Apicomplexan phosphodiesterases in cyclic nucleotide turnover: conservation, function, and therapeutic potential

Apicomplexa encompasses a large number of intracellular parasites infecting a wide range of animals. Cyclic nucleotide signaling is crucial for a variety of apicomplexan life stages and cellular processes. The cyclases and kinases that synthesize and respond to cyclic nucleotides (i.e., 3',5'-cyclic guanosine monophosphate and 3',5'-cyclic adenosine monophosphate) are highly conserved and essential throughout the parasite phylum. Growing evidence indicates that phosphodiesterases (PDEs) are also critical for regulating cyclic nucleotide signaling via cyclic nucleotide hydrolysis. Here, we discuss recent advances in apicomplexan PDE biology and opportunities for therapeutic interventions, with special emphasis on the major human apicomplexan parasite genera Plasmodium, Toxoplasma, Cryptosporidium, and Babesia. In particular, we show a highly flexible repertoire of apicomplexan PDEs associated with a wide range of cellular requirements across parasites and lifecycle stages. Despite this phylogenetic diversity, cellular requirements of apicomplexan PDEs for motility, host cell egress, or invasion are conserved. However, the molecular wiring of associated PDEs is extremely malleable suggesting that PDE diversity and redundancy are key for the optimization of cyclic nucleotide turnover to respond to the various environments encountered by each parasite and life stage. Understanding how apicomplexan PDEs are regulated and integrating multiple signaling systems into a unified response represent an untapped avenue for future exploration.

Advances in targeting Phosphodiesterase 1: From mechanisms to potential therapeutics

Phosphodiesterase 1 (PDE1) is an enzyme entrusted with the hydrolysis of the second messengers cAMP and cGMP, thereby governing a plethora of metabolic processes, encompassing ion channel modulation and cellular apoptosis. Recent advancements in the realm of small molecule structural variations have greatly facilitated the exploration of innovative applications for PDE1. Remarkably, a recent series of PDE1 inhibitors (PDE1i) have been meticulously formulated and devised, showcasing enhanced selectivity and potency. Among them, ITI-214 has entered Phase II clinical trials, holding promise for the treatment of Parkinson's disease and heart failure. Nevertheless, the majority of current PDE1 inhibitors have encountered substantial side effects in clinical trials attributable to their limited selectivity, this predicament presents a formidable obstacle in the development of specific small molecule inhibitors targeting PDE1. This Perspective endeavors to illuminate the potential design approaches, structure-activity relationships, and biological activities of current PDE1i, aiming to offer support and insights for clinical practice and the development of novel PDE1i.

An overview of phosphodiesterase 9 inhibitors: Insights from skeletal structure, pharmacophores, and therapeutic potential

Cyclic nucleotide phosphodiesterase 9 (PDE9), a specifically hydrolytic enzyme with the highest affinity for cyclic guanosine monophosphate (cGMP) among the phosphodiesterases family, plays a critical role in many biological processes. Consequently, the development of PDE9 inhibitors has received increasing attention in recent years, with several compounds undergoing clinical trials for the treatment of central nervous system (CNS) diseases such as Alzheimer's disease, schizophrenia, and psychotic disorders, as well as heart failure and sickle cell disease. This review analyzes the recent primary literatures and patents published from 2004 to 2023, focusing on the structure, pharmacophores, selectivity, and therapeutic potential of PDE9 inhibitors. It hoped to provide a comprehensive overview of the field's current state to inform the development of novel PDE9 inhibitors.

Virtual Screening-Accelerated Discovery of a Phosphodiesterase 9 Inhibitor with Neuroprotective Effects in the Kainate Toxicity In Vitro Model

In the central nervous system, some specific phosphodiesterase (PDE) isoforms modulate pathways involved in neuronal plasticity. Accumulating evidence suggests that PDE9 may be a promising therapeutic target for neurodegenerative diseases. In the current study, computational techniques were used to identify a nature-inspired PDE9 inhibitor bearing the scaffold of an isoflavone, starting from a database of synthetic small molecules using a ligand-based approach. Furthermore, docking studies supported by molecular dynamics investigations allowed us to evaluate the features of the ligand-target complex. In vitro assays confirmed the computational results, showing that the selected compound inhibits the enzyme in the nanomolar range. Additionally, we evaluated the expression of gene and protein levels of PDE9 in organotypic hippocampal slices, observing an increase following exposure to kainate (KA). Importantly, the PDE9 inhibitor reduced CA3 damage induced by KA in a dose-dependent manner in organotypic hippocampal slices. Taken together, these observations strongly support the potential of the identified nature-inspired PDE9 inhibitor and suggest that such a molecule could represent a promising lead compound to develop novel therapeutic tools against neurological diseases..

Phosphodiesterase and psychiatric disorders: a two-sample Mendelian randomization study

BACKGROUND

Phosphodiesterases (PDEs) have been associated with psychiatric disorders in observational studies; however, the causality of associations remains unestablished.

METHODS

Specifically, cyclic nucleotide PDEs were collected from genome-wide association studies (GWASs), including PDEs obtained by hydrolyzing both cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) (PDE1A, PDE2A, and PDE3A), specific to cGMP (PDE5A, PDE6D, and PDE9A) and cAMP (PDE4D and PDE7A). We performed a bidirectional two-sample Mendelian randomization (MR) analysis to investigate the relationship between PDEs and nine psychiatric disorders. The inverse-variance-weighted (IVW) method, MR-Egger, and weighted median were used to estimate causal effects. The Cochran's Q test, MR-Egger intercept test, MR Steiger test, leave-one-out analyses, funnel plot, and MR pleiotropy residual sum and outlier (MR-PRESSO) were used for sensitivity analyses.

RESULTS

The PDEs specific to cAMP were associated with higher-odds psychiatric disorders. For example, PDE4D and schizophrenia (SCZ) (odds ratios (OR) = 1.0531, PIVW = 0.0414), as well as major depressive disorder (MDD) (OR = 1.0329, PIVW = 0.0011). Similarly, PDE7A was associated with higher odds of attention-deficit/hyperactivity disorder (ADHD) (OR = 1.0861, PIVW = 0.0038). Exploring specific PDE subtypes and increase intracellular cAMP levels can inform the development of targeted interventions. We also observed PDEs (which hydrolyzes both cAMP and cGMP) was associated with psychiatric disorders [OR of PDE1A was 1.0836 for autism spectrum disorder; OR of PDE2A was 0.8968 for Tourette syndrome (TS) and 0.9449 for SCZ; and OR of PDE3A was 0.9796 for MDD; P < 0.05]. Furthermore, psychiatric disorders also had some causal effects on PDEs [obsessive-compulsive disorder on increased PDE6D and decreased PDE2A and PDE4D; anorexia nervosa on decreased PDE9A]. The results of MR were found to be robust using multiple sensitivity analysis.

CONCLUSIONS

In this study, potential causal relationships between plasma PDE proteins and psychiatric disorders were established. Exploring other PDE subtypes not included in this study could provide a more comprehensive understanding of the role of PDEs in psychiatric disorders. The development of specific medications targeting PDE subtypes may be a promising therapeutic approach for treating psychiatric disorders.

The Complexity and Multiplicity of the Specific cAMP Phosphodiesterase Family: PDE4, Open New Adapted Therapeutic Approaches

Cyclic nucleotides (cAMP, cGMP) play a major role in normal and pathologic signaling. Beyond receptors, cyclic nucleotide phosphodiesterases; (PDEs) rapidly convert the cyclic nucleotide in its respective 5′-nucleotide to control intracellular cAMP and/or cGMP levels to maintain a normal physiological state. However, in many pathologies, dysregulations of various PDEs (PDE1-PDE11) contribute mainly to organs and tissue failures related to uncontrolled phosphorylation cascade. Among these, PDE4 represents the greatest family, since it is constituted by 4 genes with multiple variants differently distributed at tissue, cellular and subcellular levels, allowing different fine-tuned regulations. Since the 1980s, pharmaceutical companies have developed PDE4 inhibitors (PDE4-I) to overcome cardiovascular diseases. Since, they have encountered many undesired problems, (emesis), they focused their research on other PDEs. Today, increases in the knowledge of complex PDE4 regulations in various tissues and pathologies, and the evolution in drug design, resulted in a renewal of PDE4-I development. The present review describes the recent PDE4-I development targeting cardiovascular diseases, obesity, diabetes, ulcerative colitis, and Crohn’s disease, malignancies, fatty liver disease, osteoporosis, depression, as well as COVID-19. Today, the direct therapeutic approach of PDE4 is extended by developing allosteric inhibitors and protein/protein interactions allowing to act on the PDE interactome.

