Dissociation of p44 and p42 mitogen-activated protein kinase activation from receptor-induced hypertrophy in neonatal rat ventricular myocytes

The Pleiotropic Role of Extracellular ATP in Myocardial Remodelling

Myocardial remodelling is a molecular, cellular, and interstitial adaptation of the heart in response to altered environmental demands. The heart undergoes reversible physiological remodelling in response to changes in mechanical loading or irreversible pathological remodelling induced by neurohumoral factors and chronic stress, leading to heart failure. Adenosine triphosphate (ATP) is one of the potent mediators in cardiovascular signalling that act on the ligand-gated (P2X) and G-protein-coupled (P2Y) purinoceptors via the autocrine or paracrine manners. These activations mediate numerous intracellular communications by modulating the production of other messengers, including calcium, growth factors, cytokines, and nitric oxide. ATP is known to play a pleiotropic role in cardiovascular pathophysiology, making it a reliable biomarker for cardiac protection. This review outlines the sources of ATP released under physiological and pathological stress and its cell-specific mechanism of action. We further highlight a series of cardiovascular cell-to-cell communications of extracellular ATP signalling cascades in cardiac remodelling, which can be seen in hypertension, ischemia/reperfusion injury, fibrosis, hypertrophy, and atrophy. Finally, we summarize current pharmacological intervention using the ATP network as a target for cardiac protection. A better understanding of ATP communication in myocardial remodelling could be worthwhile for future drug development and repurposing and the management of cardiovascular diseases.

Sympathetic Stimulation Upregulates the Ca2+ Channel Subunit, CaVα2δ1, via the β1 and ERK 1/2 Pathway in Neonatal Ventricular Cardiomyocytes

Intracellular Ca2+ overload secondary to chronic hemodynamic stimuli promotes the recruitment of Ca2+-dependent signaling implicated in cardiomyocyte hypertrophy. The present study tested the hypothesis that sympathetic-mediated hypertrophy of neonatal rat ventricular cardiomyocytes (NRVMs) translated to an increase in calcium influx secondary to the upregulation of CaV1.2 channel subunits. Confocal imaging of norepinephrine (NE)-treated NRVMs revealed a hypertrophic response compared to untreated NRVMs. L-type CaV1.2 peak current density was increased 4-fold following a 24-h stimulation with NE. NE-treated NRVMs exhibited a significant upregulation of CaVα2δ1 and CaVβ3 protein levels without significant changes of CaVα1C and CaVβ2 protein levels. Pre-treatment with the β1-blocker metoprolol failed to inhibit hypertrophy or CaVβ3 upregulation whereas CaVα2δ1 protein levels were significantly reduced. NE promoted the phosphorylation of ERK 1/2, and the response was attenuated by the β1-blocker. U0126 pre-treatment suppressed NE-induced ERK1/2 phosphorylation but failed to attenuate hypertrophy. U0126 inhibition of ERK1/2 phosphorylation prevented NE-mediated upregulation of CaVα2δ1, whereas CaVβ3 protein levels remained elevated. Thus, β1-adrenergic receptor-mediated recruitment of the ERK1/2 plays a seminal role in the upregulation of CaVα2δ1 in NRVMs independent of the concomitant hypertrophic response. However, the upregulation of CaVβ3 protein levels may be directly dependent on the hypertrophic response of NRVMs.

Tianeptine induces expression of dual specificity phosphatases and evokes rebound emergence of cortical slow wave electrophysiological activity

BACKGROUND

The precise mechanism governing the antidepressant effects of tianeptine is unknown. Modulation of brain glutamatergic neurotransmission has been however implicated, suggesting potential shared features with rapid-acting antidepressants targeting N-methyl-D-aspartate receptors (NMDAR). Our recent studies suggest that a single subanesthetic dose of NMDAR antagonists ketamine or nitrous oxide (N₂O) gradually evoke 1-4 Hz electrophysiological activity (delta-rhythm) of cerebral cortex that is accompanied by molecular signaling associated with synaptic plasticity (e.g. activation of tropomyosin receptor kinase B (TrkB) and inhibition of glycogen synthase kinase 3β (GSK3β)).

METHODS

We have here investigated the time-dependent effects of tianeptine (30 mg/kg, i.p.) on electrocorticogram, focusing on potential biphasic regulation of the delta-rhythm. Selected molecular markers associated with ketamine's antidepressant effects were analyzed in the medial prefrontal cortex after the treatment using quantitative polymerase chain reaction and western blotting.

RESULTS

An acute tianeptine treatment induced changes of electrocorticogram typical for active wakefulness that lasted for 2-2.5 h, which was followed by high amplitude delta-activity rebound. The levels of Arc and Homer1a, but not c-Fos, BdnfIV and Zif268, were increased by tianeptine. Phosphorylation of mitogen-activated protein kinase (MAPK), TrkB and GSK3β remained unaltered at 2-hours and at 3-hours post-treatment. Notably, tianeptine also increased the level of mRNA of several dual specificity phosphatases (Duspss) - negative regulators of MAPK.

CONCLUSION

Tianeptine produces acute changes of electrocorticogram resembling rapid-acting antidepressants ketamine and N₂O. Concomitant regulation of Dusps may hamper the effects of tianeptine on MAPK pathway and influence the magnitude of homeostatic emergence of delta-activity and TrkB-GSK3β signaling.

Dendrobium candidum aqueous extract attenuates isoproterenol-induced cardiac hypertrophy through the ERK signalling pathway

CONTEXT

The pharmacological functions of Dendrobium candidum Wall. ex Lindl. (Orchidaceae) in cardiac hypertrophy remains unclear.

OBJECTIVE

To evaluate whether D. candidum aqueous extract (DCAE) can attenuate experimental cardiac hypertrophy.

MATERIALS AND METHODS

Cardiac hypertrophy in SD rats was induced by subcutaneously injection of isoproterenol (2 mg/kg), once a day for ten days. Rats were gavaged with DCAE (0.13 and 0.78 g/kg) daily for one month. At the end of treatment, measurement of left ventricular systolic pressure (LVSP), heart-to-body weight ratio (HW/BW), left ventricular/tibia length (LV/TL), atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) levels, haematoxylin-eosin staining, and Masson's trichrome staining were conducted. In cultured H9c2 cells, DCAE (2 mg/mL) and U0126 (10 μM) were added 2 h before the isoproterenol (10 μM) stimulus. Phalloidin staining was used to evaluate cellular hypertrophy. The mRNA expression of ANP and BNP was measured by qRT-PCR. The expression of p-ERK was determined by immunoblotting.

RESULTS

DCAE treatment significantly reduced the following indicators in vivo: (1) the LVSP (16%); (2) HW/BW (13%); (3) LV/TL (6%); (4) ANP (39%); (5) BNP (32%). In cultured H9c2 cells, phalloidin staining showed that DCAE relieved cellular hypertrophy (53% reduction). Furthermore, immunoblotting showed that DCAE can significantly inhibit p-ERK protein expression in vivo and in vitro (39% and 27% reduction, respectively).

DISCUSSION AND CONCLUSIONS

DCAE prevents cardiac hypertrophy via ERK signalling pathway and has the potential for treatment of cardiac hypertrophy.

Role of extracellular signal-regulated kinase 1/2 signaling underlying cardiac hypertrophy

Cardiac hypertrophy is the result of increased myocardial cell size responding to an increased workload and developmental signals. These extrinsic and intrinsic stimuli as key drivers of cardiac hypertrophy have spurred efforts to target their associated signaling pathways. The extracellular signal-regulated kinases 1/2 (ERK1/2), as an essential member of mitogen-activated protein kinases (MAPKs), has been widely recognized for promoting cardiac growth. Several modified transgenic mouse models have been generated through either affecting the upstream kinase to change ERK1/2 activity, manipulating the direct role of ERK1/2 in the heart, or targeting phosphatases or MAPK scaffold proteins to alter total ERK1/2 activity in response to an increased workload. Using these models, both regulation of the upstream events and modulation of each isoform and indirect effector could provide important insights into how ERK1/2 modulates cardiomyocyte biology. Furthermore, a plethora of compounds, inhibitors, and regulators have emerged in consideration of ERK, or its MAPK kinases, are possible therapeutic targets against cardiac hypertrophic diseases. Herein, is a review of the available evidence regarding the exact role of ERK1/2 in regulating cardiac hypertrophy and a discussion of pharmacological strategy for treatment of cardiac hypertrophy.

