Monocyte/macrophage response to β₂-microglobulin modified with advanced glycation end products

We recently found that acidic β₂-microglobulin (β₂m), a major isoform of β₂m in amyloid fibrils of patients with dialysis-related amyloidosis (DRA), contained early Amadori prod ucts and advanced glycation end products (AGEs) formed nonenzymatically between sugar and protein. Further analysis revealed that acidic β₂m induces monocyte chemotaxis and macrophage secretion of bone-resorbing cytokines, suggesting the involvement of acidic β₂m in the pathogenesis of DRA. Acidic β₂m, however, is a mixture of heterogeneous molecular adducts due to various types of modification. In the present study, we investigated the modification responsible for the biological activity of acidic β₂m toward monocytes/macrophages. The presence of a fair amount of β₂m species with deamidation was detected in acidic β₂m isolated from urine of non-diabetic long-term hemodialysis patients, but deamidated β₂m had no biological activity. In contrast, normal β₂m acquired the activity upon incubation with glucose in vitro. Among the glycated β₂m, the pigmented and fluorescent β₂m that formed after a long incubation period, that is, AGE-modified β₂m, exhibited biological activity, whereas p,m modified with Amadori products, major Maillard products in acidic β₂m, had no such activity. These findings suggest that AGEs, although only a minor constituent of acidic β₂m, are responsible for monocyte chemotaxis and macrophage secretion of cytokines, implicating the contribution of AGEs to bone and joint destruction in DRA.