A high molecular weight serum protein is the carrier for amyloid-related protein SAA

Apolipoprotein A-II induces acute-phase response associated AA amyloidosis in mice through conformational changes of plasma lipoprotein structure

During acute-phase response (APR), there is a dramatic increase in serum amyloid A (SAA) in plasma high density lipoproteins (HDL). Elevated SAA leads to reactive AA amyloidosis in animals and humans. Herein, we employed apolipoprotein A-II (ApoA-II) deficient (Apoa2 ^-/- ) and transgenic (Apoa2Tg) mice to investigate the potential roles of ApoA-II in lipoprotein particle formation and progression of AA amyloidosis during APR. AA amyloid deposition was suppressed in Apoa2 ^-/- mice compared with wild type (WT) mice. During APR, Apoa2 ^-/- mice exhibited significant suppression of serum SAA levels and hepatic Saa1 and Saa2 mRNA levels. Pathological investigation showed Apoa2 ^-/- mice had less tissue damage and less inflammatory cell infiltration during APR. Total lipoproteins were markedly decreased in Apoa2 ^-/- mice, while the ratio of HDL to low density lipoprotein (LDL) was also decreased. Both WT and Apoa2 ^-/- mice showed increases in LDL and very large HDL during APR. SAA was distributed more widely in lipoprotein particles ranging from chylomicrons to very small HDL in Apoa2 ^-/- mice. Our observations uncovered the critical roles of ApoA-II in inflammation, serum lipoprotein stability and AA amyloidosis morbidity, and prompt consideration of therapies for AA and other amyloidoses, whose precursor proteins are associated with circulating HDL particles.

Acute inflammation, acute phase serum amyloid A and cholesterol metabolism in the mouse

Acute inflammation results in a profound change in the apolipoprotein composition of high density lipoprotein (HDL). Several isoforms of the serum amyloid A (SAA) family, SAA1 and SAA2, become major components of HDL. This structural relationship has suggested that acute phase SAA plays some as yet unidentified role in HDL function, possibly related to cholesterol transport, during the course of acute inflammation. Using subcutaneous AgNO3 to induce a sterile abscess changes in plasma cholesterol and SAA were monitored over the subsequent 144 h. Total plasma cholesterol began to increase within 12 h of the induction of inflammation and reached a peak in 24 h. Thereafter its plasma levels fell returning to normal values by 96-120 h. The bulk of the increase in plasma cholesterol was found in the free cholesterol fraction of HDL. This pattern of cholesterol increase corresponds to the established temporal changes for acute phase SAA (AP-SAA). AP-SAA levels increased within 8 h of the induction of inflammation and reached a peak at 24 h. They began to decrease by 48 h with small quantites still present 120 h later. In concert, but inversely, with the changes in AP-SAA the apoA-I, apoA-II, and apo-E, content of HDL decreased during the AP-SAA increases and increased as AP-SAA levels fell. The plasma appearance of cholesterol from the periphery, and central parts of the inflammatory site was assessed by the use of radiolabelled cholesterol. The peripherally placed cholesterol rapidly reached a peak plasma concentration within 24 h of injection. Cholesterol placed in the central part of the sterile abscess, a site relatively inaccessible to the vasculature required 48 h to reach its peak and was 5-times lower than that placed peripherally. The influence of AP-SAA on neutral cholesterol ester hydrolase (nCEH) activity in mouse liver homogenates, mouse peritoneal macrophage homogenates, and a purified porcine pancreatic enzyme with nCEH activity was also assessed. Following optimization with regard to pH, bile salt concentration, protein concentration and incubation time, mouse peritoneal macrophages had a significantly higher nCEH specific activity than that found in liver (7-8 fold). Purified AP-SAA, assessed over a concentration range of 0-10 microg/ml, enhanced nCEH activity at concentrations above 2 microg/ml. The nCEH activity, regardless of its source, increased by 3-7 fold in the presence of AP-SAA. Equivalent concentrations of apolipoprotein A-I (apo A-I) and bovine serum albumin (BSA) failed to alter the activity of nCEH. The effect of AP-SAA on a purified form of nCEH suggests that AP-SAA may have a direct effect on the activity of this enzyme. The temporal correlation of circulating AP-SAA and plasma cholesterol and the significant stimulation of nCEH by AP-SAA (but not apoA-I or BSA) provides further evidence that AP-SAA plays a role in cholesterol metabolism during the course of acute inflammation.

Degradation of amyloid-related serum protein SAA by a component present in rabbit and human serum

After incubation of protein SAA-containing rabbit serum at 37 degrees C overnight, the strength of the precipitation reaction against antiserum to SAA was decreased. This was at east partly due to enzymatic degradation of protein SAA. The enzymatic activity was not strongly associated with the SAA-high-density lipoprotein (HDL) complex, since it sedimented in the preparative ultracentrifuge at a density at which the SAA-HDL complex floats. The degrading component was obtained in concentrated form from rabbit and human sera by adsorption to Sepharose 4B. Degradation of human SAA by the human serum component was inhibited with disopropyl fluorophosphate, an inhibitor of serine proteases.

Degradation of protein SAA to an AA-like fragment by enzymes of monocytic origin

On incubation in cultures of blood mononuclear leucocytes from normal individuals, protein SAA is gradually degraded and yields an intermediate fragment with the same molecular weight as that of protein AA. This protein fragment has the same antigenic properties as protein AA, as judged from double-immunodiffusion analysis. Cell fractionation studies attribute the degrading capacity to the monocytes. Evidence was also found that the proteolytically active substances was released to the medium by cells in culture. The proteolytic substance, which also degraded protein AA, could be inhibited by diisopropyfluorophosphate and phenylmethyl sulfonylfluoride. This suggested that the enzyme might be a serine protease.