Transcription activator protein 1 mediates alpha- but not beta-adrenergic hypertrophic growth responses in adult cardiomyocytes

Single-nucleus RNA/ATAC-seq in early-stage HCM models predicts SWI/SNF-activation in mutant-myocytes, and allele-specific differences in fibroblasts

Hypertrophic cardiomyopathy (HCM) is associated with phenotypic variability. To gain insights into transcriptional regulation of cardiac phenotype, single-nucleus linked RNA-/ATAC-seq was performed in 5-week-old control mouse-hearts (WT) and two HCM-models (R92W-TnT, R403Q-MyHC) that exhibit differences in heart size/function and fibrosis; mutant data was compared to WT. Analysis of 23,304 nuclei from mutant hearts, and 17,669 nuclei from WT, revealed similar dysregulation of gene expression, activation of AP-1 TFs (FOS, JUN) and the SWI/SNF complex in both mutant ventricular-myocytes. In contrast, marked differences were observed between mutants, for gene expression/TF enrichment, in fibroblasts, macrophages, endothelial cells. Cellchat predicted activation of pro-hypertrophic IGF-signaling in both mutant ventricular-myocytes, and profibrotic TGFβ-signaling only in mutant-TnT fibroblasts. In summary, our bioinformatics analyses suggest that activation of IGF-signaling, AP-1 TFs and the SWI/SNF chromatin remodeler complex promotes myocyte hypertrophy in early-stage HCM. Selective activation of TGFβ-signaling in mutant-TnT fibroblasts contributes to genotype-specific differences in cardiac fibrosis.

YB-1 Is a Novel Target for the Inhibition of α-Adrenergic-Induced Hypertrophy

Cardiac hypertrophy resulting from sympathetic nervous system activation triggers the development of heart failure. The transcription factor Y-box binding protein 1 (YB-1) can interact with transcription factors involved in cardiac hypertrophy and may thereby interfere with the hypertrophy growth process. Therefore, the question arises as to whether YB-1 influences cardiomyocyte hypertrophy and might thereby influence the development of heart failure. YB-1 expression is downregulated in human heart biopsies of patients with ischemic cardiomyopathy (n = 8), leading to heart failure. To study the impact of reduced YB-1 in cardiac cells, we performed small interfering RNA (siRNA) experiments in H9C2 cells as well as in adult cardiomyocytes (CMs) of rats. The specificity of YB-1 siRNA was analyzed by a miRNA-like off-target prediction assay identifying potential genes. Testing three high-scoring genes by transfecting cardiac cells with YB-1 siRNA did not result in downregulation of these genes in contrast to YB-1, whose downregulation increased hypertrophic growth. Hypertrophic growth was mediated by PI3K under PE stimulation, as well by downregulation with YB-1 siRNA. On the other hand, overexpression of YB-1 in CMs, caused by infection with an adenovirus encoding YB-1 (AdYB-1), prevented hypertrophic growth under α-adrenergic stimulation with phenylephrine (PE), but not under stimulation with growth differentiation factor 15 (GDF15; n = 10-16). An adenovirus encoding the green fluorescent protein (AdGFP) served as the control. YB-1 overexpression enhanced the mRNA expression of the Gq inhibitor regulator of G-protein signaling 2 (RGS2) under PE stimulation (n = 6), potentially explaining its inhibitory effect on PE-induced hypertrophic growth. This study shows that YB-1 protects cardiomyocytes against PE-induced hypertrophic growth. Like in human end-stage heart failure, YB-1 downregulation may cause the heart to lose its protection against hypertrophic stimuli and progress to heart failure. Therefore, the transcription factor YB-1 is a pivotal signaling molecule, providing perspectives for therapeutic approaches.

Cardiac deficiency of P21-activated kinase 1 promotes atrial arrhythmogenesis in mice following adrenergic challenge

P21-activated kinase 1 (Pak1) signalling plays a vital and overall protective role in the heart. However, the phenotypes of Pak1 deficiency in the cardiac atria have not been well explored. In this study, Pak1 cardiac-conditional knock-out (cKO) mice were studied under baseline and adrenergic challenge conditions. Pak1 cKO mice show atrial arrhythmias including atrial fibrillation (AF) in vivo, detected during anaesthetized electrocardiography without evidence of interstitial fibrosis upon Masson's trichrome staining. Optical mapping of left atrial preparations from Pak1 cKO mice revealed a higher incidence of Ca2+ and action potential alternans under isoprenaline challenge and differences in baseline action potential and calcium transient characteristics. Type-2 ryanodine receptor (RyR2) channels from Pak1 cKO hearts had a higher open probability than those from wild-type. Reverse transcription-quantitative polymerase chain reaction and Western blotting indicated that pCamkIIδ and RyR2 are highly phosphorylated at baseline in the atria of Pak1 cKO mice, while the expression of Slc8a2 and Slc8a3 as a Na+-Ca2+ exchanger, controlling the influx of Ca2+ from outside of the cell and efflux of Na+ from the cytoplasm, are augmented. Chromatin immunoprecipitation study showed that pCreb1 interacts with Slc8a2 and Slc8a3. Our study thus demonstrates that deficiency of Pak1 promotes atrial arrhythmogenesis under adrenergic stress, probably through post-translational and transcriptional modifications of key molecules that are critical to Ca2+ homeostasis. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Current updates on arrhythmia within Timothy syndrome: genetics, mechanisms and therapeutics

Timothy syndrome (TS), characterised by multiple system malfunction especially the prolonged corrected QT interval and synchronised appearance of hand/foot syndactyly, is an extremely rare disease affecting early life with devastating arrhythmia. In this work, firstly, the various mutations in causative gene CACNA1C encoding cardiac L-type voltage-gated calcium channel (LTCC), regard with the genetic pathogeny and nomenclature of TS are reviewed. Secondly, the expression profile and function of CACNA1C gene encoding Cav1.2 proteins, and its gain-of-function mutation in TS leading to multiple organ disease phenotypes especially arrhythmia are discussed. More importantly, we focus on the altered molecular mechanism underlying arrhythmia in TS, and discuss about how LTCC malfunction in TS can cause disorganised calcium handling with excessive intracellular calcium and its triggered dysregulated excitation-transcription coupling. In addition, current therapeutics for TS cardiac phenotypes including LTCC blockers, beta-adrenergic blocking agents, sodium channel blocker, multichannel inhibitors and pacemakers are summarised. Eventually, the research strategy using patient-specific induced pluripotent stem cells is recommended as one of the promising future directions for developing therapeutic approaches. This review updates our understanding on the research progress and future avenues to study the genetics and molecular mechanism underlying the pathogenesis of devastating arrhythmia within TS, and provides novel insights for developing therapeutic measures.

