Murine monoclonal antibodies against active site epitopes on human endothelial plasminogen activator inhibitor (PAI-1)

Study of recombinant antibody fragments and PAI-1 complexes combining protein-protein docking and results from site-directed mutagenesis

Elevated plasma levels of plasminogen activator inhibitor-1 (PAI-1) have been correlated with cardiovascular diseases such as myocardial infarction and venous thrombosis. PAI-1 has also been shown to play an important role in tumor development, diabetes, and obesitas. Monoclonal antibodies MA-8H9D4 and MA-56A7C10, and their single-chain variable fragments (scFv), exhibit PAI-1-neutralizing properties. In this study, a rigid-body docking approach is used to predict the binding geometry of two distinct conformations of PAI-1 (active and latent) in complex with these antibody fragments. Resulting models were initially refined by using the dead-end elimination algorithm. Different filtering criteria based on the mutagenesis studies and structural considerations were applied to select the final models. These were refined by using the slow-cooling torsion-angle dynamic annealing protocol. The docked structures reveal the respective epitopes and paratopes and their potential interactions. This study provides crucial information that is necessary for the rational development of low-molecular weight PAI-1 inhibitors.

Epitopes on plasminogen activator inhibitor type-1 important for binding to tissue plasminogen activator

The molecular details of the rapid complex formation between tissue plasminogen activator (tPA, E.C. 3.4.21.68) and plasminogen activator inhibitor type-1 (PAI-1) are still not fully elucidated. We have used surface plasmon resonance (SPR), the BIAcore, to characterize the binding of a large panel of monoclonal antibodies to four forms of recombinant human PAI-1, including active and latent PAI-1 as well as the complex between PAI-1 and recombinant human tc tPA or the protease part of tPA, the B-chain. Antibodies that discriminate between these different forms of PAI-1 have been identified, which is reflected by differences in k(a), k(d) as well as in Kd. In addition, in a chromogenic assay with PAI-1 and tPA we determined the IC50-values for these antibodies, i.e., studied their ability to inhibit the decrease in tPA-activity caused by PAI-1. In a competition assay using SPR, we have also been able to study whether concurrent binding of these antibodies to PAI-1 was possible. We could thereby assign the antibodies to five groups according to their binding areas. Furthermore, by using this technique, we have for the first time been able to identify three distinct epitopes on PAI-1, which are all of importance for the interaction and complex-formation with tPA. Since the antibodies that bind to one of these areas all have very poor affinity for the complex between PAI-1 and tPA, we suggest that this not previously described epitope must be located near the final binding site for tPA in this complex. Altogether, this also supports the theory of a multistep reaction between PAI-1 and tPA, in which tPA interacts with different parts of the PAI-1-molecule.

Distribution of plasminogen activator inhibitor in normal liver, cirrhotic liver, and liver with metastases

AIMS

To examine the distribution of PAI-1 antigen in normal and cirrhotic liver and liver with metastases.

METHODS

Sections of normal and cirrhotic liver and liver with metastases were stained using the alkaline phosphatase antialkaline phosphatase (APAAP) technique and monoclonal antibody specific for plasminogen activator inhibitor (PAI-1).

RESULTS

PAI-1 antigen was identified as discrete granules in the cytoplasm of hepatocytes in normal liver, particularly around portal tracts and central veins of the liver lobule. In cirrhotic liver a striking reduction of PAI-1 antigen was noted. In liver with metastases increased amounts of PAI-1 antigen were concentrated in hepatocytes around the margins of malignant deposits.

CONCLUSIONS

Cirrhotic liver contains considerably less PAI-1 antigen than does normal liver, despite raised plasma concentrations of PAI-1. This may reflect release of hepatic PAI-1 into the circulation or decreased clearance of PAI-1 from the plasma. Secondary malignant deposits in the liver seem to stimulate production of PAI-1 in adjacent hepatocytes. This may influence the invasive process and may contribute to the thrombotic tendency associated with malignancy.

Distribution of plasminogen activator inhibitor (PAI-1) in tissues

Extracts of human tissue were analysed for plasminogen activator inhibitor (PAI-1) antigen and activity. PAI-1 was localised in tissues by an immunochemical method, using monoclonal antibodies. PAI-1 occurred throughout the body; its concentration and activity differed considerably from organ to organ. Extracts of liver and spleen had the greatest abundance of PAI-1, but the activity of the inhibitor was much higher in liver than in spleen: the liver may be a source of plasma PAI-1. Immunochemical staining for PAI-1 was observed in endothelium, platelets and their precursor cells, the megakaryocytes, and locations central to the process of haemostasis. PAI-1 also occurred in neutrophil polymorphs and macrophages, cells important in inflammatory and immune processes, but not in lymphocytes. Other cell types, in particular, vascular smooth muscle cells and mesangial cells, also stained positively for PAI-1 and such cells seem to represent an important reservoir of PAI-1.