An essential role for nuclear receptors SXR/PXR in detoxification of cholestatic bile acids

Regulation of rat organic anion transporters in bile salt-induced cholestatic hepatitis: effect of ursodeoxycholate

Hepatic uptake of organic anions, including bile salts, is mediated by members of the organic anion-transporting polypeptide (Oatp) family. In rat liver, Oatp1 (Slc21a1), Oatp2 (Slc21a5), and Oatp4 (Slca10) are expressed at the basolateral membrane of hepatocytes and may be differentially regulated under pathophysiologic conditions such as cholestasis. The aim of this study was to determine the effects of cholic acid (CA) and ursodeoxycholic acid (UDCA) on the expression of Oatp4 compared with Ntcp, Oatp1, and Oatp2. Wistar rats were fed with CA (0.5%) or both CA (0.5%) and UDCA (0.25%) for 3 weeks. Oatp expression was studied by Northern and Western blot analysis as well as immunofluorescence analysis. Transport function was compared measuring biliary secretion of (14)C-CA and (14)C-taurocholic acid (TCA). In CA-fed animals, biliary secretion of (14)C-CA and (14)C-TCA was markedly delayed over 40 minutes compared with controls. Accordingly, Oatp4 protein was significantly down-regulated in CA-fed animals together with Oatp1 and Ntcp. Cofeeding of CA plus UDCA prevented the impairment of (14)C-CA and (14)C-TCA secretion and the down-regulation of Oatp4. Oatp4 messenger RNA (mRNA) levels did not differ significantly between bile salt-fed groups, suggesting a posttranscriptional effect of CA on Oatp4 expression. In contrast to Oatp1 and Oatp4, Oatp2 protein expression was increased by CA feeding, indicating a differential regulation of Oatp transporters. In conclusion, we show that CA feeding may cause cholestasis associated with a posttranscriptional down-regulation of Oatp4. UDCA may prevent impairment of hepatic function by restoring hepatic transporter expression.

Nuclear receptors and the control of metabolism

The metabolic nuclear receptors act as metabolic and toxicological sensors, enabling the organism to quickly adapt to environmental changes by inducing the appropriate metabolic genes and pathways. Ligands for these metabolic receptors are compounds from dietary origin, intermediates in metabolic pathways, drugs, or other environmental factors that, unlike classical nuclear receptor ligands, are present in high concentrations. Metabolic receptors are master regulators integrating the homeostatic control of (a) energy and glucose metabolism through peroxisome proliferator-activated receptor gamma (PPARgamma); (b) fatty acid, triglyceride, and lipoprotein metabolism via PPARalpha, beta/delta, and gamma; (c) reverse cholesterol transport and cholesterol absorption through the liver X receptors (LXRs) and liver receptor homolog-1 (LRH-1); (d) bile acid metabolism through the farnesol X receptor (FXR), LXRs, LRH-1; and (e) the defense against xeno- and endobiotics by the pregnane X receptor/steroid and xenobiotic receptor (PXR/SXR). The transcriptional control of these metabolic circuits requires coordination between these metabolic receptors and other transcription factors and coregulators. Altered signaling by this subset of receptors, either through chronic ligand excess or genetic factors, may cause an imbalance in these homeostatic circuits and contribute to the pathogenesis of common metabolic diseases such as obesity, insulin resistance and type 2 diabetes, hyperlipidemia and atherosclerosis, and gallbladder disease. Further studies should exploit the fact that many of these nuclear receptors are designed to respond to small molecules and turn them into therapeutic targets for the treatment of these disorders.

Comparative analysis of CYP3A expression in human liver suggests only a minor role for CYP3A5 in drug metabolism

To study mechanisms behind the interindividual variability in CYP3A expression and the relative contribution of the different CYP3A enzymes to the overall CYP3A activity, we have analyzed CYP3A4, CYP3A5, CYP3A43, and PXR mRNA and CYP3A4 and CYP3A5 protein expression, catalytic activity, and polymorphism in the CYP3A5 gene in a panel of 46 Caucasian human livers. Protein quantification was performed by Western blotting using enzyme-specific antibodies directed to the C termini of CYP3A4 or CYP3A5, and carrier protein-coupled peptides as standards. The mRNA levels were determined by quantitative real-time PCR. CYP3A activity was measured by analysis of the rate of testosterone 6beta-hydroxylation. A correlation existed between all CYP3A and PXR mRNA transcripts measured. The interindividual variability in CYP3A4 and CYP3A5 mRNA expression was higher than that of CYP3A protein and activity. The CYP3A5 protein was expressed at quantifiable levels in 5 (10.9%) of the livers. Four of those were heterozygous for the CYP3A5*1 allele and had CYP3A5 protein at a mean level of 17% of that of total CYP3A, whereas one liver sample was from a CYP3A5*3 homozygote individual having lower amounts of CYP3A5. In total, CYP3A5 only contributed 2% of the overall CYP3A protein among all samples. In conclusion, our data indicate that CYP3A4, CYP3A5, CYP3A43, and PXR hepatic mRNA expression correlate, indicating common regulatory features, and that the contribution of CYP3A5 to hepatic drug metabolism in Caucasians is insignificant.

Protective role of hydroxysteroid sulfotransferase in lithocholic acid-induced liver toxicity

Supplement of 1% lithocholic acid (LCA) in the diet for 5-9 days resulted in elevated levels of the marker for liver damage aspartate aminotransferase and alkaline phosphatase activities in both farnesoid X receptor (FXR)-null and wild-type female mice. The levels were clearly higher in wild-type mice than in FXR-null mice, despite the diminished expression of a bile salt export pump in the latter. Consistent with liver toxicity marker activities, serum and liver levels of bile acids, particularly LCA and taurolithocholic acid, were clearly higher in wild-type mice than in FXR-null mice after 1% LCA supplement. Marked increases in hepatic sulfating activity for LCA (5.5-fold) and hydroxysteroid sulfotransferase (St) 2a (5.8-fold) were detected in liver of FXR-null mice. A 7.4-fold higher 3alpha-sulfated bile acid concentration was observed in bile of FXR-null mice fed an LCA diet compared with that of wild-type mice. Liver St2a content was inversely correlated with levels of alkaline phosphatase. In contrast, microsomal LCA 6beta-hydroxylation was not increased and was in fact lower in FXR-null mice compared in wild-type mice. Clear decreases in mRNA encoding sodium taurocholate cotransporting polypeptide, organic anion transporting polypeptide 1, and liver-specific organic anion transporter-1 function in bile acid import were detected in LCA-fed mice. These transporter levels are higher in FXR-null mice than wild-type mice after 1% LCA supplement. No obvious changes were detected in the Mrp2, Mrp3, and Mrp4 mRNAs. These results indicate hydroxysteroid sulfotransferase-mediated LCA sulfation as a major pathway for protection against LCA-induced liver damage. Furthermore, Northern blot analysis using FXR-null, pregnane X receptor-null, and FXR-pregnane X receptor double-null mice suggests a repressive role of these nuclear receptors on basal St2a expression.

