Adrenergic receptor stimulation of the mitogen-activated protein kinase cascade and cardiac hypertrophy

Palmitoylation-dependent regulation of cardiomyocyte Rac1 signaling activity and minor effects on cardiac hypertrophy

S-palmitoylation is a reversible lipid modification catalyzed by 23 S-acyltransferases with a conserved zinc finger aspartate-histidine-histidine-cysteine (zDHHC) domain that facilitates targeting of proteins to specific intracellular membranes. Here we performed a gain-of-function screen in the mouse and identified the Golgi-localized enzymes zDHHC3 and zDHHC7 as regulators of cardiac hypertrophy. Cardiomyocyte-specific transgenic mice overexpressing zDHHC3 show cardiac disease, and S-acyl proteomics identified the small GTPase Rac1 as a novel substrate of zDHHC3. Notably, cardiomyopathy and congestive heart failure in zDHHC3 transgenic mice is preceded by enhanced Rac1 S-palmitoylation, membrane localization, activity, downstream hypertrophic signaling, and concomitant induction of all Rho family small GTPases whereas mice overexpressing an enzymatically dead zDHHC3 mutant show no discernible effect. However, loss of Rac1 or other identified zDHHC3 targets Gαq/11 or galectin-1 does not diminish zDHHC3-induced cardiomyopathy, suggesting multiple effectors and pathways promoting decompensation with sustained zDHHC3 activity. Genetic deletion of Zdhhc3 in combination with Zdhhc7 reduces cardiac hypertrophy during the early response to pressure overload stimulation but not over longer time periods. Indeed, cardiac hypertrophy in response to 2 weeks of angiotensin-II infusion is not diminished by Zdhhc3/7 deletion, again suggesting other S-acyltransferases or signaling mechanisms compensate to promote hypertrophic signaling. Taken together, these data indicate that the activity of zDHHC3 and zDHHC7 at the cardiomyocyte Golgi promote Rac1 signaling and maladaptive cardiac remodeling, but redundant signaling effectors compensate to maintain cardiac hypertrophy with sustained pathological stimulation in the absence of zDHHC3/7.

The Microenvironment of the Pathogenesis of Cardiac Hypertrophy

Pathological cardiac hypertrophy is a key risk factor for the development of heart failure and predisposes individuals to cardiac arrhythmia and sudden death. While physiological cardiac hypertrophy is adaptive, hypertrophy resulting from conditions comprising hypertension, aortic stenosis, or genetic mutations, such as hypertrophic cardiomyopathy, is maladaptive. Here, we highlight the essential role and reciprocal interactions involving both cardiomyocytes and non-myocardial cells in response to pathological conditions. Prolonged cardiovascular stress causes cardiomyocytes and non-myocardial cells to enter an activated state releasing numerous pro-hypertrophic, pro-fibrotic, and pro-inflammatory mediators such as vasoactive hormones, growth factors, and cytokines, i.e., commencing signaling events that collectively cause cardiac hypertrophy. Fibrotic remodeling is mediated by cardiac fibroblasts as the central players, but also endothelial cells and resident and infiltrating immune cells enhance these processes. Many of these hypertrophic mediators are now being integrated into computational models that provide system-level insights and will help to translate our knowledge into new pharmacological targets. This perspective article summarizes the last decades' advances in cardiac hypertrophy research and discusses the herein-involved complex myocardial microenvironment and signaling components.

Role of angiotensin II in the development of subcellular remodeling in heart failure

The development of heart failure under various pathological conditions such as myocardial infarction (MI), hypertension and diabetes are accompanied by adverse cardiac remodeling and cardiac dysfunction. Since heart function is mainly determined by coordinated activities of different subcellular organelles including sarcolemma, sarcoplasmic reticulum, mitochondria and myofibrils for regulating the intracellular concentration of Ca2+, it has been suggested that the occurrence of heart failure is a consequence of subcellular remodeling, metabolic alterations and Ca2+-handling abnormalities in cardiomyocytes. Because of the elevated plasma levels of angiotensin II (ANG II) due to activation of the renin-angiotensin system (RAS) in heart failure, we have evaluated the effectiveness of treatments with angiotensin converting enzyme (ACE) inhibitors and ANG II type 1 receptor (AT1R) antagonists in different experimental models of heart failure. Attenuation of marked alterations in subcellular activities, protein content and gene expression were associated with improvement in cardiac function in MI-induced heart failure by treatment with enalapril (an ACE inhibitor) or losartan (an AT1R antagonist). Similar beneficial effects of ANG II blockade on subcellular remodeling and cardiac performance were also observed in failing hearts due to pressure overload, volume overload or chronic diabetes. Treatments with enalapril and losartan were seen to reduce the degree of RAS activation as well as the level of oxidative stress in failing hearts. These observations provide evidence which further substantiate to support the view that activation of RAS and high level of plasma ANG II play a critical role in inducing subcellular defects and cardiac dys-function during the progression of heart failure.

The common characteristics and mutual effects of heart failure and atrial fibrillation: initiation, progression, and outcome of the two aging-related heart diseases

Atrial fibrillation (AF) and heart failure (HF) are common chronic diseases noted in humans. AF and HF share several risk factors, such as age, hypertension, obesity, diabetes, and dyslipidemia. They can interact with each other, while both their morbidity and mortality have been considerably increased. And AF and HF often occur together, suggesting a strong association between the two. However, the underlying mechanism behind this association is not well understood. Among them, aging is the most significant common risk factor, which represents an aging heart and is characterized by fibrosis and decreased number of cardiomyocytes, known as senescence-related cardiac remodeling for both atria and ventricles. Finally, it is proposed that cardiac remodeling is the key link between AF and HF.

Involvement of exchange protein directly activated by cAMP and tumor progression locus 2 in IL-1β production in microglial cells following activation of β-adrenergic receptors

Endogenous noradrenaline (NA) has multiple bioactive functions and, in the central nervous system (CNS), has been implicated in modulating neuroinflammation via β-adrenergic receptors (β-ARs). Microglia, resident macrophages in the CNS, have a central role in the brain immune system and have been reported to be activated by NA. However, intracellular signaling mechanisms of the AR-mediated proinflammatory responses of microglia are not fully understood. Using a rapid and stable in vitro reporter assay system to evaluate IL-1β production in microglial BV2 cells, we found that NA and the β-AR agonist isoproterenol upregulated the IL-1β reporter activity. This effect was suppressed by β-AR antagonists. We further examined the involvement of EPAC (exchange protein directly activated by cAMP) and TPL2 (tumor progression locus 2, MAP3K8) and found that inhibitors for EPAC and TPL2 reduced AR agonist-induced IL-1β reporter activity. These inhibitors also suppressed NA-induced endogenous Il1b mRNA expression and IL-1β protein production. Our results suggest that EPAC and TPL2 are involved in β-AR-mediated IL-1β production in microglial cells, and extend our understanding of its intracellular signaling mechanism.

Norepinephrine Inhibits Synovial Adipose Stem Cell Chondrogenesis via α2a-Adrenoceptor-Mediated ERK1/2 Activation

In recent years, first evidences emerged that sympathetic neurotransmitters influence osteoarthritis (OA) manifestation. Joint-resident stem cells might contribute to cartilage repair, however, their chondrogenic function is reduced. The neurotransmitter norepinephrine (NE) was detected in the synovial fluid of trauma and OA patients. Therefore, the aim of this study was to analyse how NE influences the chondrogenesis of synovial adipose tissue-derived stem cells (sASCs). sASCs were isolated from knee-OA patients synovia. After adrenoceptor (AR) expression analysis, proliferation and chondrogenic differentiation in presence of NE and/or α- and β-AR antagonist were investigated. Cell count, viability, chondrogenic and hypertophic gene expression, sulfated glycosaminoglycan (sGAG) and type II collagen content were determined. Key AR-dependent signaling (ERK1/2, PKA) was analyzed via western blot. sASC expressed α1A-, α1B-, α2A-, α2B-, α2C-, and β2-AR in monolayer and pellet culture. NE did not affect proliferation and viability, but 10⁻⁷ and 10⁻⁶ M NE significantly reduced sGAG and type II collagen content as well as ERK1/2 phosphorylation. These effects were fully reversed by yohimbine (α2-AR antagonist). Our study confirms the important role of NE in sASC chondrogenic function and provides new insights in OA pathophysiology. Future studies might help to develop novel therapeutic options targeting neuroendocrine pathways for OA treatment.

