M. Abdelouahed et al., Accelerated Publication - Activation of platelet-transforming growth factor beta-1 in the absence of thrombospondin-1, J BIOL CHEM, 275(24), 2000, pp. 17933-17936
Thrombospondin-1 (TSP-1) has been shown to bind and activate transforming g
rowth factor-beta 1 (TGF-beta 1). This observation raises the possibility t
hat TSP-1 helps to sequester TGF-beta 1 in platelet alpha granules and acti
vates TGF-beta 1 once both proteins are secreted. Herein, we evaluated the
level of active and latent TGF-beta 1 in the plasma and in the supernatant
of thrombin-treated platelets from TSP-1 null and wild-type mice on two gen
etic backgrounds (C57BL/6 and 129Sv). The plasminogen activator inhibitor-1
/luciferase bioassay and an immunological assay were used to determine acti
ve and latent TGF-beta 1, No significant differences were observed in the l
evels of active and latent TGF-beta 1 in the supernatant of thrombin-treate
d platelets from TSP-1 null and wildtype mice. Active and latent TGF-beta 1
were significantly increased in the plasma and platelets of C57BL/6 mice a
s compared with 129Sv mice. In addition, there was an increase of plasma le
vel of latent TGF-beta 1 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-b
eta 1 was observed in the plasma of either TSP-1 null and wild-type mice. T
hese data indicate that TSP-I does not function as a chaperon for TGF-beta
1 during platelet production and does not activate significant quantities o
f secreted TGF-beta 1 despite a vast excess in the number of TSP-1 molecule
s as compared with TGF-beta 1 molecules. Because platelet releasates from T
SP-1 null mice contain active TGF-beta 1, we suggest that other important m
echanisms of physiological activation of TGF-beta 1 probably exist in plate
lets.