Renal urate handling: Clinical relevance of recent advances

Anti-Hyperuricemic Effects of Extracts from Chaenomeles speciosa (Sweet) Nakai Fruits on Hyperuricemic Rats

Chaenomeles speciosa (Sweet) Nakai (C. speciosa) fruit has medicinal and food applications and exhibits beneficial pharmacological properties. This study aimed to explore the hypouricemic effect of C. speciosa fruit extracts on hyperuricemic rats and uncover potential protective mechanisms. The rats were given hypoxanthine (HX, 100 mg/kg) and potassium oxonate (PO, 300 mg/kg) for 14 days to induce hyperuricemia. Subsequently, the rats were orally administered C. speciosa fruits total extract (CSFTE, 250, 500, and 1000 mg/kg) and allopurinol (AP, 10 mg/kg) one hour after exposure to HX and PO. The results showed that CSFTE had significant xanthine oxidase (XOD) inhibitory activity in vitro (IC50 value of 334.2 μg/mL) and exhibited hypouricemic effects in vivo, reducing uric acid (UA), creatinine (CRE), and blood urea nitrogen (BUN) levels in serum. CSFTE increased UA excretion through the regulation of URAT1, GLUT9, OAT1, and OAT3 protein expression in the kidneys of hyperuricemic rats. Additionally, CSFTE (500 and 1000 mg/kg) was more effective than AP in improving renal injury and protecting kidney function in hyperuricemic rats. Our study demonstrated that CSFTE effectively reduced UA levels and protected the kidneys by inhibiting XOD expression in vitro and regulating UA, CRE, BUN, URAT1, GLUT9, OAT1, and OAT3 proteins in vivo.

In vitro xanthine oxidase inhibitory and in vivo anti-hyperuricemic properties of sodium kaempferol-3'-sulfonate

Xanthine oxidase (XO), a key enzyme in purine catabolism, catalyzes the oxidation of xanthine to uric acid in the body, but overproduction of uric acid may lead to hyperuricemia. This study aims to investigate in vitro XO inhibitory and in vivo anti-hyperuricemic properties of sodium kaempferol-3'-sulfonate (KS). The kinetic analysis indicates that KS is a reversible competitive inhibitor and has significant inhibitory effects on XO activity with an IC50 value of 0.338 μM. Fluorescence spectra suggested that KS could cause fluorescence quenching and conformational changes of XO due to the formation of a KS-XO complex. Molecular docking studies demonstrated that KS interacted with several amino acid residues of XO by the π-π stacking, hydrogen bonds, and hydrophobic interactions. The inhibitory mechanism of KS on XO activity might be the insertion of KS into the active site of XO to prevent the entrance of the substrate xanthine and induce conformational changes of XO. The results carried out in hyperuricemic mice showed that KS reduced serum XO activity, serum uric acid (UA), creatinine (CRE), and urea nitrogen (BUN) levels, as well as alleviating renal histopathological injury. These findings suggest that KS may be a new potent XO inhibitor against hyperuricemia-related diseases.

Comparison of Benzbromarone and Allopurinol on Primary Prevention of the First Gout Flare in Asymptomatic Hyperuricemia

Objectives. Whether uric acid-lowering agent use in asymptomatic hyperuricemia can reduce the development of the first gout flare remains unsettled. The goal of the present research was to test the efficacy of benzbromarone and allopurinol on primary prevention of the first gout flare in persons with asymptomatic hyperuricemia in Taiwan. Methods. One observational cohort study was constructed to examine the 2001−2015 dataset adapted from the National Health Insurance Program of Taiwan containing the claims data of 2 million beneficiaries. Asymptomatic hyperuricemia was considered as individuals on uric acid-lowering therapy who did not have gout flares. Individuals aged 20−84 without gout flares who had the use of benzbromarone alone were assigned into a benzbromarone group. Individuals ages 20−84 without gout flares who had the use of allopurinol alone were assigned into an allopurinol group. The final study included 6111 pairs of 1:1 propensity score-matched individuals from both benzbromarone and allopurinol groups. The end point was assigned as individuals who were newly diagnosed with their first gout flare. The incidence rate of the first gout flare was estimated between the benzbromarone and allopurinol groups. A Cox proportional hazards regression model was applied to explore the hazard ratio and 95% confidence interval of the first gout flare related to benzbromarone use and allopurinol use. Results. The incidence rate of the first gout flare was lower in the benzbromarone group compared with an allopurinol group (3.29 versus 5.46 per 1000 person-months, incidence rate ratio = 0.60 and 95% confidence interval = 0.56−0.64). After adjustment for co-variables, the adjusted hazard ratio of the first gout flare was 0.63 (95% confidence interval = 0.59−0.68, p < 0.001) for the benzbromarone group when compared with the allopurinol group. Conclusion. People with asymptomatic hyperuricemia taking benzbromarone have a lower hazard of developing their first gout flare when compared with those taking allopurinol. Based on the medication safety, the therapeutic effects and the low price, with oral administration once daily, we suggest that benzbromarone should be the first drug of choice if clinicians are treating asymptomatic hyperuricemia.