Matrix Metalloproteinases Repress Hypertrophic Growth in Cardiac Myocytes

Purpose

Matrix metalloproteinases (MMPs) are identified as modulators of the extracellular matrix in heart failure progression. However, evidence for intracellular effects of MMPs is emerging. Pro- and anti-hypertrophic cardiac effects are described. This may be due to the various sources of different MMPs in the heart tissue. Therefore, the aim of the present study was to determine the role of MMPs in hypertrophic growth of isolated rat ventricular cardiac myocytes.

Methods

Cardiomyocytes were isolated form ventricular tissues of the rat hearts by collagenase perfusion. RT-qPCR, western blots, and zymography were used for expression and MMP activity analysis. Cross-sectional area and the rate of protein synthesis were determined as parameters for hypertrophic growth.

Results

MMP-1, MMP-2, MMP-3, MMP-9 and MMP-14 mRNAs were detected in cardiomyocytes, and protein expression of MMP-2, MMP-9, and MMP-14 was identified. Hypertrophic stimulation of cardiomyocytes did not enhance, but interestingly decreased expression of MMPs, indicating that downregulation of MMPs may promote hypertrophic growth. Indeed, the nonselective MMP inhibitors TAPI-0 or TIMP2 and the MMP-2-selective ARP-100 enhanced hypertrophic growth. Furthermore, TAPI-0 increased phosphorylation and thus activation of extracellular signaling kinase (ERK) and Akt (protein kinase B), as well as inhibition of glycogen synthase 3β (GSK3β). Abrogation of MEK/ERK- or phosphatidylinositol-3-kinase(PI3K)/Akt/GSK3β-signaling with PD98059 or LY290042, respectively, inhibited hypertrophic growth under TAPI-0.

Conclusion

MMPs’ inhibition promotes hypertrophic growth in cardiomyocytes in vitro. Therefore, MMPs in the healthy heart may be important players to repress cardiac hypertrophy.

Signalosome-Regulated Serum Response Factor Phosphorylation Determining Myocyte Growth in Width Versus Length as a Therapeutic Target for Heart Failure

BACKGROUND

Concentric and eccentric cardiac hypertrophy are associated with pressure and volume overload, respectively, in cardiovascular disease both conferring an increased risk of heart failure. These contrasting forms of hypertrophy are characterized by asymmetrical growth of the cardiac myocyte in mainly width or length, respectively. The molecular mechanisms determining myocyte preferential growth in width versus length remain poorly understood. Identification of the mechanisms governing asymmetrical myocyte growth could provide new therapeutic targets for the prevention or treatment of heart failure.

METHODS

Primary adult rat ventricular myocytes, adeno-associated virus (AAV)-mediated gene delivery in mice, and human tissue samples were used to define a regulatory pathway controlling pathological myocyte hypertrophy. Chromatin immunoprecipitation assays with sequencing and precision nuclear run-on sequencing were used to define a transcriptional mechanism.

RESULTS

We report that asymmetrical cardiac myocyte hypertrophy is modulated by SRF (serum response factor) phosphorylation, constituting an epigenomic switch balancing the growth in width versus length of adult ventricular myocytes in vitro and in vivo. SRF Ser¹⁰³ phosphorylation is bidirectionally regulated by RSK3 (p90 ribosomal S6 kinase type 3) and PP2A (protein phosphatase 2A) at signalosomes organized by the scaffold protein mAKAPβ (muscle A-kinase anchoring protein β), such that increased SRF phosphorylation activates AP-1 (activator protein-1)-dependent enhancers that direct myocyte growth in width. AAV are used to express in vivo mAKAPβ-derived RSK3 and PP2A anchoring disruptor peptides that block the association of the enzymes with the mAKAPβ scaffold. Inhibition of RSK3 signaling prevents concentric cardiac remodeling induced by pressure overload, while inhibition of PP2A signaling prevents eccentric cardiac remodeling induced by myocardial infarction, in each case improving cardiac function. SRF Ser¹⁰³ phosphorylation is significantly decreased in dilated human hearts, supporting the notion that modulation of the mAKAPβ-SRF signalosome could be a new therapeutic approach for human heart failure.

CONCLUSIONS

We have identified a new molecular switch, namely mAKAPβ signalosome-regulated SRF phosphorylation, that controls a transcriptional program responsible for modulating changes in cardiac myocyte morphology that occur secondary to pathological stressors. Complementary AAV-based gene therapies constitute rationally-designed strategies for a new translational modality for heart failure.

Modification of gene expression in rat cardiomyocytes by linoleic and docosahexaenoic acids 1

Regulation of cardiac fatty acid metabolism is central to the development of cardiac hypertrophy and heart failure. We investigated the effects of select fatty acids on the expression of genes involved in immediate early as well as inflammatory and hypertrophic responses in adult rat cardiomyocytes. Cardiac remodeling begins with upregulation of immediate early genes for c-fos and c-jun, followed by upregulation of inflammatory genes for nuclear factor kappa B (NF-κB) and nuclear factor of activated T-cells (NFAT). At later stages, genes involved in hypertrophic responses, such as atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), are upregulated. Adult rat cardiomyocytes were treated with palmitic acid, a saturated fatty acid; oleic acid, a monounsaturated fatty acid; linoleic acid, a polyunsaturated fatty acid belonging to the n-6 class; and docosahexaenoic acid, a polyunsaturated fatty acid belonging to the n-3 class. Linoleic acid produced a greater increase in the mRNA expression of c-fos, c-jun, NF-κB, NFAT3, ANP, and BNP relative to palmitic acid and oleic acid. In contrast, docosahexaenoic acid caused a decrease in the expression of genes involved in cardiac hypertrophy. Our findings suggest that linoleic acid may be a potent inducer of genes involved in cardiac hypertrophy, whereas docosahexaenoic acid may be protective against the cardiomyocyte hypertrophic response.

