Novel (sulfated) thyroid hormone transporters in the solute carrier 22 family

Objective

Thyroid hormone (TH) transport represents a critical first step in governing intracellular TH regulation. It is still unknown whether the full repertoire of TH transporters has been identified. Members of the solute carrier (SLC) 22 family have substrates in common with the known TH transporters of the organic anion-transporting peptide family. Therefore, we screened the SLC22 family for TH transporters.

Methods

Uptake of 1 nM of iodothyronines or sulfated iodothyronines in COS1 cells expressing SLC22 proteins was performed.

Results

We first tested 25 mouse (m) SLC22 proteins for TH uptake and found that the majority of the organic anion transporter (OAT) clade were capable of 3,3',5-triiodothyronine and/or thyroxine (T4) transport. Based on phylogenetic tree analysis of the mouse and human (h) SLC22 family, we selected eight hSLC22s that grouped with the newly identified mouse TH transporters. Of these, four tested positive for uptake of one or more substrates, particularly hSLC22A11 showed robust (3-fold over control) uptake of T4. Uptake of sulfated iodothyronines was strongly (up to 17-fold) induced by some SLC22s, most notably SLC22A8, hSLC22A9, mSLC22A27 and mSLC22A29. Finally, the zebrafish orthologues of SLC22A6/8 drOatx and drSlc22a6l also transported almost all (sulfated) iodothyronines tested. The OAT inhibitors lesinurad and probenecid inhibited most SLC22 proteins.

Conclusions

Our results demonstrated that members of the OAT clade of the SLC22 family constitute a novel, evolutionary conserved group of transporters for (sulfated) iodothyronines. Future studies should reveal the relevance of these transporters in TH homeostasis and physiology.

Pharmacokinetic Interaction of Chiglitazar with CYP3A4 Inducer or Inhibitor: An Open-Label, Sequential Crossover, Self-Control, 3-Period Study in Healthy Chinese Volunteers

Chiglitazar, a pan agonist of non-thiazolidinedione peroxisome proliferator-activated receptor, has the potential to regulate blood sugar, improve lipid metabolism, and reduce cardiovascular complications. This study aimed to examine the effect of cytochrome P450 (CYP) 3A4 inhibitors/inducers on the in vivo metabolism of chiglitazar and provide a reference for the clinical combination use of chiglitazar. A single-center, open-label, sequential crossover, and self-control study was carried out in 24 healthy subjects to determine the pharmacokinetics of chiglitazar dosed with and without CYP3A4 inhibitors and inducers. The findings showed that the CYP3A4 inhibitor itraconazole had no apparent pharmacokinetic drug interaction with chiglitazar, whereas rifampicin did. When combined with rifampicin after continuous dosing, chiglitazar exposure was not theoretically reduced but increased compared to a single dose of chiglitazar. The possible explanation may be the transporters of bile salt export pump, but this needs to be confirmed. The safety of chiglitazar in single or combination doses was well tolerated. The findings of this study provide a basis for clinical combinations of chiglitazar with CYP3A4 inhibitors or inducers.

Mirogabalin, a novel α2δ ligand, is not a substrate of LAT1, but of PEPT1, PEPT2, OAT1, OAT3, OCT2, MATE1 and MATE2-K

Mirogabalin is a α₂δ ligand as well as pregabalin. The aim of this study was to clarify whether mirogabalin is a substrate of human LAT1, which involved in absorption and disposition of pregabalin, and to investigate transporters involved in renal secretion and absorption of mirogabalin using transporter-expressing cells and fresh human kidney slices.We employed uptake assay of [³H]mirogabalin by HEK293T or HEK293 cells transiently overexpress human OAT1, OAT3, OCT2, LAT1/4F2hc, LAT2/4F2hc, PEPT1, and PEPT2 proteins. Transport assay of MDCKII cells transiently overexpress OCT2/MATE1, and OCT2/MATE2-K proteins was conducted. Contribution of transporters to renal secretion was investigated by uptake assay using human kidney slices.Uptake clearances of [³H]mirogabalin by OAT1-, OAT3-, OCT2-, PEPT1-, and PEPT2-expressing cells were higher than that by vector cells, but by LAT1/4F2hc and LAT2/4F2hc-expressing cells were not. In transport assay using OCT2/MATE1 and OCT2/MATE2-K cells, [³H]mirogabalin showed directional transport from basolateral to apical side. Contribution of OAT1, OAT3, and OCT2 was observed by uptake of [³H]mirogabalin into the kidney slices.These results indicate that mirogabalin is not a substrate of LAT1, but of PEPT1 and PEPT2 involved in absorption and of OAT1, OAT3, OCT2, MATE1 and/or MATE2-K involved in its urinary secretion.

The Prescription of Drugs That Inhibit Organic Anion Transporters 1 or 3 Is Associated with the Plasma Accumulation of Uremic Toxins in Kidney Transplant Recipients

The renal elimination of uremic toxins (UTs) can be potentially altered by drugs that inhibit organic anion transporters 1/3 (OAT1/OAT3). The objective of the present study was to determine whether the prescription of at least one OAT1/OAT3 inhibitor was associated with the plasma accumulation of certain UTs in kidney transplant recipients. We included 403 kidney transplant recipients. For each patient, we recorded all prescription drugs known to inhibit OAT1/OAT3. Plasma levels of four UTs (trimethylamine N-oxide (TMAO), indole acetic acid (IAA), para-cresylsulfate (pCS), and indoxylsulfate (IxS) were assayed using liquid chromatography-tandem mass spectrometry. Plasma UT levels were significantly higher among patients prescribed at least one OAT inhibitor (n = 311) than among patients not prescribed any OAT inhibitors (n = 92). Multivariate analysis revealed that after adjustment for age, estimated glomerular filtration rate (eGFR), plasma level of albumin and time since transplantation, prescription of an OAT1/OAT3 inhibitor was independently associated with the plasma accumulation of pCS (adjusted odds ratio (95% confidence interval): 2.11 (1.26; 3.61]). Our results emphasize the importance of understanding the interactions between drugs and UTs and those involving UT transporters in particular.

SLC22 Transporters in the Fly Renal System Regulate Response to Oxidative Stress In Vivo

Several SLC22 transporters in the human kidney and other tissues are thought to regulate endogenous small antioxidant molecules such as uric acid, ergothioneine, carnitine, and carnitine derivatives. These transporters include those from the organic anion transporter (OAT), OCTN/OCTN-related, and organic cation transporter (OCT) subgroups. In mammals, it has been difficult to show a clear in vivo role for these transporters during oxidative stress. Ubiquitous knockdowns of related Drosophila SLC22s-including transporters homologous to those previously identified by us in mammals such as the "Fly-Like Putative Transporters" FLIPT1 (SLC22A15) and FLIPT2 (SLC22A16)-have shown modest protection against oxidative stress. However, these fly transporters tend to be broadly expressed, and it is unclear if there is an organ in which their expression is critical. Using two tissue-selective knockdown strategies, we were able to demonstrate much greater and longer protection from oxidative stress compared to previous whole fly knockdowns as well as both parent and WT strains (CG6126: p < 0.001, CG4630: p < 0.01, CG16727: p < 0.0001 and CG6006: p < 0.01). Expression in the Malpighian tubule and likely other tissues as well (e.g., gut, fat body, nervous system) appear critical for managing oxidative stress. These four Drosophila SLC22 genes are similar to human SLC22 transporters (CG6126: SLC22A16, CG16727: SLC22A7, CG4630: SLC22A3, and CG6006: SLC22A1, SLC22A2, SLC22A3, SLC22A6, SLC22A7, SLC22A8, SLC22A11, SLC22A12 (URAT1), SLC22A13, SLC22A14)-many of which are highly expressed in the kidney. Consistent with the Remote Sensing and Signaling Theory, this indicates an important in vivo role in the oxidative stress response for multiple SLC22 transporters within the fly renal system, perhaps through interaction with SLC22 counterparts in non-renal tissues. We also note that many of the human relatives are well-known drug transporters. Our work not only indicates the importance of SLC22 transporters in the fly renal system but also sets the stage for in vivo studies by examining their role in mammalian oxidative stress and organ crosstalk.

Pharmacokinetics of gallic acid and protocatechuic acid in humans after dosing with Relinqing (RLQ) and the potential for RLQ-perpetrated drug-drug interactions on organic anion transporter (OAT) 1/3

CONTEXT

Relinqing granules (RLQ) are being used alone or in combination with antibacterial drugs to treat urological disorders.

OBJECTIVE

This study investigates the pharmacokinetics of RLQ in humans and the potential for RLQ-perpetrated interactions on transporters.

MATERIALS AND METHODS

Twelve healthy subjects (six women and six men) participated to compare single- and multiple-dose pharmacokinetics of RLQ. In the single-dose study, all 12 subjects received 8 g of RLQ orally. After a 7-d washout period, the subjects received 8 g of RLQ for seven consecutive days (t.i.d.) and then a single dose. Gallic acid (GA) and protocatechuic acid (PCA) in plasma and urine samples were analysed using LC-MS/MS. The transfected cells were used to study the inhibitory effect of GA (50-5000 μg/L) and PCA (10-1000 μg/L) on transporters OAT1, OAT3, OCT2, OATP1B1, P-gp and BCRP.

RESULTS

GA and PCA were absorbed into the blood within 1 h after administration and rapidly eliminated with a half-life of less than 2 h. The mean peak concentrations of GA (102 and 176 μg/L) and PCA (4.54 and 7.58 μg/L) were lower in males than females, respectively. The 24 h urine recovery rates of GA and PCA were about 10% and 5%, respectively. The steady-state was reached in 7 d without accumulation. GA was a potent inhibitor of OAT1 (IC₅₀ = 3.73 μM) and OAT3 (IC₅₀ = 29.41 μM), but not OCT2, OATP1B1, P-gp or BCRP.

DISCUSSION AND CONCLUSIONS

GA and PCA are recommended as PK-markers in RLQ-related pharmacokinetic and drug interaction studies. We should pay more attention to the potential for RLQ-perpetrated interactions on transporters.

