Multiple mechanisms of ontogenic regulation of nuclear receptors during rat liver development

Prenatal glucocorticoid administration enhances bilirubin metabolic capacity and increases Ugt1a and Abcc2 gene expression via glucocorticoid receptor and PXR in rat fetal liver

AIM

Jaundice is especially common in premature infant born before 35 weeks. Because the premature infant liver is not fully developed at birth it may be incomplete the bilirubin metabolism. The purpose was to evaluate the metabolism and the excretion of bilirubin in the premature infant rat liver following prenatal glucocorticoid (GC) administration.

METHODS

Dexamethasone (DEX) was administered subcutaneously to pregnant Wistar rats for two consecutive days on gestational days 17 and 19. The fetus were delivered by cesarean section in gestational days 19 and 21. The mRNA levels and protein levels of bilirubin-metabolic enzymes and transporters in the fetal liver tissues were analyzed using RT-PCR immunohistochemistry staining and ELISA, respectively. We evaluated that the effect of bilirubin-metabolic enzymes in the primary fetal rat hepatocytes treated with DEX after pretreated with glucocorticoid receptor (GR, Nr3c1) and Pxr (Nr1i2) siRNA.

RESULTS

Ugt1a1 and Bsep (Abcb11) mRNA levels were significantly increased in the fetuses by prenatal GC administration. The mRNA levels of nuclear transcription factors Nr1i2, Car (Nr1i3), and Rxrα (Nr2b1) were also significantly increased in the fetuses by prenatal GC administration. In addition, DEX increased Nr1i2, Ugt1a1, and Abcc2 (Mrp2) mRNA levels in the primary fetal hepatocytes. The Nr3c1 or Nr1i2 siRNA-mediated knockdown suppressed the increases of Ugt1a1, and Abcc2 mRNA levels induced by DEX, indicating that DEX are mediated by GC receptor and PXR in primary fetal hepatocytes.

CONCLUSIONS

These results suggest that prenatal GC administration increases bilirubin-metabolic ability, in the premature liver, which may prevent jaundice in neonates.

Presentation and Outcomes of Infants With Idiopathic Cholestasis: A Multicenter Prospective Study

OBJECTIVES

The aim of the study was to determine the frequency and natural history of infantile idiopathic cholestasis (IC) in a large, prospective, multicenter cohort of infants.

METHODS

We studied 94 cholestatic infants enrolled up to 6 months of age in the NIDDK ChiLDReN (Childhood Liver Disease Research Network) "PROBE" protocol with a final diagnosis of IC; they were followed up to 30 months of age.

RESULTS

Male sex (66/94; 70%), preterm birth (22/90 with data; 24% born at < 37 weeks' gestational age), and low birth weight (25/89; 28% born at <2500 g) were frequent, with no significant differences between outcomes. Clinical outcomes included death (n = 1), liver transplant (n = 1), biochemical resolution (total bilirubin [TB] ≤1 mg/dL and ALT < 35 U/L; n = 51), partial resolution (TB > 1 mg/dL and/or ALT > 35 U/L; n = 7), and exited healthy (resolved disease per study site report but without documented biochemical resolution; n = 34). Biochemical resolution occurred at median of 9 months of age. GGT was <100 U/L at baseline in 34 of 83 participants (41%).

CONCLUSIONS

Frequency of IC and of death or liver transplant was less common in this cohort than in previously published cohorts, likely because of recent discovery and diagnosis of genetic etiologies of severe/persistent cholestasis that previously were labeled as idiopathic. Preterm birth and other factors associated with increased vulnerability in neonates are relatively frequent and may contribute to IC. Overall outcome in IC is excellent. Low/normal GGT was common, possibly indicating a role for variants in genes associated with low-GGT cholestasis-this warrants further study.

Physiologically based pharmacokinetic modelling of acetaminophen in preterm neonates-The impact of metabolising enzyme ontogeny and reduced cardiac output

In preterm neonates, physiologically based pharmacokinetic (PBPK) models are suited for studying the effects of maturational and non-maturational factors on the pharmacokinetics of drugs with complex age-dependent metabolic pathways like acetaminophen (APAP). The aim of this study was to determine the impact of drug metabolising enzymes ontogeny on the pharmacokinetics of APAP in preterm neonates and to study the effect of reduced cardiac output (CO) on its PK using PBPK modelling. A PBPK model for APAP was first developed and validated in adults and then scaled to paediatric age groups to account for the effect of enzyme ontogeny. In preterm neonates, CO was reduced by 10%, 20%, and 30% to determine how this might affect APAP PK in preterm neonates. In all age groups, the predicted concentration-time profiles of APAP were within 5th and 95th percentile of the clinically observed concentration-time profiles and the predicted Cmax and AUC were within 2-folds of the reported parameters in clinical studies. Sulfation accounted for most of APAP metabolism in children, with the highest contribution of 68% in preterm neonates. A reduction in CO by up to 30% did not significantly alter the clearance of APAP in preterm neonates. The model successfully incorporated the ontogeny of drug metabolising enzymes involved in APAP metabolism and adequately predicted the PK of APAP in preterm neonates. A reduction in hepatic perfusion as a result of up to 30% reduction in CO has no effect on the PK of APAP in preterm neonates.

