GLUCOCORTICOID RECEPTORS - ATP-DEPENDENT CYCLING AND HORMONE-DEPENDENT HYPERPHOSPHORYLATION

The dependence of hormone binding to glucocorticoid receptors (GRs) on cellular ATP levels led us to propose that GRs normally traverse an A TP-dependent cycle, possibly involving receptor phosphorylation, and t hat without ATP they accumulate in a form that cannot bind hormone. We identified such a form, the null receptor, in ATP-depleted cells. GRs are basally phosphorylated, and become hyperphosphorylated after trea tment with hormone (but not RU486). In mouse receptors we have identif ied 7 phosphorylated sites, all in the N-terminal domain. Most are on serines and lie within a transactivation region. The time-course of ho rmone-induced hyperphosphorylation indicates that the primary substrat es for hyperphosphorylation are the activated receptors; unliganded an d hormone-liganded nonactivated receptors become hyperphosphorylated m ore slowly. After dissociation of hormone, most receptors appear to be recycled and reutilized in hyperphosphorylated form. From these and r elated observations, we have concluded that the postulated ATP-depende nt cycle can be accounted for by hormone-induced or spontaneous dissoc iation of receptor-Hsp90 complexes, followed by reassociation of unlig anded receptors with Hsp90 via an ATP-dependent reaction like that dem onstrated in cell-free systems. Other steroid hormone receptors might traverse a similar cycle. Four of the 7 phosphorylated sites in the N- terminal domain are in consensus sequences for p34(cdc2) kinases impor tant in cell cycle regulation. This observation, along with the known cell cycle-dependence of sensitivity to glucocorticoids and other evid ence, point to a role for receptor phosphorylation in controlling resp onses to glucocorticoids through the cell cycle.