Biological Mechanisms of Vascular Dysfunction With Age and Estrogen Deficiency
Biological Mechanisms of Vascular Dysfunction With Age and Estrogen Deficiency
As women get older and go through menopause, levels of the female reproductive hormone estradiol decrease to low levels. Also with aging, the functioning of the arteries declines. Over time this vascular dysfunction can lead to health problems such as high blood pressure and heart disease. This study is being done to help determine what causes arteries to become unhealthy in postmenopausal women, who have low levels of the female reproductive hormone estradiol. In this study we will test whether low levels of tetrahydrobiopterin (BH4), a natural substance in the body that can cause the arteries to expand, explains why arteries become unhealthy in women with low levels of estradiol. To answer this question, we will study how vascular function changes with a medication that causes a short-term increase in BH4 levels when estradiol is lowered with a medication, compared to when estradiol is normal. We will also determine whether the administration of the antioxidant vitamin C, along with the medication to increase BH4 levels, will normalize vascular health in perimenopausal and postmenopausal women, and in women who have their estradiol levels lowered.
Endothelial dysfunction, featuring reduced vascular endothelial-dependent dilation (EDD), is a significant predictor of cardiovascular (CV) events. Reduced EDD is characterized by the loss of nitric oxide (NO) production. EDD declines with aging in women at a slower rate compared to men but rapidly declines after menopause and quickly catches up to men, suggesting a beneficial effect of estrogen (E2). Our research has investigated the mechanisms underlying endothelial protection of E2 including how hormonal fluctuations during the peri-to early postmenopausal years contribute to endothelial dysfunction, and how these hormonal changes might interact with aging. Using a well described experimental model (systemic infusion of ascorbic acid; AA) to acutely reduce reactive oxygen species (ROS), our preliminary findings implicate oxidative stress as an underlying mechanism explaining the impaired EDD during the menopausal transition. Specifically we found improved EDD in response to AA in late peri- and postmenopausal women but not in pre- or early perimenopausal women. These data indicate that E2 may protect the vasculature from free radical injury during pre- and early perimenopausal years. In support of this concept, we demonstrated that AA can reverse the decrease in EDD that occurs in the setting of short-term E2 deficiency (3-days of gonadotropin releasing hormone antagonist; GnRHant) in pre-and early perimenopausal women.
Tetrahydrobiopterin (BH4) is a critical co-factor in the production of NO by endothelial nitric oxide synthase (eNOS), and in the face of deficient levels, uncoupling of eNOS results in the production of ROS instead of NO. Whether the increased ROS and impaired EDD that occurs during the peri- and early postmenopausal periods is associated with reduced vascular BH4 is at present unknown. However, our finding that AA could not improve the reduced EDD found in early perimenopausal women or restore EDD in peri-and postmenopausal women to premenopausal levels, indicates that other sources of ROS or causes of low NO are involved in the impaired EDD across the stages of the menopause transition and aging in women. In vitro studies demonstrate that the co-administration of BH4 and AA completely prevents the eNOS uncoupling and decreased NO production in endothelial cells treated with a peroxynitrite (ONOO-) donor, compared to administering either one alone. The global aim of this renewal is to follow-up on our previous findings and to determine the role of BH4 as a mediator of endothelial dysfunction [across the stages of the menopausal transition and aging in women. This global aim will be addressed by 3 specific aims (SA). Using a cross-sectional design, SA1 will measure EDD in healthy pre-, peri- and postmenopausal women: 1) under basal conditions; and 2) following an acute increase in BH4. SA2 will expand on the cross-sectional comparisons via short-term (10 days) ovarian suppression (GnRhant) in pre-and early perimenopausal women to isolate the age-independent effects of E2. SA3 will examine whether co-administration of BH4 and AA fully restore youthful EDD in peri-and postmenopausal women.
SA1: To determine the possible mechanistic involvement of BH4 deficiency in the decline in endothelial function during the peri-and early postmenopausal period and its potential relation to oxidative stress. This will be assessed by comparing brachial artery EDD and plasma and endothelial cell markers of oxidative stress and NO (see methods for details) in healthy pre-, peri-and postmenopausal women: 1) under basal conditions; and 2) in response to acute oral BH4 (vs PL) supplementation.
Hypothesis1 (H1): a) EDD will increase in response to BH4 in early peri, late peri and postmenopausal women but not premenopausal women. b) The increase in EDD in response to BH4 will be associated with a decrease in plasma markers of oxidative stress. c) The reduced EDD in peri- and postmenopausal women will be associated with lower endothelial cell markers of NO and higher oxidative Stress.
SA2: To determine whether the decrease in EDD in response to acute ovarian suppression in pre- and early perimenopausal women is related to reduced BH4 bioavailability and elevated markers of oxidative stress. This will be assessed by comparing EDD and plasma markers of oxidative stress after acute oral supplementation of BH4 (or PL) following either acute ovarian suppression with (GnRHant+E2) or without (GnRHant+PL) E2 add-back. Additionally, endothelial cell protein expression of markers of BH4 biosynthesis and oxidative stress will be measured before and after the suppression/add-back paradigm.
H2: a) The expected decrease in EDD and increase in plasma oxidative stress markers following acute ovarian suppression will be mitigated in the groups treated with oral BH4 (GnRHant+PL+BH4); there will be little or no effect of the addition of BH4 to the E2 add-back group. b) Ovarian suppression alone will reduce endothelial protein markers of BH4 biosynthesis and will increase markers of oxidative stress. These changes will not be observed following ovarian suppression with E2 add-back and oral BH4. These sets of experiments will provide evidence that the oxidative stress-related decrease in EDD in response to E2 suppression is in part, related to decreased BH4 bioavailability.
SA3: To determine whether the co-administration of BH4 with AA will completely restore EDD in peri-and postmenopausal women, and in early perimenopausal women following ovarian suppression.
H3: The co-administration of BH4 with AA will restore EDD to premenopausal levels a) in peri-and postmenopausal women, and b) in early perimenopausal following acute ovarian suppression.
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