The farnesoid X-activated receptor mediates bile acid activation of phospholipid transfer protein gene expression

Bile salt signaling and bile salt-based therapies in cardiometabolic disease

Bile salts have an established role in the emulsification and intestinal absorption of dietary lipids, and their homeostasis is tightly controlled by various transporters and regulators in the enterohepatic circulation. Notably, emerging evidence points toward bile salts as major modulators of cardiometabolic disease (CMD), an umbrella disease of disorders affecting the heart and blood vessels that is caused by systemic metabolic diseases such as Type 2 diabetes mellitus (T2DM) and metabolic dysfunction-associated steatotic liver disease (MASLD), the latter encompassing also metabolic dysfunction-associated steatohepatitis (MASH). The underlying mechanisms of protective effects of bile salts are their hormonal properties, enabling them to exert versatile metabolic effects by activating various bile salt-responsive signaling receptors with the nuclear farnesoid X receptor (FXR) and the Takeda G-protein-coupled receptor 5 (TGR5) as most extensively investigated. Activation of FXR and TGR5 is involved in the regulation of glucose, lipid and energy metabolism, and inflammation. Bile salt-based therapies directly targeting FXR and TGR5 signaling have been evaluated for their therapeutic potential in CMD. More recently, therapeutics targeting bile salt transporters thereby modulating bile salt localization, dynamics, and signaling, have been developed and evaluated in CMD. Here, we discuss the current knowledge on the contribution of bile salt signaling in the pathogenesis of CMD and the potential of bile salt-based therapies for the treatment of CMD.

A Bibliometric Analysis of Research on the Links Between Gut Microbiota and Atherosclerosis

Background

Emerging evidence has linked gut microbiota (GM) and its related metabolites to atherosclerosis (AS). This study aimed to analyze the evolution of GM in AS in the past decades, and provide valuable insights in this field.

Methods

Web of Science Core Collection (WoSCC) was applied to retrieve the publications related to GM in AS from their inception until 2 December 2021, and the data was analyzed in Microsoft Excel, Scimago Graphica, CiteSpace, and VOSviewer.

Results

In total, 560 documents were extracted from the WoSCC databases. The publications have shown rapid growth since 2008. China and Cleveland Clin were the most prolific country and institution, respectively. The journal with the most publications is Nutrients, and Nature was the most co-cited journal. Among 3556 related authors, Hazen, Stanley L., Tang, W. H. Wilson, and Wang, Zeneng were the top 3 contributing authors in this field. Aside from “gut microbiota,” “atherosclerosis,” the terms “TMAO,” “metabolite,” “obesity,” and “phosphatidylcholine” were frequently occurred in the abstract and title of articles. Burst detection of keywords indicated that “metabolic syndrome,” “acid,” and “bile acid” were hot topics in recent years. According to the co-citation analysis of references, the research focus in this area has changed over time, and recent researches focus on choline, hypertension, butyrate, and berberine.

Conclusion

Our study showed that the researches of GM in AS have been flourishing, and the content themes were constantly deepened. Human GM is critical to atherosclerotic diseases, and this hot topic is still worthy of more focus in the future.

Bile acid and receptors: biology and drug discovery for nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD), a series of liver metabolic disorders manifested by lipid accumulation within hepatocytes, has become the primary cause of chronic liver diseases worldwide. About 20%-30% of NAFLD patients advance to nonalcoholic steatohepatitis (NASH), along with cell death, inflammation response and fibrogenesis. The pathogenesis of NASH is complex and its development is strongly related to multiple metabolic disorders (e.g. obesity, type 2 diabetes and cardiovascular diseases). The clinical outcomes include liver failure and hepatocellular cancer. There is no FDA-approved NASH drug so far, and thus effective therapeutics are urgently needed. Bile acids are synthesized in hepatocytes, transported into the intestine, metabolized by gut bacteria and recirculated back to the liver by the enterohepatic system. They exert pleiotropic roles in the absorption of fats and regulation of metabolism. Studies on the relevance of bile acid disturbance with NASH render it as an etiological factor in NASH pathogenesis. Recent findings on the functional identification of bile acid receptors have led to a further understanding of the pathophysiology of NASH such as metabolic dysregulation and inflammation, and bile acid receptors are recognized as attractive targets for NASH treatment. In this review, we summarize the current knowledge on the role of bile acids and the receptors in the development of NAFLD and NASH, especially the functions of farnesoid X receptor (FXR) in different tissues including liver and intestine. The progress in the development of bile acid and its receptors-based drugs for the treatment of NASH including bile acid analogs and non-bile acid modulators on bile acid metabolism is also discussed.

Farnesoid X receptor agonist attenuates subchondral bone osteoclast fusion and osteochondral pathologies of osteoarthritis via suppressing JNK1/2/NFATc1 pathway

Osteoarthritis (OA) is a prevalent degenerative disease of the joint, featured by articular cartilage destruction and subchondral bone marrow lesions. Articular cartilage and subchondral bone constitute an osteochondral unit that guarantees joint homeostasis. During OA initiation, activated osteoclasts in subchondral bone ultimately result in impaired capacities of the subchondral bone in response to mechanical stress, followed by the degradation of overlying articular cartilage. Thus, targeting osteoclasts could be a potential therapeutic option for treating OA. Here, we observed that farnesoid X receptor (FXR) expression and osteoclast fusion and activity in subchondral bone were concomitantly changed during early-stage OA in the OA mouse model established by anterior cruciate ligament transection (ACLT). Then, we explored the therapeutic effects of FXR agonist GW4064 on the osteochondral pathologies in ACLT mice. We showed that GW4064 obviously ameliorated subchondral bone deterioration, associated with reduction in tartrate-resistant acid phosphatase (TRAP) positive multinuclear osteoclast number, as well as articular cartilage degradation, which were blocked by the treatment with FXR antagonist Guggulsterone. Mechanistically, GW4064 impeded osteoclastogenesis through inhibiting subchondral bone osteoclast fusion via suppressing c-Jun N-terminal kinase (JNK) 1/2/nuclear factor of activated T-cells 1 (NFATc1) pathway. Taken together, our results present evidence for the protective effects of GW4064 against OA by blunting osteoclast-mediated aberrant subchondral bone loss and subsequent cartilage deterioration. Therefore, GW4064 demonstrates the potential as an alternative therapeutic option against OA for further drug development.

