Cell type-specific angiotensin II-evoked signal transduction pathways: critical roles of Gbetagamma subunit, Src family, and Ras in cardiac fibroblasts

Urotensin II Induces Cardiac Fibrosis through the TGF-β/Smad Signaling Pathway during the Development of Cardiac Hypertrophy

Myocardial fibrosis is an important pathological phenomenon of cardiac remodeling that is induced by hypertension, myocardial ischemia, valvular heart disease, hypertrophic cardiomyopathy, and other heart diseases and can progress to heart failure. Urotensin II (UII) is regarded as a cardiovascular autacoid/hormone that is not only the most potent vasoconstrictor in mammals but also involved in cardiac remodeling. However, the molecular mechanisms responsible for UII-induced cardiac fibrosis have not yet been fully elucidated. Therefore, we aimed to investigate the effect of UII on myocardial fibrosis in cardiac hypertrophy and the mechanism of UII-induced cardiac fibrosis. Cardiac tissue from mice subjected to Transverse aortic constriction (TAC) was collected. Cardiac hypertrophy, myocardial fibrosis, and the expression of UII protein were assessed using echocardiography and pathological and molecular biological analyses. The effect of UII on fibrosis was evaluated in UII-treated mice and isolated rat primary cardiac fibroblasts, and the results indicated that UII induced significant myocardial fibrosis and increases in the proliferation and fibrotic responses both in mice and cultured fibroblasts. Mechanistically, UII treatment induced activation of the TGF-β/Smad signaling pathway, which was suppressed by the UII receptor antagonist. In conclusion, UII plays critical roles in cardiac fibrosis by modulating the TGF-β/Smads signaling pathway, which may be a promising therapeutic target in hypertrophic cardiomyopathy and related problems, such as cardiac remodeling and heart failure.

Precision medicine for heart failure based on molecular mechanisms: The 2019 ISHR Research Achievement Award Lecture

Heart failure is a leading cause of death, and the number of patients with heart failure continues to increase worldwide. To realize precision medicine for heart failure, its underlying molecular mechanisms must be elucidated. In this review summarizing the "The Research Achievement Award Lecture" of the 2019 XXIII ISHR World Congress held in Beijing, China, we would like to introduce our approaches for investigating the molecular mechanisms of cardiac hypertrophy, development, and failure, as well as discuss future perspectives.

Tenascin-C aggravates ventricular dilatation and angiotensin-converting enzyme activity after myocardial infarction in mice

AIMS

Tenascin-C (TN-C) is suggested to be detrimental in cardiac remodelling after myocardial infarction (MI). The aim of this study is to reveal the effects of TN-C on extracellular matrix organization and its haemodynamic influence in an experimental mouse model of MI and in myocardial cell culture during hypoxic conditions.

METHODS AND RESULTS

Myocardial infarction was induced in TN-C knockout (TN-C KO) and wild-type mice. Six weeks later, cardiac function was studied by magnetic resonance imaging and under isolated working heart conditions. Myocardial mRNA levels and immunoreactivity of TN-C, TIMP-1, TIMP-3, and matrix metalloproteinase (MMP)-9, as well as serum and tissue activities of angiotensin-converting enzyme (ACE), were determined at 1 and 6 weeks after infarction. Cardiac output and external heart work were higher, while left ventricular wall stress and collagen expression were decreased (P < 0.05) in TN-C KO mice as compared with age-matched controls at 6 weeks after infarction. TIMP-1 expression was down-regulated at 1 and 6 weeks, and TIMP-3 expression was up-regulated at 1 week (P < 0.01) after infarction in knockout mice. MMP-9 level was lower in TN-C KO at 6 weeks after infarction (P < 0.05). TIMP-3/MMP-9 ratio was higher in knockout mice at 1 and 6 weeks after infarction (P < 0.01). ACE activity in the myocardial border zone (i.e. between scar and free wall) was significantly lower in knockout than in wild-type mice 1 week after MI (P < 0.05).

CONCLUSIONS

Tenascin-C expression is induced by hypoxia in association with ACE activity and MMP-2 and MMP-9 elevations, thereby promoting left ventricular dilatation after MI.

Phosphorylation of GATA4 at serine 105 is required for left ventricular remodelling process in angiotensin II-induced hypertension in rats

In this study, we investigated whether local intramyocardial GATA4 overexpression affects the left ventricular (LV) remodelling process and the importance of phosphorylation at serine 105 (S105) for the actions of GATA4 in an angiotensin II (AngII)‐induced hypertension rat model. Adenoviral constructs overexpressing wild‐type GATA4 or GATA4 mutated at S105 were delivered into the anterior LV free wall. AngII (33.3 µg/kg/h) was administered via subcutaneously implanted minipumps. Cardiac function and structure were examined by echocardiography, followed by histological immunostainings of LV sections and gene expression measurements by RT‐qPCR. The effects of GATA4 on cultured neonatal rat ventricular fibroblasts were evaluated. In AngII‐induced hypertension, GATA4 overexpression repressed fibrotic gene expression, reversed the hypertrophic adult‐to‐foetal isoform switch of myofibrillar genes and prevented apoptosis, whereas histological fibrosis was not affected. Overexpression of GATA4 mutated at S105 resulted in LV chamber dilatation, cardiac dysfunction and had minor effects on expression of myocardial remodelling genes. Fibrotic gene expression in cardiac fibroblasts was differently affected by overexpression of wild‐type or mutated GATA4. Our results indicate that GATA4 reduces AngII‐induced responses by interfering with pro‐fibrotic and hypertrophic gene expressions. GATA4 actions on LV remodelling and fibroblasts are dependent on phosphorylation site S105.

Pharmacological management of atrial fibrillation in patients with heart failure with reduced ejection fraction: review of current knowledge and future directions

Introduction: Both heart failure with reduced ejection fraction (HFrEF) and atrial fibrillation (AF) independently cause significant morbidity and mortality. The two conditions commonly coexist and AF in the setting of HFrEF is associated with worse mortality, hospitalizations, and quality of life compared to HFrEF without AF. Despite the large burden of these conditions, there is no clear optimal management strategy for when they occur together.Areas covered: This review focuses on the pharmacological management of AF in HFrEF. Studies were identified through PubMed search of relevant keywords. The authors review key clinical trials that have influenced management strategies and guidelines. The authors focus on the classes of drugs used to treat AF for both rate and rhythm control strategies including beta-blockers, digoxin, amiodarone, and dofetilide. Additionally, the authors discuss select non-antiarrhythmic medications that affect AF in HFrEF. The authors highlight the strengths and weakness of the data supporting the use of these medications and suggest future directions.Expert opinion: The pharmacological treatment of AF in HFrEF will need further refinement alongside the emerging role of catheter ablation. Novel HF medications and antiarrhythmics offer new tools to prevent the development of AF, as well as for rate and rhythm control strategies.

Biomarkers Associated with Atrial Fibrosis and Remodeling

Atrial fibrillation is the most common rhythm disturbance encountered in clinical practice. Although often considered as solely arrhythmic in nature, current evidence has established that atrial myopathy constitutes both the substrate and the outcome of atrial fibrillation, thus initiating a vicious, self-perpetuating cycle. This myopathy is triggered by stress-induced (including pressure/volume overload, inflammation, oxidative stress) responses of atrial tissue, which in the long term become maladaptive, and combine elements of both structural, especially fibrosis, and electrical remodeling, with contemporary approaches yielding potentially useful biomarkers of these processes. Biomarker value becomes greater given the fact that they can both predict atrial fibrillation occurrence and treatment outcome. This mini-review will focus on the biomarkers of atrial remodeling (both electrical and structural) and fibrosis that have been validated in human studies, including biochemical, histological and imaging approaches.

