Abstract 1331: TGFβ1 induces breast tumor kinase overexpression in triple negative breast cancer via p38 MAPK signaling to glucocorticoid receptors

Triple negative breast cancer (TNBC) is the deadliest breast cancer (BC) subtype, accounting for 20-30% of all BCs. It has a heterogeneous pathology and pathogenicity, but it is defined by the lack of estrogen receptor, progesterone receptor, and Her2 epidermal growth factor receptor expression. Because targeted therapy through these receptors is not possible, treatment relies on chemotherapy and surgery, which are often inadequate. Thus, the identification of possible molecular targets is critically important in TNBC. Breast tumor kinase (Brk) is a soluble tyrosine kinase that is overexpressed in 85% of BCs and a driver of aggressive and metastatic phenotypes. Overexpression of Brk mRNA and protein occurs in TNBC by unknown mechanisms. The glucocorticoid receptor (GR), a very potent modulator of cytokine mediated actions of the immune system, is emerging as a mediator of chemoresistance and recurrence in TNBC. We previously demonstrated that GR signaling cooperates with physiologic stress signaling mediated by hypoxia inducible factors HIF-1a and HIF-2 to modulate the expression of Brk mRNA and protein in TNBC cells. Moreover, phosphorylation of GR at ser134 by p38 MAPK is essential for GR regulation of Brk expression. P38 is an essential Ser/Thr kinase that regulates cellular transduction of growth factors, such as Hepatocyte Growth Factor (HGF), and cytokines (e.g. TGFβ1) in TNBC cells, and was previously shown to be co-expressed with Brk in human breast tumors. Herein, we further probed mechanisms of crosstalk between key cytokines, GR, and p38 signaling in the regulation of Brk overexpression. We hypothesize that TGFβ1 signaling modulates EMT and metastasis in part by increasing the expression of Brk in TNBC. Treatment of MDA-MB-231 cells with TGFβ1 for 1, 2, 24 and 48 hours increased Brk protein expression relative to vehicle controls. Additionally, TGFβ1 increased both HIF1 and HIF2 protein levels (at 24 and 48 hours respectively). TGFβ1 regulated Brk expression at the level of mRNA, as measured using RT-PCR. Moreover, TGFβ1 synergized with activated GR to further increase Brk mRNA levels. In contrast, mRNA levels of HIF1 and HIF2 were not modulated by TGFβ1, suggesting that the observed protein increases are due to stabilization of HIFs. Finally, TGFβ1 robustly induced p38-dependent phosphorylation of GR at serine 134. This phosphorylation event promoted ligand-independent GR transcriptional activity at the Brk promoter. Human breast tumors significantly co-express active p38 MAPK and Brk. Our molecular model implicates TGFβ1 signaling (via p38 MAPKs) to phospho-GR in the aberrant overexpression of Brk in TNBC. We conclude that blocking of the TGFβ1 pathway may provide a strategy to inhibit Brk mediated TNBC tumor progression. This work was supported by NIH/NCI R01 CA192178 (to CAL) and T32 GM008244-24. Citation Format: Carlos J. Santos Perez, Tarah Regan Anderson, Carol A. Lange. TGFβ1 induces breast tumor kinase overexpression in triple negative breast cancer via p38 MAPK signaling to glucocorticoid receptors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1331. doi:10.1158/1538-7445.AM2017-1331