The estrogen receptor (ER) is well characterized as a driver of proliferation and metastasis of luminal breast cancers (BC). Here we show that ER also promotes expression of the transcription factor C/EBPd (CEBPD) and that C/EBPd has tumor-suppressor activity in ER+ BC/cells and identifies ER+ cancers with better prognosis.
Transcriptome analyses showed that CEBPD expression is downregulated with malignant progression of BC but did not identify preferential expression in specific BC subtypes. Analysis of two independent breast tumor tissue microarrays revealed that the CEBPD protein is preferentially expressed in hormone receptor positivity BC, and that CEBPD expression was associated with longer patient survival.
Mechanistic studies using the MCF-7 and T47D cell lines showed that ER supports CEBPD expression at the level of the protein without affecting its mRNA expression. Gene silencing approaches showed that CEBPD attenuates cell proliferation, motility and invasiveness. Furthermore, CEBPD promotes estrogen-dependence and cytoxicity of tamoxifen. To elucidate the molecular mechanism(s) of CEBPD’s function, we determined the CEBPD-dependent transcriptome by mRNA-Seq analysis of CEBPD-silenced and control silenced MCF-7 cells. This approach revealed that CEBPD inhibits expression of the Slug (SNAI2) transcription factor, which can promote epithelial-mesenchymal transition (EMT). Silencing of Slug reverted the increased motility of CEBPD-silenced MCF-7 cells. Furthermore, CEBPD promotes expression and nuclear localization of the p21 tumor suppressor protein by promoting p53 protein expression and attenuating ERK activation. Tamoxifen-sensitivity of CEBPD-silenced MCF-7 cells could be partially restored by a constitutively nuclear p21 protein.
Taken together, our results indicate that CEBPD expression in ER+ BC contributes to a more benign tumor phenotype. We propose that by supporting CEBPD expression, ER activates a brake on its own tumor-promoting functions. Thus, we hypothesize that mutations or signals that prevent ER from supporting CEBPD’s expression may lead to a more malignant tumor phenotype and/or resistance to endocrine therapy.