Estrogen Receptor β Activates the Human Retinoic Acid Receptorα -1 Promoter in Response to Tamoxifen and Other Estrogen Receptor Antagonists, but Not in Response to Estrogen
Aihua ZouKeith B. MarschkeKatharine E. ArnoldE. BergerPatrick FitzgeraldDale E. MaisElizabeth A. Allegretto
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Human estrogen receptor-alpha (hERalpha) or -beta (hERbeta) transfected into Hep G2 or COS1 cells each responded to estrogen to increase transcription from an estrogen-responsive element (ERE)-driven reporter vector with similar fold induction through a classical mechanism involving direct receptor binding to DNA. ER antagonists inhibited this estrogen induction through both hERalpha and hERbeta, although raloxifene was more potent through ERalpha than ERbeta, and tamoxifen was more potent via ERbeta than ERalpha. We have shown previously that estrogen stimulated the human retinoic acid receptor-alpha-1 (hRARalpha-1) promoter through nonclassical EREs by a mechanism that was ERalpha dependent, but that did not involve direct receptor binding to DNA. We show here that in contrast to hERalpha, hERbeta did not induce reporter activity driven by the hRARalpha-1 promoter in the presence of estrogen. While hERbeta did not confer estrogen responsiveness on this promoter, it did elicit transcriptional activation in the presence of 4-hydroxytamoxifen (4-OH-Tam). Additionally, this 4-OH-Tam agonist activity via ERbeta was completely blocked by estrogen. Like ERalpha, transcriptional activation of this promoter by ERbeta was not mediated by direct receptor binding to DNA. While hERalpha was shown to act through two estrogen-responsive sequences within the promoter, hERbeta acted only at the 3'-region, through two Sp1 sites, in response to 4-OH-Tam. Other ER antagonists including raloxifene, ICI-164,384 and ICI-182,780 also acted as agonists through ERbeta via the hRARalpha-1 promoter. Through the use of mutant and chimeric receptors, it was shown that the 4-OH-Tam activity via ERbeta from the hRARalpha-1 promoter in Hep G2 cells required the amino-terminal region of ERbeta, a region that was not necessary for estrogen-induced ERbeta activity from an ERE in Hep G2 cells. Additionally, the progesterone receptor (PR) antagonist RU486 acted as a weak (IC50 >1 microM) antagonist via hERalpha and as a fairly potent (IC50 approximately 200 nM) antagonist via hERbeta from an ERE-driven reporter in cells that do not express PR. Although RU486 bound only weakly to ERalpha or ERbeta in vitro, it did bind to ERbeta in whole-cell binding assays, and therefore, it is likely metabolized to an ERbeta-interacting compound in the cell. Interestingly, RU486 acted as an agonist through ERbeta to stimulate the hRARalpha-1 promoter in Hep G2 cells. These findings may have ramifications in breast cancer treatment regimens utilizing tamoxifen or other ER antagonists and may explain some of the known estrogenic or antiestrogenic biological actions of RU486.Keywords:
Estrogen receptor beta
Hormone response element
Estrogen receptor alpha
Raloxifene
Estrogen-related receptor gamma
Estrogen-related receptor alpha
Several lines of evidence demonstrate that the objective of osteoporosis treatment consists in the prophylaxis of osteoporotic fractures. With the endpoint of osteoporosis treatment thus clarified, currently, the selective estrogen receptor modulator (SERM) raloxifine represents the mainstay of therapy for osteoporosis, together with the antiresorptive agents bisphosphonates. Thus, this review has drawn mainly on the results of the MORE study to explore the efficacy of raloxifene in inhibiting bone metabolism, increasing bone mineral density effects, and preventing bone fractures. Notably, the available evidence for raloxifene suggests that the efficacy of raloxifene in preventing bone fractures has not only to do with bone mineral density but also to do with bone quality.
