Relevance of conserved lysine and arginine residues in transmembrane helices for the transport activity of organic anion transporting polypeptide 1B3

Targeting Glial Cells by Organic Anion-Transporting Polypeptide 1C1 (OATP1C1)-Utilizing l-Thyroxine-Derived Prodrugs

OATP1C1 (organic anion-transporting polypeptide 1C1) transports thyroid hormones, particularly thyroxine (T4), into human astrocytes. In this study, we investigated the potential of utilizing OATP1C1 to improve the delivery of anti-inflammatory drugs into glial cells. We designed and synthesized eight novel prodrugs by incorporating T4 and 3,5-diiodo-l-tyrosine (DIT) as promoieties to selected anti-inflammatory drugs. The prodrug uptake in OATP1C1-expressing human U-87MG glioma cells demonstrated higher accumulation with T4 promoiety compared to those with DIT promoiety or the parent drugs themselves. In silico models of OATP1C1 suggested dynamic binding for the prodrugs, wherein the pose changed from vertical to horizontal. The predicted binding energies correlated with the transport profiles, with T4 derivatives exhibiting higher binding energies when compared to prodrugs with a DIT promoiety. Interestingly, the prodrugs also showed utilization of oatp1a4/1a5/1a6 in mouse primary astrocytes, which was further supported by docking studies and a great potential for improved brain drug delivery.

Identifying Novel Inhibitors for Hepatic Organic Anion Transporting Polypeptides by Machine Learning-Based Virtual Screening

Integration of statistical learning methods with structure-based modeling approaches is a contemporary strategy to identify novel lead compounds in drug discovery. Hepatic organic anion transporting polypeptides (OATP1B1, OATP1B3, and OATP2B1) are classical off-targets, and it is well recognized that their ability to interfere with a wide range of chemically unrelated drugs, environmental chemicals, or food additives can lead to unwanted adverse effects like liver toxicity and drug-drug or drug-food interactions. Therefore, the identification of novel (tool) compounds for hepatic OATPs by virtual screening approaches and subsequent experimental validation is a major asset for elucidating structure-function relationships of (related) transporters: they enhance our understanding about molecular determinants and structural aspects of hepatic OATPs driving ligand binding and selectivity. In the present study, we performed a consensus virtual screening approach by using different types of machine learning models (proteochemometric models, conformal prediction models, and XGBoost models for hepatic OATPs), followed by molecular docking of preselected hits using previously established structural models for hepatic OATPs. Screening the diverse REAL drug-like set (Enamine) shows a comparable hit rate for OATP1B1 (36% actives) and OATP1B3 (32% actives), while the hit rate for OATP2B1 was even higher (66% actives). Percentage inhibition values for 44 selected compounds were determined using dedicated in vitro assays and guided the prioritization of several highly potent novel hepatic OATP inhibitors: six (strong) OATP2B1 inhibitors (IC₅₀ values ranging from 0.04 to 6 μM), three OATP1B1 inhibitors (2.69 to 10 μM), and five OATP1B3 inhibitors (1.53 to 10 μM) were identified. Strikingly, two novel OATP2B1 inhibitors were uncovered (C7 and H5) which show high affinity (IC₅₀ values: 40 nM and 390 nM) comparable to the recently described estrone-based inhibitor (IC₅₀ = 41 nM). A molecularly detailed explanation for the observed differences in ligand binding to the three transporters is given by means of structural comparison of the detected binding sites and docking poses.

Comparative Modelling of Organic Anion Transporting Polypeptides: Structural Insights and Comparison of Binding Modes

To better understand the functionality of organic anion transporting polypeptides (OATPs) and to design new ligands, reliable structural data of each OATP is needed. In this work, we used a combination of homology model with molecular dynamics simulations to generate a comprehensive structural dataset, that encompasses a diverse set of OATPs but also their relevant conformations. Our OATP models share a conserved transmembrane helix folding harbouring a druggable binding pocket in the shape of an inner pore. Our simulations suggest that the conserved salt bridges at the extracellular region between residues on TM1 and TM7 might influence the entrance of substrates. Interactions between residues on TM1 and TM4 within OATP1 family shown their importance in transport of substrates. Additionally, in transmembrane (TM) 1/2, a known conserved element, interact with two identified motifs in the TM7 and TM11. Our simulations suggest that TM1/2-TM7 interaction influence the inner pocket accessibility, while TM1/2-TM11 salt bridges control the substrate binding stability.

Neurosteroids: Structure-Uptake Relationships and Computational Modeling of Organic Anion Transporting Polypeptides (OATP)1A2

In this study, we investigated the delivery of synthetic neurosteroids into MCF-7 human breast adenocarcinoma cells via Organic Anionic Transporting Polypeptides (OATPs) (pH 7.4 and 5.5) to identify the structural components required for OATP-mediated cellular uptake and to get insight into brain drug delivery. Then, we identified structure-uptake relationships using in-house developed OATP1A2 homology model to predict binding sites and modes for the ligands. These binding modes were studied by molecular dynamics simulations to rationalize the experimental results. Our results show that carboxylic acid needs to be at least at 3 carbon-carbon bonds distance from amide bond at the C-3 position of the androstane skeleton and have an amino group to avoid efflux transport. Replacement of hydroxyl group at C-3 with any of the 3, 4, and 5-carbon chained terminal carboxylic groups improved the affinity. We attribute this to polar interactions between carboxylic acid and side-chains of Lys33 and Arg556. The additional amine group showed interactions with Glu172 and Glu200. Based on transporter capacities and efficacies, it could be speculated that the functionalization of acetyl group at the C-17 position of the steroidal skeleton might be explored further to enable OAT1A2-mediated delivery of neurosteroids into the cells and also across the blood-brain barrier.

