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

Expression of recombinant human serum amyloid A in mammalian cells and demonstration of the region necessary for high-density lipoprotein binding and amyloid fibril formation by site-directed mutagenesis

Site-directed mutagenesis of the acute-phase human serum amyloid A (SAA1 alpha) protein was used to evaluate the importance of the N-terminal amino acid residues, namely RSFFSFLGEAF The full-length cDNA clone of SAA1 alpha (pA1.mod.) was used to create two mutations, namely Gly-8 to Asp-8 and an 11 amino acid truncation between Arg-1 and Phe-11 respectively. Wild-type and mutant cDNAs were expressed in Chinese hamster ovary (CHO) cells under the control of the human cytomegalovirus promoter, which resulted in the secretion of the processed proteins into the culture media. Wild-type recombinant human SAA (rSAA) protein was shown to have pI values of 6.0 and 6.4, similar to the human SAA isoform SAA1 alpha and SAA1 alpha desArg found in acute-phase plasma. N-terminal sequencing of 56 residues confirmed its identity with human SAA1 alpha. The total yield of wild-type rSAA measured by ELISA was between 3.5 and 30 mg/l. The two mutations resulted in reduced expression levels of the mutant SAA proteins (3-10 mg/l). Further measurements of rSAA concentration in lipid fractions of culture medium collected at a density of 1.21 g/ml (high-density liporotein; HDL) and 1.063-1.18 g/ml (very-low-density lipoprotein/low-density lipoprotein; VLDL/LDL) showed that 76% of the wild-type protein was found in the HDL fraction and the remaining 24% in the infranatant non-lipid fraction. In contrast the relative concentration of mutant rSAA in HDL and infranatant fractions was reversed. This is consistent with the previously proposed involvement of the 11 amino acid peptide in anchoring. SAA protein on to HDL3 [Turnell, Sarra, Glover, Baum, Caspi, Baltz and Pepys (1986) Mol. Biol. Med. 3, 387-407]. Wild-type rSAA protein was shown to from amyloid fibrils in vitro under acidic conditions as shown by electron microscopy, and stained positive with Congo Red and exhibited apple-green birefringence when viewed under polarized light. Under the same conditions mutSAA(G8D) and mutSAA delta 1-11 did not form amyloid fibrils. In conclusion, replacement of Gly-8 by Asp-8 or deletion of the first 11 amino acid residues at the N-terminus of rSAA diminishes its capacity to bind to HDL and decreases amyloid fibril formation.

The molecular basis of reactive amyloidosis

Reactive amyloidosis is a disease occurring in patients suffering from chronic infections, inflammation, and certain malignant conditions that are characterized by a considerable elevation of the acute phase reactant serum amyloid A (SAA). It is defined by the presence of extracellular deposits of fibrillar material containing amyloid A (AA) as its main component. AA is an 8.5-kd protein structurally identical to the NH2-terminal of the acute phase reactant SAA. SAA consists of a group of evolutionally conserved amphipathic proteins, encoded by a large number of genes and produced abundantly during inflammation, all suggesting an important role, probably of a neutralizing (anti-inflammatory) nature. An analysis of various aspects of SAA provides no clues to the mechanism of amyloid production, its occurrence in only selected individuals, and its preferential relationship to one isotype of SAA. Until more data is available, the present view on AA amyloidogenesis remains hypothetical.

Direct evidence for SAA deposition in tissues during murine amyloidogenesis

To study the mechanism of amyloid deposition, the nature of amyloid proteins formed in experimental murine amyloidosis, was examined. Spleen specimens, 15-60 mg, were homogenized and extracted using aqueous acidic acetonitrile, in a recently developed procedure, making it possible to obtain amyloid proteins from minute amounts of tissue. The extracted material, 1.5-4 mg, was analysed by Western blotting and ELISA using antibodies recognizing differentially proteins AA and SAA. Two immunoreactive proteins of 8 and 12 KDa were isolated and subjected to amino acid analysis and N-terminal sequence determination. The results of immunochemical and chemical examination showed that the 8 and 12 KDa proteins represented proteins AA and SAA, respectively. The data obtained provide new direct evidence for SAA in tissues during murine amyloidogenesis.

