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The roles of proteinase-activated receptors (PARs) in platelet functions other than aggregation are not well understood. Among these is the release of factors that regulate the process of angiogenesis, such as endostatin and VEGF, which, respectively, inhibit and promote angiogenesis. PAR1 and PAR4 are expressed on the surface of human platelets and can be activated by thrombin. In the present study, we have attempted to determine the roles of PAR1 and PAR4 in regulating release of endostatin and VEGF from human platelets. Aggregation and endostatin release could be elicited by a specific PAR4 agonist (AYPGKF-NH2). The PAR4 agonist concentration dependently suppressed VEGF release. A selective PAR1 agonist (TFLLR-NH2) induced platelet aggregation and VEGF release but suppressed endostatin release. Thrombin did not affect endostatin or VEGF release. However, in the presence of a selective PAR1 antagonist (SCH79797), thrombin stimulated endostatin release and suppressed VEGF release. Conversely, in the presence of a selective PAR4 antagonist (transcinnamoyl-YPGKF-NH2), thrombin stimulated VEGF release. In vivo, treatment of rats with established gastric ulcers with a PAR1 antagonist each day for 1 wk resulted in a significant retardation of healing. We conclude that PAR1 and PAR4 counter-regulate the release of endostatin and VEGF from platelets. These protease-activated receptors could therefore play a crucial role in regulating angiogenesis and in turn could regulate the processes of wound healing and tumor growth.angiogenesis ͉ aggregation ͉ thrombin ͉ protease I n addition to its central roles in blood coagulation and hemostasis, thrombin participates in a variety of biological processes, including inflammation and wound healing (1). Activation of platelets by thrombin is mediated at least in part through cleavage of proteinase-activated receptors (PARs). Four distinct PARs have been identified, with PAR1, PAR3, and PAR4 acting as receptors for thrombin. Human platelets express PAR1 and PAR4, and activation of either is sufficient to trigger platelet aggregation and secretion (2-5). A variety of bioactive substances, including growth factors and chemokines (6-8), are stored in platelets and released during activation. We have reported (9) that endostatin, a potent inhibitor of angiogenesis, is contained within rat platelets and released in response to thrombin via PAR4 in an aggregation-independent manner (10). In studies in rats, we demonstrated that pharmacological manipulation of platelet and͞or serum levels of proangiogenic (VEGF) and antiangiogenic (endostatin) factors resulted in profound effects on healing of gastric ulcers (11,12).Whether human platelets contain endostatin is unknown. Moreover, the relative importance of PAR1 vs. PAR4 in regulating platelet endostatin release has not been reported. In the present study, we have demonstrated that human platelets contain endostatin, and that its release can be triggered by activation of PAR4 but not PAR1. Indeed, PAR1 activation leads to suppression of e...

Hydrogen sulfide is an endogenous mediator that relaxes vascular smooth muscle, exhibits several antiinflammatory activities, and contributes to gastric mucosal defense. This study was performed to examine the role of hydrogen sulfide in the resolution of injury; specifically, the healing of gastric ulcers. Ulcers were induced in rats by serosal application of acetic acid. This elicited a marked increase in gastric expression of the two key enzymes in hydrogen sulfide synthesis (cystathionine-beta-synthase and cystathionine-gamma-lyase) and in hydrogen sulfide synthesis. Twice-daily treatment for a week with hydrogen sulfide donors significantly increased the extent of healing of gastric ulcers as compared to vehicle-treatment. Similar treatment with L-cysteine, a precursor for hydrogen sulfide, also accelerated healing of the ulcers, and the effect was abolished by cotreatment with an inhibitor of cystathionine-gamma-lyase. The beneficial effects of hydrogen sulfide on ulcer healing were not dependent on nitric oxide synthesis, nor did they appear to occur through activation of ATP-sensitive K+ channels. These results suggest that hydrogen sulfide is produced in the gastric mucosa in response to injury and acts to promote healing. The results further suggest that drugs releasing hydrogen sulfide could be employed to accelerate healing of gastric ulcers, and possibly of other wounds.

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