Molecular aspects of renal anionic drug transport

Research Methods and New Advances in Drug-Drug Interactions Mediated by Renal Transporters

The kidney is critical in the human body's excretion of drugs and their metabolites. Renal transporters participate in actively secreting substances from the proximal tubular cells and reabsorbing them in the distal renal tubules. They can affect the clearance rates (CLr) of drugs and their metabolites, eventually influence the clinical efficiency and side effects of drugs, and may produce drug-drug interactions (DDIs) of clinical significance. Renal transporters and renal transporter-mediated DDIs have also been studied by many researchers. In this article, the main types of in vitro research models used for the study of renal transporter-mediated DDIs are membrane-based assays, cell-based assays, and the renal slice uptake model. In vivo research models include animal experiments, gene knockout animal models, positron emission tomography (PET) technology, and studies on human beings. In addition, in vitro-in vivo extrapolation (IVIVE), ex vivo kidney perfusion (EVKP) models, and, more recently, biomarker methods and in silico models are included. This article reviews the traditional research methods of renal transporter-mediated DDIs, updates the recent progress in the development of the methods, and then classifies and summarizes the advantages and disadvantages of each method. Through the sorting work conducted in this paper, it will be convenient for researchers at different learning stages to choose the best method for their own research based on their own subject's situation when they are going to study DDIs mediated by renal transporters.

Kidney Drug Transporters in Pharmacotherapy

The kidney functions not only as a metabolite elimination organ but also plays an important role in pharmacotherapy. The kidney tubule epithelia cells express membrane carriers and transporters, which play an important role in drug elimination, and can determine drug nephrotoxicity and drug-drug interactions, as well as constituting direct drug targets. The above aspects of kidney transport proteins are discussed in the review.

Regulation of Solute Carriers OCT2 and OAT1/3 in the Kidney: A Phylogenetic, Ontogenetic and Cell Dynamic Perspective

Over the course of more than 500 million years, the kidneys have undergone a remarkable evolution from primitive nephric tubes to intricate filtration-reabsorption systems that maintain homeostasis and remove metabolic end products from the body. The evolutionarily conserved solute carriers Organic Cation Transporter 2 (OCT2), and Organic Anion Transporters 1 and 3 (OAT1/3) coordinate the active secretion of a broad range of endogenous and exogenous substances, many of which accumulate in the blood of patients with kidney failure despite dialysis. Harnessing OCT2 and OAT1/3 through functional preservation or regeneration could alleviate the progression of kidney disease. Additionally, it would improve current in vitro test models that lose their expression in culture. With this review, we explore OCT2 and OAT1/3 regulation using different perspectives: phylogenetic, ontogenetic and cell dynamic. Our aim is to identify possible molecular targets to both help prevent or compensate for the loss of transport activity in patients with kidney disease, and to enable endogenous OCT2 and OAT1/3 induction in vitro in order to develop better models for drug development.

Membrane Carriers and Transporters in Kidney Physiology and Disease

The growing information suggests that chronic kidney disease may affect expression and function of membrane carriers and transporters in the kidney. The dysfunction of carriers and transporters entails deficient elimination of uremic solutes as well as xenobiotics (drugs and toxins) with subsequent clinical consequences. The renal carriers and transporters are also targets of drugs used in clinical practice, and intentional drug-drug interactions in the kidney are produced to increase therapeutic efficacy. The understanding of membrane carriers and transporters function in chronic kidney disease is important not only to better characterize drug pharmacokinetics, drug actions in the kidney, or drug-drug interactions but also to define the organ pathophysiology.

Extrahepatic Drug Transporters in Liver Failure: Focus on Kidney and Gastrointestinal Tract

Emerging information suggests that liver pathological states may affect the expression and function of membrane transporters in the gastrointestinal tract and the kidney. Altered status of the transporters could affect drug as well as endogenous compounds handling with subsequent clinical consequences. It seems that changes in intestinal and kidney transporter functions provide the compensatory activity of eliminating endogenous compounds (e.g., bile acids) generated and accumulated due to liver dysfunction. A literature search was conducted on the Ovid and PubMed databases to select relevant in vitro, animal and human studies that have reported expression, protein abundance and function of the gastrointestinal and kidney operating ABC (ATP-binding cassette) transporters and SLC (solute carriers) carriers. The accumulated data suggest that liver failure-associated transporter alterations in the gastrointestinal tract and kidney may affect drug pharmacokinetics. The altered status of drug transporters in those organs in liver dysfunction conditions may provide compensatory activity in handling endogenous compounds, affecting local drug actions as well as drug pharmacokinetics.

Influene of Pharmaceutical Excipients on the Membrane Transport of a P-glycoprotein Substrate in the Rat Small Intestine

BACKGROUND AND OBJECTIVES

Generic drugs are generally used worldwide because of affordability compared to brand-name drugs. One of the main differences between brand-name and generic drugs is pharmaceutical excipients. We previously reported the effects of pharmaceutical excipients on the membrane permeation of drugs via the paracellular and transcellular routes, which are passive transport routes. P-glycoprotein (P-gp) is a typical ATP-binding cassette transporter and is mostly responsible for drug-drug interactions involving transporters. In the present study, rhodamine 123 (Rho123) was selected as the P-gp substrate, and the effects of pharmaceutical excipients on its membrane transport in the rat jejunum and ileum were examined.

METHODS

Twenty major pharmaceutical excipients widely used in the pharmaceutical industry were selected. The in vitro diffusion chamber method using the rat jejunum and ileum was employed to investigate the effects of pharmaceutical excipients on the membrane permeation of Rho123.

