Long-term kinetics of AA amyloidosis and effects of inflammatory restimulation after disappearance of amyloid depositions in mice.

Amyloid A (AA) amyloidosis is characterized by extracellular pathogenic deposition of insoluble fibril protein in various body organs. Deposited amyloid generally remains in a variety of organs for long periods, but its disappearance has been reported after the precursor protein is diminished. The kinetics of AA deposition are not completely understood and, in particular, the roles of cells and cytokines in the deposition and clearance of amyloid remain unclear. In this study, we investigated the disappearance of amyloid depositions in mice over a 1-year period. AA amyloidosis was induced experimentally in mice by injecting amyloid-enhancing factor (AEF) and silver nitrate. Mice were killed at different time-points to examine the occurrence and disappearance of amyloid depositions. Maximum levels of amyloid depositions were observed at 20 days after inoculation. Clearance of amyloid depositions was observed from the 40th day onwards, with only minute traces of amyloid present by 240 days. A second inflammatory stimulus consisting of AEF and silver nitrate was given at 330 or 430 days, after amyloid depositions had disappeared almost completely. After that, serum amyloid A was overproduced and redeposition of amyloid was observed, indicating that all mice were primed for aggressive amyloid depositions. After administration of the inflammatory stimuli, the proinflammatory environment was found to have increased levels of interleukin (IL)-6, while anti-inflammatory conditions were established by IL-10 as regression of amyloid deposition occurred. These results suggest that the proinflammatory and anti-inflammatory status have key roles in both amyloid deposition and clearance.

Serum amyloid A uptake by feline peripheral macrophages.

Serum amyloid A (SAA) is one of the major acute phase proteins in cats and humans. SAA concentrations increase in response to the inflammatory status and secondary amyloid A amyloidosis has been documented in cats. In order to control the SAA concentration, it is important to clarify how the SAA protein is metabolized. Although the details of SAA metabolism in the body remain unknown, human and murine research indicates that macrophages play a key role in SAA uptake. The objectives of this study were to demonstrate SAA uptake by feline macrophages and to evaluate the effects of lipopolysaccharide (LPS) and dexamethasone (Dex) on SAA uptake. The concentration of recombinant feline SAA added to a feline macrophage culture was decreased in a time-dependent manner and was significantly reduced after a 24-h incubation, as demonstrated by enzyme linked immunosorbent assay (ELISA). SAA uptake into feline peripheral macrophages was demonstrated by immunofluorescence microscopy. Pretreatment to macrophages with LPS did not affect this decrease in the SAA concentration, but this was significantly blocked by Dex pretreatment. In conclusion, SAA was incorporated by feline macrophages and pretreatment with Dex inhibited SAA uptake by macrophages in this study. Further investigation is needed to determine the molecules that influence SAA uptake by macrophages and the effect of clinical glucocorticoid usage on the SAA concentration in cats.

Differential plasma clearance of murine acute-phase serum amyloid A proteins SAA1 and SAA2.

Serum amyloid A (SAA) proteins SAA1 and SAA2 are prominent acute-phase reactants which circulate in association with the high-density-lipoprotein (HDL) fraction of plasma. Plasma levels of SAA1 and SAA2 increase dramatically, by as much as 1000-fold, within 24 h of tissue injury and then rapidly decrease with cessation of the inflammatory stimulus, suggesting that SAA clearance and/or catabolism is important to the re-establishment of homoeostasis. In this context, aberrant SAA catabolism has long been considered a potential factor in the pathogenesis of reactive amyloidosis. To initiate studies aimed at understanding the differential regulation of SAA metabolism, we have produced 35S-labelled murine SAA1 and SAA2 in Escherichia coli, bound them individually to HDL, and then compared the plasma-clearance characteristics of SAA1 and SAA2 under normal and acute-phase conditions. When bound to normal HDL, SAA2 [half-life (t1/2) = 30 min] was cleared significantly faster than SAA1 (t1/2 = 75 min). Clearance of SAA1 and SAA2 was significantly slower when each was bound to acute-phase HDL as opposed to normal HDL, when clearance rates were determined in acute-phase mice versus normal mice, and when normal HDL was remodelled to contain both recombinant isotypes rather than just one of the isotypes. Thus it appears that an increased amount of SAA on HDL, or possibly the combined presence of both isotypes on HDL, is associated with a prolongation in the plasma half-life of SAA.

Serum amyloid A in the mouse. Sites of uptake and mRNA expression.

Murine serum amyloid A1 (SAA1) and serum amyloid A2 (SAA2) are circulating, acute phase, high density apolipoproteins of unknown function. To pursue issues relating to their possible function their uptake and formation were studied. Kinetics of SAA protein distribution and gene expression after acute phase stimulation by casein or lipopolysaccharide were examined using immunocytochemistry for protein and RNA blot and in situ hybridization with probes for SAA1 and SAA2 mRNA. After casein injection, interstitial cells of testes, cells of adrenal cortex, kidney proximal convoluted tubule epithelia, and some parafollicular cells of spleen took up SAA in a time pattern related to plasma SAA levels. Extrahepatic SAA1 and SAA2 mRNA were induced by lipopolysaccharide in kidney proximal and distal convoluted tubule epithelia, and SAA1 mRNA was induced in epithelial lining the mucosa of the ileum and large intestine, indicating that there may be more than one function for the apoSAA gene family related to site of and stimulus for expression.