Human kininogen gene is transactivated by the farnesoid X receptor

Anti-Inflammatory and Anti-Thrombogenic Properties of Arterial Elastic Laminae

Elastic laminae, an elastin-based, layered extracellular matrix structure in the media of arteries, can inhibit leukocyte adhesion and vascular smooth muscle cell proliferation and migration, exhibiting anti-inflammatory and anti-thrombogenic properties. These properties prevent inflammatory and thrombogenic activities in the arterial media, constituting a mechanism for the maintenance of the structural integrity of the arterial wall in vascular disorders. The biological basis for these properties is the elastin-induced activation of inhibitory signaling pathways, involving the inhibitory cell receptor signal regulatory protein α (SIRPα) and Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP1). The activation of these molecules causes deactivation of cell adhesion- and proliferation-regulatory signaling mechanisms. Given such anti-inflammatory and anti-thrombogenic properties, elastic laminae and elastin-based materials have potential for use in vascular reconstruction.

Progressive familial intrahepatic cholestasis — farnesoid X receptor deficiency due to NR1H4 mutation: A case report

BACKGROUND

Functioning farnesoid X receptor (FXR; encoded by NR1H4) is key to normal bile acid homeostasis. Biallelic mutations in NR1H4 are reported in a few children with intrahepatic cholestasis. We describe a boy with progressive familial intrahepatic cholestasis and homozygous mutation in NR1H4.

CASE SUMMARY

A boy had severe neonatal cholestasis with moderate hypercholanemia and persistently elevated alpha-fetoprotein. Despite medical treatment, coagulopathy was uncontrollable, prompting liver transplantation at age 8 mo with incidental splenectomy. The patient experienced catch-up growth with good liver function and did not develop allograft steatosis. However, 1 year after transplant, he died from an acute infection, considered secondary to immunosuppression and asplenia. A homozygous protein-truncating mutation, c.547C > T, p.(Arg183Ter), was subsequently identified in NR1H4, and both parents were shown to be heterozygous carriers. Absence of FXR and of bile salt export pump expression was confirmed by immunostaining of explanted liver.

CONCLUSION

Severe cholestasis with persistently high alpha-fetoprotein and modest elevation of serum bile acid levels may suggest FXR deficiency. Some patients with FXR deficiency may not develop allograft steatosis and may respond well to liver transplantation.

Partial abrogation of FXR-KNG1 signaling by carboxyl-terminal truncated HBx-C30 in hepatitis B virus-associated hepatocellular carcinoma

Hepatitis B virus (HBV) X protein (HBx) is a key regulator of HBV-associated hepatocarcinogenesis. C-terminal-truncated HBx is frequently detected in hepatocellular carcinoma (HCC). The role of HBx, especially C-terminal-truncated HBx, in HCC pathogenesis has been controversial. To elucidate the biological role of C-terminal-truncated HBx underlying HBV-associated hepato-oncogenesis, we constructed a vector expressing HBx-C30 (deletion of 30 aa from the C terminus of HBx) and functionally analyzed its regulation on farnesoid X receptor (FXR) signaling, which is known to inhibit hepatocarcinogenesis. In the present study, we found full-length HBx and HBx C-terminal truncation coexist in HCC, and both full length HBx and HBx-C30 can activate FXR signaling. Moreover, HBx-C30 weakly coactivates FXR-KNG1 signaling compared to full-length HBx.

Genetic aspects of biliary atresia etiology

Biliary atresia (BA) is a cholestatic disorder of infancy that is fatal if untreated. Despite years of study the etiology of BA remains unknown. Three etiopathogenic mechanisms may be involved, such as immune dysregulation, environmental factors and genetic susceptibility. Genetic predisposition is being actively studied. Candidate genes associated with BA in certain populations, genes affecting the cholangiocyte cilia function, as well as genes involved in stress responses have been identified. However, the long-term follow-up of twins with BA suggests that genotype is not of paramount importance for the disease development. Both epigenetic patterns and postzygotic somatic mutations may contribute to etiology of the disease. Recently, some evidence is being accumulated on the possible genetic predisposition to certain outcome of Kasai portoenterostomy performed in patients with BA. However, the presence of a number of factors contributing to the development of the disease makes it difficult to identify the genetic markers.

