Ross Grant

Age Related Changes in NAD+ Metabolism Oxidative Stress and Sirt1 Activity in Wistar Rats

The cofactor nicotinamide adenine dinucleotide (NAD+) has emerged as a key regulator of metabolism, stress resistance and longevity. Apart from its role as an important redox carrier, NAD+ also serves as the sole substrate for NAD-dependent enzymes, including poly(ADP-ribose) polymerase (PARP), an important DNA nick sensor, and NAD-dependent histone deacetylases, Sirtuins which play an important role in a wide variety of processes, including senescence, apoptosis, differentiation, and aging. We examined the effect of aging on intracellular NAD+ metabolism in the whole heart, lung, liver and kidney of female wistar rats. Our results are the first to show a significant decline in intracellular NAD+ levels and NAD:NADH ratio in all organs by middle age (i.e.12 months) compared to young (i.e. 3 month old) rats. These changes in [NAD(H)] occurred in parallel with an increase in lipid peroxidation and protein carbonyls (o- and m- tyrosine) formation and decline in total antioxidant capacity in these organs. An age dependent increase in DNA damage (phosphorylated H2AX) was also observed in these same organs. Decreased Sirt1 activity and increased acetylated p53 were observed in organ tissues in parallel with the drop in NAD+ and moderate over-expression of Sirt1 protein. Reduced mitochondrial activity of complex I-IV was also observed in aging animals, impacting both redox status and ATP production. The strong positive correlation observed between DNA damage associated NAD+ depletion and Sirt1 activity suggests that adequate NAD+ concentrations may be an important longevity assurance factor.

Murine Glial Cells Regenerate NAD, After Peroxide-Induced Depletion, Using Either Nicotinic Acid, Nicotinamide, or Quinolinic Acid as Substrates

Abstract: The potential for regeneration of intracellular pyridine nucleotide levels from different precursors, after peroxide‐induced NAD depletion, in cultured glial cells was investigated. Cultured murine glial cells showed a decrease in intracellular NAD levels of >40% after treatment with H2O2 (100 µM). Removal of the H2O2 followed by a 2‐h incubation did not result in NAD recovery in the absence of precursors. However, NAD levels increased significantly in these cells after the following substrate additions, at minimum effective concentrations of 1 mM for quinolinic acid (QUIN), 500 µM for nicotinamide, and 2 µM for nicotinic acid. The regeneration of significant amounts of NAD from nicotinic acid at doses 250 and 500 times lower than either nicotinamide or QUIN indicates a preferred route for NAD biosynthesis in glial cells in vitro, probably via nicotinic acid phosphoribosylation.

Neuroprotective effects of naturally occurring polyphenols on quinolinic acid‐induced excitotoxicity in human neurons

Quinolinic acid (QUIN) excitotoxicity is mediated by elevated intracellular Ca2+ levels, and nitric oxide‐mediated oxidative stress, resulting in DNA damage, poly(ADP‐ribose) polymerase (PARP) activation, NAD+ depletion and cell death. We evaluated the effect of a series of polyphenolic compounds [i.e. epigallocatechin gallate (EPCG), catechin hydrate, curcumin, apigenin, naringenin and gallotannin] with antioxidant properties on QUIN‐induced excitotoxicity on primary cultures of human neurons. We showed that the polyphenols, EPCG, catechin hydrate and curcumin can attenuate QUIN‐induced excitotoxicity to a greater extent than apigenin, naringenin and gallotannin. Both EPCG and curcumin were able to attenuate QUIN‐induced Ca2+ influx and neuronal nitric oxide synthase (nNOS) activity to a greater extent compared with apigenin, naringenin and gallotannin. Although Ca2+ influx was not attenuated by catechin hydrate, nNOS activity was reduced, probably through direct inhibition of the enzyme. All polyphenols reduced the oxidative effects of increased nitric oxide production, thereby reducing the formation of 3‐nitrotyrosine and poly (ADP‐ribose) polymerase activity and, hence, preventing NAD+ depletion and cell death. In addition to the well‐known antioxidant properties of these natural phytochemicals, the inhibitory effect of some of these compounds on specific excitotoxic processes, such as Ca2+ influx, provides additional evidence for the beneficial health effects of polyphenols in excitable tissue, particularly within the central nervous system.

