antioxidants Review Changes in Glutathione Content in Liver Diseases: An Update Mariapia Vairetti , Laura Giuseppina Di Pasqua * , Marta Cagna, Plinio Richelmi, Andrea Ferrigno * and Clarissa Berardo Unit of Cellular and Molecular Pharmacology and Toxicology, Department of Internal Medicine and Therapeutics, University of Pavia, 27100 Pavia, Italy; [email protected] (M.V.); [email protected] (M.C.); [email protected] (P.R.); [email protected] (C.B.) * Correspondence: [email protected] (L.G.D.P.); [email protected] (A.F.); Tel.: +39-0382-98687 (L.G.D.P.); +39-0382-986451 (A.F.) Abstract: Glutathione (GSH), a tripeptide particularly concentrated in the liver, is the most important thiol reducing agent involved in the modulation of redox processes. It has also been demonstrated that GSH cannot be considered only as a mere free radical scavenger but that it takes part in the network governing the choice between survival, necrosis and apoptosis as well as in altering the function of signal transduction and transcription factor molecules. The purpose of the present review is to provide an overview on the molecular biology of the GSH system; therefore, GSH synthesis, metabolism and regulation will be reviewed. The multiple GSH functions will be described, as well as the importance of GSH compartmentalization into distinct subcellular pools and inter-organ transfer. Furthermore, we will highlight the close relationship existing between GSH content and the pathogenesis of liver disease, such as non-alcoholic fatty liver disease (NAFLD), alcoholic liver disease (ALD), chronic cholestatic injury, ischemia/reperfusion damage, hepatitis C virus (HCV), hepatitis B virus (HBV) and hepatocellular carcinoma. Finally, the potential therapeutic benefits of Citation: Vairetti, M.; Di Pasqua, GSH and GSH-related medications, will be described for each liver disorder taken into account. L.G.; Cagna, M.; Richelmi, P.; Ferrigno, A.; Berardo, C. Changes in Keywords: glutathione; liver; Nrf2 Glutathione Content in Liver Diseases: An Update. Antioxidants 2021, 10, 364. https://doi.org/ 10.3390/antiox10030364 1. Introduction Glutathione (GSH), or L-γ-glutamyl-L-cysteinyl-glycine, is a tripeptide present in Academic Editor: Greg Barritt mammalian cells at surprisingly high levels (1–10 mM) and particularly concentrated in the liver [1,2]. The thiol group of cysteine is responsible for GSH biological activity: Received: 2 February 2021 GSH is considered the most important cellular redox buffer and major defender against Accepted: 24 February 2021 Published: 28 February 2021 oxidative stress. GSH is oxidized into glutathione disulfide (GSSG), which is actually two GSH molecules bound together at the sulfur atoms (Figure1). The oxidized form is Publisher’s Note: MDPI stays neutral reconverted into GSH by the NADPH-dependent enzyme glutathione reductase (GR) [3]. with regard to jurisdictional claims in The GSH:GSSG ratio is often used as a marker of cellular toxicity. Under normal conditions, published maps and institutional affil- the molar GSH:GSSG ratio is around 100:1; during stress, this ratio has been shown to iations. decrease to 10:1 and even 1:1 [4–8]. It has been demonstrated that GSH has multiple roles in cell physiology, including: (i) direct scavenging of reactive oxygen species (ROS), nitric oxide (NO) and its derivatives (RNS), resulting in the protection of the electron transport chain, DNA, lipids and proteins; (ii) indirect neutralization of intoxicants; (iii) S-gluta-thionylation of protein Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. thiol groups; (iv) regulation of cell cycle progression and apoptosis. This article is an open access article From these considerations, it emerges that GSH cannot be considered a mere free distributed under the terms and radical scavenger but that it has a weighty role in the network governing the choice conditions of the Creative Commons between survival, necrosis and apoptosis [9,10] as well as in cell signaling [10,11] and Attribution (CC BY) license (https:// metabolism [3,12]. creativecommons.org/licenses/by/ The present review will focus on the molecular biology of the GSH system. GSH 4.0/). synthesis, metabolism and regulation will be reviewed; the multiple GSH functions will be Antioxidants 2021, 10, 364. https://doi.org/10.3390/antiox10030364 https://www.mdpi.com/journal/antioxidants Antioxidants 2021, 10, x FOR PEER REVIEW 2 of 39 Antioxidants 2021, 10, 364 2 of 39 The present review will focus on the molecular biology of the GSH system. GSH syn- thesis, metabolism and regulation will be reviewed; the multiple GSH functions will be described, as well as GSH compartmentalization and interorgan interorgan transfer. Finally, the