Regulation by phosphodiesterase isoforms of protein kinase A-mediated attenuation of myocardial protein kinase D activation

Divergent off-target effects of RSK N-terminal and C-terminal kinase inhibitors in cardiac myocytes

P90 ribosomal S6 kinases (RSK) are ubiquitously expressed and regulate responses to neurohumoral stimulation. To study the role of RSK signalling on cardiac myocyte function and protein phosphorylation, pharmacological RSK inhibitors were tested. Here, the ATP competitive N-terminal kinase domain-targeting compounds D1870 and SL0101 and the allosteric C-terminal kinase domain-targeting FMK were evaluated regarding their ability to modulate cardiac myocyte protein phosphorylation. Exposure to D1870 and SL0101 significantly enhanced phospholamban (PLN) Ser16 and cardiac troponin I (cTnI) Ser22/23 phosphorylation in response to D1870 and SL0101 upon exposure to phenylephrine (PE) that activates RSK. In contrast, FMK pretreatment significantly reduced phosphorylation of both proteins in response to PE. D1870-mediated enhancement of PLN Ser16 phosphorylation was also observed after exposure to isoprenaline or noradrenaline (NA) stimuli that do not activate RSK. Inhibition of β-adrenoceptors by atenolol or cAMP-dependent protein kinase (PKA) by H89 prevented the D1870-mediated increase in PLN phosphorylation, suggesting that PKA is the kinase responsible for the observed phosphorylation. Assessment of changes in cAMP formation by FRET measurements revealed increased cAMP formation in vicinity to PLN after exposure to D1870 and SL0101. D1870 inhibited phosphodiesterase activity similarly as established PDE inhibitors rolipram or 3-isobutyl-1-methylxanthine. Assessment of catecholamine-mediated force development in rat ventricular muscle strips revealed significantly reduced EC₅₀ for NA after D1870 pretreatment (DMSO/NA: 2.33 μmol/L vs. D1870/NA: 1.30 μmol/L). The data reveal enhanced cardiac protein phosphorylation by D1870 and SL0101 that was not detectable in response to FMK. This disparate effect might be attributed to off-target inhibition of PDEs with impact on muscle function as demonstrated for D1870.

Inflammation leads through PGE/EP3 signaling to HDAC5/MEF2-dependent transcription in cardiac myocytes

The myocyte enhancer factor 2 (MEF2) regulates transcription in cardiac myocytes and adverse remodeling of adult hearts. Activators of G protein-coupled receptors (GPCRs) have been reported to activate MEF2, but a comprehensive analysis of GPCR activators that regulate MEF2 has to our knowledge not been performed. Here, we tested several GPCR agonists regarding their ability to activate a MEF2 reporter in neonatal rat ventricular myocytes. The inflammatory mediator prostaglandin E₂ (PGE₂) strongly activated MEF2. Using pharmacological and protein-based inhibitors, we demonstrated that PGE₂ regulates MEF2 via the EP₃ receptor, the βγ subunit of G_i/o protein and two concomitantly activated downstream pathways. The first consists of Tiam1, Rac1, and its effector p21-activated kinase 2, the second of protein kinase D. Both pathways converge on and inactivate histone deacetylase 5 (HDAC5) and thereby de-repress MEF2. In vivo, endotoxemia in MEF2-reporter mice induced upregulation of PGE₂ and MEF2 activation. Our findings provide an unexpected new link between inflammation and cardiac remodeling by de-repression of MEF2 through HDAC5 inactivation, which has potential implications for new strategies to treat inflammatory cardiomyopathies.

Compartmentalized cyclic nucleotides have opposing effects on regulation of hypertrophic phospholipase Cε signaling in cardiac myocytes

In cardiac myocytes activation of an exchange factor activated by cAMP (Epac) leads to activation of phospholipase Cε (PLCε)-dependent hydrolysis of phosphatidylinositol 4-phosphate (PI4P) in the Golgi apparatus a process critical for development of cardiac hypertrophy. Here we show that β-adrenergic receptor (βAR) stimulation does not stimulate this pathway in the presence of the broad spectrum phosphodiesterase (PDE) inhibitor IBMX, but selective PDE3 inhibition revealed βAR-dependent PI4P depletion. On the other hand, selective inhibition of PDE2 or PDE9A blocked endothelin-1 (ET-1) and cAMP-dependent PI4P hydrolysis by PLCε. Direct activation of protein kinase A (PKA), protein kinase G (PKG), or the atrial natriuretic factor (ANF) receptor abolished PI4P hydrolysis in response to multiple upstream stimuli. These results reveal distinct pools of cyclic nucleotides that either inhibit PLCε at the Golgi through PKA/PKG, or activate PLCε at the Golgi through Epac. These data together reveal a new mechanism by which ANF and selective PDE inhibitors can protect against cardiac hypertrophy.

