Characterization of indoleamine-2,3-dioxygenase 1, tryptophan-2,3-dioxygenase, and Ido1/Tdo2 knockout mice

Ubiquitylome study reveals the regulatory effect of α-lipoic acid on ubiquitination of key proteins in tryptophan metabolism pathway of pig liver

The decline in antioxidant defenses make it easily for human and animals to suffer from liver damage and diseases induced by oxidative stress, causing enormous losses to human health and livestock production. As one of the canonical protein post-translational modifications (PTMs), ubiquitination is widely involved in cell proliferation, apoptosis and damage/repair response, and is proven to be involved in the ability of mammals to resist oxidative stress. To explore whether α-lipoic acid (LA), a safe and efficient antioxidant, plays a role in regulating liver antioxidant status by PTMs, proteins in livers of pigs fed with LA were analyzed at the level of proteome and ubiquitylome. Based on proteome-wide enrichment of ubiquitination, a total of 7274 proteins were identified and 5326 were quantified, we also identified 1564 ubiquitination sites in 580 ubiquitinated proteins, among which there were 136 differentially ubiquitinated sites in 103 differentially ubiquitinated proteins upon LA. Further bioinformatics analysis showed that these differential proteins were mainly enriched in tryptophan metabolic pathway, and accompanied by significantly improvement of liver antioxidant capacity. We revealed the regulatory effect of LA on ubiquitination of kynurenine 3-monooxygenase (KMO) and other key proteins in tryptophan metabolism pathway of pig liver for the first time.

Adaptive immune resistance at the tumour site: mechanisms and therapeutic opportunities

Tumours employ various tactics to adapt and eventually resist immune attack. These mechanisms are collectively called adaptive immune resistance (AIR). The first defined and therapeutically validated AIR mechanism is the selective induction of programmed cell death 1 ligand 1 (PDL1) by interferon-γ in the tumour. Blockade of PDL1 binding to its receptor PD1 by antibodies (anti-PD therapy) has resulted in remission of a fraction of patients with advanced-stage cancer, especially in solid tumours. However, many clinical trials combining anti-PD therapy with other antitumour drugs conducted without a strong mechanistic rationale have failed to identify a synergistic or additive effect. In this Perspective article, we discuss why defining AIR mechanisms at the tumour site should be a key focus to direct future drug development as well as practical approaches to improve current cancer therapy.

Metabolomic analyses of COVID-19 patients unravel stage-dependent and prognostic biomarkers

The circulating metabolome provides a snapshot of the physiological state of the organism responding to pathogenic challenges. Here we report alterations in the plasma metabolome reflecting the clinical presentation of COVID-19 patients with mild (ambulatory) diseases, moderate disease (radiologically confirmed pneumonitis, hospitalization and oxygen therapy), and critical disease (in intensive care). This analysis revealed major disease- and stage-associated shifts in the metabolome, meaning that at least 77 metabolites including amino acids, lipids, polyamines and sugars, as well as their derivatives, were altered in critical COVID-19 patient's plasma as compared to mild COVID-19 patients. Among a uniformly moderate cohort of patients who received tocilizumab, only 10 metabolites were different among individuals with a favorable evolution as compared to those who required transfer into the intensive care unit. The elevation of one single metabolite, anthranilic acid, had a poor prognostic value, correlating with the maintenance of high interleukin-10 and -18 levels. Given that products of the kynurenine pathway including anthranilic acid have immunosuppressive properties, we speculate on the therapeutic utility to inhibit the rate-limiting enzymes of this pathway including indoleamine 2,3-dioxygenase and tryptophan 2,3-dioxygenase.

Tryptophan and indole metabolism in immune regulation

L-tryptophan is an essential amino acid that undergoes complex metabolic routes, resulting in production of many types of signaling molecules that fall into two types: retaining the indole ring such as serotonin, melatonin and indole-pyruvate or breaking the indole ring to form kynurenine. Kynurenines are the precursor of signaling molecules and are the first step in de novo NAD⁺ synthesis. In mammalian cells, the kynurenine pathway is initiated by the rate-limiting enzymes tryptophan-2,3-dioxygenase (TDO) and interferon responsive indoleamine 2,3-dioxygenase (IDO1) and is the major route for tryptophan catabolism. IDO1 regulates immune cell function through the kynurenine pathway but also by depleting tryptophan in microenvironments, and especially in tumors, which led to the development of IDO1 inhibitors for cancer therapy. However, the connections between tryptophan depletion versus product supply remain an ongoing challenge in cellular biochemistry and metabolism. Here, we highlight current knowledge about the physiological and pathological roles of tryptophan signaling network with a focus on the immune system.