NRF2 Activation in Autophagy Defects Suppresses a Pharmacological Transactivation of the Nuclear Receptor FXR

NF-E2-related factor 2 (NRF2), an antioxidant transcription factor, is activated in autophagy-deficient mice due to the accumulations of p62/SQSTM1 and its subsequent interaction with Kelch-like-ECH-associated protein 1 (KEAP1), an adaptor component for Cullin3-based E3 ubiquitin ligase complex. Farnesoid x receptor (FXR/NR1H4) is a ligand-dependent transcription factor that belongs to the nuclear receptor superfamily. FXR plays an essential role in bile acid synthesis and enterohepatic circulation, affecting glucose and lipid metabolism. Obeticholic acid as a potent FXR agonist has been approved to treat primary biliary cholangitis and clinical trials for its use in the treatment of other liver diseases are underway. Here we show that NRF2 activation in autophagy defects impedes a transactivation of FXR. Liver-specific Atg7 knockout mice or a treatment of autophagy inhibitor showed decreased inductions of FXR target genes upon its synthetic agonists. Moreover, enforced NRF2 activations with small molecules potently decreased the pharmacological activation of FXR in cultured cells. Finally, we demonstrate that NRF2 activation by the treatment with the food antioxidant butylated hydroxyanisole is necessary and sufficient to inhibit the pharmacological activation of FXR in vivo. These results reveal a novel function of the basal autophagy-NRF2 axis for the regulation of FXR transactivation, and shed light on a potential therapeutic strategy in metabolic disease.

Transcriptional Control of Trpm6 by the Nuclear Receptor FXR

Farnesoid x receptor (FXR) is a nuclear bile acid receptor that belongs to the nuclear receptor superfamily. It plays an essential role in bile acid biosynthesis, lipid and glucose metabolism, liver regeneration, and vertical sleeve gastrectomy. A loss of the FXR gene or dysregulations of FXR-mediated gene expression are associated with the development of progressive familial intrahepatic cholestasis, tumorigenesis, inflammation, and diabetes mellitus. Magnesium ion (Mg²⁺) is essential for mammalian physiology. Over 600 enzymes are dependent on Mg²⁺ for their activity. Here, we show that the Trpm6 gene encoding a Mg²⁺ channel is a direct FXR target gene in the intestinal epithelial cells of mice. FXR expressed in the intestinal epithelial cells is absolutely required for sustaining a basal expression of intestinal Trpm6 that can be robustly induced by the treatment of GW4064, a synthetic FXR agonist. Analysis of FXR ChIP-seq data revealed that intron regions of Trpm6 contain two prominent FXR binding peaks. Among them, the proximal peak from the transcription start site contains a functional inverted repeat 1 (IR1) response element that directly binds to the FXR-RXRα heterodimer. Based on these results, we proposed that an intestinal FXR-TRPM6 axis may link a bile acid signaling to Mg²⁺ homeostasis.

Design and identification of a new farnesoid X receptor (FXR) partial agonist by computational structure-activity relationship analysis: Ligand-induced H8 helix fluctuation in the ligand-binding domain of FXR may lead to partial agonism

Farnesoid X receptor (FXR) controls gene-expression relevant to various diseases including nonalcoholic steatohepatitis and has become a drug target to regulate metabolic aberrations. However, some side effects of FXR agonists reported in clinical development such as an increase in blood cholesterol levels incentivize the development of partial agonists to minimize side effects. In this study, to identify a new partial agonist, we analyzed the computational structure-activity relationship (SAR) of FXR agonists previously developed in our laboratories using molecular dynamics simulations. SAR analysis showed that fluctuations in the H8 helix, by ligand binding, of the ligand-binding domain (LBD) of FXR may influence agonistic activity. Based on this observation, 6 was newly designed as a partial agonist and synthesized. As a result of biological evaluations, 6 showed weak agonistic activity (40.0% relative agonistic activity to the full-agonist GW4064) and a potent EC₅₀ value (55.5 nM). The successful identification of the new potent partial agonist 6 suggested that helix fluctuation in the LBD induced by ligands could be one way to develop partial agonists.

FXR in liver physiology: Multiple faces to regulate liver metabolism

The liver is the central metabolic hub which coordinates nutritional inputs and metabolic outputs. Food intake releases bile acids which can be sensed by the bile acid receptor FXR in the liver and the intestine. Hepatic and intestinal FXR coordinately regulate postprandial nutrient disposal in a network of interacting metabolic nuclear receptors. In this review we summarize and update the "classical roles" of FXR as a central integrator of the feeding state response, which orchestrates the metabolic processing of carbohydrates, lipids, proteins and bile acids. We also discuss more recent and less well studied FXR effects on amino acid, protein metabolism, autophagic turnover and inflammation. In addition, we summarize the recent understanding of how FXR signaling is affected by posttranslational modifications and by different FXR isoforms. These modifications and variations in FXR signaling might be considered when FXR is targeted pharmaceutically in clinical applications.

The Role of Phospholipid Transfer Protein in the Development of Atherosclerosis

PURPOSE OF REVIEW

Phospholipid transfer protein (PLTP), a member of lipid transfer protein family, is an important protein involved in lipid metabolism in the circulation. This article reviews recent PLTP research progresses, involving lipoprotein metabolism and atherogenesis.

RECENT FINDINGS

PLTP activity influences atherogenic and anti-atherogenic lipoprotein levels. Human serum PLTP activity is a risk factor for human cardiovascular disease and is an independent predictor of all-cause mortality. PLTP deficiency reduces VLDL and LDL levels and attenuates atherosclerosis in mouse models, while PLTP overexpression exerts an opposite effect. Both PLTP deficiency and overexpression result in reduction of HDL which has different size, inflammatory index, and lipid composition. Moreover, although both PLTP deficiency and overexpression reduce cholesterol efflux capacity, but this effect has no impact in macrophage reverse cholesterol transport in mice. Furthermore, PLTP activity is related with metabolic syndrome, thrombosis, and inflammation. PLTP could be target for the treatment of dyslipidemia and atherosclerosis, although some potential off-target effects should be noted.

Hepatic metabolic regulation by nuclear factor E4BP4

Discovered as a b-ZIP transcription repressor 30 years ago, E4 promoter-binding protein 4 (E4BP4) has been shown to play critical roles in immunity, circadian rhythms, and cancer progression. Recent research has highlighted E4BP4 as a novel regulator of metabolisms in various tissues. In this review, we focus on the function and mechanisms of hepatic E4BP4 in regulating lipid and glucose homeostasis, bile metabolism, as well as xenobiotic metabolism. Finally, E4BP4-specific targets will be discussed for the prevention and treatment of metabolic disorders.