RESULTS

The results obtained showed that the membrane permeability of Rho123 significantly (P < 0.05) changed under certain dosage conditions of pharmaceutical excipients such as sodium carboxymethyl starch, pullulan, glyceryl monostearate and so on. Furthermore, the effects of pharmaceutical excipients were site specific in the small intestine.

CONCLUSION

The present results demonstrated that some pharmaceutical excipients altered the membrane permeability of Rho123 in the rat small intestine.

In Silico Prediction of Human Renal Clearance of Compounds Using Quantitative Structure-Pharmacokinetic Relationship Models

Renal clearance (CLr) plays an essential role in the elimination of drugs. In this study, 636 compounds were obtained from various sources to develop in silico models for the prediction of CLr. Stepwise multiple linear regression and random forest regression methods were employed to build global models and local models according to ionization state or net elimination pathways. The local models toward compounds undergoing different net elimination pathways showed good predictive power: the geometric mean fold error was close to 2, indicating the clearance of most compounds could be predicted within a 2-fold error range. Six classification methods were used to construct classification models. However, the performance of these classification models was less than satisfactory, and the mean accuracy of the top five models in test sets was 0.65. Moreover, qualitative analysis of physicochemical profiles between compounds undergoing different net elimination pathways revealed that compounds with higher lipophilicity tended to be reabsorbed more easily and showed lower CLr, while compounds with higher values of polar descriptors tended to secrete more easily and showed higher CLr.

Overview of organic anion transporters and organic anion transporter polypeptides and their roles in the liver

Organic anion transporters (OATs) and organic anion transporter polypeptides (OATPs) are classified within two SLC superfamilies, namely, the SLC22A superfamily and the SLCO superfamily (formerly the SLC21A family), respectively. They are expressed in many tissues, such as the liver and kidney, and mediate the absorption and excretion of many endogenous and exogenous substances, including various drugs. Most are composed of 12 transmembrane polypeptide chains with the C-terminus and the N-terminus located in the cell cytoplasm. OATs and OATPs are abundantly expressed in the liver, where they mainly promote the uptake of various endogenous substrates such as bile acids and various exogenous drugs such as antifibrotic and anticancer drugs. However, differences in the locations of glycosylation sites, phosphorylation sites, and amino acids in the OAT and OATP structures lead to different substrates being transported to the liver, which ultimately results in their different roles in the liver. To date, few articles have addressed these aspects of OAT and OATP structures, and we study further the similarities and differences in their structures, tissue distribution, substrates, and roles in liver diseases.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

Unraveling the functional role of the orphan solute carrier, SLC22A24 in the transport of steroid conjugates through metabolomic and genome-wide association studies

Variation in steroid hormone levels has wide implications for health and disease. The genes encoding the proteins involved in steroid disposition represent key determinants of interindividual variation in steroid levels and ultimately, their effects. Beginning with metabolomic data from genome-wide association studies (GWAS), we observed that genetic variants in the orphan transporter, SLC22A24 were significantly associated with levels of androsterone glucuronide and etiocholanolone glucuronide (sentinel SNPs p-value <1x10-30). In cells over-expressing human or various mammalian orthologs of SLC22A24, we showed that steroid conjugates and bile acids were substrates of the transporter. Phylogenetic, genomic, and transcriptomic analyses suggested that SLC22A24 has a specialized role in the kidney and appears to function in the reabsorption of organic anions, and in particular, anionic steroids. Phenome-wide analysis showed that functional variants of SLC22A24 are associated with human disease such as cardiovascular diseases and acne, which have been linked to dysregulated steroid metabolism. Collectively, these functional genomic studies reveal a previously uncharacterized protein involved in steroid homeostasis, opening up new possibilities for SLC22A24 as a pharmacological target for regulating steroid levels.

A simple method for the isolation and detailed characterization of primary human proximal tubule cells for renal replacement therapy

The main physiological functions of renal proximal tubule cells in vivo are reabsorption of essential nutrients from the glomerular filtrate and secretion of waste products and xenobiotics into urine. Currently, there are several established cell lines of human origin available as in vitro models of proximal tubule. However, these cells appeared to be limited in their biological relevance, because essential characteristics of the original tissue are lost once the cells are cultured. As a consequence of these limitations, primary human proximal tubule cells constitute a suitable and a biologically more relevant in vitro model to study this specific segment of the nephron and therefore, these cells can play an important role in renal regenerative medicine applications. Here, we describe a protocol to isolate proximal tubule cells from human nephrectomies. We explain the steps performed for an in-depth characterization of the cells, including the study of markers from others segments of the nephron, with the goal to determine the purity of the culture and the stability of proteins, enzymes, and transporters along time. The human proximal tubule cells isolated and used throughout this study showed many proximal tubule characteristics, including monolayer organization, cell polarization with the expression of tight junctions and primary cilia, expression of proximal tubule–specific proteins, such as megalin and sodium/glucose cotransporter 2, among others. The cells also expressed enzymatic activity for dipeptidyl peptidase IV, as well as for gamma glutamyl transferase 1, and expressed transporter activity for organic anion transporter 1, P-glycoprotein, multidrug resistance proteins, and breast cancer resistance protein. In conclusion, characterization of our cells confirmed presence of putative proximal tubule markers and the functional expression of multiple endogenous organic ion transporters mimicking renal reabsorption and excretion. These findings can constitute a valuable tool in the development of bioartificial kidney devices.

