Mechanism of reduced amyloid-A-degrading activity in serum of patients with secondary amyloidosis

Impaired degradation of serum amyloid A (SAA) protein by cytokine-stimulated monocytes

Secondary amyloidosis (AA amyloidosis) is a systemic disease characterized by the extracellular tissue deposition of insoluble amyloid A (AA) protein. Aberrant metabolism of serum amyloid A (SAA) by macrophages is only one of many putative mechanisms which may be important in AA amyloidogenesis. In this study, we investigated the effects of cytokines on human monocyte-mediated SAA proteolysis. Human peripheral blood mononuclear cells (PBMC) or CD14(+) monocytes were cultured with SAA, and the culture supernatants were then subjected to anti-SAA immunoblot. CD14(+) monocytes degraded SAA completely. Whereas, when CD14(+) monocytes were pretreated with IL-1 beta or IFN-gamma, increasing amounts of SAA-related derivatives were detected in culture supernatants. These findings suggest that activation of monocytes by IL-1 beta or IFN-gamma hampers the proteolysis of a precursor protein and leads to a partial degradation of SAA. This down-regulated proteolysis of SAA protein by cytokine-stimulated monocytes may play a role in the mechanism of AA amyloid formation as well as its removal.

Serum amyloid A protein (SAA) subtypes in acute and chronic inflammatory conditions

Serum amyloid A (SAA), a polymorphic high density lipoprotein associated plasma protein, is the putative circulating precursor of tissue AA protein fibrils in reactive (secondary) amyloidosis. In the present study we examined the SAA subtype pattern in various acute and chronic inflammatory states in order to find out whether disease-specific SAA isoform profiles exist. The method used to study the subtype pattern is based on electrofocusing of serum followed by immunoblotting. Our results show that the SAA subtype pattern is similar in patients with rheumatoid arthritis with or without amyloid. In addition, in amyloidotic subjects the SAA subtype response to acute tissue injury (arthroplasty) did not differ from that in patients without amyloidosis. Analysis of patients with acute and chronic infectious diseases and non-rheumatic inflammatory conditions showed similar SAA patterns in all subjects. These results suggest that the SAA subtype response to tissue injury and inflammation is similar irrespective of the initiating stimulus.

Correlation of persistently high serum amyloid A protein and C-reactive protein concentrations with rapid progression of secondary amyloidosis

The importance of serum amyloid A protein in the progression of renal failure was studied over three years in 28 patients with secondary (amyloid A type) amyloidosis predominantly due to rheumatoid arthritis. Creatinine clearance, the amount of protein in the urine, and serum amyloid A and C-reactive protein concentrations were determined regularly. Linear regression analysis showed a close correlation between the change in creatinine clearance each year and both serum amyloid A concentrations (20 patients: r= -0.83, p less than 0.001) and C-reactive protein concentrations (28 patients: r= -0.80, p less than 0.001). The correlation between serum amyloid A and C-reactive protein concentrations was also significant (317 parallel measurements: r=0.81, p less than 0.001). These findings suggest that monitoring serum amyloid A or C-reactive protein concentrations is valuable in assessing the prognosis in secondary amyloidosis and that therapeutic measures that lower serum amyloid A concentrations may reduce the formation of amyloid.