Attenuated progression of diet-induced steatohepatitis in glutathione-deficient mice

Glutathione synthesis in the mouse liver supports lipid abundance through NRF2 repression

Cells rely on antioxidants to survive. The most abundant antioxidant is glutathione (GSH). The synthesis of GSH is non-redundantly controlled by the glutamate-cysteine ligase catalytic subunit (GCLC). GSH imbalance is implicated in many diseases, but the requirement for GSH in adult tissues is unclear. To interrogate this, we have developed a series of in vivo models to induce Gclc deletion in adult animals. We find that GSH is essential to lipid abundance in vivo. GSH levels are highest in liver tissue, which is also a hub for lipid production. While the loss of GSH does not cause liver failure, it decreases lipogenic enzyme expression, circulating triglyceride levels, and fat stores. Mechanistically, we find that GSH promotes lipid abundance by repressing NRF2, a transcription factor induced by oxidative stress. These studies identify GSH as a fulcrum in the liver's balance of redox buffering and triglyceride production.

Nonalcoholic Fatty Liver Disease and Omega-3 Fatty Acids: Mechanisms and Clinical Use

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic fatty liver disease worldwide, particularly in obese and type 2 diabetic individuals. Currently, there are no therapies for NAFLD that have been approved by the US Food and Drug Administration. Herein, we examine the rationale for using ω3 polyunsaturated fatty acids (PUFAs) in NAFLD therapy. This focus is based on the finding that NAFLD severity is associated with a reduction of hepatic C20-22 ω3 PUFAs. Because C20-22 ω3 PUFAs are pleiotropic regulators of cell function, loss of C20-22 ω3 PUFAs has the potential to significantly impact hepatic function. We describe NAFLD prevalence and pathophysiology as well as current NAFLD therapies. We also present evidence from clinical and preclinical studies that evaluated the capacity of C20-22 ω3 PUFAs to treat NAFLD. Given the clinical and preclinical evidence, dietary C20-22 ω3 PUFA supplementation has the potential to decrease human NAFLD severity by reducing hepatosteatosis and liver injury.

Differential effects of bariatric surgery and caloric restriction on hepatic one-carbon and fatty acid metabolism

Weight loss interventions, including dietary changes, pharmacotherapy, or bariatric surgery, prevent many of the adverse consequences of obesity, and may also confer intervention-specific benefits beyond those seen with decreased weight alone. We compared the molecular effects of different interventions on liver metabolism to understand the mechanisms underlying these benefits. Male rats on a high-fat, high-sucrose diet underwent sleeve gastrectomy (SG) or intermittent fasting with caloric restriction (IF-CR), achieving equivalent weight loss. The interventions were compared to ad-libitum (AL)-fed controls. Analysis of liver and blood metabolome and transcriptome revealed distinct and sometimes contrasting metabolic effects between the two interventions. SG primarily influenced one-carbon metabolic pathways, whereas IF-CR increased de novo lipogenesis and glycogen storage. These findings suggest that the unique metabolic pathways affected by SG and IF-CR contribute to their distinct clinical benefits, with bariatric surgery potentially influencing long-lasting changes through its effect on one-carbon metabolism.

Glutathione supports lipid abundance in vivo

Cells rely on antioxidants to survive. The most abundant antioxidant is glutathione (GSH). The synthesis of GSH is non-redundantly controlled by the glutamate-cysteine ligase catalytic subunit (GCLC). GSH imbalance is implicated in many diseases, but the requirement for GSH in adult tissues is unclear. To interrogate this, we developed a series of in vivo models to induce Gclc deletion in adult animals. We find that GSH is essential to lipid abundance in vivo. GSH levels are reported to be highest in liver tissue, which is also a hub for lipid production. While the loss of GSH did not cause liver failure, it decreased lipogenic enzyme expression, circulating triglyceride levels, and fat stores. Mechanistically, we found that GSH promotes lipid abundance by repressing NRF2, a transcription factor induced by oxidative stress. These studies identify GSH as a fulcrum in the liver's balance of redox buffering and triglyceride production.

Inducing ferroptosis has the potential to overcome therapy resistance in breast cancer

Breast cancer is the most common type of malignancy among women. Due to the iron-dependent character of breast cancer cells, they are more sensitive to ferroptosis compared to normal cells. It is possible to reverse tumor resistance by inducing ferroptosis in breast cancer cells, thereby improving tumor treatment outcomes. Ferroptosis is highly dependent on the balance of oxidative and antioxidant status. When ferroptosis occurs, intracellular iron levels are significantly increased, leading to increased membrane lipid peroxidation and ultimately triggering ferroptosis. Ferroptotic death is a form of autophagy-associated cell death. Synergistic use of nanoparticle-loaded ferroptosis-inducer with radiotherapy and chemotherapy achieves more significant tumor suppression and inhibits the growth of breast cancer by targeting cancer tissues, enhancing the sensitivity of cells to drugs, reducing the drug resistance of cancer cells and the toxicity of drugs. In this review, we present the current status of breast cancer and the mechanisms of ferroptosis. It is hopeful for us to realize effective treatment of breast cancer through targeted ferroptosis.

NADPH Oxidases Connecting Fatty Liver Disease, Insulin Resistance and Type 2 Diabetes: Current Knowledge and Therapeutic Outlook

Nonalcoholic fatty liver disease (NAFLD), characterized by ectopic fat accumulation in hepatocytes, is closely linked to insulin resistance and is the most frequent complication of type 2 diabetes mellitus (T2DM). One of the features connecting NAFLD, insulin resistance and T2DM is cellular oxidative stress. Oxidative stress refers to a redox imbalance due to an inequity between the capacity of production and the elimination of reactive oxygen species (ROS). One of the major cellular ROS sources is NADPH oxidase enzymes (NOX-es). In physiological conditions, NOX-es produce ROS purposefully in a timely and spatially regulated manner and are crucial regulators of various cellular events linked to metabolism, receptor signal transmission, proliferation and apoptosis. In contrast, dysregulated NOX-derived ROS production is related to the onset of diverse pathologies. This review provides a synopsis of current knowledge concerning NOX enzymes as connective elements between NAFLD, insulin resistance and T2DM and weighs their potential relevance as pharmacological targets to alleviate fatty liver disease.

Persistence of improved glucose homeostasis in Gclm null mice with age and cadmium treatment

Antioxidant signaling/communication is among the most important cellular defense and survival pathways, and the importance of redox signaling and homeostasis in aging has been well-documented. Intracellular levels of glutathione (GSH), a very important endogenous antioxidant, both govern and are governed by the Nrf2 pathway through expression of genes involved in its biosynthesis, including the subunits of the rate-limiting enzyme (glutamate cysteine ligase, GCL) in GSH production, GCLC and GCLM. Mice homozygous null for the Gclm gene are severely deficient in GSH compared to wild-type controls, expressing approximately 10% of normal GSH levels. To compensate for GSH deficiency, Gclm null mice have upregulated redox-regulated genes, and, surprisingly, are less susceptible to certain types of oxidative damage. Furthermore, young Gclm null mice display an interesting lean phenotype, resistance to high fat diet-induced diabetes and obesity, improved insulin and glucose tolerance, and decreased expression of genes involved in lipogenesis. However, the persistence of this phenotype has not been investigated into old age, which is important in light of studies which suggest aging attenuates antioxidant signaling, particularly in response to exogenous stimuli. In this work, we addressed whether aging compromises the favorable phenotype of increased antioxidant activity and improved glucose homeostasis observed in younger Gclm null mice. We present data showing that under basal conditions and in response to cadmium exposure (2 mg/kg, dosed once via intraperitoneal injection), the phenotype previously described in young (<6 months) Gclm null mice persists into old age (24+ months). We also provide evidence that transcriptional activation of the Nrf2, AMPK, and PPARγ pathways underlie the favorable metabolic phenotype observed previously in young Gclm null mice.

The Role of the Transsulfuration Pathway in Non-Alcoholic Fatty Liver Disease

The prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing and approximately 25% of the global population may have NAFLD. NAFLD is associated with obesity and metabolic syndrome, but its pathophysiology is complex and only partly understood. The transsulfuration pathway (TSP) is a metabolic pathway regulating homocysteine and cysteine metabolism and is vital in controlling sulfur balance in the organism. Precise control of this pathway is critical for maintenance of optimal cellular function. The TSP is closely linked to other pathways such as the folate and methionine cycles, hydrogen sulfide (H₂S) and glutathione (GSH) production. Impaired activity of the TSP will cause an increase in homocysteine and a decrease in cysteine levels. Homocysteine will also be increased due to impairment of the folate and methionine cycles. The key enzymes of the TSP, cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE), are highly expressed in the liver and deficient CBS and CSE expression causes hepatic steatosis, inflammation, and fibrosis in animal models. A causative link between the TSP and NAFLD has not been established. However, dysfunctions in the TSP and related pathways, in terms of enzyme expression and the plasma levels of the metabolites (e.g., homocysteine, cystathionine, and cysteine), have been reported in NAFLD and liver cirrhosis in both animal models and humans. Further investigation of the TSP in relation to NAFLD may reveal mechanisms involved in the development and progression of NAFLD.

