Oral exposure to high concentrations of
hexavalent chromium [
Cr(VI)] induces intestinal redox changes, villus cytotoxicity, crypt
hyperplasia, and intestinal
tumors in mice. To assess the effects of
Cr(VI) in a cell model relevant to the intestine, undifferentiated (proliferating) and differentiated (confluent) Caco-2 cells were treated with
Cr(VI),
hydrogen peroxide or
rotenone for 2-24 hours. DNA damage was then assessed by nuclear staining intensity of
8-hydroxydeoxyguanosine (8-OHdG) and phosphorylated
histone variant H2AX (γ-H2AX) measured by high content analysis methods. In undifferentiated Caco-2, all three chemicals increased 8-OHdG and γ-H2AX staining at cytotoxic concentrations, whereas only 8-OHdG was elevated at non-cytotoxic concentrations at 24 hr. Differentiated Caco-2 were more resistant to cytotoxicity and DNA damage than undifferentiated cells, and there were no changes in apoptotic markers p53 or
annexin-V. However,
Cr(VI) induced a dose-dependent translocation of the unfolded protein response
transcription factor ATF6 into the nucleus. Micronucleus (MN) formation was assessed in CHO-K1 and A549 cell lines.
Cr(VI) increased MN frequency in CHO-K1 only at highly cytotoxic concentrations. Relative to the positive control
Mitomycin-C,
Cr(VI) only slightly increased MN frequency in A549 at mildly cytotoxic concentrations. The results demonstrate that
Cr(VI) genotoxicity correlates with cytotoxic concentrations, and that H2AX phosphorylation occurs at higher concentrations than oxidative DNA damage in proliferating Caco-2 cells. The findings suggest that in vitro genotoxicity of
Cr(VI) is primarily oxidative in nature at low concentrations. Implications for in vivo intestinal toxicity of
Cr(VI) will be discussed.