Abstract
Introduction AI’s are the front line treatment for endocrine postmenopausal breast cancer. However, response rates vary from 35%–70% and recurrence occurs in almost all advanced disease. In order to identify a target for overcoming AI treatment resistance, research has turned to the most widely expressed sex steroid receptor in breast cancer the AR. AI therapies, by their mechanism of action, create an unopposed androgenic steroid environment. 4AD is the major circulating steroid in postmenopausal women and AI therapy prevents it’s conversion to estrogens. We hypothesise that cancer cells adapt to utilise 4AD to promote resistance and survival in AI resistance. Material and methods Molecular changes in a breast cancer cell model which have transitioned from AI sensitive (MCF7 cells) to AI resistant (LetR cells) were assessed using RNA sequencing. Alterations in the steroid microenvironment occur as a result of AI therapy and responses to these steroids was confirmed using MTS, mammosphere and invasion assays. Mass spectrometry (LC–MS/MS) analysis identified androgen-mediated, AR protein interactions.The clinical relevance of AR in AI resistance was also evaluated in a cohort of primary breast cancer patients (n=375). Results and discussions LetR cell growth is AR dependent as the anti-AR drug Enzalutamide inhibits cell growth. Furthermore, Enzalutamide abrogates mammosphere formation and self-renewal of LetR cells. We have found LetR cells to be responsive to 4AD, and treatment with this steroid drives a more aggressive phenotype in vitro. RNA sequencing data highlighted a non-canonical cytoplasmic AR signalling mechanism. Our research suggests that 4AD initiates rapid second messenger signalling in the AI resistant setting. LC–MS/MS analysis identified androgen-mediated, AR protein interactors unique to our resistant model particularly associated with cell-cell adhesion and invasion. Evaluation of AR protein in a cohort of primary breast cancer patients found AR expression in 90% of patients, with 25% expressing high intensity AR staining. Analysis is ongoing into this subset and its association with clinical pathological parameters. Conclusion AR and its ligand 4AD may be a significant driving factor in AI resistance and may play a role in facilitating a more invasive phenotype. Further investigations into AR interactors identified will help elucidate mechanisms of resistance to AI therapy, and these novel AR protein partners may aid the identification of patients who would benefit from anti-AR therapy.
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