Many pathways to cardiac hypertrophy

Synergistic cardiac pathological hypertrophy induced by high-salt diet in IGF-IIRα cardiac-specific transgenic rats

Stress-induced cardiac hypertrophy leads to heart failure. Our previous studies demonstrate that insulin-like growth factor-II receptor (IGF-IIR) signaling is pivotal to hypertrophy regulation. In this study, we show a novel IGF-IIR alternative spliced transcript, IGF-IIRα (150 kDa) play a key role in high-salt induced hypertrophy mechanisms. Cardiac overexpression of IGF-IIRα and high-salt diet influenced cardiac dysfunction by increasing pathophysiological changes with up-regulation of hypertrophy markers, atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP). We found that, cardiac hypertrophy under high-salt conditions were amplified in the presence of IGF-IIRα overexpression. Importantly, high-salt induced angiotensin II type I receptor (AT1R) up regulation mediated IGF-IIR expressions via upstream mitogen activated protein kinase (MAPK)/silent mating type information regulation 2 homolog 1 (SIRT1)/heat shock factor 1 (HSF1) pathway. Further, G-coupled receptors (Gαq) activated calcineurin/nuclear factor of activated T-cells, cytoplasmic 3 (NFATc3)/protein kinase C (PKC) signaling was significantly up regulated under high-salt conditions. All these effects were observed to be dramatically over-regulated in IGF-IIRα transgenic rats fed with a high-salt diet. Altogether, from the findings, we demonstrate that IGF-IIRα plays a crucial role during high-salt conditions leading to synergistic cardiac hypertrophy.

Exosomes as New Intercellular Mediators in Development and Therapeutics of Cardiomyocyte Hypertrophy

Myocardial hypertrophy is a common cardiac condition in response to hemodynamic and neurohormonal alterations. Pathological hypertrophic growth in hearts caused the decline of cardiac functions, and finally developed into congestive heart failure. The exosomes are small membrane vesicles which are secreted by various cells and play important roles in cellular communication, migration, proliferation and differentiation. Recent studies uncovered that the exosomes from cardiac fibroblasts and other tissues participates in the development of myocardial hypertrophy. Nevertheless, cardiac progenitor cells and mesenchymal stem cells-derived exosomes confer protective action on myocardial hypertrophy. Thus, the exosomes serve as new intercellular mediators between cardiomyocytes and other cells, and show broad application potential in the diagnostic and therapy of cardiomyocyte hypertrophy.

Role of cyclooxygenase-2 in Trypanosoma cruzi survival in the early stages of parasite host-cell interaction

Chagas disease, caused by the intracellular protozoan Trypanosoma cruzi, is a serious health problem in Latin America. During this parasitic infection, the heart is one of the major organs affected. The pathogenesis of tissue remodelling, particularly regarding cardiomyocyte behaviour after parasite infection and the molecular mechanisms that occur immediately following parasite entry into host cells are not yet completely understood. When cells are infected with T. cruzi, they develop an inflammatory response, in which cyclooxygenase-2 (COX-2) catalyses rate-limiting steps in the arachidonic acid pathway. However, how the parasite interaction modulates COX-2 activity is poorly understood. In this study, the H9c2 cell line was used as our model and we investigated cellular and biochemical aspects during the initial 48 h of parasitic infection. Oscillatory activity of COX-2 was observed, which correlated with the control of the pro-inflammatory environment in infected cells. Interestingly, subcellular trafficking was also verified, correlated with the control of Cox-2 mRNA or the activated COX-2 protein in cells, which is directly connected with the assemble of stress granules structures. Our collective findings suggest that in the very early stage of the T. cruzi-host cell interaction, the parasite is able to modulate the cellular metabolism in order to survives.

Upregulation of Phospholipase D Expression and Activation in Ventricular Pressure-Overload Hypertrophy

Evidence for a role of phospholipase D (PLD) in cellular proliferation and differentiation is accumulating. We studied PLD activity and expression in normal and hypertrophic rat and human hearts. In rat heart, abdominal aortic banding (constriction to 50% of original lumen) caused hypertrophy in the left ventricle (as shown by weight index and ANP expression) by about 15% after 30 days without histological evidence of fibrosis or signs of decompensation and in the right ventricle after 100 days. The hypertrophy was accompanied by small increases of basal PLD activity and strong potentiation of stimulated PLD activity caused by 4beta-phorbol-12beta,13alpha-dibutyrate (PDB) and by phenylephrine. The mRNA expressions of both PLD1 and PLD2 determined by semiquantitative competitive RT-PCR were markedly enhanced after aortic banding. In the caveolar fraction of the rat heart, PLD2 protein determined by Western blot analysis was upregulated in parallel with the expression of caveolin-3. A similar induction of PLD mRNA and protein expression was observed in hypertrophied human hearts of individuals (39-45-year-old) who had died from non-cardiac causes. In conclusion, PLD1 and PLD2 expressions were strongly enhanced both in rat and human heart hypertrophy, which may be responsible for the coincident potentiation of the PLD activation by alpha-adrenoceptor and protein kinase C stimulation. These results are compatible with a significant role of PLD activation in cell signaling of ventricular pressure-overload hypertrophy.

