LST-2, a human liver-specific organic anion transporter, determines methotrexate sensitivity in gastrointestinal cancers

Amino acid residues in transmembrane domain 10 of organic anion transporting polypeptide 1B3 are critical for cholecystokinin octapeptide transport

Human organic anion transporting polypeptides (OATP) 1B1 and 1B3 are multispecific transporters that mediate uptake of amphipathic organic compounds into hepatocytes. The two OATPs contain 12 transmembrane domains (TMs) and share 80% amino acid sequence identity. Besides common substrates with OATP1B1, OATP1B3 specifically transports cholecystokinin octapeptide (CCK-8). To determine which structural domains and/or residues are important for the substrate selectivity of OATP1B3, we constructed a series of chimeric proteins between OATP1B3 and 1B1, expressed them in HEK293 cells, and determined rates of uptake of CCK-8 along with surface expression of the proteins. Replacing TM10 in OATP1B3 with TM10 of OATP1B1 resulted in a dramatically reduced degree of CCK-8 transport, indicating that TM10 is crucial for recognition and/or translocation of CCK-8. Using site-directed mutagenesis, we identified three key residues within TM10, namely, Y537, S545, and T550. When we replaced these residues with the corresponding amino acid residues found in OATP1B1, the level of CCK-8 transport was similarly low as for the replacement of the whole TM10. Kinetic experiments showed that the K m values for CCK-8 transport in the TM10 replacement and triple mutant were only 1.3 and 1.1 microM, respectively, as compared to 16.3 microM for wild-type OATP1B3. Similarly, the V max values dropped from 495.5 pmol (normalized mg) (-1) min (-1) for wild-type OATP1B3 to 13.3 and 19.0 pmol (normalized mg) (-1) min (-1) for the TM10 replacement and triple mutant, respectively. Molecular modeling indicated that two of the three identified residues might form hydrogen bonds with CCK-8. In conclusion, we have identified three amino acid residues (Y537, S545, and T550) in TM10 of OATP1B3 that are important for CCK-8 transport.

Profiling SLCO and SLC22 genes in the NCI-60 cancer cell lines to identify drug uptake transporters

Molecular and pharmacologic profiling of the NCI-60 cell panel offers the possibility of identifying pathways involved in drug resistance or sensitivity. Of these, decreased uptake of anticancer drugs mediated by efflux transporters represents one of the best studied mechanisms. Previous studies have also shown that uptake transporters can influence cytotoxicity by altering the cellular uptake of anticancer drugs. Using quantitative real-time PCR, we measured the mRNA expression of two solute carrier (SLC) families, the organic cation/zwitterion transporters (SLC22 family) and the organic anion transporters (SLCO family), totaling 23 genes in normal tissues and the NCI-60 cell panel. By correlating the mRNA expression pattern of the SLCO and SLC22 family member gene products with the growth-inhibitory profiles of 1,429 anticancer drugs and drug candidate compounds tested on the NCI-60 cell lines, we identified SLC proteins that are likely to play a dominant role in drug sensitivity. To substantiate some of the SLC-drug pairs for which the SLC member was predicted to be sensitizing, follow-up experiments were performed using engineered and characterized cell lines overexpressing SLC22A4 (OCTN1). As predicted by the statistical correlations, expression of SLC22A4 resulted in increased cellular uptake and heightened sensitivity to mitoxantrone and doxorubicin. Our results indicate that the gene expression database can be used to identify SLCO and SLC22 family members that confer sensitivity to cancer cells.

Targeted disruption of murine organic anion-transporting polypeptide 1b2 (Oatp1b2/Slco1b2) significantly alters disposition of prototypical drug substrates pravastatin and rifampin

Organic anion-transporting polypeptides (OATP) 1B1 and 1B3 are widely acknowledged as important and rate-limiting to the hepatic uptake of many drugs in clinical use. Accordingly, to better understand the in vivo relevance of OATP1B transporters, targeted disruption of murine Slco1b2 gene was carried out. It is noteworthy that Slco1b2(-/-) mice were fertile, developed normally, and exhibited no overt phenotypic abnormalities. We confirmed the loss of Oatp1b2 expression in liver using real-time polymerase chain reaction, Western Blot analysis, and immunohistochemistry. Expression of Oatp1a4 and Oatp2b1 but not Oatp1a1 was greater in female Slco1b2(-/-) mice, but expression of other non-OATP transporters did not significantly differ between wild-type and Slco1b2(-/-) male mice. Total bilirubin level was elevated by 2-fold in the Slco1b2(-/-) mice despite the fact that liver enzymes ALT and AST were normal. Pharmacological characterization was carried out using two prototypical substrates of human OATP1B1 and -1B3, rifampin and pravastatin. After a single intravenous dose of rifampin (1 mg/kg), a 1.7-fold increase in plasma area under the concentration-time curve (AUC) was observed, whereas the liver-to-plasma ratio was reduced by 5-fold, and nearly 8-fold when assessed at steady-state conditions after 24 h of continuous subcutaneous infusion in Slco1b2(-/-) mice. Likewise, continuous subcutaneous infusion at low (8 microg/h) or high (32 microg/h) dose rates of pravastatin resulted in a 4-fold lower liver-plasma ratio in the in Slco1b2(-/-) mice. This is the first report of altered drug disposition profile in the Slco1b2 knockout mice and suggests the utility of this model for understanding the in vivo role of hepatic OATP transporters in drug disposition.

Involvement of reactive oxygen species in Microcystin-LR-induced cytogenotoxicity

Microcystin-LR (MCLR) is a potent hepatotoxin. Oxidative stress is thought to be implicated in the cytotoxicity of MCLR, but the mechanisms by which MCLR produces reactive oxygen species (ROS) are still unclear. This study investigated the role and possible sources of ROS generation in MCLR-induced cytogenotoxicity in HepG2, a human hepatoma cell line. MCLR increased DNA strand breaks, 8-hydroxydeoxiguanosine formation, lipid peroxidation, as well as LDH release, all of which were inhibited by ROS scavengers. ROS scavengers partly suppressed MCLR-induced cytotoxicity determined by the MTT assay. MCLR induced the generation of ROS, as confirmed by confocal microscopy with 2-[6-(4'-hydroxy)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid, and upregulated the expression of CYP2E1 mRNA. In addition, CYP2E1 inhibitors chlormethiazole and diallyl dulphide inhibited both ROS generation and cytotoxicity induced by MCLR. The results suggest that ROS contribute to MCLR-induced cytogenotoxicity. CYP2E1 might be a potential source responsible for ROS generation by MCLR.

Effect of SLCO1B3 haplotype on testosterone transport and clinical outcome in caucasian patients with androgen-independent prostatic cancer

PURPOSE

The organic anion transporter OATP1B3, encoded by SLCO1B3, is involved in the transport of steroid hormones. However, its role in testosterone uptake and clinical outcome of prostatic cancer is unknown. This study examined (a) the SLCO1B3 genotype in cancer cells as well as the uptake of testosterone by cells transfected with genetic variants of SLCO1B3; (b) the expression of OATP1B3 in normal prostate, benign prostatic hyperplasia, and prostatic cancer; and (c) the role of SLCO1B3 haplotype on clinical outcome of Caucasian patients with androgen-independent prostatic cancer.

EXPERIMENTAL DESIGN

SLCO1B3 genotype was assessed in the NCI-60 panel of tumor cells by sequencing, whereas testosterone transport was analyzed in Cos-7 cells transfected with WT, 334G, and 699A SLCO1B3 variants. OATP1B3 expression in prostatic tissues was examined by fluorescence microscopy, and the relationship between SLCO1B3 haplotypes and survival was examined in patients.

