Therapeutic potential of selective estrogen receptor modulators

508

Therapeutic Jan-Ake

potential

of selective estrogen receptor modulators

Gustafsson

The hormone estradiol has effects on many tissues in both males and females. Some of these effects, such as inhibition of cancer growth and modulation of the devastating such as the effects on the breast and endometrium, The task of designing

are

drugs that would have

only the good effects of estradiol has, until recently, seemed almost impossible

because

only one estrogen

receptor and that an estrogenic

it was thought that there was

was definitively categorized antagonist.

as an estrogen

receptors

opposite

effects on gene transcription.

understood

which, under certain conditions,

because

as agonists

a single agent can be an agonist in

one tissue and an antagonist receptor

have

In addition, it is now

that agents cannot be described

or antagonists estrogen

agent

agonist or an

More recently we have learnt that there are two

estrogen

in another. The term ‘selective

modulator’ was designed

this. The idea of estrogen hope that tissue-specific

receptor

to incorporate

modulators

estrogens

has raised new

or anti-estrogens

can be

designed.

Address Department of Medical Nutrition, Karolinska Institute, Novum, S-l 4186 Current

Huddinge, Opinion

Sweden

in Chemical

Biology

1998,

2:508-51

1

http://biomednet.com/elecref/1367593100200508 0 Current Biology Publications ISSN

1367-5931

Abbreviations 3P-adiol DES

5cL-androstane-3P,l7P-diol 4,4’-dihydroxytrans-o,P-diethylstilbene

ER ERE ERKO

estrogen estrogen estrogen

GR LBD

glucocorticoid ligand-binding

MR PTH

mineralocorticoid receptor parathyroid hormone

SERM

selective estrogen

receptor response element receptor-cl knockout receptor domain

receptor

until their

effects

of aging on bone, brain, skin and bladder, are good. Others, undesirable.

but only as modulators of the estrogen receptor actions in specific tissues have been evaluated.

This new awareness of tissue-selective modulators of estrogen receptors has raised new hopes of designer drugs that could be targeted to selected tissues. Nowhere is the need for SERMs more urgent than in the post-menopausal female, where the agonistic action of estrogen is needed for bone maintenance and brain function but is not desirable in the uterus and breast. In the case of estrogen-responsive cancers, where there is also an urgent need for SERMs, the aim would be to target of malignant cells with an estrogen antagonist that would spare other estrogen-responsive tissues. The term SERM has removed some of the confusion about how to label several pharmaceuticals, plant components (phytoestrogens, which should now be phytoserms) and environmental pollutants (xenoestrogens, which should be xenoserms). It has also provided some basis for the concept that there could be ‘good estrogens’, which could selectively protect the blood vessels, brain and urogenital tract, and ‘bad estrogens’, which would affect the breast and uterus. The question of how these agents bring about their tissue-specific actions remains the focus of very active and exciting research. In the past year several key discoveries have been made that provide many new insights into the possible mechanisms of action of SERMs and have opened whole new vistas for future research. In addition, updates have been published on two clinical trials where the SERMs raloxifen and ipriflavone are being used for treatment research discoveries will be the subject

of post-menopausal osteoporosis. The and the results of the clinical studies of this review.

Estrogen receptor-a knockout, estrogen receptor f3 and understanding estrogen action modulator

Introduction The term ‘selective estrogen receptor modulator’ (SERM) was coined when it was realized that the old terms estrogen agonist and estrogen antagonist could not account for the actions of two well-known drugs, tamoxifen (Zeneca Pharmaceuticals, Macclesfield, UK) and raloxifen (Eli Lilly, Indianapolis, USA) [ 11. These drugs, which were characterized as estrogen antagonists/partial agonists on the basis of their binding to the estrogen receptor ERa, act as full estrogen agonists in bone. Furthermore, tamoxifen has estrogenic actions in the uterus but raloxifen does not [Z]. The acronym SERM takes into account the fact that the activity of these agents is tissue selective and they cannot be definitively labeled as agonists or antagonists

The development of estrogen receptor-a knockout (ERKO) mice [3] and a discovery of the second estrogen receptor, ERP [4], have contributed enormously to recent progress in understanding estrogen action. If ERP were simply a backup receptor for ERa, it should have been able to take over the function of ERa in ERKO mice. This is clearly not the case as ERKO mice are infertile [3], females do not display sexual receptivity [5’] and males do not have typical male-aggressive behavior [6*] despite the presence of ERP. The ERKO mouse has provided evidence that estrogen regulates resorption of fluid in the head of the epididymis via ERa [7’]. In the absence of ERa, sperm in the epididymis is dilute and this results in infertility. This phenotype exists despite high levels of ERP in ERKO mouse testes [8]. On the other hand, the brains of ERKO mice respond to estradiol with induction of progesterone receptors in a pattern that is similar to that of wild type animals [9”] and, in the blood

