early clinical trials with selective PDE inhibitors substantiate this approach while highlighting pharmacodynamic and toxicologic pitfalls inherent to the inhibition of specific PDE isozymes. This commentary will review our current understanding of PDE inhibitors as immunomodulatory agents.

Specific CD3ε Association of a Phosphodiesterase 4B Isoform Determines Its Selective Tyrosine Phosphorylation After CD3 Ligation

cAMP-specific phosphodiesterases (PDE) comprise an extensive family of enzymes that control intracellular levels of cAMP and thus regulate T cell responses. It is not known how the function of these enzymes is altered by TCR engagement. We have examined this issue by studying one of the PDE isozymes (PDE4B). PDE4B RNA and protein were detected in resting PBLs, and the levels of PDE4B protein increased with cell cycling. In peripheral blood T cells, two previously reported PDE4B isoforms could be detected: one was 75–80 kDa (PDE4B1) and the other was 65–67 kDa (PDE4B2). These two isoforms differed in their N-terminal sequence, with the presence of four potential myristylation sites in the PDE4B2 that are absent in PDE4B1. Consequently, only PDE4B2 was found in association with the CD3ε chain of the TCR. In addition, although both isoforms were phosphorylated in tyrosines in pervanadate-stimulated T cells, only the TCR-associated PDE4B2 was tyrosine-phosphorylated following CD3 ligation. The kinetics of phosphorylation of TCR-associated PDE4B2 correlated with changes in cAMP levels, suggesting that tyrosine phosphorylation of the TCR-associated PDE4B isoform upon engagement of this receptor may be an important regulatory step in PDE4B function. Our results reveal that selectivity of PDE4B activation can be achieved by differential receptor association and phosphorylation of the alternatively spliced forms of this PDE.

Activation of a cGMP-stimulated cAMP phosphodiesterase by protein kinase C in a liver Golgi-endosomal fraction

The ability of Ca2+/phospholipid-dependent protein kinase (protein kinase C, PKC) to stimulate cAMP phosphodiesterase (PDE) activity in a liver Golgi-endosomal (GE) fraction was examined in vivo and in a cell-free system. Injection into rats of 4 beta-phorbol 12-myristate 13-acetate, a known activator of PKC, caused a rapid and marked increase in PKC activity (+325% at 10 min) in the GE fraction, along with an increase in the abundance of the PKC alpha-isoform as seen on Western immunoblots. Concurrently, 4 beta-phorbol 12-myristate 13-acetate treatment caused a time-dependent increase in cAMP PDE activity in the GE fraction (96% at 30 min). Addition of the catalytic subunit of protein kinase A (PKA) to GE fractions from control and 4 beta-phorbol 12-myristate 13-acetate-treated rats led to a comparable increase (130-150%) in PDE activity, suggesting that PKA is probably not involved in the in-vivo effect of 4 beta-phorbol 12-myristate 13-acetate. In contrast, addition of purified PKC increased (twofold) PDE activity in GE fractions from control rats but affected only slightly the activity in GE fractions from 4 beta-phorbol 12-myristate 13-acetate-treated rats. About 50% of the Triton-X-100-solubilized cAMP PDE activity in the GE fraction was immunoprecipitated with an anti-PDE3 antibody. On DEAE-Sephacel chromatography, three peaks of PDE were sequentially eluted: one early peak, which was stimulated by cGMP and inhibited by erythro-9 (2-hydroxy-3-nonyl) adenine (EHNA); a selective inhibitor of type 2 PDEs; and two retarded peaks of activity, which were potently inhibited by cGMP and cilostamide, an inhibitor of type 3 PDEs. Further characterization of peak I by HPLC resolved a major peak which was activated (threefold) by 5 microM cGMP and inhibited (87%) by 25 microM EHNA, and a minor peak which was insensitive to EHNA and cilostamide. 4 beta-Phorbol 12-myristate 13-acetate treatment caused a selective increase (2.5-fold) in the activity associated with DEAE-Sephacel peak I, without changing the K(m) value. These results suggest that PKC selectively activates a PDE2, cGMP-stimulated isoform in the GE fraction.

Induction of signalling anergy via the T-cell receptor in cultured Jurkat T cells by pre-exposure to a filarial nematode secreted product

Filarial nematodes constitute major causes of morbidity in the Tropics. The worms have a life-span exceeding five years, a longevity which is considered to reflect at least in part, their ability to interfere with host lymphocyte responsiveness. To date the molecular mechanisms underlying this ability have not been defined but we now demonstrate that ES-62, a phosphorylcholine (PC)-containing glycoprotein released by the rodent filarial parasite Acanthocheilonema viteae, is able to render Jurkat T cells anergic to intracellular signalling via the antigen receptor (TCR). In particular, ES-62 acts by modulating activation of the tyrosine kinases Fyn, Lck and ZAP-70 leading to selective disruption of TCR coupling to the phospholipase D, protein kinase C, phosphoinositide-3-kinase and RasMAPkinase signalling cascades. These cascades are key elements in the transduction of transcriptional and proliferative signals following ligation of TCR. As PC-containing secreted products (PC-ES) are also released by human filarial parasites, our data suggest that PC-ES may play a role in the induction of T lymphocyte hyporesponsiveness observed during filarial infections.