Catechin-Rich Green Tea Extract and the Loss-of-TLR4 Signaling Differentially Alter the Hepatic Metabolome in Mice with Nonalcoholic Steatohepatitis

SCOPE

Catechin-rich green tea extract (GTE) limits inflammation in nonalcoholic steatohepatitis (NASH) consistent with a Toll-like receptor 4 (TLR4)-dependent mechanism. It is hypothesized that GTE supplementation during NASH will shift the hepatic metabolome similar to that attributed to the loss-of-TLR4 signaling.

METHODS AND RESULTS

Wild-type (WT) and loss-of-function TLR4-mutant (TLR4mut ) mice are fed a high-fat diet containing 0% or 2% GTE for 8 weeks prior to performing untargeted mass spectrometry-based metabolomics on liver tissue. The loss-of-TLR4 signaling and GTE shift the hepatic metabolome away from that of WT mice. However, relatively few metabolites are altered by GTE in WT mice to the same extent as the loss-of-TLR4 signaling in TLR4mut mice. GTE increases acetyl-coenzyme A precursors and spermidine to a greater extent than the loss-of-TLR4 signaling. Select metabolites associated with thiol metabolism are similarly affected by GTE and the loss-of-TLR4 signaling. Glycerophospholipid catabolites are decreased by GTE, but are unaffected in TLR4mut mice. Conversely, the loss-of-TLR4 signaling but not GTE increases several bile acid metabolites.

CONCLUSION

GTE limitedly alters the hepatic metabolome consistent with a TLR4-dependent mechanism. This suggests that the anti-inflammatory activities of GTE and loss-of-TLR4 signaling that regulate hepatic metabolism to abrogate NASH are likely due to distinct mechanisms.

Sophoricoside is a selective LXRβ antagonist with potent therapeutic effects on hepatic steatosis of mice

Nonalcoholic fatty liver disease (NAFLD) is a chronic liver disease characterized by the accumulation of triglycerides and associated with obesity, hyperlipidemia and insulin resistance. Currently, there is no therapy for NAFLD. Emerging evidences suggest that the inhibition of liver X receptor (LXR) activity may be a potential therapy for hepatic steatosis. Here, we identified that sophoricoside is a selective antagonist of LXRβ. Sophoricoside protected against obesity and glucose tolerance, and inhibited lipid accumulation in the liver of high-fat diet-induced obesity (DIO) mice and methionine and choline-deficient diet-induced nonalcoholic steatohepatitis mice. Furthermore, sophoricoside inhibited malondialdehyde, and increased superoxide dismutase and glutathione in the liver of the mice. In HepG2 cells, pretreatment with sophoricoside rescued GSH concentration decrease induced by H₂ O₂ treatment. Our data suggest that sophoricoside is a novel LXRβ selective antagonist and may improve glucose and lipid dysfunction, and attenuate lipid accumulation in the liver of DIO mice via anti-oxidant properties, which may be developed as a therapy for NAFLD.

Carbonyl Reductase 1 Plays a Significant Role in Converting Doxorubicin to Cardiotoxic Doxorubicinol in Mouse Liver, but the Majority of the Doxorubicinol-Forming Activity Remains Unidentified

Doxorubicin is a widely used cancer therapeutic, but its effectiveness is limited by cardiotoxic side effects. Evidence suggests cardiotoxicity is due not to doxorubicin, but rather its metabolite, doxorubicinol. Identification of the enzymes responsible for doxorubicinol formation is important in developing strategies to prevent cardiotoxicity. In this study, the contributions of three murine candidate enzymes to doxorubicinol formation were evaluated: carbonyl reductase (Cbr) 1, Cbr3, and thioredoxin reductase 1 (Tr1). Analyses with purified proteins revealed that all three enzymes catalyzed doxorubicin-dependent NADPH oxidation, but only Cbr1 and Cbr3 catalyzed doxorubicinol formation. Doxorubicin-dependent NADPH oxidation by Tr1 was likely due to redox cycling. Subcellular fractionation results showed that doxorubicin-dependent redox cycling activity was primarily microsomal, whereas doxorubicinol-forming activity was exclusively cytosolic, as were all three enzymes. An immunoclearing approach was used to assess the contributions of the three enzymes to doxorubicinol formation in the complex milieu of the cytosol. Immunoclearing Cbr1 eliminated 25% of the total doxorubicinol-forming activity in cytosol, but immunoclearing Cbr3 had no effect, even in Tr1 null livers that overexpressed Cbr3. The immunoclearing results constituted strong evidence that Cbr1 contributed to doxorubicinol formation in mouse liver but that enzymes other than Cbr1 also played a role, a conclusion supported by ammonium sulfate fractionation results, which showed that doxorubicinol-forming activity was found in fractions that contained little Cbr1. In conclusion, the results show that Cbr1 accounts for 25% of the doxorubicinol-forming activity in mouse liver cytosol but that the majority of the doxorubicinol-forming activity remains unidentified. SIGNIFICANCE STATEMENT: Earlier studies suggested carbonyl reductase (Cbr) 1 plays a dominant role in converting chemotherapeutic doxorubicin to cardiotoxic doxorubicinol, but a new immunoclearing approach described herein shows that Cbr1 accounts for only 25% of the doxorubicinol-forming activity in mouse liver cytosol, that two other candidate enzymes-Cbr3 and thioredoxin reductase 1-play no role, and that the majority of the activity remains unidentified. Thus, targeting Cbr1 is necessary but not sufficient to eliminate doxorubicinol-associated cardiotoxicity; identification of the additional doxorubicinol-forming activity is an important next challenge.

Sub-Chronic Microcystin-LR Liver Toxicity in Preexisting Diet-Induced Nonalcoholic Steatohepatitis in Rats

Microcystin-LR (MCLR) is a hepatotoxic cyanotoxin reported to cause a phenotype similar to nonalcoholic steatohepatitis (NASH). NASH is a common progressive liver disease that advances in severity due to exogenous stressors such as poor diet and toxicant exposure. Our objective was to determine how sub-chronic MCLR toxicity affects preexisting diet-induced NASH. Sprague-Dawley rats were fed one of three diets for 10 weeks: control, methionine and choline deficient (MCD), or high fat/high cholesterol (HFHC). After six weeks of diet, animals received vehicle, 10 µg/kg, or 30 µg/kg MCLR via intraperitoneal injection every other day for the final 4 weeks. Incidence and severity scoring of histopathology endpoints suggested that MCLR toxicity drove NASH to a less fatty and more fibrotic state. In general, expression of genes involved in de novo lipogenesis and fatty acid esterification were altered in favor of decreased steatosis. The higher MCLR dose increased expression of genes involved in fibrosis and inflammation in the control and HFHC groups. These data suggest MCLR toxicity in the context of preexisting NASH may drive the liver to a more severe phenotype that resembles burnt-out NASH.

Nonalcoholic fatty liver disease alters microcystin-LR toxicokinetics and acute toxicity

Microcystin-LR (MCLR) is a cyanotoxin produced by blue-green algae that causes liver and kidney toxicities. MCLR toxicity is dependent on cellular uptake through the organic anion transporting polypeptide (OATP) transporters. Nonalcoholic fatty liver disease (NAFLD) progresses through multiple stages, alters expression of hepatic OATPs, and is associated with chronic kidney disease. The purpose of this study was to determine whether NAFLD increases systemic exposure to MCLR and influences acute liver and kidney toxicities. Rats were fed a control diet or two dietary models of NAFLD; methionine and choline deficient (MCD) or high fat/high cholesterol (HFHC). Two studies were performed in these groups: 1) a single dose intravenous toxicokinetic study (20 μg/kg), and 2) a single dose intraperitoneal toxicity study (60 μg/kg). Compared to control rats, plasma MCLR area under the concentration-time curve (AUC) in MCD rats doubled, whereas biliary clearance (Clbil) was unchanged; in contrast, plasma AUC in HFHC rats was unchanged, whereas Clbil approximately doubled. Less MCLR bound to PP2A was observed in the liver of MCD rats. This shift in exposure decreased the severity of liver pathology only in the MCD rats after a single toxic dose of MCLR (60 μg/kg). In contrast, the single toxic dose of MCLR increased hepatic inflammation, plasma cholesterol, proteinuria, and urinary KIM1 in HFHC rats more than MCLR exposed control rats. In conclusion, rodent models of NAFLD alter MCLR toxicokinetics and acute toxicity and may have implications for liver and kidney pathologies in NAFLD patients.

