Cytochrome P450 metabolism of estradiol in hamster liver and kidney

Responses in reproductive organs, steroid hormones and CYP450 enzymes in female Mongolian gerbil (Meriones unguiculatus) over time after quinestrol treatment

The aim of this study was to assess the effects and reversibility of the synthetic estrogen compound, quinestrol, on the reproductive organs, steroid hormones, and drug-metabolizing enzymes CYP3A4 and CYP1A2 in liver and kidney over time after two quinestrol treatments in female Mongolian gerbils (Meriones unguiculatus). Female gerbils were treated with 4mg/kg quinestrol (9 gerbils/group, 3 treated group) (1 control group, 0mg/kg) for 3days and treated again after 25days. Animals were killed for collection of samples at 5, 10 and 15days after the second treatment ending. Two interval quinestrol treatments significantly increased uterine weight, with trend of increase over time, but no change could be detected in ovarian weights. Quinestrol treatment increased progesterone and estradiol levels, both with trend of decline over time. Quinestrol increased liver and kidney weights and total enzyme content of CYP3A4 and CYP1A2, with trend of decline over time. On the basis of reversible changes of detoxification enzymes or organs, interval quinestrol treatment effectively and reversibly influenced the reproductive hormone and organ to some extent.

Disruption of the rainbow trout reproductive endocrine axis by the polycyclic aromatic hydrocarbon benzo[a]pyrene

Successful reproduction in salmonids depends on a complex and highly regulated interplay between the pharmacokinetics and pharmacodynamics of naturally circulating sex steroids. The effects of a single intraperitoneal administration of the model PAH benzo[a]pyrene (B[a]P) on the kinetics of circulating levels of estradiol and testosterone through 7d post-injection in mature male and female rainbow trout (Oncorhynchus mykiss) in pre-spawning and spawning condition were investigated. Detailed measurements of the time course of injected E2 and excretion into the bile followed by pharmacokinetic modeling techniques were used to aid in identifying the potential mechanism of ED caused by B[a]P exposure. Plasma E2 and T concentrations were reduced significantly in both male and female trout. Administration of the GnRH analogue des-Gly(10)[D-Ala(6)]LH-RH-ethylamide, to induce spawning steroid profiles increased plasma E2 concentrations in control females, but not in B[a]P-treated fish. The mechanism underlying reductions in sex steroids in pre-spawning and spawning salmonids appears to be unrelated to the induction of P450 and related biotransformation enzymes by B[a]P. Induced biotransformation enzyme activities did not result in altered [(3)H]estradiol pharmacokinetics (e.g. terminal half-life) or elimination of steroid in bile, suggesting that B[a]P alters plasma E2 and T concentrations by other ED mechanisms in an anti-estrogenic manner.

Modulation of metabolizing enzymes by bisphenol a in human and animal models

Xenobiotics, such as contaminants and drugs, can be converted to potentially toxic reactive metabolites by phase 1 oxidizing enzymes. These metabolites are further detoxified by phase 2 conjugating enzymes and eliminated from cells by phase 3 transporters. Moreover, many of these xenobiotics are also able to induce or inhibit these enzymes, potentially modulating their own toxicity or that of other chemicals. The present review is focused on bisphenol A, a synthetic monomer used for many industrial applications and exhibiting xenoestrogen properties. The impact of this contaminant on all major classes of metabolizing enzymes (i.e., cytochromes P450, glutathione-S-transferases, sulfotransferases, UDP-glucuronyltransferases, and transporters) was reviewed, with a highlight on the modulation of cytochromes P450 involved in steroid metabolism. Interestingly, most of the studies reported in this review show that BPA is able to induce or inhibit metabolizing enzymes at high doses but also at doses compatible with human exposure.

Endocrine disrupting chemicals targeting estrogen receptor signaling: identification and mechanisms of action

Many endocrine disrupting chemicals (EDCs) adversely impact estrogen signaling by interacting with two estrogen receptors (ERs): ERα and ERβ. Though the receptors have similar ligand binding and DNA binding domains, ERα and ERβ have some unique properties in terms of ligand selectivity and target gene regulation. EDCs that target ER signaling can modify genomic and nongenomic ER activity through direct interactions with ERs, indirectly through transcription factors such as the aryl hydrocarbon receptor (AhR), or through modulation of metabolic enzymes that are critical for normal estrogen synthesis and metabolism. Many EDCs act through multiple mechanisms as exemplified by chemicals that bind both AhR and ER, such as 3-methylcholanthrene. Other EDCs that target ER signaling include phytoestrogens, bisphenolics, and organochlorine pesticides, and many alter normal ER signaling through multiple mechanisms. EDCs can also display tissue-selective ER agonist and antagonist activities similar to selective estrogen receptor modulators (SERMs) designed for pharmaceutical use. Thus, biological effects of EDCs need to be carefully interpreted because EDCs can act through complex tissue-selective modulation of ERs and other signaling pathways in vivo. Current requirements by the U.S. Environmental Protection Agency require some in vitro and cell-based assays to identify EDCs that target ER signaling through direct and metabolic mechanisms. Additional assays may be useful screens for identifying EDCs that act through alternative mechanisms prior to further in vivo study.

