This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Arnal, J.-F. Articles by Bayard, F. Search for Related Content PubMed PubMed Citation Articles by Arnal, J.-F. Articles by Bayard, F. Related Collections Gene expression Endothelium/vascular type/nitric oxide

(Circulation Research. 2002;91:759.)
© 2002 American Heart Association, Inc.

Editorial

Alteration in Endothelial Estrogen Receptor Expression

A Potential Key of Vasculoprotection by Estrogens?

Jean-François Arnal, Francis Bayard

From INSERM U397 et Physiologie médicale, Hôpital Rangueil, Toulouse Cedex, France.

Correspondence to Jean-François Arnal, INSERM U397 et Physiologie médicale, Hôpital Rangueil, TSA 50032, 31059 Toulouse Cedex 9, France. E-mail arnal{at}toulouse.inserm.fr

Key Words: endothelium • estrogen receptors • estradiol • atherosclerosis • vasculoprotection

The results of the Women’s Health Initiative, a randomized controlled primary prevention trial evaluating the risks and benefits of estrogen plus progestin in healthy postmenopausal women, were published in July 2002.¹ After a mean of 5.2 years of follow-up, the data and safety monitoring board recommended cessation of the trial because the test statistic for invasive breast cancer exceeded the stopping boundary for this adverse effect and the global index statistic supported risks exceeding benefits. More surprisingly, this trial revealed that hormone replacement therapy (HRT) does not confer vasculoprotection, but even slightly increases the risk of a cardiovascular event early after the start of HRT. These results are quite similar to those reported concerning the effect of HRT in secondary prevention (Heart and Estrogen/Progestin Replacement Study [HERS, 1998]).² These data clearly conflict with previous clinical observations and experimental studies. Indeed, the clinical rationale for these two large randomized controlled trials was based on the epidemiological evidence that women are protected against the clinical complications of atherosclerosis until menopause. However, this protection, probably attributable to sex hormones, is steadily lost within the years after menopause. In addition, all experimental studies that use animal models of atherosclerosis, by feeding monkeys, swine, or rabbits an atherogenic diet,³ have clearly demonstrated that estrogens prevent fatty streak deposit, the first step of atherosclerosis. More recently, the effect of estrogens was also studied in genetically modified mice, which allowed evaluation of the expression of various genes in physiological or various pathophysiological processes. Mice deficient in apolipoprotein E demonstrate an endogenous hypercholesterolemia on a chow diet and develop fatty streaks, which are prevented by estradiol.⁴

Clinical studies have demonstrated that estrogens increase HDL-cholesterol and decrease LDL-cholesterol, thereby improving the lipoprotein profile. However, the fatty streak deposit in experimental atherosclerosis can be prevented even in the absence of changes in circulating cholesterol.^3,4 Thus, only a minor part of the vasculoprotective effect of estrogens can be attributed to a beneficial effect on circulating lipoproteins. Accordingly, a number of recent studies strongly suggest a direct effect of estrogens on the vascular wall and, in particular, on the endothelium.⁵ The endothelium is recognized to play a crucial role in the physiology of circulation, because this cell monolayer is uniquely positioned at the interface between the blood and the vessel wall. As such, it could constitute a crucial estrogen target by regulating, for example, the generation of nitric oxide (NO)⁵ or stimulating reendothelialization of the carotid artery after endovascular injury.⁶ To date, expression of the two isoforms of estrogen receptor (ER), encoded by two different genes, ER and ERß, has been observed in endothelial cells.⁷ ER, the first to be described, mediates most of the effects of estradiol (E₂) in the process of reproduction. More recently, ERß was characterized, but its role remains unknown. Thus, estrogens can directly influence endothelial physiology through a genomic mechanism, although nongenomic mechanisms responsible for short-term effects have also been demonstrated. Their respective importance remains to be clarified.⁸

Alterations of the cellular ER levels could then be of major importance in mediating the endothelial effect of E₂. This aspect is investigated by Ihionkhan and coworkers⁹ in this issue of Circulation Research. In experiments using cultured ovine endothelial cells, they demonstrated that physiological concentrations of E₂ induce an initial (2-hour) ER downregulation. This observation fits well with the decrease initially described in the uterus after estrogen administration to rats,¹⁰ as well as in cultured uterine and breast cancer cell lines after estrogen exposure.^10,11 The mechanism of this downregulation has recently been investigated in transfected cells: ER is degraded by the ubiquitin-proteasome pathway in a hormone-dependent manner, and such degradation is actually required for ER to serve as a transcription factor.¹² Whether such a mechanism is also involved in endothelium was not investigated by Ihionkhan et al⁹ and will probably deserve further investigation in future studies. This group also reports that ER downregulation is followed by an increase in ER protein abundance that becomes evident after 6 hours of hormone exposure. Thus, the recovery of ER in endothelial cells is more rapid at reaching receptor levels greater than baseline when compared with what is seen in reproductive tissues.¹⁰ Persistently elevated ER levels under continuous estrogen treatment indicate a role of the hormone in maintaining the enhanced endothelial ER expression. To what extent this dynamic gene regulation occurs in vivo, in endothelial cells from other vascular beds, and from other species, if it is modified by an associated progestin treatment, will require further investigation. Indeed, if similar kinetics occur in vivo, the route, dosage, and mode of E₂ administration could greatly influence the biological activity of E₂ in several ways. At least three routes of administration are currently proposed: oral, percutaneous, and, recently, nasal.¹³ The oral route and, more impressively, the nasal route elicit a sharp plasma peak followed by a rapid decline of the plasma concentration of the hormone, whereas the percutaneous route induces rather plateaued plasma levels, a profile close to that given by the subcutaneous pellets used in most of the experimental studies. Thus, in future studies, it will be of great interest to assess the ER expression in tissues of the experimental models in response to both continuous and intermittent E₂ administration. The intracrine or paracrine production of estrogens in the vascular wall by aromatization of androgen precursors and local metabolism will probably also have to be considered.^14,15

