Transcriptional activation of the cholesterol 7α-hydroxylase gene (CYP7A) by nuclear hormone receptors

Cerebrotendinous Xanthomatosis, a Treatable Disorder Often Missed: Case Series of Three Patients Confirmed by Genetic Testing

ABSTRACT

Cerebrotendinous xanthomatosis (CTX) is a treatable autosomal recessive disorder with varied clinical manifestations and age of onset and is often diagnosed late. We report three cases of CTX who presented at our center with clinical features of frequent diarrhea, early cataracts, xanthomas, cognitive decline, ataxia, neuropathy, and other manifestations of CTX. Magnetic resonance imaging (MRI) brain in all three patients revealed abnormalities consistent with CTX. Diagnosis was confirmed by next-generation sequencing. Chenodeoxycholic acid (CDCA) is recommended as the drug of choice, as it can halt the disease progression and reverse some of the symptoms. In addition to late diagnosis, nonavailability of CDCA in our part of world adds to the problem of management of such patients; therefore, they are often started on alternative therapies, which are less effective.

Upregulation of Taurine Biosynthesis and Bile Acid Conjugation with Taurine through FXR in a Mouse Model with Human-like Bile Acid Composition

Taurine, the end product in the sulfur-containing amino acid pathway, is conjugated with bile acids (BAs) in the liver. The rate-limiting enzymes in both taurine synthesis and BA conjugation may be regulated by a nucleus receptor, FXR, that promotes BA homeostasis. However, it is controversial because BAs act as natural FXR agonists or antagonists in humans and mice, respectively, due to the species differences in BA synthesis. The present study evaluated the influences of different BA compositions on both pathways in the liver by comparing Cyp2a12^-/-/Cyp2c70^-/- mice with a human-like BA composition (DKO) and wild-type (WT) mice. The DKO liver contains abundant natural FXR agonistic BAs, and the taurine-conjugated BA proportion and the taurine concentration were significantly increased, while the total BA concentration was significantly decreased compared to those in the WT liver with natural FXR antagonistic BAs. The mRNA expression levels of the enzymes Bacs and Baat in BA aminations and Cdo and Fmo1 in the taurine synthesis, as well as Fxr and its target gene, Shp, were significantly higher in the DKO liver than in the WT liver. The present study, using a model with a human-like BA composition in the liver, confirmed, for the first time in mice, that both the taurine synthesis and BA amidation pathways are upregulated by FXR activation.

Effect of Total SMS Activity on LDL Catabolism in Mice

BACKGROUND

Sphingomyelin (SM) and cholesterol are 2 key lipid partners on cell membranes and on lipoproteins. Many studies have indicated the influence of cholesterol on SM metabolism. This study examined the influence of SM biosynthesis on cholesterol metabolism.

METHODS

Inducible global Sms1 KO (knockout)/global Sms2 KO mice were prepared to evaluate the effect of whole-body SM biosynthesis deficiency on lipoprotein metabolism. Tissue cholesterol, SM, ceramide, and glucosylceramide levels were measured. Triglyceride production rate and LDL (low-density lipoprotein) catabolism were measured. Lipid rafts were isolated and LDL receptor mass and function were evaluated. Also, the effects of exogenous sphingolipids on hepatocytes were investigated.

RESULTS

We found that total SMS (SM synthase) depletion significantly reduced plasma SM levels. Also, the total deficiency significantly induced plasma cholesterol, apoB (apolipoprotein B), and apoE (apolipoprotein E) levels. Importantly, total SMS deficiency, but not SMS2 deficiency, dramatically decreased LDL receptors in the liver and attenuated LDL uptake through the receptor. Further, we found that total SMS deficiency greatly reduced LDL receptors in the lipid rafts, which contained significantly lower SM and significantly higher glucosylceramide, as well as cholesterol. Furthermore, we treated primary hepatocytes and Huh7 cells (a human hepatoma cell line) with SM, ceramide, or glucosylceramide, and we found that only SM could upregulate LDL receptor levels in a dose-dependent fashion.

CONCLUSIONS

Whole-body SM biosynthesis plays an important role in LDL cholesterol catabolism. The total SMS deficiency, but not SMS2 deficiency, reduces LDL uptake and causes LDL cholesterol accumulation in the circulation. Given the fact that serum SM level is a risk factor for cardiovascular diseases, inhibiting SMS2 but not SMS1 should be the desirable approach.

My lifelong dedication to bile acid research

It is a great honor to be invited to write a reflections article on my scientific journey and lifelong bile acid research for the Journal of Biological Chemistry, in which I am proud to have published 24 articles. I have also published 21 articles in the Journal of Lipid Research, another journal of the American Society of Biochemistry and Molecular Biology. I begin my reflections from my early education in Taiwan, my coming to America for graduate study, and continue with my postdoctoral training in cytochrome P450 research, and my lifelong bile acid research career at Northeast Ohio Medical University. I have witnessed and helped in the transformation of this rural not so visible medical school to a well-funded leader in liver research. Writing this reflections article on my long and rewarding journey in bile acid research brings back many good memories. I am proud of my scientific contributions and attribute my academic success to hard work, perseverance, good mentoring, and networking. I hope these reflections of my academic career would help inspire young investigators to pursue an academic career in biochemistry and metabolic diseases.

Transcription networks in liver development and acute liver failure

Acute liver failure (ALF) is a medical emergency due to massive hepatocyte loss. In such a harsh condition, maintaining transcriptional regulation in the remaining hepatocytes while activating similar transcription factor networks in liver progenitor cells (LPCs) to ensure essential liver functions are two critical processes to rescue patients from liver failure and death. In this review, we discuss the formation and functions of transcription networks in ALF and liver development. We focus on a hierarchical network of transcription factors that responds to different pathophysiological circumstances: (1) Under normal circumstances, pioneer factor forkhead box protein A2 (FOXA2) coordinates several constitutive hepatic transcription factors, such as hepatic nuclear factor 4 alpha (HNF4α) and CCAAT-enhancer binding protein α (C/EBPα), which ensure normal liver function; (2) When the expression of both HNF4α and C/EBPα in hepatocytes are disrupted by severe inflammation, retinoic acid receptor (RAR) is the alternative transcription factor that compensates for their absence; (3) When massive hepatic necrosis occurs, a similar transcription network including FOXA2 and HNF4α, is activated as a "rescue network" in LPCs to maintain vital liver functions when hepatocytes fail, and thus ensures survival. Expression of these master transcription factors in hepatocytes and LPCs is tightly regulated by hormone signals and inflammation. The performance of this hierarchical transcription network, in particularly the "rescue network" described above, significantly affects the clinical outcome of ALF.

My lifelong dedication to bile acid research

It is a great honor to be invited to write a reflection of my lifelong bile acid research for the Journal of Biological Chemistry, the premier biochemistry journal in which I am proud to have published 24 manuscripts. I published 21 manuscripts in the Journal of Lipid Research, also a journal of American Society of Biochemistry and Molecular Biology. I started my reflection from my early education in Taiwan, my coming to America for graduate study, my postdoctoral training in cytochrome P450 research, and my lifelong bile acid research career at the not so "visible" Northeast Ohio Medical University. I have witnesses and help to transform this sleepy rural medical school to a well-funded powerhouse in liver research. Writing this reflection of my long, exciting, and rewarding journey in bile acid research brought back many good memories. I am proud of my scientific contribution. I attribute my lifelong academic success to working hard, perseverance, good mentoring, and networking. I hope that this reflection of my academic career may provide guidance to younger investigators who are pursuing academic teaching and research and might inspire the next generation of researchers in biochemistry and metabolic diseases.

Effect of Total Sphingomyelin Synthase Activity on Low Density Lipoprotein Catabolism in Mice

Background

Sphingomyelin (SM) and cholesterol are two key lipid partners on cell membranes and on lipoproteins. Many studies have indicated the influence of cholesterol on SM metabolism. This study examined the influence of SM biosynthesis on cholesterol metabolism.

Methods

Inducible global Sms1 KO/global Sms2 KO mice were prepared to evaluate the effect of whole-body SM biosynthesis deficiency on lipoprotein metabolism. Tissue cholesterol, SM, ceramide, and glucosylceramide levels were measured. TG production rate and LDL catabolism were measured. Lipid rafts were isolated and LDL receptor mass and function were evaluated. Also, the effects of exogenous sphingolipids on hepatocytes were investigated.

