Urate transport in health and disease

Protecting against ferroptosis in hyperuricemic nephropathy: The potential of ferrostatin-1 and its inhibitory effect on URAT1

Hyperuricemic nephropathy (HN) is characterized by renal fibrosis and tubular necrosis caused by elevated uric acid levels. Ferroptosis, an iron-dependent type of cell death, has been implicated in the pathogenesis of kidney diseases. The objective of this study was to explore the role of ferroptosis in HN and the impact of a ferroptosis inhibitor, ferrostatin-1 (Fer-1). The study combined adenine and potassium oxonate administration to establish a HN model in mice and treated HK-2 cells with uric acid to simulate HN conditions. The effects of Fer-1 on the renal function, fibrosis, and ferroptosis-associated molecules were investigated in HN mice and HK-2 cells treated with uric acid. The HN mice presented with renal dysfunction characterized by elevated tissue iron levels and diminished antioxidant capacity. There was a significant decrease in the mRNA and protein expression levels of SLC7A11, GPX4, FTL-1 and FTH-1 in HN mice. Conversely, treatment with Fer-1 reduced serum uric acid, serum creatinine, and blood urea nitrogen, while increasing uric acid levels in urine. Fer-1 administration also ameliorated renal tubule dilatation and reduced renal collagen deposition. Additionally, Fer-1 also upregulated the expression levels of SLC7A11, GPX4, FTL-1, and FTH-1, decreased malondialdehyde and iron levels, and enhanced glutathione in vivo and in vitro. Furthermore, we first found that Fer-1 exhibited a dose-dependent inhibition of URAT1, with the IC50 value of 7.37 ± 0.66 μM. Collectively, the current study demonstrated that Fer-1 effectively mitigated HN by suppressing ferroptosis, highlighting the potential of targeting ferroptosis as a therapeutic strategy for HN.

SLC2A9 rs16890979 reduces uric acid absorption by kidney organoids

Introduction: The excretion and absorption of uric acid (UA) by the kidneys helps regulate serum UA levels. GLUT9, encoded by SLC2A9, is mainly expressed in the renal tubules responsible for UA absorption. SLC2A9 polymorphisms are associated with different serum UA levels. However, the lack of proper in vitro models has stalled research on the mechanisms of single nucleotide polymorphisms (SNPs) that affect UA metabolism in human urate transporters. Methods: In this study, we constructed a gene-edited human embryonic stem cells-9 (ESC-H9) derived kidney organoid bearing rs16890979, an SLC2A9 missense mutation with undetermined associations with hyperuricemia or hypouricemia. Kidney organoids derived from ESC-H9 with genetical overexpression (OE) and low expression (shRNA) of SLC2A9 to serve as controls to study the function of SLC2A9. The function of rs16890979 on UA metabolism was evaluated after placing the organoids to urate-containing medium and following histopathological analysis. Results: The kidney organoids with heterozygous or homozygous rs16890979 mutations showed normal SLC2A9 expression levels and histological distribution, phenotypically similar to the wild-type controls. However, reduced absorption of UA by the kidney organoids with rs16890979 mutants was observed. This finding together with the observation that UA absorption is increased in organoids with SLC2A9 overexpression and decreased in those with SLC2A9 knockdown, suggest that GLUT9 is responsible for UA absorption, and the rs16890979 SNP may compromise this functionality. Moreover, epithelial-mesenchymal transition (EMT) was detected in organoids after UA treatment, especially in the kidney organoid carrying GLUT9OE, suggesting the cytobiological mechanism explaining the pathological features in hyperuricosuria-related renal injury. Discussion: This study showing the transitional value of kidney organoid modeling the function of SNPs on UA metabolism. With a defined genetic background and a confirmed UA absorption function should be useful for studies on renal histological, cellular, and molecular mechanisms with this organoid model.

Association of dietary inflammatory index with all-cause and cardiovascular disease mortality in hyperuricemia population: A cohort study from NHANES 2001 to 2010

