Alcoholic Liver Disease: The Gut Microbiome and Liver Cross Talk

Alcoholic liver disease (ALD) is a leading cause of morbidity and mortality worldwide. Alcoholic fatty liver disease can progress to steatohepatitis, alcoholic hepatitis, fibrosis, and cirrhosis. Patients with alcohol abuse show quantitative and qualitative changes in the composition of the intestinal microbiome. Furthermore, patients with ALD have increased intestinal permeability and elevated systemic levels of gut‐derived microbial products. Maintaining eubiosis, stabilizing the mucosal gut barrier, or preventing cellular responses to microbial products protect from experimental ALD. Therefore, intestinal dysbiosis and pathological bacterial translocation appear fundamental for the pathogenesis of ALD. This review highlights causes for intestinal dysbiosis and pathological bacterial translocation, their relationship, and consequences for ALD. We also discuss how the liver affects the intestinal microbiota.     

Intestinal Dysbiosis: A Possible Mechanism of Alcohol-Induced Endotoxemia and Alcoholic Steatohepatitis in Rats

Background:  Clinical and animal data indicate that gut-derived endotoxin and other luminal bacterial products are necessary cofactors for development of alcoholic liver disease (ALD). Although gut leakiness is clearly an important cause of endotoxemia in ALD, it cannot fully explain endotoxemia in all ALD subjects and thus other factors may be involved. One possible factor is a change in gut microbiota composition (dysbiosis). Thus, the aim of our study was to interrogate the gut bacterial microbiota in alcohol-fed rats to see if chronic alcohol consumption affects gut bacteria composition.
Method:  Male Sprague–Dawley rats were given either alcohol or dextrose intragastrically by gavage twice daily for up to 10 weeks. A subgroup of rats was also given either a probiotic (lactobacillus GG) or a prebiotic (oats) by gavage. Ileal and colonic mucosal-attached microbiota composition were interrogated by Length Heterogeneity PCR (LH-PCR) fingerprinting.
Results:  Bacterial microbiota composition in alcohol-fed rats is not different from dextrose-fed rats at weeks 4 and 6. Mucosa-associated microbiota composition in the colon is altered at 10 weeks of daily alcohol gavage. Both LGG and oats prevented alcohol-induced dysbiosis up to 10 weeks of alcohol treatment.
Conclusion:  Daily alcohol consumption for 10 weeks alters colonic mucosa-associated bacterial microbiota composition in rats. Our data showed, for the first time, that daily alcohol consumption can affect colonic microbiome composition and suggest that dysbiosis may be an important mechanism of alcohol-induced endotoxemia. Further studies are needed to determine how dysbiotic microbiota contributes to development of ALD and whether therapeutic interventions targeted towards dysbiotic microbiota can prevent complications of alcoholism like ALD.     

Intestinal microbiota in liver disease

The intestinal microbiota have emerged as a topic of intense interest in gastroenterology and hepatology. The liver is on the front line as the first filter of nutrients, toxins and bacterial metabolites from the intestines and we are becoming increasingly aware of interactions among the gut, liver and immune system as important mediators of liver health and disease. Manipulating the microbiota with therapeutic intent is a rapidly expanding field. In this review, we will describe what is known about the contribution of intestinal microbiota to liver homeostasis; the role of dysbiosis in the pathogenesis of liver disease including alcoholic and non-alcoholic fatty liver disease, cirrhosis and hepatocellular carcinoma; and the therapeutic manifestations of altering intestinal microbiota via antibiotics, prebiotics, probiotics and fecal microbiota transplantation.     