A Head-To-Head Comparison of Benzbromarone and Allopurinol on the Risk of Type 2 Diabetes Mellitus in People With Asymptomatic Hyperuricemia

Objective: The study aimed to thoroughly address the influence of benzbromarone and allopurinol on the risk of the development of type 2 diabetes mellitus (T2DM) in people with asymptomatic hyperuricemia. Methods: We conducted a retrospective cohort study to examine the 2000-2015 national dataset containing all claims data of 23 million beneficiaries in Taiwan. Subjects who already had diabetes mellitus, gout-related diseases, and any cancer prior to the index date were excluded. Asymptomatic hyperuricemia was defined as subjects taking urate-lowering drugs who never had a gout flare. Subjects aged 20-84 with asymptomatic hyperuricemia who had benzbromarone prescriptions were selected as the benzbromarone group. Sex-matched and age-matched subjects with asymptomatic hyperuricemia who had allopurinol prescriptions were identified as the allopurinol group. The maximum follow-up duration was set as 5 years in our study. The outcome was set as subjects who had a new diagnosis of T2DM. The incidence density of T2DM was calculated in the benzbromarone and allopurinol groups. The hazard ratio (HR) and 95% confidence interval (CI) for T2DM was utilized to estimate the association between medications and the risk of T2DM. Results: The incidence of T2DM among benzbromarone users was significantly lower than that of allopurinol users (7.91 versus 8.48 per 100 person-years, incidence rate ratio = 0.93, and 95% CI = 0.87-0.99). After adjustment for co-variables, the adjusted HR of T2DM would be 0.91 (95% CI = 0.85-0.98 and p = 0.008) in benzbromarone users as compared to allopurinol users. Conclusion: There is a small but statistically significant risk reduction of developing T2DM in people with asymptomatic hyperuricemia taking benzbromarone as compared to those taking allopurinol during 5 years of follow-up. It indicates a future research direction for the use of individual urate-lowering drugs on the prevention of T2DM in the general population.

Effects of genetic and nongenetic factors on hyperuricemia in Chinese patients with coronary artery disease

Aim: The relationship between hyperuricemia and polymorphisms of transporter genes in coronary artery disease (CAD) patients in China remains unclear. Materials & methods: A total of 258 hyperuricemia patients with CAD and 242 control patients with CAD were recruited in this case-control study. Twenty-four SNPs in genes of ABCG2, PDZK1, URAT1, OAT4, GLUT9, ABCC4, NPT1 and NPT4 were genotyped using direct sequencing in all subjects. Results: The mutation of ABCG2 rs2231142 locus increases the risk of hyperuricemia, and there is a gene dose effect in the influence of mutant heterozygotes and homozygotes. rs3825017 in URAT1 and rs62293298 in GLUT9 were also confirmed to be associated with hyperuricemia. Conclusion: Age, weight, creatinine clearance rate, diuretics and SNPs on ABCG2, URAT1 and GLUT9 were all risk factors of hyperuricemia.

Sex Differences in Renal Function: Participation of Gonadal Hormones and Prolactin

Kidney pathophysiology is influenced by gender. Evidence suggests that kidney damage is more severe in males than in females and that sexual hormones contribute to this. Elevated prolactin concentration is common in renal impairment patients and is associated with an unfavorable prognosis. However, PRL is involved in the osmoregulatory process and promotes endothelial proliferation, dilatation, and permeability in blood vessels. Several proteinases cleavage its structure, forming vasoinhibins. These fragments have antagonistic PRL effects on endothelium and might be associated with renal endothelial dysfunction, but its role in the kidneys has not been enough investigated. Therefore, the purpose of this review is to describe the influence of sexual dimorphism and gonadal hormones on kidney damage, emphasizing the role of the hormone prolactin and its cleavage products, the vasoinhibins.

Anthocyanins: Promising Natural Products with Diverse Pharmacological Activities

Anthocyanins are natural products that give color to plants. As natural plant pigments, anthocyanins also have a series of health-promoting benefits. Many researchers have proved that anthocyanins have therapeutic effects on diseases, such as circulatory, nervous, endocrine, digestive, sensory, urinary and immune systems. Additionally, a large number of studies have reported that anthocyanins have an anticancer effect through a wide range of anti-inflammatory and antioxidant effects. The anti-disease impact and mechanism of anthocyanins are diverse, so they have high research value. This review summarizes the research progress of anthocyanins on the pharmacological agents of different diseases to provide references for subsequent research.

Hyperuricemia and gout caused by missense mutation in d-lactate dehydrogenase

Gout is caused by deposition of monosodium urate crystals in joints when plasma uric acid levels are chronically elevated beyond the saturation threshold, mostly due to renal underexcretion of uric acid. Although molecular pathways of this underexcretion have been elucidated, its etiology remains mostly unknown. We demonstrate that gout can be caused by a mutation in LDHD within the putative catalytic site of the encoded d-lactate dehydrogenase, resulting in augmented blood levels of d-lactate, a stereoisomer of l-lactate, which is normally present in human blood in miniscule amounts. Consequent excessive renal secretion of d-lactate in exchange for uric acid reabsorption culminated in hyperuricemia and gout. We showed that LDHD expression is enriched in tissues with a high metabolic rate and abundant mitochondria and that d-lactate dehydrogenase resides in the mitochondria of cells overexpressing the human LDHD gene. Notably, the p.R370W mutation had no effect on protein localization. In line with the human phenotype, injection of d-lactate into naive mice resulted in hyperuricemia. Thus, hyperuricemia and gout can result from the accumulation of metabolites whose renal excretion is coupled to uric acid reabsorption.

Comparisons of protective effects between two sea cucumber hydrolysates against diet induced hyperuricemia and renal inflammation in mice

Differences in the anti-hyperuricemic and anti-inflammation effects between two sea cucumber hydrolysates in diet induced hyperuricemic mice.

The anti-hyperuricemic effect of four astilbin stereoisomers in Smilax glabra on hyperuricemic mice

ETHNOPHARMACOLOGICAL RELEVANCE

Smilax glabra Roxb. (SG), a Chinese medicinal herb which called "tufuling", is believed to be effective in treating hyperuricemia and gout symptoms. But the active substance and pharmacological mechanism of reducing uric acid remain unknown. This study aimed to obtain the total flavonoids including four astilbin stereoisomers and to examine their effects on reducing uric acid content in hyperuricemic mice.

