Renal Reabsorptive Transport of Uric Acid Precursor Xanthine by URAT1 and GLUT9

Xanthine and hypoxanthine are intermediate metabolites of uric acid and a source of reactive oxidative species (ROS) by xanthine oxidoreductase (XOR), suggesting that facilitating their elimination is beneficial. Since they are reabsorbed in renal proximal tubules, we investigated their reabsorption mechanism by focusing on the renal uric acid transporters URAT1 and GLUT9, and examined the effect of clinically used URAT1 inhibitor on their renal clearance when their plasma concentration is increased by XOR inhibitor. Uptake study for [³H]xanthine and [³H]hypoxanthine was performed using URAT1- and GLUT9-expressing Xenopus oocytes. Transcellular transport study for [³H]xanthine was carried out using Madin-Darby canine kidney (MDCK)II cells co-expressing URAT1 and GLUT9. In in vivo pharmacokinetic study, renal clearance of xanthine was estimated based on plasma concentration and urinary recovery. Uptake by URAT1- and GLUT9-expressing oocytes demonstrated that xanthine is a substrate of URAT1 and GLUT9, while hypoxanthine is not. Transcellular transport of xanthine in MDCKII cells co-expressing URAT1 and GLUT9 was significantly higher than those in mock cells and cells expressing URAT1 or GLUT9 alone. Furthermore, dotinurad, a URAT1 inhibitor, increased renal clearance of xanthine in rats treated with topiroxostat to inhibit XOR. It was suggested that xanthine is reabsorbed in the same manner as uric acid through URAT1 and GLUT9, while hypoxanthine is not. Accordingly, it is expected that treatment with XOR and URAT1 inhibitors will effectively decrease purine pools in the body and prevent cell injury due to ROS generated during XOR-mediated reactions.

Inhibition of the Hepatic Uptake of 99mTc-Tetrofosmin Using an Organic Cation Transporter Blocker

The accumulation of high levels of ^99mTc-tetrofosmin (^99mTc-TF) in the hepatobiliary system can lead to imaging artifacts and interference with diagnosis. The present study investigated the transport mechanisms of ^99mTc-TF and attempted to apply competitive inhibition using a specific inhibitor to reduce ^99mTc-TF hepatic accumulation. In this in vitro study, ^99mTc-TF was incubated in HEK293 cells expressing human organic anion transporting polypeptide 1B1 (OATP1B1), OATP1B3, OATP2B1, organic anion transporter 2 (OAT2), organic cation transporter 1 (OCT1), OCT2, and Na⁺-taurocholate cotransporting polypeptide with or without each specific inhibitor to evaluate the contribution of each transporter to ^99mTc-TF transportation. In vivo studies, dynamic planar imaging, and single photon emission computed tomography (SPECT) experiments with rats were performed to observe alterations to ^99mTc-TF pharmacokinetics using cimetidine (CMT) as an OCT1 inhibitor. Time-activity curves in the liver and heart were acquired from dynamic data, and the ^99mTc-TF uptake ratio was calculated from SPECT. From the in vitro study, ^99mTc-TF was found to be transported by OCT1 and OCT2. When CMT-preloaded rats and control rats were compared, the hepatic accumulation of the ^99mTc-TF was reduced, and the time to peak heart count shifted to an earlier stage. The hepatic accumulation of ^99mTc-TF was markedly suppressed, and the heart-to-liver ratio increased 1.6-fold. The pharmacokinetics of ^99mTc-TF were greatly changed by OCT1 inhibitor. Even in humans, the administration of OCT1 inhibitor before cardiac SPECT examination may reduce ^99mTc-TF hepatic accumulation and contribute to the suppression of artifacts and the improvement of SPECT image quality.

MicroRNAs in Apple-Derived Nanoparticles Modulate Intestinal Expression of Organic Anion-Transporting Peptide 2B1/SLCO2B1 in Caco-2 Cells