Mechanoactivation of the angiotensin II type 1 receptor induces β-arrestin-biased signaling through Gαi coupling

Ligand activation of the angiotensin II type 1 receptor (AT1R), a member of the G protein-coupled receptor (GPCR) family, stimulates intracellular signaling to mediate a variety of physiological responses. The AT1R is also known to be a mechanical sensor. When activated by mechanical stretch, the AT1R can signal via the multifunctional adaptor protein β-arrestin, rather than through classical heterotrimeric G protein pathways. To date, the AT1R conformation induced by membrane stretch in the absence of ligand was thought to be the same as that induced by β-arrestin-biased agonists, which selectively engage β-arrestin thereby preventing G protein coupling. Here, we show that in contrast to the β-arrestin-biased agonists TRV120023 and TRV120026, membrane stretch uniquely promotes the coupling of the inhibitory G protein (Gα_i ) to the AT1R to transduce signaling. Stretch-triggered AT1R-Gα_i coupling is required for the recruitment of β-arrestin2 and activation of downstream signaling pathways, such as EGFR transactivation and ERK phosphorylation. Our findings demonstrate additional complexity in the mechanism of receptor bias in which the recruitment of Gα_i is required for allosteric mechanoactivation of the AT1R-induced β-arrestin-biased signaling.

Diabetic Microvascular Disease: An Endocrine Society Scientific Statement

Both type 1 and type 2 diabetes adversely affect the microvasculature in multiple organs. Our understanding of the genesis of this injury and of potential interventions to prevent, limit, or reverse injury/dysfunction is continuously evolving. This statement reviews biochemical/cellular pathways involved in facilitating and abrogating microvascular injury. The statement summarizes the types of injury/dysfunction that occur in the three classical diabetes microvascular target tissues, the eye, the kidney, and the peripheral nervous system; the statement also reviews information on the effects of diabetes and insulin resistance on the microvasculature of skin, brain, adipose tissue, and cardiac and skeletal muscle. Despite extensive and intensive research, it is disappointing that microvascular complications of diabetes continue to compromise the quantity and quality of life for patients with diabetes. Hopefully, by understanding and building on current research findings, we will discover new approaches for prevention and treatment that will be effective for future generations.

STAT3 balances myocyte hypertrophy vis-à-vis autophagy in response to Angiotensin II by modulating the AMPKα/mTOR axis

Signal transducers and activators of transcription 3 (STAT3) is known to participate in various cardiovascular signal transduction pathways, including those responsible for cardiac hypertrophy and cytoprotection. However, the role of STAT3 signaling in cardiomyocyte autophagy remains unclear. We tested the hypothesis that Angiotensin II (Ang II)-induced cardiomyocyte hypertrophy is effected, at least in part, through STAT3-mediated inhibition of cellular autophagy. In H9c2 cells, Ang II treatment resulted in STAT3 activation and cellular hypertrophy in a dose-dependent manner. Ang II enhanced autophagy, albeit without impacting AMPKα/mTOR signaling or cellular ADP/ATP ratio. Pharmacologic inhibition of STAT3 with WP1066 suppressed Ang II-induced myocyte hypertrophy and mRNA expression of hypertrophy-related genes ANP and β-MHC. These molecular events were recapitulated in cells with STAT3 knockdown. Genetic or pharmacologic inhibition of STAT3 significantly increased myocyte ADP/ATP ratio and enhanced autophagy through AMPKα/mTOR signaling. Pharmacologic activation and inhibition of AMPKα attenuated and exaggerated, respectively, the effects of Ang II on ANP and β-MHC gene expression, while concomitant inhibition of STAT3 accentuated the inhibition of hypertrophy. Together, these data indicate that novel nongenomic effects of STAT3 influence myocyte energy status and modulate AMPKα/mTOR signaling and autophagy to balance the transcriptional hypertrophic response to Ang II stimulation. These findings may have significant relevance for various cardiovascular pathological processes mediated by Ang II signaling.

Atrial fibrillation in heart failure with preserved ejection fraction: Insights into mechanisms and therapeutics

Atrial fibrillation (AF) and heart failure (HF) often coexist, and the outcomes of patients who have both AF and HF are considerably worse than those with either condition in isolation. Heart failure with preserved ejection fraction (HFpEF) is a heterogeneous clinical entity and accounts for approximately one-half of current HF. At least one-third of patients with HFpEF are burdened by comorbid AF. The current understanding of the relationship between AF and HFpEF is limited, but the clinical implications are potentially important. In this review, we explore 1) the pathogenesis that drives AF and HFpEF to coexist; 2) pharmacologic therapies that may attenuate the impact of AF in HFpEF; and 3) future directions in the management of this complex syndrome.

Angiotensin II activates signal transducers and activators of transcription 3 via Rac1 in the atrial tissue in permanent atrial fibrillation patients with rheumatic heart disease

Patients with rheumatic heart disease (RHD) often experience persistent atrial fibrillation (AF) associated with adverse atrial structural remodeling (ASR) manifested by atrial fibrosis and left atrial enlargement. The aim of this study was to explore the potential molecular signaling mechanisms for atrial fibrosis and ASR. Twenty RHD patients with persistent AF and 10 RHD patients with sinus rhythm (Group A) were recruited in our study, which all underwent transthoracic echocardiography. Right atrial appendage (RAA) tissue samples were obtained from these patients during mitral/aortic valve replacement operation. The AF patients were further divided into two groups according to left atrial diameter (LAD): Group B with LAD ranging 50-65 mm and Group C with LAD >65 mm. Histological examinations were performed with hematoxylin-eosin staining and Masson's trichrome staining. Atrial angiotensin II (AngII) content was measured by ELISA. Rac1 and STAT3 protein levels were determined by Western blot analysis. Hematoxylin-eosin staining demonstrated highly organized arrangement of atrial muscles in control Group A and significant derangement in both Group B and C AF patients with reduced cell density and increased cell size. Moreover, Masson's trichrome staining showed that atrial myocytes were surrounded by large trunks of collagen fibers in both Group B and C, but not in Group A. There was a positive correlation between atrial tissue fibrosis and LAD. AngII content was markedly higher in Group C than in Group B than in Group A, which was positively correlated with LAD. Similarly, Rac1 and STAT3 protein levels were found considerably higher in Group C and B than in Group A with excellent correlation to LAD. Our study unraveled for the first time the AngII/Rac1/STAT3 signaling as a mechanism for ASR thereby AF in a particular clinical setting-RHD patients with persistent AF and indicated inhibition of this pathway may help ameliorating adverse ASR.

