Infrared plus visible light and heat from natural sunlight participate in the expression of MMPs and type I procollagen as well as infiltration of inflammatory cell in human skin in vivo.

BACKGROUND: Compared with the detailed characterization of the ultraviolet (UV) response in human skin, the effects of infrared (IR) and other regions of the sunlight are scarce.

OBJECTIVES: To determine the participation of IR/visible light and heat components of the sunlight on matrix metalloproteinases (MMPs) and type I procollagen expression, and inflammatory cell infiltration in human skin in vivo.

METHODS: The buttocks of 16 healthy volunteers (aged 24-43 years, 10 male and 6 female) were irradiated with a 1.1-3 minimal erythema dose (MED) of natural sunlight. To determine the differential effects of UV, IR/visible rays and solar heat alone, the exposed sites were covered with either a UV filter or black cloth, respectively, during irradiation. Skin samples were taken 24h later.

RESULTS: IR/visible light spectrum of sunlight significantly increased MMP-1 and MMP-9 expression and decreased type I procollagen expression. Solar heat also contributed to the increased MMP-1 expression. Only the UV region recruited neutrophils into the dermis, while UV, IR/visible light and heat contributed to macrophage infiltration.

CONCLUSIONS: IR/visible light and heat of natural sunlight, in addition to UV, play a role in modulating the expressions of MMPs and procollagen, and inflammatory cell infiltration in human skin.