Exchange protein directly activated by cAMP (epac): a multidomain cAMP mediator in the regulation of diverse biological functions

Epac Proteins and Calmodulin as Possible Arrhythmogenesis Trigger in Alcoholic Cardiomyopathy

The expression of Epac proteins (exchange protein directly activated by cAMP) and calmodulin (CaM) was assessed by the content of the corresponding mRNA in biopsy specimens of cardiac atrium, left ventricle, and thoracic aorta of rats with alcoholic cardiomyopathy. In the myocardium, overexpression of Еpac1, Ерас2, and СаМ mRNA was found. The content of Epac2 mRNA in the left ventricle was elevated by 2.9 times (p=0.000001), in the left atrium by 3.2 times (p=0.00001), in the right atrium by 3 times (p=0.00001). In contrast to the myocardial tissue, the content of CaM mRNA in the thoracic aorta was not increased, but showed a tendency to decrease, when compared to the control values, while the level of Epac1 and Epac2 mRNA was increased. The assumption is made that regulatory proteins Epac and CaM can play a key role in arrhythmogenesis development under conditions of alcoholic cardiomyopathy.

On the Mechanism of the Cardioprotective Action of σ1 Receptor Agonist Anxiolytic Fabomotizole Hydrochloride (Afobazole)

Original translational rat model of chronic heart failure provoked by experimental anterior transmural myocardium infarction was employed to examine the preventive action of anxiolytic Afobazole (15 mg/kg/day administered intraperitoneally during the first 15 days after coronary occlusion) on the development of the heart failure assessed in 3 months after infarction. Afobazole prevented the development of pathologic remodeling of the myocardium, maintained its inotropic function, and decreased the plasma level of brain natriuretic peptide known as a biochemical marker of chronic heart failure. In the myocardium, Afobazole down-regulated overexpression of the genes induced in chronic heart failure and assessed by corresponding RNA levels, which code angiotensin (AT1A-R), vasopressin (V1A-R), and glucocorticoid (GR) receptors as well as Epac2 protein. The revealed biochemical changes are consistent with the data on cardioprotective action of Afobazole.

Epac and Nociceptor Sensitization

Abstract

Primary sensory neurons are responsible for transmitting sensory information from the peripheral to the central nervous system. Their responses to incoming stimulation become greatly enhanced and prolonged following inflammation, giving rise to exaggerated nociceptive responses and chronic pain. The inflammatory mediator, prostaglandin E2 (PGE2), released from the inflamed tissue surrounding the terminals of sensory neurons contributes to the abnormal pain responses. PGE2 acts on G protein-coupled EP receptors to activate adenylyl cyclase, which catalyzes the conversion of adenosine triphosphate to cyclic adenosine 3′,5′-monophosphate (cAMP). Under normal conditions, cAMP activates primarily protein kinase A. After inflammation, cAMP also activates the exchange proteins activated by cAMP (Epacs) to produce exaggerated PGE2-mediated hyperalgesia. The role of cAMP-Epac signaling in the generation of hypersensitivity is the topic of this review.

Epac2a-knockout mice are resistant to dexamethasone-induced skeletal muscle atrophy and short-term cold stress

Exchange protein directly activated by cAMP (Epac) 2a-knockout (KO) mice exhibit accelerated diet-induced obesity and are resistant to leptin-mediated adipostatic signaling from the hypothalamus to adipose tissue, with sustained food intake. However, the impact of Epac2a deficiency on hypothalamic regulation of sympathetic nervous activity (SNA) has not been elucidated. This study was performed to elucidate the response of Epac2a-KO mice to dexamethasone-induced muscle atrophy and acute cold stress. Compared to age-matched wild-type mice, Epac2a-KO mice showed higher energy expenditures and expression of myogenin and uncoupling protein-1 in skeletal muscle (SM) and brown adipose tissue (BAT), respectively. Epac2a-KO mice exhibited greater endurance to dexamethasone and cold stress. In wild-type mice, exogenous leptin mimicked the responses observed in Epac2a-KO mice. This suggests that leptin-mediated hypothalamic signaling toward SNA appears to be intact in these mice. Hence, the potentiated responses of SM and BAT may be due to their high plasma leptin levels.

Mechanism of Selective Enzyme Inhibition through Uncompetitive Regulation of an Allosteric Agonist

Classical uncompetitive inhibitors are potent pharmacological modulators of enzyme function. Since they selectively target enzyme-substrate complexes (E:S), their inhibitory potency is amplified by increasing substrate concentrations. Recently, an unconventional uncompetitive inhibitor, called CE3F4R, was discovered for the exchange protein activated by cAMP isoform 1 (EPAC1). Unlike conventional uncompetitive inhibitors, CE3F4R is uncompetitive with respect to an allosteric effector, cAMP, as opposed to the substrate (i.e., CE3F4R targets the E:cAMP rather than the E:S complex). However, the mechanism of CE3F4R as an uncompetitive inhibitor is currently unknown. Here, we elucidate the mechanism of CE3F4R's action using NMR spectroscopy. Due to limited solubility and line broadening, which pose major challenges for traditional structural determination approaches, we resorted to a combination of protein- and ligand-based NMR experiments to comparatively analyze EPAC mutations, inhibitor analogs, and cyclic nucleotide derivatives that trap EPAC at different stages of activation. We discovered that CE3F4R binds within the EPAC cAMP-binding domain (CBD) at a subdomain interface distinct from the cAMP binding site, acting as a wedge that stabilizes a cAMP-bound mixed-intermediate. The mixed-intermediate includes attributes of both the apo/inactive and cAMP-bound/active states. In particular, the intermediate targeted by CE3F4R traps a CBD's hinge helix in its inactive conformation, locking EPAC into a closed domain topology that restricts substrate access to the catalytic domain. The proposed mechanism of action also explains the isoform selectivity of CE3F4R in terms of a single EPAC1 versus EPAC2 amino acid difference that destabilizes the active conformation of the hinge helix.

A study on the regenerative effect of platelet-rich plasma on experimentally induced hepatic damage in albino rats

Hepatic fibrosis is a worldwide health problem with significant morbidity and mortality. Currently, there is no effective therapy for hepatic fibrosis. The present study was aimed to evaluate the possible regenerative effect of platelet-rich plasma (PRP) against thioacetamide (TAA)-induced hepatic damage. Eighty albino rats were included; 40 were used for PRP preparation and 40 were randomly divided into 4 groups: group I (control group); group II (PRP control); group III (TAA-intoxicated by a dose of 200 mg/kg body mass, intraperitoneally, twice weekly for 7 weeks), and group IV (TAA intoxicated + PRP treated). Macrophage inflammatory protein-1α (MIP-1α) and cyclic adenosine monophosphate (cAMP) were immunoassayed in addition to peroxinitrite level, NADPH-quinone oxidoreductase-1 (NQO1) enzyme activity, and liver function. PRP treatment showed significant improvement in hepatic function, and decreased MIP-1α and peroxinitrite levels. Meanwhile, significant increase in NQO1 enzyme activity and cAMP level were observed. The histopathological results confirmed the laboratory results with improvement of hepatic architecture except for some inflammatory cellular infiltrates. This study shows that PRP has the ability to protect against TAA-induced liver damage, possibly by improving redox status, liver histopathological architecture, and disruption of the inflammatory and fibrotic response induced by TAA.

Pharmacological modulation of the CO2/HCO3-/pH-, calcium-, and ATP-sensing soluble adenylyl cyclase

Cyclic AMP (cAMP), the prototypical second messenger, has been implicated in a wide variety of (often opposing) physiological processes. It simultaneously mediates multiple, diverse processes, often within a single cell, by acting locally within independently-regulated and spatially-restricted microdomains. Within each microdomain, the level of cAMP will be dependent upon the balance between its synthesis by adenylyl cyclases and its degradation by phosphodiesterases (PDEs). In mammalian cells, there are many PDE isoforms and two types of adenylyl cyclases; the G protein regulated transmembrane adenylyl cyclases (tmACs) and the CO₂/HCO₃^-/pH-, calcium-, and ATP-sensing soluble adenylyl cyclase (sAC). Discriminating the roles of individual cyclic nucleotide microdomains requires pharmacological modulators selective for the various PDEs and/or adenylyl cyclases. Such tools present an opportunity to develop therapeutics specifically targeted to individual cAMP dependent pathways. The pharmacological modulators of tmACs have recently been reviewed, and in this review, we describe the current status of pharmacological tools available for studying sAC.

