Functional reconstitution, membrane targeting, genomic structure, and chromosomal localization of a human urate transporter

Galectin-9 in Gastroenterological Cancer

Immunochemotherapy has become popular in recent years. The detailed mechanisms of cancer immunity are being elucidated, and new developments are expected in the future. Apoptosis allows tissues to maintain their form, quantity, and function by eliminating excess or abnormal cells. When apoptosis is inhibited, the balance between cell division and death is disrupted and tissue homeostasis is impaired. This leads to dysfunction and the accumulation of genetically abnormal cells, which can contribute to carcinogenesis. Lectins are neither enzymes nor antibodies but proteins that bind sugar chains. Among soluble endogenous lectins, galectins interact with cell surface sugar chains outside the cell to regulate signal transduction and cell growth. On the other hand, intracellular lectins are present at the plasma membrane and regulate signal transduction by regulating receptor–ligand interactions. Galectin-9 expressed on the surface of thymocytes induces apoptosis of T lymphocytes and plays an essential role in immune self-tolerance by negative selection in the thymus. Furthermore, the administration of extracellular galectin-9 induces apoptosis of human cancer and immunodeficient cells. However, the detailed pharmacokinetics of galectin-9 in vivo have not been elucidated. In addition, the cell surface receptors involved in galectin-9-induced apoptosis of cancer cells have not been identified, and the intracellular pathways involved in apoptosis have not been fully investigated. We have previously reported that galectin-9 induces apoptosis in various gastrointestinal cancers and suppresses tumor growth. However, the mechanism of galectin-9 and apoptosis induction in gastrointestinal cancers and the detailed mechanisms involved in tumor growth inhibition remain unknown. In this article, we review the effects of galectin-9 on gastrointestinal cancers and its mechanisms.

Galectin-9 Is Critical for Mucosal Adaptive Immunity through the T Helper 17-IgA Axis

Impairment of the intestinal mucosal immunity significantly increases the risk of acute and chronic diseases. IgA plays a major role in humoral mucosal immunity to provide protection against pathogens and toxins in the gut. Here, we investigated the role of endogenous galectin-9, a tandem repeat-type β-galactoside-binding protein, in intestinal mucosal immunity. By mucosal immunization of Lgals9^-/- and littermate control mice, it was found that lack of galectin-9 impaired mucosal antigen-specific IgA response in the gut. Moreover, Lgals9^-/- mice were more susceptible to developing watery diarrhea and more prone to death in response to high-dose cholera toxin. The results indicate the importance of galectin-9 in modulating intestinal adaptive immunity. Furthermore, bone marrow chimera mice were established, and galectin-9 in hematopoietic cells was found to be critical for adaptive IgA response. In addition, immunized Lgals9^-/- mice exhibited lower expression of Il17 and fewer T helper 17 (Th17) cells in the lamina propria, implying that the Th17-IgA axis is involved in this mechanism. Taken together, these findings suggest that galectin-9 plays a role in mucosal adaptive immunity through the Th17-IgA axis. By manipulating the expression or activity of galectin-9, intestinal mucosal immune response can be altered and may benefit the development of mucosal vaccination.

Secretion of Galectin-9 as a DAMP during Dengue Virus Infection in THP-1 Cells

Damage-associated molecular patterns (DAMPs) are endogenous cellular molecules released to the extracellular environment in response to stress conditions such as virus infection. Galectins are β-galactoside-binding proteins that are widely expressed in cells and tissues of the immune system, are localized in the cell cytoplasm, and have roles in inflammatory responses and immune responses against infection. Elevated levels of galectin-9 (Gal-9) in natural human infections have been documented in numerous reports. To investigate the effect of dengue virus (DENV) infection on expression of endogenous Gal-9, monocytic THP-1 cells were infected with varying doses of DENV-3 (multiplicity of infection (MOI) 0.01, 0.03 and 0.1) and incubated at varying time points (Day 1, Day 2, Day 3). Results showed augmentation of Gal-9 levels in the supernatant, reduction of Gal-9 levels in the cells and decreased expression of LGALS9 mRNA, while DENV-3 mRNA copies for all three doses remained stable through time. Dengue virus induced the secretion of Gal-9 as a danger response; in turn, Gal-9 and other inflammatory factors, and stimulated effector responses may have limited further viral replication. The results in this pilot experiment add to the evidence of Gal-9 as a potential DAMP.

Cancer Therapy Due to Apoptosis: Galectin-9

Dysregulation of apoptosis is a major hallmark in cancer biology that might equip tumors with a higher malignant potential and chemoresistance. The anti-cancer activities of lectin, defined as a carbohydrate-binding protein that is not an enzyme or antibody, have been investigated for over a century. Recently, galectin-9, which has two distinct carbohydrate recognition domains connected by a linker peptide, was noted to induce apoptosis in thymocytes and immune cells. The apoptosis of these cells contributes to the development and regulation of acquired immunity. Furthermore, human recombinant galectin-9, hG9NC (null), which lacks an entire region of the linker peptide, was designed to resist proteolysis. The hG9NC (null) has demonstrated anti-cancer activities, including inducing apoptosis in hematological, dermatological and gastrointestinal malignancies. In this review, the molecular characteristics, history and apoptosis-inducing potential of galectin-9 are described.

Galectin-9 in Combination With EX-527 Prolongs the Survival of Cardiac Allografts in Mice After Cardiac Transplantation

Galectin-9 (Gal-9), a member of the galectin family, has a variety of biologic activities. However, its role in allografts is not fully clarified yet. The relationship between interleukin-17 (IL-17) and Gal-9 and the role of Gal-9 in T(H)17-cell differentiation also remain unclear. We built a murine cardiac transplantation model, which we treated with Gal-9 and/or EX-527, a specific Sirtuin-1 inhibitor. Afterwards, flow-cytometric analysis and reverse-transcription polymerase chain reaction were used to detect the number of T(H)17 cells and the expression of key factors involved in the differentiation of T(H)17 cells; in addition, the survival times of cardiac transplanted mice in different groups were recorded. The levels of circulating T(H)17 cells were found to increase in the peripheral blood of mice that exhibited acute rejection (AR) after heart transplantation, which was determined to be correlated with the rejection grade. Gal-9 or EX-527 can inhibit the activation and differentiation of T(H)17 cells and effectively suppress T(H)17-cell-mediated AR. These data provide new evidence for the potential regulatory effects of Gal-9 in alloimmune responses in a murine model of heart transplantation, and suggest the combined use of galectin-9 and EX-527 may be a powerful method to induce tolerance of fully mismatched murine cardiac allografts.

