The in vivo role of p38 MAP kinases in cardiac remodeling and restrictive cardiomyopathy

Complete Inhibition of Anisomycin and UV Radiation but Not Cytokine Induced JNK and p38 Activation by an Aryl-substituted Dihydropyrrolopyrazole Quinoline and Mixed Lineage Kinase 7 Small Interfering RNA

Mixed lineage kinase 7 (MLK7) is a mitogen-activated protein kinase kinase kinase (MAPKKK) that activates the pro-apoptotic signaling pathways p38 and JNK. A library of potential kinase inhibitors was screened, and a series of dihydropyrrolopyrazole quinolines was identified as highly potent inhibitors of MLK7 in vitro catalytic activity. Of this series, an aryl-substituted dihydropyrrolopyrazole quinoline (DHP-2) demonstrated an IC50 of 70 nM for inhibition of pJNK formation in COS-7 cell MLK7/JNK co-transfection assays. In stimulated cells, DHP-2 at 200 nM or MLK7 small interfering RNA completely blocked anisomycin and UV induced but had no effect on interleukin-1beta or tumor necrosis factor-alpha-induced p38 and JNK activation. Additionally, the compound blocked anisomycin and UV-induced apoptosis in COS-7 cells. Heart tissue homogenates from MLK7 transgenic mice treated with DHP-2 at 30 mg/kg had reduced JNK and p38 activation with no apparent effect on ERK activation, demonstrating that this compound can be used to block MLK7-driven MAPK pathway activation in vivo. Taken together, these data demonstrate that MLK7 is the MAPKKK required for modulation of the stress-activated MAPKs downstream of anisomycin and UV stimulation and that DHP-2 can be used to block MLK7 pathway activation in cells as well as in vivo.

Oxidant stress from nitric oxide synthase-3 uncoupling stimulates cardiac pathologic remodeling from chronic pressure load

Cardiac pressure load stimulates hypertrophy, often leading to chamber dilation and dysfunction. ROS contribute to this process. Here we show that uncoupling of nitric oxide synthase-3 (NOS3) plays a major role in pressure load-induced myocardial ROS and consequent chamber remodeling/hypertrophy. Chronic transverse aortic constriction (TAC; for 3 and 9 weeks) in control mice induced marked cardiac hypertrophy, dilation, and dysfunction. Mice lacking NOS3 displayed modest and concentric hypertrophy to TAC with preserved function. NOS3(-/-) TAC hearts developed less fibrosis, myocyte hypertrophy, and fetal gene re-expression (B-natriuretic peptide and alpha-skeletal actin). ROS, nitrotyrosine, and gelatinase (MMP-2 and MMP-9) zymogen activity markedly increased in control TAC, but not in NOS3(-/-) TAC, hearts. TAC induced NOS3 uncoupling in the heart, reflected by reduced NOS3 dimer and tetrahydrobiopterin (BH4), increased NOS3-dependent generation of ROS, and lowered Ca(2+)-dependent NOS activity. Cotreatment with BH4 prevented NOS3 uncoupling and inhibited ROS, resulting in concentric nondilated hypertrophy. Mice given the antioxidant tetrahydroneopterin as a control did not display changes in TAC response. Thus, pressure overload triggers NOS3 uncoupling as a prominent source of myocardial ROS that contribute to dilatory remodeling and cardiac dysfunction. Reversal of this process by BH4 suggests a potential treatment to ameliorate the pathophysiology of chronic pressure-induced hypertrophy.

Phosphorylation-dependent degradation of p300 by doxorubicin-activated p38 mitogen-activated protein kinase in cardiac cells

p300 and CBP are general transcriptional coactivators implicated in different cellular processes, including regulation of the cell cycle, differentiation, tumorigenesis, and apoptosis. Posttranslational modifications such as phosphorylation are predicted to select a specific function of p300/CBP in these processes; however, the identification of the kinases that regulate p300/CBP activity in response to individual stimuli and the physiological significance of p300 phosphorylation have not been elucidated. Here we demonstrate that the cardiotoxic anticancer agent doxorubicin (adriamycin) induces the phosphorylation of p300 in primary neonatal cardiomyocytes. Hyperphosphorylation precedes the degradation of p300 and parallels apoptosis in response to doxorubicin. Doxorubicin-activated p38 kinases alpha and beta associate with p300 and are implicated in the phosphorylation-mediated degradation of p300, as pharmacological blockade of p38 prevents p300 degradation. p38 phosphorylates p300 in vitro at both the N and C termini of the protein, and enforced activation of p38 by the constitutively active form of its upstream kinase (MKK6EE) triggers p300 degradation. These data support the conclusion that p38 mitogen-activated protein kinase regulates p300 protein stability and function in cardiomyocytes undergoing apoptosis in response to doxorubicin.

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.

Bigendothelin-1 via p38-MAPK-dependent mechanism regulates adult rat ventricular myocyte contractility in sepsis

We tested the hypothesis that exogenous administration of the ET-1 precursor, bigET-1, would regulate adult rat ventricular myocyte (ARVM) contractility in a p38-mitogen activated protein kinase (p38-MAPK)-dependent mechanism during sepsis. Ventricular myocytes from adult rat hearts (both sham and septic) were stimulated to contract at 0.5 Hz and mechanical properties were evaluated using an IonOptix Myocam system. Immunoblot analysis was used to determine the phosphorylation of p38-MAPK and extracellular signal-regulated kinase 1/2 (ERK1/2). ARVMs were treated with vehicle, bigET-1 and inhibitors for 24 h and then subjected to functional and biochemical estimations. Septic ARVM displayed a distorted cell membrane and irregular network within the cells along with increased cell contractility as evidenced by elevated peak shortening (PS), maximal velocity of shortening (+dL/dt) and relengthening (-dL/dt) in comparison to sham ARVM. BigET-1 treatment caused ARVM enlargement in both sham and sepsis groups. BigET-1 (100 nM) produced an increase in ARVM contractility in sham group as compared to vehicle treatment. However, septic ARVM treated with bigET-1 exhibited unaltered ARVM contractility, and upregulated ET(B) receptors as compared to respective sham group. BigET-1 increased the concentration of ET-1 and upregulated phosphorylation of p38-MAPK but not of ERK1/2 in sham and septic ARVM. Furthermore, inhibition of p38-MAPK by SB203580 (10 microM) increased ARVM contractility in sham but not in sepsis group. BigET-1 reversed SB203580-induced increase in PS in sham group but accentuated it in sepsis group. BigET-1 also reversed SB203580-induced inhibition of p38-MAPK phosphorylation in sham but not in septic ARVM. SB203580 pretreatment followed by bigET-1 administration significantly decreased p38-MAPK phosphorylation and downregulated ET(B) receptor expression as compared to bigET-1 treatment per se in sepsis group but not in sham. We concluded that a bigET-1-induced non-responsive effect on septic ARVM contractile function could be due to upregulation of p38-MAPK phosphorylation and ET(B) receptor expression.

