Human tumstatin and human endostatin exhibit distinct antiangiogenic activities mediated by αvβ3 and α5β1 integrins

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Abstract
Tumstatin and endostatin are two inhibitors of angiogenesis derived from precursor human collagen molecules known as α3 chain of type IV collagen and α1 chain of type XVIII collagen, respectively. Although both these inhibitors are noncollagenous (NC1) domain fragments of collagens, they only share a 14% amino acid homology. In the present study we evaluated the functional receptors, mechanism of action, and intracellular signaling induced by these two collagen-derived inhibitors. Human tumstatin prevents angiogenesis via inhibition of endothelial cell proliferation and promotion of apoptosis with no effect on migration, whereas human endostatin prevents endothelial cell migration with no effect on proliferation. We demonstrate that human tumstatin binds to αvβ3 integrin in a vitronectin/fibronectin/RGD cyclic peptide independent manner, whereas human endostatin competes with fibronectin/RGD cyclic peptide to bind α5β1 integrin. The activity of human tumstatin is mediated by αvβ3 integrin, whereas the activity of human endostatin is mediated by α5β1 integrin. Additionally, although human tumstatin binding to αvβ3 integrin leads to the inhibition of Cap-dependent translation (protein synthesis) mediated by focal adhesion kinase/phosphatidylinositol 3-kinase/Akt/mTOR/4E-BP1 pathway, human endostatin binding to α5β1 integrin leads to the inhibition of focal adhesion kinase/c-Raf/MEK1/2/p38/ERK1 mitogen-activated protein kinase pathway, with no effect on phosphatidylinositol 3-kinase/Akt/mTOR/4E-BP1 and Cap-dependent translation. Collectively, such distinct properties of human tumstatin and human endostatin provide the first insight into their diverse antiangiogenic actions and argue for combining them for targeting tumor angiogenesis.