Role of intracellular degradation of epidermal growth factor in mitogenesis

Addition of leupeptin, methylamine and the antitubulin agent nocodazole did not affect the initial rate of association of 125I-labelled epidermal growth factor (125I-EGF) to Swiss mouse 3T3 fibroblast cells in vitro, but continued incubation with these drugs (up to 24 h) led to an increase in cell-associated radioactivity in a time- and dose-dependent fashion. Combinations of these drugs caused additive increments in cell-associated and internalized radioactivity. Throughout the incubation period, 81-89% of the cell-associated 125I-EGF was internalized. Upon incubation of 125I-EGF with 3T3 cells in the presence or absence of the three inhibitors of degradation for periods of up to 24 h, and after removal of the surface-bound material, the internalized 125I-EGF was extracted and 42-53% was found to biochemically intact (by acid precipitation) and 56-65% was antigenically similar to native EGF (using double antibody immunoprecipitation in an EGF radioimmunoassay). The extracted internalized 125I-EGF was capable of binding to fresh 3T3 cells. Despite causing a similar increase in intact internalized 125I-EGF, leupeptin did not interfere with and nocodazole alone or in combination with leupeptin markedly enhanced EGF-stimulated DNA synthesis, whereas methylamine inhibited mitogenesis. These data indicate a dissociation between EGF degradation and DNA synthesis, and are not consistent with the hypothesis that intracellular degradation of EGF is necessary for its mitogenic effects.