Microbiota and Alcoholic Liver Disease

Molecular and cellular mechanisms of liver fibrosis and its regression

Chronic liver injury leads to liver inflammation and fibrosis, through which activated myofibroblasts in the liver secrete extracellular matrix proteins that generate the fibrous scar. The primary source of these myofibroblasts are the resident hepatic stellate cells. Clinical and experimental liver fibrosis regresses when the causative agent is removed, which is associated with the elimination of these activated myofibroblasts and resorption of the fibrous scar. Understanding the mechanisms of liver fibrosis regression could identify new therapeutic targets to treat liver fibrosis. This Review summarizes studies of the molecular mechanisms underlying the reversibility of liver fibrosis, including apoptosis and the inactivation of hepatic stellate cells, the crosstalk between the liver and the systems that orchestrate the recruitment of bone marrow-derived macrophages (and other inflammatory cells) driving fibrosis resolution, and the interactions between various cell types that lead to the intracellular signalling that induces fibrosis or its regression. We also discuss strategies to target hepatic myofibroblasts (for example, via apoptosis or inactivation) and the myeloid cells that degrade the matrix (for example, via their recruitment to fibrotic liver) to facilitate fibrosis resolution and liver regeneration.

Intestinal Epithelial Cell-Derived Extracellular Vesicles Modulate Hepatic Injury via the Gut-Liver Axis During Acute Alcohol Injury

Binge drinking, i.e., heavy episodic drinking in a short time, has recently become an alarming societal problem with negative health impact. However, the harmful effects of acute alcohol injury in the gut-liver axis remain elusive. Hence, we focused on the physiological and pathological changes and the underlying mechanisms of experimental binge drinking in the context of the gut-liver axis. Eight-week-old mice with a C57BL/6 background received a single dose (p.o.) of ethanol (EtOH) [6 g/kg b.w.] as a preclinical model of acute alcohol injury. Controls received a single dose of PBS. Mice were sacrificed 8 h later. In parallel, HepaRGs and Caco-2 cells, human cell lines of differentiated hepatocytes and intestinal epithelial cells intestinal epithelial cells (IECs), respectively, were challenged in the presence or absence of EtOH [0-100 mM]. Extracellular vesicles (EVs) isolated by ultracentrifugation from culture media of IECs were added to hepatocyte cell cultures. Increased intestinal permeability, loss of zonula occludens-1 (ZO-1) and MUCIN-2 expression, and alterations in microbiota-increased Lactobacillus and decreased Lachnospiraceae species-were found in the large intestine of mice exposed to EtOH. Increased TUNEL-positive cells, infiltration of CD11b-positive immune cells, pro-inflammatory cytokines (e.g., tlr4, tnf, il1β), and markers of lipid accumulation (Oil Red O, srbep1) were evident in livers of mice exposed to EtOH, particularly in females. In vitro experiments indicated that EVs released by IECs in response to ethanol exerted a deleterious effect on hepatocyte viability and lipid accumulation. Overall, our data identified a novel mechanism responsible for driving hepatic injury in the gut-liver axis, opening novel avenues for therapy.

Potential role of gut microbiota, the proto-oncogene PIKE (Agap2) and cytochrome P450 CYP2W1 in promotion of liver cancer by alcoholic and nonalcoholic fatty liver disease and protection by dietary soy protein

We have previously demonstrated promotion of diethylnitrosamine (DEN) initiated liver tumorigenesis after feeding diets high in fat or ethanol (EtOH) to male mice. This was accompanied by hepatic induction of the proto-oncogene PIKE (Agap2). Switch of dietary protein from casein to soy protein isolate (SPI) significantly reduced tumor formation in these models. We have linked EtOH consumption in mice to microbial dysbiosis. Adoptive transfer studies demonstrate that microbiota from mice fed ethanol can induce hepatic steatosis in the absence of ethanol suggesting that microbiota or the microbial metabolome play key roles in development of fatty liver disease. Feeding SPI significantly changed gut bacteria in mice increasing alpha diversity (P < 0.05) and levels of Clostidiales spp. Feeding soy formula to piglets also resulted in significant changes in microbiota, the pattern of bile acid metabolites and in inhibition of the intestinal-hepatic FXR/FGF19-SHP pathway which has been linked to both steatosis and hepatocyte proliferation. Moreover, feeding SPI also resulted in induction of hepatic PPARα signaling and inhibition of PIKE mRNA expression coincident with inhibition of steatosis and cancer prevention. Feeding studies in the DEN model with differing dietary fats demonstrated tumor promotion specific to the saturated fat, cocoa butter relative to diets containing olive oil or corn oil associated with microbial dysbiosis including dramatic increases in Lachnospiraceae particularly from the genus Coprococcus. Immunohistochemical analysis demonstrated that tumors from EtOH-fed mice and patients with alcohol-associated HCC also expressed high levels of a novel cytochrome P450 enzyme CYP2W1. Additional adoptive transfer experiments and studies in knockout mice are required to determine the exact relationship between soy effects on the microbiota, expression of PIKE, CYP2W1, PPARα activation and prevention of tumorigenesis.

