Aryl hydrocarbon receptor (AHR) functions in NAD+ metabolism, myelopoiesis and obesity

Diverse physiologic functions of AHR, a transcription factor discovered in studies of dioxin toxicity, are currently elucidated in many laboratories including chemical and microbial defense, immunity and myelopoiesis. Accumulating evidence suggests that AHR may also be involved in obesity and TCDD-mediated lethality in sensitive species. Underlying mechanisms include NAD⁺- and sirtuin-mediated deregulation of lipid, glucose and NAD⁺ homeostasis. Progress in NAD metabolome research suggests large consumption of NAD⁺ by NAD glycohydrolases (NADases) and NAD-dependent sirtuins. In focus are two NADases: (i) TiPARP (TCDD-induced poly(ADP-ribose) polymerase), one of several nuclear NADases, and (ii) plasma membrane-bound ectoNADase/CD38, a multifunctional enzyme and receptor. CD38 is involved in extra- and intracellular NAD degradation but acts also as differentiation marker. Both CD38 and AHR are components of a complex signalsome that enhances retinoic acid-induced differentiation of myeloid progenitor cells to granulocytes. Further advances of NAD metabolome research may lead to therapeutic options in the control of obesity and to improved risk assessment of TCDD toxicity.

NAD Metabolome Analysis in Human Cells Using ¹H NMR Spectroscopy

Nicotinamide adenine dinucleotide (NAD) and its phosphorylated form, NADP, are the major coenzymes of redox reactions in central metabolic pathways. Nicotinamide adenine dinucleotide is also used to generate second messengers, such as cyclic ADP-ribose, and serves as substrate for protein modifications including ADP-ribosylation and protein deacetylation by sirtuins. The regulation of these metabolic and signaling processes depends on NAD availability. Generally, human cells accomplish their NAD supply through biosynthesis using different forms of vitamin B3: Nicotinamide (Nam) and nicotinic acid as well as nicotinamide riboside (NR) and nicotinic acid riboside (NAR). These precursors are converted to the corresponding mononucleotides NMN and NAMN, which are adenylylated to the dinucleotides NAD and NAAD, respectively. Here, we have developed an NMR-based experimental approach to detect and quantify NAD(P) and its biosynthetic intermediates in human cell extracts. Using this method, we have determined NAD, NADP, NMN and Nam pools in HEK293 cells cultivated in standard culture medium containing Nam as the only NAD precursor. When cells were grown in the additional presence of both NAR and NR, intracellular pools of deamidated NAD intermediates (NAR, NAMN and NAAD) were also detectable. We have also tested this method to quantify NAD+ in human platelets and erythrocytes. Our results demonstrate that ¹H NMR spectroscopy provides a powerful method for the assessment of the cellular NAD metabolome.