A pulse‐chase study tracking the conversion of macrophage‐endocytosed serum amyloid A into extracellular amyloid

Abstract

Objective. To determine whether serum amyloid A (SAA) is internalized by and processed in macrophages en route to deposition as extracellular amyloid.Methods. SAA was tracked in cultures of peritoneal macrophages, using a pulse-chase protocol. Conclusion. The results of this study provide direct evidence that SAA internalized by and processed in macrophages forms extracellular amyloid. Based on the presence of 125 I-AA protein in macrophage lysates prior to the appearance of extracellular TxR-labeled amyloid, it was concluded that cleavage of SAA to AA occurs intracellularly.

Susceptibility to AA amyloidosis in rheumatic diseases: A critical overview

“…Mononuclear phagocytes are physiologically involved in SAA catabolism through endocytosis and trafficking to lysosomes, where the protein undergoes degradation. A role in initiating amyloid A fibril formation by mononuclear phagocytes was originally postulated on the basis of the presence of AA amyloid fibrils both in intracellular vesicles and close to the cell membrane in amyloid-laden tissues (4) and has been subsequently demonstrated in cell culture models (5). Studies in human monocyte cell lines have shown accumulation of newly formed AA amyloid fibrils in intracellular lysosomal compartments, indicating that aberrant processing of SAA is relevant in the pathogenesis of AA amyloidosis (6).…”

Role of Endocytic Inhibitory Drugs on Internalization of Amyloidogenic Light Chains by Cardiac Fibroblasts

“…3 These data indicate that LC toxicity may contribute to cardiac dysfunction as well as play a role in amyloid fibril deposition. Furthermore, internalization of LCs (1,6, and 3 subtypes) by cardiac fibroblasts resulted in enhanced sulfation of secreted glycosaminoglycans (GAGs) with minimal heparan-sulfated proteoglycans localized within the cytoplasm. 4 These results supported earlier studies demonstrating an increase in GAGs with amyloid deposition.…”

“…Models of AA and A␤ amyloid suggest that endocytosis and/or pinocytosis may be critical to amyloid protein internalization. 6,7 In addition, studies using mesangial cells demonstrate that internalization of LCs by cells utilizes a specific receptor complex pathway. 8,9 In neuronal cells, investigators showed that receptors of advanced glycation end products function as signal-transducing cell surface acceptors for A␤.…”

The genetics of the amyloidoses: interactions with immunity and inflammation

“…Mixed cultures of SAA-producing hepatoma cells or hepatocytes and macrophages have been reported to produce AA fibrils in tissue culture. 25 A single published mixed culture experiment with human bone marrow cells and macrophages from an amyloid patient suggested a similar process. 91 More recent studies in which L-chains isolated from patients with AL and light chain deposition disease were incubated with cultured human mesangial cells seem to demonstrate cleavage of the amyloidogenic L-chains with subsequent fibril formation, a very interesting result awaiting confirmation in other laboratories.…”

“…24,25 In this instance, there are few experimental data available regarding the details of these processes. In the case of the homotetrameric protein transthyretin (TTR), a shift in the equilibrium between tetramer and monomer leads to the enhanced population of a monomer pool predisposed to misfold.…”