3 '-Azido-3 '-deoxythymidine reduces the rate of transferrin receptor endocytosis in K562 cells

K562 cells, exposed for at least 24 h to 5 mu M 3'-azido-3'-deoxythymidine (AZT), gave rise to an overall increase in the number of cell surface trans ferrin binding receptors (18-20%). This effect was ascertained either with binding experiments by using I-125-transferrin and with immunoprecipitation by using a specific monoclonal antibody against the transferrin receptor. At higher AZT concentrations (20 and 40 mu M), a further increase was found , that is, up to 23% by binding experiments and up to 110% by immunoprecipi tation. However, Scatchard analysis of the binding data indicated that alth ough the number of cell surface transferrin receptors increased, the affini ty of transferrin for its receptor did not change (K-a = 4.0 x 10(8) M). Su rprisingly, immunoprecipitation of total receptor molecules showed that the synthesis of receptor was not enhanced by the drug treatment. The effect o f AZT on transferrin internalization and receptor recycling was also invest igated. In this case, data indicated that the increase in the number of rec eptors at the cell surface was probably due to a slowing down of endocytosi s rate rather than to an increased recycling rate of the receptor to cell s urface. In fact, the time during which half the saturated amount of transfe rrin had been endocytosed (t(1/2)) was 2.15 min for control cells and 3.41, 3.04, and 3.74 min for 5, 20, and 40 mu M AZT-treated cells, respectively. Conversely, recycling experiments did not show any significant differences between control and treated cells. A Likely mechanism through which AZT co uld interfere with the transferrin receptor trafficking, together with the relevance of our findings, is extensively discussed. (C) 1999 Elsevier Scie nce B.V. All rights reserved.