Immunohistochemical distribution and functional characterization of an organic anion transporting polypeptide 2 (oatp2)

Evidence that decreased expression of sinusoidal bile acid transporters accounts for the inhibition by rapamycin of bile flow recovery following liver ischemia

Rapamycin is employed as an immunosuppressant following organ transplant and, in patients with hepatocellular carcinoma, to inhibit cancer cell regrowth following liver surgery. Preconditioning the liver with rapamycin to induce the expression of antioxidant enzymes is a potential strategy to reduce ischemia reperfusion (IR) injury. However, pre-treatment with rapamycin inhibits bile flow, especially following ischemia. The aim was to investigate the mechanisms involved in this inhibition. In a rat model of segmental hepatic ischemia and reperfusion, acute administration of rapamycin by intravenous injection did not inhibit the basal rate of bile flow. Pre-treatment of rats with rapamycin for 24 h by intraperitoneal injection inhibited the expression of mRNA encoding the sinusoidal influx transporters Ntcp, Oatp1 and 2 and the canalicular efflux transporter Bsep, and increased expression of canalicular Mrp2. Dose-response curves for the actions of rapamycin on the expression of Bsep and Ntcp in cultured rat hepatocytes were biphasic, and monophasic for effects on Oatp1. In cultured tumorigenic H4IIE liver cells, several bile acid transporters were not expressed, or were expressed at very low levels compared to hepatocytes. In H4IIE cells, rapamycin increased expression of Ntcp, Oatp1 and Mrp2, but decreased expression of Oatp2. It is concluded that the inhibition of bile flow recovery following ischemia observed in rapamycin-treated livers is principally due to inhibition of the expression of sinusoidal bile acid transporters. Moreover, in tumorigenic liver tissue the contribution of tumorigenic hepatocytes to total liver bile flow is likely to be small and is unlikely to be greatly affected by rapamycin.

Functional Expression of P-glycoprotein and Organic Anion Transporting Polypeptides at the Blood-Brain Barrier: Understanding Transport Mechanisms for Improved CNS Drug Delivery?

Drug delivery to the central nervous system (CNS) is greatly limited by the blood-brain barrier (BBB). Physical and biochemical properties of the BBB have rendered treatment of CNS diseases, including those with a hypoxia/reoxygenation (H/R) component, extremely difficult. Targeting endogenous BBB transporters from the ATP-binding cassette (ABC) superfamily (i.e., P-glycoprotein (P-gp)) or from the solute carrier (SLC) family (i.e., organic anion transporting polypeptides (OATPs in humans; Oatps in rodents)) has been suggested as a strategy that can improve delivery of drugs to the brain. With respect to P-gp, direct pharmacological inhibition using small molecules or selective regulation by targeting intracellular signaling pathways has been explored. These approaches have been largely unsuccessful due to toxicity issues and unpredictable pharmacokinetics. Therefore, our laboratory has proposed that optimization of CNS drug delivery, particularly for treatment of diseases with an H/R component, can be achieved by targeting Oatp isoforms at the BBB. As the major drug transporting Oatp isoform, Oatp1a4 has demonstrated blood-to-brain transport of substrate drugs with neuroprotective properties. Furthermore, our laboratory has shown that targeting Oatp1a4 regulation (i.e., TGF-β signaling mediated via the ALK-1 and ALK-5 transmembrane receptors) represents an opportunity to control Oatp1a4 functional expression for the purpose of delivering therapeutics to the CNS. In this review, we will discuss limitations of targeting P-gp-mediated transport activity and the advantages of targeting Oatp-mediated transport. Through this discussion, we will also provide critical information on novel approaches to improve CNS drug delivery by targeting endogenous uptake transporters expressed at the BBB.

Quantitative Targeted Absolute Proteomics for 28 Transporters in Brush-Border and Basolateral Membrane Fractions of Rat Kidney

The purpose of the present study was to determine the absolute protein expression levels of various transporters in renal brush-border membrane (BBM) and basolateral membrane (BLM) fractions, in order to understand the quantitative differences in average transport activities among different transporters at each cellular membrane. BBM and BLM fractions of rat kidney were prepared and digested with trypsin, and simultaneous absolute quantification of 28 transporters and a BLM marker, Na(+)/K(+)-ATPase, was performed using our established quantitative-targeted absolute proteomics (QTAP) technique. In BBM fraction, the protein expression levels of bcrp, urat1, mate1, octl1, mrp4, mdr1a, and abca3 were 40.3, 22.2, 8.90, 4.85, 4.69, 3.22, and 0.976 fmol/μg protein, respectively. In BLM fraction, the protein expression levels of oat1, oat3, oct1, mrp6, and mrp1 were 10.6, 10.2, 4.59, 0.724, and 0.271 fmol/μg protein, respectively. The expression levels of abca2, abca4, abca5, abca12, abcb4, mrp5, abcc9, abcg1, abcg5, lat1, ntcp, pgt, oatp2b1, oatp1b2, oatp3a1, and oct3 were under the limit of quantification in both fractions. The quantitative transporter protein expression profiles at these membranes, as determined by QTAP analysis, should be helpful to understand the contributions of individual transporters to renal excretion of xenobiotics and endogenous compounds.

Zebrafish Oatp-mediated transport of microcystin congeners

Microcystins (MC), representing >100 congeners being produced by cyanobacteria, are a hazard for aquatic species. As MC congeners vary in their toxicity, the congener composition of a bloom primarily dictates the severity of adverse effects and appears primarily to be governed by toxicokinetics, i.e., whether transport of MCs occurs via organic anion-transporting polypeptides (Oatps). Differences in observed MC toxicity in various fish species suggest differential expression of Oatp subtypes leading to varying tissue distribution of the very same MC congener within different species. The objectives of this study were the functional characterization and analysis of the tissue distribution of Oatp subtypes in zebrafish (Danio rerio) as a surrogate model for cyprinid fish. Zebrafish Oatps (zfOatps) were cloned, and the organ distribution was determined at the mRNA level. zfOatps were transiently expressed in HEK293 cells for functional characterization using the Oatp substrates estrone-3-sulfate, taurocholate and methotrexate and specific MC congeners (MC-LR, MC-RR, MC-LF and MC-LW). Novel zfOatp isoforms were isolated. Among these isoforms, the organ-specific expression of zfOatp1d1 and of members of the zfOatp1f subfamily was identified. At the functional level, zfOatp1d1, zfOatp1f2, zfOatp1f3 and zfOatp1f4 transported at least one of the Oatp substrates, and zfOatp1d1, zfOatp1f2 and zfOatp1f4 were shown to transport MC congeners. MC-LF and MC-LW were generally transported faster than MC-LR and MC-RR. The subtype-specific expression of zfOatp1d1 and of members of the zfOatp1f subfamily as well as differences in the transport of MC congeners could explain the MC congener-dependent differences in toxicity in cyprinids.

