Affinage

SLC29A2

Equilibrative nucleoside transporter 2 · UniProt Q14542

Length
456 aa
Mass
50.1 kDa
Annotated
2026-04-28
24 papers in source corpus 11 papers cited in narrative 11 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SLC29A2 (ENT2) is a broadly selective equilibrative nucleoside transporter that mediates bidirectional flux of nucleosides, nucleobases, urate, and nicotinamide across cell membranes, thereby controlling extracellular purine signaling, intracellular NAD+ homeostasis, and purine catabolism. ENT2 is insensitive to nanomolar NBMPR (IC50 ~2.8 µM) and transports inosine and adenosine with the highest affinity among its nucleoside substrates, while hypoxanthine is its preferred nucleobase; it also transports urate with pH-dependent kinetics, and the N68K mutation dissociates urate uptake from efflux, indicating distinct transport modes (PMID:10722669, PMID:39054074, PMID:40367900). In vivo, epithelial ENT2 limits extracellular adenosine availability upstream of A2B adenosine receptor signaling: genetic deletion of Ent2 elevates tissue adenosine and confers Adora2b-dependent protection against acute lung injury and intestinal inflammation (PMID:23603835, PMID:30333323). ENT2 also functions as a nicotinamide transporter required for NAD+ biosynthesis and mitochondrial respiration, and its loss in colorectal cancer cells causes hypoxanthine accumulation, increased xanthine oxidase–derived ROS, and apoptosis (PMID:39885119, PMID:40824957).

Mechanistic history

Synthesis pass · year-by-year structured walk · 11 steps
  1. 2000 High

    Defining the substrate selectivity and pharmacological profile of ENT2 established it as a broadly selective, NBMPR-insensitive equilibrative transporter distinct from ENT1, with preferential affinity for inosine and the ability to transport the nucleobase hypoxanthine.

    Evidence Radiolabeled uptake assays and pharmacological inhibition in nucleoside-transporter-null PK15NTD cells stably expressing hENT2; PNGase F deglycosylation

    PMID:10722669

    Open questions at the time
    • No crystal structure or homology model to explain NBMPR insensitivity
    • Substrate selectivity for non-canonical substrates (e.g. urate, nicotinamide) not yet tested
  2. 2005 Medium

    Mutagenesis of a C-terminal frameshift variant demonstrated that the ENT2 C-terminus is essential for transport function, while several nonsynonymous variants retained activity, beginning to map structure–function relationships.

    Evidence Site-directed mutagenesis and inosine uptake assays in Xenopus oocytes

    PMID:16214850

    Open questions at the time
    • Only one loss-of-function variant tested; systematic domain mapping not performed
    • No structural context for C-terminal requirement
  3. 2007 Medium

    Identification of mTOR-dependent, PI3K-independent transcriptional upregulation of ENT2 by insulin in cardiac fibroblasts revealed a specific signaling axis controlling ENT2 expression, distinct from regulation of concentrative transporters.

    Evidence RT-PCR with rapamycin, wortmannin, PD98059, and cycloheximide in primary rat cardiac fibroblasts

    PMID:17537394

    Open questions at the time
    • Regulation shown only at mRNA level, not protein or transport activity
    • Relevance beyond cardiac fibroblasts not tested
    • Downstream transcription factor not identified
  4. 2013 High

    Knockout of Ent2 in mice elevated bronchoalveolar adenosine and protected against ventilator-induced lung injury in an Adora2b-dependent manner, establishing for the first time a physiological role for ENT2 in controlling extracellular adenosine signaling during tissue inflammation.

    Evidence Ent2−/− mice subjected to mechanical ventilation-induced acute lung injury; bronchoalveolar adenosine measurement; genetic epistasis with Adora2b KO

    PMID:23603835

    Open questions at the time
    • Cell-type-specific contribution of ENT2 in lung not resolved
    • Mechanism by which elevated adenosine dampens edema not fully delineated
  5. 2016 Medium

    Immunocytochemical localization in polarized Caco-2 cells showed ENT2 distributes non-polarly across the plasma membrane, unlike apically restricted ENT1, suggesting ENT2 mediates basolateral nucleoside/nucleobase transport in intestinal epithelium.