RESULTS

Cells transfected with wild-type (334T/699G) SLCO1B3, or with a vector containing either the 334G or 699A variants, actively transported testosterone, whereas its uptake was impaired in cells transfected with a gene carrying both 334G and 699A single nucleotide polymorphisms. Prostatic cancer overexpresses OATP1B3 compared with normal or benign hyperplastic tissue; patients with SLCO1B3 334GG/699AA haplotype showed longer median survival (8.5 versus 6.4 years; P = 0.020) and improved survival probability at 10 years (42% versus 23%; P < 0.023) than patients carrying TT/AA and TG/GA haplotypes.

CONCLUSIONS

The common SLCO1B3 GG/AA haplotype is associated with impaired testosterone transport and improved survival in patients with prostatic cancer.

Role of OATP transporters in the disposition of drugs

During recent years, it has become increasingly recognized that drug transporters play important roles in drug absorption and disposition. Organic anion transporting polypeptides (OATPs) are membrane transporters critically involved in the cellular uptake of drugs in tissues important for pharmacokinetics, such as the intestine, liver and kidneys. Recent advances in the pharmacogenomics of OATP1B1 have revealed that OATP transporters can play important roles in explaining interindividual variability in drug pharmacokinetics, and thus contribute to interindividual as well as interethnic variability in drug response. This article will provide an up-to-date review of human OATPs and their substrates, and a current compilation of their DNA sequence variations.

The functional consequences of genetic variations in transporter genes encoding human organic anion-transporting polypeptide family members

It is increasingly recognised that uptake transporters of the organic anion-transporting polypeptide (OATP) family play important roles in drug absorption, distribution and excretion. They are expressed in a variety of different tissues, including gut, brain, kidney and liver. Substrates of OATPs include several endogenous substances, such as bile salts and hormones, and drugs such as HMG-CoA reductase inhibitors (e.g., pravastatin), cytotoxic drugs and antibiotics. Recent advances in the pharmacogenetics of OATPs have demonstrated that variations (polymorphisms) in genes encoding human OATPs can explain parts of the interindividual variability in the pharmacokinetics of drugs and, thus, contribute to the interethnic and interindividual variability in drug response. This review focuses on consequences of these genetic variations and summarises in vivo as well as in vitro analyses demonstrating the impact of polymorphisms in genes encoding OATPs on transport and pharmacokinetics of drugs.

Mechanisms of antifolate resistance and methotrexate efficacy in leukemia cells

Antifolates are the first class of antimetabolites introduced to clinic about 6 decades ago. Now, after several years of administration of antifolates against malignancies and particularly leukemia, we are still trying to achieve a full understanding of the mechanisms of action and resistance to these agents. The present article covers different factors able to influence efficacy of antifolates on leukemic cells, the known mechanisms of resistance to methotrexate (MTX) and strategies to overcome these mechanisms. The dominant factors that are contributed to tolerance to cytocidal effects of MTX including pharmacokinetic factors, impaired transmembrane uptake as the most frequent rote of provoking resistance to MTX, augmented drug efflux, impaired intracellular polyglutamation as a determining process of drug efficacy, alterations in expression or activity of target enzymes and increased intracellular folate pools; and finally role of 7-hydroxymethotrexate on response or resistance to MTX will be discussed in more detail. Finally, strategies to overcome resistance to antifolates are discussed.

Carrier-mediated cellular uptake of pharmaceutical drugs: an exception or the rule?

It is generally thought that many drug molecules are transported across biological membranes via passive diffusion at a rate related to their lipophilicity. However, the types of biophysical forces involved in the interaction of drugs with lipid membranes are no different from those involved in their interaction with proteins, and so arguments based on lipophilicity could also be applied to drug uptake by membrane transporters or carriers. In this article, we discuss the evidence supporting the idea that rather than being an exception, carrier-mediated and active uptake of drugs may be more common than is usually assumed - including a summary of specific cases in which drugs are known to be taken up into cells via defined carriers - and consider the implications for drug discovery and development.

Rapid screening of antineoplastic candidates for the human organic anion transporter OATP1B3 substrates using fluorescent probes

A rapid screening system has been established to extract novel candidates that exhibit potent inhibition of the transport of fluorescent substrate by organic anion transporting polypeptide (OATP) 1B3. OATP1B3 is abundantly expressed in solid digestive organ cancers. Thus, the identification of new substrates leads to novel strategies for effective cancer chemotherapy with minimal adverse effects. We used an automated image acquisition and analysis system (IN Cell Analyzer 1000) to visualize the transport and subsequent accumulation of the fluorescent substrate chenodeoxycholyl-(Nepsilon-NBD)-lysine (CDCA-NBD). Antineoplastic screening demonstrated that five candidates agents, docetaxel, actinomycin D, mitoxantrone, paclitaxel, and SN-38, exhibited potent inhibitory effects on OATP1B3-mediated transport of CDCA-NBD. To clarify if these antineoplastic drugs are substrates for OATP1B3, we performed transport assays in OATP1B3-expressing cells. We determined that SN-38 is a novel substrate for OATP1B3. In conclusion, our results demonstrate that the screening system established in this study is a useful method for the rapid extraction of candidate therapeutic agents from the large numbers of compounds.

Potent cytotoxicity of the phosphatase inhibitor microcystin LR and microcystin analogues in OATP1B1- and OATP1B3-expressing HeLa cells

Microcystins are a family of cyclic peptides that are potent inhibitors of the protein phosphatase families PP1 and PP2A. Only three human proteins are thought to be able to mediate the hepatic uptake of microcystins (the organic anion-transporting polypeptides OATP1B1, OATP1B3, and OATP1A2), and the predominant hepatic expression of these transporters accounts for the liver-specific toxicity of microcystins. A significant obstacle in the study of microcystins as anticancer drugs is the requirement of specific transport proteins for cellular uptake. We report that OATP1B3 mRNA is up-regulated in non-small cell lung cancer tumors in comparison with normal control tissues. This finding led to the exploration of microcystins as potential anticancer agents. We have developed a HeLa cell model with functional OATP1B1 and OATP1B3 activity. Transiently transfected HeLa cells are over 1,000-fold more sensitive to microcystin LR than the vector-transfected control cells, showing that transporter expression imparts marked selectivity for microcystin cytotoxicity. In addition, microcystin analogues showed variable cytotoxicities in the OATP1B1- and OATP1B3-transfected cells, including two analogues with IC(50) values <1 nmol/L. Cytotoxicity of microcystin analogues seems to correlate to the inhibition of PP2A in these cells and induces rapid cell death as seen by chromatin condensation and cell fragmentation. These studies show that microcystin-induced phosphatase inhibition results in potent cytotoxicity when microcystin compounds can gain intracellular access and are a potent novel class of therapeutic agents for tumors expressing these uptake proteins.

Human liver-specific organic anion transporter-2 is a potent prognostic factor for human breast carcinoma

Human liver-specific organic anion transporter-2 (LST-2/OATP8/SLCO1B3) has been demonstrated to be expressed in various gastrointestinal carcinomas and also to play pivotal roles in the uptake of a wide variety of both endogenous and exogenous anionic compounds, including bile acids, conjugated steroids and hormones, into hepatocytes in the human liver. However, the biological significance of LST-2 in human carcinomas remains unknown. In the present study, we examined the expression of LST-2 in 102 cases of breast carcinoma using immunohistochemistry and correlated the findings with various clinicopathological parameters in order to examine the possible biological and clinical significance of LST-2. LST-2 immunoreactivity was detected in 51 cases (50.0%); of these 51 positive cases, LST-2 immunoreactivity was inversely correlated with tumor size (P = 0.0289). In addition, LST-2 immunoreactivity was significantly associated with a decreased risk of recurrence and improved prognosis by both univariate (P = 0.02 and P = 0.01) and multivariate (P = 0.03 and P = 0.01) analyses. In the estrogen receptor-positive groups, the LST-2-positive patients showed good prognoses. Considering that LST-2 transports estrone-3-sulfate, these results suggest that LST-2 overexpression is associated with a hormone-dependent growth mechanism of the breast cancer. The results of our present study demonstrate that LST-2 immunoreactivity is a potent prognostic factor in human breast cancer.