β-Adrenergic Stimulation Induces Histone Deacetylase 5 (HDAC5) Nuclear Accumulation in Cardiomyocytes by B55α-PP2A-Mediated Dephosphorylation

Background

Class IIa histone deacetylase (HDAC) isoforms such as HDAC5 are critical signal‐responsive repressors of maladaptive cardiomyocyte hypertrophy, through nuclear interactions with transcription factors including myocyte enhancer factor‐2. β‐Adrenoceptor (β‐AR) stimulation, a signal of fundamental importance in regulating cardiac function, has been proposed to induce both phosphorylation‐independent nuclear export and phosphorylation‐dependent nuclear accumulation of cardiomyocyte HDAC5. The relative importance of phosphorylation at Ser259/Ser498 versus Ser279 in HDAC5 regulation is also controversial. We aimed to determine the impact of β‐AR stimulation on the phosphorylation, localization, and function of cardiomyocyte HDAC5 and delineate underlying molecular mechanisms.

Methods and Results

A novel 3‐dimensional confocal microscopy method that objectively quantifies the whole‐cell nuclear/cytoplasmic distribution of green fluorescent protein tagged HDAC5 revealed the β‐AR agonist isoproterenol to induce β₁‐AR‐mediated and protein kinase A‐dependent HDAC5 nuclear accumulation in adult rat cardiomyocytes, which was accompanied by dephosphorylation at Ser259/279/498. Mutation of Ser259/Ser498 to Ala promoted HDAC5 nuclear accumulation and myocyte enhancer factor‐2 inhibition, whereas Ser279 ablation had no such effect and did not block isoproterenol‐induced nuclear accumulation. Inhibition of the Ser/Thr phosphatase PP2A blocked isoproterenol‐induced HDAC5 dephosphorylation. Co‐immunoprecipitation revealed a specific interaction of HDAC5 with the PP2A targeting subunit B55α, as well as catalytic and scaffolding subunits, which increased >3‐fold with isoproterenol. Knockdown of B55α in neonatal cardiomyocytes attenuated isoproterenol‐induced HDAC5 dephosphorylation.

Conclusions

β‐AR stimulation induces HDAC5 nuclear accumulation in cardiomyocytes by a mechanism that is protein kinase A‐dependent but requires B55α‐PP2A‐mediated dephosphorylation of Ser259/Ser498 rather than protein kinase A‐mediated phosphorylation of Ser279.

Emergency Spatiotemporal Shift: The Response of Protein Kinase D to Stress Signals in the Cardiovascular System

Protein Kinase D isoforms (PKD 1-3) are key mediators of neurohormonal, oxidative, and metabolic stress signals. PKDs impact a wide variety of signaling pathways and cellular functions including actin dynamics, vesicle trafficking, cell motility, survival, contractility, energy substrate utilization, and gene transcription. PKD activity is also increasingly linked to cancer, immune regulation, pain modulation, memory, angiogenesis, and cardiovascular disease. This increasing complexity and diversity of PKD function, highlights the importance of tight spatiotemporal control of the kinase via protein–protein interactions, post-translational modifications or targeting via scaffolding proteins. In this review, we focus on the spatiotemporal regulation and effects of PKD signaling in response to neurohormonal, oxidant and metabolic signals that have implications for myocardial disease. Precise targeting of these mechanisms will be crucial in the design of PKD-based therapeutic strategies.

Nanodomain Regulation of Cardiac Cyclic Nucleotide Signaling by Phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) form an 11-member superfamily comprising 100 different isoforms that regulate the second messengers cyclic adenosine or guanosine 3',5'-monophosphate (cAMP or cGMP). These PDE isoforms differ with respect to substrate selectivity and their localized control of cAMP and cGMP within nanodomains that target specific cellular pools and synthesis pathways for the cyclic nucleotides. Seven PDE family members are physiologically relevant to regulating cardiac function, disease remodeling of the heart, or both: PDE1 and PDE2, both dual-substrate (cAMP and cGMP) esterases; PDE3, PDE4, and PDE8, which principally hydrolyze cAMP; and PDE5A and PDE9A, which target cGMP. New insights regarding the different roles of PDEs in health and disease and their local signaling control are broadening the potential therapeutic utility for PDE-selective inhibitors. In this review, we discuss these PDEs, focusing on the different mechanisms by which they control cardiac function in health and disease by regulating intracellular nanodomains.

