Renal Urate Transport Research Articles

Mangiferin Inhibits Renal Urate Reabsorption by Modulating Urate Transporters in Experimental Hyperuricemia.

Mangiferin, a natural glucosyl xanthone from the leaves of Mangifera indica L., was previously shown to exert potent hypouricemic effects associated with inhibition of the activity of xanthine dehydrogenase/oxidase. The present study aimed to evaluate its uricosuric effect and possible molecular mechanisms underlying the renal urate transporters responsible for urate reabsorption in vivo. Mangiferin (1.5-24.0 mg/kg) was administered intragastrically to hyperuricemic mice and rats induced by the intraperitoneal injection of uric acid and potassium oxonate, respectively. The uricosuric effect was evaluated by determining the serum and urinary urate levels as well as fractional excretion of uric acid (FEUA). The mRNA and protein levels of renal urate-anion transporter 1 (URAT1), organic anion transporter 10 (OAT10), glucose transporter 9 (GLUT9), and PDZ domain-containing protein (PDZK1) were analyzed. The administration of mangiferin significantly decreased the serum urate levels in hyperuricemic mice in a dose- and time-dependent manner. In hyperuricemic rats, mangiferin also reduced the serum urate levels and increased the urinary urate levels and FEUA. These results indicate that mangiferin has uricosuric effects. Further examination showed that mangiferin markedly inhibited the mRNA and protein expression of renal URAT1, OAT10, and GLUT9 in hyperuricemic rats, but did not interfere with PDZK1 expression. Taken together, these findings suggest that mangiferin promotes urate excretion by the kidney, which may be related to the inhibition of urate reabsorption via downregulation of renal urate transporters.

Nuciferine restores potassium oxonate-induced hyperuricemia and kidney inflammation in mice

Nuciferine, a major aporphine alkaloid of the leaves of Nelumbo nucifera, was found to decrease serum urate levels and improved kidney function, as well as inhibited system and renal interleukin-1β (IL-1β) secretion in potassium oxonate-induced hyperuricemic mice. Furthermore, nuciferine reversed expression alteration of renal urate transporter 1 (URAT1), glucose transporter 9 (GLUT9), ATP-binding cassette, subfamily G, membrane 2 (ABCG2), organic anion transporter 1 (OAT1), organic cation transporter 1 (OCT1), and organic cation/carnitine transporters 1/2 (OCTN1/2) in hyperuricemic mice. More importantly, nuciferine suppressed renal activation of Toll-like receptor 4/myeloid differentiation factor 88/NF-kappaB (TLR4/MyD88/NF-κB) signaling and NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome to reduce serum and renal IL-1β levels in hyperuricemic mice with renal inflammation reduction. The anti-inflammatroy effect of nuciferine was also confirmed in human proximal renal tubular epithelial cells (HK-2 cells) incubated with 4mg/dl uric acid for 24h. This study firstly reported the anti-hyperuricemic and anti-inflammatory effects of nuciferine by regulating renal organic ion transporters and inflammatory signaling in hyperuricemia. These results suggest that a dietary supplement of nuciferine rich in lotus leaf may be potential for the prevention and treatment of hyperuricemia with kidney inflammation.

Total saponins from Discorea nipponica ameliorate urate excretion in hyperuricemic mice.

Rhizoma Dioscoreae Nipponicae, from Discorea nipponica, is a widely used traditional Chinese herb. It is used to treat arthroncus, arthrodynia, and arthritis. Hyperuricemia is an important foundation of gouty arthritis. The current study was aimed at investigating whether the effects of total saponins from Rhizoma Dioscoreae Nipponicae on hyperuricemia were due to renal organic ion transporters in potassium oxonate-induced hyperuricemia mice. Hyperplasia of synovial cells prepared from Wistar rats was induced by IL-1β (1 × 10(4) µg/mL). MTT was used and to screen active components in the inhibition of hyperplasia by total saponins from Rhizoma Dioscoreae Nipponica, individual pure compounds, and different combinations of these compounds. Sixty Kun Ming mice were randomly divided into six groups: normal, model, allopurinol (40 mg/kg), and three total saponins groups receiving dose (600 mg/kg), middle (300 mg/kg), and low doses (60 mg/kg). Hyperuricemic mice were induced with potassium oxonate (300 mg/kg) intragastrically. The total saponins were given six days and the positive drug allopurinol was given one day before inducing hyperuricemia. The serum and urine levels of uric acid and creatinine and the fractional excretion of uric acid were measured in normal and hyperuricemic mice treated with Rhizoma Dioscoreae Nipponicae and allopurinol. The mRNA and protein levels of the mouse urate transporter 1, glucose transporter 9, organic anion transporter 1, and organic anion transporter 3 were analyzed by real-time-PCR and Western blotting methods, respectively. Total saponins from Rhizoma Dioscoreae Nipponicae could effectively reverse potassium oxonate-induced alterations in renal mouse urate transporter 1, glucose transporter 9, organic anion transporter 1, and organic anion transporter 3 mRNA and protein levels, resulting in enhancement of renal urate excretion in mice. These findings suggested that the total saponins from Rhizoma Dioscoreae Nipponicae had a uricosuric effect on the regulation of renal organic ion transporters in hyperuricemic animals.