Predominant Contribution of Rat Organic Anion Transporting Polypeptide-2 (Oatp2) to Hepatic Uptake of β-Lactam Antibiotics

PURPOSE

To identify the rat hepatic basolateral transporters involved in the hepatic uptake of beta-lactam antibiotics using nafcillin as a model beta-lactam antibiotic that undergoes extensive biliary excretion.

MATERIALS AND METHODS

Uptake by isolated rat hepatocytes and Xenopus laevis oocytes expressing organic anion transporting peptides (Oatp1, 2, and 4) and organic anion transporter (OAT2) was evaluated.

RESULTS

Nafcillin uptake by isolated rat hepatocytes was saturable with the Km of 210 microM and was significantly inhibited by anionic compounds (estrone-3-sulfate and sulfobromophthalein), but not by cationic compounds (tetraethylammonium and 1-methyl-4-phenylpyridinium). In an in vitro uptake study by Xenopus oocytes expressing hepatic basolateral membrane transporters, nafcillin was transported by multiple Oatps with Km values of 4120 microM (Oatp1/Oatp1a1), 198 microM (Oatp2/Oatp1a4), and 1,570 microM (Oatp4/Oatp1b2), though it was not transported by hOAT2. Comparison of affinity and analysis by the relative activity factor method indicated that Oatp2 is the predominant contributor to the hepatic uptake of nafcillin. Cefadroxil, cefazolin, cefmetazole, cefoperazone, cefsulodin, and cephalexin, though not cefotaxime or ceftriaxone, were also substrates of Oatp2.

CONCLUSION

These findings suggest that Oatp2 plays a key role in the hepatic uptake of nafcillin and most other beta-lactam antibiotics in rats.

Involvement of rat and human organic anion transporter 3 in the renal tubular secretion of topotecan [(S)-9-dimethylaminomethyl-10-hydroxy-camptothecin hydrochloride]

Topotecan [(S)-9-dimethylaminomethyl-10-hydroxy-camptothecin hydrochloride] is primarily excreted into urine in humans, with approximately 49% of the dose recovered as total topotecan (topotecan lactone plus topotecan hydroxyl acid form). The renal elimination of topotecan involves tubular secretion in addition to glomerular filtration, but little is known about the molecular mechanism of the renal tubular secretion. In the present study, we investigated the transport characteristics of topotecan hydroxyl acid across the renal basolateral membrane using rat kidney slices and rat or human transporter-expressing Xenopus laevis oocytes. Pravastatin and probenecid significantly inhibited the uptake of topotecan hydroxyl acid by rat kidney slices with K(i) values of 10.6 and 8.1 microM, respectively, and p-aminohippurate was weakly inhibitory at high concentrations, whereas excess tetraethylammonium had no effect. The uptake of topotecan hydroxyl acid by oocytes injected with complementary RNA of either rat or human organic anion transporter 3 (rOAT3 or hOAT3) was greater than that of water-injected oocytes. Kinetic analysis showed that the K(m) values for rOAT3 and hOAT3 were 21.9 and 56.5 microM, respectively. Neither rOAT1 nor hOAT1 stimulated topotecan hydroxyl acid transport. These results suggest that the urinary excretion of topotecan hydroxyl acid is accounted for by transport via OAT3, as well as glomerular filtration, in both rats and humans; therefore, drug-drug interactions involving OAT3 may cause a change in clearance of topotecan.

Genetic variation in thyroid hormone transporters

Determination of the sequence of the human genome has led to a large expansion of research in this area. Several studies on single nucleotide polymorphisms, i.e. variations in the genome that occur in > 1% of the population, have been published in recent years. In the thyroid field, information about variation in relevant genes is also forthcoming, which is not surprising as these polymorphisms are thought to play a role in determining each individual's thyroid hormone set-point. So far, research has focused mainly on genetic variation in the thyroid-stimulation hormone receptor and the deiodinases, and their association with thyroid parameters and/or clinical endpoints, such as insulin resistance. However, with the characterization of specific thyroid hormone transporters, a new field of research is emerging.

Cellular entry of thyroid hormones by organic anion transporting polypeptides

Several transporter families that can transport thyroid hormones have been identified. One of these is represented by the organic anion transporting polypeptide (OATP) gene superfamily, classified within the solute carrier family SLCO. In humans, eleven different OATPs are expressed in various tissues, including the liver, kidney, brain, lung, intestine and placenta. They mediate sodium-independent transport of a variety of amphipathic organic compounds, including thyroid hormones, bile acids, steroid hormones and their conjugates, linear and cyclic peptides, prostaglandins, numerous drugs and other xenobiotics. OATP1C1, which is expressed in the blood-brain barrier and testes, is the thyroid hormone transporter with the highest apparent affinity, with K(m) values of 90.4nM for thyroxine (T(4)) and 127.7nM for reverse T(3) (rT(3)) transport, and could be essential for thyroid hormone delivery to the developing brain. OATP4A1 is expressed in the placenta and could be important for maternal thyroid hormone transport to the developing fetus.

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.

Novel liver-specific organic anion transporter OAT7 that operates the exchange of sulfate conjugates for short chain fatty acid butyrate

The liver plays an important role in the elimination of endogenous and exogenous lipophilic organic compounds from the body, which is mediated by various carrier proteins that differ in substrate specificity and kinetic properties. Here, we have characterized a novel member of the organic anion transporter family (SLC22) isolated from human liver. The transporter named organic anion transporter 7 (OAT7/ SLC22A9) showed 35% to 46% identities to those of other organic anion transporters of SLC22 family. When expressed in Xenopus oocytes, OAT7 mediated Na(+)-independent, high-affinity transport of sulfate-conjugated steroids, estrone sulfate (ES; K(m) = 8.7 microM), and dehydroepiandrosterone sulfate (K(m) = 2.2 microM). In addition, OAT7 interacted with negatively charged sulfobromophthalein, indocyanine green, and several sulfate-conjugated xenobiotics. In contrast, glucuronide and glutathione conjugates exhibited no inhibitory effects on OAT7-mediated [(3)H]ES transport. OAT7-mediated [(3)H]ES transport was trans-stimulated by three-carbon to five-carbon (C3 to C5) short-chain fatty acids. The efflux of [(14)C]butyrate (C4) via OAT7 was significantly trans-stimulated by extracellular ES. Furthermore, OAT7 mediated [(14)C]butyrate uptake and [(3)H]ES efflux in exchange for extracellular butyrate both in Xenopus oocytes and OAT7-stably expressing cells. OAT7 protein was localized in the sinusoidal membrane of hepatocytes by immunohistochemical analysis.

CONCLUSION

OAT7 is the first liver-specific transporter among members of the organic anion transporters of SLC22 family. Our findings suggest a new class of substrates for organic anion transporters and provide evidence for the transport of anionic substances such as sulfate-conjugates in exchange for butyrate in hepatocytes.

Expression profiles of 50 xenobiotic transporter genes in humans and pre-clinical species: a resource for investigations into drug disposition

Carrier-mediated transporters play a critical role in xenobiotic disposition and transporter research is complicated by species differences and their selective tissue expression. The purpose of this study was to generate a comprehensive data set of xenobiotic transporter gene expression profiles in humans and the pre-clinical species mouse, rat, beagle dog and cynomolgus monkey. mRNA expression profiles of 50 genes from the ABC, SLC and SLCO transporter superfamilies were examined in 40 human tissues by microarray analyses. Transporter genes that were identified as enriched in the liver or kidney, or that were selected for their known roles in xenobiotic disposition, were then compared in 22 tissues across the five species. Finally, as clinical variability in drug response and adverse reactions may be the result of variability in transporter gene expression, variability in the expression of selected transporter genes in 75 human liver donors were examined and compared with the highly variable drug metabolizing enzyme CYP3A4.

