Molecular physiology and the four-component model of renal urate transport

The Use of Thiazide Diuretics for the Treatment of Hypertension in Patients With Advanced Chronic Kidney Disease

The use of thiazide diuretics for the treatment of hypertension in patients with advance chronic kidney disease. Thiazides have been recommended as the first-line for the treatment of hypertension, yet their use has been discouraged in advanced chronic kidney disease (CKD), as they are suggested to be ineffective in advanced CKD. Recent data suggest that thiazide diuretics may be beneficial blood pressure control in addition to natriuresis in existing CKD. This review discusses the commercially available thiazides with a focus on thiazide pharmacology, most common adverse effects, clinical uses of thiazide diuretic, and the evidence for efficacy of thiazide use in advanced CKD.

Reversal of an unfavorable effect of hydrochlorothiazide compared to angiotensin converting enzyme inhibitor on serum uric acid and oxypurine levels by estrogen-progestin therapy in hypertensive postmenopausal women

Background: The aim was to assess the effect of estrogen-progestin therapy (EPT) on serum levels of uric acid (SUA) and its precursors xanthine (X) and hypoxanthine (HX), and on uric acid (UA) renal excretion in hypertensive postmenopausal women treated with an angiotensin-converting enzyme inhibitor (ACEI) or thiazide diuretic (HCTZ) (ClinicalTrials.gov identifier: NCT03921736, registered 19 April 2019). Methods: Postmenopausal women with untreated essential hypertension were recruited to the study. The control group consisted of 40 postmenopausal women with normal blood pressure. Hypertensive women were randomized to two groups: hydrochlorothiazide (n = 50) or perindopril (n = 50) and to a group receiving or not receiving EPT (EPT+/EPT-) due to vasomotor symptoms. The follow-up period was one year. Blood pressure measurements as well as blood tests for SUA and its precursors X and HX were performed at baseline and after 12 months. Results: In hypertensive women, baseline serum X and HX were significantly higher when compared to the group of normotensive women. Treatment with HCTZ led to a statistically significant increase in SUA in the subgroup of EPT- women. In this group concentrations of X and HX increased significantly after 12 months. UA/X significantly decreased after treatment with HCTZ. Lack of EPT resulted in a decrease of renal plasma flow in the HCTZ group. However, in the HCTZ and EPT + group, SUA decreased significantly when compared to baseline. None of these unfavorable effects was observed in the ACEI group regardless of EPT. Conclusions: 1) EPT prevents the development of hyperuricemia during antihypertensive treatment with thiazide diuretics. 2) Arterial hypertension and menopause cause impairment of UA excretion and increase the levels of SUA and its precursors X and HX. 3) EPT reduces the risk of hyperuricemia in postmenopausal women.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

Glucocorticoids Increase Renal Excretion of Urate in Mice by Downregulating Urate Transporter 1

Both nonsteroidal anti-inflammatory drugs (NSAIDs) and glucocorticoids have been widely used for the treatment of gout, a disease promoted by an excess body burden of uric acid (UA); however, their effects on the homeostasis of UA remain poorly understood. The present study showed that 1-week treatments with three NSAIDs (ibuprofen, diclofenac, and indomethacin) had little effect on UA homeostasis in mice, whereas 1-week low doses (1 and 5 mg/kg) of dexamethasone (DEX) significantly decreased serum UA by about 15%. Additionally, low doses of DEX also resulted in increases in hepatic UA concentration and urinary UA excretion, which were associated with an induction of xanthine oxidoreductase (XOR) in the liver and a downregulation of urate transporter 1 (URAT1) in the kidney, respectively. Neither 75 mg/kg DEX nor 100 mg/kg pregnenolone-16α-carbonitrile altered UA concentrations in serum and livers of mice, suggesting that the effect of DEX on UA homeostasis was not due to the pregnane X receptor pathway. Further in vitro studies demonstrated that glucocorticoid receptor (GR) was involved in DEX-mediated downregulation of URAT1. Knockdown of both p65 and c-Jun completely blocked the effect of DEX on URAT1, suggesting that GR regulates URAT1 via its interaction with both nuclear factor κB and activator protein 1 signaling pathways. To conclude, the present study identifies, for the first time, a critical role of glucocorticoids in regulating UA homeostasis and elucidates the mechanism for GR-mediated regulation of URAT1 in mice. SIGNIFICANCE STATEMENT: This study demonstrates, for the first time, a critical role of glucocorticoid receptor in regulating urate transporter 1 in mouse kidney.

