Farnesoid X receptor represses hepatic lipase gene expression

microRNA-27b regulates hepatic lipase enzyme LIPC and reduces triglyceride degradation during hepatitis C virus infection

miRNAs are short, noncoding RNAs that negatively and specifically regulate protein expression, the cumulative effects of which can result in broad changes to cell systems and architecture. The miRNA miR-27b is known to regulate lipid regulatory pathways in the human liver and is also induced by the hepatitis C virus (HCV). However, the functional targets of miR-27b are not well established. Herein, an activity-based protein profiling method using a serine hydrolase probe, coupled with stable isotope labeling and mass spectrometry identified direct and indirect targets of miR-27b. The hepatic lipase C (LIPC) stood out as both highly dependent on miR-27b and as a major modulator of lipid pathway misregulation. Modulation of miR-27b using both exogenous miRNA mimics and inhibitors demonstrated that transcription factors Jun, PPARα, and HNF4α, all of which also influence LIPC levels and activity, are regulated by miR-27b. LIPC was furthermore shown to affect the progress of the life cycle of HCV and to decrease levels of intracellular triglycerides, upon which HCV is known to depend. In summary, this work has demonstrated that miR-27b mediates HCV infection by downregulating LIPC, thereby reducing triglyceride degradation, which in turn increases cellular lipid levels.

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.

Farnesoid X receptor regulates the growth of renal adenocarcinoma cells without affecting that of a normal renal cell-derived cell line

The farnesoid X receptor (FXR) is a bile acid-activated nuclear receptor which is abundant in the liver, intestine, and kidney. FXR is a pivotal factor in cholesterol/bile acid homeostasis but is involved in the growth of hepatocellular carcinoma cells. In the present study, we investigated whether FXR is also involved in the growth of renal adenocarcinoma cells. The cell growth of renal adenocarcinoma cell line ACHN was inhibited by FXR knockdown and stimulated by FXR ligand, while that of a normal renal cell-derived cell line, HK-2, was not affected. The carcinoma-specific stimulation of cell growth by FXR was found to arise from down-regulation of p53 and p21/Cip1 mRNA expression. Our study showed that FXR stimulates proliferation of renal adenocarcinoma cells and that FXR knockdown is useful for growth suppression of renal adenocarcinoma without cytotoxicity to normal renal cells.

Activation of FXR protects against renal fibrosis via suppressing Smad3 expression

Renal fibrosis is the common pathway of most chronic kidney disease progression to end-stage renal failure. The nuclear receptor FXR (farnesoid X receptor), a multiple functional transcription factor, plays an important role in protecting against fibrosis. The TGFβ-Smad signaling has a central role in kidney fibrosis. However, it remains unclear whether FXR plays direct anti-fibrotic effect in renal fibrosis via regulating TGFβ-Smad pathway. In this study, we found that the level of FXR was negatively correlated with that of Smad3 and fibronectin (a marker of fibrosis) in human fibrotic kidneys. Activation of FXR suppressed kidney fibrosis and downregulated Smad3 expression, which was markedly attenuated by FXR antagonist. Moreover, the FXR-mediated repression of fibrosis was significantly alleviated by ectopic expression of Smad3. Luciferase reporter assay revealed that FXR activation inhibited the transcriptional activity of Smad3 gene promoter. The in vivo experiments showed that FXR agonist protected against renal fibrosis and downregulated Smad3 expression in UUO mice. These results suggested that FXR may serve as an important negative regulator for manipulating Smad3 expression, and the FXR/Smad3 pathway may be a novel target for the treatment of renal fibrosis.

Farnesoid X receptor knockdown provides significant growth inhibition in hepatocellular carcinoma cells while it does not interfere with the proliferation of primary human hepatocyte-derived cells

Identification of substances with specific toxicity for carcinoma cells promises to facilitate the development of cancer chemotherapeutics that cause minimal side effects. Here, we show that knockdown of the farnesoid X receptor (FXR) effectively suppresses the proliferation of human hepatocellular carcinoma cell lines HepG2 and HLE accompanied by elevated expression of cyclin-dependent kinase (CDK) inhibitor p16/INK4a and p21/Cip1 proteins. On the other hand, the growth of the primary human hepatocyte-derived cell line Fa2N-4 is not affected by the treatment with FXR siRNA irrespective of marked increases in the mRNAs of p16/INK4a and p21/Cip1. Surprisingly, the expression levels of p16/INK and p21/Cip1 proteins are left unchanged in Fa2N-4 cells that are subjected to the FXR siRNA treatment. Since the expression levels of these CDK inhibitor proteins in FXR-knockdown Fa2N-4 cells were elevated in the presence of proteasomal inhibitor MG132, these CDK inhibitors may be subjected to the proteasomal degradation, thereby counteracting the increased expression of their cognate mRNAs, therefore similar levels of p16 and p21 proteins were observed in control and FXR-knockdown Fa2N-4 cells. These results suggest that FXR-knockdown is effective for inhibiting the proliferation of hepatocellular carcinoma cells, not interfering with the regulatory mechanism of normal hepatocyte growth.

