Antiestrogen Binding Sites Research Articles

ICI 182,780: a pure antiestrogen that affects behaviors and energy balance in rats without acting in the brain

ICI 182,780 is one of a new class of steroidal antiestrogens that differs from nonsteroidal antiestrogens, such as tamoxifen, in a number of respects. 1) It is bound by estrogen receptors with a high affinity, similar to that for estradiol. 2) It is a "pure" antiestrogen in that it does not mimic any of the effects of estradiol. 3) This class of antiestrogens does not seem to be bound by antiestrogen binding sites. 4) ICI 182,780 may not be active in the brain after peripheral administration. Indeed, ICI 182,780 blocked in vivo cell nuclear binding of [3H]estradiol in uterus, pituitary, and adipose tissue but not in hypothalamus-preoptic area. In vitro, ICI 182,780 competed for binding by neural estrogen receptors with an affinity comparable with that for estradiol. When given to ovariectomized rats, ICI 182,780 did not mimic any of the actions of estradiol. Instead, ICI 182,780 treatment completely blocked the uterotrophic effects of estradiol and attenuated the actions of estradiol on linear growth, carcass fat content, fat pad weight, and sexual receptivity. Treatment with ICI 182,780 also attenuated the estrogenic effects of tamoxifen on food intake, body weight and composition, linear growth, and uterine weight. These findings support the concept that, in addition to its actions in the brain, estradiol can act peripherally to modulate regulatory behaviors, energy balance, and estrous behavior. They are also consistent with the hypothesis that nonsteroidal antiestrogens, such as tamoxifen, affect energy balance via estrogen receptors, rather than antiestrogen binding sites.

Presence of a specific antiestrogen binding site on human follicular thyroid carcinoma cell line (UCLA RO 82 W-1): inhibition by an endogenous ligand present in human serum.

A receptor for antiestrogens, distinct from the estrogen receptor, has been identified in several tissues including the MCF-7 breast cancer cell line. Estrogen receptors have also been found in normal and pathological thyroid tissue homogenates. We demonstrate the presence of an antiestrogen binding site (AEBS) on a pure human follicular thyroid carcinoma cell line (UCLA RO 82 W-1) using a 3H-tamoxifen (3H-TAM) binding assay. The binding of 3H-TAM to the AEBS was determined after preincubation (30 min) of the cells with excess 17 beta-estradiol (2 mumol/L). Specific and saturable binding of 3H-TAM to the cells was observed. Displacement of the tracer from its binding site was dose dependent. Scatchard analysis revealed a dissociation constant (Kd) of 73 nmol/L, indicating a binding site with moderate affinity and capacity (72 pmol/10(6) cells). Using this assay we were also able to demonstrate the presence of an endogenous ligand for the AEBS in ethanol extracts of human serum. Cell growth and 3H-thymidine incorporation by the follicular thyroid carcinoma cells were inhibited when the cells were exposed to TAM (1.5 mumol/L). In conclusion, TAM is able to bind to a specific receptor on this follicular thyroid carcinoma cell line, and a natural circulating ligand present in ethanol extracts of human serum interferes with its binding.

Presence of a specific antiestrogen binding site on human follicular thyroid carcinoma cell line (UCLA RO 82 W-1): inhibition by an endogenous ligand present in human serum

Presence of a specific antiestrogen binding site on human follicular thyroid carcinoma cell line (UCLA RO 82 W-1): inhibition by an endogenous ligand present in human serum

An Improved Method for the Synthesis of 2-(p-Halobenzyl)-3-aryl-6-methoxybenzofurans as Selective Ligands for the Antiestrogen-Binding Sites

Abstract A series of 2-(p-fluorobenzyl)-3-aryl-6-methoxybenzofurans has been prepared in good yields in a two-step sequence from the corresponding benzylidencoumaranones (aurones).

Estrogens and antiestrogens: actions and interactions with fluphenazine on food intake and body weight in rats.

