Discovery of a chalcone derivative as an anti-fibrotic agent targeting transforming growth factor-β1 signaling: Potential therapy of renal fibrosis

As a final common pathway of renal injuries, renal fibrosis leads to chronic kidney disease (CKD). Currently, there is no safe and effective therapy to prevent the progression of renal fibrosis to CKD. Inhibition of transforming growth factor-β1 (TGF-β1) pathway is proposed as one of the most promising approaches for anti-renal fibrosis therapies. This study aimed to identify novel anti-fibrotic agents using the TGF-β1-induced fibrosis in renal proximal tubule epithelial cells (RPTEC) and characterize their mechanism of action as well as in vivo efficacy. By screening 362 natural product-based compounds for their ability to reduce collagen accumulation assessed by picro-sirius red (PSR) staining in RPTEC cells, a chalcone derivative AD-021 was identified as an anti-fibrotic agent with IC50 of 14.93 μM. AD-021 suppressed TGF-β1-induced collagen production, expression of pro-fibrotic proteins (fibronectin and α-smooth muscle actin (αSMA)), and Smad-dependent and Smad-independent signaling pathways via suppression of TGF-β receptor II (TGFβRII) phosphorylation in RPTEC cells. Furthermore, TGF-β1-induced mitochondrial fission in RPTEC cells was ameliorated by AD-021 via mechanisms involving inhibition of Drp1 phosphorylation. In a mouse model of unilateral ureteral obstruction (UUO)-induced renal fibrosis, AD-021 reduced plasma TGF-β1, ameliorated renal fibrosis and improved renal function. Collectively, AD-021 represents a novel class of natural product-based anti-fibrotic agent that has therapeutic potential in the prevention of fibrosis-associated renal disorders including CKD.

Interaction of buspirone and its major metabolites with human organic cation transporters

Buspirone, a cationic drug, is an anxiolytic and antidepressant drug. However, whether buspirone and its metabolites are interacted with organic cationic transporter remains uncertain. In this study, we examined the interaction of buspirone and its major metabolites 1-(2-pyrimidinyl)piperazine (1-PP) and 6-hydroxybuspirone (6'-OH-Bu) with hOCTs using human hepatocellular carcinoma (HepG2), human colorectal adenocarcinoma (Caco-2) cells, and S2 cells expressing OCT1 (S2hOCT1), 2 (S2hOCT2), or 3 (S2hOCT3). Coadministration of buspirone and fluorescent 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP⁺ ) was examined using HepG2 cells, and [³ H]-1-methyl-4-phenylpyridinium (MPP⁺ ) transport was assessed in S2 cell overexpressing hOCTs. The results showed that ASP⁺ transport was suppressed by buspirone with an IC₅₀ of 26.3 ± 2.9 μM without any cytotoxic effects in HepG2 expressing hOCTs cells. Consistently, buspirone strongly inhibited [³ H]-MPP⁺ uptake by S2hOCT1, S2hOCT2, and S2hOCT3 cells with an IC_50s of 89.0 ± 1.3 μM, 43.7 ± 7.5 μM, and 20.4 ± 1.0 μM, respectively. Nonetheless, 6'-OH-Bu and 1-PP caused weak or no inhibition on ASP⁺ and [³ H]-MPP⁺ transport. These findings suggest the potential interaction of buspirone with organic cation drugs that are handled by hOCT3. However, further clinical relevance is needed to support these findings for preventing drug-drug interaction in patients who take prescribed drugs together with buspirone.

Nephroprotective potential of Panduratin A against colistin-induced renal injury via attenuating mitochondrial dysfunction and cell apoptosis

Colistin is a last-resort polypeptide antibiotic widely used to treat against multidrug-resistant Gram-negative bacterial infections. However, this treatment is associated with nephrotoxicity. The aim of this study was to examine the potential protective effect of panduratin A, a bioactive compound of Boesenbergia rotunda, on colistin-induced nephrotoxicity in both in vivo and in vitro models. Intraperitoneal injection of 15 mg/kg colistin for 7 days markedly promoted renal tubular degeneration, increased blood urea nitrogen (BUN) levels, and upregulated the expression of renal injury biomarker and apoptosis proteins. In addition, treatment with colistin increased oxidative stress and apoptosis in mice kidney tissues. Interestingly, these defects were attenuated when co-administered of colistin with panduratin A (2.5 or 25 mg/kg). The underlying mechanisms of panduratin A attenuating colistin toxicity was investigated in human renal proximal tubular cells (RPTEC/TERT1). The mechanisms by which colistin-triggered cytotoxicity was determined by analysis of cell death, reactive oxygen species (ROS) levels, mitochondria function as well as the expression of proteins related to apoptosis pathway. Colistin treatment (200 µg/ml) significantly increased cell apoptosis, elevated ROS production, reduced mitochondrial membrane potential, and decreased anti-apoptotic protein (Bcl-2) expression. These effects were notably suppressed by co-treatment with panduratin A (5 μM). Collectively, panduratin A exerts as a novel nephroprotective agent to protect against colistin-induced renal injury by attenuating mitochondrial damage and renal cell apoptosis.