Nelumbo nucifera Receptaculum Extract Suppresses Angiotensin II-Induced Cardiomyocyte Hypertrophy

Nelumbo nucifera Gaertn. (lotus) is an important medicinal plant, and many parts of the plant have been investigated for their therapeutic effects. However, the therapeutic effect of receptacles of lotuses on pathological cardiomyocyte hypertrophy has not been investigated yet. Therefore, the current study aimed to determine the protective effect of lotus against angiotensin II (Ang II)-induced cardiomyocyte hypertrophy in vitro. Ang II was used to induce hypertrophy of H9c2 cells. The lotus receptacle powder (MeOH extract of receptaculum Nelumbinis; MRN) used in the experiments was prepared by MeOH extraction and subsequent evaporation. To evaluate the effect of MRN on cardiomyocyte hypertrophy, cell size, protein synthesis, and hypertrophic marker expressions were examined. The antioxidant ability of MRN was determined by using CM-H₂DCFDA, a general oxidative stress indicator. Ang II-induced cardiomyocyte hypertrophy was significantly attenuated by 5 µg/mL of MRN, as confirmed by the reductions in cell size, protein synthesis, and hypertrophic marker expression. MRN also attenuated Ang II-induced excessive intracellular reactive oxygen species (ROS) production through the suppression of protein kinase C (PKC), extracellular-signal-regulated kinase (ERK), and NF-κB activation and subsequent type I angiotensin receptor (AT1R), receptor for advanced glycation end products (RAGE), and NADPH oxidase (NOX) expression. MRN exerted a significant protective effect against Ang II-induced cardiomyocyte hypertrophy through suppression of PKC–ERK signaling, and this subsequently led to attenuation of intracellular ROS production.

Natriuretic peptide receptor-C-mediated attenuation of vascular smooth muscle cell hypertrophy involves Gqα/PLCβ1 proteins and ROS-associated signaling

Hypertension is associated with vascular remodeling due to hyperproliferation and hypertrophy of vascular smooth muscle cells (VSMC). Recently, we showed the implication of enhanced expression of Gqα and PLCβ1 proteins in hypertrophy of VSMCs from 16‐week‐old spontaneously hypertensive rats (SHR). The aim of this study was to investigate whether C‐ANP_4‐23, a natriuretic peptide receptor‐C (NPR‐C) ligand that was shown to inhibit vasoactive peptide‐induced enhanced protein synthesis in A10 VSMC could also attenuate hypertrophy of VSMC isolated from rat model of cardiac hypertrophy and to further explore the possible involvement of Gqα/PLCβ1 proteins and ROS‐mediated signaling in this effect. The protein synthesis and cell volume, markers of hypertrophy were significantly enhanced in VSMC from 16‐week‐old SHR compared with age‐matched WKY rats and C‐ANP_4‐23 treatment attenuated both to WKY levels. In addition, C‐ANP_4‐23 treatment also attenuated the enhanced expression of AT1 receptor, Gqα, PLCβ1, Nox4, and p47^phox proteins, the enhanced activation of EGFR, PDGFR, IGF‐1R, enhanced phosphorylation of ERK1/2/AKT and c‐Src in VSMC from SHR. Furthermore, the enhanced levels of superoxide anion and NADPH oxidase activity exhibited by VSMC from SHR were also attenuated to control levels by C‐ANP_4‐23 treatment. These results indicate that C‐ANP_4‐23 via the activation of NPR‐C attenuates VSMC hypertrophy through decreasing the overexpression of Gqα/PLCβ1 proteins, enhanced oxidative stress, increased activation of growth factor receptors, and enhanced phosphorylation of MAPK/AKT signaling pathways. Thus, it can be suggested that C‐ANP_4‐23 may be used as a therapeutic agent for the treatment of vascular complications associated with hypertension and atherosclerosis.

Investigating β-adrenergic-induced cardiac hypertrophy through computational approach: classical and non-classical pathways

The chronic stimulation of β-adrenergic receptors plays a crucial role in cardiac hypertrophy and its progression to heart failure. In β-adrenergic signaling, in addition to the well-established classical pathway, Gs/AC/cAMP/PKA, activation of non-classical pathways such as Gi/PI3K/Akt/GSK3β and Gi/Ras/Raf/MEK/ERK contribute in cardiac hypertrophy. The signaling network of β-adrenergic-induced hypertrophy is very complex and not fully understood. So, we use a computational approach to investigate the dynamic response and contribution of β-adrenergic mediators in cardiac hypertrophy. The proposed computational model provides insights into the effects of β-adrenergic classical and non-classical pathways on the activity of hypertrophic transcription factors CREB and GATA4. The results illustrate that the model captures the dynamics of the main signaling mediators and reproduces the experimental observations well. The results also show that despite the low portion of β2 receptors out of total cardiac β-adrenergic receptors, their contribution in the activation of hypertrophic mediators and regulation of β-adrenergic-induced hypertrophy is noticeable and variations in β1/β2 receptors ratio greatly affect the ISO-induced hypertrophic response. The model results illustrate that GSK3β deactivation after β-adrenergic receptor stimulation has a major influence on CREB and GATA4 activation and consequent cardiac hypertrophy. Also, it is found through sensitivity analysis that PKB (Akt) activation has both pro-hypertrophic and anti-hypertrophic effects in β-adrenergic signaling.

Buthionine sulfoximine, an inhibitor of glutathione biosynthesis, induces expression of soluble epoxide hydrolase and markers of cellular hypertrophy in a rat cardiomyoblast cell line: roles of the NF-κB and MAPK signaling pathways

Evidence suggests that upregulation of soluble epoxide hydrolase (sEH) is associated with the development of myocardial infarction, dilated cardiomyopathy, cardiac hypertrophy, and heart failure. However, the upregulation mechanism is still unknown. In this study, we treated H9C2 cells with buthionine sulfoximine (BSO) to explore whether oxidative stress upregulates sEH gene expression and to identify the molecular and cellular mechanisms behind this upregulatory response. Real-time PCR and Western blot analyses were used to measure mRNA and protein expression, respectively. We demonstrated that BSO significantly upregulated sEH at mRNA levels in a concentration- and time-dependent manner, leading to a significant increase in the cellular hypertrophic markers, atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP). Furthermore, BSO significantly increased the cytosolic phosphorylated IκB-α and translocation of NF-κB p50 subunits, as measured by Western blot analysis. This level of translocation was paralleled by an increase in the DNA-binding activity of NF-κB P50 subunits. Moreover, our results demonstrated that pretreatment with the NF-κB inhibitor PDTC significantly inhibited BSO-mediated induction of sEH and cellular hypertrophic marker gene expression in a dose-dependent manner. Additionally, mitogen-activated protein kinases (MAPKs) were transiently phosphorylated by BSO treatment. To understand further the role of MAPKs pathway in BSO-mediated induction of sEH mRNA, we examined the role of extracellular signal-regulated kinase (ERK), c-JunN-terminal kinase (JNK), and p38 MAPK. Indeed, treatment with the MEK/ERK signal transduction inhibitor, PD98059, partially blocked the activation of IκB-α and translocation of NF-κB p50 subunits induced by BSO. Moreover, pretreatment with MEK/ERK signal transduction inhibitors, PD98059 and U0126, significantly inhibited BSO-mediated induction of sEH and cellular hypertrophic marker gene expression. These results clearly demonstrated that the NF-κB signaling pathway is involved in BSO-mediated induction of sEH gene expression, and appears to be associated with the activation of the MAPK pathway. Furthermore, our findings provide a strong link between sEH-induced cardiac dysfunction and involvement of NF-κB in the development of cellular hypertrophy.

Functional contribution of α1D-adrenoceptors in the renal vasculature of left ventricular hypertrophy induced with isoprenaline and caffeine in Wistar-Kyoto rats

This study investigated the role of α1D-adrenoceptor in the modulation of renal haemodynamics in rats with left ventricular hypertrophy (LVH). LVH was established in Wistar-Kyoto (WKY) rats with isoprenaline (5.0 mg · (kg body mass)(-1), by subcutaneous injection every 72 h) and caffeine (62 mg · L(-1) in drinking water, daily for 14 days). Renal vasoconstrictor responses were measured for noradrenaline (NA), phenylephrine (PE), and methoxamine (ME) before and immediately after low or high dose intrarenal infusions of BMY 7378, a selective α1D-adrenoceptor blocker. The rats with LVH had higher mean arterial blood pressure and circulating NA levels, but lower renal cortical blood perfusion compared with the control group (all P < 0.05). In the LVH group, the magnitude of the renal vasoconstrictor response to ME was blunted, but not the response to NA or PE (P < 0.05), compared with the control group (LVH vs. C, 38% vs. 50%). The magnitude of the drop in the vasoconstrictor responses to NA, PE, and ME in the presence of a higher dose of BMY 7378 was significantly greater in the LVH group compared with the control group (LVH vs. C, 45% vs. 25% for NA, 52% vs. 33% for PE, 66% vs. 53% for ME, all P < 0.05). These findings indicate an impaired renal vasoconstrictor response to adrenergic agonists during LVH. In addition, the α1D-adrenoceptor subtype plays a key role in the modulation of vascular responses in this diseased state.

