Review Article

Emerging Role of the Gut Microbiome in the Progression of Nonalcoholic Fatty Liver Disease and Potential Therapeutic Implications

Saumya Jayakumar; Rohit Loomba

Disclosures

Aliment Pharmacol Ther. 2019;50(2):144-158. 

In This Article

Gut Microbiome and NAFLD

The major NAFLD risk factors (ie, diet, obesity and insulin resistance) are closely connected with the gut microbiome (Figure 1).[14,62,87–96] It is reasonable to speculate that the gut microbiota and the pathophysiology of NASH are closely intertwined. One study found that the gut microbiota play a large role in the development of NAFLD, by transplanting the gut microbiota from mice with diet-induced NAFLD into germ-free mice; NAFLD developed in the initially germ-free mice.[18] In humans, characterisation of the faecal microbiomes of 86 patients with biopsy-proven NAFLD (n = 72, stages 0-2 fibrosis; n = 14, stages 3-4 [advanced] fibrosis) revealed that patients with NAFLD and advanced fibrosis had increased levels of Proteobacteria, whereas patients with mild fibrosis had increased levels of Firmicutes.[19] In addition, 37 of 40 features that were predictors of advanced fibrosis in patients with NAFLD were related to the gut microbiota.

Figure 1.

Role of the gut microbiome in NAFLD progression.14, 62, 87–96 DAMPS, damage-associated molecular patterns; LPL, lipoprotein lipase; LPS, lipopolysaccharides; NAFLD, nonalcoholic fatty liver disease; NASH, nonalcoholic steatohepatitis; PAMPs, pathogen-associated molecular patterns; TG, triglyceride; TLR, toll-like receptors; TMAO, trimethylamine-N-oxide

Patients with NAFLD have increased intestinal permeability related to disrupted tight junctions.[97] As mentioned previously, increased GI "leakiness" may allow bacterial translocation and entry of bacteria-derived products into the portal circulation.[98,99] Once in the liver, these factors may initiate proinflammatory cascades (eg, production of interleukin-6 [IL-6] and TNF-α) via interaction with the TLR present on a variety of cell types (ie, Kupffer cells, stellate cells and hepatocytes).[100] In fact, more than half of patients with NAFLD may have small-intestinal bacterial overgrowth,[97] and the presence of this comorbid condition parallels cirrhosis severity (ie, Child-Turcotte-Pugh class).[101,102]

Inflammation is a key factor in the development of NASH, and LPS produced by GI bacteria trigger proinflammatory cytokine cascades that involve TLR-4 and nuclear factor kappa B (NFκB).[92] LPS are also, as mentioned earlier, a key factor in the activation of Kupffer cells. Kupffer cells may release inflammatory cytokines in response to leptin, a hormone associated with adipocytes, thereby indirectly activating hepatic stellate cells and potentially perpetuating liver fibrosis.[103] In a mouse model, obesity-induced leptin increased liver responsiveness to LPS and enhanced progression of NASH.[104] Results from a meta-analysis of patients with NAFLD or NASH showed that circulating levels of leptin were higher in patients with NAFLD compared with healthy controls (standardised mean difference 0.64; 95% CI 0.42-0.86) and in patients with NASH compared with patients with simple steatosis (standardised mean difference 0.21; 95% CI, 0.02-0.40).[105]

Non-inflammatory bacterial products also have been implicated in the development of hepatic steatosis. Monosaccharides produced by microbial fermentation of carbohydrates in the GI tract may activate carbohydrate-responsive element-binding protein (ChREBP) and sterol-response element-binding protein 1 (SREBP1) pathways, which regulate lipid accumulation. In an obese mouse model, deficiency of ChREBP reduced hepatic fat levels, suggesting that inhibiting ChREBP could be beneficial in patients with hepatic steatosis.[106] Using stool samples and 16S ribosomal RNA gene pyrosequencing, Zhu et al[88] examined the gut bacteria of three groups of paediatric patients—healthy, obese without NASH and those with biopsy-proven NASH. The study found an association between health status and gut microbiome composition (at the phylum, family and genus levels). Both the obese and NASH groups demonstrated increased abundance of Bacteroidetes (specifically species of the genus Prevotella) and decreased abundance of Firmicutes compared with the healthy group. In addition, levels of species in the Proteobacteria phylum increased with progression from the healthy to obese to NASH groups, while the abundance of species in the Actinobacteria phylum (specifically those of the genus Bifidobacterium) decreased with worsening health status. The gut microbiome composition of obese patients and patients with NASH was similar, except for increased levels of Proteobacteria (specifically those of the Enterobacteriaceae family and the genus Escherichia; P < 0.05 for all three levels of classification) in the NASH group compared with the obese group. Increased abundance of ethanol-producing bacteria in the NASH microbiome prompted the investigators to measure serum alcohol levels in the three groups. While little to no difference was noted in ethanol levels between the healthy and obese groups, ethanol levels were significantly increased in the NASH vs the obese groups (P < 0.01). Combined with the demonstration of increased abundance of Escherichia in patients with NASH, the authors theorised a pathophysiologic mechanism linking the altered microbiome in NASH and the development of hepatic inflammation.

Although other studies have demonstrated increased blood ethanol levels associated with NASH and obesity,[94,107] the study conducted by Zhu et al was the first to demonstrate that patients with NASH have higher blood ethanol levels than obese patients, and related this finding to alcohol-producing bacteria in the gut microbiome of NASH patients.[88] The results from this study suggest that patients with NASH may be differentiated from healthy and obese patients by assessing the gut microbiome using stool samples. The authors postulated that the constant presence of bacteria-derived ethanol in the patients with NASH supplied a source of reactive oxygen species that could, in turn, increase inflammation and fibrosis.[88] Figure 2 summarises the effects of various factors involved in the development of fibrosis and cirrhosis in patients with NAFL.[19,40,41,88,108,109]

Figure 2.

Association between gut microbiome and NAFLD. Patients with NAFL can progress to fibrosis and cirrhosis through different mechanisms, including toxic bile acids,40, 41, 124 increased gut permeability,41 increased endogenous ethanol88 and gut microbiome dysbiosis19, 88 (with higher levels of Escherichia coli and Prevotella). However, patients can have an improvement in hepatic inflammation and fibrosis with lifestyle modifications that include exercise and diet (which improves gut microbiome dysbiosis) and weight loss, as both of these conditions decrease FGF-19.108, 109 FGF-19, fibroblast growth factor-19; NAFL, nonalcoholic fatty liver; NAFLD, nonalcoholic fatty liver disease

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