Transcriptional activation of the cardiac myosin light chain 2 and atrial natriuretic factor genes by protein kinase C in neonatal rat ventricular myocytes

Role of Adenosine and Purinergic Receptors in Myocardial Infarction: Focus on Different Signal Transduction Pathways

Myocardial infarction (MI) is a dramatic event often caused by atherosclerotic plaque erosion or rupture and subsequent thrombotic occlusion of a coronary vessel. The low supply of oxygen and nutrients in the infarcted area may result in cardiomyocytes necrosis, replacement of intact myocardium with non-contractile fibrous tissue and left ventricular (LV) function impairment if blood flow is not quickly restored. In this review, we summarized the possible correlation between adenosine system, purinergic system and Wnt/β-catenin pathway and their role in the pathogenesis of cardiac damage following MI. In this context, several pathways are involved and, in particular, the adenosine receptors system shows different interactions between its members and purinergic receptors: their modulation might be effective not only for a normal functional recovery but also for the treatment of heart diseases, thus avoiding fibrosis, reducing infarcted area and limiting scaring. Similarly, it has been shown that Wnt/β catenin pathway is activated following myocardial injury and its unbalanced activation might promote cardiac fibrosis and, consequently, LV systolic function impairment. In this regard, the therapeutic benefits of Wnt inhibitors use were highlighted, thus demonstrating that Wnt/β-catenin pathway might be considered as a therapeutic target to prevent adverse LV remodeling and heart failure following MI.

Pharmacological Protein Kinase C Modulators Reveal a Pro-hypertrophic Role for Novel Protein Kinase C Isoforms in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Background: Hypertrophy of cardiomyocytes (CMs) is initially a compensatory mechanism to cardiac overload, but when prolonged, it leads to maladaptive myocardial remodeling, impairing cardiac function and causing heart failure. A key signaling molecule involved in cardiac hypertrophy is protein kinase C (PKC). However, the role of different PKC isoforms in mediating the hypertrophic response remains controversial. Both classical (cPKC) and novel (nPKC) isoforms have been suggested to play a critical role in rodents, whereas the role of PKC in hypertrophy of human CMs remains to be determined. Here, we aimed to investigate the effects of two different types of PKC activators, the isophthalate derivative HMI-1b11 and bryostatin-1, on CM hypertrophy and to elucidate the role of cPKCs and nPKCs in endothelin-1 (ET-1)-induced hypertrophy in vitro.

Methods and Results: We used neonatal rat ventricular myocytes (NRVMs) and human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to study the effects of pharmacological PKC modulators and ET-1. We used quantitative reverse transcription PCR to quantify hypertrophic gene expression and high-content analysis (HCA) to investigate CM morphology. In both cell types, ET-1, PKC activation (bryostatin-1 and HMI-1b11) and inhibition of cPKCs (Gö6976) increased hypertrophic gene expression. In NRVMs, these treatments also induced a hypertrophic phenotype as measured by increased recognition, intensity and area of α-actinin and F-actin fibers. Inhibition of all PKC isoforms with Gö6983 inhibited PKC agonist-induced hypertrophy, but could not fully block ET-1-induced hypertrophy. The mitogen-activated kinase kinase 1/2 inhibitor U0126 inhibited PKC agonist-induced hypertrophy fully and ET-1-induced hypertrophy partially. While ET-1 induced a clear increase in the percentage of pro-B-type natriuretic peptide-positive hiPSC-CMs, none of the phenotypic parameters used in HCA directly correlated with gene expression changes or with phenotypic changes observed in NRVMs.

Conclusion: This work shows similar hypertrophic responses to PKC modulators in NRVMs and hiPSC-CMs. Pharmacological PKC activation induces CM hypertrophy via activation of novel PKC isoforms. This pro-hypertrophic effect of PKC activators should be considered when developing PKC-targeted compounds for e.g. cancer or Alzheimer’s disease. Furthermore, this study provides further evidence on distinct PKC-independent mechanisms of ET-1-induced hypertrophy both in NRVMs and hiPSC-CMs.

The proarrhythmic features of pathological cardiac hypertrophy in neonatal rat ventricular cardiomyocyte cultures

Different factors may trigger arrhythmias in diseased hearts, including fibrosis, cardiomyocyte hypertrophy, hypoxia, and inflammation. This makes it difficult to establish the relative contribution of each of them to the occurrence of arrhythmias. Accordingly, in this study, we used an in vitro model of pathological cardiac hypertrophy (PCH) to investigate its proarrhythmic features and the underlying mechanisms independent of fibrosis or other PCH-related processes. Neonatal rat ventricular cardiomyocyte (nr-vCMC) monolayers were treated with phorbol 12-myristate 13-acetate (PMA) to create an in vitro model of PCH. The electrophysiological properties of PMA-treated and control monolayers were analyzed by optical mapping at day 9 of culture. PMA treatment led to a significant increase in cell size and total protein content. It also caused a reduction in sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2 level (32%) and an increase in natriuretic peptide A (42%) and α1-skeletal muscle actin (34%) levels, indicating that the hypertrophic response induced by PMA was, indeed, pathological in nature. PMA-treated monolayers showed increases in action potential duration (APD) and APD dispersion, and a decrease in conduction velocity (CV; APD₃₀ of 306 ± 39 vs. 148 ± 18 ms, APD₃₀ dispersion of 85 ± 19 vs. 22 ± 7 and CV of 10 ± 4 vs. 21 ± 2 cm/s in controls). Upon local 1-Hz stimulation, 53.6% of the PMA-treated cultures showed focal tachyarrhythmias based on triggered activity (n = 82), while the control group showed 4.3% tachyarrhythmias (n = 70). PMA-treated nr-vCMC cultures may, thus, represent a well-controllable in vitro model for testing new therapeutic interventions targeting specific aspects of hypertrophy-associated arrhythmias.NEW & NOTEWORTHY Phorbol 12-myristate 13-acetate (PMA) treatment of neonatal rat ventricular cardiomyocytes (nr-vCMCs) led to induction of many significant features of pathological cardiac hypertrophy (PCH), including action potential duration prolongation and dispersion, which provided enough time and depolarizing force for formation of early afterdepolarization (EAD)-induced focal tachyarrhythmias. PMA-treated nr-vCMCs represent a well-controllable in vitro model, which mostly resembles to moderate left ventricular hypertrophy (LVH) rather than severe LVH, in which generation of a reentry is the putative mechanism of its arrhythmias.

Transcriptomic Validation of the Protective Effects of Aqueous Bark Extract of Terminalia arjuna (Roxb.) on Isoproterenol-Induced Cardiac Hypertrophy in Rats

Aqueous extract of the bark of Terminalia arjuna (TA) is used by a large population in the Indian subcontinent for treating various cardiovascular conditions. Animal experiments have shown its anti-atherogenic, anti-hypertensive, and anti-inflammatory effects. It has several bioactive ingredients with hemodynamic, ROS scavenging, and anti-inflammatory properties. Earlier we have done limited proteomic and transcriptomic analysis to show its efficacy in ameliorating cardiac hypertrophy induced by isoproterenol (ISO) in rats. In the present study we have used high-throughput sequencing of the mRNA from control and treated rat heart to further establish its efficacy. ISO (5 mg/kg/day s.c.) was administered in male adult rats for 14 days to induce cardiac hypertrophy. Standardized aqueous extract TA bark extract was administered orally. Total RNA were isolated from control, ISO, ISO + TA, and TA treated rat hearts and subjected to high throughput sequence analysis. The modulations of the transcript levels were then subjected to bio-informatics analyses using established software. Treatment with ISO downregulated 1,129 genes and upregulated 204 others. Pre-treatment with the TA bark extracts markedly restored that expression pattern with only 97 genes upregulated and 85 genes downregulated. The TA alone group had only 88 upregulated and 26 downregulated genes. The overall profile of expression in ISO + TA and TA alone groups closely matched with the control group. The genes that were modulated included those involved in metabolism, activation of receptors and cell signaling, and cardiovascular and other diseases. Networks associated with those genes included those involved in angiogenesis, extracellular matrix organization, integrin binding, inflammation, drug metabolism, redox metabolism, oxidative phosphorylation, and organization of myofibril. Overlaying of the networks in ISO and ISO_TA group showed that those activated in ISO group were mostly absent in ISO_TA and TA group, suggesting a global effect of the TA extracts. This study for the first time reveals that TA partially or completely restores the gene regulatory network perturbed by ISO treatment in rat heart; signifying its efficacy in checking ISO-induced cardiac hypertrophy.

Atrial and brain natriuretic peptides: Hormones secreted from the heart

The natriuretic peptide family consists of three biologically active peptides: atrial natriuretic peptide (ANP), brain (or B-type) natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). Among these, ANP and BNP are secreted by the heart and act as cardiac hormones. Both ANP and BNP preferentially bind to natriuretic peptide receptor-A (NPR-A or guanylyl cyslase-A) and exert similar effects through increases in intracellular cyclic guanosine monophosphate (cGMP) within target tissues. Expression and secretion of ANP and BNP are stimulated by various factors and are regulated via multiple signaling pathways. Human ANP has three molecular forms, α-ANP, β-ANP, and proANP (or γ-ANP), with proANP predominating in healthy atrial tissue. During secretion proANP is proteolytically processed by corin, resulting in secretion of bioactive α-ANP into the peripheral circulation. ProANP and β-ANP are minor forms in the circulation but are increased in patients with heart failure. The human BNP precursor proBNP is proteolytically processed to BNP_1-32 and N-terminal proBNP (NT-proBNP) within ventricular myocytes. Uncleaved proBNP as well as mature BNP_1-32 and NT-proBNP is secreted from the heart, and its secretion is increased in patients with heart failure. Mature BNP, its metabolites including BNP_3-32, BNP_4-32, and BNP_5-32, and proBNP are all detected as immunoreactive-BNP by the current BNP assay system. We recently developed an assay system that specifically detects human proBNP. Using this assay system, we observed that miR30-GALNTs-dependent O-glycosylation in the N-terminal region of proBNP contributes to regulation of the processing and secretion of proBNP from the heart.

