Inactivation of Plasminogen Activator Inhibitor-1 by RNA Aptamer Molecules
Authors: Fortenberry Yolanda , Damare Jared 
Published: October 2019

Abstract
Abstract 1107

Introduction:
The serine protease inhibitor (serpin) plasminogen activator inhibitor-1 (PAI-1), binds and inhibits the following plasminogen activators: tissue-type plasminogen activator (tPA), and urokinase-type plasminogen activator (uPA). This decreases plasmin production and triggers the dissolution of fibrin clots. Elevated levels of PAI-1 have been correlated with an increased risk for cardiovascular disease, as well as obesity and metabolic syndrome. Consequently, pharmacologically suppressing PAI-1 might prevent, or successfully treat vascular disease. Several PAI-1 small molecule inhibitors have recently been studied (PAI-039 is the best characterized). Since PAI-1 is a multifunctional protein, completely inhibiting PAI-1 may hinder its other functions. Therefore, it is important to independently develop inhibitors to the various regions of PAI-1. This can be accomplished by using small RNA molecules (aptamers) that bind with high affinity and specificity to individual protein domains. We recently published a paper showing how PAI-1 specific RNA aptamers bind to the heparin/vitronectin binding site of PAI-1 (Blake et al., 2009). We demonstrated that PAI-1 specific aptamers prevent cancer cells from detaching from vitronectin (in the presence of PAI-1), resulting in increased cell adhesion. These aptamers had no effect on PAI-1's other functions, particularly its antiproteolytic activity.

Objective:
This study's goal was to develop RNA aptamers to the active site of PAI-1; thereby, preventing the ability of PAI-1 to interact with plasminogen activators (tPA and uPA).

Methods:
The aptamers were generated by the systematic evolution of ligands by exponential enrichment (SELEX). Adopting the SELEX in vitro selection technique ensures the creation of nuclease-resistant RNA molecules that will bind to target proteins. We used in vitroassays to determine the effect of the aptamers on the interaction of PAI-1 with both tPA and uPA.

Results:
We isolated a family of aptamers that bind to wild-type PAI-1 with affinities in the nanomolar range. From this family, two aptamer clones (10–2 and 10–4) exhibited reduced binding to elastase cleaved PAI-1 and the PAI-1/tPA complex. This suggests that they bind to, or in the vicinity of, the active site. Using a chromogenic assay, we showed that the aptamer clone 10–4, and (to a lesser extent) the aptamer clone 10–2, inhibited PAI-1's antiproteolytic activity against tPA, further suggesting that these clones bind to PAI-1 within its active site region. Interestingly, neither clone was able to prevent PAI-1 from inhibiting uPA activity. Both aptamer clones disrupted PAI-1's ability to form a stable covalent complex with tPA. Increasing aptamer concentrations positively correlated with an increase in cleaved PAI-1, suggesting that these aptamer clones convert PAI-1 from an inhibitor to a substrate. Furthermore, we showed that both aptamer clones are able to inhibit PAI-1's activity in the presence of vitronectin.

Conclusions:
We have shown that we are able to inhibit one of PAI-1's functions without hindering its other functions. To our knowledge, this is the first report of an RNA molecule that is able to inhibit the antiproteolytic activity of PAI-1. We have generated two specific RNA aptamer molecules that hinder the ability of PAI-1 to interact with tPA, which has the potential to be used as an antithrombotic agent.

Disclosures:
No relevant conflicts of interest to declare.