Graduation Year

2013

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Biology (Cell Biology, Microbiology, Molecular Biology)

Major Professor

Timothy Yeatman

Co-Major Professor

David Shibata

Keywords

autophagy, Bcl-xL, GSI, Mcl-1, obatoclax, oxaliplatin

Abstract

Colorectal cancer is the third leading cause of cancer-related mortality. Much progress has been achieved in combating this disease with surgical resection and chemotherapy in combination with targeted drugs. However, most metastatic patients develop drug resistance so new modalities of treatment are needed.

Notch signaling plays a vital role in intestinal homeostasis, self-renewal, and cell fate decisions during post-development and is activated in colorectal adenocarcinomas. Under debate is its role in carcinomas and metastatic disease. In theory, blocking Notch activation using gamma-secretase inhibitors (GSIs) may show efficacy alone or in combination with chemotherapy in the treatment of colon cancer.

In Chapter Three, we tested the capacity for GSIs to synergize with oxaliplatin in colon cancer cell lines and evaluated the underlying molecular mechanisms. GSI alone had no effect on colon cancer cell lines. Surprisingly, we show that GSIs blocked oxaliplatin-induced apoptosis through increased protein levels of the anti-apoptotic Bcl-2 proteins Mcl-1 and/or Bcl-xL. Restoration of apoptosis was achieved by blocking Mcl-1 and/or Bcl-xL with obatoclax (an anti-apoptotic Bcl-2 agonist) or siRNA. An unexpected result was the induction of cell death with the combination of GSI and obatoclax.

In Chapter Four, we examined the mechanism of GSI + obatoclax-mediated cell death. We found that apoptosis played a minimal role. Rather, we identified blockage of cytoprotective autophagy played a causative role. Interestingly, we also saw autophagy induction in GSI-treated cells, which could explain the insensitivity of colon cancer cells to GSI. When autophagy was blocked in GSI-treated cells, cells became sensitive to GSI and cell death was elicited. The mechanism by which induction of autophagy occurs in GSI- treated cells is an area for further research.

Overall, our work questions the validity of the use of GSIs in the treatment of colorectal cancers. We show that GSIs may block apoptosis and induce cytoprotective autophagy simultaneously, resulting in increased drug resistance and cellular survival. Whether these two cellular survival processes occurs in patients needs to be examined before GSIs can be utilized in a clinical setting. If so, these two hurdles must be overcome.

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