Estrogen protects against intracranial aneurysm rupture in ovariectomized mice

Abstract

Clinical observations suggest that postmenopausal women have a higher incidence of aneurysmal rupture than premenopausal women. We hypothesize that a relative deficiency in estrogen may increase the risks of aneurysmal growth and subarachnoid hemorrhage in postmenopausal women. We assessed the effects of estrogen and selective estrogen receptor subtype agonists on the development of aneurysmal rupture in ovariectomized female mice. We used an intracranial aneurysm mouse model that recapitulates the key features of human intracranial aneurysms, including spontaneous rupture. Ten- to 12-week-old ovariectomized female mice received treatment with estrogen, nonselective estrogen receptor antagonist, estrogen receptor-α agonist, or estrogen receptor-β agonist starting 6 days after aneurysm induction so that the treatments affected the development of aneurysmal rupture without affecting aneurysmal formation. Estrogen significantly reduced the incidence of ruptured aneurysms and rupture rates in ovariectomized mice. Nonselective estrogen receptor antagonist abolished the protective effect of estrogen. Although estrogen receptor-α agonist did not affect the incidence of ruptured aneurysms or rupture rates, estrogen receptor-β agonist prevented aneurysmal rupture without affecting the formation of aneurysms. The protective role of estrogen receptor-β agonist was abolished by the inhibition of nitric oxide synthase. We showed that estrogen prevented aneurysmal rupture in ovariectomized female mice. The protective effect of estrogen seemed to occur through the activation of estrogen receptor-β, a predominant subtype of estrogen receptor in human intracranial aneurysms and cerebral arteries.

Keywords: estrogens; menopause; models, animal.

Figure 1

A–C. Representative intracranial aneurysms in mice. A: Normal cerebral artery. B: Unruptured aneurysm in the anterior cerebral artery. C: Ruptured aneurysm with subarachnoid hemorrhage. D. Experimental protocol to study the protective role of estrogen against the development of aneurysmal rupture. DOCA: deoxycorticosterone acetate

Figure 2. Estrogen receptors in intracranial aneurysms and control arteries in humans

Though estrogen receptor-α was almost absent in both human intracranial aneurysms and superficial temporal arteries, estrogen receptor-β was highly expressed in smooth muscle cell layers of the human intracranial aneurysms and superficial temporal arteries.

Figure 3. Estrogen protected against the development of aneurysmal rupture in ovariectomized female mice

A: Incidence of ruptured and unruptured aneurysms. B: Rupture rate. C: Survival curve. Survival curves were constructed after excluding those mice that did not have aneurysms so that the curves mimic clinical settings. * vs vehicle, # vs Estrogen + ER antagonist. ER: estrogen receptor.

Figure 4. Stimulation of estrogen receptor-β, but not of estrogen receptor-α, protected against the development of aneurysmal rupture

A: Incidence of ruptured and unruptured aneurysms. B: Rupture rate. C: Survival curve. *vs vehicle ER: estrogen receptor

Figure 5. The protective effect of estrogen receptor-β stimulation was dependent on nitric oxide production

A: Incidence of ruptured and unruptured aneurysms. B: Rupture rate. C:Survival curve. * p vs vehicle, † vs L-NAME, # vs ER-β+L-NAME.