Theophylline and PDE4 inhibitors in asthma

Phosphodiesterase in heart and vessels: from physiology to diseases

Phosphodiesterases (PDEs) are a superfamily of enzymes that hydrolyze cyclic nucleotides, including cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Both cyclic nucleotides are critical secondary messengers in the neurohormonal regulation in the cardiovascular system. PDEs precisely control spatiotemporal subcellular distribution of cyclic nucleotides in a cell- and tissue-specific manner, playing critical roles in physiological responses to hormone stimulation in the heart and vessels. Dysregulation of PDEs has been linked to the development of several cardiovascular diseases, such as hypertension, aneurysm, atherosclerosis, arrhythmia, and heart failure. Targeting these enzymes has been proven effective in treating cardiovascular diseases and is an attractive and promising strategy for the development of new drugs. In this review, we discuss the current understanding of the complex regulation of PDE isoforms in cardiovascular function, highlighting the divergent and even opposing roles of PDE isoforms in different pathogenesis.

JAK/STAT inhibitors and other small molecule cytokine antagonists for the treatment of allergic disease

OBJECTIVE

To provide an overview of janus kinase (JAK), chemoattractant receptor homologous molecule expressed on T_H2 cells (CRTH2), and phosphodiesterase 4 (PDE4) inhibitors in allergic disorders.

DATA SOURCES

PubMed literature review.

STUDY SELECTIONS

Articles included in this review discuss the emerging mechanism of action of small molecule inhibitors and their use in the treatment of atopic dermatitis (AD), asthma, and allergic rhinitis (AR).

RESULTS

Allergic diseases represent a spectrum of diseases, including AD, asthma, and AR. For decades, these diseases have been primarily characterized by increased T_H2 signaling and downstream inflammation. In recent years, additional research has identified disease phenotypes and subsets of patients with non-Th2 mediated inflammation. The increasing heterogeneity of disease has prompted investigators to move away from wide-ranging treatment approaches with immunosuppressive agents, such as corticosteroids, to consider more targeted immunomodulatory approaches focused on specific pathways. In the past decade, inhibitors that target JAK signaling, PDE4, and CRTH2 have been explored for their potential activity in models of allergic disease and therapeutic benefit in clinical trials. Interestingly, although JAK inhibitors provide an opportunity to interfere with cytokine signaling and could be beneficial in a broad range of allergic diseases, current clinical trials are focused on the treatment of AD. Conversely, both PDE4 and CRTH2 inhibitors have been evaluated in a spectrum of allergic diseases. This review summarizes the varying degrees of success that these small molecules have demonstrated across allergic diseases.

CONCLUSION

Emerging therapies currently in development may provide more consistent benefit to patients with allergic diseases by specifically targeting inflammatory pathways important for disease pathogenesis.

Severe acute asthma exacerbation in children: a stepwise approach for escalating therapy in a pediatric intensive care unit

OBJECTIVES

An increasing prevalence of pediatric asthma has led to increasing burdens of critical illness in children with severe acute asthma exacerbations, often leading to respiratory distress, progressive hypoxia, and respiratory failure. We review the definitions, epidemiology, pathophysiology, and clinical manifestations of severe acute asthma, with a view to developing an evidence-based, stepwise approach for escalating therapy in these patients.

METHODS

Subject headings related to asthma, status asthmaticus, critical asthma, and drug therapy were used in a MEDLINE search (1980-2012), supplemented by a manual search of personal files, references cited in the reviewed articles, and treatment algorithms developed within Le Bonheur Children's Hospital.

RESULTS

Patients with asthma require continuous monitoring of their cardiorespiratory status via noninvasive or invasive devices, with serial clinical examinations, objective scoring of asthma severity (using an objective pediatric asthma score), and appropriate diagnostic tests. All patients are treated with β-agonists, ipratropium, and steroids (intravenous preferable over oral preparations). Patients with worsening clinical status should be progressively treated with continuous β-agonists, intravenous magnesium, helium-oxygen mixtures, intravenous terbutaline and/or aminophylline, coupled with high-flow oxygen and non-invasive ventilation to limit the work of breathing, hypoxemia, and possibly hypercarbia. Sedation with low-dose ketamine (with or without benzodiazepines) infusions may allow better toleration of non-invasive ventilation and may also prepare the patient for tracheal intubation and mechanical ventilation, if indicated by a worsening clinical status.