    Evidence Immunocytochemistry of polarized Caco-2 cells on membrane filters with mathematical modeling

    PMID:27160886

    Open questions at the time
    • Non-polar distribution inferred from a single cell line
    • No biochemical surface biotinylation to confirm polarity
  6. 2018 High

    Tissue-specific (epithelial) Ent2 deletion recapitulated global KO protection against colitis, and epistasis with epithelial Adora2b deletion abolished this protection, pinpointing an epithelial ENT2→A2B adenosine receptor signaling axis that resolves intestinal inflammation.

    Evidence Global and epithelial-specific Ent2 KO mice; murine IBD model; pharmacological A2B blockade; epithelial Adora2b KO

    PMID:30333323

    Open questions at the time
    • Specific adenosine metabolizing enzymes contributing to the phenotype not identified
    • Whether ENT2 regulation changes during inflammation not addressed
  7. 2021 Medium

    ENT2 was shown to mediate transcytosis of an anti-DNA autoantibody (DX1) across brain endothelial cells and the blood-brain barrier, revealing an unexpected role for this nucleoside transporter in macromolecular shuttling.

    Evidence ENT2 knockdown and pharmacological inhibition in brain endothelial cells; BBB transcytosis assays; in vivo orthotopic glioblastoma model

    PMID:34128837

    Open questions at the time
    • Mechanism by which a small-molecule transporter mediates antibody transcytosis is unexplained
    • Not independently replicated
    • Structural basis for DX1 interaction with ENT2 unknown
  8. 2024 High

    Quantitative kinetic characterization in ENT1-knockout HEK293 cells confirmed that ENT2 has highest affinity for adenosine (Ki 2.6 µM) among nucleosides and identified hypoxanthine as the only nucleobase with submillimolar affinity, refining the substrate hierarchy in a clean genetic background.

    Evidence CRISPR/Cas9 ENT1 KO in HEK293 cells; [³H]2-chloroadenosine competitive inhibition kinetics; immunoblotting; qPCR

    PMID:39054074

    Open questions at the time
    • Full kinetic parameters for nicotinamide and urate not determined in this system
    • No structural explanation for adenosine preference
  9. 2024 High

    ENT2 was identified as a pH-dependent urate transporter with mechanistically separable uptake and efflux modes, as the N68K mutation selectively ablated urate uptake while preserving efflux.

    Evidence HEK293 transfection; radiolabeled and fluorescent urate uptake at varying pH; N68K mutagenesis; ENT2 knockdown in Caco-2 cells

    PMID:40367900

    Open questions at the time
    • Physiological relevance of ENT2-mediated urate transport in vivo not tested
    • Structural basis for asymmetric N68K effect on uptake vs efflux unknown
  10. 2025 High

    ENT1/2 were identified as cellular nicotinamide transporters required for NAD+ biosynthesis; their knockdown impaired mitochondrial respiration and induced senescence, linking ENT2-mediated NAM uptake to cellular energy metabolism and aging.

    Evidence ENT1/2 knockdown; NAM uptake assays; metabolomics; NAD+ quantification; mitochondrial respiration; senescence assays; NMN rescue

    PMID:39885119

    Open questions at the time
    • Individual contributions of ENT1 vs ENT2 to NAM transport not fully deconvolved
    • In vivo tissue-specific relevance for NAD+ homeostasis not established
  11. 2025 Medium

    CRISPR knockout of ENT2 in colorectal cancer cells caused intracellular hypoxanthine accumulation, elevated xanthine oxidase activity and ROS, and apoptosis, establishing ENT2 as a regulator of purine catabolism and cell survival.

