Vascular effects of estrogenic menopausal hormone therapy

Abstract

Cardiovascular disease (CVD) is more common in men and postmenopausal women (Post-MW) than premenopausal women (Pre-MW). Despite recent advances in preventive measures, the incidence of CVD in women has shown a rise that matched the increase in the Post-MW population. The increased incidence of CVD in Post-MW has been related to the decline in estrogen levels, and hence suggested vascular benefits of endogenous estrogen. Experimental studies have identified estrogen receptor ERα, ERβ and a novel estrogen binding membrane protein GPR30 (GPER) in blood vessels of humans and experimental animals. The interaction of estrogen with vascular ERs mediates both genomic and non-genomic effects. Estrogen promotes endothelium-dependent relaxation by increasing nitric oxide, prostacyclin, and hyperpolarizing factor. Estrogen also inhibits the mechanisms of vascular smooth muscle (VSM) contraction including [Ca2+]i, protein kinase C and Rho-kinase. Additional effects of estrogen on the vascular cytoskeleton, extracellular matrix, lipid profile and the vascular inflammatory response have been reported. In addition to the experimental evidence in animal models and vascular cells, initial observational studies in women using menopausal hormonal therapy (MHT) have suggested that estrogen may protect against CVD. However, randomized clinical trials (RCTs) such as the Heart and Estrogen/ progestin Replacement Study (HERS) and the Women's Health Initiative (WHI), which examined the effects of conjugated equine estrogens (CEE) in older women with established CVD (HERS) or without overt CVD (WHI), failed to demonstrate protective vascular effects of estrogen treatment. Despite the initial set-back from the results of MHT RCTs, growing evidence now supports the 'timing hypothesis', which suggests that MHT could increase the risk of CVD if started late after menopause, but may produce beneficial cardiovascular effects in younger women during the perimenopausal period. The choice of an appropriate MHT dose, route of administration, and estrogen/progestin combination could maximize the vascular benefits of MHT and minimize other adverse effects, especially if given within a reasonably short time after menopause to women that seek MHT for the relief of menopausal symptoms.

Figures

Fig. 1

Discrepancies in the vascular effects of estrogen in experimental studies and RCT. Experimental studies largely and consistently demonstrate vascular benefits of estrogen. The excitement generated from the vascular benefits of estrogen observed in initial observational studies, was followed by disappointment from the lack of vascular benefits in RCTs. Recent RCTs such as KEEPS and ELITE may resolve some of the discrepancies regarding the vascular benefits of estrogen and bridge the gap between the findings of experimental studies and RCTs. CVD, cardiovascular disease; ELITE, Early versus Late Intervention Trial with Estradiol; HERS, Heart and Estrogen/progestin Replacement Study; HDL, high density lipoprotein; KEEPS, Kronos Early Estrogen Prevention Study; LDL, low density lipoprotein; MHT, menopausal hormone therapy; NO, Nitric Oxide; VSMC, vascular smooth muscle cell; WHI, Women’s Health Initiative

Fig. 2

Genomic and nongenomic vascular effects of estrogen. In the genomic pathway in endothelial cells, E2 binds to cytoplasmic ER leading to ER dimerization and localization to the nucleus where the complex interacts with EREs to increase gene transcription and eNOS expression. In the nongenomic pathway, E2 binds to endothelial ER and activates phospholipase C (PLC), generating inositol 1,4,5-triphosphate (IP3) and DAG. IP3 causes Ca2+ release from the endoplasmic reticulum. Ca2+ forms a complex with calmodulin (CAM), which activates eNOS. E2/ER also interacts with Src and activates Modulator of Nongenomic Action of ER (MNAR). They interact with the p85 regulatory subunit of PI3-kinase (PI3K), which transforms phosphatidylinositol-4,5-bisphosphate (PIP2) into phosphatidylinositol 3,4,5-trisphosphate (PIP3), which activates Akt. ER-mediated activation of Akt or MAPK pathway causes phosphorylation and full activation of eNOS, transformation of L-arginine to L-citrulline and production of NO, which causes VSM relaxation. E2 also binds to membrane GPR30 and activates adenylate cyclase (AC) leading to increased cAMP and activation of protein kinase A (PKA), which activates eNOS and COX1 to produce NO and PGI2, respectively. E2 also induces production of EDHF. In the genomic pathway in VSM, E2 binds to ER, inhibiting growth factor (GF) receptors, which are known to activate MAPK translocation to the nucleus. E2 binding to ERs also stimulates ER translocation to the nucleus where it may affect gene transcription and VSM growth. In the non-genomic pathway, E2 binds to membrane ERs to inhibit the mechanisms of VSM contraction including [Ca2+]i, Ca2+-dependent MLC phosphorylation, protein kinase C (PKC), and Rho-kinase (Rho-K). Endothelial NO and PGI2 activate guanylate cyclase (GC) and AC, respectively, leading to increased cGMP/cAMP and increased activity of protein kinase G and A (PKG and PKA), respectively. PKG/PKA activate Ca2+ extrusion via plasmalemmal Ca2+ pump and Ca2+ uptake by SR, and inhibit Ca2+ entry through membrane Ca2+ channels. E2 may also bind plasma membrane ERs and activate K+ channels and other EDHF leading to hyperpolarization and inhibition of membrane Ca2+ channels. COX1, cyclooxygenase-1; DAG, diacylglycerol; EDHF, Endothelial derived hyperpolarizing factor; eNOS, endothelial nitric oxide synthase; ER, estrogen receptor; ERE, estrogen response elements; PGI2, prostacyclin; MAPK, mitogen-activated protein kinase; SR, sacroplasmic reticulum; VSM, vascular smooth muscle

Fig. 3

Differential protective effects of estrogenic MHT in early atherogenesis and harmful effects in established atherosclerosis. In early atherogenesis cardiovascular risk factors, hemodynamic forces, and circulating inflammatory factors cause endothelial cell injury resulting in decreased NO production and increased EC permeability. Once injured, the endothelium increases the expression of leukocyte adhesion molecules, which increases the adherence of macrophages and other leukocytes. The increased EC permeability allows entry of leukocytes and lipoproteins into the subendothelial space. Oxidized lipoproteins are taken up by macrophages and SMCs to form foam cells (fatty streak). E2 has beneficial effects on early atherosclerotic lesions by changing the plasma lipid profile, maintaining EC integrity and promoting NO production. In established atherosclerosis foam cells at the central-most position of the developing atheroma become necrotic and form the central lipid core, whereas the shoulder regions contain SMCs, macrophages, and other leukocytes. Platelet-derived growth factor and transforming growth factor-β stimulate SMC migration and collagen formation in the subendothelial space, as well as formation of the fibrous cap. E2 increases MMP expression in established atherosclerosis, causing instability of the fibrous cap and rupture of the plaque. CAM, cell adhesion molecule; EC, endothelial cell; ET-1, endothelin-1; LDL, low density lipoprotein; MCP-1, monocyte chemotactic protein-1; MMP, matrix metalloproteinase; NO, nitric Oxide; PGI2, prostacyclin; TNF-α, tumor necrosis factor-α; VSMC, vascular smooth muscle cell