Copy-number variation of functional galectin genes: studying animal galectin-7 (p53-induced gene 1 in man) and tandem-repeat-type galectins-4 and -9

Galectins are potent adhesion/growth-regulatory effectors with characteristic expression profiles. Understanding the molecular basis of gene regulation in each case requires detailed information on copy number of genes and sequence(s) of their promoter(s). Our report reveals plasticity in this respect between galectins and species. We here describe occurrence of a two-gene constellation for human galectin (Gal)-7 and define current extent of promoter-sequence divergence. Interestingly, cross-species genome analyses also detected single-copy display. Because the regulatory potential will then be different, extrapolations of expression profiles are precluded between respective species pairs. Gal-4 coding in chromosomal vicinity was found to be confined to one gene, whereas copy-number variation also applied to Gal-9. The example of rat Gal-9 teaches the lesson that the presence of multiple bands in Southern blotting despite a single-copy gene constellation is attributable to two pseudogenes. The documented copy-number variability should thus be taken into consideration when studying regulation of galectin genes, in a species and in comparison between species.

Renal transport of uric acid: evolving concepts and uncertainties

In addition to its role as a metabolic waste product, uric acid has been proposed to be an important molecule with multiple functions in human physiologic and pathophysiologic processes and may be linked to human diseases beyond nephrolithiasis and gout. Uric acid homeostasis is determined by the balance between production, intestinal secretion, and renal excretion. The kidney is an important regulator of circulating uric acid levels by reabsorbing about 90% of filtered urate and being responsible for 60% to 70% of total body uric acid excretion. Defective renal handling of urate is a frequent pathophysiologic factor underpinning hyperuricemia and gout. Despite tremendous advances over the past decade, the molecular mechanisms of renal urate transport are still incompletely understood. Many transport proteins are candidate participants in urate handling, with URAT1 and GLUT9 being the best characterized to date. Understanding these transporters is increasingly important for the practicing clinician as new research unveils their physiologic characteristics, importance in drug action, and genetic association with uric acid levels in human populations. The future may see the introduction of new drugs that act specifically on individual renal urate transporters for the treatment of hyperuricemia and gout.

Regulation of uric acid excretion by the kidney

It has been known for many years that the kidney plays a major role in uric acid homeostasis, as more than 70% of urate excretion is renal. Furthermore, hyperuricemia in gout is most commonly the result of relative urate underexcretion, as the kidney has enormous capacity for urate reabsorption. A clear understanding of the mechanisms of renal handling of urate has been hampered by the differences between humans and animal models. The power of human genetics and genome-wide association studies has now provided new insight into the molecular mechanisms of urate transport by identifying the transporters that have critical roles in urate transport. This review surveys the new evidence for a molecular model of urate transport in the renal proximal tubule and uses these data to refute the popular four-component model for urate transport that has long been in vogue. It also discusses data that help us understand the relation of diuretics to hyperuricemia, losartan-induced uricosuria, variations in uric acid levels in hyperglycemia, and the effects of dairy diets on serum urate levels. In the end, several of these clinical findings are explained, and the remaining gaps in our knowledge will become evident.

Uric Acid Nephrolithiasis: A Systemic Metabolic Disorder

Uric acid nephrolithiasis is characteristically a manifestation of a systemic metabolic disorder. It has a prevalence of about 10% among all stone formers, the third most common type of kidney stone in the industrialized world. Uric acid stones form primarily due to an unduly acid urine; less deciding factors are hyperuricosuria and a low urine volume. The vast majority of uric acid stone formers have the metabolic syndrome, and not infrequently, clinical gout is present as well. A universal finding is a low baseline urine pH plus insufficient production of urinary ammonium buffer. Persons with gastrointestinal disorders, in particular chronic diarrhea or ostomies, and patients with malignancies with a large tumor mass and high cell turnover comprise a less common but nevertheless important subset. Pure uric acid stones are radiolucent but well visualized on renal ultrasound. A 24 h urine collection for stone risk analysis provides essential insight into the pathophysiology of stone formation and may guide therapy. Management includes a liberal fluid intake and dietary modification. Potassium citrate to alkalinize the urine to a goal pH between 6 and 6.5 is essential, as undissociated uric acid deprotonates into its much more soluble urate form.

G109T polymorphism of SLC22A12 gene is associated with serum uric acid level, but not with metabolic syndrome

SLC22A12 gene, encoding urate transport 1, has been known to be responsible to urate metabolism. This study sought to determine the association between the novel G109T polymorphism in SLC22A12 with serum uric acid and the development of metabolic syndrome in Korean male subjects. A total of 132 healthy male subjects were enrolled in this study. Metabolic syndrome was determined using the modified guidelines for metabolic syndrome proposed by the National Cholesterol Education Program's Third Adult Treatment Panel. Genotyping for the SLC22A12 gene was assessed using denaturing high-performance liquid chromatography analysis. Serum uric acid and fractional excretion of uric acid (FEUA) from blood and urine samples were measured. Frequencies of the 109GG, 109GT, and 109TT genotypes were 57.6, 38.6, and 3.8%, respectively. Serum uric acid levels and FEUAs were significantly different among the three genotypes of the G109T polymorphism (P = 0.035 and P = 0.033, respectively). In addition, subjects of genotypes with the T allele had lower uric acid levels and higher FEUAs compared to those with the 109GG genotype (P = 0.007 and P = 0.031, respectively). The G109T polymorphism of the SLC22A12 gene has no association with metabolic syndrome. However, a number of metabolic syndrome components were related to serum uric acid level (r = 0.285, P = 0.001) and also significantly different between genotype with and without T allele (P = 0.008). The novel G109T polymorphism of the SLC22A12 gene is related to serum uric acid level, but not to the development of metabolic syndrome.

