Epigallocatechin-3-gallate inhibits growth of activated hepatic stellate cells by enhancing the capacity of glutathione synthesis

The roles of epigallocatechin gallate in the tumor microenvironment, metabolic reprogramming, and immunotherapy

Cancer, a disease that modern medicine has not fully understood and conquered, with its high incidence and mortality, deprives countless patients of health and even life. According to global cancer statistics, there were an estimated 19.3 million new cancer cases and nearly 10 million cancer deaths in 2020, with the age-standardized incidence and mortality rates of 201.0 and 100.7 per 100,000, respectively. Although remarkable advancements have been made in therapeutic strategies recently, the overall prognosis of cancer patients remains not optimistic. Consequently, there are still many severe challenges to be faced and difficult problems to be solved in cancer therapy today. Epigallocatechin gallate (EGCG), a natural polyphenol extracted from tea leaves, has received much attention for its antitumor effects. Accumulating investigations have confirmed that EGCG can inhibit tumorigenesis and progression by triggering apoptosis, suppressing proliferation, invasion, and migration, altering tumor epigenetic modification, and overcoming chemotherapy resistance. Nevertheless, its regulatory roles and biomolecular mechanisms in the immune microenvironment, metabolic microenvironment, and immunotherapy remain obscure. In this article, we summarized the most recent updates about the effects of EGCG on tumor microenvironment (TME), metabolic reprogramming, and anti-cancer immunotherapy. The results demonstrated EGCG can promote the anti-cancer immune response of cytotoxic lymphocytes and dendritic cells (DCs), attenuate the immunosuppression of myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs), and inhibit the tumor-promoting functions of tumor-associated macrophages (TAMs), tumor-associated neutrophils (TANs), and various stromal cells including cancer-associated fibroblasts (CAFs), endothelial cells (ECs), stellate cells, and mesenchymal stem/stromal cells (MSCs). Additionally, EGCG can suppress multiple metabolic reprogramming pathways, including glucose uptake, aerobic glycolysis, glutamine metabolism, fatty acid anabolism, and nucleotide synthesis. Finally, EGCG, as an immunomodulator and immune checkpoint blockade, can enhance immunotherapeutic efficacy and may be a promising candidate for antitumor immunotherapy. In conclusion, EGCG plays versatile regulatory roles in TME and metabolic reprogramming, which provides novel insights and combined therapeutic strategies for cancer immunotherapy.

Antioxidative, Anti-Inflammatory, Anti-Obesogenic, and Antidiabetic Properties of Tea Polyphenols-The Positive Impact of Regular Tea Consumption as an Element of Prophylaxis and Pharmacotherapy Support in Endometrial Cancer

Endometrial cancer (EC) is second only to cervical carcinoma among the most commonly diagnosed malignant tumours of the female reproductive system. The available literature provides evidence for the involvement of 32 genes in the hereditary incidence of EC. The physiological markers of EC and coexisting diet-dependent maladies include antioxidative system disorders but also progressing inflammation; hence, the main forms of prophylaxis and pharmacotherapy ought to include a diet rich in substances aiding the organism’s response to this type of disorder, with a particular focus on ones suitable for lifelong consumption. Tea polyphenols satisfy those requirements due to their proven antioxidative, anti-inflammatory, anti-obesogenic, and antidiabetic properties. Practitioners ought to consider promoting tea consumption among individuals genetically predisposed for EC, particularly given its low cost, accessibility, confirmed health benefits, and above all, suitability for long-term consumption regardless of the patient’s age. The aim of this paper is to analyse the potential usability of tea as an element of prophylaxis and pharmacotherapy support in EC patients. The analysis is based on information available from worldwide literature published in the last 15 years.

Impact of spaceflight and artificial gravity on sulfur metabolism in mouse liver: sulfur metabolomic and transcriptomic analysis

Spaceflight induces hepatic damage, partially owing to oxidative stress caused by the space environment such as microgravity and space radiation. We examined the roles of anti-oxidative sulfur-containing compounds on hepatic damage after spaceflight. We analyzed the livers of mice on board the International Space Station for 30 days. During spaceflight, half of the mice were exposed to artificial earth gravity (1 g) using centrifugation cages. Sulfur-metabolomics of the livers of mice after spaceflight revealed a decrease in sulfur antioxidants (ergothioneine, glutathione, cysteine, taurine, thiamine, etc.) and their intermediates (cysteine sulfonic acid, hercynine, N-acethylserine, serine, etc.) compared to the controls on the ground. Furthermore, RNA-sequencing showed upregulation of gene sets related to oxidative stress and sulfur metabolism, and downregulation of gene sets related to glutathione reducibility in the livers of mice after spaceflight, compared to controls on the ground. These changes were partially mitigated by exposure to 1 g centrifugation. For the first time, we observed a decrease in sulfur antioxidants based on a comprehensive analysis of the livers of mice after spaceflight. Our data suggest that a decrease in sulfur-containing compounds owing to both microgravity and other spaceflight environments (radiation and stressors) contributes to liver damage after spaceflight.

