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Glucocorticoid Receptors are Required Effectors of TGFb1-Induced p38 MAPK Signaling to Advanced Cancer Phenotypes in Triple Negative Breast Cancer

ABSTRACT: Altered signaling pathways typify breast cancer and serve as direct inputs to steroid hormone receptor sensors. We previously reported that phospho-Ser134-GR (pS134-GR) species are elevated in triple negative breast cancer (TNBC) and cooperate with hypoxia-inducible factors, providing a novel avenue for activation of GR in response to local or cellular stress. We probed GR regulation by factors (cytokines, growth factors) that are rich within the tumor microenvironment (TME). TNBC cells harboring endogenous wild-type (wt) or S134A-GR species were created by CRISPR/Cas knock-in and subjected to transwell migration, invasion, soft-agar colony formation, and tumorsphere assays. RNA-Seq was employed to identify pS134-GR target genes that are regulated both basally (intrinsic) or by TGFβ in the absence of exogenously added GR ligands. Regulation of selected basal and TGFβ-induced pS134-GR target genes was validated by qRT-PCR and chromatin immunoprecipitation assays. Bioinformatics tools were used to probe public data sets for expression of pS134-GR 24-gene signatures. In the absence of GR ligands, GR is transcriptionally activated via p38-dependent phosphorylation of Ser134 as a mechanism of homeostatic stress-sensing and regulated upon exposure of TNBC cells to TME-derived agents. The ligand-independent pS134-GR transcriptome encompasses TGFβ1 and MAPK signaling gene sets associated with TNBC cell survival and migration/invasion. Accordingly, pS134-GR was essential for TNBC cell anchorage-independent growth in soft-agar, migration, invasion, and tumorsphere formation, an in vitro readout of cancer stemness properties. Both pS134-GR and expression of the MAPK-scaffolding molecule 14-3-3ζ were essential for a functionally intact p38 MAPK signaling pathway downstream of MAP3K5/ASK1, indicative of a feed-forward signaling loop wherein self-perpetuated GR phosphorylation enables cancer cell autonomy. A 24-gene pS134-GR-dependent signature induced by TGFβ1 predicts shortened overall survival in breast cancer patients. Phospho-S134-GR is a critical downstream effector of p38 MAPK signaling and TNBC migration/invasion, survival, and stemness properties. Our studies define a ligand-independent role for GR as a homeostatic “sensor” of intrinsic stimuli as well as extrinsic factors rich within the TME (TGFβ1) that enables potent activation of the p38 MAPK stress-sensing pathway and nominate pS134-GR as a therapeutic target in aggressive TNBC.

ORGANISM(S): Homo sapiens

PROVIDER: GSE148444 | GEO | 2020/05/01

REPOSITORIES: GEO

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Project description:In response to environmental stressors and a variety of inflammatory cytokines, p38 MAPKs become directly activated. Here we report the human glucocorticoid receptor (GR) Serine 134 as a novel target for p38 MAPK. Unlike most other phosphorylation events that occur on the GR, phosphorylation of Ser134 was found to be hormone-independent in several human and rat cell types. Instead we found phosphorylation of Ser134 was induced by a variety of stress-activating stimuli, including: glucose starvation, ultraviolet irradiation, osmotic shock, and oxidative stress. Pharmacological inhibitors and shRNA-mediated knockdown experiments correlate this phosphorylation with the activation of p38 MAPK. Compared to wild-type GR, cells expressing a mutant receptor incapable of phosphorylation at Ser134 (S134A GR) had a significantly altered hormone-dependent genome-wide transcriptional response to glucocorticoids. Moreover, we show that although WT GR regulated roughly half as many genes as S134A GR, WT receptor selectively activated significantly more genes associated with endocrine and inflammatory disease than the mutant receptor, suggesting that the phosphorylation status of Ser134 is critical for modulating GR function. Phosphorylation of Ser134 did not alter either nuclear translocation or the stability of the GR protein in the absence or presence of ligand. However, phosphorylation of Ser134 significantly increased the association of the GR with the zeta isoform the 14-3-3 class of signaling proteins, resulting in a blunted hormone-dependent transcriptional response of LAD1 and IGFBP1 but not GILZ. Together these data suggest that the phosphorylation of Ser134 acts as a molecular sensor on the GR, monitoring the level of cellular stress to allow for altered 14-3-3zeta cofactor association, ultimately modifying glucocorticoid signaling in a gene-dependent manner. Our results reveal one mechanism that may allow cellular stress to dictate the transcriptional response of cells to hormone. U2OS cells, a human osteosarcoma cell line, were transfected with either WT GR or S134A GR and put under antibiotic selection to produce a stable mixed population of cells expressing comparable levels of GR. 10^6 cells were treated with 100nM Dexamethasone (DEX) or vehicle control for 6 hours. Three biological and one hybridization replicate are included for each sample.

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Glucocorticoid Receptors are Required Effectors of TGFb1-Induced p38 MAPK Signaling to Advanced Cancer Phenotypes in Triple Negative Breast Cancer

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