Role of Transporters in Hyperuricemia

Abstract

Hyperuricemia (HUA) is a metabolic disorder characterized by elevated serum uric acid levels exceeding the body’s metabolic limit. In the past two decades, the prevalence of this disease has shown an increasing trend and is becoming more common in younger individuals. As a metabolic disease, hyperuricemia has been found to correlate with cardiovascular diseases, renal diseases, and metabolic syndrome. Various complex metabolic processes are involved in the pathological process in the elevation of uric acid. Transporters are one of the most important families controlling the metabolism of uric acid. The vast majority of cases of hyperuricemia are caused by insufficient uric acid excretion and excessive reabsorption by the kidneys. Therefore, limiting the reabsorption of transport proteins is key to lowering uric acid levels. This chapter will revisit the basic situation of hyperuricemia and summarize the known mechanisms of transport proteins in HUA, as well as the therapeutic approaches developed for these transport proteins.

1. Introduction

1.1 Hyperuricemia

1.1.1 Concept

Hyperuricemia, characterized by elevated uric acid (UA) levels in the bloodstream, can arise from both exogenous (dietary) and endogenous purine conversion processes, primarily orchestrated by the liver and intestines [1]. Although kidneys and intestines typically excrete uric acid, an increase in production or impaired elimination pathways may lead to its accumulation, resulting in hyperuricemia. Elevated uric acid concentrations can foster urate crystal formation, which deposits in joints and tissues, triggering diseases like gout.

According to the “2021 Guidelines for the Diagnosis and Treatment of Hyperuricemia and Gout,” regardless of gender and under normal purine diet conditions, a fasting blood uric acid concentration exceeding 420 μmol/L on two separate occasions indicates hyperuricemia [2]. Epidemiological studies demonstrate that in males, hyperuricemia is identified by serum uric acid levels exceeding 7.0 mg/dL (>420 μmol/L), while in females, it is noted at levels surpassing 6.0 mg/dL (>360 μmol/L) [3]. These parameters offer valuable insights into the clinical assessment of hyperuricemia, forming a basis for diagnosis and management.

1.1.2 Epidemiology of hyperuricemia

During the years 2015–2016, the prevalence of hyperuricemia among adult men and women in the United States stood at 20.2 and 20.0%, respectively, and has demonstrated stability over the preceding decade [4]. As of 2021, research has unveiled a range of hyperuricemia prevalence rates across regions, with the United States displaying figures spanning from 11.3 to 47%, Europe exhibiting rates between 11.9 and 25.0%, Japan registering a prevalence of 26.8%, and China showing rates fluctuating between 13.1 and 13.3% [5]. Recent epidemiological studies suggest a relatively high prevalence of hyperuricemia with an upward trajectory [6].

1.1.3 Relationship between hyperuricemia and other diseases

Hyperuricemia is increasingly recognized as a significant health concern, often dubbed the “fourth high,” alongside elevated blood sugar, high blood lipids, and hypertension. Extensive research highlights the link between hyperuricemia and various medical conditions, such as gout, chronic kidney disease, cardiovascular diseases, and diabetes. The presence of hyperuricemia, characterized by the deposition of monosodium urate (MSU) crystals and the activation of inflammatory pathways, poses a primary risk factor for gout attacks [7].

Moreover, hyperuricemia contributes to an increased incidence of coronary heart disease (CHD) and impacts the progression and prognosis of heart failure patients [8, 9]. Investigations reveal a heightened prevalence of diabetes among individuals with hyperuricemia and gout, emphasizing the potential for controlling uric acid levels to partially prevent early-stage diabetes [10].

In the context of chronic kidney disease (CKD), hyperuricemia assumes a significant clinical role in disease onset. Uric acid itself may pose harm to CKD patients by amplifying inflammation and fostering CKD progression. Elevated serum uric acid levels can induce kidney damage through crystal-dependent pathways and non-crystal-dependent mechanisms, including inflammation, oxidative stress, and hemodynamic changes [11]. These findings underscore the multifaceted impact of hyperuricemia on various health conditions, highlighting its relevance in clinical contexts.

Exploring hyperuricemia is crucial for unraveling its associations with gout, uric acid-related kidney diseases, metabolic syndrome, and cardiovascular diseases, among others. A thorough investigation into the causes, mechanisms, and contributing factors of hyperuricemia is essential for enhancing our comprehension and enabling early diagnosis and intervention. This comprehensive research is pivotal for developing more effective treatment strategies, ultimately improving patient outcomes and contributing to overall population health enhancement.

