Extra-renal elimination of uric acid via intestinal efflux transporter BCRP/ABCG2. PLoS One. 2012;7:e30456. [PMID: 22348008 PMCID: PMC3277506 DOI: 10.1371/journal.pone.0030456]

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

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

Uromodulin in a Pathway Between Decreased Renal Urate Excretion and Albuminuria

BACKGROUND

The mechanism explaining the inverse association between renal urate and albumin excretion remains unclear. First, we evaluated the impact of candidate variants in the main urate transporter genes (i.e., SLC2A9, SLC22A12, ABCG2) on the association between fractional excretion of uric acid (FEUA) and urinary albumin/creatinine ratio (uACR). Second, we examined uromodulin and sodium excretion as mediators of the association between FEUA and uACR.

METHODS

We performed cross-sectional analysis of 737 French Canadians from the CARTaGENE cohort, a random sample of the Quebec population aged 40-69 years (a total of 20,004 individuals). Individuals with available genotyping and urinary data were obtained from a sub-study including gender-matched pairs with high and low Framingham Risk Score and vascular rigidity index. We further excluded individuals with an estimated glomerular filtration rate <60 ml/min/1.73 m2, glycosuria, and use of confounding medication. A spot urine sample was analyzed. Genotyping was performed using the Illumina Omni2.5-8 BeadChips. Genetic variants were analyzed using an additive model.

RESULTS

Final analyses included 593 individuals (45.5% of men; mean age 54.3 ± 8.6). We observed an antagonistic interaction between rs13129697 variant of the SLC2A9 gene and FEUA tertiles on uACR (P = 0.002). Using the mediation analysis, uromodulin explained 32%, fractional excretion of sodium (FENa) 44%, and uromodulin together with FENa explained 70% of the inverse relationship between FEUA and uACR. Bootstrapping process confirmed the role of both mediators.

CONCLUSIONS

Our data suggest that the association of albuminuria with decreased renal urate excretion may be modified by the transporter SLC2A9, and mediated by uromodulin and sodium handling.

The systems biology of uric acid transporters: the role of remote sensing and signaling

PURPOSE OF REVIEW

Uric acid homeostasis in the body is mediated by a number of SLC and ABC transporters in the kidney and intestine, including several multispecific 'drug' transporters (e.g., OAT1, OAT3, and ABCG2). Optimization of uric acid levels can be viewed as a 'systems biology' problem. Here, we consider uric acid transporters from a systems physiology perspective using the framework of the 'Remote Sensing and Signaling Hypothesis.' This hypothesis explains how SLC and ABC 'drug' and other transporters mediate interorgan and interorganismal communication (e.g., gut microbiome and host) via small molecules (e.g., metabolites, antioxidants signaling molecules) through transporters expressed in tissues lining body fluid compartments (e.g., blood, urine, cerebrospinal fluid).

RECENT FINDINGS

The list of uric acid transporters includes: SLC2A9, ABCG2, URAT1 (SLC22A12), OAT1 (SLC22A6), OAT3 (SLC22A8), OAT4 (SLC22A11), OAT10 (SLC22A13), NPT1 (SLC17A1), NPT4 (SLC17A3), MRP2 (ABCC2), MRP4 (ABCC4). Normally, SLC2A9, - along with URAT1, OAT1 and OAT3, - appear to be the main transporters regulating renal urate handling, while ABCG2 appears to regulate intestinal transport. In chronic kidney disease (CKD), intestinal ABCG2 becomes much more important, suggesting remote organ communication between the injured kidney and the intestine.

SUMMARY

The remote sensing and signaling hypothesis provides a useful systems-level framework for understanding the complex interplay of uric acid transporters expressed in different tissues involved in optimizing uric acid levels under normal and diseased (e.g., CKD, gut microflora dysbiosis) conditions.

What Is the Biological Function of Uric Acid? An Antioxidant for Neural Protection or a Biomarker for Cell Death

The main aim of the present study was to investigate the biological function of uric acid. The level of uric acid in different organs in normal male rats was determined with uric acid assay kits, and the expression level of genes in the organs was determined by RNA quantitative sequencing. The correlation analysis between uric acid in the organs and gene expression (measured by FPKM value) was made. Serum uric acid (SUA) in patients with breast cancer or with breast benign tumor was assayed when the diagnosis was made, and SUA in patients with breast cancer was also assayed just after chemotherapy. There were 1937 mRNAs whose expression level significantly correlated with the level of uric acid, and most of which were associated with purine or nucleoside metabolism, cellular metabolism, cell cycles, and cell death pathways. Further analysis showed that the level of uric acid was highly correlated with cell death rather than cell viability. The level of SUA in patients with breast cancer was higher than that in patients with breast benign tumor, and the SUA increased after chemotherapy. All the results suggested that uric acid was mainly synthesized from local nucleosides degraded from dead cells, and uric acid could be an important biomarker for cell death rather than an antioxidant for neural protection.

Effect of lemon water soluble extract on hyperuricemia in a mouse model

Lemon is a healthy fruit with high medicinal value.

ABC Family Transporters

The transport of specific molecules across lipid membranes is an essential function of all living organisms. The processes are usually mediated by specific transporters. One of the largest transporter families is the ATP-binding cassette (ABC) family. More than 40 ABC transporters have been identified in human, which are divided into 7 subfamilies (ABCA to ABCG) based on their gene structure, amino acid sequence, domain organization, and phylogenetic analysis. Of them, at least 11 ABC transporters including P-glycoprotein (P-GP/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2) are involved in multidrug resistance (MDR) development. These ABC transporters are expressed in various tissues such as the liver, intestine, kidney, and brain, playing important roles in absorption, distribution, and excretion of drugs. Some ABC transporters are also involved in diverse cellular processes such as maintenance of osmotic homeostasis, antigen processing, cell division, immunity, cholesterol, and lipid trafficking. Several human diseases such as cystic fibrosis, sitosterolemia, Tangier disease, intrahepatic cholestasis, and retinal degeneration are associated with mutations in corresponding transporters. This chapter will describe function and expression of several ABC transporters (such as P-GP, BCRP, and MRPs), their substrates and inhibitors, as well as their clinical significance.

Bioinformatic and Metabolomic Analysis Reveal Intervention Effects of Chicory in a Quail Model of Hyperuricemia

Background. Hyperuricemia (HUA) is a kind of a metabolic disease that seriously threatens human health worldwide. Chicory, a natural herbal medicine, has an obvious effect of reducing uric acid. The aim of this study is to explore the potential components and pharmacological pathways that may play a role in hypouricemia activity of chicory. Bioinformatics and metabonomics were applied to this research. Firstly, component-target network was used to identify possible components related to the pharmacological properties and their corresponding mechanisms pathway of chicory. Afterwards, animal pharmacodynamic experiments were performed. Blood and stool samples were collected for untargeted metabolomic analysis by dint of UHPLC-Q-TOF/MS methods, and principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) were performed for the pattern recognition and characteristic metabolites identification. Significant enriched function pathways were used in bioinformatics suggesting that chicory might have the effect of regulation of lipolysis in adipocytes. PLS-DA analysis was applied to discover differentiating metabolites, and pathway enrichment analysis indicated that chicory had powerful effects of glycosylphosphatidylinositol- (GPI-) anchor biosynthesis, inositol phosphate metabolism, glycerophospholipid metabolism, and steroid hormone biosynthesis. Combining bioinformatics and metabolomics results, we consider that chicory may develop on lowering uric acid by adjusting lipid metabolism. In addition, we chose quail as animal model innovatively and discussed the treatment of hyperuricemia with chicory in multiple methods, which may render reference for the research of HUA.

