Why some
tumors remain indolent and others progress to clinical relevance remains a major unanswered question in
cancer biology. IFN signaling in nascent
tumors, mediated by STAT1, is a critical step through which the surveilling immune system can recognize and destroy developing
tumors. In this study, we have identified an interaction between the
progesterone receptor (PR) and STAT1 in
breast cancer cells. This interaction inhibited efficient IFN-induced STAT1 phosphorylation, as we observed a decrease in phospho-STAT1 in response to IFN 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 with their PR-negative counterparts, indicating that this phenotype translates to human
tumors.
Breast cancer cells lacking PR exhibited higher levels of IFN-stimulated gene (ISG)
RNA, the transcriptional end point of IFN 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 IFN-induced STAT1 signaling, culminating in significantly abrogated activation of genes transcribed in response to IFNs. PR-positive
tumors may use downregulation of STAT1-mediated IFN 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.