1.2 Mechanisms of uric acid metabolism

Purine sources within the human body are distinguished into endogenous and exogenous categories, with 80% originating from endogenous synthesis or the oxidative breakdown of nucleic acids. The residual 20% is derived from exogenous purines, predominantly influenced by dietary factors, with notable contributions from animal proteins. Uric acid formation, arising from purine catabolism, is chiefly orchestrated through a sequence of enzymatic reactions involving xanthine oxidase, primarily occurring in the liver and intestines [2].

Under normal physiological conditions, the synthesis and excretion of uric acid maintain a dynamic equilibrium, with the kidneys serving as the principal regulatory factor for uric acid excretion [12, 13]. Approximately 70% of UA processing takes place through renal mechanisms, involving initial free filtration at the renal glomerulus, followed by secretion and reabsorption facilitated by renal tubules and associated transport proteins. The majority of circulating urate exists in a free form, exhibiting a protein binding rate of less than 5%, rendering them readily filterable by the renal glomerulus. Nevertheless, as much as 90% of urate may undergo reabsorption, returning to the bloodstream [3]. The remaining 30% of UA traverses the intestines, where transport proteins and microbial metabolism collectively contribute to its elimination (Figure 1) [14, 15, 16].

Figure 1.

Mechanisms of UA production and excretion. Figure legends: Endogenous purines produced by cell metabolism or apoptosis and exogenous purines produced by ingested food constitute an important source of purines in the human body, mainly in the liver after a series of enzymatic reactions with xanthine oxidase as an important rate-limiting enzyme to generate uric acid, about 30% of uric acid through the intestinal transport proteins and the role of the bacterial flora to the body; about 70% of uric acid through the renal filtration, after filtration, about 90% is reabsorbed back into the blood circulation with the assistance of transport proteins, and the remaining 10% of uric acid is excreted. About 70% of the uric acid is filtered through the kidneys, about 90% of the filtered uric acid is reabsorbed back into the blood circulation with the assistance of transport proteins, and the remaining about 10% of the uric acid is excreted from the body.

1.3 Transport proteins

Transporter proteins, a class pivotal in orchestrating the movement of molecules or ions within living organisms, exhibit their functional roles on cell membranes or within the cellular membrane system. Through specific structures and mechanisms, these proteins facilitate the transfer of specific molecules from one location to another. The human genome presently encompasses more than 400 annotated membrane transporter proteins [17]. Following the guidelines established by the HUGO Gene Nomenclature Committee, these transporter proteins are mainly categorized into the solute carrier (SLC) and ATP-binding cassette (ABC) superfamily [18].

The SLC proteins, constituting an exceptionally diverse superfamily with over 400 members, exhibit extensive functional variability [19]. Concurrently, the ABC transporter proteins, totaling 48 members within the transmembrane transporter protein superfamily, actively engage in transporting diverse biological substrates. The multifaceted functionalities of these transporter proteins underscore their widespread involvement in numerous facets of human biology [20].

In this context, we give a brief overview of the principal transporter proteins identified in different organs, including kidneys, liver, and intestines (Table 1).

The transporting family	Transporter	Dominant location	Primary function	References
Solute carrier transporters	OATP	Liver, kidney, intestine	Organic anion transporting polypeptide	[21, 22]
	NTCP	Liver	Sodium taurocholate cotransporting	[23]
	OAT	Liver, kidney	Organic anion transporters	[24]
	OCT	Liver, kidney	Organic Cation Transporter	[25]
	MATE	Liver, kidney	Multidrug and toxic compound extrusion	[26]
	URAT1	Kidney, intestine	Urate transporter	[27]
	OCTN	Kidney, intestine	Organic Cation Transporters Novel	[28]
	OSTα/β	Liver	Bile acids, steroid metabolites and drugs can be transported into and out of cells	[29]
	PEPT	Intestine	Transporting peptides or proteins between intracellular and extracellular spaces.	[30]
	PMAT	Intestine	Capable of transporting monoamine neurot ransmitters with low affinity and high capacity	[31]
	MCT	Liver, intestine	Catalyze the proton linking and transport of monocarboxylate across plasma membranes	[32, 33]
	GLUT	Intestine	Glucose is transported along the concentration gradient in a facilitated diffusion manner	[34]
	Pgp (MDR)	Kidney, intestine	Multidrug resistant protein	[35]
ATP-Binding Cassette (ABC) Transporter	BCRP (ABCG 2)	Liver, kidney, intestine	The breast cancer resistance protein	[36]
	MRP	Liver, kidney, intestine	Causes multidrug resistance in tumor cells by squeezing anticancer drugs out of cells	[11]
	BSEP	Liver	Eliminating unconjugated and conjugated bile acids/salts from hepatocytes into bile	[37]

Table 1.

Transport proteins in the liver, kidneys, and intestines.