Effects of Chicory on Serum Uric Acid, Renal Function, and GLUT9 Expression in Hyperuricaemic Rats with Renal Injury and In Vitro Verification with Cells

Hyperuricaemia (HUA) is an independent risk factor for chronic kidney disease. Urate crystals are deposited in the kidney and can cause renal tubular interstitial fibrosis, leading to renal dysfunction. Chicory extract (hereafter referred to as chicory) clearly reduced serum uric acid levels in rats with HUA induced by 10% fructose. This is the first study to observe the effect of chicory on serum uric acid levels and renal function in rats with HUA and renal injury. In vivo studies using hyperuricaemic rats with renal injury induced by yeast and adenine demonstrated that chicory decreased serum uric acid level, and its effect of delaying the progression of kidney injury was better than that of benzbromarone. In vitro cell experiments showed that this effect is related to the inhibition of GLUT9 protein expression in renal tubules and that lowering blood uric acid concentrations is one of the factors that alleviates renal damage. The results of this study indicate that chicory can be used as an alternative for alleviating renal dysfunction in hyperuricaemia.

Alterations of the Gut Microbiome Associated With the Treatment of Hyperuricaemia in Male Rats

Hyperuricaemia is an important risk factor for many diseases including gout, hypertension, and type II diabetes. The gut microbiota is associated with hyperuricaemia and has also been demonstrated to play significant roles in the effects of drug therapy. This study used Illumina MiSeq sequencing to explore alterations of the gut microbiome associated with allopurinol and benzbromarone treatment in the male rat with hyperuricaemia. After drug treatment, both allopurinol and benzbromarone caused an increase of the genera Bifidobacterium and Collinsella and a decrease of the genera Adlercreutzia and Anaerostipes. In addition, allopurinol and benzbromarone caused respective unique changes in genera. The genera Bilophila, Morganella, and Desulfovibrio specifically decreased due to allopurinol treatment. Decreased Butyricimonas and Ruminococcus and increased Proteus were caused by benzbromarone treatment. The PICRUST analysis indicated that allopurinol renovated the disorder of nucleotide metabolism and benzbromarone renovated the disorder of lipid metabolism in the gut microbiota of male rats with hyperuricaemia. These findings demonstrated that the gut microbiota may be altered by the treatment of hyperuricaemia with allopurinol and benzbromarone in male rats. Such alterations of the gut microbiota could be considered as indicators of the effectiveness of drug therapy.

Decreased Serum Bilirubin Levels and Increased Uric Acid Levels are Associated with Ulcerative Colitis

Background

In recent years, emerging evidence has suggested that ulcerative colitis occurs as a consequence of an imbalance between oxidative stress and antioxidant capacity. The objective of this study was to investigate whether serum total bilirubin and serum uric acid levels were associated with ulcerative colitis.

Material/Methods

We conducted a retrospective case-control study which included 170 patients with ulcerative colitis and 200 healthy individuals. Concentrations of serum total bilirubin and serum uric acid were obtained from biochemical information and segregated into quartiles. Logistic regression analysis was adopted to explore the correlations between levels of the 2 biochemical markers and the risk of ulcerative colitis.

Results

Compared with healthy controls, patients with ulcerative colitis exhibited lower levels of serum bilirubin (9.30 umol/L versus 12.49 umol/L respectively, P<0.001). Multivariate logistic regression showed that the lowest quartile of total serum bilirubin was independently associated with the occurrence of ulcerative colitis (OR=2.56, 95%CI: 1.54–4.25, P<0.001). Similarly, ulcerative colitis patients exhibited higher concentrations of serum uric acid (338 umol/L versus 300 umol/L respectively, P=0.041). Multivariate logistic regression showed that the highest quartile of serum uric acid was independently associated with ulcerative colitis risk (OR=1.20, 95%CI: 1.05–1.77, P=0.045). Furthermore, a negative association was observed between serum total bilirubin and serum uric acid in patients with ulcerative colitis.

Conclusion

Lower levels of serum total bilirubin and higher levels of serum uric acid are associated with ulcerative colitis patients compared to healthy controls.

Genetics of serum urate concentrations and gout in a high-risk population, patients with chronic kidney disease

We evaluated genetics of hyperuricemia and gout, their interaction with kidney function and medication intake in chronic kidney disease (CKD) patients. Genome-wide association studies (GWAS) of urate and gout were performed in 4941 CKD patients in the German Chronic Kidney Disease (GCKD) study. Effect estimates of 26 known urate-associated population-based single nucleotide polymorphisms (SNPs) were examined. Interactions of urate-associated variants with urate-altering medications and clinical characteristics of gout were evaluated. Genome-wide significant associations with serum urate and gout were identified for known loci at SLC2A9 and ABCG2, but not for novel loci. Effects of the 26 known SNPs were of similar magnitude in CKD patients compared to population-based individuals, except for SNPs at ABCG2 that showed greater effects in CKD. Gene-medication interactions were not significant when accounting for multiple testing. Associations with gout in specific joints were significant for SLC2A9 rs12498742 in wrists and midfoot joints. Known genetic variants in SLC2A9 and ABCG2 were associated with urate and gout in a CKD cohort, with effect sizes for ABCG2 significantly greater in CKD compared to the general population. CKD patients are at high risk of gout due to reduced kidney function, diuretics intake and genetic predisposition, making treatment to target challenging.

Probiotic supplements prevented oxonic acid-induced hyperuricemia and renal damage

Hyperuricemia is highly prevalent and especially common in subjects with metabolic, cardiovascular and renal diseases. In chronic kidney disease, hyperuricemia is extremely common, and uric acid (UA) excretion relies on gut uricolysis by gut microbiota. Current therapy for lowering serum UA includes drugs that may produce undesired secondary effects. Therefore, this pilot study was designed to evaluate the potential of two probiotic supplements to reduce systemic uric acid concentrations. Secondary objectives were to assess whether the hypouricemic effect related to a therapeutic benefit on the hyperuricemia-induced renal damage and hypertension. Analysis of fecal microbiota was also performed. Groups of 6 rats each were followed for 5 weeks and allocated in the following treatment groups: C = Control; HU-ND = Oxonic acid-induced hyperuricemia (HU) +regular diet; HU-P = HU+placebo; HU-F1 = HU+ probiotics formula 1 and HU-F2 = HU+ probiotics formula 2. We confirmed that oxonic acid-induced hyperuricemia produced hypertension and renal functional and structural changes, along with modest changes in the overall composition of fecal microbiota. Both probiotic-containing diets prevented HU, elevated UA urinary excretion and intrarenal UA accumulation induced by oxonic acid. The hypouricemic effect conferred by probiotic supplementation also prevented the renal changes and hypertension caused by hyperuricemia. However, probiotic treatment did not restore the fecal microbiota. In conclusion, we demonstrated for the first time the ability of probiotics containing uricolytic bacteria to lower serum uric acid in hyperuricemic animals with beneficial consequences on blood pressure and renal disease. As probiotics supplements are innocuous for human health, we recommend clinical studies to test if probiotic supplements could benefit hyperuricemic individuals.

