Peroxiredoxin expression and redox state in human cancers

Peroxiredoxins (Prxs) are a class of ubiquitous peroxidases that degrade hydrogen peroxide. They are found in almost all compartments of the cell: Prxs 1 and 2 in the cytosol, and Prx 3 in the mitochondria. Their role in cellular antioxidant defence is well established, but they have also been implicated in redox signalling, although the mechanisms by which they participate in this are only beginning to be revealed. Prx 1 knockout mice develop malignancies; however, high Prx expression has also been reported in many cancers, in both cell culture and tumour tissue. The role of the differential expression of Prxs in cancer is unclear, although it is commonly thought to be an adaptive response to increased production of hydrogen peroxide. Lowering the expression of Prxs was shown to be detrimental to cancer growth and progression, and the overexpression was shown to promote tumour development. In this thesis, the absolute amount of Prx 1, 2 and 3 was measured in each of the National Cancer Institute Sixty (NCI-60) panel of cell lines. The NCI-60 is a broad group of 60 cell lines derived from 9 tumour types: leukemia, lung, colon, CNS, melanoma, ovarian, renal, prostate, and breast. Substantial variation in Prx expression was seen across the panel, regardless of tumour type. Prx 1 was the most abundantly expressed, and was elevated in the panel of breast cancer cells compared to non-malignant breast cells. It is not known what controls the expression of Prxs in cells, though mutations and increased oxidative stress are known to result in the constitutive activation of Nrf-2 and the upregulation of antioxidant genes, including Prx 1. Cancer cells are often said to exist in a highly oxidising environment, which is thought to be due to alterations in metabolism, increased NADPH oxidase activity, and transient hypoxia. A lot of the evidence is from endpoint biomarkers of oxidative damage in cancer; these are not particularly sensitive. In contrast, Prxs are very sensitive to oxidation by hydrogen peroxide and can be used to detect subtle disturbances within localised areas of cell that may not result in cell damage. The redox state of Prxs was investigated in the NCI-60. With the exception of the melanoma cells, which showed high levels of oxidised Prx 2, the Prxs were predominately reduced. Prx redox state was also examined in colorectal and renal solid tumours processed immediately after surgery. High levels of oxidation were seen in both the tumour and nearby normal tissue, making it difficult to examine differences. Little difference was seen between the colorectal tumour and nearby normal tissue, but high levels of oxidation were observed in both. In renal tissue, Prxs 1 and 2 were mainly reduced, similar to the adjacent normal tissue. However, high levels of Prx 3 oxidation was measured in a group of high grade tumours. Indicating there was an increase in oxidant production from the mitochondria that did not impact the cytosolic Prxs. Aside from the melanoma cell lines and the renal tissue, it is apparent from the analysis that Prxs are not accumulating in their oxidised form or are not highly oxidised in comparison to normal tissue. This contradicts the original hypothesis that cancer cells are under increased stress and suggests they are capable of maintaining a reducing environment