Uptake and fate of class B scavenger receptor ligands in HepG2 cells

Class B scavenger receptors (SR-Bs) interact with native, acetylated and ox idized low-density lipoprotein (LDL, AcLDL and OxLDL), high-density lipopro tein (HDL3) and maleylated BSA (M-BSA). The aim of this study was to analyz e the catabolism of CD36- and LIMPII-analogous-1 (CLA-1), the human ortholo gue for the scavenger receptor class B type I (SR-BI), and CD36 ligands in HepG2 (human hepatoma) cells. Saturation binding experiments revealed moder ate-affinity binding sites for all the SR-B ligands tested with dissociatio n constants ranging from 20 to 30 mu g . mL(-1). Competition binding studie s at 4 degrees C showed that HDL and modified and native LDL share common b inding site(s), as OxLDL competed for the binding of I-125-LDL and I-125-HD L3 and vice versa, and that only M-BSA and LDL may have distinct binding si tes. Degradation/association ratios for SR-B ligands show that LDL is very efficiently degraded, while M-BSA and HDL3 are poorly degraded. The modifie d LDL degradation/association ratio is equivalent to 60% of the LDL degrada tion ratio, but is three times higher than that of HDL3. All lipoproteins w ere good cholesteryl eater (CE) donors to HepG2 cells, as a 3.6-4.7-fold CE -selective uptake ([H-3]CE association/I-125-protein association) was measu red. M-BSA efficiently competed for the CE-selective uptake of LDL-, OxLDL- , AcLDL- and HDL3-CE. All other lipoproteins tested were also good competit ors with some minor variations. Hydrolysis of [H-3]CE-lipoproteins in the p resence of chloroquine demonstrated that modified and native LDL-CE were ma inly hydrolyzed in lysosomes, whereas HDL3-CE was hydrolyzed in both lysoso mal and extralysosomal compartments. Inhibition of the selective uptake of CE from HDL and native modified LDL by SR-B ligands clearly suggests that C LA-1 and/or CD36 are involved at least partially in this process in HepG2 c ells.