Abstract
The endocytosis and recycling of the human transferrin receptor were evaluated by several experimental modalities in K562 cells perturbed with 10(-5) M monensin. The work presented is an extension of a previous study demonstrating both complete inhibition of release of internalized human transferrin and a 50% reduction in the number of cell surface transferrin binding sites in K562 cells treated with monensin (Stein, B. S., Bensch, K. G., and Sussman, H. H. (1984) J. Biol. Chem. 259, 14762-14772). The data directly reveal the existence of two distinct transferrin receptor recycling pathways. One pathway is monensin-sensitive and is felt to represent recycling of transferrin receptors through the Golgi apparatus, and the other pathway is monensin-resistant and most likely represents non-Golgi-mediated transferrin receptor recycling. A transferrin-free K562 cell culture system was developed and used to demonstrate that cell surface transferrin receptors can be endocytosed without antecedent ligand binding, indicating that there are factors other than transferrin binding which regulate receptor internalization. Evidence is presented suggesting that two transferrin receptor recycling pathways are also operant in K562 cells under ligand-free conditions, signifying that trafficking of receptor into either recycling pathway is not highly ligand-dependent.
Highlights
A transferrin-free K562 cell culture system was de- to the LDL receptor systemin cultured human fibroblasts, in veloped and used to demonstrate that cell surface transferrin receptors can be endocytosed without antecedent ligand binding, indicating that there arefactors other than transferrin binding which regulate receptor internalization
This study demonstrates a maximal50% reduction inthe amountof antitransferrin receptor antibody binding to the cell surface of K562 cells resulting from monensin treatment, while total cell transferrin receptor content remains unchanged
M monensintreatmenton cell surface transferrin receptor reduction in K562 cells grown in serum-free medium supplemented with 300 pg/ml of human transferrin, since all previously described studies used cells which were passaged in medium containing bovine transferrin
Summary
Ligand-independent internalization of the LDL receptor has been documented in human fibroblasts (14). 50% of surface LDL receptors recycle continuouslyin the absence of LDL. Previous studies with K562 cells have suggested that the transferrinreceptorisnotinternalized without prior occupancy of ligand binding sites with human transferrin (7, 15). I n the currentstudyexperiments were conductedwhichdemonstrate that the humantransferrin receptor canbe internalized in a ligand-independent fashion. Findings are presented which suggestthat two transferrin receptor recycling pathways are operant in the absence of ligand, indicating that trafficking of receptor into either pathway is not highly dependent upon transferrin
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
