Differential signaling of cyclic AMP: opposing effects of exchange protein directly activated by cyclic AMP and cAMP-dependent protein kinase on protein kinase B activation

Protein SUMOylation promotes cAMP-independent EPAC1 activation

Protein SUMOylation is a prevalent stress-response posttranslational modification crucial for maintaining cellular homeostasis. Herein, we report that protein SUMOylation modulates cellular signaling mediated by cAMP, an ancient and universal stress-response second messenger. We identify K561 as a primary SUMOylation site in exchange protein directly activated by cAMP (EPAC1) via site-specific mapping of SUMOylation using mass spectrometry. Sequence and site-directed mutagenesis analyses reveal that a functional SUMO-interacting motif in EPAC1 is required for the binding of SUMO-conjugating enzyme UBC9, formation of EPAC1 nuclear condensate, and EPAC1 cellular SUMOylation. Heat shock-induced SUMO modification of EPAC1 promotes Rap1/2 activation in a cAMP-independent manner. Structural modeling and molecular dynamics simulation studies demonstrate that SUMO substituent on K561 of EPAC1 promotes Rap1 interaction by increasing the buried surface area between the SUMOylated receptor and its effector. Our studies identify a functional SUMOylation site in EPAC1 and unveil a novel mechanism in which SUMOylation of EPAC1 leads to its autonomous activation. The findings of SUMOylation-mediated activation of EPAC1 not only provide new insights into our understanding of cellular regulation of EPAC1 but also will open up a new field of experimentation concerning the cross-talk between cAMP/EPAC1 signaling and protein SUMOylation, two major cellular stress response pathways, during cellular homeostasis.

Increased cAMP-PKA signaling pathway activation is involved in up-regulation of CTLA-4 expression in CD4+ T cells in acute SIVmac239-infected Chinese rhesus macaques

Human immunodeficiency virus-1 (HIV-1) infection can cause chronic activation, exhaustion, and anergy of the immune system. Cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) is an immune checkpoint molecule, which plays an important role in immune homeostasis and disease. CTLA-4 expression is elevated in HIV-1-infected patients and is associated with disease progression. However, the mechanism controlling expression of CTLA-4 in HIV-1 infection is poorly characterized. In this study, we used a SIV-infected Chinese rhesus macaque (ChRM) model to explore CTLA-4 expression in SIV infection. Results showed that SIV infection significantly increased CTLA-4 expression in all T cell subsets, especially central memory T cells. CTLA-4+CD4+ T cell frequency was significantly associated with disease progression markers. Activation of the cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) signaling pathway regulated CTLA-4 expression in CD4+T cells, as confirmed by stimulation with dibutyryl cyclic adenosine monophosphate, forskolin, and 3-isobutyl-1-methylxanthine, and inhibition with H-89 ex vivo. Simultaneously, cAMP concentration in PBMCs and PKA activity in both PBMCs and CD4+ T cells were increased in acute SIV-infected ChRMs, accompanied by an increase in adenylate cyclase 6 expression and a decrease in cAMP-phosphodiesterase 3A (PDE3A), PDE4B, and PDE5A expression in PBMCs. In addition, selective inhibition of PDE4B and PDE5A activity enhanced CTLA-4 expression in CD4+ T cells. These results suggest that SIV infection alters cAMP metabolism and increases cAMP-PKA signaling pathway activation, which up-regulates the expression of CTLA-4 in acute SIVmac239-infected ChRMs. Thus, regulation of the cAMP-PKA signaling pathway may be a potential strategy for the restoration of T cell function and therapy for AIDS.

Protein SUMOylation promotes cAMP-independent EPAC1 activation

Exchange protein directly activated by cAMP (EPAC1) mediates the intracellular functions of a critical stress-response second messenger, cAMP. Herein, we report that EPAC1 is a cellular substrate of protein SUMOylation, a prevalent stress-response posttranslational modification. Site-specific mapping of SUMOylation by mass spectrometer leads to identifying K561 as a primary SUMOylation site in EPAC1. Sequence and site-directed mutagenesis analyses reveal a functional SUMO-interacting motif required for cellular SUMOylation of EPAC1. SUMO modification of EPAC1 mediates its heat shock-induced Rap1/2 activation in a cAMP-independent manner. Structural modeling and molecular dynamics simulation studies demonstrate that SUMO substituent on K561 of EPAC1 promotes Rap1 interaction by increasing the buried surface area between the SUMOylated receptor and its effector. Our studies identify a functional SUMOylation site in EPAC1 and unveil a novel mechanism in which SUMOylation of EPAC1 leads to its autonomous activation. The findings of SUMOylation-mediated activation of EPAC1 not only provide new insights into our understanding of cellular regulation of EPAC1 but also will open up a new field of experimentation concerning the cross-talk between cAMP/EPAC1 signaling and protein SUMOylation, two major cellular stress response pathways, during cellular homeostasis.

Opposite regulation of glycogen metabolism by cAMP produced in the cytosol and at the plasma membrane

Cyclic AMP is produced in cells by two different types of adenylyl cyclases: at the plasma membrane by the transmembrane adenylyl cyclases (tmACs, ADCY1~ADCY9) and in the cytosol by the evolutionarily more conserved soluble adenylyl cyclase (sAC, ADCY10). By employing high-resolution extracellular flux analysis in HepG2 cells to study glycogen breakdown in real time, we showed that cAMP regulates glycogen metabolism in opposite directions depending on its location of synthesis within cells and the downstream cAMP effectors. While the canonical tmAC-cAMP-PKA signaling promotes glycogenolysis, we demonstrate here that the non-canonical sAC-cAMP-Epac1 signaling suppresses glycogenolysis. Mechanistically, suppression of sAC-cAMP-Epac1 leads to Ser-15 phosphorylation and thereby activation of the liver-form glycogen phosphorylase to promote glycogenolysis. Our findings highlight the importance of cAMP microdomain organization for distinct metabolic regulation and establish sAC as a novel regulator of glycogen metabolism.

Modulation of fast sodium current in airway smooth muscle cells by exchange protein directly activated by cAMP

Airway smooth muscle (ASM) cells from mouse bronchus express a fast sodium current mediated by NaV1.7. We present evidence that this current is regulated by cAMP. ASM cells were isolated by enzymatic dispersal and studied using the whole cell patch clamp technique at room temperature. A fast sodium current, INa, was observed on holding cells under voltage clamp at -100 mV and stepping to -20 mV. This current was reduced in a concentration-dependent manner by denopamine (10 and 30 µM), a β-adrenergic agonist. Forskolin (1 µM), an activator of adenylate cyclase, reduced the current by 35%, but 6-MB-cAMP (300 µM), an activator of protein kinase A (PKA), had no effect. In contrast, 8-pCPT-2-O-Me-cAMP-AM (007-AM, 10 µM), an activator of exchange protein directly activated by cAMP (Epac), reduced the current by 48%. The inhibitory effect of 007-AM was still observed in the presence of dantrolene (10 µM), an inhibitor of ryanodine receptors, and when cytosolic [Ca2+] was buffered by inclusion of 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, Sigma (BAPTA) (50 µM) in the pipette solution, suggesting that the inhibition of INa was not due to Ca2+-release from intracellular stores. When 007-AM was tested on the current-voltage relationship, it reduced the current at potentials from -30 to 0 mV, but had no effect on the steady-state activation curve. However, the steady-state inactivation V1/2, the voltage causing inactivation of 50% of the current, was shifted in the negative direction from -76.6 mV to -89.7 mV. These findings suggest that cAMP regulates INa in mouse ASM via Epac, but not PKA.NEW & NOTEWORTHY β-adrenergic agonists are commonly used in inhalers to treat asthma and chronic obstructive pulmonary disease. These work by causing bronchodilation and reducing inflammation. The present study provides evidence that these drugs have an additional action, namely, to reduce sodium influx into airway smooth muscle cells via fast voltage-dependent channels. This may have the dual effect of promoting bronchodilation and reducing remodeling of the airways, which has a detrimental effect in these diseases.

Resveratrol directly suppresses proteolysis possibly via PKA/CREB signaling in denervated rat skeletal muscle

Although there are reports that polyphenol resveratrol (Rsv) may cause muscle hypertrophy in basal conditions and attenuate muscle wasting in catabolic situations, its mechanism of action is still unclear. Our study evaluated the ex vivo effects of Rsv on protein metabolism and intracellular signaling in innervated (sham-operated; Sham) and 3-day sciatic denervated (Den) rat skeletal muscles. Rsv (10-4 M) reduced total proteolysis (40%) in sham muscles. Den increased total proteolysis (~40%) in muscle, which was accompanied by an increase in the activities of ubiquitin-proteasome (~3-fold) and lysosomal (100%) proteolytic systems. Rsv reduced total proteolysis (59%) in Den muscles by inhibiting the hyperactivation of ubiquitin-proteasome (50%) and lysosomal (~70%) systems. Neither Rsv nor Den altered calcium-dependent proteolysis in muscles. Mechanistically, Rsv stimulated PKA/CREB signaling in Den muscles, and PKA blockage by H89 (50μM) abolished the antiproteolytic action of the polyphenol. Rsv reduced FoxO4 phosphorylation (~60%) in both Sham and Den muscles and Akt phosphorylation (36%) in Den muscles. Rsv also caused a homeostatic effect in Den muscles by returning their protein synthesis rates to levels similar to Sham muscles. These data indicate that Rsv directly inhibits the proteolytic activity of lysosomal and ubiquitin-proteasome systems, mainly in Den muscles through, at least in part, the activation of PKA/CREB signaling.

