Endocrine Abstracts

One in 11 women develop breast cancer before the age of 75, and mortality is significant. Estrogens and progesterone (PROG) are mediators of breast cancer aetiology and progression, and are targets for both diagnostics and therapy. The actions of estrogens via estrogen receptor alpha (ERα) have been extensively studied, but the genomic actions of PROG via the progesterone receptor (PGR) are still not fully understood. This study investigates the genomic actions of PGR in breast cancer cells treated with PROG with or without 17-β estradiol (E₂).

Microarray expression analysis in breast cancer ZR-75-1 cells identified virtually no effect of PROG-E₂ co-treatment on global transcription. However, priming with E₂ markedly increased transcriptional response to subsequent PROG treatment. Analysis of PGR binding sites by ChIP-seq supports these findings, with PROG treatment alone generating <10% of the number of PGR binding sites following priming with E₂ (444 vs 4566 at equivalent peak threshold). Compared to PROG alone, the primed PGR cistrome was highly conserved amongst species, and enriched for 3-keto steroid receptor response elements and binding sites for the pioneer factor, FOXA1. Candidate ChIP confirmed these findings in 100% (12/12) of sites. Importantly, E₂ enhancement of PGR binding could be virtually eliminated by tamoxifen co-priming, but not by short-term tamoxifen co-treatment. In T-47D cells that have naturally high PGR levels, E₂ priming was not required for efficient PGR/DNA interaction.

Our data suggests that the effective engagement of PGR in genomic activity is dependent on cellular PGR levels, which is modulated by E₂ activity, and steroid receptor collaborations such as FOXA1. Further, E₂ and PROG collaboratively shape a distinctive transcriptional response in breast cancer cells. These findings suggest a need to reevaluate the meaning of ERα and PGR levels in breast cancer, and better define the context of PGR contribution to breast cancer.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This work was supported however funding details unavailable.