Galectin multimerization and lattice formation are regulated by linker region structure

Galectins regulate cellular functions by binding to glycan ligands on cell surface glycoprotein receptors. Prototype galectins, such as galectin-1, are one carbohydrate recognition domain (CRD) monomers that noncovalently dimerize, whereas tandem-repeat galectins, such as galectin-9, have two non-identical CRDs connected by a linker domain. Dimerization of prototype galectins, or both CRDs in tandem-repeat galectins, is typically required for the crosslinking of glycoprotein receptors and subsequent cellular signaling. Several studies have found that tandem-repeat galectins are more potent than prototype galectins in triggering many cell responses, including cell death. These differences could be due to CRD specificity, the presence or absence of a linker domain between CRDs, or both. To interrogate the basis for the increased potency of tandem-repeat galectins compared with prototype galectins in triggering cell death, we created three tandem-repeat galectin constructs with different linker regions joining identical galectin-1 CRDs, so that any differences we observed would be due to the contribution of the linker region rather than due to CRD specificity. We found that random-coil or rigid α-helical linkers that permit separation of the two galectin-1 CRDs facilitated the formation of higher-order galectin multimers and that these galectins were more potent in binding to glycan ligands and cell surface glycoprotein receptors, as well as triggering T cell death, compared with native galectin-1 or a construct with a short rigid linker. Thus, the increased potency of tandem-repeat galectins compared with prototype galectins is likely due to the ability of the linker domain to permit intermolecular CRD interactions, resulting in the formation of higher-order multimers with increased valency, rather than differences in CRD specificity.

Galectins: structure, function and therapeutic potential

Galectins are a family of animal lectins that bind beta-galactosides. Outside the cell, galectins bind to cell-surface and extracellular matrix glycans and thereby affect a variety of cellular processes. However, galectins are also detectable in the cytosol and nucleus, and may influence cellular functions such as intracellular signalling pathways through protein-protein interactions with other cytoplasmic and nuclear proteins. Current research indicates that galectins play important roles in diverse physiological and pathological processes, including immune and inflammatory responses, tumour development and progression, neural degeneration, atherosclerosis, diabetes, and wound repair. Some of these have been discovered or confirmed by using genetically engineered mice deficient in a particular galectin. Thus, galectins may be a therapeutic target or employed as therapeutic agents for inflammatory diseases, cancers and several other diseases.

Uric Acid Nephrolithiasis

Uric acid nephrolithiasis may be the final manifestation of various pathophysiological processes. Recent advances in renal urate transport have elucidated mechanisms by which hyperuricosuria occurs. However, in most uric acid stone formers the primary pathophysiologic defect is an excessively acidic urine pH rather than hyperuricosuria. Insulin resistance may contribute to the development of acidic urine by augmenting endogenous acid production and decreasing renal ammonium excretion. Medical management strategies focus primarily on alkali treatment or decreasing hyperuricosuria.

Insect cytokine, growth-blocking peptide, is a primary regulator of melanin-synthesis enzymes in armyworm larval cuticle

The cuticles of most insect larvae have a variety of melanin patterns that function in the insects' interactions with their biotic and abiotic environments. Larvae of the armyworm Pseudaletia separata have black and white stripes running longitudinally along the body axis. This pattern is emphasized after the last larval molt by an increase in the contrast between the lines. We have previously shown that 3,4-dihydroxy-L-phenylalanine (Dopa) decarboxylase (DDC) is activated during the molt period by preferential enhancement of its transcription in the epidermal cells beneath the black stripes. This study demonstrated that tyrosine hydroxylase (TH) expression is activated synchronously with DDC. Furthermore, enhancement of DDC and TH transcription is due to an increase in cyotoplasmic Ca(2+), which is induced by the insect cytokine, growth-blocking peptide (GBP). Enhanced gene expression for both enzymes was induced by substitution of the calcium ionophore A23187, and completely blocked by EGTA. A GBP-induced increase in cytoplasmic Ca(2+) was seen in epidermal cells under the black stripes but not those beneath the white stripes, suggesting that a difference in Ca(2+) concentration in stripe cells leads to the specific expression of DDC and TH genes. Based on the fact that epidermal cells beneath the white stripes contain abundant granules composed mainly of uric acid, which can form a complex with Ca(2+) and hence decrease its free concentration, we discuss the possibility that uric acid, as well as GBP, contributes to the difference in cytoplasmic Ca(2+) within the epidermal cells.

Uric Acid nephrolithiasis: recent progress and future directions

The prevalence of urolithiasis has been increasing for the past few decades in industrialized nations. Uric acid calculi account for a significant percentage of urinary stones. Certain risk factors may be involved in the pathogenesis of uric acid nephrolithiasis, including hyperuricosuria, low urinary volume, and persistently low urinary pH. Patients with medical conditions that promote profound hyperuricosuria are at high risk of developing uric acid calculi. These conditions include chronic diarrheal states; myeloproliferative disorders; insulin resistance, including diabetes mellitus; and monogenic metabolic disorders, such as Lesch-Nyhan syndrome. Computed tomography can provide a definitive diagnosis. Except in cases in which there is severe obstruction, progressive azotemia, serious infection, or unremitting pain, the initial treatment of patients with uric acid nephrolithiasis should be medical dissolution therapy because this approach is successful in the majority of cases. A thorough review of the epidemiology and pathophysiology of uric acid nephrolithiasis is crucial for the diagnosis, treatment, and prevention of stones in patients with this condition.

Crystal Structure of the Galectin-9 N-terminal Carbohydrate Recognition Domain from Mus musculus Reveals the Basic Mechanism of Carbohydrate Recognition

The galectins are a family of beta-galactoside-binding animal lectins with a conserved carbohydrate recognition domain (CRD). They have a high affinity for small beta-galactosides, but binding specificity for complex glycoconjugates varies considerably within the family. The ligand recognition is essential for their proper function, and the structures of several galectins have suggested their mechanism of carbohydrate binding. Galectin-9 has two tandem CRDs with a short linker, and we report the crystal structures of mouse galectin-9 N-terminal CRD (NCRD) in the absence and the presence of four ligand complexes. All structures form the same dimer, which is quite different from the canonical 2-fold symmetric dimer seen for galectin-1 and -2. The beta-galactoside recognition mechanism in the galectin-9 NCRD is highly conserved among other galectins. In the apo form structure, water molecules mimic the ligand hydrogen-bond network. The galectin-9 NCRD can bind both N-acetyllactosamine (Galbeta1-4GlcNAc) and T-antigen (Galbeta1-3GalNAc) with the proper location of Arg-64. Moreover, the structure of the N-acetyllactosamine dimer (Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAc) complex shows a unique binding mode of galectin-9. Finally, surface plasmon resonance assay showed that the galectin-9 NCRD forms a homophilic dimer not only in the crystal but also in solution.

