Abstract
Polycomb repressive complexes 1 and 2 (PRC1 and PRC2) are essential regulators of stem cell pluripotency, lineage commitment, and development. Increasing amount of evidence suggests that PRC1 and PRC2 are strongly implicated in cancer, where subunits of both complexes behave in non-canonical ways to promote the expression of cancer-driving genes. Previous studies in our lab has shown that RING1B, the E3-ligase subunit of PRC1, colocalizes with ERα at active regions of the genome such as enhancers and superenhancers to regulate the oncogenic transcriptional program driving the growth of ER+ breast cancer cells. Nevertheless, the molecular mechanisms underlying the functional interaction between RING1B and ERα is not clear. Further, how RING1B is recruited to enhancers, superenhancers, and promoters of active-transcribed genes is completely unknown.
We first discovered that RING1B is dynamically recruited to the chromatin within 8 to 12bps of ERα binding sites upon E2 stimulation. Furthermore, RING1B and ERα are mutually dependent on each other for recruitment to the chromatin, and RING1B is also dependent on chromatin-associated RNA for binding to RING1B-ERα co-target sites. Upon acute estrogen stimulation, RING1B is recruited to ERα target genes to directly participate in their transcription via mechanisms such as orchestrating enhancer-promoting looping and promoting E2-induced enhancer and superenhancer establishment. Thus, RING1B depletion severely attenuates the expression of RING1B-ERα co-target genes, decreases E2-induced R-loop formation, and abrogates E2-induced proliferation and clonogenic abilities of ER+ breast cancer cells. Although RING1B orchestrate E2-induced genome-wide chromatin accessibility changes, we found that changes in chromatin accessibility are not necessarily correlated with 3D chromatin architectural changes and transcriptional activation. Overall, our data suggest that the Polycomb protein RING1B is a key regulator of the E2-induced transcriptional activities in ER+ breast cancer.