TRPC3-Nox2 axis mediates nutritional deficiency-induced cardiomyocyte atrophy

Myocardial atrophy, characterized by the decreases in size and contractility of cardiomyocytes, is caused by severe malnutrition and/or mechanical unloading. Extracellular adenosine 5′-triphosphate (ATP), known as a danger signal, is recognized to negatively regulate cell volume. However, it is obscure whether extracellular ATP contributes to cardiomyocyte atrophy. Here, we report that ATP induces atrophy of neonatal rat cardiomyocytes (NRCMs) without cell death through P2Y₂ receptors. ATP led to overproduction of reactive oxygen species (ROS) through increased amount of NADPH oxidase (Nox) 2 proteins, due to increased physical interaction between Nox2 and canonical transient receptor potential 3 (TRPC3). This ATP-mediated formation of TRPC3-Nox2 complex was also pathophysiologically involved in nutritional deficiency-induced NRCM atrophy. Strikingly, knockdown of either TRPC3 or Nox2 suppressed nutritional deficiency-induced ATP release, as well as ROS production and NRCM atrophy. Taken together, we propose that TRPC3-Nox2 axis, activated by extracellular ATP, is the key component that mediates nutritional deficiency-induced cardiomyocyte atrophy.

Maslinic acid protects against pressure overload-induced cardiac hypertrophy in mice

Cardiac hypertrophy is characterized by myocyte hypertrophy, accumulation of cardiac collagen, and reactivation of fetal genes. Maslinic acid (MA) is a pentacyclic triterpene with abundance in olive fruit skin and possesses a number of pharmacological actions. However, its effect on pressure overload-induced cardiac hypertrophy remains unknown. Here, we were to investigate the protective effect of MA on cardiac hypertrophy and fibrosis. C57 mice were subjected to aortic banding (AB) or sham surgery. One day after surgery, all the mice were orally given MA (20 mg/kg) or vehicle for the following four weeks. MA could protect against pressure overload-induced cardiac hypertrophy and cardiac fibrosis, as indicated by decreased heart weight/tibia length, and cardiomyocytes cell area and hypertrophic and fibrotic markers. MA treatment also improved cardiac function in mice with AB surgery, as assessed by echocardiographic and hemodynamic analysis. MA reduced phosphorylation of protein kinase B and extracellular regulated protein kinases in the hypertrophic hearts. MA could decrease cardiomyocyte hypertrophy, and inhibit the activation of AKT and ERK signaling pathway in vitro. In conclusion, we found that MA protected against cardiac hypertrophy. MA has the potential to become a therapeutic drug for cardiac hypertrophy.

Role of inositol 1,4,5-trisphosphate receptor type 1 in ATP-induced nuclear Ca2+ signal and hypertrophy in atrial myocytes

Inositol 1,4,5-trisphosphate receptor type 1 (IP₃R1) is expressed in atrial muscle, but not in ventricle, and they are abundant in the perinucleus. We investigated the role of IP₃R1 in the regulations of local Ca²⁺ signal and cell size in HL-1 atrial myocytes under stimulation by IP₃-generating chemical messenger, ATP. Assessment of nuclear and cytosolic Ca²⁺ signal using confocal Ca²⁺ imaging revealed that IP₃ generation by ATP (1 mM) induced monophasic nuclear Ca²⁺ increase, followed by cytosolic Ca²⁺ oscillation. Genetic knock-down (KD) of IP₃R1 eliminated the monophasic nuclear Ca²⁺ signal and slowed the cytosolic Ca²⁺ oscillation upon ATP exposure. Prolonged application of ATP as well as other known hypertrophic agonists (endothelin-1 and phenylephrine) increased cell size in wild-type cells, but not in IP₃R1 KD cells. Our data indicate that IP₃R1 mediates sustained elevation in nuclear Ca²⁺ level and facilitates cytosolic Ca²⁺ oscillation upon external ATP increase, and further suggests possible role of nuclear IP₃R1 in atrial hypertrophy.

TRPC5-eNOS Axis Negatively Regulates ATP-Induced Cardiomyocyte Hypertrophy

Cardiac hypertrophy, induced by neurohumoral factors, including angiotensin II and endothelin-1, is a major predisposing factor for heart failure. These ligands can induce hypertrophic growth of neonatal rat cardiomyocytes (NRCMs) mainly through Ca2+-dependent calcineurin/nuclear factor of activated T cell (NFAT) signaling pathways activated by diacylglycerol-activated transient receptor potential canonical 3 and 6 (TRPC3/6) heteromultimer channels. Although extracellular nucleotide, adenosine 5'-triphosphate (ATP), is also known as most potent Ca2+-mobilizing ligand that acts on purinergic receptors, ATP never induces cardiomyocyte hypertrophy. Here we show that ATP-induced production of nitric oxide (NO) negatively regulates hypertrophic signaling mediated by TRPC3/6 channels in NRCMs. Pharmacological inhibition of NO synthase (NOS) potentiated ATP-induced increases in NFAT activity, protein synthesis, and transcriptional activity of brain natriuretic peptide. ATP significantly increased NO production and protein kinase G (PKG) activity compared to angiotensin II and endothelin-1. We found that ATP-induced Ca2+ signaling requires inositol 1,4,5-trisphosphate (IP3) receptor activation. Interestingly, inhibition of TRPC5, but not TRPC6 attenuated ATP-induced activation of Ca2+/NFAT-dependent signaling. As inhibition of TRPC5 attenuates ATP-stimulated NOS activation, these results suggest that NO-cGMP-PKG axis activated by IP3-mediated TRPC5 channels underlies negative regulation of TRPC3/6-dependent hypertrophic signaling induced by ATP stimulation.

Lactosylceramide promotes hypertrophy through ROS generation and activation of ERK1/2 in cardiomyocytes

Hypertrophy is central to several heart diseases; however, not much is known about the role of glycosphingolipids (GSLs) in this phenotype. Since GSLs have been accorded several physiological functions, we sought to determine whether these compounds affect cardiac hypertrophy. By using a rat cardiomyoblast cell line, H9c2 cells and cultured primary neonatal rat cardiomyocytes, we have determined the effects of GSLs on hypertrophy. Our study comprises (a) measurement of [(3)H]-leucine incorporation into protein, (b) measurement of cell size and morphology by immunofluorescence microscopy and (c) real-time quantitative mRNA expression assay for atrial natriuretic peptide and brain natriuretic peptide. Phenylephrine (PE), a well-established agonist of cardiac hypertrophy, served as a positive control in these studies. Subsequently, mechanistic studies were performed to explore the involvement of various signaling transduction pathways that may contribute to hypertrophy in these cardiomyocytes. We observed that lactosylceramide specifically exerted a concentration- (50-100 µM) and time (48 h)-dependent increase in hypertrophy in cardiomyocytes but not a library of other structurally related GSLs. Further, in cardiomyocytes, LacCer generated reactive oxygen species, stimulated the phosphorylation of p44 mitogen activated protein kinase and protein kinase-C, and enhanced c-jun and c-fos expression, ultimately leading to hypertrophy. In summary, we report here that LacCer specifically induces hypertrophy in cardiomyocytes via an "oxygen-sensitive signal transduction pathway."

A positive inotropic effect of UTP in the human cardiac atrium

In the cardio-vascular system extracellular UTP can induce receptor-mediated vasoconstriction via smooth muscle cells and vasodilatation via endothelial cells. We evaluated inotropic effects of UTP in preparations from human heart. Contractile effects were studied in atrial preparations from patients undergoing cardiac bypass surgery. For comparison, contractility in isolated spontaneously beating right atrial and paced left atrial preparations from mice was investigated. UTP and UTPγS concentration-dependently exerted a positive inotropic effect with a maximum at 100 µM UTP that amounted to 156% of pre-drug value (n=13) without changing time parameters of contraction. UTP was able to partially attenuate the positive inotropic effect of β-adrenoceptor stimulation. UTP did not change the beating rate in right atrial mouse preparations. The positive inotropic effect of UTP could not be blocked by the P2 purinoceptor antagonists suramin (100 µM and 500 µM), PPADS (50 µM) and reactive blue (100 µM). Likewise inhibitors of PLC activity (U73122) and of adenylyl cyclase activity (SQ22563; 10 µM each) failed to affect the effects of UTP. In summary, we describe a novel positive inotropic effect of UTP on force contraction in the isolated human atrium. We tentatively suggest that UTP might act via P2Y2- or P2Y4-like receptors.

