Review paper
Cardiovascular and metabolic effects of estrogen in men

Introduction
Differences in the clinical course of coronary artery disease (CAD) between men and women have been observed for years. Lower incidence of CAD in premenopausal women led to conclusion that female sex hormones, including estradiol (E2) are responsible for this difference. This assumption leads to a wide use of the estrogen replacement therapy in postmenopausal women, between others to decrease the risk of CAD. Nevertheless, the results of recent studies have negatively verified this assumption [1]. Estrogens are produced by the ovary and are traditionally considered as exclusively female sex hormones. For years it has been considered that these hormones have no influence in men or induce the malfunctioning of the testes. However, it has been found that in men estrogens are produced in the testes, adipose tissue brain, adrenals, liver, breast, and hair. Daily production rate and serum concentration of E2 in men is higher than in postmenopausal women. Even though the role of sex hormones in the pathogenesis of CAD was postulated years ago, little is known about the link between sex steroids secretion or action and CAD spectrum in men.
Why estrogens, the “female” sex hormones, are supposed to have biological and clinical significance in men?
The initial revealing that E2 may be physiologically important for male gonadal function was published at the end of the 80s [2] and was confirmed recently [3, 4]. The classical point of view considering estrogens as female hormone exclusively was changed also due to the discovery of estrogen receptors (ER) in the male. First papers on transgenic mice with ER gene deprivation (ER knock out – ERKO) or with knock out of the gene coding aromatase, necessary for conversion of testosterone to estrogen (ARKO), were published in the 90s [5]. Subsequently, the first studies on men with inherited mutations of gene encoding aromatase showed that in the human male, estrogens are mainly responsible for pubertal spurt in bone growth and epiphyseal closure after puberty, the effects that were previously attributed to the testosterone action [6]. Estrogen may contribute to the human male pathology as well. Estrogens are risk factors for prostatic cancer. Increased biosynthesis of estrogen in tissues unfavorably aggravates the cell proliferative potential in autoimmune diseases [for review see 7]. Recent data indicate a possible role of estrogens and ER in the cardiovascular system function. Two types of estrogen receptors have been discovered in men: ERa and ERb [8]. In a 31-year-old man with inherited mutation of ERa premature CAD was documented, suggesting a possible preventive role of estrogens for CAD [9]. Furthermore, as in women, also in men a nongenomic influence of estrogens on the cardiovascular system has been reported. In men with inherited mutation of ERa resulting in the lack of estrogen activity (estrogen resistancy), a rapid (within 5 minutes) relaxation of brachial artery after administration of E2 was observed pointing to nongenomic, ER-independent action of E2 [10]. This is related to the stimulation by E2 of calcium-dependent potassium channels in the vascular smooth muscles. Further parts of this paper will deal with clinical correlations to ascertain a potential role of estrogens in the pathogenesis of ischemic heart disease in men.
Estrogens and the metabolic risk factors for CAD Lipid profile
It has been documented that physiological levels of E2 may influence serum lipoprotein concentration in healthy men [11, 12]. Nevertheless, the results of these studies are contradictory. Shono et al. [12] demonstrated that E2 negatively correlates with the levels of low density lipoproteins fraction of cholesterol (LDL), suggesting a beneficial effect of E2. Kiel et al. [13] correlated blood E2 level with lipid profile both in healthy men and in men with angiographically confirmed CAD and showed a plausible, positive correlation between E2 level and the level of high density lipoproteins fraction of cholesterol (HDL) in both groups of men. The same study showed however, also an unfavorable (positive) correlation between the blood level of E2 and total cholesterol. Other investigators did not find any correlations between E2 and lipoprotein serum concentration in healthy men [14, 15]. In our prospective study on 111 men in the mean age of 55 years with angiographically confirmed CAD, the unfavorable correlations between E2 and total cholesterol, LDL cholesterol and triglycerides were found. Significant positive correlations between E2 blood levels and total cholesterol, LDL, total cholesterol/HDL ratio or triglycerides were present. All lipid factors are considered as important measures of atherosclerotic milieu. These correlations remained significant after adjustment for clinical covariates like age, hypertension, diabetes or prior hypertension in the multivariate logistic regression model. No associations were found between levels of the blood lipids and total testosterone or dehydroepiandrosterone sulphate (DHEAS) levels. These results indicated a possible role of E2 in the promotion of atherogenic lipid milieu development in men with CAD [16, 17]. Our studies were not in accordance with the findings based on the case reports of young men lacking E2 due to the inherited mutations of the gene encoding aromatase, where the unfavorable increased levels of LDL were associated with decreased levels of HDL [6, 18, 19]. Supplementation of E2 in these patients resulted in the normalization of lipoprotein profile [6, 19], indicating a favorable effect of estrogen. The discrepancy can be explained by the profound difference in the clinical material. Namely, while in men with inherited estrogen deficiency increased susceptibility of ER to estrogen can be present because ER is unsaturated, this unique prepubertal-like state is not present in adult men with normal secretion of E2 [17]. Thus, it seems that observations of men with inherited estrogen deficiency (due to mutation of the gene encoding aromatase), representing physiology during development, may have limited application to clinical practice that represents pathology, where estrogens are available and acts through functional ER.
