Researchers at Michigan State University’s Breast Cancer and the Environment Research Center have identified how the hormones progesterone and oestrogen interact to increase cell growth in normal mammary cells and mammary cancers.

This is a new finding that explains why postmenopausal women receiving HRT with oestrogen plus progestin are at increased risk of breast cancer but unfortunately they did not examine the role that excess oestrogen (oestrogen dominance) has to play and what happens when the balance is upset.

Why progesterone is important in cancer prevention

Dr David Zava showed many years ago that natural progesterone was actually protective against breast cancer, see his article below, and what this research has shown is both hormones must be present for the increased production of the protein amphiregulin, which binds to mammary cells and promotes cell growth.

The finding might help explain earlier results from the groundbreaking Women’s Health Initiative which showed that the risk of breast cancer is significantly greater for postmenopausal women who received HRT with combined oestrogen plus a progestin compared to women receiving oestrogen only HRT.

“Breast cancers that develop in women receiving oestrogen plus progestin are more invasive and deadlier as what is the progestin doing to increase the risk of tumor growth” said lead researcher Sandra Z. Haslam.

Along with co-investigator Anastasia Kariagina, a colleague in the College of Human Medicine and Department of Physiology, she identified the protein amphiregulin and its receptor as one potential culprit as when activated, this promotes normal cell growth and the growth of tumours.

The study was performed in rats because breast cancers in rats contain receptors for oestrogen and progesterone — similar to the human breast — and tumour growth is hormone-dependent, as are the majority of human breast cancers.

The research team also confirmed the same phenomenon in human breast cancer cell cultures.

Why progesterone is protective

Although the researchers found that the results indicate that the interactions between oestrogen, progesterone and epidermal growth factor receptor pathways may be considered relevant targets for the treatment of hormone-dependent breast cancers they focused on a combined approach of inhibiting both the hormones and the epidermal growth factor receptor

However they did not look at the considerable amount of material relating to natural progesterone and its protective effect on the breast.

It does this as it balances the excess oestrogen (oestrogen dominance) which is linked to it and certainly the breast cancer risk with HRT is well established and the main reason doctors take women off it who have been on long term use.

The maximum recommended time is now 5-6 years and the benefits of supplementing with progesterone are well known. It is the hormone that women can use to help with the flushes that can arise from having treatment or chemotherapy and/or who have a family history of breast cancer and want help at menopause.