SLC23A3 is a renal hypoxanthine transporter

LLC-PK1 renal cells show Na⁺-dependent and Na⁺-independent hypoxanthine uptake. While the latter is inhibited by adenine, neither are inhibited by xanthine. In rats, intestinal Na⁺-dependent hypoxanthine transporter Slc23a4 is not expressed in the kidney, and its action is inhibited by xanthine. This study aimed to clone Slc23a4-paralog SLC23A3 from the human kidney and investigate its hypoxanthine transport activity. We observed Na⁺-dependent 10 nM [³H]-hypoxanthine uptake in SLC23A3 RNA-injected Xenopus oocytes. Moreover, 100 μM xanthine did not inhibit Na⁺-independent 300 nM [³H]-hypoxanthine uptake, whereas 100 μM adenine did. These results confirm that SLC23A3 is a hypoxanthine transporter in the human kidney.

Predictive model for recurrence of renal cell carcinoma by comparing pre- and postoperative urinary metabolite concentrations

To improve treatment outcomes in real practice, useful biomarkers are desired when predicting postoperative recurrence for renal cell carcinoma (RCC). We collected data from patients who underwent definitive surgery for RCC and for benign urological tumor at our department between November 2016 and December 2019. We evaluated the differences in pre‐ and postoperative urinary metabolites with our precise quantitative method and identified predictive factors for RCC recurrence. Additionally, to clarify the significance of metabolites, we measured the intracellular metabolite concentration of three RCC cell lines. Among the 56 patients with RCC, nine had a recurrence (16.0%). When comparing 27 patients with T1a RCC and 10 with benign tumor, a significant difference was observed between pre‐ and postoperative concentrations among 10 urinary metabolites. In these 10 metabolites, multiple logistic regression analysis identified five metabolites (lactic acid, glycine, 2‐hydroxyglutarate, succinic acid, and kynurenic acid) as factors to build our recurrence prediction model. The values of area under the receiver operating characteristic curve, sensitivity, and specificity in this predictive model were 0.894%, 88.9%, and 88.0%, respectively. When stratified into low and high risk groups of recurrence based on this model, we found a significant drop of recurrence‐free survival rates among the high risk group. In in vitro studies, intracellular metabolite concentrations of metastatic tumor cell lines were much higher than those of primary tumor cell lines. By using our quantitative evaluation of urinary metabolites, we could predict postoperative recurrence with high sensitivity and specificity. Urinary metabolites could be noninvasive biomarkers to improve patient outcome.

Self-assembled nanostructures of phosphomolybdate, nucleobase and metal ions synthesis and their in vitro cytotoxicity studies on cancer cell lines

The ability of the multidentate nucleobases, adenine and thymine, to coordinate polyoxometalate and metal ions leading to the formation of self-assembled nanostructures and their strong cytotoxicity toward cancer cell lines have been demonstrated. A unique synthetic approach is developed to make a series of functional nanoscale hybrid materials consisting of nucleobases (adenine and thymine) and phosphomolybdic acid (PMA) through solid state chemical reaction and self-assembly process. Adenine was protonated through its ring nitrogen, while the ketone group in thymine was protonated during the addition of PMA to these nucleobases. The self-assembled nanostructures formed as a result of the electrostatic interaction between the protonated nucleobases and polyanionic PMA. To promote the base pairing between the nucleobases, chloroaurate ions and silver ions were added to each PMA/adenine and PMA/thymine nanostructures. The complexation between the nucleobases and the added metal ions was found to drive the formation of subsequent self-assembled nanostructures. All the materials were screened for their anticancer activity against breast (MDAMB-231) and prostate (PC-3) cancer cells, and non-cancerous keratinocyte cells HaCaT. PMA/adenine/[AuCl4]- and PMA/thymine/Ag+ nanostructures were found to have strong anti-cancer activity, while PMA/adenine/Ag+, PMA/thymine/[AuCl4]-, and PMA/pdenine, PMA/thymine nanostructures did not exhibit such activity. The unique redox properties of these materials and the self-assembly of the PMA and metal ions were the major factors responsible for the cytotoxicity. This unique approach of making functional nanomaterials incorporate the nucleobase, PMA and metal ions using solid state self-assembly and their anti-cancer applications are considered to be an effective approach for the development of inorganic nucleoside analogue bio-pharmaceutical agents.

The nucleobase adenine as a signalling molecule in the kidney

In 2002, the first receptor activated by the nucleobase adenine was discovered in rats. In the past years, two adenine receptors (AdeRs) in mice and one in Chinese hamsters, all of which belong to the family of G protein-coupled receptors (GPCRs), were cloned and pharmacologically characterized. Based on the nomenclature for other purinergic receptor families (P1 for adenosine receptors and P2 for nucleotide, e.g. ATP, receptors), AdeRs were designated P0 receptors. Pharmacological data indicate the existence of G protein-coupled AdeRs in pigs and humans as well; however, those have not been cloned so far. Current data suggest a role for adenine and AdeRs in renal proximal tubules. Furthermore, AdeRs are suggested to be functional counterplayers of vasopressin in the collecting duct system, thus exerting diuretic effects. We are only at the beginning of understanding the significance of this new class of purinergic receptors, which might become future drug targets.

Role of ABC transporters in fluoropyrimidine-based chemotherapy response

Since over 50 years, 5-fluorouracil (5-FU) is in use as backbone of chemotherapy treatment regimens for a wide range of cancers including colon, breast, and head and neck carcinomas. However, drug resistance and severe toxicities such as mucositis, diarrhea, neutropenia, and vomiting in up to 40% of treated patients often lead to dose limitation or treatment discontinuation. Because the oral bioavailability of 5-FU is unpredictable and highly variable, 5-FU is commonly administered intravenously. To overcome medical complications and inconvenience associated with intravenous administration, the oral prodrugs capecitabine and tegafur have been developed. Both fluoropyrimidines are metabolically converted intracellularly to 5-FU, which then needs metabolic activation to exert its damaging activity on RNA and DNA. The low response rates of 10-15% of 5-FU monotherapy can be improved by combination regimens of infusional 5-FU and leucovorin together with oxaliplatin (FOLFOX) or irinotecan (FOLFIRI), thereby increasing response rates to 30-40%. The impact of metabolizing enzymes in the development of fluoropyrimidine toxicity and resistance has been studied in great detail. In addition, membrane drug transporters, which are critical determinants of intracellular drug concentrations, may play a role in occurrence of toxicity and development of resistance against fluoropyrimidine-based therapy as well. This review therefore summarizes current knowledge on the role of drug transporters with particular focus on ATP-binding cassette transporters in fluoropyrimidine-based chemotherapy response.