Breast cancer suppression by progesterone receptors is mediated by their modulation of estrogen receptors and RNA polymerase III

Greater than 50% of estrogen receptor (ER)-positive breast cancers co-express the progesterone receptor (PR), which can directly and globally modify ER action to attenuate tumor growth. However, whether this attenuation is mediated only through PR-ER interaction remains unknown. To address this question, we assessed tumor growth in ER/PR-positive PDX models of breast cancer where both natural and synthetic progestins were found to antagonize the mitogenic effects of estrogens. Probing the genome-wide mechanisms by which this occurs, we documented that chronic progestin treatment blunted ER-mediated gene expression up to 2-fold at the level of mRNA transcripts. Unexpectedly, <25% of all ER DNA binding events were affected by the same treatment. The PR cistrome displayed a bimodal distribution. In one group, >50% of PR binding sites were co-occupied by ER, with a propensity for both receptors to coordinately gain or lose binding in the presence of progesterone. In the second group, PR but not ER was associated with a large fraction of RNA polymerase III (Pol III)-transcribed tRNA genes, independent of hormone treatment. Notably, we discovered that PR physically associated with the Pol III holoenzyme. Select pre-tRNA and mature tRNA that colocalized with PR and POLR3A at their promoters were relatively decreased in estrogen+progestin treated tumors. Our results illuminate how PR may indirectly impede ER action by reducing the bioavailability of translational molecules needed for tumor growth.

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