Effects of redox modulation by inhibition of thioredoxin reductase on radiosensitivity and gene expression

Abstract

The thioredoxin system is a promising target when aiming to overcome the problem of clinical radiation resistance. Altered cellular redox status and redox sensitive thiols contributing to induction of resistance strongly connect the ubiquitous redox enzyme thioredoxin reductase (TrxR) to the cellular response to ionizing radiation. To further investigate possible strategies in combating clinical radiation resistance, human radio-resistant lung cancer cells were subjected to a combination of single fractions of γ-radiation at clinically relevant doses and non-toxic levels of a well-characterized thioredoxin reductase inhibitor, the phosphine gold(I) compound [Au(SCN)(PEt3)]. The combination of the TrxR-inhibitor and ionizing radiation reduced the surviving fractions and impaired the ability of the U1810 cells to repopulate by approximately 50%. In addition, inhibition of thioredoxin reductase caused changes in the cell cycle distribution, suggesting a disturbance of the mitotic process. Global gene expression analysis also revealed clustered genetic expression changes connected to several major cellular pathways such as cell cycle, cellular response to stress and DNA damage. Specific TrxR-inhibition as a factor behind the achieved results was confirmed by correlation of gene expression patterns between gold and siRNA treatment. These results clearly demonstrate TrxR as an important factor conferring resistance to irradiation and the use of [Au(SCN)(PEt3)] as a promising radiosensitizing agent.

Thioredoxin system – a novel therapeutic target

“…The data clearly demonstrate an important role of TrxR in radiation resistance, and warrant further studies to elucidate the specific mechanisms of TrxR involvement in resistance development. Pharmacological inhibition of TrxR is an attractive treatment strategy to optimize the efficacy of radiation therapy and TrxR inhibition (Selenius et al 2012). …”

Anticancer Profile of a Series of Gold(III) (2‐phenyl)pyridine Complexes

“…46–49 TrxR is a key flavoprotein expressed in several cell compartments (e.g., mitochondria, nucleus, cytosol) and is involved in cellular redox homeostasis 50. The inhibition of TrxR leads to redox unbalance and induction of apoptosis 51. Gold(I) complexes have already been shown to be very potent TrxR inhibitors 1.…”

“…Instead of selenocysteine, GR has a cysteine at the C terminus of its active site 5. 50, 51 Given the differing affinities between sulfur and selenium for gold, it is possible to design selective gold‐based inhibitors. To establish if the new gold(III) complexes show selectivity toward TrxR against GR, the inhibition of GR by 1 – 6 was studied by following a procedure reported for similar complexes 48.…”

“…Inhibition of two of the main components of the cellular antioxidant network, namely TrxR and GR, by our complexes is expected to induce a strong cellular response as a result of increased reactive oxygen species (ROS) levels 50. 51 We therefore decided to study ROS generation upon treatment with 4 . For this purpose, the cell‐permeable dye 2′,7′‐dichlorofluorescein diacetate (H 2 DCF‐DA), which turns highly fluorescent upon oxidation, was used as previously reported by our group for a cytotoxic Ru II complex 38.…”

Oxidative Stress Mechanisms and their Modulation

“…Changes in the cellular redox environment by using antioxidant like N-acetyl-L-cysteine (NAC, a glutathione precursor) inhibited the prolifera-4 Oxidative Stress and Carcinogenesis tion in mouse embryonic fi broblasts, hepatic stellate cells, and vascular smooth muscle cells (Menon et al 2003 ). Altered cellular redox status and the redox-sensitive thiols through the activity of redox enzyme thioredoxin reductase contribute toward resistance to the cellular response to ionizing radiation (Selenius et al 2012 ). Altered cellular redox status and the redox-sensitive thiols through the activity of redox enzyme thioredoxin reductase contribute toward resistance to the cellular response to ionizing radiation (Selenius et al 2012 ).…”