Beta-adrenergic regulation of a myocardial actin gene via a cyclic AMP-independent pathway

Effect of β-adrenergic receptor agents on cardiac structure and function and whole-body gene expression in Daphnia magna

Propranolol (PRO), a human β-AR (β-adrenergic receptor) antagonist, is considered to result in specific effects in a non-target species, D. magna, based on our previous studies. The present study investigated the effects of β-AR agents, including an antagonist and agonist using pharmacologically relevant endpoints as well as a more holistic gene expression approach to reveal the impacts and potential mode of actions (MOAs) in the model non-target species. Results show that the responses in cardiac endpoints and gene expression in D. magna are partially similar but distinguishable from the observations in different organisms. No effect was observed on heart size growth in PRO and isoprenaline (ISO) exposure. The contraction capacity of the heart was decreased in ISO exposure, and the heart rate was decreased in PRO exposure. Time-series exposures showed different magnitudes of effect on heart rate and gene expression dependent on the type of chemical exposure. Significant enrichment of gene families involved in protein metabolism and biotransformation was observed within the differentially expressed genes, and we also observed differential expression in juvenile hormone-inducible proteins in ISO and PRO exposure, which is suspected of having endocrine disruption potential. Taken together, deviation between the effects of PRO and ISO in D. magna and other organisms suggests dissimilarity in MOAs or attributes of target bio-molecules between species. Additionally, PRO and ISO may act as endocrine disruptors based on the gene expression observation. Results in the present study confirm that it is challenging to predict ecological impact of active pharmaceutical ingredients (APIs) based on the available data acquired through human-focused studies. Furthermore, the present study provided unique data and a case study on the impact of APIs in a non-target organism.

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

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

Identification of proteins responding to adrenergic receptor subtype-specific hypertrophy in cardiomyocytes by proteomic approaches

The individual signaling pathways underlying cardiac hypertrophy, which is induced by either α or β adrenergic receptor (AR), are different. Activation of different AR subtypes couples to different G proteins and induction of specific signaling pathways, which ultimately results in subtype-specific regulation of cardiac function. We present the first proteomics study identifying proteins that are related to AR subtype-specific hypertrophy in cardiomyocytes by comparing the two-dimensional electrophoresis patterns between neonatal rat cardiomyocytes treated by phenylepinephrin (PE) and by isoproterenol (ISO). An improved 2-DE strategy was used in these comparative experiments. Twenty-five differentially expressed proteins in cardiomyocytes treated by PE or treated by ISO were successfully analyzed and identified using matrix-assisted laser desorption/ionization-time of flight mass spectrometry, especially those that might be responsible to intracellular oxidative stress such as dismutase, peroxiredoxin, and thioredoxin-like protein p46. In addition, induced reactive oxygen species were also found to be AR subtype-specifically relevant to endoplasmic reticulum proteinase ERK1/2 phosphorylation during the development of hypertrophy induced by different AR subtypes. The results will help to better understand the underlying mechanisms of different adrenergic receptor subtype-induced hypertrophy.

Modulation of myocardial stiffness by β-adrenergic stimulation--its role in normal and failing heart

The acute effects of beta-adrenergic stimulation on myocardial stiffness were evaluated. New-Zealand white rabbits were treated with saline (control group) or doxorubicin to induce heart failure (HF) (DOXO-HF group). Effects of isoprenaline (10(-10)-10(-5) M), a non-selective beta-adrenergic agonist, were tested in papillary muscles from both groups. In the control group, the effects of isoprenaline were also evaluated in the presence of a damaged endocardial endothelium, atenolol (beta(1)-adrenoceptor antagonist), ICI-118551 (beta(2)-adrenoceptor antagonist), KT-5720 (PKA inhibitor), L-NNA (NO-synthase inhibitor), or indomethacin (cyclooxygenase inhibitor). Passive length-tension relations were constructed before and after adding isoprenaline (10(-5) M). In the control group, isoprenaline increased resting muscle length up to 1.017+/-0.006 L/L(max). Correction of resting muscle length to its initial value resulted in a 28.5+/-3.1 % decrease of resting tension, indicating decreased muscle stiffness, as confirmed by the isoprenaline-induced right-downward shift of the passive length-tension relation. These effects were modulated by beta(1)- and beta(2)-adrenoceptors and PKA. In DOXO-HF group, the effect on myocardial stiffness was significantly decreased. We conclude that beta-adrenergic stimulation is a relevant mechanism of acute neurohumoral modulation of the diastolic function. Furthermore, this study clarifies the mechanisms by which myocardial stiffness is decreased.