Plant-derived nanoparticles exert cytoprotective effects on intestinal cells by delivering their cargo to intestinal tissues. We previously reported that apple-derived nanoparticles (APNPs) downregulate the mRNA of the human intestinal transporter organic anion-transporting peptide 2B1 (OATP2B1)/SLCO2B1 and that the 3'-untranslated region (3'UTR) is required for the response to APNPs. Here, we investigated the involvement of microRNAs (miRNAs) in APNPs in suppressing OATP2B1 expression to demonstrate that APNP macromolecules directly interact with intestinal tissues. Using in silico analysis, seven apple miRNAs were predicted as candidate miRNAs that interact with the SLCO2B1-3'UTR. The APNP-mediated decrease in luciferase activity of pGL3/SLCO2B1-3'UTR was abrogated by inhibitors of mdm-miR-160a-e, -7121a-c, or -7121d-h. Each miRNA mimic reduced the endogenous expression of SLCO2B1 mRNA in Caco-2 cells. The luciferase activity of the truncated pGL3/SLCO2B1-3'UTR, which contains approximately 200 bp around each miRNA recognition element (MRE), was decreased by the miR-7121d-h mimic but decreased little by the other mimics. APNP also reduced the luciferase activity of truncated pGL3/SLCO2B1-3'UTR containing an MRE for miR-7121d-h. Thus, we demonstrated that mdm-miR-7121d-h contributes to the APNP-mediated downregulation of intestinal OATP2B1. Accordingly, plant macromolecules, such as miRNAs, may directly interact with intestinal tissues via nanoparticles. SIGNIFICANCE STATEMENT: This study demonstrates that mdm-miR7121d-h contained in apple-derived nanoparticles downregulated the mRNA expression of SLCO2B1 by interacting with SLCO2B1-3'-untranslated region directly and that SLCO2B1 mRNA might also be decreased by mdm-miR160a-e and -7121a-c indirectly. This finding that the specific apple-derived microRNAs influence human intestinal transporters provides a novel concept that macromolecules in foods directly interact with and affect the intestinal function of the host.

Drug Transcellular Transport Assay Using a High Porosity Honeycomb Film

In vitro transport studies across cells grown on culture inserts are widely used for evaluating pharmacokinetic characteristics such as intestinal membrane permeability. However, measurements of the apparent permeability coefficient of highly lipophilic compounds are often limited by transport across the membrane filters, not by transport across the cultured cells. To overcome this concern, we have investigated the utility of a high-porosity membrane honeycomb film (HCF) for transcellular transport studies. Using the HCF inserts, the apparent permeability coefficient (P_app) of the drugs tested in LLC-PK1 and Caco-2 cells tended to increase with an increase in lipophilicity, reaching a maximum P_app value at Log D higher than 2. In contrast, using the commercially available Track-Etched membrane (TEM) inserts, a maximum value was observed at Log D higher than 1. The basolateral to apical transport permeability P_app(BL→AP) of rhodamine 123 across LLC-PK1 cells that express P-glycoprotein (P-gp) cultured on HCF inserts and TEM inserts was 2.33 and 2.39 times higher than the reverse directional P_app(AP→BL) permeability, respectively. The efflux ratio (P_app(B-A)/P_app(A-B)) of rhodamine 123 in LLC-PK1 expressing P-gp cells using HCF inserts was comparable to that obtained using TEM inserts, whereas the transported amount in both directions was significantly higher when using the HCF inserts. Accordingly, due to the higher permeability and high porosity of HCF membranes, it is expected that transcellular transport of high lipophilic as well as hydrophilic compounds and substrate recognition of transporters can be evaluated more accurately by using HCF inserts.

Uptake Pathway of Apple-derived Nanoparticle by Intestinal Cells to Deliver its Cargo

PURPOSE

Food-derived nanoparticles exert cytoprotective effects on intestinal cells by delivering their cargo, which includes macromolecules such as microRNAs and proteins, as well as low-molecular weight compounds. We previously reported that apple-derived nanoparticles (APNPs) downregulate the expression of human intestinal transporter OATP2B1/SLCO2B1 mRNA. To verify the involvement of the cargo of APNPs in affecting the expression of transporters, we characterized the uptake mechanism of APNPs in intestinal cells.

METHODS

The uptake of fluorescent PKH26-labeled APNPs (PKH-APNPs) into Caco-2, LS180, and HT-29MTX cells was evaluated by confocal microscopy and flow cytometry.

RESULTS

The uptake of PKH-APNPs was prevented in the presence of clathrin-dependent endocytosis inhibitors, chlorpromazine and Pitstop2. Furthermore, PKH-APNPs were incorporated by the HT29-MTX cells, despite the disturbance of the mucus layer. Additionally, the decrease in SLCO2B1 mRNA by APNPs was reversed by Pitstop 2 in Caco-2 cells, indicating that APNPs decrease SLCO2B1 by being incorporated via clathrin-dependent endocytosis.

CONCLUSIONS

We demonstrated that clathrin-dependent endocytosis was mainly involved in the uptake of APNPs by intestinal cells, and that the cargo in the APNPs downregulate the mRNA expression of SLCO2B1. Therefore, APNPs could be a useful tool to deliver large molecules such as microRNAs to intestinal cells.