Cellular Defensive Mechanisms of Tea Polyphenols: Structure-Activity Relationship

Tea is particularly rich in polyphenols, including catechins and theaflavins, thearubigins, flavonols, and phenolic acids, which are believed to contribute to the health benefits of tea. The health-promoting effects of tea polyphenols are believed to be related to their cellular defensive properties. This review is intended to briefly summarize the relationship between the chemical structures of tea polyphenols and their biological activities. Tea polyphenols appear as direct antioxidants by scavenging reactive oxygen/nitrogen species; chelating transition metals; and inhibiting lipid, protein, and DNA oxidations. They also act directly by suppressing “pro-oxidant” enzymes, inducing endogenous antioxidants, and cooperating with vitamins. Moreover, tea polyphenols regulate cellular signaling transduction pathways, importantly contributing to the prevention of chronic diseases and the promotion of physiological functions. Apparently, the features in the chemical structures of tea polyphenols are closely associated with their antioxidant potentials.

Compromised glutathione synthesis results in high susceptibility to acetaminophen hepatotoxicity in acatalasemic mice

Acatalasemia is caused by genetic defect in the catalase gene. Human achatalasemia patients are able to scavenge physiological hydrogen peroxide but are vulnerable to exogenous oxidative stress. In the present study, we used an acetaminophen-induced hepatotoxicity model in acatalasemic mice to explore this vulnerability. Interestingly, the acetaminophen-induced decrease in total glutathione levels was more prolonged in acatalasemic mice. While the subunits of glutamate-cysteine ligase, a glutathione synthase enzyme, were increased by acetaminophen in the liver of wild-type mice, their expression was lower and was further reduced by acetaminophen in acatalasemic mice. This feature was also observed in immortalized hepatocytes derived from the livers of these mice. However, when catalase was knocked down in HepG2 cells, a cultured human liver cell line, the expression of glutamate-cysteine ligase subunits was increased, suggesting that the low expression of glutamate-cysteine ligase subunits in acatalasemia may be due to other mechanism than catalase deficiency. Therefore, when other factors were investigated, it was found that transforming growth factor-β1 was up-regulated by acetaminophen in the liver of acatalasemic mice, which may inhibit the expression of glutamate-cysteine ligase subunits. The results of this study suggest a new toxic mechanism of acetaminophen-induced liver injury in patients with acatalasemia.

Protein kinase Cδ knockout mice are protected from cocaine-induced hepatotoxicity

We investigated whether protein kinase Cδ (PKCδ) mediates cocaine-induced hepatotoxicity in mice. Cocaine treatment (60 mg/kg, i.p.) significantly increased cleaved PKCδ expression in the liver of wild-type (WT) mice, and led to significant increases in oxidative parameters (i.e., reactive oxygen species, 4-hydroxylnonenal and protein carbonyl). These cocaine-induced oxidative burdens were attenuated by pharmacological (i.e., rottlerin) or genetic depletion of PKCδ. We also demonstrated that treatment with cocaine resulted in significant increases in nuclear factor erythroid-2-related factor 2 (Nrf-2) nuclear translocation and increased Nrf-2 DNA-binding activity in wild-type (WT) mice. These increases were more pronounced in the rottlerin-treated WT or PKCδ knockout mice than in the saline-treated WT mice. Although cocaine treatment increased Nrf-2 nuclear translocation, DNA binding activity, and γ-glutamyl cysteine ligases (i.e., GCLc and GCLm) mRNA expressions, while it reduced the glutathione level and GSH/GSSG ratio. These decreases were attenuated by PKCδ depletion. Cocaine treatment significantly increased alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels in the serum of WT mice signifying the hepatic damage. These increases were also attenuated by PKCδ depletion. In addition, cocaine-induced hepatic degeneration in WT mice was evident 1 d post-cocaine. At that time, cocaine treatment decreased Bcl-2 and Bcl-xL levels, and increased Bax, cytosolic cytochrome c, and cleaved caspase-3 levels. Pharmacological or genetic depletion of PKCδ significantly ameliorated the pro-apoptotic properties and hepatic degeneration. Therefore, our results suggest that inhibition of PKCδ, as well as activation of Nrf-2, is important for protecting against hepatotoxicity induced by cocaine.

Therapeutic Role of Green Tea Polyphenols in Improving Fertility: A Review

Sperm cells are highly sensitive to reactive oxygen species (ROS), which are produced during cellular oxidation. In normal cell biology, ROS levels increase with a decreasing antioxidant response, resulting in oxidative stress which threatens sperm biology. Oxidative stress has numerous effects, including increased apoptosis, reduced motion parameters, and reduced sperm integrity. In this regard, green tea polyphenols (GrTPs) have been reported to possess properties that may increase the quality of male and female gametes, mostly via the capability of catechins to reduce ROS production. GrTPs have antioxidant properties that improve major semen parameters, such as sperm concentration, motility, morphology, DNA damage, fertility rate, and gamete quality. These unique properties of green tea catechins could improve reproductive health and represent an important study area. This exploratory review discusses the therapeutic effects of GrTPs against infertility, their possible mechanisms of action, and recommended supportive therapy for improving fertility in humans and in animals.

