Steroid sulfatase and 17 beta-hydroxysteroid oxidoreductase activities in mouse tissues

Steroid sulfatase in mouse liver and testis: Characterization, ontogeny and localization

Steroid hormones often circulate in the plasma as inactive sulfated forms, such as estrone sulfate and dehydroepiandrosterone sulfate. The enzyme steroid sulfatase (STS) converts these steroids into active forms, mainly estrogens, in peripheral tissues. STS is present in most tissues, but it occurs at higher levels in certain organs, notably liver and placenta. In this study, we examined the tissue distribution of STS in a prominent laboratory model, the house mouse (Mus musculus). Tissues included were heart, liver, small intestine, skeletal muscle, and gonads of both sexes. An 3H-estrone-sulfate conversion assay was used to measure STS activity in tissue homogenates and extracts. STS activities were high for hepatic tissue homogenates of both genders. Testicular STS levels were similar to those of liver, while STS activities of ovary, small intestine, heart, and muscle were considerably lower. The specific STS inhibitors, EMATE and STX-64 virtually eliminated STS activity in hepatic microsomes and cytosols, verifying that the observed enzyme activity was due to STS. Enzyme kinetic assays showed Km values of 8.6 µM for liver and 9.1 µM for testis, using E1S as substrate. Hepatic and testicular STS activities, measured in CHAPS-extracted microsome, were low up to 5 weeks of age and were higher through 56 weeks. Western blotting, with a specific STS antibody, confirmed the presence of STS protein (65 Da) in both liver and testis. Immunofluorescence of tissue sections detected the presence of STS protein in hepatocytes, in testicular Leydig cells and in seminiferous tubules (Leydig cells and developing germ cells). These results suggest that STS may have a significant role in testicular function.

Identification of a pituitary ERα-activated enhancer triggering the expression of Nr5a1, the earliest gonadotrope lineage-specific transcription factor

Background

Gonadotrope lineage differentiation is a stepwise process taking place during pituitary development. The early step of gonadotrope lineage specification is characterized by the expression of the Nr5a1 transcription factor, a crucial factor for gonadotrope cell fate determination. Abnormalities affecting Nr5a1 expression lead to hypogonadotropic hypogonadism and infertility. Although significant knowledge has been gained on the signaling and transcriptional events controlling gonadotrope differentiation, epigenetic mechanisms regulating Nr5a1 expression during early gonadotrope lineage specification are still poorly understood.

Results

Using ATAC chromatin accessibility analyses on three cell lines recapitulating gradual stages of gonadotrope differentiation and in vivo on developing pituitaries, we demonstrate that a yet undescribed enhancer is transiently recruited during gonadotrope specification. Using CRISPR/Cas9, we show that this enhancer is mandatory for the emergence of Nr5a1 during gonadotrope specification. Furthermore, we identify a highly conserved estrogen-binding element and demonstrate that the enhancer activation is dependent upon estrogen acting through ERα. Lastly, we provide evidence that binding of ERα is crucial for chromatin remodeling of Nr5a1 enhancer and promoter, leading to RNA polymerase recruitment and transcription.

Conclusion

This study identifies the earliest regulatory sequence involved in gonadotrope lineage specification and highlights the key epigenetic role played by ERα in this differentiation process.

Electronic supplementary material

The online version of this article (10.1186/s13072-019-0291-8) contains supplementary material, which is available to authorized users.

Cloning and functional characterization of the mouse sodium-dependent organic anion transporter Soat (Slc10a6)

The sodium-dependent organic anion transporter SOAT is a member of the Solute Carrier Family SLC10. In man, this carrier is predominantly expressed in the testis and has transport activity for sulfoconjugated steroid hormones. Here, we report on cloning, expression analysis and functional characterization of the mouse Soat (mSoat) and compare its characteristics with the human SOAT carrier. Quantitative mRNA expression analysis for mSoat in male mice revealed very high expression in lung and further high expression in testis and skin. Immunohistochemical studies showed expression of the mSoat protein in bronchial epithelial cells of the lung, in primary and secondary spermatocytes as well as round spermatids within the seminiferous tubules of the testis, in the epidermis of the skin, and in the urinary epithelium of the bladder. Stably transfected mSoat-HEK293 cells revealed sodium-dependent transport for dehydroepiandrosterone sulfate (DHEAS), estrone-3-sulfate, and pregnenolone sulfate (PREGS) with apparent Km values of 60.3μM, 2.1μM, and 2.5μM, respectively. In contrast to human SOAT, which has a preference for DHEAS as a substrate, mSoat exhibits the highest transport rate for PREGS, likely reflecting differences in the steroid pattern between both species. In conclusion, although certain differences between human SOAT and mSoat exist regarding quantitative gene expression in endocrine and non-endocrine tissues, as well as in the transport kinetics for steroid sulfates, in general, both can be regarded as homologous carriers.

