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-␤ (TGF-␤). 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-␤. We also incubated endothelial cells with ACM to determine the role of TGF-␤. 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-␤ 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-␤ produced dose-dependent reductions in tPA and TM. The effects of ACM on both tPA and TM were blocked by TGF-␤ neutralizing antibody. tPA is a critical circulating fibrinolytic enzyme that proteolytically activates plasminogen to plasmin. 1 Intravenous tPA improves neurological outcome in clinical stroke. 2 Reduced expression of brain capillary tPA is associated with increased infarct size following transient middle cerebral artery occlusion in diabetic and nicotine stroke models. 3,4 Brain capillary endothelial expression of tPA is limited, [5][6][7] and understanding mechanisms underlying this limited expression has potential therapeutic value. Conclusions-TheseTM, an important antithrombotic protein, functions as a cofactor for the activation of circulating protein C. 8,9 Both TM and activated protein C offer protection against a variety of thrombotic events. Treatment with purified or recombinant TM protects against thromboembolism in animal models, 10 -14 See Editorial Comment, page 1677 while TM neutralizing antibodies potentiate thrombininduced thromboembolism. 10 Treatment with activated protein C also provides protection against thromboembolic events. 15,16 Resistance to the effects of activated protein C is closely linked to venous thrombosis in humans, 17 including cerebral venous thrombosis. 18 Low levels of circulating activated protein C are present in certain forms of ischemic stroke. 19 Moreover, brain-specific protein C activation has been demonstrated in humans in vivo 20 ; brain capillaries ex vivo also exhibit protein C activation. 21 Brain TM expression is limited, [22][23][24][25][26] and, like tPA, identification of mechanisms underlying this limited expression would be expected to have therapeutic value.Astrocytes are regulators for a wide variety of brain capillary endothelial functions. Astrocytes are responsible for ...

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Astrocytes Induce Hemeoxygenase-1 Expression in Microglia: A Feasible Mechanism for Preventing Excessive Brain Inflammation

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Role of enteric glial cells in inflammatory bowel disease

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