Gut--liver axis: the impact of gut microbiota on non alcoholic fatty liver disease

Meta-omic platforms to assist in the understanding of NAFLD gut microbiota alterations: tools and applications

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease worldwide as a result of the increasing prevalence of obesity, starting from early life stages. It is characterized by a spectrum of liver diseases ranging from simple fatty liver (NAFL) to steatohepatitis (NASH), with a possible progression to fibrosis, thus increasing liver-related morbidity and mortality. NAFLD development is driven by the co-action of several risk factors, including obesity and metabolic syndrome, which may be both genetically induced and diet-related. Recently, particular attention has been paid to the gut-liver axis, which may play a physio-pathological role in the onset and progression of the disease. The gut microbiota is intended to act as a bioreactor that can guarantee autonomous metabolic and immunological functions and that can drive functional strategies within the environment of the body in response to external stimuli. The complexity of the gut microbiota suggests that it behaves as an organ. Therefore, the concept of the gut-liver axis must be complemented with the gut-microbiota-liver network due to the high intricacy of the microbiota components and metabolic activities; these activities form the active diet-driven power plant of the host. Such complexity can only be revealed using systems biology, which can integrate clinical phenomics and gut microbiota data.

Hepatic molecular responses to Bifidobacterium pseudocatenulatum CECT 7765 in a mouse model of diet-induced obesity

BACKGROUND AND AIMS

Bifidobacterium pseudocatenulatum CECT 7765 moderates body weight gain and metabolic parameters in high-fat diet-(HFD)-fed mice but, the mechanisms of action are not yet understood. To further understand the effects of this bacterial strain, we have investigated the molecular changes in the liver of mice fed a HFD and supplemented with the bacteria.

METHODS AND RESULTS

Gene expression and protein levels were measured in the liver of C57BL/6 male mice following sub-chronic consumption of a HFD and B. pseudocatenulatum CECT 7765. Our results show that the consumption of this bacterial strain modulated the expression of key genes involved in the regulation of energy metabolism and transport of lipids that were affected by the HFD.B. pseudocatenulatum CECT 7765 significantly counteracted the effects caused by the HFD on the fatty acid transporter CD36, the transcription regulator of lipid biosynthesis EGR1 and the regulators of glucose metabolism, IGFBP2 and PPP1R3B, both at the mRNA and protein levels. The bacterial strain slightly induced the transcript levels of PNPLA2, a lipase that hydrolyses triglycerides in lipid droplets. In the standard diet (SD)-fed mice, the administration of B. pseudocatenulatum CECT 7765 donwregulated the expression of INSIG1 and HMGCR critically involved in the regulation of cholesterol levels.

CONCLUSION

B. pseudocatenulatum CECT 7765 modified the expression of key regulators of fatty acid and cholesterol metabolism and transport, lipid levels and glucose levels in the liver which supports the beneficial metabolic effects of this bacterial strain.

Rational identification of diet-derived postbiotics for improving intestinal microbiota function

The intestinal microbiota plays an important role in a wide range of functions and whole body homeostasis. Recent advances have linked microbiota dysbiosis to conditions ranging from Crohn's disease to cancer. The restoration or strengthening of the intestinal microbiota through diet-based approaches such as probiotics and prebiotics has been proposed for combating the onset or progression of these diseases. In this review, we highlight the importance of postbiotics for the manipulation of the intestinal microbiota, with special emphasis on systems biology computational tools and targeted metabolomics for the rational discovery and identification of these bioactive molecules. The identification of novel postbiotics and the pathways responsible for their production should lead to improved mechanistic understanding of the role that specific probiotics, prebiotics, and postbiotics have in restoring intestinal microbiota composition and function.

What does irritable bowel syndrome share with non-alcoholic fatty liver disease?

Non-alcoholic fatty liver disease (NAFLD) and irritable bowel syndrome (IBS) are two very common diseases in the general population. To date, there are no studies that highlight a direct link between NAFLD and IBS, but some recent reports have found an interesting correlation between obesity and IBS. A systematic PubMed database search was conducted highlighting that common mechanisms are involved in many of the local and systemic manifestations of NAFLD, leading to an increased cardiovascular risk, and IBS, leading to microbial dysbiosis, impaired intestinal barrier and altered intestinal motility. It is not known when considering local and systemic inflammation/immune system activation, which one has greater importance in NAFLD and IBS pathogenesis. Also, the nervous system is implicated. In fact, inflammation participates in the development of mood disorders, such as anxiety and depression, characteristics of obesity and consequently of NAFLD and, on the other hand, in intestinal hypersensitivity and dysmotility.