Degradation of amyloid A and serum amyloid A by red blood cell haemolysate in patients with familial mediterranean fever

Enzymatic activity for the degradation of serum amyloid A (SAA) and amyloid A (AA) was detected in erythrolysates of normal subjects and patients with familial mediterranean fever. A significant difference between the activity of normal subjects and patients was not found. Serum inhibited the SAA (but not the AA) haemolysate proteolytic activity. Interindividual variation in the susceptibility of SAA to degradation by RBC haemolysates was shown. The original digestible fraction of SAA became gradually resistant to proteolytic cleavage over a 9 month period while the susceptibility of AA to degradation remained unchanged in this time period. These findings suggest that enzymatic degradation of SAA depends on the source of SAA, as well as inhibitory activity in serum.

Occurrence of protein SAA-like material in human endothelial cells in culture and smooth-muscle cells in vessel walls

In the present study human endothelial cells were isolated from the veins of umbilical cord by collagenase treatment and kept in primary culture. Antiserum with monospecific activity against protein SAA was used in an indirect immunofluorescence study. SAA-like material was shown to be present in the cytoplasm of endothelial cells that were grown on glass discs and formed a typical uniform monolayer. It was oriented in a cytoplasmic filament pattern. SAA-like material was also demonstrated in the cytoplasm of smooth-muscle cells in vessel walls in cryostat sections from placenta and umbilical cord.

Serum amyloid A protein and C-reactive protein levels in pulmonary tuberculosis: relationship to amyloidosis

C-reactive protein and serum amyloid A protein levels were measured in 54 patients with pulmonary tuberculosis. The primary tuberculous complex was associated with an insignificant acute phase response, while post-primary tuberculosis without evidence of lung destruction caused modest increases in C-reactive protein and serum amyloid A protein. In most patients with post-primary pulmonary tuberculosis with significant pulmonary destruction there was a major acute phase response, with very high serum amyloid A protein and C-reactive protein levels. The response in these patients is most likely to be due to secondary bacterial infection in addition to infection by Mycobacterium tuberculosis. Patients with miliary tuberculosis showed a major acute phase response. Serum amyloid A protein and C-reactive protein levels decreased rapidly after initiation of treatment in the patients with post-primary tuberculosis without significant pulmonary destruction.

Rapid clearance of serum amyloid A from high-density lipoproteins

The serum amyloid A proteins (SAA) occur in plasma in six polymorphic forms that are associated with the high-density lipoproteins (HDL). We studied two of the SAA proteins, SAA1 and SAA4, which have the same amino- and carboxy-terminal residues but different solution properties and electrophoretic mobilities, to determine whether they are interconverted in plasma in vivo. They were radioiodinated in vitro, incorporated into HDL, and administered to cynomolgus monkeys. Both remained associated with HDL for at least 6 h, had similar plasma die-away curves, and retained their characteristic electrophoretic mobilities, suggesting they are not related as precursor and product. The plasma clearance of the most prevalent SAA species, SAA4, was also simultaneously compared with the human A-I and C-III-2 apolipoproteins. Human apolipoprotein A-I decayed from plasma at a rate comparable to that of monkey HDL proteins. Apolipoprotein C-III-2 was cleared more rapidly and SAA4 at an even greater rate. These findings suggest that SAA are either dissociated from HDL before clearance from plasma or that SAA are contained in an HDL subspecies with metabolic fate different from that of most HDL particles.

Isolation and characterization of senile amyloid--related antigenic substance (SASSAM) from mouse serum. Apo SASSAM is a low molecular weight apoprotein of high density lipoprotein

Sera obtained from senescence-accelerated mouse (SAM) and normal mice contained a substance that reacted with antiserum raised against ASSAM, a novel senile amyloid fibril protein isolated from the liver of SAM. This physiological substance, termed "SASSAM" (serum ASSAM-related antigenic substance), migrated to the albumin/prealbumin region in immunoelectrophoresis and the precipitation line formed with anti-ASSAM antiserum was stained positively with both Amide Black 10 B and Oil Red O/Fat Red 7B solutions, thereby suggesting that SASSAM is an alpha lipoprotein. Using Sephadex G-200 gel chromatography, SASSAM was eluted as a high mol wt form of approximately 200,000 daltons. Fractionation of lipoprotein from normal mouse serum by preparative ultra-centrifugation disclosed that SASSAM was found mainly in high density lipoprotein, HDL (the density is between 1.063 and 1.21 g/cm3). The largest amount of SASSAM was found in the HDL2 fraction (the density is between 1.063 and 1.125) and in this fraction SAA was not detected. Furthermore, ASSAM immunoreactivity appeared in the low mol wt proteins (below 10,000 daltons) of apo HDL separated in the buffer containing 8 M urea through Sephadex G-200. In 8 M urea sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE), the major components of apolipoproteins in this position, possibly corresponding to apo C proteins, have the same molecular weight, 5,200 daltons, as ASSAM and this component was labeled by anti-ASSAM antiserum after transfer to nitrocellulose paper.

