The Post-Transcriptional Regulation of Antioxidant Enzymes
thesisposted on 28.06.2013, 00:00 by Emily N. Reinke
An excess in the levels of reactive oxygen species (ROS) can contribute to the development of cancer due to the oxidative damage to molecules such as DNA, protein and cellular membranes. Antioxidant proteins, including those that belong to the glutathione peroxidase (GPx), superoxide dismutase (SOD) and thioredoxin reductase (TrxR) families, contribute to the maintenance of the cellular redox balance by detoxification of ROS. Our attention was drawn to a report indicating that the non-receptor tyrosine kinase c-Abl could phosphorylate and activate the activity of GPx1 in tissue culture cells. To investigate this phenomenon further in humans, GPx activity were determined in samples obtained from patients with chronic myelogenous leukemia (CML), a disease initiated by the rearrangement of two different chromosomes and resulting in the Bcr-Abl oncogene. Samples were obtained before and after treatment with the Bcr-Abl inhibitor imatinib mesylate, and of the seven patient sample sets obtained, four exhibited elevated GPx activity following treatment, which was contrary to expectation. In order to expand upon this observation, the CML established cell lines KU812 and MEG-01 were treated with imatinib and the effect on several antioxidant proteins was determined. The levels of GPx1, manganese superoxide dismutase (MnSOD) and thioredoxin reductase 1 (TrxR1) were each significantly increased following treatment with imatinib. This increase was not due to altered steady-state mRNA levels, and appeared to be dependent on the expression of Bcr-Abl, as no increases were observed following imatinib treatment of cells that did not express the fusion protein. The nutrient-sensing signaling protein, mammalian target of rapamycin (mTOR), can be activated by Bcr-Abl and its activity regulates the translation of several proteins. Treatment of those same cells used in the imatinib studies with rapamycin, an inhibitor of mTOR, resulted in elevated GPx1, TrxR1 and GPx4 protein levels independent of Bcr-Abl expression. These proteins all belong to the selenoprotein family of peptides that contain the UGA-encoded amino acid selenocysteine. Collectively, these data provide evidence of a novel means of regulating antioxidants of the selenoprotein family via the mTOR pathway.