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Endocrinology Vol. 143, No. 6 2189-2197
Copyright © 2002 by The Endocrine Society

NEUROENDOCRINOLOGY

Genistein Affects ERß- But Not ER{alpha}-Dependent Gene Expression in the Hypothalamus

Heather B. Patisaul, Melissa Melby, Patricia L. Whitten and Larry J. Young

Center for Behavioral Neuroscience (H.B.P., P.L.W., L.J.Y.), Department of Anthropology (M.M., P.L.W.), and Department of Psychiatry and Behavioral Sciences (L.J.Y.), Emory University, Atlanta, Georgia 30329

Address all correspondence and requests for reprints to: Heather B. Patisaul, Ph.D., Center for Behavioral Neuroscience, Emory University, 954 Gatewood Road Northeast, Atlanta, Georgia 30329. E-mail: hbeaupr{at}emory.edu.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Isoflavone phytoestrogens are growing increasingly popular because of their reported cardiovascular and anticarcinogenic properties, but the effects of these compounds in the brain are largely unknown. In a previous study, we found that an isoflavone supplement, containing a mixture of soy phytoestrogens, inhibited estrogen-dependent female sexual behavior and was antiestrogenic for both ER{alpha}- and ERß-dependent gene expression in the hypothalamus. Here we examined the impact of the soy isoflavone genistein, a major component of the supplement, on estrogen-dependent female sexual behavior and ER{alpha}- and ERß-dependent gene expression in the rat brain. Genistein, at a dietary concentration of 100 or 500 ppm had no effect on lordosis behavior in rats. However, at 500 ppm genistein had differential activity through ER{alpha} and ERß in the hypothalamus. Genistein had no effect, in either the presence or absence of 17ß-E2, on oxytocin receptor density in the ventromedial nucleus of the hypothalamus, an estrogen-dependent action thought to be regulated via ER{alpha}. However, genistein increased ERß mRNA expression in the paraventricular nucleus of the hypothalamus by 24%, whereas 17ß-E2 decreased ERß mRNA expression by 26%, a process likely mediated by ERß itself. These results suggest that at this dose, genistein has antiestrogenic action through ERß in the paraventriculr nucleus but negligible activity through ER{alpha} in the brain.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
PHYTOESTROGENS ARE NONSTEROIDAL, estrogen-like compounds produced by numerous plant species, particularly the legumes. Traditional Asian and vegetarian diets, both of which have a high soy content, contain significant quantities of the phytoestrogen isoflavonoid genistein, which has been suggested to have a myriad of health benefits including anticarcinogenic properties (1, 2). Although the potentially beneficial effects of a high phytoestrogen diet including a reduction of hot flushes (3) and hepatic cholesterol production (4, 5) have been well publicized, very little is known about the potential impact of these compounds on the brain. Traditional estrogen replacement therapy (ERT) is known to confer a variety of cognitive benefits including a decreased risk of depression (6) and improved memory (7, 8, 9). As commercially prepared phytoestrogen supplements and soy-enhanced foods are becoming more widely available, and frequently advertised as a natural alternative to ERT, it is imperative to examine the effects of phytoestrogens on the brain.

We have recently demonstrated that an isoflavone supplement containing significant quantities of genistein, as well as other phytoestrogens, was antiestrogenic for both ER{alpha}- and ERß-dependent gene transcription in the brain, and interfered with estrogen-dependent sexual behavior (10). We have also shown that the coumestan phytoestrogen, coumestrol, is antiestrogenic through both ER subtypes in the brain (11, 12). The results of these experiments suggest that genistein may have antiestrogenic actions through both ER{alpha} and ERß as well; thus, this paper investigates the action of genistein alone on female sexual behavior and estrogen-dependent gene expression in the brain.

ER{alpha} and ERß are differentially expressed in the brain (13, 14), and the ligand binding region of ERß is only 55% homologous to the ligand binding region of ER{alpha} (15), suggesting that the two receptors may have differential effects on gene expression and behavior. In general, phytoestrogens have a higher relative binding affinity for ERß than ER{alpha} (16), suggesting that they may be more bioactive through ERß than ER{alpha}. However, a variety of phytoestrogens have now been shown to bind to both ER isoforms in vitro and activate ER-dependent gene transcription (16), although at least one of these assays has demonstrated that the phytoestrogens can have mixed agonist/antagonist effects (17).

In vitro competitive binding assays have shown that ERß has a higher relative binding affinity for both the coumestan phytoestrogen, coumestrol, and the isoflavone phytoestrogen, genistein, than ER{alpha} (16, 18). The affinity of genistein to ERß is 20-fold higher than the affinity of genistein to ER{alpha}, and genistein has a substantially lower affinity to both ER{alpha} and ERß than 17ß-E2 (16, 18). Despite its extremely low relative binding affinity to ER{alpha}, and preferential affinity to ERß, genistein has been repeatedly shown to be a full agonist for ER{alpha} as well as ERß in a variety of cell culture receptor reporter-gene assays (16, 19) and proliferation (E-Screen) assays (20). Our in vivo work using both coumestrol and a genistein-rich isoflavone supplement has repeatedly demonstrated that, although the majority of phytoestrogens have been shown to be full agonists in vitro, they can act as antiestrogens in the brain in vivo (10, 11, 12). The following experiments were undertaken to determine if genistein alone is sufficient to produce the antiestrogenic neuroendocrine and behavioral effects described previously using a commercially prepared isoflavone supplement containing a variety of different phytoestrogens, including genistein (10).

