The Thyroid Hormone Receptor-RUNX2 Axis: A Novel Tumor Suppressive Pathway in Breast Cancer

Metastatic breast cancer is refractory to conventional therapies and is an end-stage disease. RUNX2 is a transcription factor that becomes oncogenic when aberrantly expressed in multiple tumor types, including breast cancer, supporting tumor progression and metastases. Our previous work demonstrated that the thyroid hormone receptor beta (TR beta) inhibits RUNX2 expression and tumorigenic characteristics in thyroid cells. As TR beta is a tumor suppressor, we investigated the compelling question whether TR beta also regulates RUNX2 in breast cancer. The Cancer Genome Atlas indicates that TR beta expression is decreased in the most aggressive basal-like subtype of breast cancer. We established that modulated levels of TR beta results in corresponding changes in the high levels of RUNX2 expression in metastatic, basal-like breast cells. The MDA-MB-231 triple-negative breast cancer cell line exhibits low expression of TR beta and high levels of RUNX2. Increased expression of TR beta decreased RUNX2 levels. The thyroid hormone-mediated suppression of RUNX2 is TR beta specific as TR alpha overexpression failed to alter RUNX2 expression. Consistent with these findings, knockdown of TR beta in non-tumor MCF10A mammary epithelial-like cells results in an increase in RUNX2 and RUNX2 target genes. Mechanistically, TR beta directly interacts with the proximal promoter of RUNX2 through a thyroid hormone response element to reduce promoter activity. The TR beta suppression of the oncogene RUNX2 is a signaling pathway shared by thyroid and breast cancers. Our findings provide a novel mechanism for TR beta-mediated tumor suppression in breast cancers. This pathway may be common to many solid tumors and impact treatment for metastatic cancers.