Index of Suspicion

The reader is encouraged to write possible diagnoses for each case before turning to the discussion. We invite readers to contribute case presentations and discussions. Please inquire first by contacting Dr. Deepak Kamat at DKamat@med.wayne.edu. An 18-year-old male with a 3-year history of alcohol abuse is referred from a rehabilitation center for fever, nausea, and nonbloody diarrhea for 3 days. Six weeks ago, he enrolled in an alcohol rehabilitation program and stopped drinking alcohol. Three weeks later, he noticed yellowing of his eyes and skin, difficulty sleeping, and dyspnea. On examination, his temperature is 100.6°F (38.1°C), blood pressure is 99/62 mm Hg, pulse is 124 beats per minute, and respiratory rate is 24 breaths per minute. He is slow to answer questions. There is no asterixis. He appears thin and jaundiced with enlarged parotid glands. Decreased breath sounds are noted on the right. A tender, enlarged liver and mild abdominal distension are noted. Trace bilateral lower-extremity edema is present. No stigmata of chronic liver disease are found. Laboratory findings reveal the following: white blood cell count, 25,600/μL (25.6 × 109/L) (87% neutrophils); hemoglobin, 8.2 g/dL (82 g/L); mean corpuscular volume, 106 fL; platelets, 40 × 103/μL (40 × 109/L); sodium, 135 mEq/L (135 mmol/L); potassium, 3.5 mEq/L (3.5 mmol/L); chloride, 106 mEq/L (106 mmol/L); bicarbonate, 15 mEq/L (15 mmol/L); blood urea nitrogen, 24 mg/dL (8.6 mmol/L); creatinine, 1.01 mg/dL (89 μmol/L); glucose, 118 mg/dL (6.6 mmol/L); alkaline phosphatase, 182 U/L; aspartate aminotransferase (AST), 194 U/L; alanine aminotransferase (ALT), 71 U/L; total bilirubin, 11.8 mg/dL (201.8 μmol/L); direct bilirubin, 7.3 mg/dL (124.9 μmol/L); albumin, 2.7 g/dL (27 g/L); prothrombin time, 19.2 seconds; and international normalized ratio, 1.4. Ethanol and acetaminophen were not detected in the serum. A chest radiograph reveals a right pleural effusion …

Untreated tophaceous gout

Gout is associated with hyperuricemia and characterized by inflammatory arthritis induced by monosodium urate crystal deposition in synovial fluid and other tissues. Tophi can occur in soft and osseous tissue, ligaments, and different organs, either in presence or absence of gouty arthritis. If left untreated, hyperuricemic patients (serum urate level = 6.8 mg/dL or 400 mol/L) can evolve from intermittent arthritis to polyarticular tophaceous gout with symptoms between attacks. Lowering serum urate levels with xanthine oxidase inhibitors or uricosuric agents prevents acute flares and tophi development. Synovial fluid crystals slowly dissolve and can disappear after normalization of serum urate levels. Uricosuric agents are, generally, ineffective in patients with renal impairment. The recommended target serum uric acid concentration is <6 mg/dL (357 mol/L). Although controversial, recommendations have been made to achieve a target serum urate level <5 mg/dL (297 mmol/L) in severe chronic gout patients, as this concentration may be associated with greater depletion of synovial fluid crystals and a reduction in tophus size. Therapy is usually continued indefinitely. Several urate-lowering agents are currently under clinical investigation. These include uricase agents and inhibitors of renal urate transporter proteins. Interventions which enhance patient education and treatment adherence are considered important in improving gout management.

