Aminoterminal Propeptide of Type III Procollagen is Cleared from the Circulation by Receptor-Mediated Endocytosis in Liver Endothelial Cells

Abstract

Intravenously administered ¹²⁵I -labelled bovine aminoterminal propeptide of typeIII procollagen (¹²⁵I -BPIIINP) had a half-life in blood of about 2 minutes. Low molecular weight degradation products appeared in the circulation about 5 minutes after injection. BPIIINP coupled to ¹²⁵I -labelled tyramine cellobiose (¹²⁵I -TC-BPIIINP) was administered intravenously to determine the cellular site of uptake. TC is non-degradable and is therefore accumulated intralysosomally. With this ligand I could show that PIIINP is taken up mainly by the liver endothelial cells (LEC), with very low uptake in other types of liver cells and at extrahepatic sites. Studies on binding and endocytosis of labelled PIIINP in cultures of purified populations of liver cells can be summarized as follows: 1) Uptake and degradation were observed mainly in LEC; 2) PIIINP associated with Kupffer and parenchymal cells, but degradation was very low; 3) Serum was not required for binding of PIIINP to LEC; 4) Binding was specific, that is, other ligands, such as collagen type III, hyaluronan, chondroitin sulfate, formaldehyde-treated albumin, and mannose, that are recognized by distinct receptors on LEC, did not compete with PIIINP for binding; 5) BPIIINP, TC-BPIIINP, and rat PIIINP (RPIIINP) were recognized with the same specificity by LEC; 6) BPIIINP bound to LEC with high affinity (dissociation constant = 1 nM), and about 4.2 × 10⁵, 3.2 × 10⁵, and 1.6 × 10⁵ molecules of BPIIINP, TC-PIIINP, and R-PIIINP, respectively were bound per cell; 7) PIIINP could not be degraded by conditioned medium from cultured Kupffer cells; 8) Leupeptin, which is a strong inhibitor of lysosomal collagenolysis, only weakly inhibited degradation of PIIINP; 9) Binding and endocytosis of PIIINP was not Ca⁺⁺-dependent; 10) Agents that inhibit the endocytic machinery inhibited uptake and degradation of PIIINP. In conclusion, the present results suggest that PIIINP is rapidly eliminated from the circulation by receptor-mediated endocytosis in LEC.