ABCG2 dysfunction increases the risk of renal overload hyperuricemia

Signaling pathways in uric acid homeostasis and gout: From pathogenesis to therapeutic interventions

Uric acid is a product of purine degradation, and uric acid may have multiple physiologic roles, including the beneficial effects as an antioxidant and neuroprotector, maintenance of blood pressure during low salt ingestion, and modulation of immunity. However, overproduction of metabolic uric acid, and/or imbalance of renal uric acid secretion and reabsorption, and/or underexcretion of extrarenal uric acid, e.g. gut, will contribute to hyperuricemia, which is a common metabolic disease. Long-lasting hyperuricemia can induce the formation and deposition of monosodium urate (MSU) crystals within the joints and periarticular structures. MSU crystals further induce an acute, intensely painful, and sterile inflammation conditions named as gout by NLRP3 inflammasome-mediated cleavage of pro-IL-1β to bioactive IL-1β. Moreover, hyperuricemia and gout are associated with multiple cardiovascular and renal disorders, e.g., hypertension, myocardial infarction, stroke, obesity, hyperlipidemia, type 2 diabetes mellitus and chronic kidney disease. Although great efforts have been made by scientists of modern medicine, however, modern therapeutic strategies with a single target are difficult to exert long-term positive effects, and even some of these agents have severe adverse effects. The Chinese have used the ancient classic prescriptions of traditional Chinese medicine (TCM) to treat metabolic diseases, including gout, by multiple targets, for more than 2200 years. In this review, we discuss the current understanding of urate homeostasis, the pathogenesis of hyperuricemia and gout, and both modern medicine and TCM strategies for this commonly metabolic disorder. We hope these will provide the good references for treating hyperuricemia and gout.

Early-onset gout

Early-onset gout (EOG) is characterized by the occurrence of the first symptoms of gout at an unusually young age, usually <40 years. The aim of this review is to provide an overview of the epidemiology, clinical presentation and prognosis, association with comorbidities and specific management of EOG. A particularly high proportion of patients with EOG come from ethnic groups with stronger genetic factors, such as populations in the Pacific and Taiwan, who therefore have the highest prevalence of gout overall. The clinical presentation and severity of gout are broadly similar between EOG and common gout, although a longer disease duration exacerbates the disease, which more often tends to become polyarticular. Patients suffering from EOG develop metabolic comorbidities commonly associated with gout earlier in life, although those tend to be less frequent at the time of diagnosis. Some international guidelines recommend early treatment of EOG patients with urate-lowering therapies.

New insight into the management of renal excretion and hyperuricemia: Potential therapeutic strategies with natural bioactive compounds

Hyperuricemia is the result of increased production and/or underexcretion of uric acid. Hyperuricemia has been epidemiologically associated with multiple comorbidities, including metabolic syndrome, gout with long-term systemic inflammation, chronic kidney disease, urolithiasis, cardiovascular disease, hypertension, rheumatoid arthritis, dyslipidemia, diabetes/insulin resistance and increased oxidative stress. Dysregulation of xanthine oxidoreductase (XOD), the enzyme that catalyzes uric acid biosynthesis primarily in the liver, and urate transporters that reabsorb urate in the renal proximal tubules (URAT1, GLUT9, OAT4 and OAT10) and secrete urate (ABCG2, OAT1, OAT3, NPT1, and NPT4) in the renal tubules and intestine, is a major cause of hyperuricemia, along with variations in the genes encoding these proteins. The first-line therapeutic drugs used to lower serum uric acid levels include XOD inhibitors that limit uric acid biosynthesis and uricosurics that decrease urate reabsorption in the renal proximal tubules and increase urate excretion into the urine and intestine via urate transporters. However, long-term use of high doses of these drugs induces acute kidney disease, chronic kidney disease and liver toxicity. Therefore, there is an urgent need for new nephroprotective drugs with improved safety profiles and tolerance. The current systematic review summarizes the characteristics of major urate transporters, the mechanisms underlying the pathogenesis of hyperuricemia, and the regulation of uric acid biosynthesis and transport. Most importantly, this review highlights the potential mechanisms of action of some naturally occurring bioactive compounds with antihyperuricemic and nephroprotective potential isolated from various medicinal plants.

