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The SIX1 oncoprotein mediates aberrant uterine basal cell development following neonatal exposure to diethylstilbestrol
Citation:
Suen, A., W. Jefferson, C. Williams, AND C. Wood. The SIX1 oncoprotein mediates aberrant uterine basal cell development following neonatal exposure to diethylstilbestrol. Society of Toxicology Annual Conference, San Antonio, Texas, March 11 - 15, 2018.
Impact/Purpose:
Early-life exposure to estrogenic chemicals in the environment has been associated with increased susceptibility to cancer and other adverse reproductive health outcomes later in life. Biological pathways driving these effects are still largely unknown. This case study investigated the role of a developmental protein called SIX1 as a molecular driver and biomarker of latent estrogenic effects in a mouse model of early-life estrogen exposure. The ultimate goal of this work is to enable more rapid identification of AOPs for cancer incorporating life stage susceptibility.
Description:
Aberrant cellular differentiation early in life can contribute to increased cancer risk later in life. In a classic model of this effect, female mice exposed neonatally to the synthetic estrogen diethylstilbestrol (DES) have a high incidence of uterine carcinoma. These cancers are strongly associated with an abnormal population of basal cells in the uterine glands that represent the earliest and most prominent precursor lesion. Sine oculis homeobox homolog 1 (SIX1) is an oncoprotein that is persistently upregulated in the uteri of mice following neonatal DES exposure and localizes to uterine basal cells within all neoplastic lesions. Based on this evidence, we hypothesize that SIX1 is necessary for uterine basal cell differentiation and carcinogenesis following neonatal DES exposure. To test this hypothesis, a conditional knockout mouse model was generated in which floxed Six1 was excised in the uterus using the PgR-cre transgene. Ablation of SIX1 was confirmed by real time RT-PCR and immunoblotting on the final day of exposure and in adults. Uterine morphology and glandular development was normal following SIX1 deletion. At 12 months of age, uterine carcinoma was observed in 9/11 (81%) of DES-exposed wildtype (Six1+/+) mice and 9/12 (75%) of DES-exposed SIX1 conditional knockout (Six1d/d) mice but not in control groups, indicating that SIX1 is not required for DES-induced uterine carcinogenesis. However, DES-exposed Six1d/d mice exhibited distinct differences in differentiation. Abnormal basal cells expressing Keratin 14 (K14) were present throughout the uterine body and horns of DES-exposed Six1+/+ mice but restricted to the uterine body of the DES-exposed Six1d/d mice. Quantitative image analysis of K14 labeling indicated a >10-fold decrease in the uterine horns of DES-exposed Six1d/d mice compared to DES-exposed Six1+/+ mice, but was similar to controls. Microarray analysis revealed 470 differentially expressed genes (P<0.05, 1.5-fold, Intensity Cutoff ≤ 100) between DES-exposed uteri from Six1+/+ and Six1d/d mice, suggesting that DES-induced SIX1 expression contributes to altered gene expression. Furthermore, several basal cell-related genes, including P63, Krt5, Krt14, and Krt15, were downregulated in DES-exposed Six1d/d mice, consistent with morphologic changes. These data indicate that SIX1 acts as a key cellular differentiation factor following early-life estrogen exposure. This abstract does not reflect U.S. EPA policy.