Heat Shock Factor 1 (HSF1) regulates ESR1 action in breast cancer

Heat shock factor 1 (HSF1) is a key regulator of transcriptional responses to proteotoxic stress. It has been recently linked to signaling of estrogen via ESR1. To study the cooperation of HSF1 and ESR1 in the transcriptional response to estrogen, we established estrogen receptor (ER)-positive breast cancer cell lines with reduced HSF1 levels using specific shRNA or CRISPR/Cas9 approach. HSF1 deficiency led to the inhibition of the mitogenic effect of estrogen in MCF7 and T47D cells. RNA-seq analyses revealed that the stimulatory effect of E2 on the transcriptome was smaller in HSF1-deficient MCF7 cells. This could partially result from the higher basal expression of E2-dependent genes in these cells as a consequence of the enhanced binding of unliganded ESR1 to chromatin, which was revealed by ChIP-seq analyses. Thus, we postulate that some fraction of ESR1 could be released from the inhibitory complex with HSP90 and gain transcriptional competence without E2-stimulation. Overall design: MCF7 cells with different status of HSF1 (HSF1+/+ and HSF1-/-) were untreated or treated with 10 nM E2 for 4 hour. Then total RNA was isolated from each sample using the Direct-ZolTM RNA MiniPrep Kit (Zymo Research), digested with DNase I (Worthington Biochemical Corporation) and next-generation sequenced using the Illumina protocol. For each experimental point, three independent biological repeats were done including RNA isolation and sequencing.