1.3.1 Transport proteins in the kidneys

In the renal domain, numerous SLC transporter proteins are present, including organic anion transporters (OAT 1/2/3/4), organic cation transporters (OCT 2/3), organic anion-transporting polypeptides (OATP 4C 1), multidrug and toxin extrusion proteins (MATE 1/2-K), urate anion exchanger protein (URAT1), and organic cation/carnitine transporters (OCTN 1/2), among others. Specifically, the SLC22 transporter protein family encompasses OAT, OCT, OCTN, and URAT1 [38].

In the renal environment, ABC transporter proteins are also present, including multidrug resistance protein (MDR 1 or PGP), breast cancer resistance protein (BCRP), and multidrug resistance-associated proteins (MRP 2/4). These transporters utilize the energy derived from ATP hydrolysis to facilitate the movement of molecules across cellular membranes [39].

1.3.2 Transport proteins in the liver

In the hepatic milieu, SLC transporter proteins prominently feature organic anion-transporting polypeptide (OATP), sodium-dependent taurocholate cotransporting polypeptide (NTCP) [23], and organic cation transporter proteins (OCT) exemplified by OCT1 and OCT2 [40], along with organic solute transporter alpha/beta (OSTα/β). OATP assumes a pivotal role as a major drug carrier protein, facilitating sodium-independent hepatic uptake of organic anions [41]. Transport facilitated by OSTα/β exhibits bidirectionality (uptake or efflux), operates in an ATP-independent manner, and relies on the electrochemical gradient [42].

The ABC transporter family primarily comprises multidrug resistance proteins (MRP) such as MRP2, multidrug resistance proteins (MDR) including MDR1/3/4, bile salt export pump (BSEP), and breast cancer resistance protein (BCRP/ABCG2). These transporter proteins are strategically positioned on the apical side of hepatocytes, emphasizing their role in hepatic transport processes [28].

1.3.3 Transport proteins in the intestine

Among the intestinal transport proteins, the extensively studied ones predominantly belong to the ABC family, featuring efflux transport proteins such as P-glycoprotein (P-gp), multidrug resistance protein 2 (MRP2), and breast cancer resistance protein (BCRP). Simultaneously, SLC transport proteins in the intestinal realm primarily encompass facilitated diffusion transport proteins (GLUT) [43], peptide transport proteins (PEPT), organic cation transporter proteins (OCT), plasma membrane monoamine transporter (PMAT), organic anion-transporting polypeptide (OATP), and monocarboxylate transporter proteins (MCT) [44]. This diversity in transport proteins underscores the intricacies of intestinal transport mechanisms, implicating their significance in various physiological processes.

1.4 Challenges

Membrane transport proteins not only play pivotal roles in physiology but also wield significant influence in pharmacology, shaping the absorption, distribution, and excretion dynamics of nutrients, signaling molecules, metabolic end products, and pharmaceuticals. However, the study of membrane transport proteins encounters several challenges:

1. Despite achieving successful structural resolutions for certain transport proteins, many, particularly intricate membrane proteins, remain only partially understood in terms of their structures.

2. The dynamics and regulatory intricacies of transport proteins manifest complexities within distinct intracellular and extracellular milieus, necessitating a deeper comprehension of these nuanced details.

3. Interactions between transport proteins and other proteins, ligands, or cargo substances are commonplace, yet the precise details of these interactions warrant further investigative efforts.

4. Although certain diseases are linked to the aberrant function of transport proteins, the exact roles they play in these diseases remain inadequately understood.

5. While some drugs target transport proteins, the development of specific and efficient drugs remains a formidable challenge, particularly given the unique structural features of membrane proteins.

Addressing these issues mandates interdisciplinary research approaches that seamlessly blend experimental and computational techniques, fostering a comprehensive understanding of the biological functions and regulatory mechanisms governing transport proteins.

2. Mechanisms and roles of transport proteins in HUA

2.1 Transport proteins in HUA

Urate transporter proteins stand as pivotal targets in the research and drug development for hyperuricemia.

The kidney plays a predominant role in maintaining serum urate levels through its excretory function, eliminating approximately 70% of daily urate production. Urate metabolism in the kidneys involves three processes: filtration, secretion, and reabsorption, primarily occurring in the renal proximal tubules. Apart from filtration, urate transporter proteins participate in the remaining processes [45, 46]. After filtration, a large portion of urate ions are reabsorbed, constituting an important component of serum uric acid levels in the body. Urate transporter 1 (URAT1) and glucose transporter 9 (GLUT9) are important transport proteins involved in urate reabsorption. In addition to these, there are also organic anion transport proteins OAT4 and OAT10 in the OAT family that play a role in urate reabsorption [47]. Transporter proteins such as ABCG2, OAT1, and OAT3 present on the epithelial cells of the renal proximal tubules serve as important outlets mediating urate excretion, while ABCC4, a known organic anion transporter protein, also plays a role in urate excretion, though its specific location remains unclear.

