Inhibition of E2F abrogates the development of cardiac myocyte hypertrophy

Regulatory Mechanisms That Guide the Fetal to Postnatal Transition of Cardiomyocytes

Heart disease remains a global leading cause of death and disability, necessitating a comprehensive understanding of the heart's development, repair, and dysfunction. This review surveys recent discoveries that explore the developmental transition of proliferative fetal cardiomyocytes into hypertrophic postnatal cardiomyocytes, a process yet to be well-defined. This transition is key to the heart's growth and has promising therapeutic potential, particularly for congenital or acquired heart damage, such as myocardial infarctions. Although significant progress has been made, much work is needed to unravel the complex interplay of signaling pathways that regulate cardiomyocyte proliferation and hypertrophy. This review provides a detailed perspective for future research directions aimed at the potential therapeutic harnessing of the perinatal heart transitions.

Characterization and Validation of ceRNA-Mediated Pathway–Pathway Crosstalk Networks Across Eight Major Cardiovascular Diseases

Pathway analysis is considered as an important strategy to reveal the underlying mechanisms of diseases. Pathways that are involved in crosstalk can regulate each other and co-regulate downstream biological processes. Furthermore, some genes in the pathways can function with other genes via the relationship of the competing endogenous RNA (ceRNA) mechanism, which has also been demonstrated to play key roles in cellular biology. However, the comprehensive analysis of ceRNA-mediated pathway crosstalk is lacking. Here, we constructed the landscape of the ceRNA-mediated pathway–pathway crosstalk of eight major cardiovascular diseases (CVDs) based on sequencing data from ∼2,800 samples. Some common features shared by numerous CVDs were uncovered. A fraction of the pathway–pathway crosstalk was conserved in multiple CVDs and a core pathway–pathway crosstalk network was identified, suggesting the similarity of pathway–pathway crosstalk among CVDs. Experimental evidence also demonstrated that the pathway crosstalk was functioned in CVDs. We split all hub pathways of each pathway–pathway crosstalk network into three categories, namely, common hubs, differential hubs, and specific hubs, which could highlight the common or specific biological mechanisms. Importantly, after a comparison analysis of the hub pathways of networks, ∼480 hub pathway-induced common modules were identified to exert functions in CVDs broadly. Moreover, we performed a random walk algorithm on the hub pathway-induced sub-network and identified 23 potentially novel CVD-related pathways. In summary, our study revealed the potential molecular regulatory mechanisms of ceRNA crosstalk in pathway–pathway crosstalk levels and provided a novel routine to investigate the pathway–pathway crosstalk in cardiology. All CVD pathway–pathway crosstalks are provided in http://www.licpathway.net/cepathway/index.html.

Mitochondrial-cell cycle cross-talk drives endoreplication in heart disease

[Figure: see text].

EHP-101 alleviates angiotensin II-induced fibrosis and inflammation in mice

Some cannabinoids showed anti-inflammatory and antifibrotic activities. EHP-101 is an oral lipidic formulation of the novel non-psychotropic cannabidiol aminoquinone VCE-004.8, which showed antifibrotic activity in murine models of systemic sclerosis induced by bleomycin. We herein examined the effect of EHP-101 on cardiac and other organ fibrosis in a mouse model induced by Angiotensin II. VCE-004.8 inhibited TGFβ- and Ang II-induced myofibroblast differentiation in cardiac fibroblasts detected by α-SMA expression. VCE-004.8 also inhibited Ang II-induced ERK 1 + 2 phosphorylation, NFAT activation and mRNA expression of IL1β, IL6, Col1A2 and CCL2 in cardiac fibroblasts. Mice infused with Ang II resulted in collagen accumulation in left ventricle, aortic, dermal, renal and pulmonary tissues; oral administration of EHP-101, Ajulemic acid and Losartan improved these phenotypes. In myocardial tissue, Ang II induced infiltration of T cells and macrophages together with the accumulation of collagen and Tenascin C; those were all reduced by either EHP-101 or Losartan treatment. Cardiac tissue RNA-Seq analyses revealed a similar transcriptomic signature for both treatments for inflammatory and fibrotic pathways. However, the gene set enrichment analysis comparing data from EHP-101 vs Losartan showed specific hallmarks modified only by EHP-101. Specifically, EHP-101 inhibited the expression of genes such as CDK1, TOP2A and MKi67 that are regulated to the E2 factor family of transcription factors. This study suggests that the oral administration of EHP-101 prevents and inhibits cardiac inflammation and fibrosis. Furthermore, EHP-101 inhibits renal, pulmonary and dermal fibrosis. EHP-101 could offer new opportunities in the treatment of cardiac fibrosis and other fibrotic diseases.

Knockout of E2F1 enhances the polarization of M2 phenotype macrophages to accelerate the wound healing process

Wound healing is a complex process, which is classically divided into inflammation, proliferation, and remodeling phases. Macrophages play a key role in wound healing, however, whether E2F1 mediates the M1/M2 polarization during the wound healing process is not known. Skin wounds were surgically induced in E2F1^-/- mice and their WT littermates. At day 2 and day 7 post-surgery, the wounded skin tissues including 2 to 3 mm normal skin were obtained. The wounded skin tissues were used for the analyses of immunofluorescence staining (CD68, iNOS, CD206), western blotting (CD68, iNOS, CD206, PPAR-γ) and Co-immunoprecipitation (E2F1-PPAR-γ interactions). E2F1^-/- mice exhibited faster wound healing process. At day 2, the M2 macrophages were remarkably increased in the E2F1^-/- mice. Surprisingly, in the border zone of the wound, E2F1^-/- mice had also more M2 macrophages and fewer M1 macrophages at day 7 post-surgery, suggesting a certain degree of polarization amongst the M1 and M2 phenotypes. Co-IP revealed that E2F1 indeed interacted with PPAR-γ, meanwhile western blotting and RT-PCR showed higher expression of PPAR-γ in the E2F1^-/- mice as compared to that in the WT mice. Therefore, the findings suggest that wound healing process could be accelerated with enhanced M2 polarization through increased PPAR-γ expression in E2F1 knockout mice.

Differential gene responses 3 days following infarction in the fetal and adolescent sheep heart

There are critical molecular mechanisms that can be activated to induce myocardial repair, and in humans this is most efficient during fetal development. The timing of heart development in relation to birth and the size/electrophysiology of the heart are similar in humans and sheep, providing a model to investigate the repair capacity of the mammalian heart and how this can be applied to adult heart repair. Myocardial infarction was induced by ligation of the left anterior descending coronary artery in fetal (105 days gestation when cardiomyocytes are proliferative) and adolescent sheep (6 mo of age when all cardiomyocytes have switched to an adult phenotype). An ovine gene microarray was used to compare gene expression in sham and infarcted (remote, border and infarct areas) cardiac tissue from fetal and adolescent hearts. The gene response to myocardial infarction was less pronounced in fetal compared with adolescent sheep hearts and there were unique gene responses at each age. There were also region-specific changes in gene expression between each age, in the infarct tissue, tissue bordering the infarct, and tissue remote from the infarction. In total, there were 880 genes that responded to MI uniquely in the adolescent samples compared with 170 genes in the fetal response, as well as 742 overlap genes that showed concordant direction of change responses to infarction at both ages. In response to myocardial infarction, there were specific changes in genes within pathways of mitochondrial oxidation, muscle contraction, and hematopoietic cell lineages, suggesting that the control of energy utilization and immune function are critical for effective heart repair. The more restricted gene response in the fetus may be an important factor in its enhanced capacity for cardiac repair.

