Rapid effects of stretched myocardial and vascular cells on gene expression of neonatal rat cardiomyocytes with emphasis on autocrine and paracrine mechanisms

Myeloid leukemia factor-1 is a novel modulator of neonatal rat cardiomyocyte proliferation

The present study focuses on the identification of the gene expression profile of neonatal rat cardiomyocytes (NRVCMs) after dynamic mechanical stretch through microarrays of RNA isolated from cells stretched for 2, 6 or 24h. In this analysis, myeloid leukemia factor-1 (MLF1) was found to be significantly downregulated during the course of stretch. We found that MLF1 is highly expressed in the heart, however, its cardiac function is unknown yet. In line with microarray data, MLF1 was profoundly downregulated in in vivo mouse models of cardiomyopathy, and also significantly reduced in the hearts of human patients with dilated cardiomyopathy. Our data indicates that the overexpression of MLF1 in NRVCMs inhibited cell proliferation while augmenting apoptosis. Conversely, knockdown of MLF1 protected NRVCMs from apoptosis and promoted cell proliferation. Moreover, we found that knockdown of MLF1 protected NRVCMs from hypoxia-induced cell death. The observed accelerated apoptosis is attributed to the activation of caspase-3/-7/PARP-dependent apoptotic signaling and upregulation of p53. Most interestingly, MLF1 knockdown significantly upregulated the expression of D cyclins suggesting its possible role in cyclin-dependent cell proliferation. Taken together, we, for the first time, identified an important role for MLF1 in NRVCM proliferation.

Stretch-induced hypertrophy activates NFkB-mediated VEGF secretion in adult cardiomyocytes

Hypertension and myocardial infarction are associated with the onset of hypertrophy. Hypertrophy is a compensatory response mechanism to increases in mechanical load due to pressure or volume overload. It is characterized by extracellular matrix remodeling and hypertrophic growth of adult cardiomyocytes. Production of Vascular Endothelial Growth Factor (VEGF), which acts as an angiogenic factor and a modulator of cardiomyocyte function, is regulated by mechanical stretch. Mechanical stretch promotes VEGF secretion in neonatal cardiomyocytes. Whether this effect is retained in adult cells and the molecular mechanism mediating stretch-induced VEGF secretion has not been elucidated. Our objective was to investigate whether cyclic mechanical stretch induces VEGF secretion in adult cardiomyocytes and to identify the molecular mechanism mediating VEGF secretion in these cells. Isolated primary adult rat cardiomyocytes (ARCMs) were subjected to cyclic mechanical stretch at an extension level of 10% at 30 cycles/min that induces hypertrophic responses. Cyclic mechanical stretch induced a 3-fold increase in VEGF secretion in ARCMs compared to non-stretch controls. This increase in stretch-induced VEGF secretion correlated with NFkB activation. Cyclic mechanical stretch-mediated VEGF secretion was blocked by an NFkB peptide inhibitor and expression of a dominant negative mutant IkBα, but not by inhibitors of the MAPK/ERK1/2 or PI3K pathways. Chromatin immunoprecipitation assays demonstrated an interaction of NFkB with the VEGF promoter in stretched primary cardiomyocytes. Moreover, VEGF secretion is increased in the stretched myocardium during pressure overload-induced hypertrophy. These findings are the first to demonstrate that NFkB activation plays a role in mediating VEGF secretion upon cyclic mechanical stretch in adult cardiomyocytes. Signaling by NFkB initiated in response to cyclic mechanical stretch may therefore coordinate the hypertrophic response in adult cardiomyocytes. Elucidation of this novel mechanism may provide a target for developing future pharmacotherapy to treat hypertension and heart disease.