Molecular Properties of Drugs Handled by Kidney OATs and Liver OATPs Revealed by Chemoinformatics and Machine Learning: Implications for Kidney and Liver Disease

In patients with liver or kidney disease, it is especially important to consider the routes of metabolism and elimination of small-molecule pharmaceuticals. Once in the blood, numerous drugs are taken up by the liver for metabolism and/or biliary elimination, or by the kidney for renal elimination. Many common drugs are organic anions. The major liver uptake transporters for organic anion drugs are organic anion transporter polypeptides (OATP1B1 or SLCO1B1; OATP1B3 or SLCO1B3), whereas in the kidney they are organic anion transporters (OAT1 or SLC22A6; OAT3 or SLC22A8). Since these particular OATPs are overwhelmingly found in the liver but not the kidney, and these OATs are overwhelmingly found in the kidney but not liver, it is possible to use chemoinformatics, machine learning (ML) and deep learning to analyze liver OATP-transported drugs versus kidney OAT-transported drugs. Our analysis of >30 quantitative physicochemical properties of OATP- and OAT-interacting drugs revealed eight properties that in combination, indicate a high propensity for interaction with "liver" transporters versus "kidney" ones based on machine learning (e.g., random forest, k-nearest neighbors) and deep-learning classification algorithms. Liver OATPs preferred drugs with greater hydrophobicity, higher complexity, and more ringed structures whereas kidney OATs preferred more polar drugs with more carboxyl groups. The results provide a strong molecular basis for tissue-specific targeting strategies, understanding drug-drug interactions as well as drug-metabolite interactions, and suggest a strategy for how drugs with comparable efficacy might be chosen in chronic liver or kidney disease (CKD) to minimize toxicity.

Evolutionary Analysis of OAT Gene Family in River and Swamp Buffalo: Potential Role of SLCO3A1 Gene in Milk Performance

The organic anion transporter (OAT) family is the subfamily of the solute carrier (SLC) superfamily, which plays a vital role in regulating essential nutrients in milk. However, little is known about the members’ identification, evolutionary basis, and function characteristics of OAT genes associated with milk performance in buffalo. Comparative genomic analyses were performed to identify the potential role of buffalo OAT genes in milk performance in this study. The results showed that a total of 10 and 7 OAT genes were identified in river buffalo and swamp buffalo, respectively. These sequences clustered into three groups based on their phylogenetic relationship and had similar motif patterns and gene structures in the same groups. Moreover, the river-specific expansions and homologous loss of OAT genes occurred in the two buffalo subspecies during the evolutionary process. Notably, the duplicated SLCO3A1 gene specific to river buffalo showed higher expression level in mammary gland tissue than that of swamp buffalo. These findings highlight some promising candidate genes that could be potentially utilized to accelerate the genetic progress in buffalo breeding programs. However, the identified candidate genes require further validation in a larger cohort for use in the genomic selection of buffalo for milk production.

Para-aminosalicylic acid significantly reduced tenofovir exposure in human subjects: Mismatched findings from in vitro to in vivo translational research

AIMS

Tenofovir and para-aminosalicylic acid (PAS) may be coprescribed to treat patients with concomitant infections of human immunodeficiency virus and Mycobacterium tuberculosis bacteria. Both drugs are known to have remarkable renal uptake transporter-mediated clearance. Owing to the lack of clinical studies on drug-drug interaction between the 2 drugs, we conducted a translational clinical study to investigate the effect of PAS on tenofovir pharmacokinetics (PK).

METHODS

Initially, we studied in vitro renal uptake transporter-mediated drug-drug interactions using stably transfected cells with human organic anion transporters (OAT1 and OAT3). Later, we estimated clinical drug interactions using static and physiologically based PK modelling. Finally, we investigated the effects of PAS-calcium formulation (PAS-Ca) on tenofovir disoproxil fumarate PK in healthy male Korean subjects.

RESULTS

PAS inhibited OAT1- and OAT3-mediated tenofovir uptake in vitro. The physiologically based PK drug-drug interaction model suggested a 1.26-fold increase in tenofovir peak plasma concentration when coadministered with PAS. By contrast, an open-label, randomized, crossover clinical trial evaluating the effects of PAS-Ca on tenofovir PK showed significantly altered geometric mean ratio (90% confidence intervals) of maximum plasma concentration (C_max ) and area under the curve (AUC_0-inf ) by 0.33 (0.28-0.38) and 0.29 (0.26-0.33), respectively.

CONCLUSION

Our study findings suggest that the PAS-Ca formulation significantly reduced systemic exposure to tenofovir through an unexplained mechanism, which was contrary to the initial prediction. Caution should be exercised while predicting in vivo PK profiles from in vitro data, particularly when there are potential confounders such as pharmaceutical interactions.

Potent Inhibitors of Organic Anion Transporters 1 and 3 From Natural Compounds and Their Protective Effect on Aristolochic Acid Nephropathy

Organic anion transporters 1 and 3 (OAT1 and OAT3) play a critical role in renal drug-drug interactions and are involved in the nephrotoxicity of many anionic xenobiotics. To date, relatively little is known about the interaction of natural compounds with OAT1 and OAT3. Of the 270 natural compounds screened in the present study, 21 compounds inhibited OAT1 and 45 compounds inhibited OAT3. Further concentration-dependent studies identified 7 OAT1 inhibitors and 10 OAT3 inhibitors with IC50 values of <10 μM, and most of them were flavonoids, the most commonly ingested polyphenolic compounds in the diet and herbal products. Computational modeling of OAT1 and OAT3 revealed the important residues for the recognition of inhibitors. The two strong OAT inhibitors, namely wedelolactone and wogonin, were evaluated for their in vivo interactions with the OAT substrate aristolochic acid I (AAI), a natural compound causing aristolochic acid-induced nephropathy (AAN) in many species. The cytotoxicity of AAI increased in two OAT-overexpressing cell lines, with more cytotoxicity in OAT1-overexpressing cells, suggesting a more important role of OAT1 than OAT3 in AAN. Both wedelolactone and wogonin markedly increased serum AAI concentrations in AAI-treated rats and ameliorated kidney injuries in AAI-treated mice. To conclude, the present findings are of significant value in understanding natural compound-drug interactions and provide a natural source for developing treatments for AAN.

Influence of vitamin D treatment on functional expression of drug disposition pathways in human kidney proximal tubule cells during simulated uremia

Effects of cholecalciferol (VitD₃) and calcitriol (1,25-VitD₃), on the expression and function of major vitamin D metabolizing enzymes (cytochrome P450 [CYP]2R1, CYP24A1) and select drug transport pathways (ABCB1/P-gp, SLCO4C1/OATP4C1) were evaluated in human kidney proximal tubule epithelial cells (hPTECs) under normal and uraemic serum conditions.hPTECs were incubated with 10% normal or uraemic serum for 24 h followed by treatment with 2% ethanol vehicle, or 100 and 240 nM doses of VitD₃, or 1,25-VitD₃ for 6 days. The effects of treatment on mRNA and protein expression and functional activity of select CYP enzymes and transporters were assessedUnder uraemic serum, treatment with 1,25-VitD₃ resulted in increased mRNA but decreased protein expression of CYP2R1. Activity of CYP2R1 was not influenced by serum or VitD analogues. CYP24A1 expression was increased with 1,25-VitD₃ under normal as well as uraemic serum, although to a lesser extent. ABCB1/P-gp mRNA expression increased under normal and uraemic serum, with exposure to 1,25-VitD₃. SLCO4C1/OATP4C1 exhibited increased mRNA but decreased protein expression, under uraemic serum + 1,25-VitD₃. Functional assessments of transport showed no changes regardless of exposure to serum or 1,25-VitD₃.Key findings indicate that uraemic serum and VitD treatment led to differential effects on the functional expression of CYPs and transporters in hPTECs.

A key role for the transporter OAT1 in systemic lipid metabolism

Organic anion transporter 1 (OAT1/SLC22A6) is a drug transporter with numerous xenobiotic and endogenous substrates. The Remote Sensing and Signaling Theory suggests that drug transporters with compatible ligand preferences can play a role in “organ crosstalk,” mediating overall organismal communication. Other drug transporters are well known to transport lipids, but surprisingly little is known about the role of OAT1 in lipid metabolism. To explore this subject, we constructed a genome-scale metabolic model using omics data from the Oat1 knockout mouse. The model implicated OAT1 in the regulation of many classes of lipids, including fatty acids, bile acids, and prostaglandins. Accordingly, serum metabolomics of Oat1 knockout mice revealed increased polyunsaturated fatty acids, diacylglycerols, and long-chain fatty acids and decreased ceramides and bile acids when compared with wildtype controls. Some aged knockout mice also displayed increased lipid droplets in the liver when compared with wildtype mice. Chemoinformatics and machine learning analyses of these altered lipids defined molecular properties that form the structural basis for lipid-transporter interactions, including the number of rings, positive charge/volume, and complexity of the lipids. Finally, we obtained targeted serum metabolomics data after short-term treatment of rodents with the OAT-inhibiting drug probenecid to identify potential drug–metabolite interactions. The treatment resulted in alterations in eicosanoids and fatty acids, further supporting our metabolic reconstruction predictions. Consistent with the Remote Sensing and Signaling Theory, the data support a role of OAT1 in systemic lipid metabolism.