Effects of Different Dietary Vegetable Lipid Sources on Health Status in Nile Tilapia (Oreochromis niloticus): Haematological Indices, Immune Response Parameters and Plasma Proteome

This study aimed to investigate the effects of DLs, including palm oil (PO; an SFAs), linseed oil (LO; n-3 PUFAs) and soybean oil (SBO; n-6 PUFAs) on the health status of Nile tilapia (Oreochromis niloticus) during adulthood. Three experimental diets incorporating PO, LO or SBO were fed to adult Nile tilapia for a period of 90 days, and haematological and innate immune parameters were evaluated. Proteome analysis was also conducted to evaluate the effects of DLs on plasma proteins. The tested DLs had no significant effects on red blood cell (RBC) count, haematocrit, haemoglobin, and total immunoglobulin and lysozyme activity. Dietary LO led to increased alternative complement 50 activity (ACH50), and proteome analysis revealed that PO and SBO enhanced A2ML, suggesting that different DLs promote immune system via different processes. Dietary LO or SBO increased the expression of several proteins involved in coagulation activity such as KNG1, HRG and FGG. Increased HPX in fish fed with PO suggests that SFAs are utilised in heme lipid-oxidation. Overall, DLs with distinct fatty acids (FAs) affect several parameters corresponding to health status in Nile tilapia, and dietary LO and SBO seemed to strengthen health in this species.

Activation of Farnesoid X Receptor by Schaftoside Ameliorates Acetaminophen-Induced Hepatotoxicity by Modulating Oxidative Stress and Inflammation

Aims: Acetaminophen (APAP) overdose leads to acute liver injury by inducing hepatic mitochondrial oxidative stress and inflammation. However, the molecular mechanisms involved are still unclear. Farnesoid X receptor (FXR) serves as a therapeutic target for the treatment of liver disorders, whose activation has been proved to protect APAP-induced hepatotoxicity. In this study, we examined whether FXR activation by schaftoside (SS), a naturally occurring flavonoid from Desmodium styracifolium, could protect mice against APAP-induced hepatotoxicity via regulation of oxidative stress and inflammation. Results: We first found that SS exhibited potent protective effects against APAP-induced hepatotoxicity in mice. The study reveals that SS is a potential agonist of FXR, which protects mice from hepatotoxicity mostly via regulation of oxidative stress and inflammation. Mechanistically, the hepatoprotective SS is associated with the induction of the genes of phase II detoxifying enzymes (e.g., UGT1A1, GSTα1), phase III drug efflux transporters (e.g., bile salt export pump, organic solvent transporter protein β), and glutathione metabolism-related enzymes (e.g., glutamate-cysteine ligase modifier subunit [Gclm], glutamate-cysteine ligase catalytic subunit [Gclc]). More importantly, SS-mediated FXR activation could fine-tune the pro- and anti-inflammatory eicosanoids generation via altering eicosanoids metabolic pathway, thereby resulting in decrease of hepatic inflammation. In contrast, FXR deficiency can abrogate the above effects. Innovation and Conclusion: Our results provided the direct evidence that FXR activation by SS could attenuate APAP-induced hepatotoxicity via inhibition of nuclear factor kappa-B signaling and fine-tuning the generation of proinflammatory mediators' eicosanoids. Our findings indicate that strategies to activate FXR signaling in hepatocytes may provide a promising therapeutic approach to alleviate liver injury induced by APAP overdose.

AKR1D1 is a novel regulator of metabolic phenotype in human hepatocytes and is dysregulated in non-alcoholic fatty liver disease

OBJECTIVE

Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome. Steroid hormones and bile acids are potent regulators of hepatic carbohydrate and lipid metabolism. Steroid 5β-reductase (AKR1D1) is highly expressed in human liver where it inactivates steroid hormones and catalyzes a fundamental step in bile acid synthesis.

