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

Adrenoceptor-stimulated inflammatory response in stress-induced serum amyloid A synthesis

RATIONALE

Stressful life events are suggested to contribute to the development of various pathologies, such as cardiovascular disorders, whose etiopathogenesis is highly associated with elevated levels of serum amyloid A (SAA) proteins. SAA synthesis in the liver is regulated by a complex network of cytokines acting independently or in concert with various hormones/stimulants including the stress-activated sympathetic nervous system.

OBJECTIVE

This study aims to investigate the underlying mechanisms that regulate the stress-induced hepatic synthesis of SAA, with particular focus on adrenoceptors (AR), major components of the sympathoadrenal response to stress.

METHODS AND RESULTS

We demonstrated that repeated stress elevates IL-1β, IL-6, and TNFα serum levels in mice, accompanied by increased synthesis and secretion of hepatic SAA1/2 and SAA3, an effect that was blocked by AR antagonists. Moreover, stimulation of α₁- and β_1/2-ARs mimics the stress effect on SAA1/2 regulation, whereas α₂-AR stimulation exhibits a relatively weak impact on SAA. In support of the essential cytokine contribution in the AR-agonist induced SAA production is the fact that the anti-inflammatory drug, sodium salicylate, prevented the AR-stimulated hepatic SAA1/2 synthesis by reducing IL-1β levels, whereas IL-1β inhibition with Anakinra mimics this sodium salicylate preventive effect, thus indicating a crucial role for IL-1β. Interestingly, the AR-driven SAA3 synthesis was elevated by sodium salicylate in a TNFα-dependent way, supporting diverse and complex regulatory roles of cytokines in SAA production. In contrast to α₁/α₂-AR, the β_1/2-AR-mediated SAA1/2 and SAA3 upregulation cannot be reversed by fenofibrate, a hypolipidemic drug with anti-inflammatory properties.

CONCLUSION

Taken together, these findings strongly support a critical role of the AR-stimulated inflammatory response in the hepatic SAA production under stressful conditions, highlighting distinct AR type-specific mechanisms that regulate the hepatic synthesis of SAA1/2 and SAA3.

Chicken amyloid arthropathy: serum amyloid A, interleukin-1beta, interleukin-6, tumour necrosis factor-alpha and nitric oxide profile in acute phase (12th hour)

Acute phase response (APR) is part of the early defense system, which is triggered by different stimuli including, infection, trauma, stres, inflammation and neoplasia. The APR complex is a reaction which induces homeostasis and recovery. In this research, serum amyloid A (SAA), interlaukin (IL)-1beta, IL-6, tumour necrosis factor alpha (TNF-alpha) and nitric oxide (NO) levels were measured 12 hours following injection. For this purpose, Thirty-two 5 weeks old laying chicken were allocated into four groups and intra-articular injections of Freund's adjuvant were used to induce amylod arthropathy in Groups II, III and IV. Vitamin A in group II, and methylprednisolone in group IV were added to enhance and to reduce the severity of amyloidosis, respectively. At the end of the research, it was observed that TNF-alpha and NO increased significantly (P < 0.05) in vitamin A and methylprednisolone groups whereas SAA decreased significantly (P < 0.05) in all groups. It was also observed that IL-6 increased (P < 0.05) in vitamin A group and decreased in all other gorups however, IL-1beta decreased in vitamin A and methylprednisolone groups, while it was increased in the control group. The results of this study suggest that there is a positive correlation between serum TNF-alpha levels in acute and chronic phase in chickens with amyloid arthropathy.

Investigation of serum amino acid and serum amyloid A concentrations in chickens with amyloid arthropathy

BACKGROUND

Increased proteolytic cleavage of serum amyloid A (SAA) may potentially contribute to the development of AA amyloid deposition.

OBJECTIVE

To study the possible relationship between amyloid artropathy and expression of SAA and some serum amino acids.