Transcriptional regulation of CYP3A4 by nuclear receptors in human hepatocytes under hypoxia

The human hepatic cytochrome P-450 3A4 (CYP3A4), recognized as a multifunctional enzyme, has a wide range of substrates including commonly used drugs. Previous investigations demonstrated that the expression of CYP3A4 in human hepatocytes could be regulated by some nuclear receptors (NRs) at transcriptional level under diverse situations. The significance of oxygen on CYP3A4-mediated metabolism seems notable while the regulatory mode of CYP3A4 in the particular case still remains elusive. Recently, striking evidence has emerged that both CYP3A4 and its regulator NR could be inhibited by exposure to hypoxia. Therefore, it is of great importance to elucidate whether and how these NRs act in the transcriptional regulation of CYP3A4 in human hepatocytes under hypoxic conditions. In this review, we mainly summarized transcriptional regulation of the pivotal enzyme CYP3A4 by NRs and explored the possible regulatory pathways of CYP3A4 via these major NRs under hypoxia, expecting to provide favorable evidence for further clinical guidance under such pathological situations.

Synthesis of Novel Farnesoid X Receptor Agonists and Validation of Their Efficacy in Activating Differentiation of Mouse Bone Marrow-Derived Mesenchymal Stem Cells into Osteoblasts

The modulators of farnesoid X receptor (FXR), a bile acid receptor, regulate various biological processes including bile acid metabolism, and are associated with the control of fatty liver and osteoporosis. Thus, the control of FXR activity and development of FXR modulators are critical not only for research, but also for clinical application. In this study, we synthesized novel FXR agonists 1–4 possessing isoxazole and N-substituted benzimidazole moieties, and compared their effects on osteoblast differentiation with the known FXR agonists, chenodeoxycholic acid and a synthetic compound, GW4064. Two (3 and 4) of the four novel FXR agonists 1–4 showed high specificities for FXR. Computer-assisted modeling suggested that the binding of the FXR agonist 3 with ligand binding domain of FXR was similar to GW4064. FXR was expressed in mouse bone marrow-derived mesenchymal stem cell (MSC)-like ST2 cells (ST-2 MSCs). The FXR agonists activated the BMP-2-induced differentiation of ST-2 MSCs into osteoblasts and enhanced the expression of RUNX2. Moreover, the potency of the FXR agonist 3 was comparable to GW4064 in promoting osteoblast differentiation of ST-2 MSCs. These results indicate that FXR activation enhanced the BMP-2-induced differentiation of MSCs into osteoblasts through activating RUNX2 expression. FXR could be a potential therapeutic target for the treatment of bone diseases such as osteoporosis.

Identification of Novel Phospholipid Transfer Protein Inhibitors by High-Throughput Screening

Atherogenesis has been recognized as a risk factor for lethal cardiovascular diseases. Plasma low-density lipoprotein levels are correlated to the occurrence of atherosclerosis, and their control is critical for both the prevention and treatment of these diseases. Phospholipid transfer protein (PLTP) is one of the key regulators of lipoprotein metabolism; PLTP-deficient mice exhibit decreased apolipoprotein B (apoB)-containing lipoprotein secretion and atherosclerosis, indicating the validity of PLTP as a promising therapeutic target. Here, we demonstrate a high-throughput screening (HTS) method to identify a novel chemotype of PLTP inhibitors. Instead of using recombinant proteins, we used human plasma as a source of enzymes in the first screening, so as to efficiently exclude promiscuous inhibitors. The selected compounds were further confirmed to target PLTP both biochemically and biophysically and were shown to inhibit apoB secretion from hepatic cells with no apparent toxicity. We believe that our approach is suitable for filtering out nonspecific inhibitors at an earlier stage of screening campaigns and that these compounds should have potential to be developed into drugs to treat dyslipidemia.

Phosphorylation of hepatic farnesoid X receptor by FGF19 signaling-activated Src maintains cholesterol levels and protects from atherosclerosis

The bile acid (BA) nuclear receptor, farnesoid X receptor (FXR/NR1H4), maintains metabolic homeostasis by transcriptional control of numerous genes, including an intestinal hormone, fibroblast growth factor-19 (FGF19; FGF15 in mice). Besides activation by BAs, the gene-regulatory function of FXR is also modulated by hormone or nutrient signaling-induced post-translational modifications. Recently, phosphorylation at Tyr-67 by the FGF15/19 signaling-activated nonreceptor tyrosine kinase Src was shown to be important for FXR function in BA homeostasis. Here, we examined the role of this FXR phosphorylation in cholesterol regulation. In both hepatic FXR-knockout and FXR-knockdown mice, reconstitution of FXR expression up-regulated cholesterol transport genes for its biliary excretion, including scavenger receptor class B member 1 (Scarb1) and ABC subfamily G member 8 (Abcg5/8), decreased hepatic and plasma cholesterol levels, and increased biliary and fecal cholesterol levels. Of note, these sterol-lowering effects were blunted by substitution of Phe for Tyr-67 in FXR. Moreover, consistent with Src's role in phosphorylating FXR, Src knockdown impaired cholesterol regulation in mice. In hypercholesterolemic apolipoprotein E-deficient mice, expression of FXR, but not Y67F-FXR, ameliorated atherosclerosis, whereas Src down-regulation exacerbated it. Feeding or treatment with an FXR agonist induced Abcg5/8 and Scarb1 expression in WT, but not FGF15-knockout, mice. Furthermore, FGF19 treatment increased occupancy of FXR at Abcg5/8 and Scarb1, expression of these genes, and cholesterol efflux from hepatocytes. These FGF19-mediated effects were blunted by the Y67F-FXR substitution or Src down-regulation or inhibition. We conclude that phosphorylation of hepatic FXR by FGF15/19-induced Src maintains cholesterol homeostasis and protects against atherosclerosis.

Pharmacological Applications of Bile Acids and Their Derivatives in the Treatment of Metabolic Syndrome

Apart from well-known functions of bile acids in digestion and solubilization of lipophilic nutrients and drugs in the small intestine, the emerging evidence from the past two decades identified the role of bile acids as signaling, endocrine molecules that regulate the glucose, lipid, and energy metabolism through complex and intertwined pathways that are largely mediated by activation of nuclear receptor farnesoid X receptor (FXR) and cell surface G protein-coupled receptor 1, TGR5 (also known as GPBAR1). Interactions of bile acids with the gut microbiota that result in the altered composition of circulating and intestinal bile acids pool, gut microbiota composition and modified signaling pathways, are further extending the complexity of biological functions of these steroid derivatives. Thus, bile acids signaling pathways have become attractive targets for the treatment of various metabolic diseases and metabolic syndrome opening the new potential avenue in their treatment. In addition, there is a significant effort to unveil some specific properties of bile acids relevant to their intrinsic potency and selectivity for particular receptors and to design novel modulators of these receptors with improved pharmacokinetic and pharmacodynamic profiles. This resulted in synthesis of few semi-synthetic bile acids derivatives such as 6α-ethyl-chenodeoxycholic acid (obeticholic acid, OCA), norursodeoxycholic acid (norUDCA), and 12-monoketocholic acid (12-MKC) that are proven to have positive effect in metabolic and hepato-biliary disorders. This review presents an overview of the current knowledge related to bile acids implications in glucose, lipid and energy metabolism, as well as a potential application of bile acids in metabolic syndrome treatment with future perspectives.

