Absence of specific binding of receptor-mediated endocytosis, and of secretion of alpha-2-macroglobulin by cultured endothelial cells.

The presence of an alpha 2-macroglobulin (alpha 2M) receptor, the receptor-mediated endocytosis of alpha 2M-protease complexes, and the secretion of alpha 2M by cultured endothelial cells derived from human umbilical vein and from bovine aorta were assessed with 125I-alpha 2M trypsin. Only very low levels of endocytosis of 125I-alpha 2M trypsin were measured, compared to the levels observed in normal human fibroblast cell layers. The absence of alpha 2M receptors on endothelial cells, as measured in binding experiments at 4 degrees C, points to the nonconcentrative character of this process. The expression of an alpha 2M receptor could not be induced by plating the cells at low densities or by growing them on collagen gels. The secretion of alpha 2M by endothelial cells could not be detected. The production of alpha 2M by human endothelial cells must, therefore, be lower than 3 ng/10(6) cells/48 h, the lower limit of the assay.