Intracellular Cyclic Nucleotide Levels Research Articles

Once upon a Testis: The Tale of Cyclic Nucleotide Phosphodiesterase in Testicular Cancers.

Phosphodiesterases are key regulators that fine tune the intracellular levels of cyclic nucleotides, given their ability to hydrolyze cAMP and cGMP. They are critical regulators of cAMP/cGMP-mediated signaling pathways, modulating their downstream biological effects such as gene expression, cell proliferation, cell-cycle regulation but also inflammation and metabolic function. Recently, mutations in PDE genes have been identified and linked to human genetic diseases and PDEs have been demonstrated to play a potential role in predisposition to several tumors, especially in cAMP-sensitive tissues. This review summarizes the current knowledge and most relevant findings regarding the expression and regulation of PDE families in the testis focusing on PDEs role in testicular cancer development.

Inhibition of multidrug resistance proteins by MK571 restored the erectile function in obese mice through cGMP accumulation.

Intracellular levels of cyclic nucleotides can also be controlled by the action of multidrug resistance protein types 4 (MRP4) and 5 (MRP5). To date, no studies evaluated the role of their inhibition in an animal model of erectile dysfunction (ED). To evaluate the effect of a 2-week treatment with MK571, an inhibitor of the efflux of cyclic nucleotides in the ED of obese mice. Mice were divided in three groups: (i) lean, (ii) obese, and (iii) obese+MK571. The corpus cavernosum (CC) were isolated, and concentration-response curves to acetylcholine (ACh), sodium nitroprusside (SNP), and tadalafil in addition to electrical field stimulation (EFS) were carried out in phenylephrine pre-contracted tissues. Expression of ABCC4 and ABCC5, intracellular levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), the protein levels for pVASPSer157 and pVASPSer239 , and the intracavernous pressure (ICP) were also determined. The intracellular and extracellular (supernatant) ratios in CC from obese and lean stimulated with a cGMP-increasing substance (BAY 58-2667) in the absence and presence of MK571 (20μM, 30min) were also assessed. The treatment with MK571 completely reversed the lower relaxing responses induced by EFS, ACh, SNP, and tadalafil observed in obese mice CC in comparison with untreated obese mice. Cyclic GMP and p-VASPSer239 expression were significantly reduced in CC from obese groups. MK571 promoted a sixfold increase in cGMP without interfering in the protein expression of p-VASPSer239 . Neither the cAMP levels nor p-VASPSer157 were altered in MK571-treated animals. The ICP was ∼50% lower in obese than in the lean mice; however, the treatment with MK571 fully reversed this response. Expressions of ABCC4 and ABCC5 were not different between groups. The intra/extracellular ratio of cGMP was similar in CC from obese and lean mice stimulated with BAY 58-2667. The MRPs inhibition by MK571 favored the accumulation of cGMP in the smooth muscle cells, thus improving the smooth muscle relaxation and the erectile function in obese mice.

Cyclic nucleotides regulate oocyte meiotic maturation and quality in mammals

Oocyte meiosis is a prolong series of events that are comprised several intermittent channels in mammals. Oocyte meiosis starts during fetal life and then gets arrested at diplotene stage of first meiotic prophase in follicular oocyte. The continuous transfer of cyclic adenosine 3’, 5’-monophosphate (cAMP) and cyclic guanosine 3’, 5’-monophosphate (cGMP) from encircling granulosa cells to the oocyte through gap junctions helps in the maintenance of their high level required to achieve the long-lasting diplotene arrest so-called germinal vesicle stage. Phosphodiesterase inhibitors have been used to elevate intracellular level of both cyclic nucleotides and prevent spontaneous resumption of meiosis in oocytes under in vitro culture conditions. On the other hand, disruption of gap junction either by pituitary gonadotropin or by physical removal of encircling granulosa cells interrupts transfer of these nucleotides to the oocyte. As a result, intraoocyte cAMP as well as cGMP levels are decreased drastically that initiate downstream pathways to destabilize maturation-promoting factor (MPF). The destabilized MPF initiates meiotic resumption from diplotene arrest in mammalian oocytes. Oocyte meiosis further progresses from metaphase I to metaphase II stage and extrudes first polar body to get converted into haploid female gamete at the time of ovulation. Indeed, high level of cAMP as well as cGMP levels maintains diplotene arrest for a long time in follicular oocytes. On the other hand, transient decrease of their levels drives resumption from diplotene arrest, thereby meiotic maturation process, which enables oocyte to achieve developmental competency. Any defect in this process directly affects oocyte quality and thereby reproductive outcome in mammals including human.