Cardiovascular catecholamine receptors in children: their significance in cardiac disease

Adrenoceptors and dopamine receptors are grouped together under the name 'catecholamine receptors.' Catecholamines and catecholaminergic drugs act on catecholamine receptors located on or near the cardiovascular system. The physiological effects of catecholamine receptor stimulation are only partly understood. The catecholaminergic drugs used in critical care medicine today are not selective, or are, at best, in part selective for the various catecholamine receptor subtypes. Many patients, however, depend on them. A variety of animal models has been developed to unravel catecholamine distribution and function. However, the identification of species heterogeneity makes it imperative to determine catecholamine receptor distribution and function in humans. In addition, age-related alterations in catecholamine receptor distribution and function have been identified in human adults. This might have implications for our understanding of the effect of catecholamines in pediatric patients. This article will focus on the pediatric population and will review currently available in vitro data on the distribution and the function of catecholamine receptors in the cardiovascular system of fetuses and children. Also discussed are relevant young animal models and in vivo hemodynamic effects of cardiotonic drugs acting on the catecholamine receptor in children requiring major cardiac surgery. A better understanding of these topics might provide clues for new, receptor subtype-selective, therapeutic approaches in newborns and children with cardiac disease.

Roles of heterotrimeric GTP-binding proteins in the progression of heart failure

Heart failure is a major cause of death in developed countries, and the development of an epoch-making cure is desired from the viewpoint for improving the quality of life and reducing the medical cost of the patient. The importance of neurohumoral factors, such as angiotensin (Ang) II and catecholamine, for the progression of heart failure has been supported by a variety of evidence. These agonists stimulate seven transmembrane-spanning receptors that are coupled to heterotrimeric GTP-binding proteins (G proteins). Using specific pharmacological tools to assess the involvement of G protein signaling pathways, we have revealed that α subunit of G(q) (Gα(q)) activates Ca(2+)-dependent hypertrophic signaling through diacylglycerol-activated transient receptor potential canonical (TRPC) channels (TRPC3 and TRPC6: TRPC3/6). In contrast, activation of Gα(12) family proteins in cardiomyocytes confers pressure overload-induced cardiac fibrosis via stimulation of purinergic P2Y(6) receptors induced by extracellular nucleotides released from cardiomyocytes. In fact, direct or indirect inhibition of TRPC3/6 or P2Y(6) receptors attenuates pressure overload-induced cardiac dysfunction. These findings will provide a new insight into the molecular mechanisms underlying pathogenesis of heart failure.

A novel chimeric natriuretic peptide reduces cardiomyocyte hypertrophy through the NHE-1-calcineurin pathway

AIMS

Natriuretic peptides (NPs) inhibit cardiomyocyte hypertrophy through a cyclic GMP (cGMP)-dependent process, although these effects are associated with substantial vasodilatation. In this study, we used CU-NP, a non-vasodilatating novel NP synthesized from the ring structure of human C-type NP (CNP) and both C- and N-termini of urodilatin, and investigated whether it can directly modulate cardiomyocyte hypertrophy.

METHODS AND RESULTS

Experiments were carried out in cultured neonatal rat ventricular myocytes exposed to phenylephrine, angiotensin II, or endothelin-1 in the absence or presence of CU-NP. CU-NP produced a concentration- and time-dependent increase in intracellular cGMP levels. The hypertrophic responses to all agonists were abrogated by 10 nM CU-NP. CU-NP treatment also prevented increased activity, gene and protein expression of sodium-hydrogen exchanger-1 (NHE-1) as well as elevations in intracellular Na(+) concentrations caused by hypertrophic agents. In addition, these effects were associated with a more than two-fold increase in activity of the Ca(2+)-dependent protein phosphatase calcineurin that peaked 6 h after addition of hypertrophic stimuli. Early (1-3 h) calcineurin activation was unaffected by CU-NP, although activation at 6 and 24 h was prevented by CU-NP as was the resultant translocation of the transcriptional factor NFAT into nuclei.

CONCLUSION

Our study demonstrates a direct anti-hypertrophic effect of the chimeric peptide CU-NP via NHE-1 inhibition, thereby preventing calcineurin activation and NFAT nuclear import. Thus, CU-NP represents a novel fusion peptide of CNP and urodilatin that has the potential to be developed into a therapeutic agent to treat cardiac hypertrophy and heart failure.

IGF-1 expression in infarcted myocardium and MGF E peptide actions in rat cardiomyocytes in vitro

Insulinlike growth factor-1 (IGF-1) expression is implicated in myocardial pathophysiology, and two IGF-1 mRNA splice variants have been detected in rodents, IGF-1Ea and mechano-growth factor (MGF). We investigated the expression pattern of IGF-1 gene transcripts in rat myocardium from 1 h up to 8 wks after myocardial infarction induced by left anterior descending coronary artery ligation. In addition, we characterized IGF-1 and MGF E peptide action and their respective signaling in H9C2 myocardial-like cells in vitro. IGF-1Ea and MGF expression were significantly increased, both at transcriptional and translational levels, during the late postinfarction period (4 and 8 wks) in infarcted rat myocardium. Measurements of serum IGF-1 levels in infarcted rats were initially decreased (24 h up to 1 wk) but remained unaltered throughout the late experimental phase (4 to 8 wks) compared with sham-operated rats. Furthermore, specific anti-IGF-1R neutralizing antibody failed to block the synthetic MGF E peptide action, whereas it completely blocked IGF-1 action on the proliferation of H9C2 cells. Moreover, this synthetic MGF E peptide did not activate Akt phosphorylation, whereas it activated ERK1/2 in H9C2 rat myocardial cells. These data support the role of IGF-1 expression in the myocardial repair process and suggest that synthetic MGF E peptide actions may be mediated via an IGF-1R independent pathway in rat myocardial cells, as suggested by our in vitro experiments.

With great power comes great responsibility: using mouse genetics to study cardiac hypertrophy and failure

Over the past 20 years generation and subsequent characterization of genetically modified mouse models has revolutionized our understanding of disease-gene relationships and suggested numerous therapeutic targets for human disease. Cardiac biology has perhaps benefited more than most fields from the advent of modern genetic approaches in the mouse by providing a 3-dimensional integrated platform for phenotypic dissection of single gene function, largely replacing the unitary relationships derived from 2-dimensional cell culture-based platforms. Indeed, cardiac hypertrophy and end-stage heart failure are whole organ phenomena that occur within a dynamic neuroendocrine milieu, a backdrop that cannot be adequately modeled in cultured myocytes. Here we advocate the use of genetically modified mouse models for studying cardiac biology and show how, if employed properly, these models will continue to provide highly reliable data sets that suggest disease-gene relationships and novel therapeutic targets. In addition to a discussion of proper technique and controls, we will highlight examples of genetic approaches in the mouse that suggest novel disease relationships and therapeutic treatments for human heart failure, insights not possible with other experimental systems. In the preceding review/editorial by Cook, Clerk and Sugden, a number of strong arguments are made detailing the potential short comings associated with genetic approaches in the mouse as a means of unraveling cardiac disease mechanisms. We take very little issue with these arguments per se, although here we attempt to put these shortcomings into a greater context that extends beyond a single experimental setting, as well as to carefully construct a counterpoint that delineates the advantages of genetic approaches in the mouse compared with any other system currently in use in cardiovascular biology.

Integrin stimulation-induced hypertrophy in neonatal rat cardiomyocytes is NO-dependent

Prolonged myocardial stretch typically leads to hypertrophy of cardiomyocytes. As integrins are cellular receptors of stretch, we hypothesize that integrin stimulation induces cardiomyocyte hypertrophy. Integrins of neonatal rat cardiomyocytes (NRCMs) were stimulated with a peptide containing the Arg-Gly-Asp (RGD) sequence for 24 h. For comparison, alpha(1)-adrenergic stimulation by phenylephrine (PE) for 24 h was applied. Saline-treated NRCMs were used as control. The hypertrophic response was quantified by measuring cell surface area (CSA). Phosphorylation of NO-synthase-1 (NOS1) was assessed by immunocytochemistry. CSA was increased by 38% (IQR 31-44%) with RGD and by 68% (IQR 64-84%) with PE versus control (both P < 0.001). NOS-1 phosphorylation was increased by 61% with RGD and by 21% with PE versus control (both P < 0.01). A general NOS-inhibitor (L-NAME) inhibited RGD-induced hypertrophy completely, but had no significant effect on PE-induced hypertrophy. Administration of NO-donor to NRCMs co-incubated with RGD + L-NAME partly restored hypertrophy (to 62% of the hypertrophic effect of RGD alone), but had no effect if incubated with PE + L-NAME. Ryanodine and BAPTA-AM inhibited RGD-induced hypertrophy completely but not that induced by PE. Integrin stimulation of NRCMs by RGD leads to hypertrophy, likely by activation of NOS-1. Abrogation of RGD-induced hypertrophic response upon NOS-inhibition and rescue of this hypertrophic effect by NO-donor suggest that integrin stimulation-induced hypertrophy of NRCMs is NO-dependent.

