Characterization of mammalian equilibrative nucleoside transporters (ENTs) by mass spectrometry

Functional characterization of human equilibrative nucleoside transporter 1

Equilibrative nucleoside transporters (ENTs), which facilitate cross-membrane transport of nucleosides and nucleoside-derived drugs, play an important role in the salvage pathways of nucleotide synthesis, cancer chemotherapy, and treatment for virus infections. Functional characterization of ENTs at the molecular level remains technically challenging and hence scant. In this study, we report successful purification and biochemical characterization of human equilibrative nucleoside transporter 1 (hENT1) in vitro. The HEK293F-derived, recombinant hENT1 is homogenous and functionally active in proteoliposome-based counter flow assays. hENT1 transports the substrate adenosine with a Km of 215 ± 34 µmol/L and a Vmax of 578 ± 23.4 nmol mg-1 min-1. Adenosine uptake by hENT1 is competitively inhibited by nitrobenzylmercaptopurine ribonucleoside (NBMPR), nucleosides, deoxynucleosides, and nucleoside-derived anti-cancer and anti-viral drugs. Binding of hENT1 to adenosine, deoxyadenosine, and adenine by isothermal titration calorimetry is in general agreement with results of the competitive inhibition assays. These results validate hENT1 as a bona fide target for potential drug target and serve as a useful basis for future biophysical and structural studies.

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

Our study confirmed that Asn⁴⁸ of hENT1 is the only N-glycosylated residue when expressed in HEK293 cells, and loss of the N-glycan resulted in less hENT1 at the plasma membrane, as well as a loss of function and protein–protein self-interaction.

Human equilibrative nucleoside transporter 1 (hENT1) transports nucleosides and nucleoside analogue drugs across cellular membranes and is necessary for the uptake of many anti-cancer, anti-parasitic and anti-viral drugs. Previous work, and in silico prediction, suggest that hENT1 is glycosylated at Asn⁴⁸ in the first extracellular loop of the protein and that glycosylation plays a role in correct localization and function of hENT1. Site-directed mutagenesis of wild-type (wt) hENT1 removed potential glycosylation sites. Constructs (wt 3xFLAG-hENT1, N48Q-3xFLAG-hENT1 or N288Q-3xFLAG-hENT2) were transiently transfected into HEK293 cells and cell lysates were treated with or without peptide–N-glycosidase F (PNGase-F), followed by immunoblotting analysis. Substitution of N48 prevents hENT1 glycosylation, confirming a single N-linked glycosylation site. N48Q-hENT1 protein is found at the plasma membrane in HEK293 cells but at lower levels compared with wt hENT1 based on S-(4-nitrobenzyl)-6-thioinosine (NBTI) binding analysis (wt 3xFLAG-ENT1 B_max, 41.5±2.9 pmol/mg protein; N48Q-3xFLAG-ENT1 B_max, 13.5±0.45 pmol/mg protein) and immunofluorescence microscopy. Although present at the membrane, chloroadenosine transport assays suggest that N48Q-hENT1 is non-functional (wt 3xFLAG-ENT1, 170.80±44.01 pmol/mg protein; N48Q-3xFLAG-ENT1, 57.91±17.06 pmol/mg protein; mock-transfected 74.31±19.65 pmol/mg protein). Co-immunoprecipitation analyses suggest that N48Q ENT1 is unable to interact with self or with wt hENT1. Based on these data we propose that glycosylation at N48 is critical for the localization, function and oligomerization of hENT1.

Equilibrative nucleoside transporter 1 (ENT1) regulates postischemic blood flow during acute kidney injury in mice

A complex biologic network regulates kidney perfusion under physiologic conditions. This system is profoundly perturbed following renal ischemia, a leading cause of acute kidney injury (AKI) - a life-threatening condition that frequently complicates the care of hospitalized patients. Therapeutic approaches to prevent and treat AKI are extremely limited. Better understanding of the molecular pathways promoting postischemic reflow could provide new candidate targets for AKI therapeutics. Due to its role in adapting tissues to hypoxia, we hypothesized that extracellular adenosine has a regulatory function in the postischemic control of renal perfusion. Consistent with the notion that equilibrative nucleoside transporters (ENTs) terminate adenosine signaling, we observed that pharmacologic ENT inhibition in mice elevated renal adenosine levels and dampened AKI. Deletion of the ENTs resulted in selective protection in Ent1-/- mice. Comprehensive examination of adenosine receptor-knockout mice exposed to AKI demonstrated that renal protection by ENT inhibitors involves the A2B adenosine receptor. Indeed, crosstalk between renal Ent1 and Adora2b expressed on vascular endothelia effectively prevented a postischemic no-reflow phenomenon. These studies identify ENT1 and adenosine receptors as key to the process of reestablishing renal perfusion following ischemic AKI. If translatable from mice to humans, these data have important therapeutic implications.

Analysis of recombinant tagged equilibrative nucleoside transporter 1 (ENT1) expressed in E. coli

Nucleoside transporters (NTs) are integral membrane proteins necessary for the cellular entry of nucleoside analog drugs used in chemotherapeutic treatment of conditions such as cancer and viral or parasitic infections. NTs are also the targets of certain drugs used in the treatment of various cardiovascular conditions. Because of the importance of NTs in drug uptake, determination of the three-dimensional structure of these proteins, particularly hENT1, has the potential to improve these treatments through structure-based design of more specifically targeted and transported drugs. In this paper, we use NMR spectroscopy to investigate the structure of the large intracellular loop between transmembrane domains 6 and 7 and we also describe a method for the successful overexpression of full-length hENT1 in a bacterial system. Recombinant tandem histidine-affinity (HAT) and 3×FLAG tagged hENT1 was overexpressed in E. coli, affinity purified, and functionally characterized by nitrobenzylthioinosine (NBTI) binding. Anti-3×FLAG immunodetection confirmed the expression of N-HAT-3×FLAG-hENT1, while increased NBTI binding (3.2-fold compared with controls) confirmed the conformational integrity of the recombinant hENT1 within the bacterial inner membrane. Yields of recombinant hENT1 using this approach were ~15 µg/L of bacterial culture and this approach provides a basis for large-scale production of protein for a variety of purposes.

The life cycle of human equilibrative nucleoside transporter 1: from ER export to degradation

Nucleoside transporters (NTs) play an essential role in the transport of nucleosides across cellular membranes. Equilibrative NTs (ENTs) allow facilitated diffusion of nucleosides and the prototypic ENT, hENT1, is primarily localized to the plasma membrane (PM). hENT1 is responsible for the uptake of nucleoside analog drugs used in treating viral infections and cancer, but despite its clinical importance, virtually nothing is known about the dynamics of the hENT1 life cycle including trafficking to the PM, endocytosis and degradation. Therefore, we followed the life cycle of tagged hENT1 (GFP- or FLAG-) transiently transfected into mammalian cells to gain insight into the sequence of events, timing and underlying mechanisms regulating the hENT1 life cycle. Protein translocation to the PM was examined using fixed and live cell confocal microscopy while endocytosis and degradation were analyzed by cell surface biotinylation and [(35)S] pulse chase analysis respectively. We determined that tagged hENT1 is trafficked to the PM in association with microtubules and incorporated in the plasma membrane where it subsequently undergoes clathrin-mediated endocytosis and recycling. Finally, internalized protein is degraded via the lysosomal pathway and observations suggest the complete life cycle of tagged hENT1 within these cells is approximately 14 hours.