Urate Secretion Research Articles

Uric Acid Metabolism, Uric Acid Transporters and Dysuricemia

Serum urate levels are determined by the balance between uric acid production and uric acid excretion capacity from the kidneys and intestinal tract. Dysuricemia, including hyperuricemia and hypouricemia, develops when the balance shifts towards an increase or a decrease in the uric acid pool. Hyperuricemia is mostly a multifactorial genetic disorder involving several disease susceptibility genes and environmental factors. Hypouricemia, on the other hand, is caused by genetic abnormalities. The main genes involved in dysuricemia are xanthine oxidoreductase, an enzyme that produces uric acid, and the urate transporters urate transporter 1/solute carrier family 22 member 12 (URAT1/SLC22A12), glucose transporter 9/solute carrier family 2 member 9 (GLUT9/SLC2A9) and ATP binding cassette subfamily G member 2 (ABCG2). Deficiency of xanthine oxidoreductase results in xanthinuria, a rare disease with marked hypouricemia. Xanthinuria can be due to a single deficiency of xanthine oxidoreductase or in combination with aldehyde oxidase deficiency as well. The latter is caused by a deficiency in molybdenum cofactor sulfurase, which is responsible for adding sulphur atoms to the molybdenum cofactor required for xanthine oxidoreductase and aldehyde oxidase to exert their action. URAT1/SLC22A12 and GLUT9/SLC2A9 are involved in urate reabsorption and their deficiency leads to renal hypouricemia, a condition that is common in Japanese due to URAT1/SLC22A12 deficiency. On the other hand, ABCG2 is involved in the secretion of urate, and many Japanese have single nucleotide polymorphisms that result in its reduced function, leading to hyperuricemia. In particular, severe dysfunction of ABCG2 leads to hyperuricemia with reduced extrarenal excretion.

Pharmacological evaluation of a novel skeleton compound isobavachin (4′,7-dihydroxy-8-prenylflavanone) as a hypouricemic agent: Dual actions of URAT1/GLUT9 and xanthine oxidase inhibitory activity

Previously we discovered a novel natural scaffold compound, isobavachin (4′, 7-dihydroxy-8-prenylflavanone), as a potent URAT1 inhibitor by shape and structure based on a virtue screening approach. In this study, further urate-lowering mechanism, pharmacokinetics and toxicities of isobavachin were conducted. Isobavachin inhibited URAT1 with an IC50 value of 0.24 ± 0.06 μM, and residues S35, F365, I481 and R477 of URAT1 contributed to high affinity for isobavachin. Isobavachin also inhibited glucose transporter 9 (GLUT9), another pivotal urate reabsorption transporter, with an IC50 value of 1.12 ± 0.26 μM. Molecular docking and MMGBSA results indicated that isobavachin might compete residues R171, L75 and N333 with uric acid, which leads to inhibition of uric acid transport of GLUT9. Isobavachin weakly inhibited urate secretion transporters OAT1 with an IC50 value of 4.38 ± 1.27 μM, OAT3 with an IC50 of 3.64 ± 0.62 μM, and ABCG2 with an IC50 of 10.45 ± 2.17 μM. Isobavachin also inhibited xanthine oxidase (XOD) activity in vitro with an IC50 value of 14.43 ± 3.56 μM, and inhibited the hepatic XOD activities at 5–20 mg/kg in vivo. Docking and MMGBSA analysis indicated that isobavachin might bind to the Mo-Pt catalyze center of XOD, which leads to inhibition of uric acid production. In vivo, isobavachin exhibited powerful urate-lowering and uricosuric effects at 5–20 mg/kg compared with the positive drugs morin (20 mg/kg) and RDEA3170 (10 mg/kg). Safety assessments revealed that isobavachin was safe and had no obvious toxicities. Isobavachin has little cell toxicity in HK2 cells as indicated by the MTT assay. In vivo, after treatment with 50 mg/kg isobavachin for 14 days, isobavachin had little renal toxicity, as revealed by serum CR/BUN levels, and no hepatotoxicity as revealed by ALT/AST levels. Further HE examination also suggests that isobavachin has no obvious kidney/liver damage. A pharmacokinetic study in SD rats indicated isobavachin had lower bioavailability (12.84 ± 5.13 %) but long half-time (7.04 ± 2.68 h) to maintain a continuous plasma concentration. Collectively, these results indicate that isobavachin deserves further investigation as a candidate anti-hyperuricemic drug with a novel mechanism of action: selective urate reabsorption inhibitor (URAT1/GLUT9) with a moderate inhibitory effect on XOD.

