Hui-Kuan Chu ,Yan Ai ,Zi-Lu Cheng ,Ling Yang,Xiao-Hua Hou

Division of Gastroenterology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China

Keywords: Gut microbiota Lipopolysaccharide Probiotics Fecal microbiota transplantation Drug-induced Liver injury

ABSTRACT Drug-induced liver injury (DILI) is caused by various drugs with complex pathogenesis,and diverse clinical and pathological phenotypes. Drugs damage the liver directly through drug hepatotoxicity,or indirectly through drug-mediated oxidative stress,immune injury and inflammatory insult,which eventually lead to hepatocyte necrosis. Recent studies have found that the composition,relative content and distribution of gut microbiota in patients and animal models of DILI have changed significantly. It has been confirmed that gut microbial dysbiosis brings about intestinal barrier destruction and microorganisms translocation,and the alteration of microbial metabolites may cause or aggravate DILI. In addition,antibiotics,probiotics,and fecal microbiota transplantation are all emerging as prospective therapeutic methods for DILI by regulating the gut microbiota. In this review,we discussed how the altered gut microbiota participates in DILI.

Introduction

Drug-induced liver injury (DILI) is defined as the liver damage induced by a variety of drugs and its associated metabolites,including natural medicine,traditional Chinese medicine,and prescription or over-the-counter chemical drugs [1] . The incidence of DILI is about 14-19 cases per 100000 people [ 2,3 ],while in China it is estimated to be 23.8 per 100000 people [4] . DILI has been identified as one of the most common causes of acute liver failure [5],which may cause jaundice and lead to fatal consequences.According to the pathogenesis,DILI is usually classified into three groups,direct,idiosyncratic,and indirect liver damage. The former means that drugs or their metabolites are intrinsically toxic,while the second involves abnormal drug metabolism,mitochondrial damage,oxidative stress,immune damage,inflammatory insult,and individual genetic differences [ 1,6 ]. Recently,an increasing number of studies have explored the relationship between gut microbiota and DILI.

The gut microbiota is referred to various microorganisms residing in human gut,comprising bacteria,fungi,viruses,and archaea.In a healthy state,the gut microbiota lives with human harmoniously,which maintains the normal biological function of the host through liberating necessary nutrients from dietary foods,regulating immune responses,and conferring protection against intestinal pathobiont colonization. However,the abundance and composition of gut microbiota are dynamic,which is related to age,diet,drugs,and psychological stress. When the gut microbiota is under dysregulation,it also involves in the initiation and progression of various liver diseases,including autoimmune hepatitis,nonalcoholic fatty liver disease (NAFLD),and alcoholic hepatitis,through altering bile acid profile,promoting endotoxin translocation,modulating immune responses,and other effects.

Up to now,numerous studies have been devoted to clarifying the role of gut microbiota in DILI. The treatment of DILI is limited,and there is no specific antidote for hundreds of drugs that cause severe liver injury other than N-acetylcysteine for acetaminophen(APAP) overdose [7] . This review mainly focuses on the changes in the composition,abundance,distribution as well as metabolites of gut microbiota in DILI,and its possible mechanisms involved in DILI. Owing to the successful utilization of fecal microbiota transplantation (FMT) or probiotics in DILI treatment,the gut-liver axis shows great therapeutic potential in DILI.

Gut-liver axis

Human intestinal tract contains various microorganisms. These microbes establish a symbiotic relationship with the host [8],most of which can confer protection to the host immune system.The gut microbiota is a hidden organ,which plays a critical role in immune development,hematopoiesis,and metabolism regulation [ 9,10 ]. About one-hundred trillion bacteria reside in human gut lumen,90% of which pertain to the phyla Bacteroidetes and Firmicutes [11] . A proportion of bacteria have an intimate association with the production of metabolites. The portal vein mediates the bidirectional communication between the liver and gut,which transports bioactive substances generated in the liver to the gut and allows the translocation of enterogenic products to the liver [12] . Once the intestinal barrier is compromised,the gut microbiota and its products,such as lipopolysaccharide (LPS),can translocate to the liver and further cause liver injury [13–16] . Better understanding the gut-liver axis helps understand the connection between gut microbiota and the occurrence and development of DILI.