The Role of the Gut Microbiota in Bile Acid Metabolism

The gut microbiota has been considered a cornerstone of maintaining the health status of its human host because it not only facilitates harvesting of nutrients and energy from ingested food, but also produces numerous metabolites that can regulate host metabolism. One such class of metabolites, the bile acids, are synthesized from cholesterol in the liver and further metabolized by the gut microbiota into secondary bile acids. These bioconversions modulate the signaling properties of bile acids through the nuclear farnesoid X receptor and the G protein-coupled membrane receptor 5, which regulate diverse metabolic pathways in the host. In addition, bile acids can regulate gut microbial composition both directly and indirectly by activation of innate immune response genes in the small intestine. Therefore, host metabolism can be affected by both microbial modifications of bile acids, which leads to altered signaling via bile acid receptors, and by alterations in the composition of the microbiota. In this review, we mainly describe the interactions between bile acids and intestinal microbiota and their roles in regulating host metabolism, but we also examine the impact of bile acid composition in the gut on the intestinal microbiome and on host physiology.

Farnesoid X receptor: A "homeostat" for hepatic nutrient metabolism

The Farnesoid X receptor (FXR) is a nuclear receptor activated by bile acids (BAs). BAs are amphipathic molecules that serve as fat solubilizers in the intestine under postprandial conditions. In the post-absorptive state, BAs bind FXR in the hepatocytes, which in turn provides feedback signals on BA synthesis and transport and regulates lipid, glucose and amino acid metabolism. Therefore, FXR acts as a homeostat of all three classes of nutrients, fats, sugars and proteins. Here we re-analyze the function of FXR in the perspective of nutritional metabolism, and discuss the role of FXR in liver energy homeostasis in postprandial, post-absorptive and fasting/starvation states. FXR, by regulating nutritional metabolism, represses autophagy in conditions of nutrient abundance, and controls the metabolic needs of proliferative cells. In addition, FXR regulates inflammation via direct effects and via its impact on nutrient metabolism. These functions indicate that FXR is an attractive therapeutic target for liver diseases.

The interrelationship between bile acid and vitamin A homeostasis

Vitamin A is a fat-soluble vitamin important for vision, reproduction, embryonic development, cell differentiation, epithelial barrier function and adequate immune responses. Efficient absorption of dietary vitamin A depends on the fat-solubilizing properties of bile acids. Bile acids are synthesized in the liver and maintained in an enterohepatic circulation. The liver is also the main storage site for vitamin A in the mammalian body, where an intimate collaboration between hepatocytes and hepatic stellate cells leads to the accumulation of retinyl esters in large cytoplasmic lipid droplet hepatic stellate cells. Chronic liver diseases are often characterized by disturbed bile acid and vitamin A homeostasis, where bile production is impaired and hepatic stellate cells lose their vitamin A in a transdifferentiation process to myofibroblasts, cells that produce excessive extracellular matrix proteins leading to fibrosis. Chronic liver diseases thus may lead to vitamin A deficiency. Recent data reveal an intricate crosstalk between vitamin A metabolites and bile acids, in part via the Retinoic Acid Receptor (RAR), Retinoid X Receptor (RXR) and the Farnesoid X Receptor (FXR), in maintaining vitamin A and bile acid homeostasis. Here, we provide an overview of the various levels of "communication" between vitamin A metabolites and bile acids and its relevance for the treatment of chronic liver diseases.

Low-Density Lipoprotein Inhibits Secretion of Phospholipid Transfer Protein in Human Trophoblastic BeWo Cells

Human plasma phospholipid transfer protein (PLTP) plays an important role in lipoprotein metabolism. In this study, we investigated the effects of lipoproteins on the secretion of PLTP in cultured BeWo choriocarcinoma cells. Low-density lipoproteins (LDLs) decreased PLTP secretion in a dose- and time-dependent manner, whereas very low density lipoproteins and high-density lipoproteins (HDLs) had little effect. LDL suppression of PLTP secretion was not altered by the inhibition of both LDL receptor and LDL receptor–related protein with receptor-associated protein. Mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor, U0126, could abolish the LDL-mediated inhibition of PLTP secretion. Furthermore, LDL, but not HDL, could stimulate the expression of MAPK phosphatase-1 (MKP-1) in BeWo cells that resulted in the inactivation of p44/p42 extracellular signal-regulated kinase (ERK) 1 and 2, the family members of MAPKs. These results support the conclusion that LDL-mediated suppression of PLTP secretion in BeWo cells is through a LDL receptor-independent MAPK signaling pathway.

Role of bile acids in the regulation of the metabolic pathways

Recent studies have revealed that bile acids (BAs) are not only facilitators of dietary lipid absorption but also important signaling molecules exerting multiple physiological functions. Some major signaling pathways involving the nuclear BAs receptor farnesoid X receptor and the G protein-coupled BAs receptor TGR5/M-BAR have been identified to be the targets of BAs. BAs regulate their own homeostasis via signaling pathways. BAs also affect diverse metabolic pathways including glucose metabolism, lipid metabolism and energy expenditure. This paper suggests the mechanism of controlling metabolism via BA signaling and demonstrates that BA signaling is an attractive therapeutic target of the metabolic syndrome.

Bile acid receptors and nonalcoholic fatty liver disease

With the high prevalence of obesity, diabetes, and other features of the metabolic syndrome in United States, nonalcoholic fatty liver disease (NAFLD) has inevitably become a very prevalent chronic liver disease and is now emerging as one of the leading indications for liver transplantation. Insulin resistance and derangement of lipid metabolism, accompanied by activation of the pro-inflammatory response and fibrogenesis, are essential pathways in the development of the more clinically significant form of NAFLD, known as nonalcoholic steatohepatitis (NASH). Recent advances in the functional characterization of bile acid receptors, such as farnesoid X receptor (FXR) and transmembrane G protein-coupled receptor (TGR) 5, have provided further insight in the pathophysiology of NASH and have led to the development of potential therapeutic targets for NAFLD and NASH. Beyond maintaining bile acid metabolism, FXR and TGR5 also regulate lipid metabolism, maintain glucose homeostasis, increase energy expenditure, and ameliorate hepatic inflammation. These intriguing features have been exploited to develop bile acid analogues to target pathways in NAFLD and NASH pathogenesis. This review provides a brief overview of the pathogenesis of NAFLD and NASH, and then delves into the biological functions of bile acid receptors, particularly with respect to NASH pathogenesis, with a description of the associated experimental data, and, finally, we discuss the prospects of bile acid analogues in the treatment of NAFLD and NASH.

