Prohypertrophic Transcription Factor Research Articles

The Binding Occupancy of Pro-Hypertrophic Transcription Factors among Cardiac Developmental Enhancers

Objective: To check which of the eight pro-hypertrophic transcription factors bind in abundance to a set of 140 functionally confirmed cardiac enhancers aiming to identify potential therapeutic targets for delaying the onset of cardiac hypertrophy. Study Design: Bioinformatics, in-Silico Study.Place and Duration of Study: The study was conducted at the Department of Biological Sciences, National University of Medical Sciences (NUMS), Rawalpindi, Pakistan, from January 2022 to December 2022.Methods: For experimentally verified human and mouse noncoding regions with gene enhancer activity measured in transgenic mice, VISTA Enhancer Browser was used to obtain 140 cardiac-specific functionally confirmed enhancers. Among these enhancers, transcription factor binding sites were computationally mapped to assess transcription factor occupancy. The binding sites for all eight transcription factors were screened in the collective set of 140 cardiac enhancers via an in-house script. Results: In this study, among a small set of eight pro-hypertrophic transcription factors, occupancy of transcription factor HAND (Heart- And Neural Crest Derivatives-Expressed Protein) was found to be maximum in the collected pool of functionally confirmed cardiac enhancers. Conclusion: The transcription factor HAND was found to have maximum binding occupancy among a set of experimentally confirmed cardiac enhancers.How to cite this: Raza RZ, Hussain I, Akhtar F, Mumtaz S. The Binding Occupancy of Pro-Hypertrophic Transcription Factors among Cardiac Developmental Enhancers. Life and Science. 2024; 5(1): 22-26. doi: http://doi.org/10.37185/LnS.1.1.464

A microRNA program regulates the balance between cardiomyocyte hyperplasia and hypertrophy and stimulates cardiac regeneration

Myocardial regeneration is restricted to early postnatal life, when mammalian cardiomyocytes still retain the ability to proliferate. The molecular cues that induce cell cycle arrest of neonatal cardiomyocytes towards terminally differentiated adult heart muscle cells remain obscure. Here we report that the miR-106b~25 cluster is higher expressed in the early postnatal myocardium and decreases in expression towards adulthood, especially under conditions of overload, and orchestrates the transition of cardiomyocyte hyperplasia towards cell cycle arrest and hypertrophy by virtue of its targetome. In line, gene delivery of miR-106b~25 to the mouse heart provokes cardiomyocyte proliferation by targeting a network of negative cell cycle regulators including E2f5, Cdkn1c, Ccne1 and Wee1. Conversely, gene-targeted miR-106b~25 null mice display spontaneous hypertrophic remodeling and exaggerated remodeling to overload by derepression of the prohypertrophic transcription factors Hand2 and Mef2d. Taking advantage of the regulatory function of miR-106b~25 on cardiomyocyte hyperplasia and hypertrophy, viral gene delivery of miR-106b~25 provokes nearly complete regeneration of the adult myocardium after ischemic injury. Our data demonstrate that exploitation of conserved molecular programs can enhance the regenerative capacity of the injured heart.

Prdm16 Deficiency Leads to Age-Dependent Cardiac Hypertrophy, Adverse Remodeling, Mitochondrial Dysfunction, and Heart Failure

Hypertrophic cardiomyopathy (HCM) is a well-established risk factor for cardiovascular mortality worldwide. Although hypertrophy is traditionally regarded as an adaptive response to physiological or pathological stress, prolonged hypertrophy can lead to heart failure. Here we demonstrate that Prdm16 is dispensable for cardiac development. However, it is required in the adult heart to preserve mitochondrial function and inhibit hypertrophy with advanced age. Cardiac-specific deletion of Prdm16 results in cardiac hypertrophy, excessive ventricular fibrosis, mitochondrial dysfunction, and impaired metabolic flexibility, leading to heart failure. We demonstrate that Prdm16 and euchromatic histone-lysine N-methyltransferase factors (Ehmts) act together to reduce expression of fetal genes reactivated in pathological hypertrophy by inhibiting the functions of the pro-hypertrophic transcription factor Myc. Although young Prdm16 knockout mice show normal cardiac function, they are predisposed to develop heart failure in response to metabolic stress. Our study demonstrates that Prdm16 protects the heart against age-dependent cardiac hypertrophy and heart failure.