Calcitonin gene related peptide (CGRP) inhibits norepinephrine induced apoptosis in cultured rat cardiomyocytes not via PKA or PKC pathways

Evidence showed overrelease of norepinephrine can induce apoptosis in ventricle myocytes. Calcitonin gene related peptide and norepinephrine could be simultaneously up-regulated in early time of acute myocardial ischemia, suggesting a co-participation of calcitonin gene related peptide and norepinephrine in the pathology. In this study, we investigated a potential anti-apoptotic effect of calcitonin gene related peptide on myocardial apoptosis induced by norepinephrine and its link with the protein kinase A (PKA) or protein kinase C (PKC) pathway in cultured neonatal rat cardiomyocytes. Cultured cardiomyocytes were exposed to one of the treatments, separately: (1) 3 ml DMEM culture medium, (2) norepinephrine (10(-5)mol/l), (3) H89 (3 x 10(-5)mol/l), a specific PKA inhibitor, with norepinephrine (10(-5)mol/l), (4) calcitonin gene related peptide at a range of concentrations (10(-9)mol/l, 10(-8)mol/l and 10(-7)mol/l) with norepinephrine (10(-5)mol/l) and (5) calcitonin gene related peptide (10(-8)mol/l) with norepinephrine (10(-5)mol/l)+CGRP(8-7) (10(-7)mol/l), a specific antagonist of calcitonin gene related peptide receptor. Then, apoptosis rate and the activity of PKA and PKC were examined. The dose of norepinephrine induced a marked increase in apoptosis of the myocytes (31+/-2%), compared to the control (17+/-4%, p<0.05). The pro-apoptotic effect of norepinephrine was attenuated by H89 (3 x 10(-5)mol/l) or by calcitonin gene related peptide which could be completely reversed by CGRP(8-37). The activities of PKA and PKC were increased by norepinephrine but no difference in the activities of PKA and PKC was detected in the presence and absence of co-treatment with calcitonin gene related peptide (10(-8)mol/l). Calcitonin gene related peptide inhibits norepinephrine induced apoptosis in cultured cardiomyocytes, which is mediated by CGRP receptor but unlikely to be mediated by PKA or PKC pathway.

Structure and torsion in the normal and situs inversus totalis cardiac left ventricle. II. Modeling cardiac adaptation to mechanical load

Mathematical models provide a suitable platform to test hypotheses on the relation between local mechanical stimuli and responses to cardiac structure and geometry. In the present model study, we tested hypothesized mechanical stimuli and responses in cardiac adaptation to mechanical load on their ability to estimate a realistic myocardial structure of the normal and situs inversus totalis (SIT) left ventricle (LV). In a cylindrical model of the LV, 1) mass was adapted in response to myofiber strain at the beginning of ejection and to global contractility (average systolic pressure), 2) cavity volume was adapted in response to fiber strain during ejection, and 3) myofiber orientations were adapted in response to myofiber strain during ejection and local misalignment between neighboring tissue parts. The model was able to generate a realistic normal LV geometry and structure. In addition, the model was also able to simulate the instigating situation in the rare SIT LV with opposite torsion and transmural courses in myofiber direction between the apex and base [Delhaas et al. (6)]. These results substantiate the importance of mechanical load in the formation and maintenance of cardiac structure and geometry. Furthermore, in the model, adapted myocardial architecture was found to be insensitive to fiber misalignment in the transmural direction, i.e., myofiber strain during ejection was sufficient to generate a realistic transmural variation in myofiber orientation. In addition, the model estimates that, despite differences in structure, global pump work and the mass of the normal and SIT LV are similar.

Sustained beta1-adrenergic stimulation modulates cardiac contractility by Ca2+/calmodulin kinase signaling pathway

A tenet of beta1-adrenergic receptor (beta1AR) signaling is that stimulation of the receptor activates the adenylate cyclase-cAMP-protein kinase A (PKA) pathway, resulting in positive inotropic and relaxant effects in the heart. However, recent studies have suggested the involvement of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in beta1AR-stimulated cardiac apoptosis. In this study, we determined roles of CaMKII and PKA in sustained versus short-term beta1AR modulation of excitation-contraction (E-C) coupling in cardiac myocytes. Short-term (10-minute) and sustained (24-hour) beta1AR stimulation with norepinephrine similarly enhanced cell contraction and Ca2+ transients, in contrast to anticipated receptor desensitization. More importantly, the sustained responses were largely PKA-independent, and were sensitive to specific CaMKII inhibitors or adenoviral expression of a dominant-negative CaMKII mutant. Biochemical assays revealed that a progressive and persistent CaMKII activation was associated with a rapid desensitization of the cAMP/PKA signaling. Concomitantly, phosphorylation of phospholamban, an SR Ca2+ cycling regulatory protein, was shifted from its PKA site (16Ser) to CaMKII site (17Thr). Thus, beta1AR stimulation activates dual signaling pathways mediated by cAMP/PKA and CaMKII, the former undergoing desensitization and the latter exhibiting sensitization. This finding may bear important etiological and therapeutical ramifications in understanding beta1AR signaling in chronic heart failure.

Reversible connexin 43 dephosphorylation during hypoxia and reoxygenation is linked to cellular ATP levels

