Thrombospondin 1 does not activate transforming growth factor beta1 in a chemically defined system or in smooth-muscle-cell cultures

The cytokine transforming growth factor beta1 (TGF-beta1) is secreted in a latent form that has no known biological activity. The conversion of latent TGF-beta1 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 extracellular matrix protein, has been shown to participate in TGF-beta1 activation. We have used a chemically defined system to examine the mechanism of TSP-1-mediated TGF-beta1 activation. However, the addition of two different preparations of TSP-1 to recombinant small latent TGF-beta1 in the test tube resulted in only a very small increase in the proportion of the TGF-beta1 able to bind to the TGF-beta type II receptor: from 0.1% to a maximum of 0.4%. This small effect was not specific for TSP-1: matrix metalloproteinase 2, tissue inhibitor of matrix metalloproteinase 2 and active plasminogen activator inhibitor 1, but not transglutaminase, human serum albumin or immunoglobulin, had quantitatively similar effects on latent TGF-beta1. Furthermore, no change in the activity associated with small latent TGF-beta1 was noted in either mink lung epithelial cell or rat aortic smooth-muscle cell culture systems in the presence of TSP-1 (or TSP-1-derived peptides). We conclude that TSP-1, either alone or in the presence of cultured smooth-muscle cells (a cell type known to activate latent TGF-beta in vitro and in vivo) is unable to activate latent TGF-beta1. Any TSP-mediated activation of TGF-beta1 must depend on additional factor(s) not present in our systems.