Excessive L-cysteine induces vacuole-like cell death by activating endoplasmic reticulum stress and mitogen-activated protein kinase signaling in intestinal porcine epithelial cells

A fluorescence turn "on-off" imaging probe for sequential detection of Al3+ and L-Cysteine in HeLa cells

At this "Aluminum Age", exposure to aluminum (metallic or ionic form) is inevitable and inestimable. The presence of aluminum in biological systems is evident but more often aluminum toxicity is less understood. Therefore, the presence of biologically reactive aluminum needs to be identified and quantified. Alongside metals, L-cysteine, an essential amino acid, plays a pivotal role in the homeostasis of cellular oxidative and reductive stress. However, excess (<7g) could be lethal and can lead to death. Thus, in-situ selective detection of aluminum and L-cysteine is of larger interest. Here we report a fluorogenic probe (R) for the sequential selective detection and quantification of Al3+ and L-cysteine in a semi-aqueous medium (3:7; water: DMSO). The probe (R) was synthesized by a one-step acid-mediated condensation reaction between pyridine-3,4-diamine and 2-hydroxy-1-napthaldehyde. The synthesized probe was characterized using 1H and 13C NMR, and HR-Mass spectroscopic techniques. The probe (R) is non-emissive in nature, but on recognition of Al3+, the probe R showed "turn-on" emission (bright yellow colour) showing two emission maxima (522 nm and 547 nm), and no naked eye observable color change. Other competing cations do not show any noticeable fluorescence outcome. The R + Al3+ ensemble can specifically detect L-cysteine among all the essential amino acids by showing a fluorescence "turn-off" response. The sensing mechanism of Al3+ is obeying the chelation-enhanced fluorescence (CHEF) effect. The binding constant of R + Al3+ is 0.3 × 104 M-1. The limit of detection (LoD) for Al3+ and L-cysteine are 2.02 × 10-7 M and 0.5 × 10-5 M respectively. The probe (R) can show maximum efficiency within the pH range (7.0-10.0). The probe is found non-toxic (>80 % cell viability with 15 µM concentration) and employed for the in-vitro fluorescence imaging in the HeLa cell.

Tong-Xie-Yao-Fang alleviates diarrhea-predominant irritable bowel syndrome in rats via the GCN2/PERK-eIF2α-ATF4 signaling pathway

BACKGROUND

Diarrhea-predominant irritable bowel syndrome (IBS-D) is a common functional gastrointestinal disease. Tong-Xie-Yao-Fang (TXYF), the traditional Chinese herbal medicine prescription, is a classic and effective prescription for the treatment of IBS-D, but its mechanism of action is not fully clarified.

OBJECTIVE

To evaluate the efficacy of TXYF in the treatment of IBS-D and to explore its potential mechanism of action.

METHODS

Changes in the serum levels of 50 free amino acids were targeted for detection by high-performance liquid chromatography (HPLC), and the expression of glucose-regulated protein 78 (GRP78), general control nonderepressible 2 (GCN2), and endoplasmic reticulum-resident kinase (PERK) was detected by immunohistochemistry examinations in healthy volunteers and IBS-D patients. The IBS-D rat was constructed by the three-factor superposition method of neonatal maternal separation, 2,4,6-trinitrobenzene sulfonic acid enema, and chronic unpredictable stress stimulation. The treatment effect of TXYF on IBS-D rats was observed by recording the body weight, grasp force, fecal water content (FWC), and abdominal withdrawal reflex (AWR) of rats before and after treatment. The effects of GCN2/PERK-eukaryotic initiation factor-2 (eIF2α) -activating transcription Factor 4 (ATF4) pathway proteins and gene expression were analyzed by western blotting, reverse transcription-polymerase chain reaction (RT-qPCR), and immunohistochemistry evaluations.

RESULTS

Compared with healthy volunteers, IBS-D patients exhibited lower levels of cysteine, γ-aminoacetic acid (GABA), homoproline, and lysine, and immunohistochemistry showed strong activation of GRP78, a marker of endoplasmic reticulum stress. Differential expression of GCN2 and PERK proteins was detected in IBS-D patients and rat colons. In the IBS-D rats, TXYF improved the body weight and grasp force, reduced the FWC, and improved the AWR score. TXYF increased the levels of p-GCN2 and GCN2 and reduced the levels of GRP78, p-PERK, PERK, p-eIF2α, and eIF2α, thereby affecting the expression of the apoptosis-related transcription factors ATF4, CHOP, Caspase-3, and Bcl-2.

CONCLUSION

Our study showed that TXYF improved IBS-D by inhibiting apoptosis. The anti-apoptosis effects were potentially mediated by regulating the GCN2/PERK-eIF2a-ATF4 signaling pathway.

