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Farnesoid X receptor (FXR) is a ligand-activated transcription factor involved in the control of bile acid (BA) synthesis and enterohepatic circulation. FXR can influence glucose and lipid homeostasis. Hepatic FXR activation by obeticholic acid is currently used to treat primary biliary cholangitis. Late-stage clinical trials investigating the use of obeticholic acid in the treatment of nonalcoholic steatohepatitis are underway. Mouse models of metabolic disease have demonstrated that inhibition of intestinal FXR signalling reduces obesity, insulin resistance and fatty liver disease by modulation of hepatic and gut bacteria-mediated BA metabolism, and intestinal ceramide synthesis. FXR also has a role in the pathogenesis of gastrointestinal and liver cancers. Studies using tissue-specific and global Fxr-null mice have revealed that FXR acts as a suppressor of hepatocellular carcinoma, mainly through regulating BA homeostasis. Loss of whole-body FXR potentiates progression of spontaneous colorectal cancer, and obesity-induced BA imbalance promotes intestinal stem cell proliferation by suppressing intestinal FXR in Apcmin/+ mice. Owing to altered gut microbiota and FXR signalling, changes in overall BA levels and specific BA metabolites probably contribute to enterohepatic tumorigenesis. Modulating intestinal FXR signalling and altering BA metabolites are potential strategies for gastrointestinal and liver cancer prevention and treatment. In this Review, studies on the role of FXR in metabolic diseases and gastrointestinal and liver cancer are discussed, and the potential for development of targeted drugs are summarized.Farnesoid X receptor (FXR) is involved in the control of bile acid synthesis and enterohepatic circulation. This Review discusses the role of FXR in metabolic diseases and gastrointestinal and liver cancers, highlighting underlying mechanisms and potential therapeutic targets.

Hypoxia-inducible factors (HIFs), a family of transcription factors activated by hypoxia, consist of three α-subunits (HIF1α, HIF2α and HIF3α) and one β-subunit (HIF1β), which serves as a heterodimerization partner of the HIFα subunits. HIFα subunits are stabilized from constitutive degradation by hypoxia largely through lowering the activity of the oxygen-dependent prolyl hydroxylases that hydroxylate HIFα, leading to their proteolysis. HIF1α and HIF2α are expressed in different tissues and regulate target genes involved in angiogenesis, cell proliferation and inflammation, and their expression is associated with different disease states. HIFs have been widely studied because of their involvement in cancer, and HIF2α-specific inhibitors are being investigated in clinical trials for the treatment of kidney cancer. Although cancer has been the major focus of research on HIF, evidence has emerged that this pathway has a major role in the control of metabolism and influences metabolic diseases such as obesity, type 2 diabetes mellitus and non-alcoholic fatty liver disease. Notably increased HIF1α and HIF2α signalling in adipose tissue and small intestine, respectively, promotes metabolic diseases in diet-induced disease models. Inhibition of HIF1α and HIF2α decreases the adverse diet-induced metabolic phenotypes, suggesting that they could be drug targets for the treatment of metabolic diseases.

Peroxisome proliferator-activated receptor (PPAR) α, β/δ, and γ modulate lipid homeostasis. PPARα regulates lipid metabolism in the liver, the organ that largely controls whole-body nutrient/energy homeostasis, and its abnormalities may lead to hepatic steatosis, steatohepatitis, steatofibrosis, and liver cancer. PPARβ/δ promotes fatty acid β-oxidation largely in extrahepatic organs, and PPARγ stores triacylglycerol in adipocytes. Investigations using liver-specific PPAR-disrupted mice have revealed major but distinct contributions of the three PPARs in the liver. This review summarizes the findings of liver-specific PPAR-null mice and discusses the role of PPARs in the liver.

