Binding of the pioneer factor FOXA1 is independent of hormonal signalling

We hormone deprived MCF-7 and ZR75-1 breast cancer cells for three days and treated them with vehicle (ethanol) or estrogen for 45 minutes. We then performed FOXA1 ChIP-seq and showed that the vast majority (>99%) of FOXA1 binding events are not affected by steroid conditions. A small number (<1%) of FOXA1 binding sites appear to be induced by estrogen, but these are not genuine de novo binding sites and represent ‘shadow’ binding sites that result from chromatin interactions at super-enhancer containing estrogen-regulated genomic regions. FOXA1 is therefore not regulated by estrogen and remains a bone fide therapeutic target that is entirely upstream of the ER complex. FOXA1 ChIPseq in hormone-deprived MCF-7 and ZR751 cells, treated with vehicle or estrogen for 45 minutes.