Weight loss studies further showed that a decrease in firmicutes and an increase of bacteroides were correlated with percent body weight loss, but not caloric intake. Even though most studies showed a shift toward firmicutes in obese and diabetic individuals, others have shown opposite results: a shift toward bacteroidetes or no difference between these two phyla. Nevertheless, several bacterial species and families have been associated with biological markers of obesity and diabetes. E. coli and Lactobacillus, for example, are linked to leptin variation independent of weight changes while the Coriobacteriaceae family is correlated with changes in hepatic TG, glucose, and glycogen levels. Because various signaling pathways may be altered in the diseased state, crosstalk between the gut microbiome and host may also be disrupted. The most studied human gut bacteria responsible for 7α-dehydroxylation of BAs belong to the genus Clostridium, which are grampositive anaerobic members of the Firmicutes family. They are capable of regulating BA synthesis in the liver by removing FXR-antagonist tauro-beta-muricholic acid in the ileum and converting primary BAs into secondary BAs. Rats fed with CA displayed increased BA levels in the colon which selected against bacteroidetes and actinobacteria in favor of firmicutes, leading to greater generation of DCA and LCA, which can be cytotoxic at higher concentrations than the physiological range of 0.046–0.210 and 0–0.03 mM, respectively. Conversely, square pot cirrhotic patients were found to have a collapse of Clostridium XIVa family because of a smaller BA pool. This direct relationship between the size of the BA pool and colonization by the Clostridium genus supports an intimate interaction between the intestinal microbiome and BA metabolism.

The harmful effects of BA dysregulation and gut dysbiosis in the gut–liver axis are summarized in Figure 2.Rou-En-Y-Gastric Bypass is an increasingly popular bariatric surgical procedure for the treatment of obese and diabetic patients because of its resulting weight loss, improved insulin resistance and glucose intolerance, as well as reduction in other comorbidities. A systematic review and meta-analysis conducted for reviews from January 1990 to April 2006 showed that a striking 86.6% of type II diabetes mellitus patients who underwent gastric bypass surgery experienced resolution or dramatically alleviated symptoms of improved glycemic index and insulin levels. The mechanisms underlying these beneficial outcomes remain subjects of intense investigation. Leptin circulation, pH changes, GLP-1 secretion, as well as BA synthesis and gut microbiome shifts are all postulated as potential mechanisms. Due to the nature of the procedure, gastric bypass patients were thought to have poorer reabsorption rate, thus increasing BA synthesis and improving their glucose tolerance. Several studies reported a positive correlation between the increased postprandial GLP-1 and insulin secretion and the significant increase in serum BA in obese individuals compared to lean controls. In an attempt to elucidate the effect of RYGB on BA synthesis, a similar technique was used in rats to study the molecular changes during intestinal adaptation. A decrease in mRNA expression of Cyp7a1 and Cyp27a1 and major BA exporters such as bile salt export pump and organic anion-transporting polypeptide was observed in rats subjected to bile diversion surgery. Ileal FXR and Fgf15 were also reduced at the mRNA level, but no changes were observed for hepatic FXR, which is consistent with previous results from ileal interposition.

Causing increased serum BA levels and reduced in ER stress markers, bile diversion improved glucose tolerance and liver steatosis in diet-induced obese rats. RYGB on obese patients, similarly, had restored blunted BA mobilization, suggesting a possible weight loss mechanism post surgery. Several studies reported that intestinal microbes quickly adapt to the starvation-like, less acidic GI environment. The transfer of the gut microbial community from RYGB mice to germ-free mice resulted in similar weight loss and adiposity observed in RYGB patients. Microbiota analysis via next generation sequencing revealed that a notable increase in the relative abundance of Escherichia and Faecalibacterium prausnitzii across the fecal content of mice, rats, and humans after receiving RYGB independent of calorie restriction. F. prausnitzii abundance was also associated with reduced low-grade inflammation and reduction in colitis. Furthermore, plasma leptin levels negatively correlated with E. coli levels and positively correlated with Lactobacillus levels. A marked decrease in blood bacterial DNA was also noted suggesting that RYGB restricted bacterial translocation in the intestines. Meanwhile, the role of Bifidobacteria in regulating GI health remains unclear. After RYGB, Bifidobacterium count increases and negatively correlated with adiponectin, a marker for insulin sensitivity and inflammation in the liver. Additional investigation at the genetic and molecular levels is needed to elucidate specific microbial and metabolomic mechanisms from which the beneficial results of RYGB are derived. Nevertheless, the aforementioned findings further associate BA homeostasis and the gut microbiome in the pathogenesis of obesity and diabetes.