Glycine represses endoplasmic reticulum stress-related apoptosis and improves intestinal barrier by activating mammalian target of rapamycin complex 1 signaling

Endoplasmic reticulum (ER) stress has been associated with the dysfunction of intestinal barrier in humans and animals. We have previously shown that oral administration of glycine to suckling-piglets improves ER stress-related intestinal mucosal barrier impairment and jejunal epithelial apoptosis. However, the underlying mechanism remains unknown. In this study, the protective effect and the mechanism of glycine on apoptosis and dysfunction in intestinal barrier induced by brefeldin A (BFA), an ER stress inducer, was explored in porcine intestinal epithelial cells (IPEC-1). The results showed that BFA treatment led to enhanced apoptosis and upregulation of proteins involved in ER stress signaling, including inositol-requiring enzyme 1α (IRE1α), activating transcription factor 6α (ATF6α), c-Jun N-terminal kinase (JNK), and C/EBP-homologous protein (CHOP). In addition, BFA induced a dysfunction in intestinal epithelial barrier, as evidenced by the increased paracellular permeability, decreased transepithelial electrical resistance (TEER), and reduced abundance of tight junction proteins (occludin, claudin-1, zonula occludens [ZO]-1, and ZO-2). These alterations triggered by BFA were significantly abolished by glycine treatment (P < 0.05), indicating a protective effect of glycine on barrier function impaired by ER stress. Importantly, we found that the regulatory effect of glycine on intestinal permeability, proteins implicated in ER stress and apoptosis, as well as the morphological alterations of the ER were reversed by rapamycin. In summary, our results indicated that glycine alleviates ER stress-induced apoptosis and intestinal barrier dysfunction in IPEC-1 cells in a mammalian target of rapamycin complex 1 (mTORC1)-dependent manner. The data provides in vitro evidence and a mechanism for the protective effect of glycine against the disruption of intestinal barrier integrity induced by ER stress.

Cysteine as a Multifaceted Player in Kidney, the Cysteine-Related Thiolome and Its Implications for Precision Medicine

In this review encouraged by original data, we first provided in vivo evidence that the kidney, comparative to the liver or brain, is an organ particularly rich in cysteine. In the kidney, the total availability of cysteine was higher in cortex tissue than in the medulla and distributed in free reduced, free oxidized and protein-bound fractions (in descending order). Next, we provided a comprehensive integrated review on the evidence that supports the reliance on cysteine of the kidney beyond cysteine antioxidant properties, highlighting the relevance of cysteine and its renal metabolism in the control of cysteine excess in the body as a pivotal source of metabolites to kidney biomass and bioenergetics and a promoter of adaptive responses to stressors. This view might translate into novel perspectives on the mechanisms of kidney function and blood pressure regulation and on clinical implications of the cysteine-related thiolome as a tool in precision medicine.

Expression of gamma-glutamyltransferase 1 in glioblastoma cells confers resistance to cystine deprivation-induced ferroptosis

Ferroptosis is an iron-dependent mode of cell death caused by excessive oxidative damage to lipids. Lipid peroxidation is normally suppressed by glutathione peroxidase 4, which requires reduced glutathione. Cystine is a major resource for glutathione synthesis, especially in cancer cells. Therefore, cystine deprivation or inhibition of cystine uptake promotes ferroptosis in cancer cells. However, the roles of other molecules involved in cysteine deprivation–induced ferroptosis are unexplored. We report here that the expression of gamma-glutamyltransferase 1 (GGT1), an enzyme that cleaves extracellular glutathione, determines the sensitivity of glioblastoma cells to cystine deprivation–induced ferroptosis at high cell density (HD). In glioblastoma cells expressing GGT1, pharmacological inhibition or deletion of GGT1 suppressed the cell density–induced increase in intracellular glutathione levels and cell viability under cystine deprivation, which were restored by the addition of cysteinylglycine, the GGT product of glutathione cleavage. On the other hand, cystine deprivation induced glutathione depletion and ferroptosis in GGT1-deficient glioblastoma cells even at an HD. Exogenous expression of GGT1 in GGT1-deficient glioblastoma cells inhibited cystine deprivation–induced glutathione depletion and ferroptosis at an HD. This suggests that GGT1 plays an important role in glioblastoma cell survival under cystine-limited and HD conditions. We conclude that combining GGT inhibitors with ferroptosis inducers may provide an effective therapeutic approach for treating glioblastoma.