Characteristic and Mechanism of Drug-Herb Interaction Between Acetylsalicylic Acid and Danhong Injection Mediated by Organic Anion Transporters

The mixture of Salvia miltiorrhiza and Carthamus tinctorius (Danhong injection, DHI) is widely prescribed in China for the treatment of cardiovascular and cerebrovascular diseases. In most cases, DHI is used in combination with acetylsalicylic acid (aspirin, ASA). However, the interaction between DHI and ASA remains largely undefined. The purpose of this study is to explore the interaction profile and mechanism between DHI and ASA. The frequency of drug combination of DHI and ASA was analyzed based on 5,183 clinical cases. The interaction characteristics were evaluated by analyzing the pharmacokinetics and disposition profile of salicylic acid (SA, the primary metabolite of ASA) in rats. The interaction mechanisms were explored through evaluating the hydrolysis of ASA regulated by ASA esterase, the tubular secretion of SA mediated by influx and efflux transporters, and the tubular reabsorption of SA regulated by urinary acidity-alkalinity. The inhibitory potential of DHI on organic anion transporters (OATs) was further verified in aristolochic acid I (AAI) induced nephropathy. Clinical cases analysis showed that DHI and ASA were used in combination with high frequency of 70.73%. In drug combination of DHI and ASA, the maximum plasma concentration of SA was significantly increased by 1.37 times, while the renal excretion of SA was significantly decreased by 32.54%. The mechanism study showed that DHI significantly inhibited the transport function, gene transcription and protein expression of OATs. In OATs mediated AAI nephropathy, DHI significantly reduced the renal accumulation of AAI by 55.27%, and alleviated renal damage such as glomerulus swelling, tubular blockage and lymphocyte filtration. In drug combination of DHI and ASA, DHI increased the plasma concentration of SA not through enhancing the hydrolysis of ASA, and the tubular reabsorption of SA was not significantly affected. Inhibition of tubular secretion of SA mediated by OATs might be the reason that contributes to the decrease of SA renal excretion.

Identification of Prognostic Organic Cation and Anion Transporters in Different Cancer Entities by In Silico Analysis

The information derived from next generation sequencing technology allows the identification of deregulated genes, gene mutations, epigenetic modifications, and other genomic events that are associated with a given tumor entity. Its combination with clinical data allows the prediction of patients’ survival with a specific gene expression pattern. Organic anion transporters and organic cation transporters are important proteins that transport a variety of substances across membranes. They are also able to transport drugs that are used for the treatment of cancer and could be used to improve treatment. In this study, we have made use of publicly available data to analyze if the expression of organic anion transporters or organic cation transporters have a prognostic value for a given tumor entity. The expression of most organic cation transporters is prognostic favorable. Within the organic anion transporters, the ratio between favorable and unfavorable organic anion transporters is nearly equal for most tumor entities and only in liver cancer is the number of unfavorable genes two times higher compared to favorable genes. Within the favorable genes, UNC13B, and SFXN2 cover nine cancer types and in the same way, SLC2A1, PLS3, SLC16A1, and SLC16A3 within the unfavorable set of genes and could serve as novel target structures.

A Pilot Study of the Maternal-Fetal Pharmacokinetics of Furosemide in Plasma, Urine, and Amniotic Fluid of Hypertensive Parturient Women Under Cesarean Section

The third trimester of pregnancy is related to physiological changes that can modify the process of absorption, distribution, metabolism, and excretion and, consequently, the efficacy and toxicity of drugs. However, little is known about furosemide pharmacokinetics and placental transfer in pregnancy. This study evaluated the maternal-fetal pharmacokinetics and distribution to amniotic fluid of furosemide in hypertensive parturient women under cesarean section. Twelve hypertensive parturient women under methyldopa (250 mg/8 h) and/or pindolol (10 mg/12 h) treatment received a 40-mg single oral dose of furosemide 1 to 10 hours before delivery by cesarean section. Blood and urine samples were collected for 12 hours after furosemide administration. At delivery, samples were obtained from maternal and umbilical cord blood (n = 8) to assess the transplacental transfer. Amniotic fluid (n = 4) was collected at the time of delivery. The following furosemide pharmacokinetic parameters were obtained as median (interquartile range): C_max , 403 ng/mL (229 to 715 ng/mL); T_max , 2.00 hours (1.50 to 4.83 hours); elimination half-life (t_1/2 ), 2.50 hours (1.77 to 2.97 hours); AUC_{0-12 h} , 1366 ng⋅h/mL (927 to 2531 ng⋅h/mL); AUC_0-∞ , 1580 ng⋅h/mL (1270 to 2881 ng⋅h/mL); CL/F 25.3 L/h (13.8 to 31.4 L/h); CLR, 2.50 L/h (1.77 to 2.97 L/h); CLNR, 22.7 L/h (12.1 to 25.6 L/h); and V_d /F 82.8 L (34.4 to 173 L). The transplacental transfer of furosemide was 0.43 (0.10 to 0.73), and the amniotic fluid concentration was 11.0 ng/mL (5.51 to 14.6 ng/mL). From a clinical point of view, these results suggest that substrates of uridine diphosphate-glucuronosyltransferase isoenzymes such as furosemide may have increased clearance during pregnancy and could require dose adjustment in this population.

Acamprosate Is a Substrate of the Human Organic Anion Transporter (OAT) 1 without OAT3 Inhibitory Properties: Implications for Renal Acamprosate Secretion and Drug-Drug Interactions

Acamprosate is an anionic drug substance widely used in treating symptoms of alcohol withdrawal. It was recently shown that oral acamprosate absorption is likely due to paracellular transport. In contrast, little is known about the eliminating mechanism clearing acamprosate from the blood in the kidneys, despite the fact that studies have shown renal secretion of acamprosate. The hypothesis of the present study was therefore that renal organic anion transporters (OATs) facilitate the renal excretion of acamprosate in humans. The aim of the present study was to establish and apply OAT1 (gene product of SLC22A6) and OAT3 (gene product of SLC22A8) expressing cell lines to investigate whether acamprosate is a substrate or inhibitor of OAT1 and/or OAT3. The studies were performed in HEK293-Flp-In cells stably transfected with SLC22A6 or SLC22A8. Protein and functional data showed that the established cell lines are useful for studying OAT1- and OAT3-mediated transport in bi-laboratory studies. Acamprosate inhibited OAT1-mediated p-aminohippuric acid (PAH) uptake but did not inhibit substrate uptake via OAT3 expressing cells, neither when applied concomitantly nor after a 3 h preincubation with acamprosate. The uptake of PAH via OAT1 was inhibited in a competitive manner by acamprosate and cellular uptake studies showed that acamprosate is a substrate for OAT1 with a K_m-value of approximately 700 µM. Probenecid inhibited OAT1-mediated acamprosate uptake with a K_i-value of approximately 13 µM, which may translate into an estimated clinically significant DDI index. In conclusion, acamprosate was identified as a substrate of OAT1 but not OAT3.

Effects of Tenofovir on the Single-Dose Pharmacokinetics of Intravenous Morinidazole in Healthy Chinese Subjects

The effects of multiple-dose administration of tenofovir disoproxil fumarate (TDF) on the pharmacokinetics of morinidazole (MOR) were compared in healthy subjects. MOR exposure was similar, with an area under the curve from 0 h to infinity (AUC_0-∞) treatment ratio for MOR+TDF/MOR of 1.01 (90% confidence interval, 0.97 to 1.06). No relevant differences were observed regarding plasma exposure of metabolites. Renal clearances of MOR and its metabolites were not affected by TDF. No unexpected safety or tolerability issues were observed.

Drosophila SLC22 Orthologs Related to OATs, OCTs, and OCTNs Regulate Development and Responsiveness to Oxidative Stress

The SLC22 family of transporters is widely expressed, evolutionarily conserved, and plays a major role in regulating homeostasis by transporting small organic molecules such as metabolites, signaling molecules, and antioxidants. Analysis of transporters in fruit flies provides a simple yet orthologous platform to study the endogenous function of drug transporters in vivo. Evolutionary analysis of Drosophila melanogaster putative SLC22 orthologs reveals that, while many of the 25 SLC22 fruit fly orthologs do not fall within previously established SLC22 subclades, at least four members appear orthologous to mammalian SLC22 members (SLC22A16:CG6356, SLC22A15:CG7458, CG7442 and SLC22A18:CG3168). We functionally evaluated the role of SLC22 transporters in Drosophila melanogaster by knocking down 14 of these genes. Three putative SLC22 ortholog knockdowns-CG3168, CG6356, and CG7442/SLC22A-did not undergo eclosion and were lethal at the pupa stage, indicating the developmental importance of these genes. Additionally, knocking down four SLC22 members increased resistance to oxidative stress via paraquat testing (CG4630: p < 0.05, CG6006: p < 0.05, CG6126: p < 0.01 and CG16727: p < 0.05). Consistent with recent evidence that SLC22 is central to a Remote Sensing and Signaling Network (RSSN) involved in signaling and metabolism, these phenotypes support a key role for SLC22 in handling reactive oxygen species.

Unique metabolite preferences of the drug transporters OAT1 and OAT3 analyzed by machine learning

The multispecific organic anion transporters, OAT1 (SLC22A6) and OAT3 (SLC22A8), the main kidney elimination pathways for many common drugs, are often considered to have largely-redundant roles. However, whereas examination of metabolomics data from Oat-knockout mice (Oat1 and Oat3KO) revealed considerable overlap, over a hundred metabolites were increased in the plasma of one or the other of these knockout mice. Many of these relatively unique metabolites are components of distinct biochemical and signaling pathways, including those involving amino acids, lipids, bile acids, and uremic toxins. Cheminformatics, together with a "logical" statistical and machine learning-based approach, identified a number of molecular features distinguishing these unique endogenous substrates. Compared with OAT1, OAT3 tends to interact with more complex substrates possessing more rings and chiral centers. An independent "brute force" approach, analyzing all possible combinations of molecular features, supported the logical approach. Together, the results suggest the potential molecular basis by which OAT1 and OAT3 modulate distinct metabolic and signaling pathways in vivo As suggested by the Remote Sensing and Signaling Theory, the analysis provides a potential mechanism by which "multispecific" kidney proximal tubule transporters exert distinct physiological effects. Furthermore, a strong metabolite-based machine-learning classifier was able to successfully predict unique OAT1 versus OAT3 drugs; this suggests the feasibility of drug design based on knockout metabolomics of drug transporters. The approach can be applied to other SLC and ATP-binding cassette drug transporters to define their nonredundant physiological roles and for analyzing the potential impact of drug-metabolite interactions.

