| Abstract: | Using a serum-free medium supplemented with hormones and growth factors, which included epidermal growth factor (EGF), we investigated the binding and processing-degradation of 125I]EGF in WI-38 cells of various in vitro ages. The binding and processing-degradation systems of these cells remained essentially unchanged throughout their lifespan. The number of specific 125I]EGF binding sites per cell increased as the cultures senesced, though the number of specific binding sites per micron 2 (surface area) remained constant. The kinetics of ligand degradation as well as the qualitative and quantitative nature of the degradation products also remained essentially unchanged throughout the life-span. The only consistent alteration in any of the binding parameters measured was the slight decrease in the apparent Kd of the ligand-receptor complex, independent of temperature. Quantitation of EGF-stimulated DNA synthesis revealed a decrease in the percentage of cells incorporating 3H]thymidine (3H]TdR) during a 30-h exposure from 45% in young cells to 0.25% in senescent cells, although 125I]EGF binding or processing-degradation did not differ significantly in young and old cells. Thus, EGF binding does not decrease in senescence. |