Data-Driven Ensemble Docking to Map Molecular Interactions of Steroid Analogs with Hepatic Organic Anion Transporting Polypeptides

Hepatic organic anion transporting polypeptides—OATP1B1, OATP1B3, and OATP2B1—are expressed at the basolateral membrane of hepatocytes, being responsible for the uptake of a wide range of natural substrates and structurally unrelated pharmaceuticals. Impaired function of hepatic OATPs has been linked to clinically relevant drug–drug interactions leading to altered pharmacokinetics of administered drugs. Therefore, understanding the commonalities and differences across the three transporters represents useful knowledge to guide the drug discovery process at an early stage. Unfortunately, such efforts remain challenging because of the lack of experimentally resolved protein structures for any member of the OATP family. In this study, we established a rigorous computational protocol to generate and validate structural models for hepatic OATPs. The multistep procedure is based on the systematic exploration of available protein structures with shared protein folding using normal-mode analysis, the calculation of multiple template backbones from elastic network models, the utilization of multiple template conformations to generate OATP structural models with various degrees of conformational flexibility, and the prioritization of models on the basis of enrichment docking. We employed the resulting OATP models of OATP1B1, OATP1B3, and OATP2B1 to elucidate binding modes of steroid analogs in the three transporters. Steroid conjugates have been recognized as endogenous substrates of these transporters. Thus, investigating this data set delivers insights into mechanisms of substrate recognition. In silico predictions were complemented with in vitro studies measuring the bioactivity of a compound set on OATP expressing cell lines. Important structural determinants conferring shared and distinct binding patterns of steroid analogs in the three transporters have been identified. Overall, this comparative study provides novel insights into hepatic OATP-ligand interactions and selectivity. Furthermore, the integrative computational workflow for structure-based modeling can be leveraged for other pharmaceutical targets of interest.

Effect of rare coding variants of charged amino acid residues on the function of human organic anion transporting polypeptide 1B3 (SLCO1B3)

Human organic anion transporting polypeptide 1B3 (OATP1B3, gene symbol SLCO1B3) is a liver-specific uptake transporter. Its function was reported to be largely affected by some positively charged amino acid residues. However, so far the effect of naturally occurring genetic variants of charged residues on OATP1B3's function has not been explored yet. Therefore, in the present study nonsynonymous single nucleotide variants that led to the replacement of charged residues of OATP1B3 were investigated. Our results demonstrated that rare coding variants c.542G > A (p.R181H) and c.592G > A (p.D198N) had a great effect on the function of OATP1B3 mainly due to their influence on protein's surface expression. Further mutation studies showed that a negatively charged residue at position 198 was indispensable to the proper expression of OATP1B3 on the plasma membrane, while a positively charged reside at position 181 was not a must. Structural modeling indicated that R181 is located at the center of putative transmembrane domain 4 (TM4) and its side chain faces towards TM2 instead of towards the substrate translocation pathway, whereas D198 is located at the border of TM4 and intracellular loop 2 and may electrostatically repulse negatively charged phospholipid head groups. In conclusion, our results indicated that rare coding variants that cause changes of charged amino acid residues might have large influence on the function and expression of OATP1B3.

Synergistic transport of a fluorescent coumarin probe marks coumarins as pharmacological modulators of Organic anion-transporting polypeptide, OATP3A1

Organic anion-transporting polypeptide 3A1 (OATP3A1) is a membrane transporter mediating the cellular uptake of various hormones such as estrone-3-sulfate, prostaglandins E1 and E2 and thyroxine. OATP3A1 is widely expressed in the human body and its presence in tissue-blood barriers, neurons and muscle cells marks it as a potential pharmacological target. Herein we demonstrate that an otherwise membrane impermeant, zwitterionic fluorescent coumarin probe, bearing a sulfonate function is a potent substrate of human OATP3A1, thus readily transported into HEK-293-OATP3A1 cells allowing functional investigation and the screen of drug interactions of the OATP3A1 transporter. At the same time, dyes lacking either the sulfonate motif or the coumarin scaffold showed a dramatic decrease in affinity or even a complete loss of transport. Furthermore, we observed a distinct inhibition/activation pattern in the OATP3A1-mediated uptake of closely related fluorescent coumarin derivatives differing only in the presence of the sulfonate moiety. Additionally, we detected a synergistic effect between one of the probes tested and the endogenous OATP substrate estrone-3-sulfate. These data, together with docking results indicate the presence of at least two cooperative substrate binding sites in OATP3A1. Besides providing the first sensitive probe for testing OATP3A1 substrate/inhibitor interactions, our results also help to understand substrate recognition and transport mechanism of the poorly characterized OATP3A1. Moreover, coumarins are good candidates for OATP3A1-targeted drug delivery and as pharmacological modulators of OATP3A1.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

The Emerging Role of the SLCO1B3 Protein in Cancer Resistance

Currently, chemotherapy is one of the mainstays of oncologic therapies. But the efficacy of chemotherapy is often limited by drug resistance and severe side effects. Consequently, it is becoming increasingly important to investigate the underlying mechanism and overcome the problem of anticancer chemotherapy resistance. The solute carrier organic anion transporter family member 1B3 (SLCO1B3), a functional transporter normally expressed in the liver, transports a variety of endogenous and exogenous compounds, including hormones and their conjugates as well as some anticancer drugs. The extrahepatic expression of SLCO1B3 has been detected in different cancer cell lines and cancer tissues. Recently, accumulating data indicates that the abnormal expression and function of SLCO1B3 are involved in resistance to anticancer drugs, such as taxanes, camptothecin and its analogs, SN-38, and Androgen Deprivation Therapy (ADT) in breast, prostate, lung, hepatic, and colorectal cancer, respectively. Thus, more investigations have been implemented to identify the potential SLCO1B3-related mechanisms of cancer drug resistance. In this review, we focus on the emerging roles of SLCO1B3 protein in the development of cancer chemotherapy resistance and briefly discuss the mechanisms of resistance. Elucidating the function of SLCO1B3 in chemoresistance may bring out novel therapeutic strategies for cancer treatment.

