A general understanding of the molecular mechanisms underlying angiogenesis is emerging from the analysis of targeted mutations in vasculature-related genes. These analyses reveal that angiopoietin signaling through the TIE2 receptor is involved in regulating angiogenesis. Recently, we and several other groups have independently identified several molecules containing a coiled-coil domain and a fibrinogen-like domain, both of which are structurally conserved in angiopoietins. Because these molecules do not bind to the angiopoietin-specific receptor, TIE2, they have been named angiopoietin-related proteins (ARPs) or angiopoietin-like proteins (Angptls). ARPs/Angptls, which are all currently orphan ligands, also have potent activity for regulating angiogenesis as proangiogenic or antiangiogenic factors, suggesting that their receptors may be expressed on endothelial cells. In addition, ARPs/Angptls show pleiotropic effects not only on vascular cells but also on cells of other lineages, such as skin and chondrocyte cells. More recent studies have proposed that ARPs/Angptls are involved in various pathologies, such as tumor angiogenesis and metabolic diseases. To summarize the current findings relating to these proteins, we focus in this review on the functions of ARPs/Angptls as new angiogenic modulating factors in the vascular system and discuss the pleiotropic functions of ARPs/Angptls in nonvascular cell lineages.
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