Steroid and thyroid hormones modulate a changing brain

Epigenetic impacts of endocrine disruptors in the brain

The acquisition of reproductive competence is organized and activated by steroid hormones acting upon the hypothalamus during critical windows of development. This review describes the potential role of epigenetic processes, particularly DNA methylation, in the regulation of sexual differentiation of the hypothalamus by hormones. We examine disruption of these processes by endocrine-disrupting chemicals (EDCs) in an age-, sex-, and region-specific manner, focusing on how perinatal EDCs act through epigenetic mechanisms to reprogram DNA methylation and sex steroid hormone receptor expression throughout life. These receptors are necessary for brain sexual differentiation and their altered expression may underlie disrupted reproductive physiology and behavior. Finally, we review the literature on histone modifications and non-coding RNA involvement in brain sexual differentiation and their perturbation by EDCs. By putting these data into a sex and developmental context we conclude that perinatal EDC exposure alters the developmental trajectory of reproductive neuroendocrine systems in a sex-specific manner.

Differences in regional brain metabolism associated with specific formulations of hormone therapy in postmenopausal women at risk for AD

Differential cerebral metabolic effects of various hormone therapy formulations, and their associations with cognitive status, remain to be established. The principal aim of the current study was to assess relationships between regional cerebral metabolism and estrogen-based hormone therapies. Postmenopausal women (n=53) at elevated risk for Alzheimer's disease (AD) were on estrogen-containing hormone therapy for at least one year prior to enrollment in a prospective, randomized clinical trial. Subjects underwent an FDG-PET scan, along with neuropsychological, medical, and demographic assessments at time of enrollment, to be repeated one year following randomization to hormone therapy continuation versus discontinuation, and results from analyses of the baseline assessments are reported here. Across all subjects, years of endogenous estrogen exposure correlated most closely with metabolism in right superior frontal gyrus (p<0.0005). Women taking 17β-estradiol (E) performed three standard deviations higher in verbal memory than women taking conjugated equine estrogen (CEE), and their verbal memory performance positively correlated with metabolism in Wernicke's (p=0.003) and auditory association (p=0.002) areas. Women taking progesterone-plus-estrogen had lower metabolism than women taking unopposed estrogen within the mesial and inferior lateral temporal regions (p<0.0005) and the inferior frontal cortex, contralateral to Broca's area (p<0.0005). In conclusion, particular areas of relatively preserved metabolism were seen in women with more years of endogenous estrogen exposure, as well as in women taking estradiol-based formulations or estrogen therapies unopposed by progesterone, together suggesting regionally specific neuroprotective estrogenic effects.

Identification of a thyroid hormone response element in the mouse Kruppel-like factor 9 gene to explain its postnatal expression in the brain

Brain development is critically dependent on thyroid hormone (T(3)). Krüppel-like factor 9 (Klf9) is a T(3)-inducible gene in developing rat brain, and several lines of evidence support that KLF9 plays a key role in neuronal morphogenesis. Here we extend our findings to the mouse and demonstrate the presence of a functional T(3) response element (T(3)RE) in the 5' flanking region of the mouse Klf9 gene. Klf9 mRNA is strongly induced in the mouse hippocampus and cerebellum in a developmental stage- and T(3)-dependent manner. Computer analysis identified a near optimal direct repeat 4 (DR-4) T(3)RE 3.8 kb upstream of the Klf9 transcription start site, and EMSAs showed that T(3) receptor (TR)-retinoid X receptor heterodimers bound to the T(3)RE with high affinity. The T(3)RE acts as a strong positive response element in transfection assays using a minimal heterologous promoter. In the mouse neuroblastoma cell line N2a[TRbeta1], T(3) caused a dose-dependent up-regulation of Klf9 mRNA. Chromatin immunoprecipitation assays conducted with N2a[TRbeta1] cells showed that TRs associated with the Klf9 T(3)RE, and this association was promoted by T(3). Treatment of N2a[TRbeta1] cells with T(3) led to hyperacetylation of histones 3 and 4 at the T(3)RE site. Furthermore, TRs associated with the DR-4 T(3)RE in postnatal d 4 mouse brain, and histone 4 acetylation was greater at this site compared with other regions of the Klf9 gene. Our study identifies a functional DR-4 T(3)RE located in the mouse Klf9 gene to explain its regulation by T(3) during mammalian brain development.

