Cyclic nucleotide phosphodiesterases from purified human CD4+ and CD8+ T lymphocytes

Phosphodiesterase inhibitors and lung diseases

Phosphodiesterase enzymes (PDE) have long been known as regulators of cAMP and cGMP, second messengers involved in various signaling pathways and expressed in a variety of cell types implicated in respiratory diseases such as airway smooth muscle and inflammatory cells making them a key target for the treatment of lung diseases as chronic obstructive pulmonary disease (COPD), asthma, cystic fibrosis, and pulmonary hypertension (PH). The first reported PDE inhibitor was the xanthine, theophylline, described as a non-specific PDE inhibitor and whilst this drug is effective, it also has a range of unwanted side effects. In an attempt to improve the therapeutic window of xanthines, a number of selective PDE inhibitors have been developed for the treatment of respiratory diseases with only the selective PDE 4 inhibitor, roflumilast, being approved for the treatment of severe COPD. However, roflumilast also has a very narrow therapeutic window due to a number of important doses limiting side effects, particularly in the gastrointestinal tract. However, there continues to be research carried out in this field to identify improved selective PDE inhibitors, both by targeting other PDE subtypes (e.g., PDE 7 found in a number of inflammatory and immune cells) and through development of selective PDE inhibitors for pulmonary administration to reduce systemic exposure and improve the side effect profile. This approach has been exemplified by the development of ensifentrine, a dual PDE 3-PDE 4 inhibitor, an inhaled drug that has recently completed two successful Phase III clinical trials in patients with COPD.

Differences in the Expression of KIR, ILT Inhibitory Receptors, and VEGF Production in the Induced Decidual NK Cell Cultures of Fertile and RPL Women

RESULTS

KIR2DL1 and ILT-2 expression on idNK cells was higher in healthy women than in RPL patients. Sildenafil enhanced NKG2A expression in RPL patients. VEGF concentration was higher in fertile woman idNK cell cultures. idNK cells were more sensitive for necrosis in RPL than in fertile women. SC did not influence VEGF production or idNK cell apoptosis.

CONCLUSIONS

A combination of hypoxia, IL-15, and AZA promotes the conversion of pbNK into idNK cells CD56+CD16--expressing KIR receptors and produces VEGF. Alterations in KIR2DL1 and ILT-2 expression as well as impaired VEGF production were associated with RPL. SC affects NKG2A expression on RPL idNK cells. SC had no effect on VEGF release or idNK cell apoptosis.

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.

Role of Phosphodiesterase 7 (PDE7) in T Cell Activity. Effects of Selective PDE7 Inhibitors and Dual PDE4/7 Inhibitors on T Cell Functions

Phosphodiesterase 7 (PDE7), a cAMP-specific PDE family, insensitive to rolipram, is present in many immune cells, including T lymphocytes. Two genes of PDE7 have been identified: PDE7A and PDE7B with three or four splice variants, respectively. Both PDE7A and PDE7B are expressed in T cells, and the predominant splice variant in these cells is PDE7A1. PDE7 is one of several PDE families that terminates biological functions of cAMP—a major regulating intracellular factor. However, the precise role of PDE7 in T cell activation and function is still ambiguous. Some authors reported its crucial role in T cell activation, while according to other studies PDE7 activity was not pivotal to T cells. Several studies showed that inhibition of PDE7 by its selective or dual PDE4/7 inhibitors suppresses T cell activity, and consequently T-mediated immune response. Taken together, it seems quite likely that simultaneous inhibition of PDE4 and PDE7 by dual PDE4/7 inhibitors or a combination of selective PDE4 and PDE7 remains the most interesting therapeutic target for the treatment of some immune-related disorders, such as autoimmune diseases, or selected respiratory diseases. An interesting direction of future studies could also be using a combination of selective PDE7 and PDE3 inhibitors.

Sildenafil Citrate Influences Production of TNF-α in Healthy Men Lymphocytes

The aim of our study was to determine whether sildenafil citrate influences the production of Th1- (TNF-α, INF-γ) or Th2-type (TGF-β, IL-10) cytokines by lymphocytes of healthy men. Sildenafil citrate (SC) is a selective blocker of phosphodiesterase 5, by competing for the binding site with cGMP. It was reported that a higher risk of sexually transmitted diseases (STD) could be correlated with a recreational use of sildenafil, especially when combined with another drug. While behavioral causes of these findings are understood, it is worth considering other causes of that phenomenon that might rely on the influence of sildenafil on the immune system. Material and Methods. Peripheral blood mononuclear cells (PBMCs) were isolated from 27 healthy men donors and cultured in the presence of SC at a concentration of 400 ng/ml. The first set of research was performed on cells stimulated, for at least 4 hours, by incubation with phorbol myristate acetate (PMA), ionomycin, and Golgi-Stop. Subsequently, we determined cytokine production in cells stimulated with phytohemagglutinin (PHA) for 12 hours in the presence of Golgi-Stop. Flow cytometry immunophenotyping of PBMC was performed towards the surface marker of T cells: CD3 and intracellular cytokine expression: TNF-α, IFN-γ, TGF-β, and IL-10. Our findings show that SC significantly decreased the percentage of T cells producing TNF-α and displayed tendency to decrease IFN-γ, when stimulated with PMA. Frequent usage of SC might strengthen this effect. That could partially explain the impaired immune response to the pathogens of men using the drug.

Targeting Adenosine in Cancer Immunotherapy to Enhance T-Cell Function

T cells play a critical role in cancer control, but a range of potent immunosuppressive mechanisms can be upregulated in the tumor microenvironment (TME) to abrogate their activity. While various immunotherapies (IMTs) aiming at re-invigorating the T-cell-mediated anti-tumor response, such as immune checkpoint blockade (ICB), and the adoptive cell transfer (ACT) of natural or gene-engineered ex vivo expanded tumor-specific T cells, have led to unprecedented clinical responses, only a small proportion of cancer patients benefit from these treatments. Important research efforts are thus underway to identify biomarkers of response, as well as to develop personalized combinatorial approaches that can target other inhibitory mechanisms at play in the TME. In recent years, adenosinergic signaling has emerged as a powerful immuno-metabolic checkpoint in tumors. Like several other barriers in the TME, such as the PD-1/PDL-1 axis, CTLA-4, and indoleamine 2,3-dioxygenase (IDO-1), adenosine plays important physiologic roles, but has been co-opted by tumors to promote their growth and impair immunity. Several agents counteracting the adenosine axis have been developed, and pre-clinical studies have demonstrated important anti-tumor activity, alone and in combination with other IMTs including ICB and ACT. Here we review the regulation of adenosine levels and mechanisms by which it promotes tumor growth and broadly suppresses protective immunity, with extra focus on the attenuation of T cell function. Finally, we present an overview of promising pre-clinical and clinical approaches being explored for blocking the adenosine axis for enhanced control of solid tumors.

