Molecular insights into the structure-function relationship of organic anion transporters OATs

Unique Binding Sites of Uricosuric Agent Dotinurad for Selective Inhibition of Renal Uric Acid Reabsorptive Transporter URAT1

Dotinurad was developed as a uricosuric agent, inhibiting urate (UA) reabsorption through the UA transporter URAT1 in the kidneys. Due to its high selectivity for URAT1 among renal UA transporters, we investigated the mechanism underlying this selectivity by identifying dotinurad binding sites specific to URAT1. Dotinurad was docked to URAT1 using AutoDock4, utilizing the AlphaFold2-predicted structure. The inhibitory effects of dotinurad on wild-type and mutated URAT1 at the predicted binding sites were assessed through URAT1-mediated [14C]UA uptake in Xenopus oocytes. Nine amino acid residues in URAT1 were identified as dotinurad-binding sites. Sequence alignment with UA-transporting organic anion transporters (OATs) revealed that H142 and R487 were unique to URAT1 among renal UA-transporting OATs. For H142, IC50 values of dotinurad increased to 62, 55, and 76 nM for mutated URAT1 (H142A, H142E, and H142R, respectively) compared with 19 nM for the wild type, indicating that H142 contributes to URAT1-selective interaction with dotinurad. H142 was predicted to interact with the phenyl-hydroxyl group of dotinurad. The IC50 of the hydroxyl group methylated dotinurad (F13141) was 165 μM, 8420-fold higher than dotinurad, suggesting the interaction of H142 and the phenyl-hydroxyl group by forming a hydrogen bond. Regarding R487, URAT1-R487A exhibited a loss of activity. Interestingly, the URAT1-H142A/R487A double mutant restored UA transport activity, with the IC50 value of dotinurad for the mutant (388 nM) significantly higher than that for H142A (73.5 nM). These results demonstrate that H142 and R487 of URAT1 determine its selectivity for dotinurad, a uniqueness observed only in URAT1 among UA-transporting OATs. SIGNIFICANCE STATEMENT: Dotinurad selectively inhibits the urate reabsorption transporter URAT1 in renal urate-transporting organic ion transporters (OATs). This study demonstrates that dotinurad interacts with H142 and R487 of URAT1, located in the extracellular domain and unique among OATs when aligning amino acid sequences. Mutations in these residues reduce affinity of dotinurad for URAT1, confirming their role in conferring selective inhibition. Additionally, the interaction between dotinurad and URAT1 involving H142 is found to mediate hydrogen bonding.

Bioinformatics prediction and experimental verification of key biomarkers for diabetic kidney disease based on transcriptome sequencing in mice

Background

Diabetic kidney disease (DKD) is the leading cause of death in people with type 2 diabetes mellitus (T2DM). The main objective of this study is to find the potential biomarkers for DKD.

Materials and Methods

Two datasets (GSE86300 and GSE184836) retrieved from Gene Expression Omnibus (GEO) database were used, combined with our RNA sequencing (RNA-seq) results of DKD mice (C57 BLKS-32w db/db) and non-diabetic (db/m) mice for further analysis. After processing the expression matrix of the three sets of data using R software "Limma", differential expression analysis was performed. The significantly differentially expressed genes (DEGs) (-logFC- > 1, p-value < 0.05) were visualized by heatmaps and volcano plots respectively. Next, the co-expression genes expressed in the three groups of DEGs were obtained by constructing a Venn diagram. In addition, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were further analyzed the related functions and enrichment pathways of these co-expression genes. Then, qRT-PCR was used to verify the expression levels of co-expression genes in the kidney of DKD and control mice. Finally, protein-protein interaction network (PPI), GO, KEGG analysis and Pearson correlation test were performed on the experimentally validated genes, in order to clarify the possible mechanism of them in DKD.

Results

Our RNA-seq results identified a total of 125 DEGs, including 59 up-regulated and 66 down-regulated DEGs. At the same time, 183 up-regulated and 153 down-regulated DEGs were obtained in GEO database GSE86300, and 76 up-regulated and 117 down-regulated DEGs were obtained in GSE184836. Venn diagram showed that 13 co-expression DEGs among the three groups of DEGs. GO analysis showed that biological processes (BP) were mainly enriched inresponse to stilbenoid, response to fatty acid, response to nutrient, positive regulation of macrophage derived foam cell differentiation, triglyceride metabolic process. KEGG pathway analysis showed that the three major enriched pathways were cholesterol metabolism, drug metabolism-cytochrome P450, PPAR signaling pathway. After qRT-PCR validation, we obtained 11 genes that were significant differentially expressed in the kidney tissues of DKD mice compared with control mice. (The mRNA expression levels of Aacs, Cpe, Cd36, Slc22a7, Slc1a4, Lpl, Cyp7b1, Akr1c14 and Apoh were declined, whereas Abcc4 and Gsta2 were elevated).

Conclusion

Our study, based on RNA-seq results, GEO databases and qRT-PCR, identified 11 significant dysregulated DEGs, which play an important role in lipid metabolism and the PPAR signaling pathway, which provide novel targets for diagnosis and treatment of DKD.

High uric acid induces liver fat accumulation via ROS/JNK/AP-1 signaling

Uric acid is the end metabolite derived from the oxidation of purine compounds. Overwhelming evidence shows the vital interrelationship between hyperuricemia (HUA) and nonalcoholic fatty liver disease (NAFLD). However, the mechanisms for this association remain unclear. In this study, we established a urate oxidase-knockout (Uox-KO) mouse model by clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology. To study the correlation between HUA and NAFLD, human HepG2 hepatoma cells were treated in culture medium with high level of uric acid. In vivo, the Uox-KO mice spontaneously developed hyperuricemia and aberrant lipid-metabolism, concomitant with abnormal hepatic fat accumulation. HUA activated c-Jun N-terminal kinase (JNK) in vivo and in vitro. Furthermore, inhibiting JNK activation by a JNK-specific inhibitor, SP600125, decreased fat accumulation and lipogenic gene expression induced by HUA. Overexpression of the lipogenic enzymes fatty acid synthase and acetyl-CoA carboxylase 1 was via activation of JNK, which was blocked by the JNK inhibitor SP600125. HUA activated AP-1 to upregulate lipogenic gene expression via JNK activation. In addition, HUA caused mitochondrial dysfunction and reactive oxygen species production. Pretreatment with the antioxidant N-acetyl-l-cysteine could ameliorate HUA-activated JNK and hepatic steatosis. These data suggest that ROS/JNK/AP-1 signaling plays an important role in HUA-mediated fat accumulation in liver.NEW & NOTEWORTHY Hyperuricemia and nonalcoholic fatty liver disease are global public health problems, which are strongly associated with metabolic syndrome. In this study, we demonstrate that uric acid induces hepatic fat accumulation via the ROS/JNK/AP-1 pathway. This study identifies a new mechanism of NAFLD pathogenesis and new potential therapeutic strategies for HUA-induced NAFLD.