Identification of Triterpene Acids in Poria cocos Extract as Bile Acid Uptake Transporter Inhibitors

Literature reports that Poria cocos reduces blood lipid levels; however, the underlying mechanism remains unclear. Blood lipid levels are closely related to the enterohepatic circulation of bile acids, where uptake transporters playing a significant role. P. cocos extract is commonly used in traditional prescriptions and food supplements in China. We investigated the effects of P. cocos and its five triterpene acids on bile acid uptake transporters, including intestinal apical sodium-dependent bile acid transporter (ASBT) and hepatic sodium/taurocholate cotransporting polypeptide (NTCP). Triterpene acids were fingerprinted by high-performance liquid chromatography-TripleTOF and quantified by ultraperformance liquid chromatography/tandem mass spectrometry. The inhibitory effect of P. cocos and its five major representative triterpene acids on ASBT and NTCP was investigated by in vitro assays using Xenopus oocytes expressing ASBT and NTCP. P. cocos extract exhibited significant inhibitory effects with half-maximum inhibition constants of 5.89 µg/ml and 14.6 µg/ml for NTCP and ASBT, respectively. Among five triterpene acids, poricoic acid A, poricoic acid B, and polyporenic acid C significantly inhibited NTCP function. Poricoic acid A, poricoic acid B, and dehydrotumulosic acid significantly inhibited ASBT function. The representative triterpene acid, poricoic acid A, was identified as a competitive inhibitor of NTCP with an inhibitory constant of 63.4 ± 18.7 µM. In conclusion, our results indicate that both P. cocos extract and its major triterpenes are competitive inhibitors of ASBT and NTCP. Accordingly, it was suggested that competitive inhibition of these bile acid transporters is one of the underlying mechanisms for the hypolipidemic effect of P. cocos. SIGNIFICANCE STATEMENT: Poria cocos, a commonly used Chinese herbal medicine and food supplement, demonstrates significantly inhibitory effects on the function of apical sodium-dependent bile acid transporter and sodium/taurocholate cotransporting polypeptide. P. cocos has potential to reduce the blood lipid through inhibition of these uptake transporters in enterohepatic circulation of bile acid.

Assessment of drug transporters involved in the urinary secretion of [99mTc]dimercaptosuccinic acid

INTRODUCTION

We clarified the renal uptake and urinary secretion mechanism of [^99mTc]dimercaptosuccinic acid ([^99mTc]DMSA) via drug transporters in renal proximal tubules.

METHODS

[^99mTc]DMSA was added to human embryonic kidney 293 cells expressing human multidrug and toxin extrusion (MATE)1 and MATE2-K, carnitine/organic cation transporter (OCTN)1 and OCTN2, and organic cation transporter (OCT)2; to Flp293 cells expressing human organic anion transporter (OAT)1 and OAT3; and to vesicles expressing P-glycoprotein (P-gp), multidrug resistance associated protein (MRP)2, MRP4, or breast cancer resistance protein with and without probenecid (OAT inhibitor for both OATs and MRPs). Time activity curves of [^99mTc]DMSA with and without probenecid were established using LLC-PK₁ cells. Biodistribution and single photon emission computed tomography (SPECT) imaging in mice were conducted using [^99mTc]DMSA with and without probenecid.

RESULTS

[^99mTc]DMSA uptake was significantly higher in Flp293/OAT3 than in mock cells. Uptake via OAT3 was inhibited by probenecid. [^99mTc]DMSA uptake into vesicles that highly expressed MRP2 was significantly higher in adenosine triphosphate (ATP) than in adenosine monophosphate (AMP), and probenecid decreased uptake to similar levels as that in AMP. In the time activity curves for [^99mTc]DMSA in LLC-PK₁ cells, probenecid loading inhibited accumulation from the basolateral side into LLC-PK₁ cells, whereas accumulation from the apical side into cells gradually increased. Transport of [^99mTc]DMSA from both sides was low. Biodistribution and SPECT imaging studies showed that [^99mTc]DMSA with probenecid loading resulted in significantly higher accumulation in blood, heart, liver, and bladder after [^99mTc]DMSA injection compared with control mice. Probenecid induced significantly lower accumulation in the kidney after [^99mTc]DMSA injection.

CONCLUSIONS

[^99mTc]DMSA accumulates in renal proximal tubular epithelial cells from blood via OAT3 on the basolateral side, and then a small volume of [^99mTc]DMSA will be excreted in urine via MRP2. ADVANCES IN KNOWLEDGE: [^99mTc]DMSA accumulates via OAT3 in renal proximal tubular epithelial cells and is slightly excreted from the cells via MRP2. IMPLICATIONS FOR PATIENT CARE: [^99mTc]DMSA may be useful for measuring renal transport function with OAT3 in patients.