Uric acid lowering in relation to HbA1c reductions with the SGLT2 inhibitor tofogliflozin

An integrated analysis was performed with data from 4 phase 2 and phase 3 studies of tofogliflozin in which patients with type 2 diabetes mellitus received the sodium-glucose cotransporter 2 inhibitor tofogliflozin for up to 24 weeks. Sex differences, baseline haemoglobin A1c (HbA1c) and serum uric acid (UA) levels, and log₁₀ -transformed urinary N-acetyl-β-D-glucosaminidase ratio were significantly correlated with the reduction in serum UA levels at both 4 and 24 weeks in multivariate analysis (respectively, P < .0001). The decrease in HbA1c levels was greatest in the group with the highest baseline HbA1c level (quartile 4; HbA1c > 8.6%) and lowest in the group with the lowest baseline HbA1c level (quartile 1; HbA1c ≤ 7.4%). The decrease in serum UA levels was greatest in the quartile 1 group and lowest in the quartile 4 group. In most groups, the maximum decrease in serum UA levels was seen in the first 4 weeks, while the maximum decrease in HbA1c was seen at week 24. Thus, serum UA levels were significantly decreased in patients with moderate HbA1c levels.

Inhibition of l-type amino acid transporter 1 activity as a new therapeutic target for cholangiocarcinoma treatment

Unlike normal cells, cancer cells undergo unlimited growth and multiplication, causing them to require massive amounts of amino acid to support their continuous metabolism. Among the amino acid transporters expressed on the plasma membrane, l-type amino acid transporter-1, a Na⁺-independent neutral amino acid transporter, is highly expressed in many types of human cancer including cholangiocarcinoma. Our previous study reported that l-type amino acid transporter-1 and its co-functional protein CD98 were highly expressed and implicated in cholangiocarcinoma progression and carcinogenesis. Therefore, this study determined the effect of JPH203, a selective inhibitor of l-type amino acid transporter-1 activity, on cholangiocarcinoma cell inhibition both in vitro and in vivo. JPH203 dramatically suppressed [¹⁴C]l-leucine uptake as well as cell growth in cholangiocarcinoma cell lines along with altering the expression of l-type amino acid transporter-1 and CD98 in response to amino acid depletion. We also demonstrated that JPH203 induced both G2/M and G0/G1 cell cycle arrest, as well as reduced the S phase accompanied by altered expression of the proteins in cell cycle progression: cyclin D1, CDK4, and CDK6. There was also cell cycle arrest of the related proteins, P21 and P27, in KKU-055 and KKU-213 cholangiocarcinoma cells. Apoptosis induction, detected by an increase in trypan blue-stained cells along with a cleaved caspase-3/caspase-3 ratio, occurred in JPH203-treated cholangiocarcinoma cells at the highest concentration tested (100 µM). As expected, daily intravenous administration of JPH203 (12.5 and 25 mg/kg) significantly inhibited tumor growth in KKU-213 cholangiocarcinoma cell xenografts in the nude mice model in a dose-dependent manner with no statistically significant change in the animal's body weight and with no differences in the histology and appearance of the internal organs compared with the control group. Our study demonstrates that suppression of l-type amino acid transporter-1 activity using JPH203 might be used as a new therapeutic strategy for cholangiocarcinoma treatment.

LAT1 acts as a crucial transporter of amino acids in human thymic carcinoma cells

L-type amino acid transporter 1 (LAT1, SLC7A5) incorporates essential amino acids into cells. Recent studies have shown that LAT1 is a predominant transporter in various human cancers. However, the function of LAT1 in thymic carcinoma remains unknown. Here we demonstrate that LAT1 is a critical transporter for human thymic carcinoma cells. LAT1 was strongly expressed in human thymic carcinoma tissues. LAT1-specific inhibitor significantly suppressed leucine uptake and growth of Ty82 human thymic carcinoma cell lines, suggesting that thymic carcinoma takes advantage of LAT1 as a quality transporter and that LAT1-specific inhibitor might be clinically beneficial in therapy for thymic carcinoma.

