N-linked glycosylation of N48 is required for equilibrative nucleoside transporter 1 (ENT1) function

Defective insulin-stimulated equilibrative nucleoside transporter-2 activity and altered subcellular transporter distribution drive the loss of adenosine homeostasis in diabetic kidney disease progression

AIM

Progression of diabetic nephropathy (DN) is linked to the dysregulated increase of adenosine and altered signaling properties. A major contribution to the maintenance of physiological extracellular adenosine levels relies on cellular uptake activity through plasma membrane nucleoside transporters. Because kidney cells are responsive to insulin, this study aims to determine how DN affects insulin regulation of the equilibrative nucleoside transporter-2 (ENT2).

METHODS

Human Podocytes and rat glomeruli were used to study ENT2 regulation. The effects of diabetes and insulin on ENT2 mediated transport activity were determined measuring the fraction of total adenosine uptake in sodium-free medium which is inhibitable by hypoxanthine. Alterations in ENT2 subcellular distribution were assessed in the kidney of people affected with DN and diabetic rats. The consequences of impaired ENT2 activity on the kidney were evaluated using dipyridamole in an animal model.

RESULTS

Insulin upregulates ENT2 uptake activity by increasing the Vmax, thus counteracting decreased adenosine uptake due to high d-glucose and achieving extracellular adenosine homeostasis. Insulin promoted ENT2 translocation to the plasma membrane dependent on PI3-kinase/Akt signaling and actin cytoskeleton integrity. However, in diabetic rats, the insulin-mediated induction of ENT2 activity was lost. Additionally, reduced Akt activation in response to insulin correlated with decreased ENT2 distribution at the plasma membrane. Kidney tissues from diabetic rats and human DN biopsies showed ENT2 redistribution to an intracellular pattern, evidencing dysfunctional adenosine uptake. Through ENT inhibition, we evidenced increased proteinuria and induced alpha-smooth muscle actin as a result of profibrotic activation of cells in the kidney.

CONCLUSION

Deficient insulin regulation of ENT2 activity contributes to chronically high adenosine levels and glomerular alterations that underline diabetic kidney disease progression.

Assembly Characterization of Human Equilibrium Nucleoside Transporter 1 (hENT1) by Inhibitor Probe-Based dSTORM Imaging

Nucleoside transporters (NTs) play an important role in the metabolism of nucleoside substances and the efficacy of nucleoside drugs. Its spatial information related to biofunctions at the single-molecule level remains unclear, owing to the limitation of the existing labeling methods and traditional imaging methods. Therefore, we synthesize the inhibitor-based fluorescent probe SAENTA-Cy5 and apply direct stochastic optical reconstruction microscopy (dSTORM) to conduct refined observation of human equilibrative nucleoside transporter 1 (hENT1), the most important and famous member of NTs. We first demonstrate the labeling specificity and superiority of SAENTA-Cy5 to the antibody probe. Then, we found different assembly patterns of hENT1 on the apical and basal membranes, which are further investigated to be caused by varying associations of membrane carbohydrates, membrane classical functional domains (lipid rafts), and associated membrane proteins (EpCAM). Our work provides an efficient method for labeling hENT1, which contributes to realize fine observation of NTs. The findings on the assembly features and potential assembly mechanism of hENT1 promote a better understanding of its biofunction, which facilitates further investigations on how NTs work in the metabolism of nucleoside and nucleoside analogues.

Novel variants provide differential stabilisation of human equilibrative nucleoside transporter 1 states

Human equilibrative nucleoside transporters represent a major pharmaceutical target for cardiac, cancer and viral therapies. Understanding the molecular basis for transport is crucial for the development of improved therapeutics through structure-based drug design. ENTs have been proposed to utilise an alternating access mechanism of action, similar to that of the major facilitator superfamily. However, ENTs lack functionally-essential features of that superfamily, suggesting that they may use a different transport mechanism. Understanding the molecular basis of their transport requires insight into diverse conformational states. Differences between intermediate states may be discrete and mediated by subtle gating interactions, such as salt bridges. We identified four variants of human equilibrative nucleoside transporter isoform 1 (hENT1) at the large intracellular loop (ICL6) and transmembrane helix 7 (TM7) that stabilise the apo-state (∆Tm 0.7–1.5°C). Furthermore, we showed that variants K263A (ICL6) and I282V (TM7) specifically stabilise the inhibitor-bound state of hENT1 (∆∆Tm 5.0 ± 1.7°C and 3.0 ± 1.8°C), supporting the role of ICL6 in hENT1 gating. Finally, we showed that, in comparison with wild type, variant T336A is destabilised by nitrobenzylthioinosine (∆∆Tm -4.7 ± 1.1°C) and binds it seven times worse. This residue may help determine inhibitor and substrate sensitivity. Residue K263 is not present in the solved structures, highlighting the need for further structural data that include the loop regions.

