Differential Effects of Chromium(VI) on Constitutive and Inducible Gene Expression in Chick Embryo Liver In Vivo and Correlation With Chromium(VI)‐Induced DNA Damage

Abstract

The effect of DNA damage induced by the carcinogen chromium(VI) on the function of DNA as a template for transcription of constitutive and inducible genes was examined in chick embryo liver in vivo. Changes in gene expression, determined using solution hybridization and northern blot analyses to measure steady-state mRNA levels and a nuclear run-off assay to measure gene transcription rates, were compared to chromium-DNA binding and to chromium(VI)-induced DNA damage as previously measured by DNA alkaline elution. Chromium(VI) treatment had little or no effect on either the steady-state mRNA levels or the transcription rates of the constitutively expressed genes for albumin, conalbumin (avian transferrin), or B-actin. In contrast, chromium(VI) treatment had significant but opposite effects on the basal and drug-inducible expression of 5-aminolevulinate synthase and cytochrome PB₁ P450. The changes in steady-state expression of these two inducible genes were similar to the changes in transcription rate, indicating that the effects of chromium were principally transcriptional. Chromium(VI) treatment increased the basal expression of both inducible genes four- to fivefold at maximum, and the time course of this effect was similar to the time course for chromium(VI)-induced DNA damage and repair. In contrast, chromium(VI) pretreatment suppressed by 60–70% at maximum the subsequent induction of these genes by glutethimide, a phenobarbital analog, and the time course of this effect also corresponded to that of chromium(VI)-induced DNA damage and repair. The time courses of the changes in expression of these genes were bimodal, with the second peak corresponding closely to that of chromium(VI)-induced DNA cross-links. However, the first peak occurred during a period when no DNA cross-links or strand breaks were detectable by alkaline elution, although significant levels of chromium were bound to DNA. This suggests that chromium(VI), like cisplatin, may initially produce a DNA monoadduct that subsequently leads to DNA cross-link formation and that both types of chromium(VI)-induced lesions have a significant effect on the expression of targeted genes.