Treatment of ovariectomized rats for 3 days with 2 micrograms estradiol benzoate (E2B), 6 micrograms ethinyl estradiol, or 1-2 mg of either of the antiestrogens nafoxidine or tamoxifen led to similar decreases in food intake, body weight gain, adipose tissue lipoprotein lipase activity, and hepatic fatty acid synthetase activity, despite their different effects on uterine growth and induction of progestin receptors in pituitary and adipose tissue. Longer-term (2 wk) treatment with tamoxifen resulted in similar transient changes in food intake and body weight gain, as did treatment with E2B. Daily administration of 50 micrograms fluphenazine (FLU) led to significant decreases in body weight, although there was no change in food intake. Concurrent administration of FLU with either E2B or tamoxifen led to additive effects on body weight and food intake change. None of the treatments had any effect on in vitro binding of [3H]tamoxifen to antiestrogen binding sites in pooled hypothalamic-preoptic area samples.

HUMAN TUMOR-CELL GROWTH MODULATORY EFFECTS OF THE AEBS/HIC-BINDING DRUGS USED AS SINGLE AGENTS AND IN COMBINATION WITH A NOVEL AMPHIBIAN OOCYTE RNASE

A novel amphibian oocyte RNase, ONCONASE(R) (ONC), has been previously shown to have synergistic tumor cell growth inhibitory activity when combined with tamoxifen (TMX) and/or trifluoperazine (TFP) in human ASPC-1 pancreatic and A-549 lung carcinoma cells, respectively. It has recently been reported that several drugs known to bind to the intracellular antiestrogen binding site (AEBS)/histamine (H(IC)) receptors, including tricyclic (amitriptyline, AMT) and non-tricyclic (fluoxetine, FLX) antidepressants, TMX, phenothiazines and the prototype H(IC)-binder DPPE, can stimulate the in vitro and in vivo growth of rodent tumor cells, while having a normal cell growth inhibitory activity, as reflected by the inhibition of the DNA synthesis. It has been presently shown that while at the clinically relevant concentrations some of these H(IC)-binding drugs mildly stimulated (up to 15%) the cell growth in the human lines studied when used as single agents, in most instances this stimulation did not exceed 10% above the control values. When used in combination with ONC, neither of these H(IC)-binding drugs demonstrated any apparent synergistic activity as judged from the ED50 values. However, the combinations of DPPE+TFP and AMT+TFP, in both the ASPC-1 and COLO 320DM lines, demonstrated a significant cell growth inhibition, while there was no difference between the effects of AMT alone and the AMT+TFP combination in the U87MG line. The most effective cell growth inhibition was obtained when ONC was combined with DPPE+TFP and/or AMT+TFP, as reflected by the significantly decreased ED50 values.

Ligands of the Antiestrogen Binding Site Block Endothelial Cell Proliferation Reversibly

Benzylphenoxyethanamine derivatives are known to display antiproliferative activities on tumor cell lines consistently with their binding affinity to the microsomal antiestrogen binding site. In the present study we show that pyrrolidinobenzylphenoxyethanamine, a new efficient compound of this series, exhibits reversible effects on exponentially growing adult bovine aortic endothelial cells inducing (1) lamellated cytoplasmic inclusions, (2) cell proliferation inhibition, (3) dose-dependent transition delay of cells in the G0-G1 phase of the cell cycle. Complete reversal of these effects is achieved only by withdrawing the drug from the medium. The ultrastructural cellular modifications disappeared, and flow cytometry and thymidine incorporation analysis showed the effect on degree of synchronization of this one-step methodology.