Changes of gut microbiota in DILI

The gut microbiota is an active part of the gut-liver axis [17],and the disturbance of its composition and activity is thought to be related to the emergence of DILI. Some probiotics,such asAkkermansia.muciniphila,can reduce liver injury through maintaining gut barrier integrity,reshaping the microbial community,and promoting short-chain fatty acid (SCFA) secretion [ 18,19 ].A.muciniphilacould significantly ameliorate APAP-induced oxidative stress and inflammatory response. And it also reduces the levels of serum proinflammatory chemokines and cytokines[macrophage inflammatory protein-1b (MIP-1b),MIP-1a,monocyte chemotactic protein-1 (MCP-1),interleukin-12p40 (IL-12p40),interferon-γ(IFN-γ),IL-2] [19] in Con A-induced liver injury in mouse.

At present,studies [20–26] have observed the reduction of gut microbiota diversity in DILI patients and animal models,and suggested that potential pathogenic groups are relatively overgrown,and that the relative abundance of potentially beneficial native groups is decreased ( Table 1 ). Moreover,the ratio of Firmicutes/Bacteroidetes is elevated in DILI conspicuously. At the phylum level,the abundance of Actinobacteria,Proteobacteria and Deferribacterota is significantly increased,whereas that of Bacteroidota,Bacteroidetes,Verrucomicrobiota,unclassified_k_norank_d_Bacteria and Fusobacteriota is decreased. At the genus level,DILI patients or animal models have an increased abundance ofA.muciniphila,Blautia,Ruminococcu_torques_group,Staphylococcus,Enterorhabdus,Enterococcus,Collinsella,Streptococcus,Aerococcus,Lactobacillus,Dorea,Lachnospiraceae_incertae_sedis,Eubacterium_rectaleandRomboutsia,and a decreased abundance ofLactobacillus,Allobaculum,norank_f_Lachnospiraceae,unclassified_f_Lachnospiraceae,norank_f_Muribaculaceae,Dubosiella,Ruminococcus,Eubacterium_xylanophilum_group,Phascolarctobacterium,Faecalibaculum,Dehalobacterium,Bacteroides,Bifidobacterium,Blautia,Eubacterium,Clostridium,Roseburia,Faecalibacterium,andClostridium_sensu_stricto_1compared with healthy controls [20–26] .Lactobacillusreuteri,LactobacillusacidophilusandLactobacillusrhamnosusall belong toLactobacilluswith antiinflammatory function [27–29] and hepatoprotective effects [30],but the abundance ofLactobacillusis decreased in DILI [ 21,23 ]. In addition,the reduction ofBacteroidesis intimately related to the increased oxidative stress and declining liver function [23] . Interestingly,there is an increase in the abundance ofA.muciniphilain wild type (WT) mice with APAP-induced liver injury,which significantly thickens colonic mucosa [20] . The increment of this beneficial bacteria may be a form of self-protection.

Table 1The changes of gut microbiota in DILI.

Besides,gut microbiota also regulates liver injury through their metabolites. Proteobacteria can generate massive gut-derived endotoxic LPS,which is increased in DILI [ 21,24 ]. Enterobacteriaceae andBacteroidesexhibitβ-glucuronidase activity,whileLactobacillusgenus inhibits bacterialβ-glucuronidase activity and is intimately related to hepatoprotection.β-glucuronidase can enhance the susceptibility of tacrine-induced liver injury [26] .

Moreover,the metabolism of some hepatotoxic drugs may have a circadian rhythm,which is putatively related to the diurnal concussion of gut microbiota [31] . Hepatotoxicity induced by APAP showed a circadian rhythm [ 32,33 ]. Compared with zeitgeber time(ZT) 0 (8:00 am),the liver injury of ZT12 (8:00 pm) was significantly enhanced [ 31,34 ]. At the same time,the composition and function of several intestinal microbiota,such asClostridiales,Lac-tobacillales,andBacteroidales,also have been confirmed to have diurnal oscillations affected by eating rhythm [35],and this change might affect the metabolism of APAP. The abundance ofEnterorhabdusandLactobacilluswas increased in ZT0,while the abundance ofRikenella,Barnesiella,Bacteroides,AlistipesandPseudoflavonifractorwere enriched in ZT12 [31] .