ANIMALS AND METHODS

Values of 15 serum amino acids and SAA were investigated in chickens with experimentally induced amyloid arthropathy. Thirty-four, 5-week-old chicks were allocated into two groups: one group was injected intra-articularly with 0.25 mL complete Freund's adjuvant at the left tibio-metatarsal joint to induce amyloid arthropathy, whereas the other group served as control. All pullets were necropsied 13 weeks after injection. Collected tissue samples were examined histopathologically. Blood samples were collected and SAA concentrations were measured with enzyme-linked immunosorbent assay. High-performance liquid chromatography was used to assess the amino acid concentrations in serum.

RESULTS

Amyloid accumulation in joints occurred only in the experimental group (89%). SAA concentrations of 166 ± 17 and 423 ± 39 (SD) ng/mL were found in the control and experimental groups, respectively (p < 0.001). In the experimental group, an increase was observed in all examined amino acid concentrations except for citrulline. The most significant (p < 0.001) increases were noticed in serine (from 159 ± 15 to 360 ± 29 µmol/L), glycine (from 151 ± 20 to 279 ± 16 µmol/L), isoleucine (from 48 ± 2 to 80 ± 6 µmol/L), and phenylalanine (from 49 ± 2 to 90 ± 3 µmol/L).

CONCLUSION

The results of this study suggest that there is a positive correlation between some serum amino acid values, especially serine, glycine, isoleucine, and phenylalanine, and the high concentrations of SAA in chickens with amyloid arthropathy.

AA-Amyloid is cleared by endogenous immunological mechanisms

OBJECTIVE

AA amyloidosis is a complication to longstanding inflammatory diseases, but reduction of amyloid mass has been reported as the inflammation ceases. Not much is known about the endogenous factors that contribute to this amyloid resolution. Herein, we describe the dynamics of amyloid degradation and resolution in experimental murine AA-amyloidosis.

METHODS

AA-amyloidosis was induced in mice with injections of amyloid enhancing factor (AEF) and by inflammation induced with injections of silver nitrate. Resolution of amyloid deposits was monitored over time.

RESULTS

Virtually all amyloid was cleared within 34 weeks. Using the ELISA-technique, antibodies directed against protein AA were detected in animals during amyloid clearance phase and macrophages were shown to internalize amyloid. Also, passive immunization with an amyloid specific monoclonal antibody, produced by a B-cell clone recovered from an animal with advanced AA-amyloidosis, reduced amyloid development in murine AA-amyloidosis.

CONCLUSION

Immunoglobulins co-localize with amyloid deposits and can contribute to amyloid degradation by Fc-receptor mediated phagocytosis, and should be considered key players in the degradation process.

Plasma total prekallikrein/kallikrein activity in rheumatoid arthritis with and without amyloidosis. Increased kaolin-stimulated activity in patients with amyloidosis

Following exposure to kaolin, plasma samples were assayed for total prekallikrein/kallikrein activity in 19 patients with rheumatoid arthritis (RA), 39 patients with RA complicated by amyloidosis, 13 patients with nonamyloid nephropathy and 54 healthy subjects. Increased total kallikrein activity was found in RA patients with amyloidosis and in patients with nonamyloid nephropathy. The concentrations of the plasma kallikrein inhibitors C1-inactivator and alpha 2-macroglobulin were normal in RA patients without amyloidosis, whereas they were increased in patients with amyloidosis as well as in patients with nonamyloid nephropathy. The results suggest that the increased activity of plasma kaolin-stimulated kallikrein in RA patients with amyloidosis is due to the nephropathy per se and probably reflects increased levels of prekallikrein.

The role of chicken IL-1beta, IL-6 and TNF-alpha in the occurrence of amyloid arthropathy

In this study, the roles of IL-1beta, IL-6 and TNF-alpha in amyloid arthropathic chickens with variable amounts (severe, moderate and mild) of amyloid accumulation were investigated. The presence and the levels of cytokines were evaluated in serum and in joint tissues by using ELISA and immunohistochemistry, respectively. One hundred brown layer chicks were allocated into four groups and intra-articular injections of Freund's adjuvant were used to induce amyloid arthropathy in Groups II, III and IV. Vitamin A in group II, and methylprednisolone in Group IV were added to enhance and to reduce the severity of amyloidosis, respectively. At the end of the study, a positive correlation was observed among the incidence and severity of amyloidosis, the serum amyloid A levels and the IL-1beta values both in the serum and tissues. Elevation in the tissue TNF-alpha levels in parallel with the severity of amyloidosis has also been noted. As a conclusion, IL-1beta appears to play an important role in avian AA amyloidosis either alone or in combination with TNF-alpha. Further investigation is needed for understanding the role of the pro-inflammatory cytokines in avian AA amyloidosis.

