Megalin-dependent cubilin-mediated endocytosis is a major pathway for the apical uptake of transferrin in polarized epithelia

Cubilin is a 460-kDa protein functioning as an endocytic receptor for intri nsic factor vitamin B-12 complex in the intestine and as a receptor for apo lipoprotein Al and albumin reabsorption in the kidney proximal tubules and the yolk sac. In the present study, we report the identification of cubilin as a novel transferrin (Tf) receptor involved in catabolism of Tf. Consist ent with a cubilin-mediated endocytosis of Tf in the kidney, lysosomes of h uman, dog, and mouse renal proximal tubules strongly accumulate Tf, whereas no Tf is detectable in the endocytic apparatus of the renal tubule epithel ium of dogs with deficient surface expression of cubilin. As a consequence, these dogs excrete increased amounts of Tf in the urine. Mice with deficie nt synthesis of megalin, the putative coreceptor colocalizing with cubilin, also excrete high amounts of Tf and fail to internalize Tf in their proxim al tubules. However, in contrast to the dogs with the defective cubilin exp ression, the megalin-deficient mice accumulate Tf on the luminal cubilin-ex pressing surface of the proximal tubule epithelium. This observation indica tes that megalin deficiency causes failure in internalization of the cubili n-ligand complex. The megalin-dependent, cubilin-mediated endocytosis of Tf and the potential of the receptors thereby to facilitate iron uptake were further confirmed by analyzing the uptake of I-125- and Fe-59-labeled Tf in cultured yolk sac cells.