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Scientists ID molecule that makes sunburn hurt

By Eleanor Spicer Rice
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  • Stay sun-safe

    As researchers work to develop a cure, sunburn pain continues to plague outdoor enthusiasts. Dr. Margaret Boyse, a dermatologist at Raleigh’s Southern Dermatology and Skin Cancer Center, warns that sunblock should be the last line of defense against sunburn.

    “My patients often rely on sunscreen for too long,” she said. “A good rule of thumb is to think of the SPF number as the number of minutes times your natural protection. So an SPF of 50 would be 50 minutes longer than your skin’s natural protection. People often apply sunblock once and assume they’re protected for the rest of the day. They should reapply often.”

    Boyse advises the best way to avoid burning is to stick to a four-point line of defense, in order of importance:

    Stay indoors. “Fifteen minutes of sun a day can be good for your skin,” she said, “but staying inside is the best way to stay sun-safe.” Boyse sees patients suffering from burns as early as March here in North Carolina. “Many people don’t realize UV-B rays can penetrate clouds and have nothing to do with temperature,” she said. “So you can get burned when it’s cold and cloudy, once the Earth is tilted the right way.”

    Seek shade. “Whenever you look down and see your shadow is shorter than you are, seek shade,” Boyse said. “That’s because the sun’s rays are strongest between the hours of 10 a.m. and 2 p.m., when it hangs almost directly overhead.” Boyse points out that you can get burned even outside this range, so hanging out in the shade is a good idea at any time of day.

    Wear sun-protective clothing. “Long-sleeved shirts like rash-guard shirts, wide-brimmed hats, pants and sunglasses can go a long way in protecting you from sunburn, the signs of aging and skin cancer,” she said.

    Think of sunblock as the last line of defense. “Don’t rely only on sunscreen to protect you from burning,” Boyse warned. When sunblock shopping, try to stay away from the spray. “Most often, burned patients who insist they use sunscreen use the spray variety. It’s easy to miss spots when you’re spraying.”

    Boyse recommends applying a shot glass’s worth of sunscreen at least once every two hours. “Sunscreen wears off over time,” she said.

    The type of sunscreen matters, Boyse added. “Non-chemical sunscreen, like those containing zinc oxide, and broad-spectrum sunscreens, work best,” she said. “Also consider water-resistant sunscreen, even if you’re not going swimming, and sunscreen with an SPF of 30 or greater.” Eleanor Spicer Rice

Researchers get to the root of sunburn pain in a Proceedings of the National Academy of Sciences study published this month. Despite application (and re-application) of sunscreen, the sun’s rays have a way of finding those hard-to-reach places, leaving us red, blistered and itchy, says a team from Duke University, the University of California and the Howard Hughes Medical Institute.

“We have uncovered a novel explanation for why sunburn hurts,” Dr. Wolfgang Liedtke, one of the senior authors of the study and associate professor of neurology and neurobiology at Duke University School of Medicine, said in a news release.

The culprit responsible for sunburn’s sting: a molecule called TRPV4 – a nightclub bouncer of the cell membrane world. Nestled into skin cells’ membranes, TRPV4 acts like a gate, blocking some molecules from entering cells and opening the door for others. Because earlier research revealed TRPV4 enables us to feel pain from tactile contact, such as a pinch or poke, Liedtke and his team chose to investigate its role in pain brought on by the sun’s rays.

From mice to men

To explore TRPV4’s role in sunburn pain, the team genetically engineered mice to lack TRPV4 in their skin cells. The skin cells of naked mice paws are similar to human skin and can help scientists examine the sun’s effects without any humans feeling the burn. They exposed these TRPV4-free mice, along with control mice, to UV-B rays – the type of sunlight that most often causes sunburn pain.

After catching some rays, the paws of normal mice were sensitive to the touch, a lot like sunbathers after an August day at the beach. But the mice without TRPV4 were a lot less sensitive, and their skin tissue was significantly less damaged.

Conversely, when the researchers applied a chemical to these mice’s paws that blocked the TRPV4 gateway, the mice proved resistant to sunburn’s negative effects. “This animal gave us the clue that TRPV4 in skin cells is critical for sunburn tissue damage and pain,” said Liedtke.

When TRPV4 is present in the skin’s cells, it reacts with the presence of UV-B rays by opening its gates, allowing calcium to rush into cells. A protein associated with pain and itching, endothelin tags along with calcium as it tumbles in through the cell wall, resulting in the itching and pain associated with prolonged exposure to the sun’s rays.

“The results position TRPV4 as a new target for preventing and treating sunburn, and probably chronic sun damage, including skin cancer or skin photo-aging, though more work must be done before TRPV4 inhibitors can become part of the sun defense arsenal, perhaps in new kinds of skin cream, or to treat chronic sun damage,” said UC’s Martin Steinhoff, co-senior author of the study.

A new ingredient

While Liedtke and his team now understand the cause of sunburn pain, he notes more research is needed before introducing a TRPV4 blocker to the market. “I think we should be cautious because we want to see what inhibition of TRPV4 will do to other processes going on in the skin,” Liedtke said.

“Once these concerns are addressed, we will need to adapt TRPV4 blockers to make them more suitable for topical application,” he added. “I could imagine it being mixed with traditional sunblock to provide stronger protections against UVB exposure.”

Margaret Boyse, a dermatologist at Raleigh’s Southern Dermatology and Skin Cancer Center, treats an influx of sunburn patients each year in the first few weeks of summer. “I see sunburn patients as early as March who are suffering from itching and pain.” Still, she believes the sun’s rays aren’t always bad, as they help repair cells, synthesize vitamin D, and can reduce depression.

Plus, Boyse points out that sunburn pain is nature’s way of telling us to seek shade. “Sunburn pain acts as a warning system,” she said. “We need that trigger to help keep us safe. When people feel uncomfortable from sunburn, they’re more likely to stay indoors and out of the sun, limiting further skin damage.”

She welcomes more research from Liedtke’s team. “If we understand sunburn better, we can understand pain better. What plagues my patients day in and day out is what temporarily affects otherwise healthy people who suffer from sunburn.”

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