Molecular interactions of acute phase serum amyloid A: possible involvement in carcinogenesis

Acute phase serum amyloid A (A-SAA) is a well-known marker of inflammation. The present review summarizes data on the regulation of A-SAA expression, signaling pathways which it is involved in, its effects, and possible influences on progression of malignant tumors.

Cellular events associated with the initial phase of AA amyloidogenesis: insights from a human monocyte model

Reactive amyloidosis is a systemic protein deposition disease that develops in association with chronic inflammation. The deposits are composed of extracellular, fibrillar masses of amyloid A (AA) protein, an N-terminal fragment of the acute-phase serum protein serum amyloid A (SAA). The pathogenic conversion of SAA into amyloid has been studied in two human cell culture models, peritoneal cells and peripheral blood monocytes. Human monocyte cultures proved more robust than either mouse or human peritoneal cells at initiating amyloid formation in the absence of a preformed nidus such as amyloid-enhancing factor and particularly well suited for examination of individual cells undergoing amyloid formation. Amyloid-producing monocyte cultures were stained with Congo red and Alcian blue for detection of amyloid and glycosaminglycans, respectively; immunocytochemistry was performed to identify SAA/AA, CD68, CD14, lysosomal protein Lamp-1, and early endosomal protein EEA1. SAA interaction with monocytes was also visualized directly via fluorescence confocal microscopy. Amyloid was initially detected only in intracellular vesicles, but with time was seen extracellularly. Morphologic changes in lysosomes were noted during the early phase of amyloid formation, suggesting that exocytosis of fibrils may occur via lysosome-derived vesicles. Cultures engaged in amyloid formation remained metabolically active; no cytotoxic effects were observed. Mimicking in vivo phenomena, amyloid formation was accompanied by increased glycosaminoglycan content and C-terminal processing of SAA. The ability of human monocytes to endocytose and intracellularly transform SAA into amyloid via a mechanism that requires and maintains, rather than compromises, metabolic activity distinguishes them as a useful model for probing earliest events in the disease process.

The effects of vitamin A, pentoxyfylline and methylprednisolone on experimentally induced amyloid arthropathy in brown layer chicks

The effects of vitamin A, pentoxyfylline and methylprednisolone on experimentally induced amyloid arthropathy were investigated. In this study, 175 1-day-old brown layer chicks were used. Throughout the study Group II (vitamin A) received high doses of vitamin A (75,000 IU/kg), whereas Group I (negative control), Group III (positive control), Group IV (pentoxyfylline) and Group V (methylprednisolone) received normal levels of vitamin A in the diet. At the fifth week, the experimental Groups II, III, IV and V were injected with Freund's adjuvant intra-articularly to induce amyloid arthropathy. Group IV received pentoxyfylline and Group V received methylprednisolone (10 mg/kg, intramuscularly) once. Joint and blood samples were examined 13 weeks after the injections. The values in Groups I, II, III, IV and V, respectively, were as follows: amyloid arthropathy formation (%), 0, 100, 87, 76, 66; serum amyloid A (ng/ml), 166+/-17, 607+/-40, 423+/-39, 342+/-27, 293+/-22; serum retinol (microg/dl): 59.75+/-3.8, 42.72+/-3, 59.24+/-3.6, 102+/-9.1, 101.3+/-12.3; heterophil/lymphocyte ratio: 0.504, 0.75, 0.75, 0.087, 0.44. In conclusion, it was observed that vitamin A enhanced the development of amyloid arthropathy and there were positive associations between amyloidosis, increased levels of serum amyloid A and increased numbers of tissue infiltrating macrophages. Methylprednisolone had a more successful inhibitory effect on amyloid arthropathy than pentoxyfylline.

