Regulation of transferrin receptor expression at the cell surface by insulin-like growth factors, epidermal growth...

Addition of platelet-derived growth factor (PDGF), recombinant insulin-like growth factor I (rIGF-I) or epidermal growth factor (EGF) to BALB/c 3T3 fibroblasts causes a marked increase in the binding of [125I]diferric transferrin to cell surface receptors. This effect is very rapid and is complete within 5 min. The effect of EGF is transient, with [125I]diferric transferrin binding returning to control values within 25 min. In contrast, PDGF and rIGF-I cause a prolonged stimulation of [125I]diferric transferrin binding that could be observed for up to 2 h. The increase in the binding of [125I]diferric transferrin caused by growth factors was investigated by analysis of the binding isotherm. Epidermal growth factor, PDGF and rIGF-I were found to increase the cell surface expression of transferrin receptors rather than to alter the affinity of the transferrin receptors. This result was confirmed in human fibroblasts by the demonstration that EGF, PDGF and rIGF-I could stimulate the binding of a monoclonal antibody directed against the transferrin receptor (OKT9) to the cell surface. Furthermore, PDGF and rIGF-I stimulated the sustained uptake of [59Fe]diferric transferrin by BALB/c 3T3 fibroblasts, while EGF transiently increased uptake. Thus the effect of these growth factors to increase the cell surface expression of the transferrin receptor appears to have an important physiological consequence.