Pillar[6]arene acts as a biosensor for quantitative detection of a vitamin metabolite in crude biological samples

Metabolic syndrome is associated with obesity, hypertension, and dyslipidemia, and increased cardiovascular risk. Therefore, quick and accurate measurements of specific metabolites are critical for diagnosis; however, detection methods are limited. Here we describe the synthesis of pillar[n]arenes to target 1-methylnicotinamide (1-MNA), which is one metabolite of vitamin B3 (nicotinamide) produced by the cancer-associated nicotinamide N-methyltransferase (NNMT). We found that water-soluble pillar[5]arene (P5A) forms host-guest complexes with both 1-MNA and nicotinamide, and water-soluble pillar[6]arene (P6A) selectively binds to 1-MNA at the micromolar level. P6A can be used as a "turn-off sensor" by photoinduced electron transfer (detection limit is 4.38 × 10^-6 M). In our cell-free reaction, P6A is used to quantitatively monitor the activity of NNMT. Moreover, studies using NNMT-deficient mice reveal that P6A exclusively binds to 1-MNA in crude urinary samples. Our findings demonstrate that P6A can be used as a biosensor to quantify 1-MNA in crude biological samples.

A Novel Fluorescence-Based Method to Evaluate Ileal Apical Sodium-Dependent Bile Acid Transporter ASBT

This study aimed to demonstrate usefulness of the fluorophore-labeled bile acid derivative, N-(24-[7-(4-N,N-dimethylaminosulfonyl-2,1,3-benzoxadiazole)]amino-3α,7α,12α-trihydroxy-27-nor-5β-cholestan-26-oyl)-2'-aminoethane sulfonate (tauro-nor-THCA-24-DBD) as a substrate of apical sodium-dependent bile acid transporter (ASBT, SLC10A2), which is expressed at distal ileum for reabsorption of bile acids and to find a novel fluorescence-based method to evaluate ASBT activity. In HPLC analysis, chromatogram of tauro-nor-THCA-24-DBD showed double peaks: R- and S-isomers of the compound. When ASBT was expressed in Xenopus laevis oocytes, their uptakes were higher than those by control oocytes, demonstrating both are transported by ASBT. Therefore, results were analyzed separately as peak 1, peak 2 and sum of them. Concentration dependent uptake of tauro-nor-THCA-24-DBD in ASBT-expressing oocytes was saturable with Km 122 μM and Vmax 1.49 pmol/oocyte/30 min for peak 1, 30.7 μM and 1.34 pmol/oocyte/30 min for peak 2, and 40.6 μM and 2.36 pmol/oocyte/30 min for sum, respectively. These uptakes were decreased in the presence of taurocholic acid and in the Na⁺ free condition. Furthermore, in Caco-2 cells, tauro-nor-THCA-24-DBD uptake was also Na⁺-dependent and saturable. Additionally, these uptakes were decreased by elobixibat, a selective ASBT inhibitor. Accordingly, it was concluded that tauro-nor-THCA-24-DBD is a substrate of ASBT and useful to evaluate the intestinal ASBT transport activity.

[The inhibitors of the apical sodium-dependent bile acid transporter (ASBT) as promising drugs]

Inhibition of the apical sodium-dependent bile acid transporter (ASBT, also known as IBAT - ileal bile acid transporter, SLC10A2) leads to disruption of the enterohepatic circulation of bile acids and their excretion with fecal masses. This is accompanied by cholesterol utilization for synthesis of new bile acids. ASBT inhibitors are promising drugs for the treatment of such diseases as non-alcoholic fatty liver disease, non-alcoholic steatohepatitis, type 2 diabetes mellitus, necrotic enterocolitis, chronic constipation, atherosclerosis. To date the most known chemically synthesized inhibitors are: A3309, SHP626, A4250, 264W94, GSK2330672, SC-435. All of them are at different stages of clinical trials, which confirm the high efficacy and good tolerance of these inhibitors. Current trends in this field also include directed chemical synthesis of ASBT inhibitors, as well as their search among substances of plant origin.

[131I]MIBG exports via MRP transporters and inhibition of the MRP transporters improves accumulation of [131I]MIBG in neuroblastoma

INTRODUCTION

¹³¹I-labeled m-iodobenzylguanidine ([¹³¹I]MIBG) has been used to treat neuroblastoma patients, but [¹³¹I]MIBG may be immediately excreted from the cancer cells by the adenosine triphosphate binding cassette transporters, similar to anticancer drugs. The purpose of this study was to clarify the efflux mechanism of [¹³¹I]MIBG in neuroblastomas and improve accumulation by inhibition of the transporter in neuroblastomas.