Oxidative stress stimulates multiple MAPK signalling pathways and phosphorylation of the small HSP27 in the perfused amphibian heart

We investigated the activation of three subfamilies of MAPKs (ERK, JNKs and p38-MAPK) by oxidative stress in the isolated perfused amphibian heart. Activation of p43-ERK by 100 micro mol l(-1) H(2)O(2) was maximally observed within 5 min, remained elevated for 30 min and was comparable with the effect of 1 micro mol l(-1) PMA. p43-ERK activation by H(2)O(2) was inhibited by PD98059 but not by SB203580. The p46 and p52 species of JNKs were maximally activated by 2.5- and 2.1-fold, respectively, by 100 micro mol l(-1) H(2)O(2) within 2 min. JNK activation was still detectable after 15 min, reaching control values at 30 min of treatment. p38-MAPK was maximally activated by 9.75-fold by 100 micro mol l(-1) H(2)O(2) after 2 min and this activation progressively declined thereafter, reaching control values within 45 min of treatment. The observed dose-dependent profile of p38-MAPK activation by H(2)O(2) revealed that 30 micro mol l(-1) H(2)O(2) induced maximal phosphorylation, whereas 100-300 micro mol l(-1) H(2)O(2) induced considerable activation of the kinase. Our studies also showed that the phosphorylation of MAPKAPK2 by H(2)O(2) followed a parallel time-dependent pattern and that SB203580 abolished this phosphorylation. Furthermore, our experiments clearly showed that 30 micro mol l(-1) H(2)O(2) induced a strong, specific phosphorylation of HSP27. Our immunohistochemical studies showed that immune complexes of phosphorylated forms of both p38-MAPK and HSP27 were strongly enhanced by 30 micro mol l(-1) H(2)O(2) in the perinuclear region as well as dispersedly in the cytoplasm of ventricular cells and that SB203580 abolished this phosphorylation. These data indicate that oxidative stress is a powerful activator of all three MAPK subfamilies in the amphibian heart. Stimulation of p38-MAPK and the consequent phosphorylation of HSP27 may be important in cardioprotection under such conditions.

Role for interleukin-1 beta in Trypanosoma cruzi-induced cardiomyocyte hypertrophy

Chagas' disease, the leading cause of heart failure in Latin America, results from infection with the intracellular protozoan parasite Trypanosoma cruzi. Host cell responses elicited in the myocardium early in the infective process are thought to be critical for establishment of infection by this pathogen; however, these changes have not been well characterized. We report here that primary cardiomyocytes undergo hypertrophy as an early response to T. cruzi infection. The T. cruzi-elicited hypertrophic response is characterized by increased expression of genes encoding the contractile proteins MyHC beta and MyHC alpha, followed by an approximately twofold increase in cell size. Hypertrophy was observed in both parasite-containing and noninfected cell populations represented in T. cruzi-infected cultures, indicating the involvement of a soluble mediator in this process. Conditioned medium harvested from T. cruzi-infected cultures, which contained significant levels of interleukin-1 beta (IL-1 beta) but not endothelin-1 or tumor necrosis factor alpha, was sufficient to induce hypertrophy in isolated cardiomyocytes. Addition of a high-affinity receptor chimera, IL-1 trap, to cardiomyocyte cultures blocked the overall increase in cell size elicited by T. cruzi. These novel findings indicate that IL-1 beta, which is rapidly induced in response to T. cruzi, promotes cardiomyocyte hypertrophy early in the infective process and may contribute to maintenance of cardiomyocyte function during establishment of T. cruzi infection in the heart.