Hepatic metabolic adaptation in a murine model of glutathione deficiency

Glutathione (GSH), the most abundant cellular non-protein thiol, plays a pivotal role in hepatic defense mechanisms against oxidative damage. Despite a strong association between disrupted GSH homeostasis and liver diseases of various etiologies, it was shown that GSH-deficient glutamate-cysteine ligase modifier subunit (Gclm)-null mice are protected against fatty liver development induced by a variety of dietary and environmental insults. The biochemical mechanisms underpinning this protective phenotype have not been clearly defined. The purpose of the current study was to characterize the intrinsic metabolic signature in the livers from GSH deficient Gclm-null mice. Global profiling of hepatic polar metabolites revealed a spectrum of changes in amino acids and metabolites derived from fatty acids, glucose and nucleic acids due to the loss of GCLM. Overall, the observed low GSH-driven metabolic changes represent metabolic adaptations, including elevations in glutamate, aspartate, acetyl-CoA and gluconate, which are beneficial for the maintenance of cellular redox and metabolic homeostasis.

Obesity Development in a Miniature Yucatan Pig Model: A Multi-compartmental Metabolomics Study on Cloned and Normal Pigs Fed Restricted or Ad Libitum High-Energy Diets

Miniature-pig models for human metabolic disorders such as obesity and metabolic syndrome are gaining popularity. However, in-depth knowledge on the phenotypic and metabolic effects of metabolic dysregulation is lacking, and ad libitum feeding is not well-characterized in these pig breeds. Therefore, an investigation was performed into the metabolome of Yucatan minipigs fed ad libitum or restricted diets. Furthermore, we used cloned and conventional minipigs to assess if cloning reflects a presumably lowered variation between subjects. For 5 months, 17 female Yucatan minipigs were fed either ad libitum or restricted Western-style diets. Serum, urine, and liver tissues were collected and analyzed by non-targeted liquid chromatography-mass spectrometry metabolomics and by biochemical analyses. Several metabolic pathways were deregulated as a result of obesity and increased energy-dense feed intake, particularly the hepatic glutathione pathway and the pantothenic acid and tryptophan metabolic pathways in serum and urine. Although cloned minipigs were phenotypically similar to wild-type minipigs, the metabolomics analysis of serum and liver tissues showed several altered pathways, such as amino acid and purine metabolism. These changes, as an effect of cloning, could limit the use of cloned models in dietary intervention studies and provides no evidence of decreased variability between subjects.

The role of the thioredoxin/thioredoxin reductase system in the metabolic syndrome: towards a possible prognostic marker?

Mammalian thioredoxin reductase (TrxR) is a selenoprotein with three existing isoenzymes (TrxR1, TrxR2, and TrxR3), which is found primarily intracellularly but also in extracellular fluids. The main substrate thioredoxin (Trx) is similarly found (as Trx1 and Trx2) in various intracellular compartments, in blood plasma, and is the cell's major disulfide reductase. Thioredoxin reductase is necessary as a NADPH-dependent reducing agent in biochemical reactions involving Trx. Genetic and environmental factors like selenium status influence the activity of TrxR. Research shows that the Trx/TrxR system plays a significant role in the physiology of the adipose tissue, in carbohydrate metabolism, insulin production and sensitivity, blood pressure regulation, inflammation, chemotactic activity of macrophages, and atherogenesis. Based on recent research, it has been reported that the modulation of the Trx/TrxR system may be considered as a new target in the management of the metabolic syndrome, insulin resistance, and type 2 diabetes, as well as in the treatment of hypertension and atherosclerosis. In this review evidence about a possible role of this system as a marker of the metabolic syndrome is reported.

Hydrogen sulfide mediates athero-protection against oxidative stress via S-sulfhydration

S-sulfhydration is a signalling pathway of hydrogen sulfide (H₂S), which is suggested as an anti-atherogenic molecule that may protect against atherosclerosis. The identification of S-sulfhydrated proteins by proteomic approach could be a major step towards understanding the mechanisms of H₂S in response to atherosclerosis. The present study studied targeted S-sulfhydrated proteins using the modified biotin switch method followed by matrix-assisted laser desorption/ionisation time of flight tandem mass spectrometry identification. The results showed that H₂S can protect against atherosclerosis by reducing body weight gain and alleviating aortic plaque formation. In addition, H₂S treatment can increase aortic protein S-sulfhydration. Seventy targeted S-sulfhydrated aortic proteins were identified, mainly involved in metabolism, stimulus response and biological regulation, as determined by gene ontology database analysis. H₂S also induced S-sulfhydration of glutathione peroxidase 1 and further reduced lipid peroxidation and increased antioxidant defence in the aorta by prompting glutathione synthesis. Our data suggest that H₂S is a cardiovascular-protective molecule that S-sulfhydrates a subset of proteins that are mainly responsible for lipid metabolism and exerts its cytoprotective effects to clear free radicals and inhibit oxidative stress through cysteine S-sulfhydration.

Dietary Methionine Restriction Regulates Liver Protein Synthesis and Gene Expression Independently of Eukaryotic Initiation Factor 2 Phosphorylation in Mice

Background: The phosphorylation of eukaryotic initiation factor 2 (p-eIF2) during dietary amino acid insufficiency reduces protein synthesis and alters gene expression via the integrated stress response (ISR).Objective: We explored whether a Met-restricted (MR) diet activates the ISR to reduce body fat and regulate protein balance.Methods: Male and female mice aged 3-6 mo with either whole-body deletion of general control nonderepressible 2 (Gcn2) or liver-specific deletion of protein kinase R-like endoplasmic reticulum kinase (Perk) alongside wild-type or floxed control mice were fed an obesogenic diet sufficient in Met (0.86%) or an MR (0.12% Met) diet for ≤5 wk. Ala enrichment with deuterium was measured to calculate protein synthesis rates. The guanine nucleotide exchange factor activity of eIF2B was measured alongside p-eIF2 and hepatic mRNA expression levels at 2 d and 5 wk. Metabolic phenotyping was conducted at 4 wk, and body composition was measured throughout. Results were evaluated with the use of ANOVA (P < 0.05).Results: Feeding an MR diet for 2 d did not increase hepatic p-eIF2 or reduce eIF2B activity in wild-type or Gcn2-/- mice, yet many genes transcriptionally regulated by the ISR were altered in both strains in the same direction and amplitude. Feeding an MR diet for 5 wk increased p-eIF2 and reduced eIF2B activity in wild-type but not Gcn2-/- mice, yet ISR-regulated genes altered in both strains similarly. Furthermore, the MR diet reduced mixed and cytosolic but not mitochondrial protein synthesis in both the liver and skeletal muscle regardless of Gcn2 status. Despite the similarities between strains, the MR diet did not increase energy expenditure or reduce body fat in Gcn2-/- mice. Finally, feeding the MR diet to mice with Perk deleted in the liver increased hepatic p-eIF2 and altered body composition similar to floxed controls.Conclusions: Hepatic activation of the ISR resulting from an MR diet does not require p-eIF2. Gcn2 status influences body fat loss but not protein balance when Met is restricted.

Current Concepts in Pediatric Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) represents a spectrum of disease. Its increasing prevalence is a direct result of historically high rates of obesity. Hepatocyte lipid accumulation is the first step in a cascade of metabolic and inflammatory events thought to precipitate NAFLD. Histologic findings provide insight into these events. Lifestyle modification remains the primary therapy in children. Current recommendations include vitamin E treatment in those with biopsy-proven NASH. Trials of novel drugs are ongoing in adults. As efficacy/safety are established, these therapies may be tenable for use in children. At the current time, biopsy-driven histology endpoints are necessary to establish whether future therapies can improve pediatric or adult-type NASH in children.