Regulation of CAR and PXR Expression in Health and Disease

Pregnane X receptor (PXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3) are members of the nuclear receptor superfamily that mainly act as ligand-activated transcription factors. Their functions have long been associated with the regulation of drug metabolism and disposition, and it is now well established that they are implicated in physiological and pathological conditions. Considerable efforts have been made to understand the regulation of their activity by their cognate ligand; however, additional regulatory mechanisms, among which the regulation of their expression, modulate their pleiotropic effects. This review summarizes the current knowledge on CAR and PXR expression during development and adult life; tissue distribution; spatial, temporal, and metabolic regulations; as well as in pathological situations, including chronic diseases and cancers. The expression of CAR and PXR is modulated by complex regulatory mechanisms that involve the interplay of transcription factors and also post-transcriptional and epigenetic modifications. Moreover, many environmental stimuli affect CAR and PXR expression through mechanisms that have not been elucidated.

The influence of the gastrointestinal microbiome on infant colic

INTRODUCTION

Although infantile colic is relatively frequent, its pathophysiology is not yet understood. The aim of this paper is to provide a better understanding of the link between infantile colic and the gastrointestinal microbiome.

AREAS COVERED

The gastro-intestinal microbiome may already start to develop in the womb and grows exponentially immediately after birth. Factors influencing the microbiome can cause dysbiosis and precipitate symptoms of colic through several mechanisms such as increased gas production and low grade gut inflammation. Other possible factors are immaturity of the enterohepatic bile acid cycle and administration of antibiotics and other medications during the perinatal period. An effective treatment for all colicky infants has yet to be discovered, but the probiotic Lactobacillus reuteri DSM17938 was shown to be effective in breastfed infants with colic. The scientific databases 'Pubmed' and 'Google scholar' were searched from inception until 02/2020. Relevant articles were selected based on the abstract.

EXPERT OPINION

Recent literature confirmed that the composition of the gastrointestinal microbiome is associated with the development of infantile colic. It can be speculated that full sequencing and bioinformatics analysis to identify the microbiome down to the species level may provide answers to the etiology and management of infantile colic.

Ontogeny of Hepatic Sulfotransferases and Prediction of Age-Dependent Fractional Contribution of Sulfation in Acetaminophen Metabolism

Cytosolic sulfotransferases (SULTs), including SULT1A, SULT1B, SULT1E, and SULT2A isoforms, play noteworthy roles in xenobiotic and endobiotic metabolism. We quantified the protein abundances of SULT1A1, SULT1A3, SULT1B1, and SULT2A1 in human liver cytosol samples (n = 194) by liquid chromatography-tandem mass spectrometry proteomics. The data were analyzed for their associations by age, sex, genotype, and ethnicity of the donors. SULT1A1, SULT1B1, and SULT2A1 showed significant age-dependent protein abundance, whereas SULT1A3 was invariable across 0-70 years. The respective mean abundances of SULT1A1, SULT1B1, and SULT2A1 in neonatal samples was 24%, 19%, and 38% of the adult levels. Interestingly, unlike UDP-glucuronosyltransferases and cytochrome P450 enzymes, SULT1A1 and SULT2A1 showed the highest abundance during early childhood (1 to <6 years), which gradually decreased by approx. 40% in adolescents and adults. SULT1A3 and SULT1B1 abundances were significantly lower in African Americans compared with Caucasians. Multiple linear regression analysis further confirmed the association of SULT abundances by age, ethnicity, and genotype. To demonstrate clinical application of the characteristic SULT ontogeny profiles, we developed and validated a proteomics-informed physiologically based pharmacokinetic model of acetaminophen. The latter confirmed the higher fractional contribution of sulfation over glucuronidation in the metabolism of acetaminophen in children. The study thus highlights that the ontogeny-based age-dependent fractional contribution (fm) of individual drug-metabolizing enzymes has better potential in prediction of drug-drug interactions and the effect of genetic polymorphisms in the pediatric population.

