J. Biol. Chem., Vol. 261, Issue 20, 9187-9195, Jul, 1986
Transferrin receptor number, synthesis, and endocytosis during erythropoietin-induced maturation of Friend virus-infected erythroid cells
ST Sawyer and SB Krantz
Erythropoietin (EP) responsive Friend virus-infected erythroid cells had
200,000 steady-state binding sites for transferrin at 37 degrees C when
isolated from the spleens of Friend virus-infected mice. Upon culture of
these cells with EP, the synthesis of transferrin receptors increased 4- to
7-fold and the number of transferrin-binding sites per cell doubled after
24 h. However, the rate of uptake of 59Fe from transferrin remained
constant at approximately 35,000 atoms of 59Fe per minute per cell during
this period in culture. The amount of 125I- transferrin internalized during
the steady-state binding did not change during this culture period while
the transferrin bound to the surface increased 3-fold. At all stages of
erythroid maturation, the maximum rate of endocytosis was determined to be
18,000 molecules of transferrin per minute per cell, and the interval that
125I-transferrin remains in the interior of the cell was calculated to be
6.9 min. After 48 h of culture with EP, the number of steady-state
transferrin-binding sites was reduced in part due to the sequestration of
surface receptors within the cell. The uptake of iron from transferrin was
limited by the level of endocytosis of transferrin during the initial phase
of culture and the number of transferrin receptors at the cell surface
during the latter stages of erythroid maturation of these cells.