Obesity and glucose levels
Overweight and obesity are well recognized risk factors for CAD in both men and women. The distribution of fatty tissue in humans shows sexual dimorphism. Females in the reproductive period of life tend to show gynoidal distribution of adipose tissue (hips, buttock), while androidal distribution predominates in men and in postmenopausal women. The former phenotype, with predominant androidal (trunk or visceral) obesity, is related to the higher risk of diabetes, insulin resistance, CAD and breast cancer development. The relationship between estrogen deficiency and the development of visceral obesity may arise from the findings that both in females and in males with inherited estrogen deficiency, androidal obesity with coexisting insulin resistance, hypercholesterolemia and hypertriglyceridemia were observed [18, 19]. Mendoza et al. [20, 21] showed higher E2 levels in healthy but obese men as compared to healthy men with normal body weight. These authors conclude that the profile of endogenous sex hormones observed in obese men (high E2, low testosterone and high E2/testosterone ratio) is similar to one found in young postinfarction men, suggesting a possible unfavorable role of E2, predisposing for CAD. Coexistence of obesity and diabetes is frequent and unfavorable for cardiovascular system. Nevertheless, the results of few studies assessing a role of E2 in the metabolism of glucose are contradictory. There are data showing that in healthy men E2 levels negatively correlate with basal blood level of glucose [12], while other authors did not confirm these results. Tchernhof et al. [15] did not find any relationship between E2 and glucose level or the degree of visceral obesity (detected on the computerized tomography). Just the opposite, in men with angiographically documented CAD an unfavorable correlation between E2 and impaired glucose tolerance was observed [11].
Endogenous estrogens versus the occurrence, clinical stage and prediction of CAD in men
Higher levels of E2 and low testosterone/E2 ratio were observed in men with CAD as compared to healthy controls, appearing together with sings of metabolic syndrome [22]. In a study of Philips et al. [23, 24] higher levels of E2 in men with CAD than in healthy men were found. Girndt et al. [25] documented higher E2 levels in men with angiographically documented CAD as compared to healthy subjects. Srzednicki et al. [26] also found significantly increased E2 levels in patients with stable and unstable CAD. Several other studies did not support the association between higher E2 and CAD. Goldberg et al. [27] and Lichtenstein et al. [28] showed that blood E2 did not differentiate healthy men and men with CAD. Hauner et al. [29] did not find any difference in E2 levels between men with angiographically documented CAD and in subjects with no coronary atherosclerosis on angiography. The lack of correlation between E2 and CAD is also supported by a study of Barrett-Connor and Khaw [30] who evaluated a cohort of over 1000 healthy men during 12 years follow up and did not find E2 level to be predictive for CAD. Similar results based on over 10 years follow up were showed by Contoreggi et al. [31]. Our recent data revealed that although in men with CAD the variability of plasma E2 is predictive for the variability of total cholesterol, LDL and HDL/total cholesterol ratio and through this way E2 may predispose toward atherogenic lipid profile, E2 blood level is not predictive for the degree of coronary artery stenosis in men [32].