Phytosterols promote liver injury and Kupffer cell activation in parenteral nutrition-associated liver disease

Parenteral nutrition-associated liver disease (PNALD) is a serious complication of PN in infants who do not tolerate enteral feedings, especially those with acquired or congenital intestinal diseases. Yet, the mechanisms underlying PNALD are poorly understood. It has been suggested that a component of soy oil (SO) lipid emulsions in PN solutions, such as plant sterols (phytosterols), may be responsible for PNALD, and that use of fish oil (FO)-based lipid emulsions may be protective. We used a mouse model of PNALD combining PN infusion with intestinal injury to demonstrate that SO-based PN solution causes liver damage and hepatic macrophage activation and that PN solutions that are FO-based or devoid of all lipids prevent these processes. We have furthermore demonstrated that a factor in the SO lipid emulsions, stigmasterol, promotes cholestasis, liver injury, and liver macrophage activation in this model and that this effect may be mediated through suppression of canalicular bile transporter expression (Abcb11/BSEP, Abcc2/MRP2) via antagonism of the nuclear receptors Fxr and Lxr, and failure of up-regulation of the hepatic sterol exporters (Abcg5/g8/ABCG5/8). This study provides experimental evidence that plant sterols in lipid emulsions are a major factor responsible for PNALD and that the absence or reduction of plant sterols is one of the mechanisms for hepatic protection in infants receiving FO-based PN or lipid minimization PN treatment. Modification of lipid constituents in PN solutions is thus a promising strategy to reduce incidence and severity of PNALD.

Establishment of metabolism and transport pathways in the rodent and human fetal liver

The ultimate fate of drugs and chemicals in the body is largely regulated by hepatic uptake, metabolism, and excretion. The liver acquires the functional ability to metabolize and transport chemicals during the perinatal period of development. Research using livers from fetal and juvenile rodents and humans has begun to reveal the timing, key enzymes and transporters, and regulatory factors that are responsible for the establishment of hepatic phase I and II metabolism as well as transport. The majority of this research has been limited to relative mRNA and protein quantification. However, the recent utilization of novel technology, such as RNA-Sequencing, and the improved availability and refinement of functional activity assays, has begun to provide more definitive information regarding the extent of hepatic drug disposition in the developing fetus. The goals of this review are to provide an overview of the early regulation of the major phase I and II enzymes and transporters in rodent and human livers and to highlight potential mechanisms that control the ontogeny of chemical metabolism and excretion pathways.

Identification of functionally relevant lysine residues that modulate human farnesoid X receptor activation

Base amino acid lysine residues play an important role in regulation of nuclear receptors [e.g., farnesyl X receptor (FXR)], leading to enhanced or suppressed biologic activity. To understand the molecular mechanisms and the subsequent effects in modulating FXR functions in diverse biologic processes, we individually replaced eight highly conserved lysine residues of human FXR (hFXR) with arginine. The effects of each mutated FXR on target gene activation, subcellular localization, protein-protein association, and protein-DNA interaction were investigated. Results demonstrated that K122R, K210R, K339R, and K460R mutants of hFXR significantly impaired target gene [organic solute transporter α/β and bile salt export pump (BSEP)] promoter reporter activity in a ligand-dependent fashion. None of the four mutants affected the nuclear localization of FXR. Protein interaction studies show that K210R slightly but significantly decreased FXR/retinoid X receptor (RXR) binding affinity but enhanced the interaction of FXR with lysine methyltransferase Set7/9 by ∼21%. K460R decreased the FXR interaction with Set7/9 by ∼45% but had no significant effects on interaction with RXR. Electrophoretic mobility shift assays demonstrated that hFXR-K210R and -K339R reduced the protein-DNA (IR1 element at hBSEP promoter) binding affinity by ∼80 and ∼90%, respectively. Computational-based protein modeling studies were consistent with these results and provided further insights into the potential underlying mechanisms responsible for these results. In conclusion, four highly conserved lysine residues of hFXR, K122, K210, K339, and K460, have been identified that play a critical role in FXR target gene regulation and molecular interaction (protein-protein and protein-DNA).