Inhibition of Senescence-Associated Genes Rb1 and Meis2 in Adult Cardiomyocytes Results in Cell Cycle Reentry and Cardiac Repair Post-Myocardial Infarction

Background Myocardial infarction results in a large-scale cardiomyocyte loss and heart failure due to subsequent pathological remodeling. Whereas zebrafish and neonatal mice have evident cardiomyocyte expansion following injury, adult mammalian cardiomyocytes are principally nonproliferative. Despite historical presumptions of stem cell-mediated cardiac regeneration, numerous recent studies using advanced lineage-tracing methods demonstrated that the only source of cardiomyocyte renewal originates from the extant myocardium; thus, the augmented proliferation of preexisting adult cardiomyocytes remains a leading therapeutic approach toward cardiac regeneration. In the present study we investigate the significance of suppressing cell cycle inhibitors Rb1 and Meis2 to promote adult cardiomyocyte reentry to the cell cycle. Methods and Results In vitro experiments with small interfering RNA-mediated simultaneous knockdown of Rb1 and Meis2 in both adult rat cardiomyocytes, isolated from 12-week-old Fischer rats, and human induced pluripotent stem cell-derived cardiomyocytes showed a significant increase in cell number, a decrease in cell size, and an increase in mononucleated cardiomyocytes. In vivo, a hydrogel-based delivery method for small interfering RNA-mediated silencing of Rb1 and Meis2 is utilized following myocardial infarction. Immunofluorescent imaging analysis revealed a significant increase in proliferation markers 5-ethynyl-2'-deoxyuridine, PH3, KI67, and Aurora B in adult cardiomyocytes as well as improved cell survivability with the additional benefit of enhanced peri-infarct angiogenesis. Together, this intervention resulted in a reduced infarct size and improved cardiac function post-myocardial infarction. Conclusions Silencing of senescence-inducing pathways in adult cardiomyocytes via inhibition of Rb1 and Meis2 results in marked cardiomyocyte proliferation and increased protection of cardiac function in the setting of ischemic injury.

E2F6 protein levels modulate drug induced apoptosis in cardiomyocytes

The E2F/Rb pathway regulates cell growth, differentiation, and death. In particular, E2F1 promotes apoptosis in all cells including those of the heart. E2F6, which represses E2F activity, was found to induce dilated cardiomyopathy in the absence of apoptosis in murine post-natal heart. Here we evaluate the anti-apoptotic potential of E2F6 in neonatal cardiomyocytes (NCM) from E2F6-Tg hearts which showed significantly less caspase-3 cleavage, a lower Bax/Bcl2 ratio, and improved cell viability in response to CoCl₂ exposure. This correlated with a decrease in the pro-apoptotic E2F3 protein levels. In contrast, no difference in apoptotic markers or cell viability was observed in response to Doxorubicin (Dox) treatment between Wt and Tg-NCM. Dox caused a rapid and dramatic loss of the E2F6 protein in Tg-NCM within 6h and was undetectable after 12h. The level of e2f6 transcript was unchanged in Wt NCM, but was dramatically decreased in Tg cells in response to both Dox and CoCl₂. This was related to an impact of the drugs on the α-myosin heavy chain promoter used to drive the E2F6 transgene. By comparison in HeLa, Dox induced apoptosis through upregulation of endogenous E2F1 involving post-transcriptional mechanisms, while E2F6 was down regulated with induction of the Checkpoint kinase-1 and proteasome degradation. These data imply that E2F6 serves to modulate E2F activity and protect cells including cardiomyocytes from apoptosis and improve survival. Strategies to modulate E2F6 levels may be therapeutically useful to mitigate cell death associated disorders.

E2F6 Impairs Glycolysis and Activates BDH1 Expression Prior to Dilated Cardiomyopathy

Rationale

The E2F pathway plays a critical role in cardiac growth and development, yet its role in cardiac metabolism remains to be defined. Metabolic changes play important roles in human heart failure and studies imply the ketogenic enzyme β-hydroxybutyrate dehydrogenase I (BDH1) is a potential biomarker.

Objective

To define the role of the E2F pathway in cardiac metabolism and dilated cardiomyopathy (DCM) with a focus on BDH1.

Methods and Results

We previously developed transgenic (Tg) mice expressing the transcriptional repressor, E2F6, to interfere with the E2F/Rb pathway in post-natal myocardium. These Tg mice present with an E2F6 dose dependent DCM and deregulated connexin-43 (CX-43) levels in myocardium. Using the Seahorse platform, a 22% decrease in glycolysis was noted in neonatal cardiomyocytes isolated from E2F6-Tg hearts. This was associated with a 39% reduction in the glucose transporter GLUT4 and 50% less activation of the regulator of glucose metabolism AKT2. The specific reduction of cyclin B1 (70%) in Tg myocardium implicates its importance in supporting glycolysis in the postnatal heart. No changes in cyclin D expression (known to regulate mitochondrial activity) were noted and lipid metabolism remained unchanged in neonatal cardiomyocytes from Tg hearts. However, E2F6 induced a 40-fold increase of the Bdh1 transcript and 890% increase in its protein levels in hearts from Tg pups implying a potential impact on ketolysis. By contrast, BDH1 expression is not activated until adulthood in normal myocardium. Neonatal cardiomyocytes from Wt hearts incubated with the ketone β-hydroxybutyrate (β-OHB) showed a 100% increase in CX-43 protein levels, implying a role for ketone signaling in gap junction biology. Neonatal cardiomyocyte cultures from Tg hearts exhibited enhanced levels of BDH1 and CX-43 and were not responsive to β-OHB.

Conclusions

The data reveal a novel role for the E2F pathway in regulating glycolysis in the developing myocardium through a mechanism involving cyclin B1. We reveal BDH1 expression as an early biomarker of heart failure and its potential impact, through ketone signaling, on CX-43 levels in E2F6-induced DCM.

E2F1, a Novel Regulator of Metabolism

In the past years, several lines of evidence have shown that cell cycle regulatory proteins also can modulate metabolic processes. The transcription factor E2F1 is a central player involved in cell cycle progression, DNA-damage response, and apoptosis. Its crucial role in the control of cell fate has been extensively studied and reviewed before; however, here, we focus on the participation of E2F1 in the regulation of metabolism. We summarize recent findings about the cell cycle-independent roles of E2F1 in various tissues that contribute to global metabolic homeostasis and highlight that E2F1 activity is increased during obesity. Finally, coming back to the pivotal role of E2F1 in cancer development, we discuss how E2F1 links cell cycle progression with different metabolic adaptations required for cell growth and survival.

Dyrk1a regulates the cardiomyocyte cell cycle via D-cyclin-dependent Rb/E2f-signalling

AIMS

Down syndrome-associated dual-specificity tyrosine-(Y)-phosphorylation-regulated kinase 1A (DYRK1A) is a ubiquitously expressed protein kinase. Up to date a variety of targets have been identified, establishing a key role for Dyrk1a in selected signalling pathways. In cardiomyocytes, Dyrk1a acts as a negative regulator of hypertrophy by phosphorylating transcription factors of the NFAT family, but its mechanistic function in the heart remains poorly understood. This study was designed to investigate a potential protective role of Dyrk1a in cardiac hypertrophy in vivo.

METHODS AND RESULTS

We generated transgenic mice with cardiac-specific overexpression of Dyrk1a. Counterintuitively, these mice developed severe dilated cardiomyopathy associated with congestive heart failure and premature death. In search for the cause of this unexpected phenotype, we found that Dyrk1a interacts with all members of the D-cyclin family and represses their protein levels in vitro and in vivo. Particularly, forced expression of Dyrk1a leads to increased phosphorylation of Ccnd2 on Thr280 and promotes its subsequent proteasomal degradation. Accordingly, cardiomyocytes overexpressing Dyrk1a display hypo-phosphorylated Rb1, suppression of Rb/E2f-signalling, and reduced expression of E2f-target genes, which ultimately results in impaired cell cycle progression.