Enhanced expression of Gqα and PLC-β1 proteins contributes to vascular smooth muscle cell hypertrophy in SHR: role of endogenous angiotensin II and endothelin-1

Vascular Gqα signaling has been shown to contribute to cardiac hypertrophy. In addition, angiotensin II (ANG II) was shown to induce vascular smooth muscle cell (VSMC) hypertrophy through Gqα signaling; however, the studies on the role of Gqα and PLC-β1 proteins in VSMC hypertrophy in animal model are lacking. The present study was therefore undertaken to examine the role of Gqα/PLC-β1 proteins and the signaling pathways in VSMC hypertrophy using spontaneously hypertensive rats (SHR). VSMC from 16-wk-old SHR and not from 12-wk-old SHR exhibited enhanced levels of Gqα/PLC-β1 proteins compared with age-matched Wistar-Kyoto (WKY) rats as determined by Western blotting. However, protein synthesis as determined by [3H]leucine incorporation was significantly enhanced in VSMC from both 12- and 16-wk-old SHR compared with VSMC from age-matched WKY rats. Furthermore, the knockdown of Gqα/PLC-β1 in VSMC from 16-wk-old SHR by antisense and small interfering RNA resulted in attenuation of protein synthesis. In addition, the enhanced expression of Gqα/PLC-β1 proteins, enhanced phosphorylation of ERK1/2, and enhanced protein synthesis in VSMC from SHR were attenuated by the ANG II AT1 and endothelin-1 (ET-1) ETA receptor antagonists losartan and BQ123, respectively, but not by the ETB receptor antagonist BQ788. In addition, PD98059 decreased the enhanced expression of Gqα/PLC-β1 and protein synthesis in VSMC from SHR. These results suggest that the enhanced levels of endogenous ANG II and ET-1 through the activation of AT1 and ETA receptors, respectively, and MAP kinase signaling, enhanced the expression of Gqα/PLC-β1 proteins in VSMC from 16-wk-old SHR and result in VSMC hypertrophy.

β-adrenergic receptor responsiveness in aging heart and clinical implications

Elderly healthy individuals have a reduced exercise tolerance and a decreased left ventricle inotropic reserve related to increased vascular afterload, arterial-ventricular load mismatching, physical deconditioning and impaired autonomic regulation (the so called "β-adrenergic desensitization"). Adrenergic responsiveness is altered with aging and the age-related changes are limited to the β-adrenergic receptor density reduction and to the β-adrenoceptor-G-protein(s)-adenylyl cyclase system abnormalities, while the type and level of abnormalities change with species and tissues. Epidemiological studies have shown an high incidence and prevalence of heart failure in the elderly and a great body of evidence correlate the changes of β-adrenergic system with heart failure pathogenesis. In particular it is well known that: (a) levels of cathecolamines are directly correlated with mortality and functional status in heart failure, (b) β1-adrenergic receptor subtype is down-regulated in heart failure, (c) heart failure-dependent cardiac adrenergic responsiveness reduction is related to changes in G proteins activity. In this review we focus on the cardiovascular β-adrenergic changes involvement in the aging process and on similarities and differences between aging heart and heart failure.

New advances in beta-blocker therapy in heart failure

The use of β-blockers (BB) in heart failure (HF) has been considered a contradiction for many years. Considering HF simply as a state of inadequate systolic function, BB were contraindicated because of their negative effects on myocardial contractility. Nevertheless, evidence collected in the past years have suggested that additional mechanisms, such as compensatory neuro-humoral hyperactivation or inflammation, could participate in the pathogenesis of this complex disease. Indeed, chronic activation of the sympathetic nervous system, although initially compensating the reduced cardiac output from the failing heart, increases myocardial oxygen demand, ischemia and oxidative stress; moreover, high catecholamine levels induce peripheral vasoconstriction and increase both cardiac pre- and after-load, thus determining additional stress to the cardiac muscle (1). As a consequence of such a different view of the pathogenic mechanisms of HF, the efficacy of BB in the treatment of HF has been investigated in numerous clinical trials. Results from these trials highlighted BB as valid therapeutic tools in HF, providing rational basis for their inclusion in many HF treatment guidelines. However, controversy still exists about their use, in particular with regards to the selection of specific molecules, since BB differ in terms of adrenergic β-receptors selectivity, adjunctive effects on α-receptors, and effects on reactive oxygen species and inflammatory cytokines production. Further concerns about the heterogeneity in the response to BB, as well as the use in specific patients, are matter of debate among clinicians. In this review, we will recapitulate the pharmacological properties and the classification of BB, and the alteration of the adrenergic system occurring during HF that provide a rationale for their use; we will also focus on the possible molecular mechanisms, such as genetic polymorphisms, underlying the different efficacy of molecules belonging to this class.

The induction of cardiac ornithine decarboxylase by β2 -adrenergic agents is associated with calcium channels and phosphorylation of ERK1/2

The role that the induction of cardiac ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis, by beta-adrenergic agents may have in heart hypertrophy is a controversial issue. Besides, the signaling pathways related to cardiac ODC regulation have not been fully elucidated. Here we show that in Balb C mice the stimulation of cardiac ODC activity by adrenergic agents was mainly mediated by β2 -adrenergic receptors, and that this induction was lower in the hypertrophic heart. Interestingly, this stimulation was abolished by the L-calcium channel antagonists verapamil and nifedipine. In addition, whereas the treatment with β2 -adrenergic agents was associated to both the increases in ODC, ODC-antizyme inhibitor 1 (AZIN1), c-fos and c-myc mRNA levels and the phosphorylation of CREB and MAP kinases ERK1 and ERK2 (ERK1/2), the co-treatment with L-calcium channel blockers differentially prevented most of these changes. These results suggest that the stimulation of cardiac ODC by β2 -adrenergic agents is associated with the activation of MAP kinases through the participation of L-calcium channels, and that by itself p-CREB does not appear to be sufficient for the transcriptional activation of ODC. In addition, post-translational mechanisms related with the induction of AZIN1 appear to be related to the increase of cardiac ODC activity.

β-Adrenergic signaling stimulates osteoclastogenesis via reactive oxygen species

Sympathetic signaling regulates bone resorption through receptor activator of nuclear factor-κB ligand (RANKL) expression via the β-adrenergic receptor (β-AR) on osteoblasts. Reactive oxygen species (ROS) are known as one type of osteoclast regulatory molecule. Here we show that an antioxidant, α-lipoic acid (α-LA), treatment prevent the β-adrenergic signaling-induced bone loss by suppressing osteoclastogenesis, and sympathetic signaling directly regulates osteoclastogenesis through β2-AR expressed on osteoclasts via intracellular ROS generation. In an in vitro study, the β-AR agonist isoprenaline increased intracellular ROS generation in osteoclasts prepared from bone marrow macrophages (BBMs) and RAW 264.7 cells. Isoprenaline enhanced osteoclastogenesis through β2-AR expressed on BMMs and RAW 264.7 cells. The antioxidant α-LA inhibited isoprenaline-enhanced osteoclastogenesis. Isoprenaline increased the expression of osteoclast-related genes such as nuclear factor of activated T cells, cytoplasmic, calcineurin-dependent 1, tartrate-resistant acid phosphatase, and cathepsin K on osteoclasts. α-LA also inhibited isoprenaline-induced increases of these gene expressions. These in vitro results led to the hypothesis that β-adrenergic signaling directly stimulates osteoclastogenesis via ROS generation. In an in vivo study, isoprenaline treatment alone caused oxidative damage in local bone and reduced bone mass because of an increase in bone resorption, and, in α-LA-treated mice, isoprenaline did not increase tibial osteoclast number even though the RANKL-to-osteoprotegerin ratio increased. These in vitro and in vivo results indicate that β-adrenergic signaling, at least in part, directly stimulates osteoclastogenesis through β2-AR on osteoclasts via ROS generation.

β-Adrenergic receptor stimulation causes cardiac hypertrophy via a Gβγ/Erk-dependent pathway

AIMS

Activation of the β(1)-adrenergic receptor and its G protein, G(s), induces cardiac hypertrophy. However, activation of classic Gα(s) effectors, adenylyl cyclases (AC) and protein kinase A, is not sufficient for induction of hypertrophy, which suggests the involvement of additional pathway(s) activated by G(s). Recently, we discovered that βγ subunits of G(q) induce phosphorylation of the extracellular regulated kinases 1 and 2 (Erk1/2) at threonine188 and thereby induce hypertrophy. Here we investigated whether β-adrenergic receptors might also induce cardiac hypertrophy via Erk(Thr188) phosphorylation.

METHODS AND RESULTS

β-Adrenergic receptor activation induced Erk(Thr188) phosphorylation in mouse hearts and in neonatal cardiomyocytes. Inhibition of Erk1/2 or overexpression of Erk(Thr188) phosphorylation-deficient mutants (Erk2(T188A) and Erk2(T188S)) significantly attenuated β-adrenergic cardiomyocyte hypertrophy in vitro. Erk activity was stimulated by both isoproterenol and the direct AC activator forskolin, but only isoproterenol induced Erk(Thr188) phosphorylation. Erk(Thr188) phosphorylation required Gβγ released from G(s) and was prevented by Gβγ inhibition. Similarly, isoproterenol, but not forskolin, induced nuclear accumulation of Erk and cardiomyocyte hypertrophy. Long-term application of isoproterenol in mice caused left ventricular hypertrophy and cardiac remodelling, and this was reduced in Erk2(T188S) transgenic mice, supporting the physiological relevance of Erk(Thr188) phosphorylation.

CONCLUSIONS

Activation of G(s) by β-adrenergic receptors leads to (i) canonical Erk1/2 activation via AC, and (ii) release of Gβγ, which then associates with activated Erk1/2 and induces Erk(Thr188) phosphorylation, causing nuclear accumulation of Erk and ultimately cardiomyocyte hypertrophy. These findings reveal a new pathway critically involved in β-adrenergically mediated cardiac hypertrophy and may yield new therapeutic strategies against hypertrophic remodelling.

