Comprehensive review of amino acid transporters as therapeutic targets

The solute carrier (SLC) family, with more than 400 membrane-bound proteins, facilitates the transport of a wide array of substrates such as nutrients, ions, metabolites, and drugs across biological membranes. Amino acid transporters (AATs) are membrane transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs participate in many important physiological functions including nutrient supply, metabolic transformation, energy homeostasis, redox regulation, and neurological regulation. Several AATs have been found to significantly impact the progression of human malignancies, and dysregulation of AATs results in metabolic reprogramming affecting tumor growth and progression. However, current clinical therapies that directly target AATs have not been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, the molecular mechanisms in human diseases such as tumors, kidney diseases, and emerging therapeutic strategies for targeting AATs.

Development and Functional Evaluation of MDR1-expressing Microvascular Endothelial-like Cells Derived from Human iPS Cells as an In vitro Blood-brain Barrier Model

In order to establish an in vitro model of the human blood-brain barrier (BBB), MDR1-overexpressing human induced pluripotent stem cells (hiPSCs) were generated, and they were differentiated to MDR1-expressing brain microvascular endothelial-like cells (MDR1-expressing hiPS-BMECs). MDR1-expressing hiPS-BMECs monolayers showed good barrier function in terms of tight junction protein expression and trans-epithelial electrical resistance (TEER). In sequential window acquisition of all theoretical fragment ion spectra mass spectrometry (SWATH-MS), MDR1 protein expression was markedly increased in MDR1-expressing hiPS-BMECs, whereas other ABC and SLC transporters showed almost identical expression between MDR1-expressing hiPS-BMECs and mock hiPS-BMECs, suggesting that MDR1 overexpression had little or no knock-on effect on other proteins. The basolateral-to-apical transport of MDR1 substrates, such as quinidine, [3H]digoxin and [3H]vinblastine, was higher than the apical-to-basolateral transport, and the efflux-dominant transport was attenuated by PSC833, an MDR1-specific inhibitor, indicating that MDR1-mediated efflux transport is functional. The robust MDR1 function was also supported by the efflux-dominant transports of [3H]cyclosporin A, loperamide, cetirizine, and verapamil by MDR1-expressing hiPS-BMECs. These results suggest that MDR1-expressing hiPS-BMECs can be used as an in vitro model of the human BBB.

A single high-dose irradiation changes accumulation of methotrexate and gene expression levels of SLC and ABC transporters in cancer cells

Chemoradiotherapy is frequently used to treat cancer. Stereotactic body radiotherapy (SBRT) is a single high-dose radiotherapy used to treat a variety of cancers. The anticancer drug methotrexate (MTX) shows affinity for solute carrier (SLC) and ATP-binding cassette (ABC) transporters. This study investigated relationships between accumulation of methotrexate and gene expression levels of solute carrier and ATP-binding cassette transporters in cancer cells after a single and high-dose X-ray irradiation. Cancer cell lines were selected from lung and cervical cancer cell line that are commonly used for stereotactic body radiotherapy and effective with methotrexate. We examined expression levels of organic anion-transporting polypeptide (OATP)1B1, OATP1B3, OATP1B7, and organic anion transporter (OAT)1 as solute carrier transporters and multidrug resistance-associated protein (MRP)1 and MRP2 as ATP-binding cassette transporters, using real-time polymerase chain reaction and accumulation of ³H-MTX in cancer cells after 10-Gy irradiation, assuming stereotactic body radiotherapy. Cells were divided into three groups: Control without irradiation; 4 h after irradiation; and 24 h after irradiation. In control, gene expression levels of OAT1 in all cells was below the limit of measurement. After irradiation, gene expression levels of OATP1B1/1B3/1B7 showed changes in each cell line. Gene expression levels of MRP1/2 tended to increase after irradiation. Gene expression levels of OATP1B1/1B3/1B7 were much lower than those of MRP1/2. Accumulation of ³H-MTX tended to decrease over time after irradiation. Irradiation of cancer cells thus alters gene expression levels of both solute carrier transporters (OATP1B1/1B3/1B7) and ABC transporters (MRP1/2) and decreases accumulation of ³H-MTX in cancer cells over time due to elevated expression of MRP1/2.

Heterologous (Over) Expression of Human SoLute Carrier (SLC) in Yeast: A Well-Recognized Tool for Human Transporter Function/Structure Studies

For more than 20 years, yeast has been a widely used system for the expression of human membrane transporters. Among them, more than 400 are members of the largest transporter family, the SLC superfamily. SLCs play critical roles in maintaining cellular homeostasis by transporting nutrients, ions, and waste products. Based on their involvement in drug absorption and in several human diseases, they are considered emerging therapeutic targets. Despite their critical role in human health, a large part of SLCs' is 'orphans' for substrate specificity or function. Moreover, very few data are available concerning their 3D structure. On the basis of the human health benefits of filling these knowledge gaps, an understanding of protein expression in systems that allow functional production of these proteins is essential. Among the 500 known yeast species, S. cerevisiae and P. pastoris represent those most employed for this purpose. This review aims to provide a comprehensive state-of-the-art on the attempts of human SLC expression performed by exploiting yeast. The collected data will hopefully be useful for guiding new attempts in SLCs expression with the aim to reveal new fundamental data that could lead to potential effects on human health.