Leflunomide increased the renal exposure of acyclovir by inhibiting OAT1/3 and MRP2

Rheumatoid arthritis patients can be prescribed a combination of immunosuppressive drug leflunomide (LEF) and the antiviral drug acyclovir to reduce the high risk of infection. Acyclovir is a substrate of organic anion transporter (OAT) 1/3 and multidrug resistance-associated protein (MRP) 2. Considering the extraordinarily long half-life of LEF's active metabolite teriflunomide (TER) and the kidney injury risk of acyclovir, it is necessary to elucidate the potential impact of LEF on the disposition of acyclovir. Here we used a specific MRP inhibitor MK571 and probenecid (OAT1/3 and MRP2 inhibitor) to assess the effects of MRP2 and OAT1/3 on the pharmacokinetics and tissue distribution of acyclovir in rats. We showed that LEF and probenecid, but not MK571 significantly increased the plasma concentration of acyclovir. However, kidney and liver exposures of acyclovir were increased when coadministered with LEF, probenecid or MK571. The kidney/plasma ratio of acyclovir was increased to approximately 2-fold by LEF or probenecid, whereas it was increased to as much as 14.5-fold by MK571. Consistently, these drugs markedly decreased the urinary excretion of acyclovir. TER (0.5-100 μmol/L) dose-dependently increased the accumulation of acyclovir in MRP2-MDCK cells with an IC₅₀ value of 4.91 μmol/L. TER (5 μmol/L) significantly inhibited the uptake of acyclovir in hOAT1/3-HEK293 cells. These results suggest that LEF/TER increased the kidney accumulation of acyclovir by inhibiting the efflux transporter MRP2, which increased its kidney/plasma ratio and renal injury risk. However, the inhibitory effects of LEF/TER on OAT1/3 reduced the tubular cells' uptake of acyclovir and increased the plasma concentration.

Activation of Protein Kinase A Stimulates SUMOylation, Expression, and Transport Activity of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays a vital role in removing a broad variety of anionic drugs from kidney, thus avoiding their possible toxicity in the body. We earlier established that activation of protein kinase C (PKC) enhances OAT3 ubiquitination, which promotes OAT3 internalization from the cell plasma membrane to intracellular endosomes and consequent degradation. As a result, OAT3 expression and transport activity are reduced. In the current study, we discovered that protein kinase A (PKA) had an opposite effect to PKC on the regulation of OAT3. We showed that activation of PKA by Bt2-cAMP stimulated OAT3 transport activity, which was largely caused by an enhanced plasma membrane expression of the transporter, kinetically reflected as an augmented maximal transport velocity V_max without notable alteration in substrate-binding affinity K_m. Additionally, we showed that PKA activation accelerated the rate of OAT3 recycling from intracellular compartments to the plasma membrane and decelerated the rate of OAT3 degradation. We further showed that OAT3 is subjected to post-translational modification by SUMO-2 and SUMO-3 not by SUMO-1. PKA activation enhanced OAT3 SUMOylation, which was accompanied by a reduced OAT3 ubiquitination. Finally, insulin-like growth factor 1 significantly stimulated OAT3 transport activity and SUMOylation through PKA signaling pathway. In conclusion, this is the first demonstration that PKA stimulated OAT3 expression and transport activity by altering the trafficking kinetics of OAT3 possibly through the crosstalk between SUMOylation and ubiquitination. Our studies are consistent with a remote sensing and signaling model for transporters (Wu et al. in Mol Pharmacol. 79(5):795-805, 2011).

From the Cover: Identification of Natural Products as Inhibitors of Human Organic Anion Transporters (OAT1 and OAT3) and Their Protective Effect on Mercury-Induced Toxicity

Mercury accumulates in kidneys and produces acute kidney injury. Semen cassiae (SC), a widely consumed tea and herbal medicine in Eastern Asia, has been reported to have protective effects on kidneys. In this study, SC extract was shown to almost abolish the histological alterations induced by mercuric chloride in rat kidneys. A total of 22 compounds were isolated from SC, and 1,7,8-methoxyl-2-hydroxyl-3-methyl-anthraquinone was detected in SC for the first time. Among the eight compounds identified in the blood of rats after SC treatment, six were strong inhibitors of human organic anion transporter 1 and 3 (OAT1 and OAT3). Inhibitory studies revealed that OAT1 and OAT3 were inhibited by SC constituents, in both a competitive and noncompetitive manner. Both OAT1- and OAT3-overexpressing cells were susceptible to the cytotoxicity of the cysteine-mercury conjugate, but only OAT1-overexpressing cells could be protected by 200 μM probenecid or 10 μM of the eight inhibitors in SC, suggesting that OAT1 is the major determinant in the cellular uptake of mercury. To facilitate the identification of inhibitors of OAT1 and OAT3, models of OAT1 and OAT3 were constructed using recently determined protein templates. By combining in silico and in vitro methods, inhibitors of OAT1 and OAT3 were predicted and validated from SC constituents. Collectively, the present study suggests that additional inhibitors of OAT1 and OAT3 can be predicted and validated from natural products by combining docking and in vitro screening, and could be a source of pharmaceutical compounds for developing treatments for mercury-induced kidney injury.

Altered methotrexate clearance in the treatment of CNS lymphoma with concurrent use of nitrofurantoin for a urinary tract infection

Methotrexate is a widely used chemotherapy agent with a propensity for drug interactions placing the patient at risk for toxicities. There are several modes for altering methotrexate clearance including concomitant drugs leading to toxic effects on the kidneys, medications producing an acidic urine pH, and agents interfering with methotrexate transporters including the organic anion transporter and breast cancer resistance protein efflux pump. We report a case of a patient with central nervous system non-Hodgkin's lymphoma receiving high-dose methotrexate and being concomitantly treated for a urinary tract infection with nitrofurantoin. Subsequently, her initial methotrexate clearance was altered by the introduction of nitrofurantoin and returned to baseline when her nitrofurantoin was discontinued. This is the first case report describing the altered methotrexate clearance from concurrent administration of methotrexate and nitrofurantoin.

Cellular Uptake of the Atypical Antipsychotic Clozapine Is a Carrier-Mediated Process

The weak base antipsychotic clozapine is the most effective medication for treating refractory schizophrenia. The brain-to-plasma concentration of unbound clozapine is greater than unity, indicating transporter-mediated uptake, which has been insufficiently studied. This is important, because it could have a significant impact on clozapine's efficacy, drug-drug interaction, and safety profile. A major limitation of clozapine's use is the risk of clozapine-induced agranulocytosis/granulocytopenia (CIAG), which is a rare but severe hematological adverse drug reaction. We first studied the uptake of clozapine into human brain endothelial cells (hCMEC/D3). Clozapine uptake into cells was consistent with a carrier-mediated process, which was time-dependent and saturable ( V_max = 3299 pmol/million cells/min, K_m = 35.9 μM). The chemical inhibitors lamotrigine, quetiapine, olanzapine, prazosin, verapamil, indatraline, and chlorpromazine reduced the uptake of clozapine by up to 95%. This could in part explain the in vivo interactions observed in rodents or humans for these compounds. An extensive set of studies utilizing transporter-overexpressing cell lines and siRNA-mediated transporter knockdown in hCMEC/D3 cells showed that clozapine was not a substrate of OCT1 (SLC22A1), OCT3 (SLC22A3), OCTN1 (SLC22A4), OCTN2 (SLC22A5), ENT1 (SLC29A1), ENT2 (SLC29A2), and ENT4/PMAT (SLC29A4). In a recent genome-wide analysis, the hepatic uptake transporters SLCO1B1 (OATP1B1) and SLCO1B3 (OATP1B3) were identified as additional candidate transporters. We therefore also investigated clozapine transport into OATP1B-transfected cells and found that clozapine was neither a substrate nor an inhibitor of OATP1B1 and OATP1B3. In summary, we have identified a carrier-mediated process for clozapine uptake into brain, which may be partly responsible for clozapine's high unbound accumulation in the brain and its drug-drug interaction profile. Cellular clozapine uptake is independent from currently known drug transporters, and thus, molecular identification of the clozapine transporter will help to understand clozapine's efficacy and safety profile.

Quantification of Hepatic Organic Anion Transport Proteins OAT2 and OAT7 in Human Liver Tissue and Primary Hepatocytes

Organic anion transporter (OAT) 2 and OAT7 were recently shown to be involved in the hepatic uptake of drugs; however, there is limited understanding of the population variability in the expression of these transporters in liver. There is also a need to derive relative expression-based scaling factors (REFs) that can be used to bridge in vitro functional data to the in vivo drug disposition. To this end, we quantified OAT2 and OAT7 surrogate peptide abundance in a large number of human liver tissue samples ( n = 52), as well as several single-donor cryopreserved human hepatocyte lots ( n = 30) by a novel, validated liquid chromatography tandem mass spectrometry (LC-MS/MS) method. The average surrogate peptide expression of OAT2 and OAT7 in the liver samples was 1.52 ± 0.57 and 4.63 ± 1.58 fmol/μg membrane protein, respectively. While we noted statistically significant differences ( p < 0.05) in hepatocyte and liver tissue abundances for both OAT2 and OAT7, the differences were relatively small (1.8- and 1.5-fold difference in median values, respectively). Large interindividual variability was noted in the hepatic expression of OAT2 (16-fold in liver tissue and 23-fold in hepatocytes). OAT7, on the other hand, showed less interindividual variability (4-fold) in the livers, but high variability for the hepatocyte lots (27-fold). A significant positive correlation in OAT2 and OAT7 expression was observed, but expression levels were neither associated with age nor sex. In conclusion, our data suggest marked interindividual variability in the hepatic expression of OAT2/7, which may contribute to the pharmacokinetic variability of their substrates. Because both transporters were less abundant in hepatocytes than livers, a REF-based approach is recommended when scaling in vitro hepatocyte transport data to predict hepatic drug clearance and liver exposure of OAT2/7 substrates.

Comparative Study of the Dose-Dependence of OATP1B Inhibition by Rifampicin Using Probe Drugs and Endogenous Substrates in Healthy Volunteers

PURPOSE

To evaluate association of the dose-dependent effect of rifampicin, an OATP1B inhibitor, on the plasma concentration-time profiles among OATP1B substrates drugs and endogenous substrates.