Benzbromarone as a possible cause of acute kidney injury in patients with urolithiasis: Two case reports

RATIONALE

Reports of acute kidney injury (AKI) associated with benzbromarone use in patients with hyperuricemia (HUA) are rare so far.

PATIENT CONCERNS

We describe 2 unique clinical patterns in which benzbromarone was a possible cause of AKI following self-medication for HUA. In case 1, a 45-year-old man developed AKI after taking 100 mg of benzbromarone. His serum creatinine (Scr) increased to 2.3 mg/dL on day 2 after benzbromarone administration. Ultrasound showed multiple small stones in both kidneys, and the 24-hour urine uric acid level was 3128 mg. In case 2, a 17-year-old male student presented with AKI after self-administration of 50 mg of benzbromarone. His Scr increased to 6.8 mg/dL on day 3 after benzbromarone administration. Ultrasound showed multiple stones in the left kidney.

DIAGNOSIS

Both patients underwent renal biopsy, with findings of acute tubular interstitial nephropathy in case 1 and acute tubular damage in case 2. Drug-induced AKI was considered.

INTERVENTIONS

Both cases were treated supportively with intravenous hydration only. In both patients, the Scr level recovered within 0.5 months and renal function was normal 3 months after discharge.

LESSONS

Oral benzbromarone is widely used in Asian counties to treat HUA and the adverse effects are mostly mild. However, clinicians should be alert for benzbromarone-induced AKI. Moreover, uricosuric drugs should only be used after exclusion of urolithiasis and other contraindications.

Influence of urate-lowering therapies on renal handling of uric acid

The purpose of this study is to investigate the effect of urate-lowering therapies (ULTs) on renal uric acid excretion in gout patients. This prospective observational study involved 106 primary gout patients and 51 healthy controls. Gout patients received ULT with either xanthine oxidase inhibitors or the uricosuric agent benzbromarone. Parameters such as 24-h urinary uric acid, creatinine clearance, uric acid clearance, glomerular filtration load of uric acid, fractional excretion of uric acid, excretion of uric acid per volume of glomerular filtration, and urinary uric acid to urinary creatinine ratio were used to evaluate the pre- and post-treatment renal capacity for uric acid clearance in gout patients and were compared with the values in the healthy controls. Compared to healthy controls, gout patients had higher glomerular filtration load of uric acid and lower uric acid clearance, creatinine clearance, and fractional uric acid excretion. After ULT, both the xanthine oxidase inhibitor group and benzbromarone group patients showed reduction in glomerular filtration load of uric acid. Creatinine clearance was significantly improved in the xanthine oxidase inhibitor group. Excretion function was remarkably enhanced in patients who reached the treatment target (serum uric acid <6 mg/dl). Changes in glomerular uric acid filtration load were significantly correlated with changes in serum urate levels. Gout patients have impaired renal uric acid excretion. ULTs reduce renal urate load and enhance the renal capacity of uric acid clearance. Xanthine oxidase inhibitors showed superiority over benzbromarone in improving renal function.

Immunohistochemical studies of organic anion transporters and urate transporter 1 expression in human salivary gland

BACKGROUND

Various substances including uric acid, organic acids and drugs are transported by organic anion transporters (OATs) in the kidney. In addition, a member of the OAT family, urate transporter 1 (URAT1), is involved in the reabsorption of uric acid from the renal tubule. Benzbromarone inhibits URAT1 to block uric acid reabsorption.

METHODS

Our group previously observed higher salivary uric acid levels than serum levels in patients taking benzbromarone, and reported the possible existence of URAT1-like uric acid excretion mechanism in the salivary gland. The purpose of this study was to elucidate the uric acid excretion mechanism in salivary gland tissues using rabbit anti-human OAT1-4 and URAT1 polyclonal antibodies with EnVision(™) system.