Potential mechanism of gut microbiota affecting DILI

As mentioned above,there is an obvious alteration of gut microbiota in DILI,which may contribute to the pathogenesis of DILI.The specific mechanisms underlying how the gut microbiota contributes to DILI have not been fully figured out. The potential mechanisms of gut microbiota affecting DILI include increasing intestinal permeability,influencing microbial metabolites production and corresponding signaling pathways,as well as modulating immune responses ( Fig. 1 ).

Fig. 1. The potential mechanism of gut microbiota affecting DILI. The microbial dysbiosis in DILI,especially the increment of Enterobacteriaceae,brings about the destruction of gut barrier and promotes the translocation of gut-derived microbiota and its associated metabolites. High level of LPS,caused by the increased Proteobacteria,translocates to the liver and stimulates the secretion of TNF- α,IL-1 β,IL-6,and MCP-1,which may aggravate the inflammatory insult in DILI via the activation of TLR4/MyD88/NFκB. Moreover,LPS also enhances ROS generation by upregulating the expression of CYP2E1,leading to hepatocyte necrosis. Necrotic hepatocyte releases DAMPs,which can activate TLRs and stimulate the secretion of massive inflammatory cytokines and chemokines,thus putatively contributing to the pathological progression of DILI. The increase of E. coli and Citrobacter freundii in DILI causes the increment of PPD,which also promotes ROS production through consuming GSH. In addition,the reduced SBA inactivates GPBAR1 and hampers NKT10 cells polarization,ultimately blocking the secretion of IL-10. Furthermore,the reduced SCFAs caused by the decrease of Bacteroidetes may exacerbate intestinal inflammation in DILI by inactivating FFAR2 and FFAR3 distributed on immune cells and GPR109A on IECs,which can repress NF- κB activation induced by LPS. DILI: drug-induced liver injury; LPS: lipopolysaccharide; IL: interleukin; SCFA: short-chain fatty acid; TLR4: Toll-like receptor 4; NF- κB: nuclear factor kappa B;MyD88: myeloid differentiation primary response 88; TNF: tumor necrosis factor; MCP-1: monocyte chemical attractant protein-1; ROS: reactive oxygen species; FFAR2:free fatty acid receptor 2; PPD: 1-phenyl-1,2-propanedione; SBA: secondary bile acid; DAMP: damage associated molecular pattern; GSH: glutathione; CYP2E1: cytochrome P4502E1; GPBAR1: G protein-coupled bile acid receptor 1; NKT: natural killer T; GPR109A: G protein-coupled receptor; NAPQI: N-acetyl-p-benzoquinonimine.

Increased intestinal permeability

The intestinal barrier is made up of physical,immunologic,and microbial constituents. Gut dysbiosis damages intestinal barrier and increases intestinal permeability. Impaired intestinal barrier results in the translocation of microorganisms and microbial metabolites to systemic circulation [ 15,36,37 ]. It has been confirmed that the increase of Enterobacteriaceae is related to the destruction of intestinal barrier [25] . The reduction ofA.muciniphilais often concomitant with a thinner mucous layer,thus damaging the integrity of intestinal barrier [18] .

Besides,part of the microbiota regulates intestinal immunology and inflammation.A.muciniphilais a kind of mucin decomposing bacteria [38] . It helps maintain a healthy intestinal barrier [ 39,40 ] by modulating immune responses and blocking inflammation,which belongs to a fundamental cause of many diseases.The alteration of this bacterial genus has an intimate association with gut barrier destruction and intestinal inflammation [ 8,41 ].

The change of microbial components is also related to the intestinal barrier. The absence of nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 6 (NLRP6) results in the disorder of intestinal barrier function and the change of microbiota composition [ 42,43 ].Nlrp6-/-mice with intestinal microbiota imbalance showed aggravated APAP and LPS-induced liver damage,which showed a strong correlation with the significant decrease of the microbial diversity in the intestine and liver and with the increase of intestinal permeability at baseline.