Human RAMP1 overexpressing mice are resistant to migraine therapies for motion sensitivity: a mouse model of vestibular migraine

Both enhanced motion-induced nausea and increased static imbalance are \observed symptoms in migraine and especially vestibular migraine (VM). Motion-induced nausea and static imbalance were investigated in a mouse model, nestin/hRAMP1, expressing elevated levels of human RAMP1 in the CNS, which enhances CGRP signaling in the nervous system. Behavioral surrogates such as the motion-induced thermoregulation and postural sway center of pressure (CoP) assays were used to assess motion sensitivity. Tail vasodilation analysis revealed that this model exhibits an increased sensitivity to CGRP's effects at lower doses compared to control mice. In addition, the nestin/hRAMP1 mice exhibit a higher dynamic range in postural sway than their wildtype counterparts, along with increased sway observed in nestin/hRAMP1 male mice that was not present in male littermate controls. Results from migraine blocker experiments were challenging to interpret, but the data suggests that olcegepant is incapable of reversing CGRP-induced alterations in the nestin/hRAMP1 mice, while rizatriptan was ineffective in both the nestin/hRAMP1 and control mice. The results indicate that overexpression of hRAMP1 leads to heightened endogenous CGRP signaling. Results also suggest that both olcegepant and rizatriptan are ineffective in reducing CGRP-triggered nausea and sway in this hypersensitive CGRP mouse model. This study suggests that hypersensitivity to CGRP may be a mouse model for difficult to treat cases of vestibular migraine.

cAMP/PKA signaling promotes AKT deactivation by reducing CIP2A expression, thereby facilitating decidualization

cAMP signaling is widely known to be indispensable for decidualization, but the details are not fully understood. Here, we show that cAMP signaling promotes AKT deactivation in endometrial stromal cells, which favors their decidualization. The deactivation of AKT is found to be a consequence of the reduced expression of several inhibitors of PP2A, the major phosphatase of AKT, with CIP2A being the most prominent. CIP2A reduction is obligatory for decidualization, as persistent CIP2A expression impairs chromatin remodeling and the expression of several decidualization markers (IGFBP1, PRL, MAOA, and IL-15). Furthermore, analyses of the responsiveness of the CIP2A promoter to cAMP signaling suggest the ETS family to be a bridge between cAMP signaling and CIP2A reduction. Our results provide novel insights into the role of cAMP signaling in decidualization and might benefit the development of novel therapies for decidualization deficiency, AKT-driven tumors, and the reverse, insulin resistance.

cAMP-EPAC-PKCε-RIM1α signaling regulates presynaptic long-term potentiation and motor learning

The cerebellum is involved in learning of fine motor skills, yet whether presynaptic plasticity contributes to such learning remains elusive. Here, we report that the EPAC-PKCε module has a critical role in a presynaptic form of long-term potentiation in the cerebellum and motor behavior in mice. Presynaptic cAMP-EPAC-PKCε signaling cascade induces a previously unidentified threonine phosphorylation of RIM1α, and thereby initiates the assembly of the Rab3A-RIM1α-Munc13-1 tripartite complex that facilitates docking and release of synaptic vesicles. Granule cell-specific blocking of EPAC-PKCε signaling abolishes presynaptic long-term potentiation at the parallel fiber to Purkinje cell synapses and impairs basic performance and learning of cerebellar motor behavior. These results unveil a functional relevance of presynaptic plasticity that is regulated through a novel signaling cascade, thereby enriching the spectrum of cerebellar learning mechanisms.

Epac as a tractable therapeutic target

In 1957, cyclic adenosine monophosphate (cAMP) was identified as the first secondary messenger, and the first signaling cascade discovered was the cAMP-protein kinase A (PKA) pathway. Since then, cAMP has received increasing attention given its multitude of actions. Not long ago, a new cAMP effector named exchange protein directly activated by cAMP (Epac) emerged as a critical mediator of cAMP's actions. Epac mediates a plethora of pathophysiologic processes and contributes to the pathogenesis of several diseases such as cancer, cardiovascular disease, diabetes, lung fibrosis, neurological disorders, and others. These findings strongly underscore the potential of Epac as a tractable therapeutic target. In this context, Epac modulators seem to possess unique characteristics and advantages and hold the promise of providing more efficacious treatments for a wide array of diseases. This paper provides an in-depth dissection and analysis of Epac structure, distribution, subcellular compartmentalization, and signaling mechanisms. We elaborate on how these characteristics can be utilized to design specific, efficient, and safe Epac agonists and antagonists that can be incorporated into future pharmacotherapeutics. In addition, we provide a detailed portfolio for specific Epac modulators highlighting their discovery, advantages, potential concerns, and utilization in the context of clinical disease entities.

Direct reprogramming of cardiomyocytes into cardiac Purkinje-like cells

Currently, there are no treatments that ameliorate cardiac cell death, the underlying basis of cardiovascular disease. An unexplored cell type in cardiac regeneration is cardiac Purkinje cells; specialized cells from the cardiac conduction system (CCS) responsible for propagating electrical signals. Purkinje cells have tremendous potential as a regenerative treatment because they may intrinsically integrate with the CCS of a recipient myocardium, resulting in more efficient electrical conduction in diseased hearts. This study is the first to demonstrate an effective protocol for the direct reprogramming of human cardiomyocytes into cardiac Purkinje-like cells using small molecules. The cells generated were genetically and functionally similar to native cardiac Purkinje cells, where expression of key cardiac Purkinje genes such as CNTN2, ETV1, PCP4, IRX3, SCN5a, HCN2 and the conduction of electrical signals with increased velocity was observed. This study may help to advance the quest to finding an optimized cell therapy for heart regeneration.

Epac1 activation by cAMP regulates cellular SUMOylation and promotes the formation of biomolecular condensates

Protein SUMOylation plays an essential role in maintaining cellular homeostasis when cells are under stress. However, precisely how SUMOylation is regulated, and a molecular mechanism linking cellular stress to SUMOylation, remains elusive. Here, we report that cAMP, a major stress-response second messenger, acts through Epac1 as a regulator of cellular SUMOylation. The Epac1-associated proteome is highly enriched with components of the SUMOylation pathway. Activation of Epac1 by intracellular cAMP triggers phase separation and the formation of nuclear condensates containing Epac1 and general components of the SUMOylation machinery to promote cellular SUMOylation. Furthermore, genetic knockout of Epac1 obliterates oxidized low-density lipoprotein-induced cellular SUMOylation in macrophages, leading to suppression of foam cell formation. These results provide a direct nexus connecting two major cellular stress responses to define a molecular mechanism in which cAMP regulates the dynamics of cellular condensates to modulate protein SUMOylation.

Urocortin 2 promotes hypertrophy and enhances skeletal muscle function through cAMP and insulin/IGF-1 signaling pathways

Objective

Although it is well established that urocortin 2 (Ucn2), a peptide member of the corticotrophin releasing factor (CRF) family, and its specific corticotrophin-releasing factor 2 receptor (CRF2R) are highly expressed in skeletal muscle, the role of this peptide in the regulation of skeletal muscle mass and protein metabolism remains elusive.

Methods

To elucidate the mechanisms how Ucn2 directly controls protein metabolism in skeletal muscles of normal mice, we carried out genetic tools, physiological and molecular analyses of muscles in vivo and in vitro.

Results

Here, we demonstrated that Ucn2 overexpression activated cAMP signaling and promoted an expressive muscle hypertrophy associated with higher rates of protein synthesis and activation of Akt/mTOR and ERK1/2 signaling pathways. Furthermore, Ucn2 induced a decrease in mRNA levels of atrogin-1 and in autophagic flux inferred by an increase in the protein content of LC3-I, LC3-II and p62. Accordingly, Ucn2 reduced both the transcriptional activity of FoxO in vivo and the overall protein degradation in vitro through an inhibition of lysosomal proteolytic activity. In addition, we demonstrated that Ucn2 induced a fast-to-slow fiber type shift and improved fatigue muscle resistance, an effect that was completely blocked in muscles co-transfected with mitogen-activated protein kinase phosphatase 1 (MKP-1), but not with dominant-negative Akt mutant (Aktmt).

Conclusions

These data suggest that Ucn2 triggers an anabolic and anti-catabolic response in skeletal muscle of normal mice probably through the activation of cAMP cascade and participation of Akt and ERK1/2 signaling. These findings open new perspectives in the development of therapeutic strategies to cope with the loss of muscle mass.

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Ucn2 overexpression promotes muscle growth due to an increase in protein synthesis.

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Ucn2 inhibits FoxO activity and autophagic-lysosomal system.

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Ucn2-induced skeletal muscle phenotype is dependent on Akt and ERK1/2.

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Ucn2 induces a fast-to-slow fiber type shift and improves fatigue resistance.

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The increase in muscle fatigue resistance is dependent on ERK1/2.