Insight into the Nucleoside Transport and Inhibition of Human ENT1

The human equilibrative nucleoside transporter 1 (hENT1) is an effective controller of adenosine signaling by regulating its extracellular and intracellular concentration, and has become a solid drug target of clinical used adenosine reuptake inhibitors (AdoRIs). Currently, the mechanisms of adenosine transport and inhibition for hENT1 remain unclear, which greatly limits the in-depth understanding of its inner workings as well as the development of novel inhibitors. In this work, the dynamic details of hENT1 underlie adenosine transport and the inhibition mechanism of the non-nucleoside AdoRIs dilazep both were investigated by comparative long-time unbiased molecular dynamics simulations. The calculation results show that the conformational transitions of hENT1 from the outward open to metastable occluded state are mainly driven by TM1, TM2, TM7 and TM9. One of the trimethoxyphenyl rings in dilazep serves as the adenosyl moiety of the endogenous adenosine substrate to competitively occupy the orthosteric site of hENT1. Due to extensive and various VDW interactions with N30, M33, M84, P308 and F334, the other trimethoxyphenyl ring is stuck in the opportunistic site near the extracellular side preventing the complete occlusion of thin gate simultaneously. Obviously, dilazep shows significant inhibitory activity by disrupting the local induce-fit action in substrate binding cavity and blocking the transport cycle of whole protein. This study not only reveals the nucleoside transport mechanism by hENT1 at atomic level, but also provides structural guidance for the subsequent design of novel non-nucleoside AdoRIs with enhanced pharmacologic properties.

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The transitions of hENT1 from the outward open to metastable occluded state are mainly driven by TM1, TM2, TM7 and TM9.

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The induce-fit action by adenosine recognition precedes. inward contraction of the extracellular side.

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Dilazep exerts its special hENT1 inhibitory function through competitive binding and allosteric regulation.

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A gating strategy of extracellular loop is revealed to ensure adenosine is firmly located in the transport cavity.

Augustine Blood Group System and Equilibrative Nucleoside Transporter 1

Augustine (AUG) is a blood group system comprising four antigens: AUG1, AUG2 (Ata), and AUG4 are of very high frequency; AUG3 is of very low frequency. These antigens are located on ENT1, an equilibrative nucleoside transporter encoded by SLC19A1. AUG antibodies are of clinical relevance in blood transfusion and pregnancy: anti-AUG2 have caused haemolytic transfusion reactions; the only anti-AUG3 was associated with severe haemolytic disease of the fetus and newborn. ENT1 is present in almost all human tissues. It facilitates the transfer of purine and pyrimidine nucleosides and is responsible for the majority of adenosine transport across plasma membranes. Adenosine transport appears to be an important factor in the regulation of bone metabolism. The AUGnull phenotype (AUG:-1,-2,-3,-4) has been found in three siblings, who are homozygous for an inactivating splice-site mutation in SLC29A1. Although ENT1 is very likely to be absent from all cells in these three individuals, they were apparently healthy with normal lifestyles. However, they suffered frequent attacks of pseudogout, a form of arthritis, in various joints with multiple calcifications around their hand joints. Ectopic calcification in the hips, pubic symphysis, and lumbar discs was present in the propositus. The three AUGnull individuals had misshapen red cells with deregulated protein phosphorylation, but no anaemia or shortening of red cell lifespan. Defective in vitro erythropoiesis in the absence of ENT1 was confirmed by shRNA-mediated knockdown of ENT1 during in vitro erythropoiesis of CD34+ progenitor cells from individuals with normal ENT1. Nucleoside transporters, such as ENT1, are vital in the uptake of synthetic nucleoside analogue drugs, used in cancer and viral chemotherapy. It is feasible that the efficacy of these drugs would be compromised in patients with the extremely rare AUGnull phenotype.

Impact of natural mutations on the riboflavin transporter 2 and their relevance to human riboflavin transporter deficiency 2

Riboflavin transporter deficiency 2 (RTD2) is a rare neurological disorder caused by mutations in the Solute carrier family 52 member 2 (Slc52a2) gene encoding human riboflavin transporter 2 (RFVT2). This transporter is ubiquitously expressed and mediates tissue distribution of riboflavin, a water-soluble vitamin that, after conversion into FMN and FAD, plays pivotal roles in carbohydrate, protein, and lipid metabolism. The 3D structure of RFVT2 has been constructed by homology modeling using three different templates that are equilibrative nucleoside transporter 1 (ENT1), Fucose: proton symporter, and glucose transporter type 5 (GLUT5). The structure has been validated by several approaches. All known point mutations of RFVT2, associated with RTD2, have been localized in the protein 3D model. Six of these mutations have been introduced in the recombinant protein for functional characterization. The mutants W31S, S52F, S128L, L312P, C325G, and M423V have been expressed in E. coli, purified, and reconstituted into proteoliposomes for transport assay. All the mutants showed impairment of function. The K_m for riboflavin of the mutants increased from about 3 to 9 times with respect to that of WT, whereas V_max was only marginally affected. This agrees with the improved outcome of most RTD2 patients after administration of high doses of riboflavin.

