BINDING AND ENDOCYTOSIS OF MURINE HIGH-DENSITY-LIPOPROTEIN FROM HEALTHY (HDL) AND INFLAMED DONORS (HDLSAA) BY MURINE MACROPHAGES IN-VITRO -A LIGHT AND ELECTRON-MICROSCOPIC INVESTIGATION

Serum amyloid A (SAA), an acute phase protein found primarily on high density lipoproteins (HDLSAA), is the precursor protein of AA-amyloido sis. Though extensively studied, the pathway by which SAA bound to HDL becomes deposited as a fibril protein in AA-amyloidosis, is unknown. Macrophages (M phi) have been implicated in the pathogenesis of AA amy loidosis and are known to bind, endocytose and retro-endocytose HDL. S tudies were therefore performed to examine whether HDLSAA is handled i n a manner similar to HDL. HDL and HDLSAA binding and internalisation experiments were performed with cultured, cholesterol-laden murine M p hi using either native or colloidal gold-labelled HDL and HDLSAA, and visualized either directly by light and electronmicroscopy, or by immu nocytochemical means. Pretreatment of M phi with heparinase examined t he potential importance of membrane-bound glycosaminoglycans on the bi nding and uptake of HDL and HDLSAA. The binding of HDL and HDLSAA by M phi depended on culture conditions. Short-term cultures (2 vs 5 days) , or cells treated with heparinase reduced the binding of both HDL and HDLSAA. Gold-labelled HDL and HDLSAA were found in compartments of th e receptor-mediated pathway, such as coated pits, coated vesicles, end osomes, outer leaflets of multivesicular bodies as well as in lipid dr oplets and compartments which appear to be part of the exocytotic path way. Only after prolonged incubation periods were significant numbers of gold-particles found in lysosomes. No differences were observed bet ween the route followed by HDL and HDLSAA. These studies are consisten t with the view that internalization of HDLSAA by cells of the reticul oendothelial system is similar to that of HDL, and that its binding ma y involve a surface heparan sulfate proteoglycan and a receptor.