Examining an Association of Single Nucleotide Polymorphisms with Hyperuricemia in Chinese Flight Attendants

Background

Both genetic and environmental factors strongly affect serum uric acid (SUA) concentrations. The incidence of hyperuricemia tends to be younger in the Chinese population. In particular, we have found a high prevalence of hyperuricemia among Chinese flight attendants, aged from 20 to 40, in our survey. This study aims to evaluate whether there is an association between gene polymorphisms and hyperuricemia among Chinese flight attendants.

Methods

A total of 532 flight attendants with high and normal serum uric acid levels were recruited. Allele-specific polymerase chain reaction (AS-PCR) was performed using blood samples of enrolled subjects.

Results

Previous studies have reported single nucleotide polymorphisms (SNPs) that are tightly associated with uric acid levels. Among them, six SNPs that are strongly associated with SUA or gout in Asians, for instance ABCG2 (rs2231142, rs72552713 and rs2231137), GCKR (rs780094), SLC2A9 (rs1014290) and SLC17A1 (rs1183201), were selected for AS-PCR analyses. We found that SNPs such as ABCG2 rs2231142, GCKR rs780094 and SLC2A9 rs1014290 are strongly associated with hyperuricemia in male flight attendants, and SLC2A9 rs1014290 among female flight attendants.

Conclusion

Our study provides evidences of an association between SNPs and hyperuricemia in the Chinese flight attendants, and highlights the significance of improving diagnostics and prevention of disease development in uric acid metabolism disorders and gout using these SNPs.

A machine learning-assisted model for renal urate underexcretion with genetic and clinical variables among Chinese men with gout

OBJECTIVES

The objective of this study was to develop and validate a prediction model for renal urate underexcretion (RUE) in male gout patients.

METHODS

Men with gout enrolled from multicenter cohorts in China were analyzed as the development and validation data sets. The RUE phenotype was defined as fractional excretion of uric acid (FE_UA) <5.5%. Candidate genetic and clinical features were screened by the least absolute shrinkage and selection operator (LASSO) with 10-fold cross-validation. Machine learning algorithms (stochastic gradient descent (SGD), logistic regression, support vector machine) were performed to construct a predictive classifier of RUE. Models were assessed by the area under the receiver operating characteristic curve (AUC) and the precision-recall curve (PRC).

RESULTS

One thousand two hundred thirty-eight and two thousand twenty-three patients were enrolled as the development and validation cohorts, with 1220 and 754 randomly chosen patients genotyped, respectively. Rs3775948.GG of SLC2A9/GLUT9, rs504915.AA of NRXN2/URAT1, and 7 clinical features (age, hypertension, nephrolithiasis, blood glucose, serum urate, urea nitrogen, and creatinine) were generated by LASSO. Two additional SNP variants (rs2231142.GG of ABCG2 and rs11231463.GG of SLC22A9/OAT7) were selected based on their contributions to gout in the development cohort and their reported effects on renal urate handling. The optimized classifiers yielded AUCs of ~0.914 and PRCs of ~0.980 using these 11 variables. The SGD model was conducted in the validation cohort with an AUC of 0.899 and the PRC of 0.957.

CONCLUSIONS

A prediction model for RUE composed of four SNPs and readily accessible clinical features was established with acceptable accuracy for men with gout.