Respiratory Cilia as a Therapeutic Target of Phosphodiesterase Inhibitors

Mucociliary clearance is an essential airway defense mechanism dependent predominantly on the proper ciliary function and mucus rheology. The crucial role of cilia is evident in `a variety of respiratory diseases, as the ciliary dysfunction is associated with a progressive decline in lung function over time. The activity of cilia is under supervision of multiple physiological regulators, including second messengers. Their role is to enable a movement in coordinated metachronal waves at certain beat frequency. Ciliary function can be modulated by various stimuli, including agents from the group of beta2 agonists, cholinergic drugs, and adenosine triphosphate (ATP). They trigger cilia to move faster in response to elevated cytoplasmic Ca2+ originated from intracellular sources or replenished from extracellular space. Well-known cilia-stimulatory effect of Ca2+ ions can be abolished or even reversed by modulating the phosphodiesterase (PDE)-mediated breakdown of cyclic adenosine monophosphate (cAMP) since the overall change in ciliary beating has been dependent on the balance between Ca2+ ions and cAMP. Moreover, in chronic respiratory diseases, high ATP levels may contribute to cAMP hydrolysis and thus to a decrease in the ciliary beat frequency (CBF). The role of PDE inhibitors in airway cilia-driven transport may help in prevention of progressive loss of pulmonary function often observed despite current therapy. Furthermore, administration of selective PDE inhibitors by inhalation lowers the risk of their systemic effects. Based on this review we may conclude that selective (PDE1, PDE4) or dual PDE inhibitors (PDE3/4) increase the intracellular level of cyclic nucleotides in airway epithelial cells and thus may be an important target in the development of new inhaled mucokinetic agents. Further research is required to provide evidence of their effectiveness and feasibility regarding their cilia-modulating properties.

Drug repurposing for targeting cyclic nucleotide transporters in acute leukemias - A missed opportunity

While current treatment regimens for acute leukemia can dramatically improve patient survival, there remains room for improvement. Due to its roles in cell differentiation, cell survival, and apoptotic signaling, modulation of the cyclic AMP (cAMP) pathway has provided a meaningful target in hematological malignancies. Several studies have demonstrated that gene expression profiles associated with increased pro-survival cAMP activity or downregulation of various pro-apoptotic factors associated with the cAMP pathway are apparent in acute leukemia patients. Previous work to increase leukemia cell intracellular cAMP focused on the use of cAMP analogs, stimulating cAMP production via transmembrane-associated adenylyl cyclases, or decreasing cAMP degradation by inhibiting phosphodiesterase activity. However, targeting cyclic nucleotide efflux by ATP-binding cassette (ABC) transporters represents an unexplored approach for modulation of intracellular cyclic nucleotide levels. Preliminary studies have shown that inhibition of cAMP efflux can stimulate leukemia cell differentiation, cell growth arrest, and apoptosis, indicating that targeting cAMP efflux may show promise for future therapeutic development. Furthermore, inhibition of cyclic nucleotide transporter activity may also contribute multiple anticancer benefits by reducing extracellular pro-survival signaling in malignant cells. Hence, several opportunities for drug repurposing may exist for targeting cyclic nucleotide transporters.

