Direct transcriptional regulation of the progesterone receptor by retinoic acid diminishes progestin responsiveness in the breast cancer cell line T-47D.

Retinoic acid (RA) treatment of T-47D human breast cancer cells results in a rapid decrease in the concentration of progesterone receptor (PR) mRNA which causes a slow loss of cellular PR protein (Clarke, C. L., Roman, S. D., Graham, J., Koga, M., Sutherland, R. L. (1990) J. Biol. Chem. 265, 12694-12700). The mechanisms involved are unknown and this study was undertaken to determine whether the decline in PR mRNA was due to transcriptional inhibition and to evaluate the functional consequences of the RA-mediated decrease in PR. The transcription rate of the PR gene was decreased by RA, and the effect was maximal 2-3 h after treatment. Cycloheximide cotreatment was unable to relieve the inhibitory effect of RA and PR transcription suggesting that the effect was not dependent on ongoing protein synthesis. There was no effect of RA on PR mRNA half-life at the times examined (0-6 h of RA treatment). To determine the functional consequence of PR down-regulation the progestin-responsive plasmid pMSG-CAT was expressed transiently in T-47D cells which were then exposed to RA for 24 h. RA-pretreated cells were then treated with the synthetic progestin ORG 2058 and the extent of progestin stimulation of chloramphenicol acetyltransferase (CAT) activity measured. ORG 2058 treatment resulted in an induction of CAT activity which was maximal at a progestin concentration of 1 nM. Interestingly, the ability of ORG 2058 to induce CAT activity was decreased in RA-pretreated cells. The diminished progestin responsiveness of RA-pretreated cells was confirmed in separate experiments which showed that the progestin inducibility of TGF-alpha mRNA was also decreased in cells treated with ORG 2058 following pretreatment with RA for 24 h. These data demonstrate that RA decreases PR mRNA concentrations by direct transcriptional inhibition, leading to decreased cellular PR concentrations. The decreased levels of PR result in impaired responsiveness to progestins and this suggests that RA derived from dietary vitamin A may have a role in modulating cellular sensitivity to progestins.