Lordosis is a reflexive sexual posture made by female rodents in response to male mounting. Females will display this posture only when sexually receptive. Ovariectomized (OVX) females can be reliably induced into behavioral estrus by administering either E2 benzoate (EB) and progesterone sequentially, or small daily doses of EB alone (for a detailed review, see Ref. 21). Female sexual behavior is absent in ER{alpha} knockout (ER{alpha}KO) but not ERß knockout (ERßKO) mice, demonstrating that ER{alpha} is critically important for the regulation of female sexual behavior, including lordosis (22, 23, 24). Thus, if genistein is antiestrogenic, particularly through ER{alpha}, it should attenuate the lordosis response in hormonally primed OVX females.

To distinguish through which ER subtype genistein is acting, we have chosen to look at two regions of the hypothalamus, each of which contains predominantly one ER subtype. Oxytocin receptor (OTR) density in the ventromedial nucleus of the hypothalamus (VMN) was used as a marker for ER{alpha} activity because this region contains little or no ERß (25), and estrogen is known to regulate OTR via an ER{alpha}-dependent mechanism (26) through an increase in OTR mRNA expression (27). If genistein is antiestrogenic through ER{alpha}, it should either decrease OTR density in the absence of estrogen, or attenuate the estrogen-dependent up-regulation of this receptor in the presence of estrogen. ERß mRNA expression in the paraventricular nucleus (PVN) was chosen as a marker of ERß activity because this region is devoid of ER{alpha}, and estrogen down-regulates ERß mRNA in this region (11). Again, if genistein is antiestrogenic through ERß, it should either have an opposite effect of estrogen on ERß mRNA expression, or interfere with the estrogen-dependent down-regulation of this receptor.

Because phytoestrogenic effects are dose specific, and higher doses are often needed in rats to produce effects comparable to those seen in humans (28), we were careful to choose a dietary dose of genistein that would produce plasma genistein levels equivalent to those seen in humans consuming an isoflavone supplemented, or traditional Asian diet. We also examined whether or not genistein is estrogenic in the uterus at the low doses selected for the neuroendocrine experiments by assessing uterine weight in juvenile rats. The juvenile rat uterotrophic assay has frequently been employed to assess the estrogenicity of various compounds, including a number of phytoestrogens (29). Because newly weaned pups have not yet reached puberty, their endogenous estrogen levels are low and their uteri immature. Administration of estrogen, or an estrogenic compound, stimulates uterine growth and thus greatly increases uterine weight. This simple test is therefore a good way to assess estrogenicity.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Juvenile rat uterotropic assay
Animal care, maintenance, and surgery were conducted in accordance with the applicable portions of the Animal Welfare Act and the U.S. Department of Health and Human Services "Guide for the Care and Use of Laboratory Animals." All juveniles were selected from an in-house colony of Long Evans rats housed in a 12-h light, 12-h dark cycle at 23 C and 50% humidity. Female pups (n = 50) were weaned on postnatal day (PND) 21 and randomly assorted into groups of 10. The dams of each pup were maintained on a semipurified, phytoestrogen-free diet (AIN-76A Test Diet, PMI Nutrition International, Inc., Richmond, IN) beginning on the day of parturition. On the day of weaning (PND 21), the pups (n = 10 per group) were placed on either the phytoestrogen-free diet, or a genistein (Indofine Chemical Company Inc., Somerville, NJ) diet at a concentration of either 500, 1000, 2000, or 4000 ppm. The diets were prepared by mixing the genistein powder into the semipurified diet at the desired concentration. On PND 26, all pups were killed by CO2 asphyxiation, the uteri removed and weighed, and blood collected and chilled over ice. Due to incorrect sexing at the time of weaning, each group ended up with only 7 or 8 females.

Estrogen RIA
Immediately after collection, all blood samples were spun for 10 min at a relative centrifugal force of 9.8 to separate the plasma. The plasma was then extracted and stored at –80 C until analysis. Each sample was ether extracted, and the estrogen content of each plasma sample was quantified by RIA with a modification of the Pantex Direct 125I Estradiol kit (Pantex, Santa Monica, CA) as described previously (30, 31). The assay has a sensitivity of 9 pg/ml.

