CHARACTERIZATION OF ALPHA(2)-MACROGLOBULIN BINDING TO HUMAN TRABECULAR MESHWORK CELLS - PRESENCE OF THE ALPHA(2)-MACROGLOBULIN SIGNALING RECEPTOR

Direct binding of receptor-recognized alpha(2)-macroglobulin (alpha(2) M) Or a cloned receptor binding fragment from rat alpha(1)-macroglobu lin (RBF) to human trabecular meshwork cells demonstrated two classes of cell surface binding sites, One class has an apparent K-d of 5.0 nM and a receptor number of 31,800 receptors/cell, The other class has a n apparent K-d of 20 pM and a receptor number of 1600 receptors/cell, Binding studies of alpha(2)M or RBF in the presence of a competitor f or binding to low-density-lipoprotein receptor-related protein/alpha(2 )M receptor (LRP/alpha(2)MR) called receptor-associated protein (RAP) show that only the lower affinity class of binding sites is susceptibl e to competition with RAP, Uptake studies demonstrate specific interna lization and degradation of alpha(2)M which is inhibitable by RAP, Ex posure of the cells to alpha(2)M and RBF (40 nM) is associated with m ean increases of 171 and 210%, respectively, in the intracellular calc ium concentration, which is not inhibitable by RAP or pertussis toxin, These studies present the first characterization of alpha(2)M and RB F signaling in a primary human cell type and suggest a role for alpha( 2)M in the physiology of the eye. (C) 1996 Academic Press, Inc.