Multiple Membrane Transporters and Some Immune Regulatory Genes are Major Genetic Factors to Gout

Gout is a common form of inflammatory arthritis caused by hyperuricemia and the deposition of Monosodium Urate (MSU) crystals. It is also considered as a complex disorder in which multiple genetic factors have been identified in association with its susceptibility and/or clinical outcomes. Major genes that were associated with gout include URAT1, GLUT9, OAT4, NPT1 (SLC17A1), NPT4 (SLC17A3), NPT5 (SLC17A4), MCT9, ABCG2, ABCC4, KCNQ1, PDZK1, NIPAL1, IL1β, IL-8, IL-12B, IL-23R, TNFA, MCP-1/CCL2, NLRP3, PPARGC1B, TLR4, CD14, CARD8, P2X7R, EGF, A1CF, HNF4G and TRIM46, LRP2, GKRP, ADRB3, ADH1B, ALDH2, COMT, MAOA, PRKG2, WDR1, ALPK1, CARMIL (LRRC16A), RFX3, BCAS3, CNIH-2, FAM35A and MYL2-CUX2. The proteins encoded by these genes mainly function in urate transport, inflammation, innate immunity and metabolism. Understanding the functions of gout-associated genes will provide important insights into future studies to explore the pathogenesis of gout, as well as to develop targeted therapies for gout.

Uric acid and cardiovascular disease

Uric acid (UA) is an end product of purine metabolism in humans and great apes. UA acts as an antioxidant and it accounts for 50% of the total antioxidant capacity of biological fluids in humans. When present in cytoplasm of the cells or in acidic/hydrophobic milieu in atherosclerotic plaques, UA converts into a pro-oxidant agent and promotes oxidative stress and through this mechanism participates in the pathophysiology of human disease including cardiovascular disease (CVD). Most epidemiological studies but not all of them suggested the existence of an association between elevated serum UA level and CVD, including coronary heart disease (CHD), stroke, congestive heart failure, arterial hypertension and atrial fibrillation as well as an increased risk for mortality due to CVD in general population and subjects with confirmed CHD. Evidence available also suggests an association between elevated UA and traditional cardiovascular risk factors, metabolic syndrome, insulin resistance, obesity, non-alcoholic fatty liver disease and chronic kidney disease. Experimental and clinical studies have evidenced several mechanisms through which elevated UA level exerts deleterious effects on cardiovascular health including increased oxidative stress, reduced availability of nitric oxide and endothelial dysfunction, promotion of local and systemic inflammation, vasoconstriction and proliferation of vascular smooth muscle cells, insulin resistance and metabolic dysregulation. Although the causality in the relationship between UA and CVD remains unproven, UA may be pathogenic and participate in the pathophysiology of CVD by serving as a bridging mechanism mediating (enabling) or potentiating the deleterious effects of cardiovascular risk factors on vascular tissue and myocardium.

Clinically relevant mutations in the ABCG2 transporter uncovered by genetic analysis linked to erythrocyte membrane protein expression

The ABCG2 membrane protein is a key xeno- and endobiotic transporter, modulating the absorption and metabolism of pharmacological agents and causing multidrug resistance in cancer. ABCG2 is also involved in uric acid elimination and its impaired function is causative in gout. Analysis of ABCG2 expression in the erythrocyte membranes of healthy volunteers and gout patients showed an enrichment of lower expression levels in the patients. By genetic screening based on protein expression, we found a relatively frequent, novel ABCG2 mutation (ABCG2-M71V), which, according to cellular expression studies, causes reduced protein expression, although with preserved transporter capability. Molecular dynamics simulations indicated a stumbled dynamics of the mutant protein, while ABCG2-M71V expression in vitro could be corrected by therapeutically relevant small molecules. These results suggest that personalized medicine should consider this newly discovered ABCG2 mutation, and genetic analysis linked to protein expression provides a new tool to uncover clinically important mutations in membrane proteins.

Uric acid and blood pressure: exploring the role of uric acid production in The Maastricht Study

OBJECTIVE

Accumulation of reactive oxygen species by increased uric acid production has been suggested as a possible underlying mechanism for the association between uric acid and high blood pressure (BP). We, therefore, investigated the association between serum uric acid concentration and 24-h urinary uric acid excretion, as proxy for uric acid production, with ambulatory 24-h blood pressure and hypertension.

METHODS

Cross-sectional analyses were conducted among 2555 individuals [52% men, mean age 60.0 ± 8.2 years; 27% type 2 diabetes (by design)] from The Maastricht Study. Multivariable regression analyses were performed to investigate the association of serum uric acid and 24-h urinary uric acid excretion with 24-h pulse pressure, 24-h mean arterial pressure (MAP), and hypertension.

RESULTS

After adjustment for traditional hypertension risk factors, serum uric acid concentration (per SD of 81 μmol/l) was associated with higher 24-h MAP [β 0.63 mmHg; confidence interval (CI) 0.27-1.00] and positively associated with hypertension (odds ratio 1.43; CI 1.27-1.61). Urinary uric acid excretion (per SD of 140 mg/day/1.73 m) was associated with higher 24-h MAP (β 0.79 mmHg; CI 0.46-1.12) and with hypertension (odds ratio 1.13; CI 1.02-1.25). There was no significant association between serum and 24-h urinary uric acid excretion with 24-h pulse pressure. There was no interaction with sex or age for the aforementioned associations.

CONCLUSION

Higher serum and urinary uric acid concentrations were associated with higher 24-h MAP and hypertension. These results suggest that serum and 24-urinary uric acid concentrations, the latter as proxy for uric acid production are, independent of each other, associated with BP and hypertension.

Effect and mechanism of dioscin from Dioscorea spongiosa on uric acid excretion in animal model of hyperuricemia

ETHNOPHARMACOLOGY RELEVANCE

Dioscin, a spirostane glycoside, the rhizoma of Dioscorea septemloba (Diocoreacea) is used for diuresis, rheumatism, and joints pain. Given the poor solubility and stability of Dioscin, we proposed a hypothesis that Dioscin's metabolite(s) are the active substance(s) in vivo to contribute to the reducing effects on serum uric acid levels.

