Expression of "relaxed" conformation of pai-2 and lrp in human term gestational tissues

The relaxed conformation of plasminogen activator inhibitor-2 (PAI2r) occurs after interaction of active PAI-2 with bovine thrombin, the plasminogen activators (uPA, tPA) or insertion of a P, M reactive site loop peptide |Aundei1 a II- wbinilied]. The presence of PAI-2r in tissues, determined using a MAb specific for PAl-2r (2H5), may be taken as indicative of in situ inhibition of serine protease-mediated proteolysis by PAI-2. In addition, it is known that relaxed serpins function as ligands for the clearance receptor, low density lipoprotein receptor-related protein (LRP), promoting the intemalisation of uPA-PAI complexes from the cell surface. Internalisation of receptor-bound proteases may be an important criterion determining the invasive cell phenotype. This study characterises the immunohistochemical localisation of PAI2r in human term gestauonal tissues (amnion, choriodecidua and placenta) and establishes its expression with other components of the uPA cascade. The results obtained indicate that PAI-2r immunoreactivity was exclusively localised to amnion epithelial cells, with only minimal staining in the underlying chorion. Although PAI-2 antigen is present (using ADI "3750), no PAI-2r immunoreactivity was detectable in any of the trophoblastic tissues examined (villous and extravillous). Immunoreactive LRP was confined to trophoblasts of the chorion, villous and extravillous tissue, whereas staining in the amnion was absent. For the first time, localisation of PAI-2r at the tissue level has been identified. In addition, the data obtained are consistent with the hypothesis that cells of invasive phenotype, although expressing all components of the uPA cascade, do not accumulate immunoreactive PAI-2r, since it is rapidly internalised from the cell surface. Conversely, cells of non-invasive phenotype may only accumulate PAI-2r immunoreactivity in the absence of LRP expression. We propose, therefore, that the presence of PAI-2r combined with the absence of LRP at the cell surface are putative markers of non-invasive phenotypes in cells that express pAl-2[Tsatas et al J. Histochem Cytochem., in press].