No abstract

An enzyme-linked immunosorbent assay for plasminogen activator inhibitor-1 (PAI-1) in biologic fluids was developed on the basis of two murine monoclonal antibodies raised against PAI-1 purified from HT- 1080 fibrosarcoma cells. The lower limit of sensitivity of the assay in plasma is 2 ng/mL. The assay is 12 times less sensitive toward the PAI- 1/human tissue-type plasminogen activator (t-PA) complex as compared with free PAI-1. The intraassay, interassay, and interdilution coefficients of variation are 5.2%, 8.0%, and 7.1%, respectively. The level of PAI-1 in platelet-poor plasma of healthy subjects is 18 +/- 10 ng/mL (mean +/- SD, n = 45). In platelet-rich plasma after freezing and thawing, 92% of PAI-1 antigen is released from platelets, whereas only 8% is found in the corresponding platelet-poor plasma. In platelet-poor plasma from healthy subjects, a linear correlation (r = 0.80) was found between PAI activity and PAI-1 antigen. In plasma approximately two thirds of the PAI-1 antigen was functionally active, whereas only 5% of the PAI-1 antigen released from platelets was active. During pregnancy a progressive increase of PAI-1 antigen levels up to three- to sixfold the control value was observed. In plasma of patients with recurrent deep vein thrombosis, PAI-1 levels were 44 +/- 20 ng/mL (mean +/- SD, n = 7), during a clinically silent phase. Four of these patients had a level above 38 ng/mL (mean +/- 2 SD of normal). The present assay, based on stable and reproducible reagents, allows the specific determination of PAI-1 antigen in biologic fluids. It may facilitate interlaboratory comparisons and be useful for further investigations of the role of PAI-1 in clinical conditions associated with impaired fibrinolysis and/or a tendency to thrombosis and investigations of the role of PAI-1 in platelets.

During pregnancy the plasma concentration of two different inhibitors of plasminogen activators (PAIs) increases. The only one found in the plasma of nonpregnant women (PAI1) is immunologically related to a PAI of endothelial cells; its plasma activity, as deduced from the inhibition of single-chain tissue-type plasminogen activator (t-PA), increased from 3.4 +/- 2.3 U/mL (mean +/- 95% confidence limits) in the plasma of nonpregnant women to 29 +/- 7 U/mL at term, and its antigen level, measured by a radioimmunoassay, increased from 54 +/- 17 ng/mL to 144 +/- 25 ng/mL. In pregnancy plasma a second PAI (PAI 2) related to a PAI found in placenta extracts was observed. Its level, quantified with a radioimmunoassay, increased from below the detection limit (approximately 10 ng/mL) in normal plasma to 260 ng/mL at term. One hour after delivery, PAI 1 activities and antigen decreased sharply, but the PAI 2 antigen levels remained constant. Three days later, the PAI 1 antigen levels had fallen to normal levels, but the PAI 2 antigen levels were still at least eightfold above the nonpregnant values. During pregnancy, the t-PA and prourokinase (u-PA) antigen concentrations increased 50% and 200%, respectively, whereas the plasminogen and alpha 2-antiplasmin levels remained constant. Despite the large variations in the levels of PAs and PAIs, the overall fibrinolytic activity as measured in diluted plasma by a radioiodinated fibrin plate assay did not change significantly. Just after delivery, a great increase in the t-PA antigen levels was observed. Three to five days after delivery most parameters of the fibrinolytic system were normal again. Our results demonstrate that during pregnancy and in the puerperium profound alterations of the fibrinolytic system occur that are characterized by increases in PAs and their inhibitors, but these alterations do not affect the overall fibrinolytic activity.

To study the effect of plasminogen activator inhibitors (PAI) on fibrinolysis it is essential to be able to specifically measure these proteins in plasma. To this end PAI-1 was purified from cortisol- stimulated HT 1080 fibrosarcoma cells and antisera raised in rabbits. The immunologic relationship of the purified inhibitor to PAI-1 in plasma and platelet extracts was established by immunoblotting and regular and reverse fibrin zymography. Furthermore, the purified product could be immunoprecipitated with antibodies to human or bovine endothelial cell-derived PAI-1. A radioimmunoassay was developed that measures both free and tissue-type plasminogen activator (t-PA)-bound PAI-1 in plasma and has an effective range of 8 to 250 ng/mL. PAI-1 antigen levels showed a twofold increase after 20 minutes of venous occlusion, partially due to hemoconcentration. Approximately one quarter of PAI-1 before and after venous occlusion is derived from platelets. After correction for hemoconcentration and the contribution of platelets to plasma PAI-1 levels, a still significant increase in PAI-1 levels was noted during venous occlusion, which suggests that the local vascular bed releases PAI-1. Concomitant with PAI-1, t-PA antigen levels increased eightfold and fibrinolytic activity 18-fold after 20 minutes of venous occlusion. PAI-1 and t-PA levels tend to augment with age: in a group of older healthy volunteers (mean age, 53 years) PAI-1 levels were twice and t-PA levels 1.7 times higher than those in a group with a mean age of 29 years. Determination of PAI-1 antigen levels before and after platelet aggregation demonstrated that 85% of PAI-1 in platelet-rich plasma is associated with platelets. The average amount of PAI-1 per platelet was 0.3 fg/platelet, ie, 4,000 molecules per platelet.

