Intracellular targeting of phosphodiesterase-4 underpins compartmentalized cAMP signaling

The role of soluble adenylyl cyclase in sensing and regulating intracellular pH

Soluble adenylyl cyclase (sAC) differs from transmembrane adenylyl cyclases (tmAC) in many aspects. In particular, the activity of sAC is not regulated by G-proteins but by the prevailing bicarbonate concentrations inside cells. Therefore, sAC serves as an exquisite intracellular pH sensor, with the capacity to translate pH changes into the regulation of localization and/or activity of cellular proteins involved in pH homeostasis. In this review, we provide an overview of literature describing the regulation of sAC activity by bicarbonate, pinpointing the importance of compartmentalization of intracellular cAMP signaling cascades. In addition, examples of processes involving proton and bicarbonate transport in different cell types, in which sAC plays an important regulatory role, were described in detail.

Physiological roles of mammalian transmembrane adenylyl cyclase isoforms

Adenylyl cyclases (ACs) catalyze the conversion of ATP to the ubiquitous second messenger cAMP. Mammals possess nine isoforms of transmembrane ACs, dubbed AC1-9, that serve as major effector enzymes of G protein-coupled receptors (GPCRs). The transmembrane ACs display varying expression patterns across tissues, giving the potential for them to have a wide array of physiological roles. Cells express multiple AC isoforms, implying that ACs have redundant functions. Furthermore, all transmembrane ACs are activated by Gαs, so it was long assumed that all ACs are activated by Gαs-coupled GPCRs. AC isoforms partition to different microdomains of the plasma membrane and form prearranged signaling complexes with specific GPCRs that contribute to cAMP signaling compartments. This compartmentation allows for a diversity of cellular and physiological responses by enabling unique signaling events to be triggered by different pools of cAMP. Isoform-specific pharmacological activators or inhibitors are lacking for most ACs, making knockdown and overexpression the primary tools for examining the physiological roles of a given isoform. Much progress has been made in understanding the physiological effects mediated through individual transmembrane ACs. GPCR-AC-cAMP signaling pathways play significant roles in regulating functions of every cell and tissue, so understanding each AC isoform's role holds potential for uncovering new approaches for treating a vast array of pathophysiological conditions.

Roflumilast, a type of phosphodiesterase 4 inhibitor, can reduce intestinal injury caused by sepsis

Sepsis is a systemic inflammatory response syndrome caused by infection, which has a complex mechanism. The gastrointestinal tract is commonly the first organ affected by sepsis, but intestinal disease itself can also induce sepsis. Roflumilast has been found to exert anti-inflammatory effects and, thus, the present study sought to examine its effect on intestinal damage caused by sepsis. In vivo studies were conducted using cecal ligation and puncture rat models, and in vitro experiments were performed using IEC-6 cells. The intestinal cells were first induced with lipopolysaccharide and the induced cells were then treated with roflumilast to evaluate its effects on phosphodiesterase (PDE)4 expression, intestinal function indices, release of inflammatory factors and cell apoptosis. The expression level of PDE4 in the small intestinal tissue of septic rats was found to be significantly higher compared with that in the normal group, suggesting that PDE4 may play a key role in intestinal injury caused by sepsis. It was found that roflumilast reduced PDE4 expression, as well as the levels of intestinal function indices, including lactate dehydrogenase, diamino oxidase and intestinal fatty acid-binding protein, in intestinal cells. Moreover, roflumilast reduced cellular damage, the release of inflammatory factors and apoptosis. In summary, the findings of the present study indicated that roflumilast can relieve the inflammation and apoptosis of intestinal cells caused by sepsis and can promote their functional recovery. These findings may promote the expansion of the clinical application of roflumilast in the future.

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions. SIGNIFICANCE STATEMENT: cAMP mediates the intracellular response to multiple hormones and neurotransmitters. Signal fidelity and accurate coordination of a plethora of different cellular functions is achieved via organization of multiprotein signalosomes and cAMP compartmentalization in subcellular nanodomains. Defining the organization and regulation of subcellular cAMP nanocompartments is necessary if we want to understand the complex functional ramifications of pharmacological treatments that target G protein-coupled receptors and for generating a blueprint that can be used to develop precision medicine interventions.

Cyclic AMP efflux through MRP4 regulates actin dynamics signalling pathway and sperm motility in bovines

Previously we demonstrated that multidrug resistance-associated protein 4 transporter (MRP4) mediates cAMP efflux in bovine spermatozoa and that extracellular cAMP (ecAMP) triggers events associated to capacitation. Here, we deepen the study of the role of MRP4 in bovine sperm function by using MK571, an MRP4 inhibitor. The incubation of spermatozoa with MK571 during 45 min inhibited capacitation-associated events. MRP4 was localized in post-acrosomal region and mid-piece at 15 min capacitation, while at 45 min it was mainly located in the acrosome. After 15 min, MK571 decreased total sperm motility (TM), progressive motility (PM) and several kinematic parameters. The addition of ecAMP rescued MK571 effect and ecAMP alone increased the percentage of motile sperm and kinematics parameters. Since actin cytoskeleton plays essential roles in the regulation of sperm motility, we investigated if MRP4 activity might affect actin polymerization. After 15 min capacitation, an increase in F-actin was observed, which was inhibited by MK571. This effect was reverted by the addition of ecAMP. Furthermore, ecAMP alone increased F-actin levels while no F-actin was detected with ecAMP in the presence of PKA inhibitors. Our results support the importance of cAMP efflux through MRP4 in sperm capacitation and suggest its involvement in the regulation of actin polymerization and motility.

