Methodology Development for the Study of Estrogen Carcinogenesis
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Evidence from clinical epidemiology and in vivo and in vitro experiments have a strong correlation between estrogen exposure and breast cancer. However, the mechanisms of estrogen carcinogenesis are highly complex. One of the proposed pathways is the hormonal pathway, by which estrogen stimulates cell proliferation which enhances the chance of genetic error during DNA replication. Another pathway, which is the focus of this project, is the chemical pathway in which estrogens are metabolized to form reactive o-quinones that covalently modify DNA resulting in DNA adduct formation. Hormone replacement therapy (HRT) has been used for the relief of postmenopausal symptoms since its introduction in the 1930s. Clinical results from the two landmark studies, the Women’s Health Initiative in 2002 and the Million Women Study in 2003, have raised questions about the use of HRT. Premarin (Wyeth-Ayerst), the most prescribed drug for HRT, is mostly made of conjugated equine estrogen. When taken into body, equine estrogen conjugates are hydrolyzed to their parent estrogens. These equine estrogens can be oxidatively metabolized by cytochrome P450s to form 2- and 4-hydroxyl metabolites. Previous studies have shown that compared to endogenous estrogens predominantly metabolized to 2-hydroxyl metabolites, increasing unsaturation in the B-ring shifts the metabolism of equine estrogens to mainly 4-hydroxylation. Both 4-hydroxyequilenin (4-OHEN) and 4-hydroxyequilin (4-OHEQ) will rapidly autoxidize to o-quinone and 4-OHEQ has been found to oxidize to 4-OHEN-o-quinone. Studies suggest that 4-OHEN-o-quinone is more reactive than endogenous estrogen quinones, and potentially more toxic and carcinogenic. Alzheimer’s disease (AD), the most common form of dementia, is a neurodegenerative disorder affecting memory, cognition, and behavior. Advancing age is considered one of the greatest risk factors to develop AD. In addition, women have been found to be at higher risk of developing AD compared to men. Postmenopausal depletion of endogenous estrogens may also contribute to increased risk. Clinical results have shown that in women, previous history of using HRT is associated with reduced risk of AD. Anastrozole, an aromatase inhibitor which has been studied for the treatment of early breast cancer in postmenopausal women, was found to cause poorer hippocampaldependent tasks such as verbal and visual learning and memory.