Basic fibroblast growth factor-induced activation of latent transforming growth factor β in endothelial cells: Regulation of plasminogen activator activity

Abstract

Exposure of bovine aortic or capillary endothelial cells to basic FGF (bFGF) for 1 h resulted in an approximately sixfold increase in plasminogen activator (PA) activity by 18 h that returned nearly to basal levels by 36 h. We hypothesized that the decrease in PA activity following bFGF stimulation was mediated by transforming growth factor β (TGF-β) formed from its inactive precursor. Conditioned medium collected from endothelial cells 36 h after a 1-h exposure to bFGF, but not control medium, inhibited basal levels of PA activity when transferred to confluent monolayers of bovine aortic endothelial cells. Antibody to TGF-β neutralized the inhibitory activity of this conditioned medium, indicating that the medium contained active TGF-β. Northern blot analysis and quantitation of acid activatable latent TGF-β in conditioned medium demonstrated that bFGF exposure did not increase the amount of transcription or secretion of latent TGF-β by the endothelial cells. Both aprotinin, an inhibitor of plasmin, and anti-urokinase type PA IgG blocked the generation of active TGF-β in cultures exposed to bFGF. These results demonstrated that plasmin generated by uPA activity is required for the activation of latent TGF-β in endothelial cell cultures treated with bFGF. Activation of TGF-β by endothelial cells exposed to bFGF appears to limit both the degree and duration of PA stimulation. Thus, in bFGF-stimulated endothelial cell cultures, PA levels are controlled by a negative feedback loop: PA, whose expression is stimulated by bFGF, contributes to the formation of TGF-β, which in turn opposes the effects of bFGF by limiting PA synthesis and activity. These studies suggest a role for TGF-β in reversing the invasive stage of angiogenesis and contributing to the formation of quiescent capillaries.