Identification of a novel human constitutive androstane receptor (CAR) agonist and its use in the identification of CAR target genes

The orphan nuclear constitutive androstane receptor (CAR) is proposed to play a central role in the response to xenochemical stress. Identification of CAR target genes in humans has been limited by the lack of a selective CAR agonist. We report the identification of 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO) as a novel human CAR agonist with the following characteristics: (a) potent activity in an in vitro fluorescence-based CAR activation assay; (b) selectivity for CAR over other nuclear receptors, including the xenobiotic pregnane X receptor (PXR); (c) the ability to induce human CAR nuclear translocation; and (d) the ability to induce the prototypical CAR target gene CYP2B6 in primary human hepatocytes. Using primary cultures of human hepatocytes, the effects of CITCO on gene expression were compared with those of the PXR ligand rifampicin. The relative expression of a number of genes encoding proteins involved in various aspects of steroid and xenobiotic metabolism was analyzed. Notably, CAR and PXR activators differentially regulated the expression of several genes, demonstrating that these two nuclear receptors subserve overlapping but distinct biological functions in human hepatocytes.

Cholestasis

In contrast with urine formation, bile flow is not dependent on hydrostatic forces, but driven by osmotic pressure of solutes secreted across the apical membrane of hepatocytes and bile duct epithelial cells. This secretory process is mediated by a set of primary active transporters that use ATP hydrolysis to pump solutes against the concentration gradient. The most important solutes in bile are bile salts, lipids, electrolytes, and organic anions. The direct consequence of the osmotic mechanism of bile formation is that impaired function of these pumps leads to impaired bile flow-that is, cholestasis. The function of these pumps is highlighted by a number of inherited cholestatic diseases, which are caused by mutations in these genes. Identification of the molecular defect in these diseases was not only important for diagnostic reasons but also emphasised that impaired transporter function has pathological consequences. Indeed, it is now becoming clear that impaired or downregulated transporter function is also involved in the pathogenesis of acquired cholestatic syndromes.

Induction of bilirubin clearance by the constitutive androstane receptor (CAR)

Bilirubin clearance is one of the numerous important functions of the liver. Defects in this process result in jaundice, which is particularly common in neonates. Elevated bilirubin levels can be decreased by treatment with phenobarbital. Because the nuclear hormone receptor constitutive androstane receptor (CAR) mediates hepatic effects of this xenobiotic inducer, we hypothesized that CAR could be a regulator of bilirubin clearance. Activation of the nuclear hormone receptor CAR increases hepatic expression of each of five components of the bilirubin-clearance pathway. This induction is absent in homozygous CAR null mice but is observed in mice expressing human CAR instead of mouse CAR. Pretreatment with xenobiotic inducers markedly increases the rate of clearance of an exogenous bilirubin load in wild-type but not CAR knockout animals. Bilirubin itself can also activate CAR, and mice lacking CAR are defective in clearing chronically elevated bilirubin levels. Unexpectedly, CAR expression is very low in livers of neonatal mice and humans. We conclude that CAR directs a protective response to elevated bilirubin levels and suggest that a functional deficit of CAR activity may contribute to neonatal jaundice.

Clinical, cellular, and molecular aspects of cancer invasion

Invasion causes cancer malignancy. We review recent data about cellular and molecular mechanisms of invasion, focusing on cross-talk between the invaders and the host. Cancer disturbs these cellular activities that maintain multicellular organisms, namely, growth, differentiation, apoptosis, and tissue integrity. Multiple alterations in the genome of cancer cells underlie tumor development. These genetic alterations occur in varying orders; many of them concomitantly influence invasion as well as the other cancer-related cellular activities. Examples discussed are genes encoding elements of the cadherin/catenin complex, the nonreceptor tyrosine kinase Src, the receptor tyrosine kinases c-Met and FGFR, the small GTPase Ras, and the dual phosphatase PTEN. In microorganisms, invasion genes belong to the class of virulence genes. There are numerous clinical and experimental observations showing that invasion results from the cross-talk between cancer cells and host cells, comprising myofibroblasts, endothelial cells, and leukocytes, all of which are themselves invasive. In bone metastases, host osteoclasts serve as targets for therapy. The molecular analysis of invasion-associated cellular activities, namely, homotypic and heterotypic cell-cell adhesion, cell-matrix interactions and ectopic survival, migration, and proteolysis, reveal branching signal transduction pathways with extensive networks between individual pathways. Cellular responses to invasion-stimulatory molecules such as scatter factor, chemokines, leptin, trefoil factors, and bile acids or inhibitory factors such as platelet activating factor and thrombin depend on activation of trimeric G proteins, phosphoinositide 3-kinase, and the Rac and Rho family of small GTPases. The role of proteolysis in invasion is not limited to breakdown of extracellular matrix but also causes cleavage of proinvasive fragments from cell surface glycoproteins.

Bile salt transporters: molecular characterization, function, and regulation

Molecular medicine has led to rapid advances in the characterization of hepatobiliary transport systems that determine the uptake and excretion of bile salts and other biliary constituents in the liver and extrahepatic tissues. The bile salt pool undergoes an enterohepatic circulation that is regulated by distinct bile salt transport proteins, including the canalicular bile salt export pump BSEP (ABCB11), the ileal Na(+)-dependent bile salt transporter ISBT (SLC10A2), and the hepatic sinusoidal Na(+)- taurocholate cotransporting polypeptide NTCP (SLC10A1). Other bile salt transporters include the organic anion transporting polypeptides OATPs (SLC21A) and the multidrug resistance-associated proteins 2 and 3 MRP2,3 (ABCC2,3). Bile salt transporters are also present in cholangiocytes, the renal proximal tubule, and the placenta. Expression of these transport proteins is regulated by both transcriptional and posttranscriptional events, with the former involving nuclear hormone receptors where bile salts function as specific ligands. During bile secretory failure (cholestasis), bile salt transport proteins undergo adaptive responses that serve to protect the liver from bile salt retention and which facilitate extrahepatic routes of bile salt excretion. This review is a comprehensive summary of current knowledge of the molecular characterization, function, and regulation of bile salt transporters in normal physiology and in cholestatic liver disease and liver regeneration.

Design, synthesis and photochemical properties of caged bile acids

Photolabile derivatives of bile acids (8-10 and 13) were synthesized via silver (I) oxide promoted selective etherification of 3alpha-hydroxyls. Quantitative production of the parent cholic acid was detected from the photolytic mixture of 3-NB-CA (8) in Tris buffered solution. Interestingly, the unexpectedly stable nitroso-hemiacetal intermediate (14) was detected when the photolysis was conducted in methanol. The enzymatic analysis using 7alpha-HSDH showed 8 and 9 could serve as caged bile acids that might be able to regulate certain biological processes upon UV irradiation.

