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Determining the Mechanism of Six1-Induced TRAIL Resistance and Its Role in Breast Cancer Metastasis

Principal Investigator: DIMBERG, LINA
Program: BCRP
Proposal Number: BC093627
Award Number: W81XWH-10-1-0350
Funding Mechanism: Postdoctoral Fellowship Award
Partnering Awards:
Award Amount: $285,440.00


I feel very fortunate to be working in the cancer research field. It allows me to penetrate fundamental cellular and molecular mechanisms that fascinate me, and it is deeply rewarding to know that my work may one day lead to discoveries that can benefit cancer patients. After having seen the devastating effects of breast cancer up close in my own family, I have decided to dedicate my scientific career to understanding and fighting breast cancer. The proposed project is ideally suited for me since I can make use of my background studying cancer and cell death, while enabling me to expand my knowledge to more specifically focus on breast cancer. In addition, I will learn several new techniques and gain an increased understanding of mouse models. I am confident that a postdoctoral fellowship in Dr. Heide Ford's laboratory working on the proposed project will enable me to realize my goal of becoming an independent breast cancer researcher.

The proposed project revolves around a developmental protein called Six1 and its role in protecting tumor cells from cell death and in promoting metastasis. Six1 belongs to a group of proteins called the homeobox proteins that all play important roles in embryonic development by regulating processes such as growth, cell type specification, and cell migration. Six1 is not needed in most tissues in the adult body and is normally not expressed in them. However, Six1 is re-expressed in 50% of primary breast tumors and 90% of metastatic lesions, suggesting that it may re-instate a developmental program in adult tissue out of context. Indeed, Six1 overexpression has been shown to play an important role in breast tumor initiation and metastasis.

We have found that one effect of Six1 overexpression is that it makes tumor cells resistant to TRAIL, a naturally occurring protein that kills cells by inducing so called cell death, or apoptosis. TRAIL-mediated apoptosis is one mechanism by which the body protects itself against tumors. It is especially important in immune surveillance, the process whereby immune cells recognize and eliminate tumor cells, and it is also important in the prevention of metastasis. Interestingly, the TRAIL pathway can be exploited in cancer therapy by administering recombinant TRAIL proteins or antibodies against TRAIL receptors to patients. A great benefit of this strategy compared to other treatment is that TRAIL typically kills tumor cells but has very little effect on normal cells. The aim of this project is to find out how Six1 can induce resistance to TRAIL and how this resistance can be circumvented.

We have already found that one protein, the TRAIL decoy receptor DcR1, which prevents TRAIL from binding to a functional receptor, is upregulated when Six1 is overexpressed. We will investigate whether this protein affects TRAIL resistance by blocking its expression in Six1-overexpressing cells using interfering RNA molecules. If blocking DcR1 reverses TRAIL resistance in Six1-overexpressing cells, it is likely that upregulation of DcR1 is at least part of the mechanism by which Six1 mediates TRAIL resistance. We will also look for other potential targets of Six1 that mediate its ability to induce resistance to TRAIL, using a targeted approach where we will examine proteins already known to affect apoptosis and using an unbiased approach in which we will screen every gene in the genome. Finally, we will use breast cancer mouse models in which Six1 induces metastasis to evaluate the importance of Six1-mediated TRAIL resistance in its ability to induce metastasis disease.

Although this is work is still some years from the clinic, the findings from these studies will lead to a better understanding of metastasis in breast cancer, and, importantly, it may help us to identify which patients would benefit from TRAIL-based cancer therapy. Ultimately, we may be able to find ways to circumvent Six1-induced TRAIL resistance, which would not only facilitate TRAIL-based therapy but may also improve the efficacy of our own immune system in eradicating breast cancer. Indeed, since we have demonstrated that Six1 can initiate breast tumorigenesis and metastasis and since this gene is overexpressed in the tumors of at least half of all breast cancer patients, finding mechanistic approaches to inhibit the actions of Six1 is highly motivated and is likely to lead to improved breast cancer therapy.