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

Tumstatin and endostatin are two inhibitors of angiogenesis derived from precursor human collagen molecules known as alpha3 chain of type IV collagen and alpha1 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-clerived inhibitors. Human turnstatin 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 turnstatin binds to alphavbeta3 integrin in a vitronectin/fibronectin/RGD cyclic peptide independent manner, whereas human endostatin competes with fibronectin/RGD cyclic peptide to bind alpha5beta1 integrin. The activity of human turnstatin is mediated by alphavbeta3 integrin, whereas the activity of human endostatin is mediated by alpha5beta1 integrin. Additionally, although human turnstatin binding to alphavbeta3 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 alpha5beta1 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 turnstatin and human endostatin provide the first insight into their diverse antiangiogenic actions and argue for combining them for targeting tumor angiogenesis.