Bile acid regulation of hepatic physiology: III. Bile acids and nuclear receptors

Bile acids are physiological detergents that facilitate excretion, absorption, and transport of fats and sterols in the intestine and liver. Recent studies reveal that bile acids also are signaling molecules that activate several nuclear receptors and regulate many physiological pathways and processes to maintain bile acid and cholesterol homeostasis. Mutations of the principal regulatory genes in bile acid biosynthetic pathways have recently been identified in human patients with hepatobiliary and cardiovascular diseases. Genetic manipulation of key regulatory genes and bile acid receptor genes in mice have been obtained. These advances have greatly improved our understanding of the molecular mechanisms underlying complex liver physiology but also raise many questions and controversies to be resolved. These developments will lead to early diagnosis and discovery of drugs for treatment of liver and cardiovascular diseases.

Predicting inductive drug-drug interactions

Until recently, inductive drug-drug interactions have proved difficult to predict prior to formal pharmacokinetic studies in man. Even then, important interactions have often gone unrecognized until clinical sequelae have occurred in the postmarketing phase. Recent advances in the molecular and cellular biology of nuclear receptors have revealed that there are 'sensors' for xenobiotics, which in turn transactivate genes involved in drug metabolism and excretion. Knowledge of these mechanisms has allowed the development of assay systems that detect the potential of drugs to cause gene induction, well before human studies are contemplated.

Molecular mechanism of nuclear translocation of an orphan nuclear receptor, SXR

The steroid and xenobiotic receptor (SXR) is an orphan nuclear receptor that plays a key role in the regulation of xenobiotic response by controlling the expression of drug metabolizing and clearance enzymes. We observed that pregnane X receptor (PXR), the mouse ortholog of SXR, was retained in the cytoplasm of hepatic cells of untreated mice, whereas PXR was translocated to the nucleus after administration of a ligand, pregnenolone 16 alpha-carbonitrile. To understand the molecular mechanisms underlying the xenochemical-dependent nuclear translocation of SXR, we identified the signal sequence of SXR that regulates its nuclear translocation; using an in vitro expression system, we allocated the nuclear localization signal (NLS) to amino acid residues 66 to 92 within the DNA binding domain of SXR. The NLS of SXR is characterized as the bipartite type, and is recognized by the three molecular species of importin alpha: Rch1 (PTAC58), NPI1, and Qip1, in the presence of PTAC97 of importin beta to target the nuclear pore. The nuclear translocation of SXR was observed as an essential regulatory event for transcription of its target genes such as CYP3A4. These results strongly suggest that the molecular mechanism of the nuclear import of SXR was different from that of another xenosensor, the constitutively active receptor, whose translocation into the nucleus is mediated by a leucine-rich xenochemical response signal in its ligand binding domain.

The expression of CYP2B6, CYP2C9 and CYP3A4 genes: a tangle of networks of nuclear and steroid receptors

Numerous chemicals increase the metabolic capability of organisms by their ability to activate genes encoding various xenochemical-metabolizing enzymes, such as cytochromes P450 (CYPs), transferases and transporters. For example, natural and synthetic glucocorticoids (agonists and antagonists) as well as other clinically important drugs induce the hepatic CYP2B, CYP2C and CYP3A subfamilies in man, and these inductions might lead to clinically important drug-drug interactions. Only recently, the key cellular receptors that mediate such inductions have been identified. They include nuclear receptors, such as the constitutive androstane receptor (CAR, NR1I3), the retinoid X receptor (RXR, NR2B1), the pregnane X receptor (PXR, NR1I2), and the vitamin D receptor (VDR, NR1I1) and steroid receptors such as the glucocorticoid receptor (GR, NR3C1). There is a wide promiscuity of these receptors in the induction of CYPs in response to xenobiotics. Indeed, this adaptive system appears now as a tangle of networks, where receptors share partners, ligands, DNA response elements and target genes. Moreover, they influence mutually their relative expression. This review is focused on these different pathways controlling human CYP2B6, CYP2C9 and CYP3A4 gene expression, and the cross-talk between these pathways.

Functional and structural comparison of PXR and CAR

The nuclear receptors pregnane X receptor (PXR, NR1I2) and constitutive active receptor (CAR, NR1I3) have both been proposed to function as xenosensors, but the details of their respective physiological roles are still being elucidated. We have contrasted these two receptors in a variety of experiments including gene expression assays, cell-based ligand profiling assays, and crystallographic/structural modeling analyses. These data highlight key differences between PXR and CAR.

Regulation of P450 genes by liver-enriched transcription factors and nuclear receptors

Cytochrome P450s (P450s) constitute a superfamily of heme-proteins that play an important role in the activation of chemical carcinogens, detoxification of numerous xenobiotics as well as in the oxidative metabolism of endogenous compounds such as steroids, fatty acids, prostaglandins, and leukotrienes. In addition, some P450s have important roles in physiological processes, such as steroidogenesis and the maintenance of bile acid and cholesterol homeostasis. Given their importance, the molecular mechanisms of P450 gene regulation have been intensely studied. Direct interactions between transcription factors, including nuclear receptors, with the promoters of P450 genes represent one of the primary means by which the expression of these genes is controlled. In this review, several liver-enriched transcription factors that play a role in the tissue-specific, developmental, and temporal regulation of P450s are discussed. In addition, the nuclear receptors that play a role in the fine control of cholesterol and bile acid homeostasis, in part, through their modulation of specific P450s, are discussed.

Identification of an endogenous ligand that activates pregnane X receptor-mediated sterol clearance

The nuclear receptor PXR (pregnane X receptor) is a broad-specificity sensor that recognizes a wide variety of synthetic drugs and xenobiotic agents. On activation by these compounds, PXR coordinately induces a network of transporters, cytochrome P450 enzymes, and other genes that effectively clear xenobiotics from the liver and intestine. Like PXR, the majority of its target genes also possess a broad specificity for exogenous compounds. Thus, PXR is both a sensor and effector in a well integrated and generalized pathway for chemical immunity. Although it is clear that PXR responds to numerous foreign compounds, it is unclear whether it possesses an endogenous ligand. To address this issue, we noted that there is substantial overlap in the substrate specificities of PXR and its critical CYP3A target gene. This prompted us to ask whether endogenous CYP3A substrates also serve as PXR ligands. We demonstrate that 5beta-cholestane-3alpha,7alpha,12alpha-triol (triol), a cholesterol-derived CYP3A substrate, is a potent PXR agonist that effectively induces cyp3a expression in mice. This defines a critical salvage pathway that can be autoinduced to minimize triol accumulation. In contrast, triol can accumulate to very high levels in humans, and unlike mice, these people develop the severe clinical manifestations of cerebrotendinous xanthomatosis. The reason for these dramatic species differences has remained unclear. We now demonstrate that triol fails to activate human PXR or induce the CYP3A-salvage pathway. This explains why humans are more susceptible to sterol accumulation and suggests that synthetic ligands for human PXR could be used to treat cerebrotendinous xanthomatosis and other disorders of cholesterol excess.