Epac1-/- and Epac2-/- mice exhibit deficient epithelial Na+ channel regulation and impaired urinary Na+ conservation

Exchange proteins directly activated by cAMP (Epacs) are abundantly expressed in the renal tubules. We used genetic and pharmacological tools in combination with balance, electrophysiological, and biochemical approaches to examine the role of Epac1 and Epac2 in renal sodium handling. We demonstrate that Epac1^–/– and Epac2^–/– mice exhibit a delayed anti-natriuresis to dietary sodium restriction despite augmented aldosterone levels. This was associated with a significantly lower response to the epithelial Na⁺ channel (ENaC) blocker amiloride, reduced ENaC activity in split-opened collecting ducts, and defective posttranslational processing of α and γENaC subunits in the KO mice fed with a Na⁺-deficient diet. Concomitant deletion of both isoforms led to a marginally greater natriuresis but further increased aldosterone levels. Epac2 blocker ESI-05 and Epac1&2 blocker ESI-09 decreased ENaC activity in Epac WT mice kept on the Na⁺-deficient diet but not on the regular diet. ESI-09 injections led to natriuresis in Epac WT mice on the Na⁺-deficient diet, which was caused by ENaC inhibition. In summary, our results demonstrate similar but nonredundant actions of Epac1 and Epac2 in stimulation of ENaC activity during variations in dietary salt intake. We speculate that inhibition of Epac signaling could be instrumental in treatment of hypertensive states associated with ENaC overactivation.

Intracellular receptor EPAC regulates von Willebrand factor secretion from endothelial cells in a PI3K-/eNOS-dependent manner during inflammation

Coagulopathy is associated with both inflammation and infection, including infections with novel severe acute respiratory syndrome coronavirus-2, the causative agent Coagulopathy is associated with both inflammation and infection, including infection with novel severe acute respiratory syndrome coronavirus-2, the causative agent of COVID-19. Clot formation is promoted via cAMP-mediated secretion of von Willebrand factor (vWF), which fine-tunes the process of hemostasis. The exchange protein directly activated by cAMP (EPAC) is a ubiquitously expressed intracellular cAMP receptor that plays a regulatory role in suppressing inflammation. To assess whether EPAC could regulate vWF release during inflammation, we utilized our EPAC1-null mouse model and revealed increased secretion of vWF in endotoxemic mice in the absence of the EPAC1 gene. Pharmacological inhibition of EPAC1 in vitro mimicked the EPAC1-/- phenotype. In addition, EPAC1 regulated tumor necrosis factor-α-triggered vWF secretion from human umbilical vein endothelial cells in a manner dependent upon inflammatory effector molecules PI3K and endothelial nitric oxide synthase. Furthermore, EPAC1 activation reduced inflammation-triggered vWF release, both in vivo and in vitro. Our data delineate a novel regulatory role for EPAC1 in vWF secretion and shed light on the potential development of new strategies to control thrombosis during inflammation.

Origin and Isoform Specific Functions of Exchange Proteins Directly Activated by cAMP: A Phylogenetic Analysis

Exchange proteins directly activated by cAMP (EPAC1 and EPAC2) are one of the several families of cellular effectors of the prototypical second messenger cAMP. To understand the origin and molecular evolution of EPAC proteins, we performed a comprehensive phylogenetic analysis of EPAC1 and EPAC2. Our study demonstrates that unlike its cousin PKA, EPAC proteins are only present in multicellular Metazoa. Within the EPAC family, EPAC1 is only associated with chordates, while EPAC2 spans the entire animal kingdom. Despite a much more contemporary origin, EPAC1 proteins show much more sequence diversity among species, suggesting that EPAC1 has undergone more selection and evolved faster than EPAC2. Phylogenetic analyses of the individual cAMP binding domain (CBD) and guanine nucleotide exchange (GEF) domain of EPACs, two most conserved regions between the two isoforms, further reveal that EPAC1 and EPAC2 are closely clustered together within both the larger cyclic nucleotide receptor and RAPGEF families. These results support the notion that EPAC1 and EPAC2 share a common ancestor resulting from a fusion between the CBD of PKA and the GEF from RAPGEF1. On the other hand, the two terminal extremities and the RAS-association (RA) domains show the most sequence diversity between the two isoforms. Sequence diversities within these regions contribute significantly to the isoform-specific functions of EPACs. Importantly, unique isoform-specific sequence motifs within the RA domain have been identified.

GPCR-mediated YAP/TAZ inactivation in fibroblasts via EPAC1/2, RAP2C, and MAP4K7

Yes-associated protein (YAP) and PDZ-binding motif (TAZ) have emerged as important regulators of pathologic fibroblast activation in fibrotic diseases. Agonism of Gαs-coupled G protein coupled receptors (GPCRs) provides an attractive approach to inhibit the nuclear localization and function of YAP and TAZ in fibroblasts that inhibits or reverses their pathological activation. Agonism of the dopamine D1 GPCR has proven effective in preclinical models of lung and liver fibrosis. However, the molecular mechanisms coupling GPCR agonism to YAP and TAZ inactivation in fibroblasts remain incompletely understood. Here, using human lung fibroblasts, we identify critical roles for the cAMP effectors EPAC1/2, the small GTPase RAP2c, and the serine/threonine kinase MAP4K7 as the essential elements in the downstream signaling cascade linking GPCR agonism to LATS1/2-mediated YAP and TAZ phosphorylation and nuclear exclusion in fibroblasts. We further show that this EPAC/RAP2c/MAP4K7 signaling cascade is essential to the effects of dopamine D1 receptor agonism on reducing fibroblast proliferation, contraction, and extracellular matrix production. Targeted modulation of this cascade in fibroblasts may prove a useful strategy to regulate YAP and TAZ signaling and fibroblast activities central to tissue repair and fibrosis.

Glucagon transiently stimulates mTORC1 by activation of an EPAC/Rap1 signaling axis

Activation of the protein kinase mechanistic target of rapamycin (mTOR) in both complexes 1 and 2 (mTORC1/2) in the liver is repressed during fasting and rapidly stimulated in response to a meal. The effect of feeding on hepatic mTORC1/2 is attributed to an increase in plasma levels of nutrients, such as amino acids, and insulin. By contrast, fasting is associated with elevated plasma levels of glucagon, which is conventionally viewed as having a counter-regulatory role to insulin. More recently an expanded role for glucagon action in post-prandial metabolism has been demonstrated. Herein we investigated the impact of insulin and glucagon on mTORC1/2 activation. In H4IIE and HepG2 cultures, insulin enhanced phosphorylation of the mTORC1 substrates S6K1 and 4E-BP1. Surprisingly, the effect of glucagon on mTORC1 was biphasic, wherein there was an acute increase in phosphorylation of S6K1 and 4E-BP1 over the first hour of exposure, followed by latent suppression. The transient stimulatory effect of glucagon on mTORC1 was not additive with insulin, suggesting convergent signaling. Glucagon enhanced cAMP levels and mTORC1 stimulation required activation of the glucagon receptor, PI3K/Akt, and exchange protein activated by cAMP (EPAC). EPAC acts as the guanine nucleotide exchange factor for the small GTPase Rap1. Rap1 expression enhanced S6K1 phosphorylation and glucagon addition to culture medium promoted Rap1-GTP loading. Signaling through mTORC1 acts to regulate protein synthesis and we found that glucagon promoted an EPAC-dependent increase in protein synthesis. Overall, the findings support that glucagon elicits acute activation of mTORC1/2 by an EPAC-dependent increase in Rap1-GTP.

Epac1 (Exchange Protein Directly Activated by cAMP 1) Upregulates LOX-1 (Oxidized Low-Density Lipoprotein Receptor 1) to Promote Foam Cell Formation and Atherosclerosis Development

OBJECTIVE

The cAMP second messenger system, a major stress-response pathway, plays essential roles in normal cardiovascular functions and in pathogenesis of heart diseases. Here, we test the hypothesis that the Epac1 (exchange protein directly activated by cAMP 1) acts as a major downstream effector of cAMP signaling to promote atherogenesis and represents a novel therapeutic target. Approach and Results: To ascertain Epac1's function in atherosclerosis development, a triple knockout mouse model (LTe) was generated by crossing Epac1^-/- mice with atherosclerosis-prone LDb mice lacking both Ldlr and Apobec1. Deletion of Epac1 led to a significant reduction of atherosclerotic lesion formation as measured by postmortem staining, accompanied by attenuated macrophage/foam cell infiltrations within atherosclerotic plaques as determined by immunofluorescence staining in LTe animals compared with LDb littermates. Primary bone marrow-derived macrophages were isolated from Epac1-null and wild-type mice to investigate the role of Epac1 in lipid uptake and foam cell formation. ox-LDLs (oxidized low-density lipoproteins) stimulation of bone marrow-derived macrophages led to elevated intracellular cAMP and Epac1 levels, whereas an Epac-specific agonist, increased lipid accumulation in wild-type, but not Epac1-null, bone marrow-derived macrophages. Mechanistically, Epac1 acts through PKC (protein kinase C) to upregulate LOX-1 (ox-LDL receptor 1), a major scavenger receptor for ox-LDL uptake, exerting a feedforward mechanism with ox-LDL to increase lipid uptake and propel foam cell formation and atherogenesis.