METHODS

[¹³¹I]MIBG was incubated in human embryonic kidney (HEK)293 cells expressing human organic anion transporting polypeptide (OATP)1B1, OATP1B3, OATP2B1, organic anion transporter (OAT)1 and OAT2, organic cation transporter (OCT)1 and OCT2, and sodium taurocholate cotransporting polypeptide, and in vesicles expressing P-glycoprotein (MDR1), multidrug resistance associated protein (MRP)1-4, or breast cancer resistance protein with and without MK-571 and probenecid (MRP inhibitors). Time activity curves of [¹³¹I]MIBG with and without MK-571 and probenecid were established using an SK-N-SH neuroblastoma cell line, and transporter expression of multiple drug resistance was measured. Biodistribution and SPECT imaging examinations were conducted using [¹²³I]MIBG with and without probenecid in SK-N-SH-bearing mice.

RESULTS

[¹³¹I]MIBG uptake was significantly higher in OAT1, OAT2, OCT1, and OCT2 than in mock cells. Uptake via OCT1 and OCT2 was little inhibited by MK-571 and probenecid. [¹³¹I]MIBG uptake into vesicles that highly expressed MRP1 or MRP4 was significantly higher in ATP than in AMP, and these inhibitors restored uptake to levels similar to that in AMP. Examining the time activity curves for [¹³¹I]MIBG in SK-N-SH cells, higher expressions of MDR1, MRP1, MRP4, and MK-571, or probenecid loading produced significantly higher uptake than in control at most incubation times. The ratios of tumors to blood or muscle in SK-N-SH-bearing mice were significantly increased by probenecid loading in comparison with normal mice.

CONCLUSIONS

[¹³¹I]MIBG exports via MRP1 and MRP4 in neuroblastoma. The accumulation and tumor-to-blood or muscle ratios of [¹³¹I]MIBG are improved by inhibition of MRPs with probenecid in neuroblastoma. ADVANCES IN KNOWLEDGE: [¹³¹I]MIBG, widely used for treatment of neuroendocrine tumors including neuroblastoma, is excreted via MRP1 and MRP4 in neuroblastoma.

IMPLICATIONS FOR PATIENT CARE

Loading with probenecid, OAT, and MRP inhibitors improves [¹³¹I]MIBG accumulation.

Toxicological implication of prostaglandin transporter SLCO2A1 inhibition by cigarette smoke in exacerbation of lung inflammation

We reported that bleomycin (BLM)-induced pulmonary fibrosis was exacerbated in the prostaglandin transporter gene (Slco2a1)-deficient mice (Slco2a1(-/-)). Because cigarette smoke (CS) contributes to creating a profibrotic milieu in the respiratory region, the present study aimed to investigate the impact of CS on SLCO2A1-associated pathogenesis in the lungs of BLM-instilled mice. Bronchoalveolar lavage (BAL) fluid cell analysis indicated more severe inflammation in Slco2a1(-/-) on day 5 after BLM intratracheal instillation, and Slco2a1 deletion increased mRNA expression of pro-inflammatory cytokines (Tnf-α and Il-1β) and chemokine (Ccl5) in BAL cells. Male Slco2a1(-/-) exhibited significantly higher amounts of released Il-1β in BAL fluid, compared with female Slco2a1(-/-), male or female Slco2a1(+/+) group. The amount of PGE₂ collected in BAL fluid tended to increase in Slco2a1(-/-) compared with Slco2a1(+/+) group, whereas the PGE₂ concentrations in lung tissues were comparable between both groups. Besides, PGE₂ accumulated more in BAL fluid of male than that of female mice. Therefore, Slco2a1-deficient male mice were found to be more susceptible to BLM-treatment. Moreover, CS extracts (CSE) significantly reduced initial PGE₂ uptake by rat type1 alveolar epithelial cell-like (AT1-L) cells and human SLCO2A1-transfected cells. Exposure of AT1-L cells to CSE resulted in decreased mRNA expression of Slco2a1, suggesting that CS modulates SLCO2A1 function. These results indicate that exacerbated lung inflammation is attributed to an increase in Il-1β peptide and PGE₂ accumulation in the alveolar space, which exhibits a male predominance. SLCO2A1 inhibition by CSE is considered to be a new rationale for the lung toxicity of CS.