A mechanistic review on GNAO1-associated movement disorder

Mutations in the GNAO1 gene cause a complex constellation of neurological disorders including epilepsy, developmental delay, and movement disorders. GNAO1 encodes Gα_o, the α subunit of G_o, a member of the G_i/o family of heterotrimeric G protein signal transducers. G_o is the most abundant membrane protein in the mammalian central nervous system and plays major roles in synaptic neurotransmission and neurodevelopment. GNAO1 mutations were first reported in early infantile epileptic encephalopathy 17 (EIEE17) but are also associated with a more common syndrome termed neurodevelopmental disorder with involuntary movements (NEDIM). Here we review a mechanistic model in which loss-of-function (LOF) GNAO1 alleles cause epilepsy and gain-of-function (GOF) alleles are primarily associated with movement disorders. We also develop a signaling framework related to cyclic AMP (cAMP), synaptic vesicle release, and neural development and discuss gene mutations perturbing those mechanisms in a range of genetic movement disorders. Finally, we analyze clinical reports of patients carrying GNAO1 mutations with respect to their symptom onset and discuss pharmacological/surgical treatments in the context of our mechanistic model.

Functional Significance of the Adcy10-Dependent Intracellular cAMP Compartments

Mounting evidence confirms the compartmentalized structure of evolutionarily conserved 3'⁻5'-cyclic adenosine monophosphate (cAMP) signaling, which allows for simultaneous participation in a wide variety of physiological functions and ensures specificity, selectivity and signal strength. One important player in cAMP signaling is soluble adenylyl cyclase (sAC). The intracellular localization of sAC allows for the formation of unique intracellular cAMP microdomains that control various physiological and pathological processes. This review is focused on the functional role of sAC-produced cAMP. In particular, we examine the role of sAC-cAMP in different cellular compartments, such as cytosol, nucleus and mitochondria.

Vascular Endothelial Growth Factor-A Exerts Diverse Cellular Effects via Small G Proteins, Rho and Rap

Vascular endothelial growth factors (VEGFs) include five molecules (VEGF-A, -B, -C, -D, and placental growth factor), and have various roles that crucially regulate cellular functions in many kinds of cells and tissues. Intracellular signal transduction induced by VEGFs has been extensively studied and is usually initiated by their binding to two classes of transmembrane receptors: receptor tyrosine kinase VEGF receptors (VEGF receptor-1, -2 and -3) and neuropilins (NRP1 and NRP2). In addition to many established results reported by other research groups, we have previously identified small G proteins, especially Ras homologue gene (Rho) and Ras-related protein (Rap), as important mediators of VEGF-A-stimulated signaling in cancer cells as well as endothelial cells. This review article describes the VEGF-A-induced signaling pathways underlying diverse cellular functions, including cell proliferation, migration, and angiogenesis, and the involvement of Rho, Rap, and their related molecules in these pathways.

cAMP Signaling Compartmentation: Adenylyl Cyclases as Anchors of Dynamic Signaling Complexes

It is widely accepted that cAMP signaling is compartmentalized within cells. However, our knowledge of how receptors, cAMP signaling enzymes, effectors, and other key proteins form specific signaling complexes to regulate specific cell responses is limited. The multicomponent nature of these systems and the spatiotemporal dynamics involved as proteins interact and move within a cell make cAMP responses highly complex. Adenylyl cyclases, the enzymatic source of cAMP production, are key starting points for understanding cAMP compartments and defining the functional signaling complexes. Three basic elements are required to form a signaling compartment. First, a localized signal is generated by a G protein-coupled receptor paired to one or more of the nine different transmembrane adenylyl cyclase isoforms that generate the cAMP signal in the cytosol. The diffusion of cAMP is subsequently limited by several factors, including expression of any number of phosphodiesterases (of which there are 24 genes plus spice variants). Finally, signal response elements are differentially localized to respond to cAMP produced within each locale. A-kinase-anchoring proteins, of which there are 43 different isoforms, facilitate this by targeting protein kinase A to specific substrates. Thousands of potential combinations of these three elements are possible in any given cell type, making the characterization of cAMP signaling compartments daunting. This review will focus on what is known about how cells organize cAMP signaling components as well as identify the unknowns. We make an argument for adenylyl cyclases being central to the formation and maintenance of these signaling complexes.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

Catecholamines facilitate VEGF-dependent angiogenesis via β2-adrenoceptor-induced Epac1 and PKA activation

Chronic stress has been associated with the progression of cancer and antagonists for β-adrenoceptors (βAR) are regarded as therapeutic option. As they are also used to treat hemangiomas as well as retinopathy of prematurity, a role of endothelial β₂AR in angiogenesis can be envisioned. We therefore investigated the role of β₂AR-induced cAMP formation by analyzing the role of the cAMP effector molecules exchange factor directly activated by cAMP 1 (Epac1) and protein kinase A (PKA) in endothelial cells (EC). Epac1-deficient mice showed a reduced amount of pre-retinal neovascularizations in the model of oxygen-induced retinopathy, which is predominantly driven by vascular endothelial growth factor (VEGF). siRNA-mediated knockdown of Epac1 in human umbilical vein EC (HUVEC) decreased angiogenic sprouting by lowering the expression of the endothelial VEGF-receptor-2 (VEGFR-2). Conversely, Epac1 activation by β₂AR stimulation or the Epac-selective activator cAMP analog 8-p-CPT-2’-O-Me-cAMP (8-pCPT) increased VEGFR-2 levels and VEGF-dependent sprouting. Similar to Epac1 knockdown, depletion of the monomeric GTPase Rac1 decreased VEGFR-2 expression. As Epac1 stimulation induces Rac1 activation, Epac1 might regulate VEGFR-2 expression through Rac1. In addition, we found that PKA was also involved in the regulation of angiogenesis in EC since the adenylyl cyclase (AC) activator forskolin (Fsk), but not 8-pCPT, increased sprouting in Epac1-depleted HUVEC and this increase was sensitive to a selective synthetic peptide PKA inhibitor. In accordance, β₂AR- and AC-activation, but not Epac1 stimulation increased VEGF secretion in HUVEC.

Our data indicate that high levels of catecholamines, which occur during chronic stress, prime the endothelium for angiogenesis through a β₂AR-mediated increase in endothelial VEGFR-2 expression and VEGF secretion.

Using cAMP Sensors to Study Cardiac Nanodomains

3′,5′-cyclic adenosine monophosphate (cAMP) signalling plays a major role in the cardiac myocyte response to extracellular stimulation by hormones and neurotransmitters. In recent years, evidence has accumulated demonstrating that the cAMP response to different extracellular agonists is not uniform: depending on the stimulus, cAMP signals of different amplitudes and kinetics are generated in different subcellular compartments, eliciting defined physiological effects. In this review, we focus on how real-time imaging using fluorescence resonance energy transfer (FRET)-based reporters has provided mechanistic insight into the compartmentalisation of the cAMP signalling pathway and allowed for the precise definition of the regulation and function of subcellular cAMP nanodomains.

Cyclic 3',5'-adenosine monophosphate (cAMP) signaling in the anterior pituitary gland in health and disease

The cyclic 3',5'-adenosine monophosphate (cAMP) was the first among the so-called "second messengers" to be described. It is conserved in most organisms and functions as a signal transducer by mediating the intracellular effects of multiple hormones and neurotransmitters. In this review, we first delineate how different members of the cAMP pathway ensure its correct compartmentalization and activity, mediate the terminal intracellular effects, and allow the crosstalk with other signaling pathways. We then focus on the pituitary gland, where cAMP exerts a crucial function by controlling the responsiveness of the cells to hypothalamic hormones, neurotransmitters and peripheral factors. We discuss the most relevant physiological functions mediated by cAMP in the different pituitary cell types, and summarize the defects affecting this pathway that have been reported in the literature. We finally discuss how a deregulated cAMP pathway is involved in the pathogenesis of pituitary disorders and how it affects the response to therapy.

