Transactivation of progestin- and estrogen-responsive promoters by 19-nor progestins in African Green Monkey Kidney CV1 cells.

New and more potent progestins and antiprogestins suitable for reproductive therapy and contraception are currently the target of intensive research. The design of such drugs has been hampered by the complex technology required for screening these compounds at the molecular level. To solve this problem, we developed an in vitro cell system that allows detection of the progestagenic effects of a given compound using a PRE2-TATA-CAT reporter vector transiently introduced in a cell line stably transfected with the rabbit progesterone receptor (PR). The African Green Monkey Kidney CV1 (AGMK-CV1) cell line was chosen because these cells do not express endogenous steroid receptors; the selected clone stably expressing the rabbit PR has been maintained in our laboratory for more than 2 yr without detectable losses in PR content and progestagenic response. The presence and function of the PR were assessed by immunohistochemical and saturation analyses as well as by monitoring transactivation of the PRE2-TATA-CAT reporter gene. In this cell line, the PR is expressed at a concentration of 0.170 fmol/mg of protein, and the receptor is localized within the cell nucleus in either the presence or absence of the potent synthetic progestin R5020. This PR-expressing cell system allowed study of the in vitro progestational activity of several 19-nor progestins. The antiprogestin RU486 inhibited CAT activity induced by R5020; norethisterone (NET), levonorgestrel (LNG), and gestodene (GSD) induced PRE2-TATA-CAT activity at concentrations similar to those of R5020, whereas NET A-ring-reduced metabolites induced CAT activity at an extent lower than (5alpha-NET) or similar (3beta,5alpha-NET) to that of the precursor compound. The PRE2-TATA-CAT induction by 17beta-estradiol was also analyzed and no crossreactivity was detected. However, when the ERE-VitA2-TK-CAT (estrogen-responsive element-vitellogenin A2-thymidine kinase promoter-CAT) reporter vector and the estradiol receptor alpha or beta were cotransfected, CAT activity was induced in the presence of 17beta-estradiol, and NET tetrahydro-reduced derivatives. The results indicate that this AGMK-CV1-PR cell assay system appears to be suitable for measuring the effects of different synthetic progestins at the transcriptional level. In this assay system, NET, LNG, and GSD exhibit potent progestational effects at the transcriptional level. In the particular case of NET, the assay system allowed us to determine that the single or multiple hormonal transcriptional effects of this compound are partially mediated by its A-ring-reduced derivatives.