Mathematical modeling analysis of hepatic uric acid disposition using human sandwich-cultured hepatocytes

Uric acid is biosynthesized from purine by xanthine oxidase (XO) mainly in the liver and is excreted into urine and feces. Although several transporters responsible for renal and intestinal handling of uric acid have been reported, information on hepatic transporters is limited. In the present study, we studied quantitative contribution of transporters for hepatic handling of uric acid by mathematical modeling analysis in human sandwich-cultured hepatocytes (hSCH). Stable isotope-labeled hypoxanthine, hypoxanthine-¹³C2,¹⁵N (HX), was incubated with hSCH and formed ¹³C2,¹⁵N-labeled xanthine (XA) and uric acid (UA) were measured by LC-MS/MS time dependently. Rate constants for metabolism and efflux and uptake transport across sinusoidal and bile canalicular membranes of HX, XA and UA were estimated in the presence of inhibitors of XO and uric acid transporters. An XO inhibitor allopurinol significantly decreased metabolisms of HX and XA. Efflux into bile canalicular lumen was negligible and sinusoidal efflux was considered main efflux pathway of formed UA. Transporter inhibition study highlighted that GLUT9 strongly and MRP4 intermediately contribute to the sinusoidal efflux of UA with minor contribution of NPT1/4. Modeling analysis developed in the present study should be useful for quantitative prediction of uric acid disposition in liver.

Contribution of Prostaglandin Transporter OATP2A1/SLCO2A1 to Placenta-to-Maternal Hormone Signaling and Labor Induction

We evaluated the contribution of organic anion transporting polypeptide 2A1 (OATP2A1/SLCO2A1), a high-affinity carrier for prostaglandins (PGs), to the parturition process. At gestational day (GD) 15.5, OATP2A1 is co-localized with 15-hydroxy-PG dehydrogenase in the mouse placental junctional zone and facilitates PG degradation by delivering PGs to the cytoplasm. Slco2a1 (+/−) females mated with Slco2a1 (−/−) males frequently showed elevated circulating progesterone at GD18.5 and delayed parturition. Progesterone receptor inhibition by RU486 treatment at GD18.5 blocked the delay of parturition. In the junctional zone, PGE₂ stimulated placental lactogen II (PL-II) production, resulting in higher expression of PL-II in Slco2a1 (−/−) placenta at GD18.5. Indomethacin treatment at GD15.5 suppressed the PL-II overproduction at GD18.5 in Slco2a1 (−/−) embryo-bearing dams, which promoted progesterone withdrawal and corrected the delayed parturition. These results suggest that extracellular PGE₂ reduction by OATP2A1 at mid-pregnancy would be associated with progesterone withdrawal by suppressing PL-II production, triggering parturition onset.

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Placental OATP2A1 promotes PGE₂ degradation by delivering PGE₂ to the cytoplasm

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Placental PGE₂ stimulates the PL-II production

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Extracellular PGE₂ decrease by placental OATP2A1 promotes progesterone withdrawal

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Feto-placental OATP2A1 is associated with the initiation of parturition

Identification of the Uptake Transporter Responsible for Distribution of Acotiamide into Stomach Tissue

The acetylcholinesterase inhibitor, acotiamide, improves gastric motility and is clinically used to treat functional dyspepsia. The present study aimed to identify the transporters involved in the distribution of acotiamide in stomach tissue. Acotiamide uptake by the gastric cancer-derived model cell line, Hs746 T, was Na⁺- and pH-independent. The initial uptake velocity of acotiamide was saturable with increasing concentrations of acotiamide and was inhibited by selective serotonin reuptake inhibitors, which are potent inhibitors of the plasma membrane monoamine transporter (PMAT). The uptake of acotiamide by PMAT gene-transfected HEK293 cells was saturable, with similar K_m (197.9 μM) values to those of uptake by Hs 746T cells (106 μM). Moreover, immunoreactivity of PMAT was found in the gastric smooth muscle and vascular endothelial cells. These results suggest that PMAT contributes to the distribution of acotiamide in the stomach, where it exerts its pharmacological effects.

ABCG2 requires a single aromatic amino acid to "clamp" substrates and inhibitors into the binding pocket

ATP-binding cassette sub-family G member 2 (ABCG2) is a homodimeric ATP-binding cassette (ABC) transporter that not only has a key role in helping cancer cells to evade the cytotoxic effects of chemotherapy, but also in protecting organisms from multiple xeno- and endobiotics. Structural studies indicate that substrate and inhibitor (ligands) binding to ABCG2 can be differentiated quantitatively by the number of amino acid contacts, with inhibitors displaying more contacts. Although binding is the obligate initial step in the transport cycle, there is no empirical evidence for one amino acid being primarily responsible for ligand binding. By mutagenesis and biochemical studies, we demonstrated that the phylogenetically conserved amino acid residue, F439, was critical for both transport and the binding of multiple substrates and inhibitors. Structural modeling implied that the π-π interactions from each F439 monomer mediated the binding of a surprisingly diverse array of structurally unrelated substrates and inhibitors and that this symmetrical π-π interaction "clamps" the ligand into the binding pocket. Key molecular features of diverse ABCG2 ligands using the π-π clamp along with structural studies created a pharmacophore model. These novel findings have important therapeutic implications because key properties of ligands interacting with ABCG2 have been disovered. Furthermore, mechanistic insights have been revealed by demonstrating that for ABCG2 a single amino acid is essential for engaging and initiating transport of multiple drugs and xenobiotics.