Ontogeny of renal, hepatic, and placental expression of ATP-binding cassette and solute carrier transporters in the rat and the rabbit

Species differences in developmental toxicity can be due to varying expression of xenobiotic transporters. Hence, knowledge on the ontogeny of these transporters, especially in human, rat and rabbit, is pivotal. Two superfamilies of transporters, the ATP-binding cassette (ABC) and the solute carrier (SLC) transporters, are well known for their role in the absorption, distribution and/or elimination of xenobiotics and endogenous substances. The aim of this study was to compare the expression levels of these xenobiotic transporters in liver, kidney and placenta of man, Wistar rat and New Zealand White rabbit during pre- and postnatal development. For this purpose, qPCR experiments were performed for rat and rabbit tissues and the gene expression profiles were compared with literature data from man, rat and rabbit. Data analysis showed large differences in transporter expression in development and between species. These results can be used to better understand developmental toxicity findings in non-clinical species and their relevance for man.

The role of SLC transporters for brain health and disease

The brain exchanges nutrients and small molecules with blood via the blood-brain barrier (BBB). Approximately 20% energy intake for the body is consumed by the brain. Glucose is known for its critical roles for energy production and provides substrates for biogenesis in neurons. The brain takes up glucose via glucose transporters GLUT1 and 3, which are expressed in several neural cell types. The brain is also equipped with various transport systems for acquiring amino acids, lactate, ketone bodies, lipids, and cofactors for neuronal functions. Unraveling the mechanisms by which the brain takes up and metabolizes these nutrients will be key in understanding the nutritional requirements in the brain. This could also offer opportunities for therapeutic interventions in several neurological disorders. For instance, emerging evidence suggests a critical role of lactate as an alternative energy source for neurons. Neuronal cells express monocarboxylic transporters to acquire lactate. As such, treatment of GLUT1-deficient patients with ketogenic diets to provide the brain with alternative sources of energy has been shown to improve the health of the patients. Many transporters are present in the brain, but only a small number has been characterized. In this review, we will discuss about the roles of solute carrier (SLC) transporters at the blood brain barrier (BBB) and neural cells, in transport of nutrients and metabolites in the brain.

Transport Characteristics of 6-Mercaptopurine in Brain Microvascular Endothelial Cells Derived From Human Induced Pluripotent Stem Cells

The likelihood of reoccurrence of acute lymphoblastic leukemia is influenced by the cerebral concentration of the therapeutic agent 6-mercaptopurine (6-MP) during treatment. Therefore, it is important to understand the blood-brain barrier (BBB) transport mechanism of 6-MP. The purpose of this study was to characterize this mechanism using human induced pluripotent stem cell-derived microvascular endothelial cells (hiPS-BMECs). The permeability coefficient of 6-MP across hiPS-BMECs monolayer in the basal-to-apical direction (B-to-A) was significantly greater than that in the opposite direction (A-to-B). The inhibition profiles of 6-MP transport in the A-to-B direction were different from those in the B-to-A direction. Transport in the A-to-B direction was mainly inhibited by adenine (an inhibitor of equilibrative nucleobase transporter 1; ENBT1), while transport in the B-to-A direction was significantly reduced by inhibitors of multidrug resistance-associated proteins (MRPs), especially zaprinast (an MRP5 inhibitor). Immunocytochemical analyses demonstrated the expression of ENBT1 and MRP5 proteins in hiPS-BMECs. We confirmed that the cellular uptake of 6-MP is decreased by ENBT1 inhibitors in hiPS-BMECs and by knockdown of ENBT1 in hCMEC/D3 cells. These results suggest that ENBT1 and MRP5 make substantial contributions to the transport of 6-MP in hiPS-BMECs and hCMEC/D3 cells.

In vitro evaluations for pharmacokinetic drug-drug interactions of a novel serotonin-dopamine activity modulator, brexpiprazole

Brexpiprazole, a serotonin-dopamine activity modulator, is indicated for the treatment of schizophrenia and also adjunctive therapy to antidepressants for the treatment of Major Depressive Disorder. To determine the drug-drug interaction risk for cytochrome P450, and SLC and ABC transporters, brexpiprazole and its metabolite, DM-3411 were assessed in this in vitro investigation.Brexpiprazole exhibited weak inhibitory effects (IC₅₀ >13 μmol/L) on CYP2C9, CYP2C19, CYP2D6 and CYP3A4 activities, but had moderate inhibitor activity on CYP2B6 (IC₅₀ 8.19 μmol/L). The ratio of systemic unbound concentration (3.8 nmol/L) to the K_i value was sufficiently low. DM-3411 had comparable inhibitory potentials with brexpiprazole only for CYP2D6 and CYP3A4. The mRNA expressions of CYP1A2, CYP2B6 and CYP3A4 were not changed by the exposure of brexpiprazole to human hepatocytes.Brexpiprazole and DM-3411 exhibited weak or no inhibitory effects for hepatic and renal transporters (OATPs, OATs, OCTs, MATE1, and BSEP), except for MATE-2K (0.156 μmol/L of DM-3411), even for which the ratio to systemic unbound concentration (5.3 nmol/L) was sufficiently low.Brexpiprazole effected the functions of P-gp and BCRP with IC₅₀ values of 6.31 and 1.16 μmol/L, respectively, however, the pharmacokinetic alteration was not observed in the clinical concomitant study on P-gp and BCRP substrates.These in vitro data suggest that brexpiprazole is unlikely to cause clinically relevant drug interactions resulting from the effects on CYPs or transporters mediating the absorption, metabolism, and/or disposition of co-administered drugs.

