Sexually dimorphic role of smooth muscle cell estrogen receptor alpha in aging‐associated arterial stiffness

Aging is associated with rising arterial stiffness (AS), an independent risk factor for cardiovascular disease. The development of aging-associated AS follows a sexually dimorphic pattern with women developing AS later in life. Estrogen and estrogen receptor alpha (ERα) are protective in the premenopausal state, potentially due to the regulation of the vasoprotective angiotensin II type 2 receptor (AT2R), however, the role of estrogen/ERα in aging-associated AS has not been defined and the mechanisms of the sexually-dimorphic development of aging-associated AS are unknown.All studies were conducted in accordance with the FASEB Statement of Principles for the use of Animals in Research and Education. We generated novel smooth muscle cell-specific ERα-KO mice and measured aortic pulse wave velocity (PWV), an index of AS, in 3, 12, and 18 month old male and female intact and KO mice. Male mice develop increases in AS from 3 to 12 months compared to 3 to 18 months in females (p<0.05). SMC-ERα-KO protected females from the aging-associated increase in AS (p<0.05). Circulating estrogens also declined in 18 month old females vs. 3 mo. (p<0.05), suggesting a role for unliganded SMC-ERα in aging-associated AS. SMC-ERα-KO females were also protected from the aging-associated decline in aortic AT2R expression observed in intact females (p<0.05). To further investigate the potential role of unliganded SMC-ERα in AS, we performed ovariectomy (OVX) or sham surgeries in young female SMC-ERα-Intact and KO mice and measured PWV. Results demonstrated an OVX-driven increase in AS that was prevented by SMC-ERα-KO (p<0.05). This was also associated with an OVX-mediated decrease in aortic expression of the AT2R, which was prevented by SMC-ERα-KO. To determine the in vivo role of AT2R in the vascular protection observed in 12 month old female mice, we treated 12 month old male and female mice with AT2R antagonist PD-123319 (3mg/kg/day) or vehicle via osmotic minipumps for 4 weeks. Chronic inhibition of the AT2R increased AS in 12 month old females (p<0.05), but not males, suggesting that AT2R-mediated protection contributes to the delay in aging-associated AS in female mice. In vitro studies also demonstrated that estrogen upregulates AT2R mRNA and protein in female mouse aortic SMC, supporting an estrogen-mediated mechanism of AT2R driven vascular protection in females.These data indicate sex differences in the time course of the development of aging-associated arterial stiffness and suggest the presence of unliganded SMC-ERα contributes specifically to aging-associated arterial stiffness in females via reductions in vascular AT2R. Further, these results implicate that AT2R agonist therapy may improve cardiovascular outcomes in aging women to retain the beneficial downstream effects of estrogen signaling, without the potential detrimental effects of hormone replacement therapy.