MATERIALS AND METHODS

The total flavonoids of S. glabra (TFSG) were purified and then analysed by HPLC-PDA-MS. The effect of TFSG on the content of serum uric acid (S_UA), Serum creatinine (S_Cr), blood urea nitrogen (BUN) and the activities of xanthine oxidase (XOD) in hyperuricemic mouse model induced by potassium oxonate were examined. Western blot and PCR method were also used to investigate whether TFSG have effect on renal transport protein organic anion transporter 1 (OAT1), organic cation/carnitine transporter 2 (OCTN2) and their mRNA in hypeuricemic mice.

RESULT

Total flavonoids were obtained from EtOAc soluble portion of S. glabra. Four compounds were identified as neoastilbin, astilbin, neoisoastilbin and isoastilbin, which accounted for 55.6% of total flavonoids. TFSG could significantly reduce the serum uric acid content in hyperuricemic mouse (p < 0.01 or p < 0.05). The activities of hepatic XOD have been reduced in hyperuricemic mouse administered with 125 mg/kg TFSG (p < 0.05). The expressions of renal OAT1, OCTN2 and their mRNA have been up-regulated in hyperuricemic mice administered with TFSG (250, 125 mg/kg) (p < 0.01or p < 0.05). TFSG (62.5 mg/kg) could also elevated the expression of renal OCTN2 (p < 0.05).

CONCLUSION

A novel and simple method for preparative separation of astilbin stereoisomers from S. glabra was developed. It was the first time to obtain total flavonoids (including four marker compounds) of S. glabra, and the total content was up to 55.6%. The results suggested TFSG has significant effect on reducing uric acid in hyperuricemic mice by inhibiting the XOD activities and up-regulating the expression of OAT1, OCTN2 and their mRNA in kidney tissue.

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.

Anti-hyperuricemic and nephroprotective effects of extracts from Chaenomeles sinensis (Thouin) Koehne in hyperuricemic mice

CSF-E possesses potent anti-hyperuricemic through inhibiting XOD activity.

Uric acid stimulates proliferative pathways in vascular smooth muscle cells through the activation of p38 MAPK, p44/42 MAPK and PDGFRβ

Hyperuricemia and angiotensin II (Ang II) may have a pathogenetic role in the development of hypertension and atherosclerosis as well as cardiovascular disease (CVD) and its prognosis. The purpose of this study was to investigate whether uric acid can induce proliferative pathways of vascular smooth muscle cell (VSMC) that are thought to be responsible for the development of CVD. The phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK), p44/42 mitogen-activated protein kinase (p44/42 MAPK) and platelet-derived growth factor receptor β (PDGFRβ) was measured by Elisa and Western blot techniques to determine the activation of proliferative pathways in primary cultured VSMCs from rat aorta. Results demonstrated that uric acid can stimulate p38 MAPK, p44/42 MAPK and PDGFRβ phosphorylation in a time- and concentration-dependent manner. Furthermore, treatment of VSMCs with the angiotensin II type I receptor (AT1R) inhibitor losartan suppressed p38 MAPK and p44/42 MAPK induction by uric acid. The stimulatory effect of uric acid on p38 MAPK was higher compared to that of Ang II. The results of this study show for the first time that uric acid-induced PDGFRβ phosphorylation plays a crucial role in the development of CVDs and that elevated uric acid levels could be a potential therapeutical target in CVD patients.

Bidirectional Association between Self-Reported Hypertension and Gout: The Singapore Chinese Health Study

It has been hypothesized that the association between hypertension and gout is bidirectional, however, few studies have examined this in a prospective cohort. We analyzed data from the Singapore Chinese Health Study (SCHS) follow-up I (1999–2004) and II (2006–2010) interviews, when both physician-diagnosed hypertension and gout were self-reported. We included participants with data for both follow-up interviews and who were free of heart disease, stroke and cancer at follow-up I. The analysis of hypertension and risk of gout included 31,137 participants when prevalent gout cases were excluded, while the analysis of gout and risk of hypertension included 20,369 participants when prevalent hypertension cases were excluded. Cox proportional hazards models were used to estimate multivariable-adjusted hazard ratios (HRs) and 95% confidence intervals (CIs). The mean age at follow-up I was 60.1 (SD 7.3) years, and the average follow-up was 6.8 (SD 1.4) years. In the analysis of hypertension and risk of gout, 682 incident cases were identified. Compared to normotensive participants, hypertensive patients had an88% increased risk of developing gout (HR 1.88; 95% CI 1.61–2.21). In the parallel analysis, 5,450 participants reported to have newly diagnosed hypertension during follow-up. Compared to participants without gout, those with gout had an18% increased risk of developing hypertension (HR 1.18; 95% CI 1.02–1.37). The bidirectional association was stronger in normal weight adults compared to overweight/obese individuals (P_interaction = 0.06 and 0.04, respectively). The hypertension to gout association was stronger in women compared to men (P_interaction = 0.04), while the gout to hypertension association was evident in women but not in men (P_interaction = 0.02). In conclusion, our results suggest that the hypertension-gout association is bidirectional in this cohort of Singapore Chinese adults. The potential interactions of the bidirectional association with obesity and sex deserve further investigations.