PDE8 controls CD4+ T cell motility through the PDE8A-Raf-1 kinase signaling complex

The levels of cAMP are regulated by phosphodiesterase enzymes (PDEs), which are targets for the treatment of inflammatory disorders. We have previously shown that PDE8 regulates T cell motility. Here, for the first time, we report that PDE8A exerts part of its control of T cell function through the V-raf-1 murine leukemia viral oncogene homolog 1 (Raf-1) kinase signaling pathway. To examine T cell motility under physiologic conditions, we analyzed T cell interactions with endothelial cells and ligands in flow assays. The highly PDE8-selective enzymatic inhibitor PF-04957325 suppresses adhesion of in vivo myelin oligodendrocyte glycoprotein (MOG) activated inflammatory CD4⁺ T effector (Teff) cells to brain endothelial cells under shear stress. Recently, PDE8A was shown to associate with Raf-1 creating a compartment of low cAMP levels around Raf-1 thereby protecting it from protein kinase A (PKA) mediated inhibitory phosphorylation. To test the function of this complex in Teff cells, we used a cell permeable peptide that selectively disrupts the PDE8A-Raf-1 interaction. The disruptor peptide inhibits the Teff-endothelial cell interaction more potently than the enzymatic inhibitor. Furthermore, the LFA-1/ICAM-1 interaction was identified as a target of disruptor peptide mediated reduction of adhesion, spreading and locomotion of Teff cells under flow. Mechanistically, we observed that disruption of the PDE8A-Raf-1 complex profoundly alters Raf-1 signaling in Teff cells. Collectively, our studies demonstrate that PDE8A inhibition by enzymatic inhibitors or PDE8A-Raf-1 kinase complex disruptors decreases Teff cell adhesion and migration under flow, and represents a novel approach to target T cells in inflammation.

The cAMP Pathway as Therapeutic Target in Autoimmune and Inflammatory Diseases

Nucleotide signaling molecules contribute to the regulation of cellular pathways. In the immune system, cyclic adenosine monophosphate (cAMP) is well established as a potent regulator of innate and adaptive immune cell functions. Therapeutic strategies to interrupt or enhance cAMP generation or effects have immunoregulatory potential in autoimmune and inflammatory disorders. Here, we provide an overview of the cyclic AMP axis and its role as a regulator of immune functions and discuss the clinical and translational relevance of interventions with these processes.

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

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

Modulation of Th1/Th2 cytokine production by selective and nonselective phosphodiesterase inhibitors administered to mice

Phosphodiesterase (PDE) inhibitors can modulate the functions of immune cells, including T lymphocytes, due to increased intracellular levels of cyclic nucleotides. The drugs (aminophylline, milrinone and sildenafil) were administered once or five times at 24 h intervals at the following doses: 20 mg/kg, i.m., 1 mg/kg, i.m. and 1 mg/kg, p.o., respectively. Th1 and Th2 cytokine levels (IL-2, IFN-γ, IL-4, IL-5, TNF) were determined 12, 24 or 72 h after the last administration of the drugs. A commercial BD™ Cytometric Bead Array Mouse Th1/Th2 Cytokine Kit (CBA) was used to determine the levels of Th1/Th2 cytokines in the serum. Neither of the PDE inhibitors under investigation administered once changed IFN-γ, TNF and IL-4 production. A single dose of aminophylline decreased the production of IL-2 (after 12 h). A single dose of milrinone did not affect Th1/Th2 cytokine secretion. Sildenafil administered once decreased the production of IL-2 (after 72 h). A temporary enhancement in the level of IL-5 was observed 12 h after a single dose of sildenafil. No changes in Th1 and Th2 cytokine production were observed after five doses of PDE inhibitors under investigation. These results indicate that nonstimulated lymphocytes Th1 and Th2 exhibited a slight sensitivity to aminophylline and sildenafil. The drugs under investigation were ineffective inhibitors of Th1/Th2 cytokine production.

PDE5 Inhibitors as Potential Tools in the Treatment of Cystic Fibrosis

Despite great advances in the understanding of the genetics and pathophysiology of cystic fibrosis (CF), there is still no cure for the disease. Using phosphodiesterase type 5 (PDE5) inhibitors, we and others have provided evidence of rescued F508del-CFTR trafficking and corrected deficient chloride transport activity. Studies using PDE5 inhibitors in mice homozygous for the clinically relevant F508del mutation have been conducted with the aim of restoring F508del-CFTR protein function. We demonstrated, by measuring transepithelial nasal potential difference in F508del mice following intraperitoneal injection of sildenafil, vardenafil, or taladafil at clinical doses are able to restore the decreased CFTR-dependent chloride transport across the nasal mucosa. Moreover, vardenafil, but not sildenafil, stimulates chloride transport through the normal CFTR protein. We developed a specific nebulizer setup for mice, with which we demonstrated, through a single inhalation of PDE5 inhibitors, local activation of CFTR protein in CF. Significant potential advantages of inhalation drug therapy over oral or intravenous routes include rapid onset of pharmacological action, reduced systemic secondary effects, and reduced effective drug doses compared to the drug delivered orally; this underlines the relevance and impact of our work for translational science. More recently, we analyzed the bronchoalveolar lavage of CF and wild-type mice for cell infiltrates and expression of pro-inflammatory cytokines and chemokines; we found that the CFTR activating effect of vardenafil, selected as a representative long-lasting PDE5 inhibitor, breaks the vicious circle of lung inflammation which plays a major role in morbi-mortality in CF. Our data highlight the potential use of PDE5 inhibitors in CF. Therapeutic approaches using clinically approved PDE5 inhibitors to address F508del-CFTR defects could speed up the development of new therapies for CF.

Phosphodiesterase inhibitors: history of pharmacology

The first pharmacological investigations of phosphodiesterase (PDE) inhibitors were developed with the clinical efficacies of drugs isolated from coffee, cacao and tea but only later their relevant ingredients were identified as xanthines that act as PDE. With its diuretic, inotropic and bronchodilating clinical efficacy, use of theophylline anticipated the clinical goals, which were later approached with the first-generation of weakly selective PDE inhibitors in the period from 1980 to 1990. Pharmacological and clinical research with these early compounds provided a vast pool of information regarding desired and adverse actions - although most of these new drugs had to be discontinued due to severe adverse effects. The pharmacological models for cardiac, vascular and respiratory indications were analysed for their PDE isoenzyme profiles, and when biochemical and molecular biological approaches expanded our knowledge of the PDE superfamily, the purified isoenzymes that were now available opened the door for more systematic studies of inhibitors and for generation of highly selective isoenzyme-specific drugs. The development of simple screening models and clinically relevant indication models reflecting the growing knowledge about pathomechanisms of disease are summarised here for today's successful application of highly selective PDE3, PDE4 and PDE5 inhibitors. The interplay of serendipitous discoveries, the establishment of intelligent pharmacological models and the knowledge gain by research results with new substances is reviewed. The broad efficacies of new substances in vitro, the enormous biodiversity of the PDE isoenzyme family and the sophisticated biochemical pharmacology enabled Viagra to be the first success story in the field of PDE inhibitor drug development, but probably more success stories will follow.