Altered gap junction coupling of cardiac myocytes during ischemia may contribute to development of lethal arrhythmias. The phosphoprotein connexin 43 (Cx43) is the major constituent of gap junctions. Dephosphorylation of Cx43 and uncoupling of gap junctions occur during ischemia, but the significance of Cx43 phosphorylation in this setting is unknown. Here we show that Cx43 dephosphorylation in synchronously contracting myocytes during ischemia is reversible, independent of hypoxia, and closely associated with cellular ATP levels. Cx43 became profoundly dephosphorylated during hypoxia only when glucose supplies were limited and was completely rephosphorylated within 30 minutes of reoxygenation. Similarly, direct reduction of ATP by various combinations of metabolic inhibitors and by ouabain was closely paralleled by loss of phosphoCx43 and recovery of phosphoCx43 accompanied restoration of ATP. Dephosphorylation of Cx43 could not be attributed to hypoxia, acid pH or secreted metabolites, or to AMP-activated protein kinase; moreover, the process was selective for Cx43 because levels of phospho-extracellular signal regulated kinase (ERK)1/2 were increased throughout. Rephosphorylation of Cx43 was not dependent on new protein synthesis, or on activation of protein kinases A or G, ERK1/2, p38 mitogen-activated protein kinase, or Jun kinase; however, broad-spectrum protein kinase C inhibitors prevented Cx43 rephosphorylation while also sensitizing myocytes to reoxygenation-mediated cell death. We conclude that Cx43 is reversibly dephosphorylated and rephosphorylated during hypoxia and reoxygenation by a novel mechanism that is sensitive to nonlethal fluctuations in cellular ATP. The role of this regulated phosphorylation in the adaptation to ischemia remains to be determined.

Regulation of protein kinase C isozyme and calcineurin expression in isoproterenol induced cardiac hypertrophy

Protein kinase C (PKC) and calcineurin are known to play a pivotal role in the development of cardiomyocyte growth. However, its role in Isoproterenol-induced (Iso) cardiac hypertrophy has not been characterized so far and were focus of the current study. After chronic beta-adrenergic stimulation of male Wistar rats with Iso (2mg/kg x day) for 2 and 7 days using osmotic minipumps, we determined a) cardiac PKC-activity, b) the expression of cardiac PKC isozymes (PKC-alpha, PKC-delta and PKC-epsilon) both at the protein and the mRNA-level and c) the expression of calcineurin using Western blot analysis. Iso-treatment for 2 and 7 days results in cardiac hypertrophy with an increase of the heart weight-to-body weight ratio by 36% and 27%. Iso-induced myocardial growth was associated with an enhanced total PKC-activity and a significant increased protein expression of cytosolic PKC-alpha (day 2: +38%; day 7: +43%), PKC-delta (day 2: 85%; day 7: +78%) and PKC-epsilon (day 7: +58%). The protein amount of calcineurin was not significantly altered by Iso compared with sham-operated controls. The increased expression of PKC-alpha, PKC-delta and PKC-epsilon in the cytosol was paralleled by a transcriptional upregulation of the absolute mRNA-levels of these PKC-isozymes as determined by quantitative RT-PCR.

Beta-adrenergic receptor activation inhibits keratinocyte migration via a cyclic adenosine monophosphate-independent mechanism

There is increasing evidence that G-protein-coupled receptors cross-talk with growth factor receptor-mediated signal transduction in a variety of cell types. We have investigated mechanisms by which the activation of beta-adrenergic receptors, classically GTP-binding proteins coupled receptors, influence the migration of cultured human keratinocytes. We found that iso-proterenol, a beta-adrenergic receptor-selective agonist, inhibited cell migration stimulated by either epidermal growth factor, or extracellular Ca2+ in a concentration-dependent manner. This was prevented by pretreatment of the cells with the beta-adrenergic receptor-selective antagonist timolol. Interestingly, isoproterenol, at a concentration of 1 nm, did not measurably increase intracellular cyclic adenosine monophosphate concentrations yet inhibited cell migration by 50%. To test further if isoproterenol's actions were mediated via activation of adenylyl cyclase, two inhibitors of its activity, 2'5'-dideoxyadenosine and SQ22536, were used. Both compounds significantly diminished iso-proterenol-induced increases in intracellular cyclic adenosine monophosphate concentrations but did not attenuate isoproterenol-induced inhibition of cell migration. Also, forskolin (1 microm) markedly increased intracellular cyclic adenosine monophosphate concentrations but did not significantly inhibit cell migration. As mitogen-activated protein kinases are known to signal growth factor-stimulated cell migration, we examined whether beta-adrenergic receptor-mediated inhibition of keratinocyte migration might occur via inactivation of mitogen-activated protein kinases. We found that isoproterenol inhibited phosphorylation of extracellular signal-regulated kinase mitogen-activated protein kinase in a concentration-dependent manner but had no effect on the phosphorylation of the stress mitogen-activated protein kinases c-jun N-terminal kinase and stress-activated protein kinase-2. Neither forskolin nor a membrane permeable cyclic adenosine monophosphate analog inhibited phosphorylation of any of these mitogen-activated protein kinases. These findings suggest that beta-adrenergic receptor-induced inhibition of keratinocyte migration is mediated through inhibition of the extracellular signal-regulated kinase mitogen-activated protein kinase signaling in a cyclic adenosine monophosphate-independent manner.

