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

Apolipoprotein A-I (apoA-I) shares with other exchangeable apolipoproteins a high level of structural plasticity. In the lipid-free state, the apolipoprotein amphipathic a-helices interact intra- and intermolecularly, providing structural stabilization by self-association.Wehave reported that lipid-free apoA-Ibecomes 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 selfassociation. The oxidized apoA-I monomers also exhibited increased cellular cholesterol release capacity and stronger associationwith macrophages,comparedto nonoxidizedapoA-I.Ofphysiologic relevance,preformed oxidizedapoAI amyloidfibrils inducedamyloid formation innonoxidized apoA-I.Thisprocesswasenhancedwhenself-association of nonoxidized apoA-Iwasdisrupted bythermal treatment.Solid stateNMRanalysis revealed that aggregates formed by seeded nonoxidized apoA-I were structurally similar to those formed by the oxidized protein, featuring a b-structure-rich amyloid fold alongside a-helices retained from the native state. In atherosclerotic lesions, the conditions thatpromote apoA-I amyloid formation are readily available:myeloperoxidase, active oxygen species, lowpH, and high concentration of lipid-free apoA-I. Our results suggest that even partialMet oxidation of apoA-I can nucleate amyloidogenesis, thus sequestering and inactivating otherwise antiatherogenic and HDL-forming apoA-I.