Insights into exchange factor directly activated by cAMP (EPAC) as potential target for cancer treatment

Cancer remains a global health problem and approximately 1.7 million new cancer cases are diagnosed every year worldwide. Although diverse molecules are currently being explored as targets for cancer therapy the tumor treatment and therapy is highly tricky. Secondary messengers are important for hormone-mediated signaling pathway. Cyclic AMP (cAMP), a secondary messenger responsible for various physiological processes regulates cell metabolism by activating Protein kinase A (PKA) and by targeting exchange protein directly activated by cAMP (EPAC). EPAC is present in two isoforms EPAC1 and EPAC2, which exhibit different tissue distribution and is involved in GDP/GTP exchange along with activating Rap1- and Rap2-mediated signaling pathways. EPAC is also known for its dual role in cancer as pro- and anti-proliferative in addition to metastasis. Results after perturbing EPAC activity suggests its involvement in cancer cell migration, proliferation, and cytoskeleton remodeling which makes it a potential therapeutic target for cancer treatments.

Epac Function and cAMP Scaffolds in the Heart and Lung

Evidence collected over the last ten years indicates that Epac and cAMP scaffold proteins play a critical role in integrating and transducing multiple signaling pathways at the basis of cardiac and lung physiopathology. Some of the deleterious effects of Epac, such as cardiomyocyte hypertrophy and arrhythmia, initially described in vitro, have been confirmed in genetically modified mice for Epac1 and Epac2. Similar recent findings have been collected in the lung. The following sections will describe how Epac and cAMP signalosomes in different subcellular compartments may contribute to cardiac and lung diseases.

Alleviation of thalamic pain by cilostazol administration: a case report

Thalamic pain is severe and treatment-resistant; however, there are few available options for improving thalamic pain. This study demonstrated that thalamic pain was alleviated by administration of cilostazol, suggesting that cilostazol may be a candidate for treating thalamic pain.

Epac1 links prostaglandin E2 to β-catenin-dependent transcription during epithelial-to-mesenchymal transition

In epithelial cells, β-catenin is localized at cell-cell junctions where it stabilizes adherens junctions. When these junctions are disrupted, β-catenin can translocate to the nucleus where it functions as a transcriptional cofactor. Recent research has indicated that PGE₂ enhances the nuclear function of β-catenin through cyclic AMP. Here, we aim to study the role of the cyclic AMP effector Epac in β-catenin activation by PGE₂ in non-small cell lung carcinoma cells.

We show that PGE₂ induces a down-regulation of E-cadherin, promotes cell migration and enhances β-catenin translocation to the nucleus. This results in β-catenin-dependent gene transcription. We also observed increased expression of Epac1. Inhibition of Epac1 activity using the CE3F4 compound or Epac1 siRNA abolished the effects of PGE₂ on β-catenin. Further, we observed that Epac1 and β-catenin associate together. Expression of an Epac1 mutant with a deletion in the nuclear pore localization sequence prevents this association. Furthermore, the scaffold protein Ezrin was shown to be required to link Epac1 to β-catenin.

This study indicates a novel role for Epac1 in PGE₂-induced EMT and subsequent activation of β-catenin.

Prostaglandin E2 suppresses hCAP18/LL-37 expression in human macrophages via EP2/EP4: implications for treatment of Mycobacterium tuberculosis infection

Prostaglandin (PG)E₂ is an arachidonic acid-derived lipid mediator that plays an important role in inflammation and immunity. In this study, we demonstrate that PGE₂ suppresses basal and 1,25-dihydroxy vitamin D₃ (VD₃)-induced expression of hCAP18/LL-37 via E prostanoid (EP)2 and EP4 receptors. In humans, VD₃ up-regulates vitamin D receptor (VDR) expression and promotes transcription of the cathelicidin hCAP18/LL-37 gene, whereas PGE₂ counteracts this effect. We find that PGE₂ induces the cAMP/PKA-signaling pathway and enhances the expression of the inhibitory transcription factor cAMP-responsive modulator/inducible cAMP early repressor, which prevents VDR expression and induction of hCAP18/LL-37 in human macrophages. The negative regulation by PGE₂ was evident in M1- and M2-polarized human macrophages, although PGE₂ displayed more profound inhibitory effects in M2 cells. PGE₂ impaired VD₃-induced expression of cathelicidin and concomitant activation of autophagy during Mycobacterium tuberculosis (Mtb) infection and facilitated intracellular Mtb growth in human macrophages. An EP4 agonist also significantly promoted Mtb survival in human macrophages. Our results indicate that PGE₂ inhibits hCAP18/LL-37 expression, especially VD₃-induced cathelicidin and autophagy, which may reduce host defense against Mtb. Accordingly, antagonists of EP4 may constitute a novel adjunctive therapy in Mtb infection.-Wan, M., Tang, X., Rekha, R. S., Muvva, S. S. V. J. R., Brighenti, S., Agerberth, B., Haeggström, J. Z. Prostaglandin E₂ suppresses hCAP18/LL-37 expression in human macrophages via EP2/EP4: implications for treatment of Mycobacterium tuberculosis infection.

Signalling transduction events involved in agonist-induced PGE2/EP4 receptor externalization in cultured rat dorsal root ganglion neurons

BACKGROUND

Prostaglandin E2 (PGE2) enriched in inflamed tissues contributes to chronic pain by sensitizing nociceptive dorsal root ganglion (DRG) neurons (nociceptors). Of four PGE2 receptors (EP1-4), EP4 plays a major role in PGE2-induced nociceptor sensitization. We have previously reported that PGE2 or EP4 agonists stimulated EP4 externalization in cultured DRG neurons and this event contributes to nociceptor sensitization. However, the signalling transduction events governing this event remain unknown.

METHODS

In this study, using antibody-based externalization assay, we examined EP subtypes and multiple signalling transduction events involved in PGE2-induced EP4 externalization in cultured DRG neurons.

RESULTS

In addition to EP4 agonist, EP2 agonist, to a lesser extent, also induced EP4 externalization while EP1 and EP3 agonists had no effect. The extracellular and intracellular calcium chelators, the inhibitors of CaMKII, cAMP, PKA, PKC, PKCε, PLC, MAPKs, PI3K and Akt suppressed agonist-induced EP4 externalization. The activator of AC, two PKA-specific cAMP analogues and one Epac-specific cAMP analogue also induced EP4 externalization. ELISA showed that double sequential exposures to EP4 agonists induced a greater release of pain peptide CGRP from cultured DRG neurons than a single exposure, an event blocked by the inhibitor of anterograde transport from ER/Golgi complex to cell surface.

CONCLUSIONS

Taken together, these data suggest that mobilization of extracellular and intracellular calcium as well as the activation of CaMKII, cAMP/PKA, cAMP/Epac, PKC/PKCε, MAPKs, PI3K-Akt and PLC signalling transduction pathways are involved in agonist-induced EP4 externalization. Agonist-enhanced EP4 externalization increases EP4 cell surface abundance and activity, thus enhancing nociceptor sensitization.

SIGNIFICANCE

This study adds mechanistic information regarding signalling transduction events involved in agonist-induced EP4 cell surface trafficking. EP4 and EP2 (to lesser extent) receptors, extra- and intracellular Ca⁺⁺ , CaKMII, cAMP, PKA, PKC, PKCε, PLC, MAPK, PI3K and Akt are involved in this event. Agonist-induced EP4 externalization contributes to enhanced nociceptor sensitization.

cAMP: From Long-Range Second Messenger to Nanodomain Signalling

How cAMP generates hormone-specific effects has been debated for many decades. Fluorescence resonance energy transfer (FRET)-based sensors for cAMP allow real-time imaging of the second messenger in intact cells with high spatiotemporal resolution. This technology has made it possible to directly demonstrate that cAMP signals are compartmentalised. The details of such signal compartmentalisation are still being uncovered, and recent findings reveal a previously unsuspected submicroscopic heterogeneity of intracellular cAMP. A model is emerging where specificity depends on compartmentalisation and where the physiologically relevant signals are those that occur within confined nanodomains, rather than bulk changes in cytosolic cAMP. These findings subvert the classical notion of cAMP signalling and provide a new framework for the development of targeted therapeutic approaches.