Pharmacokinetic/Pharmacodynamic Assessment of Selective Phosphodiesterase Inhibitors in a Mouse Model of Autoimmune Hepatitis

Autoimmune hepatitis (AIH) is a life-threatening disorder currently treated with nonspecific immunosuppressive drugs. It is postulated that phosphodiesterase (PDE) inhibitors, as agents exerting anti-inflammatory and immunomodulatory activities, may constitute a possible treatment of autoimmune disorders. This study develops a pharmacokinetic/pharmacodynamic (PK/PD) model to assess the effects of PDE-selective inhibitors, namely, cilostazol (PDE3), rolipram (PDE4), and BRL-50481 (PDE7), in a mouse model of AIH. The pharmacokinetics of the PDE inhibitors (PDEi) were assessed in male BALB/c mice after intraperitoneal administration. In pharmacodynamic studies, mice received PDEi and AIH was induced in these animals by intravenous injection of concanavalin A (ConA). Serum drug concentrations, tumor necrosis factor α (TNFα), interleukin 17 (IL-17), and aminotransferase activities were quantified. The PK/PD analysis was performed using ADAPT5 software. The PK/PD model assumes inhibition of cAMP hydrolysis in T cells by PDEi, ConA-triggered formation of TNFα and IL-17, suppression of TNFα and IL-17 production by cAMP, and stimulatory effects of TNFα and IL-17 on the hepatic release of aminotransferases. Selective blockage of PDE4 leads to the highest inhibition of cAMP degradation in T cells and amelioration of disease outcomes. However, inhibition of both PDE3 and PDE7 also contribute to this effect. The proposed PK/PD model may be used to assess and predict the activities of novel PDEi and their combinations in ConA-induced hepatitis. A balanced suppression of different types of PDE appears to be a promising treatment option for AIH; however, this hypothesis warrants testing in humans based on translation of the PK/PD model into clinical settings. SIGNIFICANCE STATEMENT: A novel PK/PD model of PDE inhibitor effects in mice with ConA-induced autoimmune hepatitis was developed involving a mechanistic component describing changes in cAMP concentrations in mouse T cells. According to model predictions, inhibition of PDE4 in T cells causes the highest cAMP elevation in T cells, but suppression of PDE3 and PDE7 also contribute to this effect. A balanced inhibition of PDE3, PDE4, and PDE7 appears to be a promising treatment strategy for AIH.

Characterization of the cAMP phosphodiesterase domain in plant adenylyl cyclase/cAMP phosphodiesterase CAPE from the liverwort Marchantia polymorpha

Cyclic AMP (cAMP) acts as a second messenger and is involved in the regulation of various physiological responses. Recently, we identified the cAMP-synthesis/hydrolysis enzyme CAPE, which contains the two catalytic domains adenylyl cyclase (AC) and cAMP phosphodiesterase (PDE) from the liverwort Marchantia polymorpha. Here we characterize the PDE domain of M. polymorpha CAPE (MpCAPE-PDE) using the purified protein expressed in E. coli. The K_m and V_max of MpCAPE-PDE were 30 µM and 5.8 nmol min^-1 mg^-1, respectively. Further, we investigated the effect of divalent cations on PDE activity and found that Ca²⁺ enhanced PDE activity, suggesting that Ca²⁺ may be involved in cAMP signaling through the regulation of PDE activity of CAPE. Among the PDE inhibitors tested, only dipyridamole moderately inhibited PDE activity by approximately 40% at high concentrations. Conversely, 3-isobutyl-1-methylxanthine (IBMX) did not inhibit PDE activity.

The regulation of myosin II in Dictyostelium

Dictyostelium conventional myosin (myosin II) is an abundant protein that plays a role in various cellular processes such as cytokinesis, cell protrusion and development. This review will focus on the signal transduction pathways that regulate myosin II during cell movement. Myosin II appears to have two modes of action in Dictyostelium: local stabilization of the cytoskeleton by myosin filament association to the actin meshwork (structural mode) and force generation by contraction of actin filaments (motor mode). Some processes, such as cell movement under restrictive environment, require only the structural mode of myosin. However, cytokinesis in suspension and uropod retraction depend on motor activity as well. Myosin II can self-assemble into bipolar filaments. The formation of these filaments is negatively regulated by heavy chain phosphorylation through the action of a set of novel alpha kinases and is relatively well understood. However, only recently it has become clear that the formation of bipolar filaments and their translocation to the cortex are separate events. Translocation depends on filamentous actin, and is regulated by a cGMP pathway and possibly also by the cAMP phosphodiesterase RegA and the p21-activated kinase PAKa. Myosin motor activity is regulated by phosphorylation of the regulatory light chain through myosin light chain kinase A. Unlike conventional light chain kinases, this enzyme is not regulated by calcium but is activated by cGMP-induced phosphorylation via an upstream kinase and subsequent autophosphorylation.

Effect of Combining Phosphodiesterase III Inhibitors With St Thomas Hospital’s Solution Used as Transplantation Preservative Solution in Isolated Rat Hearts

Improved preservation of the harvested heart with attenuation of the reperfusion injury is important for successful outcomes of cardiac transplantations. The most commonly used cardioplegic solution, to prevent ischemic changes has been St Thomas' Hospital cardioplegic solution (STHCS). However, it is neither ideal nor sufficient to prevent myocardial ischemia and reperfusion injury. Phosphodiesterase inhibitors can attenuate the damage due to the injuries of ischemia and reperfusion. In this study we sought to enrich STHCS with a phosphodiesterase inhibitor to improve preservation of cardiac functions. The harvested hearts of 24 rats were divided into four groups. All hearts were mounted on a Langendorff perfusion system. After a stabilization period, cardiac arrest was maintained by STHCS. The hearts were stored in STHCS alone or with milrinone, amrinone, or enoximone for 6 hours. The reperfusion was maintained using a modified Tyrode's solution. All hearts were compared for their preischemic and postischemic left ventricular developed pressure, +dp/dtmax, -dp/dtmax, duration of systole, ejection time, and time to reach peak systolic pressure. Coronary effluent was collected for lactate dehydrogenase (LDH) measurements. The initial values for all metrics were comparable between the groups. During the postreperfusion period, all hearts showed lower peak systolic pressures than the initial values. Although the amrinone group seemed to have higher values, the 25-minute result was at the border of significance and the 30-minute value, significantly higher. All hearts showed far lower results of maximum changes in contractility during the time period (+dp/dtmax) versus the initial values; comparisons between groups were not significant. For the parameter of maximum changes in relaxation during the time period (-dp/dtmax), while other hearts showed lower results, the amrinone group displayed values comparable to the initial ones after 20 minutes. Comparisons between groups were insignificant. While other hearts had comparable values for time of systole, the hearts applied with milrinone reached these values after 15 minutes. Group comparison for time of ejection revealed that the results at 5-minute postreperfusion were higher in the enoximone and the amrinone groups than the milrinone group. Postreperfusion 5-minute results were higher in the enoximone and the amrinone groups than the milrinone group for time to reach peak systolic pressure. LDH levels were lowest in the amrinone group. In conclusion, our study revealed that adding phosphodiesterase inhibitors to STHCS improved peak systolic pressure and maximum changes in relaxation during the time period (-dp/dtmax, mm Hg/s). It also decreased the LDH leakage, which corresponded to the degree of ischemic tissue damage. Amrinone seemed to result in more favorable results, which may be attributed to its additional effects on inflammation, including those on cytokines and leukocyte aggregation.

Intracellular role of adenylyl cyclase in regulation of lateral pseudopod formation during Dictyostelium chemotaxis

Cyclic AMP (cAMP) functions as the extracellular chemoattractant in the aggregation phase of Dictyostelium development. There is some question, however, concerning what role, if any, it plays intracellularly in motility and chemotaxis. To test for such a role, the behavior of null mutants of acaA, the adenylyl cyclase gene that encodes the enzyme responsible for cAMP synthesis during aggregation, was analyzed in buffer and in response to experimentally generated spatial and temporal gradients of extracellular cAMP. acaA- cells were defective in suppressing lateral pseudopods in response to a spatial gradient of cAMP and to an increasing temporal gradient of cAMP. acaA- cells were incapable of chemotaxis in natural waves of cAMP generated by majority control cells in mixed cultures. These results indicate that intracellular cAMP and, hence, adenylyl cyclase play an intracellular role in the chemotactic response. The behavioral defects of acaA- cells were surprisingly similar to those of cells of null mutants of regA, which encodes the intracellular phosphodiesterase that hydrolyzes cAMP and, hence, functions opposite adenylyl cyclase A (ACA). This result is consistent with the hypothesis that ACA and RegA are components of a receptor-regulated intracellular circuit that controls protein kinase A activity. In this model, the suppression of lateral pseudopods in the front of a natural wave depends on a complete circuit. Hence, deletion of any component of the circuit (i.e., RegA or ACA) would result in the same chemotactic defect.