METHODS

Eight healthy volunteers received atorvastatin (1 mg), pitavastatin (0.2 mg), rosuvastatin (0.5 mg), and fluvastatin (2 mg) alone or with rifampicin (300 or 600 mg) in a crossover fashion. The plasma concentrations of these OATP1B probe drugs, total and direct bilirubin, glycochenodeoxycholate-3-sulfate (GCDCA-S), and coproporphyrin I, were determined.

RESULTS

The most striking effect of 600 mg rifampicin was on atorvastatin (6.0-times increase) and GCDCA-S (10-times increase). The AUC_0-24h of atorvastatin was reasonably correlated with that of pitavastatin (r² = 0.73) and with the AUC_0-4h of fluvastatin (r² = 0.62) and sufficiently with the AUC_0-24h of rosuvastatin (r² = 0.32). The AUC_0-24h of GCDCA-S was reasonably correlated with those of direct bilirubin (r² = 0.74) and coproporphyrin I (r² = 0.78), and sufficiently with that of total bilirubin (r² = 0.30). The AUC_0-24h of GCDCA-S, direct bilirubin, and coproporphyrin I were reasonably correlated with that of atorvastatin (r² = 0.48-0.70) [corrected].

CONCLUSION

These results suggest that direct bilirubin, GCDCA-S, and coproporphyrin I are promising surrogate probes for the quantitative assessment of potential OATP1B-mediated DDI.

Functional transport of organic anions and cations in the murine mesonephros

The mesonephros of mammals is a transient renal structure that contributes to various aspects of mammalian fetal development, including the male reproductive system, hematopoietic stem cells, and vascular endothelial cells. The mesonephros develops from the intermediate mesoderm and forms tubules that are segmented in a similar way to the nephrons of the permanent kidney (but lacking loops of Henle). Early studies have suggested that the mesonephros in marsupials and some placental mammals may perform an excretory function, but these studies have not directly shown active transport of organic anions and cations. Excretory function in the rodent mesonephros has not been investigated. Functional characterization of the earliest stages of mammalian renal development is important for our understanding of congenital disease and may help to inform the growing field of renal tissue engineering. Here, we use live uptake and efflux assays in vitro to show that the murine mesonephros is able to transport organic anions and cations through specific transporters from early in its development. Transcript analysis suggests that there are subtle differences between the transporters involved in uptake and efflux by the murine permanent metanephric tubules and by the mesonephric tubules. These data suggest that the mammalian mesonephros can provide an excretory function for the early developing embryo, in addition to the excretory function provided by the placenta.

Inhibition of Methotrexate Uptake via Organic Anion Transporters OAT1 and OAT3 by Glucuronides of Nonsteroidal Anti-inflammatory Drugs

Combination therapy of non-steroidal anti-inflammatory drugs (NSAIDs) and methotrexate (MTX) sometimes triggers adverse effects, such as liver injury, renal failure, gastrointestinal disorders, and myelosuppression, owing to the reduction of MTX clearance. Previous reports have suggested that NSAIDs inhibit renal MTX uptake via organic anion transporters (OATs) and reduced folate transporter (RFC)-1 and efflux via multidrug resistance-associated proteins (MRPs). Recently, our laboratory found inhibitory effects of NSAIDs-glucuronide (NSAIDs-Glu), a major metabolite of NSAIDs, on MRP-mediated MTX transport as a new site of interaction between MTX and NSAIDs. However, it remains unclear that whether NSAIDs-Glu inhibit renal uptake of MTX. Therefore, the present study aimed to evaluate inhibitory effects of several NSAIDs-Glu (diclofenac, R- and S-ibuprofen, R- and S-flurbiprofen, and R- and S-naproxen) on human OAT1 and OAT3-mediated MTX transport. In this study, [³H]MTX uptake was observed by using human OAT1 and OAT3-overexpressing HEK293 cells in the presence or absence of NSAIDs-Glu. All examined NSAIDs-Glu exhibited concentration-dependent inhibitory effects on MTX uptake via OAT1 and OAT3. Our results indicated that NSAIDs-Glu are more potent (5- to 15-fold) inhibitors of OAT3 than OAT1. Moreover, stereoselective inhibitory effects of NSAIDs-Glu on OATs-mediated MTX uptake were not observed, unlike on MRPs-mediated transport. These findings suggest that inhibition of OAT1 and OAT3-mediated renal uptake of MTX by plasma NSAIDs-Glu may be one of the competitive sites underlying complex drug interaction between MTX and NSAIDs.

Organic anion transporter OAT3 enhances the glucosuric effect of the SGLT2 inhibitor empagliflozin

The sodium-glucose cotransporter SGLT2 inhibitor empagliflozin (plasma protein binding ~88%) may reach its target in the brush border of the early proximal tubule by glomerular filtration and tubular secretion. Here we determined whether empagliflozin is secreted by renal tubules in mice and whether genetic knockout of the basolateral organic anion transporter 3 ( Oat3-/-) affects its tubular secretion or glucosuric effect. Renal clearance studies in wild-type (WT) mice showed that tubular secretion accounted for 50-70% of empagliflozin urinary excretion. Immunostaining indicated that SGLT2 and OAT3 localization partially overlapped in proximal tubule S1 and S2 segments. Glucosuria in metabolic cage studies was reduced in Oat3-/- vs. WT mice for acute empagliflozin doses of 1, 3, and 10 mg/kg, whereas 30 mg/kg induced similar maximal glucosuria in both genotypes. Chronic application of empagliflozin (~25 mg·kg^-1 ·day^-1) in Oat3-/- mice was associated with lower urinary glucose-to-creatinine ratios despite maintaining slightly higher blood glucose levels than WT. On a whole kidney level, renal secretion of empagliflozin was largely unchanged in Oat3-/- mice. However, the absence of OAT3 attenuated the influence of empagliflozin on fractional glucose excretion; higher levels of plasma or filtered empagliflozin were needed to induce similar increases in fractional renal glucose excretion. We conclude that empagliflozin is excreted into the urine to similar extent by glomerular filtration and tubular secretion. The latter can occur largely independent of OAT3. However, OAT3 increases the glucosuric effect of empagliflozin, which may relate to the partial overlap of its localization with SGLT2 and thus OAT3-mediated tubular secretion of empagliflozin in the early proximal tubule.

The organic anion transporter SLCO2A1 constitutes the core component of the Maxi-Cl channel

The maxi-anion channels (MACs) are expressed in cells from mammals to amphibians with ~60% exhibiting a phenotype called Maxi-Cl. Maxi-Cl serves as the most efficient pathway for regulated fluxes of inorganic and organic anions including ATP However, its molecular entity has long been elusive. By subjecting proteins isolated from bleb membranes rich in Maxi-Cl activity to LC-MS/MS combined with targeted siRNA screening, CRISPR/Cas9-mediated knockout, and heterologous overexpression, we identified the organic anion transporter SLCO2A1, known as a prostaglandin transporter (PGT), as a key component of Maxi-Cl. Recombinant SLCO2A1 exhibited Maxi-Cl activity in reconstituted proteoliposomes. When SLCO2A1, but not its two disease-causing mutants, was heterologously expressed in cells which lack endogenous SLCO2A1 expression and Maxi-Cl activity, Maxi-Cl currents became activated. The charge-neutralized mutant became weakly cation-selective with exhibiting a smaller single-channel conductance. Slco2a1 silencing in vitro and in vivo, respectively, suppressed the release of ATP from swollen C127 cells and from Langendorff-perfused mouse hearts subjected to ischemia-reperfusion. These findings indicate that SLCO2A1 is an essential core component of the ATP-conductive Maxi-Cl channel.

SGK1/Nedd4-2 signaling pathway regulates the activity of human organic anion transporters 3

Human organic anion transporter 3 (hOAT3) is localized at the basolateral membrane of renal proximal tubule cells and facilitates the renal secretion of numerous clinical drugs, including anti-HIV therapeutics, anti-tumor drugs, antibiotics, antihypertension drugs and anti-inflammatories. The present study explored the role of serum and glucocorticoid-inducible kinase 1 (sgk1) in the regulation of hOAT3. It was shown that over-expression of sgk1 in hOAT3-expressing cells stimulated hOAT3 transport activity by enhancing the transporter expression at the plasma membrane, kinetically reflected as an increased maximal transport velocity V_max without substantial change in the substrate-binding affinity K_m . In contrast, treatment of cells with the sgk-specific inhibitor GSK650394 resulted in a dose-dependent inhibition of hOAT3 transport activity. Evidence was further provided that sgk1 regulation of hOAT3 activity was mediated by ubiquitin ligase Nedd4-2, an enzyme previously shown to have an inhibitory effect on hOAT3. It was shown that sgk1 phosphorylated Nedd4-2, weakened the association between Nedd4-2 and hOAT3, and decreased hOAT3 ubiquitination. Functionally, the sgk1-stimulated hOAT3 transport activity was attenuated in the presence of a ligase-dead mutant of Nedd4-2. In summary, the investigation established for the first time that sgk1 stimulates hOAT3 transport activity by interfering with the inhibitory effect of Nedd4-2 on the transporter.

Urinary Excretion of Tetrodotoxin Modeled in a Porcine Renal Proximal Tubule Epithelial Cell Line, LLC-PK₁

This study examined the urinary excretion of tetrodotoxin (TTX) modeled in a porcine renal proximal tubule epithelial cell line, LLC-PK₁. Time course profiles of TTX excretion and reabsorption across the cell monolayers at 37 °C showed that the amount of TTX transported increased linearly for 60 min. However, at 4 °C, the amount of TTX transported was approximately 20% of the value at 37 °C. These results indicate that TTX transport is both a transcellular and carrier-mediated process. Using a transport inhibition assay in which cell monolayers were incubated with 50 µM TTX and 5 mM of a transport inhibitor at 37 °C for 30 min, urinary excretion was significantly reduced by probenecid, tetraethylammonium (TEA), l-carnitine, and cimetidine, slightly reduced by p-aminohippuric acid (PAH), and unaffected by 1-methyl-4-phenylpyridinium (MPP+), oxaliplatin, and cefalexin. Renal reabsorption was significantly reduced by PAH, but was unaffected by probenecid, TEA and l-carnitine. These findings indicate that TTX is primarily excreted by organic cation transporters (OCTs) and organic cation/carnitine transporters (OCTNs), partially transported by organic anion transporters (OATs) and multidrug resistance-associated proteins (MRPs), and negligibly transported by multidrug and toxic compound extrusion transporters (MATEs).