A growing body of evidence has linked various inFammatory signaling pathways including TLR/MyD88, NFκB, and COX2 as bridging factors between pathogen-triggered inflammation and carcinogenesis. More recently, BA dysregulation and gut dysbiosis were also implicated in modulating the inflammatory process. Serum BA levels have increasingly served as biomarkers for liver diseases, obesity, and diabetes. BAs not only regulate hepatic de novo lipogenesis, TG export, and plasma turnover, but also hepatic gluconeogenesis and insulin sensitivity through the actions of FXR and TGR5. For instance, ursodeoxycholic acid , a weak FXR and TGR5 binding ligand, improved hepatic ER stress and insulin sensitivity in diabetic mice, while norUDCA, a short chained homologue of UDCA, lowered hepatic TG levels. BA-activated FXR inhibited bacterial overgrowth and mucosal injury in the distal small intestine through promotion of innate defense mechanisms to prevent epithelial barrier deterioration and bacterial translocation, thus reducing the risk for HCC and CRC. Conversely, BA dysregulation causes increased bacterial translocation by disrupting barrier function in the small intestine leading to systemic infection. High concentration of DCA and CDCA can damage tissue by solubilizing the cell membrane and acting as immuno suppressive agents. Indeed, BAs, especially DCA over 500 μM, can increase chloride secretion and intestinal permeability, inhibit mucosal healing, and eventually elicit intestinal inflammation in the colon often seen in irritable bowel disease patients. Similar dose-dependent findings were also found in mice feeding studies. The physiological DCA concentration under a high-fat diet can impair gut barrier function whereas DCA concentration under a low-fat diet did not disrupt gut permeability. Secondary BA UDCA and DCA exhibited opposing roles, although higher than physiological concentrations of UDCA were necessary to produce significant protective effects against DCA-induced gut permeability, especially in the colon. A high-fat diet was reported to decrease the ratio of UDCA and DCA, even though the overall fecal BA pool is larger. The enterohepatic circulation of toxic BAs such as DCA increased the senescence-associated secretory phenotype in mouse hepatic stellate cells, induced greater levels of inflammatory and tumor-promoting factors, and promoted non-alcoholic steatohepatitis and HCC development. Furthermore, square plastic planter the integrity of the intestinal mucosal barrier and immunological activities are governed by a network of nuclear receptors including FXR. Increases in intestinal FXR and TNFα expression were observed in mice fed a high fat diet. Elevated BA not only upregulated numerous pro-inflammatory mediators and NFκβ, but also shifted intestinal microbe composition toward endotoxin-producing species. Endotoxic microbes are capable of aggravating glucose tolerance and insulin resistance to elicit increased metabolic inflammation and gut permeability. Low grade inflammation and harmful metabolites generated by endotoxic microbes in the gut permitted microbial translocation into the enterohepatic circulation. Bacterial translocation through compromised tight junctions in the small intestine was found to precede intestinal bacterial overgrowth in acute liver injury caused by cholestasis, alcohol toxicity, and obesity in mice. Given that plasma endotoxin levels correlate with the severity of liver disease and that KO mice lacking toll-like receptor 2 and TNFα signaling pathways are resistant to liver injury and fibrosis, bacterial translocation appears to play a role in the progression of chronic liver disease. Although bacterial LPS may not be involved in the onset of gut barrier dysfunction, it translocates through the intestinal mucosa in the presence of a leaky gut. Consistent with murine findings, elevated bacterial and antigen uptake in the intestines of obese and diabetic individuals resulted from impaired tight junctions.