Periodic signaling controlled by an oscillatory circuit that includes protein kinases ERK2 and PKA

Self-regulating systems often use robust oscillatory circuits. One such system controls the chemotactic signaling mechanism of Dictyostelium, where pulses of adenosine 3',5'-monophosphate (cAMP) are generated with a periodicity of 7 minutes. We have observed spontaneous oscillations in activation of the mitogen-activated protein (MAP) kinase ERK2 that occur in phase with peaks of cAMP, and we show that ERK2 modulates cAMP levels through the phosphodiesterase RegA. Computer modeling and simulations of the underlying circuit faithfully account for the ability of the cells to spontaneously generate periodic pulses during specific stages of development. Similar oscillatory processes may occur in cells of many different species.

In vivo efficacy in airway disease models of N-(3,5-dichloropyrid-4-yl)-[1-(4-fluorobenzyl)-5-hydroxy-indole-3-yl]-glyoxylic acid amide (AWD 12-281), a selective phosphodiesterase 4 inhibitor for inhaled administration

N-(3,5-Dichloro-pyrid-4-yl)-[1-(4-fluorobenzyl)-5-hydroxy-indole-3-yl]-glyoxylic acid amide (AWD 12-281) is a highly potent and selective phosphodiesterase 4 (PDE4) inhibitor that was designed to have a metabolic profile that was optimized for topical administration. The aim of the current study was to explore the pharmacological profile of intratracheally administered AWD 12-281 in different models of asthma and chronic obstructive pulmonary disease (COPD) in comparison with steroids. To assess the anti-inflammatory potential of AWD 12-281, the antigen-induced cell infiltration in bronchoalveolar lavage fluid (BALF) of Brown Norway rats was determined. AWD 12-281 (ID50 of 7 microg/kg i.t.) as well as beclomethasone (0.1microg/kg i.t.) suppresses late-phase eosinophilia when administered intrapulmonary. Furthermore, AWD 12-281 has also strong anti-inflammatory properties when tested in lipopolysaccharide-induced acute lung neutrophilia in Lewis rats (ID50 of 0.02 microg/kg i.t.), ferrets (ID50 of 10 microg/kg i.t.), and domestic pigs (2-4 mg/pig i.t. or 1 mg/kg i.v.). In pigs, AWD 12-281 was as effective as beclomethasone (0.4 mg/pig i.t.) and dexamethasone (0.28 mg/kg i.v.), although at 3 to 10 times the dosage. The bronchodilatory activity of AWD 12-281 was assessed in sensitized guinea pigs. AWD 12-281 (1.5 mg/kg i.t., 1-h pretreatment) inhibited allergen-induced bronchoconstriction by 68% (parameter airway resistance). In sensitized BP-2 mice AWD 12-281 abolished the allergen-induced bronchial hyperresponsiveness and eosinophilia in BALF, showing dose dependence. When given orally, i.v. or i.t., AWD 12-281 has a considerably lower emetic potential than cilomilast in ferrets and roflumilast in pigs. When given topically by inhalation, no emesis could be induced in dogs up to the highest feasible dose (15 mg/kg in 50% lactose blend). These results indicate that AWD 12-281 is a unique potential new drug for the topical treatment of asthma and COPD.

Constitutively active protein kinase A disrupts motility and chemotaxis in Dictyostelium discoideum

The deletion of the gene for the regulatory subunit of protein kinase A (PKA) results in constitutively active PKA in the pkaR mutant. To investigate the role of PKA in the basic motile behavior and chemotaxis of Dictyostelium discoideum, pkaR mutant cells were subjected to computer-assisted two- and three-dimensional motion analysis. pkaR mutant cells crawled at only half the speed of wild-type cells in buffer, chemotaxed in spatial gradients of cyclic AMP (cAMP) but with reduced efficiency, were incapable of suppressing lateral pseudopods in the front of temporal waves of cAMP, a requirement for natural chemotaxis, did not exhibit the normal velocity surge in response to the front of a wave, and were incapable of chemotaxing toward an aggregation center in natural waves generated by wild-type cells that made up the majority of cells in mixed cultures. Many of the behavioral defects appeared to be the result of the constitutively ovoid shape of the pkaR mutant cells, which forced the dominant pseudopod off the substratum and to the top of the cell body. The behavioral abnormalities that pkaR mutant cells shared with regA mutant cells are discussed by considering the pathway ERK2 perpendicular RegA perpendicular [cAMP] --> PKA, which emanates from the front of a wave. The results demonstrate that cells must suppress PKA activity in order to elongate along a substratum, suppress lateral-pseudopod formation, and crawl and chemotax efficiently. The results also implicate PKA activation in dismantling cell polarity at the peak and in the back of a natural cAMP wave.

Pyridazines. Part XXIX: synthesis and platelet aggregation inhibition activity of 5-substituted-6-phenyl-3(2H)-pyridazinones. Novel aspects of their biological actions

A series of 6-phenyl-3(2H)-pyridazinones with a diverse range of substituents in the 5-position have been prepared and evaluated in the search for new antiplatelet agents. A significant dependence of the substituent on the inhibitory effect has been observed. The pharmacological study of these compounds confirms that modification of the chemical group at position 5 of the 6-phenyl-3(2H)-pyridazinone system influences both variations in the antiplatelet activity and the mechanism of action.

NO-cGMP pathway increases the hyperpolarisation-activated current, I(f), and heart rate during adrenergic stimulation

OBJECTIVES

The role of the nitric oxide (NO)-cGMP pathway in the autonomic modulation of cardiac pacemaking is controversial and may involve an interplay between the L-type calcium current, I(CaL), and the hyperpolarisation activated current, I(f). We tested the hypothesis that following adrenergic stimulation, the NO-cGMP pathway stimulates phosphodiesterase 2 (PDE2) to reduce cAMP dependent stimulation of I(f) and heart rate (HR).

METHODS

In the presence of norepinephrine (NE, 1 microM), the effects of the NO donor sodium nitroprusside (SNP) were evaluated in sinoatrial node (SAN)/atria preparations and isolated SAN cells from adult guinea pigs.

RESULTS

Contrary to our hypothesis, SNP (10 and 100 microM, n=5) or the membrane permeable cGMP analogue, 8Br-cGMP (0.5 mM, n=6) transiently increased HR by 5+/-1, 12+/-1 and 12+/-2 beats/min, respectively. The guanylyl cyclase inhibitor 1H-(1,2,4)-oxadiazolo-(4,3-a)-quinoxalin-1-one (ODQ, 10 microM, n=5) abolished the increase in HR to SNP (100 microM) as did the I(f) blockers caesium chloride (2 mM, n=7) and 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)-pyrimidinium chloride (ZD7288, 1 microM, n=7). Addition of SNP (10 microM) also transiently increased I(f) in SAN cells (n=5). After inhibition of PDE2 with erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA, 10 microM, n=5), the increase in HR to SNP in the presence of NE was significantly augmented and maintained. RT-PCR analysis confirmed the presence of PDE2 in addition to cGMP inhibited PDE3 mRNA in central SAN tissue.

CONCLUSIONS

These results suggest that during adrenergic stimulation, activation of the NO-cGMP pathway does not decrease HR, but has a transient stimulatory effect that is I(f) dependent, and is limited in magnitude and duration by stimulation of PDE2.

Milrinone echocardiographic viability test: a pilot study

We assessed the utility of milrinone to predict recovery of function after surgical myocardial revascularization in patients with severe baseline left ventricular systolic dysfunction caused by coronary artery disease (CAD). Prediction of viable myocardial segments that will regain function after revascularization may help in the selection of patients who will benefit from coronary artery bypass graft surgery (CABG) as well as aid in the choice of target sites for coronary revascularization. We investigated 20 consecutive patients with CAD and left ventricular ejection fraction < or = 40% who had evidence of myocardial viability by either thallium scan or dobutamine viability test and were candidates for elective CABG. Left ventricular regional wall motion and global ejection fraction were assessed by transesophageal echocardiography in the operating room. Measurements were done before and 10 minutes after milrinone infusion, and immediately after CABG. Left ventricular wall motion score was derived by means of a 12-segment model. Functional improvement for each segment was defined as a wall motion change > 1. Baseline ejection fraction was 27% +/- 5% (mean +/- SD). Ejection fraction increased to 35% +/- 5% after milrinone infusion (P < .0001) and to 36% +/- 6% after CABG (P < .0001). Post-CABG ejection fraction was significantly correlated with postmilrinone ejection fraction (r = 0.65, P < .0001). Milrinone infusion resulted in augmentation of contraction in 98 of the 209 abnormal segments (wall motion score > or = 2); 91 (92.9%) of these improved after CABG. One hundred nine of the 111 segments that showed no improvement with milrinone did not improve after revascularization (98.2%). Seventy-three segments were akinetic or dyskinetic at baseline; 46 (63.0%) of these improved with milrinone. Improvement in regional wall motion after revascularization was detected in 84.8% of the segments that improved with milrinone versus only 3.7% of the segments that did not improve with milrinone. In patients with ischemic cardiomyopathy, improvement in left ventricular function (segmental wall motion and global ejection fraction) during milrinone infusion is highly predictive of improvement after CABG.

