Paracellular transport of insulin-like growth factor-I (IGF-I) across human umbilical vein endothelial cell monolayers.

Insulin-like growth factors (IGFs) are well defined mitogens and growth promoters, which are found in blood associated with high affinity IGF binding proteins (IGFBPs). In vivo, the endothelium is potentially the primary site of uptake of IGFs or IGF-IGFBP complexes from blood for transport to the extravascular space. However, the pathway and mechanisms by which IGFs cross the endothelial cell barrier are not known. The presence of high affinity receptors for IGF-I and IGF-II on human umbilical vein endothelial (HUVE) cells was demonstrated by (i) radio-receptor assays using both IGF-I and IGF-II and (ii) affinity label cross-linking studies. In addition, Western ligand blotting and immunoblotting revealed that IGFBP-2, -3, and -4 are secreted into serum-free media conditioned by confluent HUVE cell monolayers. To study transendothelial migration of IGF-I, HUVE cells were grown on microporous membranes in a bichamber system. When compared with membranes without cells, HUVE monolayers restricted the passage of 125I-IGF-I and [3H]inulin, whereas the control Madin Darby canine kidney (MDCK) cell line virtually excluded all passage of these molecules. Transport of 125I-IGF-I across HUVE cell monolayers was not significantly different to that of [3H]inulin, a paracellular probe. Moreover, 125I-IGF-I transport was not inhibited by either excess unlabelled IGF-I or a monoclonal antibody to the type I IGF receptor at a concentration shown to inhibit 125I-IGF-I binding to HUVE cell monolayers. Our findings show that the movement of free IGF-I across HUVE cell monolayers occurs via a paracellular route and not by a receptor-mediated, transcellular pathway.