Signal transduction mechanisms and nuclear effectors in gene expression during hypertrophy of cardiac myocytes

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>During cardiac hypertrophy individual cardiac myocytes increase in size, which is accompanied by augmented protein synthesis and selective induction of a subset of genes. These phenotypic changes of myocytes are a result from altered intracellular signaling mechanisms and molecules. B-type natriuretic peptide (BNP) gene was selected as a target gene for the study of cardiac signaling mechanisms, since it is activated by mechanical, neural and humoral stimuli during myocyte hypertrophy.</p><p>To generate hypertrophy of cardiac myocytes, neonatal rat cardiac myocytes were subjected to exogenous hypertrophic agonists such as endothelin-1 (ET-1) or to cyclic mechanical stretch. The role and regulation of transcription factors were studied by utilizing promoter analysis together with site-specific mutations and measurement of DNA binding activity and phosphorylation. GATA-4 mediated signaling was inhibited by blocking DNA binding with decoy oligonucleotides or by decreasing GATA-4 synthesis via adenoviral antisense delivery. ET-1 activated GATA-4 via serine residue phosphorylation, and this effect was mediated via p38 kinase. Similarly, GATA-4 binding activity was increased by ET-1 and mechanical stretch, but it was essential for activation of BNP gene only in the latter stimulation. Importantly, downregulation of GATA-4 protein levels prevented mechanical stretch induced hypertrophy of cardiac myocytes. In contrast, separate mechanism for an ET-1 specific signaling was composed of p38 kinase regulated ETS-like transcription factor-1 (Elk-1). Finally, the effect of mechanical stretch on endogenous endothelin-1 (ET-1) synthesis in cardiac cells was studied. Intrinsic ET-1 synthesis was activated in stretched cardiac myocytes, yet the levels of ET-1 were relatively low.</p><p>This work suggests that GATA-4 transcription factor is required for mechanical stretch mediated hypertrophic program, and Elk-1 may act as a downstream effector of ET-1 in cardiac myocytes. Taken together, induction of ET-1 and BNP genes as well as activation of GATA-4 and Elk-1 transcription factors are regulated via a network of mitogen activated protein kinase pathways.</p></div>