Mechanical stretch induces endothelial nitric oxide synthase gene expression in neonatal rat cardiomyocytes

1. Mechanical stretch leads to cardiac hypertrophy and may ultimately cause heart failure. However, the effect of mechanical stretch on gene induction in cardiomyocytes remains to be determined. 2. In the present study, we compared transcript profiles of mechanically stretched neonatal rat cardiomyocytes with those of unstretched cells using cDNA microarrays. The microarrays contained probes for 480 known genes, including those involved in signal transduction, cell cycle regulation, the cytoskeleton and cell motility. Eighteen genes, including the eNOS gene, were identified as having significantly differential expression in response to mechanical stretch in cardiomyocytes. 3. Northern and western blot analysis further quantified the expression of the eNOS gene. Mechanical stretch increased constitutive NOS activity and nitric oxide (NO) production. The NO donor s-nitroso-N-acetylpenicillamine (SNAP) inhibited mechanical stretch-stimulated protein synthesis, as measured by [3H]-leucine uptake. In addition, cardiomyocytes were infected with adenoviral vectors encoding cDNA for eNOS (Ad-eNOS) and a phosphoglycerate kinase (PGK) empty vector (Ad-PGK). In contrast with Ad-PGK-infected cells, in cardiomyocytes infected with Ad-eNOS, there was increased calcium-dependent NOS activity and nitrite production. Cardiomyocytes infected with Ad-eNOS exhibited diminished mechanical stretch-stimulated protein synthesis. In contrast, in eNOS-knockdown cells, the increased eNOS protein levels and NOS activity induced by mechanical stretch were abolished, but protein synthesis was enhanced. 4. The results of the present study indicate that eNOS gene expression is induced by mechanical stretch, leading to increased constitutive NOS activity and NO production, which may be a negative regulator in cardiomyocyte hypertrophy.

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

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

Acute changes of biventricular gene expression in volume and right ventricular pressure overload

OBJECTIVE

We investigated the effects of acute volume and RV pressure overload on biventricular function and gene expression of BNP, pro-inflammatory cytokines (IL-6 and TNF-alpha), iNOS, growth factors (IGF-1, ppET-1), ACE and Ca2+-handling proteins (SERCA2a, phospholamban and calsequestrin).

METHODS

Male Wistar rats (n=45) instrumented with pressure tip micromanometers in right (RV) and left ventricular (LV) cavities were assigned to one of three protocols: i) Acute RV pressure overload induced by pulmonary trunk banding in order to double RV peak systolic pressure, during 120 or 360 min; ii) acute volume overload induced by dextran40 infusion (5 ml/h), during 120 or 360 min; iii) Sham. RV and LV samples were collected for mRNA quantification.

RESULTS

BNP upregulation was restricted to the overloaded ventricles. TNF-alpha, IL-6, ppET-1, SERCA2a and phospholamban gene activation was higher in volume than in pressure overload. IGF-1 overexpression was similar in both types of overload, but was limited to the RV. TNF-alpha and CSQ mRNA levels were increased in the non-overloaded LV after pulmonary trunk banding. No significant changes were detected in ACE or iNOS expression. RV end-diastolic pressures positively correlated with local expression of BNP, TNF-alpha, IL-6, IGF-1, ppET-1 and SERCA2a, while RV peak systolic pressures correlated only with local expression of IL-6, IGF-1 and ppET-1.

CONCLUSIONS

Acute cardiac overload alters myocardial gene expression profile, distinctly in volume and pressure overload. These changes correlate more closely with diastolic than with systolic load. Nonetheless, gene activation is also present in the non-overloaded LV of selectively RV overloaded hearts.

