High-affinity-receptor-mediated uptake and degradation of glucose-modified proteins: A potential mechanism for the removal of senescent macromolecules

Proteins that have been modified by long-term exposure to glucose accumulate advanced glycosylation end products (AGE) as a function of protein age. In these studies, we have characterized the interaction of AGE-protein with mouse peritoneal macrophages, using AGE-modified bovine serum albumin (AGE-BSA, prepared by incubation with glucose) as a probe. AGE-BSA was specifically bound to cells at 4°C and was taken up and degraded at 37°C; these processes were concentration dependent and saturable. Competition experiments with AGE-BSA, BSA incubated with phosphate-buffered saline rather than glucose, and yeast mannan demonstrated that macrophages specifically recognize AGE on proteins by a receptor that is completely distinct from the mannose/fucose receptor. Scatchard analysis of AGE-BSA binding data indicated that there are ~ 1.06 x 10⁵ receptors per macrophage, with an affinity constant of 1.75 x 10^-11 M. Specific binding of AGE-BSA to the macrophage receptor was competitively inhibited by BSA that had been chemically coupled to a synthetic analogue of the specific AGE, 2-(2-furoyl)-4(5)-2(furanyl)-1H-imidazole (FFI-BSA). FFI-BSA was also taken up by macrophages in a concentration-dependent, saturable manner. Prior incubation of macrophages with AGE-BSA failed to influence the subsequent uptake and degradation of added AGE-BSA. Thus, the AGE receptor does not appear to be down-regulated by exposure to AGE-proteins. Results from these studies suggest that AGE could act in vivo as a specific signal for recognition and degradation of senescent macromolecules. Incomplete removal of AGE-proteins by macrophages may ultimately give rise to some of the physiologic changes that occur with normal aging.