Two novel rat liver membrane proteins that bind advanced glycosylation endproducts: relationship to macrophage receptor for glucose-modified proteins

Advanced glycosylation endproducts (AGEs), the glucose-derived adducts that form nonenzymatically and accumulate on tissue proteins, are implicated in many chronic complications associated with diabetes and aging. We have previously described a monocyte/macrophage surface receptor system thought to coordinate AGE protein removal and tissue remodeling, and purified a corresponding 90-kD AGE-binding protein from the murine RAW 264.7 cell line. To identify AGE-binding proteins in normal animals, the tissue distribution of 125I-AGE rat serum albumin taken up from the blood was determined in rats in vivo. These uptake studies demonstrated that the liver was a major site of AGE protein sequestration. Using a solid-phase assay system involving the immobilization of solubilized membrane proteins onto nitrocellulose to monitor binding activity, and several purification steps including affinity chromatography over an AGE bovine serum albumin matrix, two rat liver membrane proteins were isolated that specifically bound AGEs, one migrating at 60 kD (p60) and the other at 90 kD (p90) on SDS-PAGE. NH2-terminal sequence analysis revealed no significant homology between these two proteins nor to any molecules available in sequence databases. Flow cytometric analyses using avian antibodies to purified rat p60 and p90 demonstrated that both proteins are present on rat monocytes and macrophages. Competition studies revealed no crossreactivity between the two antisera; anti-p60 and anti-p90 antisera prevented AGE-protein binding to rat macrophages when added alone or in combination. These results indicate that rat liver contains at least two novel and distinct proteins that recognize AGE-modified macromolecules, although p90 may be related to the previously described 90-kD AGE receptor isolated from RAW 264.7 cells. The constitutive expression of AGE-binding proteins on rat monocytes and macrophages, and the sequestration of circulating AGE-modified proteins by the liver, provides further evidence in support of a role for these molecules in the normal removal of proteins marked as senescent by accumulated glucose-derived covalent addition products, or AGEs.