Ihionkhan et al⁹ reported that, in contrast to the upregulation in ER after long-term E₂ administration, the expression of ERß was downregulated, which is the first evidence of contrasting ER isoform regulation by the hormone. It again raises the question of the role of ERß in vascular biology, especially considering that this isoform is upregulated in the rat carotid artery after injury.¹⁶ Using gene targeting¹⁷ to investigate the function of the two receptors in vivo, it has been observed that E₂ accelerates reendothelialization and increases endothelial production of endothelium-derived relaxing factor (EDRF)/NO in wild-type and in ERßKO mice. These effects of E₂ are completely lost in ERKO mice, unambiguously demonstrating that ER, but not ERß, mediates these endothelial effects of E₂.^18,19 The specific gene target(s) of ERß are, if any, still to be characterized. Moreover, the full-length ER at 66 kDa may not by itself mediate the complete range of activities of E₂ in the endothelium. Human umbilical cultured endothelial cells contain two prominent ER immunoreactive bands, one of the expected 66 kDa mass and the other at 45 kDa, which may separately trigger genomic or rapid signaling responses.²⁰ Tissue-specific expression of such ER mRNA variants, including aorta, have previously been described in the mouse²¹ and in humans.²² These observations in cultured cells recently found an in vivo counterpart when we demonstrated that the AF-1 activation function of ER may be dispensable to mediate the effect of estradiol on endothelial NO production in mice.²³ At this point, it is also worth noting that the cellular level of the large series of nuclear receptor coactivators may play as great a role in estrogen action as the ER concentration itself.

Finally, in addition to the endothelium, the atherosclerotic process involves many other cell populations, such as smooth muscle, monocyte macrophages, and lymphocyte populations. In each of them, conditions of estrogen deficiency after surgical or natural menopause may also be characterized by altered ER expression, thereby potentially contributing to the increase in vascular disease associated with these states.⁵ Future studies will have to precisely define their specific roles. This assessment is required to understand the mechanisms by which estrogen can be vasculoprotective (or not).