Gastrointestinal bacterial overgrowth: pathogenesis and clinical significance

Small intestinal bacterial overgrowth (SIBO) is defined as the presence of an abnormally high number of coliform bacteria in the small bowel. It is associated with a broad range of predisposing small intestinal motility disorders and with surgical procedures that result in bowel stasis. The most common symptoms associated with SIBO include diarrhea, flatulence, abdominal pain and bloating. Quantitative culture of small bowel contents and a variety of indirect tests have been used over the years in an attempt to facilitate the diagnosis of SIBO. The indirect tests include breath tests and biochemical tests based on bacterial metabolism of a variety of substrates. Unfortunately, there is no single valid test for SIBO, and the accuracy of all current tests remains limited due to the failure of culture to be a gold standard and the lack of standardization of the normal bowel flora in the small intestine. Currently, the ideal approach to treat SIBO is to treat the underlying disease, eradicate overgrowth, and address nutritional deficiencies that may be associated with the development of SIBO.

Colonic bacterial metabolites and human health

The influence of the microbial-mammalian metabolic axis is becoming increasingly important for human health. Bacterial fermentation of carbohydrates (CHOs) and proteins produces short-chain fatty acids (SCFA) and a range of other metabolites including those from aromatic amino acid (AAA) fermentation. SCFA influence host health as energy sources and via multiple signalling mechanisms. Bacterial transformation of fibre-related phytochemicals is associated with a reduced incidence of several chronic diseases. The 'gut-liver axis' is an emerging area of study. Microbial deconjugation of xenobiotics and release of aromatic moieties into the colon can have a wide range of physiological consequences. In addition, the role of the gut microbiota in choline deficiency in non-alcoholic fatty liver disease (NAFLD) and insulin resistance is receiving increased attention.

Gut-liver axis: role of inflammasomes

Inflammasomes are large multiprotein complexes that have the ability to sense intracellular danger signals through special NOD-like receptors or NLRs. They include NLRP3, NLRC4, AIM2 and NLRP6. They are involved in recognizing diverse microbial (bacteria, viruses, fungi and parasites), stress and damage signals, which result in direct activation of caspase-1, leading to secretion of potent pro-inflammatory cytokines and pyroptosis. NLRP3 is the most studied antimicrobial immune response inflammasome. Recent studies reveal expression of inflammasomes in innate immune response cells including monocytes, macrophages, neutrophils, and dendritic cells. Inflammasome deficiency has been linked to alterations in the gastrointestinal microflora. Alterations in the microbiome population and/or changes in gut permeability promote microbial translocation into the portal circulation and thus directly to the liver. Gut derived lipopolysaccharides (LPS) play a significant role in several liver diseases. Recent advancements in the sequencing technologies along with improved methods in metagenomics and bioinformatics have provided effective tools for investigating the 10(14) microorganisms of the human microbiome that inhabit the human gut. In this review, we examine the significance of inflammasomes in relation to the gut microflora and liver. This review also highlights the emerging functions of human microbiota in health and liver diseases.

Human serum proteome analysis: new source of markers in metabolic disorders

The prevalence of metabolic disorders (MDs), especially diabetes, is rapidly increasing worldwide, leading to an increasing risk of cardiovascular and other socially relevant complications. To boost MD biomarker discovery, advanced proteomics can harmonize metabolomics. Indeed, the rapid development of mass spectrometry (MS) has designated proteomics as an emerging platform to interrogate the plasma/serum proteome for the discovery of next-generation biomarkers exploitable for risk assessment, early detection and prognosis of MDs. Preanalytical plasma/serum treatment, such as combinatorial peptide ligand libraries with nano-liquid chromatography coupled with tandem MS or selected reaction monitoring coupled to triple-quadrupole time-of-flight instruments, are proven clinical laboratory techniques for quantitative analyses. New strategies, such as SWATH™ MS, which allows us to systematically characterize and quantify query sample sets of 'any protein of interest' in complex biological samples, may dramatically improve next-generation MD biomarkers, especially considering the plethora of candidates coming from the 'bioreactor' gut microbiota affecting MD onset and progression.

Fatty liver accompanies an increase in lactobacillus species in the hind gut of C57BL/6 mice fed a high-fat diet

High-fat (HF) diets can produce obesity and have been linked to the development of nonalcoholic fatty liver disease and changes in the gut microbiome. To test the hypothesis that HF feeding increases certain predominant hind gut bacteria and development of steatohepatitis, C57BL/6 mice were fed an HF (45% energy) or low-fat (LF) (10% energy) diet for 10 wk. At the end of the feeding period, body weights in the HF group were 34% greater than those in the LF group (P < 0.05). These changes were associated with dramatic increases in lipid droplet number and size, inflammatory cell infiltration, and inducible nitric oxide (NO) synthase protein concentration in the livers of mice fed the HF diet. Consistent with the fatty liver phenotype, plasma leptin and tumor necrosis factor-α concentrations were also elevated in mice fed the HF diet, indicative of chronic inflammation. Eight of 12 pairs of polymerase chain reaction (PCR) primers for bacterial species that typically predominate hind gut microbial ecology generated specific PCR products from the fecal DNA samples. The amount of DNA from Lactobacillus gasseri and/or Lactobacillus taiwanensis in the HF group was 6900-fold greater than that in the LF group. Many of these bacteria are bile acid resistant and are capable of bile acid deconjugation. Because bile acids are regulators of hepatic lipid metabolism, the marked increase of gut L. gasseri and/or L. taiwanensis species bacteria with HF feeding may play a role in development of steatohepatitis in this model.