Selective

vessels of ERKO mice, estrogen can still inhibit vascular injury caused by mechanical damage [ lO**]. These results strongly favor distinct roles for ERa and ERB and foster the hope that specific modulators of these two receptors will be discovered and will be invaluable clinical tools. It seems that ERB has an important role in the brain and the vascular system and ERa a role in the reproductive system. It is too early to determine the relative roles of these two receptors in bone. It has been clear for a long time that estrogen is necessary for maintenance of bone in women. The surprising recent finding, which came from a male with no functional ERa in the male is also [ill, was that bone maintenance ERa-dependent. In men, the plasma level of estradiol is similar to that in the post-menopausal female. The question that must be addressed is how do males maintain their bone with levels of estradiol that are not sufficient for the task in women? One possible explanation is local synthesis of estradiol in the bone. If this were the case, the question would then be why do women not have sufficient local estrogen synthesis? Another possibility is that males have an alternative estrogen. In fact, it is quite well recognized that a metabolite of dihydrotestosterone, 5a-androstane-3B,17B-diol (3B-adiol), is a good estrogen with high affinity for both ERa and ERB [lZ**]. If this is indeed the male estrogen responsible for bone maintenance, it would qualify as a novel SERM, with specific action on bone and no estrogenic action in the prostate, pituitary or brain. This tissue specificity is due to the presence in the prostate, brain and pituitary of a specific form of cytochrome P450 that is very efficient in eliminating 3B-adiol [ 13,141.

Estrogen receptors and their interactions selective estrogen receptor modulators

with

The solution of the three-dimensional structure of the ligand-binding domain of ERa crystallized in the presence of either raloxifen or estradiol has revealed what was long thought to be the case-that there is a major difference in the structure of the ligand-binding domain when it is occupied by estradiol or by raloxifen [15”]. These two ligands induce two distinct conformations of helix 12, a major transactivation domain of the receptor, and thus provide different surfaces with which the receptor interacts with other components of the transcription machinery. These components would include co-activators and co-repressors. This discovery goes a long way towards explaining the mechanism of the distinct array of cellular responses elicited by these two agents in different cells. Similar studies with the ERB ligand-binding domain are urgently

needed.

The AP-1 site and the estrogen response element as targets of estrogen receptors In addition to alteration in receptor structure by different ligands described above, another possible explanation for the tissue-specific actions of estrogen is that ligand-

estrogen

receptor

modulators

Gustafsson

509

activated estrogen receptors can bind to various DNA enhancer elements and produce distinct patterns of gene transcription at each of these sites. So far, two types of estrogen response elements (EREs) have been described. One is the classical ERE, composed of two inverted hexanucleotide repeats, and the other is the activator protein-l (AP-1) site, which is the binding site for the Fos-Jun transcription factor complex. Both ERa and ERB bind to EREs and transactivate. With regard to the AP-1 site, however, a surprising discovery was made by Peach et nl. [16”], who have shown that ERa and ERB behave very differently at the AP-1 site. With ERa in the presence of estradiol or the estrogen agonist (4,4’-dihydroxy-trans-a, B-diethylstilbene (DES), transcription is activated from this site. With ERB under the same conditions, transcription is inhibited. Even more fascinating, it was found that raloxifene, tamoxifen and ICI 182780 (Zeneca Pharmaceuticals, Macclesfield, UK) could stimulate transcription from the AP-1 site with ERB while these agents inhibited transcription in the presence bf ERa. Clearly, agonist and antagonist actions are defined by the DNA response element and by the ratio of ERa to ERB in the cell. An additional set of experiments coming from a completely independent team of investigators sheds some more light on the physiological relevance of this interaction of ERs at the AP-1 site. The AP-1 site is also called the antioxidant response element and many phytoestrogens are known to be antioxidants. Katzenellenbogen and colleagues have found that tamoxifen can induce, and estrogen repress, the enzyme quinone reductase in MCF-7 (breast cancer) cells [17”]. This antioxidant action of tamoxifen may be mediated by ERB on the AP-1 site and may indicate that part of the role of ERB may be to protect tissues from oxidative stress by inducing a battery of protective enzymes. The suppression of transcription at the AP-1 site by ERB in the presence of estradiol may indicate another, quite new, function of ERBsuppression of cell growth. The AP-1 complex is an immediate early transcription complex important for the initiation of cell growth. Repression by the ERB-estradiol complex of the site at which it binds could be a mechanism for regulating growth in tissues like the prostate. In fact, it has already been demonstrated that the response of the carotid artery to estradiol in the absence of ERa is suppression of stromal growth [lO”].