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

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

Apremilast reversed carfilzomib-induced cardiotoxicity through inhibition of oxidative stress, NF-κB and MAPK signaling in rats

Carfilzomib (CFZ), is a potent, selective second generation proteasome inhibitor, used for the treatment of multiple myeloma. The aim of the present study was to investigate the possible protective effect of apremilast (AP) on the CFZ -induced cardiotoxicity. Rats were randomly divided into four groups: Group 1, served as the control group, received normal saline. Group 2, served as the toxic group, received CFZ (4 mg/kg, intraperitoneally [i.p.]). Groups 3 and 4, served as treatment groups, and received CFZ with concomitant oral administration of AP in doses of 10 and 20 mg/kg/day, respectively. In the present study, administration of CFZ resulted in a significant increase in serum aspartate transaminase (AST), lactate dehydrogenase (LDH), creatine kinase (CK) and creatine kinase-MB (CK-MB), which were reversed by treatment with AP. CFZ resulted in a significant increase in heart malondialdehyde (MDA) contents and decrease in cardiac glutathione (GSH) level and catalase (CAT) enzyme activity which were significantly reversed by treatment with AP. Induction of cardiotoxicity by CFZ significantly increased caspase-3 enzyme activity which were reversed by treatment with AP. RT-PCR analysis revealed an increased mRNA expression of NF-κB, ERK and JNK which were reversed by treatment with AP in cardiac tissues. Western blot analysis revealed an increased expression of caspase-3 and NF-κB p65 and a decrease expression of inhibitory kappa B-alpha (Iκbα) with CFZ, which were reversed by treatment with AP. In conclusion, apremilast showed protective effect against CFZ-induced cardiotoxicity.

Loss of cAMP-specific phosphodiesterase rescues spore development in G protein mutant in dictyostelium

Cyclic AMP (cAMP) is an important intracellular signaling molecule for many G protein-mediated signaling pathways but the specificity of cAMP signaling in cells with multiple signaling pathways is not well-understood. In Dictyostelium, at least two different G protein signaling pathways, mediated by the Gα2 and Gα4 subunits, are involved with cAMP accumulation, spore production, and chemotaxis and the stimulation of these pathways results in the activation of ERK2, a mitogen-activated protein kinase that can down regulate the cAMP-specific phosphodiesterase RegA. The regA gene was disrupted in gα2(−) and gα4(−) cells to determine if the absence of this phosphodiesterase rescues the development of these G protein mutants as it does for erk2(−) mutants. There gA(−) mutation had no major effects on developmental morphology but enriched the distribution of the Gα mutant cells to the prespore/prestalk border in chimeric aggregates. The loss of RegA function had no effect on Gα4- mediated folate chemotaxis. However, the regA gene disruption in gα4(−) cells, but not in gα2(−) cells, resulted in a substantial rescue and acceleration of spore production. This rescue in sporulation required cell autonomous signaling because the precocious sporulation could not be induced through intercellular signaling in chimeric aggregates. However, intercellular signals from regA(−) strains increased the expression of the prestalk gene ecmB and accelerated the vacuolization of stalk cells. Intercellular signaling from the gα4(−)regA(−) strain did not induce ecmA gene expression indicating cell-type specificity in the promotion of prestalk cell development. regA gene disruption in a Gα4(HC) (Gα4 overexpression) strain did not result in precocious sporulation or stalk cell development indicating that elevated Gα4 subunit expression can mask regA(−) associated phenotypes even when provided with wild-type intercellular signaling. These findings indicate that the Gα2 and Gα4-mediated pathways provide different contributions to the development of spores and stalk cells and that the absence of RegA function can bypass some but not all defects in G protein regulated spore development.

Compartmentalization of distinct cAMP signaling pathways in mammalian sperm

Fertilization competence is acquired in the female tract in a process known as capacitation. Capacitation is needed for the activation of motility (e.g. hyperactivation) and to prepare the sperm for an exocytotic process known as acrosome reaction. Although the HCO3(-)-dependent soluble adenylyl cyclase Adcy10 plays a role in motility, less is known about the source of cAMP in the sperm head. Transmembrane adenylyl cyclases (tmACs) are another possible source of cAMP. These enzymes are regulated by stimulatory heterotrimeric Gs proteins; however, the presence of Gs or tmACs in mammalian sperm has been controversial. In this study, we used Western blotting and cholera toxin-dependent ADP-ribosylation to show the Gs presence in the sperm head. Also, we showed that forskolin, a tmAC-specific activator, induces cAMP accumulation in sperm from both WT and Adcy10-null mice. This increase is blocked by the tmAC inhibitor SQ22536 but not by the Adcy10 inhibitor KH7. Although Gs immunoreactivity and tmAC activity are detected in the sperm head, PKA is only found in the tail, where Adcy10 was previously shown to reside. Consistent with an acrosomal localization, Gs reactivity is lost in acrosome-reacted sperm, and forskolin is able to increase intracellular Ca(2+) and induce the acrosome reaction. Altogether, these data suggest that cAMP pathways are compartmentalized in sperm, with Gs and tmAC in the head and Adcy10 and PKA in the flagellum.

