The fibrinogen anion-binding exosite of thrombin is necessary for induction of rises in intracellular calcium and prostacyclin production in endothelial cells.

Thrombin stimulation of prostacyclin (PGI2) synthesis by cultured human umbilical vein endothelial cells (HUVEC) requires the active site of thrombin and involves rapid and transient rises in cytoplasmic free calcium [Ca2+]i. In this study, we investigated whether or not the anion-binding exosite for fibrinogen recognition of thrombin (which confers certain substrate specificities) is also necessary for the induction of rises in [Ca2+]i and PGI2 production. Thrombin variants which lack either the catalytic site (DIP-alpha-thrombin) or anion-binding exosite (gamma-thrombin) either alone or in combination failed to induce rises in [Ca2+]i or PGI2 production in HUVEC. To further study the role of the anion-binding exosite of thrombin in the activation of HUVEC, COOH-terminal fragments of hirudin were used. This portion of hirudin interacts with the anion-binding exosite of thrombin and inhibits thrombin-induced fibrinogen coagulation while leaving the catalytic activity of thrombin intact. A 21-amino acid COOH-terminal peptide of hirudin (N alpha-acetyldesulfato-hirudin45-65 or Hir45-65) inhibited thrombin-induced (0.5 U/ml) rises in [Ca2+]i and PGI2 production with IC50 of 0.13 and 0.71 microM, respectively. Similar results were obtained using shorter hirudin-derived peptides. Thus, the fibrinogen anion-binding exosite of thrombin is required for alpha-thrombin-induced rises in [Ca2+]i and PGI2 production in HUVEC.