Regulated protein degradation controls PKA function and cell-type differentiation in Dictyostelium

Cullins function as scaffolds that, along with F-box/WD40-repeat-containing proteins, mediate the ubiquitination of proteins to target them for degradation by the proteasome. We have identified a cullin CulA that is required at several stages during Dictyostelium development. culA null cells are defective in inducing cell-type-specific gene expression and exhibit defects during aggregation, including reduced chemotaxis. PKA is an important regulator of Dictyostelium development. The levels of intracellular cAMP and PKA activity are controlled by the rate of synthesis of cAMP and its degradation by the cAMP-specific phosphodiesterase RegA. We show that overexpression of the PKA catalytic subunit (PKAcat) rescues many of the culA null defects and those of cells lacking FbxA/ChtA, a previously described F-box/WD40-repeat-containing protein, suggesting CulA and FbxA proteins are involved in regulating PKA function. Whereas RegA protein levels drop as the multicellular organism forms in the wild-type strain, they remain high in culA null and fbxA null cells. Although PKA can suppress the culA and fbxA null developmental phenotypes, it does not suppress the altered RegA degradation, suggesting that PKA lies downstream of RegA, CulA, and FbxA. Finally, we show that CulA, FbxA, and RegA are found in a complex in vivo, and formation of this complex is dependent on the MAP kinase ERK2, which is also required for PKA function. We propose that CulA and FbxA regulate multicellular development by targeting RegA for degradation via a pathway that requires ERK2 function, leading to an increase in cAMP and PKA activity.

Characterization of TbPDE2A, a novel cyclic nucleotide-specific phosphodiesterase from the protozoan parasite Trypanosoma brucei

This study reports the identification and characterization of a cAMP-specific phosphodiesterase from the parasitic hemoflagellate Trypanosoma brucei. TbPDE2A is a class I phosphodiesterase. Its catalytic domain exhibits 30-40% sequence identity with those of all 11 mammalian phosphodiesterase (PDE) families, as well as with PDE2 from Saccharomyces cerevisiae, dunce from Drosophila melanogaster, and regA from Dictyostelium discoideum. The overall structure of TbPDE2A resembles that of human PDE11A in that its N-terminal region contains a single GAF domain. This domain is very similar to those of the mammalian PDE2, -5, -6, -10, and -11, where it constitutes a potential cGMP binding site. TbPDE2A can be expressed in S. cerevisiae, and it complements an S. cerevisiae PDE deletion strain. Recombinant TbPDE2A is specific for cAMP, with a K(m) of approximately 2 micrometer. It is entirely resistant to the nonselective PDE inhibitor 3-isobutyl-1-methylxanthine, but it is sensitive to trequinsin, dipyridamole, sildenafil, and ethaverine with IC(50) values of 5.4, 5.9, 9.4, and 14.2 micrometer, respectively. All four compounds inhibit proliferation of bloodstream form trypanosomes in culture, indicating that TbPDE2A is an essential enzyme.

Osmotic stress response in Dictyostelium is mediated by cAMP

DokA, a homolog of bacterial hybrid histidine kinases, is essential for hyperosmotic stress resistance in Dictyostelium: We show that a transient intracellular cAMP signal, dependent on the presence of DokA, is generated in response to an osmotic shock. This variation of cAMP levels contributes to survival under hypertonic conditions. In contrast to the low cAMP levels observed in dokA(-) strains, overexpression of the receiver domain of DokA causes an increase in cAMP levels, resulting in a rapidly developing phenotype. We present biochemical and cell biological data indicating that the DokA receiver domain is a dominant-negative regulator of a phosphorelay, which controls the intracellular cAMP phosphodiesterase RegA. The activity of the DokA receiver domain depends on a conserved aspartate, mutation of which reverses the developmental phenotype, as well as the deregulation of cAMP metabolism.

The internal phosphodiesterase RegA is essential for the suppression of lateral pseudopods during Dictyostelium chemotaxis

Dictyostelium strains in which the gene encoding the cytoplasmic cAMP phosphodiesterase RegA is inactivated form small aggregates. This defect was corrected by introducing copies of the wild-type regA gene, indicating that the defect was solely the consequence of the loss of the phosphodiesterase. Using a computer-assisted motion analysis system, regA(-) mutant cells were found to show little sense of direction during aggregation. When labeled wild-type cells were followed in a field of aggregating regA(-) cells, they also failed to move in an orderly direction, indicating that signaling was impaired in mutant cell cultures. However, when labeled regA(-) cells were followed in a field of aggregating wild-type cells, they again failed to move in an orderly manner, primarily in the deduced fronts of waves, indicating that the chemotactic response was also impaired. Since wild-type cells must assess both the increasing spatial gradient and the increasing temporal gradient of cAMP in the front of a natural wave, the behavior of regA(-) cells was motion analyzed first in simulated temporal waves in the absence of spatial gradients and then was analyzed in spatial gradients in the absence of temporal waves. Our results demonstrate that RegA is involved neither in assessing the direction of a spatial gradient of cAMP nor in distinguishing between increasing and decreasing temporal gradients of cAMP. However, RegA is essential for specifically suppressing lateral pseudopod formation during the response to an increasing temporal gradient of cAMP, a necessary component of natural chemotaxis. We discuss the possibility that RegA functions in a network that regulates myosin phosphorylation by controlling internal cAMP levels, and, in support of that hypothesis, we demonstrate that myosin II does not localize in a normal manner to the cortex of regA(-) cells in an increasing temporal gradient of cAMP.

A molecular network that produces spontaneous oscillations in excitable cells of Dictyostelium

A network of interacting proteins has been found that can account for the spontaneous oscillations in adenylyl cyclase activity that are observed in homogenous populations of Dictyostelium cells 4 h after the initiation of development. Previous biochemical assays have shown that when extracellular adenosine 3',5'-cyclic monophosphate (cAMP) binds to the surface receptor CAR1, adenylyl cyclase and the MAP kinase ERK2 are transiently activated. A rise in the internal concentration of cAMP activates protein kinase A such that it inhibits ERK2 and leads to a loss-of-ligand binding by CAR1. ERK2 phosphorylates the cAMP phosphodiesterase REG A that reduces the internal concentration of cAMP. A secreted phosphodiesterase reduces external cAMP concentrations between pulses. Numerical solutions to a series of nonlinear differential equations describing these activities faithfully account for the observed periodic changes in cAMP. The activity of each of the components is necessary for the network to generate oscillatory behavior; however, the model is robust in that 25-fold changes in the kinetic constants linking the activities have only minor effects on the predicted frequency. Moreover, constant high levels of external cAMP lead to attenuation, whereas a brief pulse of cAMP can advance or delay the phase such that interacting cells become entrained.

A novel adenylyl cyclase detected in rapidly developing mutants of Dictyostelium

Disruption of either the RDEA or REGA genes leads to rapid development in Dictyostelium. The RDEA gene product displays homology to certain H2-type phosphotransferases, while REGA encodes a cAMP phosphodiesterase with an associated response regulator. It has been proposed that RDEA activates REGA in a multistep phosphorelay. To test this proposal, we examined cAMP accumulation in rdeA and regA null mutants and found that these mutants show a pronounced accumulation of cAMP at the vegetative stage that is not observed in wild-type cells. This accumulation was due to a novel adenylyl cyclase and not to the known Dictyostelium adenylyl cyclases, aggregation stage adenylyl cyclase (ACA) or germination stage adenylyl cyclase (ACG), since it occurred in an acaA/rdeA double mutant and, unlike ACG, was inhibited by high osmolarity. The novel adenylyl cyclase was not regulated by G-proteins and was relatively insensitive to stimulation by Mn2+ ions. Addition of the cAMP phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) permitted detection of the novel adenylyl cyclase activity in lysates of an acaA/acgA double mutant. The fact that disruption of the RDEA gene as well as inhibition of the REGA-phosphodiesterase by IBMX permitted detection of the novel AC activity supports the hypothesis that RDEA activates REGA.

The histidine kinase dhkC regulates the choice between migrating slugs and terminal differentiation in Dictyostelium discoideum

An early decision that a newly formed aggregate of Dictyostelium cells must make is whether to form a migrating slug or to proceed through culmination, the process of forming the mature fruiting body. The choice between these alternative morphological pathways is influenced by external and internal cues. dhkC was identified as a potential hybrid sensor kinase possessing domains homologous to the histidine kinase and receiver motifs of two-component signaling systems. Null strains of dhkC show a rapidly developing phenotype for aggregation through finger formation, and culmination commences immediately thereafter and proceeds at a normal rate to generate typical fruiting bodies. Ammonia, an endogenous regulator of the slug versus culmination choice, results in a prolonged slug stage for wild-type strains while the dhkC- strain bypasses the slug stage in the presence or absence of ammonia. Conversely, expression in wild-type cells of a modified DHKC protein composed of only the histidine kinase domain results in normal timing through early aggregation, but subsequent development is significantly delayed. The resulting fingers, once formed, readily convert to slugs that do not undergo culmination but instead migrate until their energy sources are depleted. The slugger phenotype is dependent on the presence of a functional response regulator REGA, and it is rescued by exogenously supplied cAMP. Together, the results indicate that DHKC contributes to the integration of environmental and cellular signals so that the appropriate choice is made between slug formation and culmination. We suggest that DHKC may function as a sensor for ammonia, and that it is the initial component of a phosphorelay signaling system that may modulate the activity of cAMP-dependent protein kinase to either inhibit or promote culmination. Additionally, dhkC- spores were found to be defective in germination, indicating a role for the DHKC signaling pathway in activating spore germination.