Population pharmacokinetics and exposure-uric acid analyses after single and multiple doses of ABT-639, a calcium channel blocker, in healthy volunteers

ABT-639 is a selective T-type calcium channel blocker with efficacy in a wide range of preclinical models of nociceptive and neuropathic pain. In the current first-in-human (FIH) study, the pharmacokinetics, tolerability, and safety of ABT-639 after single- (up to 170 mg) and multiple doses (up to 160 mg BID) were evaluated in healthy volunteers in a randomized, double-blinded, placebo-controlled manner. ABT-639 demonstrated acceptable safety and pharmacokinetic profiles in human. Results from assessment of the routine laboratory variables showed an unexpected statistically significant and clinically relevant decrease in blood uric acid with the increase in ABT-639 dose, which is possibly due to inhibition in URAT1 transporter. Pharmacokinetic/pharmacodynamic models were constructed to characterize the relationship between ABT-639 exposure and uric acid response. The final model was a mechanism-based indirect response pharmacodynamic model with the stimulation of uric acid elimination by ABT-639. The model estimated K in values in males and females were 10.2 and 7.13 μmol/h, respectively. The model estimated K out was 0.033 1/h. ABT-639 concentration that can produce 50% stimulation in uric acid elimination was estimated to be 8,070 ng/mL. Based on the final model, further simulations were conducted to predict the effect of ABT-639 on uric acid in gout patients. The simulation results indicated that, if the urate-lowering response to ABT-639 in gout patients is similar to that in healthy subjects, ABT-639 BID doses of 140 mg or higher would be expected to provide clinically meaningful lowering of blood uric acid levels below the 380 μmol/L solubility limit of monosodium urate.

Drug discovery for hyperuricemia

Hyperuricemia is associated with an increased risk of developing gout. This increases with the degree and duration of hyperuricemia. Gout can be managed by dietary modification and pharmacologic urate-lowering therapies. The recent identification of the renal apical urate/anion exchanger URAT1 (SLC22A12) and several membrane proteins relevant to the transport of urate play an important role in gaining a better understanding of the mode of action of many drugs used to treat gout. As described in this review, therapeutics designed to modify URAT1 transport activities might be useful in treating pathologies associated with hyperuricemia such as gout and urolithiasis. Continuing studies into the urate transportsome hold promise for the development of new, more effective therapeutics for hyperuricemia.

Efficacy and tolerability of probenecid as urate-lowering therapy in gout; clinical experience in high-prevalence population

OBJECTIVE

Probenecid is recommended as urate-lowering therapy (ULT) in patients with gout where xanthine oxidase inhibitors are ineffective, not tolerated, or contraindicated. The aim of our study was to determine the efficacy of probenecid to achieve serum urate (SU) targets (< 0.36 mmol/l) in clinical practice.

METHODS

We identified 57 patients prescribed with probenecid from a database of 521 rheumatology clinic attenders with gout. Demographic characteristics, indications for probenecid, probenecid doses, side effects, and laboratory data including estimated glomerular filtration rate (eGFR) and SU were recorded.

RESULTS

There were 30/57 (53%) patients treated with probenecid as monotherapy and 27/57 (47%) patients treated with probenecid in combination with allopurinol. Target SU concentrations (< 0.36 mmol/l) were achieved in 10/30 (33%) of the probenecid monotherapy group and 10/27 (37%) of the combination treatment group. Baseline SU concentrations, but not eGFR or probenecid dose, independently predicted achievement of target SU. Target SU was achieved in 5/15 (33%) patients with eGFR < 50 ml/min/1.73 m(2). There was no difference in the percentage of patients achieving SU target in those with eGFR < 50 ml/min/1.73 m(2) compared with those with eGFR ≥ 50 ml/min/1.73 m(2). Adverse events attributed to probenecid were observed in 8/42 (19%) patients with eGFR ≥ 50 ml/min/1.73 m(2) and in 2/15 (13%) patients with eGFR < 50 ml/min/1.73 m(2).

CONCLUSION

Probenecid has moderate efficacy as ULT in clinical management of patients with complex gout who have a lack of efficacy or intolerance to allopurinol. Patients with chronic kidney disease may respond to probenecid with similar rates of adverse events.

Functional cooperation of URAT1 (SLC22A12) and URATv1 (SLC2A9) in renal reabsorption of urate

BACKGROUND

Serum urate (SUA) level is affected by alteration in urinary reabsorption caused by clinically important drugs; however, there are no experimental models suitable to assess their effect on renal reabsorption. We, therefore, aimed to establish an experimental system co-expressing the urate transporters URAT1 (SLC22A12) and URATv1 (SLC2A9) (designated UUv cells) at the apical and basolateral membranes, respectively.

METHODS

Apical uptake and vectorial transport of [(14)C]urate in the apical-to-basolateral direction in UUv cells were measured in the presence or absence of uricosuric benzbromarone or anti-uricosuric trans-stimulators.

RESULTS

The urate permeability in the apical-to-basolateral direction remarkably increased by 7.0-fold in UUv cells, compared with non-transfected mock cells. The apical-to-basolateral transport was cis-inhibited by benzbromarone, but trans-stimulated by pyrazinecarboxylic acid and monocarboxylates such as nicotinate and lactate. Furthermore, salicylate showed both trans-stimulation and cis-inhibition in the urate transport at low and high concentrations, respectively. Finally, coexpression of URAT1 and URATv1 in human kidney epithelial cells was exhibited immunohistochemically.

CONCLUSIONS

It is demonstrated that functional cooperation of URAT1 and URATv1 is essential for renal reabsorption of urate, and in the established system influence of drugs on SUA is reflected in the alteration of urate permeability across the renal tubular epithelial cells.

Urate transporters: an evolving field

Urate (uric acid) is the end product of purine metabolism in human beings owing to the genetic loss of hepatic urate oxidase (uricase). Despite its potential advantage as an antioxidant, sustained hyperuricemia is associated with gout, renal diseases, hypertension, and cardiovascular diseases. Because the kidney plays a dominant role in maintaining serum urate levels through its excretion, it is important to understand the molecular mechanism of renal urate handling. Although molecular identification of the urate/anion exchanger URAT1 (SLC22A12) in 2002 paved the way for successive identification of several urate transport-related proteins, the entire picture of effective renal urate handling in human beings has not yet been clarified. Recently, several genome-wide association studies have revealed close associations between serum urate levels and single nucleotide polymorphisms in at least 10 genetic loci including eight transporter-related genes. These findings led us to consider the roles of urate transporters in extrarenal tissues such as the intestine. In this review, we discuss various aspects of transmembrane transport of urate in the human body.