Variants in the SLCO1B3 Gene: Interethnic Distribution and Association with Paclitaxel Pharmacokinetics

To explore retrospectively the relationships between paclitaxel pharmacokinetics and three known, non-synonymous single-nucleotide polymorphisms (SNPs) in SLCO1B3, the gene encoding organic anion transporting polypeptide (OATP)1B3. Accumulation of [(3)H]paclitaxel was studied in Xenopus laevis oocytes injected with cRNA of Oatp1b2, OATP1A2, OATP1B1, OATP1B3, OAT1, OAT3, OCT1, and NTCP. The 334T>G (Ser112Ala), 699G>A (Met233Ile), and 1564G>T (Gly522Cys) loci of SLCO1B3 were screened in 475 individuals from five ethnic groups and 90 European Caucasian cancer patients treated with paclitaxel. Only OATP1B3 was capable of transporting paclitaxel to a significant extent (P=0.003). The 334T>G and 699G>A SNPs were less common in the African-American and Ghanaian populations (P<0.000001). Paclitaxel pharmacokinetics were not associated with the studied SNPs or haplotypes (P>0.3). The studied SNPs in SLCO1B3 appear to play a limited role in the disposition of paclitaxel, although their clinical significance in other ethnic populations remains to be investigated.

Genetic polymorphisms of drug transporters: pharmacokinetic and pharmacodynamic consequences in pharmacotherapy

There has been increasing appreciation of the role of drug transporters in pharmacokinetic and pharmacodynamic consequences in pharmacotherapy. The clinical relevance of drug transporters depends on the localisation in human tissues (i.e., vectorial movement), the therapeutic index of the substrates and inherent interindividual variability. With regard to variability, polymorphisms of drug transporter genes have recently been reported to be associated with alterations in the pharmacokinetics and pharmacodynamics of clinically useful drugs. A growing number of preclinical and clinical studies have demonstrated that the application of genetic information may be useful in individualised pharmacotherapy for numerous diseases. However, the reported effects of variants in certain drug transporter genes have been inconsistent and, in some cases, conflicting among studies. Furthermore, the incidence of almost all known variants in transporter genes tends to be racially dependent. These observations suggest the necessity of considering interethnic variability before extrapolating pharmacokinetic data obtained in one ethic group to another, especially in the early phase of drug development. This review focuses on the impact of genetic variations in the function of drug transporters (ABC, organic anion and cation transporters) and the implications of these variations for pharmacotherapy from pharmacokinetic and pharmacodynamic viewpoints.

Role of the liver-specific transporters OATP1B1 and OATP1B3 in governing drug elimination

Members of the organic anion transporting polypeptide (OATP) family are responsible for the cellular uptake of a broad range of endogenous compounds and xenobiotics in multiple tissues. This review focuses on OATP1B1 and -1B3, which are specifically expressed in the liver and considered to be of particular importance for hepatic drug elimination and drug pharmacokinetics. Recent literature has indicated that inhibition of these transporters may result in drug-drug interactions. Furthermore, genetic polymorphisms in the genes encoding OATP1B1 and -1B3 have been described that increase or decrease transport in vitro and in vivo. Alteration of transporter function by either of these mechanisms may contribute to interindividual variability in drug disposition and response. In this review an update of this rapidly emerging field is provided.

Predominant contribution of organic anion transporting polypeptide OATP-B (OATP2B1) to apical uptake of estrone-3-sulfate by human intestinal Caco-2 cells

Human organic anion transporting polypeptide OATP-B (OATP2B1) is a pH-sensitive transporter expressed in the apical membranes of small intestinal epithelial cells. In this study, we have examined the contribution of OATP-B to the uptake of [3H]estrone-3-sulfate in Caco-2 cells in comparison with those of its homologs OATP-D (OATP3A1) and OATP-E (OATP4A1). Immunocytochemical study revealed that OATP-B is expressed in the apical membranes of Caco-2 cells. The uptake of [3H]estrone-3-sulfate by Caco-2 cells was Na+-independent and inhibited by several organic anions. It showed biphasic saturation kinetics with Km values of 1.81 microM and 1.40 mM. The uptake of [3H]estrone-3-sulfate by human embryonic kidney (HEK) 293 cells stably expressing OATP-B (HEK293/OATP-B) was also Na+-independent and inhibited by several organic anions. The Km value for estrone-3-sulfate uptake by OATP-B (1.56 microM) was close to that for the high-affinity component observed in Caco-2 cells. The mRNA expression level of OATP-B was higher than that of OATP-D or OATP-E in Caco-2 cells and in human jejunum biopsies from healthy volunteers. The values of [3H]estrone-3-sulfate uptake normalized to OATP-B mRNA expression were similar in Caco-2 cells and HEK293/OATP-B cells. The specific activity of OATP-B per mRNA expression was much higher than that of OATP-D and OATP-E. [3H]Estrone-3-sulfate uptake by membrane vesicles prepared from HEK293/OATP-B cells exhibited an overshoot phenomenon in the presence of an inwardly directed H+ gradient, suggesting that an H+ gradient is the driving force of estrone-3-sulfate transport by OATP-B. These results suggest that OATP-B is predominantly responsible for the apical uptake of estrone-3-sulfate in Caco-2 cells.

Variability in mRNA expression of ABC- and SLC-transporters in human intestinal cells: Comparison between human segments and Caco-2 cells

The Caco-2 cell monolayer model is widely used as a tool for evaluating human intestinal permeability and interaction with transporters. Therefore, we determined mRNA levels for 15 of the most frequently studied uptake and efflux transporters (MDR1, MRP2-3, BCRP, OCTN2, PepT1, OATP-B, OATP8, OCT1-3, OAT1-3, MCT1) using real-time PCR in Caco-2 cells and in human jejunum and colon. The expression levels in the Caco-2 cells did not significantly vary between different passages (p29-43) and batches for any of the genes measured. However, levels increased with culture time (1-5 weeks) for PepT1, MDR1, MRP2, OATP-B and BCRP. The general rank order of the gene expression levels in Caco-2 cells was established as follows: MRP2>OATP-B>PepT1>>MDR1>MCT1 approximately MRP3 approximately BCRP approximately OCTN2>>OCT3>OCT1>OAT2. Four genes were absent: OATP8, OCT2, OAT1, and OAT3. Ranking of 11 expressed genes showed a significant correlation between human jejunum and 2-5-week-old Caco-2 cells. The expression profile in colon was, however, very different compared to both Caco-2 cells and jejunum. We conclude that the Caco-2 cells in our hands express similar transporters as the human jejunum, but are different from colon, indicating their usefulness for obtaining small intestinal transport data. In addition, we also suggest that cells with a well-defined range of culture ages should be used to minimize variability in data from experiments and even erroneous conclusions.

Involvement of transporters in the hepatic uptake and biliary excretion of valsartan, a selective antagonist of the angiotensin II AT1-receptor, in humans

Valsartan is a highly selective angiotensin II AT1-receptor antagonist for the treatment of hypertension. Valsartan is mainly excreted into the bile in unchanged form. Because valsartan has an anionic carboxyl group, we hypothesized that a series of organic anion transporters could be involved in its hepatic clearance. In this study, to identify transporters that mediate the hepatic uptake and biliary excretion of valsartan and estimate the contribution of each transporter to the overall hepatic uptake and efflux, we characterized its transport using transporter-expressing systems, human cryopreserved hepatocytes, and Mrp2-deficient Eisai hyperbilirubinemic rats (EHBRs). Valsartan was significantly taken up into organic anion-transporting polypeptide (OATP) 1B1 (OATP2/OATP-C)- and OATP1B3 (OATP8)-expressing HEK293 cells. We also observed saturable uptake into human hepatocytes. Based on our estimation, the relative contribution of OATP1B1 to the uptake of valsartan in human hepatocytes depends on the batch, ranging from 20 to 70%. Regarding efflux transporters, the ratio of basal-to-apical transcellular transport of valsartan to that in the opposite direction in OATP1B1/MRP2 (multidrug resistance-associated protein 2) double transfected cells was the highest among the three kinds of double transfectants, OATP1B1/MRP2, OATP1B1/multi-drug resistance 1, and OATP1B1/breast cancer resistance protein-expressing MDCKII cells. We observed saturable ATP-dependent transport into membrane vesicles expressing human MRP2. We also found that the elimination of intravenously administered valsartan from plasma was markedly delayed, and the biliary excretion was severely impaired in EHBR compared with normal Sprague-Dawley rats. These results suggest that OATP1B1 and OATP1B3 as the uptake transporters and MRP2 as the efflux transporter are responsible for the efficient hepatobiliary transport of valsartan.