Results

We found that total SMS depletion significantly reduced plasma SM levels. Also, the total deficiency significantly induced plasma cholesterol, apoB, and apoE levels. Importantly, total SMS deficiency, but not SMS2 deficiency, dramatically decreased LDL receptors in the liver and attenuated LDL uptake through the receptor. Further, we found that total SMS deficiency greatly reduced LDL receptors in the lipid rafts which contained significantly lower SM and significantly higher glucosylceramide as well as cholesterol. Furthermore, we treated primary hepatocytes and Huh7 cells (a human hepatoma cell line) with SM, ceramide, or glucosylceramide, and we found that only SM could up-regulate LDL receptor levels in a dose-dependent fashion.

Conclusions

Whole-body SM biosynthesis plays an important role in LDL-cholesterol catabolism. The total SMS deficiency, but not SMS2 deficiency, reduces LDL uptake and causes LDL-cholesterol accumulation in the circulation. Given the fact that serum SM level is a risk factor for cardiovascular diseases, inhibiting SMS2 but not SMS1 should be the desirable approach.

Graphic Abstract

Regulation of Hepatocyte Nuclear Factor 4α Attenuated Lipotoxicity but Increased Bile Acid Toxicity in Non-Alcoholic Fatty Liver Disease

Hepatocyte nuclear factor 4 alpha (HNF4α) is a key master transcriptional factor for hepatic fat and bile acid metabolic pathways. We aimed to investigate the role of HNF4α in non-alcoholic fatty liver disease (NAFLD). The role of HNF4α was evaluated in free fatty acid-induced lipotoxicity and chenodeoxycholic acid (CDCA)-induced bile acid toxicity. Furthermore, the role of HNF4α was evaluated in a methionine choline deficiency (MCD)-diet-induced NAFLD model. The overexpression of HNF4α reduced intracellular lipid contents and attenuated palmitic acid (PA)-induced lipotoxicity. However, the protective effects of HNF4α were reversed when CDCA was used in a co-treatment with PA. HNF4α knockdown recovered cell death from bile acid toxicity. The inhibition of HNF4α decreased intrahepatic inflammation and the NAFLD activity score in the MCD model. Hepatic HNF4α inhibition can attenuate bile acid toxicity and be more effective as a therapeutic strategy in NAFLD patients; however, it is necessary to study the optimal timing of HNF4α inhibition.

Impairment of bile acid metabolism by perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) in human HepaRG hepatoma cells

Perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) are man-made chemicals that are used for the fabrication of many products with water- and dirt-repellent properties. The toxicological potential of both substances is currently under debate. In a recent Scientific Opinion, the European Food Safety Authority (EFSA) has identified increased serum total cholesterol levels in humans as one major critical effect being associated with exposure to PFOA or PFOS. In animal studies, both substances induced a decrease of serum cholesterol levels, and the underlying molecular mechanism(s) for these opposed effects are unclear so far. In the present study, we examined the impact of PFOA and PFOS on cholesterol homoeostasis in the human HepaRG cell line as a model for human hepatocytes. Cholesterol levels in HepaRG cells were not affected by PFOA or PFOS, but both substances strongly decreased synthesis of a number of bile acids. The expression of numerous genes whose products are involved in synthesis, metabolism and transport of cholesterol and bile acids was strongly affected by PFOA and PFOS at concentrations above 10 µM. Notably, both substances led to a strong decrease of CYP7A1, the key enzyme catalyzing the rate-limiting step in the synthesis of bile acids from cholesterol, both at the protein level and at the level of gene expression. Moreover, both substances led to a dilatation of bile canaliculi that are formed by differentiated HepaRG cells in vitro. Similar morphological changes are known to be induced by cholestatic agents in vivo. Thus, the strong impact of PFOA and PFOS on bile acid synthesis and bile canalicular morphology in our in vitro experiments may allow the notion that both substances have a cholestatic potential that is connected to the observed increased serum cholesterol levels in humans in epidemiological studies.

Co-polysomy of 1p/19q in glial tumors: Retrospective analysis of 221 cases from single center

Glial tumors are malignant brain tumors that arise from glial cells of brain or spine and have genetic aberrations in their genome. 1p/19q co-deletion is associated with increased Overall Survival (OS) time with enhanced response to chemo- and radio-therapy in oligodendrogliomas. However, prognostic significance of 1p/19q co-polysomy is still unclear. We evaluated 1p/19q status of 221 patients with glial tumor by Fluorescent in situ Hybridization (FISH). Records of the patients were collected retrospectively. Our results demonstrated that 1p/19q co-polysomy was associated with decreased OS time, high P53 expression and frequently located in temporal lobe, whereas 1p/19q co-deletion was associated with increased overall survival time, low P53 expression and frontal lobe location. Furthermore, classification of patients based on both 1p/19q status and P53 expression revealed that patients with 1p/19q co-polysomy and high P53 expression had the worst prognosis. Lastly, our bioinformatic survival analysis revealed that high expression of SRM, ICMT, and FTL located in 1p36.13-p36.31 and 19q13.2-q13.33 region were related with decreased OS time in patients with Low Grade Glioma (LGG). The study demonstrated that 1p/19q co-polysomy is a poor prognostic marker for glial tumor.

Repression of hepatocyte nuclear factor 4 alpha by AP-1 underlies dyslipidemia associated with retinoic acid

All-trans retinoic acid (atRA) is used to treat certain cancers and dermatologic diseases. A common adverse effect of atRA is hypercholesterolemia; cytochrome P450 (CYP) 7A repression is suggested as a driver. However, the underlying molecular mechanisms remain unclear. We investigated CYP7A1 expression in the presence of atRA in human hepatocytes and hepatic cell lines. In HepaRG cells, atRA increased cholesterol levels dose-dependently alongside dramatic decreases in CYP7A1 expression. Lentiviral-mediated CYP7A1 overexpression reversed atRA-induced cholesterol accumulation, suggesting that CYP7A1 repression mediated cholesterol accumulation. In CYP7A1 promoter reporter assays and gene-knockdown studies, altered binding of hepatocyte nuclear factor 4 α (HNF4α) to the proximal promoter was essential for atRA-mediated CYP7A1 repression. Pharmacologic inhibition of c-Jun N-terminal kinase (JNK) and ERK pathways attenuated atRA-mediated CYP7A1 repression and cholesterol accumulation. Overexpression of AP-1 (c-Jun/c-Fos), a downstream target of JNK and ERK, repressed CYP7A1 expression. In DNA pull-down and chromatin immunoprecipitation assays, AP-1 exhibited sequence-specific binding to the proximal CYP7A1 promoter region overlapping the HNF4α binding site, and atRA increased AP-1 but decreased HNF4α recruitment to the promoter. Collectively, these results indicate that atRA activates JNK and ERK pathways and the downstream target AP-1 represses HNF4α transactivation of the CYP7A1 promoter, potentially responsible for hypercholesterolemia.

COUP-TFII revisited: Its role in metabolic gene regulation

Chicken Ovalbumin Upstream Promoter Transcription Factor II (COUP-TFII) is an orphan member of the nuclear receptor family of transcriptional regulators. Although hormonal activation of COUP-TFII has not yet been identified, rodent genetic models have uncovered vital and diverse roles for COUP-TFII in biological processes. These include control of cardiac function and angiogenesis, reproduction, neuronal development, cell fate and organogenesis. Recently, an emerging body of evidence has demonstrated COUP-TFII involvement in various metabolic systems such as adipogenesis, lipid metabolism, hepatic gluconeogenesis, insulin secretion, and regulation of blood pressure. The potential relevance of these observations to human pathology has been corroborated by the identification of single nucleotide polymorphism in the human COUP-TFII promoter controlling insulin sensitivity. Of particular interest to metabolism is the ability of COUP-TFII to interact with the Glucocorticoid Receptor (GR). This interaction is known to control gluconeogenesis, principally through direct binding of COUP-TFII/GR complexes to the promoters of gluconeogenic enzyme genes. However, it is likely that this interaction is critical to other metabolic processes, since GR, like COUP-TFII, is an essential regulator of adipogenesis, insulin sensitivity, and blood pressure. This review will highlight these unique roles of COUP-TFII in metabolic gene regulation.

