Effects of Thrombin on Interactions between β3-Integrins and Extracellular Matrix in Platelets and Vascular Cells

The β₃-integrin family consists of α _IIbβ₃ (also known as glycoprotein IIb/IIIa) and α_vβ₃. α_IIbβ₃ is found on platelets and megakaryocytes and has an essential role in hemostasis. α_vβ₃ has a broader distribution, and it functions in angiogenesis, neointimal formation after vascular injury, and leukocyte trafficking. There are important interactions between thrombin and β₃-integrins relative to both "inside-out" (integrin activation) and "outside-in" (modification of cellular events by ligand binding to integrins) signaling. Thrombin, by binding to G protein-coupled, protease-activated receptors, is a potent activator of α_IIbβ₃. Conversely, outside-in signaling through α_IIbβ₃ amplifies events initiated by thrombin and is necessary for full platelet spreading, platelet aggregation, granule secretion, and the formation of a stable platelet thrombus. In smooth muscle cells, α_vβ₃-integrins influence various responses to thrombin, including proliferation, c-Jun NH₂-terminal kinase-1 activation, and focal adhesion formation. Other interactions between β₃-integrins and thrombin include β₃-integrin promotion of the generation of thrombin by localizing prothrombin to cellular surfaces and/or enhancing the formation of procoagulant microparticles and the requirement of β₃-integrin function for platelet-dependent clot retraction. In summary, there is increasing evidence that interactions between β₃-integrins and thrombin play important roles in the regulation of hemostatic and vascular functions.