Urate Transporters in the Kidney: What Clinicians Need to Know

Uric acid or urate homeostasis in humans is determined by the balance between hepatic production and renal and intestinal excretion1). Purines are metabolized to urate in the liver, and their sources are both exogenous (dietary intake) and endogenous (cellular metabolism of nucleic acids). The enzyme xanthine oxidase (XO) acts on the final step of purine metabolism from xanthine to urate, and XO inhibitors such as allopurinol and febuxostat are clinically useful as urate-lowering therapy. In most mammals, urate is converted by uricase or urate oxidase into allantoin, which is water-soluble and easily excreted in urine. In humans and apes, however, urate is the end product of purine metabolism because of nonsense mutations in uricase, and plasma urate levels are three to ten times higher than in other mammals2).

Uric acid has two dissociable protons with an aqueous pKa1 of 5.4 and pKa2 of 9.8 (Fig. 1). Consequently, at the physiological pH of 7.4, uric acid in the extracellular fluid is predominantly (98-99%) found as a deprotonated urate anion3). The divalent urate anion is practically nonexistent in the body because of the very high pKa2, and thus the term urate is generally used to refer to monovalent urate in the biomedical literature. Because the ratio of urate-to-uric acid in the circulation remains constant with constant pH, the terms urate and uric acid are often used interchangeably to refer to the total pool of uric acid, dissociated and undissociated1).

Fig. 1
Uric acid pKa and formation of urate salts.
At physiological pH, uric acid is predominantly found as the deprotonated urate anion. Adapted from Reference 3.

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Abnormal urate homeostasis can be classified into hyperuricemia and hypouricemia. Hyperuricemia may be induced by increased urate production in the liver or decreased urate excretion through the kidney and/or gut. Although a purine-rich diet may increase hepatic production of uric acid, diet alone is generally insufficient to cause hyperuricemia. Lesch-Nyhan syndrome is caused by hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency and resultant uric acid overproduction and is presented with dystonia, gout, intellectual disability, and self-mutilation4). HGPRT is an enzyme catalyzing the conversion of hypoxanthine to inosine monophosphate and guanine to guanosine monophosphate, generating purine nucleotides through the purine salvage pathway5). Urate underexcretion through the gut and/or kidney is the other important etiology of hyperuricemia, and intestinal and renal urate transporters may be involved in these pathological conditions. Diet and genetic polymorphisms in the renal transporters of urate seem to be the main causal factors of primary gout6).

Theoretically, hypouricemia can be caused by decreased urate production in the liver or increased urate excretion through the kidney and/or gut. Malnutrition may be associated with hypouricemia because of low dietary purine intake. Molecular defects in critical pathways involving urate synthesis in the liver or urate reabsorption in the kidney may cause hypouricemia. Medications affecting urate production in the liver (e.g., allopurinol, febuxostat) and urate transport through the gut and kidney (e.g., probenecid, benzbromarone) may lower serum uric acid levels.

The kidney is the major route of urate removal, typically responsible for 70-80% of daily urate excretion, and the remainder is eliminated by intestinal secretion7). Plasma urate is freely filtered at the glomeruli, and fractional excretion of urate is approximately 10% in adult humans1). Thus, 90% of the glomerular filtrates are normally reabsorbed along the nephron, but this is the net result of reabsorption and secretion in the proximal tubule.

Figure 2 illustrates major urate transporters in the proximal tubule. Considering the usual ranges of fractional excretion of urate, transporters mediating urate reabsorption may be more influential than those mediating urate secretion. The former includes apically located urate transporter 1 (URAT1), organic anion transporter 4 (OAT4), and OAT10, and basolaterally located glucose transporter 9 (GLUT9). URAT1, encoded by the SLC22A12 gene, is the primary urate/anion exchanger responsible for luminal urate reabsorption. For the exchange of urate with organic anions, it is coupled with sodium monocarboxylate cotransporter 1 (SMCT1) and SMCT2 at the apical membrane of the proximal tubule. With URAT1, organic anions such as nicotinate and pyrazinoate have a higher affinity than other anions such as lactate, β-hydroxybutyrate, acetoacetate, chloride, and nitrate8).

Fig. 2
Major urate transporters for bidirectional pathways in the proximal tubule.
(A) Na⁺-dependent anion transport by SMCT1 and SMCT2 increases intracellular concentrations of anions that exchange with luminal urate by URAT1. GLUT9 acts as the basolateral exit for urate reabsorption. (B) OAT1 and OAT3 transport urate through the basolateral membrane in exchange with α-ketoglutarate (α-KG). At the apical membrane, urate is secreted via ABCG2, NPT1, and/or NPT4.

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OAT4 and OAT10 may act as urate/anion exchangers like URAT1. OAT10, encoded by SLC22A13, is expressed at the messenger RNA (mRNA) level in the human kidney, but not at the protein level9). Its function as a urate/monocarboxylate exchanger was demonstrated only in vitro with a lower affinity for urate than URAT110). Human OAT10 is also expressed to a weak extent in the intestine but its function remains unclear11). Unlike URAT1 and OAT10, OAT4, encoded by SLC22A11, mediates urate/dicarboxylate exchange, with a lower estimated affinity for urate compared with URAT112). Thus, URAT1 appears to play the most important role in apical entrance for urate reabsorption.