New developments in the epidemiology and genetics of gout

The prevalence of gout appears to be rapidly increasing worldwide and is no longer a disorder suffered primarily by over-fed alcohol consumers. Emerging risk factors include longevity, metabolic syndrome, and new classes of pharmacologic agents. In some ethnic populations, no obvious risk factors can explain the high incidence of hyperuricemia and gout, suggesting a genetic liability. Studies to identify genes associated with gout have included families with defects in purine metabolism, as well as families in whom the occurrence of gout is secondary to renal disorders such as juvenile hyperuricemic nephropathy and medullary cystic kidney disease. Case-control studies of isolated aboriginal cohorts suffering from primary gout have revealed several chromosomal loci that may harbor genes that are important to the development and/or progression of gout.

Renal Urate Transport

Serum uric acid is determined by a balance between production and renal excretion. Luminal reabsorption of urate by the proximal tubule from the glomerular ultrafiltrate involves coupling between sodium-anion cotransport and urate-anion exchange. Apical sodium-coupled cotransport of lactate, ketoacids, nicotinate, and pyrazinoate increases intracellular levels of these anions in proximal tubular cells, stimulating the apical absorption of luminal urate via anion exchange. Hyperuricemia occurs when plasma levels of these anions increase; for example, hyperuricemia is a well-recognized concomitant of lactic acidosis and ketoacidosis. Relevant developments in the molecular and renal physiology of urate homeostasis are reviewed.

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.

Association of the human urate transporter 1 with reduced renal uric acid excretion and hyperuricemia in a German Caucasian population

OBJECTIVE

Human urate transporter 1 (hURAT1) is a member of the organic anion transporter family (SLC22A12) that mainly regulates tubular urate reabsorption. Loss-of-function mutations result in idiopathic hypouricemia. The present case-control study was designed to analyze whether hURAT1 might also be a candidate gene for hyperuricemia with primary reduced renal urate excretion.

METHODS

DNA samples from 389 individuals with reduced fractional excretion of uric acid (FEUA) (< or =6.5%) and from 263 controls (FEUA >6.5%) were sequenced. Genotype frequencies between groups were compared by Cochran-Armitage trend test.

RESULTS

Significantly different genotype distributions could be demonstrated for the -788 T >A (promoter; P = 0.014), the C258T (exon 1; P = 0.006), and the C426T (exon 2; P = 0.0002) polymorphisms, but not for the T1309C (exon 8) and the +18 C >T (intron 9) polymorphisms. The strongest association with reduced FEUA was observed for the C426T polymorphism, with odds ratios (ORs) of 1.59 and 2.54 (P = 0.0002) for the CT and TT genotypes, respectively. Adjusted values for FEUA in the C426T genotype, were significantly reduced decreasing to 7.3%, 6.7%, and 6.3% in individuals with the CC, CT, and TT genotypes, respectively (P = 0.004). Haplotypes were constructed from the -788 T >A, C258T, and C426T polymorphisms. Individuals carrying at least 1 ACT haplotype (n = 349) had a significantly higher risk for reduced FEUA than individuals without any ACT haplotype (n = 303) (OR 1.39, P = 0.041).

CONCLUSION

These results indicate that polymorphisms in the N-terminus of the hURAT1 gene were significantly associated with reduced renal uric acid excretion. The main regulating factor seems to be located close to the C426T polymorphism or is in strong linkage disequilibrium.

Hormonal and cytokine effects of uric acid

PURPOSE OF REVIEW

Current evidence supports the role of soluble uric acid as a true mediator of injury, exerting its effects through the induction of growth factors, cytokines, hormones and autacoids. In the present review, we summarize recent studies on the mechanisms involved in the uric acid deleterious effects.

RECENT FINDINGS

Although uric acid is considered an antioxidant in plasma, recent clinical and epidemiological studies have found that hyperuricemia is associated with mortality and development of hypertension, cardiovascular and chronic renal diseases. Experimental studies suggest that uric acid induce its detrimental effects at the cellular level entering to vascular smooth muscle cells (VSMC) via an organic anion transport system, and followed by the activation of specific MAP kinases, nuclear transcription factors, with stimulation of COX-2, PDGF A and C chain, PDGF alpha receptor, and various inflammatory mediators, including C-reactive protein and monocyte chemoattractant protein-1. Physiologically, these effects translate into a rise of arterial pressure, VSMC hypertrophy, tubulointerstitial infiltration and glomerular hypertension in the setting of renal vasoconstriction. Uric acid also promotes endothelial dysfunction through inactivation of NO and arresting the proliferation of endothelial cells. Thus, arteriosclerosis induced by hyperuricemia may be a novel mechanism for the development of essential hypertension.

SUMMARY

Soluble uric acid has important biologic roles. While it acts as an antioxidant, there is also evidence that uric acid has pro-inflammatory and proliferative effects on VSMC, and causes dysfunction of endothelial cells. These cellular mechanisms may translate into why uric acid is associated with renal and cardiovascular disease.

Roles of organic anion transporters (OATs) and a urate transporter (URAT1) in the pathophysiology of human disease

Renal proximal and distal tubules are highly polarized epithelial cells that carry out the specialized directional transport of various solutes. This renal function, which is essential for homeostasis in the body, is achieved through the close pairing of apical and basolateral carriers expressed in the renal epithelial cells. The family of organic anion transporters (OATs), which belong to the major facilitator superfamily (SLC22A), are expressed in the renal epithelial cells to regulate the excretion and reabsorption of endogenous and exogenous organic anions. We now understand that these OATs are crucial components in the renal handling of drugs and their metabolites, and they are implicated in various clinically important drug interactions, and their adverse reactions. In recent years, the molecular entities of these transporters have been identified, and their function and regulatory mechanisms have been partially clarified. Workers in this field have identified URAT1 (urate transporter 1), a novel member of the OAT family that displays unique and selective substrate specificity compared with other multispecific OATs. In the OAT family, URAT1 is the main transporster responsible for human genetic diseases. In this review, we introduce and discuss some novel aspects of OATs, with special emphasis on URAT1, in the context of their biological significance, functional regulation, and roles in human disease.

Uric acid causes vascular smooth muscle cell proliferation by entering cells via a functional urate transporter

BACKGROUND

Soluble uric acid stimulates vascular smooth muscle cell (VSMC) proliferation by activating mitogen-activated protein kinases, and stimulating COX-2 and PDGF synthesis. The mechanism by which uric acid enters the VSMC is not known. We hypothesized that uric acid enters via transporters similar to that observed in the kidney.

