GSE126517
PR attentuation of interferon signaling
Why some tumors remain indolent and others progress to clinical relevance remains a major unanswered question in cancer biology. Interferon signaling in nascent tumors, mediated by STAT1, is a critical step through which the surveilling immune system can recognize and destroy developing tumors. Herein, we have identified an interaction between the progesterone receptor (PR) and STAT1 in breast cancer cells. This interaction inhibited efficient interferon-induced STAT1 phosphorylation, as we observed a decrease in phospho-STAT1 in response to interferon treatment in PR-positive breast cancer cell lines. This phenotype was further potentiated in the presence of PR ligand. In human breast cancer samples, PR-positive tumors exhibited lower levels of phospho-STAT1 as compared to their PR-negative counterparts, indicating that this phenotype translates to human tumors. Breast cancer cells lacking PR exhibited higher levels of interferon-stimulated gene (ISG) RNA, the transcriptional endpoint of interferon activation, indicating that unliganded PR alone could decrease transcription of ISGs. Moreover, the absence of PR led to increased recruitment of STAT1, STAT2 and IRF9 (key transcription factors necessary for ISG transcription) to ISG promoters. These data indicate that PR, both in the presence and absence of ligand, attenuates interferon-induced STAT1 signaling, culminating in significantly abrogated activation of genes transcribed in response to interferons. PR-positive tumors may use downregulation of STAT1-mediated interferon signaling to escape immune surveillance, leading to the development of clinically relevant tumors. Selective immune evasion of PR-positive tumors may be one explanation as to why over 65% of breast cancers are PR-positive at the time of diagnosis. Overall design: 2 different cell lines (T47D-Y [PR null] and T47D-co [PR-positive]) were treated +/- interferon and R5020 (PR ligand), in biologic triplicate. There are a total of 8 groups, 3 replicates each, making for 24 samples total