Tamoxifen-Ethynylestradiol Cotreatment of C57BL/6 Mice: Gross, Morphometric and Gene Expression Analysis of Uterine Responses.

Cora J. Fong1,2,3, Lyle D. Burgoon1,2,3, Kurt J. Williams2,4, Agnes L. Forgacs1,2 and Timothy R. Zacharewski1,2,3

1Department of Biochemistry & Molecular Biology
2National Food Safety and Toxicology Center
3Center for Integrative Toxicology
4Department of Pathobiology and Diagnostic Investigation
Michigan State University, East Lansing, MI, 48824, USA

Tamoxifen (TAM), the primary treatment for estrogen receptor positive (ER) breast cancer, has been
associated with an increased incidence of endometrial cancer in post-, but not pre-menopausal women.
TAM elicits a partial ER-mediated uterotrophic response in immature rodents when compared to ethynyl
estradiol (EE), a potent agonist. Moreover, cotreatment with 1000 µg/kg TAM antagonizes the
uterotrophic effect induced by 30 µg/kg EE. To further investigate the uterotrophic antiestrogenicity of
TAM, immature, ovariectomized C57BL/6 mice were treated with a single oral dose of EE, TAM, EE
plus TAM or vehicle (sesame oil) and uteri were harvested at 2, 4, 8, 12, 18, 24 hrs or after three daily
treatments (3x24 hrs). Significant increases in uterine wet weight (UWW) were observed at 18 hrs for
EE, TAM, and EE plus TAM. However, EE plus TAM induction of UWW was significantly less when
compared to EE-induced uterotrophy at 3x24 hrs. This inhibitory effect is reflected by the absence of an
EE-induction of luminal circumference, yet the hallmark estrogenic increase in luminal epithelial cell
height was not affected. cDNA microarray analysis using a 2x2 factorial study design, identified 310
genes differentially expressed following EE treatment. However, only a subset of EE elicited changes in
gene expression were found to be attenuated by EE plus TAM that correlate with the antagonistic
physiological response. These data indicate that TAM antagonizes EE-induced uterine wet weight by
inhibiting specific EE-induced genes, and therefore its primary mechanism of antiestrogenic activity is
not the inhibition of E2 binding to the ER.

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