Parabacteroides distasonis ( P. distasonis ) plays an important role in human health, including diabetes, colorectal cancer and inflammatory bowel disease. Here, we show that P. distasonis is decreased in patients with hepatic fibrosis, and that administration of P. distasonis to male mice improves thioacetamide (TAA)- and methionine and choline-deficient (MCD) diet-induced hepatic fibrosis. Administration of P. distasonis also leads to increased bile salt hydrolase (BSH) activity, inhibition of intestinal farnesoid X receptor (FXR) signaling and decreased taurochenodeoxycholic acid (TCDCA) levels in liver. TCDCA produces toxicity in mouse primary hepatic cells (HSCs) and induces mitochondrial permeability transition (MPT) and Caspase-11 pyroptosis in mice. The decrease of TCDCA by P. distasonis improves activation of HSCs through decreasing MPT-Caspase-11 pyroptosis in hepatocytes. Celastrol, a compound reported to increase P. distasonis abundance in mice, promotes the growth of P. distasonis with concomitant enhancement of bile acid excretion and improvement of hepatic fibrosis in male mice. These data suggest that supplementation of P. distasonis may be a promising means to ameliorate hepatic fibrosis. Parabacteroides distasonis ( P. distasonis ), part of the gut microbiome, was reported to play a role in diabetes, colorectal cancer and inflammatory bowel disease. Here the authors report that P. distasonis ameliorates liver fibrosis in studies with male mice, potentially via altered bile acid metabolism and hepatocyte pyroptosis.

Key Points Peroxisome proliferator-activated receptors (PPARs) have central roles in the regulation of glucose and lipid homeostasis through their functions as molecular sensors that respond to endogenous ligands, leading to the modulation of gene expression. PPARs also regulate cell proliferation, differentiation and inflammation. PPARα mediates hepatocarcinogenesis induced by long-term administration of PPARα agonists in rodent models, an effect that is not found in humans. The mechanism underlying species-specific hepatocarcinogenesis is through mouse PPARα-dependent regulation of the let-7c microRNA, which leads to increased expression of the oncoprotein MYC. The current interest in targeting PPARα for the prevention of certain cancers, including colon and leukaemia, is based on studies showing that PPARα agonists inhibit the proliferation of endothelial cells, increase the synthesis of PPARγ agonists and potentially interfere with the Warburg effect. The role of PPARβ/δ in carcinogenesis is controversial. Several studies have shown that PPARβ/δ is upregulated in cancer cells by the adenomatous polyposis coli (APC)–β-catenin–TCF4 pathway and has a pro-tumorigenic effect in many cancer types. However, other studies have shown that PPARβ/δ agonists can induce terminal differentiation and inhibit innate inflammation, suggesting anticancer effects. In addition, a retrospective study has shown that low expression levels of PPARβ/δ are associated with the decreased survival of patients with colorectal cancer. Therefore, there remains a need to further examine PPARβ/δ protein expression patterns quantitatively in tumour models and the putative mechanisms that are mediated by PPARβ/δ agonists associated with anti-apoptotic or growth stimulatory effects. PPARγ agonists can induce terminal differentiation, inhibit cell proliferation, promote apoptosis and inhibit innate inflammation in many cancer models. This has led to a number of clinical trials with PPARγ agonists, but these have generated mixed results. Moreover, some PPARγ agonists have been associated with pro-tumorigenic effects. Emerging evidence indicates that targeting PPARγ in combination with other chemopreventive or chemotherapeutic agents might increase the efficacy of the effects that are induced by monotherapies. Owing to similarities in the abilities of the three PPARs to improve different metabolic disorders that are known to be associated with increased cancer risk (such as diabetes, obesity, dyslipidemias and chronic inflammation), modulating the activities of the PPARs remains an attractive approach for the treatment and prevention of cancer. The challenge is to advance the discovery of molecular mechanisms of action in order to identify and characterize effective PPAR agonists with acceptable safety profiles. Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that are involved in regulating glucose and lipid homeostasis, inflammation, proliferation and differentiation. This Review discusses the roles of PPARs in cancer and focuses on PPARβ/δ and the controversies yet to be resolved. Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that are involved in regulating glucose and lipid homeostasis, inflammation, proliferation and differentiation. Although all of these functions might contribute to the influence of PPARs in carcinogenesis, there is a distinct need for a review of the literature and additional experimentation to determine the potential for targeting PPARs for cancer therapy and cancer chemoprevention. As PPAR agonists include drugs that are used for the treatment of metabolic diseases, a more complete understanding of the roles of PPARs in cancer will aid in determining any increased cancer risk for patients undergoing therapy with PPAR agonists.