CONCLUSIONS

Our study demonstrates a fundamental role of cAMP/Epac1 signaling in vascular remodeling by promoting ox-LDL uptake and foam cell formation during atherosclerosis lesion development. Therefore, Epac1 represents a promising, unexplored therapeutic target for atherosclerosis.

Inhibition of type 4 cAMP-phosphodiesterases (PDE4s) in mice induces hypothermia via effects on behavioral and central autonomous thermoregulation

Inhibitors of Type 4 cAMP-phosphodiesterases (PDE4s) exert a number of promising therapeutic benefits, including potent anti-inflammatory, memory- and cognition-enhancing, metabolic, and antineoplastic effects. We report here that treatment with a number of distinct PDE4 inhibitors, including Rolipram, Piclamilast, Roflumilast and RS25344, but not treatment with the PDE3-selective inhibitor Cilostamide, induces a rapid (10-30 min), substantial (-5 °C) and long-lasting (up to 5 h) decrease in core body temperature of C57BL/6 mice; thus, identifying a critical role of PDE4 also in the regulation of body temperature. As little as 0.04 mg/kg of the archetypal PDE4 inhibitor Rolipram induces hypothermia. As similar or higher doses of Rolipram were used in a majority of published animal studies, most of the reported findings are likely paralleled by, or potentially impacted by hypothermia induced by these drugs. We further show that PDE4 inhibition affects central body temperature regulation and acts by lowering the cold-defense balance point of behavioral (including posture and locomotion) and autonomous (including cutaneous tail vasodilation) cold-defense mechanisms. In line with the idea of an effect on central body temperature regulation, hypothermia is induced by moderate doses of various brain-penetrant PDE4 inhibitors, but not by similar doses of YM976, a PDE4 inhibitor that does not efficiently cross the blood-brain barrier. Finally, to begin delineating the mechanism of drug-induced hypothermia, we show that blockade of D2/3-type dopaminergic, but not β-adrenergic, H1-histaminergic or opiate receptors, can alleviate PDE4 inhibitor-induced hypothermia. We thus propose that increased D2/3-type dopaminergic signaling, triggered by PDE4 inhibitor-induced and cAMP-mediated dopamine release in the thermoregulatory centers of the hypothalamus, is a significant contributor to PDE4 inhibitor-induced hypothermia.

EPAC regulates von Willebrand factor secretion from endothelial cells in a PI3K/eNOS-dependent manner during inflammation

Coagulopathy is associated with both inflammation and infection, including infection with the novel SARS-CoV-2 (COVID-19). Endothelial cells (ECs) fine tune hemostasis via cAMP-mediated secretion of von Willebrand factor (vWF), which promote the process of clot formation. The e xchange p rotein directly a ctivated by c AMP (EPAC) is a ubiquitously expressed intracellular cAMP receptor that plays a key role in stabilizing ECs and suppressing inflammation. To assess whether EPAC could regulate vWF release during inflammation, we utilized our EPAC1 -null mouse model and revealed an increased secretion of vWF in endotoxemic mice in the absence of the EPAC1 gene. Pharmacological inhibition of EPAC1 in vitro mimicked the EPAC1 -/- phenotype. EPAC1 regulated TNFα-triggered vWF secretion from human umbilical vein endothelial cells (HUVECs) in a phosphoinositide 3-kinases (PI3K)/endothelial nitric oxide synthase (eNOS)-dependent manner. Furthermore, EPAC1 activation reduced inflammation-triggered vWF release, both in vivo and in vitro . Our data delineate a novel regulatory role of EPAC1 in vWF secretion and shed light on potential development of new strategies to controlling thrombosis during inflammation.

Key Point

PI3K/eNOS pathway-mediated, inflammation-triggered vWF secretion is the target of the pharmacological manipulation of the cAMP-EPAC system.

Stimulation of GLP-1 Receptor Inhibits Methylglyoxal-Induced Mitochondrial Dysfunctions in H9c2 Cardiomyoblasts: Potential Role of Epac/PI3K/Akt Pathway

Accumulation of methylglyoxal (MG) contributes to oxidative stress, apoptosis, and mitochondrial dysfunction, leading to the development of type 2 diabetes and cardiovascular diseases. Inhibition of mitochondrial abnormalities induced by MG in the heart may improve and delay the progression of heart failure. Although glucagon-like peptide-1 receptor (GLP-1R) agonists have been used as anti-diabetic drugs and GLP-1R has been detected in the heart, the cardioprotective effects of GLP-1R agonists on the inhibition of MG-induced oxidative stress and mitochondrial abnormalities have not been elucidated. Stimulation of GLP-1Rs leads to cAMP elevation and subsequently activates PKA- and/or Epac-dependent signaling pathway. However, the signaling pathway involved in the prevention of MG-induced mitochondrial dysfunctions in the heart has not been clarified so far. In the present study, we demonstrated that stimulation of GLP-1Rs with exendin-4 inhibited MG-induced intracellular and mitochondrial reactive oxygen species (ROS) production and apoptosis in H9c2 cardiomyoblasts. GLP-1R stimulation also improved the alterations of mitochondrial membrane potential (MMP) and expressions of genes related to mitochondrial functions and dynamics induced by MG. In addition, stimulation of GLP-1R exhibits antioxidant and antiapoptotic effects as well as the improvement of mitochondrial functions through cAMP/Epac/PI3K/Akt signaling pathway in H9c2 cells. Our study is the first work demonstrating a novel signaling pathway for cardioprotective effects of GLP-1R agonist on inhibition of oxidative stress and prevention of mitochondrial dysfunction. Thus, GLP-1R agonist represents a potential therapeutic target for inhibition of oxidative stress and modulation of mitochondrial functions in the heart.

Depletion of Ric-8B leads to reduced mTORC2 activity

mTOR, a serine/threonine protein kinase that is involved in a series of critical cellular processes, can be found in two functionally distinct complexes, mTORC1 and mTORC2. In contrast to mTORC1, little is known about the mechanisms that regulate mTORC2. Here we show that mTORC2 activity is reduced in mice with a hypomorphic mutation of the Ric-8B gene. Ric-8B is a highly conserved protein that acts as a non-canonical guanine nucleotide exchange factor (GEF) for heterotrimeric Gαs/olf type subunits. We found that Ric-8B hypomorph embryos are smaller than their wild type littermates, fail to close the neural tube in the cephalic region and die during mid-embryogenesis. Comparative transcriptome analysis revealed that signaling pathways involving GPCRs and G proteins are dysregulated in the Ric-8B mutant embryos. Interestingly, this analysis also revealed an unexpected impairment of the mTOR signaling pathway. Phosphorylation of Akt at Ser473 is downregulated in the Ric-8B mutant embryos, indicating a decreased activity of mTORC2. Knockdown of the endogenous Ric-8B gene in cultured cell lines leads to reduced phosphorylation levels of Akt (Ser473), further supporting the involvement of Ric-8B in mTORC2 activity. Our results reveal a crucial role for Ric-8B in development and provide novel insights into the signals that regulate mTORC2.

Gene inactivation in mice can be used to identify genes that are involved in important biological processes and that may contribute to disease. We used this approach to study the Ric-8B gene, which is highly conserved in mammals, including humans. We found that Ric-8B is essential for embryogenesis and for the proper development of the nervous system. Ric-8B mutant mouse embryos are smaller than their wild type littermates and show neural tube defects at the cranial region. This approach also allowed us to identify the biological pathways that potentially contribute to the observed phenotypes, and uncover a novel role for Ric-8B in the mTORC2 signaling pathway. mTORC2 plays particular important roles in the adult brain, and has been implicated in neurological disorders. Our mutant mice provide a model to study the complex molecular and cellular processes underlying the interplay between Ric-8B and mTORC2 in neuronal function.

Sympathetic innervation suppresses the autophagic-lysosomal system in brown adipose tissue under basal and cold-stimulated conditions

The sympathetic nervous system (SNS) activates cAMP signaling and promotes trophic effects on brown adipose tissue (BAT) through poorly understood mechanisms. Because norepinephrine has been found to induce antiproteolytic effects on muscle and heart, we hypothesized that the SNS could inhibit autophagy in interscapular BAT (IBAT). Here, we describe that selective sympathetic denervation of rat IBAT kept at 25°C induced atrophy, and in parallel dephosphorylated forkhead box class O (FoxO), and increased cathepsin activity, autophagic flux, autophagosome formation, and expression of autophagy-related genes. Conversely, cold stimulus (4°C) for up to 72 h induced thermogenesis and IBAT hypertrophy, an anabolic effect that was associated with inhibition of cathepsin activity, autophagic flux, and autophagosome formation. These effects were abrogated by sympathetic denervation, which also upregulated Gabarapl1 mRNA. In addition, the cold-driven sympathetic activation stimulated the mechanistic target of rapamycin (mTOR) pathway, leading to the enhancement of protein synthesis, evaluated in vivo by puromycin incorporation, and to the inhibitory phosphorylation of Unc51-like kinase-1, a key protein in the initiation of autophagy. This coincided with a higher content of exchange protein-1 directly activated by cAMP (Epac1), a cAMP effector, and phosphorylation of Akt at Thr³⁰⁸, all these effects being abolished by denervation. Systemic treatment with norepinephrine for 72 h mimicked most of the cold effects on IBAT. These data suggest that the noradrenergic sympathetic inputs to IBAT restrain basal autophagy via suppression of FoxO and, in the setting of cold, stimulate protein synthesis via the Epac/Akt/mTOR-dependent pathway and suppress the autophagosome formation, probably through posttranscriptional mechanisms.NEW & NOTEWORTHY The underlying mechanisms related to the anabolic role of sympathetic innervation on brown adipose tissue (BAT) are unclear. We show that sympathetic denervation activates autophagic-lysosomal degradation, leading to a loss of mitochondrial proteins and BAT atrophy. Conversely, cold-driven sympathetic activation suppresses autophagy and stimulates protein synthesis, leading to BAT hypertrophy. Given its high-potential capacity for heat production, understanding the mechanisms that contribute to BAT mass is important to optimize chances of survival for endotherms in cold ambients.

