Hypoxic injury to cardiomyocytes is a stress that causes cardiac pathology through cardiac-restricted gene expression. SRF (
serum-response factor) and
myocardin are important for cardiomyocyte growth and differentiation in response to myocardial
injuries. Previous studies have indicated that AngII (
angiotensin II) stimulates both
myocardin expression and cardiomyocyte
hypertrophy. In the present study, we evaluated the expression of
myocardin and AngII after
hypoxia in regulating gene transcription in neonatal cardiomyocytes. Cultured rat neonatal cardiomyocytes were subjected to
hypoxia, and the expression of
myocardin and AngII were evaluated. Different signal transduction pathway inhibitors were used to identify the pathway(s) responsible for
myocardin expression. An EMSA (electrophoretic mobility-shift assay) was used to identify
myocardin/SRF binding, and a
luciferase assay was used to identify transcriptional activity of
myocardin/SRF in neonatal cardiomyocytes. Both
myocardin and AngII expression increased after
hypoxia, with AngII appearing at an earlier time point than
myocardin.
Myocardin expression was stimulated by AngII and ERK (
extracellular-signal-regulated kinase) phosphorylation, but was suppressed by an ARB (AngII type 1 receptor blocker), an ERK pathway inhibitor and
myocardin siRNA (
small interfering RNA). AngII increased both
myocardin expression and transcription in neonatal cardiomyocytes. Binding of
myocardin/SRF was identified using an EMSA, and a
luciferase assay indicated the transcription of
myocardin/SRF in neonatal cardiomyocytes. Increased BNP (
B-type natriuretic peptide), MHC (
myosin heavy chain) and [(3)H]
proline incorporation into cardiomyocytes was identified after
hypoxia with the presence of
myocardin in hypertrophic cardiomyocytes. In conclusion,
hypoxia in cardiomyocytes increased
myocardin expression, which is mediated by the induction of AngII and the ERK pathway, to cause cardiomyocyte
hypertrophy. Myocardial
hypertrophy was identified as an increase in transcriptional activities, elevated hypertrophic and cardiomyocyte phenotype markers, and morphological hypertrophic changes in cardiomyocytes.