CONCLUSIONS

We identified Dyrk1a as a novel negative regulator of D-cyclin-mediated Rb/E2f-signalling. As dysregulation of this pathway with impaired cardiomyocyte proliferation leads to cardiomyopathy, dose-specific Dyrk1a expression and activity appears to be critical for the hyperplastic and hypertrophic growth of the developing heart.

CDK6 mediates the effect of attenuation of miR-1 on provoking cardiomyocyte hypertrophy

MicroRNA-1 (miR-1) is approved involved in cardiac hypertrophy, but the underlying molecular mechanisms of miR-1 in cardiac hypertrophy are not well elucidated. The present study aimed to investigate the potential role of miR-1 in modulating CDKs-Rb pathway during cardiomyocyte hypertrophy. A rat model of hypertrophy was established with abdominal aortic constriction, and a cell model of hypertrophy was also achieved based on PE-promoted neonatal rat ventricular cardiomyocytes (NRVCs). We demonstrated that miR-1 expression was markedly decreased in hypertrophic myocardium and hypertrophic cardiomyocytes. Dual luciferase reporter assays revealed that miR-1 interacted with the 3'UTR of CDK6, and miR-1 was verified to inhibit CDK6 expression at the posttranscriptional level. CDK6 protein expression was observed increased in hypertrophic myocardium and hypertrophic cardiomyocytes. Morover, miR-1 mimic, in parallel to CDK6 siRNA, could inhibit PE-induced hypertrophy of NRVCs, with decreases in cell size, newly transcribed RNA, expressions of ANF and β-MHC, and the phosphorylated pRb. Taken together, our results reveal that derepression of CDK6 and activation of Rb pathway contributes to the effect of attenuation of miR-1 on provoking cardiomyocyte hypertrophy.

Muscle ring finger-3 protects against diabetic cardiomyopathy induced by a high fat diet

BACKGROUND

The pathogenesis of diabetic cardiomyopathy (DCM) involves the enhanced activation of peroxisome proliferator activating receptor (PPAR) transcription factors, including the most prominent isoform in the heart, PPARα. In cancer cells and adipocytes, post-translational modification of PPARs have been identified, including ligand-dependent degradation of PPARs by specific ubiquitin ligases. However, the regulation of PPARs in cardiomyocytes and heart have not previously been identified. We recently identified that muscle ring finger-1 (MuRF1) and MuRF2 differentially inhibit PPAR activities by mono-ubiquitination, leading to the hypothesis that MuRF3 may regulate PPAR activity in vivo to regulate DCM.

METHODS

MuRF3-/- mice were challenged with 26 weeks 60% high fat diet to induce insulin resistance and DCM. Conscious echocardiography, blood glucose, tissue triglyceride, glycogen levels, immunoblot analysis of intracellular signaling, heart and skeletal muscle morphometrics, and PPARα, PPARβ, and PPARγ1 activities were assayed.

RESULTS

MuRF3-/- mice exhibited a premature systolic heart failure by 6 weeks high fat diet (vs. 12 weeks in MuRF3+/+). MuRF3-/- mice weighed significantly less than sibling-matched wildtype mice after 26 weeks HFD. These differences may be largely due to resistance to fat accumulation, as MRI analysis revealed MuRF3-/- mice had significantly less fat mass, but not lean body mass. In vitro ubiquitination assays identified MuRF3 mono-ubiquitinated PPARα and PPARγ1, but not PPARβ.

CONCLUSIONS

These findings suggest that MuRF3 helps stabilize cardiac PPARα and PPARγ1 in vivo to support resistance to the development of DCM. MuRF3 also plays an unexpected role in regulating fat storage despite being found only in striated muscle.

Interplay between the E2F pathway and β-adrenergic signaling in the pathological hypertrophic response of myocardium

The E2F/Pocket protein (Rb) pathway regulates cell growth, differentiation, and death by modulating gene expression. We previously examined this pathway in the myocardium via manipulation of the unique E2F repressor, E2F6, which is believed to repress gene activity independently of Rb. Mice with targeted expression of E2F6 in postnatal myocardium developed dilated cardiomyopathy (DCM) without hypertrophic growth. We assessed the mechanisms of the apparent failure of compensatory hypertrophic growth as well as their response to the β-adrenergic agonist isoproterenol. As early as 2 weeks, E2F6 transgenic (Tg) mice present with dilated thinner left ventricles and significantly reduced ejection fraction and fractional shortening which persists at 6 weeks of age, but with no apparent increase in left ventricle weight: body weight (LVW:BW). E2F6-Tg mice treated with isoproterenol (6.1 mg/kg/day) show double the increase in LVW:BW than their Wt counterparts (32% vs 16%, p-value: 0.007). Western blot analysis revealed the activation of the adrenergic pathway in Tg heart tissue under basal conditions with ~2-fold increase in the level of β2-adrenergic receptors (p-value: 8.9E-05), protein kinase A catalytic subunit (PKA-C) (p-value: 0.0176), activated c-Src tyrosine-protein kinase (p-value: 0.0002), extracellular receptor kinase 2 (ERK2) (p-value: 0.0005), and induction of the anti-apoptotic protein Bcl2 (p-value 0. 0.00001). In contrast, a ~60% decrease in the cardiac growth regulator: AKT1 (p-value 0.0001) and a ~four fold increase in cyclic AMP dependent phosphodiesterase 4D (PDE4D), the negative regulator of PKA activity, were evident in the myocardium of E2F6-Tg mice. The expression of E2F3 was down-regulated by E2F6, but was restored by isoproterenol. Further, Rb expression was down-regulated in Tg mice in response to isoproterenol implying a net activation of the E2F pathway. Thus the unique regulation of E2F activity by E2F6 renders the myocardium hypersensitive to adrenergic stimulus resulting in robust hypertrophic growth. These data reveal a novel interplay between the E2F pathway, β2-adrenergic/PKA/PDE4D, and ERK/c-Src axis in fine tuning the pathological hypertrophic growth response. E2F6 deregulates E2F3 such that pro-hypertrophic growth and survival are enhanced via β2-adrenergic signaling however this response is outweighed by the induction of anti-hypertrophic signals so that left ventricle dilation proceeds without any increase in muscle mass.

Attenuation of microRNA-16 derepresses the cyclins D1, D2 and E1 to provoke cardiomyocyte hypertrophy

Cyclins/retinoblastoma protein (pRb) pathway participates in cardiomyocyte hypertrophy. MicroRNAs (miRNAs), the endogenous small non-coding RNAs, were recognized to play significant roles in cardiac hypertrophy. But, it remains unknown whether cyclin/Rb pathway is modulated by miRNAs during cardiac hypertrophy. This study investigates the potential role of microRNA-16 (miR-16) in modulating cyclin/Rb pathway during cardiomyocyte hypertrophy. An animal model of hypertrophy was established in a rat with abdominal aortic constriction (AAC), and in a mouse with transverse aortic constriction (TAC) and in a mouse with subcutaneous injection of phenylephrine (PE) respectively. In addition, a cell model of hypertrophy was also achieved based on PE-promoted neonatal rat ventricular cardiomyocyte and based on Ang-II-induced neonatal mouse ventricular cardiomyocyte respectively. We demonstrated that miR-16 expression was markedly decreased in hypertrophic myocardium and hypertrophic cardiomyocytes in rats and mice. Overexpression of miR-16 suppressed rat cardiac hypertrophy and hypertrophic phenotype of cultured cardiomyocytes, and inhibition of miR-16 induced a hypertrophic phenotype in cardiomyocytes. Expressions of cyclins D1, D2 and E1, and the phosphorylated pRb were increased in hypertrophic myocardium and hypertrophic cardiomyocytes, but could be reversed by enforced expression of miR-16. Cyclins D1, D2 and E1, not pRb, were further validated to be modulated post-transcriptionally by miR-16. In addition, the signal transducer and activator of transcription-3 and c-Myc were activated during myocardial hypertrophy, and inhibitions of them prevented miR-16 attenuation. Therefore, attenuation of miR-16 provoke cardiomyocyte hypertrophy via derepressing the cyclins D1, D2 and E1, and activating cyclin/Rb pathway, revealing that miR-16 might be a target to manage cardiac hypertrophy.