Impaired Transport Activity of Human Organic Anion Transporters (OATs) and Organic Anion Transporting Polypeptides (OATPs) by Wnt Inhibitors

The Wnt/β-catenin signaling pathway is dysregulated in diseases and Wnt inhibitors like PRI-724 are in clinical development. This study evaluated the regulatory actions of PRI-724 and other Wnt inhibitors on the transport activity of human renal Organic anion transporters (OATs) and Organic anion transporting polypeptides (OATPs). The substrate uptake by OAT4 and OATP2B1 was markedly decreased by PRI-724 (V_max/K_m: ∼26% and ∼17% of corresponding control), with less pronounced decreases in OAT1, OAT3 and OAT1A2. PRI-724 decreased the plasma membrane expression of inhibited OATs/OATPs but didn't affect their total cellular expression. Two model Wnt inhibitors - FH535 and 21H7 - were also tested in comparative studies. Like PRI-724, they also strongly decreased the activities and membrane expression of multiple OATs/OATPs. In contrast, FH535 didn't affect the substrate uptake by organic cation transporters. In control studies, the EGFR inhibitor lapatinib did not inhibit the function of some OATs/OATPs. Together these findings suggest that Wnt inhibitors selectively modulate the function of multiple organic anions transporters, so their clinical use may have unanticipated effects on drug entry into cells. These findings are pertinent to current clinical trials that have been designed to understand the safety and efficacy of new Wnt inhibitor drugs.

Regulation of organic anion transporters: Role in physiology, pathophysiology, and drug elimination

The members of the organic anion transporter (OAT) family are mainly expressed in kidney, liver, placenta, intestine, and brain. These transporters play important roles in the disposition of clinical drugs, pesticides, signaling molecules, heavy metal conjugates, components of phytomedicines, and toxins, and therefore critical for maintaining systemic homeostasis. Alterations in the expression and function of OATs contribute to the intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs, and to many pathophysiological conditions. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. This review will present an update on the recent advance in understanding the cellular and molecular mechanisms underlying the regulation of renal OATs, emphasizing on the post-translational modification (PTM), the crosstalk among these PTMs, and the remote sensing and signaling network of OATs. Such knowledge will provide significant insights into the roles of these transporters in health and disease.

The role of solute carrier (SLC) transporters in actinomycin D pharmacokinetics in paediatric cancer patients

BACKGROUND

Actinomycin D is used for treatment of paediatric cancers; however, a large inter-patient pharmacokinetic (PK) variability and hepatotoxicity are significant limitations to its use and warrant further investigation. Elimination of actinomycin D may be mediated by transporters, as the drug does not seem to undergo significant metabolism. We investigated the role of solute carrier (SLC) transporters in actinomycin D PK.

METHODS

Fourteen key SLCs were screened through probe substrate uptake inhibition by actinomycin D in HEK293 cells. Uptake of actinomycin D was further studied in candidate SLCs by measuring intracellular actinomycin D using a validated LCMS assay. Pharmacogenetic analysis was conducted for 60 patients (Clinical trial: NCT00900354), who were genotyped for SNPs for OAT4 and PEPT2.

RESULTS

OAT4, OCT2, OCT3 and PEPT2 showed significantly lower probe substrate uptake (mean ± SD 75.0 ± 3.5% (p < 0.0001), 74.8 ± 11.2% (p = 0.001), 81.2 ± 14.0% (p = 0.0083) and 70.7 ± 5.7% (p = 0.0188)) compared to that of control. Intracellular accumulation of actinomycin D was greater compared to vector control in OAT4-transfected cells by 1.5- and 1.4-fold at 10 min (p = 0.01) and 20 min (p = 0.03), and in PEPT2-transfected cells by 1.5- and 1.7-fold at 10 min (p = 0.047) and 20 min (p = 0.043), respectively. Subsequent clinical study did not find a significant association between OAT4 rs11231809 and PEPT2 rs2257212 genotypes, and actinomycin D PK parameters such as clearance (CL) and volume of distribution (V_d).

CONCLUSION

Transport of actinomycin D was mediated by OAT4 and PEPT2 in vitro. There was a lack of clinical significance of OAT4 and PEPT2 genotypes as predictors of actinomycin D disposition in paediatric cancer patients.

Renal organic anion transporters in drug-drug interactions and diseases

The kidney plays a vital role in maintaining systemic homeostasis. Active tubular secretion and reabsorption, which are mainly mediated by transporters, is an efficient mechanism for retaining glucose, amino acids, and other nutrients and for the clearance of endogenous waste products and xenobiotics. These substances are recognized by uptake transporters located in the basolateral and apical membranes of renal proximal tubule cells and are extracted from plasma and urine. Organic anion transporters (OATs) belong to the solute carrier (SLC) 22 superfamily and facilitate organic anions across the plasma membranes of renal proximal tubule cells. OATs are responsible for the transmembrane transport of anionic and zwitterionic organic molecules, including endogenous substances and many drugs. The alteration in OAT expression and function caused by diseases, drug-drug interactions (DDIs) or other issues can thus change the renal disposition of substrates, induce the accumulation of toxic metabolites, and lead to unexpected clinically outcome. This review summarizes the recent information regarding the expression, regulation, and substrate spectrum of OATs and discusses the roles of OATs in diseases and DDIs. These findings will enables us to have a better understanding of the related disease therapy and the potential risk of DDIs mediated by OATs.

Recent advance in the pharmacogenomics of human Solute Carrier Transporters (SLCs) in drug disposition

Drug pharmacokinetics is influenced by the function of metabolising enzymes and influx/efflux transporters. Genetic variability of these genes is known to impact on clinical therapies. Solute Carrier Transporters (SLCs) are the primary influx transporters responsible for the cellular uptake of drug molecules, which consequently, impact on drug efficacy and toxicity. The Organic Anion Transporting Polypeptides (OATPs), Organic Anion Transporters (OATs) and Organic Cation Transporters (OCTs/OCTNs) are the most important SLCs involved in drug disposition. The information regarding the influence of SLC polymorphisms on drug pharmacokinetics is limited and remains a hot topic of pharmaceutical research. This review summarises the recent advance in the pharmacogenomics of SLCs with an emphasis on human OATPs, OATs and OCTs/OCTNs. Our current appreciation of the degree of variability in these transporters may contribute to better understanding the inter-patient variation of therapies and thus, guide the optimisation of clinical treatments.

Efflux proteins at the blood-brain barrier: review and bioinformatics analysis

1. Efflux proteins at the blood-brain barrier provide a mechanism for export of waste products of normal metabolism from the brain and help to maintain brain homeostasis. They also prevent entry into the brain of a wide range of potentially harmful compounds such as drugs and xenobiotics. 2. Conversely, efflux proteins also hinder delivery of therapeutic drugs to the brain and central nervous system used to treat brain tumours and neurological disorders. For bypassing efflux proteins, a comprehensive understanding of their structures, functions and molecular mechanisms is necessary, along with new strategies and technologies for delivery of drugs across the blood-brain barrier. 3. We review efflux proteins at the blood-brain barrier, classified as either ATP-binding cassette (ABC) transporters (P-gp, BCRP, MRPs) or solute carrier (SLC) transporters (OATP1A2, OATP1A4, OATP1C1, OATP2B1, OAT3, EAATs, PMAT/hENT4 and MATE1). 4. This includes information about substrate and inhibitor specificity, structural organisation and mechanism, membrane localisation, regulation of expression and activity, effects of diseases and conditions and the principal technique used for in vivo analysis of efflux protein activity: positron emission tomography (PET). 5. We also performed analyses of evolutionary relationships, membrane topologies and amino acid compositions of the proteins, and linked these to structure and function.

