Potential endogenous epoxides of steroid hormones: Initiators of breast and other malignancies?

A 'new' estrogen metabolite: an epoxide of estrone as a sulfated steroid

Current heightened recognition of the importance of sulfated steroids led to the examination of conjugates in media from incubations of estrogens in tissues from the reproductive tract of stallions. Previously, we had reported a 'new' unidentified metabolite of estrone (E1) and [3H]-E1, located between 17β-estradiol (E2) and E1 reference standards on chromatography (HPLC) and identified tentatively as a stable 5α,6α-estrone epoxide. Stallion tissues were minced and incubated for 2 h with [3H]-E1 (1 × 106 cpm). Solid-phase extraction of unconjugated and conjugated steroids from media was followed by liquid scintillation counting (LSC), where radioactivity was mostly in the conjugate fractions (>80%). HPLC of conjugated steroids used an isocratic solvent system of acetonitrile/water (8:92) at 700 µL/min with detection by LSC. A radioactive peak between reference standards of E1 and E2 sulfates was examined, after solvolysis, in a second solvent system. Sulfated steroids yielded largely E1, whereas acid treatment of the unconjugated E1 epoxide had earlier formed 6α-OH-E1 almost exclusively. With sulfatase enzyme, at neutral pH, radioactivity was also seen mostly as E1 with trace amounts of polar material. Reduction with KBH4, however, led also to desulfation; radioactivity had alignment with E2 but even more had low retention times as for 6α/6β-OH-E2. These findings point to a different hydrolysis for desulfation; even more, they reveal an additional oxygen atom at C6 and are supportive of biological formation of 5α,6α-epoxides of E1 and E2. As possible alternatives to catechol estrogens, implicated in cancer, the 'new' estrogen metabolites and their sulfated forms may have special interest.

A stable epoxide of estrone: Evidence for formation of a 'new' estrogen metabolite

Oxidative metabolism of estrogens is an important feature in liver and some non-hepatic tissues. In initial studies on estrogen metabolism in tissues from the reproductive tract of the stallion, where testicular estrogen secretion is remarkably high, a prominent radiolabeled product from [³H]-estrone (E1) was noted on chromatography; it had a retention time (Rt) between 17β-estradiol (E2) and E1. Unexpectedly, when non-radiolabeled E1 was the substrate no UV absorption at 280nm was seen at the Rt for the [³H]-labeled product-suggesting a non-aromatic ring A. The following efforts were made to reveal more about the nature of the "unknown" compound. Reduction and acetylation showed, separately, the presence of a single keto and hydroxyl group. Exposure to acid gave a single radiolabeled peak with Rt of 6α-hydroxy-E1-suggesting the presence of a third molecule of oxygen. Mass spectrometry with limited material was inconclusive but supportive for a formula of C₁₈H₂₂O_3. Thus, an epoxide involving the aromatic ring of E1 is suggested as a labile intermediate in the formation of the "unknown" metabolite. Estrogen epoxides as labile, reactive intermediates have been considered as potential precursors of the 2- and 4-hydroxy catechol estrogens with implications in breast cancer [Soloway, 2007]. Because of the association of the "unknown" metabolite with 6α-hydroxy-E1, the structural form proposed for the stable epoxide is that for 5α,6α-epoxy-estrone. This represents an alternative to the production of the 2- and 4-hydroxy-catechol estrogens. The broad range in normal tissues where the "unknown" compound was shown to be a persistent metabolite (e.g. mouse mammary glands, ovary, uterus, brain, muscle, equine conceptus, stallion and domestic boar reproductive tracts) suggests more general biological implications.

A stable epoxide as a potential endogenous estrogen metabolite: Possible significance in breast cancer?

Epoxides as reactive intermediates of estrogen metabolism have been considered to be potential precursors of the 2- and 4-hydroxy, catechol estrogens and even to be mutagenic/carcinogenic agents themselves. The labile nature of the intermediates has made proof of their existence difficult in natural biological conditions. In our studies on estrogen metabolism in vitro, in various tissues from several laboratory and domestic species, there was chromatographic evidence of formation of a stable estrogen metabolite that could be seen after incubation with radiolabeled estrone, but not with unlabeled substrate. Investigation with acid treatment of the metabolite yielded material detected as 6-hydroxy-estrone-suggesting the presence of an additional oxygen atom in the molecule. An identification of the "unknown compound" has not yet been made but, with this evidence, the properties revealed so far can best be met by assuming the presence of 5,6-epoxy-estrone. The recent favorable reports on the role of 5α,6α-epoxy-cholesterol in breast cancer has led to the hypothesis that the formation of a similar, stable epoxide of an estrogen could potentially be a compound of interest. If a metabolic pathway from estrone to 6-hydroxy-estrone through a stable epoxide has indeed been observed, it would suggest that identifying and screening for the enzymes responsible for its production, as opposed to those generating the catecholestrogens, could provide valuable information in relation to breast cancer. The balance in production of estrogen epoxides could be a key factor in determining normal health or risk of tumor development.

Macular degeneration: a possible biochemical mechanism

The possible role of labile endogenous metabolites in the cause of various chronic debilitating diseases such as macular degeneration has not been adequately explored. In the metabolism of the various retinoids, namely retinal (vitamin A aldehyde), retinol (vitamin A alcohol) and retinoic acid, each has the potential for generating labile intermediates, such as their corresponding 5,6-epoxides by the action of various cytochrome P(450)s. Such retinoid epoxides may well have the capacity for acting as toxins upon the neurons in the macula unless they are rapidly hydrolyzed by epoxide hydrolases. Since the cytochrome P(450)s responsible for epoxide formation and the various epoxide hydrolases involved in their hydrolysis are determined genetically, this may serve to explain a genetic component being involved in the causation of age-related macular degeneration.