The coming of age of our understanding of the enterohepatic circulation of bile salts

Recent advances in molecular biology have greatly accelerated knowledge relating to the significance of the enterohepatic circulation of bile salts. This review highlights the role that both oxysterols and bile salts play as ligands which, when bound to nuclear hormone receptors, activate transcription factors that set into play feed-forward catabolism of cholesterol to bile salts and feedback control of bile acid synthesis. The nuclear hormone receptors, liver X receptor (LXR) and farnesoid X receptor (FXR) both combined as heterodimers with retinoid X receptor and with oxysterols and bile salts, respectively as their ligands, initiate powerful genetic controls over cholesterol and bile acid homeostatic mechanisms. LXR/RXR signals molecular control of feed-forward catabolism of cholesterol to bile acids while FXR/RXR initiates feedback control of bile acid synthesis. An additional nuclear hormone receptor, small heterodimer partner (SHP), is required to inhibit the competence factor, liver receptor homolog-1 to achieve repression of bile acid synthesis in the liver and in so doing SHP autoregulates its own function. Additionally, while bile acid synthesis is repressed, pool size is preserved by the action of FXR/RXR at both hepatic and intestinal levels, which genetically signals enhanced hepatocyte bile salt transport by the bile salt export pump (BSEP) and the ileal bile acid binding protein (IBABP) for ileal reabsorption. During activation of cholesterol catabolism, LXR/RXR enhances reverse cholesterol transport by increasing cholesterol efflux via the ABC-1 transporter from extrahepatic cells. This cholesterol is then taken up by high-density lipoprotein (HDL) and transported back to the liver for further cholesterol catabolism and elimination in bile. The genetic coordination of nuclear hormone receptor function within the territory of the enterohepatic of bile salts allows for normal cholesterol and bile acid homeostasis thereby preventing atherosclerosis.

The farnesoid X-receptor is an essential regulator of cholesterol homeostasis

To address the importance of the farnesoid X-receptor (FXR; NR1H4) for normal cholesterol homeostasis, we evaluated the major pathways of cholesterol metabolism in the FXR-deficient (-/-) mouse model. Compared with wild-type, FXR(-/-) mice have increased plasma high density lipoprotein (HDL) cholesterol and a markedly reduced rate of plasma HDL cholesterol ester clearance. Concomitantly, FXR(-/-) mice exhibit reduced expression of hepatic genes involved in reverse cholesterol transport, most notably, that for scavenger receptor BI. FXR(-/-) mice also have increased: (i) plasma non-HDL cholesterol and triglyceride levels, (ii) apolipoprotein B-containing lipoprotein synthesis, and (iii) intestinal cholesterol absorption. Surprisingly, biliary cholesterol elimination was increased in FXR(-/-) mice, despite decreased expression of hepatic genes thought to be involved in this process. These data demonstrate that FXR is a critical regulator of normal cholesterol metabolism and that genetic changes affecting FXR function have the potential to be pro-atherogenic.

Efflux transporters of the human placenta

The use of pharmaceuticals during pregnancy is often a necessity for the health of the mother. Until recently, the placenta was viewed as a passive organ through which molecules are passed indiscriminately between mother and fetus. In reality, the placenta contains a plethora of transporters, some of which appear to be specifically dedicated to removal of xenobiotics and toxic endogenous compounds. Drug efflux transporters such as P-glycoprotein (P-gp), several multidrug resistant associated proteins (MRPs) and breast cancer resistant protein (BCRP) may provide mechanisms that protect the developing fetus. Bile acid transporters may also play a role in exporting compounds back into the maternal compartment. Steroid hormones directly influence the level of expression and function in some of these transporters. Investigating the link between the hormones of pregnancy and these drug efflux transporters is one possible key in developing strategies to deliver drugs to the mother with minimal fetal risk.

Drug transport proteins in the liver

Together with drug metabolising enzymes, transmembrane transporters are important determinants of drug metabolism and drug clearance by the liver. Hepatic uptake of organic anions, cations, prostaglandins and bile salts is supported by dedicated transporter proteins in the basolateral (sinusoidal) membrane of hepatocytes: OATPs, OATs, OCTs, PGTs and NTCP, respectively. ATP-binding cassette (ABC) transporter proteins in the canalicular membrane of hepatocytes mediate the hepatic efflux of drugs, bile salts and metabolites against a steep concentration gradient from liver to bile. This transport is driven by ATP hydrolysis. Drugs, endogenous metabolites, bile salts and cytokines affect the expression levels of these transporters. They act through a family of ligand-activated transcription factors, the nuclear hormone receptors. Consequently, the levels of the various transporter proteins are subject to genetic polymorphism in the encoding genes as well as in these transcription factors. Adverse drug reactions may be caused by genetic or disease-induced variations of transporter expression or drug-drug interactions at the level of these transporters.

The superfamily of organic anion transporting polypeptides

Organic anion transporting polypeptides (Oatps/OATPs) form a growing gene superfamily and mediate transport of a wide spectrum of amphipathic organic solutes. Different Oatps/OATPs have partially overlapping and partially distinct substrate preferences for organic solutes such as bile salts, steroid conjugates, thyroid hormones, anionic oligopeptides, drugs, toxins and other xenobiotics. While some Oatps/OATPs are preferentially or even selectively expressed in one tissue such as the liver, others are expressed in multiple organs including the blood-brain barrier (BBB), choroid plexus, lung, heart, intestine, kidney, placenta and testis. This review summarizes the actual state of the rapidly expanding OATP superfamily and covers the structural properties, the genomic classification, the phylogenetic relationships and the functional transport characteristics. In addition, we propose a new species independent and open ended nomenclature and classification system, which is based on divergent evolution and agrees with the guidelines of the Human Genome Nomenclature Committee.