Role of Mitogen-Activated Protein (MAP) Kinases in Cardiovascular Diseases

Over the last decade important advances have been made in our understanding of the molecular events underlying cellular responses to extracellular signals. Increased understanding of signal transduction mechanisms and gene regulation involved in cardiovascular diseases has created opportunities for the discovery of novel therapeutic compounds useful for the treatment of cardiovascular disorders. One of the best-studied signalling routes is the mitogen activated protein (MAP) kinase signal transduction pathway, which plays a crucial role in many aspects of cardiovascular responses. Here, our current understanding of the MAP kinase pathway is reviewed, as well as recent advances in the design of novel agents that are able to modulate the activity of these signaling cascades.

Estrogen receptor-alpha mediates acute myocardial protection in females

Sex differences in myocardial recovery have been reported after acute ischemia and reperfusion injury. Estrogen and the estrogen receptor are critical determinants of cardiovascular sex differences. However, the mechanistic pathways responsible for these differences remain unknown. We hypothesized that estrogen receptor-alpha is an important modulator of 1) myocardial functional recovery after ischemia and 2) inflammatory signaling via MAPK. To study this, adult male and female wild-type (WT) and estrogen receptor-alpha knockout (ER1KO) mouse hearts were isolated, perfused via Langendorff model, and subjected to 20 min of ischemia and 60 min of reperfusion. Myocardial contractile function (left ventricular developed pressure and positive and negative first derivative of pressure) was continuously recorded. After ischemia-reperfusion, hearts were assessed for expression of inflammatory cytokines (ELISA) and activation of MAPK and caspase-3 (Western blot analysis). Data were analyzed with two-way ANOVA or Student's t-test, and P < 0.05 was statistically significant. ER1KO females exhibited significantly less functional recovery than WT females and were similar to WT males. Activated ERK was increased in female WT hearts compared with female ER1KO. Activated JNK was decreased in female WT hearts compared with female ER1KO. No significant differences were found between male WT, female WT, male ER1KO, and female ER1KO in activated p38 MAPK, proinflammatory cytokine expression, and proapoptotic signaling. Estrogen receptor-alpha plays a role in the protection observed in the female heart. Differential activation of MAPK may mediate this protection. Further studies are necessary to delineate these mechanistic pathways.

Cardiac-specific transgenic overexpression of alpha1B-adrenergic receptors induce chronic activation of ERK MAPK signalling

Cardiomyocyte-specific overexpression of the wild-type alpha(1B)-adrenergic receptor (alpha(1B)-AR) produces a slowly progressing cardiomyopathy associated with clinical signs of heart failure and premature death around middle age (Lemire et al. 2001). In the heart, alpha(1)-AR activate the extracellular signal-regulated kinase (ERK) MAPK cascade. The aim of this project was to determine if cardiac-specific overexpression of the wild-type alpha(1B)-AR results in sustained activation of the ERK pathway. At 3 and 9 months, ERK activity was increased in alpha(1B)-AR overexpressing hearts relative to non-transgenic animals. Similarly, phosphorylation of MEK and p90(rsk) were also elevated. MAP kinase phosphatases (MKPs), which inactivate MAP kinases, are transcriptionally regulated. MKP2 mRNA levels were reduced at 3 months in alpha(1B)-AR overexpressing hearts. Interestingly, there was a general trend for reduced expression of MKP-1, -2, and -3 with increased age. In addition, expression of the modulatory calcineurin-interacting protein (MCIP) 1, an indicator of calcineurin activity, was elevated 3-fold in alpha(1B)-AR overexpressing hearts at both 3 and 9 months. These results indicate that the overexpression of the wild-type alpha(1B)-AR leads to chronic changes in the activation of signalling pathways previously shown to be associated with the hypertrophic response.

The novel angiotensin II type 1 receptor (AT1R)-associated protein ATRAP downregulates AT1R and ameliorates cardiomyocyte hypertrophy

Activation of angiotensin II (Ang II) type 1 receptor (AT1R) signaling is reported to play an important role in cardiac hypertrophy. We previously cloned a novel molecule interacting with the AT1R, which we named ATRAP (for Ang II type 1 receptor-associated protein). Here, we report that overexpression of ATRAP significantly decreases the number of AT1R on the surface of cardiomyocytes, and also decreases the degree of p38 mitogen-activated protein kinase phosphorylation, the activity of the c-fos promoter and protein synthesis upon Ang II treatment. These results indicate that ATRAP significantly promotes downregulation of the AT1R and further attenuates certain Ang II-mediated hypertrophic responses in cardiomyocytes.

Cyclic adenosine monophosphate inhibits nitric oxide-induced apoptosis of cardiac muscle cells in a c-Jun N-terminal kinase-dependent manner

Cyclic adenosine monophosphate (cAMP) modulates various agent-induced apoptosis. In this study, we observed that cAMP had a significantly protective effect on nitric oxide (NO)-induced cytotoxicity in H9c2 cardiac muscle cells. Pretreatment with DBcAMP (cAMP analogue) or forskolin (adenylyl cyclase activator) also significantly prevented the SNP-induced apoptosis in H9c2 cells. In contrast, H-89 or KT5720 (PKA inhibitor) reversed the protective effects of DBcAMP. In this study, DBcAMP or forskolin reduced SNP-induced JNK/SAPK activation to the basal level, but KT5720 reversed the inhibitory effects of these two agents. In contrast to JNK/SAPK activation, DBcAMP and forskolin significantly enhanced SNP-activated p38 MAPK phosphorylation and did not affect SNP-mediated ERK activation. KT5720 reversed the effects of DBcAMP and forskolin on p38 MAPK phosphorylation. The inhibition of the JNK pathway by transfection of a dominant negative mutant of JNK/SAPK markedly reduced the extent of SNP-induced cell death. Taken together, we suggest that JNK/SAPK is related to cAMP-protective effect in SNP-induced apoptosis. In addition, c-AMP relating agents protected SNP-induced cell death in neonatal rat ventricular cardiomyocytes. The cAMP-relating agent-induced protective effect is not restricted in H9c2 cardiac muscle cells.

Effect of angiotensin II type 2 receptor blockade on mitogen activated protein kinases during myocardial ischemia-reperfusion

Mitogen-activated protein kinases (MAPKs) have been implicated during ischemia-reperfusion (IR) and angiotensin II (AngII) type 2 receptor (AT2R) blockade has been shown to induce cardioprotection involving protein kinase Cepsilon (PKCepsilon) signaling after IR. We examined whether the 3 major MAPKs, p38, c-Jun NH2-terminal kinase (JNK-1 and JNK-2), and extracellular signal regulated kinases (ERK-1 and ERK-2) are activated after IR and whether treatment with the AT2R antagonist PD123,319 (PD) alters their expression. Isolated rat hearts were randomized to control (aerobic perfusion, 80 min), IR (no drug; 50 min of perfusion, 30 min global ischemia and 30 min reperfusion; working mode), and IR + PD (0.3 micromol/l) and left ventricular (LV) work was measured. We measured LV tissue content of p38, p-p38, p-JNK-1 (54 kDa), p-JNK-2 (46 kDa), p-ERK-1 (44 kDa), p-ERK-2 (42 kDa) and PKCepsilon proteins by immunoblotting and cGMP by enzyme immunoassay. IR resulted in significant LV dysfunction, increase in p-p38 and p-JNK-1/-2, no change in p-ERK-1/-2 or PKCepsilon, and decrease in cGMP. PD improved LV recovery after IR, induced a slight increase in p-p38 (p < 0.01 vs. control), normalized p-JNK-1, did not change p-ERK-1/-2, and increased PKCepsilon and cGMP. The overall results suggest that p38 and JNK might play a significant role in acute IR injury and the cardioprotective effect of AT2R blockade independent of ERK. The activation of p38 and JNKs during IR may be linked, in part, to AT2R stimulation.

Cardiac-specific disruption of the c-raf-1 gene induces cardiac dysfunction and apoptosis

The Raf/MEK/extracellular signal-regulated kinase (ERK) signaling pathway regulates diverse cellular processes such as proliferation, differentiation, and apoptosis and is implicated as an important contributor to the pathogenesis of cardiac hypertrophy and heart failure. To examine the in vivo role of Raf-1 in the heart, we generated cardiac muscle-specific Raf-1-knockout (Raf CKO) mice with Cre-loxP-mediated recombination. The mice demonstrated left ventricular systolic dysfunction and heart dilatation without cardiac hypertrophy or lethality. The Raf CKO mice showed a significant increase in the number of apoptotic cardiomyocytes. The expression level and activation of MEK1/2 or ERK showed no difference, but the kinase activity of apoptosis signal-regulating kinase 1 (ASK1), JNK, or p38 increased significantly compared with that in controls. The ablation of ASK1 rescued heart dysfunction and dilatation as well as cardiac fibrosis. These results indicate that Raf-1 promotes cardiomyocyte survival through a MEK/ERK-independent mechanism.

