Thrombospondin 1 does not activate transforming growth factor beta 1 in a chemically defined system or in smooth-muscle-cell cultures

The cytokine transforming growth factor beta 1 (TGF-beta 1) is secreted in a latent form that has no known biological activity. The conversion of late nt TGF-beta 1 into its biologically active 25 kDa form is thought to be an important step in the regulation of TGF-beta activity both in cell culture and in vivo. Thrombospondin (TSP)-1, a 360 kDa platelet alpha-granule and e xtracellular matrix protein, has been shown to participate in TGF-beta 1 ac tivation. We have used a chemically defined system to examine the mechanism of TSP-1-mediated TCF-beta 1 activation. However, the addition of two diff erent preparations of TSP-1 to recombinant small latent TGF-beta 1 in the t est tube resulted in only a very small increase in the proportion of the TG F-beta 1 able to bind to the TGF-beta type II receptor: from 0.1 % to a max imum of 0,4%. This small effect was not specific for TSP-1: matrix metallop roteinase 2, tissue inhibitor of matrix metalloproteinase 2 and active plas minogen activator inhibitor 1, but not transglutaminase, human serum albumi n or immunoglobulin, had quantitatively similar effects on latent TGF-beta 1. Furthermore, no change in the activity associated with small latent TGF- beta 1 was noted in either mink lung epithelial cell or rat aortic smooth-m uscle eel culture systems in the presence of TSP-1 (or TSP-1-derived peptid es). We conclude that TSP-1, either alone or in the presence of cultured sm ooth-muscle cells (a cell type known to activate latent TGF-beta in vitro a nd in vivo) is unable to activate latent TGF-beta 1. Any TSP-mediated activ ation of TGF-beta 1 must depend on additional factor(s) not present in our systems.