Stepping under the shade of an umbrella after soaking in some sun might feel like an instant cool, but your skin cells have a postsunshine hangover that lasts for hours. In fact, molecules excited by the ultraviolet (UV) rays keep on damaging the DNA in skin cells even in complete darkness, researchers have discovered. The finding could lead to a new generation of skin cancer preventives that are applied after a day in the sun and block these delayed effects.
“This is an interesting, unexpected, and very important new mechanism of skin damage by UV radiation,” says dermatologist David Fisher of Massachusetts General Hospital in Boston, who was not involved in the work.
When UV rays from sunlight—or a tanning bed—hit skin cells, the radiation damages genes, causing extra chemical bonds to form between the building blocks of DNA. Over time, these genetic changes accumulate and can make cells more prone to the skin cancer melanoma. But when radiology researcher Douglas Brash of Yale University tracked the timing of these genetic changes in isolated skin cells, he found that the extra bonds in the DNA didn’t stop forming when UV rays stopped. For more than 3 hours after cells were exposed to UV light, the DNA damage kept increasing.
“What this means is that we’ve been underestimating the amount of DNA damage that people are getting from UV exposure,” Brash says.
He and his collaborators went on to show, in both mouse and human skin cells, that the lingering damaging effects of UV rays were dependent on melanin, the pigment that gives skin its color. High levels of melanin in dark-skinned people are typically associated with protection against melanoma, because the melanin absorbs UV energy, preventing it from causing DNA damage. But Brash found a second role for the pigment. When UV rays hit skin cells, they cause a cascading reaction that puts melanin in an excited state. For hours afterward, even if the cells are in darkness, the energy from the excited melanin can keep damaging DNA, the scientists report online today in Science.
The new paper is “extraordinarily important and elegantly done,” says Frank Meyskens, a melanoma researcher at the University of California, Irvine. It finally explains a longtime clinical observation that has stumped researchers, he says: why black albinos in Africa have such low rates of melanoma. Without any melanin in their skin, they don’t get the protective effects of the pigment, yet they also don’t have the long-term damage after sun exposure that comes with excited melanin.
The research also suggests a new way to prevent melanoma. “If we can divert the energy from the excited melanin before it gets transferred, we might be able to intervene,” Brash says. In the new study, his group found that vitamin E lotion diminished some of the delayed effects of UV rays, but he suspects there are other compounds that could work even better. “If you look at the data out there on current sunscreens and skin cancer prevention, it’s not so great,” Fisher says. “But this new finding could really give us new opportunities to improve.”