Interestingly, URAT1 was also localized to the cell membrane of human vascular smooth muscle cells. This finding is compatible with the role of uric acid in cardiovascular disease, suggesting that uric acid enters the human vascular smooth muscle cell via URAT113).

GLUT9, encoded by the SLC2A9 gene, functions as the major mechanism for the basolateral exit of urate, allowing for its reabsorption into the blood10). Human GLUT9 has two splice variants with distinct N-terminal isoforms; GLUT9a is expressed in multiple tissues including the small intestine and colon, while GLUT9b (also termed GLUT9ΔN) is highly expressed in the kidney, and to a lesser extent in the liver14). Their unique N-termini are responsible for differential localization, with GLUT9a localizing to the basolateral membrane of the proximal tubule and GLUT9b located in the apical membrane of the proximal tubule14) or collecting duct15). In the murine kidney, Glut9a is expressed weakly in the proximal tubule whereas Glut9b is present in the distal convoluted tubule16). Physiologic action and species-specific intrarenal localization of GLUT9b remain to be confirmed.

Another set of transporters mediate urate secretion in the proximal tubule. ATP-binding cassette super-family G member 2 (ABCG2), also known as the breast cancer-resistant protein (BCRP), was initially identified as a transporter of drugs but has a role in the ATP-dependent urate efflux17). It is located in the apical membrane of the proximal tubule for secretion of urate, along with other ABC transporters such as multidrug resistance protein 2 (MRP2) and MRP418). ABCG2 is also highly expressed in intestinal tissue, where it excretes up to one-third of all uric acid and is thus thought to be the main extrarenal site of uric acid elimination19). To a lesser extent, MRP2 also called ATP-binding cassette sub-family C member 2 (ABCC2) and MRP4 also called ATP-binding cassette sub-family C member 4 (ABCC4) are expressed in the intestine to efflux organic anions including urate11).

NPT1, encoded by SLC17A1, is a sodium-phosphate cotransporter located in the apical membrane of the proximal tubule in the human kidney and was identified as a candidate protein in mediating the electrogenic exit of urate20). The highly related NPT4, encoded by SLC17A3, is located at the apical membrane of the proximal tubule and mediates voltage-dependent urate transport21). It was also reported to exist in the intestines and liver22). On the other hand, NPT5, encoded by SLC17A4, is expressed in the intestinal mucosae but not in the kidney23). These findings may indicate a possible involvement of the NPT homologs in urate efflux from the kidney and intestinal tract.

OAT1 and OAT3, encoded by SLC22A6 and SLC22A8, respectively, are located in the basolateral membrane of the proximal tubule for urate uptake and overall function in renal urate secretion. They exchange urate and other organic anions for intracellular dicarboxylates such as α-ketoglutarate, coupled to intracellular sodium transport through the sodium-dicarboxylate cotransporter 3 (NaDC-3) encoded by SLC13A35). OAT2, encoded by SLC22A7, similarly is located in the basolateral membrane of the proximal tubule24), and its activity as a low-affinity urate transporter was found in transfected HEK293 cells25).

Hypouricemia caused by a renal tubular defect was termed ‘renal hypouricemia’. The apical URAT1 and the basolateral GLUT9 are the main reabsorptive urate transporters, and loss-of-function mutations of the SLC22A12 and SLC2A9 gene can cause type 1 and type 2 renal hypouricemia, respectively26). Patients with renal hypouricemia can present with hematuria, urolithiasis, or exercise-induced acute kidney injury (AKI), and their fractional excretion of urate is much higher than 10% despite a very low level of serum urate. Among different mutations in the SLC22A12 gene, W258X (rs121907892) was predominant in patients with type 1 renal hypouricemia reported from both Japan27) and Korea28).

Whereas type 1 renal hypouricemia mainly occurs in Asian children, cases of type 2 renal hypouricemia were reported from various parts of the world, including Asia, Middle East, and Europe. Different mutations were clustered according to the regions, and patients with type 2 renal hypouricemia were often diagnosed during their adulthood29).

The reason why exercise-induced AKI can occur in patients with renal hypouricemia is unclear. One explanation could be that uric acid is a powerful antioxidant that scavenges singlet oxygen, oxygen radicals, and peroxynitrite7). The absence of uricase occurred approximately 30 million years ago in hominid evolution and was associated with the loss of the ability to synthesize ascorbic acid de novo30), suggesting that uric acid may have replaced ascorbic acid as an antioxidant31). Interestingly, uric acid may function as an antioxidant in plasma and may act as a pro-oxidant within the cell (in cardiovascular disease)32). In patients with hypouricemia, the antioxidant activity of uric acid is overwhelmed by the massive reactive oxygen species produced by strenuous exercise. Thus, loss of antioxidant activity may lead to vascular constriction and endothelial damages, progressing to AKI33).

