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Some forms of hexavalent chromium [Cr(VI)] are known to cause damage to respiratory tract tissue, and are thought to be human lung carcinogens. Because Cr(VI) is mutagenic and carcinogenic at doses that evoke cell toxicity, the objective of these experiments was to examine the effect of Cr(VI) on the growth, survival, and mode of cell death in normal human lung fibroblasts (HLF cells). DNA adduct formation was monitored as a marker for bioavailability of genotoxic chromium. We also examined the modulation of these endpoints by vitamins C and E. Long-term Cr(VI) exposures were employed, which decreased clonogenic cell survival by 25% to 95% in a dose-dependent manner. The predominant cellular response to Cr(VI) was growth arrest. We found that Cr(VI) caused up to 20% of HLF cells to undergo apoptosis, and documented apoptotic morphology and the phagocytosis of apoptotic bodies by neighboring cells. P53 levels increased 4- to 6-fold in chromium-treated cells. In contrast with previous studies using CHO cells, the present study using HLFs found that pretreatment with either vitamin C or E did not exhibit a significant effect on Cr-induced apoptosis or clonogenic survival. In addition, pretreatment with vitamin C did not affect the p53 induction observed after chromium treatment. Neither vitamin had any effect on Cr-DNA adduct formation. These data indicate that although pretreatment with vitamin C or E alters the spectrum of cellular and/or genetic lesions induced by chromium(VI), neither vitamin altered the initiation or progression of apoptosis in diploid human lung cells.

A novel method is described for the sensitive detection of chromium-DNA adducts. Chromium-DNA adducts were determined in 1 microgram of DNA from normal human lung fibroblasts exposed to sodium chromate using microscale flow injection analysis with a direct injection high-efficiency nebulizer and inductively coupled plasma mass spectrometry detection. The frequency of Cr-DNA adducts increased in a dose-dependent sigmoidal manner, indicating saturation and toxicity. The low detection limits (on the order of parts per trillion) allows the detection of as few as two Cr adducts per 10,000 bases, which, coupled with the small DNA sample requirement, makes this technique suitable for measuring metal-DNA adducts as biomarkers of exposure to toxic and carcinogenic metals such as Cr, in cultured cells, animals, and humans.

Some compounds of hexavalent chromium are well-established carcinogens. Chromium enters mammalian cells in the hexavalent form and is reduced to chromium (III). Treatment of purified DNA with chromium (III) produces DNA-DNA interstrand crosslinks (DDC) which obstruct the progression of DNA polymerases in vitro. DDC were also detected in chromate-treated cultured normal human lung cells using the renaturing agarose gel electrophoresis (RAGE) assay and correlated with base-specific inhibition of DNA replication. Curiously, DDC have gone undetected in studies of cultured cells using the alkaline elution (AE) technique, whereas chromium-mediated DNA-protein crosslinks (DPC) were readily detected by AE. We tested the hypothesis that AE conditions [60 mM tetraethyl ammonium hydroxide (TEA), 20 mM EDTA, pH 12.6, for 16 h at room temperature] dissociate DDC but not DPC using chromium(III)-treated plasmid DNA and the RAGE assay. Dose-dependent chromium-induced DDC were unaffected by TEA (pH 11.8) alone or by more rigorous alkaline denaturation conditions (200 mM NaOH, pH 13.5, for 16 h). DDC were, however, completely disrupted by EDTA (pH 12.6) alone or the combination of TEA and EDTA (pH 12.6). In contrast, DPC remained largely intact under these conditions. Therefore, past AE-based studies which have failed to detect chromium-induced DDC do not prove the absence of this lesion. AE may not be suitable for detecting DDC induced by EDTA-chelatable agents such as metals.