HUMAN GLIOMA U-251 CELLS CONTAIN TYPE-1 PLASMINOGEN-ACTIVATOR INHIBITOR IS A RAPIDLY RELEASABLE FORM

Because recent information suggests that the localized deposition of p rotease inhibitors is one mechanism by which cells regulate pericellul ar proteolysis during tissue invasion, the distribution of type 1 plas minogen activator inhibitor (PAI-1) associated with the invasive human glioma cell line U-251 mas investigated. Direct and reverse fibrin zy mography indicated the presence of urokinase-like plasminogen activato r (u-PA) and PAI-1 in U-251 conditioned media and cell lysates. PAI-1 antigen was detected immunologically in cytoplasmic granules present w ithin cellular processes of U-251 cells and these organelles could be isolated on Percoll density gradients in a high density band. In contr ast, u-PA activity and another secreted protein, amyloid beta-protein precursor, were only present in the low density region of the gradient s, Functional analysis of PAI-1 in the granules contained within the h igh density fractions revealed the presence of active PAI-1. Incubatio n of U-251 cells with the secretagogue, 8-bromoadenosine 3':5'-cyclic monophosphate, resulted in a 3-fold increase in the release of PAI-1 i n the media conditioned by these cells, These data suggest that the hu man glioma cell line U-251 contains PAI-1 in a rapidly releasable form , which may provide another mechanism by which these tumors could regu late proteolytic activity in a localized manner.