Mood symptoms and cognitive performance in women estrogen users and nonusers and men

OBJECTIVES

Previous studies have suggested sex differences in mood and cognition and that estrogen effects may partially explain such differences. In this study, we explore sex differences for a range of mood symptoms and for neuropsychological performance in men and postmenopausal women and assess the potential influence of estrogen on these measures.

DESIGN

Cross-sectional study of men and women examining mood, neuropsychological test data, and estrogen replacement therapy (ERT) use.

SETTING

Outpatient study at an urban teaching hospital with subjects recruited from the community.

PARTICIPANTS

All subjects (N = 96) were between the ages of 57 and 75 and included 31 women using ERT, 16 non-ERT users, and 49 men. Subjects did not have major depression and were nondemented.

MEASUREMENT

The three groups were compared according to profile of mood states and neuropsychological performance, and statistical analyses were controlled for socioeconomic status, age, and education level.

RESULTS

Female ERT users were less depressed and less angry and performed better on measures of verbal fluency and working memory than the other subject groups.

CONCLUSION

Postmenopausal estrogen use is associated with better mood and cognitive performance on tasks of fluency and working memory. These results suggest that estrogen should be examined as a potentially critical variable influencing late-life sex differences in mood and cognition.

Analysis of steroid-induced genes in the rat preoptic area-anterior hypothalamus using a differential-display reverse transcriptase-polymerase chain reaction

Steroid hormones modulate a variety of physiological functions in the hypothalamus. We attempted to identify steroid-regulated genes in the rat preoptic area-anterior hypothalamus by comparing differentially expressed mRNAs. Adult female rats were ovariectomized and, 1 week later, a silastic capsule containing 17beta-oestradiol (180 microg/ml) was subcutaneously implanted. After 2 days, a single injection of progesterone (1 mg) was administered at 10.00 h and rats were killed at 17.00 h on the same day. Differential-display polymerase chain reaction followed by Northern blot analysis showed that 10 clones were differentially regulated. Using homology search in Genbank, three genes were identified as sodium, potassium-ATPase beta1, protein kinase C-binding Nell-homologue protein and evectin-1. Further characterization of 10 clones showed that the expression patterns were tissue-specific and differentially regulated during puberty. Among these, mRNAs for protein kinase C-binding Nell-homologue protein, evectin-1 and human CGI-118 protein-like gene were induced after vagina opening, and differentially expressed during the oestrous cycle. Taken together, several steroid-regulated genes identified in the present study may play an important role in regulating hypothalamic functions, including puberty and the oestrous cycle.

Amphibian metamorphosis as a model for studying the developmental actions of thyroid hormone

The thyroid hormones L-thyroxine and triiodo-L-thyronine have profound effects on postembryonic development of most vertebrates. Analysis of their action in mammals is vitiated by the exposure of the developing foetus to a number of maternal factors which do not allow one to specifically define the role of thyroid hormone (TH) or that of other hormones and factors that modulate its action. Amphibian metamorphosis is obligatorily dependent on TH which can initiate all the diverse physiological manifestations of this postembryonic developmental process (morphogenesis, cell death, re-structuring, etc.) in free-living embryos and larvae of most anurans. This article will first describe the salient features of metamorphosis and its control by TH and other hormones. Emphasis will be laid on the key role played by TH receptor (TR), in particular the phenomenon of TR gene autoinduction, in initiating the developmental action of TH. Finally, it will be argued that the findings on the control of amphibian metamorphosis enhance our understanding of the regulation of postembryonic development by TH in other vertebrate species.

Amphibian metamorphosis as a model for studying the developmental actions of thyroid hormone

The thyroid hormones L-thyroxine and triiodo-L-thyronine have profound effects on postembryonic development of most vertebrates. Analysis of their action in mammals is vitiated by the exposure of the developing foetus to a number of maternal factors which do not allow one to specifically define the role of thyroid hormone (TH) or that of other hormones and factors that modulate its action. Amphibian metamorphosis is obligatorily dependent on TH which can initiate all the diverse physiological manifestations of this postembryonic developmental process (morphogenesis, cell death, re-structuring, etc.) in free-living embryos and larvae of most anurans. This article will first describe the salient features of metamorphosis and its control by TH and other hormones. Emphasis will be laid on the key role played by TH receptor (TR), in particular the phenomenon of TR gene autoinduction, in initiating the developmental action of TH. Finally, it will be argued that the findings on the control of amphibian metamorphosis enhance our understanding of the regulation of postembryonic development by TH in other vertebrate species.