The role of Ca2+/calmodulin-dependent protein kinase II and calcineurin in TNF-α-induced myocardial hypertrophy

We investigated whether Ca2+/calmodulin-dependent kinase II (CaMKII) and calcineurin (CaN) are involved in myocardial hypertrophy induced by tumor necrosis factor α (TNF-α). The cardiomyocytes of neonatal Wistar rats (1-2 days old) were cultured and stimulated by TNF-α (100 μg/L), and Ca2+ signal transduction was blocked by several antagonists, including BAPTA (4 µM), KN-93 (0.2 µM) and cyclosporin A (CsA, 0.2 µM). Protein content, protein synthesis, cardiomyocyte volumes, [Ca2+]i transients, CaMKIIδB and CaN were evaluated by the Lowry method, [³H]-leucine incorporation, a computerized image analysis system, a Till imaging system, and Western blot analysis, respectively. TNF-α induced a significant increase in protein content in a dose-dependent manner from 10 µg/L (53.56 µg protein/well) to 100 μg/L (72.18 µg protein/well), and in a time-dependent manner from 12 h (37.42 µg protein/well) to 72 h (42.81 µg protein/well). TNF-α (100 μg/L) significantly increased the amplitude of spontaneous [Ca2+]i transients, the total protein content, cell size, and [³H]-leucine incorporation in cultured cardiomyocytes, which was abolished by 4 µM BAPTA, an intracellular Ca2+ chelator. The increases in protein content, cell size and [³H]-leucine incorporation were abolished by 0.2 µM KN-93 or 0.2 µM CsA. TNF-α increased the expression of CaMKIIδB by 35.21% and that of CaN by 22.22% compared to control. These effects were abolished by 4 µM BAPTA, which itself had no effect. These results suggest that TNF-α induces increases in [Ca2+]i, CaMKIIδB and CaN and promotes cardiac hypertrophy. Therefore, we hypothesize that the Ca2+/CaMKII- and CaN-dependent signaling pathways are involved in myocardial hypertrophy induced by TNF-α.

Beta-adrenergic receptors, from their discovery and characterization through their manipulation to beneficial clinical application

β-Adrenergic receptors (β-AR) are central to the overall regulation of cardiac function. From the first proposed receptor/transmitter concept to the latest clinical β-blocker trials β-AR have been shown to play an important role in cardiac disease and heart failure in particular. This study provides a historical perspective, reviews the latest discoveries and beliefs, and discusses the current clinical practices of β-AR and their modulation with their associated guanine-nucleotide regulatory protein/adenylylcyclasesignal transduction pathways.

Continuous treatment with a low-dose β-agonist reduces bone mass by increasing bone resorption without suppressing bone formation

The sympathetic nervous system regulates bone remodeling through the β-adrenergic receptor (β-AR). However, the systemic roles of adrenergic actions on bone remodeling through the β-AR are largely unknown. In this study, we examined the dose effect of continuous treatment with isoprenaline, a nonspecific β-AR agonist, on bone remodeling. Male C57BL/6J mice were intrasubcutaneously administrated with four different doses (5, 25, 50, or 100 μg/g daily) of isoprenaline or vehicle using an osmotic pump for 2 weeks. The region of high-turnover cancellous bone was analyzed by microcomputed tomography (μCT). Continuous isoprenaline treatment caused a ~35.7% decline in the femoral cancellous bone volume fraction (BV/TV) at all doses (5-100 μg/g daily). Furthermore, continuous isoprenaline treatment weakened the bone mechanical properties in the trunk of lumbar vertebra 4 (L4). These parameters did not show significant differences between doses. Histomorphometric analysis revealed that isoprenaline doses of 50 μg/g daily or less did not significantly inhibit bone formation parameters, such as bone formation rate (BFR) and mineral surface/bone surface (MS/BS). Only the highest dose (100 μg/g daily) of isoprenaline significantly inhibited BFR and MS/BS. On the other hand, osteoclast number/bone surface (Oc.N/BS) was enhanced approximately 2.4-fold and osteoclast surface/bone surface (Oc.S/BS) was increased 2.0-fold by all doses of continuous isoprenaline treatment. The osteoclast parameters plateaued at the lowest dose (5 μg/g daily) of continuous isoprenaline treatment. These results indicate that chronic stimulation of β-AR with low-dose agonist treatment induces bone loss mainly via enhanced bone resorption.

Monophosphothreonyl extracellular signal-regulated kinases 1 and 2 (ERK1/2) are formed endogenously in intact cardiac myocytes and are enzymically active

ERK1 and ERK2 (ERK1/2) are central to the regulation of cell division, growth and survival. They are activated by phosphorylation of the Thr- and the Tyr- residues in their Thr-Glu-Tyr activation loops. The dogma is that dually-phosphorylated ERK1/2 constitute the principal activities in intact cells. We previously showed that, in neonatal rat cardiac myocytes, endothelin-1 and phorbol 12-myristate 13-acetate (PMA) powerfully and rapidly (maximal at ~ 5 min) activate ERK1/2. Here, we show that dually-phosphorylated ERK1/2 rapidly (< 2 min) appear in the nucleus following stimulation with endothelin-1. We characterized the active ERK1/2 species in myocytes exposed to endothelin-1 or PMA using MonoQ FPLC. Unexpectedly, two peaks of ERK1 and two peaks of ERK2 activity were resolved using in vitro kinase assays. One of each of these represented the dually-phosphorylated species. The other two represented activities for ERK1 or ERK2 which were phosphorylated solely on the Thr- residue. Monophosphothreonyl ERK1/2 represented maximally ~ 30% of total ERK1/2 activity after stimulation with endothelin-1 or PMA, and their k_cat values were estimated to be minimally ~ 30% of the dually-phosphorylated species. Appearance of monophosphothreonyl ERK1/2 was rapid but delayed in comparison with dually-phosphorylated ERK1/2. Of 10 agonists studied, endothelin-1 and PMA were most effective in terms of ERK1/2 activation and in stimulating the appearance of monophosphothreonyl and dually-phosphorylated ERK1/2. Thus, enzymically active monophosphothreonyl ERK1/2 are formed endogenously following activation of the ERK1/2 cascade and we suggest that monophosphothreonyl ERK1/2 arise by protein tyrosine phosphatase-mediated dephosphorylation of dually-phosphorylated ERK1/2.

RGS2 inhibits beta-adrenergic receptor-induced cardiomyocyte hypertrophy

The chronic stimulation of certain G protein-coupled receptors promotes cardiomyocyte hypertrophy and thus plays a pivotal role in the development of human heart failure. The beta-adrenergic receptors (beta-AR) are unique among these in that they signal via Gs, whereas others, such as the alpha1-adrenergic (alpha1-AR) and endothelin-1 (ET-1) receptors, predominantly act through Gq. In this study, we investigated the potential role of regulator of G protein signalling 2 (RGS2) in modulating the hypertrophic effects of the beta-AR agonist isoproterenol (ISO) in rat neonatal ventricular cardiomyocytes. We found that ISO-induced hypertrophy in rat neonatal ventricular myocytes was accompanied by the selective upregulation of RGS2 mRNA, with little or no change in RGS1, RGS3, RGS4 or RGS5. The adenylyl cyclase activator forskolin had a similar effect suggesting that it was mediated through cAMP production. To study the role of RGS2 upregulation in beta-AR-dependent hypertrophy, cardiomyocytes were infected with adenovirus encoding RGS2 and assayed for cell growth, markers of hypertrophy, and beta-AR signalling. ISO-induced increases in cell surface area were virtually eliminated by the overexpression of RGS2, as were increases in alpha-skeletal actin and atrial natriuretic peptide. RGS2 overexpression also significantly attenuated ISO-induced extracellular signal-regulated kinases 1 and 2 (ERK1/2) and Akt activation, which may account for, or contribute to, its observed antihypertrophic effects. In contrast, RGS2 overexpression significantly activated JNK MAP kinase, while decreasing the potency but not the maximal effect of ISO on cAMP accumulation. In conclusion, the present results suggest that RGS2 negatively regulates hypertrophy induced by beta-AR activation and thus may play a protective role in cardiac hypertrophy.

Alterations of cardiac ERK1/2 expression and activity due to volume overload were attenuated by the blockade of RAS

The activities and protein content of extracellular signal-regulated kinase (ERK)1/2 in the heart were measured in rats at 4 and 16 weeks after volume overload due to aortocaval shunt. Protein content of phosphorylated ERK1/2 was increased at both 4 and 16 weeks, whereas protein content of total ERK1/2 was increased only at 16 weeks of inducing volume overload. The ERK1/2 activities, estimated as phospho-Elk-1 content, were also increased at 4 and 16 weeks of inducing volume overload. The increased phosphorylated ERK1/2 and E-26-like (Elk)-1 protein content in 16 weeks failing hearts was much greater than that in 4 weeks hypertrophied hearts. These changes in phosphorylated ERK1/2 and Elk-1 protein content in both 4 and 16 weeks volume overloaded animals were attenuated by treatment with enalapril and/or losartan. The results indicate that activation of ERK1/2 may be involved in the development of cardiac hypertrophy and heart failure due to volume overload, and these changes are partially prevented by blockade of the renin-angiotensin system (RAS).

