Abstract 16: Amyloidogenic Modifications of Apolipoprotein A-I Promote a Strong Pro-inflammatory Response in Macrophages, but Formation of Amyloid Fibrils Abrogates the Pro-inflammatory Effect

Abstract

Inflammation is central to atherosclerosis, as inactivation of inflammation pathways strongly reduces atherosclerosis progression. Particulate matter, such as cholesterol crystals and amyloid materials, promotes release of the potent pro-inflammatory cytokine IL-1β in macrophages. Recently, it has been demonstrated that soluble substances that are precursors of particulate matter, such as free cholesterol, oxidized LDL and amyloidogenic peptides (i.e. amyloid-β, IAPP), can also induce an inflammatory response. In this study, we investigated the interplay between oxidation of apolipoprotein A-I (apoA-I), amyloid formation, and the inflammatory response of macrophages. We previously reported that oxidation of apoA-I methionines (MetO-ApoA-I) promotes protein aggregation and formation of amyloid fibrils when MetO-ApoA-I is incubated at pH 6.0. In contrast, at physiological pH, MetO-ApoA-I remains soluble and perfectly functional, as it promotes cholesterol efflux from macrophages with the same efficiency of intact-ApoA-I. However, upon incubation of mouse bone marrow derived macrophages with soluble pre-fibrillar MetO-ApoA-I, levels of pro-IL-1β synthesis were more than 2-fold higher than those induced by intact-ApoA-I. In contrast, amyloid fibrils produced by MetO-ApoA-I did not increase the levels of pro-IL-1β synthesis compared to intact-ApoA-I. Furthermore, the pro-inflammatory effect was not observed in macrophages derived from MyD88/TRIF knock-out mice, indicating that the response is membrane TLR receptors-dependent. In contrast, the >2-fold increase (MetO-ApoA-I vs. intact-apoA-I) in pro-IL-1β synthesis was maintained in macrophages derived from CD36 knock-out mice. This observation suggests that the signaling pathway is not exclusively dependent on the TLR2/TLR6/CD36 membrane complex. Thus, in atherosclerotic lesions, oxidized apoA-I species that are amyloidogenic, but otherwise functional, may induce a pro-inflammatory response in macrophages. Amyloid fibril formation in contrast, could reduce, rather than exacerbate, the inflammatory burden produced by these pro-inflammatory apoA-I species by sequestering them in the form of inert amyloids.