Transforming Growth Factors β Stimulate Both Thrombospondin-1 and CISP/Thrombospondin-2 Synthesis by Bovine Adrenocortical Cells

Abstract

We recently observed that adrenocortical cells secrete, under ACTH treatment, a large trimeric glycoprotein (CISP) presenting amino acid sequence similarity with thrombospondin-2. We also observed that the same cells synthesize and secrete thrombospondin-1 whereas under smaller amounts. The aim of this study was to investigate the regulation of these two secreted proteins by members of the TGFβ family of regulatory peptides. We developed an appropriate immunoprecipitation technique that allowed us to quantitate synthesis of thrombospondin-1 and CISP/thrombospondin-2 in a single assay. Using this assay, we observed that thrombospondin-1 and CISP/thrombospondin-2 syntheses were respectively stimulated threefold and twofold by a 24-h treatment with 2 ng/ml TGFβ1. These inductions were dose-dependent (half-maximal effect: 0.2 ng/ml) and time-dependent (detectable after 5 h and plateauing between 15 and 25 h of treatment). They were not observed when transcription was blocked by RNA polymerase inhibitors such as 5,6-dichlorobenzimidazole riboside or actinomycin D. Among members of the TGFβ family, TGFβ1 and TGFβ2 and to a lesser extent activin could stimulate thrombospondin-1 and CISP/thrombospondin-2 synthesis, whereas inhibin and Müllerian inhibiting substance were inactive. Taken together, these data represent the first study on the regulation of both thrombospondin-1 and CISP/thrombospondin-2 by TGFβs. They further support the concept that TGFβ is a local regulator of adrenocortical functions.