Understanding the role of cysteine in ferroptosis: progress & paradoxes

Cysteine is a conditionally essential amino acid required for the synthesis of proteins and many important intracellular metabolites. Cysteine depletion can trigger iron-dependent nonapoptotic cell death-ferroptosis. Despite this, cysteine itself is normally maintained at relatively low levels within the cell, and many mechanisms that could act to buffer cysteine depletion do not appear to be especially effective or active, at least in cancer cells. How do we reconcile these seemingly paradoxical features? Here, we describe the regulation of cysteine and its contribution to ferroptosis and speculate about how the levels of this amino acid are controlled to govern nonapoptotic cell death.

Amino Acids in Endoplasmic Reticulum Stress and Redox Signaling

Proteins are the chains of amino acids linked via peptide bonds. In cells, newly synthesized proteins are modified and folded in the endoplasmic reticulum (ER) and matured to be functional proteins before they are transported to other tissues or organs. In addition to protein synthesis, the ER is also a stress-sensing organelle for diverse biological functions, such as calcium storage, lipid synthesis, and cellular metabolism. Nutrient deprivation, accumulation of reactive oxygen species, and other intracellular insults can activate ER stress and unfolded protein response (UPR) to restore homeostasis. Dysfunction of the ER influences cellular physiology and metabolism, and contributes to the pathogenesis of various diseases. Amino acids are the building blocks for proteins of eukaryotic organisms. Both in vivo and in vitro studies have found that amino acids can function as signaling molecules to regulate gene expression, cell proliferation and apoptosis, immune response, and antioxidant capacity in numerous biological processes. Importantly, several lines of studies have indicated that amino acids regulate the abundances of proteins implicated in UPR and the redox state, therefore restoring the intracellular homeostasis. Amino acids play an important role in regulating ER stress and redox homeostasis in animal cells for their survival, growth, and development.

Dietary L-Tryptophan Regulates Colonic Serotonin Homeostasis in Mice with Dextran Sodium Sulfate-Induced Colitis

BACKGROUND

L-tryptophan (Trp) has been reported to regulate gut immune responses during inflammation. However, the underlying mechanisms are largely unknown.

OBJECTIVE

We investigated the role of Trp supplementation on the serotonin receptor (HTR)-mediated immune response in the colon of mice with dextran sodium sulfate (DSS)-induced colitis.

METHODS

In Experiment 1, male C57BL/6 mice were randomly assigned to 1 of 4 groups: Control (Con) or L-Trp supplementation [0.1 mg/(g body weight·d) in drinking water] (Trp) with (+DSS) or without 2% DSS in drinking water from days 8 to 14 of the 17-d study. In Experiments 2 and 3, Trp + DSS (Expt. 2) or DSS (Expt. 3) mice were treated as described above and subcutaneously administered with HTR1A or HTR4 antagonists (or their combination) or an HTR2 agonist from days 8 to 14 of the 15-d study. Changes in immune cell phenotypes, inflammatory mediators, and related cell signaling molecules were assessed by flow cytometry, real-time PCR, or Western blot. The mRNA abundances of Trp hydroxylase (Tph1), serotonin reuptake transporter (Slc6a4), and Htr in the colon were also assessed.

RESULTS

Trp supplementation before DSS treatment upregulated the expression of colonic Slc6a4 (0.49 compared with 0.30), Htr1a (1.14 compared with 0.65), and Htr4 (1.08 compared with 0.70), downregulated the expression of Htr2a (1.54 compared with 1.89), and decreased the colonic serotonin concentration (11.5 compared with 14.8 nmol/g tissue) (P < 0.01). Trp regulated the DSS-induced immune response partly through attenuating the activation of toll-like receptor 4 (TLR4)-STAT3 signaling and nucleus p-65. Either an HTR2 agonist or HTR1A and HTR4 antagonists reversed the effects of Trp.

CONCLUSIONS

In mice treated with DSS, Trp supplementation before DSS administration improved colonic immune responses partly by reducing colonic serotonin and subsequent interactions with HTR1A and HTR4, which are known to be present on neutrophils and macrophages.