Antiestrogen binding sites in brain and pituitary of ovariectomized rats

In addition to interacting with estrogen binding sites in a number of tissues, antiestrogens have recently been shown to interact with a separate, estrogen-non-competable, antiestrogen-specific binding site (AEBS). In order to better understand possible mechanisms by which the antiestrogens may effect behavioral and physiological changes, we have examined AEBS in several areas of the brain and pituitary in adult, ovariectomized rats. Single point binding assays with 2 nM [ 3H]tamoxifen (TAM) in the presence of saturating amounts (1 μM) E and ± 1μM TAM indicated the existence of specific binding to AEBS throughout the brain and pituitary. In most areas of the brain (cortex, cerebellum, amygdala, area postrema/nucleus of the solitary tract region) as well as pituitary, Scatchard analyses revealed the presence of a single AEBS with a dissociation constant ( K d = 1−4 × 10 −9M ) similar to that previously reported for other tissues. However, in both hypothalamus and preoptic area, an additional, higher affinity site ( K d = 6−9 × 10 −11M ) was found. Competitive inhibition studies revealed that there was little competition by the potent estrogen agonist, diethylstilbesterol, for AEBS binding. Antiestrogens competed in the following order: tamoxifen ≥ nafoxidine ≫ keoxifene. Additional competitive inhibition studies were run using neurotransmitter antagonists. for binding at the AEBS. Other pharmacological substances, including specific antagonists of the D 2 sites, as well as antagonists of the norepinephrine, opiate, histamine, GABA and acetylcholine systems, were ineffective competitors for [ 3H]TAM binding.

Synthesis of 2-(p-chlorobenzyl)-3-aryl-6-methoxybenzofurans as selective ligands for antiestrogen-binding sites. Effects on cell proliferation and cholesterol synthesis.

A series of nonsteroidal compounds, 2-(p-chlorobenzyl)-3-aryl-6- methoxybenzofurans derived from the 2-(p-chlorobenzyl)-6-methoxy-3(2H)-benzofuranones has been synthesized. The key steps in the synthesis were reactions of 2-(p-chlorobenzyl)-6-methoxy-3(2H)-benzofuranones with the arylorganometallic reagents followed by dehydration of the resulting carbinols. The benzofurans are ligands for antiestrogen-binding sites (AEBS) and display no significant interaction with the estrogen receptor (ER). All bind to AEBS with equivalent or greater affinity than tamoxifen. These compounds decrease [3H]thymidine incorporation in AEBS-containing EL4 lymphoid cells and MCF7 breast cancer cells in a concentration-dependent manner between 10(-8) and 10(-6) M and are generally more inhibitory than tamoxifen. In contrast, they have no effect on [3H]thymidine incorporation by an AEBS-deficient variant of the MCF7 cell line, RTx6. The present findings of (1) selective and high affinity binding of the benzofurans to AEBS, (2) their concentration-dependent inhibition of [3H]thymidine incorporation in AEBS-containing cells, and (3) their lack of antiproliferative effect in an AEBS-deficient cell line suggest a functional role for AEBS in mediating the antigrowth effect of these compounds. Two of the more active benzofuran compounds also significantly inhibited de novo cholesterol biosynthesis in EL4 cells which lack ER. This effect could be obtained after 5 h of treatment and preceded significant loss of cell viability. This is the first demonstration that selective ligands of AEBS (other than the known nonsteroidal antiestrogens) interfere with cholesterol biosynthesis-an action that may contribute to their antigrowth effect.

Stimulation of in vivo tumor growth and phorbol ester-induced inflammation by N,N-diethyl-2-[4-(phenylmethyl)phenoxy] ethanamine HCl, a potent ligand for intracellular histamine receptors

We have implicated histamine as a mediator of proliferation through its binding to novel intracellular receptors (HIC), closely associated with antiestrogen binding sites (AEBS) in microsomes and nuclei. N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine HCl (DPPE), is a potent ligand for AEBS/HIC. We now demonstrate that DPPE stimulates in vivo tumor growth (DMBA-induced mammary cancer in Sprague-Dawley rats and L5178Y leukemia in DBA/2 mice) and synergizes with phorbol-12-myristate-13-acetate (PMA) to induce inflammation and mitotic activity in mouse epidermis. Thus, ligands for intracellular histamine receptors may represent a new class of tumor promoting agents; this finding lends new credence to an important role for histamine in growth.