Cardiac Hypertrophy Is Inhibited by a Local Pool of cAMP Regulated by Phosphodiesterase 2

RATIONALE

Chronic elevation of 3'-5'-cyclic adenosine monophosphate (cAMP) levels has been associated with cardiac remodeling and cardiac hypertrophy. However, enhancement of particular aspects of cAMP/protein kinase A signaling seems to be beneficial for the failing heart. cAMP is a pleiotropic second messenger with the ability to generate multiple functional outcomes in response to different extracellular stimuli with strict fidelity, a feature that relies on the spatial segregation of the cAMP pathway components in signaling microdomains.

OBJECTIVE

How individual cAMP microdomains affect cardiac pathophysiology remains largely to be established. The cAMP-degrading enzymes phosphodiesterases (PDEs) play a key role in shaping local changes in cAMP. Here we investigated the effect of specific inhibition of selected PDEs on cardiac myocyte hypertrophic growth.

METHODS AND RESULTS

Using pharmacological and genetic manipulation of PDE activity, we found that the rise in cAMP resulting from inhibition of PDE3 and PDE4 induces hypertrophy, whereas increasing cAMP levels via PDE2 inhibition is antihypertrophic. By real-time imaging of cAMP levels in intact myocytes and selective displacement of protein kinase A isoforms, we demonstrate that the antihypertrophic effect of PDE2 inhibition involves the generation of a local pool of cAMP and activation of a protein kinase A type II subset, leading to phosphorylation of the nuclear factor of activated T cells.

CONCLUSIONS

Different cAMP pools have opposing effects on cardiac myocyte cell size. PDE2 emerges as a novel key regulator of cardiac hypertrophy in vitro and in vivo, and its inhibition may have therapeutic applications.

β-adrenergic signaling inhibits Gq-dependent protein kinase D activation by preventing protein kinase D translocation

RATIONALE

Both β-adrenergic receptor (β-AR) and Gq-coupled receptor (GqR) agonist-driven signaling play key roles in the events, leading up to and during cardiac dysfunction. How these stimuli interact at the level of protein kinase D (PKD), a nodal point in cardiac hypertrophic signaling, remains unclear.

OBJECTIVE

To assess the spatiotemporal dynamics of PKD activation in response to β-AR signaling alone and on coactivation with GqR-agonists. This will test our hypothesis that compartmentalized PKD signaling reconciles disparate findings of PKA facilitation and inhibition of PKD activation.

METHODS AND RESULTS

We report on the spatial and temporal profiles of PKD activation using green fluorescent protein-tagged PKD (wildtype or mutant S427E) and targeted fluorescence resonance energy transfer-based biosensors (D-kinase activity reporters) in adult cardiomyocytes. We find that β-AR/PKA signaling drives local nuclear activation of PKD, without preceding sarcolemmal translocation. We also discover pronounced interference of β-AR/cAMP/PKA signaling on GqR-induced translocation and activation of PKD throughout the cardiomyocyte. We attribute these effects to direct, PKA-dependent phosphorylation of PKD-S427. We also show that phosphomimetic substitution of S427 likewise impedes GqR-induced PKD translocation and activation. In neonatal myocytes, S427E inhibits GqR-evoked cell growth and expression of hypertrophic markers. Finally, we show altered S427 phosphorylation in transverse aortic constriction-induced hypertrophy.

CONCLUSIONS

β-AR signaling triggers local nuclear signaling and inhibits GqR-mediated PKD activation by preventing its intracellular translocation. PKA-dependent phosphorylation of PKD-S427 fine-tunes the PKD responsiveness to GqR-agonists, serving as a key integration point for β-adrenergic and Gq-coupled stimuli.

Histone deacetylase signaling in cardioprotection

Cardiovascular disease (CVD) represents a major challenge for health care systems, both in terms of the high mortality associated with it and the huge economic burden of its treatment. Although CVD represents a diverse range of disorders, they share common compensatory changes in the heart at the structural, cellular, and molecular level that, in the long term, can become maladaptive and lead to heart failure. Treatment of adverse cardiac remodeling is therefore an important step in preventing this fatal progression. Although previous efforts have been primarily focused on inhibition of deleterious signaling cascades, the stimulation of endogenous cardioprotective mechanisms offers a potent therapeutic tool. In this review, we discuss class I and class II histone deacetylases, a subset of chromatin-modifying enzymes known to have critical roles in the regulation of cardiac remodeling. In particular, we discuss their molecular modes of action and go on to consider how their inhibition or the stimulation of their intrinsic cardioprotective properties may provide a potential therapeutic route for the clinical treatment of CVD.

