The roles of tumor endothelial cells in cancer metastasis

Tumor metastasis is the main cause of cancer-related deaths. Tumor metastasis is orchestrated by a complex network of biological events. One such event is the formation of new blood vessels, termed as tumor angiogenesis. Tumor angiogenesis is essential for tumor progression. Without tumor angiogenesis, most solid tumors remain dormant. Apart from supplying tumors with nutrients and oxygen, tumor blood vessels provide a route for metastasis. Endothelial cells are key players in the formation of neovessels. Tumor endothelial cells that line tumor blood vessels differ from normal endothelial cells in many aspects. Tumor endothelial cells are irregular monolayers, have a higher expression of proangiogenic factors, and impaired endothelial barrier function when compared with their normal counterparts. The basement membrane thickness of tumor blood vessels is uneven and the association between pericytes and tumor endothelial cells is weak, leading to vascular leakiness. In this chapter, we highlight the role of endothelial cells in the initial steps of tumor metastasis.