Cellular and molecular basis of estrogen's neuroprotection. Potential relevance for Alzheimer's disease

Alzheimer's disease (AD) is one of the most common types of dementia among the aged population, with a higher prevalence in women. The reason for this latter observation remained unsolved for years, but recent studies have provided evidence that a lack of circulating estrogen in postmenopausal women could be a relevant factor. Moreover, follow-up studies among postmenopausal women who had received estrogen-replacement therapy (ERT), suggested that they had a markedly reduced risk of developing AD. In addition, studies among older women who already had AD indeed confirmed that a decrease in estrogen levels was likely to be an important factor in triggering the pathogenesis of the disease. In this review article, we will discuss the evidence suggesting that estrogen may have a protective role against AD, mainly through its action as: a trophic factor for cholinergic neurons, a modulator for the expression of apolipoprotein E (ApoE) in the brain, an antioxidant compound decreasing the neuronal damage caused by oxidative stress, and a promoter of the physiological nonamyloidogenic processing of the amyloid precursor protein (APP), decreasing the production of the amyloid-beta-peptide (A beta), a key factor in the pathogenesis of AD.

The effect of estrogen replacement therapy on cognitive function in women: a critical review of the literature

OBJECTIVE

To conduct a review of the available clinical trials to determine whether sufficient evidence exists to support the conclusion that estrogen replacement therapy has a beneficial effect on cognitive performance in post-menopausal women and in women with Alzheimer's disease. Studies were identified through a MEDLINE search of all English-language publications between 1970 and 1996 in which the words estrogen and cognition or estrogen and memory appeared.

DATA EXTRACTION

Data were extracted for each study, including features of subjects and eligibility criteria, duration of follow-up, and treatment regimen. Baseline characteristics were evaluated, including age; menopausal status; follicle-stimulating hormone, luteinizing hormone, and estradiol levels; mood; and measures of cognitive function. Psychological tests were evaluated for construct validity.

RESULTS

Nineteen studies were reviewed, including 10 randomized trials of estrogen replacement therapy versus placebo. Extreme heterogeneity among subjects and variability in the use of cognitive measures across the studies precluded performing a quantitative summary. Of the 10 randomized trials, eight claimed therapeutic benefits for estrogen therapy, three of which reported significant improvements in memory and two of which showed improvements in attention. These studies did not control for potential confounds such as depression and vasomotor symptoms. Of the nine observational studies, five found a significant association between estrogen use and cognitive function.

CONCLUSION

Although several observational studies provide encouraging evidence for the beneficial effect of estrogen on cognitive function, there is currently inadequate evidence available from randomized, controlled trials to support the conclusion that estrogen replacement therapy improves cognitive function in post-menopausal women or women with Alzheimer's dementia.

Distribution of adrenocorticoid receptors in the rat CNS measured by competitive PCR and cytosolic binding

Combined quantitative polymerase chain reaction (PCR) and cytosolic binding assay techniques are used to measure mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) mRNA, Kd, and Bmax in various rat central nervous system (CNS) regions, namely amygdala, hypothalamus, hippocampus, cortex, pituitary, and cervical, thoracic, and lumbar spinal cord. Two internal standards (i.s.) cDNA were cloned for quantitative PCR purposes. The i.s. templates differed from the respective wild-type (wt) templates for a single base-pair mutation introduced by PCR that generated a unique restriction site, thus allowing amplification products arising from coamplification of wt and i.s. to be distinguished. Results show that cerebellum, which displayed average Bmax values for both receptors, contained the highest level of MR and GR mRNA. Hippocampus also had a high level of MR mRNA. Low mRNA content was found in the hypothalamus for MR and GR as well as in the cortex for GR. High Bmax values for both MR and GR were found in the lumbar spinal cord, despite a modest mRNA content. The lowest Bmax values were found in the cortex for both receptors. It is, therefore, concluded that mRNA content and Bmax are not closely correlated in the rat CNS. These data suggest a differential regulation of various adrenocorticoid receptor isoforms. Moreover, this quantitative PCR method is very sensitive and can be used to assay small amounts of material in order to obtain absolute measurements of mRNA expression.