Role of gut microbiota and Toll-like receptors in nonalcoholic fatty liver disease

Emerging data have shown a close association between compositional changes in gut microbiota and the development of nonalcoholic fatty liver disease (NAFLD). The change in gut microbiota may alter nutritional absorption and storage. In addition, gut microbiota are a source of Toll-like receptor (TLR) ligands, and their compositional change can also increase the amount of TLR ligands delivered to the liver. TLR ligands can stimulate liver cells to produce proinflammatory cytokines. Therefore, the gut-liver axis has attracted much interest, particularly regarding the pathogenesis of NAFLD. The abundance of the major gut microbiota, including Firmicutes and Bacteroidetes, has been considered a potential underlying mechanism of obesity and NAFLD, but the role of these microbiota in NAFLD remains unknown. Several reports have demonstrated that certain gut microbiota are associated with the development of obesity and NAFLD. For instance, a decrease in Akkermansia muciniphila causes a thinner intestinal mucus layer and promotes gut permeability, which allows the leakage of bacterial components. Interventions to increase Akkermansia muciniphila improve the metabolic parameters in obesity and NAFLD. In children, the levels of Escherichia were significantly increased in nonalcoholic steatohepatitis (NASH) compared with those in obese control. Escherichia can produce ethanol, which promotes gut permeability. Thus, normalization of gut microbiota using probiotics or prebiotics is a promising treatment option for NAFLD. In addition, TLR signaling in the liver is activated, and its downstream molecules, such as proinflammatory cytokines, are increased in NAFLD. To data, TLR2, TLR4, TLR5, and TLR9 have been shown to be associated with the pathogenesis of NAFLD. Therefore, gut microbiota and TLRs are targets for NAFLD treatment.     

Gut–liver axis: The impact of gut microbiota on non alcoholic fatty liver disease

AimTo examine the impact of gut microbiota on non alcoholic fatty liver disease (NAFLD) pathogenesis.Data synthesisEmerging evidence suggests a strong interaction between gut microbiota and liver. Receiving approximately 70% of its blood supply from the intestine, the liver represents the first line of defence against gut-derived antigens. Intestinal bacteria play a key role in the maintenance of gut–liver axis health. Disturbances in the homeostasis between bacteria- and host-derived signals at the epithelial level lead to a break in intestinal barrier function and may foster “bacterial translocation”, defined as the migration of bacteria or bacterial products from the intestinal lumen to mesenteric lymph nodes or other extraintestinal organs and sites. While the full repertoire of gut-derived microbial products that reach the liver in health and disease has yet to be explored, the levels of bacterial lipopolysaccharide, a component of the outer membrane of Gram-negative bacteria, are increased in the portal and/or systemic circulation in several types of chronic liver diseases. Derangement of the gut flora, particularly small intestinal bacterial overgrowth, occurs in a large percentage (20–75%) of patients with chronic liver disease. In addition, evidence implicating the gut–liver axis in the pathogenesis of metabolic liver disorders has accumulated over the past ten years.ConclusionsComplex metabolic diseases are the product of multiple perturbations under the influence of triggering factors such as gut microbiota and diet, thus, modulation of the gut microbiota may represent a new way to treat or prevent NAFLD.     

Gut microbiome in primary sclerosing cholangitis: A review

Primary sclerosing cholangitis(PSC) is a chronic cholestatic liver disease characterized by biliary inflammation and stricturing. Exploration of the pathogenesis of PSC in light of its association with inflammatory bowel disease(IBD) and the "gut-liver" axis is an emerging area of interest. A growing number of studies have begun to elucidate the role of the gut microbiota, its metabolites and its influence on host immune responses in the development of PSC and PSCIBD. Studies of the fecal microbiota have highlighted enriched levels of certain species, including Veillonella, Streptococcus and Enterococcus, among others. A heightened immune response to enteric dysbiosis and bacterial translocation have also been implicated. For example, Klebsiella pneumoniae strains derived from gnotobiotic mice transplanted with PSC-IBD microbiota were found to induce pore formation in human intestinal epithelial cells and enhanced Th17 responses. Gut microbes have additionally been hypothesized to be implicated in PSC pathogenesis through their role in the synthesis of various metabolites,including bile acids(BAs), which function as signaling molecules with important gut and hepatic effects. An expanded knowledge of the gut microbiome as it relates to PSC offers critical insight into the development of microbe-altering therapeutic interventions, such as antibiotics, nutritional interventions and fecal microbial transplantation. Some of these have already shown some preliminary evidence of benefit. Despite exciting progress in the field, much work remains to be done; areas that are particularly lacking include functional characterization of the microbiome and examination of pediatric populations. In this review, we summarize studies that have investigated the microbiome in PSC and PSC-IBD as well as putative mechanisms, including the potential role of metabolites, such as BAs. We then briefly review the evidence for interventions with microbe-altering properties for treating PSC.     