Elevated mitochondrial activity distinguishes fibrogenic hepatic stellate cells and sensitizes for selective inhibition by mitotropic doxorubicin

Activation of hepatic stellate cells (HSCs) is an integral component of the wound‐healing process in liver injury/inflammation. However, uncontrolled activation of HSCs leads to constant secretion of collagen‐rich extracellular matrix (ECM) proteins, resulting in liver fibrosis. The enhanced ECM synthesis/secretion demands an uninterrupted supply of intracellular energy; however, there is a paucity of data on the bioenergetics, particularly the mitochondrial (mito) metabolism of fibrogenic HSCs. Here, using human and rat HSCs in vitro, we show that the mito‐respiration, mito‐membrane potential (Δψm) and cellular ‘bioenergetic signature’ distinguish fibrogenic HSCs from normal, less‐active HSCs. Ex vivo, HSCs from mouse and rat models of liver fibrosis further confirmed the altered ‘bioenergetic signature’ of fibrogenic HSCs. Importantly, the distinctive elevation in mito‐Δψm sensitized fibrogenic HSCs for selective inhibition by mitotropic doxorubicin while normal, less‐active HSCs and healthy human primary hepatocytes remained minimally affected if not, unaffected. Thus, the increased mito‐Δψm may provide an opportunity to selectively target fibrogenic HSCs in liver fibrosis.

Chronic Inflammatory Diseases and Green Tea Polyphenols

Chronic inflammatory diseases affect millions of people globally and the incidence rate is on the rise. While inflammation contributes to the tissue healing process, chronic inflammation can lead to life-long debilitation and loss of tissue function and organ failure. Chronic inflammatory diseases include hepatic, gastrointestinal and neurodegenerative complications which can lead to malignancy. Despite the millennial advancements in diagnostic and therapeutic modalities, there remains no effective cure for patients who suffer from inflammatory diseases. Therefore, patients seek alternatives and complementary agents as adjunct therapies to relieve symptoms and possibly to prevent consequences of inflammation. It is well known that green tea polyphenols (GrTPs) are potent antioxidants with important roles in regulating vital signaling pathways. These comprise transcription nuclear factor-kappa B mediated I kappa B kinase complex pathways, programmed cell death pathways like caspases and B-cell lymphoma-2 and intervention with the surge of inflammatory markers like cytokines and production ofcyclooxygenase-2. This paper concisely reviews relevant investigations regarding protective effects of GrTPs and some reported adverse effects, as well as possible applications for GrTPs in the treatment of chronic and inflammatory complications.

Astragaloside IV attenuates lead acetate-induced inhibition of neurite outgrowth through activation of Akt-dependent Nrf2 pathway in vitro

Recently, oxidative stress is strongly associated with lead (Pb)-induced neurotoxicity. We reported previously that Astragaloside IV (AS-IV) possesses potent antioxidant properties. Here, we evaluate the hypothesis that AS-IV attenuates lead acetate (PbAc)-mediated inhibition of neurite outgrowth might mainly result from its antioxidant property via serine/threonine protein kinase (Akt)-dependent activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway. Interestingly, AS-IV attenuates PbAc-induced inhibition of neurite outgrowth and displayed potential antioxidant properties by inhibiting reactive oxygen species (ROS). Concomitantly, AS-IV enhanced phase II detoxifying enzymes such as heme oxygenase 1 (HO-1), thioredoxin reductase (TrxR), and glutamate cysteine ligase catalytic subunit (GCLc). Conversely, AS-IV had no effect on GCL modulatory subunit (GCLm) and superoxide dismutase (SOD) activity/expression. Furthermore, AS-IV evoked Akt phosphorylation, and subsequent induced phosphorylation of glycogen synthase kinase-3β (GSK-3β) at Ser9 (that is, inactivation), which stimulated Nrf2-mediated antioxidant response element (ARE)-containing activation. Importantly, Akt locates upstream of GSK-3β and regulates phase II detoxifying enzymes gene expression through Nrf2 nuclear accumulation in PC12 cells exposed to PbAc. Noteworthy, these results were further confirmed through signalling pathway inhibitors, dominant negative mutant and short hairpin RNA technology. Collectively, these in vitro findings suggest that AS-IV attenuates PbAc-induced inhibition of neurite outgrowth attributed to its antioxidant properties and may be a promising candidate for the treatment of lead developmental neurotoxicity.

Evolving Insights on Metabolism, Autophagy, and Epigenetics in Liver Myofibroblasts

Liver myofibroblasts (MFB) are crucial mediators of extracellular matrix (ECM) deposition in liver fibrosis. They arise mainly from hepatic stellate cells (HSCs) upon a process termed “activation.” To a lesser extent, and depending on the cause of liver damage, portal fibroblasts, mesothelial cells, and fibrocytes may also contribute to the MFB population. Targeting MFB to reduce liver fibrosis is currently an area of intense research. Unfortunately, a clog in the wheel of antifibrotic therapies is the fact that although MFB are known to mediate scar formation, and participate in liver inflammatory response, many of their molecular portraits are currently unknown. In this review, we discuss recent understanding of MFB in health and diseases, focusing specifically on three evolving research fields: metabolism, autophagy, and epigenetics. We have emphasized on therapeutic prospects where applicable and mentioned techniques for use in MFB studies. Subsequently, we highlighted uncharted territories in MFB research to help direct future efforts aimed at bridging gaps in current knowledge.