Urotensin II Promotes Hypertrophy of Cardiac Myocytes via Mitogen-Activated Protein Kinases

Urotensin II and its receptor are coexpressed in the heart and up-regulated during cardiac dysfunction. In cultured neonatal cardiomyocytes, we mimicked this up-regulation using an adenovirus to increase expression of the urotensin receptor. In this model system, urotensin II promoted strong hypertrophic growth and phenotypic changes, including cell enlargement and sarcomere reorganization. Urotensin II potently activated the MAPKs, ERK1/2 and p38, and blocking these kinases with PD098059 and SB230580, respectively, significantly inhibited urotensin II-mediated hypertrophy. In contrast, urotensin II did not activate JNK. The activation of ERK1/2 and p38 as well as cellular hypertrophy was independent of protein kinase C, and calcium and phosphoinositide 3-kinase, yet dependent on the capacity of the urotensin receptor to trans-activate the epidermal growth factor receptor. Urotensin II promoted the tyrosine phosphorylation of epidermal growth factor receptors, which was inhibited by the selective epidermal growth factor receptor kinase inhibitor, AG1478. These data indicate that perturbations in cardiac homeostasis, which lead to up-regulation of urotensin II receptors, promote urotensin II-mediated cardiomyocyte hypertrophy via ERK1/2 and p38 signaling pathways in an epidermal growth factor receptor-dependent manner.

Distinct serine residues in CBP and p300 are necessary for their activation by phenylephrine

The ability of CREB binding protein (CBP) and p300 co-activators to stimulate transcription has previously been shown to be enhanced by treatment of cardiac cells with the hypertrophic agent phenylephrine (PE). This effect is dependent on activation of the mitogen activated protein kinase pathway (p42/44 MAPK). Here, we demonstrate the first identification of potential phosphorylation sites targeted by PE within the proteins CBP and p300. We show that serine 2015 of CBP and serine 89 of p300 are necessary for PE to stimulate the transcriptional activity of these proteins. Furthermore, we have shown that PE is capable of mediating phosphorylation of endogenous p300 at serine 89. This phosphorylation mediated regulation of CBP and p300 suggests a potential signal transduction pathway for the induction of cardiac cell hypertrophy by PE.

ERK1/2 and JNKs, but not p38 kinase, are involved in reactive oxygen species-mediated induction of osteopontin gene expression by angiotensin II and interleukin-1beta in adult rat cardiac fibroblasts

Osteopontin (OPN), also called cytokine Eta-1, expressed in the myocardium co-incident with heart failure plays an important role in post myocardial infarction (MI) remodeling by promoting collagen synthesis and accumulation. Angiotensin II (Ang II) and inflammatory cytokines are increased in the heart following MI. We studied the involvement of mitogen-activated protein kinases (ERK1/2, JNKs, p38 kinase) and reactive oxygen species (ROS) in Ang II- and cytokine-induced OPN gene expression in adult rat cardiac fibroblasts. Ang II alone increased OPN mRNA (3.3 +/- 0.3-folds; P < 0.05; n = 7), while interleukin-1beta (IL-1beta), tumor necrosis factor (TNF-alpha), and interferon-gamma (IFN-gamma) had no effect. A combination of Ang II with IL-1beta or TNF-alpha, not IFN-gamma, increased OPN mRNA more than Ang II alone. Nitric oxide donor, S-nitrosoacetylpenicillamine (SNAP), alone or in combination with Ang II had no effect. Diphenylene iodonium (DPI), inhibitor of NAD(P)H oxidase, and tiron, superoxide scavenger, inhibited Ang II- and Ang II+ IL-1beta-stimulated increases in OPN mRNA. Ang II activated ERK1/2 within 5 min of treatment, not JNKs. IL-1beta activated ERK1/2 and JNKs within 15 min of treatment. A combination of Ang II and IL-1beta activated ERK1/2 within 5 min of treatment. None of these stimuli activated p38 kinase. DPI almost completely inhibited Ang II + IL-1beta-stimulated activation of ERK1/2, while partially inhibiting JNKs. PD98059, ERK1/2 pathway inhibitor, and SP600125, JNKs inhibitor, partially inhibited Ang II + IL-1beta-stimulated increases in OPN mRNA. A combination of PD98059 and SP600125 almost completely inhibited Ang II + IL-1beta-stimulated increases in OPN mRNA. Thus, Ang II alone increases OPN expression, while IL-1beta and TNF-alpha act synergistically with Ang II to increase OPN mRNA possibly via NO independent mechanisms. The synergistic increase in OPN mRNA involves ROS-mediated activation of ERK1/2 and JNKs, not P38 kinase, pathways in cardiac fibroblasts.

Extracellular signal-regulated kinase 2 interacts with and is negatively regulated by the LIM-only protein FHL2 in cardiomyocytes

The mitogen-activated protein kinase (MAPK) signaling pathway regulates diverse biologic functions including cell growth, differentiation, proliferation, and apoptosis. The extracellular signal-regulated kinases (ERKs) constitute one branch of the MAPK pathway that has been implicated in the regulation of cardiac differentiated growth, although the downstream mechanisms whereby ERK signaling affects this process are not well characterized. Here we performed a yeast two-hybrid screen with ERK2 bait and a cardiac cDNA library to identify novel proteins involved in regulating ERK signaling in cardiomyocytes. This screen identified the LIM-only factor FHL2 as an ERK interacting protein in both yeast and mammalian cells. In vivo, FHL2 and ERK2 colocalized in the cytoplasm at the level of the Z-line, and interestingly, FHL2 interacted more efficiently with the activated form of ERK2 than with the dephosphorylated form. ERK2 also interacted with FHL1 and FHL3 but not with the muscle LIM protein. Moreover, at least two LIM domains in FHL2 were required to mediate efficient interaction with ERK2. The interaction between ERK2 and FHL2 did not influence ERK1/2 activation, nor was FHL2 directly phosphorylated by ERK2. However, FHL2 inhibited the ability of activated ERK2 to reside within the nucleus, thus blocking ERK-dependent transcriptional responsiveness of ELK-1, GATA4, and the atrial natriuretic factor promoter. Finally, FHL2 partially antagonized the cardiac hypertrophic response induced by activated MEK-1, GATA4, and phenylephrine agonist stimulation. Collectively, these results suggest that FHL2 serves a repressor function in cardiomyocytes through its ability to inhibit ERK1/2 transcriptional coupling.

Cardiac hypertrophy: the good, the bad, and the ugly

Cardiac hypertrophy is the heart's response to a variety of extrinsic and intrinsic stimuli that impose increased biomechanical stress. While hypertrophy can eventually normalize wall tension, it is associated with an unfavorable outcome and threatens affected patients with sudden death or progression to overt heart failure. Accumulating evidence from studies in human patients and animal models suggests that in most instances hypertrophy is not a compensatory response to the change in mechanical load, but rather is a maladaptive process. Accordingly, modulation of myocardial growth without adversely affecting contractile function is increasingly recognized as a potentially auspicious approach in the prevention and treatment of heart failure. In this review, we summarize recent insights into hypertrophic signaling and consider several novel antihypertrophic strategies.

Characterization of apoptosis signal transduction pathways in HL-5 cardiomyocytes exposed to ischemia/reperfusion oxidative stress model

During ischemia/reperfusion (I/R), cardiomyocytes are exposed to sudden lack of nutrients and successively to radical oxygen species (ROS). In the present study, we used the HL-5 cardiac atrial myocyte cell line exposed to serum/glucose depletion added or not in H(2)O(2) to mimic ROS during ischemia, then replaced in their standard culture medium to simulate reperfusion. We investigated the effects of serum/glucose depletion combined or not to ROS exposure on AKT and MAP kinases activation to address the role of each event with respect to apoptosis. We demonstrate that serum/glucose depletion per se did not induce apoptosis when compared to ROS exposure. In particular, ROS recruited p38MAPK and JNK pathways. SB202190 preventing p38MAPK activity, partially protected HL-5 from apoptosis while blocking JNK, thanks to JNKI, further enhanced apoptosis. Blocking phosphatidylinositol (PI) 3-kinase with LY294002 or ERKs with U0126 was without consequence on apoptosis. Finally, BCL-2 and BCL-X(L/S) expression levels were analyzed in cells exposed to 1 h ischemia followed by 12-h reperfusion in the presence or not of SB202190; BCL-2, but not BCL-X(L/S), expression was decreased in ROS treated cells but SB202190 failed to restore BCL-2 level. Our data suggest that p38MAPK activation primarily mediates ROS-induced apoptosis while concomitant JNK activation would represent a scavenger pathway for cells trying to escape apoptosis.