Nephron-Specific Deletion of Circadian Clock Gene Bmal1 Alters the Plasma and Renal Metabolome and Impairs Drug Disposition

The circadian clock controls a wide variety of metabolic and homeostatic processes in a number of tissues, including the kidney. However, the role of the renal circadian clocks remains largely unknown. To address this question, we performed a combined functional, transcriptomic, and metabolomic analysis in mice with inducible conditional knockout (cKO) of BMAL1, which is critically involved in the circadian clock system, in renal tubular cells (Bmal1lox/lox/Pax8-rtTA/LC1 mice). Induction of cKO in adult mice did not produce obvious abnormalities in renal sodium, potassium, or water handling. Deep sequencing of the renal transcriptome revealed significant changes in the expression of genes related to metabolic pathways and organic anion transport in cKO mice compared with control littermates. Furthermore, kidneys from cKO mice exhibited a significant decrease in the NAD+-to-NADH ratio, which reflects the oxidative phosphorylation-to-glycolysis ratio and/or the status of mitochondrial function. Metabolome profiling showed significant changes in plasma levels of amino acids, biogenic amines, acylcarnitines, and lipids. In-depth analysis of two selected pathways revealed a significant increase in plasma urea level correlating with increased renal Arginase II activity, hyperargininemia, and increased kidney arginine content as well as a significant increase in plasma creatinine concentration and a reduced capacity of the kidney to secrete anionic drugs (furosemide) paralleled by an approximate 80% decrease in the expression level of organic anion transporter 3 (SLC22a8). Collectively, these results indicate that the renal circadian clocks control a variety of metabolic/homeostatic processes at the intrarenal and systemic levels and are involved in drug disposition.

Identification and Quantitative Assessment of Uremic Solutes as Inhibitors of Renal Organic Anion Transporters, OAT1 and OAT3

One of the characteristics of chronic kidney disease (CKD) is the accumulation of uremic solutes in the plasma. Less is known about the effects of uremic solutes on transporters that may play critical roles in pharmacokinetics. We evaluated the effect of 72 uremic solutes on organic anion transporter 1 and 3 (OAT1 and OAT3) using a fluorescent probe substrate, 6-carboxyfluorescein. A total of 12 and 13 solutes were identified as inhibitors of OAT1 and OAT3, respectively. Several of them inhibited OAT1 or OAT3 at clinically relevant concentrations and reduced the transport of other OAT1/3 substrates in vitro. Review of clinical studies showed that the active secretion of most drugs that are known substrates of OAT1/3 deteriorated faster than the renal filtration in CKD. Collectively, these data suggest that through inhibition of OAT1 and OAT3, uremic solutes contribute to the decline in renal drug clearance in patients with CKD.

Nedd4-2 but not Nedd4-1 is critical for protein kinase C-regulated ubiquitination, expression, and transport activity of human organic anion transporter 1

Human organic anion transporter 1 (hOAT1) expressed at the membrane of the kidney proximal tubule cells mediates the body disposition of a diverse array of clinically important drugs, including anti-HIV therapeutics, antitumor drugs, antibiotics, antihypertensives, and antiinflammatories. Therefore, understanding the regulation of hOAT1 will provide significant insights into kidney function and dysfunction. We previously established that hOAT1 transport activity is inhibited by activation of protein kinase C (PKC) through accelerating hOAT1 internalization from cell surface into intracellular endosomes and subsequent degradation. We further established that PKC-induced hOAT1 ubiquitination is an important step preceding hOAT1 internalization. In the current study, we identified two closely related E3 ubiquitin ligases, neural precursor cell expressed, developmentally downregulated 4-1 and 4-2 (Nedd4-1 and Nedd4-2), as important regulators for hOAT1: overexpression of Nedd4-1 or Nedd4-2 enhanced hOAT1 ubiquitination, reduced the hOAT1 amount at the cell surface, and suppressed hOAT1 transport activity. In further exploring the relationship among PKC, Nedd4-1, and Nedd4-2, we discovered that PKC-dependent changes in hOAT1 ubiquitination, expression, and transport activity were significantly blocked in cells transfected with the ligase-dead mutant of Nedd4-2 (Nedd4-2/C821A) or with Nedd4-2-specific siRNA to knockdown endogenous Nedd4-2 but not in cells transfected with the ligase-dead mutant of Nedd4-1 (Nedd4-1/C867S) or with Nedd4-1-specific siRNA to knockdown endogenous Nedd4-1. In conclusion, this is the first demonstration that both Nedd4-1 and Nedd4-2 are important regulators for hOAT1 ubiquitination, expression, and function. Yet they play distinct roles, as Nedd4-2 but not Nedd4-1 is a critical mediator for PKC-regulated hOAT1 ubiquitination, expression, and transport activity.

The in vivo disposition and in vitro transmembrane transport of two model radiometabolites of DOTA-conjugated receptor-specific peptides labelled with (177) Lu

In vivo metabolism of the radiolabelled receptor-specific peptides has been described; however, information regarding the pharmacokinetic behaviour of the degradation products within the body is very scarce. The present study was designed to obtain new knowledge on the disposition and elimination of low-molecular radiometabolites of receptor-specific peptides in the organism and to reveal the potential involvement of selected membrane transport mechanisms in the cellular uptake of radiometabolites, especially in the kidney. The study compared pharmacokinetics of two radiometabolites: a final metabolite of somatostatin analogues, (177)Lu-DOTA-DPhe, and a tripeptide metabolite of (177)Lu-DOTA-minigastrin 11, (177)Lu-DOTA-DGlu-Ala-Tyr. Their pharmacokinetics was compared with that of respective parent (177)Lu-radiopeptide. Both radiometabolites exhibited relative rapid clearing from most body tissues in rats in vivo along with predominant renal excretion. The long-term renal retention of the smaller radiometabolite (177)Lu-DOTA-DPhe was lower than that of (177)Lu-DOTA-DGlu-Ala-Tyr. An uptake of (177)Lu-DOTA-DPhe by human renal influx transporter organic cation transporter 2 was found in vitro in a cellular model. The study brings the first experimental data on the in vivo pharmacokinetics of radiometabolites of receptor-specific somatostatin and gastrin analogues. The found results may indicate a negative correlation between the degree of decomposition of the parent peptide chain and the renal retention of the metabolite.

Transcriptome-based reconstructions from the murine knockout suggest involvement of the urate transporter, URAT1 (slc22a12), in novel metabolic pathways

URAT1 (slc22a12) was identified as the transporter responsible for renal reabsorption of the medically important compound, uric acid. However, subsequent studies have indicated that other transporters make contributions to this process, and that URAT1 transports other organic anions besides urate (including several in common with the closely related multi-specific renal organic anion transporters, OAT1 (slc22a6) and OAT3 (slc22a8)). These findings raise the possibility that urate transport is not the sole physiological function of URAT1. We previously characterized mice null for the murine ortholog of URAT1 (mURAT1; previously cloned as RST), finding a relatively modest decrement in urate reabsorptive capacity. Nevertheless, there were shifts in the plasma and urinary concentrations of multiple small molecules, suggesting significant metabolic changes in the knockouts. Although these molecules remain unidentified, here we have computationally delineated the biochemical networks consistent with transcriptomic data from the null mice. These analyses suggest alterations in the handling of not only urate but also other putative URAT1 substrates comprising intermediates in nucleotide, carbohydrate, and steroid metabolism. Moreover, the analyses indicate changes in multiple other pathways, including those relating to the metabolism of glycosaminoglycans, methionine, and coenzyme A, possibly reflecting downstream effects of URAT1 loss. Taken together with the available substrate and metabolomic data for the other OATs, our findings suggest that the transport and biochemical functions of URAT1 overlap those of OAT1 and OAT3, and could contribute to our understanding of the relationship between uric acid and the various metabolic disorders to which it has been linked.

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URAT1 handles multiple substrates suggesting functions beyond urate transport

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We determined metabolic constraints of gene expression changes in URAT1 null mice

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These suggest URAT1 involvement in multiple bioenergtic and biosynthetic pathways

Prostaglandin transporter in the rat brain: its localization and induction by lipopolysaccharide

Prostaglandin E₂ (PGE₂) is produced in the brain during infectious/inflammatory diseases, and it mediates acute-phase responses including fever. In the recovery phase of such diseases, PGE₂ disappears from the brain through yet unidentified mechanisms. Rat prostaglandin transporter (PGT), which facilitates transmembrane transport of PGE₂, might be involved in the clearance of PGE₂ from the brain. Here, we examined the cellular localization of PGT mRNA and its protein in the brains of untreated rats and those injected intraperitoneally with a pyrogen lipopolysaccharide (LPS) or saline. PGT mRNA was weakly expressed in the arachnoid membrane of untreated rats and saline-injected ones, but was induced in blood vessels of the subarachnoidal space and choroid plexus and in arachnoid membrane at 5 h and 12 h after LPS injection. In the same type of cells, PGT-like immunoreactivity was found in the cytosol and cell membrane even under nonstimulated conditions, and its level was also elevated after LPS injection. PGT-positive cells in blood vessels were identified as endothelial cells. In most cases, PGT was not colocalized with cyclooxygenase-2, a marker of prostaglandin-producing cells. The PGE₂ level in the cerebrospinal fluid reached its peak at 3 h after LPS, and then dropped over 50% by 5 h, which time point coincides with the maximum PGT mRNA expression and enhanced level of PGT protein. These results suggest that PGT is involved in the clearance of PGE₂ from the brain during the recovery phase of LPS-induced acute-phase responses.

