α-Thrombin Stimulates Urokinase Production and DNA Synthesis in Cultured Human Cerebral Microvascular Endothelial Cells

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Abstract
α-Thrombin regulation of endothelial cell (EC) fibrinolysis has been documented by using endothelia derived from a number of anatomic locations but not with those derived from the human cerebral vasculature. In the present study, the fibrinolytic properties of human cerebral microvascular ECs and their regulation by α-thrombin are delineated and contrasted with those of human umbilical vein and foreskin microvascular ECs. In cerebral ECs, α-thrombin elicited a unique dose-dependent increase in urokinase production and DNA synthesis. Maximal stimulation, observed with 10 nmol/L α-thrombin, resulted in a 30- to 50-fold increase in urokinase production and a concomitant fourfold increase in DNA synthesis; the increase in urokinase was reflected in higher steady-state levels of urokinase mRNA. The major urokinase product secreted is the single-chain form of the enzyme. No effect was observed with the addition of other proteases or catalytically inactive variants of α-thrombin. A thrombin receptor agonist peptide upregulated urokinase production but had no effect on DNA synthesis, suggesting that fibrinolysis is mediated by the thrombin receptor but that proliferation is regulated by a different pathway. These findings suggest the possibility that the cerebral microvasculature may be a specialized region of the vascular system in which urokinase-type plasminogen activator, not tissue-type plasminogen activator, is the key catalyst of fibrin lysis when the brain responds to thrombotic events and that α-thrombin may regulate repair of the cerebral microvascular system.