Submitted to: Society of Toxicology
Publication Type: Abstract only
Publication Acceptance Date: 12/15/2006
Publication Date: 3/15/2007
Citation: Singhal, R., Shankar, K., Badger, T.M., Ronis, M.J. 2007. Estrogenic status modulates dmba-mediated hepatic gene expression: Microarray-based analysis [abstract]. The Toxicologist. 96(1):159. Program No. 772. Interpretive Summary: Estrogens are known to play crucial role in the onset of cancer. The objective of this study was to examine the influence of estradiol (E2) on 7,12 dimethylbenz(a)anthracene (DMBA), a compound in cooked meat that binds to a protein called the aryl hydrocarbon receptor, and this interaction mediates changes on gene expression (how genes are switched on or off) thought to be responsible for cancer formation. To elucidate the overall changes in the hepatic gene expression microarray analysis was performed on the RNA taken from the livers of the rats treated with DMAB and/or E2. Using this technique we observed that there are a number of genes whose expression changes when DMBA or E2 is administered alone. Interestingly, on the co-treatment with E2 and DMBA we observed not only changes in the expression of genes changed by either treatment, but also various interactions between the two treatments. By performing real-time PCR and chromatin immunoprecipitation we found that the CYP1A1 gene is induced more in the presence of E2 + DMBA as compared to DMBA alone. Another gene of interest, AhR, was found to be expressed more in the presence of both E2 and DMBA, while either treatment did not have any effect on the expression of this gene. These analyses suggest increased DMBA-mediated mutagen activation, and hence carcinogenesis, in the presence of E2. These data expand our understanding of the synergistic, additive, and interactive effects of E2 on compounds that cause cancer. This also suggests that pre-menopausal women and women on estrogen replacement therapy are more prone to develop cancer as compared to post-menopausal women.
Technical Abstract: Estrogenic status in women influences the metabolism and toxicity of polycyclic aromatic hydrocarbons (PAH). The objective of this study was to examine the influence of estradiol (E2) on 7,12 dimethylbenz(a)anthracene (DMBA), a ligand for aryl hydrocarbon receptor, mediated changes on gene expression. Sprague-Dawley rats were ovariectomized at PND50 and infused with E2 using osmotic pumps for 14d at 5'g/kg/day. Changes in hepatic gene expression were examined 24h following gavage with 50mg/kg DMBA. To understand the mechanism of DMBA-mediated hepato-carcinogenicity in the presence of E2, microarray analysis (Rat 230 2.0 Affymetrix-GeneChip) was performed. Two hundred twenty-five genes were down-regulated, while 54 genes were up-regulated significantly (+/-2 fold, P<0.05) by DMBA treatment. Expression of 520 genes was altered by the combination of E2 and DMBA. Hierarchical clustering revealed that expression of 24 genes (including egf, leptin receptor, tgfbr1) was reversed by the co-treatment of E2 and DMBA compared to DMBA alone. Gene ontology associated regulation of 303 genes involved, for example, in transcriptional regulation (including ahr, hepatic nuclear factor4, sp1, igfbp3), protein modification, xenobiotic and stress response, uniquely with the interaction of E2 and DMBA, but not by the either treatment alone. Real-time RT-PCR confirmed a significantly (P<0.05) higher hepatic CYP1A1, a pro-carcinogen metabolizing enzyme, mRNA expression in the groups co-treated with E2 and DMBA as compared to DMBA alone, suggesting increased DMBA-mediated mutagen activation, and hence carcinogenesis, in the presence of E2. These data expand our understanding of the synergistic, additive and interactive effects of E2 on PAHs-mediated carcinogenicity.