RESULTS

In the salivary gland, OAT1 was expressed in ductal cells. OAT2 was found in both ductal cells and serous acinar cells and weak expression was also observed in several nuclei. OAT3 expression was observed in serous acinar cells and nuclei and OAT4 was expressed only in ductal cells. URAT1 expression was observed in the cytoplasm of ductal cells and strong punctuate staining was seen in part of the supra-nuclear cytoplasm. The number of cells expressing URAT1 was smaller compared with OATs. In the kidney, however, OAT1-4 and URAT1 were strongly expressed on proximal renal tubules.

CONCLUSIONS

The present study confirmed the existence of OAT1-4 and URAT1 in the salivary gland. These results may support the previous speculation that benzbromarone inhibits URAT1 to block uric acid reabsorption in the salivary gland, resulting in higher salivary uric acid levels than serum levels.

Linkage of organic anion transporter-1 to metabolic pathways through integrated "omics"-driven network and functional analysis

The main kidney transporter of many commonly prescribed drugs (e.g. penicillins, diuretics, antivirals, methotrexate, and non-steroidal anti-inflammatory drugs) is organic anion transporter-1 (OAT1), originally identified as NKT (Lopez-Nieto, C. E., You, G., Bush, K. T., Barros, E. J., Beier, D. R., and Nigam, S. K. (1997) J. Biol. Chem. 272, 6471-6478). Targeted metabolomics in knockouts have shown that OAT1 mediates the secretion or reabsorption of many important metabolites, including intermediates in carbohydrate, fatty acid, and amino acid metabolism. This observation raises the possibility that OAT1 helps regulate broader metabolic activities. We therefore examined the potential roles of OAT1 in metabolic pathways using Recon 1, a functionally tested genome-scale reconstruction of human metabolism. A computational approach was used to analyze in vivo metabolomic as well as transcriptomic data from wild-type and OAT1 knock-out animals, resulting in the implication of several metabolic pathways, including the citric acid cycle, polyamine, and fatty acid metabolism. Validation by in vitro and ex vivo analysis using Xenopus oocyte, cell culture, and kidney tissue assays demonstrated interactions between OAT1 and key intermediates in these metabolic pathways, including previously unknown substrates, such as polyamines (e.g. spermine and spermidine). A genome-scale metabolic network reconstruction generated some experimentally supported predictions for metabolic pathways linked to OAT1-related transport. The data support the possibility that the SLC22 and other families of transporters, known to be expressed in many tissues and primarily known for drug and toxin clearance, are integral to a number of endogenous pathways and may be involved in a larger remote sensing and signaling system (Ahn, S. Y., and Nigam, S. K. (2009) Mol. Pharmacol. 76, 481-490, and Wu, W., Dnyanmote, A. V., and Nigam, S. K. (2011) Mol. Pharmacol. 79, 795-805). Drugs may alter metabolism by competing for OAT1 binding of metabolites.

Update on the molecular physiology of organic anion transporters

PURPOSE OF REVIEW

Organic anion transporters (OATs) mediate the renal absorption and excretion of a wide range of metabolites and xenobiotics. We discuss the recent advances that have been made in elucidating the binding and transport characteristics of OATs, new insights into their physiological role and regulation by various factors, and pharmacogenetics.

RECENT FINDINGS

Overlapping substrate specificity among the OATs is well established. However, recent findings have suggested distinct differences in the structural binding determinants among the OATs, which have important implications for understanding drug interactions and drug design. A potential role for OATs in blood pressure regulation and remote sensing has been reported. Meanwhile, factors regulating the expression of OATs continue to be identified and characterized. The effect of renal ischemia on OAT expression and function is currently being explored. Finally, recent studies identifying various OAT polymorphisms may facilitate prediction of individual drug response and toxicity.

SUMMARY

As progress is made in unveiling the many functional aspects of the OATs, it is becoming clear that their significance is not only limited to a role in drug elimination from the body, but also extends to other vital physiological roles. Further delineation of the function and regulation of the OATs will uncover enormous potential clinical and pharmacological applications.

