Artificial Light Stimulates Cancer Growth

A study of laboratory mice has shown that exposure to artificial light at night stimulated the growth of human breast tumors by suppressing the levels of melatonin.

In contrast, extended periods of darkness at night greatly slowed the growth of the tumors. These results help explain why women who work night shifts have a higher rate of breast cancer than other women. In advanced industrial countries such as the United States, it provides a new explanation for the epidemic rise in breast cancer incidence. The study findings were reported in the December 1, 2005, issue of Cancer Research.

"We know that many tumors are largely dependent on a nutrient called linoleic acid, an essential fatty acid, in order to grow,” observed lead author David Blask, M.D., Ph.D., a neuroendocrinologist with the Bassett Research Institute (Cooperstown, NY, USA). "Melatonin interferes with the tumor's ability to use linoleic acid as a growth signal, which causes tumor metabolism and growth activity to shut down.

To test this hypothesis, the researchers injected human breast cancer cells into laboratory mice. After these cells developed into cancerous tumors, the tumors were implanted into female rats where they could continue to grow. The researchers took blood samples from 12 healthy volunteers under three different conditions: during the daytime, during the night following two hours of complete darkness, and during the night following 90 minutes of exposure to bright fluorescent light. These blood samples were then pumped directly through the developing tumors.

"The melatonin-rich blood collected from subjects while in total darkness severely slowed the growth of tumors,” said Dr. Blask. "These results are due to a direct effect of the melatonin on the cancer cells. The melatonin is clearly suppressing tumor development and growth.”

In contrast, tests with the melatonin-depleted blood from light-exposed subjects stimulated tumor growth. "We observed rapid growth comparable to that seen with administration of daytime blood samples, when tumor activity is particularly high,” Dr. Blask explained.

Melatonin exerts a strong influence on the body's circadian rhythm, according to the researchers. The disruption of the circadian clock is associated with cancer in humans, and interference with that internal timekeeping can effect a change in favor of tumor development. These results may lead to more artificial lighting and perhaps the use of lighting that has a wavelength and intensity that do not disrupt melatonin levels and internal timekeeping.

"Day workers who spend their time indoors would benefit from lighting that better mimics sunlight,” added Dr. Blask. "Companies that employ shift workers could introduce lighting that allows the workers to see without disrupting their circadian and melatonin rhythms.”




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