Humoral Proinflammatory Cytokine and SAA Generation Profiles and Spatio-Temporal Relationship Between SAA and Lysosomal Cathepsin B and D in Murine Splenic Monocytoid Cells During AA Amyloidosis

Evidence shows that tissue macrophages (MPhis), in mice undergoing AA amyloidosis, endocytose acute-phase humoral serum amyloid A (SAA) and traffic it to lysosomes where it is degraded. Incomplete degradation of SAA leads to intracellular nascent AA fibril formation. In vitro, cathepsin (Cat) B is known to generate amyloidogenic SAA derivatives, whereas Cat D generates non-amyloidogenic SAA derivatives, and interferon (IFN-gamma)-treated MPhis show selective increase in Cat B concentration, a factor conducive to AA amyloidogenesis. To understand the cumulative effect of these factors in AA amyloidosis, humoral levels of SAA, IFN-gamma, tumour necrosis factor (TNF-alpha) and granulocyte-macrophage colony-stimulating factor were determined in azocasein (AZC)-treated CD-1 mice. We correlated these responses with the spatio-temporal distribution of SAA, Cat B- and Cat D-immunoreactive splenic reticuloendothelial (RE) cells. AZC-treated CD-1 mice similar to that of A/J mice showed partial amyloid resistance; their peak humoral IFN-gamma and SAA responses overlapped during the pre-amyloid phase. Unexpectedly, Cat D immunoreactivity (IR), instead of Cat B IR, was predominant in the splenic RE cells, indicating an apparent lack of causal relationship between IFN-gamma-mediated increase in Cat B expression. Partial amyloid resistance in CD-1 mice, probably a genetic trait, may be linked to high levels of Cat D expression, causing a delay in nascent AA fibril formation.

Leukocyte responses in two breeds of layer chicken that differ in susceptibility to induced amyloid arthropathy

Amyloid arthropathy in chicken can be induced by intravenous inoculation of an arthropathic and amyloidogenic Enterococcus faecalis in susceptible breeds. The commercial brown layer hybrids (BL) are more susceptible to the disease compared to their white counterparts (WL). The precursor of amyloid-A protein, which is serum amyloid-A (SAA), is identical in WL and BL. To investigate the factors involved in the breed-restricted susceptibility to amyloid arthropathy, we studied the type of leukocyte response and inflammatory reactions in E. faecalis-induced disease. In the BL, a significant dose dependent peripheral leukocytosis mainly by heterophils, and plasma cell infiltration in arthritic joints was found. In contrast, secondary lymphoid nodular aggregates in the synovial membrane were prominent in the WL. The aggregates consisted mainly of CD8+ T cells. The high number of circulating leukocyte and prolific plasma cell responses in the BL predict extensive humoral and acute phase reactions. This is in agreement with literature data on suppressed T-cell function in casein-induced amyloid-susceptible mice strains. The difference in leukocyte response and type of inflammation between WL and BL, when arthropathic and amyloidogenic bacteria induce infection, in conjunction with susceptibility to amyloid arthropathy, is discussed in view of the murine T-helper responses.

Catabolism of lipid-free recombinant apolipoprotein serum amyloid A by mouse macrophages in vitro results in removal of the amyloid fibril-forming amino terminus

Serum amyloid A fibrils are formed when the normally rapid catabolism of the acute-phase reactant apolipoprotein serum amyloid A (apoSAA) is incomplete; thus amyloidosis may be viewed as a condition of dysregulated proteolysis. There is evidence that apoSAA is dissociated from plasma high-density lipoprotein (HDL) prior to fibril formation. The objective of this study was to investigate degradation of lipid-free apoSAA by tissue macrophages derived from amyloid-susceptible CBA/J mice in vitro. Peritoneal macrophages derived from untreated (normal) mice converted apoSAA (12 kDa) to a single 4 kDa C-terminal peptide while splenic macrophages converted apoSAA to 10, 7 and 4 kDa C-terminal peptides and a 4 kDa peptide that lacked the C- and N-terminal regions. Similar patterns of proteolysis occurred when peritoneal and splenic macrophages from amyloidotic CBA/J mice were used. Conditioned medium prepared from peritoneal, but not splenic macrophages, degraded apoSAA. Specific sites of cleavage indicated activity of cathepsin G- and elastase-like neutral proteases. The data indicate that lipid-free apoSAA can be degraded by secreted or cell-associated neutral proteases that are generated by macrophages to yield peptides that lack fibrillogenic potential.

