Abstract P6-04-16: Glucocorticoid receptor signaling elicits anti-cancer effects in endocrine resistant breast cancer via induction of AZGP1

Despite the availability of effective drugs that target ERα-positive breast cancer, ERα-positive disease still accounts for more breast cancer related deaths than any other subtype. Relapse in these patients is largely due to the development of resistance to anti-estrogen therapies such as tamoxifen. While tamoxifen and its resistance mechanisms have been extensively studied, relatively little is known about its active metabolite endoxifen. Our group has provided evidence that endoxifen is a potent and clinically relevant metabolite of tamoxifen. Therefore, characterization of endoxifen resistance mechanisms may be crucial to understanding tamoxifen resistance. We have developed novel endoxifen resistant MCF7 and T47D cell lines through chronic exposure to endoxifen over a period of 12-24 months. Using these models and their respective controls, we compared global gene expression profiles of endoxifen resistant cells to tamoxifen resistant cells and found marked differences between the two models. We subjected endoxifen resistant cells to a genome-wide, CRISPR-mediated knockout screen to identify genes involved in mediating endoxifen resistance. Subsequent analyses revealed that disruption of genes regulated by the glucocorticoid receptor (GR) enhanced cells’ ability to survive and proliferate in the presence of endoxifen, suggesting that GR signaling blocks growth of resistant cells. We provide evidence that GR activation via dexamethasone significantly inhibits the proliferation rates of endoxifen resistant cells by 50-60%, with little to no inhibitory effects in endoxifen sensitive models. Follow-up studies suggested that alpha zinc-glycoprotein 1 (AZGP1), a secreted lipolytic enzyme induced by GR, plays a role in mediating this phenotype. AZGP1 RNA and protein are highly expressed in normal breast tissue, but corresponding levels are lower in ERα-positive breast malignancies. In these tumors, low AZGP1 expression correlates with worse prognosis. Additionally, high expression of AZGP1 is associated with improved outcomes in patients with prostate, pancreatic, gastric and breast cancer. AZGP1 is expressed in endocrine sensitive MCF7 and T47D cells, but is silenced in endoxifen resistant models at the mRNA and protein level. Further, treatment of resistant cells with conditioned medium containing AZGP1 resulted in significant inhibition of cell proliferation and migration, and antibody-mediated depletion of AZGP1 from conditioned medium completely reversed these effects. While there are no reports regarding the molecular functions of AZGP1 in cancer we provide evidence that AZGP1 directly inhibits oncogenic signaling in endocrine resistant breast cancer through interaction with, and inhibition of, β6 integrin. Through the analysis of endoxifen resistant breast cancer cells, we have identified novel roles for GR, AZGP1 and β6 integrin in the development and progression of endocrine resistant breast cancer. We demonstrate that activation of GR may elicit therapeutic benefit in endocrine resistant breast cancer, specifically in tumors expressing AZGP1 and β6 integrin. Further, AZGP1 and β6 integrin may represent novel prognostic and/or predictive biomarkers in endocrine resistant disease. Citation Format: Calley J. Jones, Santiago Acero Bedoya, Anna V. Mykytyn, Matthew P. Goetz, John R. Hawse. Glucocorticoid receptor signaling elicits anti-cancer effects in endocrine resistant breast cancer via induction of AZGP1 [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-04-16.