Identification of bile acid precursors as endogenous ligands for the nuclear xenobiotic pregnane X receptor

Sterol 27-hydroxylase (CYP27A1) is required for bile acid synthesis by both the classical and alternate pathways. Cyp27a1(-/-) mice exhibit a dramatic increase in the activity of cytochrome P450 3A (CYP3A), which catalyzes side-chain hydroxylations of bile acid intermediates, thereby facilitating their excretion in the bile and urine. We examine the role of the nuclear xenobiotic receptor PXR (pregnane X receptor) in this process. We demonstrate that expression of Cyp3a11 and other established PXR target genes is increased in the Cyp27a1(-/-) mice. WhenCyp27a1(-/-) mice are fed a diet containing either cholic acid or chenodeoxycholic acid, expression of CYP7A1, which catalyzes the rate-limiting step in bile acid biosynthesis, is strongly suppressed. In parallel, the induction of Cyp3a11 observed in these mice is reversed, suggesting that bile acid intermediates serve as PXR activators. In support of this hypothesis, three potentially toxic sterols (7alpha-hydroxy-4-cholesten-3-one, 5beta-cholestan-3alpha,7alpha,12alpha-triol, and 4-cholesten-3-one), including two that are known to accumulate in Cyp27a1(-/-) mice, are efficacious activators of mouse PXR. All three compounds are more potent activators of mouse PXR than of human PXR, which may explain in part why humans who lack functional CYP27A1 do not display a corresponding increase in CYP3A activity and are stricken with the disease cerebrotendinous xanthomatosis. Taken together, these results reveal the existence of a feedforward regulatory loop by which potentially toxic bile acid intermediates activate PXR and induce their own metabolism. In addition, this study demonstrates that animal models with alterations in gene expression can be used to identify endogenous ligands for orphan nuclear receptors.

The role of the nuclear receptor CAR as a coordinate regulator of hepatic gene expression in defense against chemical toxicity

The nuclear receptor CAR (constitutive active receptor) mediates the induction of transcription of cytochrome P450 (CYP) genes by phenobarbital (PB) and PB-type inducers. A recent study using CAR-null mice has shown that CAR regulates not only the CYP genes but also other genes encoding various drug/steroid-metabolizing enzymes. In addition to coordinating these enzymes, CAR plays other roles in hepatic gene expression: CAR represses various genes including carnitine palmitoyltransferase 1a and phosphoenolpyruvate carboxykinase 1 in response to PB, and the receptor regulates the constitutive expression of genes such as squalene epoxidase. On the other hand, induction of certain genes such as amino levulinate synthase 1 by PB is not regulated by CAR. Here we describe diverse roles of CAR in hepatic gene expression with a particular focus on endogenous substances such as cholesterol, bilirubin, and steroid hormones.

Transporters and xenobiotic disposition

Metabolism alone does not adequately account for the observed intersubject variability in drug disposition or response. Carrier-mediated processes, or transporters, are increasingly recognized to be importantly involved in drug absorption, distribution, and excretion. Thus for many drugs, transport and metabolism must be considered together to better predict drug disposition in vivo. Accordingly, this review will outline relevant background information regarding drug transporters and the role of such transporters in the drug disposition process.

Liganded VDR induces CYP3A4 in small intestinal and colon cancer cells via DR3 and ER6 vitamin D responsive elements

The nuclear vitamin D receptor (VDR) mediates the effects of 1,25-dihydroxyvitamin D(3) (1,25D(3)) to alter intestinal gene transcription and promote calcium absorption. Because 1,25D(3) also exerts anti-cancer effects, we examined the efficacy of 1,25D(3) to induce cytochrome P450 (CYP) enzymes. Exposure of human colorectal adenocarcinoma cells (HT-29) to 10(-8)M 1,25D(3) resulted in >/=3-fold induction of CYP3A4 mRNA and protein as assessed by RT-PCR and Western blotting, respectively. Six vitamin D responsive element (VDRE)-like sequences in the promoter region of the CYP3A4 gene were then individually tested for their ability to enhance transcription. A canonical DR3-type element in the distal region of the promoter (-7719-GGGTCAgcaAGTTCA-7733), and a proximal, non-classical everted repeat with a spacer of 6 bp (ER6; -169-TGAACTcaaaggAGGTCA-152) were identified as functional VDREs in this CYP gene. These data suggest that 1,25D(3)-dependent, VDR-mediated induction of CYP3A4 may constitute a chemoprotective mechanism for detoxification of enteric xenobiotics and carcinogens.

Taurine modulates induction of cytochrome P450 3A4 mRNA by rifampicin in the HepG2 cell line

Taurine is not only present in foods, tonics and nutrient drinks but is also used as a medicinal agent mainly for treatment of chronic heart failure and liver disease. However, little is known about its influence on drug-metabolizing enzymes, especially cytochrome P450 (CYP), in human. We examined whether taurine could affect the expression of CYP3A4 mRNA in the presence or absence of rifampicin (RFP), which is a potent inducer of CYPs, with HepG2 cells. Taurine enhanced twice the induction of CYP3A4 mRNA by RFP, but did not affect the expression by itself. This effect was both concentration- and time-dependent. On the other hand, taurine did not affect the induction by phenobarbital. Taurine did not increase intracellular uptake of RFP. Therefore, we conclude that taurine is an enhancer for the induction of CYP3A4 by RFP.

Regulation of CYP2B6 and CYP3A expression by hydroxymethylglutaryl coenzyme A inhibitors in primary cultured human hepatocytes

The effects of treatment with the 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) inhibitors lovastatin, simvastatin, pravastatin, fluvastatin, and atorvastatin on the contents of cytochrome p450 mRNAs were examined in primary cultures of human hepatocytes prepared from three different livers. Treatment of 2- to 3-day-old human hepatocyte cultures with 3 x 10(-5) M lovastatin, simvastatin, fluvastatin, or atorvastatin for 24 h increased the amounts of CYP2B6 and CYP3A mRNA by an average of 3.8- to 9.2-fold and 24- to 36-fold, respectively. In contrast, pravastatin treatment had no effect on the mRNA level of either CYP2B6 or CYP3A, although treatment with pravastatin did produce the expected compensatory increase in HMG-CoA reductase mRNA content, indicating effective inhibition of cholesterol biosynthesis. Although treatment with the active (+), but not the inactive (-), enantiomer of atorvastatin increased the amount of HMG-CoA reductase mRNA, treatment with each enantiomer significantly induced both CYP2B6 and CYP3A mRNA levels. Treatment of primary cultured rat hepatocytes with the atorvastatin enantiomers effectively increased the amount of CYP3A mRNA, but had no effect on CYP2B or CYP4A mRNA levels, in contrast to fluvastatin, which increased both. Findings for p450 proteins by Western blotting were consistent with the mRNA results. These findings indicate that the ability of a drug to inhibit HMG-CoA reductase activity does not predict its ability to produce p450 induction in primary cultured human hepatocytes, and demonstrate that some, but not all, of the effects of these drugs that occur in primary cultured rat hepatocytes are conserved in human hepatocyte cultures.