Management of NAFLD: a stage-based approach

NAFLD is the most prevalent form of liver disease in the USA, affecting an estimated 30% of the population. The condition is associated with increased mortality related to cardiovascular disease, malignancy and liver disease. Identification of patients who might be at increased risk of adverse outcomes is critical as it is not feasible to screen all patients with suspected NAFLD. Patients with NASH, the progressive subtype of NAFLD, should be targeted for treatment, especially if they have concomitant fibrosis because such patients are more likely than those without fibrosis to have adverse outcomes. Treatment goals in patients with NAFLD vary depending on the disease stage owing to differential risk of progression and the particularities of an individual's comorbid disease. Lifestyle intervention is important for all patients irrespective of disease stage, but other therapies should be targeted to those most likely to benefit. In this Review, we highlight risk factors for disease progression and offer a stage-based treatment approach for patients with NAFLD.

Effect of the particle size of cellulose from sweet potato residues on lipid metabolism and cecal conditions in ovariectomized rats

This study aims to examine the effect of the particle size of cellulose from sweet potato residues on lipid metabolism and cecal conditions in ovariectomized rats. Forty mature female Wistar rats were divided into five groups. The sham-operated group was used as the sham control. The other four groups were double-ovariectomized and assigned to the model, ordinary cellulose (100 g kg(-1) diet), microcrystalline cellulose (100 g kg(-1) diet), and cellulose nanocrystal (100 g kg(-1) diet) groups. As the cellulose particle size decreased, the body weight gain and food intake were decreased. The plasma lipids and hepatic lipids were decreased. In addition, the mRNA levels of cholesterol 7α-hydroxylase, farnesoid X receptor, and 3-hydroxy-3-methylglutaryl coenzyme A reductase were decreased, whereas those of ileal apical sodium-dependent bile acid transporter and intestinal bile acid binding protein were increased. The cecum weight, cecum content, and short-chain fatty acid concentration and the amount of total bile acids in the small intestinal content, as well as the bile acids and neutral steroids in fecal excretion, were increased. These results indicate that as the particle size decreased, cellulose was more effective in preventing ovarian hormone deficiency-induced hyperlipidemia and in improving intestinal health.

Bacterial metabolites directly modulate farnesoid X receptor activity

BACKGROUND

The farnesoid X receptor (FXR), a ligand-activated transcription factor belonging to the adopted orphan receptor, plays an important role in maintaining health of the liver and intestine. In this study, we identified individual bacterial strains that directly modulated the activation of intestinal FXR.

METHODS

The FXR stimulatory potential of 38 bacterial strains was determined using a stable FXR reporter system derived from intestinal epithelial cells (IEC). The induction of FXR target genes by screened FXR stimulatory bacteria was determined by real-time PCR. In addition, a high fat diet (HFD)-induced obese mouse model was used to evaluate in vivo FXR stimulatory potential of bacterial metabolites screened in this study.

RESULTS

A luciferase assay with the FXR reporter cell line demonstrated that the FXR-stimulatory activity of most bacterial cell samples was less than 2-fold. The culture supernatants of Bacteroides dorei and Eubacterium limosum induced FXR activity and selectively regulated FXR target expression in the FXR reporter system. Treatment with B. dorei-derived metabolites strongly induced ileal bile acid binding protein (IBABP) (8.4-fold) and organic solute transporter (OST) α (3.1-fold) compared with E. limosum-derived metabolites. Furthermore, administration of B. dorei derived metabolites showed significant reduction in body weight gain, and both two bacterial metabolites reduced liver weight in obese mice compared to PBS-treated controls. Administration of each bacterial metabolites improved in serum levels of obesity-related metabolic biochemical markers such as ALT, AST, total cholesterol, and triglyceride. Furthermore, two bacterial metabolites enhanced the Fxr gene expression in the intestine and liver, and ileal Shp gene expression tended to be increased by treatment with the metabolites derived from B. dorei.

CONCLUSIONS

B. dorei and E. limosum secreted the bioactive substances that directly stimulate FXR in the intestinal epithelial cells. Administration of these bacterial FXR-stimulatory metabolites improves the obesity phenotype including body weight gain, liver damage, lipid metabolism in DIO mice.

The role of chicken ovalbumin upstream promoter transcription factor II in the regulation of hepatic fatty acid oxidation and gluconeogenesis in newborn mice

Chicken ovalbumin upstream promoter transcription factor II (COUP-TFII) is an orphan nuclear receptor involved in the control of numerous functions in various organs (organogenesis, differentiation, metabolic homeostasis, etc.). The aim of the present work was to characterize the regulation and contribution of COUP-TFII in the control of hepatic fatty acid and glucose metabolisms in newborn mice. Our data show that postnatal increase in COUP-TFII mRNA levels is enhanced by glucagon (via cAMP) and PPARα. To characterize COUP-TFII function in the liver of suckling mice, we used a functional (dominant negative form; COUP-TFII-DN) and a genetic (shRNA) approach. Adenoviral COUP-TFII-DN injection induces a profound hypoglycemia due to the inhibition of gluconeogenesis and fatty acid oxidation secondarily to reduced PEPCK, Gl-6-Pase, CPT I, and mHMG-CoA synthase gene expression. Using the crossover plot technique, we show that gluconeogenesis is inhibited at two different levels: 1) pyruvate carboxylation and 2) trioses phosphate synthesis. This could result from a decreased availability in fatty acid oxidation arising cofactors such as acetyl-CoA and reduced equivalents. Similar results are observed using the shRNA approach. Indeed, when fatty acid oxidation is rescued in response to Wy-14643-induced PPARα target genes (CPT I and mHMG-CoA synthase), blood glucose is normalized in COUP-TFII-DN mice. In conclusion, this work demonstrates that postnatal increase in hepatic COUP-TFII gene expression is involved in the regulation of liver fatty acid oxidation, which in turn sustains an active hepatic gluconeogenesis that is essential to maintain an appropriate blood glucose level required for newborn mice survival.