Clinical administration of estrogens to men
Chronic administration of estrogens in male-to-female transsexualism resulted in the reduction of total cholesterol, LDL and Apo-lipoproteins B levels, suggesting a plausible effects of estrogens in men [33]. The influence of E2 on vascular endothelium, causing blood vessels relaxation, indicates another plausible effect of estrogens. Chester et al. [34] observed that E2 provoked in vitro relaxation of coronary arteries which was less expressed in the arteries taken from men than from women. The direct effect of E2 on coronary arteries was also studied. Both relaxation and no influence were described [35, 36]. In the randomized, double-blind, placebo controlled cross-over study on 30 healthy men, vasodilation was not increased by E2 administration [37]. On the other hand, intracoronary injection of E2 in men undergoing coronary angioplasty significantly attenuated vasoconstriction by reducing endothelin levels [38]. “The Coronary Drug Project” [39], a clinical study with the administration of estrogens in men with CAD after myocardial infarction, was stopped before the study was complete due to increased incidence of mortality and myocardial infarctions after estrogens as compared to controls. Similarly, in men with male-to-female transexualism treated orally with estrogens and anti-androgens venous thrombo-embolism was found as one of the most frequent side effects [40]. Recent clinical experience based on the hormonal “replacement” therapy in postmenopausal women and studies with the administration of oral hormonal contraceptives to women showed an increase in thrombotic events related to estrogen therapy [1]. It can not be excluded, however, that there may exist a difference between the effects of long-term estrogen therapy, which may represent unfavorable effects of estrogen [1, 39, 40] and the effects of short term estrogen administration, that may account for non-genomic stimulation of vascular dilation [34, 35, 38]. Much less is known about the influence of estrogen therapy in men on the risk factors of CAD, that are other than lipids. It was observed that estrogen administration to men results in a decrease in homocystein level [41, 42]. In turn, a favorable effect of estrogen treatment on procoagulation activity in a human male was also shown. A decrease in the inhibitor of plasminogen activation and fibrinogen levels as well as a decrease in platelet activating factor activity were seen after estrogen treatment [42].
Sex-specific differences in estrogen influence on vascular endothelium
In 2003 Gong et al. [43] showed that in contrast to women where estrogens generate nitric oxide (NO) production in the vascular endothelium, they do not do so in men. NO is responsible for vascular dilation and inhibits lipoprotein oxydation, monocyte adhesion to the endothelium, as well as production of the inflammation mediators. All these mechanisms of NO action prevent development of arteriosclerosis. Endothelial NO synthase (eNOS) is stimulated by HDL. However, according to Gong et al. [43] eNOS is stimulated only by HDL separated from females. Female HDL is a carrier of E2 and acts together with E2 in the vascular endothelium stimulating eNOS through HDL receptor, called scavenger receptor class B type I, localised in the sinuses of vascular endothelium. As HDL separated from males is not a carrier of E2 it does not stimulate eNOS in men. This difference may explain the lack of the relaxation effect of estrogen administration on coronary blood vessels [36, 37] or lack of correlations between blood E2 concentration and the severity of CAD spectrum in men [27-32].

References
