Abstract
The mechanism by which glucocorticoids influence erythroid colony formation by murine bone marrow cells was studied in a plasma clot culture system. All glucocorticoids tested, including 9α‐fluorocortisol (> 10^−8^M), cortisol (≥ 10^−8^M), dexamethasone (> 10^−7^M), and corticosterone (≥ 10^−7^M), inhibited erythroid colony formation; none enhanced colony formation. Inhibition by a prototypic glucocorticoid, dexamethasone, was evident at all concentrations of erythropoietin (25–200 mU) in the clots. The percent dexamethasone‐sensitive cells killed by high specific activity [^3^H]‐TdR (∼ 78%) was the same as the percent erythroid colony‐forming cells killed by this treatment, and the inhibition appears to represent a direct effect on the erythroid colony‐forming cell since: (1) Cells exposed (1 hour) to dexamethasone, washed, and cultured in steroid‐free medium formed a reduced number of colonies (67% of control); (2) dexamethasone decreased colony formation by marrow depleted of adherent cells or obtained from either nude or neonatally thymectomized mice; and (3) co‐cultivation studies of normal marrow with that obtained from glucocorticoid‐treated mice, which forms a reduced number of colonies in vitro, indicated that this inhibition was not the result of enhanced suppressor cell or reduced stimulatory cell activity.
The proliferative rate of colony‐forming cells obtained from dexamethasone‐treated mice, as determined with high specific activity [^3^H]‐TdR, was reduced ( ∼ 38% of control), suggesting that a reduction in the proliferative rate of the colony‐forming cell is the major mechanism of the glucocorticoid‐mediated inhibition of erythroid colony formation. In addition, dexamethasone, added 24 hours after the initiation of the cultures, inhibited the growth of developing erythroid colonies, indicating a potential inhibitory action on the proliferation of the progeny of the colony‐forming cell.