Modulating effects of nonselective and selective phosphodiesterase inhibitors on lymphocyte subsets and humoral immune response in mice

Phosphodiesterase (PDE) inhibitors can regulate the activity of immune cells by increasing intracellular levels of cyclic nucleotides. The aim of this study was to determine the effects of milrinone, a selective PDE3 inhibitor, sildenafil, a selective PDE5 inhibitor, and aminophylline, a nonselective PDE inhibitor, on lymphocyte subsets and humoral immune response in mice when administered in vivo. Aminophylline (20 mg/kg, i.m.), milrinone (1 mg/kg, i.m.) or sildenafil (1 mg/kg, p.o.) were administered to mice either once or five times at 24 h intervals. Some mice were immunized with a sheep red blood cell (SRBC) suspension administered i.p. either 2 h after the single dose or 2 h after the second of the five doses. In non-immunized mice treated five times with PDE inhibitors, the subsets of T lymphocytes in the thymus and T and B lymphocytes in the spleen and mesenteric lymph nodes were determined 12, 24 or 72 h after the last dose. The humoral immune response was determined on days 4, 7 and 14 after SRBC injection in SRBC-immunized mice treated with PDE inhibitors. A modulating effect of the drugs on lymphocyte subpopulations was observed. The greatest impact was observed in splenocyte subpopulations, and resulted in decreased percentages of B cells (CD19(+)) and increased percentages of T cells (CD3(+), CD4(+), CD8(+)). No effect or slight influence of the drugs on anti-SRBC hemagglutinins was observed, but the number of plaque-forming splenocytes was increased. The drugs under investigation did not show a significant immunosuppressive effect.

Inhibition of PDE3, PDE4 and PDE7 potentiates glucocorticoid-induced apoptosis and overcomes glucocorticoid resistance in CEM T leukemic cells

Stimulation of the cAMP signaling pathway has been shown to induce apoptosis and augment the effects of glucocorticoids in inducing apoptosis in leukemic cells. We recently reported that in primary B cell chronic lymphocytic leukemic (B-CLL) cells, apoptosis could be induced by stimulating the cAMP signaling pathway with a phosphodiesterase4 (PDE4) inhibitor alone; while in contrast, in the CEM T leukemic cell line, PDE4 inhibitors alone were ineffective, and concurrent stimulation of adenylyl cyclase was required to see effects [Tiwari et al. (2005)]. We report here that in the CEM and Jurkat T leukemic cell lines, the most abundantly expressed PDEs are PDE3B, PDE4A, PDE4D, PDE7A, and PDE8A. Selective inhibition of PDE3, PDE4 or PDE7 alone produces little effect on cell viability, but inhibition of all three of these PDEs together dramatically enhances glucocorticoid-induced apoptosis in CEM cells, and overcomes glucocorticoid resistance in a glucocorticoid-resistant CEM cell line. These studies indicate that for some leukemic cell types, a desired therapeutic effect may be achieved by inhibiting more than one form of PDE.

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

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

ABCD of the phosphodiesterase family: interaction and differential activity in COPD

Phosphodiesterases (PDEs) are important enzymes that hydrolyze the cyclic nucleotides adenosine 3'5'-cyclic monophosphate (cAMP) and guanosine 3'5'-cyclic monophosphate (cGMP) to their inactive 5' monophosphates. They are highly conserved across species and as well as their role in signal termination, they also have a vital role in intra-cellular localization of cyclic nucleotide signaling and integration of the cyclic nucleotide pathways with other signaling pathways. Because of their pivotal role in intracellular signaling, they are now of considerable interest as therapeutic targets in a wide variety diseases, including COPD where PDE inhibitors may have bronchodilator, anti-inflammatory and pulmonary vasodilator actions. This review examines the diversity and cellular localization of the isoforms of PDE, the known and speculative relevance of this to the treatment of COPD, and the range of PDE inhibitors in development together with a discussion of their possible role in treating COPD.

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.

Soluble guanylate cyclase beta1-subunit expression is increased in mononuclear cells from patients with erectile dysfunction

The aim was to determine in circulating mononuclear cells from patients with erectile dysfunction (ED), the level of expression of endothelial nitric oxide synthase (eNOS), soluble guanylate cyclase (sGC) beta1-subunit and phosphodiesterase type-V (PDE-V). Peripheral mononuclear cells from nine patients with ED of vascular origin and nine patients with ED of neurological origin were obtained. Fourteen age-matched volunteers with normal erectile function were used as control. Reduction in eNOS protein was observed in the mononuclear cells from patients with ED of vascular origin but not in those from neurological origin. Although sGC beta1-subunit expression was increased in mononuclear cells from patients with ED, the sGC activity was reduced. However, only the patients with ED of vascular origin showed an increased expression of PDE-V. This work shows for the first time that, independently of the aetiology of ED, the expression of sGC beta1-subunit was increased in circulating mononuclear cells; however, the expression of both eNOS and PDE-V was only modified in the circulating mononuclear cells from patients with ED of vascular origin.

Phosphodiesterase (PDE) 7 in inflammatory cells from patients with asthma and COPD

In inflammatory cells, the low K(m) cyclic adenosine monophosphate (cAMP)-specific phosphodiesterase (PDE) 4 subtype is predominant in terms of expression and function, although more recently it has been suggested that PDE 7 may also play a role in regulating inflammatory cell activity. In the present study, PDE 4 and PDE 7 subtype messenger ribonucleic acid (mRNA) transcripts in CD4 and CD8 lymphocytes from healthy (n=10) and asthmatic (n=10) subjects and polymorphonuclear neutrophils (PMNs) and CD8 lymphocytes obtained from healthy (n=10) and chronic obstructive pulmonary disease (COPD) (n=7) subjects were identified and quantified. PDE 4A, PDE 4B, PDE 4D and PDE 7A mRNA were present in similar quantities in both CD4 and CD8 lymphocytes obtained from healthy and asthmatic subjects and in CD8 lymphocytes obtained from healthy and COPD subjects. Expression of PDE 4C and PDE 7B mRNA was also observed, although transcript levels were low and variable between individuals. In addition, the effects of selective PDE 7 inhibition on both phytohaemagluttinin (PHA)-induced human peripheral blood mixed mononuclear cell (HPBMNC) proliferation and fMLP-induced neutrophil elastase (NE) release were studied. HPBMNC and human neutrophils, isolated from the venous blood of healthy volunteers (n=6) were treated with either a novel selective PDE 7 inhibitor PF 0332040 alone or in combination with rolipram. Proliferation of HPBMNC was stimulated by PHA (2microgml(-1)) and assessed by [(3)H]-thymidine incorporation, while fMLP-induced (100nM) NE release was determined using a chromogenic substrate. Both rolipram (0.003-10microM) and PF 0332040 (0.003-10microM) significantly inhibited PHA-stimulated proliferation of HPBMNC ((**)P<0.01). Co-administration of rolipram (0.3-10microM) and PF 0332040 (0.003-10microM) significantly increased the degree of inhibition observed, compared to when either drug was administered alone ((**)P<0.01). PF 0332040 (0.003-10microM) had no inhibitory effect on NE release from human peripheral blood neutrophils stimulated with fMLP (100nM), while rolipram (0.003-10microM) significantly inhibited neutrophil degranulation ((**)P<0.01). These findings suggest no evidence of altered PDE 4 or PDE 7 mRNA transcript levels in inflammatory cells isolated from the peripheral venous blood of mild asymptomatic asthmatic subjects or stable COPD subjects, however, inhibition of PDE 7 may influence mononuclear cell function.