The kidney was previously believed to be the sole pathway for urate excretion; however, it is estimated that approximately one-third of urate is excreted into the intestine, where it undergoes further metabolism by intestinal bacteria [15]. Intestinal secretion is considered a significant factor in extra-renal urate elimination [48]. Intestinal transporter proteins play crucial roles in urate excretion, yet the molecular characteristics of urate transporter proteins in the intestine have not been extensively studied to date. Members of the ABC family may play a role in intestinal urate excretion, with ABCG2 located on the apical membrane of intestinal epithelial cells serving as a crucial outlet for intestinal urate excretion [48]. Given the high expression of ABCG2 in the intestine, it may primarily facilitate intestinal urate excretion rather than renal excretion (Figure 2) [49].

Figure 2.

UA transport proteins in the kidneys and intestines. Figure legends: (a) uric acid enters the kidney and undergoes filtration in the renal capsule, then excretion and reabsorption via transporter proteins in the epithelial cells of the proximal tubule of the kidney. URAT1, GLUT9, OAT4, and OAT10 are responsible for the reabsorption of UA, and ABCG2, ABCC4, OAT1, and OAT3 are responsible for the excretion of UA; (b) a small portion of uric acid is metabolized in the intestinal epithelial cells and is known to excrete UA by ABCG2, and the rest of the intestinal-associated transport proteins need to be explored further.

2.2 Key transporters

Elevated serum urate concentration stands as the paramount risk factor contributing to the onset of hyperuricemia. The journey of urate is intricately orchestrated within the renal landscape: initially freely filtered at the renal glomerulus, followed by reabsorption in the S1 segment of the proximal tubule, and subsequent secretion, potentially accompanied by reabsorption, in the S2-S3 segments of the proximal tubule [50]. A fractional portion of urate undergoes metabolism in the intestines, adding another layer of complexity to its regulation.

Through genetic approaches, an array of renal and gastrointestinal transport proteins with a significant affinity for urate have been unearthed. Notable among these are the SLC transporter protein genes, namely SLC22A12 and SLC2A9, along with the ABC transporter protein gene ABCG2. Remarkably, variations within just these three genes contribute to a substantial 5% of the variability observed in serum urate levels, surpassing the cumulative impact of all other genetic variations. These findings illuminate the intricate genetic landscape underpinning urate regulation and underscore the pivotal role played by specific transport proteins in shaping serum urate dynamics [51, 52]. Serum uric acid is regulated by urate transport proteins in the kidneys and intestines, with most of the uric acid being reabsorbed back into the bloodstream. Therefore, we will focus on the characteristics of three important transport proteins: URAT1 and GLUT9, which are associated with uric acid reabsorption, and ABCG2, which is involved in uric acid excretion [53].

2.2.1 URAT1

URAT1 stands as a pivotal apical urate exchanger within the proximal renal tubules, wielding a central role in the intricate physiological homeostasis of urate [54]. Comprising 12 transmembrane domains and 555 amino acid residues, URAT1 is encoded by the SLC22A12 gene, sharing a noteworthy 30% homology with the rat organic cation transporter 1 [55]. Originally uncovered during the exploration of organic anion transporter-like (OAT-like) molecules in the gene database, URAT1’s functional aspects were revealed through expression studies in African clawed frog oocytes [56].

URAT1, with its high affinity for uric acid, exhibits specific expression on the luminal side of the proximal renal tubules. Responsible for transporting uric acid from the lumen of the proximal renal tubule to the apical membrane through the transmembrane electric potential gradient, URAT1 plays a crucial role in mediating urate salt uptake through exchange with organic anions (such as lactate and nicotinate) and inorganic anions Cl-, thereby contributing significantly to the regulation of human serum uric acid levels. In one study, functional analysis of rat Urat1 was conducted using isolated brush border membrane vesicles (BBMVs) prepared from rat kidneys [57]. It was observed that urate uptake by Urat1 exhibited Cl − dependency, with an approximately 57% increase in Km value. Additionally, research conducted by expressing URAT1 in African clawed frog oocytes revealed that the application of inorganic anions Br and I also significantly accelerates urate uptake. Furthermore, it was found that URAT1-mediated urate uptake is not pH-dependent (i.e., no affinity for OH−) [56]. Another study, involving the cloning characterization of human SMCT2 (SLC5A12) and its expression in mouse kidney slices unveiled the remarkable ability of both low-affinity (SMCT2) and high-affinity (SMCT1) lactate transporters to efficiently assist in lactate reabsorption in the proximal tubules of the kidney [58]. Immunofluorescence staining vividly highlighted the intense expression of SMCT2 and SMCT1 in different segments of the proximal renal tubules, suggesting that Urat1 activity might be expedited through preloading lactate.

