SRP125813 PRJNA420212 GSE107476 Transition of breast cancer cells from luminal to basal-like phenotype engages a shift from genomic to non-genomic activities of estrogen receptor-alpha activity Estrogen receptor (ERa) is central in driving the development of hormone-dependent breast cancers. A major challenge in treating these cancers is to understand and struggle endocrine resistance. We have previously shown that the Megakaryoblastic Leukemia 1 (MKL1) protein, a master regulator of actin dynamic and cellular motile functions, directs down-regulation of ERa and hormonal escape of estrogen-responsive breast cancer cell lines. In the present study, we decipher the underlining mechanisms by tracking functional changes in ERa activity. We demonstrate through gene expression microarray analysis that the constitutive activation of MKL1 in ERa-positive MCF7 breast cancer cells induces a transition from a luminal to a basal-like phenotype and suppresses almost all regulations of gene expression by estrogen. Chromatin immunoprecipitation of DNA coupled to high-throughput sequencing (ChIP-Seq) shows a profound reprogramming in ERa cistrome associated with a massive loss of ERa binding sites (ERBSs) generally associated with lower ERa-binding activity. Novel ERBSs appear to be associated with EGF and RAS signaling pathways. ERa dynamic was further impacted by a displacement of its monomer/dimer equilibrium towards its monomer state, associated with a redistribution of the normally nuclear ERa protein to the entire cell volume. We next show that the combination of these remodeled dynamics of ERa alters its interactions with genomic and non-genomic partners. In particular, the formation of ERa/kinase complexes was promoted by the constitutive activation of MKL1. Hence, MKL1-induced transition of ER-positive breast cancer cells from luminal to basal-like phenotype shifts ERa activities from genomic to non-genomic function. Overall design: Whole-genome analysis of estrogen receptor (ER) binding events in MCF-7 cells ectopically expressing a dominant form of the MKL1 protein and associated control cells. GSE107476 pubmed 32113984 pubmed 33894309