Objective-To determine the role of Wnt antagonist Dickkopf (DKK) 1 in human endothelial colony-forming cells (ECFCs) in view of the emerging importance of Wnt pathways in vascular biology. Methods and Results-Endothelial progenitor cells have been proposed to be crucial in tumor neovascularization.Recombinant DKK1 has been tested in ECFC angiogenic properties in vitro. DKK1 enhanced ECFC proliferation and the capacity of ECFCs to form pseudotubes in Matrigel. These effects have been attributed to enhancement of vascular endothelial growth factor receptor 2, SDF-1, and CXCR4. DKK1 gene silencing has been realized on ECFCs and mesenchymal stem cells, and we found that DKK1 silencing in the 2 cell types decreased their angiogenic potential. We then examined the possible role of DKK1 in tumor neovasculogenesis and found that blood vessels of breast cancer tissues expressed DKK1 far more strongly in human breast tumors than in normal breast tissues. By studying 62 human breast tumors, we found a significant positive correlation between DKK1 expression and von Willebrand factor. In vivo, DKK1 strongly enhanced the vascularization of Matrigel plugs and increased tumor size in a xenograft model of human breast carcinoma in nude mice. W ingless related proteins (Wnts) are powerful regulators of cell proliferation and differentiation, and their signaling pathway involves proteins that participate directly in both gene transcription and cell adhesion. 1 The major signaling pathway of Wnt is the canonical pathway that results from Wnt binding to the frizzled and low-densitylipoprotein-receptor-related protein (LRP) families on the cell surface. Complex formation induces -catenin nuclear entry and forms a complex with transcription factor T cell factor (TCF) and/or lymphoid enhancer factor (LEF) to activate transcription of Wnt target genes. The Wnt pathway is modulated by several Wnt antagonists, including Dickkopfs (DKKs). 2 The DKK family encodes secreted proteins of 255 to 350 amino acids and comprises 4 main members in vertebrates (DKK1 to DKK4). DKK1, the most widely studied member of this family, has been implicated in various physiological and pathological processes in human adults. DKK1 mediates inflammation in atherosclerotic lesions 3 and mobilizes progenitor cells by activating the bone marrow endosteal stem cell niche. 4 Recently, serum levels of DKK1 correlated with the extent of bone disease in patients with multiple malignant neoplasms, such as breast cancer. 5 In tumoral angiogenesis, endothelial cells have distinct gene expression profiles when compared with normal endothelial cells. Notably, they express high levels of DKK3. 6,7 However, the role of DKK1 in tumoral angiogenesis and postnatal vasculogenesis by endothelial progenitor cells (EPCs) has not yet been studied.
Conclusion-DKK1 enhances angiogenic properties of ECFCs in vitro and is required for ECFCBone marrow-derived cells have been shown to contribute to tumor neovasculature. 8,9 The active cell population in bone marrow-derived cells has...