A longitudinal study of whole body, tissue, and cellular physiology in a mouse model of fibrosing NASH with high fidelity to the human condition

The sequence of events that lead to inflammation and fibrosing nonalcoholic steatohepatitis (NASH) is incompletely understood. Hence, we investigated the chronology of whole body, tissue, and cellular events that occur during the evolution of diet-induced NASH. Male C57Bl/6 mice were assigned to a fast-food (FF; high calorie, high cholesterol, high fructose) or standard-chow (SC) diet over a period of 36 wk. Liver histology, body composition, mitochondrial respiration, metabolic rate, gene expression, and hepatic lipid content were analyzed. Insulin resistance [homeostasis model assessment-insulin resistance (HOMA-IR)] increased 10-fold after 4 wk. Fibrosing NASH was fully established by 16 wk. Total hepatic lipids increased by 4 wk and remained two- to threefold increased throughout. Hepatic triglycerides declined from sixfold increase at 8 wk to threefold increase by 36 wk. In contrast, hepatic cholesterol levels steadily increased from baseline at 8 wk to twofold by 36 wk. The hepatic immune cell population altered over time with macrophages persisting beyond 16 wk. Mitochondrial oxygen flux rates of FF mice diet were uniformly lower with all the tested substrates (13-276 pmol·s^-1·ml^-1 per unit citrate synthase) than SC mice (17-394 pmol·s^-1·ml^-1 per unit citrate synthase) and was accompanied by decreased mitochondrial:nuclear gene copy number ratios after 4 wk. Metabolic rate was lower in FF mice. Mitochondrial glutathione was significantly decreased at 24 wk in FF mice. Expression of dismutases and catalase was also decreased in FF mice. The evolution of NASH in the FF diet-induced model is multiphasic, particularly in terms of hepatic lipid composition. Insulin resistance precedes hepatic inflammation and fibrosis. Mitochondrial dysfunction and depletion occur after the histological features of NASH are apparent. Collectively, these observations provide a unique overview of the sequence of changes that coevolve with the histological evolution of NASH.NEW & NOTEWORTHY This study demonstrates in a first of kind longitudinal analysis, the evolution of nonalcoholic steatohepatitis (NASH) on a fast-food diet-induced model. Key findings include 1) hepatic lipid composition changes in a multiphasic fashion as NASH evolves; 2) insulin resistance precedes hepatic inflammation and fibrosis, answering a longstanding chicken-and-egg question regarding the relationship of insulin resistance to liver histology in NASH; and 3) mitochondrial dysfunction and depletion occur after the histological features of NASH are apparent.

Oxidative Stress as a Critical Factor in Nonalcoholic Fatty Liver Disease Pathogenesis

SIGNIFICANCE

Nonalcoholic fatty liver disease (NAFLD), characterized by liver triacylglycerol build-up, has been growing in the global world in concert with the raised prevalence of cardiometabolic disorders, including obesity, diabetes, and hyperlipemia. Redox imbalance has been suggested to be highly relevant to NAFLD pathogenesis. Recent Advances: As a major health problem, NAFLD progresses to the more severe nonalcoholic steatohepatitis (NASH) condition and predisposes susceptible individuals to liver and cardiovascular disease. Although NAFLD represents the predominant cause of chronic liver disorders, the mechanisms of its development and progression remain incompletely understood, even if various scientific groups ascribed them to the occurrence of insulin resistance, dyslipidemia, inflammation, and apoptosis. Nevertheless, oxidative stress (OxS) more and more appears as the most important pathological event during NAFLD development and the hallmark between simple steatosis and NASH manifestation.

CRITICAL ISSUES

The purpose of this article is to summarize recent developments in the understanding of NAFLD, essentially focusing on OxS as a major pathogenetic mechanism. Various attempts to translate reactive oxygen species (ROS) scavenging by antioxidants into experimental and clinical studies have yielded mostly encouraging results.

FUTURE DIRECTIONS

Although augmented concentrations of ROS and faulty antioxidant defense have been associated to NAFLD and related complications, mechanisms of action and proofs of principle should be highlighted to support the causative role of OxS and to translate its concept into the clinic. Antioxid. Redox Signal. 26, 519-541.

Clinical implications of understanding the association between oxidative stress and pediatric NAFLD

Oxidative stress is central to the pathogenesis of non-alcoholic steatohepatitis. The reactive oxygen species (ROS) that characterise oxidative stress are generated in several cellular sites and their production is influence by multi-organ interactions. Areas covered: Mitochondrial dysfunction is the main source of ROS in fatty liver and is closely related to endoplasmic reticulum stress. Both are caused by lipotoxicity and together these three factors form a cycle of progressive organelle damage, resulting in sterile inflammation and apoptosis. Adipose tissue inflammation and intestinal dysbiosis provide substrates for ROS formation and trigger immune activation. Obstructive sleep apnea and abnormal divalent metal metabolism may also play a role. Expert commentary: The majority of available high-quality data originates from studies in adults and there are fewer therapeutic trials performed in pediatric cohorts, therefore conclusions are generalised to children. Establishing the role of organelle interactions, and its relationship with oxidative stress in steatohepatitis, is a rapidly evolving area of research.

Exploring the Lean Phenotype of Glutathione-Depleted Mice: Thiol, Amino Acid and Fatty Acid Profiles

BACKGROUND

Although reduced glutathione (rGSH) is decreased in obese mice and humans, block of GSH synthesis by buthionine sulfoximine (BSO) results in a lean, insulin-sensitive phenotype. Data is lacking about the effect of BSO on GSH precursors, cysteine and glutamate. Plasma total cysteine (tCys) is positively associated with stearoyl-coenzyme A desaturase (SCD) activity and adiposity in humans and animal models.

OBJECTIVE

To explore the phenotype, amino acid and fatty acid profiles in BSO-treated mice.

DESIGN

Male C3H/HeH mice aged 11 weeks were fed a high-fat diet with or without BSO in drinking water (30 mmol/L) for 8 weeks. Amino acid and fatty acid changes were assessed, as well as food consumption, energy expenditure, locomotor activity, body composition and liver vacuolation (steatosis).

RESULTS

Despite higher food intake, BSO decreased particularly fat mass but also lean mass (both P<0.001), and prevented fatty liver vacuolation. Physical activity increased during the dark phase. BSO decreased plasma free fatty acids and enhanced insulin sensitivity. BSO did not alter liver rGSH, but decreased plasma total GSH (tGSH) and rGSH (by ~70%), and liver tGSH (by 82%). Glutamate accumulated in plasma and liver. Urine excretion of cysteine and its precursors was increased by BSO. tCys, rCys and cystine decreased in plasma (by 23-45%, P<0.001 for all), but were maintained in liver, at the expense of decreased taurine. Free and total plasma concentrations of the SCD products, oleic and palmitoleic acids were decreased (by 27-38%, P <0.001 for all).

CONCLUSION

Counterintuitively, block of GSH synthesis decreases circulating tCys, raising the question of whether the BSO-induced obesity-resistance is linked to cysteine depletion. Cysteine-supplementation of BSO-treated mice is warranted to dissect the effects of cysteine and GSH depletion on energy metabolism.

Chronic Glutathione Depletion Confers Protection against Alcohol-induced Steatosis: Implication for Redox Activation of AMP-activated Protein Kinase Pathway

The pathogenesis of alcoholic liver disease (ALD) is not well established. However, oxidative stress and associated decreases in levels of glutathione (GSH) are known to play a central role in ALD. The present study examines the effect of GSH deficiency on alcohol-induced liver steatosis in Gclm knockout (KO) mice that constitutively have ≈15% normal hepatic levels of GSH. Following chronic (6 week) feeding with an ethanol-containing liquid diet, the Gclm KO mice were unexpectedly found to be protected against steatosis despite showing increased oxidative stress (as reflected in elevated levels of CYP2E1 and protein carbonyls). Gclm KO mice also exhibit constitutive activation of liver AMP-activated protein kinase (AMPK) pathway and nuclear factor-erythroid 2–related factor 2 target genes, and show enhanced ethanol clearance, altered hepatic lipid profiles in favor of increased levels of polyunsaturated fatty acids and concordant changes in expression of genes associated with lipogenesis and fatty acid oxidation. In summary, our data implicate a novel mechanism protecting against liver steatosis via an oxidative stress adaptive response that activates the AMPK pathway. We propose redox activation of the AMPK may represent a new therapeutic strategy for preventing ALD.

Kombucha tea prevents obese mice from developing hepatic steatosis and liver damage

Nonalcoholic fatty liver disease (NAFLD) is associated with the increased accumulation of hepatocellular lipids. Although Kombucha tea (KT) has emerged as a substance protecting the liver from damage, the effects of KT in NAFLD remain unclear. Hence, we investigated whether KT influenced hepatic steatosis. Db/db mice were fed either control or methionine/choline-deficient (MCD) diets for 4 weeks. The MCD diet group was treated with KT or water for 3 weeks. KT treatment alleviated macrovesicular steatosis compared to the MCD-fed group. The levels of triglyceride, ALT, and AST also decreased in the KT+MCD-treated db/db mice. RNA expression in the MCD+KT group showed reduced triglyceride synthesis and uptake of fatty acids. Immunostaining and western blot assays for active caspase-3 demonstrated a lower level of apoptosis in the MCD+KT than in the MCD group. These results demonstrate that KT attenuated lipid accumulation and protected the liver from damage, promoting liver restoration in mice.