Effects of acute ventricular volume manipulation on in situ cardiomyocyte cell membrane configuration

Effects of mechanical stimulation on cardiac electrical activity, gene expression, protein synthesis, and tissue remodelling have received increasing attention in recent years, as reviewed in this issue of PBMB. Little is known, though, about how changes in ventricular filling affect the cell configuration of cardiomyocytes in the ventricular wall. Here, we present first electron-microscopic insight into changes in cardiomyocyte cell structure in situ during acute ventricular volume manipulation. Apart from confirming the anticipated ventricular volume-related changes in cardiomyocyte sarcomere length, there is evidence of (i) unfolding of 'slack' membrane, primarily from sarcolemmal invaginations near the Z-lines, and (ii) stretch-induced incorporation of sub-membrane caveolae into the surface membrane. The functional relevance of these changes in cardiomyocyte membrane configuration-other than to cater for the length-dependent modulation of the cell surface to cell volume ratio-remains to be elucidated.

Dance band on the Titanic: biomechanical signaling in cardiac hypertrophy

Biomechanical signaling is a complex interaction of both intracellular and extracellular components. Both passive and active components are involved in the extracellular environment to signal through specific receptors to multiple signaling pathways. This review provides an overview of extracellular matrix, specific receptors, and signaling pathways for biomechanical stimulation in cardiac hypertrophy.

Stretch-induced paracrine hypertrophic stimuli increase TGF-beta1 expression in cardiomyocytes

Cardiac hypertrophy refers to the abnormal growth of cardiomyocytes, and is often caused by valvular heart disease and hypertension. It involves the activation of growth, including increased protein synthesis and changes in gene expression. Transforming growth factor-beta1 (TGF-beta1) may play a central role in protecting the heart during the hypertrophic response by helping to restore normal functions of the affected myocardium. We tested the hypothesis that cardiomyocytes respond to stretch-induced paracrine hypertrophic stimuli with increased expression of TGF-beta1. To that purpose, we investigated whether angiotensin II (All), endothelin- I (ET-1) and TGF-beta, secreted by stretched cardiac and vascular cells, are involved in the paracrine mechanisms of stretch-induced changes of TGF-beta1 mRNA expression in stationary (i.e. non-stretched) cardiomyocytes. Our results indicated that TGF-beta1 mRNA expression in stationary cardiomyocytes was increased by AII release from cardiomyocytes that had been stretched for 30-60 min. Furthermore, it is likely that ET-1 and TGF-beta were released by stretched cardiac fibroblasts and endothelial cells to induce TGF-beta1 mRNA expression in stationary cardiomyocytes. Stretched vascular smooth muscle cells did not influence TGF-beta1 mRNA expression in stationary cardiomyocytes. These results indicate that AII, ET-I and TGF-beta, released by cardiac cell types, act as paracrine mediators of TGF-beta1 mRNA expression in cardiomyocytes. Therefore, we conclude that in stretched myocardium the cardiomyocytes, cardiac fibroblasts and endothelial cells take part in intercellular interactions contributing to cardiomyocyte hypertrophy.

Molecular cloning and functional characterization of the upstream rat atrial natriuretic peptide promoter

OBJECTIVE

The upregulation of left ventricular atrial natriuretic peptide (ANP) serves as a molecular marker of cardiac hypertrophy. The precise mechanisms underlying this gene induction are unclear, since the presently cloned 3.6 kilo base (kb) rat ANP promoter failed to substantially induce coupled reporter genes in chronically hypertrophied hearts. The aim of this study was to clone and to functionally analyse the upstream ANP promoter.

DESIGN

Upstream of the known ANP promoter, a 1.5 kb segment was cloned by the promoter walker method and found to harbour a putative CCAAT-binding site as well as multiple putative transcription factor binding sites. This newly cloned segment was ligated with a reporter gene, in vivo transfected into rat myocardium, and analysed under basal conditions or after stimulation with both acute (isovolumetric contractions in the Langendorff apparatus) and chronic wall stress (aortic banding).