The raloxifen clinical trial Results of the first clinical trials on the use of raloxifen to prevent post-menopausal bone loss have been published and the results look good [18’]. Raloxifen protects the bone but does not stimulate the uterus. It also has some beneficial effects on plasma lipoproteins. Unfortunately, raloxifene is an estrogen antagonist in the vasculature and women suffer from hot flushes. The meaning of this action on the vasculature is not clear. Estrogen increases

510

Next generation

therapeutics

production of muscle relaxants, including nitric oxide, in the endothelium and this causes relaxation of the blood vessel wall. The long-term effects of raloxifen on blood vessel walls and its effects on the risk of coronary events must, therefore, be viewed with concern.

Other new selective estrogen receptor modulators Ipriflavone (7-isopropoxy-isoflavone) is a derivative of isoflavone that does not bind to either ERa or ERB and is said to be devoid of estrogenic properties. It can, however, prevent bone loss resulting from estrogen deprivation or from excess parathyroid hormone (PTH). A multicenter clinical trial is underway in Europe to test the effectiveness of ipriflavone in post-menopausal women [19’]. This drug is being called a nonhormonal agent for prevention of bone loss. Isoflavones such as genistein (4’,5,7-trihydroxy-isoflavone) and daidzein (4’,7-dihydroxy-isoflavone) are weak estrogens. Ipriflavone can be metabolized in the body by removal of the isopropyl group and hydroxylation of the 4’ position to produce daidzein. Genistein can also reduce bone loss in ovariectomized rats [ZO] and can inhibit PTH-induced bone resorption in tissue culture [al]. Ipriflavone itself, at lo-TM, can displace estradiol from its binding site in MCF-7 cells [Z?]. This indicates that it is most probably similar to genistein, a SERM with specific activity in bone. Recently, yet another dietary component, resveratrol (trans_3,4’,5_trihydroxystilbene), which bears a chemical resemblance to DES, was shown to be estrogenic in compound is abundant in MCF-7 cells [23] . This grape skins and is therefore found in wine. The average concentration of resveratrol in wine is lo-ZOpM, which is the concentration at which this compound is estrogenic. It is premature to suggest that resveratrol in wine may be related to the cardiovascular benefits of drinking wine as it is not clear how effective levels of this compound could be achieved in the body by drinking the recommended one or two glasses of wine each day. Data is needed on the pharmacokinetic disposition of this compound in the body. In addition, if resveratrol is estrogenic in a similar manner to DES, red wine may not be benign to the breast. Estrogens are not just female hormones. They have important effects on the male urogenital tract, on bone and in the brain. The prostate epithelium contains high levels of ERB, the function of which is completely unknown. In human populations there is a correlation between sperm count and birth rate [24]. Environmental estrogens have been implicated in reduction of sperm count in men [25]. This idea has been difficult to reconcile with the high birth rates and the lack of evidence that sperm counts are low in countries where the consumption of soya, which contains phytoestrogen, is high. The concept of SERMS may make it easier to understand the difference between the effects of xenoestrogens and those of phytoestrogens on the male reproductive tract. In normal mice, tamoxifen can produce

the phenotype observed in ERKO micethat is, dilute sperm that leads to infertility [7*]. It is possible that certain xenoestrogens can act as estrogen antagonists in the testis and may be causing the small but measurable reduction in sperm count. This hypothesis needs to be tested experimentally. So far no specific ERa or ERB ligands have been discovered, but some phytoestrogens are better ligands for ERB [ll]; this offers the promise that specific ligands will be found for each receptor. The amino acid homology between ERa ligand-binding domain (LBD) and ERB LBD is of the same magnitude as that between the glucocorticoid receptor (GR) LBD and mineralocorticoid receptor (MR) LBD [26]. Both GR and MR bind glucocorticoids very well but the two receptors have different functions in the body and have distinct natural ligands. If a lesson can be learnt from the GR/MR story, where tissue sensitivity to MR is determined by the capacity of the tissue to inactivate glucocorticoids [27], the story of tissue-specific estrogen action may be more complicated than two receptors, two response elements and two conformations of the LBD.