Characterization of estrogen and antiestrogen binding to the cytosol and microsomes of breast tumors

The binding of [ 3H]estradiol and [ 3H]hydroxytamoxifen to the cytosol and microsomal fractions of several human breast tumors was investigated. By washing microsomal membranes with a KCl-free or a KCl-containing medium we could distinguish between intrinsic, extrinsic and contaminant estradiol binding sites in these membranes. We observed that treatment of the microsomes with low salt medium removes about 80% of the total estradiol binding sites, whereas 20% are not extractable. The concentration of unextractable [ 3H]estradiol binding sites in the microsomes varies in proportion to the level of cytosolic estrogen receptors (ER). About 10% of the total extranuclear specific estrogen binding sites was consistently found tightly associated to the microsomal fraction, which displays an affinity for estradiol ( K d = 0.1−0.6 nM) similar to that of the cytosolic ER. The displacement of [ 3H]estradiol with unlabeled hormone or with the antiestrogens, nafoxidine, enclomiphene and tamoxifen (TAM) exhibits identical IC 50 values either in the cytosol or in the microsomal membranes. On the other hand, the microsomal fraction of breast tumors also binds [ 3H]hydroxyTAM, but with higher capacity and lower affinity than those of the cytosolic fraction. Furthermore, we did not observe correlation between the concentrations of ER and of antiestrogen binding sites (AEBS) in the tumors. These results indicate microsomal membranes of human breast tumors contain estrogen binding sites which may be related to the cytosol ER recycling and that specific AEBS are predominantly localized in this membrane system. Furthermore, it is shown that the magnitude of estradiol binding to microsomes depends on the ER positive degree of the tumors, whereas the magnitude of the antiestrogen binding to the microsomes is independent of the ER status of the tumors.

Factors Affecting Anti-Estrogen Binding Site Concentration in Rat Liver

It is known that synthetic anti-estrogens such as tamoxifen bind to specific high affinity anti-estrogen binding sites (AEBS), which are distinct from estrogen receptors. These binding sites are widely distributed in animal and human tissues, the highest concentrations being found in the liver. The physiological role of these intracellular binding sites, which are located predominantly in the microsomal fraction, is currently unknown, as is the nature and identity of their endogenous ligands. In an attempt to gain information which may provide clues to the possible physiological role of these binding sites, studies were carried out to determine whether the concentration of these binding sites in rat liver was affected by a number of physiological variables. The results of these studies indicated that in the rat (i) liver AEBS increased progressively with age; (ii) liver AEBS concentration tended to be higher among females than males after 100 days of age; (iii) there was no significant variation in liver AEBS level with different phases of the estrous cycle; (iv) liver AEBS level was not significantly affected by castration in both males and females or by estradiol replacement in castrated females; (v) liver AEBS concentration increased significantly with increases in ambient temperature; (vi) there was no clearly detectable alteration in liver AEBS levels with changes in the light:dark cycle; (vii) starvation for 24, 48, and 72 hr increased liver AEBS by approximately 1.5-, 3-, and 2-fold, respectively, while refeeding decreased its level; and (viii) liver AEBS was not affected by increasing dietary fat content from 0.5% to 20% (w/w), but was increased modestly by the addition of cholesterol (2% w/w) to the diet. These observations identify several physiological variables which are associated with changes in liver AEBS concentration and suggest possible avenues for future studies to define the physiological role of these binding sites.

Antiproliferative effects of oxygenated sterols: positive correlation with binding affinities for the antiestrogen-binding sites

Oxygenated derivatives of cholesterol have long been known to exhibit antiproliferative properties but the mechanism of this effect remains incompletely understood. Following up on a recent observation in our laboratory that certain oxysterols bind with high affinity to the microsomal antiestrogen-binding site, we attempted to determine if a relationship existed between the antiproliferative effect of oxysterols and their binding affinities for the antiestrogen-binding site. Using the human breast cancer cell line, MCF7, and 14 different compounds (4 nonsteroidal antiestrogens, 6 oxysterols with appreciable binding affinities for the antiestrogen-binding site, and 4 other sterols with little or no affinity for this site), we showed that for compounds which bound to the antiestrogen-binding site, there was a clear positive correlation between their relative cytotoxic potencies and their affinities for this binding site. Two sterols, namely 25-hydroxycholesterol and 20α-hydroxycholesterol, were exceptions to this general pattern, both clearly inhibited cell proliferation without having significant binding affinity for the antiestrogen-binding site. The same general order of antiproliferative potency of the oxysterols seen with MCF7 cells was also observed with the murine lymphoma cell line EL4. These findings suggest the possibility that the antiestrogen-binding site may be involved in mediating the antiproliferative effects of nonsteroidal antiestrogens and oxygenated sterols.