Lowering body weight in obese mice with diastolic heart failure improves cardiac insulin sensitivity and function: implications for the obesity paradox

Recent studies suggest improved outcomes and survival in obese heart failure patients (i.e., the obesity paradox), although obesity and heart failure unfavorably alter cardiac function and metabolism. We investigated the effects of weight loss on cardiac function and metabolism in obese heart failure mice. Obesity and heart failure were induced by feeding mice a high-fat (HF) diet (60% kcal from fat) for 4 weeks, following which an abdominal aortic constriction (AAC) was produced. Four weeks post-AAC, mice were switched to a low-fat (LF) diet (12% kcal from fat; HF AAC LF) or maintained on an HF (HF AAC HF) for a further 10 weeks. After 18 weeks, HF AAC LF mice weighed less than HF AAC HF mice. Diastolic function was improved in HF AAC LF mice, while cardiac hypertrophy was decreased and accompanied by decreased SIRT1 expression, increased FOXO1 acetylation, and increased atrogin-1 expression compared with HF AAC HF mice. Insulin-stimulated glucose oxidation was increased in hearts from HF AAC LF mice, compared with HF AAC HF mice. Thus lowering body weight by switching to LF diet in obese mice with heart failure is associated with decreased cardiac hypertrophy and improvements in both cardiac insulin sensitivity and diastolic function, suggesting that weight loss does not negatively impact heart function in the setting of obesity.

Biolistic transfection of freshly isolated adult ventricular myocytes

Transfection of mammalian cells has long been an extremely powerful approach for the study of the effects of specific gene expression on cell function. Until recently, however, this approach has been unavailable for the study of gene function in adult cardiac myocytes. Here, an adaptation of the biolistic method to the transfection of adult cardiac myocytes is described. DNA is precipitated onto gold particles in the absence of PVP and the particles are biolistically delivered to freshly isolated adult rat cardiomyocytes via a Bio-Rad Helios System gene gun. The myocytes are cultured in the absence of bovine serum albumin and expression of the introduced genes, in phenotypically intact myocytes, is robust within 12-24 h.

Cell-intrinsic functional effects of the α-cardiac myosin Arg-403-Gln mutation in familial hypertrophic cardiomyopathy

Human familial hypertrophic cardiomyopathy is the most common Mendelian cardiovascular disease worldwide. Among the most severe presentations of the disease are those in families heterozygous for the mutation R403Q in β-cardiac myosin. Mice heterozygous for this mutation in the α-cardiac myosin isoform display typical familial hypertrophic cardiomyopathy pathology. Here, we study cardiomyocytes from heterozygous 403/+ mice. The effects of the R403Q mutation on force-generating capabilities and dynamics of cardiomyocytes were investigated using a dual carbon nanofiber technique to measure single-cell parameters. We demonstrate the Frank-Starling effect at the single cardiomyocyte level by showing that cell stretch causes an increase in amplitude of contraction. Mutant 403/+ cardiomyocytes exhibit higher end-diastolic and end-systolic stiffness than +/+ cardiomyocytes, whereas active force generation capabilities remain unchanged. Additionally, 403/+ cardiomyocytes show slowed relaxation dynamics. These phenotypes are consistent with increased end-diastolic and end-systolic chamber elastance, as well as diastolic dysfunction seen at the level of the whole heart. Our results show that these functional effects of the R403Q mutation are cell-intrinsic, a property that may be a general phenomenon in familial hypertrophic cardiomyopathy.

Regulation of cardiac excitability by protein kinase C isozymes

Cardiac excitability and electrical activity are determined by the sum of individual ion channels, gap junctions and exchanger activities. Electrophysiological remodeling during heart disease involves changes in membrane properties of cardiomyocytes and is related to higher prevalence of arrhythmia-associated morbidity and mortality. Pharmacological and genetic manipulation of cardiac cells as well as animal models of cardiovascular diseases are used to identity changes in electrophysiological properties and the molecular mechanisms associated with the disease. Protein kinase C (PKC) and several other kinases play a pivotal role in cardiac electrophysiological remodeling. Therefore, identifying specific therapies that regulate these kinases is the main focus of current research. PKC, a family of serine/threonine kinases, has been implicated as potential signaling nodes associated with biochemical and biophysical stress in cardiovascular diseases. In this review, we describe the role of PKC isozymes that are involved in cardiac excitability and discuss both genetic and pharmacological tools that were used, their attributes and limitations. Selective and effective pharmacological interventions to normalize cardiac electrical activities and correct cardiac arrhythmias will be of great clinical benefit.

Regulation of cardiac excitability by protein kinase C isozymes

Cardiac excitability and electrical activity are determined by the sum of individual ion channels, gap junctions and exchanger activities. Electrophysiological remodeling during heart disease involves changes in membrane properties of cardiomyocytes and is related to higher prevalence of arrhythmia-associated morbidity and mortality. Pharmacological and genetic manipulation of cardiac cells as well as animal models of cardiovascular diseases are used to identity changes in electrophysiological properties and the molecular mechanisms associated with the disease. Protein kinase C (PKC) and several other kinases play a pivotal role in cardiac electrophysiological remodeling. Therefore, identifying specific therapies that regulate these kinases is the main focus of current research. PKC, a family of serine/threonine kinases, has been implicated as potential signaling nodes associated with biochemical and biophysical stress in cardiovascular diseases. In this review, we describe the role of PKC isozymes that are involved in cardiac excitability and discuss both genetic and pharmacological tools that were used, their attributes and limitations. Selective and effective pharmacological interventions to normalize cardiac electrical activities and correct cardiac arrhythmias will be of great clinical benefit.

Aborto em bovinos devido à intoxicação por Tetrapterys acutifolia (Malpighiaceae)

Esse estudo teve por objetivo demonstrar experimentalmente que Tetrapterys acutifolia Cav. (fam. Malpighiaceae) é capaz de provocar aborto em bovinos e caracterizar as alterações clínico-patológicas nas vacas e nos fetos. Estas plantas são responsáveis por significativo número de mortes em bovinos com mais de um ano de idade, especialmente nos Estados de Rio de Janeiro e São Paulo, mas até agora não havia sido comprovado experimentalmente seu efeito abortivo em bovinos. Os experimentos foram realizados no município de Barra do Piraí, RJ. Quatro vacas de descarte receberam brotos e folhas novas frescas de T. acutifolia, coletadas em propriedades vizinhas, nas doses de 2,5g/kg/dia, 5,0g/kg/dia (2 vacas) e 10g/kg/dia, até ocorrer o abortamento. O quadro clínico nas vacas caracterizou-se por arritmia cardíaca, tremores musculares, anorexia, ascite, jugular ingurgitada, edema de peito e barbela e aborto (23-76 dias após o início da ingestão da planta); todas as vacas abortaram. Das quatro vacas apenas uma (a que recebeu 10g/kg/dia) morreu 36 dias após o abortamento, com sintomas de insuficiência cardíaca. O exame necroscópico dos fetos/natimortos revelou hidrotórax, hidropericárdio, hidroperitônio e congestão hepática; ao corte do miocárdio, verificaram-se áreas pálidas. No exame histológico havia edema intersticial com fibrose incipiente. Na vaca que recebeu a maior dose e foi a óbito, bem como em outra intoxicada naturalmente, os achados de necropsia foram similares aos observados nos fetos, exceto pela dilatação dos vasos da base do coração e mais acentuada palidez do miocárdio. Observaram-se ainda edema subcutâneo nas regiões cervical e esternal, bem como veias jugulares ingurgitadas. Os achados histopatológicos foram necrose e edema intersticial com acentuada fibrose no miocárdio, espongiose da substância branca do encéfalo e, no fígado, congestão e leve fibrose. Adicionalmente, observou-se na vaca intoxicada espontaneamente, 17 dias após o aborto, arritmia cardíaca, jugular ingurgitada, edema de peito e barbela, anorexia com morte 43 dias após o aborto. Este estudo demonstra que Tetrapterys acutifolia é capaz de induzir aborto e, dependendo da dose, ainda causar a morte das vacas que abortarem.

Phorbol ester and endothelin-1 alter functional expression of Na+/Ca2+ exchange, K+, and Ca2+ currents in cultured neonatal rat myocytes

Endothelin-1 (ET-1) and activation of protein kinase C (PKC) have been implicated in alterations of myocyte function in cardiac hypertrophy and heart failure. Changes in cellular Ca2+ handling and electrophysiological properties also occur in these states and may contribute to mechanical dysfunction and arrhythmias. While ET-1 or PKC stimulation induces cellular hypertrophy in cultured neonatal rat ventricular myocytes (NRVMs), a system widely used in studies of hypertrophic signaling, there is little data about electrophysiological changes. Here we studied the effects of ET-1 (100 nM) or the PKC activator phorbol 12-myristate 13-acetate (PMA, 1 μM) on ionic currents in NRVMs. The acute effects of PMA or ET-1 (≤30 min) were small or insignificant. However, PMA or ET-1 exposure for 48-72 h increased cell capacitance by 100 or 25%, respectively, indicating cellular hypertrophy. ET-1 also slightly increased Ca2+ current density (T and L type). Na+/Ca2+ exchange current was increased by chronic pretreatment with either PMA or ET-1. In contrast, transient outward and delayed rectifier K+ currents were strongly downregulated by PMA or ET-1 pretreatment. Inward rectifier K+ current tended toward a decrease at larger negative potential, but time-independent outward K+ current was unaltered by either treatment. The enhanced inward and reduced outward currents also result in action potential prolongation after PMA or ET-1 pretreatment. We conclude that chronic PMA or ET-1 exposure in cultured NRVMs causes altered functional expression of cardiac ion currents, which mimic electrophysiological changes seen in whole animal and human hypertrophy and heart failure.