Differential in vitro interactions of the Janus kinase inhibitor ruxolitinib with human SLC drug transporters

Interactions of the Janus kinase (JAK) inhibitor ruxolitinib with solute carriers (SLCs) remain incompletely characterised. The present study was therefore designed to investigate this issue.The interactions of ruxolitinib with SLCs were analysed using transporter-overexpressing human embryonic kidney HEK293 cells. Substrate accumulation was detected by spectrofluorimetry, liquid chromatography coupled to tandem mass spectrometry or scintillation counting.Ruxolitinib was found to potently inhibit the activities of organic anion transporter 3 (OAT3), organic cation transporter 2 (OCT2), multidrug and toxin extrusion 1 (MATE1) and MATE2-K (half maximal inhibitory concentration (IC₅₀) < 10 µM). It blocked OAT1, OAT4, OATP1B1, OATP1B3, OATP2B1 and OCT3, but in a weaker manner (IC₅₀ > 10 µM), whereas OCT1 was not impacted. No time-dependent inhibition was highlighted. When applying the US Food and Drug Administration (FDA) criteria for transporters-related drug-drug interaction risk, OCT2 and MATE2-K, unlike MATE1 and OAT3, were predicted to be in vivo inhibited by ruxolitinib. Cellular uptake studies additionally indicated that ruxolitinib is a substrate for MATE1 and MATE2-K, but not for OAT3 and OCT2.Ruxolitinib in vitro blocked activities of most of SLC transporters. Only OCT2 and MATE-2K may be however clinically inhibited by the JAK inhibitor, with the caution for OCT2 that in vitro inhibition data were generated with an FDA-non recommended fluorescent substrate. Ruxolitinib MATEs-mediated transport may additionally deserve attention for its possible pharmacological consequences in MATE-positive cells.

Hepatobiliary acid-base homeostasis: Insights from analogous secretory epithelia

Many epithelia secrete bicarbonate-rich fluid to generate flow, alter viscosity, control pH and potentially protect luminal and intracellular structures from chemical stress. Bicarbonate is a key component of human bile and impaired biliary bicarbonate secretion is associated with liver damage. Major efforts have been undertaken to gain insight into acid-base homeostasis in cholangiocytes and more can be learned from analogous secretory epithelia. Extrahepatic examples include salivary and pancreatic duct cells, duodenocytes, airway and renal epithelial cells. The cellular machinery involved in acid-base homeostasis includes carbonic anhydrase enzymes, transporters of the solute carrier family, and intra- and extracellular pH sensors. This pH-regulatory system is orchestrated by protein-protein interactions, the establishment of an electrochemical gradient across the plasma membrane and bicarbonate sensing of the intra- and extracellular compartment. In this review, we discuss conserved principles identified in analogous secretory epithelia in the light of current knowledge on cholangiocyte physiology. We present a framework for cholangiocellular acid-base homeostasis supported by expression analysis of publicly available single-cell RNA sequencing datasets from human cholangiocytes, which provide insights into the molecular basis of pH homeostasis and dysregulation in the biliary system.

Bicarbonate enhances the inflammatory response by activating JAK/STAT signalling in LPS + IFN-γ-stimulated macrophages

Macrophages, which develop by changing their functions according to various environmental conditions and stimuli, defend against the pathogens and play roles in homoeostasis and disease states. Bicarbonate (HCO3-) is important in the maintenance of intracellular and extracellular pH in the body. However, the effects of bicarbonate on macrophage function have not been examined. In this study, we investigated the effects of bicarbonate on macrophage activation in lipopolysaccharide (LPS) and interferon (IFN)-γ (LPS + IFN-γ)-stimulated murine macrophage-like RAW264.7 cells. The expression of the interleukin (IL)-6, inducible nitric oxide (NO) synthase and cyclooxygenase-2 genes was enhanced by sodium bicarbonate (NaHCO3) in a concentration-dependent manner in LPS + IFN-γ-stimulated RAW264.7 cells. The production of IL-6, NO2- and prostaglandin E2 was also increased by treatment with NaHCO3 in these cells. Moreover, NaHCO3-mediated elevation of inflammatory gene expression was abrogated by solute carrier (SLC) transporter inhibitors. Furthermore, its NaHCO3-mediated activation was negated by a JAK inhibitor , tofacitinib. NaHCO3-enhanced phosphorylation of STAT1, and its enhancement was abrogated by pre-treating with SLC transporter inhibitors in LPS + IFN-γ-stimulated RAW264.7 cells. In addition, similar results were obtained in murine bone marrow-derived macrophages. These results indicate that bicarbonate enhanced the inflammatory response through the JAK/STAT signalling in LPS + IFN-γ-stimulated macrophages.

