ORF24-04 - Mineralocorticoid and Glucocorticoid Receptor Cooperation Drives Triple-Negative Breast Cancer Progression to Metastasis

Triple-negative breast cancer (TNBC) is a subtype that is characterized by the lack of expression of estrogen, progesterone and HER2 receptors and is associated with aggressive behavior, a high risk of distant metastasis, and poor survival outcomes. Our lab previously showed that glucocorticoid receptors (GR) mediate the convergence of host and cellular stress signaling to orchestrate an array of advanced cancer phenotypes associated with TNBC. In TNBC, p38 MAPK phosphorylates GR Ser134 in response to cytokines including TGFβ1. Phospho-Ser134-GR (pSer134-GR) regulates genes promoting migration, invasion, and altered metabolism. Glucocorticoids also activate closely-related mineralocorticoid receptors (MR), whose natural ligand aldosterone (aldo) mediates cardiac and vascular conditions such as hypertension, inflammation and fibrosis. While our prior studies have implicated GR/p-GR in TNBC progression to metastasis, the contribution of MR remains unknown. Analysis of the METABRIC dataset revealed significantly higher MR transcript levels in TNBC relative to luminal subtypes, with elevated MR expression predicting worse overall survival. We observed pSer134-GR- and p38-dependent rapid association of cytoplasmic MR-p-GR complexes upon treatment of TNBC cells with TGFβ1, while nuclear MR-GR complexes predominated in response to dexamethasone (dex) and/or aldo. Cytoplasmic MR-p-GR complexes translocated to the nucleus within 4 hours. MR knockdown or inhibition with MR-selective antagonists (spironolactone, finerenone) dramatically reduced aldo- or TGFβ1-induced migratory and stemness properties in TNBC models. MR-deficient TNBC models exhibited reduced migration, attenuated stem cell expansion, and greatly reduced lung metastasis by tail-vein injection, phenocopying TNBC cells harboring phospho-mutant S134A-GR. MR activation by aldo or TGFβ1 upregulated canonical MR-target genes (SGK1, ENaC1) and non-canonical fibrosis-related genes (COL1A1, ICAM1 and CTGF). Moreover, MR expression was essential for functional p38 MAPK/p-Ser134-GR signaling downstream of TGFβ1 receptor activation. Our studies define a novel activity of MR-p-GR complexes in regulation of TNBC cell migration, stemness properties, and metastasis. Pharmacological inhibition of MR with FDA-approved MR antagonists offers an exciting and rapidly translatable opportunity for improved clinical management of TNBC patients.