Epac1 inhibition ameliorates pathological angiogenesis through coordinated activation of Notch and suppression of VEGF signaling

In this study, we investigated the roles of Epac1 in pathological angiogenesis and its potential as a novel therapeutic target for the treatment of vasoproliferative diseases. Genetic deletion of Epac1 ameliorated pathological angiogenesis in mouse models of oxygen-induced retinopathy (OIR) and carotid artery ligation. Moreover, genetic deletion or pharmacological inhibition of Epac1 suppressed microvessel sprouting from ex vivo aortic ring explants. Mechanistic studies revealed that Epac1 acted as a previously unidentified inhibitor of the γ-secretase/Notch signaling pathway via interacting with γ-secretase and regulating its intracellular trafficking while enhancing vascular endothelial growth factor signaling to promote pathological angiogenesis. Pharmacological administration of an Epac-specific inhibitor suppressed OIR-induced neovascularization in wild-type mice, recapitulating the phenotype of genetic Epac1 knockout. Our results demonstrate that Epac1 signaling is critical for the progression of pathological angiogenesis but not for physiological angiogenesis and that the newly developed Epac-specific inhibitors are effective in combating proliferative retinopathy.

AMP-activated protein kinase slows D2 dopamine autoreceptor desensitization in substantia nigra neurons

Dopamine neurons in the substantia nigra zona compacta (SNC) are well known to express D2 receptors. When dopamine is released from somatodendritic sites, activation of D2 autoreceptors suppresses dopamine neuronal activity through activation of G protein-coupled K⁺ channels. AMP-activated protein kinase (AMPK) is a master enzyme that acts in somatic tissues to suppress energy expenditure and encourage energy production. We hypothesize that AMPK may also conserve energy in central neurons by reducing desensitization of D2 autoreceptors. We used whole-cell patch-clamp recordings to study the effects of AMPK activators and inhibitors on D2 autoreceptor-mediated current in SNC neurons in midbrain slices from rat pups (11-23 days post-natal). Slices were superfused with 100 μM dopamine or 30 μM quinpirole for 25 min, which evoked outward currents that decayed slowly over time. Although the AMPK activators A769662 and ZLN024 significantly slowed rundown of dopamine-evoked current, slowing of quinpirole-evoked current required the presence of a D1-like agonist (SKF38393). Moreover, the D1-like agonist also slowed the rundown of quinpirole-induced current even in the absence of an AMPK activator. Pharmacological antagonist experiments showed that the D1-like agonist effect required activation of either protein kinase A (PKA) or exchange protein directly activated by cAMP 2 (Epac2) pathways. In contrast, the effect of AMPK on rundown of current evoked by quinpirole plus SKF38393 required PKA but not Epac2. We conclude that AMPK slows D2 autoreceptor desensitization by augmenting the effect of D1-like receptors.

A peek into Epac physiology in the kidney

cAMP is a critical second messenger of numerous endocrine signals affecting water-electrolyte transport in the renal tubule. Exchange protein directly activated by cAMP (Epac) is a relatively recently discovered downstream effector of cAMP, having the same affinity to the second messenger as protein kinase A, the classical cAMP target. Two Epac isoforms, Epac1 and Epac2, are abundantly expressed in the renal epithelium, where they are thought to regulate water and electrolyte transport, particularly in the proximal tubule and collecting duct. Recent characterization of renal phenotype in mice lacking Epac1 and Epac2 revealed a critical role of the Epac signaling cascade in urinary concentration as well as in Na⁺ and urea excretion. In this review, we aim to critically summarize current knowledge of Epac relevance for renal function and to discuss the applicability of Epac-based strategies in the regulation of systemic water-electrolyte homeostasis.

Epac2 Elevation Reverses Inhibition by Chondroitin Sulfate Proteoglycans In Vitro and Transforms Postlesion Inhibitory Environment to Promote Axonal Outgrowth in an Ex Vivo Model of Spinal Cord Injury

Millions of patients suffer from debilitating spinal cord injury (SCI) without effective treatments. Elevating cAMP promotes CNS neuron growth in the presence of growth-inhibiting molecules. cAMP's effects on neuron growth are partly mediated by Epac, comprising Epac1 and Epac2; the latter predominantly expresses in postnatal neural tissue. Here, we hypothesized that Epac2 activation would enhance axonal outgrowth after SCI. Using in vitro assays, we demonstrated, for the first time, that Epac2 activation using a specific soluble agonist (S-220) significantly enhanced neurite outgrowth of postnatal rat cortical neurons and markedly overcame the inhibition by chondroitin sulfate proteoglycans and mature astrocytes on neuron growth. We further investigated the novel potential of Epac2 activation in promoting axonal outgrowth by an ex vivo rat model of SCI mimicking post-SCI environment in vivo and by delivering S-220 via a self-assembling Fmoc-based hydrogel that has suitable properties for SCI repair. We demonstrated that S-220 significantly enhanced axonal outgrowth across the lesion gaps in the organotypic spinal cord slices, compared with controls. Furthermore, we elucidated, for the first time, that Epac2 activation profoundly modulated the lesion environment by reducing astrocyte/microglial activation and transforming astrocytes into elongated morphology that guided outgrowing axons. Finally, we showed that S-220, when delivered by the gel at 3 weeks after contusion SCI in male adult rats, resulted in significantly better locomotor performance for up to 4 weeks after treatment. Our data demonstrate a promising therapeutic potential of S-220 in SCI, via beneficial effects on neurons and glia after injury to facilitate axonal outgrowth.SIGNIFICANCE STATEMENT During development, neuronal cAMP levels decrease significantly compared with the embryonic stage when the nervous system is established. This has important consequences following spinal cord injury, as neurons fail to regrow. Elevating cAMP levels encourages injured CNS neurons to sprout and extend neurites. We have demonstrated that activating its downstream effector, Epac2, enhances neurite outgrowth in vitro, even in the presence of an inhibitory environment. Using a novel biomaterial-based drug delivery system in the form of a hydrogel to achieve local delivery of an Epac2 agonist, we further demonstrated that specific activation of Epac2 enhances axonal outgrowth and minimizes glial activation in an ex vivo model of spinal cord injury, suggesting a new strategy for spinal cord repair.

Elevated cyclic-AMP represses expression of exchange protein activated by cAMP (EPAC1) by inhibiting YAP-TEAD activity and HDAC-mediated histone deacetylation

Ligand-induced activation of Exchange Protein Activated by cAMP-1 (EPAC1) is implicated in numerous physiological and pathological processes, including cardiac fibrosis where changes in EPAC1 expression have been detected. However, little is known about how EPAC1 expression is regulated. Therefore, we investigated regulation of EPAC1 expression by cAMP in cardiac fibroblasts. Elevation of cAMP using forskolin, cAMP-analogues or adenosine A2B-receptor activation significantly reduced EPAC1 mRNA and protein levels and inhibited formation of F-actin stress fibres. Inhibition of actin polymerisation with cytochalasin-D, latrunculin-B or the ROCK inhibitor, Y-27632, mimicked effects of cAMP on EPAC1 mRNA and protein levels. Elevated cAMP also inhibited activity of an EPAC1 promoter-reporter gene, which contained a consensus binding element for TEAD, which is a target for inhibition by cAMP. Inhibition of TEAD activity using siRNA-silencing of its co-factors YAP and TAZ, expression of dominant-negative TEAD or treatment with YAP-TEAD inhibitors, significantly inhibited EPAC1 expression. However, whereas expression of constitutively-active YAP completely reversed forskolin inhibition of EPAC1-promoter activity it did not rescue EPAC1 mRNA levels. Chromatin-immunoprecipitation detected a significant reduction in histone3-lysine27-acetylation at the EPAC1 proximal promoter in response to forskolin stimulation. HDAC1/3 inhibition partially reversed forskolin inhibition of EPAC1 expression, which was completely rescued by simultaneously expressing constitutively active YAP. Taken together, these data demonstrate that cAMP downregulates EPAC1 gene expression via disrupting the actin cytoskeleton, which inhibits YAP/TAZ-TEAD activity in concert with HDAC-mediated histone deacetylation at the EPAC1 proximal promoter. This represents a novel negative feedback mechanism controlling EPAC1 levels in response to cAMP elevation.

Molecular mechanisms of caffeine-mediated intestinal epithelial ion transports

BACKGROUND AND PURPOSE

As little is known about the effect of caffeine, one of the most widely consumed substances worldwide, on intestinal function, we aimed to study its action on intestinal anion secretion and the underlying molecular mechanisms.