The black sheep of class IIa: HDAC7 SIKens the heart.

Class IIa histone deacetylases (HDACs) repress cardiomyocyte hypertrophy through association with the prohypertrophic transcription factor (TF) myocyte enhancer factor-2 (MEF2). The four class IIa HDACs - HDAC4, -5, -7, and -9 - are subject to signal-dependent phosphorylation by members of the Ca2+/calmodulin-dependent protein kinase (CaMK) group. In response to stress, HDAC4, HDAC5, and HDAC9 undergo phosphorylation-induced nuclear export in cardiomyocytes, freeing MEF2 to stimulate progrowth genes; it was generally assumed that HDAC7 is also antihypertrophic. However, in this issue of the JCI, Hsu and colleagues demonstrate that, in sharp contrast to the other class IIa HDACs, HDAC7 is constitutively localized to the cardiomyocyte cytoplasm, where it promotes cardiac hypertrophy. Phosphorylation of HDAC7 by the CaMK group member salt-inducible kinase 1 (SIK1) stabilized the deacetylase, leading to increased expression of c-Myc, which in turn stimulated a pathological gene program. These unexpected findings highlight the SIK1/HDAC7 signaling axis as a promising target for the treatment of cardiac hypertrophy and heart failure.

Regulation of WNT5A and WNT11 during MSC in vitro chondrogenesis: WNT inhibition lowers BMP and hedgehog activity, and reduces hypertrophy.

Re-directing mesenchymal stromal cell (MSC) chondrogenesis towards a non-hypertrophic articular chondrocyte-(AC)-like phenotype is important for improving articular cartilage neogenesis to enhance clinical cartilage repair strategies. This study is the first to demonstrate that high levels of non-canonical WNT5A followed by WNT11 and LEF1 discriminated MSC chondrogenesis from AC re-differentiation. Moreover, β-catenin seemed incompletely silenced in differentiating MSCs, which altogether suggested a role for WNT signaling in hypertrophic MSC differentiation. WNT inhibition with the small molecule IWP-2 supported MSC chondrogenesis according to elevated proteoglycan deposition and reduced the characteristic upregulation of BMP4, BMP7 and their target ID1, as well as IHH and its target GLI1 observed during endochondral differentiation. Along with the pro-hypertrophic transcription factor MEF2C, multiple hypertrophic downstream targets including IBSP and alkaline phosphatase activity were reduced by IWP-2, demonstrating that WNT activity drives BMP and hedgehog upregulation, and MSC hypertrophy. WNT inhibition almost matched the strong anti-hypertrophic capacity of pulsed parathyroid hormone-related protein application, and both outperformed suppression of BMP signaling with dorsomorphin, which also reduced cartilage matrix deposition. Yet, hypertrophic marker expression under IWP-2 remained above AC level, and in vivo mineralization and ectopic bone formation were reduced but not eliminated. Overall, the strong anti-hypertrophic effects of IWP-2 involved inhibition but not silencing of pro-hypertrophic BMP and IHH pathways, and more advanced silencing of WNT activity as well as combined application of IHH or BMP antagonists should next be considered to install articular cartilage neogenesis from human MSCs.

Targeting transcriptional machinery to inhibit enhancer-driven gene expression in heart failure.

Pathological cardiac remodeling is induced through multiple mechanisms that include neurohumoral and biomechanical stress resulting in transcriptional alterations that ultimately become maladaptive and lead to the development of heart failure (HF). Although cardiac transcriptional remodeling is mediated by the activation of numerous signaling pathways that converge on a limited number of transcription factors (TFs) that promote hypertrophy (pro-hypertrophic TFs), the current therapeutic approach to prevent HF utilizes pharmacological inhibitors that largely target specific receptors that are activated in response to pathological stimuli. Thus, there is limited efficacy with the current pharmacological approaches to inhibit transcriptional remodeling associated with the development of HF. Recent evidence suggests that these pro-hypertrophic TFs co-localize at enhancers to cooperatively activate transcription associated with pathological cardiac remodeling. In disease states, including cancer and HF, evidence suggests that the general transcriptional machinery is disproportionately bound at enhancers. Therefore, pharmacological inhibition of transcriptional machinery that integrates pro-hypertrophic TFs may represent a promising alternative therapeutic approach to limit pathological remodeling associated with the development of HF.

