Regulation of organic anion and drug transporters of the sinusoidal membrane

Revisiting Quantification of Phenylalanine/Tyrosine Flux in the Ochronotic Pathway during Long-Term Nitisinone Treatment of Alkaptonuria

Changes in the phenylalanine (PHE)/tyrosine (TYR) pathway metabolites before and during homogentisic acid (HGA)-lowering by nitisinone in the Suitability of Nitisinone in Alkaptonuria (AKU) 2 (SONIA 2) study enabled the magnitude of the flux in the pathway to be examined. SONIA 2 was a 48-month randomised, open-label, evaluator-blinded, parallel-group study performed in the UK, France and Slovakia recruiting patients with confirmed AKU to receive either 10 mg nitisinone or no treatment. Site visits were performed at 3 months and yearly thereafter. Results from history, photographs of eyes/ears, whole body scintigraphy, echocardiography and abdomen/pelvis ultrasonography were combined to produce the Alkaptonuria Severity Score Index (cAKUSSI). PHE, TYR, hydroxyphenylpyruvate (HPPA), hydroxyphenyllactate (HPLA) and HGA metabolites were analysed by liquid chromatography/tandem mass spectrometry in 24 h urine and serum samples collected before and during nitisinone. Serum metabolites were corrected for total body water (TBW), and the sum of 24 h urine plus total body water metabolites of PHE, TYR, HPPA, HPLA and HGA were determined. The sum of urine metabolites (PHE, TYR, HPPA, HPLA and HGA) were similar pre- and peri-nitisinone. The sum of TBW metabolites and sum TBW + URINE metabolites were significantly higher peri-nitisinone (p < 0.001 for both) compared with pre-nitisinone baseline. Significantly higher concentrations of metabolites from the tyrosine metabolic pathway were observed during treatment with nitisinone. Arguments for unmasking of the ochronotic pathway and biliary elimination of HGA are put forward.

The zonula occludens protein family regulates the hepatic barrier system in the murine liver

The hepatic barrier is indispensable for the physiological functions of the liver and is impaired under various pathological conditions. Tight junctions reportedly play a central role in hepatic barrier regulation; however, there is limited direct evidence supporting this observation, with few in vivo models or confirmations of the implicated molecular mechanisms presented to date. We inactivated the tight junction component gene, Tjp2/ZO-2, and the related molecule, Tjp1/ZO-1, in mouse livers. In humans, TJP2/ZO-2 mutations have been implicated in the development of human progressive familial intrahepatic cholestasis 4 (PFIC4). The mice deficient in either ZO-1 or ZO-2 in the liver did not exhibit major abnormalities. However, the ablation of both molecules impaired the molecular architecture as well as the structure and function of hepatocyte tight junctions, which disrupted the hepatic barrier and was lethal to the mice by 6 weeks of age. In mutant mice, bile canaliculus formation and cellular polarity were compromised; also, transporter expression and localization were deregulated. Moreover, typical hepatic zonation and bile duct formation were inhibited, and sinusoidal vessels were disorganized. These findings clarify the role of tight junctions and polarity in the hepatic barrier as well as the effect that their disruption has on liver tissue. The observations also suggest that liver-specific ZO-1^-/- and ZO-2^-/- mice could be used as models for PFIC4, and this will provide new insights into liver pathophysiology and clinical applications.

Increased Renal Clearance of Rocuronium Compensates for Chronic Loss of Bile Excretion, via upregulation of Oatp2

Requirement for rocuronium upon surgery changes only minimally in patients with end-stage liver diseases. Our study consisted of both human and rat studies to explore the reason. The reduction rate of rocuronium infusion required to maintain neuromuscular blockade during the anhepatic phase (relative to paleohepatic phase) was examined in 16 children with congenital biliary atresia receiving orthotopic liver transplantation. Pharmacodynamics and pharmacokinetics of rocuronium were studied based on BDL rats. The role of increased Oatp2 and decrease Oatp1 expressions in renal compensation were explored. The reduction of rocuronium requirements significantly decreased in obstructively jaundiced children (24 ± 9 vs. 39 ± 11%). TOF50 in BDL rats was increased by functional removal of the kidneys but not the liver, and the percentage of rocuronium excretion through urine increased (20.3 ± 6.9 vs. 8.6 ± 1.8%), while that decreased through bile in 28d-BDL compared with control group. However, this enhanced renal secretion for rocuronium was eliminated by Oatp2 knock-down, rather than Oatp1 overexpression (28-d BDL vs. Oatp1-ShRNA or Oatp2-ShRNA, 20.3 ± 6.9 vs. 17.0 ± 6.6 or 9.3 ± 3.2%). Upon chronic/sub-chronic loss of bile excretion, rocuronium clearance via the kidneys is enhanced, by Oatp2 up-regulation.

Characterization of 22 Antituberculosis Drugs for Inhibitory Interaction Potential on Organic Anionic Transporter Polypeptide (OATP)-Mediated Uptake

We investigated the inhibitory interaction potential of 22 currently marketed antituberculosis (TB) drugs on organic anion-transporting polypeptide 1B1 (OATP1B1)-, OATP2B1-, and OATP1B3-mediated uptake using in vitro Xenopus oocytes and HEK cells. Rifabutin, ethambutol, amoxicillin, linezolid, p-amino salicylic acid, and rifapentine exhibited mild to moderate inhibitory effects on OATP-mediated uptake of estrone-3 sulfate, estradiol 17β-d-glucuronide, and rosuvastatin. The 50% inhibitory concentration (IC50) values of rifabutin, amoxicillin, ethambutol, p-amino salicylic acid, and linezolid were 35.4, 36.2, 57.6, 72.6, and 65.9 μM, respectively, for uptake mediated by organic anionic transporter polypeptide 1B1 (OATP1B1) and 28.8, 28.9, 53.9, 31.5, and 61.0 μM, respectively, for uptake mediated by organic anionic transporter polypeptide 1B3 (OATP1B3). Streptomycin and linezolid showed greater inhibition of organic anionic transporter polypeptide 2B1 (OATP2B1)-mediated uptake, with IC50 values of 33.2 and 35.6 μM, respectively, along with mild inhibition of other drugs. Furthermore, rifabutin, amoxicillin, and rifapentine significantly inhibited OATP1B1-mediated rosuvastatin uptake, with IC50 values of 12.3, 13.0, and 11.0 μM, respectively, which showed a similar profile to estrone-3 sulfate uptake. The calculated R values ([I]u inlet,max/Ki, where [I]u inlet,max represents the maximum estimated inhibitor concentration inlet to the liver and Ki is the inhibition constant) as the drug-drug interaction (DDI) indexes of PAS, ethambutol, and amoxicillin were 26.1, 6.5, and 4.3 for OATP1B1 and 52.0, 8.0, and 4.6 for OATP1B3, and those for streptomycin, amikacin, and linezolid were 5.0, 4.2, and 4.4 for OATP2B1, respectively, suggesting a higher possibility of in vivo DDIs. This study is the first comprehensive report to show the novel inhibitory potential of 22 marketed anti-TB drugs on OATP-mediated uptake, providing evidence for future in vivo clinical DDI studies.