Identification of the multivalent PDZ protein PDZK1 as a binding partner of sodium-coupled monocarboxylate transporter SMCT1 (SLC5A8) and SMCT2 (SLC5A12)

Sodium-coupled monocarboxylate transporters SMCT1 (SLC5A8) and SMCT2 (SLC5A12) mediate the high- and low-affinity transport of lactate in the kidney, but their regulatory mechanism is still unknown. Since these two transporters have the PDZ-motif at their C-terminus, the function of SMCTs may be modulated by a protein-protein interaction. To investigate the binding partner(s) of SMCTs in the kidney, we performed yeast two-hybrid (Y2H) screenings of a human kidney cDNA library with the C-terminus of SMCT1 (SMCT1-CT) and SMCT2 (SMCT2-CT) as bait. PDZK1 was identified as a partner for SMCTs. PDZK1 coexpression in SMCT1-expressing HEK293 cells enhanced their nicotinate transport activity. PDZK1, SMCT1, and URAT1 in vitro assembled into a single tri-molecular complex and their colocalization was confirmed in the renal proximal tubule in vivo by immunohistochemistry. These results indicate that the SMCT1-PDZK1 interaction thus plays an important role in both lactate handling as well as urate reabsorption in the human kidney.

Carbon Nanotube Reinforced Supramolecular Hydrogels for Bioapplications

Nanocomposite hydrogels based on carbon nanotubes (CNTs) are known to possess remarkable stiffness, electrical, and thermal conductivity. However, they often make use of CNTs as fillers in covalently cross-linked hydrogel networks or involve direct cross-linking between CNTs and polymer chains, limiting processability properties. Herein, nanocomposite hydrogels are developed, in which CNTs are fillers in a physically cross-linked hydrogel. Supramolecular nanocomposites are prepared at various CNT concentrations, ranging from 0.5 to 6 wt%. Incorporation of 3 wt% of CNTs leads to an increase of the material's toughness by over 80%, and it enhances electrical conductivity by 358%, compared to CNT-free hydrogel. Meanwhile, the nanocomposite hydrogels maintain thixotropy and processability, typical of the parent hydrogel. The study also demonstrates that these materials display remarkable cytocompatibility and support cell growth and proliferation, while preserving their functional activities. These supramolecular nanocomposite hydrogels are therefore promising candidates for biomedical applications, in which both toughness and electrical conductivity are important parameters.

Liver Receptor Homolog-1 Regulates Organic Anion Transporter 2 and Docetaxel Pharmacokinetics

Organic anion transporter 2 (OAT2/SLC22A7) is an uptake transporter that plays an important role in drug disposition. Here, we investigate a potential role of liver receptor homolog-1 (Lrh-1) in regulation of Oat2 and docetaxel pharmacokinetics. Hepatoma cells (Hepa1-6 and HepG2 cells) were transfected with Lrh-1/LRH-1 expression vector or siRNA. The relative mRNA and protein levels of Oat2/OAT2 in the cells or livers of Lrh-1^hep-/- mice were determined by qPCR and Western blotting, respectively. Transcriptional regulation of Oat2/OAT2 by Lrh-1/LRH-1 was investigated using luciferase reporter, mobility shift, and chromatin immunoprecipitation (ChIP) assays. Pharmacokinetic studies were performed with wild-type (Lrh-1^fl/fl) and Lrh-1^hep-/- mice after intraperitoneal injection of docetaxel. Overexpression of Lrh-1 in Hepa1-6 cells led to significant increases in Oat2 mRNA and protein. Consistently, Lrh-1 knockdown caused decreases in Oat2 mRNA and protein, as well as reduced cellular uptake of PGE2, a prototypical substrate of Oat2. Similarly, an activation effect of LRH-1 on OAT2 expression was observed in HepG2 cells. In addition, the levels of Oat2 mRNA and protein were markedly reduced in Lrh-1^hep-/- mice. Lrh-1/LRH-1 induced the transcription of Oat2/OAT2 in luciferase reporter assays. Truncation analysis revealed a potential Lrh-1 response element (-716- to -702-bp) in Oat2 promoter. Direct binding of Lrh-1 to this response element was confirmed by mobility shift and ChIP assays. Furthermore, systemic exposure of docetaxel was upregulated in Lrh-1^hep-/- mice due to reduced hepatic uptake. In conclusion, Lrh-1 transcriptionally regulates Oat2, thereby impacting tissue uptake and pharmacokinetics of Oat2 substrates.

Enhanced neuroprotective effect of mild-hypothermia with VPA against ethanol-mediated neuronal injury

INTRODUCTION

Progress in understanding pathophysiological mechanisms and the development of targeted regenerative strategies have been hampered by the lack of predictive disease models, specifically for the conditions to which affected cell types are inaccessible. The present study has aimed to unearth the role of valproic acid (VPA) and mild hypothermia (MH) as promising strategy to enhance the neuroprotective mechanisms in undifferentiated and differentiated human neural precursor cells (hNPCs) against ethanol-induced damage.

METHODS

5mM VPA alone or in combination with MH (33°C) was used to prevent the damage in proliferating and differentiating hNPCs. CD133+ve enriched hNPCs were cultured in vitro and exposed to 1M chronic ethanol concentration for 72h and followed by VPA and MH treatment for 24h. Morphometric analysis was performed to identify changes in neurospheres development and neuronal cell phenotypes. Flow cytometry and RT-qPCR analysis was performed to investigate alterations in key molecular pathways involved in cell survival and signaling.

RESULTS

Combination of VPA with MH displayed higher proportion of neuronal cell viability as compared to single treatment. Combination treatment was most effective in reducing apoptosis and reactive oxygen species levels in both the undifferentiated and differentiated hNPCs. VPA with MH significantly improved neuronal cell phenotype, active chromatin modeling, chaperon and multi-drug resistant pumps activity and expression of neuronal signaling molecules.