METHODS

Human liver biopsies were obtained from 34 obese patients and AKR1D1 mRNA expression levels were measured using qPCR. Genetic manipulation of AKR1D1 was performed in human HepG2 and Huh7 liver cell lines. Metabolic assessments were made using transcriptome analysis, western blotting, mass spectrometry, clinical biochemistry, and enzyme immunoassays.

RESULTS

In human liver biopsies, AKR1D1 expression decreased with advancing steatosis, fibrosis and inflammation. Expression was decreased in patients with type 2 diabetes. In human liver cell lines, AKR1D1 knockdown decreased primary bile acid biosynthesis and steroid hormone clearance. RNA-sequencing identified disruption of key metabolic pathways, including insulin action and fatty acid metabolism. AKR1D1 knockdown increased hepatocyte triglyceride accumulation, insulin sensitivity, and glycogen synthesis, through increased de novo lipogenesis and decreased β-oxidation, fueling hepatocyte inflammation. Pharmacological manipulation of bile acid receptor activation prevented the induction of lipogenic and carbohydrate genes, suggesting that the observed metabolic phenotype is driven through bile acid rather than steroid hormone availability.

CONCLUSIONS

Genetic manipulation of AKR1D1 regulates the metabolic phenotype of human hepatoma cell lines, driving steatosis and inflammation. Taken together, the observation that AKR1D1 mRNA is down-regulated with advancing NAFLD suggests that it may have a crucial role in the pathogenesis and progression of the disease.

Unveiling the participation of avian kinin ornithokinin and its receptors in the chicken inflammatory response

Vasoactive peptides are key early mediators of inflammation released through activation of different enzymatic systems. The mammalian kinin-kallikrein (K-KLK) system produces bradykinin (BK) through proteolytic cleavage of a kininogen precursor by enzymes named kallikreins. BK acts through specific ubiquitous G-protein coupled receptors (B1R and B2R) to participate in physiological processes and inflammatory responses, such as activation of mononuclear phagocytes. In chickens, the BK-like nonapeptide ornithokinin (OK) has been shown to promote intracellular calcium increase in embryonic fibroblasts and to be vasodilatory in vivo. Also, one of its receptors (B2R) was already cloned. However, the participation of chicken K-KLK system components in the inflammatory response remains unknown and was therefore investigated. We first showed that B1R, B2R and kininogen 1 (KNG1) are expressed in unstimulated chicken tissues and macrophages. We next showed that chicken B1R and B2R are expressed at transcript and protein levels in chicken macrophages and are upregulated by E. coli LPS or avian pathogenic E. coli (APEC) infection. Interestingly, exogenous OK induced internalization and degradation of OK receptors protein, notably B2R. Also, OK induced intracellular calcium increase and potentiated zymosan-induced ROS production and Dextran-FITC endocytosis by chicken macrophages. Exogenous OK itself did not promote APEC killing and had no pro-inflammatory effect. However, when combined with LPS or APEC, OK upregulated cytokine/chemokine gene expression and NO production by chicken macrophages. This effect was not blocked by canonical non-peptide B1R or B2R receptor antagonists but was GPCR- and PI3K/Akt-dependent. In vivo, pulmonary colibacillosis led to upregulation of OK receptors expression in chicken lungs and liver. Also, colibacillosis led to significant upregulation of OK precursor KNG1 expression in liver and in cultured hepatocytes (LMH). We therefore provide hitherto unknown information on how OK and its receptors are involved in inflammation and infection in chickens.

