Supplementation of Nicotinic Acid and Its Derivatives Up-Regulates Cellular NAD⁺ Level Rather than Nicotinamide Derivatives in Cultured Normal Human Epidermal Keratinocytes

Nicotinamide adenine dinucleotide (NAD(+)) plays a pivotal role in various physiological processes within mammalian cells, including energy metabolism, redox homeostasis, and genetic regulation. In the majority of mammalian cellular contexts, NAD(+) biosynthesis primarily relies on vitamin B3, including nicotinamide (NAM) and nicotinic acid (NA). The concept of NAD(+) augmentation therapy has recently emerged as a promising strategy to mitigate aging-associated phenomena, termed rejuvenation. Despite the involvement of diverse enzymatic cascades in NAD(+) biosynthesis, certain cellular environments exhibit deficiencies in specific enzymes, suggesting cell type-dependent variability in optimal NAD(+) precursor selection. However, the optimization of NAD(+) precursors for topical formulations has received scant attention thus far. In the present investigation, we sought to delineate the most efficacious precursor for augmenting NAD(+) levels in human skin keratinocytes. Remarkably, NA supplementation led to a significant 1.3-fold elevation in intracellular NAD(+) levels, even in the presence of nicotinamide phosphoribosyltransferase inhibition by FK866. Additionally, NA mononucleotide demonstrated a 1.5-fold increase (but not significant) in NAD(+) levels following 100 mu M application. Conversely, NAM and its derivatives failed to elicit a NAD(+) response in keratinocytes. Notably, NA supplementation elicited up-regulation of mitochondrial superoxide dismutase (SOD2) and sirtuin 3 (SIRT3), indicative of its beneficial impact on mitochondrial function. Furthermore, NA mitigated rotenone-induced mitochondrial reactive oxygen species (ROS) accumulation. Collectively, these findings advocate for the potential utility of NA in topical applications aimed at skin rejuvenation.