Altered interaction of cis-dichlorodiammineplatinum(II)-modified alpha(2)-macrooglobulin (alpha(2)M) with the low density lipoprotein receptor-related protein/alpha(2)M receptor but not the alpha(2)M signaling receptor - Evidence for interference with receptor dissociation and recycling

Receptor-recognized forms of alpha-macroglobulin (alpha(2)M*) bind to two macrophage receptors: an endocytic receptor, the low density lipoprotein receptor-related protein/alpha(2)M receptor (LRP/alpha(2)MR), and a G protein-coupled receptor, the alpha(2)M Signaling receptor (alpha(2)MSR). Binding of alpha(2)M* to LRP/alpha(2)MR but not alpha(2)MSR is inhibited by receptor-associated protein, We now present binding characteristics of alpha(2)MSR (K-d similar to 50 pM; 1,530 sites/cell) using Scatchard analysis. We also demonstrate that chemical modification of alpha(2)M* with cis-dichlorodiammineplatinum (cis-DDP) does not significantly alter binding to either receptor or signaling characteristics as compared with unmodified alpha 2M*. However, internalization by LRP/alpha(2)MR is greatly affected, Cis-DDP-modified alpha(2)M* (cis-DDP-alpha(2)M*) and alpha(2)M* show comparable internalization during a single round of endocytosis; however, cis-DDP modification of alpha(2)M* results in a greater than or equal to 82% reduction in internalization involving receptor recycling and multiple rounds of endocytosis. Results from PH 5.0 dissociation and receptor recycling experiments suggest that the mechanism of decreased internalization of cis-DDP-alpha(2)M* involves poor dissociation from the receptor in endosomes and a decrease in available surface receptors over the time of exposure to the ligand.