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Date of Award

6-2014

Document Type

Restricted Thesis: Campus only access

Degree Name

Master of Science in Biology

Department

Biology

First Reader/Committee Chair

Nicole Bournias Ph.D.

Abstract

Glioblastoma multiforme (GBM) is a grade IV astrocytoma and is the most aggressive malignant primary brain tumor in adults. Without treatment, patients are expected to survive an average of three months. Conversely, current treatment regimens only extend survival to 12-14 months. Characteristically, GBM tumors are highly proliferative, invasive and stop responding to treatments relatively fast due to therapy resistance. Interestingly, GBM also exhibits high metabolic activity but manages to maintain a low level of reactive oxygen species (ROS). These ROS neutralization capabilities are sustained by system Xc, a sodium-independent, electro neutral transporter that is found in the plasma membrane of GBM cells. System Xc is composed of a regulatory heavy subunit (4F2hc) linked to a 12 transmembrane domain catalytic light chain subunit (xCT) that mediates the uptake of L-cystine into the cell, and L-glutamate out of the cell, at a 1:1 ratio. Imported cystine is quickly reduced to L- cysteine, the rate limiting substrate in glutathione (GSH) synthesis. Glutathione is a major antioxidant in the central nervous system that is responsible for maintaining intracellular redox homeostasis by neutralizing ROS by direct and indirect methods. The function of chemo and radiation therapy is to generate significant levels of ROS that tigger the cell to undergo apoptosis. High intracellular GSH levels in cancer cells are associated with drug resistance and detoxification of alkylating agents such as temozolomide (TMZ). Therefore, system Xc represents a potential target to reduce glioma cell survival and reduce tumor progression. Sulfasalazine is an FDA approved drug in the treatment of arthritis and Crohne’s disease and has been shown to inhibit system Xc. In vitro SASP studies demonstrated a strong antitumor potential in preclinical mouse models of malignant glioma. However, two clinical trials using sulfasalazine with standard chemo and radiation therapy to treat GBM patients were terminated due to off-target effects. Both results showed high toxicity and no change in the overall survival of patients. These studies demonstrate the need for a more effective inhibitor of system Xc. To further elucidate the role of system Xc in GBM survival, stable xCT knock-down and over-expressing U251 glioma cells were generated. These lines were characterized for survival, proliferation, apoptosis and resistance to oxidative and genotoxic insult. As expected xCT-knockdown cells exhibited lower GSH levels, increased intracellular ROS and markers for apoptosis after oxidative and genotoxic insult. The xCT-over-expressing cells displayed higher levels of GSH, increased resistance to hydrogen peroxide and various chemotherapy drugs including TMZ. An interesting unforeseen result of xCT over-expression in glioma cells was an increase in the metabolic activity as a result of increased mitochondria. Using xCT-modified glioma lines stably, we demonstrate for the first time that system XC over-expression not only promotes survival under oxidative stress but may also decreases sensitivity to chemotherapy treatment and increase metabolic properties. Therefore, therapeutic manipulation of this transporter either alone or in combination with other treatments may improve clinical outcome in patients diagnosed with GBM.

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