Certain uricosuric agents, including probenecid, sulfinpyrazone, benzbromarone, and losartan, exert their influence by inhibiting urate reabsorption, binding to URAT1, and promoting urate excretion [59]. Noteworthy are the implications of mutations in URAT1, which can precipitate hypouricemia in humans [56]. Japan, in particular, reports a heightened prevalence of hereditary renal hypouricemia, with approximately 90% of Japanese hypouricemia patients attributable to non-functional variants of URAT1 [60]. Mouse studies further corroborate URAT1’s significance, revealing that knocking out the URAT1 gene induces mild hypouricemia and heightened urate salt excretion [61]. Another study indicated that Urat-Uox double knockout mice exhibit elevated ATP degradation, with uric acid excretion levels increasing 25 times that of humans, leading to decreased serum uric acid levels [62]. These findings underscore URAT1’s indispensable role in urate regulation and its potential as a therapeutic target for modulating serum urate levels.

2.2.2 GLUT9

GLUT9, encoded by the SLC2A9 gene, stands as a distinguished member within the glucose transporter (GLUT) family. Its structure aligns with the type II glutamine subtype, featuring an intricate arrangement of 12 transmembrane domains. [63]. Initially identified for its role as a glucose and/or fructose transporter, rather than a urate transporter [64], GLUT9 shares the characteristic inhibition by cell relaxin B observed in other GLUT family members. Recent revelations, however, illuminate GLUT9’s role as a high-capacity urate transporter, pivotal for the basolateral transport of urate in the proximal tubule. This function contributes to urate reabsorption by facilitating the transport of glucose [65, 66]. The findings revealed that the urate transport rate of SLC2A9 is 45 to 60 times faster than that of glucose, and it was demonstrated that the urate transport mediated by SLC2A9 is primarily facilitated by glucose, with fructose playing a lesser role [66].

GLUT9 manifests in two distinct isoforms, SLC2A9-L (540 amino acids) and SLC2A9-S (512 amino acids). Research by Kimura et al. unveils the unique locations of these isoforms, with SLC2A9-L situated on the basolateral membrane of the proximal tubule and SLC2A9-S on the apical membrane of the collecting duct [67]. Intriguingly, certain kinetic properties of the GLUT9 isoforms defy differentiation. Additionally, GLUT9, owing to its ability to induce depolarization, functions more akin to an electronic transporter rather than a urate-anion exchanger [67, 68, 69]. Preitner et al.’s study revealed that knockout of the SLC2A9 gene in mice resulted in elevated blood creatinine levels and a 20 to 30-fold increase in urate salt excretion rates. In male G9KO mice, the excretion fraction approached approximately 100%, while in female G9KO mice, the urate salt excretion fraction reached approximately 150%. These findings suggest inhibited urate salt reabsorption and increased tubular excretion in both male and female G9KO mice [70].

2.2.3 ABCG2

ABCG2, also known as the human breast cancer resistance protein (BCRP), stands among the trio of human ATP-binding cassette (ABC) transporter proteins, ubiquitously distributed throughout the body. Its role extends to facilitating the cellular efflux of a diverse array of chemically and structurally varied compounds [12, 71]. Revered as a “gatekeeper” in virtue of its function and localization, ABCG2 stands guard, thwarting the passage of endogenous or exogenous toxins and foreign substances across biological barricades into delicate tissues [49].

Expressed in various tissues, ABCG2 graces the luminal membranes of renal tubules and the intestines, where it orchestrates the secretion of an array of compounds, including urate. Notably, ABCG2 assumes a prominent role in urate excretion within the intestines, surpassing its involvement in other tissues, thus emerging as the primary conduit for extra-renal urate elimination [12, 52].

Research has illuminated the expansion of extra-renal urate salt excretion as renal function declines, highlighting its pivotal role in preserving urate homeostasis, especially in individuals grappling with renal insufficiency [72]. Notably, in a rat model of 5/6 nephrectomy, despite diminished uric acid excretion, serum uric acid levels remained unchanged, concurrently with a notable rise in intestinal ABCG2 levels [73]. These findings underscore ABCG2’s pivotal role in intestinal urate excretion, implying that compromised intestinal urate excretion due to ABCG2 dysfunction may underlie one of the mechanisms of hyperuricemia.