Pulmonary metabolism of resveratrol: in vitro and in vivo evidence

The role of pulmonary metabolism in trans-resveratrol (RES) pharmacokinetics was studied in a mouse model. Plasma concentrations of RES and its major metabolites trans-resveratrol-3-sulfate (R3S) and trans-resveratrol-3-glucuronide (R3G) were compared after administration of RES by intravenous (IV) and intra-arterial (IA) routes. Total area under the curve (AUC) of RES decreased by approximately 50% when RES was administered by the IV route compared with the IA route. The AUC of R3G was also significantly higher in mice administered RES by the IV route compared with the IA route. In vitro studies performed with mouse and human lung fractions confirmed pulmonary metabolism of RES. Interestingly, mouse-lung fractions gave rise to both R3S and R3G, whereas human lung fractions yielded R3S. This indicates marked interspecies variation in RES conjugation, especially in the context of extrapolating rodent data to humans. Taken together, the results presented here underline, for the first time, the impact of pulmonary metabolism on resveratrol pharmacokinetics and interspecies differences in RES pulmonary metabolism.

Increased Estrogen Sulfation of Estradiol 17β-D-Glucuronide in Metastatic Tumor Rat Livers

Changes in the disposition of estradiol 17beta-d-glucuronide (E(2)17G), a substrate of the organic anion-transporting polypeptide family (Oatp) and multidrug resistance-associated protein 2 (Mrp2), were examined in livers of male Wag/Rij rats that were injected with CC531 cells intraportally to induce metastatic tumors (n = 5) or with phosphate-buffered saline for sham-operated controls (n = 4). Multiple indicator dilution, single-pass liver perfusions revealed extremely high influx clearances of [(3)H]E(2)17G (>190 ml/min) in both groups. In recirculating liver perfusions, [(3)H]E(2)17G decayed monoexponentially in the reservoir perfusate, and the total (9.19 +/- 1.33 versus 8.18 +/- 0.94 ml/min) and biliary (4.94 +/- 1.07 versus 4.60 +/- 0.86 ml/min) clearances were similar in both groups (P > 0.05). The metabolic clearance of E(2)17G was higher in the tumor group (4.60 +/- 0.64 versus 3.23 +/- 0.23 ml/min, P < 0.05). E(2)3S17G, the 3-sulfate metabolite, whose identity was confirmed by mass spectrometry, appeared only in bile and not perfusate. Liver microsomal incubations of E(2)3(35)S17G and [(3)H]estrone sulfate revealed similar sulfatase activities between the tumor and sham livers, albeit the activities were much lower for E(2)3(35)S17G. Oatp1a1 and Oatp1b2 protein expression in liver membrane fragments was reduced by 42% and 38%, respectively, whereas that of cytosolic estrogen sulfotransferase (Sult1e1) was significantly increased (41%) with tumor (P < 0.05). All of the observations were captured by modeling. From modeling, we showed that reduction of the high influx clearance (546 to 283 ml/min) failed to lower the total clearance of E(2)17G, whereas up-regulation of Sult1e1 increased the E(2)17G sulfation clearance (2.56 to 3.69 ml/min) in livers with metastatic tumors.