Characterization of human serum amyloid A protein isoforms separated by two-dimensional electrophoresis by liquid chromatography/electrospray ionization tandem mass spectrometry

A detailed structural analysis of the serum amyloid A proteins (SAA) of an individual with highly active, chronic rheumatoid arthritis is reported. SAA isoforms were separated by high-resolution two dimensional (2-D) gel electrophoresis. Peptide mapping by reverse-phase chromatography/electrospray ionization tandem mass spectrometry was applied to correlate the protein(s) contained in each spot with their respective coding gene and to study the post-translational processing and modification events which might result in differential electrophoretic mobility. Nine protein spots were analyzed. The six major spots corresponded to the Arg and des-Arg forms of SAA1 alpha and SAA2 alpha, respectively, and to the glycosylated and nonglycosylated form of constitutive serum amyloid A protein (C-SAA). Two minor spots were identified as SAA1 alpha isoforms containing post-translational modifications. We suggest that these variants contained a gamma-N, N'-dimethylasparagine residue at position 83 and that one of them was additionally oxidized at Trp53 and Trp85. The ninth spot was shown to contain a mixture of SAA1 alpha and SAA2 alpha. To our knowledge, this is the first report in which analysis of peptides has been used to verify the presence of C-SAA in acute-phase serum. Furthermore, the data illustrate that extensive post-translational processing results in a structurally diverse class of acute-phase SAA proteins, which are derived from a small number of genes. Finally, the fast and conclusive technology used in this study promises to be generally useful for the comprehensive investigation of proteins at the level of the primary structure.

Serum amyloid A: an acute phase apolipoprotein and precursor of AA amyloid

Serum amyloid A is an acute phase protein complexed to HDL as an apoprotein. The molecular weight is 11.4-12.5 kDa in different species and the protein has from 104 to 112 amino acids, without or with an insertion of eight amino acids at position 72. The protein is very well conserved throughout evolution, indicating an important biological function. The N-terminal part of the molecule is hydrophobic and probably responsible for the lipid binding properties. The most conserved part is from position 38 to 52 and this part is therefore believed to be responsible for the until now unknown biological function. The protein is coded on chromosome 11p in man, and chromosome 7 in mice, and found in all mammals until now investigated, and also in the Peking duck. In the rat a truncated SAA mRNA has been demonstrated, but no equivalent serum protein has been reported. Acute phase SAA is first of all produced in hepatocytes after induction by cytokines, but extrahepatic expression of both acute phase and constitutive SAA proteins have been demonstrated. Several cytokines, first of all IL-1, IL-6 and TNF are involved in the induction of SAA synthesis, but the mutual importance of these cytokines seems to be cell-type specific and to vary in various experimental settings. The role of corticosteroids in SAA induction is somewhat confusing. In most in vitro studies corticosteroids show an enhancing or synergistic effect with cytokines on SAA production in cultured cell. However, in clinical studies and in vivo studies in animals an inhibitory effect of corticosteroids is evident, probably due to the all over anti-inflammatory effect of the drug. Until now no drug has been found that selectively inhibits SAA production by hepatocytes. Effective anti-inflammatory or antibacterial treatment is the only tool for reducing SAA concentration in serum and reducing the risk of developing secondary amyloidosis. The function of SAA is still unclear. Interesting theories, based on current knowledge of the lipid binding properties of the protein and the relation to macrophages, in the transportation of cholesterol from damaged tissues has been advanced. A putative role in cholesterol metabolism is supported by the findings of SAA as an inhibitor of LCAT. The potential that SAA is a modifying protein in inflammation influencing the function of neutrophils and platelets is interesting and more directly related to the inflammatory process itself.(ABSTRACT TRUNCATED AT 400 WORDS)