Regulation and significance of atrial and brain natriuretic peptides as cardiac hormones

Atrial and brain natriuretic peptides (ANP and BNP, respectively) are cardiac hormones. During cardiac development, their expression is a maker of cardiomyocyte differentiation and is under tight spatiotemporal regulation. After birth, however, their ventricular expression is only up-regulated in response to various cardiovascular diseases. As a result, analysis of ANP and BNP gene expression has led to discoveries of transcriptional regulators and signaling pathways involved in both cardiac differentiation and cardiac disease. Studies using genetically engineered mice have shed light on the molecular mechanisms regulating ANP and BNP gene expression, as well as the physiological and pathophysiological relevance of the cardiac natriuretic peptide system. In this review we will summarize what is currently known about their regulation and the significance of ANP and BNP as hormones derived from the heart.

Elucidating the role of reversible protein phosphorylation in sepsis-induced myocardial dysfunction

Mortality in children with sepsis is most often related to diminished cardiac output with cardiovascular collapse, resulting in impaired oxygen delivery and, ultimately, end-organ failure. Although cardiovascular "collapse" is commonly observed in individuals with septic shock, the hemodynamic causes of this differ greatly. In children, intrinsic myocardial dysfunction is most commonly present, whereas the systemic vascular resistance is typically high. This pattern is distinct from adults with sepsis where the principal hemodynamic profile shows elevated cardiac output, but substantially reduced systemic vascular resistance. Various studies support the concept that myocardial dysfunction, as occurs in pediatric septic patients, is due to intrinsic abnormalities in cardiomyocyte function and is not related to hypoperfusion as a result of low systemic vascular resistance. Importantly, when examined more closely, data from adults with septic shock also reveal that intrinsic myocardial dysfunction may play a larger role than previously appreciated. As a result, cardiovascular support, especially in pediatric sepsis, requires a treatment strategy directed at the underlying mechanism(s) responsible for this dysfunction. Thus, it is imperative to gain a better understanding of the myocardial derangements that occur during sepsis to identify targets that will ultimately influence the management of children with septic shock and favorably alter the associated mortality. We hypothesize that key signaling pathways that control myocardial calcium flux, regulated to key kinases and phosphatases, influence myocyte contractility in sepsis. Thus, we review the data relevant to the sepsis-induced intracellular alterations in calcium flux in the cardiomyocyte, with an emphasis on changes in the phosphorylation state of the contractile proteins regulated by the balance between kinases and phosphatases. We believe that therapies modulating the activity of these key proteins may provide an improvement in calcium handling and myocardial contractility and alter the clinical outcomes in sepsis.

LPS-induced autophagy is mediated by oxidative signaling in cardiomyocytes and is associated with cytoprotection

Bacterial endotoxin lipopolysaccharide (LPS) is responsible for the multiorgan dysfunction that characterizes septic shock and is causal in the myocardial depression that is a common feature of endotoxemia in patients. In this setting the myocardial dysfunction appears to be due, in part, to the production of proinflammatory cytokines. A line of evidence also indicates that LPS stimulates autophagy in cardiomyocytes. However, the signal transduction pathway leading to autophagy and its role in the heart are incompletely characterized. In this work, we wished to determine the effect of LPS on autophagy and the physiological significance of the autophagic response. Autophagy was monitored morphologically and biochemically in HL-1 cardiomyocytes, neonatal rat cardiomyocytes, and transgenic mouse hearts after the administration of bacterial LPS or TNF-alpha. We observed that autophagy was increased after exposure to LPS or TNF-alpha, which is induced by LPS. The inhibition of TNF-alpha production by AG126 significantly reduced the accumulation of autophagosomes both in cell culture and in vivo. The inhibition of p38 MAPK or nitric oxide synthase by pharmacological inhibitors also reduced autophagy. Nitric oxide or H(2)O(2) induced autophagy in cardiomyocytes, whereas N-acetyl-cysteine, a potent antioxidant, suppressed autophagy. LPS resulted in increased reactive oxygen species (ROS) production and decreased total glutathione. To test the hypothesis that autophagy might serve as a damage control mechanism to limit further ROS production, we induced autophagy with rapamycin before LPS exposure. The activation of autophagy by rapamycin suppressed LPS-mediated ROS production and protected cells against LPS toxicity. These findings support the notion that autophagy is a cytoprotective response to LPS-induced cardiomyocyte injury; additional studies are needed to determine the therapeutic implications.

Inflammatory pathways are activated during cardiomyocyte hypertrophy and attenuated by peroxisome proliferator-activated receptors PPARalpha and PPARdelta

Accumulating evidence indicates an important role for inflammation in cardiac hypertrophy and failure. Peroxisome proliferator-activated receptors (PPARs) have been reported to attenuate inflammatory signaling pathways and, as such, may interfere with cardiac remodeling. Accordingly, the objectives of the present study were to explore the relationship between cardiomyocyte hypertrophy and inflammation and to investigate whether PPARalpha and PPARdelta are able to inhibit NF-kappaB activation and, consequently, the hypertrophic growth response of neonatal rat cardiomyocytes (NCM). mRNA levels of markers of both hypertrophy and inflammation were increased following treatment with the pro-hypertrophic factor phenylephrine (PE) or the chemokine TNF-alpha. Induction of inflammatory genes was found to be fast (within 2 h after stimulation) and transient, while induction of hypertrophic marker genes was more gradual (peaking at 24-48 h). Inflammatory and hypertrophic pathways appeared to converge on NF-kappaB as both PE and TNF-alpha increased NF-kappaB binding activity as measured by electrophoretic mobility shift assay. Following transient transfection, the p65-induced transcriptional activation of a NF-kappaB reporter construct was significantly blunted after co-transfection of PPARalpha or PPARdelta in the presence of their respective ligands. Finally, adenoviral overexpression of PPARalpha and PPARdelta markedly attenuated cell enlargement and the expression of hypertrophic marker genes in PE-stimulated NCM. The collective findings reveal a close relationship between hypertrophic and inflammatory signaling pathways in the cardiomyocyte. It was shown that both PPARalpha and PPARdelta are able to mitigate cardiomyocyte hypertrophy in vitro by inhibiting NF-kappaB activation.

Molecular identity of the late sodium current in adult dog cardiomyocytes identified by Nav1.5 antisense inhibition

Late Na(+) current (I(NaL)) is a major component of the action potential plateau in human and canine myocardium. Since I(NaL) is increased in heart failure and ischemia, it represents a novel potential target for cardioprotection. However, the molecular identity of I(NaL) remains unclear. We tested the hypothesis that the cardiac Na(+) channel isoform (Na(v)1.5) is a major contributor to I(NaL) in adult dog ventricular cardiomyocytes (VCs). Cultured VCs were exposed to an antisense morpholino-based oligonucleotide (Na(v)1.5 asOligo) targeting the region around the start codon of Na(v)1.5 mRNA or a control nonsense oligonucleotide (nsOligo). Densities of both transient Na(+) current (I(NaT)) and I(NaL) (both in pA/pF) were monitored by whole cell patch clamp. In HEK293 cells expressing Na(v)1.5 or Na(v)1.2, Na(v)1.5 asOligo specifically silenced functional expression of Na(v)1.5 (up to 60% of the initial I(NaT)) but not Na(v)1.2. In both nsOligo-treated controls and untreated VCs, I(NaT) and I(NaL) remained unchanged for up to 5 days. However, both I(NaT) and I(NaL) decreased exponentially with similar time courses (tau = 46 and 56 h, respectively) after VCs were treated with Na(v)1.5 asOligo without changes in 1) decay kinetics, 2) steady-state activation and inactivation, and 3) the ratio of I(NaL) to I(NaT). Four days after exposure to Na(v)1.5 asOligo, I(NaT) and I(NaL) amounted to 68 +/- 6% (mean +/- SE; n = 20, P < 0.01) and 60 +/- 7% (n = 11, P < 0.018) of those in VCs treated by nsOligo, respectively. We conclude that in adult dog heart Na(v)1.5 sodium channels have a "functional half-life" of approximately 35 h (0.69tau) and make a major contribution to I(NaL).

Overt expression of AP-1 reduces alpha myosin heavy chain expression and contributes to heart failure from chronic volume overload

Reduced expression of alpha-MHC plays a significant role in cardiac contractile dysfunction from hemodynamic overload. Previously, Pur proteins and YY1 have been shown to play a role in alpha-MHC repression during heart failure induced by pressure overload and by spontaneous hypertension, respectively. This was not observed in volume-overload-induced heart failure, suggesting additional regulatory mechanisms for alpha-MHC repression. The present study was performed to identify volume overload responsive transcription factors involved in alpha-MHC gene regulation. DNA binding activity of several transcription factors was evaluated in a functionally characterized rat model of heart failure induced by aorto-caval shunt. After 10 weeks of shunt, severe LV dilatation and reduced LV function were accompanied by increased expression of ANF and beta-MHC, and decreased expression of alpha-MHC. This was associated with dramatic (10-fold) activation of AP-1 together with increased expression of c-fos and c-jun. AP-1 activation was not observed following 4 weeks of shunt when cardiac function was preserved. In cultured cardiomyocytes, induction of AP-1 by PMA attenuated alpha-MHC mRNA by 60%. Transient transfection assays mapped PMA responsive sequence to -582 to -588 bp of alpha-MHC promoter. Deletion or mutation of these nucleotides had minimal effect on basal promoter activity but played a dominant role in PMA-mediated repression of alpha-MHC promoter activity. Over-expression of c-fos and c-jun in cardiomyocytes inhibited alpha-MHC promoter activity in a concentration dependent manner. Data suggest a repressive role of AP-1 in alpha-MHC expression and its possible involvement in the transition from compensatory hypertrophy to heart failure in chronic volume overload.