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

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

Anti-hyperuricemic and nephroprotective effects of Smilax china L

ETHNOPHARMACOLOGICAL RELEVANCE

Smilax china L., popularly known as "Jin Gang Ten", has been widely used as a traditional herbal medicine for the treatment of gout, rheumatoid arthritis and other diseases for a long time in China.

AIM OF STUDY

The present study was carried out to investigate the effect of Smilax china L. on hyperuricemia and renal dysfunction in induced hyperuricemic animals.

MATERIALS AND METHODS

Five fractions (petroleum ether, chloroform, ethyl acetate, n-butanol and residual ethanol fraction) of Smilax china L. were orally administered to potassium oxonate-induced hyperuricemic mice for three days. The xanthine oxidase inhibitory activities and modes of action of nine compounds isolated from ethyl acetate fraction (EAF) were then examined in vitro. Finally, different dosages of EAF were administered to 10% fructose-induced hyperuricemic rats.

RESULTS

EAF (250 mg/kg) exhibited stronger anti-hyperuricemic activity in hyperuricemic mice compared with the other four fractions. Caffeic acid, resveratrol, rutin and oxyresveratrol isolated from EAF showed different inhibitory activities on xanthine oxidase in vitro, with the IC(50) values of 42.60, 37.53, 42.20 and 40.69 μM, respectively, and exhibited competitive or mixed inhibitory actions. Moreover, EAF (125, 250 and 500 mg/kg) markedly reversed the serum uric acid level (p<0.05, p<0.01 and p<0.001, respectively), fractional excretion of urate (p<0.05, p<0.01 and p<0.01, respectively) and blood urea nitrogen (p<0.05, p<0.01 and p<0.01, respectively) to their normal states, and prevented the renal damage against tubulointerstitial pathologies in hyperuricemic rats.

CONCLUSION

These findings show that Smilax china L. exhibits anti-hyperuricemic and nephroprotective activity in hyperuricemic animals.

Allopurinol to Febuxostat: How far have we come?

For decades allopurinol has been used as a xanthine oxidase inhibitor for treatment of hyperuricemia and gout. Although effective in many patients, some experience sensitivity to the drug. In some cases, this sensitivity may lead to allopurinol hypersensitivity disorder, which if untreated can be fatal. Recently the Food and Drug Administration has approved the use of febuxostat as an alternative therapy for hyperuricemia and gout. Febuxostat is a new xanthine oxidase inhibitor, but is not purine based and therefore decreases adverse reactions due to patient sensitivity. This review is a comprehensive look at the background of hyperuricemia and gout treatment with allopurinol compared to recent clinical studies with febuxostat. Each clinical study is evaluated and summarized, identifying the advances in treatment that have been made as well as the concerns that still exist with either treatment.

New insights into the epidemiology of gout

Gout is a true crystal deposition disease caused by formation of monosodium urate crystals in joints and other tissues. It is a common inflammatory arthritis that has increased in prevalence in recent decades. Gout normally results from the interaction of genetic, constitutional and environmental risk factors. It is more common in men and strongly age related. A major determinant is the degree of elevation of uric acid levels above the saturation point for urate crystal formation, principally caused by inefficient renal urate excretion. Local joint tissue factors may influence the topography and extent of crystal deposition. Recent studies have provided information on dietary risk factors for gout: higher intakes of red meat, fructose and beer are independently associated with increased risk, whereas higher intakes of coffee, low-fat dairy products and vitamin C are associated with lower risk. Several renal urate transporters have been identified including URAT1 and SLC2A9 (GLUT9) and polymorphisms in these genes are associated with an increased risk of hyperuricaemia and gout. Many drugs influence serum uric acid levels through an effect on renal urate transport. Comorbidities, including the metabolic syndrome and impaired renal function are common in gout patients. The usual initial presentation of gout is with rapidly developing acute inflammatory monoarthritis, typically affecting the first MTP joint. If left untreated it may progress with recurrent acute attacks and eventual development of chronic symptoms and joint damage. New knowledge of the modifiable risk factors for gout can be integrated into the management strategy to optimize long-term patient outcomes.

Solute carrier family 2, member 9 and uric acid homeostasis

PURPOSE OF REVIEW

The goal of this article is to review the possible physiological roles of the recently identified urate transporter, solute carrier family 2 (facilitated glucose transporter), member 9 (SLC2A9), in the renal handling of urate.

RECENT FINDINGS

Glucose transporter 9 is a high affinity hexose transporter encoded by the SLC2A9 gene found on human chromosome 4. The two splice variants SLC2A9b and SLC2A9a are expressed in the apical and basolateral membranes, respectively, of the proximal convoluted tubule. Recent reports have found significant correlations between two different sets of single nucleotide polymorphisms in SLC2A9. In one case, they are associated with increases in plasma urate levels and/or the incidence of hypertension or gout. The second set of single nucleotide polymorphisms correlate with hypouricaemia in Japanese patients. Expression of SLC2A9a and b in Xenopus laevis oocytes shows that these proteins mediate rapid urate fluxes and can exchange glucose for urate. Indirect evidence also suggests that the transporter is electrogenic.

SUMMARY

This review proposes that SLC2A9 contributes significantly in two ways to the fluxes of urate across the proximal convoluted tubule. Firstly, the apical expression of SLC2A9b secretes urate back into the urine in exchange for lumenal glucose. Secondly, the basolateral membrane SLC2A9a could be the primary route for urate movement out of the epithelium into the peritubular space.