An update of genetics, co-morbidities and management of hyperuricaemia

Hyperuricaemia (HU) caused by disorders of purine metabolism is a metabolic disease. A number of epidemiological reports have confirmed that HU is correlated with multiple disorders, such as chronic kidney diseases, cardiovascular disease and gout. Recent studies showed that the expression and functional changes of uric acid transporters, including URAT1, GLUT9 and ABCG2, were associated with HU. Moreover, a large number of genome-wide association studies have shown that these transporters' dysfunction leads to HU. In this review, we describe the recent progress of aetiology and related transporters of HU, and we also summarise the common co-morbidities possible mechanisms, as well as the potential pharmacological and non-pharmacological treatment methods for HU, aiming to provide new ideas for the treatment of HU.

The Role of ABCG2 in the Pathogenesis of Primary Hyperuricemia and Gout-An Update

Urate homeostasis in humans is a complex and highly heritable process that involves i.e., metabolic urate biosynthesis, renal urate reabsorption, as well as renal and extrarenal urate excretion. Importantly, disturbances in urate excretion are a common cause of hyperuricemia and gout. The majority of urate is eliminated by glomerular filtration in the kidney followed by an, as yet, not fully elucidated interplay of multiple transporters involved in the reabsorption or excretion of urate in the succeeding segments of the nephron. In this context, genome-wide association studies and subsequent functional analyses have identified the ATP-binding cassette (ABC) transporter ABCG2 as an important urate transporter and have highlighted the role of single nucleotide polymorphisms (SNPs) in the pathogenesis of reduced cellular urate efflux, hyperuricemia, and early-onset gout. Recent publications also suggest that ABCG2 is particularly involved in intestinal urate elimination and thus may represent an interesting new target for pharmacotherapeutic intervention in hyperuricemia and gout. In this review, we specifically address the involvement of ABCG2 in renal and extrarenal urate elimination. In addition, we will shed light on newly identified polymorphisms in ABCG2 associated with early-onset gout.

Pharmacologic inducers of the uric acid exporter ABCG2 as potential drugs for treatment of gouty arthritis

Uric acid is the end product of purine catabolism and its plasma levels are maintained below its maximum solubility in water (6–7 mg/dl). The plasma levels are tightly regulated as the balance between the rate of production and the rate of excretion, the latter occurring in urine (kidney), bile (liver) and feces (intestinal tract). Reabsorption in kidney is also an important component of this process. Both excretion and reabsorption are mediated by specific transporters. Disruption of the balance between production and excretion leads to hyperuricemia, which increases the risk of uric acid crystallization as monosodium urate with subsequent deposition of the crystals in joints causing gouty arthritis. Loss-of-function mutations in the transporters that mediate uric acid excretion are associated with gout. The ATP-Binding Cassette exporter ABCG2 is important in uric acid excretion at all three sites: kidney (urine), liver (bile), and intestine (feces). Mutations in this transporter cause gout and these mutations occur at significant prevalence in general population. However, mutations that are most prevalent result only in partial loss of transport function. Therefore, if the expression of these partially defective transporters could be induced, the increased number of the transporter molecules would compensate for the mutation-associated decrease in transport function and hence increase uric acid excretion. As such, pharmacologic agents with ability to induce the expression of ABCG2 represent potentially a novel class of drugs for treatment of gouty arthritis.

The Paradoxical Role of Uric Acid in Osteoporosis

Because of its high prevalence worldwide, osteoporosis is considered a serious public health concern. Many known risk factors for developing osteoporosis have been identified and are crucial if planning health care needs. Recently, an association between uric acid (UA) and bone fractures had been explored. Extracellular UA exhibits antioxidant properties by effectively scavenging free radicals in human plasma, but this benefit might be disturbed by the hydrophobic lipid layer of the cell membrane. In contrast, intracellular free oxygen radicals are produced during UA degradation, and superoxide is further enhanced by interacting with NADPH oxidase. This intracellular oxidative stress, together with inflammatory cytokines induced by UA, stimulates osteoclast bone resorption and inhibits osteoblast bone formation. UA also inhibits vitamin D production and thereby results in hyper-parathyroidism, which causes less UA excretion in the intestines and renal proximal tubules by inhibiting the urate transporter ATP-binding cassette subfamily G member 2 (ABCG2). At normal or high levels, UA is associated with a reduction in bone mineral density and protects against bone fracture. However, in hyperuricemia or gout arthritis, UA increases bone fracture risk because oxidative stress and inflammatory cytokines can increase bone resorption and decrease bone formation. Vitamin D deficiency, and consequent secondary hyperparathyroidism, can further increase bone resorption and aggravated bone loss in UA-induced osteoporosis.

