Uptake and processing of 125I-labelled transferrin and 59Fe-labelled transferrin by isolated human trophoblast cells

Trophoblast cells isolated from term human placenta and maintained as an adherent culture express surface receptors for transferrin as indicated by quantitative binding studies using 125I-labelled transferrin. The Kd was 5.3 x 10(-9) M. About 36 per cent of the total cell receptor population was found at the cell surface, the remainder being intracellular. Both 125I-labelled and 59Fe-labelled transferrin were internalized by receptor-mediated endocytosis with similar rates. Pulse-chase experiments showed that 125I-labelled transferrin was recycled and released back to the medium, whereas 59Fe accumulated intracellularly and was released slowly. Polyacrylamide gel electrophoresis followed by autoradiography revealed that 59Fe was accumulated by cells largely in the form of ferritin. A small intracellular pool of low molecular weight 59Fe was also detected. In the presence of monensin, the transfer of 59Fe to ferritin was greatly reduced. The nature and amount of 59Fe released from cells could be modulated by the incubation conditions. In the absence of chelating agents and iron salts, released 59Fe was found to be associated with a low molecular weight fraction as well as with transferrin and ferritin. The low molecular weight 59Fe readily formed a complex with added chelators such as apotransferrin, DTPA or desferrioxamine. The release of 59Fe could be increased by repeatedly changing the medium during the course of the incubation. 59Fe release from trophoblast cells exceeded the release of lactate dehydrogenase and also exceeded the release of 59Fe from 3T3 fibroblasts, suggesting a cell-specific process.