Blockade of IL-17 signaling reverses alcohol-induced liver injury and excessive alcohol drinking in mice

Chronic alcohol abuse has a detrimental effect on the brain and liver. There is no effective treatment for these patients, and the mechanism underlying alcohol addiction and consequent alcohol-induced damage of the liver/brain axis remains unresolved. We compared experimental models of alcoholic liver disease (ALD) and alcohol dependence in mice and demonstrated that genetic ablation of IL-17 receptor A (IL-17ra-/-) or pharmacological blockade of IL-17 signaling effectively suppressed the increased voluntary alcohol drinking in alcohol-dependent mice and blocked alcohol-induced hepatocellular and neurological damage. The level of circulating IL-17A positively correlated with the alcohol use in excessive drinkers and was further increased in patients with ALD as compared with healthy individuals. Our data suggest that IL-17A is a common mediator of excessive alcohol consumption and alcohol-induced liver/brain injury, and targeting IL-17A may provide a novel strategy for treatment of alcohol-induced pathology.

Specifically Sized Hyaluronan (35 kDa) Prevents Ethanol-Induced Disruption of Epithelial Tight Junctions Through a layilin-Dependent Mechanism in Caco-2 Cells

BACKGROUND

Specific-sized species of the carbohydrate hyaluronan elicit a variety of cellular responses mediating tissue integrity and repair, as well as regulating inflammatory responses. Orally provided hyaluronan with an average molecular weight of 35 kDa (HA35) protects mice from short-term ethanol (EtOH)-induced liver injury. This protection was associated with maintenance of the colocalization of zonula occludens-1 (ZO-1) and occludin at tight junctions in the proximal colon. However, it is not known whether HA35 also protects other regions of the intestine or whether protection is due to a direct and/or indirect interaction of HA35 with the intestinal epithelium.

METHODS

Female C57BL/6J mice were fed an EtOH containing diet or pair-fed control diet (4 days) and treated with or without HA35 via daily gavage during the last 3 days of EtOH feeding. Intestinal morphology and tight junction integrity were assessed. Differentiated Caco-2 cells were transfected or not with scrambled siRNA or siRNA targeting layilin, a hyaluronan receptor. Caco-2 cells were treated with or without HA35 prior to challenge with EtOH. Localization of tight junction proteins, fluorescein isothiocyanate (FITC)-dextran permeability, and transepithelial electrical resistance (TEER) were evaluated.

RESULTS

While short-term EtOH did not result in any apparent changes in the gross morphology of the intestine, colocalization of ZO-1 and occludin at tight junctions was decreased in the proximal and distal colon. HA35 prevented these effects of EtOH. In differentiated Caco-2 cells, EtOH decreased the localization of ZO-1 and occludin at tight junctions and increased permeability of FITC-dextran. At higher concentrations, EtOH also decreased TEER. Pretreatment with HA35 prevented these changes. When the hyaluronan receptor layilin was knocked down in Caco-2 cells, HA35 no longer protected cells from EtOH-induced loss of tight junctions.

CONCLUSIONS

Taken together, these data indicate that HA35 interacts with layilin on intestinal epithelial cells and maintains intestinal tight junction integrity during short-term EtOH exposure.

Intestinal dysbiosis and permeability: the yin and yang in alcohol dependence and alcoholic liver disease

Alcohol dependence and alcoholic liver disease represent a major public health problem with substantial morbidity and mortality. By yet incompletely understood mechanisms, chronic alcohol abuse is associated with increased intestinal permeability and alterations of the gut microbiota composition, allowing bacterial components, bacteria, and metabolites to reach the portal and the systemic circulation. These gut-derived bacterial products are recognized by immune cells circulating in the blood or residing in remote organs such as the liver leading to the release of pro-inflammatory cytokines which are considered important mediators of the liver-gut-brain communication. Although circulating cytokines are likely not the sole factors involved, they can induce liver inflammation/damage and reach the central nervous system where they favor neuroinflammation which is associated with change in mood, cognition, and drinking behavior. In this review, the authors focus on the current evidence describing the changes that occur in the intestinal microbiota with chronic alcohol consumption in conjunction with intestinal barrier breakdown and inflammatory changes sustaining the concept of a gut-liver-brain axis in the pathophysiology of alcohol dependence and alcoholic liver disease.

Hepatic Immune System: Adaptations to Alcohol

Both the innate and adaptive immune systems are critical for the maintenance of healthy liver function. Immune activity maintains the tolerogenic capacity of the liver, modulates hepatocellular response to various stresses, and orchestrates appropriate cellular repair and turnover. However, in response to heavy, chronic alcohol exposure, the finely tuned balance of pro- and anti-inflammatory functions in the liver is disrupted, leading to a state of chronic inflammation in the liver. Over time, this non-resolving inflammatory response contributes to the progression of alcoholic liver disease (ALD). Here we review the contributions of the cellular components of the immune system to the progression of ALD, as well as the pathophysiological roles for soluble and circulating mediators of immunity, including cytokines, chemokines, complement, and extracellular vesicles, in ALD. Finally, we compare the role of the innate immune response in health and disease in the liver to our growing understanding of the role of neuroimmunity in the development and maintenance of a healthy central nervous system, as well as the progression of neuroinflammation.