Cysteine and hydrogen sulphide in the regulation of metabolism: insights from genetics and pharmacology

Obesity and diabetes represent a significant and escalating worldwide health burden. These conditions are characterized by abnormal nutrient homeostasis. One such perturbation is altered metabolism of the sulphur‐containing amino acid cysteine. Obesity is associated with elevated plasma cysteine, whereas diabetes is associated with reduced cysteine levels. One mechanism by which cysteine may act is through its enzymatic breakdown to produce hydrogen sulphide (H₂S), a gasotransmitter that regulates glucose and lipid homeostasis. Here we review evidence from both pharmacological studies and transgenic models suggesting that cysteine and hydrogen sulphide play a role in the metabolic dysregulation underpinning obesity and diabetes. We then outline the growing evidence that regulation of hydrogen sulphide levels through its catabolism can impact metabolic health. By integrating hydrogen sulphide production and breakdown pathways, we re‐assess current hypothetical models of cysteine and hydrogen sulphide metabolism, offering new insight into their roles in the pathogenesis of obesity and diabetes. © 2015 The Authors. Pathological Society of Great Britain and Ireland.

Glutathione Depletion Accelerates Cigarette Smoke-Induced Inflammation and Airspace Enlargement

The study objective was to assess age-related changes in glutathione (GSH) adaptive response to cigarette smoke (CS) exposure. Older cigarette smokers show a decline (67%) in lung epithelial lining fluid (ELF) GSH and a 1.8-fold decreased GSH adaptive response to cigarette smoking with a concomitant elevation (47%) of exhaled nitric oxide compared with younger smokers. In order to isolate the changes in tissue GSH from other age-related effects, pharmacological inhibition of the rate limiting step in GSH synthesis was employed to examine the lung's response to CS exposure in young mice. The γ-glutamylcysteine ligase inhibitor L-buthionine-sulfoximine (BSO) was administered in the drinking water (20 mM) to decrease by half the in vivo GSH levels to those found in aged mice and humans. Mice were then exposed to CS (3 h/day) for 5 or 15 days. Biochemical analysis of the ELF and lung tissue revealed an inhibition of the CS-induced GSH adaptive response by BSO with a concurrent increase in mixed protein-GSH disulfides indicating increased cysteine oxidation. The prevention of the GSH adaptive response led to an increase in pro-inflammatory cytokines present in the lung. Airspace enlargement is a hallmark of lung emphysema and was observed in mice treated with BSO and exposed to CS for as little as 15 days, whereas these types of changes normally take up to 6 months in this model. BSO treatment potentiated both lung elastase and matrix metalloproteinase activity in the CS group. These data suggest that age-related decline in the GSH adaptive response can markedly accelerate many of the factors thought to drive CS-induced emphysema.

Glutathione and thioredoxin antioxidant pathways synergize to drive cancer initiation and progression

Controversy over the role of antioxidants in cancer has persisted for decades. Here, we demonstrate that synthesis of the antioxidant glutathione (GSH), driven by GCLM, is required for cancer initiation. Genetic loss of Gclm prevents a tumor's ability to drive malignant transformation. Intriguingly, these findings can be replicated using an inhibitor of GSH synthesis, but only if delivered prior to cancer onset, suggesting that at later stages of tumor progression GSH becomes dispensable potentially due to compensation from alternative antioxidant pathways. Remarkably, combined inhibition of GSH and thioredoxin antioxidant pathways leads to a synergistic cancer cell death in vitro and in vivo, demonstrating the importance of these two antioxidants to tumor progression and as potential targets for therapeutic intervention.

Metabolism of doxorubicin to the cardiotoxic metabolite doxorubicinol is increased in a mouse model of chronic glutathione deficiency: A potential role for carbonyl reductase 3

Doxorubicin is highly effective at inducing DNA double-strand breaks in rapidly dividing cells, which has led to it being a widely used cancer chemotherapeutic. However, clinical administration of doxorubicin is limited by off-target cardiotoxicity, which is thought to be mediated by doxorubicinol, the primary alcohol metabolite of doxorubicin. Carbonyl reductase 1 (CBR1), a well-characterized monomeric enzyme present at high basal levels in the liver, is known to exhibit activity toward doxorubicin. Little is known about a closely related enzyme, carbonyl reductase 3 (CBR3), which is present in the liver at low basal levels but is highly inducible by the transcription factor Nrf2. Genetic polymorphisms in CBR3, but not CBR1, are associated with differential cardiac outcomes in doxorubicin treated pediatric patients. Cbr3 mRNA and CBR3 protein are highly expressed in the livers of Gclm-/- mice (a mouse model of glutathione deficiency) relative to wild type mice. In the present study, we first investigated the ability of CBR3 to metabolize doxorubicin. Incubations of doxorubicin and purified recombinant murine CBR3 (mCBR3) were analyzed for doxorubicinol formation using HPLC, revealing for the first time that doxorubicin is a substrate of mCBR3. Moreover, hepatocytes from Gclm-/- mice produced more doxorubicinol than Gclm+/+ hepatocytes. In addition, differentiated rat myoblasts (C2C12 cells) co-cultured with primary Gclm-/- murine hepatocytes were more sensitive to doxorubicin-induced cytostasis/cytotoxicity than incubations with Gclm+/+ hepatocytes. Our results indicate a potentially important role for CBR3 in doxorubicin-induced cardiotoxicity. Because there is likely to be variability in hepatic CBR3 activity in humans (due to either genetic or epigenetic influences on its expression), these data also suggest that inhibition of CBR3 may provide protection from doxorubicinol cardiotoxicity.

3, 3'-Diindolylmethane alleviates steatosis and the progression of NASH partly through shifting the imbalance of Treg/Th17 cells to Treg dominance

This study was designed to discuss the effects of 3, 3'-diindolylmethane (DIM) on methionine-choline-deficient (MCD)-diet induced mouse nonalcoholic steatohepatitis (NASH) and the potential mechanisms. NASH mice were administrated with or without DIM at different concentrations for 8 weeks. Both the in-vivo and in-vitro effects of DIM on Treg/Th17 imbalance during NASH progression were analyzed. The in-vivo blocking of CD25 or IL-17 was performed to respectively deplete respective function of Treg or Th17 subset. Besides, with the assistance of AhR antagonist CH223191 and anti-TLR4 neutralizing antibody, we designed the in-vitro DIM-incubation experiments to discuss the roles of aryl hydrocarbon receptor (AhR) (CYP1A1, CYP1B1) and toll-like receptor 4 (TLR4) on DIM's effects when shifting Treg/Th17 imbalance. Notably, in NASH mouse models, DIM alleviated hepatic steatosis and inflammation, and shifted the Treg/Th17 imbalance from MCD diet-induced Th17 dominance to Treg dominance. In-vitro, DIM not only significantly up-regulated the mRNAs of Foxp3 (Treg-specific) in purified spleen CD4(+) T cells, but also enhanced the immunosuppressive function of these Treg cells. Besides, DIM significantly up-regulated the proteins of CYP1A1 and CYP1B1 whereas down-regulated those of TLR4 on CD4(+) T cells from MCD-diet mice. Moreover, blocking AhR attenuated while blocking TLR4 enhanced the effects of DIM when regulating Treg/Th17 imbalance. Conclusively, DIM could be used as a potential therapeutic candidate to treat NASH based on its dramatic induction of Treg dominance to alleviate intra-hepatic inflammation, suggesting us a clue that the dietary cruciferous vegetables (containing abundant DIM) might exist as a protective factor for patients with NASH-related liver diseases.

Body composition in gene knockouts of sulfur amino acid-metabolizing enzymes

Plasma concentrations of several amino acids are elevated in human obesity and insulin resistance, but there is no conclusive evidence on whether the amino acid alterations are causal. Dietary restriction of the essential SAA methionine (MR) in rats produces a hypermetabolic phenotype, with an integrated set of transcriptional changes in lipid enzymes in liver and adipose tissue. MR also induces an array of changes in methionine metabolites, including elevated plasma homocysteine and decreased cystathionine, cysteine, glutathione, and taurine. Several knockouts of enzymes acting downstream of methionine recapitulate the phenotypic results of MR, suggesting that the MR phenotype may be driven by changes distal to methionine. Here we review the changes in SAA and body composition in seven relevant knockout mouse models. All seven models feature decreased body weight, which in five of these have been further explored and shown to result from predominantly decreased fat mass. Common to several models is increased energy expenditure, enhanced insulin sensitivity, and protection against dietary obesity, as occurs in MR. A decrease in plasma total cysteine concentrations is also seen in most models. The lean phenotype could often be reversed by dietary supplementation of cysteine or choline, but not taurine, betaine or a H2S donor. Importantly, the plasma concentrations of both cysteine and choline are positively associated with fat mass in large populations studies, while taurine, betaine, and H2S are not. Collectively, the emerging data from dietary and knockout models are in harmony with human epidemiologic data, suggesting that the availability of key nutrients in the SAA pathway regulates fat storage pathways.

Methionine restriction prevents the progression of hepatic steatosis in leptin-deficient obese mice

OBJECTIVE

This study investigated the effects of dietary methionine restriction (MR) on the progression of established hepatic steatosis in the leptin-deficient ob/ob mouse.

