Estrogen receptor-mediated, nitric oxide-dependent modulation of the immunologic barrier function of the endothelium: regulation of fas ligand expression by estradiol

Estrogen-related mechanisms in sex differences of hypertension and target organ damage

Hypertension (HTN) is a primary risk factor for cardiovascular (CV) events, target organ damage (TOD), premature death and disability worldwide. The pathophysiology of HTN is complex and influenced by many factors including biological sex. Studies show that the prevalence of HTN is higher among adults aged 60 and over, highlighting the increase of HTN after menopause in women. Estrogen (E2) plays an important role in the development of systemic HTN and TOD, exerting several modulatory effects. The influence of E2 leads to alterations in mechanisms regulating the sympathetic nervous system, renin-angiotensin-aldosterone system, body mass, oxidative stress, endothelial function and salt sensitivity; all associated with a crucial inflammatory state and influenced by genetic factors, ultimately resulting in cardiac, vascular and renal damage in HTN. In the present article, we discuss the role of E2 in mechanisms accounting for the development of HTN and TOD in a sex-specific manner. The identification of targets with therapeutic potential would contribute to the development of more efficient treatments according to individual needs.

Estradiol attenuates high glucose-induced endothelial nitrotyrosine: role for neuronal nitric oxide synthase

Hyperglycemia in diabetes causes increased oxidative stress in the vascular endothelium with generation of free radicals such as superoxide. Peroxynitrite, a highly reactive species generated from superoxide and nitric oxide (NO), induces proinflammatory tyrosine nitration of intracellular proteins under such conditions. The female sex hormone estrogen appears to exert protective effects on the nondiabetic endothelium. However, several studies show reduced vascular protection in women with diabetes, suggesting alterations in estrogen signaling under high glucose. In this study, we examined the endothelial effects of estrogen under increasing glucose levels, focusing on nitrotyrosine and peroxynitrite. Human umbilical vein endothelial cells were incubated with normal (5.5 mM) or high (15.5 or 30.5 mM) glucose before addition of estradiol (E2, 1 or 10 nM). Selective NO synthase (NOS) inhibitors were used to determine the role of specific NOS isoforms. Addition of E2 significantly reduced high glucose-induced increase in peroxynitrite and consequently, nitrotyrosine. The superoxide levels were unchanged, suggesting effects on NO generation. Inhibition of neuronal NOS (nNOS) reduced high glucose-induced nitrotyrosine, demonstrating a critical role for this enzyme. E2 increased nNOS activity under normal glucose while decreasing it under high glucose as determined by its phosphorylation status. These data show that nNOS contributes to endothelial peroxynitrite and subsequent nitrotyrosine generation under high glucose, which can be attenuated by E2 through nNOS inhibition. The altered regulation of nNOS by E2 under high glucose is a potential therapeutic target in women with diabetes.

Biomarkers for predicting postmenopausal coronary heart disease

Coronary heart disease (CHD) is the main cause of death in post-menopausal women (PMW). Beyond the ‘traditional’ cardiovascular risk factors of CHD, newer biomarkers, reflecting inflammation, endothelial function and oxidative stress, have received growing consideration. We systematically reviewed the literature on the biomarkers for predicting CHD in PMW. C-reactive protein, IL-6 and Lipoprotein (a) have been consistently found to be associated with CHD risk in PMW. However, no evidence supports the existence of a causal and independent link between such biomarkers and CHD in PMW. Also, the new biomarkers only marginally improve cardiovascular risk prediction. Upcoming studies are needed to provide further evidence on the validity of the new biomarkers in PMW and to understand their relationships with hormone therapy, opening new avenues for prevention.

Sex steroids as inflammatory regulators

It is becoming increasingly clear that endogenous sex steroids are key players in a range of inflammatory contexts. Androgens and estrogens have been shown to have a profound influence on the function of inflammatory cells including macrophages and on the secretion and activation of a range of plasma-borne inflammatory mediators. The menopause and polymorphisms in estrogen receptor genes have separately been shown to affect the incidence of a range of inflammatory disorders. Sex steroids themselves have been shown to be protective in certain conditions; harmful in others. This review will summarize their documented effects on inflammatory processes, with particular focus on two areas that have received much recent attention: the antiatherosclerotic properties of estrogens in females and the wound healing effects of sex steroids.

