Tamoxifen suppresses brain metastasis of estrogen receptor-deficient breast cancer by skewing microglia polarization and enhancing their immune functions

Abstract

Background Brain metastasis of breast cancer exhibits exceedingly poor prognosis, and both triple negative (TN) 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. Methods To examine the effect of estrogen on brain metastasis progression, nude mice were implanted with brain metastatic cells and treated with either estrogen supplement, tamoxifen, or ovariectomy for estrogen depletion. For clinical validation study, brain metastasis specimens from pre- and post-menopause breast cancer patients were examined for microglia polarization by immunohistochemistry. To examine the estrogen-induced M2 microglia polarization, microglia cells were treated with estrogen, and the M1/M2 microglia polarization was detected by qRT-PCR and FACS. The estrogen receptor-deficient brain metastatic cells, SkBrM and 231BrM, were treated with conditioned medium (CM) derived from microglia that were treated with estrogen in the presence or absence of tamoxifen. The effect of microglia-derived CM on tumor cells was examined by colony formation assay and sphere forming ability. Results We found that M2 microglia were abundantly infiltrated in brain metastasis of pre-menopausal breast cancer patients. A similar observation was made in vivo, when we treated mice systemically with estrogen. Blocking of estrogen signaling either by tamoxifen treatment or surgical resection of mice ovaries suppressed M2 microglial polarization and decreased the secretion of C-C motif chemokine ligand 5, resulting in suppression of brain metastasis. The estrogen modulation also suppressed stemness in TNBC cells in vitro. Importantly, estrogen enhanced the expression of signal regulatory protein α on microglia and restricted their phagocytic ability. Conclusions Our results indicate that estrogen promotes brain metastasis by skewing polarity of M2 microglia and inhibiting their phagocytic ability, while tamoxifen suppresses brain metastasis by blocking the 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.

Tamoxifen Exerts Anticancer Effects on Pituitary Adenoma Progression via Inducing Cell Apoptosis and Inhibiting Cell Migration

“…TAM is also known to regulate phenotypic polarization of macrophages in PAs. Tumor-associated macrophages stimulate angiogenesis, tumor progression and metastasis by suppressing immune responses [ 7 , 27 ], and are polarized to the anti-inflammatory M2 phenotype [ 28 ]. The M2 macrophages are abundant in hepatocellular carcinoma tumors, and correlate with the activation of the JAK1/STAT6 signaling pathway [ 29 ].…”

Tamoxifen Twists Again: On and Off-Targets in Macrophages and Infections

“…These pathways are known to control macrophage immunometabolism and inflammatory responses, thus being essential in containing infections. Moreover, tamoxifen was shown to induce neuroprotective effects by regulating microglia activation, further expanding the tissue distribution of drug immune activity ( Barreto, Santos-Galindo, and Garcia-Segura 2014 ; Wu et al, 2021 ). Therefore, further studies are needed not only to upraise the clinical relevance of tamoxifen activity in macrophages but also to advance target identification and drug development in order to improve therapeutic options.…”

Rewiring of the Endocrine Network in Triple-Negative Breast Cancer

“…Albeit with the absence of ERα, estrogens and xenoestrogens have been identified to act on the tumor microenvironment via their corresponding receptors. E2 functions in the trigger of “don’t eat me” signaling by strengthening CD47-SIRPα interaction and skewing the antiphagocytic effect of the M2 microglia ( 5 , 11 ). In addition, E2 depletion was of therapeutic value evidenced by the reportedly increasing tumor cell tropomyosin kinase receptor B (TrkB) signal regulated by E2 in premenopausal TNBC patients and thus reduced the risk of brain metastasis (BM) ( 12 , 13 ).…”

“…Typically, endocrine agents are applied to ERα-positive breast cancer comprised of selective ER modulators (SERMs), selective ER degraders (SERDs), and aromatase inhibitors (AI), which are committed to regional recurrence suppression and long-term survival benefit. As evidenced by the fact that ERα-negative breast cancer sheltered from endocrine therapy previously acquired arousable sensitivity to tamoxifen ( 5 ), it was implied that the endocrine response promised to be a novel mechanism in TNBC, which the absence of hormone receptors could not overshadow. Thus, endocrine strategies in TNBC were rewired, and the underlying signaling cascades triggered downstream were found to be significant.…”