Methionine oxidized apolipoprotein A‐I at the crossroads of HDL biogenesis and amyloid formation

Abstract

Apolipoprotein A-I (apoA-I) shares with other exchangeable apolipoproteins a high level of structural plasticity. In the lipid-free state, the apolipoprotein amphipathic α-helices interact intra- and intermolecularly, providing structural stabilization by self-association. We have reported that lipid-free apoA-I becomes amyloidogenic upon physiologically relevant (myeloperoxidase-mediated) Met oxidation. In this study, we established that Met oxidation promotes amyloidogenesis by reducing the stability of apoA-I monomers and irreversibly disrupting self-association. The oxidized apoA-I monomers also exhibited increased cellular cholesterol release capacity and stronger association with macrophages, compared to nonoxidized apoA-I. Of physiologic relevance, preformed oxidized apoA-I amyloid fibrils induced amyloid formation in nonoxidized apoA-I. This process was enhanced when self-association of nonoxidized apoA-I was disrupted by thermal treatment. Solid state NMR analysis revealed that aggregates formed by seeded nonoxidized apoA-I were structurally similar to those formed by the oxidized protein, featuring a β-structure-rich amyloid fold alongside α-helices retained from the native state. In atherosclerotic lesions, the conditions that promote apoA-I amyloid formation are readily available: myeloperoxidase, active oxygen species, low pH, and high concentration of lipid-free apoA-I. Our results suggest that even partial Met oxidation of apoA-I can nucleate amyloidogenesis, thus sequestering and inactivating otherwise antiatherogenic and HDL-forming apoA-I.-Witkowski, A., Chan, G. K. L., Boatz, J. C., Li, N. J., Inoue, A. P., Wong, J. C., van der Wel, P. C. A., Cavigiolio, G. Methionine oxidized apolipoprotein A-I at the crossroads of HDL biogenesis and amyloid formation.

Oxidation of methionine residues in human apolipoprotein A-I generates a potent pro-inflammatory molecule

“…35) could also facilitate the interaction of apoA-I with macrophages (35) and induce their pro-inflammatory response, as shown in the present study.…”

“…In this study, we investigated how oxidation of methionine to methionine sulfoxide (Met(O)) imparts pro-inflammatory properties to lipid-free apoA-I. For our experiments, we used a reproducible and well-characterized oxidized human plasma apoA-I (Met(O)-ApoA-I), in which all three methionine residues are oxidized to Met(O) by an excess of H 2 O 2 (34,35,44).…”

“…As a simple and reproducible model for apoA-I methionine oxidation, we used a 1000-fold molar excess of H 2 O 2 to completely oxidize all three apoA-I methionine residues (Met(O)-ApoA-I) ( Table 1). Although methionine oxidation destabilizes the tertiary structure and disrupts self-association (quaternary structure) of apoA-I (35), the protein remains completely soluble until a secondary destabilizing stimulus (e.g. lowering the pH or applying sheer force) initiates protein aggregation and amyloid fibril formation (34,35,44).…”

“…In atherosclerotic lesions, macrophage-secreted myeloperoxidase targets apoA-I locally (31)(32)(33). Previously, we demonstrated that myeloperoxidase at levels found in atherosclerotic lesions oxidizes apoA-I methionine residues, thus reducing the structural stability of the protein and affording it amyloidogenic properties (34,35). In vitro, and presumably in the arteries, lipid binding stabilizes oxidized apoA-I and prevents amyloid formation (34).…”

Methionine sulfoxide reductase A attenuates atherosclerosis via repairing dysfunctional HDL in scavenger receptor class B type I deficient mice

“…37,38 Met oxidation of apoAI directly impairs its function, including inhibition of two critical early events in RCT: sterol export by ABCA1 and activation of LCAT 39 ; and reduces the stability, resulting in amyloid formation. 40 Recently, clinical studies have reported a significant increase in Met oxidation of apoAI in patients with CAD and human heterozygote carriers of SR-BI mutation. 38,41 However, the absence of highly specific antibody has impeded the direct detection of MetSO sites in apoAI in this study.…”