MATERIAL/METHODS

Ten-week-old ob/ob mice were fed diets containing 0.86% (control-fed; CF) or 0.12% methionine (MR) for 14 weeks. At 14 weeks, liver and fat were excised and blood was collected for analysis. In another study, blood was collected to determine in vivo triglyceride (TG) and very-low-density lipoprotein (VLDL) secretion rates. Liver histology was conducted to determine the severity of steatosis. Hepatic TG, free fatty acid levels, and fatty acid oxidation (FAO) were also measured. Gene expression was analyzed by quantitative PCR.

RESULTS

MR reversed the severity of steatosis in the ob/ob mouse. This was accompanied by reduced body weight despite similar weight-specific food intake. Compared with the CF group, hepatic TG levels were significantly reduced in response to MR, but adipose tissue weight was not decreased. MR reduced insulin and HOMA ratios but increased total and high-molecular-weight adiponectin levels. Scd1 gene expression was significantly downregulated, while Acadvl, Hadha, and Hadhb were upregulated in MR, corresponding with increased β-hydroxybutyrate levels and a trend toward increased FAO. The VLDL secretion rate was also significantly increased in the MR mice, as were the mRNA levels of ApoB and Mttp. The expression of inflammatory markers, such as Tnf-α and Ccr2, was also downregulated by MR.

CONCLUSIONS

Our data indicate that MR reverses steatosis in the ob/ob mouse liver by promoting FAO, increasing the export of lipids, and reducing obesity-related inflammatory responses.

Dynamics of oxidative/nitrosative stress in mice with methionine-choline-deficient diet-induced nonalcoholic fatty liver disease

Insulin resistance, oxidative stress, and proinflammatory cytokines play a key role in pathogenesis of nonalcoholic fatty liver disease (NAFLD). The aim of our study was to investigate the dynamics of oxidative/nitrosative stress in methionine-choline-deficient (MCD) diet -induced NAFLD in mice. Male C57BL/6 mice were divided into following groups: group 1: control group on standard diet; group 2: MCD diet for 2, 4, and 6 weeks (MCD2, MCD4, and MCD6, respectively). After treatment, liver and blood samples were taken for histopathology, alanine- and aspartate aminotransferase, acute phase reactants, and oxidative/nitrosative stress parameters. Liver malondialdehyde level was higher in all MCD-fed groups versus control group (p < 0.01), while nitrites + nitrates level showed a progressive increase. The activity of total superoxide dismutase and its isoenzymes was significantly lower in all MCD-fed groups (p < 0.01). Although catalase activity was significantly lower in MCD-fed animals at all intervals (p < 0.01), the lowest activity of this enzyme was evident in MCD4 group. Liver content of glutathione was lower in MCD4 (p < 0.05) and MCD6 group (p < 0.01) versus control. : Ferritin and C-reactive protein serum concentration were significantly higher only in MCD6 group. Our study suggests that MCD diet induces a progressive rise in nitrosative stress in the liver. Additionally, the most prominent decrease in liver antioxidative capacity is in the fourth week, which implies that application of antioxidants would be most suitable in this period, in order to prevent nonalcoholic steatohepatitis but not the initial NAFLD phase.

In utero exposure to benzo[a]pyrene increases adiposity and causes hepatic steatosis in female mice, and glutathione deficiency is protective

Polycyclic aromatic hydrocarbons (PAHs), including benzo[a]pyrene (BaP), are ubiquitous environmental pollutants found in tobacco smoke, air pollution, and grilled foods. Reactive metabolites and reactive oxygen species generated during PAH metabolism are detoxified by reactions involving glutathione (GSH). Early life exposures to tobacco smoke and air pollution have been linked to increased risk of obesity and metabolic syndrome. We investigated the independent and interactive effects of prenatal exposure to BaP and GSH deficiency due to deletion of the modifier subunit of glutamate cysteine ligase (Gclm), the rate-limiting enzyme in GSH synthesis, on adiposity and hepatic steatosis in adult female F1 offspring. We mated Gclm(+/-) dams with Gclm(+/-) males and treated the pregnant dams with 0, 2, or 10mg/kg/day BaP in sesame oil by oral gavage daily from gestational day 7 through 16. We analyzed metabolic endpoints in female Gclm(-/-) and Gclm(+/+) littermate F1 offspring. Prenatal BaP exposure significantly increased visceral adipose tissue weight, weight gain between 3 weeks and 7.5 months of age, hepatic lipid content measured by oil red O staining, and hepatic fatty acid beta-oxidation gene expression in Gclm(+/+), but not in Gclm(-/-), female offspring. Hepatic expression of lipid biosynthesis and antioxidant genes were decreased and increased, respectively, in Gclm(-/-) mice. Our results suggest that reported effects of pre- and peri-natal air pollution and tobacco smoke exposure on obesity may be mediated in part by PAHs. GSH deficiency is protective against the metabolic effects of prenatal BaP exposure.

Glutathione defense mechanism in liver injury: insights from animal models

Glutathione (GSH) is the most abundant cellular thiol antioxidant and it exhibits numerous and versatile functions. Disturbances in GSH homeostasis have been associated with liver diseases induced by drugs, alcohol, diet and environmental pollutants. Until recently, our laboratories and others have developed mouse models with genetic deficiencies in glutamate-cysteine ligase (GCL), the rate-limiting enzyme in the GSH biosynthetic pathway. This review focuses on regulation of GSH homeostasis and, specifically, recent studies that have utilized such GSH-deficient mouse models to investigate the role of GSH in liver disease processes. These studies have revealed a differential hepatic response to distinct profiles of hepatic cellular GSH concentration. In particular, mice engineered to not express the catalytic subunit of GCL in hepatocytes [Gclc(h/h) mice] experience almostcomplete loss of hepatic GSH (to 5% of normal) and develop spontaneous liver pathologies characteristic of various clinical stages of liver injury. In contrast, mice globally engineered to not express the modifier subunit of GCL [Gclm⁻/⁻ mice] show a less severe hepatic GSH deficit (to ≈15% of normal) and exhibit overall protection against liver injuries induced by a variety of hepatic insults. Collectively, these transgenic mouse models provide interesting new insights regarding pathophysiological functions of GSH in the liver.

Glutathione (GSH) and the GSH synthesis gene Gclm modulate plasma redox and vascular responses to acute diesel exhaust inhalation in mice

CONTEXT

Inhalation of fine particulate matter (PM₂.₅) is associated with acute pulmonary inflammation and impairments in cardiovascular function. In many regions, PM₂.₅ is largely derived from diesel exhaust (DE), and these pathophysiological effects may be due in part to oxidative stress resulting from DE inhalation. The antioxidant glutathione (GSH) is important in limiting oxidative stress-induced vascular dysfunction. The rate-limiting enzyme in GSH synthesis is glutamate cysteine ligase and polymorphisms in its catalytic and modifier subunits (GCLC and GCLM) have been shown to influence vascular function and risk of myocardial infarction in humans.

OBJECTIVE

We hypothesized that compromised de novo synthesis of GSH in Gclm⁻/⁺ mice would result in increased sensitivity to DE-induced lung inflammation and vascular effects.

MATERIALS AND METHODS

WT and Gclm⁻/⁺ mice were exposed to DE via inhalation (300 μg/m³) for 6 h. Neutrophil influx into the lungs, plasma GSH redox potential, vascular reactivity of aortic rings and aortic nitric oxide (NO•) were measured.

RESULTS

DE inhalation resulted in mild bronchoalveolar neutrophil influx in both genotypes. DE-induced effects on plasma GSH oxidation and acetylcholine (ACh)-relaxation of aortic rings were only observed in Gclm⁻/⁺ mice. Contrary to our hypothesis, DE exposure enhanced ACh-induced relaxation of aortic rings in Gclm⁻/⁺ mice.

DISCUSSION AND CONCLUSION

THESE data support the hypothesis that genetic determinants of antioxidant capacity influence the biological effects of acute inhalation of DE. However, the acute effects of DE on the vasculature may be dependent on the location and types of vessels involved. Polymorphisms in GSH synthesis genes are common in humans and further investigations into these potential gene-environment interactions are warranted.

Non-alcoholic steatohepatitis: the role of oxidized low-density lipoproteins

Non-alcoholic steatohepatitis (NASH) is hallmarked by lipid accumulation in the liver (steatosis) along with inflammation (hepatitis). The transition from simple steatosis towards NASH represents a key step in pathogenesis, as it will set the stage for further severe liver damage. Yet, the pathogenesis behind hepatic inflammation is still poorly understood. It is of relevance to better understand the underlying mechanisms involved in NASH in order to apply new knowledge to potential novel therapeutic approaches. In the current review, we propose oxidized cholesterol as a novel risk factor for NASH. Here, we summarize mouse and human studies that provide possible mechanisms for the involvement of oxidized low-density lipoproteins in NASH and consequent potential novel diagnostic tools and treatment strategies for hepatic inflammation.

