Coordinated regulation of β-tubulin isotypes and epithelial-to-mesenchymal transition protein ZEB1 in breast cancer cells.

The regulation of β-tubulin isotypes, the primary targets for antimitotic chemotherapeutic drugs like taxanes, has implications for drug response and drug resistance. Over the past 15 years, micro-RNAs have been studied widely as regulators of mRNA levels. For example, the tumor suppressor miR-200c was shown in cell culture to target mesenchymal genes, including ZEB1 [ Cochrane ( 2009 ) Mol. Cancer Ther. 8 ( 5 ), 1055 - 1066 ]. In that work, exogenous miR-200c was also shown to reduce β-tubulin class III, one of its predicted targets. Furthermore, decreased miR-200c and increased β-tubulin class III were associated with poor outcomes for ovarian cancer patients [ Cittelly , D.M. ( 2012 ) Mol. Cancer Ther. 11 ( 12 ), 2556 - 2565 ]. Because miR-200c targets the epithelial-to-mesenchymal inducer ZEB1, we wanted to know whether changes in ZEB1 parallel β-tubulin isotype changes, implicating β-tubulin isotypes in ZEB1-associated cell survival pathways. We found coordinated positive feedback regulation of mRNA for ZEB1 and β-tubulin isotype classes I, III, and IVB in MDA-MB-231 breast cancer cells, commonly used as a model for triple-negative breast cancers. Low levels of paclitaxel (40 nM) were found to significantly reduce mRNA levels for these tubulin genes along with a 2-3-fold increase in miR-200c. ZEB1 silencing also reduced β-tubulin isotype classes I, III, and IVB mRNA, whereas upregulation of ZEB1 was associated with increases in these isotype classes. Our work indicates that paclitaxel-induced reduction of ZEB1 and β-tubulin isotypes are, in part, due to increased activity of miR-200c. These results suggest that in aggressive breast cancers, as modeled by MDA-MB-231 cells, β-tubulin class III is a biomarker for cell survival mediated through ZEB1-induced tumor progression pathways.