Meta-analysis of cohort studies has identified increased risk in obese individuals =1.89, 95% confidence interval : 1.51–2.36 for developing liver cancer compared to normal weight individual while meta-analysis of prospective studies correlated a larger waist circumference and waist–hip ratio with increased risk for colon cancer. Similarly, diabetics are more than twice as likely to be diagnosed with cancer of the liver, pancreas, endometrium, and to a lesser extent, colon, breast, and bladder.5 Many possible underlying mechanisms including hyperinsulinemia, hyperglycemia, and inflammation have been proposed to explain the increased cancer incidence. Insulin, produced by b-cells in the pancreas, is released to promote cellular absorption of blood glucose and many factors including excess weight and increased plasma triglyceride levels can raise circulating levels of insulin. Chronic insulin elevation results in resistance, which then increases the biological activity of insulin-like growth factor , an endocrine and paracrine hormone regulating tissue growth and metabolism. Epidemiological studies have linked IGF-1 to several cancer types, including hepatocellular carcinoma and colorectal cancer . Moreover, excess adiposity leads to the derangement of other peptide hormones such as resistin, leptin, adiponectin, pot with drainge holes and tumor necrosis factor α , contributing to the metabolic abnormalities commonly observed in obese and diabetic individuals.
Indeed, increased leptin and reduced adiponectin have been identified as risk factors for the progression of liver steatosis, fibrosis, and tumorigenesis as well as CRC formation. Both bile acids and the intestinal microbiota have been extensively studied in the context of various health conditions, particularly obesity and type II diabetes-associated HCC and CRC. Although the exact mechanism of how gut microbes and BAs affect one another remains unclear, it is evident that the introduction of intestinal microbes increased liver cholesterol and altered BA profiles in germ-free mice. Conversely, dietary BA supplementation can modulate gut microbial profile in animal models.This review will focus on the current understanding of the complex interplay between BAhomeostasis and gut microbial profiles in regards to obesity and diabetes-associated liver and colon carcinogenesis. They are synthesized by cholesterol catabolism in the liver through both the classical and the acidic pathway , which differ in the modification order of the sterol ring and side chain oxidation. BAs synthesized in the liver are subsequently conjugated for storage in the gallbladder.
Upon ingestion of fat and protein, cholecystokinin, a peptide hormone in the small intestine will stimulate the release of bile containing digestive enzymes and primary BAs, cholic acid and chenodeoxycholic acid , from the gall bladder. These BAs will then activate farnesoid x receptor in the liver which induces the expression of small heterodimer partner to inhibit the activity of liver receptor homolog-1 responsible for upregulating the rate-limiting BA synthesis enzyme CYP7A. Intestinal FXR activity on the other hand, induces the expression of fibroblast growth factor 19 which binds hepatic fibroblast growth factor receptor 4 and activates c-Jun N-terminal kinase 1/2 and extracellular signal-regulated kinase 1/2 to inhibit BA synthesis. BAs are actively reabsorbed from the ileum by the ileal BA transporters and circulated back to the liver through the hepatic portal vein. This highly efficient process ensures that a majority of synthesized BAs are recycled with only 1-2% being converted into secondary BAs, deoxycholic acid , and lithocholic acid , by bacterial 7α-dehydroxylation in the terminal ileum and colon and excreted in feces. The regulation of BA circulation between the liver and intestines is summarized in Figure 1. In addition to modulating BA synthesis, FXR also regulates the expression of several transporters including apical sodium dependent BA transporter , fatty acidbinding protein subclass 6, and organic solute transporter α and β to control the absorption of not only BAs but also lipids, vitamins, and xenobiotics. Interestingly, recent studies have implicated FXR in the interplay between obesity-associated BA dysregulation and gut dysbiosis to potentially promote carcinogenesis in the liver and colon. Within the context of obesity and diabetes, FXR can regulate glucose and lipid homeostasis through actions at various sites along the gut-liver axis.
