Equine Estrogen Metabolite 4-hydroxyequilenin Induces Anchorage-independent Growth of Human Mammary Epithelial MCF-10A Cells: Differential Gene Expression

Long-term exposure to synthetic and endogenous estrogens has been associated with the development of cancer in several tissues. One potential mechanism of estrogen carcinogenesis involves catechol formation and these catechols are further oxidized to electrophilic/redox active o-quinones, which have the potential to both initiate and promote the carcinogenic process. Previously we showed that 4-hydroxyequilenin (4-OHEN) autoxidized to an o-quinone and caused a variety of damage to DNA. Since these deleterious effects could contribute to gene mutations, we investigated the Chinese hamster V79 cells to ascertain the relative ability of estradiol, 4-hydroxyestradiol, 17beta-hydroxyequilenin, 4,17beta-hydroxyequilenin, estrone, 4-hydroxyestrone, equilenin, and 4-hydroxyequilenin to induce the mutation of the hypoxanthine-guanine phosphoribosyltransferase (hprt) gene. All the 4-hydroxylated catechols induced significantly more colony formations in V79 cells as compared to the parent phenols at 100nM, suggesting that the catechol estrogen metabolites are more mutagenic towards the hprt gene than estrogens. Since 4-OHEN induced the highest mutation frequency, we examined a biomarker for transformation potential of this compound in MCF-10A cells using an anchorage-independent growth assay. Although 4-OHEN induced anchorage-independent growth of these cells, the isolated clones were not able to grow as tumors in vivo when injected into nude mice. These cells were assayed for genetic changes using cDNA microarrays. Real time RT-PCR confirmation of some of the differentially expressed genes showed down-regulation of metallothionein 2A, p53, BRCA1, and c-myc. Moreover, we showed the involvement of other genes important in cell transformation and oxidative stress, strengthening the hypothesis that this mechanism plays a considerable role in 4-OHEN-induced anchorage-independent growth.