Identification of Distinct Receptors for Native and Acetylated-Low-Density Lipoprotein in Human Placental Microvilli

Abstract

Human placental progesterone biosynthesis is essentially dependent upon the maternal cholesterol (Hellig et al., 1970) delivered from plasma low-density lipoprotein (LDL), the major cholesterol-carrier protein in the human (Winkel et al., 1980a; Winkel et al., 1981; Simpson and MacDonald, 1981; Gwynne and Strauss, 1982). Cultured trophoblastic cells take up and degrade LDL by a receptor mediated endocytosis process (Winkel et al., 1980b), similar to that initially described by Goldstein and Brown (1974) for human fibroblasts in culture. Placental binding sites specific for LDL have been identified simultaneously, in a crude membranous fraction (Cummings et al., 1982) and on microvillous membranes, the effective site of exchange between the maternal blood and the syncytiotrophoblast (Alsat et al., 1982). Further investigations have allowed the characterization of the LDL binding sites, which are detected on microvillous membranes as early as the 6th wk of pregnancy (Alsat et al., 1984). Their presence on term placental microvilli has been confirmed by an ultrastructural study of villi incubated with dense electron conjugates: 80% of ferritin-LDL or colloidal gold-LDL are bound to sites sparsely distributed over the membrane surface of the microvilli (Malassiné et al., 1984).