Agonist activated PKCβII translocation and modulation of cardiac myocyte contractile function

Elevated protein kinase C β_II (PKCβ_II) expression develops during heart failure and yet the role of this isoform in modulating contractile function remains controversial. The present study examines the impact of agonist-induced PKCβ_II activation on contractile function in adult cardiac myocytes. Diminished contractile function develops in response to low dose phenylephrine (PHE, 100 nM) in controls, while function is preserved in response to PHE in PKCβ_II-expressing myocytes. PHE also caused PKCβ_II translocation and a punctate distribution pattern in myocytes expressing this isoform. The preserved contractile function and translocation responses to PHE are blocked by the inhibitor, LY379196 (30 nM) in PKCβ_II-expressing myocytes. Further analysis showed downstream protein kinase D (PKD) phosphorylation and phosphatase activation are associated with the LY379196-sensitive contractile response. PHE also triggered a complex pattern of end-target phosphorylation in PKCβ_II-expressing myocytes. These patterns are consistent with bifurcated activation of downstream signaling activity by PKCβ_II.

Prostaglandin E1 facilitates inotropic effects of 5-HT4 serotonin receptors and β-adrenoceptors in failing human heart

Prostaglandins have displayed both beneficial and detrimental effects in clinical studies in patients with severe heart failure. Prostaglandins are known to increase cardiac output, but the mechanism is not clarified. Here, we tested the hypothesis that prostaglandins can increase contractility in human heart by amplifying cAMP-dependent inotropic responses. Contractility was measured ex vivo in isolated left ventricular strips and phosphodiesterase (PDE) and adenylyl cyclase (AC) activity was measured in homogenates or membranes from failing human left ventricles. PGE(1) (1 µM) alone did not modify contractility, but given prior, amplified maximal serotonin (5-HT)-evoked (10 µM) contractile responses mediated by 5-HT(4) receptors several fold (24 ± 7 % with PGE(1) vs. 3 ± 2 % above basal with 5-HT alone). The 5-HT(4)-mediated inotropic response was amplified by the PDE3 inhibitor cilostamide and further amplified in combination with PGE(1) (26 ± 6 vs. 56 ± 12 % above basal). PGE(1) reduced the time to reach 90 % of both the maximal 5-HT- and isoproterenol-evoked inotropic response compared to 5-HT or isoproterenol alone. PGE(1) did not modify PDE activity in the homogenate, either alone or when given simultaneously with PDE3 and/or PDE4 inhibitors. Neither 5-HT- nor isoproterenol-stimulated AC activity was significantly amplified by PGE(1). Sensitivity of ventricular strips to Ca(2+) was not enhanced in the presence of PGE(1). Our results show that PGE(1) can enhance cAMP-mediated responses in failing human left ventricle, through a mechanism independent of PDE inhibition, amplification of AC activity or increasing sensitivity to calcium. This effect of PGE(1) possibly contributes to the increase of cardiac output, independent of decreased afterload, observed after prostaglandin administration in humans.

Constitutive phosphorylation of inhibitor-1 at Ser67 and Thr75 depresses calcium cycling in cardiomyocytes and leads to remodeling upon aging

The activity of protein phosphatase-1 (PP1) inhibitor-1 (I-1) is antithetically modulated by the cAMP-protein kinase A (PKA) and Ca(2+)-protein kinase C (PKC) signaling axes. β-adrenergic (β-AR) stimulation results in PKA-phosphorylation of I-1 at threonine 35 (Thr35) and depressed PP1 activity, while PKC phosphorylation at serine 67 (Ser67) and/or Thr75 increases PP1 activity. In heart failure, pThr35 is decreased while pSer67 and pThr75 are elevated. However, the role of Ser67/Thr75 phosphorylation in vivo and its effects on Ca(2+)-cycling are not known. Thus, our aim was to investigate the functional significance of Ser67 and Thr75 phosphorylation in intact hearts. We generated transgenic mice (TG) with cardiac-specific overexpression of constitutively phosphorylated I-1 at Ser67 and Thr75 (S67D/T75D) and evaluated cardiac function. The S67D/T75D cardiomyocytes exhibited significantly depressed Ca(2+)-kinetics and contractile parameters, compared with wild-type (WT) cells. The decreased Ca(2+)-cycling was associated with a 27 % increase in PP1 activity, no alterations in PP2 activity and impaired phosphorylation of myosin-binding protein-C (MyBPC). Upon aging, there was cardiac remodeling associated with increases in systolic and diastolic left ventricular internal diameter dimensions (at 16 months), compared with WTs. The results indicate that phosphorylation of I-1 at Ser67 and Thr75 is associated with increased PP1 activity and depressed cardiomyocyte Ca(2+)-cycling, which manifests in geometrical alterations over the long term. Thus, hyperphosphorylation of these sites in failing hearts may contribute to deteriorative remodeling.

