Estrogen Induces the Akt-dependent Activation of Endothelial Nitric-oxide Synthase in Vascular Endothelial Cells

Abstract

Although estrogen is known to activate endothelial nitric oxide synthase (eNOS) in the vascular endothelium, the molecular mechanism responsible for this effect remains to be elucidated. In studies of both human umbilical vein endothelial cells ( The inhibitory effect of estrogen on the development of atherosclerosis has been suggested by abundant human epidemiological and animal experimental data (1-9). The incidence of atherosclerotic diseases is lower in premenopausal women than in men, steeply rises in postmenopausal women, and is reduced to premenopausal levels in postmenopausal women who receive estrogen therapy (10 -12). Until recently, the atheroprotective effects of estrogen were attributed principally to the effects on serum lipid concentrations. However, estrogeninduced alterations in serum lipids account for only approximately one-third of the observed clinical benefits of estrogen (12)(13)(14). Recent evidence suggests that the direct actions of estrogen on blood vessels contribute to the cardioprotective effects of estrogen (13, 15). There are many kinds of direct effects of estrogen on blood vessels, such as estrogen-induced increases of vasodilatation and inhibition of the response of blood vessels to injury and the development of atherosclerosis. However, the molecular mechanism underlying the estrogeninduced vasodilatation has not yet been determined. Several studies suggest that a key mediator of this vasodilator response could be the endothelium-derived relaxing factor nitric oxide (NO), and that brief treatment with estrogen increases basal NO release in endothelial cells without elevation of eNOS mRNA or protein (16). Estrogen activates endothelial nitric oxide synthase (eNOS) without altering expression of the eNOS gene in vascular endothelium (17)(18)(19)(20). However, the details of the mechanism of the estrogen-induced eNOS activation are not yet well understood.The serine/threonine kinase termed Akt or protein kinase B (PKB) 1 is an important regulator of various cellular processes, including glucose metabolism and cell survival (21, 22). Activation of receptor tyrosine kinases and G-protein-coupled receptors, and stimulation of cells by mechanical force, can lead to the phosphorylation and activation of . Akt was identified as a downstream component of survival signaling through phosphatidylinositol 3-kinase (PI3K) (26 -30). Akt may be regulated by both phosphorylation and the direct binding of PI3K lipid products to the Akt pleckstrin homology domain. Akt can then phosphorylate substrates such as glycogen synthase kinase-3, 6-phosphofructo-2-kinase, and BAD. More recently, it was found that eNOS is also an Akt substrate and is activated by Akt-dependent phosphorylation to release NO in endothelial cells (31-34).The actions of estrogen can be mediated by the classical nuclear receptors, ER␣ and ER␤ (35,36) or through other putative membrane receptors. By definition, rapid effects of estrogen that involve nongenomic mechanisms are independent of transcriptional activation by the nuclea...

Androgen depletion in humans leads to cavernous tissue reorganization and upregulation of Sirt1–eNOS axis

“…This does not seem to result from estradiol administration. Although estrogens are able to rapidly induce NO production in ECs without altering the expression of eNOS gene or protein (Caulin-Glasser et al 1997;Haynes et al 2000;Hisamoto et al 2001;Russel et al 2000;Lantin-Hermoso et al 1997), by a mechanism that involves PI3-kinase-dependent activation of Akt, neither estrogen receptors were expressed nor phospho-Akt expression was significantly changed, although Akt activation occurring in Sirt1 overexpression appears to be limited to insulinresistant conditions (Sun et al 2007). Fig.…”

Shear stress stimulates phosphorylation of eNOS at Ser635by a protein kinase A-dependent mechanism

“…It has been well documented that phosphorylation of eNOS-Ser 1179 plays a key role in stimulation of eNOS activity in response to various physiological stimuli, which activate eNOS in a Ca 2ϩ -independent manner, including shear stress, VEGF, and estrogen (3,12,17,21,25). Other stimuli that activate eNOS by Ca 2ϩ -dependent mechanisms may not require phosphorylation.…”

“…In addition to its well-known vascular functions such as vessel relaxation and inhibition of platelet aggregation, NO also inhibits some of the key steps in atherogenesis, including cell death and monocyte adhesion induced by proatherogenic factors (14,15,24,45). NO production from eNOS is stimulated by a variety of mechanical forces such as shear stress and stretching, and humoral factors ranging from growth factors and peptide hormones such as vascular endothelial growth factor (VEGF), estrogen, sphingosine-1-phosphate, acetylcholine, and bradykinin (13,17,18,25,36,37).…”

“…Phosphorylation of eNOS-Ser 1179 is regulated by phophosinositide-3-kinase (PI3K)-dependent mechanisms (19). Akt, one of the major regulatory targets of PI3K, has been shown to directly phosphorylate eNOS at Ser 1179 and activate the enzyme in response to vascular endothelial growth factor (VEGF), sphingosine-1-phosphate, and estrogen (13,17,25,36,37). However eNOS-Ser 1179 can also be phosphorylated by AMP-activated protein kinase (8), or protein kinase A (PKA), and protein kinase G (PKG) (5).…”

Functional significance of the rapid regulation of brain estrogen action: Where do the estrogens come from?

“…Among others, it has been shown that the binding of estradiol to a cytoplasmic estrogen receptor, ERα or ERβ, allows an interaction between the ER and the protein kinase Src resulting in the activation of the intracellular cascade Src/Ras/ERK [168]. Moreover, the binding of E 2 to the cytoplasmic ERα activates the protein kinase Akt via a direct interaction of ER with the protein kinase PI3K ( [112,238]; for more information see [80]). …”