EXPERIMENTAL APPROACH

Anion secretion and channel expression were examined in mouse duodenal epithelium by Ussing chambers and immunocytochemistry. Ca²⁺ imaging was also performed in intestinal epithelial cells (IECs).

KEY RESULTS

Caffeine (10 mM) markedly increased mouse duodenal short-circuit current (I_sc ), which was attenuated by a removal of either Cl^- or HCO₃ ^- , Ca²⁺ -free serosal solutions and selective blockers of store-operated Ca²⁺ channels (SOC/Ca²⁺ release-activated Ca²⁺ channels), and knockdown of Orai1 channels on the serosal side of duodenal tissues. Caffeine induced SOC entry in IEC, which was inhibited by ruthenium red and selective blockers of SOC. Caffeine-stimulated duodenal I_sc was inhibited by the endoplasmic reticulum Ca²⁺ chelator (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine), selective blockers (ruthenium red and dantrolene) of ryanodine receptors (RyR), and of Ca²⁺ -activated Cl^- channels (niflumic acid and T16A). There was synergism between cAMP and Ca²⁺ signalling, in which cAMP/PKA promoted caffeine/Ca²⁺ -mediated anion secretion. Expression of STIM1 and Orai1 was detected in mouse duodenal mucosa and human IECs. The Orai1 proteins were primarily co-located with the basolateral marker Na⁺ , K⁺ -ATPase.

CONCLUSIONS AND IMPLICATIONS

Caffeine stimulated intestinal anion secretion mainly through the RyR/Orai1/Ca²⁺ signalling pathway. There is synergism between cAMP/PKA and caffeine/Ca²⁺ -mediated anion secretion. Our findings suggest that a caffeine-mediated RyR/Orai1/Ca²⁺ pathway could provide novel potential drug targets to control intestinal anion secretion.

Enhanced β-adrenergic signalling underlies an age-dependent beneficial metabolic effect of PI3K p110α inactivation in adipose tissue

The insulin/IGF-1 signalling pathway is a key regulator of metabolism and the rate of ageing. We previously documented that systemic inactivation of phosphoinositide 3-kinase (PI3K) p110α, the principal PI3K isoform that positively regulates insulin signalling, results in a beneficial metabolic effect in aged mice. Here we demonstrate that deletion of p110α specifically in the adipose tissue leads to less fat accumulation over a significant part of adult life and allows the maintenance of normal glucose tolerance despite insulin resistance. This effect of p110α inactivation is due to a potentiating effect on β-adrenergic signalling, which leads to increased catecholamine-induced energy expenditure in the adipose tissue. Our findings provide a paradigm of how partial inactivation of an essential component of the insulin signalling pathway can have an overall beneficial metabolic effect and suggest that PI3K inhibition could potentiate the effect of β-adrenergic agonists in the treatment of obesity and its associated comorbidities.

Mechanosensing and Mechanoregulation of Endothelial Cell Functions

Vascular endothelial cells (ECs) form a semiselective barrier for macromolecules and cell elements regulated by dynamic interactions between cytoskeletal elements and cell adhesion complexes. ECs also participate in many other vital processes including innate immune reactions, vascular repair, secretion, and metabolism of bioactive molecules. Moreover, vascular ECs represent a unique cell type exposed to continuous, time-dependent mechanical forces: different patterns of shear stress imposed by blood flow in macrovasculature and by rolling blood cells in the microvasculature; circumferential cyclic stretch experienced by the arterial vascular bed caused by heart propulsions; mechanical stretch of lung microvascular endothelium at different magnitudes due to spontaneous respiration or mechanical ventilation in critically ill patients. Accumulating evidence suggests that vascular ECs contain mechanosensory complexes, which rapidly react to changes in mechanical loading, process the signal, and develop context-specific adaptive responses to rebalance the cell homeostatic state. The significance of the interactions between specific mechanical forces in the EC microenvironment together with circulating bioactive molecules in the progression and resolution of vascular pathologies including vascular injury, atherosclerosis, pulmonary edema, and acute respiratory distress syndrome has been only recently recognized. This review will summarize the current understanding of EC mechanosensory mechanisms, modulation of EC responses to humoral factors by surrounding mechanical forces (particularly the cyclic stretch), and discuss recent findings of magnitude-specific regulation of EC functions by transcriptional, posttranscriptional and epigenetic mechanisms using -omics approaches. We also discuss ongoing challenges and future opportunities in developing new therapies targeting dysregulated mechanosensing mechanisms to treat vascular diseases. © 2019 American Physiological Society. Compr Physiol 9:873-904, 2019.

Prostacyclin: A major prostaglandin in the regulation of adipose tissue development

Prostaglandins (PGs) belong to the group lipid mediators and can act as local hormones. They contain 20 carbon atoms, including a 5-carbon ring, and are biosynthesized from membrane phospholipid derived arachidonic acid through the arachidonate cyclooxygenase (COX) pathway with the help of various terminal synthase enzymes. Prostacyclin (prostaglandin I₂ ) is one of the major prostanoids produced with the help of prostacyclin synthase (prostaglandin I₂ synthase) enzyme and rapidly hydrolyzed into 6-keto-PGF_1α in biological fluids. Obesity indicates an excess of body adiposity, which is globally considered as one of the major health disasters responsible for developing complex pathological situations in the human body. Adipose tissues can produce various PGs, and thus, the level and the molecular activity of these endogenously synthesized PGs are considered critical for the development of obesity. In this regard, the involvement of prostacyclin in adipogenesis has been studied in the last few decades. The current review, along with the background of other related PGs, presents the several molecular aspects of endogenous prostaglandin I₂ in adipose tissue development. Especially, the regulation of life cycle of adipocytes, impact on terminal differentiation, activity through prostacyclin receptor (IP), autocrine-paracrine manner, thermogenic adipose tissue remodeling and some future experimental aspects of prostacyclin have been focused upon in this study. This discussion might assist to develop new drug molecules acting on the signaling pathways of prostacyclin and devise therapeutic strategies for treating obesity.

Exchange Protein Directly Activated by cAMP Modulates Ebola Virus Uptake into Vascular Endothelial Cells

Members of the family Filoviridae, including Ebola virus (EBOV) and Marburg virus (MARV), cause severe hemorrhagic fever in humans and nonhuman primates. Given their high lethality, a comprehensive understanding of filoviral pathogenesis is urgently needed. In the present studies, we revealed that the exchange protein directly activated by cAMP 1 (EPAC1) gene deletion protects vasculature in ex vivo explants from EBOV infection. Importantly, pharmacological inhibition of EPAC1 using EPAC-specific inhibitors (ESIs) mimicked the EPAC1 knockout phenotype in the ex vivo model. ESI treatment dramatically decreased EBOV infectivity in both ex vivo vasculature and in vitro vascular endothelial cells (ECs). Furthermore, postexposure protection of ECs against EBOV infection was conferred using ESIs. Protective efficacy of ESIs in ECs was observed also in MARV infection. Additional studies using a vesicular stomatitis virus pseudotype that expresses EBOV glycoprotein (EGP-VSV) confirmed that ESIs reduced infection in ECs. Ultrastructural studies suggested that ESIs blocked EGP-VSV internalization via inhibition of macropinocytosis. The inactivation of EPAC1 affects the early stage of viral entry after viral binding to the cell surface, but before early endosome formation, in a phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)-dependent manner. Our study delineated a new critical role of EPAC1 during EBOV uptake into ECs.

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.

Pregnancy-related hormones, progesterone and human chorionic gonadotrophin, upregulate expression of maternal plasma gelsolin

Plasma gelsolin (pGSN), a protein primarily involved in clearance of circulating actin filaments, is an upcoming novel biomarker. Its level changes in multiple disease and injury conditions, attributable mainly to its consumption during actin clearance; the endogenous regulation of its expression, however, remains elusive as well as unexplored. Here, we are reporting the first isolation of the promoter region of pGSN gene and investigation of its transcriptional regulation during pregnancy (a natural process associated with a well-programmed injury course of parturition). Interestingly, two of the pregnancy-related hormones, human chorionic gonadotrophin (hCG) and progesterone, significantly upregulated pGSN promoter activity in muscle cells. This action of both hormones was found to mediate through their respective cellular receptors and involved a contribution of multiple signaling pathways including those of protein kinase A, protein kinase C, epidermal growth factor receptor and prostaglandin-endoperoxidase synthase 2 in the case of hCG-mediated upregulation. This novel upregulation was further supported by elevated levels of endogenous pGSN transcripts as well as secreted protein upon hormonal treatments of muscle cells compared with untreated controls. A participation of pGSN promoter cis-elements, capable of interacting with endogenous transcription factors, Ap1, Sp1, and p300, was also observed during this hormonal upregulation. Additionally, the augmented pGSN levels observed in pregnant mice compared with the control animals further supported an upregulation of this protein during pregnancy, implicating vital role(s) played by pGSN during this period in mammals.