CONCLUSIONS

Severe asthma can be a devastating illness in children, but most patients can be managed by using serial objective assessments and the stepwise clinical approach outlined herein. Following multidisciplinary education and training, this approach was successfully implemented in a tertiary-care, metropolitan children's hospital.

Phosphodiesterases as targets for modulating T-cell responses

The cAMP-protein kinase A (PKA) signaling pathway is strongly involved in the regulation and modulation of immune responses, and cAMP is the most potent and acute inhibitor of T-cell activation. Thus, cAMP levels in the cell must be tightly regulated. Cyclic AMP-specific phosphodiesterases (PDEs) provide the only mechanism for degrading cAMP in cells, thereby functioning as key regulators of signaling. To obtain a complete immune response with optimal cytokine production and T-cell proliferation, ligation of both the T-cell receptor (TCR) and the CD28 receptor is required. However, engagement of the TCR in primary T cells is followed by rapid cAMP production in lipid rafts and activation of the cAMP- PKA-Csk pathway inhibiting proximal T-cell signaling. In contrast, TCR/CD28 costimulation leads to the recruitment of a PDE4/β-arrestin complex to rafts in a phosphatidylinositol 3-kinase (PI3K)-dependent manner, resulting in the downregulation of cAMP levels. Thus, the activities of both PKA and PDE4 seem to be important for regulation of TCR-induced signaling and T-cell function. The use of selective inhibitors has revealed that PDEs are important drug targets in several diseases with an inflammatory component where immune function is important such as asthma, chronic obstructive pulmonary disease (COPD), cardiovascular diseases, and neurological disorders. PDEs are also interesting drug targets in immunosuppression following transplantation and for modulation of immune responses.

Spatiotemporal control of cyclic AMP immunomodulation through the PKA-Csk inhibitory pathway is achieved by anchoring to an Ezrin-EBP50-PAG scaffold in effector T cells

A variety of immunoregulatory signals to effector T cells from monocytes, macrophages and regulatory T cells act through cyclic adenosine monophosphate. In the effector T cell, the protein kinase A (PKA) type I isoenzyme localizes to lipid rafts during T cell activation and modulates directly the proximal events that take place after engagement of the T cell receptor. The most proximal target for PKA phosphorylation is C-terminal Src kinase (Csk), which initiates a negative signal pathway that fine-tunes the T cell activation process. The A kinase anchoring protein Ezrin colocalizes PKA and Csk by forming a supramolecular signaling complex consisting of PKA, Ezrin, Ezrin/radixin/moesin (ERM) binding protein of 50 kDa (EBP50), phosphoprotein associated with glycosphingolipid-enriched membrane microdomains (GEMs) (PAG) and Csk.

Novel and emerging therapies for asthma

At present, there are a wide variety of novel and emerging therapeutic approaches for the treatment of asthma. Here, we will summarize these state-of-the-art approaches, including specific and nonspecific mediator inhibition-- a quest that has been on going for more than 25 years-- together with cytokine modulation in asthma (primarily attempting to modulate the Th2-Th1 balance in asthma), targeting cell recruitment, angiogenesis, signal transduction and gene transduction pathways. Finally, we will discuss the recently approved anti-IgE therapy for the treatment of allergic asthma and immune modulation using CpG oligodeoxynucleotides.

RNA silencing identifies PDE4D5 as the functionally relevant cAMP phosphodiesterase interacting with beta arrestin to control the protein kinase A/AKAP79-mediated switching of the beta2-adrenergic receptor to activation of ERK in HEK293B2 cells

