A University of Missouri study has found that hormone therapies not just increase the risk of breast cancer in post-menopausal women, they can also increase the chance of the cancer metastasising.
After menopause, women take hormone therapies, which are often a combination of estrogen and progestin, to replace hormones lost from inactive ovaries.
Progestin is a hormone that is used to counteract the potentially negative effects of estrogen therapy on the uterus.
Previous studies have shown that estrogen and progestin in hormone therapies increase the risk of breast cancer in post-menopausal women.
Now, the new study has demonstrated that progestins can also increase the chance of the cancer metastasizing, or spreading to the lymph nodes.
"In our study, we found that progestins increase the number of blood vessels that are responsible for transporting existing cancer cells," said Salman Hyder, the Zalk Endowed Professor in Tumour Angiogenesis and professor of biomedical sciences in the College of Veterinary Medicine and the Dalton Cardiovascular Research Center.
"The more the blood vessels increase, the higher the chance of cancer cell metastasizing. Progestins could even be more harmful to women who have functionally abnormal p53, a protein that acts as a tumour suppressor. In the absence of p53, progestins increase the release of a protein from tumour cells that allows formation of new blood vessels within tumours."
In the study, researchers compared the effects of several types of commonly used progestins on breast cancer tumours in an animal model. Researchers found that all types of progestin tested act in the same way and increased the risk of metastasis.
Also, results showed that estrogen and progestin acted the same way whether taken together or separately. Although Hyder said that the study was independent of whether or not the ovaries were intact, it's still unclear whether progestins have the same effects in pre-menopausal women.
"Especially if there's a family history of breast cancer, it's advisable not to take progestins. It's a difficult call that must be made on an individual basis by a physician," Hyder said.
"The next step for this research is finding a type of progestin that does not cause tumor progression but still protects the uterus. Also, we're trying to see if it's possible to give patients something in addition to estrogen and progestin that can protect the breast," Hyder added.
The study has been accepted for publication in the Journal Menopause.