Cell-type-specific receptors for alpha-fetoprotein in a mouse T-lymphoma cell line

Binding and uptake of alpha-fetoprotein (AFP) by mouse T-lymphoma YAC-1 cells exhibited the characteristics of receptor-mediated endocytosis. The binding saturation curve obtained by incubating YAC-1 cells at 4 degrees C with 125I-labeled AFP at different concentrations (50 ng/ml to 2.5 mg/ml) showed three saturation plateaus. AFP binding was inhibited by unlabeled mouse, rat, or bovine AFP and, to a lesser extent, by rat or bovine serum albumin. No significant competition was observed with transferrin, alpha 2-macroglobulin, IgG, or ovalbumin. Scatchard analysis suggested the presence of three types of receptor sites with a Kd of 2.2 X 10(-9) M (approximately equal to 700 sites per cell), 8.6 X 10(-7) M (approximately equal to 210,000 sites per cell), and 5.7 X 10(-6) M (approximately equal to 910,000 sites per cell), respectively. At 37 degrees C, AFP was rapidly internalized and could be localized in the cytoplasm after incubation of cells with fluoresceinated AFP. After a short residence time, AFP was released undegraded from the cells. Normal adult thymocytes and T lymphocytes, which are counterparts of YAC-1 cells, did not show any significant uptake of AFP. On the other hand, a small subpopulation of fetal and newborn thymocytes was labeled by fluoresceinated AFP.