Metabolism of Sulfur-Containing Amino Acids in the Liver: A Link between Hepatic Injury and Recovery

Methionine is an essential sulfur-containing amino acid that is metabolized mainly in the liver, where it is converted to S-adenosylmethionine (SAM) by methionine adenosyltransferase. Importantly, SAM is a metabolically pleiotropic molecule that participates in three types of biochemical reactions; transmethylation, transsulfuration (which results in the transfer of sulfur from methionine to serine to form cysteine), and amino propylation (to synthesize polyamines). Critical roles of SAM in the liver have been extensively studied using transgenic animals with chronically reduced or increased hepatic SAM levels. Interestingly, both models with abnormal hepatic SAM concentrations develop liver disease suggesting that SAM homeostasis plays a pivotal role in liver disease. The transsulfuration pathway is connected to the production of glutathione (GSH), which has potent antioxidant capacity in the liver. Accumulating data show that GSH depletion renders the liver vulnerable to oxidative stress and prone to progression of liver disease. In this review, we highlight the importance of homeostasis in the metabolism of sulfur-containing amino acids with a particular focus on the transsulfuration pathway which could be a promising therapeutic target in liver injury.

Targeting of Gamma-Glutamyl-Cysteine Ligase by miR-433 Reduces Glutathione Biosynthesis and Promotes TGF-β-Dependent Fibrogenesis

AIMS

Glutathione (GSH) is the main antioxidant against cell damage. Several pathological states course with reduced nucleophilic tone and perturbation of redox homeostasis due to changes in the 2GSH/GSSG ratio. Here, we investigated the regulation of the rate-limiting GSH biosynthetic heterodimeric enzyme γ-glutamyl-cysteine ligase (GCL) by microRNAs (miRNAs).

RESULTS

"In silico" analysis of the 3'- untranslated regions (UTRs) of both catalytic (GCLc) and regulatory (GCLm) subunits of GCL enabled an identification of miR-433 as a strong candidate for the targeting of GCL. Transitory overexpression of miR-433 in human umbilical vein endothelial cells (HUVEC) showed a downregulation of both GCLc and GCLm in a nuclear factor (erythroid-derived 2)-like 2 (Nrf2)-independent manner. Increases in pro-oxidant stimuli such as exposure to hydrogen peroxide or GSH depletion in endothelial and hepatic cells caused an expected increase in GCLc and GCLm protein expression and abrogation of miR-433 levels, thus supporting a cross-regulation of these pathways. Treatment of HUVEC with miR-433 resulted in reduced antioxidant and redox potentials, increased S-glutathionylation, and reduced endothelial nitric oxide synthase activation. In vivo models of renal and hepatic fibrosis were associated with transforming growth factor β1 (TGF-β1)-related reduction of GCLc and GCLm levels that were miR-433 dependent.

INNOVATION AND CONCLUSION

We describe for the first time an miRNA, miR-433, capable of directly targeting GCL and promoting functional consequences in endothelial physiology and fibrotic processes by decreasing GSH levels.

Induction of glutathione synthesis in human hepatocytes by acute and chronic arsenic exposure: differential roles of mitogen-activated protein kinases

Glutathione (GSH) is a vital component of antioxidant defense which protects cells from toxic insults. Previously we found intracellular GSH was involved in cell resistance against arsenic-induced cytotoxicity. However, molecular mechanisms of GSH homeostasis during arsenic exposure are largely undefined. Here, we investigated roles of mitogen-activated protein kinases (MAPKs) in GSH synthesis pathway with two arsenic exposure strategies by using Chang human hepatocytes. In one strategy, acute arsenic exposure (20 μM, 24 h) was applied, as MAPK signaling is generally considered to be transient. In the other one, chronic arsenic exposure (500 nM, 20 weeks) was applied, which mimicked the general human exposure to arsenic. We found that acute arsenic exposure activated extracellular signal-regulated 1/2 kinases (ERK1/2) and c-Jun N-terminal kinase (JNK) in parallel with increased transcription and nuclear translocation of factor-erythroid 2-related factor 2 (NRF2) and enhanced expression of γ-glutamyl cysteine ligase catalytic subunit (GCLC), resulting in elevated intracellular GSH levels. Specific ERK inhibitor abolished arsenic-induced NRF2 nuclear translocation and GSH synthesis. During chronic arsenic exposure which induced a malignant cellular phenotype, continuous p38 activation and NRF2 nuclear translocation were observed with enhanced GSH synthesis. Specific p38 inhibitor attenuated arsenic-enhanced GSH synthesis without changing NRF2 nuclear translocation. Taken together, our results indicate MAPK pathways play an important role in cellular GSH homeostasis in response to arsenic. However, the specific activation of certain MAPK is different between acute and chronic arsenic exposure. Furthermore, it appears that during chronic arsenic exposure, GSH synthesis is regulated by p38 at least in part independent of NRF2.

