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Other Data Relevant to an Evaluation of Carcinogenicity and Its Mechanisms

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Other Data Relevant to an Evaluation of Carcinogenicity and Its Mechanisms COMBINED ESTROGEN−PROTESTOGEN MENOPAUSAL THERAPY 263 4. Other Data Relevant to an Evaluation of Carcinogenicity and its Mechanisms 4.1 Absorption, distribution, metabolism and excretion The distribution of progestogens is described in the monograph on Combined estro- gen–progestogen contraceptives. That of estrogens is described below. 4.1.1 Humans Little more has been discovered about the absorption and distribution of estrone, estradiol and estriol products and conjugated equine estrogens in humans since the previous evaluation (IARC, 1999). Greater progress has been made in the identification and characterization of the enzymes that are involved in estrogen metabolism and excre- tion. The various metabolites and the responsible enzymes, including genotypic varia- tions, are described below (see Figures 3 and 4). Sulfation and glucuronidation are the main metabolic reactions of estrogens in humans. (a) Metabolites (i) Estrogen sulfates Several members of the sulfotransferase (SULT) gene family can sulfate hydroxy- steroids, including estrogens. The importance of SULTs in estrogen conjugation is demons- trated by the observation that a major component of circulating estrogen is sulfated, i.e. estrone sulfate (reviewed by Pasqualini, 2004). In addition to the parent hormones, estrone and estradiol, SULTs can also conjugate their respective catechols and also methoxyestro- gens (Spink et al., 2000; Adjei & Weinshilboum, 2002). The resulting sulfated metabolites are more hydrophilic and can be excreted. In postmenopausal breast cancers, levels of estrone sulfate can reach 3.3 ± 1.9 pmol/g tissue, which is five to nine times higher than the corresponding plasma concentration (equating gram of tissue with millilitre of plasma) (Pasqualini et al., 1996). In contrast, levels of estrone sulfate in premenopausal breast tumours are two to four times lower than those in plasma. Since inactive estrone sulfate can serve as a source for biologically active estradiol, it is of interest that various progestogens caused a significant decrease in the formation of estradiol when physiological concentrations of estrone sulfate were incu- bated with breast cancer cells MCF-7 and T47D (reviewed by Pasqualini, 2003). Figure 3. Pathways of the metabolism and redox cycling of estradiol, estriol and estrone 264 OH OH Glucuronides OH OH 4-Hydroxyestriol Sulfates (or 4-hydroxy 16α- Fatty-acid esters HO HO hydroxyestrone) Estriol OH Methylated + glucuronides O and sulfate O metabolites OH HO OH 2-Hydroxy 16α- lase HO hydroxyestrone VOLUME 91 IARC MONOGRAPHS roxy OH β O α-Hyd (or 2-hydroxyestriol) 17 -Hydroxysteroid 16 HO dehydrogenase 16α-Hydroxyestrone HO HO Estradiol Estrone OH OH OH O O 2 2 O HO O O HO HO 2-Hydroxyestradiol OH OH (or 2-hydroxyestrone) OH Catechol-O-methyltransferase OH OH H3CO O2 O2 HO HO OCH3 O HO HO O 2/4-Methoxyestradiol (or 2/4-methoxyestrone) OH O 4-Hydroxyestradiol (CYP oxidase/reductase redox cycling) (or 4-hydroxyestrone) Glucuronides and sulfates Catechol estrogens Semiquinones Quinones Modified from Yager & Liehr (1996) COMBINED ESTROGEN−PROTESTOGEN MENOPAUSAL THERAPY 265 Figure 4. The estrogen metabolism pathway is regulated by oxidizing phase I and conjugating phase II enzymes E2 CYP1A1 CYP1B1 CYP1B1 2-MeOE2 COMT 2-OHE2 4-OHE2 4-MeOE2 COMT 2-OH-3-MeOE2 E2-2,3-SQ E2-3,4-SQ O2 2-OHE -4-SG 2 O2 E2-2,3-Q E2-3,4-Q 4-OHE2-2-SG GSTP1 GSTP1 2-OHE2-1-SG DNA Damage Adapted from Dawling et al. (2003) COMT, catechol-O-methyltransferase; CYP, cytochrome P450; E2, estradiol; GSH, glutathione; GST, gluta- thione S-transferase; MeOE2, methoxyestradiol; OH, hydroxy; OHE2, hydroxyestradiol; Q, quinone; SG, S-gluta- thione (oxidized); SQ, semiquinone CYP1A1 and CYP1B1 catalyse the oxidation of E2 to the catechol estrogens 2-OHE2 and 4-OHE2. The catechol estrogens are either methylated by COMT to methoxyestrogens (2-MeOE2, 2-OH-3-MeOE2, 4-MeOE2) or further oxidized to semiquinones (E2-2,3-SQ, E2-3,4-SQ) and quinones (E2-2,3-Q, E2-3,4-Q). The methoxyestro- gens exert feedback inhibition on CYP1A1 and CYP1B1, as indicated by the curved arrows, and reduce the for- mation of oxidative E2 metabolites. The estrogen quinones are either conjugated by GSTP1 to GSH-conjugates − (2-OHE2-1-SG, 2-OHE2-4-SG, 4-OHE2-2-SG) or they form quinone DNA adducts (e.g. 4-OHE2-N7-guanine) or oxidative DNA adducts via quinone–semiquinone redox cycling (e.g. 8-OH-deoxyguanosine). The same path- way applies to estrone. The thicker arrows indicate preferential reactions. (ii) Estrogen glucuronides Estradiol and estrone and their respective catechols are recognized as substrates by various isoforms of the uridine-5′ diphosphate (UDP)-glucuronyltransferase (UGT) enzyme family. Several isoforms were more active towards catechol estrogens than towards the parent hormones (Albert et al., 1999; Turgeon et al., 2001). The resulting glucuronidated metabolites are more hydrophilic and can be excreted in bile and urine. (iii) Estrogen fatty acid esters Several steroids, including