RhoA/Rho kinase up-regulate Bax to activate a mitochondrial death pathway and induce cardiomyocyte apoptosis

The small G-protein RhoA regulates the actin cytoskeleton, and its involvement in cell proliferation has also been established. In contrast, little is known about whether RhoA participates in cell survival or apoptosis. In cardiomyocytes in vitro, RhoA induces hypertrophic cell growth and gene expression. In vivo, however, RhoA expression leads to development of heart failure (Sah, V. P., Minamisawa, S., Tam, S. P., Wu, T. H., Dorn, G. W., Ross, J. Jr., Chien, K. R., and Brown, J. H. (1999) J. Clin. Investig. 103, 1627-1634), a condition widely associated with cardiomyocyte apoptosis. We demonstrate here that adenoviral overexpression of activated RhoA in cardiomyocytes induces hypertrophy, which transitions over time to apoptosis, as evidenced by caspase activation and nucleosomal DNA fragmentation. The Rho kinase inhibitors Y-27632 and HA-1077 and expression of a dominant negative Rho kinase block these responses. Caspase-9, but not caspase-8, is activated, and its inhibition prevents DNA fragmentation, consistent with involvement of a mitochondrial death pathway. Interestingly, RhoA expression induces a 3-4-fold up-regulation of the proapoptotic Bcl-2 family protein Bax. RhoA also increases levels of activated Bax and the amount of Bax protein localized at mitochondria. Bax mRNA is increased by RhoA, indicating transcriptional regulation, and the ability of a dominant negative p53 mutant to block Bax up-regulation implicates p53 in this response. The involvement of Bax in RhoA-induced apoptosis was examined by treatment with a Bax-inhibitory peptide, which was found to significantly attenuate DNA fragmentation and caspase-9 and -3 activation. The dominant negative p53 also prevents RhoA-induced apoptosis. We conclude that RhoA/Rho kinase activation up-regulates Bax through p53 to induce a mitochondrial death pathway and cardiomyocyte apoptosis.

Increased Collagen Deposition and Diastolic Dysfunction but Preserved Myocardial Hypertrophy After Pressure Overload in Mice Lacking PKCε

Overexpression and activation of protein kinase C-epsilon (PKCepsilon) results in myocardial hypertrophy. However, these observations do not establish that PKCepsilon is required for the development of myocardial hypertrophy. Thus, we subjected PKCepsilon-knockout (KO) mice to a hypertrophic stimulus by transverse aortic constriction (TAC). KO mice show normal cardiac morphology and function. TAC caused similar cardiac hypertrophy in KO and wild-type (WT) mice. However, KO mice developed more interstitial fibrosis and showed enhanced expression of collagen Ialpha1 and collagen III after TAC associated with diastolic dysfunction, as assessed by tissue Doppler echocardiography (Ea/Aa after TAC: WT 2.1+/-0.3 versus KO 1.0+/-0.2; P<0.05). To explore underlying mechanisms, we analyzed the left ventricular (LV) expression pattern of additional PKC isoforms (ie, PKCalpha, PKCbeta, and PKCdelta). After TAC, expression and activation of PKCdelta protein was increased in KO LVs. Moreover, KO LVs displayed enhanced activation of p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK), whereas p42/p44-MAPK activation was attenuated. Under stretch, cultured KO fibroblasts showed a 2-fold increased collagen Ialpha1 (col Ialpha1) expression, which was prevented by PKCdelta inhibitor rottlerin or by p38 MAPK inhibitor SB 203580. In conclusion, PKCepsilon is not required for the development of a pressure overload-induced myocardial hypertrophy. Lack of PKCepsilon results in upregulation of PKCdelta and promotes activation of p38 MAPK and JNK, which appears to compensate for cardiac hypertrophy, but in turn, is associated with increased collagen deposition and impaired diastolic function.

Cytochrome c oxidase subunit IV as a marker of protein kinase Cepsilon function in neonatal cardiac myocytes: implications for cytochrome c oxidase activity

We have previously demonstrated that low concentrations of phorbol esters stimulate the selective translocation of protein kinase C (PKC) alpha and epsilon from the cell soluble to the particulate fraction in NCMs (neonatal rat cardiac myocytes). We therefore determined if the in vitro phosphorylation of substrates in these fractions could be used as assays of PKCalpha or epsilon activation. Intact cell phorbol ester treatment caused a decline in the in vitro (32)P-incorporation into several proteins in the cell-soluble fraction. These declines occurred in the presence or absence of in vitro Ca(2+) and probably reflected the exit of PKC isoenzymes from the soluble fraction. In contrast, an approx. 18 kDa protein incorporated (32)P in particulate fractions isolated from 4beta-PMA-treated cells in a Ca(2+)-independent manner. Proteomic and immunoprecipitation analyses indicated that the protein is subunit IV of the cytochrome c oxidase complex (COIV). In vitro phosphorylation of COIV was attenuated by PKC pseudosubstrate peptides. Introduction of an PKCepsilon-selective translocation inhibitor [Johnson, Gray, Chen and Mochly-Rosen (1996) J. Biol. Chem. 271, 24962-24966] into NCMs before 4beta-PMA treatments also attenuated the in vitro phosphorylation of COIV. In mitochondrial extracts from 4beta-PMA-treated NCMs, the PKCepsilon isoenzyme coimmunoprecipitated with COIV, and cytochrome c oxidase activity was enhanced 2-fold. The in vitro phosphorylation of COIV reflects a novel approach for monitoring PKCepsilon function in NCMs. Furthermore, PKCepsilon probably interacts with COIV in NCM mitochondria to enhance electron-transport chain complex IV activity.

Mechanotransduction in cardiac myocytes

Cardiac myocytes react to diverse mechanical demands with a multitude of transient and long-term responses to normalize the cellular mechanical environment. Several stretch-activated signaling pathways have been identified, most prominently guanine nucleotide binding proteins (G-proteins), mitogen-activated protein kinases (MAPK), Janus-associated kinase/signal transducers and activators of transcription (JAK/STAT), protein kinase C (PKC), calcineurin, intracellular calcium regulation, and several autocrine and paracrine factors. Multiple levels of crosstalk exist between pathways. The cellular response to changes in the mechanical environment can lead to cardiac myocyte hypertrophy, cellular growth that can be accompanied by pathological myocyte dysfunction, and tissue fibrosis. Several candidates for the primary mechanosensor in cardiac myocytes have been identified, ranging from stretch-activated ion channels in the membrane to yet-unknown mechanosensitive mechanisms in the nucleus. New and refined experimental techniques will exploit advances in molecular biology and biological imaging to study mechanotransduction in isolated cells and genetically engineered mice to explore the function of individual proteins.

Phospholipase C-delta1 rescues intracellular Ca2+ overload in ischemic heart and hypoxic neonatal cardiomyocytes

Ischemia and simulated ischemic conditions cause intracellular Ca2+ overload in the myocardium. The relationship between ischemia injury and Ca2+ overload has not been fully characterized. The aim of the present study was to investigate the expression and characteristics of PLC isozymes in myocardial infarction-induced cardiac remodeling and heart failure. In normal rat heart tissue, PLC-delta1 (about 44 ng/mg of heart tissue) was most abundant isozymes compared to PLC-gamma1 (6.8 ng/mg) and PLC-beta1 (0.4 ng/mg). In ischemic heart and hypoxic neonatal cardiomyocytes, PLC-delta1, but not PLC-beta1 and PLC-gamma1, was selectively degraded, a response that could be inhibited by the calpain inhibitor, calpastatin, and by the caspase inhibitor, zVAD-fmk. Overexpression of the PLC-delta1 in hypoxic neonatal cardiomyocytes rescued intracellular Ca2+ overload by ischemic conditions. In the border zone and scar region of infarcted myocardium, and in hypoxic neonatal cardiomyocytes, the selective degradation of PLC-delta1 by the calcium sensitive proteases may play important roles in intracellular Ca2+ regulations under the ischemic conditions. It is suggested that PLC isozyme-changes may contribute to the alterations in calcium homeostasis in myocardial ischemia.

c-Fos phosphorylation induced by H2O2 prevents proteasomal degradation of c-Fos in cardiomyocytes

Oxidants cause activation of the AP-1 transcription factor in cardiomyocytes. c-Fos, a component of the AP-1 transcription factor, is transiently induced by H2O2 and the induction is sensitive to the protein synthesis inhibitor cycloheximide. With high percentage gel electrophoresis, multiple c-Fos bands were resolved by Western blot analyses, indicating post-translational modification of newly synthesized c-Fos protein after H2O2 exposure. Treatment of immunoprecipitated c-Fos protein with the type 2 serine/threonine phosphatase A (PP2A) and immunoblotting of c-Fos protein with antibodies against phosphorylated serine or threonine demonstrated that c-Fos was phosphorylated at serine residues. A pharmacological inhibitor of JNKs inhibited the formation of multiple c-Fos bands without affecting c-fos transcription. The proteasomal inhibitor MG132 and Proteasome Inhibitor I extended the time course of c-Fos protein elevation. An increase in ubiquitin was detectable in c-Fos protein from H2O2-treated cells. Interestingly, treating the whole cell lysates with PP2A, but not calcineurin (i.e. PP2B), resulted in disappearance of c-Fos protein and MG132 was able to prevent this loss. H2O2 caused an elevation of PP2B and total phosphatase activity. The phosphatase inhibitor okadaic acid, but not PP2B inhibiter cypermethrin, extended the time course of c-Fos protein elevation after H2O2 exposure. These data suggest that JNK-mediated phosphorylation of newly synthesized c-Fos protects the protein from being degraded by the proteasome. PP2B independent dephosphorylation contributes to degradation of c-Fos protein during oxidative stress response of cardiomyocytes.