Zinc-finger protein 418 overexpression protects against cardiac hypertrophy and fibrosis

Background

This study aimed to investigated the effect and mechanism of zinc-finger protein 418 (ZNF418) on cardiac hypertrophy caused by aortic banding (AB), phenylephrine (PE) or angiotensin II (Ang II) in vivo and in vitro.

Methods

The expression of ZNF418 in hearts of patients with dilated cardiomyopathy (DCM) or hypertrophic cardiomyopathy (HCM) and AB-induced cardiac hypertrophy mice, as well as in Ang II- or PE-induced hypertrophic primary cardiomyocytes was detected by western blotting. Then, the expression of ZNF418 was up-regulated or down-regulated in AB-induced cardiac hypertrophy mice and Ang II -induced hypertrophic primary cardiomyocytes. The hypertrophic responses and fibrosis were evaluated by echocardiography and histological analysis. The mRNA levels of hypertrophy markers and fibrotic markers were detected by RT-qPCR. Furthermore, the phosphorylation and total levels of c-Jun were measured by western blotting.

Results

ZNF418 was markedly down-regulated in hearts of cardiac hypertrophy and hypertrophic primary cardiomyocytes. Down-regulated ZNF418 exacerbated the myocyte size and fibrosis, moreover increased the mRNA levels of ANP, BNP, β-MHC, MCIP1.4, collagen 1a, collagen III, MMP-2 and fibronection in hearts of AB-treated ZNF418 knockout mice or Ang II-treated cardiomyocytes with AdshZNF418. Conversely, these hypertrophic responses were reduced in the ZNF418 transgenic (TG) mice treated by AB and the AdZNF418-transfected primary cardiomyocytes treated by Ang II. Additionally, the deficiency of ZNF418 enhanced the phosphorylation level of c-jun, and overexpression of ZNF418 suppressed the phosphorylation level of c-jun in vivo and in vitro.

Conclusion

ZNF418 maybe attenuate hypertrophic responses by inhibiting the activity of c-jun/AP-1.

Molecular switches under TGFβ signalling during progression from cardiac hypertrophy to heart failure

Cardiac hypertrophy is a mechanism to compensate for increased cardiac work load, that is, after myocardial infarction or upon pressure overload. However, in the long run cardiac hypertrophy is a prevailing risk factor for the development of heart failure. During pathological remodelling processes leading to heart failure, decompensated hypertrophy, death of cardiomyocytes by apoptosis or necroptosis and fibrosis as well as a progressive dysfunction of cardiomyocytes are apparent. Interestingly, the induction of hypertrophy, cell death or fibrosis is mediated by similar signalling pathways. Therefore, tiny changes in the signalling cascade are able to switch physiological cardiac remodelling to the development of heart failure. In the present review, we will describe examples of these molecular switches that change compensated hypertrophy to the development of heart failure and will focus on the importance of the signalling cascades of the TGFβ superfamily in this process. In this context, potential therapeutic targets for pharmacological interventions that could attenuate the progression of heart failure will be discussed.

ATF3-dependent cross-talk between cardiomyocytes and macrophages promotes cardiac maladaptive remodeling

RATIONALE

Pressure overload induces adaptive remodeling processes in the heart. However, when pressure overload persists, adaptive changes turn into maladaptive alterations leading to cardiac hypertrophy and heart failure. ATF3 is a stress inducible transcription factor that is transiently expressed following neuroendocrine stimulation. However, its role in chronic pressure overload dependent cardiac hypertrophy is currently unknown.

OBJECTIVE

The objective of the study was to study the role of ATF3 in chronic pressure overload dependent cardiac remodeling processes.

METHODS AND RESULTS

Pressure overload was induced by phenylephrine (PE) mini-osmotic pumps in various mice models of whole body, cardiac specific, bone marrow (BM) specific and macrophage specific ATF3 ablations. We show that ATF3-KO mice exhibit a significantly reduced expression of cardiac remodeling markers following chronic pressure overload. Consistently, the lack of ATF3 specifically in either cardiomyocytes or BM derived cells blunts the hypertrophic response to PE infusion. A unique cross-talk between cardiomyocytes and macrophages was identified. Cardiomyocytes induce an ATF3 dependent induction of an inflammatory response leading to macrophage recruitment to the heart. Adoptive transfer of wild type macrophages, but not ATF3-KO derived macrophages, into wild type mice potentiates maladaptive response to PE infusion.

CONCLUSIONS

Collectively, this study places ATF3 as a key regulator in promoting pressure overload induced cardiac hypertrophy through a cross-talk between cardiomyocytes and macrophages. Inhibiting this cross-talk may serve as a useful approach to blunt maladaptive remodeling processes in the heart.

Small molecule inhibitors targeting activator protein 1 (AP-1)

Activator protein 1 (AP-1) is a pivotal transcription factor that regulates a wide range of cellular processes including proliferation, apoptosis, differentiation, survival, cell migration, and transformation. Accumulating evidence supports that AP-1 plays an important role in several severe disorders including cancer, fibrosis, and organ injury, as well as inflammatory disorders such as asthma, psoriasis, and rheumatoid arthritis. AP-1 has emerged as an actively pursued drug discovery target over the past decade. Excitingly, a selective AP-1 inhibitor T-5224 (51) has been investigated in phase II human clinical trials. Nevertheless, no effective AP-1 inhibitors have yet been approved for clinical use. Despite significant advances achieved in understanding AP-1 biology and function, as well as the identification of small molecules modulating AP-1 associated signaling pathways, medicinal chemistry efforts remain an urgent need to yield selective and efficacious AP-1 inhibitors as a viable therapeutic strategy for human diseases.

