PROCESSING OF TYPE-1 PLASMINOGEN-ACTIVATOR INHIBITOR (PAI-1) INTO THEREGULATED SECRETORY PATHWAY

To understand the processing of type 1 plasminogen activator inhibitor (PAI-1) into the storage granules of platelets, we utilized a eukaryo tic expression vector (pRC/CMV) to transfer the human cDNA for PAI-1 i nto AtT-20 cells, a mouse pituitary cell line known to sort proteins i n a regulated fashion. Immunofluorescence staining of PAI-1-transfecte d AtT-20 clones revealed colocalization of PAI-1 with an endogenously produced and stored hormone (i.e. adrenocorticotropic hormone, ACTH). Stimulation of PAI-1-transfected AtT-20 cells with a secretagogue resu lted in the release of both active PAI-1 and the latent form. In compa rison, PAI-1-transfected Chinese hamster ovary cells (i.e. a nonpackag ing cell line) did not release PAI-1 in response to a secretagogue and exhibited immunoreactivity for PAI-1 primarily confined to the Golgi region. Percoll density gradient fractionation of AtT-20 cells reveale d a codistribution of PAI-1 and ACTH in cellular compartments of the s ame density.The half-life of PAI-1 activity at 37-degrees-C was prolon ged in intact granules (t1/2, 5 h) in comparison with its half-life in lysed granules (t1/2, 2 h). These studies demonstrate the presence of a new functional property associated with the PAI-1 molecule that dir ects this inhibitor into the storage secretory pathway.