CANCER
March 12, 2015

Here Comes the Sun

Exposure to the sun can cause skin damage hours after you head indoors.

The spring solstice is approaching, and for those in the Northern hemisphere, it means a return to being cautious about exposure to the sun's rays. Most people know they should limit their exposure from 10 in the morning to 3 o’clock in the afternoon and wear sun block and clothing with a sun protection factor (SPF) of at least 25 or 30.

This may not be enough however. Skin damage goes on long after you head indoors. A new study of how ultraviolet (UV) light damages skin finds that the damage continues for hours afterward. With skin cancer already the most common type of cancer in the U.S., this may mean even more cause for concern. But the findings also hold the promise of new treatments to counter the effects of UV.

Melanin, the pigment in skin cells called melanocytes, helps protect the skin by absorbing some of the UV. But researchers studying melanocytes found that these cells continue to suffer damage even after exposure to UV has stopped. While melanin does protect the skin, its protection is far from perfect.

Ultraviolet (UV) light damage continues for hours after sun exposure ends.

The researchers exposed melanocytes from mice and humans to radiation from a UV lamp. This caused alterations in the DNA known as cyclobutane dimers, changes that are known to lead to permanent mutations in the DNA, mutations which can cause melanoma.

When these dimers do form, it generally happens less than a second after a UV photon impacts the DNA. But the researchers found that the dimer alterations in the lab continued to form long after the UV had been turned off, more than three hours later. There was no such dimer formation in the dark in skin cells that did not contain melanin.

UV light damages DNA because it's highly energetic radiation. Some of this energy gets transferred to the DNA, creating a new chemical linkage, the cyclobutane dimer, that does not normally exist in our DNA. Sometimes the cell can repair this alteration; if it doesn't, the genetic information is read incorrectly by the cellular machinery and can ultimately lead to mutation (a permanent change in the DNA) and cancer.

Most cases of melanoma, the deadliest type of skin cancer, come from exposure to UV, either from the sun or a tanning bed.

But the energy from UV can also cause other changes inside a cell, including generation of reactive oxygen and nitrogen compounds. Melanin in the dark can receive some of this extra energy from these compounds, creating a high-energy melanin molecule, which, in turn, transfers its extra energy to DNA. This is similar to what UV does in sunlight, although the process is much slower.

Because this energy transfer is so slow, it offers the hope of development of an “evening after” pill or treatment for UV exposure that can help counteract the UV damage that occurs in the dark.

For now, it's probably best to continue to remain cautious about exposure to the sun and to remember to use plenty of sunscreen.

The study appears in Science.

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