Internalization of Fractionated3H-Heparin by the Scavenger-Like Receptor in Rat Liver Parenchymal Cells in Primary Culture

The new general biological property of stem-like tumor cells Part I. Peculiarities of the process of the double-stranded DNA fragments internalization into stem-like tumor cells

Stem-like tumor cells of ascites carcinoma Krebs-2 and Epstein-Barr virus–induced B-lymphoma were shown to possess the innate capability of binding and internalizing the TAMRA-labeled double-stranded DNA (dsDNA) probe. The process of binding and internalizing is rather complicated and composed of the following successive stages: 1) initiating electrostatic interaction and contact of a negatively charged dsDNA molecule with a positively charged molecule(s) on the surface of a stem-like tumor cell; 2) binding of the dsDNA probe to a tumor stem cell surface protein(s) via the formation of a strong chemical/molecular bond; and 3) the very internalization of dsDNA into the cell. Binding of DNA to cell surface proteins is determined by the presence of heparin/polyanion-binding sites within the protein structure, which can be competitively blocked by heparin and/or dextran sulfate, wherein heparin blocks only the binding, while dextran sulfate abrogates both binding and internalization. The abrogation of internalization by dextran sulfate implies the role of scavenger receptors in this process. Cells were shown to uptake DNA in amounts constituting ∼0.008% of the haploid genome. Inhibitors of caveolae-dependent internalization abrogate the DNA uptake in Krebs-2 cells, and inhibitors of the clathrin/caveolar mechanism block the internalization in B-lymphoma cells. In the present report, it is shown for the first time that in contrast to the majority of committed tumor cells, stem-like tumor cells of Krebs-2 and B-lymphoma carry a general positive charge on their surface.

Are novel scavenger-like receptors involved in the hepatic uptake of heparin?

Heparin is an anionic macromolecular drug. It has been widely used as an anticoagulant, and numerous efforts to clarify the mechanism of its disposition in the body have been made to help expand its clinical applications, using its newly found biological activities, as well as to further improve its use in anticoagulant therapy. It has now been shown that heparin is taken up extensively not only by Kupffer cells but also by parenchymal cells in the liver, the major distribution organ, and a receptor-mediated endocytotic mechanism, which is shared by heparin analogs and various anionic macromolecules, is responsible for heparin uptake in both types of cells. Although the characteristics of the receptors for heparin in both cells have lots of similarities to those of scavenger receptors, the receptors in parenchymal cells do not accept acetylated low density lipoprotein (Ac-LDL) as a ligand, which is the only striking difference between them and major scavenger receptors. Although the receptors in Kupffer cells, which accept Ac-LDL as a ligand, may belong to class A scavenger receptors, this remains to be established. We therefore conclude at present that it is likely that novel scavenger-like receptors for heparin (heparin receptors) or unidentified scavenger receptors are responsible for heparin uptake in the liver.

Uptake of FH by two types of scavenger-like receptors in rat liver parenchymal cells in primary culture

Several anionic proteins that are known to be substrates of scavenger receptors documented in the literature were selected and tested for their effects on the uptake of fractionated heparin (FH), an anionic macromolecular drug. The tests were made in rat liver parenchymal cells to characterize scavenger-like receptors involved in FH uptake, probing into substrate recognition characteristics in comparison with those of scavenger receptors. Although the uptake of FH was completely inhibited by dextran sulfate, a typical substrate of scavenger receptors, suggesting that scavenger-like receptors that have affinity for some anionic macromolecules are responsible for the uptake, it was not inhibited by acetylated low density lipoprotein (Ac-LDL), another typical substrate. Thus, the scavenger-like receptors were suggested to be different from the major scavenger receptors of classes A and B that are known to be sensitive to Ac-LDL. The uptake of FH was only partially inhibited by maleylated bovine serum albumin (Mal-BSA), suggesting that the scavenger-like receptors can be classified into two types in terms of sensitivity to Mal-BSA. The Mal-BSA-sensitive receptor was also found to be sensitive to oxidized low density lipoprotein (Ox-LDL). Kinetic analysis revealed that the binding capacity (Bmax) of the Mal-BSA-insensitive receptor was significantly larger than that of the Mal-BSA-sensitive one, though their dissociation constants (Kd) and apparent internalization rate constants (kint,app) were comparable. Information obtained in this study should be helpful for understanding the disposition mechanism of FH and also of anionic macromolecules and for developing delivery strategies, although the physiological roles and molecular identity of each receptor need to be further clarified in the future.