Immunity and inflammatory signaling in alcoholic liver disease

The pathogenesis of alcoholic liver disease (ALD) is multifactorial and characterized by steatosis, steatohepatitis and cirrhosis. Several signaling pathways in different liver cell types that contribute to the development and progression of alcoholic liver injury have been identified. Among these, immune cells and signaling pathways are the most prominent and central to ALD. Both innate and adaptive immune responses contribute to ALD. The key features of inflammatory pathways in ALD including liver innate and adaptive immune cell types, signaling receptors/pathways, and pro- and antiinflammatory/protective responses are summarized here.     

Bacterial translocation and changes in the intestinal microbiome associated with alcoholic liver disease

Alcoholic liver disease progresses through several stages of tissue damage, from simple steatosis to alcoholic hepatitis, fibrosis, or cirrhosis. Alcohol also affects the intestine, increases intestinal permeability and changes the bacterial microflora. Liver disease severity correlates with levels of systemic bacterial products in patients, and experimental alcoholic liver disease is dependent on gut derived bacterial products in mice. Supporting evidence for the importance of bacterial translocation comes from animal studies demonstrating that intestinal decontamination is associated with decreased liver fibrogenesis. In addition, mice with a gene mutation or deletion encoding receptors for either bacterial products or signaling molecules downstream from these receptors, are resistant to alcohol-induced liver disease. Despite this strong association, the exact molecular mechanism of bacterial translocation and of how changes in the intestinal microbiome contribute to liver disease progression remains largely unknown. In this review we will summarize evidence for bacterial translocation and enteric microbial changes in response to alcoholic liver injury and chronic alcoholic liver disease. We will further describe consequences of intestinal dysbiosis on host biology. We finally discuss how therapeutic interventions may modify the gastrointestinal microflora and prevent or reduce alcoholic liver disease progression.     

Gut-liver axis and fibrosis in nonalcoholic fatty liver disease: An input for novel therapies

Non-alcoholic fatty liver disease is a multifactorial condition, ranging from simple steatosis to non-alcoholic steatohepatitis with or without fibrosis. In non-alcoholic fatty liver disease, alteration of gut microbiota and increased intestinal permeability increase exposure of the liver to gut-derived bacterial products: lipopolysaccharides and unmethylated CpG DNA. These products stimulate innate immune receptors, namely Toll-like receptors, which activate signalling pathways involved in liver inflammation and fibrogenesis. Currently, there are several studies on the involvement of lipopolysaccharide-activated Toll-like receptor 4 signalling in non-alcoholic fatty liver disease pathogenesis. There has been widespread interest in the study of the involvement of resident hepatic stellate cells and Kupffer cells activation in liver fibrogenesis upon TLR4 stimulation. Although the best evidence to support a role for gut microbiota in non-alcoholic fatty liver disease-induced fibrosis comes largely from animal models, data from human studies are accumulating and could lead to new therapeutic approaches. Therapeutic modulation of gut microflora may be an alternative strategy to develop an anti-fibrotic therapy.In this review, we discuss the relevant role of gut-liver axis in non-alcoholic liver disease-associated liver fibrosis and discuss the evidence on novel anti-fibrotic therapeutic approaches.     

