Research ArticleCancer

BET inhibition increases βIII-tubulin expression and sensitizes metastatic breast cancer in the brain to vinorelbine

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Science Translational Medicine  26 Aug 2020:
Vol. 12, Issue 558, eaax2879
DOI: 10.1126/scitranslmed.aax2879

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BETting against brain metastases

Brain metastases are a dreaded complication of many cancer types. Even for malignancies that are relatively treatable, such as breast cancer, brain metastases are difficult to reach and often are not susceptible to the same therapies as peripheral tumors. By comparing primary and metastatic breast cancers, Kanojia et al. identified differences in the expression of cytoskeletal protein βIII-tubulin, which was increased in tumors that metastasized to the brain and sensitized them to vinorelbine, a chemotherapy drug. Another drug type, called a BET inhibitor, promoted βIII-tubulin expression and further sensitized breast cancer metastases to vinorelbine in mouse models, demonstrating a promising therapeutic combination.

Abstract

Metastases from primary breast cancer result in poor survival. βIII-tubulin (TUBB3) has been established as a therapeutic target for breast cancer metastases specifically to the brain. In this study, we conducted a systematic analysis to determine the regulation of TUBB3 expression in breast cancer metastases to the brain and strategically target these metastases using vinorelbine (VRB), a drug approved by the U.S. Food and Drug Administration (FDA). We found that human epidermal growth factor receptor 2 (HER2) signaling regulates TUBB3 expression in both trastuzumab-sensitive and trastuzumab-resistant neoplastic cells. We further discovered that bromodomain and extra-terminal domain (BET) inhibition increases TUBB3 expression, rendering neoplastic cells more susceptible to apoptosis by VRB. Orthotopic xenograft assays using two different breast cancer cell models revealed a reduction in tumor volume with BET inhibition and VRB treatment. In addition, in vivo studies using a model of multiple brain metastasis (BM) showed improved survival with the combination of radiation + BET inhibitor (iBET-762) + VRB (75% long-term survivors, P < 0.05). Using in silico analysis and BET inhibition, we found that the transcription factor myeloid zinc finger-1 (MZF-1) protein binds to the TUBB3 promoter. BET inhibition decreases MZF-1 expression and subsequently increases TUBB3 expression. Overexpression of MZF-1 decreases TUBB3 expression and reduces BM in vivo, whereas its knockdown increases TUBB3 expression in breast cancer cells. In summary, this study demonstrates a regulatory mechanism of TUBB3 and provides support for an application of BET inhibition to sensitize breast cancer metastases to VRB-mediated therapy.

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