Contraceptives and hormone replacement therapies have been linked to an increased risk for breast cancer, and now scientists think they know why. The hormones in these drugs activate a protein that affects breast cells in a way that can ultimately cause them to become cancerous, according to a new study in mice.

However, the researchers caution the findings in mice need to be confirmed by studies in humans.

The study was published online Sept. 29 in the journal Nature.

Oral contraceptives , such as the pill, and hormone replacement drugs medications women take to relieve the symptoms of menopause contain hormones called progestins. Two previous studies involving more than 100,000 women found these hormones increase the risk of breast cancer . In fact, the One Million Women's study, conducted in the United Kingdom, found hormone replacement therapies increased the risk as much as 2-fold.

Josef Penninger, a researcher at the Institute of Molecular Biotechnology at the Austrian Academy of Sciences in Vienna, and his colleagues studied a protein known as RANKL, found in breast cells, to see if it influenced breast cancer risk . Previous research by Penninger found RANKL to be an important regulator of bone loss in the body.

Mice given doses of progestins saw a 2000-fold increase in the levels of RANKL in their mammary cells. RANKL drives the mammary cells to grow and multiply, and prevents them from dying. The more times cells divide, the more chances they have to develop genetic mutations. And if cells don't die from their DNA damage, they can turn cancerous .

"The same molecule that controls bones loss turns out to be the missing link between hormones and breast cancer," Penninger said.

Medications that block RANKL may help prevent breast cancer, Penninger said. In fact, a second study, also published Sept. 29 in Nature, found this was true in mice.

When mice in Penninger's study were given a certain dose of a progestin hormone, 100 percent of them developed breast cancer. The second nature paper, conducted by researchers at the U.S. pharamcuetial company Amgen, found that an inhibiting RANKL in these same mice could reduce their incidence of breast cancer to just 10 percent.

A drug that blocks RANKL has already been developed and approved by the U.S. Food and Drug Aministration for treatment of osteoporosis. This means researchers could begin testing the drug right away to see if it has a preventative affect on breast cancer, Penninger said.

"People could in theory start tomorrow to run clinical trials and see if this is real," he told MyHealthNewsDaily. "I believe this should really be done now."