OATP1B1, OATP1B3, and mrp2 are involved in hepatobiliary transport of olmesartan, a novel angiotensin II blocker

Hepatic uptake and biliary excretion of olmesartan, a new angiotensin II blocker, were investigated in vitro using human hepatocytes, cells expressing uptake transporters and canalicular membrane vesicles, and in vivo using Eisai hyperbilirubinemic rats (EHBR), inherited multidrug resistance-associated protein (mrp2)-deficient rats. The uptake by human hepatocytes reached saturation with a Michaelis constant (K(m)) of 29.3 +/- 9.9 microM. Both Na(+)-dependent and Na(+)-independent uptake of olmesartan by human hepatocytes were observed. The uptake by Na(+)-independent human liver-specific organic anion transporters OATP1B1 and OATP1B3 expressed in Xenopus laevis oocytes was also saturable, with K(m) values of 42.6 +/- 28.6 and 71.8 +/- 21.6 microM, respectively. The Na(+)-dependent taurocholate-cotransporting polypeptide expressed in HEK 293 cells did not transport olmesartan. The cumulative biliary excretion in EHBR was one-sixth compared with that in Sprague-Dawley rats. ATP-dependent uptake of olmesartan was observed in both human canalicular membrane vesicles (hCMVs) and MRP2-expressing vesicles. An MRP inhibitor, MK-571 ([[[3-[2-(7-chloro-2-quinolinyl)ethenyl]phenyl][3-(dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid) completely inhibited the uptake of olmesartan by hCMVs. In conclusion, the hepatic uptake and biliary excretion of olmesartan are mediated by transporters in humans. OATP1B1 and OATP1B3 are involved in hepatic uptake, at least in part, and MRP2 plays a dominant role in the biliary excretion.

Farnesoid X receptor, hepatocyte nuclear factors 1alpha and 3beta are essential for transcriptional activation of the liver-specific organic anion transporter-2 gene

BACKGROUND

We isolated the human liver-specific organic anion transporter gene, LST-2 (OATP8/SLCO1B3), which is exclusively expressed in the basolateral membrane of the hepatocytes. In this study, we analyzed the transcriptional regulation of the LST-2 gene in hepatocyte-derived cells and the effect of bile acid.

METHODS

Transcriptional activity of the LST-2 gene was measured using a human LST-2 promoter-luciferase reporter plasmid under various concentrations of bile acids. Electrophoresis mobility shift assays of farnesoid X receptor (FXR), hepatocyte nuclear factor (HNF) 1alpha, and HNF3beta were performed.

RESULTS

Luciferase analysis showed that the 5'-flanking region from -180 to -20 bp is responsible for LST-2 transcriptional activity. By site-directed mutation analysis, it was revealed that the consensus binding sites for FXR, HNF1alpha, and HNF3beta play important roles in the transcriptional activity of the LST-2 gene. By electrophoresis mobility shift assay, we observed specific protein-DNA complexes of FXR, HNF1alpha, and HNF-3beta. Luciferase activity was increased fivefold when chenodeoxycholate or deoxycholate were added. Northern blot analyses revealed that the expression of LST-2 was increased by addition of chenodeoxycholate or deoxycholate in a dose-dependent manner.

CONCLUSIONS

This study demonstrated that the transcription of the LST-2 gene is regulated by three transcription factors, FXR, HNF1alpha, and HNF3beta. HNF1alpha and HNF3beta might contribute to its liver-specific expression, and FXR might play a role in its transcriptional activation by bile acids.

Transport of fluorescent chenodeoxycholic acid via the human organic anion transporters OATP1B1 and OATP1B3

This study sought to clarify the contributions of organic anion-transporting polypeptide (OATP) 1B1 and 1B3 to the liver uptake of chenodeoxycholic acid (CDCA). We synthesized a fluorescent version of CDCA, chenodeoxychilyl-(Nepsilon-NBD)-lysine (CDCA-NBD), to characterize transporter-mediated uptake. CDCA-NBD is efficiently transported by OATP1B1 and OATP1B3 with high affinities. The Michaelis-Menten constants for CDCA-NBD uptake by OATP1B1 and OATP1B3 were 1.45 +/- 0.39 microM and 0.54 +/- 0.09 microM, respectively. By confocal laser scanning microscopy, CDCA-NBD, which is taken up by OATP1B1 and OATP1B3, was observed to localize to the cytosol. We also examined the transport of newly synthesized fluorescent bile acids. NBD-labeled bile acids, including cholic acid, deoxycholic acid, lithocholic acid, and ursodeoxycholic acid, were all transported by OATP1B1 and OATP1B3. CDCA-NBD exhibited the highest rate of transport of the five NBD-labeled bile acids examined in OATP1B1- and OATP1B3-expressing cells. Our results suggest that OATP1B1 and OATP1B3 play important roles in CDCA uptake into the liver. Fluorescent bile acids are useful tools to characterize the uptake properties of membrane transporters.

Pharmacogenomics of human OATP transporters

Organic anion transporting polypeptides (OATPs) mediate the uptake of a broad range of compounds into cells. Substrates for members of the OATP family include bile salts, hormones, and steroid conjugates as well as drugs like the HMG-CoA-reductase inhibitors (statins), cardiac glycosides, anticancer agents like methotrexate, and antibiotics like rifampicin. OATPs are expressed in a variety of different tissues, including intestine, liver, kidney, and brain, suggesting that they play a critical role in drug absorption, distribution, and excretion. The identification and functional characterisation of naturally occurring variations in genes encoding human OATP (SLCO) family members is in the focus of transporter research. As a result of their broad substrate spectrum and their wide tissue distribution, altered transport characteristics or protein localisation can contribute significantly to interindividual variations of drug effects. The analysis of the consequences of genetic variations in genes encoding transport proteins may, therefore, contribute to a better understanding of interindividual differences in drug effects and to individualise treatment regimens with drugs that are substrates for human OATP proteins. In this review, we summarise the current knowledge on genetic variations in transporter genes encoding human OATP family members and their functional consequences analysed by in vitro and in vivo studies.

Suppression of cell proliferation by inhibition of estrone-3-sulfate transporter in estrogen-dependent breast cancer cells

PURPOSE

The aim of the study is to suppress the progress of estrogen-dependent breast cancer by inhibiting the membrane transporter, which mediates the internalization of estrone-3-sulfate as estrogen precursor in the cancer cells.

METHODS

The uptake of estrone-3-sulfate by estrogen-dependent breast cancer MCF-7 cells was measured, and inhibitory study using various organic anions on estrone-3-sulfate uptake by MCF-7 cells was conducted. The effects of the inhibitor on the transcription of reporter gene and cell proliferation induced by estrone-3-sulfate were examined.

RESULTS

The uptake of estrone-3-sulfate by MCF-7 cells was saturable with Km value of 2.32 microM. The uptake was Na+-independent and was inhibited by several anionic compounds such as bromosulfophthalein. Bromosulfophthalein also significantly inhibited the transcription of reporter gene via estrogen response element and cell proliferation induced by estrone-3-sulfate. However, the transcriptional activation or cell proliferation induced by estrone was not inhibited by bromosulfophthalein. Reverse transcription-polymerase chain reaction analysis revealed the expression of mRNA of organic anion transporting polypeptide (OATP)-D and OATP-E as possible candidates to transport estrone-3-sulfate.

