Estradiol induces BDNF/TrkB signaling in triple-negative breast cancer to promote brain metastases

Abstract

Breast cancer brain metastases (BM) affect younger women disproportionally, including those lacking estrogen-receptor (ER), progesterone-receptor and HER2 (known as triple-negative breast cancer, TNBC). Previous studies in preclinical models, showed that pre-menopausal levels of estradiol (E2) promote TNBC-BM, through incompletely understood mechanisms involving reactive astrocytes. Herein, a novel mechanism involving E2-dependent upregulation of brain-derived neurotrophic factor (BDNF) in astrocytes, and subsequent activation of tumor cell tropomyosin kinase receptor B (TrkB), is identified. E2 increased experimental BM of TNBC 4T1BR5 and E0771 cells by 21 and 3.6 fold, respectively, compared to E2-depleted mice. ERα+ reactive astrocytes were found at early and late stages of BM, and E2 upregulated BDNF in ER ⁺ reactive astrocytes in vitro and in vivo. TrkB was expressed in TNBC brain-trophic cell lines, BM-patient-derived-xenografts and breast cancer BM. Conditioned media from E2-treated astrocytes (CM-E2) activated TrkB TrkB and downstream AKT, ERK and PLC-γ signaling in TNBC cells, increasing their invasiveness and tumor-initiating capability in vitro. The promotion of BM by E2-activated astrocytes was found to be more complex, involving feedback loops and other receptor tyrosine kinases. In 4T1BR5 cells, there was a positive feedback-loop whereby astrocytic BDNF induced cancer-cell BDNF translation. Upregulation of cancer-cell-BDNF was required to promote full invasiveness of 4T1BR5 in response to CM-E2, and was observed in brain metastatic cells in E2-treated mice in vivo. Moreover, the non-competitive BDNF/TrkB inhibitor ANA-12, reduced E2-induced 4T1BR5 BM to levels similar to OVX mice. BDNF also activated EGFR in TrkB ⁺EGFR ⁺ TNBC cells, suggesting that E2-action through astrocytes activates redundant pathways promoting BM. These findings have important therapeutic implications, as they provide a rationale to use E2-depletion therapies or TrkB inhibitors to prevent or delay development of BM in younger women.