Long-term exposure to
estrogens influences the development of
breast cancer in women, but the precise mechanisms involved are not clearly defined. Our working hypothesis is that
estrogen modulates this process by two separate processes. One involves the binding of
estradiol to
estrogen receptor (ER) alpha with stimulation of cell proliferation. Errors in
DNA occurring during replication result in fixed mutations when not repaired. The other process results from the formation of genotoxic metabolites of
estradiol, which can bind to
DNA, cause depurination, and result in mutations. Our collaborative group, funded by a Department of Defense Center of Excellence grant, has examined this hypothesis using a variety of experimental methods.
Estradiol and its
catechol-
estrogen metabolite 4-OH-estradiol causes mutations in cell culture systems and can transform benign MCF-10F cells, allowing them to cause
tumors in SCID mice. We have demonstrated loss of heterozygosity and gains and losses of
DNA segments by comparative genomic hybridization methodology. The depurinated
estradiol-
guanine and -
adenine adducts are measurable in MCF-7
breast cancer cells in culture and in mouse mammary tissue. The double transgenic, alpha
estrogen receptor knockout/Wnt-1 knockin mouse model allows us to dissect out the separate effects of ER-mediated and ER-independent actions of
estradiol. Knock out of the ER alpha delays the onset of
breast tumors in this model, demonstrating a role of receptor-mediated actions.
Oophorectomy delays the onset of
tumors and reduces overall incidence, providing evidence for an ER-independent effect. Taken together, these data suggest that both ER-dependent and genotoxic ER-independent effects of
estradiol mediate
breast cancer development.