A methoxyflavonoid, chrysoeriol, selectively inhibits the formation of a carcinogenic estrogen metabolite in MCF-7 breast cancer cells

A 17beta-estradiol (E(2)) is hydrolyzed to 2-hydroxy-E(2) (2-OHE(2)) and 4-hydroxy-E(2) (4-OHE(2)) via cytochrome P450 (CYP) 1A1 and 1B1, respectively. In estrogen target tissues including the mammary gland, ovaries, and uterus, CYP1B1 is highly expressed, and 4-OHE(2) is predominantly formed in cancerous tissues. In this study, we investigated the inhibitory effects of chrysoeriol (luteorin-3'-methoxy ether), which is a natural methoxyflavonoid, against activity of CYP1A1 and 1B1 using in vitro and cultured cell techniques. Chrysoeriol selectively inhibited human recombinant CYP1B1-mediated 7-ethoxyresorufin-O-deethylation (EROD) activity 5-fold more than that of CYP1A1-mediated activity in a competitive manner. Additionally, chrysoeriol inhibited E(2) hydroxylation was catalyzed by CYP1B1, but not by CYP1A1. Methylation of 4-OHE(2), which is thought to be a detoxification process, was not affected by the presence of chrysoeriol. In human breast cancer MCF-7 cells, chrysoeriol did not affect the gene expression of CYP1A1 and 1B1, but significantly inhibited the formation of 4-methoxy E(2) without any effects on the formation of 2-methoxy E(2). In conclusion, we present the first report to show that chrysoeriol is a chemopreventive natural ingredient that can selectively inhibit CYP1B1 activity and prevent the formation of carcinogenic 4-OHE(2) from E(2.).

Nuclear receptors CAR and PXR in the regulation of hepatic metabolism

The nuclear receptors CAR and PXR were first characterized as xenosensing transcription factors regulating the induction of phase I and II xenobiotic-metabolizing enzymes as well as transporters in response to exogenous stimuli. It has now become clear, however, that these receptors cross-talk with endogenous stimuli as well, which extends their regulation to various physiological processes such as energy metabolism and cell growth. As recognition of the function of these receptors has widened, the molecular mechanism of their regulation has evolved from simple protein-DNA binding to regulation by complex protein-protein interactions. Novel mechanisms as to how xenobiotic exposure alters hepatic metabolic pathways such as gluconeogenesis and beta-oxidation have emerged. At the same time, the molecular mechanism of how endogenous stimuli, such as insulin, regulate xenobiotc metabolism via CAR and PXR have also become evident.

DNA content and chromatin texture of human breast epithelial cells transformed with 17-beta-estradiol and the estrogen antagonist ICI 182,780 as assessed by image analysis

The immortalized human breast epithelial MCF-10F cell line, although estrogen receptor alpha negative, develops cell proliferating activities and invasiveness indicative of neoplastic transformation, after treatment with 17-beta-estradiol (E-2). These effects are similar to those produced by benzo[a]pyrene (BP). Since we have previously reported changes in the nuclear parameters accompanying BP-induced tumorigenesis in MCF-10F cells, we have examined whether similar alterations occur in E-2-treated cells. We therefore studied DNA amounts and other nuclear parameters in Feulgen-stained MCF-10F cells after treatment with various concentrations of E-2, BP, the estrogen antagonist ICI 182,780, and E-2 in the presence of ICI 182,780. E-2 caused a certain loss of DNA and changes in the nuclear size and chromatin supraorganization of MCF-10F cells. Many of these changes were similar to those produced by BP and were indicative of neoplastic transformation. More intense chromatin remodelling was seen with 70 nM E-2. Since these changes were not abrogated totally or partially by ICI 182,780, the neoplastic transformation of MCF-10F cells stimulated by E-2 involved a process that was independent of estrogen alpha-receptors. The changes produced by ICI 182,780 alone were attributed to effects other than its well-known anti-estrogenic activity.

Induction of oxidative stress responses by dioxin and other ligands of the aryl hydrocarbon receptor

TCDD and other polyhalogenated aromatic hydrocarbon ligands of the aryl hydrocarbon receptor (AHR) have been classically considered as non-genotoxic compounds because they fail to be directly mutagenic in either bacteria or most in vitro assay systems. They do so in spite of having repeatedly been linked to oxidative stress and to mutagenic and carcinogenic outcomes. Oxidative stress, on the other hand, has been used as a marker for the toxicity of dioxin and its congeners. We have focused this review on the connection between oxidative stress induction and the toxic effects of fetal and adult dioxin exposure, with emphasis on the large species difference in sensitivity to this agent. We examine the roles that the dioxin-inducible cytochromes P450s play in the cellular and toxicological consequences of dioxin exposure with emphasis on oxidative stress involvement. Many components of the health consequences resulting from dioxin exposure may be attributable to epigenetic mechanisms arising from prolonged reactive oxygen generation.