Hepatoprotective effect of tamoxifen on steatosis and non-alcoholic steatohepatitis in mouse models

Non-alcoholic fatty liver disease (NAFLD) is characterized by hepatic lipid accumulation that starts with steatosis and progresses to non-alcoholic steatohepatitis (NASH). Recently, the number of patients with such liver diseases has increased, but the understanding of the fundamental mechanisms and appropriate therapies are lacking. Tamoxifen (TAM) is a selective estrogen receptor modulator. We previously reported that TAM plays a protective role against drug-induced and chemical-induced acute liver injuries. However, the effects of TAM on chronic liver injury, including steatosis and NASH, remain to be addressed. We first found that the administration of TAM to mouse models of steatosis and NASH significantly decreased the plasma ALT and AST levels. The administration of TAM decreased the accumulated fat and inflammation in the livers in both mouse models. In addition, we observed decreased hepatic mRNA levels of triglyceride synthesis, acyl-CoA: diacylglycerol acyltransferase 2 (DGAT2), proinflammatory cytokines, tumor necrosis factor (TNF) α, and chemokines, monocyte chemoattractant protein (MCP) -1. TAM increased the extracellular signal-regulated kinase (ERK) phosphorylation, which is related to the proliferation and regeneration of liver and to decreased DGAT2 gene expression. Furthermore, a decrease in eukaryotic translational initiation factor (eIF2α), which is involved in apoptosis, was observed in both models. These findings suggest that TAM treatment exerts a hepatoprotective effect against steatosis and NASH, presumably via up-regulation of the ERK pathways and attenuation of eIF2α activation. These pathways represent a potential therapeutic target for steatosis and NASH in drug development.

Sustained Glutathione Deficiency Interferes with the Liver Response to TNF-α and Liver Regeneration after Partial Hepatectomy in Mice

Glutathione (GSH) is a critical intracellular antioxidant that is active in free radical scavenging and as a reducing equivalent in biological reactions. Recent studies have suggested that GSH can affect cellular function at the level of gene transcription as well, in particular by affecting NF-κB activation. Additionally, increased or decreased GSH levels in vitro have been tied to increased or decreased hepatocyte proliferation, respectively. Here, we investigated the effect of GSH on the liver's response to TNF-α injection and 2/3 partial hepatectomy (PH), using mice deficient for the modifier subunit of glutamate-cysteine ligase (GCLM), the rate-limiting enzyme in de novo GSH synthesis. We demonstrate that Gclm^-/- mice have a delay in IκBα degradation after TNF-α injection, resulting in delayed NF-κB nuclear translocation. These mice display profound deficiencies in GSH levels both before and during regeneration, and after PH, Gclm^-/- mice have an overall delay in cell cycle progression, with slower DNA synthesis, mitosis, and expression of cell cycle proteins. Moreover, there is a delay in expression of downstream targets of NF-κB in the regenerating liver in Gclm^-/- mice. These data suggest that GSH may play a role in hepatic NF-κB activation in vivo, which is necessary for accurate timing of liver regeneration.

Role of Pin1 protein in the pathogenesis of nonalcoholic steatohepatitis in a rodent model

Nonalcoholic steatohepatitis (NASH) is a disorder characterized by simultaneous fat accumulation and chronic inflammation in the liver. In this study, Pin1 expression was revealed to be markedly increased in the livers of mice with methionine choline-deficient (MCD) diet-induced NASH, a rodent model of NASH. In addition, Pin1 KO mice were highly resistant to MCD-induced NASH, based on a series of data showing simultaneous fat accumulation, chronic inflammation, and fibrosis in the liver. In terms of Pin1-induced fat accumulation, it was revealed that the expression levels of peroxisome proliferator-activated receptor α and its target genes were higher in the livers of Pin1 KO mice than in controls. Thus, resistance of Pin1 KO mice to hepatic steatosis is partially attributable to the lack of Pin1-induced down-regulation of peroxisome proliferator-activated receptor α, although multiple other mechanisms are apparently involved. Another mechanism involves the enhancing effect of hematopoietic Pin1 on the expressions of inflammatory cytokines such as tumor necrosis factor and monocyte chemoattractant protein 1 through NF-κB activation, eventually leading to hepatic fibrosis. Finally, to distinguish the roles of hematopoietic or nonhematopoietic Pin1 in NASH development, mice lacking Pin1 in either nonhematopoietic or hematopoietic cells were produced by bone marrow transplantation between wild-type and Pin1 KO mice. The mice having nonhematopoietic Pin1 exhibited fat accumulation without liver fibrosis on the MCD diet. Thus, hepatic Pin1 appears to be directly involved in the fat accumulation in hepatocytes, whereas Pin1 in hematopoietic cells contributes to inflammation and fibrosis. In summary, this is the first study to demonstrate that Pin1 plays critical roles in NASH development. This report also raises the possibility that hepatic Pin1 inhibition to the appropriate level might provide a novel therapeutic strategy for NASH.

Glutathione (GSH) and the GSH synthesis gene Gclm modulate vascular reactivity in mice

Oxidative stress has been implicated in the development of vascular disease and in the promotion of endothelial dysfunction via the reduction in bioavailable nitric oxide (NO()). Glutathione (GSH) is a tripeptide thiol antioxidant that is utilized by glutathione peroxidase (GPx) to scavenge reactive oxygen species such as hydrogen peroxide and phospholipid hydroperoxides. Relatively frequent single-nucleotide polymorphisms (SNPs) within the 5' promoters of the GSH synthesis genes GCLC and GCLM are associated with impaired vasomotor function, as measured by decreased acetylcholine-stimulated coronary artery dilation, and with increased risk of myocardial infarction. Although the influence of genetic knockdown of GPx on vascular function has been investigated in mice, no work to date has been published on the role of genetic knockdown of GSH synthesis genes on vascular reactivity. We therefore investigated the effects of targeted disruption of Gclm in mice and the subsequent depletion of GSH on vascular reactivity, NO() production, aortic nitrotyrosine protein modification, and whole-genome transcriptional responses as measured by DNA microarray. Gclm(-/+) and Gclm(-/-) mice had 72 and 12%, respectively, of wild-type (WT) aortic GSH content. Gclm(-/+) mice had a significant impairment in acetylcholine (ACh)-induced relaxation in aortic rings as well as increased aortic nitrotyrosine protein modification. Surprisingly, Gclm(-/-) aortas showed enhanced relaxation compared to Gclm(-/+) aortas, as well as increased NO() production. Although aortic rings from Gclm(-/-) mice had enhanced ACh relaxation, they had a significantly increased sensitivity to phenylephrine (PE)-induced contraction. Alternatively, the PE response of Gclm(-/+) aortas was nearly identical to that of their WT littermates. To examine the role of NO() or other potential endothelium-derived factors in differentially regulating vasomotor activity, we incubated aortic rings with the NO() synthase inhibitor L-NAME or physically removed the endothelium before PE treatment. L-NAME treatment and endothelium removal enhanced PE-induced contraction in WT and Gclm(-/+) mice, but this effect was severely diminished in Gclm(-/-) mice, indicating a potentially unique role for GSH in mediating vessel contraction. Whole-genome assessment of aortic mRNA in Gclm(-/-) and WT mice revealed altered expression of genes within the canonical Ca(2+) signaling pathway, which may have a role in mediating these observed functional effects. These findings provide additional evidence that the de novo synthesis of GSH can influence vascular reactivity and provide insights regarding possible mechanisms by which SNPs within GCLM and GCLC influence the risk of developing vascular diseases in humans.

The association of cysteine with obesity, inflammatory cytokines and insulin resistance in Hispanic children and adolescents

Context

Plasma total cysteine (tCys) independently relates to fat mass in adults. Dietary cyst(e)ine promotes adiposity and decreases glucose tolerance in some rodent models, but alleviates insulin resistance in others.

Objective

To investigate whether the association of tCys with body fat extends to children at particular risk of obesity, and whether tCys is associated with insulin resistance and obesity-associated inflammation.

Methods

We explored the cross-sectional relations of fasting plasma tCys and related metabolites with body composition measured by dual-energy X-ray absorptiometry in 984 Hispanic children and adolescents aged 4–19 years from the Viva La Familia Study. Linear and logistic regression and dose-response curves were used to evaluate relations of tCys with obesity, insulin resistance and inflammatory markers including interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), monocyte chemoattractant protein-1 (MCP-1) and C-reactive protein (CRP).

Results

tCys, methionine and total homocysteine (tHcy) increased with age. Upper tCys quartile was independently associated with a 5-fold increased risk of obesity (95% CI 3.5–8.0, P<0.001), and 2-fold risk of insulin resistance (95% CI: 1.6-5.0, P<0.001; adjusted for body fat%). Within the overweight/obese subgroup, but not in normal-weight children, tCys accounted for 9% of the variability in body fat% (partial r = 0.30, P<0.001; adjusted for age and gender). tCys correlated positively with serum non-esterified fatty acids and leptin, partly independent of body fat, but was not associated with serum IL-6, TNF-α or MCP-1. A positive correlation with CRP disappeared after adjustment for BMI.