Inhibition of isoform D of phosphodiesterase type 4 reduces cell proliferation and survival in hepatocarcinoma cell lines

Cyclic AMP phosphodiesterases (PDEs) are enzymes that regulate intracellular levels of cAMP by controlling its rate of degradation, in contrast to adenylyl cyclases that synthesize cAMP from intracellular ATP. PDEs are classified in 11 families (PDE1-PDE11). Some of these control the intracellular levels of both cyclic nucleotides, while others (PDE4, 7, 8) are specific for cAMP degradation. Previous experiments showed that isoform D of cAMP specific PDE4 (PDE4D) was aberrantly up-regulated in HCC cells, in particular in the highly tumorigenic Hep3B and Huh7 cell lines, which also showed increased PDE4 activity.

Biphasic Effect of Sildenafil on Energy Sensing is Mediated by Phosphodiesterases 2 and 3 in Adipocytes and Hepatocytes.

Sirt1 (Sirtuin 1), AMPK (AMP-activated protein kinase), and eNOS (endothelial nitric oxide synthase) modulate hepatic energy metabolism and inflammation and play a major role in the development of NASH. Cyclic nucleotide phosphodiesterases (PDEs) play an important role in signal transduction by modulating intracellular levels of cyclic nucleotides. We previously found the PDE5 inhibitor sildenafil to synergize with leucine and leucine-metformin combinations in preclinical studies of NASH and obesity. However, efficacy is diminished at higher sildenafil concentrations. Herein, we have successfully modeled the U-shaped sildenafil dose-response in vitro and utilized this model to assess potential mechanisms of this dose-response relationship. Adipocytes and liver cells were treated with leucine (0.5 mM) and different concentrations of sildenafil (1 nM to 100 µM). cAMP, cGMP, and P-AMPK protein expression were used to demonstrate the biphasic response for increasing concentrations of sildenafil. The reversal with higher sildenafil levels was blunted by PDE2 inhibition. These data indicate that sildenafil-mediated increases in cGMP inhibits PDE3 at lower concentrations, which increases cAMP. However, further increases in cGMP from higher sildenafil concentrations activate PDE2 and consequently decrease cAMP, which demonstrates crosstalk between cAMP and cGMP via PDE2, PDE3, and PDE5. These changes in cAMP concentration are further reflected in downstream effects, including AMPK activation.

Cyclic nucleotide phosphodiesterases: potential therapeutic targets for alcohol use disorder.

Alcohol use disorder (AUD), which combines the criteria of both alcohol abuse and dependence, contributes as an important causal factor to multiple health and social problems. Given the limitation of current treatments, novel medications for AUD are needed to better control alcohol consumption and maintain abstinence. It has been well established that the intracellular signal transduction mediated by the second messengers cyclic AMP (cAMP) and cyclic GMP (cGMP) crucially underlies the genetic predisposition, rewarding properties, relapsing features, and systemic toxicity of compulsive alcohol consumption. On this basis, the upstream modulators phosphodiesterases (PDEs), which critically control intracellular levels of cyclic nucleotides by catalyzing their degradation, are proposed to play a role in modulating alcohol abuse and dependent process. Here, we highlight existing evidence that correlates cAMP and cGMP signal cascades with the regulation of alcohol-drinking behavior and discuss the possibility that PDEs may become a novel class of therapeutic targets for AUD.

The role of adenosine diphosphate mediated platelet responsiveness for the stability of platelet integrity in citrated whole blood under ex vivo conditions.

BackgroundPlatelets are important for effective hemostasis and considered to be involved in pathophysiological processes, e.g. in cardiovascular diseases. Platelets provided for research or for therapeutic use are frequently separated from citrated whole blood (WB) stored for different periods of time. Although functionally intact platelets are required, the stability of platelet integrity, e.g. adenosine diphosphate (ADP) mediated responsiveness, has never been thoroughly investigated in citrated WB under ex vivo conditions.ObjectivesPlatelet integrity was evaluated at different time points in citrated WB units, collected from healthy donors and stored for 5 days at ambient temperature. The analysis included the measurement of activation markers, of induced light transmission aggregometry and of purinergic receptor expression or function. Inhibitory pathways were explored by determination of basal vasodilator-stimulated phosphoprotein (VASP)-phosphorylation, intracellular cyclic nucleotide levels and the content of phosphodiesterase 5A. Fresh peripheral blood (PB) samples served as controls.ResultsOn day 5 of storage, thrombin receptor activating peptide-6 (TRAP-6) stimulated CD62P expression and fibrinogen binding were comparable to PB samples. ADP induced aggregation continuously decreased during storage. Purinergic receptor expression remained unchanged, whereas the P2Y1 activity progressively declined in contrast to preserved P2Y12 and P2X1 function. Inhibitory pathways were unaffected except for a slight elevation of VASP phosphorylation at Ser239 on day 5.ConclusionAfter 5 days of storage in citrated WB, platelet responsiveness to TRAP-6 is sufficiently maintained. However, ADP-mediated platelet integrity is more sensitive to deterioration, especially after storage for more than 2 days. Decreasing ADP-induced aggregation is particularly caused by the impairment of the purinergic receptor P2Y1 activity. These characteristics should be considered in the use of platelets from stored citrated WB for experimental or therapeutic issues.