Starring role of toll-like receptor-4 activation in the gut-liver axis

Since the introduction of the term “gut-liver axis”, many studies have focused on the functional links of intestinal microbiota, barrier function and immune responses to liver physiology. Intestinal and extra-intestinal diseases alter microbiota composition and lead to dysbiosis, which aggravates impaired intestinal barrier function via increased lipopolysaccharide translocation. The subsequent increased passage of gut-derived product from the intestinal lumen to the organ wall and bloodstream affects gut motility and liver biology. The activation of the toll-like receptor 4 (TLR-4) likely plays a key role in both cases. This review analyzed the most recent literature on the gut-liver axis, with a particular focus on the role of TLR-4 activation. Findings that linked liver disease with dysbiosis are evaluated, and links between dysbiosis and alterations of intestinal permeability and motility are discussed. We also examine the mechanisms of translocated gut bacteria and/or the bacterial product activation of liver inflammation and fibrogenesis via activity on different hepatic cell types.     

Immunologic mechanisms of alcoholic liver injury

Clinically, the association of alcoholic liver disease (ALD) with circulating autoantibodies, hypergammaglobulinemia, antibodies to unique hepatic proteins, and cytotoxic lymphocytes reacting against autologous hepatocytes strongly suggests altered immune regulation with an increased activity toward normal self-proteins (loss of tolerance). Experimentally, there are several immune responses generated specifically recognizing self-proteins that are modified by metabolites of alcohol. These data strongly suggest that immune reactions may play a significant role in inducing and sustaining an inflammatory cascade of tissue damage to the liver. Additional support for this comes from the observation that the histological appearance of livers with ALD is that of a chronic active hepatitis-like inflammatory disease. Therefore, the hypothesis that immune mechanisms are involved in recurrent alcoholic hepatitis, although not summarily proven, is reasonable, supported by clinical and experimental evidence, and the subject of this article.     

Gut microbiota mediated molecular events and therapy in liver diseases

Gut microbiota is a community of microorganisms that reside in the gastrointestinal tract. An increasing number of studies has demonstrated that the gut-liver axis plays a critical role in liver homeostasis. Dysbiosis of gut microbiota can cause liver diseases, including nonalcoholic fatty liver disease and alcoholic liver disease. Preclinical and clinical investigations have substantiated that the metabolites and other molecules derived from gut microbiota and diet interaction function as mediators to cause liver fibrosis, cirrhosis, and final cancer. This effect has been demonstrated to be associated with dysregulation of intrahepatic immunity and liver metabolism. Targeting these findings have led to the development of novel preventive and therapeutic strategies. Here, we review the cellular and molecular mechanisms underlying gut microbiota-mediated impact on liver disease. We also summarize the advancement of gut microbiota-based therapeutic strategies in the control of liver diseases.     

Alcoholic liver disease: focus on prodromal gut health

Alcoholic liver disease (ALD) is implicated in gut disturbances, both functionally and structurally. It has been noticed that the gut–liver interaction is an important feature in the prevention of systemic inflammation as well as liver health. The optimal functioning of the gut–liver axis depends on gut health. Therefore, gut problems may be important for estimating liver inflammation, while our knowledge of ALD could also provide an insight into gut health. Gut problems accompanied by ALD include gut motility and absorption problems, mucosal damage and the dysbiosis of gut microbiota and gastrointestinal carcinogenesis. Moreover, there is emerging evidence that besides direct inflammatory injury caused by alcohol, gut problems related to ALD play a crucial role in the pathogenesis of cardiovascular and immunological disorders. In this regard, we should consider ALD in relation to both gut health and chronic systemic low‐grade inflammation. Accordingly, integrative therapeutic strategies are warranted for treating and preventing ALD and systemic inflammation as well as alcohol‐related gut problems.     