Conclusions If the concept of tissue-selective estrogen modulators holds true and the task of creating such selective molecules can be realized, there is no tissue in the body where a selective estrogen agonist or antagonist might not be of clinical use. Because of space limitations I have focused here only on bone, brain and the cardiovascular system; however, targets such as the skin, hair follicles, immune system, bladder and prostate are equally important because diseases such as benign prostatic hyperplasia, prostate cancer and autoimmune disease are serious health problems, and hair loss and aging of the skin are serious psychological ones.

Acknowledgement This

study was supported

References

by a grant from the Swedish

and recommended

Cancer

Society.

reading

Papers of particular interest, published within the annual period of review, have been highlighted as: . l

*

of special interest of outstanding interest Mitlak BH, Cohen FJ: In search of optimal long-term female hormone replacement: the potential of selective estrogen receptor modulators. Harm Res 1997, 48:155-l 63. Sato M, Rippy MK, Bryant HU: Raloxifene, tamoxifen, nafoxidine, or estrogen effects on reproductive and nonreproductive tissues in ovariectomized rats. FASEB J 1996, 10:905-912. Korach KS. Come JF Curtis SW. Washburn TF Lindzev J. Kimbro KS: Eddy EM: Migliaccio ‘S, Snedeker S’M, Lubahn’ DB et a/.: Estrogen receptor gene disruption: molecular characterization and experimental and clinical phenotypes. Recent frogr Harm Res 1996, 51 :159-l 86. Kuiper GG, Enmark E, Pelto-Huikko M, Nilsson S, Gustafsson J-A: Cloning of a novel receptor expressed in rat prostate and ovary. froc Nat/ Acad Sci USA 1996, 93:5925-5930.

Selective

5. .

Rissman EF, Early AH, Taylor JA, Korach KS, Lubahn DB: Estrogen receptors are essential for female sexual receptivity. Endocrinology 1997, 138:507-510. This paper, and [6’,7’1, are collectors’ items, since they all provide original information on the phenotype of ERLP mice. 6. .

Ogawa S, Lubahn DB, Korach KS, F’faff DW: Behavioral effects of estrogen receptor gene disruption in male mice. froc Nat/ Acad SC; USA 1997, 94:1476-l 481. This paper, and [5’,7’1, are collectors’ items, since they all provide original information on the phenotype of ERLP mice. 7. .

Hess RA, Bunick D, Lee KH, Bahr J, Taylor JA, Korach KS, Lubahn DB: A role for oestrogens in the male reproductive system. Nature 1997, 390:509-512. This paper, and [5’,6’1, are collectors’ items, since they all provide original information on the phenotype of ERLP mice. 8.

Cause JF, Lindzey J, Grandien K, Gustafsson J-O, Korach KS: Tissue distribution and quantitative analysis of estrogen receptor-alpha (ER-alpha) and estrogen receptor-beta (ERbeta) messenger ribonucleic acid in the wild-type and ERalpha-knockout mouse. Endocrinology 1997, 138:4613-4621.

9. ..

Shughrue PJ, Lubahn DB, Negro-Vilar A, Korach KS, Merchenthaler I: Responses in the brain of estrogen receptor alpha-disrupted mice. froc Nat/ Acad Sci USA 1997, 94:11008-l 1012. This paper is a uniquely original and important study showing not only that estrogen receptor u is important in the male reproductive system but also providing a mechanism through which estradiol affects male fertility. 10. ..

lafrati MD, Karas RH, Aronovitz M, Kim S, Sullivan TR Jr, Lubahn DB, O’Donnell TF Jr, Korach KS, Mendelsohn ME: Estrogen inhibits the vascular injury response in estrogen receptor alpha-deficient mice. Nat Med 1997, 3:545-548. This is a very thought provoking and important paper revealing estogen effects in the coromary arteries in estrogen receptor (ER) knockout mice and thus suggesting a function for ERP in the vascular system. 11.

Sudhir K, Chou TM, Chattetjee K, Smith EP, Williams TC, Kane JP, Malloy MJ, Korach KS, Rubanyi GM: Premature coronary artery disease associated with a disruptive mutation in the estrogen receptor gene in a man. Circulation 1997, 96:3774-3777.

12. ..

