Surface binding and internalization of platelet-derived growth factor in human fibroblasts

Surface binding and uptake of platelet-derived growth factor (PDGF) in human fibroblasts cultivated in vitro were studied by quantitative electron microscopic autoradiography using 125I-labeled PDGF and by indirect immunofluorescence using PDGF antibodies. After 120 min at 12 degrees C PDGF was found preferentially in coated regions of the plasma membrane. Warming the cells to 37 degrees C initiated rapid ingestion of the factor via small vesicles, usually lacking a membrane coat. After 10 min PDGF started to appear in lysosomes, and it showed maximal concentration within these organelles after 30 min. There were also signs of passage of PDGF through the Golgi complex, but only after 60 min. Treatment of the cells with chloroquine, a weak base that inhibits intralysosomal degradation, prevented disappearance of tracer from the lysosomes. The observations indicate that PDGF was internalized via coated regions of the plasma membrane and carried to lysosomes for degradation. Subsequent appearance of tracer within the Golgi complex could reflect receptor-ligand complexes that escaped degradation and were recirculated back to the cell surface.