Meta-analysis of 28,141 individuals identifies common variants within five new loci that influence uric acid concentrations

Elevated serum uric acid levels cause gout and are a risk factor for cardiovascular disease and diabetes. To investigate the polygenetic basis of serum uric acid levels, we conducted a meta-analysis of genome-wide association scans from 14 studies totalling 28,141 participants of European descent, resulting in identification of 954 SNPs distributed across nine loci that exceeded the threshold of genome-wide significance, five of which are novel. Overall, the common variants associated with serum uric acid levels fall in the following nine regions: SLC2A9 (p = 5.2×10⁻²⁰¹), ABCG2 (p = 3.1×10⁻²⁶), SLC17A1 (p = 3.0×10⁻¹⁴), SLC22A11 (p = 6.7×10⁻¹⁴), SLC22A12 (p = 2.0×10⁻⁹), SLC16A9 (p = 1.1×10⁻⁸), GCKR (p = 1.4×10⁻⁹), LRRC16A (p = 8.5×10⁻⁹), and near PDZK1 (p = 2.7×10⁻⁹). Identified variants were analyzed for gender differences. We found that the minor allele for rs734553 in SLC2A9 has greater influence in lowering uric acid levels in women and the minor allele of rs2231142 in ABCG2 elevates uric acid levels more strongly in men compared to women. To further characterize the identified variants, we analyzed their association with a panel of metabolites. rs12356193 within SLC16A9 was associated with DL-carnitine (p = 4.0×10⁻²⁶) and propionyl-L-carnitine (p = 5.0×10⁻⁸) concentrations, which in turn were associated with serum UA levels (p = 1.4×10⁻⁵⁷ and p = 8.1×10⁻⁵⁴, respectively), forming a triangle between SNP, metabolites, and UA levels. Taken together, these associations highlight additional pathways that are important in the regulation of serum uric acid levels and point toward novel potential targets for pharmacological intervention to prevent or treat hyperuricemia. In addition, these findings strongly support the hypothesis that transport proteins are key in regulating serum uric acid levels.

Elevated serum uric acid levels cause gout and are a risk factor for cardiovascular disease and diabetes. The regulation of serum uric acid levels is under a strong genetic control. This study describes the first meta-analysis of genome-wide association scans from 14 studies totalling 28,141 participants of European descent. We show that common DNA variants at nine different loci are associated with uric acid concentrations, five of which are novel. These variants are located within the genes coding for organic anion transporter 4 (SLC22A11), monocarboxylic acid transporter 9 (SLC16A9), glucokinase regulatory protein (GCKR), Carmil (LRRC16A), and near PDZ domain-containing 1 (PDZK1). Gender-specific effects are shown for variants within the recently identified genes coding for glucose transporter 9 (SLC2A9) and the ATP-binding cassette transporter (ABCG2). Based on screening of 163 metabolites, we show an association of one of the identified variants within SLC16A9 with DL-carnitine and propionyl-L-carnitine. Moreover, DL-carnitine and propionyl-L-carnitine were strongly correlated with serum UA levels, forming a triangle between SNP, metabolites and UA levels. Taken together, these associations highlight pathways that are important in the regulation of serum uric acid levels and point toward novel potential targets for pharmacological intervention to prevent or treat hyperuricemia.

Pharmacokinetic and pharmacodynamic interaction between allopurinol and probenecid in healthy subjects

BACKGROUND AND OBJECTIVE

Combination therapy with allopurinol and probenecid is used to treat tophaceous gout in patients who do not respond sufficiently to allopurinol alone. However, the potential interaction between these drugs has not been systematically investigated. The objective of this study was to investigate the pharmacokinetics and hypouricaemic effect of oxypurinol (the active metabolite of allopurinol) and probenecid when administered alone and in combination in healthy subjects.

METHODS

An open-label, randomized, three-way crossover clinical trial was conducted in 12 healthy adults. Subjects were randomized to receive treatment for 7 days with allopurinol (150 mg twice daily), probenecid (500 mg twice daily) or combination therapy with both drugs, with a 7-day washout period between treatments. Venous blood samples were collected predose (at 0 hours) and 1, 2, 3, 4, 6, 8, 10 and 12 hours after dosage for determination of oxypurinol and/or probenecid concentrations. Plasma and urinary urate concentrations were determined on each study day and at the end of each washout period. Pharmacokinetic and pharmacodynamic parameters were analysed using two-way ANOVA.

RESULTS

Coadministration of allopurinol and probenecid significantly reduced average steady-state plasma oxypurinol concentrations (mean+/-SD: allopurinol alone 9.7+/-2.1 mg/L vs combination 5.1+/-1.0 mg/L, p<0.001). Probenecid concentrations were unaffected. Plasma urate concentrations decreased (p<0.01) during allopurinol therapy (0.16+/-0.05 mmol/L), probenecid therapy (0.13+/-0.02 mmol/L) and combination therapy (0.09+/-0.02 mmol/L) compared with baseline (0.30+/-0.05 mmol/L).

CONCLUSION

Coadministration of allopurinol and probenecid to healthy subjects had a greater hypouricaemic effect than either allopurinol or probenecid alone, despite a reduction in plasma oxypurinol concentrations when the drugs were taken concomitantly.

[Molecular basis of primary renal hyperuricemia : role of the human urate transporter hURAT1]

In highly industrialized countries hyperuricemia is one of the most common metabolic disorders. High uric acid blood levels may lead to the manifestation of gout owing to the precipitation of urate crystals in connective tissue, the skeletal system and kidneys. A primary reduction of renal uric acid excretion can be detected in more than 90% of all cases of hyperuricemia. Despite the identification of several uric acid transporting proteins their pathogenetic role for the induction of primary reduced renal uric acid excretion has not yet been verified. As a result of a case-control study on individuals with normal and reduced renal uric acid excretion, an association of polymorphisms in the human urate transporter 1 gene (hURAT1) with primary reduced urate excretion has been demonstrated for the first time. The hURAT1 gene is an organic anion transporter (SLC22A12), which is preferentially expressed in the apical membrane of proximal renal tubule cells. Functioning as an antiporter, hURAT1 mediates the uptake of urate from the lumen into proximal tubule cells in exchange for organic and inorganic anions. Loss-of-function mutations in the hURAT1 gene are a cause of hereditary renal hypouricemia. The precisely regulated hURAT1 is a candidate gene for hyperuricemia and an important target for the development and optimization of new diagnostic approaches and pharmacological interventions of primary reduced renal uric acid excretion.