FRI0403 Arhalofenate, a novel uricosuric agent, is an inhibitor of human URIC acid transporters

BackgroundIt is estimated that the hyperuricemia in 80- 90% of gout patients is due to under excretion of uric acid. Uricouric drugs decrease serum uric acid by causing a reciprocal...

New developments in gout

Gout is the most common type of inflammatory arthritis. This review summarizes the most recent studies on newer therapeutics, disease management strategies and treatment recommendations. There are several new therapeutic agents being investigated both for the management of the acute gout symptoms, targeting interleukin-1β, as well as urate-lowering therapies including uricase and inhibitors of renal urate transporter proteins. Interventions led by pharmacists and nurses, which include patient education, lifestyle advice, monitoring and titration of urate-lowering medications have been implemented to improve gout management. Recently, the American College of Rheumatology has published guidelines for nonpharmacologic and pharmacologic therapeutic approaches for hyperuricemia and acute gouty arthritis. New therapeutic agents targeting the mechanism of inflammation (IL-1β) are under investigation. In addition, new urate-lowering medications to be used alone or in combination with allopurinol are undergoing rigorous evaluation to use for patients not responding to or unable to take current therapies. There is also increasing interest in redesigning clinical care to improve patient education, self-management training and urate-lowering medication titration. Although we await results of these investigations, the American College of Rheumatology treatment guidelines provide a framework for clinicians in order to provide optimal gout care.

Genetics of Hyperuricemia and Gout: Implications for the Present and Future

Gout is the most common inflammatory arthropathy and occurs in the setting of elevated serum urate levels. Gout is also known to be associated with multiple comorbidities including cardiovascular disease and the metabolic syndrome. Recent advances in research have increased our understanding and improved our knowledge of the pathophysiology of gout. Genome-wide association studies have permitted the identification of several new and common genetic factors that contribute to hyperuricemia and gout. Most of these are involved with the renal urate transport system (the uric acid transportasome), generally considered the most influential regulator of serum urate homeostasis. Thus far, SCL22A12, SCL2A9, and GLUT9 have been found to have the greatest variation and most influence on serum urate levels. However, genetics are only a part of the explanation in the development of hyperuricemia and gout. As results have been mixed, the role of known urate influential genes in gout's associated comorbidities remains unclear. Regardless, GWAS findings have expanded our understanding of the pathophysiology of hyperuricemia and gout, and will likely play a role in the development of future therapies and treatment of this ancient disease.

Functional Cooperation of SMCTs and URAT1 for Renal Reabsorption Transport of Urate

Urate is mainly excreted into urine in humans. Serum urate level is regulated by a urate transport system located on the renal proximal tubule. Urate transporter 1 (URAT1) is located on the apical side of the renal proximal tubule and is responsible for the reabsorption of urate from the luminal side into tubular cells. At the same site, it has been hypothesized that sodium-coupled monocarboxylate transporters (SMCTs) are responsible for the transportation of monocarboxylates such as lactate and nicotinate, which are exchanged for urate transport via URAT1. Accordingly, SMCTs could enhance URAT1-mediated urate reabsorption by providing monocarboxylates for the exchange. The present study was carried out to clarify the hypothesized functional cooperative relationship between URAT1 and SMCTs in the reabsorptive transport of urate. By preloading nicotinate in SMCT1/URAT1-coexpressing Xenopus oocytes, URAT1-mediated urate transport was stimulated. Nicotinate was taken up by SMCT1 but not by URAT1. When removing sodium ions from the uptake medium, the stimulation effect was decreased. When adding SMCT1 inhibitors, the stimulation effect was also reduced. The results from this study indicate the cooperative relationship of URAT1 and SMCT1, and that SMCT1 is a potential target for the alteration of renal handling of urate indirectly.

Anti-hyperuricemic and nephroprotective effects of Modified Simiao Decoction in hyperuricemic mice