Neurohormonal regulation of cardiac histone deacetylase 5 nuclear localization by phosphorylation-dependent and phosphorylation-independent mechanisms

RATIONALE

Myocyte enhancer factor 2 (MEF2) transcription factors drive the genetic reprogramming that precipitates pathological cardiac hypertrophy and remodeling. Class II histone deacetylase (HDAC) isoforms, such as HDAC5, act as signal-responsive repressors of MEF2 activity in cardiac myocytes and their nuclear export provides a key mechanism for the neurohormonal induction of such activity.

OBJECTIVE

To delineate the mechanism(s) through which 2 clinically relevant neurohormonal stimuli, endothelin-1 (ET1) and the β-adrenergic receptor (β-AR) agonist isoproterenol (ISO), may regulate HDAC5 nuclear localization in adult cardiac myocytes.

METHODS AND RESULTS

ET1 induced HDAC5 phosphorylation and nuclear export in ventricular myocytes from the adult rat heart. Use of a novel, highly selective protein kinase D (PKD) inhibitor and a nonphosphorylatable HDAC5 mutant revealed that PKD-mediated phosphorylation was necessary for ET1-induced HDAC5 nuclear export. In contrast, ISO reduced HDAC5 phosphorylation in the presence or absence of ET1 but still induced HDAC5 nuclear export. ISO-induced HDAC5 nuclear export occurred through a β(1)-AR-mediated oxidative process that was independent of PKD, protein kinase A, and Ca(2+)/calmodulin-dependent kinase II activities. Although ET1 and ISO shared a similar ability to induce HDAC5 nuclear export, albeit through distinct phosphorylation-dependent versus phosphorylation-independent mechanisms, ISO induced a significantly greater increase in MEF2 activity.

CONCLUSIONS

PKD-mediated HDAC5 phosphorylation and nuclear export are unlikely to be of major importance in regulating MEF2-driven cardiac remodeling in the presence of sympathetic activity with intact β(1)-AR signaling, which would not only counteract HDAC5 phosphorylation but also induce HDAC5 nuclear export through a novel phosphorylation-independent, oxidation-mediated mechanism. Inhibition of this mechanism may contribute to the therapeutic efficacy of β(1)-AR antagonists in heart failure.

Neuronal nitric oxide synthase is up-regulated by angiotensin II and attenuates NADPH oxidase activity and facilitates relaxation in murine left ventricular myocytes

Angiotensin II (Ang II) is critical in myocardial pathogenesis, mostly via stimulating NADPH oxidase. Neuronal nitric oxide synthase (nNOS) has recently been shown to play important roles in modulating myocardial oxidative stress and contractility. Here, we examine whether nNOS is regulated by Ang II and affects NADPH oxidase production of intracellular reactive oxygen species (ROS(i)) and contractile function in left ventricular (LV) myocytes. Our results showed that Ang II induced biphasic effects on ROS(i) and LV myocyte relaxation (TR(50)) without affecting the amplitude of sarcomere shortening and L-type Ca(2+) current density: TR(50) was prolonged at 30 min but was shortened after 3h (or after Ang II treatment in vivo). Correspondingly, ROS(i) was increased, followed by a reduction to control level. Quantitative RT-PCR and immunoblotting experiments showed that Ang II (3h) increased the mRNA and protein expression of nNOS and increased NO production (nitrite assay) in LV myocyte homogenates, suggesting that nNOS activity may be enhanced and involved in mediating the effects of Ang II. Indeed, n(omega)-nitro-l-arginine methyl ester (l-NAME) or a selective nNOS inhibitor, S-methyl-l-thiocitrulline (SMTC) increased NADPH oxidase production of superoxide/ROS(i) and abolished faster myocyte relaxation induced by Ang II. The positive lusitropic effect of Ang II was not mediated by PKA-, CaMKII-dependent signaling or peroxynitrite. Conversely, inhibition of cGMP/PKG pathway abolished the Ang II-induced faster relaxation by reducing phospholamban (PLN) Ser(16) phosphorylation. Taken together, these results clearly demonstrate that myocardial nNOS is up-regulated by Ang II and functions as an early adaptive mechanism to attenuate NADPH oxidase activity and facilitate myocardial relaxation.