The role of mAKAPβ in the process of cardiomyocyte hypertrophy induced by angiotensin II

Angiotensin II (AngII) is the central product of the renin-angiotensin system (RAS) and this octapeptide contributes to the pathophysiology of cardiac hypertrophy and remodeling. mAKAPβ is an A-kinase anchoring protein (AKAP) that has the function of binding to the regulatory subunit of protein kinase A (PKA) and confining the holoenzyme to discrete locations within the cell. In this study, we aimed to investigate the role of mAKAPβ in AngII‑induced cardiomyocyte hypertrophy and the possible mechanisms involved. Cultured cardiomyocytes from neonatal rats were treated with AngII. Subsequently, the morphology of the cardiomyocytes was observed and the expression of mAKAPβ and cardiomyocyte hypertrophic markers was measured. mAKAPβ‑shRNA was constructed for RNA interference; the expression of mAKAPβ and hypertrophic markers, the cell surface area and the [3H]Leucine incorporation rate in the AngII‑treated rat cardiomyocytes were detected following RNA interference. Simultaneously, changes in the expression levels of phosphorylated extracellular signal-regulated kinase (p-ERK)2 in the cardiomyocytes were assessed. The cell size of the AngII-treated cardiaomyocytes was significantly larger than that of the untreated cardiomyocytes. The expression of hypertrophic markers and p-ERK2, the cell surface area and the [3H]Leucine incorporation rate were all significantly increased in the AngII‑treated cells. However, the expression of mAKAPβ remained unaltered in this process. RNA interference simultaneously inhibited the protein expression of mAKAPβ and p‑ERK2, and the hypertrophy of the cardiomyocytes induced by AngII was attenuated. These results demonstrate that AngII induces hypertrophy in cardiomyocytes, and mAKAPβ is possibly involved in this process. The effects of mAKAPβ on AngII‑induced cardiomyocyte hypertrophy may be associated with p-ERK2 expression.

Angiotensin II type 1a receptor signalling directly contributes to the increased arrhythmogenicity in cardiac hypertrophy

BACKGROUND AND PURPOSE

Angiotensin II has been implicated in the development of various cardiovascular ailments, including cardiac hypertrophy and heart failure. The fact that inhibiting its signalling reduced the incidences of both sudden cardiac death and heart failure in several large-scale clinical trials suggests that angiotensin II is involved in increased cardiac arrhythmogenicity during the development of heart failure. However, because angiotensin II also promotes structural remodelling, including cardiomyocyte hypertrophy and cardiac fibrosis, it has been difficult to assess its direct contribution to cardiac arrhythmogenicity independently of the structural effects.

EXPERIMENTAL APPROACH

We induced cardiac hypertrophy in wild-type (WT) and angiotensin II type 1a receptor knockout (AT1aR-KO) mice by transverse aortic constriction (TAC). The susceptibility to ventricular tachycardia (VT) assessed in an in vivo electrophysiological study was compared in the two genotypes. The effect of acute pharmacological blockade of AT1R on the incidences of arrhythmias was also assessed.

KEY RESULTS

As described previously, WT and AT1aR-KO mice with TAC developed cardiac hypertrophy to the same degree, but the incidence of VT was much lower in the latter. Moreover, although TAC induced an increase in tyrosine phosphorylation of connexin 43, a critical component of gap junctional channels, and a reduction in ventricular levels of connexin 43 protein in both genotypes, the effect was significantly ameliorated in AT1aR-KO mice. Acute pharmacological blockade of AT1R also reduced the incidence of arrhythmias.

CONCLUSIONS AND IMPLICATIONS

Our findings demonstrate that AT1aR-mediated signalling makes a direct contribution to the increase in arrhythmogenicity in hypertrophied hearts independently of structural remodelling.

Non-canonical signalling and roles of the vasoactive peptides angiotensins and kinins

GPCRs (G-protein-coupled receptors) are among the most important targets for drug discovery due to their ubiquitous expression and participation in cellular events under both healthy and disease conditions. These receptors can be activated by a plethora of ligands, such as ions, odorants, small ligands and peptides, including angiotensins and kinins, which are vasoactive peptides that are classically involved in the pathophysiology of cardiovascular events. These peptides and their corresponding GPCRs have been reported to play roles in other systems and under pathophysiological conditions, such as cancer, central nervous system disorders, metabolic dysfunction and bone resorption. More recently, new mechanisms have been described for the functional regulation of GPCRs, including the transactivation of other signal transduction receptors and the activation of G-protein-independent pathways. The existence of such alternative mechanisms for signal transduction and the discovery of agonists that can preferentially trigger one signalling pathway over other pathways (called biased agonists) have opened new perspectives for the discovery and development of drugs with a higher specificity of action and, therefore, fewer side effects. The present review summarizes the current knowledge on the non-canonical signalling and roles of angiotensins and kinins.

Impaired JAK2-induced activation of STAT3 in failing human myocytes

Although angiotensin (Ang)II-induced Janus-activated kinase (JAK)2 phosphorylation was reported to be enhanced in failing human cardiomyocytes, the downstream balance between cardio-protective (signal transducer and activator of transcription-STAT3) and the pro-inflammatory (STAT2 and STAT5) response remains unexplored. Therefore STATs phosphorylation and putative genes overexpression following JAK2 activation were investigated in isolated cardiomyocytes obtained from failing human hearts (n = 16), and from non-failing(NF) hearts of humans (putative donors, n = 6) or adult rats. In NF myocytes Ang II-induced JAK2 activation was followed by STAT3 phosphorylation (186 ± 45% at 30 min), with no STAT2 or STAT5 response. The associated B cell lymphoma (Bcl)-xL overexpression (1.05 ± 0.39 fold) was abolished by both JAK2 and extracellular signal-regulated kinase (ERK)1/2 inhibitors (AG490, 10 μM, and PD98059, 30 μM, respectively), whereas Fas ligand (Fas-L) response (0.91 ± 0.21 fold) was inhibited only by p38MAPK antagonism (SB203580, 10 μM). In failing myocytes Ang II-induced JAK2 activation was followed by STAT2 (237 ± 38%) and STAT5 (222 ± 31%) phosphorylation, with no STAT3 response. No changes in Bcl-xL expression were observed, and the associated Fas-L gene overexpression (1.14 ± 0.27 fold) being abolished by p38 mitogen-activated protein kinase (MAPK) antagonism. The altered JAK2 induced STATs response in human failing cardiomyocytes may be of relevance for the progression of cardiac dysfunction in heart failure.

Gαi2 signaling: friend or foe in cardiac injury and heart failure?

Receptors coupled to G proteins have many effects on the heart. Enhanced signaling by Gα(s) and Gα(q) leads to cardiac injury and heart failure, while Gα(i2) signaling in cardiac myocytes can protect against ischemic injury and β-adrenergic-induced heart failure. We asked whether enhanced Gα(i2) signaling in mice could protect against heart failure using a point mutation in Gα(i2) (G184S), which prevents negative regulation by regulators of G protein signaling. Contrary to our expectation, it worsened effects of a genetic dilated cardiomyopathy (DCM) and catecholamine-induced cardiac injury. Gα (i2) (G184S/+) /DCM double heterozygote mice (TG9(+)Gα (i2) (G184S/+)) had substantially decreased survival compared to DCM animals. Furthermore, heart weight/body weight ratios (HW/BW) were significantly greater in TG9(+)Gα (i2) (G184S/+) mice as was expression of natriuretic peptide genes. Catecholamine injury in Gα (i2) (G184S/G184S) mutant mice produced markedly increased isoproterenol-induced fibrosis and collagen III gene expression vs WT mice. Cardiac fibroblasts from Gα (i2) (G184S/G184S) mice also showed a serum-dependent increase in proliferation and ERK phosphorylation, which were blocked by pertussis toxin and a mitogen-activated protein/extracellular signal-regulated kinase kinase inhibitor. Gα(i2) signaling in cardiac myocytes protects against ischemic injury but enhancing Gα(i2) signaling overall may have detrimental effects in heart failure, perhaps through actions on cardiac fibroblasts.