Proteomic profiling reveals comprehensive insights into adrenergic receptor-mediated hypertrophy in neonatal rat cardiomyocytes

Myocardial adrenergic receptors (ARs) play important roles in cardiac hypertrophy. However, the detailed molecular mechanism of AR-mediated cardiac hypertrophy remains elusive to date. To gain full insight into how ARs are involved in the regulation of cardiac hypertrophy, protein expression profiling was performed with comparative proteomics approach on neonatal rat cardiomyocytes. Forty-six proteins were identified as differentially expressed in hypertrophic cardiomyocytes induced by AR stimulation. To better understand the biological significance of the obtained proteomic data, we utilized the ingenuity pathway analysis tool to construct biological networks and analyze function and pathways that might associate with AR-mediated cardiac hypertrophy. Pathway analysis strongly suggested that ROS may be involved in the development of AR-mediated cardiac hypertrophy, which was then confirmed by further experimentation. The results showed that a marked increase in ROS production was detected in AR-mediated cardiac hypertrophy and blocking of ROS production significantly inhibited AR-mediated cardiac hypertrophy. We further proved that the ROS production was through NADPH oxidase or the mitochondrial electron transport chain and this ROS accumulation resulted in activation of extracellular signal-regulated kinase 1/2 leading to AR-mediated cardiac hypertrophy. These experimental results support the hypothesis, from the ingenuity pathway analysis, that AR-mediated cardiac hypertrophy is associated with the dysregulation of a complicated oxidative stress-regulatory network. In conclusion, our results provide a basis for understanding the detailed molecular mechanisms of AR-mediated cardiac hypertrophy.

Exploiting cGMP-based therapies for the prevention of left ventricular hypertrophy: NO* and beyond

Left ventricular hypertrophy (LVH), an increased left ventricular (LV) mass, is common to many cardiovascular disorders, initially developing as an adaptive response to maintain myocardial function. In the longer term, this LV remodelling becomes maladaptive, with progressive decline in LV contractility and diastolic function. Indeed LVH is recognised as an important blood-pressure independent predictor of cardiovascular morbidity and mortality. The clinical efficacy of current treatments for LVH is reduced, however, by their tendency to slow disease progression rather than induce its reversal, and thus the development of new therapies for LVH is paramount. The signalling molecule cyclic guanosine-3',5'-monophosphate (cGMP), well-recognised for its role in regulating vascular tone, is now being increasingly identified as an important anti-hypertrophic mediator. This review is focused on the various means by which cGMP can be stimulated in the heart, such as via the natriuretic peptides, to exert anti-hypertrophic actions. In particular we address the limitations of traditional nitric oxide (NO*) donors in the face of the potential therapeutic advantages offered by novel alternatives; NO* siblings, ligands of the cGMP-generating enzymes, soluble (sGC) and particulate guanylyl cyclases (pGC), and phosphodiesterase inhibitors. Further impact of cGMP within the cardiovascular system is also discussed with a view to representing cGMP-based therapies as innovative pharmacotherapy, alone or concurrent with standard care, for the management of LVH.

delta-Opioid receptor stimulation enhances the growth of neonatal rat ventricular myocytes via the extracellular signal-regulated kinase pathway

1. The aims of the present study were to determine whether delta-opioid receptor stimulation enhanced proliferation of and to investigate the role of the extracellular signal-regulated kinase (ERK) pathway in ventricular myocytes from neonatal rats. 2. At concentratins ranging from 10 nmol/L to 10 micromol/L, [D-Ala2,D-Leu5]enkephalin (DADLE) concentration-dependently promoted myocardial growth and DNA synthesis and altered the cytoskeleton. 3. At 1 micromol/L, DADLE also increased the expression and phosphorylation of ERK. 4. These effects of 1 micromol/L DADLE were abolished by 10 micromol/L naltrindole, a selective delta-opioid receptor antagonist, 10 nmol/L U0126, a selective ERK antagonist, 1 micromol/L staurosporine, an inhibitor of protein kinase (PK) C, and 100 micromol/L Rp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt hydrate (Rp-cAMPS), an inhibitor of PKA. 5. In conclusion, delta-opioid receptor stimulation enhances the proliferation and development of the ventricular myocytes of neonatal rats. The ERK pathway and related signalling mechanisms, namely PKC and PKA, are involved.

Changes in the transcriptional profile of cardiac myocytes following green fluorescent protein expression

Green fluorescent protein (GFP) and its multiple forms, such as enhanced GFP (EGFP), have been widely used as marker proteins and for tracking purposes in many biological systems, including the heart and cardiac cell systems. Despite some concerns on its toxicity under certain circumstances, GFP remains amongst the most reliable and easy-to-use markers available. Using rat full genome DNA microarrays, we have investigated the broader consequences of adenoviral-driven GFP expression in cardiac myocytes. In our transcriptional profiling analysis, we set a threshold of a twofold change. We removed possible changes resulting from adenoviral infection by comparison with transcriptional profiles of cardiac myocytes with adenoviral-driven expression of an unrelated protein, the kinase MEK. Our analysis revealed changes in the expression of 212 genes. Of these genes, 174 were upregulated and 38 were downregulated following GFP expression. Many of these genes remain unannotated, but an evaluation of those with described functions for their resulting proteins indicated that many were involved in processes, including responses to stimuli/stress and signal transduction. Our analysis thus indicates the broader consequences of GFP expression in altering gene expression profiles in cardiac cells. Care should therefore be taken when using GFP expression as a control in gene expression studies.

Transient Exposure to Hydrogen Peroxide Causes an Increase in Mitochondria-Derived Superoxide As a Result of Sustained Alteration in L-Type Ca 2+ Channel Function in the Absence of Apoptosis in Ventricular Myocytes

We sought to understand the effect of a transient exposure of cardiac myocytes to H 2 O 2 at a concentration that did not induce apoptosis. Myocytes were exposed to 30 μmol/L H 2 O 2 for 5 minutes followed by 10 U/mL catalase for 5 minutes to degrade the H 2 O 2 . Cellular superoxide was measured using dihydroethidium. Transient exposure to H 2 O 2 caused a 66.4% increase in dihydroethidium signal compared with controls exposed to only catalase, without activation of caspase 3 or evidence of necrosis. The increase in dihydroethidium signal was attenuated by the mitochondrial inhibitors myxothiazol or carbonyl cyanide p -(trifluoromethoxy)phenyl-hydrazone and when calcium uptake by the mitochondria was inhibited with Ru360. We investigated the L-type Ca 2+ channel ( I Ca-L ) as a source of calcium influx. Nisoldipine, an inhibitor of I Ca-L , attenuated the increase in superoxide. Basal channel activity increased from 5.4 to 8.9 pA/pF. Diastolic calcium was significantly increased in quiescent and contracting myocytes after H 2 O 2 . The response of I Ca-L to β-adrenergic receptor stimulation was used as a functional reporter because decreasing intracellular H 2 O 2 alters the sensitivity of I Ca-L to isoproterenol. H 2 O 2 increased the K 0.5 required for activation of I Ca-L by isoproterenol from 5.8 to 27.8 nmol/L. This effect and the increase in basal current density persisted for several hours after H 2 O 2 . We propose that extracellular H 2 O 2 is associated with an increase in superoxide from the mitochondria caused by an increase in Ca 2+ influx from I Ca-L . The effect persists because a positive feedback exists among increased basal channel activity, elevated intracellular calcium, and superoxide production by the mitochondria.

Cardiac hypertrophy induced by sustained β-adrenoreceptor activation: pathophysiological aspects

Cardiac hypertrophy is promoted by adrenergic over-activation and represents an independent risk factor for cardiovascular morbidity and mortality. The basic knowledge about mechanisms by which sustained adrenergic activation promotes myocardial growth, as well as understanding how structural changes in hypertrophied myocardium could affect myocardial function has been acquired from studies using an animal model of chronic systemic beta-adrenoreceptor agonist administration. Sustained beta-adrenoreceptor activation was shown to enhance the synthesis of myocardial proteins, an effect mediated via stimulation of myocardial growth factors, up-regulation of nuclear proto-oncogenes, induction of cardiac oxidative stress, as well as activation of mitogen-activated protein kinases and phosphatidylinositol 3-kinase. Sustained beta-adrenoreceptor activation contributes to impaired cardiac autonomic regulation as evidenced by blunted parasympathetically-mediated cardiovascular reflexes as well as abnormal storage of myocardial catecholamines. Catecholamine-induced cardiac hypertrophy is associated with reduced contractile responses to adrenergic agonists, an effect attributed to downregulation of myocardial beta-adrenoreceptors, uncoupling of beta-adrenoreceptors and adenylate cyclase, as well as modifications of downstream cAMP-mediated signaling. In compensated cardiac hypertrophy, these changes are associated with preserved or even enhanced basal ventricular systolic function due to increased sarcoplasmic reticulum Ca(2+) content and Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. The increased availability of Ca(2+) to maintain cardiomyocyte contraction is attributed to prolongation of the action potential due to inhibition of the transient outward potassium current as well as stimulation of the reverse mode of the Na(+)-Ca(2+) exchange. Further progression of cardiac hypertrophy towards heart failure is due to abnormalities in Ca(2+) handling, necrotic myocardial injury, and increased myocardial stiffness due to interstitial fibrosis.

