Glucocorticoid-Receptor Complexes in Rat Thymus Cells

Receptor-bound cortisol in rat thymus cells incubated with cortisol at 37° is shown by a number of criteria to be retained largely in the nuclei, which are obtained by disrupting the cells with hypotonic shock. In cells incubated with cortisol at 3° for up to 2 hours most of the receptor-bound cortisol is not retained in the nuclei; it is referred to as cytoplasmic, but its location is not firmly established. Upon warming of such cells from 3 to 37° there is a rapid transfer from cytoplasmic to nuclear form. This temperature-sensitive transfer, which is not reversed on cooling, appears to be an obligatory step in the formation of the nuclear cortisol-receptor complex. A chase experiment indicates that the transfer probably occurs without dissociation of cortisol from the receptor binding site. At 37° the half-time for transfer is about 30 s, at 26° about 60 s, and at 15° more than 4 min. Cortexolone, a steroid that competes with cortisol for binding to the glucocorticoid receptors but has little glucocorticoid activity, is shown to undergo the temperature-sensitive transfer from cytoplasmic to nuclear form. The inactivity of cortexolone consequently cannot be ascribed entirely to inability to induce transfer to the nucleus.