17beta-Estradiol induces protein S-nitrosylation in the endothelium

AIMS

Estrogen induces nitric oxide (NO) in the endothelium and appears to protect against inflammation and atherosclerosis. NO can induce post-translational protein modifications such as cysteine S-nitrosylation in the cellular proteins which may exert anti-inflammatory effects. However, whether estrogen can induce protein S-nitrosylation in the endothelium is not known. Given this background, we investigated the role of 17beta-estradiol (E2beta), the major form of estrogen in the body, on endothelial protein S-nitrosylation.

METHODS AND RESULTS

Experiments were performed in human umbilical vein endothelial cells (HUVECs). S-nitrosylation was detected by immunostaining for nitrosocysteine and further confirmed by biotin switch method. Ovariectomized 12-month-old Sprague-Dawley rats with/without estradiol supplementation were used for in vivo validation of findings. We found that physiologically relevant doses of E2beta increased protein S-nitrosylation in HUVECs through estrogen receptor-alpha (ERalpha) and endothelial nitric oxide synthase (eNOS). Interestingly, specific agonists for both ERalpha and ERbeta increased eNOS protein expression, while only the former could activate eNOS through phosphorylation. S-nitrosylation by E2beta prevented angiotensin II-induced upregulation of intercellular cell adhesion molecule-1, suggesting a potential anti-inflammatory mechanism. Finally, we showed that exogenous E2beta could increase endothelial S-nitrosylation in vivo in a rat model.

CONCLUSION

Our results demonstrate for the first time that E2beta increases protein S-nitrosylation in the vascular endothelium, which might be a novel pathway to mediate the protective effects on the vasculature.

High glucose-induced oxidative stress alters estrogen effects on ERalpha and ERbeta in human endothelial cells: reversal by AMPK activator

Estrogen appears to protect against cardiovascular disease in pre-menopausal women. However, these protective effects are absent in women with diabetes. The hyperglycemia and consequent oxidative stress observed in diabetes cause endothelial dysfunction, but specific effects on endothelial estrogen responses are not known. In this study, we hypothesized that high glucose conditions would alter the regulation of the estrogen receptors (ERs), ERalpha and ERbeta, in endothelial cells, possibly through increased oxidative stress. The role of the AMPK activator AICAR was examined on modulating the effects of high glucose. Cultured human endothelial cells were exposed to physiologically relevant doses of 17-beta-estradiol (E2) for 24h in presence of normal (5.5mM) and high (30.5mM) levels of glucose. Protein levels of estrogen receptors, ERalpha and ERbeta, were measured through western blotting. Oxidative stress was measured by the dihydroethidium (DHE) assay for superoxide. Under normal glucose, E2 increased the levels of ERalpha relative ERbeta; however, high glucose reversed the estrogen effects on endothelial ER expression. AMPK activation restored the physiological estrogen responses, likely through amelioration of oxidative stress. Control of oxidative stress by AMPK activation or anti-oxidants could restore normal estrogen responses even in presence of hyperglycemia.

Influence of menopause on biochemical markers of endothelial dysfunction-A case-control pilot study in North Indian population

OBJECTIVE

Menopause, an estrogen deficient state, is known to increase the cardiovascular risk. Lipid changes accompanying menopause account for only few cases of coronary artery disease (CAD). Endothelium-dependent nitric oxide-mediated vasodilatory mechanisms are also known to play a role in development of coronary artery disease, but studies in menopausal women are very few. This study was hence undertaken to see if nitric oxide (NO)-cyclic guanidine monophosphate (c-GMP) pathway is influenced by menopause.

DESIGN

This study was a hospital-based case-control study involving 100 women in age group 40-55 years. Of these, 50 women were postmenopausal and 50 were premenopausal. Women with known risk factors for CAD were excluded. Fasting blood samples from these women were collected and analyzed for estradiol levels, lipid profile, apolipoprotein B, plasma nitric oxide, c-GMP and platelet nitric oxide using standard kits and reagents. Statistical analysis was done on SPSS and two-tailed p-value <0.05 was considered significant.

