Altered Steroid Milieu in AI-Resistant Breast Cancer Facilitates AR Mediated Gene-Expression Associated with Poor Response to Therapy

Abstract

Divergent roles for androgen receptor (AR) in breast cancer have been reported. Following aromatase inhibitor (AI) treatment, the conversion of circulating androgens into estrogens can be diminished by >99%. We wished to establish whether the steroid environment can dictate the role of AR and the implications of this for subsequent therapy. This study utilizes models of AI resistance to explore responsiveness to PI3K/ mTOR and anti-AR therapy when cells are exposed to unconverted weak androgens. Transcriptomic alterations driven by androstenedione (4AD) were assessed by RNA-sequencing. AR and estrogen receptor (ER) recruitment to target gene promoters was evaluated using ChIP, and relevance to patient profiles was performed using publicly available data sets. Although BEZ235 showed decreased viability across AI-sensitive and -resistant cell lines, anti-AR treatment elicited a decrease in cell viability only in the AI-resistant model. Serum and glucocorticoid-regulated kinase 3 (SGK3) and cAMP-dependent protein kinase inhibitor b (PKIB) were confirmed to be regulated by 4AD and shown to be mediated by AR; crucially, reexposure to estradiol suppressed expression of these genes. Metaanalysis of transcript levels showed high expression of SGK3 and PKIB to be associated with poor response to endocrine therapy (HR ¼ 2.551, P ¼ 0.003). Furthermore, this study found levels of SGK3 to be sustained in patients who do not respond to AI therapy. This study highlights the importance of the tumor steroid environment. SGK3 and PKIB are associated with poor response to endocrine therapy and could have utility in tailoring therapeutic approaches.

Pre-Menopausal Women With Breast Cancers Having High AR/ER Ratios in the Context of Higher Circulating Testosterone Tend to Have Poorer Outcomes

“…Previous studies in postmenopausal breast cancer women have shown significantly high levels of circulating testosterone than the normal controls (37) and further showed the association of high testosterone levels with worse prognosis in ER positive post-menopausal women (38). Regulation of AR depends on the hormonal milieu, and it is hypothesized that the discordance between AR and ER based signaling may be regulated by relative availability of each receptor (39). Testosterone is a precursor for estrogens and is converted by aromatase to either estradiol or 5adihydrotestosterone (DHT) by the enzyme 5a reductase in the tumor microenvironment (40).…”

Androgen Receptor Is a Non-canonical Inhibitor of Wild-Type and Mutant Estrogen Receptors in Hormone Receptor-Positive Breast Cancers

“…Recent preclinical and clinical studies indicate that AR could be a growth promoter in tamoxifen-resistant breast cancer, but a growth inhibitor in tamoxifen-sensitive breast cancer. Although AR antagonist enzalutamide increased proliferation of parental breast cancer cell line MCF-7, it inhibited proliferation of tamoxifen-resistant MCF-7 cells (Creevey et al., 2019). Similarly, tamoxifen-resistant clinical specimens that had higher AR:ER ratio had aggressive disease and poor prognosis (Cao et al., 2019).…”

Steroid Ligands, the Forgotten Triggers of Nuclear Receptor Action; Implications for Acquired Resistance to Endocrine Therapy

“…All ER signaling markers were significantly decreased from baseline to 4þmonths AI therapy (ESR1 P < 0.0001, FOXA1 P < 0.0001, GATA3 P < 0.001, IL6ST P < 0.001), whereas the breast cancer-specific AR signaling markers were increased or unchanged (GHR P < 0.01, PTGER3 P < 0.01, ADRA2A n.s, AR n.s). Using our previously published 4AD-regulated gene set (7), we further found that PKIB (P < 0.01) and SYTL4 (P < 0.01) were elevated in patients whose tumors developed acquired resistance compared with dormant tumors (Fig. 4C).…”

“…5). In light of this, there is now renewed focus on alternate steroid facilitators of breast cancer progression (6,7), nongenomic steroid action (8) and also members of the nuclear receptor subfamily 3C, which includes the androgen receptor (AR; refs. [9][10][11].…”