Katsuwonus pelamis Peptide and its Complexes Protect Zebrafish and Mice From Hyperuricemia Through Promoting Kidney Excretion of Uric Acid and Inhibiting Liver Xanthine Oxidase Activity.

Katsuwonus pelamis peptide and its complexes have the effect of lowering uric acid (UA)-levels. To identify the effect and possible mechanisms, different concentrations of Katsuwonus pelamis peptide and its complexes were administered to the zebrafish and mice hyperuricemia models, and the UA level was measured. Meanwhile, the hyperuricemic mice were treated orally at 0.83, 1.67, and 5.00 mg/g body weight for 7 days with Katsuwonus pelamis peptide and the complexes groups, separately. The levels of serum UA (SUA), urinary UA (UUA), serum creatinine (SCR), blood urine nitrogen (BUN), and xanthine oxidase (XOD) activities were detected in each group. The results showed that the Katsuwonus pelamis peptide (125 μg/ml) and its complexes (83.3 and 250 μg/ml) effectively reduced UA level in zebrafish with hyperuricemia (p < 0.05). The Katsuwonus pelamis peptide at high concentration (5.00 mg/g) decreased the SUA level, SCR level, BUN level, and hepatic XOD activity, and the complexes (1.67 and 5.00 mg/g) significantly reduced the SUA level and hepatic XOD activity (p < 0.05) in the hyperuricemic mice. In addition, in a hyperuricemic mouse model, the UUA level was increased after treatment with Katsuwonus pelamis peptide and its complexes at high concentrations (p < 0.05). The total therapeutic effects in the Katsuwonus pelamis peptide complex group were better than those in the Katsuwonus pelamis peptide group. Thus, Katsuwonus pelamis peptide and its complexes may possibly be used to prevent hyperuricemia via promoting urate secretion and inhibiting XOD activity production.

Renal Transcriptional Profiles of Hyperuricemic Mouse Models Reveal Urate Dependent Alternations in Metabolic Pathways

Increased serum urate (hyperuricemia, HUA) contributes to the development of kidney stones, chronic kidney disease, cardiovascular disease, metabolic syndrome, and gout. HUA is caused by an underexcretion or an overproduction of urate. These two mechanisms result in similar serum urate levels, but critically important differences in how urate is handled in the kidney and resulting tubular urate levels. To better understand altered gene expression by divergent renal handling of urate in each subtype of HUA, we compared transcriptional profiles of kidneys of two functionally different mouse models of HUA. The underexcretion model is our well established ABCG2‐Q140K knock‐in mouse, which inserts the human pathological variant Q141K into the orthologous position of the mouse Abcg2 gene. This causes decreased secretion of urate, leading to an increase in serum urate in male mice only. The overproduction model is a novel inducible knock‐out of the urate metabolizing gene uricase (UOX‐iKO), which renders both male (M) and female (F) mice unable to metabolize urate, which increases circulating levels to a similar degree seen in HUA humans. Here, UOX‐iKO mice of both sexes were induced at 9 weeks of age, and 2 weeks later, UOX‐iKO mice had serum urate levels similar to age matched ABCG2‐Q140K M mice. These UOX‐iKO mice had increased overall urinary urate excretion (UUE), as well as increased fractional excretion of urate (FEUA) in both M and F, implying higher tubular urate, consistent with urate overproduction. In contrast, ABCG2‐Q140K mice demonstrated no change in overall UUE, with a decrease in FEUA seen in M only, implying lower tubular urate, consistent with urate underexcretion. Next, we harvested kidneys from M and F mice of both HUA models and appropriate controls, followed by RNA‐Seq. To determine pathways altered by HUA, we utilized pathway analysis (DAVID Bioinformatics Resource 6.8) on all significant differentially expressed genes (DESeq2, p &lt; 0.05, FDR &lt; 0.1). Both models showed similar alterations in gene expression, with approximately 10% of all differentially expressed genes from all 3 groups of HUA mice clustered into metabolic pathways, including genes involved in steroid and amino acid metabolism. M HUA mice of both models also shared differential expression of genes involved in glutathione and fatty acid metabolism. We hypothesize that although luminal urate differs between the two models, the intracellular urate values in the proximal tubule cells will be similarly elevated. This elevation must be influencing the observed changes in metabolism related gene expression. To begin to elucidate urate dependent metabolic regulators, we next explored which transcription factors (TFs) may be driving this differential expression (Enrichr enrichment analysis) and found 2 TFs shared by all 3 HUA groups: Gata2 and Tcf3, with 2 more TFs, Sox2 and Nanog, in both M HUA mice. In conclusion, we found that models of both classical types of HUA alter renal metabolic pathways. We have also identified 2 candidate TFs that may be directly affected by urate, which could drive transcriptional changes in response to HUA. Additional experiments are required to determine the effect of urate itself on the activity of these TFs, as well as mechanistic studies to explore the effects of altered intracellular urate levels. Further elucidation these interactions may lead to improved understanding of urate homeostasis and new insights into HUA treatment.