The structure of the apo cAMP-binding domain of HCN4 - a stepping stone toward understanding the cAMP-dependent modulation of the hyperpolarization-activated cyclic-nucleotide-gated ion channels

The hyperpolarization-activated cyclic-nucleotide-gated (HCN) ion channels control nerve impulse transmission and cardiac pacemaker activity. The modulation by cAMP is critical for the regulatory function of HCN in both neurons and cardiomyocytes, but the underlying mechanism is not fully understood. Here, we show how the structure of the apo cAMP-binding domain of the HCN4 isoform has contributed to a model for the cAMP-dependent modulation of the HCN ion-channel. This model recapitulates the structural and dynamical changes that occur along the thermodynamic cycle arising from the coupling of cAMP-binding and HCN self-association equilibria. The proposed model addresses some of the questions previously open about the auto-inhibition of HCN and its cAMP-induced activation, while opening new opportunities for selectively targeting HCN through allosteric ligands. A remaining challenge is the investigation of HCN dimers and their regulatory role. Overcoming this challenge will require the integration of crystallography, cryo electron microscopy, NMR, electrophysiology and simulations.

Involvement of cAMP/EPAC/Akt signaling in the antiproteolytic effects of pentoxifylline on skeletal muscles of diabetic rats

Advances in the knowledge of the mechanisms controlling protein breakdown in skeletal muscles have allowed the exploration of new options for treating muscle-wasting conditions. Pentoxifylline (PTX), a nonselective phosphodiesterase (PDE) inhibitor, attenuates the loss of muscle mass during catabolic conditions, mainly via inhibiting protein breakdown. The aim of this study was to explore the mechanisms by which PTX inhibits proteolysis in the soleus and extensor digitorum longus (EDL) muscles of streptozotocin-induced diabetic rats. The levels of atrogin-1 and muscle RING finger-1 were decreased, as were the activities of caspase-3 (EDL) and calpains (soleus and EDL), in diabetic rats treated with PTX, which at least partly explains the drop in the ubiquitin conjugate (EDL) levels and in proteasome activity (soleus and EDL). Treatment with PTX decreased PDE activity and increased cAMP content in muscles of diabetic rats; moreover, it also increased both the protein levels of exchange protein directly activated by cAMP (EPAC, a cAMP effector) and the phosphorylation of Akt. The loss of muscle mass was practically prevented in diabetic rats treated with PTX. These findings advance our understanding of the mechanisms underlying the antiproteolytic effects of PTX and suggest the use of PDE inhibitors as a strategy to activate cAMP signaling, which is emerging as a promising target for treating muscle mass loss during atrophic conditions. NEW & NOTEWORTHY cAMP signaling has been explored as a strategy to attenuate skeletal muscle atrophies. Therefore, in addition to β₂AR agonists, phosphodiesterase inhibitors such as pentoxifylline (PTX) can be an interesting option. This study advances the understanding of the mechanisms related to the antiproteolytic effects of PTX on skeletal muscles of diabetic rats, which involve the activation of both exchange protein directly activated by cAMP and Akt effectors, inhibiting the expression of atrogenes and calpain/caspase-3-proteolytic machinery.

Molecular mechanisms of calcium signaling in the modulation of small intestinal ion transports and bicarbonate secretion

Background and Purpose:

Although Ca²⁺ signaling may stimulate small intestinal ion secretion, little is known about its critical role and the molecular mechanisms of Ca²⁺-mediated biological action.

Key Results

Activation of muscarinic receptors by carbachol(CCh) stimulated mouse duodenal I_sc, which was significantly inhibited in Ca²⁺-free serosal solution and by several selective store-operated Ca²⁺ channels(SOC) blockers added to the serosal side of duodenal tissues. Furthermore, we found that CRAC/Orai channels may represent the molecular candidate of SOC in intestinal epithelium. CCh increased intracellular Ca²⁺ but not cAMP, and Ca²⁺ signaling mediated duodenal Cl^- and HCO₃^- secretion in wild type mice but not in CFTR knockout mice. CCh induced duodenal ion secretion and stimulated PI3K/Akt activity in duodenal epithelium, all of which were inhibited by selective PI3K inhibitors with different structures. CCh-induced Ca²⁺ signaling also stimulated the phosphorylation of CFTR proteins and their trafficking to the plasma membrane of duodenal epithelial cells, which were inhibited again by selective PI3K inhibitors.

Materials and Methods

Functional, biochemical and morphological experiments were performed to examine ion secretion, PI3K/Akt and CFTR activity of mouse duodenal epithelium. Ca²⁺ imaging was performed on HT-29 cells.

Conclusions and Implications

Ca²⁺ signaling plays a critical role in intestinal ion secretion via CRAC/Orai-mediated SOCE mechanism on the serosal side of epithelium. We also demonstrated the molecular mechanisms of Ca²⁺ signaling in CFTR-mediated secretion via novel PI3K/Akt pathway. Our findings suggest new perspectives for drug targets to protect the upper GI tract and control liquid homeostasis in the small intestine.

Lithium and an EPAC-specific inhibitor ESI-09 synergistically suppress pancreatic cancer cell proliferation and survival

Our previous studies showed that while lithium suppresses proliferation and induces apoptosis in pancreatic cancer cells, the inhibition of exchange proteins directly activated by cyclic adenosine monophosphate (cAMP) (EPAC)1 blocks pancreatic cancer cell migration and invasion. In this study, we further investigated the combinatory effects of lithium and EPAC-specific inhibitor (ESI)-09, an EPAC-specific inhibitor, on pancreatic cancer cell proliferation and viability, and explored whether lithium synergistically cooperates with EPAC inhibition in suppressing pancreatic cancer cell tumorigenicity. The cell viability of pancreatic cancer cell lines PANC-1 and MiaPaCa-2 was measured after 48 h of incubation with different dose combination of lithium and ESI-09. Flow cytometric analysis was carried out to further verify the impact of lithium and ESI-09 upon PANC-1 cell proliferation and apoptosis. To investigate the mechanism that the effects generated by lithium and ESI-09 on PANC-1 cells, the intracellular cAMP level was measured by an ELISA-based cAMP immunoassay. Our data showed that lithium and ESI-09 synergistically inhibit pancreatic cancer cell growth and survival. Furthermore, our results revealed a novel mechanism in which the synergism between lithium and ESI-09 is not mediated by the inhibitory effect of lithium toward GSK3β, but by lithium's ability to suppress cAMP/protein kinase A signaling.

Stimulation of Adenosine A2B Receptor Inhibits Endothelin-1-Induced Cardiac Fibroblast Proliferation and α-Smooth Muscle Actin Synthesis Through the cAMP/Epac/PI3K/Akt-Signaling Pathway

Background and Purpose: Cardiac fibrosis is characterized by an increase in fibroblast proliferation, overproduction of extracellular matrix proteins, and the formation of myofibroblast that express α-smooth muscle actin (α-SMA). Endothelin-1 (ET-1) is involved in the pathogenesis of cardiac fibrosis. Overstimulation of endothelin receptors induced cell proliferation, collagen synthesis, and α-SMA expression in cardiac fibroblasts. Although adenosine was shown to have cardioprotective effects, the molecular mechanisms by which adenosine A₂ receptor inhibit ET-1-induced fibroblast proliferation and α-SMA expression in cardiac fibroblasts are not clearly identified.

Experimental Approach: This study aimed at evaluating the mechanisms of cardioprotective effects of adenosine receptor agonist in rat cardiac fibroblast by measurement of cell proliferation, and mRNA and protein levels of α-SMA.

Key results: Stimulation of adenosine subtype 2B (A_2B) receptor resulted in the inhibition of ET-1-induced fibroblast proliferation, and a reduction of ET-1-induced α-SMA expression that is dependent on cAMP/Epac/PI3K/Akt signaling pathways in cardiac fibroblasts. The data in this study confirm a critical role for Epac signaling on A_2B receptor-mediated inhibition of ET-1-induced cardiac fibrosis via PI3K and Akt activation.

Conclusion and Implications: This is the first work reporting a novel signaling pathway for the inhibition of ET-1-induced cardiac fibrosis mediated through the A_2B receptor. Thus, A_2B receptor agonists represent a promising perspective as therapeutic targets for the prevention of cardiac fibrosis.

Role of Exchange Protein Directly Activated by Cyclic AMP Isoform 1 in Energy Homeostasis: Regulation of Leptin Expression and Secretion in White Adipose Tissue

Epacs (exchange proteins directly activated by cyclic AMP [cAMP]) act as downstream effectors of cAMP and play important roles in energy balance and glucose homeostasis. While global deletion of Epac1 in mice leads to heightened leptin sensitivity in the hypothalamus and partial protection against high-fat diet (HFD)-induced obesity, the physiological functions of Epac1 in white adipose tissue (WAT) has not been explored. Here, we report that adipose tissue-specific Epac1 knockout (AEKO) mice are more prone to HFD-induced obesity, with increased food intake, reduced energy expenditure, and impaired glucose tolerance. Despite the fact that AEKO mice on HFD display increased body weight, these mice have decreased circulating leptin levels compared to their wild-type littermates. In vivo and in vitro analyses further reveal that suppression of Epac1 in WAT decreases leptin mRNA expression and secretion by inhibiting cAMP response element binding (CREB) protein and AKT phosphorylation, respectively. Taken together, our results demonstrate that Epac1 plays an important role in regulating energy balance and glucose homeostasis by promoting leptin expression and secretion in WAT.

