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.

The recurrence factor

Cancer stem cells are those within a tumor that have a heightened ability to spread to other sites in the body and generate new tumors where they land. Recent studies have shown the stem cells to be more resistant to traditional cancer treatments such as chemotherapy and radiation, Vivanco said.

In recent years, some scientists have postulated that the existence of these cells explains why cancers can seem to completely disappear, only to later recur with a vengeance: A few cancer stem cells will survive a course of treatment, perhaps undetected, then sprout new tumors composed of cells largely resistant to treatments.

"If we are thinking about a way to get rid of the tumor forever, it's important to make sure that not only the bulk of the tumor disappears, but also the cancer stem cells," Vivanco said.

Vivanco and her colleagues at the biosciences center examined the effect of estrogen on breast cancer stem cells growing in lab dishes. They used cells both from tumors in patients and from established cell lines that had been grown in labs for many years.

The finding that estrogen slowed the growth of these cells is consistent with the observation that, among postmenopausal women, those with less-aggressive breast cancers have higher levels of estrogen in their blood, Vivanco said.

It's too early to make treatment recommendations from these findings, Vivanco told MyHealthNewsDaily. But they reveal a complex situation, in which the same hormone can have a different impact depending on the type of cell, Vivanco said.

"What is important is to understand in more detail how cancer stem cells react to the hormones and to other signaling molecules ... and how can that influence the progression of the tumor, and the reoccurrence as well," Vivanco said.

One way to find out if the findings apply to tumors in patients would be to examine women currently taking anti-estrogen drugs, such as tamoxifen, said Charlotte Kuperwasser, of Tufts University School of Medicine in Boston, who has researched breast cancer stem cells. These drugs are designed to block estrogen signaling, and researchers could study what impact these drugs have on breast cancer stem cells, said Kuperwasser, who was not involved in the Spanish study.

Same hormone, opposite conclusions

Previous studies have found estrogen has the opposite effect it feeds the growth of breast cancer stem cells, Kuperwasser said.

In 2010, Kuperwasser and her colleagues published a study in the journal Proceedings of the National Academy of Sciences showing that estrogen expanded the proportion of breast cancer stem cells within a tumor.

"It really is a mirror image of this study," she said.

The discrepancy between these two findings is part of scientific discourse, Kuperwasser said.

"We do, clinically, know that estrogen fuels tumor growth," Kuperwasser said. "So it is somewhat counterintuitive that this paper would show that estrogen was suppressing breast cancer stem cells, because that really kind of goes against the long history of estrogen being a tumor promoter."

Ultimately, more experiments should be carried out to see which of the two observations is consistent, she said.

Pass it on: Estrogen may have both good and bad effects on breast cancer. While it promotes proliferation of cancer cells, it may reduce the number of breast cancer stem cells.

Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @Rachael_MHND.