    Evidence CRISPR/Cas9 ENT2 KO in HT29 and DLD1 cells; hypoxanthine quantification; XO activity; ROS and apoptosis assays

    PMID:40824957

    Open questions at the time
    • Whether hypoxanthine export failure is the sole driver of ROS/apoptosis not tested with rescue
    • Relevance to non-cancer intestinal epithelium or other tissues not assessed

Open questions

Synthesis pass · forward-looking unresolved questions
  • A high-resolution structure of ENT2 is lacking, and the structural basis for its broad substrate selectivity, NBMPR insensitivity, asymmetric urate transport modes, and the reported ability to mediate macromolecular transcytosis remain unresolved.
  • No crystal or cryo-EM structure available
  • Mechanism of antibody transcytosis is mechanistically unexplained
  • Tissue-specific contributions of ENT2 vs ENT1 to NAD+ homeostasis in vivo are undefined

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0005215 transporter activity 7
Localization
GO:0005886 plasma membrane 2
Pathway
R-HSA-382551 Transport of small molecules 4 R-HSA-1430728 Metabolism 2 R-HSA-168256 Immune System 2
Partners
ADORA2B

Evidence

Reading pass · 11 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2000 hENT2 (SLC29A2) is a broadly selective, low-affinity equilibrative nucleoside transporter that is insensitive to NBMPR at nanomolar concentrations (IC50 ~2.8 µM vs ~0.4 nM for ENT1) and dipyridamole (IC50 ~356 nM vs ~5 nM for ENT1); it transports inosine with ~4-fold higher affinity than ENT1 and can transport the nucleobase hypoxanthine; hENT2 runs as 50 and 47 kDa on SDS-PAGE and is N-glycosylated (deglycosylated to 45 kDa by PNGase F). Stable transfection in nucleoside transporter-deficient PK15NTD cells; radiolabeled uptake assays; [3H]NBMPR binding; pharmacological inhibition; PNGase F/EndoH deglycosylation The Journal of biological chemistry High 10722669
2005 Protein-altering variants of ENT2 (SLC29A2) were characterized by site-directed mutagenesis and Xenopus oocyte expression; the frameshift deletion ENT2-Δ845-846 abolished inosine transport, while three nonsynonymous variants retained transport activity, demonstrating that the C-terminus is required for function. Site-directed mutagenesis; Xenopus laevis oocyte expression; radiolabeled inosine uptake assay Drug metabolism and disposition Medium 16214850
2013 ENT2 (SLC29A2) mediates adenosine uptake at the alveolar epithelium; genetic deletion of Ent2 in mice elevates bronchoalveolar adenosine levels, attenuates pulmonary edema, and improves gas exchange during acute lung injury, with protection abolished by A2B adenosine receptor (Adora2b) deletion, placing ENT2 upstream of Adora2b in a lung-protective signaling axis. Gene-targeted Ent2−/− mice; murine ALI model (high-pressure mechanical ventilation); bronchoalveolar adenosine measurement; genetic epistasis with Adora2b knockout FASEB journal High 23603835
2018 Epithelial ENT2 (SLC29A2) is the functionally relevant transporter limiting extracellular adenosine in the intestinal mucosa; tissue-specific Ent2 deletion (epithelial) recapitulates the protection seen in global Ent2−/− mice during colitis, and protection is abolished by A2B adenosine receptor blockade or epithelial Adora2b deletion, establishing an epithelial ENT2→A2B signaling pathway that resolves intestinal inflammation. Global and tissue-specific Ent2−/− mice; murine IBD model; intestinal adenosine measurement; selective ENT2 inhibitor; pharmacological A2B blockade; epithelial Adora2b KO epistasis JCI insight High 30333323
