The brain isozyme of creatine kinase (CKB) is a major component of the estrogen-induced proteins in the rat uterus. Hormonal specificity of this response was studied in cotransfection assays using the rat CKB promoter linked to the bacterial chloramphenicol acetyltransferase gene. Response was specific for estrogen as 17 beta-estradiol in the presence of estrogen receptor dramatically stimulated the CKB promoter. This induction was completely blocked by the estrogen antagonist ICI 164,384. Nuclear receptors for progesterone, androgen, glucocorticoid and vitamin D did not significantly activate the CKB promoter in the presence of their respective ligands. Creatine kinase (CK) activity was analyzed in decidualized mouse uterus to assess estrogenic activity in vivo. Upon oil stimulation, uterine horns of day 4 pseudopregnant mice underwent a dramatic outgrowth in response to endogenous progesterone. This response was accompanied by a significant decrease in CK activity from a control value of 1.44 +/- 0.25 to 0.38 +/- 0.08 IU/mg protein (P < 0.001), indicating that the action of estrogen was suppressed. Treatment of females one day prior to oil-stimulation with progesterone receptor antagonists, RU486 (Mifepristone) or ZK299 (Onapristone), or with a monoclonal antibody to progesterone (DB3), abolished decidualization, and also restored the CK activity to the control value. These results suggest that CK can be used as a specific cellular marker to detect unopposed estrogen action in the mouse uterus associated with progesterone withdrawal or receptor blockade.