HNF4α Regulates CSAD to Couple Hepatic Taurine Production to Bile Acid Synthesis in Mice

Cysteine dioxygenase 1 (CDO1) converts cysteine to cysteine sulfinic acid, which can be further converted by cysteine sulfinic acid decarboxylase (CSAD) to hypotaurine for taurine production. This cysteine catabolic pathway plays a major role in regulating hepatic cysteine homeostasis. Furthermore, taurine is used for bile acid conjugation, which enhances bile acid solubility and physiological function in the gut. Recent studies show that this cysteine catabolic pathway is repressed by bile acid signaling, but the molecular mechanisms have not been fully elucidated. The mechanisms of bile acid and farnesoid X receptor (FXR) regulation of hepatic CSAD expression were studied in mice and hepatocytes. We showed that hepatocyte nuclear factor 4α (HNF4α) bound the mouse CSAD proximal promoter and induced CSAD transcription. FXR-induced small heterodimer partner (SHP) repressed mouse CSAD gene transcription via interacting with HNF4α as a repressor. Consistent with this model, cholic acid feeding, obeticholic acid administration, and liver HNF4α knockdown reduced hepatic CSAD expression, while liver SHP knockout and apical sodium-dependent bile acid transporter (ASBT) inhibitor treatment induced hepatic CSAD expression in mice. Furthermore, TNF-α also inhibited CSAD expression, which may be partially mediated by reduced HNF4α in mouse hepatocytes. In contrast, bile acids and GW4064 did not inhibit CSAD expression in human hepatocytes. This study identified mouse CSAD as a novel transcriptional target of HNF4α. Bile acids and cytokines repress hepatic CSAD, which closely couples taurine production to bile acid synthesis in mice. The species-specific regulation of CSAD reflects the differential preference of bile acid conjugation to glycine and taurine in humans and mice, respectively.

Bile acids regulate cysteine catabolism and glutathione regeneration to modulate hepatic sensitivity to oxidative injury

Bile acids are signaling molecules that critically control hepatocellular function. Disrupted bile acid homeostasis may be implicated in the pathogenesis of chronic liver diseases. Glutathione is an important antioxidant that protects the liver against oxidative injury. Various forms of liver disease share the common characteristics of reduced cellular glutathione and elevated oxidative stress. This study reports a potentially novel physiological function of bile acids in regulating hepatic sulfur amino acid and glutathione metabolism. We found that bile acids strongly inhibited the cysteine dioxygenase type-1-mediated (CDO1-mediated) cysteine catabolic pathway via a farnesoid X receptor-dependent mechanism. Attenuating this bile acid repressive effect depleted the free cysteine pool and reduced the glutathione concentration in mouse liver. Upon acetaminophen challenge, cholestyramine-fed mice showed impaired hepatic glutathione regeneration capacity and markedly worsened liver injury, which was fully prevented by N-acetylcysteine administration. These effects were recapitulated in CDO1-overexpressing hepatocytes. Findings from this study support the importance of maintaining bile acid homeostasis under physiological and pathophysiological conditions, as altered hepatic bile acid signaling may negatively impact the antioxidant defense mechanism and sensitivity to oxidative injury. Furthermore, this finding provides a possible explanation for the reported mild hepatotoxicity associated with the clinical use of bile acid sequestrants in human patients.

Inactivation of the Nuclear Orphan Receptor COUP-TFII by Small Chemicals

Chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII/NR2F2) is an orphan member of the nuclear receptor family of transcription factors whose activities are modulated upon binding of small molecules into an hydrophobic ligand-binding pocket (LBP). Although the LBP of COUP-TFII is filled with aromatic amino-acid side chains, alternative modes of ligand binding could potentially lead to regulation of the orphan receptor. Here, we screened a synthetic and natural compound library in a yeast one-hybrid assay and identified 4-methoxynaphthol as an inhibitor of COUP-TFII. This synthetic inhibitor was able to counteract processes either positively or negatively regulated by COUP-TFII in different mammalian cell systems. Hence, we demonstrate that the true orphan receptor COUP-TFII can be targeted by small chemicals which could be used to study the physiological functions of COUP-TFII or to counteract detrimental COUP-TFII activities in various pathological conditions.

Prostaglandin E2 promotes hepatic bile acid synthesis by an E prostanoid receptor 3-mediated hepatocyte nuclear receptor 4α/cholesterol 7α-hydroxylase pathway in mice

Prostaglandin E₂ (PGE₂ ) is an important lipid mediator of inflammation. However, whether and how PGE₂ regulates hepatic cholesterol metabolism remains unknown. We found that expression of the PGE₂ receptor, E prostanoid receptor 3 (EP3) expression is remarkably increased in hepatocytes in response to hyperlipidemic stress. Hepatocyte-specific deletion of EP3 receptor (EP3^hep-/- ) results in hypercholesterolemia and augments diet-induced atherosclerosis in low-density lipoprotein receptor knockout (Ldlr^-/- ) mice. Cholesterol 7α-hydroxylase (CYP7A1) is down-regulated in livers of EP3^hep-/- Ldlr^-/- mice, leading to suppressed hepatic bile acid (BA) biosynthesis. Mechanistically, hepatic-EP3 deficiency suppresses CYP7A1 expression by elevating protein kinase A (PKA)-dependent Ser143 phosphorylation of hepatocyte nuclear receptor 4α (HNF4α). Disruption of the PKA-HNF4α interaction and BA sequestration rescue impaired BA excretion and ameliorated atherosclerosis in EP3^hep-/- Ldlr^-/- mice.

CONCLUSION

Our results demonstrated an unexpected role of proinflammatory mediator PGE₂ in improving hepatic cholesterol metabolism through activation of the EP3-mediated PKA/HNF4α/CYP7A1 pathway, indicating that inhibition of this pathway may be a novel therapeutic strategy for dyslipidemia and atherosclerosis. (Hepatology 2017;65:999-1014).

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.

Maintenance of Hepatic Functions in Primary Human Hepatocytes Cultured on Xeno-Free and Chemical Defined Human Recombinant Laminins

Refined methods for maintaining specific functions of isolated hepatocytes under xeno-free and chemical defined conditions is of great importance for the development of hepatocyte research and regenerative therapy. Laminins, a large family of heterotrimeric basement membrane adhesion proteins, are highly cell and tissue type specific components of the extracellular matrix and strongly influence the behavior and function of associated cells and/or tissues. However, detailed biological functions of many laminin isoforms are still to be evaluated. In this study, we determined the distribution of laminin isoforms in human liver tissue and isolated primary human hepatocytes by western blot analysis, and investigated the efficacy of different human recombinant laminin isoforms on hepatic functions during culture. Protein expressions of laminin-chain α2, α3, α4, β1, β3, γ1, and γ2 were detected in both isolated human hepatocytes and liver tissue. No α1 and α5 expression could be detected in liver tissue or hepatocytes. Hepatocytes were isolated from five different individual livers, and cultured on human recombinant laminin isoforms -111, -211, -221, -332, -411, -421, -511, and -521 (Biolamina AB), matrigel (extracted from Engelbreth-Holm-Swarm sarcoma), or collagen type IV (Collagen). Hepatocytes cultured on laminin showed characteristic hexagonal shape in a flat cell monolayer. Viability, double stranded DNA concentration, and Ki67 expression for hepatocytes cultured for six days on laminin were comparable to those cultured on EHS and Collagen. Hepatocytes cultured on laminin also displayed production of human albumin, alpha-1-antitrypsin, bile acids, and gene expression of liver-enriched factors, such as hepatocyte nuclear factor 4 alpha, glucose-6-phosphate, cytochrome P450 3A4, and multidrug resistance-associated protein 2. We conclude that all forms of human recombinant laminin tested maintain cell viability and liver-specific functions of primary human hepatocytes, and that recombinant laminin is a promising xeno-free and chemical defined strategy for preservation of hepatocyte specific function in vitro.

LATS-YAP/TAZ controls lineage specification by regulating TGFβ signaling and Hnf4α expression during liver development

The Hippo pathway regulates the self-renewal and differentiation of various adult stem cells, but its role in cell fate determination and differentiation during liver development remains unclear. Here we report that the Hippo pathway controls liver cell lineage specification and proliferation separately from Notch signalling, using mice and primary hepatoblasts with liver-specific knockout of Lats1 and Lats2 kinase, the direct upstream regulators of YAP and TAZ. During and after liver development, the activation of YAP/TAZ induced by loss of Lats1/2 forces hepatoblasts or hepatocytes to commit to the biliary epithelial cell (BEC) lineage. It increases BEC and fibroblast proliferation by up-regulating TGFβ signalling, but suppresses hepatoblast to hepatocyte differentiation by repressing Hnf4α expression. Notably, oncogenic YAP/TAZ activation in hepatocytes induces massive p53-dependent cell senescence/death. Together, our results reveal that YAP/TAZ activity levels govern liver cell differentiation and proliferation in a context-dependent manner.

The Hippo pathway regulates the differentiation of stem and progenitor cells, but it is unclear how it acts in liver development. Here, the authors knockout Hippo pathway components Lats1 and 2 in the liver, causing suppression of hepatocyte differentiation but promoting biliary cell differentiation.

