Pharmacokinetic analysis of scavenger receptor-mediated uptake of mucopolysaccharides in various cells

Although the scavenger receptor-mediated uptake has been qualitatively investigated in the research fields of biochemistry and pathology, pharmacokinetic characteristics of the scavenger receptors are poorly understood. In this review, we summarized basic findings on scavenger receptors reported in available literature, and introduced our recent studies on the quantitative characteristics of the scavenger receptor-mediated uptake. High molecular weight fractionated 3H]heparin (HMWFH, 16,000-24,000 Da), one of the model mucopolysaccharides, was investigated to elucidate its uptake mechanism into isolated rat Kupffer cells, isolated peritoneal macrophages and liver parenchymal cells in primary culture. The equilibrium bindings of HMWFH to isolated Kupffer cells and peritoneal macrophages were concentration-dependent with the respective dissociation constants (Kd) of 5.7 and 6.0 nM and with the respective maximum binding capacities (Bmax) of 1.5 and 1.9 pmol/10(6) cells. Several ligands of scavenger receptors inhibited the binding of HMWFH to macrophages, suggesting the involvement of scavenger receptors in the uptake of HMWFH by these macrophages. It was also suggested that the scavenger receptor-mediated uptake is different from the receptor-mediated endocytosis of polypeptides and phagocytosis, based on the evidence of the now inhibitory effects of an inhibitor of receptor-mediated endocytosis of polypeptides(phenylarsine oxide) and phagocytosis inhibitors (cytochalasine B and colchicine) on the internalization. The involvement of scavenger-like receptors was also suggested in the uptake of HMWFH by liver parenchymal cells in primary culture by demonstrating inhibitory effects of ligands for scavenger receptors. The internalization into liver parenchymal cells by scavenger-like receptors was not affected by an inhibitor of receptor-mediated endocytosis of polypeptides and phagocytosis inhibitors, similarly to the results in the macrophage scavenger receptors. The Kd of 53.5 nM and Bmax of 32.8 pmol/10(6) cells in parenchymal cells were both in the order of magnitude larger than those in isolated Kupffer cells, suggesting the binding of HMWFH to scavenger-like receptors in parenchymal cells with lower affinity and higher capacity. On the other hand, an apparent internalization rate constant (kint, app) of 0.0056 min-1 was comparable with that in Kupffer cells (0.0118 min-1). Thus, we demonstrated the involvement of scavenger receptors in the uptake of HMWFH by rat Kupffer cells, peritoneal macrophages and liver parenchymal cells, and succeeded in characterizing the uptake kinetically. These findings should provide useful information for not only establishing the rational clinical use of mucopolysaccharides but also developing new drugs such as antiatherosclerotic agents and peptides delivered to cells with scavenger receptors.