Abstract 1677: COUP-TFII, an Orphan Nuclear Receptor, Inhibits Myocardin-Induced Smooth Muscle Differentiation by Sequestrating p300 from Myocardin
COUP-TFII is a member of nuclear receptor superfamily that plays a critical role during mouse development. The COUP-TFII null mutants exhibit defects in angiogenesis and heart development. Recent studies have revealed that COUP-TFII, exclusively expressed in venous endothelial cells, plays an important role in arterial-venous differentiation by repressing Notch signaling. However, little is known about the role of COUP-TFII in vascular smooth muscle cells (VSMC) despite the fact that COUP-TFII is expressed in VSMC of both artery and vein. We first studied expression of COUP-TFII in neointima of balloon-injured rat aortas and human atherosclerotic lesion, which consist of multiple components including dedifferentiated SMC. Immunohistochemistry showed that COUP-TFII was strongly expressed in nuclei of dediffer-entiated VSMC. COUP-TFII is also known to keep cells undifferentiated state during skeletal muscle differentiation. Based on these findings, we hypothesize that COUP-TFII is involved in pathogenesis of vascular diseases by inhibiting VSMC differentiation. We next studied whether COUP-TFII affects VSMC differentiation. Myocardin, a potent co-activator of serum response factor (SRF), is capable of inducing expression of the VSMC-specific genes even in non-VSMC. We found that exogenous COUP-TFII introduced by adenovirus suppressed myocardin-dependent induction of VSMC-specific myosin heavy chain (SM-MHC) and SM22α genes in 10T1/2 fibroblasts. Moreover, reporter gene analysis revealed that overexpressed COUP-TFII dramatically reduced myocardin-dependent transactivation of SM-MHC and SM22α promoters. Furthermore, functional analyses by co-immunoprecipitation (Co-IP) and chromatin immuno-precipitation assays showed that COUP-TFII neither inhibited physical interaction between SRF and myocardin nor between SRF and CArG box, respectively. Rather, Co-IP assays revealed that COUP-TFII physically interacted with p300, a critical partner of myocardin in its promyogenic activity, thereby sequestrating p300 from myocardin. These findings suggest that COUP-TFII plays a pivotal role in pathogenesis of vascular disease by inhibiting VSMC differentiation via abrogating physical interaction between myocardin and p300.