Footnotes

The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

References

1. Risks, and benefits of estrogen plus progestin in healthy postmenopausal women: principal results from the Women’s Health Initiative randomized controlled trial. JAMA. 2002; 288: 321–333.[Abstract/Free Full Text]
2. Hulley S, Grady D, Bush T, Furberg C, Herrington D, Riggs B. Randomized trial of estrogen plus progestin for secondary prevention of coronary heart disease in postmenopausal women. JAMA. 1998; 280: 605–613.[Abstract/Free Full Text]
3. Holm P. Effect of estrogen on development of atherosclerosis: a review of experimental animal studies. Dan Med Bull. 2001; 48: 146–160.[Medline] [Order article via Infotrieve]
4. Hodgin J, Maeda N. Estrogen and mouse models of atherosclerosis. Endocrinology. In press.
5. Mendelsohn ME, Karas RH. The protective effects of estrogen on the cardiovascular system. N Engl J Med. 1999; 340: 1801–1811.[Free Full Text]
6. Krasinski K, Spyridopoulos I, Asahara T, van der Zee R, Isner JM, Losordo DW. Estradiol accelerates functional endothelial recovery after arterial injury. Circulation. 1997; 95: 1768–1772.[Abstract/Free Full Text]
7. Couse JF, Korach KS. Estrogen receptor null mice: what have we learned and where will they lead us? Endocr Rev. 1999; 20: 358–417.[Abstract/Free Full Text]
8. Mendelsohn ME. Nongenomic, ER-mediated activation of endothelial nitric oxide synthase: how does it work? What does it mean? Circ Res. 2000; 87: 956–960.[Abstract/Free Full Text]
9. Ihionkhan CE, Chambliss KL, Gibson LL, Hahner LD, Mendelsohn ME, Shaul PW. Estrogen causes dynamic alterations in endothelial estrogen receptor expression. Circ Res. 2002; 91: 814–820.[Abstract/Free Full Text]
10. Nardulli AM, Katzenellenbogen BS. Dynamics of estrogen receptor turnover in uterine cells in vitro and in uteri in vivo. Endocrinology. 1986; 119: 2038–2046.[Abstract]
11. Read LD, Greene GL, Katzenellenbogen BS. Regulation of estrogen receptor messenger ribonucleic acid and protein levels in human breast cancer cell lines by sex steroid hormones, their antagonists, and growth factors. Mol Endocrinol. 1989; 3: 295–304.[Abstract]
12. Nawaz Z, Lonard DM, Dennis AP, Smith CL, O’Malley BW. Proteasome-dependent degradation of the human estrogen receptor. Proc Natl Acad Sci U S A. 1999; 96: 1858–1862.[Abstract/Free Full Text]
13. Devissaguet JP, Brion N, Lhote O, Deloffre P. Pulsed estrogen therapy: pharmacokinetics of intranasal 17-ß-estradiol (S21400) in postmenopausal women and comparison with oral and transdermal formulations. Eur J Drug Metab Pharmacokinet. 1999; 24: 265–271.[Medline] [Order article via Infotrieve]
14. Bayard F, Clamens S, Delsol G, Blaes N, Maret A, Faye JC. Oestrogen synthesis, oestrogen metabolism and functional oestrogen receptors in bovine aortic endothelial cells. Ciba Found Symp. 1995; 191: 122–132.[Medline] [Order article via Infotrieve]
15. Mukherjee TK, Dinh H, Chaudhuri G, Nathan L. Testosterone attenuates expression of vascular cell adhesion molecule-1 by conversion to estradiol by aromatase in endothelial cells: implications in atherosclerosis. Proc Natl Acad Sci U S A. 2002; 99: 4055–4060.[Abstract/Free Full Text]
16. Makela S, Savolainen H, Aavik E, Myllarniemi M, Strauss L, Taskinen E, Gustafsson JA, Hayry P. Differentiation between vasculoprotective and uterotrophic effects of ligands with different binding affinities to estrogen receptors and ß. Proc Natl Acad Sci U S A. 1999; 96: 7077–7082.[Abstract/Free Full Text]
17. Dupont S, Krust A, Gansmuller A, Dierich A, Chambon P, Mark M. Effect of single and compound knockouts of estrogen receptors (ER) and ß (ERß) on mouse reproductive phenotypes. Development. 2000; 127: 4277–4291.[Abstract]
18. Brouchet L, Krust A, Dupont S, Chambon P, Bayard F, Arnal J-F. Estradiol accelerates reendothelialization in mouse carotid artery through estrogen receptor- but not estrogen receptor-ß. Circulation. 2001; 103: 423–428.[Abstract/Free Full Text]
19. Darblade B, Pendaries C, Krust A, Dupont S, Fouque MJ, Rami J, Chambon P, Bayard F, Arnal J-F. Estradiol alters nitric oxide production in the mouse aorta through the -, but not ß-, estrogen receptor. Circ Res. 2001; 90: 413–419.
20. Russell KS, Haynes MP, Sinha D, Clerisme E, Bender JR. Human vascular endothelial cells contain membrane binding sites for estradiol, which mediate rapid intracellular signaling. Proc Natl Acad Sci U S A. 2000; 97: 5930–5935.[Abstract/Free Full Text]
21. Kos M, O’Brien S, Flouriot G, Gannon F. Tissue-specific expression of multiple mRNA variants of the mouse estrogen receptor gene. FEBS Lett. 2000; 477: 15–20.[CrossRef][Medline] [Order article via Infotrieve]
22. Flouriot G, Brand H, Denger S, Metivier R, Kos M, Reid G, Sonntag-Buck V, Gannon F. Identification of a new isoform of the human estrogen receptor- (hER-) that is encoded by distinct transcripts and that is able to repress hER- activation function 1. EMBO J. 2000; 19: 4688–4700.[CrossRef][Medline] [Order article via Infotrieve]
23. Pendaries C, Darblade B, Rochaix P, Krust A, Chambon P, Korach KS, Bayard F, Arnal J-F. The AF-1 activation-function of ER may be dispensable to mediate the effect of estradiol on endothelial NO production in mice. Proc Natl Acad Sci U S A. 2002; 99: 2205–2210.[Abstract/Free Full Text]

This article has been cited by other articles:

				V. M. Miller and S. P. Duckles Vascular Actions of Estrogens: Functional Implications Pharmacol. Rev., June 1, 2008; 60(2): 210 - 241. [Abstract] [Full Text] [PDF]

				P. Geraldes, M. G. Sirois, and J.-F. Tanguay Specific Contribution of Estrogen Receptors on Mitogen-Activated Protein Kinase Pathways and Vascular Cell Activation Circ. Res., September 5, 2003; 93(5): 399 - 405. [Abstract] [Full Text] [PDF]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Arnal, J.-F. Articles by Bayard, F. Search for Related Content PubMed PubMed Citation Articles by Arnal, J.-F. Articles by Bayard, F. Related Collections Gene expression Endothelium/vascular type/nitric oxide