Identification of a pharmacological inhibitor of Epac1 that protects the heart against acute and chronic models of cardiac stress.

Recent studies reported that cAMP-binding protein Epac1-deficient mice were protected against various forms of cardiac stress, suggesting that pharmacological inhibition of Epac1 could be beneficial for the treatment of cardiac diseases. To test this assumption, we characterized an Epac1-selective inhibitory compound and investigated its potential cardioprotective properties. We used the Epac1-BRET (bioluminescence resonance energy transfer) for searching for non-cyclic nucleotide Epac1 modulators. A thieno[2,3-b]pyridine derivative, designated as AM-001 was identified as a non-competitive inhibitor of Epac1. AM-001 has no antagonist effect on Epac2 or protein kinase A activity. This small molecule prevents the activation of the Epac1 downstream effector Rap1 in cultured cells, in response to the Epac1 preferential agonist, 8-CPT-AM. In addition, we found that AM-001 inhibited Epac1-dependent deleterious effects such as cardiomyocyte hypertrophy and death. Importantly, AM-001-mediated inhibition of Epac1 reduces infarct size after mouse myocardial ischaemia/reperfusion injury. Finally, AM-001 attenuates cardiac hypertrophy, inflammation and fibrosis, and improves cardiac function during chronic β-adrenergic receptor activation with isoprenaline (ISO) in mice. At the molecular level, ISO increased Epac1-G protein-coupled receptor kinase 5 (GRK5) interaction and induced GRK5 nuclear import and histone deacetylase type 5 (HDAC5) nuclear export to promote the activity of the prohypertrophic transcription factor, myocyte enhancer factor 2 (MEF2). Inversely, AM-001 prevented the non-canonical action of GRK5 on HDAC5 cytoplasmic shuttle to down-regulate MEF2 transcriptional activity. Our study represents a 'proof-of-concept' for the therapeutic effectiveness of inhibiting Epac1 activity in cardiac disease using small-molecule pharmacotherapy.

In utero exposure to PM2.5 during gestation caused adult cardiac hypertrophy through histone acetylation modification.

Ambient particles with a diameter of <2.5 μm (PM2.5) is a global health concern, and exposure to PM2.5 contributes to the progression of cardiovascular morbidity and mortality. In this study, pregnant c57 mice were exposed to PM2.5 during the whole gestation (approximately 300 µg/m 3 PM2.5 for 2 hours/d). A significantly low birth weight was found after in utero PM2.5 exposure, and low body weight continued for 12 weeks after birth. In the offspring, remarkable destructions of cardiac ultrastructures were determined both in newborn and adult hearts. In adulthood, hearts of mice in the PM2.5 exposed group showed cardiac hypertrophy. Protein levels of p300, CBP (histone acetyltransferase), and acetylated histone3 lysine 9 (H3K9ac) increased in the trial group; messenger RNA (mRNA) levels of GATA binding protein 4 (GATA4) and myocyte enhancer factor 2C (Mef2c) (prohypertrophic transcription factors), and mRNA levels of the classic hypertrophic genes, such as α-MHC and β-MHC, increased significantly in the hearts of the PM2.5 exposed group. H3K9ac levels near the promoter region of GATA4 and Mef2c went up in the PM2.5 group. The binding affinities of p300/CBP with promoters of GATA4 and Mef2c increased notably. Taken together, out data indicated that maternal exposure to PM2.5 during gestation may cause a series of cardiovascular events in the offspring; histone acetylation modification may play an important role in the programming of cardiac hypertrophy.

Kruppel-Like Factor 15 Is Critical for the Development of Left Ventricular Hypertrophy.

Left ventricular hypertrophy (LVH) is an independent risk factor for adverse cardiovascular events and is often present in patients with hypertension. Treatment to reduce blood pressure and regress LVH is key to improving health outcomes, but currently available drugs have only modest cardioprotective effects. Improved understanding of the molecular mechanisms involved in the development of LVH may lead to new therapeutic targets in the future. There is now compelling evidence that the transcription factor Kruppel-like factor 15 (KLF15) is an important negative regulator of cardiac hypertrophy in both experimental models and in man. Studies have reported that loss or suppression of KLF15 contributes to LVH, through lack of inhibition of pro-hypertrophic transcription factors and stimulation of trophic and fibrotic signaling pathways. This review provides a summary of the experimental and human studies that have investigated the role of KLF15 in the development of cardiac hypertrophy. It also discusses our recent paper that described the contribution of genetic variants in KLF15 to the development of LVH and heart failure in high-risk patients.

