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To better define thrombin-receptor interactions, we synthesized human thrombin peptides and identified binding-

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 MAM).Binding studies with l251-labeled EGF (125I-EGF) suggested that dexamethasone caused this "permissive" effect by modulating cell surface receptors for EGF. Paralleling their increased responsiveness to EGF growth stimulation, dexamethasonetreated cells exhibited a 50-100% increased ability to bind physiological concentrations of 125I-EGF. A binding increase was apparent after a 4-hr dexamethasone treatment. The dexamethasone-treated cells maintained an increased ability to bind 12sI-EGF during the prolonged exposure to EGF that was required to stimulate cell division. Moreover, the increase in 125I-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 125I-EGF binding diminished with increasing concentrations of 125I-EGF, indicating that dexamethasone caused a qualitative change in the EGF receptors (possibly a change in receptor affinity or cooperativity). The alteration in 125I-EGF binding may occur as part of a far-reaching dexa-*methasone-mediated change in the cell surface, because dexamethasone treatment slightly increased the ability of HF cells to bind l251-insulin, and decreased by half their ability to bind 1251 thrombin. There are numerous reports that glucocorticoid steroid hormones affect the proliferation of animal cells in culture-in some cell types stimulating (1-3), and in others inhibiting cell division (4, 5). An intriguing property of these glucocorticoid actions is that they occur only in the presence of polypeptide growth factors or serum. Thus, it appears that glucocorticoids modulate cell growth indirectly by altering cell responsiveness to growth factors. Glucocorticoids might cause these effects by altering growth factor interaction with the cells as measured by growth factor binding (2). Alternatively, glucocorticoids could bring about these effects by altering key biochemical events subsequent to the binding of growth factors to cell surface receptors (6). Here we report that the synthetic glucocorticoid dexamethasone (dex) enhances the mitogenic action of epidermal growth factor (EGF) on human diploid foreskin (HF) cells. Moreover, we have discovered that dex also increases the specific cellular binding of EGF, suggesting that dex proThe costs of publication of this article were defrayed in part by the ...

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