Chemopreventive and hepatoprotective effects of Epigallocatechin-gallate against hepatocellular carcinoma: role of heparan sulfate proteoglycans pathway

Objective

Epigallocatechin-gallate (EGCG) claims a plethora of health benefits including protection against neoplastic diseases. Meanwhile, heparan-sulfate proteoglycans (HSPGs) have defensive role against tumour cell invasion. Therefore, the chemopreventive and hepatoprotective effects of EGCG were studied in hepatocellular carcinoma (HCC) in vivo and in vitro and compared with strong water soluble antioxidant, sodium ascorbate.

Methods

HCC was induced in SD rats by thioacetamide (200 mg/Kg). Some rats were treated with EGCG (20 mg/Kg) or sodium ascorbate (100 mg/Kg). Liver impairment was assessed by measuring serum α-fetoprotein and investigating liver sections stained with H/E. Hepatic HSPGs, syndecan-1 and matrix metalloproteinase-9 (MMP-9) were measured by ELISA. Gene expression of fibroblast growth factor (FGF)-2 was measured. Cell death was assessed by caspase-3 activity. In addition, all markers were measured in human hepatocellular carcinoma cell line (HepG2).

Key findings

EGCG increased the animal survival and decreased both α-fetoprotein and HepG2 viability. In addition, EGCG ameliorated fibrosis and massive hepatic tissue breakdown. EGCG restored HSPGs and reduced expression of MMP-9, syndecan-1 and FGF-2 in-vivo and in-vitro. Sodium ascorbate showed significantly lower results than EGCG.

Conclusions

Besides antioxidant activity, other mechanisms are involved in the chemopreventive and hepatoprotective effects of EGCG including restoration of HSPGs receptors and inhibition of vascular invasion.

(-)-Epigallocatechin-gallate (EGCG) stabilize the mitochondrial enzymes and inhibits the apoptosis in cigarette smoke-induced myocardial dysfunction in rats

The present study brings out the preventive role of (-)-epigallocatechin-gallate (EGCG) on cardiac mitochondrial metabolism and apoptosis in cigarette smoke (CS)-exposed rats. The CS-exposed rats showed significantly decreased activities of TCA cycle enzymes and mitochondrial enzymatic antioxidants, on the other hand, mitochondrial lipid peroxidation was increased and GSH level was decreased. Further, CS exposure was found to induce cardiac apoptosis through release of cytochrome c into the cytosol, cleavage of pro-caspase-3 to active caspase-3, up-regulation of pro-apoptotic (Bax) and down-regulation of antiapoptotic (Bcl-2) molecules. The CS-induced apoptosis was further confirmed by mitochondrial and nuclear ultra structural apoptotic features as evaluated by electron microscopic studies. EGCG supplementation shelters the activities of TCA cycle enzymes and antioxidant enzymes, with concomitant decrease in lipid peroxidation and increase in GSH level. EGCG administration inhibited apoptosis through the inhibition of cytochrome c release into cytosol, activation of pro-caspase-3, down regulation of Bax and significant up regulation of Bcl-2. EGCG reversed the ultra structural apoptotic alterations of mitochondria and nucleus. The present study has provided experimental evidences that the EGCG treatment enduring to cardio protection at mitochondrial level.

Regulation of nodularin-induced apoptosis by epigallocatechin-3-gallate on fish lymphocytes in vitro

Nodularin is one of the most conspicuous and widespread pollutants that elicit water ecological hazards to fish, causing serious damage on the immune system and physiological functions. Nodularin can cause oxidative stress-induced apoptosis on fish lymphocytes. The regulatory effects of epigallocatechin-3-gallate (EGCG) at 10, 100, and 1000 μg/L levels on the antioxidant defense system and apoptosis of Carassius auratus lymphocytes exposed to a high dose of nodularin (100 μg/L) were quantified in vitro. EGCG reduced nodularin-induced oxidative damage on fish immune cells. This compound significantly increased the activities of superoxide dismutase and catalase and the level of glutathione but decreased the levels of intracellular reactive oxygen species and malondialdehyde. Flow cytometry results showed that the percentages of apoptotic cells after treatment with 10, 100, and 1000 μg/L EGCG for 12 h reached 27.9%, 19.1%, and 13.7%, respectively. By contrast, the nodularin alone-induced group showed a high percentage of apoptosis (44.2%). Western blot analysis showed the increased expression of bcl-2 and the decreased expression of bax and caspase-3 in EGCG-treated fish lymphocytes. EGCG also inhibited the potential collapse of the mitochondrial membrane. Overall, EGCG can inhibit nodularin-induced apoptosis and protect the normal immunity of fish by regulating bax/bcl-2 and blocking the downstream of mitochondrial apoptosis pathway with increased intracellular antioxidant enzyme activity.