Diets with high-fat cheese, high-fat meat, or carbohydrate on cardiovascular risk markers in overweight postmenopausal women: a randomized crossover trial

BACKGROUND

Heart associations recommend limited intake of saturated fat. However, effects of saturated fat on low-density lipoprotein (LDL)-cholesterol concentrations and cardiovascular disease risk might depend on nutrients and specific saturated fatty acids (SFAs) in food.

OBJECTIVE

We explored the effects of cheese and meat as sources of SFAs or isocaloric replacement with carbohydrates on blood lipids, lipoproteins, and fecal excretion of fat and bile acids.

DESIGN

The study was a randomized, crossover, open-label intervention in 14 overweight postmenopausal women. Three full-diet periods of 2-wk duration were provided separated by 2-wk washout periods. The isocaloric diets were as follows: 1) a high-cheese (96-120-g) intervention [i.e., intervention containing cheese (CHEESE)], 2) a macronutrient-matched nondairy, high-meat control [i.e., nondairy control with a high content of high-fat processed and unprocessed meat in amounts matching the saturated fat content from cheese in the intervention containing cheese (MEAT)], and 3) a nondairy, low-fat, high-carbohydrate control (i.e., nondairy low-fat control in which the energy from cheese fat and protein was isocalorically replaced by carbohydrates and lean meat (CARB).

RESULTS

The CHEESE diet caused a 5% higher high-density lipoprotein (HDL)-cholesterol concentration (P = 0.012), an 8% higher apo A-I concentration (P < 0.001), and a 5% lower apoB:apo A-I ratio (P = 0.008) than did the CARB diet. Also, the MEAT diet caused an 8% higher HDL-cholesterol concentration (P < 0.001) and a 4% higher apo A-I concentration (P = 0.033) than did the CARB diet. Total cholesterol, LDL cholesterol, apoB, and triacylglycerol were similar with the 3 diets. Fecal fat excretion was 1.8 and 0.9 g higher with the CHEESE diet than with CARB and MEAT diets (P < 0.001 and P = 0.004, respectively) and 0.9 g higher with the MEAT diet than with the CARB diet (P = 0.005). CHEESE and MEAT diets caused higher fecal bile acid excretion than did the CARB diet (P < 0.05 and P = 0.006, respectively). The dominant type of bile acids excreted differed between CHEESE and MEAT diets.

CONCLUSIONS

Diets with cheese and meat as primary sources of SFAs cause higher HDL cholesterol and apo A-I and, therefore, appear to be less atherogenic than is a low-fat, high-carbohydrate diet. Also, our findings confirm that cheese increases fecal fat excretion. This trial was registered at clinicaltrials.gov as NCT01739153.

Bile acid activated receptors are targets for regulation of integrity of gastrointestinal mucosa

Bile acids are the end product of cholesterol metabolism. Synthesized in the liver, primary bile acids are secreted by hepatocytes and are transformed by intestinal microbiota into secondary bile acids. In addition to their role in cholesterol and lipid absorption, bile acids act as signaling molecules activating a family of nuclear and G-protein-coupled receptors collectively known as bile acid activated receptors (BARs). These receptors are expressed at high density in enterohepatic tissues, but their expression occurs throughout the body and their activation mediates regulatory functions of bile acids on lipids and glucose metabolism and immunity. In the gastrointestinal tract, BARs maintain intestinal integrity, and their deletion makes the intestine more susceptible to the damage caused by acetylsalicylic acid and nonsteroidal anti-inflammatory drugs (NSAIDs). Deficiency in farnesoid X receptor and G-protein-coupled bile acid receptor 1 genes alters the expression/activity of cystathione γ-lyase and endothelial nitric oxide synthase, two genes involved in the synthesis of hydrogen sulfide and nitric oxide, i.e., two gaseous mediators that have been shown to be essential in maintaining the intestinal homeostasis. In addition, farnesoid X receptor regulates the expression of transporters required for secretion of phospholipid by hepatocytes. Because phospholids attenuate intestinal injury caused by acetylsalicylic acid and NSAIDs, BAR agonism could be exploited to protect the intestinal mucosa against injury caused by anti-inflammatory medications. This approach might be useful in the prevention of so-called NSAID enteropathy, a common clinical condition occurring in long-term users of NSAIDs, which is not effectively prevented either by cotreatment with proton pump inhibitors or by the use of coxibs.

Maltitol Prevents the Progression of Fatty Liver Degeneration in Mice Fed High-Fat Diets

Nonalcoholic fatty liver disease (NAFLD) progresses to nonalcoholic steatohepatitis, ultimately leading to cirrhosis and liver cancer. It is important to prevent this progression during the initial stages of hepatic fatty degeneration. Maltitol is a polyol produced by the hydrogenation of maltose. We investigated the efficacy of maltitol for treating hepatic fatty degeneration in C57BL/6 male mice using a high-fat diet model. Intake of 5.0% maltitol for 8 weeks significantly suppressed weight gain, hepatic fatty degeneration, hyperglycemia, and hypercholesterolemia. With maltitol intake, sterol regulatory element-binding protein 1c (SREBP1c) mRNA expression was significantly decreased, and farnesoid X receptor (FXR), peroxisome proliferator-activated receptor α (PPARα), and hydroxymethylglutaryl-Co reductase expressions were significantly higher in the liver. The increase in SREBP1c and suppression of FXR and PPARα expressions are correlated with NAFLD. Our results suggest that maltitol may prevent steatosis of NAFLD with a high-fat diet.

Nuclear bile acid signaling through the farnesoid X receptor

Bile acids (BAs) are amphipathic molecules produced from cholesterol by the liver. Expelled from the gallbladder upon meal ingestion, BAs serve as fat solubilizers in the intestine. BAs are reabsorbed in the ileum and return via the portal vein to the liver where, together with nutrients, they provide signals to coordinate metabolic responses. BAs act on energy and metabolic homeostasis through the activation of membrane and nuclear receptors, among which the nuclear receptor farnesoid X receptor (FXR) is an important regulator of several metabolic pathways. Highly expressed in the liver and the small intestine, FXR contributes to BA effects on metabolism, inflammation and cell cycle control. The pharmacological modulation of its activity has emerged as a potential therapeutic strategy for liver and metabolic diseases. This review highlights recent advances regarding the mechanisms by which the BA sensor FXR contributes to global signaling effects of BAs, and how FXR activity may be regulated by nutrient-sensitive signaling pathways.

Role of gut barrier function in the pathogenesis of nonalcoholic Fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is one of the most common forms of chronic liver disease, and its incidence is increasing year by year. Many efforts have been made to investigate the pathogenesis of this disease. Since 1998 when Marshall proposed the conception of "gut-liver axis," more and more researchers have paid close attention to the role of gut barrier function in the pathogenesis of NAFLD. The four aspects of gut barrier function, including physical, chemical, biological, and immunological barriers, are interrelated closely and related to NAFLD. In this paper, we present a summary of research findings on the relationship between gut barrier dysfunction and the development of NAFLD, aiming at illustrating the role of gut barrier function in the pathogenesis of this disease.