Agonist activation or hepatic overexpression of FXR significantly lowered blood glucose levels in both diabetic and wild type mice. FXR stimulation also decreased blood low density lipoprotein levels and inhibited fatty acid and TG synthesis in mice fed a high sugar and fat diet. These combined effects of FXR protected mice against body weight gain, liver and muscle fat deposition, and reversed insulin resistance. At the metabolic level, FXR functions to repress hepatic gluconeogenesis, lipogenesis, and fatty acid synthesis genes. Consequently, FXR stimulation promotes glycogen synthesis and enhances insulin sensitivity in obese mice. FXR activity in pancreatic β-cell lines and human islets can regulate transcription factor Kruppellike factor 11 to increase insulin gene expression and protein kinase B-dependent phosphorylation and translocation of glucose transporter 2 at the plasma membrane of hepatocytes. By stimulating pancreatic insulin secretion and hepatic glucose uptake, FXR can effectively delay the pathological progression of insulin resistance, hyperglycemia, and glucosuria in diabetic mice. Consistently, FXR knockout mice exhibited glucose intolerance, insulin insensitivity, elevated serum TG, cholesterol, and BA levels resulting in greater hepatic fat accumulation compared to wild type mice. Hepatic FXR expression is conversely regulated by glucose levels in streptozotocininduced diabetic rats. Chromatin immunoprecipitation in mice also revealed that long-term high-glucose exposure increased histone acetylation and demethylation on the FXR-target Cyp7a1 gene promoter region leading to elevated basal expression and consequently, a larger BA pool with altered composition. These observations strongly support the existence of crosstalk between the cellular mechanisms regulating glucose, lipid, and BA homeostasis in the liver and intestines with FXR serving mediator. A link between FXR and enterohepatic cancer was firmly established when FXR KO mice were found to have markedly elevated hepatic inflammatory and oxidative stress markers compared to WT mice and a striking 100% incidence rate of spontaneous liver tumors between 13 and 15 months of age. BA-containing diet further exacerbated inflammation and oxidative stress in FXR KO mouse liver supporting that BA dysregulation subjects hepatocytes to higher oxidative stress. Hepatocyte-specific over expression of SHP failed to alter liver tumor incidence or size in FXR KO mice, large pot with drainage but did result in lower neoplasia grade, decreased cell proliferation, and increased apoptosis. Moreover, FXR stimulation can down-regulate lipopolysaccharide -induced, nuclear factor kappa-light-chainenhancer of activated B cells -mediated hepatic inflammation by suppressing the expression of proinflammatory mediators in human HCC cells and mouse primary hepatocytes. FXR KO mice displayed higher hepatic mRNA levels of inducible nitric oxide synthase, prostaglandin-endoperoxide synthase 2 , chemokine ligand 10, and interferon type II which resulted in exaggerated inflammation and necrosis after LPS exposure at a dose that failed to elicit measurable liver injury or inflammation in WT mice. The HCC in FXR KO mice was associated with sustained oncogenic Wnt/B-catenin signaling through Wnt4 and disheveled induction, E-cadherin repression, and glycogen synthase kinase-3B inactivation as the mice aged. Furthermore, microarray analysis of FXR KO mouse liver revealed altered gene expression profiles related to metabolism, inflammation, and fibrosis compared to WT liver recapitulating human HCC progression. Liver tumor bearing FXR KO mice showed elevated levels of interleukin 6 and signal transducer and activator of transcription 3 due to diminished expression of suppressor of cytokine signaling 3, a direct FXR target gene. STAT3 activation in conjunction with elevated TNFα and IL-6 levels has been shown to potentiate HCC formation.
