Metabolism of Endogenous and Exogenous Estrogens in Women

Estrogens regulate important processes in reproductive, skeletal, cardiovascular, and central nervous systems that impact women's overall health. Understanding endogenous and exogenously administered estrogen metabolism is vital to determining therapeutic estrogen levels. The present review provides an overview of estrogen metabolites formed in non-pregnant and pregnant women, and those resulting from exogenous estrogen administration. There are four principal endogenous estrogens: estrone (E1), estradiol (E2), estriol (E3), and estetrol (E4). E4, which is produced only in pregnancy, has emerged recently as an estrogen with significant therapeutic potential. E1, E2, and E3 undergo extensive metabolism primarily through phase I (hydroxylation, oxidation, reduction) and phase II (primarily conjugation) reactions, whereas E4 undergoes only phase II reactions. Exogenous estrogens commonly used for menopausal treatment and/or contraception, including micronized E2, conjugated equine estrogens, and ethinyl estradiol, also undergo phase I and phase II reactions, but differ widely in the types of metabolites formed. The mechanisms by which estrogen metabolites are formed and their excretion in urine, bile, and feces, are still poorly understood. We highlight areas that require further research to foster a better understanding of how estrogen metabolism impacts dosing of estrogens for therapeutic use, as well as the physiological regulation of endogenous estrogens.

Hepatic steroid sulfatase critically determines estrogenic activities of conjugated equine estrogens in human cells in vitro and in mice

Conjugated equine estrogens (CEEs), whose brand name is Premarin, are widely used as a hormone-replacement therapy (HRT) drug to manage postmenopausal symptoms in women. Extracted from pregnant mare urine, CEEs are composed of nearly a dozen estrogens existing in an inactive sulfated form. To determine whether the hepatic steroid sulfatase (STS) is a key contributor to the efficacy of CEEs in HRT, we performed estrogen-responsive element (ERE) reporter gene assay, real-time PCR, and UPLC-MS/MS to assess the STS-dependent and inflammation-responsive estrogenic activity of CEEs in HepG2 cells and human primary hepatocytes. Using liver-specific STS-expressing transgenic mice, we also evaluated the effect of STS on the estrogenic activity of CEEs in vivo We observed that CEEs induce activity of the ERE reporter gene in an STS-dependent manner and that genetic or pharmacological inhibition of STS attenuates CEE estrogenic activity. In hepatocytes, inflammation enhanced CEE estrogenic activity by inducing STS gene expression. The inflammation-responsive estrogenic activity of CEEs, in turn, attenuated inflammation through the anti-inflammatory activity of the active estrogens. In vivo, transgenic mice with liver-specific STS expression exhibited markedly increased sensitivity to CEE-induced estrogenic activity in the uterus resulting from increased levels of liver-derived and circulating estrogens. Our results reveal a critical role of hepatic STS in mediating the hormone-replacing activity of CEEs. We propose that caution needs to be applied when Premarin is used in patients with chronic inflammatory liver diseases because such patients may have heightened sensitivity to CEEs due to the inflammatory induction of STS activity.

Effects of different menopausal hormone replacement regimens on body composition in Chinese women

PURPOSE

The aim of this study was to determine the effects of different menopausal hormone therapy regimens on body composition in healthy postmenopausal Chinese women.

MATERIALS AND METHODS

One hundred and twenty-three healthy postmenopausal Chinese women were randomly assigned to either group A (0.625 mg conjugated equine estrogens [CEE] plus 100 mg micronized progesterone [MP]), group B (0.3 mg CEE plus 100 mg MP), or group C (0.625 mg CEE plus 10 mg dydrogesterone). Body composition was assessed by dual-energy X-ray absorptiometry.

RESULTS

One hundred and two women completed the trial at 1 year. A small but significant gain in lean body mass (619 ± 1019 g, p = 0.002) and a decrease of fat mass in all separate regions was observed in group A. A significant shift from gynoid to android fat distribution was observed in group B and group C (android/gynoid fat percentage ratios increased by 0.06 ± 0.08, p = 0.000 and 0.03 ± 0.08, p = 0.018, respectively), whereas no significant change was observed in group A (0.02 ± 0.06, p = 0.103).