Neuroanatomical distribution of androgen and estrogen receptor-immunoreactive cells in the brain of the male roughskin newt

Immunohistochemistry was used to investigate the neuroanatomical distribution of androgen and estrogen receptors in brains of adult male roughskin newts, Taricha granulosa, collected during the breeding season. Immunoreactive cells were found to be widely distributed in specific brain areas of this urodele amphibian. Androgen receptor-immunoreactive (AR-ir) cells were observed in the olfactory bulbs, habenula, pineal body, preoptic area, hypothalamus, interpeduncular nucleus, area acusticolateralis, cerebellum, and motor nuclei of the medulla oblongata. Estrogen receptor-immunoreactive (ER-ir) cells were found in the lateral septum, amygdala pars lateralis, pallium, preoptic area, hypothalamus, and dorsal mesencephalic tegmentum. This immunocytochemical study of the newt brain reveals AR-ir and ER-ir cells in several regions that have not been previously reported to contain androgen and estrogen receptors in non-mammalian vertebrates. Additionally, the distribution of AR-ir and ER-ir cells in the newt brain, in general, is consistent with previous studies, suggesting that the distribution of sex steroid receptor-containing neurons in some brain regions is relatively conserved among vertebrates.

Memory complaints associated with the use of gonadotropin-releasing hormone agonists: a preliminary study

OBJECTIVES

To study the effect of GnRH agonist (GnRH-a) treatment on memory and to assess the role of psychological factors.

DESIGN

A randomized prospective study.

SETTING

An academic teaching hospital.

PARTICIPANTS

Women with endometriosis and infertility or endometriosis alone.

MAIN OUTCOME MEASURES

Memory Observation Questionnaire, Profile of Mood States, Health Concerns scale, a weekly diary of adverse effects.

RESULTS

Perceived memory functioning decreased during GnRH-a administration and by the final week of treatment 44% of women reported moderate to marked impairment in comparison to community norms. Prospective memory was most affected and withdrawal of GnRH-a treatment resulted in a return to normal memory functioning. Impairment was not related to excessive health concerns or mood changes and was uncorrelated with other adverse effects.

CONCLUSIONS

Memory disruption may be a more common side effect of GnRH-a treatment than currently is recognized. Problems were temporary and more likely a result of rapid estrogen depletion than a consequence of mood, somatic distress, or personality factors.

Differential regulation of adrenocorticoid receptors in the hippocampus and spinal cord of adrenalectomized rats

Using multiple polymerase chain reaction assay and cytosolic receptor binding assay we studied type I, mineralocorticoid receptor (MR), and type II, glucocorticoid receptor (GR), adrenocorticoid receptors expression in rat hippocampus and spinal cord, at various times after adrenalectomy: 12 hr, 24 hr, 3 days, and 1 week. Analysis of the data demonstrates that in hippocampus the expression of MR and GR mRNA was not significantly affected by adrenalectomy. On the contrary, Bmax of MR was significantly increased at each time post-surgery, with only slight modifications of Kd. Bmax and Kd for GR showed a significant increase after 3 days and 1 week. In the spinal cord, MR mRNA was increased 12 hr after adrenalectomy, reaching a maximum at 3 days. Bmax of MR was also significantly increased after 3 days, whereas its Kd remained unchanged for the entire duration of the the study. Both GR mRNA and binding parameters were poorly affected by adrenalectomy. The results of the present experiments demonstrate that the absence of adrenocortical hormones influences differentially MR and GR expression in hippocampus and spinal cord, suggesting the existence of various and independent mechanisms of regulation of adrenocorticoid receptor.