Urate Transporters in the Kidney: What Clinicians Need to Know.

Urate is produced in the liver by the degradation of purines from the diet and nucleotide turnover and excreted by the kidney and gut. The kidney is the major route of urate removal and has a pivotal role in the regulation of urate homeostasis. Approximately 10% of the glomerular filtered urate is excreted in the urine, and the remainder is reabsorbed by the proximal tubule. However, the transport of urate in the proximal tubule is bidirectional: reabsorption and secretion. Thus, an increase in reabsorption or a decrease in secretion may induce hyperuricemia. In contrast, a decrease in reabsorption or an increase in secretion may result in hyperuricosuria. In the proximal tubule, urate reabsorption is mainly mediated by apical URAT1 (SLC22A12) and basolateral GLUT9 (SLC2A9) transporter. OAT4 (SLC22A11) also acts in urate reabsorption in the apical membrane, and its polymorphism is associated with the risk of hyperuricemia. Renal hypouricemia is caused by SLC22A12 or SLC2A9 loss-of-function mutations, and it may be complicated by exercise-induced acute kidney injury. URAT1 and GLUT9 are also drug targets for uricosuric agents. Sodium-glucose cotransporter inhibitors may induce hyperuricosuria by inhibiting GLUT9b located in the apical plasma membrane. Urate secretion is mediated by basolateral OAT1 (SLC22A6) and OAT3 (SLC22A8) and apical ATP-binding cassette super-family G member 2 (ABCG2), NPT1 (SLC17A1), and NPT4 (SLC17A3) transporter in the proximal tubule. NPT1 and NPT4 may be key players in renal urate secretion in humans, and deletion of SLC22A6 and SLC22A8 in mice leads to decreased urate excretion. Dysfunctional variants of ABCG2 inhibit urate secretion from the gut and kidney and may cause gout. In summary, the net result of urate transport in the proximal tubule is determined by the dominance of transporters between reabsorption (URAT1, OAT4, and GLUT9) and secretion (ABCG2, NPT1, NPT4, OAT1, and OAT3).

Mangiferin promotes intestinal elimination of uric acid by modulating intestinal transporters

Increasing evidence shows that the intestinal tract plays an important role in maintaining urate homeostasis and might be a potential therapeutic target for hyperuricaemia. However, uric acid-lowering drugs available in the clinic do not target intestinal excretion as a therapeutic strategy. We previously reported that mangiferin had potent hypouricaemic effects in hyperuricaemic animals. However, the underlying mechanisms are not completely clear. Here, we investigated the effects of mangiferin on the intestinal excretion of urate and its underlying mechanisms. The data revealed that mangiferin concentration-dependently promoted the intestinal secretion of endogenous urate in in situ intestinal closed loops in normal and hyperuricaemic mice, as well as inhibited the absorption of exogenous uric acid perfused into the intestinal loops in rats. Administration of mangiferin not only decreased the serum urate levels in the hyperuricaemic mice but also increased the protein expression of ATP-binding cassette transporter, subfamily G, member 2 (ABCG2) and inhibited the protein expression of glucose transporter 9 (GLUT 9) in the intestine. These findings suggested that intestinal ABCG2 and GLUT9 might be pivotal and possible action sites for the observed hypouricaemic effects. Moreover, no significant changes in intestinal xanthine oxidoreductase activities were observed, suggesting that mangiferin did not affect intestinal uric acid generation in the hyperuricaemic mice. Overall, promoting intestinal elimination of urate by upregulating ABCG2 expression and downregulating GLUT9 expression might be an important mechanism underlying mangiferin lowering serum uric acid levels. Mangiferin supplementation might be beneficial for the prevention and treatment of hyperuricaemia.

A natural complex product Yaocha reduces uric acid level in a live zebrafish model

Introduction: This study was aimed to assess uric acid (UA)-lowering effect and its possible mechanisms of a natural complex product Yaocha in a live zebrafish model. Methods: The zebrafish high UA model was established by feeding 5 dpf zebrafish with both an uricase inhibitor potassium oxonate at 10 mM and an UA synthesis precursor xanthine sodium at 0.5 mM for 24 h. Yaocha was administered to the high UA zebrafish through soaking at 3 various concentrations, with allopurinol as a positive control. UA level, xanthine oxidase (XOD) activity, and mRNA expression of hypoxanthine guanine-phosphoribosyltransferases transferase (HPRT1) and organic anion transporter 1 (OAT1) were measured. Results: Yaocha effectively reduced UA level and inhibited xanthine oxidase (XO) activity in the high UA zebrafish. Yaocha could be a potential therapeutics for hyperuricemia through up-regulating HPRT1 and OAT1 gene expression and suppressing XO activity. Discussion: These results suggested that Yaocha hold a potential for high UA prevention and therapy, possibly through inhibiting UA production and promoting urate secretion and purine conversion.