Wide expression of type I Na+-phosphate cotransporter 3 (NPT3/SLC17A2), a membrane potential-driven organic anion transporter

Membrane potential (Δψ)-driven and Cl(-)-dependent organic anion transport is a primary function of the solute carrier family 17 (SLC17) transporter family. Although the transport substrates and physiological relevance of the major members are well understood, SLC17A2 protein known to be Na(+)-phosphate cotransporter 3 (NPT3) is far less well characterized. In the present study, we investigated the transport properties and expression patterns of mouse SLC17A2 protein (mNPT3). Proteoliposomes containing the purified mNPT3 protein took up radiolabeled p-aminohippuric acid (PAH) in a Δψ- and Cl(-)-dependent manner. The mNPT3-mediated PAH uptake was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDs) and Evans blue, common inhibitors of SLC17 family members. The PAH uptake was also inhibited by various anionic compounds, such as hydrophilic nonsteroidal anti-inflammatory drugs (NSAIDs) and urate. Consistent with these observations, the proteoliposome took up radiolabeled urate in a Δψ- and Cl(-)-dependent manner. Immunohistochemistry with specific antibodies against mNPT3 combined with RT-PCR revealed that mNPT3 is present in various tissues, including the hepatic bile duct, luminal membranes of the renal urinary tubules, maternal side of syncytiotrophoblast in the placenta, apical membrane of follicle cells in the thyroid, bronchiole epithelial cells in the lungs, and astrocytes around blood vessels in the cerebrum. These results suggested that mNPT3 is a polyspecific organic anion transporter that is involved in circulation of urate throughout the body.

Down-regulation of OATP1B proteins correlates with hyperbilirubinemia in advanced cholestasis

AIM

Organic anion-transporting polypeptides OATP1B1 and OATP1B3 are sinusoidal membrane transporters mediating liver uptake of a wide range of substrates including conjugated and unconjugated bilirubin, xenobiotics and drugs. Absence of OATP1Bs in the liver causes Rotor syndrome. Our aim was to correlate OATP1B expression with hyperbilirubinemia in common liver diseases.

METHODS

Immunoreactivity of five antibodies against human OATP1Bs was tested on frozen and formalin-fixed paraffin-embedded liver tissue of mouse strains transgenic for SLCO1B1 or SLCO1B3 and on human specimens. The proportion of hepatocytes expressing OATP1Bs was then assessed immunohistologically in formalin-fixed paraffin-embedded liver samples obtained from patients with hepatocellular and primary biliary liver diseases. UGT1A1 promoter TATA-box and SLCO1B1 rs4149056 genotyping was performed to rule out individuals predisposed to hyperbilirubinemia.

RESULTS

The most specific detection of OATP1B3 was achieved with the H-52 (sc-98981) antibody. OATP1B1 was specifically recognized with the ESL (ab15441) anti-OATP1B1 antibody, but only in frozen sections. The MDQ (ab15442) anti-OATP1B1 antibody cross-reacted with both OATP1B proteins in liver tissue of the transgenic mouse strains. Expression of the OATP1B proteins was decreased in advanced liver diseases and inversely correlated with serum bilirubin levels. The reduction was more pronounced in advanced primary biliary diseases (1.9±1.1 vs. 2.7±0.6; P=0.009).

CONCLUSIONS

Down-regulation of OATP1B proteins may contribute to pathogenesis of jaundice accompanying advanced cholestatic liver diseases.

Renal human organic anion transporter 3 increases the susceptibility of lymphoma cells to bendamustine uptake

Chronic lymphatic leukemia (CLL) is often associated with nephritic syndrome. Effective treatment of CLL by chlorambucil and bendamustine leads to the restoration of renal function. In this contribution, we sought to elucidate the impact of organic anion transporters (OATs) on the uptake of bendamustine and chlorambucil as a probable reason for the superior efficacy of bendamustine over chlorambucil in the treatment of CLL. We examined the effects of structural analogs of p-aminohippurate (PAH), melphalan, chlorambucil, and bendamustine, on OAT1-mediated [(3)H]PAH uptake and OAT3- and OAT4-mediated [(3)H]estrone sulfate (ES) uptake in stably transfected human embryonic kidney-293 cells. Melphalan had no significant inhibitory effect on any OAT, whereas chlorambucil reduced OAT1-, OAT3-, and OAT4-mediated uptake of PAH or ES down to 14.6%, 16.3%, and 66.0% of control, respectively. Bendamustine inhibited only OAT3-mediated ES uptake, which was reduced down to 14.3% of control cells, suggesting that it interacts exclusively with OAT3. The IC50 value for OAT3 was calculated to be 0.8 μM. Real-time PCR experiments demonstrated a high expression of OAT3 in lymphoma cell lines as well as primary CLL cells. OAT3-mediated accumulation of bendamustine was associated with reduced cell proliferation and an increased rate of apoptosis. We conclude that the high efficacy of bendamustine in treating CLL might be partly contributed to the expression of OAT3 in lymphoma cells and the high affinity of bendamustine for this transporter.

Effect of drug transporter pharmacogenetics on cholestasis

INTRODUCTION

The liver is the central place for the metabolism of drugs and other xenobiotics. In the liver cell, oxidation and conjugation of compounds take place, and at the same time, bile formation helps in extrusion of these compounds via the biliary route. A large number of transporters are responsible for drug uptake into the liver cell and excretion into bile or efflux to the sinusoidal blood.

AREAS COVERED

Genetic variants of these transporters and their transactivators contribute to changes in drug handling and are also responsible for cholestatic syndromes of different severity. This review summarizes the current knowledge regarding the influence of these genetic changes. The review covers progressive hereditary cholestatic syndromes as well as recurrent or transient cholestatic syndromes such as drug-induced liver injury, intrahepatic cholestasis of pregnancy, and benign recurrent intrahepatic cholestasis.

EXPERT OPINION

Polymorphisms in transporter genes are frequent. For clinically relevant cholestatic syndromes, it often requires a combination of genetic variants or acquired triggers such as pregnancy or drug treatment. In combination with other pathogenetic aspects, genetic variants in drug transporters may contribute to our understanding of not only cholestatic diseases such as primary sclerosing cholangitis or primary biliary cirrhosis, but also the natural course of chronic liver disease in general.

Interaction of immunosuppressive drugs with human organic anion transporter (OAT) 1 and OAT3, and multidrug resistance-associated protein (MRP) 2 and MRP4

Renal proximal tubule transporters can play a key role in excretion, pharmacokinetic interactions, and toxicity of immunosuppressant drugs. Basolateral organic anion transporters (OATs) and apical multidrug resistance-associated proteins (MRPs) contribute to the active tubular uptake and urinary efflux of these drugs, respectively. We studied the interaction of 12 immunosuppressants with OAT1- and OAT3-mediated [(3)H]-methotrexate (MTX) uptake in cells, and adenosine triphosphate-dependent [(3)H]-MTX transport in membrane vesicles isolated from human embryonic kidney 293 cells overexpressing human MRP2 and MRP4. Our results show that at a clinically relevant concentration of 10 μM, mycophenolic acid inhibited both OAT1- and OAT3-mediated [(3)H]-MTX uptake. Cytarabine, vinblastine, vincristine, hydrocortisone, and mitoxantrone inhibited only OAT1, whereas tacrolimus, azathioprine, dexamethasone, cyclosporine, and 6-mercaptopurine had no effect on both transporters. Cyclophosphamide stimulated OAT1, but did not affect OAT3. With regard to the apical efflux transporters, mycophenolic acid, cyclophosphamide, hydrocortisone, and tacrolimus inhibited MRP2 and MRP4, whereas mitoxantrone and dexamethasone stimulated [(3)H]-MTX transport by both transporters. Cyclosporine, vincristine, and vinblastine inhibited MRP2 only, whereas 6-mercaptopurine inhibited MRP4 transport activity only. Cytarabine and azathioprine had no effect on either transporter. In conclusion, we charted comprehensively the differences in inhibitory action of various immunosuppressive agents against the 4 key renal anion transporters, and we provide evidence that immunosuppressant drugs can modulate OAT1-, OAT3-, MRP2-, and MRP4-mediated transport of MTX to different extents. The data provide a better understanding of renal mechanisms underlying drug-drug interactions and nephrotoxicity concerning combination regimens with these compounds in the clinic.

Short-term calorie restriction feminizes the mRNA profiles of drug metabolizing enzymes and transporters in livers of mice

Calorie restriction (CR) is one of the most effective anti-aging interventions in mammals. A modern theory suggests that aging results from a decline in detoxification capabilities and thus accumulation of damaged macromolecules. The present study aimed to determine how short-term CR alters mRNA profiles of genes that encode metabolism and detoxification machinery in the liver. Male C57BL/6 mice were fed CR (0, 15, 30, or 40%) diets for one month, followed by mRNA quantification of 98 xenobiotic processing genes (XPGs) in the liver, including 7 uptake transporters, 39 phase-I enzymes, 37 phase-II enzymes, 10 efflux transporters, and 5 transcription factors. In general, 15% CR did not alter mRNAs of most XPGs, whereas 30 and 40% CR altered over half of the XPGs (32 increased and 29 decreased). CR up-regulated some phase-I enzymes (fold increase), such as Cyp4a14 (12), Por (2.3), Nqo1 (1.4), Fmo2 (5.4), and Fmo3 (346), and numerous number of phase-II enzymes, such as Sult1a1 (1.2), Sult1d1 (2.0), Sult1e1 (33), Sult3a1 (2.2), Gsta4 (1.3), Gstm2 (1.3), Gstm3 (1.7), and Mgst3 (2.2). CR feminized the mRNA profiles of 32 XPGs in livers of male mice. For instance, CR decreased the male-predominantly expressed Oatp1a1 (97%) and increased the female-predominantly expressed Oatp1a4 (11). In conclusion, short-term CR alters the mRNA levels of over half of the 98 XPGs quantified in livers of male mice, and over half of these alterations appear to be due to feminization of the liver.