Interindividual Diversity in Expression of Organic Anion Uptake Transporters in Normal and Cirrhotic Human Liver

The liver plays an essential role in removing endogenous and exogenous compounds from the circulation. This function is mediated by specific transporters, including members of the family of organic anion transport proteins (OATPs) and the Na⁺-taurocholate transporting polypeptide (NTCP). In the present study, transporter protein expression was determined in liver samples from patients with cirrhosis or controls without liver disease. Five transporters (OATP1A2, OATP1B1, OATP1B3, OATP2B1, and NTCP) were studied. Transporter content in homogenates of human liver was quantified on western blots probed with transporter-specific antibodies in which a calibrated green fluorescent protein-tagged transporter standard was included. Liver samples from 21 patients with cirrhosis (hepatitis C in 17 and alcohol abuse in 4) and 17 controls without liver disease were analyzed. Expression of each of the transporters had a large spread, varying by an order of magnitude in cirrhotic and control livers. OATP1B1 was the most abundant transporter in controls (P < 0.01) but was significantly lower in cirrhotic livers as was NTCP expression (P < 0.01). There was little difference in transporter expression with respect to age or sex. Despite the large variability in transporter expression within a group, analysis in individuals showed that those with high or low expression of one transporter had a similar magnitude in expression of the others. Conclusion: Differences in transporter expression could explain unanticipated heterogeneity of drug transport and metabolism in individuals with and without liver disease.

Amino-terminal region of human organic anion transporting polypeptide 1B1 dictates transporter stability and substrate interaction

Organic anion transporting polypeptides (OATPs) are key players of drug absorption, distribution and excretion due to their broad substrate specificity, wide tissue distribution and the involvement in drug-drug interaction. OATP1B1 is specifically localized at the basolateral membrane of human hepatocytes and serves a crucial role in the drug clearance from the body. Previous studies have shown that transmembrane domains (TMs) are essential for proper functions of OATPs. In the present study, site-directed mutagenesis was performed to study the TM1 and amino-terminus of OATP1B1. Two positively charged residues, K41 and K49, as well as a hydrophobic residue I46, in TM1 were identified to be important for the proper function of the transporter. K41A and K49A exhibited altered Km value at the high and low affinity binding sites of estrone-3- sulfate (ES), respectively; while alanine substitution of I46 showed altered Km and Vmax values for both binding components of ES. Additional replacement of K41 revealed that the positively charged property at this position is important for maintaining OATP1B1 protein level and function; while the specific side-group structure of lysine at position 49 is irreplaceable for the transporter activity. Conservative replacement of I46 with leucine also recovered the function of the transporter. In addition, studies of the amino-terminus of OATP1B1 revealed that residues ranging from 19 to 27 are essential for protein stability and substrate interaction. Therefore, the amino-terminal region, which includes TM1 and the amino-terminus of OATP1B1, is important for proper function of the membrane protein.

The intracellular NPxY motif is critical in maintaining the function and expression of human organic anion transporting polypeptide 1B1

Organic anion transporting polypeptides (OATPs, gene symbol SLCO) mediate sodium-independent transport of endogenous compounds such as bile salts, hormones and their conjugates as well as toxins and drugs. OATP1B1 is the major OATP specifically expressed at the basolateral membrane of human hepatocytes and many clinically important drugs have been shown to be substrates of the transporter. According to the computer-based hydropathy analysis, a large intracellular loop 3 (IL3) is situated between transmembrane domain 6 and 7 of OATPs, in which a conserved NPxY motif is found. In the current study, HEK293 cells expressing the HA-tagged OATP1B1 was utilized to investigate the role of the NPxY motif for the function and expression of the transporter. Alanine replacement of N335 or P336 retained substantial uptake function; while simultaneous mutation of these residues resulted in a double mutant that lost almost all the transport activity. On the other hand, Y338A showed >80% reduction for estrone-3-sulfate uptake. Plasma membrane protein analysis revealed that N335/P336A completely lost its cell surface protein expression; while that of Y338A is dramatically reduced. Further investigation with pharmacological inhibitors and immunocytochemistry demonstrated that N335/336A is detained in the Golgi apparatus and Y338A exhibited accelerated protein degradation rate compared to that of the wild-type. Conservative replacement of Y338 with phenylalanine fully recovered uptake and expression of the transporter. In summary, a new role was observed for the NPxY motif located in the IL3 of OATP1B1, which may affect processing and stability of the transporter.