Identification of a new urate and high affinity nicotinate transporter, hOAT10 (SLC22A13)

The orphan transporter hORCTL3 (human organic cation transporter like 3; SLC22A13) is highly expressed in kidneys and to a weaker extent in brain, heart, and intestine. hORCTL3-expressing Xenopus laevis oocytes showed uptake of [(3)H]nicotinate, [(3)H]p-aminohippurate, and [(14)C]urate. Hence, hORCTL3 is an organic anion transporter, and we renamed it hOAT10. [(3)H]Nicotinate transport by hOAT10 into X. laevis oocytes and into Caco-2 cells was saturable with Michaelis constants (K(m)) of 22 and 44 microm, respectively, suggesting that hOAT10 may be the molecular equivalent of the postulated high affinity nicotinate transporter in kidneys and intestine. The pH dependence of hOAT10 suggests p-aminohippurate(-)/OH(-), urate(-)/OH(-), and nicotinate(-)/OH(-) exchange as possible transport modes. Urate inhibited [(3)H]nicotinate transport by hOAT10 with an IC(50) value of 759 microm, assuming that hOAT10 represents a low affinity urate transporter. hOAT10-mediated [(14)C]urate uptake was elevated by an exchange with l -lactate, pyrazinoate, and nicotinate. Surprisingly, we have detected urate(-)/glutathione exchange by hOAT10, consistent with an involvement of hOAT10 in the renal glutathione cycle. Uricosurics, diuretics, and cyclosporine A showed substantial interactions with hOAT10, of which cyclosporine A enhanced [(14)C]urate uptake, providing the first molecular evidence for cyclosporine A-induced hyperuricemia.

Gender differences in kidney function

Sex hormones influence the development of female (F) and male (M) specific traits and primarily affect the structure and function of gender-specific organs. Recent studies also indicated their important roles in regulating structure and/or function of nearly every tissue and organ in the mammalian body, including the kidneys, causing gender differences in a variety of characteristics. Clinical observations in humans and studies in experimental animals in vivo and in models in vitro have shown that renal structure and functions under various physiological, pharmacological, and toxicological conditions are different in M and F, and that these differences may be related to the sex-hormone-regulated expression and action of transporters in the apical and basolateral membrane of nephron epithelial cells. In this review we have collected published data on gender differences in renal functions, transporters and other related parameters, and present our own microarray data on messenger RNA expression for various transporters in the kidney cortex of M and F rats. With these data we would like to emphasize the importance of sex hormones in regulation of a variety of renal transport functions and to initiate further studies of gender-related differences in kidney structure and functions, which would enable us to better understand occurrence and development of various renal diseases, pharmacotherapy, and drug-induced nephrotoxicity in humans and animals.

New insights into renal transport of urate

PURPOSE OF REVIEW

This review focuses on recent progress in the understanding of various aspects of renal transport of urate.

RECENT FINDINGS

Since the molecular cloning of the renal apical urate/anion exchanger URAT1 (SLC22A12), several membrane proteins relevant to the transport of urate have been identified. The molecular identification of two sodium-coupled monocarboxylate transporters, SMCT1(SLC5A8) and SMCT2(SLC5A12), and the emerging role of PDZ (PSD-95, DglA, and ZO-1) scaffold for renal apical transporters have led to a new concept of renal urate transport: urate-transporting multimolecular complex, or 'urate transportsome', that may form an ultimate functional unit including the sodium-coupled urate transport system by linking URAT1 and sodium-coupled monocarboxylate transporters or the coordinated apical urate uptake system by balancing reabsorptive (URAT1) and efflux (NPT1/OATv1 and MRP4) transporters. In addition, genetic variations of the URAT1 gene are associated not only with idiopathic renal hypouricemia but also with reduced renal urate excretion.

SUMMARY

Although our knowledge of renal urate handling has been increased by the molecular identification of urate transport proteins and by results of genetic studies on patients with serum urate disorders, current evidence is insufficient to fully understand the precise mechanism governing the bi-directional transport of urate. Further studies are still necessary.

Human renal organic anion transporter 4 operates as an asymmetric urate transporter

Human organic anion transporter 4 (hOAT4) is located at the apical membrane of proximal tubule cells and involved in renal secretion and reabsorption of endogenous substances as well as many drugs and xenobiotics. This study reevaluated the physiologic role, transport mode, and driving forces of hOAT4. 6-Carboxyfluorescein (6-CF) uptake into HEK293 cells that stably expressed hOAT4 was saturable, resulting in a K(m) of 108 muM. 6-CF as well as [(3)H]estrone sulfate ([(3)H]ES) accumulation by HEK293-hOAT4 cells were abolished by ES, dehydroepiandrosterone sulfate, sulfinpyrazone, benzbromarone, and probenecid, whereas several OA, including p-aminohippurate (PAH), lactate, pyrazinoate, nicotinate, glutarate, and the diuretic hydrochlorothiazide (HCTZ) exhibited a slight or a NS inhibitory effect. PAH and glutarate are not taken up by HEK293-hOAT4 cells, but they trans-stimulated 6-CF and [(3)H]ES uptake, indicating an asymmetric interaction of hOAT4 with these substrates. In chloride-free medium, HEK293-hOAT4-mediated [(3)H]PAH efflux was almost abolished, whereas addition of ES restored it comparable to Ringer solution, consistent with a physiologic ES/PAH or PAH/Cl(-) exchange mode of hOAT4. Moreover, an acidification of the uptake medium increased 6-CF as well as [(3)H]ES uptake, which was reduced by nigericin, suggesting that hOAT4 also can operate as an OA/OH(-) exchanger. hOAT4 facilitates substantial uptake of [(14)C]urate, which was elevated 2.6-fold by intracellular HCTZ. Thus, hOAT4 is the long-postulated, low-affinity apical urate anion exchanger that facilitates HCTZ-associated hyperuricemia.