Angiotensin II-induced MMP-2 activity and MMP-14 and basigin protein expression are mediated via the angiotensin II receptor type 1-mitogen-activated protein kinase 1 pathway in retinal pigment epithelium: implications for age-related macular degeneration

Accumulation of various lipid-rich extracellular matrix (ECM) deposits under the retinal pigment epithelium (RPE) has been observed in eyes with age-related macular degeneration (AMD). RPE-derived matrix metalloproteinase (MMP)-2, MMP-14, and basigin (BSG) are major enzymes involved in the maintenance of ECM turnover. Hypertension (HTN) is a systemic risk factor for AMD. It has previously been reported that angiotensin II (Ang II), one of the most important hormones associated with HTN, increases MMP-2 activity and its key regulator, MMP-14, in RPE, inducing breakdown of the RPE basement membrane, which may lead to progression of sub-RPE deposits. Ang II exerts most of its actions by activating the mitogen-activated protein kinase (MAPK) signaling pathway. Herein is explored the MAPK signaling pathway as a potential key intracellular modulator of Ang II-induced increase in MMP-2 activity and MMP-14 and BSG protein expression. It was observed that Ang II stimulates phosphorylation of extracellular signal-regulated kinase (ERK) and p38 MAPK in RPE cells and ERK/p38 and Jun N-terminal kinase (JNK) in mice. These effects were mediated by Ang II type 1 receptors. Blockade of ERK or p38 MAPK abrogated the increase in MMP-2 activity and MMP-14 and BSG proteins in ARPE-19 cells. A better understanding of the molecular events by which Ang II induces ECM dysregulation is of critical importance to further define its contribution to the progression of sub-RPE deposits in AMD patients with HTN.

Neuro-endocrine regulation of blood pressure

As our understanding of the underlying aetiology of hypertension is far from adequate, over 90% of patients with hypertension receive a diagnosis of essential hypertension. This non-specific diagnosis leads to suboptimal therapeutics and a major problem with non-compliance. Understanding the normal control of blood pressure (BP) is, hence, important for a better understanding of the disease.This review attempts to unravel the present understanding of BP control. The local mechanisms of BP control, the neural mechanisms, renal-endocrine mechanisms, and a variety of other hormones that have a bearing in normal BP control are discussed and the possible role in the pathophysiology is alluded to.

Phosphoproteomic analysis of AT1 receptor-mediated signaling responses in proximal tubules of angiotensin II-induced hypertensive rats

The signaling mechanisms underlying the effects of angiotensin II in proximal tubules of the kidney are not completely understood. Here we measured signal protein phosphorylation in isolated proximal tubules using pathway-specific proteomic analysis in rats continuously infused with pressor or non-pressor doses of angiotensin II over a 2-week period. Of the 38 phosphoproteins profiled, 14 were significantly altered by the pressor dose. This included increased phosphorylation of the protein kinase C isoenzymes, PKCα and PKCβII, and the glycogen synthase kinases, GSK3α and GSK3β. Phosphorylation of the cAMP-response element binding protein 1 and PKCδ were decreased, whereas PKCɛ remained unchanged. By contrast, the phosphorylation of only seven proteins was altered by the non-pressor dose, which increased that of PKCα, PKCδ, and GSKα. Phosphorylation of MAP kinases, ERK1/2, was not increased in proximal tubules in vivo by the pressor dose, but was in proximal tubule cells in vitro. Infusion of the pressor dose decreased, whereas the non-pressor dose of angiotensin II increased the phosphorylation of the sodium and hydrogen exchanger 3 (NHE-3) in membrane fractions of proximal tubules. Losartan largely blocked the signaling responses induced by the pressor dose. Thus, PKCα and PKCβII, GSK3α and GSK3β, and cAMP-dependent signaling pathways may have important roles in regulating proximal tubular sodium and fluid transport in Ang II-induced hypertensive rats.

Pathophysiological mechanisms of atrial fibrillation: a translational appraisal

Atrial fibrillation (AF) is an arrhythmia that can occur as the result of numerous different pathophysiological processes in the atria. Some aspects of the morphological and electrophysiological alterations promoting AF have been studied extensively in animal models. Atrial tachycardia or AF itself shortens atrial refractoriness and causes loss of atrial contractility. Aging, neurohumoral activation, and chronic atrial stretch due to structural heart disease activate a variety of signaling pathways leading to histological changes in the atria including myocyte hypertrophy, fibroblast proliferation, and complex alterations of the extracellular matrix including tissue fibrosis. These changes in electrical, contractile, and structural properties of the atria have been called "atrial remodeling." The resulting electrophysiological substrate is characterized by shortening of atrial refractoriness and reentrant wavelength or by local conduction heterogeneities caused by disruption of electrical interconnections between muscle bundles. Under these conditions, ectopic activity originating from the pulmonary veins or other sites is more likely to occur and to trigger longer episodes of AF. Many of these alterations also occur in patients with or at risk for AF, although the direct demonstration of these mechanisms is sometimes challenging. The diversity of etiological factors and electrophysiological mechanisms promoting AF in humans hampers the development of more effective therapy of AF. This review aims to give a translational overview on the biological basis of atrial remodeling and the proarrhythmic mechanisms involved in the fibrillation process. We pay attention to translation of pathophysiological insights gained from in vitro experiments and animal models to patients. Also, suggestions for future research objectives and therapeutical implications are discussed.

Mitofusin 2 inhibits angiotensin II-induced myocardial hypertrophy

BACKGROUND AND OBJECTIVES

Myocardial hypertrophy is a common clinical finding leading to heart failure and sudden death. Mitofusin 2 (Mfn2), a hyperplasia suppressor protein, is downregulated in hypertrophic heart. This study examined the role of Mfn2 in myocardial hypertrophy and its potential signal pathway.

METHODS AND RESULTS

In in vitro studies, neonatal cardiac myocytes were isolated and cultured. Incubation of cultured cardiomycytes with angiotensin II (Ang II) inhibited gene expression of Mfn2; induced cell hypertrophy and protein synthesis; and activated protein kinase Akt. Pretreatment of cells with AdMfn2-a replication-deficient adenoviral vector encoding rat Mfn2 gene-upregulated Mfn2 expression and subsequently attenuated Ang II-induced cell hypertrophy; protein synthesis; and Akt activation. In in vivo studies, direct gene delivery of AdMfn2 into myocardium decreased the infusion of Ang II-induced atrial natriuretic factor (ANF, a hypertrophic marker) expression and cardiomyocyte cross-sectional area. Consistently, upregulation of Mfn2 in myocardium decreased the thicknesses of anterior and posterior walls of left ventricle (LV) and the ratio of LV mass/body weight in Ang II-treated rats. Of note, AdGFP (control for AdMfn2) did not affect the effects of Ang II in vitro or in vivo.

CONCLUSIONS

Upregulation of Mfn2 inhibits Ang II-induced myocardial hypertrophy. In this process, inhibition of Akt activation seems to play a significant role. These findings indicate Mfn2 is a critical protein in modulating myocyte hypertrophy.

Functional selectivity in adrenergic and angiotensin signaling systems

β-Adrenergic and angiotensin II type 1A receptors are therapeutic targets for the treatment of a number of common human diseases. Pharmacological agents designed as antagonists for these receptors have positively affected the morbidity and mortality of patients with hypertension, heart failure, and renal disease. Antagonism of these receptors, however, may only partially explain the therapeutic benefits of β-blockers and angiotensin receptor blockers given the emerging concept of functional selectivity or biased agonism. This new pharmacological paradigm suggests that multiple signaling pathways can be differentially modified by a single ligand-receptor interaction. This review examines the functional selectivity of β-adrenergic and angiotensin II type 1A receptors with respect to their ability to signal via both G protein-dependent and G protein-independent mechanisms, with a focus on the multifunctional protein β-arrestin. Also highlighted are the concept of "biased signaling" through β-arrestin mediated pathways, the affect of ligand/receptor modification on such biased agonism, and the implications of functional selectivity for the development of the next generation of β-blockers and angiotensin receptor blockers.

