Yale University. US: Much of the damage that ultraviolet radiation (UV) does to skin
occurs hours after sun exposure, a team of Yale-led researchers
concluded in a study that was published online Feb. 19 by the journal
Science. Exposure to UV light from the sun or from tanning beds
can damage the DNA in melanocytes, the cells that make the melanin that
gives skin its color.
This damage is a major cause of skin cancer, the
most common form of cancer in the United States. In the past, experts
believed that melanin protected the skin by blocking harmful UV light.
But there was also evidence from studies suggesting that melanin was
associated with skin cell damage.
In the current study, Douglas E.
Brash, clinical professor of therapeutic radiology and dermatology at
Yale School of Medical, and his co-authors first exposed mouse and human
melanocyte cells to radiation from a UV lamp. The radiation caused a
type of DNA damage known as a cyclobutane dimer (CPD), in which two DNA
“letters” attach and bend the DNA, preventing the information it
contains from being read correctly. To the researchers' surprise, the
melanocytes not only generated CPDs immediately but continued to do so
hours after UV exposure ended. Cells without melanin generated CPDs only
during the UV exposure.
This finding showed that melanin had both
carcinogenic and protective effects. “If you look inside adult skin,
melanin does protect against CPDs. It does act as a shield,” said Brash,
also a member of Yale Cancer Center. “But it is doing both good and bad
things.”
The researchers next tested the extent of damage that
occurred after sun exposure by preventing normal DNA repair in mouse
samples. They found that half of the CPDs in melanocytes were “dark
CPDs” — CPDs created in the dark.
In searching for an explanation
of these results, Sanjay Premi, associate research scientist in the
Brash laboratory, discovered that the UV light activated two enzymes
that combined to “excite” an electron in melanin. The energy generated
from this process — known as chemiexcitation — was transferred to DNA in
the dark, creating the same DNA damage that sunlight caused in daytime.
Chemiexcitation has previously been seen only in lower plants and
animals.
While noting that news of the carcinogenic effect of
melanin is disconcerting, the researchers also pointed to a ray of hope:
The slowness of chemiexcitation may allow time for new preventive
tools, such as an “evening-after” sunscreen designed to block the energy
transfer.
Other study authors include Yale’s Silvia Wallisch,
Camila M. Mano, Adam B. Weiner, Antonella Bacchiocchi, and Ruth Halaban;
Kazumasa Wakamatsu of Fujita Health University School of Health
Sciences in Japan; Etelvino J. H. Bechara of Universidade de São Paulo
in Brazil; and Thierry Douki Commissariat à l’Energie Atomique in
France.
The study was supported in part by Department of Defense
CDMRP grants CA093473P1 and CA093473 (D.E.B. and R.H.), and NIH grant 2
P50 CA121974 (R.H. and D.E.B.).
Citation: Science