Genetic Repair of Oxidative Damage: Effect of Estrogen

Institution: The Burnham Institute for Medical Research
Investigator(s): Nicholas Rampino, Ph.D. -
Award Cycle: 1997 (Cycle III) Grant #: 3PB-0176 Award: $606,017
Award Type: Request for Applications
Research Priorities
Biology of the Breast Cell>Biology of the Normal Breast: the starting point

Initial Award Abstract (1997)
Estrogen has been implicated in the pathogenesis of breast cancer. Since many tumors grow in response to estrogen, synthetic estrogen antagonists have been widely studied as potential therapeutic agents for hormone dependent cancers. Tamoxifen, a synthetic anti-estrogen, is among the most successful specific therapies for breast cancer. Tamoxifen, however, displays estrogen-like effects in the uterus, increasing the risk of uterine cancer. Our ultimate goal is the identification of specific anti-estrogens that can stop breast cancer without stimulating cancer in other tissues, particularly the uterus.

Estrogen strongly stimulates breast cell division. When a cell divides, it goes through a process where it makes copies of its DNA and then splits in two new cells, each of which contains a carbon copy of the original DNA. The cells pause in what is called a resting phase between each division. During the resting phase, the cells check their DNA for damage (mutations) and repair it. We hypothesize that at least one way that estrogen may drive cancer development is by causing the cells to divide quickly and rush through this resting phase, which would mean that they would not have time to repair all of the DNA damage. The damaged DNA would produce changes in the cells that eventually cause them to become cancers. We also hypothesize that anti-estrogens will keep the cells in resting phase, thus prolonging the amount of time the cell has to repair DNA damage.

To test this hypothesis, we will make use of human breast and uterine cells that we expose to an oxidative stress, which causes DNA damage, and estrogen or anti-estrogen. Identification of specific anti-estrogens that can decrease DNA damage in breast tissue without unwanted side effects in other tissues, particularly uterus, will provide highly effective anti-cancer therapies for the prevention and/or cure of breast cancer.

Final Report (2001)
We determined how particular clinically important Select Estrogen Receptor Modulators (SERMs) affect cellular biochemistry in terms of their ability to protect the human genome from oxygen free radical induced mutations. Of the three SERMs tested, raloxifene (Dristan), tamoxifen (Nolvadex) and ICI 182,780, raloxifene displayed a biochemical profile with the best anti breast cancer properties. Raloxifene manifested the most pronounced beneficial effects in stimulating the activation of DNA repair proteins. Raloxifene was found to impede the phosphorylation of the retinoblastoma (Rb) protein to a greater extent than Tamoxifen or ICI 182,780, and also provided the greatest prolonged increased production of the cyclin dependent kinase inhibitor p21 in response to oxidative stress. This prolonged increase in the p21 protein was paralleled by the largest reduction in pS3. Raloxifene was superior in halting cell cycle progression after oxidative insult, while concomitantly allowing increased DNA repair. Of the three SERMs, raloxifene had the highest ability to beneficially affect G1 checkpoint function allowing for a prolonged dwell period of DNA repair before the commencement of DNA replication. After an oxidative insult tamoxifen and ICI 183,780 were found to reduce the frequency of mutations by 30%, while raloxifene reduced the frequency by 70%. In clinical studies both tamoxifen and raloxifene have been observed to prevent breast cancer, where tamoxifen was found to reduce the risk of breast cancer by 45% in high risk women, while raloxifene, in a study of women taking it for osteoporosis, lowered the risk by approximately 62%. The biochemical differences we have uncovered explain, at least in part, raloxifene's superior ability to prevent breast cancer.

Overall, our work has provided a foundation to understand how certain clinically important SERMs function in preventing breast cancer, and will provide critical information for the development of even more potent SERMs to prevent breast cancer.