AIM OF THE STUDY

The aim of this study is to identify the active metabolite(s) of Dioscin in vivo and to explore the mechanism of its antihyperuricemic activity.

MATERIALS AND METHODS

After oral administration of Dioscin in potassium oxonate (PO) induced hyperuricemia rats and adenine-PO induced hyperuricemia mice models, serum uric acid and creatinine levels, clearance of uric acid and creatinine, fractional excretion of uric acid, and renal pathological lesions were determined were used to evaluate the antihyperuricemic effects. Renal glucose transporter-9 (GLUT-9) and organic anion transporter-1 (OAT-1) expressions were analyzed by western blotting method. Renal uric acid excretion was evaluated using stably urate transporter-1 (URAT-1) transfected human epithelial kidney cell line. Intestinal uric acid excretion was evaluated by measuring the transcellular transport of uric acid in HCT116 cells.

RESULTS

In hyperuricemia rats, both 25 and 50mg/kg of oral Dioscin decreased serum uric acid levels over 4h. In the hyperuricemia mice, two weeks treatment of Dioscin significantly decreased serum uric acid and creatinine levels, increased clearance of uric acid and creatinine, increased fractional excretion of uric acid, and reduced renal pathological lesions caused by hyperuricemia. In addition, renal GLUT -9 was significantly down-regulated and OAT-1 was up-regulated in Dioscin treated hyperuricemia mice. Dioscin's metabolite Tigogenin significantly inhibited uric acid re-absorption via URAT1 from 10 to 100μM. Diosgenin and Tigogenin increased uric acid excretion via ATP binding cassette subfamily G member 2 (ABCG2).

CONCLUSION

Decreasing effect of Dioscin on serum uric acid level and enhancing effect on urate excretion were confirmed in hyperuricemia animal models. Tigogenin, a metabolite of Dioscin, was identified as an active substance with antihyperuricemic activity in vivo, through inhibition of URAT1 and promotion of ABCG2.

Hyperuricemia enhances intracellular urate accumulation via down-regulation of cell-surface BCRP/ABCG2 expression in vascular endothelial cells

Hyperuricemia has been recognized as an independent risk factor for cardiovascular disease. Urate stimulates NADPH oxidase and induces production of reactive oxygen species (ROS); consequently, intracellular urate accumulation can induce oxidative stress leading to endothelial dysfunction. Here, we studied the mechanism involved, using human umbilical vascular endothelial cells (HUVEC) as a model. Pretreatment with 15 mg/dL unlabeled uric acid (corresponding to hyperuricemia) resulted in increased uptake of [¹⁴C]uric acid at steady-state by HUVEC, whereas pretreatment with 5 mg/dL uric acid (in the normal serum concentration range) did not. However, the initial uptake rate of [¹⁴C]uric acid was not affected by uric acid at either concentration. These results suggest that efflux transport of uric acid is decreased under hyperuricemic conditions. We observed a concomitant decrease of phosphorylated endothelial nitric oxide synthase. Plasma membrane expression of breast cancer resistance protein (BCRP), a uric acid efflux transporter, was decreased under hyperuricemia, though the total cellular expression of BCRP remained constant. Uric acid did not affect expression of another uric acid efflux transporter, multidrug resistance associated protein 4 (MRP4). Moreover, phosphorylation of Akt, which regulates plasma membrane localization of BCRP, was decreased. These uric acid-induced changes of BCRP and Akt were reversed in the presence of the antioxidant N-acetylcysteine. These results suggest that in hyperuricemia, uric acid-induced ROS generation inhibits Akt phosphorylation, causing a decrease in plasma membrane localization of BCRP, and the resulting decrease of BCRP-mediated efflux leads to increased uric acid accumulation and dysregulation of endothelial function.

ABCG2 as a therapeutic target candidate for gout

INTRODUCTION

Hyperuricemia (chronically elevated serum uric acid) is the main pathology underlying the development of gout, the most common inflammatory arthropathy. Management of these conditions therefore relies on controlling serum uric acid levels. ATP-binding cassette transporter, sub-family G, member 2 (ABCG2/BCRP) is a well-studied urate transporter expressed on apical membranes in several tissues, including the intestine, liver, and kidney. Here, we discuss the potential of future gout therapies targeting ABCG2. Areas covered: ABCG2 regulates serum uric acid via physiologically important roles in both renal and extra-renal urate excretion. ABCG2 dysfunction, which promotes onset of hyperuricemia, often results in decreased urate excretion through the extra-renal (principally intestinal), rather than the renal pathway. This review covers recent attempts to establish the basis of ABCG2 function according to genetic diathesis, its molecular structure, and the effects of medication. Furthermore, the possibility of treating gout and hyperuricemia by upregulating intestinal ABCG2 expression is examined. Expert opinion: ABCG2 holds great promise as a therapeutic target for these conditions, particularly considering its involvement in extra-renal urate excretion. Manipulation of ABCG2, including controlling the level and location of its expression, has the potential to prevent gout by promoting uric acid excretion as effectively as general uricosuric drugs.

ABBREVIATIONS

ATP-binding cassette (ABC), transmembrane domain (TMD), nucleotide binding domain (NBD), single nucleotide polymorphism (SNP), single nucleotide polymorphisms (SNPs).

Management of hyperuricemia through dietary polyphenols as a natural medicament: A comprehensive review

Hyperuricemia, a condition due to high serum uric acid level and is notorious to health. It is considered to be a potent risk factor for gout and dramatically associated in the development of many chronic diseases such as malignant tumor, cardiovascular disorders and renal failure. Modern innovative medicinal and therapeutic interventions are underlying these days to combat hyperuricemia. Previously reported studies revealed the significant impact of dietary polyphenols (e.g. anthocyanins, phenolic acids, flavonoids etc.) against hyperurecemia disorder. Dietary plant polyphenols, unlike anti- hyperuricemic agents, are not reported to have any side effects in curing hyperuricemia. The current comprehensive review figure outs the use of dietary polyphenols as a natural remedy for the management of hyperuricemia. The sources, affiliated pathways, mode of actions and factors affecting their efficiency to prevent hyperuricemia are deeply discussed in this article. Additionally, limitations and suggestions regarding previously reported studies are also highlighted.

Intestinal tract is an important organ for lowering serum uric acid in rats

The kidney was recognized as a dominant organ for uric acid excretion. The main aim of the study demonstrated intestinal tract was an even more important organ for serum uric acid (SUA) lowering. Sprague-Dawley rats were treated normally or with antibiotics, uric acid, adenine, or inosine of the same molar dose orally or intraperitoneally for 5 days. Rat’s intestinal tract was equally divided into 20 segments except the cecum. Uric acid in serum and intestinal segment juice was assayed. Total RNA in the initial intestinal tract and at the end ileum was extracted and sequenced. Protein expression of xanthine dehydrogenase (XDH) and urate oxidase (UOX) was tested by Western blot analysis. The effect of oral UOX in lowering SUA was investigated in model rats treated with adenine and an inhibitor of uric oxidase for 5 days. SUA in the normal rats was 20.93±6.98 μg/ml, and total uric acid in the intestinal juice was 308.27±16.37 μg, which is two times more than the total SUA. The uric acid was very low in stomach juice, and attained maximum in the juice of the first segment (duodenum) and then declined all the way till the intestinal end. The level of uric acid in the initial intestinal tissue was very high, where XDH and most of the proteins associated with bicarbonate secretion were up-regulated. In addition, SUA was decreased by oral UOX in model rats. The results suggested that intestinal juice was an important pool for uric acid, and intestinal tract was an important organ for SUA lowering. The uric acid distribution was associated with uric acid synthesis and secretion in the upper intestinal tract, and reclamation in the lower.

