How Sunlight Ages Skin

Take a look at a long-haul trucker sometime, a guy who's been driving for decades. Look closely at his face. One side will have more wrinkles than the other. Guess which one?The left side, the side of his face most exposed to the open window. Do you know why it has more wrinkles? Because it absorbs more direct sunlight than the right side of his face that's shaded inside the truck cab.

Look at the face of a long-haul trucker from a country like Great Britain, where people drive on the left side of the road. The right side of his face has more wrinkles because that's the side that faces the open window.

We're not talking lying in the sun here. We're not talking sunburn. We're talking chronic, long-term exposure to micro-doses of ultraviolet light that never overtly damages the skin, but over the years causes a collection of micro-scars that leaves a telling impression: wrinkles.

The epidermis, the outer layer of skin, "is as thin as a sheet of paper," says John J. Voorhees, M.D., chairman of dermatology at the University of Michigan Medical School. "Ninety percent of the mass of the skin is collagen," a large protein composed of three intertwined chains of amino acids that contributes to the form, function and strength of the skin. That also makes collagen the principal recipient of ultraviolet light damage.

But the pathway to aged skin is not straightforward. Sunlight itself does little direct damage to the collagen protein. A growing body of research shows, instead, that ultraviolet light turns genes "on" and "off"--and which genes get turned on can make all the difference.

Normal skin maintains a dynamic collagen exchange. A common type of skin cell called a fibroblast exudes new layers of collagen when collagen genes are turned on. When collagen is damaged, skin cells produce enzymes that digest and liquefy the large collagen proteins into gelatin for disposal.

Voorhees' group discovered a complex genetic pathway through which sunlight can suppress collagen production by turning off the collagen-producing genes. At the same time sunlight activates collagen digestion by stimulating production of the destructive enzymes.

Damaged skin results. The skin now carries a wound, and it needs to heal. "Anytime you cut yourself more than superficially, there is always a little bit of a scar," Voorhees says. "Our claim is that wound healing is never perfect. It could be 99.9 percent perfect, but never perfect. And that 99.9 percent [healing after sun damage] is going to lead to the slightest imperfection that is not visible to the human eye, but after thousands of these over a lifetime, the micro-scars become macro-scars. This is the UV-induced aging we call photo-aging, and it is piled on top of natural aging that has nothing to do with the sun."

Prematurely wrinkled skin results. Although FDA has approved retinoic acid to treat chronic photo-aging, prevention remains the more effective approach.

Here's the really tricky part: Most of the genetic changes and resulting photo-aging appear to come from so-called UVA, the wavelengths of ultraviolet light in the A band of the spectrum. Most sunscreens currently on the market provide excellent protection against UVB, but not all provide equally good protection against UVA. "If you put on gobs of sunscreen, it blocks" the damage, Voorhees says. "But if you don't use much, it does not block [the damage] at all."

Moreover, sunlight turns on the genetic destruction quickly, but it also stops quickly when you get out of the sun. The level of collagen production is completely back to normal in two days.

"The average person thinks, 'I didn't get pink so I have no photo-aging,'" Voorhees says. "Our data suggest that is not true. You are going to be getting the photo-aged [signals that turn genes on and off] and develop the micro-scars without getting any pinkness at all. You can get photo-aged damage long before you get pink or sunburned."