α1- and β-adrenoceptor stimulation differentially activate p38-MAPK and atrial natriuretic peptide production in the perfused amphibian heart

We investigated the activation of p38-MAPK by various adrenergic agents in the perfused Rana ridibunda heart. Phenylephrine (50 μmol l-1) rapidly induced the differential activation of all three mitogen-activated protein kinase (MAPK) subfamilies (ERK, JNKs and p38-MAPK)in this experimental system. Focusing on p38-MAPK response to phenylephrine,we found that the kinase phosphorylation reached maximal values at 30 s,declining thereafter to basal values at 15 min. p38-MAPK activation by phenylephrine was verified as exclusively α1-AR-mediated. Furthermore, SB203580 (1 μmol l-1) abolished the kinase phosphorylation by phenylephrine. Isoproterenol (50 μmol l-1)was also shown to activate p38-MAPK in a time- and temperature-dependent manner. A marked, sustained p38-MAPK activation profile was observed at 25°C, while at 18°C the kinase response to isoproterenol was modest. Isoproterenol effect on p38-MAPK stimulation was β-AR-mediated. Immunohistochemical studies revealed the enhanced presence of phosphorylated p38-MAPK and atrial natriuretic peptide (ANP) in both phenylephrine- and isoproterenol-stimulated hearts, a reaction completely blocked by the respective specific antagonists, or the specific p38-MAPK inhibitor SB203580. These findings indicate a functional correlation between p38-MAPK activation and ANP accumulation in the perfused amphibian heart.

Actin capping protein: an essential element in protein kinase signaling to the myofilaments

Actin capping protein (CapZ) binds the barbed ends of actin at sarcomeric Z-lines. In addition to anchoring actin, Z-discs bind protein kinase C (PKC). Although CapZ is crucial for myofibrillogenesis, its role in muscle function and intracellular signaling is unknown. We hypothesized that CapZ downregulation would impair myocardial function and disrupt PKC-myofilament signaling by impairing PKC-Z-disc interaction. To test these hypotheses, we examined transgenic (TG) mice in which cardiac CapZ protein is reduced. Fiber bundles were dissected from papillary muscles and detergent extracted. Some fiber bundles were treated with PKC activators phenylephrine (PHE) or endothelin (ET) before detergent extraction. We simultaneously measured Ca2+-dependent tension and actomyosin MgATPase activity. CapZ downregulation increased myofilament Ca2+ sensitivity without affecting maximum tension or actomyosin MgATPase activity. Maximum tension and actomyosin MgATPase activity were decreased after PHE or ET treatment of wild-type (WT) muscle. Fiber bundles from TG hearts did not respond to PHE or ET. Immunoblot analysis revealed an increase in myofilament-associated PKC-epsilon after PHE or ET exposure of WT preparations. In contrast, myofilament-associated PKC-epsilon was decreased after PHE or ET treatment in TG myocardium. Protein levels of myofilament-associated PKC-beta were decreased in TG ventricle. C-protein and troponin I phosphorylation was increased after PHE or ET treatment in WT and TG hearts. Basal phosphorylation levels of C-protein and troponin I were higher in TG myocardium. These results indicate that downregulation of CapZ, or other changes associated with CapZ downregulation, increases cardiac myofilament Ca2+ sensitivity, inhibits PKC-mediated control of myofilament activation, and decreases myofilament-associated PKC-beta.

Calcineurin and cardiac hypertrophy: where have we been? Where are we going?

The heart is a dynamic organ capable of adapting its size and architecture in response to alterations in workload associated with developmental maturation, physiological stimulation and pathological diseases. Such alterations in heart size typically result from the hypertrophic growth of individual myocytes, but not myocyte cellular proliferation. In recent years, a great deal of investigation has gone toward elucidating the molecular signalling machinery that underlies the hypertrophic response and manner in which increased cardiac load promotes alterations in gene expression. To this end, the Ca(2+)-calmodulin-activated phosphatase calcineurin has been proposed as a necessary component of the multi-pathway hypertrophy program in the heart. Despite initial controversy over this hypothesis due to disparate results from pharmacological inhibitory studies in animal models of hypertrophy, compelling data from genetic models with calcineurin inhibition now exist. This review will summarize many of these studies and will attempt to address a number of unanswered issues. In particular, specific downstream mediators of calcineurin signalling will be discussed, as well as the need to identify calcineurin's temporal activation profile, transcriptional targets and cross-communication with other reactive signalling pathways in the heart. Finally, we will present evidence suggesting that calcineurin, as a Ca(2+)-responsive enzyme, may function as an internal load sensor in cardiac myocytes, matching output demands to hypertrophic growth.