Moreover, mutations or dysfunction of ABCG2 may precipitate hyperuricemia or gout [74, 75]. ABCG2 knockout mice exhibit diminished intestinal urate excretion and heightened plasma urate concentrations [12]. A recent study has confirmed that the common ABCG2 variant Q141K, identified in human intervention cohort studies, is associated with an increased risk of hyperuricemia and gout. Additionally, the research team observed significant hyperuricemia and metabolic changes in male mice carrying the direct homologous Q140K Abcg2 variant. These mice exhibited elevated urinary sodium excretion fractions (FeNa+) and decreased glomerular filtration rates (GFR). Notably, there was a substantial reduction in ABCG2 abundance (from 78 to 44%) and severe functional defects observed in the intestines of these mice [52].

2.3 Transporters in the HUA network

The exchange of uric acid is mediated by various molecules expressed in the epithelial cells of the renal proximal tubules and intestinal epithelial cells, primarily including GLUT9, URAT1, and human ABCG2, OAT1, OAT3, and OAT4 (Figure 3).

Figure 3.

UA transport in the network of hyperuricemia. Figure legends: UA exchange occurs predominantly in renal and intestinal epithelial cells, where complex substance exchanges mediated by UA excretory and reabsorptive transport proteins occur to form a hyperuricemia network.

URAT1 facilitates the reabsorption of uric acid in the proximal tubular lumen by exchanging monovalent carboxylate anions or organic anions (such as lactate, pyrazinecarboxylate PZA, and nicotinate) in the epithelial cells [24, 47, 56, 76]. The intracellular concentration of these anions mainly depends on Na + −dependent uptake from the same glomerular ultrafiltration [77]. GLUT9 is a high-capacity electrogenic transporter that significantly activates membrane depolarization (uptake of extracellular [K + -Cl−] in the medium), with a specific ability to transport uric acid unidirectionally through a voltage-dependent manner, seemingly influenced by extracellular Cl-concentration but independent of Na + transmembrane gradients [78]. OAT4 appears to facilitate the reabsorption of substrates in urine through exchange with dicarboxylates (such as succinate) or hydroxyl ions [13, 77, 79]. OAT10 mediates the exchange of urate-PZA and urate-nicotinate. ABCG2 and ABCC4 utilize ATP hydrolysis to expel uric acid from the cell, and the urate efflux mediated by both depends on the intracellular urate concentration [80, 81]. While genetic variations in human MRP4 have not been linked to impacting serum uric acid levels, genetic variations in human ABCG2 have emerged as major factors in human hyperuricemia. OAT1 and OAT3 mediate the movement of substrate molecules from the blood into cells through dicarboxylate/organic anion exchange (Figure 3) [61, 82].

In addition to the previously mentioned transporter proteins, emerging research has identified associations between uric acid excretion and certain related proteins. A comprehensive investigation re-evaluated the tissue distribution of the orphan transporter protein hORCTL3 through RT-PCR, revealing robust expression in the kidneys. This confirmed hORCTL3 as a low-affinity UA transporter [83]. Notably, mutant mice with diminished expression of renal sodium/lactate cotransporter proteins (SLC5A8 and SLC5A12) were found to display impaired uric acid reabsorption, resulting in an approximately eightfold elevation in UA levels [84]. Intriguingly, the expression of URAT1, a transporter protein responsible for the apical uptake of uric acid in the renal proximal tubules, remained unaltered. These findings provide compelling in vivo evidence for a functional link between lactate reabsorption and uric acid reabsorption in the kidneys. The murine ortholog of URAT1, the renal-specific transporter protein RST, is predominantly situated on the apical surface of proximal tubules in the kidneys [55, 56, 85, 86, 87]. While exhibiting significantly reduced affinity when expressed in oocytes, RST does demonstrate uric acid transport capabilities. Knockout C57BL/6Jmice lacking RST exhibits a substantial increase in urinary uric acid (approximately two folds), underscoring the involvement of RST in renal uric acid reabsorption. However, despite the absence of RST, a notable fraction of reabsorption persists, hinting at the presence of unknown transporter proteins that contribute significantly to uric acid reabsorption [61].

3. HUA treatment options targeting transporters

3.1 Drug research targeting HUA-related transport proteins

When urate is excreted from the kidneys, approximately 90% of urate filtered by the renal glomeruli is reabsorbed back into the bloodstream, with only 10% being excreted by the kidneys. Therefore, inhibiting urate reabsorption-related transport proteins is key to uric acid-lowering therapy [54]. In individuals with hyperuricemia, the predominant cause often stems from inadequate urate excretion [14]. Agents designed to enhance urate excretion primarily operate by inhibiting the reabsorption of urate in the proximal renal tubules. Notable transport proteins implicated in urate reabsorption encompass URAT1 and GLUT9. Understanding and modulating the activity of these proteins offer potential avenues for therapeutic interventions aimed at promoting efficient urate elimination (Table 2).