Plasma genistein quantification and analysis
We examined blood levels of genistein from adult Long Evans females (Charles River Laboratories, Inc., Raleigh, NC) (n = 6). Many of these animals were ultimately used for round 1 of the behavioral experiments described below. Treatment consisted of the sequential administration of our standard lab chow (Lab Diet 5001, PMI Nutrition International, Inc.), the phytoestrogen-free diet (AIN-76A), a 100 ppm genistein diet (GEN 100), a 500-ppm genistein diet (GEN 500), and a 0.35% isoflavone supplemented diet (SUPPL) used in our previous experiments (10). The supplement is called "Super Concentrated Isoflavones with Genistein and Daidzein" (Solgar, Leonia, NJ) and is widely available to consumers via health food and grocery stores. The genistein diets were prepared by mixing the genistein powder into the semipurified diet at the specified concentration. Treatment order was randomly distributed, and each treatment was preceded by 1 wk on the semipurified diet. Blood was collected after 6 d on each of the five diets, for a total of five samples per animal. All animals were lightly anesthetized with isoflurane (Abbott Laboratories, Abbott Park, IL) and blood was collected by nicking the tail of each animal with a sterile razor and dripping one drop of blood onto each of three to six wells on sample collection filter paper (Schleicher and Schuell, Keene, NH; no. 903). Papers were allowed to dry for approximately 4 h at room temperature, placed in a zip-lock bag, then stored in a sealed container at –27 C until the time of the assay.

Concentrations of genistein in dried blood spots were determined using a time-resolved competitive-binding fluoroimmunoassay (TR-FIA) in which europium-labeled genistein competes with unlabeled genistein for (rabbit antigenistein) antibody binding sites. This assay was originally developed for measurement of genistein in plasma and serum by Wang et al. (32) and has been modified by us for use with dried blood spots (30). This method differs from the one developed by Wang et al. in that it does not involve hydrolysis or extraction of samples, and thus only measures the free (unconjugated) phytoestrogens and the 4'-monosulfates and monoglucuronides (most conjugated phytoestrogens are in this form). The biggest discrepancy between the direct TR-FIA and gas chromotography-mass spectrometry (GC-MS) methods is due to 7'-compounds.

Blood spot standards (range 0–640 ng/ml) were prepared by mixing serially diluted solutions of genistein (Sigma, St. Louis, MO) in 5% BSA Tris buffer (pH 7.75) 1:2 with (human) red blood cells that have been washed three times with normal saline (0.86 g NaCl/100 ml DI H2O). After mixing for 1 h, aliquots of blood spot standards and controls were pipetted onto sample collection papers (Schleicher and Schuell, no. 903), allowed to dry for approximately 4 h at room temperature (RT), and stored in a sealed container at –27 C (i.e. treated identically to samples).

Using a 1/8 inch hole punch, two spots (equivalent to 5.0 µl of plasma/serum or 10 µl of whole blood) from each of the blood spot standards, controls and samples were eluted in 100 µl of 0.5% BSA Tris buffer in glass tubes. Tubes were covered with Parafilm, rotated gently (50 rpm) on an automatic shaker for 1 h at room temperature, and incubated overnight at 4 C.

On the following day, after mixing at 50 rpm for 1 h at RT, standards, controls and samples (40 µl) were pipetted into prewashed coated wells (goat antirabbit IgG Microtitre Plates (Wallac, Inc. Turku, Finland). Next, 100 µl of genistein antibody (an antibody obtained from rabbits immunized with genistein-BSA conjugates, and a generous gift from H. Adlercreutz) solution (1:80,000 dilution in 0.5% BSA Tris buffer) was pipetted into each well, followed by 100 µl of europium-labeled genistein derivative [4'-O-carboxy-methyl-genistein (Wallac, Inc.)] solution (1:500,000 dilution in 0.5% BSA Tris buffer). Assay wells were covered and incubated on an automatic plate shaker for 90 min at 50 rpm and RT. After washing wells to remove unbound antigen using an automatic washer, 200 µl of DELFIA enhancement solution (Wallac, Inc.) were added to each well. Assay wells were covered and placed on an automatic plate shaker for 45 min at 50 rpm and RT. Fluorescence was measured using a 1232 DELFIA fluorometer (Wallac, Inc.). The log of the standard curve, with spline data reduction, was used for interpolation of results. Assay performance characteristics are as follows: sensitivity (8 ng/ml); average recovery (100.8–107.5%); linearity (96.3–105.2%); intraassay coefficient of variation (6.8–11.1, n = 10) and interassay coefficient of variation (11.1–17.7, n = 13). All values below the sensitivity level were reduced to zero. The majority of these values were for animals fed the phytoestrogen-free diet. One data point was identified as an outlier and dropped from the analysis.

Female sexual behavior
Round 1 (100 ppm).
Adult male (n = 8) and female (n = 32). Long Evans rats (Charles River Laboratories, Inc.) were housed in a 12-h light, 12-h dark cycle at 23 C and 50% humidity. Females were housed in groups of two, and males were individually housed. All animals were maintained on the phytoestrogen-free diet for at least 4 d before beginning the experiment. Following a 1-wk acclimation to the vivarium, all females were OVX under ketamine anesthesia. Within a few weeks after surgery, all females were induced into estrus with EB (10 µg in 0.1 ml sesame oil) and progesterone (500 µg in 0.1 ml sesame oil) and tested with males to confirm each male’s vigor and each female’s sexual responsiveness to ovarian hormones. Each female was tested at least twice with multiple males, and each male was tested at least four times. Males that did not reliably engage in sexual behavior after repeated exposures (reliability = mating in at least 80% of pairings) were not used in the study (n = 5 vigorous males). Testing began 2 wk after the completion of this initial priming, and all females were placed on the phytoestrogen-free diet 3 d before receiving treatment diet.