An efficient system for tissue-specific overexpression of transgenes in podocytes in vivo

The utility of promoter fragments isolated from the 5'-flanking region of endogenous mammalian genes to drive transgene expression in vivo is often limited by low expression levels. In this study, a bigenic system was established that allows constitutive overexpression of transgenes in a tissue-specific fashion in transgenic mice in a time- and cost-effective fashion. A modified floxed expression vector was constructed [CMVflox-enhanced green fluorescent protein (eGFP)], in which a lacZ cassette (beta-galactosidase) flanked by lox sites was placed between a CMV-promoter and the transgene of interest (eGFP). Before Cre recombination, expression of eGFP was effectively prevented by the interposed floxed lacZ cassette, whereas beta-galactosidase was strongly expressed in transiently transfected cells. Transcription of the gene of interest (eGFP) could be irreversibly activated by cotransfection with Cre recombinase. Mice transgenic for CMVflox-eGFP were generated by pronuclear injection. A rapid assay was developed to identify transgenic founders with active transgene expression by measuring transgene activity (beta-galactosidase) in tail biopsies. Transgene activity in tails correlated with transgene expression in most other tissues tested including podocytes within the kidney. To activate expression of the gene of interest in a tissue-specific fashion, founder mice were mated to the Cre mouse line 2.5P-Cre previously shown to mediate 100% Cre recombination exclusively in podocytes (Moeller MJ, Sanden SK, Soofi A, Wiggins RC, and Holzman LB. Genesis 35: 39-42, 2003). In doubly transgenic offspring, high-level eGFP expression resulting from Cre excision of the interposed lacZ cassette was detected in four of seven CMVflox-eGFP founder lines. This approach should also circumvent common limitations arising from lethality or transgene silencing as a consequence of transgene overexpression.

Direct and indirect interactions between calcineurin-NFAT and MEK1-extracellular signal-regulated kinase 1/2 signaling pathways regulate cardiac gene expression and cellular growth

MEK1, a member of the mitogen-activated protein kinase (MAPK) cascade that directly activates extracellular signal-regulated kinase (ERK), induces cardiac hypertrophy in transgenic mice. Calcineurin is a calcium-regulated protein phosphatase that also functions as a positive regulator of cardiac hypertrophic growth through a direct mechanism involving activation of nuclear factor of activated T-cell (NFAT) transcription factors. Here we determined that calcineurin-NFAT and MEK1-ERK1/2 signaling pathways are interdependent in cardiomyocytes, where they directly coregulate the hypertrophic growth response. For example, genetic deletion of the calcineurin Abeta gene reduced the hypertrophic response elicited by an activated MEK1 transgene in the heart, while inhibition of calcineurin or NFAT in cultured neonatal cardiomyocytes also blunted the hypertrophic response driven by activated MEK1. Conversely, targeted inhibition of MEK1-ERK1/2 signaling in cultured cardiomyocytes attenuated the hypertrophic growth response directed by activated calcineurin. However, targeted inhibition of MEK1-ERK1/2 signaling did not directly affect calcineurin-NFAT activation, nor was MEK1-ERK1/2 activation altered by targeted inhibition of calcineurin-NFAT. Mechanistically, we show that MEK1-ERK1/2 signaling augments NFAT transcriptional activity independent of calcineurin, independent of changes in NFAT nuclear localization, and independent of alterations in NFAT transactivation potential. In contrast, MEK1-ERK1/2 signaling enhances NFAT-dependent gene expression through an indirect mechanism involving induction of cardiac AP-1 activity, which functions as a necessary NFAT-interacting partner. As a second mechanism, MEK1-ERK1/2 and calcineurin-NFAT proteins form a complex in cardiac myocytes, resulting in direct phosphorylation of NFATc3 within its C terminus. MEK1-ERK1/2-mediated phosphorylation of NFATc3 directly augmented its DNA binding activity, while inhibition of MEK1-ERK1/2 signaling reduced NFATc3 DNA binding activity. Collectively, these results indicate that calcineurin-NFAT and MEK1-ERK1/2 pathways constitute a codependent signaling module in cardiomyocytes that coordinately regulates the growth response through two distinct mechanisms.

Molecular Mechanism of Apoptosis Induced by Mechanical Forces

In all biological systems, a balance between cell proliferation/growth and death is required for normal development as well as for adaptation to a changing environment. To affect their fate, it is essential for cells to integrate signals from the environment. Recently, it has been recognized that physical forces such as stretch, strain, and tension play a critical role in regulating this process. Despite intensive investigation, the pathways by which mechanical signals are converted to biochemical responses is yet to be completely understood. In this review, we will examine our current understanding of how mechanical forces induce apoptosis in a variety of biological systems. Rather than being a degenerative event, physical forces act through specific receptor-like molecules such as integrins, focal adhesion proteins, and the cytoskeleton. These molecules in turn activate a limited number of protein kinase pathways (p38 MAPK and JNK/SAPK), which amplify the signal and activate enzymes (caspases) that promote apoptosis. Physical forces concurrently activate other signaling pathways such as PIK-3 and Erk 1/2 MAPK, which modulate the apoptotic response. The cell phenotype and the character of the physical stimuli determine which pathways are activated and, consequently, allow for variability in response to a specific stimulus in different cell types.

Antioxidative effect of p38 mitogen-activated protein kinase inhibitor in the kidney of hypertensive rat

OBJECTIVE

Nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase is regulated by angiotensin II, interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha via p38 mitogen-activated protein kinase (MAPK). We hypothesized that p38 MAPK inhibitor, FR167653, may suppress NAD(P)H oxidase and its oxygen radical production and ameliorate renal damage in Dahl salt-sensitive rats with heart failure (DSHF).

METHODS

DSHF rats were fed with 8% NaCl diet from 6 to 18 weeks old. Eleven-week-old DSHF rats received either vehicle or FR167653 (2 mg/kg per day) for 7 weeks and the renal NAD(P)H oxidase p47phox and nitric oxide synthase (NOS), superoxide production and renal damage were evaluated in comparison with the control Dahl salt-resistant rat fed with 8% NaCl diet.

RESULTS

In the kidney of DSHF rat, phosphorylated p38 MAPK was enhanced with an increased IL-1beta and TNF-alpha production compared with control rats. Treatment with FR167653 significantly suppressed p38 MAPK, IL-1beta and TNF-alpha. Renal NAD(P)H oxidase p47phox expression and superoxide production were significantly increased in the DSHF rats and treatment with FR167653 suppressed NAD(P)H oxidase expression and reduced superoxide formation. Renal endothelial and inducible NOS were reduced in DSHF rats compared with control rats, but FR167653 increased NOS and NO production in the kidney. Proteinuria, glomerulosclerosis and interstitial macrophage migration via intercellular adhesion molecule-1 (ICAM-1) were enhanced in DSHF rat and they were ameliorated by FR167653.