The AP-1 transcription factor c-Jun prevents stress-imposed maladaptive remodeling of the heart

Systemic hypertension increases cardiac workload and subsequently induces signaling networks in heart that underlie myocyte growth (hypertrophic response) through expansion of sarcomeres with the aim to increase contractility. However, conditions of increased workload can induce both adaptive and maladaptive growth of heart muscle. Previous studies implicate two members of the AP-1 transcription factor family, junD and fra-1, in regulation of heart growth during hypertrophic response. In this study, we investigate the function of the AP-1 transcription factors, c-jun and c-fos, in heart growth. Using pressure overload-induced cardiac hypertrophy in mice and targeted deletion of Jun or Fos in cardiomyocytes, we show that c-jun is required for adaptive cardiac hypertrophy, while c-fos is dispensable in this context. c-jun promotes expression of sarcomere proteins and suppresses expression of extracellular matrix proteins. Capacity of cardiac muscle to contract depends on organization of principal thick and thin filaments, myosin and actin, within the sarcomere. In line with decreased expression of sarcomere-associated proteins, Jun-deficient cardiomyocytes present disarrangement of filaments in sarcomeres and actin cytoskeleton disorganization. Moreover, Jun-deficient hearts subjected to pressure overload display pronounced fibrosis and increased myocyte apoptosis finally resulting in dilated cardiomyopathy. In conclusion, c-jun but not c-fos is required to induce a transcriptional program aimed at adapting heart growth upon increased workload.

Inhibition of AP-1 signaling by JDP2 overexpression protects cardiomyocytes against hypertrophy and apoptosis induction

AIMS

Expression and activity of the transcription factor AP-1 are enhanced during cardiac remodelling and heart failure progression. In order to test if AP-1 inhibition may limit processes contributing to cardiac remodelling, ventricular cardiomyocytes of mice with cardiac overexpression of the AP-1 inhibitor JDP2 were analysed under stimulation of hypertrophy, apoptosis, or contractile function.

METHODS AND RESULTS

Three models of JDP2 overexpressing mice were analysed: JDP2 was overexpressed either life-long, for 7 weeks, or 1 week. Then cardiomyocytes were isolated and stimulated with β-adrenoceptor agonist isoprenaline (ISO, 50 nM). This enhanced cross-sectional area and the rate of protein synthesis in WT but not in JDP2 overexpressing cardiomyocytes. To induce apoptosis, cardiomyocytes were stimulated with 3 ng/mL TGFβ1. Again, JDP2 overexpression prevented apoptosis induction compared with WT cells. Determination of contractile function under electrical stimulation at 2 Hz revealed enhancement of cell shortening, and contraction and relaxation velocities under increasing ISO concentrations (0.3-30 nM) in WT cells. This inotropic effect was abrogated in JDP2 overexpression cells. Responsiveness to increased extracellular calcium concentrations was also impaired in JDP2 overexpressing cardiomyocytes. Simultaneously, a reduction of SERCA expression was found in JDP2 mice.

CONCLUSION

A central role of AP-1 in the induction of hypertrophy and apoptosis in cardiomyocytes is demonstrated. Besides these protective effects of AP-1 inhibition on factors of cardiac remodelling, AP-1-inhibition impairs contractile function. Therefore, AP-1 acts as a double-edged sword that mediates mal-adaptive cardiac remodelling, but is required for maintaining a proper contractile function of cardiomyocytes.

The signal transduction cascade regulating the expression of the gap junction protein connexin43 by beta-adrenoceptors

BACKGROUND AND PURPOSE

In mammalian heart, connexin43 (Cx43) represents the predominant connexin in the working myocardium. As the beta-adrenoceptor is involved in many cardiac diseases, we wanted to clarify the pathway by which beta-adrenoceptor stimulation may control Cx43 expression.

EXPERIMENTAL APPROACH

Cultured neonatal rat cardiomyocytes were stimulated with isoprenaline. Cx43 expression as well as activation of p38 mitogen-activated protein kinase (MAPK), p42/44 MAPK, JUN NH(2)-terminal kinase (JNK) and nuclear translocation of the transcription factors activator protein 1 (AP1) and CRE-binding protein (CREB) were investigated. Additionally, we assessed Cx43 expression and distribution in left ventricular biopsies from patients without any significant heart disease, and from patients with either congestive heart failure [dilated cardiomyopathy (DCM)] or hypertrophic cardiomyopathy (HCM).

KEY RESULTS

Isoprenaline exposure caused about twofold up-regulation of Cx43 protein with a pEC(50) of 7.92 +/- 0.11, which was inhibited by propranolol, SB203580 (4-(4-fluorophenyl)-2-(4-methylsulphinylphenyl)-5-(4-pyridyl)-1H-imidazole) (p38 inhibitor), PD98059 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one) (MAPK 1 kinase inhibitor) (Alexis Biochemicals, San Diego, CA, USA) or cyclosporin A. Similar findings were obtained for Cx43 mRNA. Furthermore, Cx43 up-regulation was accompanied by phosphorylation of p38, p42/44 and JNK, and by translocation of AP1 and CREB to the nucleus. Analysis of Cx43 protein and mRNA in ventricular biopsies revealed that in patients with DCM, Cx43 content was significantly lower, and in patients with HCM, Cx43 content was significantly higher, relative to patients without any cardiomyopathy. More importantly, Cx43 distribution also changed with more Cx43 being localized at the lateral border of the cardiomyocytes.

CONCLUSION AND IMPLICATION

Beta-adrenoceptor stimulation up-regulated cardiac Cx43 expression via a protein kinase A and MAPK-regulated pathway, possibly involving AP1 and CREB. Cardiomyopathy altered Cx43 expression and distribution.

AFos inhibits phenylephrine-mediated contractile dysfunction by altering phospholamban phosphorylation

Using neonatal rat ventricular myocytes, we previously reported that the expression of a dominant negative form of the c-Fos proto-oncogene (AFos) inhibited activator protein 1 activity and blocked the induction of the pathological gene profile stimulated by phenylephrine (PE) while leaving growth unaffected. We now extend these observations to the adult rat ventricular myocyte (ARVM) to understand the relationship between gene expression, growth, and function. Ventricular myocytes were isolated from adult rats and infected with adenovirus expressing beta-galactosidase (control) or AFos. The cells were subsequently treated with PE, and protein synthesis, gene program, calcium transients, and contractility were evaluated. As seen with the neonatal rat ventricular myocytes, in control cells PE stimulated an increase in protein synthesis, induced the pathological gene profile, and exhibited both depressed contractility and calcium transients. Although ARVMs expressing AFos still had PE-induced growth, pathological gene expression as well as contractility and calcium handling abnormalities were inhibited. To determine a possible mechanism of the preserved myocyte function in AFos-expressing cells, we examined phospholamban (PLB) and sarco(endo)plasmic reticulum calcium-ATPase proteins. Although there was no change in total PLB or sarco(endo)plasmic reticulum calcium-ATPase expression in response to PE treatment, PE decreased the phosphorylation of PLB at serine-16, an observation that was prevented in AFos-expressing cells. In conclusion, although PE-induced growth was unaffected in AFos-expressing ARVMs, the expression of the pathological gene profile was inhibited and both contractile function and calcium cycling were preserved. The inhibition of functional deterioration was, in part, due to the preservation of PLB phosphorylation.

