A study of the androgen mediated transcriptome and AR interactome in AI resistant breast cancer

Aromatase inhibitors (AI) are the vanguard treatment for hormone receptor positive, postmenopausal breast cancer, however, acquired resistance occurs in up to 40% of patients. AI therapies, by their mechanism of action, create a predominantly androgenic steroid environment. In this study, a multi-omic approach was undertaken to elucidate the role of the androgen receptor (AR) in AI resistant breast cancer. Evidence from this study suggests that cancer cells adapt to utilise the adrenal prohormone, androstenedione and/or its metabolites, to promote resistance and survival in AI resistance. AR protein levels are associated with better disease-free survival in a breast cancer tissue microarray (TMA), however, in an AI treated cohort this does not hold. This research has found AR

transcriptional activity, and serum levels of the steroid, androstenedione, to be elevated in patients who recur on AI therapy. RNA sequencing of AI resistant cells (LetR) identified an AR mediated gene signature arising from

the altered steroid environment associated with AI therapy resistance. This gene signature is associated with cells that have adapted to utilise androstenedione and/or its metabolites as demonstrated by transcriptional attenuation when cells are re-exposed to estradiol. LetR cell growth is AR dependent as a reduction in viability is observed with the anti-AR therapy enzalutamide. Furthermore, enzalutamide abrogates colony formation, mammosphere generation and the self-renewal capabilities of LetR cells,

which is not observed in sensitive cell counterparts. LC–MS/MS analysis identified androgen-mediated AR protein interactors unique to the AI resistant model. Further analysis of these identified a differential protein interaction network in AI resistant and sensitive cells. As breast cancer is a steroid sensitive disease, this study provides important insight into understanding the impact of alterations in the steroid microenvironment. Furthermore, the AR-mediated gene signature and AR protein partners identified in this study provide novel insight into the

dichotomous role of AR in breast cancer and could be used to better identify patients who could benefit from anti-AR therapy.