RESULT

Postmenopausal women had significantly lower estradiol, plasma NO, and c-GMP levels as compared to premenopausal women (p<0.05). Cholesterol, low-density lipoprotein (LDL) cholesterol and apolipoprotein B (apo-B) levels were higher and HDL levels were lower in postmenopausal as compared to premenopausal women (p<0.05). Plasma NO showed a significant positive correlation with estradiol, HDL levels and negative correlation with apo-B levels.

CONCLUSION

Menopause tends to downregulate NO-c-GMP pathway resulting in endothelial dysfunction. The mechanism may be directly through estrogen receptors or indirectly through potentiation of dyslipidemia.

Estrogen is a modulator of vascular inflammation

Vascular inflammation underlies the pathogenesis of atherosclerosis. Atherosclerotic changes in the vasculature lead to conditions such as coronary artery disease and stroke, which are the major causes of morbidity and mortality worldwide. Epidemiological studies in premenopausal women suggest a beneficial role for estrogen in preventing vascular inflammation and consequent atherosclerosis. However, the benefits of estrogen areabsent or even reversed in older postmenopausal subjects. The modulation of inflammation by estrogen under different conditions might explain this discrepancy. Estrogen exerts its antiinflammatory effects on the vasculature through different mechanisms such as direct antioxidant effect, generation of nitric oxide, prevention of apoptosis in vascular cells and suppression of cytokines and the renin-angiotensin system. On the other hand, estrogen also elicits proinflammatory changes under certain conditions, which are less completely understood. Some of the mechanisms underlying a possible proinflammatory role for estrogen include increased expression of the proinflammatory receptor for advanced glycation end products, increased tyrosine nitration of cellular proteins, and generation of reactive oxygen species through an uncoupled eNOS. In this review, we have presented evidence for both antiinflammatory and proinflammatory pathways modulated by estrogen and how interactions among such pathways might determine the effects of estrogen on the vascular system.

Estrogen increases apoptosis in the arterial wall in a murine atherosclerosis model

OBJECTIVE

To investigate the effect of estrogen (E) on vascular apoptosis during atherosclerotic plaque formation.

DESIGN

Laboratory study using a murine atherosclerosis model.

SETTING

Academic research center.

ANIMAL(S)

Female mice homozygous for null alleles of LDL receptor (LDL-R(-/-)) in a C57BL/6 background. LDL-R(-/-) mice develop atherosclerosis in a predictable manner when fed a high cholesterol diet.

INTERVENTION(S)

Eight-week-old female LDL-R(-/-) mice (n = 68) were ovariectomized, and implanted subcutaneously with 90-day release pellets containing 0.5 mg of 17beta-estradiol (E(2)) or placebo. Four animals were evaluated at the initiation of the study. Thereafter, four animals from each group were sacrificed weekly for 8 weeks and their aortas studied.

MAIN OUTCOME MEASURE(S)

The effect of E(2) on atherosclerotic plaque development, apoptosis, and cell proliferation was examined in the aorta of ovariectomized LDL-R(-/-) mice that were fed a high cholesterol diet.

RESULT(S)

Mice treated with E(2) displayed a delay in atherosclerotic plaque formation, associated with an increase in DNA strand breaks in the arterial wall indicative of increased apoptosis, compared to placebo-treated mice. The two groups did not differ in mitotic activity.

CONCLUSION(S)

In female LDL-R(-/-) mice fed a high cholesterol diet, ovariectomy is associated with increased atherogenesis. The effect of ovariectomy on atherogenesis is reversed by E(2) treatment. In addition to delayed atherogenesis, E(2) treatment of ovariectomized LDL-R(-/-) mice results in an increase in apoptosis in the aortic wall without an effect on the mitotic activity. Our findings suggest that vascular effects of E may be in part mediated by a proapoptotic activity.