Fatty liver diseases, bile acids, and FXR

The prevalence of nonalcoholic fatty liver disease (NAFLD) worldwide has increased at an alarming rate, which will likely result in enormous medical and economic burden. NAFLD presents as a spectrum of liver diseases ranging from simple steatosis, nonalcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and even to hepatocellular carcinoma (HCC). A comprehensive understanding of the mechanism(s) of NAFLD-to-NASH transition remains elusive with various genetic and environmental susceptibility factors possibly involved. An understanding of the mechanism may provide novel strategies in the prevention and treatment to NASH. Abnormal regulation of bile acid homeostasis emerges as an important mechanism to liver injury. The bile acid homeostasis is critically regulated by the farnesoid X receptor (FXR) that is activated by bile acids. FXR has been known to exert tissue-specific effects in regulating bile acid synthesis and transport. Current investigations demonstrate FXR also plays a principle role in regulating lipid metabolism and suppressing inflammation in the liver. Therefore, the future determination of the molecular mechanism by which FXR protects the liver from developing NAFLD may shed light to the prevention and treatment of NAFLD.

Mutations in the nuclear bile acid receptor FXR cause progressive familial intrahepatic cholestasis

Neonatal cholestasis is a potentially life-threatening condition requiring prompt diagnosis. Mutations in several different genes can cause progressive familial intrahepatic cholestasis, but known genes cannot account for all familial cases. Here we report four individuals from two unrelated families with neonatal cholestasis and mutations in NR1H4, which encodes the farnesoid X receptor (FXR), a bile acid-activated nuclear hormone receptor that regulates bile acid metabolism. Clinical features of severe, persistent NR1H4-related cholestasis include neonatal onset with rapid progression to end-stage liver disease, vitamin K-independent coagulopathy, low-to-normal serum gamma-glutamyl transferase activity, elevated serum alpha-fetoprotein and undetectable liver bile salt export pump (ABCB11) expression. Our findings demonstrate a pivotal function for FXR in bile acid homeostasis and liver protection.

Neonatal cholestasis is a result of elevated bile acid levels, and is associated with mutations in genes regulating bile acid homeostasis. Here the authors identify mutations in the bile acid sensing farnesoid X receptor in four individuals with neonatal cholestasis from two unrelated families.

The nuclear bile acid receptor FXR controls the liver derived tumor suppressor histidine-rich glycoprotein

The nuclear bile acid receptor Farnesoid X receptor (FXR) is strongly expressed in liver and intestine, controls bile acid and lipid homeostasis and exerts tumor-protective functions in liver and intestine. Histidine-rich glycoprotein (HRG) is an abundant plasma protein produced by the liver with the proposed function as a pattern recognition molecule involved in the clearance of immune complexes, necrotic cells and pathogens, the modulation of angiogenesis, the normalization of deranged endothelial vessel structure in tumors and tumor suppression. FXR recognition sequences were identified within a human HRG promoter fragment that mediated FXR/FXR-agonist dependent reporter gene activity in vitro. We show that HRG is a novel transcriptional target gene of FXR in human hepatoma cells, human upcyte® primary hepatocytes and 3D human liver microtissues in vitro and in mouse liver in vivo. Prolonged administration of the potent nonsteroidal FXR agonist PX20606 increases HRG levels in mouse plasma. Finally, daily oral administration of this FXR agonist for seven days resulted in a significant increase of HRG levels in the plasma of healthy human male volunteers during a clinical Phase I safety study. HRG might serve as a surrogate marker indicative of liver-specific FXR activation in future human clinical studies. Furthermore, potent FXR agonists might be beneficial in serious health conditions where HRG is reduced, for example, in hepatocellular carcinoma but also other solid cancers, liver failure, sepsis and pre-eclampsia.

Glucose sensing O-GlcNAcylation pathway regulates the nuclear bile acid receptor farnesoid X receptor (FXR)

