16-alpha-ethyl-21-hydroxy-19-nor-4-pregnene-3-20-dione and onapristone

Interactions between transcription factors are an important means of regulating gene transcription, leading to modifications in the pattern of gene expression and cell fate. In this study, we report that the progesterone receptor (PR) can strongly interfere with transactivation mediated by the arylhydrocarbon receptor (AhR) in T47D breast cancer cells. This interference was not only demonstrated by induction of a transfected dioxin-responsive reporter plasmid but also on the AhR-mediated up-regulation of the endogenous cytochrome P450-1A1 activity. The interference was not mutual, as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most potent activator of the AhR, did not inhibit progestin-induced promoter activity. When the isoforms of the human PR, hPR-A and hPR-B, were expressed separately in HepG-2 hepatocarcinoma cells, both negatively interfered with the AhR signaling, indicating that the effect is not restricted to T47D cells. In addition, results obtained from studies with both antiprogestins and mutant receptors indicate differences in the underlying molecular mechanisms of repression for both PR isoforms. The suppression by hPR-A does not require additional gene expression or a full transcriptional competent conformation of the receptor. For the repressive effects of hPR-B, however, additional gene expression seems to be involved, as only the agonist-bound, wild-type hPR-B could clearly repress the TCDD-induced response. In conclusion, these studies highlight different mechanisms of repression for the progesterone receptor isoforms on the AhR-mediated trans-activation and underscore the importance of interactions between transcription factors of different families in the regulation of gene transcription.

Topics: Breast Neoplasms; Cytochrome P-450 CYP1A1; Female; Gene Expression Regulation, Enzymologic; Gonanes; Hormone Antagonists; Humans; Mifepristone; Polychlorinated Dibenzodioxins; Pregnenediones; Progesterone Congeners; Receptors, Aryl Hydrocarbon; Receptors, Progesterone; Recombinant Proteins; Signal Transduction; Transcriptional Activation; Transfection; Tumor Cells, Cultured; Up-Regulation

In 1986 we reported the appearance of a progestin binding protein in the human breast cancer cell line Evsa-T, originally described as lacking both estrogen and progesterone receptors (ER and PR). In this report we show that PR of this cell line displays a binding affinity for [3H]ORG 2058 and a sucrose gradient sedimentation profile similar to those ascribed to PR from MCF-7 or T47D breast cancer cell lines. PR from Evsa-T cells is down-regulated by the progestin R-5020 as well as by the two antiprogestins, ZK 112.993 and ZK 98.299, but does not confer growth sensitivity to these compounds. ER remains undetectable by ligand binding assay, enzyme immunoassay and northern blotting. Our Evsa-T clone could be a valuable model for assessing the mechanisms leading the ER-/PR+ phenotype occurring occasionally in breast cancers and frequently in meningiomas.

Topics: Breast Neoplasms; Cell Division; Cytosol; Down-Regulation; Female; Gene Expression Regulation, Neoplastic; Gonanes; Hormone Antagonists; Humans; Mifepristone; Pregnenediones; Progestins; Receptors, Estrogen; Receptors, Progesterone; RNA, Messenger; RNA, Neoplasm; Tumor Cells, Cultured

In a human breast cancer cell line (ZR-PR-LT) we have found a poor overall correlation between affinity of progestins and anti-progestins for the progesterone receptor (PGR), concentration required for receptor down-regulation and anti-proliferative potency. Medroxyprogesterone acetate (MPA) and the anti-progestin RU 38.486, which possess glucocorticoid and antiglucocorticoid activity, respectively, cause receptor down-regulation at lower concentrations than their Kdi for [3H] ORG 2058 binding sites. In addition dexamethasone markedly down-regulates PGR at concentrations which fail to interact with PGR suggesting that heterospecific modulation of PGR occurs via the glucocorticoid receptor. In contrast the progestin ORG2058 and the anti-progestin ZK 98.299 caused 50% PGR down-regulation at a concentration (EC50) 50-fold higher than their Kdi values. ZK 112.993 was 500-fold more potent at PGR down-regulation than ZK 98.299 but had only a 5-fold higher affinity for PGR. Anti-proliferative concentrations of progestins/anti-progestins showing were generally higher than either Kdi values or EC50 values.

Topics: Binding, Competitive; Breast Neoplasms; Dexamethasone; Down-Regulation; Female; Gonanes; Humans; Medroxyprogesterone Acetate; Mifepristone; Mitosis; Pregnenediones; Progesterone Congeners; Progestins; Receptors, Glucocorticoid; Receptors, Progesterone; Tumor Cells, Cultured