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.

Bile acids reduce endocytosis of high-density lipoprotein (HDL) in HepG2 cells

High-density lipoprotein (HDL) transports lipids to hepatic cells and the majority of HDL-associated cholesterol is destined for biliary excretion. Cholesterol is excreted into the bile directly or after conversion to bile acids, which are also present in the plasma as they are effectively reabsorbed through the enterohepatic cycle. Here, we provide evidence that bile acids affect HDL endocytosis. Using fluorescent and radiolabeled HDL, we show that HDL endocytosis was reduced in the presence of high concentrations of taurocholate, a natural non-cell-permeable bile acid, in human hepatic HepG2 and HuH7 cells. In contrast, selective cholesteryl-ester (CE) uptake was increased. Taurocholate exerted these effects extracellularly and independently of HDL modification, cell membrane perturbation or blocking of endocytic trafficking. Instead, this reduction of endocytosis and increase in selective uptake was dependent on SR-BI. In addition, cell-permeable bile acids reduced HDL endocytosis by farnesoid X receptor (FXR) activation: chenodeoxycholate and the non-steroidal FXR agonist GW4064 reduced HDL endocytosis, whereas selective CE uptake was unaltered. Reduced HDL endocytosis by FXR activation was independent of SR-BI and was likely mediated by impaired expression of the scavenger receptor cluster of differentiation 36 (CD36). Taken together we have shown that bile acids reduce HDL endocytosis by transcriptional and non-transcriptional mechanisms. Further, we suggest that HDL endocytosis and selective lipid uptake are not necessarily tightly linked to each other.

Effects of a farnesoid X receptor antagonist on hepatic lipid metabolism in primates

We aimed to elucidate the mechanism underlying the anti-dyslipidemic effect of compound-T3, a farnesoid X receptor antagonist, by investigating its effects on hepatic lipid metabolism in non-human primates. We administered lipid-lowering drugs for 7 days to cynomolgus monkeys receiving a high-fat diet, and subsequently measured the levels of lipid parameters in plasma, feces, and hepatic tissue fluids. Compound-T3 (0.3 and 3mg/kg p.o.) significantly decreased the plasma levels of non-high-density lipoprotein (non-HDL) cholesterol and apolipoprotein B in a dose-dependent manner. It also decreased the mRNA levels of hepatic small heterodimer partner-1, induced the mRNA expression of hepatic cholesterol 7α-hydroxylase, reduced hepatic cholesterol and triglyceride levels, increased fecal bile acid excretion, and upregulated the expression of hepatic low-density lipoprotein (LDL) receptor. Furthermore, compound-T3 significantly increased plasma HDL cholesterol and apolipoprotein A-I levels. The mRNA expression levels of hepatic apolipoprotein A-I tended to increase after compound-T3 treatment. Compound-T3 also induced accumulation of hepatic bile acids and decreased the mRNA expression levels of the hepatic bile acid export pump. The effects of cholestyramine (300mg/kg p.o.) on the plasma and hepatic lipid parameters were similar to those of compound-T3, and it increased fecal bile acid levels without causing accumulation of hepatic bile acids. These findings suggest that LDL receptor-mediated hepatic LDL incorporation due to cholesterol catabolism catalyzed by cholesterol 7α-hydroxylase decreases plasma non-HDL cholesterol levels. Upregulation of hepatic apolipoprotein A-I mRNA expression may partially contribute to the increase in HDL cholesterol levels mediated by compound-T3.

Peptides identified in soybean protein increase plasma cholesterol in mice on hypercholesterolemic diets

The in vitro micellar cholesterol displacement assay has been used to identify peptides that may potentially reduce cholesterol in vivo. Two of these peptides, LPYPR and WGAPSL, derived from soybean protein (SP) that have been reported to displace cholesterol from micelles were tested by feeding them as a part of a hypercholesterolemic diet to mice for 3 weeks. Except reduction of very low-density lipoprotein cholesterol (VLDL-C) and triglyceride contents, the peptide-containing diets increased plasma cholesterol content with the increasing dose of the peptides. Mice fed diets supplemented with the peptides also had lower fecal bile acid excretion. Negative correlations between fecal bile acid excretion and plasma total cholesterol content (r = -0.876, P = 0.062) and non-HDL-C content (r = -0.831, P = 0.084) were observed. The mRNA levels of the genes for cholesterol and bile acid metabolism, CYP51, LDLR, CYP7A1, and LPL, were up-regulated in mice fed diets supplemented with peptides except the group fed the low dose of WGAPSL. The results suggested that higher plasma total cholesterol content possibly due to lower fecal steroid excretion as well as lower VLDL-C and triglyceride contents might due to the up-regulated expression levels of the genes CYP51, LDLR, and LPL.

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.

