Cellular NAD Status as a Regulator of Skin Photodamage

Abstract

The maintenance and regulation of cellular NAD(P)(H) content and its influence on cell function involves in many metabolic pathways which are poorly understood. Niacin deficiency in humans, which leads to low NAD status, causes sun sensitivity in skin, indicative of DNA repair deficiencies. Animal models of niacin deficiency demonstrate genomic instability and increased cancer development in sensitive tissues including skin. Therefore, we have developed a cell culture model that allows assessment of pathways regulated by NAD(P) content as a way to identify NAD-dependent signaling events that may be critical in early skin carcinogenesis. Using our model, we showed that niacin restriction, and consequent NAD depletion, reversibly alters NAD(P)(H) pools, increases apoptosis, induces G2/M cell cycle arrest, and decreases DNA stability. These alterations are affected by increased expression and activity of NOX leading to an accumulation of ROS, which may provide a survival mechanism as has been shown in cancer cells. Our data also support the hypothesis that glutamine is a likely alternative energy source during niacin deficiency. Here, we also identified the expression of all seven NAD-dependent deacetylase (SIRT) family members in skin cells. We showed that in response to photodamage, the expression of several SIRTs is altered in keratinocytes. Furthermore, we showed that SIRTs responses to photodamage differ between normal and immortalized keratinocytes, which may be indicative of alterations potentially important in skin carcinogenesis. In addition, we have shown that NAD-depleted HaCaT keratinocytes are more sensitive to photodamage. We observed that both poly(ADP-ribose) polymerases (PARPs) and SIRTs are inhibited by the unavailability of their substrate, NAD+, leading to unrepaired DNA damage upon photodamage and subsequent increase in cell death. Our data demonstrate that both SIRTs and PARPs are critical following DNA damage and identify which SIRTs are essential. Finally, we identified for the first time the expression of the nicotinic acid receptor in human skin keratinocytes, mainly in the differentiating keratinocytes of the stratum corneum in the epidermis. This study identifies new roles for niacin as a potential skin cancer prevention agent and demonstrates that niacin status is a critical resistance factor for UV damaged skin cells.