Current Understanding of the Intestinal Absorption of Nucleobases and Analogs

Yuasa, Hiroaki; Yasujima, Tomoya; Inoue, Katsuhisa

doi:10.1248/bpb.b20-00342

Cited by 6 publications

(3 citation statements)

References 68 publications

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“…In addition to ADA1, beta-cells can most likely utilize other enzymes/transporters, i. e. AMPD, which catalyzes the deamination of AMP to IMP, and ENT1/ENT2/ENBT1, which transport nucleosides over the cell membrane ( Figure 9 ), for the disposal of adenosine ( 42 – 45 ), thereby alleviating the toxic effects of high levels of intracellular adenosine/2’-deoxyadenosine. It is likely that the activities of these enzymes/transporters, depending on the particular situation, are regulated by transcriptional/post-transcriptional/post-translational mechanisms ( 43 , 44 ), and that factors or circumstances that cause suboptimal AMPD- and ENT-mediated adenosine disposal could be detrimental to beta-cells.…”

Section: Discussionmentioning

confidence: 99%

Metabolic stress-induced human beta-cell death is mediated by increased intracellular levels of adenosine

et al. 2023

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IntroductionHigh intracellular concentrations of adenosine and 2’-deoxyadenosine have been suggested to be an important mediator of cell death. The aim of the present study was to characterize adenosine-induced death in insulin-producing beta-cells, at control and high glucose + palmitate-induced stress conditions.MethodsHuman insulin-producing EndoC-betaH1 cells were treated with adenosine, 2’-deoxyadenosine, inosine and high glucose + sodium palmitate, and death rates using flow cytometry were studied.ResultsWe observed that adenosine and the non-receptor-activating analogue 2-deoxyadenosine, but not the adenosine deamination product inosine, promoted beta-cell apoptosis at concentrations exceeding maximal adenosine-receptor stimulating concentrations. Both adenosine and inosine were efficiently taken up by EndoC-betaH1 cells, and inosine counteracted the cell death promoting effect of adenosine by competing with adenosine for uptake. Both adenosine and 2’-deoxyadenosine promptly reduced insulin-stimulated production of plasma membrane PI(3,4,5)P3, an effect that was reversed upon wash out of adenosine. In line with this, adenosine, but not inosine, rapidly diminished Akt phosphorylation. Both pharmacological Bax inhibition and Akt activation blocked adenosine-induced beta-cell apoptosis, indicating that adenosine/2’-deoxyadenosine inhibits the PI3K/Akt/BAD anti-apoptotic pathway. High glucose + palmitate-induced cell death was paralleled by increased intracellular adenosine and inosine levels. Overexpression of adenosine deaminase-1 (ADA1) in EndoC-betaH1 cells, which increased Akt phosphorylation, prevented both adenosine-induced apoptosis and high glucose + palmitate-induced necrosis. ADA2 overexpression not only failed to protect against adenosine and high glucose + palmitate-activated cell death, but instead potentiated the apoptosis-stimulating effect of adenosine. In line with this, ADA1 overexpression increased inosine production from adenosine-exposed cells, whereas ADA2 did not. Knockdown of ADA1 resulted in increased cell death rates in response to both adenosine and high glucose + palmitate. Inhibition of miR-30e-3p binding to the ADA1 mRNA 3’-UTR promoted the opposite effects on cell death rates and reduced intracellular adenosine contents.DiscussionIt is concluded that intracellular adenosine/2’-deoxyadenosine regulates negatively the PI3K pathway and is therefore an important mediator of beta-cell apoptosis. Adenosine levels are controlled, at least in part, by ADA1, and strategies to upregulate ADA1 activity, during conditions of metabolic stress, could be useful in attempts to preserve beta-cell mass in diabetes.

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Section: Discussionmentioning

confidence: 99%

Metabolic stress-induced human beta-cell death is mediated by increased intracellular levels of adenosine

et al. 2023

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“…Moreover, lorlatinib is predicted to also in vivo inhibit ENT2 activity, thus suggesting that this TKI may fully abrogate facilitated transport of nucleoside analogues into target cells and, by this way, may cause potential DDIs. Lorlatinib may also be suspected to reduce intestinal absorption of ENT1 substrates, through blocking their ENT1-related efflux at the basolateral membrane of enterocytes [ 42 ]; it may additionally inhibit ENT1-mediated drug transport at the hepatic and/or placental level. Nevertheless, ENT activity inhibition may have beneficial consequences, i.e., it may be hypothesized to contribute to the antiproliferative effects of lorlatinib, through reducing cellular uptake of nucleosides in cancer cells, in a kinase-independent manner.…”