Bile acid metabolism is intimately linked to the control of energy homeostasis and glucose and lipid metabolism. The nuclear receptor farnesoid X receptor (FXR) plays a major role in the enterohepatic cycling of bile acids, but the impact of nutrients on bile acid homeostasis is poorly characterized. Metabolically active hepatocytes cope with increases in intracellular glucose concentrations by directing glucose into storage (glycogen) or oxidation (glycolysis) pathways, as well as to the pentose phosphate shunt and the hexosamine biosynthetic pathway. Here we studied whether the glucose nonoxidative hexosamine biosynthetic pathway modulates FXR activity. Our results show that FXR interacts with and is O-GlcNAcylated by O-GlcNAc transferase in its N-terminal AF1 domain. Increased FXR O-GlcNAcylation enhances FXR gene expression and protein stability in a cell type-specific manner. High glucose concentrations increased FXR O-GlcNAcylation, hence its protein stability and transcriptional activity by inactivating corepressor complexes, which associate in a ligand-dependent manner with FXR, and increased FXR binding to chromatin. Finally, in vivo fasting-refeeding experiments show that FXR undergoes O-GlcNAcylation in fed conditions associated with increased direct FXR target gene expression and decreased liver bile acid content.

CONCLUSION

FXR activity is regulated by glucose fluxes in hepatocytes through a direct posttranslational modification catalyzed by the glucose-sensing hexosamine biosynthetic pathway.

Association of X-prolyl aminopeptidase 1 rs17095355 polymorphism with biliary atresia in Thai children

AIM

  To investigate XPNPEP1 rs17095355 polymorphism in biliary atresia (BA) patients and to determine whether there is an association between XPNPEP1 gene polymorphism and susceptibility to BA in a Thai population.

METHODS

  A total of 124 cases of BA and 114 controls were genotyped for XPNPEP1 rs17095355 polymorphism. The XPNPEP1 rs17095355 C/T genotype was determined by polymerase chain reaction (PCR) and direct sequencing. Allele and genotype frequencies were established by directed counting from the sequences.

RESULTS

  Genotype distributions for the XPNPEP1 rs17095355 polymorphism tested were in Hardy-Weinberg equilibrium for both control and study groups. There were no significant differences in genotype and allele frequencies of the single nucleotide polymorphism between controls and Thai children with BA. Genotype frequencies of rs17095355 of T/T in BA were higher than those of controls (34.68% and 16.67%, P < 0.002). Also, the T allele frequencies of BA were higher than those of controls (56.85% and 42.98%, P < 0.003).

CONCLUSION

  The association between XPNPEP1 rs17095355 polymorphism and BA has been demonstrated, particularly with the T allele. We hypothesize that the XPNPEP1 rs17095355 polymorphism confers increased susceptibility to the disease.

Deciphering the nuclear bile acid receptor FXR paradigm

Originally called retinoid X receptor interacting protein 14 (RIP14), the farnesoid X receptor (FXR) was renamed after the ability of its rat form to bind supra-physiological concentrations of farnesol. In 1999 FXR was de-orphanized since primary bile acids were identified as natural ligands. Strongly expressed in the liver and intestine, FXR has been shown to be the master transcriptional regulator of several entero-hepatic metabolic pathways with relevance to the pathophysiology of conditions such as cholestasis, fatty liver disease, cholesterol gallstone disease, intestinal inflammation and tumors. Furthermore, given the importance of FXR in the gut-liver axis feedbacks regulating lipid and glucose homeostasis, FXR modulation appears to have great input in diseases such as metabolic syndrome and diabetes. Exciting results from several cellular and animal models have provided the impetus to develop synthetic FXR ligands as novel pharmacological agents. Fourteen years from its discovery, FXR has gone from bench to bedside; a novel nuclear receptor ligand is going into clinical use.

Farnesoid X receptor activation improves erectile dysfunction in models of metabolic syndrome and diabetes

The metabolic syndrome (MetS) is an insulin-resistant state characterized by a cluster of cardiovascular risk factors, including abdominal obesity, hyperglycemia, elevated blood pressure and combined dyslipidemia. In this review, we discuss the potential of farnesoid X receptor (FXR) agonists in the treatment of erectile dysfunction (ED), a multifactorial disorder often comorbid with MetS. FXR not only regulates lipid and glucose homeostasis but also influences endothelial function and atherosclerosis, suggesting a regulatory role for this hormone nuclear receptor in the cardiovascular complications associated with the MetS, including ED. MetS induces ED via several mechanisms, and in particular through endothelial dysfunction in penile vessels. In a high-fat diet rabbit model of MetS, a 3-month treatment with the potent and selective FXR agonist INT-747 restores endothelium-dependent relaxation in isolated cavernous tissue, normalizing responsiveness to acetylcholine and to electrical field stimulation. Accordingly, eNOS expression in the penis is greatly up-regulated by INT-747 treatment. Experiments in a rat model of chemically-induced type 1 diabetes further demonstrate that INT-747 treatment preserves erectile function induced by electrical stimulation of the cavernous nerve. These results add a new facet to the pleiotropic activities mediated by FXR, and reveal novel beneficial effects of FXR activation with potential clinical relevance. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Orphan nuclear receptors as targets for drug development

