Effects of Kynurenine Pathway Metabolites on Intracellular NAD⁺ Synthesis and Cell Death in Human Primary Astrocytes and Neurons

Abstract

The kynurenine pathway (KP) is a major route of L-tryptophan catabolism resulting in the production of the essential pyridine nucleotide nicotinamide adenine dinucleotide, (NAD+). Up-regulation of the KP during inflammation leads to the release of a number of biologically active metabolites into the brain. We hypothesised that while some of the extracellular KP metabolites may be beneficial for intracellular NAD+ synthesis and cell survival at physiological concentrations, they may contribute to neuronal and astroglial dysfunction and cell death at pathophysiological concentrations. In this study, we found that treatment of human primary neurons and astrocytes with 3-hydroxyanthranilic acid (3-HAA), 3-hydroxykynurenine (3-HK), quinolinic acid (QUIN), and picolinic acid (PIC) at concentrations below 100 nM significantly increased intracellular NAD+ levels compared to non-treated cells. However, a dose dependent decrease in intracellular NAD+ levels and increased extracellular LDH activity was observed in human astrocytes and neurons treated with 3-HAA, 3-HK, QUIN and PIC at concentrations >100 nM and kynurenine (KYN), at concentrations above 1 μM. Intracellular NAD+ levels were unchanged in the presence of the neuroprotectant, kynurenic acid (KYNA), and a dose dependent increase in intracellular NAD+ levels was observed for TRP up to 1 mM. While anthranilic acid (AA) increased intracellular NAD+ levels at concentration below 10 μM in astrocytes. NAD+ depletion and cell death was observed in AA treated neurons at concentrations above 500 nM. Therefore, the differing responses of astrocytes and neurons to an increase in KP metabolites should be considered when assessing KP toxicity during neuroinflammation.

Effects of sodium benzoate, a widely used food preservative, on glucose homeostasis and metabolic profiles in humans

“…Anthranilic acid accumulates in the setting of renal failure, and one cell culture study suggested that it might promote renal failure through adverse effects on mesangial cells [16]. In a separate study, treatment of cultured neurons and glial cells with anthranilic acid altered NAD+ levels and caused cytotoxicity [17]. Our metabolite profiling also revealed a drop in acetylglycine, which, like hippurate, is a product of glycine N-acyltransferase and is generated through conjugation of acetyl-CoA and glycine.…”

The Janus faces of 3-hydroxykynurenine: Dual redox modulatory activity and lack of neurotoxicity in the rat striatum

“…In addition, the KP has been associated with inflammatory responses in different neurological disorders, and this mechanism, which may be inherent to some KP metabolites, could contribute to the neurodegenerative pattern associated with these diseases (Chen et al, 2010;Tan et al, 2012). Some neuroactive metabolites are formed in the KP, including the well-known neurotoxin and glutamate agonist quinolinic acid (QUIN) (Schwarcz et al, 1984;Guidetti et al, 2006), the neuroprotectant and glutamate antagonist kynurenic acid (KYNA) (Schwarcz et al, 1983;Yu et al, 2004), and the redox active metabolites 3-hydroxykynurenine (3-HK) and 3-hydroxyanthranilic acid (3-HAAO) (Goldstein et al, 2000;Braidy et al, 2009;reviewed by Colín-González et al, 2013).…”

Tryptophan and Cell Death

“…Although 3HAA, 3HK, and QA at low concentrations signifi cantly increase intracellular NAD + levels, at concentrations exceeding 100 nM, they cause a dose-dependent decrease in intracellular NAD + levels. Likewise to NAD + depletion, higher concentrations of anthranilic acid may also cause cell death (Braidy et al 2009a ). Competitive inhibition of IDO activity with 1-methyl-l -tryptophan results in a dose-dependent decrease in intracellular NAD + levels and sirtuin deacetylase-1 (silent mating type information regulation 2 homolog-1, SIRT1) activity.…”