Breast cancer prognosis and response to endocrine therapy strongly depends on the expression of the estrogen and progesterone receptors (ER and PR, respectively). Although much is known about ERα gene (ESR1) regulation after hormonal stimulation, how it is regulated in hormone-free condition is not fully understood. We used ER-/PR-positive breast cancer cells to investigate the role of PR in ESR1 regulation in the absence of hormones. We show that PR binds to the low-methylated ESR1 promoter and maintains both gene expression and DNA methylation of the ESR1 locus in hormone-deprived breast cancer cells. Depletion of PR reduces ESR1 expression, with a concomitant increase in gene promoter methylation. The high amount of methylation in the ESR1 promoter of PR-depleted cells persists after the stable re-expression of PR and inhibits PR binding to this genomic region. As a consequence, the rescue of PR expression in PR-depleted cells is insufficient to restore ESR1 expression. Consistently, DNA methylation impedes PR binding to consensus progesterone responsive elements. These findings contribute to understanding the complex crosstalk between PR and ER and suggest that the analysis of ESR1 promoter methylation in breast cancer cells can help to design more appropriate targeted therapies for breast cancer patients.

We received funding from the Spanish Ministry of Economy and Competitiveness, Plan Nacional Project SAF 2013-42497-P; Centro de Excelencia Severo Ochoa 2013–2017; the Centre de Recerca de Catalunya (CERCA) Programme/Generalitat de Catalunya; G.V. has received funding from the Spanish Ministry of Economy and Competitiveness, “Juan de la Cierva Incorporation” fellowship (Ref. IJCI-2014-20723), the European Union Seventh Framework Programme (FP7/2007-2013) under Grant Agreement Number 299429 and the European Molecular Biology Organization (EMBO long-term fellowship ALTF 1106-2011, cofunded with the European Commission EMBOCOFUND2010, GA-2010-267146).