Current understanding of hepatic and intestinal OATP-mediated drug-drug interactions

At present, many patients are medicated with various drugs, which are, at the same time, associated with an increased risk of drug-drug interactions (DDIs). Detailed analysis of mechanisms underlying DDIs is the basis of a better prediction of adverse drug events caused by drug interactions. In the last few decades, an involvement of transporters in such processes has been more and more recognized. Indeed, uptake transporters belonging to the organic anion-transporting polypeptide (OATP) family have been shown to interact with a variety of drugs in clinical use. Particularly, the subfamily of OATP1B transporters has been extensively studied, identifying several clinical significant DDIs based on those hepatic uptake transporters. By contrast, the role of OATP2B1 in this context is rather underestimated. Therefore, in addition to known interactions based on OATP1B transporters, we have focused on DDIs probably based on OATP2B1 inhibition in the liver and those possibly owing to the inhibition of OATP2B1-mediated drug absorption in the intestine.

Characterization of organic anion-transporting polypeptide (Oatp) 1a1 and 1a4 null mice reveals altered transport function and urinary metabolomic profiles

Organic anion-transporting polypeptides (Oatp) 1a1 and 1a4 were deleted by homologous recombination, and mice were characterized for Oatp expression in liver and kidney, transport in isolated hepatocytes, in vivo disposition of substrates, and urinary metabolomic profiles. Oatp1a1 and Oatp1a4 proteins were undetected in liver, and both lines were viable and fertile. Hepatic constitutive messenger RNAs (mRNAs) for Oatp1a4, 1b2, or 2b1 were unchanged in Oatp1a1⁻/⁻ mice, whereas renal Oatp1a4 mRNA decreased approximately 50% (both sexes). In Oatp1a4⁻/⁻ mice, no changes in constitutive mRNAs for other Oatps were observed. Uptake of estradiol-17β-D-glucuronide and estrone-3-sulfate in primary hepatocytes decreased 95 and 75%, respectively, in Oatp1a1⁻/⁻ mice and by 60 and 30%, respectively, in Oatp1a4⁻/⁻ mice. Taurocholate uptake decreased by 20 and 50% in Oatp1a1⁻/⁻ and Oatp1a4⁻/⁻ mice, respectively, whereas digoxin was unaffected. Plasma area under the curve (AUC) for estradiol-17β-D-glucuronide increased 35 and 55% in male and female Oatp1a1⁻/⁻ mice, respectively, with a concurrent 50% reduction in liver-to-plasma ratios. In contrast, plasma AUC or tissue concentrations of estradiol-17β-D-glucuronide were unchanged in Oatp1a4⁻/⁻ mice. Plasma AUCs for dibromosulfophthalein increased nearly threefold in male Oatp1a1⁻/⁻ and Oatp1a4⁻/⁻ mice, increased by 40% in female Oatp1a4⁻/⁻ mice, and were unchanged in female Oatp1a1⁻/⁻ mice. In both lines, no changes in serum ALT, bilirubin, and cholesterol were noted. NMR analyses showed no generalized increase in urinary excretion of organic anions. However, urinary excretion of taurine decreased by 30-40% and was accompanied by increased excretion of isethionic acid, a taurine metabolite generated by intestinal bacteria, suggesting some perturbations in intestinal bacteria distribution.

Physiologically based pharmacokinetic modelling 2: Predicting the tissue distribution of acids, very weak bases, neutrals and zwitterions

A key component of whole body physiologically based pharmacokinetic (WBPBPK) models is the tissue-to-plasma water partition coefficients (Kpu's). The predictability of Kpu values using mechanistically derived equations has been investigated for 7 very weak bases, 20 acids, 4 neutral drugs and 8 zwitterions in rat adipose, bone, brain, gut, heart, kidney, liver, lung, muscle, pancreas, skin, spleen and thymus. These equations incorporate expressions for dissolution in tissue water and, partitioning into neutral lipids and neutral phospholipids. Additionally, associations with acidic phospholipids were incorporated for zwitterions with a highly basic functionality, or extracellular proteins for the other compound classes. The affinity for these cellular constituents was determined from blood cell data or plasma protein binding, respectively. These equations assume drugs are passively distributed and that processes are nonsaturating. Resultant Kpu predictions were more accurate when compared to published equations, with 84% as opposed to 61% of the predicted values agreeing with experimental values to within a factor of 3. This improvement was largely due to the incorporation of distribution processes related to drug ionisation, an issue that is not addressed in earlier equations. Such advancements in parameter prediction will assist WBPBPK modelling, where time, cost and labour requirements greatly deter its application.

Characterization of organic anion transporter regulation, glutathione metabolism and bile formation in the obese Zucker rat

BACKGROUND/AIMS

Alterations in hepatobiliary transporters may render fatty livers more vulnerable against various toxic insults.

METHODS

We therefore studied expression and function of key organic anion transporters and their transactivators in 8-week-old obese Zucker rats, an established model for non-alcoholic fatty liver disease.