Bile acid nuclear receptor FXR and digestive system diseases

Bile acids (BAs) are not only digestive surfactants but also important cell signaling molecules, which stimulate several signaling pathways to regulate some important biological processes. The bile-acid-activated nuclear receptor, farnesoid X receptor (FXR), plays a pivotal role in regulating bile acid, lipid and glucose homeostasis as well as in regulating the inflammatory responses, barrier function and prevention of bacterial translocation in the intestinal tract. As expected, FXR is involved in the pathophysiology of a wide range of diseases of gastrointestinal tract, including inflammatory bowel disease, colorectal cancer and type 2 diabetes. In this review, we discuss current knowledge of the roles of FXR in physiology of the digestive system and the related diseases. Better understanding of the roles of FXR in digestive system will accelerate the development of FXR ligands/modulators for the treatment of digestive system diseases.

Regulation of HDL genes: transcriptional, posttranscriptional, and posttranslational

HDL regulation is exerted at multiple levels including regulation at the level of transcription initiation by transcription factors and signal transduction cascades; regulation at the posttranscriptional level by microRNAs and other noncoding RNAs which bind to the coding or noncoding regions of HDL genes regulating mRNA stability and translation; as well as regulation at the posttranslational level by protein modifications, intracellular trafficking, and degradation. The above mechanisms have drastic effects on several HDL-mediated processes including HDL biogenesis, remodeling, cholesterol efflux and uptake, as well as atheroprotective functions on the cells of the arterial wall. The emphasis is on mechanisms that operate in physiologically relevant tissues such as the liver (which accounts for 80% of the total HDL-C levels in the plasma), the macrophages, the adrenals, and the endothelium. Transcription factors that have a significant impact on HDL regulation such as hormone nuclear receptors and hepatocyte nuclear factors are extensively discussed both in terms of gene promoter recognition and regulation but also in terms of their impact on plasma HDL levels as was revealed by knockout studies. Understanding the different modes of regulation of this complex lipoprotein may provide useful insights for the development of novel HDL-raising therapies that could be used to fight against atherosclerosis which is the underlying cause of coronary heart disease.

Lepidozenolide from the liverwort Lepidozia fauriana acts as a farnesoid X receptor agonist

Lepidozenolide is a sesquiterpenoid isolated from the liverwort Lepidozia fauriana and its possible bioactivity is unclear. The farnesoid X receptor (FXR) is a member of nuclear receptor superfamily that has been widely targeted for developing treatments for chronic liver disease and hyperglycemia. In this study, whether lepidozenolide may act as a FXR agonist was determined. Indeed, in mammalian one-hybrid and transient transfection reporter assays, lepidozenolide transactivated FXR to modulate promoter action including GAL4, CYP7A1, and PLTP promoters in a dose-dependent manner, while it exhibited slightly less agonistic activity than chenodeoxycholic acid, an endogenous FXR agonist. Through the molecular modeling docking studies lepidozenolide was shown to bind to FXR ligand binding pocket fairly well. All these results indicate that lepidozenolide acts as a FXR agonist.

Transcription factor networks regulating hepatic fatty acid metabolism

Tight regulation of lipid levels is critical for cellular and organismal homeostasis, not only in terms of energy utilization and storage, but also to prevent potential toxicity. The liver utilizes a set of hepatic transcription factors to regulate the expression of genes implicated in all aspects of lipid metabolism including catabolism, transport, and synthesis. In this article, we will review the main transcriptional mechanisms regulating the expression of genes involved in hepatic lipid metabolism. The principal regulatory pathways are composed of simple modules of transcription factor crosstalks, which correspond to building blocks of more complex regulatory networks. These transcriptional networks contribute to the regulation of proper lipid homeostasis in parallel to posttranslational mechanisms and end product-mediated modulation of lipid metabolizing enzymes. This article is part of a Special Issue entitled Linking transcription to physiology in lipodomics.

Human FXR regulates SHP expression through direct binding to an LRH-1 binding site, independent of an IR-1 and LRH-1

Background

Farnesoid X receptor/retinoid X receptor-alpha (FXR/RXRα) is the master transcriptional regulator of bile salt synthesis and transport in liver and intestine. FXR is activated by bile acids, RXRα by the vitamin A–derivative 9-cis retinoic acid (9cRA). Remarkably, 9cRA inhibits binding of FXR/RXRα to its response element, an inverted repeat-1 (IR-1). Still, most FXR/RXRα target genes are maximally expressed in the presence of both ligands, including the small heterodimer partner (SHP). Here, we revisited the FXR/RXRα-mediated regulation of human SHP.

Methods

A 579-bp hSHP promoter element was analyzed to locate FXR/chenodeoxycholic acid (CDCA)- and RXRα/9cRA-responsive elements. hSHP promoter constructs were analyzed in FXR/RXRα-transfected DLD-1, HEK293 and HepG2 cells exposed to CDCA, GW4064 (synthetic FXR ligand) and/or 9cRA. FXR-DNA interactions were analyzed by in vitro pull down assays.

Results

hSHP promoter elements lacking the previously identified IR-1 (−291/−279) largely maintained their activation by FXR/CDCA, but were unresponsive to 9cRA. FXR-mediated activation of the hSHP promoter was primarily dependent on the −122/−69 region. Pull down assays revealed a direct binding of FXR to the −122/−69 sequence, which was abrogated by site-specific mutations in a binding site for the liver receptor homolog-1 (LRH-1) at −78/−70. These mutations strongly impaired the FXR/CDCA-mediated activation, even in the context of a hSHP promoter containing the IR-1. LRH-1 did not increase FXR/RXRα-mediated activation of hSHP promoter activity.

Conclusion

FXR/CDCA-activated expression of SHP is primarily mediated through direct binding to an LRH-1 binding site, which is not modulated by LRH-1 and unresponsive to 9cRA. 9cRA-induced expression of SHP requires the IR-1 that overlaps with a direct repeat-2 (DR-2) and DR-4. This establishes for the first time a co-stimulatory, but independent, action of FXR and RXRα agonists.