Estrogen regulation of nitric oxide and inducible nitric oxide synthase (iNOS) in immune cells: implications for immunity, autoimmune diseases, and apoptosis

Nitric oxide plays a central role in the physiology and pathology of diverse tissues including the immune system. It is clear that the levels of nitric oxide must be carefully regulated to maintain homeostasis. Appropriate levels of nitric oxide derived from iNOS assist in mounting an effective defense against invading microbes. Conversely, inability to generate nitric oxide results in serious, even fatal, susceptibility to infections. Further, dysregulation or overproduction of nitric oxide has been implicated in the pathogenesis of many disorders, including atherosclerosis, neurodegenerative diseases, inflammatory autoimmune diseases, and cancer. Therefore, depending upon the levels of nitric oxide generated, the potential exists for nitric oxide to behave like a "double-edged" biological sword. Taking these issues into consideration, it is thus pivotal to understand the regulation of nitric oxide. Nitric oxide is regulated by many endogenous factors including hormones such as estrogens. While the effects of estrogen on the generation of nitric oxide in non-immune tissues are relatively well documented, the effect of estrogen on iNOS/nitric oxide in immune cells is only now becoming apparent. Our laboratory has recently shown that estrogen treatment of mice markedly upregulates the levels of iNOS mRNA, iNOS protein, and nitric oxide in activated splenocytes. This upregulation of nitric oxide is in part mediated through interferon-gamma (IFN-gamma), a pro-inflammatory cytokine that is enhanced by estrogen. These findings are important considering that estrogens are not only involved in regulation of normal immune responses, but also are implicated in many autoimmune and inflammatory diseases. To date, there are no reviews on the effects of estrogen on immune tissue-derived nitric oxide and therefore this review will address this critical gap in the literature. Given the increasing importance of immune-tissue-derived iNOS in health and disease, studies on estrogen-induced regulation of iNOS may offer a better understanding of diseases and aid in devising new therapeutic interventions.

Estrogen-mediated immunomodulation involves reduced activation of effector T cells, potentiation of treg cells, and enhanced expression of the PD-1 costimulatory pathway

Estrogen (E2)-induced immunomodulation involves dual effects on antigen-presenting cells (APC) and CD4(+)CD25(+) regulatory T cells (Treg) but not a direct effect on effector T cells. In this report, we further investigated the effects of E2 on APC and Treg function. We found that E2 treatment in vivo strongly reduced recovery of APC from the peritoneal cavity and inhibited induction of the inflammatory cytokines interleukin (IL)-12 and interferon-gamma but enhanced secretion of IL-10. Moreover, E2-conditioned bone marrow-derived dendritic cells (BM-DC) could both enhance Treg activity and directly inhibit responder T cells in the absence of Treg cells. We examined whether this E2-induced inhibitory activity of BM-DC might involve costimulation through the recently described PD-1 pathway. Both E2 and pregnancy markedly enhanced PD-1 expression in several types of APC, including macrophages, B cells, and especially dendritic cells (DC). Similarly to E2-induced enhancement of FoxP3 expression and experimental autoimmune encephalomyelitis protection, E2-induced enhancement of PD-1(+) cells was also mediated through estrogen receptor alpha (Esr1) in DC and macrophages but not in B cells. Based on antibody inhibition studies, PD-1 interaction with its ligands, PDL-1 and especially PDL-2, could mediate either positive or negative regulatory signaling in both mature and immature E2-conditioned DC, depending, respectively, on a relatively high (10:1) or low (1:1) ratio of T cells:BM-DC. These novel findings indicate that E2-induced immunomodulation is mediated in part through potentiation in BM-DC of the PD-1 costimulatory pathway.

Estrogen-induced spermatogenic cell apoptosis occurs via the mitochondrial pathway: role of superoxide and nitric oxide

The detrimental effects of estrogen on testicular function provide a conceptual basis to examine the speculative link between increased exposure to estrogens and spermatogenic cell death. Using an in vitro model, we provide an understanding of the events leading to estrogen-induced apoptosis in cells of spermatogenic lineage. Early events associated with estrogen exposure were up-regulation of FasL and increased generation of H(2)O(2), superoxide, and nitric oxide. The ability of anti-FasL antibodies to prevent several downstream biochemical changes and cell death induced by 17beta-estradiol substantiates the involvement of the cell death receptor pathway. Evidence for the amplification of the death-inducing signals through mitochondria was obtained from the transient mitochondrial hyperpolarization observed after estradiol exposure resulting in cytochrome c release. A combination of nitric oxide and superoxide but not H(2)O(2) was responsible for the mitochondrial hyperpolarization. Mn(III) tetrakis(4-benzoic acid)porphyrin chloride, an intracellular peroxynitrite scavenger, was able to reduce mitochondrial hyperpolarization and cell death. Although nitric oxide augmentation occurred through an increase in the expression of inducible nitric-oxide synthase, superoxide up-regulation was a product of estradiol metabolism. All of the above changes were mediated through an estrogen receptor-based mechanism because tamoxifen, the estrogen receptor modulator, was able to rescue the cells from estrogen-induced alterations. This study establishes the importance of the independent capability of cells of the spermatogenic lineage to respond to estrogens and most importantly suggests that low dose estrogens can potentially cause severe spermatogenic cellular dysfunction leading to impaired fertility even without interference of the hypothalamo-hypophyseal axis.

