Chronic exposure to high concentrations of
hexavalent chromium [
Cr(VI)] as
sodium dichromate dihydrate (SDD) in
drinking water induces duodenal
tumors in mice, but the mode of action (MOA) for these
tumors has been a subject of scientific debate. To evaluate the
tumor-site-specific genotoxicity and cytotoxicity of SDD in the mouse small intestine, tissue pathology and cytogenetic damage were evaluated in duodenal crypt and villus enterocytes from B6C3F1 mice exposed to 0.3-520mg/L SDD in
drinking water for 7 and 90 days. Allele-competitive blocker PCR (ACB-PCR) was used to investigate the induction of a sensitive,
tumor-relevant mutation, specifically in vivo K-Ras
codon 12
GAT mutation, in scraped duodenal epithelium following 90 days of
drinking water exposure. Cytotoxicity was evident in the villus as disruption of cellular arrangement, desquamation, nuclear atypia and blunting. Following 90 days of treatment, aberrant nuclei, occurring primarily at villi
tips, were significantly increased at ≥60mg/L SDD. However, in the crypt compartment, there were no dose-related effects on mitotic and apoptotic indices or the formation of aberrant nuclei indicating that
Cr(VI)-induced cytotoxicity was limited to the villi.
Cr(VI) caused a dose-dependent proliferative response in the duodenal crypt as evidenced by an increase in crypt area and increased number of crypt enterocytes. Spontaneous K-Ras
codon 12
GAT mutations in untreated mice were higher than expected, in the range of 10(-2) to 10(-3); however no treatment-related trend in the K-Ras
codon 12
GAT mutation was observed. The high spontaneous background K-Ras mutant frequency and
Cr(VI) dose-related increases in crypt enterocyte proliferation, without dose-related increase in K-Ras mutant frequency, micronuclei formation, or change in mitotic or apoptotic indices, are consistent with a lack of genotoxicity in the crypt compartment, and a MOA involving accumulation of mutations late in
carcinogenesis as a consequence of sustained regenerative proliferation.