Indoxyl sulfate down-regulates SLCO4C1 transporter through up-regulation of GATA3

The accumulated uremic toxins inhibit the expression of various renal transporters and this inhibition may further reduce renal function and subsequently cause the accumulation of uremic toxins. However, the precise mechanism of the nephrotoxicity of uremic toxins on renal transport has been poorly understood. Here we report that indoxyl sulfate, one of the potent uremic toxins, directly suppresses the renal-specific organic anion transporter SLCO4C1 expression through a transcription factor GATA3. The promoter region of SLCO4C1 gene has several GATA motifs, and indoxyl sulfate up-regulated GATA3 mRNA and subsequently down-regulated SLCO4C1 mRNA. Overexpression of GATA3 significantly reduced SLCO4C1 expression, and silencing of GATA3 increased SLCO4C1 expression vice versa. Administration of indoxyl sulfate in rats reduced renal expression of slco4c1 and under this condition, plasma level of guanidinosuccinate, one of the preferable substrates of slco4c1, was significantly increased without changing plasma creatinine. Furthermore, in 5/6 nephrectomized rats, treatment with oral adsorbent AST-120 significantly decreased plasma indoxyl sulfate level and conversely increased the expression of slco4c1, following the reduction of plasma level of guanidinosuccinate. These data suggest that the removal of indoxyl sulfate and blocking its signal pathway may help to restore the SLCO4C1-mediated renal excretion of uremic toxins in CKD.

Organic anion-transporting polypeptide 1a4 (Oatp1a4) is important for secondary bile acid metabolism

Organic anion transporting polypeptides (human: OATPs; rodent: Oatps) were thought to have important functions in bile acid (BA) transport. Oatp1a1, 1a4, and 1b2 are the three major Oatp1 family members in rodent liver. Our previous studies have characterized the BA homeostasis in Oatp1a1-null and Oatp1b2-null mice. The present study investigated the physiological role of Oatp1a4 in BA homeostasis by using Oatp1a4-null mice. Oatp1a4 expression is female-predominant in livers of mice, and thereby it was expected that female Oatp1a4-null mice will have more prominent changes than males. Interestingly, the present study demonstrated that female Oatp1a4-null mice had no significant alterations in BA concentrations in serum or liver, though they had increased mRNA of hepatic BA efflux transporters (Mrp4 and Ostα/β) and ileal BA transporters (Asbt and Ostα/β). In contrast, male Oatp1a4-null mice showed significantly altered BA homeostasis, including increased concentrations of deoxycholic acid (DCA) in serum, liver and intestinal contents. After feeding a DCA-supplemented diet, male but not female Oatp1a4-null mice had higher concentrations of DCA in serum and livers than their WT controls. This suggested that Oatp1a4 is important for intestinal absorption of secondary BAs in male mice. Furthermore, loss of Oatp1a4 function did not decrease BA accumulation in serum or livers of bile-duct-ligated mice, suggesting that Oatp1a4 is not likely a BA uptake transporter. In summary, the present study for the first time demonstrates that Oatp1a4 does not appear to mediate the hepatic uptake of BAs, but plays an important male-predominant role in secondary BA metabolism in mice.

Loss of organic anion transporting polypeptide 1a1 increases deoxycholic acid absorption in mice by increasing intestinal permeability

Deoxycholic acid (DCA) is a known hepatotoxicant, a tissue tumor promoter, and has been implicated in colorectal cancer. Male mice are more susceptible to DCA toxicity than female mice. Organic anion transporting polypeptide 1a1 (Oatp1a1), which is known to transport bile acids (BAs) in vitro, is predominantly expressed in livers of male mice. In addition, the concentrations of DCA and its taurine conjugate (TDCA) are increased in serum of Oatp1a1-null mice. To investigate whether Oatp1a1 contributes to the gender difference in DCA toxicity in mice, wild-type (WT) and Oatp1a1-null mice were fed a 0.3% DCA diet for 7 days. After feeding DCA, Oatp1a1-null mice had 30-fold higher concentrations of DCA in both serum and livers than WT mice. Feeding DCA caused more hepatotoxcity in Oatp1a1-null mice than WT mice. After feeding DCA, Oatp1a1-null mice expressed higher BA efflux-transporters (bile salt-export pump, organic solute transporter (Ost)α/β, and multidrug resistance-associated protein [Mrp]2) and lower BA-synthetic enzymes (cytochrome P450 [Cyp]7a1, 8b1, 27a1, and 7b1) in livers than WT mice. Intravenous administration of DCA and TDCA showed that lack of Oatp1a1 does not decrease the plasma elimination of DCA or TDCA. After feeding DCA, the concentrations of DCA in ileum and colon tissues are higher in Oatp1a1-null than in WT mice. In addition, Oatp1a1-null mice have enhanced intestinal permeability. Taken together, the current data suggest that Oatp1a1 does not mediate the hepatic uptake of DCA or TDCA, but lack of Oatp1a1 increases intestinal permeability and thus enhances the absorption of DCA in mice.

The HMG-CoA reductase inhibitor pravastatin stimulates insulin secretion through organic anion transporter polypeptides

The 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor pravastatin has been reported to have a beneficial effect on reducing the new onset of diabetes as well as lowering plasma lipids. Because pravastatin is a water-soluble organic anion, it cannot easily penetrate the lipid bilayer of the cell membrane. As the precise mechanisms of the effect of pravastatin on glucose metabolism and diabetes have not been clarified, we examined the roles of the organic anion transporter family on pravastatin-treated islet and adipocyte functions. Rat oatp1/slco1a1, oatp2/slco1a4 and oatp3/slco1a5 were expressed in the pancreas, and rat oatp3/slco1a5 was also detected in rat insulinoma cell line INS-1e. Pravastatin was transported not only by oatp1/slco1a1 and oatp2/slco1a4, but also by rat oatp3/slco1a5. Pravastatin uptake into INS-1e cells was detected and this transport was inhibited by sulfobromophthalein and rifampicin, both of which are known to inhibit oatp family-mediated uptake. In addition, pravastatin enhanced the glucose-stimulated insulin secretion from INS-1e cells. When fat-loaded db/db mice were treated with pravastatin, glucose intolerance and insulin resistance were prevented. In addition, insulin secretion from isolated islets was enhanced by pravastatin. These data suggest that pravastatin has pleiotropic effects on islets through membrane transport under high fat/glucose conditions.

Modulated function of tissue efflux transporters under hyperbilirubinemia in rats

The effect of hyperbilirubinemia on the function of tissue efflux transporters such as multidrug resistance-associated proteins (Mrps) and organic anion transporting polypeptides (Oatps) was examined by measuring tissue accumulation of 2,4-dinitrophenyl-S-glutathione (DNP-SG) after intravenous administration of 1-chloro-2,4-dinitrobenzene (CDNB), a precursor of DNP-SG, in rats. DNP-SG is known as a substrate of both Mrps and Oatps. Hyperbilirubinemia was induced by a bolus intravenous administration of bilirubin. Treatment with probenecid, an inhibitor for both Mrps and Oatps, significantly increased DNP-SG concentrations in the brain, heart, liver, kidney, jejunum, spleen and skeletal muscle as compared with those in control rats, suggesting the expression of some probenecid-sensitive efflux transporters in these tissues. Rats with more than 70 microM of unconjugated/conjugated bilirubin in plasma exhibited significantly higher DNP-SG concentrations in the brain, liver, jejunum, and skeletal muscle. These results suggested that probenecid-sensitive efflux transporters in tissues were suppressed functionally under hyperbilirubinemia. In conclusion, hyperbilirubinemia accompanied by obstructive jaundice is caused by various disease states, which may increase harmful toxicities of exogenously administered Mrps and/or Oatps substrate drugs at various tissues, by suppressing the efflux transporter's function systemically.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Pharmacokinetic analysis of factors determining elimination pathways for sulfate and glucuronide metabolites of xenobiotics. iii: mechanisms for sinusoidal efflux of 4-methylumbelliferone sulfate