CONCLUSIONS

The uptake of estrone-3-sulfate is mediated by Na+-independent transporter(s). Inhibitor of estrone-3-sulfate transporter suppressed the transcription and cell proliferation induced by estrone-3-sulfate in MCF-7 cells. The results provide the basis of a novel strategy for breast cancer treatment by focusing on the transporter responsible for the uptake of estrone-3-sulfate.

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.

Characterization of the organic cation transporter SLC22A16: A doxorubicin importer

Specific efflux transporters, such as P-glycoprotein, have been shown to confer drug resistance by decreasing the intracellular accumulation of anticancer drugs. Understanding influx transporters, as well as efflux transporters, is essential to overcome this resistance. We report the expression profile and pharmacological characterization of an organic cation transporter, SLC22A16. The results of our experiments indicate that SLC22A16 is a mediator of doxorubicin uptake in cancer cells. Quantitative real-time RT-PCR analyses show that SLC22A16 is expressed in primary samples taken from patients with acute leukemia. Xenopus oocytes injected with SLC22A16 cRNA import doxorubicin, a widely used anticancer drug for hematological malignancies, in a saturable and dose-dependent manner. The apparent Km value for doxorubicin import was 5.2+/-0.4 microM. In cytotoxic assays, stable transfectants of leukemic Jurkat cells overexpressing SLC22A16 cells became significantly more sensitive to doxorubicin (2 microM) treatment. Characterization of SLC22A16 will help in designing novel therapies targeting hematological malignancies.

The relationship between folate transport activity at low pH and reduced folate carrier function in human Huh7 hepatoma cells

Transport of folates and antifolates in both hepatocytes and Huh7 human hepatoma cells is characterized by a low-pH optimum. Studies were undertaken to determine the extent to which this transport activity is mediated by the reduced folate carrier (RFC) in Huh7 human hepatoma cells. RFC expression was ablated by chemical mutagenesis and antifolate selective pressure with PT632 resulting in the PT632(R) subline in which RFC mRNA could not be detected. Methotrexate (MTX) influx in these cells at pH 7.4 was reduced by 70%, leaving substantial residual RFC-independent influx while influx of MTX and folic acid at pH 5.5 was not significantly decreased. The influx K(t) for folic acid and MTX at pH 5.5 in PT632(R) cells was 0.36 and 1.5 microM, respectively. The affinity of the low pH transporter in PT632(R) cells was highest for pemetrexed (K(i)=140 nM), very low for PT632 (K(i)=77 microM), and was stereospecific for the natural isomer (6S) of 5-formyltetrahydrofolate. In Huh7 cells transiently transfected with an RFC siRNA, RFC expression was reduced by 60% resulting in a 40% decrease in MTX influx at pH 7.4 but only a very small (5%) reduction in MTX or folic acid influx at pH 5.5. These data indicate that MTX transport in Huh7 cells at neutral pH is mediated largely by RFC while at pH 5.5 the predominant route of transport is independent of RFC.

Hormonal regulation of hepatic organic anion transporting polypeptides

Organic anion transporting polypeptides (Oatp) mediate the transport of a wide variety of amphipathic organic substrates. Rat Oatp1b2 and human OATP1B3 are members of a liver-specific subfamily of Oatps/OATPs. We investigated whether prolactin (PRL) and growth hormone (GH) regulated Oatp1b2 and OATP1B3 gene expression via signal transducers and activators of transcription 5 (Stat5). Binding sites for Stat5 transcription factors were located in the promoters of Oatp1b2 and OATP1B3 at -209 to -201 (5'-TTCTGGGAA-3') and -170 to -162 (5'-TTCTGAGAA-3'), respectively. In primary hepatocytes from female and male rats treated with PRL or GH, Oatp1b2 mRNA measured by real-time polymerase chain reaction was significantly induced 2-fold. HepG2 cells were transiently transfected with expression vectors containing Oatp1b2 or OATP1B3 promoter fragments, cDNAs for Stat5a, and the receptors for PRL (PRLR(L)) or GH (GHR), and treated with PRL or GH. PRL and GH induction of Oatp1b2 and OATP1B3 promoter activity required cotransfection of Stat5a and PRLR(L) or GHR. Mutation of the Stat5 binding site in both promoters eliminated hormonal induction. In DNA binding assays, HepG2 cells transfected with cDNAs for Stat5a and PRLR(L) were treated with PRL, and nuclear extracts were probed with a (32)P-labeled oligomer corresponding to -177 to -157 of the OATP1B3 promoter. PRL enhanced the binding of Stat5a to the OATP1B3 promoter and DNA-protein binding was inhibited in competition assays by excess OATP1B3 and Stat5 consensus oligomers but not by mutant Stat5 oligomers. These findings indicate that PRL and GH can regulate Oatp1b2 and OATP1B3 gene expression via the Stat5 signal-transduction pathway.

Evaluation of drug-drug interaction in the hepatobiliary and renal transport of drugs

Recent studies have revealed the import role played by transporters in the renal and hepatobiliary excretion of many drugs. These transporters exhibit a broad substrate specificity with a degree of overlap, suggesting the possibility of transporter-mediated drug-drug interactions with other substrates. This review is an overview of the roles of transporters and the possibility of transporter-mediated drug-drug interactions. Among the large number of transporters, we compare the Ki values of inhibitors for organic anion transporting polypeptides (OATPs) and organic anion transporters (OATs) and their therapeutic unbound concentrations. Among them, cephalosporins and probenecid have the potential to produce clinically relevant OAT-mediated drug-drug interactions, whereas cyclosporin A and rifampicin may trigger OATP-mediated ones. These drugs have been reported to cause drug-drug interactions in vivo with OATs or OATP substrates, suggesting the possibility of transporter-mediated drug-drug interactions. To avoid adverse consequences of such transporter-mediated drug-drug interactions, we need to be more aware of the role played by drug transporters as well as those caused by drug metabolizing enzymes.

Molecular mechanisms of reduced glutathione transport: role of the MRP/CFTR/ABCC and OATP/SLC21A families of membrane proteins

The initial step in reduced glutathione (GSH) turnover in all mammalian cells is its transport across the plasma membrane into the extracellular space; however, the mechanisms of GSH transport are not clearly defined. GSH export is required for the delivery of its constituent amino acids to other tissues, detoxification of drugs, metals, and other reactive compounds of both endogenous and exogenous origin, protection against oxidant stress, and secretion of hepatic bile. Recent studies indicate that some members of the multidrug resistance-associated protein (MRP/CFTR or ABCC) family of ATP-binding cassette (ABC) proteins, as well as some members of the organic anion transporting polypeptide (OATP or SLC21A) family of transporters contribute to this process. In particular, five of the 12 members of the MRP/CFTR family appear to mediate GSH export from cells namely, MRP1, MRP2, MRP4, MRP5, and CFTR. Additionally, two members of the OATP family, rat Oatp1 and Oatp2, have been identified as GSH transporters. For the Oatp1 transporter, efflux of GSH may provide the driving force for the uptake of extracellular substrates. In humans, OATP-B and OATP8 do not appear to transport GSH; however, other members of this family have yet to be characterized in regards to GSH transport. In yeast, the ABC proteins Ycf1p and Bpt1p transport GSH from the cytosol into the vacuole, whereas Hgt1p mediates GSH uptake across the plasma membrane. Because transport is a key step in GSH homeostasis and is intimately linked to its biological functions, GSH export proteins are likely to modulate essential cellular functions.

