JOURNAL OF COSMETIC SCIENCE 62 DISCUSSION Although most sirtuin research has focused on their effects on longevity, our approach has been to measure temporal changes in sirtuins in skin cells and also as a result of environ- mental challenge because the skin is the fi rst line of defense against the environment. Deleterious changes sustained over time in skin due to environmental insult can lead to photoag ing and increased visible signs of aging. We report here that sirt1 and sirt6 in NHEK follow a defi nite temporal pattern that is similar to the pattern that we had previ- ously observed for sirt3. These data support the fi ndings of Asher e t al. (11) who showed that SIRT1 follows a circadian pattern. Further, decreases in sirtuin expression after expo- sure to a non-cytotoxic dose of UVB were also measured which disrupted their temporal pattern. Since s irt6, unlike sirt1and 3, increased after 6 h under normal conditions and then was the fi rst to recover after UVB exposure, may indicate its importance for DNA repair (12). In parallel to these changes in sirtuin expression, we also observed a similar temporal pat- tern of ATP levels in NHEK. However, after irradiation, this pattern was signifi cantly altered, as in the sirtuins. Part of the reason for this may be attributable to a reduction in SIRT3 and the inactivation of acetyl CoA synthetase, which would normally lead to in- creased ATP synthesis (13). Additio nally, if SIRT3 is unable to deacetylate SOD2, there will be a concomitant increase in ROS (14) that we also observed. These results show how less obvious targets of sun exposure, such as sirtuins, may have a more subtle and long-term effect on skin. Our data present for the fi rst time a unifi ed modality for sirtuin expression and the effects that UVB has on their expression. Intrigu- ingly, the disruption of energy levels also points to the role that sirtuins play in metabo- lism and how low levels of UVB may quickly affect metabolic regulation. Taken together, these results further emphasize the importance of protection against sun exposure and may suggest a new paradigm for determining environmentally induced aging. Finally , our results show the importance of sustaining sirtuin levels and developing new cosmetic products in support of repair and metabolic mechanisms for a healthy skin. REFERENCES (1) Y. L. Deribe, T. Pawson, and I. Dikic, Post-translational modifi cations in signal integration, Nat. Struct. Mol. Biol., 17, 666–672 (2 010). (2) M. Kaeberlein, M. McVey, and L. Guarente, The SIR2/3/4 complex and SIR2 alone promote longevity in Saccharomyces cerevisiae by two different mechanisms, Genes Dev., 13, 2570–2580 ( 1999). (3) L. Guarente, Sir2 links chromatin silencing, metabolism, and aging, Genes Dev., 14, 1021–1026 (2000). (4) K. Dong, E. Pelle, D. B. Yarosh, and N. Pernodet, Sirtuin 4 identifi cation in normal human epidermal keratinocytes and its relation to sirtuin 3 and energy metabolism under normal conditions and UVB- induced stress, Exp. Dermatol., 21, 231–233 (2012). (5) J. T. McCarthy, E. Pelle, K. Dong, K. Brahmbhatt, D. Yarosh, and N. Pernodet, Effects of ozone in normal human epidermal keratinocytes, Exp. Dermatol., 22, 360–361 (2013). (6) A. A. Sauve, C. Wolberger, V. L. Schramm, and J. D. Boeke, The biochemistry of sirtuins, Annu. Rev. Biochem., 75, 435–465 (2006). (7) E. Pelle and N. Pernodet, “Sirtuin s: Biology and Anti-Aging Benefi ts for Skin Care,” in: Harry’s Cosme- tology, 9th Ed., M. Rosen. Ed. (in press). (8) T. G. Polefka, T. A. Meyer, P. P. Agin, and R. J. Bianchini, Effects of solar radiation on the skin, J. Cosmet. Dermatol., 11, 134–143 (2012). (9) M. C. Haigis and L. P. Guarente, Mammalian sirtuins–emerging roles in physiology, aging, and calorie restriction, Genes Dev., 20, 2913–2921 (2006).

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