CONCLUSION

The inhibition of p38 MAPK by FR167653 reduced renal IL-1beta and TNF-alpha production and ameliorated renal damage in hypertensive rat via suppression of NAD(P)H oxidase and enhanced NO bioavailability.

p38alpha mitogen-activated protein kinase plays a critical role in cardiomyocyte survival but not in cardiac hypertrophic growth in response to pressure overload

The molecular mechanism for the transition from cardiac hypertrophy, an adaptive response to biomechanical stress, to heart failure is poorly understood. The mitogen-activated protein kinase p38alpha is a key component of stress response pathways in various types of cells. In this study, we attempted to explore the in vivo physiological functions of p38alpha in hearts. First, we generated mice with floxed p38alpha alleles and crossbred them with mice expressing the Cre recombinase under the control of the alpha-myosin heavy-chain promoter to obtain cardiac-specific p38alpha knockout mice. These cardiac-specific p38alpha knockout mice were born normally, developed to adulthood, were fertile, exhibited a normal life span, and displayed normal global cardiac structure and function. In response to pressure overload to the left ventricle, they developed significant levels of cardiac hypertrophy, as seen in controls, but also developed cardiac dysfunction and heart dilatation. This abnormal response to pressure overload was accompanied by massive cardiac fibrosis and the appearance of apoptotic cardiomyocytes. These results demonstrate that p38alpha plays a critical role in the cardiomyocyte survival pathway in response to pressure overload, while cardiac hypertrophic growth is unaffected despite its dramatic down-regulation.

Calcium-calcineurin signaling in the regulation of cardiac hypertrophy

Cardiac hypertrophy is a leading predicator of progressive heart disease that often leads to heart failure and a loss of cardiac contractile performance associated with profound alterations in intracellular calcium handling. Recent investigation has centered on identifying the molecular signaling pathways that regulate cardiac myocyte hypertrophy, as well as the mechanisms whereby alterations in calcium handling are associated with progressive heart failure. One potential focal regulator of cardiomyocyte hypertrophy that also responds to altered calcium handling is the calmodulin-activated serine/threonine protein phosphatase calcineurin (PP2B). Once activated by increases in calcium, calcineurin mediates the hypertrophic response through its downstream transcriptional effector nuclear factor of activated T cells (NFAT), which is directly dephosphorylated by calcineurin resulting in nuclear translocation. While previous studies have convincingly demonstrated the sufficiency of calcineurin to mediate cardiac hypertrophy and progressive heart failure, its necessity remains an area of ongoing investigation. Here we weigh an increasing body of literature that suggests a causal link between calcineurin signaling and the cardiac hypertrophic response and heart failure through the use of pharmacologic inhibitors (cyclosporine A and FK506) and genetic approaches. We will also discuss the manner in which calcineurin-NFAT signaling is negatively regulated in the heart through a diverse array of kinases and inhibitory proteins. Finally, we will discuss emerging theories as to the mechanisms whereby alterations in intracellular calcium handling might stimulate calcineurin within the context of a contractile cell continually experiencing calcium flux.

Overexpression of mitogen-activated protein kinase kinase 6 in the heart improves functional recovery from ischemia in vitro and protects against myocardial infarction in vivo

The mitogen-activated protein kinases (MAPK) have been the subject of many studies to identify signaling pathways that promote cell survival or death. In cultured cardiac myocytes, p38 MAPK promotes cell survival or death depending on whether it is activated by mitogen-activated protein kinase kinase 6 (MKK6) or MKK3, respectively. The objectives of the current study were to examine the effects of MKK6-mediated p38 activation in the heart in vivo. Accordingly, we generated transgenic (TG) mice that overexpress wild type MKK6 in a cardiac-restricted manner. Although p38 was about 17-fold more active in TG than non-transgenic (NTG) mouse hearts, TG mouse hearts were morphologically and functionally similar to those of NTG littermates. However, upon transient ischemia followed by reperfusion, the MKK6 TG mouse hearts exhibited significantly better functional recovery and less injury than NTG mouse hearts. Because MKK6 increases levels of the protective small heat shock protein, alpha B-crystallin (alpha BC), in cultured cardiac myocytes, we examined alpha BC levels in the mouse hearts. The level of alpha BC was 2-fold higher in MKK6 TG than NTG mouse hearts. Moreover, ischemia followed by reperfusion induced a 6.4-fold increase in alpha BC levels in the mitochondrial fractions of TG mouse hearts but no increase in alpha BC levels in any of the other fractions analyzed. These alterations in alpha BC expression and localization suggest possible mechanisms of cardioprotection in MKK6 TG mouse hearts.

Intracellular acidosis‐activated p38 MAPK signaling and its essential role in cardiomyocyte hypoxic injury

Activation of p38 mitogen-activated protein kinase (MAPK) plays a central role in cellular responses to a multitude of stress signals. In the heart, enhanced p38 MAPK signaling has been implicated in cardiac hypoxic and ischemic injury. However, the mechanism underlying hypoxia-induced p38 MAPK activation remains elusive. We investigated p38 MAPK activation during hypoxia in adult rat cardiomyocytes. Here, we reported that hypoxia leads to concurrent intracellular acidosis and activation of p38 MAPK and that the hypoxia-induced p38 MAPK signaling can be fully abolished by neutralizing intracellular pH, whereas intracellular acidosis (intracellular pH<7.0) per se overtly augments activation of p38 MAPK but not ERK1/2 and JNK. Furthermore, inhibition of p38 MAPK protects myocytes against hypoxic cell death, suggesting that acidosis-evoked p38 MAPK signaling plays an important role in hypoxic cell injury and cell death. These results demonstrate, for the first time, that intracellular acidosis constitutes a necessary and sufficient link responsible for hypoxia-activated p38 MAPK signaling and the subsequent hypoxic cardiomyocyte injury and death.

p38 mitogen-activated protein kinase inhibition improves cardiac function and attenuates left ventricular remodeling following myocardial infarction in the rat

OBJECTIVES

The aim of this study was to examine the effect of the p38 mitogen-activated protein kinase (MAPK) inhibitor, RWJ-67657 (RWJ), on left ventricular (LV) dysfunction and remodeling post-myocardial infarction (MI) in rats.

BACKGROUND

p38 MAPK signaling has been implicated in the progression of chronic heart failure.

METHODS

From day 7 post-MI (coronary artery ligation), rats received either RWJ (50 mg/day, by gavage, n = 8, MI+RWJ) or vehicle (by gavage, n = 8, MI+V) for 21 days. Echocardiography was performed on day 6, before the commencement of treatment, and on day 27. In vivo hemodynamic measurements were made on day 28. Sham-operated rats served as controls.

RESULTS

The LV end-diastolic pressure and lung/body weight ratio were reduced, whereas the maximum rate of rise of LV pressure was increased towards sham levels in MI+RWJ compared with MI+V. Baseline echocardiographic studies demonstrated uniform LV remodeling and dysfunction in MI rats. Fractional shortening (FS) further deteriorated in MI+V, whereas FS was preserved in MI+RWJ. Progressive LV dilation and infarct expansion observed in MI+V were inhibited in MI+RWJ. MI+RWJ also demonstrated increased myocyte hypertrophy in the peri-infarct and non-infarct zones, and reduced myocardial collagen and alpha-smooth muscle actin (SMA) immunoreactivity compared with MI+V. The antifibrotic effects of RWJ in vivo may reflect direct effects on cardiac fibroblasts, because RWJ attenuated transforming growth factor beta-1-stimulated collagen synthesis and alpha-SMA expression in isolated cardiac fibroblasts. RWJ also protected cultured myocytes from hydrogen peroxide-induced apoptosis.

