© 2000 by Oxford University Press
Journal of the National Cancer Institute, Vol. 92, No. 23, 1926-1934,
December 6, 2000
© 2000 Oxford University Press
REPORT |
Oxidative Stress and AP-1 Activity in Tamoxifen-Resistant Breast Tumors In Vivo
Affiliations of authors: R. Schiff, P. Reddy, S. G. Hilsenbeck, R. Herrera, G. C. Chamness, S. A. W. Fuqua, P. H. Brown, C. K. Osborne, The Breast Center and the Departments of Molecular and Cellular Biology and Medicine at Baylor College of Medicine, Houston, TX; M. Ahotupa, MCA Research Laboratory, Department of Physiology, University of Turku, Finland; E. Coronado-Heinsohn, M. Grim, S. Deneke (Department of Medicine), R. Lawrence (Institute for Drug Development), The University of Texas Health Science Center, San Antonio.
Correspondence to: C. Kent Osborne, M.D., The Breast Center at Baylor College of Medicine, 1 Baylor Plaza, MS: 600, Houston, TX 77030 (e-mail: kosborne{at}bcm.tmc.edu).
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 NH2-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.
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