Regulatory T-Cell-Mediated Suppression of Conventional T-Cells and Dendritic Cells by Different cAMP Intracellular Pathways

Regulatory T-cells (Tregs) mediate their suppressive action by acting directly on conventional T-cells (Tcons) or dendritic cells (DCs). One mechanism of Treg suppression is the increase of cyclic adenosine 3′,5′-monophosphate (cAMP) levels in target cells. Tregs utilize cAMP to control Tcon responses, such as proliferation and cytokine production. Tregs also exert their suppression on DCs, diminishing DC immunogenicity by downmodulating the expression of costimulatory molecules and actin polymerization at the immunological synapse. The Treg-mediated usage of cAMP occurs through two major mechanisms. The first involves the Treg-mediated influx of cAMP in target cells through gap junctions. The second is the conversion of adenosine triphosphate into adenosine by the ectonucleases CD39 and CD73 present on the surface of Tregs. Adenosine then binds to receptors on the surface of target cells, leading to increased intracellular cAMP levels in these targets. Downstream, cAMP can activate the canonical protein kinase A (PKA) pathway and the exchange protein activated by cyclic AMP (EPAC) non-canonical pathway. In this review, we discuss the most recent findings related to cAMP activation of PKA and EPAC, which are implicated in Treg homeostasis as well as the functional alterations induced by cAMP in cellular targets of Treg suppression.

Stimulation of fat storage by prostacyclin and selective agonists of prostanoid IP receptor during the maturation phase of cultured adipocytes

We have previously shown that cultured adipocytes have the ability to biosynthesize prostaglandin (PG) I₂ called alternatively as prostacyclin during the maturation phase by the positive regulation of gene expression of PGI synthase and the prostanoid IP receptor. To clarify how prostacyclin regulates adipogenesis, we investigated the effects of prostacyclin and the specific agonists or antagonists for the IP receptor on the storage of fats during the maturation phase of cultured adipocytes. Exogenous PGI₂ and the related selective agonists for the IP receptor including MRE-269 and treprostinil rescued the storage of fats attenuated by aspirin, a cyclooxygenase inhibitor. On the other hand, selective antagonists for IP such as CAY10441 and CAY10449 were effective to suppress the accumulation of fats as GW9662, a specific antagonist for peroxisome proliferator-activated receptor (PPAR)γ. Thus, pro-adipogenic action of prostacyclin can be explained by the action mediated through the IP receptor expressed at the maturation stage of adipocytes. Cultured adipocytes incubated with each of PGI₂ and MRE-269 together with troglitazone, an activator for PPARγ, exhibited additively higher stimulation of fats storage than with either compound alone. The combined effect of MRE-269 and troglitazone was almost abolished by co-incubation with GW9662, but not with CAY10441. Increasing concentrations of troglitazone were found to reverse the inhibitory effect of CAY10441 in a dose-dependent manner while those of MRE-269 failed to rescue adipogenesis suppressed by GW9662, indicating the critical role of the PPARγ activation as a downstream factor for the stimulated adipogenesis through the IP receptor. Treatment of cultured adipocytes with cell permeable stable cAMP analogues or forskolin as a cAMP elevating agent partly restored the inhibitory effect of aspirin. However, excess levels of cAMP stimulated by forskolin attenuated adipogenesis. Supplementation with H-89, a cell permeable inhibitor for protein kinase A (PKA), had no effect on the promoting action of PGI₂ or MRE-269 along with aspirin on the storage of fats, suggesting that the promotion of adipogenesis mediated by the IP receptor does not require the PKA activity.

Critical role for Epac1 in inflammatory pain controlled by GRK2-mediated phosphorylation of Epac1

cAMP signaling plays a key role in regulating pain sensitivity. Here, we uncover a previously unidentified molecular mechanism in which direct phosphorylation of the exchange protein directly activated by cAMP 1 (EPAC1) by G protein kinase 2 (GRK2) suppresses Epac1-to-Rap1 signaling, thereby inhibiting persistent inflammatory pain. Epac1(-/-) mice are protected against inflammatory hyperalgesia in the complete Freund's adjuvant (CFA) model. Moreover, the Epac-specific inhibitor ESI-09 inhibits established CFA-induced mechanical hyperalgesia without affecting normal mechanical sensitivity. At the mechanistic level, CFA increased activity of the Epac target Rap1 in dorsal root ganglia of WT, but not of Epac1(-/-), mice. Using sensory neuron-specific overexpression of GRK2 or its kinase-dead mutant in vivo, we demonstrate that GRK2 inhibits CFA-induced hyperalgesia in a kinase activity-dependent manner. In vitro, GRK2 inhibits Epac1-to-Rap1 signaling by phosphorylation of Epac1 at Ser-108 in the Disheveled/Egl-10/pleckstrin domain. This phosphorylation event inhibits agonist-induced translocation of Epac1 to the plasma membrane, thereby reducing Rap1 activation. Finally, we show that GRK2 inhibits Epac1-mediated sensitization of the mechanosensor Piezo2 and that Piezo2 contributes to inflammatory mechanical hyperalgesia. Collectively, these findings identify a key role of Epac1 in chronic inflammatory pain and a molecular mechanism for controlling Epac1 activity and chronic pain through phosphorylation of Epac1 at Ser-108. Importantly, using the Epac inhibitor ESI-09, we validate Epac1 as a potential therapeutic target for chronic pain.

The pleiotropic role of exchange protein directly activated by cAMP 1 (EPAC1) in cancer: implications for therapeutic intervention

The pleiotropic second messenger adenosine 3',5'-cyclic monophosphate (cAMP) regulates a myriad of biological processes under both physiological and pathophysiological conditions. Exchange protein directly activated by cAMP 1 (EPAC1) mediates the intracellular functions of cAMP by acting as a guanine nucleotide exchange factor for the Ras-like Rap small GTPases. Recent studies suggest that EPAC1 plays important roles in immunomodulation, cancer cell migration/metastasis, and metabolism. These results, coupled with the successful development of EPAC-specific small molecule inhibitors, identify EPAC1 as a promising therapeutic target for cancer treatments.

Targeting protein-protein interactions in complexes organized by A kinase anchoring proteins

Cyclic AMP is a ubiquitous intracellular second messenger involved in the regulation of a wide variety of cellular processes, a majority of which act through the cAMP – protein kinase A (PKA) signaling pathway and involve PKA phosphorylation of specific substrates. PKA phosphorylation events are typically spatially restricted and temporally well controlled. A-kinase anchoring proteins (AKAPs) directly bind PKA and recruit it to specific subcellular loci targeting the kinase activity toward particular substrates, and thereby provide discrete spatiotemporal control of downstream phosphorylation events. AKAPs also scaffold other signaling molecules into multi-protein complexes that function as crossroads between different signaling pathways. Targeting AKAP coordinated protein complexes with high-affinity peptidomimetics or small molecules to tease apart distinct protein–protein interactions (PPIs) therefore offers important means to disrupt binding of specific components of the complex to better understand the molecular mechanisms involved in the function of individual signalosomes and their pathophysiological role. Furthermore, development of novel classes of small molecules involved in displacement of AKAP-bound signal molecules is now emerging. Here, we will focus on mechanisms for targeting PPI, disruptors that modulate downstream cAMP signaling and their role, especially in the heart.

Inhibition of Gαs/cAMP Signaling Decreases TCR-Stimulated IL-2 transcription in CD4(+) T Helper Cells

BACKGROUND

The role of cAMP in regulating T cell activation and function has been controversial. cAMP is generally known as an immunosuppressant, but it is also required for generating optimal immune responses. As the effect of cAMP is likely to depend on its cellular context, the current study investigated whether the mechanism of activation of Gαs and adenylyl cyclase influences their effect on T cell receptor (TCR)-stimulated interleukin-2 (IL-2) mRNA levels.

METHODS

The effect of blocking Gs-coupled receptor (GsPCR)-mediated Gs activation on TCR-stimulated IL-2 mRNA levels in CD4(+) T cells was compared with that of knocking down Gαs expression or inhibiting adenylyl cyclase activity. The effect of knocking down Gαs expression on TCR-stimulated cAMP accumulation was compared with that of blocking GsPCR signaling.

RESULTS

ZM-241385, an antagonist to the Gs-coupled A2A adenosine receptor (A2AR), enhanced TCR-stimulated IL-2 mRNA levels in primary human CD4(+) T helper cells and in Jurkat T cells. A dominant negative Gαs construct, GαsDN3, also enhanced TCR-stimulated IL-2 mRNA levels. Similar to GsPCR antagonists, GαsDN3 blocked GsPCR-dependent activation of both Gαs and Gβγ. In contrast, Gαs siRNA and 2',5'-dideoxyadenosine (ddA), an adenylyl cyclase inhibitor, decreased TCR-stimulated IL-2 mRNA levels. Gαs siRNA, but not GαsDN3, decreased TCR-stimulated cAMP synthesis. Potentiation of IL-2 mRNA levels by ZM-241385 required at least two days of TCR stimulation, and addition of ddA after three days of TCR stimulation enhanced IL-2 mRNA levels.

CONCLUSIONS

GsPCRs play an inhibitory role in the regulation of TCR-stimulated IL-2 mRNA levels whereas Gαs and cAMP can play a stimulatory one. Additionally, TCR-dependent activation of Gαs does not appear to involve GsPCRs. These results suggest that the context of Gαs/cAMP activation and the stage of T cell activation and differentiation determine the effect on TCR-stimulated IL-2 mRNA levels.