CONCLUSION

The study provided an efficient and disease specific in vitro model and demonstrated that combined treatment with VPA and MH activates several neuroprotective mechanisms and provides enhanced protection against ethanol-induced damage in cultured undifferentiated and differentiated hNPCs.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

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.

Cellular Models and In Vitro Assays for the Screening of modulators of P-gp, MRP1 and BCRP

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are highly expressed in tumor cells, as well as in organs involved in absorption and secretion processes, mediating the ATP-dependent efflux of compounds, both endogenous substances and xenobiotics, including drugs. Their expression and activity levels are modulated by the presence of inhibitors, inducers and/or activators. In vitro, ex vivo and in vivo studies with both known and newly synthesized P-glycoprotein (P-gp) inducers and/or activators have shown the usefulness of these transport mechanisms in reducing the systemic exposure and specific tissue access of potentially harmful compounds. This article focuses on the main ABC transporters involved in multidrug resistance [P-gp, multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP)] expressed in tissues of toxicological relevance, such as the blood-brain barrier, cardiovascular system, liver, kidney and intestine. Moreover, it provides a review of the available cellular models, in vitro and ex vivo assays for the screening and selection of safe and specific inducers and activators of these membrane transporters. The available cellular models and in vitro assays have been proposed as high throughput and low-cost alternatives to excessive animal testing, allowing the evaluation of a large number of compounds.

ABCG2 is a potential prognostic marker of overall survival in patients with clear cell renal cell carcinoma

Background

ATP-binding cassette sub-family G member 2 (ABCG2) is a semi-transport protein that plays a major role in multidrug resistance. We aimed to evaluate the prognostic significance of ABCG2 expression in patients with clear cell renal cell carcinoma.

Methods

From 2008 to 2013, 120 patients with clear cell kidney cancer underwent surgery with paraffin-embedded specimens and necessary clinical information available. Immunohistochemistry staining was performed to grade the expression of ABCG2 as ABCG2(−): less than 10% of tumor cells stained; ABCG2(+): weak membrane staining; and ABCG2(++): moderate or strong membrane staining. The overall survival was analyzed using Kaplan-Meier method. Multivariable Cox regression evaluated the independent predictors for overall survival.

Results

ABCG2(−) was diagnosed in 57 (48%) patients, ABCG2(+) in 52 (43%) patients, and ABCG2 (++) in 11(9.2%) patients. ABCG2 expression significantly correlated with the five-year survival (p < 0.001) and distant metastasis (p = 0.001). In the multivariable analysis, besides Fuhrman grade, the ABCG2 expression was an independent prognostic marker for overall survival (p < 0.001) when incorporating other relevant tumor and clinical parameters (HR = 3.84, 95% CI: 1.92–7.70).

Conclusion

The current data suggests that ABCG2 may serve as a prognostic marker for overall survival in patients with clear cell renal cell carcinoma. Further studies with large cohorts of patients will be essential for validating these findings and defining the clinical utility of ABCG2 in the patient population.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3224-6) contains supplementary material, which is available to authorized users.

Tenofovir Disoproxil Fumarate Is a New Substrate of ATP-Binding Cassette Subfamily C Member 11

Tenofovir disoproxil fumarate (TDF), a nucleotide reverse transcriptase inhibitor, after conversion to tenofovir (TFV), is mainly eliminated by glomerular filtration and active tubular secretion. The major adverse effect of tenofovir is nephrotoxicity; however, the exact mechanism remains poorly understood. In this study, the ATP-binding cassette subfamily C member 11 (ABCC11; multidrug resistance protein 8 [MRP8]) transporter, which is abundant in proximal tubular cells, was demonstrated to act as an efflux transporter of tenofovir. Real-time PCR (RT-PCR) and indirect immunofluorescence assays were used to determine MRP8 overexpression in a continuous cell line. Tenofovir accumulations were assessed by cytotoxicity, cellular transport, and vesicular uptake assays. Substrate specificity was confirmed using MK-571, an MRP-specific inhibitor, and methotrexate, which served as a known substrate. Intracellular and intravesicular concentrations of tenofovir were determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The 50% cytotoxic concentration (CC₅₀) of TDF in MRP8-overexpressing cells was 4.78 times higher than that of parental cells. Transport assays also showed that the intracellular accumulation of tenofovir in MRP8-overexpressing cells was 55 times lower than that in parental cells and was partly reversed by MK-571. Similarly, an "inside-out" vesicular uptake assay, using Sf9 inverted membrane vesicles to allow measuring of accumulation of the substrates into the vesicles, demonstrated a higher intravesicular concentration of tenofovir in MRP8-overexpressing vesicles than in Sf9 insect control vesicles. These effects were effectively reversed by increasing concentrations of the specific inhibitor MK-571. In conclusion, tenofovir is a new substrate of the MRP8 transporter. An alteration in the activity of this efflux pump may increase the intracellular accumulation of tenofovir in proximal renal tubular cells.

Application of Caco-2 cell line in herb-drug interaction studies: current approaches and challenges

The Caco-2 model is employed in pre-clinical investigations to predict the likely gastrointestinal permeability of drugs because it expresses cytochrome P450 enzymes, transporters, microvilli and enterocytes of identical characteristics to the human small intestine. The FDA recommends this model as integral component of the Biopharmaceutics Classification System (BCS). Most dedicated laboratories use the Caco-2 cell line to screen new chemical entities through prediction of its solubility, bioavailability and the possibility of drug-drug or herb-drug interactions in the gut lumen. However, challenges in the inherent characteristics of Caco-2 cell and inter-laboratory protocol variations have resulted to generation of irreproducible data. These limitations affect the extrapolation of data from pre-clinical research to clinical studies involving drug-drug and herb-drug interactions. This review addresses some of these caveats and enumerates the plausible current and future approaches to reduce the anomalies associated with Caco-2 cell line investigations focusing on its application in herb-drug interactions.