Cross-species analysis of hepatic cytochrome P450 and transport protein expression

Most drugs and xenobiotics are metabolized in the liver. Amongst others, different cytochrome P450 (CYP) enzymes catalyze the metabolic conversion of foreign compounds, and various transport proteins are engaged in the excretion of metabolites from the hepatocytes. Inter-species and inter-individual differences in the hepatic levels and activities of drug-metabolizing enzymes and transporters result from genetic as well as from environmental factors, and play a decisive role in determining the pharmacokinetic properties of a compound in a given test system. To allow for a meaningful comparison of results from metabolism studies, it is, therefore, of utmost importance to know about the specific metabolic properties of the test systems, especially about the levels of metabolic enzymes such as the CYPs. Using a targeted proteomics approach, we, therefore, compared the hepatic levels of important CYP enzymes and transporters in different experimental systems in vivo and in vitro, namely Wistar rats, C57/Bl6 mice, mice humanized for the two xeno-sensing receptors PXR (pregnane-X-receptor) and CAR (constitutive androstane receptor), mice with human hepatocyte-repopulated livers, human HepaRG hepatocarcinoma cells, primary human hepatocytes, and human liver biopsies. In addition, the effects of xenobiotic inducers of drug metabolism on CYP enzymes and transporters were analyzed in selected systems. This study for the first time presents a comprehensive overview of similarities and differences in important drug metabolism-related proteins among the different experimental models.

Disruption of small molecule transporter systems by Transporter-Interfering Chemicals (TICs)

Small molecule transporters (SMTs) in the ABC and SLC families are important players in disposition of diverse endo- and xenobiotics. Interactions of environmental chemicals with these transporters were first postulated in the 1990s, and since validated in numerous in vitro and in vivo scenarios. Recent results on the co-crystal structure of ABCB1 with the flame-retardant BDE-100 demonstrate that a diverse range of man-made and natural toxic molecules, hereafter termed transporter-interfering chemicals (TICs), can directly bind to SMTs and interfere with their function. TIC-binding modes mimic those of substrates, inhibitors, modulators, inducers, and possibly stimulants through direct and allosteric mechanisms. Similarly, the effects could directly or indirectly agonize, antagonize or perhaps even prime the SMT system to alter transport function. Importantly, TICs are distinguished from drugs and pharmaceuticals that interact with transporters in that exposure is unintended and inherently variant. Here, we review the molecular mechanisms of environmental chemical interaction with SMTs, the methodological considerations for their evaluation, and the future directions for TIC discovery.

Structure of the Regulatory Cytosolic Domain of a Eukaryotic Potassium-Chloride Cotransporter

Cation-chloride cotransporters (CCCs) regulate the movement of chloride across membranes, controlling physiological processes from cell volume maintenance to neuronal signaling. Human CCCs are clinical targets for existing diuretics and potentially additional indications. Here, we report the X-ray crystal structure of the soluble C-terminal regulatory domain of a eukaryotic potassium-chloride cotransporter, Caenorhabditis elegans KCC-1. We observe a core α/β fold conserved among CCCs. Using structure-based sequence alignment, we analyze similarities and differences to the C-terminal domains of other CCC family members. We find that important regulatory motifs are in less-structured regions and residues important for dimerization are not widely conserved, suggesting that oligomerization and its effects may vary within the larger family. This snapshot of a eukaryotic KCC is a valuable starting point for the rational design of studies of cellular chloride regulation.

Vectorial transport of the arginine derivatives asymmetric dimethylarginine (ADMA) and L-homoarginine by OATP4C1 and P-glycoprotein studied in double-transfected MDCK cells

Elevated plasma concentrations of the uremic toxin asymmetric dimethylarginine (ADMA) and low plasma concentrations of l-homoarginine are independently associated with cardiovascular events and mortality. Key enzymes involved in the homeostasis of both arginine derivatives are expressed in proximal tubule cells of the kidney. To get access to these enzymes, transport proteins are important. One of the transporters mediating the transport of ADMA and l-homoarginine is the solute carrier superfamily (SLC) member OATP4C1, located in the basolateral membrane of proximal tubule cells. To gain insights into the role of export pumps in the transport of both substances, we established a double-transfected MDCK cell line expressing OATP4C1 and the export pump P-glycoprotein (P-gp). Using MDCK cell monolayers, we demonstrated in time-dependent and concentration-dependent vectorial transport experiments that ADMA and l-homoarginine are transported from the basolateral to the apical compartment of MDCK-OATP4C1-P-gp cells with significantly higher transport rates compared to single-transfected MDCK-OATP4C1, MDCK-P-gp and MDCK-VC (control) cells (e.g. transport ratio MDCK-OATP4C1-P-gp/MDCK-VC: for 50 µM ADMA = 2.0-fold, for 50 µM l-homoarginine  = 3.4-fold). These results indicate that both OATP4C1 and P-gp transport the arginine derivatives ADMA and l-homoarginine and are, therefore, important for the homoeostasis of both substances.

