Current Understanding of the Intestinal Absorption of Nucleobases and Analogs

Impact of the loss of slc43a3 on 6-mercaptopurine absorption and tissue distribution in mice

6-Mercaptopurine (6-MP) is a nucleobase analog used in the therapy of acute lymphoblastic leukemia and inflammatory bowel disease. It is associated with numerous side effects including myelotoxicity, hepatotoxicity, and gastrointestinal complications, which can lead to patient adherence issues or discontinuation of treatment. This is further complicated by the wide variability in plasma levels of 6-MP and the therapeutic response to a standard dose. Although a number of enzyme polymorphisms have been linked to therapeutic response, it is unclear what factors underlie the variability in plasma levels. We have established that SLC43A3-encoded equilibrative nucleobase transporter 1 mediates the transport of 6-MP into cells in both mice and humans. To determine whether this transporter is critical for 6-MP absorption and biodistribution, we examined the effect of the genetic deletion of slc43a3 in mice on the absorption and tissue distribution of orally administered 6-MP. A high-performance liquid chromatography method was developed to measure tissue levels of 6-MP and its key metabolites, 6-methylmercaptoprine, 6-thiourate, and 6-thioguanine nucleotides. The results of this study show that loss of slc43a3 dramatically reduces the absorption of 6-MP from the gastrointestinal tract and attenuates the levels achieved in peripheral tissues. Furthermore, the loss of slc43a3 decreases the tissue:blood concentration ratios of 6-MP and its metabolites, particularly in those tissues that show high levels of expression of slc43a3, such as the heart and lungs. Therefore, it is possible that differences in SLC43A3 expression in humans may contribute to the variability seen in 6-MP plasma levels and therapeutic response. SIGNIFICANCE STATEMENT: The loss of slc43a3 in mice dramatically reduces the absorption and the biodistribution of the chemotherapeutic drug 6-mercaptopurine. These data suggest that variations in SLC43A3 expression in humans may contribute to the variability in plasma levels that have been reported when using this drug therapeutically.

Unraveling Caco-2 cells through functional and transcriptomic assessments

The static Caco-2 monolayer is an extensively utilized model for predicting the permeability of small molecules during the drug development process. While these cells can differentiate and develop key functional and morphological features that emulate human enterocytes, they do not fully replicate the complexity of human intestinal physiology. In this study, we investigated functional and morphological aspects of Caco-2 cells, alongside their transcriptomic profiles, with a particular emphasis on genes encoding drug-metabolizing enzymes and drug transporters. We found that Caco-2 cells not only established a robust and bio-relevant permeable intestinal barrier but also demonstrated functional maturity and differentiation in the intestinal epithelium, substantiated by the activities of important enzymes and an efflux transporter. However, our targeted gene expression analyses revealed that substantial disparities were found in mRNA transcript levels among Caco-2 cells and human biopsy samples. These findings highlight that, although Caco-2 cells are valuable for assessing the passive transport of drugs, their accuracy for predicting active transport or small intestinal drug metabolism is constrained by their transcriptomic divergence from human intestinal tissues. This study highlights the importance of understanding the Caco-2 model's inherent limitations and provides insights that could inform its appropriate application in drug development and regulatory decision-making.

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

INTRODUCTION

High 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.

METHODS

Human 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.

RESULTS

We 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.

DISCUSSION

It 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.

Identification of New Specificity Determinants in Bacterial Purine Nucleobase Transporters based on an Ancestral Sequence Reconstruction Approach

The relation of sequence with specificity in membrane transporters is challenging to explore. Most relevant studies until now rely on comparisons of present-day homologs. In this work, we study a set of closely related transporters by employing an evolutionary, ancestral-reconstruction approach and reveal unexpected new specificity determinants. We analyze a monophyletic group represented by the xanthine-specific XanQ of Escherichia coli in the Nucleobase-Ascorbate Transporter/Nucleobase-Cation Symporter-2 (NAT/NCS2) family. We reconstructed AncXanQ, the putative common ancestor of this clade, expressed it in E. coli K-12, and found that, in contrast to XanQ, it encodes a high-affinity permease for both xanthine and guanine, which also recognizes adenine, hypoxanthine, and a range of analogs. AncXanQ conserves all binding-site residues of XanQ and differs substantially in only five intramembrane residues outside the binding site. We subjected both homologs to rationally designed mutagenesis and present evidence that these five residues are linked with the specificity change. In particular, we reveal Ser377 of XanQ (Gly in AncXanQ) as a major determinant. Replacement of this Ser with Gly enlarges the specificity of XanQ towards an AncXanQ-phenotype. The ortholog from Neisseria meningitidis retaining Gly at this position is also a xanthine/guanine transporter with extended substrate profile like AncXanQ. Molecular Dynamics shows that the S377G replacement tilts transmembrane helix 12 resulting in rearrangement of Phe376 relative to Phe94 in the XanQ binding pocket. This effect may rationalize the enlarged specificity. On the other hand, the specificity effect of S377G can be masked by G27S or other mutations through epistatic interactions.

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

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 [³H]-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 (IC₅₀ 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 (IC₅₀ 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.