Taken together,the dysregulated gut microbiota leads to intestinal barrier destruction and increased intestinal permeability by reducing the indispensable components for the intestinal barrier and modulating inflammatory and immune responses. Compromised gut barrier is conducive to the translocation of gut microbiota and its associated metabolites from the gut to the liver,where they trigger a series of inflammatory and immune responses,thus exacerbating DILI.

Production of metabolites LPS

The LPS,a component of Gram-negative bacteria [44],translocates directly to the portal vein and then activates Toll-like receptors (TLRs) in liver,ultimately resulting in liver injury [ 13-16,45 ].The severity of liver disease is often associated with the degree of endotoxemia [46] . In recent years,several studies have observed an obvious elevation of LPS in both DILI patients and animal models,suggesting that high level of LPS participates in DILI pathogenesis [47–49] . The increment of LPS level is in parallel to the increased abundance of Proteobacteria in DILI.

The increased LPS initiates a series of inflammatory responses by activating TLR4 [50] . TLR4 knockout mice are less likely to develop extrahepatic organ dysfunction after liver injury [51] . And TLR4 anti-agonists are able to significantly ameliorate liver injury induced by paracetamol and galactosamine [52] . Microbiotaderived TLR ligands regulate parenteral macrophages (M1/M2) development and function [53] . LPS first interacts with TLR4,which then activates myeloid differentiation factor 88 (MyD88) and nuclear factor-kappa B (NF- к B) sequentially,ultimately resulting in the secretion of enormous proinflammatory chemokines and cytokines,including MCP-1,IL-6,IL-1β,and tumor necrosis factor-α(TNF-α) [54] . Furthermore,LPS promotes the overexpression of cytochrome P4502E1 (CYP2E1) [55],which is mainly in charge of the generation of reactive oxygen species (ROS). ROS has the capacity to induce necrosis and apoptosis in liver,thus exacerbating hepatotoxicity [56] .

SCFAs

SCFAs pertains to one kind of fermentation products generated by the gut microbiota,which contains fewer than six carbons. SCFAs not only provide the intestinal epithelial cells with energy,but also exert great influence on its proliferation,differentiation,and function,contributing to the maintenance of the gut barrier and the modulation of host metabolism [57] . Plenty of recent studies have supported compelling evidence to link SCFAs to DILI [58–60] . Xia et al. have reported thatAkkermansiamitigated the liver damage caused by APAP partly through potentiating SCFAs production [59] . Moreover,Pirozzi et al. have establishedinvitroandinvivomodels of DILI exposed to valproate (VPA),and found that sodium butyrate was able to ameliorate liver inflammation and fibrosis induced by VPA through blocking oxidative stress and modulating fatty acid metabolism [58] . Furthermore,ampicillin aggravated APAP-induced liver damage partly via the reduction of butyrate level,which could be reversed byLactobacillussupplementation [60] . Butyrate is a nutrient for intestinal cells,which can promote cell regeneration,maintain intestinal barrier function,and has anti-inflammatory properties [61] .

To be more specific,SCFAs can activate free fatty acid receptor 2(FFAR2) that located on intestinal epithelial L cells and pancreatic isletαandβcells,which alleviates insulin resistance [62] . Moreover,FFAR2 and FFAR3 located on immune cells also have antiinflammation properties through downregulating proinflammatory chemokines and cytokines,such as MCP-1,IL-1β,IL-6,and TNFα,and upregulating anti-inflammatory cytokines,like IL-10 [ 63,64 ].Furthermore,in bothinvitroandinvivoexperiments,butyrate can activate G protein-coupled receptor (GPR109A) and further block NF-κB activation induced by LPS,thus conferring protection against intestinal inflammation [65] . Therefore,the decreased SCFAs in DILI caused by the reduced abundance of Bacteroidetes may contribute to the disease progression.