Medication	Animal models target proteins	Pharmacokinetics	Adverse drug reactions	Major contraindications
Probenecid	URAT1	no result found	hypersensitivity reactions, rash, gastrointestinal discomfort, gastrointestinal irritation, skin reactions, anorexia.	advanced kidney failure, uric acid kidney stones, blood urea, urolithiasis
Benzbromarone	Patients URAT1	In subjects with CYP2C9*1/1 and CYP2C91/3 genotypes, the plasma concentration of benzbromarone reached its maximum 2 hours after administration, while in subjects with the CYP2C93/3 genotype, it reached the maximum plasma concentration 6 hours after administration	Hepatotoxicity, allergy, gastrointestinal discomfort, abnormal liver function, urolithiasis.	Individuals with impaired liver and kidney function, those allergic to benzbromarone, patients with uric acid kidney stones, pregnant or potentially pregnant women
Lesinurad	Volunteers URAT1	Mild, moderate, and severe renal impairment increased the plasma concentration of levosulpiride by 34, 54–65, and 102%, respectively	Nephrotoxicity, headache, flulike symptoms, elevated creatinine levels, gastroesophageal reflux	Severe renal failure, tumor lysis syndrome, Lesch- Nyhan syndrome
Dotinurad	patients URAT1	Regardless of dose, age, and gender, the half-life (T1/2) is approximately 10 hours, with only mild to moderate renal impairment and hepatic impairment observed	Elevated serum creatinine levels, renal injury, and the potential to lead to the formation of stones	no result found
Sulpiride	URAT1	no result found	Nausea, vomiting, abdominal pain, diarrhea, anemia, rash	no result found

Table 2.

Limitations of marketed drugs targeting UA reabsorption-related transport proteins.

3.1.1 URAT1

Approved clinical URAT1 inhibitors for urate reabsorption, including probenecid, benzbromarone, sulfinpyrazone, and lesinurad, represent valuable tools in managing hyperuricemia [88, 89]. However, their usage is not without challenges, as these drugs may exhibit varying degrees of side effects and toxicity [90]. Probenecid, while effective in weakening active urate reabsorption, may lead to undesirable effects such as rash, gastrointestinal irritation, hypersensitivity reactions, and hemolytic anemia [91, 92]. Benzbromarone, an efficient URAT1 inhibitor, offers a reduction in serum uric acid levels in CKD patients but demands caution in those with an estimated glomerular filtration rate (eGFR) <30 mL/min due to potential liver toxicity, along with risks of uric acid kidney stones and bone marrow suppression [69, 88, 89, 91]. Sulfinpyrazone, another option, is associated with adverse effects like nausea, vomiting, abdominal pain, diarrhea, anemia, and rash [93]. Lesinurad, although effective, presents dose-dependent renal toxicity and potential cardiovascular risks, as evidenced by clinical trials [94, 95].

The pursuit of novel urate excretion agents has led to innovative approaches. Utsumi Junichiro and colleagues, for instance, devised Dotinurad (FYU-981), a phenol derivative with enhanced urate-lowering activity [96]. Dotinurad, approved in Japan, addresses structural limitations observed in benzbromarone, proving more effective in reducing plasma UA levels and exhibiting significant inhibitory activity against URAT1-mediated urate transport. Additionally, Philip K. Tan and colleagues explored the potential of verinurad, a novel, potent URAT1 inhibitor [97]. Verinurad’s specificity, demonstrated through competitive binding assays, offers promise for developing highly efficient and specific drugs to treat hyperuricemia.

Natural products, including phenolic compounds, terpenes, and fatty acids, have emerged as intriguing avenues for reducing URAT1 activity. Their potential to lower urate levels has garnered attention, providing a rich field for further exploration in the quest for effective hyperuricemia management [98, 99, 100].

3.1.2 GLUT9

The focus on URAT1 as the primary driver of urate reabsorption has led to limited exploration of drug development targeting the GLUT9 transporter for hyperuricemia treatment, with most studies conducted in conjunction with URAT1 [97]. However, recent research has identified promising compounds with potential anti-hyperuricemic effects and nephroprotective small molecule drugs and traditional Chinese medicine extracts.

Small molecule drugs include oxyberberine (OBR) and betulinic acid. In a study led by Zhong Linjiang and colleagues, OBR demonstrated effectiveness in countering hyperuricemia and protecting the kidneys in hyperuricemic mice [101]. OBR, administered orally, downregulated the renal expression of URAT1, GLUT9, NOD-like receptor 3 (NLRP3), and other factors at both mRNA and protein levels, showcasing its potential as a therapeutic agent. Betulinic acid, a flavonoid compound found in honey, propolis, and mushrooms, exhibited anti-hyperuricemic effects in a rat model induced by high-fructose syrup feeding [102]. Research by Zhang and colleagues explored the mechanism of action of salicin in combating hyperuricemia. The results indicate that salicin exhibits anti-hyperuricemic effects in a rat model of hyperuricemia induced by high-fructose syrup feeding. It significantly downregulates the protein expression of URAT1 and GLUT9 while upregulating the protein expression of OAT1 and human ATP-binding ABCG2.