RESULTS

Compared to their heterozygous littermates, obese animals showed a significant reduction in canalicular bile salt secretion, which was paralleled by significantly diminished Oatp2 mRNA and protein levels together with reduced nuclear HNF3beta, while expression of bile salt export pump, organic anion transporter (Oatp) 1 and multidrug resistance-associated protein (Mrp) 4 were unchanged. Impaired bile salt-independent bile flow in obese rats was associated with a 50% reduction of biliary secretion of the Mrp 2 model-substrates glutathione disulfide and S-(2,4-dinitrophenyl)glutathione. In line Mrp2 protein expression was reduced by 50% in obese rats.

CONCLUSIONS

Oatp2 and Mrp2 expression is decreased in fatty liver and may impair metabolism and biliary secretion of numerous xenobiotics. Reduction of bile salt secretion and absence of biliary GSH excretion may contribute to impaired bile flow and posthepatic disorders associated with biliary GSH depletion.

Thyroid hormone transporters in health and disease

Cellular entry is required for conversion of thyroid hormone by the intracellular deiodinases and for binding of 3,3',5-triiodothyronine (T(3)) to its nuclear receptors. Recently, several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have demonstrated that organic anion transporters (e.g., OATPs), and L-type amino acid transporters (LATs) facilitate thyroid hormone uptake. Among these, OATP1C1 has a high affinity and specificity for thyroxine (T(4)). OATP1C1 is expressed in capillaries throughout the brain, suggesting it is critical for transport of T(4) over the blood-brain barrier. We have also characterized a member of the monocarboxylate transporter family, MCT8, as a very active and specific thyroid hormone transporter. Human MCT8 shows preference for T(3) as the ligand. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. The MCT8 gene is located on the X chromosome. Recently, mutations in MCT8 have been found to be associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels.

Thyroid hormone transporters

Thyroid hormone is essential for the development of the brain and the nervous system. Cellular entry is required for conversion of thyroid hormones by the intracellular deiodinases and for binding of T(3) to its nuclear receptors. Several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have so far identified two categories of transporters involved in thyroid hormone uptake (i.e., organic anion transporters and amino acid transporters). Among the organic anion transporters, both Na(+) taurocholate cotransporting polypeptide (NTCP) and various members of the organic anion transporting polypeptide (OATP) family mediate transport of iodothyronines. Because iodothyronines are a particular class of amino acids derived from tyrosine residues, it is no surprise that some amino acid transporters have been shown to be involved in thyroid hormone transport. We have characterized monocarboxylate transporter 8 (MCT8) as a very active and specific thyroid hormone transporter, the gene of which is located on the X chromosome. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. MCT8 shows 50% amino acid identity with a system T amino acid transporter 1 (TAT1). TAT1, also called MCT10, has been characterized to transport aromatic amino acids but no iodothyronines. We have also found that mutations in MCT8 are associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels in young boys.

Regulation of the human organic cation transporter hOCT1

The human organic cation transporter type 1 (hOCT1) is an important transport system for small organic cations in the liver. Organic cation transporters are regulated by different signaling pathways, but the regulation of hOCT1 has not yet been studied. In this work, we have for the first time investigated the regulation of hOCT1. hOCT1 was expressed in Chinese hamster ovary cells (CHO-hOCT1) and in human embryonic kidney cells (HEK293-hOCT1). Its activity was monitored using microfluorimetry with the fluorescent organic cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP(+)) as substrate. hOCT1 expressed in CHO-cells was inhibited by protein kinase A (PKA) activation (1 microM forskolin, -58 +/- 6%, n = 12), calmodulin inhibition (0.1 microM calmidazolium, -68 +/- 3%, n = 6; 10 microM ophiobolin A, -48 +/- 10%, n = 7), calmodulin-dependent kinase II inhibition (1 microM KN62, -78 +/- 4%, n = 12), and inhibition of p56(lck) tyrosine kinase (10 microM aminogenistein, -35 +/- 7%, n = 12). The apparent affinities for TEA(+) were lower in CHO-hOCT1 than in HEK293-hOCT1, while those for TPA(+) and quinine were almost identical; the rank order of EC(50) values (TPA(+) > quinine > TEA(+)) was independent of the expression system. EC(50) values for TEA(+) in CHO-hOCT1 or HEK293-hOCT1 were increased under calmidazolium incubation (6.3 and 1.4 mM, respectively). hOCT1 was inhibited by PKA and endogenously activated by calmodulin, calmodulin-dependent kinase II, and p56(lck) tyrosine kinase. Regulation pathways were the same in the two expression systems. Since apparent substrate affinities depend on activity of regulatory pathways, the expression system plays a role in determining the substrate affinities.

Evaluation of an internalizing monoclonal antibody labeled using N-succinimidyl 3-[131i]iodo-4-phosphonomethylbenzoate ([131i]SIPMB), a negatively charged substituent bearing acylation agent

Monoclonal antibodies such as L8A4, reactive with the epidermal growth factor receptor variant III, internalize after receptor binding resulting in proteolytic degradation by lysosomes. Labeling internalizing mAbs requires the use of methodologies that result in the trapping of labeled catabolites in tumor cells after intracellular processing. Herein we have investigated the potential utility of N-succinimidyl-3-[131I]iodo-4-phosphonomethylbenzoate ([131I]SIPMB), an acylation agent that couples the corresponding negatively charged acid [131I]IPMBA to the protein, for this purpose. Biodistribution studies demonstrated that [131I]IPMBA cleared rapidly from normal tissues and exhibited thyroid levels < or =0.1% injected dose, consistent with a low degree of dehalogenation. Biodistribution experiments in athymic mice bearing subcutaneous D-256 human glioma xenografts were performed to compare L8A4 labeled using [131I]SIPMB to L8A4 labeled with 125I using both the analogous positively charged acylation agent N-succinimidyl-4-guanidinomethyl-3-[125I]iodobenzoate ([125I]SGMIB) and Iodogen. Tumor uptake of [131I]SIPMB-L8A4 (41.9+/-3.5% ID/g) was nearly threefold that of L8A4 labeled using Iodogen (14.0+/-1.1% ID/g) after 2 days, and tumor to tissue ratios remained uniformly high throughout with [131I]SIPMB-L8A4. Thyroid uptake increased for the Iodogen labeled mAb (3.55+/-0.36 %ID at 5 days) whereas that of [131I]SIPMB labeled mAb remained low (0.21+/-0.04% ID at 5 days). In the second biodistribution, L8A4 labeled using [131I]SIPMB and [125I]SGMIB showed no difference in normal tissue uptake and had nearly identical tumor uptake ([131I]SIPMB, 41.8+/-14.2% ID/g; [125I]SGMIB, 41.6+/-15.8% ID/g, at 4 days). These results suggest that [131I]SIPMB may be a viable acylation agent for the radioiodination of internalizing mAbs.

