ApoA-I cleaved by transthyretin has reduced ability to promote cholesterol efflux and increased amyloidogenicity

Márcia Almeida Liz,Cláudio M Gomes,Maria João Saraiva,Mónica Mendes Sousa

doi:10.1194/jlr.m700158-jlr200

Márcia Almeida Liz, Cláudio M Gomes + Show 2 more

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https://doi.org/10.1194/jlr.m700158-jlr200

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Abstract

A fraction of plasma transthyretin (TTR) circulates in HDL through binding to apolipoprotein A-I (apoA-I). Moreover, TTR is able to cleave the C terminus of lipid-free apoA-I. In this study, we addressed the relevance of apoA-I cleavage by TTR in lipoprotein metabolism and in the formation of apoA-I amyloid fibrils. We determined that TTR may also cleave lipidated apoA-I, with cleavage being more effective in the lipid-poor prebeta-HDL subpopulation. Upon TTR cleavage, discoidal HDL particles displayed a reduced capacity to promote cholesterol efflux from cholesterol-loaded THP-1 macrophages. In similar assays, TTR-containing HDL from mice expressing human TTR in a TTR knockout background had a decreased ability to perform reverse cholesterol transport compared with similar particles from TTR knockout mice, reinforcing the notion that cleavage by TTR reduces the ability of apoA-I to promote cholesterol efflux. As amyloid deposits composed of N-terminal apoA-I fragments are common in the atherosclerotic intima, we assessed the impact of TTR cleavage on apoA-I aggregation and fibrillar growth. We determined that TTR-cleaved apoA-I has a high propensity to form aggregated particles and that it formed fibrils faster than full-length apoA-I, as assessed by electron microscopy. Our results show that apoA-I cleavage by TTR may affect HDL biology and the development of atherosclerosis by reducing cholesterol efflux and increasing the apoA-I amyloidogenic potential.

Highlights

A fraction of plasma transthyretin (TTR) circulates in HDL through binding to apolipoprotein A-I
We previously determined that TTR has several connections to apolipoprotein A-I biology: i) under physiological conditions, a fraction of plasma TTR circulates in HDL through binding to apoA-I [3]; ii) similar to chymase [4], TTR is able to cleave the C terminus of apoA-I after phenylalanine 225 [5]; and iii) under pathological conditions, an amyloidogenic apoA-I mutation was identified in which fibrils presented both mutant apoA-I N-terminal fragments and wild-type TTR [6], suggesting that TTR might influence the generation of apoA-I fragments and apoA-I deposition
To further analyze the putative physiological consequences of this cleavage, we evaluated the ability of TTR to cleave apoA-I present in different HDL particles, namely in Reconstituted discoidal preb-migrating high density lipoproteins (rHDLs) and sHDL isolated from human plasma

Summary

Introduction

A fraction of plasma transthyretin (TTR) circulates in HDL through binding to apolipoprotein A-I (apoA-I). In addition to a structural function, apoA-I is responsible for the major functions of HDL: its central region has been implicated in the activation of LCAT [7], whereas its C-terminal domain, including the region cleaved by TTR, has been shown as especially important for promoting cholesterol efflux from cholesterolloaded macrophages [4]. These functions are pivotal for reverse cholesterol transport, in which peripheral cell cholesterol is returned to the liver for subsequent metabolism. This article is available online at http://www.jlr.org

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