Small heat-shock protein Hsp20 attenuates beta-agonist-mediated cardiac remodeling through apoptosis signal-regulating kinase 1

Chronic stimulation of the beta-adrenergic neurohormonal axis contributes to the progression of heart failure and mortality in animal models and human patients. In cardiomyocytes, activation of the beta-adrenergic pathway has been shown to result in transiently increased expression of a cardiac small heat-shock protein Hsp20. The present study shows that cardiac overexpression (10-fold) of Hsp20 may protect the heart against beta-agonist-induced cardiac remodeling, associated with isoproterenol (50 mug/g per day) infusion for 14 days. Hsp20 attenuated the cardiac hypertrophic response, markedly reduced interstitial fibrosis, and decreased apoptosis. Contractility was also preserved in hearts with increased Hsp20 levels. These beneficial effects were associated with attenuation of the ASK1-JNK/p38 (apoptosis signal-regulating kinase 1/c-Jun NH(2)-terminal kinase/p38) signaling cascade triggered by isoproterenol, whereas there was no difference in either extracellular signal-related kinase 1/2 or Akt activation. Parallel in vitro experiments supported the inhibitory role of Hsp20 on enforced ASK1-JNK/p38 activation in both H9c2 cells and adult rat cardiomyocytes. Immunostaining studies also demonstrated that Hsp20 colocalizes with ASK1 in cardiomyocytes. Taken together, our findings indicate that (1) beta-agonist-induced cardiac injury is associated with activation of the ASK1-JNK/p38 cascade; (2) increased expression of Hsp20 attenuates the induction of remodeling, dysfunction, and apoptosis in response to sustained beta-adrenergic stimulation; and (3) the beneficial effects of Hsp20 are at least partially attributable to inhibition of the ASK1-signaling cascade.

Maximal beta3-adrenergic regulation of lipolysis involves Src and epidermal growth factor receptor-dependent ERK1/2 activation

Catecholamine-stimulated lipolysis is primarily a beta-adrenergic and cAMP-dependent event. In previous studies we established that the beta(3)-adrenergic receptor (beta(3)AR) in adipocytes utilizes a unique mechanism to stimulate extracellular signal-regulated kinases 1 and 2 (ERK) by direct recruitment and activation of Src kinase. Therefore, we investigated the role of the ERK pathway in adipocyte metabolism and found that the beta(3)AR agonist CL316,243 regulates lipolysis through both cAMP-dependent protein kinase (PKA) and ERK. Inhibition of PKA activity completely eliminated lipolysis at low (subnanomolar) CL316,243 concentrations and by 75-80% at higher nanomolar concentrations. The remaining 20-25% of PKA-independent lipolysis, as well as ERK activation, was abolished by inhibiting the activity of either Src (PP2 or small interfering RNA), epidermal growth factor receptor (EGFR with AG1478 or small interfering RNA), or mitogen-activated protein kinase kinase 1 or 2 (MKK1/2 with PD098059). PD098059 inhibited lipolysis by 53% in mice as well. Finally, the effect of estradiol, a reported acute activator of ERK and lipolysis, was also totally prevented by PP2, AG1478, and PD098059. These results suggest that ERK activation by beta(3)AR depends upon Src and epidermal growth factor receptor kinase activities and is responsible for the PKA-independent portion of the lipolytic response. Together these results illustrate the distinct and complementary roles for PKA and ERK in catecholamine-stimulated lipolysis.

Gene expression profiling reveals complex changes following MEK-EE expression in cardiac myocytes

The activation of the MEK/ERK pathway has been implicated in the proliferative growth of many tissues, however in the heart it has been linked with hypertrophic growth of the individual cardiac myocytes. We have explored the transcriptional consequences of prolonged ERK1/2 activation in cardiac myocytes following the adenoviral overexpression of a constitutively active form of MEK, MEK-EE. Analysis of microarray data obtained using full rat genome arrays showed >2000 gene expression changes in response to MEK-EE overexpression for 24h. We observed similar numbers of genes upregulated and downregulated. The genes were involved in diverse processes including cell structure, metabolism and intracellular signalling. There were also changes in the pro- and ani-apoptotic genes as well as downregulation of the antioxidant enzymes, Mn superoxide dismutase, catalase and thioredoxin 2. Our results reveal the complexity of transcriptional changes that follow the activation of the ERK signalling pathway in these cells and suggest that activation of this MAPK pathway impinges on diverse cellular functions.

Contrasting actions of prolonged mitogen-activated protein kinase activation on cell survival

Activation of the ERK mitogen-activated protein kinase pathway has been implicated in pro-survival and cellular protective mechanisms, so that chronic ERK activation may be a useful therapeutic strategy. Here, we further explored the consequences of prolonged ERK activation following expression of constitutively active form of MEK, MEK-EE, in cardiac myocytes. We confirmed that chronic MEK-EE overexpression halved myocyte death following glucose deprivation, but surprisingly this was not associated with preserved intracellular ATP levels. Whilst activities of a number of antioxidant enzymes were not altered upon MEK-EE expression, paradoxically Cu/Zn superoxide dismutase activity was almost halved upon MEK-EE expression. When we then exposed myocytes to the superoxide generator menadione, we observed significantly higher death of MEK-EE expressing myocytes. Pre-incubation with U0126 inhibited menadione-induced death. Our results are the first to show that MEK-ERK signalling can act to increase or decrease cell survival, the outcome depending on the form of stress stimulus encountered.

A beta1-adrenergic receptor CaM kinase II-dependent pathway mediates cardiac myocyte fetal gene induction

Beta-adrenergic signaling plays an important role in the natural history of dilated cardiomyopathies. Chronic activation of beta-adrenergic receptors (beta1-AR and beta2-AR) during periods of cardiac stress ultimately harms the failing heart by mechanisms that include alterations in gene expression. Here, we show that stimulation of beta-ARs with isoproterenol in neonate rat ventricular myocytes causes a "fetal" response in the relative activities of the human cardiac fetal and/or adult gene promoters that includes repression of the human and rat alpha-myosin heavy chain (alpha-MyHC) promoters with simultaneous activation of the human atrial natriuretic peptide (ANP) and rat beta-MyHC promoters. We also show that the promoter changes correlate with changes in endogenous gene expression as measured by mRNA expression. Furthermore, we show that these changes are specifically mediated by the beta1-AR, but not the beta2-AR, and are independent of alpha1-AR stimulation. We also demonstrate that the fetal gene response is independent of cAMP and protein kinase A, whereas inhibition of Ca2+/calmodulin-dependent protein kinase (CaMK) pathway blocks isoproterenol-mediated fetal gene program induction. Finally, we show that induction of the fetal program is dependent on activation of the L-type Ca2+ channel. We conclude that in neonatal rat cardiac myocytes, agonist-occupied beta1-AR mobilizes Ca2+ stores to activate fetal gene induction through cAMP independent pathways that involve CaMK.

Activation pattern of MAPK signaling in the hearts of trained and untrained rats following a single bout of exercise

Since exercise training causes cardiac hypertrophy and a single bout induces mechanical stress to the heart, the present study aimed to characterize the activation patterns of multiple MAPK signaling pathways in the heart after a single bout of exercise or chronic exercises. The hearts of untrained rats received 5, 15, and 30 min of treadmill running exercise (Ex5 to Ex30) and rested for 0.5, 1, 3, 6, 12, and 24 h (PostEx0.5 to PostEx24) before subjecting them to the following different experiments. Activation of MAPKs (ERK, JNK, and p38) and MAPKKs (MEK1/2, SEK, and MKK3/6) increased immediately after acute exercise in a time-dependent manner, with ERK, JNK, and p38 peaking at Ex15, Ex15, and Ex30, respectively. Expression of immediate early genes (c-fos, c-jun, and c-myc) was augmented and activator protein-1 DNA binding activity was enhanced in untrained rats immediately after a single bout of exercise. The elevated levels of MAPKs declined to the resting levels within 24 h after exercise. In another set of experiments, following 4, 8, and 12 wk of exercise training, the rats exhibited significant cardiac hypertrophy by week 12. Activation of MAPKs in the 4-wk-trained rats increased after a 30-min single bout of exercise but decreased in the 8-wk group. Finally, the activity of MAPKs signaling in the 12-wk-trained rats exposed to an acute bout of exercise was unaltered. We conclude that exercise induces the activation of multiple MAPK (ERK, JNK, and p38) pathways in the heart, an effect that gradually declines with the development of exercise-induced cardiac hypertrophy.

