Coordinated nicotinamide metabolism by NAMPT and NNMT: Integrating metabolic, epigenetic and immune crosstalk

Cancer cells adapt to microenvironmental cues through the coordinated integration of metabolic reprogramming, epigenetic remodeling, and immune regulation. An emerging central regulator of this adaptive network is coordinated nicotinamide metabolism, which controls both nicotinamide adenine dinucleotide (NAD) biosynthesis and cellular methylation potential. Within this network, the nicotinamide-metabolizing enzymes nicotinamide phosphoribosyltransferase (NAMPT) and nicotinamide N-methyltransferase (NNMT) govern the metabolic fate of nicotinamide (NAM), thereby coupling energy metabolism to epigenetic regulation and adaptive cellular responses. NAMPT, the rate-limiting enzyme of the NAD salvage pathway, sustains intracellular NAD pools required for redox homeostasis, mitochondrial function, DNA repair, and stress responses. Conversely, NNMT diverts NAM from NAD biosynthesis by catalyzing its methylation while consuming S-adenosylmethionine (SAM), thereby creating a methylation sink that remodels chromatin organization and promotes transcriptional plasticity. Beyond intracellular metabolism, coordinated nicotinamide metabolism extends to the tumor microenvironment through extracellular mediators, including extracellular NAMPT (eNAMPT), the emerging extracellular functions of NNMT (eNNMT) and its metabolite 1-methylnicotinamide (1-MNA), which regulate stromal remodeling, immune cell function, chronic inflammation, and immune evasion. Collectively, these complementary intracellular and extracellular activities establish a multilevel adaptive network that integrates metabolic homeostasis, epigenetic remodeling, and immunometabolic communication, thereby promoting tumor plasticity, therapeutic adaptation, and resistance. Here, we propose a conceptual framework in which NAMPT and NNMT function as complementary regulators of coordinated nicotinamide metabolism operating across interconnected intracellular and extracellular levels. By integrating metabolic, epigenetic, and immune crosstalk into a unified model of tumor adaptation, this perspective provides new insights into tumor progression and therapy resistance while highlighting context-dependent opportunities for mechanism-based combination therapies in cancer.