Extracellular NAD+ Broken Down by Human Cells to Maintain Intracellular NAD+ Levels: Preclinical Findings

Synopsis

Nicotinamide adenine dinucleotide (NAD+) is vital for cellular energy and signaling, and human cells replenish NAD+ by using extracellular precursors like nicotinamide (NAM), nicotinic acid, nicotinamide riboside (NR), and nicotinic acid riboside (NAR), not collectively referred to as vitamin B3. Other precursors such as extracellular NAD+, nicotinamide mononucleotide (NMN), and related molecules might also contribute to NAD+ inside cells. However, it’s unclear whether these nucleotides enter cells directly or must first break down into smaller parts. This study used human HEK293 cells in different culture conditions to investigate how extracellular NAD+ and its derivatives are metabolized. In medium with fetal bovine serum (FBS), enzymes degrade nucleotides to nucleosides and then to bases, even without cells present, showing that FBS itself can break down NAD+ intermediates. NR was also broken down to NAM without FBS. In serum-free medium, cells cleaved NAD+ and related molecules into smaller precursors like NMN, which was stable but partly converted to NR. Blocking cell membrane transporters showed that breaking down NAD+ and NMN to NR outside the cell is needed for cells to maintain NAD+ levels. NR inside cells was further converted to NAM. The study reveals that both cells and culture medium actively convert NAD+ precursors, meaning experimental conditions must be carefully controlled in NAD+ supplementation studies.

Journal

Metabolites