Researchers from Yale University (CT, USA), have discovered that much of the damage to skin caused by ultraviolet (UV) radiation happens hours after sun exposure. The study was recently published online in the journal Science.
It is well known that UV light exposure, either from natural sunlight or tanning beds, can damage the DNA in melanocytes. This damage can ultimately lead to skin cancer, which is currently the most common cancer type in the USA. In the past, there has been conflicting evidence regarding the role of melanin in skin cancer, with some studies indicating that melanin protects skin by blocking harmful UV light, and others suggesting that melanin is associated with skin cell damage.
The Yale team exposed mouse and human melanocytes to UV radiation from a lamp and discovered that this exposure caused DNA damage in the form of cyclobutane dimers (CPD), which prevent the correct reading of DNA. They observed that melanocytes produced CPDs immediately but also for several hours after UV exposure had ceased. It was also observed that cells lacking melanin only generated CPDs during UV exposure.
These observations demonstrate that melanin has both protective and carcinogenic effects. Lead author Douglas E Brash (Yale University) commented: “If you look inside adult skin, melanin does protect against CPDs. It does act as a shield, but it is doing both good and bad things.” “
To test the extent of damage that occurred after radiation exposure, the researchers prevented normal DNA repair in mouse samples. They discovered that half of the CPDs present in melanocytes were ‘dark CPDs’ – CPDs that were created in the dark after UV exposure.
Investigating these results further, Sanjay Premi (Yale University) discovered that UV caused the activation of two enzymes that subsequently combine to excite an electron in melanin. This process, termed chemiexcitation, transfers energy to DNA in dark conditions, thus causing the same damage to skin cells that is observed in daytime conditions. Chemiexcitation has only previously been observed in lower plants and animals.
Although the carcinogenic effect of melanin concerns researchers, one positive they note is the slow-acting nature of the chemiexcitiation process. They speculate that preventative tools could be created, including an ‘evening after’ sunscreen that could block the energy transfer.