Ontogeny of novel cytochrome P450 gene isoforms during postnatal liver maturation in mice

The ontogeny of the first four families of cytochromes P450 (P450s) (i.e., Cyp1-Cyp4) can affect the biotransformation of drugs and dietary chemicals in liver, resulting in unique pharmacological reactions in children. Because genome-scale investigations have identified many novel P450 isoforms, it is critical to perform a systematic characterization of these P450s during liver development. In this study, livers were collected from C57BL/6 mice 2 days before birth and at various postnatal ages (0-45 days of age). The mRNA levels for 75 P450 isoforms (Cyp1-Cyp4) were quantified with branched DNA assays and reverse transcription-polymerase chain reaction assays. More than half of the mouse P450s are conserved in humans, but there are more isoforms in mice. The P450 mRNA levels increased after birth in mouse liver, forming four distinct ontogenic patterns. The majority of P450s form a total of eight genomic clusters, namely, Cyp1a1 and Cyp1a2 genes on chromosome 9 (cluster 1), Cyp2a, Cyp2b, Cyp2f, Cyp2g, and Cyp2t genes on chromosome 7 (cluster 2), Cyp2c genes on chromosome 19 (cluster 3), Cyp2d genes on chromosome 15 (cluster 4), Cyp2j genes on chromosome 4 (cluster 5), Cyp3a genes on chromosome 5 (cluster 6), Cyp4a, Cyp4b, and Cyp4x genes on chromosome 4 (cluster 7), and Cyp4f genes on chromosome 17 (cluster 8). Some P450 isoforms within the same genomic cluster showed similar ontogenic patterns. In conclusion, the present study revealed four patterns of ontogeny for P450s in liver and showed that many P450s within a genomic cluster exhibited similar ontogenic patterns, which suggests that some P450s within a cluster are likely regulated by a common pathway during liver development.

The Roles of PPARs in the Fetal Origins of Metabolic Health and Disease

Beyond the short-term effects on fertility, there is increasing evidence that obesity or the consumption of an inappropriate diet by the mother during pregnancy adversely affects the long-term health of her offspring. PPAR and RXR isotypes are widely expressed in reproductive tissues and in the developing fetus. Through their interactions with fatty acids, they may mediate adaptive responses to the changes in the maternal diet. In the maturing follicle, PPAR-γ has an important role in the granulosa cells that surround the maturing oocyte. After fertilisation, PPAR-γ and PPAR-β/δ are essential regulators of placentation and the subsequent development of key metabolic tissues such as skeletal muscle and adipose cells. Activation of PPAR-γ and PPAR-β/δ during fetal development has the potential to modify the growth and development of these tissues. PPAR-α is expressed at low levels in the fetal liver, however, this expression may be important, as changes in the methylation of DNA in its promoter region are reported to take place during this period of development. This epigenetic modification then programmes subsequent expression. These findings suggest that two separate PPAR-dependent mechanisms may be involved in the fetal adaptations to the maternal diet, one, mediated by PPAR-γ and PPAR-β/δ, regulating cell growth and differentiation; and another adapting long-term lipid metabolism via epigenetic changes in PPAR-α to optimise postnatal survival.

Disappearance of gender-related difference in the toxicity of benzotriazole ultraviolet absorber in juvenile rats

2-(2'-hydroxy-3',5'-di-tert-butylphenyl)benzotriazole (HDBB) is an ultraviolet absorber used in plastic resin products, such as building materials and automobile components. In oral repeated dose toxicity studies using 5- or 6-week-old rats, this chemical induced hepatic histopathological changes, such as hypertrophy accompanied with eosinophilic granular changes and focal necrosis of hepatocytes, and male rats showed nearly 25 times higher susceptibility to the toxic effects than females. Castration at approximately 4 weeks of age markedly reduced the sex-related variation in HDBB toxicity, but some difference, less than five times, remained between male and female castrated rats. Following oral HDBB administration to male and female juvenile rats from postnatal days 4-21, such gender-related difference in toxic susceptibility was not detected; therefore, it is speculated that the determinants of susceptibility to HDBB toxicity are differentiated between sexes after weaning. In young rats given HDBB, there was no gender-related difference in plasma HDBB concentration, and no metabolites were detected in the plasma of either sex. HDBB induced lauric acid 12-hydroxylase activity in the liver and this change was more pronounced in males than in females. These findings indicate that HDBB could show hepatic peroxisome proliferation activity, and the difference in the susceptibility of male and female rats to this effect might lead to marked gender-related differences in toxicity.