METHODS

We studied the uptake of uric acid into rat VSMC under polarized and depolarized conditions and in the presence of organic anion transport (OAT) inhibitors (probenecid and benzbromarone) or p-aminohippurate (PAH). We also examined the ability of probenecid to inhibit uric acid-induced VSMC proliferation and monocyte chemoattractant protein-1 (MCP-1) synthesis.

RESULTS

(14)C-Urate uptake was shown in VSMC and was enhanced under depolarized conditions. (14)C-Uric acid uptake was inhibited by probenecid and benzbromarone, as well as by unlabelled urate and PAH. Probenecid blocked VSMC proliferation and MCP-1 expression in response to uric acid. VSMC did not express rOAT1-3, rOAT-5 or URAT-1 mRNA by PCR, but did express the voltage-sensitive transporter (UAT) by both PCR and RNase protection assay.

CONCLUSIONS

Urate enters VSMC by both voltage-sensitive and OAT pathways, and the uptake, cell proliferation and MCP-1 expression can be blocked by OAT inhibitors. The specific transporter(s) responsible for the urate uptake remains to be determined.

Effect of ethanol on metabolism of purine bases (hypoxanthine, xanthine, and uric acid)

There are many factors that contribute to hyperuricemia, including obesity, insulin resistance, alcohol consumption, diuretic use, hypertension, renal insufficiency, genetic makeup, etc. Of these, alcohol (ethanol) is the most important. Ethanol enhances adenine nucleotide degradation and increases lactic acid level in blood, leading to hyperuricemia. In beer, purines also contribute to an increase in plasma uric acid. Although rare, dehydration and ketoacidosis (due to ethanol ingestion) are associated with the ethanol-induced increase in serum uric acid levels. Ethanol also increases the plasma concentrations and urinary excretion of hypoxanthine and xanthine via the acceleration of adenine nucleotide degradation and a possible weak inhibition of xanthine dehydrogenase activity. Since many factors such as the ALDH2*1 gene and ADH2*2 gene, daily drinking habits, exercise, and dehydration enhance the increase in plasma concentration of uric acid induced by ethanol, it is important to pay attention to these factors, as well as ingested ethanol volume, type of alcoholic beverage, and the administration of anti-hyperuricemic agents, to prevent and treat ethanol-induced hyperuricemia.

Elevation of galectin-9 as an inflammatory response in the periodontal ligament cells exposed to Porphylomonas gingivalis lipopolysaccharide in vitro and in vivo

Considerable evidence suggests that periodontal disease not only is caused by bacterial infection but also is associated with host susceptibility. Using in-house cDNA microarray analysis, we attempted to identify gene expression changes in human periodontal ligament (PDL)-derived cells with and without treatment with lipopolysaccharide (LPS) extracted from Porphylomonas gingivalis (P. gingivalis LPS). Of the five up-regulated genes in the PDLs treated with P. gingivalis LPS, galectin-9, which was reported to have eosinophil chemoattraction, was selected for further analyses. By semiquantitative reverse transcriptase-polymerase chain reaction (sqRT-PCR), real-time quantitative RT-PCR, and Western blot analyses, elevated galectin-9 gene expression was detected in LPS-treated PDL-derived cells. Consequently, it was confirmed that the LPS enhances the expression level of galectin-9 mRNA and protein in a time-dependent manner together with interleukin-8. In addition, strong immunoreaction for galectin-9 was detected in the PDL consisting of the periodontal pocket of a patient with severe periodontal disease. Furthermore, significant up-regulation of galectin-9 mRNA expression was detected in the mRNA from PDLs of patients with periodontal disease when compared with healthy donors (P < 0.05). These results suggest that galectin-9 expression is associated with inflammatory reactions in the PDL.

Uric acid and chronic renal disease: Possible implication of hyperuricemia on progression of renal disease

Although hyperuricemia has long been associated with renal disease, uric acid has not been considered as a true mediator of progression of renal disease. The observation that hyperuricemia commonly is associated with other risk factors of cardiovascular and renal disease, especially hypertension, has made it difficult to dissect the effect of uric acid itself. However, recent epidemiologic evidence suggests a significant and independent association between the level of serum uric acid and renal disease progression with beneficial effect of decreasing uric acid levels. Furthermore, our experimental data using hyperuricemic animals and cultured cells have provided robust evidence regarding the role of uric acid on progression of renal disease. Hyperuricemia increased systemic blood pressure, proteinuria, renal dysfunction, vascular disease, and progressive renal scarring in rats. Recent data also suggest hyperuricemia may be one of the key and previously unknown mechanisms for the activation of the renin-angiotensin and cyclooxygenase-2 (COX-2) systems in progressive renal disease. Although we must be cautious in the interpretation of animal models to human disease, these studies provide a mechanism to explain epidemiologic data that show uric acid is an independent risk factor for renal progression. Although there is no concrete evidence yet that uric acid bears a causal or reversible relationship to progressive renal disease in humans, it is time to reevaluate the implication of hyperuricemia as an important player for progression of renal disease and to try to find safe and reasonable therapeutic modalities in individual patients based on their clinical data, medication history, and the presence of cardiovascular complications.

Gene expression profiles in rat intestine identify pathways for 1,25-dihydroxyvitamin D(3) stimulated calcium absorption and clarify its immunomodulatory properties

Microarray technology has been used to discover 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) induced gene expression changes in rat small intestine in vivo. Here, we report gene expression changes related to intestinal absorption or transport, the immune system and angiogenesis in response to 1,25-(OH)(2)D(3). Vitamin D deficient rats were intrajugularly given vehicle or vehicle containing 730 ng of 1,25-(OH)(2)D(3)/kg of body weight. Intestinal mRNA was harvested from duodenal mucosa at 15 min, 1, 3, and 6 h post-injection and studied by Affymetrix microarrays. Genes significantly affected by 1,25-(OH)(2)D(3) were confirmed by quantitative RT-PCR with remarkable agreement. The most strongly affected gene in intestine was CYP24 with 97-fold increase at 6 h post-1,25-(OH)(2)D(3) treatment. Intestinal calcium absorption genes: TRPV5, TRPV6, calbindin D(9k), and Ca(2+) dependent ATPase all were up-regulated in response to 1,25-(OH)(2)D(3), supporting the currently accepted mechanism of 1,25-(OH)(2)D(3) induced transcellular calcium transport. However, a 1,25-(OH)(2)D(3) suppression of several intra-/intercellular matrix modeling proteins such as sodium/potassium ATPase, claudin 3, aquaporin 8, cadherin 17, and RhoA suggests a vitamin D regulation of tight junction permeability and paracellular calcium transport. Several other genes related to the immune system and angiogenesis whose expression was changed in response to 1,25-(OH)(2)D(3) provided evidence for an immunomodulatory and anti-angiogenic role of 1,25-(OH)(2)D(3).

