Genome-wide gene expression effects in B6C3F1 mouse intestinal epithelia following 7 and 90 days of exposure to hexavalent chromium in drinking water

Abstract

Chronic administration of high doses of hexavalent chromium [Cr(VI)] as sodium dichromate dihydrate (SDD) elicits alimentary cancers in mice. To further elucidate key events underlying tumor formation, a 90-day drinking water study was conducted in B6C3F1 mice. Differential gene expression was examined in duodenal and jejunal epithelial samples following 7 or 90days of exposure to 0, 0.3, 4, 14, 60, 170 or 520mg/L SDD in drinking water. Genome-wide microarray analyses identified 6562 duodenal and 4448 jejunal unique differentially expressed genes at day 8, and 4630 and 4845 unique changes, respectively, in the duodenum and jejunum at day 91. Comparative analysis identified significant overlap in duodenal and jejunal differential gene expression. Automated dose-response modeling identified >80% of the differentially expressed genes exhibited sigmoidal dose-response curves with EC(50) values ranging from 10 to 100mg/L SDD. Only 16 genes satisfying the dose-dependent differential expression criteria had EC(50) values <10mg/L SDD, 3 of which were regulated by Nrf2, suggesting oxidative stress in response to SDD at low concentrations. Analyses of differentially expressed genes identified over-represented functions associated with oxidative stress, cell cycle, lipid metabolism, and immune responses consistent with the reported effects on redox status and histopathology at corresponding SDD drinking water concentrations. Collectively, these data are consistent with a mode of action involving oxidative stress and cytotoxicity as early key events. This suggests that the tumorigenic effects of chronic Cr(VI) oral exposure likely require chronic tissue damage and compensatory epithelial cell proliferation.

Assessment of genotoxic potential of Cr(VI) in the mouse duodenum: An in silico comparison with mutagenic and nonmutagenic carcinogens across tissues

“…As part of our research into the MOA of Cr(VI)-induced intestinal carcinogenesis, in vivo micronucleus formation and k-ras mutations were assessed in duodenal tissue sections of mice exposed to Cr(VI) up to 90 days, and were found to be negative (Harris et al, 2012;O'Brien et al, in preparation). Toxicogenomic evaluation of responses to Cr(VI) in the mouse small intestine indicated activation of Nrf2 signaling at relatively low exposure concentrations (Kopec et al, 2012a), consistent with clear alteration in cellular redox status in similarly treated mice (Thompson et al, 2011b). Several genes involved in DNA repair were elevated by day 8 of exposure to carcinogenic concentrations of Cr(VI), and functional analysis indicated enrichment of DNA repair pathways at the highest Cr(VI) concentrations (Kopec et al, 2012a).…”

“…drinking water). To this end, a comprehensive research program was conducted to gather critical data needed to inform the MOA underlying Cr(VI)-induced intestinal carcinogenesis (Kopec et al, 2012a;Thompson et al, 2011a,b).…”

“…Toxicogenomic evaluation of responses to Cr(VI) in the mouse small intestine indicated activation of Nrf2 signaling at relatively low exposure concentrations (Kopec et al, 2012a), consistent with clear alteration in cellular redox status in similarly treated mice (Thompson et al, 2011b). Several genes involved in DNA repair were elevated by day 8 of exposure to carcinogenic concentrations of Cr(VI), and functional analysis indicated enrichment of DNA repair pathways at the highest Cr(VI) concentrations (Kopec et al, 2012a). Beyond transcriptional and functional analyses, it is of interest to scientists and risk assessors to examine whether the genomic signature/profile of Cr(VI) is similar to that of known mutagens.…”

Mechanistic Insights from the NTP Studies of Chromium

“…Significant decreases in the ratio of reduced/oxidized glutathione were reported in the oral mucosa and jejunum of the rats and the duodenum and jejunum of the mice, suggesting upregulation of a response to Cr(VI)-induced oxidative stress in these cells. As part of these studies, whole genome microarrays were used to identify gene expression profiles in duodenum and jejunum of female B6C3F1 mice (Kopec, Kim, et al 2012) and F344/N rats (Kopec, Thompson, et al 2012). In female mice, 16 genes with dose-dependent differential expression in duodenal epithelial samples were identified.…”

Integration of mechanistic and pharmacokinetic information to derive oral reference dose and margin‐of‐exposure values for hexavalent chromium

“…To inform the MOA and risk assessment of oral Cr(VI) exposure, a series of studies were conducted beginning with an overall proposed MOA (Thompson, Haws, Harris, Gatto, & Proctor, 2011), and subsequent 90‐day toxicity studies (Thompson, Proctor, et al, 2011; Thompson et al, 2012), transcriptomic analyses (Kopec et al, 2012; Kopec, Thompson, Kim, Forgacs, & Zacharewski, 2012; Rager et al, 2017), genotoxicity studies (O'Brien et al, 2013; Thompson, Seiter, et al, 2015; Thompson, Wolf, et al, 2015; Thompson, Young, et al, 2015; Thompson et al, 2017), as well as ex vivo gastric reduction studies and pharmacokinetic modeling (De Flora et al, 2016; Kirman et al, 2012; Kirman et al, 2013; Kirman et al, 2016; Proctor et al, 2012). Other genotoxicity studies were conducted in response to early drafts of the NTP 2‐year cancer bioassay (De Flora et al, 2006; De Flora et al, 2008).…”