Critical role of farnesoid X receptor for hepatocellular carcinoma cell proliferation

Farnesoid X receptor (FXR), a pivotal factor maintaining bile acid homeostasis, has been recently shown to be a critical factor required for liver regeneration. The elucidation of the mechanism how FXR controls the proliferation of hepatocellular carcinoma cells is useful to establish the therapy for liver cancer. Here, we show that FXR plays a crucial role in the proliferation of human hepatocellular carcinoma cell line, HepG2, Huh7 and HLE. The treatment of HepG2 with FXR siRNA elevates the level of p16/INK4a expression resulting in the inhibition of cell proliferation. By contrast, FXR activation reduces p16/INK4a expression and stimulates the cell proliferation. The ectopic expression of the active form of Ras that causes strong activation of extracellular signal-regulated kinase (ERK) leads to the decrease in FXR expression, suggesting that FXR expression is negatively regulated via Ras/ERK pathway. The elevation of p16/INK4a expression and the inhibition of cell proliferation by FXR knockdown are also observed in Huh7 and HLE. In this study, we have suggested a novel mechanism by which hepatocellular carcinoma cell proliferation is regulated: FXR stimulates cell proliferation by suppressing the p16/INK4a expression, whereas Ras/ERK pathway down-regulates the FXR expression, leading to the suppressed cell proliferation in hepatocellular carcinoma cell lines.

Farnesoid X receptor: from medicinal chemistry to clinical applications

The farnesoid X receptor (FXR) is a bile sensor that acts in coordination with other nuclear receptors to regulate essential steps in bile acid uptake, metabolism and excretion. In addition, FXR is an ancillary receptor involved in lipid and glucose homeostasis. Steroidal and non-steroidal FXR ligands are currently available. Both groups have shown limitations in the preclinical studies regarding absorption, metabolism, specificity of target and intrinsic toxicity. FXR ligands endowed with agonistic activity are under development for the treatment of cholestatic liver diseases, including primary biliary cirrhosis and metabolic disorders linked to insulin resistance. Despite the fact that results from preclinical models are encouraging, targeting FXR holds potential for side effects (i.e., impaired cholesterol disposal and cholestasis). Thus, results from FXR gene-ablated mice and mice administered an FXR antagonist support a role for FXR antagonists or modulators (i.e., FXR agonists that selectively activate specific subsets of FXR target genes in a tissue) or co-regulator-specific manner.

Lipoprotein distribution and serum concentrations of 7α-hydroxy-4-cholesten-3-one and bile acids: effects of monogenic disturbances in high-density lipoprotein metabolism

BA (bile acid) formation is considered an important final step in RCT (reverse cholesterol transport). HDL (high-density lipoprotein) has been reported to transport BAs. We therefore investigated the effects of monogenic disturbances in human HDL metabolism on serum concentrations and lipoprotein distributions of the major 15 BA species and their precursor C4 (7α-hydroxy-4-cholesten-3-one). In normolipidaemic plasma, approximately 84%, 11% and 5% of BAs were recovered in the LPDS (lipoprotein-depleted serum), HDL and the combined LDL (low-density lipoprotein)/VLDL (very-low-density lipoproteins) fraction respectively. Conjugated BAs were slightly over-represented in HDL. For C4, the respective percentages were 23%, 21% and 56% (41% in LDL and 15% in VLDL) respectively. Compared with unaffected family members, neither HDL-C (HDL-cholesterol)-decreasing mutations in the genes APOA1 [encoding ApoA-I (apolipoprotein A-I], ABCA1 (ATP-binding cassette transporter A1) or LCAT (lecithin:cholesterol acyltransferase) nor HDL-C-increasing mutations in the genes CETP (cholesteryl ester transfer protein) or LIPC (hepatic lipase) were associated with significantly different serum concentrations of BA and C4. Plasma concentrations of conjugated and secondary BAs differed between heterozygous carriers of SCARB1 (scavenger receptor class B1) mutations and unaffected individuals (P<0.05), but this difference was not significant after correction for multiple testing. Moreover, no differences in the lipoprotein distribution of BAs in the LPDS and HDL fractions from SCARB1 heterozygotes were observed. In conclusion, despite significant recoveries of BAs and C4 in HDL and despite the metabolic relationships between RCT and BA formation, monogenic disorders of HDL metabolism do not lead to altered serum concentrations of BAs and C4.

Ursodeoxycholic acid treatment of hepatic steatosis: a (13)C NMR metabolic study

Ursodeoxycholic acid (UDCA) is commonly used for the treatment of hepatobiliary disorders. In this study, we tested whether a 4-week treatment with this bile acid (12-15 mg/kg/day) could improve hepatic fatty acid oxidation in obese Zucker rats - a model for nonalcoholic fatty liver disease and steatosis. After 24 h of fasting, livers were perfused with physiological concentrations of [U-(13) C]nonesterified fatty acids and [3-(13) C]lactate/[3-(13) C]pyruvate. Steatosis was associated with abundant intracellular glucose, lactate, alanine and methionine, and low concentrations of choline and betaine. Steatotic livers also showed the highest output of glucose and lactate. Glucose and glycolytic products were mostly unlabeled, indicating active glycogenolysis and glycolysis after 24 h of fasting. UDCA treatment resulted in a general amelioration of liver metabolic abnormalities with a decrease in intracellular glucose and lactate, as well as their output. Hepatic betaine and methionine were also normalized after UDCA treatment, suggesting the amelioration of anti-oxidative defenses. Choline levels were not affected by the bile acid, which may indicate a deficient synthesis of very-low-density lipoproteins. The percentage contribution of [U-(13) C]nonesterified fatty acids to acetyl-coenzyme A entering the tricarboxylic acid (TCA) cycle was significantly lower in livers from Zucker obese rats relative to control rats: 23.1 ± 4.9% versus 44.1 ± 2.7% (p < 0.01). UDCA treatment did not alter significantly fatty acid oxidation in control rats, but improved significantly oxidation in Zucker obese rats to 46.0 ± 6.1% (p > 0.05), comparable with control group values. The TCA cycle activity subsequent to fatty acid oxidation was reduced in steatotic livers and improved when UDCA was administered (0.24 ± 0.04 versus 0.37 ± 0.05, p = 0.05). We further suggest that the mechanism of action of UDCA is either related to the activity of the farnesoid receptor, or to the amelioration of the anti-oxidative defenses and cell nicotinamide adenine dinucleotide (NAD(+) /NADH) ratio, favoring TCA cycle activity and β-oxidation.