C5 deficiency in A/J mice is not associated with resistance to the development of secondary amyloidosis

The aim of the study was to determine whether C5 deficiency in the mouse is associated with resistance to the development of secondary amyloidosis. Chronic inflammation was induced in the F2 progeny, derived from matings between amyloid-susceptible and amyloid-resistance mice, by daily injections of azocasein for thirty days. Using a restriction fragment length polymorphism generated by digestion of genomic DNA with the restriction enzyme HindIII, C5 sufficient and deficient DNA can be clearly differentiated. Eight mice were found to be C5 sufficient, 32 were heterozygotes and 14 were found to be C5 deficient. Grading of the splenic amyloid load from negative to 4+ was performed after staining tissue squashes with Congo red and viewing them under a polarizing microscope. Seventeen mice were noted to have negative to trace, 18 had moderate (1+ - 2+) and 19 had heavy (3+ - 4+) amyloid deposition. There was no correlation between splenic amyloid load and C5 deficiency. Based on these results it is clear that C5 deficiency and resistance to secondary amyloidosis are not associated.

Amino acid sequence variations in protein AA of cats with high and low incidences of AA amyloidosis

1. Amyloid isolated from the liver of a domestic short-haired (DSH) cat was dissolved and purified by gel filtration for amino acid sequence analysis. 2. Sequences of two major peptides corresponding to positions 18-23 and 25-75 of human amyloid protein AA were obtained when cyanogen bromide-cleaved protein was applied to an amino acid sequenator. 3. Comparison of these regions of amyloid protein from the Abyssinian cat (high incidence of AA amyloidosis) and DSH cat (low incidence of AA amyloidosis) revealed three amino acid differences, two of which occurred within regions that are completely conserved in the Abyssinian cat and all other species. 4. Secondary prediction plots showed less potential for amyloidogenicity (i.e., less beta-sheet conformation) in protein AA of the DSH cat as compared to the Abyssinian cat and other animal species. 5. These differences in protein AA of the DSH cat may, therefore, be linked to the comparatively uncommon occurrence of AA amyloidosis in the DSH cat as compared to the Abyssinian cat and other animals species.

Amyloidosis

Recent studies have provided new insight into the pathogenesis of amyloidosis and have broadened our knowledge of the mechanisms of deposition and resolution of amyloid. In particular, the structure, synthesis and plasma clearance of the inflammation-associated amyloid precursor, SAA, have been extensively studied and the precursor-product relationship between circulating SAA, protein AA and fibrillar amyloid A clarified. Information has been accumulating about the enzymatic processes involved in the cleavage of SAA and the degradation of protein AA and a new view has been presented on the possible role of amyloid P component in AA amyloidogenesis. The current model of AA pathogenesis emphasizes the dynamic character of amyloid.

Amyloid-enhancing factor (AEF) in the pathogenesis of AA-amyloidosis in the hamster

AA-amyloidosis was induced in hamsters receiving amyloid-enhancing factor (AEF) by daily subcutaneous injection with either an aged casein solution or casein supplemented with lipopolysaccharide (LPS). Both amyloid inducers gave similar results with respect to amyloid development in spleen, liver and kidneys and to serum amyloid A (SAA) concentrations and plasma cathepsin D activities. AEF was isolated from amyloid-containing tissue by the method described by Hol et al. (1985), and amyloid-enhancing material was also extracted from isolated hamster amyloid fibrils by intensive sonification. This fibril-derived amyloid-enhancing material lacked typical green birefringence after staining with Congo red and appeared as amorphous material on electron microscopy. AEF shortened the pre-amyloid phase for splenic and hepatic amyloid development and also the subsequent interval before renal amyloid deposition. This indicates that endogenous AEF, unlike passively transferred preformed AEF, is not distributed throughout the body and is probably generated at the site of amyloid deposition. Moreover, these results suggest that amyloid deposition in the kidneys, like that in the spleen and liver, involves an AEF-dependent pathway. Thus redistribution of amyloid is probably not an important cause of renal amyloid involvement. In addition to the reduction in the lag phase for splenic and hepatic amyloid deposition, AEF also speeds the changes in SAA concentration and plasma cathepsin D activity. This indicates that AEF accelerates rather than eliminates the pre-amyloid phase.