Chronic activation of extracellular-signal-regulated protein kinases by phenylephrine is required to elicit a hypertrophic response in cardiac myocytes

Extracellular-signal-regulated protein kinases (ERKs) are activated rapidly and transiently in response to phenylephrine (PE) and endothelin-1 (ET-1) in cardiac myocytes, but whether this is linked to the subsequent development of the hypertrophic phenotype remains equivocal. To investigate this, we examined the dependence of the hypertrophic response on the length of exposure to PE in neonatal myocyte cultures. In addition to the initial transient activation of ERKs (maximum at 5-10 min), PE (10 microM) induced a second, more prolonged peak of activity several hours later. The activity of a transfected atrial natriuretic factor-luciferase reporter gene was increased 10- to 24-fold by PE. This response was inhibited by the alpha(1)-antagonist prazosin (100 nM) and by U0126 (10 microM) and PD184352 (1 microM), inhibitors of ERK activation, irrespective of whether these were added before or up to 24 h after the addition of PE. Prazosin had no effect on ET-1 (50 nM)-stimulated atrial natriuretic factor-luciferase activity. Protein synthesis was enhanced by 35+/-6% by PE, and this was blocked by prazosin added 1 h after the addition of PE, but decreased only by half when added 8 h after PE. Similarly, PE (48 h) increased myocyte area by 49% and this was prevented by prazosin added 1 h after PE, but decreased only by half when added at 24 h. These results demonstrate that prolonged exposure to PE is required to elicit alterations in gene expression, protein synthesis and cell size, characteristic of hypertrophied myocytes, and they confirm that the initial peak of ERK activity is insufficient to trigger hypertrophic responses.

Involvement of extracellular signal-regulated kinases 1/2 in cardiac hypertrophy and cell death

In response to pathophysiological stress, the adult heart undergoes hypertrophic enlargement characterized by an increase in the cross-sectional area of individual myofibers. Although cardiac hypertrophy is initially a compensatory response, sustained hypertrophy is a leading predictor for the development of heart failure. At the molecular level, disease-related stimuli invoke endocrine, paracrine, and autocrine regulatory circuits, which directly influence cardiomyocyte hypertrophy, in part, through membrane bound G protein-coupled receptors and receptor tyrosine kinases. These membrane receptors activate intermediate signal transduction pathways within the cytoplasm such as mitogen-activated protein kinases (MAPKs), protein kinase C (PKC), and calcineurin, which directly modify transcriptional regulatory factors promoting alterations in cardiac gene expression. This review will weigh an increasing body of literature implicating the intermediate signaling pathway consisting of MEK1 and extracellular signal-regulated kinases (ERK1/2) as important regulators of cardiac hypertrophy and myocyte survival. The MEK1-ERK1/2 pathway likely occupies a central regulatory position in the signaling hierarchy of a cardiac myocyte given its unique ability to respond to virtually every characterized hypertrophic agonist and stress stimuli examined to date and based on its ability to promote myocyte growth in vitro and in vivo.

Relationship between alpha(1)-adrenergic receptor-induced contraction and extracellular signal-regulated kinase activation in the bovine inferior alveolar artery

The endogenous adrenergic agonists norepinephrine (NE) and epinephrine regulate vascular tone by stimulating alpha(1)-adrenergic receptors (ARs) on smooth muscle cells to cause contraction. In addition, alpha(1)-ARs also couple to growth factor pathways, through stimulation of mitogen-activated protein kinases (MAPKs). MAPKs are a family of serine-threonine kinases that include extracellular signal-regulated kinase (ERK) and a variety of other kinases that are able to activate transcription factors when stimulated. We examined alpha(1)-AR stimulation of contraction and ERK activation in the bovine inferior alveolar artery (BIAA), using in vitro contraction studies and Western blotting. Using antagonists selective for individual adrenergic receptor types, we found that only alpha(1)-ARs were coupled to ERK activation and contraction. NE stimulated contraction (EC(50) = 11 microM) and ERK activation (EC(50) = 21 microM) with similar potency. Using alpha(1)-AR subtype-selective antagonists, we identified the alpha(1)-AR subtypes coupled to each response. Affinity values for alpha(1)-AR subtype-selective antagonists were consistent with alpha(1A)-AR-mediated contraction. In contrast, simultaneous treatment with concentrations of these antagonists selective for each alpha(1)-AR subtype (alpha(1A)-, alpha(1B)-, and alpha(1D)-AR) was required to inhibit ERK activation, suggesting that all three alpha(1)-ARs activate ERK in BIAA. Transmural electrical stimulation of BIAA segments resulted in activation of ERK, which was inhibited by the alpha(1)-AR-selective antagonist BE 2254 (2-[[beta-(4-hydroxyphenyl)ethyl]aminomethyl]-1-tetralone). These data suggest that in an intact artery, NE released from sympathetic nerves stimulates alpha(1)-ARs to cause contraction and ERK activation, and that redundancy among subtypes exists for alpha(1)-AR activation of ERK.

Expression of cyclin D1 and CDK4 causes hypertrophic growth of cardiomyocytes in culture: a possible implication for cardiac hypertrophy

Differentiated cardiomyocytes have little capacity to proliferate and show the hypertrophic growth in response to alpha1-adrenergic stimuli via the Ras/MEK pathway. In this study, we investigated a role of cyclin D1 and CDK4, a positive regulator of cell cycle, in cultured neonatal rat cardiomyocyte hypertrophy. D-type cyclins including cyclin D1 were induced in cells stimulated by phenylephrine. This induction was inhibited by MEK inhibitor PD98059 and the dominant negative RasN17, but mimicked by expression of the constitutive active Ras61L. Over-expression of cyclin D1 and CDK4 using adenovirus gene transfer caused the hypertrophic growth of cardiomyocytes, as evidenced by an increase of the cell size as well as the amount of cellular protein and its rate of synthesis. However, the cyclin D1/CDK4 kinase activity was not up-regulated in cells treated by hypertrophic stimuli or in cells over-expressing the cyclin D1 and CDK4. Furthermore, a CDK inhibitor, p16, did not inhibit the hypertrophic growth of cardiomyocytes. These results clearly indicated that cyclin D1 and CDK4 have a role in hypertrophic growth of cardiomyocytes through a novel mechanism(s) which appears not to be related to its activity required for cell cycle progression.

Guanine nucleotide exchange factor-like factor (Rlf) induces gene expression and potentiates alpha 1-adrenergic receptor-induced transcriptional responses in neonatal rat ventricular myocytes

Expression of constitutively active Ras (V12Ras) in cultured neonatal rat ventricular myocytes or targeted cardiac expression of V12Ras in transgenic mice induces myocardial cell growth and expression of genes that are markers of cardiac hypertrophy including atrial natriuretic factor (ANF) and myosin light chain-2. However, the signaling pathways that modulate the effects of Ras on acquisition of the various features of cardiac hypertrophy are not known. We identified the Ral guanine nucleotide exchange factor-like factor (Rlf) in a yeast two-hybrid screen of human heart cDNA library using Ras as bait, suggesting that Ras signaling in the heart may involve Rlf. We demonstrate here that Rlf is expressed in human heart. Expression of wild type Rlf or Rlf-CAAX, a membrane-targeted mutant of Rlf, transactivated ANF and myosin light chain-2 promoters but did not activate canonical cAMP responsive elements or phorbol ester responsive elements, suggesting that Rlf expression does not lead to a generalized increase in transcription. Transfection of mutant ANF promoter-reporter gene constructs demonstrated that the proximal serum response element is both necessary and sufficient for Rlf-inducible ANF expression. Rlf-induced ANF promoter activation required Ral and Cdc42 but not RhoA, Rac1, ERK, or p38 kinase activation. In addition, Rlf potentiated alpha(1)-adrenergic receptor (alpha(1)-AR)-induced ANF expression. Prolonged activation of the alpha(1)-AR increases RalGTP levels in neonatal rat ventricular myocytes, further emphasizing a role for Ral guanine nucleotide exchange factors in alpha(1)-AR signaling. Overall, this study supports the concept that Rlf and Ral are important previously unrecognized signaling components that regulate transcriptional responses in myocardial cells.

From the sarcomere to the nucleus: role of genetics and signaling in structural heart disease

The identification of genetic mutations underlying familial structural heart disease has provided exciting new insights into how alterations in structural components of the cardiomyocyte lead to different forms of cardiomyopathy. Specifically, mutations in components of the sarcomere are frequently associated with hypertrophic cardiomyopathy, whereas mutations in cytoskeletal proteins lead to dilated cardiomyopathy. In addition, extrinsic stresses such as hypertension and valvular disease can produce myocardial remodeling that is very similar to that observed in genetic cardiomyopathy. For myocardial remodeling to occur, changes in gene expression must occur; therefore, changes in contractile function or wall stress must be communicated to the nucleus via signal transduction pathways. The identity of these signaling pathways has become a key question in molecular biology. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, G alpha(q) and downstream effectors, mitogen-activated protein kinase pathways, and the Ca(2+)-regulated phosphatase, calcineurin. In the past it has been difficult to discern which signaling molecules actually contributed to disease progression in vivo; however, the development of numerous transgenic and knockout mouse models of cardiomyopathy is now allowing the direct testing of stimulatory and inhibitory molecules in the mouse heart. From this work it has been possible to identify signaling molecules and pathways that are required for different aspects of disease progression in vivo. In particular, a number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocyte. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underly heart disease.