Bile acid

Bile acid is synthesized from cholesterol within hepatocytes mainly via the canonical pathway and alternative pathway,predominantly comprising primary bile acid [cholic acid (CA) and chenodeoxycholic acid (CDCA)] and secondary bile acid [deoxycholic acid (DCA) and lithocholic acid (LCA)] [66] . CYP7A1 serves as the rate-limiting enzyme in classic pathway,while CYP27A1 is mainly in charge of the synthesis of bile acid in alternative pathway. Under usual conditions,bile acid is deemed to be an important emulsifier,which contributes to the absorption of lipid-soluble nutrient substances. In pace with more penetrative investigations of bile acid,it has been confirmed to take an important part in modulating various biological and pathological processes. In several studies,the dysregulated bile acid metabolism has been found to correlate with the pathogenesis of many diseases,including fatty liver,obesity,as well as inflammatory bowel disease (IBD) [67–69] .

Indeed,more and more studies have indicated that the disorder of bile acid metabolism has an intimate association with DILI [ 70,71 ]. Recent metabolomic analyses have shed light on that the specific bile acid profile has great potential in serving as a biomarker for DILI and predicting the severity of DILI [70–72] .Clostridiumis rich in bacterial 7α-dehydroxylase and has the capacity to transform CA and CDCA to DCA and LCA,respectively [73] . As aforementioned,the abundance ofClostridiumis decreased in the DILI group,thus reducing secondary bile acid production in the gut,which is consistent with the increased ratio of primary bile acid to secondary bile acid in another study [71] .The evidence suggests that the gut dysbiosis in DILI can alter the composition and concentration of bile acid,which shows a strong correlation with the severity of DILI. However,the specific mechanisms by which dysregulated bile acid metabolism affects the pathological progression of DILI remain obscure,which demands more in-depth investigations.

1-phenyl-1,2-propanedione and CYP450

1-phenyl-1,2-propanedione (PPD) levels in the colon and liver are identified as prospective biomarkers of bacteria that increase APAP toxicity,which directly consumes glutathione (GSH) and significantly increases the ratio of oxidized glutathione (GSSG)/GSH.CYP2E1 is mainly responsible for the biotransformation of APAP into N-acetyl-p-benzoquinone imine [74],which can be detoxified by combining with liver GSH. If not reduced by GSH,toxic metabolites combine with mitochondrial proteins covalently,resulting in increased production of ROS. ROS has the capacity to activate the mitogen-activated protein kinase (MAPK)pathway,which can subsequently activate c-Jun N-terminal kinase (JNK) [75] . JNK then amplifies the production of mitochondrial ROS by interacting with the mitochondria [76] . In the APAP model,mitochondrial ROS level is positively related to hepatocyte necrosis and acute liver failure [77–79] . Some strains likeEnterococcusfaecalis,Clostridiumdifficile,Citrobacterfre-undii,andE.coli,generate PPD [31] . Among them,the abundance ofCitrobacterfreundiiandE.coliis increased in the DILI model group induced by APAP,thus enhancing the production of PPD and further exacerbating APAP-induced liver injury [31] .Moreover,when treated withSaccharomycescerevisiae(Yeast),the level of PPD and the liver damage in the APAP-induced DILI model mice are significantly reduced,which suggests that probiotics may ameliorate the liver damage induced by APAP partly through reducing PPD production [31] .

In summary,the altered gut microbiota in DILI patients and animal models changes the content of gut luminal metabolites,which also takes an important part in DILI pathogenesis. The increased LPS in the DILI group aggravates hepatic inflammation by activating TLR4/MyD88/NF-κB pathway and upregulating CYP2E1 expression. The decreased SCFAs in DILI inactivates FFAR2,FFAR3,and GPR109A,thus expediting the inflammatory progression of DILI.In addition,the dysregulation of bile acid metabolism induced by gut dysbiosis shows a strong correlation with the severity of DILI.PPD,a metabolite which exacerbates APAP-induced liver damage through promoting ROS generation and activating MAPK pathway,is confirmed to increase in the DILI model group induced by APAP owing to the increase ofE.coliandCitrobacterfreundiiin DILI.

