Yu Toyoda (1,2), Hiroshi Miyata (2), Hirotaka Matsuo (1), Tappei Takada (2)
Affiliation(s):
1. Department of Integrative Physiology and Bio-Nano Medicine, National Defense Medical College, 3-2 Namiki, Tokorozawa, Saitama 359-8513, Japan
2. Department of Pharmacy, The University of Tokyo Hospital, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan
Uric acid, the end product of purine metabolism in humans, is a significant substance because of its anti-oxidant activity and a causal relationship with hyperuricemia and gout. Like vitamin C (VC), another water-soluble anti-oxidant known as ascorbic acid, uric acid mainly exists as its anion form (urate) under physiological conditions; therefore, it cannot passively penetrate the plasma membrane, highlighting the fact that active transport plays a pivotal role in regulating urate handling in humans. Indeed, several physiologically important urate transporters regulate this water-soluble metabolite in our body; however, previously-identified urate transporters do not thoroughly explain such handling systems, suggesting the presence of latent machineries.
To address this issue, we herein focused on SLC23A proteins that have been identified as sodium-dependent vitamin C transporters (SVCTs) given the followings: 1) we previously identified SLC2A12 as a physiologically important urate and VC transporter [PNAS, 2020 (PMID: 32690690); iScience, 2022 (PMID: 35106468)]; 2) a homology search revealed that SLC23A1/SVCT1 and SLC23A2/SVCT2 are the closest to YgfU, a urate transporter in E. coli which belongs to nucleobase-ascorbate transporter (NAT) family, in amino acid sequence; 3) only SLC23A proteins are members of the NAT family in humans. To examine whether SVCT1 and SVCT2 transport urate, we conducted cell-based analyses using each transporter-expressing mammalian cells.
The results demonstrated that SVCT1 [1] and SVCT2 [2] are novel urate transporters characterized by their lower affinity for urate compared with already-identified urate importers. Similar results were obtained for mouse Svct1 and Svct2. Regarding SVCT1, we generated Svct1 knockout (KO) mice lacking both urate transporter 1 and uricase. In this hyperuricemic model, serum urate concentrations were lower than controls, suggesting that Svct1 disruption could reduce serum urate [1].
As Svct1 serves as a renal vitamin C re-absorber, it could also be involved in the reabsorption of urate in the kidney. Regarding SVCT2, focusing on its molecular properties as a sodium-dependent urate importer, we established a convenient mammalian cell-based urate efflux assay using SVCT2-expressing cells for urate exporter hunting [2].
Additionally, VC inhibited the urate transport activity of SVCT2 with a half-maximal inhibitory concentration of 36.59 μM, suggesting that the SVCT2-mediated urate transport may be sensitive to physiological ascorbate levels in blood. However, the physiological role of SVCT2 as a urate transporter remains to be investigated; because Svct2 knockout mice die soon after birth, conditional knockout approach will be required for this purpose.
While further studies are required to obtain deeper insights into the underlying mechanisms, our findings regarding the dual-substrate specificity of SVCT1 and SVCT2 expand the understanding of handling systems for the water-soluble antioxidants in our body as well as functional evolutionary changes in NAT family proteins.
Refrences:
1. Toyoda Y, Miyata H, Uchida N et al., Pflugers Arch. 2023, PMID: 36749388
2. Toyoda Y, Miyata H, Shigesawa R et al., J Biol Chem. 2023, PMID: 37390985