1. To elucidate the mechanisms involved in the sinusoidal efflux of sulfate and glucuronide metabolites of 4-methylumbelliferone (4MU), isolated rat liver perfusion studies were performed under several conditions. 2. The effect of sodium azide on the hepatic handling of both conjugates was examined. The net sinusoidal efflux clearance (CL(eff)) based on the unbound concentration in the liver did not change for 4MU glucuronide (4MUG) or significantly increase for 4MU sulfate (4MUS), suggesting that the sinusoidal efflux of both conjugates is not mediated by the transport systems dependent on adenosine triphosphate. 3. Under Cl(-)-depleted conditions, the CL(eff) of 4MUG significantly decreased, but the saturation of its sinusoidal efflux rather than the transport system dependent on Cl(-) might be involved because the hepatic concentration of 4MUG was extensively higher than that of the control study due to the extremely attenuated biliary excretion. The CL(eff) of 4MUS also significantly decreased, but its hepatic concentration was not different from that in the control study, suggesting that the transport system using Cl(-) is involved in the sinusoidal efflux of 4MUS. 4. The effect of glutathione was examined. CL(eff) of 4MUG was not affected by the additional glutathione, but CL(eff) of 4MUS decreased significantly, suggesting that some transport system sensitive to glutathione is involved in the sinusoidal efflux of 4MUS, but not of 4MUG. 5. Transporters such as Oatp1, Oatp2 and/or Npt1 might be involved in the sinusoidal efflux of 4MUS, but 4MUG is secreted from the sinusoidal membrane via the systems that are totally different from those for 4MUS.

Extrahepatic cholestasis downregulates Oatp1 by TNF-alpha signalling without affecting Oatp2 and Oatp4 expression and sodium-independent bile salt uptake in rat liver

Hepatic uptake of bile salts is mediated by sodium-dependent and sodium-independent transport systems. During extrahepatic cholestasis, both the function and the expression of the Na(+)/taurocholate cotransporting polypeptide (Ntcp) are downregulated. To test whether sodium-independent organic anion-transporting polypeptides are also affected by extrahepatic cholestasis, the function and expression of all three Oatps have been determined in common bile duct-ligated (CBDL) rats. Oatp1/Oatp1a1 protein mass remained unchanged after CBDL for 1 day, but then declined by 75+/-7% and 90+/-17%, respectively, after 3 and 7 days. In contrast, Oatp2/Oatp1a4 and Oatp4/Oatp1b2 protein expression was not affected by CBDL as compared with controls. After CBDL, Oatp1 mRNA was rapidly downregulated by 68+/-21% of untreated controls (P<0.05) within 24 h, and remained at similar levels at 3 and 7 days. Cytokine-inactivation studies with etanercept pretreatment demonstrated that TNF-alpha-dependent signals mediated the down-regulation of this transporter gene at both protein and mRNA levels during obstructive cholestasis. Sodium-independent uptake of taurocholate and cholate into freshly isolated hepatocyte suspensions showed neither significant differences in K(m) nor V(max) values. These results indicate that sodium-independent transport of bile salts may be mediated by Oatp2 and 4 during biliary obstruction, because its expression remains unaffected and may compensate for loss of Oatp1 expression and function in cholestatic hepatocytes.

Bile acid transporters: structure, function, regulation and pathophysiological implications

Specific transporters expressed in the liver and the intestine, play a critical role in driving the enterohepatic circulation of bile acids. By preserving a circulating pool of bile acids, an important factor influencing bile flow, these transporters are involved in maintaining bile acid and cholesterol homeostasis. Enterohepatic circulation of bile acids is fundamentally composed of two major processes: secretion from the liver and absorption from the intestine. In the hepatocytes, the vectorial transport of bile acids from blood to bile is ensured by Na+ taurocholate co-transporting peptide (NTCP) and organic anion transport polypeptides (OATPs). After binding to a cytosolic bile acid binding protein, bile acids are secreted into the canaliculus via ATP-dependent bile salt excretory pump (BSEP) and multi drug resistant proteins (MRPs). Bile acids are then delivered to the intestinal lumen through bile ducts where they emulsify dietary lipids and cholesterol to facilitate their absorption. Intestinal epithelial cells reabsorb the majority of the secreted bile acids through the apical sodium dependent bile acid transporter (ASBT) and sodium independent organic anion transporting peptide (OATPs). Cytosolic ileal bile acid binding protein (IBABP) mediates the transcellular movement of bile acids to the basolateral membrane across which they exit the cells via organic solute transporters (OST). An essential role of bile acid transporters is evident from the pathology associated with their genetic disruption or dysregulation of their function. Malfunctioning of hepatic and intestinal bile acid transporters is implicated in the pathophysiology of cholestatic liver disease and the depletion of circulating pool of bile acids, respectively. Extensive efforts have been recently made to enhance our understanding of the structure, function and regulation of the bile acid transporters and exploring new potential therapeutics to treat bile acid or cholesterol related diseases. This review will highlight current knowledge about structure, function and molecular characterization of bile acid transporters and discuss the implications of their defects in various hepatic and intestinal disorders.

Differential regulation of transport proteins in the periinfarct region following reversible middle cerebral artery occlusion in rats

Members of various transport protein families including ATP-binding cassette transporters and solute carriers were shown to be expressed in brain capillaries, choroid plexus, astrocytes or neurons, controlling drug and metabolite distribution to and from the brain. However, data are currently very limited on how the expression of these transport systems is affected by damage to the brain such as stroke. Therefore we studied the expression of four selected transporters, P-glycoprotein (Mdr1a/b; Abcb1a/b), Mrp5 (Abcc5), Bcrp (Abcg2), and Oatp2 (Slc21a5) in a rat model for stroke. Transporter expression was analyzed by real-time polymerase chain reaction in the periinfarcted region and protein localization and cellular phenotyping were done by immunohistochemistry and confocal immunofluorescence microscopy. After stroke, P-glycoprotein staining was detected in endothelial cells of disintegrated capillaries and by day 14 in newly generated blood vessels. There was no significant difference, however, in the Mdr1a mRNA amount in the periinfarcted region compared with the contralateral site. For Bcrp, a significant mRNA up-regulation was observed from days 3-14. This up-regulation was followed by the protein as confirmed by quantitative immunohistochemistry. Oatp2, located in the vascular endothelium, was also up-regulated at day 14. For Mrp5, an up-regulation was observed in neurons in the periinfarcted region (day 14). In conclusion, after stroke the transport proteins were up-regulated with a maximum at day 14, a time point that coincides with behavioral recuperation. The study further suggests Bcrp as a pronounced marker for the regenerative process and a possible functional role of Mrp5 in surviving neurons.

