(HealthDay News) — A study in rats hints that exposure to dim light at night may make human breast cancer tumors resistant to doxorubicin. The study was presented at the American Association for Cancer Research’s (AACR) International Conference on Frontiers in Cancer Prevention Research, held from Sept. 28 to Oct. 1 in New Orleans.

Prior rat-based research has found that exposure to dim light at night boosted human breast cancer tumors’ resistance to tamoxifen. In the new study, rats with human breast cancer tumors were exposed to 12 hours of normal light followed by 12 hours of dim light, simulating dim light in a nighttime environment. Half of the rodents were also given melatonin supplements during the dim light period. The researchers found that tumor growth in rats that did not receive melatonin was nearly triple that of those that did receive melatonin. In addition, tumors in rats that did not receive melatonin became completely resistant to doxorubicin, while tumors in rats that received melatonin remained sensitive to the chemotherapy drug and regressed rapidly.

“When we analyzed tumors from rats that did not receive nighttime melatonin supplementation, we detected substantially increased levels of two enzymes that break down doxorubicin to a less active form,” Steven Hill, Ph.D., chair for breast cancer research at Tulane University School of Medicine in New Orleans, explained in an AACR news release.

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Hill believes that an increase in the levels of these enzymes might speed the transport of doxorubicin away from cancer cells, “compared with tumors from rats receiving nighttime melatonin supplementation,” he said. “Tumors from rats receiving nighttime melatonin supplementation had lower levels of these enzymes. So we think that melatonin helps maintain high levels of active doxorubicin in the cancer cells, whereas suppression of circadian melatonin production by exposure to light at night has the opposite effect.”

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