Induction and repression effects on CYP and transporter protein abundance by azole mixture uptake in rat liver

Detection of mixture effects is a major challenge in current experimental and regulatory toxicology. Robust markers are needed that are easy to quantify and responsive to chemical stressors in a broad dose range. Several hepatic enzymes and proteins related to drug metabolism like cytochrome-P-450 (CYP) enzymes and transporters have been shown to be responsive to pesticide active substances in a broad dose range and are therefore good candidates to be used as markers for mixture toxicity. Even though they can be well quantified at the mRNA level, quantification on the protein level is challenging because most of these proteins are membrane bound. Here we report the development of mass spectrometry-based assays using triple-x-proteomics (TXP) antibodies in combination with targeted selected ion monitoring (tSIM) to quantify changes of protein levels due to exposure to mixtures of pesticide active substances. Our results indicate that changes on the protein level of CYP1A1, ABCB2, ABCC3 are in line with observations on the mRNA and enzyme activity level and are indicative of mixture effects. Therefore, the tests are promising to reveal effects by chemical mixture effects in toxicological studies in rats.

Effects of single and repetitive valproic acid administration on the gene expression of placental transporters in pregnant rats: An analysis by gestational period

The use of valproic acid (VPA), an antiepileptic drug, during pregnancy, is known to increase various fetal risks. Since VPA has been known to inhibit histone deacetylases (HDACs); its administration could alter gene transcription levels. However, in vivo effects of VPA administration on placental transporters have not been fully elucidated. The purpose of the present study was to comprehensively evaluate the effects of single and repetitive VPA administration on the expression of placental transporters and analyze them by gestational day. We investigated 18 transporters (8 ATP-binding cassette (ABC) and 10 solute carrier (SLC) transporters) in the placentas of pregnant rats that were orally administered 400 mg/kg/day VPA for one or four days, during mid- or late gestation. In the control rats, 4 ABC transporter genes (Abcb1a, 1b, Abcc2, Abcc4) were upregulated, 3 (Abcc3, Abcc5, Abcg2) downregulated through gestation, whereas 1 (Abcc1) was not changed. Regarding SLC transporters, 6 genes (Slc7a5, Slc16a3, Slc22a3, Slc22a4, Slco2b1, Slco4a1) were increased, 1 (Slc29a1) decreased through gestation, whereas 3 (Slc7a8, Slc22a5, Slco2a1) showed no significant change. Single VPA administration altered the expression of 9 transporters and repetitive administration, 13 transporters. In particular, VPA remarkably decreased Abcc4 and Slc22a4 in late gestation and increased Abcc5 during mid-gestation. Our findings indicated that VPA administration changed transporter expression levels in rat placenta, and suggested that sensitivity to VPA differs across gestational stages.

Repurposing Tyrosine Kinase Inhibitors to Overcome Multidrug Resistance in Cancer: A Focus on Transporters and Lysosomal Sequestration

Tyrosine kinase inhibitors (TKIs) are being increasingly used to treat various malignancies. Although they were designed to target aberrant tyrosine kinases, they are also intimately linked with the mechanisms of multidrug resistance (MDR) in cancer cells. MDR-related solute carrier (SLC) and ATB-binding cassette (ABC) transporters are responsible for TKI uptake and efflux, respectively. However, the role of TKIs appears to be dual because they can act as substrates and/or inhibitors of these transporters. In addition, several TKIs have been identified to be sequestered into lysosomes either due to their physiochemical properties or via ABC transporters expressed on the lysosomal membrane. Since the development of MDR represents a great concern in anticancer treatment, it is important to elucidate the interactions of TKIs with MDR-related transporters as well as to improve the properties that would prevent TKIs from diffusing into lysosomes. These findings not only help to avoid MDR, but also help to define the possible impact of combining TKIs with other anticancer drugs, leading to more efficient therapy and fewer adverse effects in patients.

DirectMX - One-Step Reconstitution of Membrane Proteins From Crude Cell Membranes Into Salipro Nanoparticles

Integral membrane proteins (IMPs) are central to many physiological processes and represent ∼60% of current drug targets. An intricate interplay with the lipid molecules in the cell membrane is known to influence the stability, structure and function of IMPs. Detergents are commonly used to solubilize and extract IMPs from cell membranes. However, due to the loss of the lipid environment, IMPs usually tend to be unstable and lose function in the continuous presence of detergent. To overcome this problem, various technologies have been developed, including protein engineering by mutagenesis to improve IMP stability, as well as methods to reconstitute IMPs into detergent-free entities, such as nanodiscs based on apolipoprotein A or its membrane scaffold protein (MSP) derivatives, amphipols, and styrene-maleic acid copolymer-lipid particles (SMALPs). Although significant progress has been made in this field, working with inherently unstable human IMP targets (e.g., GPCRs, ion channels and transporters) remains a challenging task. Here, we present a novel methodology, termed DirectMX (for direct membrane extraction), taking advantage of the saposin-lipoprotein (Salipro) nanoparticle technology to reconstitute fragile IMPs directly from human crude cell membranes. We demonstrate the applicability of the DirectMX methodology by the reconstitution of a human solute carrier transporter and a wild-type GPCR belonging to the human chemokine receptor (CKR) family. We envision that DirectMX bears the potential to enable studies of IMPs that so far remained inaccessible to other solubilization, stabilization or reconstitution methods.