Induction of Indolamine 2,3-Dioxygenase in Primary Human Macrophages by Human Immunodeficiency Virus Type 1 Is Strain Dependent

ABSTRACT Increased kynurenine pathway metabolism has been implicated in the etiology of AIDS dementia complex (ADC). The rate-limiting enzyme for this pathway is indolamine 2,3-dioxygenase (IDO). We tested the efficacy of different strains of human immunodeficiency virus type 1 (HIV1-BaL, HIV1-JRFL, and HIV1-631) to induce IDO in cultured human monocyte-derived macrophages (MDM). A significant increase in both IDO protein and kynurenine synthesis was observed after 48 h in MDM infected with the brain-derived HIV-1 isolates, laboratory-adapted (LA) HIV1-JRFL, and primary isolate HIV1-631. In contrast, almost no kynurenine production or IDO protein was evident in MDM infected with the highly replicating macrophage-tropic LA strain HIV1-BaL. The induction of IDO and kynurenine synthesis by HIV1-JRFL and HIV1-631 declined to baseline levels by day 8 postinfection. Abundant HIV-1 replication did not reduce the ability of exogenous gamma interferon (IFN-γ) to induce IDO and kynurenine synthesis in HIV-infected MDM. The addition of anti-IFN-γ antibody to MDM infected with HIV1-JRFL resulted in an absence of detectable IDO protein after 48 h and a decrease of 64% ± 1% in supernatant kynurenine concentration. Together, these results indicate that only selected strains of HIV-1 are capable of inducing IDO synthesis and subsequent kynurenine metabolism in MDM. The induction of IDO, while apparently independent of replication capacity, appears to be mediated by a transient production of IFN-γ in MDM responding to the initial infection with selected strains of HIV-1.

The Kynurenine Pathway in Brain Tumor Pathogenesis

Brain tumors are among the most common and most chemoresistant tumors. Despite treatment with aggressive treatment strategies, the prognosis for patients harboring malignant gliomas remains dismal. The kynurenine pathway (KP) is the principal route of L-tryptophan catabolism leading to the formation of the essential pyridine nucleotide, nicotinamide adenine dinucleotide (NAD(+)), and important neuroactive metabolites, including the neurotoxin, quinolinic acid (QUIN), the neuroprotective agent, picolinic acid (PIC), the T(H)17/Treg balance modulator, 3-hydroxyanthranilic acid (3-HAA), and the immunosuppressive agent, L-kynurenine (KYN). This review provides a new perspective on KP dysregulation in defeating antitumor immune responses, specifically bringing light to the lower segment of the KP, particularly QUIN-induced neurotoxicity and downregulation of the enzyme α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD) as a potential mechanism of tumor progression. Given its immunosuppressive effects, 3-HAA produced from the KP may also play a role in suppressing antitumor immunity in human tumors. The enzyme indoleamine 2, 3-dioxygenase (IDO-1) initiates and regulates the first step of the KP in most cells. Mounting evidence directly implicates that the induction and overexpression of IDO-1 in various tumors is a crucial mechanism facilitating tumor immune evasion and persistence. Tryptophan 2, 3-dioxygenase (TDO-2), which initiates the same first step of the KP as IDO-1, has likewise recently been shown to be a mechanism of tumoral immune resistance. Further, it was also recently shown that TDO-2-dependent production of KYN by brain tumors might be a novel mechanism for suppressing antitumor immunity and supporting tumor growth through the activation of the Aryl hydrocarbon receptor (AhR). This newly identified TDO-2-KYN-AhR signaling pathway opens up exciting future research opportunities and may represent a novel therapeutic target in cancer therapy. Our discussion points to a number of KP components, namely TDO-2, IDO-1, and ACMSD, as important therapeutic targets for the treatment of brain cancer. Targeting the KP in brain tumors may represent a viable strategy likely to prevent QUIN-induced neurotoxicity and KYN and 3-HAA-mediated immune suppression.