EPAC-RAP1 Axis-Mediated Switch in the Response of Primary and Metastatic Melanoma to Cyclic AMP

Cyclic AMP (cAMP) is an important second messenger that regulates a wide range of physiologic processes. In mammalian cutaneous melanocytes, cAMP-mediated signaling pathways activated by G-protein-coupled receptors (GPCR), like melanocortin 1 receptor (MC1R), play critical roles in melanocyte homeostasis including cell survival, proliferation, and pigment synthesis. Impaired cAMP signaling is associated with increased risk of cutaneous melanoma. Although mutations in MAPK pathway components are the most frequent oncogenic drivers of melanoma, the role of cAMP in melanoma is not well understood. Here, using the Braf(V600E)/Pten-null mouse model of melanoma, topical application of an adenylate cyclase agonist, forskolin (a cAMP inducer), accelerated melanoma tumor development in vivo and stimulated the proliferation of mouse and human primary melanoma cells, but not human metastatic melanoma cells in vitro The differential response of primary and metastatic melanoma cells was also evident upon pharmacologic inhibition of the cAMP effector protein kinase A. Pharmacologic inhibition and siRNA-mediated knockdown of other cAMP signaling pathway components showed that EPAC-RAP1 axis, an alternative cAMP signaling pathway, mediates the switch in response of primary and metastatic melanoma cells to cAMP. Evaluation of pERK levels revealed that this phenotypic switch was not correlated with changes in MAPK pathway activity. Although cAMP elevation did not alter the sensitivity of metastatic melanoma cells to BRAF(V600E) and MEK inhibitors, the EPAC-RAP1 axis appears to contribute to resistance to MAPK pathway inhibition. These data reveal a MAPK pathway-independent switch in response to cAMP signaling during melanoma progression.Implications: The prosurvival mechanism involving the cAMP-EPAC-RAP1 signaling pathway suggest the potential for new targeted therapies in melanoma. Mol Cancer Res; 15(12); 1792-802. ©2017 AACR.

Structure-Activity Relationship Studies with Tetrahydroquinoline Analogs as EPAC Inhibitors

EPAC proteins are therapeutic targets for the potential treatment of cardiac hypertrophy and cancer metastasis. Several laboratories use a tetrahydroquinoline analog, CE3F4, to dissect the role of EPAC1 in various disease states. Here, we report SAR studies with tetrahydroquinoline analogs that explore various functional groups. The most potent EPAC inhibitor 12a exists as a mixture of inseparable E (major) and Z (minor) rotamers. The rotation about the N-formyl group indeed impacts the activity against EPAC.

Network Pharmacology-Based Approach to Investigate the Analgesic Efficacy and Molecular Targets of Xuangui Dropping Pill for Treating Primary Dysmenorrhea

This study aimed to evaluate the clinical analgesic efficacy and identify the molecular targets of XGDP for treating primary dysmenorrhea (PD) by a network pharmacology approach. Analysis of pain disappearance rate of XGDP in PD treatment was conducted based on data from phase II and III randomized, double-blind, double-simulation, and positive parallel controlled clinical trials. The bioactive compounds were obtained by the absorption, distribution, metabolism, and excretion processes with oral bioavailability (OB) and drug-likeness (DL) evaluation. Subsequently, target prediction, pathway identification, and network construction were employed to clarify the mechanisms of the analgesic effect of XGDP on PD. The pain disappearance rates in phase II and III clinical trials of XGDP in PD treatment were 62.5% and 55.8%, respectively, yielding a significant difference (P < 0.05) when compared with the control group using Tongjingbao granules (TJBG). Among 331 compounds, 53 compounds in XGDP were identified as the active compounds related to PD through OB, DL, and target prediction. The active compounds and molecular targets of XGDP were identified, and our study showed that XGDP may exert its therapeutic effects on PD through the regulation of the targets related to anti-inflammation analgesia and central analgesia and relieving smooth muscle contraction.

Recruitment of Epac2A to Insulin Granule Docking Sites Regulates Priming for Exocytosis

Epac is a cAMP-activated guanine nucleotide exchange factor that mediates cAMP signaling in various types of cells, including β-cells, where it is involved in the control of insulin secretion. Upon activation, the protein redistributes to the plasma membrane, but the underlying molecular mechanisms and functional consequences are unclear. Using quantitative high-resolution microscopy, we found that cAMP elevation caused rapid binding of Epac2A to the β-cell plasma membrane, where it accumulated specifically at secretory granules and rendered them more prone to undergo exocytosis. cAMP-dependent membrane binding required the high-affinity cyclic nucleotide-binding (CNB) and Ras association domains, but not the disheveled-Egl-10-pleckstrin domain. Although the N-terminal low-affinity CNB domain (CNB-A) was dispensable for the translocation to the membrane, it was critical for directing Epac2A to the granule sites. Epac1, which lacks the CNB-A domain, was recruited to the plasma membrane but did not accumulate at granules. We conclude that Epac2A controls secretory granule release by binding to the exocytosis machinery, an effect that is enhanced by prior cAMP-dependent accumulation of the protein at the plasma membrane.

Paving the Rho in cancer metastasis: Rho GTPases and beyond

Malignant carcinomas are often characterized by metastasis, the movement of carcinoma cells from a primary site to colonize distant organs. For metastasis to occur, carcinoma cells first must adopt a pro-migratory phenotype and move through the surrounding stroma towards a blood or lymphatic vessel. Currently, there are very limited possibilities to target these processes therapeutically. The family of Rho GTPases is an ubiquitously expressed division of GTP-binding proteins involved in the regulation of cytoskeletal dynamics and intracellular signaling. The best characterized members of the Rho family GTPases are RhoA, Rac1 and Cdc42. Abnormalities in Rho GTPase function have major consequences for cancer progression. Rho GTPase activation is driven by cell surface receptors that activate GTP exchange factors (GEFs) and GTPase-activating proteins (GAPs). In this review, we summarize our current knowledge on Rho GTPase function in the regulation of metastasis. We will focus on key discoveries in the regulation of epithelial-mesenchymal-transition (EMT), cell-cell junctions, formation of membrane protrusions, plasticity of cell migration and adaptation to a hypoxic environment. In addition, we will emphasize on crosstalk between Rho GTPase family members and other important oncogenic pathways, such as cyclic AMP-mediated signaling, canonical Wnt/β-catenin, Yes-associated protein (YAP) and hypoxia inducible factor 1α (Hif1α) and provide an overview of the advancements and challenges in developing pharmacological tools to target Rho GTPase and the aforementioned crosstalk in the context of cancer therapeutics.

A cell-based, quantitative and isoform-specific assay for exchange proteins directly activated by cAMP

Extensive functional studies of the exchange protein directly activated by cAMP (EPAC) family of signaling molecules have demonstrated that EPAC proteins play a fundamental role in several physiological and pathophysiological responses, therefore are attractive drug targets. In this report, the development of a cell-based, medium to high throughput screening assay that is capable of monitoring EPAC-mediated activation of cellular Rap1 in an isoform-specific manner is described. This assay adapts a conventional ELISA format with immobilized RalGDS-RBD as a bait to selectively capture GTP-bound active Rap1. As a result, it fills an urgent need for a cell-based EPAC assay that can be conveniently performed using microtiter plates for the discovery and/or validation of isoform-specific EPAC agonists and antagonists.