Allergen challenge alters lymphocyte phosphodiesterase activity in horses with heaves

Heaves is an allergic airway disease in horses characterised by reversible airway obstruction, bronchial hyperresponsiveness and airway inflammation associated with a Th(2) response. Cyclic nucleotide-dependent signalling pathways can regulate lymphocyte function. In this study, we examined lymphocyte PDE activity comparing horses with heaves to healthy control animals. Total PDE activity and the effects of isoenzyme selective inhibitors were measured before, 5 and 24 h after the start of a 7 h allergen challenge. Allergen challenge had no effect on either total cAMP PDE activity or its inhibition by the PDE4 selective inhibitor, rolipram, and the non-selective PDE inhibitor, theophylline. In contrast, the PDE3 selective inhibitor, quazinone, caused significantly greater inhibition of cAMP PDE activity before challenge in the heaves susceptible group. Additionally, total cGMP PDE activity was significantly lower 24 h after the start of challenge in the heaves affected group (11+/-2 and 21+/-3 pmol/min/mg for heaves and control animals, respectively) and the PDE5 selective inhibitor, zaprinast, caused significantly less inhibition in the heaves group at this time point. The functional significance of these findings was explored by examining the effect of PDE3, PDE4 and PDE5 selective inhibitors on mitogen-induced mononuclear cell proliferation before and 24 h after the start of allergen challenge. Proliferation decreased after challenge in the heaves group (stimulation index=328+/-110 and 200+/-72 before and after challenge, respectively) whilst remaining constant in the control group (stimulation index=161+/-13 and 183+/-45 before and after challenge, respectively). However, all three PDE inhibitors caused a similar amount of inhibition at each time point and the effect of a combination of a PDE3 and a PDE5 inhibitor was simply additive in both groups. These results suggest differences in the control of lymphocyte PDE activity in horses with heaves.

Biochemical and functional assessment of equine lymphocyte phosphodiesterases and protein kinase C

Lymphocytes play an important role in allergic inflammation and have been implicated in the pathogenesis of equine allergic skin and respiratory disease. Targeting intracellular signalling pathways in human lymphocytes has demonstrated a role for both phosphodiesterase and protein kinase C in cell activation. The aim of this study was to measure total cyclic nucleotide hydrolysing phosphodiesterase activity and to identify the phosphodiesterase and protein kinase C isoenzymes present in equine lymphocytes. The functional significance of these isoenzymes was then investigated by examining their role in peripheral blood mononuclear cell proliferation using isoenzyme selective inhibitors. Total cyclic adenosine monophosphate hydrolysing phosphodiesterase activity was double that of cyclic guanosine monophosphate (30+/-2 pmol/min mg versus 16+/-3 pmol/min mg for cyclic adenosine and cyclic guanosine monophosphate phosphodiesterase activity, respectively). Evidence for the presence of PDE1, 3, 4 and 5 was obtained and PKCalpha, beta, delta, eta, iota, theta and zeta were identified. Selective inhibitors of PDE4, PKCdelta and conventional PKCs alpha and beta caused significant inhibition of mitogen-induced peripheral blood mononuclear cell proliferation. This study demonstrates a functional role for specific signalling isoenzymes and suggests that, in the context of allergic inflammation, targeting inflammatory cells involved in disease pathogenesis with relevant isoenzyme inhibitors may have therapeutic potential.

Phosphodiesterase-4 inhibitors in the treatment of inflammatory lung disease

Phosphodiesterases (PDE) belong to an important family of proteins that regulate the intracellular levels of cyclic nucleotide second messengers. Targeting PDE with selective inhibitors may offer novel therapeutic strategies in the treatment of various conditions, and in the context of respiratory disease these include asthma and chronic obstructive pulmonary disease (COPD). The rationale for such an approach stems, in part, from the clinical efficacy of theophylline, an orally active drug that is purportedly a nonselective PDE inhibitor. In addition, intracellular cyclic adenosine monophosphate (cAMP) levels regulate the function of many of the cells thought to contribute to the pathogenesis of respiratory diseases such as asthma and COPD, and these cells also selectively express PDE4. This has offered pharmaceutical companies the opportunity to selectively targeting these enzymes for the treatment of these diseases. Finally, the success of targeting PDE5 in the treatment of erectile dysfunction provides clinical proof of concept for the targeting of PDE in disease. Whether a 'Viagra' of the airways can be found for the treatment of asthma and COPD remains to be seen, but positive results from recent clinical studies examining the efficacy of selective PDE4 inhibitors such as cilomilast and roflumilast offer some optimism. However, one of the major issues to be resolved is the tolerability profile associated with this drug class that is a consequence of PDE4 inhibition. While cilomilast and roflumilast have low emetic potential they are not free from emesis and various strategies are being investigated in the hope of developing a PDE4 inhibitor without this adverse effect.

Regulation of T-cell activation by phosphodiesterase 4B2 requires its dynamic redistribution during immunological synapse formation

Stimulation of T cells through their antigen receptors (TCRs) causes a transient increase in the intracellular concentration of cyclic AMP (cAMP). However, sustained high levels of cAMP inhibit T-cell responses, suggesting that TCR signaling is coordinated with the activation of cyclic nucleotide phosphodiesterases (PDEs). The molecular basis of such a pathway is unknown. Here we show that TCR-dependent signaling activates PDE4B2 and that this enhances interleukin-2 production. Such an effect requires the regulatory N terminus of PDE4B2 and correlates with partitioning within lipid rafts, early targeting of this PDE to the immunological synapse, and subsequent accumulation in the antipodal pole of the T cell as activation proceeds.

T cell receptor-independent CD4 signalling: CD4-MHC class II interactions regulate intracellular calcium and cyclic AMP

CD4 is a coreceptor on T helper (Th) cells that interacts with MHC class II molecules (MHCII). The mechanisms mediating the effects of CD4 on responses by T helper cells to stimulation of the antigen-specific T cell receptor (TCR) are still poorly understood. Here, we demonstrate T cell costimulation via CD4 signalling independent of T cell receptor-mediated signals. Incubation of T helper cells with peptide mimetics of the CD4-binding region on the MHC class II beta2 domain caused intracellular calcium mobilization in the absence of antigen or other T cell receptor stimuli. Engagement of CD4 by peptide mimetics or wild-type MHC class II, but not by mutant MHC class II molecules incapable of engaging CD4, inhibited the T cell receptor-mediated increase in cyclic AMP (cAMP) concentrations in T helper cells. CD4-mediated signals activated cyclic AMP phosphodiesterases (PDEs) and inhibited adenylyl cyclase. Full activation and clonal expansion of antigen-stimulated T helper cells required the CD4-mediated regulation of cyclic AMP. Our results suggest a costimulatory mechanism of CD4 function that acts on the second messengers, calcium and cyclic AMP.

Differential effects of phosphodiesterase inhibitors on platelet activating factor (PAF)- and adenosine diphosphate (ADP)-induced equine platelet aggregation

Compounds that activate adenylate cyclase, increasing intracellular cyclic adenosine monophosphate (cAMP), inhibit equine platelet aggregation. Cyclic AMP is broken down by phosphodiesterase (PDE) and, in the present study, the effects of theophylline, a nonselective PDE inhibitor, and selective inhibitors of PDE isoenzymes PDE3, PDE4 and PDE5, on equine platelet aggregation in response to platelet activating factor (PAF) and adenosine diphosphate (ADP) have been examined. Theophylline and the PDE3 inhibitors, trequinsin and quazinone, inhibited both PAF and ADP-induced aggregation in a concentration dependent manner. The inhibition of PAF-induced aggregation was, however, significantly greater than that of the response to ADP. The inhibitory effects of theophylline and the PDE3 inhibitors on ADP- but not PAF-, induced aggregation were reversed by addition of the calcium ionophore, A23187. Rolipram and zaprinast, inhibitors of PDE4 and PDE5, respectively, had no effect on either PAF- or ADP-induced aggregation. These results demonstrate that inhibition of aggregation caused by PAF or ADP can be achieved by selective inhibition of PDE3 but suggest that there may be agonist-specific differences in the intracellular signalling pathways that regulate equine platelet aggregation.