Pharmacological properties and clinical efficacy of dotinurad (URECE® tablets), a novel hypouricemic agent

In Jan 2020, dotinurad (URECE® tablets) was approved for gout and hyperuricemia therapy in Japan. We developed a novel hypouricemic agent because benzbromarone, a commercially available uricosuric agent, has several problems, such as drug-induced liver injury or drug-drug interaction caused by CYP2C9 inhibition. In transporter-overexpressing cells, dotinurad potently inhibited URAT1 which is localized in the renal proximal tubules and functions as a urate reabsorption. On the contrary, dotinurad hardly inhibited urate secretion transporters, ABCG2 or OAT1/3. In Cebus monkeys, dotinurad dose-dependently decreased plasma urate levels at low doses compared with benzbromarone. Inhibitory effect of dotinurad on mitochondria was weaker than that of benzbromarone and there was no observation suggesting a risk of drug-induced liver injury taking into consideration the clinical dose or exposure. Dotinurad weakly inhibited CYPs and further analysis indicated there was no drug-drug interaction risk in the clinical dose. In clinical pharmacology studies, there was no difference among sex and age. Furthermore, dosage and administration are equal even in hepatic impairment patients (mild to severe) and renal impairment patients (mild to moderate). In confirmatory phase II and long-term studies, dotinurad decreased serum urate levels at low doses and almost patients using maintenance dose (2 or 4 mg) achieved a serum urate level ≤ 6.0 mg/dL. Moreover, there was no finding to raise safety concern including liver injury. In conclusion, dotinurad, a selective urate reabsorption inhibitor (SURI) could be a therapeutic option because of its more effective hypouricemic action at low doses than those of commercially available uricosuric agents.

Pharmacological Evaluation of Dotinurad, a Selective Urate Reabsorption Inhibitor.

The effect of dotinurad [(3,5-dichloro-4-hydroxyphenyl)(1,1-dioxo-1,2-dihydro-3H-1λ 6-1,3-benzothiazol-3-yl)methanone] was compared with that of commercially available uricosuric agents-namely, benzbromarone, lesinurad, and probenecid. Its effect on urate secretion transporters was evaluated using probe substrates for respective transporters. Dotinurad, benzbromarone, lesinurad, and probenecid inhibited urate transporter 1 (URAT1) with IC50 values of 0.0372, 0.190, 30.0, and 165 μM, respectively. Dotinurad weakly inhibited ATP-binding cassette subfamily G member 2 (ABCG2), organic anion transporter 1 (OAT1), and OAT3, with IC50 values of 4.16, 4.08, and 1.32 μM, respectively, indicating higher selectivity for URAT1. The hypouricemic effects of dotinurad and benzbromarone were evaluated in Cebus monkeys. Dotinurad, at doses of 1-30 mg/kg, concomitantly decreased plasma urate levels and increased fractional excretion of urate (FEUA) in a dose-dependent manner. On the contrary, benzbromarone, at a dose of 30 mg/kg, showed a modest effect on plasma urate levels. The inhibitory effect of dotinurad on urate secretion transporters was evaluated in Sprague-Dawley rats, with sulfasalazine and adefovir as probe substrates of ABCG2 and OAT1, respectively. Drugs, including febuxostat as a reference ABCG2 inhibitor, were administered orally before sulfasalazine or adefovir administration. Dotinurad had no effect on urate secretion transporters in vivo, whereas benzbromarone, lesinurad, probenecid, and febuxostat increased the plasma concentrations of probe substrates. These results suggested dotinurad is characterized as a selective urate reabsorption inhibitor (SURI), which is defined as a potent URAT1 inhibitor with minimal effect on urate secretion transporters, including ABCG2 and OAT1/3, because of its high efficacy in decreasing plasma urate levels compared with that of other uricosuric agents. SIGNIFICANCE STATEMENT: Our study on the inhibitory effects on urate transport showed that dotinurad had higher selectivity for urate transporter 1 (URAT1) versus ATP-binding cassette subfamily G member 2 (ABCG2) and organic anion transporter (OAT) 1/3 compared to other uricosuric agents. In Cebus monkeys, dotinurad decreased plasma urate levels and increased fractional excretion of urate in a dose-dependent manner. To determine the inhibitory effect of dotinurad on urate secretion transporters, we studied the movement of substrates of ABCG2 and OAT1 in rats. Dotinurad had no effect on these transporters, whereas the other uricosuric agents increased the plasma concentrations of the substrates. These results suggested dotinurad as a potent and selective urate reabsorption inhibitor is characterized by increased efficacy with decreasing plasma urate levels.