Section: Discussionmentioning

confidence: 99%

Differential Inhibition of Equilibrative Nucleoside Transporter 1 (ENT1) Activity by Tyrosine Kinase Inhibitors

Jouan

Moreau

Bruyère

et al. 2021

Eur J Drug Metab Pharmacokinet

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Background and Objectives Equilibrative nucleoside transporter (ENT) 1 is a widely-expressed drug transporter, handling nucleoside analogues as well as endogenous nucleosides. ENT1 has been postulated to be inhibited by some marketed tyrosine kinase inhibitors (TKIs). To obtain insights into this point, the interactions of 24 TKIs with ENT1 activity have been analyzed. Methods Inhibition of ENT1 activity was investigated in vitro through quantifying the decrease of [ 3 H]-uridine uptake caused by TKIs in HAP1 ENT2-knockout cells, exhibiting selective ENT1 expression. TKI effects towards ENT1-mediated transport were additionally characterized in terms of their in vivo relevance and of their relationship to TKI molecular descriptors. Putative transport of the TKI lorlatinib by ENT1/ENT2 was analyzed by LC-MS/MS. Results Of 24 TKIs, 12 of them, each used at 10 µM, were found to behave as moderate or strong inhibitors of ENT1, i.e., they decreased ENT1 activity by at least 35%. This inhibition was concentration-dependent for at least the strongest ones (IC 50 less than 10 µM) and was correlated with some molecular descriptors, especially with atom-type E-state indices. Lorlatinib was notably a potent in vitro inhibitor of ENT1/ENT2 (IC 50 values around 1.0-2.5 µM) and was predicted to inhibit these nucleoside transporters at relevant clinical concentrations, without, however, being a substrate for them. Conclusion Our data unambiguously add ENT1 to the list of drug transporters inhibited by TKIs, especially by lorlatinib. This point likely merits attention in terms of possible drug-drug interactions, notably for nucleoside analogues, whose ENT1-mediated uptake into their target cells may be hampered by co-administrated TKIs such as lorlatinib.

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“…In rats, though, rSNBT1 (SLC23a4) was found to be transporting a number of nucleobases, including uracil, guanine, urate, xanthine and hypoxanthine (Yamamoto et al, 2010b;Yasujima et al, 2018). The gene for SLC23A4 in humans was also identified, but it is not well or not at all expressed (Yuasa et al, 2020).…”

Section: Substrate Specificity Of Slc23 Transportersmentioning

confidence: 99%

Substrate translocation in SLC23 and SLC26 transporters

Holzhüter¹

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Specialized transporter proteins facilitate controlled uptake and extrusion of molecules across biological membranes that would otherwise be impermeable to them. The superfamily of solute carriers (SLC) comprises the second largest group of membrane proteins in humans, acting on a variety of small polar and non-polar molecules and ions. Because of their central role in metabolism, malfunctioning of these proteins often is pathogenic. The interest in SLC transporters as drug targets – as well as for drug delivery – has therefore increased in the past years. For many SLC subfamilies, however, structural and functional information remains scarce to date. The here presented data provides important insights into different aspects of the transport mechanism of the SLC23 and SLC26 protein families. Importantly, we show that SLC23 nucleobase transporters, in contrast to what was been previously reported, work as uniporters rather than as proton-coupled symporters. In order to do so, we developed the first and only in vitro transport assay for the SLC23 family, which enables investigation of protein function in a defined environment. Moreover, we provide a hypothesis on the role of the extremely conserved negative charged substrate binding site residue found not only in the SLC23, but also SLC4 and SLC26 families. Based on a detailed analysis of binding and transport we conclude that this conserved negative charged has a relevance for protein stability rather than for substrate binding, which explains its conservation for all three protein families that otherwise differ in their substrate specificities and modes of transport. Lastly, we investigated the relevance of oligomerization for the SLC23 and SLC26 families, highlighting the importance of the STAS domain for forming active dimers in the SLC26 anion transporter family.

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Current Understanding of the Intestinal Absorption of Nucleobases and Analogs

Cited by 6 publications

References 68 publications

Metabolic stress-induced human beta-cell death is mediated by increased intracellular levels of adenosine

Metabolic stress-induced human beta-cell death is mediated by increased intracellular levels of adenosine

Differential Inhibition of Equilibrative Nucleoside Transporter 1 (ENT1) Activity by Tyrosine Kinase Inhibitors

Substrate translocation in SLC23 and SLC26 transporters

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