Do serine proteases degrade amyloid A fibrils?

Human serum contains enzyme(s) able to degrade serum amyloid A protein (SAA) and amyloid A (AA) fibrils. On the basis of inhibition tests these enzymes are regarded as serine proteases, but further characterization of the enzymes has, however, so far not been done. Chymotrypsin, trypsin, elastase, collagenase and kallikrein, when added to SAA-containing serum, all degraded SAA to peptides within 2 h at 37 degrees C. With the exception of collagenase these enzymes also destroyed the Sirius-Red-binding ability of amyloidotic tissue and that of isolated AA fibrils. Hence, they altered the conformation of the beta-pleated structure and possibly also degraded the fibrils. These results suggest that any of these serine proteases could be responsible of the degradation of SAA in serum. The enzyme concentrations needed to degrade amyloid fibrils, however, were much higher than normally found in serum. Thus, it is unlikely that the amyloid-fibril-degrading activity in serum could be due to any of these enzymes.

Characteristics of the amyloid A fibril-degrading activity of human serum

Radial diffusion in agarose gel containing amyloid A (AA) fibrils was used to study the serum enzyme capable of degrading AA fibrils in vitro. This degradative activity was unaffected by soya bean trypsin inhibitor, tosyl-lysine chloromethyl ketone, and gold thiomalate but was inhibited by bovine pancreatic trypsin inhibitor, phenylmethylsulphonylfluoride, diisopropyl fluorophosphate, alpha 1-antitrypsin, and alpha 2-macroglobulin, indicating that the enzyme involved is a serine protease. Agarose gel electrophoresis showed the enzyme to be an acidic protein with the same electrophoretic mobility as albumin. The molecular weight, measured by gel filtration, was approximately 50,000. The optimum pH of this enzyme was 7.3, and it was fairly heat-resistant. The results suggest that the AA-fibril-degrading activity in human serum is due neither to elastase nor to cathepsin G. It has many characteristics in common with the enzymes unlike elastase that are involved in the complete degradation of serum AA protein.

In vitro enhancement of AA-degrading activity in human plasma with the plasminogen activator streptokinase

Radiolabelled protein AA was coupled to cyanogen-bromide-activated Sepharose 6 MB and used as a substrate to determine the AA-degrading activity of enzymes in solution. The applicability of the substrate was tested with an elastase preparation known to have AA-degrading activity. The substrate was used to determine the AA-degrading activity in fractions of normal human serum in the presence and absence of the plasminogen activator streptokinase. The AA-degrading activity was increased in fractions in which streptokinase-induced plasminogen activation had occurred. The increase in activity could be inhibited with antibodies to plasminogen. AA-degrading activity could also be increased in whole human plasma by the addition of streptokinase.

Degradation of amyloid proteins by different serine proteases

A protein extract was obtained from normal human serum by adsorption to unsubstituted Sepharose 4B. This extract contained one or several enzymes with SAA and AA degrading capacity. The optimal pH for degradation of SAA was about 7.3. On fractionation of the enzyme extract on Sephadex G-160, the active component was eluted in the V0 peak. The V0 fraction, which on double immunodiffusion analysis was found to contain alpha 2-macroglobulin, was also active against synthetic substrates used to determine the activity of thrombin and plasma kallikrein. Gel filtration under dissociating conditions and molecular weight estimation further indicated the presence of those enzymes in the preparation. Several serine proteases which are known to be inhibited by alpha 2-macroglobulin possessed AA and SAA degrading activity. On degradation of SAA, an intermediate split product with molecular weight similar to AA was formed. Kallikrein, plasmin and elastase were also able to degrade intact amyloid fibrils suspended in phosphate-buffered saline.