Unraveling the mysteries of endostatin

Endostatin, a 20-kDa C-terminal proteolytic fragment of collagen XVIII, is a specific endogenous angiogenesis inhibitor discovered more than a decade. The structure, stability, and mechanism of actions of endostatin have been extensively investigated during the past 12 years, among which controversial reports remain unclarified. The mysteries include the following: 1) Why controversial efficacies were observed with endostatin regarding tumor inhibition? Particularly, why does an N-terminal modified endostatin show good clinical responses in China, whereas the clinical trials of the wild type endostatin were terminated at the early stage of phase II in the USA? 2) What is the contribution of zinc-binding to the stability and biological functions of endostatin? 3) Why does insoluble endostatin shrink tumors? 4) How to ensure that endostatin is correctly refolded? 5) How does endostatin exert its biological functions? Recent progress regarding the biophysical properties, biological functions, signaling pathways, and clinical trials of endostatin are reviewed here. Surprising findings show that the integrity of the N-terminal sequence, the capability of zinc-binding, and the correct folding are three essential elements for assurance of structural stability and biological functions of endostatin. This review provides clues to understand the mysteries of endostatin and its derivatives.