Disruption of Epac1 protects the heart from adenylyl cyclase type 5-mediated cardiac dysfunction

Adenylyl cyclase isoforms 5 and 6 in the cardiovascular system: complex regulation and divergent roles

Adenylyl cyclases (ACs) are crucial effector enzymes that transduce divergent signals from upstream receptor pathways and are responsible for catalyzing the conversion of ATP to cAMP. The ten AC isoforms are categorized into four main groups; the class III or calcium-inhibited family of ACs comprises AC5 and AC6. These enzymes are very closely related in structure and have a paucity of selective activators or inhibitors, making it difficult to distinguish them experimentally. AC5 and AC6 are highly expressed in the heart and vasculature, as well as the spinal cord and brain; AC6 is also abundant in the lungs, kidney, and liver. However, while AC5 and AC6 have similar expression patterns with some redundant functions, they have distinct physiological roles due to differing regulation and cAMP signaling compartmentation. AC5 is critical in cardiac and vascular function; AC6 is a key effector of vasodilatory pathways in vascular myocytes and is enriched in fetal/neonatal tissues. Expression of both AC5 and AC6 decreases in heart failure; however, AC5 disruption is cardio-protective, while overexpression of AC6 rescues cardiac function in cardiac injury. This is a comprehensive review of the complex regulation of AC5 and AC6 in the cardiovascular system, highlighting overexpression and knockout studies as well as transgenic models illuminating each enzyme and focusing on post-translational modifications that regulate their cellular localization and biological functions. We also describe pharmacological challenges in the design of isoform-selective activators or inhibitors for AC5 and AC6, which may be relevant to developing new therapeutic approaches for several cardiovascular diseases.

Oral angiotensin-converting enzyme inhibitor captopril protects the heart from Porphyromonas gingivalis LPS-induced cardiac dysfunction in mice

Although angiotensin converting enzyme (ACE) inhibitors are considered useful for the treatment of human heart failure, some experimental failing-heart models have shown little beneficial effect of ACE inhibitors in animals with poor oral health, particularly periodontitis. In this study, we examined the effects of the ACE inhibitor captopril (Cap; 0.1 mg/mL in drinking water) on cardiac dysfunction in mice treated with Porphyromonas gingivalis lipopolysaccharide (PG-LPS) at a dose (0.8 mg/kg/day) equivalent to the circulating level in patients with periodontal disease. Mice were divided into four groups: 1) Control, 2) PG-LPS, 3) Cap, and 4) PG-LPS + Cap. After1 week, we evaluated cardiac function by echocardiography. The left ventricular ejection fraction was significantly decreased in PG-LPS-treated mice compared to the control (from 66 ± 1.8 to 59 ± 2.5%), while Cap ameliorated the dysfunction (63 ± 1.1%). The area of cardiac fibrosis was significantly increased (approximately 2.9-fold) and the number of apoptotic myocytes was significantly increased (approximately 5.6-fold) in the heart of PG-LPS-treated group versus the control, and these changes were suppressed by Cap. The impairment of cardiac function in PG-LPS-treated mice was associated with protein kinase C δ phosphorylation (Tyr-311), leading to upregulation of NADPH oxidase 4 and xanthine oxidase, and calmodulin kinase II phosphorylation (Thr-286) with increased phospholamban phosphorylation (Thr-17). These changes were also suppressed by Cap. Our results suggest that the renin-angiotensin system might play an important role in the development of cardiac diseases induced by PG-LPS.

Modulation of cardiac cAMP signaling by AMPK and its adjustments in pressure overload-induced myocardial dysfunction in rat and mouse

The beta-adrenergic system is a potent stimulus for enhancing cardiac output that may become deleterious when energy metabolism is compromised as in heart failure. We thus examined whether the AMP-activated protein kinase (AMPK) that is activated in response to energy depletion may control the beta-adrenergic pathway. We studied the cardiac response to beta-adrenergic stimulation of AMPKα2-/- mice or to pharmacological AMPK activation on contractile function, calcium current, cAMP content and expression of adenylyl cyclase 5 (AC5), a rate limiting step of the beta-adrenergic pathway. In AMPKα2-/- mice the expression of AC5 (+50%), the dose response curve of left ventricular developed pressure to isoprenaline (p<0.001) or the response to forskolin, an activator of AC (+25%), were significantly increased compared to WT heart. Similarly, the response of L-type calcium current to 3-isobutyl-l-methylxanthine (IBMX), a phosphodiesterase inhibitor was significantly higher in KO (+98%, p<0.01) than WT (+57%) isolated cardiomyocytes. Conversely, pharmacological activation of AMPK by 5-aminoimidazole-4-carboxamide riboside (AICAR) induced a 45% decrease in AC5 expression (p<0.001) and a 40% decrease of cAMP content (P<0.001) as measured by fluorescence resonance energy transfer (FRET) compared to unstimulated rat cardiomyocytes. Finally, in experimental pressure overload-induced cardiac dysfunction, AMPK activation was associated with a decreased expression of AC5 that was blunted in AMPKα2-/- mice. The results show that AMPK activation down-regulates AC5 expression and blunts the beta-adrenergic cascade. This crosstalk between AMPK and beta-adrenergic pathways may participate in a compensatory energy sparing mechanism in dysfunctional myocardium.