Conclusion

tCys is independently associated with obesity and insulin resistance in Hispanic children and adolescents, highlighting a previously underappreciated link between the sulfur amino acid metabolic pathway and obesity and cardiometabolic risk.

Mechanism and significance of changes in glutamate-cysteine ligase expression during hepatic fibrogenesis

GSH is synthesized sequentially by glutamate-cysteine ligase (GCL) and GSH synthase and defends against oxidative stress, which promotes hepatic stellate cell (HSC) activation. Changes in GSH synthesis during HSC activation are poorly characterized. Here, we examined the expression of GSH synthetic enzymes in rat HSC activation and reversion to quiescence. Expression of the GCL catalytic subunit (GCLC) fell during HSC activation and increased when activated HSCs revert back to quiescence. Blocking the increase in GCLC expression kept HSCs in an activated state. Activated HSCs have higher nuclear levels and binding activity of MafG to the antioxidant response element (ARE) of GCLC but lower Nrf2/MafG heterodimer binding to the ARE. Quiescent HSCs have a lower nuclear MafG level but higher Nrf2/MafG heterodimer binding to ARE. This occurred because of enhanced sumoylation of Nrf2 and MafG by SUMO-1, which promoted Nrf2 binding to ARE and heterodimerization with MafG. In vivo, knockdown of GCLC exacerbated bile duct ligation-induced liver injury and fibrosis. Ursodeoxycholic acid and S-adenosylmethionine are anti-fibrotic in bile duct ligation, but this effect was nearly lost if GCLC induction was blocked. In conclusion, sumoylation of Nrf2 and MafG enhances heterodimerization and increases GCLC expression, which keeps HSCs in a quiescent state. Antifibrotic agents require activation of GCLC to fully exert their protective effect.

Antitumor and immunostimulatory activity of a polysaccharide-protein complex from Scolopendra subspinipes mutilans L. Koch in tumor-bearing mice

Scolopendra subspinipes mutilans L. Koch has been used for cancer treatment in traditional Chinese medicine for hundreds of years. In this study, the effects of a polysaccharide-protein complex from Scolopendra subspinipes mutilans L. Koch (SPPC) on the tumor growth and immune function were assessed in sarcoma S180 and hepatoma H22 bearing mice. Results showed that SPPC significantly inhibited the growth of S180 transplanted in mice and prolonged the survival time of H22- bearing mice. In S180-bearing mice, it promoted specific and nonspecific immune response as evidenced by enhancing the activities of natural killer (NK) cells, cytotoxic T lymphocytes (CTL) and the ratio of Th1/Th2 cytokines, and increasing the percentages of CD4(+) T cells, B cells and NK cells. Furthermore, SPPC not only significantly inhibited mRNA expression and production of the immunosuppressive cytokines (IL-10 and TGF-β), but also diminished arachidonic acid (AA)-metabolizing enzymes (COX-2 and CYP4A) and their products (PGE(2) and 20-HETE) in tumor-associated macrophages (TAMs). Taken together, our results indicate that SPPC inhibits tumor growth in vivo by improving antitumor immune responses at least partly via downregulating AA-metabolic pathways in TAMs, and could act as an anti-tumor agent with immunomodulatory activity.

A novel murine model for non-alcoholic steatohepatitis developed by combination of a high-fat diet and oxidized low-density lipoprotein

Non-alcoholic steatohepatitis (NASH) is the hepatic manifestation of metabolic syndrome that is characterized by steatosis, inflammation, and fibrosis, and may progress to cirrhosis and carcinoma. To investigate its pathogenic processes, we established a novel murine model for NASH by combination of a high-fat diet (HFD) and oxidized low-density lipoprotein (oxLDL). Mice that received HFD for 23 weeks showed hepatic steatosis, slight fibrosis, and a high level of lipid peroxidation compared with a regular diet (RD)-fed mice. Hepatic injury and elevated tumor necrosis factor (TNF)-α mRNA expression were also detected in these mice. Moreover, oxLDL administration to HFD-fed mice during weeks 21-23 not only aggravated hepatic steatosis, fibrosis, and lipid metabolism, but also resulted in intense inflammation, including severe hepatic injury and inflammatory cell infiltration, which are the typical histological features of NASH. Inflammation was accompanied by increased gene expression of TNF-α and interleukin (IL)-6. Additionally, the livers of RD-fed animals treated with oxLDL during weeks 21-23 were characterized by foamy macrophages and inflammatory cell infiltration along with an elevated IL-6 mRNA level. These results suggest that an increased oxidative state, including HFD-induced intracellular lipid peroxidation and its extracellular source from oxLDL, is the actual trigger for hepatic inflammation in which liver injury is mediated by TNF-α and inflammatory cell accumulation is dependent on IL-6. HFD and oxLDL also induced insulin resistance in mice; additionally, oxLDL downregulated insulin secretion. In this model, CD36 overexpression was observed in the hepatocytes of HFD-fed mice and those treated with HFD and oxLDL, and in the hepatic macrophages of RD-fed mice immediately after oxLDL treatment. In vitro experiments indicated a rapid and transient elevation of CD36 on macrophage plasma membrane in response to oxLDL. Our findings demonstrate that CD36 expressed on hepatocytes and hepatic macrophages mediates the pathophysiology of NASH.

Heterozygosity in the glutathione synthesis gene Gclm increases sensitivity to diesel exhaust particulate induced lung inflammation in mice

CONTEXT

Inhalation of ambient fine particulate matter (PM₂.₅) is associated with adverse respiratory and cardiovascular effects. A major fraction of PM₂.₅ in urban settings is diesel exhaust particulate (DEP), and DEP-induced lung inflammation is likely a critical event mediating many of its adverse health effects. Oxidative stress has been proposed to be an important factor in PM₂.₅-induced lung inflammation, and the balance between pro- and antioxidants is an important regulator of this inflammation. An important intracellular antioxidant is the tripeptide thiol glutathione (GSH). Glutamate cysteine ligase (GCL) carries out the first step in GSH synthesis. In humans, relatively common genetic polymorphisms in both the catalytic (Gclc) and modifier (Gclm) subunits of GCL have been associated with increased risk for lung and cardiovascular diseases.

OBJECTIVE

This study was aimed to determine the effects of Gclm expression on lung inflammation following DEP exposure in mice.

MATERIALS AND METHODS

We exposed Gclm wild type, heterozygous, and null mice to DEP via intranasal instillation and assessed lung inflammation as determined by neutrophils and inflammatory cytokines in lung lavage, inflammatory cytokine mRNA levels in lung tissue, as well as total lung GSH, Gclc, and Gclm protein levels.

RESULTS

The Gclm heterozygosity was associated with a significant increase in DEP-induced lung inflammation when compared to that of wild type mice.

DISCUSSION AND CONCLUSION

This finding indicates that GSH synthesis can mediate DEP-induced lung inflammation and suggests that polymorphisms in Gclm may be an important factor in determining adverse health outcomes in humans following inhalation of PM₂.₅.

Lipid metabolism and body composition in Gclm(-/-) mice

In humans and experimental animals, high fat diets (HFD) are associated with risk factors for metabolic diseases, such as excessive weight gain and adiposity, insulin resistance and fatty liver. Mice lacking the glutamate-cysteine ligase modifier subunit gene (Gclm(-/-)) and deficient in glutathione (GSH), are resistant to HFD-mediated weight gain. Herein, we evaluated Gclm-associated regulation of energy metabolism, oxidative stress, and glucose and lipid homeostasis. C57BL/6J Gclm(-/-) mice and littermate wild-type (WT) controls received a normal diet or an HFD for 11 weeks. HFD-fed Gclm(-/-) mice did not display a decreased respiratory quotient, suggesting that they are unable to process lipid for metabolism. Although dietary energy consumption and intestinal lipid absorption were unchanged in Gclm(-/-) mice, feeding these mice an HFD did not produce excess body weight nor fat storage. Gclm(-/-) mice displayed higher basal metabolic rates resulting from higher activities of liver mitochondrial NADH-CoQ oxidoreductase, thus elevating respiration. Although Gclm(-/-) mice exhibited strong systemic and hepatic oxidative stress responses, HFD did not promote glucose intolerance or insulin resistance. Furthermore, HFD-fed Gclm(-/-) mice did not develop fatty liver, likely resulting from very low expression levels of genes encoding lipid metabolizing enzymes. We conclude that Gclm is involved in the regulation of basal metabolic rate and the metabolism of dietary lipid. Although Gclm(-/-) mice display a strong oxidative stress response, they are protected from HFD-induced excessive weight gain and adipose deposition, insulin resistance and steatosis.