Protein kinase D in the hypertrophy pathway

Chronic neurohormonal stimulation can have direct adverse effects on the structure and function of the heart. Heart failure develops and progresses as a result of the deleterious changes. It is well established that phosphorylation of class II HDAC5 (histone deacetylase 5) is an important event in the transcriptional regulation of cardiac gene reprogramming that results in the hypertrophic growth response. To date, experimentation on phosphorylation-mediated translocation of HDAC5 has focused on the regulatory properties of PKD (protein kinase D) within intact cells. With regard to the potential role of PKD in myocardium, recent observations raise the possibility that PKD-mediated myocardial regulatory mechanisms may represent promising therapeutic avenues for the treatment of heart failure. The present review summarizes the most recent and important insights into the role of PKD in hypertrophic signalling pathways.

The biology and therapeutic implications of HDACs in the heart

The heart responds to stresses such as chronic hypertension and myocardial infarction by undergoing a remodeling process that is associated with myocyte hypertrophy, myocyte death, inflammation and fibrosis, often resulting in impaired cardiac function and heart failure. Recent studies have revealed key roles for histone deacetylases (HDACs) as both positive and negative regulators of pathological cardiac remodeling, and small molecule HDAC inhibitors have demonstrated efficacy in animal models of heart failure. This chapter reviews the functions of individual HDAC isoforms in the heart and highlights issues that need to be addressed to enable development of novel HDAC-directed therapies for cardiovascular indications.

cMyBP-C as a promiscuous substrate: phosphorylation by non-PKA kinases and its potential significance

It is now generally accepted that phosphorylation of cMyBP-C is critically important in maintaining normal cardiac function. Although much of the work to date on phospho-regulation of cMyBP-C has focused on the role of protein kinase A (PKA, also known as cAMP-dependent protein kinase), recent evidence suggests that a number of non-PKA serine/threonine kinases, such as Ca(2+)/calmodulin-dependent protein kinase II, protein kinase C, protein kinase D and the 90-kDa ribosomal S6 kinase are also capable of targeting this key regulatory sarcomeric protein. This article reviews such evidence and proposes a hypothetical role for some of the pertinent signalling pathways in phospho-regulation of cMyBP-C in the setting of heart failure.

Phosphodiesterase type-2 and NO-dependent S-nitrosylation mediate the cardioinhibition of the antihypertensive catestatin

The chromogranin A (CHGA)-derived peptide catestatin (CST: hCHGA(352-372)) is a noncompetitive catecholamine-release inhibitor that exerts vasodilator, antihypertensive, and cardiosuppressive actions. We have shown that CST directly influences the basal performance of the vertebrate heart where CST dose dependently induced a nitric oxide-cGMP-dependent cardiosuppression and counteracted the effects of adrenergic stimulation through a noncompetitive antagonism. Here, we sought to determine the specific intracardiac signaling activated by CST in the rat heart. Physiological analyses performed on isolated, Langendorff-perfused cardiac preparations revealed that CST-induced negative inotropism and lusitropism involve β(2)/β(3)-adrenergic receptors (β(2)/β(3)-AR), showing a higher affinity for β(2)-AR. Interaction with β(2)-AR activated phosphatidylinositol 3-kinase/endothelial nitric oxide synthase (eNOS), increased cGMP levels, and induced activation of phosphodiesterases type 2 (PDE2), which was found to be involved in the antiadrenergic action of CST as evidenced by the decreased cAMP levels. CST-dependent negative cardiomodulation was abolished by functional denudation of the endothelium with Triton. CST also increased the eNOS expression in cardiac tissue and human umbilical vein endothelial cells. cells, confirming the involvement of the vascular endothelium. In ventricular extracts, CST increased S-nitrosylation of both phospholamban and β-arrestin, suggesting an additional mechanism for intracellular calcium modulation and β-adrenergic responsiveness. We conclude that PDE2 and S-nitrosylation play crucial roles in the CST regulation of cardiac function. Our results are of importance in relation to the putative application of CST as a cardioprotective agent against stress, including excessive sympathochromaffin overactivation.

