Amyloid A (AA) amyloidosis, a complication of inflammatory diseases such as tuberculosis, leprosy and rheumatoid arthritis, occurs more frequently with increasing length of unchecked disease. AA fibrils are derived from apoSAA proteins (transient, injury-specific constituents of high density lipoprotein (HDL)). At the resolution of an acute inflammatory episode, elevated apoSAA appears to be catabolized by two pathways; one is cell- associated and the other involves secreted enzymes, either extracellularly or in phagolysosomes. When normal clearance is impaired, insoluble AA fibrils accumulate extracellularly. The model of spontaneous AA amyloidosis in aging female golden Syrian hamsters is ideally suited for analysis of amyloidogenic factors in the absence of confounding factors due to the massive inflammatory stimulation required by other animal models of AA amyloidogenesis; and, in particular, for assessing the pathogenetic role of SAP, a ubiquitous constituent of all amyloid deposits. We have shown that one of at least 8 closely related species of Syrian hamster apoSAA, apoSAA3, is selectively deposited spontaneously as AA fibrils. Here we propose to study apoSAA catabolism as it relates to AA amyloidosis, using recombinant apoSAA3 and nonamyloidogenic isoforms such as apoSAA1 as controls. These apoSAA molecules will be radiolabeled and used for in vivo and in vitro studies of apoSAA catabolism in hepatocytes and macrophages from young and old, amyloidotic and nonamyloidotic, male and female hamsters. The goal is to achieve AA fibril formation in a deemed in vitro system, thereby establishing the requisite factors for AA fibril formation. The hypothesis that apoSAA clearance occurs as part of its normal function to interrupt reverse cholesterol transport will be tested. We propose that apoSAA is normally cleared by dual mechanisms: type I in which apoSAA is catabolized by enzymes(s) secreted by cells of the monocyte/macrophage lineage (monocytic cell lines and peritoneal exudate cells), and type II in which apoSAA is catabolized by cell-associated enzymes(s) during cholesterol transport (e.g. hepatoma cell lines). The ability of lipids and lipoproteins, serum amyloid P (SAP) and extracellular matrix (ECM) constituents to alter the capacity of lysosomal enzymes for complete catabolism of apoSAA will be investigated. The long range goals are to enhance the normal protective role of apoSAA in restoration of homeostasis, to prevent dysfunctions such as amyloidosis that occur as a complication of the chronic inflammatory conditions that are more prevalent with aging, and to understand in general how age- associated changes in regulated proteolysis can lead to amyloid fibril formation.
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