Therapies targeting exogenous cholesterol uptake: new insights and controversies

Exogenous cholesterol uptake involves a complex process in the intestines for the absorption of cholesterol and bile acids. This process is regulated by intestinal nuclear transcription factors such as LXR that affect sterol transporters NPC1L1, ABCG5/G8, and ABCG1, and enzymes such as ACAT-2. Plant sterol/stanols, ezetimibe, and bile acid sequestrants have a variety of effects on these various transporters, and new insights into their mechanism(s) of action have provided a plethora of exciting targets for metabolic diseases, dyslipidemia, and atherosclerosis.

Lipoprotein lipase and hepatic triglyceride lipase reduce the infectivity of hepatitis C virus (HCV) through their catalytic activities on HCV-associated lipoproteins

The effect of lipolysis by lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) on hepatitis C virus (HCV) infection was evaluated. First, medium from HuH7.5 cells bearing HCV genome replication was treated with LPL. LPL treatment led to reduced HCV infectivity, shifted HCV to higher densities, and lowered the amount of apolipoprotein E-associated HCV. The effect of endogenous HTGL secreted from HuH7.5 on HCV infectivity was next examined. Neutralization of HTGL by an anti-HTGL antibody resulted in suppression of LPL-induced reduction in infectivity of HCV-bearing medium, while knockdown of HTGL by siRNA led to increased HCV infectivity irrespective of LPL. HCV in medium from HTGL knockdown cells was found in fractions with a lower density. These results indicate that changes in the nature of HCV-associated lipoproteins by LPL and/or HTGL affect HCV infectivity, suggesting that association of HCV with specific lipoproteins is important for HCV infectivity.

Effects of farnesoid X receptor on the expression of the fatty acid synthetase and hepatic lipase

The farnesoid X receptor (FXR) is a nuclear receptor that regulates gene expression in response to bile acids (BAs). FXR plays an important role in the homeostasis of bile acid, cholesterol, lipoprotein and triglyceride. In this report, we identified fatty acid synthase (FAS) and hepatic lipase (HL) genes as novel target genes of FXR. Human hepatoma HepG2 cells were treated with chenodeoxycholic acid, the natural FXR ligand, and the messenger RNA and protein levels of FAS and HL were determined by RT-PCR and Western blot analysis, respectively. Chenodeoxycholic acid (CDCA) down-regulated the expression of FAS and HL genes in a dose and time-dependent manner in human hepatoma HepG2 cells. In addition, treatment of mice with CDCA significantly decreased the expression of FAS and HL in mouse liver and the activity of HL. These results demonstrated that FAS and HL might be FXR-regulated genes in liver cells. In view of the role of FAS and HL in lipogenesis and plasma lipoprotein metabolism, our results further support the central role of FXR in the homeostasis of fatty acid and lipid.

Improving glycemic and cholesterol control through an integrated approach incorporating colesevelam - a clinical perspective

Bile sequestrants have been used for almost 50 years to lower low density lipoprotein cholesterol (LDL-C). The advent of colesevelam in 2000 provided a more tolerable add-on LDL-C-lowering agent with an excellent safety record and with likely benefit for coronary heart disease events. Colesevelam lowers LDL-C approximately 15%, and has an additive effect when combined with statin or non-statin lipid-modifying agents. It also tends to increase triglyceride levels. The discovery that bile sequestrants also lower glucose levels led to definitive large-scale clinical trials testing the effect of colesevelam as a dual antihyperglycemic agent with LDL-C-lowering properties in type 2 diabetic subjects on metformin-, sulfonylurea- or insulin-based therapy with inadequate glycemic control. Colesevelam was found to lower hemoglobin A1c (HbA1c) by approximately 0.5% compared to placebo over the 16- to 26-week period, and had similar effects on the lipid profile in these diabetic subjects, as had previously been demonstrated in non-diabetic individuals. Colesevelam was well tolerated, with constipation being the most common adverse effect, and did not cause weight gain or excessive hypoglycemia. Colesevelam thus combines antihyperglycemic action with LDL-C-lowering properties, and should be useful in the management of type 2 diabetes.

Novel therapies for increasing serum levels of HDL

The protectiveness of elevated HDL-C against CHD and its long-term sequelae is a subject of intense investigation throughout the world. HDL has the capacity to modulate a large number of atherogenic mechanisms, such as inflammation, oxidation, thrombosis, and cell proliferation. Among lipoproteins, HDL is also unique, in that it promotes the mobilization and clearance of excess lipid via the series of reactions collectively termed "reverse cholesterol transport." Numerous therapeutic agents are being developed in an attempt to modulate serum levels of HDL-C as well as its functionality. This article discusses the development of newer treatments targeted at raising HDL-C and HDL particle numbers to reduce residual risk in patients at risk for CHD.