Inhibition of miR-208b improves cardiac function in titin-based dilated cardiomyopathy

BackgroundDilated cardiomyopathy (DCM) is the result of maladaptive cardiac remodeling, which involves microRNA regulation. In turn, microRNAs can contribute to the remodeling process by post-transcriptional modulation of gene expression networks. The exact role of microRNAs in the pathogenesis of DCM is largely unknown. Here, we used an inducible DCM mouse model that carries a human truncation mutation in the sarcomeric protein titin to dissect microRNA pathways in DCM development. Methods and resultsMicroRNA microarray studies revealed up-regulation of microRNA-208b in the myocardium of DCM mice and DCM patients (p<0.05 compared to controls). In order to investigate the effect of microRNA-208b on cardiac remodeling, loss-of-function and gain-of-function studies were performed by repetitive injections of LNA-modified microRNA-208b mimics and antimiR-208b. MiR-208b overexpression resulted in cardiac hypertrophy, whereas miR-208b antagonisation prevented transition of adaptive to maladaptive remodeling in the DCM mouse model. In vitro studies identified several pro-hypertrophic transcription factors as potential targets of miR-208b, suggesting that microRNA-208b plays an important role in cardiac development and growth. MiR-208b was also upregulated in DCM patients, but not in heart failure patients due to ischemic heart disease or myocarditis. ConclusionOur data suggests that miR-208b is involved in the remodeling process and pathogenesis of DCM by post-transcriptional gene expression modulation. MicroRNA-208b might be a novel therapeutic target for DCM.

C1q-TNF-Related Protein-9 Promotes Cardiac Hypertrophy and Failure.

Myocardial endothelial cells promote cardiomyocyte hypertrophy, possibly through the release of growth factors. The identity of these factors, however, remains largely unknown, and we hypothesized here that the secreted CTRP9 (C1q-tumor necrosis factor-related protein-9) might act as endothelial-derived protein to modulate heart remodeling in response to pressure overload. To examine the source of cardiac CTRP9 and its function during pressure overload. CTRP9 was mainly derived from myocardial capillary endothelial cells. CTRP9 mRNA expression was enhanced in hypertrophic human hearts and in mouse hearts after transverse aortic constriction (TAC). CTRP9 protein was more abundant in the serum of patients with severe aortic stenosis and in murine hearts after TAC. Interestingly, heterozygous and especially homozygous knock-out C1qtnf9 (CTRP9) gene-deleted mice were protected from the development of cardiac hypertrophy, left ventricular dilatation, and dysfunction during TAC. CTRP9 overexpression, in turn, promoted hypertrophic cardiac remodeling and dysfunction after TAC in mice and induced hypertrophy in isolated adult cardiomyocytes. Mechanistically, CTRP9 knock-out mice showed strongly reduced levels of activated prohypertrophic ERK5 (extracellular signal-regulated kinase 5) during TAC compared with wild-type mice, while CTRP9 overexpression entailed increased ERK5 activation in response to pressure overload. Inhibition of ERK5 by a dominant negative MEK5 mutant or by the ERK5/MEK5 inhibitor BIX02189 blunted CTRP9 triggered hypertrophy in isolated adult cardiomyocytes in vitro and attenuated mouse cardiomyocyte hypertrophy and cardiac dysfunction in vivo, respectively. Downstream of ERK5, we identified the prohypertrophic transcription factor GATA4, which was directly activated through ERK5-dependent phosphorylation. The upregulation of CTRP9 during hypertrophic heart disease facilitates maladaptive cardiac remodeling and left ventricular dysfunction and might constitute a therapeutic target in the future.