The Prevalence of Hyperuricemia Sharply Increases from the Late Menopausal Transition Stage in Middle-Aged Women

The impact of menopausal transition on change of serum uric acid level remains unknown. The present study evaluated the relationship of menopausal stages with prevalent hyperuricemia in middle-aged women. This cross-sectional study included 58,870 middle-aged Korean women, aged ≥40, who participated in a health examination from 2014 to 2016. Menopausal stages were obtained with a standardized, self-administered questionnaire and were categorized according to the criteria of the Stages of Reproductive Aging Workshop (STRAW+10). Hyperuricemia was defined as a serum uric acid level of ≥6 mg/dL. The prevalence of hyperuricemia increased as menopausal stage increased. The multivariable-adjusted odds ratios (95% confidence intervals) for prevalent hyperuricemia comparing early transition, late transition, and post-menopause to pre-menopause were 1.19 (0.80–1.77), 2.13 (1.35–3.36), and 1.65 (1.33–2.04), respectively. This association was stronger among non-obese compared to obese participants and in those with low high-sensitivity C-reactive protein (hsCRP) levels (<1.0 mg/L) compared to those with elevated hsCRP levels of ≥1.0 mg/L (p for interaction = 0.01). In this large sample of middle-aged women, the prevalence of hyperuricemia significantly increased from the menopausal stage of late transition, independent of potential confounders. Appropriate preventive strategies for reducing hyperuricemia and its related consequences should be initiated prior to menopause.

IL-1β functionally attenuates ABCG2 and PDZK1 expression in HK-2 cells partially through NF-ĸB activation

Long-standing untreated hyperuricemia could lead to gout. Several recent studies have demonstrated a significant decrease of serum urate during acute gout attack, which is an aseptic inflammation process focusing on IL-1β. However, how IL-1β, by itself, alters the expression and the functional activity of urate transporters in renal tubular epithelial cells is still unclear. Herein, we revealed that IL-1β could attenuate the mRNA and protein levels of ABCG2, a major urate efflux pump, in HK-2 cells by real-time PCR and Western-blot assays. Moreover, using an ABCG2 specific inhibitor and a new sensitive and specific detection system, it was found that IL-1β also reduced the ABCG2 transporter activities. Incubation with specific inhibitors of the NF-κB pathway partly dampened the inhibitory effect of IL-1β on ABCG2, indicating that IL-1β reduced the ABCG2 expression partially through the NF-ĸB pathway. Furthermore, the decreased expression of PDZK1 induced by IL-1β, which is dependent on the NF-κB pathway, could account for the imbalance between the functions and expressions of ABCG2 on this status. These findings demonstrated a new role for IL-1β, whereby it leads to the inhibition of ABCG2 in renal tubular epithelial cells; this new role probably does not encompass its involvement in the process of renal urate excretion mediated by inflammation. Therefore, other regulation mechanisms of urate reabsorption in renal tubular epithelial cells deserve to be examined in further studies.

