No one can hear the ticking of his or her brain. But we all have biological clocks in our bodies that are responsible not only for sleep and wakefulness but also the release of hormones, when you get hungry, your body temperature, fertility, aging and other functions.
Now, scientists at Tel Aviv University and the Technion-Israel Institute of Technology in Haifa have discovered that our suntan also functions according to biological clocks in our bodies.
The researchers learned from their experiments on human and mouse skin samples that a 48-hour cycle that synchronizes the protective mechanisms that arise in the skin after exposure to ultraviolet rays. The research has just been published in the journal Molecular Cell, which also dedicated the title page to their work.
“Skin sun exposure induces two protection programs – stress responses and pigmentation – the former within minutes and the latter only hours afterwards,” they wrote in the paper. Although serving the same physiological purpose, it is not known if and how these programs are coordinated.
“To our surprise, we found that exposure of once every two days, rather than daily exposure, causes optimal tanning,” declared Prof. Carmit Levy of the department of human genetics and biochemistry at TAU’s Sackler Faculty of Medicine who worked with her doctoral student, Hagar Malcov-Brog. “In fact, we have shown that the frequency of exposure is very important, and we have sought an explanation for the phenomenon.”
Together with Prof. Shai Shen-Or and doctoral student Ayelet Alpert of the Technion’s Rappaport Faculty of Medicine and Dr. Mehdi Khaled of France, they identified a process that synchronizes the skin’s natural defense mechanisms.
During evolution, when the precursors of modern humans lost the fur that protected their bodies, our skin developed natural defense mechanisms that arise from exposure to the sun’s rays, explained Levy. “One of them is the pigmentation mechanism – tanning in popular usage – that creates physical-mechanical protection on the skin. It recruits the immune system to repair DNA damage caused by UV radiation that can cause skin cancer.”
Many scientists around the world are studying the effect of the sun’s rays on the skin, but according to Levy, most studies focus on reactions related to the intensity of exposure. The Israeli researchers, however, examined the effect of another factor – frequency. To this end, they exposed samples of skin to UV rays at constant intensity – at the level of the summer sun at noon – but at different frequencies, such as daily, every other day, intermittent exposure and so on.
The researchers examined the skin samples every hour after exposure and collected diverse data on the response of the defense systems to exposure at different frequencies. Among other things, they measured levels of different proteins in the skin, levels of pigmentation, RNA and DNA repair. They found that proteins expressing certain genes appear in the skin in a neat and well-synchronized sequence during the 48 hours following exposure to UV radiation.
The team built a mathematical model that showed that the levels of MITF (melanocyte-inducing transcription factor) protein – which is responsible for the skin’s protective mechanisms – rise and fall like a wave fading with time. This activates the defense mechanisms simultaneously.
“Imagine that you threw a stone into a lake and formed waves. If you throw another stone before the first wave has faded, the wave created by the second stone will disrupt the pattern,” explained Shen-Orr. “That’s exactly what happens in the skin. Additional exposure to the sun during the first synchronization wave reduces the effectiveness of the skin protection system.”
The mathematical model was verified in a laboratory experiment. “We discovered that the biological cycle of the skin defense mechanisms is 48 hours, in contrast to the natural exposure to the sun that occurs every day – that is, in a 24-hour cycle,” Levy concluded. “
The number of cases of skin cancer increases every year. UV exposure is a major risk factor for all common skin malignancies, they concluded. MITF has been shown to play a key role in the development of melanoma. the most-deadly type of human skin cancer.
“We are now trying to understand why this may be connected to the expression of Vitamin D, which is related to exposure to the sun and reaches its peak every 48 hours. In any case, Levy said, “we recommend to everyone that to achieve optimal protection after sun exposure, let the natural mechanism in your skin finish what is has to do – and do not expose yourself too frequently to the sun’s rays.”