2-Methoxyestradiol, a promising anticancer agent

Estrogens occurring naturally in the body are metabolized to catecholestrogens (2- and 4-hydroxyestradiol) by the cytochrome P450 enzymes. 2-Hydroxy catecholestrogens are further metabolized by catechol-O-methyltransferase to 2-methoxyestradiol, which is known to be protective against tumor formation. 2-Methoxyestradiol exhibits potent apoptotic activity against rapidly growing tumor cells. It also possesses antiangiogenic activity through a direct apoptotic effect on endothelial cells. Other molecular mechanisms, including microtubule stabilization by inhibition of the colchicine-binding site, have been reported. The exact mechanism of action of 2-methoxyestradiol is still unclear, but it has been shown to be effective in preventing tumor growth in a variety of cell lines. 2-Methoxyestradiol also possesses cardioprotective activity by inhibiting vascular smooth muscle cell growth in arteries. It has a lower binding affinity for estrogen receptor alpha compared with that of estradiol, and its affinity for estrogen receptor beta is even lower than that of estrogen receptor alpha, thus it has minimal estrogenic activity. 2-Methoxyestradiol is distinct because of its inability to engage estrogen receptors as an agonist, and its unique antiproliferative and apoptotic activities are mediated independently of estrogen receptors alpha and beta. A phase I clinical trial of 2-methoxyestradiol 200, 400, 600, 800, and 1,000 mg/day in 15 patients with breast cancer showed significant reduction in bone pain and analgesic intake in some patients, with no significant adverse effects. Another phase I study of 2-methoxyestradiol 200-1,000 mg/day in combination with docetaxel 35 mg/m2/week for 4-6 weeks performed in 15 patients with advanced refractory metastatic breast cancer showed no serious drug-related adverse effects. A phase II randomized, double-blind trial of 2-methoxyestradiol 400 and 1,200 mg/day in 33 patients with hormone-refractory prostate cancer showed that it was well tolerated and showed prostate specific antigen stabilizations and declines. We have started a phase I clinical trial to explore dosages greater than 1,000 mg/day.

Pleotropic effects of dietary DHEA

We present data pertaining to some of the in vivo effects associated with dietary DHEA administration to mice and rats. Dietary DHEA leads to: (1) decrease in body weight gain; (2) relative increases in liver weight; (3) liver color change; (4) induction of hepatic peroxisomal enzymes; (5) proliferation of hepatic peroxisomes with increased cross-sectional area; (6) decreased hepatic mitochondrial cross-sectional area; (7) elevated levels of hepatic cytosolic malic enzyme; (8) slight decreases, significant decreases, or significant increases in serum triglyceride levels, depending on mouse strain; (9) increases in total serum cholesterol levels; (10) significant decreases in the hepatic rates of fatty acid synthesis; (11) significant increases in the hepatic rates of cholesterol synthesis; (12) decreases in both protein content and specific activity of hepatic mitochondrial carbamoyl phosphate synthetase-I without concomitant changes in serum urea nitrogen; (13) induction of glutathione S-transferase activity in liver; (14) decrease in hepatic endogenous protein phosphorylation; (15) increase in hepatic AMPase and GTPase activities; (16) formation of 5-androstene-3 beta,17 beta-diol as a major metabolite of DHEA by subcellular fractions of liver, which is reflected in serum and tissue levels; and (17) reduction in serum prolactin levels.

Influence of growth hormone on the hepatic mixed function oxidase and transferase systems of rainbow trout

The effect of GH treatment on hepatic cytochrome P450 content, aryl hydrocarbon hydroxylase (AHH), aminopyrine-N-demethylase (AND), testosterone hydroxylase, testosterone 5α- and 5β-reductase, UDP-glucuronyl transferase (UDPGT) and glutathione S-transferase (GST) activities in immature rainbow trout were investigated. Hepatic cytochrome P450 content, AHH and GST activities were measured in both GH implanted and GH injected animals whereas other activities were assayed in GH implanted trout only.GH implants significantly decreased cytochrome P450 content at 15 days compared to the control but no significant effect was observed at 15 or 30 d when GH was injected biweekly. In both cases, AHH activity was significantly decreased by GH treatment compared to the control whereas GST remained unchanged. Compared to the control, GH implanted fish exhibited a pronounced inhibition of AND, a decreased 6β and 16β-testosterone hydroxylation, an inhibition of UDPGT with testosterone as substrate and an enhanced 17β-testosterone oxidation.

A "soluble" form of sterol sulphate sulphohydrolase from cell nuclei of human placenta tissue--examinations with oestrone sulphate as substrate

DN-ase digestion of the nuclear envelope-chromatin complex of the cell nuclei preparations from human placenta, released a soluble form of sterolsulphohydrolase. The enzyme revealed three pH optima, at 4.0, 6.2 and 7.4. The Km value was 4.16 +/- 1.44 x 10(-5) M. The molecular mass determined by gel filtration on Bio-gel A 15 m was 406 kDa. The enzyme is sensitive to -SH group reacting reagents such as cysteine, p-chloromercuribenzoate and iodoacetamide. Oxidized and reduced forms of NAD, FAD, dithiothreitol and glutathione moderately inhibited enzyme activity. Ascorbic acid (reduced and oxidized) exerted slight activation. The enzyme was insensitive to phosphate ions.

