Involvement of Sirt1 in ultraviolet radiation mediated damages and transformation of skin keratinocytes

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Ultraviolet (UV) radiation is a well-established causative factor for non-melanoma skin cancers, precancerous conditions such as actinic keratoses and skin aging. Further, like several other cancers, skin cancer is believed to be associated with aging. Therefore, it may be useful to decipher the molecular determinant(s) that connect skin aging and skin cancer. Sirtuins are nicotinamide adenine dinucleotide (NAD(+))-dependent deacetylases which have been implicated in the process of aging. Studies are increasingly suggesting that Sirtuins may be involved in cancer development. Sirt1 is the most well studied member of sirtuin family that has been shown to be linked to cell longevity through a number of biological functions such as gene silencing, apoptosis, fatty acid metabolism, oxidative stress and regulation of cellular life span. In this study, we tested the hypothesis that Sirt1 plays a critical role in the UV response including UV-induced transformation of skin keratinocytes. To test this hypothesis, we examined the effect of UVA (0.1 to 0.4 J/cm²) and UVB (10 to 200 mJ/cm²), alone or in combination, on Sirt1 in normal human epidermal keratinocytes (NHEK), hTERT immortalized keratinocytes (N/TERT-1) and HaCaT immortalized keratinocytes. Studies have shown that repeated UVA and UVB exposures leads to a transformed phenotype in HaCaT keratinocytes. Single UVA or UVB treatment to NHEK cells resulted in an initial increase followed by a decrease in Sirt1 protein levels. Interestingly, single UVA or UVB treatment to N/TERT-1 or HaCaT cells resulted in a dose-dependent decrease in Sirt1 protein levels as well as Sirt1 activity. Further, repeated exposures to N/TERT-1 and HaCaT keratinocytes with a combination of UVA and UVB resulted in an initial decrease in Sirt1 protein. However, Sirt1 protein and activity levels were found to be appreciably upregulated when keratinocytes became transformed. This increased Sirt1 activity was associated with decrease in p53 levels in both N/TERT-1 cells which contain normal p53 as well as HaCaT cells with mutated p53. Further, treatment of UV-transformed N/TERT-1 and HaCaT keratinocytes with Sirt1 inhibitor nicotinamide resulted in an induction of cell cycle arrest and apoptosis. Taken together, our data suggested that Sirt1 plays an important role in UV response including UV-induced keratinocyte cell transformation. Based on our study, we suggest that Sirt1 manipulation should be evaluated as a strategy for the management of UV-damages including skin cancer and skin aging.