Gut-liver axis and probiotics: Their role in non-alcoholic fatty liver disease

The incidence of obesity and its related conditions, including non-alcoholic fatty liver disease (NAFLD), has dramatically increased in all age groups worldwide. Given the health consequences of these conditions, and the subsequent economic burden on healthcare systems, their prevention and treatment have become major priorities. Because standard dietary and lifestyle changes and pathogenically-oriented therapies (e.g., antioxidants, oral hypoglycemic agents, and lipid-lowering agents) often fail due to poor compliance and/or lack of efficacy, novel approaches directed toward other pathomechanisms are needed. Here we present several lines of evidence indicating that, by increasing energy extraction in some dysbiosis conditions or small intestinal bacterial overgrowth, specific gut microbiota and/or a “low bacterial richness” may play a role in obesity, metabolic syndrome, and fatty liver. Under conditions involving a damaged intestinal barrier (“leaky gut”), the gut-liver axis may enhance the natural interactions between intestinal bacteria/bacterial products and hepatic receptors (e.g., toll-like receptors), thus promoting the following cascade of events: oxidative stress, insulin-resistance, hepatic inflammation, and fibrosis. We also discuss the possible modulation of gut microbiota by probiotics, as attempted in NAFLD animal model studies and in several pilot pediatric and adult human studies. Globally, this approach appears to be a promising and innovative add-on therapeutic tool for NAFLD in the context of multi-target therapy.     

Gut microbiota in the pathogenesis of inflammatory bowel disease

Inflammatory bowel disease (IBD), including ulcerative colitis and Crohn’s disease, is a chronic and relapsing inflammatory disorder of the intestine. Although its incidence is increasing globally, the precise etiology remains unclear and a cure for IBD has yet to be discovered. The most accepted hypothesis of IBD pathogenesis is that complex interactions between genetics, environmental factors, and the host immune system lead to aberrant immune responses and chronic intestinal inflammation. The human gut harbors a complex and abundant aggregation of microbes, collectively referred to as the gut microbiota. The gut microbiota has physiological functions associated with nutrition, the immune system, and defense of the host. Recent advances in next-generation sequencing technology have identified alteration of the composition and function of the gut microbiota, which is referred to as dysbiosis, in IBD. Clinical and experimental data suggest dysbiosis may play a pivotal role in the pathogenesis of IBD. This review is focused on the physiological function of the gut microbiota and the association between the gut microbiota and pathogenesis in IBD. In addition, we review the therapeutic options for manipulating the altered gut microbiota, such as probiotics and fecal microbiota transplantation.     

Gut microbiota and liver diseases

Several studies revealed that gut microbiota are associated with various human diseases,e.g.,metabolic diseases,allergies,gastroenterological diseases,and liver diseases.The liver can be greatly affected by changes in gut microbiota due to the entry of gut bacteria or their metabolites into the liver through the portal vein,and the liver-gut axis is important to understand the pathophysiology of several liver diseases,especially non-alcoholic fatty liver disease and hepatic encephalopathy.Moreover,gut microbiota play a significant role in the development of alcoholic liver disease and hepatocarcinogenesis.Based on theseprevious findings,trials using probiotics have been performed for the prevention or treatment of liver diseases.In this review,we summarize the current understanding of the changes in gut microbiota associated with various liver diseases,and we describe the therapeutic trials of probiotics for those diseases.     

A humanized microbiota mouse model of ovalbumin-induced lung inflammation

There is increasing evidence for a role of early life gut microbiota in later development of asthma in children. In our recent study, children with reduced abundance of the bacterial genera Lachnospira, Veillonella, Faecalibacterium, and Rothia had an increased risk of development of asthma and addition of these bacteria in a humanized mouse model reduced airway inflammation. In this Addendum, we provide additional data on the use of a humanized gut microbiota mouse model to study the development of asthma in children, highlighting the differences in immune development between germ-free mice colonized with human microbes compared to those colonized with mouse gut microbiota. We also demonstrate that there is no association between the composition of the gut microbiota in older children and the diagnosis of asthma, further suggesting the importance of the gut microbiota-immune system axis in the first 3 months of life.     