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With the endpoint of osteoporosis treatment thus clarified, currently, the selective estrogen receptor modulator (SERM) raloxifene represents the mainstay of therapy for osteoporosis, together with the antiresorptive agents bisphosphonates. Thus, this review has drawn mainly on the results of the MORE study to explore the efficacy of raloxifene in inhibiting bone metabolism, increasing bone mineral density effects, and preventing bone fractures. Notably, the available evidence for raloxifene suggests that the efficacy of raloxifene in preventing bone fractures has not only to do with bone mineral density but also to do with bone quality.
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Selective estrogen receptor Modulator (SERM)은 에스트로겐 수용체와 결합하여, 조직마다 다르게 작용하여 길항이나 대항의 역할을 수행한다. 이러한 SERM 중의 하나인 raloxifene은 여성호르몬 수용체와 nanomolar 친화력을 가지고, 조직에 따라 길항제와 대항제로 작용한다. 즉 뼈와 지질에는 에스트로겐 길항제로 작용하고, 유방과 자궁에는 대항제로 작용한다. Tamoxifen은 다른 종류의 SERM으로 길항제와 대항제의 양면성을 자궁 내막에 작용한다. 그러므로 자궁근종은 에스트로겐 의존성이므로 SERM을 이용하여 사람 자궁근종 세포에 투여함으로써 치료의 가능성을 보고자 이 연구를 계획하였다.
자궁근종과 정상 자궁 근 세포를 일차 배양하여 에스트로겐의 투여 전후의 반응을 보았으며, SERM 투여후의 반응을 MTS 및 생존세포수 계산을 하였고, 세포주기회로 및 세포자멸사를 보기 위해 FACS와 Western-blot을 시행하였다.
일차 배양된 자궁근종와 정상 자궁근 세포에서 에스트로겐과 황체호르몬 수용체의 발현을 확인하고 수용체 양성군과 음성군으로 구별하였다. 에스트로겐 수용체 음성군에서는 에스트로겐을 10^-12에서 10^-6 mol/L까지 용량을 증가하면서 투여하였으나 세포의 증식은 자궁근종 세포나 정상에서는 보이지 않았다. 반대로 에스트로겐 수용체양성군에서는 에스트로겐을 용량의 증가와 투여 일수를 증가하여 세포의 증식이 용량에 비례하여 증가하는 것을 보여주었다. 자궁근종와 정상 자궁근 세포에 에스트로겐을 10^-6 mol/L을 6일간 투여 후 raloxifene을 10^-11에서 10^-7 mol/L까지 용량을 증가하면서 투여하여, 에스트로겐 수용체 음성군에서는 세포 증식의 억제가 거의 없었으나, 양성군에서는 증식의 억제를 관찰하였다. 같은 조건으로 tamoxifen을 10^-11에서 10^-7 mol/L까지 용량을 증가하면서 투여하여, 에스트로겐 수용체 음성군에서는 변화가 없었고, 수용체 양성군에서는 증식의 억제가 관찰되었으나 raloxifene의 증식 억제보다는 적었다. ELT-3 세포주에 tsmoxifen, raloxifene을 투여하여 효과를 비교하여, 두 약제에 모두 세포 증시 억제의 효과를 확인하였다. ELT-3 세포를 FACS를 이용한 세포 주기회로 분석에서 어느 특정한 주기도 증가하는 것을 보이지 않았다. ELT-3 세포를 FACS를 이용한 세포 주기회로 분석에서 어느 특정한 주기도 증가하는 것을 보이지 않았다. 세포 증식의 억제가 세포자멸사와 세포 주기 회로에 관계하는 유전자의 발현을 보았으나, 약제 투여 전이나 후의 발현의 차이가 없었다.
이상의 결과로 보아 SERM은 자궁근종의 세포 증식을 억제하는 역할을 하며, raloxifene의 억제작용이 tamoxifen보다 우수하여 raloxifene이 가임기 여성의 자궁근종의 약물 치료법에 가능성을 보여주었다고 생각한다. 향후 좀더 많은 조사 군을 상대로 다양한 용량에 따른 변화를 조사하여, SERM이 자궁근종에 미치는 기전을 더 확실하게 밝혀, raloxifene이 가임기 여성의 자궁근종 약물치료 개발에 길잡이가 될 것으로 사료된다.
