NAD+ loss, a new player in AhR biology: prevention of thymus atrophy and hepatosteatosis by NAD+ repletion

Abstract

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD) is a carcinogenic and highly toxic industrial byproduct that persists in the environment and produces a pleiotropic toxicity syndrome across vertebrate species that includes wasting, hepatosteatosis, and thymus atrophy. Dioxin toxicities require binding and activation of the aryl hydrocarbon receptor (AhR), a ligand activated transcription factor. However, after nearly 50 years of study, it remains unknown how AhR activation by dioxin produces toxic effects. Here, using the chick embryo close to hatching, a well-accepted model for dioxin toxicity, we identify NAD+ loss through PARP activation as a novel unifying mechanism for diverse effects of dioxin in vivo. We show that NAD+ loss is attributable to increased PARP activity in thymus and liver, as cotreatment with dioxin and the PARP inhibitor PJ34 increased NAD+ levels and prevented both thymus atrophy and hepatosteatosis. Our findings additionally support a role for decreased NAD+ dependent Sirt6 activity in mediating dioxin toxicity following PARP activation. Strikingly, treatment in vivo with the NAD+ repleting agent nicotinamide, a form of vitamin B3, prevented thymus atrophy and hepatosteatosis by dioxin and increased sirtuin activity, providing a therapeutic approach for preventing dioxin toxicities in vivo.

Therapeutic Potential of NAD-Boosting Molecules: The In Vivo Evidence

“…Olaparib treatment raises NAD + levels and prevents lipopolysaccharide (LPS)-induced acute hepatitis (Gariani et al, 2017). NAM and another PARP inhibitor, PJ34, are both effective in increasing NAD + levels, and preventing hepatosteatosis and thymus atrophy, in a chick embryo model of dioxin-toxicity (Diani-Moore et al, 2017). …”

Methods to Study TCDD-Inducible Poly-ADP-Ribose Polymerase (TIPARP) Mono-ADP-Ribosyltransferase Activity

“…Increased TIPARP and PARP1 activity have been proposed to be important in augmenting dioxin toxicity through the depletion of NAD + . Indeed, NAD + repletion or treatment with the pan-PARP inhibitor PJ34 can prevent dioxin-induced thymic atrophy and hepatosteatosis in a chicken embryo model (Diani-Moore, et al, 2017). However, we observed decreased hepatic NAD + levels in dioxin-treated Tiparp fl/fl Cre Alb mice, suggesting that PARP1 or an NAD + -consuming enzyme other than TIPARP is responsible for the reduced NAD + levels after dioxin treatment in mice.…”

“…9G). Because dioxin toxicity has been tightly linked with NAD + levels, its precursors and metabolites, and because TIPARP activity is dependent on NAD + , we examined NAD + metabolism (Diani-Moore et al, 2010;Diani-Moore et al, 2017;He et al, 2013). Dioxin-dependent increases in nicotinamide ribonucleoside (Fig.…”

Roles of Nicotinamide Adenine Dinucleotide (NAD+) in Biological Systems

“…CD38 levels increase during aging, exerting a high NADase activity [52][53][54]. Mitochondrial depletion of NAD + has been linked to ageing and various human diseases [8,[55][56][57][58][59][60][61]. There are relatively independent pools of NAD + in the nucleus, cytosol, and mitochondria [60] in eukaryotes.…”

“…NAD + depletion has been also linked to the activation of PARP1 and excessive PARylation following dioxin binding to the AhR nuclear receptor [56], followed by decreased SIRT6 activity: providing the cells with additional NAD + or even the precursor, NAM, was sufficient to restore cell function and vitality [57]. Various roles have been assigned to O-acetyl-ADP-ribose, in particular in the assembly of the SIR complex [59,60].…”