SMAD-proteins as a molecular switch from hypertrophy to apoptosis induction in adult ventricular cardiomyocytes

Heart failure development goes along with a transition from hypertrophic growth to apoptosis induction. In adult cardiomyocytes SMAD proteins are only activated under apoptotic, but not under hypertrophic conditions and are increased at the transition to heart failure. Therefore, SMADs could be candidates that turn the balance from hypertrophic growth to apoptosis resulting in heart failure development. To test this hypothesis we infected isolated rat ventricular cardiomyocytes with adenovirus encoding SMAD4 (AdSMAD4) and investigated the impact of SMAD4 overexpression on the development of apoptosis and hypertrophy under stimulation with phenylephrine (PE). Infection of cardiomyocytes with AdSMAD4 significantly enhanced SMAD-binding activity while apoptosis after 24 and 36 h infection did not rise. But when SMAD4 overexpressing cardiomyocytes were incubated with PE (10 microM), the number of apoptotic cells increased (Ctrl: 94.97 +/- 6.91%; PE: 102.48 +/- 4.78% vs. AdSMAD4 + PE: 118.64 +/- 3.28%). Furthermore expression of caspase 3 as well as bax/bcl2 ratio increased in SMAD4 overexpressing, PE-stimulated cardiomyocytes. In addition, the effects of SMAD4 overexpression on PE-induced hypertrophic growth were analyzed. Protein synthesis 36 h after AdSMAD4 infection was comparable to control cells, whereas the increase in protein synthesis stimulated by phyenylephrine was significantly reduced in SMAD4 overexpressing cells (134.28 +/- 10.02% vs. 100.57 +/- 8.86%). SMAD4 triggers the transition from hypertrophy to apoptosis in ventricular cardiomyocytes. Since SMADs are increased under several pathophysiological conditions in the heart, it can be assumed that it triggers apoptosis induction and therefore contributes to negative remodeling and heart failure progression.

Reciprocal regulation of transcription factors and PLC isozyme gene expression in adult cardiomyocytes

By employing a pharmacological approach, we have shown that phospholipase C (PLC) activity is involved in the regulation of gene expression of transcription factors such as c-Fos and c-Jun in cardiomyocytes in response to norepinephrine (NE). However, there is no information available regarding the identity of specific PLC isozymes involved in the regulation of c-Fos and c-Jun or on the involvement of these transcription factors in PLC isozyme gene expression in adult cardiomyocytes. In this study, transfection of cardiomyocytes with PLC isozyme specific siRNA was found to prevent the NE-mediated increases in the corresponding PLC isozyme gene expression, protein content and activity. Unlike PLC gamma(1) gene, silencing of PLC beta(1), beta(3) and delta(1) genes with si RNA prevented the increases in c-Fos and c-Jun gene expression in response to NE. On the other hand, transfection with c-Jun si RNA suppressed the NE-induced increase in c-Jun as well as PLC beta(1), beta(3) and delta(1) gene expression, but had no effect on PLC gamma(1) gene expression. Although transfection of cardiomyocytes with c-Fos si RNA prevented NE-induced expression of c-Fos, PLC beta(1) and PLC beta(3) genes, it did not affect the increases in PLC delta(1) and PLC gamma(1) gene expression. Silencing of either c-Fos or c-Jun also depressed the NE-mediated increases in PLC beta(1), beta(3) and gamma(1) protein content and activity in an isozyme specific manner. Furthermore, silencing of all PLC isozymes as well as of c-Fos and c-Jun resulted in prevention of the NE-mediated increase in atrial natriuretic factor gene expression. These findings, by employing gene silencing techniques, demonstrate that there occurs a reciprocal regulation of transcription factors and specific PLC isozyme gene expression in cardiomyocytes.

Sympathetic outflow activates the venom gland of the snakeBothrops jararacaby regulating the activation of transcription factors and the synthesis of venom gland proteins

The venom gland of viperid snakes has a central lumen where the venom produced by secretory cells is stored. When the venom is lost from the gland,the secretory cells are activated and new venom is produced. The production of new venom is triggered by the action of noradrenaline on bothα 1- and β-adrenoceptors in the venom gland. In this study, we show that venom removal leads to the activation of transcription factors NFκB and AP-1 in the venom gland. In dispersed secretory cells,noradrenaline activated both NFκB and AP-1. Activation of NFκB and AP-1 depended on phospholipase C and protein kinase A. Activation of NFκB also depended on protein kinase C. Isoprenaline activated both NFκB and AP-1, and phenylephrine activated NFκB and later AP-1. We also show that the protein composition of the venom gland changes during the venom production cycle. Striking changes occurred 4 and 7 days after venom removal in female and male snakes, respectively. Reserpine blocks this change,and the administration of α1- and β-adrenoceptor agonists to reserpine-treated snakes largely restores the protein composition of the venom gland. However, the protein composition of the venom from reserpinized snakes treated with α1- or β-adrenoceptor agonists appears normal, judging from SDS-PAGE electrophoresis. A sexual dimorphism in activating transcription factors and activating venom gland was observed. Our data suggest that the release of noradrenaline after biting is necessary to activate the venom gland by regulating the activation of transcription factors and consequently regulating the synthesis of proteins in the venom gland for venom production.