Cardiac mast cells cause atrial fibrillation through PDGF-A-mediated fibrosis in pressure-overloaded mouse hearts

Atrial fibrillation (AF) is a common arrhythmia that increases the risk of stroke and heart failure. Here, we have shown that mast cells, key mediators of allergic and immune responses, are critically involved in AF pathogenesis in stressed mouse hearts. Pressure overload induced mast cell infiltration and fibrosis in the atrium and enhanced AF susceptibility following atrial burst stimulation. Both atrial fibrosis and AF inducibility were attenuated by stabilization of mast cells with cromolyn and by BM reconstitution from mast cell-deficient WBB6F1-KitW/W-v mice. When cocultured with cardiac myocytes or fibroblasts, BM-derived mouse mast cells increased platelet-derived growth factor A (PDGF-A) synthesis and promoted cell proliferation and collagen expression in cardiac fibroblasts. These changes were abolished by treatment with a neutralizing antibody specific for PDGF alpha-receptor (PDGFR-alpha). Consistent with these data, upregulation of atrial Pdgfa expression in pressure-overloaded hearts was suppressed by BM reconstitution from WBB6F1-KitW/W-v mice. Furthermore, injection of the neutralizing PDGFR-alpha-specific antibody attenuated atrial fibrosis and AF inducibility in pressure-overloaded hearts, whereas administration of homodimer of PDGF-A (PDGF-AA) promoted atrial fibrosis and enhanced AF susceptibility in normal hearts. Our results suggest a crucial role for mast cells in AF and highlight a potential application of controlling the mast cell/PDGF-A axis to achieve upstream prevention of AF in stressed hearts.

Prostaglandin D2 induces the production of human beta-defensin-3 in human keratinocytes

The antimicrobial peptide human beta-defensin-3 (hBD-3) is produced by epidermal keratinocytes and protects the skin from infections. This peptide induces the release of a lipid mediator, prostaglandin D(2) from dermal mast cells. Prostaglandin D(2) binds to cell-surface G protein-coupled receptors, D prostanoid receptor, and chemoattractant receptor-homologous molecule expressed on T helper cell type 2 (CRTH2). Both receptors are detected on epidermal keratinocytes. It is reported that prostaglandin D(2) is involved in cutaneous allergy, however, its role in antimicrobial defense is unknown. We examined the in vitro effects of prostaglandin D(2) on hBD-3 production in normal human keratinocytes. Prostaglandin D(2) enhanced hBD-3 secretion and mRNA expression in human keratinocytes. Prostaglandin D(2)-induced hBD-3 production was suppressed by the CRTH2 antagonist ramatroban and by antisense oligonucleotides against c-Jun and c-Fos, components of a transcription factor, activator protein-1 (AP-1). Prostaglandin D(2) enhanced the transcriptional activity and DNA binding of AP-1, expression, phosphorylation, and DNA binding of c-Fos proteins in keratinocytes. Prostaglandin D(2)-induced hBD-3 production, AP-1 activity, and c-Fos expression and phosphorylation were suppressed by U0126, PP2, and pertussis toxin, which are inhibitors of mitogen-activated protein kinase kinase (MEK), src, and G(i) proteins, respectively. The phosphorylation of extracellular signal-regulated kinase (ERK), downstream kinase of MEK, was induced by prostaglandin D(2), and suppressed by ramatroban, pertussis toxin, PP2, and U0126. These results suggest that prostaglandin D(2) induces hBD-3 production in human keratinocytes by activating AP-1 through the expression and phosphorylation of c-Fos via the CRTH2/G(i)/src/MEK/ERK pathway. Prostaglandin D(2) may promote cutaneous antimicrobial activity via hBD-3.

Independent beta-arrestin2 and Gq/protein kinase Czeta pathways for ERK stimulated by angiotensin type 1A receptors in vascular smooth muscle cells converge on transactivation of the epidermal growth factor receptor

Recent studies in receptor-transfected cell lines have demonstrated that extracellular signal-regulated kinase (ERK) activation by angiotensin type 1A receptor and other G protein-coupled receptors can be mediated by both G protein-dependent and β-arrestin-dependent mechanisms. However, few studies have explored these mechanisms in primary cultured cells expressing endogenous levels of receptors. Accordingly, here we utilized the β-arrestin biased agonist for the angiotensin type 1A receptor, SII-angiotensin (SII), and RNA interference techniques to investigate angiotensin II (ANG)-activated β-arrestin-mediated mitogenic signaling pathways in rat vascular smooth muscle cells. Both ANG and SII induced DNA synthesis via the ERK activation cascade. Even though SII cannot induce calcium influx (G protein activation) after receptor stimulation, it does cause ERK activation, although less robustly than ANG. Activation by both ligands is diminished by depletion of β-arrestin2 by small interfering RNA, although the effect is more complete with SII. ERK activation at early time points but not later time points is strongly inhibited by those protein kinase C inhibitors that can block protein kinase Cζ. Moreover, ANG- and SII-mediated ERK activation require transactivation of the epidermal growth factor receptor via metalloprotease 2/9 and Src kinase. β-Arrestin2 facilitates ANG and SII stimulation of Src-mediated phosphorylation of Tyr-845 on the EGFR, a known site for Src phosphorylation. These studies delineate a convergent mechanism by which G protein-dependent and β-arrestin-dependent pathways can independently mediate ERK-dependent transactivation of the EGFR in vascular smooth muscle cells thus controlling cellular proliferative responses.

PKC-dependent extracellular signal-regulated kinase 1/2 pathway is involved in the inhibition of Ib on AngiotensinII-induced proliferation of vascular smooth muscle cells

AngiotensinII (AngII) induces vascular smooth muscle cell (VSMC) proliferation, which plays an important role in the development and progression of hypertension. AngII-induced cellular events have been implicated, in part, in the activation of protein kinase C (PKC) and extracellular signal-regulated kinases 1/2 (ERK1/2). In the present study, we investigated the effect of Ib, a novel nonpeptide AngII receptor type 1 (AT(1)) antagonist, on the activation of PKC and ERK1/2 in VSMC proliferation induced by AngII. MTT, and [(3)H]thymidine incorporation assay showed that AngII-induced VSMC proliferation was inhibited significantly by Ib. The specific binding of [(125)I]AngII to AT(1) receptors was blocked by Ib in a concentration-dependent manner with IC(50) value of 0.96nM. PKC activity assay and Western blot analysis demonstrated that Ib significantly inhibited the activation of PKC and phosphorylation of ERK1/2 induced by AngII, respectively. Furthermore, AngII-induced ERK1/2 activation was obviously blocked by GF109203X, a PKC inhibitor. These findings suggest that the suppression of Ib on AngII-induced VSMC proliferation may be attributed to its inhibitory effect on PKC-dependent ERK1/2 pathway.

Hyper-phosphorylation of alpha-enolase in hypertrophied left ventricle of spontaneously hypertensive rat

Cardiac hypertrophy is one of the main target organ damages of essential hypertension and predicts a poor prognosis of the disease. The molecules involved in this event, especially their posttranslational modifications, remain largely unknown to date. With a combination of phosphoprotein column enrichment and two-dimensional gel electrophoresis separation, here we compared the profiling of enriched phosphoproteins from the left ventricle (LV) of spontaneously hypertensive rats (SHR) to that of age-matched Wistar Kyoto rats. As a result, 19 differential proteins were found in the hypertrophied LV of SHR. Among them, we focused on a glycolysis enzyme alpha-enolase, of which the hyper-phosphorylation was shown in the hypertrophied LV but not in non-hypertrophied atrium and right ventricle of SHR. Furthermore, the alpha-enolase hyper-phosphorylation was accompanied by decreased enzymatic activity. The further investigation based on these results would provide new clues to understand the pathological process of cardiac hypertrophy in SHR.

