Endocytosis and intracellular processing of tissue-type plasminogen activator by rat liver cells in vivo

Pharmacokinetic modeling of hepatocyte growth factor in experimental animals and humans

Hepatocyte growth factor (HGF) is under development for treatment of renal failure. This study was designed to clarify changes in HGF pharmacokinetics in renal failure and to establish a pharmacokinetic model applicable to single and repeated doses. The plasma concentration profile in mice with glycerol-induced acute renal failure was similar to that in normal mice, indicating a minimal contribution of kidney to systemic clearance of HGF. Nevertheless, accumulation of fluorescein-4-isocyanate-labeled HGF in renal tubules in both cases suggests the occurrence of efficient endocytosis of HGF in kidney. A pharmacokinetic model including plasma and liver compartments was constructed, incorporating both high- and low-affinity receptors for association and subsequent endocytosis of HGF because HGF is eliminated via specific receptor c-Met and heparin-like substance. The model well explained the plasma concentration profiles at all doses examined after bolus injection in animals and humans, and those during infusion in rodents. It includes externalization of receptors, which is negatively regulated by HGF, and can explain the gradual increase in trough concentration during repeated dosing in monkeys. Overall pharmacokinetic profiles of HGF are governed by at least two receptors and are well described by this pharmacokinetic model, which should assist in safe management of clinical trials.

Comparison of receptor-mediated endocytosis kinetics between wild-type t-PA and recombinant pamiteplase in isolated rat hepatocytes and liver cell plasma membranes

1. Differences in receptor-mediated endocytosis kinetics between pamiteplase, an engineered t-PA, and an unmodified rt-PA were examined using liver cell plasma membranes and isolated rat hepatocytes. 2. Whereas the binding site of pamiteplase on hepatocytes was the same as that of rt-PA, the Kd of pamiteplase was 5.1-7.7 times larger than that of rt-PA, indicating a lower affinity of pamiteplase for the t-PA receptor. 3. ke for pamiteplase measured using parenchymal cells or non-parenchymal cells was slightly smaller than that for rt-PA, whereas kon for pamiteplase were much lower than that of rt-PA, suggesting that the interaction between pamiteplase and the receptor is slower than that of rt-PA because of its structural modification. 4. Therefore, the difference in drug disposition between pamiteplase and rt-PA is mainly due to the difference in the hepatic clearance caused by a change in the interaction rate between the ligand and its cell-surface receptor.

Effect of brefeldin A and lysosomotropic reagents on intracellular trafficking of epidermal growth factor and transferrin in Madin-Darby canine kidney epithelial cells

To regulate intracellular sorting of epidermal growth factor (EGF) or transferrin (Tf), the effect of brefeldin A (BFA) or lysosomotropic reagents was investigated. To examine the effect of them on the net transcellular transport of 125I-EGF or 125I-Tf, their transcytosis was investigated in the presence or absence of reagents. For the investigation of their fate after internalization, radiolabeled ligands were internalized at 37 degreesC, followed by extensive washing and subsequent incubation at 37 degreesC in the ligand-free medium (pulse-chase study). BFA enhanced transcytosis of 125I-Tf, but had no effect on 125I-EGF. Kinetic analysis in the pulse-chase study showed that BFA does not affect cell-surface binding or intracellular sorting of EGF, while it only increases the transcytosis rate constant of Tf. From the lysosomotropic reagents study, both ammonium chloride and monensin suppressed transcytosis and recycling as well as the degradation of EGF, while both chloroquine and bafilomycin A selectively suppressed the degradation process with only a minimal effect on transcytosis, resulting in an increase in the amount transcytosed. It is concluded the that enhancement effect of BFA on transcytosis depends upon the type of receptor targeted. Lysosomotropic reagents can be divided into two types as far as the specificity of the effect on the net amount of EGF transcytosed in Madin-Darby canine kidney (MDCK) cells is concerned.

Evidence for carbohydrate-independent endocytosis of tissue-type plasminogen activator by liver cells

In the liver, tissue-type plasminogen activator (t-PA) is endocytosed by hepatic parenchymal (PC), endothelial (EC) and Kupffer (KC) cells. Although the endocytosis is receptor-mediated, it remains a matter of discussion which receptors are involved in this catabolic process. To evaluate the role of a protein-specific receptor, as well as the possible involvement of the galactose receptor on PC and the mannose receptor on EC, we have employed different glycosylation variants of t-PA in biochemical and immunocytochemical studies. Partial or total removal of carbohydrate side-chains by endoglycosidases did not prevent clearance and hepatic endocytosis of t-PA by either of the liver cell types. Blockade of the galactose and mannose receptors by co-application of a large excess of the glycoprotein ovalbumin remained without effect on the binding and uptake of t-PA by hepatic cells. However, the contribution of different liver cell types to the hepatic clearance of t-PA was to a certain extent dependent on the type of oligosaccharide chains removed. The mannose receptor on EC is partially responsible for the clearance of t-PA by this cell type, whereas the galactose receptor does not seem to be involved in this process. The results obtained in this study further demonstrate that the major portion of the hepatic catabolism of t-PA is independent of its carbohydrate side-chains.