Mechanisms of trophoblast invasiveness and their control: the role of proteases and protease inhibitors.

Implantation and subsequent placental development in many species including the human are dependent on trophoblast invasion of the uterine epithelium, the underlying basement membrane, connective tissue and blood vessels. However, trophoblast invasion in situ is strictly controlled by the microenvironment provided by the pregnant uterus. Key mechanisms underlying various steps in trophoblast invasion of basement membrane and stroma are similar to those identified in the case of invasive tumor cells: (a) attachment to basement membrane by binding to laminin and possibly other basement membrane components; (b) detachment from the basement membrane matrix prior to its penetration, a process that requires the presence of complex-type oligosaccharides on the cell surface; (c) breakdown of basement membrane components by trophoblast-derived metalloproteases (type IV and interstitial collagenase) and serine proteases (plasminogen activator). Type IV collagenase activity is stimulated by binding to laminin, a molecule also secreted by the trophoblast. Activation of trophoblast-derived metalloproteases appears to be plasmin-dependent. Plasmin results from the cleavage of plasminogen by trophoblast-derived plasminogen activator. Control of trophoblast invasion in situ is mediated by decidua-derived transforming growth factor beta (TGF beta) which in turn induces tissue inhibitor of metalloproteases (TIMP) both in the decidua and the trophoblast. We suggest that this control of trophoblast invasiveness is regulated both spatially as well as temporally during gestation. A preprogrammed decline in trophoblast invasiveness with increasing gestational age remains an additional possibility. The nature of the loss of control of trophoblast invasiveness in choriocarcinoma remains to be identified. Refractoriness to TGF beta action remains to strong possibility.