Probiotics and the BSH-related cholesterol lowering mechanism: a Jekyll and Hyde scenario

Probiotic microorganisms have been documented over the past two decades to play a role in cholesterol-lowering properties via various clinical trials. Several mechanisms have also been proposed and the ability of these microorganisms to deconjugate bile via production of bile salt hydrolase (BSH) has been widely associated with their cholesterol lowering potentials in prevention of hypercholesterolemia. Deconjugated bile salts are more hydrophobic than their conjugated counterparts, thus are less reabsorbed through the intestines resulting in higher excretion into the feces. Replacement of new bile salts from cholesterol as a precursor subsequently leads to decreased serum cholesterol levels. However, some controversies have risen attributed to the activities of deconjugated bile acids that repress the synthesis of bile acids from cholesterol. Deconjugated bile acids have higher binding affinity towards some orphan nuclear receptors namely the farsenoid X receptor (FXR), leading to a suppressed transcription of the enzyme cholesterol 7-alpha hydroxylase (7AH), which is responsible in bile acid synthesis from cholesterol. This notion was further corroborated by our current docking data, which indicated that deconjugated bile acids have higher propensities to bind with the FXR receptor as compared to conjugated bile acids. Bile acids-activated FXR also induces transcription of the IBABP gene, leading to enhanced recycling of bile acids from the intestine back to the liver, which subsequently reduces the need for new bile formation from cholesterol. Possible detrimental effects due to increased deconjugation of bile salts such as malabsorption of lipids, colon carcinogenesis, gallstones formation and altered gut microbial populations, which contribute to other varying gut diseases, were also included in this review. Our current findings and review substantiate the need to look beyond BSH deconjugation as a single factor/mechanism in strain selection for hypercholesterolemia, and/or as a sole mean to justify a cholesterol-lowering property of probiotic strains.

Orphan nuclear receptor oestrogen-related receptor γ (ERRγ) plays a key role in hepatic cannabinoid receptor type 1-mediated induction of CYP7A1 gene expression

Bile acids are primarily synthesized from cholesterol in the liver and have important roles in dietary lipid absorption and cholesterol homoeostasis. Detailed roles of the orphan nuclear receptors regulating cholesterol 7α-hydroxylase (CYP7A1), the rate-limiting enzyme in bile acid synthesis, have not yet been fully elucidated. In the present study, we report that oestrogen-related receptor γ (ERRγ) is a novel transcriptional regulator of CYP7A1 expression. Activation of cannabinoid receptor type 1 (CB1 receptor) signalling induced ERRγ-mediated transcription of the CYP7A1 gene. Overexpression of ERRγ increased CYP7A1 expression in vitro and in vivo, whereas knockdown of ERRγ attenuated CYP7A1 expression. Deletion analysis of the CYP7A1 gene promoter and a ChIP assay revealed an ERRγ-binding site on the CYP7A1 gene promoter. Small heterodimer partner (SHP) inhibited the transcriptional activity of ERRγ and thus regulated CYP7A1 expression. Overexpression of ERRγ led to increased bile acid levels, whereas an inverse agonist of ERRγ, GSK5182, reduced CYP7A1 expression and bile acid synthesis. Finally, GSK5182 significantly reduced hepatic CB1 receptor-mediated induction of CYP7A1 expression and bile acid synthesis in alcohol-treated mice. These results provide the molecular mechanism linking ERRγ and bile acid metabolism.

PPARα-UGT axis activation represses intestinal FXR-FGF15 feedback signalling and exacerbates experimental colitis

Bile acids play a pivotal role in the pathological development of inflammatory bowel disease (IBD). However, the mechanism of bile acid dysregulation in IBD remains unanswered. Here we show that intestinal peroxisome proliferator-activated receptor α (PPARα)-UDP-glucuronosyltransferases (UGTs) signalling is an important determinant of bile acid homeostasis. Dextran sulphate sodium (DSS)-induced colitis leads to accumulation of bile acids in inflamed colon tissues via activation of the intestinal peroxisome PPARα-UGTs pathway. UGTs accelerate the metabolic elimination of bile acids, and thereby decrease their intracellular levels in the small intestine. Reduced intracellular bile acids results in repressed farnesoid X receptor (FXR)-FGF15 signalling, leading to upregulation of hepatic CYP7A1, thus promoting the de novo bile acid synthesis. Both knockout of PPARα and treatment with recombinant FGF19 markedly attenuate DSS-induced colitis. Thus, we propose that intestinal PPARα-UGTs and downstream FXR-FGF15 signalling play vital roles in control of bile acid homeostasis and the pathological development of colitis.

Bile acids have been linked to the development of inflammatory bowel diseases, such as colitis. Here the authors show that bile acid levels in mice are controlled by a circular feedback system involving the nuclear receptors PPARα and FXR, and that this system is dysregulated in colitis.

Hepatocyte-like cells derived from human amniotic epithelial cells can be encapsulated without loss of viability or function in vitro

Placenta derived human amniotic epithelial cells (hAEC) are an attractive source of stem cells for the generation of hepatocyte-like cells (HLC) for therapeutic applications to treat liver diseases. During hAEC differentiation into HLC, they become increasingly immunogenic, which may result in immune cell-mediated rejection upon transplantation into allogeneic recipients. Placing cells within devices such as alginate microcapsules can prevent immune cell-mediated rejection. The aim of this study was to investigate the characteristics of HLC generated from hAEC and to examine the effects of encapsulation on HLC viability, gene expression, and function. hAEC were differentiated for 4 weeks and evaluated for hepatocyte-specific gene expression and function. Differentiated cells were encapsulated in barium alginate microcapsules and cultured for 7 days and the effect of encapsulation on cell viability, function, and hepatocyte related gene expression was determined. Differentiated cells performed key functions of hepatocytes including urea synthesis, drug-metabolizing cytochrome P450 (CYP)3A4 activity, indocyanine green (ICG) uptake, low-density lipoprotein (LDL) uptake, and exhibited glutathione antioxidant capacity. A number of hepatocyte-related genes involved in fat, cholesterol, bile acid synthesis, and xenobiotic metabolism were also expressed showing that the hAEC had differentiated into HLC. Upon encapsulation, the HLC remained viable for at least 7 days in culture, continued to express genes involved in fat, cholesterol, bile acid, and xenobiotic metabolism and had glutathione antioxidant capacity. CYP3A4 activity and urea synthesis by the encapsulated HLC were higher than that of monolayer HLC cultures. Functional HLC can be derived from hAEC, and HLC can be encapsulated within alginate microcapsules without losing viability or function in vitro.

The COUP-TFII variant lacking a DNA-binding domain inhibits the activation of the Cyp7a1 promoter through physical interaction with COUP-TFII

The COUP-TFII (chicken ovalbumin upstream promoter-transcription factor II) nuclear receptor, which is composed of a DNA-binding domain and a ligand-binding domain, exerts pleiotropic effects on development and cell differentiation by regulating the transcription of its target genes, including Cyp7a1 (cytochrome P450, family 7, subfamily a, polypeptide 1), which plays important roles in catabolism of cholesterol in the liver. Although multiple variants of COUP-TFII exist, their roles in the regulation of Cyp7a1 expression have not been elucidated. In the present study, we investigated the roles of COUP-TFII-V2 (variant 2), which lacks a DNA-binding domain, in the regulation of the transcriptional control of the Cyp7a1 gene by COUP-TFII in hepatocellular carcinoma cells. We found that COUP-TFII-V2 was significantly expressed in Huh7 cells, in which Cyp7a1 was not expressed. Furthermore, knockdown of COUP-TFII-V2 enhanced endogenous Cyp7a1 expression in Huh7 cells. Although COUP-TFII activates the Cyp7a1 promoter through direct binding to DNA, this activation was affected by COUP-TFII-V2, which physically interacted with COUP-TFII and inhibited its DNA-binding ability. Chromatin immunoprecipitation assays showed that COUP-TFII-V2 inhibited the binding of endogenous COUP-TFII to the intact Cyp7a1 promoter. The results of the present study suggest that COUP-TFII-V2 negatively regulates the function of COUP-TFII by inhibiting its binding to DNA to decrease Cyp7a1 expression.