p-Cresyl sulfate induces osteoblast dysfunction through activating JNK and p38 MAPK pathways

Recent data suggest that several uremic toxins may contribute to the development of bone abnormalities in chronic kidney disease. p-Cresyl sulfate (PCS), the sulfate conjugate of p-cresol, is a protein-bound uremic toxin associated with the progression of chronic kidney disease, cardiovascular risk, and mortality. However, the effects of PCS on bone metabolism remain unclear. In the present study, we evaluated the toxic effects of PCS on primary mouse osteoblasts, compared with an extensively studied uremic toxin indoxyl sulfate (IS). Pre-treatment of osteoblasts with PCS at 0.125 mM and above significantly decreased parathyroid hormone (PTH)-induced cAMP production in a dose-dependent manner. PCS also induced a significant increase in intracellular production of reactive oxygen species (ROS) at 0.25 mM and above, but not at lower concentrations. PCS at 0.125 mM (a concentration that did not induce significant ROS increase) decreased cell viability by augmenting DNA fragmentation and reducing cell proliferation. Inhibition of JNK and p38 mitogen-activated protein kinase (MAPK) abolished the PCS-induced increase in DNA fragmentation and decrease in cAMP production in osteoblastic cells. Compared with PCS, IS induced ROS production at 0.05 mM but did not reduce cAMP production from 0.05 to 0.5 mM. IS induced decrease in cell viability and increase in DNA fragmentation at 0.5mM only. These results suggest that PCS damages osteoblastic cells through not only increasing ROS production but also activating JNK/p38 MAPKs, which is different from the mechanism of injury by IS. These damages of osteoblasts induced by PCS may play a critical role in impairing bone metabolism in patients with chronic kidney disease in whom PCS accumulates.

Proposed mechanistic description of dose-dependent BDE-47 urinary elimination in mice using a physiologically based pharmacokinetic model

Polybrominated diphenyl ethers (PBDEs) have been used in a wide variety of consumer applications as additive flame retardants. In North America, scientists have noted continuing increases in the levels of PBDE congeners measured in human serum. Some recent studies have found that PBDEs are associated with adverse health effects in humans, in experimental animals, and wildlife. This laboratory previously demonstrated that urinary elimination of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) is saturable at high doses in mice; however, this dose-dependent urinary elimination has not been observed in adult rats or immature mice. Thus, the primary objective of this study was to examine the mechanism of urinary elimination of BDE-47 in adult mice using a physiologically based pharmacokinetic (PBPK) model. To support this objective, additional laboratory data were collected to evaluate the predictions of the PBPK model using novel information from adult multi-drug resistance 1a/b knockout mice. Using the PBPK model, the roles of mouse major urinary protein (a blood protein carrier) and P-glycoprotein (an apical membrane transporter in proximal tubule cells in the kidneys, brain, intestines, and liver) were investigated in BDE-47 elimination. The resulting model and new data supported the major role of m-MUP in excretion of BDE-47 in the urine of adult mice, and a lesser role of P-gp as a transporter of BDE-47 in mice. This work expands the knowledge of BDE-47 kinetics between species and provides information for determining the relevancy of these data for human risk assessment purposes.

Role of P-glycoprotein in the efflux of raltegravir from human intestinal cells and CD4+ T-cells as an interaction target for anti-HIV agents

Cellular efflux and uptake transports of several anti-HIV agents are mediated by plasma membrane-localized solute transporters. However, transporters involved in raltegravir disposition have not been fully characterized. Here, we performed in vitro studies to identify transporters mediating transcellular transport of raltegravir. Transepithelial raltegravir transport was examined using porcine kidney epithelial cell line (LLC-PK1) and LLC-PK1 cells stably transfected with P-glycoprotein (also known as Multiple drug resistance 1) (L-MDR1). Transepithelial transport of raltegravir in Caco-2 cell monolayers, and intracellular accumulation of raltegravir in the MT-2 and MT-4 (CD4+ T-) cells were measured in the presence or absence of anti-HIV agents. The uptake of raltegravir was investigated in HEK-293 cells expressing each of several solute carrier family transporters. The apical-to-basal raltegravir transport was significantly decreased in L-MDR1 as compared to that in LLC-PK1 monolayers. In HEK-293 cells expressing breast cancer resistance protein (BCRP), raltegravir accumulation was lower than that in the mock-transfected cells. In Caco-2 cells, protease inhibitors including nelfinavir, ritonavir and lopinavir enhanced the apical-to-basal transport of raltegravir. By contrast, reverse transcriptase inhibitors such as zidovudine, efavirenz, and nevirapine, had no effect on raltegravir transport. The cellular accumulation of raltegravir in MT-2 cells, which express P-glycoprotein, was significantly increased in the presence of protease inhibitors. By contrast, protease inhibitors only marginally increased the accumulation of raltegravir in MT-4 cells, in which P-glycoprotein is not expressed. The present findings suggest that raltegravir is a substrate of both P-glycoprotein and BCRP. Protease inhibitors increase the absorptive transport of raltegravir in Caco-2 cells, and the cellular accumulation in T-cells, at least in part, by P-glycoprotein-mediated interaction.

Reduced renal clearance of cefotaxime in asians with a low-frequency polymorphism of OAT3 (SLC22A8)

Organic anion transporter 3 (OAT3, SLC22A8), a transporter expressed on the basolateral membrane of the proximal tubule, plays a critical role in the renal excretion of organic anions including many therapeutic drugs. The goal of this study was to evaluate the in vivo effects of the OAT3-Ile305Phe variant (rs11568482), present at 3.5% allele frequency in Asians, on drug disposition with a focus on cefotaxime, a cephalosporin antibiotic. In HEK293-Flp-In cells, the OAT3-Ile305Phe variant had a lower maximum cefotaxime transport activity, Vmax , [159 ± 3 nmol*(mg protein)(-1) /min (mean ± SD)] compared with the reference OAT3 [305 ± 28 nmol*(mg protein)(-1) /min, (mean ± SD), p < 0.01], whereas the Michaelis-Menten constant values (Km ) did not differ. In healthy volunteers, we found volunteers that were heterozygous for the Ile305Phe variant and had a significantly lower cefotaxime renal clearance (CLR ; mean ± SD: 84.8 ± 32.1 mL/min, n = 5) compared with volunteers that were homozygous for the reference allele (158 ± 44.1 mL/min, n = 10; p = 0.006). Furthermore, the net secretory component of cefotaxime renal clearance (CLsec ) was reduced in volunteers heterozygous for the variant allele [33.3 ± 31.8 mL/min (mean ± SD)] compared with volunteers homozygous for the OAT3 reference allele [97.0 ± 42.2 mL/min (mean ± SD), p = 0.01]. In summary, our study suggests that a low-frequency reduced-function polymorphism of OAT3 associates with reduced cefotaxime CLR and CL(sec) .

Human Organic Solute Transporter (hOST): protein interaction and membrane sorting process

The human organic solute transporter (hOST) is a heterodimer composed of alpha and beta subunits. Physical association of hOSTα and β subunits is essential for their polarized basolateral plasma membrane localization and function in the export of bile acids and steroids. To understand the role of carboxyl- and amino-tails of OSTβ and mechanisms underlying membrane localization of hOST, the effects of tail deletion of the hOSTβ subunit and biological reagents on membrane distribution and transport function of hOST were investigated in stably transfected MDCK cells. After deletion of 35 amino acids from the amino-tail of hOSTβ, the efflux transport activity and polarized membrane distribution of the truncated hOSTβ was abolished. A co-immunoprecipitation study verified that the amino-tail of hOSTβ is essential for the association with hOSTα subunit. Treatments with acytochalasin D (interrupting ctin-filaments), bafilomycin A1 (inhibiting vacuolar H(+)-ATPase), brefeldin A (disrupting the Golgi complex), and calphostin C (inhibiting protein kinase C), significantly disrupted the polarized membrane distribution of hOST and markedly reduced transport activity in stably transfected MDCK cells. In summary, the 35 amino acid amino-terminal fragment of hOSTβ contains critical information for interaction with the hOSTα subunit and subsequent trafficking to the plasma membrane. These studies suggest that the membrane sorting process of hOST is mediated by a bafilomycin A1-sensitive vesicular pathway that is associated with the actin-cytoskeleton network. The membrane localization of hOST is also partially mediated through a brefeldin A sensitive mechanism, which controls its transit from the ER to Golgi and is regulated by PKC.

Mutational analysis of the role of GXXXG motif in the function of human organic anion transporter 1 (hOAT1)

Human organic anion transporter hOAT1 plays a critical role in the body disposition of environmental toxins and clinically important drugs including anti-HIV therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories. hOAT1 has two GXXXG motifs in its transmembrane domains 2 and 5, a motif linked to the protein processing and oligomerization of other proteins. In the current study, we substituted glycine of these GXXXG motifs with alanine and evaluated the effect of such mutations on the expression and function of hOAT1. Mutations of GXXXG motif in the transmembrane domain 2 resulted in mutants G144A and G148A, both of which had no transport activity due to complete loss in the surface and total cell expression of the transporter protein. Treatment of G144A- and G148A-expressing cells with proteasomal inhibitor resulted in the recovery of ER-resident immature form of hOAT1, but not its surface-resident mature form, whereas treatment of these cells with lysosomal inhibitor had no effect on the expression of the mutant transporters. Mutations of GXXXG motif in the transmembrane domain 5 resulted in mutants G223A and G227A, among which only G227 had dramatic reduction of transport activity due to dramatic loss in the surface and total cell expression of the transporter. The reduction in the surface expression of G227 was consistent with the decrease in maximum transport velocity Vmax. Treatment of G227A-expressing cells with proteasomal inhibitor or lysosomal inhibitor resulted in partial recovery of both the immature form and the mature form of hOAT1 in the total cell extracts. However, such partial recovery of the mature form in total cell extracts did not lead to the partial recovery of surface expression and function of the transporter. Our data suggest that the GXXXG motifs in transmembrane domains 2 and 5 play critical roles in the stability of hOAT1.