Galectin genes: regulation of expression

In this review we have summarized the more recent studies on the expression of mammalian galectins. One interesting observation that can be made is that in most of microarrays and/or differential display analysis performed in recent years one or more galectins have been picked up. From a critical evaluation of the pertinent studies the main conclusion that can be drawn is that, although it is not yet clear whether the 14 galectins identified so far have functions in common, a striking common feature of all galectins is the strong modulation of their expression during development, differentiation stages and under different physiological or pathological conditions. This suggests that the expression of different galectins is finely tuned and possibly coordinated. In spite of these observations it is rather unexpected that very few studies have been performed on the molecular mechanisms governing the activity of galectin genes.

Galectin 9 is the sugar-regulated urate transporter/channel UAT

UAT, also designated galectin 9, is a multifunctional protein that can function as a urate channel/transporter, a regulator of thymocyte-epithelial cell interactions, a tumor antigen, an eosinophil chemotactic factor, and a mediator of apoptosis. We review the evidence that UAT is a transmembrane protein that transports urate, describe our molecular model for this protein, and discuss the evidence from epitope tag and lipid bilayer studies that support this model of the transporter. The properties of recombinant UAT are compared with those of urate transport into membrane vesicles derived from proximal tubule cells in rat kidney cortex. In addition, we review channel functions predicted by our molecular model that resulted in the novel finding that the urate channel activity is regulated by sugars and adenosine. Finally, the presence and possible functions of at least 4 isoforms of UAT and a closely related gene hUAT2 are discussed.

Exclusion of galectin 9 as a candidate gene for hyperuricosuria in the Dalmatian dog

All Dalmatian dogs have an inherited defect in purine metabolism leading to high levels of uric acid excretion in their urine (hyperuricosuria) rather than allantoin, the normal end product of purine metabolism in all other breeds of dog. Transplantation experiments have demonstrated that the defect is intrinsic to the liver and not the kidney. Uricase, the enzyme involved in the breakdown of urate into allantoin, has been shown to function in Dalmatian liver cells. Therefore, candidate genes for this defect include transporters of urate, a salt of uric acid, across cell membranes. We excluded one such urate transporter candidate, galectin 9, using a Dalmatian x Pointer backcross in which hyperuricosuria was segregating.

Human organic anion transporter MRP4 (ABCC4) is an efflux pump for the purine end metabolite urate with multiple allosteric substrate binding sites

The end product of human purine metabolism is urate, which is produced primarily in the liver and excreted by the kidney through a well-defined basolateral blood-to-cell uptake step. However, the apical cell-to-urine efflux mechanism is as yet unidentified. Here, we show that the renal apical organic anion efflux transporter human multidrug resistance protein 4 (MRP4), but not apical MRP2, mediates ATP-dependent urate transport via a positive cooperative mechanism (K(m) of 1.5 +/- 0.3 mM, V(max) of 47 +/- 7 pmol x mg(-1) x min(-1), and Hill coefficient of 1.7 +/- 0.2). In HEK293 cells overexpressing MRP4, intracellular urate levels were lower than in control cells. Urate inhibited methotrexate transport (IC50 of 235 +/- 8 microM) by MRP4, did not affect cAMP transport, whereas cGMP transport was stimulated. Urate shifted cGMP transport by MRP4 from positive cooperativity (K(m) and V(max) value of 180 +/- 20 microM and 58 +/- 4 pmol x mg(-1) x min(-1), respectively, Hill coefficient of 1.4 +/- 0.1) to single binding site kinetics (K(m) and V(max) value of 2.2 +/- 0.9 mM and 280 +/- 50 pmol x mg(-1) x min(-1), respectively). Finally, MRP4 could transport urate simultaneously with cAMP or cGMP. We conclude that human MRP4 is a unidirectional efflux pump for urate with multiple allosteric substrate binding sites. We propose MRP4 as a candidate transporter for urinary urate excretion and suggest that MRP4 may also mediate hepatic export of urate into the circulation, because of its basolateral expression in the liver.

Novel insights into the pathogenesis of uric acid nephrolithiasis

PURPOSE OF REVIEW

The factors involved in the pathogenesis of uric acid nephrolithiasis are well known. A low urinary pH is the most significant element in the generation of stones, with hyperuricosuria being a less common finding. The underlying mechanism(s) responsible for these disturbances remain poorly characterized. This review summarizes previous knowledge and highlights some recent developments in the pathophysiology of low urine pH and hyperuricosuria.

RECENT FINDINGS

Epidemiological and metabolic studies have indicated an association between uric acid nephrolithiasis and insulin resistance. Some potential mechanisms include impaired ammoniagenesis caused by resistance to insulin action in the renal proximal tubule, or substrate competition by free fatty acids. The evaluation of a large Sicilian kindred recently revealed a putative genetic locus linked to uric acid stone disease. The identification of novel complementary DNA has provided an interesting insight into the renal handling of uric acid, including one genetic cause of renal uric acid wasting.

SUMMARY

The recognition of metabolic, molecular, and genetic factors that influence urinary pH, and uric acid metabolism and excretion, will provide novel insights into the pathogenesis of uric acid stones, and open the way for new therapeutic strategies.

The W258X mutation in SLC22A12 is the predominant cause of Japanese renal hypouricemia

Recently, a urate transporter, hURAT1 (human uric acid transporter 1) encoded by SLC22A12, was isolated from the human kidney. hURAT1 is presumed to play the central role in reabsorption of urate from glomerular filtrate. In the present study, we analyzed SLC22A12 in seven unrelated Japanese patients with renal hypouricemia whose serum level of urate was less than 1.0 mg/dl, and their family members. We performed direct DNA sequencing of the exon and exon-intron boundaries of SLC22A12 using genomic DNA. Six of the seven patients (86%) possess mutations in SLC22A12. In five patients, a homozygous G to A transition at nucleotide 774 within exon 4 of SLC22A12, which forms a stop codon (TGA) at codon 258 (TGG), was identified (W258X). In one patient, the C to T transition within exon 3, which changes threonine at codon 217 to methionine (T217 M), and the W258X mutation were found (compound heterozygote). Thus, among 12 mutational alleles in six patients, 11 were the W258 X mutation (92%). Family members with the heterozygous W258X mutation (carriers) show relatively low levels of serum urate. The present study demonstrates that homozygous W258X mutation is the predominant genetic cause of idiopathic renal hypouricemia in Japanese patients.