estradiol, have been shown to undergo esterification to long- chain fatty acids in a number of mammalian tissues (Hochberg, 1998). The responsible enzyme, fatty acyl-coenzyme A (CoA):estradiol-17β-acyltransferase, has a pH optimum of 266 IARC MONOGRAPHS VOLUME 91 5–5.5, which distinguishes it from the related enzyme, acyl-CoA:cholesterol acyltransferase (optimal pH ~7.0) (Xu et al., 2001a,b). The fatty acyl-CoA:estradiol-17β-acyltransferase shows specificity for the D-ring, especially the C-17β group of the estrogen molecule. The vicinity of a bulky 16α-hydroxy group appears to hamper the accessibility to the C-17β hydroxyl, which results in a reduced rate (28%) of esterification of estriol compared with estradiol (Pahuja et al., 1991). The D-ring esterification of estradiol has two effects: (i) the bulky fatty acid moiety prevents the binding of estradiol fatty acid to the estrogen receptor; and (ii) the fatty acid moiety shields the D-ring from oxidative metabolism to estrone. Thus, estradiol fatty acid may play a role in the action of estrogen by affecting the intracellular equilibrium between estrone and estradiol. In the circulation, estradiol fatty acids are mainly bound by plasma lipoproteins; the majority (54%) are recovered in the high-density lipoprotein (HDL) and 28% in the low- density lipoprotein (LDL) fractions (Vihma et al., 2003a). They are present in very small amounts in the blood of premenopausal women, although their concentration increases 10- fold during pregnancy, from 40 pmol/L in early pregnancy to 400 pmol/L in late pregnancy (Vihma et al., 2001). Treatment of postmenopausal women with either oral or transdermal estradiol for 12 weeks resulted in a differential effect on serum estradiol fatty acids and non- esterified estradiol. Both types of application led to similar median concentrations of free (non-protein-bound) estradiol but only the oral therapy caused an increase (27%) in median serum estradiol fatty acid (Vihma et al., 2003b). The change during treatment in serum concentrations of estradiol fatty acid, but not those of non-esterified estradiol correlated positively with enhanced forearm blood flow responses in vivo. These data suggest that an increase in serum estradiol fatty acid may contribute to the effects of oral treatment with estradiol, compared with those of an equipotent transdermal dose. (iv) Oxidative metabolism Estradiol and estrone undergo extensive oxidative metabolism via the action of several cytochrome P450 (CYP) monooxygenases. Each CYP favours the hydroxylation of specific carbons, altogether, the CYP enzymes can hydroxylate virtually all carbons in the steroid molecule, with the exception of the inaccessible angular carbons 5, 8, 9, 10 and 13 (Badawi et al., 2001; Lee et al., 2001, 2002, 2003a,b; Kisselev et al., 2005). The generation of hydroxyl and keto functions at specific sites of the steroid nucleus markedly affects the biological properties of the respective estrogen metabolites, i.e. different hydroxylation reactions yield estrogenic, non-estrogenic or carcinogenic metabolites. Quantitatively and functionally, the most important reactions occur at carbons 2, 4 and 16. Catechol estrogens 2- and 4-Hydroxyestrone, -estradiol and -estriol have been shown to serve a physio- logical function, to have some hormonal activity and to be substrates in the oxidative estrogen metabolism pathway. In their physiological function, they mediate the activation of dormant blastocysts for implantation into the receptive uterus. Specifically, 4-hydroxy- estradiol produced in the uterus from estradiol mediates blastocyst activation for implanta- COMBINED ESTROGEN−PROTESTOGEN MENOPAUSAL THERAPY 267 tion in a paracrine manner. This effect is not mediated by the estrogen receptor but via prostaglandin synthesis (Paria et al., 1998, 2000). The oxidative metabolism of estrogens to catechol estrogens is generally thought to terminate the estrogenic signal, although catechol estrogens retain some binding affinity to the estrogen receptor. Treatment of MCF-7 cells with 2- and 4-hydroxyestradiol increased the rate of cell proliferation and the expression of estrogen-inducible genes such as the progesterone receptor (PR) gene and pS2. Relative to estradiol, 2- and 4-hydroxyestradiol increased proliferation rate, level of PR protein and pS2 mRNA expression by 36 and 76%, 10 and 28% and 48 and 79%, res- pectively (Schütze et al., 1993, 1994). Catechol estrogens occupy a key position in the oxidative pathway of estrogen meta- bolism (see Figures 3 and 4). They are products as well as substrates of CYP1A1 and CYP1B1 (Hachey et al., 2003; Dawling et al., 2004). Specifically, CYP1A1 converts estra- diol firstly to 2-hydroxyestradiol and then to the estradiol-2,3-semiquinone and estradiol quinone. CYP1B1 converts estradiol firstly to 2- as well as to 4-hydroxyestradiol and then to the corresponding semiquinones and quinones. Estrone is metabolized in a similar manner by CYP1A1 and CYP1B1 (Lee et al., 2003a). The catechol estrogens also serve as substrates for catechol-O-methyltransferase (COMT), which catalyses O-methylation by forming monomethyl ethers at the 2-, 3- and 4-hydroxyl groups.
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