Kunstlicht
bevordert wellicht de groei van kanker.*
Uit
een studie met muizen en ratten blijkt dat kunstlicht, ’s-nachts, de groei van
borstkankercellen doet bevorderen. Dit gebeurt omdat kunstlicht de aanmaak van
het hormoon melatonine doet verminderen, terwijl net lange perioden van
duisternis de groei van de tumoren sterk doet verminderen. Melatonine is een
lichaamshormoon dat hoofdzakelijk gevormd wordt tijdens de duisternis van de
nacht. Eerdere studie toonden al aan dat kunstlicht de vorming van melatonine
afremde. Nu blijkt ook dat daardoor de groei van kankercellen bevordert wordt.
In de studie werden borstkankercellen ingespoten in muizen. Toen deze
uitgegroeid waren tot een tumor werd deze tumor getransplanteerd in een
vrouwelijke rat, waar deze verder kon groeien. Dan werden van gezonde vrouwen
bloedstalen genomen onder drie verschillende condities te weten bij daglicht, na
enige uren duisternis en als laatste na enige uren kunstlicht. Dit bloed werd
door de tumoren gevoerd. Het bloed dat bij duisternis genomen was deed de
tumorgroei duidelijk verminderen in tegenstelling tot de andere bloedstalen die
de groei bevorderden. Wellicht zit hierin ook een deel van de verklaring waarom
in onze westerse wereld als gevolg van het vele kunstlicht veel meer kanker
voorkomt dan in ontwikkelingslanden. Verder onderzoek is noodzakelijk en als
meer bewijs gevonden wordt is het wellicht wenselijk lampen te ontwikkelen die
de productie van melatonine niet remmen.
Artificial
Light At Night Stimulates Breast Cancer Growth In Laboratory Mice
Results
from a new study in laboratory mice show that nighttime exposure to artificial
light stimulated the growth of human breast tumors by suppressing the levels of
a key hormone called melatonin. The study also showed that extended periods of
nighttime darkness greatly slowed the growth of these tumors.
The study results might explain why female night shift workers have a higher
rate of breast
cancer.
It also offers a promising new explanation for the epidemic rise in breast
cancer
incidence in industrialized countries like the United States.
The National Cancer Institute and the National Institute of Environmental Health
Sciences, agencies of the federal National Institutes of Health, provided
funding to researchers at the Bassett Research Institute of the Mary Imogene
Bassett Hospital in Cooperstown, New York and The Thomas Jefferson University in
Philadelphia, Pa. The results are published in the December 1, 2005 issue of the
scientific journal Cancer Research.
"This is the first experimental evidence that artificial light plays an
integral role in the growth of human breast
cancer,"
said NIEHS Director David A. Schwartz, M.D. "This finding will enable
scientists to develop new strategies for evaluating the effects of light and
other environmental factors on cancer growth."
"The risk of developing breast cancer is about five times higher in
industrialized nations than it is in underdeveloped countries," said Les
Reinlib, Ph.D., a program administrator with the NIEHS' grants division.
"These results suggest that the increasing nighttime use of electric
lighting, both at home and in the workplace, may be a significant factor."
Previous research showed that artificial light suppresses the brain's production
of melatonin, a hormone that helps to regulate a person's sleeping and waking
cycles. The new study shows that melatonin also plays a key role in the
development of cancerous tumors.
"We know that many tumors are largely dependent on a nutrient called
linoleic acid, an essential fatty acid, in order to grow," said David Blask,
M.D., Ph.D., a neuroendocrinologist with the Bassett Research Institute and lead
author on the study. "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. Once these cells developed into cancerous tumors,
the tumors were implanted into female rats where they could continue to grow and
develop.
The researchers then took blood samples from 12 healthy, premenopausal
volunteers. The samples were collected under three different conditions - during
the daytime, during the nighttime following 2 hours of complete darkness, and
during the nighttime 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 the tumors. "These results are due to a
direct effect of the melatonin on the cancer cells," said Blask. "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," Blask said.
According to the researchers, melatonin exerts a strong influence on the body's
circadian rhythm, an internal biological clock that regulates sleep-wake cycle,
body temperature, endocrine functions, and a number of disease processes
including heart attack, stroke and asthma.
"Evidence is emerging that disruption of one's circadian clock is
associated with cancer in humans, and that interference with internal
timekeeping can tip the balance in favor of tumor development," said Blask.
"The effects we are seeing are of greatest concern to people who routinely
stay in a lighted environment during times when they would prefer to be sleeping,"
said Mark Rollag, Ph.D., a visiting research scientist at the University of
Virginia and one of the study co-authors. "This is because melatonin
concentrations are not elevated during a person's normal waking hours."
"If the link between light exposure and cancer risk can be confirmed, it
could have an immediate impact on the production and use of artificial lighting
in this country," said Blask. "This might include lighting with a
wavelength and intensity that does not disrupt melatonin levels and internal
timekeeping."
"Day workers who spend their time indoors would benefit from lighting that
better mimics sunlight," added Blask. "Companies that employ shift
workers could introduce lighting that allows the workers to see without
disrupting their circadian and melatonin rhythms."
NIEHS, a component of the National Institutes of Health, supports research to
understand the effects of the environment on human health. For more information
on breast cancer and other environmental health topics, visit our website at www.niehs.nih.gov
.
( December 2005)