Selective inhibition of CYP27A1 and of chenodeoxycholic acid synthesis in cholestatic hamster liver

The aim of this study was to explore the regulation of serum cholic acid (CA)/chenodeoxycholic acid (CDCA) ratio in cholestatic hamster induced by ligation of the common bile duct for 48 h. The serum concentration of total bile acids and CA/CDCA ratio were significantly elevated, and the serum proportion of unconjugated bile acids to total bile acids was reduced in the cholestatic hamster similar to that in patients with obstructive jaundice. The hepatic CA/CDCA ratio increased from 3.6 to 11.0 (P<0.05) along with a 2.9-fold elevation in CA concentration (P<0.05) while the CDCA level remained unchanged. The hepatic mRNA and protein level as well as microsomal activity of the cholesterol 7alpha-hydroxylase, 7alpha-hydroxy-4-cholesten-3-one 12alpha-hydroxylase and 5beta-cholestane-3alpha,7alpha,12alpha-triol 25-hydroxylase were not significantly affected in cholestatic hamsters. In contrast, the mitochondrial activity and enzyme mass of the sterol 27-hydroxylase were significantly reduced, while its mRNA levels remained normal in bile duct-ligated hamster. In conclusion, bile acid biosynthetic pathway via mitochondrial sterol 27-hydroxylase was preferentially inhibited in bile duct-ligated hamsters. The suppression of CYP27A1 is, at least in part, responsible for the relative decreased production of CDCA and increased CA/CDCA ratio in the liver, bile and serum of cholestatic hamsters.

Therapeutic and biological importance of getting nucleotides out of cells: a case for the ABC transporters, MRP4 and 5

The energy dependent transport of drugs contributes to cellular resistance and is undoubtedly a prime suspect in chemotherapeutic failure of a variety of disease processes. Early studies focused on a single gene, the multidrug resistance gene, MDR1, as a main contributor to chemotherapeutic failure. However, the multifaceted nature of cellular resistance lead to the discovery of the MRP gene. This pivotal finding and the concurrent rapid development of gene databases lead to the expansion of the MRP gene family. The purpose of this review is to discuss two of the recently described MRP family members that were orphans until their role in drug resistance was discovered. This review will provide an overview of the current state of our understanding of MRP4 and 5.

Identification of membrane-type receptor for bile acids (M-BAR)

Bile acids play an essential role in the solubilization and absorption of dietary fat and lipid-soluble vitamins. Bile acids also modulate the transcription of various genes for enzymes and transport proteins for their own and cholesterol homeostasis through binding to nuclear receptors. Here we report a novel category of bile acid receptor, a membrane-type G protein-coupled receptor (GPCR), BG37. Bile acids induced rapid and dose-dependent elevation of intracellular cAMP levels in BG37-expressing cells, but not in mock-transfected cells, independently of nuclear receptor expression. The rank order of potency of various bile acids for BG37-expressing cells was different from that for the nuclear receptor-mediated response. These observations demonstrate the presence of two independent signaling pathways for bile acids; membrane-type GPCR for rapid signaling and nuclear receptors for delayed signaling. Expression of BG37 was detected in various specific tissues, suggesting its physiological role, although it remains to be further characterized.

Regulation of a xenobiotic sulfonation cascade by nuclear pregnane X receptor (PXR)

The nuclear receptor PXR (pregnane X receptor) protects the body from hepatotoxicity of secondary bile acids such as lithocholic acid (LCA) by inducing expression of the hydroxylating cytochrome P450 enzyme CYP3A and promoting detoxification. We found that activation of PXR also increases the activity and gene expression of the phase II conjugating enzyme dehydroepiandrosterone sulfotransferase (STD) known to sulfate LCA to facilitate its elimination. This activation is direct and appears to extend to other xenobiotic sulfotransferases as well as to 3'-phosphoadenosine 5'-phosphosulfate synthetase 2 (PAPSS2), an enzyme that generates the donor cofactor for the reaction. Because sulfation plays an important role in the metabolism of many xenobiotics, prescription drugs, and toxins, we propose that PXR serves as a master regulator of the phase I and II responses to facilitate rapid and efficient detoxification and elimination of foreign chemicals.

Bile acids stimulate invasion and haptotaxis in human colorectal cancer cells through activation of multiple oncogenic signaling pathways

Bile acids are implicated in colorectal carcinogenesis as evidenced by epidemiological and experimental studies. We examined whether bile acids stimulate cellular invasion of human colorectal and dog kidney epithelial cells at different stages of tumor progression. Colon PC/AA/C1, PCmsrc, and HCT-8/E11 cells and kidney MDCKT23 cells were seeded on top of collagen type I gels and invasive cells were counted after 24 h incubation. Activation of the Rac1 and RhoA small GTPases was investigated by pull-down assays. Haptotaxis was analysed with modified Boyden chambers. Lithocholic acid, chenodeoxycholic acid, cholic acid and deoxycholic acid stimulated cellular invasion of SRC- and RhoA-transformed PCmsrc and MDCKT23-RhoAV14 cells, and of HCT-8/E11 cells originating from a sporadic tumor, but were ineffective in premalignant PC/AA/C1 and MDCKT23 cells. Bile acid-stimulated invasion occurred through stimulation of haptotaxis and was dependent on the RhoA/Rho-kinase pathway and signaling cascades using protein kinase C, mitogen-activated protein kinase, and cyclooxygenase-2. Accordingly, BA-induced invasion was associated with activation of the Rac1 and RhoA GTPases and expression of the farnesoid X receptor. We conclude that bile acids stimulate invasion and haptotaxis in colorectal cancer cells via several cancer invasion signaling pathways.

The nuclear pregnane X receptor: a key regulator of xenobiotic metabolism

The nuclear pregnane X receptor (PXR; NR1I2) is an important component of the body's adaptive defense mechanism against toxic substances including foreign chemicals (xenobiotics). PXR is activated by a large number of endogenous and exogenous chemicals including steroids, antibiotics, antimycotics, bile acids, and the herbal antidepressant St. John's wort. Elucidation of the three-dimensional structure of the PXR ligand binding domain revealed that it has a large, spherical ligand binding cavity that allows it to interact with a wide range of hydrophobic chemicals. Thus, unlike other nuclear receptors that interact selectively with their physiological ligands, PXR serves as a generalized sensor of hydrophobic toxins. PXR binds as a heterodimer with the 9-cis retinoic acid receptor (NR2B) to DNA response elements in the regulatory regions of cytochrome P450 3A monooxygenase genes and a number of other genes involved in the metabolism and elimination of xenobiotics from the body. Although PXR evolved to protect the body, its activation by a variety of prescription drugs represents the molecular basis for an important class of harmful drug-drug interactions. Thus, assays that detect PXR activity will be useful in developing safer prescription drugs.