Current Advances in Studying Clinically Relevant Transporters of the Solute Carrier (SLC) Family by Connecting Computational Modeling and Data Science

Organic anion and cation transporting proteins (OATs, OATPs, and OCTs), as well as the Multidrug and Toxin Extrusion (MATE) transporters of the Solute Carrier (SLC) family are playing a pivotal role in the discovery and development of new drugs due to their involvement in drug disposition, drug-drug interactions, adverse drug effects and related toxicity. Computational methods to understand and predict clinically relevant transporter interactions can provide useful guidance at early stages in drug discovery and design, especially if they include contemporary data science approaches. In this review, we summarize the current state-of-the-art of computational approaches for exploring ligand interactions and selectivity for these drug (uptake) transporters. The computational methods discussed here by highlighting interesting examples from the current literature are ranging from semiautomatic data mining and integration, to ligand-based methods (such as quantitative structure-activity relationships, and combinatorial pharmacophore modeling), and finally structure-based methods (such as comparative modeling, molecular docking, and molecular dynamics simulations). We are focusing on promising computational techniques such as fold-recognition methods, proteochemometric modeling or techniques for enhanced sampling of protein conformations used in the context of these ADMET-relevant SLC transporters with a special focus on methods useful for studying ligand selectivity.

Analysis of naturally occurring mutations in the human uptake transporter NaCT important for bone and brain development and energy metabolism

The human uptake transporter NaCT is important for human brain development, brain function and energy metabolism and mediates the uptake of citrate and other intermediates of the tricarboxylic acid cycle from blood into neurons and hepatocytes. Mutations in the SLC13A5 gene encoding NaCT are associated with epileptic encephalopathy. To gain more insights into the transport mechanisms we analyzed the functional consequences of mutations in the SLC13A5 gene on NaCT-mediated transport function. Using HEK293 cells expressing wild-type and eight mutated NaCT proteins, we investigated the mRNA and protein amount as well as the protein localization of all NaCT variants. Furthermore, the impact on NaCT-mediated citrate uptake was measured. In addition, a structural model of the transport pore was generated to rationalize the consequences of the mutations on a structural basis. We demonstrated that all proteins were synthesized with an identical molecular weight as the wild-type transporter but several mutations (NaCTp.G219R, −p.G219E, −p.T227M, −p.L420P and −p.L488P) lead to a complete loss of NaCT-mediated citrate transport. This loss of transport activity can be explained on the basis of the developed structural model. This model may help in the further elucidation of the transport mechanism of this important uptake transporter.

Transmembrane Domain 1 of Human Organic Anion Transporting Polypeptide 2B1 Is Essential for Transporter Function and Stability

Organic anion transporting polypeptides (OATPs, gene symbol SLCO) are important membrane transporter proteins that mediate the uptake of wide ranges of endogenous and exogenous compounds. OATP2B1 has been found in multiple organs and tissues, including the liver, small intestine, kidney, brain, placenta, heart, skin, as well as skeletal muscle, and is proposed to be involved in the uptake of orally administered drugs. Quite a few reports have demonstrated that transmembrane domains (TMs) are crucial for proper functions of OATP family members. Comparative modeling proposed that TM1, along with TM2, 4, and 5 of the N-terminal half of OATP2B1, may be localized within the substrate interaction pocket and are important for uptake function of the transporter. Alanine scanning of the putative transmembrane domain 1 of OATP2B1 revealed that substitution of L58 with alanine dramatically altered the K_m value, and mutation of V52, H55, Q59, and L69 resulted in significantly reduced substrate turnover number, whereas A61V, Q62A, and S66A exhibited significant change in both K_m and V_max values. In addition, phenylalanine at position 51 seems to play an important role in maintaining proper folding of OATP2B1 because alanine replacement of F51 caused accelerated degradation of the transporter protein. Although proteasome and lysosome inhibitors could partially recover protein level, the mutant transporter remained nonfunctional. Taken together, the identification of nine essential amino acid residues within TM1 of OATP2B1 suggested that the transmembrane domain is important for maintaining proper function of the transporter.

Compound heterozygous SLC19A3 mutations further refine the critical promoter region for biotin-thiamine-responsive basal ganglia disease

Mutations in the gene SLC19A3 result in thiamine metabolism dysfunction syndrome 2, also known as biotin-thiamine-responsive basal ganglia disease (BTBGD). This neurometabolic disease typically presents in early childhood with progressive neurodegeneration, including confusion, seizures, and dysphagia, advancing to coma and death. Treatment is possible via supplement of biotin and/or thiamine, with early treatment resulting in significant lifelong improvements. Here we report two siblings who received a refined diagnosis of BTBGD following whole-genome sequencing. Both children inherited compound heterozygous mutations from unaffected parents; a missense single-nucleotide variant (p.G23V) in the first transmembrane domain of the protein, and a 4808-bp deletion in exon 1 encompassing the 5′ UTR and minimal promoter region. This deletion is the smallest promoter deletion reported to date, further defining the minimal promoter region of SLC19A3. Unfortunately, one of the siblings died prior to diagnosis, but the other is showing significant improvement after commencement of therapy. This case demonstrates the power of whole-genome sequencing for the identification of structural variants and subsequent diagnosis of rare neurodevelopmental disorders.

Biochemical studies on the structure-function relationship of major drug transporters in the ATP-binding cassette family and solute carrier family

Human drug transporters often play key roles in determining drug accumulation within cells. Their activities are often directly related to therapeutic efficacy, drug toxicity as well as drug-drug interactions. However, the progress for interpretation of their crystal structures is relatively slow. Hence, conventional biochemical studies together with computer modeling became useful manners to reveal essential structures of these membrane proteins. Over the years, quite a few structure-function relationship information had been obtained for members of the two major transporter families: the ATP-binding cassette family and the solute carrier family. Critical structural features of drug transporters include transmembrane domains, post-translational modification sites and domains for cell surface assembly and protein-protein interactions. Alterations at these important sites may affect protein stability, trafficking to the plasma membrane and/or ability of transporters to interact with substrates.