Increase in L-type amino acid transporter 1 expression during cholangiocarcinogenesis caused by liver fluke infection and its prognostic significance

L-type amino acid transporter 1 (LAT1) is highly expressed in various human cancers, including cholangiocarcinoma (CCA), the most common cancer in Northeast Thailand. Chronic inflammation and oxidative stress induced by liver fluke, Opisthorchis viverrini, infection has been recognized as the major cause of CCA in this area. We show here that an increased expression of LAT1 and its co-functional protein CD98 are found during carcinogenesis induced by Ov in hamster CCA tissues. We also demonstrate that oxidative stress induced by H₂O₂ is time-dependent and dramatically activates LAT1 and CD98 expression in immortal cholangiocytes (MMNK1). In addition, H₂O₂ treatment increased LAT1 and CD98 expression, as well as an activated form of AKT and mTOR in MMNK1 and CCA cell lines (KKU-M055 and KKU-M213). We also show that suppression of PI3K/AKT pathway activity with a dual PI3K/mTOR inhibitor, BEZ235, causes a reduction in LAT1 and CD98 expression in KKU-M055 and KKU-M213 in parallel with a reduction of activated AKT and mTOR. Interestingly, high expression of LAT1 in human CCA tissues is a significant prognostic factor for shorter survival. Taken together, our data show that LAT1 expression is significantly associated with CCA progression and cholangiocarcinogenesis induced by oxidative stress. Moreover, the expression of LAT1 and CD98 in CCA is possibly regulated by the PI3K/AKT signaling pathway.

Interaction of green tea catechins with renal organic cation transporter 2

1. Green tea extract (GTE) and EGCG have previously shown to increase the uptake of MPP⁺ into Caco-2 cells. However, whether GTE and its derivatives interact with renal basolateral organic cation transporter 2 (Oct2) which plays a crucial role for cationic clearance remains unknown. Thus, this study assessed the potential of drug-green tea (GT) catechins and its derivatives interactions with rat Oct2 using renal cortical slices and S2 stably expressing rat Oct2 (S2rOct2). 2. Both GTE and ECG inhibited MPP⁺ uptake in renal slices in a concentration-dependent manner (IC₅₀ = 2.71 ± 0.360 mg/ml and 0.87 ± 0.151 mM), and this inhibitory effect was reversible. Inhibition of [³H]MPP⁺ transport in S2rOct2 by either GTE or ECG (IC₅₀ = 1.90 ± 0.087 mg/ml and 1.67 ± 0.088 mM) was also observed. 3. The weak and reversible interactions of GTE and ECG with rOct2 indicate that consumption of GT beverages could not interfere with cationic drugs secreted via renal OCT2 in humans. However, the rise of therapeutic use of GTE and ECG might have to take into account the significant possibility of adverse drug-green tea catechins interactions which could alter renal organic cation drug clearance.

Distinct action of the α-glucosidase inhibitor miglitol on SGLT3, enteroendocrine cells, and GLP1 secretion

Oral ingestion of carbohydrate triggers glucagon-like peptide 1 (GLP1) secretion, but the molecular mechanism remains elusive. By measuring GLP1 concentrations in murine portal vein, we found that the ATP-sensitive K(+) (KATP) channel is not essential for glucose-induced GLP1 secretion from enteroendocrine L cells, while the sodium-glucose co-transporter 1 (SGLT1) is required, at least in the early phase (5 min) of secretion. By contrast, co-administration of the α-glucosidase inhibitor (α-GI) miglitol plus maltose evoked late-phase secretion in a glucose transporter 2-dependent manner. We found that GLP1 secretion induced by miglitol plus maltose was significantly higher than that by another α-GI, acarbose, plus maltose, despite the fact that acarbose inhibits maltase more potently than miglitol. As miglitol activates SGLT3, we compared the effects of miglitol on GLP1 secretion with those of acarbose, which failed to depolarize the Xenopus laevis oocytes expressing human SGLT3. Oral administration of miglitol activated duodenal enterochromaffin (EC) cells as assessed by immunostaining of phosphorylated calcium-calmodulin kinase 2 (phospho-CaMK2). In contrast, acarbose activated much fewer enteroendocrine cells, having only modest phospho-CaMK2 immunoreactivity. Single administration of miglitol triggered no GLP1 secretion, and GLP1 secretion by miglitol plus maltose was significantly attenuated by atropine pretreatment, suggesting regulation via vagal nerve. Thus, while α-GIs generally delay carbohydrate absorption and potentiate GLP1 secretion, miglitol also activates duodenal EC cells, possibly via SGLT3, and potentiates GLP1 secretion through the parasympathetic nervous system.