Trafficking and other regulatory mechanisms for organic anion transporting polypeptides and organic anion transporters that modulate cellular drug and xenobiotic influx and that are dysregulated in disease

Organic anion transporters (OATs) and organic anion-transporting polypeptides (OATPs), encoded by a number of solute carrier (SLC)22A and SLC organic anion (SLCO) genes, mediate the absorption and distribution of drugs and other xenobiotics. The regulation of OATs and OATPs is complex, comprising both transcriptional and post-translational mechanisms. Plasma membrane expression is required for cellular substrate influx by OATs/OATPs. Thus, interest in post-translational regulatory processes, including membrane targeting, endocytosis, recycling and degradation of transporter proteins, is increasing because these are critical for plasma membrane expression. After being synthesized, transporters undergo N-glycosylation in the endoplasmic reticulum and Golgi apparatus and are delivered to the plasma membrane by vesicular transport. Their expression at the cell surface is maintained by de novo synthesis and recycling, which occurs after clathrin- and/or caveolin-dependent endocytosis of existing protein. Several studies have shown that phosphorylation by signalling kinases is important for the internalization and recycling processes, although the transporter protein does not appear to be directly phosphorylated. After internalization, transporters that are targeted for degradation undergo ubiquitination, most likely on intracellular loop residues. Epigenetic mechanisms, including methylation of gene regulatory regions and transcription from alternate promoters, are also significant in the regulation of certain SLC22A/SLCO genes. The membrane expression of OATs/OATPs is dysregulated in disease, which affects drug efficacy and detoxification. Several transporters are expressed in the cytoplasmic subcompartment in disease states, which suggests that membrane targeting/internalization/recycling may be impaired. This article focuses on recent developments in OAT and OATP regulation, their dysregulation in disease and the significance for drug therapy.

Gene expression profiles in testis of developing male Xenopus laevis damaged by chronic exposure of atrazine

As a widely used herbicide, atrazine (AZ) has been extensively studied for its adverse effects on the reproductive system, especially feminization in male animals. However, the relationship of gene expression changes and associated toxicological endpoints remains unclear. In this study, developing Xenopus laevis tadpoles were exposed to concentration of AZ at 0.1, 1, 10 or 100 μg/L continuously. Compared with froglets in the control group, there were no significant differences in body length, body weight, liver weight and hepatosomatic index (HSI) of males in groups treated with AZ for 90 d. At 100 μg/L AZ treatment caused a significant reduction of gonad weight and gonadosomatic index (GSI) of males (p < 0.01). In addition, AZ at all dose levels caused testicular degeneration, especially in froglets from the groups with 0.1 and 100 μg/L which exhibited U-shaped dose-response trend. We further investigated the gene expression changes associated with the testicular degeneration induced by AZ. We found that the expression of 1165 genes was significantly altered with 616 upregulated and 549 downregulated compared to the expression profile of the control animals. KEGG analysis showed that genes which were significantly affected by AZ are mainly involved in arginine and proline metabolism, cell cycle, riboflavin metabolism, spliceosome, base excision repair and progesterone-mediated oocyte maturation pathway. Our results show that AZ may affect reproductive and immune systems by interference with the related gene expression changes during the male X. laevis development. The findings may help to clarify the feminization mechanisms of AZ in male X. laevis.

Clinical Pharmacogenetics of the Major Adenosine Triphosphate−Binding Cassette and Solute Carrier Drug Transporters

Interindividual variability in drug response is a significant problem in clinical practice, and it is likely that genetic variation among the drug transport genes are major contributors to such variability. Numerous genetic alterations affecting the members of the adenosine triphosphate-binding cassette (ABC) and solute carrier (SLC) families of transporters have been identified. Considerable data exist regarding how mutations in the ABCB1 gene that encodes p-glycoprotein impact drug disposition and response in vivo, but many study reports are conflicting on both the direction of any effect as well as the significance of any alteration. Many possible reasons for such discrepant study results have been identified, and efforts to improve the quality of such pharmacogenetic clinical association studies are ongoing. For most other clinically important transporters relatively, little clinical data exist regarding the significance of known genetic variants despite in vitro evidence of altered function for many of these transporters. What clinical data do exist suggest that certain mutations in ABCG2 and SLCO1B1 may be of importance clinically. Until the current uncertainties regarding the importance of genetic variants in drug transporter genes are clarified, the clinical application of existing pharmacogenetic data should be done with caution.

Posttranslational Regulation of Organic Anion Transporters by Ubiquitination: Known and Novel

Organic anion transporters (OATs) encoded by solute carrier 22 family are localized in the epithelia of multiple organs, where they mediate the absorption, distribution, and excretion of a diverse array of negatively charged environmental toxins and clinically important drugs. Alterations in the expression and function of OATs play important roles in intra- and interindividual variability of the therapeutic efficacy and the toxicity of many drugs. As a result, the activity of OATs must be under tight regulation so as to carry out their normal functions. The regulation of OAT transport activity in response to various stimuli can occur at several levels such as transcription, translation, and posttranslational modification. Posttranslational regulation is of particular interest, because it usually happens within a very short period of time (minutes to hours) when the body has to deal with rapidly changing amounts of substances as a consequence of variable intake of drugs, fluids, or meals as well as metabolic activity. This review article highlights the recent advances from our laboratory in uncovering several posttranslational mechanisms underlying OAT regulation. These advances offer the promise of identifying targets for novel strategies that will maximize therapeutic efficacy in drug development.

Chloroquine and Hydroxychloroquine Are Novel Inhibitors of Human Organic Anion Transporting Polypeptide 1A2

Chloroquine (CQ) and hydroxychloroquine (HCQ) are widely used to treat malaria and inflammatory diseases, long-term usage of which often causes severe side effects, especially retinopathy. Solute carrier transporters (SLCs) are important proteins responsible for the cellular uptake of endogenous and exogenous substances. Inhibitors competing with transporter substrates for SLCs often results in unfavorable toxicities and unsatisfactory therapeutic outcomes. We investigated the inhibitory effect of CQ and HCQ on substrate uptake mediated through a range of important SLC transporters in overexpressing human embryonic kidney (HEK293) cells. Our data revealed that both CQ and HCQ potently inhibit the uptake activity of organic anion transporting polypeptide 1A2 (OATP1A2). We recently reported OATP1A2 to be expressed in human retinal pigment epithelium (RPE), where it mediates cellular uptake of all-trans-retinol (atROL), a key step in the classical visual cycle. In this study, we demonstrate that CQ and HCQ could markedly impair atROL uptake in OATP1A2-expressing HEK293 cells and more importantly, in primary human RPE cells. Our study shows that CQ and HCQ are novel inhibitors of OATP1A2 and significantly impair OATP1A2-mediated substrate uptake, particularly transport of atROL into the RPE. This effect may compromise the function of the classic visual cycle leading to vision impairment and contribute to the retinopathy observed clinically in patients using CQ or HCQ.