Orphan nuclear receptors regulate diverse biological processes. These important molecules are ligand-activated transcription factors that act as natural sensors for a wide range of steroid hormones and xenobiotic ligands. Because of their importance in regulating various novel signaling pathways, recent research has focused on identifying xenobiotics targeting these receptors for the treatment of multiple human diseases. In this review, we will highlight these receptors in several physiologic and pathophysiologic actions and demonstrate how their functions can be exploited for the successful development of newer drugs.

Genome-wide association study identifies a susceptibility locus for biliary atresia on 10q24.2

Biliary atresia (BA) is characterized by the progressive fibrosclerosing obliteration of the extrahepatic biliary system during the first few weeks of life. Despite early diagnosis and prompt surgical intervention, the disease progresses to cirrhosis in many patients. The current theory for the pathogenesis of BA proposes that during the perinatal period, a still unknown exogenous factor meets the innate immune system of a genetically predisposed individual and induces an uncontrollable and potentially self-limiting immune response, which becomes manifest in liver fibrosis and atresia of the extrahepatic bile ducts. Genetic factors that could account for the disease, let alone for its high incidence in Chinese, are to be investigated. To identify BA susceptibility loci, we carried out a genome-wide association study (GWAS) using the Affymetrix 5.0 and 500 K marker sets. We genotyped nearly 500 000 single-nucleotide polymorphisms (SNPs) in 200 Chinese BA patients and 481 ethnically matched control subjects. The 10 most BA-associated SNPs from the GWAS were genotyped in an independent set of 124 BA and 90 control subjects. The strongest overall association was found for rs17095355 on 10q24, downstream XPNPEP1, a gene involved in the metabolism of inflammatory mediators. Allelic chi-square test P-value for the meta-analysis of the GWAS and replication results was 6.94 x 10(-9). The identification of putative BA susceptibility loci not only opens new fields of investigation into the mechanisms underlying BA but may also provide new clues for the development of preventive and curative strategies.

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.

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.

Disruption of an SP2/KLF6 repression complex by SHP is required for farnesoid X receptor-induced endothelial cell migration

The farnesoid X receptor (FXR) signaling pathway regulates bile acid and cholesterol homeostasis. Here, we demonstrate, using a variety of gain- and loss-of-function approaches, a role of FXR in the process of cell motility, which involves the small heterodimeric partner (SHP)-dependent up-regulation of matrix metalloproteinase-9. We use this observation to reveal a transcriptional regulatory mechanism involving the SP/KLF transcription factors, SP2 and KLF6. Small interference RNA-based silencing studies in combination with promoter, gel shift, and chromatin immunoprecipitation assays indicate that SP2 and KLF6 bind to the matrix metalloproteinase-9 promoter and together function to maintain this gene in a silenced state. However, upon activation of FXR, SHP interacts with SP2 and KLF6, disrupting the SP2/KLF6 repressor complex. Thus, together, these studies identify a mechanism for antagonizing Sp/KLF protein repression function via SHP, with this process regulating endothelial cell motility.

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.