Carrier-mediated hepatic uptake of a novel nonrenal excretion type uric acid generation inhibitor, Y-700

Y-700, a novel xanthine oxidase inhibitor, was recently developed for the treatment of hyperuricemia and gout. Since the major elimination route of this compound is hepatic metabolism and excretion, the aim of the present study was to characterize the uptake mechanism of Y-700 in the liver, which is also the pharmacological target of Y-700. Efficient uptake of Y-700 was observed both in the liver in vivo and in isolated rat hepatocytes. The uptake was Na(+)-dependent, saturable and inhibited both by ATP-depressants and various organic anions. Indomethacin competitively inhibited Y-700 uptake, whereas the inhibitory effect of organic cations and nucleosides was not so remarkable. Saturable and Na(+)-dependent uptake of Y-700 was also observed in freshly isolated human hepatocytes. Uptake of Y-700 by sinusoidal membrane transporters, such as organic anion transporter (Oat) 2 and organic anion transporting polypeptide (OATP)-B, OATP-C, OATP-8, and Oatp1, could not be detected although uptake of Y-700 in the oocytes expressing sodium/taurocholate cotransporting polypeptide (NTCP) was slightly observed. In conclusion, active transport system(s), which specifically recognize certain types of anionic compounds, are involved in the hepatic uptake of Y-700 and, at least partially, relevant to its elimination from the circulation as well as delivery to pharmacological target.

Acute and chronic effects of developmental iron deficiency on mRNA expression patterns in the brain

Because of the multiple biochemical pathways that require iron, iron deficiency can impact brain metabolism in many ways. The goal of this study was to identify a molecular footprint associated with ongoing versus long term consequences of iron deficiency using microarray analysis. Rats were born to iron-deficient mothers, and were analyzed at two different ages: 21 days, while weaning and iron-deficient; and six months, after a five month iron-sufficient recovery period. Overall, the data indicate that ongoing iron deficiency impacts multiple pathways, whereas the long term consequences of iron deficiency on gene expression are more limited. These data suggest that the gene array profiles obtained at postnatal day 21 reflect a brain under development in a metabolically compromised setting that given appropriate intervention is mostly correctable. There are, however, long term consequences to the developmental iron deficiency that could underlie the neurological deficits reported for iron deficiency.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters the mRNA expression of critical genes associated with cholesterol metabolism, bile acid biosynthesis, and bile transport in rat liver: a microarray study

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent hepatotoxin that exerts its toxicity through binding to the aryl hydrocarbon receptor (AhR) and the subsequent induction or repression of gene transcription. In order to further identify novel genes and pathways that may be associated with TCDD-induced hepatotoxicity, we investigated gene changes in rat liver following exposure to single oral doses of TCDD. Male Sprague-Dawley rats were administered single doses of 0.4 microg/kg bw or 40 microg/kg bw TCDD and killed at 6 h, 24 h, or 7 days, for global analyses of gene expression. In general, low-dose TCDD exposure resulted in greater than 2-fold induction of genes coding for a battery of phase I and phase II metabolizing enzymes including CYP1A1, CYP1A2, NADPH quinone oxidoreductase, UGT1A6/7, and metallothionein 1. However, 0.4 microg/kg bw TCDD also altered the expression of Gadd45a and Cyclin D1, suggesting that even low-dose TCDD exposure can alter the expression of genes indicative of cellular stress or DNA damage and associated with cell cycle control. At the high-dose, widespread changes were observed for genes encoding cellular signaling proteins, cellular adhesion, cytoskeletal and membrane transport proteins as well as transcripts coding for lipid, carbohydrate and nitrogen metabolism. In addition, decreased expression of cytochrome P450 7A1, short heterodimer partner (SHP; gene designation nr0b2), farnesyl X receptor (FXR), Ntcp, and Slc21a5 (oatp2) were observed and confirmed by RT-PCR analyses in independent rat liver samples. Altered expression of these genes implies major deregulation of cholesterol metabolism and bile acid synthesis and transport. We suggest that these early and novel changes have the potential to contribute significantly to TCDD induced hepatotoxicity and hypercholesterolemia.

Apical/Basolateral Surface Expression of Drug Transporters and its Role in Vectorial Drug Transport

It is well known that transporter proteins play a key role in governing drug absorption, distribution, and elimination in the body, and, accordingly, they are now considered as causes of drug-drug interactions and interindividual differences in pharmacokinetic profiles. Polarized tissues directly involved in drug disposition (intestine, kidney, and liver) and restricted distribution to naive sanctuaries (blood-tissue barriers) asymmetrically express a variety of drug transporters on the apical and basolateral sides, resulting in vectorial drug transport. For example, the organic anion transporting polypeptide (OATP) family on the sinusoidal (basolateral) membrane and multidrug resistance-associated protein 2 (MRP2/ABCC2) on the apical bile canalicular membrane of hepatocytes take up and excrete organic anionic compounds from blood to bile. Such vectorial transcellular transport is fundamentally attributable to the asymmetrical distribution of transporter molecules in polarized cells. Besides the apical/basolateral sorting direction, distribution of the transporter protein between the membrane surface (active site) and the intracellular fraction (inactive site) is of practical importance for the quantitative evaluation of drug transport processes. The most characterized drug transporter associated with this issue is MRP2 on the hepatocyte canalicular (apical) membrane, and it is linked to a genetic disease. Dubin-Johnson syndrome is sometimes caused by impaired canalicular surface expression of MRP2 by a single amino acid substitution. Moreover, single nucleotide polymorphisms in OATP-C/SLC21A6 (SLCO1B1) also affect membrane surface expression, and actually lead to the altered pharmacokinetic profile of pravastatin in healthy subjects. In this review article, the asymmetrical transporter distribution and altered surface expression in polarized tissues are discussed.

Molecular characterization of human and rat organic anion transporter OATP-D

We have isolated and characterized a novel human and rat organic anion transporter subtype, OATP-D. The isolated cDNA from human brain encodes a polypeptide of 710 amino acids (Mr 76,534) with 12 predicted transmembrane domains. The rat clone encodes 710 amino acids (Mr 76,821) with 97.6% amino acid sequence homology with human OATP-D. Human and rat OATP-D have moderate amino acid sequence homology with LST-l/rlst-1, the rat oatp family, the prostaglandin transporter, and moatl/MOAT1/KIAA0880/OATP-B. Phylogenetic tree analysis revealed that OATP-D is branched in a different position from all known organic anion transporters. OATP-D transports prostaglandin E1 (Km 48.5 nM), prostaglandin E2 (Km 55.5 nM), and prostaglandin F2,, suggesting that, functionally, OATP-D encodes a protein that has similar characteristics to those of the prostaglandin transporter. Rat OATP-D also transports prostaglandins. The expression pattern of OATP-D mRNA was abundant mainly in the heart, testis, brain, and some cancer cells. Immunohistochemical analysis further revealed that rat OATP-D is widely expressed in the vascular, renal, and reproductive system at the protein level. These results suggest that OATP-D plays an important role in translocating prostaglandins in specialized tissues and cells.