SUPPRESSION OF CNS INFLAMMATION BY PHOSPHODIESTERASE-1 (PDE1) INHIBITORS: TOWARD NEW TREATMENTS FOR NEURODEGENERATIVE DISEASES

Neuroinflammatory processes are implicated in the cause and progression of neurodegenerative disorders, like Alzheimer's disease. Elevated intracellular cyclic nucleotide levels suppress neuroinflammatory responses by controlling microglial activation. The second messenger cyclic adenosine monophosphate (cAMP) is a negative modulator of inflammatory cell responses, including cytokine secretion and leukocyte recruitment. Similarly, increasing intracellular cyclic guanosine monophosphate (cGMP) attenuates lipopolysaccharide (LPS)-induced responses in microglia. Hence, increasing intracellular cAMP or cGMP using analogs or phosphodiesterase (PDE) inhibitors, antagonizes changes in microglial cell morphology and production of proinflammatory cytokines in the context of inflammation. Intra-Cellular Therapies, Inc. has developed ITI-214, a potent, specific, and clinically-safe inhibitor of PDE1, a dual-specificity (cAMP/cGMP) PDE, and tested its ability to control neuroinflammation. Effects of LPS and/or ITI-214 on mRNA cytokine expression were measured by qPCR using BV2 cells or iPSC-derived astrocytes in vitro and mouse brain tissue. RNAseq was performed (GENEWIZ, LLC) followed by gene ontology using AmiGo 2 software. LPS-induced increases in TNFα, IL-1β, and Ccl2 mRNA expression were reduced by >50% by ITI-214 in BV2 cells in vitro and in mice in vivo in brain regions with high PDE1 expression (striatum, cortex and hippocampus) while brain expression of the anti-inflammatory cytokine IL-10 was increased after ITI-214. Effects of ITI-214 appeared microglial-specific as it did not attenuate induction of IL-6 levels in hiPSC-derived astrocytes after proinflammatory cytokine stimulation. We then employed RNA-Seq to identify a subset of genes whose transcript expression was significantly changed with PDE1 inhibition in LPS-treated BV2 cells. These genes were significantly enriched in cell migration and extravasation pathways as well as inflammatory pathways. Of genes induced by LPS, a subset were attenuated by PDE1 inhibition, all of which were significantly associated with inflammatory pathways. Importantly, inhibition of another PDE isoform, PDE4, attenuated a different subset of LPS-induced genes, demonstrating the unique properties of our target. The data demonstrate a key role for PDE1 inhibition in regulating inflammatory responses in microglia and support PDE1 inhibition as a viable therapeutic approach in disorders in which neuroinflammation and microglia activation are known components of the disease, like Alzheimer's disease.

Effects of fetal calf serum on cGMP pathway and oocyte lipid metabolism in vitro.