Functional Characterization of SLC Transporters Using Solid Supported Membranes

Here, we present a protocol for the functional characterization of the H⁺-coupled human peptide transporter PepT1 and sufficient notes to transfer the protocol to the Na⁺-coupled sugar transporter SGLT1, the organic cation transporter OCT2, the Na⁺/Ca²⁺ exchanger NCX, and the neuronal glutamate transporter EAAT3.The assay was developed for the commercially available SURFE²R N1 instrument (Nanion Technologies GmbH) which applies solid supported membrane (SSM)-based electrophysiology. This technique is widely used for the functional characterization of membrane transporters with more than 100 different transporters characterized so far. The technique is cost-effective, easy to use, and capable of high-throughput measurements.SSM-based electrophysiology utilizes SSM-coated gold sensors to physically adsorb membrane vesicles containing the protein of interest. A fast solution exchange provides the substrate and activates transport. For the measurement of PepT1 activity, we applied a peptide concentration jump to activate H⁺/peptide symport. Proton influx charges the sensor. A capacitive current is measured reflecting the transport activity of PepT1 . Multiple measurements on the same sensor allow for comparison of transport activity under different conditions. Here, we determine EC₅₀ for PepT1-mediated glycylglycine transport and perform an inhibition experiment using the specific peptide inhibitor Lys[Z(NO₂)]-Val.

Contributions of Drug Transporters to Blood-Placental Barrier

The placenta is the only organ linking two different individuals, mother and fetus, termed as blood-placental barrier. The functions of the blood-placental barrier are to regulate material transfer between the maternal and fetal circulation. The main functional units are the chorionic villi within which fetal blood is separated by only three or four cell layers (placental membrane) from maternal blood in the surrounding intervillous space. A series of drug transporters such as P-glycoprotein (P-GP), breast cancer resistance protein (BCRP), multidrug resistance-associated proteins (MRP1, MRP2, MRP3, MRP4, and MRP5), organic anion-transporting polypeptides (OATP4A1, OATP1A2, OATP1B3, and OATP3A1), organic anion transporter 4 (OAT4), organic cation transporter 3 (OCT3), organic cation/carnitine transporters (OCTN1 and OCTN2), multidrug and toxin extrusion 1 (MATE1), and equilibrative nucleoside transporters (ENT1 and ENT2) have been demonstrated on the apical membrane of syncytiotrophoblast, some of which also expressed on the basolateral membrane of syncytiotrophoblast or fetal capillary endothelium. These transporters are involved in transport of most drugs in the placenta, in turn, affecting drug distribution in fetus. Moreover, expressions of these transporters in the placenta often vary along with the gestational ages and are also affected by pathophysiological factor. This chapter will mainly illustrate function and expression of these transporters in placentas, their contribution to drug distribution in fetus, and their clinical significance.

Comparative expression patterns of selected membrane transporters in differentiated BeWo and human primary trophoblast cells

Although placental membrane transporters have an important impact on materno-fetal nutrient transfer, placental cell models are poorly characterized regarding transporter expression. We assessed the mRNA expression of 26 physiologically important solute carriers and ABC transporters in BeWo (b30 clone) and primary human trophoblast cells (PHT) before and after syncytialization. 77% of the transporters showed similar mRNA expression changes between BeWo and PHT after syncytialization. Selected transporters, however, were either lacking in BeWo or showed different trends after syncytialization. In conclusion, BeWo cells generally represent an apt model for transporter studies, but their suitability should be confirmed for each transporter by comparison with PHT.

Molecular dissection of the valproic acid effects on glioma cells

Many glioblastoma patients suffer from seizures why they are treated with antiepileptic agents. Valproic acid (VPA) is a histone deacetylase inhibitor that apart from its anticonvulsive effects in some retrospective studies has been suggested to lead to a superior outcome of glioblastoma patients. However, the exact molecular effects of VPA treatment on glioblastoma cells have not yet been deciphered. We treated glioblastoma cells with VPA, recorded the functional effects of this treatment and performed a global and unbiased next generation sequencing study on the chromatin (ChIP) and RNA level. 1) VPA treatment clearly sensitized glioma cells to temozolomide: A protruding VPA-induced molecular feature in this context was the transcriptional upregulation/reexpression of numerous solute carrier (SLC) transporters that was also reflected by euchromatinization on the histone level and a reexpression of SLC transporters in human biopsy samples after VPA treatment. DNA repair genes were adversely reduced. 2) VPA treatment, however, also reduced cell proliferation in temozolomide-naive cells: On the molecular level in this context we observed a transcriptional upregulation/reexpression and euchromatinization of several glioblastoma relevant tumor suppressor genes and a reduction of stemness markers, while transcriptional subtype classification (mesenchymal/proneural) remained unaltered. Taken together, these findings argue for both temozolomide-dependent and -independent effects of VPA. VPA might increase the uptake of temozolomide and simultaneously lead to a less malignant glioblastoma phenotype. From a mere molecular perspective these findings might indicate a surplus value of VPA in glioblastoma therapy and could therefore contribute an additional ratio for clinical decision making.

