UVA Rays May Play Bigger Role in Skin Cancer

Study indicates need for sunblock that protects against A and B rays

TUESDAY, March 23, 2004 (HealthDayNews) -- Ultraviolet-A (UVA) rays have long been known to cause aging of the skin, but they may play a bigger role in promoting skin cancer than previously thought, researchers report.

"Studies have shown that sunlight causes skin cancer, but have not determined which part of sunlight," says study author Gary Halliday, a professor of dermatology at the University of Sydney in Australia. "There is good evidence that UVB [ultraviolet-B] is important [in causing skin cancer], but our studies show that UVA is also very important."

The research appears in this week's online issue of the Proceedings of the National Academy of Sciences.

UVB rays have shorter wavelengths and are primarily responsible for sunburn and are known to contribute to skin cancer, including the deadliest form -- melanoma. UVA rays are longer wavelength rays that can damage the skin's connective tissue and lead to premature aging. UVA rays have been considered less carcinogenic than UVB rays.

For the new study, Halliday's team evaluated cells from biopsies taken from 16 patients with squamous cell skin cancer and solar keratosis, precancerous skin growths caused by sun damage. The researchers used a technique called laser capture microdissection and searched for "signature" DNA mutations that are characteristically caused by either UVA or UVB wavelengths.

Damage from UVA causes a different mutation in the cell than does damage from UVB, skin cancer experts say.

The researchers looked at cells called keratinocytes in the epidermis, the outermost layer of skin. They found the majority of UVA signature mutations were found in cancer cells residing in the basal keratinocyte layer of cells, the area that houses stem cells that give rise to keratinocyte cells that migrate upward. Most UVB signature mutations were found in the more superficial, upper layer of keratinocytes.

The finding in human skin cancers mirrors that of several animal studies, Halliday says. "A number of experiments in animal models show that UVA is involved in skin cancer," he says. "However, this is the first report to show UVA causes gene mutations in human skin cancer."

Another skin cancer expert, Dr. Vincent DeLeo, chairman of dermatology at St. Luke's-Roosevelt and Beth Israel medical centers in New York City, calls the new study exciting but says more research is needed to verify the results.

"What [the study] is saying essentially is that in squamous cell and solar keratosis there are changes in the DNA of the basal cells that look like the changes were made to some extent by UVA rather than UVB," he says.

"We've known UVA is a carcinogen, but most experts thought those [skin cancer] tumors were caused by UVB primarily," DeLeo adds.

Researchers talk about a UVA or UVB "footprint" or "fingerprint," and that is a special mutation that has been found in the cancer cells, suggesting which type of wavelength caused it, he says.

"They [the Australian researchers] are showing these [UVA mutations] at the base of the tumor, but then at the top show UVB mutations." That finding, he says, "warrants more study."

In future research, Halliday says he hopes to study how damage to DNA caused by UVA and UVB rays is repaired differently and how the body protects itself against the two different wavelengths.

Meanwhile, Halliday says, "Our studies indicate that it is important to protect from both UVB and UVA. Therefore the best advice is to avoid sunlight exposure as much as possible, and if this is not possible to use a sunscreen which protects for both UVB and UVA," he says.

DeLeo agrees, adding that "protecting yourself from UVB alone is not enough. Use a broad-spectrum sunscreen [that protects against both A and B]. Sunscreens in the U.S. today are not as effective in the A range as the B."

More information

For more information about skin cancer, visit the American Cancer Society and the Skin Cancer Foundation.

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