CONCLUSIONS

In healthy postmenopausal Chinese women, 0.625 mg of CEE combined with 100 mg of MP was associated with a more favorable fat distribution compared with 0.3 mg CEE plus 100 mg MP or 0.625 mg CEE plus 10 mg dydrogesterone.

Estrogen metabolism in menopausal hormone users in the women's health initiative observational study: Does it differ between estrogen plus progestin and estrogen alone?

The WHI found an unexpected reduced breast cancer risk in women using CEE alone. We hypothesized CEE alone induces estrogen hydroxylation along the 2-pathway rather than the competing 16-pathway, a pattern linked to reduced postmenopausal breast cancer risk. One thousand eight hundred and sixty-four women in a WHIOS case-control study of estrogen metabolism and ovarian and endometrial cancer were studied of whom 609 were current E + P users (351 used CEE + MPA), while 272 used E alone (162 used CEE). Fifteen EM were measured, and analyses were conducted for each metabolite, hydroxylation pathway (2-, 4-, or 16-pathway) and ratios of pathway concentrations using inverse probability weighted linear regression. Compared to E + P users, all EM were higher in E alone users (significant for unconjugated estrone, total/conjugated estradiol, total/unconjugated 2-methoxyestrone, 4-methoxyestrone and unconjugated estriol). The relative concentrations of 2- and 4-pathway EM did not differ between the MHT users (2-pathway EM comprised 15% and 4-pathway EM <2% of the total), but 16-pathway EM were lower in E alone users (p = 0.036). Ratios of 2- and 4-pathway EM compared to 16-pathway EM were significantly higher in E alone compared to E + P users. Similar but not significant patterns were observed in CEE-alone and CEE + MPA users. Our data suggest that compared to E + P users, women using E alone have more extensive metabolism via the 2- vs. the competing 16-pathway. This is consistent with epidemiologic evidence of reduced postmenopausal breast cancer risk associated with this metabolic profile and may provide a clue to the breast cancer risk reduction in CEE alone users during the WHI.

Effect of a tissue selective estrogen complex on breast cancer: Role of unique properties of conjugated equine estrogen

The Women's Health Initiative studies reported that the menopausal hormone therapy (MHT) regimen containing conjugated equine estrogen (CEE) and medroxyprogesterone acetate increased, whereas CEE alone reduced breast cancer incidence. These observations suggest the possibility that CEE might exert unique actions on breast and also suggest the need to eliminate the progestogen from MHT regimens. A MHT regimen called a tissue selective estrogen complex (TSEC), containing CEE plus bazedoxifene (BZA), to avoid the need for a progestogen, was developed and FDA approved. Our study addressed two questions regarding this TSEC: (i) whether CEE exert effects on breast cancer which differ from those of estradiol (E₂ ) and (ii) whether BZA antagonize the effects of E₂ and CEE on breast cancer? Two rodent models (NMU and ACI) were used to compare the effect of CEE with E₂ on mammary tumor formation, proliferation and apoptosis. In both the NMU and ACI models, E₂ significantly increased tumor incidence and multiplicity whereas in striking contrast CEE did not, even though the estrogenic effects of CEE and E₂ on uterine weight were identical. Mechanistically E₂ blocked whereas CEE stimulated apoptosis (cleaved caspase-3) in ACI animals and only E₂ stimulated proliferation (Ki67). BZA exerted highly potent anti-estrogenic effects on tumors by completely blocking palpable tumor formation. These data suggest that the CEE/BZA TSEC may be a safer, breast-antagonistic, MHT agent for women and might have potential to prevent breast cancer while relieving menopausal symptoms.

Differences in verbal memory performance in postmenopausal women receiving hormone therapy: 17β-estradiol versus conjugated equine estrogens

OBJECTIVE

Much controversy exists and many questions remain unanswered about the effects of hormone therapy (HT) on cognition in postmenopausal women. There is growing evidence suggesting that HT compounds containing conjugated equine estrogen (CEE) have negative effects on cognition whereas 17β-estradiol (17β-E) either has positive or neutral effects. The present study sought to further examine this issue in a sample of postmenopausal women with risk factors for Alzheimer's disease (AD).