Ethnopharmacological relevanceModified Simiao Decoction (MSD), based on clinical experience, has been used for decades and famous for its efficiency in treating hyperuricemic and gouty diseases. Aim of the studyTo investigate the effects of MSD on anti-hyperuricemic and nephroprotective effects are involved in potassium oxonate-induced hyperuricemic mice. Materials and methodsThe effects of MSD were investigated in hyperuricemic mice induced by potassium oxonate. MSD were fed to hyperuricemic mice daily at a dose of 0.45, 0.90, 1.80g/kg for 10 days, and allopurinol (5mg/kg) was given as a positive control. Serum and urine levels of uric acid and creatinine, and fractional excretion of uric acid (FEUA) were determined by colorimetric method. Its nephroprotective effects were evaluated by determining a panel of oxidative stress markers after the intervention in hyperuricemic mice. Simultaneously, protein levels of urate transporter 1 (URAT1) and organic anion transporter 1 (OAT1) in the kidney were analyzed by Western blotting. ResultsMSD could inhibit XOD activities in serum and liver, decrease levels of serum uric acid, serum creatinine and BUN, and increased levels of urine uric acid, urine creatinine, FEUA dose-dependently through down-regulation of URAT1 and up-regulation of OAT1 protein expressions in the renal tissue of hyperuricemic mice. It also effectively reversed oxonate-induced alterations on renal MDA levels and SOD activities in this model. ConclusionMSD processes uricosuric and nephroprotective actions by regulating renal urate transporters and enhancing antioxidant enzymes activities to improve renal dysfunction in hyperuricemic mice.

Protection of curcumin against fructose-induced hyperuricaemia and renal endothelial dysfunction involves NO-mediated JAK–STAT signalling in rats

Increasing evidence has demonstrated that excess fructose consumption as a risk factor for metabolic syndrome causes hyperuricaemia and renal injury. Curcumin, a natural plant phenolic food additive, lowered serum urate, creatinine and blood urea nitrogen levels, and increased urinary urate and nitrate/nitrites levels, as well as renal nitric oxide (NO) production in fructose-fed rats. Moreover, curcumin regulated urate transport-related proteins and inhibited activation of the JAK2-STAT3 cascade and overexpression of SOCS3 and TGF-β1 in the kidneys of fructose-fed rats. These results suggested that the anti-hyperuricaemic and renal protective actions of curcumin might be the result of renal NO-mediated JAK2-STAT3 signalling and TGF-β1 normality, which ameliorated renal endothelial dysfunction to improve renal urate transporter system in this model. The present study may provide the evidence for the potential use of a functional food ingredient curcumin because of its action against hyperuricaemia and renal injury induced by high fructose intake.

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.

Furocoumarins affect hepatic cytochrome P450 and renal organic ion transporters in mice

Furocoumarins are a group of natural products with many biological activities. Clinical evidences have demonstrated the important contribution of furocoumarins to the toxicity of some foods and herbs. In order to assess liver and kidney toxicity of furocoumarins, male mice were orally administrated with psoralen, isopsoralen, imperatorin, isoimperatorin and xanthotoxin at 20 and 40 mg/kg once daily for 28 days, respectively. No changes of food or water intake were observed in furocoumarins-treated mice. Only 40 mg/kg isopsoralen reduced body weight. 40 mg/kg furocoumarins altered serum activities of alanine transaminase, aspartate aminotransferase, alkaline phosphatase, and/or levels of albumin, showing hepatotoxicity. Furthermore, furocoumarins increased activity and protein expression of hepatic microsomal cytochrome P450 (CYP450) 3A11. CYP 2E1 activity and protein expression were suppressed by psoralen and isopsoralen and increased by xanthotoxin. Renal protein levels of organic cation/carnitine transporters (OCT1, OCT2 and OCTN2) and organic anion transporter 3 were increased by most furocoumarins. Renal urate transporter 1, glucose transporter 9 and multidrug resistance protein 4 were influenced by furocoumarins. These findings suggest that furocoumarins may interfere in metabolism, excretion and bioavailability of endogenous and exogenous compounds to impair liver and kidney functions mediated by affecting hepatic CYP450 and renal organic ion transport system.

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.

Quercetin regulates organic ion transporter and uromodulin expression and improves renal function in hyperuricemic mice

