Sunbathers beware! Skin damage caused by ultraviolet A (UVA) rays from the sun continues even after you have retreated indoors. In fact, half of the damage to skin due to sun exposure appears to occur up to 4 hours after you have come in out of the sun.
UVA causes damage in cells called melanocytes, which are the cells that produce the melanin that gives skin its color. This UVA damage gives rise to skin cancer, including melanoma, the deadliest type of skin cancer.
The radiation causes melanocytes to create a chemical called cyclobutane pyrimidine dimer (CPD). By examining melanocyte cells from mice and humans that were exposed to UVA light, the researchers found that the melanocytes not only generated CPDs immediately but continued to do so hours after the light exposure ended. Cells without melanin generated CPDs only during the UVA exposure.
This role for melanin in skin cancer came as a surprise, because melanin was thought to help protect skin from damage caused by UVA. It appears that melanin has both carcinogenic and protective effects in the skin. "If you look inside adult skin, melanin does protect against CPDs. It does act as a shield," said Douglas E. Brash, a clinical professor at Yale School of Medicine and a co-author of the study. "But it is doing both good and bad things."
Brash's Yale colleague Sanjay Premi found out why the cells were still producing CPDs hours later. He discovered that the UV light activated two enzymes that combined to "excite" an electron in melanin. The energy generated from this process, called chemiexcitation, was transferred to DNA in the dark, creating the same DNA damage that UVA light caused in daylight. Chemiexcitation has not been seen in human cells before.
UVA is present in sunlight and is emitted by sun lamps and tanning beds.
This finding was reported in an article in the journal Science.