Dark skin protects against photoaging

New York - Increased pigmentation in people with darker skin types appears to protect against photoaging by mitigating ultraviolet (UV) light-mediated induction of matrix-degrading metalloproteinases (MMPS), says Sewon Kang, M.D. He reported on the findings of his research at the American Academy of Dermatology's Academy '04.

To investigate the mechanism underlying the clinical observation that ethnic and racial groups with more darkly pigmented skin are relatively protected against photoaging, Dr. Kang and his colleagues at the University of Michigan evaluated changes in MMP expression in response to UV irradiation of skin in normal subjects. The results showed MMP induction decreased as skin color darkened. At the light doses studied, the most darkly pigmented individuals exhibited virtually no changes in MMP enzyme expression.

MMP an indicator "Based on the model of photoaging we have developed that implicates MMP induction as the culprit, then measurement of MMP responses to UV irradiation can be used as a surrogate for studying the risk of photoaging," says Dr. Kang, professor of dermatology, University of Michigan, Ann Arbor. "The reduced MMP induction we have observed fits with clinical observations about skin-type-related differences in photoaging and suggests that melanin in the skin acts as a very good, endogenous sunscreen, blocking UV transmission through the skin and inhibiting the UV signaling that eventuates in MMP induction."

In their study, skin color was measured with a chromometer and subjects were divided into three arbitrarily defined groups of light, moderate and dark. Their skin was exposed to UV irradiation over a range of doses, and pre- and post-treatment punch biopsies were performed to obtain skin samples for evaluating changes in MMP expression.

Induction varies The results showed significant, dose-related MMP induction in the light skin group while virtually no change was observed in the dark group. The response in the moderate color group, which included Asians, was closer to that of the dark group. However, a dose-response was observed with increases in MMP levels at higher UV doses.

Dr. Kang points out that the lack of change observed in the dark group should not be interpreted to mean that such individuals are completely protected from UV-induced MMP induction. Instead, he proposes that observation might reflect the limitations of the study design, which for practical reasons limited the maximum UV dose that could be delivered.

"While they enjoy some advantage because of the greater amount of pigment in their skin, sun protection is still important," Dr. Kang says.