| Abstract: | The specific binding of various concentrations of 125I-labeled nerve growth factor (NGF) to PC12 cells at 37 degrees C reached maxima after 90 min and then declined to 25% of maximal binding after 10 h. Decreased binding was accompanied by degradation of 125I-NGF and the appearance of acid-soluble biologically inactive 125I (mainly 125I-monoiodotyrosine) in the medium as well as a decrease in the number of surface NGF receptors. The time-dependent decrease in binding and the degradation of 125I-NGF were inhibited by low temperature and the lysosomotropic agent chloroquine while degradation was inhibited by metabolic energy inhibitors in the absence of glucose. Chloroquine also produced an increase in the accumulation of 125I-NGF which was not readily removed from the cells. These data suggest that 125I-NGF bound to PC12 cells is efficiently internalized by receptor-mediated endocytosis and degraded by the lysosomes. It appears from other data that this process does not produce the intracellular signals regulating neurite outgrowth. |