1. Grady D, Applegate W, Bush T, Furberg C, Riggs B, Hulley SB. Heart and Estrogen/progestin Replacement Study (HERS): design, methods, and baseline characteristics. Control Clin Trials 1998; 19: 314-35. 2. Kula K. Induction of precocious maturation of spermatogenesis in infant rats by human menopausal gonadotropin and inhibition by simultaneous administration of gonadotropins and testosterone. Endocrinology 1988; 122: 34-9. 3. Ebling FJ, Brooks AN, Cronin AS, Ford H, Kerr JB. Estrogenic induction of spermatogenesis in the hypogonadal mouse. Endocrinology 2000; 141: 2861-9. 4. Kula K, Walczak-Jedrzejowska R, Slowikowska-Hilczer J, Oszukowska E. Estradiol enhances the stymulatory effects of FSH on testicular maturation and contributes to precocious initiation of spermatogenesis. Mol Cell Endocrinol 2001; 178: 89-97. 5. Eddy EM, Washburn TF, Bunch DO, Goulding EH, Gladen BC, Lubahn DB, et al. Targeted disruption of the estrogen receptor gene in male mice causes alteration of spermatogenesis and infertility. Endocrinology 1996; 137: 4796-805. 6. Carani C, Qin K, Simoni M, Faustini-Fustini M, Serpente S, Boyd J, et al. Effect of testosterone and estradiol in a man with aromatase deficiency. New Engl J Med 1997; 337: 91-5. 7. Kula K, Walczak-Jedrzejowska R, Slowikowska-Hilczer J, Wranicz JK, Kula P, Oszukowska E, et al. Important functions of estrogens in men - breakthrough in contemporary medicine. Przegl Lek 2005; 62: 908-15. 8. Mosselman S, Polman J, Dijkema R. ER-beta: identification and characterization of a novel human estrogen receptor. FEBS Lett 1996; 392: 49-53. 9. Sudhir K, Chou TM, Chatterjee K, Smith EP, Williams TC, Kane JP, et al. Premature coronary artery disease associated with a disruptive mutation in the estrogen receptor gene in a man. Circulation 1997; 96: 3774-7. 10. Goetz RM, Thatte HS, Prabhakar P, Cho MR, Michel T, Golan DE. Estradiol induces the calcium-dependent translocation of endothelial nitric oxide synthase. Proc Natl Acad Sci 1999; 96: 2788-93. 11. Dai WS, Gutai JP, Kuller LH, Laporte RE, Falvo-Gerard L, Caggiula A. Relation between plasma high-density lipoprotein cholesterol and sex hormone concentrations in men. Am J Cardiol 1994; 53: 1259-63. 12. Shono N, Kumagai S, Higaki Y, Nishizumi M, Sasaki H. The relationship of testosterone, estradiol, dehydroepiandrosterone-sulfate and sex hormone-binding globulin to lipid and glucose metabolism in healthy men. J Atheroscler Thromb 1996; 3: 45-51. 13. Kiel DP, Baron JA, Plymate SR, Chute CG. Sex hormones and lipoproteins in men. Am J Med 1989; 87: 35-9. 14. Duell PB, Bierman EL. The relationship between sex hormones and high-density lipoprotein cholesterol levels in healthy adult men. Arch Intern Med 1990; 150: 2317-20. 15. Tchernof A, Labrie F, Belanger A, Prud’homme D, Bouchard C, Tremblay A, et al. Relationships between endogenous steroid hormone, sex hormone-binding globulin and lipoprotein levels in men: contribution of visceral obesity, insulin levels and other metabolic variables. Atherosclerosis 1997; 133: 235-44. 16. Wranicz JK, Cygankiewicz I, Kosmider M, Kula K. Sex steroids versus lipid profile and the degree of coronary artery stenosis in men with angiographically documented coronary artery disease. Pol Arch Med Wew 2000; 5-6: 257-65. 17. Wranicz JK, Cygankiewicz I, Rosiak M, Kula P, Kula K, Zareba W. The relationship between sex hormones and lipid profile in men with coronary artery disease. Int J Cardiol 2005; 101: 105-10. 18. Morishima A, Grumbach MM, Simpson ER, Fisher C, Qin K. Aromatase deficiency in male and female siblings caused by a novel mutation and the physiological role of estrogens. J Clin Endocrinol Metab 1995; 100: 3689-98. 19. Bilezikian JP, Morishima A, Bell J, Grumbach MM. Increased bone mass as a result of estrogen therapy in a man with aromatase deficiency. New Engl J Med 1998; 339: 599-603. 20. Mendoza SG, Zerpa A, Carrasco H, Colmenares O, Rangel A, Gartside PS, H, et al. Estradiol, testosterone, apolipoproteins, lipoprotein cholesterol, and lipolitic enzymes in men with premature myocardial infarction and angiographically assessed coronary occlusion. Artery 1983; 12: 1-23. 21. Mendoza SG, Zerpa A, Velazquez E, Schwarzenberg A, Glueck CJ, Kashyap ML. Sex hormones, lipids, lipoprotein cholesterols, and apolipoproteins in normal and obese subjects: atherogenic relationship. Int J Obes 1986; 10: 427-41. 22. Dunajska K, Milewicz A, Szymczak J, Jedrzejuk D, Kuliczkowski W, Salomon P, et al. Evaluation of sex hormone levels and some metabolic factors in men with coronary atherosclerosis. Aging Male 2004; 7: 197-204. 