Control of renal uric acid excretion and gout

PURPOSE OF REVIEW

Impaired renal uric acid excretion is the major mechanism of hyperuricemia in patients with primary gout. This review highlights recent advances in the knowledge of normal mechanisms of renal uric acid handling and derangement of these mechanisms in uric acid underexcretion.

RECENT FINDINGS

The discovery of URAT1 has facilitated identification of other molecules potentially involved in uric acid transport in the renal tubules. Some of these molecules show gender differential expression in animal experiments. Sodium-dependent monocarboxylate cotransporters have been shown to transport lactate and butyrate, and may have roles in hyperuricemia associated with diabetic ketoacidosis and alcohol ingestion. Certain polymorphisms in SLC22A12 may be associated with the development of hyperuricemia or gout, although confirmation is needed. Mechanisms of hyperuricemia associated with uric acid underexcretion in patients with familial juvenile hyperuricemic nephropathy also remain to be clarified. Distal tubular salt wasting and compensatory upregulation of the resorption of sodium and uric acid in the proximal tubule may explain the hyperuricemia associated with this disorder.

SUMMARY

Much progress has been made in understanding the mechanisms of renal uric acid handling. Elucidation of the mechanisms of hyperuricemia in patients with familial juvenile hyperuricemic nephropathy will shed light on the function of uromodulin, functional impairment of which eventually results in diminished uric acid excretion.

Multiple organic anion transporters contribute to net renal excretion of uric acid

Excretion of uric acid, a compound of considerable medical importance, is largely determined by the balance between renal secretion and reabsorption. The latter process has been suggested to be principally mediated by urate transporter 1 (URAT1; slc22a12), but the role of various putative urate transporters has been much debated. We have characterized urate handling in mice null for RST, the murine ortholog of URAT1, as well as in those null for the related organic anion transporters Oat1 and Oat3. Expression of mRNA of other putative urate transporters (UAT, MRP2, MRP4, Oatv1) was unaffected in the knockouts, as were general indexes of renal function (glomerular filtration rate, fractional excretion of fluid and electrolytes). While mass spectrometric analyses of urine and plasma revealed significantly diminished renal reabsorption of urate in RST-null mice, the bulk of reabsorption, surprisingly, was preserved. Oat1- and Oat3-null mice manifested decreased secretion rather than reabsorption, indicating that these related transporters transport urate in the "opposite" direction to RST. Moreover, metabolomic analyses revealed significant alteration in the concentration of several molecules in the plasma and urine of RST knockouts, some of which may represent additional substrates of RST. The results suggest that RST, Oat1, and Oat3 each contribute to urate handling, but, at least in mice, the bulk of reabsorption is mediated by a transporter(s) that remains to be identified. We discuss the data in the context of recent human genetic studies that suggest that the magnitude of the contribution of URAT1 to urate reabsorption might vary with ethnic background.

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.

Development of High-specificity Antibodies against Renal Urate Transporters Using Genetic Immunization

Recently three proteins, playing central roles in the bidirectional transport of urate in renal proximal tubules, were identified: two members of the organic anion transporter (OAT) family, OAT1 and OAT3, and a protein that designated renal urate-anion exchanger (URAT1). Antibodies against these transporters are very important for investigating their expressions and functions. With the cytokine gene as a molecular adjuvant, genetic immunization-based antibody production offers several advantages including high specificity and high recognition to the native protein compared with current methods. We fused high antigenicity fragments of the three transporters to the plasmids pBQAP-TT containing T-cell epitopes and flanking regions from tetanus toxin, respectively. Gene gun immunization with these recombinant plasmids and two other adjuvant plasmids, which express granulocyte/ macrophage colony-stimulating factor and FMS-like tyrosine kinase 3 ligand, induced high level immunoglobulin G antibodies, respectively. The native corresponding proteins of URAT1, OAT1 and OAT3, in human kidney can be recognized by their specific antibodies, respectively, with Western blot analysis and immunohistochemistry. Besides, URAT1 expression in Xenopus oocytes can also be recognized by its corresponding antibody with immuno-fluorescence. The successful production of the antibodies has provided an important tool for the study of UA transporters.