Unliganded Estrogen Receptor Beta cooperates with Argonaute 2 in the modulation of transcription rate and splicing of target genes in breast cancer cells. Part 4

Estrogen receptors (ERs) are primary targets for chemoprevention and endocrine therapy of breast cancer (BC) and provide prognostic and predictive information concerning tumor response to endocrine therapies. Expression of ERß reduces cancer cell proliferation and tumor growth, pointing to an anti-proliferative action of this transcription factor and its positive prognostic value. Moreover, it has emerged that unliganded ERß exhibits an active role in constitutive regulation of target genes transcription. Indeed, gene expression profiling of hormone-deprived human BC cells expressing ERß revealed a great impact of this ER subtype in the modulation of gene expression in absence of hormonal ligands. Starting from this observation, and to identify key nuclear factors acting in concert with ligand-free ERß, Tandem Affinity Purification (TAP) coupled to mass spectrometry (MS) was applied to isolate nuclear protein complexes associated to unliganded ERß in BC cells. Among the >300 proteins identified, we focused our attention on the functional significance of a complex comprising ERß and AGO2, since this argonaute protein has been implicated in key biological processes in the cell nucleus. ERß association with AGO2 and other known AGO2 interacting proteins was confirmed by co-immunoprecipitation in MCF-7 cells expressing tagged ERß. ChIP-Seq allowed the identification of a number of ERß- and AGO-binding sites to breast cancer cells genome, while AGO2 knock down resulted in significant changes in transcription rate and co-transcriptional mRNA splicing of a group of genes, comprising in particular a significant number of those modulated by unliganded ERß. In conclusion, these experimental evidences suggest that AGO2 is a functional partner of ERß involved in hormone-independent gene regulation in BC cells