Tamoxifen Inhibition of Prolactin Action in the Mouse Mammary Gland

Binding of lactogenic hormones to particulate and solubilized microsomal membranes isolated from mammary glands of lactating mice is inhibited by direct addition of 10(-10) M or greater concentrations of triphenylethylene antiestrogens [i.e. tamoxifen (TAM), 4-hydroxy-tamoxifen, and Nafoxidine] to the binding assays. Estradiol and other antiestrogens such as BPEA (-2-(4-tert-butyl-phenoxy) ethyl diethylamine hydrochloride, LY117018, and LY 156758 do not have this effect. The triphenylethylene antiestrogens bind to the membrane-associated antiestrogen binding sites (AEBS). Effectiveness of binding to the AEBS parallels the effectiveness of inhibition of the lactogen binding. The effect is selective in that binding of epidermal growth factor and insulin to these same membranes is unaffected by the antiestrogens. Binding of PRL to membranes prepared from the livers of the lactating mice is also unaffected. Both the PRL receptor and AEBS are primarily localized to the microsomal membrane fraction of cells. Maximal inhibition of PRL binding by TAM is observed in the light microsomes that contain plasma membranes. In addition to inhibition of PRL binding, TAM also prevents the PRL-induced accumulation of caseins by cultured mouse mammary explants. Thus it appears that the triphenylethylene antiestrogens, acting through the AEBS, act as antilactogens in the normal mammary gland.

Displacement by tamoxifen of the estradiol-estrogen receptor binding: A functional assay for breast cancer studies

Displacement curves with estradiol (E 2) and Tamoxifen (Tam) of the [ 3H]E 2-ER binding in 49 ER+ mammary neoplasias showed a great heterogeneity suggesting the existence of more than one population of ER+ tumors when the relative binding affinity of both ligands for the ER was considered. The (D 50E 2/D 50Tam) × 100 ratio was called Displacement Index (DI) with values assimetrically distributed from 0.05 to 2.90. The range from 0.18 to 0.54 was adopted as central interval given by the median ±2SE (median: 0.36; SE: 0.09). DI values below 0.18 (24% of the tumors in our series) were considered as “lower”, indicating that higher Tam doses would be necessary to displace the E 2-ER binding. The potency of Tam as displacer is dependent not only of its own affinity for the ER, but also of that of E 2 for the same receptor. The DI expresses their relative binding “strength”. DI values were not correlated with ER and progesterone receptor content nor with the D 50 Tam and D 50E 2 taken separately. Antiestrogen binding sites (AEBS) were determined in the cytosol (AEBS c) and in the microsomal fraction of 10 ER+ tumors from our series. The AEBS c/ER ratio was inversely correlated with the DI, that is, displacement of 3HE 2 from the E 2-ER complex by Tam would be lower in tumors with higher AEBS c/ER ratio. The DI is another parameter to be considered in the study of the sensitivity of breast neoplasias to antiestrogen treatments.

A potent and selective photoaffinity probe for the anti-estrogen binding site of rat liver.

The anti-estrogen binding site (ABS) is an apparently ubiquitous component of cells that has been shown to be intimately linked with the antiproliferative effects of certain antiestrogenic compounds, like tamoxifen, which is currently used for the treatment of breast cancer. However, the identification and in vitro study of this novel protein has been hampered to date by a lack of convenient probes that will efficiently label the molecule in nonpurified preparations. Thus, using a selective ABS ligand (4-benzylphenoxy-N-ethylmorpholine, MBPE) as starting material, we synthesized a photosensitive azido derivative, [(2-azido-4-benzyl)phenoxy]N-ethylmorpholine (azido-MBPE) that can be prepared in a tritiated form. Azido-MBPE has a high affinity for ABS (Kd = 3 nM), identical to that of tamoxifen, and covalently labels 5 and 12% of membrane-bound and detergent-solubilized ABS, respectively. Its incorporation is selectively and competitively inhibited by other ABS ligands (tamoxifen greater than nitromifen greater than hydroxytamoxifen). [3H]Azido-MBPE potently photolabels either membrane-bound or detergent-solubilized ABS as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under denaturing conditions revealing specific photoincorporation in a protein band of Mr = 40,000. This molecular weight is approximately two times lower than what we observed previously for ABS preparations studied under nondenaturing conditions and postlabeled with [3H]tamoxifen (Mr = 80,000-110,000). In chromatofocusing experiments with photolabeled ABS, a single specifically labeled protein fraction migrating with a pI of 6.4 was found to exhibit a Mr of 40,000 when subsequently electrophoresed on sodium dodecyl sulfate-polyacrylamide gels. These results indicate that [3H]azido-MBPE is a specific high affinity probe of ABS that will prove useful in the ultimate identification of this protein.