Molecular and cellular physiology of renal organic cation and anion transport

Organic cations and anions (OCs and OAs, respectively) constitute an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. Renal secretion of these compounds, which occurs principally along the proximal portion of the nephron, plays a critical role in regulating their plasma concentrations and in clearing the body of potentially toxic xenobiotics agents. The transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. It is increasingly apparent that basolateral and luminal OC and OA transport reflects the concerted activity of a suite of separate transport processes arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney now allows the development of models describing the molecular basis of the renal secretion of OCs and OAs. This review examines recent work on this issue, with particular emphasis on attempts to integrate information concerning the activity of cloned transporters in heterologous expression systems to that observed in studies of physiologically intact renal systems.

Reversible cholestasis induced by experimental partial obstruction of the bile duct

The aim of this investigation was to reproduce in rats a partial stenosis of the common bile duct to analyze early liver functional and morphometric changes. The hepatic transport kinetics of sulfobromophthalein (organic anion) and rhodamine B (organic cation) was also investigated, and compartmental analysis of both compounds was performed. The humoral parameters of liver function indicated a cholestasis after 2 days of surgery, which reverted to reach normal values on the seventh day. Tumor necrosis factor alpha serum levels showed a tendency to increase on the second day of stenosis (7 out of 14 rats) while white blood cells increased on the second day of stenosis, and turned to normal levels on the seventh day. Histological studies showed increased volume of portal areas and ductular proliferation, which did no revert during the time of the study (up to 7 days post-op). Conversely, a moderate fibrosis and leukocyte infiltrates in portal areas predominated on the second day of stenosis, but normalized on the seventh day. Bile flow was considerably diminished on the second day of partial obstruction as compared to controls. The mean recovery in bile of sulfobromophthalein after 1h of being injected was low on the second day of stenosis, but normalized on the seventh day. Conversely, that of rhodamine B was very low in all animals. Sulfobromophthalein kinetics showed that hepatic uptake and canalicular excretion were impaired during the second but normalized on the seventh day of stenosis. However, rhodamine B kinetics showed that this compound was poorly excreted in all groups although canalicular excretion increased on the second day. The results suggested a model of obstructive cholestasis induced by the experimental stenosis of the bile duct which was not only reversible but also implicates the role of hepatic inflammation.

Enterohepatic bile salt transporters in normal physiology and liver disease

The vectorial transport of bile salts from blood into bile is essential for the generation of bile flow, solubilization of cholesterol in bile, and emulsification of lipids in the intestine. Major transport proteins involved in the enterohepatic circulation of bile salts include the hepatocellular bile salt export pump (BSEP, ABCB11), the apical sodium-dependent bile salt transporter (ASBT, SLC10A2) in cholangiocytes and enterocytes, the sodium-dependent hepatocyte bile salt uptake system NTCP (SLC10A1), the organic anion transporting polypeptides OATP-C (SLC21A6), OATP8 (SLC21A8) and OATP-A (SLC21A3), and the multidrug resistance protein MRP3 (ABCC3). Synthesis and transport of bile salts are intricately linked processes that undergo extensive feedback and feed-forward regulation by transcriptional and posttranscriptional mechanisms. A key regulator of hepatocellular bile salt homeostasis is the bile acid receptor/farnesoid X receptor FXR, which activates transcription of the BSEP and OATP8 genes and of the small heterodimer partner 1 (SHP). SHP is a transcriptional repressor that mediates bile acid-induced repression of the bile salt uptake systems rat Ntcp and human OATP-C. A nuclear receptor that activates rodent Oatp2 (Slc21a5) and human MRP2 (ABCC2) is the pregnane X receptor/steroid X receptor PXR/SXR. Intracellular trafficking and membrane insertion of bile salt transporters is regulated by lipid, protein, and extracellular signal-related kinases in response to physiologic stimuli such as cyclic adenosine monophosphate or taurocholate. Finally, dysfunction of individual bile salt transporters such as BSEP, on account of genetic mutations, steric inhibition, suppression of gene expression, or disturbed signaling, is an important cause of cholestatic liver disease.

Disorders of bile formation and biliary transport

A wide range of cholestatic liver diseases result from various primary defects in bile formation. Clinical features include jaundice, pruritus, failure to thrive, fat malabsorption, cholelithiasis, and variably progressive cirrhosis. Accurate diagnosis of these disorders is essential for determination of prognosis and selection of the most appropriate therapies. Severe genetic defects in canalicular bile acid and phospholipid excretion lead to progressive liver disease that often requires liver transplantation. Defects in bile acid biosynthesis and aminophospholipid transport may be responsive to medical or non-transplant surgical approaches.

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.

Characterization and identification of steroid sulfate transporters of human placenta

Human trophoblasts depend on the supply of external precursors, such as dehydroepiandrosterone-3-sulfate (DHEA-S) and 16 alpha-OH-DHEA-S, for synthesis of estrogens. The aim of the present study was to characterize the uptake of DHEA-S by isolated mononucleated trophoblasts (MT) and to identify the involved transporter polypeptides. The kinetic analysis of DHEA-(35)S uptake by MT revealed a saturable uptake mechanism (K(m) = 26 microM, V(max) = 428 pmol x mg protein(-1) x min(-1)), which was superimposed by a nonsaturable uptake mechanism (diffusion constant = 1.2 microl x mg protein(-1) x min(-1)). Uptake of [(3)H]DHEA-S by MT was Na(+) dependent and inhibited by sulfobromophthalein (BSP), steroid sulfates, and probenecid, but not by steroid glucuronides, unconjugated steroids, conjugated bile acids, ouabain, p-aminohippurate (PAH), and bumetanide. MT took up [(35)S]BSP, [(3)H]estrone-sulfate, but not (3)H-labeled ouabain, estradiol-17beta-glucuronide, taurocholate, and PAH. RT-PCR revealed that the organic anion-transporting polypeptides OATP-B, -D, -E, and the organic anion transporter OAT-4 are highly expressed, and that OATP-A, -C, -8, OAT-3, and Na(+)-taurocholate cotransporting polypeptide (NTCP) are not or are only lowly expressed in term placental tissue and freshly isolated and cultured trophoblasts. Immunohistochemistry of first- and third-trimester placenta detected OAT-4 on cytotrophoblast membranes and at the basal surface of the syncytiotrophoblast. Our results indicate that uptake of steroid sulfates by isolated MT is mediated by OATP-B and OAT-4 and suggest a physiological role of both carrier proteins in placental uptake of fetal-derived steroid sulfates.