The flavonoid-rich fraction from rhizomes of Smilax glabra Roxb. ameliorates renal oxidative stress and inflammation in uric acid nephropathy rats through promoting uric acid excretion

Uric acid metabolic disorder is considered to be the main pathogenesis of uric acid nephropathy (UN). Smilax glabra Roxb. is a traditional Chinese herb which has been used in the treatment of gout, but the mechanism was unclear. In this study, we investigated the protective effects of the flavonoid-rich fraction from rhizomes of Smilax glabra Roxb. (SGF) on uric acid nephropathy rats and its underlying mechanisms of promoting uric acid excretion. Sprague Dawley (SD) rats were induced by high purine diet (yeast pellets + adenine) for 5 weeks. Rats were orally treated with SGF or allopurinol daily. The biochemical parameters and enzymes in different treated rats were determined by commercial kits. Kidney pathology was visualized using optical microscopy and electron microscopy. Renal inflammatory factors were detected by ELISA. Renal fibrosis factors and uric acid transporters were analyzed by real time RT-PCR and western blot. The results showed that SGF significantly improved kidney function. Histopathologic examination revealed that urate-induced renal damage was markedly reversed by SGF. Meanwhile, SGF treatment was also found to significantly inhibit renal oxidative stress. SGF treatment obviously suppressed the inflammatory factors of interleukin-6 (IL-6), tumor necrosis factor alpha (TNF-α), interleukin-1β (IL-1β), cyclooxygenase-2 (COX-2) and the profibrotic factors of basic fibroblast growth factor (bFGF), transforming growth factor-β₁ (TGF-β₁) expression in UN rats. Moreover, SGF either significantly inhibited uric acid production or promoted uric acid excretion in UN rats. The mechanism of SGF promoting uric acid excretion was related to its increase of ATP-binding cassette transporter G2 (ABCG2), organic anion transporter 1 (OAT1), organic anion transporters 2 (OCT2) and organic cation/carnitine transporters 2 (OCTN2) expression. In conclusion, SGF could ameliorate renal oxidative stress and inflammation in UN rats through promoting uric acid excretion.

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.

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.

ABCG2 contributes to the development of gout and hyperuricemia in a genome-wide association study

Although many genome-wide association studies (GWASs) of hyperuricemia or gout have been reported, the related genetic factors and the mechanisms from hyperuricemia to gouty attack remain unclear. This study aimed to identify genetic factors and pathogenesis of gout from hyperuricemia by genome-wide association study (GWAS). 747 gout patients, 747 hyperuricemia and 2071 age-matched controls were recruited and analyzed with Affymetrix 650 K chip to find the related genetic variants. The functions of the related genes were investigated in an endothelial cell (EC) with urate crystal stimulation. The GWAS results showed 36 SNPs to be strongly associated with gout compared to controls (all p-values < 10⁻⁷). Whereas the rs2231142 in ABCG2 gene had significant associations between gout and controls, between gout and hyperuricemia, and between hyperuricemia and controls (all p-values < 10⁻⁷), and the ORs were 4.34, 3.37 and 2.15 (all p-values < 0.001) after adjustment of potential confounders, respectively. The cell model showed significantly higher IL-8 release from EC combined with ABCG2 knockdown. We concluded that ABCG2 gene contributed to hyperuricemia but also gout, and that it was involved in the inflammation dysregulation via augmented IL-8 release in EC.

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).

Reverse Translational Research of ABCG2 (BCRP) in Human Disease and Drug Response

Reverse translational research takes a bedside-to-bench approach, using sophisticated basic research to explain the biological mechanisms behind observed clinical data. For transporters, which play a role in human disease and drug response, this approach offers a distinct advantage over the typical translational research, which often falters due to inadequate in vitro and preclinical animal models. Research on ABCG2, which encodes the Breast Cancer Resistance Protein, has benefited immensely from a reverse translational approach due to its broad implications for disease susceptibility and both therapeutic and adverse drug response. In this review, we describe the success of reverse translational research for ABCG2 and opportunities for further studies.

