Expanding the Therapeutic World of Tryptophan Metabolism.

Abstract

HomeCirculationVol. 145, No. 24Expanding the Therapeutic World of Tryptophan Metabolism Free AccessEditorialPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessEditorialPDF/EPUBExpanding the Therapeutic World of Tryptophan Metabolism Gregory A. Wyant, PhD and Javid Moslehi, MD Gregory A. WyantGregory A. Wyant Correspondence to: Gregory Wyant, PhD, Department of Medical Oncology, Dana-Farber Cancer Institute, 440 Brookline Avenue, Boston, MA 02215, Email E-mail Address: [email protected] https://orcid.org/0000-0002-4665-2405 Department of Medical Oncology, Dana-Farber Cancer Institute, Boston, MA (G.A.W.). Howard Hughes Medical Institute, Chevy Chase, MD (G.A.W.). Search for more papers by this author and Javid MoslehiJavid Moslehi Javid Moslehi, MD, Section of Cardio-Oncology & Immunology, Division of Cardiology and the Cardiovascular Research Institute, University of California San Francisco, 555 Mission Bay Boulevard South, Box 3118, San Francisco, CA 94143, Email E-mail Address: [email protected] https://orcid.org/0000-0001-5443-4312 Section of Cardio-Oncology and Immunology, Division of Cardiology and the Cardiovascular Research Institute, University of California San Francisco (J.M.). Search for more papers by this author Originally published13 Jun 2022https://doi.org/10.1161/CIRCULATIONAHA.122.059812Circulation. 2022;145:1799–1802This article is a commentary on the followingIndoleamine 2,3-Dioxygenase 1 Deletion–Mediated Kynurenine Insufficiency in Vascular Smooth Muscle Cells Exacerbates Arterial CalcificationArticle, see p 1784The cardiovascular system must adapt to changing nutritional supplies to meet its physiological and energetic demands. The heart, for example, needs continuous production of ATP, which is generated by metabolizing a variety of fuels, including fatty acids, glucose, lactate, ketones, and amino acids. Likewise, in the vasculature, metabolism plays a key role in homeostasis with metabolic dysregulation playing a role in vascular disease initiation and progression. Although the contributions of lipids and carbohydrates have been studied for years, there is increasing appreciation of the role amino acids play in vascular metabolism. In this issue of Circulation, a new study by Ouyang et al1 sheds light on the crucial role for tryptophan metabolism in the regulation of vascular calcification.l-Tryptophan (Trp) is an amino acid that has emerged as a critical player in cardiovascular biology. As an essential amino acid, Trp is obtained exclusively from dietary intake in humans. In addition to serving as a building block for proteins, Trp is metabolized into either the serotonin or kynurenine (Kyn) pathway. Serotonin (also known as 5-hydroxytryptamine) mediates critical functions in the gastrointestinal and central nervous systems; however, 5-hydroxytryptamine was initially isolated from serum (sero-) and recognized as a substance that could modify smooth muscle tone (-tonin), leading to vasoconstriction. Nevertheless, the majority (>95%) of free Trp is metabolized to the Kyn pathway.2 Whereas Kyn metabolism has been best studied in other tissues (specifically neural and immune systems), recent data indicate an important role for the Kyn pathway in the cardiovascular response to stress especially in ischemia.The rate-limiting step of the Kyn pathway is the enzymatic conversion of Trp to N-formylkynurenine that is mediated by 3 enzymes, IDO1 (indoleamine-2,3-dioxygenase 1), IDO2, and TDO (tryptophan-2,3-dioxygenase). N-Formylkynurenine is then converted to Kyn and other bioactive metabolites such as anthranilic acid, kynurenic acid (KynA), 3-hydroxykynurenine, xanthurenic acid, 3-hydroxyanthranilic acid, and NAD+ (nicotinamide adenine dinucleotide)2 (Figure). The IDO/TDO pathway has been associated with both protective and deleterious outcomes in cardiovascular diseases. For instance, IDO1 and KynA contribute to atherosclerosis in preclinical models (presumably by immunomodulation) with elevated levels of KynA and 3-hydroxyanthranilic acid associated with increased risk of acute myocardial infarction in patients.3 Conversely, 3-hydroxyanthranilic acid inhibits macrophage oxidized low-density lipoprotein uptake and reduces total plasma cholesterol and triglycerides in Ldlr–/– mice.4 These data point to the underlying complexity with perhaps cell- and tissue-specific perturbations of the Trp metabolism leading to divergent effects.Download figureDownload PowerPointFigure. Tryptophan metabolism, therapeutic targets, and their role in vascular calcification. Top, l-Tryptophan (Trp) is an essential amino acid that is obtained exclusively from dietary intake in humans. Although free Trp is necessary for protein synthesis and the synthesis of neuromodulators, such as serotonin, >95% is a substrate for the