NATURE MEDICINE • VOLUME 8• NUMBER 1• JANUARY 2002 27 ART I C LE S Angiogenesis, the formation of new blood vessels from pre- existing vasculature, involves coordinated endothelial-cell proliferation, migration and tube formation. This process is influenced both by growth factors, such as vascular endothe- lial growth factor (VEGF), and by cell adhesion molecules such as integrins 1,2 . Angiogenesis is a hallmark of cancer, as well as various ischemic diseases such as retinopathy of pre- maturity 3 , implying that anti-angiogenic drugs are likely to be of importance in the treatment of these diseases. Elucidating the precise molecular mechanisms of angiogenic regulation is therefore important in determining rational strategies for such anti-angiogenic approaches. VEGF has been identified as a major angiogenic factor act- ing through endothelial cell-specific receptors, including VEGF receptor-2 (VEGFR-2; also called Flk-1) 4 . The impor- tance of the VEGF/VEGFR-2 system in angiogenesis is sup- ported strongly by the lack of vascular development and early embryonic lethality in mice both heterozygous for and defi- cient in VEGFR-2 (refs. 4–6). In many tumors and ischemic diseases, VEGF production is elevated, inducing adult patho- logical angiogenesis and further emphasizing the importance of VEGF in neovascularization. Strategies to block VEGF and VEGF receptor signaling and function have resulted in signif- icant inhibition of tumor angiogenesis and such reagents are presently in clinical trials 1,7,8 . Several members of the integrin family are also implicated in angiogenesis 9–18 . The largest body of data has linked ! v " 3 and ! v " 5 integrins (both recep- tors for vitronectin and other extracellular matrix molecules) with blood-vessel development 11–18 . Particular attention has been paid to the role of ! v " 3 integrin in angiogenesis as it is prominent on proliferating vascular endothelial cells 13,14 . Furthermore, blockade of ! v " 3 integrin with monoclonal an- tibodies or low-molecular-weight antagonists inhibits blood- vessel formation in a variety of in vivo models 17 , including tumor angiogenesis 11–13 and neovascularization during oxy- gen-induced retinopathy 19 . In a recent report, a single small- molecule inhibitor of both ! v " 3 and ! v " 5 integrins inhibited tumor angiogenesis in animal models 20 . Taken together, these inhibition data suggest critical roles for ! v " 3 and ! v " 5 in an- giogenesis, and highlight their importance as potential tar- gets in anti-angiogenic therapy. In fact, the ! v " 3 -integrin antagonist, Vitaxin, is presently in clinical trials 21 . In contrast with these inhibitor studies, mice lacking ! v , " 3 or " 5 integrins exhibit extensive developmental angiogene- sis 22–24 . All ! v -null mice have extensive sprouting angiogenesis and develop normally until embryonic day 9.5 and approxi- mately 20% of them survive to birth 22 . " 3 -null mice are both viable and fertile and developmental angiogenesis, including postnatal neovascularization of the retina, appears to be " 3 - independent 23 . " 5 -null mice are also viable and fertile and have no defects in wound healing, suggesting that adult an- giogenesis is unaffected in these animals 24 . These results indi- cate that the precise role of ! v integrins in angiogenesis is likely to be more complex than initially thought and raise the question of the importance of ! v " 3 and ! v " 5 integrins in adult pathological angiogenic processes. Using genetically defi- Enhanced pathological angiogenesis in mice lacking " 3 integrin or " 3 and " 5 integrins LOUISE E. REYNOLDS 1 , LORENZA WYDER 1 , JULIE C. LIVELY 2 , DANIELA TAVERNA 2 , STEPHEN D. ROBINSON 1 , XIAOZHU HUANG 3 , DEAN SHEPPARD 3 , RICHARD O. HYNES 2 & KAIRBAAN M. HODIVALA-DILKE 1 1 Cell Adhesion and Disease Laboratory, Richard Dimbleby Department, Imperial Cancer Research Fund, St. Thomas’ Hospital, London, UK 2 Howard Hughes Medical Institute, Center for Cancer Research, MIT, Cambridge, Massachusetts 02139, USA 3 Lung Biology Center, University of California, San Francisco, California, USA L.E.R. and L.W. contributed equally to this study. Correspondence should be addressed to K.M.H.-D.; email: k.hodivala-dilke@icrf.icnet.uk Inhibition of ! v " 3 or ! v " 5 integrin function has been reported to suppress neovascularization and tumor growth, suggesting that these integrins are critical modulators of angiogenesis. Here we report that mice lacking " 3 integrins or both " 3 and " 5 integrins not only support tumorigenesis, but have enhanced tumor growth as well. Moreover, the tumors in these inte- grin-deficient mice display enhanced angiogenesis, strongly suggesting that neither " 3 nor " 5 integrins are essential for neovascularization. We also observed that angiogenic responses to hypoxia and vascular endothelial growth factor (VEGF) are augmented significantly in the ab- sence of " 3 integrins. We found no evidence that the expression or functions of other integrins were altered as a consequence of the " 3 deficiency, but we did observe elevated levels of VEGF receptor-2 (also called Flk-1) in " 3 -null endothelial cells. These data indicate that ! v " 3 and ! v " 5 integrins are not essential for vascular development or pathological angiogenesis and high- light the need for further evaluation of the mechanisms of action of ! v -integrin antagonists in anti-angiogenic therapeutics. © 2002 Nature Publishing Group http://medicine.nature.com