DESIGN

Cross-sectional neuropsychological evaluation.

SETTING

Academic research clinic.

PARTICIPANTS

68 healthy postmenopausal women (aged 49-68) receiving either 17β-E or CEE for at least one year with increased risk for AD.

MEASUREMENTS

Neuropsychological test battery of the cognitive domains of attention/working memory/processing speed, verbal memory, visual memory, and executive functioning.

RESULTS

Multivariate analyses of variance (MANOVA) showed significantly better verbal memory performance in women receiving 17β-E compared to women receiving CEE regardless of age, IQ, years of education, risk factors for AD (including APOE-ε4 carriership), duration of endogenous and exogenous estrogen exposure, concurrent progesterone use, or natural versus surgical menopause status.

CONCLUSIONS

Verbal memory performance was better in postmenopausal women receiving 17β-E compared to CEE in a sample population of women with risk factors for AD. Genetic risk for AD as well as other confounds did not affect this finding. The results suggest a differential effect of HT type on verbal memory, with 17β-E being a preferential compound. Further evaluation of HT types, regimens and duration of use on cognitive performance in postmenopausal women in a controlled longitudinal design is warranted.

Hormone therapy and physical function change among older women in the Women's Health Initiative: a randomized controlled trial

OBJECTIVE

Although estrogen may be linked to biological pathways that maintain higher physical function, the evidence is derived mostly from observational epidemiology and therefore has numerous limitations. We examined whether hormone therapy affected physical function in women 65 to 79 years of age at enrollment.

METHODS

This study involves an analysis of the Women's Health Initiative randomized controlled trials of hormone therapy in which 922 nondisabled women who had previous hysterectomies were randomized to receive estrogen therapy or a placebo and 1,458 nondisabled women with intact uteri were randomized to receive estrogen + progestin therapy or a placebo. Changes in physical function were analyzed for treatment effect, and subgroup differences were evaluated. All women completed performance-based measures of physical function (grip strength, chair stands, and timed walk) at baseline. These measures were repeated after 1, 3, and 6 years.

RESULTS

Overall, participants' grip strength declined by 12.0%, chair stands declined by 3.5%, and walk pace slowed by 11.4% in the 6 years of follow-up (all P values <0.0001). Hormone therapy, as compared with placebo, was not associated with an increased or decreased risk of decline in physical function in either the intention-to-treat analyses or in analyses restricted to participants who were compliant in taking study pills.

CONCLUSIONS

Hormone therapy provided no overall protection against functional decline in nondisabled postmenopausal women 65 years or older in 6 years of follow-up. This study did not address the influence of hormone therapy for women of younger ages.

Molecular characterization of a B-ring unsaturated estrogen: implications for conjugated equine estrogen components of premarin

Conjugated equine estrogens (CEEs) are routinely used for hormone replacement therapy (HRT), making it important to understand the activities of individual estrogenic components. Although 17beta-estradiol (17beta-E2), the most potent estrogen in CEE, has been extensively characterized, the actions of nine additional less potent estrogens are not well understood. Structural differences between CEEs and 17beta-E2 result in altered interactions with the two estrogen receptors (ERalpha and ERbeta) and different biological activities. To better understand these interactions, we have determined the crystal structure of the CEE analog, 17beta-methyl-17alpha-dihydroequilenin (NCI 122), in complex with the ERalpha ligand-binding domain and a peptide from the glucocorticoid receptor-interacting protein 1 (GRIP1) coactivator. NCI 122 has chemical properties, including an unsaturated B-ring and 17alpha-hydroxyl group, which are shared with some of the estrogens found in CEEs. Structural analysis of the NCI 122-ERalpha LBD-GRIP1 complex, combined with biochemical and cell-based comparisons of CEE components, suggests that factors such as decreased ligand flexibility, decreased ligand hydrophobicity and loss of a hydrogen bond between the 17-hydroxyl group and His524, contribute significantly to the reduced potency of CEEs on ERalpha.