Messenger RNAs encoding steroidogenic enzymes are expressed in rodent brain

Using the reverse transcription polymerase chain reaction, mRNAs encoding steroidogenic P450s as well as NADPH-cytochrome P450 reductase (P450 reductase), adrenodoxin and the transcription factor steroidogenic factor 1 (SF-1) were all detected in rodent brain, but their distribution between brain regions varied. Adrenodoxin and P450 reductase were detected in all regions, suggesting the presence of both mitochondrial and microsomal P450s throughout the brain. Messenger RNAs encoding P450scc (CYP11A1) and P45017 alpha (CYP17) were also detected in all brain regions, this being the first report of CYP17 in the brain. P450c21 (CYP21) was detected only in the brain stem. P45011 beta (CYP11B1) and P450aldo (CYP11B2) are expressed in rat brain, but not in mouse brain; CYP11B1 primarily in the cerebrum, whereas CYP11B2 was detected in all brain regions. In both species, highest levels of aromatase P450 (CYP19) mRNA were detected in the cerebrum. SF-1 expression was restricted to the cerebrum minus cortex. Thus, although SF-1 is required for high level expression of the steroidogenic enzymes in adrenals and gonads, other factors may influence the expression of these genes in the brain. If the mRNAs detected by RT-PCR are indeed translated into functional enzymes, these studies suggest that different brain regions have different capacities for local steroid hormone production and metabolism. This raises the technical challenge of locating the specific sites of synthesis as well as the function of such locally produced ligands.

Expression screening reveals an orphan receptor chick ovalbumin upstream promoter transcription factor I as a regulator of neurite/substrate-cell contacts and cell aggregation

A rat homologue of chick ovalbumin upstream promoter transcription factor I (COUP-TF I) was isolated using an expression cloning method developed to isolate neurite outgrowth inhibitors. Overexpression of COUP-TF I in 3T3 fibroblasts resulted in reduction of stable contact formation between neurites and transfected cells. Additionally, COUP-TF I enhanced retinoic acid response element-dependent reporter gene expression in 3T3 fibroblasts, indicating that COUP-TF I can modulate transcriptional activation in these cells. Our data suggest that COUP-TF transcription factors are involved in the regulation of cell surface molecules during neruogenesis.

Orphan receptor COUP-TF I antagonizes retinoic acid-induced neuronal differentiation

Chicken ovalbumin upstream promoter-transcription factors (COUP-TF) are expressed in the developing nervous system and interact with nuclear hormone receptors to regulate expression of different genes. The role of COUP-TF orphan receptors in neurogenesis is virtually unknown. To study the possible function of COUP-TF I during neuronal differentiation, we generated COUP-TF I overexpressing teratocarcinoma PCC7 cell lines and analyzed retinoic acid (RA)-induced neuronal differentiation of these cells. COUP-TF I overexpression results in the blockade of morphological differentiation after induction to differentiate. COUP-TF I represses expression of microtubule-associated protein 2 (MAP2) gene and delays induction of growth-associated protein 43 (GAP43) gene expression. In contrast, expression of the neurofilament light subunit (NF-L) gene is not affected by COUP-TF I overexpression during neuronal differentiation. Also, cells overexpressing COUP-TF I do not stop proliferating after RA and dBcAMP treatment and possess suppressed transcriptional activation from different RA response elements. These results suggest that COUP-TF I plays an important role in regulating RA-induced neuronal differentiation.

Effects of experience and juvenile hormone on the organization of the mushroom bodies of honey bees

There is an age-related division of labor in the honey bee colony that is regulated by juvenile hormone. After completing metamorphosis, young workers have low titers of juvenile hormone and spend the first several weeks of their adult lives performing tasks within the hive. Older workers, approximately 3 weeks of age, have high titers of juvenile hormone and forage outside the hive for nectar and pollen. We have previously reported that changes in the volume of the mushroom bodies of the honey bee brain are temporally associated with the performance of foraging. The neuropil of the mushroom bodies is increased in volume, whereas the volume occupied by the somata of the Kenyon cells is significantly decreased in foragers relative to younger workers. To study the effect of flight experience and juvenile hormone on these changes within the mushroom bodies, young worker bees were treated with the juvenile hormone analog methoprene but a subset was prevented from foraging (big back bees). Stereological volume estimates revealed that, regardless of foraging experience, bees treated with methoprene had a significantly larger volume of neuropil in the mushroom bodies and a significantly smaller Kenyon cell somal region volume than did 1-day-old bees. The bees treated with methoprene did not differ on these volume estimates from untreated foragers (presumed to have high endogenous levels of juvenile hormone) of the same age sampled from the same colony. Bees prevented from flying and foraging nonetheless received visual stimulation as they gathered at the hive entrance. These results, coupled with a subregional analysis of the neuropil, suggest a potentially important role of visual stimulation, possibly interacting with juvenile hormone, as an organizer of the mushroom bodies. In an independent study, the brains of worker bees in which the transition to foraging was delayed (overaged nurse bees) were also studied. The mushroom bodies of overaged nurse bees had a Kenyon cell somal region volume typical of normal aged nurse bees. However, they displayed a significantly expanded neuropil relative to normal aged nurse bees. Analysis of the big back bees demonstrates that certain aspects of adult brain plasticity associated with foraging can be displayed by worker bees treated with methoprene independent of foraging experience. Analysis of the overaged nurse bees suggests that the post-metamorphic expansion of the neuropil of the mushroom bodies of worker honey bees is not a result of foraging experience.