β2‐Adrenergic receptor activation delays wound healing

Keratinocytes migrate directionally into the wound bed to initiate re-epithelialization, necessary for wound closure and restoration of barrier function. They solely express the beta2-adrenergic receptor (beta2-AR) subtype of beta-ARs and can also synthesize beta-AR agonists generating a hormonal mediator network in the skin. Emerging studies from our laboratory demonstrate that beta-AR agonists decrease keratinocyte migration via a protein phosphatase (PP) 2A-dependent mechanism. Here we have extended our investigations to observe the effects of beta2-AR activation on keratinocyte polarization, migration, and ERK phosphorylation at the wound edge, cytoskeletal organization, phospho-ERK intracellular localization, proliferation, human skin wound re-epithelialization, wound-induced ERK phosphorylation, and murine skin wound healing. We demonstrate that in keratinocytes, beta2-AR activation is anti-motogenic and anti-mitogenic with both mechanisms being PP2A dependent. beta2-AR activation dramatically alters the organization of the actin cytoskeleton and prevents localization of phospho-ERK to the lamellipodial edge and its colocalization with vinculin. Finally, we demonstrate a beta2-AR-mediated delay in re-epithelialization and decrease in wound-induced epidermal ERK phosphorylation in human skin wounds and a delay in re-epithelialization in murine tail-clip wounds. Our work uncovers novel keratinocyte biology and a previously unrecognized role for the adrenergic hormonal mediator network in the wound repair process.

Phosphodiesterase inhibition promotes the transition from compensated hypertrophy to cardiac dilatation in rats

The cellular signaling pathways responsible for the transition from compensated left ventricular hypertrophy (LVH) to LV dilatation (remodeling) and heart failure are unclear. As chronic administration of a beta-adrenoreceptor (beta-AR) agonist mediates the premature onset of cardiac remodeling without myocyte necrosis or myocardial dysfunction in LVH, we suggest that beta-AR activation is critical in promoting the transition from compensated LVH to cardiac dilatation. However, beta-AR mediated effects in the heart can occur via either the cyclic adenosine monophosphate (cAMP) system or via cAMP independent signaling pathways. To determine the role of cAMP in promoting adverse cardiac chamber remodeling, we evaluated whether phosphodiesterase inhibition (PDEI) promotes LV dilatation in rats with compensated LVH. The impact of chronic administration of the PDEI, pentoxifylline, on LV remodeling and function was assessed in spontaneously hypertensive rats (SHR) with compensated LVH. The PDEI mediated inotropic effects and increased cAMP concentrations in SHR. This dose of the PDEI administered for 4 months to SHR did not modify LV weight or influence intrinsic myocardial systolic function (as assessed in the absence of the PDEI) in SHR. However, the PDEI mediated the development of a right shift in LV end diastolic (LVED) pressure-internal dimension and LVED pressure-volume relations, LV wall thinning, and increments in myocardial soluble (non-cross-linked) collagen concentrations. In conclusion, chronic PDEI administration induces adverse geometric and interstitial cardiac remodeling in SHR, a finding that supports the notion that the beta-AR-cAMP system is important in mediating the progression to heart failure by promoting interstitial remodeling and LV dilatation in LVH.

AFos dissociates cardiac myocyte hypertrophy and expression of the pathological gene program

BACKGROUND

Although induction of activator protein-1 (AP-1) transcription factor activity has been observed in cardiac hypertrophy, a direct role for AP-1 in myocardial growth and gene expression remains obscure.

METHODS AND RESULTS

Hypertrophy was induced in cultured neonatal rat cardiomyocytes with phenylephrine or overexpression of a constitutively active MAP3K, MKK6. In both treatment groups, induction of the pathological gene profile was observed, ie, expression of beta-myosin heavy chain (betaMHC), atrial/brain natriuretic peptides (ANP/BNP), and skeletal alpha-actin (sACT) was increased, whereas expression for alpha-myosin heavy chain (alphaMHC) and the sarcoplasmic reticulum Ca2+-ATPase (SERCA) genes was repressed. The role of AP-1 in the hypertrophic phenotype was evaluated with the use of an adenoviral construct expressing a dominant negative mutant of the c-Fos proto-oncogene (AdAFos). Although AFos did not change the myocyte growth response, it abrogated the gene profile to both agonists, including the upregulation of both alphaMHC and SERCA expression.

CONCLUSIONS

Although c-Fos/AP-1 is necessary for induction of the pathological/fetal gene program, it does not appear to be critical for cardiomyocyte hypertrophy.

Strain-dependent beta-adrenergic receptor function influences myocardial responses to isoproterenol stimulation in mice

Recently, we showed that compared with the A/J inbred mouse strain, C57BL/6J (B6) mice have an athlete's cardiac phenotype. We postulated that strain differences would result in greater left ventricular (LV) hypertrophy in response to isoproterenol in B6 than A/J mice and tested the hypothesis that a differential response could be explained partly by differences in beta-adrenergic receptor (beta-AR) density and/or coupling. A/J and B6 mice were randomized to receive daily isoproterenol (100 mg/kg sc) or isovolumic vehicle for 5 days. Animals were studied using echocardiography, tail-cuff blood pressure, histopathology, beta-AR density and percent high-affinity binding, and basal and stimulated adenylyl cyclase activities. One hundred twenty-eight mice (66 A/J and 62 B6) were studied. Isoproterenol-treated A/J mice demonstrated greater percent increases in echocardiographic LV mass/body weight (97 +/- 11 vs. 20 +/- 10%, P = 0.001) and in gravimetric heart mass/body weight versus same-strain controls than B6 mice. Histopathology scores (a composite of myocyte hypertrophy, nuclear changes, fibrosis, and calcification) were greater in isoproterenol-treated A/J vs. B6 mice (2.8 +/- 0.2 vs.1.9 +/- 0.3, P < 0.05), as was quantitation of myocyte damage (22.3 +/- 11.5 vs. 4.3 +/- 3.5%). Interstrain differences in basal beta-AR density, high-affinity binding, and adenylyl cyclase activity were not significant. However, whereas isoproterenol-treated A/J mice showed nonsignificant increases in all beta-AR activity measures, isoproterenol-treated B6 mice had lower beta-AR density (57 +/- 6 vs. 83 +/- 8 fmol/mg, P < 0.05), percent high-affinity binding (15 +/- 2 vs. 26 +/- 3%, P < 0.005), and GTP + isoproterenol-stimulated adenylyl cyclase activity (10 +/- 1.1 vs. 5.8 +/- 1.5 pmol cAMP.mg(-1).min(-1)) compared with controls. High-dose, short-term isoproterenol produces greater macro- and microscopic cardiac hypertrophy and injury in A/J than B6 mice. A/J mice, unlike B6 mice, do not experience beta-AR downregulation or uncoupling in response to isoproterenol. Abnormalities in beta-adrenergic regulation may contribute to strain-related differences in the vulnerability to isoproterenol-induced cardiac changes.

Different roles of alpha1-adrenoceptor subtypes in mediating cardiomyocyte protein synthesis in neonatal rats

1. Three different alpha1-adrenoceptor subtypes, designated alpha1A, alpha1B and alpha1D, have been cloned and identified pharmacologically in cardiomyocytes. In vitro studies have suggested that alpha1-adrenoceptors play an important role in facilitating cardiac hypertrophy. However, it remains controversial as to which subtype of alpha1-adrenoceptors is involved in this response. In the present study, we investigated the different role of each alpha1-adrenoceptor subtype in mediating cardiomyocyte protein synthesis, which is a most important characteristic of cardiac hypertrophy in cultured neonatal rat cardiomyocytes. 2. Cardiomyocyte hypertrophy was monitored by the following characteristic phenotypic changes: (i) an increase in protein synthesis; (ii) an increase in total protein content; and (iii) an increase in cardiomyocyte size. 3. The role of each alpha1-adrenoceptor subtype in mediating cardiomyocyte protein synthesis was investigated by the effect of specific alpha1-adrenoceptor subtype-selective antagonists on noradrenaline-induced [3H]-leucine incorporation. In addition, pKB values for alpha1-adrenoceptor subtype-selective antagonists were calculated and compared with the corresponding pKi values to further identify their effects. 4. Activation of alpha1-adrenoceptors by phenylephrine or noradrenaline in the presence of propranolol significantly increased [3H]-leucine incorporation, protein content and cell size. 5. Pre-incubating cardiomyocytes with 5-methyl-urapidil, RS 17053 or WB 4101 significantly inhibited noradrenaline-induced [3H]-leucine incorporation. However, there was no effect when cardiomyocytes were pre-incubated with BMY 7378. The correlation coefficients between pKB values for alpha1-adrenoceptor subtype-selective antagonists and pKi values obtained from cloned alpha1A-, alpha1B- or alpha1D-adrenoceptors were 0.92 (P <0.01), 0.66 (P >0.05) and 0.24 (P >0.05), respectively. 6. Our results suggest that the alpha1-adrenoceptor is dominantly responsible for adrenergic hypertrophy of cultured cardiomyocytes in neonatal rats. The efficiency in mediating cardiomyocyte protein synthesis is alpha1A > alpha1B >> alpha1D.

