Binding and endocytosis of murine high density lipoprotein from healthy (HDL) and inflamed donors (HDL_SAA) by murine macrophages in vitro. A light and electronmicroscopic investigation

Abstract

Serum amyloid A (SAA), an acute phase protein found primarily on high density lipoproteins (HDLsaa), is the precursor protein of AA-amyloidosis. Though extensivly studied, the pathway by which SAA bound to HDL becomes deposited as a fibril protein in AA-amyloidosis, is unknown. Macrophages (Mø) have been implicated in the pathogen-esis of AA amyloidosis and are known to bind, endocytose and retro-endocytose HDL. Studies were therefore performed to examine whether HDLsaa is handled in a manner similar to HDL

HDL and HDL_saa binding and intemalisation experiments were performed with cultured, cholesterol-laden murine MpH using either native or colloidal gold-labelled HDL and HDLsaa, and visualized either directly by light and electronmicroscopy, or by immunocytochemical means. Pre-treatment of MpH with heparinase examined the potential importance of membrane-bound glycosaminoglycans on the binding and uptake of HDL and HDL_saa

The binding of HDL and HDL_saa by MpH depended on culture conditions. Short-term cultures (2 vs 5 days), or cells treated with heparinase reduced the binding of both HDL and HDL_saa. Gold-labelled HDL and HDL_saa were found in compartments of the receptor-mediated pathway, such as coated pits, coated vesicles, endosomes, outer leaflets of multivesicular bodies as well as in lipid droplets and compartments which appear to be part of the exocytotic pathway. Only after prolonged incubation periods were signifcant numbers of gold-particles found in lysosomes. No differences were observed between the route followed by HDL and HDL_saa

These studies are consistent with the view that inter-nalization of HDL_saa by cells of the reticuloendothelial system is similar to that of HDL, and that its binding may involve a surface heparan sulfate proteoglycan and a receptor

Key Words:

• amyloid
• high density lipoprotein
• macrophage
• endocytosis