Plasminogen Activator Inhibitor Type 1 Production by Rat Type II Pneumocytes in Culture

Co-secretion of plasminogen activator inhibitor type 1 (PAH) and urokinase-type plasminogen activator was identified in short-term cultures of primary type II pneumocytes isolated from adult rats. After separation by sodium dodecyl sulfate (SDS)-PAGE and reverse fibrin autography (reverse FA) of serum-free conditioned medium (SFCM), cellular lysate, and extracellular matrix (ECM), the inhibitor was seen as a zone of spared lysis at an apparent molecular mass of 46 to 48 kD. The plasminogen activator (PA) activity could only be visualized when human instead of bovine fibrin was used in the indicator gel. It presented as a single band of lysis at an apparent molecular mass of 45 kD when tested by regular FA and was found adjacent to PAI-1 when examined by reverse FA. Immunoblot analysis of type II pneumocyte SFCM, cellular lysate, and ECM revealed two bands at 46 and 48 kD, consistent with the apparent molecular masses (Mr) reported for rat PAI-1 from HTC hepatoma cells. Type II pneumocyte PAI-1 formed SDS-resistant complexes with tissue-type and urokinase-type plasminogen activator and was found to be stable to acid, to short-term exposure to heat, and to the denaturants guanidine HCl and SDS, while being sensitive to treatment with alkali and urea. When levels of type II pneumocyte PAI-1 activity were monitored over time during short-term culture conditions, the level of PAI-1 in SFCM remained stable, whereas activity in the lysate accumulated and activity in the ECM declined. The PAI-1 present in SFCM remained active over prolonged periods of time, and only minimal latent form, which could be activated by detergent, was found. We speculate that co-secretion of stabilizing factors such as negatively charged phospholipids or the association with vitronectin keep the secreted PAI-1 in active form. PAI-1 is a novel type II pneumocyte product that may play an important role in the pathogenesis of pulmonary fibrin deposition.