Oxidative stress and AP-1 activity in tamoxifen-resistant breast tumors in vivo.
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.


CONCLUSIONS
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.
Schiff
R
R
The Breast Center and the Department of Molecular and Cellular Biology and Medicine at Baylor College of Medicine, Houston, TX, USA.
Reddy
P
P
Ahotupa
M
M
Coronado-Heinsohn
E
E
Grim
M
M
Hilsenbeck
S G
SG
Lawrence
R
R
Deneke
S
S
Herrera
R
R
Chamness
G C
GC
Fuqua
S A
SA
Brown
P H
PH
Osborne
C K
CK
D000818:Animals; D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology* / Q000627:therapeutic use; D015153:Blotting, Western; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D015500:Chloramphenicol O-Acetyltransferase / Q000032:analysis; D004273:DNA, Neoplasm / Q000187:drug effects; D004195:Disease Models, Animal; D019008:Drug Resistance, Neoplasm; D020847:Estrogen Receptor Modulators / Q000494:pharmacology* / Q000627:therapeutic use; D005260:Female; D005982:Glutathione Transferase / Q000378:metabolism; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases; D015227:Lipid Peroxidation; D051379:Mice; D008819:Mice, Nude; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D018384:Oxidative Stress; D010427:Pentose Phosphate Pathway; D010641:Phenotype; D010766:Phosphorylation; D016755:Proto-Oncogene Proteins c-jun / Q000378:metabolism; D013482:Superoxide Dismutase / Q000378:metabolism; D013629:Tamoxifen / Q000494:pharmacology* / Q000627:therapeutic use; D018808:Transcription Factor AP-1 / Q000378:metabolism*; D014158:Transcription, Genetic / Q000187:drug effects; D014183:Transplantation, Heterologous
D018931:Antineoplastic Agents, Hormonal; D004273:DNA, Neoplasm; D020847:Estrogen Receptor Modulators; D016755:Proto-Oncogene Proteins c-jun; D018808:Transcription Factor AP-1; D013629:Tamoxifen; D013482:Superoxide Dismutase; D015500:Chloramphenicol O-Acetyltransferase; D005982:Glutathione Transferase; D048031:JNK Mitogen-Activated Protein Kinases; D020928:Mitogen-Activated Protein Kinases