kynurenine (Kyn) pathway of Trp degradation to generate several bioactive metabolites. One such metabolite, Kyn, activates AhR to inhibit RUNX2 (shown in gray) as detailed in Ouyang et al. Kyn pathway metabolites have other molecular targets; for instance, kynurenic acid activates AhR and GPR35, as well, and inhibits glutamate receptors and neuronal nicotinic acetylcholine receptors. Drugs targeting Kyn pathway enzymes that are currently being tested in clinical trials are italicized. Bottom, In this issue of Circulation, Ouyang et al reveal a key role of IDO1 in VSMC calcification. AFMID indicates kynurenine formamidase; AhR, aryl-hydrocarbon receptor; GPR35, G-protein coupled receptor 35; Glut-R, glutamate receptor; IDO, indoleamine-2,3-dioxygenase; KATs, kynurenine amino transferases I–III; KMO, kynurenine 3-monooxygenase; α7nAChR, alpha7 nicotinic acetylcholine receptor; NAD+, nicotinamide adenine dinucleotide; RUNX2, Runt-like transcription factor-2; TDO, tryptophan-2,3-dioxygenase; TPH, tryptophan hydroxylase; and VSMC, vascular smooth muscle cell.Calcific vasculopathy is prevalent in advanced atherosclerotic lesions, promotes arterial stiffness, and contributes to atherosclerotic plaque formation. Ouyang and colleagues1 demonstrate that IDO1 reduces arterial calcification through Kyn production. Vascular calcification is fundamentally a gene-regulated biological process that resembles bone mineralization and involves osteoblast differentiation of vascular smooth muscle cells; IDO1 activity and Kyn metabolites have previously been shown to be essential during osteoblast differentiation.5,6 Given this overlap, Ouyang and colleagues hypothesized that IDO1 and Kyn metabolites similarly play a critical role in arterial calcification. Using apolipoprotein E–deficient ApoE–/– animal models of atherosclerosis, the authors assessed the role of IDO1 in arterial calcification by generating IDO1-deficient ApoE–/– mice. IDO1 loss markedly increased arterial calcification in both male and female ApoE–/– mice fed a high-fat diet. This effect was independent of any changes in atherosclerosis risk factors, such as serum triglycerides, cholesterol, calcium, blood glucose, or body weight. Furthermore, the investigators determined that IDO1 loss specifically in vascular smooth muscle cells, but not in macrophages, promotes arterial calcification.To further delineate the downstream genetic targets of IDO1-dependent arterial calcification, the group focused on RUNX2 (Runt-like transcription factor-2), which is both necessary and sufficient for osteogenic differentiation and vascular calcification.7 The investigators found that IDO1 loss increased RUNX2 abundance, and RUNX2 loss blocked arterial calcification mediated by IDO1 inhibition. In addition, serum Kyn levels negatively correlated with severity of atherosclerotic calcification in affected mice, and Kyn was sufficient to reduce arterial calcification and RUNX2 expression in the absence of IDO1 both in isolated vascular smooth muscle cells and in vivo. These results collectively suggested that Kyn, a product of the IDO1 enzyme, plays a crucial role in calcification inhibition by reducing RUNX2 abundance.How Kyn may regulate RUNX2 remained an open question, especially because RUNX2 undergoes many posttranslational regulations including phosphorylation, acetylation, and ubiquitination.7 First, the authors found that Kyn supplementation decreased RUNX2 protein half-life, suggesting that Kyn promotes RUNX2 degradation. Because Kyn acts as an endogenous ligand for the AhR (aryl-hydrocarbon receptor), a transcription factor that mediates the systematic inflammation response, the authors asked whether AhR mediated the Kyn-regulated stability of RUNX2. To their surprise, although AhR was necessary for Kyn to reduce RUNX2 abundance, this was independent of its canonical function as a transcription factor. Instead, AhR acts as an atypical component of a ubiquitin ligase complex containing Cul4B.8 Kyn activates AhR-Cul4B mechanistically to mediate RUNX2 degradation to regulate arterial calcification. This work highlights the importance of Trp and Kyn metabolism in arterial calcification and atherosclerosis and provides a rationale of further study of Kyn metabolism as protective molecules in this disease.These studies add to the growing appreciation for Trp metabolism in the cardiovascular system. For instance, endothelial-derived Kyn acts as a vasodilator and decreases blood pressure, contributing to the regulation of vascular tone with pharmacological IDO inhibition increasing blood pressure in rats.9 Other studies point to a tissue-protective role for the Kyn pathway, perhaps recapitulating local or remote ischemic preconditioning.10,11 Genetic or pharmacological inhibition of the EglN enzymes, which in metazoans act as O2 sensors to coordinate cellular responses