Dietary management is a crucial component of non-pharmacological treatment for hyperuricemia, yet there is a paucity of research on the impact of dietary habits on the survival outcomes of individuals with hyperuricemia. The objective of this study is to examine the association between dietary inflammatory index (DII) and the all-cause and cardiovascular disease (CVD) mortality in individuals with hyperuricemia. This study included 3093 adult participants from National Health and Nutrition Examination Survey (NHANES) 2001 to 2010. Participants were categorized into 4 groups based on quartiles of DII to demonstrate data characteristics, with sample weights considered. The relationship between DII and the risk of hyperuricemia was examined using multivariable logistic regression models. Kaplan-Meier models and Cox proportional hazards models were employed to assess the relationship between DII levels and the all-cause mortality in individuals with hyperuricemia, with the non-linear relationship tested using restricted cubic splines (RCS). Competing risk models were employed to investigate the association between DII levels and the CVD mortality in individuals diagnosed with hyperuricemia. Subgroup and sensitivity analysis were performed to confirm the robustness and reliability of the findings. Among the participants, 47.95% were aged over 60 years. A positive association observed between the highest quartile of DII level and the incidence of hyperuricemia (OR: 1.34, CI [1.13, 1.57]). Elevated DII levels were correlated with increased all-cause mortality (P value < .001) and CVD mortality (P value < .001) in participants. In comparison to the lowest quartile, the highest quartile of DII exhibited a 31% rise in all-cause mortality (HR: 1.31, CI [1.01, 1.68]) and a 50% increase in CVD mortality (HR: 1.50, CI [1.00, 2.26]). No indication of a nonlinear association between DII levels and all-cause mortality (p-non-linear = .43). These findings indicate a positive correlation between the pro-inflammatory diet and the incidence of hyperuricemia. Additionally, a pro-inflammatory diet may elevate the all-cause and CVD mortality in individuals with hyperuricemia.

Emerging Urate-Lowering Drugs and Pharmacologic Treatment Strategies for Gout: A Narrative Review

Hyperuricemia with consequent monosodium urate crystal deposition leads to gout, characterized by painful, incapacitating inflammatory arthritis flares that are also associated with increased cardiovascular event and related mortality risk. This narrative review focuses on emerging pharmacologic urate-lowering treatment (ULT) and management strategies in gout. Undertreated, gout can progress to palpable tophi and joint damage. In oral ULT clinical trials, target serum urate of < 6.0 mg/dL can be achieved in ~ 80-90% of subjects, with flare burden reduction by 1-2 years. However, real-world ULT results are far less successful, due to both singular patient nonadherence and prescriber undertreatment, particularly in primary care, where most patients are managed. Multiple dose titrations commonly needed to optimize first-line allopurinol ULT monotherapy, and substantial potential toxicities and other limitations of approved, marketed oral monotherapy ULT drugs, promote hyperuricemia undertreatment. Common gout comorbidities with associated increased mortality (e.g., moderate-severe chronic kidney disease [CKD], type 2 diabetes, hypertension, atherosclerosis, heart failure) heighten ULT treatment complexity and emphasize unmet needs for better and more rapid clinically significant outcomes, including attenuated gout flare burden. The gout drug armamentarium will be expanded by integrating sodium-glucose cotransporter-2 (SGLT2) inhibitors with uricosuric and anti-inflammatory properties as well as clinically indicated antidiabetic, nephroprotective, and/or cardioprotective effects. The broad ULT developmental pipeline is loaded with multiple uricosurics that selectively target uric acid transporter 1 (URAT1). Evolving ULT approaches include administering selected gut anaerobic purine degrading bacteria (PDB), modulating intestinal urate transport, and employing liver-targeted xanthine oxidoreductase mRNA knockdown. Last, emerging measures to decrease the immunogenicity of systemically administered recombinant uricases should simplify treatment regimens and further improve outcomes in managing the most severe gout phenotypes.

New SLC22A12 (URAT1) Variant Associated with Renal Hypouricemia Identified by Whole-Exome Sequencing Analysis and Bioinformatics Predictions

Renal hypouricemia (RHUC) is a rare hereditary disorder caused by loss-of-function mutations in the SLC22A12 (RHUC type 1) or SLC2A9 (RHUC type 2) genes, encoding urate transporters URAT1 and GLUT9, respectively, that reabsorb urate in the renal proximal tubule. The characteristics of this disorder are low serum urate levels, high renal fractional excretion of urate, and occasional severe complications such as nephrolithiasis and exercise-induced acute renal failure. In this study, we report two Spanish (Caucasian) siblings and a Pakistani boy with clinical characteristics compatible with RHUC. Whole-exome sequencing (WES) analysis identified two homozygous variants: a novel pathogenic SLC22A12 variant, c.1523G>A; p.(S508N), in the two Caucasian siblings and a previously reported SLC2A9 variant, c.646G>A; p.(G216R), in the Pakistani boy. Our findings suggest that these two mutations cause RHUC through loss of urate reabsorption and extend the SLC22A12 mutation spectrum. In addition, this work further emphasizes the importance of WES analysis in clinical settings.