Therapeutic approaches to non-alcoholic fatty liver disease: past achievements and future challenges

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is a leading cause of chronic liver injury and mortality in Western countries and China. However, as to date, there is no direct and effective therapy for this disease. The aim of this review is to analyze the key progress and challenges of main current therapeutic approaches in NAFLD.

DATA SOURCE

We carried out a PubMed search of English-language articles relevant to NAFLD therapy.

RESULTS

There are two major therapeutic strategies for NAFLD treatment: (1) lifestyle interventions (including weight reduction, dietary modification and physical exercise) and (2) pharmaceutical therapies. Lifestyle interventions, particularly chronic and moderate intensity exercise, are the most effective and recognized clinical therapies for NAFLD. For pharmaceutical therapies, although their effects and mechanisms have been extensively investigated in laboratory studies, they still need further tests and investigations in clinical human trials.

CONCLUSION

Future advancement of NAFLD therapy should focus on the mechanistic studies on cell based and animal models and human clinical trials of exercise, as well as the combination of lifestyle intervention and pharmaceutical therapy specifically targeting main signaling pathways related to lipid metabolism, oxidative stress and inflammation.

Nonalcoholic fatty liver disease: for better or worse, blame the gut microbiota?

Nonalcoholic fatty liver disease (NAFLD) is a major clinical consequence for people with obesity and metabolic syndrome and is also associated with enteral and parenteral nutrition. Early studies suggested that altered gut microbiota might contribute to obesity by affecting energy harvest from the diet and energy storage in the host. Recent evidence in humans as well as in animal models has linked gut microbiota to the development of NAFLD through the gut-liver axis. With bacterial overgrowth and increased intestinal permeability observed in patients with NAFLD and in animal models, gut-derived bacterial products such as endotoxin (lipopolysaccharide) and bacterial DNA are being delivered to the liver through the portal vein and then activate Toll-like receptors (TLRs), mainly TLR4 and TLR9, and their downstream cytokines and chemokines, leading to the development and progression of NAFLD. Given the limited data in humans, the role of gut microbiota in the pathogenesis of NAFLD is still open to discussion. Prebiotics and probiotics have been attempted to modify the microbiota as preventive or therapeutic strategies on this pathological condition. Their beneficial effects on NALFD have been demonstrated in animal models and limited human studies. However, prospective, appropriately powered, randomized, controlled clinical trials are needed to determine whether prebiotics and probiotics and other integrated strategies to modify intestinal microbiota are efficacious therapeutic modalities to treat NALFD.

Towards polypharmacokinetics: pharmacokinetics of multicomponent drugs and herbal medicines using a metabolomics approach

Determination of pharmacokinetics (PKs) of multicomponent pharmaceuticals and/or nutraceuticals (polypharmacokinetics, poly-PKs) is difficult due to the vast number of compounds present in natural products, their various concentrations across a wide range, complexity of their interactions, as well as their complex degradation dynamics in vivo. Metabolomics coupled with multivariate statistical tools that focus on the comprehensive analysis of small molecules in biofluids is a viable approach to address the challenges of poly-PK. This paper discusses recent advances in the characterization of poly-PK and the metabolism of multicomponent xenobiotic agents, such as compound drugs, dietary supplements, and herbal medicines, using metabolomics strategy. We propose a research framework that integrates the dynamic concentration profile of bioavailable xenobiotic molecules that result from in vivo absorption and hepatic and gut bacterial metabolism, as well as the human metabolic response profile. This framework will address the bottleneck problem in the pharmacological evaluation of multicomponent pharmaceuticals and nutraceuticals, leading to the direct elucidation of the pharmacological and molecular mechanisms of these compounds.