Reduction of bone loss by denbufylline, an inhibitor of phosphodiesterase 4

The effects of denbufylline, a xanthine derivative with selective inhibitory activity on the phosphodiesterase (PDE) 4 isoenzyme, on bone loss in Walker 256/S-bearing rats and on mineralized nodule formation and osteoclastlike cell formation in bone marrow culture systems were examined. Serial oral administrations of denbufylline inhibited the decrease in the bone mineral density of femurs from Walker 256/S-bearing rats, without influence on the healthy rats. Denbufylline restored the bone mass and the number of osteoclasts and osteoblasts per trabecular surface in the femur metaphysis. Among PDE inhibitors, only PDE4-selective inhibitors increased the number of mineralized nodules and decreased the number of osteoclastlike cells in the in vitro bone marrow culture systems, and dibutyryl cyclic AMP mimicked these effects in the in vitro systems. These results suggest that the PDE4 isoenzyme may play an important role in bone turnover through cyclic AMP and that its inhibitors are candidates for therapeutic drugs for the bone loss diseases.

Activation of phosphodiesterase IV during desensitization of the A2A adenosine receptor-mediated cyclic AMP response in rat pheochromocytoma (PC12) cells

Prolonged activation of an A2A adenosine receptor significantly inhibits the cellular response to subsequent stimulation (A2A desensitization). We have reported previously that activation of phosphodiesterase (PDE) contributes to A2A desensitization in PC12 cells. In the present study, we show that a type IV PDE (PDE4)-selective inhibitor (Ro 20-1724) effectively blocks the increase in PDE activity in desensitized cells. Thus, PDE4 appears to be the PDE specifically activated during A2A desensitization in PC12 cells. Prolonged treatment of PC12 cells with an A2A-selective agonist (CGS21680) leads to increased PDE4 activity in a dose-dependent manner, which can be blocked by an A2A-selective antagonist [8-(3-chlorostyryl)caffeine]. Using two PDE4 antibodies, we were able to demonstrate that the levels of two PDE4-immunoreactive bands (72 and 79 kDa) were increased significantly during A2A desensitization. Prolonged treatment with forskolin to elevate intracellular cyclic AMP contents also resulted in increased PDE4 activity. In addition, activation of PDE4 activity during A2A desensitization could be blocked by a protein kinase A (PKA)-selective inhibitor (H89) and was not observed in a PKA-deficient PC12 cell line (A123). Taken together, activation of PDE4 via a cyclic AMP/PKA-dependent pathway plays a critical role in dampening the signal of the A2A receptor.

Insulin stimulates rat calmodulin I gene transcription through activation of Sp1

We have shown previously that insulin positively regulates transcription of the rat calmodulin (CaM) I gene. This activation occurs concomitantly with the activation of the low-Km adenosine 3':5'-cyclic phosphate phosphodiesterase (PDE), which appears to be coregulated with CaM. Rat hepatoma H-411E cells were transfected with plasmids containing various lengths of the putative CaM promoter coupled to a luciferease reporter and were challenged with insulin. We demonstrate that insulin-stimulated transcription of CaM I gene is mediated by a 392-bp 5'-flanking region of the CaM I gene, encompassing 185 bp downstream and 207 bp upstream of the start site of transcription. The CaM I promoter contains three potential Sp1 sites, located at -114 through -109 [(3), +], -77 through -72 [(2), -] and at +53 through +58 [(1), +]. The gel mobility shift assays demonstrated that nuclear protein(s) associate with all three sp1 sites. We present data demonstrating the relative importance of the three Sp1 sites for the insulin effect: prCaM I 1835, 3.8x, delta 1081; prCaM I 392, 5.3x, delta 1055; prCaM I 180, 3.7x, delta 462; prCaM I 237, 1.6x, delta 478; prCaM I 139, 2.6x, delta 182; prCaM I 130, 2.1x, delta 194; and prCaM I 1463, negligible activity. In summary, the maximal insulin stimulation of CaM gene expression is seen when the promoter region contains at least two Sp1 sites.

Role of conserved histidines in catalytic activity and inhibitor binding of human recombinant phosphodiesterase 4A

To identify critical amino acids within the central conserved region of recombinant human cAMP-specific phosphodiesterase 4 subtype A (rhPDE4A), we engineered the expression of point mutants in a fully active rhPDE4A/Met201-886. When histidine residues at positions 433, 437, 473, and 477, which are highly conserved among all PDE families, were changed independently to serine residues, cAMP hydrolyzing activities were substantially reduced or abolished. The ability of these mutants to bind prototypical PDE4 inhibitors [3H]-(R)-rolipram or [3H]RP 73401 was also decreased in parallel with the loss of catalytic activity. The parallel loss of catalytic activity and inhibitor binding suggests that these changes resulted from non-localized perturbations in the structure of the enzyme. More interesting results were obtained when histidine residues at positions 505 and 506 were changed independently to aspar agines. The K(m) value for cAMP increased 3-fold in H505N (K(m) = 11 +/- 3 microM) and 11-fold in H506N (K(m) = 44 +/- 6 microM) compared with the wild-type protein (K(m) = 4 +/- 1 microM). These mutant proteins bound [3H]-(R)-rolipram and [3H]RP 73401 with K(d) values of 1.8 +/- 0.4 and 0.3 +/- 0.1 nM, respectively, for H505N, and 3.9 +/- 0.9 and 0.5 +/- 0.1 nM, respectively, for H506N. These values are nearly identical to those obtained with the wild-type rhPDE4A/Met201-886. In contrast, the IC50 values for cAMP competition with either [3H]-(R)-rolipram or [3H]RP 73401 binding increased approximately 2-fold in H505N and approximately 13-fold in H506N compared with the wild type protein. These increases are virtually identical to the changes in the K(m) value for cAMP in these mutants. We conclude that His506 and, perhaps, His505 are involved in binding of cAMP to PDE4A/Met201-886 but not in binding of PDE4-selective inhibitors.

Induction of cyclic AMP and cyclic GMP 3':5'-cyclic nucleotide phosphodiesterase activities in neuroblastoma lines under differentiating conditions

It is now widely accepted that cyclic nucleotide phosphodiesterases (PDEs) play fundamental roles in signal transduction pathways; they show a remarkable molecular complexity, different tissue distribution and complex regulatory mechanisms. Here we report PDE isoforms expression in two dibutyryl cyclic AMP differentiated murine cell lines: the hybrid neuroblastoma-glioma 108CC15 and the parental neuroblastoma N18TG2. They differ in the ability to establish functional synapses, a feature present only in the former. Ionic exchange chromatography elution profiles of N18TG2 and 108CC15 undifferentiated cell extracts show two main peaks of activity. The first one hydrolyzes cyclic GMP and is specifically inhibited by Zaprinast, thus representing a member of the PDE5 family. The second peak hydrolyzes cyclic AMP and is significantly inhibited by rolipram, as all the PDE4 family members. The induction of differentiation by dibutyryl cyclic AMP in both clonal lines results in an increase of PDE activities only after 3 hr of treatment, suggesting that protein neosynthesis is involved. Interestingly in both clones, besides the increase in cyclic AMP hydrolyzing specific activity (3.1-fold in 108CC15 and 2.5-fold in N18TG2), we also observed an increase in cyclic GMP hydrolyzing activity (1.7-fold in 108CC15 and 4.3-fold in N18TG2). While the induction of PDE4, previously reported also in other cellular systems, could be considered as a feedback response to the higher cyclic AMP levels, this is not true for the isoform that hydrolyzes cyclic GMP. These data suggest that the induction of PDE isoforms in neuroblastoma cells could be related to the activation of neuronal differentiative pathway.