Molecular evidence of organic ion transporters in the rat adrenal cortex with adrenocorticotropin-regulated zonal expression

Experimental evidence suggested that secretion of steroid hormones from adrenocortical cells involves carrier-mediated transport: Cortisol release from, and uptake of p-[3H]aminohippurate into, bovine adrenocortical cells showed properties of the renal p-[3H]aminohippurate/anion exchanger OAT1. Other poly-specific transporters such as organic anion-transporting polypeptides (oatps) and organic cation transporters (OCTs) could also be involved in steroid hormone release. A homology-cloning procedure was established to detect these transporters in rat adrenal gland cDNA. PCR revealed the presence of OAT1, oatp1, oatp2, and oatp3. In situ hybridization localized OAT1 in the outer zona fasciculata, oatp3 in the zona glomerulosa, and oatp1 and oatp2 in the inner zona fasciculata and outer zona reticularis. An OCT2-specific probe produced signals in the zona glomerulosa and outer zona fasciculata. Pretreatment of rats with ACTH increased the expression of OAT1 mRNA that spread to all zones, and hypophysectomy strongly decreased it. A less pronounced regulation was detected for OCT2 and oatp3. Specific antibodies confirmed the localization of OAT1 in the outer zona fasciculata, supporting a possible role of OAT1 in cortisol release. The zonated distribution of transporters furthermore suggest that oatp1-3 and OCT2 may be important for the endocrine function of rat adrenocortical cells.

The role of drug transporters at the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic interface between the blood and the brain. It eliminates (toxic) substances from the endothelial compartment and supplies the brain with nutrients and other (endogenous) compounds. It can be considered as an organ protecting the brain and regulating its homeostasis. Until now, many transport systems have been discovered that play an important role in maintaining BBB integrity and brain homeostasis. In this review, we focus on the role of carrier- and receptor-mediated transport systems (CMT, RMT) at the BBB. These include CMT systems, such as P-glycoprotein, multidrug-resistance proteins 1-7, nucleoside transporters, organic anion transporters, and large amino-acid transporters; RMT systems, such as the transferrin-1 and -2 receptors; and the scavenger receptors SB-AI and SB-BI.

Blood-brain barrier efflux transport

Efflux transport at the blood-brain barrier (BBB) limits the brain tissue exposure to a variety of potential therapeutic agents, including compounds that are relatively lipophilic and would be predicted to permeate the endothelial lining of the brain microvasculature. Recent advances in molecular and cell biology have led to identification of several specific transport systems at the blood-brain interface. Refinement of classical pharmacokinetic experimentation has allowed assessment of the structural specificity of transporters, the impact of efflux transport on brain tissue exposure, and the potential for drug-drug interactions at the level of BBB efflux transport. The objective of this minireview is to summarize efflux transporter characteristics (location, specificity, and potential inhibition) for transport systems identified in the BBB. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on net brain tissue uptake of substrates also are presented. The potential impact of efflux transport on the pharmacodynamics of agents acting in the central nervous system are illustrated. Finally, general issues regarding the role of identifying efflux transport as part of the drug development process are discussed.

Improved brain uptake and pharmacological activity of dalargin using a peptide-vector-mediated strategy

The blood-brain barrier restricts the passage of substances into the brain. Neuropeptides, such as enkephalins, cannot be delivered into the brain when given systemically because of this barrier. Therefore, there is a need to develop efficient transport systems to deliver these drugs to the brain. Recently, we have demonstrated that conjugation of doxorubicin or penicillin to peptide vectors significantly enhances their brain uptake. In this study, we have conjugated the enkephalin analog dalargin with two different peptide vectors, SynB1 and SynB3, to improve its brain delivery and its pharmacological effect. We show by in situ brain perfusion that vectorization markedly enhances the brain uptake of dalargin. We also show using the hot-plate model that this enhancement in brain uptake results in a significant improvement in the observed antinociceptive effect of dalargin. These results support the usefulness of peptide-mediated strategies for improving the availability and efficacy of central nervous system drugs.

Uptake of rosuvastatin by isolated rat hepatocytes: comparison with pravastatin

1. The liver is the target organ for the lipid-regulating effect of rosuvastatin, a new 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, and liver-selective uptake of this drug is therefore a desirable property. The uptake kinetics of rosuvastatin were investigated and compared with those of pravastatin using isolated rat hepatocytes. 2. Uptake for both drugs involved both active transport and passive diffusion processes. The Michaelis constant (K(m)) of uptake rate for rosuvastatin (9.17 micro M) was approximately half that for pravastatin (16.5 micro M). However, the maximum uptake rate (V(max)) and carrier-mediated uptake clearance (V(max)/K(m)) of rosuvastatin were significantly (p < 0.01) greater than those of pravastatin, and a larger contribution of carrier-mediated uptake clearance to total uptake clearance was shown for rosuvastatin (contribution ratio 0.903 versus pravastatin 0.654). 3. Sodium and chloride ions did not play a significant role in the uptake of rosuvastatin and pravastatin, but the uptake of both drugs was inhibited both by depletion of cellular ATP and by organic anions such as bromosulfophthalein. 4. Rosuvastatin competitively inhibited the uptake of pravastatin, with an inhibition constant (K(i)) (2.75 micro M) relatively similar to its K(m). 5. The results suggest that an organic anion transport protein is the main mediator of the hepatic uptake of rosuvastatin and pravastatin, which occurs in an ATP-dependent manner. Our results indicated that rosuvastatin was taken up by the hepatocytes via the same transport systems as pravastatin, but with a greater affinity and efficiency than pravastatin.

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.

In vitro study of the functional expression of organic anion transporting polypeptide 3 at rat choroid plexus epithelial cells and its involvement in the cerebrospinal fluid-to-blood transport of estrone-3-sulfate

The cerebrospinal fluid-to-blood efflux transport of estrone-3-sulfate (E(1)S) via the blood-cerebrospinal fluid barrier (BCSFB) may play an important role in regulating E(1)S levels in the brain. Here, we investigated the efflux transport of E(1)S at the BCSFB using conditionally immortalized rat choroid plexus epithelial cells (TR-CSFB) and identified the responsible transporter. The [(3)H]E(1)S uptake by TR-CSFB cells was composed of saturable and nonsaturable components, and the K(m) and V(max) values of the saturable component were determined to be 16.8 +/- 5.1 microM and 12.3 +/- 2.3 pmol/min/mg of protein, respectively. [(3)H]E(1)S uptake was inhibited by probenecid, cholate, taurocholate, sulfobromophthalein, dehydroepiandrosterone sulfate, triiodothyronine, thyroxin, and digoxin but not by p-aminohippuric acid, gamma-aminobutyric acid, or methotrexate, suggesting the involvement of organic anion transporting polypeptide (oatp) in the uptake. Reverse transcription-polymerase chain reaction analysis revealed that oatp3 was expressed in TR-CSFB cells and isolated rat choroid plexus, although oatp1 was not detected in either. Xenopus laevis oocytes expressing oatp3 exhibited [(3)H]E(1)S uptake activity with a K(m) of 8.09 +/- 2.83 microM and V(max) of 8.02 +/- 0.87 pmol/h/oocyte. Moreover, oatp3 is localized at the brush-border membrane of choroid plexus epithelial cells. These results suggest that oatp3 is involved in the E(1)S efflux transport at the BCSFB.

