Giant cell tumor of bone (GCT) is a bone tumor consisting of numerous multinucleated osteoclastic giant cells involved in bone resorption and neoplastic osteoblast-like stromal cells responsible for tumor growth. The tumor occasionally metastasizes to the lung; however, factors leading to metastasis in this tumor are unknown. The TWIST-1 protein (also referred to as TWIST) has been suggested to be involved in epithelial-mesenchymal transition (EMT) and tumor progression in some cancers. In this study we investigated the functional role of TWIST in GCT cell angiogenesis and migration. Overexpression of TWIST in neoplastic GCT stromal cells significantly increased mRNA and protein expression of VEGF and VEGFR1 in vitro, whereas knockdown of TWIST resulted in decreased VEGF and VEGFR1 expression. A stable cell line with TWIST overexpression resulted in features of EMT including increased cell migration and downregulation of E-cadherin. The results of our study indicate that TWIST may play an important role in angiogenesis and cell migration in GCT. 1. Introduction Giant cell tumor of bone (GCT) is an aggressive bone tumor characterized by the presence of an abundance of reactive multinucleated giant cells surrounded by mesenchymal stromal cells. To date, the oncogenesis of GCT remains unknown as the neoplastic stromal cells appear to be preosteoblastic cells that do not undergo terminal osteoblastic differentiation [1–4]. GCT is a highly vascular tumor and in several cases metastasizes to the lungs [2, 5, 6]. Bone tumors recruit new blood vessels from preexisting vessels of the host through factors secreted from either the tumor itself or the surrounding stromal cells [7, 8]. Tumor growth is dependent upon the growth of these new blood vessels. Angiogenesis is the process of developing new capillaries from the existing circulation via a complex multistep system regulated by a delicate balance between various angiogenic inhibitors and activators . Activators of angiogenesis include growth factors, such as vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and hypoxic conditions that activate hypoxia-inducible factor-1 (HIF-1) which in turn upregulate angiogenic proteins as well as angiogenic oncogenes . The transcription factor TWIST is known to induce an embryonic event termed epithelial-mesenchymal transition (EMT) in tumor cells . EMT is a process initially observed in embryonic development in which the cells lose epithelial cell properties and gain mesenchymal cells characteristics . In hepatocellular
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