Organic anion transporting polypeptides expressed in liver and brain mediate uptake of microcystin

Microcystins are toxins produced by freshwater cyanobacteria. They are cyclic heptapeptides that exhibit hepato- and neurotoxicity. However, the transport systems that mediate uptake of microcystins into hepatocytes and across the blood-brain barrier have not yet been identified. Using the Xenopus laevis oocyte expression system we tested whether members of the organic anion transporting polypeptide superfamily (rodent: Oatps; human: OATPs) are involved in transport of the most common microcystin variant microcystin-LR by measuring uptake of a radiolabeled derivative dihydromicrocystin-LR. Among the tested Oatps/OATPs, rat Oatp1b2, human OATP1B1, human OATP1B3, and human OATP1A2 transported microcystin-LR 2- to 5-fold above water-injected control oocytes. This microcystin-LR transport was inhibited by co-incubation with the known Oatp/OATP substrates taurocholate (TC) and bromosulfophthalein (BSP). Microcystin-LR transport mediated by the human OATPs was further characterized and showed saturability with increasing microcystin-LR concentrations. The apparent K(m) values amounted to 7 +/- 3 microM for OATP1B1, 9 +/- 3 microM for OATP1B3, and 20 +/- 8 microM for OATP1A2. No microcystin-LR transport was observed in oocytes expressing Oatp1a1, Oatp1a4, and OATP2B1. These results may explain some of the observed organ-specific toxicity of microcystin-LR. Oatp1b2, OATP1B1, and OATP1B3 are responsible for microcystin transport into hepatocytes, whereas OATP1A2 mediates microcystin-LR transport across the blood-brain barrier.

Functional characteristics and pharmacokinetic significance of kidney-specific organic anion transporters, OAT-K1 and OAT-K2, in the urinary excretion of anionic drugs

During the last decade, cDNA cloning has identified various gene families of drug transporters, and pharmacokinetic studies of drugs based on the molecular characteristics of transporters have advanced. We cloned and characterized two organic anion transporters OAT-K1 and OAT-K2 from the rat kidney. The expression of both transporters was limited to the kidney, especially the brush-border membranes of proximal tubules, with an apparent molecular mass of 40 kDa. Using MDCK or LLC-PK1 cells stably expressing OAT-K1, posttranslational cleavage was suggested to affect the membrane localization and functional characteristics; 50 kDa with multispecificity in the apical membrane of MDCK cells and 70 kDa with methotrexate specific transport in the basolateral membrane of LLC-PK1 cells. A wide variety of anionic compounds including methotrexate are bidirectionally transported via OAT-K1 and OAT-K2 across the apical membrane in the MDCK-transfectants. The urinary secretion of methotrexate was depressed in 5/6 nephrectomized rats in association with the selective loss of OAT-K1 and OAT-K2 expression, and both transporters were suggested to be target molecules for methotrexate-folinic acid rescue. In this review, recent advances in the study of OAT-K1 and OAT-K2 were summarized in comparison with other transporters.

Role of organic anion transporter OATP1B1 (OATP-C) in hepatic uptake of irinotecan and its active metabolite, 7-ethyl-10-hydroxycamptothecin: in vitro evidence and effect of single nucleotide polymorphisms

Irinotecan hydrochloride (CPT-11) is a potent anticancer drug that is converted to its active metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38), and other metabolites in liver. The disposition and gastrointestinal toxicity of irinotecan exhibit a wide interpatient variability. Here, we examined the contribution of an organic anion-transporting polypeptide, OATP1B1 (OATP-C), which transports a variety of drugs and their metabolites from blood to liver in humans, to the hepatic disposition of irinotecan, SN-38, and its glucuronide conjugate (SN-38G) by using HEK293 cells stably transfected with SLCO1B1*1a (OATP-C*1a) coding wild-type OATP1B1. We further examined the effect of single nucleotide polymorphisms in OATP1B1 by measuring uptake activity in Xenopus oocytes expressing OATP1B1*1a and three common variants. In all cases, transport activity for SN-38 was observed, whereas irinotecan and SN-38G were not transported. Moreover, SN-38 exhibited a significant inhibitory effect on OATP1B1-mediated uptake of [(3)H]estrone-3-sulfate. Among the variants examined, OATP1B1*15 (N130D and V174A; reported allele frequency 10-15%) exhibited decreased transport activities for SN-38 as well as pravastatin, estrone-3-sulfate, and estradiol-17beta-glucuronide. This study is the first to yield evidence that OATP1B1 is involved in the hepatic disposition of SN-38 and that genetic polymorphisms of OATP1B1 may contribute to the known interpatient variability in disposition of irinotecan.

The organic anion transporter (OATP) family

In the last decade, many organic anion transporters have been isolated, characterized their distribution and substrates. The recently identified organic anion transporter family OATP (organic anion transporting polypeptide)/LST (liver-specific transporter) family, transport bile acids, hormones as well as eicosanoids, various compounds (BSP, HMG-CoA reductase inhibitor, angiotensin converting enzyme inhibitor, etc.). The isolation of the family revealed that not only hydrophilic compounds, drugs and hormones of lipophilic nature need a membrane transport system to penetrate cell membrane. In this family, the nomenclature becomes very complicated and the physiological role of this family is still unclear except about few organs such as the brain, liver and kidney. Even in such organs, the co-existence of the OATP/LST family and similar substrate specificity hamper the progress and clear characterization identifying the real role of the transporter family. Here, recent progress and an insight of this field are reviewed.

Intestinal solute carriers: an overview of trends and strategies for improving oral drug absorption

A large amount of absorptive intestinal membrane transporters play an important part in absorption and distribution of several nutrients, drugs and prodrugs. The present paper gives a general overview on intestinal solute carriers as well as on trends and strategies for targeting drugs and/or prodrugs to these carriers in order to increasing oral bioavailability and distribution. A number of absorptive intestinal transporters are described in terms of gene and protein classification, driving forces, substrate specificities and cellular localization. When targeting absorptive large capacity membrane transporters in the small intestine in order to increase oral bioavailabilities of drug or prodrug, the major influence on in vivo pharmacokinetics is suggested to be dose-dependent increase in bioavailability as well as prolonged blood circulation due to large capacity facilitated absorption, and renal re-absorption, respectively. In contrast, when targeting low-capacity transporters such as vitamin transporters, dose independent saturable absorption kinetics are suggested. We thus believe that targeting drug substrates for absorptive intestinal membrane transporters could be a feasible strategy for optimizing drug bioavailability and distribution.

Identification of phalloidin uptake systems of rat and human liver

To determine whether the liver toxin phalloidin is transported into hepatocytes by one of the known bile salt transporters, we expressed the sodium-dependent Na+/taurocholate cotransporting polypeptide (Ntcp) and several sodium-independent bile salt transporters of the organic anion transporting polypeptide (OATP/SLCO) superfamily in Xenopus laevis oocytes and measured uptake of the radiolabeled phalloidin derivative [3H]demethylphalloin. We found that rat Oatp1b2 (previously called Oatp4 (Slc21a10)) as well as human OATP1B1 (previously called OATP-C (SLC21A6)) and OATP1B3 (previously called OATP8 (SLC21A8)) mediate uptake of [3H]demethylphalloin when expressed in X. laevis oocytes. Transport of increasing [3H]demethylphalloin concentrations was saturable with apparent Km values of 5.7 microM (Oatp1b2), 17 microM (OATP1B1) and 7.5 microM (OATP1B3). All other tested Oatps/OATPs as well as the rat liver Ntcp did not transport [3H]demethylphalloin. Therefore, we conclude that rat Oatp1b2 as well as human OATP1B1 and OATP1B3 are responsible for phalloidin uptake into rat and human hepatocytes.

