Transforming growth factor-beta 1 potentiated alpha 1-adrenergic and stretch-induced c-fos mRNA expression in rat myocardial cells

Hexarelin protects rat cardiomyocytes from angiotensin II-induced apoptosis in vitro

Loss of cardiomyocytes by apoptosis is proposed to cause heart failure. Angiotensin II (ANG II), an important neurohormonal factor during heart failure, can induce cardiomyocyte apoptosis. Inasmuch as hexarelin has been reported to have protective effects in this process, we examined whether hexarelin can prevent cardiomyocytes from ANG II-induced cell death. Cultured cardiomyocytes from neonatal rats were stimulated with ANG II. Apoptosis was evaluated using fluorescence microscopy, TdT-mediated dUTP nick-end labeling (TUNEL) method, flow cytometry, DNA laddering, and analysis of cell viability by (3,4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). It was found that incubation with 0.1 μmol/l ANG II for 48 h increased cardiomyocyte apoptosis. Administration of 0.1 μmol/l hexarelin significantly decreased this ANG II-induced apoptosis and DNA fragmentation and increased myocyte viability. To further investigate the underlying mechanisms, caspase-3 activity assay and mRNA expression of Bax, Bcl-2, and growth hormone secretagogue receptor (GHS-R; the supposed hexarelin binding site) were examined. GHS-R mRNA was abundantly expressed in cardiomyocytes and was upregulated after administration of hexarelin. These results suggest that hexarelin abates cardiomyocytes from ANG II-induced apoptosis possibly via inhibiting the increased caspase-3 activity and Bax expression induced by ANG II and by increasing the expression of Bcl-2, which is depressed by ANG II. Whether the upregulated expression of GHS-R induced by hexarelin is associated with this antiapoptotic effect deserves further investigation.

Transforming growth factor beta in cardiovascular development and function

Transforming growth factor betas (TGFbetas) are pleiotropic cytokines involved in many biological processes. Genetic engineering and tissue explanation studies have revealed specific non-overlapping roles for TGFbeta ligands and their signaling molecules in development and in normal function of the cardiovascular system in the adult. In the embryo, TGFbetas appear to be involved in epithelial-mesenchymal transformations (EMT) during endocardial cushion formation, and in epicardial epithelial-mesenchymal transformations essential for coronary vasculature, ventricular myocardial development and compaction. In the adult, TGFbetas are involved in cardiac hypertrophy, vascular remodeling and regulation of the renal renin-angiotensin system. The evidence for TGFbeta activities during cardiovascular development and physiologic function will be given and areas which need further investigation will be discussed.

TNF-alpha induces protein synthesis through PI3-kinase-Akt/PKB pathway in cardiac myocytes

The activation of phosphatidylinositol (PI) 3-kinase and Akt/protein kinase B (PKB) by tumor necrosis factor (TNF)-alpha and their roles on stimulation of protein synthesis were investigated in cultured neonatal rat cardiac myocytes. Treatment of cells with TNF-alpha resulted in enlargement of cell surface area and stimulation of protein synthesis without affecting myocyte viability. TNF-alpha induced marked activation of PI3-kinase and Akt/PKB, and the activation of PI3-kinase and Akt/PKB was rapid (maximal at 10 and 15 min, respectively) and concentration dependent. Akt/PKB activation by TNF-alpha was inhibited by a PI3-kinase-specific inhibitor LY-294002 and adenovirus-mediated expression of a dominant negative mutant of PI3-kinase, indicating that TNF-alpha activates Akt/PKB through PI3-kinase activation. Furthermore, TNF-alpha-induced protein synthesis was inhibited by pretreatment with LY-294002 and expression of a dominant negative mutant of PI3-kinase or Akt/PKB. These results indicate that activation of the PI3-kinase-Akt/PKB pathway plays an essential role in protein synthesis induced by TNF-alpha in cardiac myocytes.

Requirement of activation of the extracellular signal-regulated kinase cascade in myocardial cell hypertrophy

The signal transduction mechanisms mediating hypertrophic responses in myocardial cells (MCs) remain uncertain. We investigated the role of the extracellular signal-regulated kinase (ERK) cascade in myocardial cell hypertrophy by the strategy of using the adenovirus-mediated overexpression of mitogen-activated protein kinase (MAPK)/ERK kinase (MEK), which is the upstream activator of ERK. We generated recombinant adenoviruses expressing constitutively active MEK1 (MEK1 EE) and dominant negative MEK1 (MEK1 DN). Overexpression of MEK1 EE in MCs activated ERK1/2 and subsequently induced atrial natriuretic peptide (ANP) mRNA expression. In addition, MEK1 EE overexpression resulted in an increase in cell size and sarcomeric reorganization. In contrast, overexpression of MEK1 DN in MCs inhibited endothelin-1 (ET-1)-, phenylephrine (PE)-, leukemia inhibitory factor (LIF)-, isoproterenol (ISP)-, and mechanical stretch-induced ERK activation and ANP mRNA expression. MEK1 DN overexpression inhibited ET-1-, PE-, LIF-, and ISP-induced increases in cell size and sarcomeric reorganization. Consistent with the observed effects on cellular morphology, overexpression of MEK1 EE resulted in an increase in amino acid incorporation, while overexpression of MEK1 DN inhibited ET-1-, PE-, LIF-, ISP-, and mechanical stretch-induced increases in amino acid incorporation. These results indicate that the ERK cascade plays an important role in the signaling pathway leading to the development of myocardial cell hypertrophy.

Endothelin-1 activates p38 mitogen-activated protein kinase via endothelin-A receptor in rat myocardial cells

In myocardial cells (MCs), endothelin-1 (ET-1) exerts various effects such as hypertrophy, and causes cellular injury. Long-term treatment with an endothelin-A (ET(A)) receptor antagonist improves the survival of rats with heart failure, suggesting that myocardial endothelin system contributes to the progression of heart failure. p38 mitogen-activated kinase (MAPK) is a member of the MAPK family and activated by several forms of environmental stresses. We show here the effect of ET-1 on p38 MAPK activation and the role of ET-1-activated p38 MAPK on morphological changes in MCs. ET-1-stimulated p38 MAPK phosphorylation was detectable within 2 min and maximal at 5 min and was concentration dependent. The maximum effect was obtained at 10 nM. An ET(A) receptor antagonist, BQ-123, but not an endothelin-B receptor antagonist, BQ-788, inhibited these reactions. A p38 MAPK inhibitor, SB203580, failed to inhibit the morphological changes associated with ET-1-induced myocardial cell hypertrophy. These results indicate that p38 MAPK is activated by ET-1 but does not contribute to the development of ET-1-induced myocardial cell hypertrophy.

Myocardial alpha1-adrenoceptor: inotropic effect and physiologic and pathologic implications

Alpha1-adrenergic receptors have been found in myocardium of all mammalian species. Although the exact underlying mechanisms have not been conclusively determined, it would appear that the myocardial effects of alpha1-adrenoceptors may vary in importance according to the pathophysiologic process involved. In physiological conditions, this receptor system plays a role in cardiac growth, cardiac contraction, and has both an antiarrhythmic function as well as a role in cardiac adaptation to various situations. This system is also involved in some pathological processes such as ischemia/reperfusion, ischemic preconditioning, and cardiac hypertrophy. The role of alpha1-adrenoceptors in heart failure is somewhat controversial. Experimental evidence suggests that myocardial alpha1-adrenoceptors can have either beneficial or deleterious effects on the heart. It thus seems possible that the development of agents specific to certain subtypes of alpha1-adrenoceptor and a better understanding of their role in pathophysiologic states could be clinically relevant.