Gout Causing ABCG2 Mutation Results in Intestinal Net Urate Transport Defect, Hyperuricemia, &amp; Altered Metabolic Phenotype

Hyperuricemia causes gout and is implicated in metabolic syndrome, impaired glucose tolerance/type 2 diabetes, and hypertension. Individuals carrying a common loss of function variant in the dominant secretory transporter, Q141K ABCG2, have significantly increased serum urate and gout risk yet their renal fractional excretion of urate (FEUA) is largely unchanged. However, the role in ABCG2 and other urate transporters in extra‐renal secretion remains mostly unexplored. Here we sought to identify the role of ABCG2 in trans‐epithelial urate transport in the intestines and contribution of intestinal ABCG2 dysfunction to hyperuricemia and urate related phenotypes. First, we established a genetic mouse model of the human ABCG2 Q141K variant (Q140K in mouse) using CRISPR Cas9 gene editing techniques on a C57BL6 mouse background. As compared to control mice, mice possessing the Q140K Abcg2 allele were profoundly hyperuricemic, yet exhibited only modest changes in renal excretion of urate, similar to what has been reported previously in humans. To explore mechanisms, we first mapped prominent urate transporters ABCG2 and SLC2A9 along the mouse intestine and found the highest levels in the jejunum and ileum. Localization of the wild type ABCG2 protein was exclusive to the brush border (apical membrane) of the villus cells, seemingly optimized to facilitate urate secretion(JUAsm). Interestingly, we also found evidence that SLC2A9 localized predominantly to the basolateral enterocyte membrane. In Ussing chamber experiments, we functionally confirmed basolateral localization of the electrogenic SLC2A9 transporter by observing basolateral but not apical addition of 0.5 mM urate significantly stimulated Isc (24 ± 3μA/cm2) in comparison with control (10± 2.2 μA/cm2). Further, treatment of the serosal membrane with SLC2A9 inhibitors tranilast and pCMBS partially inhibited JUAsm flux of urate in an intestinal loop model. In contrast to the WT controls, Q140K mice demonstrated a significant decrease of apical ABCG2 protein abundance, with commiserate, significant loss of unidirectional urate transport (JUAsm) in jejunal loops (Q140K, 24 ± 3.38 μM/cm2/min; WT, 46 ± 1.50 μM/cm2/min), resulting in conversion of net urate secretion (−18 ± 4 μM/cm2/min) to net urate absorption (+5.99 ± 2.2 μM/cm2/min). Interestingly, these significant alterations in intestinal urate secretion in the Q140K mice correlated with evidence of altered serum insulin and non‐fasting glucose levels, not unlike what has been previously reported in mice lacking intestinal SLC2A9 strongly suggesting the establishment of SLC2A9 and ABCG2 as the intestinal urate excretion pathway.CONCLUSIONSOur findings demonstrate the pathophysiology of one of the most common hyperuricemia and gout risk variants, Q141K ABCG2, primarily affects the urate secretion in the gut, causing hyperuricemia and alterations in insulin and glucose levels with important implications for human disease.Support or Funding InformationNIDDK RO1DK114091AHA14SDG18060004This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

A model of uric acid transport in the rat proximal tubule.

The objective of the present study was to theoretically investigate the mechanisms underlying uric acid transport in the proximal tubule (PT) of rat kidneys, and their modulation by factors, including Na+, parathyroid hormone, ANG II, and Na+-glucose cotransporter-2 inhibitors. To that end, we incorporated the transport of uric acid and its conjugate anion urate in our mathematical model of water and solute transport in the rat PT. The model accounts for parallel urate reabsorption and secretion pathways on apical and basolateral membranes and their coupling to lactate and α-ketoglutarate transport. Model results agree with experimental findings at the segment level. Net reabsorption of urate by the rat PT is predicted to be ~70% of the filtered load, with a rate of urate removal from the lumen that is 50% higher than the rate of urate secretion. The model suggests that apical URAT1 deletion significantly reduces net urate reabsorption across the PT, whereas ATP-binding cassette subfamily G member 2 dysfunction affects it only slightly. Inactivation of basolateral glucose transporter-9 raises fractional urate excretion above 100%, as observed in patients with renal familial hypouricemia. Furthermore, our results suggest that reducing Na+ reabsorption across Na+/H+ exchangers or Na+-glucose cotransporters augments net urate reabsorption. The model predicts that parathyroid hormone reduces urate excretion, whereas ANG II increases it. In conclusion, we have developed the first model of uric acid transport in the rat PT; this model provides a framework to gain greater insight into the numerous solutes and coupling mechanisms that affect the renal handing of uric acid.

