Estrogen is known to promote the growth of breast cancer cells, but a new study suggests this hormone also may reduce the number of stem cells that initiate the cancer's growth.
The findings, to be published in an upcoming issue of the journal Breast Cancer Research and Treatment, indicate that estrogen can have two opposite effects on breast cancer. On the one hand, it can induce cancer cells to proliferate. On the other, it can make the tumor less aggressive, by diminishing the number of breast cancer stem cells.
The results could explain why women whose breast tumors have estrogen receptors — meaning they can respond to estrogen's signals — often have a better prognosis than those whose tumors do not have these receptors, said study researcher María Vivanco, of the Center for Cooperative Research in Biosciences in Bilbao, Spain.