A comprehensive analysis of the association of common variants of ABCG2 with gout

The objective of the present study was to determine whether there was an association between single nucleotide polymorphisms (SNPs) in ABCG2 and gout. We recruited 333 participants including 210 patients with gout and 123 controls and genotyped 45 SNPs in both cohorts. We found that 24 SNPs in ABCG2 are susceptibility loci associated with gout. Haplotype analysis revealed five blocks across the ABCG2 locus were associated with an increased risk of gout with odds ratios (ORs) from 2.59–3.17 (all P < 0.0001). A novel finding in the present study was the identification of rs3114018 in block 3 and its association with increased gout risk. We found that the rs2231142T allele in block 2 and the rs3114018C-rs3109823T (C-T) risk haplotype in block 3 conferred the greatest evidence of association to gout risk (P = 1.19 × 10⁻¹² and P = 9.20 × 10⁻¹¹, respectively). Our study provides an improved understanding of ABCG2 variations in patients with gout and, as shown by haplotype analysis, that ABCG2 may have a role in gout susceptibility.

Beyond drug-drug interactions: effects of transporter inhibition on endobiotics, nutrients and toxins

INTRODUCTION

Membrane transport proteins play a central role in regulating the disposition of endobiotics, dietary nutrients and environmental toxins. The inhibition of transporters by drugs has potential physiologic consequences. The full extent of the effect of drugs on the function of transporters is poorly understood because only a small subset of the hundreds of transporters expressed in humans - primarily those mediating the rate-determining step in the elimination of specific drugs - are assessed during clinical development. Areas covered: We provide a comprehensive overview of literature reports implicating the inhibition of transporters as the mechanism for off-target effects of drugs. Expert opinion: Transporter inhibition, the mechanism of action of many marketed drugs, appears to play an underappreciated role in a number of side effects including vitamin deficiency, edema, dyslipidemia, cholestasis and gout. Cell systems more broadly expressing transporter networks and methods like unbiased metabolomics should be incorporated into the screening paradigm to expand our understanding of the impact of drugs on the physiologic function of transporters and to allow for these effects to be taken into account in drug discovery and clinical practice.

Cichorium intybus L. promotes intestinal uric acid excretion by modulating ABCG2 in experimental hyperuricemia

Background

Excessive production and/or reduced excretion of uric acid could lead to hyperuricemia, which could be a major cause of disability. Hyperuricemia has received increasing attention in the last few decades due to its global prevalence. Cichorium intybus L., commonly known as chicory, is a perennial herb of the asteraceae family. It was previously shown to exert potent hypouricemic effects linked with decreasing uric acid formation in the liver by down-regulating the activity of xanthine oxidase, and increasing uric acid excretion by up-regulating the renal OAT3 mRNA expression. The present study aimed to evaluate its extra-renal excretion and possible molecular mechanism underlying the transporter responsible for intestinal uric acid excretion in vivo.

Methods

Chicory was administered intragastrically to hyperuricemic rats induced by drinking 10% fructose water. The uricosuric effect was evaluated by determining the serum uric acid level as well as the intestinal uric acid excretion by HPLC. The location and expression levels of ATP-binding cassette transporter, sub-family G, member 2 (ABCG2) in jejunum and ileum were analyzed.

Results

The administration of chicory decreased the serum uric acid level significantly and increased the intestinal uric acid excretion obviously in hyperuricemic rats induced by 10% fructose drinking. Staining showed that ABCG2 was expressed in the apical membrane of the epithelium and glands of the jejunum and ileum in rats. Further examination showed that chicory enhanced the mRNA and protein expressions of ABCG2 markedly in a dose-dependent manner in jejunum and ileum.

Conclusion

These findings indicate that chicory increases uric acid excretion by intestines, which may be related to the stimulation of intestinal uric acid excretion via down-regulating the mRNA and protein expressions of ABCG2.

Oral uricase eliminates blood uric acid in the hyperuricemic pig model

An elevated level of serum uric acid-hyperuricemia, is strongly associated with the development of gout and chronic kidney disease (CKD) which is often accompanied by a significantly reduced glomerular filtration rate (GFR). In the present study, we investigated the extra-renal elimination of uric acid via the intestine in a healthy pig model and the effect of oral uricase therapy on plasma uric acid concentrations in pigs with induced hyperuricemia and CKD. The experiment was conducted on eleven, ten-week-old pigs (n = 11). The porcine model of CKD was developed by performing 9/10 nephrectomy surgery on eight pigs. A stable model of hyperuricemia was established in only five of the eight nephrectomized pigs by frequent injections of uric acid (UA) into the jugular vein. All pigs (three healthy pigs and five CKD pigs) were operated for implantation of jugular vein catheters and the three healthy pigs also had portal vein catheters inserted. Blood uric acid concentrations were measured spectrophotometrically, using the Uric Acid Assay Kit (BioAssay Systems, Hayward, USA). The piglets with CKD received orally administered uricase (treatment) and served as their own controls (without uricase supplementation). Oral uricase therapy significantly decreased plasma uric acid concentrations in pigs with CKD, whereas hyperuricemia was observed in the pigs whilst not being treated with uricase. Urinary uric acid excretion was similar during both the treatment and control periods during the first 8 h and 24 h after UA infusions in the CKD pigs. To demonstrate the elimination of UA via the intestine, the healthy pigs were infused with UA into the jugular vein. The blood collected from the jugular vein represents circulating UA concentrations and the blood collected from the portal vein represents the concentration of UA leaving the intestine. The final (after 2 h) concentration of UA was significantly lower in blood collected from the portal vein compared to that collected from the jugular vein (3.34 vs. 2.43 mg/dL, respectively, p = 0.024). The latter allows us to suggest that UA is eliminated from the blood via the gut tissue.

[Effects of Clostridium butyricum on serum uric acid and inflammatory mediators in rats with hyperuricemia]

OBJECTIVE

To investigate the effects of intragastric administration of Clostridium butyricum in regulating serum uric acid, lipopolysaccharides (LPS), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) in rats with hyperuricemia rats.