Case Report : Urolith Surgical Removal in a Green Iguana (Iguana iguana) (LAPORAN KASUS : BEDAH PENGELUARAN BATU KANTUNG KEMIH PADA SEEKOR IGUANA HIJAU (IGUANA IGUANA))

Two years old male Green Iguana (Iguana iguana) weighing 1.3 kilograms presented with a swelling mass in the abdomen. Clinical signs, palpation and radiography images indicated urolith in the urinary bladder. The mass was radiopaque and had a width of 37 mm and a length of 41 mm. Clinical findings, examination and radio-imaging confirmed urolithiasis in the urinary bladder. Coeliotomy and cystotomy were performed to remove the urolith. Two days after surgery, the iguana regained its appetite and normal urate secretion was observed. Prognosis is good when surgery is performed on a healthy animal.

Jump into a New Fold—A Homology Based Model for the ABCG2/BCRP Multidrug Transporter

ABCG2/BCRP is a membrane protein, involved in xenobiotic and endobiotic transport in key pharmacological barriers and drug metabolizing organs, in the protection of stem cells, and in multidrug resistance of cancer. Pharmacogenetic studies implicated the role of ABCG2 in response to widely used medicines and anticancer agents, as well as in gout. Its Q141K variant exhibits decreased functional expression thus increased drug accumulation and decreased urate secretion. Still, there has been no reliable molecular model available for this protein, as the published structures of other ABC transporters could not be properly fitted to the ABCG2 topology and experimental data. The recently published high resolution structure of a close homologue, the ABCG5-ABCG8 heterodimer, revealed a new ABC transporter fold, unique for ABCG proteins. Here we present a structural model of the ABCG2 homodimer based on this fold and detail the experimental results supporting this model. In order to describe the effect of mutations on structure and dynamics, and characterize substrate recognition and cholesterol regulation we performed molecular dynamics simulations using full length ABCG2 protein embedded in a membrane bilayer and in silico docking simulations. Our results show that in the Q141K variant the introduced positive charge diminishes the interaction between the nucleotide binding and transmembrane domains and the R482G variation alters the orientation of transmembrane helices. Moreover, the R482 position, which plays a role the substrate specificity of the transporter, is located in one of the substrate binding pockets identified by the in silico docking calculations. In summary, the ABCG2 model and in silico simulations presented here may have significant impact on understanding drug distribution and toxicity, as well as drug development against cancer chemotherapy resistance or gout.

Stronger Uricosuric Effects of the Novel Selective URAT1 Inhibitor UR-1102 Lowered Plasma Urate in Tufted Capuchin Monkeys to a Greater Extent than Benzbromarone.

Urate-lowering therapy is indispensable for the treatment of gout, but available drugs do not control serum urate levels tightly enough. Although the uricosurics benzbromarone and probenecid inhibit a urate reabsorption transporter known as renal urate transporter 1 (URAT1) and thus lower serum urate levels, they also inhibit other transporters responsible for secretion of urate into urine, which suggests that inhibiting URAT1 selectively would lower serum urate more effectively. We identified a novel potent and selective URAT1 inhibitor, UR-1102, and compared its efficacy with benzbromarone in vitro and in vivo. In human embryonic kidney (HEK)293 cells overexpressing URAT1, organic anion transporter 1 (OAT1), and OAT3, benzbromarone inhibited all transporters similarly, whereas UR-1102 inhibited URAT1 comparably to benzbromarone but inhibited OAT1 and OAT3 quite modestly. UR-1102 at 3-30 mg/kg or benzbromarone at 3-100 mg/kg was administered orally once a day for 3 consecutive days to tufted capuchin monkeys, whose low uricase activity causes a high plasma urate level. When compared with the same dosage of benzbromarone, UR-1102 showed a better pharmacokinetic profile, increased the fractional excretion of urinary uric acid, and reduced plasma uric acid more effectively. Moreover, the maximum efficacy of UR-1102 was twice that of benzbromarone, suggesting that selective inhibition of URAT1 is effective. Additionally UR-1102 showed lower in vitro potential for mechanisms causing the hepatotoxicity induced by benzbromarone. These results indicate that UR-1102 achieves strong uricosuric effects by selectively inhibiting URAT1 over OAT1 and OAT3 in monkeys, and could be a novel therapeutic option for patients with gout or hyperuricemia.