LAT1 is a critical transporter of essential amino acids for immune reactions in activated human T cells

Activation of T cells accompanies remarkable enhancement of metabolism. Sufficient and continuous nutrient supply is therefore important to support immune reaction in T cells. However, the mechanism of the promotion of nutrient incorporation in activated T cells has not been elucidated. In this study, we show that L-type amino acid transporter 1 (LAT1) is a major transporter for essential amino acids into activated human T cells. CD3/CD28 stimulation in primary human T cells triggered dramatic induction of LAT1 expression mediated by NF-κB and AP-1. Functional disturbance of LAT1 by a specific inhibitor and by small interfering RNA in human T cells suppressed essential amino acid uptake and induced a stress response mediated by DNA damage-inducible transcript 3 to attenuate cytokine production via inhibition of NF-κB and NFAT activities. These results uncover the previously unknown mechanism by which T cells accelerate essential amino acid uptake upon activation and adapt to essential amino acid starvation. Our results also raise the possibility for application of an LAT1 inhibitor as a new drug for therapy of disease caused by exaggerated immune response.

Potent human uric acid transporter 1 inhibitors: in vitro and in vivo metabolism and pharmacokinetic studies

Human uric acid transporter 1 (hURAT1; SLC22A12) is a very important urate anion exchanger. Elevated urate levels are known to play a pivotal role in cardiovascular diseases, chronic renal disease, diabetes, and hypertension. Therefore, the development of potent uric acid transport inhibitors may lead to novel therapeutic agents to combat these human diseases. The current study investigates small molecular weight compounds and their ability to inhibit 14C-urate uptake in oocytes expressing hURAT1. Using the most promising drug candidates generated from our structure-activity relationship findings, we subsequently conducted in vitro hepatic metabolism and pharmacokinetic (PK) studies in male Sprague-Dawley rats. Compounds were incubated with rat liver microsomes containing cofactors nicotinamide adenine dinucleotide phosphate and uridine 5'-diphosphoglucuronic acid. In vitro metabolism and PK samples were analyzed using liquid chromatography/mass spectrometry-mass spectrometry methods. Independently, six different inhibitors were orally (capsule dosing) or intravenously (orbital sinus) administered to fasting male Sprague-Dawley rats. Blood samples were collected and analyzed; these data were used to compare in vitro and in vivo metabolism and to compute noncompartmental model PK values. Mono-oxidation (Phase I) and glucuronidation (Phase II) pathways were observed in vitro and in vivo. The in vitro data were used to compute hepatic intrinsic clearance, and the in vivo data were used to compute peak blood concentration, time after administration to achieve peak blood concentration, area under the curve, and orally absorbed fraction. The experimental data provide additional insight into the hURAT1 inhibitor structure-activity relationship and in vitro-in vivo correlation. Furthermore, the results illustrate that one may successfully prepare potent inhibitors that exhibit moderate to good oral bioavailability.

Atypical Leydig cell hyperplasia in adult rats with low T and high LH induced by prenatal Di(n-butyl) phthalate exposure

The present study describes atypical Leydig cell (LC) hyperplasia in 20-week-old Sprague-Dawley rats with low testosterone and high luteinizing hormone levels after prenatal administration of 100 mg/kg/day di(n-butyl) phthalate on days 12 to 21 postconception. Light microscopy revealed LC hyperplasia surrounded by severely degenerated seminiferous tubules. Aggregated LCs had large ovoid nuclei with nucleoli and abundant eosinophilic cytoplasm. Immunohistochemical analysis showed expression of proliferating cell nuclear antigen and vimentin in many hyperplastic LCs. Electron microscopy revealed atypical nuclei, abundant free ribosomes, stripped rough endoplasmic reticulum, intermediate-size filaments, elongated cytoplasmic filopodia, atypical tight junctions, and cilia formations, but smooth endoplasmic reticulum was scarcely observed.

c-Myc is crucial for the expression of LAT1 in MIA Paca-2 human pancreatic cancer cells

Tumor cells take up a massive amount of nutrition compared to normal cells for increased metabolism. Therefore, special transporters for organic materials are required to satisfy the powerful consumption of nutrition in tumor cells. L-type amino acid transporter 1 (LAT1) incorporates large neutral amino acids, most of which are also categorized as essential amino acids, into cells in a Na+-independent manner. Because of its high expression levels in a variety of cancer cells, it is speculated that LAT1 functions as a key transporter for highly effective delivery of essential amino acids into cancer cells. In this regard, LAT1 inhibitor is expected to have clinical benefit for cancer therapy. However, the molecular mechanism of enrichment of LAT1 in cancer cells remains poorly understood. Here, we show that a proto-oncogene, c-Myc, is a critical positive regulator of LAT1 expression in MIA Paca-2 human pancreatic cancer cells. The uptake of leucine, a representative neutral amino acid, was strictly dependent on LAT1 in MIA Paca-2 cells, and siRNA-mediated knockdown of LAT1 inhibited cell proliferation. Diminished c-Myc expression with siRNA resulted in severe reduction of LAT1 protein levels as well as mRNA levels, which, in turn, led to a significant defect of leucine incorporation. The LAT1 promoter has a canonical c-Myc binding sequence and overexpression of c-Myc increased LAT1 promoter activity, whereas mutation of c-Myc binding site diminished this effect. Our results suggest biological significance of LAT1 in tumor growth and molecular machinery that could explain why LAT1 is preferentially expressed in cancer cells.