Use of PET Imaging to Evaluate Transporter-Mediated Drug-Drug Interactions

Several membrane transporters belonging to the adenosine triphosphate-binding cassette (ABC) and solute carrier (SLC) families can transport drugs and drug metabolites and thereby exert an effect on drug absorption, distribution, and excretion, which may potentially lead to transporter-mediated drug-drug interactions (DDIs). Some transporter-mediated DDIs may lead to changes in organ distribution of drugs (eg, brain, liver, kidneys) without affecting plasma concentrations. Positron emission tomography (PET) is a noninvasive imaging method that allows studying of the distribution of radiolabeled drugs to different organs and tissues and is therefore the method of choice to quantitatively assess transporter-mediated DDIs on a tissue level. There are 2 approaches to how PET can be used in transporter-mediated DDI studies. When the drug of interest is a potential perpetrator of DDIs, it may be administered in unlabeled form to assess its influence on tissue distribution of a generic transporter-specific PET tracer (probe substrate). When the drug of interest is a potential victim of DDIs, it may be radiolabeled with carbon-11 or fluorine-18 and used in combination with a prototypical transporter inhibitor (eg, rifampicin). PET has already been used both in preclinical species and in humans to assess the effects of transporter-mediated DDIs on drug disposition in different organ systems, such as brain, liver, and kidneys, for which examples are given in the present review article. Given the growing importance of membrane transporters with respect to drug safety and efficacy, PET is expected to play an increasingly important role in future drug development.

The effects of dietary and herbal phytochemicals on drug transporters

Membrane transporter proteins (the ABC transporters and SLC transporters) play pivotal roles in drug absorption and disposition, and thus determine their efficacy and safety. Accumulating evidence suggests that the expression and activity of these transporters may be modulated by various phytochemicals (PCs) found in diets rich in plants and herbs. PC absorption and disposition are also subject to the function of membrane transporter and drug metabolizing enzymes. PC-drug interactions may involve multiple major drug transporters (and metabolizing enzymes) in the body, leading to alterations in the pharmacokinetics of substrate drugs, and thus their efficacy and toxicity. This review summarizes the reported in vitro and in vivo interactions between common dietary PCs and the major drug transporters. The oral absorption, distribution into pharmacological sanctuaries and excretion of substrate drugs and PCs are considered, along with their possible interactions with the ABC and SLC transporters which influence these processes.

ρ⁰ Cells Feature De-Ubiquitination of SLC Transporters and Increased Levels and Fluxes of Amino Acids

Solute carrier (SLC) transporters are a diverse group of membrane transporter proteins that regulate the cellular flux and distribution of endogenous and xenobiotic compounds. Post-translational modifications (PTMs), such as ubiquitination, have recently emerged as one of the major regulatory mechanisms in protein function and localization. Previously, we showed that SLC amino acid transporters were on average 6-fold de-ubiquitinated and increased amino acid levels were detected in ρ⁰ cells (lacking mitochondrial DNA, mtDNA) compared to parental cells. Here, we elucidated the altered functionality of SLC transporters and their dynamic ubiquitination status by measuring the uptake of several isotopically labeled amino acids in both human osteosarcoma 143B.TK- and ρ⁰ cells. Our pulse chase analysis indicated that de-ubiquitinated amino acid transporters in ρ⁰ cells were accompanied by an increased transport rate, which leads to higher levels of amino acids in the cell. Finding SLC transport enhancers is an aim of the pharmaceutical industry in order to compensate for loss of function mutations in these genes. Thus, the ubiquitination status of SLC transporters could be an indicator for their functionality, but evidence for a direct connection between de-ubiquitination and transporter activity has to be further elucidated.

Role and modulation of drug transporters in HIV-1 therapy

Current treatment of human immunodeficiency virus type-1 (HIV-1) infection involves a combination of antiretroviral drugs (ARVs) that target different stages of the HIV-1 life cycle. This strategy is commonly referred to as highly active antiretroviral therapy (HAART) or combined antiretroviral therapy (cART). Membrane-associated drug transporters expressed ubiquitously in mammalian systems play a crucial role in modulating ARV disposition during HIV-1 infection. Members of the ATP-binding cassette (ABC) and solute carrier (SLC) transporter superfamilies have been shown to interact with ARVs, including those that are used as part of first-line treatment regimens. As a result, the functional expression of drug transporters can influence the distribution of ARVs at specific sites of infection. In addition, pathological factors related to HIV-1 infection and/or ARV therapy itself can alter transporter expression and activity, thus further contributing to changes in ARV disposition and the effectiveness of HAART. This review summarizes current knowledge on the role of drug transporters in regulating ARV transport in the context of HIV-1 infection.

An Organic Anion Transporter 1 (OAT1)-centered Metabolic Network

There has been a recent interest in the broader physiological importance of multispecific "drug" transporters of the SLC and ABC transporter families. Here, a novel multi-tiered systems biology approach was used to predict metabolites and signaling molecules potentially affected by the in vivo deletion of organic anion transporter 1 (Oat1, Slc22a6, originally NKT), a major kidney-expressed drug transporter. Validation of some predictions in wet-lab assays, together with re-evaluation of existing transport and knock-out metabolomics data, generated an experimentally validated, confidence ranked set of OAT1-interacting endogenous compounds enabling construction of an "OAT1-centered metabolic interaction network." Pathway and enrichment analysis indicated an important role for OAT1 in metabolism involving: the TCA cycle, tryptophan and other amino acids, fatty acids, prostaglandins, cyclic nucleotides, odorants, polyamines, and vitamins. The partly validated reconstructed network is also consistent with a major role for OAT1 in modulating metabolic and signaling pathways involving uric acid, gut microbiome products, and so-called uremic toxins accumulating in chronic kidney disease. Together, the findings are compatible with the hypothesized role of drug transporters in remote inter-organ and inter-organismal communication: The Remote Sensing and Signaling Hypothesis (Nigam, S. K. (2015) Nat. Rev. Drug Disc. 14, 29). The fact that OAT1 can affect many systemic biological pathways suggests that drug-metabolite interactions need to be considered beyond simple competition for the drug transporter itself and may explain aspects of drug-induced metabolic syndrome. Our approach should provide novel mechanistic insights into the role of OAT1 and other drug transporters implicated in metabolic diseases like gout, diabetes, and chronic kidney disease.

