Inhibition of human pyridoxal kinase by 2-acetyl-4-((1R,2S,3R)-1,2,3,4-tetrahydroxybutyl)imidazole (THI)

Inhibiting sphingosine 1-phosphate lyase: From efficacy to mechanism

Sphingosine-1 phosphate (S1P) is a lipid metabolite regulating diverse biological processes, including proliferation, differentiation, migration, and apoptosis, highlighting its physiological and therapeutic significance. Current S1P-based therapeutic approaches primarily focus on modulating the downstream signalling via targeting S1P receptors, however, this is challenged by incomplete receptor internalisation. Sphingosine-1-phosphate lyase (SPL) is a highly conserved enzyme that "gatekeeps" the final step of S1P degradation. Cognisant of the complex ligand and receptor interaction and dynamic metabolic networks, the selective modulation of SPL activity presents a new opportunity to regulate S1P biosynthesis and reveal its role in various systems. Over the past decade, an evolving effort has been made to identify new molecules that could block SPL activity in vitro or in vivo. This review focuses on summarising the current understanding of the reported SPL inhibitors identified through various screening approaches, discussing their efficacy in diverse model systems and the possible mechanism of action. Whilst effective modulation of S1P levels via inhibiting SPL is feasible, the specificity of those inhibitors remains inconclusive, presenting a clear challenge for future implications. Yet, none of the currently available SPL inhibitors is proven effective in elevating S1P levels within the central nervous system. This review article embraces future research focusing on investigating selective SPL inhibitors with high potency and possibly blood-brain-barrier permeability, which would aid the development of new S1P-based therapeutics for neurological disorders.

Immunotoxicity of 2-Acetyl-4-tetrahydroxybutylimidazole in BALB/c mice with different vitamin B6 nutritional statuses

Caramel color is a widely used food pigment, and 2-Acetyl-4-tetrahydroxybutylimidazole (THI) is a by-products of Class III caramel color. Some studies have shown that THI can reduce the number of peripheral blood lymphocytes. However, the comprehensive mechanism of THI immunotoxicity requires further study. In this study, the effects of THI on lymphocyte count, humoral immunity, cellular immunity and nonspecific immunity were determined and the effect of the nutritional status of VB6 on THI immunotoxicity was evaluated. Female BALB/c mice were divided into 3 groups and fed chow containing different doses of VB6: VB6-normal (6 mg/kg VB6), VB6-deprived (0.5 mg/kg VB6) or VB6-enhanced (12 mg/kg VB6) feed. Each group was further divided into 4 subgroups and treated with THI (0.5, 2.5 or 12.5 mg/kg bw) or the solvent control by gavage for 30 days. The thymic cortical thickness was measured with ViewPoint; the proportions of major immune cells and T cells in peripheral blood and tissues were detected via flow cytometry; the transformation and proliferation abilities of T and B cells were detected via T and B lymphocyte proliferation assays; NK cell activity was assessed via lactate dehydrogenase assays; humoral immune function was assessed via plaque-forming cell assays; and the immune function of T lymphocytes was assessed via delayed type hypersensitivity assays. The results showed that compared with those in the corresponding control group, the white blood cell count and lymphocyte count decreased significantly in all the VB6-deprived groups, in the 2.5 and 12.5 mg/kg VB6 groups, and in the 12.5 mg/kg VB6-enhanced group. With increasing THI dose, the thymic cortical layer became thinner. In the thymus, THI increased the proportions of CD3+ T cells and mature CD8+ T cells and decreased the proportions of immature double-positive, double-negative T cells and CD69-expressing lymphocytes. The proportions of naïve T cells and Tcm (central memory T) cells related to homing decreased. The proportion of mature T cells in the spleen decreased significantly. The proliferation of T cells stimulated by ConA decreased after THI exposure. VB6-deficient mice were more sensitive to THI immunotoxicity, and supplementation with VB6 had a certain protective effect on these mice. The results of the PFC and NK cell activity assays indicated that THI exposure might not affect humoral immune or innate immune function.