The heart: mostly postmitotic or mostly premitotic? Myocyte cell cycle, senescence, and quiescence

The concept of myocyte division and myocyte-mediated regeneration has re-emerged in the past 5 years through development of sophisticated transgenic mice and carbon-dating of cells. Although recently, a couple of studies have been conducted as an attempt to intervene in myocyte division, the efficiency in adult animals remains discouragingly low. Re-enforcing myocyte division is a vision that has been desired for decades, leading to years of experience in myocyte resistance to proproliferative stimuli. Previous attempts have indeed provided a platform for basic knowledge on molecular players and signalling in myocytes. However, natural biological processes such as hypertrophy and binucleation provide layers of complexity in interpretation of previous and current findings. A major hurdle in mediating myocyte division is a lack of insight in the myocyte cell cycle. To date, no knowledge is gained on myoycte cell cycle progression and/or duration. This review will include an overview of previous and current literature on myocyte cell cycle and division. Furthermore, the limitations of current approaches and basic questions that might be essential in understanding myocardial resistance to division will be discussed.

Evaluation of the plasma micro RNA expression levels in secondary hemophagocytic lymphohistiocytosis

BACKGROUND

Hemophagocytic lymphohistiocytosis (HLH) is a life threatening hyper inflammatory disease. Micro RNAs (miRNA) are about 22 nucleotide-long, small RNAs encoded with genes, and they have regulatory functions in immune response.

OBJECTIVE

To determine the miRNA expression levels of 11 secondary HLH patients, we evaluated the associations of miRNA levels with pathogenesis, clinical presentation, and prognosis of the disease.

PATIENTS AND METHODS

Patients who were diagnosed with secondary HLH from January 2011 to December 2012 were included in this study. We profiled the expressions of 379 miRNAs in plasma of both HLH patients and healthy controls. Patients were evaluated regarding with age, clinical findings, miRNA expresions, laboratory data, treatment, and prognosis, by using descriptive statistics.

RESULTS

A total of 11 secondary HLH patients and 11 healthy children were included in this study. miR-205-5p was expressed in all case and controls and expression level of miR-205-5p was found 6.21 fold higher than control group (p=0.01). We detected the second highest expression percent in miR-194-5p with 81% of cases and controls. Expression level of miR-194-5p was found to have 163 fold higher than controls (p= 0.009). miR-30c-5p showed 77% expression percent in cases and controls together. The expression level of this miRNA was detected 9 fold decreased in HLH patients compared to healthy children (p= 0.031).

CONCLUSION

We showed that miR-205-5p, miR-194-5p and miR-30c-5p could be useful plasma biomarkers for HLH. Further research is needed in larger and homogenous study groups, especially for these miRNAs as biomarkers for HLH.

An overview of the crosstalk between inflammatory processes and metabolic dysregulation during diabetic cardiomyopathy

Metabolic disorders such as obesity, insulin resistance and type 2 diabetes mellitus are all linked to cardiovascular diseases such as cardiac hypertrophy and heart failure. Diabetic cardiomyopathy in particular, is characterized by structural and functional alterations in the heart muscle of people with diabetes that finally lead to heart failure, and which is not directly attributable to coronary artery disease or hypertension. Several mechanisms have been involved in the pathogenesis of diabetic cardiomyopathy, such as alterations in myocardial energy metabolism and calcium signaling. Metabolic disturbances during diabetic cardiomyopathy are characterized by increased lipid oxidation, intramyocardial triglyceride accumulation, and reduced glucose utilization. Overall changes result in enhanced oxidative stress, mitochondrial dysfunction and apoptosis of the cardiomyocytes. On the other hand, the progression of heart failure and cardiac hypertrophy usually entails a local rise in cytokines in cardiac cells and the activation of the proinflammatory transcription factor nuclear factor (NF)-κB. Interestingly, increasing evidences are arising in the recent years that point to a potential link between chronic low-grade inflammation in the heart and metabolic dysregulation. Therefore, in this review we summarize recent new insights into the crosstalk between inflammatory processes and metabolic dysregulation in the failing heart during diabetes, paying special attention to the role of NF-κB and peroxisome proliferator activated receptors (PPARs). In addition, we briefly describe the role of the AMP-activated protein kinase (AMPK), sirtuin 1 (SIRT1) and other pathways regulating cardiac energy metabolism, as well as their relationship with diabetic cardiomyopathy.

RhoA regulation of cardiomyocyte differentiation

Earlier findings from our laboratory implicated RhoA in heart developmental processes. To investigate factors that potentially regulate RhoA expression, RhoA gene organisation and promoter activity were analysed. Comparative analysis indicated strict conservation of both gene organisation and coding sequence of the chick, mouse, and human RhoA genes. Bioinformatics analysis of the derived promoter region of mouse RhoA identified putative consensus sequence binding sites for several transcription factors involved in heart formation and organogenesis generally. Using luciferase reporter assays, RhoA promoter activity was shown to increase in mouse-derived P19CL6 cells that were induced to differentiate into cardiomyocytes. Overexpression of a dominant negative mutant of mouse RhoA (mRhoAN19) blocked this cardiomyocyte differentiation of P19CL6 cells and led to the accumulation of the cardiac transcription factors SRF and GATA4 and the early cardiac marker cardiac α-actin. Taken together, these findings indicate a fundamental role for RhoA in the differentiation of cardiomyocytes.

Role of D-type cyclins in heart development and disease

A defining feature of embryonic cardiomyocytes is their relatively high rates of proliferation. A gradual reduction in proliferative capacity throughout development culminates in permanent cell cycle exit by the vast majority of cardiomyocytes around the perinatal period. Accordingly, the adult heart has severely limited capacity for regeneration in response to injury or disease. The D-type cyclins (cyclin D1, D2, and D3) along with their catalytically active partners, the cyclin dependent kinases, are positive cell cycle regulators that play important roles in regulating proliferation of cardiomyocytes during normal heart development. While expression of D-type cyclins is generally low in the adult heart, expression levels are augmented in association with cardiac hypertrophy, but are uncoupled from myocyte cell division. Accordingly, re-activation of D-type cyclin expression in the adult heart has been implicated in pathophysiological processes via mechanisms distinct from those that drive proliferation during cardiac development. Growth factors and other exogenous agents regulate D-type cyclin production and activity in embryonic and adult cardiomyocytes. Understanding differences in the precise intracellular mediators downstream from these signalling molecules in embryonic versus adult cardiomyocytes could prove valuable for designing strategies to reactivate the cell cycle in cardiomyocytes in the setting of cardiovascular disease in the adult heart.

Agonist-induced hypertrophy and diastolic dysfunction are associated with selective reduction in glucose oxidation: a metabolic contribution to heart failure with normal ejection fraction

BACKGROUND

Activation of the renin-angiotensin and sympathetic nervous systems may alter the cardiac energy substrate preference, thereby contributing to the progression of heart failure with normal ejection fraction. We assessed the qualitative and quantitative effects of angiotensin II (Ang II) and the α-adrenergic agonist, phenylephrine (PE), on cardiac energy metabolism in experimental models of hypertrophy and diastolic dysfunction and the role of the Ang II type 1 receptor.