Drug efflux transporters in the CNS

The central nervous system (CNS) contains important cellular barriers that maintain homeostasis by protecting the brain from circulating toxins and through the elimination of toxic metabolites generated in the brain. The barriers that limit the concentration of toxins and xenobiotics in the interstitial fluids of the CNS are the capillary endothelial cells of the blood-brain barrier (BBB) and the epithelial cells of the blood-cerebrospinal fluid barrier (BCSFB). Both of these barriers have cellular tight junctions and express transport systems which serve to actively transport nutrients into the brain, and actively efflux toxic metabolites and xenobiotics out of the brain. This review will focus on the expression and function of selected drug efflux transporters in these two barriers, specifically the multidrug resistance transporter, p-glycoprotein, and various organic anion transporters, such as multidrug resistance-associated proteins, organic anion transporter polypeptides, and organic anion transporters. These transport systems are increasingly recognized as important determinants of drug distribution to, and elimination from, different compartments of the CNS. Consequences of drug efflux transporters in barriers of the CNS include limiting the distribution of substrates that are beneficial to treat CNS diseases, and increasing the possibility of drug-drug interactions that may lead to untoward toxicities. Therefore, the study of these transporters is important in examining the various determinants of drug delivery to the CNS.

Mechanisms of impaired biliary excretion of acetaminophen glucuronide after acute phenobarbital treatment or phenobarbital pretreatment

Previous studies have demonstrated that phenobarbital (PB) significantly impairs the biliary excretion of acetaminophen glucuronide (AG) in rats. Studies also suggested that Mrp2 mediates AG biliary excretion, and Mrp3 is involved in AG basolateral export. It was hypothesized that inhibition of Mrp2-mediated AG transport by PB or PB metabolites, and PB induction of Mrp3, may contribute to the impaired biliary excretion of AG by PB. In the present study, the hepatobiliary transport of AG in single-pass isolated perfused Wistar and TR(-) rat livers was investigated. The AG biliary clearance was markedly decreased, and the AG basolateral clearance was significantly increased in TR(-) rat livers. Uptake of AG by Mrp2 and Mrp3, and inhibition of Mrp2- and Mrp3-mediated transport by PB and major PB metabolites, were investigated with rat Mrp2- or Mrp3-expressing Sf9 cell plasma membrane vesicles (Sf9-PMVs). AG was transported by Mrp3 (K(m) approximately 0.91 mM). Net ATP-dependent AG uptake into Mrp2-expressing Sf9-PMVs could not be detected directly. However, AG significantly inhibited Mrp2-mediated 5-(and 6)-carboxy-2',7'-dichlorofluorescein (CDF) transport. p-Hydroxyphenobarbital glucuronide (p-OHPBG), but not PB or p-hydroxyphenobarbital, significantly inhibited Mrp2-mediated CDF transport. The IC(50) values for p-OHPBG inhibition of Mrp2-mediated CDF uptake and Mrp3-mediated AG transport were similar (approximately 0.68 and 0.46 mM, respectively). PB treatment (80 mg/kg/day x 4 days) markedly increased hepatic Mrp3 expression in Wistar rats. In conclusion, inhibition of Mrp2-mediated AG transport by p-OHPBG provided one possible explanation for the impaired biliary excretion of AG after acute PB treatment. However, impaired biliary excretion of AG after PB pretreatment may be attributed primarily to the induction of hepatic Mrp3 by PB.

The interaction of anthocyanins with bilitranslocase

Bilitranslocase (TC 2.A.65.1.1) is an organic anion membrane carrier expressed at the sinusoidal domain of the liver plasma membrane and in epithelial cells of the gastric mucosa. Its substrates are sulfobromophthalein, bilirubin, and nicotinic acid. This work reports on the identification of a new class of bilitranslocase substrates, i.e., anthocyanins. Seventeen out thes 20 compounds tested behaved as competitive inhibitors of bilitranslocase transport activity (K(I)=1.4-22 microM). Their structure-activity relationship reveals that mono- and di-glucosyl anthocyanins, the anthocyanin species occurring in food, are better ligands than the corresponding aglycones. Moreover, the first interaction of anthocyanins with the carrier occurs through hydrophilic moieties, such as the 3-glucosyl moiety and the B ring for the monoglucosides, through the 5-glucosyl moiety and the A ring for the diglucosides, and through either the B or the A ring for the aglycones. These findings suggest that bilitranslocase could play a role in the bioavailability of anthocyanins.

Pharmacokinetic analysis of factors determining elimination pathways for sulfate and glucuronide metabolites of drugs. I: studies by in vivo constant infusion

1. The hepatic and renal handling of glucuronides and sulphates of three phenolic compounds, 4-methylumbelliferone (4-MU), p-nitrophenol (pNP) and acetaminophen (APAP), were evaluated pharmacokinetically by in vivo constant infusion experiments in rat. It was shown that the urinary excretion rate at steady-state was larger than the biliary excretion rate for both glucuronides and sulfates, and sulfates, in particular, were extensively excreted into the urine. 2. For each glucuronide, however, biliary excretion clearances (CL(b)) calculated based on the total concentration and unbound concentration in the liver were much larger than the corresponding renal excretion clearances (CL(r)). Even in the case of sulfates, there was not any large difference between CL(r) and CL(b) based on the total and unbound concentration in tissues, which could not explain their extensive urinary excretion. From these results, these excretion clearances were recognized not to reflect necessarily the actual excretion rate obtained. 3. On the other hand, the tissue-to-plasma concentration ratio (K(p)) of both glucuronides and sulfates for every phenolic compound was much higher in the kidney than that in the liver. The results suggested that one of the most important determinants for the preferential excretion of these conjugates into the bile or urine is the extent of disposition of each compound to the liver or kidney. 4. In addition, K(p) of both glucuronides and sulfates in the liver, where these conjugates are mainly formed, was small. The K(p) of sulfates was quite low, suggesting that sulfates generated in the liver were subject to extensive sinusoidal efflux.