Ubiquitous expression of phosphodiesterase 7A in human proinflammatory and immune cells

We have determined the expression of phosphodiesterase (PDE) 7A1 and PDE7A2 in human cells that have been implicated in the pathogenesis of chronic obstructive pulmonary disease and asthma. Messenger RNA transcripts were detected by RT-PCR in T lymphocytes, monocytes, neutrophils, airway and vascular smooth muscle cells, lung fibroblasts, epithelial cells, and cardiac myocytes. Human epithelial, T cell, eosinophil, and lung fibroblast cell lines were also positive for PDE7A1 and PDE7A2 mRNA transcripts. By Western immunoblot analyses the amount of PDE7A1 was greatest in T cell lines, peripheral blood T lymphocytes, epithelial cell lines, airway and vascular smooth muscle cells, lung fibroblasts, and eosinophils but was not detected in neutrophils. In contrast, PDE7A2 protein, which was identified in human cardiac myocytes, was not found in any of the other cell types investigated. Immunoconfocal analyses showed that PDE7A was expressed in neutrophils and alveolar macrophages. As the expression of PDE7A mirrors the distribution of PDE4 we speculate that this enzyme could be a target for novel anti-inflammatory drugs.

Benefit of phosphodiesterase 4 inhibitors as supplemental therapy after lung transplantation concerning their antiproliferative effects: an experimental study using a heterotopic rodent model

BACKGROUND

Recent advances in the understanding of immunomodulatory properties of phosphodiesterase 4 (PDE4) inhibitors recommend these drugs for immunosuppressive therapy after lung transplantation. The potency of three PDE4 inhibitors was tested using an established model of heterotopic tracheal transplantation in rats.

METHODS

Five allogenic groups were investigated and treated with the PDE4 inhibitors: rolipram, cilomilast (Ariflo, SB-207499, SmithKline Beecham, Munich, Germany), roflumilast (Altana Pharmacia, Bad Homburg, Germany) or cyclosporine A (CsA), or left without immunosuppression. The grafts were quantitatively analyzed for epithelial integrity, monocyte/macrophage content, cell proliferation, and tracheal obliteration by histology/immunohistochemistry (days 1, 5, 7, 21, 28; n=4-7).

RESULTS

In animals treated with the PDE4 inhibitors, the epithelium was completely lost until day 21. The epithelium was partially preserved in the rats receiving CsA until day 28. In the acute phase (days 5 and 7) the infiltration of monocytes and macrophages was significantly inhibited similarly (cilomilast) or less effective (rolipram, roflumilast) as in CsA-treated rats. In the chronic phase (day 28) the significant increase of monocytes and macrophages after CsA-treatment was not found in PDE4 inhibitor-treated rats. The PDE4 inhibitors showed lower (rolipram) or higher (cilomilast, roflumilast) potency as CsA to inhibit the cell proliferation. Only treatment with PDE4 inhibitor (Ariflo) significantly inhibited the obliteration, but to a lesser degree as CsA.

CONCLUSION

The PDE4 inhibitors tested in our study are not suitable on their own for immunosuppressive therapy after lung transplantation because of the limited protection against the epithelial disturbance, infiltration of immune cells, and luminal obliteration. The strong anti-proliferative effect of the second-generation PDE4 inhibitors, cilomilast and roflumilast, suggest a benefit for the effective inhibition of immune cell and fibroblast proliferation contributing to the development of obliterative bronchiolitis.

Potential role of phosphodiesterase 7 in human T cell function: comparative effects of two phosphodiesterase inhibitors

Even though the existence of phosphodiesterase (PDE) 7 in T cells has been proved, the lack of a selective PDE7 inhibitor has confounded an accurate assessment of PDE7 function in such cells. In order to elucidate the role of PDE7 in human T cell function, the effects of two PDE inhibitors on PDE7A activity, cytokine synthesis, proliferation and CD25 expression of human peripheral blood mononuclear cells (PBMC) were determined. Recombinant human PDE7A was obtained and subjected to cyclic AMP-hydrolysis assay. PBMC of Dermatophagoides farinae mite extract (Df)-sensitive donors were stimulated with the relevant antigen or an anti-CD3 monoclonal antibody (MoAb). PBMC produced IL-5 and proliferated in response to stimulation with Df, while stimulation with anti-CD3 MoAb induced CD25 expression and messenger RNA (mRNA) synthesis of IL-2, IL-4 and IL-5 in peripheral T cells. A PDE inhibitor, T-2585, which suppressed PDE4 isoenzyme with high potency (IC50 = 0.00013 microM) and PDE7A with low potency (IC50 = 1.7 microM) inhibited cytokine synthesis, proliferation and CD25 expression in the dose range at which the drug suppressed PDE7A activity. A potent selective inhibitor of PDE4 (IC50 = 0.00031 microM), RP 73401, which did not effectively suppress PDE7A (IC50 > 10 microM), inhibited the Df- and anti-CD3 MoAb-stimulated responses only weakly, even at 10 microM. PDE7 may play a critical role in the regulation of human T cell function, and thereby selective PDE7 inhibitors have the potential to be used to treat immunological and inflammatory disorders.

Anti-mycotics suppress interleukin-4 and interleukin-5 production in anti-CD3 plus anti-CD28-stimulated T cells from patients with atopic dermatitis

It is reported that anti-mycotic agents are effective for the treatment of patients with atopic dermatitis. We studied the in vitro effects of anti-mycotics on T helper-1 and T helper-2 cytokine production in anti-CD3 plus anti-CD28-stimulated T cells from atopic dermatitis patients and normal donors. The amounts of interleukin-4 and interleukin-5 secreted by anti-CD3/CD28-stimulated T cells were higher in atopic dermatitis patients than in normal donors. Azole derivatives, ketoconazole, itraconazole, miconazole, and nonazole terbinafine hydrochloride, and tolnaftate reduced interleukin-4 and interleukin-5 secretion without altering that of interferon-gamma and interleukin-2 in anti-CD3/CD28-stimulated T cells from both atopic dermatitis patients and normal donors. The azole derivatives were more inhibitory than nonazole anti-mycotics. These anti-mycotics reduced the anti-CD3/CD28-induced mRNA expression and promoter activities for interleukin-4 and interleukin-5. The 3',5'-cyclic adenosine monophosphate analog dibutyryl 3',5'-cyclic adenosine monophosphate reversed the inhibitory effects of the anti-mycotics on interleukin-4 and interleukin-5 secretion, mRNA expression, and promoter activities. Anti-CD3/CD28 transiently (< or = 5 min) increased intracellular 3',5'-cyclic adenosine monophosphate in T cells, and the increase was greater in atopic dermatitis patients than in normal donors. The increase of 3',5'-cyclic adenosine monophosphate by anti-CD3/CD28 correlated with interleukin-4 and interleukin-5 secretion by anti-CD3/CD28. The transient 3',5'-cyclic adenosine monophosphate increase was suppressed by anti-mycotics, and azole derivatives were more suppressive than nonazoles. Azole derivatives inhibited the activity of cyclic adenosine monophosphate-synthesizing adenylate cyclase whereas terbinafine hydrochloride and tolnaftate enhanced the activity of 3',5'-cyclic adenosine monophosphate-hydrolyzing cyclic nucleotide phosphodiesterase in atopic dermatitis and normal T cells. These results suggest that the anti-mycotics may suppress interleukin-4 and interleukin-5 production by reducing 3',5'-cyclic adenosine monophosphate signal, and stress their potential use for the suppression of T helper-2-mediated allergic reactions.