Computational Discovery and Experimental Validation of Inhibitors of the Human Intestinal Transporter OATP2B1

Human organic anion transporters (OATPs) are vital for the uptake and efflux of drugs and endogenous compounds. Current identification of inhibitors of these transporters is based on experimental screening. Virtual screening remains a challenge due to a lack of experimental three-dimensional protein structures. Here, we describe a workflow to identify inhibitors of the OATP2B1 transporter in the DrugBank library of over 5,000 drugs and druglike molecules. OATP member 2B1 transporter is highly expressed in the intestine, where it participates in oral absorption of drugs. Predictions from a Random forest classifier, prioritized by docking against multiple comparative protein structure models of OATP2B1, indicated that 33 of the 5,000 compounds were putative inhibitors of OATP2B1. Ten predicted inhibitors that are prescription drugs were tested experimentally in cells overexpressing the OATP2B1 transporter. Three of these ten were validated as potent inhibitors of estrone-3-sulfate uptake (defined as more than 50% inhibition at 20 μM) and tested in multiple concentrations to determine exact IC₅₀. The IC₅₀ values of bicalutamide, ticagrelor, and meloxicam suggest that they might inhibit intestinal OATP2B1 at clinically relevant concentrations and therefore modulate the absorption of other concomitantly administered drugs.

Molecular localization and characterization of multiple binding sites of organic anion transporting polypeptide 2B1 (OATP2B1) as the mechanism for substrate and modulator dependent drug–drug interaction

Schematic model of the relationship and locations of putative binding sites of substrates and modulators in OATP2B1. Drug–drug interaction and drug–food interaction on OATP2B1 can be predicted by clarification of multiple binding sites.

Interplay between the prostaglandin transporter OATP2A1 and prostaglandin E2-mediated cellular effects

Prostaglandins such as prostaglandin E2 (PGE2) play a pivotal role in physiological and pathophysiological pathways in gastric mucosa. Little is known about the interrelation of the prostaglandin E (EP) receptors with the prostaglandin transporter OATP2A1 in the gastric mucosa and gastric carcinoma. Therefore, we first investigated the expression of OATP2A1 and EP4 in normal and carcinoma gastric mucosa. Different PGE2-mediated cellular pathways and mechanisms were investigated using human embryonic kidney cells (HEK293) and the human gastric carcinoma cell line AGS stably transfected with OATP2A1. Colocalization and expression of OATP2A1 and EP4 were detected in mucosa of normal gastric tissue and of gastric carcinomas. OATP2A1 reduced the PGE2-mediated cAMP production in HEK293 and AGS cells overexpressing EP4 and OATP2A1. The expression of OATP2A1 in AGS cells resulted in a reduction of [(3)H]-thymidine incorporation which was in line with a higher accumulation of AGS-OATP2A1 cells in S-phase of the cell cycle compared to control cells. In contrast, the expression of OATP2A1 in HEK293 cells had no influence on the distribution in the S-phase compared to control cells. OATP2A1 also diminished the PGE2-mediated expression of interleukin-8 mRNA (IL-8) and hypoxia-inducible-factor 1α (HIF1α) protein in AGS-OATP2A1 cells. The expression of OATP2A1 increased the sensitivity of AGS cells against irinotecan which led to reduced cell viability. Taken together, these data show that OATP2A1 influences PGE2-mediated cellular pathways. Therefore, OATP2A1 needs to be considered as a key determinant for the understanding of the physiology and pathophysiology of prostaglandins in healthy and tumorous gastric mucosa.

Organic anion-transporting polypeptides

Organic anion-transporting polypeptides or OATPs are central transporters in the disposition of drugs and other xenobiotics. In addition, they mediate transport of a wide variety of endogenous substrates. The critical role of OATPs in drug disposition has spurred research both in academia and in the pharmaceutical industry. Translational aspects with clinical questions are the focus in academia, while the pharmaceutical industry tries to define and understand the role these transporters play in pharmacotherapy. The present overview summarizes our knowledge on the interaction of food constituents with OATPs and on the OATP transport mechanisms. Further, it gives an update on the available information on the structure-function relationship of the OATPs and, finally, covers the transcriptional and posttranscriptional regulation of OATPs.

Organic anion transporting polypeptides and organic cation transporter 1 contribute to the cellular uptake of the flavonoid quercetin

Flavonoids such as quercetin and kaempferol mediate several health protective effects, e.g., anticancer effects. They are inhibitors of organic anion transporting polypeptides (OATP) and organic cation transporters (e.g., OCT2). However, little is known whether such transporters contribute to the cellular uptake of flavonoids. Therefore, we investigated the cellular uptake of kaempferol and quercetin using HEK293 cell lines stably expressing different human OATPs or OCT1. Kaempferol was not a substrate of any of the investigated transporters (OATP1A2, OATP1B1, OATP1B3, OATP2A1, OATP2B1, OATP3A1, OATP4A1, OATP5A1, and OCT1). Quercetin showed a significantly higher uptake into the HEK293-OATP1A2, HEK293-OATP2A1, HEK293-OATP2B1, and HEK293-OCT1 cells compared to control cells. The OATP1A2-, OATP2B1-, and OCT1-mediated quercetin uptake was inhibited by known inhibitors such as naringin, cyclosporin A, and quinidine, respectively. The cellular accumulation of quercetin into HEK293-OATP2A1 cells was not inhibited by prostaglandin E2 and diclofenac. The ionophore carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) reduced the net uptake of quercetin by increasing the uptake in the HEK293-control cells and causing no significant change in the HEK293-OATP2B1 cells indicating that quercetin follows the FCCP-driven proton flux through the plasma membrane. In addition to passive diffusion, the SLC transporters OATP1A2, OATP2B1, and OCT1 contribute to cellular accumulation of quercetin.