Intracellular levels of cyclic nucleotides, such as cGMP, are involved in the regulation of adipocyte lipolysis. Cumulus-oocyte complexes (COCs) express enzymes that both synthesise (guanylate cyclase) and degrade (phosphodiesterase (PDE) 5A) cGMP. Because serum interferes with lipid metabolism, its effects on the cGMP pathway and lipid content in bovine COCs were examined. COCs were matured in medium containing fetal calf serum (FCS; 2% or 10%) or 0.4% bovine serum albumin (BSA; control). At both 2% and 10%, FCS decreased cGMP levels in COCs compared with BSA (0.64 and 1.04 vs 9.46 fmol per COC respectively; P<0.05) and decreased transcript levels of guanylate cyclase 1, soluble, beta 3 (GUCY1B3), whereas PDE5A levels were increased. FCS also affected the expression of genes related to lipolysis, increasing relative expression of perilipin 2 (PLIN2) and carnitine palmitoyltransferase 1B (CPT1B) in cumulus cells. Effects of FCS and cGMP on the lipid content of oocytes and embryos were evaluated by Nile red staining. COCs were matured with 10% FCS, FCS+10-5 M sildenafil (SDF), a PDE5 inhibitor, or 0.4% BSA. The lipid content was increased in oocytes matured in FCS compared with BSA (fluorescence intensity 20.1 vs 17.61 respectively; P<0.05), whereas the lipid content in oocytes matured in FCS+SDF (fluorescence intensity 16.33) was similar to that in the BSA-treated group (P>0.05). In addition, lipid content was higher in embryos from oocytes matured with FCS than BSA (fluorescence intensity 31.12 vs 22.31 respectively; P<0.05), but was increased even further in the FCS+SDF-treated group (fluorescence intensity 40.35; P<0.05), possibly due to a compensatory mechanism during embryo culture without SDF for the reduction in lipid content during IVM. The present study provides, for the first time, evidence that the cGMP pathway may be involved in lipid metabolism in bovine COCs and that this pathway is affected by FCS.

Intracellular calcium and cyclic nucleotide levels modulate neurite guidance by microtopographical substrate features.

Micro- and nanoscale surface features have emerged as potential tools to direct neurite growth into close proximity with next generation neural prosthesis electrodes. However, the signaling events underlying the ability of growth cones to respond to topographical features remain largely unknown. Accordingly, this study probes the influence of [Ca(2+) ]i and cyclic nucleotide levels on the ability of neurites from spiral ganglion neurons (SGNs) to precisely track topographical micropatterns. Photopolymerization and photomasking were used to generate micropatterned methacrylate polymer substrates. Dissociated SGN cultures were plated on the micropatterned surfaces. Calcium influx and release from internal stores were manipulated by elevating extracellular K(+) , maintenance in calcium-free media, or bath application of various calcium channel blockers. Cyclic nucleotide activity was increased by application of cpt-cAMP or 8-Br-cGMP. Elevation of [Ca(2+) ]i by treatment of cultures with elevated potassium reduced neurite alignment to physical microfeatures. Maintenance of cultures in Ca(2+) -free medium or treatment with the non-selective voltage-gated calcium channel blocker cadmium or L-type Ca(2+) channel blocker nifedipine did not signficantly alter SGN neurite alignment. By contrast, ryanodine or xestospongin C, which block release of internal calcium stores via ryanodine-sensitive channels or inositol-1,4,5-trisphosphate receptors respectively, each significantly decreased neurite alignment. Cpt-cAMP significantly reduced neurite alignment while 8-Br-cGMP significantly enhanced neurite alignment. Manipulation of [Ca(2+) ]i or cAMP levels significantly disrupts neurite guidance while elevation of cGMP levels increases neurite alignment. The results suggest intracellular signaling pathways similar to those recruited by chemotactic cues are involved in neurite guidance by topographical features. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2037-2048, 2016.

Ca(II) Binding Regulates and Dominates the Reactivity of a Transition-Metal-Ion-Dependent Diesterase from Mycobacterium tuberculosis.