UTP transactivates epidermal growth factor receptors and promotes cardiomyocyte hypertrophy despite inhibiting transcription of the hypertrophic marker gene, atrial natriuretic peptide

In neonatal rat ventricular myocytes, activation of receptors that couple to the G(q) family of heterotrimeric G proteins causes hypertrophic growth, together with expression of "hypertrophic marker" genes, such as atrial natriuretic peptide (ANP) and myosin light chain 2 (MLC2). As reported previously for other G(q)-coupled receptors, stimulation of alpha(1)-adrenergic receptors with phenylephrine (50 microM) caused phosphorylation of epidermal growth factor (EGF) receptors as well as activation of ERK1/2, cellular growth, and ANP transcription. These responses depended on EGF receptor activation. In marked contrast, stimulation of G(q)-coupled purinergic receptors with UTP caused EGF receptor phosphorylation, ERK1/2 activation, and cellular growth but minimal increases in ANP transcription. UTP inhibited phenylephrine-dependent transcription from ANP and MLC2 promoters but not transcription from myoglobin promoters or from AP-1 elements. Myocardin is a muscle-specific transcription enhancer that activates transcription from ANP and MLC2 promoters but not myoglobin promoters or AP-1 elements. UTP inhibited ANP and MLC2 responses to overexpressed myocardin but did not inhibit responses to c-Jun, GATA4, or serum response factor, all of which are active in nonmuscle cells. Thus, UTP inhibits transcriptional responses to phenylephrine only at cardiac-specific promoters, and this may involve the muscle-specific transcription enhancer, myocardin. These studies show that EGF receptor activation is necessary but not sufficient for ANP and MLC2 responses to activation of G(q)-coupled receptors in ventricular myocytes, because inhibitory mechanisms can oppose such stimulation. ANP is a compensatory and protective factor in cardiac hypertrophy, and mechanisms that reduce its generation need to be defined.

Initiation and transduction of stretch-induced RhoA and Rac1 activation through caveolae: cytoskeletal regulation of ERK translocation

The Rho family small GTPases play a crucial role in mediating cellular responses to stretch. However, it remains unclear how force is transduced to Rho signaling pathways. We investigated the effect of stretch on the activation and caveolar localization of RhoA and Rac1 in neonatal rat cardiomyocytes. In unstretched cardiomyocytes, RhoA and Rac1 were detected in both caveolar and non-caveolar fractions as assessed using detergent-free floatation analysis. Stretching myocytes for 4 min activated RhoA and Rac1. By 15 min of stretch, RhoA and Rac1 had dissociated from caveolae, and there was decreased coprecipitation of RhoA and Rac1 with caveolin-3. To determine whether compartmentation of RhoA and Rac1 within caveolae was necessary for stretch signaling, we disrupted caveolae with methyl beta-cyclodextrin (MbetaCD). Treatment with 5 mm MbetaCD for 1 h dissociated both RhoA and Rac1 from caveolae. Under this condition, stretch failed to activate RhoA or Rac1. Stretch-induced actin cytoskeletal organization was concomitantly impaired. Interestingly the ability of stretch to activate extracellular signal-regulated kinase (ERK) was unaffected by MbetaCD treatment, but ERK translocation to the nucleus was impaired. Stretch-induced hypertrophy was also inhibited. Actin cytoskeletal disruption with cytochalasin-D also prevented stretch from increasing nuclear ERK, whereas actin polymerization with jasplakinolide restored nuclear translocation of activated ERK in the presence of MbetaCD. We suggest that activation of RhoA or Rac1, localized in a caveolar compartment, is essential for sensing externally applied force and transducing this signal to the actin cytoskeleton and ERK translocation.

Direct actions of urotensin II on the heart: implications for cardiac fibrosis and hypertrophy

Urotensin II (UII) is a somatostatin-like peptide recently identified as a potent vasoconstrictor. In this study, we examined whether UII promotes cardiac remodeling through nonhemodynamic effects on the myocardium. In a rat model of heart failure after myocardial infarction (MI), increased UII peptide and UII receptor protein expression was observed in both infarct and noninfarct regions of the left ventricle compared with sham. Moreover, post-MI remodeling was associated with a significant 75% increase in UII receptor gene expression in the heart (P<0.05 versus sham controls), with this increase noted in both regions of the left ventricle. In vitro, UII (10-7 mol/L) stimulation of neonatal cardiac fibroblasts increased the level of mRNA transcripts for procollagens alpha1(I), alpha1(III), and fibronectin by 139+/-15% (P<0.01), 59+/-5% (P<0.05), and 141+/-14% (P<0.01), respectively, with a concomitant 23+/-2% increase in collagen peptide synthesis as determined by 3H-proline incorporation (P<0.01). UII had no effect on cellular hypertrophy, as determined by changes in total protein content in isolated neonatal cardiomyocytes. However, expression of recombinant rat UII receptor in neonatal cardiomyocytes resulted in significant UII-dependent activation of hypertrophic signaling as demonstrated by increased total protein content (unstimulated, 122.4+/-4.0 microg/well; rat UII, 147.6+/-7.0 microg/well; P<0.01) and activation of the hypertrophic phenotype through Galpha(q)- and Ras-dependent pathways. These results indicate that, in addition to potent hemodynamic effects, UII may be implicated in myocardial fibrogenesis through increased collagen synthesis by cardiac fibroblasts and may also be an important determinant of pathological cardiac hypertrophy in conditions characterized by UII receptor upregulation.

PKC isozyme selective regulation of cloned human cardiac delayed slow rectifier K current

Delayed rectifying K(+) channel, I(Ks), plays a vital role in normal and arrhythmogenic heart. I(Ks) is modulated by PKC but the identity of which PKC isozymes is involved in this modulation is not known. To dissect the role of individual PKC isozymes in the regulation of I(Ks), human cardiac I(Ks) channel (minK+KvLQT1) was expressed in Xenopus oocytes. Peptide PKC isozyme-specific activator and inhibitors, in addition to the general PKC activator, PMA, were used. Whole-cell I(Ks) was recorded using two-electrode voltage clamp technique. PMA and epsilon PKC specific activator peptide, but not the inactive analog, 4alphaPDD, significantly increased I(Ks). Peptide specific inhibitors for beta(II)PKC, and a general PKC inhibitor, calphostin C antagonized PMA-induced activation of I(Ks). However, control peptide, pentalysine, and specific inhibitor peptide for alphaPKC, beta(I)PKC, deltaPKC, or etaPKC did not alter PMA effect on I(Ks). The present study demonstrates that beta(II)PKC, epsilon PKC but not beta(I)PKC, alphaPKC, deltaPKC, and etaPKC, are involved in PMA-induced activation of the cloned human I(Ks) expressed in Xenopus oocyte. Furthermore, this is the first report to dissect the fine functional role of beta(II)PKC and beta(I)PKC in the regulation of I(Ks). Identification of the particular isozyme(s) that mediates the regulation of I(Ks) channels is of importance for the understanding of the mechanism of ion channel regulation and the development of new therapeutic agents.

Cocaine produces cardiac hypertrophy by protein kinase C dependent mechanisms

BACKGROUND

Chronic cocaine users can have as much as a 69% increase in left ventricular muscle mass without associated increases in arterial blood pressure, heart rate, renin, aldosterone, or cortisol. We determined whether cocaine directly increases cardiomyocyte protein content and whether protein kinase C is important in this process.

METHODS AND RESULTS

Adult rat cardiomyocytes were isolated and grown in cultures. In Series I experiments, cocaine, 10(-8) to 10(-6) M, or vehicle, in the absence or presence of phentolamine or metoprolol, was added to each culture and the cells were subsequently harvested. In Series II, cocaine, 10(-6) M, cocaine, 10(-6) M, plus bisindolylmaleimide, 10(-6) M, a protein kinase C inhibitor, or vehicle were added to each culture and the cells subsequently harvested. We determined the total protein content, the content of alpha-myosin and fetal beta-myosin heavy-chain protein, and the presence of protein kinase C isoforms in the cardiomyocyte soluble and particulate fractions. Protein kinase C translocation from the soluble to particulate fraction is indicative of activation. In Series III, we determined the cocaine effects on ERK, SAPK/JNK, and p38. In Series I, cocaine, 10(-8) to 10(-6) M, dose-dependently increased myocyte protein content by as much as 28%+/-2% (P<.001) and fetal beta-myosin heavy-chain protein content by 80%+/-2% (P<.001). Neither phentolamine nor metoprolol inhibited this process. In Series II, we determined that ventricular myocytes contain alpha (alpha), beta (beta), delta (delta), epsilon (epsilon), and zeta (zeta) protein kinase C isoforms. Cocaine, 10(-6) M, caused a 45+/-5% increase (P<.001) in protein kinase Calpha in the particulate fraction. The addition of a protein kinase C inhibitor to the myocyte cultures prevented the cocaine-induced translocation of protein kinase Calpha and limited the increase in beta-myosin heavy-chain protein content by >75% (P<.001). However, cocaine did not increase the phosphorylation of ERK, SAPK/JNK or p38 in Series III.