Epigallocatechin gallate attenuates fibrosis, oxidative stress, and inflammation in non-alcoholic fatty liver disease rat model through TGF/SMAD, PI3 K/Akt/FoxO1, and NF-kappa B pathways

PURPOSE

To investigate the protective mechanisms of an 85 % pure extract of (-) epigallocatechin gallate (EGCG) in the development of fibrosis, oxidative stress and inflammation in a recently developed dietary-induced animal model of non-alcoholic fatty liver disease (NAFLD).

METHODS

Female Sprague-Dawley rats were fed with either normal rat diet or high-fat diet for 8 weeks to develop NAFLD. For both treatments, rats were treated with or without EGCG (50 mg/kg, i.p. injection, 3 times per week). At the end, blood and liver tissue samples were obtained for histology, molecular, and biochemical analyses.

RESULTS

Non-alcoholic fatty liver disease (NAFLD) rats showed significant amount of fatty infiltration, necrosis, fibrosis, and inflammation. This was accompanied by a significant expressional increase in markers for fibrosis, oxidative stress, and inflammation. TGF/SMAD, PI3 K/Akt/FoxO1, and NF-κB pathways were also activated. Treatment with EGCG improved hepatic histology (decreased number of fatty score, necrosis, and inflammatory foci), reduced liver injury (from ~0.5 to ~0.3 of ALT/AST ratio), attenuated hepatic changes including fibrosis (reduction in Sirius Red and synaptophysin-positive stain) with down-regulation in the expressions of key pathological oxidative (e.g. nitrotyrosine formation) and pro-inflammatory markers (e.g. iNOS, COX-2, and TNF-α). EGCG treatment also counteracted the activity of TGF/SMAD, PI3 K/Akt/FoxO1, and NF-κB pathways. Treatment with EGCG did not affect the healthy rats.

CONCLUSIONS

Epigallocatechin gallate (EGCG) reduced the severity of liver injury in an experimental model of NAFLD associated with lower concentration of pro-fibrogenic, oxidative stress, and pro-inflammatory mediators partly through modulating the activities of TGF/SMAD, PI3 K/Akt/FoxO1, and NF-κB pathways. Therefore, green tea polyphenols and EGCG are useful supplements in the prevention of NAFLD.

Capsaicin induces de-differentiation of activated hepatic stellate cell

Hepatic stellate cells (HSC) play a key role in liver fibrogenesis. Activation of PPARγ and inhibition of fibrogenic molecules are potential strategies to block HSC activation and differentiation. A number of natural products have been suggested to have antifibrotic effects for the de-activation and de-differentiation of HSCs. The purpose of this study was to investigate the in vitro effects of capsaicin on HSC de-activation and de-differentiation. The results demonstrated that capsaicin induced quiescent phenotype in GRX via PPARγ activation. Significant decrease in COX-2 and type I collagen mRNA expression was observed in the first 24 h of treatment. These events preceded the reduction of TGF-β1 and total collagen secretion. Thus, capsaicin promoted down-regulation of HSC activation by its antifibrotic and anti-inflammatory actions. These findings demonstrate that capsaicin may have potential as a novel therapeutic agent for the treatment of liver fibrosis.

Glutathione synthesis

BACKGROUND

Glutathione (GSH) is present in all mammalian tissues as the most abundant non-protein thiol that defends against oxidative stress. GSH is also a key determinant of redox signaling, vital in detoxification of xenobiotics, and regulates cell proliferation, apoptosis, immune function, and fibrogenesis. Biosynthesis of GSH occurs in the cytosol in a tightly regulated manner. Key determinants of GSH synthesis are the availability of the sulfur amino acid precursor, cysteine, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL), which is composed of a catalytic (GCLC) and a modifier (GCLM) subunit. The second enzyme of GSH synthesis is GSH synthetase (GS).

SCOPE OF REVIEW

This review summarizes key functions of GSH and focuses on factors that regulate the biosynthesis of GSH, including pathological conditions where GSH synthesis is dysregulated.

MAJOR CONCLUSIONS

GCL subunits and GS are regulated at multiple levels and often in a coordinated manner. Key transcription factors that regulate the expression of these genes include NF-E2 related factor 2 (Nrf2) via the antioxidant response element (ARE), AP-1, and nuclear factor kappa B (NFκB). There is increasing evidence that dysregulation of GSH synthesis contributes to the pathogenesis of many pathological conditions. These include diabetes mellitus, pulmonary and liver fibrosis, alcoholic liver disease, cholestatic liver injury, endotoxemia and drug-resistant tumor cells.

GENERAL SIGNIFICANCE

GSH is a key antioxidant that also modulates diverse cellular processes. A better understanding of how its synthesis is regulated and dysregulated in disease states may lead to improvement in the treatment of these disorders. This article is part of a Special Issue entitled Cellular functions of glutathione.

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.