Placental 17 beta-hydroxysteroid oxidoreductase, lactate dehydrogenase and malate dehydrogenase during the latter half of pregnancy in the mouse

The specific activity of 17 beta-hydroxysteroid oxidoreductase (17-HOR) with estradiol-17 beta (E2), estrone (E1) and testosterone (T), as well as that of lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) were measured in homogenates of CF-1 mouse placenta during the latter half of pregnancy. 17-HOR activity with E2 and T increased over 100-fold between days 9 and 12, and 3- to 4-fold between days 15 and 19, with no further change to day 21. In contrast, activity with E1 increased 39-fold between days 9 and 12, 3.8-fold between days 15 and 19 but then decreased between days 19 and 21. The E2/T activity ratio was constant while the E2/E1 ratio increased between days 9 and 21. LDH increased 2-fold between days 9 and 12 with no further increase to day 19. MDH was constant from day 9 to 19. Activity with E2 was inhibited by T, 5 alpha-dihydrotestosterone (5 alpha-DHT) and DHA but not by E1, androstenedione (A) or 20 alpha-dihydroprogesterone. Activity with T was inhibited by E2, 5 alpha-DHT and DHA, but not by A. In contrast, activity with E1 was inhibited by A and DHA but not by E2, T or 5 alpha-DHT. The results suggest placental 17-HOR is developmentally regulated. Although the results are also suggestive of multiple forms of 17-HOR, a single enzyme with an ordered kinetic mechanism cannot be ruled out.

Estrone sulfate stimulates growth of nitrosomethylurea-induced breast carcinoma in vivo in the rat

Concentrations of estradiol in human breast tumors from pre- and post-menopausal women are similar whereas plasma levels are 5- to 60-fold lower in post-menopausal women. The mechanism for maintaining high tumor tissue estrogen levels in post-menopausal women is unknown but could be related to the ability of plasma estrone sulfate to serve as a precursor for estradiol synthesis in tumor tissue. Estrone sulfate plasma levels are 30-fold higher than free estradiol levels in post-menopausal women and estrone sulfatase is present in many tissues, including breast tumors, supporting this hypothesis. In this study, we examined the ability of exogenously administered estrone sulfate to stimulate growth of a carcinogen-induced, hormone-dependent rat mammary tumor and measured the rate of conversion of estrone sulfate to free estrone and estradiol. Castrate rats bearing nitrosomethylurea-induced mammary tumors were infused with estradiol as a control or estrone sulfate over a 14-day period. Estradiol at low doses significantly increased tumor volume whereas higher amounts paradoxically inhibited growth. By comparison, estrone sulfate infusions significantly increased tumor volume over that observed in castrate animals on both days 7 and 14 of infusion. To determine whether estrone sulfate was converted to free estrone and estradiol during this protocol, 3H-estrone sulfate was substituted for unlabelled steroid and castrate animals were again infused for 14 days. At 7, 10 and 14 days of infusion, 18-26% of estrone sulfate was converted to free estrone and 9-16% to free estradiol. There were no significant differences between the 2 doses used and the rates of conversion were stable over the infusion period. Conversion of estrone sulfate to free estradiol was also demonstrated by radioimmunoassay of free estradiol in plasma during estrone sulfate infusions. These data demonstrate that exogenously administered estrone sulfate can stimulate mammary tumor growth in castrate animals and support the possibility that estrone sulfate may serve as an important source of tumor tissue estradiol.

