EFFECTS OF OVEREXPRESSION OF THE TRANSFERRIN RECEPTOR ON THE RATES OFTRANSFERRIN RECYCLING AND UPTAKE OF NON-TRANSFERRIN-BOUND IRON

The possibilities that the recycling of the transferrin receptor is a rate-limiting step in the efflux of endocytosed transferrin, and that the receptor functions as a trans-membrane Fe transporter were investi gated in untransfected Ltk(-) cells and in cells transfected with diff erent levels of DNA for wild-type, mutant and chimeric human transferr in receptors. The uptake of transferrin-bound Fe and non-transferrin-b ound Fe(II), and the surface binding, endocytosis and recycling of tra nsferrin were measured, In cells that expressed increasing numbers of surface transferrin receptors, the rate of Fe uptake increased at a sl ower rate than the number of receptors. By measurement of the rates of endocytosis and recycling of transferrin it was shown that this effec t was not due to a deficiency of endocytosis, but to a slower rate of recycling as the receptor numbers increased. Hence, a restricted recyc ling rate of the transferrin receptor appeared to be responsible for t he slower rate of Fe uptake by cells with high receptor numbers, presu mably because one or more cytosolic components required for recycling were in limited supply. The rate of uptake of non-transferrin-bound Fe (II) was not influenced by the number of transferrin receptors present on the surface of the cells even though this varied more than 20-fold between the different cell lines. Hence, this investigation does not support the hypothesis that the receptors play a direct role in the tr ansport of Fe(II) across cell membranes, as has been proposed previous ly [Singer, S. J. (1989) Biol. Cell 65, 1-5].