Adipose-derived stem cells can abrogate chemical-induced liver fibrosis and facilitate recovery of liver function

Adipose-derived stem cells (ADSCs) are easy to harvest and have the ability for self-renewal and to differentiate into various cell types, including those of the hepatic lineage. Studies on the use of ADSCs for liver transplantation are, however, limited. The objective of this study was to investigate the feasibility of using human ADSCs and to better understand their mechanism of action for the repair of liver damage in a thioacetamide (TAA)-induced model of chronic liver damage in the rat. To induce liver damage, 200 mg/kg TAA was injected intraperitoneally into Wistar rats every 3 days for 60 days. For cell therapy, 1 × 10(6) human ADSCs suspended in 300 μl of phosphate-buffered saline were transplanted into each experimental rat by direct liver injection. Immunohistochemistry showed that the transplanted ADSCs differentiated into albumin- and α-fetoprotein-secreting liver-like cells 1 week after transplantation. In addition, liver function recovered significantly, as determined by biochemical analyses that analyzed total bilirubin, prothrombin time, and albumin levels. The Metavir score, derived from histopathological analysis, also showed a significant decrease in liver fibrosis and inflammatory activity after ADSC transplantation. Finally, we found a reduction in the expression of α-smooth muscle actin, a marker of hepatic stellate cells, which produce collagen fiber, and an increase in the expression of matrix metalloproteinase-9, which degrades collagen fiber, after ADSC transplantation. These findings are consistent with abrogation of liver fibrosis in the ADSC therapy group. Consequently, these results suggest that ADSC transplantation may facilitate recovery from chronic liver damage and thus may have clinical applications.

Participation of nuclear factor (erythroid 2-related), factor 2 in ameliorating lithocholic acid-induced cholestatic liver injury in mice

BACKGROUND AND PURPOSE

Lithocholic acid (LCA), the most toxic bile acid, induces cholestatic liver injury in rodents. We previously showed that LCA activates the oxidative stress-responsive nuclear factor (erythroid-2 like), factor 2 (Nrf2) in cultured liver cells, triggering adaptive responses that reduce cell injury. In this study, we determined whether Nrf2 protects the liver against LCA-induced toxicity in vivo.

EXPERIMENTAL APPROACH

Nrf2 disrupted (Nrf2(-/-) ) and wild-type mice were treated with LCA (125 mg·kg(-1) body weight) to induce liver injury. Levels of mRNA, protein and function of important Nrf2 target genes coupled with liver histology and injury biomarkers of mice were examined.

KEY RESULTS

In 4 day LCA treatments, we observed a significantly higher hepatic induction of Nrf2 target, cytoprotective genes including thioredoxin reductase 1, glutamate cysteine ligase subunits, glutathione S-transferases, haeme oxygenase-1 and multidrug resistance-associated proteins 3 and 4 in the wild type as compared with the Nrf2(-/-) mice. Moreover, basal and LCA-induced hepatic glutathione and activities of glutathione S-transferases and thioredoxin reductases were higher in wild-type than in Nrf2(-/-) mice. This reduced production of cytoprotective genes against LCA toxicity rendered Nrf2(-/-) mice more susceptible to severe liver damage with the presence of multifocal liver necrosis, inflamed bile ducts and elevation of lipid peroxidation and liver injury biomarkers, such as alanine aminotransferase and alkaline phosphatase.

CONCLUSIONS AND IMPLICATIONS

Nrf2 plays a crucial cytoprotective role against LCA-induced liver injury by orchestrating adaptive responses. The pharmacological potential of targeting liver Nrf2 in the management of cholestatic liver diseases is proposed.

Thiol redox systems and protein kinases in hepatic stellate cell regulatory processes

Hepatic stellate cells (HSC) are the major producers of collagen in the liver and their conversion from resting cells to a proliferating, contractile and fibrogenic phenotype ('activation') is a critical step, leading to liver fibrosis characterized by deposition of excessive extracellular matrix. Cytokines, growth factors, reactive oxygen and nitrogen species (ROS/RNS), lipid peroxides and their products deriving from hepatocytes, Kupffer cells and other cells converge on HSC and influence their activation. This review focuses on glutathione and thioredoxin pathways, with particular emphasis on their role in HSC. These two systems have been shown to act in the metabolism of hydrogen peroxide, control of thiol redox balance and regulation of signalling pathways. Particular attention is paid to mitochondria and NADPH oxidase. Detailed knowledge of specific signalling, redox conditions and apoptotic processes will be of help in devising proper pharmacological treatments for liver fibrosis.

Phloroglucinol (1,3,5-trihydroxybenzene) protects against ionizing radiation-induced cell damage through inhibition of oxidative stress in vitro and in vivo