Regulation of murine lymphokine production in vivo. III. The lymphoid tissue microenvironment exerts regulatory influences over T helper cell function

We investigated the capacity of murine T lymphocytes, isolated from various lymphoid organs of normal or antigen-primed donors, to produce IL-2 or IL-4 after activation with anti-CD3 or specific antigen. Our results established that T cells resident within lymphoid organs being drained by nonmucosal tissue sites (e.g., axillary, inguinal, brachial lymph nodes, or spleen) produced IL-2 as the predominant T cell growth factor (TCGF) after activation. Conversely, activated T cells from lymphoid organs being drained by mucosal tissues (Peyer's patches, and cervical, periaortic, and parathymic lymph nodes) produced IL-4 as the major species of TCGF. Analysis of the lymphoid tissues obtained from adoptive recipients of antigen-primed lymphocytes provided by syngeneic donors provided evidence that direct influences were being exerted on T cells during their residence within defined lymphoid compartments. These lymphoid tissue influences appeared to be responsible for altering the potential of resident T cells to produce distinct species of TCGF. Steroid hormones, known transcriptional enhancers and repressors of specific cellular genes, were implicated in the controlling mechanisms over TCGF production. Glucocorticoids (GCs) were found to exert a systemic effect on all recirculating T cells, evidenced by a marked dominance in IL-4 production by T cells obtained from all lymphoid organs of GC-treated mice, or after a direct exposure of normal lymphoid cells to GCs in vitro before cellular activation with T cell mitogens. Further, the androgen steroid DHEA appeared to be responsible for providing an epigenetic influence to T cells trafficking through peripheral lymphoid organs. This steroid influence resulted in an enhanced potential for IL-2 secretion after activation. Anatomic compartmentalization of the DHEA-facilitated influence appears to be mediated by differential levels of DHEA-sulfatase in lymphoid tissues. DHEA-sulfatase is an enzyme capable of converting DHEA-sulfate (inactive) to the active hormone DHEA. We find very high activities of this enzyme isolated in murine macrophages. The implications of our findings to immunobiology are very great, and indicate that T cells, while clonally restricted for antigen peptide recognition, also appear to exhibit an extreme flexibility with regards to the species of lymphokines they produce after activation. Regulation of this highly conservative mechanism appears to be partially, if not exclusively, controlled by cellular influences being exerted by distinct species of steroid hormones, supplied in an endocrine or a paracrine manner where they mediate either systemic or tissue-localized influences, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of murine lymphokine production in vivo. II. Dehydroepiandrosterone is a natural enhancer of interleukin 2 synthesis by helper T cells

Dehydroepiandrosterone (DHEA) is an adrenal steroid hormone produced in abundance by humans and most other warm-blooded animals, is uniquely sulfated (DHEAS) prior to export into the plasma, and exhibits an age-related decline in production with progressive age. No major physiological functions have been ascribed to the activity of this steroid, although DHEA is known to serve as an intermediary in sex steroid synthesis. Studies on the effects of glucocorticoids (GCS) on the immune system led us to question whether DHEA had effects on the ability of activated lymphocytes to produce interleukin 2 (IL 2). We determined that (a) lymphocytes from DHEA- or DHEAS-treated mice consistently produced much greater levels of IL 2 than normals in response to stimulation, (b) direct lymphocyte exposure to DHEA at low doses (10(-10)-10(-7) M) caused an enhanced capacity to secrete IL 2 following activation, (c) IL 2 production by activated cloned T cell lines could be augmented by DHEA treatment, and (d) GCS-induced depressions in IL 2 synthesis by T cells or T cell clones could be overcome in vitro and in vivo by exposure to the effects of DHEA. These findings suggest that DHEA, presumably through receptor-mediated mechanisms similar to other types of steroid hormones, may represent a natural and important regulator of IL 2 production in both normal and pathologic conditions.

