Date of Award

2019

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Microbiology and Immunology

Abstract

Breast cancer is the most commonly diagnosed form of cancer in women, besides skin cancer, and second only to lung cancer for cause of mortality. It is divided into three subtypes, depending on different molecular or pathologic characteristics, including estrogen or progesterone receptor-positive (ER/PR+), human epidermal growth factor receptor-2-positive (HER2+) and triple negative breast cancer (TNBC). There are several therapies used to target these subtypes, but there is still a chance that the cancer will recur into a more aggressive, resistant form even if the therapies were successful before. This recurrence is believed to be due to the Cancer Stem Cell (CSC) hypothesis which states that within the heterogeneous breast cancer tumor, there is a population of CSCs that are responsible for resistance to therapy and tumor recurrence. Evidence shows that Notch signaling, a pathway regulating several cellular processes, could be the reason these CSCs survive, so new therapies are being developed to target Notch signaling. The problem is, that Notch gene targets are not enough to provide a predictive response in breast cancer, so new potential biomarkers have been identified to predict a response to therapy-- such as DAXX protein. This protein, which has been associated in pro-apoptotic pathways and gene expression repression, has been shown to be inhibiting bulk cell proliferation in cell lines representative of all three breast cancer subtypes. In a HER2+ and TNBC cell line this has been shown even in the presence of a Notch inhibitor. Because of this, it was believed that DAXX expression may be inhibiting Notch signaling.xResults show that DAXX expression is required to limit proliferation of TNBC and other subtypes such as the HER2+ (BT474) and not ER+ (MCF-7). Interestingly, DAXX depletion using a DAXX siRNA decreases sensitivity of breast cancer cells to standard of care therapy in the TNBC subtype, MDA-MB-231 cells, only. In other TNBC cell lines, this is not the case-- showing just how heterogenous this subtype is. Further, DAXX appears to be potentially regulating PARP activity in MDA-MB-231 cells, which is key to stabilize cells that have some type of DNA damage, since depletion of DAXX results in increased PARP-1 and PAR chains expression. These results indicate that DAXX expression plays a role in limiting proliferation. The mechanism by which this occurs is largely unknown. However, preliminary findings demonstrate that a DNA damaging agent such as Carboplatin may induce JNK phosphorylation and activation. This activation of JNK seems to be dependent on DAXX expression in MDA-MB-232 cells. We assessed whether targeting JNK would be a novel therapeutic strategy in combination with carboplatin. Results showed that the JNK inhibitor (SP600125) partially prevents the increase in cell proliferation upon DAXX depletion. Results together suggest: 1. DAXX expression limits proliferation of most subtypes of breast cancer; 2. DAXX expression is required for drug sensitivity to standard of care but this differs in TNBC subtypes; 3. DAXX activates JNK to limit proliferation in the mesenchymal stem-like subtype of TNBC; and 4. DAXX may regulate poly(ADP) ribosylation (PAR) of PARP1 to possibly regulate the DNA damage response.

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This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.

Available for download on Wednesday, August 12, 2020

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