This compound mifepristone (RU486) prevented the tumors by inhibiting progesterone, a hormone involved with the female reproductive cycle, in breast tissue cells.
Mifepristone is designed to abort pregnancy in the first trimester by blocking progesterone, thereby ending the viability of the fetus. In smaller doses, it is used as an emergency contraceptive.
In the study, researchers addressed how mifepristone affects the function of mutated BRCA-1 genes in tissue. BRCA-1 is widely studied because a mutated version of this gene significantly raises the possibility of breast and ovarian cancers.
The researchers studied mice that carried the mutated BRCA-1 gene. Mice treated with mifepristone did not develop mammary tumors by the time they reached one year of age. All of the untreated mice, however, developed tumors by eight months of age.
UCI researchers found that progesterone encourages the development of cancer when the mutated BRCA-1 is present because it speeds up the division of cells. Mifepristone was found to block a binding process that is necessary for progesterone to cause the cell division.