INTRACELLULAR-DISTRIBUTION OF NATURAL AND SYNTHETIC GLUCOCORTICOIDS IN THE ATT-20 CELL

Both natural and synthetic glucocorticoids suppress ACTH production by AtT-20 mouse pituitary cells. However, the degree of suppression is not clearly related to cytosol receptor affinity. These studies were done to test whether the synthetic and natural glucocorticoids might differ in the efficiency with which they caused nuclear translocation of the cytosol receptor. If so, this parameter as well as receptor affinity might contribute importantly to steroidal potency. To accomplish these studies, a rapid method of assaying glucocorticoid binding to whole cells and to isolated nuclei was developed. Whole cell and nuclear binding of triamcinolone acetonide (9α-fluoro-11β, 16α, 17, 21-tetrahydroxypregna-1, 4- diene-3, 20-dione cyclic 16, 17-acetal with acetone), dexamethasone, and corticosterone were measured after incubation of the steroids with intact cells at 25 C. Uptake of all three steroids reached apparent equilibrium by 1 h at 25 C. A Scatchard analysis of binding over a concentration range of 0.1—50 nM indicated approximately 75, 000 binding sites/cell for each glucocorticoid. The apparent dissociation constants (Kd) were: triamcinolone acetonide, 5.3 nM; dexamethasone, 9.7 nM; and corticosterone, 16.0 nM. Over this same concentration range, 50% of the triamcinolone acetonide and dexamethasone bound to the whole cell was in the nuclear fraction even though the affinities of these steroids for the receptor differ 2-fold. In contrast, only 15—25% of bound corticosterone was in the nucleus. We conclude that the natural and synthetic glucocorticoids vary in the efficiency with which they cause nuclear translocation. Thus, the potency of a given steroid may depend on its ability to promote nuclear translocation as well as its affinity for the cytoplasmic glucocorticoid receptor. © 1979 by The Endocrine Society.