The females were divided into four treatment groups (n = 8 per group): phytoestrogen-free diet and no estrogen (control), phytostrogen-free diet and estrogen, GEN 100 diet and no estrogen, and GEN 100 diet and estrogen. All animals were given progesterone. EB (10 µg in 0.1 ml sesame oil) was sc injected 48 h before testing, and progesterone (500 µg in 0.1 ml sesame oil) was sc injected 4–5 h before testing. Animals not receiving EB were injected with sesame oil (0.1 ml) only. Testing took place in an arena identical in size and shape to the home cage of each animal, which was thoroughly cleaned between trials. The females were given 5 min to adapt to the arena before the male was introduced. Testing sessions were 30 min in length, and all interactions were videotaped under red light and scored from the tape. Receptivity was assessed using the lordosis quotient (LQ) as calculated by the number of lordosis responses in 10 min divided by the number of mount attempts made in the same 10 min multiplied by 100. To eliminate variability between subjects due to mount latency, the beginning of the 10 min scoring period was defined as 5 sec before the first mount attempt by the male.

Round 2 (500 ppm).
In the second round of testing, we used a higher dose of genistein and a behavioral screen that is more sensitive to antiestrogenic effects. In this test, rats are injected daily with low doses of EB and tested every other day. Typically, after a few days, the rats begin to show lordosis behavior even in the absence of progesterone. LQ, and thus presumably sexual receptivity, is directly proportional to the amount of estrogen administered (33). By giving small doses of EB gradually over several days together with a genistein diet, the effects of genistein on LQ can be determined across a range of estrogen levels. This technique has been successfully employed by other experimenters to determine how plasma thyroid hormone levels interact with estrogen-induced lordosis behavior (34, 35).

A new set of animals were purchased (Charles River Laboratories, Inc.) and housed in identical treatment conditions as round 1. Male (n = 12) and female (n = 26) Long Evans rats (Charles River Laboratories, Inc.) were acclimated to the vivarium for 1 wk. The females were then OVX under ketamine anesthesia. Beginning 3 wk post surgery, the animals were primed with EB and progesterone and paired with males as described in round 1 in order ensure that each animal had sexual experience. Testing began 1 wk after the completion of this initial priming, and all females were fed the phytoestrogen-free diet for the entire week before testing.

The females were divided into four treatment groups: phytoestrogen-free diet and no estrogen (n = 8; control), phytoestrogen-free diet and estrogen (n = 6), GEN 500 diet and no estrogen (n = 8), and GEN 500 diet and estrogen (n = 6). EB was administered daily (0.2 µg/0.1 ml sesame oil) for 8 d and control animals were injected with sesame oil only (0.1 ml). The treatment diet was given daily beginning on the day of the first injection. Testing took place for 30 min, every other day, beginning on d 2, under red light in the first 4 h of the dark cycle as described in round 1. Again, all behavior was recorded and scored from the videotape as described in round 1.

Brain collection and analysis
The adult female (n = 26) Long Evans rats used in round 2 of the behavioral experiments were left undisturbed in the vivarium on standard chow for 2 wk following the completion of the behavioral testing. After 3 d on the phytoestrogen-free diet, all animals were divided into the same four groups used for the behavioral experiments: phytoestrogen-free diet and no estrogen (n = 6), phytoestrogen-free diet and estrogen (n = 6), GEN 500 diet and no estrogen (n = 6), and GEN 500 diet and estrogen (n = 7). To ensure that estrogen administration was continuous across the entire treatment period, 17ß-E2 was administered by SILASTIC brand (Dow Corning, Midland, MI) capsule (6 mm in length, 1.98 mm inner diameter, 3.2 mm outer diameter). Capsules of a similar size have previously been shown to produce physiological levels of plasma E2 (10). All animals were implanted with either an estrogen-filled or empty capsule under isoflurane (Abbott Laboratories) anesthesia 2 d after the initial administration of the treatment diet. Four days after implantation, animals were killed by CO2 asphyxiation for a total of 6 d on the treatment diet and 4 d on the hormone treatment. Uteri were collected and weighed at the time the rats were killed. Blood was collected and spun down within 20 min of collection to isolate the plasma. All brains were immediately frozen on dry ice at the time the rats were killed. The plasma and brains were kept at –80 C until use. The estrogen content of each plasma sample was quantified by RIA as was done for the serum from the juveniles used for the uterotropic assay. The brains were cut on a cyrostat into sections 20 µm thick at 80-µm intervals and thaw mounted on Superfrost plus microscope slides (Fisher Scientific, Pittsburgh, PA). Serial sections were taken from the lateral septum to the caudal end of the VMN (corresponding to Bregma –0.3 mm to –3.0 mm).

In situ hybridization
ERß in situ hybridization was performed using a set of two 48-bp, 35S-labled oligonucleotide probes (GTG AGG GAC ATC ATC ATG GAG GCC TCG GTG AAG GGC ATG CTG GGA CGG, and GAG CTC CAC AAA GCC AGG GAT TTT CTT AGC CCA GCC AAT CAT GTG CAC). These oligos are complementary to nucleotides 714–762 and 784–832 of rat ERß mRNA (GenBank accession no. 2801690) and are 60% and 77% homologous to ER{alpha}, respectively.