Pharmacologic agents for urate-lowering therapy can be classified into XO inhibitors, uricosuric agents, and uricases. Among these, uricosuric agents may act on urate transporters in the proximal tubule to increase urinary urate excretion. Both the inhibition of urate-reabsorbing transporters and stimulation of urate-secreting transporters might be conceivable, but the former is the known mechanism of action of uricosuric agents.

The apical URAT1 is the main target of uricosuric action. Probenecid and benzbromarone remarkably inhibit urate transport by URAT. Basolateral GLUT9 is also downregulated by probenecid and benzbromarone49). Lesinurad is a recently introduced uricosuric drug that may be used in combination with allopurinol or febuxostat. Its mechanism of action derives from the inhibition of URAT1, although OAT4 inhibition could also be associated3).

Losartan has a uricosuric action because of the inhibition of URAT1 and GLUT98). Sodium-glucose cotransporter-2 (SGLT2) inhibitors also have a uricosuric effect, probably induced by the inhibition of GLUT9b in the collecting duct50). On the other hand, pyrazinamide may induce hyperuricemia via the stimulation of URAT11).

Serum uric acid levels are different between men and women, and the prevalence of hyperuricemia in adult males is higher than in females51). Sex hormones may play a role in regulating the expression or activity of urate transporters because SLC2A9 may have a stronger association with lower serum uric acid levels in females, whereas ABCG2 may have a stronger association with higher serum uric acid levels in males52). The ABCG2 Q140K+/+ knock-in mouse model is orthologous to the human ABCG2 Q141K variant. The Q140K+/+ mice showed hyperuricemia in males but not in females. Urinary urate excretion decreased in association with reduced renal ABCG2 protein abundance in male Q140K+/+ mice. However, urinary urate excretion and renal ABCG2 protein abundance were unaltered in female Q140K mice53).

The regulation of renal urate transporter expression by female hormones was investigated using ovariectomized mice with or without hormone replacement. Takiue et al. reported that estradiol suppressed the protein levels of URAT1, GLUT9, and ABCG2 in mouse kidneys54). On the other hand, the effect of testosterone was investigated in orchiectomized mice with or without testosterone replacement. Testosterone enhanced mRNA and protein levels of sodium-coupled monocarboxylate transporter 1 while those of GLUT9 were attenuated. Although the mRNA level of URAT1 was enhanced by testosterone, the corresponding levels of URAT1 protein remained unaffected55).

Other hormones might affect urate transporters in the kidney and gut for the regulation of urate homeostasis. Glucocorticoid was reported to increase urate excretion in mice by downregulating URAT156). Hyperuricemia may be associated with secondary hyperparathyroidism in patients with CKD. Sugimoto et al. reported that ABCG2 was downregulated by parathyroid hormone, increasing serum urate levels and these effects could be prevented by administration of the calcimimetic cinacalcet57).

Kim et al. investigated responses of urate transporters to increased uric acid intake in rat kidneys. Whereas URAT1 protein abundance was not affected, OAT1 protein abundance was increased by uric acid supplementation58). The up-regulation of OAT1 would exert stimulation of urinary urate excretion and might contribute to protection from hyperuricemia.

The oral administration of oxonic acid can induce hyperuricemia in rats. In this rat model, Nagura et al. found an increase in GLUT9 mRNA expression but no changes in URAT1 and ABCG2 mRNA in the kidney. Interestingly, ABCG2 mRNA increased in the ileum of rats with oxonic acid-induced hyperuricemia59). The lack of valid antibodies for urate transporters has limited studies at the protein level.

Because ABCG2 is localized in both the kidney and gut, the relative contribution of intestinal urate secretion will be increased in patients with CKD. Consistent with this, Yano et al. showed that ABCG2 mRNA expression increased in the ileum of rats with 5/6 nephrectomy60). In CKD, the role of intestinal urate transporters would be more important for maintaining urate homeostasis.

The kidney plays an important role in the maintenance of urate homeostasis because renal excretion of uric acid represents 70-80% of total uric acid excretion from the body. Approximately 10% of the glomerular filtered urate is normally excreted in urine after reabsorption and secretion in the proximal tubule. URAT1 (SLC22A12) is the major apical pathway for urate reabsorption, and GLUT9 (SLC2A9) is the principal pathway of basolateral urate exit from the proximal tubule cell in the human kidney. The loss-of-function mutation in either URAT1 or GLUT9 leads to renal hypouricemia. ABCG2, an apical ATP-driven efflux pump, functions in urate secretion by the proximal tubule and intestine, and ileal ABCG2 may be upregulated in CKD when renal urate transporters are downregulated. Genetic variations in SLC2A9, ABCG2, and SLC22A12 are associated with serum uric acid levels or gout (Fig. 3). Regulation of urate homeostasis at the level of urate transporters needs to be elucidated.

Fig. 3
A genome-wide association analysis of serum uric acid in 6,881 Korean individuals.
Manhattan plot depicts that the major urate transporter genes SLC22A12, ABCG2, and SLC2A9 are highly associated with hypouricemia or hyperuricemia. Adapted from Reference 40.

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