Characterization of α2-Macroglobulin Binding to Human Trabecular Meshwork Cells: Presence of the α2-Macroglobulin Signaling Receptor

Direct binding of receptor-recognized α2-macroglobulin (α2M*) or a cloned receptor binding fragment from rat α1-macroglobulin (RBF) to human trabecular meshwork cells demonstrated two classes of cell surface binding sites. One class has an apparentKdof 5.0 nmand a receptor number of 31,800 receptors/cell. The other class has an apparentKdof 20 pmand a receptor number of 1600 receptors/cell. Binding studies of α2M* or RBF in the presence of a competitor for binding to low-density-lipoprotein receptor-related protein/α2M receptor (LRP/α2MR) called receptor-associated protein (RAP) show that only the lower affinity class of binding sites is susceptible to competition with RAP. Uptake studies demonstrate specific internalization and degradation of α2M* which is inhibitable by RAP. Exposure of the cells to α2M* and RBF (40 nm) is associated with mean increases of 171 and 210%, respectively, in the intracellular calcium concentration, which is not inhibitable by RAP or pertussis toxin. These studies present the first characterization of α2M* and RBF signaling in a primary human cell type and suggest a role for α2M* in the physiology of the eye.