Endocytosis of the transferrin receptor is altered during differentiation of murine erythroleukemic cells

During terminal differentiation of murine erythroleukemic (MEL) cells, the number of surface transferrin binding sites per cell decreases dramatically, while steady-state ligand uptake and immunoblotting studies demonstrate that the total number of transferrin receptors per cell remains constant. Since the amount of protein per cell decreases 4-fold during this 4-day period, the amount of transferrin receptor protein, relative to total soluble cell protein, increases 4-fold during this time, suggesting continued synthesis of the receptor. Supporting this, we show that the amount of transferrin receptor transcript in equal amounts of total cell RNA also increases as differentiation proceeds. Uninduced cells maintain 52% of the total transferrin binding sites on the cell surface, whereas only 22% of the receptors are on the surface in 4-day induced cells. All ligand endocytosed by either uninduced or induced cells at 37 degrees C is rapidly and completely exocytosed from the cells, suggesting that all of the cellular receptors are cycling. These studies suggest that, during MEL cell differentiation, an increasing fraction of transferrin receptors are localized to the cell interior, but are nevertheless cycling to the cell surface. This observed redistribution is due to altered kinetic parameters of the receptor. Receptor-bound 125I-labeled transferrin ligand has been followed through a single endocytic cycle. Ligand internalization occurs much more rapidly in induced cells (t1/2 = 2.9 min) than in uninduced cells (t1/2 = 6.9 min). The rates for ligand movement back out to the cell surface and its subsequent release into the medium in both uninduced and induced cells are quite similar.