Load-induced focal adhesion kinase activation in the myocardium: role of stretch and contractile activity

We investigated the influence of stretch and contractile activity on load-induced activation of focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK)1/2 in isolated rat hearts. Increases of diastolic pressure from approximately 0 to approximately 15 mmHg rapidly increased FAK tyrosine phosphorylation (maximum: 2.3-fold) and binding to c-Src (maximum: 2.8-fold) and Grb2 (maximum: 3.6-fold). This was paralleled by activation (maximum: 2.8-fold) and binding of ERK1/2 to FAK. FAK and ERK1/2 were immunolocalized at sarcolemmal sites of cardiac myocytes and in the nuclei, in the case of ERK1/2. Balloon inflation to raise ventricular pressure in hearts perfused with cardioplegic solution also activated FAK and ERK1/2. However, increases in contractile activity induced by increasing calcium concentration in the perfusate (from 0.5 to 5 mM) did not activate the FAK multicomponent signaling complex or ERK1/2 in the myocardium. These results indicate that stretch rather than contractile activity induces FAK and ERK1/2 activation in the myocardium. In addition, the activation and binding of ERK1/2 to FAK suggest that FAK drives the load-induced activation of ERK1/2.

Molecular and cellular mechanisms of mechanical stress-induced cardiac hypertrophy

Congestive heart failure is one of the major issues for cardiologists. Since cardiac hypertrophy deteriorates into heart failure, it is important to elucidate the mechanisms of cardiac hypertrophy. Hemodynamic overload, namely mechanical stress, is a major cause for cardiac hypertrophy. Mechanical stress induces various hypertrophic responses such as activation of phosphorylation cascades of many protein kinases, expression of specific genes and an increase in protein synthesis. During this process, secretion and production of vasoactive peptides such as angiotensin II and endothelin-1, are increased and play critical roles in the induction of these hypertrophic responses. Recently, a Ca2+ dependent protein kinase, CaMK, and a Ca2+ dependent protein phosphatase, calcineurin, have attracted great attention as critical molecules that induce cardiac hypertrophy. In this review, we described the mechanisms by which mechanical stress induces cardiac hypertrophy, especially focusing on the role of calcineurin in the development of cardiac hypertrophy.

The transcription factor GATA4 is activated by extracellular signal-regulated kinase 1- and 2-mediated phosphorylation of serine 105 in cardiomyocytes

The zinc finger-containing transcription factor GATA4 has been implicated as a critical regulator of multiple cardiac-expressed genes as well as a regulator of inducible gene expression in response to hypertrophic stimulation. Here we demonstrate that GATA4 is itself regulated by the mitogen-activated protein kinase signaling cascade through direct phosphorylation. Site-directed mutagenesis and phospho-specific GATA4 antiserum revealed serine 105 as the primary site involved in agonist-induced phosphorylation of GATA4. Infection of cultured cardiomyocytes with an activated MEK1-expressing adenovirus induced robust phosphorylation of serine 105 in GATA4, while a dominant-negative MEK1-expressing adenovirus blocked agonist-induced phosphorylation of serine 105, implicating extracellular signal-regulated kinase (ERK) as a GATA4 kinase. Indeed, bacterially purified ERK2 protein directly phosphorylated purified GATA4 at serine 105 in vitro. Phosphorylation of serine 105 enhanced the transcriptional potency of GATA4, which was sensitive to U0126 (MEK1 inhibitor) but not SB202190 (p38 inhibitor). Phosphorylation of serine 105 also modestly enhanced the DNA binding activity of bacterially purified GATA4. Finally, induction of cardiomyocyte hypertrophy with an activated MEK1-expressing adenovirus was blocked with a dominant-negative GATA4-engrailed-expressing adenovirus. These results suggest a molecular pathway whereby MEK1-ERK1/2 signaling regulates cardiomyocyte hypertrophic growth through the transcription factor GATA4 by direct phosphorylation of serine 105, which enhances DNA binding and transcriptional activation.

Potassium channel blocker activates extracellular signal-regulated kinases through Pyk2 and epidermal growth factor receptor in rat cardiomyocytes

OBJECTIVES

We sought to determine whether potassium (K(+)) channel blockers (KBs) can activate extracellular signal-regulated kinase (ERK) and to characterize the upstream signals leading to ERK activation in cardiomyocytes.

BACKGROUND

Because KBs attenuate K(+) outward current, they may possibly prolong the duration of action potentials, leading to an increase in calcium (Ca(2+)) transient ([Ca(2+)](i)) in cardiomyocytes. Elevation of intracellular Ca(2+) levels can trigger various signaling events. Influx of Ca(2+) through L-type Ca(2+) channels after membrane depolarization induced activation of MEK and ERK through activation of Ras in neurons. Although KBs are frequently used to treat cardiac arrhythmias, their effect on signaling pathways remains unknown.

METHODS

Primary cultured rat cardiomyocytes were stimulated with four different KBs-4-aminopyridine (4-AP), E-4031, tetra-ethylammonium and quinidine-and phosphorylation of ERK, proline-rich tyrosine kinase 2 (Pyk2) and epidermal growth factor receptor (EGFR) was detected. Action potentials were recorded by use of a conventional microelectrode. (Ca(2+))(i) was monitored by the fluorescent calcium indicator Fluo-4.

RESULTS

E-4031, 4-AP, tetra-ethylammonium and quinidine induced phosphorylation of ERK. 4-Aminopyridine prolonged the duration of action potentials by 37% and increased (Ca(2+))(i) by 52% at 1 mmol/l. Pre-incubation of ethyleneglycoltetraacetic acid, 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetrakis and diltiazem completely blocked this phosphorylation, whereas flufenamic acid and benzamil did not. 4-Aminopyridine induced tyrosine phosphorylation of Pyk2 and EGFR, which peaked at 5 and 10 min, respectively. Cytochalasin D, AG1478 and dominant-negative EGFR strongly inhibited the phosphorylation of ERK, whereas calphostin C, calmidazolium and KN62 did not.

CONCLUSIONS

These findings indicate that KBs induce ERK activation, which starts with Ca(2+) entry through the L-type Ca(2+) channel in cardiomyocytes, and that EGFR and Pyk2 are involved in this activation.

c-Jun N-terminal kinase-interacting protein 1 inhibits gene expression in response to hypertrophic agonists in neonatal rat ventricular myocytes

G(q)-coupled receptor agonists, such as endothelin-1 (ET-1) and phenylephrine (PE), initiate a hypertrophic response in cardiac myocytes that is characterized by increased expression of atrial natriuretic factor (ANF), beta-myosin heavy chain (beta-MHC), skeletal muscle alpha-actin (SkalphaA) and ventricular myosin light chain-2 (vMLC2). ET-1 and PE activate both the extracellular signal-regulated kinases and c-Jun N-terminal kinases (JNKs) in cardiac myocytes, but the extent to which each contributes to the hypertrophic response is uncertain. Here we have used the JNK-binding domain of JNK-interacting protein 1 (JIP-1), a cytosolic scaffold protein that binds to JNK and inhibits its signalling when overexpressed, to assess the contribution of JNK activation to the hypertrophic response. Expression of JIP-1 inhibited the increase in ANF, beta-MHC, SkalphaA and vMLC2 reporter gene expression in response to ET-1 (by 45-86%) and PE (by 56-60%). However, activation of these reporter genes by PMA, which does not activate JNK significantly in myocytes, was much less affected by overexpression of JIP-1. JIP-1 also failed to inhibit reporter gene activation in response to constitutively active Ras or Raf, but attenuated reporter gene activation induced by a constitutively active mutant of mitogen-activated protein kinase kinase kinase 1 (MEKK1), an upstream kinase that preferentially activates JNKs, by 50%. Overexpression of JIP-1 also significantly reduced the increase in cell area in response to PE from 63% to 56%, but had no effect on the increase in cell size in response to ET-1 (38%). These results suggest that activation of the JNK pathway contributes to the transcriptional and morphological responses to G(q) receptor-coupled hypertrophic agonists.