Toward a Molecular Basis of Cellular Nucleoside Transport in Humans

Nucleosides play central roles in all facets of life, from metabolism to cellular signaling. Because of their physiochemical properties, nucleosides are lipid bilayer impermeable and thus rely on dedicated transport systems to cross biological membranes. In humans, two unrelated protein families mediate nucleoside membrane transport: the concentrative and equilibrative nucleoside transporter families. The objective of this review is to provide a broad outlook on the current status of nucleoside transport research. We will discuss the role played by nucleoside transporters in human health and disease, with emphasis placed on recent structural advancements that have revealed detailed molecular principles of these important cellular transport systems and exploitable pharmacological features.

Fluorouracil uptake in triple-negative breast cancer cells: Negligible contribution of equilibrative nucleoside transporters 1 and 2

Equilibrative nucleoside transporters (ENTs) 1 and 2 reportedly accept fluorouracil as a substrate. Here, we evaluated ENT1/2 expression at the messenger RNA (mRNA), protein, and functional levels in a panel of four triple-negative breast cancer (TNBC) cell lines, BT-549, Hs578T, MDA-MB-231, and MDA-MB-435, and we examined the relationship of the observed profiles to fluorouracil sensitivity. Nitrobenzylthioinosine (NBMPR) at 0.1 μM inhibits only ENT1, while dipyridamole at 10 μM or NBMPR at 100 μM inhibits both ENT1 and ENT2. We found that the uptake of [³ H]uridine, a typical substrate of ENT1 and ENT2, was decreased to approximately 40% by 0.1 μM NBMPR. At 100 μM, NBMPR almost completely blocked the saturable uptake of [³ H]uridine, but this does not imply a functional role of ENT2, because 10 μM dipyridamole showed similar inhibition to 0.1 μM NBMPR. Expression of ENT1 mRNA was almost 1 order of magnitude higher than that of ENT2 in all TNBC cell lines. Liquid chromatography-tandem mass spectrometry(LC-MS/MS) LC-MS/MS-based targeted protein quantification showed that ENT1 protein levels were in the range of 9.3-30 fmol/μg protein in plasma membrane fraction of TNBC cell lines, whereas ENT2 protein was below the detection limit. [³ H]Fluorouracil uptake was insensitive to 0.1 μM NBMPR and 10 μM dipyridamole, suggesting a negligible contribution of ENT1 and ENT2 to fluorouracil uptake. The levels of ENT1 mRNA, ENT1 protein, ENT2 mRNA, and ENT1-mediated [³ H]uridine uptake in the four TNBC cell lines showed no correlation with fluorouracil sensitivity. These results indicate that neither ENT1 nor ENT2 contributes significantly to the fluorouracil sensitivity of TNBC cell lines.

Oligomerization of equilibrative nucleoside transporters: a novel regulatory and functional mechanism involving PKC and PP1

Equilibrative nucleoside transporters (ENTs) translocate nucleosides and nucleobases across plasma membranes, as well as a variety of anti-cancer, -viral, and -parasite nucleoside analogs. They are also key members of the purinome complex and regulate the protective and anti-inflammatory effects of adenosine. Despite their important role, little is known about the mechanisms involved in their regulation. We conducted membrane yeast 2-hybrid and coimmunoprecipitation studies and identified, for the first time to our knowledge, the existence of protein-protein interactions between human ENT1 and ENT2 (hENT1 and hENT2) proteins in human cells and the formation of hetero- and homo-oligomers at the plasma membrane and the submembrane region. The use of NanoLuc Binary Technology allowed us to analyze changes in the oligomeric status of hENT1 and hENT2 and how they rapidly modify the uptake profile for nucleosides and nucleobases and allow cells to respond promptly to external signals or changes in the extracellular environment. These changes in hENTs oligomerization are triggered by PKC activation and subsequent action of protein phosphatase 1.-Grañe-Boladeras, N., Williams, D., Tarmakova, Z., Stevanovic, K., Villani, L. A., Mehrabi, P., Siu, K. W. M., Pastor-Anglada, M., Coe, I. R. Oligomerization of equilibrative nucleoside transporters: a novel regulatory and functional mechanism involving PKC and PP1.