LONDON (AP) - There's a new wrinkle in the old question of why smokers' faces are prematurely lined.

A report in The Lancet medical journal this week suggests smoking switches on a gene involved in destroying collagen, the structural protein that gives skin its elasticity.

The link between smoking and wrinkles has been known for years, but scientists haven't worked out exactly how cigarettes age the skin.

In the study, scientists from St. John's Institute of Dermatology in London found that the gene, one implicated in wrinkles from sunbathing, was highly active in smokers and silent in nonsmokers.

``It indicates that something in cigarettes is injuring skin in a similar way to sun, or at least through the same pathways,'' said Dr. James Leyden, a professor of dermatology at the University of Pennsylvania Medical School, who was not connected with the research. ``I think it could be part of the story of smoking and skin.''

The finding emerged by accident while the researchers were investigating whether the gene, known as matrix metalloproteinase-1, or MMP-1, gets more active when skin is exposed to artificial sunlight.

They exposed the buttocks of 33 people to artificial sunlight and tested how active the gene was before and after exposure to ultra violet rays. As that part of the body had had not been exposed to sun, it allowed for the greatest change during the experiment.

The scientists were surprised to find that before going under the sun lamp, the MMP-1 gene was very active in some bottoms, but undetectable in others. The researchers discovered the difference between the two groups was whether they smoked.

``When you smoke, there's extra activity in the lungs and that has a knock-on effect,'' said Gillian Murphy, a cellular biologist who studies MMP but was not involved in the research. ``The lung is such a big organ that molecules from the tobacco smoke and the repair process get into the bloodstream and travel to the skin.''

The effect on the skin may be similar to that from sun exposure, she said.

Collagen is part of a scaffold that includes elastin and a lubricant to keep skin intact. MMP-1 is one of a small group of enzymes that cells use to gently renew themselves. It breaks down collagen; other substances control its concentration, while some promote collagen building.

The better the collagen structure, the better the skin, while assaults on the scaffold lead to wrinkles, experts say.

Sunbathing is the biggest known culprit.

It causes oxygen damage and a mild inflammatory reaction in the skin, both of which stimulate the cells to crank out more repair chemicals than normal. Levels of MMP-1 get particularly high.

Scientists believe part of the problem is that repeated stress disturbs the natural balance of those substances. It takes longer to build collagen structures than to break them down and eventually the collagen building can't keep up.

Leyden said screwing up the face while smoking and pursing the lips to drag on a cigarette are also considered to contribute to premature wrinkling. Smoking has also been found to trigger oxygen damage to the cells and to mess up the blood flow to the skin.

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