MITF interacts with the SWI/SNF subunit, BRG1, to promote GATA4 expression in cardiac hypertrophy

The transcriptional regulation of pathological cardiac hypertrophy involves the interplay of transcription factors and chromatin remodeling enzymes. The Microphthalmia-Associated Transcription Factor (MITF) is highly expressed in cardiomyocytes and is required for cardiac hypertrophy. However, the transcriptional mechanisms by which MITF promotes cardiac hypertrophy have not been elucidated. In this study, we tested the hypothesis that MITF promotes cardiac hypertrophy by activating transcription of pro-hypertrophy genes through interactions with the SWI/SNF chromatin remodeling complex. In an in vivo model of cardiac hypertrophy, expression of MITF and the BRG1 subunit of the SWI/SNF complex increased coordinately in response to pressure overload. Expression of MITF and BRG1 also increased in vitro when cardiomyocytes were stimulated with angiotensin II or a β-adrenergic agonist. Both MITF and BRG1 were required to increase cardiomyocyte size and activate expression of hypertrophy markers in response to β-adrenergic stimulation. We detected physical interactions between MITF and BRG1 in cardiomyocytes and found that they cooperate to regulate expression of a pro-hypertrophic transcription factor, GATA4. Our data show that MITF binds to the E box element in the GATA4 promoter and facilitates recruitment of BRG1. This is associated with enhanced expression of the GATA4 gene as evidenced by increased Histone3 lysine4 tri-methylation (H3K4me3) on the GATA4 promoter. Thus, in hypertrophic cardiomyoctes, MITF is a key transcriptional activator of a pro-hypertrophic gene, GATA4, and this regulation is dependent upon the BRG1 component of the SWI/SNF complex.

Roles and post‐translational regulation of cardiac class IIa histone deacetylase isoforms

Cardiomyocyte hypertrophy is an integral component of pathological cardiac remodelling in response to mechanical and chemical stresses in settings such as chronic hypertension or myocardial infarction. For hypertrophy to ensue, the pertinent mechanical and chemical signals need to be transmitted from membrane sensors (such as receptors for neurohormonal mediators) to the cardiomyocyte nucleus, leading to altered transcription of the genes that regulate cell growth. In recent years, nuclear histone deacetylases (HDACs) have attracted considerable attention as signal-responsive, distal regulators of the transcriptional reprogramming that in turn precipitates cardiomyocyte hypertrophy, with particular focus on the role of members of the class IIa family, such as HDAC4 and HDAC5. These histone deacetylase isoforms appear to repress cardiomyocyte hypertrophy through mechanisms that involve protein interactions in the cardiomyocyte nucleus, particularly with pro-hypertrophic transcription factors, rather than via histone deacetylation. In contrast, evidence indicates that class I HDACs promote cardiomyocyte hypertrophy through mechanisms that are dependent on their enzymatic activity and thus sensitive to pharmacological HDAC inhibitors. Although considerable progress has been made in understanding the roles of post-translational modifications (PTMs) such as phosphorylation, oxidation and proteolytic cleavage in regulating class IIa HDAC localisation and function, more work is required to explore the contributions of other PTMs, such as ubiquitination and sumoylation, as well as potential cross-regulatory interactions between distinct PTMs and between class IIa and class I HDAC isoforms.

Quantitative Genome-wide Binding Profiles Reveals Distinct Kinetics of c-Myc Transcription Factor in Cardiomyocytes

c-Myc (Myc) is one of the major oncogenes and also is a well-defined pro-hypertrophic transcription factor in cardiomyocytes. Although the molecular mechanisms underlying pro-hypertrophic property of Myc are wide-ranging, the direct target genes in cardiomyocytes still remain unclear.

Essential role of sympathetic endothelin A receptors for adverse cardiac remodeling.

In preclinical studies, endothelin receptor A (ETA) antagonists (ETAi) attenuated the progression of heart failure (HF). However, clinical HF trials failed to demonstrate beneficial effects of ETAi. These conflicting data may be explained by the possibility that established HF drugs such as adrenergic receptor blockers interfered with the mechanism of ETAi action in clinical trials. Here we report that mice lacking ETA only in sympathetic neurons (SN-KO) showed less adverse structural remodeling and cardiac dysfunction in response to pathological pressure overload induced by transverse aortic constriction (TAC). In contrast, mice lacking ETA only in cardiomyocytes (CM-KO) were not protected. TAC led to a disturbed sympathetic nerve function as measured by cardiac norepinephrine (NE) tissue levels and [(124)I]-metaiodobenzylguanidine-PET, which was prevented in SN-KO. In a rat model of HF, ETAi improved cardiac and sympathetic nerve function. In cocultures of cardiomyocytes (CMs) and sympathetic neurons (SNs), endothelin-1 (ET1) led to a massive NE release and exaggerated CM hypertrophy compared with CM monocultures. ETA-deficient CMs gained a hypertrophic response through wild-type SNs, but ETA-deficient SNs failed to mediate exaggerated CM hypertrophy. Furthermore, ET1 mediated its effects indirectly via NE in CM-SN cocultures through adrenergic receptors and histone deacetylases, resulting in activation of the prohypertrophic transcription factor myocyte enhancer factor 2. In conclusion, sympathetic ETA amplifies ET1 effects on CMs through adrenergic signaling pathways. Thus, antiadrenergic therapies may blunt potentially beneficial effects of ETAi. Taken together, this may indicate that patients with β blocker intolerance or disturbed sympathetic nerve function could be evaluated for a potential benefit from ETAi.

