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Oxidative stress and AP-1 activity in tamoxifen-resistant breast tumors in vivo
1The Breast Center and the Department of Molecular and Cellular Biology and Medicine at Baylor College of Medicine, Houston, TX, USA.
Background:
Most breast cancers, even those that are initially responsive to tamoxifen, ultimately become resistant. The molecular basis for this resistance, which in some patients is thought to involve stimulation of tumor growth by tamoxifen, is unclear. Tamoxifen induces cellular oxidative stress, and because changes in cell redox state can activate signaling pathways leading to the activation of activating protein-1 (AP-1), we investigated whether tamoxifen-resistant growth in vivo is associated with oxidative stress and/or activation of AP-1 in a xenograft model system where resistance is caused by tamoxifen-stimulated growth.
Methods:
Control estrogen-treated, tamoxifen-sensitive, and tamoxifen-resistant MCF-7 xenograft tumors were assessed for oxidative stress by measuring levels of antioxidant enzyme (e.g., superoxide dismutase [SOD], glutathione S-transferase [GST], and hexose monophosphate shunt [HMS]) activity, glutathione, and lipid peroxidation. AP-1 protein levels, phosphorylated c-jun levels, and phosphorylated Jun NH(2)-terminal kinase (JNK) levels were examined by western blot analyses, and AP-1 DNA-binding and transcriptional activities were assessed by electrophoretic mobility shift assays and a reporter gene system. All statistical tests are two-sided.
Results:
Compared with control estrogen-treated tumors, tamoxifen resistant tumors had statistically significantly increased SOD (more than threefold; P=.004) and GST (twofold; P=.004) activity and statistically significantly reduced glutathione levels (greater than twofold; P<.001) and HMS activity (10-fold; P<.001). Lipid peroxides were not significantly different between control and tamoxifen-resistant tumors. We observed no differences in AP-1 protein components or DNA-binding activity. However, AP-1-dependent transcription (P=.04) and phosphorylated c-Jun and JNK levels (P<.001) were statistically significantly increased in the tamoxifen-resistant tumors.
Conclusion:
Our results suggest that the conversion of breast tumors to a tamoxifen-resistant phenotype is associated with oxidative stress and the subsequent antioxidant response and with increased phosphorylated JNK and c-Jun levels and AP-1 activity, which together could contribute to tumor growth.
Insights
Tamoxifen resistance in breast cancer is linked to oxidative stress and increased AP-1 activity. This suggests that targeting oxidative stress and AP-1 signaling may help overcome tamoxifen resistance.
Area of Science:
- Oncology
- Molecular Biology
- Biochemistry
Background:
- Tamoxifen resistance is a major challenge in breast cancer treatment.
- The molecular mechanisms driving tamoxifen resistance, particularly tamoxifen-stimulated tumor growth, remain unclear.
- Tamoxifen can induce oxidative stress, potentially activating signaling pathways like AP-1.
Purpose of the Study:
- To investigate the association between tamoxifen-resistant breast cancer growth and oxidative stress.
- To determine if tamoxifen resistance involves the activation of activating protein-1 (AP-1) signaling.
- To explore these mechanisms in a xenograft model of tamoxifen resistance.
Main Methods:
- Assessment of oxidative stress markers (antioxidant enzyme activity, glutathione, lipid peroxidation) in xenograft tumors.
- Western blot analysis of AP-1 components (c-Jun, JNK) and their phosphorylation status.
- Electrophoretic mobility shift assays and reporter gene systems to evaluate AP-1 DNA-binding and transcriptional activity.
Main Results:
- Tamoxifen-resistant tumors showed significantly increased superoxide dismutase (SOD) and glutathione S-transferase (GST) activity.
- Reduced glutathione levels and hexose monophosphate shunt (HMS) activity were observed in resistant tumors.
- While AP-1 protein levels and DNA-binding were unchanged, AP-1-dependent transcription and levels of phosphorylated c-Jun and JNK were significantly elevated.
Conclusions:
- Tamoxifen resistance in breast tumors is associated with oxidative stress and an increased antioxidant response.
- Elevated phosphorylated JNK and c-Jun levels, leading to increased AP-1 activity, accompany tamoxifen resistance.
- These molecular changes may collectively contribute to tamoxifen-resistant tumor growth.