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In a new study, biologists report that melanocyte skin cells detect ultraviolet light using a photosensitive receptor previously thought to exist only in the eye. This eyelike ability of skin to sense light triggers the production of melanin within hours, more quickly than previously thought, in an apparent rush to protect against damage to DNA.
For most people, tanning seems a simple proposition. A naturally light-skinned person lies in the sun for hours and ends up as bronzed as a Jersey Shore star. To scientists, the reaction of skin to ultraviolet light is more mysterious. A new study demonstrates that skin detects UVA radiation using a light-sensitive receptor previously found only in the eye and that this starts melanin production within a couple of hours. Until now, scientists only knew that melanin production occurred days after UVB radiation had already begun damaging DNA.
"As soon as you step out into the sun, your skin knows that it is exposed to UV radiation," says senior author Elena Oancea, assistant professor of biology in the Department of Molecular Pharmacology, Physiology, and Biotechnology at Brown University. "This is a very fast process, faster than anything that was known before."
Scientists believe that melanin protects the DNA in skin cells against damage from UVB rays by absorbing the incoming radiation. It isn't perfect, which is why people must use sunscreen. But the new study in the journal Current Biology shows that the body mounts its defense much sooner, well before it becomes apparent in the form of a tan.
In lab experiments with human melanin-producing skin cells called melanocytes, Oancea, graduate student Nadine Wicks, and their team discovered that the cells contain rhodopsin, a photosensitive receptor used by the eye to detect light. Moreover, they traced the steps of how rhodopsin unleashes calcium ion signals that instigate melanin production.