METHODS

Forty SD rats were randomized into 4 equal groups, namely the normal control group, hyperuricemia model group, benzbromarone (3 mg/kg daily) intervention group and live Clostridium butyricum group (1.5×10⁷ CFU/day). Except for those in the control group, the rats were subjected to intragastric administration of yeast extract and oteracil potassium once daily for 12 weeks to induce hyperuricemia with corresponding treatments. The changes in serum uric acid, lipopolysaccharides , IL-6 and TNF-α in each group were detected.

RESULTS

The serum level of uric acid was significantly higher in rats fed with high-purine diet than in the control rats (P<0.01), demonstrating the successful establishment of hyperuricemia models. In rats with hyperuricemia, serum uric acid level was positively correlated with the levels of LPS, IL-6 and TNF-α, and their serum levels decreased significantly and progressively with time in Benzbromarone group and Clostridium butyricum group. Benzbromarone was more effective in decreasing serum uric acid in the rats, while Clostridium butyricum produced a stronger effect in down-regulating the inflammatory mediators.

CONCLUSION

Chronic inflammatory reaction exists in rats with hyperuricemia. Intragastric administration of Clostridium butyricum can effectively decrease serum uric acid level and inhibit the inflammatory cytokines, and thus contributes to immune homeostasis in the intestines.

Fructose Intake, Serum Uric Acid, and Cardiometabolic Disorders: A Critical Review

There is a direct relationship between fructose intake and serum levels of uric acid (UA), which is the final product of purine metabolism. Recent preclinical and clinical evidence suggests that chronic hyperuricemia is an independent risk factor for hypertension, metabolic syndrome, and cardiovascular disease. It is probably also an independent risk factor for chronic kidney disease, Type 2 diabetes, and cognitive decline. These relationships have been observed for high serum UA levels (>5.5 mg/dL in women and >6 mg/dL in men), but also for normal to high serum UA levels (5-6 mg/dL). In this regard, blood UA levels are much higher in industrialized countries than in the rest of the world. Xanthine-oxidase inhibitors can reduce UA and seem to minimize its negative effects on vascular health. Other dietary and pathophysiological factors are also related to UA production. However, the role of fructose-derived UA in the pathogenesis of cardiometabolic disorders has not yet been fully clarified. Here, we critically review recent research on the biochemistry of UA production, the relationship between fructose intake and UA production, and how this relationship is linked to cardiometabolic disorders.

Combined Signature of the Fecal Microbiome and Metabolome in Patients with Gout

This study employed microbiome and metabolome analysis to explore the fecal signatures of gout patients. Fecal samples from 52 male individuals (26 healthy controls and 26 gout patients) were analyzed by ¹H NMR spectroscopy and Illumina Miseq sequencing. The signatures of microbiome showed being up-regulation of opportunistic pathogens, such as Bacteroides, Porphyromonadaceae Rhodococcus, Erysipelatoclostridium and Anaerolineaceae. The signatures of metabolome were some altered metabolites which may involve uric acid excretion, purine metabolism, and inflammatory responses. Meanwhile, the correlation between discrepant metabolites and microbial taxa indicated that they could be the combined signatures of gout. This study suggests that the combined analysis of the fecal microbiome and metabolome may effectively characterize diseases.

Serum Uric Acid and the Risk of Incident and Recurrent Gout: A Systematic Review

OBJECTIVE

Lowering serum uric acid (SUA) levels can essentially cure gout; however, this is not widely practiced. To summarize epidemiologic evidence related to this causal link, we conducted a systematic review of the published literature reporting the association between SUA level and incident and recurrent gout (i.e., gout flares).

METHODS

We systematically searched Medline, EMBASE, and the Cochrane Database of Systematic Reviews using separate search strategies for incident gout and recurrent gout. We screened 646 abstracts to identify 8 eligible articles reporting gout incidence and 913 abstracts to identify 18 articles reporting recurrent gout.

RESULTS

For both gout incidence and recurrence, a graded trend was observed where the risk was increased with higher SUA levels. Gout incidence rates per 1000 person-years from population-based studies ranged from 0.8 (SUA ≤ 6 mg/dl) to 70.2 cases (SUA ≥ 10 mg/dl). Recurrent gout risk in clinical cohorts ranged from 12% (SUA ≤ 6 mg/dl) to 61% (SUA ≥ 9 mg/dl) among those receiving urate-lowering therapy (ULT), and 3.7% (SUA 6-7 mg/dl) to 61% (SUA > 9.3 mg/dl) after successful ULT. Retrospective database studies also showed a graded relationship, although the strength of the association was weaker. Studies reporting mean flares or time-to-flare according to SUA showed similar findings.

CONCLUSION

This systematic review confirms that higher SUA levels are associated with increased risk of incident and recurrent gout in a graded manner. Although few prospective cohorts have evaluated incident and recurrent gout according to SUA, the existing evidence underscores the need to treat to SUA targets, as recommended by the American College of Rheumatology and the European League Against Rheumatism.

Uric acid transporters hiding in the intestine

CONTEXT

Hyperuricaemia is known as an abnormally increased uric acid level in the blood. Although it was observed many years ago, since uric acid excretion via the intestine pathway accounted for approximately one-third of total elimination of uric acid, the molecular mechanism of 'extra-renal excretion' was poorly understood until the finding of uric acid transporters.

OBJECTIVE

The objective of this study was to gather all information related to uric acid transporters in the intestine and present this information as a comprehensive and systematic review article.

METHODS

A literature search was performed from various databases (e.g., Medline, Science Direct, Springer Link, etc.). The key terms included uric acid, transporter and intestine. The period for the search is from the 1950s to the present. The bibliographies of papers relating to the review subject were also searched for further relevant references.

RESULTS

The uric acid transporters identified in the intestine are discussed in this review. The solute carrier (SLC) transporters include GLUT9, MCT9, NPT4, NPT homolog (NPT5) and OAT10. The ATP binding cassette (ABC) transporters include ABCG2 (BCRP), MRP2 and MRP4. Bacterial transporter YgfU is a low-affinity and high-capacity transporter for uric acid.

CONCLUSION

The present review may be helpful for further our understanding of hyperuricaemia and be of value in designing future studies on novel therapeutic pathways.

Fructose suppresses uric acid excretion to the intestinal lumen as a result of the induction of oxidative stress by NADPH oxidase activation

BACKGROUND

A high intake of fructose increases the risk for hyperuricemia. It has been reported that long-term fructose consumption suppressed renal uric acid excretion and increased serum uric acid level. However, the effect of single administration of fructose on excretion of uric acid has not been clarified.

METHODS

We used male Wistar rats, which were orally administered fructose (5g/kg). Those rats were used in each experiment at 12h after administration.

RESULTS

Single administration of fructose suppressed the function of ileal uric acid excretion and had no effect on the function of renal uric acid excretion. Breast cancer resistance protein (BCRP) predominantly contributes to intestinal excretion of uric acid as an active homodimer. Single administration of fructose decreased BCRP homodimer level in the ileum. Moreover, diphenyleneiodonium (DPI), an inhibitor of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox), recovered the suppression of the function of ileal uric acid excretion and the Bcrp homodimer level in the ileum of rats that received single administration of fructose.