A role for the extracellular signal-regulated kinase and p38 mitogen-activated protein kinases in interleukin-1 beta-stimulated delayed signal tranducer and activator of transcription 3 activation, atrial natriuretic factor expression, and cardiac myocyte morphology

We have demonstrated that two hypertrophic agents, interleukin-1 beta (IL-1 beta) and leukemic inhibitory factor (LIF), altered cardiac myocyte morphology with striking similarity and prompted us to investigate the common actions of these cytokines. We compared the phosphorylation/activation of signal tranducer and activator of transcription 3 (STAT3), extracellular signal-regulated kinase (ERK), p38(MAPK), and c-Jun N-terminal kinase mitogen-activated protein kinases (MAPKs). The phosphorylation of STAT3 by IL-1 beta was delayed (>60 min), whereas the response to LIF was rapid (<10 min) and transient. We confirmed that IL-1 beta potently stimulated all three MAPK subfamilies. In contrast, LIF promoted strong activation of ERKs, marginal activation of p38(MAPK), and no c-Jun N-terminal kinase activation. To test the roles of ERKs and p38(MAPK), myocytes were pretreated with PD98059 and SB203580. Either inhibitor alone prevented STAT3 phosphorylation, implicating ERKs and p38(MAPK) in the delayed STAT3 response to IL-1 beta. The interplay of MAPKs and STAT3 phosphorylation in regulating IL-1 beta-stimulated hypertrophy was investigated by evaluating the effect of MAPK inhibitors on atrial natriuretic factor (ANF) expression and myocyte morphology. The specific inhibition of either ERK or p38(MAPK) attenuated the IL-1 beta- or LIF-stimulated ANF expression by up to 70%. Inhibition was not further increased in the presence of both inhibitors. Furthermore, although individual inhibition of ERK or p38(MAPK) did not affect morphology, co-treatment with both inhibitors abrogated the hypertrophic morphology stimulated by IL-1 beta but not by LIF. Taken together, our data indicate that the activation of ERK and p38(MAPK) is essential in regulating a delayed STAT3 phosphorylation as well as changes in ANF expression and morphology that follow IL-1 beta treatment. Thus, the role of MAPKs in the hypertrophic response can be dictated at least partly by the nature of the hypertrophic agent employed.

Isoproterenol activates extracellular signal-regulated protein kinases in cardiomyocytes through calcineurin

BACKGROUND

Extracellular signal-regulated kinases (ERKs) and calcineurin have been reported to play important roles in the development of cardiac hypertrophy. We examined here the relation between calcineurin and ERKs in cardiomyocytes.

METHODS AND RESULTS

Isoproterenol activated ERKs in cultured cardiomyocytes of neonatal rats, and the activation was abolished by chelation of extracellular Ca(2+) with EGTA, blockade of L-type Ca(2+) channels with nifedipine, or depletion of intracellular Ca(2+) stores with thapsigargin. Isoproterenol-induced activation of ERKs was also significantly suppressed by calcineurin inhibitors in cultured cardiomyocytes as well as in the hearts of mice. Isoproterenol failed to activate ERKs in either the cultured cardiomyocytes or the hearts of mice that overexpress the dominant negative mutant of calcineurin. Isoproterenol elevated intracellular Ca(2+) levels at both systolic and diastolic phases and dose-dependently activated calcineurin. Inhibition of calcineurin also attenuated isoproterenol-stimulated phosphorylation of Src, Shc, and Raf-1 kinase. The immunocytochemistry revealed that calcineurin was localized in the Z band, and isoproterenol induced translocation of calcineurin and ERKs into the nucleus.

CONCLUSIONS

Calcineurin, which is activated by marked elevation of intracellular Ca(2+) levels by the Ca(2+)-induced Ca(2+) release mechanism, regulates isoproterenol-induced activation of ERKs in cardiomyocytes.

Cytoplasmic signaling pathways that regulate cardiac hypertrophy

This review discusses the rapidly progressing field of cardiomyocyte signal transduction and the regulation of the hypertrophic response. When stimulated by a wide array of neurohumoral factors or when faced with an increase in ventricular-wall tension, individual cardiomyocytes undergo hypertrophic growth as an adaptive response. However, sustained cardiac hypertrophy is a leading predictor of future heart failure. A growing number of intracellular signaling pathways have been characterized as important transducers of the hypertrophic response, including specific G protein isoforms, low-molecular-weight GTPases (Ras, RhoA, and Rac), mitogen-activated protein kinase cascades, protein kinase C, calcineurin, gp130-signal transducer and activator of transcription, insulin-like growth factor I receptor pathway, fibroblast growth factor and transforming growth factor beta receptor pathways, and many others. Each of these signaling pathways has been implicated as a hypertrophic transducer, which collectively suggests an emerging paradigm whereby multiple pathways operate in concert to orchestrate a hypertrophic response

Directed inhibition of nuclear import in cellular hypertrophy

Each nuclear pore is responsible for both nuclear import and export with a finite capacity for bidirectional transport across the nuclear envelope. It remains poorly understood how the nuclear transport pathway responds to increased demands for nucleocytoplasmic communication. A case in point is cellular hypertrophy in which increased amounts of genetic material need to be transported from the nucleus to the cytosol. Here, we report an adaptive down-regulation of nuclear import supporting such an increased demand for nuclear export. The induction of cardiac cell hypertrophy by phenylephrine or angiotensin II inhibited the nuclear translocation of H1 histones. The removal of hypertrophic stimuli reversed the hypertrophic phenotype and restored nuclear import. Moreover, the inhibition of nuclear export by leptomycin B rescued import. Hypertrophic reprogramming increased the intracellular GTP/GDP ratio and promoted the nuclear redistribution of the GTP-binding transport factor Ran, favoring export over import. Further, in hypertrophy, the reduced creatine kinase and adenylate kinase activities limited energy delivery to the nuclear pore. The reduction of activities was associated with the closure of the cytoplasmic phase of the nuclear pore preventing import at the translocation step. Thus, to overcome the limited capacity for nucleocytoplasmic transport, cells requiring increased nuclear export regulate the nuclear transport pathway by undergoing a metabolic and structural restriction of nuclear import.

Stage-specific differential activation of mitogen-activated protein kinases in hypertrophied and failing rat hearts

Mitogen-activated protein kinases (MAPKs) are involved in the early development of cardiac hypertrophy, but their roles in chronic left ventricular hypertrophy (LVH) are unclear. We studied the angiotensin (Ang) II-induced cardiac MAPK activation of the hypertensive Dahl salt-sensitive (DS) rats in the subacute developing LVH stage, the chronic compensated LVH stage, and the congestive heart failure (CHF) stage. In the isolated, coronary-perfused heart preparation, Ang II infusion (1x10(-6)mol/l) activated extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and p38-MAPK in the LV myocardium. No substantial differences were observed in the Ang II-induced ERK activation between the normotensive control DS rats and the hypertensive DS rats in either stage. In contrast, the Ang II-induced activation of JNK and p38-MAPK was augmented in the subacute LVH stage of the hypertensive DS rats, but then progressively attenuated in the chronic LVH and CHF stages. Chronic treatment with an angiotensin converting enzyme inhibitor, temocapril (20 mg/kg/day), ameliorated the responsiveness of the JNK/p38-MAPK activation, suggesting that the decreased JNK/p38-MAPK activation is a consequence of negative feedback regulation for the activated cardiac renin-angiotensin system in chronic LVH and CHF. Thus, the Ang II-induced activation of multiple cardiac MAPK pathways are differentially regulated, depending on the stages of chronic hypertrophic process. The JNK and p38-MAPK activation may be involved in the early development of adaptive LVH. However, the responsiveness of the cardiac JNK/p38-MAPK pathways progressively decreased in chronic LVH and CHF under the chronic activation of tissue renin-angiotensin system.

Natriuretic peptide signalling: molecular and cellular pathways to growth regulation

The natriuretic peptides (NPs) constitute a family of polypeptide hormones that regulate mammalian blood volume and blood pressure. The ability of the NPs to modulate cardiac hypertrophy and cell proliferation as well is now beginning to be recognized. The NPs interact with three membrane-bound receptors, all of which contain a well-characterized extracellular ligand-binding domain. The R1 subclass of NP receptors (NPR-A and NPR-B) contains a C-terminal guanylyl cyclase domain and is responsible for most of the NPs downstream actions through their ability to generate cGMP. The R2 subclass lacks an obvious catalytic domain and functions primarily as a clearance receptor. This review focuses on the signal transduction pathways initiated by ligand binding and other factors that help to determine signalling specificities, including allosteric factors modulating cGMP generation, receptor desensitization, the activation and function of cGMP-dependent protein kinase (PKG), and identification of potential nuclear or cytoplasmic targets such as the mitogen-activated protein kinase signalling (MAPK) cascade. The inhibition of cardiac growth and hypertrophy may be an important but underappreciated action of the NP signalling system.