The distribution of phosphodiesterase 2A in the rat brain

The phosphodiesterases (PDEs) are a superfamily of enzymes that regulate spatio-temporal signaling by the intracellular second messengers cAMP and cGMP. PDE2A is expressed at high levels in the mammalian brain. To advance our understanding of the role of this enzyme in regulation of neuronal signaling, we here describe the distribution of PDE2A in the rat brain. PDE2A mRNA was prominently expressed in glutamatergic pyramidal cells in cortex, and in pyramidal and dentate granule cells in the hippocampus. Protein concentrated in the axons and nerve terminals of these neurons; staining was markedly weaker in the cell bodies and proximal dendrites. In addition, in both hippocampus and cortex, small populations of non-pyramidal cells, presumed to be interneurons, were strongly immunoreactive. PDE2A mRNA was expressed in medium spiny neurons in neostriatum. Little immunoreactivity was observed in cell bodies, whereas dense immunoreactivity was found in the axon tracts of these neurons and their terminal regions in globus pallidus and substantia nigra pars reticulata. Immunostaining was dense in the medial habenula, but weak in other diencephalic regions. In midbrain and hindbrain, immunostaining was restricted to discrete regions of the neuropil or clusters of cell bodies. These results suggest that PDE2A may modulate cortical, hippocampal and striatal networks at several levels. Preferential distribution of PDE2A into axons and terminals of the principal neurons suggests roles in regulation of axonal excitability or transmitter release. The enzyme is also in forebrain interneurons, and in mid- and hindbrain neurons that may modulate forebrain networks and circuits.

New paradigms in cAMP signalling

Signalling through adenosine 3'5' monophosphate (cAMP) is known to be important in virtually every cell. The mapping of the human genome over the past two decades has revealed an unexpected complexity of cAMP signalling, which is shared from insects to mammals. A more recent technical advance is the ability to monitor intracellular cAMP levels at subcellular spatial resolution within the time-domains of fast biochemical reactions. Thus, new light has been shed on old paradigms, some of which turn out to be multiple new ones. The novel aspects of cAMP signalling are highlighted here: (1) agonist induced plasticity - showing how the repertory of cAMP signalling genes supports homeostatic adaptation; (2) sustained cAMP signalling after endocytosis; (3) pre-assembled receptor-Gs-adenylyl cyclase complexes. Finally, a hypothetical model of propagating neuronal cAMP signals travelling form dendrites to the cell body is presented.

Phosphodiesterase-4 inhibition alters gene expression and improves isoniazid-mediated clearance of Mycobacterium tuberculosis in rabbit lungs

Tuberculosis (TB) treatment is hampered by the long duration of antibiotic therapy required to achieve cure. This indolent response has been partly attributed to the ability of subpopulations of less metabolically active Mycobacterium tuberculosis (Mtb) to withstand killing by current anti-TB drugs. We have used immune modulation with a phosphodiesterase-4 (PDE4) inhibitor, CC-3052, that reduces tumor necrosis factor alpha (TNF-α) production by increasing intracellular cAMP in macrophages, to examine the crosstalk between host and pathogen in rabbits with pulmonary TB during treatment with isoniazid (INH). Based on DNA microarray, changes in host gene expression during CC-3052 treatment of Mtb infected rabbits support a link between PDE4 inhibition and specific down-regulation of the innate immune response. The overall pattern of host gene expression in the lungs of infected rabbits treated with CC-3052, compared to untreated rabbits, was similar to that described in vitro in resting Mtb infected macrophages, suggesting suboptimal macrophage activation. These alterations in host immunity were associated with corresponding down-regulation of a number of Mtb genes that have been associated with a metabolic shift towards dormancy. Moreover, treatment with CC-3052 and INH resulted in reduced expression of those genes associated with the bacterial response to INH. Importantly, CC-3052 treatment of infected rabbits was associated with reduced ability of Mtb to withstand INH killing, shown by improved bacillary clearance, from the lungs of co-treated animals compared to rabbits treated with INH alone. The results of our study suggest that changes in Mtb gene expression, in response to changes in the host immune response, can alter the responsiveness of the bacteria to antimicrobial agents. These findings provide a basis for exploring the potential use of adjunctive immune modulation with PDE4 inhibitors to enhance the efficacy of existing anti-TB treatment.

