Processing of type 1 plasminogen activator inhibitor (PAI-1) into the regulated secretory pathway

To understand the processing of type 1 plasminogen activator inhibitor (PAI-1) into the storage granules of platelets, we utilized a eukaryotic expression vector (pRC/CMV) to transfer the human cDNA for PAI-1 into AtT-20 cells, a mouse pituitary cell line known to sort proteins in a regulated fashion. Immunofluorescence staining of PAI-1-transfected AtT-20 clones revealed co-localization 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 resulted in the release of both active PAI-1 and the latent form. In comparison, PAI-1-transfected Chinese hamster ovary cells (i.e. a nonpackaging 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 revealed a codistribution of PAI-1 and ACTH in cellular compartments of the same density. The half-life of PAI-1 activity at 37 degrees C was prolonged 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 directs this inhibitor into the storage secretory pathway.