Identification and quantification of phosphodiesterase 4 subtypes in CD4 and CD8 lymphocytes from healthy and asthmatic subjects

In the present study, for the first time, PDE4 subtypes were identified and semi-quantified in both CD4 and CD8 lymphocytes from healthy and asthmatic individuals. CD4 and CD8 lymphocytes from healthy and mild asymptomatic asthmatic subjects (receiving beta-agonist therapy only) were isolated from peripheral venous blood using appropriate antibody coated paramagnetic beads. PDE4 subtypes and beta-actin were identified by digoxigenin (DIG)-labelling reverse transcriptase-polymerase chain reaction and semi-quantified by DIG-detection enzyme-linked immunosorbance assay. In CD4 and CD8 lymphocytes PDE4A, PDE4B and PDE4D were detected, with no significant differences observed between healthy and asthmatic groups. In CD8 lymphocytes, enzyme subtype expression was lower and showed more intersubject variability. In functional studies investigating the effects of various PDE inhibitors on PHA-induced proliferation of mononuclear cells from healthy and asthmatic subjects, CDP840 (0.03 - 10 microM), rolipram (0.1 - 10 microM) and theophylline (10 microM - 1 mM) inhibited PHA-induced proliferation of mononuclear cells from healthy and asthmatic subjects in a concentration-dependent manner, although no significant difference was observed between the groups investigated. In additional studies, total monocyte cyclic AMP PDE activity was investigated in cells isolated from asthmatic subjects both prior to and 24 h after allergen challenge. Total monocyte cyclic AMP PDE activity remained unaffected following challenge of asthmatic subjects with either house dust mite or cat dander and was inhibited in a concentration-dependent manner by rolipram (0.01 - 100 microM) both before and after allergen challenge.

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.

Phosphodiesterase 4 (PDE4) inhibitors in asthma and chronic obstructive pulmonary disease (COPD)

Phosphodiesterases (PDE) are a family of enzymes responsible for the metabolism of the intracellular second messengers cyclic AMP and cyclic GMP. PDE4 is a cyclic AMP specific PDE that is the major if not sole cyclic AMP metabolizing enzymes found in inflammatory and immune cells, and contributes significantly to cyclic AMP metabolism in smooth muscles. Based on its cellular and tissue distribution and the demonstration that selective inhibitors of this isozyme reduce bronchoconstriction in animals and suppress the activation of inflammatory cells, PDE4 has become an important molecular target for the development of novel therapies for asthma and COPD. This chapter will review the evidence demonstrating the ability of PDE4 inhibitors to modify airway obstruction, airway inflammation and airway remodelling and hyperreactivity, will present some preliminary findings obtained with theses compounds in clinical trials and and will discuss experimental approaches designed to identify novel compounds that maintain the beneficial activity of the initial selective PDE4 inhibitors but with a reduced tendency of elicit the gastrointestinal side effects observed with this class of compounds.

Characterization of the phosphodiesterase (PDE) pattern of in vitro-generated human dendritic cells (DC) and the influence of PDE inhibitors on DC function

During differentiation of human monocytes (CD14(+)/CD1a(-)) to CD14(-)/CD1a(+)dendritic cells (DC), a drastic decrease in PDE4 activity was observed, while activities of PDE1 and PDE3 substantially increased. DC released tumour necrosis factor-alpha (TNF) in response to lipopolysaccharide (LPS) challenge, which was abolished both by dexamethasone and the cyclic AMP-elevating drugs db-cAMP and PGE(2). In addition, rolipram, at PDE4-selective concentrations, blocked TNF release by 37 +/- 5% (P<0.05 vs. control). The PDE3 inhibitor motapizone only marginally influenced TNF synthesis, but a synergistic inhibitory effect was noted in combination with rolipram. Qualitatively, similar inhibitory effects were observed in DC-stimulated T cell responses. Motapizone, lacking efficacy when used alone, increased the effect of rolipram in blocking CD4(+)T lymphocyte proliferation in response to antigen (Ag) (tetanus toxoid, TT; keyhole limpet hemocyanin, KLH) presented by DC and in allogeneic mixed leukocyte reactions (MLR). However, in these coculture systems the T cells rather than the DC seem to be the major target cells of PDE-inhibitor action. In summary, PDE inhibitors can affect DC function directly as demonstrated by blocking TNF release and their efficacy reflects the changes in the PDE activity profile during differentiation from their monocyte precursors. These results together with the known efficacy of PDE3/4 inhibitors in T cells support the concept of combined PDE3/4 inhibitors for asthma therapy.

Possible Contribution of Prostaglandin E2 to the antiproliferative effect of phosphodiesterase 4 inhibitors in human mononuclear cells

Phosphodiesterase (PDE) 4, mixed PDE3/4, and non-selective PDE inhibitors have been shown to inhibit the proliferation of human peripheral blood mononuclear cells (HPBM). The aim of the present study was to examine whether endogenous prostaglandins, in particular prostaglandin E2 (PGE2), are involved in mediating the antiproliferative actions of PDE inhibitors, by comparing their effects with drugs which elevate or mimic adenosine 3',5'-cyclic monophosphate (cAMP) through mechanisms other than PDE inhibition. Indomethacin significantly reduced the antiproliferative effects of the PDE4 inhibitors rolipram and CDP840 and the mixed PDE3/4 inhibitor zardaverine, increasing the IC50 values from 2.51 microM to >10 microM, 0.81 microM to 2.82 microM, and 1.58 microM to 4.82 microM, respectively (P < 0.05), but did not alter the effects of theophylline. Forskolin, PGE2, and dibutyryl cAMP also inhibited HPBM proliferation, and in the presence of indomethacin the effects of forskolin and dibutyryl cAMP were reduced (although this was not significant), whereas PGE2 was not affected. Rolipram, CDP840, zardaverine, and dibutyryl cAMP all produced a concentration-related increase in PGE2 production (P < 0.05, ANOVA), but theophylline significantly increased PGE2 production only at the highest concentration examined, 1000 microM. The ability of indomethacin to reduce the antiproliferative effects of rolipram, CDP840, and zardaverine, together with the fact that these drugs can stimulate PGE2 production, suggests that their antiproliferative actions may be mediated in part by stimulation of endogenous PGE2 production. In contrast, it appears that endogenous PGE2 is not critical for the antiproliferative actions of theophylline, forskolin, and dibutyryl cAMP in HPBM. These results establish the importance of co-ordinated regulation of the cAMP phosphodiesterase and cyclooxygenase-PGE2 systems for the regulation of lymphocyte function in man, and have clinical implications for therapeutic approaches to diseases associated with lymphocyte dysregulation.

