Glucocorticoid-mediated alteration in growth factor binding and action: analysis of the binding change

The addition of the glucocorticoid analog dexamethasone (DX) to serum-free cultures of human fibroblasts caused a twofold enhancement of the mitogenic response to epidermal growth factor (EGF), although DX by itself was not mitogenic. A basis for this effect was suggested by studies showing that DX also increased the cellular binding of 125I-EGF. DX increased the ability of the cells to bind 125I-EGF only at low physiological concentrations of this polypeptide. Thus, data from 125 I-EGF binding to cells incubated without DX produced a linear Scatchard plot, whereas the data from 125I-EGF binding to DX-treated cells led to an upwardly curvilinear Scatchard plot. Measurements of 125I-EGF association with the cell surface and cytoplasm indicated that this binding change involved an alteration of cell surface EGF receptors. The binding change appeared not to involve negatively cooperative interactions between EGF receptors, nor a change in the number of receptors. The binding alteration could be explained by a model in which DX converted 25--30% of the cell surface EGF receptors to a form having a fourfold increased affinity.