The NRF2-mediated oxidative stress response pathway is associated with tumor cell resistance to arsenic trioxide across the NCI-60 panel

<p>Abstract</p> <p>Background</p> <p>Drinking water contaminated with inorganic arsenic is associated with increased risk for different types of cancer. Paradoxically, arsenic trioxide can also be used to induce remission in patients with acute promyelocytic leukemia (APL) with a success rate of approximately 80%. A comprehensive study examining the mechanisms and potential signaling pathways contributing to the anti-tumor properties of arsenic trioxide has not been carried out.</p> <p>Methods</p> <p>Here we applied a systems biology approach to identify gene biomarkers that underlie tumor cell responses to arsenic-induced cytotoxicity. The baseline gene expression levels of 14,500 well characterized human genes were associated with the GI₅₀ data of the NCI-60 tumor cell line panel from the developmental therapeutics program (DTP) database. Selected biomarkers were tested <it>in vitro</it> for the ability to influence tumor susceptibility to arsenic trioxide.</p> <p>Results</p> <p>A significant association was found between the baseline expression levels of 209 human genes and the sensitivity of the tumor cell line panel upon exposure to arsenic trioxide. These genes were overlayed onto protein-protein network maps to identify transcriptional networks that modulate tumor cell responses to arsenic trioxide. The analysis revealed a significant enrichment for the oxidative stress response pathway mediated by nuclear factor erythroid 2-related factor 2 (NRF2) with high expression in arsenic resistant tumor cell lines. The role of the NRF2 pathway in protecting cells against arsenic-induced cell killing was validated in tumor cells using shRNA-mediated knock-down.</p> <p>Conclusions</p> <p>In this study, we show that the expression level of genes in the NRF2 pathway serve as potential gene biomarkers of tumor cell responses to arsenic trioxide. Importantly, we demonstrate that tumor cells that are deficient for NRF2 display increased sensitivity to arsenic trioxide. The results of our study will be useful in understanding the mechanism of arsenic-induced cytotoxicity in cells, as well as the increased applicability of arsenic trioxide as a chemotherapeutic agent in cancer treatment.</p

Altered HOX gene expression in human skin and breast cancer cells

Human HOX genes are expressed in a spatio-temporal fashion during embryogenesis and early development where they act as master transcriptional regulators. HOX genes are also expressed in normal adult cells, potentially in a tissue specific manner involving maintenance of the normal phenotype. In selected oncogenic transformations, mis-expression of many HOX genes have been shown, indicating an involvement of these transcriptional regulators in carcinogenesis and metastasis. Utilising quantitative real-time RT-PCR assays, the expression of 20 HOX genes and two known HOX co-factors, PBX1B and MEIS1, were analysed in human melanoma and breast cancer cell lines, comparing results against non-malignant cells. Alterations in HOX gene expression were observed for all malignant cells examined, with some dysregulated transcript levels seemingly random, and the expression of other HOX genes apparently following the same patterns in both skin and breast cancer establishment. Furthermore, HOX gene expression was correlated with the invasive capacity of the cells. The expression of the HOX co-factors PBX1B and MEIS1 showed no marked changes from the non-malignant to the malignant phenotypes, further indicating that it is dysregulated HOX gene expression, rather than dysregulated gene expression of HOX co-factors, that potentially commit the cell to re-differentiate and undergo oncogenic transformation

Thioredoxin/thioredoxin reductase system involvement in cerebellar granule cell apoptosis

The involvement of thioredoxin/thioredoxin reductase system has been investigated in cerebellar granule cells (CGCs), a cellular system in which neurons are induced in apoptosis by the physiological stimulus of lowering extracellular potassium. Clarifying the sequence of events that occur during apoptosis is a critical issue as it can lead to the identification of those key events that, if blocked, can slow down or reverse the death process. The results reported in this work show that TrxR is involved in the early phase of CGC apoptosis with an increase in activity that coincides with the increased expression of the TrxR1 isoform and guarantees the maintenance of adequate level of Trx in its reduced, active form. However, in late apoptosis, when about 50 % of cells are dead, partial proteolysis of TrxR1 by calpain occurs and the reduction of TrxR1 mRNA, together with the overall decrease in TrxR activity, contribute to increase the levels of the oxidized form of Trx. When the reduced form of Trx is externally added to apoptotic cultures, a significant reduction in cell death is achieved confirming that a well-functioning thioredoxin/thioredoxin reductase system is required for survival of CGCs