Introduction of aromatic ring-containing substituents in cyclic nucleotides is associated with inhibition of toxin uptake by the hepatocyte transporters OATP 1B1 and 1B3

Analogs of the cyclic nucleotides cAMP and cGMP have been extensively used to mimic or modulate cellular events mediated by protein kinase A (PKA), Exchange protein directly activated by cAMP (Epac), or protein kinase G (PKG). We report here that some of the most commonly used cyclic nucleotide analogs inhibit transmembrane transport mediated by the liver specific organic anion transporter peptides OATP1B1 and OATP1B3, unrelated to actions on Epac, PKA or PKG. Several cAMP analogs, particularly with 8-pCPT-substitution, inhibited nodularin (Nod) induced primary rat hepatocyte apoptosis. Inhibition was not mediated by PKA or Epac, since increased endogenous cAMP, and some strong PKA- or Epac-activating analogs failed to protect cells against Nod induced apoptosis. The cAMP analogs inhibiting Nod induced hepatocyte apoptosis also reduced accumulation of radiolabeled Nod or cholic acid in primary rat hepatocytes. They also inhibited Nod induced apoptosis in HEK293 cells with enforced expression of OATP1B1 or 1B3, responsible for Nod transport into cells. Similar results were found with adenosine analogs, disconnecting the inhibitory effect of certain cAMP analogs from PKA or Epac. The most potent inhibitors were 8-pCPT-6-Phe-cAMP and 8-pCPT-2′-O-Me-cAMP, whereas analogs like 6-MB-cAMP or 8-Br-cAMP did not inhibit Nod uptake. This suggests that the addition of aromatic ring-containing substituents like the chloro-phenyl-thio group to the purines of cyclic nucleotides increases their ability to inhibit the OATP-mediated transport. Taken together, our data show that aromatic ring substituents can add unwanted effects to cyclic nucleotides, and that such nucleotide analogs must be used with care, particularly when working with cells expressing OATP1B1/1B3, like hepatocytes, or intact animals where hepatic metabolism can be an issue, as well as certain cancer cells. On the other hand, cAMP analogs with substituents like bromo, monobutyryl were non-inhibitory, and could be considered an alternative when working with cells expressing OATP1 family members.

The SLCO (former SLC21) superfamily of transporters

The members of the organic anion transporting polypeptide superfamily (OATPs) are classified within the SLCO solute carrier family. All functionally well characterized members are predicted to have 12 transmembrane domains and are sodium-independent transport systems that mediate the transport of a broad range of endo- as well as xenobiotics. Substrates are mainly amphipathic organic anions with a molecular weight of more than 300Da, but some of the known transported substrates are also neutral or even positively charged. Among the well characterized substrates are numerous drugs including statins, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, antibiotics, antihistaminics, antihypertensives and anticancer drugs. Based on their amino acid sequence identities, the different OATPs cluster into families (in general with more than 40% amino acid sequence identity) and subfamilies (more than 60% amino acid identity). With the sequencing of genomes from different species and the computerized prediction of encoded proteins more than 300 OATPs can be found in the databases, however only a fraction of them have been identified in humans, rodents, and some additional species important for pharmaceutical research like the rhesus monkey (Macaca mulatta), the dog (Canis lupus familiaris) and the pig (Sus scrofa). These OATPs form 6 families (OATP1-OATP6) and 13 subfamilies. In this review we try to summarize what is currently known about OATPs with respect to endogenous substrates, tissue distribution, transport mechanisms, regulation of expression, structure-function relationship and mutations and polymorphisms.

Targeted drug delivery to treat pain and cerebral hypoxia

Limited drug penetration is an obstacle that is often encountered in treatment of central nervous system (CNS) diseases including pain and cerebral hypoxia. Over the past several years, biochemical characteristics of the brain (i.e., tight junction protein complexes at brain barrier sites, expression of influx and efflux transporters) have been shown to be directly involved in determining CNS permeation of therapeutic agents; however, the vast majority of these studies have focused on understanding those mechanisms that prevent drugs from entering the CNS. Recently, this paradigm has shifted toward identifying and characterizing brain targets that facilitate CNS drug delivery. Such targets include the organic anion-transporting polypeptides (OATPs in humans; Oatps in rodents), a family of sodium-independent transporters that are endogenously expressed in the brain and are involved in drug uptake. OATP/Oatp substrates include drugs that are efficacious in treatment of pain and/or cerebral hypoxia (i.e., opioid analgesic peptides, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors). This clearly suggests that OATP/Oatp isoforms are viable transporter targets that can be exploited for optimization of drug delivery to the brain and, therefore, improved treatment of CNS diseases. This review summarizes recent knowledge in this area and emphasizes the potential that therapeutic targeting of OATP/Oatp isoforms may have in facilitating CNS drug delivery and distribution. Additionally, information presented in this review will point to novel strategies that can be used for treatment of pain and cerebral hypoxia.

