Classical and channel-like urate transporters in rabbit renal brush border membranes

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.

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.

Molecular and cellular physiology of renal organic cation and anion transport

Organic cations and anions (OCs and OAs, respectively) constitute an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. Renal secretion of these compounds, which occurs principally along the proximal portion of the nephron, plays a critical role in regulating their plasma concentrations and in clearing the body of potentially toxic xenobiotics agents. The transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. It is increasingly apparent that basolateral and luminal OC and OA transport reflects the concerted activity of a suite of separate transport processes arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney now allows the development of models describing the molecular basis of the renal secretion of OCs and OAs. This review examines recent work on this issue, with particular emphasis on attempts to integrate information concerning the activity of cloned transporters in heterologous expression systems to that observed in studies of physiologically intact renal systems.

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.

Pathophysiology of uric acid nephrolithiasis

Humans although a predominantly ureotylic organism, has preserved the ability to excrete nitrogen as uric acid and ammonia. An imbalance between these two secondary modes of nitrogen excretion has resulted in uric acid precipitation in human urine. Uric acid nephrolithiasis can arise from diverse etiologies all with distinct underlying defects converging to one or more of three defects of hyperuricosuria, acidic urine pH, and low urinary volume, originating from secondary, genetic or heretofore undefined (idiopathic) causes. A subset of idiopathic uric acid nephrolithiasis (gouty diathesis) may be the "tip of the icebergp" of a broader systemic illness characterized by insulin resistance. A novel renal manifestation of insulin resistance is a mild defect in ammonium excretion, which is not severe enough to disturb acid-base homeostasis, but is sufficient to set up the chemical milieu for uric acid nephrolithiasis.

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.

Urate/alpha-ketoglutarate exchange in avian basolateral membrane vesicles

Membrane transport pathways for transcellular secretion of urate across the proximal tubule were investigated in avian kidney. The presence of coupled urate/alpha-ketoglutarate exchange was investigated in basolateral membrane vesicles (BLMV) by [(14)C]urate and [alpha-(3)H]ketoglutarate flux measurements. An inward Na gradient induced accumulation of alpha-ketoglutarate of sufficient magnitude to suggest a Na-dicarboxylate cotransporter. An inward Na gradient also induced concentrative accumulation of urate in the presence of alpha-ketoglutarate but not in its absence, suggesting urate/alpha-ketoglutarate exchange. alpha-Ketoglutarate-dependent stimulation of urate uptake was not observed in brush-border membrane vesicles. An outward urate gradient induced concentrative accumulation of alpha-ketoglutarate. alpha-Ketoglutarate-coupled urate uptake was specific for alpha-ketoglutarate, Cl dependent, and insensitive to membrane potential. alpha-Ketoglutarate-coupled urate uptake was inhibited by increasing p-aminohippurate (PAH) concentrations, and alpha-ketoglutarate-coupled PAH uptake was observed. alpha-Ketoglutarate-coupled PAH uptake was inhibited by increasing urate concentrations, and an outward urate gradient induced concentrative accumulation of PAH. These results suggest a Cl-dependent, alpha-ketoglutarate-coupled anion exchange mechanism as a pathway for active urate uptake across the basolateral 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.

Diffusive permeability of rabbit peritoneum for small solutes in vitro: effect of gentamicin and insulin

The purpose of the in vitro study was to compare the diffusive transport of creatinine, uric acid and glucose, directed from the interstitial (I) to the mesothelial (M) side of the peritoneum and in the opposite direction, before (15-60min) and after (75-120min) application of gentamicin and insulin. The experiments were undertaken on the rabbit parietal peritoneum in a modified Ussing chamber. A mathematical model was used to calculate a diffusive permeability coefficient P (in cm/s) and a diffusive mass transport coefficient KH (in mL/min). In the basic experimental series without drugs, the dynamics of peritoneal transport for examined solutes remained constant and there were no differences between transport directions (I--> M and M--> I). Mean values of P+/-SEM (KH+/-SEM) were 3.06+/-0.32 (321.1+/-33.5), 2.09+/-0.29 (219.2+/-30.7) and 2.73+/-0.47 (286.7+/-49.6) for creatinine, uric acid and glucose, respectively. The introduction of gentamicin decreased glucose transport directed from the mesothelial to the interstitial side of the membrane by about 12%. After insulin application we observed the increase of creatinine and glucose peritoneal transport rate. For creatinine, the above augmentation was by about 31% for I--> M and 83% for M--> I direction. In these conditions, the glucose transport directed from the interstitial to the mesothelial side of membrane increased by about 24%. Generally, in vitro gentamicin decreases, but insulin increases the diffusive permeability of the peritoneum for some small solutes. We suppose that these findings may be important for the efficiency of peritoneal dialysis.

