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

TWO HDL3 high- and low-affinity binding sites are present on the human hepa toma cell line (HepG(2)). Recently, we have suggested that the high-affinit y binding sites might modulate the endocytosis of HDL through the low-affin ity binding sites [Guendouzi, K. (1998) Biochemistry 37, 14974-14980], high lighting the physiological importance of this family of HDL high-affinity b inding sites. The present data demonstrate the presence of HDL3 high-affini ty (K-d = 0.37 mu g/mL, B-max = 260 ng/mg of protein) and low-affinity (K-d = 86.2 mu g/mL, B-max = 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 (K-d = 0.2 mu g/mL and B-max = 72 ng/m g of protein). Competition experiments between I-125-labeled HDL3 and eithe r LDL, oxidized LDL, or anti-SR-BI IgG as competitors show that SR-BI is mo stly responsible (70% displacement) for the binding of HDL3 to the low-affi nity binding sites. By contrast, the same competition experiments using I-1 25-labeled free apoA-I clearly excluded SR-BI as the high-affinity binding receptor. We conclude that the binding of HDL onto hepatocyte plasma membra nes involves: (1) two low-affinity binding receptors, one being SR-BI; (2) one family of high-affinity binding sites unrelated to SR-BI.