Tuberculosis (TB) caused by Mycobacterium tuberculosis (Mtb) is a leading infectious cause of morbidity and mortality. Although current antibiotic regimens can cure TB, treatment requires at least six months for completion. Recent studies indicate that bacteria in a less metabolically active state are less responsive to antibiotic killing and suggest that this may partly explain the long duration required for TB treatment. In this study, using a rabbit model of pulmonary TB, we show that immune modulation of Mtb infected animals with CC-3052, a phosphodiesterase-4 (PDE4) inhibitor that reduces tumor necrosis factor alpha (TNF-α) production by increasing intracellular cAMP levels, resulted in the down-regulation of host genes involved in the innate immune response. Bacteria from the lungs of CC-3052 treated rabbits displayed differential expression of those genes associated with stress responses. In addition, co-treatment of INH with CC-3052 abolished the INH-induced Mtb gene expression in the infected rabbits. Importantly, bacillary clearance from the lungs of rabbits co-treated with CC-3052 and INH was improved over that in animals treated with INH alone. The results of this study provide a basis for novel use of immune modulation to improve the efficacy of antibiotic therapy and to shorten the duration of TB treatment.

The emerging role of HSP20 as a multifunctional protective agent

The small heat shock proteins (sHSPs) are a highly conserved family of molecular chaperones that are ubiquitously expressed throughout nature. They are transiently upregulated in many tissue types following stressful stimuli. Recently, one member of the sHSP family, HSP20 (HspB6), has been shown to be highly effective as a protective mediator against a number of debilitating pathological conditions, including cardiac hypertrophy and Alzheimer's disease. Hsp20 is also an important modulator of vital physiological processes, such as smooth muscle relaxation and cardiac contractility. This review focuses on the molecular mechanisms employed by HSP20 that allow it to act as an innate protector in the context of cardiovascular and neurological diseases. Emerging evidence for a possible role as an anti-cancer agent is also presented.

Molecules, signaling, and schizophrenia

Schizophrenia is one of the most common psychiatric disorders, but despite some progress in identifying the genetic factors implicated in its development, the molecular mechanisms underlying its etiology and pathogenesis remain poorly understood. However, accumulating evidence suggests that regardless of the underlying genetic complexity, the mechanisms of the disease may impact a small number of common signaling pathways. In this review, we discuss the evidence for a role of schizophrenia susceptibility genes in intracellular signaling cascades by focusing on three prominent candidate genes: AKT, PPP3CC (calcineurin), and DISC1. We describe the regulation of a number of signaling cascades by AKT and calcineurin through protein phosphorylation and dephosphorylation, and the recently uncovered functions of DISC1 in cAMP and GSK3beta signaling. In addition, we present independent evidence for the involvement of their downstream signaling pathways in schizophrenia. Finally, we discuss evidence supporting an impact of these susceptibility genes on common intracellular signaling pathways and the convergence of their effects on neuronal processes implicated in schizophrenia.

Signalling pathway alterations in pituitary adenomas: involvement of Gsalpha, cAMP and mitogen-activated protein kinases

Despite extensive research on sporadic pituitary adenomas, it is not yet possible to assign one protein alteration to one specific type of pituitary adenomas. Nevertheless, alterations of the cAMP pathway appear to be molecular hallmarks of most growth hormone (GH)-secreting adenomas. However, these alterations do not confer specific phenotypes to patients carrying these alterations. In this review, we summarise the literature regarding signalling alterations observed in GH-secreting adenomas. We focus on Gsalpha alterations and their possible cross-talk with the extracellular signal-related kinase (ERK)1/2 pathway. In the light of results obtained on human somatotroph adenoma cells in primary culture and on models of murine somatotroph cell lines, we postulate a crucial role for ERK1/2 in GH-secreting adenomas downstream of cAMP pathway alterations that might impact the tumoural phenotype.

Selective inhibition of cytokine-activated extracellular signal-regulated kinase by cyclic AMP via Epac1-dependent induction of suppressor of cytokine signalling-3

Here we demonstrate that elevation of cyclic AMP (cAMP) levels in human umbilical vein endothelial cells (HUVECs) specifically attenuates ERK1,2 activation in response to either leptin or a soluble interleukin IL-6 receptor-alpha/IL-6 (sIL-6R alpha/IL-6) trans-signalling complex but not protein kinase C activator phorbol 12-myristate 13-acetate. The inhibitory effects of cAMP on sIL-6R alpha/IL-6-stimulated phosphorylation of ERK1,2 and STAT3 were abolished by either short interfering (si) RNA-mediated knockdown or genetic ablation of suppressor of cytokine signalling-3 (SOCS-3). The inhibitory effect of cAMP could not be reversed by inhibition of cAMP-dependent protein kinase (PKA) but was blocked by depletion of the alternative intracellular cAMP sensor exchange protein activated by cAMP 1 (Epac1), which is also required to observe SOCS-3 accumulation in response to cAMP. Interestingly, the ability of cAMP elevation to inhibit IL-6 signalling was blocked by ERK inhibition. Consistent with this observation, cAMP elevation in HUVECs produced a transient yet robust activation of ERK, and subsequent phosphorylation of transcription factor C/EBP beta, both of which were resistant to PKA inhibition. However, siRNA depletion and immunoblotting experiments revealed that neither Epac1 nor Epac2 contributed to the PKA-independent activation of ERK1,2 observed following cAMP elevation. Together, these observations suggest that while SOCS-3 induction and subsequent inhibition of cytokine-mediated phosphorylation of ERK1,2 and STAT3 in response to cAMP require Epac1 and a transient PKA-independent activation of the ERK pathway, these two events are controlled by distinct mechanisms. In addition, it reveals a novel Epac-dependent mechanism by which cAMP can specifically inhibit ERK in response to cytokine receptor activation.

