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Photoaging refers to premature skin aging caused by repeated sun exposure. The mechanism of photoaging induced by ultraviolet radiation involves the activation of intracellular signal transduction pathways, epidermal growth factor receptor and cell surface cytokine receptors, followed by an increase in the synthesis of matrix metalloproteinases, degradation of fibrous connective tissue and elastosis as well as a reduction in the synthesis of collagen in dermis. Reactive oxygen species are required for ultraviolet irradiation-induced activation of downstream signaling pathway. Besides, multiple cells such as keratinocytes, fibroblasts and infiltrating cells are involved in the photoaging process.

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