In situ hybridization was performed using a well-established protocol in our laboratory (36). Following in situ, the rinsed and dried sections were exposed to Kodak BioMax MR film (Eastman Kodak Co., Rochester, NY) with 14C-labled autoradiographic standards (Amersham Pharmacia Biotech, Arlington Heights, IL) for 32 d to produce autoradiograms for quantitative analysis.

OTR autoradiography
OTR autoradiography was performed using 125I-d(CH2)5[Tyr-(Me)2, Tyr9-NH2]OVT (NEN Life Science Products, Boston, MA) as described previously (26). After air drying, the slides were exposed to BioMax MR film (Kodak) for 48 h. 125I-Labled autoradiographic standards (Amersham Pharmacia Biotech) were included in the cassette for quantification.

Data analysis
Utertropic assay.
Uterine wet weights and endogenous estrogen levels were analyzed by one-way ANOVA (SYSTAT, SPSS, Inc., Chicago, IL) and group differences were identified using Fisher’s least significant difference post hoc test.

Genistein blood assay.
The data were log transformed to correct for nonhomogeneous variability then analyzed by a one-way-repeated-measures ANOVA. Group differences were identified using Fisher’s least significant difference post hoc test.

Female sexual behavior.
All behavior was scored from the videotapes by a single investigator, then validated by a second investigator. Both investigators were blind to the treatment groups. Number of mounts by the male, and the number of lordotic responses by the female, were scored for each 10-min test session and averaged across each of the four treatment groups. Lordosis was defined as complete dorsoflexion of the spine in response to a mount by the male as previously described in detail (37). A mount was defined as placement of both front limbs on the hindquarters of the female with or without intromission or ejaculation. All data for round 1, including male mounting, was analyzed by one-way ANOVA using SYSTAT, and group differences were identified using Fisher’s least significant difference post hoc test. For round 2, the data were analyzed by a two-way ANOVA, followed by a one-way ANOVAs examining the effects of treatment and a one-way repeated measures ANOVA examining the effect of test day. Group differences were identified by Fisher’s least significant difference post hoc test.

Estrogen-dependent gene expression. ERß in situ hybridization and OTR film autoradiograms were analyzed using the public domain NIH Image for Windows program (a free downloadable program at http://rsb.info.nih.gov/nih-image/). Brain regions from three adjacent sections per subject were measured bilaterally from anatomically matched sections, and care was taken to ensure that the area of the regions selected for measurement did not differ by more than 5% between sections and subjects. For the ERß autoradiograms, optical densities were converted to nCi/g tissue equivalents, and for the OTR autoradiograms, optical densities were converted to dpm/mg tissue equivalents using 14C and 35S standards (Amersham Pharmacia Biotech) respectively. All data were analyzed by one-way ANOVA using SYSTAT and group differences were identified using Fisher’s least significant difference post hoc test.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Uterotrophic assay
Dietary exposure to genistein at 500, 1000, 2000, and 4000 ppm (100 mg/kg BW/d; 200 mg/kg BW/d; 400 mg/kg BW/d; 800 mg/kg BW/d) resulted in a significant increase (P ≤ 0.01) in uterine wet weight over the controls, and an overall trend for a decrease in endogenous estrogen levels (P ≤ 0.08; Table 1Go). Uterine weights in the 1000, 2000, and 4000 ppm group were at least or greater than 2-fold heavier than the controls.


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  		TABLE 1. Effect of dietary genistein on uterine weight and endogenous estrogen levels in intact, immature rats

 
Plasma assay
Analysis with the TR-FIA assay revealed detectable plasma genistein (free) levels in animals fed either the standard vivarium diet, the 0.35% isoflavone supplemented diet, the GEN 100 or GEN 500 diets (Table 2Go). Animals fed the phytoestrogen-free diet did not have detectable genistein levels. Females on the GEN 500 diet had the highest plasma genistein levels (1.159 µm) and plasma genistein levels produced by the GEN 100 diet (0.057 µm) were roughly equivalent to the levels produced by the supplemented diet (0.036 µm). The standard vivarium diet also produced plasma genistein levels in a range similar to those seen with the isoflavone supplemented or GEN 100 diets (0.063 µm). The GEN 100 diet was used for round 1 of the sexual behavior tests, and the GEN 500 diet was ultimately used for round 2 of the sexual behavior tests and the neuroendocrine assays.


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  		TABLE 2. Mean plasma genistein concentrations of humans and female rats after phytoestrogen consumption

 
Female sexual behavior
Round 1.
Estrogen-treated animals showed a robust induction of lordosis behavior (LQ = 76.3%) compared with the controls (LQ = 7.5%; P ≤ 0.001; Fig. 1AGo). Genistein-treated animals (LQ = 9.1%) were similar to the controls and genistein intake had no effect on estrogen-treated animals (LQ = 74%). Males showed an equal mean number of mount attempts across all treatment groups (data not shown).