Multiple nuclear receptors may regulate hepatitis B virus biosynthesis during development

Hepatitis B virus (HBV) replicates by the reverse transcription of the viral 3.5 kb pregenomic RNA. Therefore the level of expression of this transcript in the liver is a primary determinant of HBV biosynthesis. In vivo neonatal transcription of the HBV 3.5 kb pregenomic RNA is developmental regulated by hepatocyte nuclear factor 4α (HNF4α). In addition, viral biosynthesis in non-hepatoma cells can be supported directly by this nuclear receptor. However HBV transcription and replication can be supported by additional nuclear receptors including the retinoid X receptor α/peroxisome proliferator-activated receptor α (RXRα/PPARα), retinoid X receptor α/farnesoid X receptor α (RXRα/FXRα), liver receptor homolog 1 (LRH1) and estrogen-related receptors (ERR) in non-hepatoma cells. Therefore during neonatal liver development, HNF4α may progressively activate viral transcription and replication by binding directly to the proximal HNF4α recognition sequence within the nucleocapsid promoter. Alternatively, HNF4α may support viral biosynthesis in vivo indirectly by activating a network of liver-enriched nuclear receptors that, in combination, direct HBV 3.5 kb pregenomic RNA transcription and replication.

Hepatic bile acid metabolism in the neonatal hamster: expansion of the bile acid pool parallels increased Cyp7a1 expression levels

Intraluminal concentrations of bile acids are low in newborn infants and increase rapidly after birth, at least partly owing to increased bile acid synthesis rates. The expansion of the bile acid pool is critical since bile acids are required to stimulate bile flow and absorb lipids, a major component of newborn diets. The purpose of the present studies was to determine the mechanism responsible for the increase in bile acid synthesis rates and the subsequent enlargement of bile acid pool sizes (BAPS) during the neonatal period, and how changes in circulating hormone levels might affect BAPS. In the hamster, pool size was low just after birth and increased modestly until 10.5 days postpartum (dpp). BAPS increased more significantly ( approximately 3-fold) between 10.5 and 15.5 dpp. An increase in mRNA and protein levels of cholesterol 7alpha-hydroxylase (Cyp7a1), the rate-limiting step in classical bile acid synthesis, immediately preceded an increase in BAPS. In contrast, levels of oxysterol 7alpha-hydroxylase (Cyp7b1), a key enzyme in bile acid synthesis by the alternative pathway, were relatively elevated by 1.5 dpp. farnesyl X receptor (FXR) and short heterodimeric partner (SHP) mRNA levels remained relatively constant at a time when Cyp7a1 levels increased. Finally, although simultaneous increases in circulating cortisol and Cyp7a1 levels occurred, precocious expression of Cyp7a1 could not be induced in neonatal hamsters with dexamethasone. Thus the significant increase in Cyp7a1 levels in neonatal hamsters is due to mechanisms independent of the FXR and SHP pathway and cortisol.

Pharmacological activation of LXR in utero directly influences ABC transporter expression and function in mice but does not affect adult cholesterol metabolism

Cholesterol is critical for several cellular functions and essential for normal fetal development. Therefore, its metabolism is tightly controlled during all life stages. The liver X receptors-alpha (LXRalpha; NR1H3) and -beta (LXRbeta; NR1H2) are nuclear receptors that are of key relevance in coordinating cholesterol and fatty acid metabolism. The aim of this study was to elucidate whether fetal cholesterol metabolism can be influenced in utero via pharmacological activation of LXR and whether this would have long-term effects on cholesterol homeostasis. Administration of the LXR agonist T0901317 to pregnant mice via their diet (0.015% wt/wt) led to induced fetal hepatic expression levels of the cholesterol transporter genes Abcg5/g8 and Abca1, higher plasma cholesterol levels, and lower hepatic cholesterol levels compared with controls. These profound changes during fetal development did not affect cholesterol metabolism in adulthood nor did they influence coping with a high-fat/high-cholesterol diet. This study shows that the LXR system is functional in fetal mice and susceptible to pharmacological activation. Despite massive changes in fetal cholesterol metabolism, regulatory mechanisms involved in cholesterol metabolism return to a "normal" state in offspring and allow coping with a high-fat/high-cholesterol diet.

Review of the expression of peroxisome proliferator-activated receptors alpha (PPAR alpha), beta (PPAR beta), and gamma (PPAR gamma) in rodent and human development

The peroxisome proliferator-activated receptors (PPAR) belong to the nuclear hormone receptor superfamily and there are three primary subtypes, PPARalpha, beta, and gamma. These receptors regulate important physiological processes that impact lipid homeostasis, inflammation, adipogenesis, reproduction, wound healing, and carcinogenesis. These nuclear receptors have important roles in reproduction and development and their expression may influence the responses of an embryo exposed to PPAR agonists. PPARs are relevant to the study of the biological effects of the perfluorinated alkyl acids as these compounds, including perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), activate PPARalpha. Exposure of the rodent to PFOA or PFOS during gestation results in neonatal deaths, developmental delay and growth deficits. Studies in PPARalpha knockout mice demonstrate that the developmental effects of PFOA, but not PFOS, depend on expression of PPARalpha. This review provides an overview of PPARalpha, beta, and gamma protein and mRNA expression during mouse, rat, and human development. The review presents the results from many published studies and the information is organized by organ system and collated to show patterns of expression at comparable developmental stages for human, mouse, and rat. The features of the PPAR nuclear receptor family are introduced and what is known or inferred about their roles in development is discussed relative to insights from genetically modified mice and studies in the adult.