Effects of 2-acetyl-4-tetrahydroxybutylimidazole exposure during gestation and lactation on maternity and offspring immune function in Balb/c mice

2-Acetyl-4-tetrahydroxybutylimidazole (THI), a by-product of Class Ⅲ caramel color, is generally recognized to cause lymphopenia in mammals. However, it remains unknown whether THI exposure during gestation and lactation causes damage to the immune system of offspring. In this study, pregnant Balb/c mice were gavaged with 0, 0.5, 2.5 and 12.5 mg/kg THI from gestation day (GD) 6 to postanal day (PND) 21, after which we treated another batch of dams from GD6 to PND21 and the offspring for 3 weeks after weaning with 0, 2, 10, 50 mg/L THI in drinking water respectively, and investigated the immunological anomalies of dams and offspring. The results showed that lymphopenia was observed in dams but not in weaning pups on PND21, which were exposed to THI during gestation and lactation. 2 mg/L THI and 2.5 mg/kg THI began to cause a remarkable reduction of the numbers of white blood cells and lymphocytes in dams. Besides both the cellular and the humoral immune response was not affected in weaning pups, which were measured by plaque-forming cell (PFC) assay and delayed-type hypersensitivity (DTH) assay respectively. Furthermore, THI could be detected in the plasma of dams with a dose-dependent manner, but not in that of both female and male weaning pups. In both male and female offspring being treated with 10 and 50 mg/L THI for another 3 weeks after weaning, lymphocytopenia was observed and T lymphocytes including CD4+ and CD8+ cells were significantly reduced in their spleens except lymph nodes. 10 and 50 mg/L THI treatment increased CD4+ and CD8+ single positive cells in thymus of female and male weaning mice. Mitogen-induced proliferation ability of T cells in the spleen and lymph nodes was impaired in female weaning mice exposed 50 mg/L THI, while male weaning mice treated with 10 and 50 mg/L THI showed impairment in the spleen but not lymph nodes. Based on the results in this study, no observed adverse effect level (NOAEL) for 3-week THI treatment in weaning mice was considered to be 2 mg/L (0.30 mg/kg bw for female mice and 0.34 mg/kg bw for male mice). And NOAEL for THI treatment in dams might be set to 0.5 mg/kg bw/day. Collectively from the perspective of NOAEL, offspring are not more sensitive than dams or adult mice.

Sphingosine 1-phosphate lyase facilitates cancer progression through converting sphingolipids to glycerophospholipids

BACKGROUND

In addition to potent agonist properties for sphingosine 1-phosphate (S1P) receptors, intracellularly, S1P is an intermediate in metabolic conversion pathway from sphingolipids to glycerolysophospholipids (glyceroLPLs). We hypothesized that this S1P metabolism and its products might possess some novel roles in the pathogenesis of cancer, where S1P lyase (SPL) is a key enzyme.

METHODS

The mRNA levels of sphingolipid-related and other cancer-related factors were measured in human hepatocellular carcinoma (HCC), colorectal cancer, and esophageal cancer patients' tumours and in their adjacent non-tumour tissues. Phospholipids (PL) and glyceroLPLs were measured by using liquid chromatography-tandem mass spectrometry (LC-MS/MS). In-vitro experiments were performed in Colon 26 cell line with modulation of the SPL and GPR55 expressions. Xenograft model was used for determination of the cancer progression and for pharmacological influence.

RESULTS

Besides high SPL levels in human HCC and colon cancer, SPL levels were specifically and positively linked with levels of glyceroLPLs, including lysophosphatidylinositol (LPI). Overexpression of SPL in Colon 26 cells resulted in elevated levels of LPI and lysophosphatidylglycerol (LPG), which are agonists of GPR55. SPL overexpression-enhanced cell proliferation was inhibited by GPR55 silencing. Conversely, inhibition of SPL led to the opposite outcome and reversed by adding LPI, LPG, and metabolites generated during S1P degradation, which is regulated by SPL. The xenograft model results suggested the contribution of SPL and glyceroLPLs to tumour progression depending on levels of SPL and GPR55. Moreover, the pharmacological inhibition of SPL prevented the progression of cancer. The underlying mechanisms for the SPL-mediated cancer progression are the activation of p38 and mitochondrial function through the LPI, LPG-GPR55 axis and the suppression of autophagy in a GPR55-independent manner.

CONCLUSION

A new metabolic pathway has been proposed here in HCC and colon cancer, SPL converts S1P to glyceroLPLs, mainly to LPI and LPG, and facilitates cancer development.

Caramel colors in terms of scientific research, with particular consideration of their toxicity