Renal organic ion transporters and uromodulin (UMOD) play the important roles in renal urate excretion and function. Hyperuricemia is considered as a risk factor for the development of renal dysfunction. The flavonoid quercetin in diets exerts the hypouricemic and nephroprotective effects. To evaluate the effects of quercetin on renal organic ion transporters and UMOD in hyperuricemic mice. Kun-Ming mice were divided into normal and hyperuricemic groups receiving water, 25, 50 and 100 mg/kg quercetin, 5 mg/kg allopurinol, respectively. Hyperuricemic mice were orally gavaged with 250 mg/kg oxonate daily for 1 week. Quercetin and allopurinol were orally gavaged on the day when oxonate or water was given 1 h later. After 1 week, serum uric acid, creatinine and blood urea nitrogen concentrations, excretion of urate and creatinine, and fractional excretion of uric acid were measured. The mRNA and protein levels of renal urate transporter 1 (mURAT1), glucose transporter 9 (mGLUT9), organic anion transporter 1 (mOAT1) and organic cation/carnitine transporters (mOCT1, mOCT2, mOCTN1 and mOCTN2) in mice were analyzed. Simultaneously, UMOD levels in serum, urine and kidney, as well as renal UMOD mRNA expression were detected. Quercetin significantly restored oxonate-induced abnormalities of these biochemical indexes compared with normal vehicle group. Furthermore, it remarkably prevented expression changes of renal organic ion transporters and UMOD, and UMOD level alteration in hyperuricemic mice. These results suggest that quercetin has the uricosuric and nephroprotective actions mediated by regulating the expression levels of renal organic ion transporters and UMOD.

Difficult-to-Treat Gouty Arthritis

Gouty arthritis is the most common inflammatory arthritis in adults and is characterized by very painful flares. Gouty arthritis results from an elevated body uric acid pool, which leads to deposition of monosodium urate crystals, mainly in the joints. These crystals trigger the release of proinflammatory cytokines, in particular interleukin (IL)-1β, which stimulates inflammation. Gouty arthritis can progress to a chronic, deforming and physically disabling disease through the development of disfiguring tophi, joint destruction and persistent pain. Standard treatments are effective in most patients. Acutely, anti-inflammatory therapies provide rapid pain relief and resolution of flares. Chronically, urate-lowering therapies reduce serum urate levels and, in combination with anti-inflammatory prophylaxis, reduce the risk of flares. However, for a growing number of patients, current standard treatments are ineffective or are contraindicated, largely due to the presence of co-morbidities. Indeed, metabolic syndrome, hypertension, dyslipidaemia, cardiovascular disease, diabetes mellitus and renal impairment are all highly prevalent in individuals with gouty arthritis, and may lead to standard treatments being ineffective or inappropriate. Such patients with difficult-to-treat disease require alternative therapies. Gouty arthritis can have a major impact on health-related quality of life (HR-QOL), especially in patients with difficult-to-treat disease, as revealed by recent studies comparing HR-QOL for patients with gouty arthritis with that of the general population. All studies revealed clinically significant reductions in physical functioning for individuals with gouty arthritis compared with the general population. The difference was particularly marked for patients with difficult-to-treat disease. Gouty arthritis also constitutes an important economic burden through absence from work and medical costs. Again, the burden is greater in patients with difficult-to-treat disease. The development of difficult-to-treat disease reflects the short-comings of current standard treatments in a growing number of gouty arthritis patients. This has been recognized by the pharmaceutical industry and has promoted the development of innovative therapies. An appreciation of the key role of IL-1β in inflammation in gouty arthritis has led to the development of a new class of anti-inflammatory agents that block IL-1β signal transduction. The current IL-1β blockers in trials are rilonacept and canakinumab. Canakinumab, a fully human anti-IL-1β monoclonal antibody, has been shown to produce rapid and sustained pain relief from acute flares in patients with difficult-to-treat disease, and both rilonacept and canakinumab have been shown to reduce the risk of recurrent flares. Promising new therapies for reducing serum urate levels are also being developed. These include the recently approved therapies pegloticase (a pegylated form of the enzyme uricase that converts urate to allantoin), inhibitors of renal urate transporter proteins, and inhibitors of purine nucleotide phosphorylase, an enzyme involved in purine metabolism. Further studies are warranted to establish the value and role of these new therapies in the management of gouty arthritis. These new options should help reduce the growing human burden associated with gouty arthritis, lowering the tophaceous burden, minimizing the risk of flares, and enabling patients to achieve rapid and effective pain relief when flares do occur.

Urate Transport: Regulators of Serum Urate Levels in Humans

Urate (uric acid) is the final product of purine metabolism, and its antioxidant capacity has drawn attention recently for its protective role against oxidative stress. However, hyperuricemia has a known association with onset of illnesses such as gout and cardiovascular diseases. Renal urate transport mechanisms are known to be major determinants of serum urate levels, but the molecular mechanisms involved have not yet been fully elucidated. Molecular identification of a kidney-specific urate transporter SLC22A12 (URAT1) by our research group in 2002 marked the start of a subsequent compilation of information on several different molecules contributing to urate transport by the kidneys. In addition, recent genome-wide association (GWA) studies have contributed to the detection of novel candidate genes related to uric acid metabolism such as SLC2A9 (GLUT9/URATv1), ABCG2 (BCRP), SLC17A1 (NPT1), SLC17A3 (NPT4) and PDZK1. Furthermore, use of urate transporter gene-modified mice for Slc22a12 and Slc2a9 may provide clues for understanding the physiological role of each transporter. Results of recent studies on urate transport with emphasis on the relation to serum urate disorders are described in this report.