Combination of therapeutic apheresis and therapeutic ventricular assistance for end-stage heart failure patients

Dilated cardiomyopathy is a cardiac disease of unknown origin which is characterized by the gradual development of cardiac failure associated with four-chamber dilatation of the heart. Heart transplantation has been considered as the last resort for this disease. However, some patients who received support with a ventricular assist device (VAD) as a bridge-to-transplantation and then recovered without transplantation have been reported. This new concept of treating heart failure is termed bridge-to-recovery. A VAD can inhibit the heart failure compensatory mechanisms by extreme ventricular unloading. Also, heart failure is a complex neurohormonal/autocrine-paracrine syndrome, and these mechanisms consecutively lead to inflammatory response by proinflammatory cytokines; interleukin-1 alpha (IL-1 alpha), interleukin-1 beta (IL-1 beta), interleukin-2 (IL-2), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha). Furthermore, the existence of anti-beta1-adrenoceptor autoantibodies (A-beta1-AABs) in a patient with dilated cardiomyopathy has been reported. These proinflammatory cytokines and this antibody accelerate a ventricular remodeling and a contractile dysfunction over the long term. Apheresis can also inhibit the vicious cycle in heart failure by removing the factors that are produced by activated neurohormonal/autocrine-paracrine compensatory mechanisms. Therefore, we propose that the combined therapies, therapeutic VAD and therapeutic apheresis, will provide a prominent outcome for a patient who is suffering from end-stage heart failure.

Chronic clenbuterol administration negatively alters cardiac function

PURPOSE

Chronic administration of pharmacological levels of beta2-agonists have been shown to have toxic effects on the heart; however, no data exist on cardiac function after chronic clenbuterol administration. The purpose of this study was to examine the effect of therapeutic levels of clenbuterol on cardiac performance.

METHODS

Twenty unfit Standardbred mares were divided into four experimental groups: clenbuterol (2.4 microg.kg(-1) twice daily 5 d.wk(-1)) plus exercise (20 min at 50% .VO(2max)) (CLENEX; N = 6), clenbuterol (CLEN; N = 6), exercise (EX; N = 4), and control (CON; N = 4). M-mode and two-dimensional echocardiography (2.5-MHz sector scanner transducer) were used to measure cardiac size and function before and immediately after an incremental exercise test, before and after 8 wk of drug and/or exercise treatments.

RESULTS

After treatment, CLENEX and CLEN demonstrated significantly higher left ventricular internal dimension (LVD) at end diastole (+23.7 +/- 4.8%; +25.6 +/- 4.1%), LVD at end systole (+29.2 +/- 8.7%; +40.1 +/- 7.9%), interventricular septal wall thickness (IVS) at end diastole (+28.9 +/- 11.0%; +30.7 +/- 7.0%), IVS at end systole (+29.2 +/- 8.7%; +40.1 +/- 7.9%), and left ventricular posterior wall systolic thickness (+43.1 +/- 14.%; +45.8 +/- 14.1%). CLENEX and CLEN had significantly increased aortic root dimensions (+29.9 +/- 6.1%; +24.0 +/- 1.7%), suggesting increased risk of aortic rupture.

CONCLUSION

Taken together, these data indicate that chronic clenbuterol administration may negatively alter cardiac function.

Chronic administration of therapeutic levels of clenbuterol acts as a repartitioning agent

The purpose of this study was to examine the effect of therapeutic levels of clenbuterol, with and without exercise training, on body composition. Twenty-three unfit Standardbred mares were divided into four experimental groups: clenbuterol (2.4 microg/kg body wt twice daily) plus exercise (ClenEx; 20 min at 50% maximal oxygen consumption 3 days/wk; n = 6), clenbuterol only (Clen; n = 6), exercise only (Ex; n = 5), and control (Con; n = 6). Rump fat thickness was measured at 2-wk intervals by using B-mode ultrasound, and percent body fat (%fat) was calculated by using previously published methods. For Ex, body fat decreased (P < 0.05) at week 4 (-9.3%), %fat at week 6 (-6.9%), and fat-free mass (FFM) increased (P < 0.05) at week 8 (+3.2%). On the other hand, Clen had significant changes in %fat (-15.4%), fat mass (-14.7%), and FFM (+4.3%) at week 2. ClenEx had significant decreases in %fat (-17.6%) and fat mass (-19.5%) at week 2, which was similar to Clen; however, this group had a different FFM response, which significantly increased (+4.4%) at week 6. Con showed no changes (P > 0.05) in any variable at any time. These results suggest that exercise training and clenbuterol have additive effects with respect to %fat and fat mass but antagonistic effects in terms of FFM. Furthermore, chronic clenbuterol administration causes significant repartitioning in the horse, even when administered in therapeutic doses.

beta-adrenergic mechanisms in cardiac diseases: a perspective

Several lines of evidence show that neurohumoral systems, especially those involving catecholamines, play a crucial role in cardiac diseases. Changes in the beta-adrenergic receptor (beta-AR) system such as receptor down-regulation, uncoupling from G-proteins, receptor internalization and receptor degradation may account for some of the abnormalities of contractile function in this disease. Increases in the level of inhibitory G-protein subunits also appears to be involved in attenuating the beta-AR signal. Finally beta-AR signalling is strongly regulated by members of the G-protein-coupled receptor kinase family (GRKs), the best known of which is beta-adrenergic receptor kinase 1 (beta-ARK1). beta-ARK1 mRNA, protein level and enzymatic activity is increased in heart disease, further contributing to an attenuation in beta-AR signalling. The combination of these negative alterations are presumably related to the contractile dysfunction seen in human heart disease. The combination of biochemical, physiological and molecular biological studies bearing on the normal function and regulation of these various molecules should provide strategies for elucidating the pharmacological basis of the regulation of myocardial contractility in the normal and failing heart.

beta(2)-adrenergic receptor overexpression exacerbates development of heart failure after aortic stenosis

BACKGROUND

Beta-adrenergic signaling is downregulated in the failing heart, and the significance of such change remains unclear.

