Abstract Body: Background:
Plasminogen activator inhibitor-1 (PAI-1), the primary regulator of fibrinolysis, is highly abundant in human thoracic aortic aneurysms (TAA). PAI-1 is also increased in the ascending aortas of mice infused with angiotensin II (AngII) prior to overt pathology. The purpose of this study was to determine whether deletion of PAI-1 influenced the development of TAAs during AngII infusion.

Methods and Results:
Whole-body PAI-1 deficient mice (PAI-1-/-) and wild type littermates (PAI-1+/+) were infused with AngII (1,000 ng/kg/min) for 28 days to induce TAA. Despite the upregulation of PAI-1 in ascending aortas, aortic diameters were not altered by PAI-1 deficiency. However, PAI-1 deficiency augmented grossly visible and histologically evident cardiac fibrosis, predominantly within the epicardium and posterior septum. Ferric iron, indicative of prior hemorrhage, was observed coincidently with cardiac fibrosis in PAI-1-/- mice. To verify the presence of hemorrhage, we infused PAI-1+/+ and -/- mice with AngII for 1 and 7 days. Hemorrhage was observed in PAI-1-/- mice as early as 1 day by in situ imaging and TER-119 staining and was accompanied by myocyte loss defined by troponin I staining. By 7 days, cardiac hemorrhage and myocyte loss were increased in PAI-1-/- mice. The pathologies observed at 1 and 7 days of AngII were pronounced within the epicardium and posterior septum, spatially preceding observed fibrosis at 28 days of AngII. Similar hemorrhagic and fibrotic phenotypes were observed in PAI-1-/- mice infused with a pressor dose of norepinephrine (5.6 mg/kg/day in 0.2% w/v L-ascorbic acid) for 7 and 28 days, respectively. To investigate mechanisms underlying cardiac hemorrhage, PAI-1+/+ and -/- mice were infused with saline or AngII for 7 days. Mid-ventricular tissue was analyzed by bulk RNA sequencing with hemorrhage included as a binary covariate. Differential expression and gene set enrichment analyses identified upregulation of genes related to extracellular matrix organization and proteolysis in AngII-infused PAI-1-/- hearts, including Fn1, Ecm1, Lgals3, and Ctss. These results suggest that PAI-1 deficiency induced transcriptional signatures of wound healing after AngII infusion, even in the absence of coincident hemorrhage.

Conclusion:
PAI-1 deficiency does not affect TAA formation, but augments cardiac fibrosis in AngII-infused mice. Cardiac hemorrhage, in association with myocyte loss, precedes fibrosis in PAI-1 deficiency.