Receptor-mediated endocytosis of 125I-labelled transferrin by human choriocarcinoma (JAR) cells.

The binding and subsequent internalization of 125I-labelled transferrin has been studied in the human choriocarcinoma cell line, JAR. At 4 degrees C, binding was time- and concentration-dependent and exhibited saturation kinetics. The results indicate 1.6 x 10(6) binding sites per cell surface with a Kd of 4.12 x 10(-9) M. Experiments with saponin-permeabilized cells revealed a large intracellular receptor pool constituting 72 per cent of the total cell receptor population. Evidence of ligand internalization at 37 degrees C was obtained by showing the progressive resistance of cell-associated radioactivity to acid treatment. Pulse-chase experiments showed that after internalization, ligand resided within the cell for about 6 min before being released back to the medium in an intact form. Ligand release, but not internalization, was almost completely inhibited by exposure to monensin. Preincubation of cells at 37 degrees C in the presence of monensin and under transferrin-free conditions had no effect on the subsequent binding of 125I-transferrin. Pulse-chase experiments using 125I-labelled anti-transferrin receptor antibody suggest that receptors are internalized in the absence of added transferrin with a rate coefficient of 0.036 min-1. In the presence of unlabelled transferrin, the initial rate of labelled antibody internalization was increased (rate coefficient was 0.23 min-1) but the maximal amount of antibody internalized remained virtually unchanged. Internalized antibody accumulated intracellularly with no evidence of release or degradation.