Hepatocyte nuclear factor-4alpha is a central transactivator of the mouse Ntcp gene

Sodium taurocholate cotransporting polypeptide (Ntcp) is the major uptake system for conjugated bile acids. Deletions of hepatocyte nuclear factor (HNF)-1alpha and retinoid X receptor-alpha:retinoic acid receptor-alpha binding sites in the mouse 5'-flanking region corresponding to putatively central regulatory elements of rat Ntcp do not significantly reduce promoter activity. We hypothesized that HNF-4alpha, which is increasingly recognized as a central regulator of hepatocyte function, may directly transactivate mouse (mNtcp). A 1.1-kb 5'-upstream region including the mouse Ntcp promoter was cloned and compared with the rat promoter. In contrast to a moderate 3.5-fold activation of mNtcp by HNF-1alpha, HNF-4alpha cotransfection led to a robust 20-fold activation. Deletion analysis of mouse and rat Ntcp promoters mapped a conserved HNF-4alpha consensus site at -345/-326 and -335/-316 bp, respectively. p-475bpmNtcpLUC is not transactivated by HNF-1alpha but shows a 50-fold enhanced activity upon cotransfection with HNF-4alpha. Gel mobility shift assays demonstrated a complex of the HNF-4alpha-element formed with liver nuclear extracts that was blocked by an HNF-4alpha specific antibody. HNF-4alpha binding was confirmed by chromatin immunoprecipitation. Using Hepa 1-6 cells, HNF-4alpha-knockdown resulted in a significant 95% reduction in NTCP mRNA. In conclusion, mouse Ntcp is regulated by HNF-4alpha via a conserved distal cis-element independently of HNF-1alpha.

Ostalpha-Ostbeta is required for bile acid and conjugated steroid disposition in the intestine, kidney, and liver

Mice deficient in the organic solute transporter (Ost)-alpha subunit of the heteromeric organic solute and steroid transporter, Ostalpha-Ostbeta, were generated and were found to be viable and fertile but exhibited small intestinal hypertrophy and growth retardation. Bile acid pool size and serum levels were decreased by more than 60% in Ostalpha-/- mice, whereas fecal bile acid excretion was unchanged, suggesting a defect in intestinal bile acid absorption. In support of this hypothesis, when [3H]taurocholic acid or [3H]estrone 3-sulfate were administered into the ileal lumen, absorption was lower in Ostalpha-/- mice. Interestingly, serum cholesterol and triglyceride levels were also approximately 15% lower in Ostalpha-/- mice, an effect that may be related to the impaired intestinal bile acid absorption. After intraperitoneal administration of [3H]estrone 3-sulfate or [3H]dehydroepiandrosterone sulfate, Ostalpha-/- mice had higher levels of radioactivity in their liver and urinary bladder and less in the duodenum, indicating altered hepatic, renal, and intestinal disposition. Loss of Ostalpha was associated with compensatory changes in the expression of several genes involved in bile acid homeostasis, including an increase in the multidrug resistance-associated protein 3, (Mrp3)/Abcc3, an alternate basolateral bile acid export pump, and a decrease in cholesterol 7alpha-hydroxylase, Cyp7a1, the rate-limiting enzyme in bile acid synthesis. The latter finding may be explained by increased ileal expression of fibroblast growth factor 15 (Fgf15), a negative regulator of hepatic Cyp7a1 transcription. Overall, these findings provide direct support for the hypothesis that Ostalpha-Ostbeta is a major basolateral transporter of bile acids and conjugated steroids in the intestine, kidney, and liver.