Immune response

Immune response is referred to the defensive response of the organisms to pathogens or foreign matters when they pass through the immune system,which is critical for the maintenance of normal life activities. The excessive activation or suppression of immune response greatly threatens human health,and is identified as one of the main pathological links for a variety of disorders. Absolutely,immune response has also been confirmed to have an intimate association with DILI development [80] . Hepatotoxicity drugs,such as APAP,produce reactive metabolites and protein adducts,which mediate hepatocyte necrosis [80] . The damage associated molecular patterns (DAMPs) released from necrotic hepatocytes activate a series of immune cells and further promote them to secret massive inflammatory cytokines and chemokines,which regulate liver injury,liver repair,and liver regeneration [80] .

The altered gut microbiota also putatively contributes to DILI progression through regulating immune responses. On one hand,the gut leakage in DILI allows the translocation of gut microbiota and its related metabolites to liver,in which these pathogenassociated molecular patterns (PAMPs) activate hepatocytes,immune cells,as well as hepatic stellate cells (HSCs) to release proinflammatory cytokines and chemokines,thus exacerbating the disease progression of DILI [81] . On the other hand,the changed concentration of luminal contents caused by gut dysbiosis also exerts effects on immune cell activation. The reduced secondary bile acids repress the polarization of natural killer T10 (NKT10) cell from NKT cell by inactivating G protein-coupled bile acid receptor 1 (GPBAR1),thus hampering the production of IL-10 [82] . Moreover,the decreased SCFAs bring about the increment of Th1 cells and the reduction of Tregs,exacerbating inflammatory responses in immune-mediated diseases [83] . Although little direct evidence indicates that the altered gut microbiota regulates certain kind of immune cells or immune mediators in DILI,it is still reasonable to speculate that regulating immune response is one of the putative mechanisms underlying how gut microbiota affects DILI.

Treatment based on gut microbiota for DILI

DILI is difficult to diagnose,and once suspected,the most important treatment method is to stop taking the suspicious drug.The change of gut microbiota provides the possibility for intervention. Antibiotics,prebiotics,and FMT [12],are becoming prospective therapies for DILI.

Antibiotics

With a universal utilization of antibiotics in clinical practice,antibiotics have emerged as one of the main causes for DILI,especially in Western countries [84] . However,some kinds of antibiotics also can alleviate DILI by eliminating pathological bacteria in the intestine [85] . Zheng et al. [85] has found that vancomycin mitigates the liver injury induced by APAP through modulating gut microbiota and increasing the level of serum 2-hydroxybutyric acid,which exhibits hepatoprotective effects in different species.

Probiotics

Probiotics,one of the active organisms colonizing in the host,have multiple salutary effects on the human body,including reconstructing balanced gut microbiota,promoting nutrient substances digestion and absorption,and maintaining gut barrier integrity [ 61,86 ]. Owing to its multifaceted benefits,probiotics have been regarded as an effective therapeutic method for many diseases,including atopic disease,IBD,Parkinson’s disease,etc. [87–89] . As indicated above,gut dysbiosis participates in DILI development,and restoring gut microbiota seems to be a putative approach to treat DILI. Indeed,many studies have showed that probiotics significantly ameliorate the liver damage induced by drugs.Lactobacillushas been found to prevent methotrexate(MTX)-induced intestinal barrier injury and liver damage in mice.Magnesium isoglycyrrhizinate (MgIG) treatment could increase the abundance ofLactobacillus,and thus improve liver injury and restore intestinal barrier function in MTX-induced DILI [90] .

Lactobacilluswas found to inhibit LPS-related TLR4 inflammatory pathway and block neutrophils and monocytes infiltration.Moreover,during tuberculosis treatment,Lactobacilluscaseicould mitigate the aberrant increment of some liver indexes that were associated with cholestasis through reducing endotoxin levels and improving intestinal barrier dysfunction [91] . In addition,treating the mice exposed to high fat diet (HFD)/carbon tetrachloride (CCl 4 )with live and pasteurizedA.muciniphilaand its extracellular vesicles was favorable for the normalization of the altered gut microbiota,the improvement of intestinal permeability,and the prevention of liver damage [92] .A.muciniphilaadministration also strikingly ameliorated APAP-induced hepatotoxicity,which was indicated by reduced serum alanine aminotransferase and aspartate transaminase levels [ 18,19 ]. Herein,probiotics seem to be a prospective therapeutic method for APAP-induced liver damage.