ABCG2: the molecular mechanisms of urate secretion and gout

The human propensity for high levels of serum uric acid (SUA) is a trait that has defied explanation. Is it beneficial? Is it pathogenic? Its role in the human diseases like gout and kidney stones was discovered over a century ago [Richette P, Bardin T. Lancet 375: 318-328, 2010; Rivard C, Thomas J, Lanaspa MA, Johnson RJ. Rheumatology (Oxford) 52: 421-426, 2013], but today emerging new genetic and epidemiological techniques have revived an age-old debate over whether high uric acid levels (hyperuricemia) independently increase risk for diseases like hypertension and chronic kidney disease [Feig DI. J Clin Hypertens (Greenwich) 14: 346-352, 2012; Feig DI, Madero M, Jalal DI, Sanchez-Lozada LG, Johnson RJ. J Pediatr 162: 896-902, 2013; Feig DI, Soletsky B, Johnson RJ. JAMA 300: 924-932, 2008; Wang J, Qin T, Chen J, Li Y, Wang L, Huang H, Li J. PLoS One 9: e114259, 2014; Zhu P, Liu Y, Han L, Xu G, Ran JM. PLoS One 9: e100801, 2014]. Part of the mystery of the role uric acid plays in human health stems from our lack of understanding of how humans regulate uric acid homeostasis, an understanding that could shed light on the historic role of uric acid in human adaptation and its present role in human pathogenesis. This review will highlight the recent work to identify the first important human uric acid secretory transporter, ABCG2, and the identification of a common causal ABCG2 variant, Q141K, for hyperuricemia and gout.

In-vitro evidence of enhanced breast cancer resistance protein-mediated intestinal urate secretion by uremic toxins in Caco-2 cells.

It has been reported that intestinal urate excretion is increased at chronic kidney disease (CKD) state. In this report, whether uremic toxins are involved in the upregulation of intestinal breast cancer resistance protein (BCRP), an intestinal urate exporter, was examined. Uremic toxins that were increased at least 15-fold at CKD state were selected for investigation. Caco-2 cells were exposed to these uremic toxins at clinically relevant concentrations. mRNA was quantified by real-time PCR, and flow cytometry was utilized to measure BCRP protein and function in Caco-2 cells. Transcellular secretory transport of [(14) C]urate was determined utilizing Transwell studies after uremic toxin exposure. Indoxyl sulfate (IS) treatment alone resulted in ∼ 3-fold increase in BCRP mRNA in Caco-2 cells. Membrane protein expression of BCRP in Caco-2 cells also was increased by 1.8-fold after treatment with IS. Intracellular accumulation of pheophorbide A, a selective BCRP substrate, was decreased by 22% after IS treatment for 3 days. Consistent with these findings, transcellular secretory transport of urate across Caco-2 cell monolayers was increased by 22%. Intestinal urate secretion may be increased at CKD state partially by upregulation of intestinal BCRP by uremic toxins such as IS.

Urate transporters in the genomic era

Recent advances in genome technology have provided us with a list of molecules affecting urate handling in humans, many of which are unlikely to be identified through traditional physiological approach alone. Although this article is focused on urate, this can be viewed as a successful model of genomics-physiology collaboration. URATv1/GLUT9 (SLC2A9) is shown to play a critical role in urate reabsorption at the proximal tubule, probably more prominent than its partner URAT1 (SLC22A12). The major site of action of ABCG2 (ABCG2), an influential urate secretion transporter, has been shown to be the intestine rather than the kidney proximal tubule. Accordingly, hypofunction of ABCG2 leads to increased fractional excretion of urate, a finding traditionally interpreted as overproduction hyperuricemia. Some SLC17 family members secrete urate in the kidney or intestine. OAT2 (SLC22A7) may take up urate from blood to the proximal tubular cell. In addition, how a common single-nucleotide polymorphisms in ABCG2 affects its function has been elucidated. A finer grained picture of urate handling in the human body is now emerging, which will help choosing novel targets for urate-lowering therapy.

AB0632 Chronic prostatitis: a urate crystal induced disease?

BackgroundUrate crystals have been recognized anecdotally in a long list of nonarticular tissues but have not, to our knowledge, been sought or found in inflamed prostate glands. Like gouty arthritis,...