METHODS AND RESULTS

To address the role of beta-adrenergic dysfunction in heart failure (HF), aortic stenosis (AS) was induced in wild-type (WT) and transgenic (TG) mice with cardiac targeted overexpression of beta(2)-adrenergic receptors (ARs), and animals were studied 9 weeks later. The extents of increase in systolic arterial pressure (P<0.01 versus controls), left ventricular (LV) hypertrophy (TG, 94+/-6 to 175+/-7 mg; WT, 110+/-6 to 168+/-10 mg; both P<0.01), and expression of ANP mRNA were similar between TG and WT mice with AS. TG mice had higher incidences of premature death and critical illness due to heart failure (75% versus 23%), pleural effusion (81% versus 45%), and left atrial thrombosis (81% versus 36%, all P<0.05). A more extensive focal fibrosis was found in the hypertrophied LV of TG mice (P<0.05). These findings indicate a more severe LV dysfunction in TG mice. In sham-operated mice, LV dP/dt(max) and heart rate were markedly higher in TG than WT mice (both P<0.01). dP/dt(max) was lower in both AS groups than in sham-operated controls, and this tended to be more pronounced in TG than WT mice (-32+/-5% versus -16+/-6%, P=0.059), although dP/dt(max) remained higher in TG than WT groups (P<0.05).

CONCLUSIONS

Elevated cardiac beta-adrenergic activity by beta(2)-AR overexpression leads to functional deterioration after pressure overload.

alpha- and beta-adrenergic pathways differentially regulate cell type-specific apoptosis in rat cardiac myocytes

BACKGROUND

The apoptosis of cardiac myocytes may play a role in the development of heart failure. Norepinephrine is one of the factors activated in heart failure and can induce myocardial cell apoptosis in culture. However, it is unknown if alpha- and beta-adrenergic pathways coordinately or differentially regulate apoptosis and if this apoptotic pathway uses common or cell type-specific apoptotic signals.

METHODS AND RESULTS

We stimulated cultured neonatal rat cardiac myocytes with an alpha(1)-adrenergic agonist (PE, phenylephrine), a beta-adrenergic agonist (isoproterenol [Iso]) or a membrane-permeable cAMP analogue (8-Br-cAMP) in serum-free conditions for 48 hours. Iso and 8-Br-cAMP markedly increased the number of TUNEL-positive cells (%TUNEL-positive nuclei >40%) compared with saline stimulation (<10%). DNA fragmentation was also confirmed by ladder formation in agarose gels. Apoptotic myocytes were characterized by cell shrinkage and nuclear condensation, consistent with morphological features of apoptosis. The Iso-induced apoptosis was almost completely inhibited by the protein kinase A-specific inhibitor KT5720. In contrast, PE inhibited 8-Br-cAMP-induced myocardial cell apoptosis. The apoptosis-inhibitory effect by PE was negated by the alpha(1)-adrenergic receptor antagonist prazosin and the MEK-1-specific inhibitor PD098059. Interestingly, although 8-Br-cAMP markedly induced apoptosis in cardiac myocytes, it completely blocked serum depletion-induced apoptosis in PC12 cells, a rat pheochromocytoma cell line.

CONCLUSIONS

These findings indicate that alpha- and beta-adrenergic pathways differentially regulate myocardial cell apoptosis. The results also suggest that a cAMP- protein kinase A pathway is necessary and sufficient for beta-adrenergic agonist-induced apoptosis and that this apoptotic pathway is not functional in other cell types, for example, PC12 cells.

Factors contributing to pressure overload-induced immediate early gene expression in adult rat hearts in vivo

We determined the contributions of angiotensin II type 1 receptor (AT(1)) stimulation, adrenergic stimulation, and autonomic activation to pressure overload-induced c-fos expression in the adult rat heart in vivo. c-fos expression was increased in pressure-overloaded hearts created by aortic banding compared with sham-operated rats (458 +/- 100% vs. sham, P < 0.05). GR-138950, a selective AT(1) antagonist, did not blunt this expression (banding vs. banding + GR-138950: 458 +/- 100% vs. 500 +/- 125%, not significant). Atropine and hexamethonium partially decreased c-fos expression (banding vs. banding + atropine/hexamethonium: 700 +/- 67% vs. 400 +/- 67%, P < 0.05). Phentolamine had no significant effect on c-fos expression; however, propranolol inhibited the expression (banding vs. banding + propranolol: 492 +/- 108% vs. 154 +/- 15%, P < 0.05). The inhibition by propranolol was independent of the decreases in heart rate. Thus factors contributing to pressure overload-induced c-fos expression in adult rat hearts in vivo are different from those in neonatal myocytes in vitro undergoing stretch.