Familial juvenile hyperuricemic nephropathy: Detection of mutations in the uromodulin gene in five Japanese families

BACKGROUND

Familial juvenile hyperuricemic nephropathy (FJHN) is an autosomal-dominant disease characterized by hyperuricemia of underexcretion type, gout, and chronic renal failure. We previously reported linkage on chromosome 16p12 in a large Japanese family designated as family 1 in the present study. Recent reports on the discovery of mutations of the uromodulin (UMOD) gene in families with FJHN encouraged us to screen UMOD mutations in Japanese families with FJHN, including family 1.

METHODS

Six unrelated Japanese families with FJHN were examined for mutations of the UMOD gene by direct sequencing. To confirm the results of the mutation screening, parametric linkage analyses were performed using markers in 16p12 region and around other candidate genes of FJHN.

RESULTS

Five separate heterozygous mutations (Cys52Trp, Cys135Ser, Cys195Phe, Trp202Ser, and Pro236Leu) were found in five families, including family 1. All mutations were co-segregated with the disease phenotype in all families, except for family 1, in which an individual in the youngest generation was found as a phenocopy by the genetic testing. Revised multipoint linkage analysis showed that the UMOD gene was located in the interval showing logarithm of odds (LOD) score above 6.0. One family carrying no mutation in the UMOD gene showed no linkage to the medullary cystic kidney disease type 1 (MCKD1) locus, the genes of hepatocyte nuclear factor-1beta (HNF-1beta), or urate transporters URAT1 and hUAT.

CONCLUSION

Our results gave an evidence for the mutation of the UMOD gene in the majority of Japanese families with FJHN. Genetic heterogeneity of FJHN was also confirmed. Genetic testing is necessary for definite diagnosis in some cases especially in the young generation.

Uric acid transport

PURPOSE OF REVIEW

The goal of this article is to review the physiology and describe newly defined molecular mechanisms that are responsible for renal urate transport.

RECENT FINDINGS

Four complementary DNAs have recently been cloned whose expressed proteins transport urate. Two of these proteins have been localized to the apical membrane of proximal tubular cells: one, a urate transporter/channel, a galectin, is an electrogenic transporter (an ion channel); the second is a urate-anion electroneutral exchanger, a member of the organic anion transporter family. The other urate transport proteins, organic anion transporters 1 and 3, are also members of the organic anion transporter family. These proteins have been localized to the basolateral membrane of proximal tubular cells: organic anion transporter 1 is an electroneutral organic anion exchanger; the mechanism of urate transport on organic anion transporter 3 remains to be determined.

SUMMARY

The molecular definition and localization of four urate transport proteins provides a basis for developing a molecular model of the bi-directional transport of urate in renal proximal tubules. It seems likely that the urate-anion exchanger is responsible for luminal reabsorption while the urate transporter/channel permits secretion of urate from the cell into the lumen. Since organic anion transporters 1 and 3 reside in the basolateral membrane, one or both may be relevant in the reabsorptive flux of urate into the peritubular capillary as well as in the cellular uptake of urate from the peritubular space, the first step in the process of urate secretion. Knowledge of the molecular basis of urate transport should provide greater insights into states of altered transport as well as assist in development of drugs to modify urate flux.

Hyperuricemia and gout

Gout is not a new disease for clinicians; nevertheless, there are still many secrets awaiting discovery for improving knowledge with respect to uric acid metabolism and monosodium urate crystal-induced inflammation. This review of the literature will focus on new insights on the pathogenesis of idiopathic hyperuricemia, and on secondary hyperuricemia and gout. There are also important advances on the pathophysiology of acute gout, especially as a self-limited process (switch from monocyte to macrophage, peroxisome proliferator activated receptor-gamma, and nitric oxide), but also of chronic gouty arthropathy. Armaments for treating hyperuricemia and gout may be already improved by losartan or fenofibrate and, in the future, by urate oxydase-polyethylene glycol 20 and renal handling regulatory molecules. Finally, control of hyperuricemia may also be considered in the prevention and treatment of cardiovascular disease.

Atypical familial juvenile hyperuricemic nephropathy associated with a hepatocyte nuclear factor-1beta gene mutation

BACKGROUND

Familial juvenile hyperuricemic nephropathy (FJHN) is a dominantly inherited condition characterized by young-onset hyperuricemia, gout, and renal disease. The etiologic genes are unknown, although a locus on chromosome 16 has been identified in some kindreds. Mutations in the gene encoding hepatocyte nuclear factor (HNF)-1beta have been associated with dominant inheritance of a variety of disorders of renal development, particularly renal cystic disease and early onset diabetes; hyperuricemia has been reported in some kindreds.

METHODS

To assess a possible role for the HNF-1beta gene in some FJHN kindreds we sequenced the HNF-1beta gene in subjects from three unrelated FJHN families with atypical features of renal cysts or abnormalities of renal development. We also compared serum urate levels in subjects with HNF-1beta mutations with populations of controls, type 2 diabetic subjects, and subjects with mild chronic renal failure without HNF-1beta mutations.

RESULTS

A splice-site mutation in intron 2, designated IVS2+1G>T, showed complete co-segregation with FJHN in one family with diabetes. Serum urate levels were significantly higher in the HNF-1beta subjects compared with the normal control subjects (384 micromol/L vs. 264 micromol/L, P = 0.002) and the type 2 diabetic subjects (397 micromol/L vs. 271 micromol/L, P = 0.01). Comparison of serum urate levels in the HNF-1beta subjects with gender-matched subjects with renal impairment of other causes did not reach significance (402 micromol/L vs. 352 micromol/L, P = 0.2).

CONCLUSION

Hyperuricemia and young-onset gout are consistent features of the phenotype associated with HNF-1beta mutations, but the mechanism is uncertain. Families with HNF-1beta mutations may fit diagnostic criteria for FJHN. Identification of HNF-1beta patients by recognizing the features of diabetes and disorders of renal development is important in resolving the genetic heterogeneity in FJHN.