METHODS AND RESULTS

Ang II (1.5 mg·kg(-1)·day(-1)) or PE (40 mg·kg(-1)·day(-1)) was administered to 9-week-old male C57/BL6 wild-type mice for 14 days via implanted microosmotic pumps. Echocardiography showed concentric hypertrophy and diastolic dysfunction, with preserved systolic function in Ang II- and PE-treated mice. Ang II induced marked reduction in cardiac glucose oxidation and lactate oxidation, with no change in glycolysis and fatty acid β-oxidation. Tricarboxylic acid acetyl coenzyme A production and ATP production were reduced in response to Ang II. Cardiac pyruvate dehydrogenase kinase 4 expression was upregulated by Ang II and PE, resulting in a reduction in the pyruvate dehydrogenase activity, the rate-limiting step for carbohydrate oxidation. Pyruvate dehydrogenase kinase 4 upregulation correlated with the activation of the cyclin/cyclin-dependent kinase-retinoblastoma protein-E2F pathway in response to Ang II. Ang II type 1 receptor blockade normalized the activation of the cyclin/cyclin-dependent kinase-retinoblastoma protein-E2F pathway and prevented the reduction in glucose oxidation but increased fatty acid oxidation.

CONCLUSIONS

Ang II- and PE-induced hypertrophy and diastolic dysfunction is associated with reduced glucose oxidation because of the cyclin/cyclin-dependent kinase-retinoblastoma protein-E2F-induced upregulation of pyruvate dehydrogenase kinase 4, and targeting these pathways may provide novel therapy for heart failure with normal ejection fraction.

The E2F6 repressor activates gene expression in myocardium resulting in dilated cardiomyopathy

The E2F/Rb pathway regulates cardiac growth and development and holds great potential as a therapeutic target. The E2F6 repressor is a unique E2F member that acts independently of pocket proteins. Forced expression of E2F6 in mouse myocardium induced heart failure and mortality, with severity of symptoms correlating to E2F6 levels. Echocardiography demonstrated a 37% increase (P<0.05) in left ventricular end-diastolic diameter and reduced ejection fraction (<40%, P<0.05) in young transgenic (Tg) mice. Microarray and qPCR analysis revealed a paradoxical increase in E2F-responsive genes, which regulate the cell cycle, without changes in cardiomyocyte cell number or size in Tg mice. Young adult Tg mice displayed a 75% (P<0.01) decrease in gap junction protein connexin-43, resulting in abnormal electrocardiogram including a 24% (P<0.05) increase in PR interval. Further, mir-206, which targets connexin-43, was up-regulated 10-fold (P<0.05) in Tg myocardium. The mitogen-activated protein kinase pathway, which regulates the levels of miR-206 and connexin-43, was activated in Tg hearts. Thus, deregulated E2F6 levels evoked abnormal gene expression at transcriptional and post-transcriptional levels, leading to cardiac remodeling and dilated cardiomyopathy. The data highlight an unprecedented role for the strict regulation of the E2F pathway in normal postnatal cardiac function.

Activating cardiac E2F1 induces up-regulation of pyruvate dehydrogenase kinase 4 in mice on a short term of high fat feeding

A high fat diet (HFD) induces substantial cardiac metabolic alteration(s), but the initiating molecular events remain unclear. We assessed the early cardiac energy metabolic changes in C57/BJ mice fed a HFD for 10days. Carbohydrate oxidation was markedly decreased in mice on a HFD, in which up-regulation of pyruvate dehydrogenase kinase 4 (PDK4) was evident. Concurrently, E2F1, a transcription factor controlling PDK4 expression, was activated, as was cyclin D1, an upstream-molecule of E2F1, and eukaryotic initiation factor 4E (eIF4E), a modulator of cyclinD1 translation. Hence, HFD may initiate early cardiac metabolic alterations through the eIF4E/cyclin D1/E2F1/PDK4 axis.

Limitations of the MRL mouse as a model for cardiac regeneration

OBJECTIVE

Myocardial repair following injury in mammals is restricted such that damaged areas are replaced by scar tissue, impairing cardiac function. MRL mice exhibit exceptional regenerative healing in an ear punch wound model. Some myocardial repair with restoration of heart function has also been reported following cryoinjury. Increased cardiomyocyte proliferation and a foetal liver stem cell population were implicated. We investigated molecular mechanisms facilitating myocardial repair in MRL mice to identify potential therapeutic targets in non-regenerative species.

METHODS

Expressions of specific cell-cycle regulators that might account for regeneration (CDKs 1, 2, 4 and 6; cyclins A, E, D1 and B1; p21, p27 and E2F5) were compared by immunoblotting in MRL and control C57BL/6 ventricles during development. Flow cytometry was used to investigate stem cell populations in livers from foetal mice, and infarct sizes were compared in coronary artery-ligated and sham-treated MRL and C57BL/6 adult mice.

KEY FINDINGS

No differences in the expressions of cell cycle regulators were observed between the two strains. Expressions of CD34+Sca1+ckit-, CD34+Sca1+ckit+ and CD34+Sca1-ckit+ increased in livers from C57BL/6 vs MRL mice. No differences were observed in infarct sizes, levels of fibrosis, Ki67 staining or cardiac function between MRL and C57BL/6 mice.

CONCLUSIONS

No intrinsic differences were observed in cell cycle control molecules or stem cell populations between MRL and control C57BL mouse hearts. Pathophysiologically relevant ischaemic injury is not repaired more efficiently in MRL myocardium, questioning the use of the MRL mouse as a reliable model for cardiac regeneration in response to pathophysiologically relevant forms of injury.

Silencing of sodium-hydrogen exchanger 1 attenuates the proliferation, hypertrophy, and migration of pulmonary artery smooth muscle cells via E2F1

We previously found that deficiency of the sodium-hydrogen exchanger 1 (NHE1) gene prevented hypoxia-induced pulmonary hypertension and vascular remodeling in mice, which were accompanied by a significantly reduced proliferation of pulmonary artery smooth muscle cells (PASMCs), and which decreased the medial-wall thickness of pulmonary arteries. That finding indicated the involvement of NHE1 in the proliferation and hypertrophy of PASMCs, but the underlying mechanism was not fully understood. To define the mechanism by which the inhibition of NHE1 decreases hypoxic pulmonary hypertension and vascular remodeling, we investigated the role of E2F1, a nuclear transcription factor, in silencing the NHE1 gene-induced inhibition of the proliferation, hypertrophy, and migration of human PASMCs. We found that: (1) silencing of NHE1 by short, interfering RNA (siRNA) significantly inhibited PASMC proliferation and cell cycle progression, decreased hypoxia-induced hypertrophy (in terms of cell size and protein/DNA ratio) and migration (in terms of the wound-healing and migration chamber assays); (2) hypoxia induced the expression of E2F1, which was reversed by NHE1 siRNA; and (3) the overexpression of E2F1 blocked the inhibitory effect of NHE1 siRNA on the proliferation, hypertrophy, and migration of PASMCs. The present study determined that silencing the NHE1 gene significantly inhibited the hypoxia-induced proliferation, hypertrophy, and migration of human PASMCs via repression of the nuclear transcription factor E2F1. This study revealed a novel mechanism underlying the regulation of hypoxic pulmonary hypertension and vascular remodeling via NHE1.

The interplay between NF-kappaB and E2F1 coordinately regulates inflammation and metabolism in human cardiac cells

Pyruvate dehydrogenase kinase 4 (PDK4) inhibition by nuclear factor-κB (NF-κB) is related to a shift towards increased glycolysis during cardiac pathological processes such as cardiac hypertrophy and heart failure. The transcription factors estrogen-related receptor-α (ERRα) and peroxisome proliferator-activated receptor (PPAR) regulate PDK4 expression through the potent transcriptional coactivator PPARγ coactivator-1α (PGC-1α). NF-κB activation in AC16 cardiac cells inhibit ERRα and PPARβ/δ transcriptional activity, resulting in reduced PGC-1α and PDK4 expression, and an enhanced glucose oxidation rate. However, addition of the NF-κB inhibitor parthenolide to these cells prevents the downregulation of PDK4 expression but not ERRα and PPARβ/δ DNA binding activity, thus suggesting that additional transcription factors are regulating PDK4. Interestingly, a recent study has demonstrated that the transcription factor E2F1, which is crucial for cell cycle control, may regulate PDK4 expression. Given that NF-κB may antagonize the transcriptional activity of E2F1 in cardiac myocytes, we sought to study whether inflammatory processes driven by NF-κB can downregulate PDK4 expression in human cardiac AC16 cells through E2F1 inhibition. Protein coimmunoprecipitation indicated that PDK4 downregulation entailed enhanced physical interaction between the p65 subunit of NF-κB and E2F1. Chromatin immunoprecipitation analyses demonstrated that p65 translocation into the nucleus prevented the recruitment of E2F1 to the PDK4 promoter and its subsequent E2F1-dependent gene transcription. Interestingly, the NF-κB inhibitor parthenolide prevented the inhibition of E2F1, while E2F1 overexpression reduced interleukin expression in stimulated cardiac cells. Based on these findings, we propose that NF-κB acts as a molecular switch that regulates E2F1-dependent PDK4 gene transcription.