CONCLUSIONS

Cocaine increases adult cardiomyocyte protein content by protein kinase Calpha-dependent mechanisms, and this process can contribute to the cardiac hypertrophy and cardiomyopathy that results from chronic cocaine use.

Regulation of the cardiac mitochondrial membrane potential by retinoids

Cardiomyocytes suffering irreversible damage under oxidative stress during ischemia activate their suicide program. Mitochondria play a key role in this process, while they themselves are subject to regulation by a number of signaling pathways. We demonstrate here that retinoids influence mitochondrial function in cardiomyocytes. Depending on their chemical nature, retinoids can either ameliorate or exacerbate stress-related damage. Thus, vitamin A, retinol, was protective because retinol deprivation enhanced oxidative damage, as indicated by rapid loss of mitochondrial membrane potential. Supplementation with a physiological concentration of retinol reversed this effect. Anhydroretinol (AR), a known antagonist, which works by displacing retinol from the common binding sites on serine/threonine kinases, also caused mitochondrial membrane depolarization. The AR effect was both Ca(2+)-dependent and cyclosporin-sensitive, suggesting an upstream signaling mechanism rather than direct membrane effect. Our results agree with a model where retinol supports mitochondrial integrity by enabling upstream signaling processes. The consequences of disrupting these processes by AR are opening of the permeability transition pore, release of cytochrome c, and activation of the suicide program.

Peroxisome proliferator-activated receptor (PPAR) alpha and PPARbeta/delta, but not PPARgamma, modulate the expression of genes involved in cardiac lipid metabolism

Long-chain fatty acids (FA) coordinately induce the expression of a panel of genes involved in cellular FA metabolism in cardiac muscle cells, thereby promoting their own metabolism. These effects are likely to be mediated by peroxisome proliferator-activated receptors (PPARs). Whereas the significance of PPARalpha in FA-mediated expression has been demonstrated, the role of the PPARbeta/delta and PPARgamma isoforms in cardiac lipid metabolism is unknown. To explore the involvement of each of the PPAR isoforms, neonatal rat cardiomyocytes were exposed to FA or to ligands specific for either PPARalpha (Wy-14,643), PPARbeta/delta (L-165041, GW501516), or PPARgamma (ciglitazone and rosiglitazone). Their effect on FA oxidation rate, expression of metabolic genes, and muscle-type carnitine palmitoyltransferase-1 (MCPT-1) promoter activity was determined. Consistent with the PPAR isoform expression pattern, the FA oxidation rate increased in cardiomyocytes exposed to PPARalpha and PPARbeta/delta ligands, but not to PPARgamma ligands. Likewise, the FA-mediated expression of FA-handling proteins was mimicked by PPARalpha and PPARbeta/delta, but not by PPARgamma ligands. As expected, in embryonic rat heart-derived H9c2 cells, which only express PPARbeta/delta, the FA-induced expression of genes was mimicked by the PPARbeta/delta ligand only, indicating that FA also act as ligands for the PPARbeta/delta isoform. In cardiomyocytes, MCPT-1 promoter activity was unresponsive to PPARgamma ligands. However, addition of PPARalpha and PPARbeta/delta ligands dose-dependently induced promoter activity. Collectively, the present findings demonstrate that, next to PPARalpha, PPARbeta/delta, but not PPARgamma, plays a prominent role in the regulation of cardiac lipid metabolism, thereby warranting further research into the role of PPARbeta/delta in cardiac disease.

An epsilonPKC-selective inhibitor attenuates back phosphorylation of a low molecular weight protein in cardiac myocytes

We have studied epsilon PKC-mediated phosphorylation events in neonatal cardiac myocytes using back phosphorylation. 3 nM 4-beta 12-myristate-13-acetate (PMA)-intact cell treatment preferentially activates epsilon PKC in these cells (Circ. Res. 76 (1995) 654) and caused decreased 32P incorporation (back phosphorylation) into an approximately 18-kDa protein. This response required physiological levels of free Mg(2+) and short (3-5 min) incubation periods in back phosphorylation assays. Introduction of a selective epsilon PKC translocation inhibitor (epsilon V1) into these cells attenuated the 3 nM PMA-induced back phosphorylation response while translocation inhibitors to the classical PKC or deltaPKC isozymes were without effect. Pretreatment of our cells with endothelin-1 (ET1) had similar effects to 3 nM PMA albeit the magnitude of the ET1 back phosphorylation response was about one-half that of 3 nM PMA. Our results suggest that epsilon PKC phosphorylates an approximately 18-kDa protein found in the particulate cell fraction of neonatal cardiac myocytes. Epsilon PKC modulates diverse cardiac responses including contraction, ion channel functions, hypertrophy, and ischemic preconditioning. Characterization of epsilon PKC-selective phosphotransferase events may reveal novel regulatory mechanisms for this enzyme in neonatal cardiac myocytes.

Engineered calmodulins reveal the unexpected eminence of Ca2+ channel inactivation in controlling heart excitation

Engineered calmodulins (CaMs), rendered Ca2+-insensitive by mutations, function as dominant negatives in heterologous systems, and have revealed mechanisms of ion channel modulation by Ca2+/CaM. The use of these CaMs in native mammalian cells now emerges as a strategy to unmask the biology of such Ca2+ feedback. Here, we developed recombinant adenoviruses bearing engineered CaMs to facilitate their expression in adult heart cells, where Ca2+ regulation may be essential for moment-to-moment control of the heartbeat. Engineered CaMs not only eliminated the Ca2+-dependent inactivation of native calcium channels, but exposed an unexpectedly large impact of removing such feedback: the unprecedented (4- to 5-fold) prolongation of action potentials. This striking result recasts the basic paradigm for action-potential control and illustrates the promise of virally delivered engineered CaM to investigate the biology of numerous other CaM-signaling pathways.

Identification of PKCalpha isoform-specific effects in cardiac myocytes using antisense phosphorothioate oligonucleotides

Members of the mammalian protein kinase C (PKC) superfamily play key regulatory roles in multiple cellular processes. In the heart, PKC signaling is involved in hypertrophic agonist-induced gene expression and hypertrophic growth. To investigate the specific function of PKC signaling in regulating cardiomyocyte growth, we used antisense oligonucleotides to inhibit PKC alpha, the major isozyme present in the neonatal heart. Transfection of cultured neonatal cardiomyocytes with antisense PKCalpha oligonucleotides resulted in a marked reduction in both PKCalpha mRNA and protein levels. PKCalpha antisense treatment also reduced phenylephrine (PE)-induced PKC activity and perinuclear translocation of PKCalpha. Antisense inhibition of PKCalpha led to reduction of PE-induced increase in skeletal alpha-actin mRNA levels and atrial natriuretic peptide (ANP) secretion but had no significant effects on PE-induced beta-myosin heavy chain, ANP, or B-type natriuretic peptide (BNP) gene expression. On the other hand, antisense PKCalpha treatment attenuated endothelin-1-induced increase in ANP and BNP peptide secretion, whereas endothelin-1-induced gene expression of ANP and BNP remained unchanged. The hypertrophic agonist-induced growth of cardiomyocytes, characterized by increased [(3)H]leucine incorporation, was not affected with antisense PKCalpha treatment. Furthermore, we found that PE-induced increase in extracellular signal-regulated kinase (ERK) activity was partially inhibited by antisense PKCalpha treatment, implicating ERK as a downstream mediator for PKCalpha signaling. These results indicate that PKCalpha isozyme is involved in hypertrophic signaling in cardiomyocytes and provide novel strategies for future studies to identify other cellular targets controlled selectively by PKCalpha or other PKC isozymes.

Prevention of hypertrophy by overexpression of Kv4.2 in cultured neonatal cardiomyocytes

BACKGROUND

Prolonged action potentials (APs) and decreased transient outward K+ currents (I(to)) are consistent findings in hypertrophic myocardium. However, the connection of these changes with cardiac hypertrophy is unknown. The present study investigated the effects of changes in I(to) and the associated alterations in AP on myocyte hypertrophy induced by phenylephrine.

METHODS AND RESULTS

Chronic incubation of cultured neonatal ventricular rat myocytes (NVRMs) with phenylephrine (PE) reduced I(to) density and prolonged AP duration, leading to a 2-fold increase in the net Ca2+ influx per beat and a 1.4-fold increase in Ca2+-transient amplitude. PE treatment of chronically paced (2-Hz) NVRM also induced increases in cell size, protein/DNA ratio, atrial natriuretic factor mRNA expression, as well as beta/alpha myosin mRNA ratio. These hypertrophic changes were associated with a 2.4-fold increase in activation of nuclear factor of activated T-cells (NFAT), indicating increased activity of the Ca2+-dependent phosphatase calcineurin. Overexpression of Kv4.2 channels using adenovirus prevented the AP duration prolongation as well as the increases in Ca2+ influx and Ca2+-transient amplitude induced by PE. Kv4.2 overexpression also prohibited the PE-induced increases in cell size, protein/DNA ratio, atrial natriuretic factor expression, beta/alpha myosin mRNA ratio, and NFAT activation.

CONCLUSIONS

Our results demonstrate that PE-mediated hypertrophy in NRVMs seems to require I(to) reductions and AP prolongation associated with increased Ca2+ influx and Ca2+ transients as well as calcineurin activation. The clinical implications of these studies and the possible involvement of other signaling pathways are discussed.

