Transforming growth factor-β mediates astrocyte-specific regulation of brain endothelial anticoagulant factors

Abstract

Background and Purpose - Astrocytes are potent regulators of brain capillary endothelial cell function. Recently, astrocytes were shown to regulate brain capillary endothelial expression of the fibrinolytic enzyme tissue plasminogen activator (tPA) and the anticoagulant thrombomodulin (TM). To study the mechanism of this process, we examined the hypothesis that astrocyte regulation of endothelial tPA and TM is mediated by transforming growth factor-β (TGF-β). Methods - Brain capillary endothelial cells were grown in blood-brain barrier models. We examined astrocyte-endothelial cocultures, endothelial monocultures, and astrocyte-conditioned media (ACM) for the expression of TGF-β. We also incubated endothelial cells with ACM to determine the role of TGF-β. Following 24 hours of incubation, we assayed for tPA and TM mRNA, as well as tPA and TM activity. Results - Astrocyte- endothelial cocultures and ACM exhibited significantly higher levels of active TGF-β than brain endothelial monocultures and endothelial cells grown in nonconditioned media, respectively. Brain endothelial cells incubated with ACM exhibited reduced tPA and TM mRNA and activity. Treatment with exogenous TGF-β produced dose-dependent reductions in tPA and TM. The effects of ACM on both tPA and TM were blocked by TGF-β neutralizing antibody. Conclusions - These data indicate that TGF-β mediates astrocyte regulation of brain capillary endothelial expression of tPA and TM.