Hepatic transport systems

The identification of the genes responsible for various genetic liver disorders lead to a better understanding of basic physiology of hepatic transport systems. In this review we focus on transport systems involved in the generation of bile and in the maintenance of copper homeostasis. Abnormal function of these transporters results in diseases like Wilson's disease, progressive familial cholestasis syndromes, Dubin-Johnson syndrome and cystic fibrosis. Beyond these well defined diseases, functional impairments of transport proteins may predispose to non-genetic diseases ranging from intrahepatic cholestasis of pregnancy to neurodegenerative disorders including Alzheimer's disease.

Induction of short heterodimer partner 1 precedes downregulation of Ntcp in bile duct-ligated mice

Cholestasis is associated with retention of bile acids and reduced expression of the Na(+)/taurocholate cotransporter (Ntcp), the major hepatocellular bile acid uptake system. This study aimed to determine whether downregulation of Ntcp in obstructive cholestasis 1) is a consequence of bile acid retention and 2) is mediated by induction of the transcriptional repressor short heterodimer partner 1 (SHP-1). To study the time course for the changes in serum bile acid levels as well as SHP-1 and Ntcp steady-state mRNA levels, mice were subjected to common bile duct ligation (CBDL) for 3, 6, 12, 24, 72, and 168 h and compared with sham-operated controls. Serum bile acid levels were determined by radioimmunoassay. SHP-1 and Ntcp steady-state mRNA expression were assessed by Northern blotting. In addition, Ntcp protein expression was studied by Western blotting and immunofluorescence microscopy. Increased SHP-1 mRNA expression paralleled elevations of serum bile acid levels and was followed by downregulation of Ntcp mRNA and protein expression in CBDL mice. Maximal SHP-1 mRNA expression reached a plateau phase after 6-h CBDL (12-fold; P < 0.001) and preceded the nadir of Ntcp mRNA levels (12%, P < 0.001) by 6 h. In conclusion, bile acid-induced expression of SHP-1 may, at least in part, mediate downregulation of Ntcp in CBDL mice. These findings support the concept that downregulation of Ntcp in cholestasis limits intracytoplasmatic accumulation of potentially toxic bile acids.

Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.

Role of transport proteins in drug absorption, distribution and excretion

1. The molecular and functional characterization of transport proteins is emerging rapidly and significant numbers of drugs have been shown to be substrates or inhibitors. The purpose of this review is to highlight the in vivo preclinical and clinical evidence that supports a role for transport proteins in attenuating the absorption, distribution and excretion (ADE) of drugs. 2. For absorption, a clear role has emerged for P-glycoprotein in limiting permeability across the gastrointestinal tract. As a result, a wide variety of drugs suffer from incomplete, variable and non-linear absorption. Similarly, at the blood-brain barrier a range of drugs has limited brain penetration due to P-glycoprotein-mediated efflux, which can limit therapeutic effectiveness of CNS agents. In the liver, transport proteins are present on the sinusoidal membrane that can be the rate-limiting step in hepatic clearance for some drugs. Mechanistic studies clearly suggest a key role and broad substrate specificity for the OATP family of sinusoidal transporters. Mainly ATP-dependent transport proteins such as P-glycoprotein and MRP2 govern active biliary excretion. 3. Drug-drug interactions have been demonstrated involving inhibition or induction of transport proteins. Clinically significant interactions in the gastrointestinal tract and kidney have been observed with inhibitors such as ketoconazole, erythromycin, verapamil, quinidine, probenecid and cimetidine. Clinically significant inhibition at the blood-brain barrier is more difficult to demonstrate, relying on pharmacodynamic and toxicodynamic changes, but an example is quinidine increasing loperamide-induced central effects in humans. 4. This review highlights the emerging role of transport proteins in ADE of drugs and suggests these need to be considered, in drug discovery and development, with respect to variability in drug disposition and response.

Expression of hepatic transporters OATP-C and MRP2 in primary sclerosing cholangitis

BACKGROUND/AIMS

In chronic cholestatic liver diseases, biliary excretion of organic anions from blood into bile is impaired. The aim of this study was to identify the underlying mechanism.

METHODS

Expression of the basolateral organic anion transporting polypeptide OATP-C (SLC21A6) and the canalicular multidrug resistance protein 2 (MRP2) was studied in patients with primary sclerosing cholangitis (PSC) (n=4), a chronic cholestatic liver disease, and in non-cholestatic controls (n=4) (two with chronic hepatitis C, one with idiopathic liver cirrhosis and one with fatty liver). Total RNA was isolated from liver tissue, reverse transcribed and subjected to polymerase chain reaction (PCR) amplification using primers specific for OATP-C, MRP2 and beta-actin. PCR products were quantified densitometrically.

RESULTS

When normalized for beta-actin expression, the level of OATP-C mRNA in liver tissue of patients with PSC was 49% of controls (OATP-C/beta-actin 1.60+/-0.25 vs. 3.24+/-0.69; p<0.05) and the level of MRP2 mRNA was 27% of controls (MRP2/beta-actin 0.70+/-0.36 vs. 2.54+/-0.56; p<0.01).

CONCLUSIONS

Both OATP-C and MRP2 are decreased as measured by mRNA level in PSC. Downregulation of OATP-C might be the consequence of impaired canalicular secretion of organic anions and could serve to reduce the organic anion load of cholestatic hepatocytes.

