A tumour suppressor gene p53 that prompts skin to tan when exposed to sunlight also works as a protector against skin cancer, scientists found.

Scientists already knew the skin tans because it has special cells called keratinocytes that produce the dark pigment melanin and give it to other cells to shield them from DNA damage by ultraviolet (UV) light.

But the exact mechanism of UV light boosting melanin production was not known yet.

Dr Rutao Cui, of Dana-Farber and Children's Hospital, and colleagues suspected that the whole process starts when UV light triggers the gene p53. Doctors explored the possibility that p53 protects the skin against DNA damage in another way - by triggering sun seeking behaviour.

According to researchers, when people are exposed to UV light, p53 triggers a process that releases a hormone, b-endorphin, that docks in opiate receptors in brain cells and is linked to feelings of pleasure.

"The number one risk factor for melanoma is an inability to tan; people who tan easily or have dark pigmentation are far less likely to develop the disease," says Dr. David Fisher, director of the Melanoma Program at Dana-Farber and a professor in pediatrics at Children's Hospital Boston.

When UV light falls on skin cells it stimulates the keratinocytes to make and release a hormone called alpha-MSH. This hormone sticks to another type of cell that is nearby called melanocytes, and causes them to produce melanin.

P53 protein is one of the best-known tumour suppressors and DNA damage protectors in the body. It has been nicknamed the "master watchman of the genome". This dual pigmentation triggering and tumour suppressing role of p53 might also explain another phenomenon - age spots. These are dark patches on the skin that mostly appear when people get older. They result from stress of some kind, not necessarily sun exposure.

P53 is activated whenever the skin is stressed, for example by chemotherapy, and since this threatens DNA health, p53 kicks in with its protective role, triggering melanin production and delivery to cells that don't "know that it isn't sunshine that induced p53," says Dr. Fisher, adding that this research shows that p53 "has a powerful role in protecting us against sun damage in the skin."