In vivo patterns of expression of urokinase and its inhibitor PAI-1 suggest a concerted role in regulating physiological angiogenesis

To evaluate the role of plasminogen activators (PAs) in physiological angiogenesis, we have investigated the in vivo patterns of expression of urokinase-type PA (uPA) and PA-inhibitor type 1 (PAI-1) during neovascularization of ovarian follicles, the corpus luteum, and the maternal decidua. Using in situ hybridization, we detected uPA mRNA in the ovary along the route of capillary extension, originating at the existing ovarian vasculature, extending toward growing follicles, and terminating at the newly formed capillary sheaths surrounding each growing follicle. Following ovulation, uPA mRNA was expressed in capillary sprouts within the developing corpus luteum. During the process of decidual neovascularization, uPA expression was detected in endothelial cell cords traversing the maternal decidua in the direction of the newly implanted embryo. uPA mRNA was not detected in endothelial cells upon completion of neovascularization, suggesting that uPA expression is a part of the angiogenic response. During in vitro "angiogenesis" of cultured aortic explants, uPA was expressed in capillary sprouts but not in underlying endothelial cell sheets, suggesting that the expression of uPA depends on the histological context of the endothelial cell. Interestingly, during corpus luteum development and decidual neovascularization, and in aortic explants, PAI-1 expression was preferentially activated in cells in the vicinity of uPA-expressing capillary-like structures. These findings suggest a functional interplay between uPA- and PAI-1-expressing cells and support the idea that natural PA inhibitors function during angiogenesis to protect neovascularized tissues from excessive proteolysis.