role of GSH in pathogenesis and potential therapeutictherapeutic approaches will bebe alsoalso highlighted.highlighted. Figure 1. Glutathione (GSH) structure and balance between GSH and glutathione disulfide (GSSG). Figure 1. Glutathione (GSH) structure and balance between GSH and glutathione disulfide (2.GSSG Glutathione). Synthesis and Metabolism The main source of GSH is the liver, which has the unique ability to synthesize cysteine, 2. Glutathione Synthesis and Metabolism a GSH precursor, from endogenous sources through the trans-sulphuration pathway [13], or toThe obtain main it from source protein of GSH breakdown is the liver, and which food. has GSH the availability unique ability in hepatocytes to synthesize depends cys- teine,on the a membrane GSH precursor, transport from of endogenous cysteine itself, sources cystine through and methionine the trans-sulphuration [2]. In fact, a goodpath- wayamount [13] of, or GSH to obtain is indirectly it from obtained protein breakdown from methionine; and food. methionine GSH availability deficiency in has hepato- been cytesfound depends to induce on the a reduction membrane in transport GSH availability, of cysteine especially itself, cystine in the and liver methionine [14,15]. [2] As. In a fact,consequence, a good amount the liver of playsGSH is a centralindirectly role obtained in maintaining from methionine; interorgan homeostasismethionine deficiency of GSH by hasexporting been found nearly to all induce of the synthesizeda reduction in GSH GSH into availability, plasma and especially bile; in fact, in the the liver has[14,15] the. Ashighest a consequence, capacity for the GSH liver efflux plays through a central its basolateralrole in maintaining and apical interorgan poles [13]. homeostasis Consequently, of GSHan alteration by exporting of the nearly hepatic all ability of the tosynthesized synthesize GSH or export into plasma GSH has and an bile; impact in fact, on systemicthe liver hasGSH the homeostasis highest capacity [2]. for GSH efflux through its basolateral and apical poles [13]. Con- sequently,The first an stepalteration in GSH of synthesisthe hepatic is catalyzedability to bysynthesizeL-glutamate or exportL-cysteine GSHγ has-ligase an impact (GCL), onresulting systemic in theGSH formation homeostasis of γ -glutamyl-cysteine.[2]. GCL is a heterodimer constituted by a catalyticThe monomerfirst step in (GCLC) GSH synthesis and a modifier is catalyzed monomer by L-glutamate (GCLM). GCLC L-cysteine is sensitive γ-ligase to (GCL), GSH- resultinginduced feedback in the formation inhibition. of GCLMγ-glutamyl is enzymatically-cysteine. GCL inert; is however,a heterodimer in the constituted dimer it reduces by a catalyticthe inhibitory monomer feedback (GCLC) response and a tomodifier GSH [2 ].monomer GCL expression (GCLM). responds GCLC is tosensitive oxidative to stressGSH- inducedand is positively feedbackregulated inhibition. by GCLM nuclear is factorenzymatically erythroid inert; 2-related however, factor in 2the (Nrf2) dimer [16 it– 19re-]. ducesNrf2 works the inhibitory in combination feedback with response small avianto GSH musculoaponeurotic [2]. GCL expression fibrosarcoma responds to oncogeneoxidative stresshomolog and (MAF) is positively proteins: regulated together by they nuclear recognize factor and erythroid bind antioxidant 2-related factor response 2 (Nrf2) elements [16– (ARE)19]. Nrf2 in works promoters in combination of antioxidant with enzyme small avian genes musculoaponeurotic (phase 2 enzymes suchfibro assarcoma glutathione onco- geneS-Transferase homolog (GST),(MAF) UDP-glucorosyl proteins: together transferase they recognize and superoxide and bind antioxidant dismutase (SOD),response GSH el- peroxidaseements (ARE) (GPx) in promoters and catalase of antioxidant (CAT), sulfiredoxin enzyme 1genes and Thioredoxin(phase 2 enzymes reductase such 1as [16 gluta-–19]. thioneNrf2 is S inhibited-Transferase by Kelch-like(GST), UDP ECH-associated-glucorosyl transferase protein 1and (Keap1): superoxide specifically, dismu ittase increases (SOD), GSHNrf2 peroxidase proteasome-mediated (GPx) and catalase degradation (CAT) [20, s,21ulfiredoxin]. Electrophilic 1 and agentsThioredoxin and antioxidants reductase 1 [16induce–19]. conformational Nrf2 is inhibited changes by Kelch in- Keap1,like ECH thus-associated impeding protein the binding 1 (Keap1): with specifically, Nrf2 leading it increasesto the consequent Nrf2 proteasome migration-mediated of the nucleardegradation factor [20,21] in the. Electrophilic nucleus, where agents it can and exert antiox- its idantsfunction induce [22]. conformational changes in Keap1, thus impeding the binding with Nrf2 leadingThe to second the consequent step in GSH