Leukocytes from peripheral blood and bone marrow of an 8-month-old infant with acute myelomonocytic leukemia were studied for the presence of steroid receptors and hormone responsiveness in vitro, as well as for clinical response to glucocorticoid therapy. With the use of a competitive protein-binding assay on cytoplasmic extracts, it was possible to demonstrate saturable binding of radiolabeled glucocorticoid. Scatchard analysis of the binding was consistent with a single class of receptor sites with a dissociation constant of 3.5 × 10-9 m and 2.4 × 10-9 m for peripheral blood and bone marrow, respectively. The concentration of receptor sites was 187 fmol per mg protein (peripheral blood) and 122 fmol per mg protein (bone marrow) corresponding to approximately 2400 and 1500 sites/cell. Sucrose density gradient revealed the presence of macromolecules sedimenting between 6s and 8s (low ionic strength) and at 3.6s (in gradients containing 0.4 m KCI). Significant competition for radioactive triamcinolone acetonide was observed with natural and synthetic glucocorticoids, but not with other steroids. Under in vitro incubation, macromolecular synthesis was inhibited by glucocorticoid concentrations which saturated the receptor sites. Administration of either dexamethasone or prednisolone produced a sharp increase in the absolute number of circulating leukocytes, which was accompanied by an increase in the percentage of more-immature cells. A temporal relationship was also observed between the withdrawal of the drug and the drop in the leukocyte count. These results indicate that cells which contain glucocorticoid receptors and are responsive to steroids in vitro seem to be sensitive to a stimulatory effect in vivo. Moreover, the results suggest that a factor(s) other than steroid binding and responsiveness in vitro is important in determining the in vivo effects of glucocorticoids.
Presented at the John E. Fogarty International Center Conference on Hormones and Cancer, March 29 to 31, 1978, Bethesda, Md. Supported in part by Progetto Finalizzato C. N. R. Biologia della Riproduzione.