Retinal pigment epithelial cells produce a latent fibrinolytic inhibitor that is antigenically and biochemically related to type 1 plasminogen activator inhibitor produced by vascular endothelial cells

Conditioned media from retinal pigment epithelial (RPE) cells in culture contain active and latent plasminogen activator inhibitors (PAIs). Latent activity is unmasked by denaturants and accounts for the vast majority of total inhibitor activity. Activation by denaturants is an unusual characteristic previously described for PAI-1, the inhibitor produced by vascular endothelial cells. This property is not shared by PAI-2 or protease nexin. Reverse fibrin autography demonstrates that the PAI activity in RPE-conditioned media (RPE-CM) comigrates with purified endothelial cell-derived PAI-1 and has an apparent M_r of 50 000. Immunoblotting with a monospecific antiserum directed against endothelial cell-derived PAI-1 demonstrates a cross-reacting protein in RPE-CM at 50 kDa, and this same antiserum is able to immunoprecipitate a 50 kDa protein from [³⁵S]methionine-labeled RPE-CM. These data suggest that RPE cells produce a PAI that is biochemically and immunologically related to PAI-1.