The Role of Mitochondrial Phospholipid Hydroperoxide Glutathione Peroxidase in Cancer Therapy

Phospholipid hydroperoxide glutathione peroxidase (PhGPx) is a unique selenoenzyme that directly detoxifies lipid hydroperoxides in situ. It therefore plays an important role in the protection of cellular membranes. PhGPx is expressed in most mammalian tissues. It is present as a mitochondrial form (L-PhGPx) and a cytosolic form (S-PhGPx). Overexpression of PhGPx has been shown to significantly protect cells from oxidative damage. The hypothesis of this thesis is that mitochondrial PhGPx (L-PhGPx) may play an important role in the resistance of cells to certain oxidative stress-mediated cancer therapies. A human breast carcinoma MCF-7 cell line was used as a cell model system in this research. It was stably transfected with human L-PhGPx sense cDNA. Four clones (P-1, P-2, P-3, and P-4) with 3to 7-fold increases in PhGPx activity were selected for study. Overexpression of L-PhGPx did not significantly influence other cellular antioxidants, including superoxide dismutases, cytosolic glutathione peroxidase, catalase, glutathione reductase, and glutathione. However, L-PhGPx did decrease the rate of cell growth. Cell plating efficiency was inversely correlated with effective PhGPx activity, which is defined as the product of cellular PhGPx activity and total glutathione. The biological functions of L-PhGPx have been investigated in cancer treatment, including photodynamic therapy (PDT) and hyperthermia (HT). Both PDT and HT can induce oxidative stress. Overexpression of L-PhGPx in MCF-7 cells significantly increased the resistance of cells to PDTand HT-mediated cytotoxicity. The effective PhGPx activity had a remarkable inverse linear correlation (r = -0.80) to the rate of removal of lipid hydroperoxides in living cells, and correlated positively with cell survival after photooxidation (r = 0.91). L-PhGPx protected mitochondrial function by preserving the mitochondrial membrane potential. These data demonstrate that L-PhGPx provides significant protection against lipid peroxidation via removal of LOOH and maintaining the mitochondrial membrane potential, suggesting that LOOHs are major mediators in this cell injury process. It also suggests that mitochondria are important targets for PDT and HT. PhGPx activity could contribute to the resistance of tumor cells to the treatments that have oxidative stress as a component in their mechanism of action. Abstract approved: _________________________________________ Thesis supervisorapproved: _________________________________________ Thesis supervisor _________________________________________ Title and department _________________________________________ Date THE ROLE OF MITOCHONDRIAL PHOSPHOLIPID HYDROPEROXIDE GLUTATHIONE PEROXIDASE IN CANCER THERAPY