COORDINATED INDUCTION OF PLASMINOGEN-ACTIVATOR INHIBITOR-1 (PAI-1) AND INHIBITION OF PLASMINOGEN-ACTIVATOR GENE-EXPRESSION BY HYPOXIA PROMOTES PULMONARY VASCULAR FIBRIN DEPOSITION

Oxygen deprivation, as occurs during tissue ischemia, tips the natural anticoagulant/procoagulant balance of the endovascular wall to favor activation of coagulation. To investigate the effects of low ambient o xygen tension on the fibrinolytic system, mice were placed in a hypoxi c environment with pO(2) < 40 Torr, Plasma levels of plasminogen activ ator inhibitor-1 (PAI-1) antigen, detected by ELISA, increased in a ti me-dependent fashion after hypoxic exposure (increased as early as 4 h , P < 0.05 vs, normoxic controls), and were accompanied by an increase in plasma PAI-1 activity by 4 h (P < 0.05 vs. normoxic controls). Nor thern analysis of hypoxic murine lung demonstrated an increase in PAI- 1 mRNA compared with normoxic controls; in contrast, transcripts for b oth tissue-type plasminogen activator (tPA) and urokinase-type plasmin ogen activator (uPA) decreased under hypoxic conditions. Immunocolocal ization studies identified macrophages as the predominant source of in creased PAI-1 within hypoxic lung. Using a transformed murine macropha ge line, striking induction of PAI-1 transcripts occurred under hypoxi c conditions, due to both increased de novo transcription as well as i ncreased mRNA stability. Consistent with an important role of the fibr inolytic system in hypoxia-induced fibrin accumulation, PAI-1 +/+ mice exposed to hypoxia exhibited increased pulmonary fibrin deposition ba sed upon a fibrin immunoblot, intravascular fibrin identified by immun ostaining, and increased accumulation of I-125-fibrinogen/fibrin in hy poxic tissue. In contrast, mice deficient for the PAI-1 gene(PAI-1 -/- ) similarly exposed to hypoxic conditions did not display increased fi brin accumulation compared with normoxic PAI-1 +/+ controls. Furthermo re, homozygous null uPA (uPA -/-) and tPA (tPA -/-) mice subjected to oxygen deprivation showed increased fibrin deposition compared with wi ldtype controls. These studies identify enhanced expression of PAI-1 a s an important mechanism suppressing fibrinolysis under conditions of low oxygen tension, a response which may be further amplified by decre ased expression of plasminogen activators. Taken together, these data provide insight into an important potential role of macrophages and th e fibrinolytic system in ischemia-induced thrombosis.