Thrombospondin-1 (TS1) is a multidomain glycoprotein involved in wound healing, inflammation, angiogenesis, cancer and development. We have found that integrin associated protein or IAP (CD47) is a receptor for the C-terminal cell binding domain (CBD) of TS1. Anti- IAP mAbs block may integrin-dependent functions and IAP is required for integrin-initiated signal transduction. Our preliminary data indicate that the TS1-IAP interaction costimulates or augments beta 1, beta 2 and beta 3 integrins in leukocytes, platelets, endothelial cells, fibroblasts and melanoma cells leading to chemotxis, enhanced cell spreading, platelet activation and activation of leukocyte integrins required for endothelial adhesion and transmigration. All of these functions of TS1/IAP are blocked specifically by pertussis toxin indicating a requirement for a heterotrimeric Gi protein to link IAP activation by TS1 to downstream signaling events The proposed aim are: 1. To mutagenize the CBD to determine its structural features important for binding and activating IAP. Mutations of whole TS1 will be created in which other cell binding sites have been 'knocked out' in combination with those in the CBD. 2. Determination of the molecular interactions among TS1, IAP, its partner intefrins and Gi proteins necessary for IAP signaling. 3. Assessment of the role of TS1/CBD as a costimulator of alpha iota iota beta3 in platelet adhesion and aggregation. 4. Roles of TS1/IAP activation relevant to inflammation will be tested in models of leukocyte chemotaxis, beta2 integrin activation, leukocyte transmigration of endothelial monolayers and the phagocytosis of apoptotic inflammatory cells by macrophages. We now have a novel paradigm for TS1 function in many biological systems in which the affinity and signaling functions of integrins are modulated. Some of the best examples of this are in platelet activation/aggregation and the inflammatory response where circulating leukocytes become rapidly activated to adhere to inflamed endothelium and invade tissues. This work can potentially yield information and compounds of therapeutic value in hemostasis and thrombosis, wound healing, angiogenesis and inflammatory diseases such as arthritis.
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