Fatty acid modulation of antiestrogen action and antiestrogen-binding protein in cultured lymphoid cells

Nonsteroidal antiestrogens reversibly and specifically inhibited the proliferation of two estrogen receptor-negative lymphoid cell lines (EL4 and Raji) in a dose-dependent manner. [3H]Thymidine incorporation of concanavalin A-stimulated primary splenocytes was also inhibited by 10(-6) M clomiphene (1-[4-(2-diethylaminoethoxy)phenyl]-1,2-diphenyl-2-chloroethylene). The antiproliferative effect could be prevented by the simultaneous presence in the growth medium of 10(-5) M linoleic acid or 10(-5) M arachidonic acid but not by 10(-6) M estradiol. Both lymphoid cell lines had high affinity antiestrogen-binding sites whose affinity could be altered by conditions of growth. Growth of EL4 cells in RPMI 1640 medium supplemented with charcoal-pretreated 5% fetal calf serum (charcoal-stripped medium) resulted in significantly higher affinity (Kd 0.54 nM +/- 0.11 nM; n = 6) than growth in medium supplemented with untreated serum (complete medium) (Kd = 1.68 nM +/- 0.48 nM; n = 6) (p less than 0.001). This change in affinity was partly due to removal of fatty acids from the growth medium by charcoal pretreatment, since addition of 10(-5) M linoleic acid or 10(-5) M gamma-linolenic to charcoal-stripped medium decreased the affinity of the antiestrogen-binding protein. In contrast, growth in 10(-5) M stearic acid or 10(-5) M oleic acid did not significantly alter the affinity of the antiestrogen-binding protein, whereas 10(-5) M palmitic acid significantly increased its affinity. The same fatty acids were also tested for their intrinsic effects on EL4 cell proliferation. Oleic, linoleic and gamma-linolenic acids were growth stimulatory while stearic and palmitic acids were not. Thus linoleic and gamma-linolenic acids whose presence in the growth medium was associated with decreased affinity of [3H]tamoxifen (1-[4-(2-dimethylaminoethoxy)phenyl]-1,2-diphenylbut-1(Z)-ene) binding to the intracellular antiestrogen-binding protein were also growth stimulatory. Unsaturated fatty acids have previously been shown to inhibit binding of [3H]tamoxifen to the antiestrogen-binding protein in a cell-free system. The present observations demonstrate that unsaturated fatty acids also modify the affinity of the antiestrogen-binding protein in intact cells.

The anti-proliferative properties of 4-benzylphenoxy ethanamine derivatives are mediated by the anti-estrogen binding site (ABS), whereas the anti-estrogenic effects of trifluopromazine are not

We compared the anti-proliferative properties of 4-benzylphenoxy- N ethyl morpholine (mor-pho-BPE) and trifluopromazine (TFP) on both the human breast cancer cell lines, MCF 7, and its tamoxifen-resistant variant RTx 6. We found that the calmodulin antagonist trifluopromazine (TFP) which bound ABS weakly, inhibited MCF 7 cell growth but did not follow the relationship observed for diphenylmethane derivatives between MCF 7-inhibitory potencies and their K i . Regarding the tamoxifenresistant RTx 6 cells, TFP but not morpho-BPE induced inhibition of the proliferation. Using a tritiated derivative of morpho-BPE, two distinct binding sites could be demonstrated. Indeed, a low affinity binding site was present in both cell lines whereas a high affinity binding site was mainly found in MCF 7 cells although being in lower concentration (<10%) in rtx 6 cells. Both tamoxifen and TFP displaced morpho-BPE from the two binding sites. The uptake and efflux of the tritiated drug were similar in the two cell lines. The drug did not appear to be metabolized. We concluded that TFP and morpho-BPE belong to distinct classes of molecules and that ABS mediates the anti-proliferative action of diphenylmethane derivatives but not the inhibitory effect of the calmodulin antagonist TFP.

