Activation of Platelet-transforming Growth Factor β-1 in the Absence of Thrombospondin-1

Thrombospondin-1 (TSP-1) has been shown to bind and activate transforming growth factor-β1 (TGF-β1). This observation raises the possibility that TSP-1 helps to sequester TGF-β1 in platelet α granules and activates TGF-β1 once both proteins are secreted. Herein, we evaluated the level of active and latent TGF-β1 in the plasma and in the supernatant of thrombin-treated platelets from TSP-1 null and wild-type mice on two genetic backgrounds (C57BL/6 and 129Sv). The plasminogen activator inhibitor-1/luciferase bioassay and an immunological assay were used to determine active and latent TGF-β1. No significant differences were observed in the levels of active and latent TGF-β1 in the supernatant of thrombin-treated platelets from TSP-1 null and wild-type mice. Active and latent TGF-β1 were significantly increased in the plasma and platelets of C57BL/6 mice as compared with 129Sv mice. In addition, there was an increase of plasma level of latent TGF-β1 in TSP-1 null mice as compared with wild-type mice on the C57BL/6 background but not on the 129Sv background. No active TGF-β1 was observed in the plasma of either TSP-1 null and wild-type mice. These data indicate that TSP-1 does not function as a chaperon for TGF-β1 during platelet production and does not activate significant quantities of secreted TGF-β1 despite a vast excess in the number of TSP-1 molecules as compared with TGF-β1 molecules. Because platelet releasates from TSP-1 null mice contain active TGF-β1, we suggest that other important mechanisms of physiological activation of TGF-β1 probably exist in platelets.