The farnesoid X receptor regulates transcription of 3beta-hydroxysteroid dehydrogenase type 2 in human adrenal cells

Recent studies have shown that the adrenal cortex expresses high levels of farnesoid X receptor (FXR), but its function remains unknown. Herein, using microarray technology, we tried to identify candidate FXR targeting genes in the adrenal glands, and showed that FXR regulated 3beta-hydroxysteroid dehydrogenase type 2 (HSD3B2) expression in human adrenocortical cells. We further demonstrated that FXR stimulated HSD3B2 promoter activity and have defined the cis-element responsible for FXR regulation of HSD3B2 transcription. Transfection of H295R adrenocortical cells with FXR expression vector effectively increased FXR expression levels and additional treatment with chenodeoxycholic acid (CDCA) caused a 25-fold increase in the mRNA for organic solute transporter alpha (OSTalpha), a known FXR target gene. HSD3B2 mRNA levels also increased following CDCA treatment in a concentration-dependent manner. Cells transfected with a HSD3B2 promoter construct and FXR expression vector responded to CDCA with a 20-fold increase in reporter activity compared to control. Analysis of constructs containing sequential deletions of the HSD3B2 promoter suggested a putative regulatory element between -166 and -101. Mutation of an inverted repeat between -137 and -124 completely blocked CDCA/FXR induced reporter activity. Chromatin immunoprecipitation assays further confirmed the presence of a FXR response element in the HSD3B2 promoter. In view of the emerging role of FXR agonists as therapeutic treatment of diabetes and certain liver diseases, the effects of such agonists on other FXR expressing tissues should be considered. Our findings suggest that in human adrenal cells, FXR increases transcription and expression of HSD3B2. Alterations in this enzyme would influence the capacity of the adrenal gland to produce corticosteroids.

Role of bile acids and bile acid receptors in metabolic regulation

The incidence of the metabolic syndrome has taken epidemic proportions in the past decades, contributing to an increased risk of cardiovascular disease and diabetes. The metabolic syndrome can be defined as a cluster of cardiovascular disease risk factors including visceral obesity, insulin resistance, dyslipidemia, increased blood pressure, and hypercoagulability. The farnesoid X receptor (FXR) belongs to the superfamily of ligand-activated nuclear receptor transcription factors. FXR is activated by bile acids, and FXR-deficient (FXR(-/-)) mice display elevated serum levels of triglycerides and high-density lipoprotein cholesterol, demonstrating a critical role of FXR in lipid metabolism. In an opposite manner, activation of FXR by bile acids (BAs) or nonsteroidal synthetic FXR agonists lowers plasma triglycerides by a mechanism that may involve the repression of hepatic SREBP-1c expression and/or the modulation of glucose-induced lipogenic genes. A cross-talk between BA and glucose metabolism was recently identified, implicating both FXR-dependent and FXR-independent pathways. The first indication for a potential role of FXR in diabetes came from the observation that hepatic FXR expression is reduced in animal models of diabetes. While FXR(-/-) mice display both impaired glucose tolerance and decreased insulin sensitivity, activation of FXR improves hyperglycemia and dyslipidemia in vivo in diabetic mice. Finally, a recent report also indicates that BA may regulate energy expenditure in a FXR-independent manner in mice, via activation of the G protein-coupled receptor TGR5. Taken together, these findings suggest that modulation of FXR activity and BA metabolism may open new attractive pharmacological approaches for the treatment of the metabolic syndrome and type 2 diabetes.

Activation of the farnesoid X receptor represses PCSK9 expression in human hepatocytes

The purpose of this study was to determine whether bile acids (BAs) modulate hepatic pro-protein convertase subtilisin/kexin 9 (PCSK9) gene expression. Immortalized human hepatocytes were treated with various BAs. Chenodeoxycholic acid (CDCA) treatment specifically decreased both PCSK9 mRNA and protein contents. Moreover, activation of the BA-activated farnesoid X receptor (FXR) by its synthetic specific agonist GW4064 also decreased PCSK9 expression. Of functional relevance, coadministration of CDCA counteracted the statin-induced PCSK9 expression, leading to a potentiation of LDL receptor activity. This study suggests that a transcriptional repression of PCSK9 by CDCA or FXR agonists may potentiate the hypolipidemic effect of statins.

The farnesoid X receptor induces fetuin-B gene expression in human hepatocytes

FXR (farnesoid X receptor), a nuclear receptor activated by BAs (bile acids), is a key factor in the regulation of BA, lipid and carbohydrate metabolism. The recent development of synthetic FXR agonists and knockout mouse models has accelerated the discovery of FXR target genes. In the present study, we identify human fetuin-B as a novel FXR target gene. Treatment with FXR agonists increased fetuin-B expression in human primary hepatocytes and in the human hepatoma HepG2 cell line. In contrast, fetuin-B expression was not responsive to FXR agonist treatment in murine primary hepatocytes. Fetuin-B induction by FXR agonist was abolished upon FXR knockdown by siRNA (small interfering RNA). In addition to the previously described P1 promoter, we show that the human fetuin-B gene is also transcribed from an alternative promoter, termed P2. Transcription via the P2 promoter was induced by FXR agonist treatment, whereas P1 promoter activity was not sensitive to FXR agonist treatment. Two putative FXR-response elements [IR-1 (inverted repeat-1)] were identified in the region -1.6 kb upstream of the predicted P2 transcriptional start site. Both motifs bound FXR-RXR (retinoid X receptor) complexes in vitro and were activated by FXR in transient transfection reporter assays. Mutations in the IR-1 sites abolished FXR-RXR binding and activation. Taken together, these results identify human fetuin-B as a new FXR target gene in human hepatocytes.