Regulation of c-Fos and c-Jun gene expression by phospholipase C activity in adult cardiomyocytes

This study was undertaken to determine whether gene expression for transcriptional factors such as c-Fos and c-Jun is regulated by phospholipase C (PLC) activity. Norepinephrine (NE) increased PLC beta(1), beta(3), gamma(1), and delta(1) isozyme gene expression, protein contents and their activities in adult rat cardiomyocytes. Increases in PLC beta(1), beta(3), gamma(1), and delta(1) activities and gene expression in response to NE were prevented by prazosin, an alpha(1)-adrenoceptor (AR) antagonist. Furthermore, mRNA levels for c-Fos and c-Jun, unlike other transcriptional factors, were increased by both NE and phenylephrine, a specific alpha(1)-AR agonist. Increases in c-Fos and c-Jun gene expression due to NE were attenuated by both prazosin and a PLC inhibitor, U73122. Activation of protein kinase C (PKC) with phorbol myristate acetate increased c-Fos and c-Jun mRNA, whereas inhibition of PKC with bisindolylmaleimide as well as inhibition of extracellular signal-regulated kinases (ERK) 1/2 with PD98059 abolished the NE-induced increase in c-Fos and c-Jun gene expression. Reduction of c-Jun phosphorylation by SP600125, an inhibitor of JNK activity, was associated with an attenuation of the NE-induced increases in PLC gene expression. It is suggested that c-Fos and c-Jun gene expression is regulated by PLC in adult cardiomyocytes through a PKC- and ERK1/2-dependent pathway.

Enhanced SERCA2A expression improves contractile performance of ventricular cardiomyocytes of rat under adrenergic stimulation

alpha-Adrenergic stimulation results in a positive adaptation of cardiomyocytes to increased cardiac work load by induction of hypertrophy and enhanced contraction. However, sustained adrenergic stimulation causes progression to heart failure. Under simultaneous activation of alpha- and beta-adrenoceptors by the naturally occurring catecholamine noradrenaline, beta1-stimulation inhibits alpha-adrenergic-stimulated hypertrophy. If beta-adrenergic stimulation may also influence cardiomyocyte contraction is not known yet. We now demonstrate that exposure of cardiomyocytes to noradrenaline or isoprenaline for 24 h results in a reduced cell shortening at low beating frequencies (0.5 Hz). At high beating frequencies (2 Hz), cell shortening was normal. beta-adrenergic stimulation enhances SERCA2A expression at the messenger RNA and protein level. This induction of the Ca(2+) pump SERCA2A by the transcription factor NFAT is responsible for maintenance of normal cell contraction at high beating frequencies since inhibition of NFAT by decoy-oligonucleotides impairs SERCA2A expression and cell shortening after beta-adrenergic stimulation. In conclusion, although reduced cell shortening is found under low beating frequencies, we demonstrate preservation of cardiomyocyte contraction at 2 Hz after exposure to beta-adrenergic stimuli, which indicate that adrenergic stimulation a priori does not cause impaired heart function. The increase of SERCA expression indicates an even improved Ca(2+) handling of the cells.

Direct effects of the angiotensin-converting enzyme inhibitor ramiprilat on adult rat ventricular cardiomyocytes

AIM

Angiotensin-converting enzyme (ACE) inhibitors like ramiprilat bind to ACE expressed on the cell surface of endothelial cells and induce cell-specific signalling including the activation of activator protein (AP)-1. The present study addressed the question whether ramiprilat exerts a similar effect on adult ventricular cardiomyocytes, i.e. activates the AP-1 or modifies contractile performance. It was further aimed to decide whether such effects depend on bradykinin receptors or whether they are directly mediated via ACE.

METHODS

Adult rat ventricular cardiomyocytes were isolated and cultured. mRNA expression of ACE was investigated by RT-PCR, AP-1 activation by gel mobility shift assays, and cardiac contractile performance by electrical pacing of isolated cells and analysis of cell shortening via a line-camera.

RESULTS

Cardiomyocytes stably express ACE. Ramiprilat increased maximal contraction velocity and shortened the time-to-peak of contraction. In contrast to effects evoked by bradykinin, such effects caused by ramiprilat were not attenuated by HOE 140, a bradykinin-receptor antagonist. These effects were also not attenuated in the presence of l-nitro-arginine, used to mimic bradykinin-dependent signalling. In cardiomyocytes, bradykinin but not ramiprilat activated AP-1. Ramiprilat activates AP-1 in endothelial cells that are known to respond to ramiprilat in this way.

CONCLUSION

Ramiprilat exerts direct, bradykinin-receptor independent effects on cardiomyocytes that improve cellular function without a corresponding effect on AP-1 activation or induction of AP-1 dependent effects. This newly described effect of ramiprilat may contribute to the protective effects seen by application of ACE inhibitors.

Induction of hypertrophy in vitro by mechanical loading in adult rabbit myocardium

To study myocardial hypertrophy under in vitro conditions, we developed an experimental system and protocol in which mechanical conditions of isolated multicellular myocardium can be controlled while function can be continuously assessed. This in vitro culture system now allows us to investigate how mechanical overload impacts on cardiac hypertrophy in the absence of systemic factors. In this system, small right ventricular rabbit trabeculae were subjected to different modes of mechanical load, while being electrically stimulated to contract at 1 Hz at 37 degrees C. Muscles subjected to prolonged isometric contractions at high, but physiological, pre- and afterload showed a rapid induction of cardiac hypertrophy; overall muscle diameter increased by 4.3 +/- 1.4 and 17.9 +/- 4.0% after 24 and 48 h, respectively. This finding was confirmed at the cellular level; individual myocyte width significantly increased after 24 and 48 h. In muscles subjected to a low preload, or in the absence of afterload, this hypertrophic response was absent. Functionally, after 24 h of isometric contractions at high load, active developed tension had gradually increased to 168 +/- 22% of starting values. Proteomic analysis of this cultured myocardium demonstrated reproducible changes in the protein expression pattern and included an upregulation of myofilament proteins, myosin light chain isoforms, alpha-b crystalline, and breast cancer 1 protein, and a downregulation of myoglobin. We conclude that multicellular myocardium can be stressed to undergo rapid hypertrophy in vitro, and changes in function and protein expression can be investigated during the transition from healthy myocardium to early hypertrophy.