Reversible regulation of the retinoblastoma protein/E2F-1 pathway during "reverse cardiac remodelling" after ventricular unloading

BACKGROUND

Cyclin D1, the retinoblastoma (Rb) protein, and the E2F transcription factors are involved in the pathogenesis of cardiac hypertrophy. Cyclin D1/cdk4 complexes, by phosphorylation, inactivate Rb, thereby abrogating its growth-inhibitory effect. Ventricular unloading is associated with reversible regulation of numerous cardiomyocyte molecular systems and decreased hypertrophy. Accordingly, the hypothesis whether the Rb/E2F-1 pathway is altered by ventricular unloading was tested, and correlations with the cyclin D1 protein expression and cardiomyocyte diameters were explored.

METHODS

In 21 paired myocardial samples (before and after unloading) from patients with congestive heart failure (CHF), cyclin D1, phosphorylated Rb (pRb), its homologues p107 and p130 (pocket proteins), and E2F-1 were immunohistochemically investigated and morphometrically quantified. Cardiomyocyte diameters were morphometrically determined.

RESULTS

Cyclin D1 and the proteins of the Rb/E2F-1 pathway were significantly increased during CHF compared with controls and were significantly decreased after unloading. Cyclin D1, pRb, and p130 protein expression correlated significantly with cardiomyocyte diameters. A significant positive correlation was noted between the pocket proteins, E2F-1, and cyclin D1.

CONCLUSION

Increased protein expression of phosphorylated (inactivated) Rb and the pocket proteins is associated with cardiomyocyte hypertrophy in CHF. Rb inactivation might be explained by phosphorylation by increased numbers of cyclin D1/cdk4 complexes associated with cardiomyocyte hypertrophy. However, ventricular unloading can reversibly regulate this process. These data underscore the importance of cell cycle regulatory proteins in the pathogenesis of CHF-associated (maladaptive) cardiomyocyte hypertrophy and might offer novel clues for pharmacologic approaches of congestive heart failure.

Coordinated modular functionality and prognostic potential of a heart failure biomarker-driven interaction network

Background

The identification of potentially relevant biomarkers and a deeper understanding of molecular mechanisms related to heart failure (HF) development can be enhanced by the implementation of biological network-based analyses. To support these efforts, here we report a global network of protein-protein interactions (PPIs) relevant to HF, which was characterized through integrative bioinformatic analyses of multiple sources of "omic" information.

Results

We found that the structural and functional architecture of this PPI network is highly modular. These network modules can be assigned to specialized processes, specific cellular regions and their functional roles tend to partially overlap. Our results suggest that HF biomarkers may be defined as key coordinators of intra- and inter-module communication. Putative biomarkers can, in general, be distinguished as "information traffic" mediators within this network. The top high traffic proteins are encoded by genes that are not highly differentially expressed across HF and non-HF patients. Nevertheless, we present evidence that the integration of expression patterns from high traffic genes may support accurate prediction of HF. We quantitatively demonstrate that intra- and inter-module functional activity may be controlled by a family of transcription factors known to be associated with the prevention of hypertrophy.

Conclusion

The systems-driven analysis reported here provides the basis for the identification of potentially novel biomarkers and understanding HF-related mechanisms in a more comprehensive and integrated way.

Characterisation and regulation of E2F-6 and E2F-6b in the rat heart: a potential target for myocardial regeneration?

The E2F transcription factors are instrumental in regulating cell cycle progression and growth, including that in cardiomyocytes, which exit the cell cycle shortly after birth. E2F-6 has been demonstrated to act as a transcriptional repressor; however, its potential role in normal cardiomyocyte proliferation and hypertrophy has not previously been investigated. Here we report the isolation and characterisation of E2F-6 and E2F-6b in rat cardiomyocytes and consider its potential as a target for myocardial regeneration following injury. At the mRNA level, both rat E2F-6 and the alternatively spliced variant, E2F-6b, were expressed in E18 myocytes and levels were maintained throughout development into adulthood. Interestingly, E2F-6 protein expression was down-regulated during myocyte development suggesting that it is regulated post-transcriptionally in these cells. During myocyte hypertrophy, the mRNA expressions of E2F-6 and E2F-6b were not regulated whereas E2F-6 protein was up-regulated significantly. Indeed, E2F-6 protein expression levels closely parallel the developmental withdrawal of myocytes from the cell cycle and the subsequent reactivation of their cell cycle machinery during hypertrophic growth. Furthermore, depletion of E2F-6, using anti-sense technology, results in death of cultured neonatal myocytes. Taken together, abrogation of E2F-6 expression in neonatal cardiomyocytes leads to a significant decrease in their viability, consistent with the notion that E2F-6 might be required for maintaining normal myocyte growth.

Over expression of Plk1 does not induce cell division in rat cardiac myocytes in vitro

BACKGROUND

Mammalian cardiac myocytes withdraw from the cell cycle during post-natal development, resulting in a non-proliferating, fully differentiated adult phenotype that is unable to repair damage to the myocardium, such as occurs following a myocardial infarction. We and others previously have shown that forced expression of certain cell cycle molecules in adult cardiac myocytes can promote cell cycle progression and division in these cells. The mitotic serine/threonine kinase, Polo-like kinase-1 (Plk1), is known to phosphorylate and activate a number of mitotic targets, including Cdc2/Cyclin B1, and to promote cell division.

PRINCIPAL FINDINGS

The mammalian Plk family are all differentially regulated during the development of rat cardiac myocytes, with Plk1 showing the most dramatic decrease in both mRNA, protein and activity in the adult. We determined the potential of Plk1 to induce cell cycle progression and division in cultured rat cardiac myocytes. A persistent and progressive loss of Plk1 expression was observed during myocyte development that correlated with the withdrawal of adult rat cardiac myocytes from the cell cycle. Interestingly, when Plk1 was over-expressed in cardiac myocytes by adenovirus infection, it was not able to promote cell cycle progression, as determined by cell number and percent binucleation.

CONCLUSIONS

We conclude that, in contrast to Cdc2/Cyclin B1 over-expression, the forced expression of Plk1 in adult cardiac myocytes is not sufficient to induce cell division and myocardial repair.

Effects of nitroglycerin or pentaerithrityl tetranitrate treatment on the gene expression in rat hearts: evidence for cardiotoxic and cardioprotective effects

Nitroglycerin (NTG) and pentaerithrityl tetranitrate (PETN) are organic nitrates used in the treatment of angina pectoris, myocardial infarction, and congestive heart failure. Recent data show marked differences in the effects of NTG and PETN on the generation of reactive oxygen species. These differences are attributed to different effects of NTG and PETN on the expression of antioxidative proteins like the heme oxygenase-I. To analyze the expressional effects of NTG and PETN in a more comprehensive manner we performed whole genome expression profiling experiments using cardiac total RNA from NTG- or PETN-treated rats and DNA microarrays containing oligonucleotides representing 27,044 rat gene transcripts. The data obtained show that NTG and PETN together significantly modify the expression of >1,600 genes (NTG 532, PETN 1212). However, the expression of only a small group of these genes (68) was modified by both treatments, indicating marked differences in the expressional effects of NTG and PETN. NTG treatment resulted in the enhanced expression of genes that are believed to be markers for cardiotoxic processes. In addition, NTG treatment reduced the expression of genes described to code for cardioprotective proteins. In sharp contrast, PETN treatment enhanced the expression of cardioprotective genes and reduced the expression of genes believed to perform cardiotoxic effects. In conclusion, our data suggest that NTG treatment results in the induction of cardiotoxic gene expression networks leading to an activation of mechanisms that result in pathological changes in cardiomyocytes. In contrast, PETN treatment seems to activate gene expression networks that result in cardioprotective effects.