Molecular characterization of the role of orphan receptor small heterodimer partner in development of fatty liver

The orphan receptor Small Heterodimer Partner (SHP, NROB2) regulates metabolic pathways, including hepatic bile acid, lipid, and glucose homeostasis. We reported that SHP-deletion in leptin-deficient OB(-/-) mice increases insulin sensitivity, and prevents the development of fatty liver. The prevention of steatosis in OB(-/-)/SHP(-/-) double mutants is not due to decreased body weight but is associated with increased hepatic very-low-density lipoprotein (VLDL) secretion and elevated microsomal triglyceride transfer protein (MTP) mRNA and protein levels. SHP represses the transactivation of the MTP promoter and the induction of MTP mRNA by LRH-1 in hepatocytes. Adenoviral overexpression of SHP inhibits MTP activity as well as VLDL-apoB protein secretion, and RNAi knockdown of SHP exhibits opposite effects. The expression of SHP in induced in fatty livers of OB(-/-) mice and other genetic or dietary models of steatosis, and acute overexpression of SHP by adenovirus, result in rapid accumulation of neutral lipids in hepatocytes. In addition, the pathways for hepatic lipid uptake and lipogenic program are also downregulated in OB(-/-)/SHP(-/-) mice, which may contribute to the decreased hepatic lipid content.

CONCLUSION

These studies demonstrate that SHP regulates the development of fatty liver by modulating hepatic lipid export, uptake, and synthesis, and that the improved peripheral insulin sensitivity in OB(-/-)/SHP(-/-) mice is associated with decreased hepatic steatosis.

FXR: a promising target for the metabolic syndrome?

The metabolic syndrome is an insulin-resistant state that is characterized by a cluster of cardiovascular risk factors, including abdominal obesity, hyperglycemia, elevated blood pressure and combined dyslipidemia. In this review, we discuss the role of the bile-acid-activated farnesoid X receptor (FXR) in the modulation of the metabolic syndrome. Owing to its regulatory actions in lipid and glucose homeostasis, FXR is a potential pharmacological target. Moreover, the observation that FXR also influences endothelial function and atherosclerosis indicates a regulatory role in the cardiovascular complications that are associated with the metabolic syndrome. The pharmacological activation of FXR leads to a complex response that integrates beneficial actions and potentially undesirable side-effects. Thus, the identification of selective FXR modulators (selective bile acid receptor modulators) is required for the development of compounds that can be used to treat the metabolic syndrome.

International Union of Pharmacology. LXII. The NR1H and NR1I receptors: constitutive androstane receptor, pregnene X receptor, farnesoid X receptor alpha, farnesoid X receptor beta, liver X receptor alpha, liver X receptor beta, and vitamin D receptor

The nuclear receptors of the NR1H and NR1I subgroups include the constitutive androstane receptor, pregnane X receptor, farnesoid X receptors, liver X receptors, and vitamin D receptor. The newly emerging functions of these related receptors are under the control of metabolic pathways, including metabolism of xenobiotics, bile acids, cholesterol, and calcium. This review summarizes results of structural, pharmacologic, and genetic studies of these receptors.

Bile acids alter the subcellular localization of CNT2 (concentrative nucleoside cotransporter) and increase CNT2-related transport activity in liver parenchymal cells

CNT2 (concentrative nucleoside cotransporter) is a plasma membrane high-affinity Na+-coupled adenosine transporter, also localized in intracellular structures. This transporter protein may play additional roles other than nucleoside salvage, since it has recently been shown to be under purinergic control via K(ATP) channels, by a mechanism that does not seem to involve changes in its subcellular localization. In an attempt to identify the agents that promote CNT2 trafficking, bile acids were found to increase CNT2-related transport activity in a K(ATP) channel-independent manner in both Fao hepatoma and rat liver parenchymal cells. A maximum effect was recorded after treatment with hydrophilic anions such as TCA (taurocholate). However, this effect did not involve changes in the amount of CNT2 protein, it was instead associated with a subcellular redistribution of CNT2, resulting in an accumulation of the transporter at the plasma membrane. This was deduced from subcellular fractionation studies, biotinylation of plasma membrane proteins and subsequent CNT2 detection in streptavidin precipitates and in vivo confocal microscopic analysis of the distribution of a YFP (yellow fluorescent protein)-CNT2 construct. The induction of CNT2 translocation, triggered by TCA, was inhibited by wortmannin, dibutyryl-AMPc, PD98059 and colchicine, thus suggesting the involvement of the PI3K/ERK (phosphoinositide 3-kinase/extracellular-signal related kinase) pathway in microtubule-dependent activation of recombinant CNT2. These are novel effects of bile-acid physiology and provide the first evidence for short-term regulation of CNT2 translocation into and from the plasma membrane.