Kuiper GGLM, Carlsson B, Grandien K, Enmark E, Haggblad J, Nilsson S, Gustafsson J-w: Comparison of the ligand binding specificity and transcript tissue distribution of estrogen receptors alpha and beta. Endocrinology 1997, 138:863-870. This paper provides important and useful information for those interested in specific ligands for the two estrogen receptors. In vitro translated receptors were used for binding studies with a series of steroids and their metabolites, phytoestrogens and xenoestrogens. 13.

14.

15. ..

Sundin M, Warner M, Haaparanta T, Gustafsson J-p\: Isolation and catalytic activity of cytochrome P450 from ventral prostate of control rats. J Biol Chem 1987, 262:12293-l 2297. Warner M, StrGmstedt M, MGller L, Gustafsson J-p\: Distribution and regulation of Su-androstane-3!3,17P-diol hydroxylase in the rat central nervous system. Endocrinology 1989, 124:26992706. Brzozowski AM, Pike ACW, Dauter Z, Hubbard RE, Bonn T, Engstrom 0, Ohman L, Greene GL, Gustafsson J-A, Carlquist M: Molecular basis of agonism and antagonism in the oestrogen receptor. Nature 1997, 389:753-758.

estrogen

receptor

modulators

Gustafsson

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Truly a landmark paper revealing structural differences in the ligand binding domain of estrogen receptor u in the presence of agonist and antagonist. 16. ..

Paech K, Webb P, Kuiper GGJM, Nilsson S, Gustafsson JA, Kushner PJ, Scanlan TS: Differential ligand activation of estrogen receptors ER-alpha and ER-beta at AP-1 sites Science 1997, 277:1508-l 510. This is a very important paper showing for the first time that estrogen receptor (ER)u and ERP have opposite effects at the AP-1 site. 1 7. ..

Montano MM, Katzenellenbogen BS: The quinone reductase gene: a unique estrogen receptor-regulated gene that is activated by antiestrogens. froc Nat/ Acad Sci USA 1997, 94:2581-2586. The authors of this paper reveal that quinone reductase is an important marker gene which is upregulated by tamoxifen. It may be extremely useful in distinguishing estrogen receptor (ER)u and ERP effects in viva. 18. .

Delmas PD, Bjarnason NH, Mitlak BH, Ravoux AC, Shah AS, Huster WJ, Draper M, Christiansen C: Effects of raloxifene on bone mineral density, serum cholesterol concentrations, and uterine endometrium in postmenopausal women. New Engl J Med 1997, 337:1641-l 647. This paper makes important reading for those following the clinical effects of long term use of anti-estrogens. 19. .

Reginster JY, Bufalino L, Christiansen C, Devogelaer JP, Gennari C, Riis BJ, Roux C: Design for an ipriflavone multicenter European fracture study. Calc Tiss Inf 1997, 61 :S28-32. This paper makes important reading for those interested in the clinical use of ipriflavone as an alternative to hormone replacement therapy in the postmenopausal female. 20.

Yamaguchi M, Gao YH: Inhibitory effect of genistein on bone resorption in tissue culture. Biochem fbarmacoll998, 55:71-76.

21.

Dodge JA, Glasebrook AL, Magee DE: Phillips DL, Sato M, Short LL, Bryant HU: Environmental estrogens: effects on cholesterol lowering and bone in the ovariectomized rat. J Steroid Biocbem MO/ Bioll996, 59:155-l 61.

22.

Petilli M, Fiorelli G: Benvenuti S, Frediani U, Gori F, Brandi ML: Interactions between ipriflavone and the estrogen receptor. Calc Tiss Inf 1995, 56:160-l 65.

23.

Gehm BD, McAndrews JM, Chien PY, Jameson JL: Resveratrol, a polyphenolic compound found in grapes and wine, is an agonist for the estrogen receptor. froc Nat/ Acad Sci USA 1997, 94:14138-l 4143.

24.

Fisch H, Andrews H, Hendricks J, Goluboff ET, Olson JH, Olsson CA: The relationship of sperm counts to birth rates: a population based study. J Ural 1997, 157:840-843.

25.

Safe SH: Environmental and dietary estrogens and human health: is there a problem? Environ Health ferspecf 1995, 103:346-351.

26.

Kuiper GG, Gustafsson J-w: The novel estrogen receptor-beta subtype: potential role in the cell- and promoter-specific actions of estrogens and anti-estrogens. FEES Leff 1997, 410:87-90.

27.

Benediktsson R, Walker BR, Edwards CR: Cellular selectivity of aldosterone action: role of 11 beta-hydroxysteroid dehydrogenase. Curr Opin Nepbrol Hyperfen 1995, 4:41-46.