E2F3 plays an essential role in cardiac development and function

The E2F transcription factors are key downstream targets of the retinoblastoma protein tumor suppressor. They are known to regulate the expression of genes that control fundamental biological processes including cellular proliferation, apoptosis and differentiation. However, considerable questions remain about the precise roles of the individual E2F family members. This study shows that E2F3 is essential for normal cardiac development. E2F3-loss impairs the proliferative capacity of the embryonic myocardium and most E2f3(-/-) mice die in utero or perinatally with hypoplastic ventricular walls and/or severe atrial and ventricular septal defects. A small fraction of the E2f3(-/-) neonates have hearts that appear grossly normal and they initially survive. However, these animals display ultrastructural defects in the cardiac muscle and ultimately die as a result of congestive heart failure. These data demonstrate a clear role for E2F3 in myocardial and cardiac function during both development and adulthood.

Phosphorylation of pRb by cyclin D kinase is necessary for development of cardiac hypertrophy

OBJECTIVES

A number of stimuli induce cardiac hypertrophy and may lead to cardiomyopathy and heart failure. It is believed that cardiomyocytes withdraw from the cell cycle shortly after birth and become terminally differentiated. However, cell cycle regulatory proteins take part in the development of hypertrophy, and it is important to elucidate the mechanisms of how these proteins are involved in the hypertrophic response in cardiomyocytes.

MATERIALS AND METHODS, AND RESULTS

In the present study, by immunohistochemistry with a phosphorylation-specific antibody, we found that cyclin D-cdk4/6-phosphorylated retinoblastoma protein (pRb) during hypertrophy and expression of an unphosphorylatable pRb mutant impaired hypertrophic growth in cardiomyocytes. Transcription factor E2F was activated by hypertrophic elicitors but activation was impaired by pharmacological inhibition of cyclin D-cdk4/6. Inhibition of cyclin E-cdk2 complex only partly impaired E2F activity and did not prevent hypertrophic growth, but diminished endoreplication during hypertrophy.

CONCLUSIONS

These results indicate that cyclin D-cdk4/6-dependent phosphorylation of pRb and activation of E2F is necessary for hypertrophic growth in cardiomyocytes, whereas cyclin E-cdk2 kinase is not necessary for hypertrophy but regulates endoreplication in these cells. The data support the notion that hypertrophic growth of cardiomyocytes involves a partial progression through the G1 phase of the cell cycle

A cyclin D2-Rb pathway regulates cardiac myocyte size and RNA polymerase III after biomechanical stress in adult myocardium

Normally, cell cycle progression is tightly coupled to the accumulation of cell mass; however, the mechanisms whereby proliferation and cell growth are linked are poorly understood. We have identified cyclin (Cyc)D2, a G(1) cyclin implicated in mediating S phase entry, as a potential regulator of hypertrophic growth in adult post mitotic myocardium. To examine the role of CycD2 and its downstream targets, we subjected CycD2-null mice to mechanical stress. Hypertrophic growth in response to transverse aortic constriction was attenuated in CycD2-null compared with wild-type mice. Blocking the increase in CycD2 in response to hypertrophic agonists prevented phosphorylation of CycD2-target Rb (retinoblastoma gene product) in vitro, and mice deficient for Rb had potentiated hypertrophic growth. Hypertrophic growth requires new protein synthesis and transcription of tRNA genes by RNA polymerase (pol) III, which increases with hypertrophic signals. This load-induced increase in RNA pol III activity is augmented in Rb-deficient hearts. Rb binds and represses Brf-1 and TATA box binding protein (TBP), subunits of RNA pol III-specific transcription factor B, in adult myocardium under basal conditions. However, this association is disrupted in response to transverse aortic constriction. RNA pol III activity is unchanged in CycD2(-/-) myocardium after transverse aortic constriction, and there is no dissociation of TBP from Rb. These investigations identify an essential role for the CycD2-Rb pathway as a governor of cardiac myocyte enlargement in response to biomechanical stress and, more fundamentally, as a regulator of the load-induced activation of RNA pol III.

Cardiac myocyte cell cycle control in development, disease, and regeneration

Cardiac myocytes rapidly proliferate during fetal life but exit the cell cycle soon after birth in mammals. Although the extent to which adult cardiac myocytes are capable of cell cycle reentry is controversial and species-specific differences may exist, it appears that for the vast majority of adult cardiac myocytes the predominant form of growth postnatally is an increase in cell size (hypertrophy) not number. Unfortunately, this limits the ability of the heart to restore function after any significant injury. Interest in novel regenerative therapies has led to the accumulation of much information on the mechanisms that regulate the rapid proliferation of cardiac myocytes in utero, their cell cycle exit in the perinatal period, and the permanent arrest (terminal differentiation) in adult myocytes. The recent identification of cardiac progenitor cells capable of giving rise to cardiac myocyte-like cells has challenged the dogma that the heart is a terminally differentiated organ and opened new prospects for cardiac regeneration. In this review, we summarize the current understanding of cardiomyocyte cell cycle control in normal development and disease. In addition, we also discuss the potential usefulness of cardiomyocyte self-renewal as well as feasibility of therapeutic manipulation of the cardiac myocyte cell cycle for cardiac regeneration.

Different regulation of p27 and Akt during cardiomyocyte proliferation and hypertrophy

Postnatal cardiomyocytes normally grow by hypertrophy but show a limited proliferate response to certain stimuli. Although the proliferative capacity declines shortly after birth, neonatal cardiomyocytes can grow both by hypertrophy and by proliferation. Therefore, we have used neonatal cardiomyocytes to investigate the molecular differences between hypertrophic and proliferative growth of cardiomyocytes. Stimulation of neonatal cardiomyocytes with angiotensin II mainly induced hypertrophy, whereas PDGF only had a minor effect on the size of the myocytes. In contrast, PDGF induced significant proliferation in the cardiomyocyte cultures whereas angiotensin II treatment only resulted in a small increase in the number of cells. Measurement of cyclin D-dependent kinase specific phosphorylation of pRb by immunohistochemistry showed that, both stimuli activate the G1 phase of the cell cycle. By western blotting we found that PDGF-induced proliferation correlates with activation of Akt, inactivation of GSK-3beta and downregulation of the cyclin-dependent kinase inhibitor p27, whereas angiotensin II only had a small effect on Akt, GSK-3beta and p27. Our data support the hypothesis that, the hypertrophic and proliferative responses are both activated by G1 cell cycle molecules. The difference between the two responses appears to be that high amounts of p27 are present during hypertrophic growth, whereas proliferation involves downregulation of p27 and GSK-3beta activity and upregulation of Akt.

Can the cardiomyocyte cell cycle be reprogrammed?