cAMP signal transduction in the heart: understanding spatial control for the development of novel therapeutic strategies

3'-5'-Cyclic adenosine monophosphate (cAMP) is a pleiotropic intracellular second messenger generated in response to activation of G(s) protein-coupled receptors. In the heart, cAMP mediates the catecholaminergic control on heart rate and contractility but, at the same time, it is responsible for the functional response to a wide variety of other hormones and neurotransmitters, raising the question of how the myocyte can decode the cAMP signal and generate the appropriate functional output to each individual extracellular stimulus. A growing body of evidence points to the spatial organization of the components of the cAMP signalling pathway in distinct, spatially segregated signalling domains as the key feature underpinning specificity of response and data is emerging, indicating that alteration of spatial control of the cAMP signal cascade associates with heart pathology. Most of the details of the molecular organization and regulation of individual cAMP signalling compartments are still to be elucidated but future research should provide the knowledge necessary to develop and test new therapeutic strategies that, by acting on a limited subset of downstream targets, would improve efficacy and minimize off-target effects.

Dehydroepiandrosterone activates cyclic adenosine 3',5'-monophosphate/protein kinase A signalling and suppresses sterol regulatory element-binding protein-1 expression in cultured primary chicken hepatocytes

Dehydroepiandrosterone (DHEA), a steroid hormone that is secreted by the adrenal cortex in mammals, has an array of biological actions, including inhibition of fat synthesis, decreasing the number of adipocytes, and a reduction in mammalian metabolic efficiency. Recent studies showed that DHEA may decrease fat deposition in poultry, but the mechanism of action is unclear. In the present study, we demonstrate that DHEA stimulates intracellular cyclic adenosine 3',5'-monophosphate (cAMP) accumulation in chicken hepatocytes during a 30 min incubation period. Increases in intracellular cAMP are evoked by as low as 0.1 microm-DHEA. The cAMP induced by DHEA, while suppressing cAMP-specific phosphodiesterase activity, also activates cAMP-dependent protein kinase A (PKA) in chicken hepatocytes. In addition, the activation of PKA leads to down-regulation of sterol regulatory element-binding protein-1 (SREBP-1). These findings demonstrate that direct action by DHEA leads to activation of the cAMP/PKA signalling system in the modulation of lipid metabolism by repressing SREBP-1, thereby providing a novel explanation for some of the underlying effects proposed for DHEA in the prevention of fat deposition in poultry.

Targeted inhibition of cyclic AMP phosphodiesterase-4 promotes brain tumor regression

PURPOSE

As favorable outcomes from malignant brain tumors remain limited by poor survival and treatment-related toxicity, novel approaches to cure are essential. Previously, we identified the cyclic AMP phosphodiesterase-4 (PDE4) inhibitor Rolipram as a potent antitumor agent. Here, we investigate the role of PDE4 in brain tumors and examine the utility of PDE4 as a therapeutic target.

EXPERIMENTAL DESIGN

Immunohistochemistry was used to evaluate the expression pattern of a subfamily of PDE4, PDE4A, in multiple brain tumor types. To evaluate the effect of PDE4A on growth, a brain-specific isoform, PDE4A1 was overexpressed in xenografts of Daoy medulloblastoma and U87 glioblastoma cells. To determine therapeutic potential of PDE4 inhibition, Rolipram, temozolomide, and radiation were tested alone and in combination on mice bearing intracranial U87 xenografts.

RESULTS

We found that PDE4A is expressed in medulloblastoma, glioblastoma, oligodendroglioma, ependymoma, and meningioma. Moreover, when PDE4A1 was overexpressed in Daoy medulloblastoma and U87 glioblastoma cells, in vivo doubling times were significantly shorter for PDE4A1-overexpressing xenografts compared with controls. In long-term survival and bioluminescence studies, Rolipram in combination with first-line therapy for malignant gliomas (temozolomide and conformal radiation therapy) enhanced the survival of mice bearing intracranial xenografts of U87 glioblastoma cells. Bioluminescence imaging indicated that whereas temozolomide and radiation therapy arrested intracranial tumor growth, the addition of Rolipram to this regimen resulted in tumor regression.

CONCLUSIONS

This study shows that PDE4 is widely expressed in brain tumors and promotes their growth and that inhibition with Rolipram overcomes tumor resistance and mediates tumor regression.

