Substrate Dependent Regulation of the Human Equilibrative Nucleoside Transporter 1 (hENT1) in HEK293 Cells

Nucleosides and nucleoside analog drugs enter cells through nucleoside transporters, such as the human equilibrative nucleoside transporter 1 (hENT1). The regulation of nucleoside transporters is poorly understood. In this study, through fluorescence-activated cell sorting (FACS) analyses, confocal microscopy and radio-ligand binding assays, I show a decrease in hENT1 abundance at the plasma membrane (PM) in HEK cells treated in the presence of a bolus amount of cytidine (40μM) for 6 hours. Kinetic and transport assays indicate that the remaining hENT1 population at the PM has a higher Vmax and Km but there is no change in overall substrate uptake compared to untreated cells. I also show that cytidine pre-treatment leads to an increased cytotoxicity from gemcitabine (a nucleoside analog drug). These are the first data that show direct substrate dependent regulation of a nucleoside transporter by a mechanism that may involve increased recycling/internalization of the transporter.