Apical voltage-driven urate efflux transporter NPT4 in renal proximal tubule

Uric acid (urate) is the end product of purine metabolism in humans. Human kidneys reabsorb a large proportion of filtered urate. This extensive renal reabsorption, together with the fact that humans do not possess uricase that catalyzes the biotransformation of urate into allantoin, results in a higher plasma urate concentration in humans compared to other mammals. A major determinant of plasma urate concentration is renal excretion as a function of the balance between reabsorption and secretion. We previously identified that renal urate absorption in proximal tubular epithelial cells occurs mainly via apical urate/anion exchanger, URAT1/SLC22A12, and by facilitated diffusion along the trans-membrane potential gradient by the basolateral voltage-driven urate efflux transporter, URATv1/SLC2A9/GLUT9. In contrast, the molecular mechanism by which renal urate secretion occurs remains elusive. Recently, we reported a newly characterized human voltage-driven drug efflux transporter, hNPT4/SLC17A3, which functions as a urate exit pathway located at the apical side of renal proximal tubules. This transporter protein has been hypothesized to play an important role with regard to net urate efflux. An in vivo role of hNPT4 is supported by the fact that missense mutations in SLC17A3 present in hyperuricemia patients with urate underexcretion abolished urate efflux capacity in vitro. Herein, we report data demonstrating that loop diuretics and thiazide diuretics substantially interact with hNPT4. These data provide molecular evidence for loop and thiazide-diuretics-induced hyperuricemia. Thus, we propose that hNPT4 is an important transepithelial proximal tubular transporter that transports diuretic drugs and operates functionally with basolateral organic anion transporters 1/3 (OAT1/OAT3).

Increased expression of SLC2A9 decreases urate excretion from the kidney

Urate is the final metabolite of purine in humans. Renal urate handling is clinically important because under-reabsorption or underexcretion causes hypouricemia or hyperuricemia, respectively. We have identified a urate-anion exchanger, URAT1, localized at the apical side and a voltage-driven urate efflux transporter, URATv1, expressed at the basolateral side of the renal proximal tubules. URAT1 and URATv1 are vital to renal urate reabsorption because the experimental data have illustrated that functional loss of these transporter proteins affords hypouricemia. While mutations affording enhanced function via these transporter proteins on urate handling is unknown, we have constructed kidney-specific transgenic (Tg) mice for URAT1 or URATv1 to investigate this problem. In our study, each transgene was under the control of the mouse URAT1 promoter so that transgene expression was directed to the kidney. Plasma urate concentrations in URAT1 and URATv1 Tg mice were not significantly different from that in wild-type (WT) mice. Urate excretion in URAT1 Tg mice was similar to that in WT mice, while URATv1 Tg mice excreted more urate compared with WT. Our results suggest that hyperfunctioning URATv1 in the kidney can lead to increased urate reabsorption and may contribute to the development of hyperuricemia.

Human uric acid transporter 1 (hURAT1): an inhibitor structure-activity relationship (SAR) study

The current study describes the chemical synthesis of a series of (2-ethylbenzofuran-3-yl)(substituted-phenyl)methanone compounds and their subsequent in vitro testing via oocytes expressing hURAT1. The experimental data support the notion that a potent hURAT1 inhibitor requires an anion (i.e., a formal negative charge) to interact with the positively charged hURAT1 binding pocket. An anion appears to be a primary requirement in order to be a hURAT1 substrate (i.e., urate) or inhibitor. We discuss the inhibitor structure-activity relationship and how electronically donating or withdrawing groups attached to the B-ring can decrease or increase inhibitory potency, respectively.

Clinical and functional characterization of URAT1 variants

Idiopathic renal hypouricaemia is an inherited form of hypouricaemia, associated with abnormal renal handling of uric acid. There is excessive urinary wasting of uric acid resulting in hypouricaemia. Patients may be asymptomatic, but the persistent urinary abnormalities may manifest as renal stone disease, and hypouricaemia may manifest as exercise induced acute kidney injury. Here we have identified Macedonian and British patients with hypouricaemia, who presented with a variety of renal symptoms and signs including renal stone disease, hematuria, pyelonephritis and nephrocalcinosis. We have identified heterozygous missense mutations in SLC22A12 encoding the urate transporter protein URAT1 and correlate these genetic findings with functional characterization. Urate handling was determined using uptake experiments in HEK293 cells. This data highlights the importance of the URAT1 renal urate transporter in determining serum urate concentrations and the clinical phenotypes, including nephrolithiasis, that should prompt the clinician to suspect an inherited form of renal hypouricaemia.

