Binding and uptake of chylomicron remnants by primary and THP-1 human monocyte-derived macrophages: determination of binding proteins

The binding and uptake of chylomicron remnants by human macrophages was stu died in order to resolve paradoxical observations that have described the p utative mechanisms by which postprandial lipoproteins induce foam cell form ation. Chylomicron remnants bound to human monocyte-derived macrophages (HM Ms) and to the transformed monocytic cell line THP-I with high affinity (K- d of approx. 5.5 mug of chylomicron remnant protein/ml). Binding was found to be saturable for both cell types, and was strongly inhibited in the pres ence of unlabelled chylomicron remnants. Ligand blot studies with colloidal -gold-label led chylomicron remnants identified two cell surface binding si tes on both HMMs and THP-I cells, with molecular masses of approx. 128 kDa and 43 kDa. The high-molecular-mass binding site was found to be the low-de nsity lipoprotein (LDL) receptor, based on the strong inhibition of chylomi cron remnant binding in the presence of unlabelled LDL, Fab(2) antibody fra gments to the LDL receptor or calcium chelators. Competition studies sugges ted that, in HMMs, the LDL receptor appeared to facilitate approximately ha lf of the total chylomicron remnant uptake. In contrast, the LDL receptor w as not significantly involved in macrophage uptake of chylomicron remnants by THP-I cells. The identity of the 43 kDa binding site is presently unknow n, but, importantly, expression was not inhibited as a consequence of stero l loading, which was induced by incubating HMMs and THP-1 cells with 25-hyd roxycholesterol. In contrast, the expression of the LDL receptor was substa ntially attenuated following lipid loading. Collectively, our data suggest that, while the macrophage LDL receptor can bind chylomicron remnants and f acilitate uptake in nonlipid-loaded HMMs, other sterol-insensitive sites ar e responsible for the unabated uptake of chylomicron remnants by macrophage s. We propose that the 43 kDa macrophage chylomicron remnant binding protei n may be a candidate for the sterol loading of macrophages.