New developments in clinically relevant mechanisms and treatment of hyperuricemia

The prevalence of gout has increased markedly in the United States in the past two decades, and new treatments for hyperuricemia are being developed. Recent molecular identification of urate transporter-1 (URAT1) as the central mediator of renal urate reabsorption has provided novel understanding of the pathogenesis of hyperuricemia, and the target site for current and possibly future primary uricosuric agents. Recent studies have also highlighted uricosuric effects of several drugs (losartan, atorvastatin, fenofibrate) that are prescribed for primary indications other than hyperuricemia. The niche of these agents in current management of hyperuricemia is discussed. We also review the ongoing development of recombinant uricase preparations and of novel xanthine oxidase inhibitors exemplified by febuxostat. These agents should provide novel options for patients with chronic, refractory gout and hyperuricemia, particularly in association with allopurinol hypersensitivity and renal insufficiency.

The PDZ domain protein PDZK1 interacts with human peptide transporter PEPT2 and enhances its transport activity

The proton-coupled peptide transporter PEPT2 (SLC15A2) mediates the high-affinity low-capacity transport of small peptides as well as various oral peptide-like drugs in the kidney. In contrast to its well-characterized transport properties, there is less information available on its regulatory mechanism, although the interaction of PEPT2 to the PDZ (PSD-95, DglA, and ZO-1)-domain protein PDZK1 has been preliminarily reported. To examine whether PDZK1 is a physiological partner of PEPT2 in kidneys, we started from a yeast two-hybrid screen of a human kidney cDNA library with the C-terminus of PEPT2 (PEPT2 C-terminus (PEPT2-CT)) as bait. We could identify PDZK1 as one of the positive clones. This interaction requires the PDZ motif of PEPT2-CT detected by a yeast two-hybrid assay, in vitro binding assay and co-immunoprecipitation. The binding affinities of second and third PDZ domains of PDZK1 to PEPT2-CT were measured by surface plasmon resonance. Co-immunoprecipitation using human kidney membrane fraction and localization of PEPT2 in renal apical proximal tubules revealed the physiological meaning of this interaction in kidneys. Furthermore, we clarified the mechanism of enhanced glycylsarcosine (Gly-Sar) transport activity in PEPT2-expressing HEK293 cells after the PDZK1 coexpression. This augmentation was accompanied by a significant increase in the V(max) of Gly-Sar transport via PEPT2 and it was also associated with the increased surface expression level of PEPT2. These results indicate that the PEPT2-PDZK1 interaction thus plays a physiologically important role in both oligopeptide handling as well as peptide-like drug transport in the human kidney.

Analyses of 5' regulatory region polymorphisms in human SLC22A6 (OAT1) and SLC22A8 (OAT3)

Kidney excretion of numerous organic anionic drugs and endogenous metabolites is carried out by a family of multispecific organic anion transporters (OATs). Two closely related transporters, SLC22A6, initially identified by us as NKT and also known as OAT1, and SLC22A8, also known as OAT3 and ROCT, are thought to mediate the initial steps in the transport of organic anionic drugs between the blood and proximal tubule cells of the kidney. Coding region polymorphisms in these genes are infrequent and pairing of these genes in the genome suggests they may be coordinately regulated. Hence, 5' regulatory regions of these genes may be important factors in human variation in organic anionic drug handling. We have analyzed novel single nucleotide polymorphisms in the evolutionarily conserved 5' regulatory regions of the SLC22A6 and SLC22A8 genes (phylogenetic footprints) in an ethnically diverse sample of 96 individuals (192 haploid genomes). Only one polymorphism was found in the SLC22A6 5' regulatory region. In contrast, seven polymorphisms were found in the SLC22A8 5' regulatory region, two of which were common to all ethnic groups studied. Computational analysis permitted phase and haplotype reconstruction. Proximity of these non-coding polymorphisms to transcriptional regulatory elements (including potential sex steroid response elements) suggests a potential influence on the level of transcription of the SLC22A6 and/or SLC22A8 genes and will help define their role in variation in human drug, metabolite and toxin excretion. The clustering of OAT genes in the genome raises the possibility that nucleotide polymorphisms in SLC22A6 could also effect SLC22A8 expression, and vice versa.

Recent developments in our understanding of the renal basis of hyperuricemia and the development of novel antihyperuricemic therapeutics

Although dietary, genetic, or disease-related excesses in urate production may contribute to hyperuricemia, impaired renal excretion of uric acid is the dominant cause of hyperuricemia in the majority of patients with gout. The aims of this review are to highlight exciting and clinically pertinent advances in our understanding of how uric acid is reabsorbed by the kidney under the regulation of urate transporter (URAT)1 and other recently identified urate transporters; to discuss urate-lowering agents in clinical development; and to summarize the limitations of currently available antihyperuricemic drugs. The use of uricosuric drugs to treat hyperuricemia in patients with gout is limited by prior urolothiasis or renal dysfunction. For this reason, our discussion focuses on the development of the novel xanthine oxidase inhibitor febuxostat and modified recombinant uricase preparations.

Integrated physiology of proximal tubular organic anion transport

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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