Prostaglandin E(2) couples through EP(4) prostanoid receptors to induce IL-8 production in human colonic epithelial cell lines

BACKGROUND AND PURPOSE

Prostaglandin (PG) E(2) and interleukin (IL)-8 are simultaneously increased during the inflammation that characterizes numerous pathologies such as inflammatory bowel disease. IL-8 is a potent neutrophil chemo-attractant and activator, and can initiate and/or exacerbate tissue injury. PGE(2) signals principally through prostanoid receptors of the EP(2) and/or EP(4) subtypes to promote cAMP-dependent cellular functions. The aim of this study was to identify the role of the EP(2) and EP(4) receptor subtype(s) on two human colonic epithelial cell lines (Caco-2 and T84), in regulating PGE(2)-induced IL-8 production.

EXPERIMENTAL APPROACH

To identify the causative receptor, we knocked-down and over-expressed EP(2) and EP(4) receptor subtypes in colonic epithelial cells and studied the effect of several selective EP(2)/EP(4) receptor agonists and antagonists. The inductions of IL-8 and EP receptor mRNA and protein expression were determined by real-time PCR and western blot analysis. The affinity of PGE(2) and Bmax values for the EP(2) and EP(4) receptor on colonic epithelial cells were determined by radioligand-binding assays with [(3)H]PGE(2).

KEY RESULTS

PGE(2) had the highest affinity for the EP(4) receptor subtype and promoted a robust stimulation of cAMP-dependent IL-8 synthesis. This effect was mimicked by a selective EP(4) receptor agonist, ONO-AE1-329, and abolished by silencing the EP(4) receptor gene by using siRNA techniques, a selective EP(4) receptor antagonist (ONO-AE3-208) and a selective inhibitor (Rp-cAMP) of cAMP-dependent protein kinase.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that initiation and progression of colonic inflammation induced by IL-8 could be mediated, at least in part, by PGE(2) acting via the EP(4) receptor subtype.

Regulation of aquaporin-2 trafficking

Principal cells lining renal collecting ducts control the fine-tuning of body water homeostasis by regulating water reabsorption through the water channels aquaporin-2 (AQP2), aquaporin-3 (AQP3), and aquaporin-4 (AQP4). While the localization of AQP2 is subject to regulation by arginine-vasopressin (AVP), AQP3 and AQP4 are constitutively expressed in the basolateral plasma membrane. AVP adjusts the amount of AQP2 in the plasma membrane by triggering its redistribution from intracellular vesicles into the plasma membrane. This permits water entry into the cells and water exit through AQP3 and AQP4. The translocation of AQP2 is initiated by an increase in cAMP following V2R activation through AVP. The AVP-induced rise in cAMP activates protein kinase A (PKA), which in turn phosphorylates AQP2, and thereby triggers the redistribution of AQP2. Several proteins participating in the control of cAMP-dependent AQP2 trafficking have been identified; for example, A kinase anchoring proteins (AKAPs) tethering PKA to cellular compartments; phosphodiesterases (PDEs) regulating the local cAMP level; cytoskeletal components such as F-actin and microtubules; small GTPases of the Rho family controlling cytoskeletal dynamics; motor proteins transporting AQP2-bearing vesicles to and from the plasma membrane for exocytic insertion and endocytic retrieval; SNAREs inducing membrane fusions, hsc70, a chaperone, important for endocytic retrieval. In addition, cAMP-independent mechanisms of translocation mainly involving the F-actin cytoskeleton have been uncovered. Defects of AQP2 trafficking cause diseases such as nephrogenic diabetes insipidus (NDI), a disorder characterized by a massive loss of hypoosmotic urine.This review summarizes recent data elucidating molecular mechanisms underlying the trafficking of AQP2. In particular, we focus on proteins involved in the regulation of trafficking, and physiological and pathophysiological stimuli determining the cellular localization of AQP2. The identification of proteins and protein-protein interactions may lead to the development of drugs targeting AQP2 trafficking. Such drugs may be suitable for the treatment of diseases associated with dysregulation of body water homeostasis, including NDI or cardiovascular diseases (e.g., chronic heart failure) where the AVP level is elevated, inducing excessive water retention.

Caveolae as organizers of pharmacologically relevant signal transduction molecules

Caveolae, a subset of membrane (lipid) rafts, are flask-like invaginations of the plasma membrane that contain caveolin proteins, which serve as organizing centers for cellular signal transduction. Caveolins (-1, -2, and -3) have cytoplasmic N and C termini, palmitolylation sites, and a scaffolding domain that facilitates interaction and organization of signaling molecules so as to help provide coordinated and efficient signal transduction. Such signaling components include upstream entities (e.g., G protein-coupled receptors (GPCRs), receptor tyrosine kinases, and steroid hormone receptors) and downstream components (e.g., heterotrimeric and low-molecular-weight G proteins, effector enzymes, and ion channels). Diseases associated with aberrant signaling may result in altered localization or expression of signaling proteins in caveolae. Caveolin-knockout mice have numerous abnormalities, some of which may reflect the impact of total body knockout throughout the life span. This review provides a general overview of caveolins and caveolae, signaling molecules that localize to caveolae, the role of caveolae/caveolin in cardiac and pulmonary pathophysiology, pharmacologic implications of caveolar localization of signaling molecules, and the possibility that caveolae might serve as a therapeutic target.