Inositol polyphosphate 1-phosphatase is a novel antihypertrophic factor

Activation of G(q)-coupled alpha(1)-adrenergic receptors leads to hypertrophic growth of neonatal rat ventricular cardiomyocytes that is associated with increased expression of hypertrophy-related genes, including atrial natriuretic peptide (ANP) and myosin light chain-2 (MLC), as well as increased ribosome synthesis. The role of inositol phosphates in signaling pathways involved in these changes in gene expression was examined by overexpressing inositol phosphate-metabolizing enzymes and determining effects on ANP, MLC, and 45 S ribosomal gene expression following co-transfection of appropriate reporter gene constructs. Overexpression of enzymes that metabolize inositol 1,4,5-trisphosphate did not reduce ANP or MLC responses, but overexpression of the enzyme primarily responsible for metabolism of inositol 4,5-bisphosphate (Ins(1,4)P(2)), inositol polyphosphate 1-phosphatase (INPP), reduced ANP and MLC responses associated with alpha(1)-adrenergic receptor-mediated hypertrophy. Similarly overexpressed INPP reduced ANP and MLC responses associated with contraction-induced hypertrophy. In addition, overexpression of INPP reduced the increase in ribosomal DNA transcription associated with both hypertrophic models. Hypertrophied cells from both cell models as well as ventricular tissue from mouse hearts hypertrophied by pressure overload in vivo contained heightened levels of Ins(1,4)P(2), suggesting reduced INPP activity in three different models of hypertrophy. These studies provide evidence for an involvement of Ins(1,4)P(2) in hypertrophic signaling pathways in ventricular myocytes.

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

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

Neuropeptide Y Y(1) receptor regulates protein turnover and constitutive gene expression in hypertrophying cardiomyocytes

Increased levels of neuropeptide Y correlate with severity of left ventricular hypertrophy in vivo. At cardiomyocyte level, hypertrophy is characterised by increased mass and altered phenotype. The aims were to determine the contributions of increased synthesis and reduced degradation of protein to neuropeptide Y-mediated increase in mass, assess effects on gene expression, and characterise neuropeptide Y Y receptor subtype involvement. Neuropeptide Y (10 nM) increased protein mass of adult rat ventricular cardiomyocytes maintained in culture (24 h) (16%>basal) and de novo protein synthesis (incorporation of [(14)C]phenylalanine) (18%>basal). Neuropeptide Y (100 nM) prevented degradation of existing protein at 8 h. Actinomycin D (5 microM) attenuated increases in protein mass to neuropeptide Y (< or = 1 nM) but not to neuropeptide Y (10 nM). [Leu(31), Pro(34)]neuropeptide Y (10 nM), an agonist at neuropeptide Y Y(1) receptors, increased protein mass (25%>basal) but did not stimulate protein synthesis. Neuropeptide Y-(3-36) (10 nM), an agonist at neuropeptide Y Y(2) receptors, increased protein mass (29%>basal) and increased protein synthesis (13%>basal), respectively. Actinomycin D (5 microM) abolished the increase in protein mass elicited by neuropeptide Y-(3-36) but not that by [Leu(31), Pro(34)]neuropeptide Y. BIBP3226 [(R)-N2-(diphenylacetyl)-N-(4-hydroxyphenylmethyl)-D-arginine amide] (1 microM), a neuropeptide Y Y(1) receptor subtype-selective antagonist, and T(4) [neuropeptide Y-(33-36)](4), a neuropeptide Y Y(2) receptor subtype-selective antagonist, attenuated the increase in protein mass to 100 nM neuropeptide Y by 68% and 59%, respectively. Neuropeptide Y increased expression of the constitutive gene, myosin light chain-2 (MLC-2), maximally at 12 h (4.7-fold>basal) but did not induce (t< or = 36 h) expression of foetal genes (atrial natriuretic peptide (ANP), skeletal-alpha-actin and myosin heavy chain-beta). This increase was attenuated by 86% and 51%, respectively, by BIBP3226 (1 microM) and T(4) [neuropeptide Y-(33-36)](4) (100 nM). [Leu(31), Pro(34)]neuropeptide Y (100 nM) (2.4-fold>basal) and peptide YY-(3-36) (100 nM) (2.3 fold>basal) increased expression of MLC-2 mRNA at 12 h. In conclusion, initiation of cardiomyocyte hypertrophy by neuropeptide Y requires activation of both neuropeptide Y Y(1) and neuropeptide Y Y(2) receptors and is associated with enhanced synthesis and attenuated degradation of protein together with increased expression of constitutive genes but not reinduction of foetal genes.

Statins as antioxidant therapy for preventing cardiac myocyte hypertrophy

Cardiac hypertrophy is a major cause of morbidity and mortality worldwide. The hypertrophic process is mediated, in part, by small G proteins of the Rho family. We hypothesized that statins, inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase, inhibit cardiac hypertrophy by blocking Rho isoprenylation. We treated neonatal rat cardiac myocytes with angiotensin II (AngII) with and without simvastatin (Sim) and found that Sim decreased AngII-induced protein content, [3H] leucine uptake, and atrial natriuretic factor (ANF) promoter activity. These effects were associated with decreases in cell size, membrane Rho activity, superoxide anion (O2*-) production, and intracellular oxidation, and were reversed with L-mevalonate or geranylgeranylpyrophosphate, but not with farnesylpyrophosphate or cholesterol. Treatments with the Rho inhibitor C3 exotoxin and with cell-permeable superoxide dismutase also decreased AngII-induced O2*- production and myocyte hypertrophy. Overexpression of the dominant-negative Rho mutant N17Rac1 completely inhibited AngII-induced intracellular oxidation and ANF promoter activity, while N19RhoA partially inhibited it, and N17Cdc42 had no effect. Indeed, Sim inhibited cardiac hypertrophy and decreased myocardial Rac1 activity and O2*- production in rats treated with AngII infusion or subjected to transaortic constriction. These findings suggest that statins prevent the development of cardiac hypertrophy through an antioxidant mechanism involving inhibition of Rac1.

Evidence for functional role of epsilonPKC isozyme in the regulation of cardiac Na(+) channels

Investigation of the role of individual protein kinase C (PKC) isozymes in the regulation of Na(+) channels has been largely limited by the lack of isozyme-selective modulators. Here we used a novel peptide-specific activator (epsilonV1-7) of epsilonPKC and other peptide isozyme-specific inhibitors in addition to the general PKC activator phorbol 12-myristate 13-acetate (PMA) to dissect the role of individual PKCs in the regulation of the human cardiac Na(+) channel hH1, heterologously expressed in Xenopus oocytes. Peptides were injected individually or in combination into the oocyte. Whole cell Na(+) current (I(Na)) was recorded using two-electrode voltage clamp. epsilonV1-7 (100 nM) and PMA (100 nM) inhibited I(Na) by 31 +/- 5% and 44 +/- 8% (at -20 mV), respectively. These effects were not seen with the scrambled peptide for epsilonV1-7 (100 nM) or the PMA analog 4alpha-phorbol 12,13-didecanoate (100 nM). However, epsilonV1-7- and PMA-induced I(Na) inhibition was abolished by epsilonV1-2, a peptide-specific antagonist of epsilonPKC. Furthermore, PMA-induced I(Na) inhibition was not altered by 100 nM peptide-specific inhibitors for alpha-, beta-, delta-, or etaPKC. PMA and epsilonV1-7 induced translocation of epsilonPKC from soluble to particulate fraction in Xenopus oocytes. This translocation was antagonized by epsilonV1-2. In native rat ventricular myocytes, PMA and epsilonV1-7 also inhibited I(Na); this inhibition was antagonized by epsilonV1-2. In conclusion, the results provide evidence for selective regulation of cardiac Na(+) channels by epsilonPKC isozyme.

The signal transduction pathway causing the synergistic hypertrophic effects of neuropeptide Y and norepinephrine on primary cardiomyocyte

To investigate the synergistic hypertrophic effects of neuropeptide Y (NPY) and norepinephrine (NE) and its possible signal transduction pathway on primary cardiomyocytes, neonatal cardiomyocytes were exposed to NPY, NE or angiotensin II (AnII). All three agonists induced hypertrophic effects, stimulated protein kinase C (PKC) and activated mitogen-activated protein kinase (MAPK). Moreover, NPY at sub-optimal concentration potentiated NE-, not AnII-, induced all of the above effects. Pretreatment with phorbol 12-myristate 13-acetate (PMA) completely abolished these effects for both NE and NPY. NPY potentiation was calcium-independent and pertussis toxin (PTX)-sensitive, and was different from NPY direct hypertrophic effect on cardiomyocyte, as PTX only partially abolished NPY-induced hypertrophic effects. Taken together, NPY participated in the development of cardiac hypertrophy by potentiating NE effects. The signal pathway involves PTX-sensitive G protein, PKC, MAPK, and does not require the presence of calcium.

Binding specificity for RACK1 resides in the V5 region of beta II protein kinase C

Identification of selective anchoring proteins responsible for specialized localization of specific signaling proteins has led to the identification of new inhibitors of signal transduction, inhibitors of anchoring protein-ligand interactions. RACK1, the first receptor for activated C kinase identified in our lab, is a selective anchoring protein for betaII protein kinase C (betaIIPKC). We previously found that at least part of the RACK1-binding site resides in the C2 domain of betaIIPKC (Ron, D., Luo, J., and Mochly-Rosen, D. (1995) J. Biol. Chem. 270, 24180-24187). Here we show that the V5 domain also contains part of the RACK1-binding site in betaIIPKC. In neonatal rat cardiac myocytes, the betaIIV5-3 peptide (amino acids 645-650 in betaIIPKC) selectively inhibited phorbol 12-myristate 13-acetate (PMA)-induced translocation of betaIIPKC and not betaIPKC. In addition, the betaIIV5-3 peptide inhibited cardiac myocyte hypertrophy in PMA-treated cells. Interestingly, betaIV5-3 (646-651 in betaIPKC), a selective translocation inhibitor of betaIPKC, also inhibited PMA-induced cardiac myocyte hypertrophy, demonstrating that both betaI- and betaIIPKC are essential for this cardiac function. Therefore, the betaIIV5 domain contains part of the RACK1-binding site in betaIIPKC; a peptide corresponding to this site is a selective inhibitor of betaIIPKC and, hence, enables the identification of betaIIPKC-selective functions.

