Thompson CM, Proctor DM, Suh M, Wolf JC, Haws LC, Seiter JM, Chappell MA, Harris MA. 2014. X-ray fluorescence microspectroscopic analysis of duodenal mucosae following Cr(VI) exposure in drinking water. Presented at the Society of Toxicology’s 53rd Annual Meeting, March 23-27, Phoenix, AZ.
Mutagenic and nonmutagenic modes of action (MOAs) have been proposed to explain duodenal tumors in B6C3F1 mice following oral exposure to hexavalent chromium [Cr(VI)]. Evidence against a mutagenic MOA includes an absence of Cr(VI)-induced crypt micronuclei or kras codon 12 mutations in the duodenum after exposure to ≤180 mg/L Cr(VI). Herein, we show that these findings can be explained, in part, by the distribution of Cr within the duodenal epithelium. Specifically, X-ray fluorescence microscopy was conducted on formalin-fixed, paraffin-embedded duodenal sections from mice exposed to 0 or 180 mg/L Cr(VI) for 90 days. Sections from treated mice indicated much higher Cr fluorescence in the duodenal villi than in adjacent crypts on the same section. Cr speciation analysis indicated that all Cr in the tissue was in the trivalent form, Cr(III). Immunostaining of phosphorylated H2AX (γ-H2AX), a marker of DNA double strand breakage, was elevated (relative to control) in the duodenal villi of mice exposed to 180 mg/L Cr(VI) for 90 days. Duodenal crypts from treated mice were visibly taller than untreated mice (190.5 vs 98.3 μm; p ≤ 0.001) due to the increase in the number of crypt enterocytes visible in cross-sections (63.9 vs 38.4, p ≤ 0.01). Despite the increase in crypt enterocytes due to proliferation, the overall scoring of crypt γ-H2AX staining did not differ between treated and untreated mice. The absence of treatment-related changes in γ-H2AX immunostaining, micronuclei, or kras mutations in the mouse duodenum is consistent with the low levels of Cr fluorescence in the crypt. These data support a nonmutagenic MOA whereby Cr(VI) enters anion transporters expressed on villous enterocytes, induces dose-dependent cytotoxicity and subsequent regenerative crypt hyperplasia. Such chronic compensatory crypt proliferation increases the lifetime risk of intestinal carcinogenesis.