Caramel colors, the most common food additives in the world, are divided into four classes (IIV), marked with the symbols E150 a-d, respectively. Individual classes of caramel colors differ from each other in physico-chemical properties and the method of preparation, which affects the formation of various compounds that are important for the assessment of food safety A number of studies on all caramel classes of have been performed, including toxicokinetic, genotoxic, carcinogenic and reproductive and developmental toxicity studies, which have not shown harmful effects of these additives at doses not exceeding ADI. However, there is an increasing number of scientific reports of the possible toxic effects present in caramels of low-molecular compounds. Currently, three compounds are considered to be toxicologically important and resulting from the possible concentration in the final product: 5-HMF (present in all classes), 4(5)-MeI (present in caramel classes III and IV) or THI (present in caramel class III). 4(5)-MeI has a neurotoxic effect and was considered in 2011 as a possible human carcinogen (class 2B, according to IARC). In the case of THI, studies have confirmed its lymphopenic activity, probably secondary to its immunosuppressive effect. Consequently, in the 1980s, JECFA set acceptable levels 4(5)-MeI and THI, for the caramel classes in which these compounds may be present. The toxicity of 5-HMF has not been confirmed unequivocally, but studies have shown that this compound is not neutral to living organisms. Currently, most international organizations and scientific institutes recognize these additives as safe for consumers, but at the same time scientists emphasize the need for further research.

Pyridoxal kinase inhibition by artemisinins down-regulates inhibitory neurotransmission

The antimalarial artemisinins have also been implicated in the regulation of various cellular pathways including immunomodulation of cancers and regulation of pancreatic cell signaling in mammals. Despite their widespread application, the cellular specificities and molecular mechanisms of target recognition by artemisinins remain poorly characterized. We recently demonstrated how these drugs modulate inhibitory postsynaptic signaling by direct binding to the postsynaptic scaffolding protein gephyrin. Here, we report the crystal structure of the central metabolic enzyme pyridoxal kinase (PDXK), which catalyzes the production of the active form of vitamin B6 (also known as pyridoxal 5'-phosphate [PLP]), in complex with artesunate at 2.4-Å resolution. Partially overlapping binding of artemisinins with the substrate pyridoxal inhibits PLP biosynthesis as demonstrated by kinetic measurements. Electrophysiological recordings from hippocampal slices and activity measurements of glutamic acid decarboxylase (GAD), a PLP-dependent enzyme synthesizing the neurotransmitter γ-aminobutyric acid (GABA), define how artemisinins also interfere presynaptically with GABAergic signaling. Our data provide a comprehensive picture of artemisinin-induced effects on inhibitory signaling in the brain.

Component of Caramel Food Coloring, THI, Causes Lymphopenia Indirectly via a Key Metabolic Intermediate

Caramel color is widely used in the food industry, and its many variations are generally considered to be safe. It has been known for a long time that THI (2-acetyl-4-(tetrahydroxybutyl)imidazole), a component of caramel color III, causes lymphopenia in animals through sphingosine 1-phosphate (S1P) lyase (S1PL) inhibition. However, this mechanism of action has not been fully validated because THI does not inhibit S1PL in vitro. To reconcile this situation, we examined molecular details of THI mechanism of action using "smaller" THI derivatives. We identified a bioactive derivative, A6770, which has the same lymphopenic effect as THI via S1PL inhibition. In the case of A6770 we observe this effect both in vitro and in vivo, and demonstrate that A6770 is phosphorylated and inhibits S1PL in the same way as 4-deoxypyridoxine. In addition, A6770 was detected in rat plasma following oral administration of THI, suggesting that A6770 is a key metabolic intermediate of THI.

Molecular mechanisms of the non-coenzyme action of thiamin in brain: biochemical, structural and pathway analysis

Thiamin (vitamin B1) is a pharmacological agent boosting central metabolism through the action of the coenzyme thiamin diphosphate (ThDP). However, positive effects, including improved cognition, of high thiamin doses in neurodegeneration may be observed without increased ThDP or ThDP-dependent enzymes in brain. Here, we determine protein partners and metabolic pathways where thiamin acts beyond its coenzyme role. Malate dehydrogenase, glutamate dehydrogenase and pyridoxal kinase were identified as abundant proteins binding to thiamin- or thiazolium-modified sorbents. Kinetic studies, supported by structural analysis, revealed allosteric regulation of these proteins by thiamin and/or its derivatives. Thiamin triphosphate and adenylated thiamin triphosphate activate glutamate dehydrogenase. Thiamin and ThDP regulate malate dehydrogenase isoforms and pyridoxal kinase. Thiamin regulation of enzymes related to malate-aspartate shuttle may impact on malate/citrate exchange, responsible for exporting acetyl residues from mitochondria. Indeed, bioinformatic analyses found an association between thiamin- and thiazolium-binding proteins and the term acetylation. Our interdisciplinary study shows that thiamin is not only a coenzyme for acetyl-CoA production, but also an allosteric regulator of acetyl-CoA metabolism including regulatory acetylation of proteins and acetylcholine biosynthesis. Moreover, thiamin action in neurodegeneration may also involve neurodegeneration-related 14-3-3, DJ-1 and β-amyloid precursor proteins identified among the thiamin- and/or thiazolium-binding proteins.