Identification of residues in the PXR ligand binding domain critical for species specific and constitutive activation

The cytochrome P450 family of enzymes has long been known to metabolize a wide range of compounds, including many of today's most common drugs. A novel nuclear receptor called PXR has been established as an activator of several of the cytochrome P450 genes, including CYP3A4. This enzyme is believed to account for the metabolism of more than 50% of all prescription drugs. PXR is therefore used as a negative selector target and discriminatory filter in preclinical drug development. In this paper we describe the design, construction and characterization by transient transfection of mutant receptors of the human and mouse PXR ligand binding domains. By modeling the human PXR ligand binding domain we have identified and mutated two polar residues in the putative ligand binding pocket which differ between the human and the mouse receptor. The first residue (Q285 in human/I282 in mouse) was mutated between the two species with the corresponding amino acids. These mutants showed that this residue is important for the species specific activation of PXR by the ligand pregnenolone-16alpha-carbonitrile (PCN), while having a less pronounced role in receptor activation by rifampicin. The second residue to be mutated (H407 in human/Q404 in mouse) unexpectedly proved to be important for the basal level of activation of PXR. The H407A mutant of the human receptor showed a high level of constitutive activity, while the Q404H mutant of the mouse receptor demonstrated a sharply decreased basal activity compared to wild-type.

Putative role of the orphan nuclear receptor SXR (steroid and xenobiotic receptor) in the mechanism of CYP3A4 inhibition by xenobiotics

Cytochrome P450 monooxygenase 3A4 (CYP3A4) is responsible for the metabolism of endogenous steroids and drugs in liver. Many inducers of human CYP3A4, such as rifampicin, bind to the orphan nuclear receptor SXR (steroid and xenobiotic receptor) as ligands and stimulate transcription on xenobiotic response elements located in the CYP3A4 promoter. Conversely, it is not known whether SXR mediates the transcriptional repression. We thus examined transcriptional repression of SXR and its interaction with corepressors, NCoR (nuclear receptor corepressor) and SMRT (silencing mediator for retinoid and thyroid receptors) using reporter assays in the absence and presence of ligand. Cotransfection of SMRT, but not NCoR, inhibited not only basal but also rifampicin-induced transcriptional activity of SXR on the CYP3A4 promoter through specific SMRT-SXR interaction in HepG2 cells. Interestingly, rifampicin also increased the interaction of SXR with SMRT as well as with coactivator SRC-1. On the other hand, the anti-fungal agent ketoconazole decreased SXR interaction with both SRC-1 and SMRT. Ketoconazole partially inhibited corticosterone-induced SXR-mediated transcription on the CYP3A4 promoter. Taken together, our results suggest that the differential interaction of coactivators and corepressors induced by various xenobiotics may alter SXR-mediated transcription. Further, the effects of ketoconazole on the CYP3A4 gene suppression may explain, in part, drug-induced inhibition of the CYP3A4 action at the transcriptional level.

Nuclear pregnane x receptor and constitutive androstane receptor regulate overlapping but distinct sets of genes involved in xenobiotic detoxification

The nuclear pregnane X receptor (PXR) and constitutive androstane receptor (CAR) play central roles in protecting the body against environmental chemicals (xenobiotics). PXR and CAR are activated by a wide range of xenobiotics and regulate cytochrome P450 and other genes whose products are involved in the detoxification of these chemicals. In this report, we have used receptor-selective agonists together with receptor-null mice to identify PXR and CAR target genes in the liver and small intestine. Our results demonstrate that PXR and CAR regulate overlapping but distinct sets of genes involved in all phases of xenobiotic metabolism, including oxidative metabolism, conjugation, and transport. Among the murine genes regulated by PXR were those encoding PXR and CAR. We provide evidence that PXR regulates a similar program of genes involved in xenobiotic metabolism in human liver. Among the genes regulated by PXR in primary human hepatocytes were the aryl hydrocarbon receptor and its target genes CYP1A1 and CYP1A2. These findings underscore the importance of these two nuclear receptors in defending the body against a broad array of potentially harmful xenobiotics.

Lithocholic acid decreases expression of bile salt export pump through farnesoid X receptor antagonist activity

Bile salt export pump (BSEP) is a major bile acid transporter in the liver. Mutations in BSEP result in progressive intrahepatic cholestasis, a severe liver disease that impairs bile flow and causes irreversible liver damage. BSEP is a target for inhibition and down-regulation by drugs and abnormal bile salt metabolites, and such inhibition and down-regulation may result in bile acid retention and intrahepatic cholestasis. In this study, we quantitatively analyzed the regulation of BSEP expression by FXR ligands in primary human hepatocytes and HepG2 cells. We demonstrate that BSEP expression is dramatically regulated by ligands of the nuclear receptor farnesoid X receptor (FXR). Both the endogenous FXR agonist chenodeoxycholate (CDCA) and synthetic FXR ligand GW4064 effectively increased BSEP mRNA in both cell types. This up-regulation was readily detectable at as early as 3 h, and the ligand potency for BSEP regulation correlates with the intrinsic activity on FXR. These results suggest BSEP as a direct target of FXR and support the recent report that the BSEP promoter is transactivated by FXR. In contrast to CDCA and GW4064, lithocholate (LCA), a hydrophobic bile acid and a potent inducer of cholestasis, strongly decreased BSEP expression. Previous studies did not identify LCA as an FXR antagonist ligand in cells, but we show here that LCA is an FXR antagonist with partial agonist activity in cells. In an in vitro co-activator association assay, LCA decreased CDCA- and GW4064-induced FXR activation with an IC(50) of 1 microm. In HepG2 cells, LCA also effectively antagonized GW4064-enhanced FXR transactivation. These data suggest that the toxic and cholestatic effect of LCA in animals may result from its down-regulation of BSEP through FXR. Taken together, these observations indicate that FXR plays an important role in BSEP gene expression and that FXR ligands may be potential therapeutic drugs for intrahepatic cholestasis.

Bosentan, a dual endothelin receptor antagonist, activates the pregnane X nuclear receptor

Recent clinical studies have shown that bosentan, a dual endothelin receptor antagonist, decreases the exposure to various substrates of cytochrome P450 (CYP) isoenzymes 2C9 and 3A4. The aim of the study was to investigate the effect of bosentan, its metabolites and glibenclamide on the activity of the pregnane X receptor, a nuclear receptor that regulates the transcription of CYP3A4. CV-1 monkey kidney cells were transiently transfected with a luciferase reporter plasmid containing three copies of the ER6 response element of CYP3A4 and the human or mouse pregnane X receptor. Subsequently, the cells were incubated with the test compounds and the activity of luciferase determined. Bosentan activated the human pregnane X receptor with an EC(50) of 19.9 microM, whereas rifampicin had an EC(50) value of 1.9 microM. Ro 47-8634 (4-tert-butyl-N-[6-(2-hydroxy-ethoxy)-5-(2-hydroxy-phenoxy)-2,2'-bipyrimidin-4-yl]-benzenesulfonamide), a metabolite of bosentan, and glibenclamide also activated the pregnane X receptor. The findings provide a molecular mechanism for the interactions observed between bosentan and several drugs.