Expression of organic anion transporting polypeptide E (OATP-E) in human placenta

PURPOSE

To examine the expression of multifunctional Na(+)-independent organic anion transporting polypeptide, termed OATP-E, involved in the transport of thyroid hormone in human placenta.

METHODS

Western blot analysis was performed using a specific antibody against OATP-E in human placenta to confirm the expression of OATP-E at the protein level. Immunohistochemistry was also performed using the specific antibody against OATP-E on frozen sections of human placenta.

RESULTS

By Western blot analysis, a single band for OATP-E was observed in human placenta. Immunohistochemistry revealed that OATP-E was predominantly expressed at the apical surface of the syncytiotrophoblasts in placenta.

CONCLUSION

These results reveal that OATP-E is expressed in human placenta, suggesting a functional role for the transplacental transfer of thyroid hormone.

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.

Enterohepatic circulation: physiological, pharmacokinetic and clinical implications

Enterohepatic recycling occurs by biliary excretion and intestinal reabsorption of a solute, sometimes with hepatic conjugation and intestinal deconjugation. Cycling is often associated with multiple peaks and a longer apparent half-life in a plasma concentration-time profile. Factors affecting biliary excretion include drug characteristics (chemical structure, polarity and molecular size), transport across sinusoidal plasma membrane and canniculae membranes, biotransformation and possible reabsorption from intrahepatic bile ductules. Intestinal reabsorption to complete the enterohepatic cycle may depend on hydrolysis of a drug conjugate by gut bacteria. Bioavailability is also affected by the extent of intestinal absorption, gut-wall P-glycoprotein efflux and gut-wall metabolism. Recently, there has been a considerable increase in our understanding of the role of transporters, of gene expression of intestinal and hepatic enzymes, and of hepatic zonation. Drugs, disease and genetics may result in induced or inhibited activity of transporters and metabolising enzymes. Reduced expression of one transporter, for example hepatic canalicular multidrug resistance-associated protein (MRP) 2, is often associated with enhanced expression of others, for example the usually quiescent basolateral efflux MRP3, to limit hepatic toxicity. In addition, physiologically relevant pharmacokinetic models, which describe enterohepatic recirculation in terms of its determinants (such as sporadic gall bladder emptying), have been developed. In general, enterohepatic recirculation may prolong the pharmacological effect of certain drugs and drug metabolites. Of particular importance is the potential amplifying effect of enterohepatic variability in defining differences in the bioavailability, apparent volume of distribution and clearance of a given compound. Genetic abnormalities, disease states, orally administered adsorbents and certain coadministered drugs all affect enterohepatic recycling.

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.

Drug interactions with patient-controlled analgesia

Patient-controlled analgesia (PCA) has become standard procedure in the clinical treatment of pain. Its widespread use in patients with all kinds of diseases opens a variety of possible interactions between analgesics used for PCA and other drugs that might be administered concomitantly to the patient. Many of these drug interactions are of little clinical importance. However, some drug interactions have been reported to result in serious clinical problems. Drug interactions can either predominantly affect the pharmacokinetics or pharmacodynamics of the drug. Most important pharmacokinetic drug interactions occur at the level of drug metabolism or protein binding. Acceleration of methadone metabolism caused by cytochrome P450 (CYP) 3A4 induction by antiretroviral drugs or rifampicin (rifampin) has caused methadone withdrawal symptoms. Lack of morphine formation from codeine as a result of CYP2D6 inhibition by quinidine results in an almost complete loss of the analgesic effects of codeine. Alterations of methadone protein binding caused by an inhibition of alpha1-acid glycoprotein synthesis by alkylating substances are another possibility for predominantly pharmacokinetically based drug interactions during PCA. Furthermore, inhibition of P-glycoprotein by anticancer drugs could result in altered transmembrane transport of morphine, methadone or fentanyl, although this has not been shown to be of clinical relevance. Synergistic effects of systemically administered opioids with spinally or topically delivered opioids or anaesthetics have been reported frequently. The same is true for the opioid-sparing effects of coadministered non-opioid analgesics. Antidepressants, anticonvulsants or alpha2-adrenoreceptor agonists have also been shown to exert additive analgesic effects when administered together with an opioid. Inconsistent findings, however, are reported regarding the treatment of patients with opioid-induced nausea and sedation, since coadministration of antiemetics either increased or decreased the respective adverse effects or revealed additional unwanted drug effects.

Induction of rat organic anion transporting polypeptide 2 by pregnenolone-16alpha-carbonitrile is via interaction with pregnane X receptor

The rat organic anion transporting polypeptide 2 (oatp2; Slc21a5) is a liver transporter that mediates the uptake of a variety of structurally diverse compounds, and has a high affinity for cardiac glycosides. Treatment of rats with pregnenolone-16alpha-carbonitrile (PCN), a ligand for the rodent pregnane X receptor (PXR), significantly enhances the rat oatp2 gene expression. To understand the molecular mechanism of oatp2 induction by PCN, rat oatp2 gene was cloned. The rat oatp2 gene consists of 16 exons; alternative splicing of the second noncoding exon gives rise to the two published rat oatp2 cDNAs. Approximately 8700 base pairs (bp) of the 5'-flanking region of the rat oatp2 gene were linked to the luciferase reporter gene and used in transient transfection assays in H4IIE cells. Treatment of PCN induced the expression of the reporter gene in a dose-dependent manner. Four potential PXR response elements (PXREs) were identified in the 5'-flanking region of the rat oatp2 gene. One element (DR3-1) is located approximately -5000 bp with three more (DR3-2, DR3-3, and DR3-4) clustered at about -8000 bp. Results from electrophoretic mobility shift assays showed that the PXR-retinoid X receptor alpha heterodimer binds to the DR3-2 with the highest affinity, to the DR3-4 and DR3-1 with a lower affinity, and weakly or not at all to the DR3-3. Furthermore, a series of partial deletions of the 5'-flanking region illustrated that both the proximal and distal clusters of PXREs are required for maximal induction of rat oatp2 by PCN. In conclusion, these data elucidate the molecular mechanism by which PCN treatment induces rat oatp2 gene expression. In addition, this study identifies rat oatp2 as a direct PXR-targeted gene and further supports the hypothesis that activation of PXR affects a network of genes that is involved in either metabolism or transport of drugs, steroids, and bile acids.

Effect of 17 beta-estradiol-D-17 beta-glucuronide on the rat organic anion transporting polypeptide 2-mediated transport differs depending on substrates

Rat organic anion transporting polypeptide 2 (rOatp2) is a member of the OATP family. It exhibits broad substrate specificity and accepts amphipathic organic anions, cardiac glycosides (digoxin and ouabain; a neutral compound), and organic cations (rocuronium and N-(4,4-azo-n-pentyl)-21-deoxyajamalinium). In the present study, kinetic analyses were carried out to investigate whether taurocholate (TCA), digoxin, and 17beta-estradiol-D-17beta-glucuronide (E(2)17betaG) share the same recognition site on rOatp2 for their transport. The transport of TCA and digoxin was mutually inhibited, and the K(i) values of digoxin and TCA for the transport of TCA and digoxin were 0.58 and 160 microM, respectively. The K(m) and V(max) values of TCA and digoxin were 190 microM and 140 pmol/min/mg of protein and 1.1 microM and 6.6 pmol/min/mg of protein, respectively. The K(m) and K(i) values were consistent. In addition, digoxin (1 microM) and TCA (100 microM) increased the K(m) values of TCA and digoxin, respectively, but they did not affect the V(max) values, suggesting that their inhibition is competitive. The transport of digoxin via rOatp2 was inhibited slightly by E(2)17betaG, whereas the uptake of TCA was stimulated by E(2)17betaG in a concentration-dependent manner. These results suggest that rOatp2 has at least two substrate recognition sites, one for TCA and digoxin and the other for E(2)17betaG.