Expression, purification, and characterization of human cAMP-specific phosphodiesterase (PDE4) subtypes A, B, C, and D

Although four members (A, B, C, and D) of the cAMP-specific phosphodiesterase (PDE4) family have been cloned by different groups, no study comparing the characteristics of purified human PDE4 subtypes has been published. In this study, we have expressed human PDE4 A, B, C, and D in insect (SF9) cells by using the baculovirus expression system, purified the expressed proteins, and compared their characteristics. The recombinant PDE4 subtypes all showed catalytic activity for cAMP with a K(m) of 1-5 microM. V(max) values differed significantly among these subtypes with the following order: C > B > A > D. PDE4 A, B, C, and D showed a very similar Mg2+ dependence profile. PDE4 B and C showed similar pH profiles with the optimal pH being 8.0. The pH profiles of PDE4 A and D were very different from each other and from those of B and C, with the optimal pH being 6.5 and 7.5, respectively. Furthermore, although PDE4 A, B, C, and D were all inhibited by the standard PDE4 inhibitors rolipram, Ro20-1724, and etazolate, the inhibitory potency varied. Thus, by several criteria including kinetics, pH dependency, and inhibitor sensitivity, various PDE4 subtypes differ significantly from one another.

Novel heterocyclic-fused pyridazinones as potent and selective phosphodiesterase IV inhibitors

A series of 6-aryl-4,5-heterocyclic-fused pyridazinones were designed and synthesized as selective phosphodiesterase (PDE) IV inhibitors. Biological evaluation of these compounds demonstrated a good selectivity profile toward the PDE IV family and greatly attenuated affinity for the Rolipram high-affinity binding site that seems to be responsible for undesiderable side effects. Structure-activity relationships (SARs) studies showed that the presence of an ethyl group at pyridazine N-2 is associated with the best potency and selectivity profile.

Phosphodiesterase (PDE)4 inhibitors: anti-inflammatory drugs of the future?

Phosphodiesterase type 4 (PDE4) plays a major role in modulating the activity of virtually all cells involved in the inflammatory process. Inhibitors of this enzyme family display impressive anti-inflammatory and disease-modifying effects in a variety of experimental models. In this review, Mauro Teixeira, Robert Gristwood, Nicola Cooper and Paul Hellewell examine the capacity of PDE4 inhibitors to exert anti-inflammatory actions in vivo and discuss the potential of this class of drugs to take their place as novel therapeutic agents for a variety of inflammatory diseases.

Proposal for pharmacologically distinct conformers of PDE4 cyclic AMP phosphodiesterases

cAMP-specific phosphodiesterase inhibitors display a range of activities in vitro and in vivo which suggest they may be useful in the treatment of inflammatory diseases. However, these compounds elicit a number of side-effects which may limit their therapeutic potential. Certain side-effects of PDE4 inhibitors such as emesis and gastric acid secretion are associated with their actions at a high affinity rolipram binding site (HARBS). In contrast, a number of anti-inflammatory actions of PDE4 inhibitors are better correlated with inhibition of PDE4 catalytic activity than with displacement of [3H] rolipram from HARBS. This suggests that native PDE4s in different cell-types can be discriminated pharmacologically. Although known to be associated with PDE4, the nature of HARBS is uncertain. The majority of evidence suggests it represents particular conformational states of PDE subtypes with which rolipram interacts with high potency (KD approximately 2 nM) (High-affinity PDE4, HPDE4). Rolipram is generally moderately or weakly active (IC50-200 nM-2000 nM) in inhibiting catalytic activity of the majority of crude, partially-purified or recombinant PDE4-preparations (Low-affinity PDE4, LPDE4). Solubilization or V/GSH treatment of particulate eosinophil PDE4, cAMP-dependent kinase activation of RNPDE4D3 and membrane association of HSPDE4A4 increase the potencies of some (e.g., rolipram) but not other (e.g., trequinsin) inhibitors. In eosinophils, the changes in enzyme properties brought about by solubilization result in a close correlation between the potency order of compounds in inhibiting cAMP hydrolysis and displacing [3H] rolipram from HARBS. The identification of distinct pharmacological PDE4 forms may have therapeutic consequences since it may be possible to synthesize potent inhibitors of LPDE4 with low affinity for HARBS which should, theoretically, be less emetic. Most inhibitors synthesized to date (rolipram, denbufylline nitraquazone, etc.) display high-affinity for HARBS but are much weaker in inhibiting cAMP hydrolysis. Other compounds (RP 73401, trequinsin, CDP 840) display slightly higher potency against LPDE4 or do not discriminate between the two putative PDE4 forms. Recently, inhibitors have been synthesized which are considerably more active against LPDE4 than HPDE4. Such compounds with appropriate pharmacokinetic properties may retain anti-inflammatory activity but have a reduced capacity to cause nausea and emesis and, consequently, have a wider therapeutic window than compounds currently undergoing clincial evaluation.

Protein kinase A-dependent activation of PDE4 (cAMP-specific cyclic nucleotide phosphodiesterase) in cultured bovine vascular smooth muscle cells

Incubation of cultured bovine vascular smooth muscle cells (VSMC) with forskolin increased cAMP as measured by an increase in cAMP-dependent protein kinase (PKA) activation (PKA ratio). Forskolin also produced a concentration- and time-dependent increase in activity (3-5-fold within 15 min) of a PDE4 (cAMP-specific cyclic nucleotide phosphodiesterase). The increase in PDE4 activity was not affected by cycloheximide and thus not likely due to increased synthesis of the enzyme. Activation, which was preserved during partial purification of the enzyme by chromatography on Sephacryl S-200 and MonoQ, was most likely due to a covalent modification. Incubation of cell homogenates with the catalytic subunit of PKA (PKA(c)) induced a approximately 5-fold activation of PDE4 with a time course similar to that in intact cells after forskolin addition. The forskolin-mediated activation was reversed during incubation of homogenates at room temperature for two hours. Addition of PKA(c) resulted in rapid reactivation of PDE4. These data are consistent with the hypothesis that rapid, reversible activation of PDE4 in cultured VSMC is mediated by PKA.

The phosphodiesterase type 4 (PDE4) inhibitor CP-80,633 elevates plasma cyclic AMP levels and decreases tumor necrosis factor-alpha (TNFalpha) production in mice: effect of adrenalectomy

Rolipram was previously reported to elevate plasma cyclic adenosine 3',5'-monophosphate (cAMP) and inhibit serum tumor necrosis factor-alpha (TNF-alpha) production in mice. CP-80,633, a new cyclic nucleotide phosphodiesterase (PDE4) inhibitor, has been shown to augment intracellular cAMP levels and to inhibit TNFalpha release from human monocytes in vitro. This study was undertaken to determine the effect of p.o. CP-80,633 on plasma cAMP levels and lipopolysaccharide-induced TNFalpha production in mice with and without adrenal glands. CP-80,633 dose-dependently (3-32 mg/kg p.o.) elevated plasma cAMP levels and decreased systemic TNFalpha production in response to i.p. injection of lipopolysaccharide. Elevated plasma cAMP levels can be detected for up to 4 hr. CP-80,633 (10 mg/kg p.o.) caused a 6-fold increase in the plasma cAMP level, a 2-fold increase in the plasma epinephrine level and a greater than 95% reduction in TNFalpha production. Unlike CP-80,633, neither vinpocetine, dipyridamole, SKB-94,120 nor zaprinast, at 100 mg/kg p.o., modified the cAMP response, which suggests that this response is mediated by inhibition of PDE4. Adrenalectomy reduced the cAMP response and completely blocked the epinephrine response; however, the levels of plasma cAMP in the CP-80,633-treated mice (10 mg/kg p.o.) remained elevated (vehicle: 47.3 +/- 6.8 vs. CP-80,633: 98.4 +/- 10.3 pmol/ml, n = 7, P < .05). This effect is mimicked by treatment of control mice with propranolol, which demonstrates that beta adrenoreceptors contribute to the cAMP response. Removal of adrenal glands significantly increased the LPS-induced elevation of serum TNFalpha. The ability of CP-80,633 to block the TNFalpha response was only slightly affected by adrenalectomy (ED50 = 1.2 mg/kg in controls vs. 3.9 mg/kg in adrenalectomized mice). Taken together, these results show that CP-80,633, when given p.o. to mice, is capable of elevating plasma cAMP and inhibiting TNFalpha production and that adrenal catecholamines contribute significantly to the effect of CP-80,633 on the cAMP response but only slightly to its effect on the systemic TNFalpha response.