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Abstract Selective estrogen receptor modulators (SERMs) exhibit a pharmacologic profile characterized by estrogen agonist activity in some tissues with estrogen antagonist activity in other tissues. These compounds were initially called “antiestrogens,” but it was subsequently recognized that this inadequately described their spectrum of activities. The first widely used SERM, tamoxifen, has estrogen antagonist activity in breast tissue but shows estrogen-like activity in other tissues. Raloxifene is another SERM in clinical use, and it was developed to avoid some of the undesirable estrogen agonist actions of other SERMs to improve the drug safety profile. Raloxifene has been introduced for clinical use in treatment and prevention of postmenopausal osteoporosis. This review will explore the preclinical and clinical pharmacology of raloxifene, and compare it to other SERMs currently available for clinical use.
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Selective estrogen receptor modulators (SERMs) comprise a group of structurally diverse compounds which are distinguished from estrogens by their ability to interact with the estrogen receptor but to act as either an estrogen agonist or antagonist depending on the target tissue and hormonal milieu. The mechanisms by which SERMs elicit tissue-specific responses are being intensively investigated, and recently a novel pathway for estrogen receptor-mediated gene activation by the SERM raloxifene has been demonstrated. The tissue-specific activity of SERMs suggests that they may be clinically useful as, for example, potential substitutes for long-term female hormone replacement therapy. Large-scale clinical testing currently in progress for raloxifene, and soon to begin for other SERMs, will evaluate this important potential.
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The estrogen-related receptor alpha (ERRalpha) is an orphan member of the nuclear receptor superfamily that is closely related to estrogen receptor alpha (ERalpha). ERRalpha binds an estrogen response element (ERE), directly competes with ERalpha for binding ERE, and represses ERE-dependent transcription in MCF-7 cells, ER-positive breast cancer cells.We investigated whether ERRalpha modulate some ER-dependent activities in endometrial cancer.We investigated protein and mRNA expression of ERRalpha in endometrial cancer using immunohistochemistry and RT-PCR, respectively. After transient transfection using the ERRalpha expression vector (pCI-ERRalpha) or ERRalphaSi, which suppressed the expression of endogenous ERRalpha, Ishikawa cells were assayed for ERE-dependent luciferase activity. Cells stably overexpressing ERRalpha were generated and compared with estrogen-dependent and -independent cell growth.ERRalpha was detected in human endometrial cancer tissues by immunohistochemistry. An RT-PCR study showed that mRNA of ERRalpha was expressed in four endometrial cancer cell lines (Ishikawa, Hec1a, KLE, and SNGII) and 11 human endometrial tissues. Overexpression of ERRalpha repressed estrogen-induced ERE-dependent transcriptional activity in Ishikawa cells. After transfection with ERRalphaSi1, the expression of endogenous ERRalpha decreased to 0.5-fold, and estrogen-induced ERE luciferase activity increased to 1.5-fold. The cells stably overexpressing ERRalpha grew up more slowly than control cells in the presence of 10 nm estradiol.ERRalpha is expressed in human endometrial cancer tissues and cell lines and suppresses ERE-dependent transcriptional activity in the presence of estrogen. ERRalpha modulates estrogen-induced activity in estrogen-dependent endometrial cancer.
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Selective receptor modulators (SERMs) are drugs which act via the estrogen receptors and possess tissue specific estrogenic or anti-estrogenic properties. The bone and cardiovascular effects of SERMs are estrogen-like, while they have an effect as estrogen antagonist in the mammary tissues. Raloxifene is the first representative of selective estrogen receptor modulators which does not cause estrogenic effects in the uterus. Based on numerous recently completed controlled clinical trials, the authors characterize the clinical features of raloxifene to assess its therapeutic potency.
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