Metabolites of Estrogen Derivatives, DNA Damage, and Cancer

Hormone replacement therapy

It is well established that there is a link between estrogens and an increased risk of developing cancer in women, especially in tissues of the endometrium and breast, as was recently described in the press and scholarly journals (1,2).  Estrogens are of course present naturally in the human body (endogenous estrogens).  In addition, estrogens can be  ingested by post-menopausal  women (exogenous estrogens) undergoing hormone replacement therapy (hpt).  One of the most popular hpt preparation (premarin) that has been prescribed to over 40 million women in the United States alone, contains estrogens extracted from horses.  Two of these compounds, equilin and equilinenin (Figure 1), resemble human estrogens, but their chemical reactions are quite different because of the additional one or two C-C double bonds, respectively.  The risks and benefits of hpt, as well as the metabolism of equine estrogens and associated cancer risk have been described in detail by J.L.  Bolton and her associates (3).

Metabolism of Equine Estrogens

As described by Judy Bolton (3), both equilin and equilenin are readily metabolized in mammalian cells to catechols that are easily converted by various oxidation mechanisms to ortho-quinones (Figure 2). The o-quinone derivatives react chemically with all of the DNA bases except thymidine (3) to form covalent DNA adducts.  We are particularly interested in these covalent DNA adducts and have prepared a serious of site-specific DNA adducts derived from the reactions of the catechol 4-hydroxyequilenin (4-OHEN) in aqueous aerated solutions in the presence of oligonucleotides.  A particular example is depicted in Figure 3.   We have found that in the oligonucleotide shown, the dominant adducts involve reactions at the cytidine bases shown (C*).   The structures of these adducts have been established by Judy Bolton and her group, and exhibit a very interesting and  unusual cyclic conformation.   As is evident, from the structure of the cytidine adduct shown (Figure 3), different stereoisomeric conformations are possible.  We have already identified at least two different types  of diastereomeric DNA adducts of cytidine using circular dichroism methods. The catechol 4-OHEN is readily oxidized to the o-quinones generating the ROS dioxygen superoxide anion radical (O₂·-) that is known to give rise to oxidative DNA adducts via complex reactions involving trace amounts of hydrogen peroxides and transition metal ions. The latter  can damage the DNA bases and even rupture the DNA backbone.    These are very strong redox-active derivatives that undergo redox cycling reactions in which the catechol 4-hydroxyequilenin is oxidized, thus generating the ortho-quinone and various reactive oxidizing species (ROS).

Our Projects 

We were attracted to studies of these estrogen derivatives by their importance to human health, and the chemically interesting stereoisomeric adducts that are found. As in the case of the PAH diol epoxide derivatives, interesting structure-biological activity correlations are expected (Figure 4). These studies will provide novel and exciting insights into the mechanisms utilized by DNA repair enzymes to excise these lesions and their possible role in mutagenesis when damaged DNA is replicated.  To date, we have successfully synthesized site-specifically modified oligonucleotides with defined adducts of cytidine (Figure 3). Studies of DNA repair and replication activities are presently under way. The detailed objectives are similar to those listed in our descriptions of ongoing stuies with the PAH-DNA adducts described elsewhere on this website.

References

1. The New York Times, July 17, 2002.

2. Risks and benefits of estrogen plus progestinin healthy post-menopausal women. Principal results from the Women’s Health Initiative Randomized Controlled Trial.   J. Am. Med. Assoc. 288, 321-333 (2002).  

3.  Role of Quinoids in Estrogen Carcinogenesis.

Judy L. Bolton, Emily Pisha, Fagen Zhang, and Shengxiang Qiu

Chem. Res. Toxicol. 11, 1113 – 1127.         

    Gel Images