Epac Signaling Is Required for Cocaine-Induced Change in AMPA Receptor Subunit Composition in the Ventral Tegmental Area

Exchange protein directly activated by cAMP (Epac) and protein kinase A (PKA) are intracellular receptors for cAMP. Although PKA and its downstream effectors have been studied extensively in the context of drug addiction, whether and how Epac regulates cellular and behavioral effects of drugs of abuse remain essentially unknown. Epac is known to regulate AMPA receptor (AMPAR) trafficking. Previous studies have shown that a single cocaine exposure in vivo leads to an increase in GluA2-lacking AMPARs in dopamine neurons of the ventral tegmental area (VTA). We tested the hypothesis that Epac mediates cocaine-induced changes in AMPAR subunit composition in the VTA. We report that a single cocaine injection in vivo in wild-type mice leads to inward rectification of EPSCs and renders EPSCs sensitive to a GluA2-lacking AMPAR blocker in VTA dopamine neurons. The cocaine-induced increase in GluA2-lacking AMPARs was absent in Epac2-deficient mice but not in Epac1-deficient mice. In addition, activation of Epac with the selective Epac agonist 8-CPT-2Me-cAMP (8-CPT) recapitulated the cocaine-induced increase in GluA2-lacking AMPARs, and the effects of 8-CPT were mediated by Epac2. We also show that conditioned place preference to cocaine was impaired in Epac2-deficient mice and in mice in which Epac2 was knocked down in the VTA but was not significantly altered in Epac1-deficient mice. Together, these results suggest that Epac2 is critically involved in the cocaine-induced change in AMPAR subunit composition and drug-cue associative learning.

SIGNIFICANCE STATEMENT

Addictive drugs, such as cocaine, induce long-lasting adaptions in the reward circuits of the brain. A single intraperitoneal injection of cocaine leads to changes in the composition and property of the AMPAR that carries excitatory inputs to dopamine neurons. Here, we provide evidence that exchange protein directly activated by cAMP (Epac), a cAMP sensor protein, is required for the cocaine-induced changes of the AMPAR. We found that the effects of cocaine were mimicked by activation of Epac but were blocked by genetic deletion of Epac. Furthermore, cocaine-cue associative learning was impaired in mice lacking Epac. These findings uncovered a critical role of Epac in regulating the cellular and behavioral actions of cocaine.

Acquired Exchange Protein Directly Activated by Cyclic Adenosine Monophosphate Activity Induced by p38 Mitogen-activated Protein Kinase in Primary Afferent Neurons Contributes to Sustaining Postincisional Nociception

BACKGROUND

The molecular mechanisms responsible for sustained pain after tissue injury are largely unknown. The aim of this study was to clarify the role of exchange protein directly activated by cyclic adenosine monophosphate (EPAC) in sustained postincisional nociception, using tissue injury-induced nociceptor priming, and involvement of p38 mitogen-activated protein kinase (p38MAPK) in EPAC-mediated nociceptor priming.

METHODS

Plantar incisions were made in the hind paws of Sprague-Dawley rats (n = 144). Nociceptor priming was confirmed by behavior testing followed by prostaglandin E2 injection 14 to 21 days after the incision. ESI-09, a selective EPAC inhibitor, was administered to assess its effects on nociceptor priming. Expression of two isoforms of EPAC (EPAC1/EPAC2) in dorsal root ganglions from naive rats and those 14 days after the incision was detected by immunohistochemistry and Western blotting. Separately, FR167653, a selective p38MAPK inhibitor, was administered to assess its effect on EPAC1/EPAC2 expression and the development of nociceptor priming.

RESULTS

Prostaglandin E2 injection 14 to 21 days after the plantar incision induced persistent mechanical hyperalgesia for 7 days. EPAC1/EPAC2 expression in dorsal root ganglion neurons was trivial in naive rats (7.7 ± 4.8% for EPAC1; 6.3 ± 4.1% for EPAC2) but markedly increased 14 days after the incision (21.0 ± 9.4% and 20.1 ± 3.8%, respectively). ESI-09 treatment inhibited prostaglandin E2-induced persistent mechanical hypersensitivity but had no effect on incision-induced acute nociceptive hypersensitivity. Treatment with FR167653 before the incision inhibited the development of nociceptor priming and incision-induced EPAC1/EPAC2 expression (8.5 ± 5.4% and 7.6 ± 3.3%, respectively).

CONCLUSIONS

Transient inflammatory stimulation causes long-lasting nociceptive hypersensitivity via nociceptor priming during the subacute period after incision. Acquired EPAC activity by p38MAPK in the dorsal root ganglion neurons is a key for this event.

The microRNA-7-mediated reduction in EPAC-1 contributes to vascular endothelial permeability and eNOS uncoupling in murine experimental retinopathy

AIMS

To investigate the consequences of oxidative stress and hypoxia on EPAC-1 expression during retinopathy.

METHODS

Oxygen-induced retinopathy was induced in mice and EPAC-1 expression investigated by immunofluorescence. In silico analyses were used to identify a link between EPAC-1 expression and microRNA-7-5p in endothelial cells and confirmed by western blot analyses on cells expressing microRNA-7-5p. In vitro, endothelial cells were either incubated at 2% oxygen or transfected with microRNA-7-5p, and the effects of these treatments on EPAC-1 expression, endothelial hyperpermeability and NO production were assessed. In the Ins2Akita mouse model, levels of EPAC-1 expression as well as microRNA-7-5p were assessed by qPCR. Endothelial nitric oxide synthase was assessed by immunoblotting in the Ins2Akita model.

RESULTS

Hypoxia induces the expression of microRNA-7-5p that translationally inhibits the expression of EPAC-1 in endothelial cells, resulting in hyperpermeability and the loss of eNOS activity. Activation of EPAC-1 by the cAMP analogue 8-pCPT-2'-O-Me-cAMP reduced the sensitivity of EPAC-1 to oxidative stress and restored the endothelial permeability to baseline levels. Additionally, 8-pCPT-2'-O-Me-cAMP rescued eNOS activity and NO production. In mouse models of retinopathy, i.e., oxygen-induced retinopathy and the spontaneous diabetic heterozygous Ins2^Akita mice, EPAC-1 levels are decreased which is associated with an increase in microRNA-7-5p expression and reduced eNOS activity.

CONCLUSION/INTERPRETATION

In retinopathy, EPAC-1 expression is decreased in a microRNA-7-mediated manner, contributing to endothelial dysfunction. Pharmacological activation of remnant EPAC-1 rescues endothelial function. Collectively, these data indicate that EPAC-1 resembles an efficacious and druggable target molecule for the amelioration of (diabetic) retinopathy.

The cellular and molecular bases of leptin and ghrelin resistance in obesity

Obesity, a major risk factor for the development of diabetes mellitus, cardiovascular diseases and certain types of cancer, arises from a chronic positive energy balance that is often due to unlimited access to food and an increasingly sedentary lifestyle on the background of a genetic and epigenetic vulnerability. Our understanding of the humoral and neuronal systems that mediate the control of energy homeostasis has improved dramatically in the past few decades. However, our ability to develop effective strategies to slow the current epidemic of obesity has been hampered, largely owing to the limited knowledge of the mechanisms underlying resistance to the action of metabolic hormones such as leptin and ghrelin. The development of resistance to leptin and ghrelin, hormones that are crucial for the neuroendocrine control of energy homeostasis, is a hallmark of obesity. Intensive research over the past several years has yielded tremendous progress in our understanding of the cellular pathways that disrupt the action of leptin and ghrelin. In this Review, we discuss the molecular mechanisms underpinning resistance to leptin and ghrelin and how they can be exploited as targets for pharmacological management of obesity.