Identification, characterization, and functional role of phosphodiesterase type IV in cerebral vessels: effects of selective phosphodiesterase inhibitors

The role of the phosphodiesterase type IV isozyme (PDE IV) in the regulation of cerebrovascular tone was investigated in the canine basilar artery in vitro and in vivo. The PDE isozymes extracted from the canine basilar artery were isolated by diethylaminoethanol (DEAE)-Sepharose affinity chromatography and identified based on sensitivity to isozyme-selective PDE inhibitors. [3H]cAMP hydrolysis was observed in one major and one minor peak of activity. The predominant peak was inhibited by the addition of cGMP (25%), siguazodan (26%), rolipram (39%), and the combination of siguazodan and rolipram (95%). Selective PDE IV inhibitors BRL 61063, rolipram, and denbufylline were equieffective inhibitors of [3H]-ccAMP hydrolysis mediated by PDE IV isolated from the canine basilar artery [concentrations producing 50% inhibition (IC50S) = 0.21 +/- 0.05 microM, 0.67 +/- 0.23 microM, and 0.73 +/- 0.16 microM, respectively]. In precontracted isolated ring segments of the canine basilar artery, selective PDE IV inhibitors produced potent and complete relaxation (IC50S < 150 nM). In contrast, zaprinast (a selective PDE V inhibitor) and siguazodan (a selective PDE III inhibitor) produced only weak relaxation of the basilar artery (IC50S = 4.5 microM and > 10 microM, respectively). Vasorelaxation produced by PDE IV inhibitors was not altered by removing the endothelium, 1-NAME, or adenosine receptor antagonism. In a canine model of acute cerebral vasospasm, all three selective PDE IV inhibitors reversed basilar artery spasm produced by autologous blood without altering mean arterial blood pressure. In contrast, prolonged treatment with BRL 61063 failed to alter the development of basilar spasm in the two hemorrhage canine models of chronic cerebral vasospasm. Denbufylline-induced relaxation in vitro was also significantly impaired in basilar arteries obtained from the model of chronic vasospasm. In conclusion, PDE IV appears to be the predominant isozyme regulating vascular tone mediated by cAMP hydrolysis in cerebral vessels. In addition, vasorelaxation modulated by PDE IV is compromised in chronic cerebral vasospasm associated with subarachnoid hemorrhage.

Differential efficacy of lymphocyte- and monocyte-selective pretreatment with a type 4 phosphodiesterase inhibitor on antigen-driven proliferation and cytokine gene expression

Elevations of intracellular cyclic AMP, achieved with the use of phosphodiesterase (PDE) inhibitors, cause functional downregulation of most inflammatory cells. Rolipram, an inhibitor selective for the PDE4 isozyme, can markedly downregulate antigen-driven proliferation and cytokine gene expression of unfractionated human peripheral blood mononuclear cells. However, it is unclear whether PDE4 inhibitors in a mixed-cell system exert their immunosuppressive effect on the lymphocyte or on the monocyte fraction. We have used an adherence-based protocol for separating peripheral blood mononuclear cells, isolated from atopic individuals, into lymphocyte and monocyte fractions and have selectively treated these populations with rolipram prior to reconstituting the cell cultures to their original lymphocyte/monocyte proportions. Cellular responses to both ragweed and tetanus toxoid were analyzed for both proliferation and gene expression of proinflammatory cytokines. A dose-dependent downregulation of ragweed- and tetanus toxoid-driven proliferative responses was achieved by pretreatment of lymphocytes from peripheral blood with rolipram. This downregulation was significantly greater than that achieved with pretreatment of monocytes. Pretreatment of both populations failed to show synergistic downregulation of proliferation. Lymphocyte pretreatment with rolipram also resulted in marked downregulation of gene expression for IL-4, IL-5, and interferon-gamma compared to monocyte pretreatment in both ragweed- and tetanus toxoid-driven systems. Interestingly, monocyte pretreatment in these systems resulted in significant downregulation of IL-2 gene expression compared to lymphocyte pretreatment. Flow cytometric analysis failed to show alterations in any of a panel of surface activation and signal transducing molecules by rolipram treatment with or without antigen stimulation. We conclude that, in a mixed cell system, PDE4 inhibitors downregulate antigen-driven proliferation and gene expression of proinflammatory cytokines predominantly through their effects on lymphocytes rather than monocytes.

Expression of mRNA encoding cAMP-specific phosphodiesterase isoforms in rat parotid glands

In previous reports, we have shown that cAMP-specific phosphodiesterase (PDE4) is the major PDE in the rat parotid gland, and that PDE4 is activated by phosphorylation. In this study, we investigated the expression of PDE4 isoform genes and alternative splicing variants of PDE4D in the rat parotid gland using reverse transcriptase-polymerase chain reaction (RT-PCR). PDE4A, PDE4B, PDE4C and PDE4D of PDE4 subfamily were expressed. PDE4D was found to be the dominant PDE4 isoform. A weak band of PDE4C was detectable. Three alternative splicing variants (PDE4D1, PDE4D2 and PDE4D3) derived from the rat PDE4D gene were expressed in the parotid gland. These data suggested that the intracellular cAMP level is regulated by multiple response mechanisms through the activations of the PDE by phosphorylation and gene expression in the rat parotid gland.

Activation of a cyclic nucleotide phosphodiesterase 4 (PDE4) from rat thymocytes by phosphatidic acid

The cytosolic cyclic nucleotide phosphodiesterase (PDE) activity from rat thymocytes was resolved into five peaks by HPLC. Only two forms of the cAMP-specific PDE4 were found to be sensitive to physiologically relevant phosphatidic acid (PA) concentrations. PA activated the PDE4-peak 3 form, the fatty acid composition and unsaturation degree determining the efficiency of PA. The PDE4-peak 2 form was inhibited only by PA with saturated fatty acyl groups. PDE4 activation was specific of anionic phospholipids, a free phosphate group in the phospholipid molecule being required for maximum activation. These results suggest that PA may contribute to the lowering of cAMP level required in the early steps of a lympho-proliferative response, thus regulating immune functions through PDE4 activation.

Inhibitory effect of salmeterol on the respiratory burst of adherent human neutrophils

Human neutrophils, plated in fibronectin-coated wells and stimulated with N-formyl-methionylleucyl-phenylalanine (fMLP), were found to undergo a massive and prolonged respiratory burst, as measured by monitoring superoxide production. The beta 2-agonist salmeterol inhibited the respiratory burst in a dose-dependent manner. In contrast, salbutamol was ineffective. Moreover, the neutrophil respiratory burst was partially suppressed by prostaglandin E2 (PGE2) and the phosphodiesterase type IV (PDE-IV) inhibitor RO 20-1724. When salmeterol was used in combination with PGE2 or RO 20-1724, additive inhibitory effects were observed. The inhibitory activity of salmeterol was not reversed in the presence of the beta-blocker propranolol, and did not correlate with its ability of increasing cyclic AMP (cAMP) levels. Finally, the compounds used did not affect neutrophil adherence to fibronectin-coated wells. The results suggest that salmeterol is capable of down-regulating the neutrophil oxidative response to fMLP, also of co-operating with PGE2 and PDE-IV inhibitor RO 20-1724 in a manner not related to its beta 2-receptor binding activity. In other words, salmeterol displays neutrophil-directed effects, susceptible to be amplified by natural mediators such as PGE2 or PDE-IV inhibitors, consistent with possible anti-inflammatory properties of the drug.

Mapping the functional domains of human recombinant phosphodiesterase 4A: structural requirements for catalytic activity and rolipram binding

To identify functional domains of the 886-amino acid human recombinant cAMP-specific phosphodiesterase (PDE) subtype A (rhPDE4A), we engineered the expression of seven mutant proteins containing both NH2- and COOH-terminal truncations. The level of rhPDE4A protein expression in yeast was monitored by immunoblotting using enzyme-specific antisera. Biochemical profiles of the mutant proteins were compared with those of the full-length protein or a fully active truncated form of the enzyme (rhPDE4A Met265-886), lacking the first 264 amino acids. The smallest catalytically active fragment generated was Met332-722, which at 45 kDa is less than half the mass of the full-length enzyme (approximately 110 kDa) but spans the most highly conserved region of the PDE superfamily. Two prototypical PDE4 inhibitors, rolipram and RP 73401, inhibited cAMP hydrolyzing activity of all truncated forms of the enzyme, with IC50 values of 70-2000 nM and 0.2-0.6 nM, respectively. [3H](R)-Rolipram bound to two sites on Met265-886, a high affinity site (Kd1 = 0.7 +/- 0.3 nM) and a low affinity site (Kd2 = 34 +/- 10 nM). Interestingly, [3H](R)-rolipram failed to bind to Met332-886 with high affinity, indicating that high affinity binding is not required for inhibition of enzyme activity. Low affinity rolipram binding was still present in Met332-886 (Kd = 101 +/- 7 nM). In contrast to [3H](R)-rolipram, [3H]RP 73401 bound to a single class of high affinity sites on Met265-886 (Kd = 0.4 +/- 0.1 nM). Further truncation of the enzyme to Met332-886 had no effect on [3H]RP 73401 binding (Kd = 0.2 +/- 0.03 nM). We conclude that the catalytic center of rhPDE4A lies between amino acids 332 and 722. Furthermore, amino acids 265-332 may form a high affinity binding site for rolipram that is outside of the catalytic domain. As a more likely alternative, these amino acids may not form a distinct binding site but instead may be required for the recombinant enzyme to assume a conformation that binds rolipram at the catalytic domain with a high affinity.

