Equilibrative nucleoside transporter 2 is expressed in human umbilical vein endothelium, but is not involved in the inhibition of adenosine transport induced by hyperglycaemia

Human equilibrative, Na+-independent nucleoside transport is mediated by membrane proteins sensitive (system es, hENT1) or insensitive (system ei, hENT2) to nitrobenzylthioinosine (NBMPR). Gestational diabetes and elevated extracellular concentrations of d-glucose reduce adenosine transport in human umbilical vein endothelium (HUVEC). We studied hENT2 and hENT1 expression in HUVEC, and the effect of d-glucose on their activity and expression in HUVEC preincubated with 25 mM d-glucose (24 h). hENT2 and hENT1 mRNA were quantified by real-time reverse transcription polymerase chain reaction, and their proteins were detected by Western blotting. hENT2 and hENT1 proteins are co-expressed in HUVEC and are located at the plasma membrane, however, hENT2 was mainly cytoplasmatic and perinuclear in location. d-Glucose reduced hENT1 and hENT2 mRNA expression, but only hENT1 protein abundance at the plasma membrane. Adenosine transport was inhibited by d-glucose and NMBPR (1 μM) in intact cells and membrane vesicles. Hypoxanthine inhibited adenosine transport in the absence or in the presence of 1 μM NBMPR. d-Glucose reduced NBMPR maximal binding in intact cells, membrane vesicles, and plasma membrane fractions. In conclusion, the present study demonstrates that hENT2 and hENT1 are co-expressed in HUVEC, and even when adenosine transport is also mediated by hENT2, the hENT2-mediated transport activity is not involved in the d-glucose-induced down-regulation of total adenosine transport.