Quantitative structure-pharmacokinetic relationships for the prediction of renal clearance in humans

Renal clearance (CLR), a major route of elimination for many drugs and drug metabolites, represents the net result of glomerular filtration, active secretion and reabsorption, and passive reabsorption. The aim of this study was to develop quantitative structure-pharmacokinetic relationships (QSPKR) to predict CLR of drugs or drug-like compounds in humans. Human CLR data for 382 compounds were obtained from the literature. Step-wise multiple linear regression was used to construct QSPKR models for training sets and their predictive performance was evaluated using internal validation (leave-one-out method). All qualified models were validated externally using test sets. QSPKR models were also constructed for compounds in accordance with their 1) net elimination pathways (net secretion, extensive net secretion, net reabsorption, and extensive net reabsorption), 2) net elimination clearances (net secretion clearance, CLSEC; or net reabsorption clearance, CLREAB), 3) ion status, and 4) substrate/inhibitor specificity for renal transporters. We were able to predict 1) CLREAB (Q(2) = 0.77) of all compounds undergoing net reabsorption; 2) CLREAB (Q(2) = 0.81) of all compounds undergoing extensive net reabsorption; and 3) CLR for substrates and/or inhibitors of OAT1/3 (Q(2) = 0.81), OCT2 (Q(2) = 0.85), MRP2/4 (Q(2) = 0.78), P-gp (Q(2) = 0.71), and MATE1/2K (Q(2) = 0.81). Moreover, compounds undergoing net reabsorption/extensive net reabsorption predominantly belonged to Biopharmaceutics Drug Disposition Classification System classes 1 and 2. In conclusion, constructed parsimonious QSPKR models can be used to predict CLR of compounds that 1) undergo net reabsorption/extensive net reabsorption and 2) are substrates and/or inhibitors of human renal transporters.

Renal transport of organic anions and cations

Organic anions and cations (OAs and OCs, respectively) comprise an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. The kidney, primarily the renal proximal tubule, plays a critical role in regulating the plasma concentrations of these organic electrolytes and in clearing the body of potentially toxic xenobiotics agents, a process that involves active, transepithelial secretion. This transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. Basolateral and luminal OA and OC transport reflects the concerted activity of a suite of separate proteins arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney, now allows the development of models describing the molecular basis of the renal secretion of OAs and OCs. New information on naturally occurring genetic variation of many of these processes provides insight into the basis of observed variability of drug efficacy and unwanted drug-drug interactions in human populations. The present review examines recent work on these issues.

Genomic and expression analysis of a solute carrier protein (CcSLC25a5) gene from Cyprinus carpio Linnaeus

Using the Genefishing method, we identified seven potential regulatory genes involved in the process of scale morphogenesis in fishes. We further characterized a novel solute carrier protein gene (CcSLC), from the common carp which is differentially expressed in mirror carp and Jianli. The ORF encodes a peptide of 298 amino acids with a molecular mass of 31.5 kDa and a theoretical isoelectric point of 7.49. ScanProsite analysis indicated that it is a putative solute carrier protein that contains a substrate binding site. CcSLC was detected in carp embryos by in situ hybridization in the 70%-epiboly, 6-somite, and 14-somite embryonic stages. Gene expression stopped at the long pec stage. However, CcSLC25a5 was re-expressed during the initiation of scale formation in the regions that were scale covered. These findings provide novel insights into the features of early carp embryo and scale development.

Drug discovery for hyperuricemia

Hyperuricemia is associated with an increased risk of developing gout. This increases with the degree and duration of hyperuricemia. Gout can be managed by dietary modification and pharmacologic urate-lowering therapies. The recent identification of the renal apical urate/anion exchanger URAT1 (SLC22A12) and several membrane proteins relevant to the transport of urate play an important role in gaining a better understanding of the mode of action of many drugs used to treat gout. As described in this review, therapeutics designed to modify URAT1 transport activities might be useful in treating pathologies associated with hyperuricemia such as gout and urolithiasis. Continuing studies into the urate transportsome hold promise for the development of new, more effective therapeutics for hyperuricemia.

The Effects of Ergoloid Mesylates and Ginkgo Biloba on the Pharmacokinetics of Ticlopidine

Ticlopidine is sometimes coadministered with ergoloid mesylates or ginkgo biloba in clinical situations. Our objective was to examine the effect of ergoloid mesylates and ginkgo biloba on ticlopidine pharmacokinetics. Ticlopidine, ergoloid mesylates, and ginkgo biloba significantly inhibited the organic anion transporting polypeptide (OATP-B)-mediated uptake of [(3)H]-estrone-3-sulfate in a concentration-dependent manner. When ergoloid mesylates was coadministered with ticlopidine, the ticlopidine area under the plasma drug concentration-time profile (AUC) from 0 to 12 hours was decreased 30% and the peak plasma drug concentration (C(max)) was decreased 29%, compared with ticlopidine administration alone. There were no significant changes in the pharmacokinetic parameters of ticlopidine when it was coadministered with ginkgo biloba. In summary, ergoloid mesylates is a more potent inhibitor of OATP-B than is ginkgo biloba, and it can reduce the oral bioavailability of drugs transported by OATP-B. Ergoloid mesylates markedly decreased the AUC and C(max) of ticlopidine, probably by inhibiting the OATP-B-mediated uptake of ticlopidine during the intestinal absorption phase. The results support a new model of intestinal drug-drug interaction.