OATPs, OATs and OCTs: the organic anion and cation transporters of the SLCO and SLC22A gene superfamilies

The human organic anion and cation transporters are classified within two SLC superfamilies. Superfamily SLCO (formerly SLC21A) consists of organic anion transporting polypeptides (OATPs), while the organic anion transporters (OATs) and the organic cation transporters (OCTs) are classified in the SLC22A superfamily. Individual members of each superfamily are expressed in essentially every epithelium throughout the body, where they play a significant role in drug absorption, distribution and elimination. Substrates of OATPs are mainly large hydrophobic organic anions, while OATs transport smaller and more hydrophilic organic anions and OCTs transport organic cations. In addition to endogenous substrates, such as steroids, hormones and neurotransmitters, numerous drugs and other xenobiotics are transported by these proteins, including statins, antivirals, antibiotics and anticancer drugs. Expression of OATPs, OATs and OCTs can be regulated at the protein or transcriptional level and appears to vary within each family by both protein and tissue type. All three superfamilies consist of 12 transmembrane domain proteins that have intracellular termini. Although no crystal structures have yet been determined, combinations of homology modelling and mutation experiments have been used to explore the mechanism of substrate recognition and transport. Several polymorphisms identified in members of these superfamilies have been shown to affect pharmacokinetics of their drug substrates, confirming the importance of these drug transporters for efficient pharmacological therapy. This review, unlike other reviews that focus on a single transporter family, briefly summarizes the current knowledge of all the functionally characterized human organic anion and cation drug uptake transporters of the SLCO and the SLC22A superfamilies.

Identification of amino acids essential for estrone-3-sulfate transport within transmembrane domain 2 of organic anion transporting polypeptide 1B1

As an important structure in membrane proteins, transmembrane domains have been found to be crucial for properly targeting the protein to cell membrane as well as carrying out transport functions in transporters. Computer analysis of OATP sequences revealed transmembrane domain 2 (TM2) is among those transmembrane domains that have high amino acid identities within different family members. In the present study, we identify four amino acids (Asp70, Phe73, Glu74, and Gly76) that are essential for the transport function of OATP1B1, an OATP member that is specifically expressed in the human liver. A substitution of these four amino acids with alanine resulted in significantly reduced transport activity. Further mutagenesis showed the charged property of Asp70 and Glu74 is critical for proper function of the transporter protein. Comparison of the kinetic parameters indicated that Asp70 is likely to interact with the substrate while Glu74 may be involved in stabilizing the binding site through formation of a salt-bridge. The aromatic ring structure of Phe73 seems to play an important role because substitution of Phe73 with tyrosine, another amino acid with a similar structure, led to partially restored transport function. On the other hand, replacement of Gly76 with either alanine or valine could not recover the function of the transporter. Considering the nature of a transmembrane helix, we proposed that Gly76 may be important for maintaining the proper structure of the protein. Interestingly, when subjected to transport function analysis of higher concentration of esteone-3-sulfate (50 µM) that corresponds to the low affinity binding site of OATP1B1, mutants of Phe73, Glu74, and Gly76 all showed a transport function that is comparable to that of the wild-type, suggesting these amino acids may have less impact on the low affinity component of esteone-3-sulfate within OATP1B1, while Asp 70 seems to be involved in the interaction of both sites.

Interaction of three regiospecific amino acid residues is required for OATP1B1 gain of OATP1B3 substrate specificity

The human organic anion-transporting polypeptides OATP1B1 (SLCO1B1) and OATP1B3 (SLCO1B3) are liver-enriched membrane transporters of major importance to hepatic uptake of numerous endogenous compounds, including bile acids, steroid conjugates, hormones, and drugs, including the 3-hydroxy-3-methylglutaryl Co-A reductase inhibitor (statin) family of cholesterol-lowering compounds. Despite their remarkable substrate overlap, there are notable exceptions: in particular, the gastrointestinal peptide hormone cholecystokinin-8 (CCK-8) is a high affinity substrate for OATP1B3 but not OATP1B1. We utilized homologous recombination of linear DNA by E. coli to generate a library of cDNA containing monomer size chimeric OATP1B1-1B3 and OATP1B3-1B1 transporters with randomly distributed chimeric junctions to identify three discrete regions of the transporter involved in conferring CCK-8 transport activity. Site-directed mutagenesis of three key residues in OATP1B1 transmembrane helices 1 and 10, and extracellular loop 6, to the corresponding residues in OATP1B3, resulted in a gain of CCK-8 transport by OATP1B1. The residues appear specific to CCK-8, as the mutations did not affect transport of the shared OATP1B substrate atorvastatin or the OATP1B1-specific substrate estrone sulfate. Regions involved in gain of CCK-8 transport by OATP1B1, when mapped to the crystal structures of bacterial transporters from the major facilitator superfamily, are positioned to suggest these regions could readily interact with drug substrates. Accordingly, our data provide new insight into the molecular determinants of the substrate specificity of these hepatic uptake transporters with relevance to targeted drug design and prediction of drug-drug interactions.