Human oligopeptide transporter 2 (PEPT2) mediates cellular uptake of polymyxins

OBJECTIVES

Polymyxins are a last-line therapy to treat MDR Gram-negative bacterial infections. Nephrotoxicity is the dose-limiting factor for polymyxins and recent studies demonstrated significant accumulation of polymyxins in renal tubular cells. However, little is known about the mechanism of polymyxin uptake into these cells. Oligopeptide transporter 2 (PEPT2) is a solute carrier transporter (SLC) expressed at the apical membrane of renal proximal tubular cells and facilitates drug reabsorption in the kidney. In this study, we examined the role of PEPT2 in polymyxin uptake into renal tubular cells.

METHODS

We investigated the inhibitory effects of colistin and polymyxin B on the substrate uptake mediated through 15 essential SLCs in overexpressing HEK293 cells. The inhibitory potency of both polymyxins on PEPT2-mediated substrate uptake was measured. Fluorescence imaging was employed to investigate PEPT2-mediated uptake of the polymyxin fluorescent probe MIPS-9541 and a transport assay was conducted with MIPS-9541 and [(3)H]polymyxin B1.

RESULTS

Colistin and polymyxin B potently inhibited PEPT2-mediated [(3)H]glycyl-sarcosine uptake (IC50 11.4 ± 3.1 and 18.3 ± 4.2 μM, respectively). In contrast, they had no or only mild inhibitory effects on the transport activity of the other 14 SLCs evaluated. MIPS-9541 potently inhibited PEPT2-mediated [(3)H]glycyl-sarcosine uptake (IC50 15.9 μM) and is also a substrate of PEPT2 (Km 74.9 μM). [(3)H]polymyxin B1 was also significantly taken up by PEPT2-expressing cells (Km 87.3 μM).

CONCLUSIONS

Our study provides the first evidence of PEPT2-mediated uptake of polymyxins and contributes to a better understanding of the accumulation of polymyxins in renal tubular cells.

The altered renal and hepatic expression of solute carrier transporters (SLCs) in type 1 diabetic mice

Diabetes mellitus is a chronic metabolic disorder that significantly affects human health and well-being. The Solute carrier transporters (SLCs), particularly the Organic anion/cation transporters (Oats/Octs/Octns), Organic anion transporting polypeptides (Oatps) and Oligopeptide transporters (Pepts) are essential membrane proteins responsible for cellular uptake of many endogenous and exogenous substances such as clinically important drugs. They are widely expressed in mammalian key organs especially the kidney and liver, in which they facilitate the influx of various drug molecules, thereby determining their distribution and elimination in body. The altered expression of SLCs in diabetes mellitus could have a profound and clinically significant influence on drug therapies. In this study, we extensively investigated the renal and hepatic expression of twenty essential SLCs in the type 1 diabetic Ins2Akita murine model that develops both hyperglycemia and diabetes-related complications using real-time PCR and immunoblotting analysis. We found that the renal expression of mOatp1a1, mOatp1a6, mOat1, mOat3, mOat5, mOct2 and mPept2 was decreased; while that of mPept1 was increased at the mRNA level in the diabetic mice compared with non-diabetic controls. We found up-regulated mRNA expression of mOatp1a4, mOatp1c1, mOctn2, mOct3 and mPept1 as well as down-regulation of mOatp1a1 in the livers of diabetic mice. We confirmed the altered protein expression of several SLCs in diabetic mice, especially the decreased renal and hepatic expression of mOatp1a1. We also found down-regulated protein expression of mOat3 and mOctn1 in the kidneys as well as increased protein expression of mOatp1a4 and mOct3 in the livers of diabetic mice. Our findings contribute to better understanding the modulation of SLC transporters in type 1 diabetes mellitus, which is likely to affect the pharmacokinetic performance of drugs that are transported by these transporters and therefore, forms the basis of future therapeutic optimization of regimens in patients with type 1 diabetes mellitus.

Human organic anion transporting polypeptide 1A2 (OATP1A2) mediates cellular uptake of all-trans-retinol in human retinal pigmented epithelial cells

BACKGROUND AND PURPOSE

Vision depends on retinoid exchange between the retinal pigment epithelium (RPE) and photoreceptors. Defects in any step of the canonical visual cycle can lead to retinal degenerations. All-trans-retinol (atROL) plays an important role in visual signal transduction. However, how atROL enters human RPE from the apical membrane remains unclear. This study investigated the role of human organic anion transporting polypeptide 1A2 (OATP1A2) in atROL uptake in human RPE.

EXPERIMENTAL APPROACH

Immunoblotting and immunostaining elucidated the expression and localization of OATP1A2 in human RPE. Transporter functional studies were conducted to assess the interaction of OATP1A2 with atROL.

KEY RESULTS

Our study revealed OATP1A2 is expressed in human RPE, mainly at the apical membrane. Our data also indicated atROL inhibited the uptake of the typical OATP1A2 substrate, oestrone-3-sulfate (E3S), in over-expressing cells. Studies on the uptake of (3) H-atROL in these over-expressing cells revealed atROL is a substrate of OATP1A2. We confirmed these findings in human primary RPE cells. The transport of E3S and atROL was significantly reduced in human primary RPE cells with OATP1A2 siRNA silencing.

CONCLUSION AND IMPLICATIONS

Our data provides the first evidence of OATP1A2 expression in human RPE and more importantly, its novel role in the cellular uptake of atROL, which might be essential to the proper functioning of the canonical visual cycle. Our findings contribute to the understanding of the molecular mechanisms involved in retinoid transport between the RPE and photoreceptors and provide novel insights into potential pharmaceutical interventions for visual cycle disruption associated with retinal degenerations.

Probenecid as a sensitizer of bisphosphonate-mediated effects in breast cancer cells

Background

Anti-resorptive bisphosphonates (BP) are used for the treatment of osteoporosis and bone metastases. Clinical studies indicated a benefit in survival and tumor relapse in subpopulations of breast cancer patients receiving zoledronic acid, thus stimulating the debate about its anti-tumor activity. Amino-bisphosphonates in nM concentrations inhibit farnesyl pyrophosphate synthase leading to accumulation of isopentenyl pyrophosphate (IPP) and the ATP/pyrophosphate adduct ApppI, which induces apoptosis in osteoclasts. For anti-tumor effects μM concentrations are needed and a sensitizer for bisphosphonate effects would be beneficial in clinical anti-tumor applications. We hypothesized that enhancing intracellular pyrophosphate accumulation via inhibition of probenecid-sensitive channels and transporters would sensitize tumor cells for bisphosphonates anti-tumor efficacy.

Method

MDA-MB-231, T47D and MCF-7 breast cancer cells were treated with BP (zoledronic acid, risedronate, ibandronate, alendronate) and the pyrophosphate channel inhibitors probenecid and novobiocin. We determined cell viability and caspase 3/7 activity (apoptosis), accumulation of IPP and ApppI, expression of ANKH, PANX1, ABCC1, SLC22A11, and the zoledronic acid target gene and tumor-suppressor KLF2.

