Roles of angiopoietin-like proteins in regulation of stem cell activity.

Various types of stem cells reside in the body and self-renew thoughout an organism's lifetime. Such self-renewal is essential for maintenance of tissue homeostasis and is coordinately regulated by stem cell-intrinsic signals and signals from stem cell niche. Angiopoietin is a niche-derived signaling molecule well known to contribute to maintenance of hematopoietic stem cells. Angiopoietin-like proteins (ANGPTLs) are structurally similar to angiopoietin, and recent studies reveal that they function in angiogenesis, lipid and energy metabolism, and regulation of inflammation. However, unlike angiopoietins, activities of ANGPTLs in stem cell maintenance have remained unclear. Recently, several studies have reported an association of ANGPTL signaling with stem cell maintenance. Here, we summarize those findings with a focus on hematopoietic stem cells (HSCs), intestinal stem cells (ISCs), neural stem cells (NSCs), and cancer stem cells (CSCs) and discuss mechanisms underlying ANGPTL-mediated stem cell maintenance.