Inhibitory crosstalk between ERK and AMPK in the growth and proliferation of cardiac fibroblasts

Extracellular signal-regulated kinase (ERK) is one of the key protein kinases that regulate the growth and proliferation in cardiac fibroblasts (CFs). As an energy sensor of cellular metabolism, AMP-activated protein kinase (AMPK) is found recently to be involved in myocardial remodeling. In this study, we investigated the crosstalk between ERK and AMPK in the growth and proliferation of CFs. In neonatal rat cardiac fibroblasts (NRCFs), we found that serum significantly inhibited basal AMPK phosphorylation between 10min and 24h and also partially inhibited AMPK phosphorylation by AMPK activator, 5-aminoimidazole-4-carboxamide-ribonucleoside (AICAR). Furthermore, ERK inhibitor could greatly reverse the inhibition of AMPK by serum. Conversely, activation of AMPK by AICAR also showed a significant inhibition of basal and serum-induced ERK phosphorylation but it showed a delayed and steadfast inhibition which appeared after 60min and lasted until 12h. Moreover, inhibition of ERK could repress the activation of p70S6K, an important kinase in cardiac proliferation, and AICAR could also inhibit p70S6K phosphorylation. In addition, under both serum and serum-free medium, AICAR significantly inhibited the DNA synthesis and cell numbers, and reduced cells at S phase. In conclusion, AMPK activation with AICAR inhibited growth and proliferation in cardiac fibroblasts, which involved inhibitory interactions between ERK and AMPK. This is the first report that AMPK could be a target of ERK in growth factors-induced proliferation, which may give a new mechanism that growth factors utilize in their promotion of proliferation in cardiac fibroblasts.

Epigallocatechin-3 gallate prevents cardiac hypertrophy induced by pressure overload in rats

Pressure overload diseases, such as valvular stenosis and systemic hypertension, manifest morphologically in patients as cardiac concentric hypertrophy. Prevention of cardiac remodeling due to increased pressure overload is important to reduce morbidity and mortality. Epigallocatechin-3 gallate (EGCG) is a major bioactive polyphenol present in green tea which has been found to be a nitric oxide-mediated vasorelaxant and to be cardioprotective in myocardial ischemia-reperfusion injury. Therefore, we investigated whether EGCG supplementation could reduce in vivo pressure overload-mediated cardiac hypertrophy. Cardiac hypertrophy was induced by suprarenal transverse abdominal aortic constriction (AC) in rats. Three weeks after AC surgery, heart to body weight ratio increased in the AC group by 34% compared to the sham group. EGCG administration suppressed the load-induced increase in heart weight by 69%. Attenuation of cardiac hypertrophy by EGCG was associated with attenuation of the increase in myocyte cell size and fibrosis induced by aortic constriction. Despite abolition of hypertrophy by EGCG, transstenotic pressure gradients did not change. Echocardiogram revealed that increased left ventricular systolic dimensions and deteriorated systolic function were relieved by EGCG. These results suggest that EGCG prevents the development of left ventricular concentric hypertrophy by pressure overload and may be a useful therapeutic modality to prevent cardiac remodeling in patients with pressure overload myocardial diseases.

Hyperthermia treatment prevents angiotensin II-mediated atrial fibrosis and fibrillation via induction of heat-shock protein 72

We tested the hypothesis that atrial fibrosis and atrial fibrillation (AF) evoked by angiotensin II (AII) could be prevented by the induction of heat-shock protein 72 (HSP72) by hyperthermia (HT). In cultured atrial fibroblasts isolated from male Sprague-Dawley rats, HT (42 degrees C) was applied for 30 min. AII (100 nmol/L) was added to the medium 8 h later. HT induced the expression of HSP72, which was associated with the attenuation of AII-induced extracellular signal-regulated kinase (ERK1/ERK2) phosphorylation, alpha-smooth muscle actin (alpha-SMA) expression, transforming growth factor-beta(1) secretion, collagen synthesis, and expression of collagen type I and tissue inhibitor of metalloproteinases-1. A small interfering RNA targeting HSP72 abolished these anti-fibrotic effects of HT. In male Sprague-Dawley rats in vivo, an osmotic mini-pump was subcutaneously implanted for continuous infusion of AII (400 ng/kg/min). Whole-body HT (43 degrees C, 20 min) was applied 24 h before and 7, 14, and 21 days after the start of the AII infusion. Repeated HT led to the induction of HSP72 expression, which resulted in an attenuation of AII-induced left atrial fibrosis. In an electrophysiological study using isolated perfused heart, continuous AII caused slowing of interatrial conduction without affecting atrial refractoriness. In AII-treated hearts, extrastimuli from the right atrial appendage resulted in a high incidence of repetitive atrial responses, which were suppressed by treatment with HT. Our results suggest that HT treatment is effective in suppressing AII-mediated atrial fibrosis and AF via induction of HSP72 at least in parts, and is thus expected to be a novel strategy for prevention of AF.

ANG II-induced cell proliferation is dually mediated by c-Src/Yes/Fyn-regulated ERK1/2 activation in the cytoplasm and PKCζ-controlled ERK1/2 activity within the nucleus

High-affinity binding of angiotensin II (ANG II) to the ANG II type 1 receptor (AT1R) results in the activation of ERK1/2 mitogen-activated protein kinases (MAPK). However, the precise mechanism of ANG II-induced ERK1/2 activation has not been fully characterized. Here, we investigated the signaling events leading to ANG II-induced ERK1/2 activation using a c-Src/Yes/Fyn tyrosine kinase-deficient mouse embryonic fibroblast (MEF) cell line stably transfected with the AT1R (SYF/AT1). ERK1/2 activation was reduced by ∼50% within these cells compared with wild-type controls (WT/AT1). The remaining ∼50% of intracellular ERK1/2 activation was dependent upon heterotrimeric G protein and protein kinase C zeta (PKCζ) activation. Therefore, ANG II-induced ERK1/2 activation occurs via two independent mechanisms. We next investigated whether a loss of either c-Src/Yes/Fyn or PKCζ signaling affected ERK1/2 nuclear translocation and cell proliferation in response to ANG II. ANG II-induced cell proliferation was markedly reduced in SYF/AT1cells compared with WT/AT1cells ( P < 0.01), but interestingly, ERK2 nuclear translocation was normal. ANG II-induced nuclear translocation of ERK2 was blocked via pretreatment of WT/AT1cells with a PKCζ pseudosubstrate. ANG II-induced cell proliferation was significantly reduced in PKCζ pseudosubstrate-treated WT/AT1cells ( P < 0.01) and was completely blocked in SYF/AT1cells treated with this same compound. Thus ANG II-induced cell proliferation appears to be regulated by both ERK1/2-driven nuclear and cytoplasmic events. In response to ANG II, the ability of ERK1/2 to remain within the cytoplasm or translocate into the nucleus is controlled by c-Src/Yes/Fyn or heterotrimeric G protein/PKCζ signaling, respectively.

