Beta3-tubulin is induced by estradiol in human breast carcinoma cells through an estrogen-receptor dependent pathway.

Microtubules are involved in a variety of essential cell functions. Their role during mitosis has made them a target for anti-cancer drugs. However development of resistance has limited their use. It has been established that enhanced beta3-tubulin expression is correlated with reduced response to antimicrotubule agent-based chemotherapy or worse outcome in a variety of tumor settings. However little is known regarding the regulation of beta3-tubulin expression. We investigated the regulatory mechanisms of expression of beta3-tubulin in the MCF-7 cell line, a model of hormone-dependent breast cancer. Exposure of MCF-7 cells to estradiol was found to induce beta3-tubulin mRNA as well as beta3-tubulin protein expression. Conversely, we did not observe induction of beta3-tubulin mRNA by estradiol in MDA-MB-231 cells which are negative for the estrogen receptor (ER). In order to determine whether beta3-tubulin up-regulation is mediated through the ER pathway, MCF-7 cells were exposed to two ER modulators. Exposure to tamoxifen, a selective estrogen receptor modulator, completely abolished the beta3-tubulin mRNA induction due to estradiol in MCF-7 cells. This result was confirmed with fulvestrant, a pure antagonist of ER. These results demonstrate that the effect of estradiol on beta3-tubulin transcription is mediated through an ER dependent pathway.

Vinorelbine induces beta3-tubulin gene expression through an AP-1 Site.

Although the correlation between beta3-tubulin expression level in tumors has been well correlated with clinical outcome in patients receiving microtubule-targeted agents, the regulation of this protein remains poorly understood. Recently, new insight of regulatory processes was offered with the cloning of the gene promoter. In this study beta3-tubulin gene expression was induced in response to exposure to various antimicrotubule agents such as vinorelbine and paclitaxel. The exploration of the beta3-tubulin gene promoter by successive deletions followed by site-directed mutagenesis led to the localization of a vinorelbine-responsive element containing an activator-protein 1 (AP-1) site. Among the various antimicrotubule agents tested, it appeared that the implicated AP-1 site was activated only after exposure to vinorelbine. This study confirms the inducible nature of the beta3-tubulin gene promoter.