PDE4B and PDE4D provide >90% of PDE4 cAMP phosphodiesterase activity in human embryonic kidney (HEK293B2) cells. Their selective small interference RNA (siRNA)-mediated knockdown potentiates isoprenaline-stimulated protein kinase A (PKA) activation. Whereas endogenous PDE4D co-immunoprecipitates with beta arrestin, endogenous PDE4B does not, even upon PDE4D knockdown. Ectopic overexpression of PDE4B2 confers co-immunoprecipitation with beta arrestin. Knockdown of PDE4D, but not PDE4B, amplifies isoprenaline-stimulated phosphorylation of the beta2-adrenergic receptor (beta2-AR) by PKA and activation of extracellular signal-regulated kinase (ERK) through G(i). Isoform-selective knockdown identifies PDE4D5 as the functionally important species regulating isoprenaline stimulation of both these processes. Ht31-mediated disruption of the tethering of PKA to AKAP scaffold proteins attenuates isoprenaline activation of ERK, even upon PDE4D knockdown. Selective siRNA-mediated knockdown identifies AKAP79, which is constitutively associated with the beta2-AR, rather than isoprenaline-recruited gravin, as being the functionally relevant AKAP in this process. Isoprenaline-stimulated membrane recruitment of PDE4D is ablated upon beta arrestin knockdown. A mutation that compromises interactions with beta arrestin prevents catalytically inactive PDE4D5 from performing a dominant negative role in potentiating isoprenaline-stimulated ERK activation. Beta arrestin-recruited PDE4D5 desensitizes isoprenaline-stimulated PKA phosphorylation of the beta2-AR and the consequential switching of its signaling to ERK. The ability to observe a cellular phenotype upon PDE4D5 knockdown demonstrates that other PDE4 isoforms, expressed at endogenous levels, are unable to afford rescue in HEK293B2 cells.

Selective phosphodiesterase-4 inhibitors in chronic obstructive lung disease

PURPOSE OF REVIEW

Chronic obstructive pulmonary disease (COPD) is a disease state characterized by airflow limitation that is usually progressive. In addition, an abnormal inflammatory response of the lungs to noxious particles or gases can be seen throughout the airways, parenchyma, and pulmonary vasculature. So far, anti-inflammatory medications (eg, inhaled corticosteroids) have failed to show a major effect on the decline of lung function in COPD patients. Novel anti-inflammatory therapies such as selective phosphodiesterase 4 (PDE4) inhibitors are in clinical development. Their potential role in the management of COPD is described in this review.

RECENT FINDINGS

Some of the selective PDE4 inhibitors have demonstrated in vitro and in vivo anti-inflammatory activity on cells commonly linked to airway inflammation in COPD, such as neutrophils. While these agents seem to offer only a modest improvement in lung function compared with other bronchodilators, their anti-inflammatory effects appear to provide some substantial benefits in reducing exacerbations and improving health-related quality of life.

SUMMARY

Based on the available data, the second generation of selective PDE4 inhibitors will likely provide additional therapeutic options for the management of COPD. These agents may become an important tool in the treatment of this disease, since they target three important components of COPD: airway obstruction, inflammation, and structural changes.

Asthma treatment in the 21st century: what's next?

Asthma treatment is evolving as we enter the 21st century. This review focuses on several different areas of asthma treatment now in evolution. These include: (1) the proper role of various asthma controllers--either already approved or under investigation--besides inhaled corticosteriods in asthma therapy; (2) the potential role for immune and cytokine modulation for asthma therapy; (3) the potential role for pharmacogenetics in asthma therapy; and (4) whether single-inhaler therapy with a combination of an inhaled corticosteriod and a long-acted beta-agonist could be used for both maintenance and rescue in patients with asthma.

Effects of allergen challenge on airway epithelial cell gene expression

Allergen exposure induces the airway epithelium to produce chemoattractants, proallergic interleukins, matrix-modifying proteins, and proteins that influence the growth and activation state of airway structural cells. These proteins, in turn, contribute to the influx of inflammatory cells and changes in structure that characterize the asthmatic airway. To use the response of the airway epithelium to allergen to identify genes not previously associated with allergic responses, we compared gene expression in cytokeratin-positive cells before and after segmental allergen challenge. After challenge with concentrations of allergen in the clinically relevant range, 755 (6%) of the detectable sequences had geometric mean fold-changes in expression, with 95% confidence intervals that excluded unity. Using a prospectively defined conservative filtering algorithm, we identified 141 sequences as upregulated and eight as downregulated, with confirmation by conventional polymerase chain reaction in all 10 sequences studied. Using this approach, we identified asthma-associated sequences including interleukin (IL-)-3, IL-4, and IL-5 receptor subunits, the p65 component of nuclear factor-kappaB, and lipocortin. The genomic response of the human airway to concentrations of allergen in the clinically relevant range involves a greater number of genes than previously recognized, including many not previously associated with asthma that are differentially expressed after airway allergen exposure.