Results

Treatment of MDA-MB-231 with BP induced caspase 3/7 activity, with zoledronic acid being the most effective. In MCF-7 and T47D either BP markedly suppressed cell viability with only minor effects on apoptosis. Co-treatment with probenecid enhanced BP effects on cell viability, IPP/ApppI accumulation as measurable in MCF-7 and T47D cells, caspase 3/7 activity and target gene expression. Novobiocin co-treatment of MDA-MB-231 yielded identical results on viability and apoptosis compared to probenecid, rendering SLC22A family members as candidate modulators of BP effects, whereas no such evidence was found for ANKH, ABCC1 and PANX1.

Conclusions

In summary, we demonstrate effects of various bisphosphonates on caspase 3/7 activity, cell viability and expression of tumor suppressor genes in breast cancer cells. Blocking probenecid and novobiocin-sensitive channels and transporters enhances BP anti-tumor effects and renders SLC22A family members as good candidates as BP modulators. Further studies will have to unravel if treatment with such BP-sensitizers translates into preclinical and clinical efficacy.

Electronic supplementary material

The online version of this article (doi:10.1186/1476-4598-13-265) contains supplementary material, which is available to authorized users.

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.

A genome wide study of genetic adaptation to high altitude in feral Andean Horses of the páramo

Background

Life at high altitude results in physiological and metabolic challenges that put strong evolutionary pressure on performance due to oxidative stress, UV radiation and other factors dependent on the natural history of the species. To look for genes involved in altitude adaptation in a large herbivore, this study explored genome differentiation between a feral population of Andean horses introduced by the Spanish in the 1500s to the high Andes and their Iberian breed relatives.

Results

Using allelic genetic models and Fst analyses of ~50 K single nucleotide polymorphisms (SNPs) across the horse genome, 131 candidate genes for altitude adaptation were revealed (Bonferoni of p ≤ 2 × 10^–7). Significant signals included the EPAS1 in the hypoxia-induction-pathway (HIF) that was previously discovered in human studies (p = 9.27 × 10^-8); validating the approach and emphasizing the importance of this gene to hypoxia adaptation. Strong signals in the cytochrome P450 3A gene family (p = 1.5 ×10^-8) indicate that other factors, such as highly endemic vegetation in altitude environments are also important in adaptation. Signals in tenuerin 2 (TENM2, p = 7.9 × 10^-14) along with several other genes in the nervous system (gene categories representation p = 5.1 × 10^-5) indicate the nervous system is important in altitude adaptation.

Conclusions

In this study of a large introduced herbivore, it becomes apparent that some gene pathways, such as the HIF pathway are universally important for high altitude adaptation in mammals, but several others may be selected upon based on the natural history of a species and the unique ecology of the altitude environment.

Primary porcine proximal tubular cells as an alternative to human primary renal cells in vitro: an initial characterization

Background

A good in vitro model should approximate an in vivo-like behavior as closely as possible in order to reflect most likely the in vivo situation. Regarding renal physiology of different species, humans are more closely related to pigs than to rodents, therefore primary porcine kidney cells (PKC) and their subsequent cell strain could be a valid alternative to primary human cells for renal in vitro toxicology. For this PKC must display inherent characteristics (e.g. structural organization) and functions (e.g. transepithelial transport) as observed under in vivo conditions within the respective part of the kidney.

Results

We carried out a comprehensive characterization of PKC and their subsequent cell strain, including morphology and growth as well as transporter expression and functionality. The data presented here demonstrate that PKC express various transporters including pMrp1 (abcc1), pMrp2 (abcc2), pOat1 (slc22a6) and pOat3 (slc22a8), whereas pMdr1 (abcb1) and pOatp1a2 (slco1a2) mRNA could not be detected in either the PKCs or in the porcine cortical tissue. Functionality of the transporters was demonstrated by determining the specific PAH transport kinetics.

Conclusions

On the basis of the presented results it can be concluded that PKC and to some extent their subsequent cell strain represent a valuable model for in vitro toxicology, which might be used as an alternative to human primary cells.

The inhibitory effects of the bioactive components isolated from Scutellaria baicalensis on the cellular uptake mediated by the essential solute carrier transporters

Solute carrier transporters (SLCs), in particular the organic anion transporters (OATs), OAT polypeptides (OATPs), and organic cation transporters (OCTs/OCTNs), are the important membrane proteins responsible for the cellular influx of various drugs. Baicalein (BA), baicalin (BG), and wogonin (WG) are the three major bioactive components of Scutellaria baicalensis. In this study, we evaluated the inhibitory effects of BA, BG, and WG on the cellular uptake of specific substrates mediated by the essential SLCs in human embryonic kidney-293 cells. Our data demonstrated that BA and WG significantly inhibit the OAT1-, OAT3-, and OATP1B3-mediated uptake; BG effectively reduces the influx of substrates of OAT3, OAT4, OATP1B3, and OATP2B1; WG is a potent inhibitor of OCT3. Our further kinetic analysis derived the IC50 values of these compounds with pronounced inhibitory effects on SLCs, particularly the inhibitions of WG on OAT1 and OCT3 and that of BA and WG on OAT3. Our study comprehensively evaluated the inhibitory effects of three bioactive components of Scutellaria baicalensis on the uptake of specific substrates mediated by the essential SLC transporters, which suggested that precautions will be needed when coadministrating drugs with Scutellaria baicalensis so as to prevent the unfavorable drug-drug/herb interactions in human.

Multispecific drug transporter Slc22a8 (Oat3) regulates multiple metabolic and signaling pathways

Multispecific drug transporters of the solute carrier and ATP-binding cassette families are highly conserved through evolution, but their true physiologic role remains unclear. Analyses of the organic anion transporter 3 (OAT3; encoded by Slc22a8/Oat3, originally Roct) knockout mouse have confirmed its critical role in the renal handling of common drugs (e.g., antibiotics, antivirals, diuretics) and toxins. Previous targeted metabolomics of the knockout of the closely related Oat1 have demonstrated a central metabolic role, but the same approach with Oat3 failed to reveal a similar set of endogenous substrates. Nevertheless, the Oat3 knockout is the only Oat described so far with a physiologically significant phenotype, suggesting the disturbance of metabolic or signaling pathways. Here we analyzed global gene expression in Oat3 knockout tissue, which implicated OAT3 in phase I and phase II metabolism (drug metabolizing enzymes or DMEs), as well as signaling pathways. Metabolic reconstruction with the recently developed "mouse Recon1" supported the involvement of Oat3 in the aforementioned pathways. Untargeted metabolomics were used to determine whether the predicted metabolic alterations could be confirmed. Many significant changes were observed; several metabolites were tested for direct interaction with mOAT3, whereas others were supported by published data. Oat3 thus appears critical for the handling of phase I (hydroxylation) and phase II (glucuronidation) metabolites. Oat3 also plays a role in bioenergetic pathways (e.g., the tricarboxylic acid cycle), as well as those involving vitamins (e.g., folate), steroids, prostaglandins, gut microbiome products, uremic toxins, cyclic nucleotides, amino acids, glycans, and possibly hyaluronic acid. The data seemingly consistent with the Remote Sensing and Signaling Hypothesis (Ahn and Nigam, 2009; Wu et al., 2011), also suggests that Oat3 is essential for the handling of dietary flavonoids and antioxidants.