Reciprocal regulation of beta-adrenergic receptor mRNA stability by mitogen activated protein kinase activation and inhibition

Genes encoding numerous proto-oncogenes and cytokines, as well as a number of G-protein coupled receptors, are regulated post-transcriptionally at the level of mRNA stability. A common feature of all of these genes is the presence of A + U-rich elements (AREs) within their 3' untranslated regions. We, and others, have demonstrated previously that mRNAs encoding beta-adrenergic receptors (beta-ARs) are destabilized by agonist stimulation of the beta-AR/Galphas/adenylylcyclase pathway. However, in addition to PK-A, beta-ARs can also activate or inhibit mitogen activated kinase (MAPK) cascades, in a cell-type dependent basis. Recent evidence points to an important role for MAPKs in regulating the turnover of cytokine mRNAs, such as TNFalpha. We hypothesized that activation of MAPK's may also regulate beta-AR mRNA stability. The studies conducted herein demonstrate that generalized stimulation of MAPKs (JNK, p38) with anisomycin resulted in marked stabilization of beta-AR mRNA. Reciprocally, selective inhibition of JNK with SP600125 significantly decreased beta-AR mRNA half-life. Similarly, inhibition of the MEK/ERK pathway with either PD98059 or U0126 decreased beta-AR mRNA stability substantially. However, inhibition of p38 MAPK with SB203580 produced destabilization of beta-AR mRNA only at higher, non pharmacologically selective concentrations. In contrast to their effects on several other ARE containing mRNAs, inhibition of tyrosine kinases by genistein or PI3K by wortmannin, had no detectable effect on beta-AR mRNA stability. In summary, these results demonstrate for the first time that modulation of MAPK pathways can bi-directionally influence beta-AR mRNA stability.

Glycogen synthase kinase-3beta is involved in the process of myocardial hypertrophy stimulated by insulin-like growth factor-1

BACKGROUND

Glycogen synthase kinase-3 beta (GSK-3beta) is involved in many cellular processes, such as metabolism, apoptosis, differentiation and proliferation. Insulin-like growth factor-1 (IGF-1), which is well known to have a hypertrophic effect on cardiomyocytes, inactivates (phosphorylates) GSK-3beta in some cell types. The role of GSK-3beta in cardiomyocytes as a negative regulator of cardiac hypertrophy has been recently reported and the present study investigated the role of GSK-3beta in the cardiac hypertrophy of cultivated neonatal rat cardiomyocytes induced by IGF-1.

METHODS AND RESULTS

First, the IGF-1 induced signal transduction leading to GSK-3beta in neonatal rat cardiomyocytes was examined. The phosphatidylinositol (PI) 3-kinase/Akt/GSK-3 beta signaling induced by IGF-1 was investigated using inhibitors of PI 3-kinase and Ad AktAA, a dominant negative form of Akt. Furthermore, using Ad MEK DN, a dominant negative form of MEK, it was found that MEK negatively regulates Akt phosphorylation upon IGF-1 stimulation. Next, it was examined whether GSK-3beta acts as a negative regulator in the cardiac hypertrophy induced by IGF-1. Sustained stimulation by IGF-1 caused cardiac hypertrophy in protein synthesis and cellular morphology, and overexpression of unphosphorylatable GSK-3beta (Ad GSK-3beta S9A) repressed these hypertrophic effects of IGF-1.

CONCLUSIONS

GSK-3beta may play an important role as a negative regulator of cardiac hypertrophy induced by IGF-1.

Angiotensin II-induced ventricular hypertrophy and extracellular signal-regulated kinase activation are suppressed in mice overexpressing brain natriuretic peptide in circulation

Atrial and brain (B-type) natriuretic peptides (ANP and BNP, respectively) are known to exert various cardioprotective effects. For instance, knocking out the expression of ANP, BNP, or their receptor, guanylyl cyclase-A, induces cardiac hypertrophy and/or fibrosis. The cardiac effects of elevated circulating natriuretic peptides are less well understood, however. We therefore compared angiotensin (Ang) II-induced cardiac hypertrophy and fibrosis in BNP-transgenic (Tg) mice, in which circulating BNP levels were elevated by increased secretion from the liver, and their non-Tg littermates. Left ventricular expression of Ang II type 1a receptor was similar in BNP-Tg and non-Tg mice, and there was no significant difference in the elevation of blood pressure elicited by chronic infusion or acute injection of Ang II. Nevertheless, cardiac hypertrophy and fibrosis were significantly diminished in BNP-Tg mice chronically infused with Ang II. In addition, ventricular activation of extracellular signal-regulated kinase (ERK) induced by acute injection of Ang II was also diminished in BNP-Tg mice, as was activation of ERK kinase (MEK). Conversely, expression of mitogen-activated protein kinase phosphatase (MKP) was significantly increased in the ventricles of BNP-Tg mice. Based on these findings, we conclude that elevated circulating BNP exerts cardioprotective effects via inhibition of a ventricular ERK pathway. The mechanism responsible for this inhibition likely involves 1) increased ventricular MKP expression and 2) inhibition of transduction mediators situated upstream of ERK.

PP2A activation by beta2-adrenergic receptor agonists: novel regulatory mechanism of keratinocyte migration

Understanding the mechanisms that regulate cell migration is important for devising novel therapies to control metastasis or enhance wound healing. Previously, we demonstrated that beta2-adrenergic receptor (beta2-AR) activation in keratinocytes inhibited their migration by decreasing the phosphorylation of a critical promigratory signaling component, the extracellular signal-related kinase (ERK). Here we demonstrate that beta2-AR-induced inhibition of migration is mediated by the activation of the serine/threonine phosphatase PP2A. Pretreating human keratinocytes with the PP2A inhibitor, okadaic acid, prevented the beta2-AR-induced inhibition of migration, either as isolated cells or as a confluent sheet of cells repairing an in vitro "wound" and also prevented the beta2-AR-induced reduction in ERK phosphorylation. Similar results were obtained with human corneal epithelial cells. In keratinocytes, immunoprecipitation studies revealed that beta2-AR activation resulted in the rapid association of beta2-AR with PP2A as well as a 37% increase in association of PP2A with ERK2. Finally, beta2-AR activation resulted in a rapid and transient 2-fold increase in PP2A activity. Thus, we provide the first evidence that beta2-AR activation in keratinocytes modulates migration via a novel pathway utilizing PP2A to alter the promigratory signaling cascade. Exploiting this pathway may result in novel therapeutic approaches for control of epithelial cell migration.

Role of mitogen-activated protein kinase in cardiac hypertrophy and heart failure

BACKGROUND

Mitogen-activated protein kinases (MAPKs) are involved in the regulation of various cellular responses including cell proliferation, differentiation and survival. Although MAPKs are activated by MAPK kinase and inactivated by phosphatases, different types of MAPKs, including extracellular signal-regulated kinases (ERK1 and 2), c-jun N-terminal protein kinases (JNK) and p38 kinases are known to participate in different signalling pathways. This article will review some salient features of the regulation and function of different forms of MAPKs in the heart. Furthermore, the status of cardiac MAPKs under different pathophysiological conditions will be described.

OBSERVATIONS

A wide variety of external stimuli are known to activate MAPKs, which are then translocated from the cytoplasm to the nucleus and regulate cardiac gene expression by phosphorylating various transcriptional factors. By virtue of the involvement of ERK1/2 in hypertrophic response and of the stress-activated JNKs and p38 kinases in the process of apoptosis, MAPKs are considered to be intimately involved in cardiac remodelling. Both growth factors and phorbol esters have been shown to strongly activate ERK1/2, whereas the activation of JNKs and p38 kinases by these agents is weak. Although ischemia-reperfusion activates all types of MAPKs, JNKs and p38 kinases are mainly proapoptotic, whereas ERK1/2 are antiapoptotic.

CONCLUSIONS

The activation of ERK1/2 is involved in signal transduction pathways associated with cardiac hypertrophy; however, the exact status of MAPKs in heart failure remains to be clearly defined. While both JNKs and p38 kinases appear to participate in the genesis of ischemia-reperfusion injury, ERK1/2 are considered to be cytoprotective.

Calreticulin inhibits the MEK1,2-ERK1,2 pathway in alpha 1-adrenergic receptor/Gh-stimulated hypertrophy of neonatal rat cardiomyocytes

In cardiac myocytes, stimulation of alpha(1)-adrenoceptor (AR) leads to a hypertrophic phenotype. The G(h) protein (transglutaminase II, TGII) is tissue type transglutaminase and transmits the alpha(1B)-adrenoceptor signal with GTPase activity. Recently, it has been shown that the calreticulin (CRT) down-regulates both GTP binding and transglutaminase activities of TGII. To elucidate whether G(h) mediates norepinephrine-stimulated intracellular signal transductions leading to activation of extracellular signal-regulated kinases (ERKs) and neonatal rat cardiomyocyte hypertrophy, we examined the effects of G(h) on the activation of ERKs and inhibitory effects of CRT on alpha(1)-adrenoceptor/G(h) signaling. In neonatal rat cardiomyocytes, norepinephrine-induced ERKs activation was inhibited by an alpha(1)-adrenoceptor blocker (prazosin), but not by an beta-adrenoceptor blocker (propranolol). Overexpression of the G(h) protein stimulated norepinephrine-induced ERKs activation, which was inhibited by alpha-adrenoceptor blocker (prazosin). Co-overexpression of G(h) and CRT abolished norepinephrine-induced ERKs activation. Taken together, norepinephrine induces hypertrophy in neonatal rat cardiomyocytes through alpha(1)-AR stimulation and G(h) is partly involved in norepinephrine-induced MEK1,2/ERKs activation. Activation of G(h)-mediated MEK1,2/ERKs was completely inhibited by CRT.