TCR- and CD28-mediated recruitment of phosphodiesterase 4 to lipid rafts potentiates TCR signaling

Ligation of the TCR along with the coreceptor CD28 is necessary to elicit T cell activation in vivo, whereas TCR triggering alone does not allow a full T cell response. Upon T cell activation of human peripheral blood T cells, we found that the majority of cAMP was generated in T cell lipid rafts followed by activation of protein kinase A. However, upon TCR and CD28 coligation, beta-arrestin in complex with cAMP-specific phosphodiesterase 4 (PDE4) was recruited to lipid rafts which down-regulated cAMP levels. Whereas inhibition of protein kinase A increased TCR-induced immune responses, inhibition of PDE4 blunted T cell cytokine production. Conversely, overexpression of either PDE4 or beta-arrestin augmented TCR/CD28-stimulated cytokine production. We show here for the first time that the T cell immune response is potentiated by TCR/CD28-mediated recruitment of PDE4 to lipid rafts, which counteracts the local, TCR-induced production of cAMP. The specific recruitment of PDE4 thus serves to abrogate the negative feedback by cAMP which is elicited in the absence of a coreceptor stimulus.

Expression, intracellular distribution and basis for lack of catalytic activity of the PDE4A7 isoform encoded by the human PDE4A cAMP-specific phosphodiesterase gene

PDE4A7 is an isoform encoded by the human PDE4A cAMP-specific phosphodiesterase gene that fails to hydrolyse cAMP and whose transcripts are widely expressed. Removal of either the N- or C-terminal unique portions of PDE4A7 did not reconstitute catalytic activity, showing that they did not exert a chronic inhibitory effect. A chimera (Hyb2), formed by swapping the unique N-terminal portion of PDE4A7 with that of the active PDE4A4C form, was not catalytically active. However, one formed (Hyb1) by swapping the unique C-terminal portion of PDE4A7 with that common to all active PDE4 isoforms was catalytically active. Compared with the active PDE4A4B isoform, Hyb1 exhibited a similar K(m) value for cAMP and IC50 value for rolipram inhibition, but was less sensitive to inhibition by Ro-20-1724 and denbufylline, and considerably more sensitive to thermal denaturation. The unique C-terminal region of PDE4A7 was unable to support an active catalytic unit, whereas its unique N-terminal region can. The N-terminal portion of the PDE4 catalytic unit is essential for catalytic activity and can be supplied by either highly conserved sequence found in active PDE4 isoforms from all four PDE4 subfamilies or the unique N-terminal portion of PDE4A7. A discrete portion of the conserved C-terminal region in active PDE4A isoforms underpins their aberrant migration on SDS/PAGE. Unlike active PDE4A isoforms, PDE4A7 is exclusively localized to the P1 particulate fraction in cells. A region located within the C-terminal portion of active PDE4 isoforms prevents such exclusive targeting. Three functional regions in PDE4A isoforms are identified, which influence catalytic activity, subcellular targeting and conformational status.

Remodelling of the PDE4 cAMP phosphodiesterase isoform profile upon monocyte-macrophage differentiation of human U937 cells

Monocytes and macrophages provide key targets for the action of novel anti-inflammatory therapeutics targeted at inhibition of PDE4 cAMP-specific phosphodiesterases. PDE4 enzymes provide the dominant cAMP phosphodiesterase activity in U937 human monocytic cells. Differentiation of U937 monocytic cells to a macrophage-like phenotype causes a marked reduction in total cellular PDE4 activity. Monocytic U937 cells express the long PDE4A4, PDE4D5 and PDE4D3 isoforms plus the short PDE4B2 isoform. Differentiation of U937 cells to a macrophage-like phenotype causes a marked downregulation of PDE4D3 and PDE4D5, elicits a marked upregulation of PDE4B2 and induces the novel PDE4A10 long isoform. Comparable patterns are found in human peripheral blood monocytes and macrophages differentiated from them. Immunopurification of PDE4 subfamilies identifies long PDE4D isoforms as providing the major PDE4 activity in U937 monocytic cells. In U937 macrophage-like cells, the activity of the short PDE4B2 isoform predominates. No indication of either the expression or induction of PDE4C was evident. Activation of ERK exerts an inhibitory effect on total PDE4 activity in monocytic U937 cells, where the activity of long PDE4 isoforms predominates. The effect of ERK activation is switched to one of overall stimulation of total PDE4 activity in macrophage U937 cells, where the activity of the short PDE4B2 isoform predominates.10 The profound differentiation-induced changes in PDE4 isoform profile identified here suggests that the development of inhibitors specific for particular PDE4 isoforms may allow for selective effects on monocytes and macrophages to be achieved.