Polycystic ovary syndrome (PCOS) is characterized by androgen excess, ovulatory dysfunction and polycystic ovaries1, and is often accompanied by insulin resistance2. The mechanism of ovulatory dysfunction and insulin resistance in PCOS remains elusive, thus limiting the development of therapeutics. Improved metabolic health is associated with a relatively high microbiota gene content and increased microbial diversity3,4. This study aimed to investigate the impact of the gut microbiota and its metabolites on the regulation of PCOS-associated ovarian dysfunction and insulin resistance. Here, we report that Bacteroides vulgatus was markedly elevated in the gut microbiota of individuals with PCOS, accompanied by reduced glycodeoxycholic acid and tauroursodeoxycholic acid levels. Transplantation of fecal microbiota from women with PCOS or B. vulgatus-colonized recipient mice resulted in increased disruption of ovarian functions, insulin resistance, altered bile acid metabolism, reduced interleukin-22 secretion and infertility. Mechanistically, glycodeoxycholic acid induced intestinal group 3 innate lymphoid cell IL-22 secretion through GATA binding protein 3, and IL-22 in turn improved the PCOS phenotype. This finding is consistent with the reduced levels of IL-22 in individuals with PCOS. This study suggests that modifying the gut microbiota, altering bile acid metabolism and/or increasing IL-22 levels may be of value for the treatment of PCOS.

The anti-hyperglycemic effect of metformin is believed to be caused by its direct action on signaling processes in hepatocytes, leading to lower hepatic gluconeogenesis. Recently, metformin was reported to alter the gut microbiota community in humans, suggesting that the hyperglycemia-lowering action of the drug could be the result of modulating the population of gut microbiota. However, the critical microbial signaling metabolites and the host targets associated with the metabolic benefits of metformin remained elusive. Here, we performed metagenomic and metabolomic analysis of samples from individuals with newly diagnosed type 2 diabetes (T2D) naively treated with metformin for 3 d, which revealed that Bacteroides fragilis was decreased and the bile acid glycoursodeoxycholic acid (GUDCA) was increased in the gut. These changes were accompanied by inhibition of intestinal farnesoid X receptor (FXR) signaling. We further found that high-fat-diet (HFD)-fed mice colonized with B. fragilis were predisposed to more severe glucose intolerance, and the metabolic benefits of metformin treatment on glucose intolerance were abrogated. GUDCA was further identified as an intestinal FXR antagonist that improved various metabolic endpoints in mice with established obesity. Thus, we conclude that metformin acts in part through a B. fragilis –GUDCA–intestinal FXR axis to improve metabolic dysfunction, including hyperglycemia. Metformin decreases the levels of Bacteroides fragilis while increasing the bile acid GUDCA to antagonize intestinal FXR and improves the metabolic health of humans and mice.