CK2 is a key regulator of SLC4A2-mediated Cl-/HCO3- exchange in human airway epithelia

Transepithelial bicarbonate secretion by human airway submucosal glands and surface epithelial cells is crucial to maintain the pH-sensitive innate defence mechanisms of the lung. cAMP agonists stimulate HCO₃^- secretion via coordinated increases in basolateral HCO₃^- influx and accumulation, as well as CFTR-dependent HCO₃^- efflux at the luminal membrane of airway epithelial cells. Here, we investigated the regulation of a basolateral located, DIDS-sensitive, Cl^-/HCO₃^- exchanger, anion exchanger 2 (AE2; SLC4A2) which is postulated to act as an acid loader, and therefore potential regulator of HCO₃^- secretion, in human airway epithelial cells. Using intracellular pH measurements performed on Calu-3 cells, we demonstrate that the activity of the basolateral Cl^-/HCO₃^- exchanger was significantly downregulated by cAMP agonists, via a PKA-independent mechanism and also required Ca²⁺ and calmodulin under resting conditions. AE2 contains potential phosphorylation sites by a calmodulin substrate, protein kinase CK2, and we demonstrated that AE2 activity was reduced in the presence of CK2 inhibition. Moreover, CK2 inhibition abolished the activity of AE2 in primary human nasal epithelia. Studies performed on mouse AE2 transfected into HEK-293T cells confirmed almost identical Ca²⁺/calmodulin and CK2 regulation to that observed in Calu-3 and primary human nasal cells. Furthermore, mouse AE2 activity was reduced by genetic knockout of CK2, an effect which was rescued by exogenous CK2 expression. Together, these findings are the first to demonstrate that CK2 is a key regulator of Cl^--dependent HCO₃^- export at the serosal membrane of human airway epithelial cells.

Null EPAC mutants reveal a sequential order of versatile cAMP effects during Drosophila aversive odor learning

Here, we define a role of the cAMP intermediate EPAC in Drosophila aversive odor learning by means of null epac mutants. Complementation analysis revealed that EPAC acts downstream from the rutabaga adenylyl cyclase and in parallel to protein kinase A. By means of targeted knockdown and genetic rescue we identified mushroom body Kenyon cells (KCs) as a necessary and sufficient site of EPAC action. We provide mechanistic insights by analyzing acquisition dynamics and using the "performance increment" as a means to access the trial-based sequential organization of odor learning. Thereby we show that versatile cAMP-dependent mechanisms are engaged within a sequential order that correlate to individual trials of the training session.

Exchange proteins directly activated by cAMP (EPACs): Emerging therapeutic targets

Exchange proteins directly activated by cAMP (EPACs) are critical cAMP-dependent signaling pathway mediators. The discovery of EPAC proteins has significantly facilitated understanding on cAMP-dependent signaling pathway and efforts along this line open new avenues for developing novel therapeutics for cancer, diabetes, heart failure, inflammation, infections, neurological disorders and other human diseases. Over the past decade, important progress has been made in the identification of EPAC agonists, antagonists and their biological and pharmacological applications. In this review, we briefly summarize recently reported novel functions of EPACs and the discovery of their small molecule modulators. The challenges and future perspectives are also discussed.

Dynamic monitoring of Gi/o-protein-mediated decreases of intracellular cAMP by FRET-based Epac sensors

Analysis of G-protein-coupled receptor (GPCR) signaling, in particular of the second messenger cAMP that is tightly controlled by G_s- and G_i/o-proteins, is a central issue in biomedical research. The classical biochemical method to monitor increases in intracellular cAMP concentrations consists of a radioactive multicellular assay, which is well established, highly sensitive, and reproducible, but precludes continuous spatial and temporal assessment of cAMP levels in single living cells. For this purpose, Förster resonance energy transfer (FRET)-based Epac cAMP sensors are well suitable. So far, the latter sensors have been employed to monitor G_s-induced cAMP increases and it has remained elusive whether Epac sensors can reliably detect decreased intracellular cAMP levels as well. In this study, we systematically optimize experimental strategies employing FRET-based cAMP sensors to monitor G_i/o-mediated cAMP reductions. FRET experiments with adrenergic α_2A or μ opioid receptors and a set of different Epac sensors allowed for time-resolved, valid, and reliable detection of cAMP level decreases upon G_i/o-coupled receptor activation in single living cells, and this effect can be reversed by selective receptor antagonists. Moreover, pre-treatment with forskolin or 3-isobutyl-1-methylxanthine (IBMX) to artificially increase basal cAMP levels was not required to monitor G_i/o-coupled receptor activation. Thus, using FRET-based cAMP sensors is of major advantage when compared to classical biochemical and multi-cellular assays.

Identification of novel 2-(benzo[d]isoxazol-3-yl)-2-oxo-N-phenylacetohydrazonoyl cyanide analoguesas potent EPAC antagonists

Two series of novel EPAC antagonists are designed, synthesized and evaluated in an effort to develop diversified analogues based on the scaffold of the previously identified high-throughput (HTS) hit 1 (ESI-09). Further SAR studies reveal that the isoxazole ring A of 1 can tolerate chemical modifications with either introduction of flexible electron-donating substitutions or structurally restrictedly fusing with a phenyl ring, leading to identification of several more potent and diversified EPAC antagonists (e.g., 10 (NY0617), 14 (NY0460), 26 (NY0725), 32 (NY0561), and 33 (NY0562)) with low micromolar inhibitory activities. Molecular docking studies on compounds 10 and 33 indicate that these two series of compounds bind at a similar site with substantially different interactions with the EPAC proteins. The findings may serve as good starting points for the development of more potent EPAC antagonists as valuable pharmacological probes or potential drug candidates.

Epac-Rap1-activated mesenchymal stem cells improve cardiac function in rat model of myocardial infarction

INTRODUCTION

Rap1, a member of Ras superfamily of small GTP-binding proteins, is involved in cardiovascular biology in numerous ways. It is an evolutionary conserved regulator of adhesion, polarity, differentiation and growth.

AIMS

Our aim was to analyze Rap1-activated rat bone marrow mesenchymal stem cells (MSCs) for their potential role in adhesion and cardiac differentiation.

METHODS

Myocardial infarction (MI) was produced in Sprague Dawley (SD) rats through occlusion of the left anterior descending coronary artery. MSCs were treated with 8-pCPT-2'-O-Me-cAMP (CPT) to activate Rap1. Normal (untreated) and CPT-treated MSCs were transplanted through intramyocardial injection in respective groups. Cardiac function was assessed by echocardiography at 2 and 4 weeks after cell transplantation. Histological analysis was performed to observe changes at tissue level.

RESULTS

Homing of CPT-treated MSCs was significantly (***P<.001) higher as compared to normal MSCs in the infarcted hearts. This may be due to increase in the gene expression of some of the cell adhesion molecules as evident by qRT-PCR analysis. Significant (***P<.001) improvement in the restoration of heart function in terms of left ventricular diastolic and systolic internal diameters (LVIDd, LVIDs), % ejection fraction, % fraction shortening and end-systolic and end-diastolic volumes were observed in CPT-treated MSCs as compared to the MI model. Histological analyses showed significant (***P<.001) reduction in scar formation in the CPT-treated group. Differentiation of treated MSCs into functional cardiomyocytes was evident through immunohistochemical staining. LV wall thickness was also preserved significantly (***P<.001). Blood vessel formation was more pronounced in CPT-treated group although both cell therapy groups showed significant increase as compared to MI model.

CONCLUSION

Our findings showed that pharmacological activation of Epac-Rap1 improves cardiac function through better survival, adhesion and differentiation of transplanted cells. Transplantation of these MSCs in the infarct area restored functional myocardium.

Interrogating cyclic AMP signaling using optical approaches

Optical reporters for cAMP represent a fundamental advancement in our ability to investigate the dynamics of cAMP signaling. These fluorescent sensors can measure changes in cAMP in single cells or in microdomains within cells as opposed to whole populations of cells required for other methods of measuring cAMP. The first optical cAMP reporters were FRET-based sensors utilizing dissociation of purified regulatory and catalytic subunits of PKA, introduced by Roger Tsien in the early 1990s. The utility of these sensors was vastly improved by creating genetically encoded versions that could be introduced into cells with transfection, the first of which was published in the year 2000. Subsequently, improved sensors have been developed using different cAMP binding platforms, optimized fluorescent proteins, and targeting motifs that localize to specific microdomains. The most common sensors in use today are FRET-based sensors designed around an Epac backbone. These rely on the significant conformational changes in Epac when it binds cAMP, altering the signal between FRET pairs flanking Epac. Several other strategies for optically interrogating cAMP have been developed, including fluorescent translocation reporters, dimerization-dependent FP based biosensors, BRET (bioluminescence resonance energy transfer)-based sensors, non-FRET single wavelength reporters, and sensors based on bacterial cAMP-binding domains. Other newly described mammalian cAMP-binding proteins such as Popdc and CRIS may someday be exploited in sensor design. With the proliferation of engineered fluorescent proteins and the abundance of cAMP binding targets in nature, the field of optical reporters for cAMP should continue to see rapid refinement in the coming years.

