Receptor-Mediated Endocytosis of Alpha2 -Macroglobulin and Transferrin in Rat Caput Epididymal Epithelial Cells In Vitro

Publisher Website
Google Scholar
Abstract
The endocytic activity of epithelial cells from the rat epididymis in vitro has been examined by following the uptake of tracer compounds conjugated to proteins. Transferrin-gold and .alpha.2-macroglobulin-gold were taken up initially and coated pits, internalized and sequestered into tubular-vesicular structures, multivesicular bodies and, in the case of .alpha.2-macroglobulin, into lysosomes. Uptake could be prevented by an excess of unlabeled protein. Studies using 125I-.alpha.2-macroglobulin and 125I-transferrin also showed that the uptake of these proteins was specific and could be displaced with increasing amounts of unlabeled protein. Binding of 125I-transferrin to cells was saturable at 4.degree. C. Transferrin and .alpha.2-macroglobulin are taken up by receptor-mediated endocytosis. A fluid phase marker, bovine serum albumin-gold (BSA-gold), was initially taken up predominantly in uncoated caveolae rather than coated pits, and could not be displaced with excess BSA. By virtue of their charge, polycationized ferritin and unlabeled colloidal gold were taken up and internalized by adsorptive endocytosis, a pathway which is similar to fluid phase endocytosis. The uptake and internalization of .alpha.2-macroglobulin and transferrin differed in a number of respects. Uptake and internalization of .alpha.2-macroglobulin but not of transferrin was dependent on extracellular Ca. Only .alpha.2-macroglobulin was transferred into lysosomes, whereas transferrin was recycled to the cell surface. Although the proton ionophore, monensin and the transglutaminase inhibitor, dansylcadaverine, did not stop uptake and internalization of either .alpha.2-macroglobulin or transferrin, they did prevent the transfer of .alpha.2-macroglobulin to lysosomes.