Expression of nuclear receptor mRNA and liver X receptor-mediated regulation of ABC transporter A1 at rat blood-brain barrier

The aim of the present study was to investigate the expression of nuclear receptor mRNA and regulation of the expression of ATP-binding cassette (ABC) transporters by nuclear receptor agonists in rat brain capillary endothelial cells, which form the blood-brain barrier, by using rat brain capillary fraction from 8-week-old rats and a conditionally immortalized brain capillary endothelial cell line (TR-BBB13). RT-PCR analysis revealed that liver X receptor alpha and beta, retinoid X receptor alpha and beta and peroxisome proliferator-activating receptor alpha and beta mRNAs were expressed in the rat brain capillary endothelial cells and TR-BBB cells. In contrast, pregnane X receptor, farnesoid X receptor and constitutive androstane receptor were not detected. Furthermore, treatment with a liver X receptor agonist increased the ABCA1 mRNA level in TR-BBB13 cells, while ABCG2 mRNA expression was not affected. Treatment with a rat pregnane X receptor agonist did not affect the ABCB1 mRNA level in TR-BBB13 cells. These results demonstrate that the rat blood-brain barrier has an expressional regulation mechanism via sterol-related nuclear receptor, and indicate that the blood-brain barrier in 8-week-old rats lacks ABCB1 regulation via pregnane X receptor.

Expression of Liver X Receptor α in Rat Fetal Tissues at Different Developmental Stages

The liver X receptor (LXR) is a nuclear receptor that acts as a sterol sensor and metabolic regulator of cholesterol and lipid homeostasis. Using a novel LXRα-specific antibody for immunohistochemistry, we evaluated cellular expression of LXRα in fetal rat tissues. In the fetal liver, LXRα-positive macrophages appeared at 12 days and their number peaked at 18 days of gestation. In contrast, hepatocytes expressed LXRα during the later stage of gestation, suggesting the functional development of the liver during ontogeny. Later, macrophages in spleen and thymus expressed LXRα, and some mononuclear cells in the vascular lumen compatible to primitive/fetal macrophages in the fetal circulation were found to express LXRα. In vitro, rat monocytes did not express LXRα, but monocyte-derived macrophages cultured in the presence of macrophage-colony stimulating factor revealed the distinct expression of LXRα in nucleoli. These findings suggest that LXRα plays a role in the differentiation of fetal macrophages, particularly hepatic macrophages, in rat development.

Bile salt excretory pump: biology and pathobiology

Bile acids are the major determinant and driving force for the generation of bile flow. Bile acid transport across the canalicular membrane is primarily an ATP-dependent process. The predominant transporter is the bile salt excretory pump (BSEP, ABCB11), a member of the adenosine triphosphate-binding cassette (ABC) family of transporters. Regulatory mechanisms that can coordinate the genes encoding bile acid transport proteins are critically important to avoid hepatocyte damage from intracellar accumulation of bile acids. Bile salts are natural ligands for several nuclear hormone receptors expressed in liver and intestine. Nuclear receptors are transcription factors that bind specific ligands such as bile acids and regulate gene expression according to the metabolic requirements of the cell. In cloning of the BSEP gene, we found a binding site in the promoter for the farnesoid X receptor (FXR), a nuclear receptor for bile acids. FXR activity requires heterodimerization with the 9-cis retinoid receptor (RXR alpha), and when bound by bile acids and retinoic acid, the complex effectively activates the transcription of BSEP. There is a growing body of evidence for the activation of nuclear hormone receptors through the remodeling of chromatin by histone modification involving acetylation, in concert with methylation of H3 and H4 histones. We have recently demonstrated a role for the coactivator-associated arginine methyltransferase 1 (CARM1), as a coactivator of the FXR/RXR receptor and regulator of FXR responsive genes such as BSEP. Chromatin immunoprecipitation showed that the bile acid-dependent activation of the human BSEP is associated with a simultaneous increase of FXR and CARM1 occupation of the BSEP promoter. The increased occupation of the BSEP locus by CARM1 also corresponds with the increased deposition of Arg-17 methylation and Lys-9 acetylation of histone H3 within the FXR DNA-binding element of BSEP. Our work on the role of nuclear receptors in regulation of bile acid homeostasis has led to an increased understanding of the pathogenesis of the disorder, progressive familial intrahepatic cholestasis, type 1 (PFIC1) or Byler disease. The gene mutated in PFIC1 is called FIC1 and codes for a type IV P-type ATPase whose function is unknown. Increased ileal apical sodium-dependent bile acid transporter messenger RNA (mRNA) expression was detected in 3 patients with PFIC1. Ileal FXR and short heterodimer partner (an inhibitory nuclear receptor) messenger RNA levels were reduced in the same 3 patients. In studies of cells after antisense-mediated knock-down of endogenous FIC1, the activity of the ileal apical bile acid transporter promoter was enhanced, whereas the activities of the human FXR and BSEP promoters were reduced. Nuclear but not cytoplasmic localization of FXR is markedly decreased in FIC1-negative cells, indicating that FIC1 is necessary for posttranslational modifications necessary for the nuclear translocation of FXR. This defect leads to enhanced ileal bile salt uptake and impaired canalicular bile salt secretion by BSEP. In PFIC1, an increased load of bile acids is retained in the liver leading to cholestasis and progressive liver injury.