Exercise and diabetes have opposite effects on the assembly and O-GlcNAc modification of the mSin3A/HDAC1/2 complex in the heart

BackgroundExercise causes physiological cardiac hypertrophy and benefits the diabetic heart. Mammalian switch-independent 3A (mSin3A) and histone deacetylases (HDACs) 1 and 2 regulate hypertrophic genes through associations with the DNA binding proteins repressor element-1 silencing transcription factor (REST) and O-linked β-N-acetylglucosamine transferase (OGT). O-linked β-N-acetylglucosamine (O-GlcNAc) is a glucose derivative that is chronically elevated in diabetic hearts, and a previous study showed that exercise reduces cardiac O-GlcNAc. We hypothesized that O-GlcNAc and OGT would physically associate with mSin3A/HDAC1/2 in the heart, and that this interaction would be altered by diabetes and exercise.Methods8-week-old type 2 diabetic db/db (db) and non-diabetic C57 mice were randomized to treadmill exercise or sedentary groups for 1 or 4 weeks.ResultsO-GlcNAc was significantly higher in db hearts and increased with exercise. Db hearts showed lower levels of mSin3A, HDAC1, and HDAC2 protein, but higher levels of HDAC2 mRNA and HDAC1/2 deacetylase activity. Elevated HDAC activity was associated with significantly blunted expression of α-actin and brain natriuretic peptide in db hearts. In sedentary db hearts, co-immunoprecipitation assays showed that mSin3A and OGT were less associated with HDAC1 and HDAC2, respectively, compared to sedentary C57 controls; however, exercise removed these differences.ConclusionsThese data indicate that diabetes and exercise oppositely affect interactions between pro-hypertrophic transcription factors, and suggest that an increase in total cardiac O-GlcNAc is a mechanism by which exercise benefits type 2 diabetic hearts.

Epac enhances excitation–transcription coupling in cardiac myocytes

Epac is a guanine nucleotide exchange protein that is directly activated by cAMP, but whose cardiac cellular functions remain unclear. It is important to understand cardiac Epac signaling, because it is activated in parallel to classical cAMP-dependent signaling via protein kinase A. In addition to activating contraction, Ca2+ is a key cardiac transcription regulator (excitation–transcription coupling). It is unknown how myocyte Ca2+ signals are decoded in cardiac myocytes to control nuclear transcription. We examine Epac actions on cytosolic ([Ca2+]i) and intranuclear ([Ca2+]n) Ca2+ homeostasis, focusing on whether Epac alters [Ca2+]n and activates a prohypertrophic program in cardiomyocytes. Adult rat cardiomyocytes, loaded with fluo-3 were viewed by confocal microscopy during electrical field stimulation at 1Hz. Acute Epac activation by 8-pCPT increased Ca2+ sparks and diastolic [Ca2+]i, but decreased systolic [Ca2+]i. The effects on diastolic [Ca2+]i and Ca2+ spark frequency were dependent on phospholipase C (PLC), inositol 1,4,5 triphosphate receptor (IP3R) and CaMKII activation. Interestingly, Epac preferentially increased [Ca2+]n during both diastole and systole, correlating with the perinuclear expression pattern of Epac. Moreover, Epac activation induced histone deacetylase 5 (HDAC5) nuclear export, with consequent activation of the prohypertrophic transcription factor MEF2. These data provide the first evidence that the cAMP-binding protein Epac modulates cardiac nuclear Ca2+ signaling by increasing [Ca2+]n through PLC, IP3R and CaMKII activation, and initiates a prohypertrophic program via HDAC5 nuclear export and subsequent activation of the transcription factor MEF2.