Genetic background of uric acid metabolism in a patient with severe chronic tophaceous gout

Hyperuricemia depends on the balance of endogenous production and renal excretion of uric acid. Transporters for urate are located in the proximal tubule where uric acid is secreted and extensively reabsorbed: secretion is principally ensured by the highly variable ABCG2 gene. Enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) plays a central role in purine metabolism and its deficiency is an X-linked inherited metabolic disorder associated with clinical manifestations of purine overproduction. Here we report the case of a middle-aged man with severe chronic tophaceous gout with a poor response to allopurinol and requiring repeated surgical intervention. We identified the causal mutations in the HPRT1 gene, variant c.481G>T (p.A161S), and in the crucial urate transporter ABCG2, a heterozygous variant c.421C>A (p.Q141K). This case shows the value of an analysis of the genetic background of serum uric acid.

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.

Genetic analysis of ABCG2 and SLC2A9 gene polymorphisms in gouty arthritis in a Korean population

BACKGROUND/AIMS

Gout is a common inf lammatory arthritis triggered by the crystallization of uric acid in the joints. Serum uric acid levels are highly heritable, suggesting a strong genetic component. Independent studies to confirm the genetic associations with gout in various ethnic populations are warranted. We investigated the association of polymorphisms in the ABCG2 and SLC2A9 genes with gout in Korean patients and healthy individuals.

METHODS

We consecutively enrolled 109 patients with gout and 102 healthy controls. The diagnosis of gout was based on the preliminary criteria of the America College of Rheumatology. Genomic DNA was extracted from whole blood samples. We identified single nucleotide polymorphism (SNP) changes in the ABCG2 and SLC2A9 genes using a direct sequencing technique. rs2231142 in ABCG2 and rs6449213 and rs16890979 in SLC2A9 and nearby regions were amplified by polymerase chain reaction.

RESULTS

Patients with gout had significantly higher A/A genotype (29.3% vs. 4.9%, respectively) and A allele (52.8% vs. 26.5%, respectively) frequencies of rs2231142 in ABCG2 than did controls (χ(2) = 29.42, p < 0.001; odds ratio, 3.32; 95% confidence interval, 2.11 to 5.20). We found novel polymorphisms (c.881A>G and c.1002+78G>A) in the SLC2A9 gene. The univariate logistic regression analysis revealed that the c.881A>G and c.1002+78G>A SNPs were significantly higher in patients than in controls.

CONCLUSIONS

We demonstrated a significant association between rs2231142 in the ABCG2 gene and gout and identified novel SNPs, c.881A>G and c.1002+78G>A, in the SLC2A9 gene that may be associated with gout in a Korean population.

Proton pump inhibitors are associated with lower gastrointestinal tract bleeding in low-dose aspirin users with ischaemic heart disease

BACKGROUND

Impact of acid suppressants on lower gastrointestinal bleeding remains unclear in low-dose aspirin users; we aimed to investigate this relationship.

METHODS

Retrospective cohort study of low-dose aspirin users who underwent coronary angiography for ischaemic heart disease in our institution between October 2005 and December 2006; patients were evaluated for upper or lower gastrointestinal bleedings within 3 years post-angiography.

RESULTS

538 patients were enrolled (males, 74.4%; mean age 67.4±10.6 years). Risk for upper gastrointestinal bleeding decreased with concomitant use of statins (HR, 0.37; 95% CI, 0.15-0.89), calcium channel blockers (HR, 0.29; 95% CI, 0.10-0.85), and histamine-2 receptor antagonists (HR, 0.26; 95% CI, 0.08-0.89). Concomitant use of proton pump inhibitors tended to decrease risk of upper gastrointestinal bleeding (HR, 0.27; 95% CI, 0.06-1.18). Risk for lower gastrointestinal bleeding increased with both concomitant use of warfarin (HR, 15.68; 95% CI, 4.43-55.53) and proton pump inhibitors (HR, 6.55; 95% CI, 2.01-21.32), but not with histamine-2 receptor antagonists. Hyperuricemia lowered risk for lower gastrointestinal bleeding (HR, 0.12; 95% CI, 0.02-0.88).

CONCLUSIONS

In low-dose aspirin users, concomitant use of proton pump inhibitors increased lower gastrointestinal bleeding risk, independent from effects on upper gastrointestinal bleeding.

