Absence of Specific Binding of Receptor-Mediated Endocytosis, and of Secretion of Alpha-2-Macroglobulin by Cultured Endothelial Cells

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Abstract
The presence of an α2-macroglobulin (α2M) receptor, the receptor-mediated endocytosis of α2M-protease complexes, and the secretion of α2M by cultured endothelial cells derived from human umbilical vein and from bovine aorta were assessed with 125I-α2M trypsin. Only very low levels of endocytosis of 125I-α2M trypsin were measured, compared to the levels observed in normal human fibroblast cell layers. The absence of α2M receptors on endothelial cells, as measured in binding experiments at 4 °C, points to the nonconcentrative character of this process. The expression of an α2M receptor could not be induced by plating the cells at low densities or by growing them on collagen gels. The secretion of α2M by endothelial cells could not be detected. The production of α2M by human endothelial cells must, therefore, be lower than 3 ng/l06 cells/48 h, the lower limit of the assay.