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  		FIG. 1. Effect of dietary genistein on lordosis behavior in ovariectomized (OVX) female rats in (A) round 1 and (B) round 2. Animals in round 1 (A) were given a single injection of 10 µg EB or an equivalent injection of sesame oil followed 48 h later by a single injection of 500 µg progesterone then tested 4 h following the progesterone injection. Genistein treated animals received the GEN 100 diet. Animals in round 2 (B) were given daily injections of 2 µg EB or an equivalent injection of sesame oil for 8 d. Testing took place every other day beginning on d 2 of the injections. Genistein treated animals received the GEN 500 diet. A, The GEN 100 diet did not affect lordosis behavior in either the presence or absence of estrogen after the sequential administration of estrogen and progesterone. B, OVX females exposed daily to 2 µg EB and fed the GEN 500 diet had the same lordosis quotient as females exposed daily to the same dose of EB but fed a genistein-free diet. OVX females treated with genistein alone did not differ from controls. (Values are means ± sem; *, P ≤ 0.001.)

 
Round 2.
A steady increase in LQ was seen in animals given EB, while no increase in LQ was seen in animals injected with oil only. The GEN 500 diet had no effect on LQ when given in either the presence or absence of estrogen (Fig. 1BGo). Again, males showed an equal number of mount attempts across all treatment groups on each day.

OTR binding
Binding with the 125I-d(CH2)5[Tyr(Me) 2,Tyr9-NH2]OVT ligand resulted in a strong signal in the VMN (Fig. 2AGo). Quantitative analysis of the signal on the film autoradiograms revealed that treatment with 17ß-E2 resulted in a 165% increase in OTR binding in the VMN compared with the controls (P ≤ 0.001), and treatment with the GEN 500 diet did not alter this effect (P ≤ 0.721; Fig. 2BGo). Treatment with the GEN 500 diet in the absence of estrogen produced no significant change in OTR expression compared with the controls.



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  		FIG. 2. Effects of estrogen and GEN 500 diet on OTR binding in the VMN of the adult, female, ovariectomized rat brain. A, Film autoradiograms showing OTR binding in the VMN of rats treated with no 17ß-E2 and no genistein (E–G–), no 17ß-E2 and GEN 500 diet (E–G+), 17ß-E2 and no genistein (E+G–) and both 17ß-E2 and 500 ppm dietary genistein (E+G+). B, Estrogen increased OTR binding in the VMN, and the GEN 500 diet did not alter this effect. The GEN 500 diet had no effect on OTR binding in the VMN in either the presence or absence of estrogen. (Results are expressed as the mean ± sem; *, P ≤ 0.001 compared with the controls.)

 
ERß mRNA expression
Hybridization with the antisense oligonucleotide probe for ERß mRNA resulted in strong signals for ERß mRNA in the PVN (Fig. 3AGo) but no detectable levels in the VMN (not shown). This is consistent with previous reports of ERß distribution in the brain (13, 14, 38, 39). Quantitative analysis of the signal on the film autoradiograms showed that treatment with 17ß-E2 resulted in a 26% decrease in ERß mRNA expression in the PVN (Fig. 3BGo; P ≤ 0.02) compared with the control group. In contrast, ingestion of the GEN 500 diet resulted in a 24% increase (P ≤ 0.02) compared with the control group. Treatment with both 17ß-E2 and the GEN 500 diet resulted in a 34% decrease in ERß mRNA signal compared with the control group (P ≤ 0.002), which was not significantly different from the group treated with 17ß-E2 alone (P ≤ 0.398).



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  		FIG. 3. A GEN 500 diet had the opposite effect of estrogen on ERß mRNA expression in the PVN of the adult, female, ovariectomized rat brain. A, Film autoradiograms showing ERß mRNA expression in the PVN of rats treated with no 17ß-E2 and no genistein (E–G–), no 17ß-E2 and the GEN 500 diet (E–G+), 17ß-E2 and no genistein (E+G–) and both 17ß-E2 and the GEN 500 diet (E+G+). B, Estrogen decreased, but the GEN 500 diet increased, ERß mRNA expression in the PVN. The GEN 500 diet had no effect on ERß mRNA expression in the presence of estrogen. (Results are expressed as the mean ± sem; *, P ≤ 0.02 compared with the controls.)

 
Plamsa estrogen concentrations and uterine responses
E2 levels in animals given blank capsules averaged 3.67 pg/ml compared with an average of 244.1 pg/ml for those given the implants containing 17ß-E2. Gonadally intact rat females typically have serum 17ß-E2 levels around 86.5 pg/ml during proestrus and 26.4 pg/ml during diestrus (40). Thus, the E2 levels achieved using the implants were about 2.5 times higher than physiological levels during proestrus, and the E2 levels in the OVX animals given blank capsules were even lower than physiological levels seen during diestrus.