Bis(L-cysteinato)zincate(lI) as a coordination compound that induces metallothionein gene transcription without inducing cell-stress-related gene transcription

Zinc is an essential micronutrient, deficiency of which results in growth retardation, immunodeficiency, and neurological diseases such as dysgeusia. Several zinc coordination compounds are used for zinc supplementation; however, supplemented zinc ions have no specificity and interact with various groups of molecules. Here, we found that, from a library of 30 zinc coordination compounds, bis(L-cysteinato)zincate(II), designated Z01, functioned as a metallothionein (MT) inducer. Z01 induced MT expression mediated by the transcription factor MTF-1, without inducing cell-stress-related heme oxygenase-1 gene expression at specific concentration. The zinc ion was necessary for the MT induction. (65)Zn incorporation following treatment with (65)Zn-labeled Z01 suggested that Z01 did not act as zinc ionophore despite its hydrophilicity. Electrophoretic mobility shift assays revealed that Z01 facilitates MTF-1-MRE complex formation, and, by inference, transfer of zinc from Z01 to MTF-1. Phosphorylated ERK levels were increased by ZnSO(4) treatment but not by Z01. Although our data do not definitely prove that Z01 is an MTF-1-specific activator, our observations suggest that zinc coordination compounds can regulate zinc distribution and act as zinc donors for specific molecules.

Conformational changes of the multispecific transporter organic anion transporter 1 (OAT1/SLC22A6) suggests a molecular mechanism for initial stages of drug and metabolite transport

The solute carrier (SLC) family of transporters play key roles in the movement of charged organic ions across the blood-urine, blood-cerebrospinal fluid, and blood-brain barriers and thus mediate the absorption, disposition, and elimination of many common pharmaceuticals (i.e., nonsteroidal anti-inflammatory drug (NSAIDs), antibiotics, and diuretics). They have also been proposed to participate in a remote sensing and signaling network involving small molecules. Nevertheless, other than possessing a 12-transmembrane α-helical topology comprised of two six-helix hemidomains interacting through a long loop, the structural and mechanistic details for these transporters remains unclear. Recent crystallographic studies of bacterial homologs support the idea of a "switching" mechanism, which allows for periodic changes in the overall transporter configuration and cyclic opening of the transporter to the extracellular or cytoplasmic sides of the membrane. To investigate this, computational modeling based on our recent study of glycerol-3-phosphate transporter (GlpT) (Tsigelny et al. J Bioinform Comput Biol 6:885-904, 2008) was performed for organic anion transporter 1 (OAT1/SLC22A6, originally identified as NKT), the prototypical member of this family. OAT1 was inserted into an artificial phospholipid bilayer and the positional change of the six-helix hemidomains relative to each other was followed for 100 ns. The hemidomains were found to tilt relative to each other while their configuration is mostly inflexible. Since the modeling was performed for about 100 ns, the data suggest that this tilting mechanism might explain the early steps in the transport of organic anionic metabolites, drugs, and toxins by this clinically important transporter.

Pharmacogenomics of drug metabolizing enzymes and transporters: implications for cancer therapy

The new era of personalized medicine, which integrates the uniqueness of an individual with respect to the pharmacokinetics and pharmacodynamics of a drug, holds promise as a means to provide greater safety and efficacy in drug design and development. Personalized medicine is particularly important in oncology, whereby most clinically used anticancer drugs have a narrow therapeutic window and exhibit a large interindividual pharmacokinetic and pharmacodynamic variability. This variability can be explained, at least in part, by genetic variations in the genes encoding drug metabolizing enzymes, transporters, or drug targets. Understanding of how genetic variations influence drug disposition and action could help in tailoring cancer therapy based on individual's genetic makeup. This review focuses on the pharmacogenomics of drug metabolizing enzymes and drug transporters, with a particular highlight of examples whereby genetic variations in the metabolizing enzymes and transporters influence the pharmacokinetics and/or response of chemotherapeutic agents.

Glutaric aciduria type 1 metabolites impair the succinate transport from astrocytic to neuronal cells

The inherited neurodegenerative disorder glutaric aciduria type 1 (GA1) results from mutations in the gene for the mitochondrial matrix enzyme glutaryl-CoA dehydrogenase (GCDH), which leads to elevations of the dicarboxylates glutaric acid (GA) and 3-hydroxyglutaric acid (3OHGA) in brain and blood. The characteristic clinical presentation of GA1 is a sudden onset of dystonia during catabolic situations, resulting from acute striatal injury. The underlying mechanisms are poorly understood, but the high levels of GA and 3OHGA that accumulate during catabolic illnesses are believed to play a primary role. Both GA and 3OHGA are known to be substrates for Na(+)-coupled dicarboxylate transporters, which are required for the anaplerotic transfer of the tricarboxylic acid cycle (TCA) intermediate succinate between astrocytes and neurons. We hypothesized that GA and 3OHGA inhibit the transfer of succinate from astrocytes to neurons, leading to reduced TCA cycle activity and cellular injury. Here, we show that both GA and 3OHGA inhibit the uptake of [(14)C]succinate by Na(+)-coupled dicarboxylate transporters in cultured astrocytic and neuronal cells of wild-type and Gcdh(-/-) mice. In addition, we demonstrate that the efflux of [(14)C]succinate from Gcdh(-/-) astrocytic cells mediated by a not yet identified transporter is strongly reduced. This is the first experimental evidence that GA and 3OHGA interfere with two essential anaplerotic transport processes: astrocytic efflux and neuronal uptake of TCA cycle intermediates, which occur between neurons and astrocytes. These results suggest that elevated levels of GA and 3OHGA may lead to neuronal injury and cell death via disruption of TCA cycle activity.

Glycosylation of the OCTN2 carnitine transporter: study of natural mutations identified in patients with primary carnitine deficiency

Primary carnitine deficiency is caused by impaired activity of the Na(+)-dependent OCTN2 carnitine/organic cation transporter. Carnitine is essential for entry of long-chain fatty acids into mitochondria and its deficiency impairs fatty acid oxidation. Most missense mutations identified in patients with primary carnitine deficiency affect putative transmembrane or intracellular domains of the transporter. Exceptions are the substitutions P46S and R83L located in an extracellular loop close to putative glycosylation sites (N57, N64, and N91) of OCTN2. P46S and R83L impaired glycosylation and maturation of OCTN2 transporters to the plasma membrane. We tested whether glycosylation was essential for the maturation of OCTN2 transporters to the plasma membrane. Substitution of each of the three asparagine (N) glycosylation sites with glutamine (Q) decreased carnitine transport. Substitution of two sites at a time caused a further decline in carnitine transport that was fully abolished when all three glycosylation sites were substituted by glutamine (N57Q/N64Q/N91Q). Kinetic analysis of carnitine and sodium-stimulated carnitine transport indicated that all substitutions decreased the Vmax for carnitine transport, but N64Q/N91Q also significantly increased the Km toward carnitine, indicating that these two substitutions affected regions of the transporter important for substrate recognition. Western blot analysis confirmed increased mobility of OCTN2 transporters with progressive substitutions of asparagines 57, 64 and/or 91 with glutamine. Confocal microscopy indicated that glutamine substitutions caused progressive retention of OCTN2 transporters in the cytoplasm, up to full retention (such as that observed with R83L) when all three glycosylation sites were substituted. Tunicamycin prevented OCTN2 glycosylation, but it did not impair maturation to the plasma membrane. These results indicate that OCTN2 is physiologically glycosylated and that the P46S and R83L substitutions impair this process. Glycosylation does not affect maturation of OCTN2 transporters to the plasma membrane, but the 3 asparagines that are normally glycosylated are located in a region important for substrate recognition and turnover rate.