Positive regulation of osteogenesis by bile acid through FXR

Farnesoid X receptor (FXR) is a nuclear receptor that functions as a bile acid sensor controlling bile acid homeostasis. We investigated the role of FXR in regulating bone metabolism. We identified the expression of FXR in calvaria and bone marrow cells, which gradually increased during osteoblastic differentiation in vitro. In male mice, deletion of FXR (FXR(-/-) ) in vivo resulted in a significant reduction in bone mineral density by 4.3% to 6.6% in mice 8 to 20 weeks of age compared with FXR(+/+) mice. Histological analysis of the lumbar spine showed that FXR deficiency reduced the bone formation rate as well as the trabecular bone volume and thickness. Moreover, tartrate-resistant acid phosphatase (TRACP) staining of the femurs revealed that both the osteoclast number and osteoclast surface were significantly increased in FXR(-/-) mice compared with FXR(+/+) mice. At the cellular level, induction of alkaline phosphatase (ALP) activities was blunted in primary calvarial cells in FXR(-/-) mice compared with FXR(+/+) mice in concert with a significant reduction in type I collagen a1(Col1a1), ALP, and runt-related transcription factor 2 (Runx2) gene expressions. Cultures of bone marrow-derived macrophages from FXR(-/-) mice exhibited an increased number of osteoclast formations and protein expression of nuclear factor of activated T cells, cytoplasmic 1 (NFATc1). In female FXR(-/-) mice, although bone mineral density (BMD) was not significantly different from that in FXR(+/+) mice, bone loss was accelerated after an ovariectomy compared with FXR(+/+) mice. In vitro, activation of FXR by bile acids (chenodeoxycholic acid [CDCA] or 6-ECDCA) or FXR agonists (GW4064 or Fexaramine) significantly enhanced osteoblastic differentiation through the upregulation of Runx2 and enhanced extracellular signal-regulated kinase (ERK) and β-catenin signaling. FXR agonists also suppressed osteoclast differentiation from bone marrow macrophages. Finally, administration of a farnesol (FOH 1%) diet marginally prevented ovariectomy (OVX)-induced bone loss and enhanced bone mass gain in growing C57BL/6J mice. Taken together, these results suggest that FXR positively regulates bone metabolism through both arms of the bone remodeling pathways; ie, bone formation and resorption.

Proprotein convertases in high-density lipoprotein metabolism

The proprotein convertase subtilisin/kexins (PCSKs) are a serine endopeptidase family. PCSK members cleave amino acid residues and modulate the activity of precursor proteins. Evidence from patients and animal models carrying genetic alterations in PCSK members show that PCSK members are involved in various metabolic processes. These studies further revealed the molecular mechanism by which genetic alteration of some PCSK members impairs normal molecular and physiological functions, which in turn lead to cardiovascular disease. High-density lipoprotein (HDL) is anti-atherogenic as it removes excessive amount of cholesterol from blood and peripheral tissues. Several PCSK members are involved in HDL metabolism. PCSK3, PCSK5, and PCSK6 process two triglyceride lipase family members, endothelial lipase and lipoprotein lipase, which are important for HDL remodeling. Recent studies in our lab found evidence that PCSK1 and PCSK9 are also involved in HDL metabolism. A mouse model carrying an amino acid substitution in PCSK1 showed an increase in serum apolipoprotein A1 (APOA1) level. Another mouse model lacking PCSK9 showed a decrease in APOE-containing HDL. In this review, we summarize the role of the five PCSK members in lipid, glucose, and bile acid (BA) metabolism, each of which can influence HDL metabolism. We propose an integrative model in which PCSK members regulate HDL metabolism through various molecular mechanisms and metabolic processes and genetic variation in some PCSK members may affect the efficiency of reverse cholesterol transport. PCSK members are considered as attractive therapeutic targets. A greater understanding of the molecular and physiological functions of PCSK members will improve therapeutic strategies and drug efficacy for cardiovascular disease where PCSK members play critical role, with fewer adverse effects.

The role of retinoic acid in hepatic lipid homeostasis defined by genomic binding and transcriptome profiling

Background

The eyes and skin are obvious retinoid target organs. Vitamin A deficiency causes night blindness and retinoids are widely used to treat acne and psoriasis. However, more than 90% of total body retinol is stored in liver stellate cells. In addition, hepatocytes produce the largest amount of retinol binding protein and cellular retinoic acid binding protein to mobilize retinol from the hepatic storage pool and deliver retinol to its receptors, respectively. Furthermore, hepatocytes express the highest amount of retinoid x receptor alpha (RXRα) among all the cell types. Surprisingly, the function of endogenous retinoids in the liver has received very little attention.

Results

Based on the data generated from chromatin immunoprecipitation followed by sequencing, the global DNA binding of transcription factors including retinoid x receptor α (RXRα) along with its partners i.e. retinoic acid receptor α (RARα), pregnane x receptor (PXR), liver x receptor (LXR), farnesoid x receptor (FXR), and peroxisome proliferator-activated receptor α (PPARα) has been established. Based on the binding, functional annotation illustrated the role of those receptors in regulating hepatic lipid homeostasis. To correlate the DNA binding data with gene expression data, the expression patterns of 576 genes that regulate lipid homeostasis were studied in wild type and liver RXRα-null mice treated with and without RA. The data showed that RA treatment and RXRα-deficiency had opposite effects in regulating lipid homeostasis. A subset of genes (114), which could clearly differentiate the effect of ligand treatment and receptor deficiency, were selected for further functional analysis. The expression data suggested that RA treatment could produce unsaturated fatty acids and induce triglyceride breakdown, bile acid secretion, lipolysis, and retinoids elimination. In contrast, RXRα deficiency might induce the synthesis of saturated fatty acids, triglyceride, cholesterol, bile acids, and retinoids. In addition, DNA binding data indicated extensive cross-talk among RARα, PXR, LXR, FXR, and PPARα in regulating those RA/RXRα-dependent gene expression levels. Moreover, RA reduced serum cholesterol, triglyceride, and bile acid levels in mice.

Conclusions

We have characterized the role of hepatic RA for the first time. Hepatic RA mediated through RXRα and its partners regulates lipid homeostasis.

Regulation of human class I alcohol dehydrogenases by bile acids

Class I alcohol dehydrogenases (ADH1s) are the rate-limiting enzymes for ethanol and vitamin A (retinol) metabolism in the liver. Because previous studies have shown that human ADH1 enzymes may participate in bile acid metabolism, we investigated whether the bile acid-activated nuclear receptor farnesoid X receptor (FXR) regulates ADH1 genes. In human hepatocytes, both the endogenous FXR ligand chenodeoxycholic acid and synthetic FXR-specific agonist GW4064 increased ADH1 mRNA, protein, and activity. Moreover, overexpression of a constitutively active form of FXR induced ADH1A and ADH1B expression, whereas silencing of FXR abolished the effects of FXR agonists on ADH1 expression and activity. Transient transfection studies and electrophoretic mobility shift assays revealed functional FXR response elements in the ADH1A and ADH1B proximal promoters, thus indicating that both genes are direct targets of FXR. These findings provide the first evidence for direct connection of bile acid signaling and alcohol metabolism.