Exposure of cells to gamma-rays induces the production of reactive oxygen species (ROS) that play a main role in ionizing radiation damage. We have investigated the radioprotective effect of phloroglucinol (1,3,5-trihydroxybenzene), phlorotannin compound isolated from Ecklonia cava, against gamma-ray radiation-induced oxidative damage in vitro and in vivo. Phloroglucinol significantly decreased the level of radiation-induced intracellular ROS and damage to cellular components such as the lipid, DNA and protein. Phloroglucinol enhanced cell viability that decreased after exposure to gamma-rays and reduced radiation-induced apoptosis via inhibition of mitochondria mediated caspases pathway. Phloroglucinol reduced radiation-induced loss of the mitochondrial membrane action potential, reduced the levels of the active forms of caspase 9 and 3 and elevated the expression of bcl-2. Furthermore, the anti-apoptotic effect of phloroglucinol was exerted via inhibition of mitogen-activated protein kinase kinase-4 (MKK4/SEK1), c-Jun NH(2)-terminal kinase (JNK) and activator protein-1 (AP-1) cascades induced by radiation exposure. Phloroglucinol restored the level of reduced glutathione (GSH) and protein expression of a catalytically active subunit of glutamate-cysteine ligase (GCL), which is a rate-limiting enzyme in GSH biosynthesis. In in vivo study, phloroglucinol administration in mice provided substantial protection against death and oxidative damage following whole-body irradiation. We examined survival with exposure to various radiation doses using the intestinal crypt assay and determined a dose reduction factor (DRF) of 1.24. Based on our findings, phloroglucinol may be possibly useful as a radioprotective compound.

Dysregulation of glutathione synthesis during cholestasis in mice: molecular mechanisms and therapeutic implications

Glutathione (GSH) provides important antioxidant defense and regulates multiple critical processes including fibrogenesis. There are conflicting literature studies regarding changes in GSH during cholestasis. Here we examined changes in the GSH synthetic enzymes during bile duct ligation (BDL) in mice and how treatment with ursodeoxycholic acid (UDCA) and/or S-adenosylmethionine (SAMe) affects the expression of these enzymes and liver injury. The hepatic expression of glutamate-cysteine ligase (GCL) subunits and GSH synthase (GS) increased transiently after BDL but fell to 50% of baseline by 2 weeks. Nuclear factor-erythroid 2-related factor 2 (Nrf2) trans-activates gene expression by way of the antioxidant response element (ARE), which controls the expression of all three genes. Despite increased Nrf2 nuclear levels, Nrf2 nuclear binding to ARE fell 2 weeks after BDL. Nuclear levels of c-Maf and MafG, which can negatively regulate ARE, were persistently induced during BDL and the dominant proteins bound to ARE on day 14. UDCA and SAMe induced the expression of GCL subunits and raised GSH levels. They increased nuclear Nrf2 levels, prevented c-Maf and MafG induction, and prevented the fall in Nrf2 nuclear binding to ARE. Combined treatment had additive effects, reduced liver cell death, and prevented fibrosis.

CONCLUSION

GSH synthesis falls during later stages of BDL due to lower expression of GSH synthetic enzymes. UDCA and SAMe treatment prevented this fall and combined therapy was more effective on preserving GSH levels and preventing liver injury.

Structure, function, and post-translational regulation of the catalytic and modifier subunits of glutamate cysteine ligase

Glutathione (GSH) is a tripeptide composed of glutamate, cysteine, and glycine. The first and rate-limiting step in GSH synthesis is catalyzed by glutamate cysteine ligase (GCL, previously known as gamma-glutamylcysteine synthetase). GCL is a heterodimeric protein composed of catalytic (GCLC) and modifier (GCLM) subunits that are expressed from different genes. GCLC catalyzes a unique gamma-carboxyl linkage from glutamate to cysteine and requires ATP and Mg(++) as cofactors in this reaction. GCLM increases the V(max) and K(cat) of GCLC, decreases the K(m) for glutamate and ATP, and increases the K(i) for GSH-mediated feedback inhibition of GCL. While post-translational modifications of GCLC (e.g. phosphorylation, myristoylation, caspase-mediated cleavage) have modest effects on GCL activity, oxidative stress dramatically affects GCL holoenzyme formation and activity. Pyridine nucleotides can also modulate GCL activity in some species. Variability in GCL expression is associated with several disease phenotypes and transgenic mouse and rat models promise to be highly useful for investigating the relationships between GCL activity, GSH synthesis, and disease in humans.

Regulation of glutathione synthesis

Glutathione (GSH) is a ubiquitous intracellular peptide with diverse functions that include detoxification, antioxidant defense, maintenance of thiol status, and modulation of cell proliferation. GSH is synthesized in the cytosol of all mammalian cells in a tightly regulated manner. The major determinants of GSH synthesis are the availability of cysteine, the sulfur amino acid precursor, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL). GCL is composed for a catalytic (GCLC) and modifier (GCLM) subunit and they are regulated at multiple levels and at times differentially. The second enzyme of GSH synthesis, GSH synthase (GS) is also regulated in a coordinated manner as GCL subunits and its up-regulation can further enhance the capacity of the cell to synthesize GSH. Oxidative stress is well known to induce the expression of GSH synthetic enzymes. Key transcription factors identified thus far include Nrf2/Nrf1 via the antioxidant response element (ARE), activator protein-1 (AP-1) and nuclear factor kappa B (NFkappaB). Dysregulation of GSH synthesis is increasingly being recognized as contributing to the pathogenesis of many pathological conditions. These include diabetes mellitus, pulmonary fibrosis, cholestatic liver injury, endotoxemia and drug-resistant tumor cells. Manipulation of the GSH synthetic capacity is an important target in the treatment of many of these disorders.