Plasminogen activator (PAs) are enzymes that convert the zymogen plasminogen into the trypsin-like protease plasmin, which degrades extracellular matrix proteins and fibrin in the course of fibrinolysis, embryogenesis, tissue remodeling and in tumor metastasis. Plasminogen activator inhibitors (PAIs) are important modulators of PA activity. Several proteins have been identified which inhibit at fast rates urokinase (u-PA) and tissue-type PA (t-PA). In the order of inhibition rate constants these are: a) PAI-1, present in human plasma and platelet extracts and purified from human endothelial cell, fibrosarcoma cell and melanoma cell conditioned media; b) PAI-2, first identified in extracts of human placenta and later also in extracts and conditioned media of human granulocytes and monocytes; and c) protease nexin, a broad specificity protease inhibitor that was first identified and purified from human fibroblasts. We have chosen to use phorbol myristate acetate (30 ng/ml) stimulated histiocytic lymphoma cells (U-937) for the purification of PAI-2. The concentration of PAI-2 in the conditioned media after three days culture in the absence of fetal calf serum is 5 mg/1 and PAI-2 represents 3% of total protein. PAI-2 was purified by a two step procedure consisting of isoelectric focusing and affinity chromatography on Cibacron-Blue agarose. Two forms of PAI-2 were identified: a 47 kDa, nonglycosylated, pi 5.0 form and a 60 kDa glycosylated, pi 4.4 form. Immunctolot analysis and in vivo protein labeling studies under culture conditions that assure 100% viability of the cells showed that the glycosylated Torn is secreted, whereas the 47 kDa, nonglycosylated form remains intracellular. The glycosylation does not affect the activity of the inhibitors since both forms of PAI-2 react with the same rate with u-PA. PAI-2 is a fast inhibitor of u-PA (kl=9×l05M−1s−1) and two-chain t-PA (kl=2×l05) and a rather slow inhibitor of one chain t-PA (kl=l×l02) and of plasmin (kl×l02), but does not inhibit glandular and plasma kallikrein or thrombin. The inhibition spectrum and the kinetics of inhibition clearly distinguish PAI-2 from PAI-1 (kl of reaction with u-PA and two and one chain t-PA above 107) and from protease nexin, that is an efficient inhibitor also of thrombin and plasmin.We have cloned a 1880 Ip fragment of PAI-2 cDNA and determined its nucleotide sequence. The derived acid sequence reveals that PAI-2 is like PAI-1 and protease nexin a member of the serpin family of proteins and contains arginine at its putative active site. In an attenpt to identify parts of the inhibitor proteins that are responsible for conferring PA specificity to PAI-1 and PAI-2 we have compared the primary structures of PAI-1 and PAI-2 with each other and with antithrombin III (AT III). Surprisingly, PAI-2 exhibits no homology with PAI-1 in the region close to the active site except for the active site arginine, whereas, in that region, AT III showed three and seven conserved aminoacids when compared to PAI-1 and PAI-2, respectively. This finding suggests that other regions than those close to the active site contribute to the specificity of PAIs.Plasma concentrations of PAI-2 were measured by a specific radioimmunoassay in over 50 healthy individuals, PAI-2 levels were below detection limit (15 ng/ml) in half of the saitples. Maximal concentrations encountered were in the 30 ng/ml range. PAI-2 measurements in over 300 hospitalized patients demonstrated significantly elevated PAI-2 concentrations only in pregnant women. Measurements in various stages of pregnancy showed a steady increase of PAI-2 from below detection limit in nonpregnant women to values of 250 ng/ml at term and of PAI-1 frcm 25 ng/ml to 150 ng/ml. Unlike to PAI-1 concentrations that normalize rapidly after delivery, PAI-2 concentrations remain significantly elevated for several days.