Recent advances in renal urate transport: characterization of candidate transporters indicated by genome-wide association studies

Humans have higher serum uric acid levels than other mammalian species owing to the genetic silencing of the hepatic enzyme uricase that metabolizes uric acid into allantoin. Urate (the ionized form of uric acid) is generated from purine metabolism and it may provide antioxidant defense in the human body. Despite its potential advantage, sustained hyperuricemia has pathogenetic causes in gout and renal diseases, and putative roles in hypertension and cardiovascular diseases. Since the kidney plays a dominant role in maintaining plasma urate levels through the excretion process, it is important to understand the molecular mechanism of renal urate handling. Although the molecular identification of a kidney-specific urate/anion exchanger URAT1 in 2002 paved the way for successive identification of several urate transport-related proteins, the entire picture of effective renal urate handling in humans has not yet been clarified. Recently, several genome-wide association studies identified a substantial association between uric acid concentration and single nucleotide polymorphisms in at least ten genetic loci including eight transporter-coding genes. In 2008, we functionally characterized the facilitatory glucose transporter family member SLC2A9 (GLUT9), one of the candidate genes for urate handling, as a voltage-driven urate transporter URATv1 at the basolateral side of renal proximal tubules that comprises the main route of the urate reabsorption pathway, in tandem with URAT1 at the apical side. In this review, recent findings concerning these candidate molecules are presented.

Sex-associated difference in estrogen receptor β expression in N-methyl-N'-nitro-N-nitrosoguanidine-induced gastric cancers in rats

Epidemiologic studies indicate that the incidence of gastric cancer is higher in males than in females. Although the mechanisms mediating this difference are unclear, a role for estrogens has been proposed. We used Western blotting to evaluate the role of estrogen receptor (ER) subtypes ERα and ERβ and proliferating cell nuclear antigen (PCNA) in N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced gastric carcinogenesis in Wistar rats; ERα and ERβ mRNA levels also were analyzed by quantitative real-time RT-PCR analysis. The incidence of gastric cancer was significantly higher in male than female rats. In both sexes, ERα expression was similar in MNNG-treated cancerous and noncancerous tissues and normal gastric tissue. However, ERβ expression in MNNG-treated cancerous and noncancerous tissues was significantly lower in male rats and higher in female rats than that in normal gastric tissue; MNNG-induced cancerous tissue showed the highest ERβ expression. PCNA expression in MNNG-treated cancerous tissues was higher than that in noncancerous tissues, and was higher in male rats than female rats. Western blotting results were consistent with the mRNA changes determined by quantitative real-time RT-PCR. The present study provides evidence of a sex-associated difference in ERβ and PCNA expression in MNNG-induced gastric cancers in Wistar rats.

A novel human organic anion transporter NPT4 mediates the transport of ochratoxin A

In the present study, we investigated the transport of nephrotoxic mycotoxin ochratoxin A (OTxA) by a novel human organic anion transporter hNPT4 using the Xenopus oocyte expression system. hNPT4 mediated time- and concentration-dependent uptake of OTxA (K(m): 802.8 µM) in a pH- and voltage-sensitive manner. Cis-inhibition experiments suggest that the substrate selectivity of hNPT4 is similar to that of hOAT4. The fact that the K(m) of OTxA for the efflux transporter hNPT4 was much higher than those for the uptake transporters hOAT1 and hOAT3 may favor the accumulation of OTxA in the tubular cell and lead to nephrotoxicity.

Human urate transporter 1 (hURAT1) mediates the transport of orotate

Orotate is a precursor of pyrimidine synthesis. The kidney uses exogenous orotate for the synthesis of uridine diphosphosugars, which are used in the glycosylation of collagen in glomerular and tubular basement membranes. Orotate uptake occurs in the liver and kidney, but its molecular mechanism is largely unknown. Since orotate has been shown to be a substrate of the renal urate/anion exchanger in brush border membrane vesicle studies, we investigated whether human URAT1 (hURAT1) mediates the transport of orotate using HEK293 cells expressing hURAT1 (HEK-hURAT1). hURAT1 mediated a time- and dose-dependent uptake of orotate (K (m) 5.2 μM). hURAT1-mediated [(3)H]orotate transport was inhibited strongly by non-labeled (cold) orotate and the uricouric agent benzbromarone, and moderately inhibited by urate, nicotinate, and another uricouric agent, probenecid. This is the first report demonstrating that hURAT1 mediates the transport of orotate. hURAT1 may function as one of the entrance pathways in renal proximal tubular cells.

