Receptor-mediated endocytosis and recycling of alpha-fetoprotein in human B-lymphoma and T-leukemia cells.

The kinetics of iodinated human alpha-fetoprotein (AFP) binding and uptake by 2 human neoplastic lymphoid cell lines (CEM and RAJI) have been studied. Three saturation plateaus were obtained by incubating CEM and RAJI cells at 4 degrees C with 125I-AFP at different concentrations. Scatchard analysis suggested the presence of 3 types of receptor site with different affinities and capacities on cells of both lines. AFP binding was inhibited by unlabelled human and bovine AFP, and to a lesser extent by human serum albumin (SAH); no significant competition was observed with human transferrin (Tf) or ovalbumin (Ova). Pulse-chase experiments showed that 125I-AFP was released practically undegraded from the cells. Covalent conjugates of AFP and Tf with horseradish peroxidase (HRP) were used to follow the endocytosis and intracellular pathway of these serum proteins by electron microscopy. Both proteins were observed in coated vesicles, endosomes and a tubular vesicular network localized in the Golgi-centrosphere region. SAH-HRP was internalized to a much lesser extent. Ova-HRP was poorly internalized and was observed in lysosome-like organelles.