The metabolism of tryptophan to nicotinamide adenine dinucleotide (NAD+) through the kynurenine pathway is increasingly linked to aging and age-associated disease. Kynurenine pathway enzymes and metabolites influence a range of molecular processes critical to healthy aging, including regulation of inflammatory and immune responses, cellular redox homeostasis, and energy production. Aberrant kynurenine metabolism is observed during normal aging and has been implicated in a range of age-associated pathologies, including chronic inflammation, atherosclerosis, neurodegeneration, and cancer. In previous work, we and others identified three genes-kynu-1, tdo-2, and acsd-1-encoding kynurenine pathway enzymes for which decreasing expression extends lifespan in invertebrate models. Here we report that knockdown of haao-1, a fourth kynurenine pathway gene encoding the enzyme 3-hydroxyanthranilic acid dioxygenase (HAAO), extends lifespan by ~30% and delays age-associated decline in health in Caenorhabditis elegans. This lifespan extension is mediated by increased physiological levels of the HAAO substrate 3-hydroxyanthranilic acid (3HAA). 3HAA increases resistance to oxidative stress during aging by directly degrading hydrogen peroxide and activating the Nrf2/SKN-1 oxidative stress response. Aging mice fed a diet supplemented with 3HAA are similarly long-lived. Our results identify HAAO and 3HAA as novel therapeutic targets for age-associated disease. This works provides a foundation for more detailed examination of the molecular mechanisms underlying the benefits of 3HAA, and how these mechanisms interact with other interventions both within and beyond the kynurenine pathway. We anticipate that these findings will bolster growing interest in developing pharmacological strategies to target tryptophan metabolism to improve health aging.
Metabolism of tryptophan by the kynurenine pathway is increasingly linked to aging. Kynurenine pathway enzymes and metabolites influence a range of molecular processes critical to healthy aging, including regulation of inflammatory and immune responses, cellular redox homeostasis, and energy production. Aberrant kynurenine metabolism occurs during normal aging and is implicated in many age-associated pathologies including chronic inflammation, atherosclerosis, neurodegeneration, and cancer. We and others previously identified three kynurenine pathway genes—kynu-1, tdo-2, and acsd-1—for which decreasing expression extends lifespan in invertebrates. More recently we discovered that knockdown of haao-1, a fourth kynurenine pathway gene encoding the enzyme 3-hydroxyanthranilic acid dioxygenase (HAAO), extends lifespan by ~30% and delays age-associated health decline in Caenorhabditis elegans. Lifespan extension is mediated by increased physiological levels of the HAAO substrate 3-hydroxyanthranilic acid (3HAA). Aging mice fed a diet supplemented with 3HAA are similarly long-lived. The mechanism of action liking 3HAA to aging is complex and partially overlaps with multiple pathways previously implicated in aging. We recently identified activation of the Nrf2/SKN-1 oxidative stress response and alterations to iron homeostasis as key players in the benefits 3HAA. Ongoing work explores the relationship between 3HAA, Nrf2/SKN-1, and iron in C. elegans and mammalian aging, age-associated immune decline, and cancer. This works provides a foundation for detailed examination of the molecular mechanisms underlying the benefits of 3HAA, and how these mechanisms interact with other anti-aging interventions. We anticipate that these findings will bolster growing interest in developing pharmacological strategies to target tryptophan metabolism to improve health aging.
Aging is characterized by a progressive decline in the normal physiological functions of an organism, ultimately leading to mortality. Metabolic changes throughout the aging process disrupt the balance and homeostasis of the cell. The kynurenine metabolic pathway is the sole de novo biosynthetic pathway for producing NAD+ from ingested tryptophan. Altered kynurenine pathway activity is associated with both aging and a variety of age-associated diseases, and kynurenine-based interventions can extend lifespan in Caenorhabditis elegans. Our laboratory recently demonstrated knockdown of the kynurenine pathway enzymes kynureninase (KYNU) or 3-hydroxyanthranilic acid dioxygenase (HAAO) increases lifespan by 20-30% in C elegans. However, the mechanism of how these interventions may modulate response against different stressors during the aging process has yet to be explored. Fluorescent reporter strains show the stress-responsive transcription factors skn-1 (ortholog of NRF2/NFE2L2; oxidative stress response) and hif-1 (ortholog of HIF1A; hypoxic stress response) to be highly upregulated when the kynurenine pathway is inhibited. We also demonstrated the increase expression of gst-4 and gcs-1 (transcriptional targets skn-1), which are involved in production of the antioxidant glutathione (GSH), as well as upregulation of cysl-2 (transcriptional target of hif-1), a regulator of cysteine biosynthesis from serine. We hypothesize that lifespan extension resulting from kynurenine pathway inhibition is mediated, at least in part, by upregulation of these transcription factors, providing elevated defense against oxidative stress and hypoxic stress.
3-hydroxyanthranilate 3,4-dioxygenase (HAAO) is an intermediate enzyme in the conversion from tryptophan (TRP) to nicotinamide adenine dinucleotide (NAD+) via the kynurenine pathway. The kynurenine pathway is the sole »de novo» NAD+ biosynthetic pathway from ingested tryptophan. Inhibition of several enzymatic steps in the kynurenine pathway increases lifespan in Drosophila melanogaster, Saccharomyces cerevisiae, and Caenorhabditis elegans. Knockout or knockdown of haao-1, the C. elegans gene encoding HAAO, or supplementation of its substrate metabolite 3-hydroxyanthranilic acid (3HAA), has been shown to promote healthy lifespan extension; however, the underlying mechanism remains unknown. In the present study, we report that haao-1 knockdown induces oxidative stress resistance against several reactive oxygen species (ROS) inducing agents by activating the Nrf2/SKN-1 oxidative stress response pathway. An examination of the redox state of animals with reduced haao-1 suggests that activation of the Nrf2/SKN-1 pathway is mediated by shifting the balance toward generation of ROS, generating a hormetic effect. Our results identify a novel mechanism for an endogenous metabolite (3HAA) that activates the oxidative stress response. These results provide a conceptual basis by which modulation of the kynurenine pathway can promote healthy aging and enhanced stress resistance.
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