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.

Transformation of amyloid precursor SAA to protein AA and incorporation in amyloid fibrils in vivo

Experimental amyloidosis was induced in mice by intraperitoneal injections of endotoxin (lipopolysaccharide (LPS)). In addition to LPS, a group of mice received high-density lipoprotein (HDL)-SAA complexes isolated from human acute-phase serum, whereas a group of control mice received saline in addition to LPS. Isolated amyloid fibrils from the mice given HDL-SAA contained human AA protein, as shown by immunodiffusion, immunoblot, and enzyme-linked immunosorbent assay techniques, in addition to mouse AA. In contrast, amyloid from the control mice contained exclusively AA of mouse origin. Thus, the experiments provided solid evidence that SAA is the precursor for amyloid fibril protein AA.

Human amyloid P component: an elastase inhibitor

Amyloid deposits almost invariably contain a pentagonal-shaped protein (a so-called pentraxin), termed amyloid P component (AP), in close apposition to the amyloid fibrils. AP is also detected alongside normal elastin fibres in skin and basement membrane. In the present studies, purified human AP was shown to inhibit the activity of porcine pancreatic elastase. The inhibition of elastolytic activity was not abolished by heating AP to 70 degrees C. Furthermore, two other human serum proteins used as controls did not inhibit elastase activity: albumin, which has a similarly acidic pI, and C-reactive protein, which is a pentraxin, sharing 50% sequence homology with AP. Enzyme kinetic studies showed that elastase treated with AP had a slower elastolytic rate than untreated elastase. The inhibitory effect of AP was reversed by high substrate (fivefold) concentration. These observations suggest that AP may function in vivo to protect elastin and amyloid fibrils from proteolytic cleavage. Indeed, this may in part account for the relative resistance of amyloid deposits to resorption and proteolysis.

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.

Reactive (secondary) amyloidosis and its pathogenesis

Recent advances in amyloid research have broadened our understanding of amyloidogenesis in connection with chronic inflammatory and infectious conditions. Experimental and clinical studies have clarified many of the mechanisms of induction, synthesis and regulation of the amyloid-related serum component SAA and have shed light on the enzymatic processes involved in the cleavage of SAA and degradation of fibrillar AA protein. The current pathogenetic model emphasizes the dynamic character of amyloid disease.

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.

Heterogeneity of Human Amyloid Protein AA and Its Related Serum Protein, SAA

The heterogeneity of the human amyloid proteins SAA and AA was studied. Both proteins could be separated into several fractions by ion-exchange chromatography. Amino acid analysis of the ion-exchange-chromatographed fractions of protein AA showed that the main difference was in the length of the polypeptide. Thus, it seems that the original AA preparation consists of a mixture of AA proteins with length ranging from 66 to 78 amino acid residues. By enzymatic degradation of three different forms of SAA with kallikrein, fragments were formed with a molecular weight very similar to that of protein AA.

Degradation of amyloid proteins with protease I from Aspergillus oryzae. In vivo increase in SAA clearance rate after enzyme infusion

The effect of the thrombolytic enzyme protease I from Aspergillus oryzae (brinase) on the amyloid protein AA was investigated. The effect of the enzyme on purified, low molecular weight protein AA was very high. Protein AA in intact amyloid fibrils suspended in neutral buffer was also degraded by the enzyme, although at a much lower rate. Amyloid fibrils in tissue sections could also be attacked and removed, leaving the tissue structure fairly intact. The in vivo effect of brinase on protein SAA was demonstrated in rabbits by an increase in the clearance rate of SAA after enzyme infusion.

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.