IDO induction in IFN-γ activated astroglia: A role in improving cell viability during oxidative stress

Abstract In the central nervous system (CNS), astrocytes play an integral role in the maintenance of neuronal viability and function. Inflammation within the CNS increases the concentration of oxidative metabolites and, therefore, the potential for NAD depletion through increased poly-(ADP-ribose) polymerase (PARP) activity. However, the activity of indoleamine 2,3-dioxygenase (IDO), the rate limiting enzyme for de novo NAD synthesis, is also markedly increased in astrocytes during inflammation. This study investigated the role of IDO induction in the maintenance of intracellular NAD and its relationship to improved cell viability under conditions of increased oxidative stress in the human astroglioma cell line, HTB-138. Treatment with the pro-inflammatory cytokine IFN-γ increased IDO activity in these cells. Intracellular NAD levels also increased significantly after treatment with IFN-γ in the presence of a PARP inhibitor. Pretreatment of astroglial cells with IFN-γ significantly moderated both the drop in intracellular NAD concentration and cell death following exposure to hydrogen peroxide. These results suggest that induction of IDO and subsequent de novo NAD synthesis may contribute to the maintenance of intracellular NAD levels and cell viability under conditions of increased oxidative stress.

NAD+ metabolism and oxidative stress: the golden nucleotide on a crown of thorns

Abstract In the twentieth century, NAD+ research generated multiple discoveries. Identification of the important role of NAD+ as a cofactor in cellular respiration and energy production was followed by discoveries of numerous NAD+ biosynthesis pathways. In recent years, NAD+ has been shown to play a unique role in DNA repair and protein deacetylation. As discussed in this review, there are close interactions between oxidative stress and immune activation, energy metabolism, and cell viability in neurodegenerative disorders and ageing. Profound interactions with regard to oxidative stress and NAD+ have been highlighted in the present work. This review emphasizes the pivotal role of NAD+ in the regulation of DNA repair, stress resistance, and cell death, suggesting that NAD+ synthesis through the kynurenine pathway and/or salvage pathway is an attractive target for therapeutic intervention in age-associated degenerative disorders. NAD+ precursors have been shown to slow down ageing and extend lifespan in yeasts, and protect severed axons from degeneration in animal models neurodegenerative diseases.

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

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.

Changes in kynurenine pathway metabolism in the brain, liver and kidney of aged female Wistar rats.

The kynurenine pathway of tryptophan catabolism plays an important role in several biological systems affected by aging. We quantified tryptophan and its metabolites kynurenine (KYN), kynurenine acid (KYNA), picolinic acid (PIC) and quinolinic acid (QUIN), and activity of the kynurenine pathway enzymes indoleamine 2,3‐dioxygenase (IDO), tryptophan 2,3‐dioxygenase (TDO) and quinolinic acid phosphoribosyltransferase (QPRTase), in the brain, liver and kidney of young, middle‐aged and old female Wistar rats. Tryptophan levels and TDO activity decreased in all tissues with age. In contrast, brain IDO activity increased with age, while liver and kidney IDO activity decreased with age. The levels of KYN, KYNA, QUIN and PIC in brain all increased with age, while the levels of KYN in the liver and kidney showed a tendency to decrease. The levels of KYNA in the liver did not change, but the levels of KYNA in the kidney increased. The levels of PIC and QUIN increased significantly in the liver but showed a tendency to decrease in the kidney. QPRTase activity in both brain and liver decreased with age but was elevated in the kidney in middle‐aged (12‐month‐old) rats. These age‐associated changes in tryptophan metabolism have the potential to impact upon major biological processes, including lymphocyte function, pyridine (NAD(P)(H)) synthesis and N‐methyl‐d‐aspartate (NMDA)‐mediated synaptic transmission, and may therefore contribute to several degenerative changes of the elderly.

Promotion of cellular NAD(+) anabolism: therapeutic potential for oxidative stress in ageing and Alzheimer's disease.

Oxidative imbalance is a prominent feature in Alzheimer’s disease and ageing. Increased levels of reactive oxygen species (ROS) can result in disordered cellular metabolism due to lipid peroxidation, protein-cross linking, DNA damage and the depletion of nicotinamide adenine dinucleotide (NAD+). NAD+ is a ubiquitous pyridine nucleotide that plays an essential role in important biological reactions, from ATP production and secondary messenger signalling, to transcriptional regulation and DNA repair. Chronic oxidative stress may be associated with NAD+ depletion and a subsequent decrease in metabolic regulation and cell viability. Hence, therapies targeted toward maintaining intracellular NAD+ pools may prove efficacious in the protection of age-dependent cellular damage, in general, and neurodegeneration in chronic central nervous system inflammatory diseases such as Alzheimer’s disease, in particular.