INHIBITION OF GENE-EXPRESSION BY STEROID-HORMONE RECEPTORS VIA A NEGATIVE GLUCOCORTICOID RESPONSE ELEMENT - EVIDENCE FOR THE INVOLVEMENT OFDNA-BINDING AND AGONISTIC EFFECTS OF THE ANTIGLUCOCORTICOID ANTIPROGESTIN RU486

We have used a negative glucocorticoid response element (nGRE) from th e bovine prolactin promoter linked to the gene for chloramphenicol ace tyltransferase (PRL3CAT) to study the inhibition of gene expression by steroid hormone receptors. This nGRE increased basal expression from a heterologous promoter in COS-7 cells. In the presence of cotransfect ed glucocorticoid (GR), androgen, or progesterone receptor (PR) expres sion vectors and their cognate ligands, the expression of PRL3CAT coul d be repressed, indicating that these steroid receptor subfamily membe rs could function through the same negative response element. No repre ssion was observed with the estrogen receptor, showing that the repres sive effect was specific for members of the GR-subfamily. Mutation of three amino acids within the GR-DNA binding domain that determine the specificity of GR-GRE interaction abolished the ability of the GR to i nhibit the expression of PRL3CAT, demonstrating the requirement for DN A binding of the GR in the mechanism of repression. The antiglucocorti coid/antiprogestin RU486 when bound to PR or GR also repressed the exp ression of the PRL3CAT, but higher concentrations of RU486 were requir ed to obtain an effect with the GR when compared to the PR. RU486 was unable to antagonize the effect of progestins on PRL3CAT and only part ially antagonized the glucocorticoid repression. Thus, regarding the r epression of PRL3CAT, RU486 acted as an agonist when bound to the PR a nd as a partial agonist when bound to the GR.