Cardiac repair following myocardial injury is restricted due to the limited proliferative potential of adult cardiomyocytes. The ability of mammalian cardiomyocytes to proliferate is lost shortly after birth as cardiomyocytes withdraw from the cell cycle and differentiate. We do not fully understand the molecular and cellular mechanisms that regulate this cell cycle withdrawal, although if we could it might lead to the discovery of novel therapeutic targets for improving cardiac repair following myocardial injury. For the last decade, researchers have investigated cardiomyocyte cell cycle control, commonly using transgenic mouse models or recombinant adenoviruses to manipulate cell cycle regulators in vivo or in vitro. This review discusses cardiomyocyte cell cycle regulation and summarises recent data from studies manipulating the expressions and activities of cell cycle regulators in cardiomyocytes. The validity of therapeutic strategies that aim to reinstate the proliferative potential of cardiomyocytes to improve myocardial repair following injury will be discussed.

Nucleolin is regulated both at the level of transcription and translation

Nucleolin is a multi-functional protein that is located to the nucleolus. In tissue culture cells, the stability of nucleolin is related to the proliferation status of the cell. During development, rat cardiomyocytes proliferate actively with increases in the mass of the heart being due to both hyperplasia and hypertrophy. The timing of this shift in the phenotype of the myocyte from one capable of undergoing hyperplasia to one that can grow only by hypertrophy occurs within 4 days of post-natal development. Thus, cardiomyocytes are an ideal model system in which to study the regulation of nucleolin during growth in vivo. Using Western blot and quantitative RT-PCR (TaqMan) we found that the amount of nucleolin is regulated both at the level of transcription and translation during the development of the cardiomyocyte. However, in cells which had exited the cell cycle and were subsequently given a hypertrophic stimulus, nucleolin was regulated post-transcriptionally.

Interplay between the retinoblastoma protein and LEK1 specifies stem cells toward the cardiac lineage

The molecular mechanisms governing early cardiogenesis are still largely unknown. Interestingly, the retinoblastoma protein (Rb), a regulator of cell cycle, has recently emerged as a new candidate regulating cell differentiation. Rb-/- mice die at midgestation and mice lacking E2f1/E2f3, downstream components of the Rb-dependent transcriptional pathway, die of heart failure. To gain insight into the function of Rb pathway in early cardiogenesis, we used Rb-/- embryonic stem (ES) cells differentiating into cardiomyocytes. Rb-/- cells displayed a dramatic delay in expression of cardiac-specific transcription factors and in turn in the whole process of cardiac differentiation. The phenotype of Rb-/- ES cell-derived cardiomyocytes was rescued by reintroducing Rb in cardiac progenitors, by stimulating the BMP-dependent cardiogenic pathway or by overexpression of Nkx2.5. ES cells deficient in the recently identified factor LEK1, a murine homolog of the cardiomyogenic factor 1, or specific disruption of Rb-LEK1 interaction into the nucleus of differentiating ES cells recapitulated the delay in cardiac differentiation of Rb-/- ES cells. Thus, we provide evidence for a novel Rb/LEK1-dependent and BMP-independent transcriptional program, which plays a pivotal role in priming ES cells toward a cardiac fate.

The role of voltage gated T-type Ca2+ channel isoforms in mediating "capacitative" Ca2+ entry in cancer cells

The mechanism by which Ca2+ enters electrically non-excitable cells is unclear. The sensitivity of the Ca2+ entry pathway in electrically non-excitable cells to inhibition by extracellular Ni2+ was used to direct the synthesis of a library of simple, novel compounds. These novel compounds inhibit Ca2+ entry into and, consequently, proliferation of several cancer cell lines. They showed stereoselective inhibition of proliferation and Ca2+ influx with identical stereoselective inhibition of heterologously expressed Cav3.2 isoform of T-type Ca2+ channels. Proliferation of human embryonic kidney (HEK)293 cells transfected with the Cav3.2 Ca2+ channel was also blocked. Cancer cell lines sensitive to our compounds express message for the Cav3.2 T-type Ca2+ channel isoform, its delta25B splice variant, or both, while a cell line resistant to our compounds does not. These observations raise the possibility that clinically useful drugs can be designed based upon the ability to block these Ca2+ channels.

Forced expression of the cyclin B1-CDC2 complex induces proliferation in adult rat cardiomyocytes

Repair of the mature mammalian myocardium following injury is impaired by the inability of the majority of cardiomyocytes to undergo cell division. We show that overexpression of the cyclin B1-CDC2 (cell division cycle 2 kinase) complex re-initiates cell division in adult cardiomyocytes. Thus strategies targeting the cyclin B1-CDC2 complex might re-initiate cell division in mature cardiomyocytes in vivo and facilitate myocardial regeneration following injury.

E2F-1 Regulates the Expression of a Subset of Target Genes during Skeletal Myoblast Hypertrophy

Cellular hypertrophy, or growth without division, is an adaptive response to various physiological and pathological stimuli in postmitotic muscle. We demonstrated previously that angiotensin II stimulates hypertrophy in C2C12 myoblasts by transient activation of the cyclin-dependent kinase 4 complex, subsequent phosphorylation of retinoblastoma protein, release of histone deacetylase 1 from the retinoblastoma protein inhibitory complex, and partial activation of the transcription factor E2F-1. These observations led us to propose a model in which partial inactivation of the retinoblastoma protein complex leads to the derepression of a subset of E2F-1 targets necessary for cell growth without division during hypertrophy. We now present data that support this model and suggest the mechanism by which E2F-1 regulates hypertrophy. We examined expression profiles of angiotensin II-stimulated myoblasts and identified a subset of E2F-1 target genes that are specifically regulated during the hypertrophic response. We showed that the expression of E2F-1 targets involved in G1/S transit, DNA replication, and mitosis is not altered during the hypertrophic response, while the expression of E2F-1-regulated genes controlling early G1 progression, cytoskeletal organization, protein synthesis, mitochondrial function, and programmed cell death is up-regulated. Furthermore, we demonstrated that activation of cytochrome c oxidase genes occurs during the development of hypertrophy and that cytochrome c oxidase IV is a direct transcriptional target of E2F-1. These studies demonstrated that E2F-1 activity at specific promoters is dependent on physiological circumstances and that E2F-1 should be considered a potential target in the treatment of pathologic hypertrophy.

Vgl-4, a Novel Member of the Vestigial-like Family of Transcription Cofactors, Regulates α1-Adrenergic Activation of Gene Expression in Cardiac Myocytes

Cardiac and skeletal muscle genes are regulated by the transcriptional enhancer factor (TEF-1) family of transcription factors. In skeletal muscle, TEF-1 factors interact with a skeletal muscle-specific cofactor called Vestigial-like 2 (Vgl-2) that is related to the Drosophila protein Vestigial. Here, we characterize Vgl-4, the only member of the Vestigial-like family expressed in the heart. Unlike other members of the Vgl family that have a single TEF-1 interaction domain called the tondu (TDU) motif, Vgl-4 has two TDU motifs in its carboxyl-terminal domain. Like other Vgl factors, Vgl-4 physically interacts with TEF-1 in an immunoprecipitation assay. Vgl-4 functionally interacts with TEF-1 and also with myocyte enhancer factor 2 in a mammalian two-hybrid assay. Overexpression of Vgl-4 in cardiac myocytes interfered with the basal expression and alpha1-adrenergic receptor-dependent activation of a TEF-1-dependent skeletal alpha-actin promoter. In cardiac myocytes cultured in serum and in serum-free medium, a myc-tagged Vgl-4 protein was located in the nucleus and cytoplasm but was exported from the nucleus when cells were treated with alpha1-adrenergic receptor agonist. A chimeric nuclear-retained Vgl-4 protein inhibited alpha1-adrenergic receptor-dependent activation. In contrast, deletion of the TDU motifs of Vgl-4 prevented Vgl-4 nuclear localization, relieved Vgl-4 interference of basal activity, and enhanced alpha1-adrenergic up-regulation of the skeletal alpha-actin promoter. Nuclear export of Vgl-4 is dependent on the nuclear exportin CRM-1. These results suggest that Vgl-4 modulates the activity of TEF-1 factors and counteracts alpha1-adrenergic activation of gene expression in cardiac myocytes.