There is increasing clinical interest in progestins for two reasons: one, the use of progesterone receptor (PR) measurements to mark hormone-dependent breast, endometrial, ovarian and prostatic cancers; and two, the use of progestins and antiprogestins for contraception and for endocrine therapies of cancer. We have developed a stoichiometric nuclear exchange assay for PR and have used it to show have progesterone can regulate the levels of its own receptors freed of the interfering effects of estrogens. We have also shown that a widely used synthetic experimental progestin, R5020, has chronic suppressive effects on PR levels. This, if extrapolated to the clinical setting, suggests that PR in patients taking synthetic progestins may be incorrectly assigned. These studies have been possible because of the availability of permanent human breast cancer cell lines, particularly one called T47D, that synthesizes enormous levels of PR without requiring estrogen induction. Our aim in this application is to study in detail, the biology, metabolism and receptor mechanisms of progesterone, synthetic progestins, and a new contraceptive antiprogestin RU38 486. We plan to use T47D cells (estrogen independent PR) MCF-7 cells (estrogen-dependent PR) and BT-20 cells (PR negative): 1) To develop three biological responses to mark progestin action. These are inhibition of cell growth, induction of insulin receptors, and synthesis of secreted proteins. 2) We will use gas chromatography and mass spectrometry to study cell mediated progestin metabolism and identify the ultimate receptor-bound compound responsible for biological activity. 3) We plan to study the molecular biology of PR and their interaction with progestins: the regulation of PR levels by DNA methylation, histone acetylation and hormone resistance; holoreceptor and nuclear receptor structures by chromatography and electrophoresis; PR activation, translocation nuclear matrix PR binding sites by immunofluorescence; endo and exonuclease activity. 4) We propose to study the mechanisms of antiprogestin action using labeled and unlabeled RU38 486.