cAMP signaling increases histone deacetylase 8 expression via the Epac2-Rap1A-Akt pathway in H1299 lung cancer cells

This study was performed to investigate the signaling pathway that mediates cyclic AMP (cAMP)-induced inhibition of histone deacetylase 8 (HDAC8) degradation, and the effect and underlying mechanisms of the resulting increase in HDAC8 expression on cisplatin-induced apoptosis in lung cancer cells. cAMP signaling increased HDAC8 expression via a protein kinase A (PKA)-independent pathway in H1299 non-small cell lung cancer cells. However, treatment with a selective activator of an exchange protein that was activated by cAMP (Epac) increased HDAC8 expression, and Epac2 inhibition abolished the isoproterenol (ISO)-induced increase in HDAC8 expression. ISO and the Epac activator activated Rap1, and Rap1A activation increased HDAC8 expression; moreover, inhibition of Rap1A with a dominant negative Rap1A or by shRNA-mediated knockdown abolished the ISO-induced increase in HDAC8 expression. Activation of cAMP signaling and Rap1A decreased the activating phosphorylation of Akt. Akt inhibition with a pharmacological inhibitor or expression of a dominant negative Akt inhibited the MKK4/JNK pathway and increased HDAC8 expression. The Akt inhibitor-induced increase in HDAC8 expression was abolished by pretreatment with proteasomal or lysosomal inhibitors. The ISO treatment increased cisplatin-induced apoptosis, which was abolished by HDAC8 knockdown. Exogenous HDAC8 expression increased cisplatin-induced apoptosis and decreased TIPRL expression, and the knockdown of TIPRL increased the apoptosis of cisplatin-treated cells. The ISO treatment decreased cisplatin-induced transcription of the TIPRL gene in a HDAC8-dependent manner. In conclusion, the Epac–Rap1–Akt pathway mediates cAMP signaling-induced inhibition of JNK-dependent HDAC8 degradation, and the resulting HDAC8 increase augments cisplatin-induced apoptosis by repressing TIPRL expression in H1299 lung cancer cells.

Exchange proteins directly activated by cAMP induce the proliferation of rat anterior pituitary GH3 cells via the activation of extracellular signal-regulated kinase

Cyclic adenosine 3'-5'-monophosphate (cAMP) plays a crucial role in regulating pituitary cell proliferation and hormone synthesis. Recent evidence suggests that exchange proteins directly activated by cAMP (Epacs) may mediate the effects of cAMP. Here we used rat anterior pituitary GH3 cells as the experiment model to demonstrate that forskolin increased the proliferation of GH3 cells and the phosphorylation of ERK1/2, and these effects were inhibited by PKA or Epac inhibitors. Epac activator 8-pCPT-2'-O-Me-cAMP increased GH3 cell proliferation and this was blocked by ESI-09, an Epac inhibitor. In contrast, forskolin-induced phosphorylation of CREB was unaffected by Epac inhibition. Notably, increased phosphorylation of ERK1/2 was correlated with increased cyclin D3 expression in GH3 cells. Furthermore, knockdown of Epac as well as B-Raf and MEK inhibitors blocked 8-pCPT-2'-O-Me-cAMP induced proliferation of GH3 cells and the phosphorylation of ERK1/2. In conclusion, our study suggests that Epac mediates cAMP induced pituitary cell proliferation via B-Raf and MAPK dependent mechanism.

The Epac-Phospholipase Cε Pathway Regulates Endocannabinoid Signaling and Cocaine-Induced Disinhibition of Ventral Tegmental Area Dopamine Neurons

Exchange protein directly activated by cAMP (Epac) is a direct effector for the ubiquitous second messenger cAMP. Epac activates the phospholipase Cε (PLCε) pathway. PLCβ has been linked to the synthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG). Here, we report that Epac facilitates endocannabinoid-mediated retrograde synaptic depression through activation of PLCε. Intracellular loading of a selective Epac agonist 8-CPT-2Me-cAMP into ventral tegmental area (VTA) dopamine neurons enabled previously ineffective stimuli to induce depolarization-induced suppression of inhibition (DSI) and long-term depression of IPSCs (I-LTD) in the VTA. DSI and I-LTD are mediated by 2-AG since they were blocked by a diacylglycerol lipase inhibitor. The effects of 8-CPT-2Me-cAMP on DSI and I-LTD were absent in Epac2 and PLCε knock-out mice, but remained intact in Epac1 knock-out mice. These results identify a novel mechanism for on-demand synthesis of retrograde signaling 2-AG by the Epac2-PLCε pathway. We investigated the functional significance of Epac2-PLCε-2-AG signaling in regulating inhibitory synaptic plasticity in VTA dopamine neurons induced by in vivo cocaine exposure. We showed that cocaine place conditioning led to a decrease in the frequency and amplitude of spontaneous IPSCs and an increase in action potential firing in wild-type mice, but not in Epac2 or PLCε knock-out mice. Together, these results indicate that the Epac2-PLCε-2-AG signaling cascade contributes to cocaine-induced disinhibition of VTA dopamine neurons.SIGNIFICANCE STATEMENT 2-arachidonoylglycerol (2-AG) is an endogenous cannabinoid that depresses synaptic transmission through stimulation of CB₁ receptors. Among the six isoforms of phospholipase C (PLC; PLCβ, PLCγ, PLCδ, PLCε, PLCζ, PLCη), only PLCβ has been linked to 2-AG synthesis. Here we demonstrate that 8-CPT-2Me-cAMP, a selective agonist of the cAMP sensor protein Epac, enhances 2-AG-mediated synaptic depression in ventral tegmental area (VTA) dopamine neurons via activation of PLCε. These results identify a novel mechanism for 2-AG synthesis via activation of the Epac-PLCε pathway. Furthermore, we show that cocaine-induced conditioned place preference and disinhibition of VTA dopamine neurons were impaired in mice lacking Epac or PLCε. Thus, the Epac-PLCε signaling pathway contributes to cocaine-induced disinhibition of VTA dopamine neurons and formation of drug-associated memories.

Functional and cardioprotective effects of simultaneous and individual activation of protein kinase A and Epac

Background and Purpose

Myocardial cAMP elevation confers cardioprotection against ischaemia/reperfusion (I/R) injury. cAMP activates two independent signalling pathways, PKA and Epac. This study investigated the cardiac effects of activating PKA and/or Epac and their involvement in cardioprotection against I/R.

Experimental Approach

Hearts from male rats were used either for determination of PKA and PKC activation or perfused in the Langendorff mode for either cardiomyocyte isolation or used to monitor functional activity at basal levels and after 30 min global ischaemia and 2 h reperfusion. Functional recovery and myocardial injury during reperfusion (LDH release and infarct size) were evaluated. Activation of PKA and/or Epac in perfused hearts was induced using cell permeable cAMP analogues in the presence or absence of inhibitors of PKA, Epac and PKC. H9C2 cells and cardiomyocytes were used to assess activation of Epac and effect on Ca²⁺ transients.

Key Results

Selective activation of either PKA or Epac was found to trigger a positive inotropic effect, which was considerably enhanced when both pathways were simultaneously activated. Only combined activation of PKA and Epac induced marked cardioprotection against I/R injury. This was accompanied by PKCε activation and repressed by inhibitors of PKA, Epac or PKC.

Conclusion and Implications

Simultaneous activation of both PKA and Epac induces an additive inotropic effect and confers optimal and marked cardioprotection against I/R injury. The latter effect is mediated by PKCε activation. This work has introduced a new therapeutic approach and targets to protect the heart against cardiac insults.