A combination of transcriptomics and metabolomics uncovers enhanced bile acid biosynthesis in HepG2 cells expressing CCAAT/enhancer-binding protein β (C/EBPβ), hepatocyte nuclear factor 4α (HNF4α), and constitutive androstane receptor (CAR)

The development of hepatoma-based in vitro models to study hepatocyte physiology is an invaluable tool for both industry and academia. Here, we develop an in vitro model based on the HepG2 cell line that produces chenodeoxycholic acid, the main bile acid in humans, in amounts comparable to human hepatocytes. A combination of adenoviral transfections for CCAAT/enhancer-binding protein β (C/EBPβ), hepatocyte nuclear factor 4α (HNF4α), and constitutive androstane receptor (CAR) decreased intracellular glutamate, succinate, leucine, and valine levels in HepG2 cells, suggestive of a switch to catabolism to increase lipogenic acetyl CoA and increased anaplerosis to replenish the tricarboxylic acid cycle. Transcripts of key genes involved in bile acid synthesis were significantly induced by approximately 160-fold. Consistently, chenodeoxycholic acid production rate was increased by more than 20-fold. Comparison between mRNA and bile acid levels suggest that 12-alpha hydroxylation of 7-alpha-hydroxy-4-cholesten-3-one is the limiting step in cholic acid synthesis in HepG2 cells. These data reveal that introduction of three hepatocyte-related transcription factors enhance anabolic reactions in HepG2 cells and provide a suitable model to study bile acid biosynthesis under pathophysiological conditions.

Bile acid and sterol metabolism with combined HMG-CoA reductase and PCSK9 suppression

Proprotein convertase subtilisin-kexin-9 (PCSK9) inhibition markedly augments the LDL lowering action of statins. The combination is being evaluated for long-term effects on atherosclerotic disease outcomes. However, effects of combined treatment on hepatic cholesterol and bile acid metabolism have not yet been reported. To study this, PCSK9-Y119X mutant (knockout) and wild-type mice were treated with or without atorvastatin for 12 weeks. Atorvastatin progressively lowered plasma LDL in each group, but no differences in liver cholesterol, cholesterol ester, or total bile acid concentrations, or in plasma total bile acid levels were seen. In contrast, atorvastatin increased fecal total bile acids (≈ 2-fold, P < 0.01) and cholesterol concentrations (≈ 3-fold, P < 0.01) versus controls for both PCSK9-Y119X and wild-type mice. All 14 individual bile acids resolved by LC-MS, including primary, secondary, and conjugated species, reflected similar increases. Expression of key liver bile acid synthesis genes CYP7A1 and CYP8B1 were ≈ 2.5-fold higher with atorvastatin in both strains, but mRNA for liver bile acid export and reuptake transporters and conjugating enzymes were not unaffected. The data suggest that hepatocyte cholesterol and bile acid homeostasis is maintained with combined PCSK9 and HMG-CoA reductase inhibition through efficient liver enzymatic conversion of LDL-derived cholesterol into bile acids and excretion of both, with undisturbed enterohepatic recycling.

Prox1 directly interacts with LSD1 and recruits the LSD1/NuRD complex to epigenetically co-repress CYP7A1 transcription

Cholesterol 7α-hydroxylase (CYP7A1) catalyzes the first and rate-limiting step in the classical pathway of bile acids synthesis in liver and is crucial for maintaining lipid homeostasis. Hepatocyte nuclear factor 4α (HNF4α) and α₁-fetoprotein transcription factor (FTF) are two major transcription factors driving CYP7A1 promoter activity in hepatocytes. Previous researches have shown that Prospero-related homeobox (Prox1) directly interacts with both HNF4α and FTF and potently co-represses CYP7A1 transcription and bile acid synthesis through unidentified mechanisms. In this work, mechanisms involved in Prox1-mediated co-repression were explored by identifying Prox1-associated proteins using immunoprecipitation followed by mass spectrometry (IP-MS) methodology. Multiple components of the epigenetically repressive lysine-specific demethylase 1 (LSD1)/nucleosome remodeling and histone deacetylase (NuRD) complex, most notably LSD1 and histone deacetylase 2 (HDAC2), were found to be associated with Prox1 and GST pulldown assay demonstrated that Prox1 directly interacts with LSD1. Sequential chromatin immunoprecipitation (ChIP) assays showed that Prox1 co-localizes with HNF4α, LSD1 and HDAC2 on CYP7A1 promoter in HepG2 cells. Furthermore, by using ChIP assay on HepG2 cells with endogenous Prox1 knocked down by RNA interference, Prox1 was shown to recruit LSD1 and HDAC2 onto CYP7A1 promoter and cause increased H3K4 demethylation. Finally, bile acids treatment of HepG2 cells, which significantly repressed CYP7A1 transcription, resulted in increased Prox1 and LSD1/NuRD complex occupancy on CYP7A1 promoter with a concurrent increase in H3K4 demethylation and H3/H4 deacetylation. These results showed that Prox1 interacts with LSD1 to recruit the repressive LSD1/NuRD complex to CYP7A1 promoter and co-represses transcription through epigenetic mechanisms. In addition, such Prox1-mediated epigenetic repression is involved in the physiologically essential negative feedback inhibition of CYP7A1 transcription by bile acids.

The effect of taurine on cholesterol metabolism

The elevated plasma cholesterol level, in particular, LDL cholesterol is regarded as an important risk factor for the development of atherosclerosis and coronary artery disease. A number of studies provide the evidence that taurine has the efficient action to reduce plasma and liver cholesterol concentrations, especially to decrease VLDL and LDL cholesterol in hypercholesterolemia animal induced by high cholesterol diet. Cholesterol lowering effect of taurine is actually involved in the regulatory mechanism of cholesterol and bile acid homeostasis that mediated by CYP7A1, which has become a biomarker for cholesterol metabolism and itself is also regulated by several factors and nuclear receptors. This review summarizes the change of cholesterol concentration in metabolism observed in feeding studies of hypercholesterolemia animal dealing with taurine, and then, addresses the possible metabolic and molecular mechanisms of cholesterol lowering effect by taurine in three aspects, cholesterol clearance from blood circulation, bioconversion of cholesterol to bile acid in liver, and excretion of cholesterol and bile acid from intestine.

Taurine is a liver X receptor-α ligand and activates transcription of key genes in the reverse cholesterol transport without inducing hepatic lipogenesis

SCOPE

Taurine, which is abundant in seafood, has antiatherogenic activities in both animals and humans; however, its molecular target has been elusive. We examined whether taurine could activate liver X receptor-α (LXR-α), a critical transcription factor in the regulation of reverse cholesterol transport in macrophages.

METHODS AND RESULTS

Taurine bound directly to LXR-α in a reporter gene assay, time-resolved fluorescence resonance energy transfer analysis, and limited protease digestion experiment. Macrophage cells incubated with taurine showed reduced cellular cholesterol and induced medium cholesterol in a dose-dependent manner with the induction of ATP-binding cassette transporter A1 and G gene and protein expression. In hepatocytes, taurine significantly induced Insig-2a levels and delayed nuclear translocation of the sterol regulatory element-binding protein 1 (SREBP-1) protein, resulting in a dose-dependent reduction in the cellular lipid levels without inducing the expression of fatty acid synthesis genes.

CONCLUSION

Taurine is a direct LXR-α ligand, represses cholesterol accumulation, and modulates the expression of genes involved in reverse cholesterol transport in macrophages, without inducing hepatic lipogenesis. The induction of Insig-2a suppressed the nuclear translocation of SREBP-1c.

The effect of CYP7A1 polymorphisms on lipid responses to fenofibrate

INTRODUCTION

CYP7A1 encodes cholesterol 7α-hydroxylase, an enzyme crucial to cholesterol homeostasis. Its transcriptional activity is downregulated by fenofibrate. The goal of this study was to determine the effect of CYP7A1 polymorphisms on lipid changes in response to fenofibrate.

METHODS

We examined the associations of 3 tagging single nuclear polymorphisms (i6782C>T, m204T>G, 3U12536A>C) at CYP7A1 with triglyceride (TG) and high-density lipoprotein cholesterol (HDL)-C responses to a 3-week treatment with 160 mg/d of fenofibrate in 864 US white participants from the Genetics of Lipid Lowering Drugs and Diet Network study.

RESULTS

The m204T>G variant was significantly associated with TG and HDL-C responses with fenofibrate. Individuals homozygous for the common T allele of m204T>G single nuclear polymorphism displayed both the greater reduction of TG (-32% for TT, -28% for GT, -25% for GG, P = 0.004) and an increase of HDL-C response compared with noncarriers (4.1% for TT, 3.4% for GT, 1.2% for GG, P = 0.01). Conversely, individuals homozygous for the minor allele of i6782C>T showed a greater increase in the HDL-C response compared with noncarriers (2.8% CC, 4.5% for CT, 5.8% for TT, P = 0.02), albeit no significant effect on TG response.

CONCLUSIONS

Our data suggest that common variants at the CYP7A1 locus modulate the TG-lowering and HDL-C-raising effects of fenofibrate, and contribute to the interindividual variation of the drug responses.

