Renal hypouricemia is an inherited disorder characterized by impaired renal urate (uric acid) reabsorption and subsequent low serum urate levels, with severe complications such as exercise-induced acute renal failure and nephrolithiasis. We previously identified SLC22A12, also known as URAT1, as a causative gene of renal hypouricemia. However, hypouricemic patients without URAT1 mutations, as well as genome-wide association studies between urate and SLC2A9 (also called GLUT9), imply that GLUT9 could be another causative gene of renal hypouricemia. With a large human database, we identified two loss-of-function heterozygous mutations in GLUT9, which occur in the highly conserved "sugar transport proteins signatures 1/2." Both mutations result in loss of positive charges, one of which is reported to be an important membrane topology determinant. The oocyte expression study revealed that both GLUT9 isoforms showed high urate transport activities, whereas the mutated GLUT9 isoforms markedly reduced them. Our findings, together with previous reports on GLUT9 localization, suggest that these GLUT9 mutations cause renal hypouricemia by their decreased urate reabsorption on both sides of the renal proximal tubules. These findings also enable us to propose a physiological model of the renal urate reabsorption in which GLUT9 regulates serum urate levels in humans and can be a promising therapeutic target for gout and related cardiovascular diseases.

No abstract

L-3-18 F-a-methyl tyrosine ( 18 F-FAMT) has been developed as a PET radiotracer for tumor imaging. Clinical studies have demonstrated the usefulness of 18 F-FAMT PET for the prediction of prognosis and the differentiation of malignant tumors and benign lesions. 18 F-FAMT exhibits higher cancer specificity in peripheral organs than other amino acid PET tracers and 18 F-FDG. The accumulation of 18 F-FAMT is strongly correlated with the expression of L-type amino acid transporter 1 (LAT1), an isoform of system L highly upregulated in cancers. In this study, we examined the interaction of 3-fluoro-L-a-methyltyrosine (FAMT) with amino acid transporters to assess the mechanisms of 18 F-FAMT uptake in PET. Methods: We applied in vitro assays using established mammalian cell lines stably expressing LAT1 or a non-cancer-type system L isoform LAT2. The inhibitory effect on L-14 C-leucine uptake and the induction effect on efflux of preloaded L-14 C-leucine were examined for FAMT and other amino acid tracers. FAMT transport was compared among cell lines with varied LAT1 expression level. Results: FAMT prominently inhibited LAT1-mediated L-14 C-leucine uptake in a competitive manner but had less of an effect on LAT2. In the efflux experiments, FAMT induced the efflux of preloaded L-14 C-leucine through LAT1, indicating that FAMT is transported by LAT1 and not by LAT2. Among amino acid-related compounds examined in this study, including those used for PET tracers, the compounds with an a-methyl group such as FAMT, 2-fluoro-L-a-methyl-tyrosine, 3-iodo-L-a-methyl-tyrosine, and L-a-methyl-tyrosine were well transported by LAT1 but not by LAT2. However, L-methionine, L-tyrosine, 3-fluoro-L-tyrosine, 2-fluoro-L-tyrosine, and O-(2-fluoroethyl)-L-tyrosine were transported by both LAT1 and LAT2, suggesting that the a-methyl moiety is responsible for the LAT1 selectivity of FAMT. FAMT transport rate and LAT1 protein level were well correlated, supporting the importance of LAT1 for the cellular uptake of FAMT. Conclusion: Distinct from other amino acid PET tracers, because of its a-methyl moiety, FAMT is selective to LAT1 and not transported by LAT2. This property of FAMT is proposed to contribute to highly tumorspecific accumulation of 18 F-FAMT in PET.

Although long-studied in the central nervous system, there is increasing evidence that dopamine (DA) plays important roles in the periphery including in metabolic regulation. Insulin-secreting pancreatic β-cells express the machinery for DA synthesis and catabolism, as well as all five DA receptors. In these cells, DA functions as a negative regulator of glucose-stimulated insulin secretion (GSIS), which is mediated by DA D 2 -like receptors including D 2 (D2R) and D 3 (D3R) receptors. However, the fundamental mechanisms of DA synthesis, storage, release, and signaling in pancreatic β-cells and their functional relevance in vivo remain poorly understood. Here, we assessed the roles of the DA precursor L-DOPA in β-cell DA synthesis and release in conjunction with the signaling mechanisms underlying DA’s inhibition of GSIS. Our results show that uptake of L-DOPA is essential for establishing intracellular DA stores in β-cells. Glucose stimulation significantly enhances L-DOPA uptake, leading to increased DA release and GSIS reduction in an autocrine/paracrine manner. Furthermore, D2R and D3R act in combination to mediate dopaminergic inhibition of GSIS. Transgenic knockout mice in which β-cell D2R or D3R expression is eliminated exhibit diminished DA secretion during glucose stimulation, suggesting a new mechanism where D 2 -like receptors modify DA release to modulate GSIS. Lastly, β-cell-selective D2R knockout mice exhibit marked postprandial hyperinsulinemia in vivo . These results reveal that peripheral D2R and D3R receptors play important roles in metabolism through their inhibitory effects on GSIS. This opens the possibility that blockade of peripheral D 2 -like receptors by drugs including antipsychotic medications may significantly contribute to the metabolic disturbances observed clinically.

The efficacy of boron neutron capture therapy relies on the selective delivery of boron carriers to malignant cells. p-Boronophenylalanine (BPA), a boron delivery agent, has been proposed to be localized to cells through transporter-mediated mechanisms. In this study, we screened aromatic amino acid transporters to identify BPA transporters. Human aromatic amino acid transporters were functionally expressed in Xenopus oocytes and examined for BPA uptake and kinetic parameters. The roles of the transporters in BPA uptake were characterized in cancer cell lines. For the quantitative assessment of BPA uptake, HPLC was used throughout the study. Among aromatic amino acid transporters, ATB0,+, LAT1 and LAT2 were found to transport BPA with Km values of 137.4 ± 11.7, 20.3 ± 0.8 and 88.3 ± 5.6 μM, respectively. Uptake experiments in cancer cell lines revealed that the LAT1 protein amount was the major determinant of BPA uptake at 100 μM, whereas the contribution of ATB0,+ became significant at 1000 μM, accounting for 20–25% of the total BPA uptake in MCF-7 breast cancer cells. ATB0,+, LAT1 and LAT2 transport BPA at affinities comparable with their endogenous substrates, suggesting that they could mediate effective BPA uptake in vivo. The high and low affinities of LAT1 and ATB0,+, respectively, differentiate their roles in BPA uptake. ATB0,+, as well as LAT1, could contribute significantly to the tumor accumulation of BPA at clinical dose.