Developing potent human uric acid transporter 1 (hURAT1) inhibitors

The kidneys are a vital organ in the human body. They serve several purposes including homeostatic functions such as regulating extracellular fluid volume and maintaining acid-base and electrolyte balance and are essential regarding the excretion of metabolic waste. Furthermore, the kidneys play an important role in uric acid secretion/reabsorption. Abnormalities associated with kidney transporters have been associated with various diseases, such as gout. The current study utilized Xenopus oocytes expressing human uric acid transporter 1 (hURAT1; SLC22A12) as an in vitro method to investigate novel compounds and their ability to inhibit (14)C-uric acid uptake via hURAT1. We have prepared and tested a series of 2-ethyl-benzofuran compounds and probed the hURAT1 in vitro inhibitor structure-activity relationship. As compared to dimethoxy analogues, monophenols formed on the C ring showed the best in vitro inhibitory potential. Compounds with submicromolar (i.e., IC(50) < 1000 nM) inhibitors were prepared by brominating the corresponding phenols to produce compounds with potent uricosuric activity.

Functional analysis of human sodium-phosphate transporter 4 (NPT4/SLC17A3) polymorphisms

We analyzed the functional properties of five nonsynonymous single nucleotide polymorphisms (SNPs) in the sodium-phosphate transporter NPT4 gene (SLC17A3) using the Xenopus oocyte expression system. NPT4 variants carrying SNP V257F, G279R, or P378L exhibited reduced transport of [(14)C]para-aminohippurate, [(3)H]bumetanide, [(3)H]estrone sulfate, and [(14)C]urate, when each variant clone was expressed in the plasma membrane of oocytes. This study suggests the possibility that the genetic variation of NPT4 contributes to inter-individual differences in disposition of anionic drugs such as diuretics as well as certain endogenous organic anions such as urate.

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

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

Molecular mechanism of ochratoxin a transport in the kidney

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

A novel transporter of SLC22 family specifically transports prostaglandins and co-localizes with 15-hydroxyprostaglandin dehydrogenase in renal proximal tubules

We identified a novel prostaglandin (PG)-specific organic anion transporter (OAT) in the OAT group of the SLC22 family. The transporter designated OAT-PG from mouse kidney exhibited Na(+)-independent and saturable transport of PGE(2) when expressed in a proximal tubule cell line (S(2)). Unusual for OAT members, OAT-PG showed narrow substrate selectivity and high affinity for a specific subset of PGs, including PGE(2), PGF(2alpha), and PGD(2). Similar to PGE(2) receptor and PGT, a structurally distinct PG transporter, OAT-PG requires for its substrates an alpha-carboxyl group, with a double bond between C13 and C14 as well as a (S)-hydroxyl group at C15. Unlike the PGE(2) receptor, however, the hydroxyl group at C11 in a cyclopentane ring is not essential for OAT-PG substrates. Addition of a hydroxyl group at C19 or C20 impairs the interaction with OAT-PG, whereas an ethyl group at C20 enhances the interaction, suggesting the importance of hydrophobicity around the omega-tail tip forming a "hydrophobic core" accompanied by a negative charge, which is essential for substrates of OAT members. OAT-PG-mediated transport is concentrative in nature, although OAT-PG mediates both facilitative and exchange transport. OAT-PG is kidney-specific and localized on the basolateral membrane of proximal tubules where a PG-inactivating enzyme, 15-hydroxyprostaglandin dehydrogenase, is expressed. Because of the fact that 15-keto-PGE(2), the metabolite of PGE(2) produced by 15-hydroxyprostaglandin dehydrogenase, is not a substrate of OAT-PG, the transport-metabolism coupling would make unidirectional PGE(2) transport more efficient. By removing extracellular PGE(2), OAT-PG is proposed to be involved in the local PGE(2) clearance and metabolism for the inactivation of PG signals in the kidney cortex.

Xenopus laevis oocytes expressing human P-glycoprotein: probing trans- and cis-inhibitory effects on [3H]vinblastine and [3H]digoxin efflux

P-glycoprotein (P-gp; MDR1) recognizes and actively transports many structurally diverse compounds (hydrophobic neutral and cationic). We studied MDR1-mediated drug transport using a high-throughput (96-well) oocyte expression system. MDR1-expressing oocytes contained sufficient ATP levels to conduct fundamental efflux studies; the optimal experimental temperature was 25 degrees C. [(3)H]Vinblastine efflux by MDR1-expressing oocytes was detectable and afforded a K(m) of 145.5+/-25.4microM. [(3)H]Vinblastine (5.6+/-0.3microM) and [(3)H]digoxin (1.0+/-0.1microM) were individually injected into MDR1-expressing oocytes and their efflux monitored. Quinidine and verapamil, known MDR1 substrates/inhibitors, showed trans-inhibition on MDR1-mediated [(3)H]vinblastine and [(3)H]digoxin efflux. Conversely, doxorubicin demonstrated cis-inhibition without trans-inhibition on MDR1-mediated [(3)H]vinblastine efflux. The MDR1-expressing oocyte system offers researchers with an alternative in vitro method to screen compounds and may allow one to probe P-gp drug-drug and/or drug-inhibitor interactions.