Analysis of Tryptophan and Its Metabolites by High-Performance Liquid Chromatography

Tryptophan is a nutritionally essential amino acid for both humans and animals. Besides acting as a building block for protein synthesis, tryptophan (Trp) and its metabolites are crucial for maintaining neurological function, immunity, and homeostasis in the body. To uncover the regulatory role of Trp and its metabolites in cell nutrition, metabolism and physiology, various analytical methods, including high-performance liquid chromatography (HPLC), have been developed to determine key Trp metabolites. Here we describe a rapid and sensitive method for the simultaneous analysis of Trp and its metabolites along with other amino acids by HPLC involving in-line pre-column derivatization with o-phthaldialdehyde (OPA) and dual-channel fluorescence detection. OPA reacts very rapidly (within 1 min) with Trp, 5-hydroxytryptophan, 5-hydroxytryptamine, and tryptamine at room temperature (e.g., 20-25 °C) in an autosampler. Their derivatives are immediately injected into the HPLC column without the need for extraction. Trp metabolites that cannot react with OPA but are fluorescent can be detected by setting the excitation and emission wavelengths of the fluorescence detector in another detection channel. The autosampler is programmed to mix Trp and its metabolites with OPA for 1 min to generate highly fluorescent derivatives for HPLC separation and detection (Channel A, excitation = 270 nm and emission = 350 nm; Channel B, excitation = 340 nm and emission = 450 nm). The detection limit for Trp and its metabolites is 30 pmol/mL or 150 fmol/injection. The total time for chromatographic separation (including column regeneration) is 55 min for each sample. Our HPLC method can be used for the analysis of amino acids (including Trp) in alkaline protein hydrolysates and of Trp and its metabolites in biological samples.

Subchronic methionine load induces oxidative stress and provokes biochemical and histological changes in the rat liver tissue

The aim of this study was to assess the effects of L-cysteine (Cys) (7 mg/kg) and N-acetyl-L-cysteine (NAC) (50 mg/kg) in the rat liver caused by subchronic i.p. application of methionine (Met) (0.8 mmol/kg) during 21 days. Malondialdehyde (MDA) concentration, glutathione content (GSH), catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), and acetylcholinesterase (AchE) activities were determined in the liver tissue and activities of liver enzymes (AST, ALT, ALP, and GGT) and concentrations of total proteins and albumin were determinated in plasma/serum. Catalase, superoxide dismutase, and acetylcholinesterase activities were increased by Cys and NAC. Met caused periportal mononuclear infiltration and rare focal necrosis of hepatocytes. In Cys- and NAC-supplemented groups, intracellular edema and microvesicular fatty changes without necrosis were noticed. We observed decrease of AST, ALT, and ALP activity in the methionine-treated group. Our results indicate that Cys and NAC application can increase activity of antioxidative enzymes and prevent intensive histological changes in liver in condition of subchronic methionine exposure.

Analysis of Glutathione in Biological Samples by HPLC Involving Pre-Column Derivatization with o-Phthalaldehyde

Glutathione (GSH) forms conjugates with polyamines in prokaryotes and eukaryotes. There is also evidence suggesting cross-talk between GSH and polyamines to regulate cellular homeostasis and function, particularly under the conditions of oxidative stress. Because of its versatile roles in cell metabolism and function, a number of high performance liquid chromatography (HPLC) methods have been developed for glutathione analysis. Here, we describe our rapid and sensitive method for the analysis of GSH and the oxidized form of glutathione (GS-SG) in animal tissues and cells by HPLC involving pre-column derivatization with o-phthalaldehyde (OPA). OPA reacts very rapidly (within 1 min) with S-carboxymethyl-glutathione at room temperatures (e.g., 20-25 °C) in an autosampler, and their derivatives are immediately injected into the HPLC column without any need for extraction. This method requires two simple steps (a total of 15 min) before samples are loaded into the autosampler: (a) the conversion of GS-SG into GSH by 2-mercaptoethanol; and (b) the oxidation of GSH by iodoacetic acid to yield S-carboxymethyl-glutathione. The autosampler is programmed to mix S-carboxymethyl-glutathione with OPA for 1 min to generate a highly fluorescent derivative for HPLC separation and detection (excitation wavelength 340 nm and emission wavelength 450 nm). The detection limit for GSH and GS-SG is 15 pmol/ml or 375 fmol/injection. The total time for chromatographic separation (including column regeneration) is 16 min for each sample. Our routine HPLC technique is applicable for analyses of cysteine and cystine, as well as polyamines and GSH-polyamine conjugates in biological samples.

Drug toxicity profiling of a Saccharomyces cerevisiae deubiquitinase deletion panel shows that acetaminophen mimics tyrosine