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

Human serum contains amyloid-A-degrading (AADP) activity. This activity is reduced in amyloidosis associated with rheumatoid arthritis. Since AADP co-migrates with albumin on agarose-gel electrophoresis, the relationship between these serum factors was studied in patients with amyloidosis and patients with amyloidosis and patients with altered albumin synthesis and/or distribution. AADP activity correlated positively with albumin levels in patients with rheumatoid arthritis complicated by amyloidosis. A weaker association was noted between serum activity and prealbumin levels. The AADP activity in patients with rheumatoid arthritis without signs of amyloidosis and patients with liver cirrhosis was also positively associated with serum albumin level. During the acute-phase reaction after surgery serum AADP activity fell in parallel with serum albumin level. Purified albumin preparations displayed AADP activity. The results show that serum albumin level reflects AADP activity. It is suggested that the development of hypoalbuminaemia in patients with amyloidosis may give rise to a vicious circle which leads to an accelerated reduction in AADP activity and accelerated amyloidogenesis.

The role of interleukin 1 in acute phase serum amyloid A (SAA) and serum amyloid P (SAP) biosynthesis

The acute phase SAA and SAP profiles have been compared for localized and endotoxin induced inflammation in LPS responder and nonresponder strains of mice. The SAP profile can reflect a delay with respect to the start of the increase. Its maximum is on the order of ten times the nonacute phase concentration and elevated concentrations are sustained 24 to 48 hours after SAA concentration is rapidly decreasing to normal. The role of Interleukin 1, known to have an essential role in SAA production, was investigated for SAP production. Purified mouse IL 1 and rabbit IL 1 produced a minimal elevation of SAP concentration above normal values, especially when compared with their effects on SAA concentration. BCG infection was shown to synergistically augment SAA induction by LPS and was shown to enhance IL 1 production by macrophages in response to LPS. Unlike SAA synthesis, BCG-preinfection fails to synergistically augment the LPS-induced SAP response. BCG infection alone produced highly elevated and sustained increases in SAP concentration, whereas, the effect on SAA concentration was minimal. Macrophages appear to play an important role in SAP acute phase elevation, but the mechanism in different from that of SAA elevation.

The biology of SAA: identification of the inducer, in vitro synthesis, and heterogeneity demonstrated with monoclonal antibodies

Continued studies of the macrophage-derived mediator of SAA synthesis (SAA Stimulating Factor) confirm our previous observations that SAASF copurified with leukocytic pyrogen (LP) and lymphocyte activating factor (LAF). Moreover, new data demonstrate three separate isoelectric points for human LP-LAF-SAASF each of which possess the three biological activities. During the purification of 15,000 MW LP from crude stimulated mononuclear cell supernatants, only those fractions with pyrogenic activity in rabbits caused augmented stimulation of lymphocytes (LAF) and induced SAA synthesis in mice. Purified human LP stimulated isolated mouse hepatocytes in vitro to synthesize SAA in a dose-responsive manner. Colchicine treatment of hepatocytes led to decreased secretion of SAA into the medium and to an intracellular accumulation of SAA. Messenger RNA was isolated from the livers of endotoxin-stimulated mice and translated in a wheat-germ cell-free system. A major product was identified at 13-14,000 MW. Immunoprecipitation with anti-mouse AA identified several bands on autoradiography of polyacrylamide gels. These larger SAA precursors may account for the previously noted heterogeneity of human SAA, comprising at least 6 SAA isomers, of similar molecular weight but different solubility and electrophoretic charge characteristics. Two monoclonal antibodies (IgM-K and IgG1-K) have been prepared using standard cell hybridization techniques. They are directed at the variable COOH terminal region of SAA since they detect differences between the 6 human SAAs but do not react with human, monkey, dog or mouse AA proteins, human AP, C-reactive protein, IgG nor albumin. These antibodies will be useful in examining the origin, structure and function of SAA.

The heterogeneity of protein AA in secondary (reactive)systemic amyloidosis

In secondary systemtic amyloidosis, amyloid fibrils have protein AA as a main subunit protein. As judged from gel chromatography and electrophoresis, this protein is rather homogeneous. In the present paper it is shown, however, that protein AA is very heterogeneous and composed of many peptides with different isoelectric points. However, their antigenic properties and amino acid compositions vary only little. It is concluded that protein AA is as heterogeneous as its postulated precursor, the acute phase reactant serum AA and that a theory that only one or a few serum protein AA's can give rise to amyloid fibrils, might be wrong.

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.

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.