CONCLUSIONS

RWJ-67657 treatment post-MI had beneficial effects on LV remodeling and dysfunction, supporting a key role for p38 MAPK in pathologic cell signaling in these processes and its inhibition as a novel therapy.

PI3Kgamma modulates the cardiac response to chronic pressure overload by distinct kinase-dependent and -independent effects

The G protein-coupled, receptor-activated phosphoinositide 3-kinase gamma (PI3Kgamma) mediates inflammatory responses and negatively controls cardiac contractility by reducing cAMP concentration. Here, we report that mice carrying a targeted mutation in the PI3Kgamma gene causing loss of kinase activity (PI3KgammaKD/KD) display reduced inflammatory reactions but no alterations in cardiac contractility. We show that, in PI3KgammaKD/KD hearts, cAMP levels are normal and that PI3Kgamma-deficient mice but not PI3KgammaKD/KD mice develop dramatic myocardial damage after chronic pressure overload induced by transverse aortic constriction (TAC). Finally, our data indicate that PI3Kgamma is an essential component of a complex controlling PDE3B phosphodiesterase-mediated cAMP destruction. Thus, cardiac PI3Kgamma participates in two distinct signaling pathways: a kinase-dependent activity that controls PKB/Akt as well as MAPK phosphorylation and contributes to TAC-induced cardiac remodeling, and a kinase-independent activity that relies on protein interactions to regulate PDE3B activity and negatively modulates cardiac contractility.

Transgenic mice with cardiac-specific over-expression of MLK7 have increased mortality when exposed to chronic β-adrenergic stimulation

Mixed lineage kinase 7 (MLK7) is a recently identified mitogen-activated protein kinase kinase kinase with enriched expression in skeletal muscle and heart. When over-expressed in cardiac myocytes, MLK7 activates both the p38 and c-Jun N-terminal kinase (JNK) stress-activated pathways and induces a cellular phenotype characteristic of cardiac hypertrophy, including a fetal gene expression pattern and increased protein synthesis. We sought to determine the effect of MLK7 on cardiac function in vivo by generating transgenic (Tg) mice with cardiac restricted over-expression of the enzyme. The mice were viable and demonstrated no visible signs of distress at rest. Microscopic examination of the hearts showed myocardial fibrosis and hypertrophy. Hemodynamic analysis of the Tg mice revealed impaired systolic function and significant diastolic dysfunction. Furthermore, significant mortality was observed in MLK7 Tg mice following 24-48 h of isoproterenol administration. Isoproterenol activation of JNK and p38, but not extracellular signal-regulated kinase, was significantly greater in the MLK7 Tg mice compared to littermate controls. These data indicate that MLK7 is an important signal transducer in cardiac compensation. Simultaneous activation of JNK and p38 by MLK7 may contribute to cardiac decompensation during the periods of acute cardiac stress.

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.

Mitogen-activated Protein Kinases p38 and ERK 1/2 Mediate the Wall Stress-induced Activation of GATA-4 Binding in Adult Heart

The zinc finger transcription factor GATA-4 has been implicated as a critical regulator of inducible cardiac gene expression and as a potential mediator of the hypertrophic program. However, the precise intracellular mechanisms that regulate the DNA-binding activity of GATA-4 are not fully understood. The aim of the present study was to examine the role of mitogen-activated protein kinases (p38 kinase, extracellular signal-regulated protein kinase, and c-Jun N-terminal protein kinase) in the left ventricular wall stress-induced activation of GATA-4 DNA binding in adult heart. Isolated perfused rat hearts were subjected to increased left ventricular wall stress by inflating a balloon in the ventricle. Gel mobility shift assays were used to analyze the transacting factors that interact with the GATA motifs of the B-type natriuretic peptide promoter. The left ventricular wall stress rapidly activated GATA-4 DNA binding and significantly increased the levels of phosphorylated p38 kinase, extracellular signal-regulated protein kinase, and c-Jun N-terminal protein kinase. The wall stress-induced increase in the DNA-binding activity of GATA-4 was abolished both in the presence of the p38 inhibitor SB239063 and MEK1/2 inhibitor U0126. In contrast, the inhibition of c-Jun N-terminal protein kinase by CEP11004 had no effect on the baseline or stretch-induced GATA-4 DNA binding. Moreover, GATA-4 DNA binding was up-regulated by mechanical stretch in the isolated rat atria via p38 and extracellular signal-regulated protein kinase. In conclusion, the present study demonstrates that both p38 and extracellular signal-regulated protein kinase are required for the stretch-induced GATA-4 binding in intact heart.

Role of Cardiac Myosin Binding Protein C in Sustaining Left Ventricular Systolic Stiffening

Despite advances in the molecular biology of cardiac myosin binding protein-C (cMyBP-C), little is understood about its precise role in muscle contraction, particularly in the intact heart. We tested the hypothesis that cMyBP-C is central to the time course and magnitude of left ventricular systolic elastance (chamber stiffening), and assessed mechanisms for this influence in intact hearts, trabeculae, and skinned fibers from wild-type (+/+) and homozygous truncated cMyBP-C (t/t) male mice. cMyBP-C protein was not detected by gel electrophoresis or Western blot in t/t myocardium. cMyBP-C t/t ventricles displayed reduced peak elastance, but more strikingly a marked abbreviation of the systolic elastance time course, which peaked earlier (27.6±2.1 ms) than in +/+ controls (47.8±1.6 ms). Control hearts reached only 42±4% of maximum elastance at the onset of ejection, with substantial further stiffening during ejection. This contrasted to t/t mutants, which reached 77±3% of peak elastance before ejection of peak. These unusual findings were not observed in alternative models involving severe cardiomyopathy, but were recapitulated in a cMyBP-C null mouse. The abbreviated elastance time course and lower peak were consistent with earlier time-to-peak trabecular tension, increased unloaded shortening velocity in t/t skinned muscle strips, and dramatically reduced myofilament stiffness at diastolic calcium concentrations. These results provide novel insights into the role of cMyBP-C in myocardial systolic mechanics. Abnormal sarcomere shortening velocity and abbreviated muscle stiffening may underlie development of cardiac dysfunction associated with deficient incorporation of cMyBP-C.

