The Functions And Mechanisms Of The Longevity Gene Sirt1 In The Process Of Human Skin Photoaging

The functions and mechanisms of the longevity gene SIRT1 in the process of human skin pHotoagingSkin pHotoaging is a long term process that results mainly from chronic exposure to solar UVA irradiation. Due to its higher penetration, about 35-50%of UVA reaches to dermis and plays a major role in dermal damage of collagen and elastic fibers. UVA irradiation not only contributes to pHotoaging but also to pHotocarcinogenesis. It has been the focus at present to study the mechanisms, preventions and treatments of skin pHotoaging.SIRT1, which has been called the "human longevity gene" protein, plays a key role in aging process. SIRT1 is a NAD+-dependent deacetylases with both p53 and histones as its pHysiological substrates. Current findings suggest that the effect of SIRT1 may be dependent on both cell type and the context of cellular stress. In our study, the expression of SIRT1 induced by UVA irradiation will be investigated in human skin. To further explore the mechanisms of SIRT1 in skin pHotoaging we asked whether SIRT1 regulates p53 signaling pathway by deacetylation and regulate the length of telomere in vitro. As the close relationship between cellular senescence and carcinogenesis, the further study of SIRT1 expression and effects will be helpful to demonstrate the mechanisms of skin aging and give clues to apoptosis and carcinogenesis.Materials and methodsTwo parts are included in this study. The study in vivo is performed in buttocks of 10 volunteers. Three test sites are as follows:a control site without UVA irradiation; a site with low UVA dose of 50 J/cm2 and a site with high UVA dose of 1000 J/cm2, both exposed to UVA light sources three times a week for 13 weeks. The water of stratum corneum, transepidermal water loss, pH values, the values of L*a*b and M and E are measured pre and post UVA irradiation. Skin samples were obtained after the last time UVA irradiation to study the following parameters as the thickness of stratum corneum and epidermis, changes of collagen and elastic fibers and the expression of MMP-1, SIRT1 and p53 by HE staining, elastic fiber staining and immunohistostaining:The human skin fibroblasts are exposed to low dose UVA once a day for 5 days with total dose of 5 J/cm2. Cellular viability and senescence was evaluated by MTT and SAβ-gal staining, SIRT1 and p53 expression were measured by Western-blot, telomere length by RT-PCR for fibroblasts, which were incubated only or plus resveratrol or nicotinamide.Results1. The results in vivo show:The water of corneum decreased, trans epidermal water loss increases, the values of a*b and M and E increased and L* decreased after UVA irradiation. The thickness of corneum and epidermis increased, collagen and elastic fibers showed morpHologic changes with strong expression of MMP-1.2 The expressions of SIRT1 were found in normal skin. A dose-dependent increase was observed within the range of 50 J/cm2-1000 J/cm2UVA.3. SIRT1 is functionally expressed in cultured skin fibroblasts. The sublethal dose (5 J/cm2) UVA irradiation up-regulates SIRT1 in a dose dependent manner, but the lethal dose (11 J/cm2) UVA down-regulates its expression.4. The acetylation of p53 induced by UVA is abolished by SIRT1 activator resveratrol.5. UVA-induced short telomere length is enhanced by SIRT1 activator resveratrolConclusion1. A dose-dependent skin pHotoaging can be induced by UVA irradiation.2. The expression of SIRT1 increases in the process of skin pHotoaging induced by UVA irradiation. The up-regulated expression of SIRT1 may represent a feedback to UVA irradiation.3. SIRT1 plays a protective role in skin pHotoaging by negative regulation of p53 acetylation.4. SIRT1 functions to inhibit cell senescence may via protection of telomere length.