G-protein-coupled receptor-signaling components in membrane raft and caveolae microdomains

The efficiency of signal transduction in cells derives in part from subcellular, in particular plasma membrane, microdomains that organize signaling molecules and signaling complexes. Two related plasma membrane domains that compartmentalize G-protein coupled receptor (GPCR) signaling complexes are lipid (membrane) rafts, domains that are enriched in certain lipids, including cholesterol and sphingolipids, and caveolae, a subset of lipid rafts that are enriched in the protein caveolin. This review focuses on the properties of lipid rafts and caveolae, the mechanisms by which they localize signaling molecules and the identity of GPCR signaling components that are organized in these domains.

Stimulatory phosphorylation of cAMP-specific PDE4D5 by contractile agonists is mediated by PKC-dependent inactivation of protein phosphatase 2A

Smooth muscle of the gut undergoes rhythmic cycles of contraction and relaxation. Various constituents in the pathways that mediate muscle contraction could act to cross-regulate cAMP or cGMP levels and terminate subsequent relaxation. We have previously shown that cAMP levels are regulated by PKA-mediated phosphorylation of cAMP-specific phosphodiesterase 3A (PDE3A) and PDE4D5; the latter is the only PDE4D isoform expressed in smooth muscle. In the present study we have elucidated a mechanism whereby cholecystokinin (CCK) and, presumably, other contractile agonists capable of activating PKC can cross-regulate cAMP levels. Forskolin stimulated PDE4D5 phosphorylation and PDE4D5 activity. CCK significantly increased forskolin-stimulated PDE4D5 phosphorylation and activity and attenuated forskolin-stimulated cAMP levels. The effect of CCK on forskolin-induced PDE4D5 phosphorylation and activity and on cAMP levels was blocked by the inhibitors of PLC or PKC and in cultured muscle cells by the expression of Galpha(q) minigene. The effects of CCK on PDE4D5 phosphorylation, PDE4D5 activity, and cAMP levels were mimicked by low (1 nM) concentrations of okadaic acid, but not by a low (10 nM) concentration of tautomycin, suggesting involvement of PP2A. Purified catalytic subunit of PP2A but not PP1 dephosphorylated PDE4D5 in vitro. Coimmunoprecipitation studies demonstrated association of PDE4D5 with PP2A and the association was decreased by the activation of PKC. In conclusion, cAMP levels are cross-regulated by contractile agonists via a mechanism that involves PLC-beta-dependent, PKC-mediated inhibition of PP2A activity that leads to increase in PDE4D5 phosphorylation and activity and inhibition of cAMP levels.

Direct AKAP-mediated protein-protein interactions as potential drug targets

A-kinase-anchoring proteins (AKAPs) are a diverse family of about 50 scaffolding proteins. They are defined by the presence of a structurally conserved protein kinase A (PKA)-binding domain. AKAPs tether PKA and other signalling proteins such as further protein kinases, protein phosphatases and phosphodiesterases by direct protein-protein interactions to cellular compartments. Thus, AKAPs form the basis of signalling modules that integrate cellular signalling processes and limit these to defined sites. Disruption of AKAP functions by gene targeting, knockdown approaches and, in particular, pharmacological disruption of defined AKAP-dependent protein-protein interactions has revealed key roles of AKAPs in numerous processes, including the regulation of cardiac myocyte contractility and vasopressin-mediated water reabsorption in the kidney. Dysregulation of such processes causes diseases, including cardiovascular and renal disorders. In this review, we discuss AKAP functions elucidated by gene targeting and knockdown approaches, but mainly focus on studies utilizing peptides for disruption of direct AKAP-mediated protein-protein interactions. The latter studies point to direct AKAP-mediated protein-protein interactions as targets for novel drugs.

Suppression of inflammatory and immune responses by the A(2A) adenosine receptor: an introduction

The purine nucleoside adenosine has been described as a 'retaliatory metabolite' by virtue of its ability to function in an autocrine manner to modify the activity of a range of cell types following its extracellular accumulation during cell stress or injury. These effects are largely protective and are triggered by the binding of adenosine to any of four G-protein-coupled adenosine receptors. Most of the anti-inflammatory effects of adenosine have been assigned to the adenosine A(2A) receptor subtype, which is expressed in many immune and inflammatory cells. In this brief article, we will outline the growing evidence to support the hypothesis that the development of agonists selective for the A(2A) receptor is an effective strategy for suppressing the exaggerated inflammatory responses associated with many diseases by virtue of the receptor's ability to inhibit multiple pro-inflammatory signalling cascades.

Regulating gene transcription in response to cyclic AMP elevation

Many of the effects of prototypical second messenger cyclic adenosine 3',5'-monophosphate (cAMP) on complex processes such as the regulation of fuel metabolism, spermatogenesis and steroidogenesis are mediated via changes in target gene transcription. A large body of research has defined members of the cAMP-response element binding (CREB) protein family as the principal mediators of positive changes in gene expression in response to cAMP following phosphorylation by cAMP-dependent protein kinase (PKA). However, persistent observations of cAMP-mediated induction of specific genes occurring via PKA-independent mechanisms have challenged the generality of the PKA-CREB pathway. In this review, we will discuss in detail both PKA-dependent and -independent mechanisms that have been proposed to explain how cAMP influences the activation status of multiple transcription factors, and how these influence critical biological processes whose defective regulation may lead to disease.