Targeted inhibition of p38 mitogen-activated protein kinase antagonizes cardiac injury and cell death following ischemia-reperfusion in vivo

The p38 branch of the mitogen-activated protein kinase (MAPK) signaling cascade has been implicated as a regulator of cardiomyocyte apoptosis in culture as well as in the adult heart. However, considerable disagreement persists as to the functional effects attributed to p38 signaling, given that both pro- and anti-apoptotic regulatory roles have been reported. To address this area of uncertainty in the literature, we investigated the cell death effects associated with p38 inactivation in both cultured neonatal cardiomyocytes and the adult heart. In vitro, adenoviral-mediated gene transfer of two different dominant-negative-encoding p38 vectors reduced apoptosis induced by 2-deoxyglucose treatment, whereas overexpression of wild-type p38alpha or an activated mitogen-activated protein kinase kinase (MKK)6 mutant each enhanced cell death. In vivo, transgenic mice expressing a dominant-negative MKK6 mutant or a dominant-negative p38alpha mutant were each significantly protected from ischemia-reperfusion injury, as assessed by infarct area measurements, DNA laddering, terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling, and functional assessment of ventricular performance. Similarly, transgenic mice overexpressing the p38-inactivating dual specificity phosphatase MAPK phosphatase-1 (MKP-1) were also partially protected, whereas MKP-1 gene-targeted mice showed greater injury after ischemia-reperfusion injury. Mechanistically, inhibition of p38 signaling promoted a dramatic up-regulation of Bcl-2 in the hearts of transgenic mice. In primary neonatal cardiomyocyte cultures, adenoviral-mediated gene transfer of a p38 inhibitory mutant up-regulated Bcl-2, whereas expression of an activated p38 mutant down-regulated Bcl-2 protein levels. Collectively, these results indicate that p38 functions as a pro-death signaling effector in both cultured myocytes as well as in the intact heart.

Targeted activation of c-Jun N-terminal kinase in vivo induces restrictive cardiomyopathy and conduction defects

The stress-activated protein kinase, c-Jun N-terminal kinase (JNK), has been implicated in the process of cardiac hypertrophy and apoptosis, yet the specific roles of JNK in heart failure are unclear. To determine the effects of JNK activation in intact heart, we established transgenic animals using a Cre/loxP-mediated gene switch approach to achieve targeted expression of an upstream activator, mitogen-activated protein kinase kinase 7 (D) (MKK7D), in ventricular myocytes. MKK7D expression led to significant JNK activation, robust induction of the fetal gene program, and contractile dysfunction. The animals died approximately 7 weeks after birth with signs of congestive heart failure. Doppler mode echocardiography revealed a marked stiffening of JNK-activated hearts that was associated with the remodeling of specific extracellular matrix components. Gene expression analysis of MKK7D hearts revealed up-regulation of transforming growth factor beta signaling, offering a potential molecular mechanism underlying changes in extracellular matrix composition. In addition, we demonstrated that JNK activation led to specific loss of connexin 43 protein and gap junctions without affecting the expression or localization of other key intercalated disc proteins. This specific and localized gap junction remodeling resulted in significant slowing of ventricular electrical conduction in JNK-activated hearts. These results represent the first characterization of JNK-mediated cardiac pathology in vivo and support an important role for JNK signaling in specific aspects of cardiac remodeling in the pathogenesis of cardiac disease.

Sarcoplasmic reticulum calcium defect in Ras-induced hypertrophic cardiomyopathy heart

The small G protein Ras-mediated signaling pathway has been implicated in the development of hypertrophy and diastolic dysfunction in the heart. Earlier cellular studies have suggested that the Ras pathway is responsible for reduced L-type calcium channel current and sarcoplasmic reticulum (SR) calcium uptake associated with sarcomere disorganization in neonatal cardiomyocytes. In the present study, we investigated the in vivo effects of Ras activation on cellular calcium handling and sarcomere organization in adult ventricular myocytes using a newly established transgenic mouse model with targeted expression of the H-Ras-v12 mutant. The transgenic hearts expressing activated Ras developed significant hypertrophy and postnatal lethal heart failure. In adult ventricular myocytes isolated from the transgenic hearts, the calcium transient was significantly depressed but membrane L-type calcium current was unchanged compared with control littermates. The expressions of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)2a and phospholamban (PLB) were significantly reduced at mRNA levels. The amount of SERCA2a protein was also modestly reduced. However, the expression of PLB protein and gross sarcomere organization remained unchanged in the hypertrophic Ras hearts, whereas Ser(16) phosphorylation of PLB was dramatically inhibited in the Ras transgenic hearts compared with controls. Hypophosphorylation of PLB was also associated with a significant induction of protein phosphatase 1 expression. Therefore, our results from this in vivo model system suggest that Ras-induced contractile defects do not involve decreased L-type calcium channel activities or disruption of sarcomere structure. Rather, suppressed SR calcium uptake due to reduced SERCA2a expression and hypophosphorylation of PLB due to changes in protein phosphatase expression may play important roles in the diastolic dysfunction of Ras-mediated hypertrophic cardiomyopathy.

c-Jun N-terminal kinases (JNK) antagonize cardiac growth through cross-talk with calcineurin-NFAT signaling

The c-Jun N-terminal kinase (JNK) branch of the mitogen-activated protein kinase (MAPK) signaling pathway regulates cellular differentiation, stress responsiveness and apoptosis in multicellular eukaryotic organisms. Here we investigated the functional importance of JNK signaling in regulating differentiated cellular growth in the post-mitotic myocardium. JNK1/2 gene-targeted mice and transgenic mice expressing dominant negative JNK1/2 were determined to have enhanced myocardial growth following stress stimulation or with normal aging. A mechanism underlying this effect was suggested by the observation that JNK directly regulated nuclear factor of activated T-cell (NFAT) activation in culture and in transgenic mice containing an NFAT-dependent luciferase reporter. Moreover, calcineurin Abeta gene targeting abrogated the pro-growth effects associated with JNK inhibition in the heart, while expression of an MKK7-JNK1 fusion protein in the heart partially reduced calcineurin-mediated cardiac hypertrophy. Collectively, these results indicate that JNK signaling antagonizes the differentiated growth response of the myocardium through direct cross-talk with the calcineurin-NFAT pathway. These results also suggest that myocardial JNK activation is primarily dedicated to modulating calcineurin-NFAT signaling in the regulation of differentiated heart growth.

Stress kinase phosphorylation is increased in pacing-induced heart failure in rabbits

In hearts with chronic left ventricular (LV) systolic dysfunction secondary to hypertension or myocardial infarction, MAPK phosphorylation and/or activity are increased. Whether other settings of LV dysfunction not associated with ischemia-reperfusion are also characterized by increased MAPK phosphorylation or activity is unknown. After 3 wk of rapid LV pacing (400 beats/min), eight rabbits displayed clinical signs of heart failure (HF), and echocardiography revealed an increase in LV end-diastolic diameter from 15.6 +/- 0.7 (means +/- SE) to 18.8 +/- 0.7 mm and a reduced shortening fraction from 31 +/- 1to10 +/- 2% (both P < 0.05). Morphological alterations in HF included increased numbers of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)-positive cardiomyocytes, extent of fibrosis, and cross-sectional cardiomyocyte area. Total p38 MAPK did not differ between failing and normal hearts (n = 8). However, p38 MAPK phosphorylation [164,488 +/- 29,323 vs. 43,565 +/- 14,817 arbitrary units (AU), P < 0.05, densitometry] and the activities of p38 MAPK-alpha and -beta were increased in failing compared with normal hearts (149,441 +/- 38,381 and 170,430 +/- 32,952 vs. 68,815 +/- 28,984 and 81,788 +/- 22,774 AU, respectively, both P < 0.05). In failing compared with normal hearts, total and phosphorylated JNK46 and JNK54 MAPK were increased, whereas total and phosphorylated ERK MAPK remained unchanged. In pacing-induced HF, p38 and JNK MAPK phosphorylation as well as p38 MAPK activity was increased. Further studies will have to define whether or not chronic specific blockade of MAPK activity can interfere with apoptosis/fibrosis and thereby attenuate the progression of HF.