Cyclic nucleotide phosphodiesterases (PDE) 3 and 4 in normal, malignant, and HTLV-I transformed human lymphocytes

Intracellular cyclic AMP, determined in part by cyclic nucleotide phosphodiesterases (PDEs), regulates proliferation and immune functions in lymphoid cells. Total PDE, PDE3, and PDE4 activities were measured in phytohemagglutinin (PHA)-activated peripheral blood mononuclear cells (PBMC-PHA), normal natural killer (NK) cells, Jurkat and Kit225-K6 leukemic T-cells, T-cell lines transformed with human T-lymphotropic virus (HTLV)-I (a retrovirus that causes adult T-cell leukemia/lymphoma) and HTLV-II (a nonpathogenic retrovirus), normal B-cells, and B-cells transformed with Epstein-Barr virus (EBV). All cells exhibited PDE3 and PDE4 activities but in different proportions. In EBV-transformed B cells, PDE4 was much higher than PDE3. HTLV-I+ T-cells differed significantly from other T-lymphocyte-derived cells in also having a higher proportion of PDE4 activities, which apparently were not related to selective induction of any one PDE4 mRNA (judged by reverse transcription-polymerase chain reaction) or expression of the HTLV-I regulatory protein Tax. In MJ cells (an HTLV-I+ T-cell line), Jurkat cells, and PBMC-PHA cells, the tyrosine kinase inhibitor herbimycin A strongly inhibited PDE activity. Growth of MJ cells was inhibited by herbimycin A and a protein kinase C (PKC) inhibitor, and was arrested in G1 by rolipram, a specific PDE4 inhibitor. Proliferation of several HTLV-I+ T-cell lines, PBMC-PHA, and Jurkat cells was inhibited differentially by forskolin (which activates adenylyl cyclase), the selective PDE inhibitors cilostamide and rolipram, and the nonselective PDE inhibitors pentoxifylline and isobutyl methylxanthine. These results suggest that PDE4 isoforms may be functionally up-regulated in HTLV-I+ T-cells and may contribute to the virus-induced proliferation, and that PDEs could be therapeutic targets in immune/inflammatory and neoplastic diseases.

Differential effects of phosphodiesterase type 4-specific inhibition on human autoreactive myelin-specific T cell clones

Proinflammatory cytokines, secreted by autoreactive CD4+ T lymphocytes may contribute to the pathogenesis of several human autoimmune diseases, including multiple sclerosis (MS). Since the antigen specificities of these T cells are not known at present, therapeutic strategies aiming at common effector pathways, in particular cytokine secretion, may be more feasible in the near future. We have studied the influence of the isoenzyme-specific phosphodiesterase inhibitor rolipram on the proliferation and cytokine secretion of human myelin basic protein-specific T cell clones. The inhibition of proliferation correlated with interference with the IL-2/IL-2 receptor system, while the effects of rolipram on several T helper 1-(TNF-alpha, TNF-beta, IFN-gamma) and T helper 2-like cytokines (IL-4, IL-13) as well as IL-10 revealed an interesting drug profile, with preferential inhibition of TNF-beta, TNF-alpha and IL-10. This profile suggest that rolipram differs from other currently used immunomodulatory drugs.

Selective phosphodiesterase inhibitors for the treatment of bronchial asthma and chronic obstructive pulmonary disease

Theophylline is commonly used in the treatment of obstructive airway diseases. The identification and functional characterization of different phosphodiesterase (PDE) isoenzymes has led to the development of various isoenzyme-selective inhibitors as potential anti-asthma drugs. Considering the distribution of isoenzymes in target tissues, with high activity of PDE3 and PDE4 in airway smooth muscle and inflammatory cells, selective inhibitors of these isoenzymes may add to the therapy of chronic airflow obstruction. However, initial data from clinical trials with selective PDE3 and PDE4 inhibitors have been somewhat disappointing and have tempered the expectations considerably since these drugs had limited efficacy and their use was clinically limited through side effects. The improved understanding of the molecular biology of PDEs enabled the synthesis of novel drugs with an improved risk/benefit ratio. These 'second generation' selective drugs have produced more promising clinical results not only for the treatment of bronchial asthma but also for the treatment of chronic obstructive pulmonary disease.

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.

Protective effect of docosahexaenoic acid against hydrogen peroxide-induced oxidative stress in human lymphocytes

Oxidatively stressed lymphocytes exhibit decreased proliferative response to mitogenic stimulation. Although several sensitive targets involved in lymphocyte suppression have already been identified, little is known about the influence of oxidative stress on cyclic nucleotide phosphodiesterases (PDE) (EC 3.1.4.17), thought to play a major role in the control of cyclic AMP (cAMP) level, a well-recognized negative effector of lymphoproliferation. Although the polyunsaturated fatty acid content of membrane phospholipids is thought to be directly related to the extent of oxidant-induced lipid peroxidation, some n-3 fatty acids also seem to have antioxidant effects, depending on the concentration used and the overall redox status of the cells in question. Results of the present study showed that human peripheral blood mononuclear cells (PBMC) as well as rat thymocytes were relatively resistant to a short-term exposure (10 min) to hydrogen peroxide (H2O2). Indeed, H2O2-induced lipid peroxidation, estimated by malondialdehyde (MDA) production, was only 2-fold increased by H2O2 concentrations lower than 2 mM, whereas a larger increase (10-fold) could be observed in PBMC at the highest dose (5 mM). Previous enrichment of PBMC with 5 microM docosahexaenoic acid (22:6n-3), brought to the cells as a fatty acid-albumin complex (ratio 1), significantly reduced MDA production induced by low doses of H2O2, the protective effect no longer being observed at the highest doses. In contrast, eicosapentaenoic acid (20:5n-3) did not have any protective effect. Cytosolic PDE activities of both human PBMC and rat thymocytes were significantly inhibited (40-50%) after H2O2 treatment of the cells, whereas particulate PDE activities were not modified. Different responses of PDE activities to H2O2 treatment were observed when PBMC were first enriched with 22:6n-3 prior to H2O2 addition. In 22:6n-3-treated cells, the H2O2-induced inhibition of both cAMP- and cGMP-PDE cytosolic activities was abolished, whereas the particulate activities were increased by the highest H2O2 concentration used (5 mM). At the same time, the glutathione peroxidase (glutathione: oxidoreductase, EC 1.11.1.9) (GSH-Px) activity of PBMC and thymocytes was only marginally inhibited by H2O2 addition (20%), and pretreatment of the cells with 22:6n-3 did not modify the slight inhibitory effect of H2O2. Collectively, these results suggest that lymphocytes are relatively resistant to H2O2-induced lipid peroxidation due to their high GSH-Px content, and that low doses of 22:6n-3 are able to prevent some of the H2O2-induced alterations such as lipid peroxidation and PDE inhibition. Docosahexaenoic acid might thus offer some protection against oxidant-induced lymphocyte suppression.

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.