Cholesterol and bile acids regulate xenosensor signaling in drug-mediated induction of cytochromes P450

Cytochromes P450 (CYP) constitute the major enzymatic system for metabolism of xenobiotics. Here we demonstrate that transcriptional activation of CYPs by the drug-sensing nuclear receptors pregnane X receptor, constitutive androstane receptor, and the chicken xenobiotic receptor (CXR) can be modulated by endogenous cholesterol and bile acids. Bile acids induce the chicken drug-activated CYP2H1 via CXR, whereas the hydroxylated metabolites of bile acids and oxysterols inhibit drug induction. The cholesterol-sensing liver X receptor competes with CXR, pregnane X receptor, or constitutive androstane receptor for regulation of drug-responsive enhancers from chicken CYP2H1, human CYP3A4, or human CYP2B6, respectively. Thus, not only cholesterol 7 alpha-hydroxylase (CYP7A1), but also drug-inducible CYPs, are diametrically affected by these receptors. Our findings reveal new insights into the increasingly complex network of nuclear receptors regulating lipid homeostasis and drug metabolism.

Bile acid regulation of gene expression: roles of nuclear hormone receptors

Bile acids derived from cholesterol and oxysterols derived from cholesterol and bile acid synthesis pathways are signaling molecules that regulate cholesterol homeostasis in mammals. Many nuclear receptors play pivotal roles in the regulation of bile acid and cholesterol metabolism. Bile acids activate the farnesoid X receptor (FXR) to inhibit transcription of the gene for cholesterol 7alpha-hydroxylase, and stimulate excretion and transport of bile acids. Therefore, FXR is a bile acid sensor that protects liver from accumulation of toxic bile acids and xenobiotics. Oxysterols activate the liver orphan receptors (LXR) to induce cholesterol 7alpha-hydroxylase and ATP-binding cassette family of transporters and thus promote reverse cholesterol transport from the peripheral tissues to the liver for degradation to bile acids. LXR also induces the sterol response element binding protein-1c that regulates lipogenesis. Therefore, FXR and LXR play critical roles in coordinate control of bile acid, cholesterol, and triglyceride metabolism to maintain lipid homeostasis. Nuclear receptors and bile acid/oxysterol-regulated genes are potential targets for developing drug therapies for lowering serum cholesterol and triglycerides and treating cardiovascular and liver diseases.

Loss of nuclear receptor SHP impairs but does not eliminate negative feedback regulation of bile acid synthesis

The in vivo role of the nuclear receptor SHP in feedback regulation of bile acid synthesis was examined. Loss of SHP in mice caused abnormal accumulation and increased synthesis of bile acids due to derepression of rate-limiting CYP7A1 and CYP8B1 hydroxylase enzymes in the biosynthetic pathway. Dietary bile acids induced liver damage and restored feedback regulation. A synthetic agonist of the nuclear receptor FXR was not hepatotoxic and had no regulatory effects. Reduction of the bile acid pool with cholestyramine enhanced CYP7A1 and CYP8B1 expression. We conclude that input from three negative regulatory pathways controls bile acid synthesis. One is mediated by SHP, and two are SHP independent and invoked by liver damage and changes in bile acid pool size.

Redundant pathways for negative feedback regulation of bile acid production

The orphan nuclear hormone receptor SHP has been proposed to have a key role in the negative feedback regulation of bile acid production. Consistent with this, mice lacking the SHP gene exhibit mild defects in bile acid homeostasis and fail to repress cholesterol 7-alpha-hydroxylase expression in response to a specific agonist for the bile acid receptor FXR. However, this repression is retained in SHP null mice fed bile acids, demonstrating the existence of compensatory repression pathways of bile acid signaling. We provide evidence for two such pathways, based on activation of the xenobiotic receptor PXR or the c-Jun N-terminal kinase JNK. We conclude that redundant mechanisms regulate this critical aspect of cholesterol homeostasis.

Nuclear receptors. I. Nuclear receptors and bile acid homeostasis

Bile acids are required for the absorption of lipids and fat-soluble vitamins. The hepatic biosynthesis of bile acids is a major pathway for the catabolism and removal of cholesterol from the body. Because of their intrinsic toxicity, bile acid synthesis, transport, and metabolism must be tightly regulated. It is now apparent that members of the nuclear receptor family of lipid-activated transcription factors are key regulators of these physiological processes. A greater understanding of these receptors should afford novel opportunities for therapeutic intervention in chronic diseases such as cholestasis and dyslipidemia.

Vitamin D receptor as an intestinal bile acid sensor

The vitamin D receptor (VDR) mediates the effects of the calcemic hormone 1alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3]. We show that VDR also functions as a receptor for the secondary bile acid lithocholic acid (LCA), which is hepatotoxic and a potential enteric carcinogen. VDR is an order of magnitude more sensitive to LCA and its metabolites than are other nuclear receptors. Activation of VDR by LCA or vitamin D induced expression in vivo of CYP3A, a cytochrome P450 enzyme that detoxifies LCA in the liver and intestine. These studies offer a mechanism that may explain the proposed protective effects of vitamin D and its receptor against colon cancer.

Bile salt transporters

Bile salts are the major organic solutes in bile and undergo extensive enterohepatic circulation. Hepatocellular bile salt uptake is mediated predominantly by the Na(+)-taurocholate cotransport proteins Ntcp (rodents) and NTCP (humans) and by the Na(+)-independent organic anion-transporting polypeptides Oatp1, Oatp2, and Oatp4 (rodents) and OATP-C (humans). After diffusion (bound by intracellular bile salt-binding proteins) to the canalicular membrane, monoanionic bile salts are secreted into bile canaliculi by the bile salt export pump Bsep (rodents) or BSEP (humans). Both belong to the ATP-binding cassette (ABC) transporter superfamily. Dianionic conjugated bile salts are secreted into bile by the multidrug-resistance-associated proteins Mrp2/MRP2. In bile ductules, a minor portion of protonated bile acids and monomeric bile salts are reabsorbed by non-ionic diffusion and the apical sodium-dependent bile salt transporter Asbt/ASBT, transported back into the periductular capillary plexus by Mrp3/MRP3 [and/or a truncated form of Asbt (tAsbt)], and subjected to cholehepatic shunting. The major portion of biliary bile salts is aggregated into mixed micelles and transported into the intestine, where they are reabsorbed by apical Oatp3, the apical sodium-dependent bile salt transporter (ASBT), cytosolic intestinal bile acid-binding protein (IBABP), and basolateral Mrp3/MRP3 and tAsbt. Transcriptional and posttranscriptional regulation of these enterohepatic bile salt transporters is closely related to the regulation of lipid and cholesterol homeostasis. Furthermore, defective expression and function of bile salt transporters have been recognized as important causes for various cholestatic liver diseases.