The diesterase Rv0805 from Mycobacterium tuberculosis is a dinuclear metallohydrolase that plays an important role in signal transduction by controlling the intracellular levels of cyclic nucleotides. As Rv0805 is essential for mycobacterial growth it is a promising new target for the development of chemotherapeutics to treat tuberculosis. The in vivo metal-ion composition of Rv0805 is subject to debate. Here, we demonstrate that the active site accommodates two divalent transition metal ions with binding affinities ranging from approximately 50 nm for Mn(II) to about 600 nm for Zn(II) . In contrast, the enzyme GpdQ from Enterobacter aerogenes, despite having a coordination sphere identical to that of Rv0805, binds only one metal ion in the absence of substrate, thus demonstrating the significance of the outer sphere to modulate metal-ion binding and enzymatic reactivity. Ca(II) also binds tightly to Rv0805 (Kd ≈40 nm), but kinetic, calorimetric, and spectroscopic data indicate that two Ca(II) ions bind at a site different from the dinuclear transition-metal-ion binding site. Ca(II) acts as an activator of the enzymatic activity but is able to promote the hydrolysis of substrates even in the absence of transition-metal ions, thus providing an effective strategy for the regulation of the enzymatic activity.

Abstract 4360: Validation of phosphodiesterase 10A as a cancer target

Abstract Phosphodiesterase 10A (PDE10) is a cAMP and cGMP degrading PDE isozyme that is highly expressed in the brain striatum where it plays an important role in cognition and psychomotor activity. PDE10 inhibitors are being developed for the treatment of schizophrenia and Huntington's disease and are generally well tolerated, likely because of low expression levels in peripheral tissues. We recently reported high levels of PDE10 in tumors and that genetic silencing by siRNA inhibits tumor cell growth with a high degree of selectivity over normal cells (Li et al., Oncogene 2014). These observations suggest that PDE10 may have an unrecognized role in tumorigenesis and represents a novel cancer target. To further test this possibility, we studied the effects of a highly specific PDE10 inhibitor, Pf-2545920 (MP-10) on colon tumor cell growth. Here we show that Pf-2545920 selectively inhibits tumor cell growth, causes G1 cell cycle arrest, and induces apoptosis. The concentration range by which Pf-2545920 inhibits tumor cell growth parallels the concentration range required to increase intracellular cyclic nucleotide levels and activate PKA and PKG. Moreover, PDE10 knockdown by siRNA reduces the sensitivity of tumor cells to the growth inhibitory activity of Pf-2545920. Using the crystal structure of PDE10 to design novel inhibitors, a series of compounds were synthesized and screened for tumor cell growth inhibitory activity and PDE10 isozyme specificity. A lead compound, ADT-030 was found to inhibit tumor cell growth and PDE10 enzymatic activity with IC50 values in the nanomolar range, but did not significantly affect the growth of normal cells. Unlike Pf-2545920, ADT-030 exhibits high selectivity for activating PKG signaling without affecting PKA signaling. Inhibitors of PKA and PKG were used to confirm that the tumor cell growth inhibitory activity associated with PDE10 inhibition involves PKG activation, while PKA activation appears to be ancillary. These findings serve to validate PDE10 as a cancer target, whereby novel inhibitors can be designed to specifically activate cGMP/PKG signaling with a high degree of tumor cell selectivity. Supported by NIH grants 1R01CA155638 and 1R01CA131378 (Piazza). Citation Format: Kevin Lee, Nan Li, Xi Chen, Bing Zhu, Larry Yet, Luciana Madeira da Silva, Suzanne Russo, Adam B. Keeton, Michael R. Boyd, Gary A. Piazza. Validation of phosphodiesterase 10A as a cancer target. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4360. doi:10.1158/1538-7445.AM2015-4360

A cyclic nucleotide sensitive promoter reporter system suitable for bacteria and plant cells.