Phosphodiesterase 4 and tolerance to beta 2-adrenoceptor agonists in asthma

beta 2-Adrenoceptor agonists provide a mainstay in the treatment of asthma worldwide. However, despite their ability to provide symptomatic relief, chronic or repeated exposure to beta 2-adrenoceptor agonists does not resolve asthmatic inflammation, because of the rapid development of tolerance by pro-inflammatory and immune cells of the lung. The prevailing belief is that tolerance to the so-called non-bronchodilator actions of beta 2-adrenoceptor agonists is largely attributable to direct receptor desensitization mediated by G protein receptor-coupled kinases and/or cAMP-dependent protein kinase. Here, Mark Giembycz suggests another, largely ignored, explanation for beta 2-adrenoceptor desensitization that is based on the accelerated degradation of cAMP by phosphodiesterase.

The in vivo pharmacology of CP-80, 633, a selective inhibitor of phosphodiesterase 4

The following studies were conducted to characterize the bron-chodilatory and antiinflammatory activity of the novel, selective phosphodiesterase-IV inhibitor, CP-80,633 (2'S)5-[3-(2'-exobicyclo[2.2.1]heptyloxy-4-methoxy-phenyl]te trahydro- 2(1H)-pyrimidone, a compound in clinical development for atopic disease. In IgG1 passively sensitized guinea pigs, aerosolized ovalbumin challenge increases both pulmonary eosinophil peroxidase levels and airway obstruction. CP-80,633, administered before ovalbumin challenge, significantly attenuated both the increase in tissue eosinophil peroxidase levels (ED50 = 1.4 mg/kg, p.o.) and airway obstruction (ED50 = 0.93 +/- 0.14 mg/kg,p.o.) 10 to 30 times more potently than theophyl-line. Intraarterially administered CP-80,633 also reversed an established bronchoconstriction initiated by continuous infusion of histamine to guinea pigs (ED50 of 8.2 micrograms/kg vs. 5.6 mg/kg for theophylline). The antiinflammatory effect of CP-80,633 was also examined in atopic monkeys challenged with Ascaris suum (Ag) aerosol. CP-80,633 (1 mg/kg, qid, s.c., 1 hr before antigen challenge) significantly reduced antigen-induced increases in bronchoalveolar lavage neutrophils (72.8 +/- 15.8% inhibition) and eosinophils (61.1 +/- 5.7% inhibition) 4 hr postchallenge, but did not reduce leukocytes 24 hr postchallenge. CP-80,633 did not inhibit antigen-induced increases in BAL levels of interleukin-1 beta, -6 or -8 as measured by enzyme-linked immunosorbant assay. These results indicate that CP-80,633 possesses bronchodilatory activity in guinea pigs and some antiinflammatory effects in both guinea pigs and monkeys.

In vitro pharmacology of the novel phosphodiesterase type 4 inhibitor, CP-80633

We present the in vitro pharmacology of a novel adenosine 3'-5' -cyclic monophosphate-specific phosphodiesterase (PDE) type 4 inhibitor, CP-80633 ((2'S)5-[3-(2' -exobicyclo[2.2.1]-heptyloxy)4-methoxyphenyl] tetrahydro-2(1H)-primidone), which has shown efficacy in phase II clinical trials for atopic dermatitis. CP-80633 inhibits PDE4 isozymes (human lung IC50 = 1.27 microM) in the absence of effects on PDE1, PDE2, PDE3 and PDE5 isozymes (IC50 > 100 microM). It exhibits no significant selectivity for any single cloned PDE4A, B, C or D isoform. CP-80633 inhibits adenosine 3'-5'-cyclic monophosphate hydrolysis in partially purified human peripheral blood monocyte cytosol (IC50 = 3.52 microM), eosinophil membrane (IC50 = 1.10 microM) and T cell membrane (IC50 = 2.28 microM) preparations. Inhibition of eosinophil PDE4 adenosine 3'-5'-cyclic mono-phosphate hydrolysis by CP-80,633 occurs in a noncompetitive manner. Unlike theophylline, CP-80,633 is inactive against ratrain adenosine (A1,A2) receptors. Consistent with its action as a PDE4 inhibitor in whole cells, CP-80633 potentiates PGE1 dependent increases in adenosine 3'-5'-cyclic monophosphate levels in human U937 cells, and in human eosinophils, monocytes and T cells (EC200 approximately 1.0 microM). Consequently, CP-80633 inhibits many inflammatory cell functions including 1) human eosinophil superoxide anion production (IC50 < 0.6 microM), 2 C5a-(IC50 = 0.40 microM) and LTB4-(IC50 = 0.20 microM) mediated guinea pig peritoneal eosinophil chemotaxis and 3) lipopolysac-charide-induced tumor necrosis factor-alpha release from human monocytes (IC50 = 0.219 microM). These data clearly demonstrate that CP-80633 is a selective inhibitor of PDE4 isozymes, and support its potential use as a therapeutic agent for a number of inflammatory and immune disorders.

In vitro down-regulation of antigen-induced IL-5 gene expression and protein production by cAMP-specific phosphodiesterase type 4 inhibitor

The effects of cAMP-elevating agents on antigen-induced IL-5 (interleukin-5) messenger RNA expression and protein production were examined in vitro in an antigen-driven system of splenocytes from ovalbumin sensitized BALB/c mice. IL-5 production was inhibited by rolipram, a type 4 phosphodiesterase (PDE4) inhibitor, dose-dependently (maximally at 10(-5) M) and by dibutyryl-cAMP (db-cAMP) (3 x 10(-4) M), but not by the type 3 and type 5 PDE inhibitors milrinone and zaprinast (10(-5) M), respectively. Forskolin (10(-5) M), an adenylate cyclase activator, was noninhibitory alone but potentiated inhibition by rolipram. Inhibition was associated with a decrease in IL-5 mRNA expression. Cycloheximide 10(-6) M and actinomycin 2 micrograms/ml abolished IL-5 production and mRNA expression. We conclude that in splenocytes from sensitized mice, IL-5 production and mRNA expression depend on antigen stimulation. The time course of IL-5 protein production is closely related to IL-5 mRNA expression and depends on de novo protein synthesis. db-cAMP and a selective PDE4 inhibitor, alone or in combination with forskolin, are the only cAMP-elevating agents that dose-dependently inhibited antigen-induced IL-5 mRNA expression and protein production. These results are in agreement with in vivo inhibition by a selective PDE4 inhibitor of antigen-induced pulmonary eosinophil infiltration and IL-5 production in sensitized mice, and they suggest that PDE4 inhibitors have potential for treating respiratory allergy.

Phosphodiesterase 4 in macrophages: relationship between cAMP accumulation, suppression of cAMP hydrolysis and inhibition of [3H]R-(-)-rolipram binding by selective inhibitors

A perplexing phenomenon identified in several tissues is the lack of correlation between inhibition of phosphodiesterase 4 (PDE4) and certain functional responses such as smooth muscle relaxation, gastric acid secretion and cAMP accumulation. Interpretation of these data is complicated further by the finding that function correlates with the ability of PDE4 inhibitors to displace [3H]rolipram [4-(3-cyclopentenyloxy-4-methoxyphenyl)-2-pyrrolidone] from a high-affinity site in rat brain that is apparently distinct from the catalytic centre of the enzyme. We have investigated this discrepancy by using guinea pig macrophages as a source of PDE4 and have confirmed that the ability of a limited range of structurally dissimilar PDE inhibitors (Org 20241, nitraquazone and the enantiomers of rolipram and benafentrine) to increase cAMP content did not correlate with their potency as inhibitors of partly purified PDE4, whereas a significant linear and rank order correlation was found when cAMP accumulation was related to the displacement of [3H]R-(-)-rolipram from a specific site identified in macrophage lysates. An explanation for these data emerged from the finding that the IC50 values and rank order of potency of these compounds for inhibition of partly purified PDE4 and the native (membrane-bound) form of the same enzyme were distinct. Similarly, no correlation was found when membrane-bound PDE4 was compared with the same enzyme that had been solubilized with Triton X-100. These unexpected results were attributable to a selective decrease in the potency of those inhibitors [nitraquazone, R-(-)- and S-(+)-rolipram] that interacted preferentially with the rolipram binding site. Indeed, if membrane-bound PDE4 was used as the enzyme preparation, excellent linear and rank order correlations between inhibition of cAMP hydrolysis, displacement of [3H]R-(-)-rolipram and cAMP accumulation were found, which improved further in the presence of the vanadyl (Vo)/2. GSH complex. Moreover, using Vo/2.GSH-treated membranes, the IC50 values of nitraquazone and the enantiomers of rolipram for the inhibition of PDE4 approached their affinity for the rolipram binding site. Collectively, these data suggest that the rolipram binding site and the catalytic domain on CPPDE4 might represent part of the same entity. In addition, these results support the concept that PDE4 can exist in different conformational states [Barnett, Manning, Cieslinski, Burman, Christensen and Torphy (1995) J. Pharmcol. Exp. Ther. 273, 674-679] and provide evidence that the cAMP content in macrophages is regulated primarily by a conformer of PDE4 for which rolipram has nanomolar affinity.