Inhibition of hepatic uptake transporters by flavonoids

Members of the human SLC superfamily such as organic anion transporting polypeptide 1B1 (OATP1B1), OATP1B3, and organic cation transporter 1 (OCT1) are drug uptake transporters that are localised on the basolateral membrane of hepatocytes mediating the uptake of drugs such as atorvastatin and metformin into hepatocytes. Ingredients of food such as flavonoids influence the effects of drugs, e.g. by inhibition of drug transporters. Therefore, we investigated the impact of the Ginkgo biloba flavonoids apigenin, kaempferol, and quercetin, and the grapefruit flavonoids naringenin, naringin, and rutin on the OATP1B1, OATP1B3, and OCT1 transport activity. Transporter expressing HEK293 cell lines were used with [3H]sulfobromophthalein ([3H]BSP) as substrate for OATP1B1 and OATP1B3, [3H]atorvastatin as substrate for OATP1B1, and [3H]1-methyl-4-phenylpyridinium ([3H]MPP(+)) as substrate for OCT1. The G. biloba flavonoids showed a competitive inhibition of the OATP1B1- and OATP1B3-mediated [3H]BSP and the OATP1B1-mediated [3H]atorvastatin uptake. Quercetin was the most potent inhibitor of the OATP1B1- and OATP1B3-mediated [3H]BSP transport with K(i)-values of 8.8±0.8μM and 7.8±1.7μM, respectively. For the inhibition of the OATP1B1-mediated [3H]atorvastatin transport, apigenin was the most potent inhibitor with a K(i) value of 0.6±0.2μM. Among the grapefruit flavonoids, naringenin was the most potent inhibitor of the OATP1B1- and OATP1B3-mediated [3H]BSP transport with IC(50)-values of 81.6±1.1μM and 101.1±1.1μM, respectively. All investigated flavonoids showed no significant inhibition of the OCT1-mediated [3H]MPP(+) uptake. Taken together, these in vitro studies showed that the investigated flavonoids inhibit the OATP1B1- and OATP1B3-mediated drug transport, which could be a mechanism for food-drug interactions in humans.

Functional and structural relevance of conserved positively charged lysine residues in organic anion transporting polypeptide 1B3

The human organic anion transporting polypeptide 1B3 (OATP1B3), located in the basolateral membrane of hepatocytes, mediates the uptake of endogenous substrates such as taurocholate and drugs from blood into hepatocytes. The transport activity of OATP1B3 is influenced by positively charged amino acids, which are facing the central pore. Molecular modeling was performed to select conserved positively charged amino acids, which may influence transport activity and anchoring of OATP1B3 in the plasma membrane. The modeling revealed that Lys361 faces the pore, and Lys399 is oriented to the plasma membrane. Therefore, the mutants L361>A, L361>R, L399>A, and L399>R were generated using site-directed mutagenesis to investigate the impact of the positive charges on transport activity and anchoring in the membrane. Transport kinetic analyses for the substrates sulfobromophthalein and taurocholate showed a loss of function for the L361>A mutant, whereas the transport activity was maintained by the L361>R mutant, indicating that the positive charge at position 361 is important for transport activity of OATP1B3. Comparative modeling with OATP1A2 and OATP2B1 revealed that the pore size around this lysine residue is larger in OATP1A2 and smaller in OATP2B1 compared with OATP1B3, which could be related to the respective substrate spectra. Cell surface expression of L399>A and L399>R was decreased to 16 and 72% compared with wild-type OATP1B3 (p < 0.001), respectively, indicating that the positive charge of lysine at position 399 is necessary for an unimpaired cell surface expression. Furthermore, we provide a summary of amino acids, which influence the transport activity of OATP1B3.

Uptake transporters of the human OATP family: molecular characteristics, substrates, their role in drug-drug interactions, and functional consequences of polymorphisms

Organic anion transporting polypeptides (OATPs, gene family: SLC21/SLCO) mediate the uptake of a broad range of substrates including several widely prescribed drugs into cells. Drug substrates for members of the human OATP family include HMG-CoA-reductase inhibitors (statins), antibiotics, anticancer agents, and cardiac glycosides. OATPs are expressed in a variety of different tissues including brain, intestine, liver, and kidney, suggesting that these uptake transporters are important for drug absorption, distribution, and excretion. Because of their wide tissue distribution and broad substrate spectrum, altered transport kinetics, for example, due to drug-drug interactions or due to the functional consequences of genetic variations (polymorphisms), can contribute to the interindividual variability of drug effects. Therefore, the molecular characteristics of human OATP family members, the role of human OATPs in drug-drug interactions, and the in vitro analysis of the functional consequences of genetic variations in SLCO genes encoding OATP proteins are the focus of this chapter.

Several conserved positively charged amino acids in OATP1B1 are involved in binding or translocation of different substrates

OATP1B1 and 1B3 are related transporters mediating uptake of numerous compounds into hepatocytes. A putative model of OATP1B3 with a "positive binding pocket" containing conserved positively charged amino acids was predicted (Meier-Abt et al. J Membr Biol 208:213-227, 2005). Based on this model, we tested the hypothesis that these positive amino acids are important for OATP1B1 function. We made mutants and measured surface expression and uptake of estradiol-17β-glucuronide, estrone-3-sulfate and bromosulfophthalein in HEK293 cells. Two of the mutants had low surface expression levels: R181K at 10% and R580A at 30% of wild-type OATP1B1. A lysine at position 580 (R580K) rescued the expression of R580A. Mutations of several amino acids resulted in substrate-dependent effects. The largest changes were seen for estradiol-17β-glucuronide, while estrone-3-sulfate and bromosulfophthalein transport were less affected. The wild-type OATP1B1 K (m) value for estradiol-17β-glucuronide of 5.35 ± 0.54 μM was increased by R57A to 30.5 ± 3.64 μM and decreased by R580K to 0.52 ± 0.18 μM. For estrone-3-sulfate the wild-type high-affinity K (m) value of 0.55 ± 0.12 μM was increased by K361R to 1.8 ± 0.47 μM and decreased by R580K to 0.1 ± 0.04 μM. In addition, R580K reduced the V (max) values for all three substrates to <25% of wild-type OATP1B1. Mutations at intracellular K90, H92 and R93 mainly affected V (max) values for estradiol-17β-glucuronide uptake. In conclusion, the conserved amino acids R57, K361 and R580 seem to be part of the substrate binding sites and/or translocation pathways in OATP1B1.