Transferrin receptors of human fibroblasts. Analysis of receptor properties and regulation

Normal human skin fibroblasts cultured in vitro exhibit specific binding sites for 125I-labelled transferrin. Kinetic studies revealed a rate constant for association (Kon) at 37 degrees C of 1.03×10(7) M-1 X min-1. The rate constant for dissociation (Koff) at 37 degrees C was 7.9×10(-2) X min-1. The dissociation constant (KD) was 5.1×10(-9) M as determined by Scatchard analysis of binding and analysis of rate constants. Fibroblasts were capable of binding 3.9×10(5) molecules of transferrin per cell. Binding of 125I-labelled diferric transferrin to cells was inhibited equally by either apo-transferrin or diferric transferrin, but no inhibition was evident with apo-lactoferrin, iron-saturated lactoferrin, or albumin. Preincubation of cells with saturating levels of diferric transferrin or apo-transferrin produced no significant change in receptor number or affinity. Preincubation of cells with ferric ammonium citrate caused a time- and dose-dependent decrease in transferrin binding. After preincubation with ferric ammonium citrate for 72 h, diferric transferrin binding was 37.7% of control, but no change in receptor affinity was apparent by Scatchard analysis. These results suggest that fibroblast transferrin receptor number is modulated by intracellular iron content and not by ligand-receptor binding.