Abstract

Cardiac myocytes react to increased workload and hypertrophic neurohumoral stimuli by increasing protein synthesis, reinitiating expression of fetal forms of structural genes, α-skeletal actin (α-SkA) and β-myosin heavy chain (β-MHC), and by increasing expression and secretion of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP). Initially, the response is beneficial, but when prolonged, it leads to pathological cardiomyocyte hypertrophy. In this study, cardiomyocyte hypertrophy was initiated by hypertrophic agonists, endothelin-1 (ET-1) and phenylephrine (PE), and by increased stretching of atrial wall.

Transcription factor GATA4 was studied to identify the mechanism leading to increased gene expression of BNP. In BNP promoter, GATA4 binds to cis elements mediating hypertrophic response. Eliminating GATA4 binding by using the decoy approach, basal BNP gene expression was reduced. To identify mechanisms regulating GATA4, the roles of mitogen-activated protein kinases (MAPKs) were studied. Activation of p38 MAPK increased GATA4 binding to BNP gene and led to increased GATA4 dependent BNP gene expression. p38 MAPK was required for ET-1 induced GATA4 binding, whereas extracellular signal-regulated kinase (ERK) was required for maintaining basal GATA4 binding activity. PE and ET-1 activated protein kinase C (PKC) signaling in cardiac myocytes. Antisense oligonucleotide inhibition of PKCα markedly reduced PE induced ANP secretion and ET-1 induced BNP secretion, whereas gene expression of natriuretic peptides was not affected. Antisense PKCα treatment inhibited PE induced expression of α-SkA, while increased protein synthesis or β-MHC gene expression were not affected. Sretching of the perfused rat atria increased BNP, c-fos and BNP gene expression via mechanism involving p38 MAP kinase activation of transcription factor Elk-1. In cultured neonatal rat atrial myocytes stretch induced BNP gene expression was dependent upon transcription factor Elk-1 binding sites within the BNP gene promoter.

In conclusion, hypertrophic signaling in cardiac myocytes involves multiple signaling cascades. Activation of p38 MAPK is required for the development of ET-1 induced hypertrophic phenotype and GATA4 mediated BNP gene expression in cultured ventricular myocytes, and for stretch induced Elk-1 dependent BNP gene expression in atrial myocytes. PKCα is involved in PE induced hypertrophic response and PE induced switch in gene programming inducing expression of α-SkA, the fetal form of cardiac α-actin.