Restoring NAD by NAMPT is essential for the SIRT1/p53-mediated survival of UVA- and UVB-irradiated epidermal keratinocytes.

Nicotinamide adenine dinucleotide (NAD) is a crucial coenzyme in energy production. The imbalance of NAD synthesis has been found to trigger age-related diseases, such as metabolic disorders, cancer, and neurodegenerative diseases. Also, UV irradiation induces NAD depletion in the skin. In mammals, nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the NAD salvage pathway and essential for NAD homeostasis. However, but few studies have focused on the role of NAMPT in response to UV irradiation. Here, we show that NAMPT prevents NAD depletion in epidermal keratinocytes to protect against the mild-dose UVA and UVB (UVA/B)-induced proliferation defects. We showed that poly(ADP-ribose) polymerase (PARP) inhibitor rescued the NAD depletion in UVA/B-irradiated human keratinocytes, confirming that PAPR transiently exhausts cellular NAD to repair DNA damage. Notably, the treatment with a NAMPT inhibitor exacerbated the UVA/B-induced loss of energy production and cell viability. Moreover, the NAMPT inhibitor abrogated the sirtuin-1 (SIRT1)-mediated deacetylation of p53 and significantly inhibited the proliferation of UVA/B-irradiated cells, suggesting that the NAMPT-NAD-SIRT1 axis regulates p53 functions upon UVA/B stress. The supplementation with NAD intermediates, nicotinamide mononucleotide and nicotinamide riboside, rescued the UVA/B-induced phenotypes in the absence of NAMPT activity. Therefore, NAD homeostasis is likely essential for the protection of keratinocytes from UV stress in mild doses. Since the skin is continuously exposed to UVA/B irradiation, understanding the protective role of NAMPT in UV stress will help prevent and treat skin photoaging.