Modulation of cytokine-induced cardiac myocyte apoptosis by nitric oxide, Bak, and Bcl-x

-Cytokine-induced NO production depresses myocardial contractility and has been shown to be cytotoxic to cardiac myocytes. However, the mechanisms of cytokine-induced cardiac myocyte cell death are unclear. To analyze these mechanisms in detail, we treated neonatal cardiac myocytes in serum-free culture with a combination of the macrophage-derived cytokines interleukin-1beta, tumor necrosis factor-alpha, and interferon-gamma. These cytokines caused a time-dependent induction of cardiac myocyte apoptosis, but not necrosis, beginning 72 hours after treatment, as determined by nuclear morphology, DNA internucleosomal cleavage, and cleavage of poly(ADP-ribose) polymerase, reflecting caspase activation. Apoptosis was preceded by a >50-fold induction of inducible NO synthase mRNA and the release of large amounts (5 to 8 nmol/ microgram protein) of NO metabolites (NOx) into the medium. Cell death was completely blocked by an NO synthase inhibitor and attenuated by antioxidants (N-acetylcysteine and DTT) and the caspase inhibitor ZVAD-fmk. Cytokines also mediated an NO-dependent, sustained increase in myocyte expression of the Bcl-2 homologs Bak and Bcl-x(L). The NO donor S-nitrosoglutathione also induced apoptosis and cell levels of Bak, but not of Bcl-x(L). All effects of cytokines, including poly(ADP-ribose) polymerase cleavage, could be attributed to interleukin-1beta; interferon-gamma and tumor necrosis factor-alpha had no independent effects on apoptosis or on NOx production. We conclude that cytokine toxicity to neonatal cardiac myocytes results from the induction of NO and subsequent activation of apoptosis, at least in part through the generation of oxygen free radicals. The rate and extent of this apoptosis is modulated by alterations in the cellular balance of Bak and Bcl-x(L), which respond differentially to cytokine-induced and exogenous NO and by the availability of oxidant species.

Medical therapy of chronic heart failure. Role of ACE inhibitors and beta-blockers

Heart failure has long been considered to have a progressive downhill course leading inexorably to an early demise. This course often occurs silently, in the absence of any obvious cardiac insults. The reason for this is a combination of cell loss, myocyte dysfunction, impaired energetics, and pathologic remodeling of the chamber. Improved clinical outcome should result from strategies that reduce the biologic signals responsible for myocyte growth, dysfunction, and loss and chamber remodeling. Clinicians should no longer attempt to treat chronic heart failure with pharmacologic growth and remodeling process. In time, it may be possible for the clinician to view the treatment of heart failure largely as a matter of improving the biologic function of the myocardium.

Impact of beta-adrenergic agonist on Na+ channel and Na+-K+-ATPase expression in alveolar type II cells

It has been shown that short-term (hours) treatment with beta-adrenergic agonists can stimulate lung liquid clearance via augmented Na+ transport across alveolar epithelial cells. This increase in Na+ transport with short-term beta-agonist treatment has been explained by activation of the Na+ channel or Na+-K+-ATPase by cAMP. However, because the effect of sustained stimulation (days) with beta-adrenergic agonists on the Na+ transport mechanism is unknown, we examined this question in cultured rat alveolar type II cells. Na+-K+-ATPase activity was increased in these cells by 10(-4) M terbutaline in an exposure time-dependent manner over 7 days in culture. This increased activity was also associated with an elevation in transepithelial current that was inhibited by amiloride. The enzyme's activity was also augmented by continuous treatment with dibutyryl-cAMP (DBcAMP) for 5 days. This increase in Na+-K+-ATPase activity by 10(-4) M terbutaline was associated with an increased expression of alpha1-Na+-K+-ATPase mRNA and protein. beta-Adrenergic agonist treatment also enhanced the expression of the alpha-subunit of the epithelial Na+ channel (ENaC). These increases in gene expression were inhibited by propranolol. Amiloride also suppressed this long-term effect of terbutaline and DBcAMP on Na+-K+-ATPase activity. In conclusion, beta-adrenergic agonists enhance the gene expression of Na+-K+-ATPase, which results in an increased quantity and activity of the enzyme. This heightened expression is also associated with augmented ENaC expression. Although the cAMP system is involved, the inhibition of enhanced enzyme activity with amiloride suggests that increased Na+ entry at the apical surface plays a role in this process.

Effect of clenbuterol on sarcoplasmic reticulum function in single skinned mammalian skeletal muscle fibers

We examined the effect of the beta2-agonist clenbuterol (50 microM) on depolarization-induced force responses and sarcoplasmic reticulum (SR) function in muscle fibers of the rat (Rattus norvegicus; killed by halothane overdose) that had been mechanically skinned, rendering the beta2-agonist pathway inoperable. Clenbuterol decreased the peak of depolarization-induced force responses in the extensor digitorum longus (EDL) and soleus fibers to 77.2 +/- 9.0 and 55.6 +/- 5.4%, respectively, of controls. The soleus fibers did not recover. Clenbuterol significantly and reversibly reduced SR Ca2+ loading in EDL and soleus fibers to 81.5 +/- 2.8 and 78.7 +/- 4.0%, respectively, of controls. Clenbuterol also produced an approximately 25% increase in passive leak of Ca2+ from the SR of the EDL and soleus fibers. These results indicate that clenbuterol has direct effects on fast- and slow-twitch skeletal muscle, in the absence of the beta2-agonist pathway. The increased Ca2+ leak in the triad region may lead to excitation-contraction coupling damage in the soleus fibers and could also contribute to the anabolic effect of clenbuterol in vivo.