Soluble Fas: a novel predictor of atherosclerosis in dialysis patients

BACKGROUND

Cardiovascular disease (CVD) is the leading cause of death in patients with end-stage renal disease (ESRD). Disregulation of apoptosis within the vessel wall and upregulation of the Fas/Fas-ligand (Fas-L) system contribute to the development of atherosclerosis. Cross-sectional studies have suggested that elevated plasma levels of the soluble form of Fas (sFas) are associated with CVD. However, the role of sFas and sFas-L in predicting future cardiovascular events has yet to be defined.

METHODS

We evaluated the role of plasma sFas and sFas-L levels as predictors of CVD in a prospective cohort of 107 chronic hemodialysis patients.

RESULTS

During the study period (27 months), 53 patients (49.5%) presented with at least one cardiovascular end point. On univariate analysis, baseline sFas levels were significantly associated with the occurrence of cardiovascular end points, whereas sFas-L levels were not. Using Cox proportional hazards, increased sFas levels were associated with a significantly greater risk for cardiovascular end points (P = 0.03). This effect was independent of baseline CVD history, classic risk factors for atherosclerosis (diabetes, hypercholesterolemia, hypertension, and smoking), and markers of inflammation (C-reactive protein [CRP], soluble intercellular adhesion molecule-1). Increased CRP levels also were associated with cardiovascular end points (P = 0.04). In addition, increased cardiovascular mortality was found in patients in the highest sFas tertile compared with those in the lowest tertile (27.8% versus 8.6%; P = 0.04).

CONCLUSION

Increased plasma sFas levels are predictive of future CVD. These results suggest that sFas is a novel and independent predictor of active atherosclerotic disease in patients with ESRD.

Tumour Fas ligand:Fas ratio greater than 1 is an independent marker of relative resistance to tamoxifen therapy in hormone receptor positive breast cancer

BACKGROUND

The objective of the present study was to examine the prognostic and predictive significance of the apoptosis-related marker Fas ligand (FasL):Fas ratio in breast cancer.

METHODS

Tumour biopsies from 215 primary invasive breast cancer patients were examined for the expression of FasL and Fas mRNA transcripts by quantitative real-time RT-PCR. Their prognostic and predictive impact on patient survival was determined in univariate and multivariate survival analyses.

RESULTS

Using a cutoff value of 1, a FasL:Fas ratio greater than 1 was found to have significant prognostic value for disease-free survival among the total population (median follow up 54 months). It was associated with a significantly decreased disease-free survival (P = 0.022) and with a tendency toward increased mortality (P = 0.14) in univariate analysis. Hormone receptor positive women exclusively treated with tamoxifen (n = 86) and with a FasL:Fas ratio greater than 1 had a significantly decreased disease-free survival (P = 0.008) and overall survival (P = 0.03) in univariate Kaplan-Meier analysis. Furthermore, tumour size and FasL:Fas ratio were of independent predictive significance in the multivariate model for disease-free and overall survival in that subgroup. Among postmenopausal patients (n = 148) both of those factors retained independent prognostic significance in the multivariate model for disease-free survival. In contrast, FasL:Fas ratio had no significant predictive value in patients exclusively treated with chemotherapy.

CONCLUSION

The data presented indicate that FasL:Fas ratio may be useful not only as a prognostic factor but also as a predictive factor for projecting response to the antioestrogen tamoxifen. The results strongly support a correlation between FasL:Fas ratio greater than 1 and lack of efficacy of tamoxifen in hormone receptor positive patients.