Exposure to sunlight has become a tricky balancing act; too much can give you skin cancer, too little leads to vitamin D deficiency and depression. Aesthetic considerations may complicate things further for those with a preference between tan and fair. It may be possible to get the best of both worlds, however, with the unexpected discovery that our skin's sun response functions on a 48-hour cycle.
What we call sunburn is the skin's stress reponse that includes inflammation, cell proliferation and repair of DNA damaged by ultraviolet radiation. Unfortunately this painful reaction can start within minutes of cell damage, while the production of the dark pigment melanin, which serves to protect against subsequent damaging bouts, takes hours or days.
Professor Carmit Levy of Tel Aviv University sought to explore why this difference in timing occurs. Levy’s team had mice spend sessions under UV lights for 60 days, with some being exposed every day, while others were on longer rotations. However, all mice received equal cumulative radiation.
The mice that were on a cycle of every second day exposure developed darker coloured skin cells than either those who encountered UV every day, and those who got it every third day. Yet the mice on a day on/day off cycle suffered less DNA damage, a precursor to skin cancer, than either of the other groups.
"The results were so surprising," Levy said in a statement. “We expected daily synchronization of the cell's protective cycles." There may be better models for sun exposure than a nocturnal and fur-covered species. However, Levy reports in Molecular Cell the optimum response for cultured human skin patches was also to exposure every 48 hours.
In both mice and humans a protein known as microphthalmia-associated transcription factor (MITF) controls the production of melanin and its spread to neighboring cells, where it acts as a sort of microscopic parasol. In both her models Levy found MITF also induced sunburn as short term cell protection, and operated most effectively with exposure every 48 hours.
Why this would have been the case when our ancestors experienced sunlight everday is unclear. “We do know that vitamin D, which the skin produces upon exposure to the sun, is stable in the blood for 48 hours post-exposure. Perhaps there is a link,” Levy said.
If the pattern is confirmed to work in living humans, it could prove useful for more than just safe tanning. The authors think chemotherapy treatments for existing skin cancers may also work best if administered on a 48-hour cycle.
Before that, trials will need to be run with people exposed to different light regimes. Sunbaking for science? That's a clinical trial we'll sign up for.