Gut microbiota in alcoholic liver disease: Pathogenetic role and therapeutic perspectives

Alcoholic liver disease(ALD) is the commonest cause of cirrhosis in many Western countries and it has a high rate of morbidity and mortality. The pathogenesis is characterized by complex interactions between metabolic intermediates of alcohol. Bacterial intestinal flora is itself responsible for production of endogenous ethanol through the fermentation of carbohydrates. The intestinal metabolism of alcohol produces a high concentration of toxic acetaldehyde that modifies gut permeability and microbiota equilibrium. Furthermore it causes direct hepatocyte damage. In patients who consume alcohol over a long period, there is a modification of gut microbiota and, in particular, an increment of Gram negative bacteria. This causes endotoxemia and hyperactivation of the immune system. Endotoxin is a constituent of Gram negative bacteria cell walls. Two types of receptors, cluster of differentiation 14 and Toll-like receptors-4, present on Kupffer cells, recognize endotoxins. Several studies have demonstrated the importance of gut-liver axis and new treatments have been studied in recent years to reduce progression of ALD modifying gut microbiota. It has focused attention on antibiotics, prebiotics, probiotics and synbiotics.     

Gut microbiota and host metabolism in liver cirrhosis

The gut microbiota has the capacity to produce a diverse range of compounds that play a major role in regulatingthe activity of distal organs and the liver is strategically positioned downstream of the gut. Gut microbiota linked compounds such as short chain fatty acids, bile acids, choline metabolites, indole derivatives, vitamins, polyamines, lipids, neurotransmitters and neuroactive compounds, and hypothalamic-pituitary-adrenal axis hormones have many biological functions. This review focuses on the gut microbiota and host metabolism in liver cirrhosis. Dysbiosis in liver cirrhosis causes serious complications, such as bacteremia and hepatic encephalopathy, accompanied by small intestinal bacterial overgrowth and increased intestinal permeability. Gut dysbiosis in cirrhosis and intervention with probiotics and synbiotics in a clinical setting is reviewed and evaluated. Recent studies have revealed the relationship between gut microbiota and host metabolism in chronic metabolic liver disease, especially, non-alcoholic fatty liver disease, alcoholic liver disease, and with the gut microbiota metabolic interactions in dysbiosis related metabolic diseases such as diabetes and obesity. Recently, our understanding of the relationship between the gut and liver and how this regulates systemic metabolic changes in liver cirrhosis has increased. The serum lipid levels of phospholipids, free fatty acids, polyunsaturated fatty acids, especially, eicosapentaenoic acid, arachidonic acid, and docosahexaenoic acid have significant correlations with specific fecal flora in liver cirrhosis. Many clinical and experimental reports support the relationship between fatty acid metabolism and gut-microbiota. Various blood metabolome such as cytokines, amino acids, and vitamins are correlated with gut microbiota in probioticstreated liver cirrhosis patients. The future evaluation of the gut-microbiota-liver metabolic network and the intervention of these relationships using probiotics, synbiotics, and prebiotics, with sufficient nutrition could aid the development of treatments and prevention for liver cirrhosis patients.     

Cytolysin‐positive Enterococcus faecalis is not increased in patients with non‐alcoholic steatohepatitis

Several studies show associations between gut bacterial dysbiosis and chronic liver diseases, but causative mechanisms are largely unclear. We recently identified cytolysin, a bacterial exotoxin expressed and secreted by Enterococcus faecalis to cause liver damage in the setting of alcohol‐related liver disease. Cytolysin was increased and highly correlated with liver disease severity and mortality in alcoholic hepatitis patients. In this study, we investigated if faecal cytolysin‐positivity can be linked to non‐alcoholic fatty liver disease, a highly prevalent disease where new biomarkers and treatment targets are urgently needed. In contrast to what we observed in alcoholic hepatitis, only seven out of 96 non‐alcoholic fatty liver disease patients were cytolysin‐positive, and these patients did not have increased liver disease activity compared with cytolysin‐negative patients. These results indicate that the association of cytolysin carriage with worse clinical outcome might be specific for alcoholic hepatitis.