Efflux transporters in blood-brain interfaces of the developing brain

The cerebral microvessel endothelium forming the blood-brain barrier (BBB) and the epithelium of the choroid plexuses forming the blood-CSF barrier (BCSFB) operate as gatekeepers for the central nervous system. Exposure of the vulnerable developing brain to chemical insults can have dramatic consequences for brain maturation and lead to life-long neurological diseases. The ability of blood-brain interfaces to efficiently protect the immature brain is therefore an important pathophysiological issue. This is also key to our understanding of drug entry into the brain of neonatal and pediatric patients. Non-specific paracellular diffusion through barriers is restricted early during development, but other neuroprotective properties of these interfaces differ between the developing and adult brains. This review focuses on the developmental expression and function of various classes of efflux transporters. These include the multispecific transporters of the ATP-binding cassette transporter families ABCB, ABCC, ABCG, the organic anion and cation transporters of the solute carrier families SLC21/SLCO and SLC22, and the peptide transporters of the SLC15 family. These transporters play a key role in preventing brain entry of blood-borne molecules such as drugs, environmental toxicants, and endogenous metabolites, or else in increasing the clearance of potentially harmful organic ions from the brain. The limited data available for laboratory animals and human highlight transporter-specific developmental patterns of expression and function, which differ between blood-brain interfaces. The BCSFB achieves an adult phenotype earlier than BBB. Efflux transporters at the BBB appear to be regulated by various factors subsequently secreted by neural progenitors and astrocytes during development. Their expression is also modulated by oxidative stress, inflammation, and exposure to xenobiotic inducers. A better understanding of these regulatory pathways during development, in particular the signaling pathways triggered by oxidative stress and xenobiotics, may open new opportunities to therapeutic manipulation in view to improve or restore neuroprotective functions of the blood-brain interfaces in the context of perinatal injuries.

Transport mechanisms of hepatic uptake and bile excretion in clinical hepatobiliary scintigraphy with 99mTc-N-pyridoxyl-5-methyltryptophan

INTRODUCTION

In clinical hepatobiliary scintigraphy, (99m)Tc-N-pyridoxyl-5-methyltryptophan ((99m)Tc-PMT) is an effective radiotracer among the (99m)Tc-pyridoxylaminates. However, the mechanisms of human hepatic uptake and bile excretion transport of (99m)Tc-PMT have not been determined. We thus investigated the transport mechanisms of human hepatic uptake and bile excretion in hepatobiliary scintigraphy with (99m)Tc-PMT.

METHODS

Four solute carrier (SLC) transporters involved in hepatic uptake were evaluated using human embryonic kidney (HEK) and HeLa cells with high expression of SLC transporters (organic anion transporting polypeptide (OATP)1B1, OATP1B3, OATP2B1, organic anion transporters (OAT)2 and organic cation transporters (OCT)1) after 5 min of (99m)Tc-PMT incubation. Metabolic analysis of (99m)Tc-PMT was performed using pooled human liver S9. Adenosine triphosphate (ATP)-binding cassette (ABC) transporters for bile excretion were examined using hepatic ABC transporter vesicles human expressing multiple drug resistance 1 (MDR1), multidrug resistance-associated protein 2 (MRP2), breast cancer resistance protein or bile salt export pump. (99m)Tc-PMT was incubated for 1, 3 and 5 min with ATP or adenosine monophosphate and these vesicles. SPECT scans were performed in normal and Eisai hyperbilirubinemic (EHBR) model rats, deficient in Mrp2 transporters, without and with verapamil (rat Mdr1 and human MDR1 inhibitor) after intravenous injection of (99m)Tc-PMT.

RESULTS

Uptake of (99m)Tc-PMT in HEK293/OATP1B1 and HeLa/OATP1B3 was significantly higher than that in HEK293- and HeLa-mock cells. (99m)Tc-PMT was not metabolized in the human liver S9. In vesicles with high expression of ABC transporters, uptake of MDR1 or MRP2 was significantly higher at all incubation times. Bile excretion of (99m)Tc-PMT was also identified by comparison between normal and EHBR rats with and without verapamil on in-vivo imaging.

CONCLUSIONS

Human hepatic uptake of (99m)Tc-PMT was transferred by OATP1B1 and OATP1B3, and excretion into bile canaliculi via MDR1 and MRP2. (99m)Tc-PMT hepatobiliary scintigraphy may be a useful ligand as a noninvasive method of visualizing and quantifying hepatobiliary transporter functionality, which could predict drug pharmacokinetics.