Vidarabine, an anti-herpes agent, improves Porphyromonas gingivalis lipopolysaccharide-induced cardiac dysfunction in mice

In this work, we examined the involvement of type 5 adenylyl cyclase (AC5) in cardiac dysfunction induced in mice given Porphyromonas gingivalis lipopolysaccharide (PG-LPS) at a dose equivalent to the circulating levels in periodontitis (PD) patients. Cardiac function was significantly decreased in mice given PG-LPS compared to the control, but treatment for 1 week with the AC5 inhibitor vidarabine ameliorated the dysfunction. Cardiac fibrosis and myocyte apoptosis were significantly increased in the PG-LPS group, but vidarabine blocked these changes. The PG-LPS-induced cardiac dysfunction was associated with activation of cyclic AMP/Ca2+-calmodulin-dependent protein kinase II signaling and increased phospholamban phosphorylation at threonine 17. These results suggest that pharmacological AC5 inhibition may be a promising approach to treat PD-associated cardiovascular disease.

Epac: A Promising Therapeutic Target for Vascular Diseases: A Review

Vascular diseases affect the circulatory system and comprise most human diseases. They cause severe symptoms and affect the quality of life of patients. Recently, since their identification, exchange proteins directly activated by cAMP (Epac) have attracted increasing scientific interest, because of their role in cyclic adenosine monophosphate (cAMP) signaling, a well-known signal transduction pathway. The role of Epac in cardiovascular disease and cancer is extensively studied, whereas their role in kidney disease has not been comprehensively explored yet. In this study, we aimed to review recent studies on the regulatory effects of Epac on various vascular diseases, such as cardiovascular disease, cerebrovascular disease, and cancer. Accumulating evidence has shown that both Epac1 and Epac2 play important roles in vascular diseases under both physiological and pathological conditions. Additionally, there has been an increasing focus on Epac pharmacological modulators. Therefore, we speculated that Epac could serve as a novel therapeutic target for the treatment of vascular diseases.

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.

The Epac1 Protein: Pharmacological Modulators, Cardiac Signalosome and Pathophysiology

The second messenger 3′,5′-cyclic adenosine monophosphate (cAMP) is one of the most important signalling molecules in the heart as it regulates many physiological and pathophysiological processes. In addition to the classical protein kinase A (PKA) signalling route, the exchange proteins directly activated by cAMP (Epac) mediate the intracellular functions of cAMP and are now emerging as a new key cAMP effector in cardiac pathophysiology. In this review, we provide a perspective on recent advances in the discovery of new chemical entities targeting the Epac1 isoform and illustrate their use to study the Epac1 signalosome and functional characterisation in cardiac cells. We summarize the role of Epac1 in different subcompartments of the cardiomyocyte and discuss how cAMP–Epac1 specific signalling networks may contribute to the development of cardiac diseases. We also highlight ongoing work on the therapeutic potential of Epac1-selective small molecules for the treatment of cardiac disorders.

Translationally controlled tumor protein (TCTP) plays a pivotal role in cardiomyocyte survival through a Bnip3-dependent mechanism

Prevention of cardiomyocyte death is an important therapeutic strategy for heart failure. In this study, we focused on translationally controlled tumor protein (TCTP), a highly conserved protein that is expressed ubiquitously in mammalian tissues, including heart. TCTP plays pivotal roles in survival of certain cell types, but its function in cardiomyocytes has not been examined. We aimed to clarify the role of TCTP in cardiomyocyte survival and the underlying mechanism. Here, we demonstrated that downregulation of TCTP with siRNA induced cell death of cardiomyocytes with apoptotic and autophagic features, accompanied with mitochondrial permeability transition pore (mPTP) opening. TCTP loss did not induce cell death of cardiac fibroblasts. Bcl-2/adenovirus E1B 19-kDa interacting protein 3 (Bnip3) was found to mediate the TCTP-loss-induced cardiomyocyte death. In exploring the clinical significance of the TCTP expression in the heart, we found that DOX treatment markedly downregulated the protein expression of TCTP in cultured cardiomyocytes and in mouse heart tissue. Exogenous rescue of TCTP expression attenuated DOX-induced cardiomyocyte death. In mice, cardiomyocyte-specific overexpression of TCTP resulted in decreased susceptibility to DOX-induced cardiac dysfunction, accompanied with attenuated induction of Bnip3. Dihydroartemisinin, a pharmacological TCTP inhibitor, induced development of heart failure and cardiomyocyte death in control mice, but not in mice with cardiomyocyte-specific TCTP overexpression. Our findings revealed TCTP has a pivotal role in cardiomyocyte survival, at least in part through a Bnip3-dependent mechanism. TCTP could be considered as a candidate therapeutic target to prevent DOX-induced heart failure.

