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Decreased ability of cells overexpressing MYC proteins to reduce peroxide and hydroperoxides
S Tuttle1, R Muschel, E Bernhard
1University of Pennsylvania School of Medicine, Philadelphia 19104, USA.
Abstract:
Hydroperoxides are reduced in mammalian cells by a coupled enzyme pathway involving glutathione peroxidase, glutathione reductase and the oxidative limb of the pentose cycle. Oxidation of glucose-6-phosphate by the pentose cycle yields two molecules of NADPH, which can reduce two hydroperoxide molecules to the corresponding alcohol. Rat embryo fibroblasts (REF) transfected with v-myc reduce hydroperoxides slower than the primary REF cell line-measured both as real time peroxide loss and as increased glucose oxidation via the pentose cycle. The v-myc transfected cell line is 50-fold more sensitive to the toxic effects of tBu-OOH. The decreased reduction of peroxides by v-myc transfected cells is not due to changes in the activities of GSH reductase or the enzymes of the oxidative pentose cycle, since diamide stimulates PC activity equally in both cell lines. In addition, the activities of these enzymes, measured in cell homogenates do not differ significantly between the cell lines. Also total GSH peroxidase activity, assayed in cell homogenates, is not significantly different between the cell lines. Two human tumour cell lines which overexpress myc family proteins: NCI-H69, a small-cell lung cancer line which expresses elevated levels of N-myc, and HL-60 cells which overexpress c-myc, also exhibit low levels of pentose cycle stimulation in the presence of tBu-OOH, and a decreased capacity to reduce hydrogen peroxide by peroxide electrode.
Insights
The v-myc oncogene impairs mammalian cell hydroperoxide reduction by slowing the pentose phosphate pathway, increasing sensitivity to oxidative stress. This impacts cancer cell biology and drug development.
Area of Science:
- Cellular Biology
- Biochemistry
- Oncology
Background:
- Mammalian cells detoxify hydroperoxides via a pathway involving glutathione peroxidase, glutathione reductase, and the pentose phosphate pathway.
- The pentose phosphate pathway generates NADPH, crucial for reducing hydroperoxides to alcohols.
Purpose of the Study:
- To investigate the impact of v-myc oncogene expression on hydroperoxide reduction in rat embryo fibroblasts (REF).
- To explore the relationship between v-myc, pentose phosphate pathway activity, and cellular sensitivity to oxidative stress.
Main Methods:
- Comparison of hydroperoxide reduction rates and pentose cycle activity between primary REF and v-myc transfected REF.
- Enzyme activity assays for glutathione reductase, pentose cycle enzymes, and glutathione peroxidase in cell homogenates.
- Assessment of cellular sensitivity to tert-butyl hydroperoxide (tBu-OOH).
- Evaluation of pentose cycle stimulation by diamide and tBu-OOH in human tumor cell lines overexpressing myc proteins.
Main Results:
- v-myc transfected REF exhibited slower hydroperoxide reduction and reduced pentose cycle activity compared to primary REF.
- v-myc transfected cells showed a 50-fold increased sensitivity to tBu-OOH toxicity.
- Enzyme activities of glutathione reductase, pentose cycle enzymes, and glutathione peroxidase were not significantly different between cell lines.
- Human tumor cell lines overexpressing myc proteins (NCI-H69, HL-60) displayed diminished pentose cycle stimulation and lower hydrogen peroxide reduction capacity.
Conclusions:
- v-myc oncogene expression impairs the cellular capacity to reduce hydroperoxides, likely by affecting pentose phosphate pathway regulation.
- This impairment leads to increased sensitivity to oxidative stress, a potential vulnerability in myc-driven cancers.
- The findings suggest a link between myc oncogenes, redox balance, and cancer cell survival.