2013 Robert W. Berliner Award for Excellence in Renal Physiology

Announcement2013 Robert W. Berliner Award for Excellence in Renal PhysiologyPublished Online:01 May 2013https://doi.org/10.1152/ajprenal.00163.2013This is the final version - click for previous versionMoreSectionsPDF (97 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmail William H. Dantzler, MD, PhD, was selected by the Renal Section as the recipient of the 2013 Robert W. Berliner Award. Bill is an internationally recognized leader in physiology. He is especially noted for his research in comparative renal physiology, where he defined the crucial mechanism of urate secretion in reptilian and avian proximal tubules. His work in birds also provided key information that furthered our understanding of loop of Henle function in the mammalian kidney. He has published over 140 peer-reviewed articles as well as numerous book chapters, reviews, and similar contributions. Bill has been a stalwart leader in the American Physiological Society (APS) and at the University of Arizona College of Medicine, where he has been a faculty member in the Department of Physiology since 1968, and Head of that department from 1991 until he became an Emeritus Professor in 2005.Dr. William H. DantzlerDownload figureDownload PowerPointAfter graduating from Princeton, Dr. Dantzler received his MD from Columbia University in 1961, and his PhD in Zoology from Duke in 1964. At Duke he worked in the laboratory of Dr. Bodil Schmidt-Nielsen, who was the ideal mentor to foster Bill's research and teaching career. He went directly from her laboratory to an Assistant Professorship in the Pharmacology Department of Columbia, which he left in 1968 to accept an Associate Professorship in Physiology at Arizona.Dr. Dantzler's research on the physiology of reptilian and avian nephrons established the functional importance of the anatomic organization of the renal medulla. Comparison of renal concentrating and diluting capacity in these animal classes with that in mammals provided seminal insights into the operation of the mammalian concentrating and diluting mechanisms. Even after achieving emeritus status, Bill, in collaboration with Tom Pannebecker, has used three-dimensional computer reconstruction of immunohistochemical images to provide an elegant picture of the organization of both nephron segments and critical transporters in the mammalian inner medulla.Throughout his career, Bill used innovative technologies such as these to address key questions in renal physiology. In the early 1970s he was one of the first investigators to isolate and perfuse nephron segments in vitro. By measuring unidirectional fluxes of urate in proximal nephron segments from snake kidneys, Bill characterized the mechanism of active urate secretion, which is essential for both nitrogen excretion and water conservation in reptiles and birds. In a long-standing collaboration with Stefan Silbenagl, Bill also used in vivo micropuncture and microperfusion to demonstrate recycling of amino acids between vasa recta and Henle's loops in the rat renal inner medulla.The corpus of Bill's research has been recognized by numerous awards, his long-standing support from the National Institutes of Health and National Science Foundation, his editorship of AJP-Regulatory, Integrative. and Comparative Physiology, and his herculean task of assembling and editing the Handbook of Comparative Physiology.Dr. Dantzler is an outstanding teacher and has received numerous teaching awards from the University of Arizona. He also has been a leader in the APS, serving as President (1993–94) and chairperson of both the Renal and Water and Electrolyte sections. Bill is truly a most deserving recipient of the Berliner Award, which was established to recognize distinguished lifetime achievement in renal physiological research.This year's Robert W. Berliner Award Committee was composed of Heddwen Brooks (Renal Section Chair), Thomas Kleyman (Editor of AJP-Renal Physiology), and Volker Vallon (Chair of Renal Awards Committee).This article has no references to display. Previous Back to Top FiguresReferencesRelatedInformation More from this issue > Volume 304Issue 9May 2013Pages F1230-F1230 Copyright & PermissionsCopyright © 2013 the American Physiological Societyhttps://doi.org/10.1152/ajprenal.00163.2013History Published online 1 May 2013 Published in print 1 May 2013 PDF download Metrics Downloaded 64 times

The genetics of hyperuricaemia and gout

Gout is a common and very painful inflammatory arthritis caused by hyperuricaemia. This review provides an update on the genetics of hyperuricaemia and gout, including findings from genome-wide association studies. Most of the genes that associated with serum uric acid levels or gout are involved in the renal urate-transport system. For example, the urate transporter genes SLC2A9, ABCG2 and SLC22A12 modulate serum uric acid levels and gout risk. The net balance between renal urate absorption and secretion is a major determinant of serum uric acid concentration and loss-of-function mutations in SLC2A9 and SLC22A12 cause hereditary hypouricaemia due to reduced urate absorption and unopposed urate secretion. However, the variance in serum uric acid explained by genetic variants is small and their clinical utility for gout risk prediction seems limited because serum uric acid levels effectively predict gout risk. Urate-associated genes and genetically determined serum uric acid levels were largely unassociated with cardiovascular-metabolic outcomes, challenging the hypothesis of a causal role of serum uric acid in the development of cardiovascular disease. Strong pharmacogenetic associations between HLA-B*5801 alleles and severe allopurinol-hypersensitivity reactions were shown in Asian and European populations. Genetic testing for HLA-B*5801 alleles could be used to predict these potentially fatal adverse effects.