Online

Bolf, Eric L.

Cellular, Molecular, and Biomedical Sciences Graduate Program

2020

Ph. D.

Dysregulation of the tumor suppressor thyroid hormone receptor beta (TR[Beta]) is a frequent event in a number of solid tumors. As a nuclear hormone receptor, the primary function of TR[Beta] is to regulate transcriptional activity in a ligand-dependent manner. However, TR[Beta] activity is not well-defined and the pathways it regulates are not yet fully delineated. The two cancer types where TR[Beta] is best studied are thyroid cancer and breast cancer. Interestingly, thyroid cancer is a risk factor for breast cancer and breast cancer is a risk factor for thyroid cancer, suggestive of an etiological link. Determining the molecular mechanisms of a tumor suppressor shared by both diseases may reveal critical tumor suppressive pathways. First, in order to discover novel pathways that are regulated by TR[Beta], TR[Beta] was stably overexpressed in an anaplastic thyroid cancer (ATC) cell line through lentiviral transduction. TR[Beta] has not previously been overexpressed in ATC and, through triiodothyronine (T3) stimulation, TR[Beta] repressed pro-malignant phenotypes including cellular growth, migration, and stemness. Additionally, TR[Beta] and T3 promoted apoptosis. RNA-sequencing and subsequent analysis revealed novel TR[Beta]-mediated STAT1 stimulation. Additionally, unmodified anaplastic thyroid cancer cells were sensitive to treatment with a STAT1 agonist. Next, to determine if there is common transcriptional signaling between different types of solid tumors, the TR[Beta]-RUNX2 pathway was examined in breast cell lines. In thyroid cancer, TR[Beta] represses expression of the pro-metastatic transcription factor RUNX2. In addition to promoting thyroid cancer, aberrant overexpression of RUNX2 is a driver of breast carcinogenesis. This informed the hypothesis that TR[Beta]-mediated repression of RUNX2 and resultant signaling is common to thyroid and breast cancers. T3 treatment reduced RUNX2 expression in breast cell lines. Additionally, TR[Beta] bound to thyroid hormone response elements in the RUNX2 promoter, the same mechanism of action for T3-mediated repression as in thyroid cells. Lastly, to more broadly address mechanisms mediated by TR[Beta], TR[Beta] was stably overexpressed in a basal-like aggressive breast cancer cell line. TR[Beta] is already known to be able to repress tumor growth and metastasis in this type of cancer. However, the specific molecular mechanisms driving the anti-tumor phenotype are not well delineated. In order to determine which molecular signaling pathways are regulated by TR[Beta], RNA-sequencing was performed. TR[Beta] induced a more epithelial profile of these cells, in contrast to the typically mesenchymal nature of basal-like breast cancer. Novel TR[Beta]-mediated repression of pro-invasive cytokeratins was found. The genes regulated by TR[Beta] in these cells were also compared to the findings in ATC. The set of common genes included chromatin remodelers and enzymes involved in stearate biosynthesis; the latter is known to have anti-cancer properties and may be an important driver of TR[Beta]-mediated tumor suppression in both breast and thyroid cancers.