Evidence that dephosphorylation inactivates glucocorticoid receptors

Highly purified alkaline phosphatase [orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1] from calf intestine inactivates the glucocorticoid-binding capacity of soluble preparations from mouse fibroblasts (L cells) and rat liver. The unbound receptor is sensitive to inactivation whereas the steroid-bound receptor is unaffected. The ability of the enzyme preparation to inactivate the receptor, like its ability to dephosphorylate p-nitrophenyl phosphate, is dependent on zinc and inhibited by arsenate. Both the dephosphorylating and receptor inactivating activities coelute during DEAE-cellulose purification of the enzyme. There is no detectable proteolytic activity in the purified alkaline phosphatase preparation. In a mixed system containing both glucocorticoid and estrogen receptors, the glucocorticoid receptor is selectively inactivated. Although these observations do not prove that the receptor molecule itself is the substrate, they are consistent with the proposal that the glucocorticoid receptor can be inactivated by dephosphorylation and that only the phosphorylated form of the molecule is capable of binding steroid. A phosphorylation-dephosphorylation mechanism may be responsible for determining the level of active receptor in the cell.