Role of PAR1-Egr1 in the Initiation of Thoracic Aortic Aneurysm in Fbln4-Deficient Mice.

Objective: Remodeling of the extracellular matrix plays a vital role in cardiovascular diseases. Using a mouse model of postnatal ascending aortic aneurysms (termedFbln4(SMKO)), we have reported that abnormal mechanosensing led to aneurysm formation inFbln4(SMKO)with an upregulation of the mechanosensitive transcription factor, Egr1 (Early growth response 1). However, the role of Egr1 and its upstream regulator(s) in the initiation of aneurysm development and their relationship to an aneurysmal microenvironment are unknown. Approach and Results: To investigate the contribution of Egr1 in the aneurysm development, we deletedEgr1inFbln4(SMKO)mice and generated double knockout mice (DKO,Fbln4(SMKO);Egr1(-/-)). Aneurysms were prevented inDKOmice (42.8%) andFbln4(SMKO);Egr1(+/-)mice (26%). Ingenuity Pathway Analysis identified PAR1 (protease-activated receptor 1) as a potential Egr1 upstream gene. Protein and transcript levels of PAR1 were highly increased inFbln4(SMKO)aortas at postnatal day 1 before aneurysm formed, together with active thrombin and MMP (matrix metalloproteinase)-9, both of which serve as a PAR1 activator. Concordantly, protein levels of PAR1, Egr1, and thrombin were significantly increased in human thoracic aortic aneurysms. In vitro cyclic stretch assays (1.0 Hz, 20% strain, 8 hours) using mouse primary vascular smooth muscle cells induced marked expression of PAR1 and secretion of prothrombin in response to mechanical stretch. Thrombin was sufficient to induce Egr1 expression in a PAR1-dependent manner. Conclusions: We propose that thrombin, MMP-9, and mechanical stimuli in theFbln4(SMKO)aorta activate PAR1, leading to the upregulation of Egr1 and initiation of ascending aortic aneurysms.