BackgroundCyclic AMP (cAMP) and cyclic GMP (cGMP) have roles in relaying external signals and modifying gene expression within cells in all phyla. Currently there are no reporter systems suitable for bacteria and plant cells that measure alterations in downstream gene expression following changes in intracellular levels of cyclic nucleotides. As the plant protein OLIGOPEPTIDE TRANSPORTER X (OPTX) is upregulated by cGMP, we fused the OPTX promoter to a luciferase reporter gene (OPTX:LUC) to develop a plant cell reporter of cGMP-induced gene expression. We prepared a second construct augmented with three mammalian cGMP response elements (OPTXcGMPRE:LUC) and a third construct containing five gibberellic acid response elements (OPTXGARE:LUC). All three constructs were tested in bacteria and isolated plant protoplasts.ResultsMembrane permeable cGMP enhanced luciferase activity of OPTX:LUC and OPTXGARE:LUC in protoplasts. Treatment with the plant hormone gibberellic acid which acts via cGMP also generated downstream luciferase activity. However, membrane permeable cAMP induced similar responses to cGMP in protoplasts. Significantly increased luciferase activity occurred in bacteria transformed with either OPTXcGMPRE:LUC or OPTXGARE:LUC in response to membrane permeable cAMP and cGMP. Bacteria co-transformed with OPTXcGMPRE:LUC or OPTXGARE:LUC and the soluble cytoplasmic domain of phytosulfokine receptor1 (PSKR1; a novel guanylate cyclase) had enhanced luciferase activity following induction of PSKR1 expression.ConclusionsWe have developed promoter reporter systems based on the plant OPTX promoter that can be employed in bacteria and isolated plant cells. We have shown that it can be used in bacteria to screen recombinant proteins for guanylate cyclase activity as increases in intracellular cGMP levels result in altered gene transcription and luciferase activity.

Cyclic nucleotide dependent regulation of fibroblast migration by multi drug resistant protein 4

Role of cyclic nucleotides and cyclic nucleotide‐dependent signaling molecules in cell migration has long been known. However, the concept that it is not just the absence or presence, but a highly coordinated balance between cyclic nucleotides that regulates cell migration, is new and revolutionary. In this study, we used multi‐drug resistance protein 4 (MRP4)‐expressing cell lines and MRP4 knockout mice as model systems and wound healing assays as the experimental system to explore this unique and emerging concept. MRP4, an ATP binding cassette transporter protein, localizes to the plasma membrane and functions as a nucleotide efflux transporter and plays a role in the regulation of intracellular cyclic nucleotide levels. Here we demonstrate that mouse embryonic fibroblasts (MEFs) isolated from Mrp4−/− mice have higher intracellular cyclic nucleotide levels and migrate faster compared to MEFs from Mrp4+/+ mice. In contrast to these moderate increases seen in the absence of MRP4, a robust increase in cAMP levels was observed following treatment with forskolin and IBMX, which inhibits fibroblasts migration. In response to externally added cell‐permeant cyclic nucleotides (cpt‐cAMP and cpt‐cGMP), MEF migration appears to be biphasic. Altogether our data provide the first experimental evidence supporting the novel concept that balance between cyclic nucleotides is critical for fine‐tuned cell migration.

Multi-drug Resistance Protein 4 (MRP4)-mediated Regulation of Fibroblast Cell Migration Reflects a Dichotomous Role of Intracellular Cyclic Nucleotides

It has long been known that cyclic nucleotides and cyclic nucleotide-dependent signaling molecules control cell migration. However, the concept that it is not just the absence or presence of cyclic nucleotides, but a highly coordinated balance between these molecules that regulates cell migration, is new and revolutionary. In this study, we used multidrug resistance protein 4 (MRP4)-expressing cell lines and MRP4 knock-out mice as model systems and wound healing assays as the experimental system to explore this unique and emerging concept. MRP4, a member of a large family of ATP binding cassette transporter proteins, localizes to the plasma membrane and functions as a nucleotide efflux transporter and thus plays a role in the regulation of intracellular cyclic nucleotide levels. Here, we demonstrate that mouse embryonic fibroblasts (MEFs) isolated from Mrp4(-/-) mice have higher intracellular cyclic nucleotide levels and migrate faster compared with MEFs from Mrp4(+/+) mice. Using FRET-based cAMP and cGMP sensors, we show that inhibition of MRP4 with MK571 increases both cAMP and cGMP levels, which results in increased migration. In contrast to these moderate increases in cAMP and cGMP levels seen in the absence of MRP4, a robust increase in cAMP levels was observed following treatment with forskolin and isobutylmethylxanthine, which decreases fibroblast migration. In response to externally added cell-permeant cyclic nucleotides (cpt-cAMP and cpt-cGMP), MEF migration appears to be biphasic. Altogether, our studies provide the first experimental evidence supporting the novel concept that balance between cyclic nucleotides is critical for cell migration.

