Dexamethasone modulates binding and action of epidermal growth factor in serum-free cell culture

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Abstract
Experiments probing the mechanism by which glucocorticoids modulate cell proliferation were carried out on serum-free cell cultures of quiescent human diploid foreskin (HF) cells. Added alone, the synthetic glucocorticoid dexamethasone had no effect on cell number. However, dexamethasone enhanced the mitogenic response of HF cells to epidermal growth factor (EGF) by 50% at all EGF concentrations. The mitogenic action of EGF was maximally promoted by a dexamethasone concentration of 100 ng/ml (0.25 μM). Binding studies with 125 I-labeled EGF ( 125 I-EGF) suggested that dexamethasone caused this “permissive” effect by modulating cell surface receptors for EGF. Paralleling their increased responsiveness to EGF growth stimulation, dexamethasone-treated cells exhibited a 50-100% increased ability to bind physiological concentrations of 125 I-EGF. A binding increase was apparent after a 4-hr dexamethasone treatment. The dexamethasone-treated cells maintained an increased ability to bind 125 I-EGF during the prolonged exposure to EGF that was required to stimulate cell division. Moreover, the increase in 125 I-EGF binding exhibited a dexamethasone dose-dependence similar to that for the enhancement of EGF mitogenesis, suggesting a relationship between the dexamethasone effects on binding and growth. An investigation of the binding increase showed that it was specific for glucocorticoids, and required protein synthesis. The enhancement of 125 I-EGF binding diminished with increasing concentrations of 125 I-EGF, indicating that dexamethasone caused a qualitative change in the EGF receptors (possibly a change in receptor affinity or cooperativity). The alteration in 125 I-EGF binding may occur as part of a far-reaching dexamethasone-mediated change in the cell surface, because dexamethasone treatment slightly increased the ability of HF cells to bind 125 I-insulin, and decreased by half their ability to bind 125 I-thrombin.