NAD in skin: therapeutic approaches for niacin.

The maintenance and regulation of cellular NAD(P)(H) content and its influence on cell function involves many metabolic pathways, some of which remain poorly understood. Niacin deficiency in humans, which leads to low NAD status, causes sun sensitivity in skin, indicative of deficiencies in responding to UV damage. Animal models of niacin deficiency demonstrate genomic instability and increased cancer development in sensitive tissues including skin. Cell culture models of niacin deficiency have allowed the identification of NAD-dependent signaling events critical in early skin carcinogenesis. Niacin restriction in immortalized keratinocytes leads to an increased expression and activity of NADPH oxidase resulting in an accumulation of ROS, providing a potential survival mechanism as has been shown to occur in cancer cells. Niacin deficient keratinocytes are more sensitive to photodamage, as both poly(ADP-ribose) polymerases and Sirtuins are inhibited by the unavailability of their substrate, NAD+, leading to unrepaired DNA damage upon photodamage and a subsequent increase in cell death. Furthermore, the identification of the nicotinic acid receptor in human skin keratinocytes provides a further link to niacin's role as a potential skin cancer prevention agent and suggests the nicotinic acid receptor as a potential target for skin cancer prevention agents. The new roles for niacin as a modulator of differentiation and photo-immune suppression and niacin status as a critical resistance factor for UV damaged skin cells are reviewed here.