Abstract 312: Amyloids of Oxidized Apolipoprotein A-I Induce Amyloid Fibril Formation in Intact Apolipoprotein A-I

Abstract

Background: Amyloid deposition in atherosclerotic plaques increases with aging. Although a correlation between arterial amyloid deposits and cardiovascular events is yet to be established, the high incidence of amyloids associated with aortic intima and with atherosclerotic lesions indicates that amyloid deposits may contribute to atherosclerosis progression. Remarkably, apolipoprotein A-I (apoA-I) is the main component of these amyloids. We previously demonstrated that oxidation of apoA-I methionines by myeloperoxidase, at a concentration similar to that produced by activated macrophages in atherosclerotic lesions, promotes apoA-I amyloid fibril formation. Furthermore, recent studies revealed a hundred-fold increase in the amount of lipid-free apoA-I in atherosclerotic arteries compared to normal arteries. Notably, this apoA-I is heavily oxidized. Thus in the atherosclerotic plaques, high concentration of lipid-free apoA-I and an oxidative milieu are favorable conditions for apoA-I amyloid formation. Hypothesis: We tested the hypothesis that amyloid fibrils constituted of oxidized apoA-I can transfer the dysfunctional phenotype to intact apoA-I. Methods: Pre-formed amyloid fibrils constituted of oxidized apoA-I were incubated with a 10-fold excess of intact apoA-I at 37 °C, pH 6.0, with continuous vortexing. Kinetics of amyloid fibril formation by the pool of intact apoA-I were derived by measuring Thioflavin-T (ThT) fluorescence over a 6-day period. Results: After a lag-phase of 24-48 h, fibril formation proceeded with typical sigmoidal kinetics and reached plateau levels after about 6 days. In control samples, in which intact apoA-I was incubated in the absence of pre-formed fibrils, no significant changes in ThT fluorescence were detected for the same time course. Conclusions: Oxidized apoA-I amyloid fibrils can catalyze the aggregation of a large excess of intact protein. This observation bears important pathophysiological implications. In vivo , a small amount of amyloid fibrils could be produced by oxidized apoA-I in specific microenvironments of the atheroma; when transferred to the surrounding tissues, these amyloid seeds could induce extended amyloid formation in the large available pool of lipid-free apoA-I.