Abstract 3296: Tumor suppressive effects of tristetraprolin expression in metastatic breast cancer

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA The purpose of this study is to define mechanisms by which the mRNA-destabilizing factor tristetraprolin (TTP) impacts tumorigenic phenotypes in an aggressive model of metastatic breast cancer. Widespread suppression of TTP levels in human tumors and cancer cell lines relative to non-transformed tissues suggests that TTP may function as a tumor suppressor in diverse neoplastic contexts, and low TTP expression is a negative prognostic indicator in breast cancer. TTP is a tandem zinc finger protein that binds to AU-rich elements (AREs) and targets their associated mRNAs for degradation. AREs are potent cis-acting determinants of cytoplasmic mRNA turnover in mammalian cells, and are essential for limiting cellular production of many clinically important gene products including regulators of inflammation, cell proliferation, and apoptosis. Surveys of patient-derived samples showed that TTP expression is diminished in many neoplastic tissues, particularly those from aggressive cancers. In this study, we show that restoration of TTP levels attenuates several tumorigenic phenotypes in the cultured metastatic breast cancer line MDA-MB-231, consistent with a tumor suppressor role for TTP. Notably, TTP-expressing cell lines replicate approximately 70% slower than non-transfected controls. Inhibition of cell proliferation did not result from apoptosis but rather by a delay at the G1/S checkpoint. TTP expression also significantly reduced the formation of cultured mammospheres based on both sphere-forming efficiency frequency and extreme limiting dilution analyses, indicating that this protein reduces stemness and non-adherent growth potential. Cell motility was also suppressed by TTP, demonstrated using wound healing assays. To identify potential mechanisms linking TTP to diminution of tumorigenic phenotypes, we surveyed the expression of genes encoding selected pro-tumorigenic factors, including several known to encode TTP-targeted mRNAs. Expression of cyclins D1 and E as well as the angiogenic factor VEGF were significantly reduced in TTP-expressing MDA-MB-231 cells, however, this was not mediated by accelerated mRNA decay. Interestingly, we observed that the mRNA-destabilizing function of TTP is abrogated by the constitutively active ERK signaling pathway in this cell model, indicating that the mRNA-destabilizing activity of TTP may be dispensable for at least a subset of its tumor suppressive properties. Based on subsequent analyses of TTP effects on the expression of selected transcription factors at both the mRNA and protein levels, we have developed a working model whereby TTP may suppress diverse tumorigenic phenotypes in this metastatic cancer cell model by repressing the function of a strongly pro-oncogenic transcriptional regulatory circuit, even in the absence of its canonical mRNA-destabilizing activity. Citation Format: Christina R. Ross, Gerald M. Wilson. Tumor suppressive effects of tristetraprolin expression in metastatic breast cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3296. doi:10.1158/1538-7445.AM2014-3296