Receptor-mediated vectorial transcytosis of epidermal growth factor by Madin-Darby canine kidney cells

Receptor-mediated transport of peptide hormones and its importance in the overall hormone disposition in the body

A remarkable feature of the pharmacokinetics of polypeptide hormones is the contribution of specific binding sites (receptors) to the polypeptide hormone distribution and clearance in the body. The concept of "transport receptor" is now well established, and receptor-mediated endocytosis (RME) is recognized as a general mechanism in the uptake of biologically important peptide hormones. This article focuses on the kinetic analysis of the RME of polypeptides, based mainly upon the observations of the kinetics of epidermal growth factor in the liver. The following points are emphasized: (1) How can we determine the existence and the kinetic constants of polypeptide RME in vivo and in the perfused liver system? A liver perfusion method, the single-pass multiple-indicator dilution technique, has been shown to be suitable for analyzing the dynamics of interaction of peptide hormones with their cell surface receptors. (2) What is the importance of down-regulation of transport receptors to the overall kinetics of polypeptides in vivo? Time profiles of polypeptide plasma concentrations and their surface receptors in the liver after iv administration of epidermal growth factor were simulated with a physiologic pharmacokinetic model that includes kinetic constants representing the interaction of polypeptides and their receptors.

Intracellular pathways of insulin transport across vascular endothelial cells

Processing and transport of hormones across vascular endothelial cells may modulate hormone action at subendothelial tissue sites. Insulin was transported across cultured rat capillary and bovine aortic endothelial cells, after a delay of 5-10 min, at a constant rate for 60 min at 37 degrees C. 125I-labeled insulin transport was inhibited by 88 +/- 11% (SE, n = 4) and 75 +/- 18% (SE, n = 4) in the presence of anti-insulin receptor antibody and unlabeled insulin (at 10(-7) M), respectively. Reverse phase high-performance liquid chromatography showed 88% of the 125I-insulin transported over 60 min was indistinguishable from the 125I-insulin added to the cells at 4 degrees C. In aortic endothelial cells preincubated with 2.3 x 10(-9) M of insulin for 24 h, insulin receptor binding was downregulated by 67%, and 125I-insulin transport was decreased by 52 +/- 11%. The proton ionophore monensin (0.05 mM) increased the internalized insulin in bovine aortic endothelial cells by 78%, with a corresponding decrease in 125I-insulin released by 76 +/- 2% (SE, n = 4). 125I-insulin transport across the aortic endothelial cell monolayer was similarly decreased (54 +/- 12%, SE, n = 4) by monensin. In contrast, the lysosomal protease inhibitor leupeptin had no effect. Degradation and transport were similarly dissociated by low temperature. At 15 degrees C, no significant insulin degradation was detected, whereas 125I-insulin release from the cells continued at 30 +/- 3% of the rate at 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

High affinity binding sites for epidermal growth factor (EGF) in renal membranes

The kidney produces large quantities of EGF but the role of the kidney in binding and degradation of EGF is unknown. We studied 125I-EGF binding and degradation by highly purified cortical luminal and cortical basolateral membranes of rabbit renal cortex, and by medullary plasma membranes. Specific binding for 125I-EGF was found for the first time in cortical basolateral and medullary plasma membranes (60-80% of total binding) but not in cortical luminal membranes. There was little degradation (less than 4%) of EGF by any of the membranes. Scatchard analysis of 125I-EGF binding by cortical basolateral membranes revealed two distinct classes of binding sites: high and low affinity. The existence of high specific binding sites in cortical basolateral and in medullary plasma membranes suggests a physiologic role of EGF in the kidney.