In vivo high-efficiency transcoronary gene delivery and Cre-LoxP gene switching in the adult mouse heart

High-efficiency somatic gene transfer in adult mouse heart has not yet been achieved in vivo. Here, we demonstrate high-efficiency in vivo transcoronary gene delivery to the adult murine myocardium using a catheter-based technique with recombinant adenovirus (AdV) and adeno-associated virus (AAV) vectors in normal and genetically engineered mice. The method involves immersion hypothermia followed by transient aortic and pulmonary artery occlusion with proximal intra-aortic segmental injection of cardioplegic solution containing substance P and viral vectors. Gene expression measured using a LacZ marker gene was observed throughout both ventricles. The expression efficiency of a cytoplasmic LacZ marker gene in the left ventricular myocardium was 56.4+/-14.5% (mean+/-s.d.) at 4 days with an AdV vector, and with an AAV vector it was 81.0+/-5.9% at 4 weeks. Following AAV gene transfer, no gene expression was found in kidney, brain, lung, and spleen, but there was slight expression in liver. In addition, we demonstrate temporally controlled genetic manipulation in the heart with an efficiency of 54.6+/-5.2%, by transferring an AdV vector carrying Cre recombinase in ROSA26 flox-LacZ reporter mice. Procedure-related mortality was 16% for AdV and zero for AAV transfer. Thus, this method provides efficient, relatively homogeneous gene expression in both ventricles of the adult mouse heart, and offers a novel approach for conditional gene rescue or ablation in genetically engineered mouse models.

Diverse mechanisms of myocardial p38 mitogen-activated protein kinase activation: evidence for MKK-independent activation by a TAB1-associated mechanism contributing to injury during myocardial ischemia

The ischemic activation of p38alpha mitogen-activated protein kinase (p38alpha-MAPK) is thought to contribute to myocardial injury. Under other circumstances, activation is through dual phosphorylation by MAPK kinase 3 (MKK3). Therefore, the mkk3-/- murine heart should be protected during ischemia. In retrogradely perfused mkk3-/- and mkk3+/+ mouse hearts subjected to 30 minutes of global ischemia and 120 minutes of reperfusion, infarction/risk volume was similar (50+/-5 versus 51+/-4, P=0.93, respectively), as was intraischemic p38-MAPK phosphorylation (10 minutes ischemia as percent basal, 608+/-224 versus 384+/-104, P=0.43, respectively). This occurred despite undetectable activation of MKK3/6 in mkk3-/- hearts. However, tumor necrosis factor (TNF)-induced p38-MAPK phosphorylation was markedly diminished in mkk3-/- vs mkk3+/+ hearts (percent basal, 127+/-23 versus 540+/-267, respectively, P=0.04), suggesting an MKK-independent activation mechanism by ischemia. Hence, we examined p38-MAPK activation by TAB1-associated autophosphorylation. In wild-type mice and mkk3-/- mice, the p38-MAPK catalytic site inhibitor SB203580 (1 micromol/L) diminished phosphorylation during ischemia versus control (10 minutes ischemia as percent basal, 143+/-2 versus 436+/-96, P=0.003, and 122+/-25 versus 623+/-176, P=0.05, respectively) and reduced infarction volume (infarction/risk volume, 57+/-5 versus 36+/-3, P<0.001, and 50+/-5 versus 29+/-3, P=0.003, respectively) but did not alter TNF-induced activation, although in homogenates of ischemic hearts but not TNF-exposed hearts, p38-MAPK was associated with TAB1. Furthermore, adenovirally expressed wild-type and drug-resistant p38alpha-MAPK, lacking the SB203580 binding site, was phosphorylated when H9c2 myoblasts were subjected to simulated ischemia. However, SB203580 (1 micromol/L) did not prevent the phosphorylation of resistant p38alpha-MAPK. These findings suggest the ischemic activation of p38-MAPK contributing to myocardial injury is by TAB1-associated autophosphorylation.

Temporal alterations in protein signaling cascades during recovery from muscle atrophy

Currently, the repertoire of cellular and molecular pathways that control skeletal muscle atrophy and hypertrophy are not well defined. It is possible that intracellular regulatory signaling pathways are active at different times during the muscle hypertrophy process. The hypothesis of the given experiments was that cellular signals related to protein translation would be active at early time points of skeletal muscle regrowth, whereas transcriptional signals would be active at later time points of skeletal muscle regrowth. The phosphorylation status of p38 MAPK and JNK increased at the end of limb immobilization but returned to control values at recovery day 3. Transient increases in phosphorylation and in protein concentration occurred during recovery of soleus muscle mass. Phosphorylation of Akt, p70S6k, and signal transducer and activator of transcription 3 (STAT3) peaked on recovery day 3 compared with day 0. Glycogen synthase kinase (GSK)-3beta phosphorylation was increased on the sixth and fifteenth recovery day. In addition, transient peaks in seven protein concentrations occurred at different times of recovery: STAT3, calcineurin A (CaNA), CaNB, and beta4E-BP1 protein concentrations peaked on the third recovery day; p70S6k, STAT3, Akt, and GSK3-beta peaked on the sixth recovery day; and GSK3-beta peaked on the fifteenth recovery day. The apexes of STAT3 and GSK3-beta protein concentrations remained elevated for two recovery time points. Thus the time course of increase in molecules of signaling pathways differed as the young rat soleus muscle regrew from an atrophied state.

Signaling through PI3K/Akt mediates stretch and PDGF-BB-dependent DNA synthesis in bladder smooth muscle cells

PURPOSE

Smooth muscle cells (SMC) of the bladder undergo hypertrophy and hyperplasia following exposure to sustained mechanical overload. Although superficial similarities in the response of the heart and bladder to hypertrophic stimuli suggest that similar molecular mechanisms may be involved, this remains to be demonstrated. In this study we compared signal transduction pathway activation in primary culture bladder SMC and cardiac myofibroblasts in response to cyclic stretch. The effects of growth factor stimulation on pathway activation in bladder SMC were also investigated.

MATERIALS AND METHODS

Primary culture rodent bladder SMC or cardiac myofibroblasts were subjected to cyclic stretch-relaxation in the absence or presence of pharmacologic inhibitors of the phosphoinositide-3-kinase, (PI3K)/Akt, extracellular signal-regulated kinase-mitogen activated protein kinase (Erk-MAPK) or the p38 stress-activated protein kinase-2 (SAPK2) pathways. In parallel experiments human bladder SMC were treated with platelet-derived growth factor-BB (PDGF-BB), heparin-binding EGF-like growth factor (HB-EGF) or fibroblast growth factor-2 (FGF-2). In each case the extent of DNA synthesis was determined by uptake of tritiated thymidine, and activation of specific signaling intermediates was determined by immunoblot analysis using antibodies to the non-phosphorylated and phosphorylated (activated) forms of Akt, p38 and Erk1/2.

