Most Americans like tanned skin

Recommended: morning sun

The key role in the repair team is played by a protein called XPA? a short form for the very unwieldy name "Xeroderma Pigmentosum Group A Protein". Earlier studies had shown that the internal clock in particular determines how much of it is produced in the liver or the brain, for example. In contrast, it has so far been unclear whether the amount and activity of XPA in the skin also increases and decreases over the course of the day, write the researchers led by molecular biologist Shobhan Gaddameedhi, who works at the University of North Carolina. So you examined mouse skin samples every three hours? and actually found a clear change in activity: the skin cells produced the least XPA at four in the morning and the most at four in the afternoon.

In the next step, the team tested whether these fluctuations had an influence on the incidence of pale skin cancer in the animals. To do this, they irradiated one group of mice with UV radiation three times a week at four in the morning and a second group at four in the afternoon. Here, too, there was a clear result: didn't the morning group just develop skin cancer two weeks earlier? especially squamous cell carcinoma? but also a lot more tumors. The researchers write that the risk was increased by a factor of five compared to the afternoon group.

Of course, do you need further tests and controls to ensure this result? after all, cancer development is a very complex process in which various other factors are involved. Nevertheless, the scientists believe that they have identified a key player in XPA and its various activities throughout the day. The exciting thing about it: Even in humans, different amounts of XPA can be detected in the skin at different times of the day. In this case, it is quite legitimate to assume that there is an effect similar to that in mice in humans, according to the researchers. The basic function of the internal clock is in fact quite comparable in humans and mice: in both of them, it follows roughly a 24-hour cycle, even if the two clocks are offset by around 12 hours due to their different lifestyles. Accordingly, while XPA production in the mouse reaches a peak in the afternoon, in humans it is highest in the early morning hours.

However, there is one special feature in humans: even if the most XPA is produced in the skin on average at seven in the morning, the individual values ​​can vary greatly depending on the personal disposition. For “larks”, people who are fit and productive early in the morning, the optimal time for sunbathing could be before seven o'clock. In the case of the “owls” that are more slowly getting going, it may be significantly later. If this assumption is confirmed, everyone would first have to have their chronotype determined in order to know when they can go into the sun. Fortunately, this is expensive and a little time-consuming, but not very painful: Researchers only need about ten hairs on the head, including the follicles, to determine their personal type.

Shobhan Gaddameedhi (University of North Carolina, Chapel Hill) et al .: PNAS, online pre-publication, doi: 10.1073 / pnas.1115249108 Ilka Lehnen-Beyel
October 25, 2011