CONCLUSIONS

Single administration of fructose decreases in BCRP homodimer level, resulting in the suppression the function of ileal uric acid excretion. The suppression of the function of ileal uric acid excretion by single administration of fructose is caused by the activation of Nox. The results of our study provide a new insight into the mechanism of fructose-induced hyperuricemia.

Hyperuricemia, the kidneys, and the spectrum of associated diseases: a narrative review

Hyperuricemia (elevated serum uric acid) is prevalent, and an important mediator of gout, an increasingly common condition. In addition, hyperuricemia is associated with metabolic syndrome, diabetes, hypertension, and kidney and cardiovascular diseases. Although it remains controversial whether hyperuricemia is a causal factor for kidney disease, the kidneys play a major role in the regulation of serum uric acid levels. Approximately two-thirds of the uric acid produced in humans is excreted by the kidneys. The handling of urate in the renal proximal tubule is extensive, as uric acid undergoes filtration, reabsorption, and secretion. Variations in renal urate handling have been shown to influence the risk of gout. In observational studies, hyperuricemia has been shown to predict kidney disease onset and progression, with a variety of mechanisms implicated. Because of this close association between hyperuricemia and kidney disease, and due to limited studies on the topic, it is important to conduct future studies on the treatment of hyperuricemia to slow kidney disease progression and improve cardiovascular survival in patients with chronic kidney disease. Furthermore, it is important to monitor for gout in patients with kidney disease and to follow the guidelines for treatment of hyperuricemia in this group of patients. This narrative review provides an in-depth discussion of the link between serum uric acid levels, renal handling of uric acid, and diseases associated with dysfunction in uric acid homeostasis.

Hyperuricemia in acute gastroenteritis is caused by decreased urate excretion via ABCG2

To clarify the physiological and pathophysiological roles of intestinal urate excretion via ABCG2 in humans, we genotyped ABCG2 dysfunctional common variants, Q126X (rs72552713) and Q141K (rs2231142), in end-stage renal disease (hemodialysis) and acute gastroenteritis patients, respectively. ABCG2 dysfunction markedly increased serum uric acid (SUA) levels in 106 hemodialysis patients (P = 1.1 × 10(-4)), which demonstrated the physiological role of ABCG2 for intestinal urate excretion because their urate excretion almost depends on intestinal excretion via ABCG2. Also, ABCG2 dysfunction significantly elevated SUA in 67 acute gastroenteritis patients (P = 6.3 × 10(-3)) regardless of the degree of dehydration, which demonstrated the pathophysiological role of ABCG2 in acute gastroenteritis. These findings for the first time show ABCG2-mediated intestinal urate excretion in humans, and indicates the physiological and pathophysiological importance of intestinal epithelium as an excretion pathway besides an absorption pathway. Furthermore, increased SUA could be a useful marker not only for dehydration but also epithelial impairment of intestine.

SIRT1 prevents hyperuricemia via the PGC-1α/PPARγ-ABCG2 pathway

Silent information regulator T1 (SIRT1) plays several key roles in the regulation of lipid and glucose homoeostasis. In this study, we investigated the potential role of SIRT1 in hyperuricemia and explored possible mechanisms. Significant hyperuricemia was detected in C57BL/6 mice treated with oxonate and yeast polysaccharide. Resveratrol (RSV), a specific SIRT1 activator, was administered to the mice. SIRT1 suppressed the increased serum uric acid level but up-regulated the expression of urate transporter ATP-binding cassette subfamily G member 2 (ABCG2) in the ileum of hyperuricemic mice. In a human colon carcinoma cell line, SIRT1 promoted ABCG2 production through the deacetylation of peroxisome proliferator-activated receptor (PPAR) γ co-activator 1α (PGC-1α), which then activated the effectors of PPARγ. Interestingly, the SIRT1-induced up-regulation of ABCG2 was significantly inhibited when PGC-1α or PPARγ was blocked by siRNA transfection. Our data demonstrated that SIRT1 and its activator, RSV, have clear anti-hyperuricemia functions in this mouse model. One possible mechanism is the activation of ABCG2 in the ileum through the PGC-1α/PPARγ pathway.

Time to target uric acid to retard CKD progression

Uric acid (UA) remains a possible risk factor of chronic kidney disease (CKD) but its potential role should be elucidated given a fact that multidisciplinary treatments assure a sole strategy to inhibit the progression to end-stage renal disease (ESRD). In clinical setting, most observational studies showed that elevation of serum uric acid (SUA) independently predicts the incidence and the development of CKD. The meta-analysis showed that SUA-lowering therapy with allopurinol may retard the progression of CKD but did not reach conclusive results due to small-sized studies. Larger scale, randomized placebo-controlled trials to assess SUA-lowering therapy are needed. Our recent analysis by propensity score methods has shown that the threshold of SUA should be less than 6.5 mg/dL to abrogate ESRD. In animal models an increase in SUA by the administration of oxonic acid, uricase inhibitor, or nephrectomy can induce glomerular hypertension, arteriolosclerosis including afferent arteriolopathy and tubulointerstitial fibrosis. The ever-growing discoveries of urate transporters prompt us to learn UA metabolism in the kidney and intestine. One example is that the intestinal ABCG2 may play a compensatory role at face of decreased renal clearance of UA in nephrectomized rats, the trigger of which is not a uremic toxin but SUA itself. This review will summarize the recent knowledge on the relationship between SUA and the kidney and try to draw a conclusion when and how to treat asymptomatic hyperuricemia accompanied by CKD. Finally we will address a future perspective on UA study including a Mendelian randomization approach.

Analysis of ABCG2 and other urate transporters in uric acid homeostasis in chronic kidney disease: potential role of remote sensing and signaling

Background

In the setting of chronic kidney disease (CKD), altered extra-renal urate handling may be necessary to regulate plasma uric acid. The Remote Sensing and Signaling Hypothesis (Nigam S. What do drug transporters really do? Nat Rev Drug Discov 2015; 14: 29–44) suggests that multispecific solute carrier (SLC) and ATP-binding cassette (ABC) drug transporters in different tissues are part of an inter-organ communication system that maintains levels of urate and other metabolites after organ injury.

Methods

Data from the Chronic Renal Insufficiency Cohort (CRIC; n = 3598) were used to study associations between serum uric acid and single nucleotide polymorphisms (SNPs) on the following uric acid transporters: ABCG2 (BRCP), SLC22A6 (OAT1), SLC22A8 (OAT3), SLC22A10 (OAT5), SLC22A11 (OAT4), SLC22A12 (URAT1), SLC22A13 (OAT10), SLC17A1-A3 (NPTs), SLC2A9 (GLUT9), ABCC2 (MRP2) and ABCC4 (MRP4). Regression models, controlling for principal components age, gender and renal function, were run separately for those of European (EA) and African ancestry (AA), and P-values corrected for multiple comparisons. A twin cohort with participants of EA and normal renal function was used for comparison.