Cyclic nucleotide phosphodiesterase 1A: a key regulator of cardiac fibroblast activation and extracellular matrix remodeling in the heart

Cardiac fibroblasts become activated and differentiate to smooth muscle-like myofibroblasts in response to hypertension and myocardial infarction (MI), resulting in extracellular matrix (ECM) remodeling, scar formation and impaired cardiac function. cAMP and cGMP-dependent signaling have been implicated in cardiac fibroblast activation and ECM synthesis. Dysregulation of cyclic nucleotide phosphodiesterase (PDE) activity/expression is also associated with various diseases and several PDE inhibitors are currently available or in development for treating these pathological conditions. The objective of this study is to define and characterize the specific PDE isoform that is altered during cardiac fibroblast activation and functionally important for regulating myofibroblast activation and ECM synthesis. We have found that Ca(2+)/calmodulin-stimulated PDE1A isoform is specifically induced in activated cardiac myofibroblasts stimulated by Ang II and TGF-β in vitro as well as in vivo within fibrotic regions of mouse, rat, and human diseased hearts. Inhibition of PDE1A function via PDE1-selective inhibitor or PDE1A shRNA significantly reduced Ang II or TGF-β-induced myofibroblast activation, ECM synthesis, and pro-fibrotic gene expression in rat cardiac fibroblasts. Moreover, the PDE1 inhibitor attenuated isoproterenol-induced interstitial fibrosis in mice. Mechanistic studies revealed that PDE1A modulates unique pools of cAMP and cGMP, predominantly in perinuclear and nuclear regions of cardiac fibroblasts. Further, both cAMP-Epac-Rap1 and cGMP-PKG signaling was involved in PDE1A-mediated regulation of collagen synthesis. These results suggest that induction of PDE1A plays a critical role in cardiac fibroblast activation and cardiac fibrosis, and targeting PDE1A may lead to regression of the adverse cardiac remodeling associated with various cardiac diseases.

A bioluminescence method for direct measurement of phosphodiesterase activity

We have adapted bioluminescence methods to be able to measure phosphodiesterase (PDE) activity in a one-step technique. The method employs a four-enzyme system (PDE, adenylate kinase (AK) using excess CTP instead of ATP as substrate, pyruvate kinase (PK), and firefly luciferase) to generate ATP, with measurement of the concomitant luciferase-light emission. Since AK, PK, and luciferase reactions are coupled to recur in a cyclic manner, AMP recycling maintains a constant rate of ATP formation, proportional to the steady-state AMP concentration. The cycle can be initiated by the PDE reaction that yields AMP. As long as the PDE reaction is rate limiting, the system is effectively at steady state and the bioluminescence kinetics progresses at a constant rate proportional to the PDE activity. In the absence of cAMP and PDE, low concentrations of AMP trigger the AMP cycling, which allows standardizing the system. The sensitivity of the method enables detection of <1 μU (pmol/min) of PDE activity in cell extracts containing 0.25-10 μg protein. Assays utilizing pure enzyme showed that 0.2 mM IBMX completely inhibited PDE activity. This single-step enzyme- and substrate-coupled cyclic-reaction system yields a simplified, sensitive, reproducible, and accurate method for quantifying PDE activities in small biological samples.

β-Adrenergic receptor stimulation and activation of protein kinase A protect against α1-adrenergic-mediated phosphorylation of protein kinase D and histone deacetylase 5

INTRODUCTION

Chronic activation of β(1)-adrenergic receptor (β(1)-AR) signaling can have deleterious effects on the heart, and animal models overexpressing β(1)-ARs develop a dilated cardiomyopathy and heart failure. In the classic β-AR pathway, receptor occupancy by an agonist results in increased cyclic adenosine monophosphate (cAMP) levels and activation of protein kinase A (PKA). However, the role of PKA-dependent signaling in the development and progression of cardiomyopathies and heart failure is controversial, because β-AR signal transduction is generally desensitized in the failing heart and PKA activity is not increased.

METHODS AND RESULTS

Neonatal rat ventricular myocytes were acutely (15 minutes) or chronically (48 hours) treated with isoproterenol, and phosphorylation of protein kinase D (PKD) and histone deacetylase 5 (HDAC5) was measured. Acute β(1)-AR stimulation or expression of constitutively active (CA) PKA reduced α(1)-adrenergic-mediated phosphorylation of HDAC5 and PKD by activation of a phosphatase. Overexpression of CA-PKA also reduced α(1)-adrenergic-mediated increased expression of contractile protein fetal isoforms and promoted repression of adult isoforms, but had no effect on α(1)-adrenergic-mediated cellular hypertrophy.

CONCLUSIONS

These data indicate that the PKA-dependent arm of β-AR signaling can be antihypertrophic and presumably beneficial, through dephosphorylation of PKD and HDAC5 and reduction of hypertrophic fetal isoform gene expression.

