Regular Article VASCULAR BIOLOGY Interaction of kindlin-2 with integrin b3 promotes outside-in signaling responses by the aVb3 vitronectin receptor

Integrins are transmembrane receptors composed ofa andb subunits that mediate bidirectional signaling between the extracellular matrix and the cell interior. Among the best studied integrins is aIIbb3, which is required for hemostasis and platelet involvement in thrombosis. The related b3 integrin, aVb3, is expressed at low levels in platelets and better expressed in several other hematopoietic cells and in cells of the vasculature, particularly proliferative endothelial cells (ECs). Studies of aVb3 function in vitro and in genetically modified mice appear somewhat contradictory. Antibodies or small molecules that block the binding of vitronectin and other matrix ligands to aVb3 can inhibit aVb3-dependent cell migration and angiogenesis. However, b3 knockout mice show increased tumor growth and angiogenesis, in part fromelevated expression of vascular endothelial growth factor receptor-2 (VEGFR-2) inb3 ECs. Furthermore, acute but not long-term depletion of b3 in ECs inhibits tumor growth and angiogenesis in mice. Global or tissuespecific ablation of b3 may lead to altered expression of nontargeted genes, VEGFR-2 being one example. Generation of knock-inmutations in b3 may avoid this problem, as exemplified by normal VEGFR-2 expression in “b3(DiY.F)” knock-in mice, in which 2 b3 cytoplasmic tail tyrosines (747 and 759) are mutated to phenylalanine, resulting in reduced tumor angiogenesis. This result was attributed to effects on bidirectional integrin signaling and to disruption of normal interactions between aVb3 and VEGFR-2. Thus, further studies of b3 knock-in mice might lead to a deeper understanding of aVb3 function. Integrin signaling encompasses “inside-out” regulation of adhesive ligand binding to integrins and ligand-dependent “outside-in” regulation of cellular responses. In both contexts, signaling can be influenced by interactions of integrin cytoplasmic tails with specific enzymes and adapters. Among proteins capable of interacting with the b3 cytoplasmic tail are the Src family protein tyrosine kinases (SFKs) and the adapters talin and kindlin. One function of SFKs in platelets and ECs is the phosphorylation of b3 Y747 and Y759, resulting in reduced interaction of b3 with talin and kindlins, respectively. Direct interaction between c-Src and the extreme C terminus of the b3 cytoplasmic tail appears to be required for normal aIIbb3-dependent platelet functions in vivo and for the aVb3dependent functions of stem cells and tumor cells. Consequently, binding of c-Src to the b3 tail in ECs might regulate aVb3 signaling. Indeed, “b3(DiY.F)” knock-in mice with phenylalanine substitutions at the 2 putative SFK phosphorylation sites exhibit reduced angiogenesis. The kindlin family of adapters (kindlin-1, kindlin-2, and kindlin-3) plays key roles in multiple cell types to promote bidirectional signaling involving b1, b2, and b3 integrins. For example, kindlin-3 cooperates with talin to effect b2 activation in leukocytes and b3