Interferon tau regulates PGF2alpha release from the ovine endometrial epithelial cells via activation of novel JAK/EGFR/ERK/EGR-1 pathways

In ruminants, pulsatile release of prostaglandin F2α (PGF(2α)) from the endometrium is transported to the ovary and induces luteolysis thereby allowing new estrous cycle. Interferon tau (IFNT), a type 1 IFN secreted by the trophoblast cells of the developing conceptus, acts on endometrial luminal epithelial (LE) cells and inhibits pulsatile release of PGF(2α) and establishes pregnancy. One of the unknown mechanisms is that endometrial pulsatile release of PGF(2α) is inhibited whereas basal release of PGF(2α) is increased in pregnant compared with nonpregnant sheep. We have recently found that pulsatile release of PGF(2α) from the endometrium is regulated by prostaglandin transporter (PGT)-mediated mechanisms. We hypothesize that modulation in the endometrial pulsatile vs. basal release of PGF(2α) likely requires PGT-mediated selective transport, and IFNT interacts with PGT protein and modulates pulsatile vs. basal release of PGF(2α). The new findings of the present study are: 1) IFNT activates novel JAK-SRC kinase-EGFR-RAS-RAF-ERK1/2-early growth response (EGR)-1 signaling module in LE cells; 2) IFNT increases interactions between PGT and ERK1/2 or EGR-1 proteins and alters phosphorylation of PGT protein; 3) IFNT precludes action of protein kinase C and Ca(2+) on PGT function; and 4) IFNT inhibits 80% PGT-mediated but not 20% simple diffusion-mediated release of PGF(2α) from the endometrial LE cells through this novel signaling module. The results of the present study provide important new insights on IFNT signaling and molecular control of PGT-mediated release of PGF(2α) and unravel the underlying mechanisms responsible for the increased basal release of PGF(2α) at the time of establishment of pregnancy in ruminants.

Vitamin E status and metabolism in adult and aged aryl hydrocarbon receptor null mice

The aryl hydrocarbon receptor (AhR) is involved in regulation of mechanisms for detoxification of xenobiotics, as well as vitamin A metabolism. Vitamin E is a fat-soluble nutrient whose metabolism is initialized via the cytochrome P450 system. Thus, AhR absence could alter hepatic regulation of α-tocopherol metabolism. To test this hypothesis, we assessed vitamin E status in adult (2-5 m) and old (21-22 m), wild-type and AhR-null mice. Plasma α-tocopherol concentrations in AhR-null mice (2.3±1.2 μmol/L, n=19) were lower than those of wild-type mice (3.2±1.2, n=17, P=.0131); those in old mice (3.2±1.2, n=20) were higher than those of adults (2.2±1.0, n=16, P=.0075). Hepatic α-tocopherol concentrations were not different between genotypes, but were nearly double in old (32±8 nmol/g, n=20) as compared with adult mice (17±2, n=16, P<.0001). Hepatic Cyp3a concentrations in AhR-null mice were greater than those in wild-type mice (P=.0011). Genotype (P=.0047), sex (P<.0001) and age (P<.0001) were significant modifiers of liver α-tocopherol metabolite (α-CEHC) concentrations. In general, Cyp3a concentrations correlated with hepatic α-tocopherol (r=0.3957, P<.05) and α-CEHC (r=0.4260, P<.05) concentrations. Since there were no significant genotype differences in the hepatic α- or γ-tocopherol concentrations, AhR-null mice did not have dramatically altered vitamin E metabolism. Since they did have higher hepatic α-CEHC concentrations, these data suggest metabolism was up-regulated in the AhR-null mice in order to maintain the hepatic tocopherol concentrations similar to those of wild-type mice.

Regulation of human organic anion transporter 4 by protein kinase C and NHERF-1: altering the endocytosis of the transporter

PURPOSE

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs. We have previously shown that the activity of hOAT4 was down-regulated by activation of PKC and up-regulated by PDZ protein NHERF-1. Here, we investigated the mechanisms underlying such regulations.

METHODS

COS-7 cells expressing hOAT4 were treated with PKC activator phorbol 12-myristate 13-acetate (PMA) or transfected with dominant negative mutants of dynamin-2 or Eps15 or transfected with NHERF-1. The internalization and the function of hOAT4 were then determined.

RESULTS

We showed that hOAT4 constitutively internalized from and recycled back to plasma membrane. Transfection of dominant negative mutants of dynamin-2 or Eps15 into the cells, all of which block clathrin-dependent endocytotic pathway, significantly blocked hOAT4 internalization. Treatment of cells with PMA accelerated hOAT4 internalization, whereas transfection of cells with NHERF-1 attenuated hOAT4 internalization.

CONCLUSION

Our studies demonstrated that i) hOAT4 undergoes constitutive trafficking between cell surface and intracellular compartments, ii) hOAT4 internalization partly occurs through clathrin-dependent pathway, iii) the down-regulation of hOAT4 activity by activation of PKC and the up-regulation of hOAT4 activity by NHERF-1 are mediated through alteration of hOAT4 internalization.

Functional characterization of nonsynonymous single nucleotide polymorphisms in the human organic anion transporter 4 (hOAT4)

BACKGROUND AND PURPOSE

The human organic anion transporter (hOAT) family of transmembrane carrier proteins mediate the cellular flux of anionic substances, including certain hormones and anti-cancer drugs. hOAT4 is highly expressed at the apical membrane of the renal tubular cell and facilitates drug re-absorption in the kidney. In the present study, the impact of 10 nonsynonymous single nucleotide polymorphisms (SNPs) of hOAT4 on transport function in COS-7 cells was characterized.

EXPERIMENTAL APPROACH

Transport uptake assay was used to assess the function of the variant transporters. Cell surface biotinylation and western blot analysis were used to investigate the expression characteristics of the transporter proteins. Comparative modelling was used to interpret the influence of nonsynonymous changes in terms of hOAT4 structure.

KEY RESULTS

Four naturally occurring hOAT4 variants (L29P, R48Y, V155G and T392I) exhibited a significant loss of function. Substitution of leucine-29, which is a conserved residue in OATs, with a proline residue, impaired the synthesis or the apparent stability of the transporter and membrane insertion was disrupted in the R48Y variant. In the case of the V155G and T392I variants, impaired function was due to decreased affinity of the transporter for oestrone sulphate and impaired transporter-substrate turnover respectively. The T392I variant was inhibited more extensively than the wild-type transporter by the cationic substrate tetraethyl ammonium.