Treatment with the GEN 500 diet did not significantly increase uterine weight (67 ± 5 mg) compared with the controls (69 ±2 mg), or diminish the uterotropic effects of estrogen (352 ±27 mg) when given in combination (354 ±30 mg). This is markedly different from the results of an immature rat uterotropic bioassay, which showed that the GEN 500 diet significantly increased uterine weight in juvenile females compared with the controls.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
This is the first study to demonstrate that the isoflavone phytoestrogen, genistein, has differential activity on ER{alpha}-and ERß-dependent gene expression in the hypothalamus, and the first to show that genistein is antiestrogenic for ERß-dependent gene expression in vivo. A GEN 500 diet had the opposite effect of 17ß-E2 on ERß mRNA expression in the PVN but no effect on the estrogen-dependent up-regulation of OTR binding in the VMN in either the presence or absence of estrogen. The absence of activity through ER{alpha} in the VMN may partially explain why neither a 100 ppm genistein diet (GEN 100) nor the GEN 500 diet altered lordosis behavior. These results are in contrast to the antiestrogenic activity through both ER{alpha} and ERß seen with a soy isoflavone dietary supplement containing a mixture of phytoestrogens, including genistein (10) and suggest that the antiestrogenic actions of the supplement on female sexual behavior and OTR density are mediated by some other component of the supplement than genistein.

These genistein doses used in these experiments were too low to cause uterine growth in adult OVX rats but substantially increased uterine weight in juvenile animals. This increase was coupled with a trend for a decline in endogenous estrogen levels, suggesting that genistein both interferes with endogenous estrogen production, and has estrogenic activity in the uterus. Although the increase in uterine weight with genistein treatment in juveniles was substantial, there was no dose response relationship. This may be because the majority of doses used for this study were high enough to produce maximal increases in uterine weight (equal to or greater than 2-fold).

The juvenile uterotropic assay revealed that a significant increase in uterine weight occurred at a dose of at least 500 ppm but gave no indication as to whether this was a relatively high or low dose for a rat. To choose a physiologically relevant dose for these neuroendocrine studies, we had to determine what dietary dose of genistein produces plasma levels equivalent to those found in humans consuming an isoflavone supplemented or traditional Asian diet. We used a modified version of the TR-FIA genistein assay developed by Wang et al. (32). Plasma levels have traditionally been measured using GC-MS or HPLC. TR-FIA is more sensitive than either of those two methods, although less specific. Circulating genistein is present in both conjugated and unconjugated forms. The unconjugated or "free" genistein (aglycone form) is the hormonally active form, and the TR-FIA method primarily detects the "free" form. Wang et al. have validated the TR-FIA method against GC-MS using human plasma and serum samples and have given us the opportunity to modify it for use with dried blood spot samples from humans and rats. Thus, this is the first study to use TR-FIA for rat dried blood spots.

The serology study was conducted primarily to determine what dietary concentration of pure genistein would produce plasma levels of genistein (free) equivalent to those produced by the 0.35% isoflavone supplemented diet used in earlier experiments (10). The GEN 100 diet produced plasma levels equivalent to those produced on the 0.35% isoflavone supplemented diet, and thus was chosen for the first round of behavioral experiments. It is interesting to note that animals fed the standard vivarium chow also had plasma levels in this range. This is not totally unexpected, however, given that the primary ingredient in this diet is soy meal. The TR-FIA used for the analysis is highly sensitive but can cross-react with daidzein (2.5%) and other phytoestrogens (32). Thus, the values for the vivarium diet and the isoflavone supplemented diet may be artificially high.

The plasma values obtained from this study are all in the range of human plasma levels (Table 2Go). The GEN 100 diet produced plasma genistein levels slightly higher than plasma levels in humans consuming a traditional Western diet (41). The 500 ppm genistein diet produced plasma genistein levels higher than plasma levels in humans consuming an isoflavone supplemented diet, but similar to plasma levels in humans consuming a traditional Asian diet and lower than plasma levels in infants consuming soy infant formula (42).

OTR binding density in the VMN is thought to be mediated by ER{alpha} (26) through an increase in OTR mRNA expression (27). Our previous work suggests that ERß mRNA expression in the PVN is likely mediated by ERß (11). Although the GEN 500 diet produced no change in lordosis behavior or OTR regulation in the VMN, there was a significant increase in ERß-mRNA expression in the PVN. Collectively, these results suggest that the regulation of ERß-mRNA expression in the PVN does not significantly impact lordosis behavior, but that the estrogen-dependent gene expression regulated via ER{alpha} probably does. This is consistent with multiple studies that have conclusively demonstrated that ER{alpha} is required for the normal expression of both male and female sexual behavior using ER{alpha}KO mice (22, 43, 44). Although the significance of ER{alpha} on estrogen-dependent behavior has been well documented, the role of ERß is virtually unknown. Reproductive behavior is relatively normal in both male and female ERßKO mice (23), indicating that the role of ERß in the elicitation and mediation of reproductive behaviors is minimal at best.

Although the expression and regulation of ER{alpha} mRNA in the brain have been well characterized (45, 46, 47), little is known about the regulation of ERß mRNA expression in the brain. This is now the third time we have demonstrated that a phytoestrogen, or combination of phytoestrogens, alter ERß mRNA expression in the PVN in the opposite direction of estrogen. Given that both in situ hybridization (38, 39, 48) and immunocytochemical studies (14) have failed to detect ER{alpha} in the rat PVN, this effect is likely to be mediated exclusively via ERß. Collectively, this suggests that, as a group, the phytoestrogens may be generally antiestrogenic through ERß in vivo. Given how little is currently known about the role of ERß in the brain, it is difficult to predict what behavioral or neuroendocrine endpoints disruption of ERß by phytoestrogens is likely to affect. However, our results clearly demonstrate that the phytoestrogens are not merely mimicking the effects of endogenous estrogen in the brain.

