We have previously reported that in ovine fetal pulmonary venous smooth muscle cells (FPVSMC), decreased expression of
cGMP-dependent protein kinase (PKG) by
hypoxia could explain
hypoxia-induced SMC phenotype modulation. In this study, we investigated the role of
myocardin, a possible downstream effector of PKG, in SMC phenotype modulation induced by 1 and 24 h of
hypoxia.
Hypoxia for 1 h induced the phosphorylation of E-26-like
protein 1 (Elk-1), indicating a quick activation of Elk-1 after
hypoxia. Either
hypoxia (1 h) or treatment with DT-3, a PKG inhibitor, increased associations of Elk-1 with
myosin heavy chain (MHC) gene and
serum response factor (SRF), which was paralleled by a decrease in association of
myocardin with MHC gene and SRF. Exposure to
hypoxia of FPVSMC for 24 h significantly decreased the promoter activity of multiple SMC marker genes, downregulated
protein and
mRNA expression of
myocardin, and upregulated
mRNA expression of Elk-1, but had no significant effects on the phosphorylation of Elk-1. Inhibition of
myocardin by
siRNA transfection downregulated the expression of SMC marker
proteins, while overexpression of
myocardin prevented the
hypoxia-induced decrease in expression of SMC marker
proteins. Inhibition of PKG by
siRNA transfection downregulated the expression of
myocardin, but upregulated that of Elk-1. Overexpression of PKG prevented
hypoxia-induced effects on
protein expression of
myocardin and Elk-1. These data suggest that PKG induces displacement of
myocardin from SRF and upregulates
myocardin expression, thus activating the SMC genes transcription. The inhibitory effects of
hypoxia on PKG may explain
hypoxia-induced SMC phenotype modulation by decreasing the effects of PKG on
myocardin.