Xenobiotic-sensing nuclear receptors involved in drug metabolism: a structural perspective

Xenobiotic compounds undergo a critical range of biotransformations performed by the phase I, II, and III drug-metabolizing enzymes. The oxidation, conjugation, and transportation of potentially harmful xenobiotic and endobiotic compounds achieved by these catalytic systems are significantly regulated, at the gene expression level, by members of the nuclear receptor (NR) family of ligand-modulated transcription factors. Activation of NRs by a variety of endo- and exogenous chemicals are elemental to induction and repression of drug-metabolism pathways. The master xenobiotic sensing NRs, the promiscuous pregnane X receptor and less-promiscuous constitutive androstane receptor are crucial to initial ligand recognition, jump-starting the metabolic process. Other receptors, including farnesoid X receptor, vitamin D receptor, hepatocyte nuclear factor 4 alpha, peroxisome proliferator activated receptor, glucocorticoid receptor, liver X receptor, and RAR-related orphan receptor, are not directly linked to promiscuous xenobiotic binding, but clearly play important roles in the modulation of metabolic gene expression. Crystallographic studies of the ligand-binding domains of nine NRs involved in drug metabolism provide key insights into ligand-based and constitutive activity, coregulator recruitment, and gene regulation. Structures of other, noncanonical transcription factors also shed light on secondary, but important, pathways of control. Pharmacological targeting of some of these nuclear and atypical receptors has been instituted as a means to treat metabolic and developmental disorders and provides a future avenue to be explored for other members of the xenobiotic-sensing NRs.

FXR agonist GW4064 increases plasma glucocorticoid levels in C57BL/6 mice

Since high expression of farnesoid X receptor (FXR) has been detected in glucocorticoid-producing adrenocortical cells, we evaluated the potential role of FXR in adrenal glucocorticoid production. FXR agonist GW4064 increased fasting plasma corticosterone levels (+45%; P<0.01) in C57BL/6 mice, indicative of enhanced adrenal steroidogenesis. GW4064 treatment did not affect plasma ACTH levels, adrenal weight, or adrenal expression of steroidogenic genes. Scavenger receptor BI (SR-BI) mRNA and protein expression, respectively, increased 1.9-fold (P<0.01) and 1.5-fold, which suggests a stimulated lipoprotein-associated cholesterol uptake into the adrenals upon GW4064 treatment. In line with an enhanced flux of cellular cholesterol into the steroidogenic pathway, adrenal unesterified and esterified cholesterol stores were 21-41% decreased (P<0.01) upon GW4064 treatment. In conclusion, we have shown that the FXR agonist GW4064 stimulates plasma corticosterone levels in C57BL/6 mice. Our findings suggest a novel role for FXR in the modulation of adrenal cholesterol metabolism and glucocorticoid synthesis in mice.

Bile acid receptors as targets for the treatment of dyslipidemia and cardiovascular disease

Dyslipidemia is an important risk factor for cardiovascular disease (CVD) and atherosclerosis. When dyslipidemia coincides with other metabolic disorders such as obesity, hypertension, and glucose intolerance, defined as the metabolic syndrome (MS), individuals present an elevated risk to develop type 2 diabetes (T2D) as well as CVD. Because the MS epidemic represents a growing public health problem worldwide, the development of therapies remains a major challenge. Alterations of bile acid pool regulation in T2D have revealed a link between bile acid and metabolic homeostasis. The bile acid receptors farnesoid X receptor (FXR) and TGR5 both regulate lipid, glucose, and energy metabolism, rendering them potential pharmacological targets for MS therapy. This review discusses the mechanisms of metabolic regulation by FXR and TGR5 and the utility relevance of natural and synthetic modulators of FXR and TGR5 activity, including bile acid sequestrants, in the treatment of the MS.

Non-alcoholic Fatty liver disease: the bile Acid-activated farnesoid x receptor as an emerging treatment target

Non-alcoholic fatty liver disease (NAFLD) is currently evolving as the most common liver disease worldwide. It may progress to liver cirrhosis and liver cancer and is poised to represent the most common indication for liver transplantation in the near future. The pathogenesis of NAFLD is multifactorial and not fully understood, but it represents an insulin resistance state characterized by a cluster of cardiovascular risk factors including obesity, dyslipidemia, hyperglycemia, and hypertension. Importantly, NAFLD also has evolved as independent risk factor for cardiovascular disease. Unfortunately thus far no established treatment does exist for NAFLD. The bile acid-activated nuclear farnesoid X receptor (FXR) has been shown to play a role not only in bile acid but also in lipid and glucose homeostasis. Specific targeting of FXR may be an elegant and very effective way to readjust dysregulated nuclear receptor-mediated metabolic pathways. This review discusses the body's complex response to the activation of FXR with its beneficial actions but also potential undesirable side effects.

Hepatic FXR: key regulator of whole-body energy metabolism

The farnesoid X receptor (FXR) is a nuclear receptor whose activation leads to alterations in pathways involved in energy metabolism. For example, it serves as a bile acid receptor in tissues such as the liver, and as an energy metabolism regulator in liver, muscle and adipose tissue. However, the effects of FXR activation are not exclusive to the tissue where it is present, because receptor crosstalk affects tissues throughout the body. It has been demonstrated that FXR regulates the metabolism of not just bile acids, but also of fats and hydrocarbon metabolites. FXR is currently under study as a therapeutic target for the treatment of diseases of excess, such as diabetes. Here we review the effects of FXR activation in the response of an organism to excess energy.

Lipid-sensing nuclear receptors in the pathophysiology and treatment of the metabolic syndrome

Metabolic syndrome (MS) is a cluster of different diseases, namely central obesity, hypertension, hyperglycemia, and dyslipidemia, together with a pro-thrombotic and pro-inflammatory state. These metabolic abnormalities are often associated with an increased risk for cardiovascular disease (CVD) and cancer. Dietary and lifestyle modifications are currently believed more effective than pharmacological therapies in the management of MS patients. Nevertheless, the relatively low grade of compliance of patients to these recommendations, as well as the failure of current therapies, highlights the need for the discovery of new pharmacological and nutraceutic approaches. A deeper knowledge of the patho-physiological events that initiate and support the MS is mandatory. Lipid-sensing nuclear receptors (NRs) are the master transcriptional regulators of lipid and carbohydrate metabolism and inflammatory responses, thus standing as suitable targets. This review focuses on the physiological relevance of the NRs (peroxisome proliferator-activated receptors, liver X receptors, and farnesoid X receptor) in the control of whole-body homeostasis, with a special emphasis on lipid and glucose metabolism, and on the relationships between metabolic unbalances, systemic inflammation, and the onset of CVD. Future perspectives and possible clinical applications are also presented.