How do sex and stress hormones affect nerve cells?

Gonadal and adrenal hormones regulate both structure and neurochemical function in neurons that express receptors for them. Gonadal hormone effects mediate sexual differentiation of the brain and reproductive tract, and their actions during early development program groups of cells in the nervous system to respond in male- or female-typical ways to hormones in adulthood. Induction of synapse formation is one consequences of brain sexual differentiation, but hormonally directed synaptic plasticity is by no means confined to early development and in fact occurs cyclically during reproductive cycles in a number of brain regions of the female rat, including the hippocampus. The hippocampus responds to adrenal steroids as well and undergoes changes in dendritic branching as a result of repeated stress. Implications of hormonally directed changes in brain structure and neurochemistry are discussed with respect to human pathophysiology.

Estradiol and progesterone regulate neuronal structure and synaptic connectivity in adult as well as developing brain

Until recently, it has been widely believed that the adult brain does not undergo changes in its structure, particularly in relation to the actions of circulating hormones. It has now become clear that estradiol and progesterone have important effects on adult brain structure and function. Single section Golgi silver staining and electron microscopy have been used to analyze numbers of spines on dendrites and to count synapses on dendritic spines. In the adult female rat brain, we find that dendrites of neurons in the ventromedial hypothalamus and CA1 region of the hippocampus sprout increased numbers of spines on dendrites and then lose them during the 4- or 5-day estrous cycle. Increased spine numbers are accompanied by increased numbers of synapses on spines. In the hippocampus, the loss of spines and spine synapses occurs during a 24-h period between the time of maximum sexual receptivity on the day of proestrus and the next day, the day of estrus. This loss is not due solely to the decline in estradiol; however, giving progesterone speeds up the decline, and administering the antiprogestin, Ru486, on proestrus blocks the natural decline of synapse density. The changes of synaptic density in the hypothalamus are responsible, at least in part, for the cyclicity of sexual behavior, whereas the cyclicity of synapses in the hippocampus may subserve functions related to spatial learning and memory. In human subjects, cyclic fluctuations in gonadal hormones are associated with cyclic changes in performance on a variety of cognitive and motor tasks.

Gene expression during metamorphosis: an ideal model for post-embryonic development

The precocious induction in vivo and in culture of insect and amphibian metamorphosis by exogenous ecdysteroids and thyroid hormones, and its retardation or inhibition by juvenile hormone and prolactin, respectively, has allowed the analysis of such diverse processes of post-embryonic development as morphogenesis, tissue remodelling, functional reorganization, and programmed cell death. Metamorphosis in vertebrates also shares many similarities with mammalian development in the late foetal and perinatal period. This review describes the regulation of expression of some of the 'adult' gene products during metamorphosis in invertebrates and vertebrates. Recent studies on metamorphosis have revealed the important role played by auto-induction of hormone receptor genes, based on which a model will be presented to explain the activation of 'downstream' genes which give rise to the adult phenotype. It will also be argued that metamorphosis is an ideal model for analyzing some of the major mechanisms governing post-embryonic development.

P450 enzymes of estrogen metabolism

Endogenous and exogenous estrogens undergo extensive oxidative metabolism by specific cytochrome P450 enzymes. Certain drugs and xenobiotics have been found to be potent inducers of estrogen hydroxylating enzymes with C-2 hydroxylase induction being greater than that of C-16 hydroxylase. Oxygenated estrogen metabolites have different biological activities, with C-2 metabolites having limited or no activity and C-4 and C-16 metabolites having similar potency to estradiol. Pathophysiological roles for some of the oxygenated estrogen metabolites have been proposed, e.g. 16 alpha-hydroxyestrone and 4-hydroxyestrone. These reactive estrogens are capable of damaging cellular proteins and DNA and may be carcinogenic in specific cells.