Identification of a binding motif specific to HNF4 by comparative analysis of multiple nuclear receptors

Nuclear receptors (NRs) regulate gene expression by binding specific DNA sequences consisting of AG[G/T]TCA or AGAACA half site motifs in a variety of configurations. However, those motifs/configurations alone do not adequately explain the diversity of NR function in vivo. Here, a systematic examination of DNA binding specificity by protein-binding microarrays (PBMs) of three closely related human NRs—HNF4α, retinoid X receptor alpha (RXRα) and COUPTF2—reveals an HNF4-specific binding motif (H4-SBM), xxxxCAAAGTCCA, as well as a previously unrecognized polarity in the classical DR1 motif (AGGTCAxAGGTCA) for HNF4α, RXRα and COUPTF2 homodimers. ChIP-seq data indicate that the H4-SBM is uniquely bound by HNF4α but not 10 other NRs in vivo, while NRs PXR, FXRα, Rev-Erbα appear to bind adjacent to H4-SBMs. HNF4-specific DNA recognition and transactivation are mediated by residues Asp69 and Arg76 in the DNA-binding domain; this combination of amino acids is unique to HNF4 among all human NRs. Expression profiling and ChIP data predict ∼100 new human HNF4α target genes with an H4-SBM site, including several Co-enzyme A-related genes and genes with links to disease. These results provide important new insights into NR DNA binding.

Study of FoxA pioneer factor at silent genes reveals Rfx-repressed enhancer at Cdx2 and a potential indicator of esophageal adenocarcinoma development

Understanding how silent genes can be competent for activation provides insight into development as well as cellular reprogramming and pathogenesis. We performed genomic location analysis of the pioneer transcription factor FoxA in the adult mouse liver and found that about one-third of the FoxA bound sites are near silent genes, including genes without detectable RNA polymerase II. Virtually all of the FoxA-bound silent sites are within conserved sequences, suggesting possible function. Such sites are enriched in motifs for transcriptional repressors, including for Rfx1 and type II nuclear hormone receptors. We found one such target site at a cryptic “shadow” enhancer 7 kilobases (kb) downstream of the Cdx2 gene, where Rfx1 restricts transcriptional activation by FoxA. The Cdx2 shadow enhancer exhibits a subset of regulatory properties of the upstream Cdx2 promoter region. While Cdx2 is ectopically induced in the early metaplastic condition of Barrett's esophagus, its expression is not necessarily present in progressive Barrett's with dysplasia or adenocarcinoma. By contrast, we find that Rfx1 expression in the esophageal epithelium becomes gradually extinguished during progression to cancer, i.e, expression of Rfx1 decreased markedly in dysplasia and adenocarcinoma. We propose that this decreased expression of Rfx1 could be an indicator of progression from Barrett's esophagus to adenocarcinoma and that similar analyses of other transcription factors bound to silent genes can reveal unanticipated regulatory insights into oncogenic progression and cellular reprogramming.

FoxA transcriptional regulatory proteins are “pioneer factors” that engage silent genes, helping to endow the competence for activation. About a third of the DNA sites we found to be occupied by FoxA in the adult liver are at genes that are silent. Analysis of transcription factor binding motifs near the FoxA sites at silent genes revealed a co-occurrence of motifs for the transcriptional repressors Rfx1 and type II nuclear hormone receptors (NHR-II). Further analysis of one such region downstream of the Cdx2 gene shows that it is a cryptic enhancer, in that it functions poorly unless Rfx1 or NHR-II binding is prevented, in which case FoxA1 promotes enhancer activity. Cdx2 encodes a transcription factor that promotes intestinal differentiation; ectopic expression of Cdx2 in the esophagus can help promote metaplasia and cancer. By screening numerous staged samples of human tissues, we show that Rfx1 expression is extinguished during the progression to esophageal adenocarcinoma and thus may serve as a marker of cancer progression. These studies exemplify how the analysis of pioneer factors bound to silent genes can reveal a basis for the competence of cells to deregulate gene expression and undergo transitions to cancer.

Tumor suppressor p53 regulates bile acid homeostasis via small heterodimer partner

Metabolic changes in cancer have been observed for almost a century. The mechanisms underlying these changes have begun to emerge from the recent studies implicating the tumor suppressor p53 in multiple metabolic pathways. The ability of p53 to regulate metabolism may also play important roles in the physiology of normal cells and organs. Here we demonstrate that p53 lowers bile acid (BA) levels under both normal and stressed conditions primarily through up-regulating expression of small heterodimer partner, a critical inhibitor of BA synthesis. Our results uncover a unique metabolic regulatory axis that unexpectedly couples p53 to BA homeostasis. Our results also warrant future studies to investigate a possible role of this axis in the tumor suppression by p53, because excessive quantities of BAs are cytotoxic and can cause liver damage and promote gastrointestinal cancers.

Disruption of the murine protein kinase Cbeta gene promotes gallstone formation and alters biliary lipid and hepatic cholesterol metabolism

The protein kinase C (PKC) family of Ca(2+) and/or lipid-activated serine-threonine protein kinases is implicated in the pathogenesis of obesity and insulin resistance. We recently reported that protein kinase Cβ (PKCβ), a calcium-, diacylglycerol-, and phospholipid-dependent kinase, is critical for maintaining whole body triglyceride homeostasis. We now report that PKCβ deficiency has profound effects on murine hepatic cholesterol metabolism, including hypersensitivity to diet-induced gallstone formation. The incidence of gallstones increased from 9% in control mice to 95% in PKCβ(-/-) mice. Gallstone formation in the mutant mice was accompanied by hyposecretion of bile acids with no alteration in fecal bile acid excretion, increased biliary cholesterol saturation and hydrophobicity indices, as well as hepatic p42/44(MAPK) activation, all of which enhance susceptibility to gallstone formation. Lithogenic diet-fed PKCβ(-/-) mice also displayed decreased expression of hepatic cholesterol-7α-hydroxylase (CYP7A1) and sterol 12α-hydroxylase (CYP8b1). Finally, feeding a modified lithogenic diet supplemented with milk fat, instead of cocoa butter, both increased the severity of and shortened the interval for gallstone formation in PKCβ(-/-) mice and was associated with dramatic increases in cholesterol saturation and hydrophobicity indices. Taken together, the findings reveal a hitherto unrecognized role of PKCβ in fine tuning diet-induced cholesterol and bile acid homeostasis, thus identifying PKCβ as a major physiological regulator of both triglyceride and cholesterol homeostasis.

Clinical Significance of FGF-23 in Patients with CKD

FGF23 is a bone-derived hormone that plays an important role in the regulation of phosphate and 1,25-dihydroxy vitamin D metabolism. FGF23 principally acts in the kidney to induce urinary phosphate excretion and suppress 1,25-dihydroxyvitamin D synthesis in the presence of FGF receptor 1 (FGFR1) and its coreceptor Klotho. In patients with chronic kidney disease (CKD), circulating FGF23 levels are progressively increased to compensate for persistent phosphate retention, but this results in reduced renal production of 1,25-dihydroxyvitamin D and leads to hypersecretion of parathyroid hormone. Furthermore, FGF23 is associated with vascular dysfunction, atherosclerosis, and left ventricular hypertrophy. This paper summarizes the role of FGF23 in the pathogenesis of mineral, bone, and cadiovascular disorders in CKD.

Discovering networks of perturbed biological processes in hepatocyte cultures

The liver plays a vital role in glucose homeostasis, the synthesis of bile acids and the detoxification of foreign substances. Liver culture systems are widely used to test adverse effects of drugs and environmental toxicants. The two most prevalent liver culture systems are hepatocyte monolayers (HMs) and collagen sandwiches (CS). Despite their wide use, comprehensive transcriptional programs and interaction networks in these culture systems have not been systematically investigated. We integrated an existing temporal transcriptional dataset for HM and CS cultures of rat hepatocytes with a functional interaction network of rat genes. We aimed to exploit the functional interactions to identify statistically significant linkages between perturbed biological processes. To this end, we developed a novel approach to compute Contextual Biological Process Linkage Networks (CBPLNs). CBPLNs revealed numerous meaningful connections between different biological processes and gene sets, which we were successful in interpreting within the context of liver metabolism. Multiple phenomena captured by CBPLNs at the process level such as regulation, downstream effects, and feedback loops have well described counterparts at the gene and protein level. CBPLNs reveal high-level linkages between pathways and processes, making the identification of important biological trends more tractable than through interactions between individual genes and molecules alone. Our approach may provide a new route to explore, analyze, and understand cellular responses to internal and external cues within the context of the intricate networks of molecular interactions that control cellular behavior.