Adenovirus E1A inhibits cardiac myocyte-specific gene expression through its amino terminus

Adenovirus E1A oncoproteins inhibit muscle-specific gene expression and myogenic differentiation by suppressing the transcriptional activating functions of basic helix-loop-helix proteins. As one approach to identifying cardiac-specific gene regulatory proteins, we analyzed the functional regions of E1A proteins that are required for muscle gene repression in cardiac cells. Myocyte-specific promoters, including the alpha-actins and alpha-myosin heavy chain, were selectively and potently inhibited (>90%) by E1A, while the ubiquitously expressed beta-actin promoter was only partially ( approximately 30%) repressed; endogenous gene expression was also affected. Distinct E1A protein binding sites mediated repression of muscle-specific and ubiquitous actin promoters. E1A-mediated inhibition of beta-actin required both an intact binding site for the tumor repressor proteins pRb and p107 and a second E1A domain (residues 15-35). In contrast, cardiac-specific promoter repression required the E1A amino-terminal residues 2-36. The proximal skeletal actin promoter (3' to base pair -153) was a target for repression by E1A. Although E1A binding to p300 was not required for inhibition of either promoter, co-expression of p300 partially reversed E1A-mediated transcriptional repression. We conclude that cardiac-specific and general promoter inhibition by E1A occurs by distinct mechanisms and that cardiac-specific gene expression is modulated by cellular factors interacting with the E1A p300/CBP-binding domain.

Atrial natriuretic peptide induces apoptosis in neonatal rat cardiac myocytes

Early heart failure is characterized by elevated plasma atrial natriuretic peptide (ANP) levels, but little is known about the direct effects of ANP on cardiac myocytes. In neonatal rat cardiac myocytes, ANP induced apoptosis in a dose-dependent and cell type-specific manner. Maximum effects occurred at 1 microM ANP, with a 4-5-fold increase in apoptotic cells, reaching a maximum apoptotic index of 19%. In contrast, the maximum apoptotic index of ANP-treated non-myocytes was 1.1 +/- 0.2%, equivalent to control cultures. ANP treatment also sharply reduced levels of Mcl-1 mRNA, a Bcl-2 homologue, coincident with the increase in the incidence of apoptosis. ANP induction of apoptosis was receptor-dependent and mediated by cyclic GMP: the effect was mimicked by 8-bromo-cGMP, a membrane-permeable analog, and by sodium nitroprusside, an activator of soluble guanylyl cyclase, and was potentiated by a cGMP-specific phosphodiesterase inhibitor, zaprinast. Interestingly, norepinephrine, a myocyte growth factor, inhibited ANP-induced apoptosis via activation of the beta-adrenergic receptor and elevation of cyclic AMP. These results show that ANP is a specific effector of cardiac myocyte apoptosis in culture via receptor-mediated elevation of cGMP. Furthermore, at least in this model, ANP and norepinephrine may have opposing roles in the modulation of cardiac myocyte growth and survival.

Norepinephrine induces the raf-1 kinase/mitogen-activated protein kinase cascade through both alpha 1- and beta-adrenoceptors

BACKGROUND

Although norepinephrine induces cardiac hypertrophy by activating protein kinase A and C through beta- and alpha 1-adrenoceptors, respectively, protein kinase A has been reported to inhibit cell growth in many other cell types.

METHODS AND RESULTS

To elucidate the molecular mechanism of norepinephrine-induced hypertrophic responses, we examined the effects of protein kinase A and protein kinase C on the activities of raf-1 kinase and mitogen-activated protein (MAP) kinases and on protein synthesis rates using cultured cardiomyocytes of neonatal rats. Norepinephrine-induced activation of MAP kinases was partially inhibited by either an alpha 1-adrenoceptor blocker (prazosin) or a beta-adrenoceptor blocker (propranolol) and was completely abolished by both blockers. Both a beta-adrenoceptor agonist, isoproterenol, and an alpha 1-adrenoceptor agonist, phenylephrine, increased the activities of raf-1 kinase and MAP kinases and phenylalanine incorporation into proteins. Furthermore, isoproterenol and phenylephrine synergistically activated these kinases and protein synthesis. Similar synergistic activation of MAP kinases was observed when other protein kinase A-activating agents such as forskolin, dibutyryl cAMP, and isobutyl-methylxanthine were used with a protein kinase C-activating agent at the same time. Chelation of extracellular Ca2+ completely abolished isoproterenol- and phenylephrine-evoked MAP kinase activation.

CONCLUSIONS

Norepinephrine activates the raf-1 kinase/MAP kinase cascade through both alpha 1- and beta-adrenergic stimulation, and signaling pathways from the two receptors synergistically induce cardiomyocyte hypertrophy.