SVCT2/SLC23A2 is a sodium-dependent urate transporter: functional properties and practical application

Urate transporters play a pivotal role in urate handling in the human body, but the urate transporters identified to date do not account for all known molecular processes of urate handling, suggesting the presence of latent machineries. We recently showed that a urate transporter SLC2A12 is also a physiologically important exporter of ascorbate (the main form of vitamin C in the body) that would cooperate with an ascorbate importer, sodium-dependent vitamin C transporter 2 (SVCT2). Based on the dual functions of SLC2A12 and cooperativity between SLC2A12 and SVCT2, we hypothesized that SVCT2 might be able to transport urate. To test this proposal, we conducted cell-based analyses using SVCT2-expressing mammalian cells. The results demonstrated that SVCT2 is a novel urate transporter. Vitamin C inhibited SVCT2-mediated urate transport with a half-maximal inhibitory concentration of 36.59 μM, suggesting that the urate transport activity may be sensitive to physiological ascorbate levels in blood. Similar results were obtained for mouse Svct2. Further, using SVCT2 as a sodium-dependent urate importer, we established a cell-based urate efflux assay that will be useful for identification of other novel urate exporters as well as functional characterization of nonsynonymous variants of already-identified urate exporters including ATP-binding cassette transporter G2. While more studies will be needed to elucidate the physiological impact of SVCT2-mediated urate transport, our findings deepen understanding of urate transport machineries.

Pathogenic Variants of SLC22A12 (URAT1) and SLC2A9 (GLUT9) in Spanish Patients with Renal Hypouricemia: Founder Effect of SLC2A9 Variant c.374C>T; p.(T125M)

Renal hypouricemia (RHUC) is a rare inherited disorder characterized by impaired urate reabsorption in the proximal tubule resulting in low urate serum levels and increased urate excretion. Some patients may present severe complications such as exercise-induced acute renal failure and nephrolithiasis. RHUC is caused by inactivating mutations in the SLC22A12 (RHUC type 1) or SLC2A9 (RHUC type 2) genes, which encode urate transporters URAT1 and GLUT9, respectively. In this study, our goal was to identify mutations associated with twenty-one new cases with RHUC through direct sequencing of SLC22A12 and SLC2A9 coding exons. Additionally, we carried out an SNPs-haplotype analysis to determine whether the rare SLC2A9 variant c.374C>T; p.(T125M), which is recurrent in Spanish families with RHUC type 2, had a common-linked haplotype. Six intragenic informative SNPs were analyzed using PCR amplification from genomic DNA and direct sequencing. Our results showed that ten patients carried the SLC22A12 mutation c.1400C>T; p.(T467M), ten presented the SLC2A9 mutation c.374C>T, and one carried a new SLC2A9 heterozygous mutation, c.593G>A; p.(R198H). Patients carrying the SLC2A9 mutation c.374C>T share a common-linked haplotype, confirming that it emerged due to a founder effect.

Identification of a dysfunctional exon-skipping splice variant in GLUT9/SLC2A9 causal for renal hypouricemia type 2

Renal hypouricemia (RHUC) is a pathological condition characterized by extremely low serum urate and overexcretion of urate in the kidney; this inheritable disorder is classified into type 1 and type 2 based on causative genes encoding physiologically-important urate transporters, URAT1 and GLUT9, respectively; however, research on RHUC type 2 is still behind type 1. We herein describe a typical familial case of RHUC type 2 found in a Slovak family with severe hypouricemia and hyperuricosuria. Via clinico-genetic analyses including whole exome sequencing and in vitro functional assays, we identified an intronic GLUT9 variant, c.1419+1G>A, as the causal mutation that could lead the expression of p.Gly431GlufsTer28, a functionally-null variant resulting from exon 11 skipping. The causal relationship was also confirmed in another unrelated Macedonian family with mild hypouricemia. Accordingly, non-coding regions should be also kept in mind during genetic diagnosis for hypouricemia. Our findings provide a better pathogenic understanding of RHUC and pathophysiological importance of GLUT9.

The Good, the Bad and the New about Uric Acid in Cancer

Simple Summary

The concentration of uric acid in blood is sex-, age- and diet-dependent and is maintained close to its maximal solubility, indicating that it plays some important role. Indeed, it has been demonstrated that, at physiological concentrations, uric acid is a powerful antioxidant and is a scavenger of singlet oxygen and radicals. At high intracellular concentration, uric acid has been demonstrated to act as a pro-oxidant molecule. Recently, uric acid has been reported to affect the properties of several proteins involved in metabolic regulation and signaling, and the relationship between uric acid and cancer has been extensively investigated. In this review, we present the most recent results on the positive and negative effects played by uric acid in cancer and some new findings and hypotheses about the implication of this metabolite in the pathogenesis of several diseases such as metabolic syndrome, diabetes, and inflammation, thus favoring the development of cancer.

