Plasma proteins are produced and secreted at high levels by hepatocytes. The most abundant is albumin. The remainder are a heterogeneous group of glycoproteins, several of which have important homeostatic functions. Both angiostatin and endostatin are polypeptides that inhibit endothelial cell proliferation, angiogenesis, and tumor growth in experimental models of cancer. Hepatocytes appear to be the main source of these proteins, normally synthesizing and secreting them into plasma, and thus may influence extrahepatic endothelial growth.
Tumors also depend on angiogenesis.
1 Neovascularization is required for tumor growth and for migration of metastatic cells. When angiogenesis is reduced, growth and metastasis decrease significantly. Proliferation ensues regulated by various stimulatory and inhibitory factors. 2 Prominent among the proangiogenic factors are vascular endothelial growth factor, also known as vascular permeability factor, and fibroblast growth factors. These are present in most highly vascularized tumors.