that promote adaptation to hypoxia and ischemia, is sufficient to mediate local and remote ischemic preconditioning in multiple organs. In the kidney, pharmacological EglN inhibition promotes Kyn and reduces kidney injury and inflammation through preservation of NAD+. In this context, IDO1 loss abolishes renoprotection provided by EglN inhibition.10 In our studies of cardioprotection, genetic or pharmacological EglN inhibition drives the accumulation of circulating α-ketoglutarate, which promotes the hepatic production and secretion of KynA, the downstream metabolite of Kyn. KynA, in this context, is necessary and sufficient to mediate cardiac ischemic protection.11 Whereas KynA can act as an agonist for AhR, similar to Kyn, a ligand for various receptors necessary for neurotransmission, such as N-methyl-d-aspartate and neuronal cholinergic α7 nicotine receptors, and an agonist for the orphan G-coupled protein receptor, additional studies are required to determine how KynA protects the heart.The cardiovascular studies implicating Trp metabolism are especially important because the Kyn pathway has emerged as an important drug target with the development of several small molecules that are being tested in neurological conditions and cancer (Figure).2 Kyn metabolite imbalances result in an excess of metabolites with neuroactive properties and are thought to contribute to neuropsychiatric disorders.2 Accordingly, strategies of either increasing or decreasing KynA abundance in neuropsychiatric disorders are being tested clinically. In oncology, IDO1/IDO2/TDO have tumor-promoting capabilities with a growing body of evidence supporting key roles of the IDO1/IDO2/TDO enzymes in the clinical response to immunotherapy. These observations have led to tremendous enthusiasm for the potential use of IDO1 inhibitors for cancer treatment. Although some of the early studies have been disappointing, multiple clinical trials combining IDO inhibitors with immune checkpoint inhibitors are currently underway.12 In addition, preclinical data suggest the potential for combining IDO inhibitors with BTK (Bruton tyrosine kinase) inhibitors.13 Given that immune checkpoint inhibitors and BTK each have unique cardiovascular sequelae, the potential for combination therapy with IDO inhibitors would have relevance in cardio-oncology where the cardiovascular care of the growing numbers of patients who have cancer and survivors has emerged as an important consideration in patient care.14The lack of a clear understanding of the interdependency of the various arms of Trp metabolism and the exact downstream effects of Kyn metabolites ultimately present major challenges in developing novel therapeutics targeting Trp degradation. This issue has already emerged in oncology with the clinical testing of IDO1 inhibitors. The recent phase 3 clinical trial of epacadostat, an oral IDO1 inhibitor, was negative despite efficient blockade of the Kyn pathway.12 However, a follow-up study showed a metabolic adaption that shunted Trp metabolism toward the serotonin pathway, resulting in elevated NAD+ which reduced T-cell proliferation and antitumor efficacy.15 Despite these shortcomings, the relevance of Trp degradation as a critical mechanism across numerous pathophysiological conditions is clear. Further studies are necessary to understand the relevance of individual Kyn metabolites to predict therapeutic outcomes of inhibition of their necessary enzymes.Article InformationSources of FundingDr Wyant is a HHMI Fellow of the Jane Coffin Childs Memorial Fund for Medical Research. Dr Moslehi is supported by National Institutes of Health grants (R01HL141466, R01HL155990, R01HL156021).Disclosures Dr Wyant has no competing interests to declare. Dr Moslehi has served on advisory boards for Pfizer, Novartis, Bristol-Myers Squibb, Deciphera, Audentes Pharmaceuticals, Takeda, Myokardia, AstraZeneca, GlaxoSmithKline, Boston Biomedical, ImmunoCore, Janssen, Myovant, Silverback Therapeutics, Amgen, Kurome Therapeutics, Kiniska Pharmaceuticals, Daiichi Sankyo, CRC Oncology, BeiGene, Star Therapeutics, ProteinQure, Pharmacyclics, Mallinckrodt Pharmaceuticals, Boehringer, and Cytokinetics.FootnotesThe opinions expressed in this article are not necessarily those of the editors or of the American Heart Association.Circulation is available at www.ahajournals.org/journal/circFor Sources of Funding and Disclosures, see page 1801.Correspondence to: Gregory Wyant, PhD, Department of Medical Oncology, Dana-Farber Cancer Institute, 440 Brookline Avenue, Boston, MA 02215, Email [email protected]harvard.eduJavid Moslehi, MD, Section of Cardio-Oncology & Immunology, Division of Cardiology and the Cardiovascular Research Institute, University of California San Francisco, 555 Mission Bay Boulevard South, Box 3118, San Francisco, CA 94143, Email javid.[email protected]edu