Gene expression profiling of murine hepatic steatosis induced by tamoxifen

Tamoxifen is an antiestrogenic agent used widely in the treatment of estrogen receptor-positive breast cancer. However, hepatic steatosis has been reported during clinical trials of tamoxifen. To explore the mechanism responsible for this tamoxifen-induced hepatic steatosis, we used microarray analysis to profile the gene expression pattern of mouse liver after tamoxifen treatment. Tamoxifen was administered orally as a single dose of 10mg/kg (low dose), 50mg/kg (medium dose), or 100mg/kg (high dose) to C57BL/6 mice, and the livers were removed 2h, 4h, 8h, and 24h later. From microarray data obtained from the liver samples, 414 genes were selected as tamoxifen-responsive genes (P<0.05, two-way ANOVA; cutoff ≥ 1.5-fold response). These genes were classified into three groups: 308 of the 414 genes showed a time-dependent response, nine genes showed a dose-dependent response, and 97 genes showed a time- and dose-dependent response. Most of the 308 time-dependent-responsive genes were associated predominantly with the biological processes involved in lipid metabolism. Overrepresented transcription factor binding site analysis showed that the following nuclear receptors that are important in lipid and carbohydrate metabolism were overrepresented: the androgen receptor (AR), nuclear receptor subfamily 2 group F member 1 (NR2F1), hepatocyte nuclear factor 4α (HNF4α), and retinoic acid receptor-related orphan receptor alpha 1 (RORα1). Reporter gene analysis further revealed that tamoxifen repressed the 5α-dihydrotestosterone-induced activation of the AR and the intrinsic transactivation function of RORα1, HNF4α, and NR2F1. Taken together, these data provide a better understanding of the molecular mechanism underlying tamoxifen-induced steatogenic hepatotoxicity and useful information for predicting steatogenic hepatotoxicity.

Hepatocyte nuclear factor 4alpha regulation of bile acid and drug metabolism

The hepatocyte nuclear factor 4alpha (HNF4alpha) is a liver-enriched nuclear receptor that plays a critical role in early morphogenesis, fetal liver development, liver differentiation and metabolism. Human HNF4alpha gene mutations cause maturity on-set diabetes of the young type 1, an autosomal dominant non-insulin-dependent diabetes mellitus. HNF4alpha is an orphan nuclear receptor because of which the endogenous ligand has not been firmly identified. The trans-activating activity of HNF4alpha is enhanced by interacting with co-activators and inhibited by corepressors. Recent studies have revealed that HNF4alpha plays a central role in regulation of bile acid metabolism in the liver. Bile acids are required for biliary excretion of cholesterol and metabolites, and intestinal absorption of fat, nutrients, drug and xenobiotics for transport and distribution to liver and other tissues. Bile acids are signaling molecules that activate nuclear receptors to control lipids and drug metabolism in the liver and intestine. Therefore, HNF4alpha plays a central role in coordinated regulation of bile acid and xenobiotics metabolism. Drugs that specifically activate HNF4alpha could be developed for treating metabolic diseases such as diabetes, dyslipidemia and cholestasis, as well as drug metabolism and detoxification.

Regulation of bile acid synthesis by fat-soluble vitamins A and D

Bile acids are required for proper absorption of dietary lipids, including fat-soluble vitamins. Here, we show that the dietary vitamins A and D inhibit bile acid synthesis by repressing hepatic expression of the rate-limiting enzyme CYP7A1. Receptors for vitamin A and D induced expression of Fgf15, an intestine-derived hormone that acts on liver to inhibit Cyp7a1. These effects were mediated through distinct cis-acting response elements in the promoter and intron of Fgf15. Interestingly, transactivation of both response elements appears to be required to maintain basal Fgf15 expression levels in vivo. Furthermore, whereas induction of Fgf15 by vitamin D is mediated through its receptor, the induction of Fgf15 by vitamin A is mediated through the retinoid X receptor/farnesoid X receptor heterodimer and is independent of bile acids, suggesting that this heterodimer functions as a distinct dietary vitamin A sensor. Notably, vitamin A treatment reversed the effects of the bile acid sequestrant cholestyramine on Fgf15, Shp, and Cyp7a1 expression, suggesting a potential therapeutic benefit of vitamin A under conditions of bile acid malabsorption. These results reveal an unexpected link between the intake of fat-soluble vitamins A and D and bile acid metabolism, which may have evolved as a means for these dietary vitamins to regulate their own absorption.

Clinical relevance of FGF-23 in chronic kidney disease

Fibroblast growth factor (FGF)-23 is a recently discovered regulator of calcium-phosphate metabolism. Whereas other known FGFs mainly act in a paracrine manner, FGF-23 has significant systemic effects. Together with its cofactor Klotho, FGF-23 enhances renal phosphate excretion in order to maintain serum phosphate levels within the normal range. In patients with chronic kidney disease (CKD), FGF-23 levels rise in parallel with declining renal function long before a significant increase in serum phosphate concentration can be detected. However, in cross-sectional studies increased FGF-23 levels in patients with CKD were found to be associated not only with therapy-resistant secondary hyperparathyroidism but were also independently related to myocardial hypertrophy and endothelial dysfunction after adjustment for traditional markers of calcium-phosphate metabolism. Finally, in prospective studies high serum FGF-23 concentrations predicted faster disease progression in CKD patients not on dialysis, and increased mortality in patients receiving maintenance hemodialysis. FGF-23 may therefore prove to be an important therapeutic target in the management of CKD.

1alpha,25-Dihydroxyvitamin D(3) triggered vitamin D receptor and farnesoid X receptor-like effects in rat intestine and liver in vivo

1alpha,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), a natural ligand of the vitamin D receptor (VDR), was found to increase the rat ileal Asbt and bile acid absorption. The effects of VDR, whose expression is low in liver, on hepatic transporters and enzymes are unknown. Protein and mRNA levels of target genes in the small intestine, colon and liver after intraperitoneal dosing of 1,25(OH)(2)D(3) (0-2.56 nmol/kg/day for 4 days) to the rat were determined by Western blotting and qPCR, respectively. The 1,25(OH)(2)D(3) treatment increased total Cyp3a protein and Cyp3a1 mRNA expressions in the proximal small intestine, and the short heterodimer partner (SHP), the fibroblast growth factor 15 (FGF15), organic solute transporter (Ostalpha and Ostbeta) mRNA and Asbt protein expressions in the ileum. About 50% higher portal bile acid concentration (65.1+/-14.9 vs 41.9+/-7.8 microm, p<0.05) and elevated expressions of the hepatic farnesoid X receptor (FXR) and SHP mRNA resulted with 1,25(OH)(2)D(3) treatment. Increased Bsep and Ostalpha mRNA expressions in liver and a>50% reduction in the Cyp7a1 protein level (p<0.05) and cholesterol metabolism in rat liver microsomes (p=0.002), likely consequences of the bile acid-FXR-SHP cascade and activation of the signaling pathway for Cyp7a1 inhibition by FGF15, were found. Increased hepatic multidrug resistance-associated protein (Mrp3) and multidrug resistance protein 1a (Mdr1a) mRNA and P-gp protein were also observed. It was concluded that the changes in hepatic transporters and enzymes in the rat were indirect, secondary effects of the liver FXR-SHP cascade due to increased intestinal absorption of bile acids and elevated levels of FGF15, events that led to the activation of FXR.

Mechanisms for increased expression of cholesterol 7alpha-hydroxylase (Cyp7a1) in lactating rats

Cholesterol 7alpha-hydroxylase (Cyp7a1) and the bile acid pool size are increased 2 to 3-fold in lactating postpartum rats. We investigated the interaction of nuclear receptors with the Cyp7a1 proximal promoter and the expression of regulatory signaling pathways in postpartum rats at day 10 (PPd10) versus female controls to identify the mechanisms of increased expression of Cyp7a1, which is maximal at 16 hours. Liver X receptor (LXRalpha) and RNA polymerase II (RNA Pol II) recruitment to Cyp7a1 chromatin were increased 1.5- and 2.5-fold, respectively, at 16 hours on PPd10. Expression of nuclear receptors farnesoid X receptor (FXR), LXRalpha, liver receptor homolog (LRH-1), hepatocyte nuclear factor 4alpha (HNF4alpha), and short heterodimer partner (SHP) messenger RNA (mRNA) and coactivator peroxisome proliferators-activated receptor gamma coactivator-1alpha (PGC-1alpha) mRNA was unchanged in PPd10 versus controls at 16 hours, whereas chicken ovalbumin upstream transcription factor II (COUP-TFII) was decreased 40% at 16 hours. Investigation of a repressive signaling pathway, the c-Jun-N-terminal kinase (JNK) signaling pathway in PPd10 versus controls, showed decreased mRNA expression of hepatocyte growth factor (HGF; decreased 60% at 16 hours) and tyrosine kinase receptor c-Met (decreased 44%-50% at 16 hours), but these were not accompanied by decreased expression of phosphorylated c-Jun. Importantly, expression of fibroblast growth factor 15 (FGF15) mRNA in the ileum was decreased 70% in PPd10 versus controls, whereas phosphorylated mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2 (Erk1/2) protein expression in liver was decreased 88% at 16 hours.