Localization and topology of a urate transporter/channel, a galectin, in epithelium-derived cells

Recombinant protein produced from a cDNA cloned in our laboratory (UAT) functions in lipid bilayers as a urate transporter/channel. Because UAT is a galectin, a family of proteins presumed to be soluble, the localization and topology of UAT were assessed in living cells. UAT was targeted to plasma membrane in multiple epithelium-derived cell lines and, in polarized cells, was targeted to both apical and basolateral membranes. The amino and carboxy termini of UAT were both detected on the cytoplasmic side of plasma membranes, whereas cell surface biotinylation studies demonstrated that UAT is not merely a cytosolic membrane-associated protein but contains at least one extracellular domain. Madin-Darby canine kidney cells were shown both functionally and immunologically to contain an apparent homolog of UAT; however, transfection with UAT did not modify urate uptake. Because coimmunoprecipitation studies revealed that UAT is capable of forming both homo- and heteromultimers, it is proposed that monomers of endogenous channels are in part replaced by monomers of the protein expressed subsequent to transfection, thereby maintaining constancy of urate uptake at basal levels.

Expression of the urate transporter/channel is developmentally regulated in human kidneys

Recombinant protein prepared from cDNA cloned from rat kidney and its human homolog function as urate transporter/channels in lipid bilayers. Using the antibody (anti-uricase) that detected the rat cDNA clone, we now demonstrate that normal human kidneys contain an immunoreactive protein of identical size to that in rat kidney (36-37 kDa), presumably the human urate transporter/channel (hUAT). The amount of hUAT in kidney homogenates increases progressively from 13 wk of gestation to the early postnatal period. During gestation, hUAT expression is confined to the cytoplasm of proximal tubules of Stage III and/or IV nephrons. However, at 1 yr of age hUAT is primarily located subapically and within brush borders of proximal tubules. Xenopus laevis oocytes and differentiated A6 cells injected with cRNA and transfected with cDNA of hUAT, respectively, demonstrated a similar pattern: hUAT is not detected in oocytes but is abundantly expressed in cytoplasm and plasma membranes of A6 cells. These data imply that different developmental factors regulate the initiation of cytoplasmic hUAT expression and subsequent insertion into human proximal tubule brush-border membranes.

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

Elevated serum levels of uric acid have been associated with an increased risk for gout, hypertension, cardiovascular disease, and renal failure. The molecular mechanisms for the diminished excretion of urate in these disorders, however, remain poorly understood. Human galectin 9, which is highly homologous to the rat urate transporter rUAT, has been reported to be a secreted or cytosolic protein. We provide data that galectin 9 is hUAT, the first identified human urate transporter. hUAT is a highly selective urate ion channel when inserted in lipid bilayers. When expressed in renal epithelial cells it is an integral plasma membrane protein with at least two transmembrane domains. The gene for hUAT consists of 11 exons and is mapped to chromosome 17; a highly homologous gene, hUAT2, maps to a nearby region of chromosome 17 and is also likely to be a urate transporter. hUAT is expressed in a wide variety of tissues and is present in at least three isoforms; hUAT2 is less widely expressed at severalfold lower levels than hUAT. Further knowledge about the functions of hUAT, its isoforms, and hUAT2, as well as mutational analysis of hUAT1 and hUAT2 in individuals or families with hyperuricemia, should significantly improve our understanding of the molecular mechanisms of urate homeostasis.

Functional properties and physiological roles of organic solute channels

Membrane channels provide routes for the rapid, passive movement of solutes across plasma and intracellular membranes. It is generally assumed that the major physiological role of membrane channels is to transport inorganic ions for processes such as transepithelial salt absorption and secretion, cell volume regulation, signal transduction, and control of membrane electrical properties. Increasing evidence indicates, however, that channels play an important role in organic solute transport in a wide variety of cell types and organisms. Some of the major physiological roles of organic solute channels include uptake of nutrients, excretion of metabolic waste products, volume-regulatory organic osmolyte transport, and control of mitochondrial metabolism. This article reviews the functions and characteristics of channels that participate in the transport and regulation of both charged and electroneutral organic solutes.

Molecular cloning and functional reconstitution of a urate transporter/channel

Maintenance of urate homeostasis requires urate efflux from urate-producing cells with subsequent renal and gastrointestinal excretion. The molecular basis for urate transport, however, has not been identified. A novel full-length cDNA encoding a 322-amino acid protein, designated UAT (urate transporter), has been cloned from a rat renal cDNA library by antibody screening. UAT mRNA transcripts that approximate 1.55 kilobases are present, but differentially expressed in various rat tissues. Recombinant UAT protein that was expressed from the cloned cDNA in Escherichia coli and purified via immobilized metal affinity chromatography has been functionally reconstituted as a highly selective urate transporter/channel in planar lipid bilayers. The IgG fraction of the polyclonal antibody that was used to select the UAT clone from the cDNA library, but not nonimmune IgG, blocked urate channel activity. Based on the wide tissue distribution of the mRNA for UAT we propose that UAT provides the molecular basis for urate flux across cell membranes, allowing urate that is formed during purine metabolism to efflux from cells and serving as an electrogenic transporter that plays an important role in renal and gastrointestinal urate excretion.