Colchicine inhibits taurodeoxycholate transport in pericentral but not in periportal hepatocytes

Indirect evidence for a microtubule-dependent vesicular hepatocellular transport of bile acids has accumulated. Since inhibition of this transport by colchicine can be achieved only at high but not at low bile acid infusion rates we were wondering whether this transport pathway shows a hepatic zonation or not. To answer this question we perfused isolated rat livers antegradely or retrogradely, respectively, with unlabeled and labeled taurocholate or taurodeoxycholate. Inhibition of microtubule-dependent bile acid transport was aimed at co-infusion of colchicine. Periportal cells eliminated the likewise hydrophobic taurodeoxycholate as fast as the more hydrophilic taurocholate. In contrast, pericentral cells excreted taurodeoxycholate much slower than taurocholate. Colchicine did not change the biliary taurocholate excretion profile in periportal and pericentral cells. However, colchicine reduced significantly taurodeoxycholate excretion in pericentral but not in periportal cells. It is concluded that a microtubule-dependent vesicular, colchicine-sensitive transport pathway seems to be involved in the translocation of taurodeoxycholate in pericentral but not in periportal cells. Since such a vesicular bile acid transport is regarded to be much slower than transcellular transport by diffusion, this observation may explain the much slower excretion of hydrophobic bile acids like taurodeoxycholate in pericentral than in periportal cells under physiological conditions.

Endotoxin-induced reduction in biliary indocyanine green excretion rate in a chronically catheterized rat model

Using a nonstressed chronically catheterized rat model in which the common bile duct was cannulated, we studied endotoxin-induced alterations in hepatic function by measuring changes in the maximal steady-state biliary excretion rate of the anionic dye indocyanine green (ICG). Biliary excretion of ICG was calculated from direct measurements of biliary ICG concentrations and the bile flow rate during a continuous vascular infusion of ICG. Despite significant elevations in mean peak serum tumor necrosis factor-alpha (TNF-alpha) concentrations (90.9 +/- 16.2 ng/ml), there was no effect on mean rates of bile flow or biliary ICG clearance after administration of 100 microg/kg endotoxin at 6 or 24 h. Significant differences from mean baseline rates of bile flow and biliary ICG excretion did occur after administration of 1,000 microg/kg endotoxin (mean peak TNF-alpha 129.6 +/- 24.4 ng/ml). Furthermore, when rats were treated with up to 16 microg/kg of recombinant TNF-alpha, there was no change in mean rates of bile flow or ICG biliary clearance compared with baseline values. These data suggest that the complex regulation of biliary excretion is not mediated solely by TNF-alpha.

Staphylococcal enterotoxin B potentiates LPS-induced hepatic dysfunction in chronically catheterized rats

Most models of liver dysfunction in sepsis use endotoxin (lipopolysaccharide; LPS) to induce a pathophysiological response. In our study published in this issue (Beno DWA, Uhing MR, Goto M, Chen Y, Jiyamapa-Serna VA, and Kimura RE. Am J Physiol Gastrointest Liver Physiol 280: G858-G865, 2001), the adverse effect of LPS on hepatic function in vivo was only significant at relatively high LPS doses despite high tumor necrosis factor-alpha concentrations. However, many patients with sepsis are exposed to multiple bacterial toxins that may augment the immune response, resulting in increased hepatic dysfunction. We have developed a model of polymicrobial sepsis by parentally administering a combination of staphylococcal enterotoxin B (SEB) and LPS. Using this model, we demonstrate that SEB (50 microg/kg) potentiates the effect of LPS-induced hepatic dysfunction as measured by decreased rates of biliary indocyanine green clearance and bile flow. These increases were most pronounced with doses of 10 and 100 microg/kg LPS, doses that by themselves do not induce hepatic dysfunction. This may explain the seemingly increased incidence and severity of liver dysfunction in sepsis, and it suggests that the exclusive use of LPS for replicating septic shock may not be relevant for studies of hepatic dysfunction.

Organic anion-transporting polypeptide B (OATP-B) and its functional comparison with three other OATPs of human liver

BACKGROUND & AIMS

Hepatic uptake of cholephilic organic compounds is mediated by members of the organic anion-transporting polypeptide (OATP) family. We aimed to characterize the novel OATP-B with respect to tissue distribution and hepatocellular localization and to compare its substrate specificity with those of OATP-A, OATP-C, and OATP8.

METHODS

Tissue distribution and hepatocellular localization of OATP-B were analyzed by Northern blotting and immunofluorescence, respectively. Transport of 16 substrates was measured for each individual human OATP in complementary RNA-injected Xenopus laevis oocytes.

RESULTS

Expression of OATP-B was most abundant in human liver, where it is localized at the basolateral membrane of hepatocytes. OATP-B, OATP-C, and OATP8 mediated high-affinity uptake of bromosulphophthalein (K(m), approximately 0.7, 0.3, and 0.4 micromol/L, respectively). OATP-B also transported estrone-3-sulfate but not bile salts. Although OATP-A, OATP-C, and OATP8 exhibit broad overlapping substrate specificities, OATP8 was unique in transporting digoxin and exhibited especially high transport activities for the anionic cyclic peptides [D-penicillamine(2,5)]enkephalin (DPDPE; opioid-receptor agonist) and BQ-123 (endothelin-receptor antagonist).

CONCLUSIONS

OATP-B is the third bromosulphophthalein uptake system localized at the basolateral membrane of human hepatocytes. OATP-B, OATP-C, and OATP8 account for the major part of sodium-independent bile salt, organic anion, and drug clearance of human liver.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Oatp2 mediates bidirectional organic solute transport: a role for intracellular glutathione

One member of the OATP family of transporters, rat Oatp1, functions as an anion exchanger that is driven in part by the glutathione (GSH) electrochemical gradient, indicating that other OATP-related transporters may also be energized by this mechanism. The present study examined whether rat Oatp2 is also an anion exchanger, and, if so, whether it is energized by the GSH electrochemical gradient. As with Oatp1, uptake of 10 microM [(3)H]taurocholate in Oatp2-expressing Xenopus laevis oocytes was trans-stimulated by intracellular 0.2 mM unlabeled taurocholate, indicating bidirectional transport. Interestingly, [(3)H]taurocholate uptake in Oatp2-expressing oocytes was also trans-stimulated when oocytes were preloaded with GSH, S-methylglutathione, S-sulfobromophthalein-glutathione, S-dinitrophenyl glutathione, or ophthalmic acid (a GSH analog) but not by glutarate or N-acetylcysteine, suggesting that GSH derivatives and conjugates may function as intracellular substrates for Oatp2. Support for this hypothesis was provided by the demonstration of enhanced [(3)H]GSH and [(3)H]S-(2,4-dinitrophenyl)-glutathione efflux in Oatp2-expressing oocytes. However, in contrast to Oatp1, extracellular GSH failed to cis-inhibit uptake of [(3)H]taurocholate or [(3)H]digoxin in Oatp2-expressing oocytes, indicating that the stimulatory effect of high intracellular GSH concentrations is not due to a coupled exchange mechanism. Taken together, the results indicate that Oatp2 mediates bidirectional transport of organic anions by a GSH-sensitive facilitative diffusion mechanism and suggest that this transporter may play a role in cellular export of specific organic molecules.