Medical therapy can improve the biological properties of the chronically failing heart. A new era in the treatment of heart failure

Myocardial failure has been considered to be an irreversible and progressive process characterized by ventricular enlargement, chamber geometric alterations, and diminished pump performance. However, more recent evidence has suggested that certain types of medical therapy may lead to retardation and even reversal of the cardiomyopathic process. In the failing heart, long-term neurohormonal/autocrine-paracrine activation results in abnormalities in myocyte growth, energy production and utilization, calcium flux, and receptor regulation that produce a progressively dysfunctional, mechanically inefficient heart. Interventions such as ACE inhibition and beta-blockade result in a reduction in the harmful long-term consequences of neurohormonal/autocrine-paracrine effects and retard the progression of left ventricular dysfunction or ventricular remodeling. Furthermore, in subjects with idiopathic dilated or ischemic cardiomyopathy, antiadrenergic therapy with beta-blocking agents appears to be able to partially reverse systolic dysfunction and ventricular remodeling. Although the precise mechanisms underlying this latter effect have not yet been elucidated, the general mechanism appears to be via improvement in the biological function of the cardiac myocyte. Such an improvement in the intrinsic defect(s) responsible for myocardial failure will likely translate into important clinical benefits.

The effect of angiotensin II on myosin heavy chain expression in cultured myocardial cells

Angiotensin II (AII), the principal mediator of the renin-angiotensin system, is an important regulator of vascular and cardiac homeostasis. AII has also been shown to be a regulator of cardiac hypertrophy and of the corresponding changes in amount and composition of certain tissue proteins. We examined the trophic effects of AII on cultured myocytes derived from neonatal rat ventricles and followed, by Northern blot analysis and polyacrylamide gel electrophoresis, the expression of alpha- and beta-myosin heavy chain iso-mRNAs and isoproteins. Our findings show that a single administration of AII is sufficient to induce a trophic response in cultured beating myocytes and to enhance the expression of beta-myosin heavy chain iso-mRNA and isoprotein, having no effect on alpha-myosin heavy chain. Induction of alpha-myosin heavy chain expression by thyroid hormone before AII was administered showed that AII could not potentiate a shift from alpha- to beta-myosin heavy chain predominance. We suggest that the potency of AII to regulate the expression of myosin heavy chain isogenes is restricted to the beta isoform and is overridden by thyroid hormone.

Clenbuterol induces hypertrophy of the latissimus dorsi muscle and heart in the rat with molecular and phenotypic changes

BACKGROUND

Skeletal muscle assistance of the circulation for patients in end-stage heart failure requires electrical training of the latissimus dorsi flap to produce fatigue resistance. This process of electrical transformation and the development of postmobilization atrophy results in a profound loss in peak power generated. The beta 2-adrenoceptor agonist clenbuterol was used to investigate its potential to selectively induce skeletal muscle hypertrophy, particularly the latissimus dorsi muscle (LDM), independent of adverse effects on cardiac muscle.

METHODS AND RESULTS

Forty-one male Sprague-Dawley rats were divided into four groups and used in this study. Clenbuterol 2 micrograms.g body wt-1.d-1 was administered subcutaneously for a period of either 5 weeks (group A) or 2 weeks (group A1). Groups B and B1 (controls) were injected with 0.5 mL normal saline once daily. At the end of the experimental period, all rats were weighed and terminally anesthetized for removal of the left LDM, left gastrocnemius-plantaris-soleus (GPS) muscles, and heart. The results showed that the increase in body weight did not differ significantly between the clenbuterol-treated and control groups (P > .5). The ratio of LDM to tibial length (hypertrophic index) for groups A and A1 was significantly greater than controls (P < .01), which represented a 20% to 29% increase. The hypertrophy was more pronounced for hindlimb skeletal muscle (21% to 35% for GPS), and the effects of this relatively high dose of clenbuterol on the heart were less marked (18% to 20% hypertrophy). RNA analyses indicate that ventricles of clenbuterol-treated rats express elevated levels of mRNA to atrial natriuretic factor without a concomitant increase in skeletal alpha-actin and beta-myosin heavy chain, consistent with a "physiological" form of cardiac hypertrophy.

CONCLUSIONS

Clenbuterol induces significant hypertrophy of the LDM associated with specific changes in cardiac gene expression.

The cAMP response element binding protein is expressed and phosphorylated in cardiac myocytes

Cardiac cells grow in response to a number of stimuli that activate intracellular signaling pathways. The cAMP-signaling pathway mediates the activation of gene transcription in other cell types by the cAMP response element binding protein (CREB-P). Our aim was to explore the physiological role of CREB-P in response to elevated cAMP in cardiac cells by determining if phosphorylation of CREB-P (to phosphoCREB-P) rapidly induces transcription in culture. Primary embryonic chick heart cultures were used in which cAMP was raised by forskolin (5 mumol/L) or isoproterenol (10 mumol/L) treatment. Since both these agents have inotropic effects, tension production was controlled with 2,3-butanedione monoxime (BDM). This allowed us to determine whether the cAMP-signaling pathway or the contractile state was regulating phosphorylation and transcription. The responses for time periods up to 2 hours were assayed with antibodies to detect phosphoCREB-P and by quantitative filter hybridization for creb gene expression. The staining intensity of the phosphoprotein increased in myocyte nuclei after 10 minutes and persisted for 1 hour with either forskolin or isoproterenol treatment. An increase in creb mRNA abundance was also detected, with the maximum level of expression being at 1 hour with forskolin treatment. These changes are independent of the contractile state, because BDM itself caused no change. BDM plus forskolin induced the same pattern of creb expression as observed with forskolin alone. Therefore, we conclude that elevation of cAMP leads to phosphorylation of CREB-P and an increase in creb mRNA abundance.