2016 In polarized Caco-2 intestinal epithelial cells, ENT1 localizes apically while ENT2 (SLC29A2) is distributed integrally (non-polarized) across the plasma membrane, as determined by immunocytochemical staining, with ENT2 contributing to basolateral hypoxanthine transport. Immunocytochemical staining of polarized Caco-2 cells grown on membrane filters; mathematical modeling of transporter distribution Bulletin of experimental biology and medicine Medium 27160886
2007 In rat cardiac fibroblasts, insulin upregulates ENT2 (SLC29A2) mRNA through an mTOR-dependent, PI3K- and MEK-independent pathway that also requires new protein synthesis, distinct from the pathways by which insulin regulates CNT1 and CNT2. Primary rat cardiac fibroblast culture; RT-PCR; pharmacological inhibitors of mTOR (rapamycin), PI3K (wortmannin), MEK (PD98059), and protein synthesis (cycloheximide) Archives of biochemistry and biophysics Medium 17537394
2021 ENT2 (SLC29A2) mediates transcytosis of the anti-DNA autoantibody DX1 across brain endothelial cells and the blood-brain barrier; knockdown or inhibition of ENT2 blocked DX1 penetration, identifying ENT2 as a transporter that can shuttle macromolecular cargo across the BBB. ENT2 knockdown in brain endothelial cells; BBB transcytosis assays; mechanistic studies with DX1 antibody; in vivo orthotopic glioblastoma model JCI insight Medium 34128837
2025 ENT1 and ENT2 (SLC29A1/SLC29A2) function as cellular nicotinamide transporters; ENT1/2 knockdown reduces intracellular NAD+ levels, impairs mitochondrial respiration, and accelerates cellular senescence, with these effects rescued by NMN supplementation, placing ENT2-mediated NAM uptake upstream of NAD+ biosynthesis and mitochondrial function. ENT1/2 knockdown in cell lines; NAM uptake assays; metabolomics; NAD+ quantification; mitochondrial respiration assays; senescence assays; NMN rescue experiment Nature communications High 39885119
2024 ENT2 (SLC29A2) transports urate as a substrate, with efficient uptake at acidic pH (pH 5.5, Km = 1.64 mM) inhibited by nucleoside substrates and ENT2 inhibitors; the N68K mutation abolishes urate uptake while preserving urate efflux activity, suggesting distinct uptake and efflux transport modes; ENT2 knockdown in Caco-2 cells reduces urate uptake confirming endogenous function. Transient transfection of HEK293 cells; radiolabeled/fluorescent urate uptake assays at varying pH; site-directed mutagenesis (N68K); ENT2 knockdown in Caco-2 cells; co-transfection with SNBT1 Drug metabolism and pharmacokinetics High 40367900
2024 In CRISPR/Cas9-generated ENT1-knockout HEK293 cells expressing only endogenous ENT2, the affinity of ENT2 for adenosine (Ki = 2.6 µM) was highest among tested nucleosides, hypoxanthine was the only nucleobase with submillimolar affinity (Ki = 320 µM), and ticagrelor showed the highest affinity among analogs (Ki = 8.6 µM); loss of ENT1 did not alter ENT2 expression or function. CRISPR/Cas9 ENT1 knockout in HEK293 cells; [3H]2-chloroadenosine uptake assays; competitive inhibition kinetics; immunoblotting; qPCR Drug metabolism and disposition High 39054074
2025 CRISPR/Cas9 knockout of ENT2 in colorectal cancer cell lines (HT29, DLD1) elevates intracellular hypoxanthine and xanthine oxidase activity, increases reactive oxygen species, and induces apoptosis, establishing ENT2 as a regulator of purine catabolism and cell survival in colorectal cancer cells. CRISPR/Cas9 gene knockout; hypoxanthine quantification; xanthine oxidase activity assay; ROS measurement; apoptosis assays; cell proliferation assays PloS one Medium 40824957