Whatever happened to "neonatal hepatitis"?

'Idiopathic neonatal hepatitis' is a term that has traditionally been used to denote a clinical syndrome manifest by prolonged jaundice in the neonate. This description is now used much less frequently because recent studies unite well-defined clinical, biochemical and molecular features of intrahepatic cholestasis into specific syndromes. Advances in the understanding of the molecular basis of cholestatic syndromes now enable the classification of syndromes based on biology and offer an opportunity to develop new diagnostic approaches and treatment strategies that take into account the genetic make-up of the child with cholestasis.

Pregnane X receptor-mediated transcription

The pregnane X receptor (PXR, receptor NR1I2) is a ligand-activated transcription factor that is activated by structurally diverse endogenous steroids and foreign chemicals and serves as an important steroid and xenobiotic sensor. This member of the nuclear receptor superfamily is highly expressed in liver and in the gastrointestinal tract, where it regulates transcription of a large set of genes that contribute to foreign compound metabolism and to the metabolism and transcellular transport of steroid hormones, bile acids, and other endogenous substances. This chapter summarizes studies of PXR and its biological functions and describes a cell culture-based luciferase reporter gene assay for determination of PXR transcriptional activity. This assay can be used to identify novel drugs and environmental chemicals that serve as PXR ligands and thereby modulate PXR activity and may aid in the prediction of drug-drug interactions and foreign chemical-induced toxicities associated with the activation of PXR transcriptional responses.

Gene-nutrient interactions during fetal development

Eukaryotic cells have evolved a complex series of nutrient sensors that protect them from damage caused by acute deficiencies and also mediate adaptive responses to prolonged excess or deficiency of particular nutrients. In adults gene expression is regulated by nutrients interacting with pathways involving mammalian target of rapamycin (mTOR), CCAAT/ enhancer-binding proteins (C/EBPs) and peroxisome proliferator activator proteins (PPARs). These systems are also present in key cells of the developing oocyte, embryo and fetus. In this review we will consider the role of interactions between genes and nutrients during reproduction with a particular emphasis on their possible involvement in the prenatal programming of glucose metabolism in the adult.

Perinatal lipopolysaccharide exposure downregulates pregnane X receptor and Cyp3a11 expression in fetal mouse liver

The pregnane X receptor (PXR) is a member of the nuclear receptor superfamily that regulates cytochrome P450 3A (CYP3A) gene transcription in a ligand-dependent manner. Lipopolysaccharide (LPS)-induced downregulation on PXR and cyp3a11 in adult mouse liver has been well characterized. In this study, we investigated the effects of maternal LPS exposure on PXR and cyp3a11 expression in fetal mouse liver. Pregnant ICR mice were injected intraperitoneally with different doses of LPS (0.1 approximately 0.5 mg/kg) on gestational day (GD) 17. PXR and cyp3a11 mRNA levels were determined using RT-PCR. Erythromycin N-demethylase (ERND) activity was used as an indicator of CYP3A expression in this study. Results showed that LPS significantly downregulated PXR and cyp3a11 mRNA levels and ERND activity in fetal liver in a dose-dependent manner. LPS-induced downregulation of PXR and cyp3a11 mRNA expression and ERND activity was attenuated after pregnant mice were pretreated with alpha-phenyl-N-t-butylnitrone (PBN), a free radical spin trapping agent. Additional experiment revealed that LPS significantly increased lipid peroxidation in fetal liver, which was also attenuated by PBN pretreatment. Furthermore, LPS-induced downregulation of PXR and cyp3a11 mRNA expression and ERND activity was prevented by maternal pretreatment with N-acetylcysteine (NAC). Maternal pretreatment with NAC also inhibited LPS-initiated lipid peroxidation and GSH depletion in fetal liver. However, maternal LPS treatment did not affect nitrite plus nitrate concentration in fetal liver. Correspondingly, aminoguanidine, a selective inhibitor of inducible nitric oxide synthase (iNOS), has no effect on LPS-induced downregulation of PXR and cyp3a11 expression and ERND activity in fetal liver. These results indicated that maternal LPS exposure downregulates PXR and cyp3a11 in fetal mouse liver. Reactive oxygen species (ROS) may be involved in LPS-induced downregulation of PXR and cyp3a11 in fetal mouse liver.