Receptor-mediated endocytosis of transferrin in developmentally totipotent mouse teratocarcinoma stem cells.

The transferrin-mediated pathway of iron uptake has been characterized in mouse teratocarcinoma stem cells in culture. These cells were chosen for the characterization because of their unique capacity to be converted to normalcy and to undergo complete and orderly differentiation into all tissues after microinjection into early mouse embryos. Therefore, the cells provide the possibility of experimentally analyzing in vitro and in vivo the progressive developmental expression of genes, both normal and mutant, controlling the transferrin pathway. In a new developmentally totipotent euploid cell line, the teratocarcinoma cells were found to have 5.7 lo3 high affinity (Kd = 6.7 n ~ ) specific cell-surface receptors for transferrin. Surface binding of transferrin to the receptors was documented at 4 “C, when the cells do not take in significant amounts of iron. At 37 “C, both surface binding and internalization occur, yielding large amounts of intracellular iron. Receptor-bound transferrin is internalized rapidly, at the rate of one full complement of cell-surface receptors every 6 min, and is an energy-requiring process. The lysosomotropic agents W C l and chloroquine inhibit iron uptake into the cells without inhibiting internalization of transferrin, a result suggesting that the lysosome is an important intermediate in the iron-uptake pathway. The low pH in the lysosome can account for dissociation of iron from the apoprotein and of the transferrin from its receptor. The apotransferrin molecule escapes lysosomal degradation and is released intact into the medium by the cells, thereby becoming available for another cycle of iron transport. It is now possible to design selection strategies for the isolation of mutant teratocarcinoma stem cells carrying different genetic lesions affecting the transferrin-mediated pathway as described here. The genetic variants would enable construction of mice carrying specific iron transport defects.