Induction profile of rat organic anion transporting polypeptide 2 (oatp2) by prototypical drug-metabolizing enzyme inducers that activate gene expression through ligand-activated transcription factor pathways

Knowledge of regulation of transporters would aid in predicting pharmacokinetics and drug-drug interactions. Treatment of rats with pregnenolone-16alpha-carbonitrile (PCN) and phenobarbital increases hepatic uptake of cardiac glycosides. Rat organic anion transporting polypeptide 2 (oatp2; Slc21a5) transports cardiac glycosides with high affinity. Levels of rat hepatic oatp2 protein and mRNA are regulated by PCN and phenobarbital treatment; however, the effects of other microsomal enzyme inducers on oatp2 have not been investigated. Therefore, the purpose of this study was to further determine whether oatp2 is regulated by a broader scale of drug-metabolizing enzyme inducers that are ligands or activators for the aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptor (PPAR), and antioxidant/electrophile response element (ARE/EpRE). Oatp2 protein levels determined by Western blot were decreased 56 to 72% by the AhR ligands, increased 84 to 132% by the CAR ligands, and increased 230 to 360% by PXR ligands. The PPAR ligands and ARE/EpRE activators generally had minimal effects on oatp2 protein levels. Oatp2 mRNA levels, determined by the bDNA technique, generally did not show a correlation with the altered oatp2 protein levels, e.g., among PXR ligands, only PCN increased oatp2 mRNA levels, but spironolactone and dexamethasone did not. Furthermore, only PCN, but not spironolactone and dexamethasone, increased the transcription of the oatp2 gene as the amount of hnRNA was increased when determined by reverse transcription-polymerase chain reaction. In conclusion, some drug-metabolizing enzyme inducers regulate oatp2 protein levels, especially the CYP3A inducers. However, there is no correlation between their ability to increase levels of oatp2 protein and mRNA, suggesting that regulation of oatp2 by drug-metabolizing enzyme inducers occurs at both the transcriptional and post-translational levels.

Functional characterization of human organic anion transporting polypeptide B (OATP-B) in comparison with liver-specific OATP-C

PURPOSE

To assess the functional characteristics of human organic anion transporter B (OATP-B) in comparison with those of the known, liver-specific OATP-C.

METHODS

OATP-B or -C was expressed in HEK293 cells or Xenopus oocytes, and uptakes of estradiol-17beta-glucuronide and estrone-3-sulfate were measured using radiolabeled compounds.

RESULTS

OATP-C transported both estrone-3-sulfate and estradiol-17beta-glucuronide, whereas OATP-B transported only the former. OATP-C-mediated uptake of estrone-3-sulfate exhibited biphasic saturation kinetics, whereas transports of estradiol-17beta-glucuronide by OATP-C and estrone-3-sulfate by OATP-B followed single-saturation kinetics. Inhibition kinetics showed that only the high-affinity site for estrone-3-sulfate on OATP-C was shared with glucuronide conjugates. Uptake of [3H]estrone-3-sulfate by OATP-B was inhibited by sulfate conjugates but not by glucuronide conjugates, whereas its uptake by OATP-C was inhibited by both types of conjugates.

CONCLUSIONS

OATP-B accepted sulfate conjugates of steroids but not glucuronide conjugates, whereas OATP-C transported both types of steroid conjugates. Transport of estrone-3-sulfate by OATP-B and -C followed single- and biphasic-saturation kinetics, respectively, and the high-affinity site on OATP-C was the same as that for estradiol-17beta-glucuronide. Other OATPs, OATP-A and OATP-8, reportedly exhibit different preferences for steroid conjugates, and the specific recognition of sulfate conjugates seems to be unique to OATP-B.

Transport of bile acids in hepatic and non-hepatic tissues

Bile acids are steroidal amphipathic molecules derived from the catabolism of cholesterol. They modulate bile flow and lipid secretion, are essential for the absorption of dietary fats and vitamins, and have been implicated in the regulation of all the key enzymes involved in cholesterol homeostasis. Bile acids recirculate through the liver, bile ducts, small intestine and portal vein to form an enterohepatic circuit. They exist as anions at physiological pH and, consequently, require a carrier for transport across the membranes of the enterohepatic tissues. Individual bile acid carriers have now been cloned from several species. Na(+)-dependent transporters that mediate uptake into hepatocytes and reabsorption from the intestine and biliary epithelium and an ATP-dependent transporter that pumps bile acids into bile comprise the classes of transporter that are specific for bile acids. In addition, at least four human and five rat genes that code for Na(+)-independent organic anion carriers with broad multi-substrate specificities that include bile acids have been discovered. Studies concerning the regulation of these carriers have permitted identification of molecular signals that dictate eventual changes in the uptake or excretion of bile acids, which in turn have profound physiological implications. This overview summarizes and compares all known bile acid transporters and highlights findings that have identified diseases linked to molecular defects in these carriers. Recent advances that have fostered a more complete appreciation for the elaborate disposition of bile acids in humans are emphasized.

Uptake and efflux of the peptidic delta-opioid receptor agonist

P-glycoprotein (P-gp) and organic anion transporting polypeptides (Oatp) are expressed at the blood-brain barrier (BBB). There is little functional evidence for Oatp-mediated transport at the BBB. The peptidic delta opioid-receptor agonist [D-penicillamine(2,5)]-enkephalin (DPDPE) is a substrate of mdr1a P-gp and Oatp2. The present study evaluated the influence of these transporters on brain uptake of DPDPE by in situ perfusion in mice. Brain uptake was increased approximately 12-fold in mice lacking P-gp in the BBB, but the P-gp inhibitor dexverapamil did not increase uptake in P-gp-competent mice. In P-gp-deficient mice, DPDPE uptake was saturable (K(m) approximately 24 mM), and was inhibited by dexverapamil and the Oatp2 substrates digoxin, estradiol-17beta-glucuronide and fexofenadine. These results confirm P-gp-mediated efflux of DPDPE, and suggest functional uptake transport of DPDPE by Oatp, at the murine BBB.