Impaired cardiac hypertrophic response in Calcineurin Abeta -deficient mice

Calcineurin is a calcium-calmodulin-regulated, serine-threonine phosphatase that functions as a key inducer of stress responsive gene expression in multiple cell types through a direct activation of nuclear factor of activated T cells and myocyte enhancer factor 2 transcription factors. In cardiomyocytes, calcineurin signaling has been implicated in the regulation of the hypertrophic response caused by pressure overload or neuroendocrine stimulation. Three separate genes encode the catalytic subunit of calcineurin in mammalian cells, CnAalpha, CnAbeta, and CnAgamma. To evaluate the necessary function of calcineurin as a hypertrophic regulatory factor, the CnAbeta gene was disrupted in the mouse. CnAbeta-deficient mice were viable, fertile, and overtly normal well into adulthood, but displayed a 80% decrease in calcineurin enzymatic activity in the heart that was associated with a 12% reduction in basal heart size. CnAbeta-deficient mice were dramatically impaired in their ability to mount a productive hypertrophic response induced by pressure overload, angiotensin II infusion, or isoproterenol infusion. Analysis of marker genes associated with the hypertrophic response revealed a partial defect in the molecular program of hypertrophy. Collectively, these data solidify the hypothesis that calcineurin functions as a central regulator of the cardiac hypertrophic growth response in vivo.

Differential activation of mitogen-activated protein kinase cascades and apoptosis by protein kinase C epsilon and delta in neonatal rat ventricular myocytes

Protein kinase C (PKC) epsilon and PKCdelta translocation in neonatal rat ventricular myocytes (NRVMs) is accompanied by subsequent activation of the ERK, JNK, and p38(MAPK) cascades; however, it is not known if either or both novel PKCs are necessary for their downstream activation. Use of PKC inhibitors to answer this question is complicated by a lack of isoenzyme specificity, and the fact that many PKC inhibitors stimulate JNK and p38(MAPK) activity. Therefore, replication-defective adenoviruses (Advs) encoding constitutively active (ca) mutants of PKCepsilon and PKCdelta were used to test if either or both of these PKCs are sufficient to activate ERKs, JNKs, and/or p38(MAPK) in NRVMs. Adv-caPKCepsilon infection (1 to 25 multiplicities of viral infection (MOI); 4 to 48 hours) increased total PKCepsilon levels in a time- and dose-dependent manner, with maximal expression observed 8 hours after Adv infection. Adv-caPKCepsilon induced a time- and dose-dependent increase in phosphorylated p42 and p44 ERKs, as compared with a control Adv encoding beta-galactosidase (Adv-nebetagal). Maximal ERK phosphorylation occurred 8 hours after Adv infection. In contrast, JNK was only minimally activated, and p38(MAPK) was relatively unaffected. Adv-caPKCdelta infection (1 to 25 MOI, 4 to 48 hours) increased total PKCdelta levels in a similar fashion. Adv-caPKCdelta (5 MOI) induced a 29-fold increase in phosphorylated p54 JNK, and a 15-fold increase in phosphorylated p38(MAPK) 24 hours after Adv infection. In contrast, p42 and p44 ERK were only minimally activated. Whereas neither Adv induced NRVM hypertrophy, Adv-caPKCdelta, but not Adv-caPKCepsilon, induced NRVM apoptosis. We conclude that the novel PKCs differentially regulate MAPK cascades and apoptosis in an isoenzyme-specific and time-dependent manner.

Activation of NF-kappa B is required for hypertrophic growth of primary rat neonatal ventricular cardiomyocytes

The transcription factor NF-kappaB regulates expression of genes that are involved in inflammation, immune response, viral infection, cell survival, and division. However, the role of NF-kappaB in hypertrophic growth of terminally differentiated cardiomyocytes is unknown. Here we report that NF-kappaB activation is required for hypertrophic growth of cardiomyocytes. In cultured rat primary neonatal ventricular cardiomyocytes, the nuclear translocation of NF-kappaB and its transcriptional activity were stimulated by several hypertrophic agonists, including phenylephrine, endothelin-1, and angiotensin II. The activation of NF-kappaB was inhibited by expression of a "supersuppressor" IkappaBalpha mutant that is resistant to stimulation-induced degradation and a dominant negative IkappaB kinase (IKKbeta) mutant that can no longer be activated by phosphorylation. Furthermore, treatment with phenylephrine induced IkappaBalpha degradation in an IKK-dependent manner, suggesting that NF-kappaB is a downstream target of the hypertrophic agonists. Importantly, expression of the supersuppressor IkappaBalpha mutant or the dominant negative IKKbeta mutant blocked the hypertrophic agonist-induced expression of the embryonic gene atrial natriuretic factor and enlargement of cardiomyocytes. Conversely, overexpression of NF-kappaB itself induced atrial natriuretic factor expression and cardiomyocyte enlargement. These findings suggest that NF-kappaB plays a critical role in the hypertrophic growth of cardiomyocytes and may serve as a potential target for the intervention of heart disease.

