Heterochromatin Protein 1 Alpha (HP1α: CBX5) Is a Key Regulator in Differentiation of Endothelial Progenitor Cells to Endothelial Cells

As the ability to control the differentiation of endothelial stem/progenitor cells (EPCs) into vascular endothelial cell lineages could be useful for promoting neovascularization, it is important to obtain a deeper understanding of the epigenetic mechanisms that regulate EPC differentiation and neovascularization. Heterochromatin protein 1 alpha (HP1 alpha) is known to be involved in the epigenetic regulation of gene silencing. However, recent reports demonstrate that HP1 alpha can also activate gene expression during cell differentiation. In this study, microarray analysis revealed that HP1 alpha expression was induced during EPC differentiation and is associated with the expression of outgrowing endothelial cell (OEC)-specific protein markers. To explore the role of HP1 alpha in the differentiation of EPCs to OECs, its expression was knocked-down or over-expressed in differentiating EPCs. Overexpression of HP1 alpha promoted the differentiation and angiogenic activity of EPCs in vitro and in vivo, whereas knockdown of HP1 alpha led to a defect in OEC migration, tube formation, and angiogenic sprouting activity. Gene expression profiling showed increased expression of angiogenic genes, including NOTCH1, cadherin-5, and angiopoietin-like-2, and decreased expression of progenitor cell marker genes, including CD133, CXCR4, and C-KIT, in HP1 alpha-overexpressing EPCs. Also, increased HP1 alpha at an early stage of EPC differentiation may regulate angiogenic gene transcription by interacting with chromatin that modifies epigenetic factors such as the methyl-CpG binding domain, Polycomb group ring finger 2, and DNA methyltransferases. Our findings demonstrate, for the first time, that HP1 alpha plays an important role in the differentiation and angiogenic function of EPCs by regulating endothelial gene expression.