Transporters involved in the elimination of drugs in the kidney: organic anion transporters and organic cation transporters

Transporters in the kidney mediate the secretion or reabsorption of many compounds and thereby influence the plasma levels of their substrates. Organic anion transporters and organic cation transporters are two major classes of secretory transporters in the mammalian kidney. During the past decade, significant progress has been made in the cloning, functional expression, and initial characterization of these transporters. To date, five organic cation transporters and nine organic anion transporters have been cloned. In this review, we summarize the available data on organic anion and organic cation transporters, focusing in particular on their molecular characteristics, tissue distribution, and inhibitor and substrate selectivities. Currently we have a good understanding of the inhibitor selectivities for most of these transporters, and with the development of more robust assays, we will soon have a better understanding of their substrate selectivities. Based on the available data, summarized in this review, it appears that many compounds interact with multiple transporters. Furthermore, there appears to be substantial overlap in the selectivities of organic cation transporters, and the same appears true for organic anion transporters. At the present time, it is unclear what the roles of these multiple transporters are in renal drug elimination. With the development of new assays, reagents, and experimental methods, we will soon have a better understanding of the roles of each transporter isoform in the renal elimination of drugs.

Conditionally immortalized cell lines as a new in vitro model for the study of barrier functions

Conditionally immortalized brain and retinal capillary endothelial and choroid plexus epithelial cell lines were established from a transgenic rat (Tg rat) and mouse (Tg mouse) harboring the temperature-sensitive simian virus 40 (ts SV 40) large T-antigen. These cell lines exhibit temperature-sensitive cell growth due to the expression of ts SV 40 large T-antigen. Mouse brain (TM-BBB) and rat brain (TR-BBB) and rat retinal (TR-iBRB) capillary endothelial cell lines appear to have a spindle-fiber shaped morphology and exhibit the typical endothelial markers, such as von Willebrand factor and acetylated low-density lipoprotein uptake. These cell lines express in vivo influx and efflux transporters, such as P-glycoprotein (P-gp) and GLUT1, which is capable of 3-O-methyl-D-glucose transport. TM-BBB cells are able to undergo efflux transport of cyclosporin A, which is a substrate for P-gp transport activity. They may also express oatp2 and exhibit dehydroepiandrosterone sulfate and digoxin uptake activity. TR-BBB cells express the mRNA of multidrug resistance associated protein 1 (MRP1) and a large neutral amino acid transporter, which consists of LAT1 and 4F2hc. TR-iBRB cells exhibit pH-dependent L-lactic acid transport activity and express the mRNA of monocarboxylate transporter (MCT) 1 and 2. The choroid plexus epithelial cell line (TR-CSFB) has polygonal cell morphology, expresses the typical choroid plexus epithelial cell marker, transthyretin, and has Na+, K+-ATPase located on the apical side. TR-CSFB cells also exhibit amino acid transport activity which has been observed in vivo. These barrier cell lines established from the Tg rat and Tg mouse have in vivo transport functions and are good in vitro models for drug transport to the brain and retina and as a screen for drugs which might be capable of delivery to the brain and retina.

Expression and regulation of hepatic drug and bile acid transporters

Transport across hepatocyte plasma membranes is a key parameter in hepatic clearance and usually occurs through different carrier-mediated systems. Sinusoidal uptake of compounds is thus mediated by distinct transporters, such as Na(+)-dependent or Na(+)-independent anionic transporters and by some cationic transporters. Similarly, several membrane proteins located at the apical pole of hepatocytes have been incriminated in the excretion of compounds into the bile. Indeed, biliary elimination of anionic compounds, including glutathione S-conjugates, is mediated by MRP2, whereas bile salts are excreted by a bile salt export pump (BSEP) and Class I-P-glycoprotein (P-gp) is involved in the secretion of amphiphilic cationic drugs, whereas class II-P-gp is a phospholipid transporter. The expression of hepatic transporters and their activity are regulated in various situations, such as ontogenesis, carcinogenesis, cholestasis, cellular stress and after treatment by hormones and xenobiotics. Moreover, a direct correlation between a defect and the absence of transporter with hepatic disease has been demonstrated for BSEP, MDR3-P-gp and MRP2.

Blood-brain barrier is involved in the efflux transport of a neuroactive steroid, dehydroepiandrosterone sulfate, via organic anion transporting polypeptide 2

We have investigated the transport characteristics of dehydroepiandrosterone sulfate (DHEAS), a neuroactive steroid, at the blood-brain barrier (BBB) in a series of functional in vivo and in vitro studies. The apparent BBB efflux rate constant of [(3)H]DHEAS evaluated by the brain efflux index method was 2.68 x 10(-2) min(-1). DHEAS efflux transport was a saturable process with a Michaelis constant (K:(m)) of 32.6 microM: Significant amounts of [(3)H]DHEAS were determined in the jugular venous plasma by HPLC, providing direct evidence that most of the DHEAS is transported in intact form from brain to the circulating blood across the BBB. This efflux transport of [(3)H]DHEAS was significantly inhibited by common rat organic anion-transporting polypeptide (oatp) substrates such as taurocholate, cholate, sulfobromophthalein, and estrone-3-sulfate. Moreover, the apparent efflux clearance of [(3)H]DHEAS across the BBB (118 microl/min-g of brain) was 10.4-fold greater than its influx clearance estimated by the in situ brain perfusion technique (11.4 microl/min-g of brain), suggesting that DHEAS is predominantly transported from the brain to blood across the BBB. In cellular uptake studies using a conditionally immortalized mouse brain capillary endothelial cell line (TM-BBB4), [(3)H]DHEAS uptake by TM-BBB4 cells exhibited a concentration dependence with a K:(m) of 34.4 microM: and was significantly inhibited by the oatp2-specific substrate digoxin. Conversely, [(3)H]digoxin uptake by TM-BBB4 cells was significantly inhibited by DHEAS. Moreover, the net uptake of [(3)H]DHEAS at 30 min was significantly increased under ATP-depleted conditions, suggesting that an energy-dependent efflux process may also be involved in TM-BBB4. RT-PCR and sequence analysis suggest that an oatp2 is expressed in TM-BBB4 cells. In conclusion, DHEAS efflux transport takes place across the BBB, and studies involving in vitro DHEAS uptake and RT-PCR suggest that there is oatp2-mediated DHEAS transport at the BBB.

The pathophysiology of cholestasis with special reference to primary biliary cirrhosis

Cholestasis in primary biliary cirrhosis results from impairment of bile flow either by reduced transport at the level of the canaliculi or by disturbed bile flow through damaged intrahepatic bile ductules. Whatever its cause, the expression of hepatic transport proteins will be affected. In cholestatic rats: the expression of the multispecific organic anion transporter mrp2 is decreased; the bile salt export pump bsep and the phospholipid transporter mdr2 are less affected; the carrier protein for hepatic uptake of bile salts ntcp is sharply down-regulated; Mrp3, a basolateral ATP-dependent transporter for glucuronides and bile salts, is upregulated. Thus, bile salts that cannot exit the hepatocyte because of the cholestasis are effectively removed across the basolateral membrane. These may be adaptive responses in defence against overloading of hepatocytes with cytotoxic bile salts. These responses show that the expression of hepatic transporter proteins is highly regulated. This occurs by transcriptional and post-transcriptional mechanisms. Primary biliary cirrhosis starts as a disease of the small intrahepatic bile ducts and therefore the experimental evidence for 'cross-talk' between hepatocytes and cholangiocytes is of great interest for this disease and needs to be further investigated. New insights in bile physiology may enable the development of new therapies for cholestatic liver diseases as primary biliary cirrhosis.