Activation of an alpha-fetoprotein (AFP)/receptor autocrine loop in HT-29 human colon carcinoma cells

Immunological and morphological approaches have been used to demonstrate, respectively, alpha-fetoprotein (AFP) synthesis and receptor expression in the HT-29 human colon carcinoma parental cell line. HT-29 cells cultivated in the presence of glucose synthesized and secreted AFP in the medium from 48 to 96 hr after seeding, as revealed by monoclonal antibodies (MAbs) to human AFP. Light microscopic observations of cells incubated with fluoresceinated AFP showed that the protein was specifically bound to the cell surface at 4 degrees C, and was internalized in the cytoplasm at 37 degrees C. At the ultrastructural level, horseradish peroxidase (HRP)-conjugated AFP, as well as HRP-transferrin (Tf) (used as a control), appeared to be internalized via coated pits and vesicles before being delivered to endosomes, from which they were apparently recycled back to the cell surface via small vesicles. Our results suggest that a AFP/receptor autocrine pathway might operate in these cells, be preferentially active at the beginning of the exponential phase of cell culture growth, and contribute to cell proliferation.