DIFFERENTIAL EFFECT OF FORM-A AND FORM-B OF HUMAN PROGESTERONE-RECEPTOR ON ESTRADIOL-DEPENDENT TRANSCRIPTION

In addition to stimulation of the target gene fatty-acid synthetase, t he synthetic progestin R5020 strongly inhibited estradiol-induced pS2 and cathepsin D mRNA levels in MCF7 human breast cancer cells as shown by Northern blot analysis. Inhibition was half-maximal with 30 pM R50 20, and the antiprogestin RU486 had only a weak effect. Two human prog esterone receptor isoforms have been described; isoform A is a truncat ed form of isoform B and lacks the 164 N-terminal amino acids. We hypo thesized that the two isoforms could have a differential capacity to t ransrepress estrogen-induced responses. Therefore, in MDA-MB231 cells containing no progesterone and estrogen receptors, we transiently tran sfected progesterone receptor expression vectors coding for form B (hP R1 or hPR0) or form A (hPR2) along with the estrogen receptor expressi on vector HEO. We show that R5020 inhibited estradiol-induced transcri ption of the pS2-CAT reporter plasmid only in cells selectively expres sing isoform B. The same results were obtained when progesterone recep tor isoforms were overexpressed in MCF7, Ishikawa, HeLa, or NIH-3T3 ce lls. Transrepression was dependent on the promoter context since the e xtent of inhibition by isoform B was higher when evaluated with pS2 or cathepsin D nonpalindromic estrogen-responsive element-mediated trans cription than with the perfect palindromic form of the vitellogenin ge ne. Isoform A was inefficient regardless of the reporter construct use d. Inhibition varied with the isoform ratio, and isoform B had a domin ant effect, with >70% inhibition measured in cells transfected with th e same amount of both progesterone receptor iso forms. Progestin repre ssed only one of the two transcription activation functions of the est rogen receptor, AF-2, which corresponds to the hormone-binding domain. We conclude that differential expression of progesterone receptor iso forms could be responsible for a tissue-specific inhibition of estroge n target genes by progestins.