Transcriptional regulation of the human hepatic lipase (LIPC) gene promoter

Hepatic lipase (HL) plays a key role in the metabolism of plasma lipoproteins, and its level of activity requires tight regulation, given the association of both low and high levels with atherosclerosis and coronary artery disease. However, little is known about the factors responsible for HL expression. Here, we report that the human hepatic lipase gene (LIPC) promoter is regulated by hepatocyte nuclear factor 4alpha (HNF4alpha), peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha), apolipoprotein A-I regulatory protein-1 (ARP-1), and hepatocyte nuclear factor 1alpha (HNF1alpha). Reporter analysis showed that HNF4alpha directly regulates the LIPC promoter via two newly identified direct repeat elements, DR1 and DR4. PGC-1alpha is capable of stimulating the HNF4alpha-dependent transactivation of the LIPC promoter. ARP-1 displaces HNF4alpha from the DR1 site and blocks its ability to activate the LIPC promoter. Induction by HNF1alpha requires the HNF1 binding site and upon cotransfection with HNF4alpha leads to an additive effect. In addition, the in vivo relevance of HNF4alpha in LIPC expression is shown by the ability of the HNF4alpha antagonist Medica 16 to repress endogenous LIPC mRNA expression. Furthermore, disruption of Hnf4alpha in mice prevents the expression of HL mRNA in liver. The overall effect these transcription factors have on HL expression will ultimately depend on the interplay between these various factors and their relative intracellular concentrations.

Beneficial effect of laserpitin, a coumarin compound fromAngelica keiskei, on lipid metabolism in stroke‐prone spontaneously hypertensive rats

Recently, we found that 4-hydroxyderricin, one of the major chalcones in Angelica keiskei extract (an ethyl acetate extract from the yellow liquid of stems), suppressed increases in systolic blood pressure and reduced both serum very low-density lipoprotein levels and liver triglyceride content in stroke-prone spontaneously hypertensive rats (SHRSP). In the present study, we have isolated laserpitin, a characteristic coumarin, from the A. keiskei extract and examined the effect of dietary laserpitin on blood pressure and lipid metabolism in SHRSP. Six-week-old male SHRSP were fed diets containing 0.1% laserpitin for 7 weeks with free access to the diet and water. Bodyweight gain was reduced by dietary laserpitin after 4 weeks through to 7 weeks without any significant change in daily food intake. Serum total cholesterol, phospholipid and apolipoprotein (apo) E levels were significantly increased, which was due to significant increases in cholesterol, phospholipid and apoE contents in the low- and high-density lipoprotein (LDL and HDL, respectively) fractions. These results suggest that dietary laserpitin increases serum apoE-HDL levels. In the liver, significant decreases in relative liver weight and triglyceride content were found after treatment with laserpitin for 7 weeks. An investigation of hepatic mRNA expression of proteins involved in lipid metabolism indicated that a significant decrease in hepatic triglyceride lipase may be responsible for the increase in serum HDL levels and also indicated that a marked decrease in adipocyte determination and differentiation factor 1 may be responsible, at least in part, for the decrease in hepatic triglyceride content. In conclusion, dietary laserpitin produces increases in serum HDL levels, especially apoE-HDL, and decreases in the hepatic triglyceride content in SHRSP.

LXRS and FXR: the yin and yang of cholesterol and fat metabolism

Liver X receptors (LXRs) and farnesoid X receptor (FXR) are nuclear receptors that function as intracellular sensors for sterols and bile acids, respectively. In response to their ligands, these receptors induce transcriptional responses that maintain a balanced, finely tuned regulation of cholesterol and bile acid metabolism. LXRs also permit the efficient storage of carbohydrate- and fat-derived energy, whereas FXR activation results in an overall decrease in triglyceride levels and modulation of glucose metabolism. The elegant, dual interplay between these two receptor systems suggests that they coevolved to constitute a highly sensitive and efficient system for the maintenance of total body fat and cholesterol homeostasis. Emerging evidence suggests that the tissue-specific action of these receptors is also crucial for the proper function of the cardiovascular, immune, reproductive, endocrine pancreas, renal, and central nervous systems. Together, LXRs and FXR represent potential therapeutic targets for the treatment and prevention of numerous metabolic and lipid-related diseases.

C. elegans DAF-12, Nuclear Hormone Receptors and human longevity and disease at old age

In Caenorhabditis elegans, DAF-12 appears to be a decisive checkpoint for many life history traits including longevity. The daf-12 gene encodes a Nuclear Hormone Receptor (NHR) and is member of a superfamily that is abundantly represented throughout the animal kingdom, including humans. It is, however, unclear which of the human receptor representatives are most similar to DAF-12, and what their role is in determining human longevity and disease at old age. Using a sequence similarity search, we identified human NHRs similar to C. elegans DAF-12 and found that, based on sequence similarity, Liver X Receptor A and B are most similar to C. elegans DAF-12, followed by the Pregnane X Receptor, Vitamin D Receptor, Constitutive Andosteron Receptor and the Farnesoid X Receptor. Their biological functions include, amongst others, detoxification and immunomodulation. Both are processes that are involved in protecting the body from harmful environmental influences. Furthermore, the DAF-12 signalling systems seem to be functionally conserved and all six human NHRs have cholesterol derived compounds as their ligands. We conclude that the DAF-12 signalling system seems to be evolutionary conserved and that NHRs in man are critical for body homeostasis and survival. Genomic variations in these NHRs or their target genes are prime candidates for the regulation of human lifespan and disease at old age.