Source papers

Stage 0 corpus · 24 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2000 Kinetic and pharmacological properties of cloned human equilibrative nucleoside transporters, ENT1 and ENT2, stably expressed in nucleoside transporter-deficient PK15 cells. Ent2 exhibits a low affinity for guanosine and cytidine but a high affinity for inosine. The Journal of biological chemistry 271 10722669
2013 Crosstalk between the equilibrative nucleoside transporter ENT2 and alveolar Adora2b adenosine receptors dampens acute lung injury. FASEB journal : official publication of the Federation of American Societies for Experimental Biology 93 23603835
2018 Coordination of ENT2-dependent adenosine transport and signaling dampens mucosal inflammation. JCI insight 57 30333323
1999 Distribution of mRNA encoding a nitrobenzylthioinosine-insensitive nucleoside transporter (ENT2) in rat brain. Brain research. Molecular brain research 57 10407178
2005 Functional characterization and haplotype analysis of polymorphisms in the human equilibrative nucleoside transporter, ENT2. Drug metabolism and disposition: the biological fate of chemicals 34 16214850
1995 A mammalian delayed-early response gene encodes HNP36, a novel, conserved nucleolar protein. Biochemical and biophysical research communications 24 7639753
2014 Dilazep analogues for the study of equilibrative nucleoside transporters 1 and 2 (ENT1 and ENT2). Bioorganic & medicinal chemistry letters 16 25454272
2021 ENT2 facilitates brain endothelial cell penetration and blood-brain barrier transport by a tumor-targeting anti-DNA autoantibody. JCI insight 14 34128837
2018 Plagiochianins A and B, Two ent-2,3- seco-Aromadendrane Derivatives from the Liverwort Plagiochila duthiana. Organic letters 14 30289265
2025 SLC29A1 and SLC29A2 are human nicotinamide cell membrane transporters. Nature communications 13 39885119
2023 Increased ENT2 expression and its association with altered purine metabolism in cell lines derived from different stages of colorectal cancer. Experimental and therapeutic medicine 12 37123217
2022 PF-07321332 (Nirmatrelvir) does not interact with human ENT1 or ENT2: Implications for COVID-19 patients. Clinical and translational science 12 35505633
2016 Localization and Expression of Nucleoside Transporters ENT1 and ENT2 in Polar Cells of Intestinal Epithelium. Bulletin of experimental biology and medicine 12 27160886
2019 Quantification of ENT1 and ENT2 Proteins at the Placental Barrier and Contribution of These Transporters to Ribavirin Uptake. Journal of pharmaceutical sciences 10 31520644
2015 Effect of Hypoxanthine on Functional Activity of Nucleoside Transporters ENT1 and ENT2 in Caco-2 Polar Epithelial Intestinal Cells. Bulletin of experimental biology and medicine 10 26593410
2007 Different signaling pathways utilized by insulin to regulate the expression of ENT2, CNT1, CNT2 nucleoside transporters in rat cardiac fibroblasts. Archives of biochemistry and biophysics 9 17537394
2021 Candida albicans ENT2 Contributes to Efficient Endocytosis, Cell Wall Integrity, Filamentation, and Virulence. mSphere 8 34585966
1994 Regional assignment of seven loci to 12p13.2-pter by PCR analysis of somatic cell hybrids containing the der(12) or the der(X) chromosome from a mesothelioma showing t(X;12)(q22;p13). Genomics 6 8020938
2013 Crystallographic analysis of the ENTH domain from yeast epsin Ent2 that induces a cell division phenotype. Protein science : a publication of the Protein Society 5 23553749
1997 The human HNP36 gene is localized to chromosome 11q13 and produces alternative transcripts that are not mutated in multiple endocrine neoplasia, type 1 (MEN I) syndrome. Genomics 5 9192854
2023 Ent2 Governs Morphogenesis and Virulence in Part through Regulation of the Cdc42 Signaling Cascade in the Fungal Pathogen Candida albicans. mBio 4 36809010
2024 Development of a Novel HEK293 Cell Model Lacking SLC29A1 to Study the Pharmacology of Endogenous SLC29A2-Encoded Equilibrative Nucleoside Transporter Subtype 2. Drug metabolism and disposition: the biological fate of chemicals 2 39054074
2025 CRISPR/Cas9 mediated ENT2 gene knockout altered purine catabolic pathway and induced apoptosis in colorectal cell lines. PloS one 1 40824957
2024 Functional characteristics of equilibrative nucleoside transporter 2 (ENT2/SLC29A2) for the transport of urate as a newly identified substrate. Drug metabolism and pharmacokinetics 1 40367900