Traditional Chinese medicine drugs include palmatine (Pal) and apigenin (API). Pal is a major alkaloid from Coptis chinensis, traditionally used for hyperuricemia-related diseases, and showed promise in a hyperuricemia mouse model [103]. Pal effectively reduced the protein levels of GLUT9 and URAT1 while the group Pal (100 mg/kg) increased the expression levels of OAT1 and ABCG2 by nearly twofold compared to the HUA group, suggesting its potential therapeutic role in hyperuricemia. API is a natural flavonoid, demonstrated significant anti-hyperuricemic effects in mice with hyperuricemia nephropathy (HN) [104]. API promoted uric acid excretion, inhibited URAT1 and GLUT9, and showed competitive inhibition in vitro, highlighting its potential as a treatment for hyperuricemia.

Compounds like BDEO (Compound 9), dual inhibitor 83 (Compound 21), and CDER167 (Compound 27) exhibited dual inhibitory effects on URAT1 and XO or GLUT9 [105, 106, 107]. These compounds demonstrated excellent uric acid-lowering capabilities with higher safety profiles, suggesting potential therapeutic avenues for hyperuricemia.

3.1.3 Summary

Presently, there is a growing inclination within the industry toward the development of pharmaceuticals featuring dual-action mechanisms or employing combination therapy approaches. Notable examples in this domain include ACQT-1127 and THDBH151, both functioning as dual inhibitors targeting xanthine oxidase and URAT1 [108]. A pioneering initiative by Westlake Therapeutics involves the utilization of its REDx platform to craft a cell-based therapy [109, 110]. Engineered red blood cells serve as carriers for this innovative approach, specifically designed to target both uricase (UOX) and URAT1. Researchers across the field share a common commitment to ensuring the safety and effectiveness of these developments, particularly in the pursuit of creating highly efficient, multi-target drugs. This emphasis reflects the collaborative effort to advance pharmaceutical innovation with a focus on achieving optimal outcomes for patients.

3.2 Other factors of influencing hyperuricemia

Beyond the ongoing exploration of transporter protein inhibitors, various factors contribute to the modulation of transporter protein activity, influencing serum uric acid levels within the human body.

Lifestyle adjustments, such as dietary modifications and weight loss through exercise, have been identified as effective strategies to reduce serum uric acid levels and mitigate the risk of gout. However, it is crucial to note that abrupt weight loss can induce ketosis, a process that, via the URAT1, enhances uric acid reabsorption, consequently elevating serum uric acid levels [111]. Abdominal obesity-induced insulin resistance and hyperinsulinemia exacerbate uric acid reabsorption, facilitated by the sodium and monocarboxylate transporter (SMCT) exchanging with URAT1, leading to an upsurge in serum uric acid levels.

4. Conclusion

Hyperuricemia is a metabolic syndrome characterized by a persistently high serum uric acid level, primarily due to inadequate uric acid excretion by the kidneys and excessive reabsorption. The excretion and reabsorption of uric acid are important processes in uric acid metabolism, with several transport proteins, particularly URAT1, GLUT9, and ABCG2, found in the proximal tubules of the kidneys and intestines, playing a crucial role in the transmembrane transport of large-molecule urate salts and contributing significantly to the balance of serum uric acid.

The process of transporting uric acid by transport proteins is complex, involving exchanges of energy, potential, and surrounding substances. The deeper mechanisms of this process require further exploration. Transport proteins serve as crucial hubs in uric acid metabolism, and inhibitors targeting these proteins, particularly those involved in reabsorption, have long been one of the most important therapeutic approaches for managing hyperuricemia. Marketed drugs like probenecid, benzbromarone, sulfinpyrazone, and lesinurad effectively impede transporter proteins but may elicit varying degrees of adverse reactions in the body. New agents with better efficacy and safety are on the way for development.

Presently, the field is witnessing the development and modification of numerous compounds and natural products. Striking a balance between enhancing drug specificity and ensuring safety, particularly in drugs targeting critical transporter proteins, is paramount. As more novel modalities have been successfully delivered as new drugs, we would like to see whether the new modalities targeting transporters will benefit more patients. Additionally, the exploration of unknown uric acid transporter proteins and addressing other factors influencing these proteins represent the industry’s concerted efforts to elevate drug effectiveness in the treatment of hyperuricemia.

Acknowledgments

We appreciate the assistance and support provided by the Nanjing Municipal Food and Drug Supervision and Testing Institute for this article.