Characterization of Two High-Density Lipoprotein Binding Sites on Porcine Hepatocyte Plasma Membranes: Contribution of Scavenger Receptor Class B Type I (SR-BI) to the Low-Affinity Component

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Abstract
Two HDL3 high- and low-affinity binding sites are present on the human hepatoma cell line (HepG2). Recently, we have suggested that the high-affinity binding sites might modulate the endocytosis of HDL through the low-affinity binding sites [Guendouzi, K. (1998) Biochemistry37, 14974−14980], highlighting the physiological importance of this family of HDL high-affinity binding sites. The present data demonstrate the presence of HDL3 high-affinity (Kd = 0.37 μg/mL, Bmax = 260 ng/mg of protein) and low-affinity (Kd = 86.2 μg/mL, Bmax = 14 300 ng/mg of protein) binding sites on purified porcine hepatocyte plasma membranes. By contrast, free apoA-I was strictly specific to the high-affinity sites (Kd = 0.2 μg/mL and Bmax = 72 ng/mg of protein). Competition experiments between 125I-labeled HDL3 and either LDL, oxidized LDL, or anti-SR-BI IgG as competitors show that SR-BI is mostly responsible (70% displacement) for the binding of HDL3 to the low-affinity binding sites. By contrast, the same competition experiments using 125I-labeled free apoA-I clearly excluded SR-BI as the high-affinity binding receptor. We conclude that the binding of HDL onto hepatocyte plasma membranes involves: (1) two low-affinity binding receptors, one being SR-BI; (2) one family of high-affinity binding sites unrelated to SR-BI.