Induction of phosphodiesterases 3B, 4A4, 4D1, 4D2, and 4D3 in Jurkat T-cells and in human peripheral blood T-lymphocytes by 8-bromo-cAMP and Gs-coupled receptor agonists. Potential role in beta2-adrenoreceptor desensitization

In this study, a potential mechanism of beta2-adrenoreceptor desensitization has been explored that is based upon the enhanced degradation of cAMP by phosphodiesterase (PDE). Pretreatment of Jurkat T-cells with 8-bromo cAMP (8-Br-cAMP) or prostaglandin E2 increased PDE3 and PDE4 activity in an actinomycin D- and cycloheximide-sensitive manner. This effect was associated with increased expression of HSPDE3B, HSPDE4A4, HSPDE4D1, HSPDE4D2, and HSPDE4D3 mRNA transcripts. Western analysis reproducibly labeled a band of immunoreactivity in vehicle-treated cells that corresponded to HSPDE4A4 (125 kDa). Although the intensity of this band was unchanged in cells treated with 8-Br-cAMP, additional 68-72-kDa proteins (HSPDE4D2, HSPDE4D1) were labeled that were not detected after vehicle. Similar results were obtained with T-lymphocytes exposed to 8-Br-cAMP and fenoterol. However, in those experiments HSPDE4A4 and HSPDE4D1 appeared to be equally expressed in vehicle- and treated cells, whereas HSPDE4D2 (72 kDa) was detected only after 8-Br-cAMP. The up-regulation of PDE activity in Jurkat T-cells abolished the ability of isoproterenol to elevate cAMP, which was partially reversed by the non-selective PDE inhibitor, 3-isobutyl-1-methylxanthine, and by the PDE3 and PDE4 inhibitors, Org 9935 and rolipram, respectively. Collectively, these data suggest that chronic treatment of T-cells with cAMP-elevating agents compromises beta2-adrenoreceptor-mediated cAMP accumulation by increasing the expression of HSPDE3B and HSPDE4D gene products.

Expression and regulation of mRNA for distinct isoforms of cAMP-specific PDE-4 in mitogen-stimulated and leukemic human lymphocytes

We reported previously that the gene for PDE-1B1 is induced in isolated human peripheral blood lymphocytes (HPBL) following mitogenic stimulation (Jiang, X., Li, J., Paskind, M., and Epstein, P.M. [1996] Proc. Natl. Acad. Sci. USA 93, 11,236-11,241). Using reverse transcription-polymerase chain reaction (RT-PCR), we investigated possible changes in the expression of the four genes for cAMP-specific phosphodiesterase (PDE-4A-D) in HPBL under the same conditions. Isolated, quiescent HPBL express mRNA for PDE-4B as the principal transcript. Following mitogenic stimulation with phytohemagglutinin (PHA), mRNA for PDE-4A and PDE-4D are clearly induced. HPBL appear not to express PDE-4C under resting or stimulated conditions. The PHA induced increase in PDE-1B1, PDE-4A, and PDE-4D mRNA is mimicked by incubation of HPBL with dibutyryl cAMP (dBcAMP) and 1-methyl-3-isobutylxanthine (IBMX). The B-lymphoblastoid cell line, RPMI 8392, and the T-leukemic cell line, Molt 4, express PDE-4A mRNA as the most abundant transcript, but incubation with dBcAMP and IBMX induces an increase in the expression of mRNA for PDE-4B in both of these cell lines, and in PDE-4D3 in the RPMI 8392 cell line. These studies demonstrate that expression of mRNA for PDE-1B1 and some of the subtypes of PDE-4 are induced in HPBL following mitogenic stimulation, possibly secondarily to elevation of cAMP induced by the mitogen. As already indicated for PDE-1B1, some of these subtypes of PDE-4 might also provide additional therapeutic targets for treatment of immunoproliferative disorders and immune dysfunction.

Phosphodiesterase profile of human B lymphocytes from normal and atopic donors and the effects of PDE inhibition on B cell proliferation

1. CD19+ B lymphocytes were purified from the peripheral blood of normal and atopic subjects to analyse and compare the phosphodiesterase (PDE) activity profile, PDE mRNA expression and the importance of PDE activity for the regulation of B cell function. 2. The majority of cyclic AMP hydrolyzing activity of human B cells was cytosolic PDE4, followed by cytosolic PDE7-like activity; marginal PDE3 activity was found only in the particulate B cell fraction. PDE1, PDE2 and PDE5 activities were not detected. 3. By cDNA-PCR analysis mRNA of the PDE4 subtypes A, B (splice variant PDE4B2) and D were detected. In addition, a weak signal for PDE3A was found. 4. No differences in PDE activities or mRNA expression of PDE subtypes were found in B cells from either normal or atopic subjects. 5. Stimulation of B lymphocytes with the polyclonal stimulus lipopolysaccharide (LPS) induced a proliferative response in a time- and concentration-dependent manner, which was increased in the presence of interleukin-4 (IL-4). PDE4 inhibitors (rolipram, piclamilast) led to an increase in the cellular cyclic AMP concentration and to an augmentation of proliferation, whereas a PDE3 inhibitor (motapizone) was ineffective, which is in accordance with the PDE profile found. The proliferation enhancing effect of the PDE4 inhibitors was partly mimicked by the cyclic AMP analogues dibutyryl (db) cyclic AMP and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3',5'-cyclic monophosphorothioate, Sp-isomer (dcl-cBIMPS), respectively. However, at concentrations exceeding 100 microM db-cyclic AMP suppressed B lymphocyte proliferation, probably as a result of cytotoxicity. Prostaglandin E2 (PGE2, 1 microM) and forskolin (10 microM) did not affect B cell proliferation, even when given in combination with rolipram. 6. Inhibition of protein kinase A (PKA) by differentially acting selective inhibitors (KT 5720, Rp-8-Br-cyclic AMPS) decreased the proliferative response of control cells and reversed the proliferation enhancing effects of rolipram. 7. Importantly, PDE4 activity in LPS/IL-4-activated B lymphocytes decreased by about 50% compared to unstimulated control values. 8. We conclude that an increase in cyclic AMP, mediated by down-regulation of PDE4 activity, is involved in the stimulation of B cell proliferation in response to LPS/IL-4. B cell proliferation in response to a mitogenic stimulus can be further enhanced by pharmacological elevation of cyclic AMP.

A selective type 4 phosphodiesterase inhibitor, T-440, modulates intracellular cyclic AMP level and interleukin-2 production of Jurkat cells

Effect of a selective type 4 phosphodiesterase (PDE4) inhibitor, T-440, on intracellular cyclic AMP (cAMP) level and interleukin (IL)-2 production of Jurkat cells was investigated. T-440 suppressed both cAMP-PDE activities in cytosolic and membrane fractions of Jurkat cells. Intracellular cAMP level in Jurkat cells was elevated by PGE2 and forskolin but not by T-440. T-440, however, significantly enhanced the increase of cAMP by PGE2. PGE2 and forskolin inhibited IL-2 production of Jurkat cells stimulated with concanavalin A. T-440 by itself did not affect IL-2 production, but significantly enhanced the effect of PGE2 on IL-2 production. The increase of intracellular cAMP by T-440, PGE2, forskolin and T-440 plus PGE2 was well correlated with the inhibition of IL-2 production. These results indicate that IL-2 production of T cells is regulated by cAMP-PDE activity. Immunomodulatory effects of PDE4 inhibitors like T-440 should further be explored in vivo.