Tissue type plasminogen activator (tPA) is a key enzyme in the fibrinolytic cascade. In this paper we report that tPA contains 2 independent epitopes that exert opposite effects on blood vessel tone. Low concentrations of tPA (1 nM) inhibit the phenylephrine (PE)-induced contraction of isolated aorta rings. In contrast, higher concentrations (20 nM) stimulate the contractile effect of PE. The 2 putative vasoactive epitopes of tPA are regulated by the plasminogen activator inhibitor-1 (PAI-1) and by a PAI-1-derived hexapeptide that binds tPA. TNK-tPA, a tPA variant in which the PAI-1 docking site has been mutated, stimulates PE-induced vasoconstriction at all concentrations used. The stimulatory, but not the inhibitory, effect of tPA on the contraction of isolated aorta rings was abolished by anti-low-density lipoprotein receptor-related protein/␣ 2 -macroglobulin receptor (LRP) antibodies. Administering tPA or TNK-tPA to rats regulates blood pressure and cerebral vascular resistance in a dose-dependent mode. In other in vivo experiments we found that the vasopressor effect of PE is more pronounced in tPA knockout than in wildtype mice. Our findings draw attention to a novel role of tPA and PAI-1 in the regulation of blood vessel tone that may affect the course of ischemic diseases.

Accumulation of low-density lipoprotein (LDL)-derived cholesterol by macrophages in vessel walls is a pathogenomic feature of atherosclerotic lesions. Platelets contribute to lipid uptake by macrophages through mechanisms that are only partially understood. We have previously shown that platelet factor 4 (PF4) inhibits the binding and degradation of LDL through its receptor, a process that could promote the formation of oxidized LDL (ox-LDL). We have now characterized the effect of PF4 on the binding of ox-LDL to vascular cells and macrophages and on the accumulation of cholesterol esters. PF4 bound to ox-LDL directly and also increased ox-LDL binding to vascular cells and macrophages. PF4 did not stimulate ox-LDL binding to cells that do not synthesize glycosaminoglycans or after enzymatic cleavage of cell surface heparan and chondroitin sulfates. The effect of PF4 on binding ox-LDL was dependent on specific lysine residues in its C terminus. Addition of PF4 also caused an ϳ10-fold increase in the amount of ox-LDL esterified by macrophages. Furthermore, PF4 and ox-LDL co-localize in atherosclerotic lesion, especially in macrophage-derived foam cells. These observations offer a potential mechanism by which platelet activation at sites of vascular injury may promote the accumulation of deleterious lipoproteins and offer a new focus for pharmacological intervention in the development of atherosclerosis.

Urokinase plasminogen activator (uPA) is a multifunctional protein that has been implicated in several physiological and pathological processes involving cell adhesion and migration in addition to fibrinolysis. In a previous study we found that two-chain urokinase plasminogen activator (tcuPA) stimulates phenylephrine-induced vasoconstriction of isolated rat aortic rings. In the present paper we report that uPA ؊/؊ mice have a significantly lower mean arterial blood pressure than do wild type mice and that aortic rings from uPA ؊/؊ mice show an attenuated contractile response to phenylephrine. In contrast, the blood pressure of urokinase receptor knockout (uPAR ؊/؊ ) mice and the response of their isolated aortic rings to phenylephrine were normal, indicating that the effect of uPA on vascular contraction is independent of uPAR. Addition of mouse and human uPA almost completely reversed both the impaired vascular contractility and the lower arterial blood pressure in vivo. The in vitro and in vivo effects of infused uPA on aortic contractility and the restoration of normal blood pressure in uPA ؊/؊ mice were prevented by antibody to low-density lipoprotein receptor-related protein/␣ 2 -macroglobulin receptor (LRP). A modified form of uPA that lacks the kringle failed to restore the blood pressure in uPA ؊/؊ mice, notwithstanding having a longer half-life in the circulation. Ligands that regulate the interaction of uPA with LRP, such as PAI-1 or the PAI-1-derived peptide (EEIIMD), abolished the vasoactivity of tcuPA in vitro and in vivo. These studies identify a novel signal transducing cellular receptor pathway involved in the regulation of vascular contractility.Urokinase plasminogen activator (uPA) 1 is a multifunctional protein that has been implicated in several physiological and pathological processes, including fibrinolysis. Transgenic mice with a targeted disruption in the uPA gene (uPA Ϫ/Ϫ ) are prone to form thrombi when exposed to endotoxin (1) or hypoxia (2, 3) or when the uPA gene is disrupted in otherwise healthy tPA Ϫ/Ϫ mice (1, 4). We recently reported that clearance of pulmonary microemboli in uPA Ϫ/Ϫ mice is delayed, despite the presence of an intact tPA system (5).uPA has also been implicated in other pathophysiological processes, such as pulmonary inflammation and repair, in which the relationship to fibrinolysis is less clear. For example, uPA Ϫ/Ϫ mice are more susceptible to lethal pulmonary infection (6) and to the development of pulmonary fibrosis (7), endpoints that might reflect contribution of uPA in cell adhesion (8) and migration to these phenotypes. However, the mechanism by which uPA is involved in these processes has not been established.In a previous study we found that uPA enhances phenylephrine and endothelin-induced vasoconstriction of aortic rings isolated from rats (9). The possibility that uPA contributes to the regulation of vascular tone may help to explain some of the phenotypic changes described in uPA Ϫ/Ϫ mice and perhaps provide a broader understanding of the role pl...

We have previously identified ␣-defensin in association with medial smooth muscle cells (SMCs) in human coronary arteries. In the present paper we report that ␣-defensin, at concentrations below those found in pathological conditions, inhibits phenylephrine (