Induction of cellular oxidative stress by aryl hydrocarbon receptor activation

The aryl hydrocarbon receptor (AHR) has long been associated with the induction of a battery of genes involved in the metabolism of foreign and endogenous compounds. Depending on experimental conditions, AHR can mediate either activation or amelioration of chemical toxicity. For the past decade, evidence has mounted that AHR is associated with a cellular oxidative stress response that must be considered when evaluating the mechanism of action of xenobiotics capable of activating AHR, or capable of metabolic activation by enzymes encoded by genes under control of AHR. In this review, we have evaluated the diverse mechanisms by which AHR generates an oxidative stress response, including inflammation, antioxidant and prooxidant enzymes and cytochrome P450. A review of the regulation of Ahr transcription and functional polymorphisms especially related to oxidative stress is also included. We have carefully avoided placing a value judgment on the degree of toxicity produced by such a response, in view of the realization that an oxidative response is involved in many normal physiological processes. Since the interface between physiological, adaptive and toxicological responses elicited by the AHR-mediated oxidative stress response is not clearly defined, it behooves the researcher to evaluate both toxicological and physiological features of the response.

Catechol estrogen formation in liver microsomes from female ACI and Sprague-Dawley rats: comparison of 2- and 4-hydroxylation revisited

Estradiol (E(2))-hydroxylation was studied in liver microsomes from ACI and Sprague-Dawley female rats, which differ markedly in their susceptibility to E(2)-induced formation of mammary tumors. NADPH-dependent oxidation of E(2) by liver microsomes from ACI and Sprague-Dawley rats produced several metabolites of which 2-hydroxyestradiol (2-OH-E(2)), estrone (E(1)), and 2-hydroxyestrone (2-OH-E(1)) were predominant. Incubations with either low (9 nM) or high (50 microM) concentrations of radiolabeled E(2) and with varying amounts of microsomal protein indicated the formation of only small amounts of 4-hydroxyestradiol (4-OH-E(2)). The ratio of 2-OH-E(2) to 4-OH-E(2) formed with the low concentration of E(2) was about 10:1 regardless of the amount of microsomal protein used, and about 20:1 using a high concentration of E(2). Thus, oxidation of E(2) by liver microsomes from female ACI and Sprague-Dawley rats occurs primarily via 2-hydroxylation, and 4-hydroxylation is only a minor pathway. These results are in disagreement with a recent report indicating substantial 4-hydroxylation of E(2) by liver microsomes from female ACI rats.

Role of DNA adducts in hormonal carcinogenesis

In this review, a mechanism of estrogen-induced cancer has been examined which features a dual role of estrogen as hormone and carcinogen. Evidence exists that estrogens are metabolically activated to 4-hydroxylated metabolites by a specific cytochrome P450 in tissues prone to estrogen-induced cancer. These metabolites and their semiquinone/quinone oxidation products may cause various types of DNA damage. Preliminary data also exist that estrogens induce various genetic mutations. Tumors may develop from cells carrying such mutations and responding to receptor-mediated proliferation signals.

Is estradiol a genotoxic mutagenic carcinogen?

The natural hormone 17 beta-estradiol (E2) induces tumors in various organs of rats, mice, and hamsters. In humans, slightly elevated circulating estrogen levels caused either by increased endogenous hormone production or by therapeutic doses of estrogen medications increase breast or uterine cancer risk. Several epigenetic mechanisms of tumor induction by this hormone have been proposed based on its lack of mutagenic activity in bacterial and mammalian cell test systems. More recent evidence supports a dual role of estrogen in carcinogenesis as a hormone stimulating cell proliferation and as a procarcinogen inducing genetic damage. Tumors may be initiated by metabolic conversion of E2 to 4-hydroxyestradiol catalyzed by a specific 4-hydroxylase (CYP1B1) and by further activation of this catechol to reactive semiquinone/quinone intermediates. Several types of direct and indirect free radical-mediated DNA damage are induced by E2, 4-hydroxyestradiol, or its corresponding quinone in cell-free systems, in cells in culture, and/or in vivo. E2 also induces various chromosomal and genetic lesions including aneuploidy, chromosomal aberrations, gene amplification, and microsatellite instability in cells in culture and/or in vivo and gene mutations in several cell test systems. These data suggest that E2 is a weak carcinogen and weak mutagen capable of inducing genetic lesions with low frequency. Tumors may develop by hormone receptor-mediated proliferation of such damaged cells.