KINETIC-ANALYSIS OF TRANSCYTOSIS OF EPIDERMAL GROWTH-FACTOR IN MADIN-DARBY CANINE KIDNEY EPITHELIAL-CELLS

Purpose. The aim of this study is to clarify the intracellular fare an d a rate limiting step in transcytosis of epidermal growth factor (EGF ) in Madin-Darby Canine Kidney (MDCK) epithelial cells. Methods. The k inetics of transcytosis of I-125-EGF was investigated. To examine the fate of EGF molecules bound to its receptor on the cell surface, I-125 -EGF was prebound to the basal surface at 4 degrees C, followed by ext ensive washing and subsequent incubation at 37 degrees C in EGF-free m edium. Results. Saturable transport of I-125-EGF through the cell mono layer could only be observed from the basal to apical side. Most (simi lar to 90%) of the EGF molecules bound to the surface receptor are int ernalized with a half-life of 1-3 min, followed by intracellular degra dation with a half-life of 20-50 min. The exocytosis of internalized E GF into the apical medium is much slower with a half-life of 130-250 m in. Even when I-125-EGF was incubated with MDCK cells at 37 degrees C and washed with acid to remove cell-surface I-125-EGF, intact I-125-EG F appeared in the basal medium with a half life of 160-170 min. Conclu sions. The exocytosis of internalized EGF into the apical medium is a rate limiting step in EGF transcytosis. At least a small amount of int ernalized EGF is recycled.