Role of the GATA3 Transcription Factor on AR Signalling in Breast Cancer

Abstract

Almost 80 % of all breast cancers (BCs) are dependent on the estrogen receptor alpha (ER) for their growth (termed ER+ BC), while the rest are negative for the expression of ER (ER- BC) (~ 20-25 %). Most ER+ BCs (more than 90 %) and around 50 % of ER- BCs express the androgen receptor (AR). AR is an important BC biomarker, with prognostic and therapeutic potential. AR is a tumor-suppressor in ER+ BC, where it inhibits estrogen-stimulated growth of tumours through down-regulation of key ER-regulated cell cycle genes and activation of good outcome genes. AR has been suggested to play a role in promoting a basal to luminal lineage transition in normal mouse mammary epithelial cells. Despite the role of AR in ER- BCs being controversial, AR positivity has been shown to be associated with improved disease-free survival and more benign clinical and pathologic factors (e.g. lower tumor grade and smaller tumor size) in ER- BCs. In the recent past, the GATA3 transcription factor (TF) has been characterized as an important regulator of ER signalling and mediates normal mammary gland development and cell lineage determination. GATA3 is expressed exclusively in the luminal epithelial cell population, plays an essential role in mammary development and specification, and actively maintains the luminal epithelial differentiation in the adult mammary gland. To date, the role of GATA3 in AR signalling in the context of BC, in the presence or absence of ER, has not been investigated. Therefore, the aim of this thesis was to further investigate and characterise the cross-talk between AR and GATA3 in the presence or absence of ER, using a variety of cell-line models, clinical samples and where possible, patient-derived xenografts (PDXs). I first investigated the functional interplay between GATA3 and AR in inducing the luminal epithelial lineage in breast cancer cells regardless of ER expression. Since the inhibitory role of AR in ER+ BCs is well established, I then investigated whether GATA3 is involved in AR-induced growth inhibition in ER+ BC models in response to ER and/or AR activation. Findings of this PhD thesis showed: • GATA3 is a novel AR interacting protein independent of ER expression in normal mammary tissues and different BC subtypes. • Stimulation of BC cells with estrogen or androgen hormones reprograms the GATA3 cistrome in ER+ and ER- BC cell lines and ER+ PDX models. • GATA3 and AR co-regulate the expression of essential luminal epithelial markers in both ER+ and ER- cell lines and ER+ PDXs, indicating a key collaborative role for GATA3 and AR in luminal epithelial differentiation of the breast cells regardless of the ER expression. • AR requires the presence of GATA3 at genomic loci associated with AR-mediated growth inhibition in ER+ BC to inhibit the E2-stimulated growth in these models. Collectively, the generated data from this thesis suggest a cooperative role for AR and GATA3 TFs in suppressing the tumour growth of ER+ BCs and in driving the luminal-lineage identity in mammary glands and BC contexts. Also, the findings of this thesis provide novel insight into the cross-talk between ER, GATA3, and AR in BC, highlight the signalling complexity of TFs in this disease and provide the basis for further investigations into AR and GATA3 co-operative genomic activity and the direct consequences of this for BC progression and differentiation.