Identification of organic anion transporting polypeptide 4 (Oatp4) as a major full-length isoform of the liver-specific transporter-1 (rlst-1) in rat liver

A novel organic anion transporting polypeptide (Oatp)4(1) was isolated from rat liver that is 35 amino acids longer than the reported rat liver specific organic anion transporter (rlst)-1 and exhibits a 64% amino acid sequence identity with the human OATP-C (LST-1/OATP2; gene symbol SLC21A6). When expressed in Xenopus laevis oocytes, Oatp4 (Slc21a10) mediated polyspecific uptake of a variety of organic anions including taurocholate (K(m) approximately 27 microM), bromosulfophthalein (K(m) approximately 1.1 microM) and steroid conjugates. Based on nuclease protection analysis Oatp4 appears to be the predominant transcript in rat liver indicating that rlst-1 plays a minor role in overall hepatic organic anion uptake.

Mechanisms of cholestasis

From the multiple mechanisms of cholestasis presented in this article, a unifying hypothesis may be deduced by parsimony. The disturbance of the flow of bile must inevitably lead to the intracellular retention of biliary constituents. Alternatively, the lack of specific components of bile unmasks the toxic potential of other components, as in the case of experimental mdr2 deficiency. In the sequence of events that leads to liver injury, the cytotoxic action of bile salts is pivotal to all forms of cholestasis. The inhibition of the bsep by drugs, sex steroids, or monohydroxy bile salts is an example of direct toxicity to the key mediator in canalicular bile salt excretion. In other syndromes, the dysfunction of distinct hepatocellular transport systems is the primary pathogenetic defect leading to cholestasis. Such dysfunctions include the genetic defects in PFIC and the direct inhibition of gene transcription by cytokines. Perturbations in the short-term regulation of transport protein function are exemplified by the cholestasis of endotoxinemia. The effect of bile salts on signal transduction, gene transcription, and transport processes in hepatocytes and cholangiocytes has become the focus of intense research in recent years. The central role of bile salts in the pathogenesis of cholestasis has, ironically, become all the more evident from the improvement of many cholestatic syndromes with oral bile salt therapy.

Cholestatic syndromes

Continued advances in the field of liver cell biology and molecular biology have provided further insights into the normal physiology of bile secretion and the pathogenesis and therapy for cholestatic syndromes. Important new data have also been published about pathogenesis, clinical features, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, drug-induced cholestasis, and cholestatic syndromes caused by viral infections.

Pathology of the liver

At the close of the 20th century, a selection of articles published in 1999 with relevance to liver pathology reflects the wealth of technological and intellectual progress made during the span of the century. Immunohistochemical staining for hepatitis B virus antigens focused attention on a correlation between cytoplasmic expression of core antigen in individuals with precore mutants and higher activity of hepatitis. Infection of ducklings with a presurface mutant strain of duck hepatitis B virus produced cytopathic liver cell damage. Fibrosing cholestatic hepatitis, originally described as an unusual form of recurrent hepatitis B after liver transplantation, has now been described in hepatitis C virus-positive patients with renal transplants. It may be related to the emergence or selection of hepatitis C virus quasispecies. In biliary tract disease, researchers investigated the canal of Hering as a possible source of hepatic stem cells, sporadic mutations in the JAGGED1 gene (involved in cell differentiation) in Alagille syndrome, and several models of nonsuppurative destructive cholangitis. Further work was accomplished on nonalcoholic steatohepatitis, including a proposal of a grading and staging system as well as its detection in workers exposed to volatile petrochemicals. Among hepatic neoplasms and proliferative disorders, epithelioid hemangioendothelioma, angiomyolipoma and Langerhans' cell histiocytosis received coverage in articles describing the diagnostic pathology in collected series of patients.

Hepatobiliary transport

The alterations of hepatobiliary transport that occur in cholestasis can be divided into primary defects, such as mutations of transporter genes or acquired dysfunctions of transport systems that cause defective canalicular or cholangiocellular secretion, and secondary defects, which result from biliary obstruction. The dysfunction of distinct biliary transport systems as a primary cause of cholestasis is exemplified by the genetic defects in progressive familial intrahepatic cholestasis or by the direct inhibition of transporter gene expression by cytokines. In both, the hepatocellular accumulation of toxic cholephilic compounds causes multiple alterations of hepatocellular transporter expression. In addition, lack of specific components of bile caused by a defective transporter, as in the case of mdr2/MDR3 deficiency, unmasks the toxic potential of other components. The production of bile is critically dependent upon the coordinated regulation and function of sinusoidal and canalicular transporters, for instance of Na+-taurocholate cotransporting polypeptide (NTCP) and bile salt export pump (BSEP). Whereas the downregulation of the unidirectional sinusoidal uptake system NTCP protects the hepatocyte from further intracellular accumulation of bile salts, the relative preservation of canalicular BSEP expression serves to uphold bile salt secretion, even in complete biliary obstruction. Conversely, the strong downregulation of canalicular MRP2 (MRP, multidrug resistance protein) in cholestasis forces the hepatocyte to upregulate basolateral efflux systems such as MRP3 and MRP1, indicating an inverse regulation of basolateral and apical transporters The regulation of hepatocellular transporters in cholestasis adheres to the law of parsimony, since many of the cellular mechanisms are pivotally governed by the effect of bile salts. The discovery that bile salts are the natural ligand of the farnesoid X receptor has shown us how the major bile component is able to regulate its own enterohepatic circulation by affecting transcription of the genes critically involved in transport and metabolism.