We report the effect of UVA irradiation on collagen metabolism of fibroblasts, including both synthesis of the collagen degrading enzyme collagenase and de novo synthesis of type I collagen as the major structural component of the dermis. For this purpose confluent fibroblast monolayers were irradiated under standardized conditions (5, 15, 35, 60 J/cm2 using UVASUN 3000, Mutzhas, Munich, FRG, and UV source Sellas sunlight type 2.001, Sellas, Gevelsberg, FRG). Subsequently, total RNA was isolated and subjected to dot blot and northern blot analysis using oligolabelled cDNA clones for human type I collagen, collagenase and beta-actin. Collagen type I and beta-actin mRNA levels remained unaltered following irradiation, suggesting that the synthetic pathway of collagen metabolism at the pretranslational level is not affected by short-term UVA irradiation. However, collagenase mRNA was found to be dose-dependently induced in fibroblasts after irradiation, thus probably contributing to the actinic damage to the dermis. These in vitro data were confirmed in vivo using in situ hybridization on frozen sections of biopsy material obtained from UVA irradiated patients.

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UVA irradiation leads to photoaging including clinical features such as wrinkle formation, reduced recoil capacity and blister formation of the skin. Besides synthesis of the extracellular matrix, its regulated degradation by various matrix-metalloproteinases (MMPs) determines the amount and the composition of the extracellular matrix within the dermis and the basement membrane of the dermo-epidermal junction. In this study we therefore ascertained whether UV irradiation could modulate the synthesis of MMPs with substrate specificities for dermal (collagen I, III, V) and basement membrane compounds (collagen IV, VII, proteoglycans, laminin) and whether synthesis of the counteracting tissue inhibitor of metalloproteinases (TIMP-1) was also affected. Following UVA irradiation specific mRNAs of MMPs 1, 2 and 3 were induced concomitantly up to 5-fold compared to mock irradiated controls. In contrast, TIMP-1 mRNA levels remained unaltered. Immunoprecipitation indicated that after UVA irradiation synthesis and secretion of MMPs 1, 2 and 3 into the supernatant increased. Taken together, our data show that UVA irradiation coordinately induced MMPs 1, 2 and 3 implying similar mechanisms in their regulatory pathways, while TIMP-1 synthesis was not altered. Hence, unbalanced synthesis of MMPs potentially contributes to the dissolution of dermal and basement membrane compounds finally leading to blister formation and cutaneous photoaging.