A new study by Tulane University School of Medicine cancer researchers says exposure to light at night, which shuts off nighttime production of the hormone melatonin, causes breast cancer to become resistant to tamoxifen, a widely used breast cancer drug.

The study, “Circadian and Melatonin Disruption by Exposure to Light at Night Drives Intrinsic Resistance to Tamoxifen Therapy in Breast Cancer,” published in the journal Cancer Research, is the first to show that melatonin is essential to the success of tamoxifen in treating breast cancer.

Melatonin by itself slowed down the formation of tumors and greatly slowed their growth, but tamoxifen caused a dramatic regression of tumors in animals with either high nighttime levels of melatonin during complete darkness or those receiving melatonin supplementation during dim light at night exposure.

These findings have potentially huge implications for women being treated with tamoxifen and also regularly exposed to light at night due to working night shifts, sleep problems, or exposure to light from computer and TV screens.

“High melatonin levels at night put breast cancer cells to ‘sleep’ by turning off key growth mechanisms. These cells are vulnerable to tamoxifen. But when the lights are on and melatonin is suppressed, breast cancer cells ‘wake up’ and ignore tamoxifen,” said David Blask, principal investigator and co-leader of the study.

The study could make light at night a risk factor for developing resistance to tamoxifen and other anticancer drugs and make the use of melatonin in combination with tamoxifen, administered at the most favorable time of night or day, standard treatment for breast cancer patients…… sciencedaily.com    7/25/14

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