FMT

FMT means transplanting functional bacteria from the healthy donator’s stool into the sick recipient’s gastrointestinal tract to normalize recipient’s gut microbiota composition. Distinct from antibiotics and prebiotics,FMT can directly provide the recipient with the bacteria which cannot be cultured artificially. With the successful utilization of FMT inClostridiumdifficileinfection,more and more researchers have explored the possibility and feasibility of FMT in treating human diseases. Indeed,a growing body of animal and clinical studies have confirmed that FMT is able to serve as an effective therapeutic approach for various diseases,including IBD,neurological disorders,and NAFLD [93–95] . Hong et al. [96] transferred the fecal microbiota from the Oridonin + APAP treatment group and single APAP group to the recipient mice which previously received antibiotics treatment,respectively. And they found that the liver injury in the Oridonin + APAP group was less severe than that in the single APAP group. Moreover,in another animal study,Liu et al. [97] confirmed that FMT from healthy subjects was able to strikingly mitigate the liver damage induced by Dgalactosamine partly through modulating the gut microbiota. The evidence supports the perspective that FMT has the potential to treat DILI.

In a nutshell,restoring the altered gut microbiota in DILI seems to be an effective method to treat DILI. Antibiotics,like vancomycin,probiotics,such asLactobacillusandA.muciniphila,as well as FMT,all have great potential in ameliorating the liver damage induced by hepatotoxic drugs through modulating the composition and abundance of gut microbiota in several animal studies.However,corresponding clinical studies are scarce,which demand further studies.

Conclusions

DILI is classified into three groups,direct,idiosyncratic,and indirect liver damage. The direct DILI is the focal point in this review. There are thousands of drugs causing direct DILI with various mechanisms. In DILI patients and animal models,an increasing number of studies have suggested that there are obvious changes in the abundance and diversity of gut microbiota. The proportion of pathogenic microbiota is increased,while the proportion of beneficial microbiota is decreased. Gut dysbiosis contributes to DILI in many ways,including increasing intestinal permeability,inducing microorganisms translocation,as well as altering the contents of microbial metabolites. These microbial products,including LPS,SCFAs,bile acid,and PPD,regulate oxidative stress,immune injury,and inflammatory insult,and eventually lead to hepatocyte necrosis. The treatment of DILI is limited,and the most important treatment method is to stop using suspected drugs. But notably,owing to the recent studies with respect to the gut microbiota and DILI,gut microbiota has become a prospective therapeutic target for DILI. Antibiotics,probiotics,and FMT increase the abundance of beneficial bacteria,regulate bile acid metabolism,and repair immune injury.

However,there are also differences in the changes of the same bacterial strain among different studies about DILI,which may be related to the sample size,individual difference,region,diet,heredity,model category,etc. Therefore,a review of different studies is of great benefit to figure out specific alterations of gut microbiota in DILI,explicitly clarify the mechanisms underlying how the altered gut microbiota affects DILI,and provide compelling theoretical support for the establishment of novel and effective therapy for DILI.

Acknowledgments

None.

CRediT authorship contribution statement

Hui-KuanChu:Conceptualization,Funding acquisition,Writing– review & editing.YanAi:Investigation,Writing – original draft.Zi-LuCheng:Investigation,Writing – original draft.LingYang:Funding acquisition,Writing – review & editing.Xiao-HuaHou:Conceptualization,Funding acquisition,Writing – review & editing.

Funding

This study was supported by grants from the National Natural Science Foundation of China ( 82000561,81974078,81570530,81370550,81974062,81720108006 ),the Natural Science Foundation of Hubei Province ( 2019ACA1333 ) and the Science Foundation of Union Hospital ( 2021xhyn005 ).

Ethical approval

Not needed.

Competing interest

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.