RESULTS

Reporter gene constructs carrying the newly cloned segment conferred only little promoter activity. In hearts exposed to acute wall stress, the previously cloned 3.6 kb ANP promoter as well as a constitutive promoter (pGL3 promoter vector) were active but markedly suppressed after extension with the newly cloned upstream promoter (-88.1 and -85.5%; P < 0.05 respectively). Site directed mutagenesis of two AP-2 transcription factor binding sites (base pairs -3946 to -3954 or -4192 to -4200) eliminated this silencing effect. In hearts with chronic pressure overload hypertrophy as well as in normal, unstimulated hearts the activity of the 3.6 kb ANP promoter was weak and also abolished after ligation with the 1.5 kb upstream segment. Moreover, both putative AP-2 binding sites within the upstream rat ANP promoter bound specifically to nuclear proteins of unstimulated, acute and chronic pressure overloaded hearts as demonstrated by electrophoresis mobility shift assays.

CONCLUSION

Novel silencer elements were cloned, localized to two AP-2 binding sites in the upstream ANP promoter, and functionally characterized. Given that the putative upregulation of left ventricular ANP by the extensively studied 3.6 kb proximal promoter region is substantially diminished by the newly cloned segment, the functional significance of regulatory elements within the proximal promoter region should be re-evaluated. The molecular mechanism causing ANP mRNA induction in left ventricular hypertrophy remains obscure.

Mechanical signaling through connective tissue: a mechanism for the therapeutic effect of acupuncture

The mechanism of action of acupuncture remains largely unknown. The reaction to acupuncture needling known as 'de qi', widely viewed as essential to the therapeutic effect of acupuncture, may be a key to understanding its mechanism of action. De qi includes a characteristic needling sensation, perceived by the patient, and 'needle grasp' perceived by the acupuncturist. During needle grasp, the acupuncturist feels pulling and increased resistance to further movement of the inserted needle. We hypothesize that 1) needle grasp is due to mechanical coupling between the needle and connective tissue with winding of tissue around the needle during needle rotation and 2) needle manipulation transmits a mechanical signal to connective tissue cells via mechanotransduction. Such a mechanism may explain local and remote, as well as long-term effects of acupuncture.

The role of angiotensin II, endothelin-1 and transforming growth factor-beta as autocrine/paracrine mediators of stretch-induced cardiomyocyte hypertrophy

Cardiac hypertrophy is a compensatory response of myocardial tissue upon increased mechanical load. Of the mechanical factors, stretch is rapidly followed by hypertrophic responses. We tried to elucidate the role of angiotensin II (AII), endothelin-1 (ET-1) and transforming growth factor-beta (TGF-beta) as autocrine/paracrine mediators of stretch-induced cardiomyocyte hypertrophy. We collected conditioned medium (CM) from stretched cardiomyocytes and from other stretched cardiac cells, such as cardiac fibroblasts, endothelial cells and vascular smooth muscle cells (VSMCs). These CMs were administered to stationary cardiomyocytes with or without an AII type 1 (AT1) receptor antagonist (losartan), an ET-1 type A (ET(A)) receptor antagonist (BQ610), or anti-TGF-beta antibodies. By measuring the mRNA levels of the proto-oncogene c-fos and the hypertrophy marker gene atrial natriuretic peptide (ANP), the molecular phenotype of the CM-treated stationary cardiomyocytes was characterized. Our results showed that c-fos and ANP expression in stationary cardiomyocytes was increased by All release from cardiomyocytes that had been stretched for 60 min. Stretched cardiomyocytes, cardiac fibroblasts and endothelial cells released ET-1 which led to increased c-fos and ANP expression in stationary cardiomyocytes. ET-1 released by stretched VSMCs, and TGF-beta released by stretched cardiac fibroblasts and endothelial cells, appeared to be paracrine mediators of ANP expression in stationary cardiomyocytes. These results indicate that AII, ET-1 and TGF-beta (released by cardiac and vascular cell types) act as autocrine/paracrine mediators of stretch-induced cardiomyocyte hypertrophy. Therefore, it is likely that in stretched myocardium the cardiomyocytes, cardiac fibroblasts, endothelial cells and VSMCs take part in intercellular interactions contributing to cardiomyocyte hypertrophy.