Familial juvenile hyperuricaemic nephropathy (FJHN): linkage analysis in 15 families, physical and transcriptional characterisation of the FJHN critical region on chromosome 16p11.2 and the analysis of seven candidate genes

Familial juvenile hyperuricaemic nephropathy (FJHN) is an autosomal dominant renal disease characterised by juvenile onset of hyperuricaemia, gouty arthritis, and progressive renal failure at an early age. Recent studies in four kindreds showed linkage of a gene for FJHN to the same genomic interval on chromosome 16p11.2, where the gene for the phenotypically similar medullary cystic disease type 2 (MCKD2) has been localised. In this study we performed linkage analysis in additional 15 FJHN families. Linkage of FJHN to 16p11.2 was confirmed in six families, which suggests that, in a large proportion of FJHN kindreds, the disease is likely to be caused by a gene or genes located outside of 16p11.2. Haplotype analysis of the new and previously analysed families provided two non-overlapping critical regions on 16p11.2-FJHN1, delimited by markers D16S499-D16S3036 and FJHN2, delimited by markers D16S412-D16S3116. Considering MCKD2 to be a distinct molecular entity, the analysis suggests that as many as three kidney disease genes may be located in close proximity on 16p11.2. From genomic databases we compiled integrated physical and transcription maps of whole critical genomic region in which 45 known genes and 129 predicted loci have been localised. We selected, analysed and found no pathogenic mutations in seven candidate genes. The linkage and haplotype analysis reported here demonstrates the genetic heterogeneity of FJHN. The report of integrated physical and mostly in-silico predicted transcription maps of the FJHN critical region provides a basis for precise experimental annotation of the current transcript map, which is essential for final identification of the FJHN gene(s).

Urate transport via human PAH transporter hOAT1 and its gene structure

BACKGROUND

We recently cloned the human organic anion transporter 1 (hOAT1) as a p-aminohippurate (PAH) transporter. Whether urate is transported by the PAH transporter in humans remains unclear. Familial juvenile gouty nephropathy (FJGN) is thought to develop as a result of an abnormality in the urate transporter.

METHODS

To determine if hOAT1 transported urate, the cellular uptakes of PAH and urate were determined, as were the inhibition profiles of inorganic anions, and uricosuric and antiuricosuric agents using a mouse S2 cell line expressing hOAT1. The hOAT1 gene was cloned from a genomic library using full-length hOAT1-1 cDNA as a probe. The coding regions of the hOAT1 genes of two sisters with FJGN were sequenced. Also, immunohistochemical fluorescence analysis of hOAT1 in the kidney of the younger sister with FJGN was performed.

RESULTS

The Km and Vmax values of urate transport via hOAT1 were 943 +/- 84 micromol/L and 1286 +/- 162 pmol/mg protein/min, respectively. The order of the IC50 of urate transport via hOAT1 was benzbromarone < probenecid < salicylate or pyrazine carboxylic acid. The 10.9 kb hOAT1 gene was found to be interrupted by nine introns. Mutations in the coding region of the hOAT1 gene from the two sisters with FJGN were undetectable. Immunohistochemical fluorescent staining showed that hOAT1 in the kidney of the younger sister was similar to that of control individuals.

CONCLUSIONS

Our data show that hOAT1 transports urate, and the inhibition profiles of uricosuric and antiuricosuric agents are defined. hOAT1 is not responsible for FJGN in the two sisters examined in this study.

Selective eosinophil adhesion to fibroblast via IFN-gamma-induced galectin-9

Among galectin family members, galectin-9 was first described as a potent eosinophil chemoattractant derived from Ag-stimulated T cells. In the present study a role of galectin-9 in the interaction between eosinophils and fibroblasts was investigated using a human lung fibroblast cell line, HFL-1. RT-PCR, real-time PCR, and Western blot analyses revealed that both galectin-9 mRNA and protein in HFL-1 cells were up-regulated by IFN-gamma stimulation. On the one hand, IL-4, known as a Th2 cytokine, did not affect the galectin-9 expression in HFL-1 cells. We further confirmed that IFN-gamma up-regulated the expression of galectin-9 in primary human dermal fibroblasts. Flow cytometric analysis revealed that IFN-gamma up-regulated surface galectin-9 expression on HFL-1 cells. Stimulation of HFL-1 cells with IFN-gamma up-regulated adhesion of eosinophils, but not neutrophils, to HFL-1 cells. This adherence of eosinophils to HFL-1 cells was inhibited by both lactose and anti-galectin-9 Ab. These findings demonstrate that IFN-gamma-induced galectin-9 expression in fibroblasts mediates eosinophil adhesion to the cells, suggesting a crucial role of galectin-9 in IFN-gamma-stimulated fibroblasts as a physiological modulator at the inflammatory sites.

Facilitated diffusion of urate in avian brush-border membrane vesicles

Membrane transport pathways mediating transcellular secretion of urate across the proximal tubule were investigated in brush-border membrane vesicles (BBMV) isolated from avian kidney. An inside-positive K diffusion potential induced a conductive uptake of urate to levels exceeding equilibrium. Protonophore-induced dissipation of membrane potential significantly reduced voltage-driven urate uptake. Conductive uptake of urate was inhibitor sensitive, substrate specific, and a saturable function of urate concentration. Urate uptake was trans-stimulated by urate and cis-inhibited by p-aminohippurate (PAH). Conductive uptake of PAH was cis-inhibited by urate. Urate uptake was unaffected by an outward alpha-ketoglutarate gradient. In the absence of a membrane potential, urate uptake was similar in the presence and absence of an imposed inside-alkaline pH gradient or an outward Cl gradient. These observations suggest a uniporter-mediated facilitated diffusion of urate as a pathway for passive efflux across the brush border membrane of urate-secreting proximal tubule cells.

Functional analysis and molecular model of the human urate transporter/channel, hUAT

Recombinant protein, designated hUAT, the human homologue of the rat urate transporter/channel (UAT), functions as a highly selective urate channel in lipid bilayers. Functional analysis indicates that hUAT activity, like UAT, is selectively blocked by oxonate from its cytosolic side, whereas pyrazinoate and adenosine selectively block from the channel's extracellular face. Importantly, hUAT is a galectin, a protein with two beta-galactoside binding domains that bind lactose. Lactose significantly increased hUAT open probability but only when added to the channel's extracellular side. This effect on open probability was mimicked by glucose, but not ribose, suggesting a role for extracellular glucose in regulating hUAT channel activity. These functional observations support a four-transmembrane-domain structural model of hUAT, as previously predicted from the primary structure of UAT. hUAT and UAT, however, are not functionally identical: hUAT has a significantly lower single-channel conductance and open probability is voltage independent. These differences suggest that evolutionary changes in specific amino acids in these highly homologous proteins are functionally relevant in defining these biophysical properties.