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 (Kd= 5.90 +/- 0.70 x 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.