Pharmacokinetic/pharmacodynamic modeling and simulation of dotinurad, a novel uricosuric agent, in healthy volunteers

This study aimed to investigate the pharmacokinetic and pharmacodynamic (PK/PD) profiles of dotinurad, a novel uricosuric agent, and to construct a PK/PD model to predict serum urate (SUA) levels after dotinurad administration in healthy men. PK/PD model was constructed using single-dose study data considering the physiological features of urate handling. Model validation was performed by comparing the predicted SUA levels with the SUA levels in a multiple-dose study. Dotinurad was absorbed rapidly, and its exposure increased proportionally in the tested dose ranges (0.5-20 mg) after a single-dose administration. The PK model after oral administration was described using a one-compartment model with first-order absorption. Effects on SUA and renal urate excretion of dotinurad increased with dose escalation but were apparently saturable at a dose >5 mg. The simple maximal effect (Emax) model was selected as the PD model of dotinurad on renal urate reabsorption, resulting in an estimated Emax of 0.51. The plasma concentration at the half-maximal effect of dotinurad was 196 ng/mL. Other PD parameters were calculated from the change in SUA level or urinary excretion of urate before and after dotinurad administration. The predicted SUA levels, using the PK/PD model, were well-fitted with the observed values. The constructed PK/PD model of dotinurad appropriately described the profiles of dotinurad plasma concentrations and SUA level in multiple administration study.

Quantification of Prostaglandin E2 Concentration in Interstitial Fluid from the Hypothalamic Region of Free-moving Mice

Prostaglandin E₂ (PGE₂) is a well-established chemical mediator for the generation of the fever at the hypothalamus of the brain. PGE₂ mediates fever generation via PGE receptor 3 (i.e., EP3) on neurons in the preoptic area. The role of PGE₂ has been analyzed by measuring PGE₂ concentration in cerebrospinal fluid (C_csf); however, local PGE₂ concentration at the hypothalamus may not necessarily be consistent with C_csf. In this protocol, we introduce our method to measure directly the alteration in PGE₂ concentration in interstitial fluid in the hypothalamus (C_isf) of awake (free-moving) mice using a microdialysis technique. Male mice (c57BL/6J) were anesthetized and fixed in the stereotaxic instrument, and a microdialysis probe was inserted into the hypothalamus through a guide cannula. On the fifth postoperative day, C_isf was monitored in free-moving mice that were intraperitoneally (i.p.) injected with lipopolysaccharide (LPS). PGE₂ and other eicosanoids recovered in Krebs-Ringer phosphate buffer and defused through a microdialysis probe were extracted into ethyl acetate/formic acid and then quantified with LC-MS/MS. Our method is useful to understand the role of key regulators of prostaglandin concentration such as those of transporters, which have been unappreciated in inflammation-based brain diseases.

Changes of drug pharmacokinetics mediated by downregulation of kidney organic cation transporters Mate1 and Oct2 in a rat model of hyperuricemia

The effects of hyperuricemia on the expression of kidney drug transporters and on the pharmacokinetics of several substrate drugs were examined. We first established a rat model of hyperuricemia without marked symptoms of chronic kidney failure by 10-day co-administration of oxonic acid (uricase inhibitor) and adenine (biosynthetic precursor of uric acid). These hyperuricemic rats showed plasma uric acid concentrations of up to 6 mg/dL, which is similar to the serum uric acid level in hyperuricemic humans, with little change of inulin clearance. The mRNA levels of multidrug and toxin extrusion 1 (Mate1, Slc47a1), organic anion transporter 1 (Oat1, Slc22a6), organic cation transporter 2 (Oct2, Slc22a2), urate transporter 1 (Urat1, Slc22a12) and peptide transporter 1 (Pept1, Slc15a1) were significantly decreased in kidney of hyperuricemic rats. Since Oct2, Mate1 and Oat1 are important for renal drug elimination, we next investigated whether the pharmacokinetics of their substrates, metformin, cephalexin and creatinine, were altered. The plasma concentration of metformin was not affected, while its kidney tissue accumulation was significantly increased. The plasma concentration and kidney tissue accumulation of cephalexin and the plasma concentration of creatinine were also increased. Furthermore, the protein expression of kidney Mate1 was decreased in hyperuricemic rats. Accordingly, although multiple factors may influence renal handling of these drugs, these observations can be accounted for, at least in part, by downregulation of Mate1-mediated apical efflux from tubular cells and Oct2-mediated basolateral uptake. Our results suggest that hyperuricemia could alter the disposition of drugs that are substrates of Mate1 and/or Oct2.