Adrenomedullin inhibits angiotensin II-induced oxidative stress via Csk-mediated inhibition of Src activity

We have demonstrated that adrenomedullin (AM) protects against angiotensin II (ANG II)-induced cardiovascular damage through the attenuation of increased oxidative stress observed in AM-deficient mice. However, the mechanism(s) that underlie this activity remain unclear. To address this question, we investigated the effect of AM on ANG II-stimulated reactive oxygen species (ROS) production in cultured rat aortic vascular smooth muscle cells (VSMCs). ANG II markedly increased ROS production through activation of NADPH oxidase. This effect was significantly attenuated by AM in a concentration-dependent manner. This effect was mimicked by dibutyl-cAMP and blocked by pretreatment with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide hydrochloride (H-89), a protein kinase A inhibitor, and CGRP(8-37), an AM/CGRP receptor antagonist. This inhibitory effect of AM was also lost following the expression of a constitutively active Src. Moreover, AM intersected ANG II signaling by inducing COOH-terminal Src kinase (Csk) activation that, in turn, inhibits Src activation. These data, for the first time, demonstrate that AM attenuates the ANG II-induced increase in ROS in VSMCs via activation of Csk, thereby inhibiting Src activity.

Quercetin-supplemented diets lower blood pressure and attenuate cardiac hypertrophy in rats with aortic constriction

Quercetin (Q), a flavonoid found in berries and onions, can reduce blood pressure in hypertensive animals and inhibit signal transduction pathways in vitro that regulate cardiac hypertrophy. We hypothesized that quercetin could prevent cardiovascular complications in rats with abdominal aortic constriction (AAC). Rats consumed standard or Q-supplemented chow (1.5 g Q/kg chow) for 7 days before AAC or sham surgery (SHAM, n = 15; AAC, n = 15; SHAMQ, n = 15; AACQ, n = 14). Fourteen days after surgery, plasma and liver Q concentrations were elevated (P < 0.05) and hepatic lipid oxidation was reduced (P < 0.05) in Q-treated versus untreated rats. Carotid arterial blood pressure and cardiac hypertrophy were attenuated (P < 0.05), and cardiac protein kinase C betaII translocation was normalized (P < 0.05) in AACQ versus AAC. Expression of cardiac beta-myosin heavy-chain mRNA was also reduced in AACQ versus AAC (P < 0.05). However, extracellular regulated kinase 1/2 phosphorylation was similar in AAC versus AACQ. The level of aortic endothelial dysfunction (wire myography) was also similar between AAC and AACQ, in spite of reduced aortic thickening in AACQ. Importantly, Q-treated rats did not show any deleterious changes in myocardial function (echocardiography). Our data supports an antihypertensive and antihypertrophic effect of Q in vivo in the absence of changes concerning vascular and myocardial function.

Foxo transcription factors blunt cardiac hypertrophy by inhibiting calcineurin signaling

BACKGROUND

Cellular hypertrophy requires coordinated regulation of progrowth and antigrowth mechanisms. In cultured neonatal cardiomyocytes, Foxo transcription factors trigger an atrophy-related gene program that counters hypertrophic growth. However, downstream molecular events are not yet well defined.

METHODS AND RESULTS

Here, we report that expression of either Foxo1 or Foxo3 in cardiomyocytes attenuates calcineurin phosphatase activity and inhibits agonist-induced hypertrophic growth. Consistent with these results, Foxo proteins decrease calcineurin phosphatase activity and repress both basal and hypertrophic agonist-induced expression of MCIP1.4, a direct downstream target of the calcineurin/NFAT pathway. Furthermore, hearts from Foxo3-null mice exhibit increased MCIP1.4 abundance and a hypertrophic phenotype with normal systolic function at baseline. Together, these results suggest that Foxo proteins repress cardiac growth at least in part through inhibition of the calcineurin/NFAT pathway. Given that hypertrophic growth of the heart occurs in multiple contexts, our findings also suggest that certain hypertrophic signals are capable of overriding the antigrowth program induced by Foxo. Consistent with this, multiple hypertrophic agonists triggered inactivation of Foxo proteins in cardiomyocytes through a mechanism requiring the PI3K/Akt pathway. In addition, both Foxo1 and Foxo3 are phosphorylated and consequently inactivated in hearts undergoing hypertrophic growth induced by hemodynamic stress.

CONCLUSIONS

This study suggests that inhibition of the calcineurin/NFAT signaling cascade by Foxo and release of this repressive action by the PI3K/Akt pathway are important mechanisms whereby Foxo factors govern cell growth in the heart.

Arachidonic acid induces ERK activation via Src SH2 domain association with the epidermal growth factor receptor

Within the kidney, angiotensin II type 2 (AT(2)) receptor mediates phospholipase A(2) (PLA(2)) activation, arachidonic acid release, epidermal growth factor (EGF) receptor transactivation, and mitogen-activated protein kinase activation. Arachidonic acid mimics this transactivation by an undetermined mechanism. The role of c-Src in mediating angiotensin II and arachidonic acid signaling was determined by employing immunocomplex kinase assay, Western blotting analysis, and protein immunoblotting on co-precipitated EGF receptor (EGFR) proteins and agarose conjugates of glutathione S-transferase fusion proteins containing the c-Src homology 2 (SH2) and SH3 domains. Angiotensin II induced extracellular signal-regulated kinase (ERK) activation in primary cultures of rabbit proximal tubule cells via the activation of c-Src and association of the EGFR with the c-Src SH2 domain, effects that were mimicked by arachidonic acid and its inactive analogue eicosatetraynoic acid. Inhibition of PLA(2) by mepacrine and methyl arachidonyl fluorophosphate, AT(2) receptor by PD123319, Src family kinases by, 1-(tert-butyl)-3-(4-chlorophenyl)-4-aminopyrazolo[3,4-d] pyrimidine (PP2) and c-Src by overexpression of a dominant-negative mutant of c-Src abrogated these effects. However, inhibitors of arachidonic acid metabolic pathways did not block these effects. The present work provides a new and novel paradigm for transactivation of a kinase receptor linked to a fatty acid, which may apply to activation of a variety of phospholipases and accompanying arachidonic acid release.

Prevention of atrial fibrillation in patients with symptomatic chronic heart failure by candesartan in the Candesartan in Heart failure: assessment of Reduction in Mortality and morbidity (CHARM) program

BACKGROUND

Atrial fibrillation (AF) is frequent in patients with chronic heart failure (CHF). Experimental and small patient studies have demonstrated that blocking the renin-angiotensin-aldosterone system may prevent AF. In the CHARM program, the effects of the angiotensin receptor blocker candesartan on cardiovascular mortality and morbidity were evaluated in a broad spectrum of patients with symptomatic CHF. CHARM provided the opportunity to prospectively determine the effect of candesartan on the incidence of new AF in this CHF population.

METHODS

7601 patients with symptomatic CHF and reduced or preserved left ventricular systolic function were randomized to candesartan (target dose 32 mg once daily, mean dose 24 mg) or placebo in the 3 component trials of CHARM. The major outcomes were cardiovascular death or CHF hospitalization and all-cause mortality. The incidence of new AF was a prespecified secondary outcome. Median follow-up was 37.7 months. A conditional logistic regression model for stratified data was used.

RESULTS

6446 patients (84.8%) did not have AF on their baseline electrocardiogram. Of these, 392 (6.08%) developed AF during follow-up, 177 (5.55%) in the candesartan group and 215 (6.74%) in the placebo group (odds ratio 0.812, 95% CI 0.662-0.998, P = .048). After adjustment for baseline covariates, the odds ratio was 0.802 (95% CI 0.650-0.990, P = .039). There was no heterogeneity of the effects of candesartan in preventing AF between the 3 component trials (P = .57).

CONCLUSIONS

Treatment with the angiotensin receptor blocker candesartan reduced the incidence of AF in a large, broadly-based, population of patients with symptomatic CHF.

