STIMULATION OF ACTIVIN-A EXPRESSION IN RAT AORTIC SMOOTH-MUSCLE CELLSBY THROMBIN AND ANGIOTENSIN-II CORRELATES WITH NEOINTIMAL FORMATION IN-VIVO

Vasoactive GTP-binding protein-coupled receptor agonists (e.g., angiot ensin II [ALI] and alpha-thrombin) stimulate the production of mitogen ic factors from vascular smooth muscle cells, In experiments to identi fy mitogens secreted from AII- or alpha-thrombin-stimulated rat aortic smooth muscle (RASM) cells, neutralizing antibodies directed against several growth factors (e.g., PDGF and basic fibroblast growth factor [basic FGF]) failed to inhibit the mitogenic activity of conditioned m edia samples derived from the cells, In this report, we found that pol yclonal neutralizing antibodies directed against purified human placen tal basic FGF reduced the mitogenic activity of AII-stimulated RASM ce ll-conditioned media and in immunoblot experiments identified a 26-kD protein (14 kD under reducing conditions) that was distinct from basic FGF, After purification from RASM cell-conditioned medium, amino acid sequence analysis identified the protein as activin A, a member of th e TGF-beta superfamily, Increased activin A expression was observed af ter treatment of the RASM cells with AII, alpha-thrombin, and the prot ein kinase C agonist PMA. In contrast, PDGF-BB or serum caused only a minor induction of this protein. Although activin A alone only weakly stimulated RASM cell DNA synthesis, it demonstrated a potent comitogen ic effect in combination with either EGF or heparin-binding EGF-like g rowth factor in the RASM cells, increasing DNA synthesis by up to four fold. Furthermore, in a rat carotid injury model, activin A mRNA was u pregulated within 6 h after injury followed by increases in immunoreac tive protein detected in the expanding neointima 7 and 14 d later. Tak en together, these results indicate that activin A is a vascular smoot h muscle cell-derived factor induced by vasoactive agonists that may, either alone or in combination with other vascular derived growth fact ors, have a role in neointimal formation after arterial injury.