Chronic cardiac pressure overload induces adrenal medulla hypertrophy and increased catecholamine synthesis

Increased activity of the sympathetic system is an important feature contributing to the pathogenesis and progression of chronic heart failure. While the mechanisms and consequences of enhanced norepinephrine release from sympathetic nerves have been intensely studied, the role of the adrenal gland in the development of cardiac hypertrophy and progression of heart failure is less well known. Thus, the aim of the present study was to determine the effect of chronic cardiac pressure overload in mice on adrenal medulla structure and function. Cardiac hypertrophy was induced in wild-type mice by transverse aortic constriction (TAC) for 8 weeks. After TAC, the degree of cardiac hypertrophy correlated significantly with adrenal weight and adrenal catecholamine storage. In the medulla, TAC caused an increase in chromaffin cell size but did not result in chromaffin cell proliferation. Ablation of chromaffin α(2C)-adrenoceptors did not affect adrenal weight or epinephrine synthesis. However, unilateral denervation of the adrenal gland completely prevented adrenal hypertrophy and increased catecholamine synthesis. Transcriptome analysis of microdissected adrenal medulla identified 483 up- and 231 downregulated, well-annotated genes after TAC. Among these genes, G protein-coupled receptor kinases 2 (Grk2) and 6 and phenylethanolamine N-methyltransferase (Pnmt) were significantly upregulated by TAC. In vitro, acetylcholine-induced Pnmt and Grk2 expression as well as enhanced epinephrine content was prevented by inhibition of nicotinic acetylcholine receptors and Ca(2+)/calmodulin-dependent signaling. Thus, activation of preganglionic sympathetic nerves innervating the adrenal medulla plays an essential role in inducing adrenal hypertrophy, enhanced catecholamine synthesis and induction of Grk2 expression after cardiac pressure overload.

Disruption of the cyclic AMP phosphodiesterase-4 (PDE4)-HSP20 complex attenuates the β-agonist induced hypertrophic response in cardiac myocytes

The small heat shock protein HSP20 is known to be cardioprotective during times of stress and the mechanism underlying its protective abilities depends on its phosphorylation on Ser16 by PKA (protein kinase A). Although the external stimuli that trigger Ser16 phosphorylation have been well studied, the events that modulate spatial and temporal control of this modification remain to be clarified. Here, we report that inhibition of cAMP phosphodiesterase-4 (PDE4) induces the phosphorylation of HSP20 in resting cardiac myocytes and augments its phosphorylation by PKA following β-adrenergic stimulation. Moreover, using peptide array technology, in vitro binding studies, co-immunoprecipitation techniques and immunocytochemistry, we show that HSP20 binds directly to PDE4 within a region of the conserved catalytic domain. We also show that FRET-based, genetically-encoded cAMP reporters anchored to HSP20 exhibit a larger response to PDE4 inhibition compared to free cytosolic cAMP reporters, suggesting that the interaction with PDE4 is crucial in modulating the highly localised pool of cAMP to which HSP20 is exposed. Using information gleaned from peptide array analyses, we developed a cell-permeable peptide that serves to inhibit the interaction of PDE4 with HSP20. Disruption of the HSP20-PDE4 complex, using this peptide, suffices to induce phosphorylation of HSP20 by PKA and to protect against the hypertrophic response measured in neonatal cardiac myocytes following chronic β-adrenergic stimulation.

Conserved expression and functions of PDE4 in rodent and human heart

PDE4 isoenzymes are critical in the control of cAMP signaling in rodent cardiac myocytes. Ablation of PDE4 affects multiple key players in excitation–contraction coupling and predisposes mice to the development of heart failure. As little is known about PDE4 in human heart, we explored to what extent cardiac expression and functions of PDE4 are conserved between rodents and humans. We find considerable similarities including comparable amounts of PDE4 activity expressed, expression of the same PDE4 subtypes and splicing variants, anchoring of PDE4 to the same subcellular compartments and macromolecular signaling complexes, and downregulation of PDE4 activity and protein in heart failure. The major difference between the species is a fivefold higher amount of non-PDE4 activity in human hearts compared to rodents. As a consequence, the effect of PDE4 inactivation is different in rodents and humans. PDE4 inhibition leads to increased phosphorylation of virtually all PKA substrates in mouse cardiomyocytes, but increased phosphorylation of only a restricted number of proteins in human cardiomyocytes. Our findings suggest that PDE4s have a similar role in the local regulation of cAMP signaling in rodent and human heart. However, inhibition of PDE4 has ‘global’ effects on cAMP signaling only in rodent hearts, as PDE4 comprises a large fraction of the total cardiac PDE activity in rodents but not in humans. These differences may explain the distinct pharmacological effects of PDE4 inhibition in rodent and human hearts.