Role of Mouse Organic Anion Transporter 3 (mOat3) as a Basolateral Prostaglandin E2 Transport Pathway

Renal organic anion transporters play an important role in the handling of a number of endogenous and exogenous anionic substances in the kidney. In this study, we investigated prostaglandin E(2) (PGE(2)) transport properties and intrarenal localization of mouse organic anion transporter 3 (mOat3). When expressed in Xenopus oocytes, mOat3 mediated the time- and concentration-dependent transport of PGE(2) (K(m): 1.48 microM). PGE(2) transport mediated by mOat3 was trans-stimulated by intracellular glutarate injected into the oocytes. PGE(2) efflux via mOat3 was also trans-stimulated by extracellular glutarate. Thus, mOat3 was shown to mediate the bidirectional transport of PGE(2), partly coupled to the dicarboxylate exchange mechanism. Immunohistochemical study revealed that mOat3 protein was localized at the basolateral membrane of renal proximal and distal tubules. Furthermore, diffuse expression of mOat3, including expression in the basolateral membrane in macula densa (MD) cells, was observed. These results indicate that mOat3 plays an important role as a basolateral transport pathway of PGE(2) in the distal nephron including MD cells that may constitute one of the indispensable steps for renin release and regulation of the tubuloglomerular feedback mechanism.

Modulation of Renal Apical Organic Anion Transporter 4 Function by Two PDZ Domain–Containing Proteins

Human organic anion transporter 4 (OAT4) is an apical organic anion/dicarboxylate exchanger in the renal proximal tubules and mediates high-affinity transport of steroid sulfates such as estrone-3-sulfate (E1S) and dehydroepiandrosterone sulfate. Here, two multivalent PDZ (PSD-95/Discs Large/ZO-1) proteins PDZK1 and NHERF1 were examined as interactors of OAT4 by a yeast two-hybrid assay. These interactions require the extreme C-terminal region of OAT4 and the first and fourth PDZ domains of PDZK1 and the first PDZ domain of NHERF1. These interactions were confirmed by surface plasmon resonance assays (K(D): 36 nM, 1.2 microM, and 41.7 microM, respectively). In vitro binding assays and co-immunoprecipitation studies revealed that the OAT4 wild-type but not a mutant lacking the PDZ motif interacted directly with both PDZK1 and NHERF1. OAT4, PDZK1, and NHERF1 proteins were shown to be localized at the apical membrane of renal proximal tubules. The association with PDZK1 or NHERF1 enhanced OAT4-mediated E1S transport activities in HEK293 cells (1.2- to 1.4-fold), and the deletion of the OAT4 C-terminal PDZ motif abolished this effect. The augmentation of the transport activity was accompanied by alteration in V(max) of E(1)S transport via OAT4 and was associated with the increased surface expression level of OAT4 protein. This study indicates that the functional activity of OAT4 is modulated through the PDZ interaction with the network of PDZK1 and NHERF1 and suggests that OAT4 is involved in the regulated apical organic anion handling in the renal proximal tubules, provided by the PDZ scaffold.

Integrated physiology of proximal tubular organic anion transport

PURPOSE OF REVIEW

Renal organic anion transport proteins play important roles in the reabsorption and the secretion of endogenous and exogenous compounds. This review focuses on the interpretation of the physiological integration of identified transport molecules in the renal proximal tubules.

RECENT FINDINGS

To date, molecular identification of organic anion transport proteins is still continuing: rodent organic anion transporter 5, organic anion-transporting polypeptide 4C1, voltage-driven organic anion transporter 1, multidrug resistance-associated protein 4, and sodium-coupled monocarboxylate transporter have yielded additional information in this field. In addition, particularly at the apical membrane of the proximal tubules, the importance of the PDZ (PSD-95, DglA, and ZO-1) binding domain proteins has emerged in the formation of the multimolecular complex as a functional unit of membrane transport. Finally, discovery of dicarboxylate receptors in the renal tubular cells raises the possibility that dicarboxylate anions function as intrarenal signaling molecules. This novel aspect of renal organic anion transport, the potential modulation of signaling via dicarboxylate receptors, may be of significant relevance to renovascular hypertension and other renal diseases.

SUMMARY

Comprehensive understanding of the multimolecular complex, which is composed of transporters and their related signaling elements and is supported by the scaffold proteins underneath the plasma membrane, may be useful in clarifying complex transport phenomena such as renal apical organic anion handling. In addition to the recent proteomics approaches and conventional molecular physiology, it is necessary to develop novel methods to analyze the overall function of the multimolecular complex for the post-genomic era.