β-Nicotinamide Mononucleotide Enhances Skin Barrier Function and Attenuates UV-B-Induced Photoaging in Mice

Ultraviolet B (UV-B) radiation significantly contributes to skin photoaging, which is characterized by epidermal thickening, collagen degradation, wrinkle formation, barrier dysfunction, and oxidative stress. Nicotinamide mononucleotide (NMN), a key precursor of nicotinamide adenine dinucleotide, regulates cellular energy metabolism and antioxidant defense and demonstrates anti-aging effects in animal models. Here, we investigated the protective effects of oral NMN supplementation against UV-B-induced photoaging in SKH-1 hairless mice. Over a 10-week experimental period, oral NMN administration significantly alleviated epidermal hypertrophy, reduced wrinkle formation and skin surface roughness, improved hydration and elasticity, and restored transepidermal water loss to near-normal levels. Histological analyses revealed marked preservation of collagen fiber density and attenuation of dermal matrix degradation. Furthermore, NMN supplementation inhibited the phosphorylation of MAPK signaling components (ERK, JNK, and p38), suppressed pro-inflammatory cytokine (TNF-α and IL-6) and matrix-degrading enzyme (MMP-1) expression, and restored hyaluronan synthase (HAS-1 and HAS-2) expression. Additionally, NMN enhanced the systemic antioxidant defense, as indicated by the restored superoxide dismutase activity. Thus, NMN has multi-layered protective effects against UV-B-induced skin aging by modulating oxidative stress, inflammatory signaling, extracellular matrix remodeling, and hyaluronic acid metabolism.