on Steroid Responsiveness Lymphoblasts following Hydroxyurea Treatment: Lack of Effect Elevated Glucocorticoid Receptor Binding in Cultured Human

While studying the effects of chemotherapy on glucocorticoid receptor (GR) binding levels in hentatological malignancies, we observed a sizable increase in nuclear GR binding of (3H)dexamethasone in peripheral leukocytes from a chronic basophilic leukemia patient following treat ment with hydroxyurea plus prednisone, but not after prednisone alone. This apparent clinical effect of hydroxyurea led to an examination of hydroxyurea effects on GR binding and sensitivity in the glucocorticoidsensitive human lymphoblast cell line GM4672A. GR binding levels in GM4672A cells were measured following a 3-day exposure to 50 ¿IM hydroxyurea, a concentration chosen to have a minimal but measurable effect on cellular growth rates with little or no effect on cellular viability. Under these conditions, nuclear (3H]dexamethasone receptor binding measured by Scatchard analysis using a whole-cell assay was elevated 2.4-fold over control values (P < 0.05), while cytosolic residual receptor binding (measured at 37°C) remained unchanged. Thus, the total cellular content of measurable GR was increased, and this increase was totally accounted for by GR capable of nuclear binding. Hydroxyurea treatment of GM4672A cells had no effect on the affinity of nuclear or cytosolic GR for |3H)dexamethasone. The increase in measurable nuclear-bound receptors occurred in a time-dependent manner over a period of 3 days and was fully reversible within 3 days following removal of hydroxyurea. The increase in receptor binding could not be explained by the slight alterations in cell cycle kinetics which occur at this low level of hy droxyurea. Despite increased receptor binding, cellular glucocorticoid responsiveness was unaltered as assessed by dexamethasone inhibition of cell growth and dexamethasone inhibition of a urokinase-like plasminogen activator. Thus, increased nuclear and total cellular GR binding levels in hydroxyurea-treated GM4672A cells are not associated with increased glucocorticoid responsiveness.