The transcription factors GATA4 and GATA6 regulate cardiomyocyte hypertrophy in vitro and in vivo

The zinc finger-containing transcription factors GATA4 and GATA6 are important regulators of basal and inducible gene expression in cardiac and smooth muscle cell types. Here we demonstrate a direct functional role for GATA4 and GATA6 as regulators of cardiomyocyte hypertrophic growth and gene expression. To model the increase in endogenous GATA4 and GATA6 transcriptional activity that occurs in response to hypertrophic stimulation, each factor was overexpressed in cardiomyocytes using recombinant adenovirus. Overexpression of either GATA4 or GATA6 was sufficient to induce cardiomyocyte hypertrophy characterized by enhanced sarcomeric organization, a greater than 2-fold increase in cell surface area, and a significant increase in total protein accumulation. In vivo, transgenic mice with 2.5-fold overexpression of GATA4 within the adult heart demonstrated a slowly progressing increase in heart to body weight ratio, histological features of cardiomyopathy, and activation of hypertrophy-associated genes, suggesting that GATA factors are sufficient regulators of cardiomyocyte hypertrophy in vitro and in vivo. To evaluate the requirement of GATA factors as downstream transcriptional mediators of hypertrophy, a dominant negative GATA4-engrailed repressor fusion-encoding adenovirus was generated. Expression of GATA4-engrailed blocked GATA4- and GATA6-directed transcriptional responses and agonist-induced cardiomyocyte hypertrophy, demonstrating that cardiac-expressed GATA factors are necessary mediators of this process.

Enhanced Ca2+ channel currents in cardiac hypertrophy induced by activation of calcineurin-dependent pathway

Cardiac-specific expression of an activated calcineurin protein in the hearts of transgenic (CLN) mice produces a profound hypertrophy that rapidly progresses to heart failure. While calcineurin is regulated by Ca2+, the potential effects of calcineurin on cardiac myocyte Ca2+ handling has not been evaluated. To this end, we examined L-type Ca2+ currents (I(Ca)) in left ventricular myocytes. CLN myocytes had larger (approximately 80%) cell capacitance and enhanced I(Ca) density (approximately 20%) compared with non-transgenic (NTG) littermates, but no change in the current-voltage relationship, single-channel conductance or protein levels of alpha 1 or beta 2 subunit of L-type Ca2+ channels. Interestingly, the kinetics of I(Ca) inactivation was faster (approximately two-fold) in CLN myocytes compared with NTG myocytes. Ryanodine application slowed the rate of I(Ca) inactivation in both groups and abolished the kinetic difference, suggesting that Ca2+ dependent inactivation is increased in CLN myocytes due to altered SR Ca2+ release. Treatment of CLN mice with Cyclosporine A (CsA), a calcineurin inhibitor, prevented myocyte hypertrophy and changes in I(Ca) activity and inactivation kinetics. However, there was no direct effect of CsA on I(Ca) in either NTG or CLN myocytes, suggesting that endogenous calcineurin activity does not directly regulate Ca2+ channel activity. This interpretation is consistent with the observation that I(Ca) density, inactivation kinetics and regulation by isoproterenol were normal in cardiac-specific transgenic mice expressing calcineurin inhibitory protein domains from either Cain or AKAP79. Taken together these data suggest that chronic activation of calcineurin is associated with myocyte hypertrophy and a secondary enhancement of intracellular Ca2+ handling that is tied to the hypertrophy response itself.

The dual-specificity phosphatase MKP-1 limits the cardiac hypertrophic response in vitro and in vivo

Mitogen-activated protein kinase (MAPK) signaling pathways are important regulators of cell growth, proliferation, and stress responsiveness. A family of dual-specificity MAP kinase phosphatases (MKPs) act as critical counteracting factors that directly regulate the magnitude and duration of p38, c-Jun N-terminal kinase (JNK), and extracellular signal-regulated kinase (ERK) activation. Here we show that constitutive expression of MKP-1 in cultured primary cardiomyocytes using adenovirus-mediated gene transfer blocked the activation of p38, JNK1/2, and ERK1/2 and prevented agonist-induced hypertrophy. Transgenic mice expressing physiological levels of MKP-1 in the heart showed (1) no activation of p38, JNK1/2, or ERK1/2; (2) diminished developmental myocardial growth; and (3) attenuated hypertrophy in response to aortic banding and catecholamine infusion. These results provide further evidence implicating MAPK signaling factors as obligate regulators of cardiac growth and hypertrophy and demonstrate the importance of dual-specificity phosphatases as counterbalancing regulatory factors in the heart.