Thrombospondins selectively activate one of the two latent forms of transforming growth factor-beta present in adrenocortical cell-conditioned medium.

Transforming growth factor-beta (TGF beta) has been shown previously to be a potent inhibitor of bovine adrenocortical cell steroidogenic functions. However, it is present in the culture medium of these cells in a latent form. In this study, we analyzed in detail the biochemical composition of this latent TGF beta. Two distinct complexes could be separated chromatographically by gel filtration on Sephacryl S-300, and their composition was studied using immunochemical methods. The results indicate that one form (peak I) is a complex between alpha 2-macroglobulin (alpha 2M) and either the unprocessed TGF beta precursor or the mature form of TGF beta. In a major fraction of this complex, TGF beta is covalently linked to alpha 2 M, whereas in a minor fraction, it is noncovalently bound and, therefore, activatable. The second form of latent TGF beta (peak II) is a complex among latent TGF beta-binding protein (LTBP), latency-associated protein, and mature TGF beta and a complex between LTBP and unprocessed TGF beta. We investigated the ability of thrombospondins (TSP1 and TSP2) to activate these latent forms of TGF beta. TSP1 and TSP2 were equally potent at activating the LTBP-latency-associated protein-TGF beta complex in the absence of cell contact, but were ineffective on the alpha 2M-TGF beta complex. Therefore, TGF beta may act as an autocrine regulator of adrenocortical steroidogenic functions. Its activity appears to be controlled by TSPs, the local production of which is regulated by systemic ACTH.