Wuling San protects kidney dysfunction by inhibiting renal TLR4/MyD88 signaling and NLRP3 inflammasome activation in high fructose-induced hyperuricemic mice

ETHNOPHARMACOLOGICAL RELEVANCE

Wuling San, a famous prescription in Chinese medicine, is composed of Polyporus, Poria, Alismatis rhizoma, Cinnamomi cortex and Atractylodis macrocephalae rhizoma, and promotes kidney function and diuresis. The main purpose of this study was to investigate its renal protective effect in high fructose-induced hyperuricemic mice.

MATERIALS AND METHODS

ICR mice were fed with 30% fructose in drinking water for 6 weeks to induce hyperuricemia and renal dysfunction. Then mice were orally administrated for other 6 weeks with Wuling San (987, 1316, 1755 and 2340mg/kg), allopurinol (5mg/kg) and water daily, respectively. Serum and urine levels of uric acid, creatinine and blood urea nitrogen (BUN) were measured. Hematoxylin and eosin staining was used to assess renal histological changes. Renal interleukin (IL)-1β concentrations were measured using ELISA kit. Renal protein levels of organic ion transporters, as well as toll-like receptor 4 (TLR4)/myeloid differentiation factor 88 (MyD88) signaling and pyrin domain containing 3 (NLRP3) inflammasome were determined by Western blot assay.

RESULTS

Wuling San significantly decreased serum uric acid, creatinine and BUN levels, increased fractional excretion of uric acid (FEUA) in fructose-fed mice. It restored fructose-induced dysregulation of renal urate transporter 1 (URAT1), glucose transporter 9 (GLUT9), ATP-binding cassette subfamily G member 2 (ABCG2) and organic anion transporter 1 (OAT1), as well as organic cation transporter 1 (OCT1) and OCT2 in mice. Wuling San obviously alleviated infiltration of inflammation cells in kidney glomerulus of fructose-fed mice. Moreover, Wuling San suppressed the activation of TLR4/ MyD88 signaling to inhibit nuclear factor κB (NF-κB) signaling and mitogen-activated protein kinases (MAPKs) activation in fructose-fed mice. Additionally, Wuling San decreased NLRP3 inflammasome activation and IL-1β secretion in the kidney of fructose-fed mice.

CONCLUSION

Wuling San exerts renal protective effect by modulating renal organic ion transporters in fructose-induced hyperuricemic mice. The molecular mechanism of its action may be associated with the suppression of TLR4/MyD88 signaling and NLRP3 inflammasome activation to reduce IL-1β production in high fructose-induced hyperuricemic mice.

Linking uric acid metabolism to diabetic complications

Hyperuricemia have been thought to be caused by the ingestion of large amounts of purines, and prevention or treatment of hyperuricemia has intended to prevent gout. Xanthine dehydrogenase/xanthine oxidase (XDH/XO) is rate-limiting enzyme of uric acid generation, and allopurinol was developed as a uric acid (UA) generation inhibitor in the 1950s and has been routinely used for gout prevention since then. Serum UA levels are an important risk factor of disease progression for various diseases, including those related to lifestyle. Recently, other UA generation inhibitors such as febuxostat and topiroxostat were launched. The emergence of these novel medications has promoted new research in the field. Lifestyle-related diseases, such as metabolic syndrome or type 2 diabetes mellitus, often have a common pathological foundation. As such, hyperuricemia is often present among these patients. Many in vitro and animal studies have implicated inflammation and oxidative stress in UA metabolism and vascular injury because XDH/XO act as one of the major source of reactive oxygen species Many studies on UA levels and associated diseases implicate involvement of UA generation in disease onset and/or progression. Interventional studies for UA generation, not UA excretion revealed XDH/XO can be the therapeutic target for vascular injury and renal dysfunction. In this review, the relationship between UA metabolism and diabetic complications is highlighted.