Regulation of cAMP homeostasis by the efflux protein MRP4 in cardiac myocytes

Recent studies indicate that members of the multidrug-resistance protein (MRP) family belonging to ATP binding cassette type C (ABCC) membrane proteins extrude cyclic nucleotides from various cell types. This study aimed to determine whether MRP proteins regulate cardiac cAMP homeostasis. Here, we demonstrate that MRP4 is the predominant isoform present at the plasma membrane of cardiacmyocytes and that it mediates the efflux of cAMP in these cells. MRP4-deficient mice displayed enhanced cardiac myocyte cAMP formation, contractility, and cardiac hypertrophy at 9 mo of age, an effect that was compensated transiently by increased phosphodiesterase expression at young age. These findings suggest that cAMP extrusion via MRP4 acts together with phosphodiesterases to control cAMP levels in cardiac myocytes.

Kidney regeneration and resident stem cells

Given its complexity, high metabolic activity and excretory functions, the kidney is particularly susceptible to acute ischemic and toxin-mediated injury. Current therapies do not facilitate kidney regeneration, and there is an increasing interest in newer therapies that are based on cellular sources of kidney regeneration, such as stem cell therapy. Our understanding of cellular sources for kidney regeneration and stem cells present in the adult kidney has dramatically evolved over the recent years. Herein, we discuss the current understanding of kidney stem cells present in the adult mammalian kidney and their role in kidney regeneration. We have also summarized the best available evidence supporting the role of stem cells in kidney regeneration.

Aristolochic acid I induced oxidative DNA damage associated with glutathione depletion and ERK1/2 activation in human cells

Aristolochic acid I (AAI) has been widely found in herbal remedies and linked to the development of nephropathy and urothelial carcinoma in humans. This study elucidated the mechanism of oxidative stress and DNA damage mediated by AAI in human cells. Treatment of human promyelocytic leukemia cells (HL-60) and human renal proximal tubular cells (HK-2) with AAI led to a dose-dependent increase of reactive oxygen species (ROS). AAI also elevated the levels of DNA strand breaks and 8-hydroxy guanosine in HL-60 and HK-2 cells. Antioxidants, including Tiron, N-acetyl-l-cysteine (NAC) and glutathione (GSH), effectively suppressed the AAI-induced ROS and AAI-elicited genotoxicity, indicating that AAI induced the DNA damage through oxidative stress. GSH depletion was also found in AAI-treated cultures and proceeded prior to ROS formation. Exposure of HL-60 cells with AAI activated both ERK1/2 and p38 kinase phosphorylation, while only MEK1/2 inhibitor, U0126, significantly decreased AAI-mediated ROS. Preincubation of cells with thiol-containing compounds (NAC and GSH) inhibited the caspase 3 activity triggered by AAI, but non-thiol Tiron did not show a similar effect. This study demonstrated that AAI treatment results in oxidative stress-related DNA damage through GSH depletion and ERK1/2 activation; AAI-induced apoptosis is associated with GSH loss, but is independent of ROS generation.

Molecular biology of the blood-brain and the blood-cerebrospinal fluid barriers: similarities and differences

Efficient processing of information by the central nervous system (CNS) represents an important evolutionary advantage. Thus, homeostatic mechanisms have developed that provide appropriate circumstances for neuronal signaling, including a highly controlled and stable microenvironment. To provide such a milieu for neurons, extracellular fluids of the CNS are separated from the changeable environment of blood at three major interfaces: at the brain capillaries by the blood-brain barrier (BBB), which is localized at the level of the endothelial cells and separates brain interstitial fluid (ISF) from blood; at the epithelial layer of four choroid plexuses, the blood-cerebrospinal fluid (CSF) barrier (BCSFB), which separates CSF from the CP ISF, and at the arachnoid barrier. The two barriers that represent the largest interface between blood and brain extracellular fluids, the BBB and the BCSFB, prevent the free paracellular diffusion of polar molecules by complex morphological features, including tight junctions (TJs) that interconnect the endothelial and epithelial cells, respectively. The first part of this review focuses on the molecular biology of TJs and adherens junctions in the brain capillary endothelial cells and in the CP epithelial cells. However, normal function of the CNS depends on a constant supply of essential molecules, like glucose and amino acids from the blood, exchange of electrolytes between brain extracellular fluids and blood, as well as on efficient removal of metabolic waste products and excess neurotransmitters from the brain ISF. Therefore, a number of specific transport proteins are expressed in brain capillary endothelial cells and CP epithelial cells that provide transport of nutrients and ions into the CNS and removal of waste products and ions from the CSF. The second part of this review concentrates on the molecular biology of various solute carrier (SLC) transport proteins at those two barriers and underlines differences in their expression between the two barriers. Also, many blood-borne molecules and xenobiotics can diffuse into brain ISF and then into neuronal membranes due to their physicochemical properties. Entry of these compounds could be detrimental for neural transmission and signalling. Thus, BBB and BCSFB express transport proteins that actively restrict entry of lipophilic and amphipathic substances from blood and/or remove those molecules from the brain extracellular fluids. The third part of this review concentrates on the molecular biology of ATP-binding cassette (ABC)-transporters and those SLC transporters that are involved in efflux transport of xenobiotics, their expression at the BBB and BCSFB and differences in expression in the two major blood-brain interfaces. In addition, transport and diffusion of ions by the BBB and CP epithelium are involved in the formation of fluid, the ISF and CSF, respectively, so the last part of this review discusses molecular biology of ion transporters/exchangers and ion channels in the brain endothelial and CP epithelial cells.

