Modulation of cell surface iron transferrin receptors by cellular density and state of activation.

This report describes investigations of plasma membrane transferring receptors on a variety of lymphoid cell lines and normal peripheral blood lymphocytes during activation and cell growth cycles. Transformed lymphoid cell lines have as many as 1,000 times the number of receptors found on normal resting lymphocytes. The number of iron transferrin receptors on continuous cell lines as well as normal human fibroblasts is down-regulated during the transition from log-phase growth to stationary plateau growth. When normal lymphocytes are transformed by mixed lymphocyte culture or mitogens, they rapidly express a 50-fold increase in the number of transferrin binding sites. This appearance of iron transferrin receptors anticipates nuclear changes during cell activation and subsequent mitosis of normal cells.