Identification of differentially expressed genes induced by angiotensin II in rat cardiac fibroblasts

1. Cardiac fibroblasts play an important regulatory role in cardiac remodelling by undergoing proliferation, differentiation and upregulating various gene products, including some cytokines and extracellular matrix (ECM) proteins. A highly potent mediator of cardiac remodelling is angiotensin (Ang) II. 2. In the present study, the suppression subtractive hybridization method was used to identify differentially expressed cDNAs in adult rat cardiac fibroblasts induced by AngII. 3. Following mRNA isolation of non-stimulated and AngII-stimulated cells, cDNAs of both populations were prepared and subtracted by suppression polymerase chain reaction. Sequencing of the partially enriched cDNAs identified 36 genes differentially expressed, including ECM proteins (pro-alpha(1) collagen type III, fibronectin), structural protein (spectrin), enzyme (GTP-specific succinyl-CoA synthetase), transcriptional regulators (glucocorticoid-induced leucine zipper, inhibitor of DNA binding 3) and proteins involved in cell division control (cdc2) or cell signalling (insulin-like growth factor binding protein-3, mutant p53-binding protein, grp75, CGI-121, protein phosphatase type 2A, tspan-2 and Sam68). 4. The diversity of genes identified in the present study further emphasises the central role of AngII in the regulation of cardiac remodelling.

Extracellular signal-regulated kinase as an inducer of non-apoptotic neuronal death

Extracellular signal-regulated kinase (ERK) is a versatile protein kinase, which has been implicated in signaling numerous biological functions ranging from embryonic development to memory formation. Recent reports, including ours, indicate that ERK plays a central role in promoting neuronal degeneration in various neuronal systems including neurodegenerative diseases. Mechanisms involved in ERK-induced neuronal degeneration are beginning to emerge. In this review, we summarize evidence suggesting ERK to be a predominant inducer of a non-apoptotic mode of neuronal death. Further, we discuss the mechanisms and the putative molecular inter-players associated with ERK-mediated neuronal death.

Targeting the renin-angiotensin-aldosterone-system in atrial fibrillation: a shift from electrical to structural therapy?

Despite its increasing incidence and prevalence, treatment options in atrial fibrillation (AF) are far from ideal and often limited. After decades of focus on the electrical aspects of AF with unsatisfactory results, recent research is focusing increasingly on the atrial structural remodelling that underlies the development of AF in different pathological conditions, such as hypertension, heart failure, diabetes mellitus and coronary artery disease. The aim of this review is to provide a comprehensive overview of the role of the renin-angiotensin-aldosterone-system in AF and to highlight the clinical evidence on renin-angiotensin-aldosterone-system blockade as a therapeutic option in AF.

G alpha 12/13- and reactive oxygen species-dependent activation of c-Jun NH2-terminal kinase and p38 mitogen-activated protein kinase by angiotensin receptor stimulation in rat neonatal cardiomyocytes

In the present study, we examined signal transduction mechanism of reactive oxygen species (ROS) production and the role of ROS in angiotensin II-induced activation of mitogen-activated protein kinases (MAPKs) in rat neonatal cardiomyocytes. Among three MAPKs, c-Jun NH(2)-terminal kinase (JNK) and p38 MAPK required ROS production for activation, as an NADPH oxidase inhibitor, diphenyleneiodonium, inhibited the activation. The angiotensin II-induced activation of JNK and p38 MAPK was also inhibited by the expression of the Galpha(12/13)-specific regulator of G protein signaling (RGS) domain, a specific inhibitor of Galpha(12/13), but not by an RGS domain specific for Galpha(q). Constitutively active Galpha(12)- or Galpha(13)-induced activation of JNK and p38 MAPK, but not extracellular signal-regulated kinase (ERK), was inhibited by diphenyleneiodonium. Angiotensin II receptor stimulation rapidly activated Galpha(13), which was completely inhibited by the Galpha(12/13)-specific RGS domain. Furthermore, the Galpha(12/13)-specific but not the Galpha(q)-specific RGS domain inhibited angiotensin II-induced ROS production. Dominant negative Rac inhibited angiotensin II-stimulated ROS production, JNK activation, and p38 MAPK activation but did not affect ERK activation. Rac activation was mediated by Rho and Rho kinase, because Rac activation was inhibited by C3 toxin and a Rho kinase inhibitor, Y27632. Furthermore, angiotensin II-induced Rho activation was inhibited by Galpha(12/13)-specific RGS domain but not dominant negative Rac. An inhibitor of epidermal growth factor receptor kinase AG1478 did not affect angiotensin II-induced JNK activation cascade. These results suggest that Galpha(12/13)-mediated ROS production through Rho and Rac is essential for JNK and p38 MAPK activation.

Cysteinyl-leukotrienes are released from astrocytes and increase astrocyte proliferation and glial fibrillary acidic protein via cys-LT1 receptors and mitogen-activated protein kinase pathway

Cysteinyl-leukotrienes (cys-LTs), potent mediators in inflammatory diseases, are produced by nervous tissue, but their cellular source and role in the brain are not very well known. In this report we have demonstrated that rat cultured astrocytes express the enzymes (5'-lipoxygenase and LTC(4) synthase) required for cys-LT production, and release cys-LTs in resting condition and, to a greater extent, in response to calcium ionophore A23187, 1 h combined oxygen-glucose deprivation or 2-methyl-thioATP, a selective P2Y(1)/ATP receptor agonist. MK-886, a LT synthesis inhibitor, prevented basal and evoked cys-LT release. In addition, 2-methyl-thioATP-induced cys-LT release was abolished by suramin, a P2 receptor antagonist, or by inhibitors of ATP binding cassette proteins involved in cys-LT release. We also showed that astrocytes express cys-LT(1) and not cys-LT(2) receptors. The stimulation of these receptors by LTD(4) activated the mitogen-activated protein kinase (MAPK) pathway. This effect was: (i) insensitive to inhibitors of receptor-coupled Gi protein (pertussis toxin) or tyrosine kinase receptors (genistein); (ii) abolished by MK-571, a cys-LT(1) selective receptor antagonist, or PD98059, a MAPK inhibitor; (iii) reduced by inhibitors of calcium/calmodulin-dependent kinase II (KN-93), Ca(2+)-dependent and -independent (GF102903X) or Ca(2+)-dependent (Gö6976) protein kinase C isoforms. LTD(4) also increased astrocyte proliferation and glial fibrillary acidic protein content, which are considered hallmarks of reactive astrogliosis. Both effects were counteracted by cell pretreatment with MK-571 or PD98059. Thus, cys-LTs released from astrocytes might play an autocrine role in the induction of reactive astrogliosis that, in brain injuries, contributes to the formation of a reparative glial scar.

The biochemical response of the heart to hypertension and exercise

Mechanical stress on the heart can lead to crucially different outcomes. Exercise is beneficial because it causes heart muscle cells to enlarge (hypertrophy). Chronic hypertension also causes hypertrophy, but in addition it causes an excessive increase in fibroblasts and extracellular matrix (fibrosis), death of cardiomyocytes and ultimately heart failure. Recent research shows that stimulation of physiological (beneficial) hypertrophy involves several signaling pathways, including those mediated by protein kinase B (also known as Akt) and the extracellular-signal-regulated kinases 1 and 2 (ERK1/2). Hypertension, beta-adrenergic stimulation and agonists such as angiotensin II (Ang II) activate not only ERK1/2 but also p38 and the Jun N-terminal kinase (JNK), leading to pathological heart remodeling. Despite this progress, the mechanisms that activate fibroblasts to cause fibrosis and those that differentiate between exercise and hypertension to produce physiological and pathological responses, respectively, remain to be established.