Insights into the fate of the N‐terminal amyloidogenic polypeptide of ApoA‐I in cultured target cells

Apolipoprotein A‐I (ApoA‐I) is an extracellular lipid acceptor, whose role in cholesterol efflux and high‐density lipoprotein formation is mediated by ATP‐binding cassette transporter A1 (ABCA1). Nevertheless, some ApoA‐I variants are associated to systemic forms of amyloidosis, characterized by extracellular fibril deposition in peripheral organs. Heart amyloid fibrils were found to be mainly constituted by the 93‐residue N‐terminal fragment of ApoA‐I, named [1–93]ApoA‐I. In this paper, rat cardiomyoblasts were used as target cells to analyse binding, internalization and intracellular fate of the fibrillogenic polypeptide in comparison to full‐length ApoA‐I. We provide evidence that the polypeptide: ( i ) binds to specific sites on cell membrane (K d = 5.90 ± 0.70 × 10 −7 M), where it partially co‐localizes with ABCA1, as also described for ApoA‐I; ( ii ) is internalized mostly by chlatrin‐mediated endocytosis and lipid rafts, whereas ApoA‐I is internalized preferentially by chlatrin‐coated pits and macropinocytosis and ( iii ) is rapidly degraded by proteasome and lysosomes, whereas ApoA‐I partially co‐localizes with recycling endosomes.  Vice versa , amyloid fibrils, obtained by  in vitro  aggregation of [1–93]ApoA‐I, were found to be unable to enter the cells. We propose that internalization and intracellular degradation of [1–93]ApoA‐I may divert the polypeptide from amyloid fibril formation and contribute to the slow progression and late onset that characterize this pathology.