Disrupted in schizophrenia 1 and phosphodiesterase 4B: towards an understanding of psychiatric illness

Disrupted in schizophrenia 1 (DISC1) is one of the most convincing genetic risk factors for major mental illness identified to date. DISC1 interacts directly with phosphodiesterase 4B (PDE4B), an independently identified risk factor for schizophrenia. DISC1-PDE4B complexes are therefore likely to be involved in molecular mechanisms underlying psychiatric illness. PDE4B hydrolyses cAMP and DISC1 may regulate cAMP signalling through modulating PDE4B activity. There is evidence that expression of both genes is altered in some psychiatric patients. Moreover, DISC1 missense mutations that give rise to phenotypes related to schizophrenia and depression in mice are located within binding sites for PDE4B. These mutations reduce the association between DISC1 and PDE4B, and one results in reduced brain PDE4B activity. Altered DISC1-PDE4B interaction may thus underlie the symptoms of some cases of schizophrenia and depression. Factors likely to influence this interaction include expression levels, binding site affinities and the DISC1 and PDE4 isoforms involved. DISC1 and PDE4 isoforms are targeted to specific subcellular locations which may contribute to the compartmentalization of cAMP signalling. Dysregulated cAMP signalling in specific cellular compartments may therefore be a predisposing factor for major mental illness.

Daily rhythm in pineal phosphodiesterase (PDE) activity reflects adrenergic/3',5'-cyclic adenosine 5'-monophosphate induction of the PDE4B2 variant

The pineal gland is a photoneuroendocrine transducer that influences circadian and circannual dynamics of many physiological functions via the daily rhythm in melatonin production and release. Melatonin synthesis is stimulated at night by a photoneural system through which pineal adenylate cyclase is adrenergically activated, resulting in an elevation of cAMP. cAMP enhances melatonin synthesis through actions on several elements of the biosynthetic pathway. cAMP degradation also appears to increase at night due to an increase in phosphodiesterase (PDE) activity, which peaks in the middle of the night. Here, it was found that this nocturnal increase in PDE activity results from an increase in the abundance of PDE4B2 mRNA (approximately 5-fold; doubling time, approximately 2 h). The resulting level is notably higher (>6-fold) than in all other tissues examined, none of which exhibit a robust daily rhythm. The increase in PDE4B2 mRNA is followed by increases in PDE4B2 protein and PDE4 enzyme activity. Results from in vivo and in vitro studies indicate that these changes are due to activation of adrenergic receptors and a cAMP-dependent protein kinase A mechanism. Inhibition of PDE4 activity during the late phase of adrenergic stimulation enhances cAMP and melatonin levels. The evidence that PDE4B2 plays a negative feedback role in adrenergic/cAMP signaling in the pineal gland provides the first proof that cAMP control of PDE4B2 is a physiologically relevant control mechanism in cAMP signaling.

The effects of increased cAMP content on inflammation, oxidative stress and PDE4 transcripts during Brucella melitensis infection

Cyclic AMP (cAMP) is a key intracellular second messenger which at increased levels has been shown to have anti-inflammatory and tissue-protective effects. Its concentration is determined by the activities of both adenylate cyclase (AC) and the phosphodiesterase (PDE) enzymes. The aim of this study was to compare the effects of increased cAMP and glucocorticoid dexamethasone administration on B. melitensis-induced lipid peroxidation, Brucella suppressed antioxidant enzyme activities and PDE4 transcripts in rats. Intracellular cyclic AMP level was elevated by two different approaches; activation of AC and inhibition of PDE activities. Rats were inoculated with B. melitensis for seven days then a single dose of nonselective PDE inhibitor 3-isobutyl-1-methylxanthine (IBMX), the adenylate cyclase activator forskolin and dexamethasone were administrated to each infected group, and animals were challenged for 48 h. Brucella-induced lipid peroxidation was significantly reduced by the cAMP elevating agents as well as dexamethasone administration in plasma, liver and spleen. The antioxidant enzymes glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) activities were significantly decreased by the pathogen. Whilst suppressed GSH-Px activity was reversed by cAMP elevating agents, SOD activity was not restored. Superoxide generating enzyme xanthine oxidase activity was not altered at the end of the infection period. Brucella infection increased plasma IL-12 level and this effect was also suppressed by the cAMP elevating agents, whereas TNF-alpha, IFN-gamma and IL-10 levels were unchanged. Intracellular cAMP levels are entirely hydrolyzed by cAMP-specific PDE 4 isozymes (PDE4s) in inflammatory and immunocompetent cells. Brucella reduced mRNA transcript levels for PDE4A by 40%, though PDE4B and 4D transcriptions were being unaffected in spleen. It was concluded that B. melitensis infection decreased activity of the antioxidant defence system, induced lipid peroxidation and suppressed PDE4A transcription. Administration of cAMP elevating agents exhibited similar affect with dexamethasone on lipid peroxidation, IL-12 production and antioxidant enzyme activities in Brucella infection.