TbPDE1, a novel class I phosphodiesterase of Trypanosoma brucei

Cyclic nucleotide specific phosphodiesterases (PDEs) are important components of all cAMP signalling networks. In humans, 11 different PDE families have been identified to date, all of which belong to the class I PDEs. Pharmacologically, they have become of great interest as targets for the development of drugs for a large variety of clinical conditions. PDEs in parasitic protozoa have not yet been extensively investigated, despite their potential as antiparasitic drug targets. The current study presents the identification and characterization of a novel class I PDE from the parasitic protozoon Trypanosoma brucei, the causative agent of human sleeping sickness. This enzyme, TbPDE1, is encoded by a single-copy gene located on chromosome 10, and it functionally complements PDE-deficient strains of Saccharomyces cerevisiae. Its C-terminal catalytic domain shares about 30% amino acid identity, including all functionally important residues, with the catalytic domains of human PDEs. A fragment of TbPDE1 containing the catalytic domain could be expressed in active form in Escherichia coli. The recombinant enzyme is specific for cAMP, but exhibits a remarkably high Km of > 600 microm for this substrate.

Localized effects of cAMP mediated by distinct routes of protein kinase A

More than 20% of the human genome encodes proteins involved in transmembrane and intracellular signaling pathways. The cAMP-protein kinase A (PKA) pathway is one of the most common and versatile signal pathways in eukaryotic cells and is involved in regulation of cellular functions in almost all tissues in mammals. Various extracellular signals converge on this signal pathway through ligand binding to G protein-coupled receptors, and the cAMP-PKA pathway is therefore tightly regulated at several levels to maintain specificity in the multitude of signal inputs. Ligand-induced changes in cAMP concentration vary in duration, amplitude, and extension into the cell, and cAMP microdomains are shaped by adenylyl cyclases that form cAMP as well as phosphodiesterases that degrade cAMP. Different PKA isozymes with distinct biochemical properties and cell-specific expression contribute to cell and organ specificity. A kinase anchoring proteins (AKAPs) target PKA to specific substrates and distinct subcellular compartments providing spatial and temporal specificity for mediation of biological effects channeled through the cAMP-PKA pathway. AKAPs also serve as scaffolding proteins that assemble PKA together with signal terminators such as phosphatases and cAMP-specific phosphodiesterases as well as components of other signaling pathways into multiprotein signaling complexes that serve as crossroads for different paths of cell signaling. Targeting of PKA and integration of a wide repertoire of proteins involved in signal transduction into complex signal networks further increase the specificity required for the precise regulation of numerous cellular and physiological processes.

The crystal structure of AMP-bound PDE4 suggests a mechanism for phosphodiesterase catalysis

Cyclic nucleotide phosphodiesterases (PDEs) regulate the intracellular concentrations of cyclic 3',5'-adenosine and guanosine monophosphates (cAMP and cGMP, respectively) by hydrolyzing them to AMP and GMP, respectively. Family-selective inhibitors of PDEs have been studied for treatment of various human diseases. However, the catalytic mechanism of cyclic nucleotide hydrolysis by PDEs has remained unclear. We determined the crystal structure of the human PDE4D2 catalytic domain in complex with AMP at 2.4 A resolution. In this structure, two divalent metal ions simultaneously interact with the phosphate group of AMP, implying a binuclear catalysis. In addition, the structure suggested that a hydroxide ion or a water bridging two metal ions may serve as the nucleophile for the hydrolysis of the cAMP phosphodiester bond.

Anti-TNF-alpha therapies: the next generation

The functioning of the immune system is finely balanced by the activities of pro-inflammatory and anti-inflammatory mediators or cytokines. Unregulated activities of these mediators can lead to the development of serious inflammatory diseases. In particular, enhanced tumour-necrosis factor-alpha (TNF-alpha) synthesis is associated with the development of rheumatoid arthritis, psoriatic arthritis and inflammatory bowel disease. Inhibiting TNF-alpha activities in these diseases has been remarkably successful. However, the current injectable protein therapies have associated risks and limitations. An oral, small molecule that regulates TNF-alpha biology could either replace the injectables or provide better disease control when used alone or in conjunction with existing therapies. In this review, we discuss briefly the present understanding of TNF-alpha-mediated biology and the current injectable therapies in clinical use, and focus on some of the new therapeutic approaches with oral, small-molecule inhibitors.