Platelet-derived growth factor treatment decreases the affinity of the epidermal growth factor receptors of Swiss 3T3 cells.

Platelet-derived growth factor (PDGF) inhibits markedly (90%) the binding of a low concentration (0.5 ng/ml) of 125I-labeled epidermal growth factor (125I-EGF) to Swiss 3T3 cells in a dose, time, and temperature-dependent manner. In contrast, PDGF inhibits the binding of a high concentration (20 ng/ml) of 125I-EGF by only 20%. Detailed Scatchard analysis of 125I-EGF binding to Swiss 3T3 cells at 4 degrees C, after a 2-h incubation with a saturating level of PDGF at 37 degrees C, demonstrates that this PDGF pretreatment does not change the number of EGF receptors (70,000 sites/cell), but causes a large decrease in the affinity of the receptors, from a mixed population of 0.5-12 ng/ml, to an affinity of 13 ng/ml. A parallel treatment at 37 degrees C with EGF itself causes a 70% decrease in the number of EGF receptors. Therefore, we conclude, in contrast to previous reports, that PDGF (unlike EGF) does not cause down-regulation of EGF receptors. Rather PDGF induces a potent, temperature-dependent change in the affinity of the EGF receptor population ("transmodulation").