A protein called MITF coordinates skin-darkening melanin production with other skin protection mechanisms in response to UV light, molecular geneticist Carmit Levy of Tel Aviv University and her colleagues discovered. The team shone UV-B light on mice every 24, 48 or 72 hours for 60 days. Mice exposed to UV-B radiation on the 48-hour schedule developed darker skin and had less DNA damage than mice in the other groups. Mouse and human skin cells grown in lab dishes that were exposed to UV light every other day also made more melanin than cells that were irradiated daily.
Other experiments with skin cells in dishes suggest that within minutes of UV exposure, MITF turns on genes involved in skin cell survival. Those genes make proteins engaged in inflammation, DNA repair and recruitment of immune cells to the skin. Only later does MITF give the OK for melanin production to begin. Hitting cells with daily UV interrupts melanin production, leaving skin without its protective shield, the researchers found.