Probiotic as a novel treatment strategy against liver disease

CONTEXT

A symbiotic relationship between the liver and intestinal tract enables the healthy status of both organs. Microflora resident in intestinal lumen plays a significant role in hepatocytes function. Alterations to the type and amount of microorganisms that live in the intestinal tract can result in serious and harmful liver dysfunctions such as cirrhosis, nonalcoholic fatty liver disease, alcoholic liver disease, and hepatic encephalopathy. An increased number of pathogens, especially enterobacteriaceae, enterococci, and streptococci species causes the elevation of intestinal permeability and bacterial translocation. The presence of high levels of lipopolysaccharide (LPS) and bacterial substances in the blood result in a portal hypertension and ensuing hepatocytes damage. Several methods including the usage of antibiotics, prebiotics, and probiotics can be used to prevent the overgrowth of pathogens. Compared to prebiotic and antibiotic therapy, probiotics strains are a safer and less expensive therapy. Probiotics are "live microorganisms (according to the FAO/WHO) which when administered in adequate amounts confer a health benefit on the host".

EVIDENCE ACQUISITIONS

Data from numerous preclinical and clinical trials allows for control of the flora bacteria quantity, decreases in compounds derived from bacteria, and lowers proinflammatory production such as TNF-α, IL-6 and IFN-γ via down-regulation of the nuclear factor kappa B (NF-κ B).

RESULTS

On the other hand, probiotic can reduce the urease activity of bacterial microflora. Furthermore, probiotic decreases fecal pH value and reduces ammonia adsorption. In addition, the serum level of liver enzymes and other substances synthesized by the liver are modulated subsequent to probiotic consumption.

CONCLUSIONS

According to our knowledge, Probiotic therapy as a safe, inexpensive and a noninvasive strategy can reduce pathophysiological symptoms and improve different types of liver diseases without side effects.

Probiotics and liver disease

Intestinal microbiota play an important role in health and disease. The gut-liver axis provides for an interaction between bacterial components like lipopolysaccharide and hepatic receptors (Toll-like receptors). Dysbiosis and altered intestinal permeability may modulate this interaction and therefore result in hepatic disorders or worsening of hepatic disorders. Administration of health-promoting microbial strains may help ameliorate these harmful interactions and hepatic disorders. This review focuses on changes in gut microbiota in the context of liver disease and possible roles of probiotics, prebiotics, and synbiotics in liver disease.

Nutrition of the critically ill - emphasis on liver and pancreas

About 25 million individuals undergo high risk surgery each year. Of these about 3 million will never return home from hospital, and the quality of life for many of those who return is often significantly impaired. Furthermore, many of those who manage to leave hospital have undergone severe life-threatening complications, mostly infections/sepsis. The development is strongly associated with the level of systemic inflammation in the body, which again is entirely a result of malfunctioning GI microbiota, a condition called dysbiosis, with deranged composition and function of the gastrointestinal microbiota from the mouth to the anus and impaired ability to maintain intact mucosal membrane functions and prevent leakage of toxins-bacterial endotoxins and whole or debris of bacteria, but also foods containing proteotoxins gluten, casein and zein and heat-induced molecules such as advanced glycation end products (AGEs) and advanced lipoxidation end products (ALEs). Markedly lower total anaerobic bacterial counts, particularly of the beneficial Bifidobacterium and Lactobacillus and higher counts of total facultative anaerobes such as Staphylococcus and Pseudomonas are often observed when analyzing the colonic microbiota. In addition Gram-negative facultative anaerobes are commonly identified microbial organisms in mesenteric lymph nodes and at serosal "scrapings" at laparotomy in patients suffering what is called "Systemic inflammation response system" (SIRS). Clearly the outcome is influenced by preexisting conditions in those undergoing surgery, but not to the extent as one could expect. Several studies have for example been unable to find significant influence of pre-existing obesity. The outcome seems much more to be related to the life-style of the individual and her/his "maintenance" of the microbiota e.g., size and diversity of microbiota, normal microbiota, eubiosis, being highly preventive. About 75% of the food Westerners consume does not benefit microbiota in the lower gut. Most of it, refined carbohydrates, is already absorbed in the upper part of the GI tract, and of what reaches the large intestine is of limited value containing less minerals, less vitamins and other nutrients important for maintenance of the microbiota. The consequence is that the microbiota of modern man has a much reduced size and diversity in comparison to what our Palelithic forefathers had, and individuals living a rural life have today. It is the artificial treatment provided by modern care, unfortunately often the only alternative, which belongs to the main contributor to poor outcome, among them; artificial ventilation, artificial nutrition, hygienic measures, use of skin penetrating devices, tubes and catheters, frequent use of pharmaceuticals, all known to significantly impair the total microbiome of the body and dramatically contribute to poor outcome. Attempts to reconstitute a normal microbiome have often failed as they have always been undertaken as a complement to and not an alternative to existing treatment schemes, especially treatments with antibiotics. Modern nutrition formulas are clearly too artificial as they are based on mixture of a variety of chemicals, which alone or together induce inflammation. Alternative formulas, based on regular food ingredients, especially rich in raw fresh greens, vegetables and fruits and with them healthy bacteria are suggested to be developed and tried.