Abstract. The interactions between transferrin (Tf)and transferrin receptor (Tfr) as they occur during biosynthesis were studied in the human hepatoma cell line HepG2, which synthesizes both. Early during biosynthesis the Tfr monomer is converted to a disulfidelinked Tfr dimer. The Tfr monomer is not able to bind Tf, but Tf binding is observed as soon as the covalent Tfr dimer is formed and can take place in the ER. The Tf-Tfr complex is transported through the Golgi reticulum and trans-Golgi reticulum (TGR) and is ultimately delivered to an acidic compartment, where Tf releases its Fe 3÷. We did not observe conversion of Tf to apoTf in the TGR, showing that the part of the TGR passed by secreted Tf has a pH higher than 5.5.We conclude that when a ligand-receptor combination is synthesized by one and the same cell, ligand and receptor can interact during biosynthesis and be transported to the cell surface.
INTERACTIONS of ligands with receptors are essential in the communication of cells with their surroundings, and furthermore supply the cell with nutrients. When a cell produces both a soluble polypeptide ligand and its receptor, they usually follow the same biosynthetic route. Examples of receptors and corresponding ligands synthesized by the same cell include the couples IL-2/IL-2 receptor, transferrin/transferrin receptor (Tf/Tfr), 1 apolipoprotein E/LDL (ApoE/LDL) receptor, and transforming growth factor or/ epidermal growth factor receptor (TGFc~/EGFR) (for reviews see references 5,7,17,22,33). In principle, the simultaneous production of receptor and ligand can result in their binding in the course of biosynthesis. Its consequences may be several, depending on the type of ligand and receptor. When the ligand is a growth factor, binding of the ligand to its receptor during biosynthesis can result in arrival of the ligand-receptor complex at the cell surface, and may produce a growth stimulus for the cell. This situation constitutes a special case of an autocrine loop: secretion of the otherwise released growth factor is not necessary. Autocrine loops have been postulated to operate for TGF, EGF, and PDGF with their respective receptors in different cell lines, and figure prominently amongst the models explaining tumor growth (8).When the ligand supplies a cell with certain nutrients (e.g., Tf, LDL), binding of ligand and receptor during bio-1. Abbreviations used in this paper: alAT, alphat-antitrypsin; apoE, apolipoprotein E; apoTf, apotransferrin; ASGR, asialoglycoprotein receptor; dNM, 1-deoxynojirimycin; EGFR, epidermal growth factor receptor; 1D-IEF, one-dimensional isoelectric focussing; NANAse, neuraminidase; PDGF, platelet-derived growth factor; Tf, transferrin; Tfr, transferrin receptor; TGFa, transforming growth factor a; TGR, trans-Golgi reticulum. synthesis can likewise result in appearance of the complex at the cell surface followed by its internalization. The ligand will thus be delivered back to the cell from which it originated.To our knowledge, the interaction of receptors with their solubl...