Abstract

Uric acid is the final product of purine catabolism in man and apes. The serum concentration of uric acid is sex-, age- and diet-dependent and is maintained close to its maximal solubility, indicating that it plays some important role. Indeed, it has been demonstrated that, at physiological concentrations, uric acid is a powerful antioxidant, while at high intracellular concentrations, it is a pro-oxidant molecule. In this review, we describe the possible causes of uric acid accumulation or depletion and some of the metabolic and regulatory pathways it may impact. Particular attention has been given to fructose, which, because of the complex correlation between carbohydrate and nucleotide metabolism, causes uric acid accumulation. We also present recent results on the positive and negative effects played by uric acid in cancer and some new findings and hypotheses about the implication of this metabolite in a variety of signaling pathways, which can play a role in the pathogenesis of diseases such as metabolic syndrome, diabetes, and inflammation, thus favoring the development of cancer. The loss of uricase in Homo sapiens and great apes, although exposing these species to the potentially adverse effects of uric acid, appears to be associated with evolutionary advantages.

Probiotics, bioactive compounds and dietary patterns for the effective management of hyperuricemia: a review

Hyperuricemia is closely linked with an increased risk of developing hypertension, diabetes, renal failure and other metabolic syndromes. Probiotics, bioactive compounds and dietary patterns are safe cost-efficient ways to control hyperuricemia, whereas comprehensive reviews of their anti-hyperuricemic mechanisms are limited. This review summarizes the roles of probiotics, bioactive compounds and dietary patterns in treating hyperuricemia and critically reviews the possible mechanisms by which these interventions exert their activities. The dietary patterns are closely related to the occurrence of hyperuricemia through the indirect action of gut microbiota or the direct effects of host purine metabolism. The Mediterranean and Dietary Approaches to Stop Hypertension diets help reduce serum uric acid concentrations and thus prevent hyperuricemia. Meanwhile, probiotics alleviate hyperuricemia by ways of absorbing purine, restoring gut microbiota dysbiosis and inhibiting xanthine oxidase (XO) activity. Bioactive compounds such as polyphenols, peptides and alkaloids exert various anti-hyperuricemic effects, by regulating urate transporters, blocking the active sites of XO and inhibiting the toll-like receptor 4/nuclear factor kappa B signaling pathway and NOD-, LRR- and pyrin domain-containing protein 3 signaling pathway. This review will assist people with hyperuricemia to adopt a healthy diet and contribute to the application of natural products with anti-hyperuricemic activity.

OAT10/SLC22A13 Acts as a Renal Urate Re-Absorber: Clinico-Genetic and Functional Analyses With Pharmacological Impacts

Dysfunctional missense variant of organic anion transporter 10 (OAT10/SLC22A13), rs117371763 (c.1129C>T; p.R377C), is associated with a lower susceptibility to gout. OAT10 is a urate transporter; however, its physiological role in urate handling remains unclear. We hypothesized that OAT10 could be a renal urate re-absorber that will be a new molecular target of urate-lowering therapy like urate transporter 1 (URAT1, a physiologically-important well-known renal urate re-absorber) and aimed to examine the effect of OAT10 dysfunction on renal urate handling. For this purpose, we conducted quantitative trait locus analyses of serum urate and fractional excretion of uric acid (FE_UA) using samples obtained from 4,521 Japanese males. Moreover, we performed immunohistochemical and functional analyses to assess the molecular properties of OAT10 as a renal urate transporter and evaluated its potential interaction with urate-lowering drugs. Clinico-genetic analyses revealed that carriers with the dysfunctional OAT10 variant exhibited significantly lower serum urate levels and higher FE_UA values than the non-carriers, indicating that dysfunction of OAT10 increases renal urate excretion. Given the results of functional assays and immunohistochemical analysis demonstrating the expression of human OAT10 in the apical side of renal proximal tubular cells, our data indicate that OAT10 is involved in the renal urate reabsorption in renal proximal tubules from urine. Additionally, we found that renal OAT10 inhibition might be involved in the urate-lowering effect of losartan and lesinurad which exhibit uricosuric effects; indeed, losartan, an approved drug, inhibits OAT10 more strongly than URAT1. Accordingly, OAT10 can be a novel potential molecular target for urate-lowering therapy.