Light at Night Makes Breast Cancer Resistant to Tamoxifen

Exposure to light at night makes breast cancer resistant to tamoxifen, a new study suggests.

Researchers at the Tulane University School of Medicine have found that melatonin is important in the battle against breast cancer. Melatonin is a hormone associated with circadian rhythm. Tamoxifen is a widely-used drug to treat common types of breast cancer.

The study was based on rats that were implanted with human breast cancer cells. Scientists analyzed rats living in either normal light/dark cycle - 12 hours of light followed by 12 hours of darkness or in conditions where 12 hours of light was followed by 12 hours of dim light.

The team found that melatonin levels in the blood of rats rose at night in rats living under normal day/night cycle. However, in rats exposed to dim light, melatonin levels remained low throughout the light/dark cycle.

Researchers said that dim light used in the study was "equivalent to faint light coming under a door."

According to the team, melatonin delayed formation of tumors and significantly slowed their growth. Tamoxifen was able to reduce cancer growth in rats that lived in 12 hours of darkness or those receiving melatonin supplementation.

"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 from Tulane's Circadian Cancer Biology Group, in a news release.

Researchers said that the study shows dim light at night as a serious risk factor for developing resistance to tamoxifen and other anti-cancer drugs. The research could also help make melatonin supplementation a standard treatment for tamoxifen resistance.

"Resistance to tamoxifen is a growing problem among patients with hormone receptor-positive breast cancer," said Steven M. Hill, PhD, from the Tulane University School of Medicine in New Orleans. "Our data, although they were generated in rats, have potential implications for the large number of patients with breast cancer who are being treated with tamoxifen, because they suggest that nighttime exposure to light, even dim light, could cause their tumors to become resistant to the drug by suppressing melatonin production."

Exposure to dim light has earlier been linked to depression. In fact, light pollution is a widely studied topic to understand how being exposed to street lights or ambient lighting can affect sleep-wake cycles in people.

According to the first World Atlas of the artificial night sky brightness, about two-thirds of the World population live in places that have light levels at night is above the threshold set for polluted status. A map by "The night sky in the World" also shows how bright our world looks.

Researchers in the present study have cautioned against using melatonin supplements without medical supervision. "Melatonin is produced by our bodies exclusively during darkness at night, and taking melatonin supplements at the wrong time of day would potentially disrupt the circadian [sleep-wake cycle] system, particularly the natural melatonin cycle, which may, in itself, impair breast cancer responsiveness to tamoxifen," Hill told the Telegraph. 

The study is published in the journal Cancer Research.