G-proteins in growth and apoptosis: lessons from the heart

The acute contractile function of the heart is controlled by the effects of released nonepinephrine (NE) on cardiac adrenergic receptors. NE can also act in a more chronic fashion to induce cardiomyocyte growth, characterized by cell enlargement (hypertrophy), increased protein synthesis, alterations in gene expression and addition of sarcomeres. These responses enhance cardiomyocyte contractile function and thus allow the heart to compensate for increased stress. The hypertrophic effects of NE are mediated through Gq-coupled alpha(1)-adrenergic receptors and are mimicked by the actions of other neurohormones (endothelin, prostaglandin F(2alpha) angiotensin II) that also act on Gq-coupled receptors. Activation of phospholipase C by Gq is necessary for these responses, and protein kinase C and MAP kinases have also been implicated. Gq stimulated cardiac hypertrophy is also evident in transgenic mouse models. In contrast, stimulation of G(s)-coupled beta-adrenergic receptors or G(i)-coupled receptors do not directly effect cardiomyocyte hypertrophy. Apoptosis is also induced by G-protein-coupled receptor stimulation in cardiomyocytes. Sustained or excessive activation of either Gq- or Gs-signaling pathways results in apoptotic loss of cardiomyocytes both in vitro and in vivo. Apoptosis is associated with decreased ventricular function in the failing heart. Cardiomyocytes provide an ideal model system for understanding the basis for G-protein mediated hypertrophy and apoptosis, and the mechanisms responsible for the transition from compensatory to deleterious levels of signaling. This information may prove critical for designing interventions that prevent the pathophysiological consequences of heart failure.

Role of protein kinase C-epsilon in hypertrophy of cultured neonatal rat ventricular myocytes

Using adenovirus (Adv)-mediated overexpression of constitutively active (ca) and dominant-negative (dn) mutants, we examined whether protein kinase C (PKC)-epsilon, the major novel PKC isoenzyme expressed in the adult heart, was necessary and/or sufficient to induce specific aspects of the hypertrophic phenotype in low-density, neonatal rat ventricular myocytes (NRVM) in serum-free culture. Adv-caPKC-epsilon did not increase cell surface area or the total protein-to-DNA ratio. However, cell shape was markedly affected, as evidenced by a 67% increase in the cell length-to-width ratio and a 17% increase in the perimeter-to-area ratio. Adv-caPKC-epsilon also increased atrial natriuretic factor (ANF) and beta-myosin heavy chain (MHC) mRNA levels 2.5 +/- 0.3- and 2.1 +/- 0.2-fold, respectively, compared with NRVM infected with an empty, parent vector (P < 0.05 for both). Conversely, Adv-dnPKC-epsilon did not block endothelin-induced increases in cell surface area, the total protein-to-DNA ratio, or upregulation of beta-MHC and ANF gene expression. However, the dominant-negative inhibitor markedly suppressed endothelin-induced extracellular signal-regulated kinase (ERK) 1/2 activation. Taken together, these results indicate that caPKC-epsilon overexpression alters cell geometry, producing cellular elongation and remodeling without a significant, overall increase in cell surface area or total protein accumulation. Furthermore, PKC-epsilon activation and downstream signaling via the ERK cascade may not be necessary for cell growth, protein accumulation, and gene expression changes induced by endothelin.

Modulation of MLC-2v gene expression by AP-1: complex regulatory role of Jun in cardiac myocytes

Hypertrophic stimulation of cardiac myocytes results in rapid induction of a number of transcription factors, including members of the AP-1 family, which is followed by a programmed alteration in the pattern of gene expression. In the ventricular cardiocytes there is re-expression of the fetal atrial natriuretic factor (ANF) gene and upregulation of its myosin light chain-2 (MLC-2v). The mechanism(s) by which the induction ofAP-1 is coupled to the promoters of these target genes is largely unknown. In this report, we demonstrate that in transient co-transfection assay, c-Jun inhibited while Jun B stimulated the MLC-2v promoter activity. Mutant c-Jun recombinants, in which the activation domains were deleted, still remained inhibitory, but a specific mutation in the leucine zipper, which changes the alignment of Jun with its dimerization partner, caused a reversal of its effect on the target MLC-2v promoter. Based on these findings, we propose that in chicken cardiac myocytes, the regulation of MLC-2v promoter by Jun may occur via its interaction with other proteins, possibly of the leucine zipper family.

Reactive oxygen species mediate alpha-adrenergic receptor-stimulated hypertrophy in adult rat ventricular myocytes

Norepinephrine (NE) causes hypertrophic growth of cardiac myocytes via stimulation of alpha1-adrenergic receptors (alpha1-AR). Reactive oxygen species (ROS) can act as signaling molecules for cell growth. Accordingly, we tested the hypothesis that ROS mediate alpha1-AR-stimulated hypertrophic growth in adult rat ventricular myocytes (ARVM). NE increased the level of intracellular ROS as assessed by lucigenin chemiluminescence or cytochrome c reduction, and this effect was prevented by the superoxide dismutase (SOD)-mimetic MnTMPyP. NE also caused the induction of MnSOD mRNA. alpha1-AR stimulation with NE (1 microM) in the presence of propranolol (2 microM) for 48-96 h caused a hypertrophic growth phenotype characterized by a 36+/-3% increase in 3H-leucine incorporation, a 49+/-14% increase in protein accumulation, a six-fold induction of atrial natriuretic peptide mRNA, actin filament reorganization, and the induction of MnSOD mRNA. These responses were all prevented by pretreatment with the alpha1-AR-selective antagonist prazosin (100 n M) or the SOD-mimetics MnTMPyP (50 microM) and Euk-8 (100 microM). MnTMPyP had no effect on alpha1-AR-stimulated 3H-inositol phosphate turnover or the hypertrophic phenotype caused by the protein kinase C activator phorbol-12-myristate-13-acetate. Thus, ROS play a critical role in mediating the hypertrophic growth response to alpha1-AR-stimulation in ARVM.

Chronic elevation of calmodulin in the ventricles of transgenic mice increases the autonomous activity of calmodulin-dependent protein kinase II, which regulates atrial natriuretic factor gene expression

Although isoforms of Ca2+/calmodulin-dependent protein kinase II (CaMKII) have been implicated in the regulation of gene expression in cultured cells, this issue has yet to be addressed in vivo. We report that the overexpression of calmodulin in ventricular myocytes of transgenic mice results in an increase in the Ca2+/calmodulin-independent activity of endogenous CaMKII. The calmodulin transgene is regulated by a 500-bp fragment of the atrial natriuretic factor (ANF) gene promoter which, based on cell transfection studies, is itself known to be regulated by CaMKII. The increased autonomous activity of CaMKII maintains the activity of the transgene and establishes a positive feed-forward loop, which also extends the temporal expression of the endogenous ANF promoter in ventricular myocytes. Both the increased activity of CaMKII and transcriptional activation of ANF are highly selective responses to the chronic overexpression of calmodulin. These results indicate that CaMKII can regulate gene expression in vivo and suggest that this enzyme may represent the Ca2+-dependent target responsible for reactivation of the ANF gene during ventricular hypertrophy.

Isoenzyme-specific protein kinase C and c-Jun N-terminal kinase activation by electrically stimulated contraction of neonatal rat ventricular myocytes

Previous studies from our laboratory and others indicate that contraction-induced mechanical loading of cultured neonatal rat ventricular myocytes produces many of the phenotypic changes associated with cardiomyocyte hypertrophy in vivo, and that these changes occur via the activation of serine-threonine protein kinases. These may include the extracellular regulated protein kinases (ERK1 and ERK2), the c-Jun N-terminal kinases (JNK1, JNK2, and JNK3), and one or more isoenzymes of protein kinase C. In this study, we assessed whether one or more of these kinases are activated by stimulated contraction, and whether activation was isoenzyme-specific. Low-density, quiescent cultures of neonatal rat ventricular myocytes were maintained in serum-free medium, or electrically stimulated to contract (3 Hz) for up to 48 h. ERK and JNK activation was assessed by Western blotting with polyclonal antibodies specific for the phosphorylated forms of both kinases. PKC activation was analysed by subcellular fractionation, detergent extraction, and Western blotting using isoenzyme-specific monoclonal antibodies. Stimulated contractile activity produced myocyte hypertrophy, as indicated by increased cell size, a 15+/-5% increase in total protein/DNA ratio, and induction of ANF and beta MHC gene transcription. Electrical pacing did not cause ERK1/2 or JNK1 activation, but increased JNK2 and JNK3 phosphorylation by;two-fold. Subcellular fractionation revealed a time-dependent increase in PKC delta, and to a much lesser extent PKC xi, in a Triton X-100-soluble membrane fraction within 5 min of the onset of stimulated contraction. PKC alpha was not activated by electrical pacing. These results indicate that contraction-induced mechanical loading acutely activates some but not all of the specific isoenzymes of JNKs and PKCs in cardiomyocytes.