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

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 <i>TUBB3</i> 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 <i>TUBB3</i> expression in both trastuzumab-sensitive and trastuzumab-resistant neoplastic cells. We further discovered that bromodomain and extra-terminal domain (BET) inhibition increases <i>TUBB3</i> 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, <i>P</i> < 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 <i>TUBB3</i> promoter. BET inhibition decreases <i>MZF-1</i> expression and subsequently increases <i>TUBB3</i> expression. Overexpression of <i>MZF-1</i> decreases <i>TUBB3</i> expression and reduces BM in vivo, whereas its knockdown increases <i>TUBB3</i> expression in breast cancer cells. In summary, this study demonstrates a regulatory mechanism of <i>TUBB3</i> and provides support for an application of BET inhibition to sensitize breast cancer metastases to VRB-mediated therapy.