23. Phillips GB, Castelli WP, Abbott RD, McNamara PM. Association of hyperestrogenemia and coronary heart disease in men in the Framingham cohort. Am J Med 1983; 74: 863-9. 24. Phillips GB, Pinkernell BH, Jing TY. The association of hyperestrogenemia with coronary thrombosis in men. Arterioscler Thromb Vasc Biol 1996; 16: 1383-7. 25. Girndt J, Calkanis Z, Kreuzer H, Seidel D, Scheler F. Oestrogen as a risk factor for the vascular system. Dtsch Med Wochenschr 1980; 105: 1601-3. 26. Srzednicki M, Sadowski Z, Slowinska-Srzednicka J, Zgliczynski S. Sex hormones in men with coronary arteriosclerosis. Pol Tyg Lek 1990; 45: 509-11. 27. Goldberg RJ, Gore JM, Zive M, Brady P, Klaiber E, Broverman D, et al. Serum estradiol and coronary artery disease. Am J Med 1987; 82: 1-4. 28. Lichtenstein MJ, Yarnell JW, Elwood PC, Beswick AD, Sweetnam PM, Marks V, et al. Sex hormones, insulin, lipids, and prevalent ischaemic heart disease. Am J Epidemiol 1987; 126: 647-57. 29. Hauner H, Stangl K, Burger K, Busch U, Blomer H, Pfeiffer EF. Sex hormone concentrations in men with angiographically assessed coronary artery disease – relationship to obesity and body fat distribution. Klin Wochenschr 1991; 69: 664-8. 30. Barrett-Connor E, Khaw KT. Endogenous sex hormones and cardiovascular disease in men. A prospective population-based study. Circulation 1988; 78: 539-45. 31. Contoreggi CS, Blackman MR, Andres R, Muller DC, Lakatta EG, Fleg JL, et al. Plasma levels of estradiol, testosterone, and dheas do not predict risk of coronary artery disease in men. J Androl 1990; 11: 460-70. 32. Wranicz JK, Cygankiewicz I, Kula P, Walczak-Jedrzejowska R, Slowikowska-Hilczer J, Kula K. Endogenous estradiol and bioavailable testosterone may predispose toward atherogenic lipid milieu but higher blood levels of testosterone is associated with lower number of coronary stenoses in men with coronary artery disease. Int J Biomedical Sci 2006; in press. 33. Sosa M, Jodar E, Arbelo E, Dominguez C, Saavedra P, Torres A, et al. Serum lipids and estrogen receptor polymorphism in male-to-female transsexuals: effects of estrogen treatment. Eur J Intern Med 2004; 15: 231-7. 34. Chester AH, Jiang C, Borland JA, Yacoub MH, Collins P. Oestrogen relaxes human epicardial coronary arteries through non-endothelium-dependent mechanisms. Coron Artery Dis 1995; 6: 417-22. 35. Blumenthal RS, Heldman AW, Brinker JA, Resar JR, Coombs VJ, Gloth ST, et al. Acute effects of conjugated estrogens on coronary blood flow response to acetylcholine in men. Am J Cardiol 1997; 80: 1021-4. 36. Reis SE, Bhoopalam V, Zell KA, Counihan PJ, Smith AJ, Pham S, et al. Conjugated estrogens acutely abolish abnormal cold-induced coronary vasoconstriction in male cardiac allografts. Circulation 1998; 97: 23-5. 37. New G, Berry KL, Cameron JD, Harper RW, Meredith IT. Long-term oestrogen treatment does not alter systemic rate compliance and haemodynamics in biological males. Coron Artery Dis 2000; 11: 253-9. 38. Lee TM, SU SF, Tsai CH. Oestrogen attenuates coronaryvasoconstriction alter angioplasty:role of endothelin-1. Eur J Clin Invest 2002; 32: 141-7. 39. The Coronary Drug Project Research Group. The coronary drug project: findings leading to discontinuation of the 2.5 mg day estrogen group. JAMA 1973; 226: 652-7. 40. Van Kesteren PJ, Asscheman H, Megens JA, Gooren LJ. Mortality and morbidity in transsexual subjects treated with cross-sex hormones. Clin Endocrinol 1997; 47: 337-42. 41. Giltay EJ, Verhoef P, Geleijnse JM, Stehouwer CD. Oral and transdermal estrogens both lower plasma total homocysteine in male-to-female transsexuals. Atherosclerosis 2003; 168: 139-46. 42. Giri S, Thompson PD, Taxel P, Contois JH, Otvos J, Allen R, et al. Oral estrogen improves serum lipids, homocysteine and fibrinolysis in elderly men. Atherosclerosis 1998; 137: 359-66. 43. Gong M, Wilson M, Kelly T, Su W, Dressman J, Kincer J, et al. HDL-associated estradiol stimulates endothelial NO synthase and vasodilatation in an SR-BI-dependent manner. J Clin Invest 2003; 111: 1579-87.