PARACELLULAR TRANSPORT OF INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) ACROSSHUMAN UMBILICAL VEIN ENDOTHELIAL-CELL MONOLAYERS

Insulin-like factors (IGFs) are well defined mitogens and growth promo ters, which are found in blood associated with high affinity IGF bindi ng proteins (lGFBPs). In vivo, the endothelium is potentially the prim ary site of uptake of IGFs or IGF-IGFBP complexes from blood for trans port to the extravascular space. However. the pathway and mechanisms b y which IGFs cross the endothelial cell barrier are not known. The pre sence of high affinity receptors for IGF-I and IGF-II on human umbilic al vein endothelial (HUVE) cells was demonstrated by (i) radio-recepto r assays using both IGF-I and IGF-II and (ii) affinity label crosslink ing studies. In addition, Western ligand blotting and immunoblotting r evealed that IGFBP-2, -3, and -4 are secreted into serum-free media co nditioned 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 I-125-IGF-I and [H-3]inulin, wher eas the control Madin Darby canine kidney (MDCK) cell line virtually e xcluded all passage of these molecules. Transport of I-125-IGF-I acros s HUVE cell monolayers was not significantly different to that of [H-3 ]inulin, a paracellular probe. Moreover, I-125-IGF-I transport was not inhibited by either excess unlabelled ICF-I or a monoclonal antibody to the type ICF receptor at a concentration shown to inhibit I-125-IGF -I binding to HUVE cell monolayers. Our findings show that the movemen t of free IGF-I across HUVE cell monolayers occurs via a paracellular route and not by a receptor-mediated, transcellular pathway. (C) 1997 Wiley-Liss, Inc.