A protein that helps suppress cancer also protects against sun damage by promoting a tan.

"The number one risk factor for melanoma is an inability to tan," said Dr. David Fisher, the study's senior author and director of the Melanoma Program in Medical Oncology at Dana-Farber Cancer Institute in Boston.

"People who tan easily or have dark pigmentation are far less likely to develop the disease."

In Friday's issue of the journal Cell, Fisher and his team said they found that p53, a tumour-suppressor protein, also protects skin from sun damage.  Because of its protective effects against cancer, p53 is also known as the "guardian of the genome."

The researchers studied how keratinocyte cells near the surface of skin react to sun exposure. Normally when keratinocytes are exposed to the sun's rays, they produce a hormone that leads to the manufacture of a skin-bronzing pigment.

When keratinocytes from humans and mice were exposed to UV radiation in the experiment, p53 and another protein called pro-opiomelanocortin or POMC, went up.

But in mice keratinocytes that lacked p53, POMC was never produced and the rodents did not tan, which suggests that it is p53 that causes POMC to rise.

Process may explain sun seekers' addictive behaviour

The same process also leads to the production of beta-endorphin, which is linked to feelings of pleasure.

"This might explain addictive behaviors associated with sun-seeking or the use of tanning salons," Fisher said.

Understanding the suntan process could lead to products that allow safe tanning without exposure to damaging UV radiation, even in people who don't otherwise tan, the researchers said.

The findings could also provide a way to identify drugs that restore the response of p53 when it is defective.

Melanoma, the most dangerous type of skin cancer, makes up only one or two per cent of all skin cancers. It is the fastest-increasing form of cancer worldwide, according to the American Cancer Society.