Role of β-adrenergic signaling in masseter muscle

In skeletal muscle, the major isoform of β-adrenergic receptor (β-AR) is β2-AR and the minor isoform is β1-AR, which is opposite to the situation in cardiac muscle. Despite extensive studies in cardiac muscle, the physiological roles of the β-AR subtypes in skeletal muscle are not fully understood. Therefore, in this work, we compared the effects of chronic β1- or β2-AR activation with a specific β1-AR agonist, dobutamine (DOB), or a specific β2-AR agonist, clenbuterol (CB), on masseter and cardiac muscles in mice. In cardiac muscle, chronic β1-AR stimulation induced cardiac hypertrophy, fibrosis and myocyte apoptosis, whereas chronic β2-AR stimulation induced cardiac hypertrophy without histological abnormalities. In masseter muscle, however, chronic β1-AR stimulation did not induce muscle hypertrophy, but did induce fibrosis and apoptosis concomitantly with increased levels of p44/42 MAPK (ERK1/2) (Thr-202/Tyr-204), calmodulin kinase II (Thr-286) and mammalian target of rapamycin (mTOR) (Ser-2481) phosphorylation. On the other hand, chronic β2-AR stimulation in masseter muscle induced muscle hypertrophy without histological abnormalities, as in the case of cardiac muscle, concomitantly with phosphorylation of Akt (Ser-473) and mTOR (Ser-2448) and increased expression of microtubule-associated protein light chain 3-II, an autophagosome marker. These results suggest that the β1-AR pathway is deleterious and the β2-AR is protective in masseter muscle. These data should be helpful in developing pharmacological approaches for the treatment of skeletal muscle wasting and weakness.

Usefulness of Exchanged Protein Directly Activated by cAMP (Epac)1-Inhibiting Therapy for Prevention of Atrial and Ventricular Arrhythmias in Mice

BACKGROUND

It has been suggested that protein directly activated by cAMP (Epac), one of the downstream signaling molecules of β-adrenergic receptor (β-AR), may be an effective target for the treatment of arrhythmia. However, there have been no reports on the anti-arrhythmic effects or cardiac side-effects of Epac1 inhibitors in vivo. Methods and Results: In this study, the roles of Epac1 in the development of atrial and ventricular arrhythmias are examined. In addition, we examined the usefulness of CE3F4, an Epac1-selective inhibitor, in the treatment of the arrhythmias in mice. In Epac1 knockout (Epac1-KO) mice, the duration of atrial fibrillation (AF) was shorter than in wild-type mice. In calsequestrin2 knockout mice, Epac1 deficiency resulted in a reduction of ventricular arrhythmia. In both atrial and ventricular myocytes, sarcoplasmic reticulum (SR) Ca²⁺ leak, a major trigger of arrhythmias, and spontaneous SR Ca²⁺ release (SCR) were attenuated in Epac1-KO mice. Consistently, CE3F4 treatment significantly prevented AF and ventricular arrhythmia in mice. In addition, the SR Ca²⁺ leak and SCR were significantly inhibited by CE3F4 treatment in both atrial and ventricular myocytes. Importantly, cardiac function was not significantly affected by a dosage of CE3F4 sufficient to exert anti-arrhythmic effects.

CONCLUSIONS

These findings indicated that Epac1 is involved in the development of atrial and ventricular arrhythmias. CE3F4, an Epac1-selective inhibitor, prevented atrial and ventricular arrhythmias in mice.

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.

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.

HD Physiology Project-Japanese efforts to promote multilevel integrative systems biology and physiome research

The HD Physiology Project is a Japanese research consortium that aimed to develop methods and a computational platform in which physiological and pathological information can be described in high-level definitions across multiple scales of time and size. During the 5 years of this project, an appropriate software platform for multilevel functional simulation was developed and a whole-heart model including pharmacokinetics for the assessment of the proarrhythmic risk of drugs was developed. In this article, we outline the description and scientific strategy of this project and present the achievements and influence on multilevel integrative systems biology and physiome research.

Epac activation inhibits IL-6-induced cardiac myocyte dysfunction

Pro-inflammatory cytokines are released in septic shock and impair cardiac function via the Jak-STAT pathway. It is well known that sympathetic and thus catecholamine signaling is activated thereafter to compensate for cardiac dysfunction. The mechanism of such compensation by catecholamine signaling has been traditionally understood to be cyclic AMP-dependent protein kinase (PKA)-mediated enforcement of cardiac contractility. We hypothesized that the exchange protein activated by cAMP (Epac), a newly identified target of cAMP signaling that functions independently of PKA, also plays a key role in this mechanism. In cultured cardiac myocytes, activation of Epac attenuated the inhibitory effect of interleukin-6 on the increase of intracellular Ca²⁺ concentration and contractility in response to isoproterenol, most likely through inhibition of the Jak-STAT pathway via SOCS3, with subsequent changes in inducible nitric oxide synthase expression. These findings suggest a new role of catecholamine signaling in compensating for cardiac dysfunction in heart failure. Epac and its downstream pathway may be a novel target for treating cardiac dysfunction in endotoxemia.