At first glance, the effects of genistein on estrogen-dependent gene expression and behavior described in this study may appear to be in contrast to our earlier work demonstrating that a commercially available, phytoestrogen supplement containing a mixture of phytoestrogens, including genistein, antagonized both ER{alpha}- and ERß-mediated gene expression in the female rat brain, and attenuated estrogen-dependent female sexual behavior. However, in vitro assays have demonstrated that the relative estrogenic potency of genistein for ERß is 30 times higher than for ER{alpha}, and the binding affinity of genistein for ER{alpha} is 20-fold lower than for ERß (16). The results of the present study suggest that genistein alone could be responsible for the down-regulation of ERß mRNA expression in the PVN by the isoflavone supplement used in previous experiments (10), but that other elements in the supplement are likely responsible for the effects on ER{alpha}-dependent gene expression and behavior.

Given the relatively low binding affinity of genistein for ER{alpha}, it is not surprising that genistein had no effect on the estrogen-dependent regulation of OTR in the VMN, an effect known to be mediated by ER{alpha} (26). It could be that higher doses are needed to elicit an effect. The dose chosen for this experiment was selected because it produced plasma genistein levels equivalent to those seen in animals consuming the phytoestrogen supplement used in our earlier experiments. High micromolar concentrations of genistein are needed in vivo to inhibit aromatase (3.5 µm) and a variety of kinases (2.6–111 µm) (for a review see Ref. 28) and blood levels for this experiment are much lower (1.159 µm; Table 2Go). Additionally, although genistein deposition is abundant in digestive and reproductive tissues, particularly the liver and the prostate, genistein may not be detectable at even picogram levels in the brain at dietary doses below 500 ppm (49, 50), further suggesting that genistein activity in the brain at low physiological doses may be limited.

Although genistein has been shown to have transcriptional activity through both ER{alpha} and ERß in cell transfection assays, the data from this study suggest that at a dietary dose of 500 ppm, genistein is only active via ERß in the brain. The different effects found in the transfection-reporter assays and our own in vivo findings, may be due to metabolism of genistein after consumption, or to interactions of the ligand-receptor complex with accessory binding proteins not present in the transfection assay tissue, but found in the PVN.

Given our findings, it is possible that genistein is only active through ERß and not ER{alpha}. If this proves to be the case, it would make genistein an extremely useful compound for exploring the individual and specific roles of ER{alpha} and ERß in the brain. However, it is essential that the effects of genistein over a much wider range of doses be explored before this assessment can be confirmed.

Genistein is one of the most common and abundant isoflavones, found in thousands of plant species, particularly the legumes. Soy and soy-based foods, including soy infant formula, contain the highest concentrations of genistein (51, 52). Soy and isoflavone supplements, like the one used in these experiments, also contain significant quantities of genistein (53). These and other phytoestrogen-rich foods are being heavily advertised as a healthy alternative to traditional ERT and are increasing in popularity as their beneficial effects on the heart and bones become more widely publicized.

We have now demonstrated that genistein, coumestrol, and a commercially prepared isoflavone supplement are antiestrogenic in the brain (10, 11). The supplement produced these effects in adult, OVX, rats at levels lower than those needed to cause uterine proliferation in juvenile rats. All of the doses used for these studies were low enough to produce plasma levels in rats that equate to plasma levels of phytoestrogens commonly seen in humans (Table 2Go). Although effects on humans are difficult to predict from studies on rats, the results of these studies suggest that although phytoestrogens may confer a variety of benefits, including a reduced risk of heart disease and cancer due to their estrogenic activity in the periphery, their antiestrogenic actions in the brain may indicate that they do not confer the same mental health benefits as traditional ERT. (6, 7, 8, 9, 53)


   Acknowledgments
 
We gratefully acknowledge Herman Adlercreutz (University of Helsinki, Helsinki, Finland) and his research team for collaborating with us on the TR-FIA analysis of the rat serum samples, Catherine Sharer and Betsy Russell (Emory University, Atlanta, GA), for their technical assistance, Isadora Rapoport and Margarette Shegog (Emory University) for their assistance with behavioral testing and scoring, and especially Eleni Antzoulatos Aultman (Emory University) for her assistance with many aspects of this project.


   Footnotes
 
This work was funded by the National Science Foundation and Technology Center for Behavioral Neuroscience, Emory University (IBN 9876754), and through the generous financial support of the ARCS Foundation.

Abbreviations: EB, E2 benzoate; ER{alpha}KO, ER{alpha} knockout; ERT, estrogen replacement therapy; GC-MS, gas chromotography-mass spectrometry; GEN 500, 500-ppm genistein diet; LQ, lordosis quotient; OTR, oxytocin receptor; OVX, ovariectomized; PND, postnatal day; PVN, paraventricular nucleus; RT, room temperature; TR-FIA, time-resolved competitive-binding fluoroimmunoassay; VMN, ventromedial nucleus of the hypothalamus.

Received December 3, 2001.

Accepted for publication February 8, 2002.


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 Introduction
 Materials and Methods
 Results
 Discussion
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