Uric acid analogue as a possible xenobiotic marker of uric acid transporter Urat1 in rats

The inhibitor of uric acid reabsorptive transporter URAT1 in kidney is drawing attention as a drug target for hyperuricemia. However, it is difficult to evaluate efficacy of URAT1 inhibitors in vivo using laboratory animals due to species difference in uric acid metabolism. In the present study, the usefulness of exogenously administering uric acid analogues resistant to uricase was investigated for in vivo evaluation of transport activity of rUrat1 in rats. Uptake of examined four uric acid analogues by rUrat1-expressing Xenopus oocytes was significantly higher than that by water-injected oocytes. In metabolism studies, disappearance of these compounds was negligible, while uric acid was significantly decreased. When oxypurinol was administered to rats, fractional excretion (FE) was 0.4, suggesting reabsorption of oxypurinol. Moreover, FE of oxypurinol was tended to be increased, but not statistically different, by co-administration of a uricosuric agent FYU-981, while plasma concentration of oxypurinol was not affected. These results suggested that oxypurinol is a potential uric acid analogue, although it was not suitable as a probe of uric acid in in vivo study. Our findings may contribute to discovery and development of novel uricosuric agent targeting URAT1.

Identification of MRP2 as a targetable factor limiting oxaliplatin accumulation and response in gastrointestinal cancer

Oxaliplatin is important for the clinical treatment of colorectal cancer and other gastrointestinal malignancies, but tumour resistance is limiting. Several oxaliplatin transporters were previously identified but their relative contributions to determining oxaliplatin tumour responses and gastrointestinal tumour cell sensitivity to oxaliplatin remains unclear. We studied clinical associations between tumour expression of oxaliplatin transporter candidate genes and patient response to oxaliplatin, then experimentally verified associations found with MRP2 in models of human gastrointestinal cancer. Among 18 oxaliplatin transporter candidate genes, MRP2 was the only one to be differentially expressed in the tumours of colorectal cancer patients who did or did not respond to FOLFOX chemotherapy. Over-expression of MRP2 (endogenously in HepG2 and PANC-1 cells, or induced by stable transfection of HEK293 cells) decreased oxaliplatin accumulation and cytotoxicity but those deficits were reversed by inhibition of MRP2 with myricetin or siRNA knockdown. Mice bearing subcutaneous HepG2 tumour xenografts were sensitised to oxaliplatin antitumour activity by concurrent myricetin treatment with little or no increase in toxicity. In conclusion, MRP2 limits oxaliplatin accumulation and response in human gastrointestinal cancer. Screening tumour MRP2 expression levels, to select patients for treatment with oxaliplatin-based chemotherapy alone or in combination with a MRP2 inhibitor, could improve treatment outcomes.

Effect of endogenous multidrug resistance 1 and P-glycoprotein expression on anticancer drug resistance in colon cancer cell lines

P-glycoprotein (P-gp, multidrug resistance 1 (MDR1)) overexpression confers multidrug resistance to cancer cells, and P-gp in cell lines transfected with MDR1 or selected with chemotherapeutics significantly affect the anticancer drug efficacy. Although human cancer cell line panels consisting of defined tumor cell lines expressing endogenous P-gp have been used to screen drugs in pharmaceutical industries, endogenous P-gp affecting in vitro anticancer drug efficacy is unclear. The impact of P-gp expression on anticancer drug efficacy was assessed by using five colon cancer cell lines expressing varying endogenous P-gp levels and by selecting from the Cancer Cell Line Encyclopedia (CCLE). mRNA expression of MDR1 was considered as a surrogate of the protein expression of its gene product, P-gp, in CL-11, C2BBe1 and RKO cells, whereas P-gp protein expression in plasma membranes or crude membrane fractions was lower than expected from mRNA expression in CW-2 and CL-40 cells. The EC₅₀ of paclitaxel and vinorelbine decreased in the presence of a P-gp inhibitor in CW-2 and CL-11 cells that highly express P-gp. No significant alterations in EC₅₀ were observed in the CL-40, C2BBe1 and RKO cells, which show lower P-gp expression. Accordingly, the apparent in vitro efficacy of anticancer drugs could be underestimated if the endogenous P-gp expression is higher than in CL-11 cells. The effect of P-gp needs to be carefully evaluated in cell lines that highly express P-gp, which account for 1.5% of cancer cell lines, including all cancer types, and 14.5% of colon cancer cell lines in CCLE, considering the protein expression levels in plasma membranes.