Background: Abdominal aortic aneurysm (AAA) is a potentially lethal vascular disease lacking non-surgical treatments. AAA is associated with decreased expression of estrogen receptor alpha (ERα) in the aorta. The significance of this finding and the mechanisms by which lack of ERα signaling contributes to AAA pathogenesis have not been examined. In this investigation, we test the hypothesis that ERα signaling protects against AAA by decreasing endothelin-1 (ET-1) production and regulating inflammation.

Methods: AAA was induced in endothelial cell-specific ERα knockout (eERαKO) male mice and their littermates (ERαFL2) (n=6-11) by administering β-Aminopropionitrile (BAPN) via drinking water (1 mg/ml, 0.15 g/kg/d) 3 days before angiotensin II (Ang II; 1 µg/kg/day) infusion for 2 weeks. Development of AAA was assessed by in vivo ultrasound measurement of aortic luminal dilation. Relevant molecular mechanisms were examined in abdominal aortic tissues, human aortic endothelial cells (HAECs) and mouse bone marrow-derived macrophages (BMDMs).

Results: BAPN/Ang II increased abdominal aortic diameter in ERαFL2 and eERαKO mice compared to control. Lack of ERα further augmented the BAPN/Ang II-induced increase in aortic diameter and AAA incidence compared to ERαFL2 mice. Immunohistochemical staining revealed that BAPN/Ang II co-infusion significantly upregulates ET-1 expression and macrophage infiltration of the abdominal aorta in eERαKO mice compared to ERαFL2 mice. In vitro studies in HAECs demonstrated that treatment with ERα agonist, β-estradiol (100 nM, 24 h), decreases TNFα (15 ng/ml)-induced ET-1 production, while treatment with ERα-specific antagonist, methylpiperidinopyrazole (1 µM, 24 h), increases ET-1 production. Additionally, BMDMs obtained from the eERαKO mice exhibited increased production of the pro-inflammatory cytokine monocyte chemoattractant protein-1 compared to macrophages from ERαFL2 mice.

Conclusion: Our findings demonstrate a protective effect of endothelial ERα signaling against AAA in male mice likely through decreased ET-1 production and reduced pro-inflammatory macrophage activation of the aorta. Further studies are needed to delineate the crosstalk between endothelial ET-1 with macrophage phenotype and their impact in AAA pathogenesis.