Abstract 2715: Tamoxifen suppresses brain metastasis of estrogen receptor deficient breast cancer by skewing microglia polarization and enhancing their immune functions
Authors: Wu Shih-Ying 
Published: July 2021

Abstract
Brain metastasis of breast cancer exhibits exceedingly poor prognosis, and both triple negative and Her2+ subtypes have the highest incidence of brain metastasis. Although estrogen blockers are considered to be ineffective for their treatment, recent evidence indicates that estrogen blockade using tamoxifen showed certain efficacy. However, how estrogen affects brain metastasis of triple negative breast cancer (TNBC) remains elusive. The communications between neoplastic cells and microglia, especially M2 type pro-tumor microglia, affect brain metastasis progression. Therefore, we hypothesize that estrogen skews microglial polarization to M2 phenotype which promotes brain metastasis. We found that M2 microglia were abundantly infiltrated in brain metastatic lesions of pre-menopausal TNBC patients. To test the effect of estrogen on brain metastasis, we transplanted the SKBrM brain metastatic cell TNBC line into nude mice by intra-cardiac injection followed by with or without 17β-estradiol (E2) treatment. We found that brain metastasis-free survival was decreased, while infiltration of M2 microglia was increased in E2-treated group. Blocking estrogen signaling either by tamoxifen treatment or surgical resection of ovaries suppressed M2 microglial polarization and brain metastasis. To test the effect of E2-driven microglia on tumor progression, brain metastatic cells were treated with conditioned medium (CM) generated from microglia that were treated with or without E2. We found that CM promoted cell growth and increased the population of cancer stem cells. Blocking E2-mediated M2 polarization by tamoxifen suppressed M2 polarization and decreased the secretion of C-C motif chemokine ligand 5, resulting in significant suppression of brain metastasis. To examine the mechanism by which estrogen suppresses anti-tumor function of microglia, we measured the expression of “do not eat me” signal, SIRP on microglia and CD47 on tumor cells. When SKBrM and microglial cells were treated with E2, we found that the protein expression of SIRP and CD47 were up-regulated on microglia and SKBrM, respectively, by E2 treatment. Furthermore, we used flow cytometry to detect the phagocytic ability of microglia by co-culturing SKBrM with microglia. We found that the phagocytic activity of microglia was significantly decreased upon E2 treatment. In contrast, tamoxifen restored E2-suppressed phagocytic ability. In conclusion, our results show that estrogen promotes brain metastasis by skewing polarity of M2 microglia and inhibiting their phagocytic ability, while tamoxifen suppresses brain metastasis by blocking M2 polarization of microglia and increasing their anti-tumor phagocytic ability. Our results also highlight a potential therapeutic utility of tamoxifen for treating brain metastasis of hormone receptor deficient breast cancer.
Citation Format: Shih-Ying Wu. Tamoxifen suppresses brain metastasis of estrogen receptor deficient breast cancer by skewing microglia polarization and enhancing their immune functions [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2715.