Results

Among those of EA in CRIC, statistically significant signals were observed for SNPs in ABCG2 (rs4148157; beta-coefficient = 0.68; P = 4.78E-13) and SNPs in SLC2A9 (rs13125646; beta-coefficient = −0.30; P = 1.06E-5). Among those of AA, the strongest (but not statistically significant) signals were observed for SNPs in SLC2A9, followed by SNPs in ABCG2. In the twin study (normal renal function), only SNPs in SLC2A9 were significant (rs4481233; beta-coefficient=−0.45; P = 7.0E-6). In CRIC, weaker associations were also found for SLC17A3 (NPT4) and gender-specific associations found for SLC22A8 (OAT3), SLC22A11 (OAT4), and ABCC4 (MRP4).

Conclusions

In patients of EA with CKD (CRIC cohort), we found striking associations between uric acid and SNPs on ABCG2, a key transporter of uric acid by intestine. Compared with ABCG2, SLC2A9 played a much less significant role in this subset of patients with CKD. SNPs in other SLC (e.g. SLC22A8 or OAT3) and ABC (e.g. ABCC4 or MRP4) genes appear to make a weak gender-dependent contribution to uric acid homeostasis in CKD. As renal urate transport is affected in the setting of declining kidney function, extra-renal ABCG2 appears to play a compensatory role—a notion consistent with animal studies and the Remote Sensing and Signaling Hypothesis. Overall, the data indicate how different urate transporters become more or less important depending on renal function, ethnicity and gender. Therapies focused on enhancing ABCG2 urate handling may be helpful in the setting of CKD and hyperuricemia.

Intestinal Microbiota Distinguish Gout Patients from Healthy Humans

Current blood-based approach for gout diagnosis can be of low sensitivity and hysteretic. Here via a 68-member cohort of 33 healthy and 35 diseased individuals, we reported that the intestinal microbiota of gout patients are highly distinct from healthy individuals in both organismal and functional structures. In gout, Bacteroides caccae and Bacteroides xylanisolvens are enriched yet Faecalibacterium prausnitzii and Bifidobacterium pseudocatenulatum depleted. The established reference microbial gene catalogue for gout revealed disorder in purine degradation and butyric acid biosynthesis in gout patients. In an additional 15-member validation-group, a diagnosis model via 17 gout-associated bacteria reached 88.9% accuracy, higher than the blood-uric-acid based approach. Intestinal microbiota of gout are more similar to those of type-2 diabetes than to liver cirrhosis, whereas depletion of Faecalibacterium prausnitzii and reduced butyrate biosynthesis are shared in each of the metabolic syndromes. Thus the Microbial Index of Gout was proposed as a novel, sensitive and non-invasive strategy for diagnosing gout via fecal microbiota.

Reactive oxygen species derived from xanthine oxidase interrupt dimerization of breast cancer resistance protein, resulting in suppression of uric acid excretion to the intestinal lumen

The prevalence of hyperuricemia/gout increases with aging. However, the effect of aging on function for excretion of uric acid to out of the body has not been clarified. We found that ileal uric acid clearance in middle-aged rats (11-12 months) was decreased compared with that in young rats (2 months). In middle-aged rats, xanthine oxidase (XO) activity in the ileum was significantly higher than that in young rats. Inosine-induced reactive oxygen species (ROS), which are derived from XO, also decreased ileal uric acid clearance. ROS derived from XO decreased the active homodimer level of breast cancer resistance protein (BCRP), which is a uric acid efflux transporter, in the ileum. Pre-administration of allopurinol recovered the BCRP homodimer level, resulting in the recovering ileal uric acid clearance. Moreover, we investigated the effects of ROS derived from XO on BCRP homodimer level directly in Caco-2 cells using hypoxanthine. Treatment with hypoxanthine decreased BCRP homodimer level. Treatment with hypoxanthine induced mitochondrial dysfunction, suggesting that the decreasing BCRP homodimer level might be caused by mitochondrial dysfunction. In conclusion, ROS derived from XO decrease BCRP homodimer level, resulting in suppression of function for uric acid excretion to the ileal lumen. ROS derived from XO may cause the suppression of function of the ileum for the excretion of uric acid with aging. The results of our study provide a new insight into the causes of increasing hyperuricemia/gout prevalence with aging.

Genome-wide association study of clinically defined gout identifies multiple risk loci and its association with clinical subtypes

OBJECTIVE

Gout, caused by hyperuricaemia, is a multifactorial disease. Although genome-wide association studies (GWASs) of gout have been reported, they included self-reported gout cases in which clinical information was insufficient. Therefore, the relationship between genetic variation and clinical subtypes of gout remains unclear. Here, we first performed a GWAS of clinically defined gout cases only.

METHODS

A GWAS was conducted with 945 patients with clinically defined gout and 1213 controls in a Japanese male population, followed by replication study of 1048 clinically defined cases and 1334 controls.

RESULTS

Five gout susceptibility loci were identified at the genome-wide significance level (p<5.0×10(-8)), which contained well-known urate transporter genes (ABCG2 and SLC2A9) and additional genes: rs1260326 (p=1.9×10(-12); OR=1.36) of GCKR (a gene for glucose and lipid metabolism), rs2188380 (p=1.6×10(-23); OR=1.75) of MYL2-CUX2 (genes associated with cholesterol and diabetes mellitus) and rs4073582 (p=6.4×10(-9); OR=1.66) of CNIH-2 (a gene for regulation of glutamate signalling). The latter two are identified as novel gout loci. Furthermore, among the identified single-nucleotide polymorphisms (SNPs), we demonstrated that the SNPs of ABCG2 and SLC2A9 were differentially associated with types of gout and clinical parameters underlying specific subtypes (renal underexcretion type and renal overload type). The effect of the risk allele of each SNP on clinical parameters showed significant linear relationships with the ratio of the case-control ORs for two distinct types of gout (r=0.96 [p=4.8×10(-4)] for urate clearance and r=0.96 [p=5.0×10(-4)] for urinary urate excretion).

CONCLUSIONS

Our findings provide clues to better understand the pathogenesis of gout and will be useful for development of companion diagnostics.

ABCG2 variant has opposing effects on onset ages of Parkinson's disease and gout

Uric acid (urate) has been suggested to play a protective role in Parkinson's disease onset through its antioxidant activity. Dysfunction of ABCG2, a high-capacity urate exporter, is a major cause for early-onset gout based on hyperuricemia. In this study, the effects of a dysfunctional ABCG2 variant (Q141K, rs2231142) were analyzed on the ages at onset of gout patients (N = 507) and Parkinson's disease patients (N = 1015). The Q141K variant hastened the gout onset (P = 0.0027), but significantly associated with later Parkinson's disease onset (P = 0.025). Our findings will be helpful for development of more effective prevention of Parkinson's disease.

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

OBJECTIVES

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.

METHODS

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.

KEY FINDINGS

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%.

CONCLUSION

Intestinal urate secretion may be increased at CKD state partially by upregulation of intestinal BCRP by uremic toxins such as IS.