Tobacco smoking is positively correlated with non-alcoholic fatty liver disease (NAFLD) 1 – 5 , but the underlying mechanism for this association is unclear. Here we report that nicotine accumulates in the intestine during tobacco smoking and activates intestinal AMPKα. We identify the gut bacterium Bacteroides xylanisolvens as an effective nicotine degrader. Colonization of B. xylanisolvens reduces intestinal nicotine concentrations in nicotine-exposed mice, and it improves nicotine-exacerbated NAFLD progression. Mechanistically, AMPKα promotes the phosphorylation of sphingomyelin phosphodiesterase 3 (SMPD3), stabilizing the latter and therefore increasing intestinal ceramide formation, which contributes to NAFLD progression to non-alcoholic steatohepatitis (NASH). Our results establish a role for intestinal nicotine accumulation in NAFLD progression and reveal an endogenous bacterium in the human intestine with the ability to metabolize nicotine. These findings suggest a possible route to reduce tobacco smoking-exacerbated NAFLD progression. Nicotine accumulates in the intestine during tobacco smoking and accelerates the progression of non-alcoholic fatty liver disease to non-alcoholic steatohepatitis (NASH), but it can be degraded effectively by the human symbiont Bacteroides xylanisolvens.

A novel, selective and sensitive single-ion monitoring (SIM) gas chromatography-mass spectrometry (GCMS) method was developed and validated for the determination of energy metabolites related to glycolysis, the tricarboxylic acid (TCA) cycle, glutaminolysis, and fatty acid β-oxidation. This assay used N-tert-butyldimethylsilyl-N-methyltrifluoroacetamide (MTBSTFA) containing 1% tert-butyldimethylchlorosilane (TBDMCS) as derivatizing reagent and was highly reproducible, sensitive, specific and robust. The assay was used to analyze liver tissue and serum from C57BL/6N obese mice fed a high-fat diet (HFD) and C57BL/6N mice fed normal chow for 8 weeks. HFD-fed mice serum displayed statistically significantly reduced concentrations of pyruvate, citrate, succinate, fumarate, and 2-oxoglutarate, with an elevated concentration of pantothenic acid. In liver tissue, HFD-fed mice exhibited depressed levels of glycolysis end-products pyruvate and lactate, glutamate, and the TCA cycle intermediates citrate, succinate, fumarate, malate, and oxaloacetate. Pantothenate levels were 3-fold elevated accompanied by a modest increased gene expression of Scl5a6 that encodes the pantothenate transporter SLC5A6. Since both glucose and fatty acids inhibit coenzyme A synthesis from pantothenate, it was concluded that these data were consistent with downregulated fatty acid β-oxidation, glutaminolysis, glycolysis, and TCA cycle activity, due to impaired anaplerosis. The novel SIM GCMS assay provided new insights into metabolic effects of HFD in mice.

Amyloid precursor protein (APP) derivative β-amyloid (Aβ) plays an important role in the pathogenesis of Alzheimer’s disease (AD). Sequential proteolysis of APP by β-secretase andγ-secretase generates Aβ. Conversely, the α-secretase “a disintegrin and metalloproteinase” 10 (ADAM10) cleaves APP within the eventual Aβ sequence and precludes Aβ generation. Therefore, up-regulation of ADAM10 represents a plausible therapeutic strategy to combat overproduction of neurotoxic Aβ. Peroxisome proliferator-activated receptor α (PPARα) is a transcription factor that regulates genes involved in fatty acid metabolism. Here, we determined that theAdam10promoter harbors PPAR response elements; that knockdown of PPARα, but not PPARβ or PPARγ, decreases the expression ofAdam10; and that lentiviral overexpression of PPARα restored ADAM10 expression inPpara −/−neurons. Gemfibrozil, an agonist of PPARα, induced the recruitment of PPARα:retinoid x receptor α, but not PPARγcoactivator 1α (PGC1α), to theAdam10promoter in wild-type mouse hippocampal neurons and shifted APP processing toward the α-secretase, as determined by augmented soluble APPα and decreased Aβ production. Accordingly,Ppara −/−mice displayed elevated SDS-stable, endogenous Aβ and Aβ1–42relative to wild-type littermates, whereas 5XFAD mice null for PPARα (5X/α−/−) exhibited greater cerebral Aβ load relative to 5XFAD littermates. These results identify PPARα as an important factor regulating neuronal ADAM10 expression and, thus, α-secretase proteolysis of APP.