Epac activation induces histone deacetylase nuclear export via a Ras-dependent signalling pathway

Epac (Exchange protein directly activated by cAMP) is a sensor for cAMP and represents a novel mechanism for governing cAMP signalling. Epac is a guanine nucleotide exchange factor (GEF) for the Ras family of small GTPases, Rap. Previous studies demonstrated that, in response to a prolonged β-adrenergic stimulation Epac induced cardiac myocyte hypertrophy. The aim of our study was to further characterize Epac downstream effectors involved in cardiac myocyte growth. Here, we found that Epac led to the activation of the small G protein H-Ras in primary neonatal cardiac myocytes. A Rap GTPase activating protein (RapGAP) partially inhibited Epac-induced H-Ras activation. Interestingly, we found that H-Ras activation involved the GEF domain of Epac. However, Epac did not directly induce exchange activity on this small GTPase protein. Instead, the effect of Epac on H-Ras activation was dependent on a signalling cascade involving phospholipase C (PLC)/inositol 1,3,5 triphosphate receptor (IP3R) and an increase intracellular calcium. In addition, we found that Epac activation induced histone deacetylase type 4 (HDAC4) translocation. Whereas HDAC5 alone was unresponsive to Epac, it became responsive to Epac in the presence of HDAC4 in COS cells. Consistent with its effect on HDAC cytoplasmic shuttle, Epac activation also increased the prohypertrophic transcription factor MEF2 in a CaMKII dependent manner in primary cardiac myocytes. Thus, our data show that Epac activates a prohypertrophic signalling pathway which involves PLC, H-Ras, CaMKII and HDAC nuclear export.

The mAKAP complex participates in the induction of cardiac myocyte hypertrophy by adrenergic receptor signaling

Maladaptive cardiac hypertrophy can progress to congestive heart failure, a leading cause of morbidity and mortality in the United States. A better understanding of the intracellular signal transduction network that controls myocyte cell growth may suggest new therapeutic directions. mAKAP is a scaffold protein that has recently been shown to coordinate signal transduction enzymes important for cytokine-induced cardiac hypertrophy. We now extend this observation and show mAKAP is important for adrenergic-mediated hypertrophy. One function of the mAKAP complex is to facilitate cAMP-dependent protein kinase A-catalyzed phosphorylation of the ryanodine receptor Ca2+-release channel. Experiments utilizing inhibition of the ryanodine receptor, RNA interference of mAKAP expression and replacement of endogenous mAKAP with a mutant form that does not bind to protein kinase A demonstrate that the mAKAP complex contributes to pro-hypertrophic signaling. Further, we show that calcineurin Abeta associates with mAKAP and that the formation of the mAKAP complex is required for the full activation of the pro-hypertrophic transcription factor NFATc. These data reveal a novel function of the mAKAP complex involving the integration of cAMP and Ca2+ signals that promote myocyte hypertrophy.

The CRM1 Nuclear Export Receptor Controls Pathological Cardiac Gene Expression

Diverse pathological insults trigger a cardiac remodeling process during which myocytes undergo hypertrophy, with consequent decline in cardiac function and eventual heart failure. Multiple transcriptional regulators of pathological cardiac hypertrophy are controlled at the level of subcellular distribution. For example, prohypertrophic transcription factors belonging to the nuclear factor of activated T cells (NFAT) and GATA families are subject to CRM1-dependent nuclear export but are rapidly relocalized to the nucleus in response to cues for hypertrophic growth. Here, we demonstrate that the antihypertrophic chromatin-modifying enzyme histone deacetylase 5 (HDAC5) is shuttled out of the cardiomyocyte nucleus via a CRM1-mediated pathway in response to diverse signals for hypertrophy. CRM1 antagonists block the agonist-mediated nuclear export of HDAC 5 and repress pathological gene expression and associated hypertrophy of cultured cardiomyocytes. Conversely, CRM1 activity is dispensable for nonpathological cardiac gene activation mediated by thyroid hormone and insulin-like growth factor 1, agonists that fail to trigger the nuclear export of HDAC5. These results suggest a selective role for CRM1 in derepression of pathological cardiac genes via its neutralizing effects on antihypertrophic factors such as HDAC5. Pharmacological approaches targeting CRM1-dependent nuclear export in heart muscle may have salutary effects on cardiac function by suppressing maladaptive changes in gene expression evoked by stress signals.