Post-translational protein modification by addition or removal of the small polypeptide ubiquitin is involved in a range of critical cellular processes, like proteasomal protein degradation, DNA repair, gene expression, internalization of membrane proteins, and drug sensitivity. We recently identified genes important for acetaminophen (APAP) toxicity in a comprehensive screen and our findings suggested that a small set of yeast strains carrying deletions of ubiquitin-related genes can be informative for drug toxicity profiling. In yeast, approximately 20 different deubiquitinating enzymes (DUBs) have been identified, of which only one is essential for viability. We investigated whether the toxicity profile of DUB deletion yeast strains would be informative about the toxicological mode of action of APAP. A set of DUB deletion strains was tested for sensitivity and resistance to a diverse series of compounds, including APAP, quinine, ibuprofen, rapamycin, cycloheximide, cadmium, peroxide and amino acids and a cluster analysis was performed. Most DUB deletion strains showed an altered growth pattern when exposed to these compounds by being either more sensitive or more resistant than WT. Toxicity profiling of the DUB strains revealed a remarkable overlap between the amino acid tyrosine and acetaminophen (APAP), but not its stereoisomer AMAP. Furthermore, co-exposure of cells to both APAP and tyrosine showed an enhancement of the cellular growth inhibition, suggesting that APAP and tyrosine have a similar mode of action.

Therapeutic Potential of Amino Acids in Inflammatory Bowel Disease

Inflammatory bowel disease (IBD), which includes both ulcerative colitis and Crohn’s disease, is a chronic relapsing inflammation of the gastrointestinal tract, and is difficult to treat. The pathophysiology of IBD is multifactorial and not completely understood, but genetic components, dysregulated immune responses, oxidative stress, and inflammatory mediators are known to be involved. Animal models of IBD can be chemically induced, and are used to study etiology and to evaluate potential treatments of IBD. Currently available IBD treatments can decrease the duration of active disease but because of their adverse effects, the search for novel therapeutic strategies that can restore intestinal homeostasis continues. This review summarizes and discusses what is currently known of the effects of amino acids on the reduction of inflammation, oxidative stress, and cell death in the gut when IBD is present. Recent studies in animal models have identified dietary amino acids that improve IBD, but amino acid supplementation may not be adequate to replace conventional therapy. The animal models used in dietary amino acid research in IBD are described.

Comparisons of l-cysteine and d-cysteine toxicity in 4-week repeated-dose toxicity studies of rats receiving daily oral administration

Two 4-week repeated-dose toxicity studies were conducted to evaluate the potential toxicity of l-cysteine and d-cysteine. In one study, three groups of 6 male rats were each administered l-cysteine once daily by gavage at doses of 500, 1,000, or 2,000 mg/kg/day for 28 consecutive days. The control group was administered a 0.5% methylcellulose vehicle solution. The other study followed a similar protocol except that the experimental groups received d-cysteine. Toxicological observations showed that the l-cysteine-treated groups exhibited renal injuries such as basophilic tubules with eosinophilic material in the lumen, and there were increased numbers of basophilic tubules in all treated groups. In 1,000 or 2,000 mg/kg/day-treated groups, salivation and necropsy findings indicative of focal erosion in the stomach mucosa were found. Increases in reticulocyte counts were observed in the 2,000 mg/kg/day-treated group. Toxicological findings obtained for the d-cysteine-treated groups included anemia and renal injuries such as basophilic tubules with eosinophilic material in the lumen, increased numbers of basophilic tubules, and crystal deposition in the medulla in the 2,000 mg/kg/day-treated group. Additional findings included sperm granuloma in the epididymis, necropsy findings suggestive of focal erosion in the stomach mucosa, and salivation in the 1,000 or 2,000 mg/kg/day-treated groups. One rat in the 2,000 mg/kg/day-treated group died due to renal failure. In conclusion, the no-observed-adverse-effect levels (NOAELs) were estimated to be less than 500 mg/kg/day for l-cysteine and 500 mg/kg/day for d-cysteine under our study conditions. The toxicological profiles were similar for l-cysteine and d-cysteine; however, there were slight differences in the dose responses. The mechanisms underlying these differences remain to be determined.

Roles of amino acids in preventing and treating intestinal diseases: recent studies with pig models

Animal models are needed to study and understand a human complex disease. Because of their similarities in anatomy, structure, physiology, and pathophysiology, the pig has proven its usefulness in studying human gastrointestinal diseases, such as inflammatory bowel disease, ischemia/reperfusion injury, diarrhea, and cancer. To understand the pathogenesis of these diseases, a number of experimental models generated in pigs are available, for example, through surgical manipulation, chemical induction, microbial infection, and genetic engineering. Our interests have been using amino acids as therapeutics in pig and human disease models. Amino acids not only play an important role in protein biosynthesis, but also exert significant physiological effects in regulating immunity, anti-oxidation, redox regulation, energy metabolism, signal transduction, and animal behavior. Recent studies in pigs have shown that specific dietary amino acids can improve intestinal integrity and function under normal and pathological conditions that protect the host from different diseases. In this review, we summarize several pig models in intestinal diseases and how amino acids can be used as therapeutics in treating pig and human diseases.