Genetic and Environmental Risk Factors in Hyperuricaemia and Common Gout

Gout results from hyperuricaemia. The most important cause of hyperuricaemia is reduced excretion of uric acid in the urine. Genome-wide association scans for genes regulating serum urate concentrations have identified two major regulators – the renal urate transporters SLC2A9 and ABCG2. The risk variants at each gene approximately double the risk for gout in people of Caucasian ancestry, with the urate and fructose transporter SLC2A9 also resulting in higher risk for gout in people of Polynesian ancestry, a diverse population characterized by a high prevalence of gout. Ongoing genetic association studies are identifying and confirming other genes (URAT1, OAT4, NPT1, PDZK1, GCKR) controlling serum urate concentrations; although genome-wide association studies in gout per se await recruitment of suitable case sample sets. The recent increase in gout incidence can only be explained by a change in the environment. One factor that fulfills most of the requirements for confirmation of an etiological role in gout is fructose. Fructose raises serum urate levels, which is exacerbated in people with genetic variants that reduce renal urate excretion, thus increasing the risk of gout. Use of GWAS approaches and application of new genomics technologies such as next-generation sequencing to very large well-phenotyped gout sample sets will enable identification of further genetic risk factors in gout. Intervention studies in cohorts characterized for genetic risk factors are needed to prove a direct link for environmental agents such as fructose in gout etiology. Keywords: Gout, gene, urate transporter, association, genome, SLC2A9, monosodium urate, Caucasian, Hyperuricaemia, polymorphisms, glucokinase

The Effect of Female Hormones Upon Urate Transport Systems in the Mouse Kidney

Increased levels of serum urate in postmenopausal women are thought to be caused by a change in renal urate elimination associated with the loss of female hormones. In this study, we investigated the regulation of renal urate transporter expression by female hormones using ovariectomized mice with or without hormone replacement. Estradiol suppressed the protein levels of urate reabsorptive transporters urate transporter 1 and glucose transporter 9 (Urat1 and Glut9), and that of urate efflux transporter ATP-binding cassette sub-family G member 2 (Abcg2). Progesterone suppressed protein levels of sodium-coupled monocarboxylate transporter 1 (Smct1). However, neither estradiol nor progesterone influenced the respective levels of mRNA.

Interactions of urate transporter URAT1 in human kidney with uricosuric drugs

Hyperuricaemia is a significant factor in a variety of diseases, including gout and cardiovascular diseases. The kidney plays a dominant role in maintaining plasma urate levels through the excretion process. Human renal urate transporter URAT1 is thought to be an essential molecule that mediates the reabsorption of urate on the apical side of the proximal tubule. In this study the pharmacological characteristics and clinical implications of URAT1 were elucidated. Madin-Darby canine kidney (MDCK) cells stably expressing URAT1 (MDCK-URAT1) were established and examined the interactions of URAT1 with various drugs such as benzbromarone and its metabolites including 6-hydroxybenzbromarone, angiotensin-converting enzyme inhibitors, non-steroidal anti-inflammatory drugs and urate transport inhibitors including E3040 and probenecid. MDCK-URAT1 cells exhibited a time- and dose-dependent increase in urate uptake, with a Km value of 570.7 µmol/L. When an URAT1-green fluorescent protein fusion protein construct was expressed in MDCK cells, the protein was sorted mainly to the apical side of the membrane. The drugs except for captoril dose-dependently inhibited urate uptake mediated by URAT1, with half maximal inhibitory concentration (IC(50) ) values ranging 0.05-716 µmol/L. Comparing these IC(50) values with intratubular concentrations of unbound drugs in humans, it is thought that URAT1 is a target molecule of uricosuric drugs, including 6-hydroxybenzbromarone, probenecid, indomethacin and salicylate, to inhibit urate reabsorption in vivo. In addition, a cell line that stably expressing URAT1 could be a useful tool for the development of uricosuric drugs.