Activation of the nuclear receptor FXR induces fibrinogen expression: a new role for bile acid signaling

Three genes, fibrinogen-alpha (FBGalpha), -beta, and -gamma, encode proteins that make up the mature FBG protein complex. This complex is secreted from the liver and plays a key role in coagulation in response to vascular disruption. We identified all three FBG genes in a screen designed to isolate genes that are regulated by the farnesoid X receptor (FXR; NR1H4). Treatment of human hepatoma cells with either naturally occurring or synthetic [3-(2,6-dichlorophenyl)-4-(3'-carboxy-2-chloro-stilben-4-yl)-oxymethyl-5-isopropyl-isoxazole] FXR ligands resulted in the induction of transcripts for all three genes. The induction of FBGbeta mRNA in response to activated FXR appears to be a primary transcriptional response, as it is blocked by actinomycin D but not by cycloheximide. Four FXR isoforms were recently identified that differ either at their N termini and/or by the presence of four amino acids in the hinge region. Interestingly, the activities of the human FBGbeta promoter-reporter constructs were highly induced by FXR isoforms that lack the four amino acid insert. The observation that all three FBG subunits are induced by specific FXR isoforms, in response to FXR ligands, suggests that bile acids and FXR modulate fibrinolytic activity.

Polyunsaturated fatty acids are FXR ligands and differentially regulate expression of FXR targets

Polyunsaturated fatty acids (PUFAs) have been previously reported as agonists of peroxisome proliferatoractivated receptor and antagonists of the liver X receptor. The activities on these two nuclear receptors have been attributed to their beneficial effects such as improvement of dyslipidemia and insulin sensitivity and decrease of hepatic lipogenesis. Here we report that PUFAs are ligands of farnesoid X receptor (FXR), a nuclear receptor for bile acids. In a conventional FXR binding assay, arachidonic acid (AA, 20:4), docosahexaenoic acid (DA, 22:6), and linolenic acid (LA, 18:3) had an affinity of 2.6, 1.5, and 3.5 microM, respectively. In a cell-free coactivator association assay, AA, DA, and LA decreased FXR agonist-induced FXR activation with IC(50)s ranging from 0.9 to 4.7 microM. In HepG2 cells, PUFAs regulated the expression of two FXR targets, BSEP and kininogen, in an opposite fashion, although both genes were transactivated by FXR. All three PUFAs dose-dependently enhanced FXR agonist-induced BSEP expression but decreased FXR agonist-induced human kininogen mRNA. Saturated fatty acids such as stearic acid (SA, 18:0) and palmitic acid (PA, 16:0) did not bind to FXR and did not change BSEP or kininogen expression. The pattern of BSEP and kininogen regulation by PUFAs is closely similar to that of the guggulsterone, previously reported as a selective bile acid receptor modulator. Our results suggest that PUFAs may belong to the same class of FXR ligands as guggulsterone, and that the selective regulation of FXR targets may contribute to the beneficial effects of PUFAs in lipid metabolism.

The nuclear bile acid receptor FXR is activated by PGC-1alpha in a ligand-dependent manner

The nuclear bile acid receptor FXR (farnesoid X receptor) is one of the key factors that suppress bile acid biosynthesis in the liver. PGC-1alpha [PPARgamma (peroxisome-proliferator-activated receptor gamma) co-activator-1alpha] is known to control energy homoeostasis in adipose tissue, skeletal muscle and liver. We performed cell-based reporter assays using the expression system of a GAL4-FXR chimaera, the ligand-binding domain of FXR fused to the DNA-binding domain of yeast GAL4, to find the co-activators for FXR. We found that the transcriptional activation of a reporter plasmid by a GAL4-FXR chimaera was strongly enhanced by PGC-1alpha, in a ligand-dependent manner. Transcriptional activation of the SHP (small heterodimer partner) gene by the FXR-RXRalpha (retinoid X receptor alpha) heterodimer was also enhanced by PGC-1alpha in the presence of CDCA (chenodeoxycholic acid). Co-immunoprecipitation and pull-down studies using glutathione S-transferase-PGC-1alpha fusion proteins revealed that the ligand-binding domain of FXR binds PGC-1alpha in a ligand-influenced manner both in vivo and in vitro. Furthermore, our studies revealed that SHP represses its own transcription, and the addition of excess amounts of PGC-1alpha can overcome the inhibitory effect of SHP. These observations indicate that PGC-1alpha mediates the ligand-dependent activation of FXR and transcription of SHP gene.

