Selective Promotion of Plasminogen Activator Inhibitor-1 Secretion by Activation of Proteinase-Activated Receptor-1 in Cultured Human Brain Microvascular Pericytes: Comparison with Endothelial Cells

Abstract

Fibrinolysis is the phenomenon by with plasmin degrades fibrin. The fibrinolytic activity depends on the balance between tissue-and urokinase-type plasminogen activators (t-PA and u-PA, respectively) and plasminogen activator inhibitor-1 (PAI-1).1) Both t-PA and u-PA convert plasminogen to plasmin that degrades fibrin. On the other hand, PAI-1 is a common inhibitor of t-PA and u-PA; a large part of PAI-1 in the liquid phase is a latent form, 2) and the remainding active form inhibits t-PA and u-PA by forming inactive complexes.3)The fibrinolytic proteins are expressed in vascular composing cells, and regulation of the synthesis and secretion are different among cell types and origins. 4,5) Microvessels are composed of endothelial cells that cover the inner surface in a monolayer and pericytes that wrap around and along endothelial cells.6) The regulation of fibrinolysis in vascular endothelial cells and pericytes is important for the removal of intravascular thrombi and the maintenance of hemostasis, respectively, in microvessels. Since cerebrovascular accident is a serious public health problem in advanced countries including Japan, it appears to be important to clarify the regulation of fibrinolysis mediated by brain microvascular endothelial cells and pericytes. Immunohistochemical studies have shown that fibrinolytic proteins such as t-PA and u-PA are expressed in brain microvascular endothelium in vivo, 7) although the regulation of fibrinolytic protein synthesis has been incompletely understood. Furthermore, little is known about the expression of fibrinolytic proteins and its regulation in pericytes.Thrombin not only converts fibrinogen to fibrin in the blood coagulation-fibrinolytic system but also regulates vascular cell functions. Proteinase regulates the synthesis and secretion of fibrinolytic proteins in vascular endothelial cells 8,9) by activation of proteinase-activated receptor-1 (PAR-1).10,11) Thrombin stimulation occurs in different patterns depending on the origin of the cells. For example, thrombin upregulates the expression of t-PA, u-PA, and PAI-1 in both cultured renal 12) and foreskin 13) microvascular endothelial cells. At that time, the activity of t-PA is enhanced in the renal endothelial cells but suppressed in the foreskin endothelial cells. It has been shown that thrombin upregulates the expression of fibrinolytic proteins in cultured human brain microvascular endothelial cells 14) ; however, the subsequent change in fibrinolytic activity is unknown.The purpose of the present study is to determine the plasminogen activator activity and the expression of fibrinolytic proteins in cultured human brain microvascular pericytes and endothelial cells after exposure to thrombin. We found that thrombin increases the secretion of t-PA, u-PA, and PAI-1, resulting in an enhancement of plasminogen activator activity in endothelial cells, whereas proteinase increased the secretion only of PAI-1, resulting in suppression of plasminogen activator activity in pericytes. MATERIALS AND METHODS Materia...

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