Steroid metabolism by epidermal keratinocytes

The metabolism of various radiolabeled steroids by cultured human epidermal keratinocytes was studied in an attempt to identify the steroid-metabolizing enzymes present in these cells. Sulfatase activity was demonstrated in keratinocytes with either E1S or DS as substrates. The products of sulfatase action were E1 and DHEA, respectively. The specific activity of the enzyme was approximately 5- to 14-fold greater with E1S as the substrate compared with DS, and the rates of hydrolysis were linear with incubation time up to 3 h. The metabolism of DHEA by the keratinocyte 17 beta-HSOR-catalyzed reaction resulted in the predominant formation of 5-androstene-3 beta,17 beta-diol. The rate of formation of 5-androstene-3 beta,17 beta-diol was linear with time of incubation up to 18 h, and the specific activity of 17 beta-HSOR, with DHEA as the substrate, was greater in keratinocytes maintained in culture for 4 weeks compared with keratinocytes kept in culture for 1 week. Androstenedione was a minor product of DHEA metabolism. The metabolism of DHT by epidermal keratinocytes resulted in the formation of 5 alpha-androstanedione, 5 alpha-androstane-3 alpha,17 beta-diol, 5 alpha-androstane-3 beta,17 beta-diol, androsterone, and isoandrosterone: the rates of formation of 5 alpha-androstane-3 alpha,17 beta-diol and 5 alpha-androstane-3 beta,17 beta-diol were linear with incubation time up to 24 h, and the specific activities of 3 alpha-HSOR and 3 beta-HSOR did not appear to change with keratinocyte time in culture up to 3 weeks. The metabolism of DOC by epidermal keratinocytes resulted in 5 alpha-dihydrodeoxycorticosterone production: the rate of formation of this metabolite was linear with incubation time up to 4 h. The metabolism of E1 by epidermal keratinocytes yielded E2, and that of E2 resulted in the formation of E1. The rate of E1 formation from E2, was approximately 10-fold greater than the rate of formation of E2 from E1; these rates were linear with incubation time up to 4 h. Epidermal keratinocytes maintained in culture did not metabolize androstenedione to either E1 or E2, and pregnenolone was not metabolized by these cells. This study serves to ascertain that epidermal keratinocytes express steroid 5 alpha-reductase, 17 beta-HSOR, 3 beta-HSOR, 3 alpha-HSOR, 3 beta-hydroxysteroid oxidoreductase-delta 5----4-isomerase, and sulfatase activities.

Estrone sulfate sulfohydrolase in the developing brain and pituitary of rat, mouse and guinea pig

The pattern of estrone sulfate sulfohydrolase (estrogen sulfatase) development in the brain of rat, mouse and guinea pig has been established by assaying whole homogenates. Activity was measurable in each species from the fetal state to adulthood. Maximum brain content was reached at about 20 days of age in rat, 14 days in mouse and 15 days in guinea pig. A considerable decrease occurred between 14 days and adulthood in mouse and lesser decreases were seen in rat and guinea pig. The subcellular distribution of enzyme in rat and mouse brain appeared to change from the immature to the adult state. No major differences in enzyme activity occurred between the sexes at any age. Tissue concentration of enzyme in the hypothalamic-preoptic area of rat and mouse was similar to that in the remainder of the brain. In guinea pig the brain concentration was slightly lower than that of the hypothalamic-preoptic region. Sulfatase content of the pituitary was low in all 3 species but the tissue concentration was considerably higher than that of brain, particularly in rat and mouse. Apparent Km values for brain sulfatase were in the range 6-17 microM, with no striking sex difference. Apparent Km's for pituitary sulfatase of immature rat and guinea pig were similar to those for brain in the same animals but that for mouse pituitary (0.9 microM) was much lower. It is unlikely that brain or pituitary sulfatase is by itself, a major factor in making available potentially active estrogen for use during differential sex development in these species.

17 beta-Hydroxysteroid oxidoreductase: a ubiquitous enzyme. Interconversion of estrone and estradiol-17 beta in BALB/c mouse tissues

A survey was conducted to define the sites of 17 beta-hydroxysteroid oxidoreductase activity in organs and tissues of male and female BALB/c mice, as well as the favored direction of the oxidoreductase reaction in intact tissues. The enzyme activity was assayed by use of radiolabeled estrone and estradiol-17 beta as substrates. Estrone formation from estradiol-17 beta was demonstrated in all tissues. The formation of estradiol-17 beta from estrone was demonstrated in most tissues, however, it was barely detected or was undetectable in the glandular stomach, small intestine, cecum, and large intestine. Thus, 17 beta-hydroxysteroid oxidoreductase activity is expressed in all BALB/c mouse organs and tissues. Approximately two-thirds of the tissues and organs examined, including those of the reproductive tracts, favored the conversion of estrone to estradiol-17 beta rather than the reverse reaction. The results of this study, however, represent qualitative estimates of 17 beta-hydroxysteroid oxidoreductase activity in BALB/c mouse tissues that are uncorrected for conversion to hydroxylated metabolites. These in vitro findings suggest that the 17 beta-hydroxysteroid oxidoreductase catalyzed reduction of estrone may contribute to the maintenance of physiologic levels of estradiol-17 beta in estrogen responsive tissues.