Cholestatic syndromes

PURPOSE OF REVIEW

This review highlights recent developments in the molecular pathogenesis of cholestasis as well new aspects of pathogenesis and management of clinical cholestatic disorders.

RECENT FINDINGS

Highlights include the role of nuclear receptors including FXR ligands as potential therapeutic agents, new genetic defects for pediatric cholestasis and sclerosing cholangitis, and novel infections and environmental agents as etiologies for primary biliary cirrhosis. Important clinical studies have been published in the area of pediatric cholestatic syndromes, intrahepatic cholestasis of pregnancy, primary biliary cirrhosis, primary and secondary sclerosing cholangitis, cholestasis of sepsis, viral cholestatic syndromes, and drug-induced cholestasis.

SUMMARY

These advances continue to improve understanding of the pathophysiology, diagnosis, and management of cholestatic liver disease.

Structure and expression of two kininogen genes in mice

Kininogens serve dual functions by forming a scaffold for the assembly of the protein complex initiating the surface-activated blood coagulation cascade and as precursors for the kinin hormones. While rats have three kininogen genes, for mice, cattle, and humans only one gene has been described. Here, we present sequence and expression data of a second mouse kininogen gene. The two genes, kininogen-I and kininogen-II, are located in close proximity on chromosome 16 in a headtohead arrangement. In liver and kidney, both genes are expressed and for each gene three alternative splice variants are synthesized. Two of them are the expected high and low molecular weight isoforms known from all mammalian kininogens. However, for both genes also a third, hitherto unknown splice variant was detected which lacks part of the high molecular weight mRNA due to splicing from the low molecular weight donor site to alternative splice acceptor sites in exon 10. The physiological functions of the six kininogen isoforms predicted by these findings need to be determined.

The enterohepatic nuclear receptors are major regulators of the enterohepatic circulation of bile salts

Recent studies have established that bile salts are signaling molecules, besides their classic function in dietary lipid absorption and cholesterol metabolism. Bile salts signal by activating mitogen-activated protein kinase (MAPK) pathways and nuclear receptors like farnesoid X receptor-alpha (FXRalpha). FXRalpha activation increases the expression of direct FXRalpha target genes involved in bile salt transport and detoxification, and decreases expression of indirect FXRalpha target genes involved in bile salt biosynthesis and uptake. These actions prevent toxic accumulation of bile salts in the enterohepatic organs. A network of interactions with other nuclear receptors and MAPK pathways may protect the liver against pathological elevation of bile salts and cholestasis. Therefore treatment of cholestasis might benefit from the development of FXRalpha agonists.

Inhibitory effect of the small heterodimer partner on hepatocyte nuclear factor-4 mediates bile acid-induced repression of the human angiotensinogen gene

Bile acids function as transcriptional regulators for the genes important in bile acid synthesis and cholesterol homeostasis. In this study, we identified angiotensinogen (ANG), the precursor of vasoactive octapeptide angiotensin II, as a novel target gene of bile acids. In human ANG transgenic mice, administration of cholic acid resulted in the down-regulation of human ANG gene expression in the liver. ANG gene expression in HepG2 cells was also repressed by chenodeoxycholic acid. Because the expression of small heterodimer partner (SHP) mRNA was induced by chenodeoxycholic acid in HepG2 cells, we analyzed the effects of SHP on the human ANG promoter. Promoter mutation analysis demonstrated that SHP repressed human ANG promoter activity through the element, which has been previously determined as a binding site for hepatocyte nuclear factor-4 (HNF-4). SHP repressed human ANG promoter activity only when the HNF-4 expression vector was cotransfected in HeLa cells. Furthermore, we found that SHP bound to the HNF-4 N-terminal region including the DNA-binding domain and activation function-1 and that SHP prevented HNF-4 from binding to the human ANG promoter. These results suggest that bile acids negatively regulate the human ANG gene through the inhibitory effect of SHP on HNF-4.