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.