Nicotinamide adenine dinucleotide (NAD⁺) is the fundamental molecule that performs numerous biological reactions and is crucial for maintaining cellular homeostasis. Studies have found that NAD⁺ decreases with age in certain tissues, and age-related NAD⁺ depletion affects physiological functions and contributes to various aging-related diseases. Supplementation of NAD⁺ precursor significantly elevates NAD⁺ levels in murine tissues, effectively mitigates metabolic syndrome, enhances cardiovascular health, protects against neurodegeneration, and boosts muscular strength. Despite the versatile therapeutic functions of NAD⁺ in animal studies, the efficacy of NAD⁺ precursors in clinical studies have been limited compared with that in the pre-clinical study. Clinical studies have demonstrated that NAD⁺ precursor treatment efficiently increases NAD⁺ levels in various tissues, though their clinical proficiency is insufficient to ameliorate the diseases. However, the latest studies regarding NAD⁺ precursors and their metabolism highlight the significant role of gut microbiota. The studies found that orally administered NAD⁺ intermediates interact with the gut microbiome. These findings provide compelling evidence for future trials to further explore the involvement of gut microbiota in NAD⁺ metabolism. Also, the reduced form of NAD⁺ precursor shows their potential to raise NAD⁺, though preclinical studies have yet to discover their efficacy. This review sheds light on NAD⁺ therapeutic efficiency in preclinical and clinical studies and the effect of the gut microbiota on NAD⁺ metabolism.

Decreased nicotinamide adenine dinucleotide (oxidized form) (NAD ⁺ ) levels are reportedly associated with several aging-related disorders. Thus, supplementation with NAD ⁺ precursors, such as nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR), exhibits beneficial effects against these disorders. However, the in vivo metabolic pathways of NMN and NR remain to be elucidated. In this study, we comprehensively analyzed the fate of orally and intravenously administered NMN and NR in mice using NAD ⁺ metabolomics. We found that only a small portion of orally administered NMN and NR was directly absorbed from the small intestine and that most of them underwent gut microbiota–mediated deamidation and conversion to nicotinic acid (NA). Moreover, intravenously administered NMN and NR were rapidly degraded into nicotinamide and secreted to bile followed by deamidation to NA by gut microbiota. Thus, enterohepatic circulated NA is preferentially used in the liver. These findings showed that NMN and NR are indirectly converted to NAD ⁺ via unexpected metabolic pathways.

Background: Apolipoprotein C-III (APOC3) plays a crucial role in triglyceride metabolism and is closely associated with cardiovascular disease risk. Elevated APOC3 levels contribute to higher plasma triglycerides and increased risk of atherosclerosis, making APOC3 expression an attractive and logical therapeutic target. Methods: While studying various APOC3 transcript isoforms expressed in hepatoma cell lines (HepG2, Huh7) and healthy liver tissue using publicly available long-read RNA sequencing, we found three novel APOC3 isoforms. These isoforms were validated through RT-PCR and Sanger sequencing. Results: All three novel isoforms are splicing variants of the MANE transcript, APOC3-201. Isoforms 1 and 2 exhibit splicing patterns similar to APOC3-201 from exons 2–4; however, isoform 1 shares its exon 1 splicing pattern with APOC3-203, while isoform 2 features an extended exon 1 that includes exon 1a, the adjacent intronic region, and exon 1b. The third isoform closely resembles APOC3-201, but lacks exon 2, which contains the translation start codon. Remarkably, similar APOC3 splicing patterns and transcript variants were observed in Caco-2 cells, a model of the small intestine, indicating that these isoforms are not liver-specific. Conclusions: This study identifies three novel APOC3 isoforms and highlights their expression in both hepatic and intestinal cell models. Further studies are needed to elucidate the functional roles of these novel isoforms and their contribution to the regulation of APOC3 gene expression.