The role of basic leucine zipper protein-mediated transcription in physiological and pathological myocardial hypertrophy

Accumulating evidence suggests that nuclear transcription factors from the basic leucine zipper (bZIP) family play an important role in cardiac development and function. This class includes the CREB/ATF family of transcription factors, namely CREB, cAMP response element modulator (CREM), ATF, and the related AP-1 and C/EBP families. An effort has been made to elucidate the role of specific bZIP members in the heart. Unfortunately, little insight could be gained from knockout experiments, either due to embryonic lethal phenotypes or functional compensation by other bZIP family members. Surprisingly, cardiac overexpression of several inhibitory transcription factors from the bZIP family, such as a nonphosphorylatable form of CREB (CREB(ser133)), a nonfunctional isoform of CREM, or ATF3 resulted in massive atrial dilatation. In order to try and characterize this pathway we have expressed the potent bZIP inhibitory protein, Jun dimerization protein 2 (JDP2), specifically in the mouse heart in a temporally controlled manner. Expression of JDP2 resulted in massive biatrial dilatation; loss of connexin 40 (Cx40), connexin43 (Cx43), and myosin light chain 2 (MLC2a) expression; atrioventricular defects in conduction; and a lethal phenotype. All these effects were independent of any developmental events acquired during adulthood, and were totally reversible upon abolishing the bZIP inhibition. The results of this article suggest that bZIP inhibition is sufficient to cause atrial dilation, that this dilatation is acquired postnatally, and that it is reversible upon the relief of inhibition. Thus, bZIP repressors may serve as novel drug targets for the prevention of atrial dilatation a major risk of atrial fibrillation (AF).

Contractile performance of adult ventricular rat cardiomyocytes is not directly jeopardized by NO/cGMP-dependent induction of pro-apoptotic pathways

The activation of NO/cGMP pathways can induce pro-apoptotic pathways in cardiomyocytes although only a small number of cardiomyocytes fulfill the criteria of apoptosis. The same pathways reduce the contractile performance of cardiomyocytes. In the present study, we tested the hypothesis that exposure of cells to NO/cGMP for 24 h decrease their contractile performance due to an activation of pro-apoptotic pathways. Experiments were performed on freshly isolated and cultured adult ventricular rat cardiomyocytes. Cells were incubated with 8-bromo-cyclo-GMP (100 nmol/L-1 micromol/L), the NO donor SNAP (1 nmol/L-100 micromol/L), or the guanylyl cyclase activator YC-1 (3 micromol/L). Cell shortening, contraction and relaxation velocities, and diastolic cell lengths were determined at beating frequencies of 0.5, 1, and 2 Hz 24 h later. The activation of pro-apoptotic pathways was determined by staining of cardiomyocytes with an antibody directed against active caspase-3 and quantification of the number of apoptotic cells (annexin staining). Caspase-3 activation and an increase in the number of apoptotic cells was observed, but only at the highest concentrations tested (8-bromo-cyclo-GMP: 1-10 mmol/L; SNAP: 1-100 micromol/L). At these concentrations, none of the drugs decreased the mean cell shortening of cardiomyocytes. However, at concentrations lower than those required for induction of apoptotic cell death, the diastolic cell lengths and sarcomere lengths increased but cell shortening decreased. In conclusion, low concentrations of either NO or cGMP cause a desensitization of myofibrils, as indicated by elongated cell shapes, increased sarcomere lengths and reduced load-free cell shortening. High concentrations of NO/cGMP induce caspase-3 activation and increase the number of cells fulfilling the criteria of apoptotic cell death but did not impair cell function. Therefore, induction of apoptotic cell death per se seems not to contribute to the loss of contractile efficiency on the cellular level.

Repression of anti-apoptotic genes via AP-1 as a mechanism of apoptosis induction in ventricular cardiomyocytes

Nitric oxide (NO) is increased under several pathophysiological, mainly inflammatory processes in the heart and has been characterized as an inducer of apoptosis in cardiomyocytes. The transcription factor activating protein-1 (AP-1) has been identified as a mediator of NO-induced apoptosis. Genes that are regulated by AP-1 under apoptotic conditions have not been identified yet. Therefore, we performed a microarray analysis with subsequent real-time polymerase chain reaction (PCR) to identify genes regulated by AP-1 in NO-induced ventricular cardiomyocytes of rats and tested the functional role of these genes in apoptosis. Cardiomyocytes were transformed with AP-1 decoy oligonucleotides for inhibition of AP-1 activity. These, as well as non-transformed control cells, were stimulated with the NO donor (+/-)-S-nitroso-N-acetylpenicillamine (SNAP, 100 microM) for 2 h. Some of the genes with differential gene expression on microarrays were further analysed by real-time PCR. Genes that are induced by SNAP were not identified. However, four genes, pyridoxal kinase, heat shock protein 10 (Hsp10), antigen identified by monoclonal antibodies 4F2 (4F2) and myosin light chain 2, were downregulated by SNAP in presence of AP-1. Pyridoxal kinase, Hsp10 and 4F2 have anti-apoptotic effects in unstimulated cells because downregulation of their expression by antisense oligos induced apoptosis in cardiomyocytes. An involvement of these genes in NO-induced apoptosis could only be proven for pyridoxal kinase. In conclusion, using microarray technology, we identified three anti-apoptotic genes (Hsp10, 4F2 and pyridoxal kinase) in ventricular cardiomyocytes, which may help the cells to resist some apoptotic stimuli. The downregulation of these genes results in cardiomyocyte apoptosis. Prevention of their downregulation may protect cardiomyocytes against apoptotic stimuli, and this may be of therapeutic benefit.

Regulation of c-jun mRNA expression in adult cardiocytes by MAP kinase interacting kinase-1 (MNK1)

Hypertrophic growth of adult myocardium is associated with increased expression of the early response gene c-jun. The purpose of this study was to determine whether eukaryotic initiation factor (elF) 4E (eIF4E) regulates translational efficiency of c-jun mRNA as measured by flux into polysomes. Adult feline cardiomyocytes in primary culture were treated with 0.2 microM 12-O-tetradecanoylphorbol 13-acetate (TPA), and c-jun mRNA was quantified in total, monosome, and polysome fractions by real-time polymerase chain reaction. After 1 h, TPA increased total c-jun mRNA by 10.5-fold. The corresponding flux into polysomes was significantly lower (5-fold). Adenoviral-mediated overexpression of either eIF4E or a nonphosphorylatable mutant (S209/A) did not affect total c-jun mRNA or its flux between monosomes and polysomes. Similar results were obtained following overexpression of the eIF4E kinase Mnk1. Thus, translational efficiency of c-jun mRNA was not affected by changes in activity or amount of eIF4E. In contrast, a kinase-deficient Mnk1 mutant significantly reduced total c-jun mRNA from 9.8-fold to 6.0-fold while flux between monosomes and polysomes remained constant. The decrease in total c-jun mRNA resulted from increased decay of c-jun mRNA incorporated into the polysomes. We conclude that Mnk1 activity stabilizes c-jun mRNA in polysomes independent of eIF4E phosphorylation.