Additionally, FXR can epigenetically silence the promoter of gankyrin, a proteasome subunit responsible for the degradation of retinoblastoma, p53, hepatic nuclear factor 4 alpha, and CCAAT /enhancer-binding tumor suppressor proteins. The loss of FXR in mice increased gankyrin expression to promote tumorigenesis. Interestingly, long-lived little mice with high basal FXR expression do not develop liver cancer with age or carcinogen administration due to insufficient gankyrin induction. FXR activation in human hepatocytes and hepatoma cells protected against cytotoxicity induced by cisplatin and other DNA-damaging agents. These findings support that in addition to its metabolic regulation, FXR also functions to modulate oxidative stress, inflammation, and cell proliferation to inhibit cancer development. Evidence also exists to suggest that FXR may act as a modulator of intestinal inflammation and a link between BA homeostasis and the intestinal microbiome. In the small intestine, FXR negatively regulates the expression of transporters involved in BA reabsorption while inducing the production and secretion of FGF19/Fgf15 to inhibit hepatic BA synthesis. Colon inflammation in Crohn’s disease patients and rodent colitis models is correlated with reduced FXR mRNA levels. The progression of colon inflammation is exacerbated in FXR KO mice while treatment with FXR agonist attenuated colonic tissue damage and immune cell activation. Conversely, FXR stimulation protected WT mice from chemical-induced colitis by reducing epithelial permeability, ulceration, and inflammatory cell infiltration. Moreover, FXR agonist-treated WT mice and differentiated enterocyte-like cells displayed lower pro-inflammatory cytokines and better preserved epithelial barrier function. In addition to its beneficial effects on intestinal function and inflammation, a connection between FXR and intestinal microbes was observed when ampicillin-treated mice had inhibited ileal expression of FXR, SHP, and FGF19/Fgf15. Expression of FXR and its target genes levels were rescued by combination treatment with CA, but not taurocholic acid, in ampicillin-treated mice suggesting that enterobacteria can enhance BA-mediated FXR activity via taurocholic acid deconjugation. Furthermore, intestinal inflammation in mice down-regulated FXR expression in a toll-like receptor 9 -dependent manner since the FXR promoter contains a response element to interferon regulatory factor 7, a TLR9- regulated factor. These preliminary findings suggest a possible role of intestinal FXR as a mediator between BA homeostasis, the gut microbiome, and host immunity to prevent excessive inflammation and maintain GI health. Examination of FXR in human HCC samples and cell lines has yielded further evidence to support its protective role against cancer formation. Marked reduction in FXR levels and activity were observed in human HCC samples compared to normal liver tissue. This reduction resulted from inhibition of hepatic nuclear factor 1 alpha activity on the FXR genepromoter by elevated pro-inflammatory mediators. The 3′ untranslated region of FXR mRNA was found to be a target of miR-421 and FXR downregulation by miR-421 promoted proliferation, migration, and invasion in human HCC cells. Decreased FXR levels in HCC cells also correlated with overexpression of active Ras resulting in strong activation of ERK1/2, a common characteristic of malignant cells. However, additional studies are required to determine whether dysregulated FXR activity increases the risk of HCC and CRC. One possible underlying cause of FXR insufficiency in humans is genetic variation in the gene itself resulting in diminished expression or function. Indeed, sequencing analysis of FXR in intrahepatic cholestasis of pregnancy patients revealed four functional heterozygous variants, three of which demonstrated functional defects in either translation efficiency or signaling activity. Additionally, FXR polymorphism identification analysis of European-, African-, Chinese-, and Hispanic-Americans identified a common, hypomorphic single nucleotide polymorphism with population allelic frequencies ranging from 2.5% to 12.1% . The in vitro transactivation activity of this hypomorphic SNP was lower relative to that of WT allele and human carriers of this allele showed significantly reduced hepatic SHP levels. Furthermore, the global FXR haplotype distribution between inflammatory bowel disease and healthy individuals was significantly different which emphasizes the link between FXR-mediated BA signaling and intestinal inflammation.48 Since chronic inflammation is widely considered a predisposition to cancer development, enhancement of FXR signaling appears to be a promising clinical target to not only normalize the BA dysregulation seen in obese and diabetic individuals but also combat chronic hepatic and intestinal inflammation. The appropriate circulation of BAs between the liver and small intestine is crucial to the maintenance of BA homeostasis and consequently, normal GI physiology. The ileum is where approximately 90% of secreted BAs are actively reabsorbed into the bloodstream by ASBT for transport back to the liver through the hepatic portal vein. Because of its predominantly ileal expression and central role in enterohepatic cycling of BAs, ASBT is another potential participant in the interplay between BA dysregulation and gut dysbiosis. In Caco-2 cells, 25-hydroxycholesterol and CDCA treatments greatly reduced ASBT promoter activity and mRNA levels through the actions of FXR, SHP, retinoic acid receptor, and retinoid x receptor . Mice fed a cholesterol-enriched diet exhibited downregulation of ASBT at both the mRNA and protein levels, decreased ileal BA uptake, and elevated fecal BA excretion. Interestingly, exposure of Caco-2 cells to pro-inflammatory factor IL-1B also caused a 65% reduction in ASBT mRNA level. Elevated levels of cholesterol in the intestinal lumen and pro-inflammatory mediators in the intestinal epithelium appear to down-regulate ASBT activity, thereby disrupting enterohepatic BA circulation.