Human sodium phosphate transporter 4 (hNPT4/SLC17A3) as a common renal secretory pathway for drugs and urate

The evolutionary loss of hepatic urate oxidase (uricase) has resulted in humans with elevated serum uric acid (urate). Uricase loss may have been beneficial to early primate survival. However, an elevated serum urate has predisposed man to hyperuricemia, a metabolic disturbance leading to gout, hypertension, and various cardiovascular diseases. Human serum urate levels are largely determined by urate reabsorption and secretion in the kidney. Renal urate reabsorption is controlled via two proximal tubular urate transporters: apical URAT1 (SLC22A12) and basolateral URATv1/GLUT9 (SLC2A9). In contrast, the molecular mechanism(s) for renal urate secretion remain unknown. In this report, we demonstrate that an orphan transporter hNPT4 (human sodium phosphate transporter 4; SLC17A3) was a multispecific organic anion efflux transporter expressed in the kidneys and liver. hNPT4 was localized at the apical side of renal tubules and functioned as a voltage-driven urate transporter. Furthermore, loop diuretics, such as furosemide and bumetanide, substantially interacted with hNPT4. Thus, this protein is likely to act as a common secretion route for both drugs and may play an important role in diuretics-induced hyperuricemia. The in vivo role of hNPT4 was suggested by two hyperuricemia patients with missense mutations in SLC17A3. These mutated versions of hNPT4 exhibited reduced urate efflux when they were expressed in Xenopus oocytes. Our findings will complete a model of urate secretion in the renal tubular cell, where intracellular urate taken up via OAT1 and/or OAT3 from the blood exits from the cell into the lumen via hNPT4.

Fluo-cAMP is transported by multidrug resistance-associated protein isoform 4 in rat choroid plexus

The choroid plexuses (CP) are responsible for transport of micronutrients into brain and clearance of toxic compounds, in addition to its barrier function and production of CSF. Multidrug resistance-associated protein (Mrp) 4 is one transport protein highly expressed in CP tissue and is characterized as a versatile pump for toxicants and signalling molecules. Aim of the study was to determine transport characteristics of a fluorescent cAMP analog in rat CP and to define whether fluo-cAMP can be used for analyses of function, substrate/inhibitor specificity and regulation of Mrp4. Confocal imaging was used to analyze transport mechanisms in absence and presence of various modulators of organic anion transport in freshly isolated and functionally intact CP. Fluo-cAMP transport was saturable, selective, concentrative and metabolism-dependent, following an active two-step mechanism composed of apical uptake into epithelial cells and basolateral efflux. Uptake included a Na(+) -dependent and a Na(+) -independent component and was inhibited by estrone sulfate, taurocholate and sildenafil indicating involvement of organic anion transporting polypeptide Oatp1a5. Efflux was composed of an indirect Na(+) -dependent component and a component inhibitable by, for example, the MRP4 substrates/inhibitors, sulindac sulfide and 4-(2-aminoethyl) benzenesulfonyl fluoride. Therefore, fluo-cAMP can be used as fluorescent model compound for studying involvement of Mrp4 in signalling pathways and neuroprotection in CP.

Characterization of immortalized choroid plexus epithelial cell lines for studies of transport processes across the blood-cerebrospinal fluid barrier

BACKGROUND

Two rodent choroid plexus (CP) epithelial cell lines, Z310 and TR-CSFB, were compared with primary rat CP epithelial cells and intact CP tissue with respect to transport protein expression, function and tight junction (TJ) formation.

METHODS

For expression profiles of transporters and TJ proteins, qPCR and western blot analysis were used. Uptake assays were performed to study the functional activity of transporters and TJ formation was measured by trans-epithelial electrical resistance (TEER) and visualized by electron microscopy.

RESULTS

The expression of known ATP-binding cassette (Abc) transporter and solute carrier (Slc) genes in CP was confirmed by qPCR. Primary cells and cell lines showed similar, but overall lower expression of Abc transporters and absent Slc expression when compared to intact tissue. Consistent with this Mrp1, Mrp4 and P-gp protein levels were higher in intact CP compared to cell lines. Functionality of P-gp and Mrp1 was confirmed by Calcein-AM and CMFDA uptake assays and studies using [3H]bis-POM-PMEA as a substrate indicated Mrp4 function. Cell lines showed low or absent TJ protein expression. After treatment of cell lines with corticosteroids, RNA expression of claudin1, 2 and 11 and occludin was elevated, as well as claudin1 and occludin protein expression. TJ formation was further investigated by freeze-fracture electron microscopy and only rarely observed. Increases in TJ particles with steroid treatment were not accompanied by an increase in transepithelial electrical resistance (TEER).

CONCLUSION

Taken together, immortalized cell lines may be a tool to study transport processes mediated by P-gp, Mrp1 or Mrp4, but overall expression of transport proteins and TJ formation do not reflect the situation in intact CP tissue.

Targeting drug transporters - combining in silico and in vitro approaches to predict in vivo

Transporter proteins are expressed throughout the human body in different vital organs. They play an important role to various extents in determining absorption, distribution, metabolism, excretion, and toxicity (ADME/Tox) properties of therapeutic molecules. Over the past decade, numerous drug transporters have been cloned and considerable progress has been made toward understanding the molecular characteristics of individual transporters. In this chapter several in vitro and in silico techniques are described with applications to understand transporter behavior. These include employing new techniques to rapidly identify novel ligands for transporters. Ultimately these methods should lead to a greater overall appreciation of the role of transporters in vivo.