It is uncertain whether microbial profile shift is the cause or consequence of increased intestinal permeability, but it is clear that the intestinal microbiome significantly impacts intestinal and colonic health. Individuals suffering from gut dysbiosis, characterized by a reduction in beneficial, anti-inflammatory Bifidobacterium and Lactobacillus and elevation in aerobic, pro-inflammatory Enterobacter and Clostridium species, are at higher risk for liver diseases. Hydrophobic, taurine-conjugated BAs enhanced the growth of sulfatereducing gut bacteria, antigen and bacterial translocation resulting in ulcerative colitis and CRC. One study positively correlated fecal levels of CDCA with members of the Enterobacteriaceae family while another found a positive linkage between Enterobacteriaceae, endotoxemia, and hepatic inflammation. On the other hand, F. prausnitzii has been consistently associated with anti-inflammatory properties and a marked decrease in F. prausnitzii is seen in IBD patients compared to healthy individuals. Intestinal microbes not only play a role in gut permeability regulation and innate immunity, but also influence the expression of genes involved in regulating inflammation and energy homeostasis. A significant increase in gramnegative proteobacteria and the disappearance of Bifidobacteria were observed in the gut of mice under a high-fat diet. Inflammasome, a multiprotein oligomer that activates pro-inflammatory cytokines, has recently been proposed to mediate liver injury progression. In the gut, inflammasome protects against gram-negative bacterial infection whereas in the liver, it aggravates hepatic steatosis and inflammation through activation of TLR4 and TLR9, driving tumorigenesis. LPS can induce inflammasome through activation of caspase-11 and specific toll-like receptors. Interestingly, mice infused with LPS for 4 weeks displayed a similar phenotype to those under a high-fat diet exhibiting increased weight gain, insulin resistance, hepatic TG content, and adipose tissue inflammation. Furthermore, circulating LPS correlated positively with insulin levels, adipose macrophage infiltration, and fasting blood glucose in diabetic patients. Impaired tight junctions as previously discussed and chylomicron-facilitated transport are two mechanisms proposed for how LPS promotes inflammation in the liver and intestines. FXR KO mice with dysregulated BAs and increased inflammation also exhibit impaired glucose tolerance and decreased insulin sensitivity in a manner similar to diabetic mice and patients with type 2 diabetes mellitus. The role of FXR in the regulating glucose homeostasis was demonstrated from multiple murine studies and a Phase 2 clinical study for type 2 diabetes mellitus and NAFLD in which FXR stimulation improves glucose tolerance, hepatic steatosis, and insulin sensitivity. The lack of FXR up-regulation by elevated BA levels in mice fed a high-fat diet may suggest a disrupted feedback system in BA synthesis. Intriguingly, the intestinal microbiota may be a potential contributor to the development of NAFLD by modulating lipid and glucose metabolism through FXR and TGR5 signaling. NAFLD is characterized by an accumulation of TG in hepatocytes, which increase inflammation and progresses into fibrosis, cirrhosis, and ultimately HCC. The condition is associated with low-grade chronic inflammation, a common feature in obese individuals with high visceral adiposity. Hepatocyte apoptosis resulting from increased fat in the liver and insulin resistance are common pathological signs of NAFLD. Elevated serum free fatty acid can impair insulin signaling, increase hepatic glucose production while hepatic fatty acid overload induces inflammatory cytokines, fibrogenic cytokines, and oxidative stress. Hepatic BA accumulation due to hepatic steatosis can up-regulate NFκb signaling along with major inflammatory markers IL-6, IL-8, and TNFα in the progression of liver carcinogenesis. Similar to the liver and the intestines, adipose tissue in obese and diabetic patients also exhibit increased expression of proinflammatory cytokines. Hypertrophic adipocytes and macrophage infiltration exacerbate this chronic inflammatory microenvironment and enhance the survival and proliferation of neoplastic liver and intestinal cells. As previously mentioned, obese and diabetic individuals suffer from abnormally elevated BA levels, particularly hydrophobic secondary BA such as DCA and LCA. A growing body of evidence exists to support that hydrophobic secondary BA can increase intracellular production of reactive oxygen and nitrogen species resulting in elevated oxidative stress and DNA damage. Insights into the mechanism of BA induced-genotoxicity were obtained when treatment of human Barrett’s esophagus tissues with a combination of BAs led to significant elevation in oxidative stress biomarker 8-hydroxy-deoxyguanosine and reactive oxygen species . Moreover, esophageal cells treated with DCA showed increased generation of reactive nitrogen species and DNA damage which were inhibited by cotreatment with nitric oxide scavenger or pretreatment with nitric oxide synthase inhibitor.