RESULTS

Akt and p38 were rapidly phosphorylated in stretched bladder SMC and cardiac myofibroblasts, and stretch-induced DNA synthesis in these cells was ablated with inhibitors of PI3K or p38 but not Erk-MAPK. Similarly, PDGF-BB up-regulated DNA synthesis in bladder SMC in a p38 and Akt-dependent manner.

CONCLUSIONS

We conclude that distinct stimuli, such as mechanical stretch and PDGF-BB, promote DNA synthesis in bladder SMC through shared downstream signaling pathways. Furthermore, phenotypically similar cells from the bladder and heart show comparable pathway activation in response to stretch. These findings suggest that similar molecular mechanisms underlie the altered growth responses of the bladder and heart to mechanical overload. This study also provides the first report of Akt activation in bladder SMC and suggests that Akt, consistent with its pivotal role in cardiac hypertrophy, may also be a key regulator of remodeling in the SMC compartment of the bladder exposed to hypertrophic/hyperplastic stimuli in vivo.

Targeted inhibition of p38 MAPK promotes hypertrophic cardiomyopathy through upregulation of calcineurin-NFAT signaling

The MAPKs are important transducers of growth and stress stimuli in virtually all eukaryotic cell types. In the mammalian heart, MAPK signaling pathways have been hypothesized to regulate myocyte growth in response to developmental signals or physiologic and pathologic stimuli. Here we generated cardiac-specific transgenic mice expressing dominant-negative mutants of p38alpha, MKK3, or MKK6. Remarkably, attenuation of cardiac p38 activity produced a progressive growth response and myopathy in the heart that correlated with the degree of enzymatic inhibition. Moreover, dominant-negative p38alpha, MKK3, and MKK6 transgenic mice each showed enhanced cardiac hypertrophy following aortic banding, Ang II infusion, isoproterenol infusion, or phenylephrine infusion for 14 days. A mechanism underlying this enhanced-growth profile was suggested by the observation that dominant-negative p38alpha directly augmented nuclear factor of activated T cells (NFAT) transcriptional activity and its nuclear translocation. In vivo, NFAT-dependent luciferase reporter transgenic mice showed enhanced activation in the presence of the dominant-negative p38alpha transgene before and after the onset of cardiac hypertrophy. More significantly, genetic disruption of the calcineurin Abeta gene rescued hypertrophic cardiomyopathy and depressed functional capacity observed in p38-inhibited mice. Collectively, these observations indicate that reduced p38 signaling in the heart promotes myocyte growth through a mechanism involving enhanced calcineurin-NFAT signaling.

Mitogen-activated protein kinases: a new therapeutic target in cardiac pathology

Eukaryotic cells respond to different external stimuli by activation of mechanisms of cell signaling. One of the major systems participating in the transduction of signal from the cell membrane to nuclear and other intracellular targets is the highly conserved mitogen-activated protein kinase (MAPK) superfamily. The members of MAPK family are involved in the regulation of a large variety of cellular processes such as cell growth, differentiation, development, cell cycle, death and survival. Several MAPK subfamilies, each with apparently unique signaling pathway, have been identified in the mammalian myocardium. These cascades differ in their upstream activation sequence and in downstream substrate specifity. Each pathway follows the same conserved three-kinase module consisting of MAPK, MAPK kinase (MAPKK, MKK or MEK), and MAPK kinase kinase (MAPKKK, MEKK). The major groups of MAPKs found in cardiac tissue include the extracellular signal-regulated kinases (ERKs), the stress-activated/c-Jun NH2-terminal kinases (SAPK/JNKs), p38-MAPK, and ERK5/big MAPK 1 (BMK1). The ERKs are strongly activated by mitogenic and growth factors and by physical stress, whereas SAPK/JNKs and p38-MAPK can be activated by various cell stresses, such as hyperosmotic shock, metabolic stress or protein synthesis inhibitors, UV radiation, heat shock, cytokines, and ischemia. Activation of MAPKs family plays a key role in the pathogenesis of various processes in the heart, e.g. myocardial hypertrophy and its transition to heart failure, in ischemic and reperfusion injury, as well in the cardioprotection conferred by ischemia- or pharmacologically-induced preconditioning. The following approaches are currently utilized to elucidate the role of MAPKs in the myocardium: (i) studies of the effects of myocardial processes on the activity of these kinases; (ii) pharmacological modulations of MAPKs activity and evaluation of their impact on the (patho)physiological processes in the heart; (iii) gene targeting or expression of constitutively active and dominant-negative forms of enzymes (adenovirus-mediated gene transfer). This review is focused on the regulatory role of MAPKs in the myocardium, with particular regard to their involvement in pathophysiological processes, such as myocardial hypertrophy and heart failure, ischemia/reperfusion injury, as well as in the mechanisms of cardioprotection. In addition, it summarizes current information on pharmacological modulations of MAPKs activity and their impact on the cardiac response to pathophysiological processes.

Temporal activation of c-Jun N-terminal kinase in adult transgenic heart via cre-loxP-mediated DNA recombination

Using a cre-loxP-mediated gene-switch approach, we achieved targeted JNK activation in adult hearts. A transgenic model is established carrying a floxed gene-switch construct that directs GFP marker gene expression in the absence of DNA recombination between two loxP sites. A tamoxifen-inducible Cre recombinase was introduced in the transgenic heart by breeding with previously established Mer-Cre-Mer transgenic mice. Upon tamoxifen administration in double transgenic adult animals, cre-loxP-mediated DNA recombination efficiently switches "off" the loxP-flanked GFP expression unit in cardiomyocytes and switches "on" the expression of the target gene, MKK7D, a constitutively activated upstream activator of c-Jun N-terminal kinases (JNK). Expression of MKK7D in adult hearts resulted in significant activation of JNK activities and causes progressive cardiomyopathy in transgenic animals. This unique animal model of cardiac-specific and temporally regulated JNK activation will provide a powerful tool to investigate the functional role of the JNK pathway in the development of heart failure. Our data also demonstrated that the inducible gene-switch approach reported here may also be applicable in other studies to achieve efficient, tissue-specific, and temporally regulated genetic manipulation in intact animals.

The role of the Grb2-p38 MAPK signaling pathway in cardiac hypertrophy and fibrosis

Cardiac hypertrophy is a common response to pressure overload and is associated with increased mortality. Mechanical stress in the heart can result in the integrin-mediated activation of focal adhesion kinase and the subsequent recruitment of the Grb2 adapter molecule. Grb2, in turn, can activate MAPK cascades via an interaction with the Ras guanine nucleotide exchange factor SOS and with other signaling intermediates. We analyzed the role of the Grb2 adapter protein and p38 MAPK in cardiac hypertrophy. Mice with haploinsufficiency of the Grb2 gene (Grb2(+/-) mice) appear normal at birth but have defective T cell signaling. In response to pressure overload, cardiac p38 MAPK and JNK activation was inhibited and cardiac hypertrophy and fibrosis was blocked in Grb2(+/-) mice. Next, transgenic mice with cardiac-specific expression of dominant negative forms of p38alpha (DN-p38alpha) and p38beta (DN-p38beta) MAPK were examined. DN-p38alpha and DN-p38beta mice developed cardiac hypertrophy but were resistant to cardiac fibrosis in response to pressure overload. These results establish that Grb2 action is essential for cardiac hypertrophy and fibrosis in response to pressure overload, and that different signaling pathways downstream of Grb2 regulate fibrosis, fetal gene induction, and cardiomyocyte growth.