Renal membrane transport of glutathione in toxicology and disease

Membrane transport processes, at both the plasma membranes and intracellular membranes, play critical roles in renal function and are a determining factor in the susceptibility of renal epithelial cells to blood-borne drugs and toxic chemicals. Proximal tubular epithelial cells possess a large array of transport proteins for organic anions, organic cations, and peptides on both basolateral and brush-border plasma membranes. Although these transporters function in excretion of waste products and reabsorption of nutrients, they also play a role in the susceptibility of the kidneys to drugs and other toxicants in the blood. The proximal tubules are typically the primary target cells because they are the first epithelial cell population exposed to such chemicals in either the renal plasma or glomerular filtrate and because of their large array of membrane transporters. Besides transport across the basolateral and brush-border plasma membranes, transport across intracellular membranes such as the mitochondrial inner membrane is a critical determinant of metabolite distribution. To illustrate the function of these transporters, carrier-mediated processes for transport of the tripeptide and antioxidant glutathione across the basolateral, brush-border, and mitochondrial inner membranes of the renal proximal tubule are reviewed. Studies are summarized that have identified the involvement of specific carrier proteins and characterized the role of these transporters in glutathione metabolism and turnover, susceptibility of the proximal tubules to oxidative and other stresses, and modulation in disease and other pathological processes.

Molecular mechanism of ochratoxin a transport in the kidney

The mycotoxin, ochratoxin A (OTA), is thought to be responsible for Balkan endemic nephropathy. OTA accumulates in several tissues, especially in the kidneys and liver. The excretion of OTA into urine is thought to be mainly by tubular secretion, presumably via the organic anion transport system. Recently, several families of multispecific organic anion transporters have been identified: organic anion transporters (OATs), organic anion-transporting polypeptides (OATPs), oligopeptide transporters (PEPTs), and ATP-binding cassette (ABC) transporters, such as MRP2 and BCRP. These renal transporters mediate the transmembrane transport of OTA and play a pivotal role in the development of OTA-induced nephrotoxicity.

Organic anion transporter 6 (Slc22a20) specificity and Sertoli cell-specific expression provide new insight on potential endogenous roles

Organic anion transporter 6 (Oat6; Slc22a20), a member of the OAT family, was demonstrated previously to mediate the transport of organic anions (Am J Physiol Renal Physiol 291:F314-F321, 2006). In the present study, we sought to further delineate the function of murine Oat6 (mOat6) by analyzing the effect of select organic anions on mOat6-mediated transport by using a Chinese hamster ovary (CHO) cell line stably expressing mOat6 (CHO-mOat6). When examined, kinetic analysis demonstrated that the mechanism of inhibition of mOat6 and mOat3 was competitive. Homovanillic acid, 5-hydroxyindole acetic acid, 2,4-dihydroxyphenylacetate, hippurate, and dehydroepiandrosterone sulfate (DHEAS) each significantly reduced mOat6 activity with inhibitory constant (K(i)) values of 3.0 +/- 0.5, 48.9 +/- 10.3, 61.4 +/- 7.1, 59.9 +/- 4.9, and 38.8 +/- 3.1 microM, respectively. Comparison to K(i) values determined for mOat3 (67.8 +/- 7.2, 134.5 +/- 27.0, 346.7 +/- 97.9, 79.3 +/- 4.0, and 3.8 +/- 1.1 microM, respectively) revealed that there are significant differences in compound affinity between each transporter. Fluoroquinolone antimicrobials and reduced folates were without effect on mOat6-mediated uptake. Investigation of testicular cell type-specific expression of mOat6 by laser capture microdissection and quantitative polymerase chain reaction revealed significant mRNA expression in Sertoli cells, but not in Leydig cells or spermatids. Overall, these data should aid further refinements to the interpretation and modeling of the in vivo disposition of OAT substrates. Specifically, expression in Sertoli cells suggests Oat6 may be an important determinant of blood-testis barrier function, with Oat6-mediated transport of estrone sulfate and DHEAS possibly representing a critical step in the maintenance of testicular steroidogenesis.

CUG2, a novel oncogene confers reoviral replication through Ras and p38 signaling pathway

As we have recently found a novel oncogene, the cancer-upregulated gene 2 (CUG2), which was elevated in a variety of tumor tissues such as the ovary, liver, lung and pancreas, we examined whether reovirus could efficiently induce cytolysis in cancer cells expressing CUG2 and thus be used as a potential cancer therapeutic agent. In this study, we describe experiments in which we use reovirus to treat NIH3T3 cells stably expressing either CUG2 (NIH-CUG2) or vector only (NIH-Vec). NIH-CUG2 cells readily support reoviral proliferation and undergo apoptosis, whereas NIH-Vec cells are highly resistant to reoviral infection and virus-induced apoptosis. This notable result may be explained by the observation that CUG2 expression inhibits PKR activation, leading to reoviral proliferation in nonpermissive NIH3T3 cells. Furthermore, reovirus infection results in almost complete regression of tumorgenic NIH-CUG2 cells in transplanted nude mice. As we found that CUG2 enhances activation of MAPK (ERK, JNK and p38), Src kinase and Ras, we examined whether CUG2 confers reoviral replication independent of the Ras or p38 MAPK signaling pathway. From these experiments we found that either inhibition of p38 MAPK or Ras blocks reoviral proliferation even in the presence of CUG2 but inhibition of ERK, JNK and Src kinase does not, indicating that activation of p38 MAPK and Ras has critical roles in reoviral replication in CUG2-expressing tumor cells. Accordingly, we propose that reovirus can be useful in the treatment of transformed cells expressing CUG2, which is commonly detected in various tumor tissues.