The role of Epac in the heart

As one of the most important second messengers, 3',5'-cyclic adenosine monophosphate (cAMP) mediates various extracellular signals including hormones and neurotransmitters, and induces appropriate responses in diverse types of cells. Since cAMP was formerly believed to transmit signals through only two direct target molecules, protein kinase A and the cyclic nucleotide-gated channel, the sensational discovery in 1998 of another novel direct effecter of cAMP [exchange proteins directly activated by cAMP (Epac)] attracted a great deal of scientific interest in cAMP signaling. Numerous studies on Epac have since disclosed its important functions in various tissues in the body. Recently, observations of genetically manipulated mice in various pathogenic models have begun to reveal the in vivo significance of previous in vitro or cellular-level findings. Here, we focused on the function of Epac in the heart. Accumulating evidence has revealed that both Epac1 and Epac2 play important roles in the structure and function of the heart under physiological and pathological conditions. Accordingly, developing the ability to regulate cAMP-mediated signaling through Epac may lead to remarkable new therapies for the treatment of cardiac diseases.

Long-term consumption of an obesogenic high fat diet prior to ischemia-reperfusion mediates cardioprotection via Epac1-dependent signaling

BACKGROUND

Obesity is still considered a risk factor for cardiovascular disease, although more recent knowledge also suggests obesity to be associated with reduced morbidity and mortality - the "obesity paradox". This study explores if long-term feeding of an obesogenic high fat diet renders the myocardium less susceptible to ischemic-reperfusion induced injury via Epac-dependent signaling.

METHODS

Wild type (wt), Epac1 (Epac1^-/-) and Epac2 (Epac2^-/-) deficient mice were fed a high fat (HFD) or normal chow diet (ND) for 33 ± 1 weeks. Six experimental groups were included: (1) control wt ND (wt ND), (2) control wt HFD (wt HFD), (3) Epac1^-/- mice on ND (Epac1^-/-ND), (4) Epac1^-/- mice on HFD (Epac1^-/-HFD), (5) Epac2^-/- mice on ND (Epac2^-/-ND), and (6) Epac2^-/- mice on HFD (Epac2^-/-HFD). Isolated ex vivo mice hearts were perfused in a constant pressure Langendorff mode, and exposed to 30min of global ischemia (GI) and 60min of reperfusion. Endpoints were infarct size and functional recovery.

RESULTS

All groups fed a HFD presented with significantly enhanced body weight, visceral fat content and reduced glucose clearance compared to corresponding ND groups. Although the HFD cohorts presented with an overall comparable systemic capability to clear glucose, the Epac1^-/- HFD group presented with glucose levels slightly above the human diabetes criteria at the end of the intraperitoneal glucose tolerance test (ipGTT). Moreover, the HFD significantly reduced infarct size in both wild type (wt HFD 41.3 ± 5.5% vs. wt ND 58.0 ± 9.8%, p < 0.05) and Epac2^-/- cohorts (Epac2^-/-HFD 34.4 ± 7.2% vs. Epac2^-/-ND 56.5 ± 3.8%, p < 0.05). Interestingly, however, the HFD did not reduce infarct size in Epac1^-/- deficient mice hearts (Epac1^-/-HFD 65.1 ± 5.1% vs. Epac1^-/-ND 56.1 ± 3.5%, ns.).

CONCLUSION

Epac1-dependent signaling is involved in mediating the cardioprotection afforded by long-term feeding of an obesogenic high fat diet in mice hearts.

Epac2a-null mice exhibit obesity-prone nature more susceptible to leptin resistance

BACKGROUND

The exchange protein directly activated by cAMP (Epac), which is primarily involved in cAMP signaling, has been known to be essential for controlling body energy metabolism. Epac has two isoforms: Epac1 and Epac2. The function of Epac1 on obesity was unveiled using Epac1 knockout (KO) mice. However, the role of Epac2 in obesity remains unclear.

METHODS

To evaluate the role of Epac2 in obesity, we used Epac2a KO mice, which is dominantly expressed in neurons and endocrine tissues. Physiological factors related to obesity were analyzed: body weight, fat mass, food intake, plasma leptin and adiponectin levels, energy expenditure, glucose tolerance, and insulin and leptin resistance. To determine the mechanism of Epac2a, mice received exogenous leptin and then hypothalamic leptin signaling was analyzed.

RESULTS

Epac2a KO mice appeared to have normal glucose tolerance and insulin sensitivity until 12 weeks of age, but an early onset increase of plasma leptin levels and decrease of plasma adiponectin levels compared with wild-type mice. Acute leptin injection revealed impaired hypothalamic leptin signaling in KO mice. Consistently, KO mice fed a high-fat diet (HFD) were significantly obese, presenting greater food intake and lower energy expenditure. HFD-fed KO mice were also characterized by greater impairment of hypothalamic leptin signaling and by weaker leptin-induced decrease in food consumption compared with HFD-fed wild-type mice. In wild-type mice, acute exogenous leptin injection or chronic HFD feeding tended to induce hypothalamic Epac2a expression.

CONCLUSIONS

Considering that HFD is an inducer of hypothalamic leptin resistance and that Epac2a functions in pancreatic beta cells during demands of greater work load, hypothalamic Epac2a may have a role in facilitating leptin signaling, at least in response to higher metabolic demands. Thus, our data indicate that Epac2a is critical for preventing obesity and thus Epac2a activators may be used to manage obesity and obesity-mediated metabolic disorders.

Epac1 interacts with importin β1 and controls neurite outgrowth independently of cAMP and Rap1

Exchange protein directly activated by cAMP-1 (Epac1) is a cAMP sensor that regulates multiple cellular functions including cellular migration, proliferation and differentiation. Classically, Epac1 is thought to exert its effects through binding of cAMP leading to a conformational change in Epac1 and its accumulation at the plasma membrane (PM) where it activates Rap1. In search for regulators of Epac1 activity, we show here that importin β1 (impβ1) is an Epac1 binding partner that prevents PM accumulation of Epac1. We demonstrate that in the absence of impβ1, endogenous as well as overexpressed Epac1 accumulate at the PM. Moreover, agonist-induced PM translocation of Epac1 leads to dissociation of Epac1 from impβ1. Localization of Epac1 at the PM in the absence of impβ1, requires residue R82 in its DEP domain. Notably, the PM accumulation of Epac1 in the absence of impβ1 does not require binding of cAMP to Epac1 and does not result in Rap1 activation. Functionally, PM accumulation of Epac1, an Epac1 mutant deficient in cAMP binding, or an Epac1 mutant tethered to the PM, is sufficient to inhibit neurite outgrowth. In conclusion, we uncover a cAMP-independent function of Epac1 at the PM and demonstrate that impβ1 controls subcellular localization of Epac1.

White Adipocyte Adiponectin Exocytosis Is Stimulated via β3-Adrenergic Signaling and Activation of Epac1: Catecholamine Resistance in Obesity and Type 2 Diabetes

We investigated the physiological regulation of adiponectin exocytosis in health and metabolic disease by a combination of membrane capacitance patch-clamp recordings and biochemical measurements of short-term (30-min incubations) adiponectin secretion. Epinephrine or the β₃-adrenergic receptor (AR) agonist CL 316,243 (CL) stimulated adiponectin exocytosis/secretion in cultured 3T3-L1 and in primary subcutaneous mouse adipocytes, and the stimulation was inhibited by the Epac (Exchange Protein directly Activated by cAMP) antagonist ESI-09. The β₃AR was highly expressed in cultured and primary adipocytes, whereas other ARs were detected at lower levels. 3T3-L1 and primary adipocytes expressed Epac1, whereas Epac2 was undetectable. Adiponectin secretion could not be stimulated by epinephrine or CL in adipocytes isolated from obese/type 2 diabetic mice, whereas the basal (unstimulated) adiponectin release level was elevated twofold. Gene expression of β₃AR and Epac1 was reduced in adipocytes from obese animals, and corresponded to a respective ∼35% and ∼30% reduction at the protein level. Small interfering RNA-mediated knockdown of β₃AR (∼60%) and Epac1 (∼50%) was associated with abrogated catecholamine-stimulated adiponectin secretion. We propose that adiponectin exocytosis is stimulated via adrenergic signaling pathways mainly involving β₃ARs. We further suggest that adrenergically stimulated adiponectin secretion is disturbed in obesity/type 2 diabetes as a result of the reduced expression of β₃ARs and Epac1 in a state we define as "catecholamine resistance."