Endostatin, a proteolytic fragment of
collagen XVIII, is a potent inhibitor of angiogenesis and
suppresses neovascularization and tumor growth.
However, the inhibitory mechanism of endostatin in
human endothelial cells has not been characterized yet.
Electron microscopic analysis revealed that endostatin
induced formation of numerous autophagic vacuoles in
endothelial in 6 to 24 h after treatment. Moreover, there
was only a 2- to 3-fold increase in intracellular reactive
oxygen species after endostatin treatment. Endostatininduced
cell death was not prevented by antioxidants
(vitamin C, vitamin E, or propyl gallate) or caspase
inhibitors, suggesting that the increase of oxidative stress
or the activation of caspases may not be the crucial
factors in the anti-angiogenic mechanism of endostatin.
However, the cytotoxicity of endostatin was significantly
reduced by 3-methyladenine (a specific inhibitor of
autophagy) and serine and cysteine lysosomal protease
inhibitors (leupeptin and aprotinin). Taken together,
these results suggest that in human endothelial cells: (1)
endostatin predominantly causes autophagic, rather than
apoptotic, cell death, (2) endostatin-induced autophagic
cell death occurs in the absence of caspase activation
and through an oxidative-independent pathway, and (3)
endostatin-induced ‘autophagic cell death or ‘type 2
physiological cell death’ is regulated by serine and
cysteine lysosomal proteases