ChemInform Abstract: Synthesis of 3-(p-Fluorophenyl)-4-arylchrom-3-enes as Selective Ligands for Antiestrogen-Binding Sites.

ChemInformVolume 21, Issue 20 Heterocyclic Compounds ChemInform Abstract: Synthesis of 3-(p-Fluorophenyl)-4-arylchrom-3-enes as Selective Ligands for Antiestrogen-Binding Sites. C.-C. TEO, C.-C. TEO Dep. Chem., Natl. Univ. Singapore, Kent Ridge, Singapore 0511Search for more papers by this authorO.-L. KON, O.-L. KON Dep. Chem., Natl. Univ. Singapore, Kent Ridge, Singapore 0511Search for more papers by this authorK.-Y. SIM, K.-Y. SIM Dep. Chem., Natl. Univ. Singapore, Kent Ridge, Singapore 0511Search for more papers by this author C.-C. TEO, C.-C. TEO Dep. Chem., Natl. Univ. Singapore, Kent Ridge, Singapore 0511Search for more papers by this authorO.-L. KON, O.-L. KON Dep. Chem., Natl. Univ. Singapore, Kent Ridge, Singapore 0511Search for more papers by this authorK.-Y. SIM, K.-Y. SIM Dep. Chem., Natl. Univ. Singapore, Kent Ridge, Singapore 0511Search for more papers by this author First published: May 15, 1990 https://doi.org/10.1002/chin.199020191Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume21, Issue20May 15, 1990 RelatedInformation

High-affinity binding sites for oxygenated sterols in rat liver microsomes: possible identity with antiestrogen binding sites

Oxygenated derivatives of cholesterol are known to exhibit a number of biological activities including the inhibition of cholesterol biosynthesis and of cell proliferation, but their mechanism of action remains unclear. Previous studies have identified a cytosolic protein which binds 25-hydroxycholesterol, as well as several other oxysterols, with high affinity, possibly mediating some of their effects. We now report the existence of a high-affinity oxysterol binding site in rat liver microsomes which is distinct from the cytosolic binding protein. Among the oxygenated sterols examined, 5α-cholestan-3β-ol-7-one (7-katecholestanol) had the highest affinity for this microsomal binding site ( K d = 2.7 nM). Using 7-keto[ 3H]cholestanol as the radioactive ligand, we found that binding of this oxysterol to the microsomal binding site was saturable and reversible and was displaceable by the following oxysterols in descending order of potency: 7-ketocholestanol > 6-ketocholestanol > 7 β-hydroxycholesterol = 7-ketocholesterol > cholesten-3 β,5 α6 β-triol = 7 α-hydroxycholesterol > 4-cholesten-3-one. One other sterols studied, including, notably, 25-hydroxycholesterol, had little or no inhibitory effect on 7-keto[ 3H]cholestanol binding. Additional studies revealed that the microsomal oxysterol binding site was probably identical to the antiestrogen binding site described by other workers. First, saturation analysis and kinetic studies demonstrated that the antiestrogen tamoxifen competed directly with 7-keto[ 3H]cholestanol for the same binding site. Second, tha ability of different oxysterols and antiestrogens to inhibit 7-keto[ 3H]cholestanol binding to the microsomal binding site paralleled their ability to inhibit [ 3H]tamoxifen binding to the antiestrogen binding site. Third, the tissue distribution of binding sites for 7-keto[ 3H]cholestanol was similar to that antiestrogen binding site. We conclude that: (1) in rat liver microsomes there are high-affinity oxysterol binding sites whose ligand specificity is different from that of the cytosolic oxysterol binding protein; and (2) the microsomal oxysterol binding site is probably identical to the antiestrogen binding site. The biological significance of these observations remains to be explored.