Risk of Intracranial Hemorrhage With Protease-Activated Receptor-1 Antagonists

See related article, p 3189. Acute clinical syndromes caused by atherosclerotic disease of the arterial wall are among the world’s leading causes of death and disability.1 These conditions include acute coronary syndromes, ischemic stroke/transient ischemic attack, and symptomatic peripheral arterial disease. As average life expectancy continues to rise, the burden posed by these diseases is expected to increase.2 Antiplatelet therapy plays a pivotal role in primary and secondary prevention of acute atherothrombotic events by targeting platelet activation, a mechanism that mediates both the beneficial (hemostatic) and detrimental (thrombotic) effect of platelets. This activation process occurs through several pathways, each targeted by different classes of drugs.3 Platelet activation can be triggered by three main extracellular agonists: thromboxane A2, adenosine diphosphate, and thrombin. These agonists activate the platelet integrin glycoprotein IIb/IIIa, a main receptor for both adhesion and aggregation. Endothelial-derived factors limit platelet aggregation by increasing intracellular levels of cyclic nucleotides (cyclic guanosine monophosphate and cyclic adenosine monophosphate).4 The most commonly used oral antiplatelet medications are inhibitors of the thromboxane A2 (aspirin) and adenosine diphosphate (P2Y12 inhibitors: clopidogrel, ticlopidine, and prasugrel) pathways. A third group of drugs acts on the cyclic nucleotide-dependent regulation step by inhibiting the phosphodiesterase responsible for the degradation of cyclic nucleotides (dipyridamole and cilostazol), and a fourth group directly inhibits the glycoprotein IIb/IIIa (abciximab, eptifibatide, and tirofiban).5 In …

New insights into myosin phosphorylation during cyclic nucleotide-mediated smooth muscle relaxation

Nitrovasodilators and agonists, via an increase in intracellular cyclic nucleotide levels, can induce smooth muscle relaxation without a concomitant decrease in phosphorylation of the regulatory light chains (RLC) of myosin. However, since cyclic nucleotide-induced relaxation is associated with a decrease in intracellular [Ca2+], and hence, a decreased activity of MLCK, we tested the hypothesis that the site responsible for the elevated RLC phosphorylation is not Ser19. Smooth muscle strips from gastric fundus were isometrically contracted with ET-1 which induced an increase in monophosphorylation from 9 ± 1 % under resting conditions (PSS) to 36 ± 1 % determined with 2D-PAGE. Electric field stimulation induced a rapid, largely NO-mediated relaxation with a half time of 8 s, which was associated with an initial decline in RLC phosphorylation to 18 % within 2 s and a rebound to 34 % after 30 s whereas relaxation was sustained. In contrast, phosphorylation of RLC at Ser19 probed with phosphospecific antibodies declined in parallel with force. LC/MS and western blot analysis with phosphospecific antibodies against monophosphorylated Thr18 indicate that Thr18 is significantly monophosphorylated during sustained relaxation. We therefore suggest that (i) monophosphorylation of Thr18 rather than Ser19 is responsible for the phosphorylation rebound during sustained EFS-induced relaxation of mouse gastric fundus, and (ii) that relaxation can be ascribed to dephosphorylation of Ser19, the site considered to be responsible for regulation of smooth muscle tone.

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

AbstractPhosphodiesterase (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 24h intervals at the following doses: 20mg/kg, im, 1mg/kg, im and 1mg/kg, po, respectively.Th1 and Th2 cytokine levels (IL-2, IFN-γ, IL-4, IL-5, TNF) were determined 12, 24 or 72h after the last administration of the drugs. A commercial BDTM 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 aminophyl-line decreased the production of IL-2 (after 12h). A single dose of milrinone did not affect Th1/Th2 cytokine secretion. Sildenafil administered once decreased the production of IL-2 (after 72h). A temporary enhancement in the level of IL-5 was observed 12h 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.