CONCLUSIONS AND IMPLICATIONS

Several naturally occurring SNPs encode variant hOAT4s that may impair the renal tubular re-absorption of important drug substrates.

Functional characterization of nonsynonymous single nucleotide polymorphisms in the human organic anion transporter 4 (hOAT4)

BACKGROUND AND PURPOSE

The human organic anion transporter (hOAT) family of transmembrane carrier proteins mediate the cellular flux of anionic substances, including certain hormones and anti-cancer drugs. hOAT4 is highly expressed at the apical membrane of the renal tubular cell and facilitates drug re-absorption in the kidney. In the present study, the impact of 10 nonsynonymous single nucleotide polymorphisms (SNPs) of hOAT4 on transport function in COS-7 cells was characterized.

EXPERIMENTAL APPROACH

Transport uptake assay was used to assess the function of the variant transporters. Cell surface biotinylation and western blot analysis were used to investigate the expression characteristics of the transporter proteins. Comparative modelling was used to interpret the influence of nonsynonymous changes in terms of hOAT4 structure.

KEY RESULTS

Four naturally occurring hOAT4 variants (L29P, R48Y, V155G and T392I) exhibited a significant loss of function. Substitution of leucine-29, which is a conserved residue in OATs, with a proline residue, impaired the synthesis or the apparent stability of the transporter and membrane insertion was disrupted in the R48Y variant. In the case of the V155G and T392I variants, impaired function was due to decreased affinity of the transporter for oestrone sulphate and impaired transporter-substrate turnover respectively. The T392I variant was inhibited more extensively than the wild-type transporter by the cationic substrate tetraethyl ammonium.

CONCLUSIONS AND IMPLICATIONS

Several naturally occurring SNPs encode variant hOAT4s that may impair the renal tubular re-absorption of important drug substrates.

Dependence of multidrug resistance protein-mediated cyclic nucleotide efflux on the background sodium conductance

Anterior pituitary cells fire action potentials and release cyclic nucleotides both spontaneously and in response to agonist stimulation, but the relationship between electrical activity and cyclic nucleotide efflux has not been studied. In these cells, a tetrodotoxin-resistant background N(+) conductance is critical for firing of action potentials, and multidrug resistance proteins (MRPs) MRP4 and MRP5 contribute to cyclic nucleotide efflux. Here, we show that abolition of the background Na(+) conductance in rat pituitary cells by complete or partial replacement of extracellular Na(+) with organic cations or sucrose induced a rapid and reversible hyperpolarization of cell membranes and inhibition of action potential firing, accompanied by a rapid inhibition of cyclic nucleotide efflux. Valinomycin-induced hyperpolarization of plasma membranes also inhibited cyclic nucleotide efflux, whereas depolarization of cell membranes induced by the inhibition of Ca(2+) influx or stimulation of Na(+) influx by gramicidin was accompanied by a facilitation of cyclic nucleotide efflux. In contrast, inhibition of cyclic nucleotide efflux by probenecid did not affect the background Na(+) conductance. In human embryonic kidney 293 cells stably transfected with human MRP4 or MRP5, replacement of bath Na(+) with organic cations also hyperpolarized the cell membranes and inhibited cyclic nucleotide efflux. In these cells, the Na(+)/H(+) antiporter monensin did not affect the membrane potential and was practically ineffective in altering cyclic nucleotide efflux. In both pituitary and MRP4- and MRP5-expressing cells, 3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid (MK571) inhibited cyclic nucleotide efflux. These results indicate that the MRP4/5-mediated cyclic nucleotide efflux can be rapidly modulated by membrane potential determined by the background Na(+) conductance.

Toward a systems level understanding of organic anion and other multispecific drug transporters: a remote sensing and signaling hypothesis

Organic anion transporters (Oats) are located in the barrier epithelia of diverse organs, where they mediate the absorption and excretion of a wide range of metabolites, signaling molecules, and xenobiotics. Although their interactions with a broad group of substrates have been extensively studied and described, the primary physiological role of Oats remains elusive. The presence of overlapping substrate specificities among the different Oat isoforms, together with recent metabolomic data from the Oat1, Oat3, and renal-specific transporter (RST/URAT1) knockout mice, suggests a possible role in remote signaling wherein substrates excreted through one Oat isoform in one organ are taken up by another Oat isoform located in a different organ, thereby mediating communication between different organ systems, or even between different organisms. Here we further develop this "remote sensing and signaling hypothesis" and suggest how the regulation of SLC22 subfamily members (including those of the organic cation, organic carnitine, and unknown substrate transporter subfamilies) can be better understood by considering the organism's broader need to communicate between epithelial and other tissues by simultaneous regulation of transport of metabolites, signaling molecules, drugs, and toxins. This systems biology perspective of remote signaling (sensing) could help reconcile an enormous array of tissue-specific data for various SLC22 family genes and, possibly, other multispecific transporters, such as those of the organic anion transporting polypeptide (OATP, SLC21) and multidrug resistance-associated protein (MRP) families.

Update on the molecular physiology of organic anion transporters

PURPOSE OF REVIEW

Organic anion transporters (OATs) mediate the renal absorption and excretion of a wide range of metabolites and xenobiotics. We discuss the recent advances that have been made in elucidating the binding and transport characteristics of OATs, new insights into their physiological role and regulation by various factors, and pharmacogenetics.

RECENT FINDINGS

Overlapping substrate specificity among the OATs is well established. However, recent findings have suggested distinct differences in the structural binding determinants among the OATs, which have important implications for understanding drug interactions and drug design. A potential role for OATs in blood pressure regulation and remote sensing has been reported. Meanwhile, factors regulating the expression of OATs continue to be identified and characterized. The effect of renal ischemia on OAT expression and function is currently being explored. Finally, recent studies identifying various OAT polymorphisms may facilitate prediction of individual drug response and toxicity.

SUMMARY

As progress is made in unveiling the many functional aspects of the OATs, it is becoming clear that their significance is not only limited to a role in drug elimination from the body, but also extends to other vital physiological roles. Further delineation of the function and regulation of the OATs will uncover enormous potential clinical and pharmacological applications.

Physiology, structure, and regulation of the cloned organic anion transporters

1. The transport of negatively charged drugs, xenobiotics, and metabolites by epithelial tissues, particularly the kidney, plays critical roles in controlling their distribution, concentration, and retention in the body. Thus, organic anion transporters (OATs) impact both their therapeutic efficacy and potential toxicity. 2. This review summarizes current knowledge of the properties and functional roles of the cloned OATs, the relationships between transporter structure and function, and those factors that determine the efficacy of transport. Such factors include plasma protein binding of substrates, genetic polymorphisms among the transporters, and regulation of transporter expression. 3. Clearly, much progress has been made in the decade since the first OAT was cloned. However, unresolved questions remain. Several of these issues--drug-drug interactions, functional characterization of newly cloned OATs, tissue differences in expression and function, and details of the nature and consequences of transporter regulation at genomic and intracellular sites--are discussed in the concluding Perspectives section.