CONCLUSION

The increased recruitment of LXRalpha, a Cyp7a1 stimulatory pathway, and decreased expression of FGF15 and phosphorylated Erk1/2, a Cyp7a1 repressive pathway, combined to increase Cyp7a1 expression during lactation.

Selenoprotein P regulation by the glucocorticoid receptor

Maintenance of the antioxidant activity of selenoproteins is one potential mechanism of the beneficial health effects of selenium. Selenoprotein P is the primary selenium distribution protein of the body as well as the major selenium containing protein in serum. The transcriptional regulation of selenoprotein P is of interest since the extrahepatic expression of this gene has demonstrated differentiation-dependent expression in development as well as under different disease states. SEPP1 displays patterned expression in numerous tissues during development and the loss of SEPP1 expression has been observed in malignancy. In addition, factors that influence inflammatory processes like cytokines and their regulators have been implicated in selenoprotein P transcriptional control. Herein, we identify a retinoid responsive element and describe a mechanism where the glucocorticoid receptor negatively regulates expression of selenoprotein P. Luciferase reporter assays and quantitative PCR were used to measure selenoprotein P transcription in engineered HEK-293 cells. When stimulated with ecdysone analogs, selenoprotein P expression was increased with the use of a fusion transcription factor that contains the glucocorticoid receptor DNA binding domain, an ecdysone ligand-binding domain, and a strong transactivation domain as well as the retinoid X receptor. The native glucocorticoid receptor inhibited selenoprotein P transactivation, and selenoprotein P was further attenuated in the presence of dexamethasone. Our results may provide insight into a potential mechanism by which selenium is redistributed during development, differentiation or under conditions of critical illness, where glucocorticoid levels are typically increased.

Fasting-induced hepatic production of DHEA is regulated by PGC-1alpha, ERRalpha, and HNF4alpha

The transcriptional coactivator peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1alpha is involved in the coordinate induction of changes in gene expression in the liver that enable a homeostatic response to alterations in metabolic state, environmental cues, and nutrient availability. In exploring the specific pathways under PGC-1alpha regulation in the liver, we have made the surprising observation that this coactivator can induce the expression of CYP11A1 and CYP17A1, key rate-limiting enzymes involved in the initial steps of steroidogenesis. Both of these enzymes function to produce C(19)-steroids, converting cholesterol into pregnenolone, and then to dehydroepiandrosterone (DHEA). Estrogen-related receptor (ERR)-alpha mediates PGC-1alpha's induction of CYP11A1 and binds within the first intron of the CYP11A1 gene. Both ERR-alpha and hepatocyte nuclear factor-4alpha are required for PGC-1alpha-mediated induction of CYP17A1, and specific binding sites for these receptors have been identified in the regulatory regions of this gene. The potential physiological significance of these observations was highlighted in rats where fasting induced hepatic expression of PGC-1alpha and CYP17A1 and was associated with an increase in hepatic levels of DHEA. These data suggest that DHEA could be playing a role as an intracellular signaling molecule involved in modulating hepatic activity in response to fasting conditions.

Bile acids: regulation of synthesis

Bile acids are physiological detergents that generate bile flow and facilitate intestinal absorption and transport of lipids, nutrients, and vitamins. Bile acids also are signaling molecules and inflammatory agents that rapidly activate nuclear receptors and cell signaling pathways that regulate lipid, glucose, and energy metabolism. The enterohepatic circulation of bile acids exerts important physiological functions not only in feedback inhibition of bile acid synthesis but also in control of whole-body lipid homeostasis. In the liver, bile acids activate a nuclear receptor, farnesoid X receptor (FXR), that induces an atypical nuclear receptor small heterodimer partner, which subsequently inhibits nuclear receptors, liver-related homolog-1, and hepatocyte nuclear factor 4alpha and results in inhibiting transcription of the critical regulatory gene in bile acid synthesis, cholesterol 7alpha-hydroxylase (CYP7A1). In the intestine, FXR induces an intestinal hormone, fibroblast growth factor 15 (FGF15; or FGF19 in human), which activates hepatic FGF receptor 4 (FGFR4) signaling to inhibit bile acid synthesis. However, the mechanism by which FXR/FGF19/FGFR4 signaling inhibits CYP7A1 remains unknown. Bile acids are able to induce FGF19 in human hepatocytes, and the FGF19 autocrine pathway may exist in the human livers. Bile acids and bile acid receptors are therapeutic targets for development of drugs for treatment of cholestatic liver diseases, fatty liver diseases, diabetes, obesity, and metabolic syndrome.

Mechanism of vitamin D receptor inhibition of cholesterol 7alpha-hydroxylase gene transcription in human hepatocytes

Lithocholic acid (LCA) is a potent endogenous vitamin D receptor (VDR) ligand. In cholestasis, LCA levels increase in the liver and intestine. The objective of this study is to test the hypothesis that VDR plays a role in inhibiting cholesterol 7alpha-hydroxylase (CYP7A1) gene expression and bile acid synthesis in human hepatocytes. Immunoblot analysis has detected VDR proteins in the nucleus of the human hepatoma cell line HepG2 and human primary hepatocytes. 1alpha, 25-Dihydroxy-vitamin D(3) or LCA acetate-activated VDR inhibited CYP7A1 mRNA expression and bile acid synthesis, whereas small interfering RNA to VDR completely abrogated VDR inhibition of CYP7A1 mRNA expression in HepG2 cells. Electrophoretic mobility shift assay and mutagenesis analyses have identified the negative VDR response elements that bind VDR/retinoid X receptor alpha in the human CYP7A1 promoter. Mammalian two-hybrid, coimmunoprecipitation, glutathione S-transferase pull-down, and chromatin immunoprecipitation assays show that ligand-activated VDR specifically interacts with hepatocyte nuclear factor 4alpha (HNF4alpha) to block HNF4alpha interaction with coactivators or to compete with HNF4alpha for coactivators or to compete for binding to CYP7A1 chromatin, which results in the inhibition of CYP7A1 gene transcription. This study shows that VDR is expressed in human hepatocytes and may play a critical role in the inhibition of bile acid synthesis, thus protecting liver cells during cholestasis.

Liver receptor homolog-1 regulates bile acid homeostasis but is not essential for feedback regulation of bile acid synthesis

Liver receptor homolog 1 (LRH-1), an orphan nuclear receptor, is highly expressed in liver and intestine, where it is implicated in the regulation of cholesterol, bile acid, and steroid hormone homeostasis. Among the proposed LRH-1 target genes in liver are those encoding cholesterol 7alpha-hydroxylase (CYP7A1) and sterol 12alpha-hydroxylase (CYP8B1), which catalyze key steps in bile acid synthesis. In vitro studies suggest that LRH-1 may be involved both in stimulating basal CYP7A1 and CYP8B1 transcription and in repressing their expression as part of the nuclear bile acid receptor [farnesoid X receptor (FXR)]-small heterodimer partner signaling cascade, which culminates in small heterodimer partner binding to LRH-1 to repress gene transcription. However, in vivo analysis of LRH-1 actions has been hampered by the embryonic lethality of Lrh-1 knockout mice. To overcome this obstacle, mice were generated in which Lrh-1 was selectively disrupted in either hepatocytes or intestinal epithelium. LRH-1 deficiency in either tissue changed mRNA levels of genes involved in cholesterol and bile acid homeostasis. Surprisingly, LRH-1 deficiency in hepatocytes had no significant effect on basal Cyp7a1 expression or its repression by FXR. Whereas Cyp8b1 repression by FXR was also intact in mice deficient for LRH-1 in hepatocytes, basal CYP8B1 mRNA levels were significantly decreased, and there were corresponding changes in the composition of the bile acid pool. Taken together, these data reveal a broad role for LRH-1 in regulating bile acid homeostasis but demonstrate that LRH-1 is either not involved in the feedback regulation of bile acid synthesis or is compensated for by other factors.