Selective Preservation of Pemetrexed Pharmacological Activity in HeLa Cells Lacking the Reduced Folate Carrier

A methotrexate (MTX)-resistant HeLa subline (R5), developed in this laboratory, with impaired transport due to a genomic deletion of the reduced folate carrier (RFC) was only 2-fold resistant to pemetrexed (PMX), but 200- and 400-fold resistant to raltitrexed (ZD1694) and N(alpha)-(-4-amino-4-deoxypteroyl)-N(delta)-hemiphthaloyl-1-ornithine (PT523), respectively, compared with parental HeLa cells when grown with 2 microM folic acid. When folic acid was replaced with the more physiological 25 nM 5-formyltetrahydrofolate, R5 cells were 2-fold collaterally sensitive to PMX but still 40- and 200-fold resistant to ZD1694 and PT523, respectively. Sensitivity to PT523 and PMX could be completely restored, and sensitivity to ZD1694 nearly restored, by transfection of RFC cDNA into R5 cells, indicating that the defect in drug transport was the only, or major, factor in resistance. The preserved PMX activity in R5 cells could not be related to the very low expression of folate receptors. Rather, retained PMX activity in R5 cells was associated with residual transport by another process that exhibits good affinity for PMX (Kt = 12 microM) with much lower affinities for ZD1694, MTX, and PT523 (Kis of approximately 90, 100, and 250 microM, respectively). PMX transported by this route was rapidly converted to higher polyglutamates and, when grown with 25 nM 5-formyl-tetrahydrofolate, the rate of formation of these derivatives and their net accumulation in R5 cells was comparable to that of wild-type cells. These data suggest that selective preservation of PMX pharmacological activity in RFC-null R5 cells is due, in part, to partial preservation of transport by secondary process with a higher affinity for PMX than the other antifolates evaluated.

Isolation and characterization of a digoxin transporter and its rat homologue expressed in the kidney

Digoxin, which is one of the most commonly prescribed drugs for the treatment of heart failure, is mainly eliminated from the circulation by the kidney. P-glycoprotein is well characterized as a digoxin pump at the apical membrane of the nephron. However, little is known about the transport mechanism at the basolateral membrane. We have isolated an organic anion transporter (OATP4C1) from human kidney. Human OATP4C1 is the first member of the organic anion transporting polypeptide (OATP) family expressed in human kidney. The isolated cDNA encodes a polypeptide of 724 aa with 12 transmembrane domains. The genomic organization consists of 13 exons located on chromosome 5q21. Its rat counterpart, Oatp4c1, is also isolated from rat kidney. Human OATP4C1 transports cardiac glycosides (digoxin, K(m) = 7.8 microM and ouabain, K(m) = 0.38 microM), thyroid hormone (triiodothyronine, K(m) = 5.9 microM and thyroxine), cAMP, and methotrexate in a sodium-independent manner. Rat Oatp4c1 also transports digoxin (K(m) = 8.0 microM) and triiodothyronine (K(m) = 1.9 microM). Immunohistochemical analysis reveals that rat Oatp4c1 protein is localized at the basolateral membrane of the proximal tubule cell in the kidney. These data suggest that human OATP4C1/rat Oatp4c1 might be a first step of the transport pathway of digoxin and various compounds into urine in the kidney.

The human organic anion transporting polypeptide 8 (SLCO1B3) gene is transcriptionally repressed by hepatocyte nuclear factor 3beta in hepatocellular carcinoma

BACKGROUND/AIMS

The organic anion transporting polypeptides (OATPs) mediate the uptake of numerous amphipathic compounds into hepatocytes. Our aim was to study the expression and regulation of OATP8 (OATP1B3, SLC21A8/SLCO1B3) and OATP-C (OATP1B1, SLC21A6/SLCO1B1) in hepatocellular carcinomas (HCC).

METHODS

RNA and protein levels in 13 paired HCC and adjacent non-tumor liver samples were quantified by real-time polymerase chain reaction or Western blot, respectively. The OATP8 and OATP-C gene promoters were characterized by luciferase reporter assays and electrophoretic mobility shift assays (EMSA).

RESULTS

The expression of OATP8 was decreased in 60% of HCC compared to surrounding non-tumor liver tissue, on both the mRNA and protein levels. Expression of the liver-enriched transcription factor hepatocyte nuclear factor 3beta (HNF3beta) was increased in 70% of HCC and correlated inversely with OATP8 mRNA (r=-0.75, P<0.05) and protein. In contrast to OATP8, expression of OATP-C was not significantly decreased in HCC. In transfected Huh7 cells, OATP8 promoter activity was inhibited by 70% when HNF3beta was cotransfected. An HNF3beta binding site was located at nt -39/-23 by EMSA. The OATP-C promoter was not inhibited by HNF3beta.

CONCLUSIONS

HNF3beta represses transcription of the OATP8 but not the OATP-C gene, providing a mechanism for reduced expression of OATP8 in HCC.

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.

Resistance to antifolates

The antifolates were the first class of antimetabolites to enter the clinics more than 50 years ago. Over the following decades, a full understanding of their mechanisms of action and chemotherapeutic potential evolved along with the mechanisms by which cells develop resistance to these drugs. These principals served as a basis for the subsequent exploration and understanding of the mechanisms of resistance to a variety of diverse antineoplastics with different cellular targets. This section describes the bases for intrinsic and acquired antifolate resistance within the context of the current understanding of the mechanisms of actions and cytotoxic determinants of these agents. This encompasses impaired drug transport into cells, augmented drug export, impaired activation of antifolates through polyglutamylation, augmented hydrolysis of antifolate polyglutamates, increased expression and mutation of target enzymes, and the augmentation of cellular tetrahydrofolate-cofactor pools in cells. This chapter also describes how these insights are being utilized to develop gene therapy approaches to protect normal bone marrow progenitor cells as a strategy to improve the efficacy of bone marrow transplantation. Finally, clinical studies are reviewed that correlate the cellular pharmacology of methotrexate with the clinical outcome in children with neoplastic diseases treated with this antifolate.

Organic anion-transporting polypeptide (OATP) transporter family and drug disposition

Drug transporters are increasingly recognized as a key determinant of drug disposition. Recent studies have revealed that targeted expression of drug uptake and efflux transporters to specific cell membrane domains allows for the efficient directional movement of many drugs in clinical use. While the role of certain efflux transporters such as MDR1 (P-glycoprotein) in drug disposition has been extensively studied, emerging evidence suggests that uptake transporters may also be important to the intestinal absorption and renal or hepatic elimination of drugs. Members of the organic anion-transporting polypeptide (OATP) family of drug uptake transporters have been found capable of transporting a large array of structurally divergent drugs. Moreover, expression of OATP isoforms in the gastrointestinal tract, liver and kidney, as well as at the level of the blood-brain barrier, has important implications for our understanding of the factors governing drug absorption, elimination and tissue penetration.

Organic anion transporting polypeptides of the OATP/ SLC21 family: phylogenetic classification as OATP/ SLCO superfamily, new nomenclature and molecular/functional properties

The organic anion transporting polypeptides (rodents: Oatps, human: OATPs) form a superfamily of sodium-independent transport systems that mediate the transmembrane transport of a wide range of amphipathic endogenous and exogenous organic compounds. Since the traditional SLC21 gene classification does not permit an unequivocal and species-independent identification of genes and gene products, all Oatps/OATPs are newly classified within the OATP/ SLCO superfamily and subdivided into families (>/=40% amino acid sequence identity), subfamilies (>/=60% amino acid sequence identity) and individual genes and gene products according to their phylogenetic relationships and chronology of identification. Implementation of this new classification and nomenclature system occurs in agreement with the HUGO Gene Nomenclature Committee (HGNC). Among 52 members of the OATP/ SLCO superfamily, 36 members have been identified so far in humans, rat and mouse. The latter are clustered within 6 (out of 12) families (OATP1-OATP6) and 13 subfamilies. Oatps/OATPs represent 12 transmembrane domain proteins and contain the superfamily signature D-X-RW-(I,V)-GAWW-X-G-(F,L)-L. Although species divergence, multispecificity and wide tissue distribution are common characteristics of many Oatps/OATPs, some members of the OATP/ SLCO superfamily are highly conserved during evolution, have a high substrate specificity and exhibit unique cellular expression in distinct organs. Hence, while Oatps/OATPs with broad substrate specificity appear to play an important role in the bioavailability, distribution and excretion of numerous exogenous amphipathic organic anionic compounds, Oatps/OATPs with a narrow spectrum of transport substrates may exhibit more specific physiological functions in distinct organs.