AbstractPlasminogen activators (PAs) regulate a variety of processes involved in tissue morphogenesis and differentiation. We used a coculture system in which microvascular endothelial cells are induced by glial cells to form capillary‐like structures in order to examine the role of urokinase‐type PA (uPA) during microvessel morphogenesis within the central nervous system (CNS). Endotheliaderived uPA activity decreased sevenfold within glial‐endothelial cocultures when capillary‐like structures were formed. Incubation of cocultures with concentrations of phorbol 12‐myristate 13‐acetate (0.1 and 1.0 nM) that induced endothelial uPA activity (45–210%) inhibited endothelial differentiation (25–70%). Furthermore, incubation of cocultures with proteolytically active low molecular weight uPA (5–500 IU/ml) inhibited endothelial differentiation (37–75%), whereas the amino terminal cell‐binding fragment of uPA had minimal effect. Inhibition of plasminogen activation in cocultures with the serine protease/plasmin inhibitors aprotinin and soybean trypsin inhibitor increased glia‐induced capillary‐like structure formation (96–98%). These findings establish a paracrine/autocrine function for urokinase and its inhibitors in regulating endothelial responses to perivascular glia and provide insight into mechanisms of microvascular reactions to CNS pathol
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