BACKGROUND: 75% of breast cancers (BCa) are driven by the estrogen receptor α (ER+). Tumours lacking ER (ER-) are more aggressive and have the poorest prognosis. Although commonly associated with prostate cancer (PCa), the androgen receptor (AR) is also widely expressed in BCa, present in 90% of primary tumours. FOXA1 is a pioneer factor required for oncogenic AR signalling in PCa but its role in AR signaling in ER-BCa is not clear. We previously showed that cell growth is increased when FOXA1 is overexpressed in AR-driven PCa and BCa cell lines, suggesting that FOXA1 enhances their growth. To further address the role of FOXA1 in ER- BCa, we examined the consequence of FOXA1 loss on AR-chromatin interactions (cistrome) in a well characterized model of AR driven ER-BCa.
HYPOTHESIS: The AR cistrome is reprogrammed in the absence of FOXA1 in ER- BCa.
METHODOLOGIES: Genome-wide chromatin binding profiles for AR and specific AR interactors were performed using ChIP-seq in the ER- AR+ MDA-MB-453 BCa cell line. The AR protein interactome was interrogated using the SILAC-RIME proteomic technique.
RESULTS: Depletion of FOXA1 inhibited the growth of MDA-MB-453 cells, suggesting a requirement for FOXA1 to sustain cell growth. AR recruitment was increased at a large number of sites (73%) in the absence of FOXA1, suggesting that AR chromatin binding is reprogrammed in the absence of FOXA1. Proteomic analysis in the absence of FOXA1 revealed an increased interaction of AR with several proteins, including TFAP2A. Motif analysis indicated that the gained AR binding sites were enriched for TFAP2A binding motifs; co-IP analyses confirmed the interaction between AR and TFAP2A. Co-localisation of core TFAP2A, AR and FOXA1 binding events (20% overlap) was identified.
CONCLUSION: In the absence of FOXA1, the AR cistrome is reprogrammed in ER-AR+ MDA-MB-453 cells. The gained genomic AR binding sites appear to be dependent on a novel factor, TFAP2A, which could to be critical for AR signalling in the absence of FOXA1.