| Abstract |
We have investigated the receptor-mediated binding and uptake of low-density lipoprotein (LDL) and its insoluble complex with dextran sulfate to determine the contribution of positively charged sites in LDL to receptor-mediated interactions. Rabbit plasma LDL derivatized with FITC retained its sedimentation ability with dextran sulfate, as well as intact LDL, and binding of fluorescent LDL and its complex to liver cells was assayed by flow cytometry. Flow cytometry revealed that the binding of complex LDL with dextran sulfate, as well as that of pure LDL, increased on rat liver parenchymal cells treated with estrogen, which enhanced the expression of LDL receptors, and decreased on Hep G2 and Chang Liver cells treated with a monoclonal antibody against LDL receptors. The binding of pure LDL and complex LDL to hepatocytes was depressed by pretreatment with unlabeled LDL in a similar manner, but not with asialoglycoprotein, a ligand of asialoglycoprotein receptors on liver cells. Furthermore, we carried out a stable primary culture of rat hepatocytes, and then pure LDL and complex LDL were applied to the cultured hepatocytes. Hepatic-differentiated functions such as albumin and bile acid secretion decreased on the uptake of pure LDL and complex LDL in a similar manner. Consequently, comparative studies using pure LDL and complex LDL allowed us to determine that the complex formation with dextran sulfate had no influence on the receptor-mediated binding or uptake of LDL, and that LDL possessed binding domains for LDL receptors and sulfonic carbohydrates, containing positively charged amino acids.
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