Vascular calcification is a prominent feature of atherosclerosis and closely correlated with cardiovascular morbidity and mortality. In this study, we hypothesize that Notch signaling plays an important role in osteogenic conversion of smooth muscle cells (SMCs) and vascular calcification.
<Methods and Results> Either Notch ligand-expressing cells or overexpression of Notch intracellular domains (NICDs) induced expression of Msx2, a key regulator of osteogenic conversion, in human aortic SMCs (HASMCs). In addition, overexpression of Notch1 intracellular domain (N1-ICD) markedly upregulated alkaline phosphatase (ALP) activity and matrix mineralization of HASMCs. A knockdown experiment with a small interfering RNA confirmed that Msx2, but not Runx2/Cbfa1, another key osteogenic transcription factor, is responsible for Notch1-induced osteogenic conversion of HASMCs. Furthermore, this Notch1-Msx2 pathway was independent of bone morphogenetic protein-2 (BMP-2), an osteogenic morphogen upstream of Msx2. The transcriptional activity of the Msx2 promoter was significantly enhanced by Notch ligands stimulation, whereas it was abrogated by a specific Notch signaling inhibitor. The RBP-Jk binding element within the Msx2 promoter was critical to Notch1-induced Msx2 gene expression, and correspondingly, neither N1-ICD overexpression nor Notch ligands stimulation increase the Msx2 expression or transcriptional activity of the Msx2 promoter, respectively, in RBP-Jk-deficient fibroblasts. Immunohistochemistry of human artery specimens revealed colocalization of Notch1 and Msx2 within atherosclerotic plaques, indicating a role of Notch1-Msx2 pathway in vascular calcification in vivo. These results suggest that Notch signaling directly targets Msx2, thus accelerating osteogenic conversion of HASMCs and, as a result, a formation of vascular calcification.
Characterization of smooth muscle cells from human atherosclerotic lesions and their responses to Notch signaling