Atrial chamber-specific expression of sarcolipin is regulated during development and hypertrophic remodeling

Intracellular Ca2+ regulation is critical in the normal cardiac function and development of pathologic hearts. Phospholamban, an endogenous inhibitor of sarcoplasmic reticulum Ca2+ ATPase in the sarcoplasmic reticulum, plays an important role in Ca2+ cycling in heart. Recently, sarcolipin has been identified as having a similar function as phospholamban in skeletal muscle. Because phospholamban is differentially expressed in atrial and ventricular myocardia and its expression is often altered in diseased hearts, we investigated the cardiac chamber specificity of sarcolipin expression and its regulation during development and hypertrophic remodeling. Northern blot analysis revealed that the expression of mouse sarcolipin mRNA was most abundant in the atria and was undetectable in the ventricles, indicating an atrial chamber-specific expression pattern. Atrial chamber-specific expression of sarcolipin mRNA was increased during development. These findings were confirmed by in situ hybridization studies. In addition, sarcolipin expression was down-regulated in the atria of hypertrophic heart when induced by ventricular specific overexpression of the activated H-ras gene. In humans, sarcolipin mRNA was also expressed in the atria but not detected in the ventricles, although sarcolipin expression was most abundant in skeletal muscle. Taken together, sarcolipin is likely to be an atrial chamber-specific regulator of Ca2+ cycling in heart.

c-Jun N-terminal kinase activation mediates downregulation of connexin43 in cardiomyocytes

Loss of gap junctions and impaired intercellular communication are characteristic features of pathological remodeling in heart failure as a result of stress or injury, yet the underlying regulatory mechanism has not been identified. Here, we report that in cultured myocytes, rapid loss of the gap junction protein connexin43 (Cx43) occurs in conjunction with the activation of c-Jun N-terminal kinase (JNK), a stress-activated protein kinase, on stress stimulation. To investigate the specific role of JNK activation in the regulation of connexin in cardiomyocytes, an activated mutant of mitogen-activated protein kinase kinase 7 (mutant D), a JNK-specific upstream activator, was expressed in myocytes by adenovirus-mediated gene transfer. JNK activation in infected cardiomyocytes resulted in significant reduction of Cx43 expression at both mRNA and protein levels and impaired cell-cell communication. To evaluate the role of JNK in the regulation of Cx43 expression and gap junction structure in vivo, a Cre-LoxP-mediated gene-switch system was used to establish a transgenic animal model with targeted activation of JNK in ventricular myocardium. The transgenic hearts exhibited significant downregulation of Cx43 expression and loss of gap junctions in myocardium that may contribute to the cardiac dysfunction and premature death phenotype. Our report represents the first evidence, both in vitro and in vivo, implicating JNK as an important mediator of stress-induced Cx43 downregulation and impaired intercellular communication in the failing heart.

Integration of pathways that signal cardiac growth with modulation of myofilament activity

We review evidence for integrated effects of the signals that promote cardiac growth and remodeling and that modify the processes of excitation-contraction coupling. We have focused on integration of alterations in myofilament function with cell growth on the basis of genetic linkage analysis demonstrating that sarcomeric mutations are causal in hypertrophic cardiomyopathies. This evidence argues strongly for a path of communication between the intrinsic functional changes associated with a sarcomeric protein mutation and nuclear events. Our hypothesis is that this communication is also essential to the transduction of extrinsic signals leading to hypertrophy and failure. Understanding this network of signaling pathways is certain to lead to better diagnostic and treatment approaches to heart failure.

Cardiac expression and subcellular localization of the p38 mitogen-activated protein kinase member, stress-activated protein kinase-3 (SAPK3)

Despite the interest in the roles that mitogen-activated protein kinases (MAPKs) play in the heart, the role of the different MAPK isoforms has been relatively poorly defined. A third isoform of p38 MAPK, known variously as stress-activated protein kinase-3 (SAPK3), p38- gamma or ERK6, has been previously shown to differ from p38- alpha/ beta both in its molecular weight and its lack of inhibition by the compound SB203580. We have generated monoclonal antibodies with specificity for SAPK3 demonstrated by immunoblot analysis, immunofluorescence studies, and cloning of SAPK3 from a rat heart cDNA expression library. By immunoblotting, we confirmed high expression of SAPK3 in fast, slow and mixed fibre types of murine skeletal muscle and observed significant expression restricted to heart, lung, thymus and testes. In addition to expression in normal heart (human, mouse, rat, dog and pig), we observed constant expression in diseased human heart, as well as control and hypertrophic cultured neonatal rat cardiac myocytes. Immunolocalization in cultured cardiac myocytes followed by confocal microscopy showed punctate, non-nuclear SAPK3 staining. In contrast, p38- alpha/ beta staining was non-punctate and distributed throughout the cytosol and nucleus. Whereas treatment with Leptomycin B to prevent nuclear export processes promoted higher levels of p38- alpha/ beta staining in cardiac myocyte nuclei, there was no apparent change in SAPK3 localization under these conditions. These differences between p38- alpha/ beta and SAPK3 probably reflect the specialized functions of SAPK3 and emphasize the need to evaluate SAPK3 upstream activators and downstream targets in the heart.

p38 Mitogen-activated protein kinase mediates a negative inotropic effect in cardiac myocytes

p38 Mitogen-activated protein kinase (MAPK) is one of the most ancient signaling molecules and is involved in multiple cellular processes, including cell proliferation, cell growth, and cell death. In the heart, enhanced activation of p38 MAPK is associated with ischemia/reperfusion injury and the onset of heart failure. In the present study, we investigated the function of p38 MAPK in regulating cardiac contractility and its underlying mechanisms. In cultured adult rat cardiomyocytes, activation of p38 MAPK by adenoviral gene transfer of an activated mutant of its upstream kinase, MKK3bE, led to a significant reduction in baseline contractility, compared with uninfected cells or those infected with a control adenoviral vector (Adv-beta-galactosidase). The inhibitory effect of MKK3bE on contractility was largely prevented by coexpressing a dominant-negative mutant of p38 MAPK or treating cells with a p38 MAPK inhibitor, SB203580. Conversely, inhibition of endogenous p38 MAPK activity by SB203580 rapidly and reversibly enhanced cell contractility in a dose-dependent manner, without altering L-type Ca(2+) currents or Ca(2+)(i) transients. MKK3bE-induced p38 activation had no significant effect on pH(i), whereas SB203580 had a minor effect to elevate pH(i). Furthermore, activation of p38 MAPK was unable to increase troponin I phosphorylation. Thus, we conclude that the negative inotropic effect of p38 MAPK is mediated by decreasing myofilament response to Ca(2+), rather than by altering Ca(2+)(i) homeostasis and that the reduced myofilament Ca(2+) sensitivity is unlikely attributable to troponin I phosphorylation or alterations in pH(i). These findings reveal a novel function of p38 MAPK and shed a new light on our understanding of the coincidence of p38 MAPK activation and the onset of heart failure.