Angiotensin II stimulates apoptosis via TGF-beta1 signaling in ventricular cardiomyocytes of rat

Elevations in angiotensin II (AngII) and transforming growth factor (TGF-beta1) levels are often found under conditions leading to progression of heart failure. From several studies, it is evident that AngII enhances TGF-beta1 expression via activator protein 1 (AP-1) activation, and that this pathway is involved in hypertrophic growth of the heart muscle and in the development of cardiac fibrosis. We now continued characterization of the signaling pathway stimulated by AngII in ventricular cardiomyocytes of rat and analyzed if the enhancement of TGF-beta1 expression by AngII may also contribute to apoptosis induction, which is another predictor of heart failure progression. Stimulation of cardiomyocytes with 100 nM AngII for 2 h activated the transcription factors AP-1 and GATA by 68.6+/-23.9 or 70.7+/-9.8%. Induction of both factors was mediated by p38 mitogen-activated protein kinase (MAPK) because it was totally blocked using a specific inhibitor of the kinase (SB202190). When GATA was inhibited by transformation of cardiomyocytes with decoy oligonucleotides, AngII could not enhance TGF-beta1 expression. This inhibition was observed on the mRNA level in real-time polymerase chain reaction and on the protein level in Western blots. As a consequence, upon AngII stimulation for 24 h, release of TGF-beta1 from cardiomyocytes was also reduced from 240.5+/-50.4 to 130.5+/-22.1% (p<0.05). In contrast to the early induction of GATA and AP-1, the transcription factor similar to mothers against decapentaplegic homolog (SMAD) was induced by AngII after 24 h. This stimulation was dependent on TGF-beta1 because it was blocked by antibodies specific for TGF-beta1. Twenty-four hours after AngII addition, the number of apoptotic cardiomyocytes increased by 6.5+/-1.2%, and this apoptosis induction was blocked when SMAD activity was inhibited by transformation of cardiomyocytes with SMAD decoy oligonucleotides. In conclusion, the transcription factors AP-1 and GATA are activated by p38 MAPK upon AngII stimulation, and both are needed to enhance TGF-beta1 expression in ventricular cardiomyocytes. TGF-beta1 acts in an autocrine loop on the cells to induce apoptosis via SMAD signaling. Thus, the often-found correlation between AngII, TGF-beta1, AP-1, and SMAD in pathogenesis of heart disease reflects the proapoptotic signaling pathway induced by AngII in cardiomyocytes.

Down-Regulation of Calpain 9 is Linked to Hypertensive Heart and Kidney

Calpains are a family of 14 intracellular calcium-dependent proteases, which have been implicated in cardiovascular diseases. We aimed to analyze specifically the expressional regulation of the different calpain isoforms in hypertensive target organ damage. Using real-time PCR, we found calpain 6 and 9 down-regulated by more than 50% and the endogenous calpain inhibitor calpastatin up-regulated by 225%, respectively, in the hearts of Dahl salt-sensitive rats on a high salt (4% NaCl) compared to normal salt diet. On the protein level, calpain 9 but not calpastatin was regulated in the hypertensive target organs heart and kidney. Moreover, the myocardial expression of calpain 9 protein was inversely linked to left ventricular mass (r= -0.93, p<0.01), and renal expression of calpain 9 protein correlated inversely with albuminuria (r= -0.82, p<0.05). In the aorta, there was no regulation of calpain 9 on the protein level. We conclude that differential regulation of calpain 9 may play a role in hypertensive target organ damage.

Slc26a6: a cardiac chloride-hydroxyl exchanger and predominant chloride-bicarbonate exchanger of the mouse heart

Bicarbonate facilitate more than 50% of pH recovery in the acidotic myocardium, and have roles in cardiac hypertrophy and steady-state pH regulation. To determine which bicarbonate transporters are responsible for this activity, we measured the expression levels of all known HCO3(-)-anion exchange proteins in mouse heart, by quantitative real time RT-PCR. Bicarbonate-anion exchangers are members of either the SLC4A or the SLC26A gene families. In neonatal and adult myocardium, AE1 (Slc4a1), AE2 (Slc4a2), AE3 (Slc4a3) (AE3fl and AE3c variants), Slc26a3 and Slc26a6 were expressed. Adult hearts expressed Slc26a3 and Slc4a1-3 mRNAs at similar levels, while Slc26a6 mRNA was about seven-fold higher than AE3, which was more abundant than any other. Immunohistochemistry revealed that Slc26a6 and AE3 are present in the plasma membrane of ventricular myocytes. Slc26a6 expression levels were higher in ventricle than atrium, whereas AE3 was detected only in ventricle. Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange activity of SLC26A6 and AE3 were investigated in transfected HEK293 cells, using intracellular fluorescence measurements of 2',7'-bis (2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), to monitor intracellular pH (pH(i)). Rates of pH(i) change were measured under HCO3(-)-containing (Cl(-)-HCO(3)(-)) or nominally HCO3(-)-free (Cl(-)-OH(-)) conditions. HCO3(-) fluxes were similar for cells expressing AE3fl, SLC26A6 or Slc26a3, suggesting that they have similar transport activity. However, only SLC26A6 and Slc26a3 functioned as Cl(-)-OH(-) exchangers. Activation of alpha-adrenergic receptors, which stimulates protein kinase C, inhibited SLC26A6 Cl(-)-HCO(3)(-) exchange activity. We conclude that Slc26a6 is the predominant Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchanger of the myocardium and that Slc26a6 is negatively regulated upon alpha-adrenergic stimulation.